Converter blowing method

Abstract

To provide a converter blowing method improved in dephosphorization capacity more than conventional methods, when performing first dephosphorization blowing, a first intermediate slag discharging, a second dephosphorization blowing, a second intermediate slag discharging, and a decarburization blowing, with one converter.SOLUTION: In a converter blowing method carried out in the following order: Step 1 (charging molten iron into the converter), Step 2 (first dephosphorization blowing), Step 3 (first intermediate slag discharge), Step 4 (second dephosphorization blowing), Step 5 (second intermediate slag discharge), and Step 6 (decarburization blowing), in Step 2, use CaO-containing flux and Al2O3 source as flux, and the slag composition at the end of Step 2 (calculated value) should be CaO / SiO2 (mass ratio)=1.7 to 2.3, %Al2O3=2 to 16 mass%. The intermediate slag rate for the total of Step 3 and Step 5 should be adjusted to ensure that the intermediate slag rate, as indicated by a formula (1), is 65% or more.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a converter blowing method for refining steel.

Background Art

[0002] In an integrated steelmaking process of hot metal, hot metal tapped from a blast furnace contains high concentrations of C, Si, P, and S as impurity components. These impurities are removed by refining the molten steel in the steelmaking process. Among the impurity components, C, Si, and P are refined and removed by refining using a top-bottom blown converter. In a top-bottom blown converter, molten iron is refined by blowing bottom blowing gas from the bottom of the converter while blowing oxygen from an upper blowing lance at the upper part of the converter.

[0003] After charging main raw materials such as hot metal into the converter, in converter blowing, oxygen is blown at high speed from the upper blowing lance. At this time, Si in the charged hot metal is oxidized and removed, and then the dephosphorization reaction proceeds. Thereafter, the dephosphorization refining efficiency can be increased by separating the slag with a high phosphorus concentration from the hot metal. Further thereafter, for the purpose of removing residual phosphorus in the hot metal and decarburizing / heating up, a new refining material is added to the hot metal, and decarburization blowing is performed by blowing oxygen at high speed from the upper blowing lance of the converter and blowing bottom blowing gas from the bottom of the converter.

[0004] As a method for producing low-phosphorus steel in a conventional converter blowing method using a top-bottom blown converter and performing dephosphorization blowing, slag removal, and decarburization blowing in the same converter, there is a method in which the slag is left in the converter after dephosphorization blowing, 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 blowing 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.

[0005] As a method of discharging the slag after dephosphorization blowing 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 dephosphorization blowing, 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 leaving 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, it is not possible to remove all the slag present in the converter. 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.

[0006] As a method for improving the MURC method, a method is known in which dephosphorization blowing is carried out in two stages, the first intermediate slag discharge is carried out after the first dephosphorization blowing, then the second dephosphorization blowing is carried out and then the second intermediate slag discharge is carried out, 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 manufactured 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-stage dephosphorization blowing, two-stage intermediate slag discharge, and subsequent decarburization blowing are carried out in one converter is called "W-MURC".

[0007] In Patent Document 1, the slag basicity (CaO / SiO2 (mass ratio)) at the end of the first dephosphorization blowing is 1.6 to 1.9 in the examples, and the slag basicity at the end of the second dephosphorization blowing is 1.4 to 2.0 in the examples, and a CaO-based flux and a SiO2-based flux are added in the second dephosphorization blowing.

[0008] In Patent Document 2, a CaO source is added so that the slag basicity at the end of the first dephosphorization blowing becomes 0.8 to 1.3, and in the second dephosphorization blowing, the CaO source is not charged or is 5 kg / ton or less so that the slag basicity is in the range of 0.8 to 1.3.

[0009] Patent Document 3 discloses a method for dephosphorizing molten iron, which involves blowing the total mass of CaO as a dephosphorizing agent in powder form together with top-blown oxygen gas onto the molten iron contained in a bottom-blown converter-type reaction vessel for dephosphorization treatment. As the dephosphorizing agent, a mixture of converter dephosphorization and decarburization slag and ladle slag is used. By using a powder with a basicity (CaO mass / SiO2 (mass ratio)) adjusted to 4.5 to 7.5, an Al2O3 concentration of 5 to 9% by mass, and a maximum particle size of 0.15 mm or less, the slag composition at the end of the dephosphorization treatment is adjusted to a basicity of 2.2 to 3.2, Al2O3 of 4.5 to 7.5% by mass, and T.Fe of 7 to 13% by mass for dephosphorization. When subjecting the dephosphorized molten iron to dephosphorization and decarburization treatment in the next step, about 285 t of the dephosphorized molten iron obtained by the above-described molten iron dephosphorization treatment, about 15 t of scrap, and the bottom-blown converter are charged to produce molten steel.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Non-Patent Documents

[0011]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0012] In the inventions described in Patent Documents 1 and 2 above, a method for reducing the P concentration in molten steel after decarburization blowing is shown by performing first dephosphorization blowing, first intermediate slag removal, second dephosphorization blowing, second intermediate slag removal, and decarburization blowing using one converter.

[0013] On the other hand, recently, the realization of a converter blowing method with further improved dephosphorization ability has been desired. An object of the present invention is to provide a converter blowing method with further improved dephosphorization ability than before.

Means for Solving the Problems

[0014] That is, the gist of the present invention is as follows. [1] When refining steel using a top-bottom blown converter, in the first step, hot metal is charged into the converter, in the second step, a flux is added to perform the first dephosphorization blowing, in the third step, the first intermediate slag removal is performed, in the fourth step, the second dephosphorization blowing is performed, in the fifth step, the second intermediate slag removal is performed, and in the sixth step, decarburization blowing is performed. In the converter blowing method, In the second step, a CaO-containing flux and an Al2O3 source are added as a flux, The slag composition (calculated value) at the end of the second step is CaO / SiO2 (mass ratio) = 1.7 to 2.3, %Al2O3 = 2 to 16% by mass, The total intermediate slag removal rate of the third step and the fifth step is adjusted so that the intermediate slag removal rate represented by the following formula (1) is 50% or more A converter blowing method characterized by the above.

Number

Advantages of the Invention

[0015] When performing the first dephosphorization blowing, the first intermediate slag removal, the second dephosphorization blowing, the second intermediate slag removal, and the decarburization blowing using one converter, an Al2O3 source is added in the first dephosphorization blowing and the CaO / SiO2 (mass ratio) is increased. As a converter blowing method without tapping or recharging, it has achieved a high level of compatibility between promoting the dephosphorization reaction and improving the intermediate slag removal rate, and realized a converter blowing method with improved dephosphorization ability compared to the conventional method.

Embodiments for Carrying Out the Invention

[0016] In a blowing method (W-MURC) that performs two dephosphorization blowings, two intermediate slag removals, and subsequent decarburization blowing in one converter, in order to further improve the dephosphorization ability, it is considered effective to increase the basicity of the slag in the dephosphorization blowing. However, if the basicity of the slag is made too high in the dephosphorization blowing, it becomes difficult to perform sufficient slag removal in the subsequent intermediate slag removal, and the dephosphorization ability cannot be improved when viewed as a whole refining process. This is because when the slag is made highly basic, the slag-forming height decreases, raising concerns about the deterioration of slag removability. Therefore, in the conventional MURC method, the blowing is performed with a low basicity of the slag in the dephosphorization blowing.

[0017] The inventors conceived the idea of adding an Al2O3 source to the slag during the dephosphorization blowing process of W-MURC. However, when Al2O3 is contained in a low basicity slag, the slag viscosity increases, the slag forming during the dephosphorization blowing process progresses rapidly, the slag overflows from the converter, making it difficult to continue the blowing process, and there is concern that sufficient oxygen supply cannot be achieved and dephosphorization becomes insufficient. In contrast, the inventors considered that by combining an increase in the basicity of the slag and the addition of Al2O3 to the slag, it should be possible to increase the slag viscosity despite the high slag basicity, and by obtaining an appropriate forming state, both the promotion of the dephosphorization reaction and the improvement of the intermediate slag discharge rate can be achieved at a high level.

[0018] It is known that when an appropriate amount of Al2O3 is added to a slag mainly composed of a CaO-SiO2 system, the formation of the liquid phase is promoted, and it is considered possible to be effective in promoting the slagging of CaO. However, Al2O3 alone has no effect of promoting the dephosphorization reaction, and there is a problem that slopping occurs early due to an increase in the slag amount and the promotion of liquid phase formation, ending the dephosphorization blowing process, and as a result, a high dephosphorization rate cannot be obtained. Therefore, the inventor considered that while adding an appropriate amount of Al2O3 during the dephosphorization blowing process, setting the slag basicity (CaO / SiO2 (mass ratio)) higher, discharging a part of the excessive slag when slopping occurs, and performing the dephosphorization blowing process again to invalidate the adverse effects of slopping due to the addition of Al2O3 and obtain the effect of improving the dephosphorization ability by promoting the slagging of CaO. In addition, this method can also be used when slopping does not pose a problem, such as when the hot metal [Si] is low or when using a relatively large reaction vessel.

[0019] Based on the above concept, when the slag was made highly basic and an Al2O3 source was added during the first dephosphorization blowing process of the W-MURC method, it was found that the dephosphorization efficiency was improved due to the high basicity, and an appropriate intermediate slag discharge rate was ensured, improving the dephosphorization efficiency in the second dephosphorization blowing process. The details will be described below.

[0020] When refining steel using a top-bottom blown converter, in the first step, hot metal is charged into the converter. In the second step, flux is added to perform the first dephosphorization blowing. In the third step, the first intermediate slag discharge is carried out. In the fourth step, the second dephosphorization blowing is performed. In the fifth step, the second intermediate slag discharge is carried out. In the sixth step, decarburization blowing is performed (W-MURC method). After the sixth step, in the seventh step, the molten steel is tapped into a ladle, and in the eighth step, the treatment of the remaining slag in the converter is carried out.

[0021] In the present invention, the range of the slag composition (calculated value) CaO / SiO2 (mass ratio) in the first dephosphorization blowing is set to be 1.7 or more and 2.3 or less. By setting CaO / SiO2 (mass ratio) to be 1.7 or more, it is possible to improve the dephosphorization ability in the first dephosphorization blowing. It is preferable that CaO / SiO2 (mass ratio) is 1.9 or more. On the other hand, when CaO / SiO2 (mass ratio) is too high, it is difficult to maintain a high foaming height as the slag viscosity decreases. However, by using Al2O3 addition to the slag in combination, since both the slag viscosity and the foaming height can be maintained high even when the slag basicity is high, it has become possible to widen the upper limit of CaO / SiO2 (mass ratio) to 2.3. It is more preferable that the upper limit of CaO / SiO2 (mass ratio) is 2.2.

[0022] In the first dephosphorization blowing of the W-MURC method, the calculation method of the slag composition (calculated value) CaO / SiO2 (mass ratio) is as follows. Calculate the total amount of CaO added from the CaO source added into the converter, calculate the total amount of SiO2 added from the SiO2 generated by the combustion of the contained Si in the charged main raw materials and the SiO2 source added, and calculate the slag composition (calculated value) CaO / SiO2 (mass ratio) as the mass ratio of the CaO addition amount / SiO2 addition amount.

[0023] In the W-MURC method, as described below, after the tapping of the previous heat (the 7th step) is completed, in the slag treatment in the converter (the 8th step), part or all of the slag in the converter may be left in the converter, and then the hot metal may be charged into the converter in the first step of this heat. When leaving the slag of the previous heat in the converter, the amounts of CaO and SiO2 are estimated from the amount of the remaining slag and the slag analysis value or the slag composition calculation value, and are calculated by adding them to the amounts of CaO added and SiO2 added, respectively. The amount of slag in the converter after the tapping of the previous heat (the 7th step) is completed can be calculated using the amount of CaO charged in each step of the previous heat, the intermediate slag discharge rate calculated by the above formula (1), and the CaO analysis value in the slag after the 6th step. When leaving part of the slag in the converter in the previous heat, the amount of the remaining slag can first be adjusted by determining the tilting angle of the converter at the time of slag discharge. From the slag component analysis value after the decarburization blowing (the 6th step) of the previous heat, the tilting angle of the converter at the time of slag discharge, the amounts of the CaO source and SiO2 source added in the second step of this heat, and the slag component analysis value after the dephosphorization blowing, a mass balance equation for two components (for example, CaO and SiO2) is established and the equation is solved to calculate the amount of the remaining slag in the case of the tilting angle. After grasping the relationship between the tilting angle of the converter at the time of slag discharge and the amount of the remaining slag in advance, the amount of the remaining slag can be adjusted by determining the tilting angle of the converter at the time of slag discharge of the previous heat. Second, the amount of the remaining slag can also be determined by weighing in the slag discharge ladle that discharges the slag in the 8th step of the previous heat and subtracting the amount of the slag in the weighed slag discharge ladle from the amount of the slag in the converter after the completion of the 7th step. When leaving all of the slag in the converter in the previous heat, the amount of the slag in the converter after the completion of the 7th step of the previous heat calculated above can be used as the amount of the remaining slag.

[0024] In the present invention, the Al2O3 content of the slag blend (calculated value) in the first dephosphorization blowing is set to the range of 2 to 16 mass%. By setting the Al2O3 content to 2 mass% or more, it is possible to increase the basicity of the slag as described above. On the other hand, if the Al2O3 content exceeds 16 mass%, the CaO concentration of the slag becomes diluted and the dephosphorization ability weakens, so the upper limit is set to 16 mass%. The Al2O3 content is preferably 3% or more. Moreover, the Al2O3 content is more preferably 10% or less. In the addition of the Al2O3 source, alumina bricks, calcium aluminate, agglomerate slag, and other alumina-containing raw materials can be used as the Al2O3 source. Although it is convenient to use a lump-shaped raw material as the Al2O3 source and charge it into the converter, a powder-shaped raw material may also be used.

[0025] The Al2O3 content of the slag blend (calculated value) is calculated as follows. The unit consumption of the CaO source, SiO2 source, MgO source, and Al2O3 source added as auxiliary materials is added together with the unit consumption of SiO2 generated by the combustion of Si contained in the main raw materials charged, and the total is divided by 0.75, taking into account FeO in the slag, etc., to calculate the unit consumption of slag. The Al2O3 content (mass%) of the slag blend (calculated value) is calculated by dividing the unit consumption of Al2O3 addition by the unit consumption of slag amount and multiplying the result by 100. If slag from the previous heat is left in the converter, the amounts of CaO, SiO2, MgO, and Al2O3 are estimated from the amount of remaining slag and the slag analysis value or the calculated slag blend value, and are added to the CaO source, SiO2 source, MgO source, and Al2O3 source, respectively, to perform the above calculation.

[0026] In the present invention, the intermediate slag ratio in the third and fifth steps is adjusted so that the intermediate slag ratio shown in the above formula (1) is 50% or more. First, the derivation of formula (1) will be explained. Hereinafter, basicity CaO / SiO2 (mass ratio) will also be referred to as C / S. Here, CaO B1 : CaO input up to the 4th step [t], CaO B2 :6th step input CaO [t], SiO 2B1 : SiO2 generated / input up to the 4th process [t], SiO 2B2: Input SiO2[t] and C / S in the 6th step B2 : Let the CaO / SiO2 (mass ratio) of the slag after blowing in the 6th step be [-], and α be the CaO slag conversion rate [-] in the 6th step. The CaO / SiO2 (mass ratio) of the slag after blowing in the 6th step (C / S B2 ) is expressed by the following formula. C / S B2 =(CaO B1 ×(1 - intermediate slag discharge rate)+CaO B2 ×slag conversion rate α) / (SiO 2B1 ×(1 - intermediate slag discharge rate)+SiO 2B2 ) By transforming the above formula, the following formula (1) can be obtained. Regarding the slag conversion rate α, based on the analysis results of the decarburization operation, α = 0.8 is adopted.

Equation

[0027] By setting the intermediate slag discharge rate in the above formula (1) to 50% or more, the P content transferred to the slag during the first phosphorus removal blowing and the second phosphorus removal blowing can be sufficiently discharged out of the furnace together with the slag discharge slag, and as a result of the decarburization blowing in the 6th step, the phosphorus concentration in the molten iron can be sufficiently reduced. The intermediate slag discharge rate in formula (1) is preferably 55% or more, more preferably 60% or more, more preferably 65% or more, and even more preferably 75% or more.

[0028] In the present invention, since 2% or more of Al2O3 is blended in the slag in the first dephosphorization blowing, the viscosity of the slag can be increased and the forming height can be maintained high despite the high basicity of the slag. Therefore, in either the first intermediate slag tapping or the second intermediate slag tapping, the intermediate slag tapping rate can be increased as intended. In Patent Document 3, the Al2O3 concentration in the slag is increased and a high-basicity slag is used in the dephosphorization blowing. However, as is clear from the fact that Patent Document 3 describes charging hot metal for dephosphorization into a top-bottom blown converter and performing the subsequent decarburization blowing, it targets a process in which intermediate slag tapping after dephosphorization blowing is not performed. Also, a powdery auxiliary material containing Al2O3 is blown into the converter together with oxygen gas and added. That is, the purpose of adding the Al2O3 source is different between the present invention and the invention described in Patent Document 3.

[0029] In the first intermediate slag tapping in the third step, preferably, by setting the intermediate slag tapping final angle as the target range determined in advance as the converter tilting angle, a part of the slag generated in the second step is left in the converter and slagged off. By intentionally leaving a part of the slag in the converter in the first intermediate slag tapping, the remaining slag can be contributed as a dephosphorization flux in the second dephosphorization blowing. As a method for determining the target range determined in advance for the intermediate slag tapping final angle, first, the molten iron outflow start angle at which molten iron flows out is obtained. As a method for obtaining the molten iron outflow start angle, in the intermediate slag tapping before the previous charge, the molten iron outflow start angle at which molten iron flows out from the furnace can be measured. By setting the final tilting angle during intermediate slag tapping to be at least 3 degrees shallower than the measured starting angle of molten iron outflow, it is possible to ensure the amount of slag remaining in the converter, and sufficiently ensure the dephosphorization ability during the second dephosphorization blowing. On the other hand, if the final tilting angle during intermediate slag tapping is made too shallow compared to the measured starting angle of molten iron outflow, the amount of slag remaining in the converter will become too large, and slopping from the furnace mouth due to slag formation will occur before the target oxygen amount for the second dephosphorization blowing is finished, increasing the risk that the dephosphorization blowing cannot be continued. By making the final tilting angle during intermediate slag tapping deeper than 7 degrees shallower than the measured starting angle of molten iron outflow, it becomes possible to set the amount of slag remaining in the furnace within a suitable range.

[0030] As described above, a predetermined amount of slag remains in the converter after the first intermediate slag tapping. Since the remaining slag still retains the dephosphorization ability, in the second dephosphorization blowing of the fourth step, it is not necessary to additionally add a new CaO source. Therefore, in the present invention, in the second dephosphorization blowing of the fourth step, it is not necessary to perform the flux input of any one or all of the CaO-based, SiO2-based, and Al2O3-based fluxes.

[0031] Since Al2O3 is contained in the slag after the second dephosphorization blowing in the fourth step, the slag can be sufficiently formed during the dephosphorization blowing, and the slag can be sufficiently discharged during the second intermediate slag tapping in the fifth step. Therefore, it becomes possible to make the total intermediate slag tapping rate of the third step and the fifth step, calculated by the formula (1), 50% or more.

[0032] In the second dephosphorization blowing of the fourth step, it is preferable to input 5 kg / ton or more of iron oxide. By inputting an iron oxide source during the second dephosphorization blowing, the T.Fe concentration in the slag can be increased, the dephosphorization ability can be improved, slag formation can be promoted, and the slag tapping rate during the second intermediate slag tapping in the fifth step can be further stabilized at a higher level. It is preferable to use sinter ore or sinter fines as the iron oxide.

[0033] In the present invention, preferably, in the second dephosphorization blowing in the fourth step, the molten metal temperature at the end of the dephosphorization blowing is set to 1380°C or lower. Since the dephosphorization reaction is more advantageous at lower temperatures, a decrease in dephosphorization efficiency can be suppressed.

[0034] In the present invention, preferably, in the second dephosphorization blowing in the fourth step, the C concentration in the molten metal at the end of the dephosphorization blowing is set to 3.0 mass% or higher. Since the dephosphorization reaction is more advantageous at higher C concentrations in the molten metal, a decrease in dephosphorization efficiency can be suppressed.

[0035] In the decarburization blowing in the sixth step, it is preferable that the CaO / SiO2 (mass ratio) of the slag is 2.5 to 5.0. Thereby, decarburization can be performed while further promoting dephosphorization in the decarburization step. By setting the CaO / SiO2 (mass ratio) of the slag in the decarburization blowing to 2.5 or higher, a slag with high dephosphorization ability can be generated. Also, by setting the CaO / SiO2 (mass ratio) to 5.0 or lower, an appropriate slag formation rate can be achieved at the end-point temperature of the decarburization blowing, and the utilization efficiency of CaO can be increased. Regarding the CaO / SiO2 (mass ratio) in the decarburization blowing, it may be set in consideration of the differences in the end-point C and end-point temperature depending on the target steel grade.

[0036] After completing the decarburization blowing in the sixth step, the converter is tilted and the molten steel in the converter is tapped into a ladle while leaving the slag. The molten steel is accommodated in the ladle on the ladle car. This step is called the seventh step.

[0037] When the tapping is completed, slag remains in the converter. In the eighth step, this remaining slag is processed. The converter may be tilted and all of the slag in the converter may be discharged into a slag pan. Alternatively, all of the slag in the converter may be left in the converter, and then the process may proceed to the first step of the next heat. Further, only a part of the slag in the converter may be left and the rest may be slagged off, and the process may proceed to the first step of the next heat while leaving a part of the slag.

[0038] In the 8th step of the previous heat, when all the slag in the converter is discharged, the flux used in the 2nd step of this heat will be the flux added to the converter at the start of the 2nd step. Also, in the 8th step of the previous heat, when part or all of the slag is left in the converter, the flux used in the 2nd step will be the flux generated in the 6th step of the previous heat before the 1st step, with part or all of it left in the converter.

Example

[0039] A converter blowing method was implemented using a 350-ton upper-bottom blown converter. In the 1st step, hot metal was charged into the converter. In the 2nd step, flux was added for the first dephosphorization blowing. In the 3rd step, the first intermediate slag discharge was carried out. In the 4th step, the second dephosphorization blowing was performed. In the 5th step, the second intermediate slag discharge was carried out. In the 6th step, decarburization blowing was performed.

[0040] After the 6th step, in the 7th step, the molten steel was tapped into a ladle, and in the 8th step, the remaining slag in the converter was processed. In some examples, in the 8th step of the previous heat, part of the slag in the converter was left in the converter, and then in the 1st step of this heat, hot metal was charged into the converter. When leaving part of the slag in the converter, the relationship between the tilting angle of the converter at the time of slag discharge and the amount of remaining slag was grasped in advance, and the amount of remaining slag was adjusted by determining the tilting angle of the converter at the time of slag discharge in the previous heat. From the slag component analysis values after decarburization blowing (6th step) in the previous heat, the tilting angle of the converter at the time of slag discharge, the addition amounts of the CaO source and SiO2 source in the 2nd step of this heat, and the slag component analysis values after dephosphorization blowing, a mass balance equation for two components (CaO and SiO2) was established and the equation was solved to calculate the relationship between the tilting angle and the amount of remaining slag. The original unit of the remaining slag amount was described in the "Previous Heat Remaining Slag Original Unit" column of Table 1. In the examples described as "0" in this column, the slag in the previous heat was processed without being left.

[0041] The refining conditions and refining results are shown in Table 1. Nos. 1 to 5 in Table 1 are comparative examples, and Nos. 6 to 15 are examples (examples of the present invention). In Table 1, numerical values outside the scope of the present invention are underlined.

[0042]

Table 1

[0043] Table 1 shows the Si concentration, P concentration, and hot metal temperature of the hot metal charged into the converter.

[0044] In the first dephosphorization blowing in the second step, quicklime was added as the CaO source. Table 1 describes the CaO source addition unit as the T.CaO unit. The SiO2 unit was calculated from SiO2 generated by the combustion of contained Si in the charged main raw materials, and the slag composition (calculated value) CaO / SiO2 (mass ratio) was calculated as the mass ratio of T.CaO unit / SiO2 unit and described as "calculated C / S" in Table 1.

[0045] In addition, in the example of the present invention, alumina bricks were added as the Al2O3 source by charging them from the scrap chute. The comparative example did not add an Al2O3 source. The addition unit of the Al2O3 source is described as the Al2O3 unit in Table 1. The units of the CaO source, SiO2 source, MgO source, and Al2O3 source charged as auxiliary materials, together with the SiO2 unit generated by the combustion of contained Si in the charged main raw materials, were totaled, and the slag amount unit was calculated by dividing the total value by 0.75. The Al2O3 content (mass%) of the slag composition (calculated value) was calculated by dividing the Al2O3 addition unit by the slag amount unit and multiplying by 100.

[0046] The de-Si external oxygen unit in the second step listed in Table 1 is the oxygen unit obtained by subtracting the oxygen unit consumed for de-Si from the oxygen unit added in the second step, and is used as an index of the oxygen unit that can contribute to the dephosphorization reaction.

[0047] In the first dephosphorization blowing in the third step, in the example of the present invention, the blowing was terminated immediately before the slag started to form and the slag discharge from the furnace mouth began, and the first intermediate slag discharge in the third step was carried out. In advance, in the intermediate slag discharge before the previous charge, the molten iron outflow start angle at which the molten iron flows out of the furnace was measured. In the first intermediate slag discharge, the final tilting angle was set to an angle 5 degrees shallower than the measured molten iron outflow start angle to ensure the amount of slag remaining in the converter after the first intermediate slag discharge.

[0048] In the second dephosphorization blowing in the fourth step, in both the example of the present invention and the comparative example, no CaO source was added. On the other hand, in the example of the present invention, sinter powder was added as iron oxide at the unit shown in Table 1, while in the comparative example, no sinter powder was added. In the example of the present invention, the blowing was terminated immediately before the slag started to form and the slag discharge from the furnace mouth began, and the second intermediate slag discharge in the fifth step was carried out. In the comparative example, the blowing was terminated when the slag forming height reached a certain value, and the second intermediate slag discharge in the fifth step was carried out. The C concentration and P concentration after blowing at the end of the second dephosphorization blowing are shown in Table 1. It is clear that the P concentration after blowing in the example of the present invention is lower compared to the comparative example.

[0049] In the second intermediate slag discharge in the fifth step, in both the example of the present invention and the comparative example, the intermediate slag discharge was carried out with the molten iron outflow angle as the final tilting angle. The intermediate slag discharge rate in the example of the present invention was high, while that in the comparative example was low.

[0050] The results of the dephosphorization blowing in the second to fifth steps as a whole are summarized in Table 1. The dephosphorization rate is (charged hot metal P concentration - P concentration after blowing in the fourth step) / charged hot metal P concentration × 100 (%). The intermediate slag discharge rate is the intermediate slag discharge rate calculated by the above formula (1). The in-furnace P residue rate is (total amount of P in the molten iron after blowing in the fourth step and the amount of P remaining in the slag remaining in the furnace after the fifth step) / amount of P in the charged hot metal × 100 (%). Compared with the comparative example, in the example of the present invention, the dephosphorization rate is improved, the intermediate slag discharge rate is high, and the in-furnace P residue rate is low, and the superiority of the present invention is clear.

[0051] After the second intermediate slag removal in the fifth step, decarburization blowing in the sixth step was carried out. The P concentration after blowing in the sixth step and the temperature after blowing are shown in Table 1. The metal P residual rate (%) = (P concentration after blowing in the sixth step / P concentration in hot metal) × 100 is shown in Table 1. It is clear that the metal P residual rate in the example of the present invention is lower compared with the comparative example.

Claims

1. When refining steel using a top-bottom blown converter, in the first step, hot metal is charged into the converter, in the second step, flux is added to perform the first dephosphorization blowing, in the third step, the first intermediate slag discharge is performed, in the fourth step, the second dephosphorization blowing is performed, in the fifth step, the second intermediate slag discharge is performed, and in the sixth step, decarburization blowing is performed. In the converter blowing method, In the second step, a CaO-containing flux and an Al ２ O ３ source are added, The slag composition (calculated value) at the end of the second step is CaO / SiO ２ (mass ratio) = 1.7 to 2.3, %Al ２ O ３ = 2 to 16% by mass, and The total intermediate slag discharge rate in the third step and the fifth step is adjusted so that the intermediate slag discharge rate represented by the following formula (1) is 50% or more. A converter blowing method characterized by this. 【Number 1】 CaO Ｂ１ : Input CaO content [t] up to the 4th step, CaO Ｂ２ : Input CaO content [t] in the 6th step, SiO ２Ｂ１ : Generated SiO up to the 4th step ２ [t], SiO ２Ｂ２ : Input SiO in the 6th step ２ [t], C / S Ｂ２ : CaO / SiO in the slag after blowing in the 6th step ２ (mass ratio) [-], α = 0.8: CaO conversion rate in the 6th step [-]

2. In the first intermediate slag discharge in the third step, by setting the intermediate slag discharge final angle as the target range determined in advance as the converter tilting angle, a part of the slag generated in the second step is left in the converter for slag discharge. The converter blowing method according to claim 1, characterized by this.

3. As a method for determining the target range determined in advance of the intermediate slag discharge final angle, in the intermediate slag discharge before the previous charge, the molten iron outflow start angle at which molten iron flows out of the furnace is measured in advance, and in the first intermediate slag discharge in the third step, an angle shallower by 3 degrees or more and 7 degrees or less than the molten iron outflow start angle is set as the target range determined in advance of the intermediate slag discharge final angle. The converter blowing method according to claim 2, characterized by this.

4. In the second dephosphorization blowing in the fourth step, 5 kg / ton or more of iron oxide is charged. The converter blowing method according to any one of claims 1 to 3, characterized by this.

5. The converter blowing method according to claim 4, wherein sinter ore or sinter fines are used as the iron oxide.

6. After the tapping of the previous heat is completed, a part or all of the slag in the converter is left in the converter, and then hot metal is charged into the converter in the first step of this heat. The amount of CaO, SiO ２ amount, MgO amount, Al ２ O ３ amount are estimated from the amount of the remaining slag and the slag analysis value or the slag blending calculation value, and part or all of the auxiliary materials to be charged as the CaO source, SiO ２ source, MgO source, Al ２ O ３ source are replaced. The converter blowing method according to any one of claims 1 to 3, characterized in that.

7. After the tapping of the previous heat is completed, a part or all of the slag in the converter is left in the converter, and then hot metal is charged into the converter in the first step of this heat. The amount of CaO, SiO ２ amount, MgO amount, Al ２ O ３ amount are estimated from the amount of the remaining slag and the slag analysis value or the slag blending calculation value, and a part or all of the auxiliary materials charged as the CaO source, SiO ２ source, MgO source, Al ２ O ３ source are replaced. The converter blowing method according to claim 5, characterized in that.

8. In the second dephosphorization blowing in the fourth step, the molten metal temperature at the end of the dephosphorization blowing is 1380°C or lower, and the C concentration in the molten metal is 3.0 mass% or more. The converter blowing method according to any one of claims 1 to 3, characterized by this.

9. In the second dephosphorization blowing in the fourth step, the molten metal temperature at the end of the dephosphorization blowing is 1380°C or lower, and the C concentration in the molten metal is 3.0 mass% or more. The converter blowing method according to claim 4, characterized by this.

10. In the second dephosphorization blowing in the fourth step, the molten metal temperature at the end of the dephosphorization blowing is 1380°C or lower, and the C concentration in the molten metal is 3.0 mass% or more. The converter blowing method according to claim 5, characterized by this.

11. In the second dephosphorization blowing in the fourth step, the method for converter blowing according to claim 6, characterized in that the molten metal temperature at the end of the dephosphorization blowing is 1380 °C or lower, and the C concentration in the molten metal is 3.0 mass% or higher.

12. In the second dephosphorization blowing in the fourth step, the method for converter blowing according to claim 7, characterized in that the molten metal temperature at the end of the dephosphorization blowing is 1380 °C or lower, and the C concentration in the molten metal is 3.0 mass% or higher.

Images