Converter blowing method

Abstract

To provide a converter blowing method improved in dephosphorization capacity more than ever, when performing a first dephosphorization blowing, a first intermediate slag discharging, a second dephosphorization blowing, a second intermediate slag discharging, and a decarburization blowing, using one converter.SOLUTION: A converter blowing method is characterized by performing: 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 this order, and in Step 2, as a flux, CaO-containing flux and a MnO source are used as the flux, and the slag composition at the end of Step 2 (calculated value) is CaO / SiO2 (mass ratio)=1.7 to 2.3, and MnO input amount index=0.10 to 0.25. Additionally, the total intermediate slag discharge rate for Steps 3 and 5 is adjusted to ensure that the intermediate slag discharge rate, as indicated by the formula (1) below, is 50% or higher.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 for 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-blown 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 and 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-blown 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 tapping 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 slag discharge 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, and further dephosphorization and decarburization are carried out (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.

[0006] As a method of improving the MURC method, there is known a method 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 stated that in this way, 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 blowings, two intermediate slag discharges, and subsequent decarburization blowing are carried out in one converter is referred to as "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 is 0.8 to 1.3, and in the second dephosphorization blowing, 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.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0010]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0011] In the inventions described in Patent Documents 1 and 2 above, by using one converter to perform the first dephosphorization blowing, the first intermediate slag removal, the second dephosphorization blowing, the second intermediate slag removal, and decarburization blowing, a method for reducing the P concentration in the molten steel after the decarburization blowing is shown.

[0012] On the other hand, recently, the realization of a converter blowing method with further improved dephosphorization ability has been desired. The purpose of the present invention is to provide a converter blowing method with further improved dephosphorization ability than before.

Means for Solving the Problems

[0013] 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, 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 carried out, in the fifth step, the second intermediate slag discharge is carried out, and in the sixth step, decarburization blowing is carried out. In the converter blowing method, in the second step, a CaO-containing flux and an MnO source are added as the flux, the slag composition (calculated value) at the end of the second step is CaO / SiO2 (mass ratio) = 1.7 to 2.3, and the MnO input amount index = 0.10 to 0.25, the total intermediate slag discharge rate of the third step and the fifth step is adjusted so that the intermediate slag discharge rate shown by the following formula (1) is 50% or more This is a converter blowing method characterized by the above.

Number

Advantages of the Invention

[0014] When performing the first dephosphorization blowing, the first intermediate slag removal, the second dephosphorization blowing, the second intermediate slag removal, and decarburization blowing using one converter, by adding an MnO source during the first dephosphorization blowing and increasing the CaO / SiO2 (mass ratio), a converter blowing method that does not perform tapping or recharging is realized, which achieves both the promotion of the dephosphorization reaction and the improvement of the intermediate slag removal rate at a high level, and improves the dephosphorization ability compared to the conventional method.

Embodiments for Carrying Out the Invention

[0015] 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 from the stage of the first dephosphorization blowing. However, if the basicity of the slag is made too high during the first dephosphorization blowing, it becomes difficult to perform sufficient slag removal in the subsequent first 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.

[0016] It is known that when an appropriate amount of MnO is added to slag mainly composed of the CaO-SiO₂ system, the formation of the liquid phase is promoted, and it was considered possible to be effective in promoting the slagging of CaO. However, MnO alone has a weak effect of promoting the dephosphorization reaction, and there was a concern that slopping would occur early due to an increase in the amount of slag and promotion of liquid-phase formation, ending the dephosphorization blowing and resulting in an inability to obtain a high dephosphorization rate. Therefore, the inventor considered that while adding an appropriate amount of MnO during dephosphorization blowing, setting a higher CaO / SiO₂ (mass ratio) and discharging a part of the excessive slag when slopping occurs and then performing dephosphorization blowing again would invalidate the adverse effects of slopping due to the addition of MnO and obtain the effect of improving the dephosphorization ability by promoting the slagging of CaO. Also, this method can be used when slopping does not pose a problem, such as when the [Si] in the hot metal is low or when using a relatively large reaction vessel. Hereinafter, the CaO / SiO₂ (mass ratio) of the slag is also referred to as the basicity of the slag.

[0017] Based on the above idea, when the slag was made highly basic and an MnO source was added in the first dephosphorization blowing of the W-MURC method, it was found that the dephosphorization efficiency was improved due to the high basicity. On the other hand, despite the high basicity, it was possible to ensure the intermediate slag discharge rate and improve the dephosphorization efficiency in the second dephosphorization blowing. The details will be described below.

[0018] The present invention targets a converter blowing method (W-MURC method) in which when refining steel using a top-bottom blown converter, hot metal is charged into the converter in the first step, flux is added in the second step to perform the first dephosphorization blowing, the first intermediate slag discharge is performed in the third step, the second dephosphorization blowing is performed in the fourth step, the second intermediate slag discharge is performed in the fifth step, and decarburization blowing is performed in the sixth step. After the sixth step, the molten steel is tapped into a ladle in the seventh step, and the remaining slag in the converter is treated in the eighth step.

[0019] 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 to set CaO / SiO2 (mass ratio) to be 1.9 or more. On the other hand, when CaO / SiO2 (mass ratio) is too high, it is difficult to maintain the slag viscosity and keep the forming height high. However, by using MnO addition to the slag in combination, even when the slag basicity is high, both the slag viscosity and the forming height can be maintained high. Therefore, it is possible to widen the upper limit of CaO / SiO2 (mass ratio) to 2.3. It is more preferable to set the upper limit of CaO / SiO2 (mass ratio) to 2.2.

[0020] In the first dephosphorization blowing of the W-MURC method, the calculation method of the slag composition (calculated value) CaO / SiO2 (mass ratio) shall be 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 Si contained 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 to the SiO2 addition amount.

[0021] In the W-MURC method, as described below, after the tapping (seventh step) of the previous heat is completed, in the slag treatment (eighth step) in the converter, a 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, estimate the amount of CaO and SiO2 from the amount of the remaining slag and the slag analysis value or the slag composition calculated value, and calculate by adding them to the CaO addition amount and the SiO2 addition amount respectively. The amount of slag in the converter after the tapping (seventh step) of the previous heat is completed can be calculated using the amount of CaO input 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 sixth step. When a part of the slag in the converter is left in the converter during the previous heat, the amount of the remaining slag can be adjusted first by determining the tilting angle of the converter at the time of slag discharge. From the slag component analysis values after decarburization blowing (the sixth 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 second step of this heat, and the slag component analysis values after dephosphorization blowing, a mass balance equation for two components (for example, CaO and SiO2) can be established and the equation can be 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 in the previous heat. Second, the amount of the remaining slag can also be determined by weighing in the slag pot that discharges the slag in the eighth step of the previous heat and subtracting the amount of the slag in the weighed slag pot from the amount of the slag in the converter after completion of the seventh step. When the entire amount of the slag in the converter is left in the converter during the previous heat, the amount of the slag in the converter after completion of the seventh step in the previous heat calculated above can be used as the amount of the remaining slag.

[0022] In the present invention, the amount of MnO added to the slag in the first dephosphorization blowing is adjusted by the MnO input amount index. The MnO input amount index of the slag composition (calculated value) is calculated as follows. The unit amounts of the CaO source, SiO2 source, MgO source, Al2O3 source, and MnO source input as auxiliary materials are added to the SiO2 unit amount obtained by converting the contained Si in the charged main material into SiO2 and the MnO unit amount obtained by converting the contained Mn into MnO to obtain the total unit amount. When the slag from the previous heat is left in the converter, the amounts of CaO, SiO2, MgO, Al2O3, and MnO are estimated from the amount of the remaining slag and the slag analysis value or the slag composition calculation value, and added to the unit amounts of the CaO source, SiO2 source, MgO source, Al2O3 source, and MnO source input as auxiliary materials respectively (the same applies hereinafter). The unit amount of the MnO source input as an auxiliary material and the MnO unit amount obtained by converting the contained Mn in the charged main material into MnO are added together to obtain the total MnO unit amount. The MnO input amount index is calculated as MnO total unit amount / total unit amount, and the MnO input amount index (-) is calculated. Note that the MnO unit amount obtained by converting the contained Mn in the charged main material into MnO is simply calculated by converting the total amount of the contained Mn in the charged main material into MnO as almost all Mn is oxidized and slagged in the temperature of the dephosphorization blowing and the slag composition.

[0023] In the present invention, the MnO input amount index is in the range of 0.10 to 0.25. By setting the MnO input amount index to 0.10 or more, it becomes possible to increase the CaO / SiO2 (mass ratio) of the slag as described above. On the other hand, when the MnO input amount index exceeds 0.25, the CaO concentration of the slag decreases and the dephosphorization ability weakens, so the upper limit is set to 0.25. It is preferable that the MnO input amount index is 0.12 or more, and further preferably 0.15 or more. Also, it is more preferable that the MnO input amount index is 0.24 or less. In the addition of the MnO source, manganese ore or ferromanganese ore, other MnO-containing raw materials or Mn-containing raw materials can be used as the MnO source. As the MnO source, a method of charging into the converter using a massive raw material is simple, but a method of blowing using a powdered raw material may also be used.

[0024] In the present invention, for the total intermediate slag discharge rate of the third step and the fifth step, the intermediate slag discharge rate represented by the above formula (1) is adjusted to be 50% or more. First, the derivation of formula (1) will be explained. Hereinafter, the basicity CaO / SiO2 (mass ratio) will also be referred to as C / S. Here, CaO B1 : The amount of CaO input up to the fourth step [t], CaO B2 : The amount of CaO input in the sixth step [t], SiO 2B1 : The generated and input SiO2 up to the fourth step [t], SiO 2B2 : The amount of SiO2 input in the sixth step [t], C / S B2 : The CaO / SiO2 (mass ratio) of the slag after blowing in the sixth step [-], α: The CaO slagification rate in the sixth step [-]. The CaO / SiO2 (mass ratio) (C / S B2 ) of the slag after blowing in the sixth step is expressed by the following formula. C / S B2 =(CaO B1 ×(1 - intermediate slag discharge rate)+CaO B2 ×slagification rate α) / (SiO 2B1 ×(1 - intermediate slag discharge rate)+SiO 2B2 ) By transforming the above formula, the following formula (1) is obtained. For the slagification rate α, based on the analysis results of the decarburization operation, α = 0.8 is adopted.

Equation

[0025] By setting the intermediate slag discharge rate of 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 outside the furnace together with the slag discharge slag, and as a result of the decarburization blowing in the sixth step, the phosphorus concentration in the molten iron can be sufficiently reduced. The intermediate slag discharge rate of formula (1) is preferably 55% or more, more preferably 60% or more, still more preferably 65% or more, and even more preferably 75% or more.

[0026] In the present invention, since an MnO source is blended into the slag so that the MnO input amount index becomes 0.10 in the first dephosphorization blowing, even though the basicity of the slag is high, the viscosity of the slag can be increased, and the forming height can be maintained high. Therefore, in either the first intermediate slag tapping or the second intermediate slag tapping, the intermediate slag tapping rate can be increased as intended.

[0027] In the first intermediate slag tapping in the third step, preferably, by setting the intermediate slag tapping final angle as a 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 made to contribute as a dephosphorization flux in the second dephosphorization blowing. As a method for determining the predetermined target range of the intermediate slag tapping final angle, first, the molten iron outflow start angle at which the 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 the molten iron flows out from the furnace can be measured. By setting the final tilting angle in the intermediate slag tapping to an angle shallower by 3 degrees or more than the measured molten iron outflow start angle, the amount of slag remaining in the converter can be ensured, and the dephosphorization ability in the second dephosphorization blowing can be sufficiently ensured. On the other hand, if the final tilting angle in the intermediate slag tapping is made too shallow compared to the measured molten iron outflow start angle, the amount of slag remaining in the converter becomes too large, and slopping from the furnace mouth due to slag forming occurs before the target oxygen amount in the second dephosphorization blowing is blown out, increasing the risk that the dephosphorization blowing cannot be continued. By making the final tilting angle in the intermediate slag tapping deeper than an angle 7 degrees shallower than the measured molten iron outflow start angle, it becomes possible to set the amount of slag remaining in the furnace to a suitable range.

[0028] As described above, a predetermined amount of slag remains in the converter after the first intermediate slag discharge. Since the remaining slag still retains its phosphorus removal ability, in the second phosphorus removal blowing, it is not necessary to additionally add a new CaO source. Therefore, in the present invention, in the second phosphorus removal blowing in the fourth step, it is not necessary to perform the flux input of any or all of the CaO-based, SiO2-based, Al2O3-based, and MnO-based fluxes.

[0029] Since MnO is contained in the slag after the second phosphorus removal blowing in the fourth step, the slag can be sufficiently formed during the phosphorus removal blowing, and the slag can be sufficiently discharged in the second intermediate slag discharge in the fifth step.

[0030] In the second phosphorus removal blowing in the fourth step, it is preferable to input 5 kg / ton or more of iron oxide. By inputting an iron oxide source in the second phosphorus removal blowing, the T.Fe concentration in the slag can be increased, the phosphorus removal ability can be improved, slag forming can be promoted, and the slag discharge rate in the second intermediate slag discharge in the fifth step can be further stabilized at a higher level. It is preferable to use sintered ore or sinter fines as the iron oxide.

[0031] Preferably in the present invention, in the second phosphorus removal blowing in the fourth step, the molten metal temperature at the end of the phosphorus removal blowing is set to 1380 °C or lower. Since the phosphorus removal reaction is more advantageous at a lower temperature, a decrease in the phosphorus removal efficiency can be suppressed.

[0032] Preferably in the present invention, in the second phosphorus removal blowing in the fourth step, the C concentration in the molten metal at the end of the phosphorus removal blowing is set to 3.0 mass% or more. Since the phosphorus removal reaction is more advantageous at a higher C concentration in the molten metal, a decrease in the phosphorus removal efficiency can be suppressed.

[0033] 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 more, a slag with high dephosphorization ability can be generated. Also, by setting the CaO / SiO2 (mass ratio) to 5.0 or less, 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 concentration and end point temperature depending on the target steel grade.

[0034] 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.

[0035] When the tapping is completed, slag remains in the converter. The remaining slag is processed in the eighth step. The converter may be tilted and all the slag in the converter may be discharged into the slag pan. Alternatively, all 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. Furthermore, 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.

[0036] In the eighth step of the previous heat, when all the slag in the converter is discharged, the flux used at the start of the second step, which is added to the converter at the start of the second step, will be used as the flux used in the second step of this heat. Also, in the eighth step of the previous heat, when a part or all of the slag is left in the converter, the flux generated in the sixth step in the previous heat before the first step, which is a part or all left in the converter, will be used as the flux used in the second step.

Example

[0037] Using a 350-ton top-bottom blown converter, pig iron was charged into the converter in the first step, flux was added in the second step to perform the first dephosphorization blowing, the first intermediate slag discharge was carried out in the third step, the second dephosphorization blowing was carried out in the fourth step, the second intermediate slag discharge was carried out in the fifth step, and the decarburization blowing was carried out in the sixth step. A converter blowing method was implemented.

[0038] After the sixth step, in the seventh step, the molten steel was tapped into a ladle, and in the eighth step, the remaining slag in the converter was processed. In some examples, in the eighth step of the previous heat, a part of the slag in the converter was left in the converter, and then the pig iron was charged into the converter in the first step of this heat. When leaving a 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 value after the decarburization blowing (the sixth 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 the SiO2 source in the second step of this heat, and the slag component analysis value after the dephosphorization blowing, a mass balance equation of 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 column of "Previous Heat Remaining Slag Original Unit" in Table 1. In the examples described as "0" in this column, the slag in the previous heat was processed without leaving it.

[0039] 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 11 are examples (examples of the present invention). In Table 1, the numerical values outside the scope of the present invention are underlined.

[0040]

Table 1

[0041] The Si concentration, P concentration, Mn concentration, and pig iron temperature of the charged pig iron charged into the converter are shown in Table 1.

[0042] In the first dephosphorization blowing in the second step, quicklime was added as the CaO source. In Table 1, the addition unit of the CaO source is described as the unit of T.CaO. The SiO2 unit was calculated from SiO2 generated by the combustion of Si contained 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.

[0043] In addition, in the present invention example, Mn ore was added as the MnO source by charging it from the scrap chute in the second step. In the comparative example, no MnO source was added. The addition unit of the MnO source is described as the Mn unit in Table 1. The units of the CaO source, SiO2 source, MgO source, Al2O3 source, and MnO source charged as auxiliary raw materials in the second step, together with the SiO2 unit generated by the combustion of Si contained in the charged main raw materials and the MnO unit generated by the combustion of contained Mn, were totaled as the overall total unit. The unit of the MnO source charged as an auxiliary raw material and the MnO unit generated by the combustion of contained Mn in the charged main raw materials were totaled as the Mn total unit. The Mn input amount index = Mn total unit / overall total unit was calculated to obtain the Mn input amount index (-).

[0044] The de-Si external oxygen unit in the second step and the de-Si external oxygen unit in the fourth step listed in Table 1 are the oxygen units obtained by subtracting the oxygen units consumed for de-Si from the oxygen units added in the second step and the fourth step respectively, and are used as indicators of the oxygen units that can contribute to the dephosphorization reaction.

[0045] In the first dephosphorization blowing in the third step, in the present invention example, the blowing was terminated immediately before the slag started to form and the slag discharge started from the furnace mouth, and the first intermediate slag discharge in the third step was performed. 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.

[0046] In the second dephosphorization blowing in the fourth step, neither the present invention example nor the comparative example added a CaO source or an MnO source. On the other hand, the present invention example added sinter powder as iron oxide in the unit shown in Table 1, while the comparative example did not add sinter powder. In the present invention example, blowing was terminated immediately before slag formation started and slag discharge began from the furnace mouth, and the second intermediate slag discharge in the fifth step was carried out. In the comparative example, 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. Table 1 shows the C concentration and P concentration after blowing at the end of the second dephosphorization blowing. It is clear that the P concentration after blowing in the present invention example is lower compared with the comparative example.

[0047] In the second intermediate slag discharge in the fifth step, both the present invention example and the comparative example carried out intermediate slag discharge with the molten iron outflow angle as the final tilting angle. Although the basicity of the present invention example was high, the intermediate slag discharge rate was high.

[0048] The results of the entire dephosphorization blowing in the second to fifth steps 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 + amount of P remaining in the slag that could not be completely discharged during intermediate slag discharge) / amount of P in the charged hot metal × 100 (%). Compared with the comparative example, in the present invention example, the dephosphorization rate is improved, the intermediate slag discharge rate is ensured without decrease, and the in-furnace P residue rate is at a low value, and the superiority of the present invention is clear.

[0049] After the second intermediate slag discharge in the fifth step, decarburization blowing in the sixth step was carried out. Table 1 shows the P concentration after blowing in the sixth step and the temperature after blowing. The metal P residue rate (%) = (P concentration after blowing in the sixth step / hot metal P concentration) × 100 is shown in Table 1. It is clear that the metal P residue rate in the present invention example is low 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, fluxes are added for the first dephosphorization blowing, in the third step, the first intermediate slag removal is carried out, in the fourth step, the second dephosphorization blowing is carried out, in the fifth step, the second intermediate slag removal is carried out, and in the sixth step, decarburization blowing is carried out. In the converter blowing method, in the second step, a CaO-containing flux and an MnO source are added as fluxes, The slag composition (calculated value) at the end of the second step is CaO / SiO ２ (mass ratio) = 1.7 to 2.3, and the MnO input amount index = 0.10 to 0.

25. the total intermediate slag removal rate in the third step and the fifth step is adjusted so that the intermediate slag removal rate shown by the following formula (1) is 50% or more. A converter blowing method characterized by this. 【Number 1】 CaO Ｂ１ : CaO input up to the 4th process [t], CaO Ｂ２ : CaO input in the 6th process [t], SiO ２Ｂ１ : SiO generated up to the 4th process ２ [t], SiO ２Ｂ２ : SiO input in the 6th process ２ [t], C / S Ｂ２ : CaO / SiO in the slag after blowing in the 6th process ２ (mass ratio) [-], α = 0.8: CaO conversion rate in the 6th process [-] Regarding the MnO input amount index (-), for the unit amounts of the CaO source, SiO ２ source, MgO source, Al ２ O ３ source, and MnO source input as auxiliary raw materials in the second step, further add the unit amount of Si contained in the charged main raw material converted to SiO ２ converted SiO ２ unit amount, and the unit amount of MnO converted from the contained Mn in the charged main raw material to calculate the total overall unit amount. Then, sum up the unit amount of the MnO source input as an auxiliary raw material and the unit amount of MnO generated by the combustion of the contained Mn in the charged main raw material to obtain the total MnO unit amount. Calculate the MnO input amount index = total MnO unit amount / total overall unit amount.

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

3. As a method for determining the determined target range of the intermediate slag removal final angle, in the intermediate slag removal 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 removal 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 determined target range of the intermediate slag removal 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 is 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. Based on the amount of the remaining slag and the slag analysis value or the slag blending calculation value, the amounts of CaO, SiO ２ amount, MgO amount, Al ２ O ３ amount, and MnO amount are estimated, and part or all of the auxiliary materials charged as the CaO source, SiO ２ source, MgO source, Al ２ O ３ source, and MnO 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, and MnO amount are estimated, and a part or all of the auxiliary materials to be charged as the CaO source, SiO ２ source, MgO source, Al ２ O ３ source, and MnO 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 set to 1380°C or lower, and the C concentration in the molten metal is set to 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 set to 1380°C or lower, and the C concentration in the molten metal is set to 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 set to 1380°C or lower, and the C concentration in the molten metal is set to 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.

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