JP6037933B2 - Hot metal dephosphorization method with high iron yield - Google Patents

Description

  The present invention relates to a hot metal dephosphorization method for increasing the iron yield when performing dephosphorization of hot metal in an upper bottom blown converter type vessel.

Conventionally, in a steel factory, dephosphorization treatment for reducing the phosphorus concentration in the hot metal is performed on the hot metal discharged from the blast furnace. In the hot metal dephosphorization treatment, the hot metal is charged into the container and the treatment is performed by blowing oxygen gas onto the hot metal. In performing dephosphorization of hot metal, various techniques as shown in Patent Documents 1 to 4 have been developed.
In Patent Document 1, in the hot metal dephosphorization method, the basicity of the hot metal is 2.5 to 3.5 before the start of the desiliconization period, and the height from the hot metal surface of the hot metal to the outlet of the upper blowing lance is set to 1. In the desiliconization period, the oxygen flow rate from the top blowing lance is set to 3.0 to 4.0 Nm ³ / min · t, the bottom blowing stirring power is set to 100 to 300 W / t. In the period, the oxygen flow rate from the top blowing lance is 1.3 to 1.6 Nm ³ / min · t, and the bottom blowing stirring power is 100 to 300 W / t. In the dephosphorization period, the oxygen flow rate from the top blowing lance is 1 .3 to 1.6 Nm ³ / min · t, and bottom blowing stirring power is set to 500 W / t or more.

In Patent Document 2, in the hot metal dephosphorization process, the gas flow rate supplied from the bottom blowing nozzle is set to 0.1 to 0.3 Nm ³ / min · t, and the collision pressure of the top blowing oxygen gas on the hot metal surface is adjusted. doing.
In Patent Document 3, in the molten steel refining method, the converter bottom blowing agitation energy is set to 1.0 kW / t · s or more, and the converter top blowing agitation energy is 2.0 kW / t until the end of the second step. -It is set as the top blowing stirring energy made into s or more, and is reduced to 1.5 kW / t.s or less at the end of the second process.

In Patent Document 4, the top blow lance is used under the condition that the mass% ratio of CaO to SiO ₂ obtained by analyzing slag collected from the converter after completion of dephosphorization is 1.8 to 2.4. The ratio of the hot spot area formed by oxygen blown from the hot metal bath surface to the hot metal bath surface area in the converter is 0.15 or more.

JP 2009-052070 A Japanese Patent Laid-Open No. 08-104912 JP 2003-064411 A JP 2010-095785 A

  The dephosphorization process in Patent Documents 1 to 4 is a technique for increasing the phosphorus reduction rate (dephosphorization efficiency) with respect to hot metal or shortening the process time of the dephosphorization process. Is not a technology to improve Therefore, when performing dephosphorization processing using a converter type vessel, even if the techniques of Patent Documents 1 to 4 are used, it is difficult to improve the iron yield.

  Accordingly, in view of the above problems, an object of the present invention is to provide a hot metal dephosphorization method capable of increasing the iron yield after hot metal dephosphorization treatment using a converter type vessel.

In order to achieve the above-described object, the present invention takes the following technical means.
In the hot metal dephosphorization method with high iron yield according to the present invention, when the hot metal dephosphorization process is performed in the converter type vessel, when the [Si] in the hot metal is 0.10% by mass or more, First input energy ε _T by oxygen gas is 500 W / t or more and 3000 W / t or less, second input energy ε _B by bottom blowing gas is 400 W / t or more and 2000 W / less or less, and ε _T / ε _B is 0.5 or more and 3 or less. When the [Si] in the hot metal exceeds 0.00 mass% and less than 0.10 mass%, the first input energy ε _T is decreased by 300 W / t or more, and the first input after the decrease is made The energy ε _{T is set} to 50 W / t or more and 400 W / t or less, the second input energy ε _B is increased by 200 W / t or more, and the increased second input energy ε _{B is set} to 1000 W / t or more and 3000 W / t or less. , Ε _T / ε _B is 0.02 or more and 0.3 or less. In addition, the composition of the hot metal is C: 4.34 to 4.75 [mass%], Si: 0.25 to 0.58 [mass%], Mn: 0.20 to 0.28 [mass%], P: 0.103 to 0.134 [mass%], The auxiliary raw materials used in the dephosphorization treatment are calcined lime: 2950-6960 kg, mill scale: 10970-18110 kg, iron ore: 1910-2030 kg, and the height from the hot water surface of the lance for blowing the above-blown oxygen gas is , 2.0m to 3.2m.

  ADVANTAGE OF THE INVENTION According to this invention, the iron yield after the dephosphorization process of the hot metal by a converter type | mold container can be made high.

It is a general view of a converter type vessel which performs dephosphorization processing, and an enlarged view of an upper blowing gas nozzle. A first input energy epsilon _T is a graph showing the relationship between the second input energy epsilon _B. It is the figure which put together the relationship between the iron yield and charge number in an Example and a comparative example.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
As shown in FIG. 1, in the hot iron dephosphorization method with high iron yield of the present invention, hot metal 1 is charged into an upper bottom blown converter type vessel (referred to as a converter type vessel) 2 to perform a dephosphorization process. Is targeted.
In the hot metal dephosphorization method of the present invention, the hot metal may be vigorously stirred at 1000 W / t or more as will be described later. Since there is a possibility that the mixture may be discharged to the outside due to stirring, the dephosphorization process using these kneading vehicles and hot metal transfer containers is not targeted. That is, in the present invention, the dephosphorization process is performed using a converter type vessel having a sufficient volume with respect to the molten iron to be charged.

  When the dephosphorization process is performed by the converter-type container 2, the hot metal 1 is charged into the furnace body 3 and auxiliary materials such as a slagging material are charged. Further, an upper blowing gas lance 5 is inserted into the furnace port 4 of the furnace body 3, and oxygen gas is blown onto the hot metal 1 by the upper blowing gas lance 5, and from the bottom blowing tuyere 7 provided at the bottom portion 6 of the furnace body 3. Dephosphorization is performed by blowing a gas such as an inert gas into the hot metal.

In the present invention, the dephosphorization process can be performed at a high iron yield by optimizing the energy of the oxygen gas blown from the top blowing gas lance 5 to the molten iron 1 or the gas blown from the bottom blowing tuyere 7 to the molten iron 2. ing.
Hereinafter, the hot metal dephosphorization method having a high iron yield will be described in detail.
The hot metal discharged from the blast furnace generally contains 0.3 mass% or more of Si. In the present invention, this hot metal is directly charged into a converter-type vessel to perform dephosphorization treatment, or after desiliconization of the molten iron [Si] to about 0.2 to 0.3 mass% before dephosphorization treatment. Dephosphorization treatment is performed by charging the converter type container.

In the dephosphorization process, when [Si] in the hot metal is 0.1 mass% or more, the dephosphorization reaction does not proceed, and the dephosphorization reaction proceeds after [Si] is less than 0.1 mass%. For this reason, in the present invention, the processing conditions are changed at the stage where the hot metal [Si] is 0.1 mass% or more and the stage where [Si] is less than 0.1 mass%.
Specifically, when [Si] in the hot metal is 0.1 mass% or more, the first input energy ε _T by the top blowing oxygen gas, that is, oxygen gas is blown to the hot metal 1 by the top blowing gas lance 5. and the first range input energy epsilon _T following 500 W / t or more 3000W / t when the.

In the stage where the hot metal [Si] is 0.1 mass%, the desiliconization reaction takes precedence over the decarburization reaction or the dephosphorization reaction. At this time, if the energy applied to the hot metal surface of the oxygen gas is too small, the proportion of the supplied oxygen gas that contributes to the desiliconization reaction (desiliconization oxygen efficiency) is significantly reduced. That is, when the first input energy epsilon _T is less than 500 W / t is decreased de silicate oxygen efficiency.

Therefore, the first input energy ε _T when oxygen gas is blown onto the hot metal 1 by the top blowing gas lance 5 is 500 W / t or more, preferably 600 W / t or more, more preferably 700 W / t or more. However, if the first input energy epsilon _T too, scattering of the molten iron is increased, since the yield decreases, and the upper limit of the first input energy epsilon _T and 3000W / t. The upper limit of the first input energy epsilon _T is preferably, 2800W / t, more preferably preferably set to 2600W / t.

The first input energy ε _T can be calculated using Equation (1).

Equation (1) is expressed as follows: “Sai Kai, Kazuo Okouchi, Mitsuo Higuchi, Masazumi Hirai,“ Examination by Cold Model of Top Blow Converter Characteristics ”; Iron and Steel, Vol. 69 (1983), No. 2, 235 Page ".
The distance x from the tip of the top blowing gas lance 5 to the hot metal (hot metal surface) 1 is obtained from the measured value of the height of the molten iron irradiated with microwaves before dephosphorization starts, that is, when the hot metal is stationary. Value. As the gas molecular weight M, the molecular weight “32” of pure oxygen was used. The oxygen gas blown onto the hot metal 1 may be any gas containing oxygen, and may be, for example, a mixed gas in which oxygen and argon are mixed, a mixed gas in which oxygen and nitrogen are mixed, or air. In this case, the gas molecular weight M may be determined according to the gas composition.

The number n of nozzle holes of the top blowing gas lance is the number of oxygen gas nozzle holes 8 provided in the top blowing gas lance 5, as shown in FIG. Is the angle ξ of the nozzle hole 8 with respect to the longitudinal direction (axial direction). The outlet diameter of the nozzle hole is the diameter d of the nozzle hole 8.
Now, in the stage where the hot metal [Si] is 0.1 mass% or more (desiliconization period), the desiliconization reaction proceeds by supplying oxygen from the top blowing gas lance 5, but the desiliconization reaction is efficiently performed. In order to proceed (in order to shorten the time required for hot metal [Si] to reach 0% by mass as quickly as possible), it is necessary to blow gas (bottom blowing gas) from the tuyere 7 and stir the hot metal. It is. In the present invention, the second input energy ε _B by the bottom blowing gas is set to 400 W / t or more, so that the hot metal is efficiently stirred and the desiliconization reaction proceeds. Efficient desiliconizing reaction by agitation of the bottom blowing gas to saturate when the second input energy epsilon _B exceeds 2000 W / t, the upper limit value of the second input energy epsilon _B is in the 2000 W / t .

The second input energy ε _B can be calculated using Equation (2).

Equation (2) is described in “Kazumi Mori, Masamichi Sano,“ Dynamics of Injection Metallurgy ”; Iron and Steel, Vol. 67 (1981), No. 6, page 687”.
The hot metal temperature T changes every moment during the dephosphorization process. In this embodiment, the temperature immediately before the hot metal is charged into the converter type vessel is used. The molten iron density (hot metal density) ρ varies depending on the hot metal component and the hot metal temperature, but in this embodiment, 7000 kg / m ³ was used. Moreover, the molten iron depth (molten metal depth) h varies depending on the amount of molten iron charged into the converter type vessel (the amount of molten iron charged) and the wear state of the refractory applied in the converter type vessel. In this embodiment, the amount of molten iron charged was 262 t, which is an average value of actual operation, and the molten iron depth h was uniformly 2 m based on the profile immediately after the construction of the refractory in the converter type vessel. Note that the constants of 9.8 m / s ² and 101325 Pa were used for the gravitational acceleration g and the atmospheric pressure p, respectively.

Further, in the de珪期[Si] in the molten iron is in 0.1% by mass or more, and the first energy input epsilon _T, the balance between the second input energy epsilon _B is also important. In the present invention, ε _T / ε _B that is a ratio of the first input energy ε _T and the second input energy ε _B is set to 0.5 or more and 3 or less. When the ratio of the first input energy ε _T to the second input energy ε _B is increased (ε _T / ε _B > 1), the desiliconization reaction efficiency is improved by the first input energy ε _T. However, if the ratio of the first input energy ε _T to the second input energy ε _B is too large (ε _T / ε _B > 3), the scattered hot metal will not return while being taken into the slag. Since the grain iron loss increases, the upper limit of ε _T / ε _B needs to be “3”.

When ε _T / ε _B is less than 0.5, the first input energy ε _T has little contribution to the desiliconization reaction, so the lower limit of ε _T / ε _B is set to 0.5.
As described above, in the dephosphorization treatment of the hot metal, in the desiliconization period when [Si] of the hot metal is 0.1 mass% or more, the first input energy ε _T = 500 W / t to 3000 W / t, the second input energy ε _B = 400 W / t to 2000 W /, and 0.5 ≦ ε _T / ε _B ≦ 3. That is, in the de-珪期, as shown in FIG. 2, the first input energy epsilon _T and a second input energy epsilon _B, are in the area indicated by de珪領region A.

Now, during the dephosphorization process, when [Si] of the hot metal becomes less than 0.1 mass%, the dephosphorization period proceeds from the desiliconization period to the dephosphorization reaction. In the dephosphorization stage, the decarburization reaction also proceeds. Especially, when the [Si] in the hot metal becomes zero, the decarburization reaction becomes the main, so the processing conditions are changed until [Si] in the hot metal becomes 0% by mass. There is a need to.
During the dephosphorization process, the processing conditions are changed depending on the hot metal [Si]. However, after the hot metal is sampled during the dephosphorization process and the concentration of [Si] is analyzed, the switching timing is changed. Therefore, [Si] in the dephosphorization process is obtained using the formula (3). In addition to gaseous oxygen, oxygen supplied into the furnace includes oxygen contained in scales and iron ore, which will be described later, but this is not considered.

Specifically, first, in the period (dephosphorization period) in which [Si] in the hot metal determined by the formula (3) exceeds 0.00 mass% and is less than 0.1 mass%, first input energy the epsilon _T with decreasing above 300 W / t, and the first input energy epsilon _T after reduction to less 50 W / t or 400W / t.
In the dephosphorization period, it is necessary to increase the FeO concentration in the slag and promote the dephosphorylation reaction. Here, when the first input energy ε _T is increased at the stage of entering the dephosphorization period as in the desiliconization period, the FeO concentration in the slag is difficult to increase. by reducing or 300 W / t than the input energy epsilon _T de珪期, is set to be short term increase the FeO concentration in the slag. That is, to improve the dephosphorizing capacity of slag by a reduction of the first input energy epsilon _T.

Further, in the dephosphorization period, the decarburization reaction is likely to proceed and CO gas is likely to be generated. In the dephosphorization period, if the first input energy ε _T is kept as high as in the desiliconization period, a large amount of CO gas is generated by the decarburization reaction, and the granular iron in the hot metal rises with dust by the CO gas. since the iron losses and, after reduction of the first input energy epsilon _T is in the first input energy epsilon _T below 400W / t. In dephosphorization stage, when the first input energy epsilon _T will be less than 50 W / t, since the slag of oxygen is not sufficiently supplied to, it does not proceed sufficiently dephosphorization reaction by reduction of FeO concentration, first input energy the upper limit value of ε _T it is necessary to 50W / t.

As described above, in the dephosphorization period, the FeO concentration of the slag is increased by lowering the first input energy ε _{T so} that the dephosphorization reaction proceeds efficiently. However, by stirring the hot metal with the bottom blowing gas, Therefore, it is necessary to actively promote the dephosphorylation reaction. Therefore, in the present invention, in the dephosphorization period, the second input energy ε _B is increased contrary to the first input energy ε _T. Specifically, the second input energy ε _B is increased by 200 W / t or more, and the second input energy ε _B after the increase is set to 1000 W / t or more and 3000 W / t or less.

In dephosphorization stage, as described above, since the lowering of the first input energy epsilon _T, stirring power of the entire hot metal is weakened. If the second input energy ε _B is increased by at least 200 W / t, the stirring force due to the decrease in the first input energy ε _T can be supplemented by the bottom blowing gas. Since the force from which the energy from the bottom blowing gas acts on the stirring is stronger than the force by the top blowing oxygen gas, the increase amount of the second input energy ε _B is smaller than the decrease amount of the first input energy ε _T. Even if it is small, the stirring force can be supplemented.

In the dephosphorization period, the dephosphorization reaction proceeds efficiently if the power for stirring the hot metal by the bottom blowing gas is large. If the second input energy ε _B is at least 1000 W / t or more, the stirring power of the hot metal by the bottom blowing gas is strong and the dephosphorization reaction proceeds. On the other hand, if the second input energy ε _B is made larger than 3000 W / t, the hot metal stirring is too strong, and FeO in the slag is excessively reduced by the carbon C in the hot metal, thereby reducing the dephosphorization ability of the slag. to have the upper limit of the second energy input epsilon _B and 3000W / t.

Further, in the dephosphorization stage, it is necessary to reduce the proportion of the first input energy epsilon _T for the second input energy epsilon _B, is the first input energy epsilon _T ratio of second input energy epsilon _B epsilon _T / epsilon is 0.02 to 0.3 of _B.
When ε _T / ε _B = 0.3, the first input energy ε _T is neither too strong nor too weak, so the FeO concentration in the slag is dephosphorized by oxygen gas from the top blowing gas lance. To the concentration required for However, if ε _T / ε _B > 0.3, the FeO concentration in the slag can be increased, but the FeO concentration is higher than the concentration necessary for the dephosphorization process and excessively increased. When viewed as the entire vessel, the Fe content in the converter type vessel is taken in as FeO, resulting in iron loss. Therefore, ε _T / ε _B needs to be 0.3 or less. On the other hand, if ε _T / ε _B is too small, it becomes difficult to maintain the FeO concentration in the slag at a concentration necessary for the dephosphorization treatment, so the lower limit value of ε _T / ε _B needs to be 0.02. is there.

As described above, in the dephosphorization process of the hot metal, the first input energy ε _T is reduced by 300 W / t or more during the period when [Si] in the hot metal exceeds 0.00 mass% and is less than 0.10 (dephosphorization period). The first input energy ε _T = 50 W / t to 400 W / t, the second input energy ε _B is increased by 200 W / t or more, and the second input energy ε _B = 1000 W / t to 3000 W / t, 0.02 ≦ ε _T / ε _B ≦ 0.3. That is, in the dephosphorization period, as shown in FIG. 2, the first input energy ε _T and the second input energy ε _B are set to a region indicated by a dephosphorization region B.

  Tables 1-2 summarize the examples in which the dephosphorization treatment was performed by the hot iron dephosphorization method of the present invention and the comparative examples in which the dephosphorization treatment was performed by a method different from the present invention. is there.

  In the examples and comparative examples, the following treatment was performed in the previous step of performing the dephosphorization treatment. Specifically, after the hot metal was discharged in the blast furnace, desiliconization treatment was performed as needed from the time the hot metal was discharged until the hot metal was received in the kneading car. For example, when [Si] at the time of brewing is 0.4 mass% or more, desiliconization treatment was performed. Note that the desiliconization process is optional, and may be directly received by a kneading vehicle without performing the desiliconization process after brewing.

In addition, after the hot metal was charged into the kneading car, desulfurization treatment was performed on the hot metal in the kneading car, and [S] in the hot metal was reduced to 0.001 mass% to 0.007 mass%. The hot metal desulfurization treatment was carried out on all chaotic vehicles.
The components of the hot metal before dephosphorization are [C] = 4.2 to 4.8 mass%, [Si] = 0.25 to 0.58 mass%, [Mn] = 0.20 to 0.31 mass. %, [P] = 0.101 to 0.134 mass%. Also, the hot metal temperature before dephosphorization treatment was set to 1372 to 1384 ° C. The hot metal temperature and the component values are those when a part of the hot metal is sampled when the hot metal is charged into a container (a ladle) that conveys the hot metal from the kneading wheel to the converter type container.

The amount of hot metal charged in the converter-type vessel is 261.7 to 262.3 tons, and 2650 to 7370 kg of calcined lime, 9520 to 18110 kg of mill scale, and 1890 to 2420 kg of iron ore are used as auxiliary materials during the dephosphorization process. did. As the oxygen gas during the dephosphorization treatment, 2200-3500 Nm ^{3 was} used. The dephosphorization time was 7 to 14 minutes. The components of the hot metal after the dephosphorization treatment are [C] = 3.3 to 3.9% by mass, [Si] = 0.01% by mass, [Mn] = 0.02 to 0.06% by mass, [P ] = 0.011-0.058 mass%. The hot metal temperature after dephosphorization was set to 1309 to 1358 ° C. The hot metal temperature and the component values are those when a part of the hot metal was collected immediately after the dephosphorization treatment.

  In Examples and Comparative Examples, the iron yield was determined and evaluated using the formula (4).

  Formula (4) considers the iron content contained in the auxiliary material etc. which were supplied at the time of dephosphorization besides the iron content contained in the hot metal. For example, in Examples and Comparative Examples, in addition to hot metal, mill scale and iron ore are charged into the converter-type vessel as an auxiliary material, so the iron content in these auxiliary materials is added to the iron content in the hot metal. And all iron. In addition, in the calculation of the total iron content charged in the furnace, the iron concentration in the mill scale and the iron concentration in the iron ore are values that are the operation results, and the iron concentration in the mill scale is 50% (0.5), The iron content in the iron ore was 63% (0.63).

In addition, when there exists iron content other than having shown in this embodiment, it is desirable to obtain | require the total iron content charged in the furnace by adding the other charged iron content. For example, when iron raw materials other than hot metal, such as scrap iron and cold iron, recycled slag, dust collection dust, etc. are used as auxiliary raw materials, the iron content included in these is also included in the calculation of the iron yield.
In Examples 1 to 19, in the stage where [Si] in the hot metal is 0.10% by mass or more (the column of the desiliconization period), the first input energy ε _T is 500 W / t or more and 3000 W / t or less, the second The input energy ε _B is 400 W / t or more and 2000 W / less or less, and ε _T / ε _B is 0.5 or more and 3 or less.

Further, as shown in the column of [Si] when switching from the desiliconization period to the dephosphorization period, the first input is performed during a period in which [Si] in the molten iron exceeds 0.00 mass% and is less than 0.10. The energy ε _T and the second input energy ε _B are to be changed. Then, in the dephosphorization period after the change, the first input energy ε _T is decreased by 300 W / t or more (the column of the decrease in ε _T from the desiliconization period to the dephosphorization period), and the first input energy ε after the decrease _{T is set} to 50 W / t or more and 400 W / t or less. In addition, in the dephosphorization period after the change, the second input energy ε _B is increased by 200 W / t or more (the column of the increase in ε _B from the desiliconization period to the dephosphorization period), and the second input energy ε after the increase is increased. _{B is set} to 1000 W / t or more and 3000 W / t or less. Furthermore, ε _T / ε _{B is set} to 0.02 or more and 0.3 or less.

On the other hand, in Comparative Examples 20 to 38, the conditions in the desiliconization period (the value of the first input energy ε _T , the value of the second input energy ε _B , the value of ε _T / ε _B ), or the conditions in the dephosphorization period ( The first input energy ε _T decreases, the second input energy ε _B increases, the first input energy ε _T , the second input energy ε _B , and ε _T / ε _B ). The specified condition is not met.

FIG. 3 summarizes the relationship between the iron yield and the number of charges in the examples and comparative examples. When all the conditions defined in the present invention are satisfied as in the embodiment, the iron yield can be surely increased to 97% or more. In the case of a comparative example that did not meet even one of the conditions defined in the present invention, the iron yield was less than 97%.
As mentioned above, according to this invention, the iron yield after the dephosphorization process of the hot metal by a converter type | mold container can be made high.

  The embodiment disclosed this time should be considered as illustrative in all points and not restrictive. In particular, in the embodiment disclosed this time, matters that are not explicitly disclosed, for example, operating conditions and operating conditions, various parameters, dimensions, weights, volumes, and the like of a component deviate from a range that a person skilled in the art normally performs. Instead, values that can be easily assumed by those skilled in the art are employed.

DESCRIPTION OF SYMBOLS 1 Hot metal 2 Converter type vessel 3 Furnace body 4 Furnace port 5 Top blowing gas lance 6 Bottom part 7 Bottom blowing tuyere 8 Nozzle hole

Claims (1)

When dephosphorizing hot metal in a converter-type vessel,
At the stage where [Si] in the hot metal is 0.10% by mass or more, the first input energy ε _T by the top blowing oxygen gas is 500 W / t or more and 3000 W / t or less, and the second input energy ε _B by the bottom blowing gas is set. 400 W / t or more and 2000 W / less or less, and ε _T / ε _B is 0.5 or more and 3 or less,
When [Si] in the hot metal exceeds 0.00 mass% and less than 0.10 mass%, the first input energy ε _T is decreased by 300 W / t or more, and the first input energy ε after the decrease _{T is set} to 50 W / t or more and 400 W / t or less, the second input energy ε _B is increased by 200 W / t or more, the increased second input energy ε _{B is set} to 1000 W / t or more and 3000 W / t or less, and ε TetsufuTome high hot metal dephosphorization method characterized by the _T / epsilon _B 0.02 to 0.3.
In addition, the composition of the hot metal is C: 4.34 to 4.75 [mass%], Si: 0.25 to 0.58 [mass%], Mn: 0.20 to 0.28 [mass%], P: 0.103 to 0.134 [mass%], The auxiliary raw materials used in the dephosphorization treatment are calcined lime: 2950-6960 kg, mill scale: 10970-18110 kg, iron ore: 1910-2030 kg, and the height from the hot water surface of the lance for blowing the above-blown oxygen gas is , 2.0m to 3.2m.