JP7614566B2 - Method for dephosphorizing molten iron - Google Patents

Description

The present invention relates to a method for removing phosphorus from molten iron by contacting the molten iron filled in a refining vessel such as a converter, ladle, or torpedo car with slag formed on the molten iron.

In recent years, from the viewpoint of preventing global warming, the steel industry has also been developing technologies to reduce the consumption of fossil fuels and reduce _CO2 gas emissions. In conventional integrated steelworks, iron ore is reduced with carbon to produce molten pig iron. To produce this molten pig iron, a carbon source of about 500 kg per ton of molten pig iron is required for the reduction of iron ore, etc. On the other hand, when a cold iron source such as iron scrap is used as a raw material for molten steel, the carbon source required for the reduction of iron ore is not required, and even when the energy required to melt the cold iron source is taken into account, the emission of _CO2 gas can be significantly reduced, so the use of the cold iron source is expected.

Molten pig iron contains carbon (C), silicon (Si), phosphorus (P), etc., among which phosphorus is considered to be a harmful component that deteriorates the performance of metallic materials. Therefore, in the conventional steelmaking process, a dephosphorization flux is added at the molten pig iron stage to form slag mainly composed of CaO, _SiO2 , and FeO, and the slag is brought into contact with the molten pig iron to remove phosphorus from the molten pig iron. This is because, at the molten pig iron stage, the carbon contained in the molten pig iron is close to its saturation concentration and the temperature is relatively low (approximately 1250 to 1450°C), which is a favorable condition for the dephosphorization reaction to proceed thermodynamically.

As mentioned above, if we switch from the conventional method of reducing iron ore with carbon using blast furnaces and converters to a method of melting cold iron sources such as scrap iron and reduced iron, the carbon concentration obtained will be lower and the temperature will be higher (1550°C or higher). This creates less favorable conditions for dephosphorization than before, and there is a risk that the dephosphorization capacity will decrease.

In addition, in the conventional steelmaking process using blast furnaces and converters, about 2 tons of _CO2 is generated per ton of molten iron, and there is an urgent need to reduce this _CO2 emission. Therefore, instead of iron ore, a cold iron source such as iron scrap is used as a raw material to ensure the amount of heat required to melt the cold iron source (hereinafter referred to as "thermal tolerance"), and the molten iron blending ratio, which represents the ratio of molten iron to the amount of crude steel, is reduced to reduce the amount of _CO2 emission. In this case, it is necessary to increase the thermal tolerance by using the exothermic reaction of Si + _O2 = _SiO2 , either unavoidably due to a decrease in the blast furnace's iron output or intentionally by changing the blast furnace operation conditions to increase the Si in the molten iron.

The amount of dephosphorization is considered to be proportional to the basicity (hereinafter referred to as "basicity C/ _S "), which is the ratio (mass%CaO/mass% _SiO2 ) of the CaO concentration (mass%CaO) and the _SiO2 concentration (mass%SiO2) of the slag. As described above, if the _SiO2 in the molten iron increases, the basicity C/S decreases, and the amount of dephosphorization decreases. If a CaO-containing substance is added to increase the basicity C/S to avoid this, the amount of dephosphorization can be secured, but the amount of slag increases. For example, if the target basicity C/S is 2.0 and the Si concentration in the molten iron is doubled, the amount of slag to obtain the same amount of dephosphorization will double.

This increase in the amount of slag causes two problems. The first is that the increase in the amount of slag makes it easier for a phenomenon called slopping to occur, in which the slag in the refining vessel overflows outside the furnace, significantly impairing the stability of the refining. The second is that, even if the thermal margin is increased by increasing the amount of _SiO2 in the molten iron as described above, the amount of heat is lost in melting the slag, so the thermal margin for melting the cold iron source (scrap), which was the original purpose, is reduced.

For this reason, various technologies and inventions have been proposed to solve the above problems regarding the method of dephosphorization from molten iron using slag. For example, Non-Patent Document 1 proposes a technology for dephosphorization of molten steel using a soda-based flux, which has a higher dephosphorization ability than CaO-based flux.

Furthermore, Non-Patent Document 2 proposes a dephosphorization technology in which the total iron concentration T.Fe (mass%) in the slag is set to 15-20%, so that the basicity C/S at the end of the treatment is 0.9-1.1, and the phosphorus distribution ratio L _P = (%P)/[%P](-), which is the ratio of the phosphorus concentration in the slag (hereinafter, indicated as "(%P)") to the phosphorus concentration in the molten iron (hereinafter, indicated as [%P]), is set to about 1.5 to about 2.0 powers of 10, which allows a certain degree of dephosphorization.

Furthermore, Non-Patent Document 3 discloses the behavior and stable existence limit of the phosphorus-concentrated phase when the slag basicity C/S is changed to the low basicity side.

Patent Document 1 also proposes a dephosphorization technology in which oxygen gas and powdered CaO source are injected from a top lance, and the lance height, the ratio of the powdered CaO injection rate to the oxygen gas flow rate F _CaO /F _O2 (kg/Nm ³ ), and the powdered CaO injection rate per total cross-sectional area of the nozzle hole (kg/(min·t·m ² )) are specified, making it possible to reduce the basicity C/S to 1.4 at the end of the treatment.

In addition, Patent Document 2 proposes a technology for dephosphorization of molten pig iron by blowing and adding oxygen gas and at least a part of a dephosphorization solvent from a top blowing lance, in which the minimum value of the basicity C/S in the furnace from the start to the end of the dephosphorization process is set to 0.8 or more and less than 1.2, thereby achieving efficient dephosphorization.

Patent Document 3 also proposes a technology for reducing the amount of undissolved lime remaining in hot metal dephosphorization treatment by setting the slag basicity to 0.8-1.8, the T.Fe content in the slag to 8-19 mass%, and setting the unit consumption of lumpy lime source of 10 mm or more to 10 kg or less per ton of hot metal.

JP 2017-226874 A JP 2015-42780 A JP 2002-105526 A

Marukawa et al.: Tetsu-to-Haganen, 67 (1981) No. 2, 323-332 Ogawa et al.: Tetsu-to-Haganen, 87 (2001) No. 1, 21-28 Uchida et al.: Iron and Steel, 102 (2016) No. 12, 691-697

However, the technology disclosed in Non-Patent Document 1 has problems with the treatment of the slag that is generated. Because soda-based slag is water-soluble, highly alkaline water is generated during slag treatment. In the steelmaking process, there has been a history of aiming to make slag sodium-free due to environmental considerations, so it is difficult to use soda-based flux. Furthermore, soda-based flux is highly reactive, which may cause wear on the refractories used in refining vessels, leading to increased operating costs.

The technology disclosed in Non-Patent Document 2 has a problem in that it is necessary to increase the total iron concentration (T.Fe) in the slag in order to obtain a certain level of dephosphorization ability. In order to increase the total iron concentration (T.Fe) in the slag, it is necessary to oxidize iron by the reaction Fe + 1/2O ₂ = FeO. At least a part of the high FeO slag in the furnace thus obtained is discharged outside the furnace, which reduces the iron yield of the operation.

According to the technology disclosed in Non-Patent Document 3, the phosphorus concentration in the slag decreases when the basicity of the slag after desiliconization reaches a border of about 0.8, but the relationship between the dephosphorization ability of the slag on the low basicity side and the total iron concentration T.Fe is not disclosed.

In the technology described in Patent Document 1, although it is said that the basicity C/S can be reduced to 1.4, only the case where the basicity C/S is 1.8 is disclosed in the examples. In the process where the basicity C/S is less than 1.8, the operation may become unstable due to the slopping phenomenon, and dephosphorization failure may occur, and no measures against these are disclosed. Furthermore, it is necessary to control the height from the tip of the top blowing lance to the surface of the molten iron to 3 m or more and less than 5 m. At that time, the atmospheric temperature becomes high due to the heat generated by the secondary combustion of the CO gas and the blowing oxygen by CO + 1/2O ₂ = CO ₂ , and the refractory used in the refining vessel is worn out. In particular, when combined with the above-mentioned slopping, the wear of the refractory may be further accelerated, leading to an increase in operating costs.

The technology disclosed in Patent Document 2 has a problem in that it is unclear to what extent the basicity C/S can be reduced at the end of processing. In the examples, although it is possible to confirm that the basicity C/S at the end of processing is around 2.0, there is no data showing a basicity C/S of less than 2.0 or a minimum basicity C/S of 0.8 or more.

The technology disclosed in Patent Document 3 only discloses a slag composition for increasing the saturated CaO concentration and reducing undissolved lime in the slag while suppressing refractory corrosion, but does not disclose a slag composition for increasing the dephosphorization ability of the slag, that is, the phosphorus distribution ratio L _P = (% P)/[% P](-), which is expressed as the ratio of the phosphorus concentration in the slag (% P) to the phosphorus concentration in the molten steel [% P] in an equilibrium state.

From the above, the conventional techniques have been unable to solve the problems of the transition to molten steel conditions with high temperatures and reduced carbon sources, or the decrease in dephosphorization ability associated with an increase in the Si concentration in molten iron, or the decrease in thermal tolerance for melting the cold iron source (scrap) due to an increase in the amount of slag to compensate for this, making it difficult to achieve a reduction in _CO2 emissions in the steelmaking process.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for dephosphorizing molten iron, which can efficiently dephosphorize molten iron using a small amount of slag by contacting the molten iron with slag formed by a flux added to the molten iron, and can reduce _CO2 emissions in the steelmaking process.

In view of these problems, the inventors have conducted extensive research and found that, in slags mainly composed of CaO, _SiO2 , and FeO, a region in which the phosphorus distribution ratio L _P can be maintained at a high value appears in a low C/S basicity region, and further, the range of C/S basicity in which the phosphorus distribution ratio L _P can be maintained at a high value becomes wider as the total iron concentration T.Fe in the slag decreases. This overturns the common sense of the conventional steelmaking process that the phosphorus distribution ratio L _P of slag decreases due to a decrease in the basicity C/S or the total iron concentration T.Fe, as reported in, for example, Non-Patent Document 2.

The present invention provides a method for dephosphorizing molten iron by contacting molten iron filled in a refining vessel with slag formed by a flux added onto the molten iron, and is characterized in that the basicity C/S of the slag at the end of the dephosphorization process is controlled to be within the range of the following formula (1) using the total iron concentration T.Fe (mass%) in the slag, where the basicity C/S of the slag is defined as the ratio of the CaO concentration (mass%CaO) to the _SiO2 concentration (mass% _SiO2 ) in the slag.
0.83+0.018×T. Fe≦C/S≦1.86-0.0455×T. Fe...
(1)

The method for dephosphorizing molten iron according to the present invention further comprises the steps of:
(1) The total iron concentration (T.Fe) of the slag is set to 4 mass% or more and less than 15 mass%;
(2) Controlling the basicity C/S of the slag during the dephosphorization treatment so that it falls within the range of the above formula (1) using the total iron concentration T.Fe (mass%) in the slag;
(3) When at least a portion of the flux comprising a CaO-containing substance is blown onto the bath surface of the molten iron together with blowing oxygen from a top blowing lance, the ratio F CaO /F O2 (kg/Nm 3 ) of the supply rate F _O2 (Nm ³ /(min·t)) of the blowing _oxygen to the supply rate F _CaO (kg/(min·t)) of the CaO-containing _substance converted into pure CaO is set to 3.5 or ^less ;
(4) The flux is continuously sprayed onto the bath surface of the molten iron during the dephosphorization treatment of the molten iron;
However, this is thought to be a more preferable solution.

According to the present invention, when dephosphorizing molten iron using slag, phosphorus can be efficiently removed with a small amount of slag in the low C/S range. Furthermore, even when a cold iron source such as scrap is used as a raw material, the thermal margin for melting the raw material can be secured, and _CO2 emissions can be reduced.

FIG. 1 is a schematic diagram showing an example of an apparatus for carrying out the method of the present invention. 1 is a graph showing the effect of a dephosphorization method on the relationship between basicity C/S and phosphorus distribution ratio _LP in a low slag total iron concentration T.Fe region. 1 is a graph showing the effect of a dephosphorization method on the relationship between basicity C/S and phosphorus distribution ratio _LP in a high slag total iron concentration T.Fe region. 1 is a graph showing the upper and lower limits of the C/S basicity at which a high phosphorus distribution ratio _LP region appears on the low basicity C/S side, using the total iron concentration T.Fe in the slag. 1 is a graph showing the effect of the ratio (F _CaO /F _O2 ) of the supply rate F _CaO of CaO sprayed from a top blowing lance to the supply rate F _O2 of oxygen gas on the phosphorus distribution ratio L _P.

The following is a detailed description of the embodiments of the present invention. Note that the drawings are schematic and may differ from the actual product. The following embodiments are illustrative of devices and methods for embodying the technical ideas of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical ideas of the present invention can be modified in various ways within the technical scope described in the claims.

Figure 1 shows an example of an apparatus for carrying out the molten iron dephosphorization method of the present invention, in which a converter is used as a refining vessel. The converter equipment 1 includes a converter 2 whose outer shell is made of a steel shell 3 and refractory material 4 is applied to the inside of the steel shell 3, and a top blowing lance 5 inserted inside the converter 2 and movable in the vertical direction.

At the top of the converter 2, a tapping port 6 is provided for tapping the treated molten iron 16 after the dephosphorization process is completed. In addition, at the bottom of the converter 2, a bottom-blowing tuyere 7 is provided for blowing in stirring gas. The bottom-blowing tuyere 7 is connected to a gas introduction pipe (not shown).

Above the converter 2, there is provided a hood 8 for collecting exhaust gas generated from the converter 2, and a raw material addition device 9 for feeding various refining agents into the converter 2. The raw material addition device 9 is composed of, for example, a hopper 10, a discharge device 11 installed at the bottom of the hopper 10, and a chute 12 connected to the discharge device 11 and passing through the hood 8. In FIG. 1, as an example, only one hopper 10 that contains a premelt dephosphorization flux 19 is provided, but in reality, multiple hoppers are provided, and one of them contains a dephosphorization flux whose main component is CaO.

The top-blowing lance 5 is connected to an oxygen gas supply pipe 13 for supplying oxygen gas (industrial pure oxygen gas) for dephosphorization refining, a cooling water supply and drainage pipe (not shown) for supplying and discharging cooling water for cooling the top-blowing lance 5, and a flux supply pipe 14 for supplying a flux 18 for dephosphorization. The oxygen gas supply pipe 13 and the flux supply pipe 14 join at the top of the top-blowing lance 5.

The powdered dephosphorization flux 18, which is mainly composed of CaO such as quicklime, is pushed out from the pressurized dispenser 15 into the flux supply pipe 14 and is sprayed and added from the tip of the top blowing lance 5 toward the molten iron 16 in the furnace together with oxygen gas supplied from the oxygen gas supply pipe 13 by the carrier gas flowing through the flux supply pipe 14. In this embodiment, the dephosphorization flux 18 is added together with the oxygen gas to the position where it collides with the bath surface of the molten iron 16 (hereinafter referred to as the "fire point").

When a converter 2 is used as a refining vessel, the dephosphorization process for the molten iron 16 is carried out by (a) charging the converter 2 with a cold iron source such as iron scrap as necessary, then charging the molten iron, and while blowing in an inert gas such as Ar gas or nitrogen gas as a stirring gas from the bottom blowing tuyeres 7, spraying and adding a powdered dephosphorization flux 18 mainly composed of CaO from the top blowing lance 5 together with oxygen gas onto the molten iron 16, or (b) spraying and adding a powdered dephosphorization flux 18 together with oxygen gas from the top blowing lance 5, and adding a lump of dephosphorization flux 19 from the raw material addition device 9 onto the bath surface of the molten iron 16.

In the dephosphorization reaction from the molten iron 16, first, the phosphorus contained in the molten iron 16 is oxidized by the oxygen gas blown from the top blowing lance 5 to form phosphorus oxide (P ₂ O ₅ ). The phosphorus oxide is fixed as a compound with CaO in the slag 17 formed by the slagification of the powdered dephosphorization flux 18 and the lump-shaped dephosphorization flux 19 added to the furnace, and the dephosphorization reaction from the molten iron 16 progresses.

Regarding such dephosphorization treatment, the inventor first performed dephosphorization treatment using a dephosphorization apparatus using a small induction melting furnace similar to that shown in Figure 1, and conducted a detailed study on the influence of basicity C/S and total iron concentration T.Fe on the phosphorus distribution ratio _LP of slag.
The dephosphorization process is as follows:
<Dephosphorization method 1> A method in which only oxygen gas is blown onto the molten iron 16 from the top blowing lance 5, and lump CaO is added as a dephosphorization flux 19 in the initial stage all at once.
<Dephosphorization method 2> A method in which oxygen gas and powdered CaO as a dephosphorization flux 18 are simultaneously blown onto the molten iron 16 from the top blowing lance 5, and lump CaO is added all at once as a dephosphorization flux 19 at the initial stage.
I went in two ways.

In the dephosphorization method 1 and the dephosphorization method 2, the total amount of CaO supplied: T.CaO (kg/t) was the same. The total iron concentration in the slag, T.Fe, was controlled by changing the supply rate of the oxygen gas supplied from the top blowing lance 5 and the supply rate of the Ar gas supplied from the bottom blowing tuyere 7.
The experimental temperature was 1300° C. in all cases, and the phosphorus distribution ratio L _P was determined from the phosphorus concentration [% P] (mass %) in the molten iron 16 and the phosphorus concentration (% P) (mass %) in the slag 17 at the time when the phosphorus concentration in the molten iron 16 stopped changing.

As a result, when the total iron concentration T.Fe in the slag is 3.6 to 5.8 mass%, as shown in Fig. 2, it was found that in both dephosphorization method 1 and dephosphorization method 2, a region in which the phosphorus distribution ratio L _P is high exists in the low region of C/S of 0.8 to 1.8, and the phosphorus distribution ratio L _P is extremely high, near 10 to the power of 2 in dephosphorization method 1 and near 10 to the power of 4 in dephosphorization method 2.

3, even when the total iron concentration T.Fe in the slag is 15-18 mass%, both dephosphorization method 1 and dephosphorization method 2 have a region where the phosphorus distribution ratio L _P is high in the low C/S region of 1.0-1.4, and the phosphorus distribution ratio L _P is extremely high, near 10 to the power of 2 in dephosphorization method 1 and near 10 to the power of 4 in dephosphorization method 2.

From the above results, it was found that in both the dephosphorization method 1 and the dephosphorization method 2, there exists a region where the phosphorus distribution ratio _LP is high in a region where the basicity C/S is low, and the lower the total iron concentration T.Fe in the slag, the wider the range of basicity C/S can be, the higher the phosphorus distribution ratio _LP can be maintained.

2 and 3, it was also found that a higher phosphorus distribution ratio L _P can be obtained by performing dephosphorization treatment using the dephosphorization method 2 than by the dephosphorization method 1. The reason for this is not clear at this point, but the inventors speculate as follows.
Dephosphorization method 2 is a method in which powdered CaO is supplied to the fire point where the entire amount of molten iron is considered to have turned to FeO by heating at 2000°C or higher, so CaO-FeO slag with a high phosphorus distribution ratio L P is formed at the fire point, while CaO-SiO ₂ -FeO slag with a lower phosphorus distribution ratio L _P than the slag at the fire point is formed in the surrounding area. Therefore, the issue with dephosphorization of the entire system is whether phosphorus can be retained in the slag around the fire point. The difference in the phosphorus distribution ratio L _P of the slag between the fire point and the surrounding area of the fire point is considered to be influenced by the oxygen partial pressure of the slag _.
On the other hand, in the dephosphorization method 1, no CaO-FeO slag with a high phosphorus distribution ratio L _P is formed at the hot spot, and dephosphorization proceeds through the interface equilibrium between the CaO-SiO ₂ -FeO slag and molten iron. At that time, it is believed that the oxygen partial pressure at the interface between the slag and molten iron (metal) has an effect.
In general, the oxygen partial pressure of slag is higher than that of the interface between slag and molten iron (metal), and the oxygen partial pressure of the interface is said to be intermediate between those of slag and molten iron. Therefore, the difference in the phosphorus distribution ratio _LP between dephosphorization method 1 and dephosphorization method 2 confirmed this time is considered to be influenced by this difference in oxygen partial pressure (whether the oxygen partial pressure that determines the dephosphorization of the entire system is the oxygen partial pressure of the slag or the oxygen partial pressure of the interface).

Next, the inventors investigated the influence of the total iron concentration T.Fe of the slag on the high phosphorus distribution ratio _LP region appearing on the low basicity C/S side. Specifically, the phosphorus distribution ratio _LP obtained in the range of general C/S basicity: 2.0 to 2.5 in dephosphorization treatment was used as a standard, and the upper and lower limits of the basicity C/S that are equal to or higher than the phosphorus distribution ratio _LP on the low basicity C/S side were obtained using the total iron concentration T.Fe of the slag.

The results are shown in Figure 4. As shown in Figure 4, the upper and lower limits of the basicity C/S at which the phosphorus distribution ratio L _P is equal to or greater than the value obtained when the basicity C/S is in the range of 2.0 to 2.5 for both dephosphorization method 1 and dephosphorization method 2 can be expressed by a linear regression equation using the total iron concentration T.Fe of the slag, and it was found that a high phosphorus distribution ratio L _P can be obtained by controlling the basicity C/S after dephosphorization so as to satisfy the range of the following formula (1).
0.83+0.018×T. Fe≦C/S≦1.86-0.0455×T. Fe...
(1)
Here, the value of the phosphorus distribution ratio L _P obtained when the basicity C/S is in the range of 2.0 to 2.5 is the average value of the phosphorus distribution ratio L _P obtained in this range, the maximum value in this range, etc., and is not particularly limited.

In addition, the higher the total iron concentration T.Fe of the slag, the narrower the upper and lower limits of the basicity C/S become. When the total iron concentration T.Fe exceeds 16 mass%, it is not possible to obtain a phosphorus distribution ratio L _P or higher that is obtained when the basicity C/S is in the range of 2.0 to 2.5.

When the total iron concentration T.Fe of the slag is 15 mass% or more, there is a region where the phosphorus distribution ratio L _P once drops in the middle of the upper and lower limits of the basicity C/S obtained from the linear regression equation in Figure 4, and as shown in Figure 3, there are two peaks where the phosphorus distribution ratio L _P is near the fourth power of 10, at the basicity C/S of around 1.0 and 1.2. Therefore, the range of basicity C/S where the phosphorus distribution ratio L _P is equal to or greater than the value obtained in the range of basicity C/S = 2.0 to 2.5 becomes extremely narrow. Therefore, in actual operation where there is some deviation between the basicity C/S calculated from the CaO content charged into the refining vessel and the SiO ₂ content generated and charged and the actual basicity C/S, it is difficult to stably reach the basicity C/S within the target range due to the largeness of the variation, and therefore it is preferable that the total iron concentration T.Fe of the slag is less than 15 mass%.

Furthermore, by controlling the basicity C/S during the dephosphorization treatment so as to satisfy the range of the above formula (1), a higher phosphorus distribution ratio L _P can be obtained after the dephosphorization treatment.

Next, the inventors changed F CaO /F O2 (kg/Nm 3 ), which is the ratio of the supply rate F _O2 (Nm ³ /(min·t)) of oxygen for blowing from the top lance to the supply rate F _CaO (kg/(min·t)) of CaO-containing material from the top lance in terms of pure CaO, in the range of 0.1 to 4.0 within the range of ^basicity C/S determined by the linear regression equation in Fig. 4, and investigated the effect of F _CaO /F _O2 on the phosphorus distribution ratio L _P. The results are shown in Fig. 5. As shown in Fig. 5, it was found that when F _CaO / _{F O2} was 2.5 to 3.0, the phosphorus distribution ratio L _P was highest, and _when it was less than 2.5 or more than 3.0, the phosphorus distribution ratio L _P decreased.

The reason for this is that the dephosphorization reaction is as follows:
2P+3CaO+5FeO=3CaO・P ₂ O ₅ +5Fe
Fe+1/2O ₂ =FeO
Considering that the reaction formula is composed of the above, the ratio of CaO to _O2 when dephosphorization is performed, CaO/ _O2 , is stoichiometrically 3.0 kg/Nm3. Since _FCaO / _F02 is synonymous with CaO/ _O2 , it is considered that when _FCaO / _F02 is greater than 3.0, excess CaO that does not contribute to the dephosphorization reaction is supplied to ^the hot point, and the amount of slag increases, resulting in a decrease in the phosphorus distribution ratio L _P. On the other hand, when _FCaO / _F02 is less than 3.0, it is considered that the phosphorus distribution ratio L _P decreases due to a decrease in CaO that contributes to the reaction. However, when F _CaO /F _O2 is greater than 3.0 and when F _CaO /F _O2 is less than 3.0, the decrease in the phosphorus distribution ratio L _p is considered to be only the same as the variation when the phosphorus distribution ratio L _p is stable at a high level near the fourth power of 10 as shown in Fig. 2. Therefore, when a CaO-containing substance is blown onto molten iron together with blowing oxygen from a top lance (dephosphorization method 2), F _CaO /F _O2 is preferably 3.5 or less, more preferably 3.0 or less, and even more preferably in the range of 0.5 to 3.0. By setting _{F CaO} /F _O2 to 3.5 or less, an increase in the amount of slag is suppressed, a high phosphorus distribution ratio L _p can be obtained, and high dephosphorization performance can be exhibited. The lower limit of _{F CaO} /F _O2 is not particularly limited, but it is preferably 0.5 or more so that the phosphorus distribution ratio L _p is about the fourth power of 10.

In addition, when a CaO-containing substance is blown onto molten iron together with the blowing oxygen from a top blowing lance (dephosphorization method 2), it is preferable to continue the blowing of CaO throughout the treatment without interrupting it midway. This is because, if the blowing of CaO is interrupted, the phosphorus distribution ratio L _P drops from the value in dephosphorization method 2 shown in Figures 2 and 3 to the value in dephosphorization method 1, and the dephosphorization reaction rate drops to the value shown in the following formula:
d[%P]/dt=K{[%P]-(%P)/L _P }
This is because it leads to a decrease in the dephosphorization rate.

In the present invention, the slag used may contain other components such as _Al2O3 , MgO, MnO, and S as long as CaO, _SiO2 , and FeO are the main components, and it is more preferable that the total concentration of CaO, _SiO2 , and FeO is 60 mass _% or more. If the FeO concentration is not analyzed, the total iron concentration T.Fe of the slag may be treated as the total amount of FeO. It has also been confirmed that the high phosphorus distribution ratio _LP region seen on the low basicity C/S side occurs regardless of whether it is molten iron or molten steel.

In this example, 300 t of molten iron with a Si concentration of 0.5 mass% and a P concentration of 0.15 mass% was charged into a converter, and slag with a C/S basicity of 1.0 and lump CaO were added to the molten iron from a hopper. At the same time, oxygen gas was blown from the top lance and _N2 gas was blown from the bottom tuyeres while stirring the mixture for desiliconization.
After the desiliconization treatment, the converter was tilted to remove some of the slag from the furnace, and then the converter was turned upright again to perform dephosphorization treatment for 4.5 min. The Si concentration of the hot metal before the dephosphorization treatment was 0.05 mass%, the P concentration was 0.127 mass%, the residual slag in the furnace was 20 kg/t-hot metal, and the basicity C/S was 1.0.

During the dephosphorization blowing, the oxygen gas supply rate F _O2 from the top lance was set to 2.2 Nm ³ /(min·t), and the height from the top lance nozzle tip to the molten iron surface was controlled to 2.0-2.5 m. In addition, N ₂ gas was injected into the molten iron from the bottom tuyeres at a supply rate of 0.12-0.26 Nm ³ /(min·t) to stir the molten iron. According to the above, the dephosphorization treatment was performed for the following Test Nos. 1 to 6, and the basicity C/S after the dephosphorization treatment, the total iron concentration T.Fe in the slag (mass%), the ratio of the oxygen supply rate to the CaO supply rate during the dephosphorization treatment: F _CaO /F _O2 (kg/Nm ³ ), and the P concentration in the molten iron (mass%) were obtained. The results are shown in Table 1.

In Test No. 1, at the start of the dephosphorization process, lump CaO was added from the hopper so that the basicity C/S after the dephosphorization process would be 2.0. At the end of the dephosphorization process, the total iron concentration T.Fe in the slag was 18 mass%, resulting in a decrease in iron yield. In addition, the temperature was 1350°C, the P concentration in the molten iron was 0.144 mass%, and 0.017% rephosphorization occurred during the dephosphorization process.

In Test No. 2, at the start of the dephosphorization process, lump CaO was added from the hopper so that the post-dephosphorization basicity C/S would be 1.1. At the end of dephosphorization, the total iron concentration T.Fe in the slag was 14.5 mass%, the temperature was 1350°C, and the P concentration in the molten iron was 0.088 mass%. Despite the basicity C/S being reduced by almost half from Test No. 1, the total iron concentration T.Fe decreased and the amount of dephosphorization increased by 0.056 mass%.

In Test No. 3, at the start of the dephosphorization process, lump CaO was added from the hopper so that the post-dephosphorization basicity C/S was 1.45. At the end of dephosphorization, the total iron concentration T.Fe in the slag was 8.0 mass%, the temperature was 1350°C, and the P concentration in the molten iron was 0.033 mass%. Despite the reduction in basicity C/S from Test No. 1, the total iron concentration T.Fe was almost halved and the amount of dephosphorization increased by 0.111 mass%.

In Test No. 4, during the dephosphorization process, powdered CaO was continuously blown onto the molten pig iron at a supply rate of 1.96 kg/min/t together with oxygen gas from the top blowing lance so that the basicity C/S after the dephosphorization process would be 1.8. The total iron concentration T.Fe in the slag at the end of the dephosphorization process was 18 mass%, resulting in a decrease in iron yield. In addition, the temperature was 1360°C, the P concentration in the molten pig iron was 0.030 mass%, and 0.097 mass% dephosphorization occurred during the dephosphorization process.

In test No. 5, powdered CaO was sprayed from the top lance at a supply rate of 1.5 kg/(min·t) together with oxygen gas onto the molten iron during dephosphorization so that the basicity C/S after dephosphorization would be 1.45, but spraying of CaO was stopped midway through the dephosphorization process. At the end of the dephosphorization process, the total iron concentration T.Fe of the slag was 8 mass%, the temperature was 1380°C, and the P concentration in the molten iron was 0.015 mass%. Despite the basicity C/S being lower than in test No. 4, the total iron concentration T.Fe of the slag was almost halved, the P concentration in the molten iron after dephosphorization was halved, and the temperature after dephosphorization increased by 20°C, resulting in improved thermal tolerance.

In test No. 6, powdered CaO was sprayed from the top lance at a supply rate of 1.5 kg/(min·t) along with oxygen gas onto the molten iron during dephosphorization so that the basicity C/S after dephosphorization was 1.45. Powdered CaO was sprayed throughout the entire dephosphorization period. At the end of dephosphorization, the total iron concentration T.Fe of the slag was 8 mass%, the temperature was 1380°C, and the P concentration in the molten iron was 0.010 mass%. Despite the basicity C/S being lower than in test No. 4, the total iron concentration T.Fe of the slag was almost halved, the P concentration in the molten iron after dephosphorization was 1/3, and the temperature after dephosphorization increased by 20°C, resulting in improved thermal tolerance.

INDUSTRIAL APPLICABILITY The method for dephosphorization of molten iron according to the present invention can contribute to reducing _CO2 emissions in the steelmaking process and reducing steelmaking costs.

Reference Signs List 1 Converter equipment 2 Converter 3 Steel shell 4 Refractory material 5 Top blowing lance 6 Tap hole 7 Bottom blowing tuyeres 8 Hood 9 Raw material addition device 10 Hopper 11 Cutting device 12 Chute 13 Oxygen gas supply pipe 14 Fluxing agent supply pipe 15 Dispenser 16 Molten iron 17 Slag 18 Dephosphorization flux 19 Dephosphorization flux

Claims (2)

1. A method for dephosphorizing molten iron by contacting the molten iron charged in a refining vessel with slag formed by a flux added onto the molten iron, comprising:
a ratio FCaO/F02 (kg/Nm3) of a supply rate of the blowing oxygen F02 ₍ Nm3 ^/ (min.t)) to a supply rate of the CaO-containing substance converted into a pure CaO FCaO (kg/(min.t)) is set to 3.5 or less when at least a portion of the flux comprising a CaO-containing substance is blown onto a bath surface of the molten iron together with blowing oxygen from a top blowing lance, thereby _controlling a basicity of ^the _slag C _/ S at the end of a dephosphorization treatment of the molten iron to be within the range of the following formula (1) using a total iron concentration T.Fe (mass %) in the slag:
0.83+0.018×T. Fe≦C/S≦1.86-0.0455×T. Fe
...(1)
Here, the basicity of the slag, C/S, is defined as the ratio of the CaO concentration (mass% CaO) to the SiO2 concentration (mass% SiO2 _{) in} the slag , and the total iron concentration T.Fe in the slag is set to 4 mass% or more and less than 15 mass%. 2. The method for dephosphorizing molten iron according to claim 1 , wherein the flux is sprayed onto the bath surface of the molten iron continuously during the dephosphorization treatment of the molten iron.

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