JP2023049462A - Dephosphorization of hot metal - Google Patents

Abstract

To provide a dephosphorization method of hot metal for the inexpensive and efficient melting of low phosphorus steel.SOLUTION: In a dephosphorization blowing, the charged basicity, which is the ratio of the mass of SiO2 produced by the oxidation of Si in hot metal and SiO2 contained in a sub-material containing the lime source to be fed before dephosphorization blowing, to the mass of CaO pure in the lime source, is controlled to 1.7 -2.2. As converter slag, 10-30 kg per ton of hot metal is used, which is composed of CaO/SiO2 of 3.0 or more, total concentration of CaO and SiO2 of 50 mass% or more, P2O5 concentration of less than 2.0 mass%, and the remainder of FetO, Al2O3, MnO, MgO, S, and inevitable impurities.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a method for dephosphorizing hot metal, particularly in operations with intermediate waste.

Conventionally, when melting low-phosphorus steel with a P concentration of less than 0.02% by mass, rephosphorization occurs due to P ₂ O ₅ contained in recycled slag. In smelting, the slag basicity is adjusted using all new lime without using recycled slag. Therefore, when melting low-phosphorus steel, a large amount of lime is required, resulting in high costs.

Therefore, in dephosphorization blowing, various techniques using recycled slag have been proposed. In Patent Document 1, high basicity slag produced by dephosphorizing low phosphorus hot metal with a P concentration of less than 0.02% by mass is returned to another steel type dephosphorization blowing, and the basicity is 2 or less. Techniques for performing dephosphorization blowing under certain conditions have been disclosed. In Patent Document 2, when recycling the converter slag in dephosphorization blowing, the converter slag is adjusted to 5 to 50 mm, the slag basicity after blowing is 2 or more, and the T.E. A technique of blowing so that Fe becomes 10 to 15% by mass is disclosed. In addition, Patent Document 3 discloses a technique of leaving the slag after decarburization blowing, adding a solidifying material such as lime, dolomite, and ingot-making slag to solidify the decarburization slag, and performing dephosphorization blowing as it is. It is

JP-A-10-237525 JP 2004-124145 A Japanese Patent No. 6421634

In general, when recycled slag is used for dephosphorization blowing, rephosphorization can be suppressed by using recycled slag containing as little P ₂ O ₅ as possible. However, especially in an operation such as the MURC process in which intermediate slag is discharged, blowing cannot be continued when slag overflow (hereinafter referred to as slopping) due to slag bubbling occurs, and before the intermediate slag is discharged, Since it is difficult to make the P concentration sufficiently low, there are many restrictions on operating conditions. Therefore, low-phosphorus steel cannot be efficiently smelted only by using recycled slag with a small amount of P ₂ O ₅ .

The method described in Patent Document 1 is premised on a process in which hot metal is dephosphorized in a dephosphorization furnace and then transferred to a decarburization furnace for decarburization, and the slag composition for facilitating intermediate slag is considered. Therefore, it is not possible to stably obtain low-phosphorus steel in an operation in which intermediate slag is discharged after dephosphorization and decarburization is performed in the same furnace. Similarly, the method described in Patent Document 2 does not assume an operation that performs intermediate waste, and does not consider the slag composition for facilitating intermediate waste. It is not possible to smelt low-phosphorus steel well. Furthermore, in Patent Document 3, the process of performing intermediate slag after dephosphorization treatment and decarburization treatment in the same furnace is basically used. Ingot-making slag containing a slag solidifying agent and Al ₂ O ₃ is charged to solidify the slag, and the slag is recycled while preventing the slag from bumping when the hot metal is charged for the next dephosphorization treatment. is described. However, the method of adding a slag solidification agent to hot recycled slag to solidify it while adding ingot-making slag containing Al ₂ O ₃ to promote melting has difficulty in controlling slag solidification and melting. The addition of ingot-making slag containing Al ₂ O ₃ increases the amount of slag and accelerates slopping in the dephosphorization treatment. do not have.

SUMMARY OF THE INVENTION An object of the present invention is to provide a hot metal dephosphorization method for producing low-phosphorus steel efficiently and inexpensively.

The present inventors have earnestly studied the conditions for efficiently performing dephosphorization blowing using recycled slag. As a result, it was found that there are appropriate basicity, composition and amount of converter slag, etc. in the operation of performing intermediate waste, leading to the present invention.

The present invention is as follows.
(1)
charging molten iron into a converter-type refining apparatus equipped with a top-blowing lance, introducing a lime source containing at least converter slag, and blowing oxygen from the top-blowing lance to dephosphorize the molten iron; After part of the generated dephosphorization slag is discharged by tilting the furnace, decarburization blowing is performed by adding a lime source, and the P concentration in the molten steel after decarburization blowing is reduced to a low level of less than 0.020% by mass. A hot metal dephosphorization method for melting phosphorous steel,
In the dephosphorization blowing, the mass of SiO ₂ generated by the oxidation of Si in the hot metal, the mass of SiO ₂ contained in the secondary material containing the lime source, which is put in before the dephosphorization blowing, and CaO in the lime source The charged basicity, which is the ratio to the mass of the pure component, is 1.7 to 2.2,
As the converter slag, CaO/SiO ₂ is 3.0 or more, the total concentration of CaO and SiO ₂ is 50% by mass or more, the P ₂ O ₅ concentration is less than 2.0% by mass, and the balance is Fe _t O (oxidized (sum of iron (FeO, Fe ₂ O ₃ )), Al ₂ O ₃ , MnO, MgO, S and unavoidable impurities. Phosphorus method.
(2)
The method for dephosphorizing molten iron according to (1) above, wherein the converter slag has a particle size of 5 to 30 mm at a rate of 80% by mass or more.

According to the present invention, it is possible to provide a hot metal dephosphorization method for producing low-phosphorus steel efficiently and inexpensively.

It is a figure for demonstrating the improvement method of dephosphorization efficiency. FIG. 4 is a diagram showing the relationship between charged basicity and the amount of oxygen removed from Si.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings.
In this embodiment, in a converter-type refining apparatus equipped with a top-blowing lance, dephosphorization blowing of hot metal is first carried out, and part of the produced dephosphorization slag is subjected to intermediate slag by tilting the furnace. Then, decarburization blowing is performed by adding a lime source later, and low phosphorus steel having a P concentration in the molten steel after decarburization blowing of less than 0.020% by mass is produced. In the dephosphorization blowing of hot metal, converter slag and new lime (such as quicklime) are added as lime sources.

In dephosphorization blowing in which the dephosphorization treatment and the decarburization treatment are separated and intermediate slag is discharged, the P concentration in the hot metal is higher than the apparent equilibrium P concentration obtained from the state of the slag and hot metal in the operation. Become. Therefore, improving the chemical equilibrium by changing the slag composition and hot metal temperature has little effect on improving dephosphorization. Therefore, as shown in FIG. 1, it is effective to (1) increase the amount of oxygen supplied or (2) promote the mass transfer of P to improve the dephosphorization efficiency.

Therefore, in order to extend the time of dephosphorization blowing and increase the amount of oxygen supplied, it is important to delay the timing of occurrence of slopping, which is a bottleneck in securing blowing time. Fig. 2 shows charged CaO/SiO ₂ (charged basicity) and amount of oxygen removed from Si (the amount of oxygen excluding the oxygen consumed for oxidizing hot metal Si, out of the amount of top-blown oxygen). indicates a relationship with The external oxygen amount for desiliconization in FIG. 2 represents the oxygen supply amount from the start of dephosphorization blowing to the occurrence of slopping. In addition, the charge basicity is the mass of _SiO2 generated by the oxidation of Si in hot metal and the mass of _SiO2 contained in the secondary material charged before dephosphorization blowing, and the mass of pure CaO in the lime source. represents a ratio. Here, secondary materials include recycled slag (converter furnace slag), silica stone, and the like, which are introduced during dephosphorization blowing. Lime sources include not only the converter slag but also new lime (quicklime, etc.). As shown in FIG. 2, the greater the charged basicity, the later the timing of occurrence of slopping, and the more oxygen supply can be increased.

On the other hand, if the basicity of the dephosphorization slag is high, the solid phase increases and the viscosity of the molten slag decreases, making slopping less likely to occur, resulting in a marked reduction in the slag rate. In addition, due to the increase in slag basicity, the liquid phase ratio of slag in the initial stage of dephosphorization blowing also decreases, and mass transfer of P also becomes stagnant. Therefore, in this embodiment, converter slag is used as a lime source as a method of promoting the mass transfer of P while keeping the charging basicity high in order to secure the dephosphorization time. However, converter furnace slag contains P ₂ O ₅ , and in order to prevent rephosphorization as much as possible, it is a prerequisite to separate slag having a low P ₂ O ₅ concentration as much as possible.

Here, when quicklime and converter slag are used as the lime source, the dephosphorization behavior, slag basicity, and slag liquid phase ratio when changing the ratio of converter slag were investigated. As compared with the case of using BOF slag, the liquid phase ratio increased and the dephosphorization behavior improved. As a factor for this, it is considered that the dissolution rate of converter slag is slower than that of quicklime, so that the slag basicity remained low and the amount of liquid phase could be maintained.

As described above, in the present embodiment, the oxygen supply amount is increased to secure the blowing time, and the converter slag is used as the lime source to promote the mass transfer of P, thereby making it inexpensive and efficient. Dephosphorization reaction can be accelerated.

Next, detailed conditions for dephosphorization blowing in this embodiment will be described.

(Charging basicity: 1.7 to 2.2)
As shown in FIG. 2, the higher the charge basicity, the more delayed the slopping during dephosphorization blowing, and the more oxygen supply can be increased. Therefore, in order to produce a low phosphorus steel with a P concentration of less than 0.02% by mass, the charging basicity is set to 1.7 or more. On the other hand, if the charged basicity exceeds 2.2, slopping hardly occurs and the slag rate is significantly deteriorated. Therefore, the upper limit of the charged basicity is set to 2.2.

(Converter slag basicity: 3.0 or more)
When the basicity of the converter slag charged as a lime source is less than 3.0, a large amount of dicalcium silicate (hereinafter referred to as C ₂ S) is generated as a structure inside the dephosphorization slag, and it is charged in the dephosphorization blowing. The actual dissolution rate is remarkably lowered. In addition, a large amount of converter slag may be required in order to adjust the basicity (charged basicity) of the dephosphorization slag to the above range. On the other hand, if the basicity is 3.0 or more, CaO is present in excess relative to SiO ₂ , so CaO—FeO with a low melting point exists as a slag structure, and a constant dissolution rate can be secured. Although the upper limit of the basicity of the converter slag is not particularly limited, there is a range in the charging basicity as described above, and there is also an adjustment in the amount of converter slag input as described later. , there is a substantial upper limit.

(Total concentration of CaO and _SiO2 in converter slag: 50% by mass or more)
If the total concentration of CaO and SiO ₂ is less than 50% by mass, a large amount of converter slag is required to adjust the basicity of the _{dephosphorization} slag. FeO, the sum of Fe ₂ O ₃ ) increases. As a result, the dissolution rate becomes too high, the actual basicity of the dephosphorization slag increases, and the liquid fraction of the dephosphorization slag decreases. Therefore, the total concentration of CaO and SiO ₂ in the converter slag is set to 50% by mass or more.

(P ₂ O ₅ concentration in converter slag: less than 2.0% by mass)
If the P ₂ O ₅ concentration in the converter slag is high, the amount of phosphorus returned to the hot metal increases, and most of it is carried over to decarburization blowing. As a result, the total dephosphorization performance of the converter blowing is lowered. Therefore, the P ₂ O ₅ concentration in converter slag is set to less than 2.0% by mass.

Here, a method for separating converter slag that satisfies the composition range as described above will be described. Since the P ₂ O ₅ concentration of general decarburized slag is around 3% by mass, in order to make the P ₂ O ₅ concentration in the converter slag less than 2% by mass, for example, blowing for low phosphorus steel It is necessary to separate the slag by focusing on the decarburized slag produced in The slag separation method is not particularly limited, and slag produced by multiple blows may be mixed, and converter slag may be used in any manner as long as it satisfies the above composition range.

(Other Components of Converter Slag)
Other components of converter slag include Fe _t O, Al ₂ O ₃ , MnO, MgO, S and incidental impurities. These concentrations are not particularly limited as long as the total concentration of CaO and SiO ₂ and the concentration of P ₂ O ₅ satisfy the above conditions.

(Amount of converter slag: 10 to 30 kg/t)
If the amount of the converter slag to be charged is less than 10 kg/t, the liquid phase ratio of the dephosphorization slag will be low, and the dephosphorization improvement effect will not be obtained. On the other hand, when the amount of charged converter slag exceeds 30 kg/t, it becomes difficult to adjust the charging basicity within the above range while suppressing the amount of dephosphorization slag. The amount of phosphorus reversion also increases. In addition, when trying to adjust the charge basicity within the above range, the amount of converter slag increases and it takes time to dissolve it, so the time when the slag basicity is low increases, delaying dephosphorization and early slopping. Shortage of dephosphorization time due to generation is likely to occur.

(Particle size of converter slag: 5 to 30 mm for 80% by mass or more of converter slag)
Considering the actual crushing and classification load and the dephosphorization efficiency, the particle size of the converter slag is preferably 30 mm or less. However, if the particle size is too small, the addition yield of the converter slag will decrease due to dust collection loss. Therefore, the particle size of the converter slag is preferably 5 to 30 mm in 80% by mass or more of the converter slag.

(other conditions)
The range of the top-blown oxygen supply rate during dephosphorization blowing is set to general blowing conditions in consideration of the timing of occurrence of slopping, specifically, 80 to 150 Nm ³ /h/t. preferable. In addition to the converter slag, lime such as quicklime is added as a lime source to adjust the charging _basicity to the above range. You may

Next, an example of the present invention will be described, but this condition is an example of conditions for confirming the feasibility and effect of the present invention, and the present invention is limited to the description of this example. isn't it. The present invention can be implemented in various ways to achieve the objects of the present invention without departing from the gist of the present invention.

300 tons of molten iron having the composition shown in Table 1 was charged into a converter-type refining vessel, and converter slag and quicklime having the amount and composition shown in Table 1 were charged and fed from a top-blowing lance at 80 to 150 Nm ³ /h/t. Dephosphorization blowing was performed by blowing oxygen at an acid rate. After dephosphorization blowing, intermediate slag was carried out to discharge part of the dephosphorization slag. Table 1 shows the intermediate waste rate. After that, secondary materials such as quicklime were recharged, and oxygen was blown from a top blowing lance at an oxygen feeding rate of 200 to 220 Nm ³ /h/t to carry out decarburization blowing. Further, decarburized slag was used as the converter slag, crushed and classified to 30 to 90 mm or 5 to 30 mm, and decarburized slag having 80% by mass or more of these ranges was prepared. When the decarburized slag classified to 30 to 90 mm was charged, it was charged from the scrap chute of the converter, and when the decarburized slag classified to 5 to 30 mm was charged, it was charged from the furnace bunker of the converter. .

Regarding the effects of the invention, it was determined that the P concentration in molten steel after decarburization blowing was less than 0.020% by mass was effective, and that the effects of the invention were significantly obtained at less than 0.015% by mass.

The underlines in Table 1 indicate that the conditions of the present invention are not satisfied. Ch. No. 1 to 8, the P concentration in the molten steel was less than 0.020% by mass, and the effect of the invention was obtained. Especially Ch. In Nos. 6 to 8, converter slag with a finer grain size was used, so the P concentration in the molten steel was less than 0.015% by mass, and the effects of the invention were obtained more remarkably.

On the other hand, Ch. No. In 9, since the basicity of the converter slag was too low, a large amount of dicalcium silicate (hereinafter referred to as C ₂ S) was generated and the dissolution rate was too low. It is had. Ch. In No. 10, the total concentration of CaO and SiO ₂ in the converter slag was too low, so Fe _t O increased relatively, and the dissolution rate became too high and the liquid phase ratio decreased. The concentration of P in the molten pig iron has become high. Ch. In No. 11, since the P ₂ O ₅ concentration in the converter slag was too high, the P concentration in the hot metal after the dephosphorization blowing ended increased due to the effect of rephosphorization.

Ch. In No. 12, since the amount of converter slag was small, the liquid phase ratio of the dephosphorization slag could not be sufficiently increased, so the P concentration in the hot metal after the completion of dephosphorization blowing increased. Ch. In No. 13, since the amount of converter slag was too large, the period of low slag basicity was prolonged, and dephosphorization was delayed and early slopping occurred, resulting in defective P removal.

Ch. In No. 14, since the charged basicity was too low, slopping was accelerated and sufficient blowing time could not be secured, resulting in a high P concentration in the hot metal after dephosphorization blowing. Ch. In No. 15, since the charge basicity was too high, the dephosphorization slag hardened and slopping hardly occurred, resulting in a low intermediate slag rate. Therefore, although the P concentration in molten iron after dephosphorization blowing was low, the P concentration in molten steel after decarburization blowing was high.

Claims (2)

charging molten iron into a converter-type refining apparatus equipped with a top-blowing lance, introducing a lime source containing at least converter slag, and blowing oxygen from the top-blowing lance to dephosphorize the molten iron; After part of the generated dephosphorization slag is discharged by tilting the furnace, decarburization blowing is performed by adding a lime source, and the P concentration in the molten steel after decarburization blowing is reduced to a low level of less than 0.020% by mass. A hot metal dephosphorization method for melting phosphorous steel,
In the dephosphorization blowing, the mass of SiO ₂ generated by the oxidation of Si in the hot metal, the mass of SiO ₂ contained in the secondary material containing the lime source, which is put in before the dephosphorization blowing, and CaO in the lime source The charged basicity, which is the ratio to the mass of the pure component, is 1.7 to 2.2,
As the converter slag, CaO/SiO ₂ is 3.0 or more, the total concentration of CaO and SiO ₂ is 50% by mass or more, the P ₂ O ₅ concentration is less than 2.0% by mass, and the balance is Fe _t O (oxidized (sum of iron (FeO, Fe ₂ O ₃ )), Al ₂ O ₃ , MnO, MgO, S and unavoidable impurities. Phosphorus method. 2. The method for dephosphorizing hot metal according to claim 1, wherein the converter slag has a particle size of 5 to 30 mm at a rate of 80% by mass or more.

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