KR100860656B1 - Refining method of molten steel and method of producting extra-low-carbon steel using thereof - Google Patents

Abstract

A refining method of molten steel is provided to reduce heating time of molten steel and temperature rise of the molten steel by injecting aluminum into molten steel in a primary refining furnace of an electric furnace or a converter before discharging molten steel from the primary refining furnace and injecting aluminum and oxygen into molten steel in a subsequent decompressing process, and a method of producing extra low carbon steel using the refining method of molten steel is provided. A method of producing extra low carbon steel using a refining method of molten steel comprises the steps of: primarily refining molten steel in a primary refining furnace of an electric furnace or a converter, and injecting 1.0 to 2.5 kg of aluminum per ton of molten steel into an upper part of the primarily refined molten steel in the primary refining furnace; discharging molten steel from the primary refining furnace, and supplying aluminum and oxygen into molten steel while molten steel is decarburized in a decompression apparatus to increase temperature of molten steel and decarburize the molten steel at the same time; and deoxidizing the decarburized molten steel with aluminum, and injecting calcium oxide into the deoxidized molten steel to maintain the content of iron oxide(T.Fe)+manganese oxide(MnO) in slag to 8.0% by weight or less, and control a ratio of weight percent of alumina(Al2O3) to that of calcium oxide(CaO) to a range of 0.8 to 2.8.

Description

REFINING METHOD OF MOLTEN STEEL AND METHOD OF PRODUCTING EXTRA-LOW-CARBON STEEL USING THEREOF}

1 is a graph showing the relationship between the total oxygen concentration in the steel and the oxide of the slag before the molten steel is sent to the reproducing process after the deoxidation and bubbling is completed after vacuum treatment in the method for producing ultra low carbon steel according to the present invention.

Figure 2 is a graph showing the relationship between the weight ratio of calcium oxide / alumina weight percent of the slag of the present invention and the total oxygen concentration in the steel.

The present invention relates to a method for refining molten steel for the production of ultra low carbon steel, and in particular, aluminum is added during the first refining process in an electric furnace to produce carbon steel with a carbon concentration of less than 300 ppm and a nitrogen concentration of 40 ppm. Refining the molten steel to reduce the treatment time and refractory consumption by supplying the oxygen and oxygen, and to improve the decarburization efficiency during the decompression refining process of the molten steel in the ladle to shorten the refining time and reduce the carbon concentration And a method for producing ultra low carbon steel using refining of molten steel.

In general, the processability of carbon steel is greatly affected by the carbon (C) concentration in the steel, the lower the carbon concentration is the quality of the product is improved.

Accordingly, in order to obtain a material having good workability and good surface quality, ultra low carbon aluminum-killed steel having a carbon concentration of 300 ppm or less is mainly used.

Existing ultra low carbon steel is molten steel produced through the primary refining process in an electric furnace or a furnace such as a converter, and is subjected to secondary refining to be suitable for a subsequent casting process by tapping into a storage vessel called a ladle.

At this time, the molten steel tapping out of the primary refining furnace or the converter contains more than 200ppm of carbon, which contains carbon in the molten iron which is the main raw material of the converter or the scrap iron raw material of the electric furnace. This is because it is not removed below politics.

In addition, even after the molten steel is manufactured in the primary refining furnace, carbon components are also included in various ferroalloys such as ferro-manganese, subsidiary materials, and the like, which are added for steel composition adjustment or deoxidation, and thus carbon concentration increases.

Therefore, it is not easy to maintain the carbon concentration below 300 ppm by the existing steelmaking method, and in the case of manufacturing ultra low carbon steel of 300 ppm or less, a separate treatment process for reducing the carbon concentration for molten steel rolled out by the ladle Is required.

It is most effective to remove carbon in molten steel in the form of CO gas by vacuum refining due to its physicochemical characteristics. Currently, a depressurization device is used for decarburization at an ultra low carbon steel level. Gas apparatus RH, fractional type degassing apparatus DH, vacuum degassing apparatus VD, vacuum decarburization apparatus VOD, vacuum tank degassing apparatus VTD, etc. are mentioned.

In the decarburization process of removing carbon (C) in the steelmaking and steelmaking process, carbon is reacted with oxygen in steel (O) to remove carbon monoxide (CO) gas as shown in Scheme 1 below.

Scheme 1

[C] + [O] → CO (gas)

In this case, the higher the oxygen concentration in the steel, the faster the decarburization reaction rate and the lower the carbon concentration remaining in the steel after the reaction.

For this reason, in the production of ultra low carbon steel, the oxygen concentration in the steel is maintained at a high level, so that the introduction of a material having a high bonding strength with oxygen such as aluminum (hereinafter referred to as a deoxidizer) is avoided as much as possible.

This is because when a deoxidizer such as aluminum is introduced into the steel, it reacts with the oxygen in the steel and is separated from the steel while being converted into an oxide state, thereby lowering the oxygen concentration in the steel and being disadvantageous in subsequent decarburization treatment operations.

However, when tapping molten steel with a ladle in a primary refining furnace, an in-house slag containing a high content of iron oxide (T.Fe) + manganese oxide (MnO) is mixed, and the mixed in-house slag is a reactant produced during tapping. New slag is formed by mixing with the added subsidiary materials.

At this time, the higher the content of iron oxide (T.Fe) + manganese oxide (MnO) in the slag, the higher the content of inclusions in the steel and the lower the cleanliness, resulting in increased defects in the final product. If the content of) + manganese oxide (MnO) is too low, there is an effect of reducing the oxygen in the steel, there is a disadvantage that the decarburization rate is lowered during the decompression treatment to lower the vacuum treatment efficiency.

Therefore, in the manufacture of ultra low carbon steel, the content of slag iron oxide (T.Fe) + manganese oxide (MnO) is sufficiently high before decarburization in a vacuum treatment apparatus, and again slag iron oxide (T) when decarburization is completed. It is desirable to increase the cleanliness of the molten steel by lowering the content of .Fe) + manganese oxide (MnO) again.

On the other hand, since ultra-low carbon steel is softness with good workability, it is preferable to keep nitrogen concentration low besides carbon concentration, and usually 40 ppm or less is common.

Since about 80% of the nitrogen is contained in the atmosphere, the molten steel is easily incorporated into the molten steel as it comes into contact with the atmosphere, and the amount of nitrogen is increased as the oxygen concentration in the molten steel is low.

When nitrogen is mixed in the existing molten steel, it is difficult to remove it again. Therefore, in order to keep the nitrogen concentration low, it is preferable to suppress and block the incorporation of nitrogen into the molten steel during refining and tapping.

After the decarburization is completed, the molten steel is made into slabs or ingots after casting after completion of post-treatment processes such as temperature control, deoxidation and removal of non-metallic inclusions. The casting operation requires a certain range of molten steel temperature depending on the type of steel. For ultra low carbon steel, about 1550 ° C is appropriate. The molten steel is cooled for the time required for the operation, and especially during the decarburization reaction, the cooling speed of the molten steel becomes faster.

In order to compensate the cooling of the molten steel, the temperature of the molten steel should be increased before and after the decarburization operation, and the molten steel temperature in the primary refining furnace should be raised to the ladle side in consideration of the cooling amount during the subsequent treatment.

On the other hand, it is known that the temperature of molten steel when tapping in a primary refining furnace should be 1680 ° C or higher during the manufacture of ordinary ultra low carbon steel, and it takes a long time to dissolve to produce molten steel at such a high temperature. In addition to the problems, there was a problem such as increasing the amount of refractory loss in the refinery.

In particular, in the electric furnace, the end point temperature is about 1620 ° C in consideration of power supply rate, refractory erosion, and interprocess processing time. To send molten steel to the casting facility after decarburization and post-treatment, the molten steel must be heated in consideration of the cooling amount of the molten steel, and the required temperature for heating is at least 100 ℃, and the time required for heating the molten steel is about 40 ~ 50 It takes about minutes, which makes continuous casting difficult.

Therefore, it is possible to ensure the molten steel temperature required for the subsequent treatment without increasing the end temperature in the primary refining furnace, thereby reducing the time and heat source energy required to heat the molten steel after leaving the primary refining furnace. There is a need for a way to do this.

Recently, in order to increase the temperature of molten steel discharged from primary refining furnaces such as electric furnaces and converters, the LF device, which is an off-furnace refining facility, is heated and refined by using an electric arc. Not only this has to be extended, there is a problem of using electric power, which is a relatively expensive energy to heat up the molten steel.

Accordingly, although a method of injecting aluminum when molten steel other than an electric furnace is in a ladle has been proposed in the past, such a method can reduce the amount of drop in which the molten steel temperature is lowered, but the introduced aluminum has oxygen as described above. Combined with and lowering the oxygen concentration, the nitrogen concentration is increased, there is a problem in reducing the decarburization reaction rate during the subsequent vacuum treatment.

The present invention has been proposed to solve this problem in view of the above-mentioned problems, the purpose of which is to put the aluminum to raise the temperature of the molten steel in the first refining furnace such as an electric furnace or converter in the subsequent decompression process In addition, it is to provide a method for producing ultra low carbon steel using refining of molten steel to shorten the time required for heating and heating the molten steel by adding aluminum and oxygen to increase the temperature to the required temperature.

In addition, another object of the present invention is to increase the temperature of the molten steel by injecting aluminum into the upper molten steel in the primary refining furnace for the molten steel refined in the primary refining furnace while the iron oxide (T.Fe) in the slag of the furnace The present invention provides a method for refining molten steel to reduce the content of MnO.

The present invention for achieving the above object is the primary refining of molten steel in the primary refining furnace of the electric furnace or converter,

Injecting aluminum into the upper side of the molten steel to increase the temperature of the molten steel with respect to the molten steel in which the primary refining is completed;

And after deducting from the primary refining furnace, supplying aluminum and oxygen during decarburization in a depressurizing device to increase the temperature of molten steel and at the same time decarburizing;

The decarburization-treated molten steel is deoxidized with aluminum, and quicklime is added to maintain the content of iron oxide (T.Fe) + manganese oxide (MnO) in the slag to 8.0 wt% or less, and alumina (Al ₂ O ₃ ) Characterized in that the weight ratio of calcium oxide (CaO) to weight percent by weight to 0.8 to 2.8.

In addition, in the step of injecting aluminum into the primary refining furnace is to inject 1.0 to 2.5kg per tonne of molten steel.

In the depressurization apparatus, 0.5-3.0 kg of aluminum is added per tonne of molten steel in the step of supplying aluminum and oxygen during the decarburization process, and the oxygen supplies 1.0-1.8 Nm ³ of oxygen per 1 kg of aluminum used. It is preferable.

On the other hand, another feature of the present invention, characterized in that the 1.0 to 2.5kg of aluminum per tonne of molten steel is added to the molten steel in the primary refining furnace for the molten steel refined in the primary refining furnace of the electric furnace or converter. do.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

In the method of refining molten steel according to the present invention, after refining of molten steel is completed in a primary refining furnace such as an electric furnace or converter, 1.0 to 2.5 kg of aluminum per tonne of molten steel is added to the upper layer of the molten steel.

In more detail, the amount of aluminum input is not enough calories when the aluminum input is less than 1.0 kg per mol of molten steel, and more than 2.5 kg causes not only unnecessary aluminum consumption, but also excessively high temperature of molten steel. I will go.

The reason why aluminum is added to the upper layer of the molten steel is that the slag is located in the upper layer of the molten steel because the specific gravity of the slag in the molten steel is different.

For this reason, after aluminum is thrown into the slag layer, the temperature of molten steel can be raised without supplying oxygen by reacting with oxygen in the furnace slag instead of reacting with oxygen in the furnace molten steel.

In addition, by injecting aluminum immediately before the tapping of the molten steel, the lower oxide content of the slag layer is reduced, and the slag layer has a function of protecting the molten steel by blocking nitrogen components in the atmosphere.

The addition of aluminum reduces the content of iron oxide (T.Fe) + manganese oxide (MnO), which is a lower oxide in the slag of the electric furnace, which keeps the oxidation degree of slag low and the slag layer It is possible to reduce the mixing of atmospheric nitrogen components during the tapping.

As described in the prior art, the lower the content of the lower oxide in the slag, the lower the oxygen concentration (T. [O]), which is a measure of inclusion in the steel, and the result is as shown in Table 1 below. You will get.

TABLE 1

Aluminum input amount (kg / t) Molten steel temperature drop after tapping (℃) Absorption mass (ppm) Post Lecture [O] (ppm) Remarks After refining in the furnace In ladle while in class Case 1 0 0 96 8 980 Comparative example Case 2 3 0 42 12 356 Invention Case 3 2.5 0 40 10 412 Invention Case 4 2 0 50 12 436 Invention Case 5 1.5 0 55 9 520 Invention Case 6 1.0 0 60 10 564 Invention Case 7 0.5 0 80 8 530 Comparative example Case 8 0 3 43 36 215 Prior art Case 9 0 2.5 41 35 235 Prior art Case 10 0 2.0 45 31 281 Prior art Case 11 0 1.5 56 27 312 Prior art

Table 1 shows the change in molten steel temperature and the increase of nitrogen concentration according to the aluminum input and the aluminum input during the tapping in the 150 ton electric furnace, and the molten steel temperature drop is the ladle temperature of 1600 ~ 1620 ℃ at the end of the electric furnace. After tapping on, the molten steel temperature was measured and the difference was calculated. The absorption mass represents the difference in nitrogen concentration of the molten steel sample simultaneously with the temperature measurement. However, the nitrogen concentration in the furnace end molten steel was adjusted in the range of 25 to 40 ppm.

As shown in Table 1 above, case 1 without aluminium has a temperature drop of nearly 100 ° C, so the molten steel will solidify because the temperature after tapping is about 1520 ° C and close to the melting point of pure iron (1536 ° C). Since there is a concern, it is necessary to raise the molten steel immediately.

On the other hand, case 8, 9, 10, 11 of the conventional method of injecting aluminum into the molten steel in the ladle, since the nitrogen concentration increases by about 30ppm during the tapping, the nitrogen concentration after tapping is about 50 ~ 70ppm, the production of ultra-low carbon steel On the other hand, the method of the present invention not only increases the nitrogen concentration and decreases the oxygen concentration less than the conventional method, but also has the advantage that the temperature drop amount can be maintained at a similar level to the existing method.

In addition, the method for producing ultra low carbon steel using the refining of molten steel according to the present invention relates to a method for producing ultra low carbon steel using the refining method of molten steel described above. Referring to FIGS. 1 and 2, 1 Before tapping the molten steel of the primary refining furnace, 1.0-2.5 kg of aluminum is added to the upper layer of the molten steel in the primary refining furnace to raise the temperature of the molten steel and subsequently depressurize the molten steel from the primary refining furnace. During the decarburization treatment in the treatment apparatus, adding 0.5 to 3.0 kg of aluminum per ton of molten steel and supplying oxygen amount of 1.0 to 1.8 Nm ³ of oxygen per 1 kg of aluminum used to decarburize the molten steel while raising the molten steel; The content of iron oxide (T.Fe) + manganese oxide (MnO), which is a lower oxide, in the slag of the molten steel after the decarburization treatment is 8.0 wt% or less, and the weight of calcium oxide (CaO) to the wt% of alumina (Al ₂ O ₃ ). %of It consists of deoxidizing with aluminum and adding quicklime so that the weight ratio is 0.8 ~ 2.8.

In the present invention, just before the molten steel is pushed out in the primary refining furnace, that is, before the decarburization treatment is performed, aluminum is added to the upper layer of the molten steel to lower the oxidation degree of the molten steel slag while raising the temperature of the molten steel, and then the molten steel is removed and decarburized. By injecting aluminum and oxygen together in a subsequent decompression device made of, it is to reduce the mixing of nitrogen components in the atmosphere as the aluminum and oxygen are combined.

Determination of the amount of aluminum in the primary refining furnace is the same as described above, so redundant description will be omitted. Also, since deoxidation and quicklime input using aluminum are well known contents, the aluminum and oxygen in the subsequent decompression device will be described below. The input phase will be described in detail.

The amount of aluminum that is added to the subsequent depressurizer to raise the molten steel temperature is appropriately added according to the required temperature increase, and the reason for supplying the oxygen amount of 1.0 to 1.8 Nm ³ per 1 kg of aluminum when the aluminum is added is the oxygen in the steel when the aluminum is added. The amount of oxygen required for the decarburization reaction may be insufficient.

Table 2 below shows changes in oxygen concentration and decarburization rate (amount of carbon concentration decrease / hour) in the steel according to the oxygen consumption.

TABLE 2

Oxygen blowing amount (Nm3) Oxygen increase (ppm) Decarburization Speed (ppm / min) division One 0.6 -245 19 Comparative example 2 0.8 -111 18 Comparative example 3 1.0 -60 29 Invention 4 1.5 + 21 31 Invention 5 1.8 +38 35 Invention 7 2.0 + 93 35 Comparative example 8 2.2 + 123 37 Comparative example

As shown in Table 2 above, 1.0 Nm ³ per kg of aluminum If it is below, the oxygen concentration in the river is severely reduced and the decarburization rate is decreased, while the oxygen concentration is 1.8 Nm ^3. If exceeded, the oxygen concentration in the steel is increased to become an impurity factor in the steel, and the consumption of the alloying material is increased.

Accordingly, the aluminum input and the oxygen input amount in the decompression device are 1.0 to 1.8 Nm ³ per kg of aluminum. It can be seen that the oxygen content of is suitable.

On the other hand, supplying the oxygen can supply a known oxygen supply lance by injecting oxygen at a position spaced upwardly from the upper surface of the molten steel.

After the decarburization operation is completed in the vacuum decompression treatment device, the step of deoxidizing molten steel to aluminum and adding quicklime is the same as the conventional method, and specifically, iron oxide (T.Fe) + manganese oxide (MnO), which is a lower oxide in slag. ) To 0.8 to 3.0% by weight or less, and the weight ratio of calcium oxide / alumina to weight ratio of 0.8 to 3.5.

Figure 1 shows the total oxygen concentration in the steel as a function of iron oxide (T.Fe) + manganese oxide (MnO) of the slag before deoxidation and bubbling after vacuum treatment and before the molten steel is sent to the regeneration process. When the concentration of iron oxide (T.Fe) + manganese oxide (MnO) is 8 wt% or less, clean steel having a total oxygen concentration of 20 ppm or less is obtained, whereas above that, the cleanness is deteriorated due to high oxygen concentration in the steel. .

2 shows the relationship between the weight ratio of the calcium oxide / alumina weight percent of the slag and the total oxygen concentration in the steel when the value in the slag is 8 wt% or less, and the weight ratio of the calcium oxide / alumina weight percent of the slag is 0.8˜. In the range of 2.8, it is shown that it is possible to manufacture clean steel with a total oxygen concentration of 20 ppm or less.

As described above, the present invention, unlike the conventional method, by introducing aluminum into the upper layer of molten steel for the molten steel refined in the primary refining furnace, and by adding aluminum and oxygen to the molten steel in the subsequent decompression device, ultra-low carbon steel It is to have a technical idea to produce a clean steel that satisfies all the conditions of molten steel that can be produced, the present invention is expressed only in the order of electric furnace ⇒ vacuum decompression treatment ⇒ playing process, vacuum decompression treatment In the secondary refining apparatus, which is a former furnace refining plant called LF, vacuum decompression treatment may be performed after the molten steel temperature is raised.

In addition, oxygen may be added together in the step of injecting aluminum immediately before the molten steel in the primary refining furnace taps. At this time, the amount of oxygen may be lower than that of the depressurization apparatus, but this is only an option. will be.

The present invention as described above is the time required for heating by adding aluminum and oxygen in order to increase the temperature to the required temperature in the subsequent decompression treatment process before the molten steel is pulled out in the primary refining furnace such as an electric furnace or a converter and The present invention relates to a method for refining molten steel and a method for producing ultra-low carbon steel using refining of molten steel so as to shorten the molten steel heating rate. In addition to reducing the processing time, power energy, and refractory erosion of the primary refining furnace, it is possible to smoothly treat ultra low carbon steel using an electric furnace and to increase the number of annual rings.

In addition, by injecting aluminum into the molten steel upper layer in the primary electric furnace, there is an advantage that the nitrogen concentration in the atmosphere during the tapping can be suppressed to keep the nitrogen concentration in the steel low.

Claims (4)

Primary refining of molten steel in the primary refining furnace of the electric furnace or converter, Injecting 1.0 ~ 2.5kg per ton of molten steel to the upper side of the molten steel in the primary refining furnace for the molten steel that the primary refining is completed, And after deducting from the primary refining furnace, supplying aluminum and oxygen during decarburization in a depressurizing device to increase the temperature of molten steel and at the same time decarburizing; The decarburization-treated molten steel is subjected to aluminum deoxidation, and quicklime is added to maintain the content of iron oxide (T.Fe) + manganese oxide (MnO) in the slag to 8.0 wt% or less, and weight of alumina (Al ₂ O ₃ ). Method for producing ultra-low carbon steel using the refining of molten steel, characterized in that it comprises a step of so that the weight ratio of calcium oxide (CaO) weight% to% is 0.8 ~ 2.8. delete The method according to claim 1, In the step of supplying aluminum and oxygen during the decarburization treatment in the depressurization device, 0.5 to 3.0 kg of aluminum is added per ton of molten steel, The oxygen is a method of producing ultra-low carbon steel using the refining of molten steel, characterized in that for supplying the amount of oxygen of 1.0 ~ 1.8N ㎥ per 1kg usage of aluminum. Refining method of the molten steel, characterized in that for the molten steel refined in the primary refining furnace of the electric furnace or converter, 1.0 ~ 2.5kg of aluminum per tonne of molten steel to the molten steel in the primary refining furnace.

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