KR101160020B1 - Producting method of low carbon steel and low phosphorus by mini-mill - Google Patents

Abstract

The present invention relates to a method for producing low carbon low lining steel by a mini mill. In the present invention, the raw material is added to the electric furnace before tapping the molten steel of the electric furnace to adjust the molten steel tapping temperature to a range of 1640-1650 ° C, and the deoxidant is added to the ladle while tapping the molten steel of the electric furnace to perform early deoxidation. Steel making process.

The present invention has the advantage that it is possible to manufacture low-carbon, low-lining steel by reducing the molten steel tap temperature to minimize the slag outflow causing the abdominal phenomenon, and shortening the desulfurization time through early deoxidation.

Mini Mill, Low Carbon, Low Lin

Description

Production method of low carbon steel and low phosphorus by mini-mill}

The present invention relates to a method for manufacturing low-carbon low-lining steel by mini-mill, and more particularly, to a mini-mill to produce steel having low carbon (C) and low-lin (P) characteristics in an electric furnace process using scrap as a main raw material. It relates to a method for producing low carbon low lining steel.

In general, when carbon is present in steel excessively, the workability of the steel is degraded, when excessively phosphorus is present, deterioration of weldability and material deviation occurs, and when sulfur is present excessively, cracking occurs.

Therefore, with the exception of some steels for special purposes, C, P and S should be as low as possible.

By the way, mini-mill process that leads to electric furnace-secondary refining (VD) -continuous casting (CC) -hot rolling does not remove C, P, S well below a certain level because scrap is used as a raw material. It is difficult to produce low carbon, low salt, low flow steel.

In general, delineation is carried out in an electric furnace, and decarburization is carried out by reaction with oxygen in a secondary pressure refining process (VD), which is a reduced pressure atmosphere, and degassing reaction is a reaction between molten steel and slag in a reducing atmosphere after decarburization in a secondary refining process. Is done by.

However, decarburization and degassing are performed in the secondary refining process, resulting in a long operation time and a temperature drop. If the temperature drop occurs in the secondary refining process, it is difficult to adjust the composition and may cause problems in the surface quality of the steel.

Therefore, the temperature is raised in the electric furnace in consideration of the temperature drop in the secondary refining process. However, if the elevated temperature of the electric furnace is high, the delinquency reaction efficiency is lowered, the slag fluidity is improved, and molten steel and slag flow out together when the molten steel of the electric furnace is ladleed to promote the abdominal phenomenon. If the compound is promoted, there is a problem that the content of the final phosphorus of the molten steel increases.

Therefore, the present invention is to solve the problems described above, the minimization of the bokrin phenomenon to produce low-carbon low-lining steel, and by mini mill that can minimize the temperature drop by shortening the secondary refining process (VD) processing time It is to provide a method for producing low carbon low lining steel.

According to a feature of the present invention for achieving the object as described above, the present invention is to put the secondary raw material into the electric furnace before tapping the molten steel of the electric furnace to the ladle to minimize the abdominal phenomenon to the molten steel tapping temperature in the range of 1640 ~ 1650 ℃ And a steelmaking process in which a deoxidizer is added to the ladle for early deoxidation to promote desulfurization during tapping of the molten steel of the electric furnace.

The secondary raw material is introduced into the slag of the furnace 3 to 5 minutes before the molten steel of the furnace is pulled out to the ladle.

The secondary raw material is quicklime, 4600 ~ 5000kg is injected per 150 tons of molten steel.

The deoxidizer is aluminum, 100 ~ 150kg per 150ton molten steel is added.

The present invention to adjust the molten steel in the slag before tapping the molten steel of the electric furnace in the slag to adjust the molten steel tap temperature in the range of 1640 ~ 1650 ℃, to put the aluminum in the ladle during the tapping of the electric furnace to make early deoxidation. .

The method reduces the slag outflow, thereby minimizing the compounding phenomena, so that the production of low coal and low lean steel is possible.

In addition, early deoxidation allows the slag formation required for desulfurization to occur early, thereby shortening the desulfurization time and reducing the total aluminum content used for deoxidation.

Reducing the desulfurization time shortens the processing time of the secondary refining process (eg, VD process), thereby reducing power and cost.

Hereinafter, a preferred embodiment of the method for producing low carbon low lining steel by the mini-mill according to the present invention will be described in detail.

The present invention adjusts the molten steel tapping temperature to the range of 1640 ~ 1650 ℃ by putting the auxiliary raw material in the slag before tapping the molten steel of the electric furnace to the ladle, and deoxidizing the molten steel in the electric furnace by adding a deoxidizer to the ladle during tapping.

The subsidiary material is added as a preparation for producing basic slag for the purpose of delineation and desulfurization. As a raw material, quicklime (CaO) is used. Quicklime removes CO ₂ by heating limestone in a kiln for more than 1173K. It is easier to dissolve than limestone, reacts faster and has less heat loss.

The Tallinn reaction promotes basic slag formation and is promoted by keeping the molten steel temperature as low as possible. The desulfurization reaction is promoted as the slag basicity is higher and the slag oxidation degree is lower. In addition, when the quicklime is added, the pyrolysis reaction is an endothermic reaction, so that heat loss occurs and the molten steel temperature is kept low, thereby facilitating the delineation reaction.

Tallinn reaction of molten steel according to the calcium oxide added is the P in the molten steel is oxidized to P ₂ O ₅ it proceeds to react to form a complex oxide such as 4CaO? P ₂ O ₅ or 3CaO? P ₂ O _5. At this time, the desulfurization reaction by quicklime along with the delineation reaction also proceeds to some extent.

The molten steel tapping temperature is maintained at 1640 ~ 1650 ℃ to minimize slag flow. When the molten steel in the electric furnace is pulled out to the ladle, the molten steel and the slag flow out together to increase the contact opportunity between the molten steel and the slag, thereby promoting the desulfurization reaction. However, in the reduction phase, P ₂ O ₅ in the slag is reduced to molten steel, causing the abdominal phenomenon.

Therefore, in the present embodiment, the molten steel of the electric furnace is preliminarily added to the electric furnace before tapping into the ladle, and dissolved by using an arc, thereby controlling the basicity of the slag to perform the delineation reaction and lowering the molten steel tap temperature to reduce the abdominal phenomenon Minimize the resulting slag outflow.

If the molten steel tap temperature is lower than 1640 ℃, the molten steel temperature is lowered in the secondary refining process (eg VD, Vacuum Degassing) after ladle (LF, Laddle Furnace) due to lack of molten steel temperature to increase working time and Causes shortage. If the molten steel tapping temperature is higher than 1650 ° C., the slag flow rate increases rapidly, and the abdominal phenomenon (P pick-up phenomenon) increases rapidly.

FIG. 1 graphically shows the relationship between the molten steel tap temperature and the P pick-up of an electric furnace.

As shown in FIG. 1, when the tapping temperature of the molten steel is higher than 1650 ° C., the P pick-up phenomenon is rapidly increased. Here, the P pick-up phenomenon refers to the compounding phenomenon, ΔP means the final P component of the electric furnace.

This quicklime is introduced into the slag of the furnace three to five minutes before the molten steel of the furnace is tapped. This is to dissolve quicklime using an arc and to secure a delineation reaction time.

Such quicklime is difficult to obtain a desired molten steel tap temperature if the input time is later than the above range, and thus, dephosphorization reaction may not be sufficient or a temperature drop may occur in the ladle, thereby increasing the time required for the secondary refining. In addition, the quicklime is faster than the above range, the molten steel temperature is lowered to the oxidizer, the operation of the furnace may be longer.

Quicklime is charged 4600 ~ 5000kg per 150ton of molten steel.

Quicklime is less than 4600kg per 150tons of molten steel, so slag formation is insufficient to promote the delinquency reaction.If it is added more than 5000kg per 150tons of molten steel, the deliming reaction is promoted, but the temperature of molten steel is lowered below the set value. have.

On the other hand, deoxidizer is added for early deoxidation. The deoxidizer is introduced into the ladle during the tapping of the molten steel in the electric furnace. These deoxidizers perform early deoxidation in the ladle to promote desulfurization and thus reduce secondary refining time.

Aluminum is used as the deoxidizer. Aluminum is charged 100 ~ 150kg per 150ton molten steel.

If aluminum is used in less than 100kg per 150ton of molten steel, the deoxidation reaction is insufficient and the second refining time is long, and if it is added in excess of 150kg per 150ton, it may affect the decarburization rate.

The decarburization reaction is a reaction in which C in molten steel is oxidized in the form of CO gas to be discharged out of the furnace and requires oxygen. Therefore, aluminum is added in a range that does not affect the decarburization rate so that decarburization is performed.

It is effective to add aluminum to the molten steel in the ladle approximately 20-50% of the time of tapping. Since aluminum is surrounded by an oxide film in the atmosphere, aluminum is injected into molten steel and reacts slowly with oxygen.

However, when 20 ~ 50% of tapping is completed, it is added to the molten steel in the ladle and turns into liquid with the tapping of the molten steel and is present in the form of activated aluminum in the molten steel. Activated aluminum facilitates the reaction with oxygen, speeds up the deoxidation reaction, and forms the slag suitable for the desulfurization reaction early.

In addition, when aluminum is added to the molten steel in the ladle at the time of completion of the molten steel 20 to 50%, the aluminum is prevented from fusion to the bottom of the ladle, and the ratio of aluminum to the molten steel is increased by strong stirring by the falling energy of the molten steel.

Of course, bubbling by gas blowing can also be performed. Bubbling is a method of stirring molten steel using the expansion energy and the kinetic energy of the gas blown into the steel. Bubbling can be carried out mainly at the point when the molten steel has been tapped into the ladle. Falling energy and bubbling of such molten steel have an effect of promoting desulfurization reaction together with deoxidation.

This method accelerates the desulfurization reaction and shortens the treatment time of the secondary refining process.

Hereinafter, the method for producing low carbon low lining steel by the mini-mill of the present invention will be described in detail through Examples and Comparative Examples.

(Example)

The molten steel of the electric furnace was poured 4800 kg of quicklime into the slag of the electric furnace 5 minutes before the tapping into the ladle and dissolved using an arc, and the molten steel tapping temperature was 1650 ° C. And the molten steel of the electric furnace was put into the ladle while the aluminum was put in the range of 100 ~ 150kg. Thereafter, decarburization was performed at the end of the secondary refining process, and 350 to 450 kg of aluminum was further added to perform deoxidation to prepare molten steel.

The contents of final carbon and phosphorus contained in the molten steel were measured. Molten steel is based on 150 tons.

(Comparative Example)

Before the operation of the furnace, 3 to 5 tons of quicklime per 150 tons of molten steel were added to the furnace and dissolved using an arc. The molten steel tapping oxygen was 800-1000 ppm and the tapping temperature was 1670 ° C. 800kg of quicklime was added to the ladle and decarburized in the secondary refining process.

In order to remove the dissolved oxygen remaining after the decarburization treatment, aluminum was added about 560-600 kg, and quicklime was added to adjust the slag basicity. After desulfurization by argon gas stirring, the secondary refining process was completed at 1580 ° C. .

Then, the content of the final carbon and phosphorus contained in the molten steel was measured.

Table 1 shows the final carbon and final phosphorus content of the molten steel produced by the comparative examples and examples. The experiment was performed up to 5th order to obtain accurate results, and the results were averaged.

(150ton of molten steel) division
Comparative example Example Primary Secondary Third 4th 5th Average Primary Secondary Third 4th 5th Average Furnace Quicklime
input
(kg) 4000 4000 4000 4000 4000 4000 4800 4800 4800 4800 4800 4800 Tap temperature
(℃) 1670 1670 1670 1670 1670 1670 1650 1650 1650 1650 1650 1650 Quicklime input
(Going out)
(kg) 800 800 800 800 800 800 - - - - - - Al input amount
(Going out)
(kg) - - - - - - 120 130 110 150 125 127 Al input amount
(VD process)
(kg) 560 600 580 570 590 580 410 390 430 360 405 399 Total Al input amount
(kg) 560 600 580 570 590 580 530 520 540 510 530 526 Final C component
(ppm) 53 57 62 59 56 57 55 65 62 57 57 59 Final P Component
(ppm) 83 79 89 85 77 83 45 50 49 52 50 49

As shown in Table 1, before the tapping of the molten steel of the electric furnace into the ladle, the quicklime is added to the electric furnace to adjust the molten steel tapping temperature to a range of 1640 to 1650 ° C, and the aluminum is added to the ladle while tapping the electric furnace into the ladle. It can be seen from the deoxidation that low-carbon low-lining steel production is possible.

The carbon and phosphorus standards for the low carbon and low phosphorus steels required in this example are less than 60 ppm carbon and phosphorus 60 ppm per 150 tons of molten steel.

In the case of the Example, the total amount of aluminum input decreased compared with the comparative example. This may be due to the decrease in slag outflow due to the change in tap temperature, which reduces the amount of aluminum used for deoxidation.

In summary, the present invention reduces the slag outflow by adjusting the molten steel temperature and changing the slag characteristics by pre-injecting quicklime into the slag of the electric furnace, thereby minimizing the abdominal phenomena.

In addition, the temperature drop was reduced by not adding quicklime during tapping of the molten steel in the electric furnace.

In addition, by introducing molten steel from the electric furnace into the ladle while tapping into the ladle, slag formation is performed early, thereby shortening the desulfurization time.

This makes it possible to produce low-carbon, low-lining steel in mini mills.

It is to be understood that the invention is not limited to the disclosed embodiments, but is capable of many modifications and alterations, all of which are within the scope of the appended claims. It is self-evident.

1 is a graph showing the relationship between the molten steel tap temperature and P pick-up of an electric furnace.

Claims (4)

In order to reduce slag outflow and minimize the abdominal phenomenon, the molten steel in the electric furnace is inputted to the electric furnace before tapping the ladle, and the molten steel tapping temperature is adjusted to the range of 1640 ~ 1650 ℃. A method for producing low carbon low-lining steel by mini-mill, comprising: a steelmaking process in which an early deoxidation is performed by introducing a deoxidizer into the ladle to promote desulfurization during tapping of the molten steel of the electric furnace. The method according to claim 1, The secondary raw material is a low-carbon low-lining steel manufacturing method by a mini-mill, characterized in that the molten steel of the electric furnace is introduced into the slag of the electric furnace 3-5 minutes before the tapping. The method according to claim 1 or 2, The secondary raw material is quicklime, a method for producing low carbon low-lining steel by mini-mill, characterized in that 4600 ~ 5000kg per 150 tons of molten steel. The method according to claim 1, The deoxidizer is aluminum, the production method of low-carbon low-lining steel, characterized in that 100 ~ 150kg per 150 tons of molten steel.

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