KR100833267B1 - Method for refining low nitrogen steel - Google Patents

Abstract

A method for refining a low nitrogen steel is provided to reduce the degassing process time in a vacuum degassing process, accomplish cleaning of molten steel, and improve quality and productivity of molten steel. In a method for refining a low nitrogen steel containing not more than 50 ppm of nitrogen, the method comprises the steps of: maintaining the speed of an IDF(Induced Draft Fan) to 330 revolutions per minute before starting a blowing operation; maintaining the speed of the IDF to 75% of the maximum speed of the IDF for 30 seconds right after starting the blowing operation; sealing a converter with a skirt after a lapse of 30 seconds after starting the blowing operation, and automatically controlling the IDF by a hood pressure according to the flue gas amount; lifting the skirt from the converter after a time point when 95% of the blowing operation has been completed; and controlling the speed of the IDF to 60 to 75% of the maximum speed of the IDF. The method comprises the step of controlling the speed of the IDF to 40 to 55% of the minimum speed of the IDF in the blowing operation. After performing the step of starting the blowing operation, the method further comprises the steps of: recovering the flue gas when flue gas contains at least 19% of CO, and a gas storage tank has a level of at least 84%; and completing the recovery of the flue gas if it is after a time point when 90% of the blowing operation has been completed, or the gas storage tank has the level of at least 92%.

Description

Method for refining low nitrogen steel

1 is a schematic diagram illustrating a converter and flue gas installation;

Figure 2 is a process flow chart showing the exhaust gas treatment process in the conventional refining process.

3 is a graph showing the behavior of the exhaust gas generated during the refining process.

4 is a graph showing the nitrogen concentration in the molten steel according to the progress of the refining process.

Figure 5 is a process flow chart showing the exhaust gas treatment process in the refining process according to the present invention.

Figure 6 is a graph showing the nitrogen concentration in the molten steel when the refining process according to the prior art and the present invention.

<Explanation of symbols for the main parts of the drawings>

1: converter

2: lance

3: skirt

4: EC

5: EP

6: IDF

7: stack

8: gas cooler

9: gas storage tank

The present invention relates to a method for refining low nitrogen steels, and more particularly, to a method for refining low nitrogen steels by preventing the absorption of molten steel due to air suction at the end of the blowing process.

Generally, the steelmaking process proceeds in the order of molten iron pretreatment, converter refining, secondary refining, and continuous casting.

In the converter refining process, carbon and impurity elements in the molten iron are introduced by charging hot metal and scrap, which are the main raw materials, into the converter, blowing high purity oxygen (O ₂ ) gas through the lance, and simultaneously supplying subsidiary materials. Means the operation of removing CO in the form of oxides of CO gas or slag. The molten iron from which impurities are removed through this process is called molten steel.

On the other hand, referring to Figure 1 showing the converter and the exhaust gas installation, in the converter refining process, the high pressure oxygen supplied from the lance 1 and the carbon in the molten iron reacts to generate the exhaust gas. The generated flue-gas removes the dust contained in the flue-gas first by the EC 4 through the skirt 3 by the suction force in the IDF 6. The first dust removed exhaust gas is secondly fine dust is removed from the EP (5), thereby high-purity converter by-product gas is stored in the gas storage tank (9) through the gas cooler (8). When the purity of the exhaust gas is low or when the gas capacity of the gas storage tank 9 is 84% or more, it is dissipated to the atmosphere through the gas stack 7.

Figure 2 is a process flow chart showing the exhaust gas treatment process in the conventional refining process.

The flue-gas treatment of the refining process is controlled by the IDF, with the IDF speed maintained at 330 RPM during the preparation phase. That is, since there is no exhaust gas or very small amount which generate | occur | produces while not carrying out a blow, it is a thing which inhales a constant air at an optimum speed. When initiation is started, the speed of the IDF is increased to 1300 RPM and maintained for 30 seconds. This is for controlling a large amount of exhaust gas because the amount of exhaust gas is rapidly increased by the CO reaction at the beginning of the blow.

30 seconds after the start of the blow, the IDF is automatically controlled by the hood pressure in accordance with the amount of exhaust gas, and the hood pressure is controlled between -70 and 70. If the hood pressure is positive (+), it means that the amount of exhaust gas is generated relatively, which causes gas and dust containing CO gas out of the furnace. Therefore, the rotation speed of IDF is increased to set the hood pressure to zero. do.

In addition, when the hood pressure is negative, air is sucked in, and the recovery of high purity LDG gas is difficult, so that the rotational speed of the IDF decreases in contrast to the above. In addition, 30 seconds after the start of blowing, the skirt seals the converter. This closed operation is desirable to recover high purity LDG gas and to prevent inhalation of air. In addition, when 95% of the elapsed time elapsed, the refining process was performed by fixing the IDF speed at 1000 RPM.

In this process, the minimum speed of the IDF is set to 800 RPM. This is at least 800 (Nm ³ / H) flue gas during decarburization, and when the flue gas is stagnant in the converter, the hood pressure is negative due to the decrease of the flue gas volume, and the IDF rate decreases. Since the control may be difficult, the minimum speed is set as described above.

As described above, the IDF controls the behavior of the flue gas and proceeds to blow. When the CO content of the flue gas is higher than 19%, the gas is recovered and stored in the gas storage tank, and the purity of the flue gas is At low or gas storage tank levels above 84%, they are released into the atmosphere through the gas stack. The stored CO gas is used for power generation in the power plant, and after 90% of the elapsed time or when the level of the gas storage tank reaches 92% or more, the gas recovery tank may be stopped and dissipated to prevent an accident such as rupture or gas ejection of the gas storage tank. do.

In order to manufacture low nitrogen steel, in this converter refining process, argon gas is blown as a low odor gas until the start of blowing and the tapping is completed. In addition, manufacturing techniques such as conducting a closed operation, increasing the use of molten iron, or injecting dolomite in the event of overdose during the drilling are used.

The nitrogen concentration in molten iron taken out from the blast furnace is about 40 to 60 ppm, but the nitrogen concentration in the molten steel at the end of the blowdown is lowered to 10 to 20 ppm. This is due to the fact that nitrogen is separated and removed by forming a TiN compound with titanium during the blowing operation. In addition, in the process of the carbon monoxide bubbles formed in the molten steel during the decarburization reaction, nitrogen in the molten steel diffuses into the carbon monoxide bubbles and is discharged together with the carbon dioxide bubbles.

However, as shown in FIG. 3, which is a graph illustrating the behavior of the flue gas generated during the refining process, the amount of flue gas is reduced at the end of the blow, and thus, outside air penetrates through the furnace and thus the nitrogen concentration in the molten steel as shown in FIG. 4. Will rise. Accordingly, in the case of producing steel grades in which the nitrogen content is regulated in a low amount, it is necessary to perform pressure reduction refining such as RH or VOD in the secondary refining process to remove nitrogen. However, the method of refining low nitrogen steel according to the reduced pressure refining has problems such as increasing the workload and increasing the manufacturing cost due to the drop in bath temperature and the extension of the treatment time.

The present invention is to solve the above problems, by preventing the absorption of the molten steel by the suction of air at the end of the blow, shortening the degassing process time in the vacuum degassing process to achieve the cleanliness of the molten steel and It is an object of the present invention to provide a method for refining low nitrogen steel, which can improve productivity.

According to the technical idea of the present invention for achieving the above object, in the refining method of low nitrogen steel, the skirt is lifted from the converter after the step of starting the blow in the converter with molten iron and scrap metal and after 95% of the time Is achieved by a method of refining low nitrogen steels comprising the step of controlling the IDF at 60-75% of the maximum speed in the set state.

Here, it is preferable that the IDF is controlled at 40 to 55% of the minimum speed during the blowing.

In addition, the speed of the IDF is fixed at 1,300 RPM for 30 seconds immediately after the start of the blow, and then the speed of the IDF is preferably controlled automatically by the hood pressure.

In addition, after the start of the blow, if the CO in the exhaust gas is 19% or more and the level of the gas storage tank is 84% or more, recovering the exhaust gas and 90% after the blow or the level of the gas storage tank is If more than 92%, it is preferable to further include the step of completing the recovery of the exhaust gas.

Hereinafter, with reference to the accompanying drawings will be described in detail with respect to the refining method of the low nitrogen steel according to the present invention.

The present invention is for the production of low nitrogen steel, such as low nitrogen steel containing less than 50ppm nitrogen content of the final steel species, injecting high purity oxygen gas into the converter containing the molten iron and scrap iron as the main raw material and at the same time injecting the secondary raw material in the molten iron It is a converter refining process that removes carbon and impurities.

The converter refining process can be divided into three stages: initial, middle and late stages depending on the decarburization reaction. The initial stage is the time when the element which has stronger affinity with oxygen (O ₂ ) than carbon (C) is oxidized and removed. For example, since silicon (Si) reacts first in molten iron, the decarburization reaction is not vigorous. Gradually increases. The middle stage is a section called decarburization strong group, and silicon in molten iron is completely removed, and all of the oxygen blown through the lance reacts with carbon in molten iron to generate CO gas as shown in Scheme 1 below. The decarburization rate depends on the blowing rate of oxygen.

[C] + 1 / 2O ₂ = CO (g)

As a result, the flow rate of the exhaust gas is also increased, and high purity CO gas can be obtained.

Meanwhile, nitrogen (N) in molten iron is accompanied and removed when decarburized by Scheme 1 during converter refining. The reaction for removing nitrogen in molten iron is the same as in Scheme 2 below, and the denitrification rate depends on the pressure and the generation rate of CO gas generated in the furnace during the blowing.

[N] = 1 / 2N ₂

In the late stage, carbon concentration in molten iron decreases and the decarburization rate gradually decreases. Oxygen injected through the lance reacts with carbon and also iron in molten iron, thereby reducing the amount of CO gas generated. For example, at the 80% elapsed time, since about 0.4 to 0.6% of carbon in molten iron remains, the oxygen supplied from the lance does not react with the carbon at all, thereby reducing the decarburization rate, which is also referred to as the rate-limiting step of carbon. . At the rate rate step, the amount of exhaust gas generated by reaction with carbon is about 900 Nm ³ / H.

In this late stage, the suction of air easily occurs in the molten steel, and as shown in FIG. 4, the nitrogen component in the molten steel that has fallen below 10 ppm increases as the nitrogen in the air is sucked into the molten steel. That is, from the start of the drilling to the point of about 85% of the elapsed time, since the inside of the furnace is always kept at a static pressure higher than atmospheric pressure due to the generation of CO gas during the eruption, air intake is not made and it is not absorbed. As carbon concentration in the molten iron decreases, the carbonization in the furnace is reduced due to the weakening of the decarburization reaction, and the air is sucked into the converter, which is absorbed by the molten steel and the nitrogen component of the molten steel rises.

The present invention is to prevent the intake of nitrogen at the end of the refining process, while controlling the speed of the IDF while performing the blow in the same manner as the usual blow pattern and the subsidiary feed conditions to prevent the absorption of the molten steel due to the suction of air. .

5 is a process flow chart showing the exhaust gas treatment process in the refining process according to the present invention.

The IDF speed is maintained at 330 RPM before starting the cleaning. That is, since there is no exhaust gas or very small amount which generate | occur | produces while not carrying out a blow, it is a thing which inhales a constant air at an optimum speed. When initiation is started, the speed of the IDF is increased to 1300 RPM and maintained for 30 seconds. This is to control a large amount of exhaust gas because the amount of exhaust gas is rapidly increased by the CO reaction at the beginning of blowing. 30 seconds after the start of the blow, the IDF is automatically controlled by the hood pressure according to the amount of exhaust gas. In addition, 30 seconds after the start of blowing, the skirt seals the converter. This closed operation is desirable to recover high purity LDG gas and to prevent inhalation of air.

In the process of blowing, if the CO content in the flue gas is higher than 19% and the level of gas storage tank is 84% or less, the gas is recovered and stored in the gas storage tank. Alternatively, if the CO content in the flue gas is lower than 19% or low, or the level of the gas storage tank is 84% or more, it is discharged to the atmosphere through the gas stack. The stored CO gas is used for power generation in the power plant, and after 90% of the elapsed time or when the level of the gas storage tank reaches 92% or more, the gas recovery tank may be stopped and dissipated to prevent an accident such as rupture or gas ejection of the gas storage tank. do.

The present invention sets the minimum speed of the IDF to 850 to 950 RPM (40 to 45% of the minimum IDF speed) in this process. As mentioned above, since the exhaust gas amount is adjusted by the hood pressure in the state in which the converter was sealed with the skirt until 95% of the elapsed time of blowing, normally, the speed of the IDF is automatically adjusted to maintain the hood pressure at zero.

However, as the exhaust gas amount decreases, the IDF speed also decreases, and when the minimum speed is reached, the hood pressure remains negative, and air is sucked between the skirt and the converter furnace by the oxygen jet, which may cause nitrogen to rise. The present invention sets the minimum speed of the IDF higher than conventionally so that the air sucked by the oxygen jet is sucked into the exhaust gas by the IDF. Accordingly, the reaction between air and molten steel can be minimized, and the absorption of molten steel can be reduced.

At 95% of the elapsed time, the exhaust gas flow rate is reduced to 50000 Nm ³ / H or less, so that there is a risk of blowing emergency stop. Thus, the blowing is performed while the skirt is raised. However, since the amount of CO gas generated in the converter is small at this time and air is sucked by the supersonic oxygen jet, nitrogen in the air can be sucked into the molten steel. Therefore, the present invention increases the exhaust gas flow rate by setting the IDF speed higher than the conventional 95% of the time point after the blow operation. That is, the speed of the IDF is automatically adjusted by the hood pressure until 95% of the elapsed time, and when the elapsed time of 95%, the IDF speed is 1100 RPM or more (60% or more of the maximum IDF speed), for example, 1100 to 1300 RPM (IDF). Proceed to the refining process by fixing at 60 ~ 75% of the maximum speed).

Table 1 shows the exhaust gas flow rate according to the IDF speed.

Referring to Table 1, when the IDF speed is 1000 RPM or less, the exhaust gas flow rate is 81000 to 96000 Nm ³ / H, while when the IDF speed is 1100 RPM, the exhaust gas flow rate is significantly increased to 120000 Nm ³ / H or more. Can be. Since there is no big fluctuation in flue gas flow rate at 1100 RPM or more, it is preferable to adjust the speed of IDF to 1100 RPM after 95% of the elapsed time.

Thus, by adjusting the exhaust gas flow rate in accordance with the speed of the IDF, it is possible to reduce the amount of nitrogen in the air is absorbed by the molten steel.

The following equation calculates the amount of nitrogen that reacts with molten steel.

Equation 1)

y = amount of nitrogen reacted with molten steel

a = amount of flue gas generated after 95% of the time

b =% of air in the flue-gases drawn into the furnace during the blow

c = nitrogen ratio in flue gas

d = proportion of nitrogen reacted with molten steel during blowing

In addition, the following formula calculates the amount of nitrogen absorbed by molten steel.

Equation 2)

x = amount of nitrogen adsorbed on molten steel

H = molten steel

The amount of nitrogen absorbed in the molten steel can be calculated using the above equations 1 and 2 above.

Table 2 below is a comparison of the exhaust gas flow rate according to the conventional IDF rate and the exhaust gas flow rate according to the IDF rate of the present invention, according to the above equation 1 and 2 the amount of nitrogen reacted with the molten steel and the amount of nitrogen absorbed in the molten steel Was calculated. In this case, the total blowing time is 1080 seconds, and the time from the 95% of the time when the blowing is completed to the completion of the blowing is 54 seconds.

Referring to Table 2, in the conventional case, the exhaust gas flow rate is 90,000 Nm ³ / H by fixing the IDF speed at 1,000 RPM after 95% of the elapsed time. By fixing the IDF speed at 1,100 RPM, the exhaust gas flow rate was increased to 120,000 Nm ³ / H. In addition, in the case of the present invention, since the air sucked by the oxygen jet is sucked into the exhaust gas by the relatively high velocity of the IDF, it can be seen that the amount of nitrogen adsorbed in the molten steel is reduced by minimizing the reaction between the air and the molten steel.

In fact, referring to Figure 6 showing the concentration (ppm) of the nitrogen component of the molten steel when the converter is refined while controlling the IDF rate according to the prior art and the present invention, in the case of the present invention, It can be seen that by reducing the concentration of nitrogen components at the end of the converter is reduced than before.

Hereinafter, the present invention will be described in more detail with reference to Examples.

The IDF speed is maintained at 330 RPM before starting the cleaning. When the blow start is started, the blow speed is increased to 1300 RPM and maintained for 30 seconds while the blow is performed in the same manner as in the usual blow pattern and the sub-raw feeding conditions. 30 seconds after the start of the blow, the skirt seals the converter and the IDF is automatically controlled by the hood pressure in accordance with the amount of exhaust gas.

Thereafter, in Comparative Examples 1 to 10, the IDF is fixed at 1,000 RPM in a state where the skirt is raised after 95% of the blow time. In addition, in this refining process, the minimum speed of the IDF is set to 800 RPM.

On the other hand, in Examples 1 to 10, the IDF speed is fixed at 1,100 RPM while the skirt is raised after 95% of the elapsed time. In addition, in this refining process, the minimum speed of the IDF is set to 900 RPM.

Table 3 below compares the concentration (ppm) of the terminal nitrogen components of the Comparative Examples and Examples.

Referring to Table 3, in Comparative Examples 1 to 10, the nitrogen concentration is 22 to 25 ppm, whereas in Examples 1 to 10, the nitrogen concentration is 10 to 17 ppm. That is, according to the present invention, the nitrogen concentration at the end of the converter can be significantly reduced to 17 ppm or less. This adjusts the minimum speed of the IDF to a relatively high speed of 900 RPM and adjusts the speed of the IDF to a relatively high speed of 1,100 RPM after 95% of the elapsed time, so that the air sucked into the furnace is sucked into the exhaust gas by the IDF. This is because it minimizes the reaction between air and molten steel and prevents absorption of molten steel.

As such, the present invention can prevent the absorption of molten steel at the end of the converter refining, thereby maintaining the nitrogen concentration at the converter end point significantly lower than the conventional 17 ppm, and thus the low nitrogen steel containing the nitrogen content of the final steel species at 50 ppm or less. It can be manufactured stably.

As mentioned above, although this invention was demonstrated in detail using the preferable Example, the scope of the present invention is not limited to a specific Example and should be interpreted by the attached Claim. In addition, those skilled in the art should understand that many modifications and variations are possible without departing from the scope of the present invention.

The present invention can prevent the absorption of the molten steel by the intake of air during the converter refining process, it is possible to shorten the degassing process time in the vacuum degassing process, to achieve the cleaning of the molten steel and to improve the quality and productivity of the molten steel.

Claims (4)

In the refining method of low nitrogen steel containing nitrogen in 50 ppm or less, Maintaining the IDF speed at 330 rpm before initiation of blowing; Maintaining the IDF at 75% of maximum speed for 30 seconds immediately after initiating the blowing; Closing the converter with a skirt 30 seconds after the start of the blowing, and automatically controlling the IDF by hood pressure according to the amount of exhaust gas; Raising the skirt from the converter after the 95% elapsed time of the blowing; And Controlling the IDF at 60-75% of maximum speed; Refining method of low nitrogen steel comprising a. The method according to claim 1, Refining method of low nitrogen steel comprising the step of controlling the IDF at 40 ~ 55% of the minimum speed of the drilling. The method according to claim 1 or 2, After starting the blowing, Recovering the exhaust gas when the CO in the exhaust gas is at least 19% and the level of the gas storage tank is at least 84%; And After 90% of the elapsed time elapsed or when the level of the gas storage tank is 92% or more, the method of refining the low nitrogen steel further comprises the step of completing the recovery of the exhaust gas. delete

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