KR20010028813A - A method for controlling carbon contents in molten steel when refining the ultra low carbon steel - Google Patents

Abstract

PURPOSE: A method for controlling dissolved carbon during refining of molten steel for very low carbon steel is provided to efficiently control dissolved carbon not only by analyzing constituents of an exhaust gas discharged but also by measuring a degree of refining vacuum, and increase productivity by shortening a carburization time. CONSTITUTION: The method for controlling dissolved carbon in the molten steel (13) comprises the steps of measuring a degree of refining vacuum (υ) by installing a vacuum gauge (30) in an RH vacuum degassing equipment; obtaining a total content (η:) by summing those contents after obtaining each contents of carbon dioxide (Xco2) and carbon monoxide (Xco) in a discharged exhaust gas through a mass spectrometer (40) installed in the equipment; judging whether dissolved carbon in the molten steel is appropriate or not by comparing the obtained degree of refining vacuum (υ) and total content (η:) with a preset target degree of refining vacuum (υ°) and a target total content (η°:) respectively; and continuously judging whether dissolved carbon in the molten steel is appropriate or not by continuously repeating the above steps in refining molten steel (13) for very low carbon steel using an RH vacuum degassing equipment consisting of a ladle (10), a deposition pipe (15), a vacuum vessel (20), a gas cooler (35) and a gas outlet (33).

Description

A method for controlling carbon contents in molten steel when refining the ultra low carbon steel}

The present invention relates to a method for controlling dissolved carbon during ultra low carbon steel refining, and more particularly, in the ultra low carbon steel molten steel refining in the RH degassing apparatus, which is a secondary steelmaking refining process. Rather, the present invention relates to a method for controlling dissolved carbon amount, which measures the degree of vacuum in the apparatus in real time.

In general, the carbon content of the steel composition is less than 70ppm is called ultra low carbon steel, and such ultra low carbon steel is widely used as a material requiring high deep drawability such as automobile materials. Such ultra low carbon steels include carbon content of 70 ppm or less (DQ), 50 ppm or less (DDQ), 25 ppm or less (EDDQ), and 20 ppm or less (SEDDQ). The molten steel refining of the ultra low carbon steel is performed in an RH vacuum degassing apparatus (hereinafter referred to as RH) in a secondary refining process located in the middle of a converter and a continuous casting. This is the final refining process that is carried out in the company, and the final steel composition adjustment is made to meet the demand. In other words, the process confirms the dissolved carbon content in the RH. If the carbon content is higher than the target value, further correction is performed. Therefore, in the production of ultra low carbon steel, it is important to effectively control the amount of dissolved carbon during secondary refining in RH. As a method for confirming the amount of dissolved carbon in the RH, conventionally, a method of directly collecting and analyzing a molten steel sample has been adopted. However, in this method, if the analysis of molten steel sample is delayed, it must be transferred to the continuous casting process without performing the correction work on dissolved carbon. Therefore, there is a problem of deteriorating productivity as well as degrading the quality of the final product in terms of carbon content.

Examples of the prior art for solving the above problems are described in Korean Patent Nos. 97830 and 117024, and Japanese Patent Laid-Open No. 97-202913. The above documents suggest to control the dissolved carbon amount by measuring the flow rate of the RH exhaust gas and the CO, CO ₂ content contained in these exhaust gases. However, in the above method, the exhaust gas flow rate analyzer becomes aging according to the aging of RH, and the exhaust gas flow rate frequently becomes zero (0) as shown in FIG. have. In addition, the method does not have any means to correct the result even if an error result is obtained from the exhaust gas flow analyzer which detects the exhaust gas flow rate discharged from the RH. There is a problem that is not easy.

The present invention is to solve the above problems, in the refining of ultra low carbon steel using the RH vacuum degassing apparatus, it is accurate by measuring the refining vacuum degree as well as the content of carbon dioxide and carbon monoxide in the exhaust gas discharged from the apparatus Its purpose is to provide a method for effectively controlling the amount of dissolved carbon.

1 is a graph showing an abnormality of the exhaust gas appearing when controlling the amount of dissolved carbon during ultra low carbon steel refining using the conventional method;

2 is a schematic diagram of an RH vacuum degassing apparatus conforming to the method for controlling dissolved carbon amount of the present invention;

3 is a graph showing the decarburization time in the method of the present invention in comparison with the conventional method.

Explanation of symbols on the main parts of the drawings

10 ....... ladle 13 ....... molten steel

15 ....... Sedimentation pipe 20 ....... Vacuum chamber

30 ....... Vacuum meter 33 ....... Gas outlet

35 ....... Gas Cooler 40 ....... Mass Spectrometer

50 ....... Arithmetic unit 60 ....... Monitor

Therefore, the present invention for achieving the above object, in refining molten steel for ultra-low carbon steel by using a RH vacuum degassing apparatus composed of a ladle, a deposition tube, a vacuum tank, a gas cooler and a gas outlet,

Installing a vacuum gauge in the apparatus to measure the refinement vacuum degree (υ);

Obtaining carbon dioxide content (X _{CO 2} ) and carbon monoxide content (X _CO ) in the exhaust gas discharged through a mass spectrometer installed in the apparatus, respectively, and calculating a total content (η :) by summing these contents;

Comparing the refining vacuum degree (υ) and the electric charge content (η :) obtained from the above to the predetermined target refining vacuum degree (υ ^o ) and the target electric charge content (η ^o :), respectively, to determine whether the dissolved carbon content in the molten steel is appropriate. Step; and

And continuously determining whether the amount of dissolved carbon in the molten steel is appropriate by repeatedly performing the above steps.

Hereinafter, the present invention will be described in detail.

In general, refining of molten steel for ultra low carbon steel using an RH apparatus means refining molten steel sufficiently supplied with carbon and oxygen under reduced pressure. At this time, carbon and oxygen dissolved in molten steel are removed by generating carbon monoxide and carbon dioxide by the following reaction formula 1.

[C] + [O] = CO (g) or CO ₂ (g)

Where [C] and [O] are carbon and oxygen dissolved in molten steel, respectively, and CO (g) and CO2 (g) are respectively carbon dioxide and carbon dioxide produced by the reaction of carbon and oxygen in a reduced pressure RH apparatus. Means carbon monoxide gas.

As can be seen from Scheme 1, the amount of dissolved carbon in the molten steel can be estimated from the contents of carbon dioxide and carbon monoxide emitted from the RH apparatus. In addition, Scheme 1 shows that the amount of dissolved carbon can vary depending on the degree of vacuum imparted in the RH apparatus. In detail, as the degree of vacuum in the RH apparatus increases, the forward reaction of Scheme 1 proceeds, thereby reducing the amount of dissolved carbon in the molten steel.

Therefore, the present invention is characterized by precisely controlling the dissolved carbon content in molten steel by measuring the refined vacuum degree as well as the carbon dioxide and carbon monoxide content in the exhaust gas discharged in the refining of the molten steel for ultra carbon steel using the RH apparatus.

That is, in the method for controlling the dissolved carbon amount of the present invention, a vacuum gauge is first installed in the RH apparatus to measure the refinement vacuum degree (υ). At this time, the vacuum gauge is irrelevant to the vacuum measuring principle or the type thereof, and may be anything that can accurately measure the vacuum degree in the range of 0.01 to 100 Torr. In addition, although the measurement period of the refined vacuum degree can be arbitrarily determined, most preferably, the measurement period is set to 1 to 2 seconds. This is because if the measurement period is less than 1 second, it is difficult to obtain an accurate vacuum degree, and if it exceeds 2 seconds, an accurate vacuum degree can be obtained, but the time for predicting the amount of dissolved carbon is relatively delayed.

The carbon dioxide content (X _{CO 2} ) and carbon monoxide content (X _CO ) in the flue gas discharged from the device are respectively measured through a mass spectrometer installed in the RH device.

As described above, the measured vacuum degree (υ), the carbon dioxide content (X _CO2 ) and the carbon monoxide content (X _CO ) are all transmitted to the computing device. First, the calculation unit adds the transferred carbon dioxide content (X _CO2 ) and carbon monoxide content (X _CO ) to measure the total content (η :), and the measurement period is controlled to be consistent with the above-described vacuum degree measurement cycle. It is more preferable for the determination of the amount of dissolved carbon.

Next, the arithmetic unit determines whether the dissolved carbon content in molten steel is appropriate by comparing the refined vacuum degree υ and the electric charge content η: with the target refined vacuum degree υ ^o and the target electric charge value η ^o :, respectively. do. Here, the target refining vacuum degree (υ ^o ) refers to the refining vacuum degree required to secure the required level of dissolved carbon for each ultra low carbon steel, and the target charge content (η ^o :) is required to secure the dissolved carbon level at the above-mentioned level. It refers to the sum of the CO content and the CO ₂ content in the flue gas (ie, Xco ^o + Xco ₂ ^o ), all of which are built in the operation unit by a predetermined program.

On the other hand, the target refining vacuum (υ ^o ) and the target electric charge (η ^o :) may vary depending on the process or the ultra-low carbon steel grade, in the present invention, the quality of the final product by controlling the amount of dissolved carbon In consideration of the following limitations are most preferred.

Ultra low carbon steel type DQ DDQ EDDQ SEDDQ Carbon content ≤70ppm ≤50ppm ≤40ppm ≤25ppm ≤20ppm Target Refining Vacuum (υ ^o ) ≤2 Torr ≤2 Torr ≤2 Torr ≤2 Torr ≤2 Torr Target battle content (η ^o :) 15 10 8 5 2

As can be seen from Table 1, the refined vacuum degree (υ) and the total content (η :) measured according to the steel grade of the ultra low carbon steel are the target refinement vacuum degree (υ ^o ) and the target electric charge value (η ^o : ), The amount of dissolved carbon in the molten steel is controlled to the desired level. For example, when refining a DQ grade ultra low carbon steel with a carbon content of 70 ppm or less, as shown in Table 1 above, the actual measured vacuum degree (υ) is 2 Torr or less, and the sum of CO and CO _{2 in} the flue gas, that is, electric charge When (η) reaches 15, the amount of dissolved carbon in the molten steel is controlled to 25 ppm or less. Therefore, when the actual measured value reaches Table 1 according to the type of ultra low carbon steel, the calculating device advances the follow-up work by notifying the outside through an external display means such as a monitor that the decarburization reaction is completed. If not, it is possible to efficiently control the amount of dissolved carbon in the molten steel by repeatedly performing the above-described steps.

Hereinafter, the present invention will be described with reference to the accompanying drawings.

2 is a schematic view showing an RH vacuum degassing apparatus conforming to the dissolved carbon amount control method of the present invention. As can be seen in FIG. 2, the apparatus is largely composed of a ladle 10, a deposition tube 15, a vacuum chamber 20, a gas outlet 33, a gas cooler 35, and a condenser 45. The molten steel 13 in the ladle 10 is introduced into the vacuum chamber by depressurizing the vacuum chamber 20 to start the decarburization reaction.

In the present invention, in order to control the amount of dissolved carbon in the molten steel introduced into the vacuum chamber 20, a vacuum gauge 30 is installed to measure the vacuum degree, and the vacuum gauge 30 includes the vacuum chamber 20 and the gas cooler. Preferably, it is provided above the gas outlet 33 between the 35. When the vacuum system is installed in the vacuum chamber 20, a high temperature splash (splash 23) of about 1600 ° C may pop out during refining under vacuum, and the vacuum system may be damaged. 35) In the case of installation at the rear end, it is difficult to accurately measure the degree of refined vacuum degree, and when it is installed at the lower end of the gas outlet 33, dust 25 is accumulated and it is difficult to measure the accurate degree of refined vacuum degree.

And a mass spectrometer (40) to the gas cooler 35 is installed at the rear end is generated from the above reaction scheme 1 for refining a molten steel content in the gas discharged CO and CO ₂ (that is, X _CO and X _CO2) in the present invention Measure Usually, as an instrument for analyzing the contents of CO and CO _{2 in} the flue gas, there is a gas chromatograph using infrared spectroscopy, a mass spectrometer using a magnetic field principle, etc. In the present invention, the gas cooler ( 35) Since the temperature of exhaust gas reaches 300 ℃ at the rear stage, it is preferable to select mass spectrometer. If the mass spectrometer 40 is installed in front of the gas cooler, the temperature of the exhaust gas reaches a maximum of 1300 degrees Celsius, and it is difficult to accurately analyze the component due to the attachment of dust. In addition, the mass spectrometer 40 may be installed at any point A between the condenser 45 and the stack 47, but it is required to reach the installation point of the exhaust gas by 2 to 3 minutes, thereby reducing the amount of dissolved carbon. It is not preferable because accurate control is difficult.

On the other hand, the refining vacuum degree (υ) measured in the vacuum system 30 described above and the content of CO and CO2 in the exhaust gas analyzed by the mass spectrometer 40 are all transmitted to the computing device (50). First, the calculation device 50 calculates the total content (η :) by adding the transferred carbon dioxide content (X _CO2 ) and carbon monoxide content (X _CO ), and then calculates the refining vacuum degree (υ) and the total content (η :). The appropriate amount of dissolved carbon in the molten steel is judged by comparing the target refinement vacuum (υ ^o ) and the target electric charge (η ^o :) respectively. In other words, the monitor 60 indicates that the decarburization reaction is completed when the actual measured vacuum degree υ and the electric charge content η: reach the target degree of vacuum υ ^o and the target electric charge value η ^o :. It is displayed to the outside through the following procedure, and if the target value is not reached by repeating the above procedure it is possible to effectively control the amount of dissolved carbon in the molten steel to the desired level.

Hereinafter, the present invention will be described in detail through examples.

(Example 1)

Molten steels for ultra low carbon steels used for refining DQ, DDQ, EDDQ and SEDDQ were respectively prepared from LD converter. In the 300 ton RH vacuum degassing apparatus provided with a vacuum gauge at the upper end of the gas outlet and a mass spectrometer at the rear of the gas cooler, the molten steel for the ultra low carbon steel was refined with different refining vacuum degrees. At this time, the refining vacuum degree was measured at 1.5 second intervals, and the sum of the contents of CO and CO ₂ , that is, the η value was measured by analyzing the contents of CO and CO _{2 in} the flue gas at the same cycle. Molten steel samples were collected according to the refinement vacuum degree and η value obtained during the refining of molten steel for ultra low carbon steels having different types as described above, and these were analyzed using the carbon / sulfur simultaneous analyzer to analyze the dissolved carbon content in the molten steel. The results compared with the small amount is shown in Table 2 below.

division Ultra low carbon steel type Target dissolved carbon content Measures Actual dissolved carbon (ppm) Refining Vacuum (torr) η () Inventive Example One DQ ≤70ppm 2 15 65 2 DDQ ≤50ppm 1.5 9 44 3 2 10 46 4 ≤40ppm 2 7.5 36 5 One 8 32 6 EDDQ ≤25ppm 2 4.6 21 7 2 5 22 8 SEDDQ ≤20ppm 2 2 18 Comparative example One DQ ≤70ppm 3 14 73 2 2 16 72 3 DDQ ≤50ppm 3.5 9 52 4 2 11 55 5 ≤40ppm 3.5 7.5 46 6 1.5 8.6 43 7 EDDQ ≤25ppm 3.5 4.6 28 8 1.5 5.7 27 9 SEDDQ ≤20ppm 3.5 1.8 22 10 1.5 2.5 21

As can be seen from Table 2, Inventive Examples (1 to 8) effectively control the amount of dissolved carbon at a desired level for each ultra-low carbon steel because both the actual measured vacuum and electric content (η :) are within the scope of the present invention. It can be seen that.

On the other hand, Comparative Examples (1, 3, 5, 7 and 9) is the degree of dissolved carbon at a desired level even if the measured vacuum degree is outside the scope of the present invention, even if the total content (η :) is within the scope of the present invention. Could not get

In Comparative Examples (2, 4, 6, 8 and 10), the refined vacuum degree measured for each ultra low carbon steel is within the scope of the present invention, but its total content (η :) is outside the range of the present invention to obtain a desired level of dissolved carbon. It can be seen that.

(Example 2)

The experiment of Example 1 was repeated to show the average decarburization time for each steel type of the ultra low carbon steel in FIG. 3. Here, the conventional method is an experimental result in the case of controlling the amount of dissolved carbon by analyzing only the contents of CO and CO _{2 in} the exhaust gas without considering the refinement vacuum degree. As can be seen from Figure 3, in the case of the present invention method for refining the ultra-low carbon steel molten steel compared to the conventional method it can be seen that the decarburization time for each steel type can be shortened by 2-3 minutes. This is a general result of excessive decarburization in consideration of the measurement error that occurs frequently in the case of the conventional method. On the contrary, in the case of the present invention, it is possible to accurately and effectively control the amount of dissolved carbon by controlling the amount of exhaust gas as well as the refined vacuum at the same time in controlling the amount of dissolved carbon, thereby reducing the decarburization time. Able to know.

As described above, the present invention enables accurate and efficient control of the dissolved carbon amount by measuring the refinement vacuum as well as the component analysis of the exhaust gas discharged during the refining of the molten steel for ultra low carbon steel, thereby reducing the decarburization time, thereby improving the productivity of the steelmaking industry. It is useful to uplift.

Claims (4)

In refining molten steel for ultra low carbon steel using RH vacuum degassing apparatus composed of ladle, immersion pipe, vacuum tank, gas cooler and gas outlet, Installing a vacuum gauge in the apparatus to measure the refinement vacuum degree (υ); Obtaining carbon dioxide content (X _{CO 2} ) and carbon monoxide content (X _CO ) in the exhaust gas discharged through a mass spectrometer installed in the apparatus, respectively, and calculating a total content (η :) by summing these contents; Comparing the refining vacuum degree (υ) and the electric charge content (η :) obtained from the above to the predetermined target refining vacuum degree (υ ^o ) and the target electric charge content (η ^o :), respectively, to determine whether the dissolved carbon content in the molten steel is appropriate. Step; and Continuously determining whether the amount of dissolved carbon in the molten steel is appropriate by repeatedly performing the above steps; The method of claim 1, wherein the target vacuum degree (υ)^o) Is 2torr or less; The target battle content (η^o)silver In the case of ultra low carbon steel, which requires 70ppm or less of carbon, 15, 50ppm or less is required, 10, 40ppm or less is required, 8, 25ppm or less is required, and 2 or 20ppm is required. Control method characterized in that The method of claim 1 or 2, wherein the measurement cycle of the refinement vacuum degree (υ) is 1 to 2 seconds, and the measurement cycle of the total content (η :) is coincident with the cycle of the refinement vacuum degree (υ). Control method The control method according to claim 1 or 2, wherein the vacuum gauge is installed at an upper end of a gas outlet between the vacuum chamber and the gas cooler, and the mass spectrometer is installed at a rear end of the gas cooler.