JP2002363635A - Method for determining decarburization terminating point in vacuum degassing apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To determine the final point of decarburization with excellent accuracy when decarburizing a molten steel in a vacuum degassing apparatus. SOLUTION: When performing the decarburization of the molten steel in the vacuum degassing apparatus, the final point of the decarburization is determined to be: the time at which carbon concentration in the molten steel estimated on the basis of the correlation between the carbon concentration in the molten steel and the carbon concentration in the exhaust gas from the vacuum degassing apparatus reaches a target value; or the time at which the carbon concentration in the molten steel obtained by taking a sample for analysis from the molten steel during the carburization to analyze the carbon concentration, successively integrating the carbon quantity in the exhaust gas from the time when the sample for analysis is sampled, and estimating the carbon concentration on the substance balance between the carbon analysis value of the sample for analysis and the integrated carbon quantity reaches the target value, whichever the earlier.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for accurately determining the end point of decarburization refining when performing decarburization refining of molten steel in a vacuum degassing apparatus.

[0002]

2. Description of the Related Art In recent years, the demand for decarburization and refining in the molten steel stage has become increasingly severe, as represented by the ultra-low carbonization of steel components accompanying continuous annealing of thin steel sheets. As a result, the ratio of steel types that require degassing and refining is increasing. From such a background, in a vacuum degassing device typified by an RH vacuum degassing device, it is required more than ever to perform decarburization refining more quickly and surely and treat a large amount of molten steel. It became so.

In order to quickly and surely perform decarburization refining in a vacuum degassing apparatus, it is extremely important to reliably grasp the carbon concentration of molten steel during decarburization refining that changes every moment. If the carbon concentration of the molten steel is not reliably grasped, the decarburization refining is continued even though the carbon concentration of the molten steel is at the target concentration, or the carbon concentration does not decrease to the target concentration. For example, the decarburization refining is required again, which causes a delay in the decarburization refining. If a sample for analysis is taken from molten steel and subjected to chemical analysis, the carbon concentration can be accurately grasped, but the chemical analysis requires several minutes, and the analysis result is clear even after the decarburization refining is completed It does not contribute to speeding up refining.

Therefore, the following method has been proposed as a means for quickly and reliably performing decarburization refining in a vacuum degassing apparatus.

[0005] Japanese Patent Application Laid-Open No. 1-222018 (hereinafter referred to as "prior art 1") discloses that CO contained in a gas in a vacuum degassing tank.
There is disclosed a method of detecting a gas concentration and performing decarburization refining while estimating the carbon concentration in the molten steel from the correlation between the CO gas concentration and the carbon concentration in the molten steel.

Japanese Unexamined Patent Application Publication No. 9-272913 (hereinafter referred to as “prior art 2”) discloses a CO gas concentration, a CO ₂ gas concentration, and a flow rate of exhaust gas discharged from a vacuum degassing tank. Calculate the amount of carbon emitted by the exhaust gas,
A method for estimating the carbon concentration in molten steel based on the mass balance of carbon while correcting the obtained carbon amount by the carbon analysis value of an analytical sample collected from molten steel before and during decarburization refining is disclosed. I have.

[0007]

However, in prior art 1, if the operating conditions during degassing refining, for example, the amount of inert gas blown, change, the correlation between the CO gas concentration in the exhaust gas and the carbon concentration in the molten steel. Since the relationship changes, the end point may not be determined if the CO gas concentration in the exhaust gas is high, even though the actual carbon concentration in the molten steel is below the target value. Does not lead to Incidentally, the end point in the present invention is the end point of the decarburization refining.

In the prior art 2, when the analysis value becomes higher than the true value due to carbon contamination at the time of collecting the carbon analysis sample, segregation of carbon in the analysis sample, etc. In some cases, the end point is not determined even though the carbon concentration in the molten steel is below the target value.
Furthermore, when the difference between the analysis value and the true value is large, the end point is not determined forever, and a sample for analysis needs to be collected again. This will lead to adverse effects such as prolonged time and increased manufacturing costs.

The present invention has been made to solve these problems, and an object of the present invention is to provide an end point of decarburization refining when performing decarburization refining of molten steel in a vacuum degassing apparatus. An object of the present invention is to provide a method for determining with high accuracy.

[0010]

Means for Solving the Problems The present inventors have intensively studied to solve the above problems. The research results are described below.

As a result of examining the relationship between the carbon concentration in the molten steel and the carbon concentration in the exhaust gas discharged from the vacuum degassing tank of the RH vacuum degassing device during the decarburization refining in the RH vacuum degassing device, the prior art 1 As described in the above, there is a certain degree of correlation between the two, but the operating conditions and the conditions of the RH vacuum degassing device itself are slightly different between the heats to be processed, causing a variation in the correlation. When used alone, it was found that highly accurate end point determination was not possible as described above.

On the other hand, at any time during the decarburization refining in the RH vacuum degassing apparatus, a sample for analysis is taken from the molten steel to analyze the carbon concentration, and the analysis value of the carbon concentration and the time when the sample for analysis is taken By obtaining a material balance with the amount of carbon emitted together with the exhaust gas thereafter, it was possible to estimate the carbon concentration in molten steel successively, and this led to the finding that the end point of decarburization refining could be determined. Was done.

However, in some cases, the estimated value thus obtained is higher than the actual concentration, and the cause is as follows.
It was found that the analysis value of the carbon concentration of the sample for analysis was higher than the true value due to contamination of the sample for analysis and segregation of carbon in the sample for analysis. In this case, there is an adverse effect that the estimated value does not reach the target value of the carbon concentration or that it is necessary to collect a sample for analysis again.
At that time, it was found that such an unnecessary process can be minimized by performing the end point determination based on the above-described correlation between the carbon concentration in the molten steel and the carbon concentration in the exhaust gas. Was.

The present invention has been made based on the above findings, and the method for determining the decarburization end point in the vacuum degassing apparatus according to the present invention provides a method for determining the carbon concentration in molten steel when decarburizing and refining molten steel in the vacuum degassing apparatus. When the carbon concentration in the molten steel estimated based on the correlation between the carbon concentration in the exhaust gas discharged from the vacuum degassing device reaches the target value, and the carbon concentration obtained by collecting a sample for analysis from the molten steel during the decarburization refining From the time when this analysis sample was collected, the carbon amount in the exhaust gas was sequentially integrated, and the carbon concentration in the molten steel estimated based on the material balance between the carbon analysis value of the analysis sample and the integrated carbon amount was set as the target. It is characterized in that the earlier of the time when the value becomes the value and the end time of the decarburization refining is determined.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic diagram of a vacuum degassing apparatus suitable for performing the decarburization end point determination method according to the present invention.

As shown in FIG. 1, a gas supply device 2 is connected to the vacuum degassing device 1, and the gas supply device 2
Gases such as r gas and O ₂ gas are supplied. Ar gas is used as a gas for stirring molten steel at the time of decarburization refining in the vacuum degassing device 1, and is used as a reflux gas when the vacuum degassing device 1 is an RH vacuum degassing device. O ₂ gas is used as an oxygen source for decarburization during decarburization refining. However, in the case of vacuum decarburization refining, O ₂
Even if no gas is used, the decarburization reaction proceeds only with the dissolved oxygen present in the molten steel, so that the O ₂ gas is not necessarily used.

In the gas flow path between the vacuum degassing device 1 and the gas supply device 2, a flow regulating valve 6 and a flow meter 7 are installed.
Flow rate control and flow rate measurement of the supplied gas are performed. As the vacuum degassing device 1, RH vacuum degassing device, DH vacuum degassing device, VAD furnace, VOD furnace, ASEA-SKF furnace,
A vacuum induction furnace or the like can be used.

The vacuum degassing device 1 is connected to an ejector 4 via an exhaust duct 3, so that CO gas, CO ₂ gas generated by the vacuum degassing device 1, Ar gas supplied from the gas supply device 2, etc. Gas as exhaust gas in exhaust duct 3
And is discharged by the ejector 4. Exhaust duct 3
Is provided with a gas analyzer 5 for analyzing the CO gas concentration, CO ₂ gas concentration, O ₂ gas concentration, N ₂ gas concentration and the like in the exhaust gas.

The molten steel obtained by refining in a converter or an electric furnace is tapped into a ladle, and the molten steel contained in the ladle is decarburized and refined by a vacuum degassing apparatus 1. When the molten steel is in a non-deoxidized state and the dissolved oxygen is sufficiently present in the molten steel, the decarburization reaction proceeds only by depressurizing the vacuum degassing apparatus 1 and exposing the molten steel to the reduced pressure. When the amount of molten steel is small or the molten steel is in a deoxidized state, it is necessary to expose the molten steel under reduced pressure and simultaneously supply O ₂ gas to the molten steel.

The CO gas and CO ₂ gas generated by the decarburization refining under such reduced pressure are exhausted through the exhaust duct 3. When the relationship between the carbon concentration in the exhaust gas and the carbon concentration in the molten steel obtained from the CO gas concentration and the CO ₂ gas concentration in the exhaust gas is investigated, a correlation having a variation as illustrated in FIG. 2 is obtained. From this correlation, the carbon concentration in the molten steel is estimated in consideration of the variation error, and the carbon concentration in the exhaust gas at which the carbon concentration in the molten steel becomes a target value is set as “final exhaust gas carbon concentration”. FIG. 2 is a diagram illustrating the relationship between the carbon concentration in molten steel and the carbon concentration in exhaust gas during decarburization and refining in an RH vacuum degassing apparatus. In FIG. In this case, the carbon concentration in the final exhaust gas is about 6%.

On the other hand, at any time during the decarburization refining, a sample for analysis is collected from the molten steel, and the carbon concentration is chemically analyzed. From the time when the sample for analysis is collected, the amount of carbon discharged as exhaust gas, that is, the amount of degassed gas, is determined based on the CO gas concentration and CO ₂ gas concentration in the exhaust gas and the gas flow rate of Ar gas and the like supplied from the gas supply device 2. The amount of carbon removed from the molten steel by the charcoal reaction is integrated, and the carbon concentration in the molten steel is sequentially estimated based on the material balance of the integrated amount of carbon and the analytical value of the carbon concentration of the analysis sample collected from the molten steel.

The point at which the estimated carbon concentration estimated from the material balance of the carbon content in the exhaust gas reaches the target carbon concentration is defined as the end point of the decarburization refining. However, if the carbon concentration in the exhaust gas reaches the aforementioned “final carbon concentration in the exhaust gas” before the estimated carbon concentration estimated from the material balance of the carbon amount in the exhaust gas reaches the target carbon concentration, the point in time is skipped. The end point of charcoal refining. After the decarburization refining quantity, deoxidation of molten steel and component adjustment are performed as necessary.

By determining the end point of the decarburization refining in the vacuum degassing apparatus 1 in this manner, the end point of the decarburization refining can be determined with high accuracy. As a result, the amount of treatment in the vacuum degassing device 1 can be increased by shortening the decarburization refining time, and the product quality can be made uniform by reducing the variation in the carbon concentration in the molten steel, and the production cost can be significantly reduced. Becomes

[0024]

[Example] The carbon concentration of the steel output from the converter was 0.02 to 0.02.
An example of the present invention in which 0.06 mass% of about 250 tons of undeoxidized molten steel is decarburized and refined by an RH vacuum degassing apparatus by applying the decarburization end point determination method according to the present invention (hereinafter referred to as "Example of the present invention") Will be described.

The processing conditions in this case are as follows: the ultimate degree of vacuum in the vacuum degassing tank is 60 to 270 Pa; the Ar gas flow rate for reflux is 3000 Nl / min; and the target molten steel carbon concentration is 20 ppm.
And Decarburization refining was performed without using O ₂ gas as an oxygen source for decarburization. Further, the correlation between the carbon concentration in the molten steel and the carbon concentration in the exhaust gas was examined in advance under these processing conditions, and the “final exhaust carbon concentration” was set to 6%.

Vacuum decarburization refining was performed under the above processing conditions, and a sample for analysis was collected from the molten steel at the time when 5 minutes had passed since the start of the decarburization processing. And from the time when the sample for analysis was collected,
The carbon amount discharged together with the exhaust gas was calculated from the exhaust gas component and the flow rate of the injected Ar gas, and the carbon concentration in the molten steel was estimated based on the material balance between the calculated carbon amount and the carbon concentration of the sample for analysis. The decarburization refining is determined as the end point of the decarburization refining at the time when the estimated carbon concentration in the molten steel reaches the target carbon concentration or when the carbon concentration in the exhaust gas reaches 6%, whichever is earlier. Finished. In this way, decarburization refining of multiple heats was performed.

FIG. 3 shows the frequency of the variation of the decarburization refining end time in the decarburization refining, and FIG. 4 shows the frequency of the variation of the carbon concentration in the molten steel at the end of the treatment in the RH vacuum degassing apparatus. As shown in these figures, in the example of the present invention, decarburization smelting can be performed in an average processing time of 13.2 minutes,
Moreover, the average value of the carbon concentration in the molten steel after the decarburization treatment could be set to 18.7 ppm, which is extremely close to the target value of 20 ppm.

For comparison, the other conditions were the same as those of the present invention, and based on the correlation between the carbon concentration in the molten steel and the carbon concentration in the exhaust gas, only when the carbon concentration in the exhaust gas reached 6%. Operation that defines the end point of decarburization refining (Comparative Example 1)
5 minutes after the start of the decarburization treatment, a sample for analysis was collected from the molten steel, and from the time when the sample for analysis was collected, the amount of carbon discharged together with the exhaust gas was discharged into the exhaust gas component and A
An operation in which only the point in time when the carbon concentration in molten steel estimated from the material balance between the calculated carbon amount and the carbon concentration of the analytical sample reaches the target carbon concentration is determined as the end point of decarburization refining. (Comparative Example 2) was also performed.

FIG. 5 shows the frequency of the variation of the decarburization refining end time in Comparative Example 1, and FIG. 6 shows the frequency of the variation of the carbon concentration in the molten steel at the end of the treatment in the RH vacuum degassing apparatus. FIG. 7 shows the frequency of the variation of the decarburization refining end time in Comparative Example 2, and FIG. 8 shows the frequency of the variation of the carbon concentration in the molten steel at the end of the treatment in the RH vacuum degassing apparatus.

As shown in FIGS. 5 to 8, the average decarburization time was 15.8 minutes in Comparative Example 1, and 13.7 minutes in Comparative Example 2.
, And both of them take a long time as compared with the present invention. The average carbon concentration in the molten steel after the decarburization treatment is 15.4 ppm in the comparative example and 18.1 pp in the comparative example 2.
m, and the separation from the target value became larger as compared with the examples of the present invention. The dispersion of the average decarburization treatment time and the carbon concentration in the molten steel after the decarburization treatment also became larger than those of the present invention.

From these results, Comparative Example 1 and Comparative Example 2
Then it was found that the decarburization treatment was continued excessively. On the other hand, in the example of the present invention, the end point of the decarburization refining can be determined with high accuracy, and as a result, it has been found that the decarburization refining time can be reduced and the carbon concentration in the molten steel can be controlled in a narrow range. .

[0032]

As described above, according to the present invention,
When performing decarburization refining of molten steel in a vacuum degassing device, the carbon concentration in the molten steel, the correlation between the carbon concentration in the molten steel and the carbon concentration in the exhaust gas, and the substance between the carbon content in the exhaust gas and the carbon content in the molten steel Estimation is made by two types of means of income and expenditure, and the point when either of the estimated values reaches the target value is determined as the end point of decarburization refining, so the end point determination of decarburization refining can be performed with high accuracy,
As a result, the amount of treatment in the vacuum degassing device is increased by shortening the decarburization refining time, and the product quality is uniformized by reducing the variation in the carbon concentration in the molten steel, and an industrially beneficial effect is achieved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a vacuum degassing apparatus suitable for performing a decarburization end point determination method according to the present invention.

FIG. 2 is a diagram illustrating the relationship between the carbon concentration in molten steel and the carbon concentration in exhaust gas during decarburization refining in an RH vacuum degassing device.

FIG. 3 is a diagram showing the frequency of variation in the decarburization refining end time in the example of the present invention.

FIG. 4 is a diagram showing the frequency of variation in the carbon concentration in molten steel in an example of the present invention.

FIG. 5 is a diagram showing the frequency of variation in the decarburization refining end time in Comparative Example 1.

FIG. 6 is a diagram showing the frequency of variation in carbon concentration in molten steel in Comparative Example 1.

FIG. 7 is a diagram showing the frequency of variation in the decarburization refining end time in Comparative Example 2.

FIG. 8 is a diagram showing the frequency of variation in carbon concentration in molten steel in Comparative Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Vacuum degassing device 2 Gas supply device 3 Exhaust duct 4 Ejector 5 Gas analyzer 6 Flow control valve 7 Flowmeter

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Eiji Sakurai 1-2-2 Marunouchi, Chiyoda-ku, Tokyo F-term (reference) 4K013 BA02 CA02 CA04 CC01 CE00 CE01 CE02 CE04 CE05 CE08 CF12 FA02 FA03 FA11

Claims (1)

[Claims] When performing decarburization and refining of molten steel in a vacuum degassing apparatus, the carbon concentration in the molten steel estimated based on the correlation between the carbon concentration in the molten steel and the carbon concentration in the exhaust gas discharged from the vacuum degassing apparatus is reduced. At the time when the target value was reached, during the decarburization refining, a sample for analysis was taken from the molten steel to analyze the carbon concentration, and from the time when the sample for analysis was taken, the carbon amount in the exhaust gas was successively integrated, and The point at which the carbon concentration in the molten steel estimated based on the material balance between the carbon analysis value of the sample and the integrated carbon amount has reached the target value, whichever is earlier, is determined to be the end point of the decarburization refining. A method for determining a decarburization end point in a vacuum degassing apparatus.