JPH0428813A - Production of dead-soft carbon steel - Google Patents

Abstract

PURPOSE:To securely obtain the desired carbon content by previously determining the computed values of CO concentration in exhaust gas in the case where the desired carbon content is presumed to be obtained and finishing decarburization at the time when the measured values of CO concentration in exhaust gas with respect to respective decarburization treatment times reach the computed values or below, respectively. CONSTITUTION:A dead-soft carbon steel is produced by using vacuum degassing equipment. At the time of carrying out decarburizing refining, the computed values of CO concentration in exhaust gas in the case where the desired carbon content is presumed to be obtained are previously determined with respect to respective decarburization treatment times. Decarburization is finished at the time when the measured values of CO concentration in exhaust gas with respect to respective decarburization treatment times reach the computed values or below, respectively. By this method, decarburization treatment time can be shortened, and this method can greatly contribute to the reduction in manufacturing costs.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for manufacturing ultra-low carbon steel. More specifically, the present invention eliminates the need for an unnecessary increase in processing time during decarburization refining when producing ultra-low carbon steel with a carbon content of, for example, 0.01% by weight or less using vacuum degassing equipment. The present invention relates to a method for producing ultra-low carbon steel that allows the carbon content in molten steel to reach a target value.

(Prior Art) In the DH vacuum degassing method and the vacuum degassing treatment method of the RH vacuum degassing ring, not only degassing but also decarburization treatment is performed.

In recent years, various proposals have been made to improve the accuracy and efficiency of vacuum degassing treatment for the purpose of degassing and decarburization.

■Unexamined Japanese Patent Publication No. 59-116315 describes ``Vacuum degassing refining method J'', and JP-A No. 61-19726 describes ``Vacuum degassing refining method for molten steel''. A technique is disclosed for achieving a target carbon content by sampling the copper composition, understanding the (C) bond, and estimating the subsequent required decarburization time.

■Unexamined Japanese Patent Publication No. 62-174317 describes a method for controlling vacuum decarburization of molten steel, in which the amount of oxygen in molten steel is measured using an oxygen sensor, and the decarburization reaction is controlled based on this measured value. , techniques have been proposed to achieve targeted carbon content.

■The applicant had implemented a method in which the amount of carbon in molten steel was brought to a target value halfway by using only the components of the exhaust gas during the decarburization process.

(Problems to be Solved by the Invention) However, such conventional techniques have the following problems.

The proposal shown in (2) above, that is, the method of sampling molten steel during the decarburization process, requires 3 to 5 minutes for the process of sampling → sample processing and analysis, and is not suitable for the current high-efficiency decarburization process. In charcoal processing, it is not possible to respond in time after sampling, which tends to lead to an unnecessary increase in decarburization processing time. Furthermore, since it involves extra work such as sampling during decarburization and scouring, it also leads to an increase in the number of personnel, resulting in an increase in costs.

In addition, the proposal shown in (2) above, that is, the method using an oxygen sensor, shows an example of end-point carbon content control in the example, but according to subsequent studies by the present inventors, depending on the oxygen content, , it may not be possible to obtain a large difference in the decarburization rate, and it is not easy to apply it to actual operations.

Furthermore, the method of using only exhaust gas components (hereinafter referred to as "conventional method") implemented by the present applicant also tends to lead to unnecessary extension of processing time. The reason for this will be explained with reference to FIG. 4, which is a graph showing the relationship between the CO concentration in the exhaust gas and the decarburization time in the manufacturing process of ultra-low carbon steel when the conventional method is implemented.

In Fig. 4, the determination of completion of decarburization in the conventional method is as follows:
Decarburization was considered to have ended when the CO concentration in the exhaust gas reached a certain predetermined value (for example, 5% by volume), that is, when the point A in FIG. 4 was reached.

This is because the decarburization rate is not constant in actual operation, so the CO concentration in the exhaust gas for determining the completion of decarburization is set to a low value from the perspective of preventing components from falling out. For example, this is shown by the broken line in Figure 4. When the decarburization rate is fast, the amount of CO gas generated/unit time also increases, so the CO concentration in the exhaust gas increases even at the same achieved (C) level, causing an unnecessary extension of treatment time.

In other words, in any conventional method,
When producing ultra-low carbon steel, for example, ultra-low carbon steel with a carbon content of 0.01% by weight or less, the target carbon content can be achieved without unnecessary extension of processing time during decarburization scouring. It was not easy to do so.

The purpose of the present invention is to provide a method that can solve all of the above problems, that is, to accurately grasp the time when the target carbon content is reached, without sampling during the decarburization process. without extending processing time.
The object of the present invention is to provide a method for producing ultra-low carbon steel that can obtain a target carbon content.

(Means for Solving the Problems) The present inventors have conducted various studies to solve the above problems. First, we focused on the decarburization reaction and the carbon concentration in the ladle. - In boat fishing, the decarburization reaction is expressed by the following formula.

[CI = [CI0exp (Kc4) ...■ However, [CI: carbon concentration in ladle (ppm) subscript [64 represents the initial value] Kc: rate constant (win-') T: decarburization treatment time (sin ) Here, by substituting the target carbon content of the product on the left side and the [CI value before decarburization treatment into [C]' on the right side, the target carbon content can be calculated for each decarburization time T. It is possible to calculate the Kc value required to obtain the desired value.

If the Kc value can be calculated in this way, the amount of decarburization at each decarburization time T can be calculated from this, and the amount of CO gas that can be generated in the exhaust gas at that time can be obtained.

In addition, if we take into account the amount of gas added to the vacuum degassing equipment (for example, R) (if we consider the vacuum degassing equipment, reflux Ar gas and Ar gas from the IJT tuyere) and the amount of leakage, The C06 degrees present in can be calculated.

Therefore, for each treatment time T, the target carbon content [CI
A calculated value of the CO concentration in the exhaust gas is obtained assuming that the CO concentration in the exhaust gas is reached, and at each treatment time during the decarburization treatment, when the measured value of CO:IA degree in the exhaust gas reaches the calculated value, All that is needed is to finish decarburization.

Based on this knowledge, after further consideration,
The present invention has now been completed.

Here, the gist of the present invention is that in a method for producing ultra-low carbon steel by decarburization refining using vacuum degassing equipment, when performing the above decarburization refining, the goal is achieved at each decarburization treatment time. The calculated value of the CO concentration in the exhaust gas is calculated assuming that the amount of carbon can be obtained, and decarburization is started when the measured value of the CO concentration in the exhaust gas at each decarburization treatment time is less than or equal to the calculated value. This is a method for producing ultra-low carbon steel, characterized by the following steps:

In the present invention, "ultra-low carbon steel" refers to steel that is a material for moldable copper plates, etc., which are typified by automobile exterior materials, and specifically, has a carbon content of 0.01% by weight or less. Includes steel that is.

That is, the present invention is a method for producing ultra-low carbon steel that can achieve the target carbon content without unnecessary extension of processing time even at any decarburization rate,
As mentioned above, the principle is to use the decarburization rate equation and carbon mass balance to predict the CO concentration in the exhaust gas at the end of decarburization for each treatment time, and then compare the measured CO concentration and the predicted CO concentration. In this method, the decarburization end time is accurately determined at the time when the decarburization end times coincide with each other, and ultra-low carbon steel having a target carbon content is manufactured based on the decarburization end time determined in this way.

(effect) Hereinafter, the present invention will be explained in detail together with its effects.

As already mentioned, the decarburization reaction in the RH vacuum degassing process can generally be expressed by equation 0.

[CI = [C]'exp(KcT) ...■ By substituting the target [CI concentration] on the left side of this equation 0 and the initial [Cl0e4 degree] at the start of decarburization of molten steel on the right side, each The decarburization rate Kc required to reach the target CI concentration for each treatment time is determined.

In addition, by integrating the decarburization rate Kc [obtained in this way over a time period of 1 minute in the vicinity for each treatment time T, the amount of decarburization at each treatment time T can be determined. It is possible to calculate the amount of CO gas generated in the belt as a result of decarburization.

Next, consider the material balance of the exhaust gas, that is, add the leakage amount to the amount of gas added to the vacuum degassing equipment (for example, in the RH degassing equipment, the reflux Ar gas and the total gas from the tuyere), and then Since the total amount of gas can be obtained by adding the amount of CO gas generated calculated as follows, it is also necessary to calculate the amount of gas in the exhaust gas assuming that the A calculated value of CO concentration can be obtained.

Therefore, the end of decarburization may be determined when the actual measured value of the CO concentration in the exhaust gas becomes equal to or less than this calculated value during each decarburization time.

Next, the calculation of the exhaust gas material balance described above will be explained in more detail. Information regarding exhaust gas material balance includes (7) or 1) below.

(7) CO gas = (t @ steel station x amount of decarburization in one minute around a certain processing time) = amount of molten steel X ((C30exp(-Kc・(d-0,
5)) B [C]'exp(-Kc・(T+0.5))) X(
The degree of coa in the exhaust gas can be calculated from the following four items: Inflow return Ar gas amount (ri) υT Tuyere Ar gas amount (1) Leakage amount.

For example, the amount of refluxed Ar gas = 2 Nm'/sin,
LIT feather tuyere r gas amount -7.6Nm'/min, leakage amount 9.2 kg/win, (C1' -400pp
If the condition of (C1=20 ppm or less) is calculated, the range as shown in FIG. 1 is obtained.

Therefore, it is possible to obtain the relationship between the CO concentration in the exhaust gas and the decarburization time T (the shaded range in FIG. 1), assuming that the target CI value is reached.

In this way, the optimal timing for finishing decarburization can be determined simply by checking the CO concentration in the exhaust gas at each decarburization treatment time T, without increasing the number of samplings during rolling. This means that ultra-low carbon steel with a high carbon content can be obtained.

Furthermore, the method for producing ultra-low carbon steel according to the present invention can be carried out without major modifications or changes to the control device or decarburization process, and has great practical advantages such as low cost for modification of existing equipment. .

Further, the present invention will be explained in detail with reference to Examples, but these are merely illustrative of the present invention and are not intended to limit the present invention.

The results of application to actual operation are shown in FIGS. 2 and 3. Second
The figure shows a comparison between the time to reach IC]=20pp+* (from the sampling results) and the time to complete decarburization (decarburization termination time) when aiming for actual rc]=20pp+m.

With the conventional method, the excessive decarburization time was about 8 minutes, but with the method of the present invention, it was possible to significantly shorten the time to 3 minutes.

However, this 3 minutes was due to the fact that the decarburization time was extended by 3 minutes in advance in order to prevent components from coming off, and the present invention was able to adjust the carbon content almost accurately.

In addition, FIG. 3 shows [C] of the product obtained by the method of the present invention.
1 distribution, but it can be seen that the results are good.

(Effects of the Invention) As detailed above, the present invention eliminates the need for a large control system and reduces manufacturing costs in the production of ultra-low carbon steel with a carbon content of 0.01% or less, for example. It became possible to accurately obtain the target carbon content without any increase in carbon content.

Furthermore, it has become possible to significantly shorten the decarburization treatment time in, for example, the RH vacuum degassing method, and it has also become possible to greatly contribute to a reduction in manufacturing costs.

The significance of the present invention having such effects is extremely significant.

[Brief explanation of the drawing]

Figure 1 is a graph showing the range of (C) = 20 ppm or less determined by the CO concentration in the exhaust gas and the RH decarburization time used in the method for producing ultra-low carbon steel according to the present invention. , A graph showing a comparison of the occurrence of excessive decarburization time in an example of the present invention and a comparative example in the method for manufacturing ultra-low carbon steel according to the present invention; Part 3 is a finished product [C] in an example of the present invention. FIG. 4 is a graph showing the relationship between the cod degree in exhaust gas and the decarburization time in the conventional manufacturing method of ultra-low carbon steel.

Claims (1)

[Claims] In the method of manufacturing ultra-low carbon steel by decarburization refining using vacuum degassing equipment, when performing the above decarburization refining, exhaust gas assuming that the target amount of carbon is obtained at each decarburization treatment time A calculated value of the CO concentration in the exhaust gas is determined in advance, and decarburization is terminated when the measured value of the CO concentration in the exhaust gas at each decarburization treatment time becomes equal to or less than the calculated value. Production method.