JPH06192724A - Method for controlling carbon concentration at end point in rh degassing treatment - Google Patents

Abstract

PURPOSE:To shorten decarburizing treatment time by quickly and precisely estimating the carbon concn. attainment in an extra low carbon steel having a specific value or lower of carbon concn. CONSTITUTION:Exhaust gas quantity is measured by using a gas flow meter 8 arranged an exhaust gas duct 7 during the decarburizing treatment of the molten steel 2 in a vacuum vessel 5. From the exhaust gas flowing difference between this measured exhaust gas quantity and the actual exhaust gas quantity at the completion of the decarburization by RH gas treatment in immediate neighborhood, the carbon concn. is estimated at this point. After confirming that this estimated carbon concn. has attained the aimed carbon concn., the decarburizing treatment is completed. By this method, since the carbon concn. can directly be estimated from the exhaust gas quantity, quick refining is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention accurately estimates the ultimate carbon concentration of molten steel and finishes decarburization when smelting ultra-low carbon steel having a carbon concentration of 50 ppm or less using an RH degasser. The present invention relates to a method for controlling the time carbon concentration.

[0002]

2. Description of the Related Art Generally, when melting extremely low carbon steel having a carbon concentration of 50 ppm or less in vacuum refining such as RH degassing, if the end point of decarburization cannot be accurately determined, excessive decarburization is required. It will be necessary and hinder the promotion of rationalization. Further, in order to precisely control the deep drawability or strength of the product, it is necessary to control the ultimate carbon concentration with high accuracy. From this, various control methods have been proposed as methods for controlling the carbon concentration of molten steel in vacuum refining.

[0003] These methods are roughly classified into two types. One is to set the decarburization rate as a primary equation and to specify the influence of operating factors by a regression equation or the like to perform [C] estimation (for example, JP-A-61-195913). The other method is to determine the residual amount of carbon content in steel by integrating the decarburization amount by measuring CO (CO ₂ ) by analyzing the exhaust gas flow rate and gas in the exhaust gas, or removing from the (CO + CO ₂ ) concentration. Estimating the charcoal speed and predicting the [C] value from the correlation equation estimated in advance (for example, Japanese Patent Laid-Open No.
No. 180424 and JP-A-3-199306).

[0004]

The above-mentioned methods have the following problems. First, the method of defining the influence of operating factors by a regression equation is static control in which changes in conditions during refining are not included in control conditions, and end point determination varies. Further, in the method of obtaining the decarburization amount by integration and obtaining the [C] value, measurement errors are accumulated.

Further, in the method of obtaining the decarburization rate from the change in the concentration of (CO + CO ₂ ), the measurement is required to be highly accurate, but the analyzer installation position is relatively far from the vacuum refining furnace. There is a delay in the time it takes for the exhaust gas to move, resulting in a lack of accuracy. If it is installed near the refining furnace, it will be very difficult to manage the equipment due to clogging of pipes due to dust, etc., and it will not be a technology at the actual operation level.

The present invention is capable of more accurately and reliably determining the end point of decarburization in the melting of ultra-low carbon steel having a carbon concentration of 50 ppm or less, avoiding excessive decarburization, and less than the target range. It is an object of the present invention to provide a method for controlling the end-point carbon concentration in RH degassing treatment, which confirms that the carbon concentration has dropped and finishes the degassing treatment.

[0007]

Means for Solving the Problems The present invention for achieving the above-mentioned object is to provide a carbon at the end of decarburization when smelting a low carbon steel having a carbon concentration of 50 ppm or less using an RH degasser. A method of controlling the concentration, in which the amount of exhaust gas is measured during the decarburization treatment of molten steel by the RH degasser, and the difference between this measured exhaust gas amount and the actual exhaust gas amount at the end of decarburization by the latest RH degassing process The RH characterized by terminating the decarburization treatment of molten steel by estimating the carbon concentration at that point in time from the exhaust gas flow rate difference and confirming that the estimated carbon concentration has reached the target carbon concentration.
It is a method of controlling the end point carbon concentration in the degassing process.

[0008]

[Function] Since the decarburization reaction in low carbon formation (carbon concentration is about 400 ppm or less) apparently follows the first-order reaction equation, the following equation (1) is established.

[0009]

[Equation 1]

However, to be exact, the coefficient K _C changes depending on the [C] value, and is expressed by the following equation (2).

[0011]

[Equation 2]

If the equations (1) and (2) are combined,

[0013]

[Equation 3]

The following relation holds and is transformed,

[0015]

[Equation 4]

It is expressed as Here, in vacuum refining, gas inflow and outflow are strictly controlled, and accurate information on exhaust gas can be obtained in real time. Further, as the inlet gas, there are reflux gas and CO gas generated by decarburization. Of these, the reflux gas is constant, so the change in the exhaust gas amount is considered to be the change in the CO gas generation amount. That is, the change in the amount of exhaust gas within a certain period of time (that is, the exhaust gas velocity) means the CO gas generation rate = the decarburization rate. D [C] / on the right side of equation (4)
dt is the decarburization rate, and the decarburization rate d is calculated from the change in the amount of exhaust gas
If [C] / dt is obtained, the coefficient α can be regarded as almost constant, so that the carbon concentration in steel [C] (ppm) can be obtained.

As described above, it is possible to accurately determine the [C] value of [C] ≦ 50 ppm by measuring the change in the amount of exhaust gas. Therefore, it is possible to perform accurate estimation without using exhaust gas analysis or the like, and reduce variations.

[0018]

EXAMPLES Examples of the present invention will be described below. FIG. 1 is a schematic view showing an implementation state of a control method by the RH degassing apparatus of the present invention, in which molten steel 2 is contained in a ladle 1, and two dipping pipes 3 are provided above the ladle 1. A vacuum chamber 5 having 4 is provided.

The tips of the two dipping tubes 3 and 4 are ladle 1.
It is immersed in the molten steel 2 inside. Further, a gas supply pipe 6 is connected to one of the dip pipes 3 for introducing a reflux Ar gas into the pipe. Further, the vacuum chamber 5 is connected to a vacuum exhaust device (not shown) via an exhaust duct 7, and the inside of the vacuum chamber 5 is maintained in a vacuum state. A gas flow meter 8 for measuring the amount of exhaust gas is installed in the exhaust gas duct 7. Further, a timer (not shown) for measuring the decarburization treatment time of the molten steel is provided.

Ar gas is blown from the gas supply pipe 6 into one of the dipping pipes 3 and the ladle 1 is operated on the principle of the gas lift pump.
The molten steel 2 inside is circulated as shown by the arrow, and the molten steel 2 is exposed to vacuum in the vacuum tank 5 to perform a vacuum decarburization treatment. In the present embodiment, the treatment conditions are as follows: amount of treated molten steel = 250 tons, immersion pipe 3,
The inner diameter of 4 was 750 mm, the amount of circulating gas was 2000 to 4000 Nl / min, and the control value of the carbon concentration at the end of decarburization was 50 ppm or less.

When the [C] low carbon steel of less than 50 ppm is melted by the RH degasser, in the stable period just before the end point of the degassing process.
Since the generation of CO gas is extremely small and can be neglected, it is considered that only Ar gas for reflux is the inlet gas of the exhaust gas. However, in reality, there are leaks that enter the tank from some junctions of the vacuum tank 5, and under this influence, the amount of exhaust gas during the stable period of the vacuum degassing process is not constant for each heat but changes according to the leak. Will be done.

FIG. 2 shows the exhaust gas flow rate kg / hr measured by the gas flow meter 8 with respect to the degassing treatment time (minutes) in the latest charge of the current charge when smelting ultra-low carbon steel using the RH degasser. An example of a trend is shown. As shown by the curve in Fig. 2, the exhaust gas flow rate is high in the high carbon region, but as the decarburization progresses, the exhaust gas flow rate decreases sharply and CO gas is not generated at the end of the degassing process in the extremely low carbon region. Is a circulating gas (Ar) containing leak gas
The flow rate in the stable period is equivalent to, and the flow rate is almost constant.

In the present invention, the amount of recirculating gas (Ar) containing leak gas at a treatment time of 20 minutes capable of producing an extremely low carbon steel of 30 ppm or less is used as the actual exhaust gas amount at the end of decarburization by the degassing process. In FIG. 2, for example, the difference between the exhaust gas amount at the point A on the curve and the recirculating gas amount (Ar) containing the leak gas, that is, the exhaust gas flow amount difference due to decarburization is regarded as the CO gas generation amount at the time point A.

By the way, in order to actually use the equation (4), the coefficient α must be obtained, which is shown in FIG.
As shown in, the relationship between the difference between the exhaust gas flow rate (kg / hr) at the time of decarburization treatment obtained as described above and the actual value of [C] is obtained in advance to obtain the [C] value from the exhaust gas flow rate. Can be asked. That is, for example, as shown in FIG. 3, the decarburization process can be reliably performed to the target [C] ≦ 30 ppm by ending the decarburization process when the difference (kg / hr) in the exhaust gas amount is 100 kg / hr.

As a decarburization determination method that does not use the decarburization determination based on the exhaust gas flow rate of the present invention, it is possible to confirm that the target carbon value is obtained by sampling during decarburization or to control the decarburization treatment time on a time basis. Conceivably, in the case of sampling, at least 5 to 6 minutes are required for the analysis value to be determined, and this time is overdecarburization. In the case of time management, the decarburization rate varies depending on the initial conditions such as the carbon concentration before decarburization and the oxygen concentration in steel for each charge, and therefore there is variation, and the time for overdecarburization is set.

Table 1 shows a comparison of the reached carbon concentrations before and after the time-controlled decarburization determination of the present invention and after the decarburization determination utilizing the amount of exhaust gas of the present invention.

[0027]

[Table 1]

As described above, according to the present invention, the excessive decarburization is eliminated, the variation in the reached carbon concentration is reduced, and the decarburization treatment time is shortened. It should be noted that the reason that the average C = 21.0 ppm after completion of decarburization with an exhaust gas flow rate difference of 100 kg / hr is considered to be that decarburization still proceeds to some extent while the deoxidizer (Al) is being added.

[0029]

As described above, according to the present invention, the RH
When melting ultra-low carbon steel with a degasser of 50 ppm or less, it is possible to accurately and quickly estimate the carbon concentration at that time using the amount of exhaust gas generated during decarburization of molten steel. As a result, excessive decarburization due to a delay in determination can be prevented, the hit rate of the ultimate carbon concentration can be improved, and the degassing treatment time can be shortened.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an RH vacuum degassing apparatus used in a control method according to the present invention.

FIG. 2 is a graph showing changes in exhaust gas flow rate.

FIG. 3 is a graph showing the relationship between the difference in exhaust gas amount (kg / hr) and [C] (ppm).

[Explanation of symbols]

 1 Ladle 2 Molten Steel 3 Immersion Pipe (Upward Side) 4 Immersion Pipe (Downward Side) 5 Vacuum Tank 6 Supply Pipe 7 Exhaust Duct 8 Gas Flow Meter

Claims (1)

[Claims] 1. The carbon concentration is 50 ppm using an RH degasser.
A method of controlling the carbon concentration at the end of decarburization when melting ultra-low carbon steel, which is the following, by measuring the amount of exhaust gas during the decarburization treatment of molten steel by the RH degasser, and measuring this exhaust gas The amount of carbon dioxide at the time is estimated from the difference between the amount of exhaust gas and the actual amount of exhaust gas at the end of decarburization by the latest RH degassing process, and it is confirmed that this estimated carbon concentration has reached the target carbon concentration. A method for controlling the end point carbon concentration in RH degassing treatment, which comprises terminating the decarburization treatment of molten steel.