JP2021152191A - Method for decarburizing molten steel in rh vacuum degassing system - Google Patents

Abstract

To provide a molten steel decarburizing method in a RH vacuum degassing system in which decarburizing progress estimation accuracy can be improved.SOLUTION: During a decarburization process a decarburization reaction rate is determined as a differential equation of the first order reaction using a decarburization rate constant KC, a decarburization capacity coefficient ak is calculated using a CO gas generation rate k, the decarburization rate constant KC is calculated using the decarburization capacity coefficient ak, and the carbon concentration in the molten steel is estimated using a mathematical model expressed by the determined differential equation. When determining the decarburization end at the timing when the estimated carbon concentration in the molten steel reaches a predetermined target value, a regression equation for obtaining the CO gas generation rate k from the molten steel information and the exhaust gas information is prepared in advance based on the relationship between the CO gas generation rate k, the molten steel information and the exhaust gas information in the past decarburization processing results. The CO gas generation rate k is calculated by the prepared regression equation using the molten steel information and the exhaust gas information in the decarburization process.SELECTED DRAWING: Figure 1

Description

The present invention relates to a molten steel decarburization method in an RH vacuum degassing apparatus.

When decarburizing molten steel using a vacuum degassing device that circulates molten steel between a vacuum tank and a ladle represented by an RH type degassing device, the carbon concentration after completion is from the upper and lower limits of the standard. If it comes off, it will need to be redissolved or assigned to another product. From the viewpoint of productivity, it is necessary to stop the decarburization treatment at an appropriate timing to increase the number of treatment charges per hour. In order to solve the above problems, it is necessary to accurately estimate the carbon concentration in the molten steel during the decarburization process and stop the decarburization process when the target value is reached. Also, some methods have been proposed (for example, Patent Document 1).

Patent Document 1 describes the rate of change of carbon concentration in molten steel in a vacuum chamber and the rate of change of carbon concentration in molten steel in a pan, the carbon concentration in molten steel in a vacuum chamber, the carbon concentration in molten steel in a pan, and the vacuum chamber. We constructed a model to calculate the outflow rate of carbon from the entire molten steel using a mathematical model expressed by a differential equation using the pressure inside, the oxygen concentration in the molten steel, the mass of the molten steel in the pan, and the mass of the molten steel in the vacuum chamber. , The value obtained by multiplying the error between the calculated carbon outflow rate from the entire molten steel by the model and the calculated carbon outflow rate in the exhaust gas by a coefficient, the rate of change in the carbon concentration in the molten steel in the vacuum chamber of the model and the inside of the pan. The carbon concentration in the pan is estimated over time by an observer that corrects the rate of change in the carbon concentration in the molten steel.

Japanese Unexamined Patent Publication No. 2006-104521

Iron and Steel, Vol.84 (1998), No.10, pp709-714 Revised 5th Edition Chemical Engineering Handbook, p. 1334 Recommended equilibrium value for steelmaking reaction Revised and expanded, Japan Society for the Promotion of Science, Steelmaking 19th Committee (1984)

The method described in Patent Document 1 uses the _{decarburization capacity coefficient ak for calculating the carbon outflow rate q COM} , and the calculation of _{this ak} uses the CO gas production rate k as a constant. The CO gas generation rate k varies depending on the conditions for each treatment such as the vacuum chamber and the degree of vacuum. Therefore, when the CO gas generation rate k is used as a constant that does not vary from decarburization, since the not using the correct CO gas generation rate k of the decarburization, the calculation accuracy of the decarburization capacity coefficient a _k Will be low. Therefore, it was found that the method described in Patent Document 1 cannot estimate the molten steel carbon concentration during the decarburization treatment with sufficient accuracy.
An object of the present invention is to provide a molten steel decarburization method in an RH vacuum degassing apparatus capable of improving the decarburization progress estimation accuracy.

That is, the gist of the present invention is as follows.
[1] In an RH vacuum degassing device in which molten steel is circulated between a ladle and a vacuum tank by supplying recirculating _{gas, exhaust gas containing CO and CO 2} is discharged from the vacuum tank, and the molten steel is decarburized. In the molten steel decarburization method
The decarburization reaction rate during the decarburization process is defined as the differential equation of the first-order reaction expressed by the product of the linear equation of the carbon concentration C in the molten steel and the decarburization rate constant K _{C.
The decarburization capacity coefficient a k} is calculated using the carbon concentration C and oxygen concentration O in the molten steel, the pressure P in the vacuum chamber, and the CO gas generation rate k, and the decarburization capacity coefficient a k is used by the ring flow rate Q and the decarburization capacity coefficient a _k. The rate constant K _C is calculated, the carbon concentration in the molten steel is estimated using the mathematical model expressed by the above-defined differential equation, and the decarburization end determination is made when the estimated carbon concentration in the molten steel reaches a predetermined target value. When doing
In advance, a regression equation for obtaining the CO gas generation rate k from the molten steel information and the exhaust gas information is created based on the relationship between the actual CO gas generation rate k in the past decarburization treatment, the molten steel information, and the exhaust gas information.
A method for decarburizing molten steel in an RH vacuum degassing apparatus, which comprises calculating a CO gas generation rate k by the regression equation created above using the molten steel information and exhaust gas information at the time of the decarburization treatment.
However, the decarburization capacity coefficient _ak is a parameter representing the rate at which CO gas bubbles are generated from the molten steel in the vacuum chamber, and the oxygen concentration O in the molten steel, the carbon concentration C in the molten steel in the vacuum chamber, and the pressure P in the vacuum chamber. It can be expressed as a function that depends on.
[2] The differential equation of the first-order reaction is the following equation (1), the equation _{for calculating the decarburization capacity coefficient ak} is the following equation (2), and the equation for calculating the _{decarburization rate constant K C is as follows.} The method for decarburizing molten steel in the RH vacuum degassing apparatus according to [1], which is characterized by the equation (3).
dC / dt = -K _C・ C ・ ・ ・ (1)
a _k = A ・ k ・ K {C ・ O- (P + P _CO ^* ) / K} / (2 ・ ρ ・ g) ・ ・ ・ (2)
K _C = (Q / W) ・ ρ ・ a _k / (Q + ρ ・ a _k ) ・ ・ ・ (3)
Here, t: Time, A: Reaction interfacial area, K: equilibrium constant of the oxidation reaction of _{carbon, P} ^CO *: ^CO bubbles critical pressure, [rho: molten steel Density, g: gravitational acceleration, Q: cyclic flow of the molten steel, W : amount of molten steel in the ladle [3] time during decarburization (t = t _S) respectively by measuring the carbon concentration and the oxygen concentration in the molten steel C in _S, and O _S, these values as initial values The RH vacuum degassing apparatus according to [1] or [2], wherein the carbon concentration C in the molten steel at each time after _{t = t S} is calculated by numerically solving the differential equation. Molten steel decarbonization method.

When using the molten steel decarburization method in the RH vacuum degassing device in the melting method of ultra-low carbon steel, it is possible to shorten the processing time by accurately estimating the decarburization progress and increasing the target decarburization concentration. It becomes.

It is a figure which shows the structural example of the RH vacuum degassing apparatus for carrying out the molten steel decarburization of this invention. It is a flowchart of the decarburization process of this invention. It is a graph which shows the distribution of the time from the start to the stop of the decarburization treatment in the example of the present invention and the conventional example.

<< Equipment Overview >>
The present invention will be described with reference to the drawings showing embodiments thereof. FIG. 1 is a configuration example of an RH vacuum degassing device for carrying out the molten steel decarburization method of the present invention.

The RH vacuum degassing device shown in FIG. 1 has a vacuum tank 1, a ladle 2, and a vacuum exhaust facility 3. The vacuum tank 1 is connected to the vacuum exhaust facility 3 via the exhaust hole 4, and the inside thereof is in a vacuum state (~ 2torr). The ladle 2 houses the refined molten steel 5, and an ascending pipe 6 and a descending pipe 7 are attached to the lower end of the vacuum chamber 1. The tips thereof are immersed in the molten steel 5 in the pan 2, and the inert gas (hereinafter referred to as recirculation gas) such as _{Ar gas or N 2 gas is transferred to the molten steel 5 from the recirculation gas blowing device 8 attached to the riser pipe 6.} It has been blown.

By blowing the recirculated gas into the molten steel 5 from the recirculation gas blowing device 8, the molten steel 5 is recirculated by the gas lift pump action in the order of the ladle 2, the ascending pipe 6, the vacuum tank 1, and the descending pipe 7 as indicated by the arrows. .. At this time, the vacuum decarburization treatment is performed in the vacuum tank 1 which is in a vacuum state.

The recirculation gas blowing device 8 is provided with a recirculation gas flow meter 9 for measuring the flow rate of the recirculation gas to be blown. Further, an oxygen concentration measuring device 10 is provided which measures the dissolved oxygen concentration in the molten steel 5 by immersing the tip in the molten steel 5 in the ladle 2. Further, a pressure gauge 11 for measuring the pressure in the vacuum chamber 1 is provided. _{Further, the vacuum exhaust facility 3 includes a component analyzer 12 for analyzing the CO, CO 2} , H ₂ , and O ₂ concentrations in the exhaust gas discharged from the vacuum tank 1, and an exhaust gas flow meter 13 for measuring the flow rate of the exhaust gas. And are provided.
In the present specification, the oxygen concentration means the dissolved oxygen concentration unless otherwise specified.

The measurement results, measurement results and analysis results obtained by the recirculation gas flow meter 9, the oxygen concentration measuring device 10, the pressure measuring meter 11, the component analyzer 12 and the exhaust gas flow meter 13 are transmitted to the carbon concentration estimation unit 14. .. From these input results, the carbon concentration estimation unit 14 estimates the carbon concentration in the molten steel 5 in the ladle 2 according to the method of the present invention, and outputs the estimated value to the decarburization end determination unit 15. The decarburization end determination unit 15 compares the input estimated value of carbon concentration with a predetermined value (target value), and determines that the decarburization process should be terminated when the estimated value reaches a predetermined value. And output an instruction to end.

<< Factors governing decarburization reaction >>
Decarburization of molten steel in an RH vacuum degassing device, in which molten steel is circulated between a pan and a vacuum tank by supplying recirculating _{gas, exhaust gas containing CO and CO 2} is discharged from the vacuum tank, and the molten steel is decarburized. The carbon dioxide reaction can be grasped as a primary reaction in which the decarburization reaction rate is proportional to the carbon concentration in the molten steel. Therefore, the decarburization reaction equation during the decarburization process can be defined as a differential equation of the linear reaction represented by the product of the linear equation of the carbon concentration C in the molten steel and the decarburization rate constant K _C.

Specifically, the differential equation to be grasped as a first-order reaction can be described as the following equation (1) in relation to the time t (see equation (1) of Non-Patent Document 1).
dC / dt = -K _C・ C ・ ・ ・ (1)

The decarburization rate constant K _C can be calculated using the ring flow rate Q and the decarburization capacity coefficient a _k. Specifically, as in Non-Patent Document 1, decarburization rate constant K _C is represented by the following equation (3) using a decarburization capacity coefficient a _k (Non-Patent Document 1 (4) reference ).
K _C = (Q / W) ・ ρ ・ a _k / (Q + ρ ・ a _k ) ・ ・ ・ (3)
Here, Q: the ring flow rate of the molten steel, W: the amount of molten steel in the ladle, and ρ: the density of the molten steel.

The decarburization capacity coefficient _ak is a parameter representing the rate at which CO gas bubbles are generated from molten steel in the vacuum chamber, and is defined as an oxygen concentration O in the molten steel, a carbon concentration C in the molten steel in the vacuum chamber, and a pressure P in the vacuum chamber. It can be expressed as a dependent function. _{The decarburization capacity coefficient a k} can be calculated by using the carbon concentration C and the oxygen concentration O in the molten steel, the pressure P in the vacuum chamber, and the CO gas generation rate k. Specifically, the decarburization capacity coefficient a _k is expressed by the following equation (2) using the CO gas generation rate k (see equation (13) of Non-Patent Document 1).
a _k = A ・ k ・ K {C ・ O- (P + P _CO ^* ) / K} / (2 ・ ρ ・ g) ・ ・ ・ (2)
Here, O: oxygen concentration in molten steel (dissolved oxygen), A: reaction boundary area, K: equilibrium constant of carbon oxidation reaction, P: vacuum chamber pressure, _PCO ^* : CO bubble critical pressure.

<< How to determine the CO gas generation rate k >>
In Non-Patent Document 1, the calculation is performed using the average value of k obtained in the experiment of 0.8 (s ^-1). Further, in Patent Document 1, k is treated as a constant, following Non-Patent Document 1. However, as described above, the CO gas generation rate k varies depending on the conditions for each treatment such as the vacuum chamber and the degree of vacuum. Therefore, when the CO gas generation rate k is used as a constant that does not vary from decarburization, since the not using the correct CO gas generation rate k of the decarburization, the calculation accuracy of the decarburization capacity coefficient a _k Will be low. Therefore, it was found that the method described in Patent Document 1 cannot estimate the molten steel carbon concentration during the decarburization treatment with sufficient accuracy.

The CO gas generation rate k has a property of fluctuating depending on the molten steel information and exhaust gas information used in each decarburization treatment, and can be treated as a constant value (constant) during the decarburization treatment, but the decarburization treatment Each can have a different value. In order to accurately estimate the carbon concentration in molten steel using a mathematical model, it is necessary to accurately estimate the CO gas generation rate k in the decarburization process until the decarburization process is completed. ..

Time t = t _F decarburization process ends at, the actual carbon concentration C _F after decarburization is known, it is possible to accurately calculate the value of the CO gas generation rate k in the decarburization. Specifically, the carbon concentration and the oxygen concentration at time t = _{t F} with a carbon concentration _{C S} and the oxygen concentration _{O S} at time t = _{t S} in decarburization, the (1) to (3) was calculated by numerical solution, the value of the calculated carbon concentration and the oxygen concentration is, the actually measured time t _F actual concentration of carbon C _F, so that as much as possible close to the actual oxygen concentration O _F, CO gas generation rate k The value of is determined by fitting. By such a method, the value of the actual CO gas generation rate k in the decarburization treatment can be determined.

Next, the CO gas generation rate k calculated in the past decarburization treatment is used as the dependent variable, and the molten steel information and exhaust gas information in each decarburization treatment are used as independent variables, and the CO gas generation rate is obtained from the molten steel information and exhaust gas information. Create a regression equation to find k. Regarding the molten steel information and the exhaust gas information to be selected, it is preferable to perform a simple regression analysis with the respective molten steel information and the exhaust gas information as the independent variables and the CO gas generation rate k as the dependent variable, and select the independent variables having a large correlation. As the molten steel information includes carbon concentration C _S and the oxygen concentration O _S at time t _S, the time t _S of at least RH processing start (time t = 0) to the molten steel sampled, as an exhaust gas information, at least the molten steel sampled It is preferable to include the exhaust gas flow rate Q _gas and the (CO + CO _{2) concentration in the exhaust gas.}

<< Numerical solution of differential equations describing decarburization reaction >>
In the above equation (1), which is a differential equation describing the decarburization reaction _{, if K C} is constant in the process of processing, the equation (1) is transformed into an indefinite integral form as in the equation (4) for analysis. Can be solved to derive equation (5). In equation (5), the C analysis value at _{t = t 0 is set to C 0} , and the constant of integration is determined.
∫dC / C = -K _C ∫dt (4)
C = C ₀ · exp (-K _{C (tt 0} )) (5)

However, K _C is a function of _ak as shown in the above equation (3). As _{shown in Eq. (2), a k} is a function of k as well as a function of carbon concentration C and oxygen concentration O. Therefore, K _C changes in the process of processing. Therefore, use Eq. (4) above. I can't.

<First numerical solution>
_{It is considered that K C} can be regarded as a constant value within the minute time Δt (for example, 0.1 second), and the equation (5) holds. _{Therefore, K C} can be calculated based on the value of C and the value of O at time t, and the value of C at time t + Δt can be calculated by the following equation (6) which is a modification of equation (5). Here, C (t) and K _C (t) are the values _{of C and K C at time t.} K _C (t) is calculated each time from a value such as C (t) at time t.
C (t + Δt) = C (t) ・ exp (−K _C (t) ・ Δt) (6)
By the above equation (6), during sampling _{(t S)} the carbon concentration in the _{C S,} the oxygen concentration _{O S} and the initial value, successively calculates the C (t) at time Δt pitch, C from the time _{t S} as a result The time change of (t) can be calculated. The decarburization process is completed when C (t) reaches the target carbon concentration.

<Second numerical solution>
The above equation (1) is transformed into a difference equation.
ΔC / Δt = −K _C・ C ・ ・ ・ (7)
When Δt is transferred to the right side and ΔC is decomposed, the following equation (8) is obtained.
C (t + Δt) -C (t) = K _C (t), C (t), Δt (8)
_{That is, it is possible to calculate K C} (t) at time t based on the value C (t) of C at time t, and to calculate the value C (t + Δt) of C at time t + Δt by the equation (8). Become. Here, C (t) and K _C (t) are the values _{of C and K C at time t.} K _C (t) is calculated by Eqs. (2) and (3) each time from a value such as C (t) at time t.

<Third numerical solution>
The Runge-Kutta method is known as a numerical method for solving ordinary differential equations (Non-Patent Document 2 and Patent Document 1). By using the above equation (1) as the basic ordinary differential equation and using the Runge-Kutta method, the equation can be solved numerically with high accuracy as an initial value problem.

<Calculation of each parameter used in the calculation by the numerical solution method>
In _{Eq. (2) for obtaining} a k, C is sequentially calculated as C (t) by a numerical solution method. In addition, since the value of O and the value of P change with the passage of time, it is necessary to determine the handling of these numerical values.
During the decarburization process
C + O = CO (gas) (9)
Reaction progresses. Therefore, the carbon concentration at the time of sampling at time _{t S C S,} and the oxygen concentration _{O S,} carbon concentration C calculated at time t _(t), the carbon reduces the amount represented by the C S -C (t) The oxygen concentration O (t) at time t can be calculated assuming that the amount of O that is stoichiometrically equal to that of is reduced. It is also possible to use the oxygen concentration measurement value measured every moment using the oxygen concentration measuring device 10.
As for the pressure P in the vacuum chamber, the value of P actually measured by the pressure gauge 11 at each time can be used as it is.
Among the parameters used in the equations (2) and (3), the reaction boundary area A is the surface area of the molten steel in the vacuum chamber 1, and the amount of molten steel W in the ladle 2 is determined by the shape of the ladle. The equilibrium constant K of the carbon oxidation reaction is based on Non-Patent Document 3.
K ^{= 10 (1160 / (molten steel temperature (℃) +273) +2.003)} × defined as the atmospheric pressure, CO bubble critical pressure _{P CO} ^* is the value described in Non-Patent Document 1 (= 0.7 × ¹⁰ 3 Pa ). The ring flow rate Q of the molten steel is determined by the following equation (10) (see equation (5) of Non-Patent Document 1).
Q = η ・ D ^4/3・ G ^1/3・ T ・ ln (P ₀ / P) (10)
However, D: RH immersion tube diameter, G: recirculated gas flow rate, T: temperature of molten steel, _{P 0:} static pressure at blowing position, P: vacuum chamber ambient pressure, eta: constant (7.44 × 10 ³⁾ be.

<< Numerical analysis of differential equations with CO gas generation rate k defined >>
As described above, in the conventional numerical analysis, the CO gas generation rate k is set as a constant, and the change with time of the carbon concentration in the molten steel during the decarburization treatment is calculated.
On the other hand, in the present invention, a regression equation for obtaining the CO gas generation rate k from the molten steel information and the exhaust gas information in advance based on the relationship between the CO gas generation rate k, the molten steel information, and the exhaust gas information, which has been actually obtained in the past decarburization treatment. Is prepared, and the CO gas generation rate k is calculated by the regression equation prepared above using the molten steel information and the exhaust gas information of the decarburization treatment.

In the method of the present invention, in the molten steel decarburization treatment using the RH vacuum degassing apparatus, the numerical values of each parameter used in the above regression equation are collected from the start of the decarburization treatment to the middle of the treatment, and these numerical values are substituted into the regression equation. Then, the CO gas generation rate k in the decarburization treatment is estimated.
Estimated CO gas generation rate k, reaction boundary area A, equilibrium constant K of carbon oxidation reaction, carbon concentration C in molten steel 5 during treatment, dissolved oxygen concentration O in molten steel 5 during treatment, pressure P in vacuum chamber , CO Bubble critical pressure _PCO ^* , molten steel density ρ, and gravitational acceleration g are used to calculate the _{decarburization capacity coefficient ak by the following equation (2).}
a _k = A ・ k ・ K {C ・ O- (P + P _CO ^* ) / K} / (2 ・ ρ ・ g) ・ ・ ・ (2)
Using the calculated decarburization capacity coefficient a _k , the ring flow rate Q of the molten steel 5, the weight W of the molten steel 5 in the ladle 2, and the molten steel 5 density ρ, the decarburization rate constant K _C is calculated by the following equation (3). ..
K _C = (Q / W) ・ ρ ・ a _k / (Q + ρ ・ a _k ) ・ ・ ・ (3)

Hereinafter, the first numerical solution method is adopted for the numerical analysis including the examples. And Delta] t = 0.1 sec, the carbon concentration _{C S} during sampling _{(t S),} the oxygen concentration _{O S} as an initial value, the (6), to calculate the C (t) at time Delta] t pitch, as a result The time change of C (t) from the _{time t S can be calculated.} The oxygen concentration O (t) at time t was stoichiometrically calculated from the value of _OS , assuming that C and O are undergoing a CO formation reaction in the molten steel. As the pressure P in the vacuum chamber, the measured value of the pressure gauge 11 at each time can be used.

According to the result of the numerical solution method, the carbon concentration estimation unit 14 estimates the carbon concentration in the molten steel 5 in the ladle 2, and outputs the estimated value to the decarburization end determination unit 15.
The decarburization end determination unit 15 compares the input estimated value of carbon concentration with a predetermined value, determines that the decarburization process should be ended when the estimated value reaches a predetermined value, and ends the decarburization process. Output the instruction to the operator.

FIG. 2 shows a flowchart up to the end of decarburization. The decarburization process of the molten steel 5 is started by the RH vacuum degassing device, the values of the parameters appearing on the right side of the regression equation are determined and substituted into the regression equation, and the CO gas generation rate k is calculated. In addition, by using the carbon and oxygen concentration and the vacuum chamber 1 in the pressure and the CO gas generation rate k in the molten steel 5 calculates the decarburization capacity coefficient a _k, using a recirculation flow Q and decarburization capacity coefficient a _k de The coal rate constant K _C is calculated, and the carbon concentration in the molten steel 5 is estimated by the carbon concentration estimation unit 14 using a mathematical model expressed by a differential equation. The estimated carbon concentration in the molten steel 5 is output to the decarburization end determination unit 15, and if the target value of the carbon concentration is not reached, the carbon concentration estimation unit 14 estimates the carbon concentration again and carbon in the molten steel 5. Recalculate until the concentration reaches the target value. When the target value of the carbon concentration is reached, the decarburization end determination unit 15 determines that the decarburization process should be completed, and outputs an instruction to the operator to end the decarburization process.

Examples of concrete implementation of the present invention will be described. After performing one or more hot metal pretreatments such as desiliconization treatment, dephosphorization treatment or desulfurization treatment of hot metal discharged from the blast furnace with a torpedo car, desulfurization treatment is carried out with a hot metal pretreatment facility, and the hot metal is converted to 400 tons. It was charged into a furnace and subjected to primary refining such as desulfurization, dephosphorization, and decarburization. The molten steel obtained by the primary refining in the converter was stored in a pan and transported to a vacuum tank for vacuum degassing. Under the conditions that the flow rate of Ar gas for recirculation was 3000 NL / min and the pressure in the vacuum chamber was 300 Pa, vacuum decarburization treatment was performed so that an extremely low carbon steel having a standard upper limit of carbon concentration of 30 ppm could be obtained.

Estimated carbon concentration in the ladle by the conventional method (model in which the CO gas production rate k is determined as a constant without changing for each process) and the method of the present invention (model in which the CO gas production rate k is calculated for each process). Reached the target value of 20 ppm of carbon concentration after the decarburization treatment, and after an arbitrary time elapsed, an Al alloy was put into the vacuum chamber to stop the decarburization treatment. Comparing the carbon concentration of the molten steel collected from the pan after the treatment with the estimated carbon concentration at the time of stopping decarburization by the conventional method and the method of the present invention, the standard deviation of the conventional method is 2.9 ppm, and that of the present invention is 2. The carbon concentration in the molten steel could be estimated at 1 ppm.

Taking into consideration the upper limit of the carbon concentration standard and the standard deviation of the model, the target value of the carbon concentration after the completion of decarburization was set. When the conventional method is used, the standard deviation of the model is large, so the target value of the carbon concentration after the completion of decarburization is set to a low target value of 22 ppm. As a result of conducting 46 tests to stop decarburization when the estimated value reached the target value, the average distribution of the time from the start of treatment to the stop of decarburization was 16.7 minutes.
On the other hand, when the method of the present invention is used, since the standard deviation of the model is small, the target value of the carbon concentration after the completion of decarburization is set to a high target value of 25 ppm. As a result of conducting 20 tests to stop decarburization when the estimated value reached the target value, the average distribution of the time from the start of treatment to the stop of decarburization was 15.1 minutes.

FIG. 3 is a graph showing the average distribution of the time from the start of treatment to the stop of decarburization in the example of the present invention and the conventional example. The "I" -shaped line in the figure represents fluttering. It was confirmed that the decarburization treatment time can be shortened by using the method of the present invention.

1 Vacuum tank 2 Topping pot 3 Vacuum exhaust equipment 4 Exhaust hole 5 Molten steel 6 Ascending pipe 7 Down pipe 8 Circulating gas blowing device 9 Circulating gas flow meter 10 Oxygen concentration measuring device 11 Pressure measuring meter 12 Component analyzer 13 Exhaust gas flow meter 14 Carbon concentration estimation unit 15 Decarburization end determination unit

Claims (3)

Molten steel is circulated between the ladle and the vacuum tank by supplying recirculation _{gas, and exhaust gas containing CO and CO 2} is discharged from the vacuum tank to perform decarburization treatment of the molten steel. In the method
The decarburization reaction rate during the decarburization process is defined as the differential equation of the first-order reaction expressed by the product of the linear equation of the carbon concentration C in the molten steel and the decarburization rate constant K _{C.
The decarburization capacity coefficient a k} is calculated using the carbon concentration C and oxygen concentration O in the molten steel, the pressure P in the vacuum chamber, and the CO gas generation rate k, and the decarburization capacity coefficient a k is used by the ring flow rate Q and the decarburization capacity coefficient a _k. The rate constant K _C is calculated, the carbon concentration in the molten steel is estimated using the mathematical model expressed by the above-defined differential equation, and the decarburization end determination is made when the estimated carbon concentration in the molten steel reaches a predetermined target value. When doing
In advance, a regression equation for obtaining the CO gas generation rate k from the molten steel information and the exhaust gas information is created based on the relationship between the actual CO gas generation rate k in the past decarburization treatment, the molten steel information, and the exhaust gas information.
A method for decarburizing molten steel in an RH vacuum degassing apparatus, which comprises calculating a CO gas generation rate k by the regression equation created above using the molten steel information and exhaust gas information at the time of the decarburization treatment.
However, the decarburization capacity coefficient _ak is a parameter representing the rate at which CO gas bubbles are generated from the molten steel in the vacuum chamber, and the oxygen concentration O in the molten steel, the carbon concentration C in the molten steel in the vacuum chamber, and the pressure P in the vacuum chamber. It can be expressed as a function that depends on. Differential equation of the first order reaction is the following equation (1), equation for calculating the decarburization capacity coefficient a _k is the following equation (2), equations for calculating decarburization rate constant K _C is below (3) The method for decarburizing molten steel in the RH vacuum degassing apparatus according to claim 1, wherein the equation is used.
dC / dt = -K _C・ C ・ ・ ・ (1)
a _k = A ・ k ・ K {C ・ O- (P + P _CO ^* ) / K} / (2 ・ ρ ・ g) ・ ・ ・ (2)
K _C = (Q / W) ・ ρ ・ a _k / (Q + ρ ・ a _k ) ・ ・ ・ (3)
Here, t: Time, A: Reaction interfacial area, K: equilibrium constant of the oxidation reaction of _{carbon, P} ^CO *: ^CO bubbles critical pressure, [rho: molten steel Density, g: gravitational acceleration, Q: cyclic flow of the molten steel, W : Amount of molten steel in the pan Time during decarburization (t = t _S) respectively C _S by measuring the carbon concentration and the oxygen concentration in the molten steel _in, and O _S, by solving the differential equation numerically these values as initial values The method for decarburizing molten steel in the RH vacuum degassing apparatus according to claim 1 or 2, wherein the carbon concentration C in the molten steel at each time after t = t _{S is calculated.}

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