JPH0841528A - Vacuum degassing refining method - Google Patents

Abstract

PURPOSE:To shorten the treating time, to reduce the refining cost and to improve the hitting ratio of product components by executing the refining, by estimating the change in the conc. of carbon with the specific equatious from the point of time when the carbon content in molten steel becomes a specific estimated value through the degassing refining. CONSTITUTION:At the time of executing the degassing refining treatment by this vacuum degassing refining method, after starting the treatment or after sampling the molten steel during the treatment, the carbon content in the molten steel is continuously estimated based on the components in an exhaust gas and the vol. of the exhaust gas. Since an optional time when the estimated value of the carbon content in this molten steel becomes 100ppm to 30ppm, the process of change in carbon concentration is estimated with the equations to execute the refining dc/dt=-KC1''(C-C*')-KC2'' (C-C*), C*'=(P+P '/(KXO)), C*=P/(KXO) (wherein, if C<C*, KC1''=O, C and O are carbon and oxygen contents in a ladle, respectively, P is pressure in the vacuum vessel, K is equilibrium constant between carbon and oxygen, KC1'' and KC2'' are decarburizing rate constants obtd. with fitting, P' is bubble producing pressure and (t) is time).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a vacuum degassing refining method for molten metal.

[0002]

2. Description of the Related Art A conventional vacuum degassing refining method is disclosed in Japanese Patent Application Laid-Open No.
For the purpose of decarburization, as introduced in 1-81722, the vacuum degassing refining is started by setting an appropriate vacuum pattern predetermined before refining and a target decarburization transition pattern obtained based on the refining time, The carbon content in the molten steel is measured or measured from the component analysis value in the exhaust gas at any point on the way, and the difference between the measured value and the target decarburization transition value at that time is used to determine the oxygen supply rate, the degree of vacuum, and the environment. By controlling the flow rate, the decarburization rate that matches the existing target transition value is controlled.

[0003]

The control of decarburization as described above utilizes a model formula for estimating the transition of decarburization.
The higher the estimation accuracy in this model, the shorter the processing time, the lower the refining cost, and the higher the accuracy rate of the components of the product.

A conventional vacuum degassing refining method is disclosed in Japanese Unexamined Patent Publication No. 61-61.
As described in Japanese Patent Laid-Open No. 19726, there is a model formula (formula (4) below) configured by a reaction rate formula of C, 0. However, in the recent production of ultra-low carbon steel, especially in the ultra-low carbon region where C is 20 ppm or less, the decarburization rate sharply decreased, and the decarburization transition value did not match the actual results. ^{_{ln ((C t -C *)}} / (C 0 -C *)) = - K t ... (4) C t: C value at time t C ^*: equilibrium reached C C _0: t = ₀ in C K _t : Decarburization rate constant

For the decarburization rate reduction phenomenon in this extremely low coal area, mathematically (5), according to the estimated mechanism,
There is a model formula such as formula (6) (Watanabe et al.
CAMP-ISIJ Vol. 1 (1988) -23
4). However, equations (5) and (6) are differential equations,
Furthermore, since it is necessary to solve these problems simultaneously, it is necessary to solve the formula by a mathematically special solution method such as Runge-Kutta. To use this for estimation of decarburization in actual operation, a considerable amount of memory was required, so it was not adopted because it puts a heavy load on the field computer. Wd [C] _IN / dt = Q ([C] _IN - [C] _OUT) ... (5) wd [C] _OUT / dt = -Q ([C] _IN - [C] _OUT) + _ρAK '([C ] _OUT −C ^* ) −ρAK _a ∫ ₀ ^Hm ([C] _OUT −C ^* (h)) dh (6) Hm = 1.48 × (K × C × _OP ′) (7) W : Amount of molten steel in pot, w: Amount of molten steel in vacuum chamber, Q: Circulation flow rate, [C] _IN : C in pot, [C] _OUT : C in vacuum chamber

[0006]

The feature of the present invention is that when the degassing refining process is carried out by the vacuum degassing refining method, the carbon content of the molten steel is estimated from the components and the amount of the exhaust gas from the start of the processing. At any time when the estimated value of the carbon content of the molten steel is 100 ppm to 30 ppm, the subsequent carbon transition is estimated by the following equations (1) to (3) and refining is performed, and the carbon content prediction means is characterized. And it is a refining method to make an operation plan. dC / dt = -K _C1 ″ × (C−C ^{* ′} ) − K _C2 ″ × (C−C ^* ) (1) However, C ^{* ′} = (P + P ′) / (K × O) (2) C ^* = P / (K × O) (3) (However, when C <C ^{* ′} , K _C1 ″ = 0) C, O: Carbon in pot, oxygen amount P: Pressure in vacuum chamber K; Carbon, Equilibrium constants of oxygen K _C1 ″, K _C2 ″: Decarburization rate constants obtained by fitting
P ': Bubble generation pressure t: Time

Alternatively, when the degassing refining process is carried out by the vacuum degassing refining method, the molten steel during the process is sampled and the carbon amount after the sampling is estimated from the components and the amount of the exhaust gas. It is characterized in that, at any time when the estimated value of the carbon content of the molten steel changes from 100 ppm to 30 ppm, the subsequent carbon transition is estimated by the following equations (1) to (3) and refining is performed. For predicting carbon content and refining method for making operation plan. dC / dt = -K _C1 ″ × (C−C ^* ′) − K _C2 ″ × (C−C ^* ) (1) However, C ^{* ′} = (P + P ′) / (K × O) (2) C ^* = P / (K × O) (3) (However, when C <C ^{* ′} , K _C1 ″ = 0) C, O: Carbon in pot, oxygen amount P: Pressure in vacuum chamber K; Carbon, Equilibrium constants of oxygen K _C1 ″, K _C2 ″: Decarburization rate constants obtained by fitting
P ': Bubble generation pressure t: Time

[0008]

[Function] There are many theories about the decarburization reaction, the reaction mechanism, and the reaction site of the vacuum degassing refining of molten steel (for example, Watanabe et al. CAMP-ISIJ Vol. 1 (1988)-
234). The present inventors have decarburized a phenomenon that can be observed as CO gas generated from the inside of molten steel by decarburization of C and O that progresses without interfering with the gas-liquid interface, and the gas-liquid interface (free surface in the vacuum chamber and molten steel). (1) was constructed by considering decarburization that occurs on the inner gas surface).

Here, the first term on the right side of the equation (1) represents the decarburization rate of C and O that proceeds without disturbing the gas-liquid interface, and the second term on the right side thereof occurs at the gas-liquid interface. It represents the decarburization rate. Here, K _C1 ″ and K _C2 ″ are decarburization rate constants obtained by fitting, and are constants that are composed of elements such as reaction rate, ring flow rate, and interfacial area, and are unique to each process and level. It has a value.

The present invention accurately estimates the transition of [C] during processing when the operation is carried out by a pre-assembled processing method, and the target [C] can be obtained at a predetermined end time. ]
If it does not reach the limit, the processing time is extended. Further, if the target [C] is reached earlier than the target [C] within a predetermined end time, it is determined that the decarburization process should be ended at a time that is considered to reach the target [C].

When carrying out these operations, the transition of decarburization can be accurately estimated by the model formula represented by the formula (1). Expression (1) is a differential expression, which can be easily used by mathematically differentiating the program. Here, C ^{* '} was reported by Watanabe et al. (CAMP-I
SIJ Vol. 1 (1988) -234) P '= 20 [Torr] at the bubble generation pressure may be used to calculate by the equation (2). Oxygen cannot be obtained by calculation because oxygen flows from the slag into the molten steel. Therefore, based on the actual results, the oxygen change pattern during processing may be used by entering it in the program.

Further, the prediction in the equation (1) is more accurate than the prediction in the low carbon section. On the other hand, the method of estimating the carbon content of molten steel from the components and the amount of exhaust gas (hereinafter referred to as mass spectrometry) has a high estimation accuracy of the decarburization amount in the high carbon region, but the accuracy is low in the low carbon region, It has the drawback of being difficult to predict. Specifically, as shown in Table 1, from 300 ppm to 1
When the transition of 00 ppm is compared with the mass spectrometric method, the estimation by the formula (1), and the test for 10 ch, the former has higher estimation accuracy. Further, as shown in Table 2, the carbon content of the molten steel is 10
Comparing the transition of the carbon content from 0 ppm to 30 ppm by the mass spectrometry method, the estimation method by the formula (1) and the test for 10 ch, the two have the same accuracy. Furthermore, as shown in Table 3, the transition of the carbon content of molten steel from 30 ppm to 10 ppm is analyzed by the mass spectrometric method, the estimation method by the equation (1) and the 10c
Comparing the tests for h, the latter has higher estimation accuracy.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

[Table 3]

Therefore, the estimation of a relatively high carbon content is estimated by the mass spectrometry method, and the estimation is switched by the equation (1) at an arbitrary point of 100 to 30 ppm when the estimation accuracy by the equation (1) begins to become high. By doing so, it becomes possible to improve the estimation accuracy.

Further, a relatively high carbon content is estimated by estimating the carbon content of the molten steel after the sampled analysis value by a mass spectrometric method, and the estimation accuracy according to the equation (1) begins to become high and the carbon content of 100 to 30 ppm. It is possible to improve the estimation accuracy by switching and estimating with the equation (1) at an arbitrary time point.

[0018]

EXAMPLES Examples are shown in Table 4. Example 1 is a mass spectrometric method and an estimation by the formula (1). Specifically, the mass spectrometric method is used from the start of the degassing process to an arbitrary time when the amount of carbon is 100 to 30 ppm, and the subsequent steps are (1). It is an example predicted by the estimation by the formula.

The second embodiment is an estimation based on the sampling, the mass spectrometric method and the equation (1). Specifically, the carbon amount is 15
This is an example of prediction from the sampling result at the time of 0 to 140 ppm by the mass spectrometric method from the time when the carbon content is 100 to 30 ppm and the subsequent estimation by the estimation by the equation (1).

Comparative Example 1 is sampling and estimation based on the equation (1). Specifically, the carbon amount is 150 to 140 ppm.
This is an example of predicting the subsequent steps from the sampling result at the time point of by the estimation by the equation (1).

Comparative Example 2 is an estimation by sampling and a mass spectrometric method. Specifically, the carbon amount is 150 to 140.
This is an example in which the subsequent sampling is predicted by the mass spectrometry method from the sampling results at the ppm point.

K _C1 ″ and K _C2 ″ in the formula (1) used in these examples and comparative examples are 0.6 and 0.06 [l / min, respectively].
], And P'has a value of 20 [Torr]. As for the accuracy, the estimation accuracy at the time when the carbon content of molten steel was 20 and 8 ppm was examined by 15 ch and compared, and the estimated average value m and variance value σ were obtained.

In all the examples, the estimated average value m is approximately the same, but there is a difference in the variance value σ, and the variance value σ is smaller and the estimation accuracy is higher in the examples than in the comparative examples 1 and 2. I understand. The comparison of the above-mentioned accuracy is remarkable in the comparison of the extremely low carbon steel 8ppm.

From the above, it was found that the present invention is effective particularly for producing molten steel of [C] = 8 ppm.

[0025]

[Table 4]

[0026]

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to accurately estimate the carbon content of molten steel during vacuum refining processing, shorten the processing time, reduce refining costs, and improve the accuracy of the components of the product.

Claims (2)

[Claims] 1. When performing degassing refining treatment by a vacuum degassing refining method, the carbon content of molten steel is estimated from the components and the amount of exhaust gas from the start of the treatment, and the estimated carbon content of molten steel is 100 ppm. From 30 ppm to 30 ppm, the carbon amount prediction means for estimating the carbon transition after that by the following equations (1) to (3) and refining, and the vacuum for making an operation plan by it. Degassing refining method. dC / dt = -K _C1 ″ × (C−C ^{* ′} ) − K _C2 ″ × (C−C ^* ) (1) However, C ^{* ′} = (P + P ′) / (K × O) (2) C ^* = P / (K × O) (3) (However, when C <C ^{* ′} , K _C1 ″ = 0) C, O: Carbon in pot, oxygen amount P: Pressure in vacuum chamber K; Carbon, Equilibrium constants of oxygen K _C1 ″, K _C2 ″: Decarburization rate constants obtained by fitting
P ': Bubble generation pressure t: Time 2. When the degassing refining process is performed by the vacuum degassing refining method, the molten steel during the process is sampled, and the carbon amount after the sampling is estimated from the components and the amount of the exhaust gas. It is characterized in that, at any time when the estimated value of the carbon content of the molten steel changes from 100 ppm to 30 ppm, the subsequent carbon transition is estimated by the following equations (1) to (3) and refining is performed. For predicting carbon content and method for vacuum degassing and refining for making an operation plan therefor. dC / dt = -K _C1 ″ × (C−C ^{* ′} ) − K _C2 ″ × (C−C ^* ) (1) However, C ^{* ′} = (P + P ′) / (K × O) (2) C ^* = P / (K × O) (3) (However, when C <C ^{* ′} , K _C1 ″ = 0) C, O: Carbon in pot, oxygen amount P: Pressure in vacuum chamber K; Carbon, Equilibrium constants of oxygen K _C1 ″, K _C2 ″: Decarburization rate constants obtained by fitting
P ': Bubble generation pressure t: Time