JPH11172323A - Method for controlling carbon concentration in rh vacuum degassing treatment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently control the carbon concn. at the end point to a target value, in the case of melting an extra-low carbon steel having <= about 30 ppm carbon concn. by using an RH vacuum degassing apparatus. SOLUTION: The starting time tO for treating the extra-low carbon rang is decided by estimating the carbon concn. [C]cal.br (r is a subscript showing times of measuring of the carbon concn., r=1, 2,..., s) with time based on a prescribed decarburize-model equation in the low carbon range. Result of rapid carbon analysis of molten steel in a ladle is quickly obtd. as [C]on site and a carbon estimating concn. equation is decided as the carbon concn. [C]O at the initial stage of the prescribed decarburize-model equation in the extra-low carbon range by using this value [C]on site and the carbon concn. at the end point is hit to the target value by adjusting the operational condition and/or the treating time tf .

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a process for vacuum decarburization of molten steel by an RH vacuum degassing apparatus.
Estimate the carbon concentration of molten steel during decarburization based on the decarburization model using the equipment conditions and operating conditions of vacuum degassing and the results of component composition analysis of molten steel and exhaust gas component analysis during decarburization. At the same time, the carbon concentration of the molten steel is quickly analyzed, and the carbon concentration of the molten steel at the end point of the vacuum decarburization process (end-point carbon concentration) is determined based on the carbon concentration analysis value based on the decarburization model in the extremely low carbon region. It relates to a method of controlling to a target value.

[0002]

2. Description of the Related Art As shown in the demand for ultra-low carbonization required for continuous annealing of thin steel sheets, steel having an extremely low carbon concentration is required by decarburizing molten steel.
In addition, the production of thin steel sheets requiring decarburization of molten steel is increasing. Thus, the vacuum degassing rate of the molten steel has been significantly increased. In order to cope with such a situation, it is important to shorten the vacuum degassing time of molten steel, accurately estimate and control the carbon concentration during the decarburization process, and not to perform excessive decarburization process. It has become a challenge.

[0003] However, it is difficult to shorten the vacuum degassing treatment time and to hit the carbon concentration at the end of the decarburization treatment of the molten steel to the target value. Therefore, conventionally, a method of estimating the carbon concentration in the molten steel at the end point of the vacuum decarburization process using information such as the component composition of the exhaust gas generated from the vacuum degassing tank and the flow rate of the exhaust gas during the decarburization process of the molten steel has been taken. I have.

For example, JP-A-59-185720 discloses a method for dynamically estimating the carbon content in molten steel based on information on exhaust gas in an exhaust duct during operation of a vacuum refining furnace. A method is disclosed in which the integrated carbon amount transferred into exhaust gas is subtracted from the carbon amount in molten steel to calculate the carbon amount in molten steel at a certain time and obtain the concentration thereof (hereinafter referred to as prior art).

[0005]

The prior art method dynamically controls the carbon concentration in molten steel during the vacuum refining process, and can control the carbon concentration at the end of vacuum refining with relatively high accuracy. . However, since the amount of carbon in the molten steel is obtained by integrating the amount of carbon in the exhaust gas, the accumulation of measurement errors such as the composition of the molten steel and the analysis of exhaust gas cannot be avoided. There is a problem to get inside.

[0006] When ultra-low carbon steel having a carbon concentration of about 30 ppm or less is melted by vacuum decarburization using an RH vacuum degassing apparatus, the carbon concentration is dynamically controlled to set the end-point carbon concentration to a target value. In the case where the carbon concentration is to be approximated, an accurate analysis value of the carbon concentration of the molten steel during the decarburization process after the carbon concentration has decreased to a region close to extremely low carbon is required. In the prior art, after the above-mentioned halfway analysis result is found, there is no time to adjust the operating conditions for hitting the target carbon concentration at the end point, and there is no means for appropriately determining the timing of collecting the molten steel sample for halfway analysis. Was. In addition, during the decarburization process, a molten steel sample is taken directly from the ladle to quickly analyze the carbon concentration. Based on this value, the carbon concentration by vacuum decarburization in the extremely low carbon region is estimated, and the target carbon There is no method to approach the concentration. As described above, in the prior art, in the vacuum decarburization process of molten steel by the RH vacuum degassing apparatus, when the extremely low carbon concentration is set as the end point target value, the carbon concentration is dynamically controlled and accurately set to the target value. There is no disclosure of the technology to hit.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to solve the above-mentioned problems, to accurately estimate the carbon concentration in the vicinity of extremely low carbon during decarburization, and to reduce the carbon concentration at the end of decarburization to the target carbon concentration. R that can efficiently approach the concentration
An object of the present invention is to provide a method for controlling the carbon concentration of molten steel by an H vacuum degassing apparatus.

[0008]

Means for Solving the Problems The present inventors have made intensive studies from the above viewpoints. In order to increase the hit ratio of the target component of carbon in the extremely low carbon molten steel having a carbon concentration of about 30 ppm or less by the RH vacuum degassing apparatus, the following three points were focused on.

Immediately before the carbon concentration of the molten steel drops to the extremely low carbon region, for example, when the carbon concentration falls to within the range of about 40 to 60 ppm, a means for quickly and accurately estimating the carbon concentration of the molten steel is taken. thing.

When the carbon concentration of the molten steel falls to the above level, at least a carbon analysis sample of the molten steel is collected during the decarburization treatment, and measures are taken to quickly obtain the carbon analysis result.

[0011] By using the results of the rapid determination of the carbon concentration of the molten steel sample described above and controlling the carbon concentration of the molten steel from the time of sample collection by operating conditions for the decarburization treatment, the end point of the decarbonization treatment is completed. Take measures that can approach the target carbon concentration.

To this end, various measurement data obtained by a plurality of past decarburization heats in the RH vacuum degassing facility are effectively used in order to take the following measures.
In order to quickly obtain accurate exhaust gas information using a mass spectrometer, etc., in order to develop a technique for rapidly collecting on-site samples of molten steel, and a technique for rapidly analyzing carbon, In order to take measures, it was found that it was effective to use the results of the rapid analysis described above for an appropriate decarburization model near extremely low carbon.

The present invention has been made based on the above findings, and a method for controlling carbon concentration according to claim 1 has the following features. In the method of controlling the carbon concentration of molten steel while vacuum decarburizing the molten steel in the ladle by the RH vacuum degassing device, the estimated carbon concentration [C] _{cal, f} at the end point of the vacuum decarburization process is reduced by the vacuum decarburization operation. Condition adjustment and / or
Alternatively, the following steps are performed in order to approach the target carbon concentration [C] _a at the end point of the vacuum decarburization processing by correcting the vacuum decarburization processing time. Here, the adjustment of the vacuum decarburization processing operation conditions is mainly performed by adjusting the flow rate of argon gas for circulating molten steel. However, when there is room in the operation of the degassing apparatus or when the value of the target carbon concentration [C] _a is extremely large. For example, when the temperature is low, the operating conditions may be kept as it is, and the processing time may be extended.

First, the elapsed time required for the vacuum decarburization treatment is provided separately for the first half of the treatment, the middle of the treatment, and the second half of the treatment. The reason why the treatment time is divided in this way is that the rate-determining conditions for decarburization differ depending on the level of carbon concentration, and the carbon concentration is 300 to
In the region of 50 ppm, the mixing rate of decarburized molten steel and undecarburized molten steel is rate-determining. However, in the area where the carbon concentration is very low carbon and below 50 ppm, the reaction rate changes, so a decarburization model suitable for this is adopted. That's why.

Step (a): An intermediate target carbon concentration [C] _{int, a} of the molten steel being processed at an intermediate point between the start and the end of the vacuum decarburization treatment is set in advance, and the vacuum decarburization is performed. Formula E for estimating carbon concentration in molten steel is created based on one decarburization model of molten steel by the treatment. In the first half of the processing, one or more times (n
Times), the carbon concentration of molten steel [C] _{cal, br} (where r is a suffix representing the number of times, r = 1, 2,..., S, the same applies hereinafter) and the s-th estimated Carbon concentration of molten steel [C]
_{When cal and bs} drop to, for example, about 45 to 60 ppm, the end point of the first half of the process is immediately determined. The reason for estimating the carbon concentration a plurality of times in this way is to more accurately determine the transition point from the first half of the treatment to the second half of the treatment based on the estimated carbon concentration. This transition time depends on the target carbon concentration at the end point, but when the target value is 30 ppm or less, it is preferably about 45 to 55 ppm.

Step (b): Immediately after the end of the first half of the treatment determined in (a), a treatment intermediate time is determined, and at the treatment intermediate time thus determined, a sample for carbon analysis of molten steel is taken from the ladle. Then, the carbon concentration [C] _{on site} is measured promptly, preferably within 2 minutes.

Step (c): The time when the sample for carbon analysis of the molten steel is collected in (b) is defined as the start time of the latter half of the treatment. The reason why the molten steel analysis sample is taken directly from the ladle during the decarburization process is to obtain high-precision information by analyzing the carbon concentration of the molten steel in the ladle, and as quickly as possible, preferably within 2 minutes. In order to complete the analysis of the carbon concentration [C] _{on site} , when the target carbon concentration of the end point is 30 ppm or less, the estimated carbon concentration [C] _{cal, f} is increased from about 45 to 55 ppm to the target carbon concentration [C] _a This is in order to secure the minimum required time left for taking an action for adjustment so as to approach. Then, the period from the time when the analysis sample is collected to the end of the vacuum decarburization process is defined as the latter stage of the decarburization process.

Step (d): Based on another decarburization model of molten steel by vacuum decarburization, formula B for estimating carbon concentration in molten steel in the latter stage of decarburization is prepared, and the carbon measured in step (b) is measured. Let the concentration [C] _{on site be} the initial carbon concentration [C] _{0 in the} carbon concentration estimation formula B. That is, the decarburization start time t
[C] ₀ is adopted as the carbon concentration at = 0. By the carbon concentration estimation formula B thus obtained, the estimated carbon concentration [C] _{cal, t} during the decarburization treatment is represented by a function H (t) of the decarburization time t in the latter stage of the treatment. The vacuum decarburization model adopted here should be limited to a model created on the premise of the reaction rate control of C + O → CO. The following equation (1) thus obtained: [C] _{cal, t} = H (t) -------------------------- (1) And the decarburization time at the end of the vacuum decarburization treatment, t =
The estimated end point carbon concentration [C] _{cal, f} at t _f is _{calculated by the following} equation (2): [C] _{cal, f} = H (t _f ) --------- Find in (2).

According to a second aspect of the present invention, there is provided a method for controlling a carbon concentration according to the first aspect of the present invention, which has the following features. That is, in the above step (a), the carbon concentration estimation formula A
Is calculated _{, the} estimated carbon concentration [C] _{cal, br} is calculated, and the s-th estimated carbon concentration [C] _{cal, bs} is calculated by the following steps (a) to (e). Ask. Then, in the above step (d), as a method of obtaining the estimated carbon concentration [C] _{cal, t} in the molten steel in the latter half of the decarburization treatment, the carbon concentration [C] _{on site} is _calculated by the carbon concentration estimation formula B as described above. By setting the initial carbon concentration [C] to ₀ and further substituting the specifications of the degassing device to be used and the vacuum decarburization processing operating conditions into the carbon concentration estimation formula B, a function H (t) of the processing time t is obtained. Ask.

However, the above steps (a) to (e) are as follows. Step (a): The equation for estimating the carbon concentration of the molten steel during the vacuum decarburization treatment is determined using the vacuum decarburization model and the mass balance equation of carbon in the entire vacuum decarburization treatment system, and is determined as shown in the following equation (3). Expression (5), which is a function expression including a function F _i not including a coefficient and a function G _{j including} only an undetermined coefficient shown in the following expression (4) and including the carbon concentration of the molten steel to be estimated as a factor: Configure in the form shown.

Step (b): In the equation (3), a plurality of heats of the vacuum decarburization process which have been performed in the past with the same equipment as the vacuum degassing equipment for processing the heat for estimating the carbon concentration of the molten steel. By substituting the operating conditions, the composition of the molten steel, the composition of the exhaust gas from the degassing tank, and the measured value of the exhaust gas flow rate, the value F _i ′ of the function F _i not including the undetermined coefficient (where i = 2, 3 ,..., M) are calculated.

Step (c): The value F _i ′ of the function F _i obtained in the above step (b) (where i = 2, 3,..., M)
Is substituted into the function formula (5), and the value G _j ′ of the function G _j consisting of only undetermined coefficients (where j = 1, 2, 3,...,
Determine n).

Step (d): The function value F _i ′ (i = 2, 3,..., M) obtained in the above step (b) and the value G _j determined in the above step (c) '(However, j = 1,
, N) into the function formula (5), the carbon concentration of the molten steel of the heat to be estimated is set as a dependent variable, and the vacuum decarburization operation conditions and the carbon of the molten steel are determined. A function formula (5 ′) is determined in which the measured values of the component composition to be removed, the component composition of the exhaust gas from the degassing tank, and the flow rate of the exhaust gas are independent variables.

Step (e): The vacuum decarburization operation conditions of the heat for estimating the carbon concentration of the molten steel and the carbon of the molten steel are determined based on the function equation (5 ′) determined in the above step (d). The carbon composition of the molten steel during the vacuum decarburization treatment of the heat [C] _{cal, br} (where r = 1, using the measured values of the composition of the components removed, the composition of the exhaust gas from the degassing tank, and the flow rate of the exhaust gas)
2,..., S).

Here, F _i = f _i (G _c , [C], [O], P, G _Ar , T, d, S) --- (3) (i = 1, 2,...) .., M) G _j = g _j (β _k ) (4) (j = 1, 2,..., N), (k = 1, 2,..., N) , M ≧ n, G _c : carbon content in exhaust gas (t / min),
[C]: Carbon concentration of molten steel (wt.%), [O]: Oxygen concentration of molten steel (wt.%), P: Vacuum tank pressure (atm), G _Ar : Argon gas flow rate for molten steel reflux (Nm ³ / Min), T: molten steel temperature (K), d: immersion pipe diameter (m), S: lower tank cross-sectional area (m ² ), β _k : undetermined coefficient F ₁ = G ₁ F ₂ + G ₂ F ₃ + · .. + G _j F _i +... --- (5) F ₁ = G ₁ 'F ₂ ' + G ₂ 'F ₃ ' + ... + G _j 'F _i + ...- ------- (5 ') (i = 1, 2,..., M) (j = 1, 2,..., N) In addition, derivation of equation (5') and estimation of carbon Concentration [C]
Derivation of _{cal, br and} the like are performed by a computer by prior input of various data and input by on-site transmission.

[0026]

Next, a method for controlling the carbon concentration of molten steel according to the present invention will be described with reference to the drawings. [1] Equipment flow FIG. 1 shows a process of vacuum decarburization of molten steel by the method of the present invention,
It is a schematic longitudinal cross-sectional view which shows an example of RH vacuum degassing equipment used when the end point carbon concentration hits a target value in an extremely low carbon concentration region. In the figure, 1 is a ladle, 2 'and 2 "are molten steel, 3 is a vacuum tank, 4 is an ascending pipe, 5 is a descending pipe, and the ascending pipe 4 and the descending pipe 5 are molten steel in the ladle 1. The vacuum tank 3 is evacuated by a vacuum exhaust device 7 through an exhaust pipe 6, and the pressure in the tank decreases, and the level of the molten steel 2 ″ in the tank increases according to the pressure in the tank. . on the other hand,
Ar gas 9 for circulating molten steel is applied to molten steel 2 ′ in riser 4 by Ar.
When blown from the gas supply pipe 8, the molten steel 2 in the ladle 1 rises through the riser pipe 4, enters the vacuum chamber 3, and is exposed to a vacuum atmosphere. The molten steel 2 ″ in the tank descends through the downcomer pipe 5 and flows into the ladle 1. Thus, the molten steel 2 in the ladle 1
Circulates in the order of riser 4, vacuum vessel 3, descender 5 and ladle 1. In the vacuum decarburization process of the molten steel 2,
When the molten steel 2 ″ is exposed to a vacuum atmosphere, CO and CO are generated by a reaction between dissolved oxygen atoms and carbon atoms in the molten steel.
₂ is generated and decarburization proceeds.

On the other hand, an exhaust gas analyzer 10 is attached to the outlet of the exhaust pipe 6, and an exhaust gas take-out pipe is provided in communication with the exhaust gas analyzer 10. The exhaust gas analyzer 10 controls the exhaust gas 11 discharged from the vacuum chamber 3 by the exhaust gas analyzer 10. Component composition analysis and measurement of exhaust gas flow rate. Examples of the exhaust gas analyzer 10 include an infrared analyzer and a mass spectrometer. The mass spectrometer is preferable because the pressure range of the atmosphere to be measured is wider and the analysis speed and accuracy are better. The measured value is transmitted from the exhaust gas analyzer 10 to the arithmetic and control unit 12. The arithmetic and control unit 12 includes:
It is for estimating the carbon concentration of the molten steel during the vacuum decarburization process, and for controlling the operating conditions and / or the remaining vacuum decarburization process time based on the estimated value, and includes an arithmetic unit and a control unit. .
The calculation unit calculates the carbon concentration of the molten steel based on a predetermined calculation expression for estimating the carbon concentration and the input data. Based on the calculated carbon concentration, the timing of entry into the extremely low carbon concentration region is determined, and various data are substituted into a predetermined carbon concentration estimation formula in the extremely low carbon concentration region to estimate the carbon concentration. Is adjusted to match the target carbon concentration by controlling the operating conditions and / or the remaining vacuum decarburization processing time to control the end-point carbon concentration.

[2] Method of Controlling Carbon Concentration Control of carbon concentration during vacuum decarburization of molten steel by the RH degassing equipment is performed as follows. In the removal of carbon in molten steel by vacuum decarburization, the rate-determining stage of decarburization differs depending on the carbon concentration range. Therefore, the decarburization model used for estimating the carbon concentration uses an equation suitable for the carbon concentration region. When the carbon concentration is relatively high in the first half of the vacuum decarburization period,
After being subjected to vacuum decarburization treatment in the vacuum chamber 3, the mixing control of the decarburized molten steel and the undecarburized molten steel that has been recirculated to the ladle is the rate of decarburization, whereas in the extremely low carbon concentration region, , C + O = CO
The reaction is rate-limiting. Therefore, first, the elapsed time period of the vacuum decarburization process is divided into the first half of the treatment, the middle of the treatment, and the second half of the treatment.

In the first low carbon concentration region, the mixing law
Based on the predetermined decarburization model at the high speed stage,
Combined with the mass balance equation of carbon in the whole science system,
Further, the above determined using predetermined data as described above
Function formula (5 ') formula: F₁= G ₁'F_Two'+ G_Two'F_Three’+
.. + G_j'F_i'+ ... at a given time t
Concentration of molten steel [C]_{cal, br}Is estimated. In this way
Based on the specified carbon concentration,
Determine the processing intermediate time, which is the time to enter the density range, and then
In the extremely low carbon concentration region,
Based on the decarburization model, use the specified data as described above.
Equation (1) of the carbon concentration estimation equation determined above: [C]
_{cal, t}= H (t) gives the carbon concentration of molten steel as a function of time
And the end-point carbon concentration is controlled based on this.

[3] Actual Decarburization Curve and Decarburization Estimation Curve A method of controlling the end-point carbon concentration will be described below with reference to the drawings. FIG. 2 is a schematic diagram illustrating a method for controlling the concentration of carbon in molten steel during vacuum decarburization processing to an end point target value according to the method of the present invention. In the figure, the solid line represents the actual decarburization curve of the molten steel, and the dashed line represents the decarburization estimation curve based on the decarburization model. For example, consider a case where the end point target carbon concentration is 30 ppm or less. In this case, the processing intermediate time point is determined based on the time point when the estimated carbon concentration falls within the range of 45 to 55 ppm. Therefore, the estimated carbon concentration [C] _{cal, br} (where r is a suffix representing the number of times the carbon concentration is measured and r = 1, 2,..., S) is calculated by the function formula (5 ′) at a predetermined time interval. S-th estimated carbon concentration [C] _{cal, bs}
There Once in the above-mentioned range, the point in time t = after the -t _s soon as the molten steel sample of the ladle were taken (at the time were taken t =
t ₀ ), the carbon concentration is analyzed quickly. This “on-site rapid analysis” is performed with the most reliable analysis method in order to quickly and accurately grasp the actual carbon concentration at the present time. As described above, the initial carbon concentration is accurately obtained by obtaining the carbon concentration in two stages, and the decarburization estimation curve in the extremely low carbon region is approximated to the actual decarburization curve. The time t = t _{0 at} which the molten steel is sampled from the ladle is defined as the start time of the vacuum decarburization treatment in the latter half of the treatment, and the carbon concentration estimation formula (1) in the extremely low carbon concentration region
Let t = 0 in the equation. Then, the carbon concentration analysis value [C] _{on site} of the molten steel is determined as the initial carbon concentration [C] _{0 of the} equation (1). That is, in FIG. 2, a decarburization estimation curve in an extremely low carbon region is obtained by the function formula (1): [C] _{cal, t} = H (t). Incidentally, in FIG. 2, 45 ≦ [C] _{cal, bs} ≦ 55 ppm, a -t _s ≒ 0. Further, after the carbon concentration analysis of molten steel sampled (within t _d) rapidly desirably performed within 2 minutes (t _d <
2 min).

Therefore, there are two methods for matching the end point carbon concentration [C] _{cal, f} to the end point target carbon concentration [C] _a . One method is to continue the vacuum decarburization process under the same conditions and obtain a time t = t _f when the estimated end-point carbon concentration [C] _{cal, f} matches the end-point target carbon concentration [C] _a , This is the method for continuing the processing until then. t _f is the equation:
[C] _a = H (t) may be solved for t.

Another method is to determine the end point time of the vacuum decarburization treatment and adjust the estimated end point carbon concentration [C] _{cal, f to} become the end point target carbon concentration [C] _a at that time.
In this case, the argon gas flow rate G _{Ar for} circulating molten steel or the pressure P in the vacuum chamber is adjusted. This adjustment involves the equation:
[C] _a = H (t _f ) is changed to the argon gas flow rate G for circulating molten steel.
_What is necessary is just to solve for _Ar or the pressure P in the vacuum chamber.

When the process is completed by controlling the end-point carbon concentration in this way, the actual carbon concentration of the molten steel is, as can be seen from FIG. 2, the point [C] _a 'on the actual decarburization curve represented by the solid line. And the difference from the target value [C] _a is Δ [C].

[4] Decarburization Estimation Curve [4] -1 Decarburization Estimation Formula in Low Carbon Region A method for estimating decarburization in the low carbon region will be described using a desirable decarburization model. The decarburization reaction in the RH-type vacuum decarburization process proceeds only in the vacuum chamber, and it is assumed that the downflow from the immersion pipe is instantaneous uniform mixing. If it is considered that after being sucked up and reacting in the vacuum chamber and returning to the ladle again with the carbon concentration [C] ', the mass balance of the entire system relating to carbon is:
It is expressed by the following equations (6) and (7).

D [C] / dt = Q ([C] ′ − [C]) / W (6) Q = 114G _Ar ^1/3 d ^{4 / 3} {ln (1 / P)} ^1/3 ------------ (7) where [C] ': carbon concentration in molten steel after passing through the vacuum chamber (wt.%) [ C]: Carbon concentration of molten steel Q: Molten steel ring flow rate (t / min) W: Total amount of molten steel (t) G _Ar : Argon gas flow rate for molten steel recirculation (Nm ³ / mi)
n) d: diameter of immersion tube (m) P: pressure in vacuum chamber (atm) The decarburization rate is expressed by the following equation (8).

D [C] / dt = β ₁ G _C / W --------------- (8) where G _C : Carbon content in exhaust gas (t / min) β ₁ : unknown coefficient From the equations (6) to (8), the following equation (9) is obtained.

Β ₁ G _C = Q ([C] ′ − [C]) --------------- (9) The decarburization reaction from the molten steel in the vacuum chamber was represented by the following equation (10). [C] '= P / K [O] + {K [O] / β ₂ + K [O] / (K [C] [O] -P)} ^-1 ----------- --- (10) Here, K = exp (2671 / T + 4.612) -------------- (11) where, K: equilibrium constant of CO reaction [O] : Oxygen concentration of molten steel T: molten steel temperature (K) β ₂ : undetermined coefficient From the equations (9) and (10), the following equation (11) is obtained.

Β ₁ G _C = −Q × [([C] −P / K [O]) ² / {([C] −P / K [O]) + β ₂ / K [O])}] − ------------- (12) Further, the carbon content in the exhaust gas is represented by the following (13).

G _C = 12G _Ar ([CO] + [CO ₂ ]) / 22400 [Ar] -------------- (13) However, [CO], [CO ₂ ] , [Ar]: of each gas in the exhaust gas
vol.% Then, substituting equation (13) into equation (12) gives (1
4) Equation is obtained.

12β ₁ G _Ar ([CO] + [CO ₂ ]) / 22400 [Ar] = − Q × [([C] −P / K [O]) ² / ｛([C] −P / K [O]) + β ₂ / K [O])｝] -------------- (14) In the present invention, equation (14) is transformed into the following equation (15). did.

F ₁ = G ₁ F ₂ + G ₂ F ₃ (15) where: F ₁ , F ₂ , and F ₃ are functions that do not include the undetermined coefficients described below. F ₁ = −Q × ([C] −P / K [O]) ² F ₂ = 12 G _Ar ([CO] + [CO ₂ ]) ([C] −P / K [O]) / 22400 [Ar F ₃ = 12 G _Ar ([CO] + [CO ₂ ]) / 22400 [Ar] K [O] G ₁ = β ₁ G ₂ = β ₁ β ₂ Thus, the decarburization estimation formula (15) is obtained. Was done.

After calculating the values of the functions F ₁ , F ₂ and F ₃ in the decarburization estimation formula of the above formula (15) using the measured values in the past three heats or more, the statistical calculation is performed from the formula (15). , G ₁ and G ₂ are calculated, and G ₁ and G thus obtained are calculated.
_2, and the dissolved oxygen content of the molten steel of the heat for which the carbon concentration is to be estimated, the pressure in the vacuum chamber, the Ar gas flow rate for the molten steel reflux,
The total amount of molten steel, molten steel temperature and gas composition in exhaust gas
By substituting into the equation (5), the carbon concentration [C] of the molten steel can be estimated.

In the above description, the above equation (10) was used as a decarburization reaction model, but the carbon concentration may be estimated using another appropriate decarburization reaction model. [4] -2 Formula for Estimating Decarburization in Extremely Low Carbon Region A method for estimating decarburization in the extremely low carbon region will be described using a decarburization model on the assumption that the reaction rate is limited to C + O → CO. As a decarburization model, the following general formula (16) was used, and (1)
7) to (19) are obtained.

[C] = [C] ₀ exp (−K _C t) -------------------- (16) Here, K _C = (Q / W) {ak / (Q / ρ + ak)} ---------- (17) Q = 114 G ^1/3 d ^4/3 {ln (760 / P)} ^1/3 S --- --- (18) ak = exp (-15.3) G ^2/3 [C] ^1.76 [O] ^0.75 × {ln (760 / P)} ^2/3 S ---------- (19 However, [C]: carbon concentration in molten steel [C] ₀ : carbon concentration before treatment K _C : apparent decarburization rate constant t: decarburization time Q: molten steel ring flow rate ak: capacity coefficient W: molten steel amount ρ: molten steel Density G: Flow rate of gas for recirculating molten steel d: Diameter of immersion pipe [O]: Oxygen concentration of molten steel P: Pressure in vacuum chamber S: Reaction area in vacuum chamber Based on the above formulas (16) to (19), past vacuum degassing The measured data in the coal heat, the oxygen concentration [O] of the molten steel obtained in the heat, and the trueness of the heat Using intracisternal pressure P and other operating conditions, determine the molten steel ring flow Q and the capacity coefficient ak, followed by obtaining a K _C,
The equation for estimating the carbon concentration [C] in the molten steel is obtained from equation (16).

[0045]

Next, the present invention will be further described with reference to examples. The end-point carbon concentration of vacuum decarburization was controlled based on the method described in the above embodiment. The low-carbon molten steel discharged from the converter to the ladle is subjected to a molten steel processing capacity of 300 t / ladle RH.
Using a vacuum degassing apparatus, extremely low carbon steel having a carbon concentration of molten steel of 30 ppm or less was produced. Carbon concentration before decarburization treatment is 200
８００800 ppm, oxygen concentration 300-600 ppm, and molten steel temperature 1620-1640 ° C. The main conditions such as vacuum decarburization treatment are as follows.

Pressure in vacuum chamber P: 1 (Torr) Argon gas flow rate for circulating molten steel G _Ar : 3 to 4 (Nm ³ /
min) Immersion tube diameter d: 0.65 (m) Lower cross-sectional area inside the vacuum chamber S: 4.64 (m ² ) In order to obtain information on the exhaust gas during the vacuum decarburization process, a mass spectrometer was used as the exhaust gas analyzer. The exhaust gas was taken out from the canopy of the vacuum chamber and analyzed. The analysis accuracy of the used mass spectrometer is 0.1%, the analyzable vacuum degree is 1 to 760 Torr,
The time required for analyzing the exhaust gas is 20 seconds.

Vacuum decarburization treatment (Example) was performed by a method within the scope of the present invention including the above conditions. On the other hand, as the vacuum decarburization treatment of the comparative example, the estimated carbon concentration in the low carbon region was extremely measured without measuring the rapid carbon analysis value [C] _{on site} of the molten steel in the ladle at the intermediate point of the treatment in the present invention. A decarburization estimation curve in an extremely low carbon region was obtained by adopting it as an initial carbon concentration in a low carbon region, and based on this, the case where the carbon concentration was controlled to an end-point target carbon concentration was performed. Regarding the control method of the end point carbon concentration, a method of setting the decarburization treatment time to a constant value of 15 minutes and setting the end point carbon concentration to 30 ppm or less was tested.

FIGS. 3 and 4 show examples (n = 30), respectively.
5 shows a group of estimated decarburization curves in Comparative Example (n = 30 charges) and Comparative Example (n = 30 charges). However, in each of the examples and comparative examples, [C] is a decarburization estimation curve only for the charge whose actual analysis value was 18 ppm at a decarburization time of 15 minutes. Shows only the graphs of the slowest and the fastest, and the graphs of intermediate decarburization estimated speeds fall in between them, but the notation is omitted.

As apparent from FIGS. 3 and 4, when the end point carbon concentration is controlled while the vacuum decarburization treatment time is fixed at 15 minutes, the average value of the end point carbon concentration is 2 in the comparative example.
At 0.5 ppm, the estimation accuracy was ± 5 ppm. In contrast, in the example, the average value of the end point carbon concentration is 18.
At 5 ppm, the estimation accuracy was ± 3 ppm. As described above, according to the present invention, the estimated concentration of the end point carbon became closer to the actual analysis value, and the variation thereof was reduced and improved.
In the above decarburization test, the end point carbon concentration was estimated by setting the decarburization time to a constant value of 15 minutes. On the other hand, the target value of the end-point carbon concentration is determined to be a certain value, the end-point carbon concentration is estimated, and the minimum and maximum values of the decarburization time required for the estimated value to reach the target value, and the difference therebetween Is estimated using the test results of the above Examples and Comparative Examples.

The target value of the end point carbon concentration is set to 25.5 ppm, which is the maximum value of the estimated carbon concentration at a decarburization time of 15 minutes in FIG. 4 of the comparative example. In FIG. 4 of the comparative example,
From the intersection of the two decarburization curve in the estimation carbon concentration 25.5 ppm, decarburization minimum time estimate of the carbon concentration reaches 25.5 ppm is 9 minutes 48 seconds at the point X _1,
Also decarburization longest time is 15 minutes 00 seconds at the point X _2. And the difference is 5 minutes and 12 seconds. On the contrary,
Even when it estimated similarly in FIG. 3 embodiment, the intersection of the two decarburization curve in the estimation carbon concentration 25.5 ppm, at point X ₃ decarburization shortest time to estimate the carbon concentration reaches 25.5 ppm a 9 minutes 05 seconds, is also decarburization maximum time of 10 minutes 27 seconds at the point X _4. The difference is 1 minute and 22 seconds. Comparing the estimated values in the example and the comparative example, it is found that, even in the case of the decarburization treatment with the target carbon concentration set, the decarburization time required according to the present invention is more stably reduced than in the comparative example. I understand.

[0051]

As described above, according to the present invention,
Since the end-point carbon concentration of the molten steel can be accurately controlled to the target value by the RH vacuum decarburization treatment, the carbon concentration is not excessively lowered and the degassing time is not extended. In this way, it is possible to provide a method for controlling the carbon concentration of molten steel by RH vacuum degassing, which can efficiently produce extremely low carbon molten steel, and has an industrially useful effect.

[Brief description of the drawings]

FIG. 1 is a flow chart of an example of RH vacuum degassing equipment used in the method of the present invention.

FIG. 2 is an explanatory diagram of a method for controlling the carbon concentration of molten steel during vacuum decarburization processing to a target end point value according to the method of the present invention. .

FIG. 3 is a graph showing a decarburization estimation curve group according to an example.

FIG. 4 is a graph showing a group of estimated decarburization curves according to a comparative example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ladle 2, 2 ', 2 "molten steel 3 Vacuum tank 4 Rise pipe 5 Down pipe 6 Exhaust pipe 7 Vacuum exhaust system 8 Ar gas supply pipe 9 Ar gas 10 Exhaust gas analyzer 11 Exhaust gas 12 Operation / control device

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Hirohisa Nakajima 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd.

Claims (2)

[Claims] 1. A method for controlling the carbon concentration of molten steel in a ladle while performing vacuum decarburization treatment on the molten steel in a ladle using an RH vacuum degassing apparatus, wherein the following (2) obtained by the following steps (a) to (d): )
The estimated end-point carbon concentration at the end of the vacuum decarburization process, represented by [C] _{cal, f} in the equation, is _calculated by adjusting the vacuum decarburization process operating conditions and / or correcting the vacuum decarburization process time. End point target carbon concentration at the end of processing [C]
_a method for controlling the carbon concentration of molten steel in RH vacuum degassing, wherein the method is adjusted to approach _a . Step (a): An intermediate target carbon concentration [C] _{int, a} of the molten steel being processed at an intermediate point between the start and the end of the vacuum decarburization treatment is set in advance, and the molten steel by the vacuum decarburization treatment is set. Formula A for estimating the carbon concentration in molten steel is created based on one of the decarburization models described above, and the estimated carbon concentration [C] _{cal, br} (where , R is a subscript representing the number of times of measuring the carbon concentration, and r = 1, 2,..., S) is calculated, and the estimated carbon concentration [C] _{cal, bs} and the intermediate target carbon concentration [ C] The end point of the first half of the decarburization treatment is determined based on _{int, a} . Step (b): Immediately after the end of the first half of the decarburization treatment determined in the above step (a), the treatment intermediate time is determined, and a carbon analysis sample of molten steel is collected from the ladle at the treatment intermediate time. Immediately after the sampling, the carbon concentration [C]
Measure _{on site} . Step (c): The time at which the sample for carbon analysis of the molten steel was collected in the above step (b) is defined as the start time of the vacuum decarburization treatment in the latter half of the decarburization treatment, and decarburization is performed from this time to the end of the vacuum decarburization treatment. Determined as the late stage of processing. Step (d): Based on another decarburization model of the molten steel by the vacuum decarburization treatment, formula B for estimating the carbon concentration in the molten steel in the latter stage of the decarburization treatment is prepared, and the carbon concentration [C The _on-site is assumed to be the initial carbon concentration [C] ₀ of the carbon concentration estimation formula B, and the estimated carbon concentration [C] _{cal, t} during the decarburization process is calculated by the above carbon concentration estimation formula B to _{obtain the} decarburization time t in the latter stage of the treatment.
The function H (t) is represented by the following equation (1)
Formula: [C] _{cal, t} = H (t) -------------------------- (1) The decarburization time at the end, t =
The estimated end point carbon concentration [C] _{cal, f} at t _f is _{calculated by the following} equation (2): [C] _{cal, f} = H (t _f ) --------- Find in (2). 2. In the step (a), the carbon concentration
An estimation formula A is created, and the estimated carbon concentration [C]_{cal, br}Is calculated
And the s-th estimated carbon concentration [C]_{cal, bs}
Is determined by the following steps (a) to (e).
In the step (d), the molten steel in the late stage of the decarburization treatment
Estimated carbon concentration [C] _{cal, t}The method for determining
Concentration [C]_{on site}Is the initial carbon of the carbon concentration estimation formula B.
Concentration [C]₀And the specifications and specifications of the degassing device
Substitute the vacuum decarburization operating conditions into the carbon concentration estimation formula B
To obtain a function H (t) of the processing time t.
The RH vacuum degassing process according to claim 1, wherein
Method of controlling the carbon concentration of molten steel in steelmaking. However, the above process
(A) to (e) are as follows. Step (a): Estimating the carbon concentration of molten steel during vacuum decarburization
Is applied to the vacuum decarburization model and the entire vacuum decarburization processing system.
Undetermined as shown in the following equation (3)
Function F without coefficient_iAnd the undecided relation shown in the following equation (4)
Function G consisting only of numbers_jAnd try to estimate
Is a functional expression containing the carbon concentration of molten steel as a factor
Step (b): Estimate the carbon concentration of the molten steel in equation (3).
The same equipment as the vacuum degassing equipment that processes heat
In several heats of vacuum decarburization
Operating conditions, composition of molten steel, exhaust gas from degassing tank
Substituting the measured values of the gas composition and exhaust gas flow rate,
Function F without coefficient_iThe value of F_i’(Where i = 2,
3,..., M), and step (c): the function F obtained in the above step (b)_iThe value of the
F_i′ (Where i = 2, 3,..., M) is
Substituting into equation (5), a function G consisting only of undetermined coefficients_jof
Value G_j’(Where j = 1, 2, 3,..., N)
Step (d): the function value F obtained in the above step (b)_i’
(Where i = 2, 3,..., M) and the above step (c)
G determined by_j’(Where j = 1, 2, 3,...
, N) by substituting it into function expression (5),
The dependent variable is the carbon concentration of the molten steel of the heat to be determined.
And vacuum decarburization operating conditions, and components excluding carbon in molten steel
Composition, composition of exhaust gas from degassing tank and exhaust gas flow rate
A function formula (5 ') is determined using the measured value of the above as an independent variable. Step (e): The function formula determined in the above step (d)
Attempts to estimate the carbon concentration of molten steel based on equation (5 ')
Heat, vacuum decarburization operating conditions and carbon in molten steel
Component composition, component composition of exhaust gas from degassing tank and exhaust gas
Using the measured flow rate during the vacuum decarburization of the heat
Carbon concentration of molten steel [C]_{cal, br}(However, r = 1, 2, ...
, S). Where F_i= F_i(G_c, [C], [O], P, G_Ar, T, d, S) --- (3) (i = 1, 2,..., M) G_j= G_j(Β_k) (4) (j = 1, 2,..., N), (k = 1, 2,..., N) where m ≧ n G_c: Carbon content in exhaust gas (t / min) [C]: Carbon concentration of molten steel (wt.%) [O]: Oxygen concentration of molten steel (wt.%) P: Pressure in vacuum chamber (atm) G_Ar: Argon gas flow rate for molten steel reflux (Nm^Three/ Min) T: Temperature of molten steel (K) d: Diameter of submerged pipe (m) S: Cross-sectional area of lower tank (m)^Two) Β_k： Undetermined coefficient F₁= G₁F_Two+ G_TwoF_Three+ ... + G_jF_i+ ・ ・ ・ -------- (5) F₁= G₁'F_Two'+ G_Two'F_Three'+ ... + G_j'F_i'+ ... -------- (5') (i = 1, 2,..., M) (j = 1, 2,..., N)