JPS63128114A - Method for predicting decarburizing end point in vod process - Google Patents

Abstract

PURPOSE:To predict the end point of finishing of decarburization in high Cr molten steel by finding the existing carbon concentration from O2 parameter measured at inlet of exhaust tube of VOD furnace, the existing molten steel temp. parameter and Cr concentration parameter in the molten steel. CONSTITUTION:The high Cr molten steel is charged in the VOD furnace in vacuum chamber 1,and blowing is executed by oxygen lance 6 and bottom blowing nozzle 4, to decarburize the molten steel. Then, a sensible part 7 of oxygen gas sensor having a dust removing filter is arranged at the inlet of exhaust tube 3 in the VOD furnace 2, to obtain the O2 parameter. By using this O2 parameter and the existing molten steel temp. parameter assumed from the exhaust temp. near the sensible part 7 and molten steel temp. before blowing oxygen, and the Cr concentration parameter in the molten steel before blowing oxygen, the existing carbon concentration is obtained from the prescribed equation. In this way, the predication of end point of decarburization is simply executed at high accuracy and inexpensive method.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for predicting the end point of a decarburization reaction in a VOD process that performs final decarburization by blowing acid, and specifically relates to a method for predicting the end point of a decarburization reaction in a VOD process that performs final decarburization by blowing acid. The present invention relates to an end point prediction method that can improve the end point prediction accuracy when performing final decarburization.

[Conventional technology] The VOD furnace was developed as a powerful device for refining stainless steel, and because it has a vacuum function, it is effective in refining ultra-low carbon steel and ultra-low sulfur steel. At the same time, it is applied to a wide range of steel manufacturing fields, including heat-resistant and corrosion-resistant high-alloy steel. One example is the finish decarburization of high Cr molten steel, in which oxygen gas is injected into the steel bath or onto the surface of the molten steel while stirring the steel bath.
is discharged from the system by vacuuming, performing advanced decarburization. The chemical reactions in molten steel caused by blown acid under vacuum are relatively complex, but the following chemical reactions occur: ■C, St, Cr,
It is believed that a reaction as shown in the following equation is proceeding between 5in2°Cr 203 in the slag bath and CO in the atmosphere.

S i +2CO2C+S i 02 (1
)2Cr+3CO:3C+Cr203 (2) The above (1) and (2) are equilibrium reactions, and the initial stage of blowing acid is S.
As a result of the oxidation reaction of T and Cr proceeding preferentially, St and Cr in the molten steel decrease and 5i02 and C are added to the slag.
r,O, increases.

However, since the above reaction is carried out under vacuum,
CO in the system is gradually exhausted, and therefore (1)
, The reaction in equation (2) proceeds from right to left. Needless to say, at this time, the stirring proceeds at a speed commensurate with the stirring state of the molten steel. In the end, the above reaction can be summarized as follows.

(1) Formation of 5i02 and Cr2O by oxidation of SL and Cr; (2) Return of St and Cr to molten steel through the progress of the reduction reaction of SiO2 and Cr2O with C; (3) Simultaneous generation and exhaust of CO. As a series of reactions proceed continuously, St and Cr in the molten steel hardly change in appearance, only C preferentially decreases and decarburization progresses. Because of the temperature rise, the equilibrium points of each reaction in equations (1) and (2) will shift, and St and Cr will decrease slightly by that amount, but this is not a concern here.] In this way, decomposition by blowing acid As the charcoal reaction progresses, the C concentration rapidly decreases due to the rate-limiting oxygen supply at the initial stage of acid blowing.
As the temperature decreases, the decarburization reaction becomes rate-limited by C diffusion. As a result, the decarburization rate decreases and eventually (1).

The equilibrium of equation (2) changes from left to right. That is, the oxidation reaction of SL and Cr, especially the oxidation reaction of Cr, progresses and the concentration in the molten steel decreases. Since such oxidation loss of Cr is fatal to the refining of high-Cr molten steel (deterioration of the basic unit of reducing agent, instability of composition adjustment and temperature adjustment, fluctuation of quality, etc.), in order to prevent overblowing, In this case, determining the end point of decarburization becomes very important.

Therefore, technology to predict the end point of decarburization has been studied in the past, and
．． The decarburization end point is determined by determining each concentration change (composition change) of 02 and considering the relationship with the degree of vacuum.

[Problems to be solved by the invention] In conventional VOD furnace exhaust gas analysis, since the exhaust gas contains a large amount of dust, it is necessary to place part of the analyzer's sensor near the outlet of the vacuum tank. cannot be installed. Therefore, it is necessary to filter the dust at the final stage on the exhaust gas outlet side of the vacuum device before collecting the sample gas, or to sample the exhaust gas near the tank outlet and then remove the dust with a filter. Therefore, in addition to the analyzer, an exhaust gas sampling device under vacuum and a dust removal device are required, and the device is not only large-scale and complicated, but also very expensive and requires a great deal of effort and expense to maintain. Furthermore, with current analysis techniques, it takes 20 to 30 seconds or more for the exhaust gas generated in the ladle to travel from the vacuum tank to the sampling position and to the analyzer, and for the analysis results to be obtained. For this reason, even if a change occurs in the decarburization reaction, it will take 20 to 30 seconds or more before it can be detected, and oxidation loss of Cr during this time cannot be prevented.

The present invention aims to provide a highly accurate end point prediction method that does not require the use of the above-mentioned complicated and expensive equipment, is very simple and inexpensive, has good quick response, and has little Cr oxidation loss. .

[Means for solving the problem] The method for predicting the end point of the VOD process according to the present invention is to
The 0□ parameter obtained by placing the furnace near the exhaust pipe inlet (vacuum tank outlet), the current molten steel temperature parameter estimated from the exhaust gas temperature near the sensitive part and the molten steel temperature before blowing acid, and the molten steel temperature parameter before blowing acid. The gist of this method is to calculate the current carbon concentration and predict the end point of decarburization by solving the relational expression between the above three parameters and carbon concentration using the Cr concentration parameter in molten steel.

[Operations and Examples] FIG. 1 is an explanatory diagram showing the implementation status of the present invention, in which a VOD furnace 2 is arranged in a vacuum chamber 1 to which an exhaust pipe 3 is connected. The arrow drawn on the exhaust pipe 3 indicates the exhaust direction.
The inside of the vacuum chamber 1 is maintained at a vacuum by a vacuum pump (not shown).

5 is a storage tank for metal added for component adjustment, and 6 is a lance for blowing acid. The VODO2O shown in the diagram
A bottom blow nozzle 4 for bottom blowing Ar or the like is provided in the third part. The first feature of the present invention is that the exhaust pipe 3
Since the sensing part 7 of the oxygen gas sensor is placed in the population of VODO2O3, the oxygen concentration in the exhaust gas discharged from VODO2O3 can be detected immediately after discharge, so the decarburization reaction progressing inside the VOD furnace and the oxygen concentration can be detected. The time difference between detections is extremely small, and the delay time that was a problem with conventional technology has been almost completely eliminated. In addition, since the present invention only measures oxygen concentration, a commercially available inexpensive oxygen gas sensor can be used, eliminating the need for a gas sampler or individual gas analyzer, which can prevent high costs. No futa.

There is no need to place any special restrictions on the oxygen gas sensor used in the present invention as long as it has a removal filter, such as a metal oxide semiconductor gas sensor, a solid electrolyte oxygen sensor (such as a zirconia oxygen sensor), a galvanic oxygen sensor, etc. Non-limiting examples include battery oxygen sensors and magnetic gas sensors. It goes without saying that the design of the mounting method to the exhaust pipe can be changed depending on the type of oxygen gas sensor, and the technical scope of the present invention may not be deviated from depending on the type of oxygen gas sensor or the mounting method of the sensing part. Can not.

Next, the case of using a solid electrolyte oxygen sensor will be explained in more detail. In order to minimize the above-mentioned delay time, it would be sufficient to provide the sensing section 7 within the vacuum chamber 1 shown in Fig. 1 or at its outlet, but as a practical matter, There is a problem in that the dust contained in the sensor and the dust entrained in the backflowing atmosphere at the time of vacuum break contaminate the sensitive section 7 and reduce measurement accuracy. To prevent this, a ceramic filter (second
There is also a method of providing 8) in the figure, which can of course be adopted in the present invention, but it is also an inconvenient problem that the filter 8 becomes severely clogged and the service life decreases.

Therefore, Figure 2 (^), (B) [Figure 2 (B) is the second
A dust-proof tube 10 with a slit 9 is attached to the tip of the probe 17 which has a built-in sensitive part 7 in birch shown in the sectional view taken along the line B-B in Figure (A).
In addition, the gas flow direction [Fig. 2 (A), (
B) A structure in which dust reflectors 118° 11b are installed at the top and bottom is recommended, taking into account the vertical direction in the inside.

That is, the gas flowing from the top to the bottom or from the bottom to the top as shown in FIGS. 2A and 2B enters the dustproof cylinder 10 through the slit 9, passes through the ceramic filter 8, and comes into contact with the sensitive part 7.

On the other hand, most of the gas flows back toward the exhaust pipe 3 through the slit 9; , the dust in the exhaust gas is collected primarily by the dustproof tube 10 and secondarily by the dust reflector 1.
1a and llb, the amount hitting the surface of the filter 8 is considerably reduced, and the problem of the filter 8 becoming clogged in a short period of time is solved.

In the southern countries, we have shown a design that takes into consideration the flow direction of exhaust gas and the backflow direction at the time of vacuum break, but it is also possible to take dust countermeasures by devising the exhaust pipe 3 itself for backflow at the time of vacuum break, so in that case, One of the dust reflectors 11a and llb may be omitted. In any case, the dustproof measures for the filter 8 itself are not the gist of the present invention, and any design changes can be made depending on the situation.

If the sensing part of the oxygen sensor as described above is installed at the entrance of the exhaust pipe 1, the decarburization state ( Here, the 02 parameter) can be caught. It is assumed that the 02 parameter when a solid electrolyte oxygen sensor is used is determined from the electromotive force according to the NernSt equation shown below, and the pressure is corrected.

In Nernst's equation (cell output of solid electrolyte), em
f: Electromotive force of oxygen probe (mV) R: Gas constant C: Coulomb equivalent Po: Oxygen partial pressure of reference gas (ATM or TORR) PS: Oxygen partial pressure of sample gas (ATM or TORR) 02 Parameter pressure correction ( Pressure correction formula for sample gas and reference gas) P in the formula. stomach. □1: Total pressure of reference gas PS-Total: Total pressure of sample gas (degree of vacuum) Next, the second parameter, the current molten steel temperature parameter, will be explained. S shown in equations (1) and (2)
The equilibrium equation for the oxidation reaction of t and Ci can be expressed as follows, and it can be said that the molten steel temperature is currently an important factor in predicting the end point of the decarburization reaction.

Equation (1) Equation Equilibrium Equation (2) Equation Equation K, 2: Equilibrium constant a: Activity P: Partial pressure T: Current molten steel temperature Therefore, it is important to know the current molten steel temperature. However, since the temperature of molten steel in the VOD furnace cannot be directly measured, it must be estimated by some means.

The estimation in the present invention is calculated using the exhaust gas temperature near the point where the sensitive section is installed and the molten steel temperature before acid blowing. The fact that the current molten steel temperature can be accurately estimated from these two data means that the Ar
The blowing flow rate, the exhaust volume by the vacuum pump, and the amount of blown acid from the lance 6 change during the VOD process, and if the amount of temperature rise in the exhaust gas can be known, the temperature of the molten steel before blowing acid will change due to the blowing acid. The extent to which it has increased can be estimated using the following equation. However, the following calculation formula does not limit the present invention, and even if another calculation formula is used, it is included within the technical scope of the present invention.

In the formula, ΔT: Temperature rise during blowing acid (1) (Temperature of molten steel in blowing acid - initial temperature) Ti: Initial temperature of molten steel (before blowing acid) ΔTg: Temperature rise of exhaust gas (ΔTg = Tg - T,, Tg: Exhaust gas temperature, T; Exhaust gas temperature at the start of acid blowing) t: Elapsed time of acid blowing Finally, the degree of Cra in the acid blowing molten steel can be determined by actual measurement. By the way, it is known that [C] and [Cr] in molten steel at the end point of decarburization can be organized as follows depending on the 00 partial pressure and molten steel temperature at that point (Hil
ty formula).

In the formula, [C96]: 0% of the decarburization end point [Cr profit: Cr% of the decarburization end point T: Molten steel temperature at the decarburization end point Pco Partial pressure of CO at the C decarburization end point In the gas phase in the ladle in the vacuum tank, CO, O7゜CO2
It is thought that the following equilibrium relationship holds between

CO+ 02 CO2,ΔG (6-1)2
← In the formula, K: Equation (8-1) equilibrium constant ΔG: Reaction free energy ΔG=RTun R: Gas constant T: Gas phase temperature On the other hand, the following equilibrium relationship is considered to hold in the exhaust gas of the oxygen sensor sensitive part. It will be done.

C+CO2″2CO,AG■■ (6-5) In the formula
K1. , □: (6-3), (Equilibrium constant II IIIPll ""PO2"Go' Go2 of equation 6-51.

co co□ Partial pressure of exhaust gas (:0.
II I II stands so P
Co Co Go where P=P holds
2 CO2.

,', From equation (6-6), (6-7), From equation (6-4), I 13A P =K -K ・(P')
(6-8) Co I II O
2 is derived.

■ On the other hand, assuming that Pco is in a functional relationship (6-2)
From formula and formula (6-8) ■ PCo=F(Pco) (6-9)
holds, the following equation can be found as an inverse function of equation (6-9).

P co= F '-' (P co) (8-10) (
The following relational expression can be obtained from equation 6-10) and equation (It-8).

It can be expressed as [Cr%] in formula (6) is approximately equivalent to the Cr content before the start of decarburization, since the purpose of the present invention is to control the oxidation loss of Cr to a minimum. I can think. Therefore, (
If the molten steel temperature can be predicted using equation 6), a value of 0% can be predicted. A prediction formula for C% in molten steel at the final stage of oxygen blowing can be generalized from the above relational formula and can be expressed by formula (6-12).

C! ni=f+ (Crk) 10f2 (Poz)+f3
(T) (6-12) where Pa,
:O, Parameter T: Temperature parameter of molten steel C "%: Cr content in molten steel before the start of decarburization The present invention applies and develops this idea, and calculates the 02 parameter during the VOD process, which is determined as described above, and the temperature of molten steel. We decided to solve the relational expression between these parameters and the degree of CvA in the current molten steel by using the parameters and the Cr concentration parameter before the start of acid blowing.Here, although the above relational expression does not control the present invention, it is a representative As a practical formula, the following formula (7) can be cited.

IJ-Ao”A1 + (Pa2)”+A2・(T)”
+A, -(Cr\) (7) In formula 0%: 0% at the final stage of decarburization A,, A,, A2: Coefficient m, n: Index of PO2, T (constant value) PO2: Measured with an oxygen probe 02 Parameter T: Molten steel temperature predicted from equation (5) If a prediction equation is given for each m pole, A3=0 can be set.

Tuesday A week (i) Prediction test of C% in molten steel at the end of acid blowing High Cr molten steel (SO5316°321.304) melted in an electric furnace was
The OD furnace is loaded, and as shown in FIG. 1, an oxygen probe (sensing part 7 of the oxygen sensor) is installed in the flue (exhaust pipe 3) near the vacuum chamber 1. The temperature of the exhaust gas during acid blowing,
The pressure and cell output of the oxygen probe in the exhaust gas were continuously measured. An example of measurement is shown in Figure 3. Temperature measurements and sampling were conducted before and after acid blowing to confirm chemical composition. From these measured data, calculate the 02 parameters5), (7)
Based on the formula, the coefficients of each prediction formula were determined by statistical analysis, the amount of rise in molten steel temperature and the blown acid end point of 0% were predicted, and the results were compared with actual data.

Note that the calculation of equation (7) was performed as follows.

C02-+Jtop = Ao"A+ 'PO2+
'A2 ・T (8) Ao"0.0004 ^1-2X 10-' A2-8.295851 X 10'Po2:02 potential (parameter, Po2aKO2/100
) T: Molten steel temperature at 02 stop (7-TO2-stop
X 10-') The prediction results of the molten steel temperature rise are shown in Fig. 4, and the prediction results of the acid blowing end point are shown in Fig. 5. From FIG. 4, it was found that the temperature rise of molten steel during acid blowing can be predicted at σ=11°C using the molten steel temperature before blowing acid and the temperature of exhaust gas during blowing acid as parameters. 2σ
When estimated with a prediction error of ±1.3 for 1700℃
% error.

The predicted value of 0% at the blowing acid end point is σ = 0.009%
It was found that even a prediction error of 2σ is within ±0.02% and can be put to practical use. This error, even on the positive side, is within a range that can be adjusted by subsequent vacuum processing.

[Effects of the Invention] The present invention makes it possible to predict the end point of decarburization without delay, improves the basic unit of auxiliary raw materials by preventing Cr overoxidation, improves productivity by shortening the acid blowing time, and reduces the basic unit of refractories. Improvement 1: It became possible to stabilize quality.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the operating status of the VOD process, Figure 2
Figures (^) and (B) are cross-sectional views showing the dust-proof structure of the sensing part of the oxygen sensor, Figure 3 is a graph showing measurement examples in Examples, Figure 4 is a graph showing predicted results of molten steel temperature rise, FIG. 5 is a graph showing the prediction results of C concentration. 2... VOD furnace 3... Exhaust pipe 7... Sensing part of oxygen sensor

Claims (1)

[Claims] (1) When performing final decarburization by blowing high Cr molten steel in the VOD furnace, the sensitive part of the oxygen gas sensor with a dust removal filter is
The O_2 parameter obtained by placing it near the exhaust pipe inlet of the OD furnace, the current molten steel temperature parameter estimated from the exhaust gas temperature near the sensitive part and the molten steel temperature before blowing acid, and the Cr concentration parameter in the molten steel before blowing acid. A method for predicting the end point of decarburization in a VOD process, characterized in that the current carbon concentration is determined by solving the relational expression between the above three parameters and the carbon concentration, and the end point of decarburization is predicted.