JP3221787B2 - Vacuum decarburization end point control method for molten steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a simple control method for controlling the end point of the decarburization treatment of molten steel or a molten steel alloy (hereinafter referred to as molten steel).

[0002]

2. Description of the Related Art A method for controlling decarburization end point by vacuum degassing of molten steel is, for example, a method based on an analysis value of [C] at an arbitrary time and a material balance by exhaust gas analysis (JP-A-51-8).
No. 1722), and a method based on the calculation of the analytical value of [C] and the reaction rate equation (Japanese Patent Application Laid-Open No. 61-19726) are known, all of which are used to predict the decarburization up to the extremely low-carbon region. On the other hand, its accuracy is a problem, and there is a problem in its use as a control method for extremely low-carbon molten steel.

[0003]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems of the conventional endpoint control method, and provides an efficient and economical decarburization endpoint determination method for extremely low carbon molten steel. It is for the purpose.

[0004]

According to the present invention, molten steel is sampled at an arbitrary time after the start of evacuation, and the carbon analysis value of the sampled molten steel, the temperature of the molten steel and the solid electrolyte measured at the same time are used. On the basis of the value of the oxygen concentration obtained by the oxygen concentration sensor, the time at which the target carbon concentration is achieved is estimated by the difference method of the following equation (1), and the decarburization treatment end time is determined.

(Equation 3)

Furthermore, in the above method, when determining the end point of the decarburization treatment using the RH degassing equipment, the value of the equipment specific constant k _CO is determined by the following equation (3), and the pressure constant P
This is a method for controlling decarburization of molten steel, wherein a value of _co ^* = 15 (mmHg) is used.

(Equation 4)

[0006]

In the ordinary decarburization treatment, the pressure in the vacuum chamber is greatly changed, and the oxygen concentration [O] of the molten steel is different from each other. Circulation forms also differ greatly. We have:
As a result of repeated studies and studies, it was found that the change with time of [C] in the vacuum decarburization process can be reproduced by the equation (1). In other words, the values of k _CO and k _S are rate constants specific to equipment that do not depend on the evacuation speed and the concentration of [O], and the value of k _{CO is} a value uniquely determined by the molten steel stirring force or the molten steel recirculation speed. , K _S are values uniquely determined by the stirring force of the molten steel or the reflux velocity of the molten steel and the gas injection method. The value of P _co ^* is a supersaturation pressure for generating CO bubbles, which is a pressure constant determined by the physical properties of molten steel, and is usually in a range of 12.5 to 20.0 (mmHg). I found a thing.

[0007] Therefore, the decarburization treatment is carried out in advance using the same equipment under the same stirring conditions, and the values of k _CO , k _S and P _co ^* which describe the actual change with time of [C] are set to the above (1). )
If it is determined based on the formula, even when the evacuation speed and the [O] concentration are different, it is determined by the analysis of the molten steel sample at an arbitrary time t and the measurement using the solid electrolyte [C], [O] ] and based on the value of the molten steel temperature, by substituting the variation of P _t in equation (1), (1) in the calculation by the difference method of expression required in order to decarburization to [C] concentration to target Processing time can be determined. Calculation of the decarburization processing time based on the difference method of the equation (1) can be easily determined in a short time by inputting a simple program to a computer. However, as the equilibrium constant K, a known value (recommended equilibrium value of steelmaking reaction: edited by the Japan Society for the Promotion of Science, 19th Committee of Steelmaking) is used.

[0008] As a special case, calculation formula of the value of the molten steel circulating rate Q _m in the normal RH of ISIJ, Vol.28 (1988), as described in page 311., has already been elucidated, the present invention Is calculated by the above equation (4). The present inventors have discovered that the value of k _CO is 0.8 times the value of the reflux ratio Q _m / W _m . Therefore, for the end point control in the decarburization treatment by RH, the value calculated by the equation (4) may be used for the k _CO of the equation (1).

[0009]

【Example】

(Example 1) Decarburization treatment was performed in a ladle-type degassing furnace with a molten steel treatment amount of 270 tons. The Ar flow rate for stirring added to the molten steel was 1800 (Nl / min), and the molten steel was blown using a lance. Advance, based on the time course of [O] and P _t in another process in the same degassing furnace, found it best described the time course of _{[C] k CO = 0.38 (} 1 / min),
The values of k _S = 0.068 (1 / min) and P _co ^* = 17 (mmHg) were determined using equation (1). However, the shape of the ladle and the flow rate and the blowing method of Ar for stirring added to the molten steel are the same.

[0010] At the time of 7.5 minutes after the start of the decarburization treatment, molten steel was sampled, and the values of [C] and [O] and the temperature of the molten steel were determined by chemical analysis and measurement by the solid electrolyte method, respectively. The changes in value and P _t is input to the computer, showing the time course of [C] concentration was calculated by the method of the present invention in FIG. 1 by solid lines. The analytical value of [C] of the RH processing results is also indicated by a black circle. According to the method of the present invention, the change with time of the result [C] could be estimated very well.

(Embodiment 2) Molten steel processing amount is 100 to 350
The decarburization treatment was carried out in the RH degassing equipment which is a ton. The inner diameter of the dip tube ranges from 40 to 75 cm. Molten steel reflux ratio (Q _m
/ W _m ) is in the range of 0.38 to 0.95 (l / min). After starting the decarburization process, the molten steel is collected in 8 to 10 minutes,
The values of [C], [O] and the temperature of the molten steel were each subjected to chemical analysis,
It was determined by measurement by the solid electrolyte method. The changes in value and P _t is input to a computer, a processing target [C] concentration was calculated by the method of the present invention, the correspondence between the RH processing results endpoint [C] concentration is shown in Figure 2. By the treatment time estimation by the method of the present invention, the treatment target [C] concentration and the RH treatment end point [C] concentration matched very well, and the efficiency of the decarburization treatment could be achieved.

[0012]

As described above, by carrying out the method of the present invention, the achievement of the decarburization treatment target [C] concentration became extremely easy.

[Brief description of the drawings]

FIG. 1C is a drawing showing the correspondence between the measured value and the estimated value of the change over time of the concentration.

FIG. 2 is a drawing showing a correspondence between a processing target [C] density and an RH processing performance end point [C] density.

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21C 7/10 C21C 7/00 C21C 7/068

Claims (2)

(57) [Claims] 1. A method for producing ultra-low carbon concentration molten steel based on determination of an end point of a vacuum decarburization treatment of molten steel, wherein a carbon analysis value of a molten steel sample obtained by collecting molten steel at an arbitrary time after evacuation is started, Based on the measured temperature of the molten steel and the value of the oxygen concentration measured by the oxygen concentration sensor, the time required to achieve the target carbon concentration at the end of decarburization is estimated by the difference method of equation (1), and the decarburization treatment end time is determined. A method for controlling decarburization of molten steel characterized by the following: (Equation 1) When determining the end point of the decarburization treatment using the RH degassing equipment, the value of the equipment specific constant k _CO is set to (3)
2. The method for controlling decarburization of molten steel according to claim 1, wherein the value is determined by an equation, and a value of a pressure constant _Pco ^* = 15 (mmHg) is used. (Equation 2)