JP2018109212A - Dehydrogenation refining method for molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a dehydrogenation refining method for a molten steel where dehydrogenation treatment time to reach a pre-set hydrogen concentration in a molten steel is beforehand predicted at a high precision, and dehydrogenation treatment for the molten steel is performed without exaggeration and without omission.SOLUTION: Provided is a dehydrogenation refining method for a molten steel using a vacuum degassing apparatus, an alloy and Al are charged to a molten steel to control the free oxygen concentration in the molten steel to 10 ppm or lower, thereafter, the hydrogen concentration in the molten steel is measured, based on the measurement result and pre-obtained dehydrogenation rate constants, using a dehydrogenation prediction model equation, dehydrogenation treatment time is calculated till the pre-set hydrogen concentration in the molten steel is obtained, and dehydrogenation treatment for the molten steel is performed for the dehydrogenation treatment time.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a method for dehydrogenating and refining molten steel using a vacuum degassing apparatus.

  The molten steel primarily refined in the converter is subjected to degassing treatment such as hydrogen and nitrogen, final adjustment of alloy component concentration, and the like by a vacuum degassing apparatus. At this time, if the vacuum degassing treatment time is excessive, the life of the refractory in the vacuum degassing tank is reduced, and utilities such as electric power and reflux gas are wasted. In particular, since the vacuum degassing time of a thick plate material that is strict regarding the hydrogen content is rate-determined by the dehydrogenation treatment, it is required to perform the dehydrogenation treatment without excess or deficiency.

  For example, Patent Document 1 discloses a dehydrogenation refining method using a vacuum degassing apparatus. In the dehydrogenation refining method described in Patent Document 1, the refining period is divided into three periods according to the progress, and the dehydrogenation rate constant in each period is determined in advance. At the time of dehydrogenation refining, the hydrogen concentration in the molten steel before treatment is directly measured by an on-line hydrogen rapid analyzer, and the dehydrogenation treatment time is determined for each refining heat by the dehydrogenation rate constant of each of the three periods.

JP-A-5-70821

  In the dehydrogenation refining method described in Patent Document 1, the final hydrogen concentration is estimated by dividing the dehydrogenation refining into three sections and determining the dehydrogenation rate constant for each section. The timing of switching to is defined by the change in the dehydrogenation rate constant value. However, in actual operation, it is difficult to accurately grasp the switching timing, which causes a deterioration in prediction accuracy. Further, in addition to the necessity of conducting a preliminary investigation on a plurality of dehydrogenation rate constants for each operating condition, variations due to a change from the operating condition subjected to the preliminary investigation are not fully considered.

  The present invention has been made in view of such circumstances, and predicts the dehydrogenation processing time until the hydrogen concentration in the molten steel reaches a preset high accuracy, and can perform the dehydrogenation processing of the molten steel without excess or deficiency. An object of the present invention is to provide a dehydrogenation refining method.

In order to achieve the above object, the present invention provides a method for dehydrogenating and refining molten steel using a vacuum degassing apparatus.
The alloy and Al are introduced into the molten steel, the free oxygen concentration in the molten steel is reduced to 10 ppm or less, the hydrogen concentration in the molten steel is measured, and dehydrogenation prediction is made based on the measurement result and the dehydrogenation rate constant determined in advance. The dehydrogenation processing time until the hydrogen concentration in the molten steel reaches a preset hydrogen concentration is calculated using the model formula, and the dehydrogenation processing of the molten steel is performed during the dehydrogenation processing time.

  By measuring the hydrogen concentration in the molten steel after introducing the alloy and Al into the molten steel, the influence of hydrogen mixed into the molten steel from the alloy or the like can be reduced. In addition, since the dehydrogenation reaction rate changes depending on the free oxygen concentration in the molten steel, fluctuations in the dehydrogenation rate constant are suppressed by measuring the hydrogen concentration in the molten steel after setting the free oxygen concentration in the molten steel to 10 ppm or less. be able to.

In the molten steel dehydrogenation refining method according to the present invention, it is preferable that the dehydrogenation prediction model equation is the equation (1).
t = −1 / k × Ln (([H] − [H] ′) / ([H] ₀ − [H] ′)) (1)
here,
t: Dehydrogenation treatment time (min), k: Dehydrogenation rate constant (min ⁻¹ ), [H]: Hydrogen concentration in molten steel (ppm) set in advance, [H] ₀ : Hydrogen in molten steel before dehydrogenation treatment Concentration (ppm), [H] ': Equilibrium hydrogen concentration (ppm) at the molten steel interface in the vacuum degassing tank, Ln: Natural logarithm

  In the said structure, the equilibrium hydrogen concentration in the molten steel interface in a vacuum degassing tank is considered. Thereby, the saturated hydrogen concentration in the low hydrogen region is reflected in the calculation of the dehydrogenation treatment time, and the dehydrogenation treatment time can be predicted with higher accuracy than the conventional dehydrogenation prediction model formula.

  In the dehydrogenation refining method for molten steel according to the present invention, the hydrogen concentration in the molten steel is measured after the alloy and Al are introduced into the molten steel so that the free oxygen concentration in the molten steel is 10 ppm or less. The influence of the concentration on the hydrogen concentration in the molten steel is reduced, and the dehydrogenation processing time can be predicted with high accuracy. As a result, the molten steel can be dehydrogenated without excess or deficiency.

It is the graph which showed the influence which the difference in alloy injection amount has on dehydrogenation rate constant k. It is the graph which showed the influence which the difference in the free oxygen concentration (Free [O]) in molten steel exerts on the dehydrogenation rate constant k. It is the graph which showed the relationship between the free oxygen concentration (Free [O]) in molten steel, and the dehydrogenation rate constant k. It is process drawing which showed the procedure of the dehydrogenation refining method of the molten steel which concerns on one embodiment of this invention.

  Next, embodiments of the present invention will be described with reference to the accompanying drawings to provide an understanding of the present invention.

[Measurement timing of hydrogen concentration in molten steel]
There has been a model that predicts the dehydrogenation time until the hydrogen concentration in the molten steel reaches a preset value based on the measurement results of the hydrogen concentration in the molten steel, but it is difficult to predict with high accuracy, or for each steel type Therefore, it was necessary to obtain the dehydrogenation rate constant in advance.

  When the present inventors investigated the conventional dehydrogenation refining method, in the conventional method, the measurement timing of the hydrogen concentration in molten steel was before the vacuum degassing process or the first half of the vacuum degassing process. In the vacuum degassing process, an alloy may be charged during the process or Al may be charged for deoxidation. The present inventors changed the dehydrogenation rate constant due to the difference in hydrogen mixed from the alloy and the like to be introduced into the molten steel and the free oxygen concentration (Free [O]) in the molten steel, and the hydrogen in the molten steel after the vacuum degassing treatment. It was found to affect the concentration.

  In the present invention, the hydrogen concentration in the molten steel is measured after the alloy and Al are introduced into the molten steel and the free oxygen concentration in the molten steel becomes 10 ppm or less. Based on the measurement result, the hydrogen concentration in the molten steel is set in advance. Calculate the dehydrogenation processing time until. Hereinafter, the basis of the present invention will be described.

  FIG. 1 is a graph showing the effect of the difference in alloy input amount on the dehydrogenation rate constant k. When performing dehydrogenation processing on about 400 tons of molten steel using an RH vacuum degassing apparatus, the hydrogen concentration in the molten steel before the vacuum degassing treatment and 15 minutes after the start of treatment is changed by changing the amount of Nb alloy introduced during the treatment. Was measured respectively. In the figure, in the heat (charge) indicated by the solid line, 1500 kg of Nb alloy is introduced between 6 minutes and 8 minutes from the start of the vacuum degassing treatment, and in the heat indicated by the broken line, 1300 kg of Nb alloy is supplied in the same time zone. I put it in.

As a result of this test, there was a difference between 2.7 ppm and 1.9 ppm after the treatment even though the hydrogen concentration in the molten steel before the vacuum degassing treatment was all equal to 4.9 ppm. In the figure, the value of the dehydrogenation rate constant k calculated using the generally known dehydrogenation equation (2) based on the experimental results is shown.
[H] = [H] ₀ · exp (−k · t) (2)
Here, [H]: hydrogen concentration after treatment (ppm), [H] ₀ : hydrogen concentration before treatment (ppm),
k: Dehydrogenation rate constant (min ⁻¹ ), t: Refining time (min)
It can be seen from the figure that the dehydrogenation rate constant k varies depending on the amount of hydrogen contained in the alloy. From this, it can be seen that if the end point is predicted from the hydrogen concentration in the molten steel before the vacuum degassing process, the prediction accuracy is deteriorated, which is not desirable.

FIG. 2 is a graph showing the effect of the difference in free oxygen concentration (Free [O]) in molten steel on the dehydrogenation rate constant k. When dehydrogenation processing is performed on about 400 tonnes of molten steel using an RH vacuum degassing device, the amount of Al to be added is changed about 10 minutes after the start of processing, and the free oxygen concentration in the molten steel is changed to perform vacuum degassing processing. The hydrogen concentration in the molten steel was measured before and 15 minutes after the start of treatment.
In the figure, in the heat indicated by the broken line, the hydrogen concentration in the molten steel before treatment was 5.0 ppm, the free oxygen concentration in the molten steel after Al was added was 34 ppm, and the hydrogen concentration in the molten steel after treatment was 1.5 ppm. . On the other hand, in the heat indicated by the solid line, the hydrogen concentration in the molten steel before the treatment was 4.9 ppm, the free oxygen concentration after the addition of Al was 10 ppm, and the hydrogen concentration in the molten steel after the treatment was 2.7 ppm.
In the figure, the value of the dehydrogenation rate constant k calculated using the formula (2) based on the experimental results is shown. Even if the hydrogen concentration in the molten steel before the vacuum degassing treatment is equivalent, the free hydrogen in the molten steel It can be seen from FIG. 2 that there is a difference in the value of the dehydrogenation rate constant k due to the difference in [O].

  FIG. 3 is a graph showing the relationship between Free [O] in molten steel and the dehydrogenation rate constant k. From the figure, the value of the dehydrogenation rate constant k decreases with the decrease in Free [O] in the molten steel, and when the Free [O] in the molten steel falls below 10 ppm, the dehydrogenation rate constant k becomes almost constant. Recognize.

[Method for Dehydrogenating Refining Molten Steel According to One Embodiment of the Present Invention]
FIG. 4 shows the procedure of the molten steel dehydrogenation refining method according to the embodiment of the present invention.
The vacuum degassing apparatus used in the molten steel dehydrogenation refining method according to the present embodiment is an RH type vacuum degassing apparatus having two dip tubes in the lower part of the vacuum degassing tank. At the time of degassing, the air in the vacuum degassing tank is discharged out of the tank by a pump or the like to 0.52 kPa or less, and the molten steel in the ladle is lifted up to the vacuum degassing tank. Moreover, degassing from molten steel is accelerated | stimulated by blowing in circulating gas, such as Ar gas, from one dip tube, and circulating molten steel.

  The alloy and Al are introduced into the molten steel within 5 to 15 minutes after the start of the vacuum degassing treatment. Alloys to be introduced are Mn alloy, Ti alloy, Nb alloy and the like.

When the free oxygen concentration in the molten steel becomes 10 ppm or less, the hydrogen concentration in the molten steel is measured. For measuring the hydrogen concentration in the molten steel, for example, an analyzer such as “HYDRIS” (registered trademark) manufactured by Heraeus Electronite Co., Ltd. can be used. This analyzer collects and analyzes nitrogen gas blown into the molten steel. The hydrogen concentration in the molten steel is calculated using Sieverts' law (see equation (3)) in which the hydrogen concentration in the nitrogen gas increases as the hydrogen concentration in the molten steel increases. Note that the nitrogen gas in this embodiment has a purity of 99.97% or more.
[H] = K / f × √P _H2 (3)
here,
[H]: Hydrogen concentration in molten steel (ppm), K: Dissolution reaction constant of hydrogen gas in molten steel (−), f: Activity coefficient of hydrogen, P _H2 : Hydrogen partial pressure in reflux gas

  Based on the measurement result of the hydrogen concentration in the molten steel and the dehydrogenation rate constant k, a dehydrogenation prediction model equation is used to calculate the dehydrogenation processing time until the preset hydrogen concentration in the molten steel is reached. And dehydrogenation processing of molten steel is performed during the dehydrogenation processing time.

Here, the dehydrogenation prediction model formula will be described.
The dehydrogenation treatment of the molten steel can be expressed by a differential equation represented by equation (4).
−d [H] / dt = k ([H] − [H] ′) (4)
here,
t: Dehydrogenation treatment time (min), [H]: Hydrogen concentration in molten steel (ppm) at dehydrogenation treatment time t, k: Dehydrogenation rate constant (min ⁻¹ ), [H] ′: In vacuum degassing tank Equilibrium Hydrogen Concentration (ppm) at Molten Steel Interface

By integrating equation (4), equation (5) is obtained.
[H] − [H] ′ = Ce ^−kt (5)
Where C: integration constant

Placing molten steel in a hydrogen concentration [H] when the t = 0 and [H] _0, [H] ₀ - because it is [H] '= C, ( 5) formula is as follows.
([H] − [H] ′) / ([H] ₀ − [H] ′) = e ^−kt (6)

By transforming equation (6), the following dehydrogenation prediction model equation is obtained.
t = −1 / k × Ln (([H] − [H] ′) / ([H] ₀ − [H] ′)) (7)
However, Ln is a natural logarithm.

On the other hand, in the conventional dehydrogenation prediction model formula, the following formula is used by approximating [H] ′ = 0 ppm.
t = −1 / k × Ln ([H] / [H] ₀ ) (8)
However, since [H] ′ ≠ 0 ppm when t = ∞ under actual operating conditions, it is possible to predict with higher accuracy by using Equation (7) considering [H] ′.
The dehydrogenation rate constant k and the equilibrium hydrogen concentration [H] ′ at the molten steel interface in the vacuum degassing tank must be obtained in advance from the past actual operation results.

  Although one embodiment of the present invention has been described above, the present invention is not limited to the configuration described in the above-described embodiment, and is within the scope of matters described in the claims. Other possible embodiments and modifications are also included. For example, in the said embodiment, although the vacuum degassing apparatus was made into RH type, it is not limited to this, You may use the apparatus etc. which make the whole ladle a vacuum state.

A verification test carried out to verify the effects of the present invention will be described.
Verification tests were conducted on five examples and one conventional example.
Dehydrogenation treatment was performed on the molten steel amount of about 400 tons using an RH vacuum degassing apparatus with a degree of vacuum of less than 0.26 kPa. Nb alloy, Mn alloy, Ti alloy, and Al were added 5 to 12 minutes after the start of the treatment. The free oxygen concentration after addition of Al was 7 ppm to 9 ppm.
Table 1 shows the hydrogen concentration in the molten steel measured after the treatment, the results of the dehydrogenation treatment time, and the results of evaluation based on them.

The “reference” treatment time in the table is the dehydrogenation treatment time for each target hydrogen concentration determined from the past results, and was determined from the hydrogen concentration before treatment, the ultimate vacuum of the vacuum degassing apparatus, and the target hydrogen concentration. . In the conventional example, the dehydrogenation treatment was performed in this “reference” treatment time. In the examples, the hydrogen concentration in the molten steel after adding the alloy and Al was measured using the analyzer described above, and the dehydrogenation treatment was performed to obtain the dehydrogenation treatment time necessary to obtain the target hydrogen concentration.
Example 1 calculates | requires dehydrogenation processing time using the conventional dehydrogenation prediction model formula shown by (8) Formula, and Examples 2-5 are dehydrogenation concerning this invention shown by (7) Formula. The dehydrogenation time was obtained using the prediction model formula.
The “predicted” treatment time in the table is the sum of the dehydrogenation treatment time calculated using the dehydrogenation prediction model formula and the degassing treatment time elapsed until the measurement of the hydrogen concentration in the molten steel (total treatment time) Is shown.

  In the evaluation, the case where the hydrogen concentration in the molten steel was significantly lower than the target value x (impossible), the prediction accuracy of the dehydrogenation treatment time was high, but the hydrogen concentration in the molten steel was slightly lower than the target value ◯ (good), the accuracy of dehydrogenation treatment time prediction is high, and the case where the hydrogen concentration in the molten steel is within the target value is marked ◎ (excellent).

Table 1 shows the following.
-Although the prediction accuracy of the dehydrogenation processing time was high in Example 1, since the conventional dehydrogenation prediction model formula was used, the hydrogen concentration in molten steel was slightly lower than the target value.
In Examples 2 to 5, it was confirmed that by using the dehydrogenation prediction model formula according to the present invention, excessive dehydrogenation treatment was prevented, and the hydrogen concentration in the molten steel became the target value. Moreover, even if there was a variation in the hydrogen concentration in the molten steel before the degassing treatment, the dehydrogenation treatment time could be predicted with high accuracy.
・ In the conventional example, the hydrogen concentration in the molten steel was significantly lower than the target value, resulting in excessive dehydrogenation treatment.

Claims (2)

In the dehydrogenation refining method for molten steel using vacuum degassing equipment,
The alloy and Al are introduced into the molten steel, the free oxygen concentration in the molten steel is reduced to 10 ppm or less, the hydrogen concentration in the molten steel is measured, and dehydrogenation prediction is made based on the measurement result and the dehydrogenation rate constant determined in advance. A dehydrogenation refining method for molten steel characterized by calculating a dehydrogenation treatment time until a hydrogen concentration in the molten steel reaches a preset level using a model formula, and performing dehydrogenation treatment of the molten steel during the dehydrogenation treatment time . 2. The method for dehydrogenating and refining molten steel according to claim 1, wherein the dehydrogenation prediction model equation is equation (1).
t = −1 / k × Ln (([H] − [H] ′) / ([H] ₀ − [H] ′)) (1)
here,
t: Dehydrogenation treatment time (min), k: Dehydrogenation rate constant (min ⁻¹ ), [H]: Hydrogen concentration in molten steel (ppm) set in advance, [H] ₀ : Hydrogen in molten steel before dehydrogenation treatment Concentration (ppm), [H] ': Equilibrium hydrogen concentration (ppm) at the molten steel interface in the vacuum degassing tank, Ln: Natural logarithm

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