JPH05263122A - Treatment of molten steel in rh degassing vessel - Google Patents

Abstract

PURPOSE:To enable degassing-refining of molten steel to extremely low hydrogen concn. range by arranging a tuyere at the bottom part of a vacuum vessel in a degassing apparatus and blowing gas in a specific condition at the time of degassing- refining the molten steel in the RH type degassing apparatus. CONSTITUTION:Both immersing tubes 16 in the vacuum degassing apparatus 14 are dipped into the molten steel 12 in a ladle 10, and the molten steel 12 is circulated into the vacuum vessel 18 through this immersion tubes 16 to execute the degassing- refining of C and H2. In this case, the tuyere is arranged by approaching to the opening part of the immersion tube 16 at the bottom part of the vacuum vessel 18 and the gas is blown to promote the degassing. In the case the pressure P in the vacuum vessel 18 is <=P<=50Torr, the degassing treatment is executed in the condition of satisfying the equation I and the inequalities II, III and becoming >=30Nm<3>/sec gas adding quantity QN per one ton of the molten steel. In the case of P>50Torr, the degassing treatment is executed in the condition of the equation I and the inequality II at P=50Torr and reducing QN, and after reaching to O<=P<=50Torr, the degassing treatment is executed in the condition of increasing QN, e.g. >=30Nm<3>/sec gas adding ing quantity per one ton of the molten steel in the degassing apparatus in the range of satisfying the equation and the inequalities I, II, III.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten steel treatment method for efficiently performing degassing such as decarburization and dehydrogenation by an RH degassing treatment device.

[0002]

2. Description of the Related Art Crude molten steel produced in a steelmaking furnace (converter, electric furnace, etc.) is degassed in a secondary refining process (RH, DH, VOD, etc.) according to the intended composition and cleanliness. (Decarburization, dehydrogenation) Treatment is performed. In particular, recently, the required properties for steel materials have become strict, and more advanced degassing treatment is required. However, in the region where the concentration is lower than [C] = 20 to 50 ppm and [H] = 1 to 2 ppm, there is a problem that the degassing rate rapidly decreases and high-purity molten steel cannot be obtained within a predetermined time. It was

In order to solve this problem, in the RH degasser, a tuyere is installed in the vacuum chamber, and an inert gas is blown into the molten steel through the tuyere to reduce the degassing rate in the low concentration region. Ways to improve are being tried. However, the operating standard including the design of the tuyere is not always clear, and in fact, when the tuyere is installed and operated, the adhesion of metal in the vacuum tank due to the blow-through of the gas blown from the tuyere becomes a problem. There is.

Introducing a large amount of gas into the vacuum chamber increases the load on the vacuum exhaust system and deteriorates the time to reach high vacuum and the degree of vacuum reached. It was not possible to obtain a sufficient degassing effect.

[0005]

The object of the present invention is to establish an operation method capable of stably improving the degassing effect even in an extremely low concentration range, that is, a method for treating molten steel in an RH degassing tank. It is to be.

[0006]

The present inventors have found that there is a certain correlation between the number of tuyere, the tuyere diameter, and the flow rate of gas blown into the vacuum tank of the RH degassing apparatus. , Knowing that the above-mentioned object can be achieved by changing the flow rate of the blown gas according to the degree of vacuum in the vacuum chamber,
The present invention has been completed.

The gist of the present invention is to provide a molten steel treatment method in which a gas is blown from a tuyere provided at the bottom of a vacuum tank of an RH degassing apparatus to degas, and the pressure in the vacuum tank is set to P.
Is 0 ≦ P ≦ 50 (Torr):

[0008]

[Equation 2]

However, P: pressure in the vacuum tank (Torr) h: bath depth of the vacuum tank (cm) h ₁ : ladle level to vacuum tank bottom (cm) ρ _l : molten steel density (g / cm ³ ) ρ _g : Gas density (g / cm ³ ) g: Acceleration of gravity (cm / sec ² ) x: Tuyere diameter (cm) Q _N : Total amount of gas blown into the vacuum chamber (Ncm ³ / sec) N: Number of tuyere above Formulas (1) to (3) are satisfied, and the amount of gas added per ton of molten steel treated in the RH degasser is 30 Ncm ³ / sec · ton.
If degassing is performed under the above conditions and the pressure in the vacuum chamber is P> 50 Torr: Q _N is reduced while satisfying the conditions of the above formulas (1) and (2) at P = 50 Torr, and 0 ≦ after P ≦ 50 (Torr) reaches above (1) to (3) increase progressively Q _N in a range satisfying, gas addition amount of processing molten steel per 1ton in the RH degasser is 30Ncm ³ / sec · R characterized by performing degassing treatment under the condition of ton or more
It is a method for treating molten steel in an H 2 degasser.

[0010]

Next, the reason for limiting the degassing conditions in the present invention as described above will be described in detail with reference to the accompanying drawings. FIG. 1 is a schematic view of an RH degassing apparatus used in the present invention. Molten steel 12 contained in a ladle 10 is provided with two immersion pipes 16 and 16 of an RH degassing apparatus 14 installed above the ladle. It is sucked from one side, returned from the other side, and circulated in the RH degassing device. The air pressure inside the RH degasser 14 is P, and the tuyere is provided at the bottom of the vacuum chamber 18 in the vicinity of the opening of the dip tube 16, although not shown. Other,
Tubing is also provided on the dip tube 16 itself for circulation,
This is a conventional one, and the amount of blown gas is not particularly limited in the present invention. However, in the case of the total amount of gas blown, the amount of gas supplied from the tuyere provided in such a dip tube is also summed up, and in the case of just the amount of gas blown, the tuyere provided at the bottom of the vacuum chamber. It is the amount blown from.

In the figure, h indicates the bath depth of the vacuum tank 18, and h ₁ indicates the ladle level to the vacuum tank bottom. (1) Bath depth h in the vacuum chamber: 1 atm is equivalent to 76 cm of mercury, so with iron pillars

[0012]

[Equation 3]

[0013] Therefore, assuming that the distance from the molten steel surface of the ladle to the vacuum tank bottom is h ₁ (cm), considering the balance between the iron column and atmospheric pressure from the relationship shown in FIG. 1,
The above equation (1) is derived as follows.

[0014]

[Equation 4]

(2) Molten Steel Penetration into Tuyere: Molten steel must not penetrate into the tuyere during the degassing process by gas blowing, and according to the hydrodynamic examination of the conditions therefor,
The following equation holds. That is, dynamic pressure of molten steel + static pressure of molten steel =
From the conditions of dynamic pressure of gas + static pressure of gas (: constant),

[0016]

[Equation 5]

Where P ₁ = static pressure of molten steel at the tuyere outlet = ρ _l · g · h, V ₁ = flow velocity of molten steel at the tuyere outlet = 0, P ₂ = static pressure of gas at the tuyere outlet = 0 Therefore, the following equation is derived from the above equation.

[0018]

[Equation 6]

Therefore, by substituting the equation (6) into the equation (5), the following equation is obtained.

[0020]

[Equation 7]

Since this equation (2) 'is an equilibrium condition when molten steel enters the tuyere, in order to prevent the molten steel from entering, it is necessary that the pressure on the gas side blown at the tuyere outlet is superior. Is derived.

[0022]

[Equation 8]

(3) Gas blow-through: Next, the blow-in gas causes so-called blow-through when the depth of the vacuum chamber is too shallow, and there is an adverse effect such as adhesion of metal in the chamber. Therefore, it is necessary to set the amount of gas blown in and the depth of the molten steel bath so that blow-through does not occur. The relational expression that defines the minimum gas injection amount at this time is described in the document “Iron and Steel” 68 (19)
82) It is known as partly cited in p.1964, but this relates to the gas injection from the bottom tuyere in the converter and, as the writer himself of the document says, gas. It is suggested that the calculated results cannot be obtained, as it is influenced by the reaction with the steel bath and the mutual interference of tuyere. However, according to the experimental results of the present inventors this time, it was found that such a relational expression is well applied in a vacuum chamber such as an RH degasser.

[0024]

[Equation 9]

As described above, the tuyere conditions are such that the molten steel does not enter the tuyere and the gas does not blow through the tuyere.
Equations (1) to (3) were obtained. Further, in a general RH degassing apparatus, gas is blown from a dipping pipe due to molten steel recirculation, and the amount thereof is about 100 to 150 Ncm ³ / sec · ton. Therefore, the amount of gas to be additionally introduced is set to ³⁰ Ncm ³ / sec · ton or more, which is an increase of 20 to 30%.

The number of tuyere is counted as 1 in the case of a single tube and N in the case of a collective tuyere in which N tucks are gathered in one block.

Further, more desirably, it is more appropriate to install 0.02 or more tuyere per ton of treated molten steel for promoting degassing. This is because the dispersed blowing method is advantageous. By the way, in the RH degassing apparatus, the molten steel is refluxed into the vacuum chamber by depressurizing the vacuum chamber. Therefore, when the degree of vacuum is extremely low, the molten steel has not reached the inside of the vacuum chamber, and it is meaningless to consider the blow-through condition shown in equation (3).

Further, in the range where the pressure is reduced from 760 to 50 Torr, the degassing rate is high. Therefore, adding gas from the bottom of the tank unnecessarily only increases the load of the vacuum exhaust system, which increases the processing time. Does not contribute to shortening. Therefore, when P> 50 Torr, the gas addition amount Q _N is kept low within the range where the formulas (1) and (2) are satisfied, and when 0 ≦ P ≦ 50 Torr, the Q _N is gradually changed to the formulas (1) to (3). Molten steel treated within the applicable range 1
It is desirable to increase the amount of gas added per ton to ³⁰ Ncm ³ / sec · ton or more. Next, the present invention will be described in more detail by way of examples, but it should be understood that the examples shown here are merely examples, and the present invention is not limited thereto.

[0029]

【Example】

(Example 1) Treated molten steel amount = 300 tons, immersion pipe diameter = 75 cm, reflux Ar = 110 Ncm ³ / sec · ton RH degasser vacuum chamber bottom, tuyere diameter = 0.1 cm, 16 blades I installed my mouth. Note that the tuyere was a collective tuyere that formed one block with four tuyere. As an example of the present invention, h ₁ = 115 cm, the gas species is Ar, and the initial [C] = 280-320 ppm, the initial a _O = 500 under the conditions that satisfy the above formulas (1) to (3). Degassing treatment of ~ 700ppm molten steel (N number = 8ch).

On the other hand, as a comparative example, an example was shown in which molten steel was treated by the conventional treatment method in which no gas was added from the bottom of the RH degasser (N number = 10 ch). Examples and comparative examples, 2.
Reached 50 Torr in 7-3.5 minutes. In the embodiment, Q _N = 12000 Ncm ³ / sec until reaching 50 Torr, Q _N = 20000 Ncm ³ / sec when 5 ≦ P ≦ 50 Torr, Q _N = 35000 Nc when P <5 Torr
Increased to m ³ / sec. 5.0 to 6.0 to reach P = 5 Torr
It took a minute.

In each of the examples and comparative examples, the amount of gas blown from the dip tube was 33000 Ncm ³ / sec. From equation (1), h ₁ = 11
Table 1 shows the relationship between h and P at 5 cm.

[0032]

[Table 1]

Here, the condition under which the molten steel does not enter the tuyere will be examined by the formula (2) with the tuyere diameter of 0.1 cm and the number of tuyere = 16 (conditions of the embodiment). (2) In the formula, x = 0.1 cm, N
= 16, the critical values of h and Q _N are calculated. The critical h for Q _N is shown in Table 2.

[0034]

[Table 2]

Furthermore, the condition (3) which does not cause blow through
Consider by formula. Substituting x = 0.1 cm and N = 16 into the equation (3) and determining the critical h for Q _N , the results are shown in Table 3 below.

[0036]

[Table 3]

The contents of the above equations (1) to (3) are summarized in a graph as shown in FIG.

As can be seen from the results shown in FIG. 2, according to the present invention, the above equation (2) regarding the penetration into the tuyere is as follows.
I had enough time. On the other hand, there was a margin of difference of 5 cm to 5 to 50 Torr with respect to the equation (3) regarding blow-through. When the degassing process was performed under the above conditions, both Example and Comparative Example reached 50 Torr in 2.7 to 3.5 minutes.

In the embodiment, Q _N = 12000 until reaching 50 Torr.
Ncm ³ / sec, Q _N = 20000 Ncm ³ / s when 5 ≦ P ≦ 50 Torr
It was increased to Q _N = 35000 Ncm ³ / sec at ec and P <5 Torr. It took 5.0 to 6.0 minutes to reach P = 5 Torr. In addition,
The amount of gas blown from the dip tube was 33000 Ncm in both Examples and Comparative Examples.
It was ³ / sec. Next, a sample was taken every 5 minutes after the start of the treatment, and the transition of [C] was measured. Table 4 shows the average values of Examples and Comparative Examples.

[0040]

[Table 4]

In the example, the time to reach the same [C] level was shortened by 5 minutes or more at [C] ≦ 15 ppm. It was also found that the arrival [C] at 25 minutes was about 5 ppm lower.
In the examples, when the treatment for 2 charges was performed for 30 minutes, 3 and 4 ppm of [C] were obtained. In addition,
[H] in the molten steel was 0.5 ± 0.2 ppm. (Example 2) Also in this example, the amount of treated molten steel is 30 as in Example 1.
0 ton, immersion pipe diameter = 75 cm, reflux Ar = 110 Ncm ³ / sec ・ ton
At the bottom of the vacuum chamber of the RH degassing device, tuyere diameter = 0.1 cm, number
16 tuyeres were installed. Note that the tuyere was a collective tuyere that formed one block with four tuyere. As an example of the present invention, h ₁ = 115c under the condition that the above-mentioned expressions (1) to (3) are satisfied.
m was used as the gas species, and Ar was used as the gas species to degas the molten steel at the initial [H] = 3.0 to 5.2 ppm (N number = 9 ch).

On the other hand, as a comparative example, an example was shown in which molten steel was treated by the conventional treatment method in which no gas was added from the bottom of the RH degasser (N number = 10 ch) (initial [H] = 2.9 to 5.0). ppm)
. In both Example and Comparative Example, it reached 50 Torr in 2.7-3.5 minutes. In the example, Q _N until reaching 50 Torr = 12000 Ncm ³ / s
ec, and if 5 ≦ P ≦ 50 Torr, Q _N = 20000 Ncm ³ / sec, P
At <5 Torr, Q _N was increased to 35000 Ncm ³ / sec. P
It took 5.0 to 6.0 minutes to reach 5 Torr.

The amount of gas blown from the dip tube was 33000 Ncm ³ / sec in both the examples and comparative examples. Next, a sample was taken every 5 minutes after the start of the treatment, and the transition of [C] was measured.
Table 5 shows the average values of Examples and Comparative Examples.

[0044]

[Table 5]

In the examples, it was found that [H] ≦ 1.0 ppm could be easily obtained. As can be seen from these results, according to the present invention, stable degassing can be performed,
In addition, the arrivals [C] and [H] are 6 ppm and 0.5 ppm, respectively, which is sufficient to meet the most demanding requirements today.

[0046]

By applying this method, it is possible to promote degassing in a RH degassing apparatus stably and efficiently.

[Brief description of drawings]

FIG. 1 is a schematic explanatory view showing an RH degassing device and each variable.

FIG. 2 is a graph showing a summary of the results of Examples.

Claims (1)

[Claims] 1. A molten steel treatment method for degassing by blowing gas from a tuyere provided at the bottom of a vacuum tank of an RH degasser, wherein the vacuum tank pressure P is 0 ≦ P ≦ 50 (Torr). Case: [Equation 1] However, P: pressure in the vacuum tank (Torr) h: vacuum tank bath depth (cm) h ₁ : ladle level to vacuum tank bottom (cm) ρ _l : molten steel density (g / cm ³ ) ρ _g : Gas density (g / cm ³ ) g: Acceleration of gravity (cm / sec ² ) x: Tuyere diameter (cm) Q _N : Total amount of gas blown into the vacuum chamber (Ncm ³ / sec) N: Number of tuyere above (1) (3) is satisfied and the amount of gas added per 1 ton of molten steel treated in the RH degasser is 30 Ncm ³ / sec · ton.
If degassing is performed under the above conditions and the pressure in the vacuum chamber is P> 50 Torr: Q _N is reduced while satisfying the conditions of the above formulas (1) and (2) at P = 50 Torr, and 0 ≦ after P ≦ 50 (Torr) reaches above (1) to (3) increase progressively Q _N in a range satisfying, gas addition amount of processing molten steel per 1ton in the RH degasser is 30Ncm ³ / sec · R characterized by performing degassing treatment under the condition of ton or more
Molten steel treatment method in H 2 degasser.