JPH07179930A - Method for top-blowing oxygen into vacuum refining furnace by using straight barrel type immersion tube - Google Patents

Abstract

PURPOSE:To enable the efficient decarburizing and oxygen-enriched operation and the removal of metal stuck to a vessel by top-blowing the oxygen under the specific conditions and specifying the overlapping area between bubble activating surface zone of stirring gas blown from the bottom part and the top-blowing gas. CONSTITUTION:Molten steel 5 tapped from a refining furnace is incorporated in a ladle 2, and an immersion tube 1 is dipped into the ladle. Successively, the pressure in the immersion tube 1 is reduced and the oxygen gas or oxidizing gas through a top-blowing lance 4 is top-blown from the upper part onto the surface of the molten steel exposed in the vacuum, and also, the inert gas is blown from a porous brick 3 at the bottom part to stir the molten steel. At this time, the top-blowing gas is blown so that a parameter Q shown by the equation Q=(F/n(hXdXp<1/2>) (F: flow rate of the top-blowing gas, h: distance between the tip part of the lance and the stationary molten steel surface, d: diameter of the lance nozzle, n: number of the nozzle holes, p: atmospheric pressure) becomes 0.05-5.0. Further, the overlapping area on the molten steel surface of the bubble activating surface zone generated by the gas 6 for stirring from the bottom part and the top-blowing gas is defined as 30-90% of the cross-sectional area of the immersion tube 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for supplying oxygen gas to a vacuum refining furnace, which enables efficient decarburization, enrichment of oxygen, and removal of metal adhered in a tank. .

[0002]

2. Description of the Related Art Supplying oxygen in a vacuum refining furnace has three purposes. One is to directly react oxygen with carbon in a steel bath for decarburization, which is used for finish decarburization of stainless steel, for example. The second is to increase the oxygen concentration in the molten steel, and in the case of ordinary steel refining, when the blowing carbon of the converter is increased to reduce the converter refractory cost, and when decarburizing in a vacuum furnace, oxygen is added at the initial stage of refining. Is used to make up for the lack of. The third is to remove the metal attached to the tank, and an exothermic reaction when the CO gas generated by decarburization in the tank space is burned to CO ₂ (secondary combustion reaction).
Is used. Of these, for the first and second purposes, it is more efficient for oxygen gas to come into direct contact with molten steel, whereas for the third purpose, oxygen gas is directly Is more effective when not in contact with molten steel.

On the other hand, Japanese Examined Patent Publication No. Sho 49-12810 discloses a method in which oxygen is blown onto a high chromium steel from above using an RH degassing furnace. In addition, in JP-A-60-145316, the RH
A method of blowing oxygen to a high chromium steel from a position lower than the molten metal surface position during undeoxidized molten steel treatment and higher than the molten metal surface position during deoxidized molten steel treatment using a degassing furnace is disclosed. . However, in either method, decarburization proceeds efficiently, but since the secondary combustion shown in the third purpose is not taken into consideration, a large amount of splash occurs and they adhere to the tank. Therefore, there was a problem that the operation was seriously hindered.

Further, Japanese Patent Laid-Open No. 2-77518 discloses that
There is disclosed a method in which the secondary combustion reaction is positively utilized by spraying oxygen gas from above the molten steel surface at a predetermined position so that the pressure reaches the molten steel surface in an appropriate range. In this case, if the ultimate pressure is too high, the secondary combustion cannot be used, and conversely, if it is too low, there is a problem that the heat deposition efficiency is deteriorated, and the appropriate operating conditions are greatly restricted. is there.

Here, the reason why the heat deposition efficiency is deteriorated in the case of so-called soft blow is that the region where the secondary combustion reaction occurs is far from the molten metal surface. This is because the heat generated in the area where the secondary combustion reaction occurs is not applied through the metal droplets but is applied to the molten steel by the radiation from the secondary combustion area, so it is necessary to react it directly above the molten metal surface. Because there is. Also, what the secondary combustion rate is decreased in the case of so-called hard blow, it is to return to the vigorously for splash occurs, react with CO ₂ in the splash secondary combustion zone (decarburization) and CO gas.

[0006]

[Problems to be Solved by the Invention] As shown above,
JP-B-49-12810 and JP-A-60-1453
The technique disclosed in Japanese Patent No. 16 has a problem in that a large amount of splashes are generated, and moreover, they are attached to the inside of the tank, which seriously hinders the operation. Further, in the technique disclosed in Japanese Patent Laid-Open No. 2-77518, secondary combustion cannot be used when the ultimate pressure is too high, and conversely, when the ultimate pressure is too low, heat deposition efficiency is deteriorated, and therefore it is appropriate. There was a problem that the operating conditions were greatly restricted. Therefore, the object of the present invention is to
It is to make it possible to operate under a wide range of appropriate conditions with technology that makes it difficult for a hard splash to occur and a secondary combustion rate to drop during hard blow, and conversely to prevent a significant decrease in heat deposition efficiency during soft blow. .

[0007]

The inventors of the present invention utilize a surface area (bubble active surface) in which gas bubbles are blown into a bath, and blow oxygen gas onto the surface region so that a wide range of operating conditions can be achieved. We have obtained the knowledge that the heat compensation and the efficient decarburization reaction can be achieved by the secondary combustion. The present invention is based on this finding. The gist of this is that a large-diameter, straight-body-shaped container is immersed in molten steel in a ladle that has been tapped from a refining furnace such as a converter or an electric furnace. In the method for vacuum refining molten steel in which the pressure is reduced, the parameter (Q) corresponding to the dynamic pressure of the top-blown gas defined by the following equation is 0 from the top on the surface of the molten steel exposed to vacuum. It is sprayed so as to be 0.05 to 5.0, and the bubble activated surface area generated by the stirring gas blown from the lower part of the ladle and the overlapping area of the upper blown gas on the steel bath surface are the molten steel in the immersion tank. This is a method for blowing oxygen gas onto a vacuum refining furnace using a straight-body type immersion pipe, which enables efficient oxygen supply by setting the cross-sectional area of the immersion tank at the surface position to 30 to 90%. Q = (F / n) / (h × d × P ^1/2 ), where F is the upper blowing gas flow rate (Nm ³ / Hr), h is the distance (cm) between the tip of the lance and the stationary molten metal surface, d Is the lance nozzle diameter (cm), n is the number of nozzle holes, P is the atmospheric pressure (T
orr).

[0008]

The present invention focuses on the effect of the bubble activating surface on the heat deposition behavior. Here, the bubble active surface is defined as a region in which the blown gas bubbles float on the surface, and the determination method is based on the observation results of water model, mercury model, or actual machine. The bubble active area (A _N ) for the gas is given by the equation (1), and the bubble active area (A _V ) for the gas blown in the horizontal direction is given by the equation (2). _{A N = 3.14 × (0.212 ×} T) 2 ···· (1) A V = 3.14 × (7 × M 0.67) 2/2 ···· (2) where, T is Distance from injection position to molten steel surface (m)
And M is the gas injection amount per nozzle (Nm ³
/ S).

The present inventors have found that heat generation, splash generation, and 2
As a result of detailed investigation of the effect of the bubble activation surface on the secondary combustion, the following conclusions were obtained. 1) There are two types of heat generated by secondary combustion on molten steel: one is by radiation and the other is by using metal droplets scattered in the secondary combustion area as a medium. If the bubble active surface is narrow, it is blown into the molten steel. Since the generated gas concentrates in a narrow area, the metal droplet rises higher than necessary, and it becomes a so-called splash that only adheres to the wall surface of the tank and is difficult to act as a heat medium. The metal droplets are generated from a wide range, and the ratio of returning to the molten steel again without adhering to the wall surface of the tank becomes very large, and thus it becomes an efficient heat medium. Therefore, when the bubble active surface is widened, heat can be applied without being affected by radiation, so that the secondary combustion region is set at a relatively high position up to the height at which the metal droplets acting as a heat medium are scattered. However, high heat-transfer efficiency can be obtained.

As this condition, in addition to the dynamic pressure of the top-blown gas on the molten metal surface, the overlapping area of the bubble active surface region and the collision region of the top-blown gas on the steel bath surface is important. If it is 30 to 90% of the cross-sectional area of the dipping tank in 1, the high heat-transfer efficiency can be obtained even with a considerable soft blow. Here, the dynamic pressure of the top-blown gas at the molten metal surface is V (m ³ / Hr), the nozzle outlet flow velocity of the gas, the nozzle diameter is d (cm), and the distance between the tip of the lance and the stationary molten metal surface is h (cm). , The gas density ρ
(G / Nm ³ ), gravitational acceleration g (m / s ² ), and atmospheric pressure P (Torr), (ρ × P) × (d × V
/ H) ² / (2g), which is rewritten as an index by rewriting the oxygen flow rate (F: Nm ³ / Hr) expressed in the standard state and the number of nozzles (n) to obtain Q. According to various experiments shown in FIG. 1, a high heat-transfer efficiency can be obtained up to a value of 0.05. It decreased and the heat transfer efficiency deteriorated. On the other hand, when Q is larger than 5.0, the droplets that have exchanged heat in the secondary combustion region do not return to the metal bath but scatter out of the system as they are, and the heat deposition efficiency deteriorates. Q = (F / n) / (h × d × P ^1/2 )

2) In the so-called hard blow, the splash is often generated by the kinetic energy of the top blowing gas and also by the rapid generation of CO gas just below the surface. This rapid CO gas generation is due to an explosive decarburization reaction at a locally high concentration of [C] or [O] when microscopic nonuniformity of concentration occurs just below the surface. It is a phenomenon. Further, when a large amount of splash is generated in the secondary combustion atmosphere, the carbon in the splash is oxidized by CO ₂ and the reverse reaction of returning to CO occurs, so that the secondary combustion rate decreases.

In order to solve these problems, it is important to prevent the occurrence of non-uniformity of the microscopic concentration just below the surface and to prevent the splash from being generated in a larger amount than the amount acting as the heat medium. . Here, since the bubble active surface is constantly updated with the gas that floats from below the molten metal surface, it is unlikely that microscopic non-uniformity of concentration will occur.By combining a wide bubble active surface and the upper blowing position, a small explosion will occur. Splash is less likely to occur. Even in this case, the metal droplets enter the secondary combustion zone, but in the case of the reverse reaction of CO ₂ , the mass transfer of [C] in the droplets is rate-determining, so compared to the heat conduction to the droplets described above. Requires a long time.

Therefore, in the state where the heat deposition efficiency is improved, C
It is possible to suppress the reverse reaction of O ₂ , and the condition is that the overlapping area of the bubble active surface area and the collision area of the upper blowing gas on the steel bath surface is the geometric surface area of the molten steel in the immersion tank. When it is 30% or more, the generation of a small explosive splash is suppressed, and as shown in FIG. 2, by setting Q to 5.0 or less, the generation of a splash due to the kinetic energy of the upper blowing gas is suppressed. Will be done. If these two conditions are not satisfied, the splash will be severe and the splash particles will stay in the secondary combustion region for a long time, so that a reverse reaction will occur and the secondary combustion rate will decrease.

[0014]

[Example] In the example, as shown in FIG.
Immerse the dip tube to reduce the pressure inside the tube and install it on the bottom of the furnace.
A vacuum cleaner that supplies gas from porous bricks and stirs the steel bath.
It was carried out with a smelting device. About 350 tons of undeoxidized molten steel is about 20
Process for minutes, carbon concentration of about 300ppm to 10ppm
Supplying oxygen gas from the top blowing lance during decarburization
did. The secondary combustion rate (PCR) is C in the gas above the immersion tank.
O₂/ (CO + CO ₂) × 100. Also wear
The thermal efficiency (η) of CO is CO₂Out of the heat of reaction that becomes meta
It was calculated as the percentage of the heat used to raise the temperature of the fuel cell. Fruit
The test conditions were to change the top blowing lance variously and change the Q significantly.
In addition to the above, the bubble activated surface and the top blowing gas shown in FIG.
Area of the overlapping area with the bath surface collision area (A: Geometric total
The calculated value) was also changed. Here, the dip tank cross-sectional area is G
It was Here, F is the top blowing gas flow rate (Nm³ / Hr), h
Is the distance between the tip of the lance and the stationary surface (cm), d is the lance
Nozzle diameter (cm), P indicates atmospheric pressure (Torr)
You

Table 1 shows examples and comparative examples. As shown in Table 1, Examples 1 to 12 illustrate the invention. A / G is 0.3 even when the top blowing gas flow rate, the distance between the tip of the lance and the stationary molten metal surface, the diameter of the lance nozzle, and the atmospheric pressure are changed.
It is understood that when the above is satisfied and Q is in the range of 0.05 to 5.0, a high secondary combustion rate and high heat transfer efficiency are obtained. Especially, those having Q in the range of 0.5 to 3.0 have particularly good results. On the other hand, 13 to 19 show comparative examples. Nos. 13 to 15 are cases in which Q is less than 0.05, but secondary combustion occurs, but the heat deposition efficiency is extremely deteriorated. 16 and 19 are cases where Q is larger than 0.5, but the secondary combustion rate decreases due to the reverse reaction, and the heat transfer efficiency decreases because the granular iron that is the heat medium scatters out of the system. 17
And 18 are when A / G is smaller than 0.3.
The test conditions are similar to the case where oxygen is supplied from above from H, but Q has a poor heat deposition efficiency even in the proper range.

[0016]

[Table 1]

[0017]

EFFECTS OF THE INVENTION By using the present invention, it is possible to prevent the occurrence of a severe splash and a decrease in the secondary combustion rate during hard blow, and conversely, a technique in which a significant decrease in the heat deposition efficiency during a soft blow does not occur. Under a wide range of appropriate conditions,
Oxygen addition at the initial stage of decarburization, improvement of decarburization speed by blowing oxygen, and prevention of metal adhesion by secondary combustion during decarburization became possible.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between Q and heat transfer efficiency,

FIG. 2 is a diagram showing the relationship between Q and the secondary combustion rate,

FIG. 3 is a schematic diagram when the present invention is implemented,

FIG. 4 is a diagram showing a cross section taken along the line aa ′ of FIG. 3;

[Explanation of symbols]

 1 Immersion pipe 2 Ladle 3 Porous brick installed at the bottom of ladle 4 Top blowing lance 5 Molten steel 6 Jet gas of top blowing gas 7 Plume zone of bottom blowing gas A Overlapping area of S and B B Collision area of top blowing gas S Bubbles Active surface

Claims (1)

[Claims] 1. A vacuum refining method for molten steel in which a straight-body-shaped dipping tank is immersed in molten steel and molten steel is depressurized in the ladle from the refining furnace. Oxygen or oxidizing gas is sprayed from above onto the surface of the molten steel so that the parameter Q defined by the equation (1) is 0.05 to 5.0, and from the bottom of the ladle. The overlapping area of the bubble active surface area generated by the stirring gas blown into the steel bath surface of the top blown gas is 30% or more and 90% or less of the immersion tank cross-sectional area at the molten steel surface position in the immersion tank. A method for blowing oxygen gas onto a vacuum refining furnace using a straight body type immersion tube, which is characterized in that Q = (F / n) / (h × d × P ^1/2 ) ... (1) Here, F is the top blowing gas flow rate (Nm ³ / Hr), and h is the distance between the tip of the lance and the stationary molten metal surface. (Cm), d is the lance nozzle diameter (cm), n is the number of nozzle holes, P is the atmospheric pressure (T
orr).