JPH0696738B2 - Vacuum degassing apparatus for ultra-low carbon steel production and operating method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a vacuum degassing apparatus for producing ultra-low carbon steel and a method of operating the same, and particularly applicable to high-speed continuous casting [C] ≦
The present invention relates to a vacuum degassing apparatus and a method of operating the same, which enables a highly efficient RH vacuum degassing treatment operation of ultra low carbon steel of 20 ppm.

[Conventional technology]

The conventional method for producing ultra-low carbon steel is to carry out secondary refining of crude decarburized low carbon steel that has been first refined in a converter or the like in an RH vacuum degassing furnace to a level of [C] ≤ 20 ppm. However, [C]
There is a problem that the decarburization rate becomes slow in the extremely low carbon region of ≤20 ppm, and [C] ≤20 ppm cannot be achieved unless it is treated for a long time.

Therefore, in the case of recent high-efficiency continuous casting machines, for example, one-charge 300-t class, the casting pitch of the continuous casting machine is one charge 25 minutes at the shortest. On the other hand, in the conventional RH vacuum degassing device, 1 charge
In the case of 300t class, one charge 35 minutes is required even in the shortest processing time, and there is a problem that the productivity of the high efficiency continuous casting machine cannot be dealt with.

As a method for solving the above problems, many studies have been made in the past and many techniques have been disclosed.

For example, JP-A-58-213819, JP-A-59-85815, and JP-A-SHO-6
In addition to 0-63311, JP-A-63-213617, and JP-A-64-28320, many disclosures have been made. The outline of the above disclosure is as follows.

JP-A-58-213819: This invention is intended to improve the degassing ability by increasing the area of the reaction interface of the molten steel in the degassing tank to the vacuum, and the summary thereof is as follows. That is, "a gas stirring port for a bath stirring made of a porous refractory provided in a tank bottom area surrounded by a straight line parallel to the target axis, passing through the center of the pair of molten steel communicating pipes with the vacuum layer bottom diameter as the target axis. Through a circulating gas degassing method for the molten steel in which the gas for bath stirring is blown into the vacuum tank bath based on a gas blowing pattern preset according to the steel type.

Japanese Patent Laid-Open No. 59-85815: The present invention discloses, "In a reflux type degassing apparatus equipped with a molten steel suction pipe having a circulating gas blow-in port at the bottom of a vacuum degassing tank and a molten steel downcomer pipe, It is a reflux type degassing device provided with a suction pipe and a downcomer having an oval hole with an enlarged hole diameter in a direction intersecting with the adjacent direction ".

Japanese Unexamined Patent Publication No. 60-63311: This invention discloses that "the depressurization rate is 120 Torr / min or more at an average rate of 760 Torr to 1 Torr, and the recirculation inert gas blowing amount Q (Nm ³ / min) is Q ≧ 3.9. X10 ^-2 x (10D) 203 ⁰³ RH degassing method for producing ultra-low carbon steel. "That is, the present invention improves the degree of vacuum in the RH vacuum degassing tank at an early stage to reduce CO gas emissions. It is intended to promote the generation, limit the amount of the inert gas such as Ar blown into a large amount, and enhance the stirring to promote the decarburization reaction.

JP-A-63-213617: The present invention is a method for producing an extremely low carbon steel in which the use of reflux is gradually increased according to the promotion of decarburization based on the steel [C] in the flow rate of molten steel during RH treatment. That is, (a) 150t / from the start of processing until [C] reaches 100ppm.
Less than min (b) 150t / from when [C] is less than 100ppm to 30ppm
Min or more and less than 200 t / min (C) 200 t / min or more when [C] is less than 30 ppm JP-A-64-28320 This invention describes "when vacuum degassing steel into a reflux type degassing apparatus, Carbon concentration 0.01% or less, oxygen concentration 0.
In the range of 005 to 0.002%, magnesio-bustite powder was blown together with the gas under the molten steel surface in the vacuum tank to maintain the oxygen concentration near the gas-molten steel interface at 0.005 to 0.02% and denitrify at the same time as decarburization. Ultra low carbon and low nitrogen steel melting method to proceed. It is.

Although these conventional methods have their respective characteristics, they are still unsatisfactory in degassing efficiency, and there is still room for improvement in order to obtain an ultra low carbon steel of [C] ≦ 20 ppm. It is left. In particular, in the method of blowing a large amount of inert gas at a pressure reduction rate of 120 Torr / min or more as in JP-A-60-63311, molten steel splattering is severely generated in the vacuum tank, and the adhesion to the furnace wall refractory is significant. ,
In order to remove this, there is no choice but to stop the operation, which not only becomes an impeding factor in terms of production, but also in terms of the quality of the molten steel, there is a high risk that the composition will be poorer than that of C pick-up.

[Problems to be solved by the invention]

The object of the present invention is to eliminate the problems of the above-mentioned conventional techniques in the production of ultra-low carbon steel, and yet to achieve high-efficiency RH degassing vacuum degassing capable of accommodating the productivity of recent high-efficiency continuous casting machines. The present invention seeks to provide an apparatus and a preferred method of operating the same.

[Means for solving problems]

The gist of the vacuum degassing apparatus according to the present invention is as follows. That is, "a ladle for containing molten steel, an ascending and descending dip pipe for dipping the tip of the molten steel in the ladle, a reflux gas blowing device provided in the ascending dip pipe, and an upper part of the dipping pipe. A vacuum degassing apparatus comprising: a vacuum degassing tank provided; and an oxygen and / or inert gas blowing lance penetrating an upper lid of the vacuum degassing tank and hung up and down in the tank. In the gas apparatus, the vacuum degassing tank and the ascending / descending dip tube, both of which are formed in an oval shape and whose longitudinal central axes intersect each other at right angles, and the oval longitudinal central axis in the lower center of the ascending dipping tube. A bridge made of a refractory provided at right angles to each other, a plurality of high-pressure gas erected on the upper part of the bridge, and a powdery oxide blowing porous thin tube if necessary, and respectively communicating with the porous thin tube. High-pressure gas supply equipment And a powdery oxide supply device. A vacuum degassing device for the production of extremely low carbon steel. "

The gist of the method of operating the vacuum degassing apparatus according to the present invention is as follows. That is, "a vacuum degassing apparatus for decarburizing crude decarburized low carbon steel produced in a refining furnace such as a converter by degassing by blowing oxygen upward in a vacuum degassing treatment tank having an ascending and descending dip pipe. In the operating method, the step of refluxing the molten steel from the ladle by the elliptical vacuum degassing tank and the dip tube whose longitudinal central axes cross each other at right angles, and the refractory in the lower center of the ascending dip tube The step of injecting high-pressure gas and, if necessary, powdery oxide from a plurality of perforated capillaries erected on the upper part of the bridge, the ultra low carbon steel according to claim (1). Operation method of vacuum degassing equipment for manufacturing. "

The requirements for achieving the object of the present invention are listed below. That is, (a) increase the area of reaction of the molten steel in the vacuum degassing tank to the vacuum.

(B) Maintaining a high degree of vacuum in the reaction area of the vacuum degassing tank.

(C) To increase the oxygen concentration in the molten steel to promote the decarburization reaction.

(D) To lower the [C] value of the received crude decarburized low carbon steel.

(E) To increase the flow rate of molten steel under reflux without increasing the amount of reflux gas.

(F) Eliminate the local stagnation of reflux molten steel.

The present invention has been obtained as an apparatus satisfying the above requirements, and the structure of the vacuum degassing tank according to the present invention will be described first.

An overall view of the vacuum degassing apparatus according to the present invention is as shown in FIG. 1, and its characteristic features will be described with reference to the accompanying drawings such as FIGS. 2 and 3. First, Fig. 2 shows ladle 3.
FIG. 6 is a cross-sectional view showing the shapes of a vacuum degassing tank and a dip tube according to the present invention to which is added. Each of the vacuum degassing tank 2, the ascending dip pipe 4, and the descending dip pipe 6 has an oval cross section,
The oval longitudinal central axes of the vacuum degassing tank 2 and the dipping tubes 4 and 6 intersect each other at right angles.

The elliptical shape of only the dipping tube is disclosed in the above-mentioned Japanese Patent Laid-Open No. 58-85815, but in the present invention, both the vacuum degassing tank 2 and the dipping tubes 4 and 6 are made oblong, and the elliptical shape of these mutually oblong shapes. By combining the longitudinal central axes so as to intersect at a right angle, the stagnation area of the reflux molten steel can be eliminated and the reaction interfacial area in the vacuum layer 2 can be increased.

That is, the cross-sectional shapes of the conventional vacuum layer 2 and the dipping tubes 4 and 6 are a combination of perfect circles and perfect circles as shown in FIGS. Since it flows straight from the pipe 4 to the descending dipping pipe 6, the molten steel flow 8
Stagnation area 10 occurs on both sides of. However, in the combination of the ellipse and the ellipse shown in FIGS.
The distance between the ascending dip pipe 4 and the descending dip pipe 6 became long, the stagnation area 10 was eliminated, and the stay time of the molten steel 8 in vacuum was extended, and therefore the reaction interface area could be increased. Further, by increasing the cross-sectional areas of the immersion pipes 4 and 6 as compared with the conventional one, the flow velocity can be made equal to or slower than that of the conventional perfect circular immersion pipe despite the dramatic increase in the ring flow rate of the molten steel 8. Yes, and by extension of residence time in vacuum [C]
It was also found to be effective in eliminating the stagnation of the decarburization reaction in the extremely low carbon region of ≤20 ppm.

As shown in FIGS. 5 (A) and 5 (B), the layout relationship between the conventional ladle and the degassing tank is such that the center of the ladle 3 is displaced forward of the center of the vacuum degassing tank 2. It is normal. As described above, the reason for providing the distance α between the ladle 3 and the vacuum degassing tank 2 is that the temperature measuring sampling device for the molten steel 8 and the visibility of the flanges of the dipping pipes 4 and 6 from the operation room are secured. Therefore, α requires a certain size. On the other hand, in a situation where the outer diameter of the ladle 3 cannot be increased, in order to increase the cross-sectional area of the vacuum degassing tank 2 and the dipping tubes 4 and 6 in actual operation, the vacuum degassing tank 2 It is most effective that both the immersion pipes 4 and 6 are oval, and the longitudinal central axes of the immersion pipes 4 and 6 intersect each other at right angles. The vacuum degassing tank 2 and the immersion pipe 4 according to the present invention are most effective. , 6 are both oval, and the longitudinal central axes thereof are arranged to intersect each other at a right angle, which is a secondary effect.

Next, other important requirements for the structure of the vacuum degassing apparatus according to the present invention will be described with reference to FIGS. 6 (A) and 6 (B).

A refractory bridge 12 is provided at a lower portion of the ascending dip tube 4 having an oval cross section according to the present invention so as to intersect the central axis of the ellipsometer at a right angle, and a plurality of high pressure gases are provided above the bridge 12. Further, a porous thin tube 14 for injecting powdery oxide is erected upright and opened toward the upper part. The high-pressure gas supply device 16 and the powdery oxide supply device 18 which are independently provided to the porous thin tube 14 respectively communicate with the high-pressure gas blowing porous thin tube 14A and the powdery oxide blowing porous thin tube 14B through the pipes 20. However, it is configured such that an inert high-pressure gas such as Ar and a powdery oxide such as iron oxide are blown into each. Therefore, in addition to the conventional porous plug 22 for injecting a circulating gas, a high pressure gas injecting porous thin tube 14 is weighted and included.

[Action]

The operation and operation method of the vacuum degassing tank according to the present invention configured as described above will be described.

In the vacuum degassing tank 2 according to the present invention, the cross-sectional shape is oval, and the ascending dip pipe 4 and the descending dip pipe 6 are provided.
Also, as shown in FIGS. 2 and 3, the ellipse of the vacuum degassing tank 2 and the ellipses of the dipping pipes 4 and 6 have their central axes in the longitudinal direction intersecting each other at right angles. In addition, ascending dip tube 4
Porous capillary tube 14A for injecting high pressure gas and porous capillary tube 1 for injecting powdery oxide provided on a bridge 12 provided on the short central axis of
Since high-pressure inert gas and powdery oxide are injected from 4B, respectively, the stagnation area 10 as in the conventional case is eliminated, and a porous inert gas injecting porous material provided in the side wall of the normal ascending dip tube 4 is further eliminated. By using upward high-pressure Ar gas from the porous thin tube 14 provided on the bridge 12 outside the plug 22, the molten steel 8 in the ladle 3 is driven from the center of the ascending dip tube 14 by the propulsive force due to the edged effect. It spouts upward as it flows. As a result, the flow rate of molten steel ring could be remarkably increased even if the amount of Ar gas blown was the same as the conventional one. This is also clear from the comparison of the rising height of the molten steel 8 in the vacuum degassing tank 2.

The principle of the molten steel recirculation in the RH vacuum degassing apparatus is that a circulating gas such as Ar is blown from the side wall of the ascending dip tube 4 and the molten steel 8 is circulated by the principle of the gas lift pump. In addition to this, by injecting upward high-pressure inert gas from the porous thin tube 14 provided in the bridge 12 together with powdered oxide if necessary, and adding a high-pressure jet flow, the principle of the gas lift pump plus the ejector effect is given. As a result, the molten steel ring flow rate could be greatly increased.

The operation method of the vacuum degassing apparatus according to the present invention is to operate so that the above-described operational effects of the present invention can be sufficiently exhibited. That is, as shown in the overall view of FIG. 1, the exhaust gas duct 24 is evacuated to maintain a high degree of vacuum, and the auxiliary evacuation device 26 is used to adjust the pressure to 0.5 to 2.0 torr, so that the ascending dip pipe 4 of the conventional device can be used. In addition to blowing inert gas such as Ar from the porous plug 22 provided on the side wall, the vacuum degassing tank 2
The operation method according to the present invention is applied to the conventional normal operation in which oxygen is upwardly blown from the lance 30 which can be moved up and down through the upper lid 28 of the upper part of the above, and a mixed gas of oxygen and Ar is blown from the lance 30 if necessary. It is a thing. That is, the molten steel 8 from the ladle 3 is circulated by the elliptical vacuum degassing tank 2 and the dipping pipes 4 and 6 whose longitudinal central axes intersect each other at right angles, and is provided at the lower center of the ascending dipping pipe 4. Fritz 12
A high-pressure inert gas is injected from a plurality of perforated thin tubes 14 attached together with the powdery oxide, if necessary, and this gives an ejector effect, and the molten steel ring flow rate is greatly increased. It is a thing.

〔Example〕

Table 1 shows a comparison between the vacuum degassing apparatus according to the present invention and the conventional degassing apparatus.

Comparative tests were carried out using a vacuum degassing tank according to the present invention as shown in Table 1 and a conventional degassing apparatus.

In the operation of the vacuum degassing tank according to the present invention, the auxiliary vacuum exhaust device 26 was used for reinforcement. The results of the comparative test are shown in FIGS. 7 and 8. As is apparent from FIG. 8, in the case of the conventional method using the conventional device, [C] ≦ 20 pp
The decarburization rate is slow in the low carbon area of m
In contrast to the minimum C value reached by [C] = 15 ppm, which is required by the minute, according to the apparatus of the present invention and the operating method of the present invention,
The decarburization rate in the low carbon region of [C] ≦ 20 ppm was also accelerated, and the minimum value of [C] = 7 to 8 ppm could be reached in a total treatment time of 25 minutes.

〔The invention's effect〕

In the vacuum degassing apparatus according to the present invention, both the vacuum chamber and the dip tube are oval, the central axes of the respective longitudinal directions intersect at right angles, and the lower part of the ascending dip tube is made of refractory material in the minor axis direction of the oval. A bridge is provided, and a plurality of perforated capillaries for injecting a high-pressure gas and a powdery oxide are provided on the upper surface of this bridge, and an inert high-pressure gas such as Ar is injected at the same time as the powdery oxide. I was able to achieve the effect.

(B) The stagnation area of the molten steel, which has been conventionally generated, does not occur on both sides of the molten steel recirculation direction from the ascending dip pipe to the descending dip pipe in the vacuum tank, and the entire molten steel can be uniformly recirculated.

(B) [C] The decarburization stagnation area of ≤20 ppm has been resolved, and the smooth decarburization reaction has been promoted.

(C) The lowest value of [C] is [C] = 15 in processing time of 35 minutes
In the case of the present invention, [C] = 7 to 8 ppm was reached in 25 minutes from the conventional value of ppm.

(D) Since the RH vacuum degassing process time for ultra-low carbon steel production could be shortened to 25 minutes, it is possible to secure productivity that can handle continuous casting with a continuous casting machine capable of high speed casting without waiting time. It became like this.

Due to the effects of (e) and (b), the conventional pick-up of C was eliminated, and the appropriateness ratio of the components of the treated molten steel could be improved.

[Brief description of drawings]

FIG. 1 is a sectional view showing the whole vacuum degassing apparatus for producing ultra-low carbon steel according to the present invention, and FIG. 2 shows the arrangement and shape of a ladle, a degassing tank, and a dip tube in the vacuum degassing apparatus according to the present invention. Fig. 3 is a plan sectional view showing the degassing tank according to the present invention, the shape of the ascending dip pipe and molten steel flow, and Figs. 4 (A) and 4 (B) are conventional degassing tanks, ascending and descending. Schematic plan sectional views showing the shape of the immersion pipe and the molten steel stagnation state, FIGS. 5 (A) and 5 (B) show a conventional RH vacuum degassing device, (A) a front sectional view, and (B) a plane section. FIGS. 6 (A) and 6 (B) show an ascending dip tube in the vacuum degassing apparatus according to the present invention, where (A) is a front sectional view and (B) is an arrow BB of FIG. FIG. 8 is a sectional view, FIG. 7 is a diagram showing changes in molten steel [C] with changes in decarburization treatment time and pressure in the vacuum chamber according to the embodiment of the present invention. Is a graph showing the change in the decarburization time and the molten steel [C] in the comparative test of the present invention method and the conventional method. 2 ... Vacuum degassing tank, 3 ... Ladle 4 ... Ascending dip tube, 6 ... Down dip tube 8 ... Molten steel, 10 ... Stagnation area 12 ... Refractory bridge, 14 ... Perforated capillary tube 16 ... High pressure gas supply device 18 ... Powder Oxide supply device 20 ... Piping, 22 ... Porous plug 24 ... Vacuum exhaust duct, 26 ... Auxiliary exhaust device 28 ... Top lid, 30 ... Top blowing lance

Claims (2)

[Claims] 1. A ladle for accommodating molten steel, an ascending and descending dip pipe for dipping a tip into the molten steel in the ladle, a reflux gas blowing device provided in the ascending dip pipe, and an immersion pipe It comprises a vacuum degassing tank arranged at the upper part, and an oxygen and / or inert gas blowing lance penetrating the upper lid of the vacuum degassing tank and hung up and down in the tank. In the vacuum degassing apparatus, the vacuum degassing tank and the ascending / descending dip tube, both of which are formed in an oval shape and whose longitudinal central axes intersect each other at right angles, and the longitudinal direction of the oval shape at the lower center of the ascending dip tube A bridge made of a refractory provided at right angles to the central axis, a plurality of high-pressure gas erected on the upper part of the bridge, and a powdery oxide blowing porous capillary if necessary, and the porous capillary, respectively. High-pressure gas supply equipment in communication And powdery oxide feeder and, ultra-low carbon steel for manufacturing vacuum degassing apparatus characterized by having a. 2. A vacuum degassing process for decarburizing crude decarburized low carbon steel melted in a refining furnace such as a converter in a vacuum degassing tank having ascending and descending dip pipes by blowing oxygen upward. In the operating method of the gas apparatus, a step of circulating molten steel from the ladle without stagnation by an elliptic vacuum degassing tank and dip tube whose longitudinal central axes cross each other at right angles, and the lower center of the ascending dip tube A step of injecting high-pressure gas and, if necessary, powdery oxide from a plurality of perforated capillaries erected on the upper part of the refractory bridge, the method (1).
2. A method of operating a vacuum degassing apparatus for producing ultra-low carbon steel according to.