JP2773883B2 - Melting method of ultra low carbon steel by vacuum degassing - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to vacuum degassing of undeoxidized or weakly deoxidized molten steel melted in a steelmaking furnace by using RH method, DH method, VOD method and the like. TECHNICAL FIELD The present invention relates to a method for producing ultra-low carbon steel by vacuum degassing, which can be charged into a tank and rapidly obtain ultra-low carbon steel without impairing the operability of the apparatus.

[Conventional technology]

From the viewpoint of continuation of the annealing process of a cold-rolled steel sheet and improvement of efficiency, adoption of a continuous annealing step equipment has been active in recent years. Ultra-low carbon steel with a carbon content of 10 ppm to several ppm has been required as a material suitable for this.

Conventionally, extremely low carbon steel has been used in converters for C: 0.02-0.05.
2. Description of the Related Art Molten steel that has been decarburized to a weight percent (hereinafter abbreviated as%) has been produced by a technique of decarburizing under reduced pressure using a vacuum degassing apparatus such as an RH method. Considerable studies have been made on the decarburization process under reduced pressure. The main theory is that the decarburization rate is as follows (1), as shown in “Iron and steel vol. 69 (1983) A37”. It is shown by the formula.

d [C] / dt = [C] ₀ exp (−Kct) (1) The rate constant Kc is expressed by the following equation (2).

Kc = {Q '/ v｝ ｛ak / (Q' + ak)} (2) where [C] ₀ : [C] concentration at the processing start time Kc: Apparent rate constant (min ^-1 ) ak: Mass transfer capacity coefficient in vacuum chamber (m ³ / s) Q ': Circulating flow rate of molten steel (m ³ / s) V: molten steel amount (m ³ ) Therefore, when trying to improve decarburization efficiency,
It is known that measures should be taken to increase Q 'and ak.

In order to increase the annular flow rate Q 'of the molten steel, a method of increasing the reflux pipe diameter and a method of increasing the amount of Ar gas blown for reflux are adopted.
The problem is that the life of the reflux tube is short, and the latter has the problem of impairing the degree of high vacuum required in the extremely low carbon region. The adhesion of the bullion inside increases,
Naturally, there is a limit to the amount of gas that can be injected due to operational restrictions.
It has also been shown that increasing the flow rate in the low-carbon region where the decarburization rate is low is not very effective.

On the other hand, to increase the mass transfer capacity coefficient ak, C
In the ultra-low carbon region of ≤50 ppm, the mass transfer process to the reaction site of C controls the reaction.
It is essential to increase (m ² ). As reaction sites, gas bubbles in molten steel / molten steel interface, steel bath surface in vacuum chamber,
Splashes accompanying gas bubbles leaving the steel bath are assumed, but the contribution of each is not necessarily clear, and increasing the amount of reflux Ar gas would be effective for the above three points. From the viewpoint, the technology that injects a large amount of Ar gas as much as 5000Nl / min is still used.

By the way, when such a large amount of Ar gas is blown, there is no means to cope with the adhesion of a large amount of splash to the inner surface of the vacuum tank, and there is a problem that the operability is impaired, and the carbon content is 10 ppm. From the viewpoint of the following rapid decarburization technology, it is still technically inadequate.

Attempts to blow Ar, a non-oxidizing gas, from the top blowing lance or to blow Ar into the vacuum chamber are shown in Kobe Steel Technical Report 36 (1986), p.40. It is shown that the effect is small compared to the method of increasing.

[Problems to be solved by the invention]

The present invention quickly performs decarburization in an extremely low carbon region up to a carbon content of 10 ppm or less and 10 ppm which was difficult to reach conventionally.
It is intended to provide a technique for stably obtaining the following ultra-low carbon steel.

At this time, it is an object of the present invention to provide a method capable of achieving the above-mentioned problem without causing a problem of metal adhesion in a vacuum chamber due to an increase in splash caused by a conventional means such as an increase in reflux Ar. .

[Means for solving the problem]

The technical means of the present invention is a method for vacuum degassing undeoxidized or weakly deoxidized molten steel melted in a steelmaking furnace, wherein the blades disposed on the side wall of the vacuum chamber furnace wall into molten steel in the vacuum degassing tank. The treatment is performed while blowing an inert gas from the mouth, and when the carbon concentration in the steel bath becomes 30 ppm or less, the flow velocity at the tuyere tip becomes 150 ppm.
This is a method for melting ultra-low carbon steel by vacuum degassing, characterized by blowing an inert gas at a rate of 0 Nm / sec or more. At that time, when the carbon concentration in the steel bath became 30 ppm or less, the blow gas flow per ton of molten steel in the vacuum chamber was increased to 0.1 Nm ³
/ Min or more, furthermore, the tuyere diameter arranged on the furnace wall side of the vacuum chamber is 3 mm or less, and the pressure of the blown gas at the time when the carbon concentration in the steel bath becomes 30 ppm or less. It is preferably at least 20 kgf / cm ² .

[Action]

The present inventors have found that the decarburization reaction is particularly stagnant [C] <50 ppm
Attempt to improve the decarburization reaction at the same time and obtained the following findings. [C]
Regarding the decrease in the decarburization reaction at <50 ppm, the stirring power of the molten steel is insufficient due to the decrease in the amount of generated CO gas, and the decarburization reaction is rate-controlled by the chemical reaction (the decarburization reaction is rate-controlled by the gas-liquid interface reaction). This is the reason, but the reason is not clear. However, in any case, an increase in the area of the gas-liquid reaction interface is effective for improving the decarburization reaction.

However, simply increasing the amount of gas blown from the reflux tube monotonically increases the bubble diameter of gas bubbles when the molten steel leaves the molten steel, and most of the molten steel droplets accompanying the bubbles are also large in diameter. The present inventors have found that the ratio of Also, the amount of splash becomes large, and the adhesion of metal to the vacuum tank becomes large. If the diameter of the droplet is large, in the extremely low carbon region, the reaction rate-determining process is C rate-limiting in the molten steel and cannot sufficiently contribute to the decarburization reaction.

In addition, we examined a method of blowing Ar into the vacuum chamber, which is considered as a means to increase the gas-liquid boundary area in the high-carbon region. It was confirmed that the effect was small as compared with the method of causing the above. The reason is that in the high-carbon region, the decarburization reaction is controlled by the flow rate of RH. That's why.

When Ar was further injected, the cause of the small increase in the gas-liquid interfacial area was investigated, and the following points became clear. That is, as shown in FIG. 2 (a), when gas is blown into the molten iron from the lateral direction, the gas immediately floats from the tuyere, and as a result, the gas travels along the vacuum chamber furnace wall and floats. Become. For this reason, an increase in the gas-liquid boundary area due to gas injection was not obtained as expected.

Therefore, the present inventors have repeatedly improved and devised the means by gas injection, and found that there is room for improvement in the tuyere 6 used for gas injection and the gas pressure. That is, by further reducing the diameter of the tuyere 6 and increasing the gas injection pressure, that is, by increasing the flow velocity at the tip of the tuyere at the same gas flow rate, as shown in FIG. As a result, the injected gas is dispersed into small bubbles in the molten steel in the vacuum chamber, resulting in a significant gas-liquid interface area. It was found that the increase in For this reason, it is not good that the blowing position is too deep in order to maintain fine bubbles.

The effect described above is expected to be higher as the tuyere diameter is smaller and the pressure is higher. However, as shown in Fig. 1, Ar is laterally blown into the vacuum chamber using a 230-ton RH vacuum degasser. As shown in Fig. 3, the gas flow rate at the tuyere tip was 1500 Nm / sec or more by changing the tuyere diameter and gas pressure at the same gas flow rate (6 Nm ³ / min in this experiment) as shown in Fig. 3. It was found that the decarburization rate constant Kc in the extremely low-carbon region was improved. This is due to the difference between the trajectories of the gas blown into the vacuum chamber and the remarkable increase in the gas-liquid boundary area.

However, this effect naturally depends on the amount of gas blown into the vacuum chamber. The present inventors also investigated this point, and as a result, as shown in FIG. 4, in the extremely low carbon concentration region, the gas injection amount in the vacuum chamber was 0.1 Nm ³ / min per molten steel amount t in the vacuum chamber. It became clear that the above was essential. However, in FIG. 4, the gas flow velocity is a constant value of 2000 Nm / sec.

Further, the present inventors have also found that the influence of the gas injection on the decarburization is correlated with the carbon concentration in the steel bath. In other words, in the range of [C]> 50 ppm, which has been conventionally attempted, the above-mentioned injection of the inert gas is not effective in promoting decarburization. It became clear that it was not good for the decarburization reaction. Therefore, in the method of the present invention, the amount of inert gas to be blown is reduced at a time when decarburization is rate-determined by the ring flow rate, and the required amount of inert gas is blown only to the extremely low carbon region.
Also in this case, the method of the present invention is much more advantageous than the conventional method.

That is, in the conventional method, if an attempt is made to secure the amount of inert gas required in the extremely low carbon region, the gas cannot be reduced in a region having a higher carbon concentration where the injection of inert gas hinders decarburization. This is because it is not possible to secure a necessary gas line flow velocity in order to prevent molten steel from entering. On the other hand, in the method of the present invention, when the tuyere diameter is reduced and the gas flow rate is increased, the method of blowing gas at a high pressure is used, so that the gas flow rate can be easily reduced.

In a normal steelmaking process, the tuyere diameter that satisfies the above conditions and the pressure of the gas to be blown in the ultra-low carbon region are each 3 mm.
It is desirable that the diameter be φ or less and 20 kgf / cm ² or more.

In addition, in areas where the carbon concentration is high (when CO gas frequently occurs), injecting such a large flow rate of gas causes a large amount of splash,
Metal adhesion to the vacuum chamber is also large, but in the extremely low carbon region, there is little splash even when a large flow of Ar gas is injected,
It was confirmed that there was no problem with ingot adhesion.

〔Example〕

An example in which a 230 t RH vacuum degassing tank is used is shown.

As shown in FIG. 1, the reflux tube 4 at the lower end of the vacuum chamber 3 is
A known RH method is a method in which the molten steel is immersed in the upper part of 230 tons of molten steel 2 in the ladle 1, blows Ar gas through the Ar gas introduction pipe 5 for reflux, and flows the molten steel into the vacuum chamber by the lift pump effect.

Now, four 3 mm tuyeres 6 were attached to the side of the furnace wall in the vacuum chamber. [C] = 400ppm, [O] = 450ppm molten steel
Rimed decarburization was performed by the treatment method. In the high coal area, gas was supplied from tuyeres at 300 Nl / min.

Normal decarburization treatment is performed, and when the decarburization amount is estimated from the generation rates of CO and CO _{2 in} the exhaust gas and it becomes equivalent to [C] = 30 ppm, the blow gas pressure is increased to 25 kg / cm ² , Tuyere 6
The processing was continued by increasing the flow rate of Ar from the furnace to 4 Nm ³ / min.

FIG. 5 shows the time transition of [C] during processing. It is clear that the decarburization reaction in the extremely low carbon region can be advanced more quickly than the conventional ordinary treatment.

[Effects of the Invention] According to the method of the present invention, decarburization in an extremely low carbon region can be rapidly performed, and an extremely low carbon steel having a carbon content of 10 ppm or less can be stably obtained. Splash generation in the vacuum chamber can be reduced, and thus adhesion to the vacuum chamber can be prevented.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing one embodiment of equipment for implementing the present invention,
FIG. 2 is an explanatory view showing the gas penetration distance at the tuyere tip, FIG.
Fig. 4 is a graph showing the relationship between the tuyere tip gas line flow velocity and the decarburization rate constant, Fig. 4 is a graph illustrating the relationship between the gas injection amount and the rate constant, and Fig. 5 illustrates the effect of the present invention. It is a graph. 1 ... Molten steel ladle 2 ... Molten steel 3 ... Vacuum tank 4 ... Reflux pipe 5 ... Reflux Ar gas inlet pipe 6 ... Tuyere

──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Toshikazu Sakuraya 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corporation Research and Development Headquarters (72) Inventor Tetsuya Fujii 1-Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Corp. (56) Reference R & D Kobe Steel Engineering Reports Vol. 36, No. 1, (1986) PP. 40-42 (58) Field surveyed (Int. Cl. ⁶ , DB name) C21C 7/10 C22B 9/04

Claims (3)

(57) [Claims] 1. A method for vacuum degassing undeoxidized or weakly deoxidized molten steel melted in a steelmaking furnace, comprising: forming a molten steel in a vacuum degassing tank through a tuyere disposed on a side of a vacuum tank furnace wall; The treatment was performed while blowing inert gas, and when the carbon concentration in the steel bath became 30 ppm or less, the flow velocity at the tuyere tip became 1
A method for melting ultra-low carbon steel by vacuum degassing, characterized by blowing an inert gas at a flow rate of 500 Nm / sec or more. 2. When the carbon concentration in the steel bath becomes 30 ppm or less, the flow rate of the blown gas per ton of molten steel in the vacuum chamber is set to 0.1 Nm ³ /
The method for melting ultra-low carbon steel by vacuum degassing according to claim 1, wherein the amount is increased to at least one minute. 3. The tuyere diameter provided on the side of the vacuum chamber furnace wall is 3 m.
m or less, and the pressure of the blown gas at the time when the carbon concentration in the steel bath is 30 ppm or less is set to 20 kgf / cm ² or more. Melting method of carbon steel.