JPH051319A - Vacuum treatment apparatus for steel - Google Patents

Abstract

PURPOSE:To shorten the necessary treating time in an RH vacuum treatment apparatus. CONSTITUTION:This RH vacuum treatment apparatus sets gas blowing pipes 4 for circulating molten steel in an uptake tube 2a as diagonally upward to inner part of the uptake tube. The inclined angle - or + is desirably 20 to 50 deg.. The molten steel circulating rate can be increased even if the gas for circulating is not increased or the inner diameter of immersion tubes is not enlarged, etc. The vacuum treatment time of molten steel is shortened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an RH vacuum processing apparatus used for melting high purity steel.

[0002]

2. Description of the Related Art As shown in FIG. 1, an RH vacuum processing apparatus comprises a vacuum chamber 1 and two immersion pipes (a rising pipe 2a and a descending pipe 2b) provided under the vacuum chamber 1. In using this, the ascending pipe 2a and the descending pipe 2b are immersed in the crude molten steel (M) contained in the ladle 3, the inside of the vacuum tank 1 is decompressed, and the molten steel is pulled up into the vacuum tank. Then, a gas such as Ar is blown from the gas blowing pipe 4 provided in the rising pipe 2a, the molten steel is lifted by the principle of the gas lift pump, and it is sent into the vacuum tank for vacuum treatment, and returned from the falling pipe 2b into the ladle. . in this way,
Vacuum processing is continuously performed by circulating molten steel.

In order to increase the processing speed in this RH vacuum processing, it is necessary to increase the flow rate of molten steel per unit time.

The annular flow rate W (t / min) of molten steel is obtained by the following equation.

W = k · n ^0.25 · D ^1.5 · Qg ^0.2 · h ^0.5 (1) where k: constant n: number of gas injection tubes D: inner diameter of immersion tube (m) Qg: reflux gas flow rate (Nm ³ / min) h: Distance (m) from the opening of the gas blowing pipe to the molten metal surface in the vacuum tank.

From the equation (1), it is found that increasing the inner diameter (D) of the immersion pipe is the most effective for increasing the flow rate of molten steel. However, although it has been confirmed that the molten steel ring flow rate increases with an increase in the inner diameter of the immersion pipe, at the same time, there arises a problem that the melting loss of the refractory on the inner wall of the immersion pipe becomes faster.
This increase in refractory melting loss is due to the following causes. That is, the flow of molten steel in the immersion pipe is not a fully developed Poiseuille flow, but an advancing flow having a trapezoidal velocity shape. In this run-up flow, the velocity near the inner wall increases as the inner diameter of the immersion pipe increases. That is, it has the characteristic of increasing the velocity gradient. The increase in the velocity gradient increases the shearing force acting on the refractory surface of the inner wall of the dip tube and promotes the melting loss of the refractory. Therefore, the expansion of the inner diameter (D) of the dip tube goes against the desire to extend the life of the apparatus by reducing the melting loss of the refractory on the inner surface of the dip tube as much as possible.

The molten steel ring flow rate also increases with an increase in the number (n) of gas blowing pipes and an increase in the distance (h) from the opening of the gas blowing pipes to the molten metal surface in the vacuum tank, but their contribution is (1 As you can see from the equation)
Since it is small, it cannot be expected to have a great effect. In addition, such a measure causes a large modification of the vacuum processing apparatus and causes an increase in equipment cost. Furthermore, the contribution of the circulating gas flow rate (Qg) to the increase in the molten steel ring flow rate is also small at the power of 0.2, as can be seen from Eq. (1). Moreover, if the circulating gas flow rate is increased above a certain critical value, the molten steel circulating flow rate will be decreased.

[0008]

In order to increase the molten steel treatment rate in RH, it is essential to increase the molten steel ring flow rate. However, the measures that can be conceived from the above formula (1) are not practical because they are accompanied by adverse effects such as increased melting loss of the refractory material or the effects are insufficient.

The present invention has been made with the object of developing a practical technique capable of increasing the flow rate of molten steel ring without the above-mentioned harmful effects.

[0010]

The gist of the present invention is that "a gas injection tube for molten steel recirculation of an ascending pipe is arranged obliquely upward toward the inside of the ascending pipe.
Vacuum processing equipment ”. The inclination angle of the gas blowing tube is preferably in the range of 20 ° to -50 ° with respect to the horizontal plane.

The present invention is based on the idea of increasing the recirculation flow rate of molten steel by effectively blowing recirculation gas without being bound by the factors involved in the above equation (1). That is, when the reflux gas is blown obliquely upward toward the inside of the rising pipe of the RH vacuum treatment, a velocity component in the same direction as the reflux velocity of the molten steel can be imparted to the gas.
The molten steel ring flow rate per unit time can be increased.

FIG. 1 is a vertical sectional view of a vacuum processing apparatus of the present invention,
FIG. 2 is an enlarged sectional view of the main part.

As shown in the figure, in this apparatus, the gas blowing pipe 4 of the rising pipe 2a is provided obliquely upward toward the inside. The angle (θ) between the gas blowing pipe and the horizontal surface (the surface perpendicular to the molten steel recirculation direction) is effective even if it is small, but it is set to 20 ° to 50 ° as shown in Examples described later. Is good.

In the present invention, various factors of the above formula (1), namely, the number of gas blowing pipes, the inner diameter of the dipping pipe, the circulating gas flow rate, and the distance from the opening of the gas blowing pipe to the molten metal surface in the vacuum tank You may use it together with the measure to increase one or more of the above.

[0015]

In general, the circulating gas blown into the rising pipe of the RH vacuum processing apparatus is blown vertically to the inner wall of the dip pipe, so it is also blown vertically to the molten steel flow. . Therefore, until the rising speed of the gas bubbles becomes equal to or higher than the circulating (raising) speed of the molten steel, the gas bubbles rather hinder the circulating (raising) of the molten steel.

In the apparatus of the present invention, since the circulating gas can be blown obliquely upward into the rising pipe, the circulating gas can be given a velocity in the same direction as the molten steel flow to promote the circulating flow of the molten steel. In this case, if the reflux gas blowing angle (θ) is too small, the effect of promoting the reflux of molten steel is small. On the other hand, if θ is too large, it is difficult for the bubbles of the circulating gas to reach the center of the rising pipe, only the rising of the molten steel near the inner wall of the rising pipe is promoted, and the circulation of the molten steel near the center of the rising pipe is not promoted.

By blowing the circulating gas obliquely upward, the projected area of the gas bubble region occupying the horizontal cross section of the rising pipe becomes small near the opening in the rising pipe of the gas blowing pipe.

Therefore, the effective cross-sectional area of the molten steel that recirculates increases, which is more advantageous for promoting the circulation of the molten steel.

[0019]

EXAMPLE In the RH vacuum processing apparatus shown in FIGS. 1 and 2, the required processing time was compared by changing the angle θ in FIG. 2 in the range of 0 to 60 °. The molten steel used is carbon steel,
The various conditions are as shown below.

Amount of molten steel: 250 ton / initial charge [C]: about 300 ppm Temperature of molten steel: 1590 ° C to 1630 ° C Required treatment time: Table 1 shows the time test results until the [C] in the molten steel becomes 15 ppm or less. Comparative example No. 1 and No. 2
Is the one in which the blowing tube is horizontally attached as in the conventional case with θ = 0. In No. 2, the reflux gas flow rate is increased.

[0021]

[Table 1]

In No. 1 of the comparative example, the value of [C] in molten steel was 1
The processing time required to reduce the concentration to 5 ppm or less was 20 minutes. Reflux 2.0 Nm ³ / min from the necessary processing time even try to increase the 2.6 Nm ³ / min as No.2 in the gas flow rate Qg No.1 is only was shortened by one minute. Since the amount of expensive Ar gas used increases and the degree of vacuum in the vacuum chamber deteriorates, this method of increasing the reflux gas flow rate is not practical.

Nos. 3 to 8 are examples of the present invention. θ is 1
No. 3, which is as small as 0 °, does not sufficiently promote the circulation of molten steel and the effect of shortening the treatment time is small, but it is as effective as increasing the reflux gas flow rate as in Comparative Example No. 2. .

No. 4 with a circulating gas injection angle of 20 ° to 50 °
In Nos. 7 to 7, the reflux of molten steel is promoted, and the treatment time can be shortened to 16 to 17 minutes. In Example No. 4, the treatment time could be shortened to about 16 minutes by setting the circulating gas injection direction at an angle of about 20 ° from the center of the cross section of the dipping pipe and swirling the molten steel. all right.

When No. 8 is obtained in which the angle θ is further increased to 60 °, the processing time is conversely lengthened to 19 minutes. This is θ
The horizontal reaching distance of the bubbles of the circulating gas
(Distance that affects the direction of the center of the riser) becomes shorter,
It is considered that the region where the reflux velocity of the molten steel flow was small increased near the center of the rising pipe.

From the above results, it was confirmed that the molten steel ring flow rate in the RH vacuum processing apparatus can be increased by blowing the circulating gas obliquely upward. It was also found that the effect was further enhanced by setting the angle of the circulating gas injection to 20 ° to 50 °.

[0027]

According to the present invention, the molten steel ring is not required to be increased in the amount of recirculating gas or the inner diameter of the immersion pipe only by changing the mounting angle of the reflux gas blowing pipe of the rising pipe. The flow rate can be increased. Since the vacuum processing time of molten steel can be shortened by using the apparatus of the present invention, there are great advantages such as improvement in working efficiency and reduction in molten steel temperature decrease.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing an embodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view of a main part of FIG.

Claims (2)

[Claims] 1. A gas injection pipe for molten steel circulation of an ascending pipe,
An RH vacuum processing apparatus, which is arranged obliquely upward toward the inside of the rising pipe. 2. The RH vacuum processing apparatus according to claim 1, wherein the inclination angle of the rising pipe with respect to the horizontal plane is in the range of 20 ° to 50 °.