JPH06212241A - Method for vacuum-refining molten steel by using large diameter immersion tube - Google Patents

Abstract

PURPOSE:To promote the effective contact of a refining agent with molten steel and to improve the refining effect by supplying stirring gas from a fixed depth from the molten steel surface and adding the refining agent on bubble activating surface. CONSTITUTION:A large diameter immersion tube 2 reducing the pressure is dipped into the molten steel 4 in a ladle 1 and stirring gas 5 is supplied from a porous plug 3 at the bottom part of the ladle 1. Then the stirring gas is supplied from the position being deeper than 0.5 H to the molten steel height H. Further, the bubble activating area (a) formed in the range diameter immersion tube 2 is made to be 40-95% and also, the flux for refining is added onto the molten steel surface in the large diameter immersion tube from a refining agent blowing lance 6. The refining agent grains are sufficiently stirred together with the molten steel in the large diameter immersion tube 2 and floated up and separated to the outer part from the lower end of the immersion tube 2 in order. Therefore, the refining agent for dephosphorization is added in the non-oxidizing condition, and after passing the several time, the deoxidizer is added and successively, even if the refining agent for desulfurization is added, without developing rephosphorization, the sulfurizing reaction can be progressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for desulfurizing and dephosphorizing molten steel in ladle refining using a large diameter immersion pipe.

[0002]

2. Description of the Related Art Conventionally, it has been difficult to obtain an ultra-low sulfur steel only by desulfurizing hot metal in producing ultra-low carbon / ultra-low sulfur and ultra-low phosphorus steel, especially in the case of desulfurization. . Therefore, it is necessary to perform desulfurization at the secondary refining stage after tapping steel from the steelmaking furnace. Therefore, when manufacturing ultra-low carbon / ultra-low sulfur steel, for example, molten steel refined in a converter is decarburized by vacuum degassing treatment such as RH method, and then deoxidized by adding Al, Si, etc., Subsequently, a flux containing CaO and CaF ₂ as main components is added in the same equipment and stirred to produce ultra-low carbon / ultra-low sulfur molten steel. However, the addition of the above-mentioned flux not only increases the melting loss of the refractory material in the vacuum degassing tank, but also increases the refractory cost, and the melting loss makes stable operation difficult, resulting in the production of ultra-low sulfur steel. There is a problem that it becomes difficult. Further, in general, the refining agent addition method by the RH method is basically considered to have a weak downflow,
In the RH method, this is a bubble pump that uses the head difference based on the bubble density difference between the ascending pipe side and the descending pipe side as the driving force.
This is because the flow velocity of the downflow is limited even if the amount of gas for circulation is increased. Therefore, the flux added in the vacuum tank does not reach the deep position of the molten steel in the ladle. As a method of increasing the efficiency for this purpose, for example, as in JP-A-61-30618, while a reverse liquid preventing gas is jetted from a J-shaped immersion lance provided in the lower portion of the rising pipe, a desulfurizing agent is added together with a carrier gas. It is to start the blow-in vacuum treatment, prevent the molten steel from scattering to the nozzle hole diameter, and perform the vacuum degassing injection operation with high capacity. Further, in JP-A-61-1815, by blowing a desulfurizing agent together with a carrier gas of an inert gas from a tuyere provided in a lower portion of an RH vacuum chamber, a molten steel is efficiently contacted with a powder refining agent. It is possible to refine the impurities by reacting them. Further, in JP-A-60-184618 and JP-A-57-150545, a RH apparatus is used under a reduced pressure, and at the time of this pressure reduction, an oxidizer powder is blown up.
Decarburize this, promote denitrification that occurs at this time, shorten the processing time, prevent the molten steel temperature from lowering due to this,
And a decompression refining device for injecting a refining agent by providing a decompression device with a dissolution tank, and providing a long-powder lance capable of moving up and down in the dissolution tank from above the decompression device.

[0003]

When the refining agent is blown into the vacuum refining apparatus as described above, the probability that the refining agent floats while being trapped in the bubbles formed by the carrier gas is extremely high. Moreover, particularly in the case of the RH method, the flux after floating in the tank rides on the circulating flow and is discharged to the outside of the tank within a short time, so that the molten steel and the refining agent do not contact effectively. For example, in JP-A-61-30618 and JP-A-61-12815, there is a problem that a violent splash occurs. Therefore, the effect of spraying the powder on the molten steel in the tank by the RH method can be utilized for denitrification, in which the reaction is instantaneously performed immediately below the surface and thus the effect is improved.
Although there is a technique like Japanese Patent No. 84618, there is a drawback that it cannot be applied to desulfurization refining. Also, the actual exploitation Sho 57-1505
When the refining agent is blown in a ladle as disclosed in Japanese Patent No. 45, the gas feed position is shallow because there are many splashes and the blowing position and the stirring gas feed hole are the same, and the floated flux is less likely to be suspended in molten steel again, which is effective. There are various drawbacks that are bad.

[0004]

DISCLOSURE OF THE INVENTION The inventors of the present invention have made detailed investigations and experiments to eliminate the drawbacks of the conventional degassing methods for molten steel, and as a result, in a vacuum refining apparatus, A refining agent is added to the surface of the molten steel in the vacuum tank while supplying a stirring gas from a deep position with a certain depth to the height of the molten steel and forming a certain bubble activation surface in the immersion pipe. By doing so, it has been newly found that the refining agent effectively contacts the molten steel and improves the refining effect.
The gist is (1) in a vacuum refining method in which a large-diameter immersion pipe is immersed in molten steel in a ladle to decompress the large-diameter immersion pipe, and a stirring gas is supplied from the bottom of the ladle. The stirring gas is supplied from a position deeper than 0.5H with respect to the molten steel height (H) in the ladle,
In addition, the bubble activation area formed in the large diameter immersion pipe is 40
A vacuum refining method for molten steel using a large-diameter dip tube in which the refining flux is added to the surface of the molten steel in the large-diameter dip tube while adjusting to 95%. (2) Add a dephosphorizing refining agent to the undeoxidized molten steel to proceed the dephosphorization reaction, then add a deoxidizing agent to deoxidize it, and then add a desulfurizing refining agent to desulfurize The method of vacuum refining molten steel using a large-diameter immersion pipe according to claim 1 for carrying out.

[0005]

The present invention will be described in detail below with reference to the drawings. FIG. 1 is an explanatory view of an embodiment of the present invention, in which molten steel 4
Is accommodated in the ladle 1, and the large-diameter dip tube 2 is immersed in the molten steel 4 in the ladle 1 and stopped. The large diameter immersion pipe 2 communicates with the exhaust pipe,
The molten steel 4 is sucked up into the large diameter immersion pipe 2 according to the degree of vacuum in the large diameter immersion pipe 2. Then, the inert gas 5 is blown into the molten steel from the porous plug 3 arranged at the bottom of the ladle 1 in which the lower cross section of the large diameter immersion pipe 2 hits vertically downward, and the molten steel 4
Are mixed by stirring. On the other hand, a refining agent blowing lance 6 is provided from above the large-diameter immersion pipe 2, and powder and carrier gas are blown from the refining agent blowing lance. In this case, as a result of repeated experiments using a water model and numerical calculations on the flow condition of molten steel in a ladle from a deep position under gas agitation, when the molten steel head is high, the molten steel circulating force due to the buoyancy of the bubbles is extremely opposite. We obtained a new finding that a strong downward flow is formed in the water. Therefore, it was found that almost all of the refining agent added to the surface of the molten steel in the vacuum tank due to the formation of this downward flow was suspended in the molten steel and had the same effect as when it was blown.
Further, in this case, since the refining agent is not trapped by the carrier gas, the refining agent and molten steel are in good contact with each other, which improves desulfurization efficiency. Moreover, the converter slag with a high iron oxide concentration flows out into the gap between the dipping tank and the ladle by stirring for a short time, even if the refining agent after the desulfurization reaction flows out of the large diameter dipping tank. Since this gap portion is hardly agitated, the reaction speed is slow, and therefore the vulcanization reaction does not occur.

Therefore, as a result of various experiments on the supply position of the stirring gas for generating the strong downward flow as described above, when the molten steel height in the ladle (H) is set, the molten steel height in the ladle is determined. It is necessary to supply the stirring gas from a position deeper than 0.5H, and the gas amount is 0.6 to 1.5 Nl (m
It has been confirmed that it is necessary to set in. Further, in order for the refining agent on the vacuum surface to be caught in the molten steel, the bubble active area needs to be 40 to 95%. Here, the bubble active area is defined as a region where blown gas bubbles float on the surface. Regarding the bubble active area, the bubble active area (An) for the gas blown in the vertical direction is expressed by the equation (1) according to the observation result in the water model, the mercury model, or the actual machine. The bubble active area (Au) is given by the equation (2). An = 3.14 × (0.212 × H ) 2 ‥‥‥‥ (1) Au = 3.14 × (7 × Q 0 · 67) 2/2 ‥‥‥‥ (2) where, H is Distance from injection position to molten steel surface (m)
And Q is the gas injection amount per nozzle (Nm ³
/ S). Thus, the bubble active area is (1)
It can be represented by (2). Further, the bubble active area ratio is expressed as (bubble active area / vacuum surface area) × 100%.
Here, the vacuum surface area means the area of the vacuum surface area A shown in FIGS. 1 and 2, and the bubble active area is the area where the gas blown by the stirring gas from the bottom of the ladle floats and floats on the surface of the vacuum molten steel. Display with a. This bubble active area is 40-95%
Need. The reason for this is that in the bubble active area, the refining flux on the vacuum surface is separated as fine particles when the bubbles burst and is caught in the molten steel. As a result, the fine flux and S and P in the molten steel come into active contact with each other to promote the reaction with the flux. This relationship is shown in FIG. FIG. 3 shows the relationship between the bubble active area ratio and the S removal rate. In this case, the bubble active area ratio is 40%.
If it is less than%, the bubble activating surface is small, and the flux is made finer and the entrainment thereof is small, so that the S removal efficiency is extremely poor. On the other hand, if the bubble active area ratio is too large and exceeds 95%, the entrained downflow becomes small, and therefore the reaction does not proceed.
% To 95%.

Further, as a preferable condition, a position below the surface of the vacuum chamber is taken as a boundary, and the inside of the large-diameter immersion tank above the vacuum chamber is expanded to cause a small reversal flow at that portion, and the entrained flux is involved. It was found that a better and better effective reaction can be obtained by increasing the residence time in molten steel. Regarding the flow state of the molten steel, there is a horizontal flow toward the wall on the surface of the vacuum chamber, which hits the wall surface and becomes a downward flow. The refining agent is dispersed in the molten steel by riding on this downward flow. When the refining agent particles thus entrained pass through the lower end of the large-diameter dipping tank, they are carried by the flow to the bottom of the ladle, and are further entrained and particles that float to the outside of the large-diameter dipping tank by buoyancy. Divide. But,
In this case, if the downward flow is extremely weak, the probability of going out of the large diameter immersion tank increases, but if the gas flow rate is reduced to weaken the downward flow, the refining agent suddenly becomes difficult to be involved. . FIG. 2 is an explanatory view showing another embodiment according to the present invention. It is basically the same as that of FIG. 1, but is characterized in that an upper expanded portion 7 of the large diameter immersion pipe 2 is provided. That is, by slightly expanding the upper part of the large-diameter immersion pipe 2 in this way, a downward flow is weakened without weakening the flow for entraining the refining agent by creating a vortex of a reversal flow near the wall surface of this part and increasing the large diameter. The probability of flowing out of the immersion pipe can be reduced. Further, since the refining agent is trapped in this reversal flow, the refining agent can efficiently contact with the molten steel, and the refining efficiency is further improved. As the refining flux according to the present invention, it is preferable to use quick lime or a mixture of quick lime and fluorite. Further, as a method of supplying the refining flux, there is a method of charging from above, preferably a method of spraying from a top blowing lance which can add fine particle flux without generation of splash.

As described above, in the case of the present invention, the refining agent particles once caught are floated and separated to the outside of the large diameter immersion pipe with a certain probability when passing through the lower end of the large diameter immersion pipe. Therefore, most of the refining agent will flow out of the large-diameter dip pipe after a certain time has passed after the addition of the refining agent. On the other hand, the progress of the reaction is extremely slow because there is little stirring in the gap between the outside of the large diameter dip tube and the ladle. For this reason, the dephosphorizing refining agent is added in an undeoxidized state, and after a certain period of time, deoxidizing is performed by adding the deoxidizing agent, and even if the desulfurizing refining agent is subsequently added, it is added first. Most of the refining agents for desulfurization thus obtained are floated to the outside of the large-diameter immersion pipe, and therefore, the desulfurization reaction can proceed without causing rephosphorization. Here, the undeoxidized state defines a state in which the concentration of dissolved oxygen in the molten steel is 200 ppm or more. As the dephosphorizing refining agent, there are quicklime and iron oxide, or quicklime, and a mixture of fluorite and iron oxide.

There are the following two specific steps described above. First, after adding the dephosphorization refining agent in a vacuum state, the dephosphorization refining agent in the large-diameter immersion pipe is stirred for a period of time just discharged to the outside of the tank, and then deoxidizing agents such as Al, Si, and Mn are added. The step of adding the desulfurization refining agent in a vacuum state, and the second step is to add the refining agent for dephosphorization in a vacuum state to allow the dephosphorization reaction to proceed sufficiently, then reduce the degree of vacuum and dip it in a large diameter. Stir in a state where the distance between the molten steel surface and the lower end of the large diameter immersion under vacuum in the pipe is agitated to make it easier for the dephosphorizing agent to flow out of the large diameter immersion pipe, and then make the vacuum state again to remove it by adding a deoxidizing agent. With acid,
There is a method of subsequently performing a desulfurization step by adding a desulfurizing agent.

[0010]

【Example】

Example 1 Using a 175 ton ladle, the vacuum refining furnace shown in FIG. 1 was used. In any case, the carbon concentration before decarburization is 40
It is 0 ppm, and the carbon concentration after decarburization is 5 to 20 pp.
After deoxidizing by adding Al to the molten steel in the range of m, various fluxes of the stirring gas were changed and the flux mixed with quick lime and fluorite at a weight ratio of 8: 2 to 5: 5 was sprayed on the powder,
Table 1 shows the desulfurization rates as a result of the powder injection or the injection of the lumps together with the comparative examples. As a result, the desulfurization rate is very high in the case of the method of the present invention as compared with the comparative example in which the stirring gas blowing position is less than 0.5H or the RH method and the conditions in which the bubble active area is less than 40%. I understand.

[0011]

[Table 1]

Example 2 As in Example 1, using a 175-ton ladle, the operation was carried out in the vacuum refining furnace shown in FIG. In each case, the carbon concentration before treatment was 400 ppm, and the dissolved oxygen concentration was 500 pp.
m, the carbon concentration after the treatment was in the range of 5 to 20 ppm, the stirring gas blowing position was 1.0H, and the mixed powder of quick lime, fluorite, and iron oxide was blown to the molten steel to perform dephosphorization treatment. , Al were added for deoxidation. Then, the desulfurization rate and the desulfurization rate when the flux mixed with the weight ratio of quicklime and fluorite at a weight ratio of 8: 2 to 5: 5 was sprayed while changing the distance between the molten steel surface in the tank before deoxidation and the lower end of the large diameter immersion pipe The phosphorus ratio is shown in Table 2 together with the comparative example. As a result, the desulfurization rate and the desulfurization rate according to the method of the present invention were compared with those of the comparative example in which the stirring gas blowing position was less than 0.5H, and the ladle degassing method and the comparative example in the case where the bubble active area was less than 40%. It can be seen that the phosphorus rates are very high.

[0013]

[Table 2]

[0014]

As described above, by carrying out the present invention, it is possible to proceed desulfurization reaction and dephosphorization reaction with extremely high efficiency without causing a problem in operation due to violent splash generation. Therefore, it has an extremely excellent industrial effect as an efficient refining method.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an embodiment according to the present invention,

FIG. 2 is an explanatory view showing another embodiment according to the present invention,

FIG. 3 is a diagram showing a relationship between a bubble active area ratio and a de-S rate.

[Explanation of symbols]

 1 ladle, 2 large diameter immersion pipe, 3 porous plug, 4 molten steel, 5 inert gas, 6 refining agent blowing lance, 7 large diameter immersion pipe lower spreading part, A vacuum surface area, a bubble active area.

Claims (2)

[Claims] 1. A molten steel in a ladle in a vacuum refining method in which a large-diameter immersion pipe is immersed in molten steel in the ladle to decompress the large-diameter immersion pipe, and a stirring gas is supplied from a lower portion of the ladle. The stirring gas is supplied from a position deeper than 0.5H with respect to the height (H), and the bubble active area formed in the large diameter immersion pipe is set to 40 to 95% and the inside of the large diameter immersion pipe is A vacuum refining method for molten steel using a large diameter immersion pipe, characterized in that a refining flux is added to the surface of the molten steel. 2. A dephosphorizing refining agent is added to undeoxidized molten steel to allow the dephosphorization reaction to proceed, and then a deoxidizing agent is added to deoxidize, and subsequently a desulfurizing refining agent is added. The method for vacuum refining molten steel with a large-diameter dip tube according to claim 1, wherein desulfurization is performed.