JPH11315319A - Method for blowing oxygen into molten metal under reduced pressure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the spattered quantity of molten iron itself such as splash and dust generated from a collided part of a jet stream and to improve the trouble such as the contamination of the molten iron caused by skull sticking by using a top-blown lance where the nozzle shape at the tip part thereof becomes a slit type. SOLUTION: In a method for blowing gaseous oxygen to molten metal under a reduced pressure from a top-blown lance, the top-blown lance in which the shape of a nozzle 1 t the tip part of the lance becomes a slit type, a prescribed rectangular type slit nozzle is arranged and a ratio Se/St of cross sectional area Se at the outlet part to cross sectional area St at a throat part 2 of the nozzle 1 at the top part of the lance is <1.1, is used. Further, desirably, the reducing effect of a jetting speed is remarkable by making the ratio B/h of the length B of the long side and the length (h) of the short side to >=4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a refining method for blowing oxygen gas onto a molten metal under reduced pressure from an upper blowing lance.

[0002]

2. Description of the Related Art During decarburization and refining of molten iron in a vacuum refining vessel such as RH and VOD, a method of blowing oxygen required for decarburization from an upper blowing lance has been used. In RH used for refining ordinary steel, oxygen is appropriately supplied for the purpose of improving decarburization efficiency, shortening the decarburization time, raising the temperature of molten steel, etc., but in VOD used for refining stainless steel, Since a part of oxygen in molten steel is consumed for burning Cr, oxygen supply is essential.

[0003] However, under reduced pressure, the ratio of the pressure on the nozzle entrance side to the ambient pressure increases, and the flow velocity during discharge of the lance increases, so that the kinetic energy of the jet impinging on the surface of the molten iron increases as compared with the atmospheric pressure. . Therefore, the amount of splashing of molten iron such as splash and dust caused by the collision of the jet increases, the operability is deteriorated due to the adhesion of metal to the lance, the inner wall of the container and the exhaust gas duct, the yield is reduced due to dust generation, and the molten metal is contaminated by the adhered metal. The problem has arisen.

[0004] In order to solve these problems, various methods and apparatuses have been conventionally disclosed. For example, a method of injecting an inert gas from an upper blowing lance to prevent splash from adhering to the inner wall of the container (described in JP-A-3-226515), a method in which a Laval nozzle (= medium-thin type nozzle) at the tip of the lance and A method of separately increasing the decarburization efficiency and increasing the heat by secondary combustion by changing the degree of vacuum with respect to the nozzle pressure ratio determined from the area ratio of the discharge holes (Japanese Patent Application Laid-Open No. 5-9555) Lance having a fuel gas supply hole at the end of the oxygen blowing section (described in JP-A-6-73431), and oxygen for secondary combustion of exhaust gas separately from oxygen from the upper blowing lance to molten steel. A method of blowing so as not to hit directly (JP-A-7-126737)
Described in Japanese Unexamined Patent Publication (KOKAI) No. 2000-214, etc.).

[0005]

However, Japanese Patent Laid-Open Publication No. Hei 3-2
The method described in Japanese Patent No. 26515 has problems such as an increase in the cost of the inert gas and an increase in the load on the vacuum exhaust due to the blowing of the inert gas. On the other hand, JP-A-5-9555,
In each of the methods described in JP-A-6-73431 and JP-A-7-126737, the inner wall of the container is heated to a high temperature by actively promoting secondary combustion and the heat of combustion of the fuel gas to prevent the adhesion of splash. This is a method in which the adhered metal is melted. These methods have problems such as adverse effects on the refractories in the container at high temperature, high temperature of exhaust gas, deterioration of oxygen gas efficiency, and increase in fuel gas cost.

In the present invention, the problem of the prior art described above is set apart from the concept of preventing the splash adhesion of the prior art, and the amount of molten iron scattered such as splash or dust generated from the collision portion of the jet flow. It is an object to provide a method for reducing itself. In addition, this will not only increase gas costs and adversely affect refractories, but will also degrade operability due to sticking of metal to the lance, vessel inner wall, and exhaust gas duct, reduce yield due to dust, and contaminate molten iron with the sticking metal. The aim is to improve.

[0007]

Means for Solving the Problems In order to solve the above-mentioned problems, the inventors of the present invention have described a jet flow velocity under reduced pressure.
As a result of intensive research, they have found the following.

[0008] Compared with a circular nozzle normally used for an upper-blown oxygen lance, a slit-shaped nozzle can maintain a low jet flow velocity on the surface of molten steel. this is,
This is because, in the slit type nozzle, the jet flow spreads greatly, and the flow velocity distribution spreads in the horizontal direction and becomes smoother and the flow velocity becomes lower than that of a circular nozzle having the same cross-sectional area at the same distance from the nozzle tip at the same position. . In vacuum refining vessels, the upper limit of the distance between the top-blown oxygen nozzle and the surface of the molten steel is restricted, and the jet flow velocity on the molten steel surface is significantly higher than under normal pressure, so the effect of the slit type nozzle is exhibited. Is done.

In order to maintain a low jet flow velocity over a wide range of conditions of vacuum and oxygen gas flow by refining under reduced pressure in which the degree of vacuum and oxygen gas flow greatly fluctuate, the cross-sectional area St of the throat 2 and the outlet Sectional area Se of part 3 (see FIG. 1)
Nozzles having a smaller ratio Se / St than nozzles having a larger ratio are more suitable. This is because, in a nozzle having a small Se / St, the jet flow is always discharged in an insufficiently expanded state at the outlet, so that an expansion wave is generated and energy is lost. On the other hand, if a nozzle having a large ratio Se / St between the cross-sectional area St of the throat portion and the cross-sectional area Se of the outlet portion is used, the jet flows in the outlet portion in an over-expanded state under certain conditions, so that a shock wave is generated. Although the energy loss is large, the refining under reduced pressure, where the degree of vacuum and the oxygen gas flow rate fluctuate greatly, is designed to design a nozzle in which the jet at the outlet is over-expanded over a wide range of conditions. The cross-sectional area of the outlet part becomes too large, so that it cannot be arranged on the lance, and measures such as enlarging the lance diameter must be taken.

Therefore, specific means of the present invention based on these findings are as follows. (1) A refining method in which oxygen gas is blown from an upper blowing lance to a molten metal under reduced pressure, wherein an upper blowing lance having a slit-shaped nozzle at the tip of the lance is used. Oxygen blowing method. (2) Cross-sectional area St of the throat of the nozzle at the tip of the lance
The method according to (1), wherein an upper blowing lance having a ratio Se / St of 1.1 or less in sectional area Se of the outlet and the outlet is used.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Here, the term "under reduced pressure" means 0.1 to 600.
It is defined as Torr. FIG. 1 and FIG. 2 are views showing examples of an upper blowing lance used in the present invention. FIG. 1 shows an upper blowing lance having a rectangular slit nozzle, and FIG. 2 shows an upper blowing lance having an annular slit nozzle. (A) is a view of the nozzle 1 in the AA section, and (b) is a plan view of the nozzle 1 at the tip of the lance.

In the present invention, by blowing oxygen gas onto the molten metal under reduced pressure using the upper blowing lance having such a nozzle, the jet flow velocity of the oxygen gas on the surface of the molten steel is significantly reduced, and Problems such as deterioration in operability due to the occurrence of dust, deterioration in yield due to generation of dust, and contamination of molten iron by the deposited metal can be improved. Desirably, the ratio B / h of the length B of the long side and the length h of the short side of the nozzle shown in the drawing is 4 or more, so that the effect of reducing the jet flow velocity becomes remarkable.

Further, the ratio Se / St of the cross-sectional area St of the throat section 2 to the cross-sectional area Se of the outlet section 3 is set to 1.1 or less, so that the jet flow at the outlet section is always discharged in an insufficiently expanded state,
Due to the energy loss due to the generation of the expansion wave, the jet flow velocity reduction effect is improved.

[0014]

EXAMPLE A refining test of molten stainless steel was performed using a vacuum refining furnace, and a comparison was made between the method according to the present invention and the conventional method using a circular nozzle. Table 1 shows the design conditions of the nozzle, and FIG. 3 shows a drawing of the nozzle.

[0015] Only the design conditions of the nozzles of the present invention and comparative examples are different, and other operating conditions are almost the same. The initial molten steel amount is 500 kg, and the molten steel composition has an initial [C] of about 0.1 kg.
7%, initial [Cr] is about 11%, and [C] is about 0.1%.
Refining was performed until the content reached 01%. Acid transfer rate, lance
The distance between molten steels and the degree of vacuum were changed as shown in Table 2 according to [C] during refining. In addition, the amount of ingots generated during refining was evaluated by converting the amount of metal adhered to the refractory plate suspended on the furnace inner wall over the entire furnace inner wall, and the yield of molten steel before and after refining was calculated. The amount of dust generated was evaluated based on the amount of dust collected on the filter paper installed in the exhaust gas duct. Table 3 shows the test results.

[0016]

[Table 1]

[0017]

[Table 2]

[0018]

[Table 3]

As shown in Table 3, compared to the comparative example, in the present invention example, both the amount of ingot adhesion and the amount of dust generation were reduced, and the scattering of molten iron was suppressed, so that the molten steel yield was improved. further,
The ratio Se of the cross-sectional area St of the throat to the cross-sectional area Se of the outlet
When / St is 1.1 or less, the effect is more remarkable. On the other hand, according to the method of the present invention, the decarbonation efficiency and C
There was no tendency for the amount of r loss to deteriorate.

[0020]

According to the method of the present invention, it is possible to remarkably reduce the jet flow velocity by blowing oxygen gas onto the molten metal under reduced pressure. Thereby, scattering of molten iron can be suppressed, and problems such as deterioration of operability due to adhesion of metal, deterioration of yield due to generation of dust, and contamination of molten iron by the adhered metal can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a nozzle shape of a lance tip used in the present invention.

FIG. 2 is a view showing an example of a nozzle shape of a lance tip used in the present invention.

FIG. 3 is a view showing a nozzle shape of each lance described in the embodiment.

[Explanation of symbols]

 1 Nozzle 2 Throat 3 Exit (discharge hole)

Claims (2)

[Claims] 1. A refining method for blowing oxygen gas onto a molten metal under reduced pressure from an upper blowing lance, wherein an upper blowing lance having a slit-shaped nozzle at the tip of the lance is used. Oxygen blowing method for molten metal. 2. The ratio Se / St of the sectional area St of the throat part of the nozzle at the tip of the lance to the sectional area Se of the outlet part is 1.1.
2. The method for blowing oxygen under molten metal under reduced pressure according to claim 1, wherein the following blow lance is used.