JPH059554A - Oxygen refining method for molten metal under reduced pressure - Google Patents

Abstract

(57) [Summary] [Objective] To achieve a refining method capable of selectively performing efficient decarburization of molten steel under reduced pressure in an RH degassing tank or heating of a steel bath. A process mainly composed of decarburization in which the pressure in the degassing tank 1 is less than 130 Torr during the oxygen gas top blowing time from the straight lance 4 of the molten steel 3 under a reduced pressure in the RH degassing tank 1. Proper decarburization and heat-up are carried out by carrying out the steps mainly or mainly of the secondary combustion of CO gas in the degassing tank 1 where the pressure in the degassing tank 1 is 130 Torr or more.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for oxygen refining molten metal under reduced pressure.

[0002]

2. Description of the Related Art When decarburizing a steel bath with an oxygen blowing lance under reduced pressure, there is a technique disclosed in JP-A-54-137421 as a method for increasing the decarburizing efficiency in a low carbon region. . Further, as a method for improving the decarburization efficiency by making the nozzle tip of the top blowing oxygen lance into a Laval shape, it is disclosed in Japanese Patent Laid-Open No. 57-1374
There are techniques disclosed in JP-A No. 15-43717, JP-A No. 52-43717, JP-A No. 50-104714, and JP-A No. 2-54714.

In the technique disclosed in Japanese Patent Laid-Open No. 54-137421, when decarburizing a high Cr molten steel in a vacuum, the effective opening cross-sectional area of the lance nozzle is changed according to the C concentration in the molten steel. Then, decarburization to an extremely low C concentration is performed. But,
In this method, considering the actual operation, it is impossible to change the diameter of the lance tip during the operation with a single-pipe single-pipe lance, and if the double-lance or multiple-pipe lance is used, the equipment becomes complicated. In addition, the type having the valve body and its drive system at the tip of the lance tip and changing the cross-sectional area has a problem that an excessive cooling structure is required due to high heat.

The technique disclosed in Japanese Laid-Open Patent Publication No. 57-137415 and the like efficiently decarburizes by designing so that a predetermined Mach number of a Laval-type nozzle can be obtained at the upper limit of the tank pressure during blowing. Is the way to do it. However, the tank pressure is 76
Since the normal pressure of 0 Torr changes to a vacuum of about 0.1 Torr, it is difficult to determine which pressure should be used as the design pressure. In addition, according to this method, among the actions of decarburization and heating,
Only decarburization can be performed efficiently, but heating cannot be performed efficiently. Furthermore, since the degree of vacuum is not controlled, the efficiency of the Laval nozzle cannot be achieved as designed, and the design Mach number fluctuates when the oxygen pressure is changed, which tends to deviate from the design operation range of the Laval nozzle.

[0005]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems of the prior art and efficiently decarburizes under reduced pressure,
Or, it was made to provide an oxygen refining technique capable of selectively achieving two of the heating of a steel bath.

[0006]

According to the present invention, oxygen of molten metal under reduced pressure is used for decarburizing the molten metal by upwardly blowing a gas containing at least oxygen gas onto the surface of the molten metal bath in a vacuum vessel. In the refining method, the following (A),
A method for oxygen refining a molten metal under reduced pressure, which comprises selectively performing the step (B) alone or in combination.

(A) A step of mainly decarburizing the molten metal with the top-blown gas by setting the pressure in the decompression container to less than 130 Torr during the top-blown time of the gas containing at least oxygen gas. (B) A step of causing secondary combustion mainly with the top-blown gas by setting the pressure in the decompression container to 130 Torr or more during the top-blowing time of the gas containing at least oxygen gas.

In the present invention, of course, in the above, the step (A) can be continuously performed after the step (B).

[0009]

[Operation] In order to carry out decarburization efficiently, (1) keep the straightness of the top-blown oxygen gas jet and allow oxygen to reach the bath surface as fast as possible, or (2) use a decompression container. It is effective to control the pressure with oxygen gas and blow the oxygen gas onto the bath surface under the condition that the carbon monoxide gas generated from the bath surface does not burn easily. In the latter case (2), the straightness of the oxygen gas jet is not so necessary and the flow velocity does not have to be high.

On the other hand, in order to efficiently raise the temperature of the molten metal bath, (1) carbon monoxide generated by decarburization from the molten metal bath by diffusing an oxygen jet while maintaining a certain degree of decarburization efficiency. It is effective to efficiently contact the gas and oxygen gas to carry out the secondary combustion of the carbon monoxide gas, and (2) control the pressure in the depressurized range to the combustible range of the carbon monoxide and the oxygen gas.

In order to quantify the explosion limit of carbon monoxide gas under reduced pressure, the present inventors use the explosion method of the fuel gas Hempel analysis method defined in Japanese Industrial Standard JIS-2301 as an experimental device. Then, it was sought to find out how the vacuum pressure at which explosion occurs changes within the range of the exhaust gas composition generated during vacuum decarburization. The result is shown in FIG. As shown in Fig. 2, it was found through this experiment that if the pressure inside the vessel is less than 130 Torr, almost no combustion is possible even if the CO concentration is as high as about 50% or more. Therefore, the following will be established by this knowledge.

(1) If the vacuum is maintained at a degree of vacuum of less than 130 Torr while the oxygen gas is being blown, the secondary combustion does not occur in the space inside the decompression container, and the blown O ₂ is preferentially used for decarburization. (2) If the vacuum is maintained at 130 Torr or more during the top blowing of oxygen gas, the secondary combustion of carbon monoxide occurs in the space inside the decompression container, and the secondary fuel heat can be effectively used for raising the temperature of the metal bath.

(3) By continuously shifting from (1) to (2) above, it is possible to efficiently perform both heat raising and decarburization.

[0014]

1 is a schematic sectional view of an RH apparatus used in the method of the present invention. As shown in FIG. 1, a straight lance 4 is vertically movable from the upper part of the RH degassing tank 1. When it is necessary to supply oxygen to the molten steel 3, the straight lance 4 is lowered to an appropriate height and blows oxygen gas onto the steel bath surface. Not only oxygen but also gases other than oxygen, such as argon gas, can be supplied through the lance 4. Two
Indicates a ladle containing molten steel 3.

Circulating flow rate of 230 tons / heat molten steel that has been primary decarburized and smelted in a converter in 2 minutes is shown in FIG.
Using a degassing facility, oxygen blowing was performed by top blowing at an oxygen flow rate of 15 Nm ³ / min. Example (1) The pressure in the RH degassing tank was fixed at 125 Torr by controlling the exhaust capacity by controlling the number of stages of the steam booster used and / or the leak air flow rate using a straight lance. The oxygen gas top blowing time was 20 minutes.

Example (2) The exhaust capacity was controlled using a straight lance and the pressure in the RH degassing tank was fixed at 135 Torr. Oxygen gas top blowing time is
20 minutes. Example (3) A straight lance was used to control the exhaust capacity and 135 Torr
Oxygen gas was blown for 10 minutes at 125 torr for 10 minutes.

Comparative Example Laval Lance 20 top blows between 300 and 100 Torr
Minutes. The pressure in the container was not controlled to be constant. The results of each operation are compared and shown in Table 1.

[0018]

[Table 1] In Example (1), which is the main purpose of decarburization, the carbon concentration after the treatment is lower than that of the comparative example, and the variation σ also decreases. In Example (2) whose main purpose is to raise the temperature, a reduction in the variation σ in the post-treatment carbon concentration and an increase in the degree of raising the temperature of about 14 ° C. are obtained as compared with the comparative example. In the example (3) of the composite method, the post-treatment carbon concentration is lower, the variation σ is reduced, and the amount of heat rise is increased by about 7 ° C. as compared with the comparative example.

[0019]

As described above, according to the method of the present invention, by controlling the pressure in the decompression container to be less than 130 Torr or 130 Torr or more, decarburization or monoxidation by top-blown oxygen gas is performed. It is possible to selectively raise the heat by secondary combustion of carbon gas. Also within the same beat 1
By performing top blowing of oxygen gas above 30 torr and top blowing below 130 torr, both decarburization and heating effect can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of an RH degassing apparatus used in an example of the present invention.

FIG. 2 is a diagram showing the results of a combustion test of a carbon monoxide-containing gas with oxygen gas under reduced pressure according to the Hempel analysis method of JIS-2301.

[Explanation of symbols]

1 RH degassing tank 2 ladle 3 Molten steel 4 straight lance

Claims (2)

[Claims] 1. A method for oxygen refining a molten metal under reduced pressure for decarburizing the molten metal by upwardly blowing a gas containing at least oxygen gas onto a surface of a molten metal bath in a decompression container, the method comprising: A method for oxygen refining a molten metal under reduced pressure, wherein the steps (B) and (B) are selectively performed individually or in combination. (A) A step of mainly decarburizing the molten metal with the top-blown gas by setting the pressure in the decompression container to less than 130 Torr during the top-blown time of the gas containing at least oxygen gas. (B) A step of causing secondary combustion mainly with the top-blown gas by setting the pressure in the decompression container to 130 Torr or more during the top-blowing time of the gas containing at least oxygen gas. 2. The method for oxygen refining a molten metal under reduced pressure according to claim 1, wherein step (A) is continuously performed after step (B).