JPH06330148A - Method for melting low n steel in vacuum refining furnace - Google Patents

Abstract

PURPOSE:To produce molten steel having low N content by efficiently executing reduction- refining of molten steel with stirred small grain Al with inert gas under the reduced atmospheric pressure after executing decarburize-refining by oxygen-blowing refining of the molten steel in the low vacuum range and finish refining under the reduced atmospheric pressure. CONSTITUTION:At the time of producing a low carbon stainless steel by executing the decarburize-refining to a raw high Cr steel with a VOD method, at first C in the molten steel is oxidized by the oxygen-blowing in the low vacuum atmosphere to execute the decarburization, and in order to lower further C content, the oxygen-blowing is executed in the low CO partial pressure at <=10Torr to execute the finish decarburize-refining to the aimed low carbon grade. Successively, in the condition of holding the refining atmosphere to high vacuum at <=10Torr, the blowing quantity of inert gas of Ar, etc., from the bottom blowing tuyere is increased and the stirring energy Eg shown in the equation is made to be >=400W/ton of molten steel to stir the molten steel. On the stirred molten steel surface, the small grain Al having <=30mm grain diameter is added to reduce the oxide of Cr, etc., produced by oxygen-blowing. As this operation is executed in the high vacuumn atmosphere, absorbing action of nitrogen from the air is not executed and the extra low N stainless steel can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing molten steel in a vacuum refining furnace, and particularly to a method for economically producing low N steel.

[0002]

2. Description of the Related Art Recently, the demand for high-purity steel is increasing. For example, high chromium steel such as ferritic stainless steel containing 5 to 35% of Cr has low N content because of high N solubility.
Despite being a disadvantageous steel type, extremely low N is required.

Such ultra-low N stainless steel has hitherto been used.
It is manufactured by decarburizing and refining by the VOD method. The VOD decarburization refining consists of a degassing period and a reducing period. In the degassing period, after decarburizing and denitrifying high carbon stainless molten steel under reduced pressure,
During the reduction period, alloying and reducing Al, etc. and a slag forming agent are added to the molten steel to promote the reduction of the chromium oxide generated during the degassing period and the deoxidation of the molten steel.

What is extremely important in the melting of ultra-low N steel is to prevent the absorption of nitrogen from the atmospheric gas during the reduction period and to add Al in a good yield.

As a conventional technique, for example, Japanese Unexamined Patent Publication No.
As disclosed in Japanese Patent No. 211216, after the Al is dipped and added under the surface of the molten steel under normal pressure, a low vacuum of 40 torr or more is maintained until a sufficient time for Al to diffuse into the molten steel elapses. There is a way.

[0006]

The above-mentioned prior art has the following drawbacks.

Exposing the surface of the molten steel under normal pressure for dipping and adding Al and maintaining it for a long time under a low vacuum of 40 torr are extremely low N steel because nitrogen in the atmospheric gas is absorbed in the molten steel. Is inconvenient for melting. After decarburization under reduced pressure, the pressure is returned to normal pressure and Al is added.
Since the treatment time becomes long when the pressure is reduced and the reduction treatment is carried out, variation of yield due to evaporation of Al and deterioration of productivity are caused, and melting loss of refractory material is promoted, leading to cost increase.

The present invention has been made to solve such drawbacks.

[0009]

According to the present invention, in melting low N steel in a vacuum degassing and refining furnace, blast acid refining is performed in a low vacuum region, and then final decarburization is performed under reduced pressure of 10 torr or less. , The degree of vacuum is maintained below 10 torr, the amount of bottom-blown inert gas is increased, and the stirring energy E _g is 400 W.
/ Ton or more, by adding Al having a particle size of 30 mm or less to the exposed surface of molten steel produced by stirring, and subsequently performing reduction refining at the vacuum degree in a low N steel melting process by a vacuum refining furnace. is there.

[0010]

[Equation 2]

However, Q _g : bottom blown inert gas flow rate (Nl / min) T ₁ : molten steel temperature (K) ρ ₁ : specific gravity of molten steel (kg / m ³ ) Ζ: molten steel depth (mm) p: vacuum furnace Internal pressure (torr) W ₁ : Molten steel weight (ton) Here, finishing decarburization at 10 torr or less is completed by holding the molten steel under high vacuum to lower the equilibrium carbon concentration and promote high purification. Also, the time for adjusting the degree of vacuum for entering the reduction step is omitted. The amount of bottom blowing inert gas is increased to increase the stirring energy E _g to 400
The reason why it is set to W / ton or more is to increase the exposed surface of the molten steel so that Al does not ride on the slag, and to quickly wind the injected Al into the molten steel. By this method Al
Since is melted and diffused in the molten steel, evaporation of Al can be prevented. The reason why Al having a particle diameter of 30 mm or less is used is to prevent the Al caught in the molten steel from floating and evaporating before melting. The reason why the reduction refining is performed under a reduced pressure of 10 torr or less is to prevent the nitrogen in the atmosphere from melting into the molten steel by lowering the equilibrium nitrogen concentration.

According to this method, the added Al diffuses into the molten steel instantly when it is pushed into the molten steel, so that there is no yield loss due to evaporation, and since it is a process under a high vacuum, the partial pressure of the atmosphere N ₂ is reduced. Is low, and N absorption can be prevented. Further, by adding Al at the same degree of vacuum as that of the finish decarburization and starting the reduction treatment, the reduction treatment time can be shortened, therefore the productivity is improved and it is also advantageous in suppressing the melting loss of refractory materials. .

[0013]

EXAMPLE 19Cr stainless steel was melted by the method of the present invention and the conventional method under the conditions shown in FIG. In the method of the present invention, all treatments were carried out under the same conditions for the refining of blown acid and finish degassing, and the reduction was carried out under the conditions where the combination of vacuum degree, stirring energy and Al particle size was changed. Further, in the conventional method, Al was dipped and added under the surface of the molten steel bath under atmospheric pressure after refining with blown acid, maintained under a vacuum of 40 torr for 3 minutes, and then stirred under a vacuum of 1 torr for 2 minutes. These results are summarized in Table 1, and it is clear that the method of the present invention is effective in preventing N absorption, shortening the reduction treatment time, and improving Al yield.

[0014]

[Table 1]

[0015]

As described above, according to the method of the present invention, it is possible to prevent N absorption in the reduction treatment step in the vacuum degassing and refining furnace and to stably manufacture the low N steel. In addition, since the yield of Al and the like can be improved and the treatment can be performed in a short time, the Al unit consumption and the refractory cost can be reduced and a large economic effect can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing patterns of vacuum degree and stirring energy in a reducing period in a typical example of a conventional method and a method of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Sugano 1-1, Toibata-cho, Tobata-ku, Kitakyushu City Nippon Steel Yawata Works Co., Ltd.

Claims (1)

[Claims] 1. When melting low N steel in a vacuum degassing refining furnace, it is smelted with blown acid in a low vacuum region and then 10 to
After finishing decarburization under reduced pressure of rr or less, the degree of vacuum is 10t.
The amount of the bottom-blown inert gas was increased to set the stirring energy E _g to 400 W / ton or more, and the particle diameter of the molten steel exposed surface generated by stirring was 30 mm or less.
1. Addition of 1 and subsequent reduction refining at the above-mentioned degree of vacuum, a method for melting low N steel in a vacuum refining furnace. However, Q _g : Bottom blown inert gas flow rate (Nl / min) T ₁ : Molten steel temperature (K) ρ ₁ : Molten steel specific gravity (kg / m ³ ) Ζ: Molten steel depth (mm) p: Vacuum furnace pressure ( torr) W ₁ : Molten steel weight (ton)