JP2000087128A - Secondary refining method of molten steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the cleanliness in a cast slab without developing the clogging of a nozzle in a continuous casting by executing the addition of metallic element during decarburizing treatment and thereafter, executing the deoxidizing treatment. SOLUTION: In a secondary refining of molten steel, the alloy element is added during decarburizing treatment and after deoxidizing treatment, a stirring treatment is executed. In this result, the deoxidizer is oxidized in the molten steel, and inclusions, such as Al2O3, TiO2 or the like are not increased. Oxygen caused by the oxygen, moisture, oxides contained in the added alloy agent, is used mainly to the decarburizing refining after charging into the molten steel, and does not form the inclusion. Even, temporarily non-metallic inclusion is formed but since immediately this inclusion is reacted with C in the molten steel with the stirring force and C-O boiling in the molten steel in the vacuum treatment and reduced, etc., this inclusion is not left till continuous casting stage. The additional alloy agent is desirable to have no deoxidizing performance and concretely, single element, such as P, Ni, Co or alloy form with Fe can be added.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for secondary refining of molten steel, and more particularly to a method for secondary refining of molten steel suitable for producing clean steel.

[0002]

2. Description of the Related Art As a method of further refining molten steel produced by primary refining in a converter or the like and then tapping, a vacuum degree of 10 to 300 mmHg is applied and vacuum degassing is performed. To adjust the components (hereinafter referred to as vacuum treatment). In this treatment process, generally, decarburization, deoxidation, addition of a component adjusting alloy, and stirring are sequentially performed.

[0003] As a specific example, Japanese Patent Application Laid-Open No. 57-73118 discloses a method in which a desired alloy element is divided or collectively added to molten steel in a vacuum degassing tank during the vacuum degassing treatment of molten steel. After the final addition of the desired alloying element, at least a time t represented by (V / F) × a
It describes that the molten steel is stirred and oscillated at ₁ (minute) and a degree of vacuum of 10 to 300 (mmHg). Where: V: amount of molten steel
(t), F: annular flow rate (t / min), a: constant (range: 1 to 10) determined by cleanliness required for the product. In this method, the large agitation force of the vacuum degassing method or the large rocking force of the DH vacuum degassing process is applied to the collisional aggregation (coalescence) of inclusions.
Utilized, the intention is to increase the cleanliness by increasing the floating efficiency up to inclusions minimum, it is obtained by regulating the time t ₁ as the time required for the combined flying.

[0004]

However, for example, in the case of high-strength steel or silicon steel for automobile outer panels, secondary refining is performed by the conventional method and solidified by a continuous casting process to obtain slag or other slabs. When passing through the immersion nozzle between the continuous casting tundish and the mold, there are problems such as inclusions in the molten steel accumulating in the nozzle and inducing nozzle clogging. Because of the occurrence of defects, the cleanliness of the slab was still not sufficient.

The present invention solves the above-mentioned problems of the prior art,
An object of the present invention is to provide a method for secondary refining of molten steel capable of sufficiently increasing the cleanliness of a slab without inducing nozzle clogging in continuous casting.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a method for secondary refining of molten steel, which comprises decarburizing, deoxidizing, and adding alloying elements to molten steel by a vacuum degassing apparatus. This is a secondary refining method for molten steel, which is performed during a decarburization process and then performed a deoxidation process. The alloying agent for adding an alloying element preferably has no deoxidizing ability.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Conventionally, in secondary refining using a vacuum degassing apparatus, the addition of alloying elements into molten steel includes Al,
It is usually performed after adding a deoxidizing agent such as Ti to molten steel. However, the present inventors have closely tracked the behavior of inclusions in molten steel from the time of addition of the alloying element to the time of casting, and the number of inclusions increases immediately after the addition of the alloying element or during the casting from the addition of the alloying element. I found that there was a tendency. After investigating the cause, moisture, oxygen, and oxides contained in the alloying agent for adding alloying elements, or moisture and oxides adhering to the alloying agent entered the molten steel with the addition of the alloying agent to the molten steel. Oxidizes deoxidizers such as Al and Ti in molten steel to form Al ₂ O
It became clear that inclusions such as ₃ and TiO ₂ were increased.

Therefore, in the present invention, the addition of the alloying agent having an oxygen source is performed not after the deoxidizing treatment but at the time of decarburizing and refining molten steel in a vacuum degassing apparatus. The decarburization and refining of the molten steel is carried out in a primary smelting furnace such as a converter by removing rimmed steel (non-deoxidized or only a small amount of a deoxidizing agent and leaving oxygen in the molten steel). The degassing is performed by reducing the pressure in the degassing device and decarburizing by the reaction of dissolved C 2 and O , or by supplying oxygen gas to the molten steel from a lance or tuyere under the reduced pressure.

According to the present invention, the alloying element is removed during the decarburization process.
After the addition and deoxidation, the mixture was agitated.
The above-mentioned problem that has occurred in the conventional method of adding an alloy element after the acid,
In other words, melting by the oxygen source accompanying the added alloy element
Deoxidizer in steel is oxidized to Al _TwoO_ThreeAnd TiO_TwoInclusions increase
The problem of adding is resolved. Added in the present invention
Oxygen due to oxygen, moisture and oxides contained in alloying agents
After entering the molten steel, it is mainly used for decarburization and refining of the molten steel.
Will be spent and will not form inclusions. Provisional
Even if non-metallic inclusions are temporarily formed in
The stirring power of the steel and the stirring of the molten steel by the CO boil
Immediately reacts with C in molten steel and is reduced or coagulated
As it moves up during the slag immediately,
That is, it remains as a nonmetallic inclusion until the continuous casting stage).
And not.

In the present invention, the alloying agent added during the decarburization refining preferably has no deoxidizing ability. That is, in the case of an element having a strong deoxidizing ability, such as Al and Ti, Al ₂ O ₃ which is hard to be reduced by being combined with dissolved oxygen in molten steel at the same time of addition.
Non-metallic inclusions, such as TiO ₂ and a large amount generated. As a result, the amount of dissolved oxygen in the molten steel is reduced at once, and the decarburization reaction is stopped, making it difficult to obtain an ultra-low carbon steel. Since it remains until the casting stage, the quality of the slab is impaired and the immersion nozzle is closed, which is not preferable. Also, in the case of a weak deoxidizing element such as Si and Mn, SiO ₂ and MnO in combination with dissolved oxygen in the molten steel
Lower oxides, etc., which migrate into the slag, and after the addition of Al and Ti, which are strong deoxidizing elements, work slowly to supply oxygen into the molten steel, Al and Ti in the molten steel are oxidized during the process from refining to continuous casting, which is a source of increasing nonmetallic inclusions, which is not preferable.

Specific examples of the alloying agent having no decarburizing ability include P, Ni, Co, Cu, Ag, Mo, W and the like.
These elements may be added alone or in the form of an alloy with iron.

[0012]

[Example] C: 19 ppm, Si: 0.01%, Mn: 0.33%, P:
High P containing 0.095%, S: 0.007%, Al: 0.034%
In the process of smelting ultra-low carbon steel, the process of primary smelting in a converter and the secondary smelting of molten steel discharged into a ladle using an RH vacuum degasser is performed using the conventional "decarburization → deoxidation → alloying agent". When the "decarburization (addition of alloying agent (Fe-P) during processing) → deoxidation → stirring" process of the present invention was adopted instead of the (Fe-P) addition → stirring "process, as shown in FIG. , Representative total oxygen (definition: 40-
The total oxygen concentration contained in the sample collected from the molten steel in the tundish at the time of 60% casting) was significantly reduced compared to the conventional method, and in the continuous casting process after the secondary refining, as shown in FIG. Increases from 4 charges to 7 charges in the past, and the productivity is dramatically improved.
Further, as shown in FIG. 3, the surface defects (defective rate index) of the product coil obtained by subjecting the continuous cast slab to hot rolling-cold rolling-annealing-surface treatment are reduced by about 40% as compared with the related art. The remarkable effect was obtained.

[0013]

As described above, according to the present invention, clogging of the immersion nozzle in continuous casting is eliminated and the productivity of continuous casting is improved.
An excellent effect of reducing surface defects after annealing-surface treatment-pressing is achieved.

[Brief description of the drawings]

FIG. 1 is a graph showing representative total oxygen of the present invention and a conventional one.

FIG. 2 is a graph showing productivity of the present invention and the conventional continuous casting.

FIG. 3 is a graph showing a defect rate index in a product stage according to the present invention and a conventional product.

Continued on the front page (72) Inventor Hiroshi Nomura 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba F-term (reference) 4K013 AA00 BA02 BA08 BA14 CA04 CE01 DA02 DA08 DA12 DA13 EA18 EA19

Claims (2)

[Claims] In a secondary refining method for molten steel, wherein decarburization, deoxidation, and addition of alloying elements to molten steel by a vacuum degassing apparatus, the alloying elements are added during the decarburization process, A secondary refining method for molten steel, which comprises performing a deoxidation treatment. 2. The method according to claim 1, wherein the alloying agent for adding an alloying element has no deoxidizing ability.