JPH0625731A - Deoxidation method of molten steel - Google Patents

Abstract

PURPOSE:To stably produce a molten steel excellent in cleanness by pouring a molten steel in the vessel lined with an Al2O3 refractory material, covering the surface with the mixture of a CaO or CaO-Al2O3 flux and an Al deoxidiser and deoxidizing by means of weak agitation of the molten steel. CONSTITUTION:The molten steel 2 is poured in the vessel 1 using an Al2O3 refractory material as a lining material and on the surface of the vessel the mixture 3 consisting of metal Al or Al ash as a deoxidiser, a CaO or CaO-Al2O3 as a flux and CaF2, SiO2 as a slagging accelerator of flux is sprayed to cover. The inert gas of Ar, etc., is blown in the molten steel from the porous plug or lance 5 at the vessel bottom to weakly agitate the flux 3 and the molten steel. The dissolved oxygen in the molten steel is reacted with Al to be deoxidized. The Al2O3 generated by deoxidation is removed with a CaO or CaO-Al2O3 flux as slag. A clean steel, having a high purity with a very low content of remaining Al2O3, is easily obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for deoxidizing molten steel for producing steel having high cleanliness.

[0002]

2. Description of the Related Art In the steelmaking process, as a deoxidation method for molten steel, generally, pure Al or Al ash is added to deoxidized molten steel to deoxidize it. In this deoxidation method, a large amount of Al ₂ O ₃ -based oxide is generated as a deoxidation product. The Al ₂ O ₃ microscopically suspended in the molten steel agglomerates and coalesces with each other, and floats due to the difference in density with the molten steel, and is removed from the molten steel. However, if Al ₂ O ₃ is insufficient in agglomeration, coalescence, and floating removal, in the continuous casting process of steel,
It is trapped by the cast slab and becomes an internal defect of inclusion properties, which significantly deteriorates the cleanliness of the slab and the quality of the steel material. Therefore, in the processing steps from deoxidation to continuous casting, Al ₂ O ₃
The following secondary refining process is performed in order to aggregate, coalesce, and float.

For example, (a) Al ₂ O ₃ suspended in molten steel is agitated by a reflux type vacuum degassing apparatus, an inert gas blowing stirring apparatus, an electromagnetic induction stirring apparatus, or the like to collide, aggregate, or combine. How to let it rise. (B) CaO, CaO-SiO 2, CaO-Al 2 O
_{A method} of injecting a flux of ₃ , CaF ₂ or the like into molten steel, causing Al ₂ O ₃ suspended in the molten steel to collide with, adsorb, and coalesce with the flux to float as a large complex oxide having a low melting point 58-93810).

(C) In the above methods (a) and (b),
The molten steel surface contains components other than iron oxide and manganese oxide
Synthetic slag is added to prevent the oxidation of molten steel.
To the Al₂ O₃ Method for adsorbing hydrogen
No. 10016). However, pure Al is added to the molten steel.
In the deoxidation method, the deoxidation product Al₂ O ₃ A large amount
And then dispersed, and then (a), (b), (c)
Even if the secondary refining process is performed, there is a probability theory of collision in molten steel.
Since it is a dynamic phenomenon, Al₂ O₃ Adsorption, aggregation, and coalescence of
Sufficient, complete with fine inclusions dispersed in the steel
Removal is difficult. In addition, because it is strongly stirred, it floats once
Done Al₂ O₃ Or the injected flux
When it is caught as a large inclusion and remains as it is
There is also.

Collision, agglomeration of deoxidized products formed in steel,
Of molten steel that improves the probability of coalescence and produces steel with high cleanliness
As a deoxidizing method, there are disclosed in Japanese Unexamined Patent Publication Nos. 62-7816 and
JP-A-3-47910 discloses a deoxidizing agent such as metallic Al and Ca.
O, CaO-SiO₂ , CaF ₂ In molten steel in a mixed state
A deoxidized product is obtained by adding injection.
Generated Al₂ O₃ Immediately in molten steel CaO-Al₂ 
O₃ There is a method of levitating and separating and controlling the morphology.

The method of mixing the deoxidizing agent and the flux for deoxidizing is to make the flux exist around the deoxidizing agent in the molten steel at a high density as compared with the conventional single Al deoxidizing and flux injection. Therefore, the generated deoxidized product and the flux have a high probability of colliding and coalescing with each other, and it is possible to form a large inclusion having good floating property.

[0007]

[0005] In the method of deacidifying agent of metal such as Al mentioned above and CaO, molten steel was added injected into molten steel in a mixed state CaO-SiO _2, CaF ₂ deoxidation, conventional Al The probability of collision, aggregation, and coalescence of the deoxidized products is higher than that in the method of performing stirring and injection after single deoxidation. However, the deoxidation product is
Similar to single deoxidation, it is suspended in molten steel in a fine form, and even if the probability of collision, agglomeration, and coalescence is increased and the inclusions are large, the reaction region is a deep region in molten steel. Therefore, it takes time to float up to the surface of the molten steel, and it cannot be completely floated up and remains in the molten steel as inclusions. As a result, defects caused by Al ₂ O ₃ in the cast slab still occur and the eradication is not yet achieved.

[0008]

The present invention has been made to solve the above problems, and its technical means are as follows. That is, when deoxidizing molten steel in a holding container made of Al ₂ O ₃ refractory, CaO-based flux and CaO-Al ₂ O ₃ are coated so as to cover the surface of the molten steel.
A method for deoxidizing molten steel, which comprises adding a mixture of a system flux or a flux obtained by adding CaF ₂ or SiO ₂ thereto and Al or Al ash which is a deoxidizing agent, and stirring the molten steel.

[0009]

In the present invention, in the process of deoxidizing molten steel, a deoxidizing agent and a flux are mixed to form a mixture, and the mixture is added to the molten steel so as to cover the surface of the molten steel. By this,
The deoxidation reaction region is near the interface between the mixture and the surface of molten steel. In the conventional method of adding a mixture to molten steel by injection or the like, even if the deoxidation product and the flux collide and coalesce to increase the diameter of the inclusions and promote floating, the inclusions suspended in the molten steel You can't surface almost everything. Further, in order to promote collision and coalescence, it is necessary to strongly stir the molten steel, and on the contrary, due to the strong stirring, slag on the molten steel surface or inclusions floating on the molten steel surface may be caught in the molten steel. However, the inclusions that cannot be floated and remain are relatively large inclusions due to agglomeration and coalescence, and are prone to defects in inclusion properties.

In the present invention, the deoxidation product is generated only near the slag-molten steel interface, which is near the surface of the molten steel, and is immediately absorbed in the flux within a short ascent time, so that it is widely dispersed and suspended in the molten steel. However, only minute inclusions remain and molten steel with extremely high cleanliness can be obtained. As a flux to be added to and coated on the surface of molten steel in a mixed state with a deoxidizing agent, CaO-based, CaO-Al ₂ that easily binds to the deoxidation product in order to prevent the deoxidation product from being caught in the molten steel.
O ₃ system is suitable, and further, a synthetic flux containing CaF ₂ and SiO ₂ is preferable in order to lower the melting point of the flux and promote slag formation. The addition amount is preferably 2.5 kg / ton or more in order to sufficiently coat the surface of the molten steel without oxidizing it.

1 to 4 show a deoxidizing method for molten steel according to the present invention. FIG. 1 shows the surface of molten steel 2 coated with CaO-based, CaO-Al ₂ O ₃ -based, or a mixture 3 in which CaF ₂ , SiO ₂ is mixed with a deoxidizer Al or Al ash in a flux. As shown in the figure, a method for deoxidizing the molten steel at the slag-molten steel interface while stirring the molten steel with an inert gas after adding the same to the ladle 1 and installing the porous plug 4 on the bottom of the ladle 1. .

In FIG. 2, after the mixture 3 of the flux and the deoxidizer is added to the surface of the molten steel 2, the lance 5 is immersed in the molten steel 2 and the molten steel is stirred with an inert gas at the slag-molten steel interface. It shows a method of performing deoxidation. FIG. 3 shows a method of performing deoxidation at the slag-molten steel interface while refluxing the molten steel 2 by the reflux type vacuum degassing device 6 after adding the mixture 3 of the flux and the deoxidizer to the surface of the molten steel 2. It is a thing.

FIG. 4 shows a method in which a mixture 3 of flux and a deoxidizer is added to the surface of the molten steel 2 and then deoxidized at the slag-molten steel interface while stirring the molten steel with an electromagnetic induction stirring device 7. It is a thing. The stirring force in each of the above means is
It is important to carry out weak stirring so that the deoxidation product generated at the flux-molten steel interface does not get caught in the molten steel. Further, for the slag formation of the flux, a flux having a melting point lower than the treatment temperature of the molten steel is used, and an inert gas is blown to supply heat to the flux while moving a lance 5 having a shallow molten steel immersion depth as shown in FIG. In combination with the method, a method of immersing the electrode 8 in the mixture 3 of the flux and the deoxidizer by using an electrode heating type refining furnace equipped with the electrode 8 for electrically heating as shown in FIG. May be.

Al ₂ is used as a refractory material constituting the ladle.
O ₃ type refractory shall be used. This is because when a silica refractory is used, SiO ₂ in the refractory is reduced by Al in the steel and enters into the steel as Al ₂ O ₃ .

[0015]

【Example】

[Example 1] As Example 1, Al-killed steel for bearing steel was melted. Fe-Si, Fe-Mn, Fe are used when tapping molten steel into a ladle after decarburizing and refining in a converter using blast furnace hot metal with low P and low S concentration that has been subjected to hot metal pretreatment. −
Alloys such as Cr have a molten steel composition of [C]: 1.0%, [S
i]: 0.25%, [Mn]: 0.40%, [Cr]:
1.35% and [Mo]: 0.02% were added. The tapping amount was 200 tons and the tapping temperature was 1650 ° C.

The converter slag flowing out into the ladle was removed to minimize it, and 2000 kg of synthetic flux and 1000 kg of Al ash as a deoxidizer were mixed and added so as to coat the surface of the molten steel. The composition of synthetic flux is 80
% CaO-20% CaF ₂ , Al ash composition is 50% Al
-50% Al ₂ O ₃ . The refractory material of the ladle was high alumina.

An Ar gas was blown into the molten steel from a porous plug installed at the bottom of the ladle to stir the molten steel while heating the flux using an electrode heating type refining furnace (LF apparatus). The Ar gas flow rate was 100 liters / min, and the processing time was 60 min. The [Al] concentration in the molten steel after the treatment became a desired 0.02%, and Al in the flux was diffused in the molten steel and the molten steel was deoxidized. The total of both (FeO) and (MnO) concentrations in the slag of the ladle after the treatment was 0.5% or less, and the (SiO ₂ ) concentration was 3% or less.

On the other hand, as Comparative Example 1, after the slag was removed under the above molten steel composition and tapping conditions, the synthetic flux 15
To the molten steel in which 00 kg was added so as to cover the surface of the molten steel and the flux was heated using the electrode heating type refining furnace (LF device), 1000 kg of synthetic flux and Al which is a deoxidizing material.
1000 liters / mi mixed with 500 kg
The injection was performed with n Ar gas. Injection time is 10 min /, then 5 from porous plug
Only Ar gas was blown for 30 minutes at 00 liter / min. The [Al] concentration in the molten steel after the treatment was 0.02%.

The molten steels of Example 1 and Comparative Example 1 were continuously cast into 400 mm x 560 mm bloom slabs, immediately rolled into 150 mm x 150 mm billets, and further rolled into round bars with a diameter of 60 mm. As shown in FIG. 7, the total oxygen concentration of the round bars was 5 ppm or less. The cross sections of the round bars of Example 1 and Comparative Example 1 were examined with a microscope, and the distribution of inclusions per 100 mm ^{2 of the} test area was compared. The distribution of inclusions is shown in FIG. In Comparative Example 1, 20
The number of large inclusions having a size of μm or more was 3.2 / 100 mm ² , but in Example 1, the number was 0.2, which was extremely small. The number of inclusions of 2 to 20 μm was also reduced in Example 1 as compared with Comparative Example 1. The rolling fatigue life of these steel bars was examined with a thrust type rolling fatigue tester. As a result, the rolling fatigue life of Example 1 was 10 times or more that of Comparative Example 1.

Example 2 As Example 2, low carbon Al
The steel plate for killed thin plates was melted. After decarburizing and refining in a converter using blast furnace hot metal with low P and low S concentration that has been subjected to hot metal pretreatment, molten steel is tapped into a ladle and Fe-Mn, Fe-
An alloy such as Ti was added during tapping. The tapping amount was 300 tons and the tapping temperature was 1650 ° C. The refractory material of the ladle was high alumina. The molten steel was subjected to CO decarburization riming treatment for 20 minutes using a reflux type vacuum degassing device,
The molten steel has a [C] concentration of 15 ppm and a [Mn] concentration of 0.
20%, [P] concentration 0.010%, [S] concentration 0.
It was set to 005%. After that, 2000 kg of synthetic flux
And 1200 kg of Al ash, which is a deoxidizer, were mixed and added so as to coat the surface of the molten steel. Synthesis composition of the flux is _{80% Ca-20% CaF 2} , the composition of the Al ash 50% A
1-50% Al ₂ O ₃ . After the mixture was added, vacuum degassing treatment was continued for 20 minutes. The [Al] concentration in the molten steel after the treatment was a desired 0.025%, and Al in the flux was diffused in the molten steel and the molten steel was deoxidized.
The total of both the concentrations of (FeO) and (MnO) in the ladle slag after the treatment was 1% or less.

On the other hand, as Comparative Example 2, after decarburizing and refining in a converter, molten steel was tapped into a ladle and Fe was tapped at the time of tapping.
Alloys such as —Mn and Fe—Ti were added, and 1000 kg of synthetic flux was added. The tapping amount was 300 tons and the tapping temperature was 1650 ° C. The refractory material of the ladle was high alumina. The molten steel was subjected to CO decarburization riming treatment for 20 minutes using a reflux type vacuum degassing device,
The [C] concentration in the molten steel is 15 ppm, and the [Mn] concentration is 0.
02%, [P] concentration is 0.050%, and [S] concentration is 0.
It was set to 002%. Then, the synthetic flux and Al as a deoxidizer were mixed and injected from the lower tank of the vacuum degassing device. The composition of the mixture of comparative materials is 20% CaO-
5% CaF ₂ -75% Al was added and a total of 1000 kg was added. After the mixture is added, the vacuum degassing process is continued to 2
I went for 0 min. The concentration of [Al] in the molten steel after the treatment was 0.025%.

The molten steels of Example 2 and Comparative Example 2 were continuously cast into 220 mm × 1800 mm slab slabs. As shown in FIG. 7, the oxygen concentration of the slab slab was 15 p in Example 2.
pm, 25 ppm in Comparative Example 2, which is a large decrease in Example 2. Then, the slab is hot-rolled and cold-rolled to obtain a product thickness of 0.
"Blistering" and "sliver" on 5mm hot-dip galvanized steel sheet
The defect occurrence ratio was comparatively investigated. The hot rolled sheet after pickling was visually inspected, and a comparison of occurrence of "blister" defects is shown in FIG. The surface of the product after cold rolling and hot dip galvanizing was visually inspected, and a comparison of occurrence of "sliver" defects is shown in FIG. As can be seen from FIGS. 9 and 10, Example 2 is different from Comparative Example 2 in that “blister” and “sliver” are caused by large aggregate inclusions of Al ₂ O ₃ type.
The incidence of defects has decreased.

As can be seen from Examples 1 and 2, in the present invention, the deoxidized organisms are generated only in the vicinity of the slag-molten steel interface, which is the vicinity of the surface of the molten steel, and are immediately absorbed in the flux in a short floating time. For this reason, only minute inclusions remain, without being widely dispersed or suspended in the molten steel, and molten steel with extremely high cleanliness can be obtained.

[0024]

As described above, according to the present invention, when deoxidizing molten steel, as described above, a CaO-based material, a CaO-Al ₂ O ₃ -based material, or a flux containing CaF ₂ and SiO ₂ and a deoxidizing agent are used. A certain Al or Al ash is mixed, the mixture is added so as to cover the surface of the molten steel, and the stirring method of the molten steel for promoting the deoxidation, bottom-blown inert gas stirring, top-blown inert gas stirring, Electromagnetic induction stirring, vacuum degassing device 1 or 2
Using more than one means, and as a holding container for molten steel, Al
By using a ₂ O ₃ -based refractory, steel with high cleanliness can be manufactured.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an application example of the deoxidizing method of the present invention.

FIG. 2 is a schematic diagram showing an application example of the deoxidizing method of the present invention.

FIG. 3 is a schematic diagram showing an application example of the deoxidizing method of the present invention.

FIG. 4 is a schematic diagram showing an application example of the deoxidizing method of the present invention.

FIG. 5 is a schematic diagram showing a method of slagging flux according to the present invention.

FIG. 6 is a schematic diagram showing a method of slagging flux according to the present invention.

FIG. 7 is a graph comparing the oxygen amounts of the example of the present invention and the comparative example.

FIG. 8 is a graph comparing the distribution of inclusions in the products of the bearing steels of the example of the present invention and the comparative example.

FIG. 9 is a graph comparing the number of defects in the products of the extremely low carbon steels of the example of the present invention and the comparative example.

FIG. 10 is a graph comparing the number of defects in the products of the extremely low carbon steels of the example of the present invention and the comparative example.

[Explanation of symbols]

1 Ladle 2 Molten Steel 3 Mixture of Flux and Deoxidizer 4 Porous Lug 5 Lance 6 Vacuum Degasser 7 Electromagnetic Induction Stirrer 8 Electrode for Electric Heating

Claims (1)

[Claims] 1. When deoxidizing molten steel in a holding vessel made of an Al ₂ O ₃ refractory, CaO-based flux, CaO-Al ₂ O ₃ -based flux or CaF so as to cover the surface of the molten steel. _{2. A} method for deoxidizing molten steel, which comprises adding a mixture of a flux containing ₂ , SiO ₂ and a deoxidizing agent, Al or Al ash, and stirring the molten steel.