JPH0819455B2 - Refining of stainless steel - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for refining stainless steel, and more particularly to a method for melting stainless steel having excellent cleanliness.

[Prior Art] Stainless steel is added with Al at the molten steel stage according to the purpose of obtaining it at the product stage.
Al ₂ O ₃ -based inclusions, especially cluster-shaped inclusions are generated,
During continuous casting, these deposits, deposits, and peel off on the tundish nozzle repeatedly, making the molten metal level in the mold unstable, inducing drooling of the oscillation marks and deteriorating the slab surface quality. This part was a surface defect such as a beard.

By the way, in stainless steel to which Al is added,
Inclusions such as ₂ O ₃ clusters cause surface flaws such as linear flaws at the product stage, and as a countermeasure, for example, after adding Al during VOD treatment as shown in JP-A-58-16018, CaO-Ca
By adding F ₂ -based flux on the surface of molten steel and then stirring it under a reduced pressure with Ar gas from a lance soaked in the molten steel from above, Al ₂ O ₃ -based inclusions are reduced in CaO-Al ₂ O ₃ -based alloy. A method has been proposed in which the inclusions are rendered harmless by changing to melting point inclusions.

However, these methods are methods for detoxifying the Al ₂ O ₃ -based inclusions generated during the VOD treatment, especially when Al is added, and those that are generated after the component adjustment and after the VOD treatment are continuously performed. It has no effect on the detoxification or reduction of Al ₂ O ₃ -based inclusions generated by the reaction between slag and [Al] in the molten steel until the casting is completed. Unless the Al ₂ O ₃ -based inclusions that are formed during this period are generated, or even if they are generated, they cannot be detoxified or reduced, the above-mentioned problems occur, and the slab surface needs to be cared for. Of course, the flaw was very economically disadvantageous.

[Problems to be Solved by the Invention] The present invention has been made in view of the background as described above, and the slag composition is controlled by simply stirring molten steel and slag after VOD treatment, and continuous casting is performed after VOD treatment. We provide a refining method that suppresses Al ₂ O ₃ inclusions that are generated by the completion of the process, that the treatment method is also extremely simple, that has a high degree of cleanliness, and that produces stainless steel that has an excellent surface property. To do.

[Means for Solving Problems] The present invention will be described in detail below. While a Cr-containing molten metal that has been roughly decarburized in a refining furnace such as a converter is received in a ladle and stirred with Ar gas from a porous plug. The ladle is evacuated, and the lance is blown from above to decarburize to the target decarburization area. After that, degassing treatment is performed by stirring Ar for 5 to 10 minutes in a vacuum state.
-Si is added and deoxidized, and CaO-based flux is added to perform slag refining. Then, under atmospheric pressure, Al, Fe-Si, and other alloys are added up to the target component values. This is the conventional VOD refining method. Decarburization in this VOD treatment,
After degassing, Al ₂ O ₃ is added when deoxidized with Al, Fe-Si, etc.
System inclusions are generated. However, the present inventors have Al ₂ O ₃ inclusions by Ar stirred by porous plug from subsequent refining and VOD process started by the CaO-type flux is added or is absorbed in the slag, CaO-Al ₂ O ₃ system It has been found that it can be a low melting point inclusion. However, since Al, Fe—Si and other alloys are added even after slag refining to adjust to the target component value, Al ₂ O ₃ type inclusions are regenerated at this point as well. In addition, CaO, SiO ₂ and Al ₂ O ₃ as well as metal oxides such as FeO, MnO and Cr ₂ O ₃ are present in the slag formed by adding the CaO-based flux. Since this slag is always present on the surface of the molten steel even after VOD treatment, [A
l] reduces SiO ₂ , FeO, MnO, Cr ₂ O _3, etc., resulting in Al ₂ O ₃
System inclusions are also formed after VOD treatment.

Therefore, even if the A ₂ O ₃ -based inclusions generated in the VOD treatment stage are converted to CaO-Al ₂ O ₃ -based inclusions at that time to be harmless, the Al ₂ O ₃ -based inclusions are again regenerated by the slag-metal reaction. Therefore, the tundish nozzle is squeezed during continuous casting, and the slab surface quality is not improved at all.

Therefore, after performing the conventional VOD treatment, continuously immerse the Ar lance in the molten steel under the atmosphere, or by mixing and stirring the molten steel and the VOD-treated slag by inhaling Ar gas using a porous plug, the slag is formed. Within a specific value. That is, [Al] in the molten steel and SiO ₂ , FeO, MnO, in the slag after VOD treatment
Cr ₂ O ₃ etc. are forcibly stirred, and S in molten steel [Al]
iO ₂ , FeO, MnO, Cr ₂ O ₃ etc. are reduced. By this treatment, [Al] in molten steel reduces slag and its value decreases, so it is more preferable to make it higher than the upper limit of the target component in advance, but a trace amount of metal Al or Al alloy may be added. . Since the stirring is performed in the atmosphere, the operation is easy, and since the slag thickness after VOD treatment is 100 mm or more, the molten steel does not contact with the outside air to be oxidized.

This stirring promotes the slag metal reaction, and [Al] in the molten steel reduces SiO ₂ , FeO, MnO, Cr ₂ O _3, etc.,
It is dissolved as Si, Mn, Fe, Cr in the molten steel, and [Al] becomes Al ₂ O ₃ . A large amount of Al ₂ O ₃ is generated when this stirring is performed, but this Al ₂ O ₃ is absorbed by the slag and disappears, and it hardly exists in the molten steel. That is, [Al] is SiO ₂ , F in the slag.
Contact with eO, MnO, Cr ₂ O ₃ at the slag metal interface and a reduction reaction with [Al] occurs, but the strength of stirring determines the supply rate of [Al] to the slag metal interface and reacts with SiO _2, etc. The speed is determined. It is desirable that [Al] is supplied at a speed higher than the supply speed of oxides such as SiO ₂ from the slag. This is oxygen [Al] is not Azukara the reaction of an oxide such as SiO ₂ without worth the quantitative small amount of SiO ₂ supply slow the oxide such as SiO ₂ of [Al] Al ₂ O supplied to molten steel and reacts with [Al] in other molten steel
_Will generate ₃ . Therefore, as an index of this stirring force, 0.015 to 0.025 Nm ³ /min.ton of Ar gas is preferable.
However, if the amount of Ar gas is more than 0.025 Nm ³ /min.ton and the stirring is too strong, it is not preferable because the molten steel comes into contact with the atmosphere even if the slag thickness after VOD treatment is thick due to stirring under the atmosphere. on the other hand
When the amount of Ar gas is 0.015 Nm ³ /min.ton or less, weak agitation results.
l] supply speed is insufficient. When immersing and stirring the lance, not only stirring molten steel and slag,
In order to eliminate segregation in molten steel, the immersion depth should be 20 below the surface of molten steel.
It is preferable to secure about 00 mm. As a result, the amount of SiO ₂ in the slag after VOD treatment becomes 10 wt% or less, the activity of SiO ₂ drops extremely, and the supply of oxygen from the SiO ₂ in the slag after this treatment until the end of continuous casting. Almost never
Even if [Al] and slag come into contact with each other, the reduction reaction of SiO ₂ etc. by [Al] cannot proceed and generation of Al ₂ O ₃ does not occur.

Therefore, the SiO ₂ content in the slag should be 10 wt% or less. Also, during the VOD process, VO is added to add CaO flux.
Slag after D treatment has a CaO-SiO ₂ -Al ₂ O ₃ slag, it is generally basicity is used to represent the oxygen supply capacity of the slag. 10 wt% of SiO _{2 in the} above slag
In addition to the following,
When it is 4.5 or more, generation of Al ₂ O ₃ is further suppressed.

Furthermore, metal oxides (MnO) other than SiO ₂ during VOD treatment,
_{(FeO), (Cr 2 O} 3) Al on SiO ₂ or more for a strong ability to provide oxygen to the molten steel than SiO ₂ if and remain in the slag
₂ O ₃ easily generated. Therefore, in addition to reducing SiO ₂ to 10 wt% or less by the stirring treatment after the VOD treatment, (FeO),
The total concentration of metal oxides formed of metal elements such as (MnO) and (Cr ₂ O ₃ ) that have a lower affinity for oxygen than Si is 3 wt.
The oxygen supply from the slag can be completely cut off by setting the content to not more than%. If the total amount of this metal oxide is more than 3%, Al ₂ O ₃ increases as a result. Further, by forming such a slag composition after the VOD treatment, the viscosity of the slag is changed and the slag easily absorbs Al ₂ O _3, and the treatment effect of the present invention is further improved.

[Examples] Next, examples of the present invention will be described to clarify the effects thereof.

16% Cr molten steel with [C] 0.30% in 175t oxygen top-bottom converter
145 t was blown with oxygen under vacuum, followed by degassing, then Al 600 kg, Fe-Si 754 kg were charged, and then CaO 2
Inputting 300kg, [C] 0.023%, [Si] 0.42%, [Mn]
0.10%, [Cr] 16.27% and [Al] 0.038% were obtained. Then, under atmospheric pressure, high-carbon FeCr 300 kg, to reach the target component value,
High carbon Mn (158 kg) and Al (300 kg) were added, and the VOD treatment was completed. At this point, [C] 0.054%, [Si] 0.49%, [Mn] 0.19
%, [Cr] 16.20%, and [Al] 0.138%. The slag composition after VOD treatment is (CaO): 52.46%, (SiO ₂ ): 14.43
%, (Al ₂ O ₃ ): 23.02%, (T.Fe): 0.24%, (Cr ₂ O ₃ ):
It was 0.04% and (MnO): 0.16%. During this VOD treatment, Ar was flowed from the porous plug at 300 to 500 / min. After that, under atmospheric pressure, the lance is immersed and Ar gas is reduced to 0.
The molten steel and the slag were directly stirred for 5 minutes at 021 Nm ³ / min / ton, and the slag was reduced by [Al] in the steel. As a result, the slag composition changed as follows.

(CaO): 56.05%, ( SiO 2): 7.61%, (Al 2 O 3): 28.62
%, (T.Fe): 0.23%, (Cr ₂ O ₃ ): 0.04%, (MnO): 0.1
Five%. The results of this treatment are shown in comparison with the conventional method. As shown in FIG. 1, the VO according to the present invention is obviously
It can be seen that the inclusions did not increase after the stirring treatment after D. The reason for this is that the floating of the inclusions was promoted together with the slag reduction, and the reformation of Al ₂ O ₃ was suppressed. Further, as shown in FIG. 2 and FIG. 3, the tundish nozzle aperture can be reduced by the method of the present invention, and the flaws on the surface of the product are significantly reduced, and the method of the present invention is extremely effective in improving the cleanliness of stainless steel. It can be seen that

[Advantages of the Invention] As described above, by stirring the slag and molten steel after the VOD treatment according to the present invention, Al ₂ O ₃ -based inclusions in the molten steel can be significantly reduced, and stainless steel with extremely high cleanliness can be produced. it can. This refining is an excellent refining method that not only improves product properties but also greatly reduces fixed costs by eliminating the need for slab maintenance and omits steps in the lower process.

[Brief description of drawings]

FIG. 1 shows the time elapsed after VOD treatment or after the present invention and Al ₂
FIG. 2 is a diagram showing a relationship between O ₃ inclusion indices, FIG. 2 is a diagram showing a comparison of the number of times of tundish nozzle aperture generation, and FIG. 3 is a diagram showing a comparison of product stagnation index.

Claims (1)

[Claims] 1. A molten Cr-containing steel that has been roughly decarburized in a converter by adding Al
After the treatment with COD, the molten steel containing Al and the slag on the molten steel bath surface after the VOD treatment are directly stirred to reduce the SiO ₂ in the slag by the Al in the molten steel and the SiO in the slag. ₂
A method for refining stainless steel, characterized in that the concentration is 10 wt% or less, and the total amount of metal oxides of metals having a smaller affinity for oxygen than silicon is 3% or less in the slag.