JPH07173515A - Decarburization refining method of stainless steel - Google Patents

Abstract

PURPOSE:To provide a refining method capable of suppressing the formation of Cr oxidation loss at the time of oxygen blowing decarburization and efficiently executing decarburization down to an extra-low carbon region at the time of self-decarburization under a vacuum in decarburization refining of a stainless steel. CONSTITUTION:The basicity (CaO/SiO2) of the slag compsn. of Cr oxide (Cr2O3)- contg. slag is specified to 2.5 to 5 and the Al2O3-contg. concn. thereof to 10 to 20% at the time of executing oxygen blowing decarburization refining of the stainless steel having the Cr-contg. concn. of >=5% in a refining furnace under the atm. pressure or vacuum or at the time of executing reduction refining of Cr2O3 by in-steel carbon by stirring of the molten steel under the high vacuum after stopping of oxygen blowing in the vacuum refining furnace. As a result, the liquid phase rate of the slag is maintained at a high level and the activity of the Cr2O3 in the slag is maintained at a high level. The decarburization reaction is thus accelerated down to the extra-low carbon region without inducing the Cr oxidation.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a stainless steel which enables efficient reduction of Cr ₂ O ₃ by carbon in steel after the termination of blowing acid in the blowing acid decarburization refining and vacuum refining of stainless steel. The present invention relates to a decarburizing and refining method.

[0002]

2. Description of the Related Art As a blown acid decarburization refining furnace for molten steel, there are a top blowing converter, a bottom blowing converter, and a top bottom blowing converter (LD-OB furnace, LD-CB furnace).
Furnace, etc.), AOD and VOD, and for the blown acid decarburization and refining of stainless steel, the main raw material is stainless steel scrap in an electric furnace.
After alloying components such as Fe-Cr and Fe-Ni are added to this to produce stainless steel crude molten steel with high carbon content, or using a top-bottom converter, Cr ore is melt-reduced and refined to produce high-carbon stainless steel. After smelting the molten steel, a method of smelting the product by ABR or a bottom-blown converter to smelt it with deoxycarburized decarburization. Also, it is carried out mainly by Japanese blast furnace manufacturers. A method of decarburizing propellant acid while charging in a bottom blowing converter and gradually adding an alloy such as Fe-Cr from the top of the furnace after heating, and further, decarburizing decarburizing in the upper and lower blowing converters to 0. In the middle carbon region of 25 to 0.8%, decarburization below this is generally carried out by blowing acid decarburization under reduced pressure such as VOD.

In these methods, for the purpose of recovering Cr ₂ O ₃ produced during decarburization of propellant acid, a reducing agent such as Fe-Si or Al is added from the furnace after stopping the propellant acid, Cr ₂ O _{3 in} the slag is reduced. Also,
When melting high-purity stainless steel in a vacuum furnace such as VOD, for the purpose of stabilizing the carbon concentration at a low level, bottom blowing stirring under high vacuum after stopping the blowing acid and before adding the reducing alloy. In this way, Cr ₂ O ₃ is reduced by carbon in steel (self-decarburization).

Generally, as a method for improving the decarburization reaction efficiency in a blown acid decarburizing and refining furnace such as a converter and performing the decarburizing and refining efficiently in a low carbon region, a bottom blowing function is added to the converter.
The steel bath is vigorously stirred to promote the supply of carbon in the molten steel to the hot spot of the acid spray, which is the most active reaction area (Japanese Patent Publication No. 62-62-
14602), shifting the critical carbon concentration from the oxygen supply rate-controlling zone that maintains the decarboxylation rate to a high level to the carbon diffusion rate-controlling zone where the decarboxylation rate decreases, to the low carbon side, and decarburization. By controlling the blowing acid rate such as decreasing the blowing acid rate in the end-stage carbon diffusion rate control region, the decarboxylation efficiency was prevented from decreasing (Iron and Steel, 68th (1982), p1946. ).

As a refining method for molten Cr-containing metal, as disclosed in Japanese Examined Patent Publication No. 1-54409, a non-oxidizing gas such as Ar is introduced below the bath surface of the molten metal to stir the molten metal at the same time. A method has been proposed in which a mixed gas of oxygen and a non-oxidizing gas is sprayed onto the bath surface, and the ratio of the non-oxidizing gas to oxygen in the mixed gas sprayed onto the bath surface is increased as the carbon concentration in the molten steel decreases. ing.

Further, in JP-A-57-41312, when Fe-Si is added as required when smelting high Cr steel in a vacuum refining furnace, one of the excess Cr oxides in the slag is removed. Part is reduced to make Cr oxide proper, and then CaO, CaF ₂ , or Ca in a region below the critical carbon concentration.
Add one or more kinds of O-SiO ₂ based fluxes to CaO
/ SiO ₂ ratio of 1.0 to 1.5 forms a low melting point Cr oxide slag having a high fluidity to ensure slag-metal contact stirring, and decarburization in a region below the critical carbon concentration. A method of accelerating the reaction and melting an extremely low carbon and high Cr steel has been proposed.

[0007]

DISCLOSURE OF THE INVENTION As described above, when carrying out blown acid decarburization refining of a molten metal containing Cr in a refining furnace such as a converter,
As a method of improving the efficiency of decarboxylation, that is, suppressing the oxidation loss of Cr, there are methods disclosed in Japanese Patent Publication No. 62-14602, iron and steel, and Japanese Patent Publication No. 1-54409. However, in these methods C
Although the oxidative loss of r is reduced, it is difficult to suppress the content concentration of Cr ₂ O ₃ produced during decarburization of blown acid in the slag to 20% or less, which causes the fluidity of the slag to be extremely low. However, it was difficult to dramatically increase the slag-metal interface. Further, due to the slag property having poor fluidity, there is a problem that dust loss in propellant acid is increased and molten steel yield is reduced.

According to the method disclosed in Japanese Patent Application Laid-Open No. 57-41312, the fluidity of slag is improved, but this method does not sufficiently suppress Cr oxidation during decarburization of blown acid, and Cr in slag is not sufficiently suppressed. _In order to optimize the ₂ O ₃ concentration, Fe-Si
Inducing an increase in the basic unit of the reducing agent and increasing the basicity of 1.0 to 1.5 promotes melting loss of the refractory, which causes a problem in the life of the refining furnace. there were.

Therefore, the object of the present invention is to promote the decarburization reaction in the blown acid decarburization refining under atmospheric pressure or reduced pressure without causing excessive Cr oxidation and increase in dust loss up to an extremely low carbon region. And to enable self-decarburization under high vacuum after stopping the blowing acid in a vacuum refining furnace while efficiently preventing melting of refractory materials. .

[0010]

SUMMARY OF THE INVENTION The inventors of the present invention prevent the Cr oxidation loss in the blown acid decarburization refining of stainless steel, that is, improve the efficiency of decarboxylation and self-deoxidation after stopping the blowing acid in a vacuum refining furnace. As a result of earnest research on a method capable of simultaneously improving the decarburization reaction efficiency at the time of charcoal and preventing melting of refractory materials, the effect is remarkable by controlling the slag composition to an appropriate composition. Found.

The present invention has been made based on this finding.
The main point is that it is under atmospheric pressure or under vacuum.
Stainless steel with a Cr content of 5% or more in a refining furnace
When carrying out the decarburization refining of the acid of Cr, the Cr oxide (Cr
₂O₃) The slag composition of the slag containing the basicity (CaO /
SiO₂) Is 2.5 to 5 and Al₂O₃Content concentration
By setting 10 to 20%, the liquid phase ratio of slag is high
And maintain Cr in slag ₂O₃To maintain high activity
It is possible to maintain the excess amount of Cr acid even in the extremely low carbon range.
Specially promoting the decarburization reaction without causing
Method for decarburizing and refining stainless steel and vacuum refining
In a smelting furnace, convert stainless steel with a Cr content of 5% or more
The molten steel under stirring in a high vacuum after stopping the blowing acid
Cr by element₂O ₃When reducing and refining₂O
₃The slag composition of the contained slag is CaO / SiO₂2.
5-5 and Al₂O₃The content concentration should be 10-20%
By maintaining the liquid phase rate of slag at a high level, and
Cr in slag₂O₃It is possible to maintain high activity of
And a stainless steel characterized by accelerating the decarburization reaction.
It is in the decarburization refining method of stainless steel.

According to the present invention, it becomes possible to suppress Cr oxidation loss during blown acid decarburization refining under atmospheric pressure and under vacuum, and to maintain a high decarburization reaction efficiency during self decarburization during vacuum refining. In addition, it is possible to suppress excessive melting loss of the refractory material. The present invention is based on controlling the slag composition to an appropriate value during decarburization refining of stainless steel described below.

Generally, the concentration of Cr ₂ O ₃ generated in the slag decarburization refining of molten Cr-containing metal is 20% or more, and the melting point of Cr ₂ O ₃ -containing slag is as high as 2000 ° C. or more. The phase ratio is 50% or less, which means that the fluidity is extremely poor. The decarburization reaction mechanism of Cr-containing molten metal by blown acid decarburization, whether under atmospheric pressure or in vacuum, is that the Cr oxide generated on the bath surface is once in the hot spot of blown acid and in the steel bath The oxide that has been generated by reacting with the carbon and cannot contribute to decarburization floats on the bath surface and forms a solidified slag. At this time, although the Cr oxide transferred to the slag reacts with the carbon in the bath at the slag-metal interface, the reaction interface area is very small due to the solidified slag property, and the solid phase As the rate increases, the decarburization reaction rate also decreases. The above phenomenon is the same when self-decarburizing in a vacuum refining furnace.

Further, the reduction reaction of Cr oxide is carried out in the slag.
Cr₂O₃The higher the activity, the more advantageous it is, but fire resistance
The melting loss of MgO, which is the main component in the slag, increases and M in slag increases.
As the gO concentration increases, the magnesia
Achromite (MgO / Cr ₂O₃) Is formed, Cr₂
O₃Will significantly reduce the activity of. Therefore, large
In a vacuum refining furnace and under pressure and decarburization of blown acid under vacuum.
Decarburization reaction efficiency in both cases
To improve, improve the fluidity of slag,
The contact area is increased and the saturated MgO concentration is
It is an effective means to form slag with a composition as low as possible
Can be.

Cr₂O₃Improves fluidity of contained slag
Means for improving the liquid phase ratio of slag
As for Cr in slag₂O₃Concentration is about 10% or less
It is most effective to lower the
Of Cr₂O₃
There is a limit to reducing the concentration. Also, Cr₂O₃Dark
Even if the degree is 20% or more, CaO / SiO₂≤1.5
And CaF as slag slagging accelerator₂Nozono
To improve the liquid phase ratio of slag by adding
Is possible, but in this case, the saturated MgO concentration is increased.
Increase in the amount of refractory melting and MgO / Cr₂O₃of
Cr by forming₂O ₃There is a problem with the decrease in activity.

[0016] Therefore, the present inventors have proposed a process for promoting slag formation.
Al in slag as a component in lag ₂O₃Focus on concentration
And CaO / SiO₂2.5 to 5, preferably 2.5
~ 4.5 and Al₂O₃To be 10 to 20%
By, Cr₂O₃Is 20% or more
The lag-metal interface area is secured, and the saturated MgO concentration
MgO / Cr due to increase₂O₃To avoid the formation of C
r₂O₃Issue a method capable of maintaining high activity
Revealed

In this case, as the flux for adjusting the Al ₂ O ₃ concentration, even if Al ₂ O ₃ itself is added, the Al alloy is added to the propellic acid to generate Al ₂ O ₃ by the oxidation reaction of Al. You may let me. Here, if the Al ₂ O ₃ concentration is less than 10%, the liquid phase ratio of the slag is insufficient, and it becomes difficult to secure sufficient decarburization reaction efficiency. Further, even if the Al ₂ O ₃ concentration exceeds 20%, the effect of improving the liquid phase ratio is not so great (Fig. 1), and the melting loss of the refractory due to the increase of the Al ₂ O ₃ concentration which is an acid oxide Is a problem (Fig. 2).

Further, the Al ₂ O ₃ concentration range is 10 to 20.
%, If CaO / SiO ₂ is less than 2.5, the saturated MgO concentration in the slag will remarkably increase and the refractory unit consumption will be deteriorated (FIG. 3). Also, CaO
When / SiO ₂ exceeds 5, the saturated MgO concentration does not increase, but the deterioration of the liquid phase ratio of slag becomes a problem. Therefore,
The range of slag composition is CaO / SiO ₂ : 2.5
5, preferably 2.5 to 4.5, Al ₂ O ₃ concentration: 1
The optimum value is 0 to 20%.

[0019]

[Example] In Table 1, in a 150t converter (LD-OB furnace),
The results of melting stainless steel melt are shown below. In this case, decarburization is carried out to 0.7% as the blowing carbon concentration in both cases.
Blowing conditions include blowing acid velocity: 3.0 Nm ³ / min.
t, bottom blowing gas flow rate: 0.2 Nm ³ / min · t. In addition, Table 2 shows the results of self-decarburizing treatment by bottom blowing stirring after stopping the blowing acid at 150 tVOD.
In this case, the treatment conditions are as follows: molten steel temperature: 165
0 ° C, vacuum degree: 1 Torr, bottom blowing gas flow rate: 0.06N
The treatment was performed at l / min · t for 10 minutes.

As can be seen from Tables 1 and 2, the method according to the present invention is a very excellent method for decarburizing and refining stainless steel.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

INDUSTRIAL APPLICABILITY By using the present invention, in refining stainless steel, the problem of increase in melt loss of refractory materials can be avoided, excessive Cr oxidation loss at the time of decarburization of propellant acid can be prevented, and under vacuum. During the self-decarburization in, it became possible to efficiently perform refining to the extremely low carbon concentration range.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an Al ₂ O ₃ concentration and a liquid phase ratio of slag.

FIG. 2 is a diagram showing the relationship between Al ₂ O ₃ concentration and refractory melting loss index.

FIG. 3 is a diagram showing a relationship between CaO / SiO ₂ and a refractory melting loss index.

Claims (2)

[Claims] 1. A refining furnace under atmospheric pressure or under vacuum,
When carrying out the blown acid decarburization refining of stainless steel having a Cr content concentration of 5% or more, the slag composition of Cr oxide (Cr ₂ O ₃ ) containing slag is adjusted to a basicity (CaO / SiO ₂ ) of 2.5 to
5. A method for decarburizing and refining stainless steel, characterized in that the Al ₂ O ₃ content is 5 to 20%. 2. A vacuum refining furnace having a Cr content of 5% or less.
In the above stainless steel, under high vacuum after stopping the blowing acid
Cr due to carbon in steel by stirring molten steel₂O₃Reduction and refining of
When doing, Cr₂O₃The slag composition of the contained slag is C
aO / SiO ₂2.5 to 5 and Al₂O₃Concentration of content
Of 10 to 20% of stainless steel
Decarburization refining method.