JPH08109410A - Finish decarburization refining of stainless steel - Google Patents

Abstract

PURPOSE: To provide a method for efficiently executing a finish decarburization while keeping the decarburizing efficiency by oxygen to high grade, in the oxygen-blowing decarburization refining of a stainless steel under vacuum. CONSTITUTION: In the vacuum decarburization refining, in which a straight barrel type immersion tube 2 is dipped into the molten steel 4 having >=5% Cr concn. in a ladle and the pressure in the immersion tube is reduced, and also, stirring gas 5 is supplied from the bottom part of the ladle and the oxygen blowing is executed in the range of 1.0-0.01% of the carbon concn., the vacuum degree is made to be <=50Torr of high vacuum degree and Ar gas is mixed to the gaseous oxygen at 3.0-0.2 oxygen/Ar ratio. Further, the oxygen gas blowing speed is made to be 0.04-0.40 (Nm<3> /hr.cm<2> ) per gas bubble active surface under condition of >=10% gas bubble active area to the whole molten steel surface area and to >=100% gaseous oxygen blowing surface. By this method, Cr oxidation during the oxygen-blowing decarburization is restrained and the decarburizing efficiency of the oxygen can be kept to a high grade.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an efficient method for finishing decarburization and refining of stainless steel by a ladle refining furnace.

[0002]

2. Description of the Related Art Molten iron containing chromium, typified by stainless steel, is more likely to undergo an oxidation reaction of chromium than a decarburization reaction in a region where the carbon concentration is low. Have proposed various methods for decarburizing carbon to the required carbon concentration. Above all, AOD
And VOD are widely known.

Of these, AOD is a method in which oxygen gas diluted with Ar is blown into the bath, and VOD is a method in which oxygen is blown upward under vacuum. In either case, CO produced by decarburization reaction It is characterized by lowering the partial pressure of gas and prioritizing the decarburization reaction over the oxidation reaction of chromium.
Of these, VOD is generally used because depressurization refining is indispensable for producing low carbon stainless steel having a carbon concentration of 500 ppm or less.

However, VOD is a method in which the entire ladle is placed in a vacuum container or a method in which the entire ladle is placed in a vacuum by covering the upper part of the ladle. There is a problem that the operation is hindered by the splash, and even when the gas flow rate for stirring is increased to promote decarburization that suppresses chromium oxidation,
Fluctuation of molten steel and splash due to bottom blowing gas increase,
Operation was hindered.

On the other hand, Japanese Patent Laid-Open No. 61-37912 discloses a method of sucking molten steel in a ladle into a vacuum tank through a large-diameter dip tube and supplying a stirring gas from a lower portion. Has been done. Further, in Japanese Patent Laid-Open No. 1-156416, the bottom blowing nozzle is eccentrically positioned with respect to the center of the dip tube and the top blowing oxygen is a floating region of the bottom blowing gas. A method of striking an active surface is disclosed.

Although these methods have solved the problem that the upper space of VOD is narrow, there is a problem that the suppression of chromium oxidation is insufficient especially in the low carbon region only by these methods.

[0007]

SUMMARY OF THE INVENTION According to the present invention, since the upper space of the VOD is small, the swing and splash of molten steel may hinder the operation.
Decarburization that enables efficient refining without the problem of insufficient suppression of chromium oxidation in the low carbon region that the methods disclosed in JP 37912 and JP-A-1-156416 have It is intended to provide a method.

[0008]

SUMMARY OF THE INVENTION The gist of the present invention is to immerse a straight barrel type immersion pipe in molten steel in a ladle having a Cr concentration of 5% or more, reduce the pressure in the immersion pipe, and lower the ladle. A stirring gas is supplied from the part and the carbon concentration is 1.0 to 0.0.
In the vacuum decarburizing refining in which the oxygen gas is blown from above in the range of 1%, the degree of vacuum is set to a high vacuum of 50 Torr or more, and the oxygen / Ar ratio of the refining oxygen gas is 3.0 to 0.2. Ar gas is mixed in and the oxygen gas spraying rate is 0.04 to 0.40 (Nm ³ / hr.
cm ² ), the bubble active area is 10% or more of the total molten steel surface area,
In addition, there is a finish decarburizing and refining method for stainless steel that suppresses Cr oxidation during the decarburization of blown acid and maintains the efficiency of decarboxylation at a high level by making the oxygen gas blowing surface 100% or more.

[0009]

[Function] It is known that the decarburization reaction of molten chromium-containing steel proceeds by reducing the chromium oxide produced by the blown oxygen gas by the carbon in the steel in the hot spot region or in the bath (( 1) and (2)). 2Cr + 3 / 2O ₂ → Cr ₂ O ₃ (1) Cr ₂ O ₃ +3 C → 2Cr + 3CO ↑ (2) Conventionally, as a method for promoting the decarburization reaction, that is, the formula (2), the CO partial pressure of the atmosphere is used. It is known that reducing (P _co ) is effective, and AOD and V
The principle of OD is also based on this finding.

As a result of various detailed experiments, the present inventors
In order to accelerate the decarburization reaction more than ever, that is, in order to accelerate the reduction of the generated chromium oxide (Equation (2)), it is effective to promote the reduction in the high temperature flash point region. I found out. The present invention was made based on this finding, and by mixing Ar gas with refining oxygen gas, the CO partial pressure in the fire point region, which is the main reaction site in the blowing acid decarburization reaction, is reduced. At the same time, it is based on accelerating the renewal of the blowing acid fire point region by forming the blowing acid fire point on the bubble active surface, which has a large effect of increasing the effective boundary area.

Usually, the temperature in the blowing acid burn point region is about 2400 ° C. Therefore, the standard free-form energy in the equation (2) is also small, which is a very advantageous reaction site for promoting the reaction. Therefore, in the present invention, as a method capable of accelerating the reaction in the flash point region (reduction of chromium oxide) to the limit,
The feature is that Ar gas, which is an inert gas, is mixed with the refining oxygen gas.

This is due to the fact that in addition to the conventionally known effect of depressurization, P _co in the fire point region, which is the main decarburization reaction site.
It is possible to maximize the driving force of the reaction by lowering the reaction temperature and reducing the equilibrium carbon concentration in the hot spot region as much as possible by the synergistic effect with the hot spot temperature. Further, the present invention is characterized in that a blowing acid fire point is formed on the bubble active surface. Here, the bubble active area, which is the region where the inert gas bubbles blown from the lower part in the bath float and burst on the free surface, based on the knowledge of the water model etc., a ladle degassing equipment, In the case of a method of immersing a straight barrel type immersion pipe in molten steel in a ladle and depressurizing the inside of the immersion pipe, it is assumed that the gas blown from the lower part rises upward from the nozzle at an angle of 12 ° on one side. Defined as the calculated geometric area. Also, the strength of the bubble active surface is defined by the gas flow rate, the gas injection depth, and the degree of vacuum.The actual bubble active area is several times the geometric area due to the bursting effect on the surface of the gas bubble. It has an effective boundary area of.

The significance of forming a blowing acid hot spot on the bubble active surface lies in the refinement of the chromium oxide produced at the blowing acid hot spot and an increase in the reduction reaction interfacial area. In other words, the synergistic effect of the bubble activation effect and the surface disturbance caused by the top-blown jet remarkably promotes the interface renewal effect in the hot spot region. Therefore, since the chromium oxide formed in this hot spot region is fine and the effective interfacial area is extremely large, the reduction reaction in the vicinity of the hot spot region is significantly promoted.

In these cases, the flow rate of the inert gas supplied from the lower part is 0.03 to 0.17.
A range of (NL / min · ton) is desirable. Specifically, the degree of vacuum is set to a high vacuum of 50 Torr or more, the refining oxygen gas is mixed with Ar gas in a ratio such that the oxygen / Ar ratio is 3.0 to 0.2, and the oxygen gas blowing rate is 0.04 to 0.40 (Nm ³ / hr · c
m ² ), the bubble active area is 10% or more of the total molten steel surface area and 100% or more of the oxygen gas sprayed surface,
The above effect is most remarkable.

Among these, if the degree of vacuum is a low vacuum of less than 50 Torr, reduction in the hot spot region is promoted due to insufficient reduction of P _co itself and insufficient bubble breaking effect of the bubble active surface. There is a problem of not getting. Furthermore, when the oxygen / Ar ratio is greater than 3.0, Ar in the flash point region
Due to the insufficient diluting effect due to, the effect of lowering P _co cannot be obtained, and conversely when it is less than 0.2, A
Since the hot spot is cooled by the r gas and the hot spot temperature is lowered, the effect itself of the hot spot reaction becomes small.

Further, spraying of oxygen gas per bubble active surface
The speed is 0.04 (Nm³/ Hr · cm²Less than)
Since the oxygen supply rate itself is slow,
Will significantly hinder productivity,
0.40 (Nm³/ Hr · cm ²) Is exceeded
Chromium oxide is generated due to excessive supply rate
There is a problem that the amount increases and the decarboxylation efficiency decreases.
Occurs.

The bubble active surface is 10% of the total molten steel surface area.
If it is smaller, the reduction of the reduction reaction site itself leads to a decrease in reaction efficiency. Further, if the bubble active area is less than 100% of the oxygen spraying area, the chromium oxide generation site becomes a bubble active surface. There is an area other than
The generated chromium oxide grows in a united state without being miniaturized, and there arises a problem that the reduction efficiency in the blowing acid hot spot region decreases.

Here, an excessive reaction (bumping phenomenon) accompanied by a large amount of splash may occur especially at the start of the treatment due to the high vacuum of 50 Torr or more during the treatment. In the invention, since there is no freeboard restriction, there is no problem in operation even if bumping occurs.

[0019]

EXAMPLES The examples were carried out using a 175 ton scale vacuum degasser. In the converter, [% C] is about 0.7%, [C
r] in an amount of 5% or more (mainly 10 to 20%) is melted, and then [%] in a vacuum degassing furnace having the shape shown in FIG.
C] = 0.01%, and blown acid decarburization refining was implemented. Also,
At this time, the inert gas supply rate from the lower part is uniformly 0.
It carried out at 07 (NL / min * ton). The results are shown in Table 1 and Table 2 (continued from Table 1).

[0020]

[Table 1]

[0021]

[Table 2]

Test numbers 1 to 8 are examples according to the present invention. On the other hand, Test No. 9 shows a case where the degree of vacuum is low, but the decarboxylation efficiency is low and the chromium oxidation loss is large due to the insufficient effect of lowering P _co . Further, Test No. 10 is when the oxygen / Ar ratio is high and Test No. 13 is when the oxygen supply amount per bubble active surface is excessive. In these cases, the oxygen supply becomes excessive, resulting in a large chromium oxidation loss. Becomes On the contrary, Test No. 11 is a case where the oxygen / Ar ratio is low, but in this case, P _co does not decrease due to the temperature decrease due to the hot spot cooling, which causes an increase in chromium oxidation loss. Furthermore, test number 12
Is the case where the oxygen supply amount per bubble active surface is too small,
In this case, since the oxygen supply amount itself becomes insufficient, it takes a long time to perform the treatment, resulting in a decrease in productivity. Further, in Test Nos. 14 and 15, the formation of the bubble active surface itself is insufficient, so that the decarboxylation efficiency is low and the chromium oxidation loss is large.

Therefore, as is clear from Tables 1 and 2, the method of the present invention keeps the efficiency of decarboxylation at a high level by the synergistic effect of the decrease of P _{co at the} reaction interface and the increase of the reduction reaction site due to the formation of the bubble active surface. It turns out that this is an excellent method.

[0024]

Industrial Applicability According to the present invention, it becomes possible to carry out finish decarburization refining of stainless steel which maintains the decarboxylation efficiency at a high level without impairing operability, and moreover, stable refining with less chromium oxidation. Was achieved.

[Brief description of drawings]

FIG. 1 is a diagram showing an example of an aspect of a refining method according to the present invention.

FIG. 2 is a diagram showing the relationship between the oxygen / Ar mixing ratio and the decarboxylation efficiency.

[Explanation of symbols]

 1 Ladle 2 Dipping pipe 3 Porous plug 4 Molten chromium-containing steel 5 Inert gas 6 Top blowing lance 7 Oxygen / Ar mixed gas

Claims (1)

[Claims] 1. A straight-body type immersion pipe is immersed in molten steel in a ladle having a Cr concentration of 5% or more, the interior of the immersion pipe is decompressed, and a stirring gas is supplied from the lower part of the ladle, and the carbon concentration is low. 1.0
In the vacuum decarburization refining in which the oxygen gas is blown from above, the degree of vacuum is set to a high vacuum of 50 Torr or more, and the oxygen gas for refining has an oxygen / Ar ratio of 3.0 to 0.01%.
Ar gas was mixed at a ratio of 0.2 and the oxygen gas spraying rate was 0.04 to 0.40 (Nm ³
/ Hr · cm ² ), bubble active area is 10 of total molten steel surface area
%, And 100% or more of the oxygen gas sprayed surface, a method for finishing decarburization and refining of stainless steel.