JP2767674B2 - Refining method of high purity stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for efficiently refining high-purity stainless steel using a ladle refining furnace.

[0002]

2. Description of the Related Art Chromium-containing molten iron typified by stainless steel is more susceptible to oxidation of chromium than decarburization in a region where the carbon concentration is low. Various methods have been proposed for decarburizing to a carbon concentration required by standards. Above all, AO
D and VOD are widely known. Of these, AOD is A
This is a method in which oxygen gas diluted with r is blown into the bath.
OD is a method in which oxygen is blown upward in a vacuum, but in any case, the partial pressure of CO gas generated in the decarburization reaction is reduced, and the decarburization reaction is prioritized over the chromium oxidation reaction. And Of these, the carbon concentration is 100 ppm
In order to melt ultra-low carbon steel as described below, decompression treatment after blowing acid refining is indispensable. Therefore, VOD is generally used. This decompression treatment promotes decarburization by oxygen in the molten steel, and is called a self-decarburization period.

However, VOD is a method in which the entire ladle is placed in a vacuum vessel, or a method in which the upper part of the ladle is evacuated by covering the upper part of the ladle, so that the upper space is narrow and the gas flow rate for stirring is reduced. When it is increased, there is a problem that the operation is hindered by the rocking of the steel bath or the splash by the bottom-blown gas. Also, in the self-decarburization period, it is important to secure a free surface area (bubble activated area) violently stirred by the bottom blown bubbles, but in VOD, slag generated during blown acid decarburization covers the surface. However, there is a problem that the bubble active area cannot be secured. Therefore, conventionally, as described in Kawasaki Steel Technical Report, No. 12, (1980), page 561 and thereafter, slag having good fluidity containing chromium oxide is suspended in a bath by vigorous stirring, and the slag in the slag is removed. The reaction between chromium oxide and carbon promoted decarburization. However, in this method, since the reaction takes place in the bath, the CO partial pressure increases due to the molten steel static pressure, and the reaction between chromium oxide and carbon hardly proceeds as easily understood from the free energy. It took a very long time to decarburize to extremely low carbon.

On the other hand, Japanese Patent Application Laid-Open No. 61-37912 discloses a method in which molten steel in a ladle is sucked up into a vacuum chamber through a large-diameter immersion pipe, and a stirring gas is supplied from a lower part. Have been. Further, in Japanese Patent Application Laid-Open No. 1-156416, while the position of the bottom blowing nozzle is eccentric to an appropriate range with respect to the center of the immersion tube, the top blowing oxygen collides with the bubble active surface, which is the floating region of the bottom blowing gas. There is disclosed a method for causing this to occur. Although these methods have solved the problem that the head space of the VOD is narrow, there is no description about the production of ultra-low carbon steel, and this method alone generates a large amount of chromium oxide in the immersion tube, so that it is stable. Was unable to melt the ultra-low carbon steel.

[0005]

SUMMARY OF THE INVENTION The present invention has a problem that the operation is hindered by the swing of the steel bath or the splash by the bottom-blown gas due to the small upper space of the VOD, The problem that the bubble active area cannot be secured for covering, and JP-A-61-37912,
A method for efficiently refining high-purity stainless steel without causing the problem that the method disclosed in Japanese Patent Application Laid-Open No. 1-156416 cannot stably produce ultra-low carbon steel. The purpose is to do so.

[0006]

SUMMARY OF THE INVENTION According to the present invention, a straight-body immersion pipe is immersed in molten steel in a ladle containing 5% or more of Cr, the inside of the immersion pipe is depressurized, and a stirring gas is supplied from a lower part of the ladle. In vacuum refining of stainless steel, the amount of C in molten steel is reduced to 0.
Oxygen blowing from 06 to 0.01% was performed, and then the acid supply was stopped, the degree of vacuum was set to P (Torr), the bath depth at the gas blowing position was set to H (m), and the flow rate of the blown gas was set to Q (NL).
/(Min.ton)), (H / (5 + P)) ×
By setting Q to be 0.3 or more, the chromium oxide-containing slag on the vacuum surface generated during the acid feeding and blowing is drawn into the bath, and then the immersion pipe tip surface and the molten steel surface outside the immersion pipe are melted. Interval Z
By setting (m) and Q to 0.25 or less as Z / Q, the entrained slag is allowed to flow out of the immersion tube, and decarburization on the bubble activated surface is promoted to the utmost. The refining method for high purity stainless steel is summarized.

Here, C in the slag generated in the first step
The r ₂ O ₃ 25% or less, 0.5 the CaO / Al ₂ O ₃
It is desirable that the slag be 2.0 and the viscosity is low.

[0008]

The present invention is based on the novel finding that the decarburization reaction in the extremely low carbon region is mainly a reaction on a free surface exposed to a vacuum atmosphere even in a high chromium molten steel. This is because, in order to achieve ultra-low carbon, Cr ₂
This is a very different fact from the fact that the reaction between molten steel and slag containing O _{3 and} having good fluidity must be promoted. Furthermore, the factor that governs the rate of this reaction is not the stirring energy density, which was thought to govern various reaction rates until now, but the inert gas bubbles blown from the lower part of the bath rise. The present inventors have clarified based on a number of experiments and detailed analysis that the area and strength of the bubble active surface, which is a region that ruptures on the free surface exposed to the vacuum, are determined. Here, the bubble active area is based on the knowledge of the water model, etc., in the case of a ladle degassing device, or a method of immersing a straight-body immersion pipe in molten steel in a ladle and depressurizing the immersion pipe, It is defined as the geometric area calculated by assuming that the gas blown from the bottom rises upward from the nozzle at an angle of 12 degrees on one side.
Further, the strength of the bubble active surface is defined by the gas flow rate, the gas blowing depth, and the degree of vacuum.

In order to effectively achieve the decarburization reaction in the bubble active area, it is essential to remove slag on the free surface. High chromium steel represented by stainless steel is decarburized under a vacuum in the region with high carbon concentration, so a large amount of slag containing chromium oxide is generated,
It is thick and covers the free surface. Eliminating this slag in a short time is an essential condition of the present invention. As a result of various tests and theoretical studies, the present inventors have clarified that slag can be discharged in a short time by satisfying the following requirements.

[0010] A straight-body-type immersion pipe is immersed in molten steel in a ladle,
A vacuum refining furnace for reducing the pressure in the immersion tube and supplying a stirring gas from the lower part of the ladle is used. The degree of vacuum is P (Torr), the bath depth at the gas injection position is H (m), and the flow rate of the injected gas is Q (NL / (minute · to).
n)), (H / (5 + P)) × Q should be 0.3 or more.

Under the conditions described above, the distances Z (m) and Q between the tip surface of the immersion tube and the molten steel surface outside the immersion tube are 0.2
5 or more. Hereinafter, the reason why each of the above requirements is required will be described. First,
The purpose of the above is to discharge the slag generated in the immersion tube during the blowing acid decarburization to the outside of the immersion tube, and this is only possible with such an apparatus configuration.

The conditions for promoting the entrainment of slag into the bath are as follows. To increase the flow velocity on the free surface and entrain the slag on the surface into the bath, the buoyancy of the blown gas is used. Energy needs to be maximized. The buoyancy energy depends on the ratio of the static pressure at the blowing position to the degree of vacuum at the surface and the gas flow rate, and the results shown in FIG. 2 were obtained by various experiments using a water model and molten steel. That is, it was discovered that slag was caught in the bath by setting (H / (5 + P)) × Q, which is an index corresponding to the relationship between the static pressure, the degree of vacuum, and the gas flow rate, to 0.3 or more.
Although this condition can be satisfied simply by lowering the pressure, equipment having an excessive exhaust capacity is required. In addition, simply trying to increase the gas flow rate requires an excessively large gas flow rate, which causes an operational problem due to a severe splash. Therefore,
By introducing the index of (H / (5 + P)) × Q,
It is possible to create conditions under which slag can be involved without causing the above problems. The term of the degree of vacuum is (5+
The reason for P) is based on the experimental result that the static pressure at a depth of 1 cm below the surface corresponds better to the surface flow velocity for slag entrainment, rather than the free surface.

Here, it is desirable to lower the viscosity of the slag in order to efficiently involve the slag. The above (H
/ (5 + P)) × Q, Cr ₂ O in the slag
₃ 25% or less, the CaO / Al ₂ O ₃ 0.5~2.0
It is more effective when a low-viscosity slag which secures a liquid phase ratio at 1600 ° C. of 75% or more is used. On the other hand, the figure shows conditions for removing the slag that has been caught up to the outside of the dip tube after reaching the lower end of the dip tube. If there is slag entrained at the lower end of the immersion tube, in principle, if the sum of the static pressure received from the molten steel inside the immersion tube and the kinetic energy of the slag particles is greater than the static pressure received from the molten steel outside the immersion tube, It will be discharged out of the dip tube. Here, the difference between the molten steel surface inside the dip tube and the molten steel surface outside the dip tube is h, and the distance between the lower end of the dip tube and the molten steel surface outside the dip tube is Z.
Then, the static pressure received from the molten steel in the immersion tube is almost ρgh
(Ρ: molten steel density, g: gravitational acceleration). Further, the static pressure received from the molten steel outside the immersion pipe is ρgZ + P0 (P0: atmospheric pressure). Here, since h is a value determined only by the degree of vacuum inside the immersion tube, the ratio of the static pressure received from the molten steel inside the immersion tube to the static pressure received from the molten steel outside the immersion tube is a function of only Z. On the other hand, the kinetic energy of the slag particles is equal to the circulation velocity of the molten steel, which is a value substantially dependent on the gas flow rate (Q). FIG. 3 shows the experimental results obtained for the slag discharge conditions. Z represents the ratio of the static pressure received from the molten steel inside the immersion pipe to the static pressure received from the molten steel outside the immersion pipe, and Q represents the kinetic energy of the slag particles. When the relationship is arranged, it is understood that the slag flows out of the tube when the Z / Q is 0.25 or less.

In the present invention, the content of Cr is set to 5% or more because it is intended for molten steel that contains Cr that oxidizes preferentially over carbon and that needs to be refined and refined under vacuum. In addition, since the decarburization reaction on the bubble activated surface is a reaction between carbon and oxygen in the molten steel, the oxygen concentration needs to be sufficiently high. For this reason, oxygen blowing under vacuum has a C content of 0.06 in molten steel.
%. However, when oxygen is blown until the C content in the molten steel becomes lower than 0.01%, there is a problem that oxidization loss of chromium in the blowing acid increases.

Further, Cr in the slag, which is defined as a slag composition necessary to promote slag entrainment,
₂ O ₃ 25% or less, CaO / Al ₂ O ₃ 0.5 to
2.0, the concentration of Cr ₂ O ₃ under the condition of a low-viscosity slag having a liquid phase ratio of 75% or more at 1600 ° C. greatly depends on the concentration of carbon at which acid supply is stopped. If the deoxidation is performed until the concentration becomes lower than 0.01%, the concentration of Cr ₂ O ₃ necessarily becomes higher than 25%. Therefore, the slag is hardly discharged and the bubble activated surface is hardly exposed, so that the decarburization speed is reduced even when the treatment is performed under the same conditions. On the other hand, when the blowing acid decarburization was stopped at a concentration of C in the molten steel higher than 0.06%, Cr ₂ O ₃
Is as low as about 15%, but since the temperature does not reach 1600 ° C., the slag is lower than 75% in terms of the liquid phase ratio of the slag, and cannot be a low-viscosity slag. Therefore, also in this case, the slag is hardly discharged, and the decarburization speed is reduced.

[0016]

EXAMPLES Examples were carried out using a 175-ton vacuum degassing apparatus. [C] is about 0.5% in the converter, [Cr]
Of molten steel containing 5% or more (mainly 10 to 25%) was refined in a vacuum degassing furnace having the shape shown in FIG. Refining is divided into the first stage and the second stage. In the first stage, Ar gas is supplied from the bottom blown porous brick, stirred and oxygen gas is blown from the top blow lance, and the degree of vacuum is about 100 Torr. Decarburized. In the second stage, the supply of acid from the top blowing lance was stopped and the degree of vacuum was reduced to 10T.
orr or less, and decarbonized by stirring with Ar.

Tables 1 and 2 show the results, where P is the degree of vacuum (Torr), H is the bath depth at the gas blowing position (m), and Q is
Represents the flow rate of the Ar gas for stirring (NL / (minute · ton)), and Z represents the distance (m) between the tip surface of the immersion tube and the molten steel surface outside the immersion tube. The decarburization rate is defined as C1 (%) and C2 (%) at the start and end of the second period, and the time of the second period is t1.
In the case of (minute), it is defined by the expression (1).

KC = (lnC1-lnC2) / t1 (1) Table 1 shows the results of refining performed in a vacuum degassing furnace having the shape shown in FIG. 13 are examples of the present invention. No. 14 is the case where the first-stage blow-off carbon concentration is reduced, but the first-stage chromium loss is large and not economical, and conversely, as in the case of No. 15, the first-stage blow-off carbon concentration is high. Has a low oxygen content in molten steel
Decarburization rate is low. Numbers 16 and 17 are (H / (5+
When P)) × Q is small, Nos. 18 and 19 are cases where Z / Q is large, but it can be seen that the slag discharge is inadequate and the bubble activated surface cannot be secured, and the decarburization rate is low.

Table 2 shows the refining by the vacuum degassing furnace having the shape shown in FIG. 1, the ladle degassing method in which the upper surface of the ladle was simply covered and the inside thereof was evacuated (so-called VOD; b). It is a result of comparison with refining by the RH method (so-called RH-OB; c) which can supply oxygen gas. In the case of the ladle degassing method, the slag cannot be discharged, so that the bubble activated surface cannot be secured and the decarburization rate is low. In the case of the RH method, the expansion of the bubbles is caused because Ar gas is supplied into the immersion tube. The decarburization rate is low because the bubble active area is small and cannot be sufficiently large.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

According to the present invention, high-purity stainless steel with an extremely low carbon concentration can be efficiently refined without causing the problem of rocking of the steel bath and generation of splash due to a large amount of bottom-blown gas. .

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an embodiment of the present invention using a vacuum degassing furnace.

[Explanation of symbols]

 A: Ladle B: Immersion tube C: Molten steel D: Lance for oxygen top blowing E: Porous brick for supplying stirring gas A: Bubble activated surface B: Slag C: Slag caught in bath D: Outside of immersion tube Discharged slag H: depth of gas injection Z: immersion pipe immersion depth h: gap between the inside and outside of the immersion pipe

FIG. 2 is a diagram showing a relationship between (H / (5 + P)) × Q and an index of slag entrainment in a bath, where P is a degree of vacuum (Torr),
H represents the bath depth (m) at the gas blowing position, and Q represents the Ar flow rate for stirring (NL / (minute · ton)).

FIG. 3 is a diagram showing an experimental result of obtaining slag discharge conditions.
Q represents the gas flow rate for stirring (NL / (minute · ton)), and Z represents the distance (m) between the tip surface of the immersion tube and the molten steel surface outside the immersion tube.

Continuation of the front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C21C 7/10 C21C 7/068

Claims (1)

(57) [Claims] In a vacuum refining of stainless steel, a straight-body type immersion pipe is immersed in molten steel in a ladle containing 5% or more of Cr, and the inside of the immersion pipe is depressurized and a stirring gas is supplied from a lower part of the ladle. Oxygen blowing is performed until the C content in the molten steel is 0.06 to 0.01%, and then the acid supply is stopped, the degree of vacuum is set to P (Torr), the bath depth at the gas injection position is set to H (m),
When the flow rate of the blown gas is Q (NL / (minute · ton)), by setting (H / (5 + P)) × Q to 0.3 or more, the oxidation on the vacuum surface generated during the acid blowing is performed. After the chromium-containing slag is rolled into the bath, the slag is rolled up by setting the distance Z (m) and Q between the tip surface of the dip tube and the molten steel surface outside the dip tube to 0.25 or less as Z / Q. A method for refining high-purity stainless steel, characterized in that slag is flowed out of a dip tube to promote decarburization on the bubble activated surface to the utmost.