JPH06228629A - Method for refining high purity stainless steel - Google Patents

Abstract

PURPOSE:To stably and efficiently refine molten steel to ultra low carbon and nitrogen concns. by using a vacuum refining furnace and specifying the relation among vacuum degree and gas blowing position and flow rate and the interval between the tip surface of an immersion tube and the molten steel surface at the outside of the immersion tube. CONSTITUTION:Into the molten steel (c) containing >=5% Cr in a ladle (a), the straight barrel type immersion tube (b) is dipped and the pressure in the immersion tube (b) is reduced, and stirring gas is supplied from the bottom part of the ladle (a) to execute the refining. In this method, oxygen blowing is executed to 0.06-0.01% C content in the molten steel. Thereafter, oxygen supply is stopped and in the case of using P (Torr) for the vacuum degree, H (m) for bath depth at the gas blowing position and Q (NL/min ton) for the blowing gas flow rate, it is necessary to satisfy the relation (H/(5+P))XQ>=0.3. Chromium oxide-containing slag B on the vacuum surface produced during oxygen blow-refining is entrapped into the bath and also, the interval Z (m) between the tip surface of the immersion tube and the molten steel surface at the outside of the immersion tube and Q is made to be <=0.25 of Z/Q. Then, the entrapped slag C is flowed out (D) to the outside of the immersion tube and the decarburization at the bubble activating surface is promoted to the limitation.

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 refining high-purity stainless steel using 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. Various methods have been proposed for decarburization to the carbon concentration required from the standard. Above all, AO
D and VOD are widely known. Of these, AOD is A
It is a method of blowing oxygen gas diluted with r into the bath.
OD is a method in which oxygen is blown upward under vacuum, but both methods reduce the partial pressure of CO gas generated in the decarburization reaction and give priority to the decarburization reaction over the oxidation reaction of chromium. I am trying. Of these, the carbon concentration is 100 ppm
VOD is generally used because the depressurization process after the smelting of blown acid is indispensable for producing the following ultra-low carbon steels. This depressurization process promotes decarburization by oxygen in the molten steel, and is called a self-decarburizing period.

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 vacuumed by covering the upper part of the ladle, so that the upper space is narrow and the gas flow rate for stirring is small. If the amount is increased, there is a problem that the operation is hindered due to the rocking of the steel bath and the splash due to the gas blown from the bottom. In addition, during the self-decarburization period, it is important to secure a free surface area (bubble active area) that is vigorously agitated by bottom-blown air bubbles. However, there is a problem that this bubble active area cannot be secured. Therefore, conventionally, as described in Kawasaki Steel Technical Report, No. 12 (1980), page 561 and subsequent pages, a slag having good fluidity containing chromium oxide is suspended in a bath by vigorous stirring, The decarburization was promoted by the reaction between the chromium oxide and carbon. However, in this method, the CO partial pressure is increased due to the static pressure of molten steel because the reaction occurs in the bath, and the reaction between chromium oxide and carbon is difficult to proceed as can be easily understood from the viewpoint of free energy. It took a very long time to decarburize to extremely low carbon.

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 pipe and supplying a stirring gas from a lower portion. Has been done. Further, in JP-A-1-156416, the bottom blowing nozzle position is eccentric with respect to the center of the immersion pipe in an appropriate range, and the top blowing oxygen collides with the bubble activating surface, which is the floating region of the bottom blowing gas. A method of causing is disclosed. These methods have solved the problem that the VOD has a small upper space, but there is no description about the melting of ultra-low carbon steel, and only this method produces a large amount of chromium oxide in the dip tube, which results in stable stability. Then, it was not possible to melt the ultra low carbon steel.

[0005]

SUMMARY OF THE INVENTION The present invention has a problem that the VOD has a narrow upper space and causes problems such as rocking of the steel bath and splash due to bottom-blown gas which hinders the operation. The problem that the bubble active area cannot be secured because of the covering, and JP-A-61-37912 and
Provided is a method for efficiently refining high-purity stainless steel without the problem that the method disclosed in Japanese Patent Application Laid-Open No. 1-156416 has the inability to stably produce an extremely low carbon steel. That is the purpose.

[0006]

According to the present invention, a straight barrel type immersion pipe is immersed in molten steel in a ladle containing 5% or more of Cr, the interior of the immersion pipe is decompressed, and a stirring gas is supplied from a lower portion of the ladle. In the vacuum refining of stainless steel, the amount of C in molten steel is 0.
Oxygen is blown up to 06 to 0.01%, then the oxygen supply is stopped, the degree of vacuum is P (Torr), the bath depth at the gas blowing position is H (m), and the flow rate of the blowing gas is Q (NL).
/ (Min · ton)), (H / (5 + P)) ×
By setting Q to 0.3 or more, the chromium oxide-containing slag on the vacuum surface generated during the acid-sending blowing is rolled into the bath, and then the tip surface of the immersion pipe and the molten steel surface outside the immersion pipe Interval Z
By setting (m) and Q to be 0.25 or less as Z / Q, the entrained slag is caused to flow out of the immersion pipe, and decarburization on the bubble activated surface is accelerated to the maximum. The main point is the refining method of pure stainless steel.

Here, C in the slag produced in the first step
r ₂ O ₃ is 25% or less, CaO / Al ₂ O ₃ is 0.5 to
It is desirable to set it to 2.0 and to make it a low viscosity slag.

[0008]

The present invention is based on the novel finding that even in high chromium molten steel, the decarburization reaction in the extremely low carbon region is mainly the reaction on the free surface exposed to the vacuum atmosphere. Up to now, it has been necessary to use Cr ₂ for extremely low carbonization.
This is a very different fact from what was said to contain the O ₃ and to promote the reaction between the slag with good fluidity and the molten steel. Furthermore, the factor that controls the reaction rate is not the stirring energy density, which was previously thought to control various reaction rates, but the inert gas bubbles blown from the lower part of the bath float up. The present inventors have clarified the area and strength of the bubble active surface, which is the region that bursts on the free surface exposed to vacuum, based on many experiments and detailed analysis. Here, the bubble active area, based on the knowledge of the water model and the like, in the case of a ladle degassing device or a method of immersing a straight barrel type immersion pipe in molten steel in the ladle and depressurizing the inside of 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 each side.
Further, the strength of the bubble active surface is defined by the gas flow rate, the gas injection depth, and the degree of vacuum.

In order to effectively achieve the decarburization reaction in the bubble active area, it is essential to eliminate the slag on the free surface. High-chromium steel, typified by stainless steel, is decarburized with propellant acid under vacuum in a high carbon concentration region, so a large amount of slag containing chromium oxide is generated,
It thickly 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 revealed that the slag can be discharged in a short time by satisfying the following requirements.

Immersing a straight barrel type immersion pipe in molten steel in a ladle,
Use a vacuum refining furnace that decompresses the inside of the dipping pipe and supplies a stirring gas from the bottom of the ladle. The degree of vacuum is P (Torr), the bath depth at the gas blowing position is H (m), and the flow rate of the blowing gas is Q (NL / (min · to.
n)), (H / (5 + P)) × Q should be 0.3 or more.

Under the above conditions, the distances Z (m) and Q between the tip surface of the dip tube and the molten steel surface outside the dip tube are set to 0.2 as Z / Q.
Must be 5 or more. Hereinafter, the reason why each of the above requirements is essential will be described. First,
The above is intended to discharge the slag generated in the dip pipe during the decarburization of propellant acid to the outside of the dip pipe, and it becomes possible only with such a device configuration.

[0012] is a condition for promoting the entrainment of the slag into the bath, but in order to increase the flow velocity on the free surface and cause the slag on the surface to be entrained in the bath, the buoyancy of the blown gas is required. You need to maximize your energy. 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 the 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.
This condition can be satisfied by merely reducing the pressure, but a facility having an excessive exhaust capacity is required. Further, if the gas flow rate is simply increased to satisfy the requirement, an excessively high gas flow rate is required, which causes a problem in operation due to violent splash. Therefore,
By introducing the index (H / (5 + P)) × Q,
It is possible to create a condition in which the slag can be wound without causing the above problems. In addition, the degree of vacuum is (5+
P) is based on the experimental result that the static pressure at a depth position of 1 cm below the surface, rather than the free surface, better corresponds to the surface flow velocity for slag inclusion.

Here, it is desirable to reduce the viscosity of the slag in order to efficiently wind the slag. Above (H
/ (5 + P)) × Q, Cr ₂ O in the slag
₃ to 25% or less, CaO / Al ₂ O ₃ 0.5 to 2.0
It is more effective when the low-viscosity slag has a liquid phase ratio of 1600 ° C. of 75% or more. On the other hand, (1) shows the conditions for removing the entrained slag from the immersion pipe after reaching the lower end of the immersion pipe. When there is slag caught at the lower end of the immersion pipe, in principle, if the total static pressure received from the molten steel inside the immersion pipe and the kinetic energy of the slag particles is greater than the static pressure received from the molten steel outside the immersion pipe, the slag will be It will be discharged to the outside of the immersion pipe. Here, the difference between the molten steel inside the immersion pipe and the molten steel outside the immersion pipe is h, and the distance between the lower end of the immersion pipe and the molten steel outside the immersion pipe is Z.
Then, the static pressure received from the molten steel in the immersion pipe is approximately ρgh.
(Ρ: molten steel density, g: gravitational acceleration). 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 in the immersion pipe, 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 is a function of Z only. On the other hand, the kinetic energy of the slag particles becomes equal to the circulation flow velocity of the molten steel, which has a value that almost depends on the gas flow rate (Q). FIG. 3 is an experimental result for obtaining the slag discharge condition. It shows a ratio Z between the static pressure received from the molten steel inside the immersion pipe and the static pressure received from the molten steel outside the immersion pipe, and Q representing the kinetic energy of the slag particles. When arranged in a relationship, it can be seen that slag flows out of the pipe when Z / Q is 0.25 or less.

In the present invention, the purpose is to provide molten steel containing Cr which is preferentially oxidized over carbon and which needs to be subjected to finish refining under vacuum. Therefore, Cr is set to 5% or more. Further, since the decarburization reaction on the bubble activation surface is a reaction between carbon and oxygen in the molten steel, the oxygen concentration needs to be sufficiently high. Therefore, oxygen blowing under vacuum produces a C content of 0.06 in the molten steel.
It is necessary to carry out up to%. However, when oxygen blowing is performed until the amount of C in the molten steel becomes lower than 0.01%, there arises a problem that the oxidation loss of chromium in blowing acid increases.

Further, Cr in the slag, which is defined as the slag composition necessary for promoting the inclusion of the slag,
₂ O ₃ 25% or less, CaO / Al ₂ O ₃ 0.5-
Under the conditions of 2.0 and a low viscosity slag that secures a liquid phase ratio of 75% or more at 1600 ° C., the concentration of Cr ₂ O ₃ largely depends on the carbon concentration at which oxygen transfer is stopped, and the amount of C When the acid is decarburized to a concentration lower than 0.01%, the concentration of Cr ₂ O ₃ inevitably becomes higher than 25%. For this reason, the slag is difficult to be discharged and the air bubble active surface is hard to be exposed, so that the decarburization rate is reduced even when treated under the same conditions. On the other hand, when the blown acid decarburization is stopped at a concentration of C in the molten steel higher than 0.06%, Cr ₂ O ₃
However, since the temperature does not reach 1600 ° C., the liquid phase ratio of the slag is lower than 75%, and it cannot be a low viscosity slag. Therefore, also in this case, the slag is difficult to be discharged and the decarburization rate is reduced.

[0016]

EXAMPLES The examples were carried out using a 175 ton scale vacuum degasser. About 0.5% of [C] in the converter, [Cr]
5% or more (mainly 10 to 25%) of molten steel was smelted, and then refined in a vacuum degassing furnace having the shape shown in FIG. Refining is divided into the first phase and the second phase. In the first phase, Ar gas is supplied from a bottom-blown porous brick to stir and oxygen gas is sprayed from a top-blown lance, and the degree of vacuum is about 100 Torr. Decarburized. In the second phase, the acid supply from the top blowing lance was stopped and the vacuum degree was set to 10T.
The temperature was adjusted to or or less, and Ar was stirred to decarburize.

The results are shown in Tables 1 and 2, where P is the degree of vacuum (Torr), H is the bath depth (m) at the gas blowing position, and Q is
Is the Ar gas flow rate for stirring (NL / (min · ton)), and Z is the interval (m) between the tip surface of the immersion pipe and the molten steel surface outside the immersion pipe. The decarburization rate is C1 (%) and C2 (%) at the beginning and end of the second period, and the second period time is t1.
When it is set to (minutes), it is defined by the equation (1).

KC = (lnC1-lnC2) / t1 (1) Table 1 shows the results of refining in a vacuum degassing furnace having the shape shown in FIG. Up to 13 are examples of the present invention. No. 14 is the case where the blowout carbon concentration in the first period was lowered, but when the chromium loss in the first period is large and it is not economical, conversely when the blowout carbon concentration in the first period is high as in number 15. Second, because there is little oxygen in the molten steel
Decarburization rate is low. Numbers 16 and 17 are (H / (5+
P)) × Q is small, and Nos. 18 and 19 are large Z / Q, but it can be seen that the bubble activated surface cannot be secured and the decarburization rate is low due to poor slag discharge.

Table 2 shows refining by a vacuum degassing furnace having the shape shown in FIG. 1 and a ladle degassing method (so-called VOD; b) in which the top of the ladle is simply covered and the inside is evacuated. It is the result of comparison with refining by the RH method (so-called RH-OB; c) capable of supplying oxygen gas. In the case of the ladle degassing method, since the slag cannot be discharged, the bubble active surface cannot be secured and the decarburization rate is low. The decarburization rate is low because it is small and the bubble active area is not large enough.

[0020]

[Table 1]

[0021]

[Table 2]

[0022]

According to the present invention, it is possible to efficiently smelt highly pure stainless steel having an extremely low carbon concentration without causing the problems of rocking of the steel bath and generation of splash due to a large amount of bottom-blown gas. .

[Brief description of drawings]

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

[Explanation of symbols]

 A: Ladle b: Immersion pipe c: Molten steel d: Oxygen top blowing lance e: Porous brick for supplying stirring gas A: Bubble activated surface B: Slag C: Slag caught in the bath D: Outside the immersion pipe Discharged slag H: Gas injection depth Z: Immersion pipe immersion depth h: Distance between molten metal surface height inside and outside the immersion pipe

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

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

Claims (1)

[Claims] 1. Vacuum smelting of stainless steel in which a straight barrel type dip tube is immersed in molten steel in a ladle containing 5% or more of Cr, the pressure in the dip tube is reduced, and a stirring gas is supplied from the bottom of the ladle. Oxygen is blown until the amount of C in the molten steel is 0.06 to 0.01%, then the oxygen transfer is stopped, the degree of vacuum is P (Torr), the bath depth at the gas blowing position is H (m),
When the flow rate of the blown gas is Q (NL / (min · ton)), by setting (H / (5 + P)) × Q to 0.3 or more, the oxidation on the vacuum surface generated during the acid sending blowing can be performed. After the chromium-containing slag is rolled up in the bath, the gaps Z (m) and Q between the tip surface of the dipping pipe and the surface of the molten steel outside the dipping pipe are set to Z / Q of 0.25 or less so that the slag is drawn in A slag refining method for high-purity stainless steel, characterized in that the slag is allowed to flow out of the immersion pipe to accelerate the decarburization on the bubble activated surface to the maximum.