JPH08104916A - Method for melting high purity stainless steel - Google Patents

Abstract

PURPOSE: To suppress Cr oxidation loss and to effectively obtain a high purity steel by suitably combining an oxygen feeding speed per bubble-active surface corresponding to the C concn., a surface stirring force and a vacuum degree, in a vacuum decarburization-refining of a Cr-containing molten steel. CONSTITUTION: The pressure in a straight-barrel type tube immersed into molten steel having >=5% Cr in a ladle is reduced and the stirring gas is supplied and O2 gas is blown from the upper part at 1.0-0.01% C concn. to execute the vacuum decarburization-refining. At this time, R in the equation I related to the vacuum degree P (Torr) oxygen feeding speed F (Nm<3> /hr.ton), gas bubble active area S (m<2> ), blowing gas flowing rate Q (NL/min.ton) and carbon concn. [%C] in the molten steel in the case of being >=10% the total molten steel surface area to the gas bubble active surface, >=100% the O2 , blowing surface and <=0.1% C concn., is made to be 1.5-3.7 to execute the oxygen-blowing decarburization while lowering R by -ΔR/Δt=0.13-0.40. Further, after stopping the oxygen-blowing, at the time of executing the vacuum-decarburization with the stirring only by blowing the inert gas from the bottom part of the ladle at the distance H (m) from the molten steel surface in the immersion tube at <=0.01 C concn., K in the equation II is controlled to 0.5-3.5.

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 smelting high-purity stainless steel in 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 required carbon concentration, among which AOD
And VOD are widely known methods.

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 decompression refining after blowing acid refining is indispensable for melting ultra-low carbon steel with a carbon concentration of 100 ppm or less. Decarburization is promoted by the oxygen contained therein, which is called the self-decarburization period or degassing 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 placed in a vacuum by covering the upper part of the ladle. There was a problem that the operation was hindered by the splash. Also, 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, but in VOD, the slag generated in propellic acid covers the surface, so there is a problem that this air bubble active area cannot be secured. there were.

Therefore, conventionally, as described in Kawasaki Steel Technical Report No. 12 (1980), page 561 and thereafter,
A slag having a good fluidity containing chromium oxide was suspended in the bath by vigorous stirring, and decarburization was promoted by the reaction between chromium oxide and carbon in the slag. However, in this method, since the reaction takes place in the bath, the CO partial pressure increases due to the static pressure of molten steel, and the reaction between chromium oxide and carbon is difficult to proceed even considering the change in free energy. Therefore, it took a very long time to decarburize to the extremely low carbon region.

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 dip tube in an appropriate range, and the top blowing oxygen is made to collide with the bubble active surface which is the floating region of the bottom blowing gas. A method is disclosed. Although these methods have solved the problem that the upper space of VOD is narrow, the suppression of chromium oxidation particularly in the low carbon region is insufficient only by these methods, and a large amount of chromium oxide is contained in the immersion pipe. Because of the formation, it was not possible to stably manufacture the ultra low carbon steel.

[0007]

SUMMARY OF THE INVENTION The present invention has a problem that the operation of the molten steel is hindered by the swing and splash of molten steel because the upper space of the VOD is narrow.
-37912 and Japanese Unexamined Patent Publication (Kokai) No. 1-156416 have insufficient suppression of chromium oxidation in the low carbon region, which is caused by slag covering the surface during self-decarburization. It is an object of the present invention to provide a method for efficiently producing high-purity stainless steel without the problem that an active surface cannot be secured and stable ultra-low carbon steel cannot be produced. is there.

[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. Part is supplied with an inert gas for stirring and the carbon concentration is 1.0
In a vacuum decarburization refining in which oxygen gas is sprayed from above, 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, and the carbon concentration is within the range of 0.01%. Is 0.1% or less, the degree of vacuum P
(Torr), acid transfer rate F (Nm ³ / hr · ton),
Bubble active area S (m ² ), blowing gas flow rate Q (NL / m
in ・ ton), carbon concentration in molten steel [% C] is R = lo
g {(F / S) ・ P / (Q ・ [% C])}: 1.5-
Within the range of 3.7 and the decreasing rate of R (-ΔR / Δt: 1
/ Min) is reduced to R of 0.13 to 0.40 while performing decarburization with propellant acid, and after stopping the propellant acid, the level of carbon in the dip tube is 0.01% or less. When performing decarburization by stirring only by injecting an inert gas from the bottom of the ladle at a distance of 1 to H (m), K = log {SH
By controlling the Q / P} to 0.5 to 3.5, there is a method for producing high-purity stainless steel that efficiently makes the carbon concentration 20 ppm or less.

[0009]

The inventors of the present invention have conducted various experiments and found that even in molten steel containing chromium, the oxygen feeding rate per bubble activated surface, the surface stirring force, and the degree of vacuum, which are suitable for the respective carbon concentrations, were appropriately combined. , Showed that the formation of chromium oxide can be suppressed to the utmost limit. The present invention is based on this finding.

Generally, the carbon concentration is 0.
It is known that the decarburization rate is represented by the following first-order reaction equation in an extremely low carbon region of 1% or less. -D [% C] / dt = (A · k / V) · ([% C] − [% C] _e ) (1) A is the reaction interface area, k is the mass transfer coefficient, V is the molten steel volume,
[% C] _e is the equilibrium carbon concentration at the reaction interface. Therefore, in the range of [% C] ≦ 0.1%, the rate-determining process of the reaction is the mass transfer of carbon in the molten steel, and the decarburization reaction rate decreases as the carbon concentration decreases.

Further, the decarburizing reaction of molten chromium-containing steel is carried out by chromium oxide (C
It is known that r ₂ O ₃ ) is caught in the hot spot region or in the bath and reduced by the carbon in the steel bath to proceed. Therefore, in order to efficiently proceed the decarburization reaction and suppress chromium oxidation loss as much as possible, Cr ₂ due to carbon in the steel bath should be used.
It is important to improve the reduction rate of O ₃ and to secure an oxygen supply rate commensurate with the reduction rate.

As a method for maximizing the reduction reaction rate, the present invention provides a method for producing a Cr ₂ O ₃ production site and a main reduction site of propellant acid in the decarburization of propellant acid on the bubble active surface. It is based on forming it. here,
The bubble active surface is a region where bubbles blown from the lower part of the ladle are broken on the surface, and the actual bubble active area is defined by the gas flow rate, the gas blowing depth, and the vacuum degree.
It has an effective interfacial area which is several times the bubble active area calculated geometrically. Therefore, by forming the blowing acid fire point on the bubble active surface, the generated Cr ₂ O ₃ can be made finer by the bubble breaking effect, and the reaction boundary area of the reduction reaction can be made extremely large.

Further, since the blowing acid fire point is a high temperature region of about 2400 ° C., the equilibrium carbon concentration ([%
C] _e ) also has an extremely small value, which is advantageous for the progress of the reduction reaction. Specifically, the degree of vacuum P (Torr), the rate of oxygen transfer F (Nm ³ / hr · ton), the bubble active area S (m
² ), blowing gas flow rate Q (NL / min · ton), carbon concentration in molten steel [% C], R = log {(F / S) · P / (Q · [% C])} ( 2), the value of R is in the range of 1.5 to 3.7, and R
Decrease rate (-ΔR / Δt: 1 / min) is 0.13 ~
It is to carry out decarburization of propellant acid while lowering R to 0.40.

At this time, R exceeds 3.7, -ΔR
If / Δt is smaller than 0.13, the oxygen supply is excessive or the bubble active area is insufficiently secured, resulting in an insufficient reduction rate and chromium oxidation. Further, the value of R is less than 1.5 and -Δ
When R / Δt is greater than 0.40, although chromium oxidation can be suppressed, the oxygen supply rate itself becomes insufficient, which causes an extension of the processing time and causes a problem of reduced productivity. .

Whether the operation of decreasing -ΔR / Δt is performed continuously or stepwise within a limited time,
The effect is almost the same. Furthermore, carbon concentration 20pp
When smelting high-purity stainless steel of m or less, self-decarburization treatment is required after decarburization with blown acid. In order to efficiently perform this self-decarburization treatment, a bubble activated surface is secured, And it is important to maintain the interface renewal on the bubble active surface.

In securing the bubble active surface, what is particularly important is that even if a small amount of chromium oxide formed during the decarburization of blown acid remains on the bubble active surface, the surface decarburization is hindered and the decarburization rate is increased. Therefore, it is necessary to completely discharge the chromium oxide out of the immersion pipe during self-carburizing. Therefore, as conditions for maintaining the interface renewal on the bubble active surface and completely discharging the chromium oxide out of the immersion tube, the vacuum degree P (Torr), the bubble active area S (m ² ),
Injected gas flow rate Q (NL / min · ton), distance from the molten metal surface in the immersion pipe to the gas injection position was H (m), and K = log {S · H · Q / P} (3) In this case, it is important to control the value of K within the range of 0.5 to 3.5.

In this case, if the value of K is smaller than 0.5,
Due to inadequate renewal of the bubble active surface and insufficient discharge of chromium oxide, the decarburization rate will decrease,
On the contrary, when the value of R is larger than 3.5, there is almost no further renewal effect of the bubble active surface, and problems such as wear of refractory due to excessive supply of the blowing gas flow rate will occur. .

[0018]

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. Furthermore, after stopping the blowing acid, just by stirring with an inert gas from the lower part,
Self-decarburization treatment was performed for 30 minutes.

Table 1 shows the examples of the present invention in the decarburizing stage of propellant acid together with comparative examples. Test number 9 is the case where the value of R becomes smaller than 1.5 during the operation, and test number 11 is the case where the decreasing rate of R (-ΔR / Δt) exceeds 0.40. In each case, although the chromium oxidation loss generation amount is small, the processing time becomes long due to the insufficient oxygen supply amount required for decarburization, and the productivity is reduced.

Table 2 shows examples of the present invention in the self-decarburizing period together with comparative examples. Exam number 1
No. 9 is the case where the K value exceeds 3.5, but the area of the bubble activation surface and the stirring strength are sufficiently maintained, and although the arrival [C] is low, it is caused by an increase in the blowing gas supply amount and the like. It is not practical because it accelerates the wear of refractory materials. As is clear from Tables 1 and 2, due to the effect of the present invention in maintaining a proper balance between the oxygen supply rate and the reduction rate in the blowing acid period, chromium oxidation loss is reduced, and bubbles are generated in the degassing period. By maintaining the active area and stirring strength,
It can be seen that this is an excellent method for efficiently producing high-purity stainless steel.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

EFFECTS OF THE INVENTION According to the present invention, it is possible to suppress chromium oxidation during the blowing acid period and to efficiently produce high-purity stainless steel during the self-decarburizing period without impairing operability. It was

[Brief description of drawings]

FIG. 1 is a diagram showing an example of a mode of a refining method according to the present invention in a blowing acid period.

FIG. 2 is a diagram showing an example of a mode of a refining method according to the present invention during a self-decarburizing period.

FIG. 3 is a diagram showing a relationship between R and a production amount of Cr oxidation loss.

FIG. 4 is a diagram showing a relationship between K and a decarburization rate constant.

[Explanation of symbols]

 1 Ladle 2 Immersion Pipe 3 Porous Plug 4 Molten Steel Containing Chromium 5 Inert Gas 6 Top Blowing Lance 7 Oxygen Gas 8 Slag 9 Chromium Oxide

Claims (1)

[Claims] 1. A straight barrel 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 an inert gas for stirring is supplied from the lower part of the ladle, and In the vacuum decarburization refining in which the carbon concentration is in the range of 1.0 to 0.01% and the oxygen gas is sprayed from above, the bubble active area is 10% or more of the total molten steel surface area and 100% of the oxygen gas sprayed surface.
% Or more and the carbon concentration is 0.1% or less, the degree of vacuum P
(Torr), acid transfer rate F (Nm ³ / hr · ton),
Bubble active area S (m ² ), blowing gas flow rate Q (NL / m
in · ton) and carbon concentration in molten steel [% C], R obtained from the following formula is in the range of 1.5 to 3.7, and the following −ΔR / Δt is 0.13 to 0.40. Decarburization while lowering R, and after stopping the blowing acid, from the bottom of the ladle at a distance of H (m) from the surface of the molten metal in the dip pipe in the range where the carbon concentration is 0.01% or less. A method for producing high-purity stainless steel, which comprises controlling K obtained from the following equation to 0.5 to 3.5 when performing vacuum decarburization treatment by stirring only by blowing an inert gas. However, R is an index showing the balance of oxygen supply per bubble active area and reduction rate of chromium oxide, and R = log {(F / S) · P /
(Q · [% C])}, and −ΔR / Δt is the rate of decrease of R per minute, ΔR is the change amount of R, and Δt is the time change (min). Further, K is an index showing the disturbance intensity of the bubble active surface, and K = log {S · H · Q / P}.