JP2582316B2 - Melting method of low carbon steel using vacuum refining furnace - 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 producing low carbon steel by vacuum refining of molten steel in a ladle.

[0002]

2. Description of the Related Art Conventionally, in order to melt ultra-low carbon steel,
The molten steel decarbonized to a certain degree in a converter etc. receives steel in a vessel such as a ladle and then depressurizes a part of the molten steel using a process with an exhaust device such as a vacuum degassing device such as RH method or DH method. A method of reducing carbon in molten steel by disposing in a gas atmosphere and reducing the pressure on the gas side has been used. However, when the carbon concentration is further reduced, there has been a problem that the decarburization speed is stagnant and a long time treatment is required. In order to solve this, a method of increasing the recirculation gas flow rate in RH, increasing the diameter of the immersion pipe, or increasing the recirculation velocity of molten steel due to an increase in the ascending and descending speed of the tank in the DH method, etc., are usually taken. However, the increase in the ring flow rate has a limit in terms of equipment, and the increase in the tank elevating speed also has a limit due to the followability of molten steel. To solve these problems, a method of blowing O ₂ into a vacuum chamber has been proposed to increase the decarburization rate.
As disclosed in Japanese Patent Application Laid-Open No. 51-152212, a multi-tube tuyere for blowing oxidizing gas is provided near the bath surface immersed in the molten steel on the side wall of the vacuum tank, and the oxidizing gas is directly introduced into the circulating molten steel under reduced pressure. RH degassing equipment that blows gas into the furnace, or as described in Japanese Patent Application Laid-Open No. 60-184819, molten steel with a carbon concentration of 0.1% or more melted in a converter under reduced pressure. A method for producing low-nitrogen steel by blowing up gaseous oxygen and decarburizing it to promote the denitrification that occurs at this time, and as disclosed in JP-A-1-52016,
Inert high-chromium steel in a vacuum ladle refining furnace by blowing inert gas from the bottom of the ladle with vigorous stirring.Oxygen gas is blown from the top blowing lance onto the steel bath surface in the ladle to decarbonize. There is disclosed an apparatus in which an argon gas or a nitrogen gas is mixed with an oxygen gas from a blowing lance so that a flow rate of an upper blown mixed gas is equal to or higher than a limit flow rate of an oxygen gas. Further, JP-A-61-37
No. 912 discloses that the ratio between the inner diameter of the immersion pipe and the inner diameter of the ladle is set to a value within a specific range, and that the specific depth is set via a blowing pipe from below the ladle below the projection plane of the immersion pipe. In this method, high-speed vacuum decarburization is performed by blowing an inert gas from a blowing position of an inert gas and blowing an oxidizing gas onto a surface of molten steel in a vacuum chamber through an upper blowing lance.

[0003]

However, according to the method disclosed in Japanese Patent Laid-Open No. 51-151212, RH
Therefore, the reflux does not start unless the degree of vacuum is set to a strong degree of vacuum of 300 Torr or less. In this state, the decarburization reaction starts abruptly, so that a violent splash is generated and the treatment from high carbon cannot be performed. Further, in the case of denitrification by blowing oxygen into an RH vacuum chamber disclosed in Japanese Patent Application Laid-Open No. 60-184819, it is necessary to suppress the acid supply rate to an extremely low level because the production of iron oxide is severe. In addition, as long as RH is assumed, reflux does not start unless the degree of vacuum rises to a certain degree, so that the decarburization reaction starts rapidly in this state. There is a problem that the speed cannot be increased. Next, JP-A-1-5201
In the method of producing stainless steel by blowing oxygen and argon over VOD, which is disclosed in Japanese Unexamined Patent Publication No. 6, the stirring of the surface is small due to the small bubble active surface in the case of ladle degassing. Since it is always present on the surface, there is a problem that the area where the oxygen gas contacts the molten steel is small and the decarburizing effect is extremely poor. Further, Japanese Patent Application Laid-Open No. 61-37912 discloses a method in which an inert gas is blown into a ladle below a projection surface of an immersion tube from below through a blowing tube. Determining the inert gas injection position as the depth of injection from the surface of the molten steel is considered to be the idea of accelerating the decarburization reaction due to an increase in the reflux velocity of the molten steel and an increase in the residence time of the inert gas. No new knowledge of the bubble active area is disclosed. This method is intended to positively improve the reflux of molten steel.This method cannot stably decarburize to the extremely low carbon region, and cannot stably suppress the generation of splash during processing. In addition, there is a problem that it is difficult to smelt highly clean steel stably. Accordingly, it is an object of the present invention to solve these problems and to provide a method capable of efficiently producing low-carbon steel in a state in which iron with low iron oxide production is high and splash is less generated.

[0004]

The present inventors have SUMMARY OF THE INVENTION, the test for a method of blowing these conventional immersion tube under vacuum O ₂ top-blown and from the bottom into the ladle through the blowing tubes inert gas However, it was not possible to perform decarburization to a stable extremely low carbon region. Therefore, as a result of further research, the basic factors for promoting decarburization under reduced pressure are not the flow rate of molten steel and the residence time of the injected inert gas, but the bubble activation. I got a new finding that it is an area. The present invention has been made based on this finding. The point is that a single straight-body-shaped immersion pipe is immersed in molten steel in a ladle refined to a carbon concentration of 0.1 to 1.0% by a converter, and the degree of vacuum in the immersion pipe is reduced. Is continuously reduced to a low vacuum of 100 Torr or more without restoring pressure, and an inert gas is supplied into the immersion tube to form a region (bubble activated surface) in which the gas bubbles blown rise on the surface and the gas lift is used. Using the molten steel reflux, the bubble active area is set to 15 to 95% of the vacuum surface in the immersion tube, while the degree of vacuum (P) in the immersion tube, oxygen gas (O) blown from the upper portion of the immersion tube, and inert gas (N ) Is expressed by the following equation .
In such a case, the constant a is in a certain range of 900 to 2500.
Then, according to the degree of vacuum (P) in the immersion tube that changes every moment,
Mixing of oxygen gas (O) and inert gas (N) blown from above
A method for melting low carbon steel using a vacuum refining furnace, wherein the ratio (R) is changed.

[0005]

The present invention will be described below in detail with reference to the drawings. FIG. 1 is a sectional view of a vacuum refining apparatus according to the present invention,
The molten steel 4 is accommodated in the ladle 1, and the immersion pipe 2 is immersed and stopped in the molten steel 4 in the ladle 1. The immersion pipe 2 communicates with the exhaust pipe, and depending on the degree of vacuum in the immersion pipe 2, molten steel 4
Is sucked up. Then, an inert gas 5 is blown into the molten steel from a porous plug 3 disposed at the bottom of the ladle 1 in which the lower section of the immersion pipe 2 is vertically downward, and the molten steel 4 is stirred and mixed. On the other hand, an oxygen blowing lance 6 is provided from above the immersion tube 2, and a mixed gas of O ₂ and an inert gas is blown into the surface of the immersion tube vacuum tank from the oxygen blowing lance. In this case, the flow condition of the molten steel in the ladle from the deep position under gas agitation was examined in detail by repeating experiments using water models and mercury models.As a result, when the molten steel head was high, the molten steel circulation force due to the buoyancy of bubbles was It is extremely large and has the strongest upward flow in the bubble floating region on the surface. On the other hand, on the surface other than the bubble floating region, the flow is directed toward the furnace wall in a direction horizontal to the surface, and this flow collides with the furnace wall and changes to a downward flow. Of these flows, the speed of the flow toward the furnace wall in the horizontal direction has been found by the present inventors to correspond to the so-called stirring energy and the reflux speed.

In addition, for the decarburization reaction under vacuum,
The large upward flow in the area where the injected gas floats is overwhelmingly more important than the velocity of the flow toward the horizontal furnace wall. It has been clarified that the factor controlling the charcoal properties is the bubble active area defined as follows. That is, the bubble active area is defined as a region where the blown gas bubbles float on the surface. Regarding the bubble active area, the bubble active area (An) for the gas blown in the vertical direction is expressed by the formula (1) according to the observation result of the water model, the mercury model, or the actual machine. Bubble active area (Au)
Is given by equation (2). An = 3.14 × (0.212 × H ) 2 ‥‥‥‥ (1) Au = 3.14 × (7 × Q 0 · 67) 2/2 ‥‥‥‥ (2) where, H is Distance from injection position to molten steel surface (m)
And Q is the gas blowing amount per nozzle (Nm ³
/ S).

The bubble active area defined by the equation (1) is a major factor that governs the decarburization rate. The reason is considered as follows. 1) The free surface where the decarburization reaction occurs has little resistance to metal flow due to the absence of slag. Therefore, the flow on the metal phase surface is extremely easy as compared with the reaction between the slag and the metal. Therefore, the mass transfer rate which is greatly affected by the surface flow rate can be sufficiently increased only by stirring with a small amount of gas, and even if it is further increased, the influence on the reaction rate is small. This is the reason why the index determined by the speed of the flow toward the furnace wall in the horizontal direction, such as the stirring energy and the recirculation speed, does not relate to the decarburization reaction speed. 2) In order to increase the speed of the decarburization reaction, the most important factor is not the increase in the mass transfer rate but the increase in the reaction surface area. By the way, considering a series of processes in which bubbles rise and break on the surface, the bubbles float on the surface after floating due to the difference in adhesion with the molten steel, and then the surrounding molten steel surface is wavy. Among them, the moment when the bubble bursts on the surface, the largest surface area is formed, and the surface area is hardly increased by the waves generated afterwards. On the other hand, the speed of the upward flow at the outermost surface formed by the floating of the bubbles is affected by the gas blowing speed and the stirring energy, which is the kinetic energy that causes the droplet to scatter to a high level.
And does not significantly affect the morphology of the free surface at the moment when an individual bubble bursts on the surface. Therefore, the free surface formed when each bubble bursts on the surface is almost constant, and it is important to increase the number of bubbles bursting on the surface in order to effectively increase the surface area of the entire reaction vessel. . For this purpose, it is necessary to float the bubbles over a wide area so that the coalescence of the bubbles can be suppressed as much as possible. Therefore, the size of the bubble active surface is important.

FIG. 2 is a graph in which the decarburization rate constants are arranged by the ratio of the bubble active area to the vacuum surface area using dip tubes having various cross-sectional areas. From this figure, it is understood that the decarburization rate decreases when the bubble active surface exceeds 95% of the vacuum surface area. This is because if air bubbles float on the entire surface under vacuum, the generation of a downward flow is hindered, and the reflux is extremely deteriorated, so that the molten steel decarburized on the surface under vacuum and the molten steel inside the ladle cannot be replaced. This is due to being sufficient. Also, when the bubble active surface is less than 15% of the vacuum surface, the decarburization rate decreases, but this is because the bubble active area becomes small and an effective reaction surface area cannot be secured.

Further, the active area of the bubble is adjusted to 15 to 9 of the vacuum surface.
In addition to the condition of 5%, the carbon concentration in the converter is 0.1 ~
It is necessary to use molten steel refined to 1.0%. The reason for this is that if the blow-off carbon concentration in the converter exceeds 1.0%, there is a problem that the refining time in the secondary refining furnace becomes longer, casting becomes impossible one after another, and the productivity is reduced. If the supply rate of the top-blown oxygen gas is increased in order to shorten the time, problems such as generation of splash and generation of iron oxide occur. Also, the blow-off carbon concentration in the converter is 0.1%.
If it is less than the above, iron oxide generation in the converter becomes intense,
Problems such as a decrease in iron yield, refractory erosion due to a rapid temperature rise, and an increase in nitrogen absorption occur.

Next, the fact that the mixed gas of the oxygen gas and the inert gas is blown from the upper part of the dip tube according to the present invention is as follows.
If oxygen gas is blown alone, the formation of iron oxide is unavoidable and the nitrogen lowering behavior is also inadequate. This problem can be solved by blowing a mixed gas of oxygen gas and inert gas. Because it is done. The reason is considered as follows. 1) Even when the degree of vacuum is the same, in the case of oxygen gas top blowing, the nitrogen partial pressure of the atmosphere in the immersion tube is high because the value is determined by the balance between the generated CO gas and the amount of leak air. .
On the other hand, when a mixed gas with an inert gas is blown, a large amount of the inert gas is blown to the interface, so that the nitrogen partial pressure of the atmosphere is kept low, and denitrification is promoted. 2) Even when the same flow rate of oxygen gas is blown, mixing with the inert gas increases the total gas flow rate coming out of the upper blow nozzle, so that the dynamic pressure of the gas on the molten steel surface is increased, so that the ignition point The temperature rises and the production of iron oxide is suppressed,
The decarburization reaction is promoted. On the other hand, in a high carbon concentration region where supply of oxygen is required, a high vacuum atmosphere is not required, and the minimum necessary vacuum determined from an equilibrium relationship in which the decarburization reaction has priority over the iron oxide formation reaction. Degree is fine.
This value depends on the carbon concentration range, but at least 100 Torr
If the vacuum is higher than this, the volume expansion at the time of generation of CO gas becomes large, so that the splash is generated intensely, and the generated gas rises rapidly in the immersion tube tank, so that the generated splash is generated. Is raised to a high position. From these facts, the mixed gas of oxygen gas and inert gas was blown from the upper part of the immersion tube, and the degree of vacuum in the upper tank of the immersion tube was determined to be a low degree of vacuum of 100 Torr or more.

Furthermore, the relationship between the ratio (R) of the pressure (P) and oxygen gas (O) and an inert gas to the total flow rate of the inert gas (N) (N), and P = 760-a × R I do. However, it is desirable to change the mixing ratio of the inert gas so that a becomes 900 to 2500. Here, when the inert gas is smaller than this ratio, iron oxide is generated and the iron yield is reduced, and slag composed of iron oxide is generated on the surface. And the rate of decarburization is reduced. Conversely, if the amount of inert gas is larger than this ratio, the supply of oxygen becomes insufficient, and decarburization will not proceed. In other words, when oxidizing gas is blown onto molten steel,
As the charcoal reaction mechanism, as is well known, the following (1),
It is established by the chain of equation (2). Fe + 1 / 2O ₂ → FeO (1) FeO + C → Fe + CO (2) At this time, the equilibrium carbon concentration ([% C] ′) is expressed by the equation (3).
expressed. [% C] '= Pco / K (3) (K is an equilibrium constant and is uniquely determined by temperature.
The decarburization reaction rate equation is expressed by the following equation (4).
Is done. −d [C] / dt = (A · k · / V) · ([% C] − [% C] ′) (4) (A is the effective reaction area, k is the material coefficient, and V is the volume of molten steel )
Therefore, the decarburization reaction is promoted (the decarburization rate is kept at a high level).
To increase the effective reaction area,
Increasing the mass transfer coefficient and driving the reaction
The value of the force term ([% C]-[% C] ') is kept large.
It is important that Where the effective reaction area of
In order to keep the mass transfer coefficient large, the above equation (2)
It is very useful to place the reaction site of
It is effective. This is because, as mentioned above, the bubbles burst.
Therefore, the effective reaction area is significantly increased.
Therefore, the decarburization rate expressed by the equation (4) can progress at a high speed.
And Furthermore, regarding the driving force
As the carbon concentration itself decreases with
It will decrease as the response progresses. Therefore, the reaction speed
When considering only keeping the degree itself high,
The elemental concentration ([% C] ') is always kept low. That is, P
It is important to keep co low. Lower this Pco
One of the effective means to keep
It is possible to keep. However, the communication described above
Especially in areas where the carbon concentration is relatively high between the start of treatment and the middle of treatment.
Under high vacuum of 100 Torr or more.
The occurrence of the splash is severe, and the operation is severely hindered.
And other problems. Therefore, the occurrence of splash
To suppress decarbonization and promote decarburization, "
Under the conditions of
It is important to control to positive Pco ". This is actually
Carbon concentration in the driving force term ([% C]-[% C] ')
Degree ([% C]) and the equilibrium carbon concentration at that time ([% C]
)) ([% C] / [% C] ') within a certain range
Control to prevent excessive splash
And maintain a high decarburization rate at the same time. others
Effective means for spraying the reaction site of equation (4)
Mix an inert gas into the oxidizing gas.
Dilution) to reduce Pco at the reaction interface
It is a method. However, in practice, Pco is a vacuum
Because it is a factor greatly affected by the degree, specific control
As means, the mixing ratio of oxygen gas and inert gas is
It is necessary to change as follows. Furthermore, this mixed
For the range of ratios, there is less inert gas than the stated ratio.
In this case, iron oxide (FeO) is produced, and the iron yield is low.
A slag of iron oxide is generated on the surface on the lower side
The subsequent contact between oxygen gas and molten steel is impeded,
Causes a decrease in Conversely, if there is more inert gas than this ratio
In this case, the supply of oxygen will be insufficient and decarburization will not proceed.
You.

[0012]

【Example】

Example 1 Molten steel refined to a carbon concentration of 0.1 to 1.0% in an oxygen top-blowing converter was carried out in a vacuum refining furnace shown in FIG. 1 using a 175-ton ladle. Table 1 shows the conditions at that time. R is represented by N / (O + N) at the oxygen gas supply rate (O) and the inert gas supply rate (N), and O + N is 3
９9 Nm ³ / (Hr · ton). B in the table represents (bubble active area) / (vacuum surface area) × 100. In this case, Comparative Examples 8 and 9 which do not satisfy the bubble active area ratio B to those satisfying the conditions of the present invention.
Indicate that the decarburization rate constant is low and the decarburization efficiency is poor. Similarly, Comparative Examples 6 and 7 in which the R value is 0 or 1.0, ie, only oxygen gas blowing and only inert gas blowing, have low decarburization rate constants, which also indicate poor decarburization efficiency. I have.

[0013]

[Table 1]

Example 2 As in Example 1, a carbon concentration of 0.1 in an oxygen top-blowing converter was used.
Using a 175-ton ladle, molten steel refined to ~ 1.0%
It carried out in the vacuum refining furnace shown in FIG. Table 2 shows the conditions at that time. The vacuum patterns in Table 2 are shown in FIG. This figure is a relationship diagram between the degree of vacuum P (Torr) and the ratio (R) of the inert gas (N) to the total flow rate of the oxygen gas (O) and the inert gas (N).
Assuming that the pressure is equal to or higher than Torr, the hatched area A indicates the range of the present invention. The straight line B indicates the relationship with the degree of vacuum when the R value is low, and corresponds to Comparative Example 7 in Table 2. Further, a straight line C indicates a case in which the inert gas having a high R value is extremely large, and corresponds to Comparative Example 8. These all indicate that the decarburization rate constant is low and the decarburization efficiency is poor.
Note that a straight line A ₁ in the region A indicating the embodiment is the same as that of the _first embodiment.
3, 4, 5, and 6, and the straight line A2 corresponds to the _second embodiment. It can be seen that all of these have high rate constants. This means that the decarburization efficiency is extremely high.

[0015]

[Table 2]

[0016]

As described above, the practice of the present invention enables extremely efficient refining of high-purity steel without causing any operational problems due to generation of severe splashes and generation of iron oxide. Thus, an industrially excellent effect is obtained.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a vacuum refining apparatus according to the present invention,

FIG. 2 is a diagram showing a ratio of a bubble active area to a surface area under vacuum with respect to a decarburization rate constant according to the present invention;

FIG. 3 is a diagram showing the relationship between the degree of vacuum and the ratio of inert gas to the total flow rate of oxygen gas and inert gas.

[Explanation of symbols]

 1 Ladle, 2 Dipping tube, 3 Porous plug, 4 Molten steel, 5 Inert gas, 6 Oxygen and inert gas injection lance.

Continuation of front page (56) References JP-A-52-52110 (JP, A) JP-A-2-194116 (JP, A) JP-B-59-19967 (JP, B2)

Claims (1)

(57) [Claims] 1. A single straight-body-shaped immersion pipe is immersed in molten steel in a ladle refined in a converter to a carbon concentration of 0.1 to 1.0%, and the degree of vacuum in the immersion pipe is restored. 1 continuously without pressure
A low vacuum of at least 00 Torr is supplied, and an inert gas is supplied into the immersion tube to form a region (bubble active surface) in which the gas bubbles blown and float on the surface, and the molten steel is recirculated by gas lift. The active area is 15 to 95% of the vacuum surface in the dip tube, while the inert gas is based on the degree of vacuum (P) in the dip tube and the total flow rate of oxygen gas (O) and inert gas (N) blown from the top of the dip tube. Ratio (R)
Is expressed by the following equation , the constant a is within a certain range 90
0 to 2500 in the dip tube that changes every moment
Oxygen gas (O) blown from above according to the degree of vacuum (P)
A method of melting low carbon steel using a vacuum refining furnace, wherein the mixing ratio (R) of nitrogen and inert gas (N) is changed.