JP2728184B2 - Oxygen top-blowing vacuum decarburization of molten 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 of vacuum decarburization of molten steel in a ladle, and more particularly to a method of vacuum decarburization of molten steel in which molten steel is sucked into a vacuum chamber and oxygen gas is blown from at least an upper blowing lance. .

[0002]

2. Description of the Related Art Decarburization under reduced pressure is widely used for decarburizing stainless steel or decarburizing ordinary steel to an extremely low-carbon region. General. However, the problem of the decarburization method under reduced pressure is that if the supply rate of top-blown oxygen is increased in order to increase the decarburization rate in a high-carbon region where the decarburization reaction proceeds with the oxygen supply rate being controlled, the inside of the vacuum chamber Splash generation is severe, and the molten steel in the vacuum chamber is usually bare and not covered with slag. There was a problem that it was difficult.

On the other hand, Japanese Patent Application Laid-Open No. 61-136613 discloses a technique for spraying a powder containing iron oxide or nickel oxide together with top-blown oxygen for the purpose of preventing splash and accelerating decarburization. This technology seeks to reduce or reduce the supply of gaseous oxygen by obtaining the required amount of oxygen as the sum of solid oxygen and gaseous oxygen, and to improve the decarburization rate without increasing the generation of splash by the gaseous oxygen jet. is there. However, although this technology is excellent in preventing splash, the effect of improving the decarburization rate is expected only for the reaction between the oxides that have penetrated into the molten steel and the carbon in the molten steel. Insufficient, and considering the thermal compensation of the steel bath by top-blown oxygen, there is no slag on the molten steel surface and the atmosphere gas in the tank is in contact with the steel bath surface with a lower heat absorption rate than the slag This is a significant disadvantage.

When decarburization is performed under reduced pressure as described above, iron, manganese and the like are partially oxidized, and oxidizing components such as iron oxide and MnO are accumulated in the ladle slag, and the decarburization under reduced pressure is completed. Thereafter, there is a problem that oxygen is supplied to the molten steel from these oxidizing components and dissolved oxygen and nonmetallic inclusions in the molten steel increase.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to prevent a splash and at least a secondary gas of CO gas in a vacuum decarburization method using at least top-blown oxygen gas. By effectively heating the heat from the combustion to the steel bath, decarburization and temperature compensation are effectively performed.
It is another object of the present invention to provide a technique capable of reducing the concentration of impurities such as dissolved oxygen and nonmetallic inclusions in a steel bath.

[0006]

SUMMARY OF THE INVENTION According to the present invention, a part of a lower reflux pipe of a vacuum tank of a reflux degassing apparatus is immersed in molten steel in a ladle, and the vacuum tank is depressurized to suck up the molten steel. ,
Oxygen gas is blown to the molten steel in the tank via an upper blowing lance to depressurize and decarburize.Then, the blowing of oxygen gas is stopped and degassing is performed to obtain a predetermined carbon concentration in the molten steel. In the above, a slag-making agent and / or slag is added to the molten steel surface in the vacuum chamber to slag the oxidizing slag, and oxygen gas is blown. This is a vacuum decarburization method of blowing molten oxygen on oxygen, characterized by adding a reducing flux to the molten steel surface of the ladle and performing vacuum decarburization treatment.The lower part of the cylindrical vacuum tank is formed in the molten steel in the ladle A part of the refractory immersion pipe to be immersed, the inside of the immersion pipe is depressurized to suck up molten steel, an inert gas is blown from the bottom of the ladle, and an oxygen gas is blown into the molten steel in the tank via a top blowing lance. After decompression and decarburization, stop blowing oxygen gas and reduce In the vacuum decarburization method of molten steel to obtain a predetermined carbon concentration in molten steel by performing decarburization, a slag-making agent and / or slag is added to the molten steel surface in a vacuum chamber to form oxidized slag, and oxygen gas is discharged. After spraying and then oxygen gas,
A vacuum decarburization method for blowing molten oxygen on oxygen, characterized by adding a reducing flux to the molten steel surface or the ladle molten steel surface in a cylindrical vacuum tank and subjecting it to a vacuum decarburization treatment, and more preferably a reducing flux. When the addition of steel, the vacuum treatment is interrupted, most of the molten steel and the slag-making agent are discharged into a ladle, and then the flux is added to the ladle, followed by a vacuum decarburization treatment. This is a top blow vacuum decarburization method.

[0007]

[Operation] In the present invention, slag is formed on the surface of molten steel under reduced pressure, so that slopping due to oxygen blown upward is reduced. This is because the momentum of the oxygen jet is attenuated in the cover slag, and the steel bath splash is less likely to occur. FIG. 3 shows the relationship between the volume fraction of cover slag and the splash index (1.0 without cover slag). From this figure, it is clear that a splash prevention effect can be obtained with a cover slag of about 5%.

In addition, since the radiant heat absorption rate of the molten steel + slag surface by the cover slag is about twice as large as that without the slag, the amount of heat of secondary combustion of CO gas by the top-blown oxygen to the steel bath is remarkably increased. To increase. FIG. 4 shows the relationship between the volume fraction of the cover slag and the temperature rise of the molten steel during the 10-minute treatment. The temperature rise of the molten steel increases up to a cover slag volume fraction of about 30%, and saturates above that. This is because the entire surface of the steel bath surface was covered at 30% or more even if the steel bath flow was present.

FIG. 5 shows the relationship between the volume fraction of the cover slag and the decarburization rate. It is clear that the decarburization rate has been improved by increasing the amount of cover slag. The properties described above are the effect of the oxidizing slag-making agent, however, the presence of slag during the vacuum treatment and the presence of iron oxides and MnO in the slag is disadvantageous in terms of cleaning the molten steel. Therefore, in the present invention, a reducing flux is introduced into the slag from the time when decarburization, heat generation and splash prevention are no longer required, thereby reducing the total Fe in the slag on the steel bath to make it harmless. As a result, clean molten steel is finally obtained.

In the case of ultra-clean molten steel which is not desirable even in the presence of slag in the vacuum vessel, the treatment is interrupted once, the slag under reduced pressure is entirely discharged into the ladle, the slag is reduced in the ladle, and the pressure is reduced again. Decarburization is more advantageous for molten steel quality.
Examples of reducing flux include C containing Al, Si, Ti, etc.
A flux consisting of a mixture with at least one of aCO ₃ , Ca (OH) ₂ , CaO and MgO can be used.

FIG. 1 is a schematic diagram showing an example of an apparatus configuration suitable for carrying out the present invention and the state of implementation of the present invention. 1
a is a molten steel housed in a ladle 2 in which a part of a reflux pipe (rising pipe) 10a and a reflux pipe (downcoming pipe) 10b attached to the lower part of a vacuum tank 3 of an RH type reflux degassing device. Is immersed and the inside of the vacuum chamber 3 is depressurized from the exhaust pipe 6 by a vacuum pump (not shown).
(Molten steel in the tank) as shown in the vacuum tank.

An Ar gas introducing pipe 7 for circulating molten steel is attached to the riser pipe 10a. When Ar gas is blown into the molten steel in the riser pipe 10a through this conduit, the inside of the ladle is introduced by the principle of a gas lift pump. The molten steel is sucked up into the vacuum chamber 3 through the rising pipe 10a as shown by the arrow in FIG. 1, and the molten steel 1b in the vacuum tank returns to the ladle 2 through the down pipe 10b. An oxygen gas jet 12 is sprayed onto the surface of the molten steel 1b in the tank from above the vacuum tank 3 via an oxygen lance 8, and is formed on the surface of the molten steel 1b in the tank via a slag forming hopper 4a from a slag forming hopper 4a. Slag and / or slag is added to form cover slag 9.

FIG. 2 is a schematic diagram showing another example of the apparatus configuration suitable for carrying out the present invention and the state of implementation of the present invention.
Reference numeral 2 denotes a ladle and 13 denotes an immersion tube. The cylindrical vacuum tank 3 is disposed in the molten steel 1a in the ladle 2 so that the lower end of the opening is immersed. The inner surface of the immersion pipe 13 is lined with a refractory, and the other opening is connected to a vacuum pump (not shown) through the exhaust pipe 6. Reference numeral 14 denotes an Ar bottom blown porous plug, which is disposed at the center of the bottom surface of the ladle 2 in this embodiment. The Ar gas is blown into the molten steel 1a in the ladle, and the molten steel is stirred by the bubbles 11 thereof. Used. Reference numeral 8 denotes an upper blowing lance, which functions to blow an oxygen-containing gas toward the surface of the molten steel 1b that has risen into the immersion pipe 13. The immersion tube 13 is provided with a slag-making agent feeding chute 4b and a slag-making agent hopper 4a as in the case of FIG. 1, and the slag-making agent 5 is injected from these devices into the surface of the molten steel 1b in the tank. Then, a cover slug 9 is formed.

According to the present invention, after the oxidizing slag is used, that is, at the time point when the decarburization of oxygen is almost completed, the brand in the slag hopper 4a is switched to the reducing flux, and the oxidizing slag is introduced. Achieve slag detoxification. The reducing flux may be charged directly into the bath surface of the ladle 2, or the reduced pressure treatment may be stopped and almost all of the molten steel in the tank or in the immersion pipe may be discharged to the ladle 2 and added.

[0015] Then, by performing the decarburization treatment under reduced pressure, the cleaning of the molten steel is further achieved.

[0016]

Example 1 and Conventional Example 1 230T molten steel was subjected to decompression treatment using a vacuum tank 3 for decompression treatment of a RH type reflux degassing apparatus shown in FIG. At that time, oxygen gas was blown in the upper blowing lance 8 while the C concentration was lowered to 100 ppm. After oxygen blowing, the mixture was decarburized under reduced pressure until the C concentration became 20 ppm, and was supplied to the next step.

In carrying out the method of the present invention, the slag forming hopper 4a and the slag forming chute 4b are provided on the side wall of the vacuum chamber 3.
Was installed so that solid slag and the like could be charged. Also,
Brands in the hopper 4a can be switched. In both the conventional method and the method of the present invention, the oxygen deblunting and decarburization treatment was performed using molten steel controlled at a tapping C concentration of 520 ppm and an O concentration of 300 ppm before vacuum treatment.

In the embodiment, 10% of converter slag (T. Fe 10%) was added in a vacuum tank during the top blowing decompression treatment.
Then, after the end of the upper-blowing process, a 70% Si-containing flux is injected into the vacuum chamber at a volume fraction of 2.5%, and the reduced pressure vessel and ladle slag are reduced, and the total Fe in the slag is reduced.
The (T.Fe) concentration was 3% or less. Table 1 shows the results. The oxygen blowing was performed up to C = 100 ppm in both the examples and the conventional example. However, in the method of the present invention, the arrival time up to 100 ppm was shortened by 2 minutes as compared with the conventional method. That is, the decarburization rate increased by 1.7 times. Moreover, despite the short blowing time,
The heat of heating the steel bath is increased by 4 ° C. as compared with the conventional method, and it is clear that the contribution to the improvement of the heating efficiency of the cover slag.

The reduced pressure treatment time (RH treatment time) was reduced by 2 minutes, and the decarburizing effect in the low carbon region was improved. In addition, the slopping was halved as a result of the observation in the tank after the treatment. Molten steel yield improved by 1.5% due to reduced slopping and recovery of iron in slag. The temperature drop of the molten steel during the RH treatment was reduced by 8 ° C. by shortening the time. As an index of the cleanliness of the molten steel after the treatment, the immersion nozzle clogging index in the next step, continuous casting, was shown, but it was observed that it was almost halved.

Example 2 and Conventional Example 2 Using a dip tube 13 shown in FIG. At this time, the oxygen gas is supplied to the upper blowing lance 8 at a C concentration of 10%.
It was blown up to 0 ppm. After oxygen sparging, the mixture was decarburized under reduced pressure to a C concentration of 20 ppm and supplied to the next step. In carrying out the method of the present invention, a slag-making agent hopper 4a and a slag-making agent charging chute 4b are provided on the side wall of the immersion tube 13 so that solid slag can be charged. Both the conventional method and the present invention have a tapping C concentration of 520pp.
m, vacuum over-pressure decarburization treatment was performed using molten steel controlled to an O concentration of 300 ppm before vacuum treatment.

In Example 2, converter slag (T. Fe 10%) was added at 10% by volume fraction in the vacuum tank during top-blowing decompression treatment. Further, after the end of the upper blowing treatment, a flux containing 70% of Al is injected into the vacuum chamber at a volume fraction of 2.4%, and the reduced pressure vessel and the ladle slag are reduced, and T / Fe in the slag is reduced to 3%.
It was as follows. Example 3 The same method as in Example 2 was used, except that a reducing flux was used.
At the end of the upper blowing, the pressure was returned to normal once, and almost all of the molten steel and slag in the immersion tube was returned to the ladle using the flux containing 70% Al, and the flux was added. Thereafter, the pressure reduction treatment was continuously performed.

The results are shown in Table 2. In the embodiment and the conventional example, the oxygen blowing was performed up to a C concentration of 100 ppm, but the arrival time up to 100 ppm was 2 min compared to the conventional method.
Shortened. That is, the decarburization rate increased 1.6 times.
Despite the short top blow time, the heat of steel bath heating increased by 4 ° C. as compared with the conventional method, and it is clear that the contribution to the improvement of the heat transfer efficiency of the cover slag is apparent.

The reduced pressure treatment time was shortened by 2 to 4 minutes in the embodiment, and the decarburizing effect in the low carbon region was improved. The slopping was reduced by half as a result of observation in the tank after the treatment. The molten steel yield is 1.0 to 1.2 due to the decrease in slopping and T / Fe recovery in slag.
% Improved. The temperature drop of the molten steel during the decompression treatment was reduced by 6 to 10 ° C. by shortening the time. As an index of the degree of cleanliness of the molten steel after the treatment, the clogging index of the immersion nozzle in the continuous casting, which is the next step, was shown, but it was halved in Example 2 and sharply reduced to 1/5 in Example 3.

[0024]

According to the method of the present invention, slapping can be prevented by forming slag on the surface of molten steel in the vacuum chamber, and the heat of molten steel deposition is increased. In addition to using the converter slag as a slag-making agent, the slag-making cost was low, and the decarburization rate was greatly improved due to the contribution of iron oxide in the slag to decarburization.

Further, the iron oxide in the slag was recovered as metallic iron in the molten steel, and the yield was improved. Furthermore, the cleanliness of the molten steel was improved, and the clogging of the immersion nozzle due to deoxidation products during continuous casting was reduced.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an embodiment of the present invention using an RH type reflux degassing apparatus.

FIG. 2 is an explanatory diagram showing an implementation state using another device suitable for implementing the present invention.

FIG. 3 is a characteristic diagram showing a relationship between a cover slag volume fraction and a splash index.

FIG. 4 is a characteristic diagram showing a relationship between a cover slag volume fraction and a temperature rise of molten steel.

FIG. 5 is a characteristic diagram showing a relationship between vacuum processing time and C concentration in molten steel using a cover slag volume fraction as a parameter.

[Explanation of symbols]

 1a Molten steel in ladle 1b Molten steel in tank 2 Ladle 3 Vacuum tank 4a Slag hopper 4b Slag hopper chute 5 Charge agent (slag maker) 6 Exhaust pipe 7 Ar gas inlet pipe for reflux 8 Upper lance 9 Cover Slag 10a Reflux pipe (rising pipe) 10b Reflux pipe (downcoming pipe) 11 Bubbles 12 Oxygen gas jet 13 Immersion pipe 14 Ar bottom blown porous plug

Continuation of the front page (56) References JP-A-2-30711 (JP, A) JP-A-62-146213 (JP, A) JP-A-59-56514 (JP, A) JP-A-4-80316 (JP) , A)

Claims (3)

(57) [Claims] 1. A part of a lower reflux tube of a vacuum tank of a reflux degassing apparatus is immersed in molten steel in a ladle, and the vacuum tank is depressurized to suck up the molten steel. In the vacuum decarburization method for molten steel to obtain a predetermined carbon concentration in the molten steel by continuously blowing the oxygen gas and performing vacuum decarburization after blowing oxygen gas through the Add slag-forming agent and / or slag to make oxidized slag, spray oxygen gas, and then spray oxygen gas, then reduce on the molten steel surface in the vacuum chamber or ladle molten steel surface. A vacuum decarburization method for blowing molten oxygen on oxygen, characterized by adding a flux and performing a vacuum decarburization treatment. 2. A part of a refractory immersion pipe forming a lower portion of a cylindrical vacuum tank is immersed in molten steel in a ladle, and the inside of the immersion pipe is depressurized to suck up the molten steel, and the molten steel is removed from the bottom of the ladle. After blowing active gas and blowing oxygen gas into the molten steel in the tank through an upper blowing lance to decompress and decarbonize, then continuously stop blowing oxygen gas and decompress to reduce the carbon concentration in the specified molten steel. In the vacuum decarburization method of the obtained molten steel, a slag-making agent and / or slag is added to the molten steel surface in the vacuum chamber to produce oxidized slag, oxygen gas is blown, and then oxygen gas is blown. A method of decarburizing oxygen-blown molten steel by adding reducing flux to the molten steel surface or ladle molten steel surface in a vacuum tank and subjecting it to vacuum decarburization treatment. 3. When adding a reducing flux, the depressurization treatment is interrupted, most of the molten steel and the slag-making agent are discharged to a ladle, and then the flux is added to the ladle, followed by a vacuum decarburization treatment. 3. The method of claim 1 or 2, wherein the molten steel is blown under oxygen under reduced pressure.