KR100368239B1 - A process of refining molten steel for high clean steel - Google Patents

Abstract

The present invention relates to a method for refining high-purity steel, the purpose of which is to put the quicklime powder into molten steel that is not deoxidized in the converter to control the aluminum inclusions with calcium aluminate that is easy to float, thereby obtaining a high-purity steel To provide a refining method.

The present invention having the above object, by blowing the quick lime powder (Flux Powder) in the oxidized molten steel from the converter and deoxidized with aluminum, and then magnetically injuries the calcium aluminate inclusions having a low specific gravity generated at this time or The technical gist of the refining method of high-clean steel which can improve the cleanliness of steel by capturing and removing floating by argon bubble at the time of pressure reduction refining at RH is made into the technical summary.

The present invention can achieve a significant inclusion reduction effect by obtaining 8-13 ppm at the level of 20 ppm or more of the conventional cast iron, and also, in the molten steel refined according to the present invention, calcium alumina instead of the existing alumina inclusions. Since there is a nitrate inclusion, there is an effect that can greatly reduce the nozzle clogging during continuous casting.

Description

A PROCESS OF REFINING MOLTEN STEEL FOR HIGH CLEAN STEEL}

The present invention relates to a method for refining high-purity steel, and more particularly, to a method for refining high-purity steel by injecting quicklime powder into molten steel which has not been deoxidized in a converter and controlling the aluminum inclusions with calcium aluminate that is easy to float. It is about.

The basic construction of the steelmaking industry consists mainly of charter pre-treatment → converter → secondary refining → continuous casting process. In the converter operation, oxygen is injected into molten iron to remove impurities such as carbon (C), manganese (Mn), silicon (Si), and phosphorus (P) (hereinafter referred to as 'refining'). The finished molten steel contains several hundred ppm of free oxygen. In order to remove this dissolved oxygen, aluminum, silicon, manganese, etc. are added to the converter steel, and as a result, the deoxidation product is suspended and dispersed in the molten steel. Since the deoxidation product remains in the continuous casting cast product and the final product (rolling material), it causes surface defects. Therefore, it is necessary to thoroughly remove the floating steel from the molten steel before the continuous casting is performed.

Due to this need, various studies have been conducted to manufacture high-purity steel, and the representative technology is to add slag modifiers during and after the converter's tapping to change slag properties or prevent reoxidation. A number of methods have been proposed (Korean Patent Registration Nos. 37363, 93299, 118956, 110322, Patent Application Nos. 95-45965, 95-56445, 96-67564). Although these methods contribute to the production of high clear steel to some extent, the addition of 1-10 kg of slag modifier or slag oxidizer per ton of molten steel has the drawback of further increasing slag generation. The slag generated is classified as a kind of waste and is also a source of environmental pollution. Therefore, an increase in the amount of slag generated means an increase in the amount of waste generated, which is disadvantageous in terms of the environment, and also requires reprocessing of the slag with a harmless substance, which requires various facility investments and costs.

In another method, in the RH vacuum degassing apparatus (hereinafter referred to as 'RH'), the length of the deposition pipe consisting of two rising pipes and a down pipe for refluxing molten steel is different from each other to improve the molten steel stirring force. A method of promoting coarsening (Korean Patent Registration No. 95381) has been proposed. This method has a drawback that the refractory erosion of a long length of sedimentation pipe is severe and the maintenance of the sedimentation pipe is very difficult and the maintenance cost is increased.

The present inventors have completed the present invention through continuous research and experiment to solve the problems of the prior art as described above and maximize the economics of steelmaking industry and stably produce high-clean molten steel, and came to suggest this.

That is, the present invention injects quicklime powder into mithalated molten steel and controls the inclusions, which were conventionally alumina alone, with calcium aluminate inclusions that are easily separated from magnetic injuries, so that the oxygen in the molten steel is preferably 15 ppm or less. To provide a method of refining steel, which has a purpose.

1 is a schematic diagram of a powder blowing device (PI process) used in the present invention

2 is a schematic diagram illustrating a mechanism for transition of inclusions by powder.

3 is a schematic view of the RH refining apparatus used in the present invention

* Description of the symbols for the main parts of the drawings *

10 ... ladle 16 ... slag

18. Inclusions 20.

22 ... Powder 26,36 ... Argon Bubble

30.Vacuum control 39 ... Melting

38.Shape controlled inclusions (calcium aluminate)

Refining method of the present invention for achieving the above object, the step of tapping the oxidized molten molten steel with a ladle; Blowing the quicklime powder into the molten steel received in the ladle;

Injecting aluminum deoxidizer into the molten steel powder is blown; And refining the molten steel to which the deoxidizer is injected, by refluxing into a vacuum chamber of RH.

Hereinafter, the present invention will be described in detail with reference to the drawings.

The present invention manufactures molten steel as a high clean steel through a series of processes, such as converter refining process, powder blowing process, RH refining process, this invention will be described in each step of the process.

Converter refining process

First, in the converter, the molten steel is refined in the usual manner and the ladle is pulled out. In this case, the molten steel is pulled out without deoxidation. As needed, ferroalloy (FeSi, FeMn) may be added.

[Powder blowing process]

The molten steel taken out of the converter refining process and received in the ladle is first transferred to a powder blowing process. The powder blowing process is a process equipped with a device that can blow the powder on the upper ladle, for example, ladle (LF, Ladle Furnace), BAP (BAP, Bubbling, Al-wire feeding & Powder Iniection), PAI ( PI, Powder Injection). 1 is a powder blowing process using the PI device as an example.

In the present invention, by lowering the lance 20 in the powder blowing process to be deposited on the molten steel 12, the quicklime powder 22 is blown together with the carrier gas through the lance 20, the existing alumina It is to ensure the cleanliness of molten steel by shaping control of the inclusions that existed alone with calcium aluminate that is easy to float. The mechanism of the present invention will be described with reference to Scheme (1-3) below and FIG. 3.

When the quicklime powder 22 is blown into the mithalated molten steel 12 according to the present invention, the blown powder 22 reacts with iron oxide (FeO) in the molten steel to produce calcium ferrite (calcium) by reaction of the following formula (2). -ferrite, CaO · FeO). This calcium ferrite is produced as calcium aluminate (calcium-aluminates, mCaO.nAl ₂ O ₃ ) by the reaction of equation 3 when the molten steel is deoxidized to aluminum in a subsequent step. This generation process is schematically shown in FIG. In this process, the oxide, which is an inclusion in molten steel, changes from (FeO) → (CaO · FeO) → mCaO · nAl ₂ O ₃ (liquid), and the specific gravity of the oxide decreases to about 5.5 → 4.1 → 3.3, and the size of the inclusion itself gradually increases. Increases.

Fe + [0] = (FeO)

CaO (solid) + (FeO) = (CaOFeO)

m (CaOFeO) + n [Al] = mCaOnAl ₂ O ₃ (Liquid)

Since the calcium aluminate 24 produced by the above reaction exists as a liquid at the steelmaking temperature (1570-1600 ° C.), the specific gravity is lowered to 2.3-2.7, which is much lower than 3.3, so that the floating separation from the molten steel becomes very easy.

In the present invention, the quicklime-based powder blown to produce calcium aluminate, which is easy to injure, contains a large amount of quicklime, for example, a powder containing Ca: 68-87% and CaF ₂ : 10-30%. There is this. Such powder may have a size suitable for blowing, and preferably has a particle size of 0.5 mm or less. In case of more than 0.5mm, it is easy to penetrate into molten steel when blowing powder, but it has low interface efficiency with molten steel, so it has low utilization efficiency due to low reaction efficiency and collision probability with inclusions. In particular, care should be taken when injecting only the transport gas and the powder is not likely to be blown, which may cause clogging of the blown lance.

The amount of powder blown into the molten steel is controlled according to the cleanliness of the target molten steel. If the total oxygen in the molten steel is less than 15 ppm, it is blown at 0.5-2.0 kg per tonne of molten steel. If the amount of powder blown is less than 0.5kg per ton of molten steel, it is difficult to secure the target cleanliness due to insufficient absolute amount compared to the amount of suspended inclusions in the molten steel.When it is more than 2.0kg, the amount of powder blown is large, which affects the temperature drop of molten steel. In addition to the rising cost of molten steel, the flocculated powder remains on the molten steel to increase the amount of slag, thereby increasing the amount of waste generated.

The blowing amount as described above is blown at a constant speed, preferably blown at a rate of 50-100kg / min. If the powder blowing rate is 50kg / min or less, it takes too much time to blow the target powder and the temperature of molten steel drops. In addition, if the pressure is 100kg / min or more, the pressure loss is great in invading and dispersing the powder in the molten steel at the same flow rate. Therefore, there is a problem of increasing the pressure of the transport gas through the remodeling of the equipment. It is disadvantageous because the efficiency used to impinge, flocculate and coalesce becomes low.

As the transport gas of such powder, it is preferable to have an inert property since it is used in the manufacture of high clean steel, for example, argon gas. At this time, the transport gas is preferably controlled in the range of pressure 5.0-10.0kg / cm ² , flow rate 1.0-2.0Nm ³ / min. When the pressure is 5.0kg / cm ² or less, clogging of the lance may occur, so it is difficult to blow the powder, and when it is 10.0kg / cm ² or more, it is too high and the molten steel is likely to jump out of the ladle. Exacerbate extremely. When transporting gas flow 1.0Nm ³ / min or less, the linear velocity of the lance tip end of my slow invasiveness of the steel powder is poor or are likely to cause nozzle clogging, 2.0Nm ³ / min or more is molten steel scattering workability extremely Worsen.

After blowing a certain amount of powder for obtaining the cleanliness of the target molten steel as mentioned above, aluminum deoxidizer is thrown in, but the injection amount may be blown in accordance with normal operating conditions.

[RH Refining Process]

The ladle 10 in which the molten steel is blown with powder is transferred to RH including a vacuum tank 30, a rising pipe 32a, a falling pipe 32b, and a reflux gas supply device 34 as shown in FIG. And reflux the molten steel. The internal pressure of the vacuum vessel 30 is lowered to a certain atmospheric pressure while the rising reflux pipe 32a and the down reflux pipe 32b are deposited on the molten steel 12 in the ladle 10, and the molten steel is blown while blowing argon gas into the rising pipe. Force reflux.

In order to facilitate the reflux of molten steel, the molten steel reflux gas controls the flow rate of 0.35-0.85 Nm ³ / hr per ton of molten steel and the pressure of 5.0-10.0 kg / cm ² . If the reflux gas flow rate is less than 0.35Nm ³ / hr, it is difficult to reflux the molten steel effectively due to the lack of floating capacity necessary for reflux of the molten steel.If the flow rate is more than 0.85Nm ³ / hr, the molten steel reflux is easy but a large amount of It is attached and the loss of nozzle part is severe and it becomes an obstacle during operation. Pressure 5.0kg / cm ² or less, injection is difficult and, 10.0kg / cm ² or more in the reflux gas is injected too much greater than the target flow rate of a large amount of the attachment or the nozzle refractory material melting portion now inside the vacuum chamber of the reflux gas is There is a drawback of being severe.

By refluxing molten steel in RH as described above, the floating separation of inclusions 18 in molten steel 12 is separated by the difference in specific gravity of the molten steel and the inclusions, which can be explained by Stokes equation (4) below. That is, it can be seen that the larger the particles of the inclusions and the smaller the density of the inclusions, the faster the floating speed.

[Equation 4]

V = (2/9) * {g * r ² * (ρ _Fe -ρ _inclu )} / η

V is the floating speed of nonmetallic inclusions, g is the acceleration of gravity (= 9.8 m / sec ² ), r is the inclusion radius (m), ρ _Fe and ρ _inclu are the density of molten steel and inclusions (ton / m ³ ), and η is It means the viscosity coefficient of molten steel (= 0.005kg / m-sec).

The argon bubble 36 generated in the molten steel 12 in the ladle collides with the surrounding inclusions 18 and is captured, and the floating speed of the captured inclusions has a value similar to that of the argon bubble 36. Will have That is, since the inclusions 38 trapped in the argon bubble are greatly increased in volume, and the density is greatly reduced, the floating speed of the inclusions is greatly increased according to Equation (4).

The present invention is to increase the floating speed of the inclusions in the molten steel by the above method, it is possible to effectively produce high clean steel.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

For the molten steel of 0.02-0.20% by weight of carbon, 0.30% by weight of silicon, 0.10-1.00% by weight of manganese, and 0.02-0.08% by weight of aluminum, Songsan refinement is completed in 300 ton converter, and the steel is not deoxidized at the time of tapping. Only a few ferroalloys (FeSi, FeMn, etc.) were added, and the ladle containing the molten steel was transferred to a PI refining apparatus, and the lance was lowered from the top to be deposited on the molten steel and the quicklime powder was blown. At this time, a representative example of the composition and particle size distribution of the blown powder is shown in Table 1 (actually, limestone powder containing CaO: 68-87% and CaF ₂ : 10-30% was added for each composition. The difference according to the composition of the powder is not large, only a representative example thereof is shown in Table 1.

Powder composition (% by weight) CaO CaF ₂ SiO ₂ H ₂ O 84.58 14.87 0.54 0.01 Powder Particle Size (%) 0.01mm or less 0.01-0.03mm 0.03-0.05mm 0.05mm or more 27% 53% 11% 10%

The powder blowing amount was 0 to 2.5 kg per ton of molten steel as shown in Table 2, and the powder blowing rate was controlled to 50-100 kg per minute. At this time, argon was used as the transport gas, the transport gas pressure was 8.0kg / cm ² , the flow rate was adjusted to 0.75Nm ³ / min. Subsequently, 300-500 kg of aluminum was directly added to molten steel to deoxidize the molten steel, and iron alloy was added to adjust the molten steel component.

Then, the ladle containing the molten steel was transferred to the RH vacuum degassing apparatus, and the molten steel was refluxed. When refluxing the molten steel, argon gas was controlled to 0.40 Nm ³ / min per ton of flow rate molten steel at a pressure of 8.0 kg / cm ² while lowering the internal pressure of the vacuum chamber to 0.005 atm or less. The molten steel treated in RH was manufactured through a continuous casting process, in which the specimens were collected and analyzed in Table 2 to confirm the effects of the present invention.

division Conventional example Inventive Example 1 Inventive Example 2 Inventive Example 3 Inventive Example 4 Inventive Example 5 Powder blowing amount (kg / t) 0 0.3 0.5 1.2 2.0 2.5 Blowing time (minutes) - 3 3 3 5 5 Powder Blowing Speed (kg / min) - 30 50 120 120 250 Oxygen content in cast steel (ppm) 23 18 13 10 8 12

As shown in Table 2, in the case of the conventional example in which the quicklime powder is not blown, the total oxygen is 23 ppm, which is very high, and thus it is not suitable for manufacturing 15 ppm steel of high clean steel.

On the other hand, when the quicklime powder is blown from 0.3kg to 2.5kg per tonne of molten steel, the total oxygen content can be seen to be improved compared to the conventional example, in particular, the powder is 0.5-2.0kg per tonne of molten steel for 3-5 minutes. Invented Example (2) (3) was found to have a significantly low oxygen content of 8-10 ppm in the cast steel. This resulted in the presence of calcium ferrite in the molten steel by blowing the powder in the PI refining device, after which the calcium aluminate inclusions produced by aluminum deoxidation had a low specific gravity and the effect of easily changing the floating separation was reflected as it was. The argon bubbles blown by the heavy metals collide with the low specific gravity inclusions in the molten steel to collect the inclusions and separate them.

On the other hand, when 0.3 kg of powder was injected per tonne of molten steel, it showed a relatively good inclusion reduction effect in terms of reducing the amount of oxygen, but the total oxygen was less than 15 ppm. In addition, when the powder was blown at 2.5 kg per ton of molten steel, the total oxygen was 12 ppm, and a good result was obtained. However, when the powder was blown, there was a problem of poor workability due to the intermittent molten steel scattering and the temperature drop of the molten steel. In addition, the powder blowing amount did not appear to be effective enough. In this way, when the powder is excessively blown, the molten steel temperature is lowered at a given steelmaking temperature, which causes the entire operation to become unstable.

As described above, in the present invention, the effect of reducing the total oxygen content in the cast steel after the treatment by injecting the quicklime powder into the mithalated molten steel is 8-13 ppm at the level of 20 ppm or more, thereby achieving a significant inclusion reduction effect. It became. In addition, although the inclusions remaining in the cast steel are conventionally present as alumina alone, the present invention exists as calcium aluminate inclusions, thereby realizing the effect of greatly reducing the nozzle clogging during continuous casting.

Claims (3)

In the refining method of high clean steel including converter refining process and RH refining process, Tapping the converter mithalated molten steel into ladles; Blowing calcium oxide powder as a transport gas into mithal acid molten steel received by the ladle to form calcium ferrite (CaO · FeO) in the molten steel; Deoxidizing by adding an aluminum deoxidizer to the molten steel on which calcium ferrite (CaO · FeO) is formed as described above, and forming calcium aluminate (mCaO · nAl ₂ O ₃ ) by the reaction of aluminum and calcium ferrite (CaO · FeO); And Refining the molten steel in which calcium aluminate (mCaO.nAl ₂ O ₃ ) formed as described above is refluxed into a vacuum chamber of the RH to refine the high clean steel. The method of claim 1, wherein the quicklime powder has a particle size of 0.5 mm or less, and contains CaO: 68-87% and CaF ₂ : 10-30%. The method according to claim 1, wherein the quicklime powder is blown at 0.5-2.0kg / tonne molten steel at a rate of 50-100kg / min.