KR20190072245A - Method of refining molten steel - Google Patents

Abstract

The present invention provides a refining method of molten steel, which is a refining method for removing an inclusion of molten steel. The present invention implements at least one process of an LF process and an RH process after implementing a bubbling process, wherein the bubbling process includes at least three bubbling in which at least one of process conditions including a blow-in gas flow rate and blow-in time is different.

Description

TECHNICAL FIELD The present invention relates to a method of refining molten steel,

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a refining method of molten steel, and more particularly, to a refining method of molten steel capable of minimizing the content of inclusions in molten steel.

Generally, the steelmaking process is carried out in the order of the iron pre-treatment process, the converter process, the refining process, and the continuous casting process.

In the converter process, hot metal and scrap are charged to the converter and high-purity oxygen (O ₂ ) gas is blown to remove carbon and impurities in the charcoal in the form of oxide of CO gas or slag. Through this process, the molten steel in which impurities are removed is called molten steel. That is, impurity elements such as carbon (C), silicon (Si), manganese (Mn), phosphorus (P), and sulfur (S) present in the charcoal are substituted with Fe atoms in the mid- It exists as an intrusion between atoms. Therefore, in the converter process, oxygen is blown by stirring the gas and various kinds of additives are added to produce molten steel. That is, in the converter process, the main raw material is charged into the converter, oxygen is supplied to the furnace while the additive is supplied to control the reaction characteristics between the molten steel and the slag, and the impurities C, Si, Mn, P and S contained in the molten iron are oxidized Molten steel is produced by reacting with added additives to remove impurities.

However, although the amount of impurity elements can be reduced through the conversion process, the amount of oxygen is increased, and the molten steel after the conversion process is completed contains several hundred ppm of free oxygen. That is, dissolved oxygen is contained by the oxygen taken in during the oxidation process in the converter process. Such dissolved oxygen reacts with impurities to form oxides such as SiO ₂ , Al ₂ O ₃ , P ₂ O ₅ , MnO, MgO, FeO and TiO ₂ to produce slag, and combined with burnt lime, (CaO-MgO-FeO-SiO ₂ -MnO-P ₂ O ₅ ). The oxide or multicomponent slag is mixed with the molten steel in the ladle in the process of introducing the molten steel into the ladle, thereby causing many adverse effects.

In order to remove such dissolved oxygen, various kinds of ferro-alloys such as Fe-Mn, Fe-Si and Fe-Al, that is, a deoxidizing agent, are introduced while introducing molten steel into a ladle. However, inclusions containing alumina (Al ₂ O ₃ ) as a main component are produced in the molten steel by the added alloying iron, especially aluminum alloy iron. Alumina has a very high melting point and is composed of fine particles, making it difficult to separate the magnetic flakes. In addition, alumina can cause nozzle clogging in a continuous casting process, and may cause residual surface defects in the final product. Therefore, the alumina inclusions must be removed from the molten steel prior to the continuous casting, and the refining process is performed for this purpose. The refining process consists of bubbling (refining slag) by supplying gas, ladle furnace (LF) stirring the molten steel using electricity, Rheinstahl-Heraus (vacuum degassing) Process. In order to remove alumina, which is a nonmetallic inclusion, the slag is reformed and the deoxidation product is separated by flotation and then the bottom bubbling is carried out in the LF process. And the vacuum process is performed at 2 Torr or less in the RH process.

However, even after the refining process, inclusions in the molten steel remain, causing clogging of the nozzle in the continuous casting process, and can cause surface defects of the final product.

Korean Patent Publication No. 2001-0011732 Korea Patent Publication No. 2017-0106073

The present invention provides a molten steel refining method capable of minimizing the amount of inclusions in molten steel.

The present invention provides a molten steel refining method capable of minimizing inclusions in molten steel through bubbling, LF and RH processes.

The refining method for molten steel according to the embodiments of the present invention is a refining method for removing inclusions in molten steel, wherein at least one of the LF process and the RH process is performed after the bubbling process is performed, At least one of the process conditions including the gas flow rate and the blow time includes at least three bubbling processes different from each other.

Wherein the LF process includes at least three bubbling processes and at least one raising step at least one of which includes at least one of process conditions including a blown gas flow rate and a blowing time and wherein the RH process includes a blowing gas flow rate, And at least one of the process conditions including at least one of the process conditions.

Wherein the bubbling process includes primary to tertiary bubbling wherein the primary bubbling introduces a blowing gas flow rate that is greater or equal to that of the secondary bubbling for a longer or the same time, Blows for a longer time or at the same time with a smaller blowing gas flow rate as compared with the bubbling.

Each of the primary to tertiary bubbling is performed under different process conditions depending on the type of steel.

The secondary bubbling is carried out under different process conditions depending on the amount of iron alloy input.

The secondary bubbling increases the blowing time as the amount of the molten iron is increased.

The primary to tertiary bubbling different bubbling positions.

The secondary bubbling bubbles at a lower position than the primary bubbling, and the tertiary bubbling bubbles at a position lower than the secondary bubbling.

Wherein the LF process includes fourth to sixth bubbling, wherein the fourth bubbling injects a blowing gas flow rate greater than or equal to the fifth bubbling for a shorter or the same time, Blows for a longer time or at the same time with a smaller blowing gas flow rate as compared with the bubbling.

The LF step carries out at least one heating step between the fourth to sixth bubbling steps.

The bubbling step after the temperature raising step makes at least one of the blowing gas flow rate and blowing time different depending on the temperature increase amount and the steel type.

The larger the temperature increase amount, the larger the blowing gas flow rate and the blowing time.

The RH process differs at least one of the blowing gas flow rate, blowing time and pressure depending on the type of steel.

The molten steel refining method according to an embodiment of the present invention includes a bubbling process, and may further include at least one of an LF process and an RH process. The bubbling step may be carried out at least three times by changing the process conditions, and the LF step and the RH step may be carried out at least three times or more by changing the process conditions. By performing at least one of the bubbling process, the LF process, and the RH process at a plurality of times with different process conditions, the inclusions in the molten steel can be separated and the content of inclusions in the molten steel can be minimized. Therefore, it is possible to prevent or minimize the occurrence of nozzle clogging in the subsequent continuous casting process, and to prevent or minimize surface defects of the final product.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a flow chart of a steelmaking process including a molten steel refining method according to the present invention; FIG.
FIG. 2 is a process flow chart for explaining a molten steel refining method according to an embodiment of the present invention; FIG.
3 is a schematic view for explaining an aspect of the inclusion flotation separation.
FIG. 4 is a graph showing the results of analysis of inclusions up to the cast steel after the refining process of the conventional example and the embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but is capable of other various forms of implementation, and that these embodiments are provided so that this disclosure will be thorough and complete, It is provided to let you know completely.

FIG. 1 is a process flow chart of a steelmaking process including a molten steel refining method according to the present invention, and FIG. 2 is a process flowchart for explaining a molten steel refining method according to an embodiment of the present invention. 3 is a schematic view for explaining aspects of the inclusion flotation separation.

1 and 2, a steelmaking process including a molten steel refining method according to the present invention includes a preliminary ironing process (S100), a transforming process (S200), a refining process (S300), and a continuous casting process (S400) can do. The refining step S300 may include a bubbling step (S310), an LF step (S320), and an RH step (S330). At this time, the bubbling process (S310), the LF process (S320), and the RH process (S330) may be performed in reverse order. For example, the bubbling process (S310), the RH process (S330), and the LF process (S320) may be performed in this order. In addition, the molten steel refining method according to the present invention may include at least one of the LF process (S320) and the RF process (S330) including the bubbling process (S310). That is, only one of the LF step (S320) and the RF step (S330) may be performed after the bubbling step (S310), or the bubbling step (S320) (S310), the secondary refining step may be performed. However, it is preferable that the present invention performs both the bubbling step (S310), the LF step (S320), and the RH step (S330).

The molten steel refining method according to the present invention minimizes the inclusions in the molten steel by removing inclusions in the molten steel that has been preheated. The inclusion of molten steel is proportional to the content of dissolved oxygen. That is, if the amount of dissolved oxygen in the molten steel is large, the amount of non-metallic inclusion is increased. On the contrary, if the amount of dissolved oxygen is small, the amount of non-metallic inclusion is reduced. The inclusions in the molten steel can be generated by the selective reaction of dissolved oxygen with elements (Al, Ca, etc.) having a strong affinity for oxygen when iron alloy is fed during the laminating process. In order to ensure the cleanliness of the final product produced by the steelmaking process, it is important to prevent the re-oxidation of molten steel, to absorb the inclusions, to reduce the oxygen potential in the slag, and to control the slag properties (viscosity and melting point). The inclusion removing mechanism of molten steel having completed the lubrication can be roughly classified into the following four types.

First, as shown in FIG. 3 (a), on the separating portion of the inclusion due to the difference in specific gravity between the inclusions and the molten steel, the floating rate follows the Stoke's Law of Equation (1).

Where g is the gravitational acceleration, p is the specific gravity, η is the viscosity of the molten steel, and r is the radius of the inclusion. That is, the floating rate may vary depending on the size of the inclusions and the specific gravity difference. The larger the difference in the specific gravity between the molten steel and the inclusions, the larger the size of the inclusions.

Secondly, as shown in Fig. 3 (b), it is a separation part by incorporation of inclusions. If two or more liquid inclusions collide, the coalescence process, which is one large lump, proceeds to have a lower surface tension, increasing the particle size and determining the rate of flotation according to the Stokes' law. That is, two or more inclusions collide to increase the size of the inclusion particles, and the larger inclusion particles can be more easily lifted than the smaller inclusion particles.

Thirdly, as shown in Fig. 3 (c), it is a separation bubble. The collision probability between bubbles and inclusion particles increases as the size of the bubbles is small and the size of the inclusion particles is large. The incorporation of the inclusions is influenced by the size of the bubbles and inclusions. When the inclusion particle size is 20 to 50 μm, the minimum bubble size is about 2 to 3 mm. The removal of inclusions is theoretically increased by decreasing the size of the bubbles, and the size of the bubbles optimal for the floating of the inclusions is known to be 0.5 to 2.0 mm.

Fourth, it is removal of inclusions by refractories. It depends on the kind of refractories, but it has a high inclusion adsorbability if it has strong chemical affinity with inclusions.

The present invention removes inclusions contained in molten steel based on the above-described theory. Since the iron wire preliminary processing step S100, the converting step S200, and the continuous casting step S400 are performed in a general steel making process, a detailed description thereof will be omitted, and a refining step S300 according to an embodiment of the present invention will be described. Will be described in detail. That is, the bubbling process (S310), the LF process (S320), and the RH process (S330) will be described in detail as follows.

1. Bubbling process

The bubbling step (S310) can be performed at least three times. That is, the bubbling process (S310) may include a primary bubbling process (S311), a secondary bubbling process (S312), and a tertiary bubbling process (S313). Of course, the bubbling step (S310) may further perform the fourth bubbling step, or may further perform the fifth bubbling step. That is, the bubbling step (S310) can be performed at least three times or more. The bubbling step (S310) may be performed at least three times by supplying an inert gas containing argon (Ar) gas.

At least one of the first to third bubbling processes (S311 to S313) may be carried out with different flow rates of the feed gas. For example, the primary and secondary bubbling processes (S311 and S312) may be performed at the same flow rate, and the tertiary bubbling process (S313) may be performed at a different flow rate. As a specific example, the primary and secondary bubbling processes (S311 and S312) are performed at a blowing rate of 150 to 230 Nm 3 / Hr, and the tertiary bubbling process (S313) is performed at a blowing rate of 20 to 50 Nm 3 / . That is, the primary and secondary bubbling processes (S311 and S312) are performed with the same flow rate of inert gas, and the tertiary bubbling process (S313) is performed in the same manner as in the primary and secondary bubbling processes (S311 and S312) It is possible to reduce the amount of the inert gas. Of course, the primary and secondary bubbling processes (S311 and S312) may also be performed with different amounts of inert gas. At this time, the amount of gas may be larger in the primary bubbling step (S311) than in the secondary bubbling step (S312). Further, the primary to tertiary bubbling processes (S311 to S313) may be performed for the same time, or one of them may be performed for another time. For example, the primary bubbling process (S311) is performed for 2 to 10 minutes, the secondary bubbling process (S312) is performed for 1 to 5 minutes, the tertiary bubbling process (S313) Min. ≪ / RTI > As another example, the primary bubbling process (S311) is performed for 3 to 5 minutes, the secondary bubbling process (S312) is performed for 1 to 3 minutes, and the tertiary bubbling process (S313) Min. ≪ / RTI > That is, the secondary bubbling step (S312) may be performed for a shorter or equal period of time than the primary bubbling step (S311), and the tertiary bubbling step (S313) Time. The primary to tertiary bubbling steps (S311 to S313) may be performed for the same period of time. The supply flow rate and process time of the inert gas in the first to third bubbling processes (S311 to S313) may vary depending on the amount of molten iron, the type of steel, and the like.

Here, the primary bubbling step (S311) can be performed for a different time depending on the type of steel. For example, deoxidized steel using single deoxidized steel, that is, aluminum, can be conducted at an argon flow rate of 170 to 230 Nm 3 / Hr for 3 to 5 minutes, and deoxidized steel using composite deoxidized steel, It can be carried out at an argon flow rate of 230 Nm 3 / Hr for 4 to 7 minutes. That is, the bubbling time of the composite deoxidized steel may be longer than that of the single deoxidized steel at the same gas flow rate. In this case, for example, a flow rate of 170 to 230 Nm 3 / Hr for 4 to 7 minutes means that an inert gas supplied for 170 to 230 Nm 3 for one hour is supplied for 4 to 7 minutes. Of course, the primary bubbling step (S311) may be performed at the same gas flow rate for the same time or at the same gas flow rate for the same time depending on the type of steel. The first bubbling step (S311) is performed by a strong bubbling method to uniformly mix the iron alloy and the deasphalted material charged during the passage and to separate the alumina inclusions floating in the tunnel. Table 1 shows the process conditions according to the type of steel in the primary bubbling process (S311). On the other hand, component analysis can be performed after the primary bubbling process (S311).

The large alumina inclusions can be removed into the slag by the strong agitation energy in the primary bubbling step (S311). However, due to reoxidation due to the occurrence of bubbling, fine alumina inclusions are generated, and a small amount of ferroalloys is added to increase the amount of minute inclusions due to the inclusion of slag. Therefore, a secondary bubbling process (S312) is performed for homogeneous mixing of iron alloy and floating separation of fine alumina inclusions for fine adjustment in the component analysis after the primary bubbling process (S311). The secondary bubbling step (S312) may be carried out for a different time depending on the amount of alloyed iron. For example, at an argon flow rate of 150 to 200 Nm.sup.3 / Hr, it is possible to carry out the annealing for 1 to 3 minutes at less than 500 kg of alloy iron, 2 to 5 minutes at 500 kg to 700 kg, and 3 to 7 minutes at 700 kg or more . That is, the secondary bubbling step (S312) can be performed for a longer time as the amount of the molten iron is increased.

The tertiary bubbling step (S313) can be performed to remove large alumina inclusions not removed by the secondary bubbling step (S312). The third bubbling step (S313) can be carried out for 3 to 7 minutes at an argon flow rate of 20 to 50 Nm 3 / Hr, for example. Therefore, generation of the slag and slag inclusion can be minimized, and the residual alumina inclusions can be separated by floating.

On the other hand, the positions of the bubbling may be different in the first to third bubbling processes (S311 to S312). That is, in the first to third bubbling processes (S311 to S313), the positions of the lances injected into the lathes may be different. For example, the distance from the bottom of the ladle to the bottom of the lance may be 500 mm in the primary bubbling process (S311), 250 mm in the secondary bubbling process (S312) S313) may be 100 mm. That is, the position of the bubbling can be lowered from the primary bubbling step (S311) to the tertiary bubbling step (S313). Therefore, in the third bubbling step (S313), inclusions that have settled on the bottom of the raisers can float.

division Primary bubbling Purpose of processing Flow rate (Nm 3 / Hr) Time (minutes) Lance Location  Single deoxidized river Molten steel uniform 170 to 230 3 to 5
500 mm Complex deoxidizing steel 170 to 230 4 to 7

division Secondary bubbling Purpose of processing Ferroalloy
Input (kg) flux
(Nm 3 / Hr) Time (minutes) Lance Location
Single deoxidized river
Molten steel uniform
> 500
150-200
1-3
250 mm
Complex deoxidizing steel 500 to 700 2 to 5 700 < 3 to 7

division
Tertiary bubbling Purpose of processing Flow rate (Nm 3 / Hr) Time (minutes) Lance Location Single deoxidized river Inclusion
Separation injury
20 to 50
3 to 7
150 mm Complex deoxidizing steel

2. LF process

The LF process (S320) can be performed at least three times. The LF process S320 according to the embodiment of the present invention may include a primary bubbling process S321, a temperature raising process S322, a secondary bubbling process S323, and a tertiary bubbling process S324 . That is, the LF process (S320) may include at least three bubbling processes and at least one heating process. Here, at least one of the primary to tertiary bubbling processes (S321, S323, S324) can be performed at different flow rates and different times. That is, the primary to tertiary bubbling processes (S321, S323, S324) may be performed at the same flow rate for the same time, or at least one of them may be performed at another flow rate for another time. For example, the primary bubbling process (S321) may be performed by top bubbling for 2 to 5 minutes at a flow rate of 150 to 200 Nm3 / Hr, and the secondary bubbling (S323) may be performed by 100 The steel bubbling can be performed at a flow rate of ~ 150 Nm 3 / Hr for 4 minutes to 7 minutes. Also, the tertiary bubbling step (S324) can be performed by bottom bubbling for 5 minutes to 7 minutes at a flow rate of 20 to 40 Nm 3 / Hr. The reaction formula by the LF step (S320) is as follows.

_{Ca + Al 2 O 3 → CaO} + Al

_{mCaO + nAl 2 O 3 → mCaO} · nAl 2 O 3

On the other hand, CaO and the slag conditioning agent may be put in 200 to 300 kg respectively for slag reforming before the primary bubbling step (S321) of the LF step (S320). The primary bubbling step (S321) is carried out to separate inclusions in the molten steel, and the molten steel and slag can be reformed by injecting 500 kg ± 50 kg of CaO.

The temperature rising step (S322) can be performed during the LF step (S320). For example, the temperature raising step (S322) may be carried out after the first bubbling step (S321) and before the second bubbling step (S323). The temperature raising step (S322) can be carried out in accordance with a decrease in the temperature of the molten steel after the alloying iron is charged for component adjustment. In the heating step (S322), an arc is generated to raise the temperature of the slag to about 2000 to 3000 占 폚, and to raise the temperature of the molten steel by raising the temperature of the slag.

The secondary bubbling process (S323) may be performed to equalize the molten steel after the molten steel has risen to an appropriate temperature. At this time, the secondary bubbling step (S323) may be carried out by injecting a conditioning agent and flux (B-Flux) according to the type of steel and the amount of heating. For example, when the energy required to raise the same temperature (10 ° C), that is, the temperature rise energy, is less than 4000 kWh, the single deoxidized steel and the composite deoxidized steel can be supplied with 200 kg ± 50 kg of the coagulant and the flux respectively. When the energy required to raise the same temperature is 4000 to 10000 kWh, the single deoxidized steel can be supplied with 300 kg ± 50 kg and 200 kg ± 50 kg of the conditioning agent and the flux, respectively, 100 kg + 50 kg and 150 kg + 50 kg, respectively. When the energy required for raising the temperature is more than 10000 kWh, the single deoxidized steel can be supplied with 300 kg ± 50 kg and 250 kg ± 50 kg of the conditioning agent and flux, respectively, 200 kg ± 50 kg and 200 kg ± 50 kg, respectively. In the secondary bubbling step (S323), the supply amount of argon gas may be different depending on the type of steel and the temperature rising energy. For example, if the heating energy is less than 4000 kWh, argon can be supplied at 100 Nm3 / 5Hr for single deoxidation and 130 Nm3 / 5Hr for argon. That is, the single deoxidized steel can be supplied for a predetermined time in an amount of supplying 100 Nm 3 for 5 hours, and the composite deoxidized steel can be supplied for a predetermined time in an amount of supplying 130 Nm 3 for 5 hours. When the heating energy is in the range of 4000 to 10000 kWh, argon can be supplied 120Nm3 / 6Hr for single deoxidized steel, and 140Nm3 / 5Hr for argon for complex deoxidized steel. When the heating energy exceeds 10000 kWh, the single deoxidized steel can supply 130 Nm 3 / 7Hr of argon and the complex deoxidized steel can supply 150 Nm 3 / 6Hr of argon.

The tertiary bubbling step (S324) can be performed to raise and lower the inclusions after completion of the molten steel agitation by the addition of the molten iron. A tertiary bubbling step (S324) is performed to promote detoxification and separation of alumina inclusions. In the tertiary bubbling step (S324), the supply amount of argon gas may be different depending on the type of steel and the temperature increase amount (that is, temperature increase energy). For example, when the heating energy is less than 4000 kWh, the single deoxidized steel can supply 20 Nm 3 / 5Hr of argon, and the combined deoxidized steel can supply 30 Nm 3 / 6Hr of argon. When the heating energy is 4000 ~ 10000kWh, argon can be supplied 30Nm3 / 6Hr for single deoxidized steel, and 40Nm3 / 6Hr for argon for complex deoxidized steel. When the heating energy is higher than 10000 kWh, the single deoxidized steel and the combined deoxidized steel can supply 40 Nm 3 / 7Hr of argon, respectively.

Table 4 shows the addition amount of coagulant and flux depending on the heating energy in the secondary bubbling process (S323), and the Ar flow rate and time according to the heating energy are shown in Table 5. The Ar flow rate and the time for the molten steel uniformization after the third bubbling step (S324) and the alloying iron injection are shown in Table 6.

division
LF Heating energy (KWh) ≤ 4000 4000 to 10000 ≥10000 Additives (kg) Flux (kg) Additives (kg) Flux (kg) Additives (kg) Flux (kg) Single deoxidized river 200 ± 50 200 ± 50 300 ± 50 200 ± 50 300 ± 50 250 ± 50 Complex deoxidizing steel - - 100 ± 50 150 ± 50 200 ± 50 200 ± 50 CaO input 500kg ± 50

division
LF Heating energy (KWh) ≤ 4000 4000 to 10000 ≥10000 Ar flow rate (Nm 3 / Hr) Ar flow rate (Nm 3 / Hr) Ar flow rate (Nm 3 / Hr) Single deoxidized river 100/5 120/6 130/7 Complex dehydration phase 130/5 140/5 150/6

division
LF Heating energy (KWh) ≤ 4000 4000 to 10000 ≥10000 Ar flow rate (Nm 3 / Hr) Ar flow rate (Nm 3 / Hr) Ar flow rate (Nm 3 / Hr) Single deoxidized river 20/5 30/6 40/7 Complex deoxidizing steel 30/6 40/6 40/7

3. RH Process

The RH process (S330) may include at least three reflux processes. That is, the RH process (S330) may include a first reflux process (S331), a second reflux process (S332), a third reflux process (S333), and a fourth reflux process (S334).

The primary reflux process (S331) is performed to remove hydrogen, nitrogen, oxygen, and the like contained in molten steel. The primary reflux process (S331) can be performed by supplying reflux gas of molten steel at a flow rate of 140 to 200 Nm 3 / Hr and a pressure of 1 to 2 Torr for 10 to 20 minutes. The primary reflux process (S331) can be carried out by varying the supply amount of the reflux gas according to the type of the steel. For example, a single deoxidized steel may be supplied at a flow rate of 140 Nm 3, a composite deoxidized steel may be supplied at a flow rate of 180 Nm 3, and a steel containing 10 to 30 ppm of carbon may be supplied at a flow rate of 200 Nm 3. The reflux time and pressure may be the same depending on the type of steel.

On the other hand, if the molten steel is refluxed in the vacuum state, the inclusions are reduced. Therefore, the second and third reflux steps (S332 and S333) can be performed after the iron alloy for fine adjustment is put in the RF process. The secondary and tertiary reflux processes (S332, S333) can be performed by adjusting the flow rate, pressure, and time of the reflux gas to the steel species. The second reflux process (S332) is carried out at a flow rate of 150 to 200 Nm 3 / Hr at a pressure of 0.5 to 5 Torr for 20 minutes to 25 minutes and a third reflux process (S333) at a flow rate of 100 to 200 Nm 3 / It can be carried out at a pressure of 0.5 to 2 Torr for 15 minutes to 25 minutes. That is, the tertiary reflux process (S333) can be carried out for the same or a shorter time at the same or a lower pressure with the same or less gas flow rate than the second reflux process (S332). Of course, the tertiary reflux process (S333) may be performed at a higher gas pressure for a longer period of time than the secondary reflux process (S332). Here, the single carbon steel was subjected to a secondary reflux process (S332) at a flow rate of 150 Nm 3 / Hr at a pressure of 5 Torr for 20 minutes, followed by a tertiary reflux process at a pressure of 2 Torr at a flow rate of 100 Nm 3 / ) Can be performed. The composite carbon steel was subjected to a secondary reflux process (S332) at a flow rate of 180 Nm 3 / Hr at a pressure of 5 Torr for 20 minutes and then subjected to a tertiary reflux process (S333) at a flow rate of 100 Nm 3 / Hr at a pressure of 2 Torr for 15 minutes . However, the steel containing 10 to 30 ppm of carbon can be carried out at a flow rate of 200 Nm 3 / Hr at a pressure of 0.5 Torr for 25 minutes without separately performing the secondary and tertiary reflux processes (S332 and S333). By performing the secondary and tertiary reflux processes (S332 and S333), inclusions and bubbles in the molten steel are brought into contact with each other, thereby facilitating the removal of the inclusions from the flocculation and separation of alumina inclusions.

The fourth reflux process (S334) is carried out to remove the alumina inclusions separated off. That is, the fourth reflux process (S334) is the final treatment for the inclusion separation. The fourth reflux process (S334) can be performed at a pressure of 1.5 to 2 Torr for about 3 hours. At this time, the fourth reflux process (S334) can be performed under different conditions depending on the type of steel. For example, a single deoxidized steel can be conducted at a pressure of 1.5 Torr for 3 hours, and a composite deoxidized steel can be conducted at a pressure of 2 Torr for 3 hours.

 [Table 7] shows the process conditions according to the steel type in the RH process (S330).

Target grade
Primary reflux Secondary reflux Tertiary reflux 4th reflux Nm 3 / Torr / min Nm 3 / Torr / min Nm 3 / Torr / min Torr / min Single deoxidized river 140/2/15 150/5/20 100/2/15 1.5 / 3 Complex dehydration phase 180/2/15 180/5/20 100/2/15 2/3 10 to 30 ppm carbon steel 200/2/15 200 / 0.5 / 25 - -

The calculation formula of the molten steel reflux amount by the RH process is as follows.

Q = 114 x G ^1/3 x D ^4/3 x [Ln (Po / P)] ^1/3

Where Q is the molten steel reflux (ton / min), G is the reflux gas (Nm 3 / min), D is the plumbing diameter (0.73 m), Po is the atmospheric pressure (torr) and P is the vacuum pressure (torr). Since the reflux velocity after the alloying iron injection is directly related to the reflux amount, the amount of circulation of the molten steel is expressed by the reflux velocity (W = Ton / Min) and is changed by the amount of the reflux gas.

As described above, the lubrication refining method according to an embodiment of the present invention may include at least one of the LF process (S320) and the RH process (S330), including the bubbling process (S310). The bubbling step (S310) may be carried out at least three times by changing the process conditions, and the LF step (S330) and the RH step (S330) may be carried out at least three times or more with varying process conditions. By performing the bubbling process (S310), the LF process (S320) and the RH process (S330) by dividing the process condition plural times, the inclusions in the molten steel can be separated and the content of inclusions in the molten steel can be minimized . Therefore, it is possible to prevent or minimize the occurrence of nozzle clogging in the subsequent continuous casting process, and to prevent or minimize surface defects of the final product.

Next, embodiments of the present invention and refining method of molten steel according to a conventional example will be described, and the results thereof will be described as follows.

Conventional example

As a conventional example, the bubbling process was performed for 6 minutes by supplying Ar at a flow rate of 180 Nm 3 / Hr. After the bubbling process, the LF process was carried out for 3 minutes at a flow rate of 60 Nm 3 / Hr with a buttom bubbling. The temperature was raised to 2500 kw / h per 10 ° C., and the flow rate of 40 Nm 3 / Weak bubbling was performed. The RH process was first refluxed at a pressure of 7 Torr at a flow rate of 200 Nm 3 / Hr, secondary refluxed for 15 minutes at a flow rate of 200 Nm 3 / Hr at a pressure of 50 Torr, and then refluxed for 3 minutes at a pressure of 10 Torr Respectively.

Example

The bubbling process was divided into primary, secondary and tertiary bubbling processes. The primary bubbling was carried out for 200 minutes at 200Nm3 / Hr for 3 minutes in both the single deoxidized steel and the combined deoxidized steel for the uniformity of the molten steel, and the lance position was 500mm from the bottom of the ladle to the bottom of the ladle. The secondary bubbling was carried out at a flow rate of 170 Nm 3 / Hr in both the single deoxidized steel and the combined deoxidized steel for uniformity of the molten steel, and the treatment time was varied according to the amount of iron alloy input. That is, 1 minute was performed for less than 500 kg, 2 minutes for 500 kg or more and less than 700 kg, and 3 minutes for 700 kg or more. At this time, the lance position was 250 mm from the bottom of the lath to the bottom of the lance. The third bubbling was carried out for 5 minutes at a flow rate of 30 Nm 3 / Hr in both the single deoxidized steel and the composite deoxidized steel for the purpose of flotation of inclusions, and the lance position was 150 mm from the bottom of the ladle to the bottom of the ladle. That is, the bubbling process was performed a plurality of times differently from the conventional one.

In addition, in the LF step, 200-300 kg of CaO and slag conditioning agent were added, respectively, and 150 Nm 3 / Hr of Ar and 500 kg ± 50 kg of CaO were injected for 3 minutes at the bottom for separating the inclusions in the molten steel, And then subjected to primary bubbling with top bubbling. Subsequently, the alloy steel for adjusting the composition was charged, and the temperature of the molten steel was lowered, thereby generating an arc, thereby raising the temperature of the slag to 2000 to 3000 ° C to raise the temperature of the molten steel. Then, Ar was supplied at a flow rate of 150 Nm 3 / Hr to uniformize the molten steel, and the molten steel was stirred by the addition of the alloying iron, The bottom bubbling was subjected to secondary bubbling with an Ar flow rate of 20 to 40 Nm &lt; 3 &gt; for 5 to 7 minutes according to the heating rate. Subsequently, both the single deoxidized steel and the composite deoxidized steel were bubbled at 20 Nm &lt; ₃ &gt; for 5 minutes to perform tertiary bubbling to separate the Al ₂ O ₃ inclusions. That is, the LF process was performed a plurality of times differently from the conventional process.

In the RH process, a primary reflux treatment was performed at 2 Torr for 15 minutes in order to remove hydrogen, nitrogen, oxygen, and the like contained in the molten steel. At this time, the single deoxidized steel, the composite deoxidized steel and the carbon steel of 10 to 30 ppm were respectively supplied with the flow rates of the reflux gas of 140 Nm 3 / Hr, 180 Nm 3 / Hr and 200 Nm 3 / Hr. Subsequently, the single deoxidized steel and the composite deoxidized steel were subjected to a secondary reflux treatment for 20 minutes at a pressure of 5 Torr with the reflux gas flow rates set at 150 Nm 3 / Hr and 180 Nm 3 / Hr, respectively, after the fine adjustment alloy iron was charged. Subsequently, the single deoxidized steel and the composite deoxidized steel were subjected to a tertiary reflux treatment at a pressure of 2 Torr for 15 minutes at a flow rate of the reflux gas of 100 Nm 3 / Hr, respectively. On the other hand, the steel containing 10 to 30 ppm of carbon was refluxed at 200 Nm 3 / Hr at a pressure of 0.5 Torr for 25 minutes. The single deoxidized steel and the composite deoxidized steel were subjected to tertiary reflux for three minutes at a pressure of 1.5 Torr and 2 Torr, respectively. That is, the RH process was performed four times at a lower pressure than the conventional one.

4 is a graph showing the number of inclusions from the bubbling to the playing process according to the conventional example and the embodiment of the present invention. In the case of the conventional example (A), approximately 251 inclusions are contained in the molten steel subjected to the bubbling, LF and RH processes. However, in the case of the present invention, the amount of inclusions is reduced as compared with the conventional example. That is, when bubbling and LF processes are performed (B), 154 inclusions remain in the molten steel, and when bubbling and RH process are performed (C), 172 inclusions remain in the molten steel. In addition, the inclusions may be changed while the molten steel according to the conventional example and the present invention is subjected to the continuous casting process. In the case of the conventional example, (A) the inclusions in the tundish are 254, the inclusions in the mold are 134 , And 78 pieces remain in the cast. However, in the case of the bubbling and LF step (B) of the present invention, the inclusions in the tundish are 106, the inclusions in the mold are 90, and 57 pieces remain in the cast. Further, when the bubbling and RH processes of the present invention are carried out, (C) the inclusions in the tundish are 153, the inclusions in the mold are 70, and the remaining 48 in the cast. Therefore, in the case of the present invention, the amount of inclusions can be reduced compared to the conventional example.

In the meantime, when the bubbling process was performed only on the size of the inclusions generated in the refining process according to the embodiment of the present invention, the inclusions were found to be 95% in size of less than 10 탆 and 0.90% in size of less than 20 탆. That is, 95% of the size of the generated inclusions was less than 10 μm and 0.90% of the sizes of 20 μm or less. However, when the bubbling process and the LF process were performed, and when the bubbling process and the RH process were performed, the inclusion size was found to be 99.07% for the size of 10 μm or less and 0.93% for the size of 20 μm or less. That is, no inclusions of 20 mu m or more were found in the molten steel through the refining process according to the present invention. Therefore, in the succeeding continuous casting process, the inclusion of Al ₂ O ₃ is less likely to be adhered to the immersion nozzle for discharging the molten steel, thereby preventing nozzle clogging and preventing surface defects of the final product.

The present invention is not limited to the above-described embodiments, but may be embodied in various forms. In other words, the above-described embodiments are provided so that the disclosure of the present invention is complete, and those skilled in the art will fully understand the scope of the invention, and the scope of the present invention should be understood by the appended claims .

Claims (13)

A refining method for removing inclusions in molten steel,
After the bubbling process is performed, at least one of the LF process and the RH process is performed,
Wherein the bubbling process includes at least three bubbling processes different in at least one of the process conditions including the blow-in gas flow rate and the blow-in time.
[2] The method according to claim 1, wherein the LF process includes at least three bubbling processes and at least one temperature-raising process in which at least one process condition among process conditions including an input gas flow rate and a blow-
Wherein the RH process includes at least three reflux processes in which at least one process condition among the process conditions including the blown gas flow rate, the blow time and the pressure is different.
The method of claim 1 or 2, wherein the bubbling step includes primary to tertiary bubbling, wherein the primary bubbling blows a blowing gas flow rate that is greater or equal to that of the secondary bubbling, Wherein the third bubbling is carried out for a longer time or at the same time with a smaller blowing gas flow rate than the secondary bubbling.
4. The refining method of molten steel according to claim 3, wherein each of the primary to tertiary bubbling is performed under different process conditions depending on the type of steel.
5. The refining method of molten steel according to claim 4, wherein the secondary bubbling is performed under different process conditions depending on the amount of iron alloy input.
[6] The method of claim 5, wherein the secondary bubbling increases the blowing time as the amount of iron alloy input increases.
The method for refining molten steel according to claim 4, wherein the primary to tertiary bubbling different bubbling positions.
The refining method of molten steel according to claim 7, wherein the secondary bubbling is bubbled at a lower position than the primary bubbling, and the tertiary bubbling is bubbled at a position lower than the secondary bubbling.
The method of claim 2, wherein the LF process includes fourth to sixth bubbling, wherein the fourth bubbling injects a blowing gas flow rate that is greater than or equal to the fifth bubbling time for a shorter or the same time, Wherein the bubbling is carried out for a long time or at the same time with a small blowing gas flow rate as compared with the fifth bubbling.
10. The refining method of molten steel according to claim 9, wherein the LF step is performed at least once during the fourth to sixth bubbling steps.
11. The method according to claim 10, wherein the bubbling step after the temperature raising step is carried out by varying at least one of a blowing gas flow rate and a blowing time depending on a heating rate and a steel type.
The method according to claim 11, wherein the higher the amount of the increase in temperature, the greater the flow rate of the blowing gas and the blowing time.
The method according to claim 2, wherein the RH process differentiates at least one of a blowing gas flow rate, a blowing time, and a pressure according to a steel type.

Images