KR101363927B1 - Refining method of the molten steel - Google Patents

Abstract

The present invention relates to a refining method of molten steel, the refining method of molten steel comprising: a step of manufacturing molten steel by melting a steel material charged in an electric furnace; a first refining step of denitrifying the molten steel in the electric furnace; a step of securing the molten steel over a predetermined temperature; and a second refining step of deoxidizing and desulfurizing the molten steel by putting additives into the molten steel just after tapping the molten steel from the electric furnace to a vacuum degassing equipment; wherein the step of tapping the molten steel, which is secured over a predetermined temperature, to the vacuum degassing facility is to tap the molten steel only if the temperature of the molten steel finished with denitrification is over the predetermined temperature, after the temperature of the molten steel is measured; otherwise to raise the temperature of the molten steel finished with denitrification over the predetermined temperature. A refining method of molten steel according to an embodiment of the present invention is provided to prevent the carbon concentration of the molten steel from increasing and nitrogen in the air from being put into the molten steel, wherein the increasing carbon concentration and the nitrogen input occur during an existing ladle furnace (LF) operation, and to reduce the working hours and processes for deoxidation by skipping an oxygen bubbling (OB) operation. [Reference numerals] (AA) START; (BB) END; (S100(S110,S120)) Electric furnace (denitrification, securing the temperature of molten steel); (S200(S210,S220)) Vacuum degassing facility (decarbonization, desulfurization, deoxidation)

Description

Description of the Related Art Refining method of the molten steel [

The present invention relates to a method for refining molten steel, and more particularly, to a method for refining molten steel that can easily refine the molten steel and shorten the refining step.

Recently, ultra-low carbon steel has been rapidly increasing demand in high value-added industries such as automotive steel sheet due to its excellent physical and mechanical properties.

Conventional manufacturing method for manufacturing such ultra-low carbon steel is to melt the scrap in the electric furnace to produce molten steel, step out the molten steel from the electric furnace, to raise the molten steel in LF (Ladle Furnace), vacuum degassing equipment ( VTD or RH) or BAP (Bubbling, Alloying, Powder injection) stand and bubbling oxygen in the molten steel to raise the molten steel and decarburization and deoxidation in a vacuum degassing facility (VTD or RH), and then , The second deoxidation step, the molten steel after the refining is traveled to the tundish, and the step of casting into a slab through a mold.

Here, the molten steel pulled out of the electric furnace is heated at any one of LF (Ladle Furnace), vacuum degassing facility (VTD or RH), and BAP (Bubbling, Alloying, Powder injection) stand, when the molten steel arrives at the tundish. The temperature is further increased to maintain a temperature (1550 ° C.) that makes slab easy to manufacture. That is, since molten steel is pulled out of the electric furnace and subjected to refining process, and the temperature decreases while moving to tundish, LF (Ladle Furnace), vacuum degassing equipment (VTD or RH), BAP (Bubbling, Alloying, Powder injection) The molten steel is heated up by operating any one of the stands.

Meanwhile, in a typical LF (Ladle Furnace) operation, the slag layer covering the molten steel is heated by using a high temperature arc heat using an electrode made of carbon, and the heat of the slag layer heated to a high temperature is transferred to the molten steel. By the way, since the electrode rod used in LF (Ladle Furnace) operation consists of carbon, carbon of an electrode rod melt | dissolves in molten steel, the nitrogen component in air | atmosphere decomposes, and enters molten steel. Thus, the problem of increasing the carbon and nitrogen concentration in molten steel during LF (Ladle Furnace) operation occurs. For this reason, high quality low carbon steel with low concentrations of nitrogen and carbon cannot be produced.

As described above, before decarburization and deoxidation are performed in the vacuum degassing facility (VTD or RH), the vacuum degassing facility (VTD or RH) or BAP (Bubbling, Alloying, Powder injection) stand is transferred to oxygen in molten steel. By bubbling, decarburization and deoxidation are performed while the molten steel is raised. However, the oxygen concentration in the molten steel is too high due to oxygen bubbling, there is a problem that secondary deoxidation must proceed.

Meanwhile, Korean Patent No. 1085817 discloses a method of raising and refining in LF (Ladle Furnace) before vacuum degassing.

Korea Patent Registration 1085817

The present invention provides a method for refining molten steel that can be easily refined and can shorten the refining step.

The present invention also provides a method for refining molten steel for the production of ultra low carbon steels that prevents nitrogen and carbon concentrations from increasing.

The present invention is a method for refining molten steel for producing low carbon steel, the process of melting the iron source charged in the electric furnace to produce molten steel; A first refining process of denitrifying molten steel in the electric furnace; Securing the molten steel above a predetermined temperature; After the tapping of the molten steel of the electric furnace to the vacuum degassing facility, and the second refining process of the deoxidation, desulfurization and deoxidation by immediately adding an additive, the process of securing the molten steel above a predetermined temperature and tapping into the vacuum degassing facility The temperature of the molten steel is measured, and if the molten steel is equal to or higher than a predetermined temperature, the molten steel from which the denitrification is completed is discharged as it is to a vacuum degassing facility as it is. Adjust to be above temperature.

At least one operation of the LF operation and the oxygen bubbling operation of the molten steel is omitted.

LF operation is omitted between the first refining process in the electric furnace and the second refining process in the vacuum degassing facility and after the second refining process in the vacuum degassing facility, and decarburization, desulfurization and Before the deoxidation second refining step, the oxygen bubbling operation in the vacuum degassing plant is omitted.

The predetermined temperature of the molten steel is 1650 ° C.

In the second refining process of the molten steel,

The desulfurization and deoxidation is carried out after decarburization of the molten steel.

In the decarburization process of the molten steel, gas is injected into the molten steel to reflux the molten steel, and the pressure in the vacuum degassing facility is set to 20 mbar or less.

In the desulfurization and deoxidation process,

The additive includes a desulfurization agent and a deoxidizer, and the ratio of desulfurization agent to deoxidizer (desulfurizer / deoxidizer) to be added is preferably 2-3.

In the process of desulfurization and deoxidation of the molten steel, the pressure in the vacuum degassing facility is preferably set to 100 mbar or more.

Including a carbon-based subsidiary material before charging the iron source into the electric furnace, and the mixed gas of LNG and oxygen is blown in the melting process.

In the embodiments of the present invention, molten steel is secured to a temperature higher than a predetermined temperature in an electric furnace, and the steel is discharged to a vacuum degassing facility, and additives are added to molten steel withdrawn to the vacuum degassing facility to perform refining (decarburization, desulfurization, and deoxidation). That is, the temperature of the molten steel in the electric furnace to ensure a temperature higher than a predetermined temperature, so that the temperature of the molten steel to maintain a temperature that is easy for casting when the tundish arrives. Thus, before the refining step in the vacuum degassing plant, an additional step of heating up the molten steel through LF operation and oxygen bubbling can be omitted.

Therefore, according to the embodiment of the present invention, it is possible to prevent the increase in the carbon concentration in the molten steel and the inflow of nitrogen into the molten steel by the LF operation as in the prior art. In addition, since it does not need to go through oxygen bubbling (OB (Oxygen Bubbling)) operation, there is an effect that the time and step for deoxidation thereafter is shortened.

And, in the present invention, by adjusting the amount of desulfurization agent: deoxidizer to 2-3 in the desulfurization and deoxidation step in the vacuum degassing equipment, there is an effect that can increase the desulfurization efficiency, reducing the unnecessary amount of material used.

1 and 2 are flowcharts sequentially showing a method for refining molten steel according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various forms, and only the embodiments are intended to complete the disclosure of the present invention and to those skilled in the art. It is provided for complete information.

1 and 2 are flowcharts sequentially showing a method of refining molten steel according to an embodiment of the present invention. Here, FIG. 1 shows the refining method of molten steel in order of the apparatus (or equipment) in which refining is performed, and FIG. 2 shows in order of the refining process.

Although not shown, the electric furnace used in the present invention includes a burner for blowing a mixed gas of LNG and oxygen, and assists the arc heat to use a raw material charged into the electric furnace by using oxidation heat of the mixed gas of LNG and oxygen. Dissolve. In addition, the electric furnace may be provided with a lance for blowing oxygen, it is provided with a dust collector for exhausting the air inside the electric furnace to the outside of the electric furnace.

The method for refining molten steel according to an embodiment of the present invention is a method for producing molten steel for low carbon steel, preferably for ultra low carbon steel. 1 and 2, the method for refining molten steel according to an embodiment of the present invention is a molten steel treatment step (S100: S110, 120) in an electric furnace and molten steel treatment step (S200: S210, in a vacuum degassing facility) 220). Here, the molten steel treatment step (S100) in the electric furnace includes a first refining step (S110) for denitrifying the molten steel in the electric furnace, and a step (S120) to secure the molten steel that has been denitrified above a predetermined temperature, in a vacuum degassing facility. The molten steel molten steel treatment step (S200) is a second refining step, divided into a decarburization step (S210) and desulfurization and deoxidation step (S220).

In the method for refining molten steel according to an embodiment of the present invention, a first refining step (denitrification) using an electric furnace and a second refining step (decarburization, desulfurization and deoxidation) in a vacuum degassing plant or a second in a vacuum degassing plant After the refining step (decarburization, desulfurization and deoxidation), the molten steel heating step in the conventional ladle furnace (LF) is omitted. In addition, oxygen bubbling operation (for example, VTD-OB (Oxygen Bubbling)) in a vacuum degassing facility is omitted.

Hereinafter, a method of refining molten steel according to an embodiment of the present invention will be described in detail.

Denitrification step (S110) is a step of removing N (nitrogen) in the molten steel, in the embodiment so that N is less than 30ppm of the entire molten steel. To this end, first, the carbon-based subsidiary material is charged into the electric furnace, and then the scrap is charged, and the input amount of the carbon-based subsidiary material is preferably 5 kg to 40 kg per ton of scrap (primary charging process). The carbonaceous subsidiary material may contain carbon powder, coke, bituminous coal, anthracite coal, and a carbonaceous agent as a material containing carbon and serving as a source of carbon in molten steel. When charging of the carbon-based subsidiary material and scrap is completed as described above, power is applied to the electrode of the electric furnace, and a mixed gas of LNG and oxygen is blown into the electric furnace through a burner. Accordingly, carbon-based subsidiary materials and scraps are melted by the arc heat generated from the electrode rod and oxidative heat by oxygen injection to produce molten steel (primary melting process), and the content of carbon increases by dissolution of the carbon-based subsidiary materials. In addition, in the electric furnace, as the carbon-based subsidiary material and scrap are dissolved, the decarburization reaction is vigorously generated by the oxygen blown, thereby promoting the denitrification reaction. The decarburization reaction is vigorous because the carbonaceous secondary raw material introduced with the scrap is high in the molten steel and reacts vigorously with the blown oxygen. This vigorous decarburization reaction promotes denitrification.

The reason for charging the carbonaceous subsidiary material into the electric furnace before scrap is that the density of the carbonaceous subsidiary material is considerably lower than that of scrap and molten molten steel. For example, when the carbon-based subsidiary material is injected into the molten steel in the molten state as in the prior art, the carbon-based subsidiary material rises on the slag, the reaction rate with the molten steel is slowed, there is a problem that the efficiency of denitrification reaction and denitrification reaction is lowered. . Thus, in the present invention, the carbon-based subsidiary material is charged before the scrap, and the carbon-based subsidiary material prior to melting is disposed below the scrap, so that the scrap prevents the carbon-based subsidiary from rising. Therefore, as the carbon-based subsidiary materials are dissolved together with the dissolution of the scrap, they may be mixed with each other to improve the reaction rate. In addition, when the carbon-based subsidiary material is added to the molten steel in a molten state as in the prior art, the carbon-based subsidiary material introduced into the molten steel reacts with the hot molten steel and slag to generate a flame. However, in the present invention, since the carbon-based subsidiary material is not charged into the molten steel in the liquid state, it is possible to prevent the generation of a flame generated by the charging of the carbon-based subsidiary material.

Moreover, when carbon content in molten steel increases, the equilibrium nitrogen concentration of molten steel will fall. When the carbon content increases in this way, the nitrogen concentration of the molten steel is lowered because the decarburization reaction induces and promotes the dedusting reaction.

The amount of carbonaceous subsidiary material introduced into the electric furnace is 5 kg to 40 kg per ton of scrap, as described above. On the other hand, when the input amount of carbonaceous subsidiary material is less than the suggested amount, the effect of adding the carbonaceous subsidiary material is insignificant. On the contrary, when the input amount of the carbonaceous subsidiary material is larger than the suggested amount, the concentration of carbon in molten steel is excessively increased, and then decarburization is performed. There is a problem that the time required for operation is excessively increased.

Subsequently, an iron source, i.e., scrap or molten iron, or direct induced iron (DRI) is charged into the molten steel of the electric furnace (secondary charging process), and the iron source is dissolved using an arc heat and an oxidation heat following the burner (secondary melting process). .

On the other hand, the nitrogen concentration of the molten steel is affected by the nitrogen concentration in the atmosphere. Therefore, in the present invention, the flow rate of the gas blown into the electric furnace during the first melting process and the second melting process, that is, the mixing flow rate of the mixed gas of LNG and oxygen blown into the burner and the exhaust gas exhausted from the electric furnace, by adjusting the By preventing the outside air from flowing into the inside to suppress the mixing of nitrogen contained in the outside air into the molten steel. At this time, the dust collection flow rate of the exhaust gas is preferably controlled while being adjusted in the range of 1 to 10 times compared to the blowing flow rate of the blown gas. The reason for controlling the dust collection flow rate within the range of 1 to 10 times the blown flow rate is to maintain the total amount of gas blown into the electric furnace and the dust flow rate of the exhaust gas exhausted from the inside of the furnace at a constant level. However, the reason why the dust collection flow rate is maintained relatively high compared to the blowing flow rate is to take into account the volume of the gas blown into the electric furnace to expand in the electric furnace. Therefore, it is preferable to control the dust collection flow rate in proportion to the fluctuation value of the blowing flow rate, and also control it in proportion to the volume of the blowing gas expanding inside the electric furnace.

If the dust collection flow rate is controlled less than the blown flow rate, the exhaust gas generated inside the electric furnace is discharged to the atmosphere, which may cause air pollution. If the dust collection flow rate is set too high, In addition, the inflow of the outside atmosphere into the interior does not allow the nitrogen to be kept low in the interior atmosphere.

When denitrification is completed in the electric furnace in this manner, the nitrogen concentration in the molten steel is maintained at 30 ppm or less and oxygen at 800 ppm. In the present invention, after the denitrification is completed in the electric furnace, the molten steel is secured to a predetermined temperature or more and the tapping is performed in a vacuum degassing facility.

The temperature obtaining step (S120) of the molten steel is a step of securing the molten steel at a predetermined temperature or more in the electric furnace, and the tapping and additional temperature increase are determined according to the temperature of the molten steel after denitrification. In the embodiment, after denitrification, the temperature of the molten steel is maintained at 1650 ° C. or higher, preferably 1670 ° C. or higher, and the steel is discharged to the vacuum degassing facility. Here, the process of securing the temperature of the molten steel to 1650 ℃ or more step of measuring the temperature of the molten steel denitrification is completed, if the temperature of the molten steel is 1650 ℃ or more, the molten steel is pulled out as a vacuum degassing facility, the temperature of the molten steel If less than 1650 ℃ molten steel in the electric furnace. In this case, additionally raising the molten steel may be controlled by adjusting the size of the power applied to the electrode of the electric furnace.

Maintaining the temperature of the molten steel at the time of tapping is more than 1650 ° C, so that the molten steel tapping out of the electric furnace does not undergo a separate heating step through conventional ladle furnace (LF) and oxygen bubbling. This is to maintain a temperature of 1550 ° C or more easily. In more detail, the molten steel that has been refined is charged into a tundish and then transferred to a mold to be manufactured as a slab. At this time, the molten steel injected into the tundish is maintained at 1550 ° C. or more to easily manufacture the slab. Do. Thus, in the embodiment of the present invention, the temperature of molten steel in which denitrification is completed is secured to a predetermined temperature, that is, 1650 ° C. or more, so that the temperature at the time of arrival of the tundish is 1550 ° C. or more. Thus, after the molten steel is pulled out from the electric furnace as in the prior art, it is possible to omit the separate heating step (LF) and oxygen bubbling operation. Therefore, an increase in the concentration of C by the electrode rod made of carbon can be prevented, and infiltration of nitrogen in the atmosphere into molten steel can be prevented. In addition, since it does not go through the LF (Ladle Furnace), there is an effect that the overall operation time is shortened.

On the other hand, when the temperature of molten steel from the electric furnace is lower than 1650 ℃, the temperature of the molten steel drops to less than 1500 ℃ when the tundish arrives, it is not easy to maintain the temperature for casting. Therefore, after denitrification in the electric furnace, oxygen bubbling is performed in a conventional ladle furnace (LF) operation or a vacuum degassing facility to raise the molten steel.

In a typical LF (Ladle Furnace) operation, the slag layer covering the molten steel is heated by using a high temperature arc heat using an electrode made of carbon, and the heat of the slag layer heated to a high temperature is transferred to the molten steel. By the way, since the electrode rod used in LF (Ladle Furnace) operation consists of carbon, carbon of an electrode rod melt | dissolves in molten steel, the nitrogen component in air | atmosphere decomposes, and enters molten steel. Thus, there is a problem that the concentration of C and N in the molten steel increases during the operation of LF (Ladle Furnace). At this time, since the N concentration is increased after the denitrification step in the electric furnace, there is a problem that the denitrification should be carried out again.

In addition, the oxygen concentration is maintained at 800 ppm or more immediately before or during the tapping of the molten steel from the electric furnace in order to maintain the amount of oxygen to participate in the decarburization in the subsequent decarburization step. For example, when the temperature of oxygen is lower than 800 ppm immediately before or during tapping, the decarburization efficiency is low and thus the amount of carbon cannot be lowered to a desired amount.

When the temperature of the molten steel in the electric furnace is secured to a predetermined temperature, that is, 1650 ° C. or more, the molten steel is tapped into a vacuum degassing facility, and then an additive is immediately added to perform the second refining (S200). Here, the additives include decarburizing agents, desulfurizing agents and deoxidizers for decarburization, desulfurization and deoxidation.

In the second refining step in the vacuum degassing equipment first decarburization is performed (S210). In the following, a vacuum tank degassing apparatus (hereinafter referred to as VTD (Vacuum Tank Degasser)) is described as a vacuum degassing plant. When the furnace is secured at a temperature of 1650 ° C. or higher of the molten steel, the molten steel is pulled out of the electric furnace and placed in a ladle. At this time, since the molten steel withdrawn from the electric furnace is 1650 ℃ or more, the temperature of the molten steel is maintained at 1600 ℃ or more when the VTD arrives. Then, the entire ladle is charged into a tank of the VTD, and the tank is covered with a cover, and then the composition is set to 20 mbar or less, preferably 10 mbar, which is high vacuum. The molten steel contained in the ladle is refluxed for 15 to 25 minutes to perform decarburization. In the embodiment, a bottom bubbling method is used. That is, bottom bubbling is performed in which gas, for example, argon (Ar), is blown into the bottom of the ladle charged in the tank and bubbled. Therefore, molten steel and slag are refluxed by bubbling argon. By forming the pressure at 20 mbar or less as described above, the volume of bubbles in the molten steel can be increased to increase the reflux efficiency. At this time, the carbon and oxygen in the molten steel reacts to become CO, so that the decarburization in which the carbon concentration in the molten steel is lowered proceeds. Since the concentration of oxygen in the molten steel is 800 ppm or more before the start of reflux of the molten steel, a reaction between the carbon and the oxygen in the molten steel occurs sufficiently, so that the concentration of the carbon can be lowered to 40 ppm or less.

On the other hand, if the pressure in the decarburization operation is higher than 20mbar, a problem that the volume of the bubble is small so that reflux or stirring between molten steel and slag does not occur vigorously.

When the decarburization operation time is less than 15 minutes, the concentration of carbon cannot be lowered to 40 ppm or less, and when the decarburization operation time is more than 25 minutes, the decarburization operation time is too long, causing the temperature to drop to less than 1600 ° C.

In the above, the bottom bubbling method has been described as an example of refluxing molten steel, but various methods, for example, a reflux method using a separate stirrer may be used.

When decarburization is completed, desulfurization and deoxidation are performed using a vacuum degassing facility (S220). That is, after decarburization is completed in VTD, desulfurization and deoxidation are performed in the VTD. To this end, the pressure inside the tank is converted to at least 100 mbar, preferably 200 mbar to 300 mbar, in vacuum, and deoxidizer and desulfurizer are introduced. In this embodiment, an alloy containing Al (aluminum) is used as the deoxidizer, and a material containing CaO (quick lime) is used as the desulfurization agent. 3) is. More preferably, the input ratio of CaO: Al is 2-3 (CaO / Al = 2-3). Accordingly, oxygen in the molten steel reacts with Al of the deoxidizer to be introduced to form Al ₂ O ₃ (alumina), and S (sulfur) in the molten steel reacts with CaO of the desulfurization agent to be removed to remove CaS. The concentration of S in molten steel is 30 ppm or less, and the concentration of oxygen is adjusted to 400 ppm or less by the addition to these desulfurizing agents and deoxidizers.

On the other hand, when the pressure inside the VTD is lower than 100 mbar in the deoxidation and decarburization step, desulfurization and deoxidation may not be performed efficiently. On the contrary, when the pressure is higher than 300 mbar, a problem may arise in that external nitrogen is introduced into the molten steel. In addition, when the CaO: Al input ratio is less than 2, the desulfurization effect is insignificant, and when it exceeds 3, there is a problem that a lot of materials are used unnecessarily.

The deoxidizer is not limited to Al or the alloy containing above, and may be an alloy containing any one of Si, Mn, Ca, Mg, and Ti. In addition, a desulfurization agent is not limited to CaO, Any one of CaC2, CaCN2, and CaF2 can be used.

In the above description, the VTD is described as a vacuum degassing facility in which decarburization, desulfurization, and deoxidation are performed. However, the present invention is not limited thereto, and decarburization, desulfurization, and deoxidation may be performed in a RF (Rheinstaal Huttenwerke und Heraus) facility.

As such, the embodiment of the present invention does not perform VTD-OB operation of bubbling oxygen into the VTD facility. That is, the temperature raising operation of molten steel performed by blowing oxygen and bubbling separately into molten steel in a VTD facility which is a vacuum degassing installation, and stirring molten steel and slag is abbreviate | omitted. This ensures that the temperature at the time of tapping the molten steel from the electric furnace is 1650 ° C or higher, so that the temperature of the molten steel is 1600 ° C or higher when the VTD arrives, so that the molten steel maintains 1550 ° C or more, which is easy to manufacture slabs upon arrival of the tundish. This is because no additional temperature is required. In addition, since N in the molten steel is sufficiently removed in the denitrification step of the electric furnace and LF operation is not performed, denitrification through OB (Oxygen Bubbling) (OB) can be omitted since N in the atmosphere is prevented from entering the molten steel. After denitrification in the electric furnace, the concentration of S in the VTD is set to 2 to 3, so that the concentration of S can be lowered to 30 ppm or less.

Table 1 is a table showing the N, C, S concentration of the molten steel produced by the refining method of the molten steel according to an embodiment of the present invention and the molten steel prepared by the refining method of the molten steel according to the first to third comparative examples.

The molten steel according to the examples and the first to third comparative examples is a molten steel for ultra low carbon steel, and includes Fe, C, Si, Mn, P, S, Al, Ti, Sn, and the like.

The molten steel according to the embodiment is the first refining step of performing denitrification in the electric furnace as described above, the step of securing the temperature of the molten steel in the electric furnace to 1650 ℃ or more, after tapping the molten steel of the electric furnace with the vacuum degassing equipment VTD It was prepared through a second refining step of refining by adding an additive. More specifically, scrap is introduced into an electric furnace, and then carbonaceous subsidiary materials are added and melted, and denitrification is performed by blowing a mixed gas of LNG and oxygen. At this time, the amount of the carbon-based subsidiary material per tonne of scrap is added to 5kg to 40kg. In addition, the flow rate of the intake of the mixed gas of LNG and oxygen and the dust collection flow rate of the exhaust gas exhausted from the electric furnace are controlled to prevent external air from flowing into the electric furnace to suppress the mixing of nitrogen contained in the external air into the molten steel. . At this time, the dust collection flow rate of exhaust gas is adjusted in 1 to 10 times the range compared with the blowing flow volume of blown gas. Then, the temperature of the molten steel in the electric furnace to 1650 ℃ or more, the oxygen concentration of 800ppm to secure the molten steel from the electric furnace and put in the ladle (Ladle), and the ladle is charged to the vacuum degassing facility VTD. Thereafter, the pressure inside the VTD is set to 10 mbar to 20 mbar, and Ar is blown to reflux to decarburize for 15 to 25 minutes. Subsequently, the inside of the VTD is composed of 200 mbar to 300 mbar, and desulfurization and deoxidation are performed by adding a deoxidizer containing Al and a desulfurizing agent containing CaO. At this time, the CaO: Al ratio is 2 to 3 (CaO / Al = 2 to 3).

On the other hand, in the case of the first to third comparative examples, denitrification is carried out in the same manner as in the example in the electric furnace. However, the first comparative example is molten steel when the CaO: Al ratio input for desulfurization and deoxidation in VTD is 4 (CaO / Al = 4). In addition, the second comparative example is a molten steel that the temperature of the molten steel is lower than 1650 ℃ at the time of tapping in the electric furnace, the molten steel is lower than 1600 ℃ at the arrival of VTD (temperature failed to secure), through the LF operation after decarburization. And, in the third comparative example, the temperature of the molten steel is lower than 1650 ℃ at the time of tapping in the electric furnace, the molten steel is kept lower than 1600 ℃ when the VTD arrives (failed to secure the temperature), the molten steel that the temperature was raised by LF operation before decarburization in VTD to be.

N concentration [ppm] C concentration [ppm] S concentration [ppm] Example 30 26 32 Comparative Example 1 35 39 123 2nd comparative example 77 185 29 Comparative Example 3 62 44 31

Referring to Table 1, in the molten steel according to the embodiment, the concentration of N is 30ppm, the concentration of C is 26ppm, and the concentration of S is 32ppm. When casting is performed using such molten steel, ultra low carbon steel with low concentrations of N, C, and S can be produced.

On the other hand, the molten steel according to the first comparative example is higher than each of the concentration of each of the N, C, S, in particular, the concentration of S is 123ppm about 4 times higher than the embodiment. This is because the CaO: Al ratio introduced at the desulfurization and deoxidation stage becomes 4 (CaO / Al = 4), and the desulfurization efficiency is not good, and thus sufficient desulfurization has not been achieved.

In addition, the molten steel according to the second comparative example has a higher concentration of N and C than the embodiment, in particular, the concentration of C is 185 ppm, about 7 times higher than the comparative example, and the N concentration is 77 ppm about 2.5 times higher. This is because LF operation was performed to raise the temperature of molten steel after decarburization in VTD. More specifically, the electrode used to raise the temperature of molten steel in LF is made of carbon, and the carbon of the electrode melts into the molten steel. In addition, the concentration of N is 77 ppm, which is higher than that of the example, because the nitrogen component in the atmosphere enters the molten steel at the time of the elevated temperature of the molten steel in the LF. Therefore, when performing LF operation after decarburization in VTD, it is not possible to produce low nitrogen, low carbon steel with low nitrogen content as in the example.

In addition, the molten steel according to the third comparative example has a high concentration of each of N, C, and S, in particular, the concentration of N is 62 ppm, which is higher than that of the example. This is because, after denitrification in the electric furnace, LF operation was performed to raise the molten steel before moving to the VTD. That is, nitrogen component in air enters molten steel at the time of temperature rising of molten steel in LF. On the other hand, as described above, although the carbon of the electrode rod melts in the molten steel during the temperature increase using LF, the concentration of C is lower than that of the second comparative example because decarburization operation is performed in VTD after LF operation. However, the concentration of C is higher than that of the examples.

In the above description, the method of manufacturing molten steel according to the embodiment was applied to manufacturing molten steel for ultra low carbon steel. However, the present invention is not limited to ultra low carbon steel but may be applied to various carbon steels such as various steels, high carbon steels, and medium carbon steels.

S110: furnace denitrification step S120: temperature obtaining step of molten steel
S200: second refining step in vacuum degassing plant

Claims (9)

As a refining method of molten steel for producing low carbon steel,
Melting molten iron source into an electric furnace to manufacture molten steel;
A first refining process of denitrifying molten steel in the electric furnace;
Securing a temperature of the molten steel in the electric furnace to maintain the temperature of 1550 ° C. or more when the molten steel arrives at the tundish;
After the tapping of the molten steel of the electric furnace to a vacuum degassing facility, a second refining process of directly adding an additive to decarburization, desulfurization and deoxidation,
The process of securing the temperature of the molten steel and tapping to the vacuum degassing facility,
By measuring the temperature of the molten steel, when the molten steel is 1650 ℃ or more, the molten steel is completed as the vacuum degassing equipment as it is,
When the molten steel is less than 1650 ° C, the molten steel having the denitrification completed is adjusted to be at least 1650 ° C,
LF operation is omitted between the first refining process in the electric furnace and the second refining process in the vacuum degassing facility and after the second refining process in the vacuum degassing facility,
A method of refining molten steel that omits oxygen bubbling operation in the vacuum degassing facility before the second refining step of decarburizing, desulfurizing and deoxidizing in the vacuum degassing facility. delete delete delete The method according to claim 1,
In the second refining process of the molten steel,
The desulfurization and deoxidation of the molten steel after the decarburization of the molten steel. The method according to claim 1 or 5,
In the decarburization process of the molten steel,
Injecting gas into the molten steel to reflux the molten steel,
A method of refining molten steel for forming a pressure in the vacuum degassing facility to 20 mbar or less. The method according to claim 1 or 5,
In the desulfurization and deoxidation process,
The additive includes a desulfurization agent and a deoxidizer, and the ratio of desulfurization agent to deoxidizer (desulfurization agent / deoxidizer) to be added is 2 to 3. The method of claim 7,
In the desulfurization and deoxidation process of the molten steel,
A method of refining molten steel for forming a pressure in the vacuum degassing facility to 100 mbar or more. The method according to claim 1,
Including the carbon-based subsidiary material before charging the iron source into the electric furnace,
The refining method of molten steel which blows the mixed gas of LNG and oxygen in the said dissolution process.

Images