EP3666416A1

EP3666416A1 - Casting facility and casting method

Info

Publication number: EP3666416A1
Application number: EP17920988.7A
Authority: EP
Inventors: Sung Jool Kim; Wook Kim; Young Ju Lee; Yong Hwan Kim; Seong Yeon Kim
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2017-08-08
Filing date: 2017-12-19
Publication date: 2020-06-17
Also published as: JP2020530399A; KR101969111B1; WO2019031661A1; CN111107953A; EP3666416A4; KR20190016345A

Abstract

In accordance with an exemplary embodiment, a slab casting method includes disposing a ladle in which molten steel is accommodated at each of an upper side and an outer side of a tundish, performing casting by supplying the molten steel of the ladle disposed at a casting position of the upper side of the tundish, and blowing an inert gas into the ladle disposed at the casting position.

Thus, in accordance with an exemplary embodiment, the inert gas is blown when the ladle is in a stand-by position on the turret device and when casting of supplying the molten steel to the tundish is performed. Thus, inclusions may be reduced in comparison with the related art, and clean steel may be produced. That is, generation of the inclusions may be reduced by performing micro-bubbling after the ladle is opened when the ladle is at the stand-by position. Also, the inclusions in the molten steel in the ladle may be reduced during casting by blowing the inert gas into the ladle during casting.

Description

TECHNICAL FIELD

The present disclosure relates to a casting facility and a casting method, and more particularly, to a casting facility capable of producing clean steel.

BACKGROUND ART

In general, inclusions such as alumina (Al₂O₃) are produced in molten steel in a ladle during a steel manufacturing and treatment process due to a reaction between aluminum (Al) and oxygen (O₂). The inclusions are coagulated with the molten steel during casting of a slab to cause a failure of a product in a rolling process.
In order to remove or prevent generation of the inclusions such as alumina (Al₂O₃) in the molten steel in the ladle, the inclusions are removed by using a reinstahl huten werke heraus (RH) and a ladle furnace (LF) or by blowing an inert gas such as an argon (Ar) gas into the molten steel in a tundish process during a casting operation.
The ladle accommodating the molten steel, which is refined by using the reinstahl huten werke heraus (RH) or in which temperature increase is completed by using the ladle furnace (LF), is supported by the ladle turret and disposed at the upper side of the tundish. That is, the ladle turret includes a supporting part, on which the ladle is seated, at each of both sides of a swing tower, and the ladle is seated and supported by the supporting part. Also, two ladles are alternately transferred to an upper side of the tundish by a rotation operation of the swing tower. Here, the ladle disposed at the upper side of the tundish among the two ladles is a ladle participating in casting, and the ladle disposed at an outer side of the tundish is a ladle in a stand-by state for next casting.
However, although the refinement is performed by using the reinstahl huten werke heraus (RH), the temperature increase is completed by using the ladle furnace (LF), and the argon gas is blown into the tundish, the inclusions are generated during casting or when the ladle is in the stand-by state on the ladle turret, and a defect due to the inclusions is still generated.
To this end, a feature of blowing the argon gas into the ladle, which is not participated in casting or in the stand-by state, is disclosed in Korean Publication Utility Model No. 1998-033102 . In case of the disclosed method, although the inclusions existing in the molten steel may be separated and floated, the generation of naked molten steel at a molten steel surface increases, and thus generation of reoxidative inclusions is accelerated.
Also, since the argon gas is not blown during the casting, inclusions are still generated in the molten steel in the ladle participating the casting.
(Patent document 1) Korean Registered Utility Model No. KR0332894Y1

DISCLOSURE OF THE INVENTION

TECHNICAL PROBLEM

The present disclosure provides a casting facility capable of reducing inclusions and a casting method.
The present disclosure provides a casting facility capable of reducing generation of inclusions by blowing a gas into a ladle in a stand-by state or a casting state on a turret and a casting method.
The present disclosure provides a casting facility capable of restricting or preventing generation of naked molten steel and a casting method.

TECHNICAL SOLUTION

In accordance with an exemplary embodiment, a slab casting method includes: disposing a ladle in which molten steel is accommodated at each of an upper side and an outer side of a tundish; performing casting by supplying the molten steel of the ladle disposed at a casting position of the upper side of the tundish; and blowing an inert gas into the ladle disposed at the casting position.
The blowing of the inert gas into the ladle disposed at the casting position may include: opening the ladle at the casting position by blowing the inert gas with a first flow rate into the ladle at the casting position; and bubbling by blowing the inert gas with a smaller flow rate than the first flow rate when the casting of supplying the molten steel to the tundish is initiated after the ladle at the casting position is opened.
The bubbling of the ladle at the casting position may include reducing a gas blowing flow rate of the inert gas as a height of the molten steel in the ladle at the casting position decreases.
The reducing of the gas blowing flow rate of the inert gas as the height of the molten steel in the ladle at the casting position decreases may include supplying with a flow rate (m₁) calculated by a mathematical equation 1 using a ratio of a current molten steel height (L₁) with respect to an initial molten steel height (L₀) before the molten steel in the ladle at the casting position is supplied to the tundish and an initial gas blowing flow rate (m₀) when the molten steel in the ladle at the casting position is initially supplied to the tundish
The initial gas blowing flow rate (m₀) may be equal to or greater than approximately 1 LPM and equal to or less than approximately 20 LPM.
The slab casting method may further include blowing the inert gas into the ladle disposed at a stand-by position of the outer side of the tundish.
The blowing of the inert gas into the ladle disposed at the stand-by position may include: opening the ladle at the stand-by position by blowing the inert gas with a first flow rate into the ladle at the stand-by position; and bubbling by blowing the inert gas with a second flow rate, which is less than the first flow rate, when casting of supplying the molten steel to the tundish is initiated after the ladle at the stand-by position is opened.
The first flow rate may be equal to or greater than approximately 80 LPM and equal to or less than approximately 200 LPM, and the second flow rate may be equal to or greater than approximately 1 LPM and equal to or less than approximately 20 LPM.
In accordance with another exemplary embodiment, a casting facility includes: a tundish configured to temporarily store molten steel; a turret device including one pair of supporting parts configured to support one pair of ladles in which the molten steel is accommodated, respectively, and configured to alternately disposing the one pair of supporting parts at a casting position of an upper side of the tundish and a stand-by position of an outer side of the tundish; a mold disposed below the tundish to coagulate the molten steel supplied from the tundish; and a gas blowing device connected to each of the ladle at the stand-by position and the ladle at the casting position so that an inert gas is blown into each of the ladle supported at the stand-by position and the ladle supported at the casting position on the turret device.
The gas blowing device may include: a first blowing line connected to the ladle supported at the stand-by position; a second blowing line connected to the ladle supported at the casting position; a first supply part connected to the first blowing line to selectively supply the inert gas to the first blowing line with a first flow rate for opening the ladle at the stand-by position and a second flow rate less than the first flow rate; and a second supply part connected to the second blowing line to selectively supply the inert gas to the second blowing line with a first flow rate for opening the ladle at the casting position and a flow rate less than the first flow rate.
The first supply part may supply the inert gas to the first blowing line with a first flow rate equal to or greater than approximately 80 LPM and equal to or less than approximately 200 LPM so that a blowing hole of the ladle at the stand-by position is opened, and supply the inert gas to the first blowing line with a second flow rate equal to or greater than approximately 1 LPM and equal to or less than approximately 20 LPM after the ladle at the stand-by position is opened, thereby bubbling the ladle at the stand-by position.
The second supply part may supply the inert gas to the second blowing line with a first flow rate equal to or greater than approximately 80 LPM and equal to or less than approximately 200 LPM so that a blowing hole of the ladle at the casting position is opened, and reduce an inert gas blowing flow rate to the second blowing line according to decrease in height of the molten steel in the ladle at the casting position with a flow rate less than the first flow rate when the molten steel in the ladle at the casting position is initially supplied to the tundish after the ladle at the casting position is opened.

ADVANTAGEOUS EFFECTS

In accordance with the exemplary embodiment, the inert gas is blown when the ladle is at the stand-by position on the turret device and when the casting of supplying the molten steel to the tundish is performed. Thus, the inclusions may be reduced in comparison with the related art, and the clean steel may be produced. That is, when the ladle is at the stand-by position, as the micro-bubbling is performed, the generation of the inclusions during the stand-by may be reduced. Also, the inclusions in the molten steel in the ladle during the casting may be reduced by blowing the inert gas into the ladle during the casting.
Also, as the gas blowing flow rate decreases according to the decrease in molten steel height during the casting, the bubbling may be performed with an appropriate amount, and the generation of the naked molten steel because of the inert gas may be restricted or prevented. That is, when the gas is blown with an excessively large flow rate with respect to the molten steel amount or the molten steel height, the naked molten steel, which generates an empty space of the slag of the molten steel surface due to a vortex generation, may be generated. In accordance with the exemplary embodiment, the gas blowing flow rate may be adjusted in correspondence to the decrease in molten steel height in the ladle L during the casting, thereby restricting or preventing the generation of the naked molten steel caused by the gas blowing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a main portion of a casting facility in accordance with an exemplary embodiment.
FIG. 2 is a view illustrating a ladle in accordance with an exemplary embodiment.
FIG. 3 is a schematic view illustrating a gas blowing device in accordance with an exemplary embodiment.
FIG. 4 is a graph representing a method of blowing a gas into a ladle in a ready state in accordance with an exemplary embodiment.
FIG. 5 is a graph representing a method of blowing a gas into a ladle in a casting state in accordance with an exemplary embodiment.
FIG. 6 is a view showing a result in which naked molten steel is generated when bubbling is performed by a method in accordance with a comparative example.
FIG. 7 is a graph indicating an amount of inclusions in each of operations as an inclusion index.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, specific embodiments will be described in detail with reference to the accompanying drawings. The present disclosure may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art.
The present disclosure provides a casting facility reducing or restricting inclusions and naked molten steel by blowing a gas into a ladle, which is in a ready state or a casting state, of a turret device, and a slab casting method using the same.
FIG. 1 is a view illustrating a main portion of a casting facility in accordance with an exemplary embodiment. FIG. 2 is a view illustrating a ladle in accordance with an exemplary embodiment. FIG. 3 is a schematic view illustrating a gas blowing device in accordance with an exemplary embodiment. FIG. 4 is a graph representing a method of blowing a gas to a ladle in a ready state in accordance with an exemplary embodiment. FIG. 5 is a graph representing a method of blowing a gas to a ladle in a casting state in accordance with an exemplary embodiment.
Referring to FIGS. 1 and 3, the casting facility in accordance with an exemplary embodiment includes: a turret device 100 capable of supporting one pair of ladles L in which molten steel is accommodated and moving the pair of ladles L by a rotation operation; a tundish T receiving the molten steel from the ladle L moving in an upper side direction and temporarily storing the received molten steel; a mold M receiving the molten steel that is temporarily stored in the tundish T and primarily cooling and initially coagulating the molten steel into a predetermined shape; a plurality of segments 20 provided below the mold M to perform a series of molding operations while secondarily cooling the primarily cooled slab; and a gas blowing device 200 blowing an inert gas to each of the ladle L disposed at an upper side the tundish T in a corresponding manner and the ladle L in a ready state at an outer side of the tundish T. Also, the casting facility includes: a shroud nozzle SN supplying molten steel to the tundish; a gate controlling communication between the ladle L and the shroud nozzle SN; a nozzle mounting unit 30 disposed at one side of the tundish T to connect between a top nozzle and the shroud nozzle.
Since the above-described ladle L, the turret device 100, the tundish T, the mold M, and the segments 20 are similar to or the same as those of a general continuous casting facility, detailed description thereof will be omitted or simply described.
The turret device 100 includes a swing tower 110 that is rotatably driven and one pair of supporting parts 120 extending in both directions by using the swing tower 110 as a center or disposed at both sides by using the swing tower 110 as a center to support the ladles L or allow the ladles L to be seated thereon, respectively. According to the turret device 100, the one pair of supports part 120 alternately move to the upper side of the tundish by rotation of the swing tower 110. That is, one supporting part 120 of the one pair of support parts 120 and the ladle L supported by the corresponding supporting part 120 are disposed at the upper side of the tundish T by the rotation of the swing tower 110. Here, the other supporting part 120 and the corresponding ladle L are disposed at the outer side of the tundish T.
However, the exemplary embodiment is not limited to thereto. The turret device 100 may include various configurations capable of supporting the one pair of ladles L and alternately moving the ladles L at the upper side of the tundish T or to a ready position.
As illustrated in FIG. 2, the ladle L includes: a main body 310 forming an appearance, having an inner space capable of accommodating molten steel, and including an opening (hereinafter, referred to as an output hole 321) defined at a lower portion to discharge the molten steel and an opening (hereinafter, referred to as a blowing hole 322) through which a gas passes; and a top nozzle TN installed to the main body 310 to communicate with the outlet hole 321. Also, the ladle L may further include a plug 330 inserted into the main body 310 to communicate with the inside of the main body 310.
In an exemplary embodiment, inclusions and generation of the naked molten steel are reduced or restricted by blowing an inert gas to each of the one pair of ladles L supported by the turret device 100. That is, the inert gas is blown to each of the ladle L disposed on and supported by the supporting part 120 disposed at the upper side of the tundish T in a corresponding manner, the ladle L supplying molten steel to the tundish T and participating in casting, and the ladle L disposed on and supported by the supporting part 120 disposed at the outer side of the tundish T of the one pair of support parts 120 of the turret device 100. When this is described in terms of a side surface of one ladle L, while one tundish T is supported by the turret device 100 and ready at the outer side of the tundish T, the inert gas is blown into the ladle T, and thereafter, the ladle L moves at the upper side of the tundish to blow the inert gas to the ladle L when the molten steel is supplied to the tundish T (i.e., when participating in casting).
Also, in an exemplary embodiment, when the inert gas is blown into the ladle L in a ready state, after the blowing hole 322 of the ladle L is opened, a relatively small amount of inert gas is blown to reduce or restrict the inclusions and the generation of the naked molten steel. Also, when the inert gas is blown into the ladle L in a casting state, after the blowing hole 322 of the ladle L is opened, at the time the casting is initiated, the inert gas is blown while an amount of the inert gas gradually decreases as a molten steel height or a bath surface height decreases to reduce or restrict the inclusions and the generation of the naked molten steel.
To this end, the gas blowing device 200 blowing the inert gas to each of the ladle L seated on the turret device 100 and in the ready state and the ladle L participating the casting and control a gas blowing amount.
Referring to FIG. 3, the gas blowing device 200 includes: a first blowing line 210a capable of being connected to the ladle L supported at a ready position; a second blowing line 210b capable of being connected to the ladle L supported at a casting position; a first gas storage 220a supplying a high pressure inert gas, a second gas storage 220b supplying a low pressure inert gas; a first supply part 230a supplying an inert gas of each of the first and second gas storages 220a and 220b to the first blowing line 210a by connecting the first blowing line 210a and the first and second gas storages 220a and 220b; and a second supply part 230b supplying the inert gas of each of the first and second gas storages 220a and 220b to the second blowing line 210b by connecting the second blowing line 210b and the first and second gas storages 220a and 220b.
Here, the first blowing line 210a may be connected to and separated from the blowing hole 322 of the ladle L at the ready position at the outer side of the tundish T, and the second blowing line 210b may be connected to and separated from the blowing hole 322 of the ladle L at the casting position at the upper side of the tundish T. Each of the first blowing line 210a and the second blowing line 210b may have a pipe shape through which the inert gas is movable.
Also, a first blowing valve 211a is installed on an extension path of the first blowing line 210a, and a second blowing valve 211b is installed on an extension path of the second blowing line 210b.
The first supply part 230a in accordance with an exemplary embodiment includes: a first supply line 231a having one end connected to the first gas storage 220a; a second supply line 234a having one end connected to the second gas storage 220b and the other end connected to the first blowing line 210a; a first supply valve 232a and a first flow control part 233a installed on an extension path of the first supply line 231a; and a second supply valve 235a and a second flow control part 236a installed on an extension path of the second supply line 234a. Here, the other end of the first supply line 231a may be connected to the second supply line 234a so as to be disposed at a front end of the second supply valve 235a.
Each of the first and second supply lines 231a and 234a may have a pipe shape through which the inert gas is movable.
For example, the first supply valve 232a in accordance with an exemplary embodiment may be a motor valve. Also, the first flow control part 233a may be installed at a rear end of the first supply valve 232a, and the second flow control part 236a may be installed at a rear end of the second supply valve 235a.
The second supply part 230b in accordance with an exemplary embodiment includes: a third supply line 231b having one end connected to the first gas storage 220a; a fourth supply line 234b having one end connected to the second gas storage 220b and the other end connected to the second blowing line 210b; a third supply valve 232b and a third flow control part 233b installed on an extension path of the third supply line 231b; and a fourth supply valve 235b and a fourth flow control part 236b installed on an extension path of the fourth supply line 234b. Here, the other end of the third supply line 231b may be connected to the fourth supply line 234b so as to be disposed at a front end of the fourth supply valve 235b.
Each of the third and fourth supply lines 231b and 234b may have a pipe shape through which the inert gas is movable.
For example, the third supply valve 232a in accordance with an exemplary embodiment may be a motor valve. Here, the third flow control part 233b may be installed at a rear end of the third supply valve 232b, and the fourth flow control part 236b may be installed at a rear end of the fourth supply valve 235b.
In an exemplary embodiment, by using the above-described gas blowing device 200, the inclusions and the generation of the naked molten steel are reduced or restricted by blowing the inert gas, e.g., an argon (Ar) gas, when the ladle L supported by the turret device is disposed at both the ready and casting positions.
However, the exemplary embodiment is not limited to thereto. The gas blowing device 200 may include various configurations capable of supplying the inert gas to each of the first and second blowing lines 210a and 210b by regulating a pressure and a flow rate.
Hereinafter, a casting method including a process of blowing an inert gas to each of the ladles L positioned at the ready and casting positions on the turret device by using the gas blowing device 200 will be described with reference to FIGS. 3 to 5. Here, the inert gas will be described as the argon gas.
A casting method in accordance with an exemplary embodiment includes: a process of disposing the ladle in which molten steel is accommodated at each of the upper side of the tundish and the outer side of the tundish; a process of performing casting by supplying the molten steel in the ladle disposed at the casting position of the upper side of the tundish to the tundish; and a process of blowing an inert gas into the ladle disposed at the casting position.
Hereinafter, the casting method in accordance with an exemplary embodiment will be described in detail.
First, the ladles L in which the molten steel is accommodated are disposed on and supported by one pair of supporting parts 120 of the turret device 100, respectively. Among the one pair of supporting parts 120, one supporting part 120 disposed at the upper side of the tundish T in a corresponding manner participates in casting by supplying the molten steel in the ladle L to the tundish T and the other supporting part 120 stands by at the outer side of the tundish T for a subsequent charging.
First, referring to FIG. 4, a method for blowing the inert gas, e.g., the argon gas, into the stand-by ladle L will be explained in detail.
When the argon gas is blown into the stand-by ladle, the argon gas is firstly blown with a first flow rate for opening the ladle L. Here, the opening of the ladle L represents that a gas passes through the main body 310 or the plug 330 of the ladle L and is supplied into the ladle. After the ladle L is opened, the gas may be blown into the ladle L although the gas blowing flow rate is reduced.
The opening of the ladle L is performed during a predetermined time since the gas is initially blown, e.g., within 10 seconds from the time of initiating gas blowing, and this section may be referred to as a blowing beginning section.
In an exemplary embodiment, when the inert gas is blown with the first flow rate into the ladle L, the gas is blown with the first flow rate in a range from approximately 80 LPM to approximately 200 LPM (5 to 5 Nm3/h) to open the blowing hole 322 of the ladle L.
Also, when the ladle L is opened, or when the inert gas is blown with the first flow rate, a gas pressure may be adjusted greater than 10 bar and equal to or less than approximately 20 bar, and supplied with a relatively higher pressure than that of the gas blown after opened.
For example, when the first flow rate is less than approximately 80 LPM, the blowing hole 322 may not be opened, and thus the argon gas may not be introduced into the ladle L. On the contrary, when the flow rate of the inert gas at the beginning of blowing exceeds approximately 200 LPM, although the blowing hole 322 of the ladle L is opened, instability of a molten steel surface of the ladle L may be caused. This may cause operation insecurity and increase a generated area of the naked molten steel.
When the ladle L is opened, the inert gas is blown with a second flow rate for reducing the inclusions and the generation of the naked molten steel of the molten steel. Here, the gas blowing flow rate is relatively less than the first flow rate when the ladle L is opened.
In an exemplary embodiment, when the stand-by ladle is opened, the gas may be blown with the second flow rate in a range from approximately 1 LPM to approximately 20 LPM to micro-bubble the molten steel, thereby reducing the inclusions and preventing the generation of the naked molten steel.
Here, a gas pressure is less than that when the ladle L is opened. In an exemplary embodiment, the gas pressure is in a range from approximately 2 bar to approximately 10 bar.
Also, when the second flow rate blown after the ladle L is opened is less than approximately 1 LPM, an effect of reducing the inclusions may be low or may not be realized due to inert gas bubbling. On the contrary, when the second flow rate blown after the ladle L is opened is greater than approximately 10 LPM, an effect of restricting the generation of the naked molten steel at the molten steel surface may not be achieved, or an area of the naked molten steel may increase. When the area of the naked molten steel is large, the inclusions are mixed with the molten steel through the naked molten steel, and thus clean steel may not be produced.
When a gas is blown into the ladle L at the stand-by position as described above, a case of using the gas blowing device 200 in FIG. 3 will be described below. First, when the first supply valve (e.g., the motor valve) 232a and the first blowing valve 211a are opened in a state in which the second supply valve 235a is closed in order to open the ladle L, the gas in the first gas storage 220a moves through the first supply line 231a, the second supply line 234a, and the first blowing line 210a and is blown to the blowing hole 322 of the ladle L at the stand-by position. Here, as a gas having a high pressure greater than 10 bar and equal to or less than approximately 20 bar instantly flows along the first supply line 231a as soon as the motor valve is opened, the first flow control part 233a is adjusted to allow a gas having a flow rate in a range from approximately 80 LPM to approximately 200 LPM to flow. Thus, as the argon gas having the pressure greater than 10 bar and equal to or less than approximately 20 bar and the flow rate in the range from approximately 80 LPM to approximately 200 LPM is blown into the stand-by ladle L, the ladle L is opened.
When the ladle L is opened, the first supply valve 232a is closed, and an operation of the first flow control part 233a is stopped. Also, as the second supply valve 235a is opened, and the second flow control part 236a operates, the argon gas having a pressure in a range from 2 bar to 10 bar and a flow rate in a range from approximately 1 LPM to approximately 20 LPM is supplied to the second supply line 234a and the first blowing line 210a, and thus the argon gas is blown into the stand-by ladle L. By the above-described blowing of the argon gas, the molten steel in the stand-by ladle L may be micro-bubbled, and the inclusions in the molten steel in the stand-by ladle may be reduced, thereby restricting the generation of the naked molten steel.
While the molten steel in the ladle L at the stand-by position is bubbled, the ladle L at the casting position participates the casting by continuously supplying the molten steel to the tundish T.
Also, when the casting is finished at the casting position, the swing tower 110 of the turret device 100 rotates to move the ladle, which is bubbled at the stand-by position as described above, to the upper side of the tundish T, i.e., the stand-by position.
Thereafter, the shroud nozzle SN and the top nozzle TN of the ladle L are mutually coupled, and the shroud nozzle SN and the top nozzle TN communicate by an operation of the gate. As the molten steel in the ladle L is supplied to the tundish through the shroud nozzle SN, and a nozzle 40 (a submerged nozzle) of the tundish T moves to the mold M, the slab having a predetermined shape is cast.
When the casting is performed by supplying the molten steel in the ladle L at the upper side of the tundish T to the tundish T, the argon gas is blown into the ladle L participating the casting, i.e., the ladle at the casting position, to bubble the molten steel.
To this end, the second blowing line 210b is connected to the blowing hole of the ladle L moving to the casting position. Thereafter, the argon gas with the first flow rate for opening the ladle L is supplied. The opening of the ladle L is maintained during a predetermined time from the gas blowing initiation time, e.g., within ten seconds from the gas blowing initiation time.
In an exemplary embodiment, when the inert gas is blown with the first flow rate into the ladle L at the casting position, the first flow rate may be in a range from approximately 80 LPM to approximately 200 LPM (5 to 5 Nm3/h), and through this, the blowing hole 322 of the ladle L is opened.
Also, when the gas is blown with the first flow rate, the gas pressure is greater than 10 bar and equal to or less than 20 bar. Thus, the gas may be supplied with a relatively higher pressure than that of the gas blown after opened.
When the first flow rate blown to the ladle L at the casting position is less than approximately 80 LPM as an example, the blowing hole 322 may not be opened, and thus the argon gas may not be introduced into the ladle L. On the contrary, when the first flow rate exceeds approximately 200 LPM, although the blowing hole 322 of the ladle L is opened, instability of the molten steel surface of the ladle L may be caused. This may cause operation insecurity and increase the generated area of the naked molten steel.
When the ladle L at the casting position is opened, the inert gas is blown to reduce the inclusions of the molten steel and the generation of the naked molten steel. Here, the gas blowing flow rate may be relatively less than the first flow rate when the ladle L is opened.
In an exemplary embodiment, when the ladle L at the casting position is opened, the gas is blown with a smaller flow rate than that when opened to bubble the molten steel, thereby reducing the inclusions and restricting the generation of the naked molten steel.
Here, the gas pressure is less than that when the ladle L is opened. In an exemplary embodiment, the gas pressure is in a range from approximately 2 bar to approximately 10 bar.
When the casting is performed by initiating the supply of the molten steel in the ladle L at the casting position to the tundish T, since the molten steel in the ladle L is continuously supplied to the tundish, the molten steel in the ladle L participating the casting has a height that decreases as a casting time elapses. Thus, in an exemplary embodiment, when the inert gas is blown into the ladle L, the argon gas blowing flow rate is varied as the height of the molten steel in the ladle L or the height of the molten steel surface decreases from the initiation of the casting. More particularly, after the ladle L at the casting position is opened, the inert gas is blown into the ladle when the molten steel in the ladle L is supplied to the tundish, and the gas flow rate at this time is referred to as an 'initial gas blowing flow rate (m₀)'. Also, in an exemplary embodiment, the gas is blown with a flow rate (m₁) that is lower than the initial gas blowing flow rate (m₀) as the molten steel height decreases while the casting is performed. That is, according to the real-time current molten steel height or the current molten steel height (L₁) on the basis of the molten steel height at the initiation of the casting (hereinafter, referred to as an initial molten steel height (L₀)), the gas is blown with the flow rate (m₁) that is lower than the initial gas blowing flow rate (m₀). Here, this feature may be expressed by a mathematical equation as stated below, i.e., a mathematical equation 1. Also, this feature may be illustrated by a graph, i.e., FIG. 5. $Inertgasblowing flowrate (m_{1}) = \frac{Currentmoltensteelheight (L_{1})}{Initial molten steelheight (L) (_{0})} \times Initial gasblowing flowrate (m) (_{0})$
Here, the current molten steel height (L₁) in the ladle may be real-time calculated through a molten steel height before the molten steel is discharged, i.e., the initial molten steel height (L₀), and a molten steel discharge velocity. Also, when the molten steel in the ladle at the casting position begins to be supplied to the tundish, the initial gas blowing flow rate (m₀) supplied to the ladle may be in a range from approximately 1 LPM to approximately 20 LPM, and the pressure in the ladle L may be constantly maintained in a range from 2 bar to 10 bar.
When the initial gas blowing flow rate (m₀) exceeds approximately 20 LPM, the naked molten steel may be generated in the molten steel in the ladle when the casting is initiated.
Also, when the inert gas is blown with a constant flow rate regardless of decrease in molten steel height during the casting, the effect of reducing the inclusions may not be obtained, the generation of the naked molten steel may not be restricted, or the naked molten steel may be largely formed. That is, when the argon gas is blown with a small flow rate with respect to an molten steel amount in the ladle L, i.e., the current molten steel height (L₁), the effect of reducing the inclusions by the inert gas may not be obtained, and thus the clean steel may not be produced. On the contrary, when the argon gas is blown with a large flow rate with respect to the current molten steel height L₁, because of the large amount of blown gas, the naked molten steel may be generated at the molten steel surface, or the area of the naked molten steel may increase.
As described above, when the gas is blown into the ladle L at the casting position, a case of using the gas blowing device 200 in FIG. 3 will be described below.
First, when the third supply valve (e.g., the motor valve) 232b and the second blowing valve 211b are opened in a state in which the fourth supply valve 235b is closed in order to open the ladle L, the gas in the first gas storage 220a moves through the third supply line 231b, the fourth supply line 234b, and the second blowing line 210b and is blown to the blowing hole 322 of the ladle L at the casting position. Here, as a gas having a high pressure greater than 10 bar and equal to or less than 20 bar instantly flows along the third supply line 231b as soon as the motor valve is opened, the third flow control part 233b is adjusted to allow a gas having a flow rate in a range from approximately 80 LPM to approximately 200 LPM to flow. Thus, as the argon gas having the pressure greater than 10 bar and equal to or less than 20 bar and the flow rate in a range from approximately 80 LPM to approximately 200 LPM is blown into the ladle L at the casting position, the ladle L is opened.
When the ladle L is opened, the molten steel in the ladle L at the casting position begins to be supplied to the tundish T. Here, the third supply valve 232b is closed, and an operation of the third flow control part 233b is stopped. Also, as the fourth supply valve 235b is opened, and the fourth flow control part 236b operates, the argon gas having a pressure in a range from approximately 2 bar to 10 bar and a flow rate equal to or less than approximately 20 LPM is supplied to the fourth supply line 234b and the second blowing line 210b, and thus the argon gas is blown into the ladle L at the casting position.
Here, the gas flow rate supplied to the ladle L according to a height variation of the molten steel in the ladle L is adjusted from the initiation to the end of the casting by using the fourth flow control part 236b. That is, as in the mathematical equation 1 and FIG. 5, the flow rate decreases with respect to the initial gas blowing flow rate according to the current molten steel height on the basis of the molten steel height at the initiation of the casting.
By the above-described blowing of the argon (Ar) gas, the molten steel in the stand-by ladle L may be micro-bubbled, and the inclusions in the molten steel in the stand-by ladle may be reduced, thereby restricting the generation of the naked molten steel.
Hereinafter, results of treating the molten steel in the ladle by using a molten steel treatment method in accordance with a comparative example and an exemplary embodiment will be explained with reference to FIGS. 6 and 7.
FIG. 6 is a view showing a result in which the naked molten steel is generated when the ladle participating in casting is bubbled by a method in accordance with a comparative example. In case of the ladle in FIG. 6, one blowing hole is provided, and two outlet holes are provided. Thus, when the ladle in FIG. 6 is correspondingly disposed at the upper side of the tundish, while the molten steel is discharged from the two outlet holes, the argon gas is blown through one plug. Here, in case of the comparative example of FIG. 6, the argon gas is supplied with a constant amount regardless of decrease in molten steel height, (a) of FIG. 6 shows a result when a flow rate is approximately 10 Nm3/h, and (b) of FIG. 6 shows a result when a flow rate is 5 Nm3/h, and whether the naked molten steel is generated and a degree of generation of the naked molten steel are illustrated through a slag concentration.
Referring to (a) and (b) of FIG. 6, it may be known that the naked molten steel, which is generated by separation between slags at the molten steel surface is generated.
In accordance with an exemplary embodiment, when the argon gas is blown into the ladle at the casting position or participating in the casting, an appropriate amount of argon gas is blown according to decrease in molten steel height. Thus, an effect of restricting the generation of the naked molten steel is obtained in comparison with the related art.
FIG. 7 is a graph indicating an amount of inclusions in each operation as an inclusion index. Here, the amount of inclusions is calculated by a total content of oxygen in the molten steel, and the calculated amount of inclusions is compared.
FIG. 7 is a graph representing an inclusion index in the molten steel when treated according to a molten steel treatment method in accordance with a comparative example and an exemplary embodiment.
The comparative example includes: a process of deoxidizing in a vacuum degassing facility; a process of blowing the argon gas into the ladle and bubbling while increasing a temperature of the molten steel in a ladle furnace after the deoxidizing is finished; a process of standing-by the ladle in which the molten steel is accommodated at the stand-by position of the turret device; and a process of moving the stand-by ladle to the upper side of the tundish and supplying the molten steel to the tundish, thereby initiating casting. Here, the argon gas is blown through a submerged nozzle to perform the bubbling during the casting, the molten steel in the tundish is supplied to the mold, and the inclusions of the molten steel in the mold are measured.
The exemplary example includes: a process of deoxidizing in a vacuum degassing facility; a process of blowing the argon gas into the ladle and bubbling while increasing the temperature of the molten steel in the ladle furnace after the deoxidizing is finished; a process of bubbling by supporting the ladle in which the molten steel is accommodated by the turret device and then blowing the argon gas into the ladle at the stand-by position; and a process of moving the ladle at the stand-by position to the upper side of the tundish and supplying the molten steel to the tundish, thereby initiating casting. Here, the argon gas is blown through the submerged nozzle to perform the bubbling during the casting, the molten steel in the tundish is supplied to the mold, and the inclusions of the molten steel in the mold are measured.
The amount of inclusions of the molten steel at each process during the operation in accordance with the comparative example and the exemplary embodiment is measured. Here, the 'tundish' of FIG. 7 represents the amount of inclusions of the molten steel in the tundish without performing additional bubbling.
Also, the amount of inclusions at each process during the operation is calculated by the total oxygen content in the molten steel. Also, the inclusion index is calculated on the basis of the amount of inclusions in the molten steel of the vacuum degassing facility.
Referring to FIG. 7, in case of first to third embodiments, the inclusion index is reduced in comparison with first to second comparative examples. More particularly, when the amounts of inclusions in the molten steel in the mold are compared, the amount of inclusions of the embodiment is reduced by 30% than the comparative example. That is, although the bubbling while increasing the temperature by using the ladle furnace and the bubbling by the submerged nozzle are performed equally in the comparative example and the embodiment, the embodiment of performing the bubbling the ladle at the stand-by and the casting in the turret device has the amount of inclusions less than that of the comparative example of not performing the same. Thus, in case of using the casting method in accordance with the exemplary embodiment, steel in which a crack caused by the inclusions is generated less than the comparative example, i.e., clean steel, may be produced.
As described above, in case of the casting method in accordance with the exemplary embodiment, the inert gas is blown during the casting of supplying the molten steel to the tundish T. Thus, the inclusions may be reduced in comparison with the related art, and the clean steel may be produced. That is, when the ladle L is in the stand-by position, as the micro-bubbling is performed, the generation of the inclusions during the stand-by may be reduced. The inclusions in the molten steel in the ladle L during the casting may be reduced by blowing the inert gas into the ladle L during the casting.
Also, as the gas blowing flow rate decreases according to the decrease in molten steel height during the casting, the bubbling may be performed with an appropriate amount, and the generation of the naked molten steel because of the inert gas may be restricted or prevented. That is, when the gas is blown with an excessively large flow rate with respect to the molten steel amount or the molten steel height, the naked molten steel, which generates an empty space of the slag of the molten steel surface due to a vortex generation, may be generated. In accordance with the exemplary embodiment, the gas blowing flow rate may be adjusted in correspondence to the decrease in molten steel height in the ladle L during the casting, thereby restricting or preventing the generation of the naked molten steel caused by the gas blowing.

INDUSTRIAL APPLICABILITY

In accordance with the exemplary embodiment, the inert gas is blown when the ladle is disposed at the stand-by position on the turret device and when the casting of supplying the molten steel to the tundish is performed. Thus, the inclusions may be reduced in comparison with the related art, and the clean steel may be produced. That is, when the ladle is disposed at the stand-by position, as the micro-bubbling is performed after the ladle is opened, the generation of the inclusions during the stand-by may be reduced. Also, the inclusions in the molten steel in the ladle during the casting may be reduced by blowing the inert gas into the ladle during the casting.

Claims

A slab casting method comprising:
disposing a ladle in which molten steel is accommodated at each of an upper side and an outer side of a tundish;

performing casting by supplying the molten steel of the ladle disposed at a casting position of the upper side of the tundish; and

blowing an inert gas into the ladle disposed at the casting position.
The slab casting method of claim 1, wherein the blowing of the inert gas into the ladle disposed at the casting position comprises:
opening the ladle at the casting position by blowing the inert gas with a first flow rate into the ladle at the casting position; and

bubbling by blowing the inert gas with a smaller flow rate than the first flow rate when the casting of supplying the molten steel to the tundish is initiated after the ladle at the casting position is opened.
The slab casting method of claim 2, wherein the bubbling of the ladle at the casting position comprises reducing a gas blowing flow rate of the inert gas as a height of the molten steel in the ladle at the casting position decreases.
The slab casting method of claim 3, wherein the reducing of the gas blowing flow rate of the inert gas as the height of the molten steel in the ladle at the casting position decreases comprises supplying with a flow rate (m₁) calculated by a mathematical equation 1 using a ratio of a current molten steel height (L₁) with respect to an initial molten steel height (L₀) before the molten steel in the ladle at the casting position is supplied to the tundish and an initial gas blowing flow rate (m₀) when the molten steel in the ladle at the casting position is initially supplied to the tundish. $Inertgasblowing flowrate (m_{1}) = \frac{Currentmoltensteelheight (L_{1})}{Initial molten steelheight (L_{0})} \times Initial gasblowing flowrate (m_{0})$
The slab casting method of claim 4, wherein the initial gas blowing flow rate (m0) is equal to or greater than approximately 1 LPM and equal to or less than approximately 20 LPM.
The slab casting method of claim 1, further comprising blowing the inert gas into the ladle disposed at a stand-by position of the outer side of the tundish.
The slab casting method of claim 6, wherein the blowing of the inert gas into the ladle disposed at the stand-by position comprises:
opening the ladle at the stand-by position by blowing the inert gas with a first flow rate into the ladle at the stand-by position; and

bubbling by blowing the inert gas with a second flow rate, which is less than the first flow rate, when casting of supplying the molten steel to the tundish is initiated after the ladle at the stand-by position is opened.
The slab casting method of claim 7, wherein the first flow rate is equal to or greater than approximately 80 LPM and equal to or less than approximately 200 LPM, and
the second flow rate is equal to or greater than approximately 1 LPM and equal to or less than approximately 20 LPM.
A casting facility comprising:
a tundish configured to temporarily store molten steel;

a turret device comprising one pair of supporting parts configured to support one pair of ladles in which the molten steel is accommodated, respectively, and configured to alternately disposing the one pair of supporting parts at a casting position of an upper side of the tundish and a stand-by position of an outer side of the tundish;

a mold disposed below the tundish to coagulate the molten steel supplied from the tundish; and

a gas blowing device connected to each of the ladle at the stand-by position and the ladle at the casting position so that an inert gas is blown into each of the ladle supported at the stand-by position and the ladle supported at the casting position on the turret device.
The casting facility of claim 9, wherein the gas blowing device comprising:
a first blowing line connected to the ladle supported at the stand-by position;

a second blowing line connected to the ladle supported at the casting position;

a first supply part connected to the first blowing line to selectively supply the inert gas to the first blowing line with a first flow rate for opening the ladle at the stand-by position and a second flow rate less than the first flow rate; and

a second supply part connected to the second blowing line to selectively supply the inert gas to the second blowing line with a first flow rate for opening the ladle at the casting position and a flow rate less than the first flow rate.
The casting facility of claim 10, wherein the first supply part supplies the inert gas to the first blowing line with a first flow rate equal to or greater than approximately 80 LPM and equal to or less than approximately 200 LPM so that a blowing hole of the ladle at the stand-by position is opened, and supplies the inert gas to the first blowing line with a second flow rate equal to or greater than approximately 1 LPM and equal to or less than approximately 20 LPM after the ladle at the stand-by position is opened, thereby bubbling the ladle at the stand-by position.
The casting facility of claim 10, wherein the second supply part supplies the inert gas to the second blowing line with a first flow rate equal to or greater than approximately 80 LPM and equal to or less than approximately 200 LPM so that a blowing hole of the ladle at the casting position is opened, and reduces an inert gas blowing flow rate to the second blowing line according to decrease in height of the molten steel in the ladle at the casting position with a flow rate less than the first flow rate when the molten steel in the ladle at the casting position is initially supplied to the tundish after the ladle at the casting position is opened.