JP6784349B1 - Highly clean steel manufacturing method - Google Patents

Abstract

In the method of continuously casting steel, in a method of preventing the reoxidation of the molten steel by creating a non-oxidizing atmosphere in the tundish before injecting the molten steel into the tundish, the following equations (1) and (2) (in the equation, ρ: non-existent) Active gas density (kg / m3), Q: Total amount of inert gas blown (Nm3 / s), μ: Inert gas viscosity (Pa · s), PTD: Gas substitution region circumference (m), V : Gas blowing region volume (m3), T: Atmospheric temperature in the tundish (K), tmax: Representing the gas blowing time (s)) toward the bottom of the tundish, an inert gas heavier than air A method for producing highly clean steel, which is characterized by blowing in. 4 ・ ρ ・ Q / (μ ・ PTD) ≦ 2000 ・ ・ ・ (1) 3 (V / Q) / (T / 298) ≦ tmax ・ ・ ・ (2)

Description

The present invention relates to a method for producing highly clean steel by efficiently substituting the atmosphere inside the tundish with an inert gas when continuously casting steel.

When continuously casting steel, a long nozzle attached to the bottom of the ladle is used when injecting molten steel from the ladle into the tundish. Then, by immersing the tip in the molten steel in the tundish, the injected molten steel is shielded from the air and flux is added on the molten steel in the tundish to prevent oxidation. However, since the tip of the long nozzle is not immersed in the molten steel in the tundish at the start of injection of molten steel from the ladle into the tundish, the molten steel is exposed to air and undergoes reoxidation in which it is oxidized.

When oxides produced by reoxidation at the start of injection of molten steel from such a ladle into a tundish, that is, non-metal inclusions, are trapped in a continuously cast slab, they are surfaced by a steel plate produced from the slab. It causes defects such as flaws. Therefore, it has become an issue to reduce non-metal inclusions at the start of molten steel injection from the ladle to the tundish, and various anti-reoxidation measures have been conventionally proposed.

As a measure to prevent reoxidation at the start of molten steel injection from the ladle to the tundish, for example, Patent Document 1 states that the space between the tundish and the lid is completely reduced in order to reduce the oxygen concentration in the tundish before injection. A method of introducing Ar gas into the tundish while sealing is disclosed. Further, in Patent Document 2, the inner diameter of the nozzle for blowing the inert gas into the tundish is set to 40 mm or more, and the penetration depth of the nozzle into the tundish is secured to be 1/4 or more of the tundish depth. , A method of reducing the gas flow velocity at the time of blowing to prevent air entrainment is disclosed.

JP-A-63-188460 JP-A-9-168846

However, the method described in Patent Document 1 has a problem that in order to inject molten steel into the tundish, a space into which a nozzle for injecting molten steel can be inserted is required in the tundish lid. Further, considering the thermal deformation of the tundish lid and the tundish body, there is a problem that it is difficult to completely seal the tundish. When the inert gas is blown into the tundish by the method described in Patent Document 1 in a situation where the tundish cannot be sealed, air is entrained through the injection point and the gap of the tundish lid, and the substitution of the inert gas is insufficient. The problem arises. Further, in the method described in Patent Document 2, since the flow velocity changes as the flow rate of the blown gas changes, there is a problem that the replacement with the inert gas is not sufficiently performed depending on the capacity of the tundish and the time when the gas can be blown. is there.

The present invention has been made in view of such circumstances, and reoxidation of the molten steel in the tundish by air, which is the main cause of reducing the cleanliness of the molten steel at the start of injection of the molten steel into the tundish, is performed. In order to prevent this, it is an object of the present invention to propose a method for producing highly clean steel by rapidly and efficiently reducing the oxygen concentration in the atmosphere inside the tundish regardless of conditions such as the capacity of the tundish.

The inventors have found that the atmosphere in the tundish can be efficiently replaced by controlling the flow velocity and the depth of the inert gas to be blown, and the gas flow in the tundish, and have developed the present invention. The method for producing high-clean steel of the present invention, which advantageously solves the above problems, is a method for preventing reoxidation of molten steel by creating a non-oxidizing atmosphere in the tundish before injecting molten steel into the tundish when continuously casting steel. The present invention is characterized in that an inert gas heavier than air is blown toward the bottom of the tundish under the conditions satisfying the following equations (1) and (2).
4 ・ ρ ・ Q / (μ ・ P _TD ) ≦ 2000 ・ ・ ・ (1)
3 (V / Q) / (T / 298) ≤ t _max ... (2)
Here, ρ: Inert gas density (kg / m ³ ),
Q: Total amount of inert gas blown (Nm ³ / s),
μ: Inert gas viscosity (Pa · s),
_PTD : _{Perimeter of} gas replacement region (m),
V: Gas blowing area volume (m ³ ),
T: Atmospheric temperature inside the tundish (K),
t _max : Gas blowing time (s),
Represents.

The method for producing high-clean steel according to the present invention is as follows.
When the inert gas is blown into the tundish from one or more nozzles, the inert gas is blown into each of the nozzles under the condition satisfying the following equation (3).
Can be a more preferred solution.
5 ≦ Q _n・ T / {74.5π (2H _n tan (12 °) + d _n ) ² } ≦ 20 ・ ・ ・ (3)
Q ₁ + Q ₂ + ... + Q _n = Q ... (4)
Here, Q _n : the amount of gas blown from the nth nozzle (Nm ³ / s),
H _n : Height (m) from the bottom of the tundish of the nth nozzle to the bottom of the gas blowing nozzle,
d _n : Gas blowing nozzle inner diameter (m) of the nth nozzle,
n: Represents an integer greater than or equal to 1.

Further, the method for producing high-clean steel according to the present invention is:
The tundish has a weir for controlling the flow of molten steel, and each of the tundish regions divided by the weir is set as a separate gas blowing region, and one or more gas blowing nozzles are provided in each region. The inert gas is blown into each region under the conditions satisfying the above equations (1) and (2) or the above equations (1) to (3).
Can be a more preferred solution. Here, the boundary of the "divided tundish area" is the position of the upper end of the weir.

According to the present invention, when continuously casting steel, the atmosphere inside the tundish is quickly and efficiently replaced with an inert gas before the molten steel is injected from the ladle into the tundish. The oxygen concentration of the injected molten steel is reduced, and the reoxidation of the injected molten steel by air is suppressed. Therefore, it is possible to suppress the amount of non-metal inclusions produced, and it is possible to produce highly clean steel.

It is a schematic diagram of the tundish used in one embodiment of the present invention, and shows (a) a cross-sectional view and (b) a perspective view. It is a schematic diagram of the tundish used in another embodiment of the present invention, and shows (a) a sectional view and (b) a perspective view. It is a schematic diagram which shows the cross section of the tundish used in Example 1. FIG. It is a schematic diagram which shows the cross section of the tundish used in Example 2. It is a schematic diagram which shows the cross section of the tundish used in Example 3.

In solving the problem, the inventors thought as follows.
When continuously casting steel, it is effective to prevent reoxidation of the molten steel by creating a non-oxidizing atmosphere in the tundish before injecting the molten steel into the tundish. Here, the non-oxidizing atmosphere preferably has an oxygen concentration of 2.0 vol% or less, and more preferably 1.0 vol% or less. In order to create a non-oxidizing atmosphere in the tundish, it is effective to replace the air in the tundish with an inert gas. Therefore, in order to efficiently replace the air in the tundish with the inert gas, it is necessary to control the flow velocity and depth of the Blowed inert gas and the gas flow in the tundish. I thought. First, the inert gas was made heavier than air, and the direction in which the gas was blown was set toward the bottom of the tundish. This is because the bottom of the tundish is gradually replaced with an inert gas. In addition, considering that if the gas flow in the tundish becomes turbulent, the mixing of the atmosphere inside the tundish and the outside air will be promoted, so we decided to select the conditions that do not cause turbulence. Examples of the inert gas heavier than air include Ar gas, carbon dioxide gas, a mixed gas thereof, and a gas in which nitrogen is partially mixed.

Here, in order to evaluate the flow of gas in the tundish, it was decided to use the Re number (Reynolds number). The Re number is expressed by the following equation (5).
Re = 4 ・ ρ ・ Q / (μ ・ P) ・ ・ ・ (5)
Here, ρ: Inert gas density (kg / m ³ ),
Q: Amount of inert gas blown (Nm ³ / s),
μ: Inert gas viscosity (Pa · s),
P: Representative length (m),
Represents.

Generally, it is said that the flow changes from laminar flow to turbulent flow when the Re number is 2000 to 4000. When the oxygen concentration in the tundish was measured by changing the amount of the inert gas blown in, the inventors measured the representative length P as the peripheral length _PTD (m) of the gas replacement region, that is, the upper surface of the tundish. It has been found that when the number of Res exceeds 2000 when the circumference is set, the oxygen concentration is less likely to decrease as compared with the case where the number of Res exceeds 2000 or less. As a result, the following equation (1) was obtained.
4 ・ ρ ・ Q / (μ ・ P _TD ) ≦ 2000 ・ ・ ・ (1)

Next, since the time during which the inside of the tundish can be replaced with the inert gas before the start of casting is limited in terms of operation, it is considered necessary to inject the amount of gas for which the replacement is completed within that time. The fully mixed model requires three times the volume of gas. Further, since the tundish is preheated, the gas blown into the tundish expands, so the gas flow rate Q (Nm ³ / s) required for complete replacement is expressed by the following equation (2).
3 (V / Q) / (T / 298) ≤ t _max ... (2)
Here, V: the volume of the gas blowing region (m ³ ),
T: Atmospheric temperature (K) in the tundish,
t _max : Gas blowing time (s),
Represents.

Furthermore, regarding the flow velocity of the gas to be blown, the influence of the flow velocity when the gas collides with the bottom of the tundish was examined. If the flow velocity of the gas when it collides with the bottom is too large, it collides with the bottom and becomes a reverse flow, and the flow flows out to the upper part of the tundish. Therefore, there is a possibility that the entire tundish cannot be replaced efficiently due to the formation of a stagnant portion or the suction of air from the gap between the tundish and the lid. On the other hand, if the flow velocity of the gas is too small, the inert gas may not spread to every corner of the bottom of the tundish and may not be completely replaced. Therefore, it was considered that there is an appropriate value for the flow velocity when colliding with the bottom. However, since it is difficult to measure the gas flow velocity when colliding with the bottom, it was decided to calculate the gas flow velocity reaching the bottom of the tundish from the gas flow rate, the inner diameter of the gas blowing nozzle, and the distance from the bottom of the tundish.

The area A (m ² ) of the region where the gas collides with the bottom of the tundish is expressed by the following equation (6).
A = (π / 4) × (2Htan (θ) + d) ² ... (6)
Here, H: height (m) from the bottom of the tundish to the lower end of the gas blowing nozzle,
θ: Spread angle (°) of blown gas,
d: Gas blowing nozzle inner diameter (m),
Represents.

Here, the spread angle θ of the blown gas was set to 12 °, which is generally said. Further, the average gas flow velocity v (m / s) when a gas having a flow rate q (Nm ³ / s) collides with the region A of the atmospheric temperature T (K) is determined by the following (6) in consideration of the above equation (6). It is expressed by equation 7).
v = q · (T / 298) / A
= Q · T / {74.5π (2Htan (12 °) + d) ² } ・ ・ ・ (7)

Here, when the gas replacement in the container was measured under various conditions, it was found that the gas replacement can be efficiently performed if the average gas flow velocity v is in the range of 5 to 20 m / s. Therefore, the nozzle diameter and height conditions for gas blowing can be expressed by the following equation (8). Here, if the average gas flow velocity v is less than 5 m / s, the gas flow rate may be too small and it may take too much time to replace the gas in the tundish. On the other hand, if it exceeds 20 m / s, turbulence may occur.
5 ≦ q ・ T / {74.5π (2Htan (12 °) + d) ² } ≦ 20 ・ ・ ・ (8)

When the gas blowing position is 1 or 2 or more, it is necessary to satisfy the above equation (8) at each position, and the total gas blowing flow rate from the gas blowing position is the total gas blowing flow rate Q (Nm ³ / s). ), Therefore, the equation (8) can be expressed by the following equations (3) and (4).
5 ≦ Q _n × T / {74.5π (2H _n tan (12 °) + d _n ) ² } ≦ 20 ・ ・ ・ (3)
Q ₁ + Q ₂ + ... + Q _n = Q ... (4)
Here, Q _n : the amount of gas blown from the nth nozzle (Nm ³ / s),
H _n : Height (m) from the bottom of the tundish of the nth nozzle to the bottom of the gas blowing nozzle,
d _n : Gas blowing nozzle inner diameter (m) of the nth nozzle,
n: Represents an integer greater than or equal to 1.

Therefore, in order to prevent the molten steel from being reoxidized by creating a non-oxidizing atmosphere in the tundish, the amount Q of the inert gas that is heavier than the air blown into the tundish is replaced with gas so as to satisfy the above equation (1). Highly clean steel can be manufactured by adjusting the circumference of the region to be _PTD according to the PTD. Further, it is necessary to adjust the gas blowing amount Q according to the gas blowing time t _max so as to satisfy the above equation (2). When gas is blown using one or more nozzles, the nozzle height H _n , the nozzle inner diameter d _n , and the gas blowing are so as to satisfy the above equation (3). It is more preferable to adjust the amount Q _n . Furthermore, when the tundish has a weir for adjusting the flow, gas is blown into each of the tundish regions divided by the position of the upper end of the weir as a boundary, so that the gas replacement of the atmosphere inside the tundish is more even. It is preferable because it is performed in. At that time, it is preferable to satisfy the above equations (1) and (2), or the above equations (1) to (3) for each tundish region.

Hereinafter, the present invention will be described with reference to the accompanying drawings. 1 and 2 are schematic views of an apparatus used in a method for carrying out the present invention (hereinafter referred to as the present method), which are (a) schematic cross-sectional view and (b) perspective view, respectively.

In FIG. 1, reference numeral 1 denotes a tundish body, in which a nozzle 2 for supplying molten steel to two continuous casting molds (not shown) is arranged, and a dipping nozzle 4 is attached to a lower portion thereof via a sliding nozzle 3. There is. Further, the tundish main body 1 is covered with a lid 5, and an opening 6 for a long nozzle is installed in the central portion of the lid 5. Further, on FIG. 1, burner openings 7 for preheating the tundish body 1 are provided on both sides of the lid 5 with the opening 6 interposed therebetween. Note that FIG. 1 shows a state after the burner (not shown) is inserted through the burner opening 7 to preheat the tundish body 1. The broken line in FIG. 1B shows the inner method of the upper surface of the tundish, where L represents the length and W represents the width. Circumferential length _{P TD} of the above-described gas replacement region can be calculated by 2L + 2W.

Further, a gas blowing nozzle 8 is inserted into the tundish main body 1 through the openings 6 and 7 of the tundish lid 5. The number, inner diameter, and installation height of the gas blowing nozzles 8 are not particularly limited as long as the above equations (1) and (2), preferably equation (3) are satisfied, and a plurality of nozzles 8 are used. When blowing, it is not necessary that the conditions for blowing are the same for each nozzle.

Further, as shown in FIG. 2, when the weir 9 is installed in the tundish main body 1, it is preferable to install the gas blowing nozzle 8 in each of the regions divided by the weir 9. The height of the weir 9, the shape of the openings, the shape such as the number, and the installation position are not particularly limited. Broken line in FIG. 2 (b) shows the clear width of the tundish top, two-dot chain line in FIG. 2 (a) shows the boundaries of the divided regions the position of the upper end of the weir as a boundary, L ₁ , L ₂ represent the length of each of the region 1 and the region 2, and W represents the width thereof. The circumference P _TD1 of the gas replacement region 1 described above can be calculated by 2L ₁ + 2W, and the circumference P _TD2 of the gas replacement region 2 can be calculated by 2L ₂ + 2W. In this case, each region needs to satisfy the above equations (1) and (2), preferably equation (3).

Next, the operation of this method will be described.
The gas blowing nozzle 8 is inserted into the tundish body 1 after the preheating is completed through the opening of the lid 5. At that time, the sliding nozzle 3 is closed. Further, it does not matter whether or not the immersion nozzle 4 is attached.

When the gas is blown, it may be blown at a constant flow rate from the start to the end of the blow, and if the above equations (1) and (2), preferably the equation (3) are satisfied, for example, from the start of replacement of the atmosphere. The flow rate may be changed stepwise or continuously until the end.

When the predetermined gas flow rate has been blown, the gas blowing nozzle 8 is discharged, and if the immersion nozzle 4 is not installed, it is installed, and it is longer in the tundish body 1 than the ladle containing the smelted molten steel. The molten steel is injected through the nozzle. At the same time, the sliding nozzle 3 is opened and continuous casting is started.

By controlling the inside of the tundish to a non-oxidizing atmosphere by using the above method, it is possible to suppress the formation of inclusions due to reoxidation and obtain a highly clean steel.

<Example 1>
As shown in FIG. 3, capacity 30t of 1 strand continuous casting machine tundish 1 (inner volume V = 5.8 m ^3, perimeter _P TD = 10.5 m, the ambient temperature T = 873 K) the lid 5 and Above, Ar gas was blown from the gas blowing nozzle 8 installed in the opening 6 for the long nozzle of the tundish lid 5. Ar gas is an inert gas having a higher density than air. Table 1 shows the number of nozzles 8, the inner diameter, the installation height, the Ar gas blowing amount, and the blowing time. Under the conditions satisfying the equations (1) and (2) (treatments Nos. 1 to 3 and 6 in Table 1), the oxygen concentration in the tundish 1 can be set to 2.0 vol% or less, and (1) to (3). Under the conditions satisfying the formula (treatments Nos. 1 to 3 in Table 1), the oxygen concentration in the tundish 1 could be 1.0 vol% or less. On the other hand, under the condition that neither of the equations (1) and (2) was satisfied (treatments Nos. 4, 5 and 7 in Table 1), the oxygen concentration in Tundish 1 exceeded 2.0 vol%. Under the above conditions, low carbon steel with C = 0.03% was cast. The number of oxides in steel from the casting start position to the slab 2 m position is measured, the target number of oxides is set to 1.0, and the ratio of the number of oxides in the slab to that is the index of bottom slab cleanliness. It was shown to. As a result, in the example of the present invention, the number of oxides could be lower than that of the comparative example. As described above, it was found that the gas replacement in the tundish can be effectively performed by this method.

<Example 2>
As shown in FIG. 4 installed capacity tundish for 2 strand continuous casting machine 70 t 1 (inner volume V = 12.3 m ^3, perimeter _P TD = 19.3 m, the ambient temperature T = 923 K) the weir 9 Then, it was divided into an injection side and a non-injection side area from the ladle. The non-injection side region including the mold molten steel supply nozzle 2 was designated as a tundish region 1 (Zone 1), and the region on the injection side from the ladle was designated as a tundish region 2 (Zone 2). The left and right regions 1 on FIG. 4 are plane-symmetrical and have the same volume and the same peripheral length. Tables 2-1 and 2 show the volumes V ₁ , V ₂ , and the peripheral lengths P _{TD 1} and P _{TD 2} of each region. After the lid 5 was put on the tundish lid 5, Ar gas was blown from the gas blowing nozzle 8 installed in the opening 6 for the long nozzle and the opening 7 for the burner of the tundish lid 5. Tables 2-1 and 2 show the number of nozzles 8, the inner diameter, the installation height, the amount of Ar gas blown, and the blowing time. Table 2-3 shows the evaluation results of the atmospheric oxygen concentration in the tundish after gas replacement and the cleanliness of the bottom slab. Under the conditions satisfying the equations (1) to (3) in all the regions (treatments No. 8 and 9 in Tables 2-3), the oxygen concentration in Tundish 1 could be 1.0 vol% or less. In addition, the oxygen concentration in the tundish 1 can be set to 2.0 vol% or less under the conditions (Tables 2-3 to No. 8 to 11 in the middle treatments) that satisfy the equations (1) and (2) in all regions. It was. On the other hand, when there is a region under the condition that neither of the equations (1) and (2) is satisfied (Tables 2-3 to Treatment Nos. 12 to 14), the oxygen concentration in the tundish 1 in that region Was over 2.0 vol%. Under the above conditions, ultra-low carbon steel with C = 0.002% was cast. The number of oxides in the steel at the position 2 m from the casting start position is measured, the target number of oxides is 1.0, and the ratio of the number of oxides in the slab to that is the index of bottom slab cleanliness. Shown in 3. As a result, in the example of the present invention, the number of oxides could be lower than that of the comparative example. It was found that the gas replacement in the tundish was effectively performed by this method.

<Example 3>
As shown in FIG. 5, a weir 9 is installed in a tundish 1 for a 4-strand continuous casting machine having a capacity of 20 tons (internal volume 4.4 m ³ , peripheral length 16.6 m, atmospheric temperature T = 873 K), and from a ladle. It was divided into the injection side and non-injection side regions. The non-injection side region including the mold molten steel supply nozzle 2 was designated as a tundish region 1 (Zone 1), and the region on the injection side from the ladle was designated as a tundish region 2 (Zone 2). The left and right regions 1 on FIG. 5 are plane-symmetrical and have the same volume and the same peripheral length. The volumes V ₁ , V ₂ , peripheral lengths P _{TD 1} , and P _{TD 2} of each region are shown in Tables 3-1 and 2. Table 3-3 shows the evaluation results of the atmospheric oxygen concentration in the tundish after gas replacement and the cleanliness of the bottom slab. After the lid 5 was put on the tundish lid 5, Ar gas was blown from the gas blowing nozzle 8 installed in the opening 6 for the long nozzle and the opening 7 for the burner of the tundish lid 5. Tables 3-1 and 2 show the number of nozzles 8, the inner diameter, the installation height, the amount of Ar gas blown, and the blowing time. Under the conditions that satisfy the equations (1) and (2) in all regions (Tables 3-1 to 3 in Treatment Nos. 15 to 18), the oxygen concentration in Tundish 1 can be reduced to 2.0 vol% or less in all regions. did it. Further, under the conditions (Tables 3-1 to 3 in Treatment Nos. 15 and 16) that satisfy the equations (1) to (3) in all regions, the oxygen concentration in Tundish 1 is set to 1.0% or less in all regions. I was able to. On the other hand, when there is a region under the condition that does not satisfy the formula (1) (Tables 3-1 to 3 in Treatment Nos. 19 and 20), the oxygen concentration in Tundish 1 in that region exceeds 2.0 vol%. It was. Under the above conditions, high carbon steel with C = 1.0% was cast. The number of oxides in steel at the position 2 m from the casting start position is measured, the target number of oxides is set to 1.0, and the ratio of the number of oxides in bloom to that is shown in Table 3 as an index of bottom slab cleanliness. Indicated. As a result, in the example of the present invention, the number of oxides could be lower than that of the comparative example. It was found that the gas replacement in the tundish was effectively performed by this method.

The present invention is not limited to the above-exemplified examples, and when continuously casting steel, the atmosphere can be quickly and efficiently replaced with an inert gas before the start of injecting molten steel into the tundish. It is suitable for application to the production of. This method can be applied not only to tundish but also to devices or methods that require gas replacement of the atmosphere.

1 Tandish 2 Nozzle for supplying molten steel 3 Sliding nozzle 4 Immersion nozzle 5 Lid 6 Opening for long nozzle 7 Opening for burner 8 Gas blowing nozzle 9 Weir Zone1 Tandish area 1
Zone2 Tandish area 2

Claims (4)

In the method of continuously casting steel, the inside of the tundish is made to have an non-oxidizing atmosphere before the molten steel is injected into the tundish to prevent the reoxidation of the molten steel. A method for producing highly clean steel, which comprises blowing an inert gas heavier than air toward the steel.
4 ・ ρ ・ Q / (μ ・ P _TD ) ≦ 2000 ・ ・ ・ (1)
3 (V / Q) / (T / 298) ≤ t _max ... (2)
Here, ρ: density of the inert gas (kg / m ³ ),
Q: Total amount of inert gas blown (Nm ³ / s),
μ: Inert gas viscosity (Pa · s),
_PTD : _{Perimeter of} gas replacement region (m),
V: Gas blowing area volume (m ³ ),
T: Atmospheric temperature inside the tundish (K),
t _max : Gas blowing time (s),
Represents. The first aspect of claim 1, wherein when the inert gas is blown into the tundish from one or more nozzles, the inert gas is blown into each of the nozzles under the condition satisfying the following equation (3). Manufacturing method of highly clean steel.
5 ≦ Q _n・ T / {74.5π (2H _n tan (12 °) + d _n ) ² } ≦ 20 ・ ・ ・ (3)
Q ₁ + Q ₂ + ... + Q _n = Q ... (4)
Here, Q _n : the amount of gas blown from the nth nozzle (Nm ³ / s),
H _n : Height (m) from the bottom of the tundish of the nth nozzle to the bottom of the gas blowing nozzle,
d _n : Gas blowing nozzle inner diameter (m) of the nth nozzle,
n: Represents an integer greater than or equal to 1. The tundish has a weir for controlling the flow of molten steel, and each of the tundish regions divided by the weir is used as a separate gas blowing region, and one or more gas blowing nozzles are provided in each region. The method for producing highly clean steel according to claim 1, wherein an inert gas is blown into each region under the conditions satisfying the above equations (1) and (2). The tundish has a weir for controlling the flow of molten steel, and each of the tundish regions divided by the weir is used as a separate gas blowing region, and one or more gas blowing nozzles are provided in each region. The method for producing high-clean steel according to claim 2, wherein the inert gas is blown into each region under the conditions satisfying the above equations (1) to (3).

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