JP7035870B2 - Melting method of high-clean steel - Google Patents

Description

The present invention relates to a method for melting high-clean steel, and more particularly to a method for melting high-clean steel by Al deoxidation.

In a refining container such as a converter, the molten steel after the completion of primary refining produced by blow acid decarburization under atmospheric pressure has a high dissolved oxygen concentration in the steel, so the components are adjusted by deoxidizing and adding alloys. After being cast, it has the characteristics of a product.
In deoxidation, elements that combine with oxygen to form oxides are generally added. In addition to Al (aluminum), Si (silicon), C (carbon), Ti (titanium), and Ca (calcium) ), Zr (zulyl), REM (rare earth metal) and the like are known to be used as deoxidizing materials.
Of these, Al used as a deoxidizing material is inexpensive and has a strong deoxidizing effect, and steel materials manufactured using this have a track record of use in applications such as beverage cans and automobile parts materials. , Highly versatile.

However, the alumina (Al ₂ O ₃ ) produced after the deoxidation reaction with Al remains as inclusions in the solidified steel material (shards obtained by continuous casting), and the product is said to have a coarse particle size. It may cause a significant loss of quality. For example, since it causes cracking during can manufacturing when used as a material for beverage cans, it is necessary to eliminate the adverse effects of alumina inclusions in order to improve the quality.
Further, when a large amount of alumina is present in the molten steel, adhesion and aggregation of alumina on the inner surface of the dipping nozzle is promoted during casting, resulting in uneven flow generation in the mold and blockage of the dipping nozzle. , The amount of fluctuation in the molten metal surface becomes large, which causes quality deterioration due to the mixing of mold powder (powder-based inclusions).

Therefore, the following method has been proposed.
For example, in Patent Document 1, following the decarburization treatment in the RH vacuum degassing device (circulation type degassing device), the pressure inside the vacuum chamber is constant or further reduced to add Al and the inside of the ladle. It is described that the immersion depth of the immersion tube in the molten steel is made shallow. In general, the operation of shallowing the immersion depth of the immersion tube is not performed during the treatment, but in Patent Document 1, the molten steel depth in the vacuum chamber can be shallowed by this operation, and the recirculation treatment is performed in this state. It has been described that this promotes agglomeration and levitation of non-metal inclusions. Specifically, it is described as an efficient inclusion removal condition that the depth of the molten steel in the vacuum chamber during the recirculation treatment is in the range of 50 mm or more and less than 100 mm by the operation of making the immersion depth of the immersion tube shallow. ing.

In Patent Document 2, the degree of vacuum in the vacuum chamber of the RH vacuum degassing device is reduced to 10 Torr or less for refining, and then the degree of vacuum in the vacuum chamber is maintained above 10 Torr, and nitrogen gas is contained in the vacuum chamber. A method for melting high nitrogen steel to be smelted by blowing in is disclosed. Specifically, the treatment under reduced pressure is described. In the first half of the treatment, the inclusions are floated and separated by treating at 10 Torr or less (low pressure vacuum) for 2 minutes or more, and in the latter half of the continuous treatment, more than 10 Torr (high pressure vacuum). ), It is described that nitrogen gas is blown onto the molten steel in the vacuum chamber to perform the nitrogen treatment for 19 to 20 minutes. Regarding the latter half of the processing, the paragraph [0026] states, "... Moreover, since the degree of vacuum in the vacuum chamber is low (atmospheric pressure is high), the ring flow rate of the molten steel 3 is lowered. The effect of floating and separating non-metal inclusions is impaired. "It is also stated that the cleaning effect is impaired because the ring flow rate of the molten steel decreases under high-pressure vacuum conditions.

Japanese Unexamined Patent Publication No. 2016-40400 JP-A-2015-427777

However, although the method described in Patent Document 1 described above can obtain a corresponding cleaning effect, further improvement in the cleanliness of the molten steel is desired.
Further, the method described in Patent Document 2 is the same as the method described in Patent Document 1, and since the treatment is performed under a low pressure vacuum of, for example, 10 Torr or less, a corresponding cleaning effect can be obtained. However, since the conditions of the nitrogen treatment are optimized for nitrogen, the present inventors have found that there are the following problems from the viewpoint of floating and removing inclusions.
Since the above-mentioned nitriding treatment of about 20 minutes has a long treatment time, the frequency of redissolution of the bare metal adhering to the inside of the tank and the loss of refractory during the low-pressure vacuum treatment increases. Inclusions of various particle sizes are also contained in this bullion, and the defect of the refractory itself becomes an inclusion of a foreign system.

The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a method for melting high-clean steel, which can melt and cast high-clean steel having less alumina inclusions than the conventional technique. The purpose.

The present inventors have conducted various experiments to determine the recirculation height of molten steel determined by the distance between the molten steel surface in the vacuum chamber and the bottom surface of the ladle for storing the molten steel in the recirculation treatment in the RH vacuum degassing device. By lowering the value, the effect of promoting the aggregation of inclusions and the effect of improving the strength of inclusions can be obtained, and by continuously casting this molten steel using a tundish under predetermined conditions, the inclusions in the molten steel are destroyed. It was found that the surface can be removed without levitation.
The present invention has been made based on the above findings, and the gist thereof is as follows.

The method for melting high-clean steel according to the present invention according to the above object is to add metallic aluminum to molten steel that has undergone primary refining by blowing acid decarburization under atmospheric pressure to reduce the concentration of dissolved oxygen in the molten steel to 40 ppm or less. Immerse the dipping tube of the RH vacuum degassing device in the molten steel in the pan, blow inert gas from the rising tube of the dipping tube, and between the vacuum tank of the RH vacuum degassing device and the pan. When performing vacuum degassing treatment to recirculate molten steel in
In the first half of the vacuum degassing treatment, the inside of the vacuum chamber is made into a low-pressure vacuum atmosphere of 1.3 kPa or less, and then the degassing treatment is performed for 15 to 45 minutes.
In the latter half of the vacuum degassing treatment, the inside of the vacuum chamber is made into a high-pressure vacuum atmosphere of 20 to 40 kPa, and then the degassing treatment is performed for 5 to 15 minutes.
A weir is provided inside to separate the hot water receiving part that receives the molten steel and the hot water discharging part that injects the molten steel into the mold for continuous casting, and one or more molten steel flow paths that communicate the hot water receiving part and the hot water discharging part are provided. The height position of the opening formed in the weir and located on the hot water receiving portion side of the molten steel flow path from the bottom surface of the hot water receiving portion is 0.2 times or less the depth of the molten steel of the hot water receiving portion. The molten steel that has been vacuum degassed is poured into the tundish, and the molten steel flowing through the molten steel flow path is induced and heated.

As described above, the first feature of the present invention is that the degassing treatment (decarburization treatment) is performed in a low pressure vacuum atmosphere in the first half of the vacuum degassing treatment, and as the subsequent treatment, the high pressure is applied in the latter half of the vacuum degassing treatment. It is to perform degassing treatment in a vacuum atmosphere. In the latter half of this vacuum degassing treatment, the vacuum chamber is changed to a high-pressure vacuum atmosphere (pressure is increased), the ring flow rate of the molten steel is reduced, and the molten steel is recirculated in a narrow range. At this time, it is not necessary (may be constant) to change the immersion depth of the immersion tube in the molten steel in the ladle.
On the other hand, the method described in Patent Document 1 described above performs a special operation of shallowing the immersion depth of the immersion tube in the molten steel in the ladle without changing the pressure in the vacuum chamber (while maintaining the low pressure vacuum atmosphere). It is described, and it is not described that a high pressure vacuum atmosphere is used as in the present invention.

Further, Patent Document 2 describes a decarburization treatment under a low pressure vacuum (corresponding to a degassing treatment in a low pressure vacuum atmosphere of the present invention), and then describes the treatment under a high pressure vacuum. This treatment is a description of nitriding, suggesting that it is not effective for high cleaning.
Further, as described above, Patent Document 2 describes the nitrogenation treatment time as 19 to 20 minutes, exemplifying the conditions different from the degassing treatment in the high-pressure vacuum atmosphere of the present invention, and blows the inert gas. It is substantially described that the degassing treatment in a high pressure vacuum atmosphere, which reduces the filling amount as compared with the degassing treatment in a low pressure vacuum atmosphere, impairs the cleaning effect.

The second feature of the present invention is a condition for floating and removing inclusions (incidents whose strength has been improved by agglomeration and coalescence) after the vacuum degassing treatment described above, that is, a molten steel flow path. It is to use a tundish provided with a weir having a.

In the method for melting highly clean steel according to the present invention, in the first half of the vacuum degassing treatment, the inside of the vacuum chamber is set to a low pressure vacuum atmosphere of 1.3 kPa or less, and then the degassing treatment is performed for 15 to 45 minutes to create a vacuum. In the latter half of the degassing treatment, the vacuum tank is evacuated to a high pressure vacuum atmosphere of 20 to 40 kPa (after increasing the pressure) and then degassed for 5 to 15 minutes to perform the vacuum degassing treatment. The circulation amount of the molten steel in the latter half is reduced as compared with the first half, and the molten steel is recirculated in a narrow range. As a result, the effect of promoting the aggregation of inclusions and the effect of improving the strength of inclusions can be obtained.
A tundish is provided inside with a weir that separates the molten steel into a hot water receiving part and a hot water draining part, and a molten steel flow path that connects the hot water receiving part and the hot water discharging part is formed at a predetermined height position of this weir. Since it is continuously cast while inducing and heating in the molten steel flow path, the inclusions in the molten steel whose aggregation is promoted and the strength is improved by the above-mentioned vacuum degassing treatment can be suppressed and removed by floating. ..
Therefore, it is possible to produce highly clean steel with less alumina inclusions than the conventional technology, and the number of alumina inclusions with a particle size (major axis) of 20 μm class, which was difficult with the conventional technology, is reduced and the total oxygen content is reduced. It is possible to stably cast extremely highly clean steel having (T. [O] value) of, for example, 10 ppm or less.

It is explanatory drawing of the melting method of the high-clean steel which concerns on one Embodiment of this invention. It is explanatory drawing of the tundish to which the melting method of the high clean steel is applied. It is explanatory drawing of the tundish to which the melting method of the high-clean steel which concerns on a comparative example is applied.

Subsequently, an embodiment embodying the present invention will be described with reference to the attached drawings, and the present invention will be understood.
First, the process of coming up with the method for melting high-clean steel of the present invention will be described.

(New findings of the present inventors)
Although the conventional techniques such as Patent Documents 1 and 2 described above have a certain degree of cleaning effect, they are all treated by a single vacuum degassing step (RH vacuum degassing device). Therefore, for example, steel products. In response to the production of steel materials that require extremely strict cleanliness (for example, total oxygen content (T. [O] value) ≤ 10 ppm) while reducing the number of inclusions with a particle size of 20 μm class remaining inside. It was difficult (there was no mention of further cleaning).

In Patent Document 1 described above, after the addition of Al following the decarburization treatment, the depth of the molten steel in the vacuum chamber is in the range of 50 mm or more and less than 100 mm by a special operation of shallowing the immersion depth of the immersion pipe in the molten steel in the ladle. It is stated that.
In this method, the amount of molten steel in the vacuum chamber is small, the stirring force per unit volume of the molten steel in the vacuum chamber is large, and the time required for the molten steel to float is shortened. It is said that aggregation and levitation are promoted.
However, if a small volume of molten steel in the vacuum chamber is stirred too violently, some of the agglutinated particles will collapse due to shearing immediately after the coalescence, and the particles re-fine due to the collapse will enter the ladle. Will be discharged.

Further, in this technique, in order to adjust the molten steel depth in the vacuum chamber by a special operation of shallowing the immersion depth of the immersion pipe after decarburization, the molten steel in the vacuum chamber is removed from the bottom of the ladle for storing the molten steel. The distance to the molten metal surface (molten steel head) has not changed since the decarburization process, and since the pressure inside the vacuum chamber is constant (the low-pressure vacuum remains), the suction force of the molten steel is also constant, and stirring in the ladle. (Recirculation velocity) is also almost the same.
For this reason, regarding the molten steel that circulates in the vacuum chamber and the ladle, the height difference during circulation is constant, and the amount of molten steel that circulates per unit time is also constant. In addition to the coalescence, it was thought that high purification would be difficult to proceed due to the collapse of inclusions due to the shearing force of the circulating recirculation.

Therefore, in the present invention, when the vacuum degassing treatment is performed by the RH vacuum degassing device, the treatment in the high pressure vacuum atmosphere is provided after the agglomeration and coalescence of inclusions and the floating removal during the decarburization treatment in the low pressure vacuum atmosphere. The conditions for weakening the stirring of molten steel (high pressure vacuuming) were specified.
That is, the above-mentioned treatment in the high-pressure vacuum atmosphere lowers the molten steel level in the vacuum chamber, shortens the distance from the bottom surface of the pan to the molten steel surface in the vacuum chamber, and shortens the distance of the molten steel during recirculation. By shortening the distance in the circulation height direction and reducing the potential energy, the stirring energy is weakened, and by weakening the stirring (high pressure vacuuming), the stirring energy is also weakened, thereby preventing the disintegration of aggregated inclusions. , Slowly promotes the aggregation and coalescence of inclusions. In addition, the strength of the aggregated inclusions is improved by treating for a certain period of time (5 to 15 minutes).

(Conventional knowledge on removal of inclusions by RH vacuum degassing treatment)
The RH vacuum degassing device (hereinafter, also simply referred to as a degassing device) 10 used in the RH method shown in FIG. 1 is conventionally known, and is a vacuum chamber 11 and two immersions communicating with the lower portion of the vacuum chamber 11. It has a pipe, that is, a dipping pipe 12 and 13 on the ascending side and the descending side of the molten steel. In use, the molten steel in the ladle 14 is sucked into the vacuum tank 11 through the two dipping pipes 12 and 13, and the inert gas is blown from the ascending side dipping pipe (rising pipe) 12 (usually 5, 5). ~ 15 NL / min / ton. Gas injection amount per minute for 1 ton of molten steel) is performed, and due to the gas lift effect, between the ladle 14 and the vacuum tank 11 through the immersion pipes 12 and 13 on the ascending side and the descending side. Circulate with.

It is known that cleaning (removal of inclusions) in the vacuum degassing treatment is determined by the balance between the agglutination of inclusions sucked up in the vacuum chamber 11 and the discharge of the agglutination out of the tank (floating in the ladle). There is.
Regarding the agglutination and coalescence of the inclusions, "the inclusion particles collide with the refractory wall to promote the aggregation of the inclusions on the wall surface" and "the inclusions in the turbulent component in the molten steel flow". Phenomena such as "promotion of agglutination and coalescence by collision between particles" have been advocated.
In general, it is known that in vacuum degassing treatment, an increase in the flow rate of the molten steel ring promotes agglutination and floating removal of inclusions up to a certain level, and the effect is low pressure vacuum treatment. Is remarkable.
The present invention is characterized in that, by performing a high-pressure vacuum treatment in the latter half of the degassing treatment in addition to the above-mentioned treatment, it is possible to prevent the inclusions from collapsing and improve the strength while promoting gradual aggregation and coalescence. At this time, the floating removal of some of the inclusions proceeds, but in the continuous casting step following the refining treatment, the inclusions are finally lifted and removed by the tundish.

(Knowledge about tundish)
In continuous casting, the molten steel is poured into the tundish in an amount corresponding to the continuous casting speed (for example, an amount of about 8 tons / minute or less), so that the flow rate of the molten steel in the tundish is the ladle. It is smaller than the stirring flow rate of molten steel in gas stirring, and the effect of agglomeration and coalescence of inclusions is difficult to expect.
In addition, when the molten steel temperature drops in the tundish, the decrease in solubility product leads to the formation of new fine alumina (2 Al + 3 O → Al ₂ O ₃ ), and the increase in alumina inclusions in the cast slab is remarkable. May become.
On the other hand, by heating the molten steel in the tundish, the effect of suppressing the formation of new alumina inclusions can be expected. In addition, when a weir (partition wall) is erected inside the tundish to generate an ascending flow in the molten steel in the tundish (generated in the molten steel after heating), the slag existing on the molten metal surface in the tundish is agitated. With the effect suppressed, the effect of floating inclusions in the molten steel having a particle size of about 30 to 50 μm and capturing them by the slag can be expected.

Although the strength of the inclusions (aggregated and coalesced inclusions) having a particle size of about 30 to 50 μm obtained by the vacuum degassing treatment of the present invention is improved, they may be broken by the shearing force of the molten steel. By using a tundish that heats the molten steel provided with the molten steel flow path described above, it is possible to promote the floating of inclusions in which fracture is suppressed. This causes, for example, the use of an upper weir that partitions the upper part of the molten steel in the tundish, which causes a shearing force to act on the inclusions when the molten steel flow once becomes a downward flow and then becomes an upward flow. This is because such shearing force does not occur when heating in the molten steel flow path is used.
Therefore, a weir that divides the hot water receiving part and the hot water discharging part (arranged independently) is erected inside the tundish, and one or more of the weirs that communicate the hot water receiving part and the hot water discharging part are connected to this weir. A hollow refractory material forming a molten steel flow path is provided, and the molten steel is heated in the region of this hollow refractory material.

Based on the above findings, the present inventors have come up with a method for melting high-clean steel, which can melt and cast high-clean steel with less alumina inclusions than the conventional technique.
That is, as shown in FIGS. 1 and 2, in the method for melting high-clean steel according to an embodiment of the present invention, metallic aluminum is added to molten steel subjected to primary refining by blowing acid decarburization under atmospheric pressure. Then, when the molten steel in the ladle 14 having the dissolved oxygen concentration in the molten steel of 40 ppm or less is subjected to vacuum degassing treatment using the RH vacuum degassing device 10, a low-pressure vacuum atmosphere is obtained in the first half of the vacuum degassing treatment. This is a method in which the degassing treatment is performed in the above step, and then the degassing treatment is performed in a high-pressure vacuum atmosphere in the latter half of the vacuum degassing treatment, and then the hot water is poured into the tundish 15 for continuous casting.
Hereinafter, it will be described in detail.

First, molten steel that has been subjected to primary refining (typical example: blowing in a converter) for decarburization by blowing acid under atmospheric pressure is supplied to a ladle 14.
Normally, the dissolved oxygen concentration in the molten steel that has been decarburized by blowing acid is about 100 to 800 ppm, so it is necessary to deoxidize it.
In the present invention, it is premised that the dissolved oxygen concentration in the molten steel is 40 ppm or less by adding metallic aluminum (including adding one containing metallic aluminum).
By the above treatment, aluminum oxide (alumina: hereinafter also referred to as inclusions) is present in the molten steel.

Degassing treatment is used as a refining step for performing each of the above-mentioned treatments for floating removal, aggregation and coalescence, and destruction prevention of inclusions generated by the addition of metallic aluminum, that is, a high cleaning treatment.
As the means for high purification, the above-mentioned RH vacuum degassing device 10 is used.
Specifically, as shown in FIG. 1, the immersion pipes 12 and 13 of the RH vacuum degassing device 10 are immersed in the molten steel in the ladle 14 having the dissolved oxygen concentration in the molten steel of 40 ppm or less, and the ascending side. Inert gas is blown from the dipping pipe 12 of the above, and a vacuum degassing treatment is performed to recirculate the molten steel between the vacuum tank 11 and the ladle 14.
This vacuum degassing treatment is performed separately in the first half and the second half as follows.

First, in the first half of the vacuum degassing treatment (hereinafter, also referred to as the first half treatment or the low pressure vacuum treatment), the inside of the vacuum chamber 11 is set to a low pressure vacuum atmosphere of 1.3 kPa (9.75 Torr) or less, and then for 15 to 45 minutes. Perform degassing treatment.
Since the primary purpose of this degassing treatment is to adjust the carbon concentration of molten steel (decarburization), it is necessary to adopt the above conditions. Here, when the pressure in the vacuum chamber 11 exceeds 1.3 kPa, the decarburization reaction is delayed and the processing time is delayed, which causes the temperature of the molten steel to drop.
If the pressure is in the vacuum chamber 11 described above, the degassing treatment can be completed in about 15 to 45 minutes.

The present inventors have found that the behavior of inclusions is shown below depending on the above-mentioned treatment conditions. Since the behavior of inclusions is characteristic depending on their size, typical particle sizes are described as three types: 70 μm or more, 30 to 50 μm, and 20 μm or less.

(70 μm or more)
It is presumed that inclusions having a size of 70 μm or more due to agglomeration and coalescence have a high inertial force in the flow in the molten steel, deviate from the molten steel flow (circulation) circulating in the vacuum chamber 11 and the ladle 14, and float in the ladle. There is a strong tendency.
Therefore, it is presumed that by remaining in the recirculation, it is less likely to be destroyed by the shearing force.

(30-50 μm)
It is presumed that the inclusions having a size of 30 to 50 μm due to the agglomeration and coalescence were able to increase the particle size (aggregation and coalescence), but the remarkable floating removal was unlikely to occur, and the inclusions tended to remain in the circulation.
Therefore, it was considered that the inclusions tended to be destroyed by the shearing force.
Shear force is inevitably generated due to the presence of a molten steel flow, and the conditions for generating it are when degassing treatment is performed for a long time, when gas is blown from the bottom of the ladle during transportation of molten steel, and when gas is blown from the bottom of the ladle. For example, when molten steel is supplied from a ladle to a tundish by a falling flow.

(20 μm or less)
It is considered that inclusions of 20 μm or less are unlikely to undergo remarkable floating removal as in the case of 30 to 50 μm even after agglomeration and coalescence, and tend to remain in the gyre. Further, it is considered that the inclusions having a size of 30 to 50 μm tend to be destroyed by the shearing force.

In the latter half of the vacuum degassing treatment (hereinafter, also referred to as the latter half treatment or the high pressure vacuum treatment) following the first half treatment described above, the inside of the vacuum chamber 11 is set to a high pressure vacuum atmosphere of 20 kPa (150 Torr) or more and 40 kPa (300 Torr) or less. Degas for 5 to 15 minutes. In this way, we aim for the effects of agglutination and strength improvement of inclusions (strength improvement to the extent that they are not destroyed by shearing force).
The present inventors considered that the following mechanism worked to obtain the effect of high purification of molten steel by performing such a degassing treatment.

Compared to the first half treatment whose main purpose is decarburization, the second half treatment is a high-pressure vacuum (20 kPa to 40 kPa), and the distance (hot water) from the bottom surface of the ladle 14 to the molten steel surface (molten steel head) in the vacuum tank 11. The surface height and recirculation height) can be shortened (the dipping depths of the dipping pipes 12 and 13 in the molten steel in the ladle 14 are the same). As a result, the stirring energy of the molten steel that circulates and recirculates in the vacuum chamber 11 and the ladle 14 can be weakened by reducing the distance in the circulation height direction of the recirculation to reduce the potential energy.
Further, by setting the high pressure vacuum, the suction amount of the molten steel can be reduced, the recirculation speed can be reduced, and the stirring energy can be weakened.
Here, when the pressure in the vacuum chamber is a low pressure vacuum of less than 20 kPa, the stirring energy is large, and the remarkable effect of suppressing the destruction of the aggregated inclusions cannot be obtained. On the other hand, when the pressure in the vacuum chamber exceeds 40 kPa, it may be difficult to suck the molten steel into the vacuum chamber, that is, to recirculate the molten steel itself, and the treatment itself may not be possible.

Due to the pressure in the vacuum chamber 11 described above, it is possible to prevent the agglutinated inclusions from collapsing, to promote the gradual agglutination of the inclusions by continuing the circulation, and to improve the strength of the agglutinated inclusions. However, for that purpose, it is necessary to set the processing time under high pressure vacuum to 5 to 15 minutes.
Specifically, the inclusions immediately after being aggregated and coalesced by the above-mentioned low-pressure vacuum treatment have low strength and may be destroyed by the shearing force of the molten steel flow. Therefore, the processing time is preferably 15 minutes or less. On the other hand, if the treatment is for 5 minutes or more, the strength of the aggregated inclusions can be improved.
As a result, it is possible to suppress the destruction of inclusions until the treatment with the tundish described later (the floating removal with the tundish becomes possible).

The behavior of inclusions according to the particle size is as follows.
(70 μm or more)
The levitation in the ladle 14 is almost completed during the low-pressure vacuum treatment (first half treatment), and even if part of it remains in the molten steel, it is also in the ladle 14 during the high-pressure vacuum treatment (second half treatment). It can be thought of as emerging.
(30-50 μm)
Agglutinations of 30 to 50 μm due to agglutination during low-pressure vacuum treatment tend to remain in the recirculation, but they are also present in the recirculation of molten steel during high-pressure vacuum treatment, and the strength is improved while suppressing fracture. It was thought to be.
As a result, the inclusions will be present in the molten steel transported to the process after the vacuum degassing treatment without the progress of fracture, but these inclusions will lead to the floating removal in the tundish described later. Can be done.

(20 μm or less)
It is considered that inclusions of 20 μm or less even after the agglomeration and coalescence by the low-pressure vacuum treatment proceed slowly by the coagulation and coalescence by the recirculation treatment while preventing fracture in the high-pressure vacuum treatment, and the strength is also improved.
Therefore, it is considered that the molten steel supplied after the vacuum degassing treatment has a reduced inclusions of 20 μm or less, for example, agglomerates and coalesces to about 30 to 50 μm, and the strength thereof is also improved. It is surfaced and removed with a tundish.

Inclusions of 70 μm or more are floated and removed from the molten steel that has undergone the above-mentioned high-pressure vacuum treatment.
Further, since the inclusions having a size of about 30 to 50 μm are not destroyed by the above-mentioned degassing treatment as compared with the prior art, the abundance ratio can be maintained at a high level and the strength is further improved, so that the abundance ratio is high. The molten steel can be transported to the tundish at a high level.
Further, inclusions having a size of 20 μm or less are subjected to the above-mentioned refining treatment (primary refining to vacuum degassing treatment) to cause agglomeration and coalescence in which fracture is suppressed (for example, coagulation and coalescence to 30 μm or more), and are present as compared with the conventional technique. The molten steel can be transported to the tundish in a state where the ratio is reduced (or the increase of inclusions of 20 μm or less is suppressed).

Subsequently, the vacuum degassed molten steel is poured from the ladle 14 (molten steel pan) into the tundish 15 via the long nozzle 16 (see FIG. 2).
The tundish 15 has a hot water receiving section 18 that receives molten steel from a ladle 14 via a long nozzle 16 and a hot water draining section 19 that injects the molten steel into a mold (not shown) for continuous casting. It is divided into. A dipping nozzle 20 is provided at the bottom of the hot water draining section 19, and the molten steel in the hot water draining section 19 is injected into the mold via the dipping nozzle 20.
The weir 17 that divides the hot water receiving portion 18 and the hot water discharging portion 19 is provided with a hollow refractory material 22 that forms a molten steel flow path 21 that communicates the hot water receiving portion 18 and the hot water discharging portion 19. The molten steel flow path 21 (hollow refractory material 22) receives molten steel from the opening 23 on the hot water receiving portion 18 side, and discharges the molten steel from the opening 24 on the hot water discharging portion 19 side to the hot water discharging portion 19. The molten steel flowing through the molten steel flow path 21 is induced and heated by an induction heating device (here, an induction heating coil 25).

In order to prevent molten steel from remaining in the hot water receiving portion 18 after the end of continuous casting, the hot water receiving portion 18 of the opening 23 (lower end of the opening 23) located on the hot water receiving portion 18 side of the molten steel flow path 21. The height position from the bottom surface 26 of the hot water receiving portion 18 is set to 0.2 times (0.2 × H) or less of the molten steel depth (bath depth) H of the hot water receiving portion 18 (the lower limit is, for example, 0 times (0 × H). ), That is, the position where the opening 23 is in contact with the bottom surface 26 of the hot water receiving portion 18).
Here, the number of molten steel flow paths 21 provided in the weir 17 may be, for example, one or a plurality of two or more depending on the casting conditions. When there are a plurality of molten steel flow paths 21, the height positions of the openings located on the hot water receiving portions side of all the molten steel flow paths 21 from the bottom surface of the hot water receiving portions satisfy the above conditions. Adjust so that. The length of the molten steel flow path 21 (thickness of the weir 17) is, for example, about 500 to 1500 mm.
The weir 17 and the hollow refractory 22 are both made of a refractory, but they may be made of the same material or different materials depending on the intended use.
Further, the molten steel flow path 21 is inclined downward from the hot water receiving portion 18 to the hot water discharging portion 19, but may be horizontal. Further, the depth position of the bottom surface 27 of the hot water draining portion 19 is deeper than the depth position of the bottom surface 26 of the hot water receiving portion 18, but the same depth may be used.
The molten steel flow path is not limited to being formed by a hollow refractory material, and may be formed, for example, by penetrating a hole through a weir (through hole).

As described above, in order to make the ascending flow of molten steel work effectively in the tundish 15, a weir 17 having a molten steel flow path 21 provided inside the tundish 15 is erected, and the hot water receiving portion 18 and the hot water draining portion 18 are erected. It is necessary to clearly divide the space (chamber) of the portion 19 (the position of the molten steel surface in the tundish 15 (hot water receiving portion 18 and hot water discharging portion 19) is lower than the upper surface of the weir 17).
In general, the molten steel temperature on the surface layer of the hot water draining section 19 drops in the tundish 15, so that the molten steel temperature on the surface layer of the hot water draining section 19 is lower than the molten steel temperature of the hot water receiving section 18, and the depth of the hot water draining section 19 is deep. There is a temperature difference in the molten steel in the vertical direction. Therefore, even if the molten steel discharged from the molten steel flow path 21 to the hot water draining portion 19 is not induced and heated in the molten steel flow path 21, convection (rising flow) of the molten steel occurs due to the above-mentioned temperature difference. By convection, inclusions in the molten steel discharged from the molten steel flow path 21 to the hot water draining portion 19 are floated and removed.
As the molten steel flow discharged from the molten steel flow path 21, there is a flow that goes straight through the hot water draining section 19 in addition to the above-mentioned ascending flow, and this flow becomes the main body of the molten steel flow in the hot water draining section 19. At this time, the inclusions aggregated and coalesced by the above-mentioned low-pressure vacuum treatment and high-pressure vacuum treatment maintain a high proportion of inclusions of about 30 to 50 μm, so that even if the molten steel flow travels straight, the inclusions are self-sustaining. It has a levitation force and can be levitation removed.

However, even if an ascending flow is formed in the tundish 15, the particle size of inclusions that can be lifted and removed is only a coarse diameter of about 30 to 50 μm or more, and it is difficult to lift and remove small diameter inclusions of about 5 to 20 μm. be.
Further, when the casting time becomes long and the molten steel temperature decreases in the tundish 15, the buoyancy of the inclusions weakens due to the increase in the viscosity of the molten steel, which causes the buoyancy efficiency of the inclusions to deteriorate and the alumina formation reaction (2). There is a concern that the solubility product of Al + 3 O → Al ₂ O ₃ ) will decrease, and fine Al ₂ O ₃ of less than 20 μm will be newly generated (secondary formation).

Therefore, as described above, by performing the low-pressure vacuum treatment and the high-pressure vacuum treatment as the vacuum degassing treatment, the coagulation and coalescence of fine Al ₂ O ₃ is promoted to be coarsened and its collapse is suppressed, and the inside of the tundish 15 is suppressed. It is important to carry out continuous casting while suppressing the formation of new fine Al ₂ O ₃ in.
Further, in order to promote the floating of the above-mentioned inclusions and suppress the formation of new fine Al ₂ O3 _, a weir 17 for separating the hot water receiving portion 18 and the hot water discharging portion 19 is provided in the tundish 15, and the hot water receiving portion 17 is provided. The portion 18 and the hot water draining portion 19 are communicated with each other by a molten steel flow path 21 provided in the weir 17, and the molten steel in the molten steel flow path 21 is induced and heated.

As a result, convection is generated in the molten steel in the hot water draining portion 19 of the tundish 15, and the agglomerated and coalesced alumina inclusions having a particle size of about 30 to 50 μm are efficiently levitated while being suppressed, and the hot water is floated. The effect of capturing the slag on the surface can be obtained. Further, by inducing and heating the molten steel in the molten steel flow path 21 to avoid a temperature drop of the molten steel, it is possible to suppress the formation of new fine alumina in the hot water draining portion 19.
Therefore, by continuously casting the obtained molten steel, it is possible to produce highly clean steel with fewer alumina inclusions than in the past, and the number of alumina inclusions with a particle size of 20 μm class, which was particularly difficult with the conventional technology, can be reduced. It is reduced, and it becomes possible to stably cast extremely highly clean steel having a total oxygen content (T. [O] value) of, for example, 10 ppm or less.

Next, an example carried out for confirming the action and effect of the present invention will be described.
Here, based on the following method, each condition was changed at the actual machine level, and the cleanliness of the stationary slab after casting was evaluated. Here, the stationary portion slab means a substantially central portion (a portion where the quality is stable) of the continuous casting length of the charge to be cast. The steel grade to be evaluated was the steel grade of bar wire (steel for gears), which is required to have high cleanliness.

After primary refining in a 90-ton converter, the molten steel (carbon concentration: 0.20 to 0.22% by mass, dissolved oxygen concentration in the molten steel: 100 to 300 ppm by mass) was discharged into the ladle. (Constant in) was moved to the ladle refining facility (LF) and the ladle refining process was performed. At that time, metallic aluminum was added to the molten steel in the ladle at a total of 3.0 to 9.0 kg per ton of molten steel, and deoxidation treatment and slag refining were performed to obtain T.I. The concentration of [O] was adjusted to be substantially constant at 30 to 40 ppm.
After that, the ladle was further moved, and a vacuum degassing treatment was carried out by an RH vacuum degassing device. At this time, the immersion depth of the immersion tube of the RH vacuum degassing device in the molten steel was maintained at a constant height position with respect to the ladle without changing from the start to the end of the treatment.
Then, the molten steel in the ladle was poured into a tundish to carry out continuous casting.
The test conditions and their results and evaluations are shown in Table 1.

Table 1 shows the treatment conditions (“time” and “pressure in the vacuum chamber” for the first half (“first half of degassing treatment”) and the second half (“second half of degassing treatment”) of the vacuum degassing treatment by the RH vacuum degassing device. ) Is described.
Here, the above-mentioned treatment conditions are described in Examples 1 to 6 and Comparative Examples 1 to 6, but in the conventional method, the high-pressure vacuum treatment in the latter half of the vacuum degassing treatment is not performed until the treatment is completed. Since the degassing treatment is performed in a low-pressure vacuum atmosphere (1.3 kPa or less), the latter half of the vacuum degassing treatment is described as "(no treatment)". The casting conditions for the tundish, which will be described later, to be performed after the vacuum degassing treatment of the conventional method are the same as those of the first embodiment.

In the "Tandish" column, "shape of weir", "presence or absence of induction heating", and "position of molten steel flow path" are described.
Here, the "weir shape" is the structure of the weir arranged in the tundish, and "A" is the structure of the weir that divides the inside of the tundish into a hot water receiving part and a hot water draining part (see FIG. 2). , "B" refer to the structure of the upper weir 31 installed in the tundish 30 shown in FIG. 3, respectively. The upper weir 31 has only the upper portion (hot water surface portion) "0.3 × h (0.3 times h)" in the depth direction, where the molten steel depth on the hot water receiving portion 32 side is h (about 1 m). Is a weir that separates the hot water receiving portion 32 side and the hot water discharging portion 33 side. In this case, the molten steel flow flowing in the upper part in the depth direction of the molten steel causes a forced flow that wraps around the lower side of the upper weir 31 along the upper weir 31 (after a forced downward flow is generated, the upper weir is generated. Passing under 31 to generate a forced ascending current).

"Presence / absence of induction heating" describes the presence / absence of induction heating for the molten steel flowing in the two molten steel flow paths provided in the weir, and "Yes" indicates the presence / absence of induction heating using the above-mentioned induction heating tundish. It refers to the case where the molten steel is subjected to induction heating (heating that raises the temperature of the molten steel received at the hot water receiving part by 1 to 10 ° C.). The diameter of this molten steel flow path is assumed to be 100 to 300 mm by converting the cross-sectional shape into a circle, but in this test, it was set to 100 mm.
When the upper weir is used, induction heating is not performed because the molten steel flow path is not provided.
The "position of the molten steel flow path" is the height position from the bottom surface of the hot water receiving portion at the lower end of the opening on the hot water receiving portion side of the molten steel flow path, and the molten steel depth of the hot water receiving portion is H (about 1 m). ), Three levels of 0 × H (bottom surface), 0.2 × H (0.2 times H), and 0.4 × H (0.4 times H) were used. When the upper weir is used, the immersion depth is set to 0.3 × h as described above.

In the column of "cleanliness of slab", "evaluation of the number of inclusions detected in 20 μm or more" and "T. [O]" are described.
For "evaluation of the number of inclusions detected of 20 μm or more", a sample (rectangle with a side of approximately 30 mm) cut out from the representative position of the stationary slab was examined with an optical microscope after mirror polishing, and alumina with a major axis of 20 μm or more was examined. The number of inclusions (converted to the number of detections of alumina inclusions per unit area) was used. In Table 1, the number of inclusions with a major axis of 20 μm or more obtained under the test conditions of Comparative Example 4 is set to “1.00”, and the number of inclusions detected under other test conditions is indexed. An index of 1.00 or more was evaluated as "fail", and an index of less than 1.00 was evaluated as "pass".
In the "T. [O]" column, the total oxygen concentration (total oxygen content) at the representative position of the stationary piece is measured, and if the value is 10 ppm or less, it is evaluated as ○ (passed), and it is 11 ppm or more. The case was marked as × evaluation (failure). It should be noted that, among the comparative examples of × evaluation, the value with the lowest total oxygen concentration was in Comparative Example 4, and since this Comparative Example 4 was 14 ppm, it was about 30% of the Comparative Example in the Example. Further improvement effect was seen.

"Comprehensive evaluation" is a combination of ○ evaluation (pass) when the column of "evaluation of the number of inclusions detected of 20 μm or more" is pass evaluation and the column of "T. [O]" is ○ evaluation. Was judged as × evaluation (failure).

In Examples 1 to 6 in Table 1, the RH vacuum degassing device 10 was used to degas in a low-pressure vacuum atmosphere in the first half of the vacuum degassing treatment (pressure in the vacuum chamber: 1.3 kPa or less, treatment time: 15). After performing the degassing treatment (pressure in the vacuum chamber: 20 to 40 kPa, processing time: 5 to 15 minutes) in a high-pressure vacuum atmosphere in the latter half of the vacuum degassing treatment (0 to 45 minutes), the appropriate range (0). This is the result of continuous casting by pouring hot water into a tundish equipped with a molten steel flow path located at (2 × H or less).
In this case, the vacuum degassing treatment (degassing treatment in a low-pressure vacuum atmosphere and the high-pressure vacuum atmosphere) has the effect of promoting the aggregation of inclusions, the effect of improving the strength of the aggregated inclusions, and the aggregated and coalesced alumina by the tundish. The effect of removing floating inclusions was obtained.
As a result, as shown in Table 1, both "evaluation of the number of inclusions detected in 20 μm or more" and "T. [O]" were good, and the cleanliness of the slab was good (comprehensive evaluation: ◯). ).

On the other hand, Comparative Example 1 is a result when the condition of the time in the latter half of the degassing treatment is more than the upper limit of the appropriate range (20 minutes) with respect to the condition of Example 1, and as shown in Table 1, casting. The number of alumina inclusions present in the piece increased, and the cleanliness of the slab deteriorated (comprehensive evaluation: ×).
It is considered that this is because the treatment time in the latter half of the degassing treatment becomes too long and the agglomerated inclusions are destroyed, so that the floating removal of the inclusions in the tundish is insufficient.

Comparative Example 2 is a result when the condition of the time in the latter half of the degassing treatment is set to less than the lower limit of the appropriate range (1 minute) with respect to the condition of Example 4, and as shown in Table 1, in the slab. The number of alumina inclusions present in the slab increased, and the cleanliness of the slab deteriorated (comprehensive evaluation: ×).
This is because the treatment time in the latter half of the degassing treatment was insufficient and the strength of the agglomerated inclusions was insufficiently improved. It is probable that this was due to insufficient removal.

Comparative Example 3 is a result when the condition of the pressure in the vacuum chamber in the latter half of the degassing treatment was set to less than the lower limit of the appropriate range (10 kPa) with respect to the condition of Example 1, and as shown in Table 1, casting. The number of alumina inclusions present in the piece increased, and the cleanliness of the slab deteriorated (comprehensive evaluation: ×).
This results in the pressure in the vacuum chamber becoming too low, resulting in insufficient shortening of the distance from the bottom of the ladle to the molten steel surface in the vacuum chamber for the first half of the degassing treatment, resulting in insufficient reduction of stirring energy. It is probable that this was due to the lack of floating removal in the tundish, which led to the destruction of the aggregated inclusions.
This Comparative Example 3 is a condition close to the condition of Patent Document 1 described above (a condition in which the immersion depth of the immersion tube is made shallow without changing the low pressure vacuum state in the vacuum chamber). Therefore, it is presumed that the method of Patent Document 1 causes the agglomerated inclusions to be destroyed and the floating removal in the tundish is insufficient.
In contrast to the condition of Example 5, setting the pressure condition in the vacuum chamber in the latter half of the degassing treatment to exceed the upper limit value (40 kPa) in the appropriate range means that the molten steel is sucked up into the vacuum chamber as described above. It is not described because it may be difficult and the processing itself may not be possible.

Comparative Example 4 is a result of the case where induction heating was not performed in the molten steel flow path with respect to the conditions of Example 1, and as shown in Table 1, the number of alumina inclusions present in the slab is large. As a result, the cleanliness of the slabs deteriorated (comprehensive evaluation: ×).
This is because even if the treatment according to the vacuum degassing treatment condition of the present invention is carried out, when the ascending flow of the molten steel due to the heating of the molten steel does not occur when the inclusions are lifted and removed in the tundish, the ascending flow of the molten steel is not generated from the molten steel flow path. Since the inclusions are levitated only by the molten steel flow discharged to the hot water discharge part side, it is presumed that the rate of destruction of the agglomerated inclusions has increased.

Comparative Example 5 is a result when the position of the molten steel flow path in the height direction exceeds the upper limit value (0.4 × H) of the appropriate range with respect to the conditions of Example 1, and is shown in Table 1. , The number of alumina inclusions present in the slab increased, and the cleanliness of the slab deteriorated (comprehensive evaluation: ×).
It is probable that this is because the inclusions in the hot water draining portion did not have enough time to ascend, so that the improvement in cleanliness was insufficient, and the inclusions once floated and removed were re-engaged in the molten steel.

Comparative Example 6 is a result in the case where the structure of the weir of the tundish is different from the condition of Example 1 (the upper weir shown in FIG. 3 is adopted) and the induction heating of the molten steel is not carried out. This is the result when only the structure of the tundish weir is different for the above conditions.
In Comparative Example 6, the number of inclusions detected increased and the total oxygen concentration (T. [O] ppm) also increased as compared with Comparative Example 4 (comprehensive evaluation: ×).
In Comparative Example 6, a molten steel flow flowing in the upper part in the depth direction of the molten steel causes a forced flow around the lower side of the upper weir along the upper weir (after a forced downward flow is generated, the upper weir It is presumed that the shearing force applied to the molten steel is larger than that of Example 1 and Comparative Example 4 because a forced ascending flow is generated by passing through the lower side, which causes the fracture of agglomerated inclusions and the tongue. It was speculated that the floating removal of inclusions in the dish was insufficient.
In addition, the evaluation of the number of inclusions of 20 μm or more in the slab of the stationary part was obtained in Comparative Example 6 more than in Example 1, but the inclusions of about 20 μm which were difficult to float and remove by tundish were obtained. The number also tended to be larger in Comparative Example 6. Therefore, it is presumed that the upper weir, which is the condition of Comparative Example 6, has a larger shearing force for collapsing inclusions than that of Example 1.

In the conventional method, as described above, the ladle in which the molten steel is stored before the continuous casting is placed near (near) the continuous casting machine without performing the latter half treatment of the vacuum degassing treatment for the condition of the first embodiment. This is the result of continuous casting after allowing 10 minutes to stand still. As shown in Table 1, the number of alumina inclusions present in the slab increases, and the cleanliness of the slab is improved. It got worse (comprehensive evaluation: ×).
It is considered that this is because the effect of improving the strength of the aggregated inclusions and the effect of further promoting the aggregated coalescence while preventing the collapse of the aggregated inclusions were insufficient, and the floating removal by the tundish was insufficient. ..

Therefore, by using the method for melting high-clean steel of the present invention, it is possible to produce high-clean steel with fewer alumina inclusions than in the conventional technique, and in particular, the number of alumina inclusions having a particle size of 20 μm class is reduced. It was confirmed that steel having an extremely high degree of cleanliness having a total oxygen content (T. [O] value) of, for example, 10 ppm or less can be stably cast.

Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the configuration described in the above-described embodiments, and the matters described in the claims. It also includes other embodiments and variations that may be considered within the scope. For example, the case where a method for melting the high-clean steel of the present invention is formed by combining some or all of the above-described embodiments and modifications thereof is also included in the scope of rights of the present invention.

10: RH vacuum degassing device, 11: vacuum tank, 12, 13: immersion pipe, 14: ladle, 15: tundish, 16: long nozzle, 17: dam, 18: hot water receiving part, 19: hot water draining part , 20: Immersion nozzle, 21: Molten steel flow path, 22: Hollow refractory, 23, 24: Opening, 25: Induction heating coil, 26, 27: Bottom, 30: Tundish, 31: Upper dam, 32: Receiving Yube, 33: Hot water drain

Claims (1)

The immersion tube of the RH vacuum degassing device is placed in the molten steel in the pan where the concentration of dissolved oxygen in the molten steel is 40 ppm or less by adding metallic aluminum to the molten steel that has undergone primary refining by blowing acid decarburization under atmospheric pressure. When the vacuum degassing treatment is performed by immersing the metal and blowing an inert gas from the rising pipe of the dipping tube to recirculate the molten steel between the vacuum tank of the RH vacuum degassing device and the pan.
In the first half of the vacuum degassing treatment, the inside of the vacuum chamber is made into a low-pressure vacuum atmosphere of 1.3 kPa or less, and then the degassing treatment is performed for 15 to 45 minutes.
In the latter half of the vacuum degassing treatment, the inside of the vacuum chamber is made into a high-pressure vacuum atmosphere of 20 to 40 kPa, and then the degassing treatment is performed for 5 to 15 minutes.
A weir is provided inside to separate the hot water receiving part that receives the molten steel and the hot water discharging part that injects the molten steel into the mold for continuous casting, and one or more molten steel flow paths that communicate the hot water receiving part and the hot water discharging part are provided. The height position of the opening formed in the weir and located on the hot water receiving portion side of the molten steel flow path from the bottom surface of the hot water receiving portion is 0.2 times or less the depth of the molten steel of the hot water receiving portion. A method for melting high-clean steel, which comprises pouring the molten steel that has been subjected to the vacuum degassing treatment into the tundish to induce and heat the molten steel flowing through the molten steel flow path.

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