JP6135462B2 - Method for preventing outflow of non-metallic inclusions in molten metal - Google Patents

Description

  The present invention relates to a method for preventing outflow of non-metallic inclusions contained in molten steel at the time of molten steel injection from a tundish into a mold in continuous casting of molten metal such as molten steel (hereinafter simply referred to as molten steel). .

  In continuous casting of steel, it is common to inject molten steel from a ladle into a mold through a long nozzle and then pour it into the tundish and then from the tundish through an immersion nozzle. .

Non-metallic inclusions such as Al ₂ O ₃ contained in molten steel cause product surface flaws and cause clogging of the immersion nozzle used when pouring molten steel in the tundish into the mold. When molten steel is poured from the tundish into the mold, it is necessary to prevent the outflow.

  As a method to prevent nonmetallic inclusions in molten steel from flowing out into the mold, a method of providing a weir in the tundish, a method of providing a refractory filter in the tundish, blowing inert gas and swirling flow A method for promoting the floating separation of non-metallic inclusions has been proposed.

  Between the molten steel injection part from the ladle and the molten steel pouring part into the mold in the tundish, for example, in Patent Document 1, a partition weir having a circulation port is arranged, and in Patent Document 2, a lower weir, A method of removing non-metallic inclusions in molten steel by arranging an upper weir and a lower weir in order has been proposed.

  It can be said that the effect of providing the weir lies in the prevention of slag entrainment by preventing the slag present on the molten steel surface in the tundish from flowing out to the molten steel pouring portion into the mold. Therefore, in the case of the methods proposed in Patent Documents 1 and 2 in which weirs are arranged, the acceleration of the flying speed by condensation / combination of nonmetallic inclusions cannot be expected, and the positive removal effect of nonmetallic inclusions is small. I must say.

  However, in Patent Document 1, since inert gas blowing is used in addition to the weir, aggregation and coalescence of nonmetallic inclusions occur, and levitation promotion can be expected by increasing the size of nonmetallic inclusions. However, since the inert gas is blown from the bottom brick of the tundish, the blowing method is complicated and repair is difficult, which is not suitable for practical use.

  Moreover, in patent document 3, the filter made from the calcareous refractory which installs between the molten steel injection | pouring part from a ladle and the molten steel pouring part to a casting_mold | template in a tundish, and adsorb | sucks the nonmetallic inclusion in molten steel. Proposed. This filter made of calcareous refractory is excellent in the effect of removing non-metallic inclusions, but is expensive and difficult to be practically used for continuous casting of mass-produced carbon steel.

  Moreover, in patent document 4, the method of installing an alumina-type refractory in either or both of the floor part and side wall in a tundish is proposed. The alumina-based refractory used as the filter is less expensive than the calcareous refractory filter. However, due to its structure, the amount of molten steel that is in direct contact with the filter is small, and the flow velocity in the tundish is as small as 0.17 m / second or less, so the trapping efficiency is poor and the removal effect of nonmetallic inclusions is small.

  Patent Document 5 proposes a method in which an inert gas is blown into molten steel imparted with a swirling flow to make the gas bubbles fine, and the gas bubbles that adsorb nonmetallic inclusions are floated and separated. In this method, gas bubbles are refined by swirling flow to enhance the trapping effect of nonmetallic inclusions. However, since separation of nonmetallic inclusions trapped in the bubbles is left to levitation, it is suspended in the molten steel. It is difficult to remove turbid non-metallic inclusions. Further, in a region where the flow velocity of the molten steel outlet and the vicinity of the inlet is high, the gas bubbles flow downstream before rising to the bath surface.

  Patent Document 6 discloses a method for promoting the floating of nonmetallic inclusions by providing a stirrer in the tundish. However, in the case of this method, there is a possibility that a problem of entrapping nonmetallic inclusions in the molten steel may occur by stirring the inside of the tundish. Coarse, non-metallic inclusions are a cause of serious product defects and are not a method that can be introduced in actual operation.

JP-A 63-157745 Japanese Patent No. 2938323 JP 59-156556 A JP 2010-179340 A JP 60-257955 A JP 58-103946 A

  The problem to be solved by the present invention is that the effect is unstable due to the change in the flow of molten steel in the tundish when the removal of non-metallic inclusions is left to floating separation as in Patent Documents 1, 2, 5, and 6. The non-metallic inclusions that are easily formed and suspended are difficult to trap.

  Also, the calcareous refractory filter proposed in Patent Document 3 is difficult to put into practical use for mass production of carbon steel, while the alumina-based refractory filter proposed in Patent Document 4 has poor trapping efficiency and is non-metallic. The effect of removing inclusions is small.

The present invention
To facilitate the agglomeration and enlargement of nonmetallic inclusions in the molten steel in the tundish and make it easier to float and separate, in order to be able to capture nonmetallic inclusions that cannot float,
A method for preventing non-metallic inclusions contained in molten steel in a tundish from flowing into a mold,
A cylindrical weir is provided so as to surround the molten steel pouring portion to the mold in the tundish, and a swirling flow is formed in the molten steel in the cylindrical weir,
The molten steel in the tundish is poured into the mold while the trapping device for non-metallic inclusions is maintained at the center of the swirling flow so that the tip is at a height position that secures a flow path to the molten steel pouring part. In this case, the most important thing is to raise the trapping device in accordance with the increase in the adhesion thickness of non-metallic inclusions on the trapping device and to maintain a constant amount of molten metal injected from the tundish into the mold. It is a feature.

  The present invention collects non-metallic inclusions in the molten steel at the center of the swirling flow by centripetal force by forming a swirling flow in the molten steel in the weir provided so as to surround the molten steel pouring portion into the mold in the tundish. Accelerate the agglomeration and enlargement of non-metallic inclusions so that they do not flow out of the molten steel pouring part.

  In addition, by arranging the trapping device at the central portion of the swirling flow, the nonmetallic inclusions gathered at the center of the swirling flow can be efficiently attached to the trapping device. The capturing device is a structure similar to a stopper rod used for flow control, and is inserted from above directly above the immersion nozzle in the tundish.

  In the present invention, when an inert gas is blown into the molten steel from the tip of the trapping device, the bubbles refined by the swirling flow are stirred simultaneously with the nonmetallic inclusions, so the nonmetallic inclusions collide with each other due to the turbulent flow effect. As a result, the particles are aggregated and coalesced, and the frequency of attachment to the capturing device increases.

Further, in the present invention, since increasing the capture device in accordance with an increase in the deposition thickness of the non-metallic inclusions to the capture device, it is possible to maintain the injection rate of molten steel from the tundish into the mold constant.

  In the present invention, a swirl flow is formed in the tundish, and a trapping device is provided at the center of the swirl flow, so that nonmetallic inclusions in the molten steel can be efficiently captured in the tundish. At that time, when an inert gas is blown into the molten steel from the tip of the trapping device, the bubbles refined by the swirling flow are stirred simultaneously with the non-metallic inclusions, and the frequency of attachment to the trapping device increases. Therefore, when extracting molten steel from a tundish to a mold, the outflow of non-metallic inclusions can be effectively prevented, so that the immersion nozzle can be prevented from being blocked and the surface flaw of the product can be suppressed.

Furthermore, since raising the capture device in accordance with an increase in the deposition thickness of the non-metallic inclusions to the capture device, no blocking of the channel due to non-metallic inclusions adhering to the capture device, the molten steel from the tundish into the mold The injection amount can be kept constant.

It is the figure which showed typically an example of the structure of the continuous casting apparatus which enforces the method of this invention. It is a figure which shows the 1st example of the cylindrical dam installed in the continuous casting apparatus which implements the method of this invention, (a) is a front view, (b) is AA sectional drawing of (a). It is a figure similar to FIG. 2 which shows the 2nd example of the cylindrical weir installed in the continuous casting apparatus which enforces this invention method. It is a figure which shows an example of the capture | acquisition apparatus installed in the continuous casting apparatus which implements a method of this invention, (a) is a thing of the structure which can blow in an inert gas from a front-end | tip, (b) does not blow in an inert gas. It is of a possible configuration. It is the figure which showed typically an example of the structure of the continuous casting apparatus which implements the comparative example of this invention method. It is the figure which showed typically an example of the structure of the continuous casting apparatus which implements the other comparative example of this invention method. It is a figure which shows the example of the cylindrical weir installed in the continuous casting apparatus which implements the comparative example of this invention method, (a) is a front view, (b) is AA sectional drawing of (a).

  An object of the present invention is to promote the agglomeration and enlargement of nonmetallic inclusions in molten steel in a tundish to facilitate floating separation and to capture nonmetallic inclusions that cannot be lifted.

  And, the capture device that defines the shape and installation position of the weir to be installed in the tundish, and attaches and captures non-metallic inclusions at the center of the swirl flow to be formed in the molten steel in the weir This was achieved by maintaining the flow path to the dispensing part while maintaining the height.

  Hereinafter, an example of the configuration of a continuous casting apparatus for carrying out the method of the present invention will be described with reference to FIG. 1, and then non-metallic inclusions contained in the molten steel by the method of the present invention using the apparatus shown in FIG. A method for preventing the outflow of objects will be described.

  1 is a ladle, and the molten steel 2 in the ladle 1 is poured into the tundish 4 through a long nozzle 3 installed at the bottom of the ladle 1. The molten steel injected into the tundish 4 is transferred to the mold 8 via the upper nozzle 5 installed in the laying part 4a of the tundish 4 and the sliding gate 6 and the immersion nozzle 7 provided continuously below the upper nozzle 5. Injected.

  The molten steel 2 injected into the mold 8 is cooled from the inner surface of the mold 8 (primary cooling) to form a solidified shell 9 on the outer periphery. As the solidified shell 9 is pulled out to the exit side of the mold 8, its thickness increases. After the solidified shell 9 is pulled out from the mold 8, it is secondarily cooled and completely solidified into a slab. In addition, 10 in FIG. 1 is a mold powder supplied to the upper surface of the molten steel 2 in the mold 8.

  In the present invention, a cylindrical refractory weir 11 is placed on the floor portion 4a of the tundish 4 so as to be coaxial with the upper nozzle 5 and the immersion nozzle 7, for example, a molten steel pouring portion into the mold of the tundish 4. Are provided upright. A horizontal swirling flow is formed in the molten steel 2 flowing into the inside from the outer peripheral side of the weir 11 through the inlet 11 a provided on the side wall of the weir 11.

  As a configuration for forming a swirl flow in the molten steel 2 inside the weir 11, for example, as shown in FIG. This can be realized by providing two shaped inflow ports 11a. In addition, as shown in FIG. 3, four rectangular inflow ports 11 a may be provided at a point-symmetric equiangular position deviated from the axial center below the intermediate portion of the weir 11 in the axial direction.

  The number, shape, opening area (swirl speed), opening position, and inclination from the outer surface to the inner surface of the weir 11 are determined so that the predetermined swirling flow can be formed. However, as long as a predetermined swirl flow can be formed, the number, shape, opening area (swirling speed), opening position, and inclination from the outer surface to the inner surface of the weir 11 are as shown in FIGS. Not limited to those shown.

  By forming a horizontal swirling flow in the molten steel 2 flowing into the weir 11 in this way, nonmetallic inclusions in the molten steel gather at the center of the swirling flow due to centripetal force, and agglomeration of nonmetallic inclusions is promoted. Thus, it can be prevented from floating and flowing out from the molten steel pouring portion into the mold 8.

  In addition, according to the present invention, the capturing device 12 for capturing non-metallic inclusions attached to the central portion of the swirl flow is disposed. The capturing device 12 is a structure similar to a stopper rod used for flow control, for example, and is inserted from above the tundish 4 into the axial center position of the weir 11 which is the central portion of the swirling flow.

  The position in the height direction of the tip 12a at the time of insertion from the upper side of the capturing device 12 is a position that can maintain a state where a flow path to the molten steel pouring portion can be secured. By disposing the capture device 12 at this position, non-metallic inclusions that cannot be lifted and collected at the center of the swirling flow can be efficiently attached to the capture device 12.

  In the present invention, when the trapping device 12 formed in a cylindrical shape is used and an inert gas is blown into the molten steel 2 from the tip 12a, bubbles refined by the swirl flow are stirred simultaneously with non-metallic inclusions. Is done. Therefore, non-metallic inclusions collide with each other due to the turbulent flow effect and coalesce and coalesce, and the frequency of attachment to the capturing device 12 increases.

  The amount of the inert gas blown is not particularly limited, but is preferably 10 N liters / minute or less according to the results of the inventors' investigation. When an inert gas exceeding 10 Nl / min is blown, the pressure loss increases and the casting speed has to be slowed, or the inert gas that cannot be lifted flows out into the mold and traps in the solidified shell This is because problems such as being easily made occur.

  In the case of the present invention, the non-metallic inclusions 13 adhere to the surface of the trapping device 12 as shown in FIG. 4 regardless of whether or not the inert gas is blown from the tip 12a of the trapping device 12, and adhere to the surface. Thickness increases with the continuous casting operation.

  When the adhesion thickness of the non-metallic inclusions 13 on the capturing device 12 increases, the distance between the upper nozzle 5 serving as the molten steel pouring portion and the tip 12a of the capturing device 12 becomes narrow, and the amount of molten steel injected into the mold 8 is reduced. It will decrease and interfere with operation.

  Therefore, in the present invention, even if the adhesion thickness of the nonmetallic inclusions to the capturing device 12 increases, the amount of nonmetallic inclusions to the capturing device 12 does not change so that the injection amount of the molten steel 2 into the mold 8 does not change. It is desirable to raise the capture device 12 as the deposition thickness increases.

  Next, the results of experiments conducted to confirm the effects of the present invention will be described. The experiment was conducted on an example satisfying the requirements of the present invention and a comparative example not satisfying the requirements of the present invention.

  The experiment was performed by casting 600 to 640 tons of an aluminum killed ultra-low carbon steel having a chemical composition shown in Table 1 below using a tundish having an inner dimension of 5100 mm in length, 900 mm in width, and 1450 mm in height.

  The embodiment of the present invention uses a continuous casting apparatus having the configuration shown in FIG. 1 provided with the weir 11 shown in FIG. 2 provided with two rectangular inflow ports 11a at point-symmetric positions deviated from the axial center. Carried out. At that time, the case where the capture device 12 configured to be able to blow inert gas from the tip 12a shown in FIG. 4A is used, and the case where the inert gas cannot be blown shown in FIG. 4B. It implemented about the case where the capture | acquisition apparatus 12 of a structure was used. In both examples, the capturing device 12 was raised in accordance with the increase in the thickness of the non-metallic inclusions 13 adhered to the surface of the capturing device 12 so that the amount of molten metal poured into the mold 8 did not change. .

  On the other hand, in the comparative example which does not satisfy the requirements of the present invention, the continuous casting apparatus having the configuration shown in FIG. 5 or FIG. 6 was used. 5 replaces the weir 11 of the continuous casting apparatus used in the embodiment, and forms a swirling flow with four inlets 14a provided at equiangular positions on the radius in the axial intermediate portion shown in FIG. The weir 14 is not used. Moreover, FIG. 6 does not provide the weir 11 of the continuous casting apparatus used in the embodiment, but only the capturing device 12 for blowing an inert gas from the tip 12a.

  Table 2 below shows experimental results of Examples and Comparative Examples.

  The average adhesion thickness of non-metallic inclusions in Table 2 is the number of measurement points measured at 5 points every 50 mm from the tip 12a of the capture device 12 to the height of 200 mm, and 10 points at both ends of the central longitudinal section. Is averaged.

-About Example 1 and Comparative Examples 1 and 2 In Comparative Example 1 in which weirs that do not form a swirl flow are arranged, the average adhesion thickness of non-metallic inclusions to the trapping device is 12 mm, and in Comparative Example 2 in which no weirs are arranged, The average thickness of non-metallic inclusions on the capture device was 10 mm.

  On the other hand, in Example 1 in which a cylindrical weir was arranged and a swirling flow was generated inside the weir, the average adhesion thickness of the non-metallic inclusions to the trapping device was greatly increased to 35 mm.

Since the average adhesion thickness of the nonmetallic inclusions in Comparative Examples 1 and 2 was similar, it is considered that the increase in the average adhesion thickness of the nonmetallic inclusions in Example 1 was not due to the weir effect. . That is, the average adhesion thickness of the nonmetallic inclusions in Example 1 increased because the nonmetallic inclusions such as Al ₂ O ₃ having a specific gravity lighter than the molten steel gathered at the center of rotation due to the centripetal force due to the swirling flow. This is considered to be an effect caused by contact with

Examples 2 to 4 and Comparative Example 3 Example 2 in which a cylindrical weir was arranged to generate a swirling flow inside the weir and 0.40 Nl / min Ar gas was blown from the tip of the trapping device. Then, the average adhesion thickness of the nonmetallic inclusions to the trapping device was 39 mm, which was slightly increased as compared with Example 1 in which Ar gas was not blown.

  In Example 3 and Example 4 in which the Ar gas injection rate was increased to 9.00 N liters / minute and 11.00 N liters / minute from Example 2, the average adhesion thickness of nonmetallic inclusions on the trapping device was 37 mm. And 36 mm, which is substantially equivalent to Example 2. However, in Example 4, the pressure loss increased, and it was necessary to lower the casting speed than in Example 3, and the casting amount was 600 tons.

  On the other hand, in Comparative Example 3 in which the same amount of Ar gas as in Example 2 was blown from the tip of the trapping device without forming a swirl flow without arranging a weir, the average adhesion thickness of nonmetallic inclusions on the trapping device Was 10 mm, the same level as in Comparative Example 2, and thinner than Example 2.

  The effect of promoting the adhesion of non-metallic inclusions to the trapping device is small by simply blowing Ar gas. By blowing Ar gas into the swirling flow field, gas bubbles that trap non-metallic inclusions are trapped by centripetal force. This is thought to be due to the effect of promoting the adhesion of non-metallic inclusions by adhering to the apparatus.

  The present invention is not limited to the above example, and it goes without saying that the embodiments may be changed as appropriate within the scope of the technical idea described in each claim.

  For example, in the above example, an example in which the cylindrical weir 11 is arranged is described, but a square tube may be used as long as it is cylindrical. Moreover, the shape of the inflow port 11a provided in the weir 11 is not limited to a rectangular shape, and may be an oval shape, an elliptical shape, or the like.

2 Molten steel 4 Tundish 8 Mold 11 Weir 11a Inlet 12 Trapping device 12a Tip 13 Non-metallic inclusions

Claims (2)

A method for preventing nonmetallic inclusions contained in molten metal in a tundish from flowing out into a mold,
A cylindrical weir is provided so as to surround the molten metal pouring portion to the mold in the tundish, and a swirling flow is formed in the molten metal in the cylindrical weir,
The molten metal in the tundish is transferred to the mold in a state where the trapping device for non-metallic inclusions is maintained at the center of the swirl flow so that the tip is at a height position that secures a flow path to the molten metal pouring portion. When dispensing, the trapping device is raised in accordance with an increase in the thickness of non-metallic inclusions attached to the trapping device, and the amount of molten metal injected from the tundish into the mold is kept constant. A method for preventing non-metallic inclusions from flowing out of molten metal. Wherein the distal end of the capture device viewed Included blowing an inert gas into the molten metal, the molten metal in claim 1, characterized in that to promote the adhesion of non-metallic inclusions to the capture device, including the distal end Of preventing non-metallic inclusions from flowing out.

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