JP2000000655A - Device for discharging molten metal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a molten metal discharging device which can prevent the clogging of a molten metal passing-through hole caused by the solidification of the molten metal in the molten metal passing-through hole and prevent the deterioration of quality of a mold product. SOLUTION: The molten metal discharging hole is provided with gas equalized zones 8 arranged in the annular form over the whole periphery along the periphery wall part 12 of the molten metal passing-through hole 2 in a fixed plate 3 and divided into plural parts, gas introducing holes 10c, 10a connected with each gas equalized zone and introducing inert gas into the equalized zones 8 and small holes 13 for supplying the gas arranged so as to communicate the equalized zones 8 and the molten metal passing-through hole 2. The inert gas is supplied into an arbitrary gas equalized zone 8 through the gas introducing hole 10c, 10a and this inert gas is supplied into the molten metal passing- through hole 2 from the arbitrary portion on the peripheral wall part 12 of the molten metal passing-through hole 2 through the small holes 13.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten metal discharge device, and more particularly to an apparatus for blowing an inert gas into a molten metal passage hole to prevent the molten metal passage hole from being blocked by solidification of the molten metal, and to provide a molded product. The present invention relates to a molten metal discharging device that does not cause quality deterioration.

[0002]

2. Description of the Related Art In general, continuous casting equipment is provided with a molten metal discharge device for adjusting the flow rate of molten metal at the bottom of a ladle or tundish for storing molten metal.

[0003] This molten metal discharging device is composed of a fixed plate made of refractory material and separately provided with a molten metal passage hole, and a sliding plate provided with a communication hole.

The sliding board is slidably mounted on one surface of the fixed board, and slides the sliding board to appropriately open and close a molten metal passage hole of the fixed board to adjust the flow rate of the molten metal. is there.

In such a molten metal discharger,
Small holes are provided in the peripheral wall of the molten metal passage hole of the fixed plate, and inert gas is blown into the molten metal to agitate the molten metal.

This gas stirring has two purposes.

The purpose of the first gas stirring is to prevent clogging,
This is to prevent the molten metal from passing through the molten metal passage hole due to solidification of the molten metal and adhesion of the oxide during casting, and to blow an inert gas through the small hole into the molten metal passage hole.

[0008] The purpose of the second gas agitation is gas opening,
This gas opening closes the molten metal passage hole with the sliding disk and stores the molten metal in the molten metal passage hole and in the tundish while the molten metal and other metals contained in the molten metal in the molten metal passage hole. Since the molten metal passage hole is closed by solidification of the metal oxide, an inert gas is blown into the molten metal passage hole from the small hole.

The above-described first purpose of preventing blockage is also performed during casting in which the molten metal passage hole is opened. Therefore, bubbles may be generated during casting depending on an excessive gas injection amount or an opening position of the small hole. There is a case where a defect called a pinhole occurs in the molten steel or the like by intrusion. Therefore, in order to prevent the blockage, it is necessary to appropriately set the gas amount and the installation position of the small hole, and a relatively small gas amount of 5 to 10 l / min is required.

[0010] The above-mentioned second object gas opening is performed by blowing an inert gas from a gas injection small hole installed at an arbitrary place of the molten metal discharging device with the molten metal passage hole closed. Gas stirring is performed so that the metal does not solidify, and the molten metal is smoothly discharged when the molten metal passage hole is opened.

This gas opening is performed particularly in a tundish or ingot casting. In a tundish, the quality of a product is degraded due to foreign matter mixed in a slab. In the ingot casting, a molten metal passage hole is completely closed or fully opened. This is done in order to improve the safety of production or to improve the production yield. Therefore, for gas stirring for the purpose of this gas opening,
Generally, a large gas amount of 80 to 150 l / min is required.

Conventionally, there have been various methods for blowing an inert gas from a small hole into a molten metal passage hole of a fixed plate for gas stirring.

For example, as shown in FIG. 6, a ring-shaped pressure equalizing band 62 provided along the circumferential direction of a molten metal passage hole 61 is provided inside a fixed platen 60, and the molten metal communicating with the pressure equalizing band 62 is provided. There is a molten metal discharging device 65 in which a plurality of small holes 63 for blowing out an inert gas, which are formed in the passage hole 61, are provided on the entire peripheral wall 64 of the molten metal passage hole 61. The molten metal discharge device 65 blows out an inert gas from the entire peripheral wall 64 of the molten metal passage hole 61. In the case of the gas opening described above, the molten metal is efficiently stirred.
When the slide 1 is closed, the molten metal passage 61 closes in the direction in which the sliding board 66 moves (arrow d).
Since the small holes 63c for gas supply are also provided at the left side of the molten metal passage hole 61 in FIG. I have.

Further, as shown in FIG.
No. 93 discloses that the molten metal passage hole 71 of the sliding plate 70 is closed at the anti-molten metal passage hole closing direction side A (the molten metal passage hole 71 in FIG. 7).
A molten metal discharging device 74 in which a plurality of small holes 73a for blowing out inert gas are provided only on the wall portion 72 (on the right side of FIG. 2).

In this molten metal discharging device 74, a plurality of small holes 73a are provided only on the wall surface 72 of the sliding plate 70 on the side A of the anti-molten metal passage hole closing direction, so that air bubbles enter the slab. Although there is little possibility that a defect such as a pinhole may occur, the blowing of the inert gas is blown out of the entire peripheral wall 72 of the molten metal passage hole 71 only from the wall portion 72 on the side A of the anti-molten metal passage hole closing direction. In the case of gas opening, the molten metal is not efficiently stirred.

Further, as shown in FIG.
No. 543 discloses that, in order to efficiently perform gas stirring by the inert gas blown out from the small holes 81, the small holes 81 have a diameter such that the inert gas forms a swirling flow and blows out to the molten metal passage holes 82. A molten metal discharging device 84 is disclosed which is formed in the entire periphery of the peripheral wall portion 83 of the molten metal passage hole 82 so as to be inclined with respect to the direction.

In the molten metal discharge device 84, small holes 81 are provided on the entire peripheral wall portion 83 of the molten metal passage hole 82 so as to be inclined with respect to the radial direction. Since the small holes 81c for gas supply are also provided on the molten metal passage hole closing direction side C of 85, there is a high possibility that problems such as pinholes in which air bubbles have entered the slab may occur.

The present invention has been made in consideration of the above circumstances, and in a molten metal discharging apparatus in which molten metal gas is stirred, prevention of clogging of a molten metal passage hole due to solidification of molten metal and deterioration of quality of a molded product. It is an object of the present invention to provide a molten metal discharging device capable of preventing the molten metal.

[0019]

Means for Solving the Problems In order to achieve the above object, the invention of claim 1 of the present invention is directed to a dense refractory having a molten metal passage hole attached to the bottom of a container for accommodating molten metal. In a molten metal discharging device having a fixed plate and a sliding plate slidably provided on one surface of the fixed plate for opening and closing the molten metal passage hole, the fixed plate has a peripheral wall portion of the molten metal passage hole therein. A gas pressure equalization zone provided in a ring shape over the entire circumference along the circumference and divided into a plurality of gas equalization zones, and gas introduction holes respectively connected to the gas equalization zones and introducing an inert gas into the gas equalization zones. Having a gas supply small hole provided to communicate the pressure equalizing chamber and the molten metal passage hole,
An inert gas is supplied to any of the gas pressure equalization zones through the gas introduction holes, and the gas is supplied to the molten metal passage holes from any portion of the peripheral wall portion through the small holes. The gist of the present invention is a molten metal discharging device.

According to the second aspect of the present invention, the gas pressure equalizing zone has a semicircular shape divided into two by a center line of the molten metal passage hole orthogonal to the moving direction of the sliding board and has a gas equalizing zone on the side opposite to the closing direction. The gist of the present invention is a molten metal discharge device according to claim 1, wherein the molten metal discharge device comprises a gas pressure equalizing zone on a closing direction side.

According to the third aspect of the present invention, when the molten metal passage hole is in an open state, an inert gas is supplied only to the gas pressure equalizing zone on the side opposite to the closing direction through the gas introducing hole, and the inert gas is supplied to the small hole. The molten metal passage hole is supplied to the molten metal passage hole from a portion of the peripheral wall portion on the side opposite to the closing direction, and in the closed state of the molten metal passage hole, an inert gas is supplied to both of the gas pressure equalizing zones via the gas introduction holes. The molten metal discharge device according to claim 1 or 2, wherein the inert gas is supplied to the molten metal passage hole from the entire periphery of the peripheral wall portion through the small hole. And

According to the invention of claim 4 of the present application, a fixed refractory fixing plate attached to the bottom of a container for storing the molten metal and provided with a molten metal passage hole for passing the molten metal; A movable plate that is movably provided with a sliding plate that opens and closes the molten metal passage hole; wherein the fixed plate is provided inside the fixed plate in the molten metal flow direction of the molten metal passage hole, and a peripheral wall is provided. A gas pressure equalizing band provided in a ring shape over the entire circumference along the portion, a partial gas pressure equalizing band provided in a part along the peripheral wall portion, and a gas equalizing band provided in the gas equalizing band. A gas introduction hole for introducing an inert gas into the gas pressure equalization zone, and a gas supply small hole provided so as to communicate the molten metal passage hole with the both pressure equalization chambers; Through the hole to the whole gas equalization zone and the partial gas equalization zone When or supplying an inert gas to any of the gas pressure equalization zones, the inert gas is supplied from the entire periphery of the peripheral wall portion of the molten metal passage hole or an arbitrary portion through the small hole through the molten metal passage hole. The gist of the present invention is that the molten metal discharging device is characterized in that it is supplied to a molten metal discharging device.

According to a fifth aspect of the present invention, the partial gas pressure equalizing zone is provided substantially at half the circumference of the peripheral wall portion of the molten metal passage hole on the side opposite to the closing direction. The gist is that it is a discharge device.

The molten metal according to claim 4 or 5, wherein the gas introduction holes are provided intensively on the side opposite to the molten metal passage hole closing direction of the molten metal passage hole. The gist is that it is a discharge device.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a molten metal discharging device according to the present invention will be described below with reference to the accompanying drawings.

As shown in FIG. 1, a molten metal discharging apparatus 1 according to the present invention comprises a flat plate-shaped upper fixed plate 3 provided with a molten metal passage hole 2 and a sliding plate 5 provided with a communication hole 4 and a molten plate. It is composed of a lower fixing plate 7 provided with a metal passage hole 6.

As shown in FIGS. 1 and 2, the upper fixing plate 3 is formed of a dense refractory, and has a ring-shaped gas uniform along the peripheral wall portion 12 of the molten metal passage hole 2. A pressure band 8 is provided. The gas pressure equalizing zone 8 is divided into a plurality of pieces 8a and 8b. For example, the molten metal passes in a semicircular arc shape divided into two by a center line L of the molten metal passing hole 2 orthogonal to the moving direction of the sliding board 5. The gas pressure equalizing zone 8c in the closing direction side C (on the left side of the molten metal passing hole 2 in FIG. 1) with respect to the moving direction of the molten metal passing hole 2 (the left side of the molten metal passing hole 2 in FIG. It is divided by a partition wall 9 made of, for example, mortar into a gas pressure equalizing zone 8a on the opposite side to the moving direction C on the opposite side in the closing direction. The upper fixed platen 3 has a gas pressure equalizing zone 8c,
Gas introduction holes 10c and 10a communicating with the gas introduction pipes 8c and 11a, respectively, are provided, and gas introduction pipes 11c and 11a are connected to the gas introduction holes 8c.

Further, the upper fixed platen 3 has a through hole 2 for molten metal.
A number of small holes 13c and 13a are provided along the peripheral wall portion 12 to communicate the molten metal passage hole 2 with the gas pressure equalizing zone 8c on the closing direction side and the gas equalizing zone 8a on the opposite side of the closing direction.

As shown in FIG. 3, the molten metal discharging device 1 is provided between a mold 15 and a container for accommodating the molten metal M, like the conventional molten metal discharging device. An immersion nozzle 16 is attached below and is used to regulate the flow of the molten metal M.

Since the molten metal discharging device 1 according to the present invention has the above-described structure, as shown in FIG. 4 (a), the communicating hole 4 has the molten metal passing hole 2 communicating with the molten metal passing hole 2. 4B or the half-opened state during drawing casting as shown in FIG.
In the open state, the molten metal flows out through the molten metal passage hole 2, the communication hole 4, and the molten metal passage hole 6.

The molten metal passage hole 2 is in the open state,
That is, during the casting, solidification of the molten metal and adhesion of the oxide may occur, particularly easily on the peripheral wall portion 12 on the side A opposite to the closing direction.

Therefore, for the purpose of preventing blockage, the gas introduction hole 1
0a, the inert gas is supplied only to the gas pressure equalizing zone 8a on the opposite side of the closing direction through the small hole 13a. 2 is supplied in an appropriate amount of 5 to 10 l / min.

By blowing the inert gas, it is possible to prevent solidification of the molten metal during casting and blockage of the molten metal passage hole due to adhesion of oxide. Further, the inert gas is supplied only from the peripheral wall portion 12 on the side A opposite to the closing direction via the gas introduction pipe 10a, and the inert gas is not supplied from the side C on the closing direction. There is no entanglement in the mold 16 and no entanglement of the mold powder or generation of pinholes due to the inert gas.

As shown in FIG. 4C, when the molten metal passage hole 2 is closed by the sliding board 5, the molten metal is stored in the molten metal passage hole 2 and the tundish 14.

In the closed state of the molten metal passage 2, the molten metal passage 2 is closed by solidification of the molten metal or an oxide of another metal contained in the molten metal in the molten metal passage 2. There is a fear.

Therefore, for the purpose of gas opening for preventing solidification of the molten metal in the molten metal passage 2 and facilitating initial opening,
Both gas pressure equalizing zones 8 are provided via the gas introduction holes 10c and 10a.
An inert gas is supplied to c and 8a, and this inert gas is further supplied to the molten metal passage hole 2 from the entire periphery of the peripheral wall portion 12 through the small hole 13 to the molten metal passage hole 2 at 80 to 150 l / min.

By blowing a large amount of inert gas from the entire circumference of the peripheral wall portion 2, solidification of the molten metal in the molten metal passage hole 2 is reliably prevented, and the initial opening of the molten metal passage hole 2 is extremely reduced. Can be easier.

As described above, when the molten metal discharge device 1 is in the open state of the molten metal passage hole 2, an appropriate amount of inert gas is supplied to the molten metal passage hole 2 at the portion of the peripheral wall portion 12 on the anti-closing direction side A. To prevent solidification of molten metal and adhesion of oxides, and furthermore, mold 1 having an excessive gas supply.
In addition, it is possible to prevent the mold powder from being entangled into the mold 5 and to prevent the mold powder from being entangled or to generate a pinhole due to an inert gas.

When the molten metal passage hole 2 is closed by the slide plate 3, a large amount of inert gas is blown into the molten metal passage hole 2 from the entire periphery of the peripheral wall portion 12, and the molten metal passage hole 2 is closed.
The solidification of the molten metal in the inside is surely prevented and the molten metal passage hole 2
Greatly facilitates the initial opening of.

Next, another embodiment will be described with reference to FIG.

The molten metal discharger 30 is attached to a container (not shown) for accommodating the molten metal, and is a flat plate-shaped upper fixed plate 32 provided with a molten metal passage hole 31 and a slide provided with a communication hole 33. It is composed of a board 34 and a lower fixed board (not shown) provided with a molten metal passage hole.

The fixed plate 32 is provided with a ring-shaped gas equalizing band 36w provided in a ring shape along the peripheral wall 35 in the molten metal flow direction D of the molten metal passage hole 31 along the peripheral wall inside the fixed plate 32. And a partial gas pressure equalizing zone 36p provided in a part thereof along the peripheral wall portion 35. The partial gas pressure equalizing zone 36p is formed on the peripheral wall 35 of the molten metal passage hole 31.
Is provided substantially half way around the side A opposite to the closing direction.

Gas introduction holes 37w and 37p for introducing an inert gas into the gas pressure equalization bands 36w and 36p are provided in the gas pressure equalization bands 36w and 36p in a concentrated manner on the side A of the molten metal passage hole 31 in the non-closing direction. The pressure equalizing chambers 36w, 36w
Small hole 3 for gas supply which communicates p with molten metal passage hole 31
8w and 38p are provided.

Since the metal discharging device 30 of the present embodiment has the above-described structure, when the molten metal passage hole 31 is in the open state, the molten metal passes through the molten metal passage hole 31 and the communication hole 3.
Outflow via 3.

In such a state, for the purpose of preventing blockage, an inert gas is supplied only to the partial gas pressure equalizing zone 36p through the gas introduction hole 38p, and the inert gas is further supplied to the small hole 38p.
A suitable amount of 5 to 10 l / min is supplied to the molten metal passage hole 31 from the portion of the peripheral wall portion 35 on the side opposite to the closing direction via the through hole.

By blowing the inert gas, solidification of the molten metal during casting and blockage of the molten metal passage hole due to adhesion of oxides can be prevented. Further, the inert gas is supplied only from the peripheral wall portion 35 on the anti-closing direction side A via the partial gas pressure equalizing zone 36p, and the inert gas is not supplied from the closing direction side C. There is no excessive entanglement in the mold, no entanglement of the mold powder, and no generation of pinholes due to the inert gas.

When the molten metal passage 31 is closed, the molten metal is stored in the molten metal passage 31 and the tundish.

In such a state, for the purpose of gas opening, an inert gas is supplied to the entire circumferential gas pressure equalizing zone 36w through the gas introducing hole 37w, and the inert gas is further supplied to the peripheral wall through the small hole 38w. An appropriate amount of 80 to 150 l / min is supplied to the molten metal passage hole 31 from the entire circumference of the section 35.

Blowing of a large amount of inert gas from the entire circumference of the peripheral wall portion 31 and solidification of the molten metal in the molten metal passage hole 31 are reliably prevented, and the initial opening of the molten metal passage hole 31 is extremely easy. Can be In addition, when performing gas stirring for the purpose of gas opening as needed, the whole circumference gas equalizing zone 3
6w and partial pressure equalizing zone 36p
The inert gas may be supplied to the molten metal passage hole 31 via w and 36p.

In the molten metal discharging device 30 of the present embodiment, solidification of the molten metal and adhesion of oxides in the molten metal passage hole 31 are prevented as in the molten metal discharging device 1 described above. Further, it is possible to prevent an excessive amount of gas to be supplied from being caught in the mold, to prevent the mold powder from being caught or to generate a pinhole due to gas, and to allow the gas introduction holes 37w and 37p to be provided in the molten metal passage hole 31.
Are provided in a concentrated manner on the side A opposite to the closing direction, and the molten metal discharging device 30 can be made compact.

[0051]

The molten metal discharger according to the present invention changes the supply amount and the blowing position of the inert gas in accordance with the open / closed state of the molten metal passage hole. Prevents sticking of inert gas into the mold and prevents excessive inert gas from being caught in the mold, preventing mold powder from being caught and generating pinholes due to the inert gas. Then, the initial opening of the molten metal passage hole can be made extremely easy.

Further, in the molten metal discharge device provided with a gas pressure equalizing zone around the entire circumference and a partial gas equalizing pressure band in the flow direction of the molten metal in the molten metal passage hole, solidification of the molten metal, adhesion of oxides, etc. In addition to preventing the occurrence of pinholes, the gas introduction holes are provided in a concentrated manner on the side opposite to the closing direction of the molten metal passage hole, so that the molten metal discharging device can be made compact.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing one embodiment of a molten metal discharging device according to the present invention.

FIG. 2 is a sectional view taken along line XX of FIG. 1;

FIG. 3 is an explanatory view showing a use state of the molten metal discharging device according to the present invention.

FIG. 4 is an explanatory view of the operation of the molten metal discharging device according to the present invention.

FIG. 5 is a cross-sectional view of a conventional molten metal discharging device.

FIG. 6 is a cross-sectional view of another conventional molten metal discharging device.

FIG. 7 is a sectional view of another conventional molten metal discharging device.

FIG. 8 is a cross-sectional view of another conventional molten metal discharging device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Molten metal discharge device 2 Molten metal passage hole 3 Lower fixed plate 4 Communication hole 5 Sliding plate 6 Molten metal passage hole 7 Lower fixed plate 8 Gas equalizing band 8a Anti-closing direction side gas equalizing band 8c Closing direction side gas equalizing band Pressure zone 9 Partition wall 10a Gas introduction hole 10c Gas introduction hole 11a Gas introduction tube 11c Gas introduction tube 12 Peripheral wall 13a Small hole 13c Small hole 14 Tundish 15 Mold 16 Immersion nozzle 30 Molten metal discharge device 31 Molten metal passage hole 32 Fixed on top Board 33 Communication hole 34 Sliding board 35 Peripheral wall 36w All around gas equalizing band 36p Partial gas equalizing band 37w Gas introduction hole 37p Gas introduction hole 38w Small hole 38p Small hole

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kazuo Ito 1 Minami Fuji, Ogakie-cho, Kariya City, Aichi Prefecture Toshiba Cellular Co., Ltd. (72) Inventor Isao Watanabe 1 Minami Fuji, Ogakie-cho, Kariya City, Aichi Prefecture Toshiba Group Lamix Corporation Kariya Factory F-term (reference) 4E004 FA01 HA00 4E014 MA06 MA20

Claims (6)

[Claims] 1. A fixed board for a dense refractory which is attached to the bottom of a container for accommodating a molten metal and is provided with a through hole for a molten metal, and a through hole which is slidably provided on one surface of the fixed board. In the molten metal discharger having a sliding plate that opens and closes, the fixed plate is provided in a ring shape all around the peripheral wall of the molten metal passage hole along the peripheral wall of the molten metal passing hole, and is divided into a plurality of gas equalizers. A pressure zone, a gas introduction hole which is respectively connected to the gas pressure equalization zone and introduces an inert gas into the gas pressure equalization zone, and a gas supply provided so as to communicate the pressure equalization chamber with the molten metal passage hole. For supplying an inert gas to any one of the gas pressure equalizing zones through the gas introduction holes, and supplying the gas from any portion of the peripheral wall portion through the small holes to the molten metal. A molten metal discharging device, wherein the molten metal is supplied to a passage hole. 2. The gas equalizing band is a semi-circular shape divided into two parts by a center line of a molten metal passage hole orthogonal to the moving direction of the sliding board, and is a gas equalizing band on the side opposite to the closing direction and the gas equalizing band on the closing direction. The molten metal discharging device according to claim 1, comprising a pressure band. 3. In the open state of the molten metal passage hole, an inert gas is supplied only to the gas pressure equalizing zone on the side opposite to the closing direction via the gas introduction hole, and the inert gas is supplied through the small hole in the direction opposite to the closing direction. The molten metal passing hole is supplied from the portion of the peripheral wall portion on the side, and in a state where the molten metal passing hole is closed, an inert gas is supplied to both of the gas pressure equalizing zones via gas introduction holes. The molten metal discharge device according to claim 1, wherein gas is supplied to the molten metal passage hole from the entire periphery of the peripheral wall portion through the small hole. 4. A fixed plate made of a dense refractory, which is attached to the bottom of a container for storing the molten metal and has a molten metal passage hole for passing the molten metal, and is slidably provided on one surface of the fixed plate. In the molten metal discharging device having a sliding plate for opening and closing the molten metal passage hole, the fixed plate is provided inside the fixed plate in the molten metal flow direction of the molten metal passage hole, and is entirely along the peripheral wall portion. A gas pressure equalizing band provided in a ring shape over the circumference, a partial gas pressure equalizing band provided partially along the peripheral wall, and the gas equalizing band provided in each of the gas pressure equalizing bands. A gas introduction hole for introducing an inert gas, and a small hole for gas supply provided so as to communicate the molten metal passage hole with both of the pressure equalizing chambers, and the gas introduction hole through the gas introduction hole Simultaneously or before any of the circumferential gas equalization zone and the partial gas equalization zone Supplying an inert gas to the gas pressure equalizing zone, and supplying the inert gas to the molten metal passage hole from the entire periphery of the peripheral wall portion of the molten metal passage hole or an arbitrary portion through the small hole. Characterized molten metal discharge device. 5. The molten metal discharging device according to claim 4, wherein the partial gas pressure equalizing zone is provided on substantially a half of the peripheral wall portion of the molten metal passage hole on the side opposite to the closing direction. 6. The molten metal discharging device according to claim 4, wherein the gas introduction holes are provided in a concentrated manner on the side opposite to the molten metal passage hole closing direction of the molten metal passage hole.