JPH08276254A - Method for continuously casting high cleanliness steel by using tundish provided with openable/closable weir - Google Patents

Abstract

PURPOSE: To stably supply molten steel having high cleanliness into a mold for continuous casting in spite of its being the stationary part or unstationary part while using a tundish enabling reuse in the hot condition. CONSTITUTION: At the time of supplying the molten steel into the mold for continuous casting through the tundish provided with the tundish body 10 having opening space without a fixed weir in the inner part and a tundish weir 20 erected freely ascendable/descendable on the bottom wall of the tundish body, inert gas is introduced into the molten steel from the side surface at the upstream side of the tundish weir 20. The inert gas can be introduced from the upper surface of the tundish weir 20 in some case. Inclusions are efficiently floated up and separated from the molten steel by combining the opening/closing weir 21 freely ascendable/descendable and the introduction of the inert gas, and the molten steel having high cleanliness is poured into the mold for continuous casting.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for continuously casting high cleanliness steel using a tundish that is hot-reusable and has excellent workability for refractory materials, while efficiently floating and separating inclusions.

[0002]

2. Description of the Related Art Molten steel melted in a smelting furnace such as a converter or an electric furnace is received in a ladle, passed through a secondary refining process such as RH vacuum degassing, and then continuously cast through a tundish. It is sent to a casting mold and manufactured into a continuous cast slab. In order to improve the cleanliness of the slab, various improvements have been made regarding the operating conditions in the smelting furnace and the refining conditions in the ladle. In the ladle, various refining agents are added as needed to remove the impurity elements contained in the molten steel. Further, the molten steel may be degassed by the vacuum treatment. The molten steel whose cleanliness has been improved in this way is poured into a continuous casting mold through a tundish. However, the molten steel contacts the atmospheric gas and the refractory lining while passing through the tundish, and is easily contaminated by gas absorption, elution of the lining material, and the like. Further, in the molten steel supplied from the ladle to the tundish, inclusions such as Al ₂ O ₃ refined by the refining reaction remain without being removed from the molten steel.

The inclusions contained in the molten steel cause the immersion nozzle and the like to be blocked during continuous casting, making the casting conditions unstable. If inclusions are brought into the continuous casting slab, they will cause defects in the subsequent rolling stage and reduce the yield. Therefore, in order to remove inclusions contained in the molten steel in the tundish, various proposals have been conventionally made. For example, in Japanese Laid-Open Patent Publication No. 1-224152, a plurality of molten steels are forcibly changed in the tundish to form an upward flow to promote floating separation of inclusions contained in the molten steel. Tundish with a weir is introduced. Further, Japanese Patent Laid-Open No. 63-72452 introduces a tundish weir having a hole inclined upward with respect to the molten metal flow direction.

[0004]

When the weir is provided inside the tundish, the cleanliness of the molten steel is certainly improved by the floating separation effect. However, if the internal space of the tundish is partitioned intricately by the upper weir and the lower weir, maintenance will be troublesome, and a great deal of labor will be required to replace the weir. Also,
There is a risk that the molten steel will be contaminated by the weir, and the amount of molten steel that flows into the continuous casting mold as an unsteady flow at the beginning and end of pouring will increase, and the production yield of steel products with the required cleanliness will decrease. In addition, in a tundish with a weir, in order to discharge molten steel from the tundish at the end of pouring, an opening commonly called "mouse passage" is provided between the bottom of the tundish and the lower part of the weir. However, there are many inclusions that pass through the mouse and are discharged in a short time, which is a cause of significantly lowering the floating separation effect of the weir.

Recently, in order to improve the productivity and reduce the cost of refractory materials, it has been considered to use the tundish in the hot state for the next charge. in this case,
If there are multiple weirs in the tundish, it takes man-hours and time to repair the tundish, and the tundish cannot be ready for the next use because it is still hot. In this respect, the tundish weir is required to have a structure as simple as possible. However, the molten steel poured from the ladle contains a large amount of various inclusions. In particular, at the end of pouring, the pollution is significant as it is affected by the slag floating in the ladle. When these contaminated molten steels are supplied to the continuous casting mold, the quality of the obtained continuous casting slab is deteriorated. The tundish weir is extremely effective as a means for reducing the amount of contaminated molten steel flowing from the tundish into the continuous casting mold as much as possible, and there is a demand for a tundish having a simplified structure after securing its action.

In order to meet such demands, the present inventors have developed a tundish in which a tundish weir is provided on a bottom wall of a main body so that the tundish can be raised and lowered, and filed as Japanese Patent Application No. 7-30214. In this tundish, the tundish weir is separated from the tundish main body in an unsteady state, so that the inside of the tundish is largely opened, and the repair work is extremely easy. Therefore, it becomes possible to prepare for the next charge without changing the heat. The present invention is a further improvement of the tundish proposed in the prior application. By blowing an inert gas into the molten steel from the upstream side surface and / or upper surface of the tundish weir, agglomeration and inclusion of inclusions and floating separation are performed. The purpose of this invention is to pour molten steel with enhanced cleanliness into a casting mold for continuous casting to obtain a continuous casting piece that is sound and stable with high quality.

[0007]

In order to achieve the object, the method for continuously casting high-cleanliness steel according to the present invention has a tundish main body having an open space without a fixed weir inside and a bottom wall of the tundish main body. When supplying molten steel to a continuous casting mold through a tundish equipped with a tundish weir that is vertically movable, an inert gas is introduced into the molten steel from the upstream side surface of the tundish weir. And The inert gas can be blown into the molten steel from the upper surface of the tundish weir. Furthermore, an inert gas may be introduced from the upstream side surface and the upper surface of the tundish weir.

Hereinafter, the present invention will be described in detail together with its operation with reference to the drawings. In the tundish used in the present invention, as shown in FIG. 1, a tundish weir 20 is provided so as to be able to move up and down on a tundish main body 10 which is widened upward. The tundish body 10 has a refractory lining 12 on a furnace wall 11 constructed of refractory bricks, and has a trapezoidal cross section that widens upward. A support wall 13 is formed in the middle of the furnace wall 11 with its side surface standing up substantially vertically. The inner space of the tundish main body 10 is basically an open space, except for the supporting wall 13 which projects. The support wall 13 hardly projects from the side wall 15 near the bottom wall 14 of the tundish body 10. The inner surface 17 of the support wall 13 rises almost vertically from the bottom wall 14, so that the upper part of the support wall 13 is
As shown in FIG. 2, it projects inward from the side wall 15 that is inclined outward. The tundish of the present embodiment has a structure in which the tundish weir 20 disposed inside is supported by the support wall 13, but a structure in which the tundish weir 20 is supported by the groove portion provided in the side wall 15 instead of the support wall 13. Can also be adopted.

The tundish weir 20 is provided with an opening / closing weir 21 that moves up and down along the support wall 13 in order to prevent molten steel from remaining inside the tundish. The upstream side surface of the opening / closing weir 21 is composed of a porous refractory material 24, and the weir support column 2
3 has a structure in which an inert gas such as Ar is introduced through a gas pipe provided inside 3. In addition, opening and closing weir 2
The upper surface of No. 1 may also be constructed of a porous refractory so that an inert gas is similarly introduced. Further, a combination of the depth adjusting weir 22 (see FIG. 4) and the opening / closing weir 21 is also used so that the height from the weir top surface to the molten steel surface can be adjusted as needed. Similarly to the opening / closing weir 21, the depth adjusting weir 22 is also constructed of a porous refractory on the upstream side surface or upper surface, and has a structure into which an inert gas can be introduced. As shown in FIG. 3, the tundish body 10 is mounted on the tundish car 30 and reciprocates between the casting position and the retracted position. A drive motor 31 is loaded on the tundish car 30, and the power from the drive motor 31 raises and lowers the lift mechanism 32.
And transmitted to the tundish weir 20 via the support arm 33. The tundish car 30 is provided with a work deck 34 in which an operation panel and the like are arranged in order to move the tundish car 30 and move the tundish weir 20 up and down.

The opening / closing position of the opening / closing weir 21, and further the depth adjusting weir 22, is adjusted by the amount of movement of the lifting mechanism 32. In a steady state in which the molten steel 2 supplied from a ladle (not shown) through the long nozzle 1 is fed into the continuous casting mold (not shown) through the immersion nozzle 3, the opening / closing weir 21 is shown in FIG. As described above, the tundish body 10 is erected while being in contact with the bottom wall 14. On the other hand, the height of the depth adjusting weir 22 is adjusted according to the level of the molten steel 2 so that the depth from the level of the molten steel 2 to the top surface is optimized. The distance from the molten metal surface to the top surface of the depth adjusting weir 22 is preferably set to 1/3 or less of the depth from the molten metal surface to the bottom wall 14 of the tundish. When the depth of the top surface of the depth adjusting weir 22 exceeds 1/3 of the depth of the bottom wall 14 of the tundish, the original damming effect of the weir is weakened and a sufficient upward flow is not formed for floating inclusions. . When supplying the molten steel 2 to the tundish, the lower end surfaces of the opening / closing weir 21 and the depth adjusting weir 22 are separated from the surface of the bottom wall 14, and the immersion nozzle 3 is closed by the stopper 4. Molten steel 2 sent in accumulates in the tundish,
When the level of the molten steel 2 rises to some extent, the opening / closing weir 21 and the depth adjusting weir 22 are lowered and brought into contact with the surface of the bottom wall 14.

In this state, the inside of the tundish is divided into an upstream region and a downstream region by the tundish weir 20. The molten steel 2 supplied from the long nozzle 1 becomes an upflow 5 along the tundish weir 20 in the upstream region and flows to the vicinity of the molten metal surface, as shown by the arrow in FIG. In this process, the inclusions contained in the molten steel 2 float and separate from the molten steel 2 due to the difference in specific gravity. At this time, since Ar gas is introduced from the porous refractory material 24 of the opening / closing weir 21 and the depth adjusting weir 22, the inclusions are reliably captured by the bubbles generated inside the molten steel 2 and the gas bubbles The levitation drive force is applied to further promote levitation separation. When the flux layer 6 is suspended on the molten metal surface, the floating inclusions are efficiently captured by the flux layer 6. The molten steel 2 from which the inclusions have been floated and separated becomes a descending flow 7 having high cleanliness, flows into the downstream region, and is supplied to the continuous casting mold through the immersion nozzle 3.

In order to efficiently carry out the floating separation, the upward flow 5 needs to pass as close to the molten metal 2 molten metal surface as possible. However, when the amount of hot water supplied from the long nozzle 1 is larger than the amount of pouring molten metal into the continuous casting mold, the molten metal level in the tundish rises. Further, when the molten steel 2 supplied from the ladle is low or when the ladle is replaced, the molten steel in the tundish decreases as continuous casting continues, and the molten metal level decreases. In this case, the height position of the depth adjusting weir 22 is adjusted by the elevating operation of the elevating mechanism 32 according to the fluctuation of the molten steel 2 level. For example, in a tundish in which the molten steel 2 is stored at a depth of 1200 mm in a steady state, the depth from the molten metal surface of the molten steel 2 to the top surface of the depth adjusting weir 22 is maintained at about 600 mm. The height position of the depth adjusting weir 22 is adjusted. As a result, the molten steel 2 rising along the tundish weir 20 flows into the downstream region from the upstream region under certain conditions suitable for floating separation of inclusions.

When the continuous casting work for one cast is completed and the next cast is prepared, the opening / closing weir 21 and the depth adjusting weir 2 are prepared.
2 together with the lifting mechanism 32, the tundish body 10
Pull up from. The tundish main body 10 in which the opening / closing weir 21 and the depth adjusting weir 22 are pulled up is an open space in which only the support wall 13 projects, so that repair work and the like becomes extremely simple. Therefore, it is possible to prepare for the next cast without changing the heat. In the above example, the depth adjusting weir 2
Although the case of using the tundish weir 20 including 2 has been described, the depth adjusting weir 22 may be omitted.
In this case, it is preferable to use the opening / closing weir 21 having a height of 1/3 to 4/5 of the depth from the bottom wall 14 to the molten metal surface in the steady state. As a result, an appropriate upward flow is formed, the inclusions are efficiently floated and separated, and the adverse effects such as the intrusion of the floating inclusions and the slag due to the extremely disturbed surface of the molten metal are prevented.

[0014]

EXAMPLE A low carbon Al killed steel produced in a converter-RH vacuum degassing process was continuously cast via a tundish shown in FIG. Tundish has a steady bath depth of 1
200 mm, molten steel amount is about 65 tons. The height of the weir is
The length was constantly set to 600 mm, and when the height of the molten metal in the tundish fluctuated, the distance from the molten metal to the top surface of the depth adjusting weir 22 was adjusted to 600 mm. Further, during the molten steel injection, Ar gas was introduced from the upstream side surface of the opening / closing weir 21 and the depth adjusting weir 22 at a flow rate of 100 Nl / min. For comparison, as shown in FIG. 5, the central weir 41, the upper weir 42, and the outer lower weir 43 are sequentially arranged from the upstream side to the downstream side, and the mouse 44 is provided in the central weir 41 and the outer lower weir 43. The same volume of tundish with a triple weir 40 is used. Then, a low carbon Al killed steel that was melted in the same manner as in the example was continuously cast.

Molten steel was sampled at the tundish outlet at the time of replacement of the stationary part and the ladle, and the total oxygen content T. [O] _TD was determined. The total oxygen content T.
[O] _TD and RH Total oxygen content in molten steel after vacuum degassing T.
The ratio with [O] _RH was calculated as the inclusion emission rate η. The inclusion discharge rate η is shown in FIG. 6 in comparison between the example and the comparative example. As shown in FIG. 6, in the stationary part of the embodiment, η =
An inclusion discharge rate of 0.2 was obtained, and η = in the stationary part of the comparative example.
It can be seen that the inclusions brought into the continuous casting mold are halved compared to 0.4. Further, when the ladle was replaced, η = 0.5 in the comparative example, which was slightly inferior to the stationary part in cleanliness, but in the example, the inclusion discharge rate was η = similarly to the stationary part.
It was stable at a low value of 0.2. From this, it was confirmed that molten steel with high cleanliness can be stably supplied to the continuous casting mold according to the present invention.

[0016]

As described above, the tundish of the present invention uses the tundish in which the tundish weir that divides the inside into the upstream region and the downstream region is provided so as to be openable and closable.
Inert gas is blown into the molten steel from the upstream side surface or upper surface of the tundish weir. Therefore, the inclusions that accompany the molten steel flow rising along the tundish weir are efficiently captured by the gas bubbles, and the floating separation is promoted.
Moreover, the molten steel rises with the distance from the molten metal surface to the top surface of the weir kept constant, and is subjected to the action of separating the inclusions by blowing the inert gas, so that the molten steel with high cleanliness is maintained regardless of the steady or unsteady part. Stable supply to the continuous casting mold. Further, in the state where the tundish weir is removed, the internal space of the tundish is largely opened, and the repair work and the like can be extremely simplified, and the next cast can be prepared while it is hot.

[Brief description of drawings]

[Figure 1] Tundish equipped with an openable and closable tundish weir

FIG. 2 is a perspective view of the tundish body.

[Figure 3] Tundish car equipped with a tundish

FIG. 4 is a diagram for explaining molten steel flow in a tundish.

FIG. 5: Tundish with triple weir with mouse loop

FIG. 6 is a graph comparing inclusion emission rates between an example and a comparative example.

[Explanation of symbols]

1: Long nozzle 2: Molten steel 3: Immersion nozzle
4: Stopper 5 :: Upflow 6: Flux layer 7: Downflow 10: Tundish body 11: Furnace wall 12: Refractory lining 13: Support wall 14: Bottom wall 15: Side wall 17: Inner surface of support wall 20: Tundish weir 21: Opening and closing weir 22: Depth adjusting weir
23: Weir support column 24: Porous refractory material 30: Tundish car 31: Drive motor 32: Lifting mechanism 33: Support arm 34: Work deck 4
0: Mie weir 41: Central lower weir 42: Upper weir 4
3: Outer weir 44: Mouse through

Claims (2)

[Claims] 1. A casting mold for continuous casting through a tundish provided with a tundish main body having an open space without a fixed weir inside, and a tundish weir vertically erected on a bottom wall of the tundish main body. When supplying molten steel, an inert gas is introduced into the molten steel from the upstream side surface of the tundish weir, the continuous casting method for high-cleanliness steel. 2. A continuous casting mold is obtained through a tundish provided with a tundish body having an open space without a fixed weir inside and a tundish weir that is erected vertically on a bottom wall of the tundish body. A continuous casting method for high cleanliness steel, characterized in that an inert gas is introduced into the molten steel from the upper surface of the tundish weir when the molten steel is supplied.