JPH09122852A - Continuous casting method for high cleanness steel with using tundish arranged with gate capable of opening/ closing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To pour the highly stabilized high cleanness steel with using a tundish reusable while being at hot state. SOLUTION: When continuously casting with using the tundish arranged with a tundish gate 20 erected elevatable on the bottom wall of a tundish main body 10 in which the inside is open space without a fixed gate, the set position and height of tundish gate 20 are controlled under the condition that the relation (0.007×L<=D<=0.02×L) and the relation 0.33×H<=h<=0.67×H are satisfied. In the relations, D is a horizontal distance (mm) from molten steel pouring flow of ladle to tundish gate, (h) is a height (mm) from tundish bottom wall to top face of tundish gate, L is a horizontal distance (mm) from pouring flow of ladle to outlet hole to mold for continuous casting, H is a depth of steel bath in tundish.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention uses a tundish that is hot-reusable and has excellent refractories workability, and efficiently floats and separates inclusions in molten steel to continuously separate high-cleanliness steel. Regarding the method of casting.

[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 increase the cleanliness of the slab, various improvements have been made regarding the operating conditions in the refining furnace, the refining conditions in the ladle, and the like. 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 comes into contact with the atmosphere 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. In the molten steel supplied from the ladle to the tundish, inclusions such as Al ₂ O ₃ produced by the refining reaction remain without being removed from the molten steel.

[0003] Inclusions contained in the molten steel cause blocking of an immersion nozzle and the like during continuous casting, and make 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 cleanliness of the molten steel that flows into the continuous casting mold as an unsteady flow at the beginning of pouring and the end of pouring is low. 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 quite a lot of inclusions that pass through the mouse and are discharged in a short time.
This is a cause of a significant decrease in 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, and in particular, at the end of the pouring, it is affected by slag floating in the ladle and is significantly contaminated. 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 out from the tundish into the continuous casting mold as much as possible, and a tundish having a simplified structure after ensuring its action is desired.

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 is possible to prepare for the next charge without changing the heat. The present invention was found in the process of continuous casting using the previously proposed tundish weir, and by optimizing the installation position and height of the tundish weir, agglomeration of inclusions The purpose of the present invention is to further enhance the cleanliness of molten steel by further promoting coalescence and floating separation, and to obtain a continuous cast piece that is sound and stable with high quality.

[0007]

In order to achieve the object, the continuous casting method of the present invention is erected vertically on a bottom wall of a tundish body having an open space without a fixed weir. When continuously casting using a tundish equipped with a tundish weir, it is characterized in that the installation position and height of the tundish weir are adjusted under conditions satisfying the expressions (1) and (2). . 0.07 × L ≦ D ≦ 0.20 × L (1) 0.33 × H ≦ h ≦ 0.67 × H (2)

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 in 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. In the middle part of the furnace wall 11,
A support wall 13 whose side surface rises up substantially vertically is formed. The internal space of the tundish body 10 is the support wall 1
There is nothing protruding except for 3, and it is basically an open space. The support wall 13 hardly projects from the side wall 15 near the bottom wall 14 of the tundish body 10. The inner side surface 17 of the support wall 13 rises substantially vertically from the bottom wall 14, so that the upper part of the support wall 13 projects inward from the outwardly sloping side wall 15, as shown in FIG. . This tundish 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 a groove portion provided in the side wall 15 instead of the support wall 13 is adopted. You can also

In order to prevent molten steel from remaining inside the tundish, the tundish weir 20 moves up and down along the support wall 13 as shown in FIG. 4, and the distance from the weir top surface to the molten steel level. Depth adjustment weir 22 so that
(See FIG. 4). Tundish body 10
Is mounted on the tundish car 30 as shown in FIG. 3, and reciprocates between the casting position and the standby position. A drive motor 31 is loaded on the tundish car 30, and power from the drive motor 31 is transmitted to the tundish weir 20 via the elevating mechanism 32 and 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 distance from the level of the molten steel 2 to the top surface is appropriate. In general, the height and installation position of the tundish weir have a great influence on the floating property of inclusions in the tundish, and it is very important to optimize them for obtaining high cleanliness steel. Therefore, the present inventors conducted a water model experiment using a hollow spherical silica balloon as a simulated inclusion in order to understand the situation in which the simulated inclusion injected from the inlet of the long nozzle 1 flows out into the mold. In the water model experiment, as shown in FIG. 5, from the ladle molten steel injection flow discharged from the long nozzle 1, the tundish weir 2
The horizontal distance D up to 0 and the height h from the tundish and tundish bottom wall 14 to the top surface of the tundish weir are variously changed to investigate the influence of the horizontal distance D and the height h on the outflow rate of the simulated inclusions. did.

The survey results are shown in FIG. In Fig. 6, the horizontal axis represents the ratio D / L of the horizontal distance D from the ladle molten steel injection flow to the tundish weir and the horizontal distance L from the ladle molten steel injection flow to the outflow hole to the continuous casting mold. The vertical axis represents the ratio h / H of the height h from the bottom wall to the top surface of the tundish weir and the depth H of the steel bath in the tundish. Then, the ratio (%) of the amount of outflow inclusions when the weir was used to the amount of outflow inclusions when the weir was not provided was arranged in the relationship of D / L-h / H.
As is clear from FIG. 6, D / L = 0.07 to 0.20
And when the weir was installed under the condition of h / H = 0.33 to 0.67, the outflow rate of inclusions was suppressed to half or less as compared with the case where the weir was not provided.

When D / L and h / H deviate from the above ranges, the outflow rate of inclusions increases. The reason for this is presumed as follows from the results of visual observation during the water model experiment. That is, when D / L does not reach 0.07 with respect to the installation position of the weir, the ladle molten steel injection flow that flows in via the long nozzle 1 overflows to the outside of the weir 20, so that the damming effect is partially impaired. Be done. On the other hand, when D / L exceeds 0.20, the stirring / floating upstream utilizing the energy of the ladle molten steel injection flow diffuses, weakening its effect and the outflow hole 3 to the continuous casting mold.
Disturbances occur in the flow of molten steel in the vicinity, and inclusions easily flow out. Regarding the height of the weir, if h / H is less than 0.33, the weiring effect becomes insufficient and the proportion of inclusions brought to the downstream side of the weir increases. Conversely, h / H is 0.67
When it exceeds, the weir top surface approaches the molten steel surface, and the flow velocity of the molten steel 7 passing over the weir top surface increases. As a result, the molten steel surface is roughened as a result. Tundish slag 6 invading or floating on the surface
Are involved and contaminate the molten steel 2. Thus, the height of the tundish weir, that is, the tundish bottom wall 14
To the top surface of the depth adjusting weir 22 and the horizontal distance D from the ladle molten steel injection flow to the tundish weir are appropriately adjusted to maintain the floating effect of inclusions in the tundish at a high level. can do.

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 attached to the bottom wall 1.
4 and the immersion nozzle 3 is closed by a stopper 4. When the fed molten steel 2 accumulates in the tundish and the molten metal 2 has a high molten metal surface 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, if a part of the opening / closing weir 21 and the depth adjusting weir 22 is made of porous brick and Ar gas is introduced from the porous brick, inclusions are reliably captured in the gas bubbles and the levitation driving force of the gas bubbles is added. Further, floating separation is promoted. In addition, the flux layer 6 on the molten metal surface
When the particles are suspended, the floating inclusions are efficiently absorbed by the flux layer 6. The molten steel 2 in which the inclusions are floated and separated becomes a descending flow 7 having high cleanliness and flows into the downstream region,
It is supplied to the continuous casting mold through the immersion nozzle 3.

In order to efficiently carry out the floating separation, it is necessary that the ascending flow 5 passes 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. Therefore, it is preferable to adjust the height position of the depth adjusting weir 22 by the elevating operation of the elevating mechanism 32 according to the fluctuation of the molten steel 2. 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 2 to the top surface of the depth adjusting weir 22 is 600.
The height position of the depth adjusting weir 22 is adjusted so that it is maintained at about mm. 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 you have finished the continuous casting work for one cast and are preparing for the next cast,
Both the opening / closing weir 21 and the depth adjusting weir 22 are pulled up from the tundish main body 10 by the elevating mechanism 32. Opening and closing weir 2
1 and the tundish body 10 in which the depth adjusting weir 22 is pulled up is an open space in which only the support wall 13 projects, so that the repair work and the like becomes extremely simple. Therefore, it is possible to prepare for the next charge without changing the heat.

[0015]

EXAMPLE A tundish shown in FIG. 7 was used to continuously cast low carbon Al killed steel melted in a converter-RH vacuum degassing process. Mold width is 1200mm, slab thickness is 250
mm, casting speed was 1.4 m / min. In the tundish, the horizontal distance from the ladle molten steel injection flow to the outflow hole to the continuous casting mold was L = 3000 mm, the steel bath depth in the steady state was H = 1200 mm, and the molten steel amount of about 65 tons was poured. Regarding the tundish weir, the horizontal distance from the ladle molten steel injection flow to the tundish weir was set to D = 300 mm, and the height to the top of the weir in a steady state was set to h = 600 mm.
When the height of the molten metal in the tundish fluctuates, the distance from the molten metal to the top surface of the depth adjusting weir 22 is 600
The depth adjusting weir 22 was moved up and down so that the depth was adjusted to mm. Under this condition, D / L = 0.10 and h / H = 0.50. For comparison, using the same tundish,
The horizontal distance D and the height h were changed as shown in Table 1, and continuous casting was performed under the same conditions.

[0016]

As Comparative Example 5, a central lower weir 41, an upper weir 42 and an outer lower weir 43 are arranged in this order from the upstream side to the downstream side as shown in FIG. Using a tundish of the same capacity in which a triple weir 50 provided with a screw thread 44 provided in 43 was installed, a low carbon Al killed steel produced in the same manner as in the example was continuously cast. Molten steel was sampled at the tundish outlet when the stationary part and the ladle were replaced, and the total oxygen content T. [O] _TD and RH
Total oxygen content in molten steel after vacuum degassing T. [O] The ratio to _RH was calculated as the inclusion outflow rate η. A large difference in the inclusion outflow rate η was observed between the example and the comparative example, as shown in comparison with FIG. 8. That is, in the stationary part of the example, the inclusion outflow rate η = 0.2 was obtained, and the comparative example 5 using the triple weir was used.
It was found that the amount of inclusions discharged into the mold was halved compared to η = 0.4 in the stationary part of. Further, when the ladle of Comparative Example 5 was replaced, η = 0.5, which was inferior to the stationary part in cleanliness, but when the ladle of Example was replaced, η = 0.
It was stable at 2 and low. Further, in Comparative Examples 1 to 4 in which the installation position and height of the weir were changed, η = 0.4 to 0.6, which was equivalent to or slightly inferior to the triple weir. Summarizing the above results, by setting the installation condition of the weir to an appropriate range, a tundish having a simpler structure than the triple weir is used, and the effect of floating inclusions that surpasses the triple weir is achieved. It can be seen that a highly clean slab is produced.

[0018]

As described above, the present invention uses the tundish in which the tundish weir that divides the inside into the upstream region and the downstream region can be opened and closed and the height can be adjusted,
By appropriately adjusting the installation position and height of the tundish weir, the floating separation of inclusions is promoted and the steel cleanliness steel can be supplied to the continuous casting mold. Therefore, it is possible to continuously cast highly clean high-cleanliness steel not only in a steady state but also in a non-steady state such as ladle exchange. Further, in the state in which 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 charge can be prepared while being hot.

[Brief description of the 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 is a diagram showing an arrangement of a weir and a tundish according to the present invention.

FIG. 6 is a graph showing the influence of the weir installation position and height on the outflow rate of inclusions.

FIG. 7: Tundish with triple weir with mouse loop

FIG. 8 is a graph showing inclusion outflow rate η in Examples and Comparative Examples.

[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 adjustment weir 30: Tundish car 3
1: Drive motor 32: Lifting mechanism 33: Support arm 34: Work deck

Front page continuation (72) Inventor Yoshihiro Murakami 11-1 Showa-cho, Kure City, Hiroshima Prefecture Nisshin Steel Co., Ltd. Technical Research Institute

Claims (1)

[Claims] 1. When continuously casting using a tundish equipped with a tundish weir that is vertically erected on a bottom wall of a tundish body having an open space without a fixed weir, A continuous casting method for high-cleanliness steel, which comprises adjusting the installation position and height of a tundish weir under conditions satisfying (1) and (2). 0.07 × L ≦ D ≦ 0.20 × L (1) 0.33 × H ≦ h ≦ 0.67 × H (2)