JPH10249498A - Method for continuously casting high cleanliness steel with tundish providing field weir closing bottom part - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently float up and separate inclusion by using a tundish excellent in workability to refractory. SOLUTION: This casting method uses the tundish 10 having the fixed weirs 20 closing the bottom parts and dips and immersion tube 30 surrounding a long nozzle 1 on the molten steel surface at the upper part of the center between the fixed weirs. At this time, the inner diameter D and the dipping depth (h) of the immersion tube are regulated to the condition satisfying the inequalities. 0.70×L<=D<=1.30×L, 0.10×H<=h<=0.30×H. In the inequalities, D is the inner diameter of the immersion tube (mm), (h) is the dipping depth of the immersion tube (mm), L is the horizontal distance between the fixed weirs (mm) and H is the height from the bottom wall of the tundish to the top surface of the fixed weir (mm).

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, which can efficiently float and separate inclusions using a tundish excellent in refractory workability.

[0002]

2. Description of the Related Art Molten steel produced in a refining furnace such as a converter or an electric furnace is received in a ladle, passes through a secondary refining process such as RH vacuum degassing, etc., and then is passed through a tundish for continuous casting. It is fed into a 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. The molten steel whose purity is improved is poured into a continuous casting mold via 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 addition, to molten steel supplied to the tundish from the ladle,
Inclusions such as Al ₂ O ₃ generated 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. When the inclusions are brought into the continuous casting slab, they cause flaws in the subsequent rolling stage, and lower the yield. Various proposals have been made to remove inclusions contained in molten steel in a tundish. For example, Japanese Patent Application Laid-Open No. 63-72452 discloses a fixed weir provided with a hole that is inclined upward with respect to the flow direction of the molten metal. In addition, a fixed weir is installed between the tundish body and the molten steel injection part from the ladle. Also, Japanese Patent Application Laid-Open No. 1-22
In Japanese Patent No. 4152, a lower weir and an upper weir form a triple weir, and the flow direction of molten steel from a passage hole provided below the lower weir is forcibly changed to generate an upward flow, which is included in the molten steel. A tundish that promotes flotation of inclusions is introduced.

[0004]

When the weir is provided inside the tundish, the cleanliness of the molten steel is certainly improved by the flotation effect. However, even if the inclusions float up to the surface of the molten steel due to the weir, there are quite a lot of inclusions that do not stay at the surface of the molten steel but are drawn back into the tundish together with the molten steel flow, which is problematic in terms of separation efficiency. Was. Some of the slag generated in the refining stage, which remains in the ladle, flows out together with the molten steel at the end of pouring from the ladle, and the slag is brought into the mold without being floated and separated in the tundish. If so, there is a problem that it leads to contamination of molten steel and further deterioration of quality of the continuous cast slab.

If the internal space of the tundish is partitioned in an intricate manner by an upper weir and a lower weir, maintenance and management are troublesome and replacement of the weir requires a great deal of trouble. In addition, in a tundish with a lower weir, an opening commonly called "mouse through" is provided between the bottom of the tundish and the lower part of the weir in order to discharge molten steel at the end of pouring from the tundish. . There are quite a lot of inclusions that pass through the rat and are discharged in a short period of time, causing a significant decrease in the floating separation effect of the weir. not only that,
After the end of one cast, the remaining lump is not separated by the weir due to the rat and becomes a huge series of lump. As a result, it takes a lot of trouble and time to discharge and dispose of the residual lump.

Recently, in order to increase productivity and reduce refractory costs, it has been studied to use a tundish in a hot state for the next casting. in this case,
If there is more than one weir in the tundish, or if the weir has a complex shape, it will take time and effort to repair the tundish, and it will be possible to use it while it is still hot and ready for the next use. difficult. In this regard, weirs are required to have as simple a structure as possible. However, molten steel poured from a ladle contains a large amount of various inclusions, and is particularly affected by slag floating in the ladle at the end of pouring from the ladle, and is significantly contaminated. When the contaminated molten steel is fed into the continuous casting mold, the quality of the resulting continuously cast slab is reduced. A fixed weir is extremely effective as a means of minimizing contaminated molten steel flowing out of the tundish into a continuous casting mold, and a tundish having a simplified structure while securing the function of the fixed weir is desired.

[0007] The present invention has been devised in order to meet such a demand. The present invention simplifies the internal structure of a tundish while maintaining the function of a fixed weir for efficiently floating and separating inclusions and slag. To make management easier,
The purpose is to enable hot reuse of the tundish.

[0008]

SUMMARY OF THE INVENTION In order to achieve the object, the present invention provides a continuous casting method using a tundish having a fixed weir with a closed bottom, and a long casting surface above the fixed weir. By immersing the dip tube so as to surround the nozzle, the floating inclusions are efficiently separated from the molten steel,
In addition, it is characterized in that slag brought from the ladle is prevented from being mixed into the mold. More desirably, the immersion depth and the inner diameter of the immersion tube are regulated to conditions satisfying the equations (1) and (2). 0.70 × L ≦ D ≦ 1.30 × L (1) 0.10 × H ≦ h ≦ 0.30 × H (2) where D: inner diameter of immersion tube (mm) h : Immersion depth of the dip tube (mm) L: Horizontal distance between fixed weirs (mm) H: Height from tundish bottom wall to fixed weir top surface (m)
m)

[0009]

The present invention will be described below in detail with reference to the drawings. In the tundish used in the present invention, as shown in FIG. 1, a fixed weir 20 is fixed to a tundish main body 10 spreading upward, and the bottom of the tundish is sealed by the fixed weir 20. Tundish body 1
Numeral 0 indicates that the refractory lining 12 is provided on the furnace wall 11 on which the refractory bricks have been constructed, and has a trapezoidal cross section that expands upward. FIG. 2 shows an outline of a tundish used in the present invention. Fixed weir 2 provided on tundish body 10
Molten steel 2 is poured from a ladle (not shown) through a long nozzle 1 at substantially the center of 0 and 20. An immersion tube 30 is immersed in the periphery so as to surround the long nozzle 1 at the center. The immersion tube 30 may be cylindrical or rectangular. The flux layer 6 is suspended on the molten steel surface. The molten steel 2 is injected into the continuous casting mold from the immersion nozzle 3 while being adjusted by the stopper 4 and a sliding nozzle (not shown).

When the molten steel 2 is supplied to the tundish, at the start of pouring from the ladle, the fed molten steel 2 accumulates inside the fixed weir 20. Then, when the molten metal surface of the molten steel 2 reaches the top surface of the fixed weir 20, it flows out of the fixed weir 20. In this state, the inside of the tundish is
Is divided into an upstream area and a downstream area. Long nozzle 1
As shown by the arrows in FIG. 2, the molten steel 2 supplied from the upstream becomes a rising flow 5 along the fixed weir 20 in the upstream region and flows to the vicinity of the molten metal surface. In the process of ascent, the inclusions contained in the molten steel 2 float and separate from the molten steel 2 due to the specific gravity difference. In addition, the slag brought from the ladle also has a lower specific gravity than molten steel, so it quickly floats and floats and separates inside the immersion pipe. At this time, when a part of the fixed weir 20 is made of porous brick and Ar gas is introduced from the porous brick, inclusions are surely captured by the gas bubbles, and the levitation driving force of the gas bubbles is applied to further promote the levitation separation. You. Moreover, when the flux layer 6 is floated on the surface of the molten metal, the floating inclusions and slag are efficiently absorbed by the flux layer 6. The molten steel 2 from which inclusions and slag have been separated flows downflow 7 while maintaining high cleanliness, flows into the downstream region, and is supplied to the continuous casting mold via the immersion nozzle 3. At the end of pouring from the ladle, the molten steel 2 supplied from the ladle is significantly contaminated by the influence of slag and the like. However, the contaminated molten steel 2
Since it does not accumulate inside the immersion tube 30 and is not brought into the continuous casting mold, the quality of the continuous cast slab obtained is not reduced.

When the continuous casting operation for one cast is completed and the next cast is prepared, the remaining mass in the tundish is taken out and the refractory is repaired. At this time, since there is no generation of a huge residual mass connected over the entire surface of the tundish bottom which is generated by a fixed weir having a ratchet formed therein, the treatment of the residual mass becomes extremely easy. In addition, the number of fixed weirs is as small as necessary, one per strand, and has a simple structure such as no mouse penetration, so that refractory construction work is extremely simple. Also, since the dip tube is independent of the tundish, inspection and repair can be performed separately from the tundish. If the degree of deformation and wear is slight, it can be reused after removing the deposits.

Generally, the influence of the size and immersion depth of the immersion tube on the floating separation of inclusions and slag in the tundish is great, and it is necessary to optimize the size and immersion depth of the immersion tube to achieve high cleanliness. It is very important in obtaining steel. Therefore, in order to grasp the situation in which inclusions and slag flow out into the mold in a tundish using the long nozzle 1 as shown in FIG. A water model experiment was performed with a simulated slag floating on an object and a tundish surface. In the water model experiment, the inner diameter D of the immersion tube and the immersion depth h of the immersion tube were variously changed, and the influence of the inner diameter D of the immersion tube and the immersion depth h on the outflow ratio of the simulated inclusion and the slag was investigated.

FIG. 3 shows the results of the investigation. In FIG. 3, the horizontal axis represents the ratio D / L of the inner diameter D of the immersion tube to the horizontal distance L between the fixed weirs, and the immersion depth h of the immersion tube and the height H from the bottom wall of the tundish to the top surface of the fixed weir. The ratio h / H with respect to the vertical axis is plotted on the vertical axis. Then, the ratio (%) of the (inclusion + slag) outflow amount when using the immersion tube to the (inclusion + slag) outflow amount when the immersion tube is not provided is arranged in a relation of D / L−h / H, (Inclusion + slag) Shown by the contour line of the outflow rate η. As is clear from FIG. 3, D / L = 0.70 to 1.30 and h / H = 0.30.
When the immersion tube is used for a tundish with a fixed weir under the condition of 10 to 0.30, the outflow ratio of the simulated inclusions and the simulated slag is suppressed to 70% or less as compared with the case where the immersion tube is not provided. Was.

When D / L and h / H are out of the above-mentioned ranges, the (inclusion + slag) outflow rate increases. (Inclusion +
The cause of the increase in the slag flow rate is estimated as follows from the results of visual observation during the water model experiment. That is, regarding the inner diameter D of the immersion pipe, when the D / L does not reach 0.70, the molten steel flow turned upward by the fixed weir reaches the vicinity of the molten metal surface outside the immersion pipe, and slag and floating on the molten metal surface are raised. The entangled inclusion is wrapped deep inside the tundish again. Conversely, if the D / L exceeds 1.30, the upflow utilizing the energy of the ladle molten steel injection flow disturbs the molten metal surface in the immersion pipe, and involves inclusions and slag that have been dammed inside the immersion pipe. And it will flow out of the dip tube. Regarding the immersion depth of the immersion tube, if h / H is less than 0.10, the damming effect becomes insufficient, and the proportion of inclusions and slag brought outside the immersion tube increases. Conversely, if h / H exceeds 0.30, the flow path of the molten steel between the immersion pipe and the fixed weir becomes narrow, and the flow velocity of the molten steel passing between the immersion pipe and the fixed weir increases. As a result of the surface being roughened in the opposite direction, the floating inclusions enter the molten steel again, or the slag floating on the surface is involved and contaminates the molten steel. As described above, by appropriately adjusting the inner diameter D and the immersion depth h of the immersion tube, the inclusion floating effect and the slag separation effect in the tundish can always be maintained at a high level.

[0015]

Next, an embodiment in which molten steel is continuously cast will be described. Using the tundish shown in FIG.
The low carbon Al killed steel melted in the vacuum degassing process was continuously cast. Mold width is 1200mm, slab thickness is 250mm,
The casting speed was 1.4 m / min. The tundish has a bath depth of 1200 mm in a steady state and contains about 65 tons of molten steel. The fixed weir was installed at a horizontal distance of 300 mm downstream from the center of the long nozzle. That is, the horizontal distance between the fixed weirs is 600 mm. The height from the tundish bottom wall to the fixed weir top surface was 600 mm. On the other hand, for the immersion tube, the inner diameter was 600 mm and the immersion depth was 120 mm.
mm and arranged above the center between the fixed weirs. Under these conditions, D / L = 1.00 and h / H = 0.20. As Comparative Examples 1 to 4, using the same tundish, the inner diameter D of the immersion tube and the immersion depth h were changed as shown in Table 1, and continuous casting was performed under the same conditions.

[0016]

As a comparative example 5, a central lower weir 41 and an upper weir 4 from the upstream side to the downstream side as shown in FIG.
2 and an outer lower weir 43 in this order, using a tundish of the same capacity in which a triple weir 40 in which a ratchet 44 is provided in the central lower weir 41 and the outer lower weir 43 is used. A carbon Al killed steel was continuously cast. At regular times and when the ladle is replaced, the molten steel is sampled at the outlet of the tundish, and the total oxygen content T. [O] _TD and RH Total oxygen content in molten steel after vacuum degassing. [O] The ratio to _RH was calculated as the (inclusion + slag) outflow rate η. A large difference was found between the example and the comparative example in the outflow rate η (inclusion + slag) as shown in FIG. That is, in the steady state of the embodiment, the (inclusion + slag) outflow rate η
= 0.2 or less, and the amount of (inclusions + slag) flowing out to the mold was reduced to 2/3 as compared with η = 0.3 or more in the steady state of Comparative Example 5 using the triple weir. I knew it was there. In addition, when the ladle was replaced in Comparative Example 5, the cleanliness was η = 0.4, which was lower than that in the steady state. It was low and stable. Comparative Examples 1-4 in which the inner diameter and the immersion depth of the immersion tube were changed.
In this case, η was 0.3 to 0.6, which was equivalent to or slightly inferior to that of the triple weir. Summarizing the above results, by setting the installation conditions of the immersion pipe in the appropriate range, using a tundish with a simpler structure than the triple weir,
Moreover, it can be seen that the effect of separating inclusions and slag surpasses that of the triple weir and that a slab with high cleanliness is manufactured.

[0018]

As described above, the present invention uses a tundish having a fixed weir with a closed bottom, and immerses a dip tube in the upper surface of the fixed weir so as to surround the long nozzle. Thereby, the floating inclusions can be efficiently separated from the molten steel, and the slag brought from the ladle can be prevented from being mixed into the mold. Inner diameter D of immersion tube
By limiting the immersion depth h to an appropriate condition, the penetration of inclusions and slag into the mold can be suppressed low, and a continuously cast slab with extremely high cleanliness can be manufactured. According to the present invention, it is possible to continuously cast a highly clean steel with a high degree of stability, not only in a steady state but also in an unsteady state such as when a ladle is replaced. Further, even when the remaining lump in the tundish is processed after the end of a series of castings, the lump is divided into small lump by the fixed weir, so that the extraction of the remaining lump becomes extremely easy. Furthermore, it is only necessary to construct a minimum number of weirs having a simple structure without a ratchet and without a circulation hole, which is simple from the viewpoint of refractory workability.

[Brief description of the drawings]

Fig. 1 Tundish with a fixed weir that seals the bottom

[Figure 2] Outline of tundish

FIG. 3 is a graph showing the effect of the inner diameter of the immersion tube and the immersion depth on the outflow ratio of inclusions and slag.

Fig. 4 Tundish with triple weir with mouse through

FIG. 5 is a graph showing the (inclusion + slag) 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 14: Bottom wall 15: Side wall 20: Fixed weir 30: Immersion pipe 40: Triple weir 41: Central lower weir 42: Upper weir
43: Outer lower weir 44: Mouse rattle

Claims (2)

[Claims] 1. A continuous casting method using a tundish having a fixed weir with a closed bottom, wherein a dip tube is immersed in a molten metal above the fixed weir so as to surround a long nozzle.
High cleanliness steel continuous casting method. 2. A method for continuously casting high cleanliness steel, wherein the inner diameter and the immersion depth of the immersion tube according to claim 1 are regulated to conditions satisfying the formulas (1) and (2). 0.70 × L ≦ D ≦ 1.30 × L (1) 0.10 × H ≦ h ≦ 0.30 × H (2) where D: inner diameter of immersion tube (mm) h : Immersion depth of the dip tube (mm) L: Horizontal distance between fixed weirs (mm) H: Height from tundish bottom wall to fixed weir top surface (m)
m)