JP3470537B2 - Inclusion removal method in tundish for continuous casting - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing inclusions in a tundish for continuous casting, which is capable of reliably and inexpensively removing non-metallic inclusions in molten steel injected into a tundish. .

[0002]

2. Description of the Related Art Non-metallic inclusions in molten steel (hereinafter referred to as "inclusions") cause defects such as surface flaws in the final product, so it is necessary to remove them as much as possible from the molten steel. . Therefore, the technique of reducing inclusions is an important technique for obtaining a high quality slab by the continuous casting method.
Various measures have been taken as seen in magnetic field flow control in the mold.

However, in the recent operation mode in which the slab drawing speed is increased to improve the productivity, there is a limit to the separation / removal of inclusions in the mold, and further, the required quality in recent years. In consideration of the strictness of the steel, it is extremely important to improve the cleanliness of molten steel before supplying it into the mold as a measure for reducing inclusions. Therefore, various measures to reduce inclusions have been proposed for tundish.

For example, Japanese Patent Laid-Open No. 6-7904 (hereinafter referred to as "Prior Art 1") discloses a method of blowing an inert gas into molten steel in a tundish and separating / removing inclusions by gas bubbles. It is, JP-a-7-132353
Japanese Patent Publication (hereinafter referred to as "Prior Art 2") discloses a method of providing three weirs in a tundish, and separating / removing inclusions by these weirs, and JP-A-7-132.
Japanese Patent No. 354 (hereinafter, referred to as "Prior Art 3") discloses a method of separating and removing inclusions by forming the bottom surface of a tundish into a concave and convex shape with deep injection and outflow portions. .

[0005]

However, in the prior art 1, although the effect of reducing inclusions by gas bubbles can be expected, refractory materials and equipment for blowing gas are required.
In addition, maintenance and maintenance are also required for stable gas injection, and these costs increase the manufacturing cost.

Further, in the prior art 2, since the generation of the circulating flow region of molten steel described later can be prevented by the weir, the effect of reducing inclusions can be expected, but since the weir made of refractory is a consumable item, the cost of the tundish refractory is low. In addition, when the tundish for the purpose of reducing the tundish cost is continuously reused while it is hot without any repair, the weir cannot be manually installed, and a dedicated installation facility is required. It becomes necessary and the manufacturing cost is increased as in the case of the prior art 1.

In Prior Art 3, since the bottom surface of the tundish has a highly uneven shape, a circulating flow region of molten steel described later is less likely to occur and the effect of reducing inclusions is excellent. However, in actual operation, at the end of casting. Factors that increase manufacturing costs, such as the problem of residual molten steel treatment in the injection part of the steel, the problem of tundish refractory construction, and the problem of heat loss of molten steel due to the large contact area between molten steel and refractory There are many and it is not realistic.

As described above, the conventional method for removing inclusions in the tundish can be expected to be effective, but it is difficult to inexpensively manufacture a slab containing few inclusions.

On the other hand, the inventors of the present invention compared and examined the quality of the slab and the casting conditions in an actual machine, and confirmed that the measures that were conventionally considered to improve the effect of removing inclusions are not necessarily effective. That is, the concept of removing inclusions in a conventional tundish is that the amount of molten steel retained in the tundish (hereinafter, referred to as “amount of molten steel retained in the tundish”) is supplied to the mold from the tundish per unit time. The average residence time, which is obtained by dividing by the amount (hereinafter referred to as “molten steel passage amount”), is extended by increasing the capacity of the tundish, and the floating time of inclusions is secured and removed. However, in actual machines, the average residence time and quality are not necessarily in a proportional relationship.

Therefore, the flow of molten steel in the tundish was investigated by numerical analysis, and as a result, the following phenomenon was found.

1. The molten steel flow in the tundish is due to the inertial force of the injection flow that falls by gravity from the ladle, the convection due to the temperature difference between the molten steel immediately after pouring from the ladle and the molten steel that remains in the tundish, and the tundish. The flow resulting from the feed flow to the mold at the outlet is formed in mutual and complex relation.

2. In the tundish, there is a circulating flow region, so-called dead water region, and the dead water region is largely influenced by the tundish shape, molten steel temperature, and the amount of molten steel passing through.

3. Since the dead water area is a stagnant area,
The injected molten steel does not pass through the dead water area and is discharged from the tundish outlet. Therefore, the generation of the dead water region reduces the actual residence time of molten steel in the tundish (hereinafter, referred to as “actual residence time”) and hinders the floating and separation of inclusions.

4. The actual residence time becomes shorter than the average residence time due to the occurrence of dead water region. 5. In order to improve the removal efficiency of inclusions in the tundish, the dead water area is reduced and the effective retention amount (effective retention amount is the molten steel amount obtained by removing the molten steel amount in the dead water region from the molten steel retention amount in the tundish. ) Is important.

The present invention was made on the basis of the above findings, and the purpose thereof is to increase the effective retention amount and prolong the actual retention time, thereby making it possible to remove inclusions inexpensively and reliably. The present invention provides a method for removing inclusions in a tundish that can be performed.

[0016]

A method for removing inclusions in a tundish for continuous casting according to the first aspect of the present invention is a tundish for continuous casting in which molten steel injected from a ladle is relay-supplied to a mold. The molten steel storage depth is 0.5 m to 2.5 m, the molten steel storage width of the tundish is 0.5 m to 2.0 m, and the value obtained by dividing the molten steel retention amount in the tundish by the molten steel passage amount per minute is
It is characterized in that the amount of molten steel retained in the tundish or the amount of molten steel passed is controlled so as to be 12 to 20.

The method for removing inclusions in the tundish for continuous casting according to the second aspect of the present invention is the method for removing inclusions in the tundish for continuous casting according to the first aspect of the invention , wherein the molten steel retention amount in the tundish is 30 tons. Through 10
It is characterized by being 0 ton.

Under various conditions of molten steel retention amount and molten steel passage amount in the tundish, three-dimensional molten steel flow in the tundish is numerically analyzed using the Navier-Stokes equation, and the molten steel flow pattern in the tundish is analyzed. investigated. In addition, numerical analysis was performed by mixing inclusions in the molten steel injected into the tundish, and the floating separation efficiency of inclusions was also investigated. The numerical analysis was performed under the condition that the same molten steel amount as the molten steel passage amount was injected into the tundish and the molten steel retention amount in the tundish was kept constant.

When the capacity of the tundish is increased and the molten steel retention amount in the tundish is increased, the problem is that the dead water region described above becomes large. This dead water area
Since the molten steel is inactive, the actual residence time may be shortened but has no effect of extending the actual residence time, and further, the molten steel temperature is lowered. As a result of numerical analysis, it was found that the causes of the dead water region were as follows.

The molten steel in the tundish is cooled by conduction transfer with the tundish refractory and radiation transfer at the surface of the molten steel in the tundish. Therefore, there is always a temperature difference between the molten steel immediately after being poured into the tundish and the molten steel that stays in the tundish, and normally, the molten steel immediately after being poured is 5 to 10 ° C. higher than the molten steel that stays. This temperature difference causes a difference in the density of the molten steel. Therefore, the molten steel immediately after being poured into the tundish is hotter and lighter than the surrounding molten steel, and therefore receives buoyancy.

FIG. 2 schematically shows a molten steel flow pattern in the tundish under the condition that the amount of molten steel passing obtained by the numerical analysis is appropriate. The molten steel injected from the ladle through the long nozzle collides with the bottom of the tundish and then tries to flow horizontally toward the tundish outlet due to the inertial force caused by the gravity drop from the ladle. Upon receipt, it rises along the tundish wall. Therefore, the molten steel flows toward the tundish outlet while rising, then descends toward the tundish outlet near the tundish outlet and is supplied into the mold from the tundish outlet. At that time, since molten steel is a viscous fluid, an associated flow is formed due to the viscosity. This means that more molten steel than is supplied to the mold forms a flow toward the tundish outlet. However, since only a necessary amount flows out at the tundish outlet, most of the accompanying flow generated due to viscosity cannot flow out and turns back toward the molten steel pouring part from the ladle. That is, this is the cause of the circulating flow, and a dead water region occurs at the bottom of the tundish.

Further, the results of analyzing the influence of the molten steel passage amount on the occurrence of the dead water region are as follows.

FIG. 3 is a diagram schematically showing the molten steel flow pattern in the tundish under the condition that the molten steel passage amount is small, which is obtained by numerical analysis. In this case, since the injection flow rate from the ladle is small, after colliding with the bottom of the tundish, the inertial force due to gravity drop from the ladle attempts to flow horizontally from the bottom of the tundish toward the tundish outlet. Since the flow is weak, the injected molten steel rises to the surface side of the molten steel due to the buoyancy due to the difference in molten steel temperature. Furthermore, the flow that tends to flow horizontally toward the tundish outlet collides with the circulating flow that flows from the tundish outlet toward the molten steel injection part and is attenuated, resulting in the dead water region at the bottom of the tundish. Will greatly expand.

FIG. 4 is a diagram schematically showing the molten steel flow pattern in the tundish under the condition that the amount of molten steel passing is too large, which is obtained by numerical analysis. In this case, the heat removal in the tundish is relatively small, so the influence of buoyancy is small, and because the horizontal inertial force of the injection flow toward the tundish outlet is also large, the molten steel flow colliding with the bottom of the tundish Flows as it is toward the tundish outlet from the bottom of the tundish, and as a result, a small dead water region occurs on the molten steel surface side.

As described above, when the molten steel passage amount is relatively small with respect to the molten steel retention amount in the tundish, the dead water region expands and the effective retention amount decreases, so that the actual retention time becomes short and In addition, when the molten steel passage amount is relatively large, the dead water region becomes small and the effective retention amount increases. However, since the molten steel passage amount increases, the actual residence time becomes short. That is, it was found that there was an appropriate molten steel passage amount depending on the molten steel retention amount in the tundish.

From the results of investigation of the floating separation efficiency of inclusions, it is possible to secure a small dead water area and an actual residence time.
It was found that the molten steel flow pattern of No. 1 had the highest inclusion separation efficiency. Then, when the value obtained by dividing the molten steel retention amount in the tundish by the molten steel passage amount per minute was 12 to 20, it was found that the molten steel flow pattern shown in FIG. 2 was obtained in the tundish. This is because if the division value is less than 10, the molten steel passing amount is too small, and if it exceeds 20, the molten steel passing amount is too large.

From the analysis results of inclusion removal efficiency and tundish shape, the molten steel storage depth of the tundish was 0.5 m to 2.5 m, and the molten steel storage width of the tundish was 0.5 m to 2.0 m. Then, it was found that the inclusion removal efficiency was further improved. Molten steel storage depth is 0.5m
If it is less than 2, the surface area of molten steel in the tundish increases and the temperature drop of molten steel becomes large, and if it exceeds 2.5 m, the levitation distance of inclusions becomes too long, and the separation efficiency of inclusions decreases. . When the molten steel storage width is less than 0.5 m, the contact area with the tundish refractory increases and the temperature drop of the molten steel becomes large, and when it exceeds 2.0 m, the molten steel retention amount in the tundish reaches the upper limit (100 tons). This is because the required molten steel storage length cannot be secured even in the case of) and the efficiency of separating inclusions is reduced.

Further, in the present invention, it is preferable that the molten steel retention amount in the tundish is 30 to 100 tons. If the molten steel retention amount in the tundish is less than 30 tons, the absolute amount of molten steel is small, the heat loss to the tundish refractory becomes large, which is disadvantageous in the floatability of inclusions.
This is because the capacity exceeding 0 ton is too large compared with the furnace volume of the current steelmaking furnace and the refractory cost increases, which is not realistic.

[0029]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described with reference to the drawings.
FIG. 1 is a schematic view of a tundish for continuous casting to which the present invention is applied, (a) is a front sectional view, and (b) is a side sectional view.

A ladle 2 is arranged above a rectangular parallelepiped tundish 1 whose inner surface is made of refractory, and molten steel 3 in the ladle 2 has a long nozzle 7 installed at the bottom of the ladle 2. It is injected into the molten steel injection part 10 in the tundish 1 through the. The injected molten steel 3 stays in the tundish 1 with the molten steel storage depth H, the molten steel storage width W, the molten steel storage length L, and the molten steel retention amount in the tundish being V.

The tundish 1 has a molten steel storage depth (H).
Is 0.5 m to 2.5 m, and the molten steel storage width (W) is 0.5 m to 2.0 m. At that time, the molten steel storage depth (H), the molten steel storage width (W), so that the molten steel retention amount (V) in the tundish becomes 30 to 100 tons.
It is preferable to determine the molten steel storage length (L).

At the bottom of the tundish 1, a tundish outlet 5 is installed on the opposite side of the molten steel injecting section 10. The molten steel 3 injected into the tundish 1 is supplied from the tundish outlet 5 to the tundish outlet 5. The molten steel passing amount per minute is set as Q and supplied into the mold 4 through the immersion nozzle 6 connected to the mold 4. Then, the mold 4 is cooled and solidified to form a cast piece 8. At that time, 1 from ladle 2
The molten steel injection amount (Q ′) per minute is controlled to be substantially equal to the molten steel passage amount (Q), and the molten steel retention amount in the tundish is controlled to be a constant value of V.

The positions of the molten steel injecting portion 10 and the tundish outlet 5 in the longitudinal direction of the tundish 1 are the distances from the tundish side walls 9, 9a with respect to the molten steel storage length (L) of the tundish 1. It is preferably within L / 5. This is because when the distance from the tundish side walls 9 and 9a exceeds L / 5, dead water is introduced between the molten steel injection part 10 and the tundish side wall 9 and between the tundish outlet 5 and the tundish side wall 9a. This is because a region may be generated.

During casting, the molten steel retention amount in the tundish (V) is divided by the molten steel passage amount (Q) per minute (Q) to obtain a value of 12 to 20, preferably 12 to 16, The amount (V) or the molten steel passage amount (Q) per minute is controlled. This is because the best result can be obtained in the range where the divided value is 12 to 16, as described later.

The tundish 1 shown in FIG. 1 has a rectangular parallelepiped shape, but when the cross section is trapezoidal, the molten steel accommodation width (W) has a maximum width of 2.0 m or less and a minimum width of 0. 5m
If there is a difference in the molten steel accommodation depth (H) in the tundish 1, the deepest position may be 2.5 m or less and the shallowest position may be 0.5 m or more. or,
Although the tundish shown in FIG. 1 is a single strand, it goes without saying that the same can be applied to the case of multiple strands.

[0036]

EXAMPLES The present invention was carried out in a rectangular parallelepiped tundish shown in FIG. The tundish used had a molten steel storage depth (H) and a molten steel storage width (W) of both 1.5 m, a molten steel storage length (L) of 4.9 m, and a molten steel retention amount in the tundish of 77 tons. The molten steel injection part from the long nozzle was installed at a position 0.8 m from the tundish side wall, and the tundish outlet was installed at a position 0.6 m from the tundish side wall. And low carbon Al killed steel, thickness 2
It was cast as a single strand with a slab size of 50 mm and a width of 1850 mm.

The slab drawing speed was 1.0 m / min.
4m / min, 1.8m / min, and 2.2m / mi
4 conditions of n. Converting the molten steel passage amount per minute from this slab withdrawal rate by converting the slab specific gravity to 7.85, it was 3.63 tons, 5.08 tons, 6.54 tons, and 7.99 tons, respectively. The values obtained by dividing the internal molten steel retention amount by the molten steel passage amount per minute are 21.2, 15.2, 11.8, and 9.6, respectively.

A slab cast under these conditions was rolled into a thin steel sheet, and surface defects due to inclusions in the thin steel sheet were investigated. The result is shown in FIG. In this example,
The quality evaluation is represented by a quality index, and the quality index is indicated by a larger value as the number of surface defects due to inclusions is smaller. As shown in FIG. 5, the slab drawing speed within the range of the present invention was 1.4 m / min (Example) and 1.8.
In the case of m / min (Example), the quality evaluation is good,
In particular, the quality index was 10 and was good when the cast strip drawing speed was 1.4 m / min. On the other hand, the slab drawing speed, which is outside the scope of the present invention, is 1.0
In the case of m / min (comparative example) and 2.2 m / min (comparative example), the quality index was low and was about 3, and the quality evaluation was poor.

Casting under various conditions of molten steel retention amount and molten steel passage amount in the tundish including the above, the obtained slab is rolled into a thin steel sheet, and surface defects due to inclusions in the thin steel sheet are investigated to determine the quality. An evaluation was performed. FIG. 6 shows the results obtained by organizing the relationship between the two in the range of the present invention and the comparative example, with the value obtained by dividing the molten steel retention amount in the tundish divided by the molten steel passage amount as the horizontal axis and the quality index as the vertical axis. is there. As shown in FIG. 6, the range of the present invention has a good quality index higher than 5, and within the range of the present invention, the range of 12 to 16 is stable and the quality index is high. Was the best.

FIG. 7 shows a case in which the slab drawing speed was lowered to 1.0 m / min (the molten steel passage amount per minute was 3.63 tons) when the ladle was replaced during continuous casting, and the steady state. 1.4 m / min (The molten steel passage amount per minute is 5.0
It is the figure which compared the change of the said quality index by changing the molten steel retention amount in a tundish in the case of a cast strip drawing speed of 8 tons. The numbers in the figure are values obtained by dividing the molten steel retention amount in the tundish by the molten steel passage amount.
As shown in FIG. 7, when the slab drawing speed was 1.4 m / min, the molten steel retention in the tundish was 77 tons, which was the best quality, but the slab drawing speed was 1.0 m / min.
In the case of min, the quality result was the best when the molten steel retention amount in the tundish was reduced from the steady state to 56 tons.

As described above, when the casting speed is changed, the molten steel retention amount in the tundish is controlled to an appropriate range each time, and a good quality slab can be manufactured with stable quality. .

[0042]

EFFECTS OF THE INVENTION According to the present invention, the tundish shape is simply made into a shape with a high efficiency of separating / removing inclusions without taking measures to reduce inclusions such as installation of weirs and gas injection, and Inclusion removal efficiency can be improved by controlling the molten steel retention amount and the molten steel passage amount within appropriate ranges, so it is possible to inexpensively and stably manufacture high quality slabs with few inclusions. Becomes

[Brief description of drawings]

FIG. 1 is a schematic view of a tundish for continuous casting to which the present invention is applied, in which (a) is a front sectional view and (b) is a side sectional view.

FIG. 2 is a diagram schematically showing a molten steel flow pattern in a tundish under conditions in which the amount of molten steel passing obtained by numerical analysis is appropriate.

FIG. 3 is a diagram schematically showing a molten steel flow pattern in a tundish under a condition where the molten steel passage amount is small, which is obtained by numerical analysis.

FIG. 4 is a diagram schematically showing a molten steel flow pattern in a tundish under a condition where the amount of molten steel passing is too large, which is obtained by numerical analysis.

FIG. 5 is a view showing an influence of a cast strip drawing speed on a cast quality in an example and a comparative example, with a retained molten steel amount in a tundish being constant.

FIG. 6 is a diagram summarizing the effects on the quality of the value obtained by dividing the molten steel retention amount in the tundish by the molten steel passage amount in the example and the comparative example.

FIG. 7 is a diagram showing that an appropriate molten steel retention amount in a tundish changes according to a change in a cast slab drawing speed according to an embodiment of the present invention.

[Explanation of symbols]

1 tundish 2 ladle 3 Molten steel 4 molds 5 Tundish exit 6 immersion nozzle 7 Long nozzle

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Keiji Yoshioka 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (56) References JP-A-2-55649 (JP, A) JP-A-7 -155911 (JP, A) Proceedings of the Iron and Steel Institute of Japan (Materials and Processes), CAMP-ISIJ, Vol. 4 (1991), p. 1322 (58) Fields surveyed (Int.Cl. ⁷ , DB name) B22D 11/10 310 B22D 11/11 B22D 43/00

Claims (2)

(57) [Claims] 1. In a tundish for continuous casting, which feeds molten steel injected from a ladle to a mold by relay, the molten steel storage depth of the tundish is 0.5 m to 2.5 m and the molten steel storage width of the tundish is 0. 5m to 2.0m,
A tundish for continuous casting, characterized in that the molten steel retention amount in the tundish or the molten steel passage amount is controlled so that the value obtained by dividing the molten steel retention amount in the tundish by the molten steel passage amount per minute is 12 to 20. For removing inclusions in. 2. The method for removing inclusions in a tundish for continuous casting according to claim 1, wherein the molten steel retention amount in the tundish is 30 to 100 tons.