JPH10193049A - Method for removing inclusion in continuos casting tundish - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for removing inclusion in a tundish by which the inclusion can inexpensively and surely be removed. SOLUTION: In the continuous casting tundish 1 for intermediately supplying molten steel 3 poured from a ladle 2 into a mold 4, the molten steel storing depth H of the tundish is defined as 0.5-2.5m and the molten steel storing width W of the tundish is defined as 0.5-2.0m. The molten steel resident quantity or the molten steel passing-through quantity in the tundish is controlled so that the value obtained by dividing the molten steel resident quantity V in the tundish by the molten steel passing-through quantity Q per one min becomes 10-20. At this time, the molten steel resident quantity in the tundish is desirable to be 30-100 ton.

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 can reliably and inexpensively remove nonmetallic inclusions in molten steel poured into a tundish. .

[0002]

2. Description of the Related Art Non-metallic inclusions (hereinafter, referred to as "inclusions") in molten steel cause defects such as surface flaws in a final product, and must be removed as much as possible from the molten steel. . Therefore, the technology for reducing inclusions is an important technology for obtaining high-quality slabs by the continuous casting method.
Various countermeasures have been implemented, as seen in flow control by a magnetic field in a mold.

However, in recent operation modes in which the slab drawing speed is increased in order to improve productivity, there is a limit in separating and removing inclusions in a mold, and furthermore, the quality required in recent years is reduced. Taking account of the stricter requirements, it is extremely important to improve the cleanliness of molten steel before supplying it into a mold as a measure to reduce inclusions. Therefore, various measures for reducing inclusions in tundish have been proposed.

For example, Japanese Unexamined Patent Publication No. 6-7904 (hereinafter referred to as "prior art 1") discloses a method of blowing an inert gas into molten steel in a tundish to separate and remove inclusions by gas bubbles. Japanese Patent Application Laid-Open No. Hei 7-132353 (hereinafter referred to as "prior art 2") provides three weirs in a tundish and separates inclusions by these weirs.
Japanese Unexamined Patent Application Publication No. 7-132354 (hereinafter referred to as “prior art 3”) discloses a method for removing inclusions by forming the bottom surface of a tundish with a deep unevenness in an injection portion and an outflow portion. A method of separating and removing is disclosed.

[0005]

However, in the prior art 1, although the effect of reducing inclusions due to gas bubbles can be expected, refractories and equipment for injecting gas are required.
In addition, in order to stably inject gas, maintenance and maintenance are also required, and these costs increase the manufacturing cost.

In the prior art 2, the effect of reducing inclusions can be expected because the occurrence of a circulating flow region of molten steel, which will be described later, can be prevented by the weir. However, since the refractory weir is a consumable product, the cost of the tundish refractory is low. When the tundish is continuously reused without any repairs while it is still hot, the weirs cannot be installed manually and dedicated installation equipment is required. It becomes necessary, and the manufacturing cost increases as in the prior art 1.

In the prior art 3, since the bottom surface of the tundish is formed to have a highly irregular shape, a circulating flow region of molten steel described later is unlikely to be generated, and the effect of reducing inclusions is excellent. 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 not realistic.

As described above, the effect of the conventional method for removing inclusions in a tundish can be expected, but it is difficult to produce inexpensively a slab with few inclusions.

[0009] On the other hand, the present inventors have compared and examined the slab quality and the casting conditions in an actual machine, and confirmed that measures conventionally considered to improve the effect of removing inclusions are not always effective. That is, the concept of the inclusion removal in the conventional tundish is based on the idea that the amount of molten steel retained in the tundish (hereinafter referred to as “the retained amount of molten steel in the tundish”) is defined as the amount of molten steel supplied from the tundish to the mold per unit time. The average residence time obtained by dividing by the amount (hereinafter, referred to as “amount of molten steel passing”) is extended by increasing the capacity of the tundish, and the floating time of the inclusions is secured and removed. However, in an actual machine, the average residence time and the 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 events were found.

1. The flow of molten steel in the tundish is caused by the inertial force of the injection flow that falls from the ladle by gravity, the convection caused by the temperature difference between the molten steel immediately after pouring from the ladle and the molten steel remaining in the tundish, and the tundish. The flow resulting from the supply flow to the mold at the outlet is formed in a mutually and complexly related manner.

2. In the tundish, there is a circulating flow region, a so-called dead water region, and the dead water region is greatly affected by the shape of the tundish, the temperature of the molten steel, and the amount of the molten steel passed.

3. Since the dead water area is a stagnant area,
The injected molten steel is discharged from the tundish outlet without passing through the dead water area. Therefore, the generation of the dead water region decreases the actual residence time of the molten steel in the tundish (hereinafter, referred to as “actual residence time”), and hinders floating and separation of inclusions.

4. The actual residence time is shorter than the average residence time due to the occurrence of dead water areas. 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 amount of molten steel in the tundish excluding the amount of molten steel in the dead water area from the retained amount of molten steel in the tundish. It is important to increase).

The present invention has been made on the basis of the above-mentioned findings. It is an object of the present invention to increase the effective residence time and extend the actual residence time so that inclusions can be reliably removed at low cost. The present invention provides a method for removing inclusions in a tundish that can be performed.

[0016]

According to a first aspect of the present invention, there is provided a method for removing inclusions in a tundish for continuous casting, wherein the molten steel injected from a ladle is relayed to a mold. The molten steel accommodation depth is 0.5 m to 2.5 m, the molten steel accommodation width of the tundish is 0.5 m to 2.0 m, and the value obtained by dividing the retained amount of molten steel in the tundish by the amount of molten steel passing per minute is 1
It is characterized in that the amount of molten steel retained in the tundish or the amount of molten steel passing therethrough is controlled to be 0 to 20.

The method for removing inclusions in a tundish for continuous casting according to claim 2 of the present invention is the method for removing inclusions in a tundish for continuous casting according to claim 1.
The amount of molten steel retained in the tundish is 30 to 100 tons.

Under various conditions of the amount of molten steel retained in the tundish and the amount of molten steel passing therethrough, a three-dimensional molten steel flow in the tundish was numerically analyzed using the Navier-Stokes equation, and the molten steel flow pattern in the tundish was analyzed. investigated. In addition, a numerical analysis was conducted in which inclusions were mixed with molten steel injected into the tundish, and the flotation efficiency of inclusions was also investigated. The numerical analysis was performed under the condition that the same amount of molten steel as the amount of molten steel passed was injected into the tundish and the amount of retained molten steel in the tundish was kept constant.

If the capacity of the tundish is increased by increasing the capacity of the molten steel in the tundish, the problem is that the dead water region described above becomes large. This dead water area
Since it is an inert molten steel, the actual residence time can be shortened, but has no effect of extending the actual residence time, and further causes a decrease in the molten steel temperature. As a result of numerical analysis, it was found that the cause of the occurrence of the dead water region was as follows.

The molten steel in the tundish is cooled by conduction transmission with the refractory in the tundish and radiant transmission on the surface of the molten steel in the tundish. Therefore, there is always a temperature difference between the molten steel immediately after being injected into the tundish and the molten steel staying in the tundish. Usually, the molten steel immediately after being injected is 5 ° C. to 10 ° C. higher than the molten steel that stays. This temperature difference causes a difference in the density of the molten steel, so that the molten steel immediately after being poured into the tundish receives buoyancy because it is hotter and lighter than the surrounding molten steel.

FIG. 2 schematically shows the flow pattern of molten steel in a tundish under the condition that the amount of molten steel passed through obtained by numerical analysis is appropriate. After the molten steel injected from the ladle through the long nozzle collides with the bottom of the tundish, it tries to flow horizontally to the tundish outlet due to the inertial force due to gravity drop from the ladle, but the influence of buoyancy Ascends and rises along the tundish wall. Therefore, the molten steel flows toward the tundish outlet while rising, and then descends toward the tundish outlet near the tundish outlet, and is supplied into the mold from the tundish outlet. At that time, since the molten steel is a viscous fluid, an accompanying flow is formed due to the viscosity. This means that more molten steel than is supplied to the mold forms a flow towards the outlet of the tundish. However, since only a required amount flows out at the outlet of the tundish, most of the accompanying flow generated due to the viscosity cannot be flowed out, but turns around in the direction of the molten steel injection portion from the ladle and returns. That is, this is a cause of the circulation flow, and a dead water region is generated at the bottom of the tundish.

The result of analyzing the effect of the amount of molten steel passing on the generation of the dead water region is as follows.

FIG. 3 is a diagram schematically showing a flow pattern of molten steel in a tundish under a condition where the amount of molten steel passing through is small, obtained by numerical analysis. In this case, since the injection flow rate from the ladle is small, after colliding with the tundish bottom, the tundish bottom tends to flow horizontally toward the tundish outlet by the inertial force due to gravity drop from the ladle. The flow is weak, so the injected molten steel rises to the molten steel surface side due to the buoyancy caused by the molten steel temperature difference. Furthermore, the flow which tends to flow in the horizontal direction toward the outlet of the tundish collides with the circulating flow flowing from the outlet of the tundish toward the molten steel injection portion and is attenuated. Greatly expands.

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

As described above, when the amount of molten steel passing through is relatively small with respect to the amount of molten steel retained in the tundish, the dead water area is enlarged and the effective retained amount is reduced. In addition, when the amount of molten steel passing is relatively large, the dead water region becomes small and the effective retention amount increases, but the amount of molten steel passing increases and the actual residence time becomes short. That is, it was found that there was an appropriate amount of molten steel passing through in accordance with the amount of molten steel retained in the tundish.

From the results of the investigation of the efficiency of flotation separation of inclusions, the dead water area is small and the actual residence time can be secured.
It was found that the flow pattern of molten steel showed the best inclusion separation efficiency. Then, when the value obtained by dividing the retained amount of molten steel in the tundish by the amount of molten steel passing per minute was 10 to 20, it was found that the molten steel flow pattern in the tundish was as shown in FIG. If the division value is less than 10, the amount of molten steel passing through is too small, and if it exceeds 20, it becomes too large.

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

Further, in the present invention, the retained amount of molten steel in the tundish is preferably 30 to 100 tons. If the retained amount of molten steel in the tundish is less than 30 tons, the absolute amount of molten steel is small, heat loss to the refractory of the tundish becomes large, and the floating of inclusions becomes disadvantageous.
This is because the capacity exceeding 0 tons is too large compared to the furnace volume of the current steelmaking furnace, and the refractory cost increases, which is not practical.

[0029]

DESCRIPTION OF THE PREFERRED EMBODIMENTS 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 solid tundish 1 having an inner surface made of a refractory, and molten steel 3 in the ladle 2 has a long nozzle 7 installed at the bottom of the ladle 2. Through the tundish 1 into the molten steel injection section 10. The poured molten steel 3 stays in the tundish 1 with the molten steel storage depth being H, the molten steel storage width being W, the molten steel storage length being L, and the molten steel retention amount in the tundish being V.

The tundish 1 has a molten steel storage depth (H).
Is selected from the range of 0.5 m to 2.5 m, and the molten steel accommodation width (W) is selected from the range of 0.5 m to 2.0 m, and the shape is determined. At that time, the molten steel storage depth (H), the molten steel storage width (W), and the molten steel retention amount (V) in the tundish become 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 provided on the opposite side of the molten steel injection section 10, and the molten steel 3 injected into the tundish 1 is supplied from the tundish outlet 5 to the tundish outlet 5 Is supplied into the mold 4 with the amount of molten steel passing per minute as Q, and then cooled and solidified in the mold 4 to form a slab 8. At that time, 1 from ladle 2
The molten steel injection amount per minute (Q ') 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 longitudinal position of the tundish 1 at the molten steel injection part 10 and the tundish outlet 5 is different from the molten steel storage length (L) of the tundish 1 from the respective tundish side walls 9 and 9a. 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 exists 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 an area may be generated.

During the casting, the molten steel staying in the tundish is adjusted so that the value obtained by dividing the molten steel staying amount (V) in the tundish by the molten steel passing amount per minute (Q) is 10 to 20, preferably 12 to 16. The amount (V) or the amount of molten steel per minute (Q) is controlled. This is because the range in which the divided value is 12 to 16 provides the best results, as described later.

Although the shape of the tundish 1 shown in FIG. 1 is a rectangular parallelepiped, when the cross section is a trapezoid, the maximum width of the molten steel accommodation width (W) is 2.0 m or less and the minimum width of the molten steel is 0.1 mm. 5m
If there is a difference in the molten steel accommodation depth (H) in the tundish 1, the deepest position may be set to 2.5 m or less and the shallowest position may be set to 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]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention was carried out using a rectangular parallelepiped tundish shown in FIG. The used tundish had a molten steel storage depth (H) and a molten steel storage width (W) of 1.5 m, a molten steel storage length (L) of 4.9 m, and a retained amount of molten steel in the tundish of 77 tons. The molten steel injection part from the long nozzle was set at a position of 0.8 m from the tundish side wall, and the tundish outlet was set at a position of 0.6 m from the tundish side wall. Then, a low carbon Al killed steel is applied with a thickness of 2
A single strand was cast with a slab size of 50 mm and a width of 1850 mm.

The slab drawing speed was set at 1.0 m / min.
4 m / min, 1.8 m / min, and 2.2 m / mi
n conditions. Converting the amount of molten steel passing per minute from the slab pulling rate assuming a slab specific gravity of 7.85 gives 3.63 tons, 5.08 tons, 6.54 tons, and 7.99 tons, respectively. The values obtained by dividing the amount of retained molten steel by the amount of molten steel passing per minute are 21.2, 15.2, 11.8, and 9.6, respectively.

The slab cast under these conditions was rolled into a thin steel sheet, and the thin steel sheet was examined for surface defects due to inclusions. The result is shown in FIG. In this embodiment,
The quality evaluation is indicated 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 scope of the present invention is 1.4 m / min (Example) and 1.8.
m / min (Example), the quality evaluation is good,
In particular, when the slab drawing speed was 1.4 m / min, the quality index was 10, which was good. On the other hand, when the slab drawing speed which is out of the range 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, about 3, and the quality evaluation was inferior.

Casting was carried out under the conditions of various amounts of molten steel in the tundish including the above and the amount of molten steel passing therethrough. The obtained slab was rolled into a thin steel sheet, and the quality of the thin steel sheet was investigated by examining surface defects due to inclusions. An evaluation was performed. FIG. 6 is a result obtained by summarizing the value obtained by dividing the molten steel retained amount in the tundish by the molten steel passage amount on the horizontal axis and the quality index on the vertical axis, and summarizing the relationship between the scope of the present invention and the comparative example. is there. As shown in FIG. 6, in the range of the present invention, the quality index is higher than 5 and the quality index is higher than 5. Was the best.

FIG. 7 shows the case where the slab withdrawal speed is reduced to 1.0 m / min (the amount of molten steel passing per minute is 3.63 tons) when the ladle is replaced during continuous casting, and the steady state. 1.4m / min (the amount of molten steel passing per minute is 5.0
FIG. 9 is a diagram showing a comparison of the change in the quality index by changing the retained amount of molten steel in the tundish when the slab drawing speed is 8 tons). The numbers in the figure are values obtained by dividing the retained amount of molten steel in the tundish by the amount of molten steel passing therethrough.
As shown in FIG. 7, when the slab withdrawal speed was 1.4 m / min, the quality was the best with the retained amount of molten steel in the tundish being 77 tons, but the slab withdrawal speed was 1.0 m / min.
In the case of min, the best quality result was obtained when the retained amount of molten steel in the tundish was reduced from the steady state to 56 tons.

As described above, when the casting speed is changed, the stagnation amount of the molten steel in the tundish is controlled to an appropriate range each time, so that a good cast piece with stable quality can be manufactured. .

[0042]

According to the present invention, the tundish shape can be simply changed to a shape with high efficiency in separating and removing inclusions without taking measures such as installing a weir or injecting gas into the tundish. By controlling the amount of retained molten steel and the amount of molten steel passing in appropriate ranges, the efficiency of inclusion removal can be increased, and high-quality slabs with few inclusions can be stably manufactured at low cost. Becomes

[Brief description of 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.

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

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

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

FIG. 5 is a diagram showing the effect of the slab drawing speed on the slab quality with the amount of molten steel retained in the tundish constant, for an example and a comparative example.

FIG. 6 is a diagram summarizing the influence on the quality of a value obtained by dividing the retained amount of molten steel in the tundish by the amount of molten steel passing through the example and the comparative example.

FIG. 7 is a diagram showing that an appropriate amount of molten steel retained in a tundish is changed by changing a slab drawing speed according to the example of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Tundish 2 Ladle 3 Molten steel 4 Mold 5 Tundish outlet 6 Immersion nozzle 7 Long nozzle

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Keiji Yoshioka 1-2-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd.

Claims (2)

[Claims] 1. In a tundish for continuous casting in which molten steel injected from a ladle is relayed to a mold, 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 by controlling the amount of molten steel retained in a tundish or the amount of molten steel passing therethrough so that the value obtained by dividing the amount of molten steel retained in the tundish by the amount of molten steel passing per minute is 10 to 20. Inclusion removal method in the above. 2. The method for removing inclusions in a tundish for continuous casting according to claim 1, wherein the retained amount of molten steel in the tundish is 30 to 100 tons.