JP2020182965A - Cast completion-controlling method - Google Patents

Abstract

To provide a completion method that prevents productivity from being reduced without causing steel leakage, when roll intervals are reduced or enlarged.SOLUTION: A method includes: a first step of setting the time when cast completion control begins; a second step of controlling the amount of secondary cooling water in a casting path to be 70% or more and 90% or less of the amount of secondary cooling water at steady casting time and controlling a casting speed to be 1.5 m/min or more and 1.8 m/min or less, from the time when the cast completion control begins to cast completion; and cooling an unsolidified cast piece with the amount of secondary cooling water set in the second step and then controlling the casting speed to be 1.2 m/min or more and 1.4 m/min or less, by the time when an end of the unsolidified cast piece on an upstream side in a casting direction reaches a horizontal level from time of the cast completion.SELECTED DRAWING: Figure 3

Description

The present invention relates to a casting end control method in continuous casting.

In continuous casting, the molten steel in the radle is continuously cast into the mold via a tundish, a solidified shell is formed on the outer circumference of the molten steel by a water-cooled mold, and the solidified shell is supported by the following guide roll group between the rolls. The solidified shell is grown by the cooling spray of the above, and the completely solidified slab is pulled out with a pinch roll, but since the supply of molten steel is cut off at the end of casting of such continuous casting, special control different from that during steady casting It is necessary to finish casting with.

Conventionally, as this casting end control method, the casting speed is reduced at the end of casting to stop casting, and a cooling material is put into the rearmost end (bottom bottom) of the residual molten steel in the mold to solidify it. After that, a deceleration casting end method for pulling out the slab at a predetermined drawing speed was known, but this deceleration casting end method has many problems due to deceleration / stop, temperature drop, bottom processing work, etc. Recently, for the purpose of improving the productivity of continuous casting machines, improving the quality of slabs, producing high-temperature slabs, reducing the workload, etc., casting is completed while maintaining the normal casting speed without slowing down the casting speed. A constant speed drawing and casting end method is being considered.

In the conventional method 1 (Patent Documents 1 to 3), first casting is performed in order to prevent steel leakage from the rearmost end (bottom bottom) of the slab at the end of casting or during drawing of the slab after the casting. Depending on the residual steel weight or residual steel level in the tundish before the end, the casting speed is appropriately reduced by a predetermined deceleration pattern, and casting is stopped leaving a predetermined small amount of molten steel in the tundish. Then, after the coolant (metal particles, metal pieces, water, etc.) is put into the bottommost part and solidified, the bottom treatment work is performed, and then the drawing speed is appropriately increased to pull out.

In the conventional method 2 (Patent Documents 4 and 5), the casting is finished while maintaining the normal casting speed without decelerating or stopping the casting speed and processing the bottom portion before the end of casting, and the end of the slab. The bottommost part, which is the end part, is solidified with the secondary cooling water directly under the mold, and after the solidification of the bottommost part is completed, the drawing speed is increased to enable high-speed casting to be completed.
In addition, as a technique for preventing steel leakage at the end of high-speed casting, unsolidified cavities are formed at the bottom of the molten steel in the mold, which is partitioned by several horizontal solidified layers. Even if the molten steel is blown up by squeezing out, it is blocked by the plurality of solidified layers to prevent leakage of steel. Such a hierarchical structure is formed by stagnating and solidifying the molten metal surface by applying a pressing force to the slab with a guide roll directly under the mold, and repeating this several times.

The conventional method 3 (Patent Document 6) is a drawing method capable of stably and surely preventing steel leakage without being affected by various conditions which was a problem of the conventional method 2. In the conventional method 3, casting is completed while maintaining the normal casting speed, the slab is pulled out, hot water supply to the inside of the mold is stopped, and at the same time, the roll interval of the lower portion of the unsolidified portion in the rearmost slab is expanded. The outer shell of the slab is intentionally bulged, the bulging absorbs the squeezed out of molten steel due to solidification shrinkage, and then the widened portion is reduced by a roll in the subsequent stage to reach a predetermined slab thickness at the completely solidified portion. This is the end method.

Japanese Unexamined Patent Publication No. 62-124056 Japanese Unexamined Patent Publication No. 62-203652 Japanese Unexamined Patent Publication No. 62-2444848 Japanese Unexamined Patent Publication No. 6-262323 Japanese Unexamined Patent Publication No. 5-261501 JP-A-10-244347

When the conventional method 1 is implemented, it is effective in preventing steel leakage, but since the casting speed is reduced before the end of casting, the productivity of the continuous casting machine is reduced and the tongue is reduced. The casting time of the residual steel of the dish becomes long, the temperature of the residual molten steel drops significantly, and the quality of the final slab may deteriorate. Further, as described above, since the bottom treatment work is required, the work load is large, the quality of the final slab is deteriorated due to the mixing of the coolant, and the cost of purchasing the coolant is high.

When the conventional method 2 is carried out, the bottommost steel leakage rate does not reach 0% because the bottommost steel leakage occurs depending on the casting speed until the end of casting, the casting width, the powder melting condition in the mold, and the like. .. There is a risk that the roll replacement at the position where the leaked steel occurs due to the leaked steel at the bottom and the surface flaws on the slab due to the leaked steel metal will greatly hinder productivity.

When the conventional method 3 is implemented, the risk of steel leakage is eliminated. In addition, since the casting is completed and the slab is pulled out while maintaining the normal casting speed, the productivity does not decrease. However, if the roll interval cannot be reduced or expanded in terms of equipment, the conventional method 3 cannot be performed.

Therefore, it is an object of the present invention to provide a termination method in which there is no risk of steel leakage and the decrease in productivity is suppressed when the roll interval is not reduced or expanded.

As a result of diligent studies by the present inventor in order to solve the above problem, the casting end control start time is set before the casting end time, and a predetermined time is set from the casting end control start time to the casting end time. The continuous casting machine is controlled under the conditions, and the continuous casting machine is controlled under the predetermined conditions from the end of casting to the time when the upstream end of the unsolidified slab in the casting direction reaches the horizontal part of the continuous casting machine. We have found that there is no risk of steel leakage and it is possible to suppress the decrease in productivity without reducing or expanding the roll interval, and completed the present invention.

That is, in the first aspect of the present invention for solving the above problems, the casting of continuous casting for producing slabs is completed by using a vertical bending type or curved type continuous casting machine having a horizontal portion in the casting path. In the control method, when A is half the length of the distance from the meniscus to the horizontal part along the casting path during steady casting, a slab with a length of 0.8A or more and 1.2A or less. The casting end control start amount was set from the range of the amount of molten metal required for manufacturing, and the amount of molten metal in the tundish of the continuous casting machine at the time of steady casting became the casting end control start amount. From the first step of setting the time as the start of casting end control and from the start of casting end control to the end of casting, the amount of secondary cooling water in the casting path is 70% or more of the amount of secondary cooling water during steady casting. The second step of controlling the ratio to% or less and controlling the casting speed to 1.5 m / min or more and 1.8 m / min or less, and the upstream end of the unsolidified slab from the end of casting in the casting direction. A third unit that cools the unsolidified slab with the amount of secondary cooling water set in the second step and controls the casting speed to 1.2 m / min or more and 1.4 m / min or less until the portion reaches the horizontal portion. It is a casting end control method including a step.

In the casting end control method, the length of the slab in the plate width direction is preferably 1450 mm or less. Further, the degree of heating of the molten metal of the molten metal in the tundish is preferably 10 ° C. or higher and 45 ° C. or lower. Further, it is preferable not to increase or decrease the roll interval of the rolls provided in the continuous casting machine.

According to the present invention, there is no risk of steel leakage and it is possible to suppress a decrease in productivity without reducing or increasing the roll interval.

It is sectional drawing of the continuous casting machine 10. It is an enlarged view of the part shown by II of FIG. It is a flowchart of a casting end control method 1. It is a figure which showed an example of the relationship between the casting speed and time in the casting end control method 1.

The present invention is a casting end control method in continuous casting. Hereinafter, the casting end control method 1 according to the embodiment of the present invention will be described in detail.

First, a process in which the molten steel 20 supplied from the ladle is pulled out as a slab 30 by using the continuous casting machine 10 for performing the casting end control method 1 will be described with reference to FIGS. 1 and 2. FIG. 1 is a schematic cross-sectional view of the continuous casting machine 10. FIG. 2 is an enlarged view of II of FIG.

The continuous casting machine 10 is a vertical bending type continuous casting machine, and is a radle (not shown), a tundish 11, a dipping nozzle 12, a mold 13, a plurality of rolls 14, and a plurality of secondary cooling sprays 15. And, the pinch roll 16 is provided, and the molten steel 20 supplied from the radle can be continuously cast to produce the slab 30. Further, the continuous casting machine 10 includes a vertical portion, a bent portion, a curved portion, a straightening portion, and a horizontal portion from the upstream side to the downstream side in the casting direction. Such a configuration is a general configuration of a vertical bending type continuous casting machine.

The "vertical portion" is a portion where the casting path is vertical (radius of curvature is infinite). The "bent portion" is a portion where the radius of curvature of the casting path is smaller than infinity and larger than the radius of curvature of the curved portion. The "curved portion" is a portion where the radius of curvature of the casting path is the minimum, and a fixed value is determined for each machine. The "corrected portion" is a portion where the radius of curvature of the casting path is larger than the curved portion and smaller than infinity. The "horizontal portion" is a portion where the casting path is horizontal (radius of curvature is infinite).
The "casting direction" is a moving direction from when the molten steel 20 in the continuous casting machine 10 is supplied to the mold until it is pulled out by the pinch roll 16. The "upstream side in the casting direction" means the mold 13 side, and the "downstream side in the casting direction" means the pinch roll 16 side.
The "casting path" is a path along a line passing through the midpoint of the distance between the axial centers of the roll 14.

The drawing of the slab is as shown in FIGS. 1 and 2. First, the molten steel 20 (molten metal) supplied from the ladle (not shown) is injected into the mold 13 from the immersion nozzle 12 via the tundish 11. At this time, since the outer periphery of the injected molten steel 20 is in contact with the water-cooled mold 13, a solidified shell 21 is formed on the outer periphery of the molten steel 20. Next, the molten steel 20 (unsolidified slab 22) having the solidified shell 21 formed on the outside is supported by a plurality of rolls 14 arranged in a pair of upper and lower layers, and each roll moves to the downstream side in the casting direction. It is cooled by the spray water sprayed from the secondary cooling spray 15 installed between 14. Then, the molten steel 20 is completely solidified into slabs 30, which are pulled out from the continuous casting machine 10 by the pinch roll 16.

In this way, the molten steel 20 is continuously cast to produce the slab 30, but unlike the steady casting in which the molten steel 20 is supplied as described above, at the end of casting, the slab 30 is produced. Since the supply of the molten steel 20 ends at the rearmost end (upstream side in the casting direction), the unsolidified molten steel inside the molten steel 20 may leak. Therefore, after the casting is completed, it is necessary to perform the casting end control different from that at the time of steady casting.

“Leaked steel” means that molten steel leaks from the rearmost end of a slab due to squeezing of molten steel due to solidification shrinkage or reduction of a bulging portion. Also called bottom leak steel.

[Casting end control method 1]
The casting end control method 1 sets the start time of the casting end control for the purpose of suppressing the decrease in productivity without the risk of steel leakage even if the roll interval is not reduced or expanded after the casting is completed. The first step S1 (hereinafter, may be referred to as "first step S1") and the second step of controlling the continuous casting machine 10 under predetermined conditions from the start of casting end control to the end of casting. In step S2 (hereinafter, may be referred to as "second step S2") and when the end of the uncured slab 22 on the upstream side in the casting direction reaches the horizontal portion of the continuous casting machine 10 from the end of casting. It is characterized by including a third step S3 (hereinafter, may be referred to as "third step S3") that controls the continuous casting machine 10 under predetermined conditions. FIG. 3 shows a flowchart of the casting end control method 1.
The first step S1 to the third step S3 will be described below. Further, FIG. 4 shows a graph for explaining one example of the casting end control method 1. The vertical axis of FIG. 4 is the casting speed, and the horizontal axis is time.

(First step S1)
In the first step S1, set the cast end control at the start _{T 1.} Specifically, T ₁ is set at the start of casting end control by performing the following steps before the end of casting and after the final supply of the molten steel 20 from the ladle to the tundish 11. To do. The “final supply” means the first supply when the molten steel 20 is supplied from the ladle to the tundish 11 once, and the last supply when the molten steel 20 is supplied a plurality of times.
The setting method is as follows.

First, let A be half the length of the distance from the meniscus to the horizontal portion along the casting path during steady casting. "Until reaching the horizontal portion" specifically means until reaching the upstream end of the horizontal portion in the casting direction. In FIG. 1, it is until the boundary between the straightening portion and the horizontal portion is reached.
Next, the casting end control start amount is set from the range of the amount of molten metal required to produce a slab having a length of 0.8 A or more and 1.2 A or less. Then, set when the amount of molten metal in tundish 11 of a continuous casting machine 10 during steady-state casting has become the casting end control starting amount as cast end control at the start T _1.
Generally, the period from the start of steady casting to the end of casting is called the steady casting, but the "steady casting" in the casting end control method 1 is from the start of steady casting to the start of casting end control. It is between. As described later, the casting end control method 1, the cast end control at the start T ₁ after the operating conditions, in order to use different operating conditions from the steady state casting.

When the casting end control start amount is set to the amount of molten metal required to produce a slab with a length of less than 0.8A, the casting end control start due to the operation fluctuations (1) to (3) below. Further, the decrease in the molten metal level of the unsolidified molten steel after the second step is reduced, so that the risk of steel leakage increases. Further, if the casting end control start amount is set to the amount of molten metal required to produce a slab having a length exceeding 1.2A, the steel leakage prevention function is saturated, but the slab is originally produced. Since the number of slabs manufactured under cooling conditions different from the cooling conditions during steady casting, which is the optimum cooling condition, increases, many slabs with unsteady casting (slabs whose quality is inferior to that of the steady part) are generated. To do.
Casting according to (1) casting width, (2) steady casting end timing (how many tons of molten metal is left in the tundish 11), (3) casting speed (affected in the case of a machine with two or more strands), etc. Since T ₁ changes at the start of the end control, the amount of molten metal in the tundish 11 has the above range.

Here, the "meniscus" is the molten steel surface in the mold 13. Since A is defined as half the length of the distance from the meniscus during steady casting to reach the horizontal portion along the casting path, it is determined to be a unique length according to the continuous casting machine.

(Second step S2)
In the second step S2, the continuous casting machine 10 is controlled under predetermined conditions from T ₁ at the start of casting end control to T ₂ at the end of casting. Specifically, the amount of secondary cooling water in the casting path is controlled to a ratio of 70% or more and 90% or less of the amount of secondary cooling water before the start of casting end control, and the casting speed is 1.5 m /. Control to min or more and 1.8 m / min or less.
In FIG. 4, the casting speed is set to 1.7 m / min.

“At the end of casting” means when the molten steel supplied from the tundish 11 into the mold 13 is exhausted. Since "at the end of casting" is defined by the presence or absence of molten steel, slag remains in the tundish 11 at the end of casting. The remaining amount is about 1.0 to 5.0 ton, depending on the operating conditions. The "secondary cooling water amount" is the amount of water sprayed from the secondary cooling spray 15.

In the second step S2, by controlling the amount of secondary cooling water in the casting path to a ratio of 70% or more and 90% or less of the amount of secondary cooling water during steady casting, the solidified shell 21 becomes thinner and the shrinkage cavity length becomes shorter. Elongation increases the volume of unsolidified molten steel at the end of casting. When unsolidified molten steel to increase the volume, since the molten metal surface of the unsolidified molten steel is decreased by solidification shrinkage of the casting at the end T _2, it is possible to reduce the probability of occurrence of Mohagane.

Further, in the second step S2, by controlling the casting speed to 1.5 m / min or more and 1.8 m / min or less, it is possible to prevent cracking at the center of the slab due to an excessive shrinkage cavity length, and shrinkage without steel leakage. The nest length can be secured.

A "shrinkage cavity" is a hole generally formed by shrinkage of molten steel due to solidification, and is generated at the rearmost end of a slab.

(Third step S3)
In the third step S3, the continuous casting machine 10 is operated under predetermined conditions from T ₂ at the end of casting to T ₃ when the upstream end of the uncoiled slab 22 in the casting direction reaches the horizontal portion of the continuous casting machine 10. To control. Specifically, the unsolidified slab 22 is cooled by the amount of secondary cooling water set in the second step S2, and the casting speed is controlled to 1.2 m / min or more and 1.4 m / min or less.
In FIG. 4, the casting speed is set to 1.3 m / min.

In the third step, the unsolidified slab 22 is cooled by the amount of secondary cooling water set in the second step S2, and the casting speed is controlled to 1.2 m / min or more and 1.4 m / min or less. When the end portion of the solidified slab 22 on the upstream side in the casting direction is tilted to an angle close to horizontal to the ground, the unsolidified portion becomes small, so that the probability of steel leakage is reduced. In FIG. 1, for example, the probability of steel leakage near the straightening portion decreases.

As described above, the casting end control method 1 includes the first step S1 to the third step S3, so that there is no risk of steel leakage and the productivity is reduced without reducing or expanding the roll interval. It is possible to suppress it. That is, when the casting end control method 1 is carried out, it is not necessary to increase or decrease the roll interval of the roll 14 provided in the continuous casting machine 10.

Further, in the casting control method 1, the degree of heating of the molten metal in the tundish 11 is preferably 10 ° C. or higher and 45 ° C. or lower. As a general theory of continuous casting operation, the molten steel heating degree of the molten metal in the tundish 11 is about 10 to 60 ° C., but if the molten steel heating degree is too low, the solidification becomes too fast and the immersion nozzle is clogged. Operation failure will occur and smooth operation will not be possible. On the other hand, if the degree of heating of the molten steel is too high, the solidification of the molten steel is not completed and the risk of steel leakage increases. Therefore, in the casting control method 1, it is preferable to set the degree of heating of the molten metal in the tundish 11 to 10 ° C. or higher and 45 ° C. or lower.
Since the molten steel heating degree of the molten metal in the tundish 11 is 10 ° C. or higher and 45 ° C. or lower, the risk of poor operation and steel leakage is reduced, and the shrinkage cavity length depends only on the casting speed and the amount of secondary cooling water. Can be in a closed state (the length of the shrinkage nest can be controlled).

The "molten steel heating degree" means a temperature difference obtained by subtracting the liquidus temperature obtained from an equilibrium state diagram or the like from the actually measured molten steel temperature. The "liquidus line temperature" is the temperature at which the material transforms from the solid-liquid coexistence region to the liquid phase. The liquidus temperature differs depending on the composition of each component. The liquidus temperature of steel can be found from well-known equilibrium diagrams and thermodynamic data. The melting point is the point at which a pure substance transforms from a solid to a liquid as the temperature of the material is raised. Since steel is not a pure substance, it has a solid-liquid coexistence zone between solid and liquid.

Further, in the casting end control method 1, the length of the slab to be manufactured in the plate width direction is preferably 1450 mm or less. As a result, steel leakage can be prevented without adding a coolant.

From the above, the casting end control method 1 performed by using the vertical bending type continuous casting machine 10 has been described. As described above, the casting end control method of the present invention is applicable to the vertical bending type continuous casting machine. However, the continuous casting machine applicable to the present invention is not limited to this. For example, a curved continuous casting machine having a horizontal portion in the casting path may be used. Even if a curved continuous casting machine is used, the casting end control method of the present invention has the same effect.

Using the casting end control method described above, a slab having a length in the plate width direction of 1450 mm or less was produced. Various conditions were tried for conditions other than the length of the slab in the plate width direction. At this time, the degree of heating of the molten metal of the molten metal in the tundish was set to be 10 ° C. or higher and 45 ° C. or lower. In addition, the test was conducted without increasing or decreasing the roll interval of the rolls of the continuous casting machine.

As a result, the number of steel leaks was 0 in the 3000 casting operations.
Since Patent Document 4 describes that the molten steel generation rate is 1%, it was found that the casting end control method of the present invention has a very low risk of steel leakage.

10 Continuous casting machine 11 Tandish 12 Immersion nozzle 13 Mold 14 Roll 15 Secondary cooling spray 16 Pinch roll 20 Molten steel 21 Solidified shell 22 Unsolidified slab 30 Shard

Claims (4)

A casting end control method for continuous casting in which slabs are manufactured using a vertical bending type or curved type continuous casting machine having a horizontal portion in the casting path.
When the length of half of the distance from the meniscus to reach the horizontal portion along the casting path during steady casting is A, the slab having a length of 0.8A or more and 1.2A or less is manufactured. When the casting end control start amount is set from the range of the amount of molten metal required for the casting end control, and the amount of molten metal in the tundish of the continuous casting machine at the time of the steady casting reaches the casting end control start amount. In the first step, which is set as the start of casting end control,
From the start of the casting end control to the end of casting, the amount of secondary cooling water in the casting path is controlled to a ratio of 70% or more and 90% or less of the amount of secondary cooling water in the steady casting, and the casting speed. The second step of controlling the speed to 1.5 m / min or more and 1.8 m / min or less, and
From the end of casting to the time when the upstream end of the unsolidified slab in the casting direction reaches the horizontal portion, the unsolidified slab is cooled with the secondary cooling water amount set in the second step. It also includes a third step of controlling the casting speed to 1.2 m / min or more and 1.4 m / min or less.
Casting end control method. The casting end control method according to claim 1, wherein the length of the slab in the plate width direction is 1450 mm or less. The casting end control method according to claim 1 or 2, wherein the molten steel heating degree of the molten metal in the tundish is 10 ° C. or higher and 45 ° C. or lower. The casting end control method according to any one of claims 1 to 3, wherein the roll interval of the rolls provided in the continuous casting machine is not expanded or reduced.