JPH09141408A - Secondary cooling method for continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably enable the control of cooling capacity, particularly to a zone which is conventionally impossible to reach in the low side, by adjusting water flowing rate and also, changing the position of spray nozzles to control the cooling capacity. SOLUTION: The position of the spray nozzles 4 is executed so that the center of the spraying comes to roll 3 side at the time of weakly cooling to plactically use the indirect cool of the roll to the max. limit. By this method, the cooling capacity of the direct cool of the spray is reduced and also, the cooling capacity of the indirect cool of the roll is intensed. Therefore since the low cooling capacity which is conventionally impossible to reach because spraying ineffective range is developed in the case of using only the water quantity adjustment, is unforcedly obtd. and the stable operation can be obtd. with this cooling capacity, such problems as the excess cooling of a cast slab 10, the bending of the roll and the surface crack of the roll, can be solved at a stroke. Therefore, since the lower limit of the water quantity due to a nozzle characteristic is unnecessary to worry, even the nozzle having large upper limit value can be selected and high speed casting can be operated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary cooling method for continuous casting, and more particularly to continuous casting suitable for casting a steel type which is likely to have surface defects in a slab or a bloom by surface supercooling in continuous casting of steel. Secondary cooling method.

[0002]

2. Description of the Related Art FIG. 2 is an explanatory view of a conventional secondary casting secondary cooling method. As shown in FIG. 2, in order to prevent the so-called bulging phenomenon in which the solidified shell 2 bulges from the gaps (roll gaps) 7 between the plurality of arranged rolls 3 due to the pressure of the molten steel 1 inside the slab 10, the roll 3 Roll gap 7 to 30
They are arranged as close as possible to 50 mm. Then, the water supplied to the spray header 5 is sprayed as fine spray water droplets 6 through the roll gap 7 onto the solidification shell 2 by using the spray nozzle 4, to promote solidification, to appropriately control the surface temperature, and to prevent bulging. We are trying to prevent it.

As long as the continuous casting operation is always carried out at a constant speed, if a spray nozzle of a suitable model number is selected, the problem of secondary cooling does not occur.
However, in the actual operation, there are many cases in which the casting must be stopped or the drawing speed (casting speed) must be extremely reduced due to the replacement work of the immersion nozzle. Therefore, it is necessary to control the cooling capacity of the spray so that the amount of heat removed from the solidified shell 2 is increased during high speed casting and decreased during low speed or during stoppage. In particular, if the amount of heat removed at low speed is excessive, supercooling of the surface of the solidified shell 2 will progress, and eventually cracking defects will occur, and if the amount of heat removed at low speed is too small, the roll 3 will bend due to thermal deformation. It is well known that cracking occurs.

The cooling capacity of the spray is generally controlled by adjusting the pressure of water in the spray header 5 to change the amount of spray water. FIG. 3 is a characteristic diagram showing the relationship between the spray water amount and the cooling capacity, which is a conventional spray of water only (water spray) and a spray of mixed water (mist spray) which has been recently used. Is mentioned. As shown in FIG. 3, the cooling capacity of the spray increases monotonically as the amount of spray water increases. FIG.
In, a is a spray non-establishment region where the pressure is too low to form a spray, and b is a water amount capping region where the water amount does not increase so much even if the pressure is further increased.

[0005]

As can be seen from FIG. 3, in the spray nozzle, there is naturally an upper limit and a lower limit in the usable pressure. Regarding the water spray, for example, regarding a certain nozzle, when the pressure falls below 0.5 kg / cm ² , it does not function as a spray because it enters the non-spray area a, and the maximum pressure is usually about 10 kg / cm ² .
According to the well-known relationship (√ (10 / 0.5) ≒ 4.5), the variable range of the water amount in this variable pressure range is a range from the minimum amount to 4 to 5 times that which is never wide. .
Even if the pressure could be further increased, the effect of increasing the amount of water is small because the amount of water is effective at √. That is, the amount of water reaches the region b.

On the other hand, it is known that the cooling capacity Q in the secondary cooling by spraying in the continuous casting has a relationship of Q∝W ⁿ ; n = 0.5 to 0.7 ... (1) with respect to the water amount W. . According to the equation (1), when the variable range of the water amount W is 4 to 5 times, the variable range of the cooling capacity Q is 2 to 3 times. This means that even if the pressure is increased by 20 times (10 / 0.5 above), the cooling capacity increases only 2-3 times, and conversely, if the pressure is reduced to 1/20, the cooling capacity is 1 /
It shows that it is only reduced to 2 to 1/3.

As described above, recently, mist spray cooling in which air and water are simultaneously sprayed has been often performed in contrast to water spray cooling having a narrow control range. In the mist spray nozzle used for this, water droplets can be ejected by the force of air even at low water pressure. Therefore, as shown in FIG. Move to. That is, the control range of the amount of water capable of stably forming the spray is expanded.

However, contrary to the popular theory and theories that mist spray is slow cooling, in reality, as shown in FIG. 3, the mist spray has a higher cooling capacity than a water spray having the same amount of water. Moreover, the difference in cooling capacity between the two increases as the amount of water decreases. For this reason, the mist spray can expand the control range of the cooling capacity even if it can expand the control range of the water amount.

Due to such circumstances, in the prior art, regardless of the cooling method such as water spray or mist spray, if the secondary cooling equipment is designed with a specification that gives the required high cooling capacity at the maximum casting speed, it is possible to design the secondary cooling equipment at low speed casting. It was difficult to obtain the required low cooling capacity without difficulty. That is, if water is forcibly squeezed in order to obtain a low cooling capacity because of fear of overcooling, as a result of entering the spray failure region a, cooling becomes unstable and uneven, and not only the solidified shell but also the rolls are sparsely splashed with water. It often disappeared, and problems such as roll bending and roll surface cracking often occurred.

In view of the above-mentioned problems, it is an object of the present invention to provide a secondary cooling method for continuous casting, which can stably control the cooling capacity especially on the low side to a region where the conventional cooling capacity cannot be reached.

[0011]

SUMMARY OF THE INVENTION The present invention provides a secondary cooling method for continuous casting, in which a slab is secondarily cooled by spraying through a roll gap using a spray nozzle.
This is a secondary cooling method for continuous casting characterized by controlling the cooling capacity by adjusting the position of the spray nozzle in addition to adjusting the amount of water.

Further, in the present invention, it is preferable that the position of the spray nozzle is changed so that the center of the spray is placed in the center of the roll gap during the strong cooling and the center of the spray is placed on the roll side during the weak cooling. Then, such position change can be suitably performed by sliding and / or rotating the spray header.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an explanatory diagram of a secondary cooling method for continuous casting according to the present invention. The same members as those shown in FIG. 2 are designated by the same reference numerals and the description thereof will be omitted. As shown in FIG.
The secondary cooling method for continuous casting of the present invention is the spray nozzle 4
When the slab 10 is secondarily cooled by spraying through the roll gap 7 using, the cooling capacity is controlled by changing the position of the spray nozzle 4 in addition to adjusting the amount of water. In FIG. 1, regarding the position change of the spray nozzle 4, the strong cooling is shown by a dotted line, and the weak cooling is shown by a solid line. That is, in the present invention, the position of the spray nozzle 4 is changed so that the center of the spray is the roll gap 7 during strong cooling.
It is preferable to do so so that it comes to the center of the roll and, at the time of weak cooling, it comes to the roll 3 side. It should be noted that FIG. 1 discloses an example in which the position of the spray nozzle 4 is changed by slidingly shifting the spray header 5 to which the spray nozzle 4 is fixed along the surface of the solidification shell 2. You may rotate and go.

The reason why the above problems can be suitably solved by this configuration will be described below. Up to now, in the secondary cooling of continuous casting, how to effectively apply the spray water droplets 6 to the slab 10 has been emphasized, and it has always been the case that the spray water droplets 6 are not applied to the roll 3 as much as possible. That is, in many cases, the amount of water applied to the roll 3 is treated as an amount of water that is ineffective for cooling the slab.

However, the cooling of the slab proceeds by a combination of direct cooling by the spray water droplets 6 (spray direct cooling) and indirect cooling by contact with the roll cooled by the spray water droplets 6 (roll indirect cooling). Normally, the cooling capacity of direct spray cooling is greater, but for example, if the casting speed is very slow and spray direct cooling is completely stopped, direct spraying of water on the roll will protect the slab. As can be seen from the fact that it is cooled a little, the effect of indirect roll cooling is not negligible.

In the present invention, in order to make maximum use of such indirect roll cooling, the position of the spray nozzle is changed so that the center of the spray comes to the roll side during weak cooling. By this measure, the cooling capacity of the direct spray cooling is reduced and the cooling capacity of the indirect roll cooling is strengthened.
The low cooling capacity, which could not be reached because it enters the spray non-establishment area a, is reasonably obtained, and the cooling capacity enables stable operation, resulting in overcooling of the cast slab and roll bending / roll. The problem of surface cracks can be solved at once.

Further, according to the present invention, since it is not necessary to care about the lower limit of the water amount due to the nozzle characteristics, it is possible to select a nozzle having a larger lower limit value of the water amount, that is, a larger upper limit value, and further high speed casting becomes possible. .

[0018]

EXAMPLE The present invention was carried out in a slab continuous casting machine of a scale capable of continuously casting a slab having a thickness of 230 mm and a width of 800 to 1900 mm. The captain of this continuous casting machine (distance from the mold surface to the final roll) is 32 m, and 88 pairs of rolls are arranged after the mold. The roll gap is 32 mm in a narrow place and 45 mm in a wide place. The radius of curvature is 12m, and there is a correction point about 19m along the length of the captain.

The section for changing the position of the spray nozzle is
12m directly below the mold where the cooling water volume is high
The position of the spray nozzle is changed within the range of 20 mm in the casting direction for each roller apron segment composed of 5 pairs of rolls. Although not shown, the position of the spray nozzle was changed by moving a spray header integrated with each roller apron segment using a cylinder.

When a thick plate material for pipes containing Nb, V, etc., which is highly susceptible to surface cracking, is selected as a steel type, and when this is continuously cast, the casting is temporarily stopped, the immersion nozzle is exchanged for 3 minutes, and then the casting is restarted. The operation to do was done. During this stop, the spray water pressure was conventionally lower than the allowable lower limit of 0.5 kg / c.
It just dropped to m ² and continued spraying. The conventional method was used as a comparative example, and the pressure of the spray water was the same as that of the comparative example, but the position of the spray nozzle according to the present invention was changed under the above-described conditions.

Comparing the temperature of the lowest temperature part in the center of the slab width on the outlet side of the continuous casting machine (where the 88th roll exited), in the comparative example, the temperature decreased to 320 ° C. and was overcooled. However, in Example, the temperature was stopped at 560 ° C. and the progress of supercooling could be suppressed. Along with this, the number of fine cracks generated on the widthwise surface of the slab was 43 in the comparative example, but was drastically reduced to 2 in the example.

[0022]

EFFECTS OF THE INVENTION According to the present invention, in the secondary cooling of continuous casting, it is possible to reasonably obtain a low cooling capacity which was unattainable only by the conventional water amount adjustment. The surface defects of the slab due to overcooling are reduced, (2)
Roll bending and roll surface cracks are reduced, and (3)
Since it is not necessary to care about the lower limit of the water amount due to the nozzle characteristics, it is possible to select a nozzle having a larger lower limit value of the water amount, that is, a larger upper limit value, and it is possible to achieve further remarkable effects that further high speed casting is possible.

[Brief description of the drawings]

FIG. 1 is an explanatory view of a secondary cooling method for continuous casting according to the present invention.

FIG. 2 is an explanatory diagram of a conventional secondary casting secondary cooling method.

FIG. 3 is a characteristic diagram showing a relationship between a spray water amount and a cooling capacity.

[Explanation of symbols]

 1 Molten Steel 2 Solidified Shell 3 Roll 4 Spray Nozzle 5 Spray Header 6 Spray Water Drop 7 Roll Gap 10 Cast Piece a No Spray Area b Water Volume Heading Area

Claims (3)

[Claims] 1. In a secondary cooling method of continuous casting, in which a slab is secondarily cooled by spraying through a roll gap using a spray nozzle, the cooling capacity is controlled by changing the position of the spray nozzle in addition to adjusting the amount of water. A secondary cooling method for continuous casting, characterized by: 2. The position of the spray nozzle is changed so that the center of the spray is placed at the center of the roll gap during the strong cooling and the center of the spray is placed on the roll side during the weak cooling. Secondary casting secondary casting method. 3. The secondary cooling method for continuous casting according to claim 2, wherein the position of the spray nozzle is changed by sliding and / or rotating the spray header.