JP2011251290A - Secondary cooling method in continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide secondary cooling technology in continuous casting, allowing prevention of occurrence of a surface crack of a slab caused by supercooling even if casting speed is increased.SOLUTION: In this secondary cooling method in the continuous casting, the surface of the slab having surface oxide scales and the mold powder remainder is cooled by water jetted from a cooling nozzle in the whole or a part of secondary cooling of the slab in the continuous casting for casting steel material. At least one of an average droplet diameter and an average water temperature of the jetted water is set to a state in which a slab surface temperature is higher than an MHF (Minimum Heat Flux) point that is a temperature point at which film boiling changes to transition boiling, in the cooling by the cooling nozzle. For example, the average droplet diameter of the water jetted from the cooling nozzle is set to be 200 μm or below. Alternatively, the average droplet diameter of the water jetted from the cooling nozzle is set to be 200 μm or above, and the water temperature is set to be 50°C or above.

Description

  The present invention relates to a secondary cooling technique in continuous casting of steel, and more particularly to a secondary cooling technique suitable for uniform cooling of a slab during high-speed casting.

The steel continuous casting equipment is equipment for solidifying molten steel and continuously producing steel (slab). That is, the continuous casting equipment includes a tundish that pushes molten steel into a mold (mold), a mold that initially solidifies the molten steel, and then a secondary cooling zone that performs secondary cooling that solidifies the steel material to the center while gradually cooling. .
The secondary cooling is generally performed at a steel surface temperature of about 700 to 1000 ° C. However, with the recent increase in casting speed, the secondary cooling capacity is enhanced and the surface temperature during cooling tends to decrease. As the cooling capacity increases as the casting speed increases in this way, in recent years, supercooling has occurred where the surface of the cast piece is locally below 700 ° C. When supercooling occurs, it causes a slab surface crack as shown in FIG. For this reason, there is a limit to the increase in casting speed.

  On the other hand, Patent Documents 1 to 5 disclose cooling techniques using a cooling nozzle that prevents surface cracks of a slab.

Japanese Patent Laid-Open No. 50-103426 Japanese Patent No. 3779194 JP-A-57-91857 Japanese Patent Laid-Open No. 7-9191 JP-A-9-225599

According to the results of previous studies by the present inventors, it has been found that when the casting speed is increased, supercooling and surface cracks occur frequently.
As a result of intensive studies by the inventors, the reason is as follows. That is, when the casting speed is increased, the slab must be solidified quickly, so that the cooling capacity of the secondary cooling is increased, and accordingly, the amount of cooling water or mist injected from the cooling nozzle needs to be increased. The reason is that when the cooling capacity is increased and the cooling capacity is increased, the MHF (Minum Heat Flux) point, which is a point that changes from film boiling to transition boiling, increases. In addition, the collision pressure of the cooling water sprayed from the cooling spray onto the slab surface increases, and a film boiling state in which a vapor film exists between the slab surface and the cooling water causes the cooling water to steam to the slab surface. This is to enter transition boiling that breaks the membrane and comes into contact (see FIG. 2).

  Here, in Patent Document 1, a flat spray nozzle provided with a plurality of injection ports at the tip is used to reduce abrupt thermal amplitude between rolls during slab cooling and prevent surface cracking. It is disclosed. That is, the cooling area is expanded to suppress sudden supercooling. However, as long as the flat spray nozzle is used, the collision pressure during cooling is strong and the amount of cooling water is large, so that residual water cannot be prevented, and overcooling may occur as the casting speed increases.

  The technique described in Patent Document 2 uses a nozzle with a wider spray angle in the thickness direction than that of Patent Document 1 mainly including a flat spray nozzle, and performs secondary cooling with a cooling nozzle provided with a plurality of spray ports. It is a technology to do. In this technique, the collision pressure can be reduced as compared with cooling with a flat spray nozzle. However, since the generation of residual water cannot be prevented as long as it is performed with a normal cooling nozzle, overcooling may occur as the casting speed is increased.

In Patent Document 3, the cooling water is jetted at a high pressure of 25 to 100 kg / cm ² to prevent the stagnant water from being generated, and the uniform cooling is performed. The generation of the stagnant water (residual water) is caused by the high pressure water. It is intended to achieve uniform cooling with a certain effect. However, in the technique of Patent Document 3, since the cooling zone for secondary cooling is 20 m or more, and it is as long as 50 m, it is expensive to cool the entire cooling zone with high-pressure water, even in the upstream part. Even if it is narrowed down to just that, it is not practical because of the increased operating costs.

Here, for example, Patent Documents 4 and 5 disclose the droplet diameter of the cooling nozzle.
Patent Document 4 discloses secondary cooling with a droplet diameter of 100 μm or more. However, Patent Document 4 is a conceptional technique in which the droplet diameter of mist cooling is increased in order to prevent overcooling, and the droplet diameter is decreased in order to increase the cooling capacity. That is, in Patent Document 4, the idea is to reduce the droplet diameter in order to increase the cooling capacity, and there is no description or suggestion of the idea to reduce the droplet diameter to lower the MHF point as in the present invention. . Moreover, there is no description about the point which carries out secondary cooling of the slab surface which has a surface oxide scale and mold powder remainder.

  Patent Document 5 describes that the top portion (final casting portion) is cooled with a droplet diameter of 10 μm. At this time, the air-to-water ratio is raised to a range of 100 to 300 in order to temporarily strengthen the cooling at the top portion, but if the droplet diameter becomes 50 μm or less, it becomes floating (mist state) due to air resistance. Not practical. Moreover, even if it exists in patent document 5, there is no description or suggestion about the point which supercooling generate | occur | produces below MHF point, and the point which carries out secondary cooling of the slab surface which has a surface oxide scale and mold powder remainder. .

As described above, the references 4 and 5 do not disclose the idea of controlling the secondary cooling so that the surface temperature exceeds the MHF point in order to suppress overcooling. Also, no consideration is given to whether or not the surface of the slab has surface oxide scale and mold powder residue.
The present invention has been made paying attention to the above points, and even when the casting speed is increased, secondary cracking in continuous casting that can prevent the occurrence of surface cracks in the slab caused by supercooling. Provide cooling technology.

In order to solve the above-mentioned problems, the invention described in claim 1 of the present invention has a surface oxide scale and a mold powder residue in all or part of secondary cooling of a slab in continuous casting in which a steel material is cast. A secondary cooling method in continuous casting in which the slab surface is cooled by water sprayed from a cooling nozzle,
The average droplet diameter of the jetted water and the slab surface temperature are higher than the MHF (Minum Heat Flux) point, which is a temperature point at which film boiling changes to transition boiling. At least one of the average water temperatures is set.

Next, the invention described in claim 2 is characterized in that an average droplet diameter of water ejected from the cooling nozzle is set to 200 μm or less.
Next, the invention described in claim 3 is characterized in that the average droplet diameter of water ejected from the cooling nozzle is set to 200 μm or more and the water temperature is set to 50 ° C. or more.

  According to the present invention, by controlling to perform secondary cooling in a state where the slab surface temperature is higher than the MHF point, even if the casting speed is increased, surface cracks of the slab caused by supercooling are caused. It is possible to provide a secondary cooling technique in continuous casting that can prevent generation.

  Further, according to the knowledge obtained by the inventors through research and the like, the slab surface (hereinafter, also simply referred to as a slab scale surface) having a surface oxide scale and mold powder residue has an MHF point as compared with a normal scale surface. Is expensive. The scale thickness of a general rolled material is about 5 to 50 μm, but the scale thickness of the slab surface may be about 50 to 1000 μm, or may be 1000 μm or more with powder remaining. On the other hand, the present invention can prevent the occurrence of surface cracking of the slab even if it is intended for secondary cooling of the slab that normally has a high MHF point and cracks.

In addition, the inventors of the present invention have made it possible to increase the MHF point by controlling at least one of the average droplet diameter and the average water temperature of the jetted water when the slab is secondarily cooled by various experiments and the like. It was found that it was possible to keep the temperature below ℃.
And according to the invention which concerns on Claim 2, the raise of a MHF point can be suppressed to 600 degrees C or less.

Similarly, the invention according to claim 3 can suppress an increase in MHF point to 700 ° C. or lower. In addition, the average droplet diameter of a normal cooling nozzle is 2000 μm or less.
The rise in MHF point can be further suppressed to 600 ° C. or lower by setting the water temperature to 50 ° C. or higher and the average droplet diameter to 430 μm or lower, or the water temperature to 60 ° C. or higher.
As a result, even if the casting speed is increased, cooling in the film boiling point region can be ensured. As a result, even if the casting speed is increased, it is possible to provide a secondary cooling technique in continuous casting that can prevent the occurrence of surface cracks in the slab due to overcooling.

It is a figure explaining the cooling nonuniformity of a slab, and the part becoming a surface crack. It is a conceptual diagram explaining MHF point and temperature nonuniformity generation | occurrence | production. It is a figure explaining the influence on the MHF point by a droplet diameter and water temperature. It is a figure explaining the influence on the MHF point by a droplet diameter and water temperature. It is a figure explaining the continuous casting installation which concerns on embodiment based on this invention. FIG. 3 is a diagram illustrating Example 1. FIG. 6 is a diagram illustrating Example 2. FIG. 6 is a diagram for explaining a third embodiment.

Next, an embodiment according to the present invention will be described with reference to the drawings.
In FIG. 5, the block diagram of the continuous casting equipment of steel materials is shown.
As shown in FIG. 5, the continuous casting equipment includes a tundish 1 that pushes molten steel into the mold 6, a mold 6 that initially solidifies the molten steel, a plurality of support rolls 3 that guide the slab 2 while gradually cooling, and a moving torch. A cutter 5 is provided. Also. A cooling nozzle 4 for secondary cooling is inserted between adjacent support rolls 3. FIG. 4 illustrates the case where the secondary cooling zone is partitioned into first to fourth zones.

In the present embodiment, a slab whose surface is a slab scale surface is a secondary cooling target.
And in this embodiment, a micro droplet cooling nozzle is employ | adopted as said each cooling nozzle 4, and the average droplet diameter of the water sprayed from a nozzle is set to 200 micrometers or less.
Alternatively, the average droplet diameter of water sprayed from the cooling nozzle 4 is set to 200 μm or more, and the water temperature is set to 50 ° C. or more.

By doing in this way, the MHF point which is a temperature point which changes from film | membrane boiling to transition boiling can be set so that it may become 700 degrees C or less.
Furthermore, the slab surface temperature is higher than the MHF point, for example, the slab surface temperature is higher than 700 ° C. and the cooling condition is within the range of 1000 ° C. or less, the casting speed, the temperature of the molten steel, etc. The amount of water sprayed on the slab surface is adjusted based on the above.

Thereby, even if it is a slab scale surface slab whose MHF point becomes high by inventors' knowledge, generation | occurrence | production of the surface crack of the slab caused by overcooling can be prevented.
Here, as described above, when cooling is performed in a state where the surface temperature of the slab is lower than the MHF point, temperature unevenness (supercooling) occurs, and the temperature unevenness and the surface crack are closely related. In secondary cooling, it is necessary to always cool above the MHF point.

From the investigations by the inventors so far, it has been found that the slab scale surface has a higher MHF point than the normal scale surface. FIG. 3 shows a result of comparing the MHF point when the slab scale surface is cut out with an average droplet diameter of 430 μm and reheated, and the MHF point when the same material is milled and reheated. In this investigation, the scale of the slab scale surface was about 320 μm thick, and alumina (Al ₂ O ₃ ) and silica (SiO ₂ ) were contained in the scale. The milled scale surface produced scales of up to 11 microns upon reheating at 1000 ° C. for 10 minutes. From the results of FIG. 3, the inventors found that the slab scale surface has a higher MHF point than the normal scale surface. The slab scale surface is thicker than the normal scale surface, and the powder contains aluminum oxide. It was found that this is due to the high affinity with water due to the inclusion of silicon oxide.

  Therefore, the present inventors paid attention to the diameter of a droplet ejected from a cooling nozzle such as a mist spray nozzle. The result of the MHF point when the slab scale surface is cooled by changing the droplet diameter is shown in FIG. As shown in FIG. 4, it can be seen that the MHF point decreases if the droplet diameter ejected from a cooling nozzle such as a mist spray nozzle is small, and the MHF point decreases if the water temperature during cooling is high. It was.

Here, as shown in FIG. 4, if the average droplet diameter is set to 200 μm or less, the MHF point can be set to 600 ° C. or less with respect to the slab whose surface is a slab scale surface.
As shown in FIG. 4, when the water temperature is 50 ° C. or higher, the MHF point is 700 ° C. or lower if the average droplet diameter is 2000 μm or less. The average droplet diameter of a normal cooling spray is 2000 μm or less. Furthermore, if the water temperature is set to 50 ° C. or more and the average droplet diameter is set to 430 μm or less, or the water temperature is set to 60 ° C. or more and the average droplet diameter is 2000 μm or less, the MHF point may be set to 600 ° C. or less. I can do it.

  As described above, if secondary cooling is performed by lowering the MHF point, supercooling hardly occurs. If the MHF point is 700 ° C. or lower, preferably 600 ° C. or lower, temperature unevenness hardly occurs.

"Example 1"
Example 1 of the present invention using the continuous casting equipment described in the above embodiment will be described.
As the continuous casting equipment, a continuous casting equipment capable of casting a slab having a length of 45 m and a width of 2 m was used. In the secondary cooling zone downstream from the mold 6, the cooling by the normal cooling nozzle having an average droplet diameter of 400 μm and the micro droplet cooling nozzle having an average droplet diameter of 200 μm according to the present invention can be switched and used. Arranged.

Here, the droplet diameter of the liquid being ejected from the cooling nozzle was investigated by a moving Doppler method (PDA: Phase Doppler Anemometer). The average droplet diameter was Sauder average diameter (D32), and measurement was performed 5 times under each condition, and the average value was used. Strictly speaking, the ratio between the total volume of the measured droplets and the total surface area (D32 = Σ (n _i · d _i ³ ) / Σ (n _i · d _i ² ), n _i is the number of particles, d _i is the diameter) called the Sauder average diameter. The same applies to other embodiments.

First, casting was started at a cooling water temperature of 27 ° C. by cooling with a normal cooling nozzle having a width of 2000 mm, a plate thickness of 250 mm, and a casting speed of 1.5 mpm and an average droplet diameter of 400 μm. When the temperature was measured by a temperature profile meter (not shown) installed between the third zone and the fourth zone at the beginning of casting, the temperature deviation was uniform within about 40 ° C. as shown in the temperature distribution shown in FIG. Secondary cooling was performed. However, when the casting speed was increased to 2.0 mpm and the secondary cooling water was increased under the same conditions, the temperature deviation expanded to 250 ° C. or more. Thus, when the temperature deviation increased, cooling water was supplied to the about 200 μm micro droplet cooling nozzle of the present invention. The temperature deviation decreased about 5 minutes after the start of supply, and the temperature deviation became within 45 ° C. after about 10 minutes (see FIG. 6).
Thus, it can be seen that by applying the present invention, the occurrence of supercooling can be prevented even if the casting speed is increased.

"Example 2"
The same continuous casting equipment as in Example 1 was used. Then, in the secondary cooling zone downstream of the mold 6, the cooling by the normal cooling nozzle having an average droplet diameter of 400 μm and the micro droplet cooling nozzle of about 200 μm according to the present invention were arranged so as to be used.

First, casting was started at a cooling water temperature of 27 ° C. by cooling with a normal cooling nozzle having a width of 2000 mm, a plate thickness of 250 mm, and a casting speed of 1.5 mpm and an average droplet diameter of 400 μm. At the beginning of casting, the temperature was measured with a temperature profile meter (not shown) installed between the third zone and the fourth zone, and as shown in the temperature distribution shown in FIG. Although secondary cooling was performed, when the casting speed was increased to 2.0 mpm and the secondary cooling water was increased, supercooling occurred and the temperature deviation expanded to 250 ° C. or more. Thus, when the temperature deviation increased, cooling water was supplied to the third and fourth zones to the micro droplet cooling nozzle of about 200 μm of the present invention. Even after 10 minutes from the start of supply, the temperature deviation was 200 ° C. or more. Therefore, the cooling water was supplied to the first and second zones where supercooling appears to have occurred, to the fine droplet nozzle of about 200 μm of the present invention. The temperature deviation decreased about 5 minutes after the start of supply, and the temperature deviation became within 45 ° C. after about 10 minutes (see FIG. 7).
Thus, it can be seen that by applying the present invention, the occurrence of supercooling can be prevented even if the casting speed is increased.

"Example 3"
The same continuous casting equipment as in Example 1 was used. Then, cooling by a normal cooling nozzle having an average droplet diameter of 400 μm is installed in the secondary cooling zone downstream from the mold 6, and further arranged so that hot water of 50 ° C. or more can be supplied.

  First, casting was started at a cooling water temperature of 27 ° C. by cooling with a normal cooling nozzle having a width of 2000 mm, a plate thickness of 250 mm, and a casting speed of 1.5 mpm and an average droplet diameter of 400 μm. At the beginning of casting, the temperature was measured with a temperature profile meter (not shown) installed between the third and fourth zones, and as shown in the temperature distribution of FIG. Although cooling was performed, when the casting speed was increased to 2.0 mpm and the secondary cooling water was increased, supercooling occurred and the temperature deviation expanded to 250 ° C. or more. Thus, when the temperature deviation has increased, cooling water having a water temperature of 50 ° C. was supplied to the first and second zones. The temperature deviation decreased about 5 minutes after the start of supply, and the temperature deviation became within 45 ° C. after about 10 minutes (see FIG. 8).

Thus, it can be seen that by applying the present invention, the occurrence of supercooling can be prevented even if the casting speed is increased.
As can be seen from the above, by performing secondary cooling with a fine droplet nozzle of 200 μm or less, and in the case of a cooling nozzle of 200 μm or more, by supplying cooling water of 50 ° C. or higher to the secondary cooling water, Generation | occurrence | production of cooling can be suppressed and generation | occurrence | production of the surface crack of the slab caused by overcooling can be prevented. In addition, since high-speed casting is possible, it is possible to increase the production of slabs and contribute to cost reduction.

1 Tundish 2 Slab 3 Support roll 4 Cooling nozzle 5 Moving torch cutter 6 Mold

Claims (3)

Secondary cooling in continuous casting in which cooling is performed by water sprayed from a cooling nozzle on the surface of the slab having a surface oxide scale and mold powder residue in all or part of the secondary cooling of the slab in continuous casting for casting steel materials. A method,
The average droplet diameter of the jetted water and the slab surface temperature are higher than the MHF (Minum Heat Flux) point, which is a temperature point at which film boiling changes to transition boiling. A secondary cooling method in continuous casting, wherein at least one of the average water temperatures is set.   The secondary cooling method in continuous casting according to claim 1, wherein an average droplet diameter of water sprayed from the cooling nozzle is set to 200 µm or less.   2. The secondary cooling method in continuous casting according to claim 1, wherein an average droplet diameter of water ejected from the cooling nozzle is set to 200 μm or more and a water temperature thereof is set to 50 ° C. or more.

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