JP4760303B2 - Secondary cooling method for continuous cast slabs - Google Patents

Description

  The present invention relates to a method for cooling a slab in a secondary cooling zone of a continuous casting machine, and more particularly to a method for efficiently cooling a slab by air mist spraying.

  In continuous casting of steel, molten steel poured from a ladle into a tundish is poured into a water-cooled mold through an immersion nozzle installed at the bottom of the tundish, and then the solidified shell formed by the mold is used. The cast slab as the outer shell is continuously drawn below the mold while being cooled, and a continuous cast slab is manufactured. In this case, first, in the mold, the molten steel is cooled by contacting the mold to form a solidified shell. After that, the slab that has passed through the mold is supported by a cooling grid or a support roll immediately below the mold, and further below it is drawn by a pinch roll while being supported by a guide roll. These cooling grids, support rolls, and guide rolls are called slab support / guide devices. The slab is supported by the slab support / guide device to prevent the slab from bulging in the thickness direction (referred to as “bulging”). The slab support / guide device is provided with a spray nozzle, and the slab is pulled out while being cooled by cooling water sprayed from the spray nozzle, and eventually solidification to the center is completed. Then, it cut | disconnects to predetermined length with the slab cutting machine installed in the machine end of the continuous casting machine, and a continuous casting slab is manufactured. The range of the slab support / guide device in which the spray nozzle is arranged is called a secondary cooling zone.

  By the way, in recent years, in order to reduce the manufacturing cost, improvement of productivity has been demanded more than ever, and in the continuous casting process, the speed of the production line, that is, the drawing speed of the slab is increased. . In order to realize the high drawing speed, various problems need to be solved, and among them, a technique for cooling the slab more efficiently is required. Under high-speed casting, the thickness of the solidified shell immediately below the mold becomes thin, and this solidified shell breaks and breakout occurs, or the solidified shell does not break. Due to the pressure, bulging occurs, and the molten steel surface in the mold fluctuates up and down, and mold powder is caught in the solidified shell, resulting in problems such as quality defects. That is, in order to prevent bulging in the secondary cooling zone, a method for efficiently cooling the slab is required.

  Conventionally, as a general method for cooling a slab in a secondary cooling zone, a two-fluid system in which water-only air-spray (hereinafter referred to as “water spray”) is mixed with water and air is used. Two types of spray (hereinafter referred to as “air mist spray”) are used. In the water spray, a nozzle tip is attached to the tip of a cooling water pipe, pressurized water is sprayed from the nozzle tip toward the cast piece, and the cast piece is cooled. Air mist spray, on the other hand, joins a pipe to which cooling water is supplied and a pipe to which air is supplied, mixes pressurized water and air into a mist state, and directs this from the nozzle tip toward the slab. It is sprayed and cooled.

  In general, a water spray has a simple structure, but there is a problem that the particle size of water to be sprayed is large, and cooling is likely to be non-uniform compared to an air mist spray. In addition, there is a problem that water spray tends to clog dust and the like in the nozzle tip, and in recent years, air mist spray has become mainstream. Air mist spray is known to have a large changeable range of water supply without disrupting the flow distribution, so-called turndown is larger than water spray, and in high speed casting, the amount of change in the slab drawing speed is large. That is, the amount of change in the amount of cooling water is large, and this is also the reason why air mist sprays that can provide a stable water amount distribution even if the amount of change in water amount is large are adopted in recent continuous casting machines.

  It is known that the cooling capacity of the water spray is affected by the water density (see, for example, Non-Patent Document 1), and that the cooling capacity of the air mist spray is affected by the water density and the air flow density. (See, for example, Non-Patent Document 2). From these, it can be seen that in order to increase the cooling capacity of the water spray and air mist spray, the water density should be increased. In the case of air mist spray, it is effective to increase the air flow density. However, according to Non-Patent Document 2, it can be seen that the influence of the air flow density is smaller than the water flow density.

  However, in order to increase the water density, not only the pumps but also the water circulation treatment facilities such as the sedimentation tank, cooling tower, and water supply tank need to be strengthened, which increases the cost of equipment modification. A problem occurs. In addition, if the nozzle tip is changed to a high water flow type, in the case of air mist spray, it is necessary to make changes including the arrangement of the nozzle. In that case, not only hardware modification but also software for water volume control is required. Since remodeling is also necessary, huge capital investment is also required.

Under such circumstances, Patent Document 1 injects high-pressure water at 25 kg / cm ² (2.5 MPa) or more and 100 kg / cm ² (9.8 MPa) or less as means for improving the cooling capacity of the secondary cooling spray. A water spray has been proposed. In this case, it is necessary to make all the cooling water pipes up to the tip of the nozzle tip into high-pressure pipes, which is not realistic in view of equipment costs. Further, in the case of water spray, the above-described turndown range is narrow, so that it is not suitable for a continuous casting machine that requires a large change in water amount for high-speed casting.

Further, Patent Document 2, in order to improve the cooling capacity, 300L / min · m and ^more 600L / min · m ² or less of the cooling water, 3Nm ³ / min · m ² or more 18Nm ³ / min · m ² An air mist spray having the following air flow rate has been proposed. However, under this spraying condition, if the nozzle tip structure is not appropriate, only a large amount of air will flow, and as a result, the collision pressure of water particles on the slab cannot be increased. There is a problem that the diameter may become too small to be a mist that contributes to cooling the slab.
JP-A-57-91857 JP-A-10-109150 Mitsuka, iron and steel, vol.54 (1968), No.14. p. 1457-1470 Teshima et al., Iron and steel, vol.74 (1988), No.7. p. 1282-1289

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for efficiently cooling a slab by air mist spraying in a secondary cooling zone of a continuous casting machine.

  The method for secondary cooling of a continuous cast slab according to the first aspect of the present invention for solving the above-described problem is that when cooling the slab using an air mist spray installed in a secondary cooling zone of a continuous casting machine, The pressure of the air immediately before mixing the air and air is in the range of 0.5 MPa to 1 MPa.

  A secondary cooling method for a continuous cast slab according to a second invention is the method according to the first invention, wherein the impingement pressure of the atomized water particles on the slab immediately below the nozzle tip of the air mist spray is 2 kPa or more. The spraying thickness of the mist-like water particles in is set to 40 mm or more.

  According to the present invention, when cooling a slab using an air mist spray installed in a secondary cooling zone of a continuous casting machine, the pressure of air immediately before mixing water and air is 0.5 MPa or more and 1 MPa or less. Therefore, the slab can be efficiently cooled even with the same amount of cooling water. As a result, even in continuous casting operations where the slab drawing speed has been increased in recent years, it is possible to stably cast high-quality slabs without causing operational troubles, which has industrially beneficial effects. Brought about.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. In FIG. 1, the schematic of the condition which is cooling the slab by air mist spray is shown.

  As shown in FIG. 1, the air mist spray 3 is supplied by a water supply pipe 7 that supplies water, an air supply pipe 8 that supplies air, a water supply pipe 7, and an air supply pipe 8. A mixing unit 6 in which water and air are mixed, a mixing pipe 5 serving as a flow path for the mixed water and air, and a nozzle tip 4 attached to the tip of the mixing pipe 5 are provided. The nozzle tip 4 is arranged in the gap between the slab support rolls 1 that are adjacent to the casting direction and is a support / guide device for the slab 2, and is sprayed from the nozzle tip 4 with mist-like water particles 11 (hereinafter referred to as “mist”). The slab 2 is cooled by colliding with the surface of the slab 2. A water pressure measurement pressure gauge 9 for measuring the pressure of the supplied water is installed in the water supply pipe 7, and an air pressure measurement pressure gauge 10 for measuring the pressure of the supplied air is installed in the air supply pipe 8. Has been. The water supply pipe 7 is connected to a pump (not shown) for supplying water, and the air supply pipe 8 is connected to a pump (not shown) for supplying air. Moreover, in FIG. 1, the slab support roll 1 and the air mist spray 3 are installed only on the left side of the slab 2, but in FIG. 1, the right side of the slab 2 is omitted, and the slab is also on the right side. A support roll 1 and an air mist spray 3 are installed.

  In the air mist spray 3 configured as described above, it is the structure of the nozzle tip 4 that determines the spray characteristics (water amount distribution, collision pressure distribution, water particle size distribution) of the mist 11, and therefore, the cooling efficiency is excellent. In order to develop the air mist spray 3, the structure of the nozzle tip 4 was examined.

  As a method for controlling the spray characteristics of the mist 11, two methods are used in actual operation: a method of controlling the flow rate and a method of controlling the pressure. In many cases, the control method is performed by simple pressure control. In the present invention, the spray characteristics are controlled by the method of controlling the pressure of the supplied water and air. The pressure of the fluid flowing in the pipe has a problem of pressure loss, and may show a different value depending on the measurement location. Therefore, in the present invention, as shown in FIG. 1, the pressures of water and air are measured by the water supply pipe 7 and the air supply pipe 8 immediately before the water and air are mixed. That is, a conventional pressure-reducing valve (not shown) attached to a pump or supply pipe for supplying water or air so that the pressure measured by the water pressure measurement pressure gauge 9 and the air pressure measurement pressure gauge 10 becomes a predetermined value. ) And thereby controlled to optimum spray conditions.

  However, it is known that the pressure of the fluid flowing in the pipe does not cause a large pressure loss unless the inner diameter of the pipe is enlarged. That is, the positions of the water pressure measurement pressure gauge 9 and the air pressure measurement pressure gauge 10 are preferably closer to the mixing unit 6, but there is no problem even if the pipe diameters are the same. Further, even if the pipe diameters are not the same, it is known that there is no large pressure loss if the pressure gauge is installed in a pipe larger than the pipe diameter of the mixing unit 6, so this is not a problem.

  Among the spray characteristics of the mist 11, the water amount distribution in the width direction of the mist 11 can be measured as shown in FIG. 2, for example. That is, the collection box 12 is arranged in the width direction of the mist 11 below the nozzle tip 4 of the air mist spray 3 installed so as to have a predetermined nozzle height, and the mist 11 is disposed from the nozzle tip 4 for a predetermined time. By spraying and quantifying the amount of water accumulated in each collection box 12, the water amount distribution in the width direction of the mist 11 can be obtained. For example, if the location where the accumulated water amount is the largest is 100% and the ratio to the water amount is displayed as a percentage, the water amount distribution in the width direction can be obtained. Here, the water amount distribution in the width direction of the mist 11 is a water amount distribution in the direction orthogonal to the casting direction of the slab 2, that is, in the same direction as the width direction of the slab 2. FIG. 2 is a schematic diagram illustrating a method for measuring the water amount distribution in the width direction of the mist 11.

  Similarly, the water amount distribution in the thickness direction of the mist 11 can be obtained, for example, as shown in FIG. That is, the collection box 12 is arranged side by side in the thickness direction of the mist 11 below the nozzle tip 4 of the air mist spray 3 installed so as to have a predetermined nozzle height, and the mist 11 is removed from the nozzle tip 4 for a predetermined time. By spraying and quantifying the amount of water accumulated in each collection box 12, the water amount distribution in the thickness direction of the mist 11 can be obtained. For example, the distribution of the amount of water in the thickness direction can be obtained by setting the most accumulated water amount as 100% and displaying the ratio to the amount of water as a percentage. The spray thickness of the mist 11 can be obtained from the water amount distribution in the thickness direction. Here, the spray thickness of the mist 11 refers to the length in the thickness direction of the region where the water amount distribution is 25% or more. Since this spray thickness greatly varies depending on the position of the mist 11 in the width direction, the spray thickness immediately below the nozzle tip 4 is defined as the spray thickness of the air mist spray 3 in the present invention. The water amount distribution in the thickness direction of the mist 11 is a water amount distribution in the same direction as the casting direction of the slab 2. FIG. 3 is a schematic diagram showing a method for measuring the water amount distribution in the thickness direction of the mist 11.

Of the spray characteristics of the mist 11, the collision pressure distribution of the mist 11 can be determined as shown in FIGS. 4 and 5, for example. That is, a collision pressure measuring sensor 13 composed of a commercially available strain gauge is disposed below the nozzle tip 4 of the air mist spray 3 installed to have a predetermined nozzle height, and the mist 11 is removed from the nozzle tip 4. Spraying and moving the collision pressure measurement sensor 13 in the width direction and the thickness direction of the mist 11 makes it possible to obtain the collision pressure distribution of the mist 11. If the size of the collision pressure measuring sensor 13 is 1 cm both vertically and horizontally, the collision pressure per cm ³ can be directly measured. In the present invention, the collision pressure measurement sensor 13 is moved in the width direction and the thickness direction of the mist 11, and the maximum value of the obtained collision pressure is defined as the collision pressure of the air mist spray 3. 4 and 5 are schematic views showing a method for measuring the collision pressure distribution of the mist 11. Even if the collision pressure measuring sensor 13 is fixed and the nozzle tip 4 is moved, the collision pressure distribution of the mist 11 can be obtained.

  Among the spray characteristics of the mist 11, it is known from experience that the water particle size distribution has a small amount of change when the pressure of the supplied air becomes 0.2 MPa or more. The spray characteristics of the mist 11 were evaluated based on the water amount distribution of the mist 11 and the collision pressure.

  The prototype nozzle tip 4 is sprayed by the above-described method, and the water amount distribution in the width direction, the water amount distribution in the thickness direction, and the collision pressure distribution of the prototype nozzle tip 4 are measured. The spray thickness and the collision pressure of the mist 11 were determined.

  In addition, the prototype nozzle tip 4 was measured for cooling ability by the following hot experiment to verify the cooling effect.

  As a method for laboratory evaluation of the cooling capacity, a method is generally used in which sprayed water is sprayed on the heated steel material to cool it, and the heat transfer coefficient is obtained from the temperature history of the steel material. A hole was provided on the side opposite to the surface to be cooled, a thermocouple was embedded therein, and the temperature history was measured with the thermocouple.

  The experiment was carried out using an experimental apparatus that simulated the arrangement of air mist spray 3 in actual operation as shown in FIG. FIG. 6 simulates the vertical part of a continuous casting machine. A slab support roll 1 is brought into contact with a steel material 14 instead of a slab, and a nozzle tip 4 is arranged in a gap between the slab support roll 1. The steel material 14 was cooled by spraying the mist 11 from 4. The specifications of the nozzle tip 4 used are a water volume of 6.6 to 48 L / min and an air pressure of 0.3 to 1.2 MPa. FIG. 6 is a schematic diagram of an experimental apparatus for comparing and evaluating the cooling ability of the air mist spray 3.

  As the steel material 14 to be heated, a carbon steel material having a width of 280 mm, a height of 560 mm, a thickness of 20 mm, and a carbon concentration of 0.2% by mass is used. Seven holes 8 mm deep and 18 mm deep were made, and a K-type sheathed thermocouple 15 having a diameter of 1.6 mm was embedded therein. As shown in FIG. 6, the thermocouple 15 is embedded at positions of 0 mm, 25 mm, 50 mm, 100 mm, and 150 mm on the upper side in the thickness direction of the mist 11 with respect to the position immediately below the nozzle chip 4, and the thickness of the mist 11. There are seven locations at 37.5 mm and 75 mm on the lower side in the direction. In FIG. 6, the upper side in the thickness direction of the mist 11 is indicated by “+”, and the lower side in the thickness direction is indicated by “−”.

  In the experiment, the steel material 14 was heated for about 1 hour in an electric furnace maintained at 1200 ° C., and the uniformly heated steel material 14 was taken out and fixed to the experimental apparatus, and cooling was started. During the cooling of the steel material 14, the temperature measurement values at seven points by the thermocouple 15 were taken into a personal computer every 0.1 second. After the experiment, the heat transfer coefficient at the position of each thermocouple 15 is calculated from the measured temperature history, the average heat transfer coefficient between the slab support rolls 1 including radiation cooling is calculated, and the cooling capacity is calculated based on this. evaluated. In the present invention, the value of the heat transfer coefficient including conduction, convection, and radiation is defined as the overall heat transfer coefficient. In the present invention, the cooling capacity was compared using the overall heat transfer coefficient at 850 ° C., which is estimated to be close to the average surface temperature of the continuous cast slab.

  A hot experiment was conducted by changing the air pressure from a range of 0.3 MPa to 1.2 MPa with the water amount set at two levels of 23 L / min and 48 L / min. FIG. 7 shows the relationship between the overall heat transfer coefficient at 850 ° C. and the air pressure measured by this experiment. As shown in FIG. 7, it has been found that the cooling capacity is higher when the air pressure is 0.5 MPa or more and 1.0 MPa or less, compared to the case where the air pressure is 0.3 MPa which is normally used. Generally, when the air pressure is increased, the water pressure needs to be increased accordingly. From the viewpoint of preventing the water pressure from becoming too high, the air pressure is preferably 0.5 MPa or more and 0.8 MPa or less.

  Moreover, it was found from experiments in which the spray thickness of the mist 11 was changed that the spray thickness of the mist 11 is preferably 40 mm or more in order to maintain a high cooling capacity. When the spray thickness of the mist 11 is less than 40 mm, it is considered that the cooling time of the slab 2 is shortened and the cooling capacity is lowered. The upper limit of the spray thickness of the mist 11 does not need to be specified in particular, but it is sufficient that it is within the gap between adjacent slab support rolls 1, for example, about 200 mm.

  Furthermore, from experiments in which the collision pressure of the mist 11 was changed, it was found that the collision pressure of the mist 11 is preferably 2 kPa or more in order to maintain a high cooling capacity. When the collision pressure of the mist 11 is less than 2 kPa, the collision pressure is too low and the cooling capacity is reduced. The upper limit of the collision pressure of the mist 11 is naturally determined according to the pressure of the supplied air.

  That is, in order to improve the cooling capacity by the air mist spray 3, it is necessary to use the nozzle tip 4 in which the pressure of the supplied air is 0.5 MPa or more and 1.0 MPa or less. It has been found that it is preferable to use the nozzle tip 4 having a spray thickness of 40 mm or more and a mist 11 collision pressure of 2 kPa or more. In other words, it has been found that it is necessary to design the nozzle tip 4 that satisfies these conditions. If the pressure of the air supplied by attaching the nozzle tip 4 to the mixing pipe 5 based on such a design concept is 0.5 MPa or more and 1.0 MPa or less, the slab 2 can be efficiently cooled. The

  In designing the nozzle tip 4, it is necessary to grasp the spray amount and pressure conditions that can be supplied from each air mist spray 3. Here, the spray amount refers to the amount of water and air supplied from a pump that supplies water or air, and the pressure condition refers to the pressure of the supplied water and air. When the structure of the nozzle tip 4 is different, the pressure conditions are different even when the same amount of water and air are supplied. When the supply pressure of air does not become 0.5 MPa or more and 1.0 MPa or less, the present invention cannot be implemented unless the equipment is remodeled to satisfy this condition.

  It is also necessary to set an area to be cooled by one air mist spray 3. When cooling a bloom slab whose width of the slab 2 is approximately 300 to 500 mm, the entire width direction of the bloom slab is cooled only by installing one air mist spray 3 in the width direction of the slab 2. However, when cooling a slab slab whose width of the slab 2 exceeds 2000 mm, the entire slab width direction cannot be cooled by only one air mist spray 3, and therefore, a plurality of air mist sprays 3. Must be installed in the slab width direction. In this case, it is necessary to determine the spray angle in the width direction in consideration of the width that can be cooled by one air mist spray 3. In that case, it is necessary to arrange so that the water amount distribution is uniform in the width direction in consideration of a portion where the mists 11 sprayed from the plurality of air mist sprays 3 overlap.

  As described above, according to the present invention, it is possible to efficiently cool the slab 2 even with the same amount of cooling water, and as a result, it is possible to perform continuous casting operation with an increased slab drawing speed. However, stable casting of high quality slabs without causing operational troubles is achieved.

  Four types of nozzle chips (nozzle chips A to D) were made as prototypes, and a steel material cooling experiment was performed using the experimental apparatus shown in FIG. The nozzle tip A and the nozzle tip C are nozzle tips designed with the pressure of the supplied air being 0.5 MPa or less, while the nozzle tip B and the nozzle tip D are designed with the pressure of the supplied air being 0.5 MPa or more. Nozzle tip. For these nozzle tips, the water amount distribution in the width direction of the mist, the water amount distribution in the thickness direction, and the collision pressure were measured in advance.

  In the experiment of the nozzle tip A and the nozzle tip B, the nozzle height was 126 mm and the water amount was 47.9 L / min (test No. 1 to 3). In the nozzle tip A experiment (test No. 1), the air pressure was 0.28 MPa, and in the nozzle tip B experiment (test No. 2-3), the air pressure was 0.50 MPa and 0.58 MPa. . Moreover, in the experiment of the nozzle chip C and the nozzle chip D, the nozzle height was set to 110 mm and the water amount was set to 23.3 L / min (test Nos. 4 to 6). In the nozzle tip C experiment (test No. 4), the air pressure was 0.45 MPa, and in the nozzle tip D experiment (test Nos. 5 to 6), the air pressure was 0.51 MPa and 0.55 MPa. .

  The steel material was heated for about 1 hour in an electric furnace maintained at 1200 ° C., and the uniformly heated steel material was taken out and fixed to an experimental apparatus, and cooling was started under the above conditions. During the cooling of the steel material, the temperature measurement value of 7 points by the thermocouple is taken into the personal computer every 0.1 second, and after the experiment, the heat transfer coefficient at each thermocouple position is calculated from the measured temperature history, and the radiation cooling And the overall heat transfer coefficient at 850 ° C. was calculated.

  Table 1 shows the test conditions, the mist spray thickness at that time, the collision pressure, and the overall heat transfer coefficient at 850 ° C.

As shown in Table 1, in Test No. 1 which is outside the scope of the present invention, the overall heat transfer coefficient at 850 ° C. was 1117 kcal / m ² · hr · ° C., but this test No. In No. 2, it was 1517 kcal / m ² · hr · ° C., and in Test No. 3, it was 1854 kcal / m ² · hr · ° C. Similarly, in Test No. 4, which is outside the scope of the present invention, the overall heat transfer coefficient at 850 ° C. was 537 kcal / m ² · hr · ° C., but in Test No. 5, which is within the scope of the present invention, 880 kcal / m ² · hr · ° C., and in test No. 6, it was 907 kcal / m ² · hr · ° C.

  Thus, it was confirmed that the slab can be efficiently cooled without increasing the amount of cooling water according to the present invention.

It is the schematic which shows the condition which is cooling the slab by air mist spray. It is the schematic which shows the water quantity distribution measuring method of the width direction of mist. It is the schematic which shows the water quantity distribution measuring method of the thickness direction of mist. It is the schematic which shows the collision pressure distribution measuring method of mist. It is the schematic which shows the collision pressure distribution measuring method of mist. It is the schematic of the experimental apparatus for comparing and evaluating the cooling capability of air mist spray. It is a figure which shows the relationship between the general heat transfer coefficient in 850 degreeC measured by the hot experiment, and an air pressure.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Casting piece support roll 2 Casting piece 3 Air mist spray 4 Nozzle tip 5 Mixing pipe 6 Mixing part 7 Water supply pipe 8 Air supply pipe 9 Pressure gauge for water pressure measurement 10 Pressure gauge for air pressure measurement 11 Mist 12 Collection box 13 Collision pressure measurement Sensor 14 Steel 15 Thermocouple

Claims (2)

  When cooling the slab using the air mist spray installed in the secondary cooling zone of the continuous casting machine, the pressure of the air immediately before the water and air are mixed should be in the range of 0.5 MPa to 1 MPa. A secondary cooling method for continuously cast slabs, which is characterized.   The collision pressure of the mist water particles on the slab immediately below the nozzle tip of the air mist spray is 2 kPa or more, and the spray thickness of the mist water particles on the slab is 40 mm or more. Secondary cooling method of continuous cast slab as described.

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