JP5428499B2 - Secondary cooling method for continuous cast slabs with two-fluid mist spray nozzle - Google Patents

Description

  The present invention relates to a method for secondary cooling of a steel slab during casting using a two-fluid mist spray nozzle in a continuous casting machine.

  In continuous casting of steel, in order to promote the solidification of the slab drawn from the mold, in the installation range of the slab support roll immediately below the mold, from the spray nozzle such as water spray nozzle or two-fluid mist spray nozzle Forced cooling by water spraying, so-called secondary cooling is performed. In order to improve the productivity of continuous casting or to improve the solidification quality of the slab, stronger secondary cooling strength is often required. As means for increasing the cooling strength by spraying, usually, the amount of the cooling medium per unit time and per unit slab surface area is increased.

  As a further contrivance, for example, in Patent Document 1, the spray area from one spray nozzle to the slab is increased, and the time ratio during which the spray is sprayed on the slab during drawing of the slab is increased. Thus, a technique for increasing the cooling strength is disclosed. Patent Document 2 discloses a technique for increasing the cooling pressure by increasing the spray pressure of the spray to increase the collision pressure of the spray droplets on the slab surface.

JP 2004-050121 A JP 2004-167521 A

  The present inventors tested the techniques disclosed in the above two patent documents for the purpose of obtaining a stronger secondary cooling strength. However, in both Patent Document 1 and Patent Document 2, when the amount of spray water is increased, the rate of increase in the cooling strength becomes asymptotic to the increase in the amount of spray water, and further cooling strength is increased in the large water amount region. When trying to strengthen, it turned out to be a technology that requires a great deal of spray water and is very inefficient in terms of equipment.

  The present invention has been made in view of the above circumstances, and its purpose is to secondary-cool the steel slab during continuous casting with a two-fluid mist spray nozzle of a conveying gas and a cooling liquid. It is to provide a secondary cooling method for continuously cast slabs by a two-fluid mist spray nozzle that can be cooled with high cooling efficiency and can obtain high cooling strength.

  The secondary cooling method for continuously cast slabs by a two-fluid mist spray nozzle according to the present invention for solving the above-described problems is a two-fluid consisting of a conveying gas and a cooling liquid for steel slabs being cast by a continuous casting machine. When performing secondary cooling with the mist spray sprayed from the mist spray nozzle, the gas-liquid ratio (V / W), which is the ratio of the flow rate of the conveying gas (V) and the flow rate of the cooling liquid (W), is set to 15 or more. The diameter of the droplet sprayed from the two-fluid mist spray nozzle is controlled to be equal to or smaller than the droplet diameter d calculated by the following equation (1).

Where d is the droplet diameter (m) of the mist spray, σ is the surface tension (N / m) of the cooling liquid, ρ is the density of the cooling liquid (kg / m ³ ), and S is Spray area (m ² ) of a two-fluid mist spray nozzle, V is a flow rate of carrier gas (Nm ³ / sec), W is a flow rate of cooling liquid (m ³ / sec), and _RT is a constant (= 0.5) ).

  According to the present invention, cooling with high cooling efficiency is possible, and cooling the slab with high cooling strength is realized. Further, when the flow rate of the cooling liquid, the flow rate of the conveying gas, and the spray area are determined, the droplets sprayed from the two-fluid mist spray nozzles coalesce and aggregate before reaching the slab surface, or the slab surface becomes water. Since the critical droplet diameter covered with the membrane can be predicted, it is possible to set the operating conditions of the mist spray nozzle with high cooling efficiency, especially the operating conditions when increasing the amount of water to obtain high cooling strength. It becomes possible.

It is a figure which shows typically the case where droplet space occupation rate _Rd is 1 and the case of 0.5. It is a schematic sectional drawing of the vertical bending type slab continuous casting machine to which this invention is applied. It is the schematic which shows the structure of a 2nd cooling zone. It is the schematic which shows the example of an air mist spray apparatus. It is a figure which shows the result of having calculated the idle time ratio _RT from the amount of water and the droplet diameter. It is a figure which shows the measurement result of the heat transfer coefficient in the slab surface just under a nozzle tip.

  Hereinafter, the present invention will be specifically described. First, the background to the present invention will be described. The inventors of the present invention have studied and researched for the purpose of cooling with high cooling efficiency and high cooling strength when the continuous casting slab is secondarily cooled with a two-fluid mist spray nozzle of a carrier gas and a cooling liquid. Went.

  As described above, in Patent Document 1 and Patent Document 2, the cooling strength becomes asymptotic even if the amount of spray water is increased, and it is difficult to obtain a further increase in cooling strength. The cause of this is considered as follows. .

  The first cause is that when the amount of water is increased to increase the cooling strength, the number of droplets increases, and as a result, the droplets collide in the air before they reach the slab surface. This means that there is a high possibility of coalescence. If the droplets merge, the droplet diameter increases, and the cooling efficiency decreases.

  The second cause is that when the amount of spray water is large, water droplets are continuous on the surface of the slab and the surface of the slab is covered with a water film. In spray cooling, sprayed fine droplets contact the slab surface, and heat transfer from the slab to the droplets causes sensible heat when the droplet temperature rises, or the droplets themselves vaporize. The slab is cooled by the heat of vaporization. Here, the droplet that has reached the surface of the slab stays on the surface of the slab for a time determined by the size and physical properties of the droplet, and then rebounds elastically and leaves the surface of the slab. At the same time, new droplets reach the slab surface from one to the next. Therefore, if the next droplet reaches the slab surface before the previously arriving droplet leaves the slab surface, the slab surface is covered with a large number of spray droplets. And when there is much spray water quantity, water droplets will continue on the surface of a slab, and the surface of a slab will be covered with a water film. When the slab surface is covered with a water film, a vapor film is formed between the water film and the slab surface, and as a result, the heat flux is lowered.

  To prevent the surface of the slab from being covered with a water film, when the spray droplet reaches the slab surface, the previous liquid is not allowed to reach the slab surface. It is desirable that the droplets detach from the slab surface, and at least after the next droplet arrives, before the entire slab surface is covered with water, the previously reached droplet may detach from the slab surface. This is the minimum necessary condition. Below, the conditions of the spray nozzle required in order to eliminate these causes are shown.

  First, the first cause will be described.

It can be seen that in order to prevent the droplets from coalescing in the air, it is necessary to keep the interval between the droplets at a certain level or more. As a quantitative index of the interval between droplets, a droplet space occupancy ratio R _d represented by the following equation (2) was used.

However, in the formula (2), d is the diameter (m) of the droplet, and L is a three-dimensionally equidistant interval in the carrier gas at a flow rate V (Nm ³ / sec) per unit time. In this case, the distance between adjacent droplets, that is, the average particle interval (m). This particle average interval L is calculated by the following equation (3).

However, in the equation (3), N is the number of droplets per unit time (number / sec) generated from the spray nozzle. The number N of droplets is expressed by the following equation (4) when the flow rate of the cooling liquid in the two-fluid mist spray is W (m ³ / sec).

Here, as shown schematically in FIG. 1 when the droplet space occupancy R _d is 1 and 0.5, when the droplet space occupancy R _d is 1, Are in point contact, and the droplets coalesce and aggregate. In order to prevent the droplets from coalescing and aggregating, the flow of the carrier gas is turbulent. Therefore, assuming that the velocity of the droplets fluctuates to about twice the average velocity, the interval between adjacent droplets is It is considered that at least the droplet diameter needs to be vacant. In other words, the droplet space occupancy R _d should be 0.5 or less. Therefore, in the present invention, the maximum value of the droplet space occupancy R _d is set as shown in the following equation (5). Set to 0.5.

By substituting Equations (3) and (4) into Equation (5), where the droplet space occupancy _Rd is 0.5, the flow rate V (Nm ³ / sec) of the carrier gas and the cooling liquid Solving the gas-liquid ratio (V / W), which is the ratio with the flow rate W (m ³ / sec), the following (6) is obtained.

  That is, in the two-fluid mist spray nozzle, the gas-liquid ratio (V / W) is set to 15 or more so that the droplets do not coalesce and aggregate before the droplets reach the slab surface. I found it necessary.

  Next, the second cause will be described.

  When the droplet reaches the surface of the slab and the next droplet arrives on the surface of the slab before the droplet detaches from the surface of the slab, the coalescence of the droplets occurs and this occurs continuously. The surface of the slab is covered with a water film, and the heat removal performance of the spray is lowered. Therefore, the conditions for generating such a water film were derived below.

First, the average interval of time for the droplets to reach the slab surface was determined. The linear velocity u (m / sec) of the carrier gas at the time when the carrier gas collides with the slab is expressed by the following equation (7), where S (m ² ) is the spray area of the spray.

  Therefore, the average time interval ΔT (sec) of the time required for the droplets to reach the slab surface is a value obtained by dividing the particle average interval L (m) by the linear velocity u of the carrier gas. From the formula (7), it can be obtained by the following formula (8).

Next, the time _Tr during which the droplets that reached the slab surface stay on the slab surface was estimated. The droplets that have reached the slab surface are elastically deformed and subsequently detached from the slab surface. Here, it is known that “the contact time of the droplet dropped on the solid surface on the solid surface is equal to the primary period of the free vibration of the droplet”. The primary period of this free vibration is the period when n = 2 of the free vibration period T _n expressed by the following equation (9). Therefore, the residence time T _r of the spray droplets on the slab surface is as follows. (Sec) is expressed by the following equation (10). In the equations (9) and (10), σ is the surface tension (N / m) of the cooling liquid, and ρ is the density (kg / m ³ ) of the cooling liquid.

  Using the parameters derived above, the conditions under which spray droplets do not form a water film on the slab surface are considered.

Here, the ratio between the residence time _Tr of one droplet on the slab and the average time interval ΔT at which the spray droplets arrive one after another is defined as “free time ratio R _T (−)”, and (11)

Here, the condition that the water film does not occur is that the next droplet reaches the slab after the droplet that has reached the slab surface is detached. Moreover, as described above, since the individual linear velocity of the droplet is expected to fluctuate to about twice the average linear velocity u, the condition that does not produce a water film, that is, the maximum value of the idle time rate _RT Becomes 0.5 as shown in the following equation (12).

Therefore, the following formula (13) is obtained by substituting the formula (8) and the formula (10) into ΔT of the denominator of the formula (12) and _Tr of the numerator, respectively.

Here, since the maximum value of the idle time rate _RT is to be obtained, if the maximum value is set for the flow rate V of the carrier gas and the flow rate W of the cooling liquid among the parameters of the equation (13), The maximum amount of the cooling medium itself is set, which is inconvenient for the purpose of improving the heat removal performance of the spray nozzle. Therefore, by setting a maximum value for the droplet diameter d among the parameters, the idle time rate _RT is controlled so as not to exceed the maximum value.

That is, Equation (13) is solved for the droplet diameter d (m) to obtain the following Equation (1). In Equation (1), the droplet diameter d when the idle time rate R _T = 0.5 is obtained. The droplet diameter was controlled as the maximum value of the droplet diameter from the two-fluid mist spray nozzle. In the formula (1), the surface tension σ of the cooling liquid and the density ρ of the cooling liquid are uniquely determined from the physical properties of the cooling liquid to be used, and the spray area S of the spray is determined from the spray nozzle characteristics. The gas flow rate V and the cooling liquid flow rate W are determined from the secondary cooling conditions at that time.

  As described above, if the expressions (6) and (1) are satisfied, the droplets sprayed from the two-fluid mist spray nozzle may coalesce and aggregate before reaching the slab surface, or water on the slab. It can be seen that no film is formed. Therefore, it has been found that it is only necessary to use a mist spray nozzle tip having a structure satisfying this critical droplet diameter and to set the flow rate ratio between the cooling liquid and the carrier gas to a predetermined value or more.

  The present invention has been made based on the above examination results, and secondary cooling of the steel slab being cast by the continuous casting machine with a mist spray sprayed from a two-fluid mist spray nozzle of a transport gas and a cooling liquid. In doing so, the gas-liquid ratio (V / W), which is the ratio of the flow rate (V) of the carrier gas and the flow rate (W) of the cooling liquid, is 15 or more, and the liquid sprayed from the two-fluid mist spray nozzle The droplet diameter is controlled to be equal to or smaller than the droplet diameter d calculated by the above equation (1).

  Hereinafter, a specific implementation method of the present invention will be described with reference to the accompanying drawings. FIG. 2 is a schematic sectional view of a vertical bending type slab continuous casting machine to which the present invention is applied.

  As shown in FIG. 2, a continuous casting machine 1 for casting a slab slab is provided with a mold 4 for injecting molten steel 20 and solidifying it, and a ladle is placed above the mold 4. A tundish 2 that receives the molten steel 20 from (not shown) and supplies the received molten steel 20 to the mold 4 through the immersion nozzle 3 is disposed. On the other hand, a pair of opposed rolls 1 is provided below the mold 4. A plurality of slab support rolls 5 are installed as a set. And on the downstream side of the slab support roll 5, a plurality of transport rolls 6 and a gas cutter 7 positioned above the transport roll 6 and synchronized with the casting speed of the slab 21 are installed. Further, in the range in which the slab support roll 5 is arranged, the first cooling zones 8 and 8, the second cooling zones 9 and 9, the third cooling zones 10 and 10, A secondary cooling zone comprising cooling zones divided into a total of twelve locations including the fourth cooling zones 11 and 11, the fifth cooling zones 12 and 12, and the sixth cooling zones 13 and 13 is installed.

  The molten steel 20 poured into the mold 4 from the tundish 2 via the immersion nozzle 3 is cooled by the mold 4 to form a solidified shell 22, and a slab support roll is formed as a slab 21 having an unsolidified portion 23 inside. 5 is continuously pulled out while being supported by 5. The slab 21 is cooled in the secondary cooling zone while passing through the slab support roll 5 to increase the thickness of the solidified shell 22 and eventually complete the solidification to the center. The slab 21 after completion of solidification is cut by the gas cutter 7 to become a slab 21a.

  As illustrated in FIG. 3, as an example of the configuration of the second cooling zone 9, a transport gas supply pipe 14 for supplying a transport gas for transporting the cooling liquid is installed in each cooling zone. A nozzle tip 16 facing the surface of the slab 21 is installed at each branched end of the supply pipe 14. Further, cooling liquid supply pipes 15 for supplying a cooling liquid for secondary cooling are installed side by side, and the branched ends of the cooling liquid supply pipes 15 join the respective transport gas supply pipes 14. doing. A flow rate adjustment valve 17 is arranged in the transfer gas supply pipe 14, and a flow rate adjustment valve 18 is arranged in the cooling liquid supply pipe 15, and the flow rate of the transfer gas is adjusted by adjusting the flow rate adjustment valve 17. The flow rate of the cooling liquid is adjusted by adjusting the flow rate adjusting valve 18. Usually, industrial water is used as the cooling liquid, and air is used as the carrier gas.

  FIG. 4 shows an example of an air mist spray device that uses industrial water as the cooling liquid and air as the carrier gas. An orifice 19 is arranged in each of the transfer gas supply pipe 14 for supplying air and the cooling liquid supply pipe 15 for supplying industrial water, and the air and water throttled by the orifice 19 are speeded up and passed through the orifice 19. Later, they are mixed vigorously with each other. The mixed air and water are jetted from the nozzle tip 16 toward the surface of the slab 21.

  In practicing the present invention, it is not necessary to arrange such an air mist spray device in all the secondary cooling zones, and a conventional water spray may be used, but at least one place is such an air mist spray. It is preferable to arrange such an air mist spraying device in the upstream half of the secondary cooling zone in which the device is disposed, and particularly where the cooling strength needs to be increased. Although the total number of cooling zones is 12 in FIG. 2, it may be divided into several according to the length of the continuous casting machine 1.

  When cooling the slab 21 during casting with an air mist spray device, the gas-liquid ratio (V / W) represented by the formula (6) is 15 or more, and the droplet diameter sprayed from the nozzle tip 16 is Control is performed so as to be equal to or smaller than the droplet diameter d calculated by the equation (1).

  In this case, setting the gas-liquid ratio (V / W) to 15 or more can be easily performed by adjusting the flow rate adjusting valve 17 and the flow rate adjusting valve 18. However, the nozzle tip 16 needs to be sprayable at a gas-liquid ratio (V / W) of 15 or more.

  On the other hand, the diameter of the droplet sprayed from the nozzle tip 16 varies depending on the characteristics of the nozzle tip 16. Therefore, in order to control the droplet diameter sprayed from the nozzle tip 16 to be equal to or smaller than the droplet diameter d (m) calculated by the equation (1), the target is set in advance. Based on the secondary cooling conditions, the droplet diameter d is calculated using the equation (1), and the nozzle tip 16 that sprays droplets equal to or smaller than the calculated droplet diameter d is used as the secondary cooling zone. It is necessary to arrange. Since the droplet diameter d calculated from the equation (1) differs depending on the secondary cooling condition, even if the nozzle tip 16 does not satisfy the present invention under the strong cooling condition, the present invention is achieved when the cooling strength is lowered. Satisfying can also occur.

  As described above, the liquid can be sprayed at a gas-liquid ratio (V / W) of 15 or more, and the liquid droplets to be sprayed are calculated by the equation (1) at the gas-liquid ratio (V / W) of 15 or more. A nozzle tip 16 that is equal to or smaller than the droplet diameter d is placed in advance in an air mist spraying device in the secondary cooling zone, and this nozzle tip 16 is used to adjust the gas-liquid ratio (V / W) to 15 or more. Then, the cast slab 21 is cooled.

  By cooling the slab 21 in this way, cooling with high cooling efficiency is possible, and cooling the slab 21 with high cooling strength is realized. Further, in the present invention, when the flow rate of the cooling liquid, the flow rate of the carrier gas, and the spray area are determined, the droplets sprayed from the two-fluid mist spray nozzle may coalesce and aggregate before reaching the slab surface, Since the critical droplet diameter at which the slab surface is covered with a water film can be predicted, the operating conditions of the mist spray nozzle with high cooling efficiency, especially the operating conditions when increasing the amount of water to obtain high cooling strength, are efficient. It is possible to set well.

  In a vertical bending type slab continuous casting machine, a test was performed in which a slab being cast was cooled using nozzle tips having different droplet diameters. Table 1 shows the specifications of the vertical bending slab caster that was tested.

  In this example, the air mist spray nozzle for the third cooling zone on the upstream side of the secondary cooling zone was tested. The conditions of the mist spray nozzle used for this cooling zone are as follows.

W: Water amount per nozzle tip = 6 to 48 L / min
V: Air amount per nozzle tip = 720 NL / min (Set so that the gas-liquid ratio is 15 when the water amount is 48 L / min, the maximum)
Distance from nozzle tip to slab surface = 126mm
Nozzle tip spray angle = 115 °
S: Spray area = 0.01495 m ² (Since a fan-type spray type was used as the nozzle tip, the spray area was considered as an ellipse, and the spray area was calculated from the maximum water density within the spray range to ½ of the maximum value. )
Under the above conditions, W was 6, 12, 24, 48, 96 L / min, the droplet diameter d was in the range of 1 to 10,000 μm, and the idle time rate _RT was calculated using equation (13). The calculation results are shown in FIG. As can be seen from FIG. 5, at 48 L / min set as the maximum water flow rate in this test, the free time ratio _RT satisfies 0.5 or less when the droplet diameter d is in the range of about 190 μm or less. .

  Therefore, three types of nozzle chips having different shapes were prepared such that the Sauta average droplet diameter was 100 μm, 200 μm, and 500 μm under the conditions of a water amount of 48 L / min and an air amount of 720 NL / min. And using these nozzle tips, the heat transfer coefficient on the slab surface directly under the nozzle tips was measured when the amount of water was increased to 12 to 48 L / min. Further, the test was carried out under the condition that the amount of water was 96 L / min. The measurement result of the heat transfer coefficient is shown in FIG.

In the case of a nozzle tip having a droplet diameter of 500 μm, as shown in FIG. 5, the idle time rate _RT is about 0.7 under the condition that the amount of water is 24 L / min, which exceeds 0.5. In fact, as shown in FIG. 6, even when the heat transfer coefficient immediately below the nozzle is seen, the nozzle tip having a droplet diameter of 500 μm is the lowest among the three nozzle tips tested.

In the nozzle tip having a droplet diameter of 200 μm, as shown in FIG. 5, the idle time rate _RT is slightly higher than 0.5 under the condition of a water amount of 48 L / min. Therefore, as shown in FIG. The heat transfer coefficient at / min is larger than that of a nozzle tip having a droplet diameter of 500 μm, but smaller than that of a nozzle tip having a droplet diameter of 100 μm. Further, in the nozzle tip having a droplet diameter of 200 μm, when the water transfer rate is increased from 12 → 24 L / min and when the water transfer rate is increased from 24 → 48 L / min, the rate of increase in the heat transfer coefficient is compared. It can be seen that when increasing from 24 to 48 L / min, the rate of increase of the heat transfer coefficient decreases. That is, it can be seen that the rate of increase in the cooling intensity is asymptotic to the increase in the amount of spray water.

On the other hand, in the nozzle chip having a droplet diameter of 100 μm, the idle time rate _RT is 0.5 or less even under the condition of the water amount of 48 L / min as shown in FIG. As such, the heat transfer coefficient is highest for the three nozzle tips tested. Moreover, it can be seen that the cooling strength increases in proportion to the amount of water up to a range of 48 L / min. However, when the amount of water is increased to 96 L / min with this nozzle tip having a droplet diameter of 100 μm, the free time rate _RT exceeds 0.5, so the rate of increase of the heat transfer coefficient with respect to the amount of water is slow.

  Thus, it turns out that a slab can be cooled efficiently by performing secondary cooling using an appropriate nozzle tip according to secondary cooling conditions.

DESCRIPTION OF SYMBOLS 1 Continuous casting machine 2 Tundish 3 Immersion nozzle 4 Mold 5 Casting piece support roll 6 Transport roll 7 Gas cutting machine 8 1st cooling zone 9 2nd cooling zone 10 3rd cooling zone 11 4th cooling zone 12 5th cooling zone 13 Sixth cooling zone 14 Transport gas supply pipe 15 Cooling liquid supply pipe 16 Nozzle tip 17 Flow rate adjusting valve 18 Flow rate adjusting valve 19 Orifice 20 Molten steel 21 Cast piece 22 Solidified shell 23 Unsolidified part

Claims (2)

When the steel slab being cast by the continuous casting machine is secondarily cooled by the mist spray sprayed from the two-fluid mist spray nozzle of the transport gas and the cooling liquid, the flow rate (V) of the transport gas and the cooling liquid The gas-liquid ratio (V / W), which is the ratio to the flow rate (W), is 15 or more, and the Sauta average diameter of droplets sprayed from the two-fluid mist spray nozzle is 15 or more. In the range, secondary cooling is performed using a two-fluid mist spray nozzle having a droplet diameter d or less calculated using the following formula (1), and continuous casting by the two-fluid mist spray nozzle Secondary cooling method of the piece.

Where d is the droplet diameter (m) of the mist spray, σ is the surface tension (N / m) of the cooling liquid, ρ is the density of the cooling liquid (kg / m ³ ), and S is Spray area of the two-fluid mist spray nozzle ( spray area is the spray area in the range from the maximum water density within the spray range to 1/2 of the maximum value: m ² ), V is the flow rate of the carrier gas (Nm ³ / sec), W is the flow rate of the cooling liquid (m ³ / sec), R _T is the idle time rate, and R _T is limited to 0.5 in equation (1) . The Sauter average diameter of droplets sprayed from the two-fluid mist spray nozzle is such that the flow rate of the cooling liquid is the maximum value of the use conditions in the continuous casting machine, and the gas-liquid ratio (V / W) is 15. 2. The two-fluid according to claim 1, wherein the secondary fluid is cooled by using a two-fluid mist spray nozzle having a droplet diameter d or less calculated by using the formula (1). Secondary cooling method for continuous cast slabs by mist spray nozzle.

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