TWI764216B - Secondary cooling device and secondary cooling method for continuous casting slab - Google Patents

Abstract

To obtain a cooling device and method for continuous casting of steel, which can suppress equipment investment or operating costs, can be applied even in environments with severe equipment constraints, and has high controllability of cooling capacity. The secondary cooling device (1) of the continuous casting slab of the present invention is connected to the secondary cooling zone of the continuous casting machine, and uses a single-fluid water spray to cool the slab (1) supported and guided by the guide rollers (3). 5), and have: two or more types of water mist nozzles (7), the flow characteristics of which are different; and switching valves (11, 13, 15), which are used to switch the water supply line (9) used; two or more types of water mist nozzles (7) with different flow characteristics are located between the guide rollers. The gap has cooling areas arranged in a row in the direction parallel to the guide roller (3).

Description

Secondary cooling device and secondary cooling method for continuous casting slab

The present invention relates to a secondary cooling device and a secondary cooling method for continuous casting of slabs.

Taking a vertical bending type continuous casting equipment as an example, the manufacturing method of a general continuous casting slab will be described with reference to FIG. 4 .

The molten steel injected into the casting mold 21 from a tundish (not shown) is first cooled in the casting mold 21, and becomes a flat slab 5 having a solidified shell formed thereon. The shape falls on the vertical belt 23 and advances towards the curved belt 27 . Then, in the curved portion 25 on the input side of the curved belt 27, the cast piece 5 is guided by a plurality of rollers (not shown) and curved while maintaining a constant curvature radius.

After that, the straightening portion 29 is bent backward (corrected) while sequentially increasing the radius of curvature, and immediately after leaving the straightening portion 29 , the cast piece 5 becomes a flat plate again and advances toward the horizontal belt 31 . After the solidification of the horizontal belt 31 is completed, the slab 5 is cut into a predetermined length by a gas cutting machine 33 installed on the output side of the continuous casting machine.

After the slab 5 leaves the casting mold 21, a water spray (water single-fluid spray or water-air two-fluid mixed spray (water-single-fluid spray or water-air two-fluid mixed spray) is applied in order to complete the solidification from the vertical belt 23 to the horizontal belt 31 to the center part. mist spray)) secondary cooling.

Generally, secondary cooling is performed by spraying a large flow of water in the vertical belt 23 directly below the casting mold 21 to perform strong cooling to ensure the strength of the shell. Conversely, cooling is weakened after the belt 27 is bent, and the surface temperature is raised (reheated) by heat conduction from the high temperature portion inside. Then, the correction part 29 is adjusted so that the surface temperature becomes equal to or higher than the embrittlement temperature range, and the occurrence of transverse cracks is avoided.

After the start of continuous casting until the casting speed reaches the maximum speed, or during the period when the injection of molten steel into the mold is stopped and the continuous casting is terminated, the casting speed greatly changes. In this case, it is necessary to control the cooling conditions of the secondary cooling zone in accordance with changes in the casting speed.

When the cooling conditions are not properly controlled, for example, if the cooling is excessive in the vertical belt, embrittlement due to the III region of the steel (from the γ low temperature region to the γ/α transformation temperature region) occurs when passing through the straightening portion. Surface cracks (transverse cracks) caused by the embrittlement of steel.

When the temperature of the cast slab is excessively lowered, problems such as cutting failure and casting speed adjustment accompanying the cutting failure occur when gas cutting is performed on the output side of the continuous casting machine.

On the other hand, the following methods are also adopted: in order to improve the production efficiency, the casting speed is increased, the central part of the cast slab is corrected in a state where the central part is not solidified, and the horizontal belt in the final stage of the continuous casting process is subjected to strong cooling, thereby using This completes the solidification. The application of this method will vary with steel grades. In order to prevent excessive and insufficient cooling, the thickness or speed of the slab must be used to control the range of the strong cooling zone or the amount of cooling water.

As described above, in the secondary cooling of the slab in continuous casting, it is necessary to greatly change the cooling conditions. As a method corresponding to this, for example, Patent Document 1 proposes a technique for obtaining stable water even when the amount of water is greatly changed by the two-fluid spray of water and compressed air. spray state.

In Patent Document 2, there is proposed a technique of introducing cooling water of two systems whose pressures and flow rates are independently controlled to a single injection port, and supplying the cooling water according to the cooling conditions in the cooling of a single fluid of water. Traffic varies substantially.

In Patent Document 3, there is proposed a technique of changing the supply flow rate of cooling water by separately using a water single-fluid spray and a water-air two-fluid spray according to the water volume area.

Furthermore, Patent Document 4 proposes a technique in which two rows of single-fluid sprays of water are provided between the rolls, and the rows of sprayed water are switched unilaterally or in both directions according to a change in casting speed to cool. [Prior Art Literature] [Patent Literature]

Patent Document 1: Japanese Patent Laid-Open No. 2016-7602 Patent Document 2: Japanese Patent Application Laid-Open No. 5-220550 Patent Document 3: Japanese Patent Laid-Open No. 2004-58117 Patent Document 4: Japanese Patent Laid-Open No. 52-128836

[The problem to be solved by the invention]

In the technique of Patent Document 1, a single nozzle (nozzle) can obtain a stable spray distribution in a wide range of the amount of cooling water, but since the supply pressure of the cooling water needs to be greatly changed, particularly in The pressure loss will become larger under large flow conditions. In this case, since a large amount of compressed air is required, it is necessary to provide a large-capacity air compressor, and the equipment cost and operation cost become large.

In the technique of Patent Document 2, compressed air is not required, and the control range of the amount of water sprayed from the nozzle is expanded by supplying cooling water of two systems with different pressures and flow rates.

In the case of water single-fluid sprays, it is generally known that the amount of sprayed water is proportional to the square root of the pressure, although the amount of sprayed water can be controlled by the pressure of the water supply to the nozzle. When increasing the turndown ratio, it is necessary to increase the pressure ratio. For example, to achieve a turndown ratio of 40 times the minimum to maximum pressure ratio would be 1600 times and exceeds the control capability of the pump.

Also, when the supply pressure is lowered to reduce the amount of water, there is a risk that the spray angle of the water becomes smaller than the design value. As a result, there is a problem that the flow distribution of the water impinging on the cooling surface becomes non-uniform, cooling non-uniformity occurs, and surface cracks (longitudinal cracks) due to thermal stress caused by surface temperature non-uniformity occur.

On the other hand, in the technique of patent document 3, like patent document 1, the consumption amount of air is suppressed by using a two-fluid spray only in a low-flow area, and there are problems in terms of equipment and operating costs. Although the pressure loss is suppressed and the stability of the spray distribution is ensured by using two types of pipes and nozzles, the pipes of the water double system and the air single system must be arranged in the same space between the rolls, and the continuous casting machine will be increased. Design load and manufacturing cost.

In the technique of Patent Document 4, although the design is simplified by using a water single-fluid double system, since two rows of sprayers are arranged between the rollers, it is difficult to reduce the distance between the rollers. It is not possible to reduce the roller interval, which is not conducive to the suppression of bulging in the central part of the cast slab width due to the static pressure of the unsolidified molten steel in the central part of the cast slab, and leads to poor internal quality of the cast slab.

As mentioned above, it is expected to develop a secondary cooling device and a secondary cooling method that do not require large equipment investment or operating costs, have high cooling capacity controllability, and are stable against large changes in casting speed. It can realize high-speed casting, and then obtain excellent surface properties or internal quality.

Therefore, the present invention has been made in view of the above-mentioned problems, and its object is to obtain a continuous casting of steel with high cooling capacity controllability, which can be applied even in an environment with severe equipment restrictions while suppressing equipment investment and operating costs. Secondary cooling device and method in the invention. [Means of Solving Problems]

(1) A secondary cooling device for continuous casting of slabs, which is attached to a secondary cooling zone of a continuous casting machine, and uses a single-fluid water spray to cool the continuous casting of slabs supported and guided by a plurality of guide rollers. The secondary cooling device for sheets is characterized by comprising: two or more types of water spray nozzles with different flow characteristics; and a plurality of water supply lines for supplying water to each of the water spray nozzles The water with the flow rate corresponding to the flow rate characteristic; and the switching device, which is to switch the water supply line used; the water mist nozzles of two or more types with different flow characteristics are in the gap between the guide rollers. The cooling area is arranged in a row in the direction parallel to the rotation axis of the rollers. (2) The secondary cooling device for continuous casting of slabs according to (1), wherein the number of the water supply lines is the same as the number of the types of the water mist nozzles. (3) The secondary cooling device for continuous casting of slabs according to (1) or (2), wherein among the two or more types of water mist nozzles, the water sprayed by the spray nozzle with the largest spray flow rate The water mass density is more than 20 times the water mass density of the water sprayed by the spray nozzle with the least jet flow rate. (4) The secondary cooling device for continuous casting of slabs according to any one of (1) to (3), wherein among the two or more types of water mist nozzles, the spray nozzle with the largest spray flow rate is used for The water volume density of the sprayed water is 500L/(m ² ×min) or more and 2000L/(m ² ×min) or less, and the water volume density of the water sprayed by the spray nozzle with the smallest spray flow rate is 50L/(m ² × min) or more and less than 500 L/(m ² ×min). (5) The secondary cooling device for continuous casting of slabs according to any one of (1) to (4), wherein on the upstream side from the casting direction, a vertical belt, a curved portion, a curved belt, a straightening portion, In the secondary cooling zone of the continuous casting machine constituted by the sequence of the horizontal zones, the cooling zone is provided in one or more zones in the horizontal zone. (6) A method for secondary cooling of continuous casting slabs, which is a method for secondary cooling of continuous casting slabs using the secondary cooling device for continuous casting slabs described in (5), characterized in that In order to: take the upstream side section in the casting direction in the aforementioned horizontal belt as a strong water cooling section, and this strong water cooling section is to spray water under the condition that the sprayed water becomes a nucleate boiling state on the surface of the cast slab to cool the slab; and A section further downstream in the casting direction than the strong water cooling section to the end of the horizontal belt is defined as a section in which the water density is lower than that in the strong water cooling section and the boiling state of the cooling liquid on the surface of the slab is reduced. maintained at nucleate boiling. [Inventive effect]

In the present invention, there are provided: two or more types of water mist nozzles with different flow characteristics; and a plurality of water supply lines for supplying water with a flow rate corresponding to the flow characteristics of the respective water mist nozzles; and a switching device for It is to switch the water supply line used; two or more types of water mist nozzles with different flow characteristics are the gaps between the guide rollers, and are arranged in a row in the direction parallel to the rotation axis of the guide rollers. This has high controllability of cooling capacity without increasing the gap between the guide rolls, and can produce cast slabs stably without causing quality degradation or problems even when the casting speed is changed.

[Embodiment 1]

FIG. 1 is an explanatory diagram illustrating a main part of a secondary cooling apparatus according to an embodiment of the present invention. FIG. 2 is an explanatory diagram illustrating the arrangement and spray pattern of the water mist nozzles in the secondary cooling device according to the embodiment of the present invention.

As shown in FIGS. 1 and 2 , the secondary cooling device 1 of the continuous casting slab of the present embodiment is attached to the secondary cooling zone of the continuous casting machine, and is cooled by a single-fluid water spray through a plurality of guide rollers 3 Supported and guided cast 5. The secondary cooling device 1 is provided with: two or more types (four types in this embodiment) of water mist nozzles 7A, 7B, 7C, and 7D having different spray flow rates as flow characteristics; There are four water supply lines 9a, 9b, 9c, 9d of the same number as the type of the water mist nozzles 7, which supply the water of the flow rate corresponding to the flow rate characteristic of each water mist nozzle 7; Three of a switching valve 11 , a second switching valve 13 , and a third switching valve 15 are used for switching the water supply line 9 . The water mist nozzles 7A, 7B, 7C, and 7D are arranged in a line in the direction parallel to the guide rollers 3 in the gap between the guide rollers 3 and constitute a cooling area.

The guide roller 3 rotates while sandwiching the cast sheet 5 up and down, thereby providing the cast sheet 5 with a pulling force in the casting direction. A plurality of guide rollers 3 are arranged at predetermined intervals in one segment. A predetermined gap is provided between the guide rolls 3 adjacent to each other in the casting direction, and a water mist nozzle 7 is provided in the gap. Although it also depends on the scale of the equipment, for example, on a horizontal belt, approximately 100 guide rolls 3 are arranged at predetermined intervals in the casting direction, and a plurality of (for example, 10) guide rolls 3 are formed as one zone. segment and can be used as a unit for flow control. In the horizontal belt, for example, 10 sections are provided.

As shown in FIG. 2 , four types of water mist nozzles 7A, 7B, 7C, and 7D (water mist nozzle groups) are arranged in a row in the direction parallel to the rotation axis of the guide roller 3 in the gap between the guide rollers 3 . set up. The area cooled by such a water mist nozzle group is called a cooling area. In the horizontal belt, the tie is provided with more than one area of this cooling area. In FIGS. 1 and 2 , there are two water mist nozzles 7A, three water mist nozzles 7B, two water mist nozzles 7C, and four water mist nozzles 7D. . However, these numbers do not all show the number of nozzles to be installed, but a part is omitted. In fact, even if one of the water mist nozzles 7 is selected, the number of nozzles that can cover the width direction of the slab 5 is still the same. The number of each water mist nozzle 7 is set in the way of full width.

In this embodiment, since a plurality of types of water mist nozzles 7 having different flow characteristics are arranged in a row, the positions arranged in the width direction of the slab 5 are different for each type. As shown in FIG. 2 , the spray angles of the water mist nozzles 7 are made different so that they can be covered without gaps in the width direction of the slab 5 even when which type of the water mist nozzles 7 is selected in different arrangements. .

The water mist nozzle 7 used is preferably used, and the water sprayed will expand into a fan shape or a full cone shape or a full angle cone, and the cooling surface (the upper and lower surfaces of the casting sheet sandwiched by the two guide rollers 3) Nozzles with higher uniformity of water density distribution. Therefore, in order to uniformly spray the cooling water to the surface to be cooled received by the row of the water mist nozzles 7, it is preferable that the water sprayed from the water mist nozzles 7 does not interfere with the water sprayed from the other water mist nozzles 7. Adjust each water mist nozzle 7 in the way of water. For example, when using the water mist nozzle 7 in which the water spreads in a fan shape, it is preferable to adjust the injection direction so that the injection surfaces of the water mist nozzle 7 are not aligned on a straight line. For example, in the case of using the water mist nozzle 7 in which the water to be sprayed is a full cone or full cone shape, it is preferable to use the water sprayed from the water mist nozzle 7 and the water sprayed from the other water mist nozzles 7 The arrangement interval of each of the water mist nozzles 7 is adjusted so as to minimize the interference of water.

FIG. 3 is a graph illustrating the control range of the water density in the secondary cooling apparatus according to the embodiment of the present invention. The flow rate characteristic of each of the four types of water mist nozzles 7 will be described using FIG. 3 . The vertical axis of FIG. 3 is the water mass density (L/(m ² ×min)), and the horizontal axis is the supply pressure (MPa). The water density refers to a value calculated by dividing the area (m ² ) of the cooled surface received by the row of water mist nozzles 7 by the total amount of water (L/min) sprayed from the row of water mist nozzles 7 .

If it is assumed that each type of water mist nozzles 7, for example, three water mist nozzles 7A are provided, the water mass density shown in FIG. 3 is the average water mass density of the three. The water density of the water mist nozzles 7A, 7B, 7C and 7D is in the range of the supply pressure of 0.1 to 0.5 (MPa), respectively A: 50 to 150 (L/(m ² ×min)), B: 150 to 370 (L/(m ² ×min)), C: 370 to 880 (L/(m ² ×min)), D: 880 to 2000 (L/(m ² ×min)).

Therefore, when the water mist nozzle 7A is selected and the supply pressure is set to 0.1 (MPa), the minimum water mass density is 50 (L/(m ² ×min)), and when the water mist nozzle 7D is selected and the supply pressure is set to When it is 0.5 (MPa), it becomes the maximum water density of 2000 (L/(m ² ×min)). That is, in the water mist nozzle 7 of the present embodiment, the pressure ratio is 5 times and the adjustment ratio can be set to 40 times.

The water supply line 9 is provided in this embodiment in order to supply water with a flow rate corresponding to the flow rate characteristics of the four types of the water mist nozzles 7A, 7B, 7C, and 7D.

For example, the water supply line 9a for supplying water to the water mist nozzle 7A includes a head 9a1 which is directly or indirectly connected to the main supply line 17, and a plurality of branch pipes 9a2 whose base ends are connected to the head 9a1 And the water mist nozzle 7A is connected to the front-end|tip. Then, in the head 9a1 and each branch pipe 9a2, the capacity of the head 9a1 and the diameter of the branch pipe 9a2 are set according to the flow rate characteristic of the water mist nozzle 7A. The same applies to the water supply lines 9b, 9c, and 9d for supplying water to the water mist nozzles 7B, 7C, and 7D.

A supply pump (not shown) is connected to the main supply line 17 for supplying water to each of the water supply lines 9 . Normally, the cooling water is supplied from the supply pump at a fixed pressure equal to or higher than the pressure capable of ejecting the maximum flow rate for each of the four types of water mist nozzles 7A, 7B, 7C, and 7D. Then, the pressure of the cooling water supplied to each of the water mist nozzles 7 is changed by controlling the opening degrees of the valves provided in the water supply lines 9a, 9b, 9c, and 9d that supply water to the water mist nozzles 7A, 7B, 7C, and 7D. . The pressure of the supply water can also be changed by changing the discharge pressure of the supply pump.

Since the required supply pressure is different depending on the type of the selected water mist nozzle 7 and the required water density, it can be controlled by valves already installed in the water supply lines 9a, 9b, 9c, 9d. The opening degree control or the drive control of the supply pump can be changed.

The first switching valve 11 to the third switching valve 15 are valves for switching which water supply line 9 to flow the water to, and are constituted by four-way valves. By configuring with a four-way valve, the flow path can be switched so that only one water supply line 9 is supplied with water, and water is not supplied to the other three water supply lines 9 .

In the present embodiment configured as described above, the water mist nozzles 7A, 7B, 7C, and 7D arranged in the respective gaps of the guide rollers 3 are selected according to the cooling conditions, that is, the required water density. And the water of the flow rate corresponding to the selected water mist nozzle 7 is supplied from the supply pump via the water supply lines 9a, 9b, 9c, 9d.

For example, when the required water density is 50 (L/(m ² ×min)), cooling water at a pressure of 0.1 (MPa) is supplied to the water supply line 9a and discharged from the water mist nozzle 7A. The cooling water system is supplied from the supply pump to the water supply line 9a at a discharge pressure of 0.5 (MPa), and is controlled to narrow the opening degree of a valve (not shown) installed in the water supply line 9a, and the pressure is reduced to 0.1 (MPa). ) and supplied to the water mist nozzle 7A. The operations of the first switching valve 11 to the third switching valve 15 are performed manually or automatically. In the case of automatic operation, the first switching valve 11 to the third switching valves 11 to The valve 15 can be switched.

According to the present embodiment, even if the casting speed is changed, the water mist nozzles 7A to 7D can be switched from the weak cooling condition to the strong cooling condition as shown in the relationship between the supply pressure and the water density in FIG. 3 . While suppressing the pressure ratio to 5 times, the turndown ratio of 40 times was realized. In this way, high-speed casting can be stably achieved without requiring large equipment investment or operating costs, thereby obtaining excellent surface properties or internal quality of the cast slab 5 .

In the above description, although the configuration of the water mist nozzles 7A to 7D and the water supply lines 9a to 9d provided in the guide roll 3 and one of the gaps between the guide rolls 3 has been described, in order to efficiently perform the operation of the continuous casting machine For maintenance, the design of each section is preferably common. Therefore, a secondary cooling device that can control a wide range of water density with the same configuration, in other words, can control the ratio of the minimum value to the maximum value of the water density of the cooling water (adjustment ratio) is required. Secondary cooling device.

Therefore, for example, it is preferable to set a common specification for all the sections constituting the horizontal belt, so that cooling can be performed at a large turndown ratio in any section, and even in any position in the casting direction, the Cooling control can be performed similarly.

According to the review by the inventors, it is found that the adjustment ratio that can correspond to the above-mentioned wide cooling conditions is preferably 20 times or more (50 to 1000 (L/(m ² ×min)), more preferably about 40 times (50 to 2000 (L/(m ² ×min)) [Embodiment 2] Next, a secondary cooling method using the secondary cooling device 1 described in the above-described Embodiment 1 will be described.

By using the secondary cooling device 1 described in the first embodiment, even when the casting speed is changed, a preferable secondary cooling method can be realized, but it can be further improved by using the secondary cooling device 1 The casting speed is increased while reducing the amount of cooling water. This point will be explained below.

As described in the prior art, after the slab 5 leaves the mold, water spray (water single-fluid spray or water-air two-fluid mixture) is applied to complete solidification from the vertical belt 23 to the horizontal belt 31 to the center portion. spray) secondary cooling. The secondary cooling system ensures the strength of the shell by spraying a large flow of water into the vertical belt 23 until it enters the bending belt 27 from directly below the casting mold to perform strong cooling. Conversely, cooling is weakened after the belt 27 is bent, and the surface temperature is raised (reheated) by heat conduction from the high temperature portion inside. Then, it is adjusted in the straightening part so that the surface temperature becomes equal to or higher than the embrittlement temperature range, and the occurrence of transverse cracks is avoided.

On the other hand, the center portion passes through the straightening portion 29 in an unsolidified state when the casting speed is increased in order to improve productivity. Therefore, when the casting speed is further increased, it is necessary to increase the amount of water in the front stage of the horizontal belt 31 and to cool it under strong cooling conditions in order to solidify the horizontal belt with certainty.

In particular, in order to realize strong cooling conditions after stabilization, it is preferable that the cooling water be in a nucleate boiling state on the surface of the slab 5 . Here, the so-called nucleate boiling refers to a boiling state in which bubbles are generated with the foaming point as the core, and the cooling liquid can take very high heat away from the object to be cooled. A boiling state that does not reach nucleate boiling is called film boiling. Film boiling refers to a boiling state in which a film of vapor is generated at the boundary between the coolant and the object to be cooled.

The inventors have examined the water density for the slab 5 to enter the strong cooling zone and rapidly realize the nucleate boiling state, and found that 500 L/(m ² ×min) or more is required. It is also understood that in the nucleate boiling state, the flow rate dependence of the cooling capacity is reduced, so there is no need to increase the cooling water supply capacity excessively, and the water density may be set to 2000 L/(m ² ×min) or less.

Furthermore, as long as the nucleate boiling state can be temporarily achieved with a large flow of cooling water and the surface temperature of the slab can be lowered, the nucleate boiling can be maintained even without spraying a large amount of cooling water. For this reason, when the total amount of cooling water that can be used in the entire continuous casting machine is limited, etc., the water density in the rear section of the strong cooling zone may be reduced to less than 500 L/(m ² ×min). However, as a result of examination by the inventors, it has been found that the nucleate boiling state cannot be stably maintained unless the water density is 50 L/(m ² ×min) or more.

In this way, in the horizontal belt 31, strong cooling in a nucleate boiling state is performed at a large flow rate in the front stage, and intensive cooling with a small flow rate to maintain nucleate boiling is performed in the rear stage, thereby reducing the amount of cooling water while reducing the amount of cooling water. Increase casting speed.

That is, the secondary cooling method of the continuous casting slab of the present embodiment is constituted by the vertical belt 23, the curved portion 25, the curved belt 27, the straightening portion 29, and the horizontal belt 31 in this order from the upstream side in the casting direction. In the secondary cooling zone of the continuous casting machine, the continuous casting slab cooling method for secondary cooling the slab 5 using the secondary cooling device 1 for the continuous casting slab described in Embodiment 1 is used. The section on the upstream side in the casting direction in the horizontal zone 31 of the secondary cooling zone refers to a strong water-cooling section in which cooling water is sprayed to cool the slab 5 under the condition that the sprayed cooling water is in a nucleate boiling state on the surface of the slab, And the section that is further downstream in the casting direction than the strong water cooling section and reaches the end of the horizontal belt 31 is a section that reduces the water density of the cooling water and maintains the boiling state of the cooling liquid on the surface of the slab at nucleate boiling. Weak water cooling range. By setting the strong water cooling zone and the weak water cooling zone in this way, it is possible to increase the casting speed while reducing the amount of cooling water.

As a strong water cooling section for realizing nucleate boiling, it is only necessary to have a water volume control section in which the minimum water volume density can be individually controlled at the most upstream portion of the horizontal belt 31 . If nucleate boiling is realized, cooling by nucleate boiling can be performed stably by cooling with a minimum water density for maintaining nucleate boiling in the subsequent section.

When the cooling water comes into contact with the surface of the slab and boils, it vaporizes and becomes water vapor. Hot air (water mist) after the water vapor condenses in the air can be observed. Here, in the nucleate boiling state, the cooling water that has come into contact with the surface of the cast slab is vigorously foamed and a large amount of water vapor is generated, so that the amount of water mist generated increases. On the other hand, in the film boiling state, since the foaming of the boiling cooling water is small, the generation amount of water vapor and water mist is also reduced.

Then, a camera is installed in each section, and the generation amount of water mist is monitored by observation by visual observation or measurement by a transmissometer. The threshold value of the generation amount of waterpipe that distinguishes nucleate boiling and film boiling is obtained by experiment in advance, and it is confirmed whether the generation amount of the water mist exceeds the threshold value. range achieved. Then, adjust to increase the amount of cooling water if the nucleate boiling state cannot be achieved. Thereby, the nucleate boiling state can be surely achieved and maintained.

In convective heat transfer including boiling, the temperature of the fluid and the solid become equal locally at the point of contact between the two. Since the temperature of water in a liquid state only rises up to the boiling point under atmospheric pressure, it is considered that the surface temperature of the slab will be about 100°C as long as nucleate boiling is achieved. Therefore, the nucleate boiling state can be confirmed by measuring the temperature of the surface of the slab and the surrounding cooling water using a contact thermometer with a small probe, and confirming that the temperature is stable at around 100°C. whether it can be achieved. Then, adjust to increase the amount of cooling water if the nucleate boiling state cannot be achieved. Thereby, the nucleate boiling state can be surely achieved and maintained.

By making the design of each section constituting the horizontal belt 31 common, it is possible to control a wide range of the amount of injected water with the same configuration, and to maintain the continuous casting machine more efficiently.

Since the range of the strong water cooling section varies depending on the thickness of the slab and the type of steel, it is preferable that it can be flexibly changed in the casting direction, that is, in the longitudinal direction of the slab 5 .

For this reason, as described in Embodiment 1, it is preferable that the secondary cooling device 1 having a larger adjustment ratio is installed in the entire section constituting the horizontal belt 31 . [Example]

The production of the slab 5 by using the vertical bending type continuous casting machine (refer to FIG. 4 ) in which the secondary cooling apparatus 1 (refer to FIGS. 1 and 2 ) of the first embodiment is installed in the horizontal belt 31 will be explained, and the present invention will be confirmed. Example of efficacy.

Although the water mist nozzles 7 are designed to use four types, the types of nozzles to be used are limited in some embodiments. The length of the continuous casting machine is 45m, and the end of the machine is provided with a thermometer and a gas cutter 33 for measuring the temperature distribution on the surface of the slab.

The slab 5 was manufactured by changing cooling conditions, casting speed, and slab thickness, and evaluated the temperature unevenness during cooling, surface properties after casting, internal defects, and manufacturing cost.

The manufacturing conditions and evaluation are shown in Table 1 below. In the table, those within the scope of the examples of the present invention are referred to as Examples 1 to 5, and those within the scope of the present invention are referred to as Comparative Examples 1 to 7.

In some of the comparative examples, as a result of the review by the previous numerical analysis, in fact, the equipment was not produced due to the problem of cost, and the production was not carried out.

在比較例1及實施例1、2中，係將235mm厚度的鑄片5分別以先前技術的條件與應用了本發明之技術的條件來製造。

In Comparative Example 1 and Examples 1 and 2, the slab 5 having a thickness of 235 mm was produced under the conditions of the prior art and the conditions to which the technique of the present invention was applied, respectively.

In Comparative Example 1, the cooling was performed by a single-fluid spray, and the cooling was performed on the way under a large flow rate condition in order to increase the casting speed. The minimum value of the set water density was set to 10 L/(m ² ×min), the maximum value was set to 100 L/(m ² ×min), and the target turndown ratio was set to 10 times.

However, in this comparative example 1, the supply pressure of the cooling water becomes 100 times and the adjustment as the target cannot be achieved due to pressure loss. For this reason, the maximum casting speed is limited to 1.8 mpm. Under large flow conditions, the cooling pattern of the sprayer becomes non-uniform, and temperature non-uniformity occurs on the surface of the slab due to cooling. As a result, uneven thermal stress was generated on the surface, and surface cracks were confirmed when the surface was inspected after casting.

On the other hand, in Example 1, the technology of the present invention is applied, and two types of water mist nozzles and water supply lines 9 are used, and the regulation ratio of 20 times (minimum 50 L/(m ² ×min) to max. 1000 L/(m ² ×min)). The upper limit of the casting speed can be increased from 1.8 mpm to as far as 2.7 mpm by stably realizing the strong cooling condition of a larger flow rate than that produced using the prior art. Defects such as cracks were not observed upon inspection after manufacture. In addition, since it is a single-fluid spray, an air compressor is not required, and the introduction cost and operating cost of the equipment can be reduced.

In Example 2, four types of nozzles and piping were used, and the regulation ratio of 40 times (minimum 50 L/(m ² ×min) to maximum 2000 L/(m ² ) could be stably achieved with a water supply pressure ratio of 5 times ×min)). The upper limit of the casting speed can be further increased from 1.8 mpm to 3.0 mpm by stably realizing the strong cooling condition of a larger flow rate than that produced using the prior art. Defects such as cracks were not observed upon inspection after manufacture.

Comparative Examples 2 and 3 and Examples 3 and 4 are examples in which the thickness of the plate was changed to 200 mm and 260 mm under the same cooling conditions as those of Comparative Example 1 and Example 2, respectively.

When the thickness of the plate was reduced from 235 mm to 200 mm, although the casting speed could be increased, in Comparative Example 2, as in Comparative Example 1, surface cracks were generated due to uneven cooling. On the other hand, in Embodiment 3, no defect occurred, and the casting speed was also a maximum of 3.3 mpm, which was faster than the maximum of 2.3 mpm in Comparative Example 2.

When the thickness of the flat plate was increased from 235 mm to 260 mm, the maximum casting speed was limited, and in Comparative Example 3, as in Comparative Example 1, surface cracks were generated due to uneven cooling.

On the other hand, in Example 4, the maximum casting speed was 2.8 mpm, and the casting could be performed at a higher speed than the maximum 1.3 mpm in Comparative Example 3, and the production could be performed without causing defects.

The comparative example 4 is the examination result of the case where one type of two-fluid spray was used. In the stage of the capability evaluation of the nozzle, it is understood that the turndown ratio becomes 20 times (minimum 10L/(m ² ×min) to maximum 200L/(m ² ×min)) with a cooling water supply pressure ratio of 30 times, And compared with Comparative Examples 1 to 3, high regulation can be achieved with a smaller pressure ratio.

However, since it is necessary to supply high-pressure, large-flow compressed air, and the cost of introducing the air compressor and the energy cost during operation are greater than those of Example 1 using only a single-fluid spray, the introduction of equipment has been delayed.

On the other hand, Comparative Example 5 is a method of switching between the single-fluid spray and the two-fluid spray. Although a higher turndown ratio (20 times) can be achieved at a lower pressure ratio (5 times) than that of Comparative Example 4, but In this case, the introduction of the device is delayed because the cost is still worse than the exemplary embodiment.

Comparative Example 6 is an example in which two types of single-fluid nozzles are used and the pressure ratio is 5 times to achieve a turndown ratio of 20 times (minimum 50 L/(m ² ×min) to maximum 1000 L/(m ² ×min)). In this example, the interval between the guide rollers is widened, and two types of nozzles are arranged in two rows parallel to the rollers. Only a part of the section in which the high turndown ratio was obtained within the secondary cooling zone was introduced into the cooling device and an experiment was carried out. Although the range of the water density and the adjustment ratio are the same as in Example 1, when the spray nozzles are arranged in two rows, the interval between the rollers becomes wider and the amount of swelling becomes larger. Therefore, internal cracks are seen when the cast slab 5 after casting is inspected, and the degree of center segregation is also deteriorated.

Example 5 is an example in which two types of single-fluid nozzles are used and the pressure ratio is 2 times to set the adjustment ratio to 40 times (minimum 50 L/(m ² ×min) to maximum 2000 L/(m ² ×min)). However, in this example, since the control range of cooling capacity is limited to two levels of weak cooling and strong cooling, the supply pressure ratio can be reduced from 5 times by narrowing the flow control range of each water mist nozzle. to 2 times.

On the other hand, the control range of the flow rate of the whole cooling apparatus becomes intermittent. In this example, compared with Example 1 or Example 2, although the controllability of the cooling capacity with respect to the variation of the casting speed is poor, high-speed casting can be performed at the same level, and the cast slab 5 after production can also be seen No defects.

Example 6 is an example in which two types of single-fluid spray nozzles are used and the pressure ratio is 2 times to set the adjustment ratio to 11 times (minimum 50 L/(m ² ×min) to maximum 550 L/(m ² ×min)).

In this example, since the maximum water density is reduced to a range that can maintain nucleate boiling, the upper limit of the casting speed is 2.5 mpm. Casting speed increase has been reduced. There were no operational problems, and no defects were seen in the cast slab 5 after manufacture.

Example 7 is an example in which two types of single-fluid spray nozzles are used and the pressure ratio is 5 times to set the adjustment ratio to 5 times (minimum 400 L/(m ² ×min) to maximum 2000 L/(m ² ×min)).

In this example, since the minimum water density was increased, the cooling of the slab was rapid, the upper limit of the casting speed was 3.0 mpm, and the injection of cooling water was stopped at the base end side of the weak water cooling section. There were no operational problems, and no defects were seen in the cast slab 5 after manufacture.

Example 8 is an example in which two types of single-fluid spray nozzles are used and the pressure ratio is 5 times to set the adjustment ratio to 20 times (minimum 45 L/(m ² ×min) to maximum 900 L/(m ² ×min)). In this example, since the minimum water density was reduced, nucleate boiling could not be rapidly achieved, and a temperature deviation occurred in the width direction of the slab. Therefore, the upper limit of the casting speed has become 2.6 mpm although slight surface cracks have occurred in the cast slab.

As described above, it has been confirmed that by using the secondary cooling device 1 which can take a large turndown ratio, the controllability of the secondary cooling with respect to the variation of the casting speed is improved, and high quality can be achieved even though the casting speed is increased. Manufacture of ingot 5. Although the above-mentioned embodiment has shown an example of applying the present invention to the horizontal belt 31, it can also be applied to other cooling belts on the upstream side than the horizontal belt 31, and can also be applied across a plurality of cooling belts.

1: Secondary cooling device 3: guide roller 5: Casting 7A~7D: Water mist nozzle 9a~9d: Water supply lines 9a1~9d1: head 9a2~9d2: branch pipe 11: The first switching valve 13: Second switching valve 15: The third switching valve 17: Main Supply Line 21: Mold 23: Vertical belt 25: Bending part 27: Bending Band 29: Correction Department 31: Horizontal belt 33: Gas cutting machine

FIG. 1 is an explanatory diagram illustrating a main part of a secondary cooling device according to an embodiment of the present invention. FIG. 2 is an explanatory diagram illustrating the arrangement and spray pattern of the water mist nozzles in the secondary cooling device according to the embodiment of the present invention. [ Fig. 3] Fig. 3 is a graph illustrating the control range of the water density in the secondary cooling device according to the embodiment of the present invention. [ Fig. 4] It is an explanatory diagram illustrating the outline of a general continuous casting facility.

1: Secondary cooling device

5: Casting

7A~7D: Water mist nozzle

9a~9d: Water supply lines

9a1~9d1: head

9a2~9d2: branch pipe

11: The first switching valve

13: Second switching valve

15: The third switching valve

17: Main Supply Line

Claims (6)

A secondary cooling device for continuous casting slabs, which is attached to the secondary cooling zone of a continuous casting machine, and uses a single fluid water spray to cool the two continuous casting slabs of the continuous casting slabs supported and guided by a plurality of guide rollers. The secondary cooling device is characterized by comprising: two or more types of water mist nozzles having different flow characteristics; and a plurality of water supply lines for supplying flow rates corresponding to the flow characteristics of the water sprayed from the respective water mist nozzles Water, this flow rate characteristic is from the intense cooling condition that cools the slab under the condition that the slab surface is in a nucleate boiling state to the boiling of the coolant on the slab surface with the water density lower than the aforementioned intense cooling condition. The state is maintained until the weak cooling condition of nucleate boiling; and the switching device is to switch the water supply line used; the above-mentioned two or more types of water mist nozzles with different flow characteristics are in the gap between the guide rollers. Cooling areas arranged in a row in a direction parallel to the rotation axis of the guide roller. The secondary cooling device for continuous casting slabs according to claim 1, wherein the number of the water supply lines is the same as the number of the types of the water mist nozzles. The secondary cooling device for continuous casting slabs according to claim 1 or 2, wherein, among the above two or more types of water mist nozzles, the water density of the water sprayed by the spray nozzle with the largest spray flow rate is the amount of water sprayed by the spray nozzle. The water density of the water sprayed by the spray nozzle with the least flow rate is more than 20 times. The secondary cooling device for continuous casting slabs according to claim 1 or 2, wherein, among the above two or more types of water mist nozzles, the water density of the water sprayed by the spray nozzle with the largest spray flow rate is 500 L/(m ² ×min) or more and 2000L/(m ² ×min) or less, and the water density of the water sprayed by the spray nozzle with the smallest jet flow rate is 50L/(m ² ×min) or more and less than 500L/(m ² ×min). The secondary cooling apparatus for continuously casting slabs according to claim 1 or 2, wherein the continuous casting machine is formed in the order of vertical belt, curved portion, curved belt, straightening portion, and horizontal belt on the upstream side from the casting direction. In the second cooling zone, the cooling zone is provided with one or more zones in the horizontal zone. A secondary cooling method for continuous casting slabs, which is a continuous casting slab cooling method for secondary cooling of the slabs by using the secondary cooling device for continuous casting slabs described in claim 5, characterized in that: The section on the upstream side in the casting direction in the horizontal belt is defined as a strong water-cooling section that cools the slab by spraying water under the condition that the sprayed water becomes a nucleate-boiling state on the surface of the slab; The strong water cooling section is also the section from the downstream side of the casting direction to the end of the horizontal belt as the weak water cooling section. maintained at nucleate boiling.

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