TW202108264A - Secondary cooling method and device for continuously cast slab - Google Patents

Abstract

The objective of the present invention is to obtain a secondary cooling method and device for a continuously cast slab with which it is possible to maintain the surface properties of the slab without impairing productivity and without the need for large additional energy costs. In this secondary cooling method for a continuously cast slab, in which a slab 5 is cooled by spraying cooling water thereon in a secondary cooling zone 7 and solidification of the slab 5 is completed in a sector extending to the end of a horizontal zone 17, a sector within the horizontal zone 17, on the upstream side thereof in the casting direction, is set as a strong water-cooling sector in which the slab 5 is cooled by spraying the cooling water under conditions whereby the sprayed cooling water adopts a nucleate boiling state in all positions across the entire width direction of the surface of the slab, and a sector extending to the end of the horizontal zone 17 on the downstream side, in the casting direction, of the strong water-cooling sector is set as a non-water-cooling sector in which the spray of cooling water is stopped, and as a result, the surface temperature of the slab at the end of the horizontal zone 17 is set to lie within a prescribed range while the surface temperature of the slab is allowed to increase in the casting direction, after the strong water-cooling sector, up to the end of the horizontal zone 17.

Description

Secondary cooling method and device for continuous casting cast slab

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

Regarding the general manufacturing method of continuous casting cast slabs, a vertical bending type continuous casting equipment is taken as an example, and the description is based on Figures 3 and 4.

The molten steel poured into the mold 3 from a feed tank (not shown) is cooled by the mold 3 once to become a flat cast piece 5 with a solidified shell formed in a flat shape, which descends on the vertical belt 9 and advances toward the curved belt 13 . Furthermore, in the curved portion 11 on the entrance side of the curved belt 13, the cast sheet 5 is guided by a plurality of rollers (not shown) while maintaining a certain radius of curvature while being bent.

Then, the slab 5 is straightened (corrected) while increasing the radius of curvature in the correcting part 15 in order, and the cast piece 5 becomes a flat plate again at the position where it is separated from the correcting part 15 and moves toward the horizontal belt 17. After the horizontal belt 17 is completely solidified, the cast piece 5 is cut into a predetermined length by a gas cutting machine 23 installed on the outlet side of the continuous casting machine.

The gas cutting machine 23 moves the torch in the width direction while moving in the casting direction in synchronization with the conveying speed of the cast piece 5. Furthermore, while the cast sheet 5 is heated by the preheating flame of the gas torch, oxygen for cutting is injected, and the cast sheet 5 is melted and cut by the oxidation heat of the oxygen and steel.

When the casting speed is too fast or the temperature of the slab is too low, the cutting pitch of the gas cutter 23 cannot be synchronized with the casting speed, and problems such as casting speed limitation and poor cutting may occur. Therefore, the setting of the casting speed that matches the cutting ability and the temperature management of the cast slab 5 become important. The cast slab 5 cut by the gas cutter 23 is transported to the cast slab refining factory and rolling factory in the next process.

After the cast piece 5 is separated from the mold 3, it is from the vertical belt 9 to the horizontal belt 17. In order to make the center part solidify, a water jet (water single fluid jet, water-air two-fluid mixed sprayer) is used for the second time. cool down.

Generally, in the secondary cooling, a large flow of water is sprayed on the vertical belt 9 immediately below the mold 3 to increase the cooling rate of the cast slab 5 (in this specification, the increase in the cooling rate of the cast slab is called "strong cooling" ), thereby ensuring the strength of the solidified shell. After the belt 13 is bent, the cooling is weakened instead, and the surface temperature of the cast piece 5 rises (reheating) by heat conduction from the high temperature part inside. Next, in the correction part 15, it is adjusted so that the surface temperature becomes higher than the embrittlement temperature region, so as to avoid the occurrence of lateral cracks in the cast slab 5.

The cast slab 5 that has passed through the straightening part 15 is cooled in the horizontal belt 17 so that even the center part is completely solidified. When the solidification speed is slower than the casting speed, the solidification completion position cannot be restrained in the continuous casting machine. During gas cutting, the molten steel flows out from the cross section, which may cause major disasters such as equipment damage and work stoppage. Conversely, when the solidification is completed prematurely, not only the cooling water after the solidification is wasted, but the temperature of the cast slab 5 decreases and the cutting as described above becomes difficult. Therefore, the setting of the cooling conditions in the horizontal belt 17 has a great influence on the assurance of productivity and manufacturing stability.

FIG. 4 is a graph showing the results of numerical analysis that reproduces the temperature history of the cast slab 5 of the previous general continuous casting method. The vertical axis represents the temperature, and the horizontal axis represents the distance from the meniscus (the molten steel level in the mold).

In the upper part of the graph, the code of the area corresponding to the area after the mold 3 shown in FIG. 3 is written.

In the graph, the solid line is the temperature history at the center of the surface width of the cast slab, the dashed line is the temperature history at the corners (corners) of the cast slab, and the one-point chain line is the temperature history at the center of the cast slab section. In the graph, the lowest temperature that can be cut is indicated by a thin dashed line. As long as it is a higher temperature region (refer to the arrow), it is the temperature that can be cut. Furthermore, in the graph, the solidification completion position is indicated by A, and the machine end of the continuous casting machine is indicated by B.

As shown by the temperature history in the center of the surface width of the cast slab, from immediately below the mold 3 to the vertical belt 9, the strong cooling by a large-flow water jet increases the shell thickness. Next, starting from the curved portion 11 and the curved belt 13, the cooling rate is reduced to reheat from the inside of the cast slab, and the surface temperature of the cast slab is controlled to be higher than the embrittlement temperature region 25 when passing through the straightening portion 15. As a result, a cast piece 5 with good surface properties can be obtained.

Then, cooling is continued in the horizontal belt 17, and when the solidification of the center part of the slab is completed at point A, the temperature drop in the center part of the slab becomes larger. Then, at point B, it passes through the machine end of the continuous casting machine, is cut into a predetermined length by the gas cutter 23, and is transported to the next process. In this example, the solidification completion position is sufficiently upstream of the machine end of the continuous casting machine, and the slab corner temperature is also sufficiently higher than the cuttable temperature, so it can be cut without any problems.

As the above-mentioned general problems in the manufacturing process of cast slabs, surface defects such as vertical cracks and lateral cracks can be cited. Among them, the characteristic of lateral cracking is that it occurs in the vicinity of the corners of the upper surface of the cast slab in equipment including bending correction such as curved and vertical curved continuous casting machines. When passing through the correction section, if the surface temperature of the cast slab is in the embrittlement (region III embrittlement) region of the steel from the γ low temperature region to the γ/α transformation temperature region, it occurs due to the tensile stress on the surface generated during the correction. Horizontal cracking. As a method of preventing this lateral cracking, for example, Non-Patent Document 1 discloses that the secondary cooling of the cast slab is slowly cooled, and the embrittlement area is avoided to the high temperature side during straightening, thereby preventing the cracking.

In addition, the technique disclosed in Patent Document 1 is to reduce or stop the amount of cooling water for secondary cooling in the correction section near the final correction point, that is, near the entrance of the horizontal belt to allow the surface of the cast slab to reheat, thereby preventing surface turbulence. crack.

However, the method of avoiding the embrittlement temperature to the high temperature side will increase the average temperature of the slab section on the exit side of the correction part. As a result, due to the delay in the completion of solidification at the center of the cast slab, in order to complete the solidification in the continuous casting machine, the machine length of the continuous casting machine must be increased or the casting speed must be restricted, which may hinder productivity.

In response to this, in the technique disclosed in Patent Document 2, in order to suppress the solidification completion position in the machine, an adjustment cooling device is installed in the horizontal belt downstream of the straightening section to perform cooling.

However, in Patent Document 2, there is no specific description about the cooling conditions. Therefore, depending on the cooling conditions, significant temperature unevenness may occur in the width direction of the surface. On the surface of the flat embryo, there is a risk of surface cracks (longitudinal cracks) caused by thermal stress caused by the temperature unevenness. In the width direction, the solidification completion position is inconsistent, and there is a risk of uneven internal quality.

On the other hand, Patent Document 3 discloses a technique for suppressing uneven cooling of secondary cooling. In the collision range of the water ejector, the boiling state of the water is maintained in the film boiling state in the front stage of the cooling zone, and the nucleate boiling state is maintained in the rear stage, thereby stabilizing the cooling.

Generally speaking, if the cooling conditions are set constant in the width direction, since the corners of the cast slab also have heat dissipation from the side surfaces, the cooling rate is higher than that of the central portion of the cast slab width. In addition, when the cooling starts in the film boiling state, if the temperature of the cooled surface drops, the phenomenon of transition to the nucleate boiling state can be seen. Therefore, if the film boiling state is to be maintained as in Patent Document 3, the corners of the cast slab with a rapid temperature drop will first shift to the nucleate boiling state, resulting in a more rapid temperature drop. Such a rapid temperature difference is the cause of cracks on the surface of the cast slab caused by thermal stress. In addition, the drop in the temperature of the corners of the cast slab will cause the cutting performance of the gas cutter on the output side of the continuous casting machine to decrease and the cutting time to increase. In view of this problem, Patent Document 3 does not specifically discuss it, and the temperature control method on the output side of the continuous casting machine is not yet clear.

On the other hand, Patent Document 4 discloses a technique of preheating the corners of the cast slab for cutting in order to ensure the cutting performance on the gas cutter side. However, in the case of strong cooling based on nucleate boiling as described above, the temperature of the cast piece drops greatly, and the preheating time must be longer than usual. Furthermore, depending on the thickness of the cast slab and the type of steel, when the casting speed increases, the gas cutting speed cannot catch up and the casting speed must be limited, and huge energy must be input for preheating. [Prior Technical Literature] [Patent Literature]

Patent Document 1: Japanese Patent No. 4690995 Patent Document 2: JP 62-064462 Patent Document 3: Japanese Patent No. 6079387 Patent Document 4: Japanese Patent No. 2605329 [Non-Patent Literature]

Non-Patent Document 1: Ogi Lin et al.: "Mechanical Behavior of Continuous Casting", Joint Research Group of the Iron and Steel Association of the Iron and Steel Association, l985, p184

[The problem to be solved by the invention]

As explained above, it is not clear that the secondary cooling conditions can ensure the surface properties without impairing productivity and without adding a huge energy cost.

The present invention was developed in view of the above problems, and its purpose is to provide a continuous casting cast slab secondary cooling method and device that can ensure the surface properties of the cast slab without impairing productivity and without adding huge energy costs. [Technical means to solve the problem]

(1) The secondary cooling method of the continuous casting slab of the present invention is the secondary cooling of a continuous casting machine consisting of a vertical belt, a bent part, a bent belt, a straightening part, and a horizontal belt in order from the upstream side in the casting direction. In the cooling zone, cooling water is sprayed on the cast slab, and the solidification of the cast slab is completed in the section up to the end of the horizontal zone. The section on the upstream side in the casting direction in the horizontal zone is set as a strong water cooling zone In this strong water cooling zone, the cooling water is sprayed under the condition that the sprayed cooling water becomes a nucleate boiling state at the position of the entire width direction of the surface of the cast slab to cool the cast slab, and will be cooled more than the strong water The section from the downstream side of the casting direction to the end of the horizontal belt is set as a non-water-cooling section where the injection of the cooling water is stopped, so that from the strong water-cooling section to the end of the horizontal belt, let in the casting direction The surface temperature of the casting slab rises, and the surface temperature of the casting slab at the end of the horizontal belt is within a predetermined range. (2) In addition, in the secondary cooling method of the continuous casting slab described in (1) above, the horizontal band is divided into n (n: integer, 3≦n) sections in the casting direction, and the nith ~ n(i:integer, 0≦i<n-1) is the aforementioned non-water-cooling interval, the first to ni-1 is the aforementioned strong water-cooling interval, and the first to ni-1 is the aforementioned strong water-cooling interval In the interval, the water volume density per unit time of the aforementioned cooling water in the interval 1~j (j: integer, 1≦j<ni-1) is higher than that of the cooling water per unit time in the interval j+1~ni-1 The density of water is greater. (3) In addition, in the secondary cooling method of continuous casting slab described in (2) above, in the aforementioned strong water cooling zone of the aforementioned first to ni-1 zone, the first to j (j: integer, 1 ≦j<ni-1) The aforementioned cooling water density of the aforementioned cooling water is 500 L/(m ² ･min) (where min is the time unit) above 2000 L/(m ² ･min), the jth The aforementioned water density of the aforementioned cooling water in the interval between +1 and ni-1 is 50 L/(m ² ･min) or more and less than 500 L/(m ² ･min). (4) In addition, in the continuous casting slab secondary cooling method described in any one of (1) to (3) above, the surface temperature of the slab at the end of the horizontal belt is in the width direction of the slab The position where the lowest temperature appears is 350°C or higher. (5) The secondary cooling device for continuous casting slabs of the present invention is the secondary cooling zone of a continuous casting machine consisting of a vertical belt, a curved belt, and a horizontal belt from the upstream side of the casting direction. Cooling is performed by spraying cooling water, and the solidification of the cast slab is completed in the section up to the end of the horizontal band, characterized in that the horizontal band is divided into n (n: integer, 3≦n) sections in the casting direction The secondary cooling device has a plurality of nozzles, water supply means, and water supply control devices. The plurality of nozzles are arranged in each of the aforementioned sections of the horizontal belt. The water supply means and the water supply control device can control each of the aforementioned sections from the aforementioned The spraying and stopping of the cooling water from a plurality of nozzles, and the water volume density of the cooling water per unit time, the water supply control device is calculated from the upstream side in the casting direction from 1 to ni-1 (i: integer, 0≦ i<n-1) section, in which the cooling water is sprayed from the nozzle so as to become a strong water-cooling section. In this strong water-cooling section, the position of the sprayed cooling water in the entire width direction of the surface of the cast slab becomes In the nucleate boiling state, in the section ni~n (i: integer, 0≦i<n-1), the injection of the cooling water from the nozzle is stopped so as to become a non-water cooling section. (6) In addition, in the continuous casting slab secondary cooling device described in (5) above, the water supply control device controls the injection of the cooling water from the nozzle to be in the first to ni-1 In the aforementioned strong water-cooling interval of the interval, the water volume density per unit time of the aforementioned cooling water in the interval 1~j (j: integer, 1≦j<ni-1) is higher than that of the interval j+1~ni-1 The density of cooling water per unit time is greater. (7) In addition, in the continuous casting slab secondary cooling device described in (6) above, the water supply control device controls the injection of the cooling water from the nozzle to be in the first to ni-1 Among the aforementioned strong water-cooling intervals of the interval, the aforementioned water density of the aforementioned cooling water in the interval 1~j (j: integer, 1≦j<ni-1) is 500 L/(m ² ･min) or more 2000 L/(m) ² ･min) (where min is the minute of the time unit) or less, the aforementioned water density of the aforementioned cooling water in the interval j+1~ni-1 is 50 L/(m ² ･min) or more and less than 500 L/ (m ² ･min). [Effects of Invention]

In the present invention, the upstream section in the casting direction of the horizontal zone is set as a strong water cooling section, and in this strong water cooling section, the sprayed cooling water is sprayed under the condition that the position of the sprayed cooling water in the entire width direction of the surface of the cast slab becomes a nucleate boiling state. The slab is cooled by cooling water, and the section on the downstream side in the casting direction to the end of the horizontal zone is set as a non-water cooling section where the injection of cooling water is stopped than the strong water cooling section. After the interval to the end of the horizontal belt, the surface temperature of the cast slab is increased in the casting direction, and the surface temperature of the cast slab at the end of the horizontal belt is within a predetermined range. Therefore, it does not hinder productivity and does not need to add huge energy costs. It can ensure the surface properties of the cast piece.

The continuous casting machine used in the secondary cooling method of the continuous casting slab of the present embodiment will be schematically described based on FIG. 1.

The continuous casting machine 1, as shown in FIG. 1, is supported by rollers (not shown) by molten steel injected into the mold 3 from a feed tank (not shown), and is supported by a cooling jet installed between the rollers. (Not shown) The cast slab 5 is pulled out while performing secondary cooling.

The secondary cooling zone 7 for the secondary cooling of the cast slab 5 is divided into a vertical zone 9, a curved part 11, a curved zone 13, a straightening part 15, and a horizontal zone 17, as shown in FIG. 1. The secondary cooling method of the present invention, The main feature is the cooling method of the cast slab 5 of the horizontal belt 17.

The secondary cooling zone 7 of the continuous casting machine 1 is equipped with a strong cooling device 21. The strong cooling device 21 divides the horizontal belt 17 into n (n: integer, 3≦n) sections, and has cooling water control in each section. Water supply means and water supply control device 19 for ON/OFF and cooling water volume.

The number of n can be set in advance according to the equipment, and which of the n sections is set to be a strong water cooling section or a non-cooling section can be appropriately set by the water supply control device 19.

In the horizontal belt 17, almost 100 rollers are arranged at predetermined intervals along the casting direction, although the scale of the equipment is different. The nozzles for spraying cooling water are arranged between the rollers, and the rollers are arranged along the width direction of the cast slab. Multiple nozzles.

The strong cooling equipment 21 of the present embodiment uses nozzles provided between a plurality of rolls (for example, ten rolls) in the casting direction as a unit, and divides the horizontal belt 17 into n sections.

Therefore, in each section, a plurality of nozzles form a unit, and a large flow rate of cooling water can be injected in order to quickly stabilize the boiling state of the cooling water in the nucleate boiling state.

In addition, in each section, it is possible to switch between nozzles and piping to be used in order to cope with not only large flow conditions but also small flow conditions.

The nozzle used here is not limited to a water single-fluid ejector as long as it can realize the water density per unit time described later, and a water-air two-fluid mixed spray nozzle or the like may also be used.

The secondary cooling method of the continuous casting cast slab of this embodiment is to cast the cast slab 5 in the continuous casting machine 1 with a vertical belt 9, a curved part 11, a curved belt 13, a straightening part 15, and a horizontal belt 17. The second cooling zone 7 sprays cooling water on the cast slab 5 to cool it. The section up to the end of the horizontal zone allows the cast slab 5 to solidify, and the upstream section of the horizontal zone 17 in the casting direction is set as a strong water cooling section. In this strong water cooling zone, the cooling water is sprayed on the condition that the sprayed cooling water becomes a nucleate boiling state on the surface of the cast slab to cool the cast slab 5, and at the aforementioned level on the downstream side in the casting direction than the aforementioned strong water cooling zone The zone up to the end of the belt is a non-water cooling zone in which the injection of cooling water is stopped.

Furthermore, after the strong water cooling zone to the end of the horizontal belt, the surface temperature of the cast slab is increased in the casting direction, and the surface temperature of the cast slab at the end of the horizontal belt is within a predetermined range.

FIG. 2 shows the results of numerical analysis that reproduced the temperature history of the surface of the cast slab manufactured using the above-mentioned continuous casting machine 1. In Figure 2, the temperature history at the center of the surface width of the cast slab is represented by a solid line, the temperature history at the corner (corner portion) of the cast slab is represented by a dotted line, and the temperature history at the center of the section of the cast slab is represented by a chain line. The lowest temperature that can be cut is indicated by a thin dashed line. In addition, in Fig. 2, the solidification completion position is indicated by A', and the machine end of the continuous casting machine is indicated by B. Fig. 2 also shows the solidification completion position A of the previous example shown in Fig. 4.

Cooling from immediately below the mold 3 until passing through the straightening part 15 is performed in the same manner as in the prior art, and the surface temperature of the cast slab 5 in the straightening part 15 is higher than the embrittlement temperature region 25.

On the other hand, if it enters the horizontal belt 17 and starts cooling by the strong cooling device 21, the horizontal belt 17 on the downstream side of the casting direction after entering the water jets installed between the first rollers of the horizontal belt 17, by the large Flow rate water jet to achieve uniform nucleate boiling state in the width direction. As a result, the temperature of the center of the slab width and the corners of the slab can be simultaneously dropped to a temperature close to the water temperature and stabilized.

Then, strong cooling is continued to maintain the nucleate boiling state, and after solidification is completed at point A', the internal temperature begins to drop. After the internal solidification is completed, or even before the solidification is completed, after the temperature has been sufficiently lowered and the solidification has been completed up to the end of the machine, cooling is not necessary. Therefore, the ejection of the injector is stopped in the i+1 region of the n-i~n (i: integer, 0≦i<n-1), and the surface of the cast slab is reheated after the point C. As a result, at point B, the temperature of the corner portion of the cast slab becomes higher than the cuttable temperature, and cutting can be performed without any problem.

Generally speaking, for the temperature control such as the fluctuation of the casting speed of the cast slab 5, the temperature control is mostly performed by changing the flow rate of the cooling water. It is not the strong cooling based on the viewpoint of cooling stabilization as in the present invention. In the case near room temperature, the flow rate is controlled based on the viewpoint of nucleate boiling maintenance. Therefore, as previously described, cooling must be stopped in a part of the cooling interval to adjust the water cooling time and control the cooling end temperature.

In the case of applying the present invention, by implementing strong cooling in the horizontal belt 17, when the casting speed is the same as that of the prior art, compared to the position A in the case of using the prior art, the solidification completion position A'is toward the continuous casting machine 1 Because of the movement on the upstream side, the casting speed becomes higher than the previous conditions. At this time, because the casting speed increases, the time to pass through the cooling zone decreases and the cooling time is shortened. Therefore, the number i+1 of the non-water cooling zone where cooling is stopped is reduced, and the length of the cooling zone where cooling is performed is increased, so that solidification can be completed in the continuous casting machine 1 with certainty.

On the other hand, the casting speed decreases at the beginning and end of casting. In this case, the following control can be performed: increasing the number of non-water cooling sections i+1 to prevent the temperature of the entire cast slab 5 from falling and making the temperature of the corners of the cast slab lower than the cuttable temperature.

Regarding the cooling water injection conditions (water density per unit time) of the present invention, nucleate boiling can be quickly achieved across the entire width regardless of the variation in casting speed, the manufacturing conditions such as the steel type, and the equipment conditions such as the arrangement interval of the injectors. As a result of the research, it is found that it must be 500 L/(m ² ･min) (where min is the minute of the time unit) or more. Here, the water volume density per unit time is a value obtained by dividing the cooling water volume (L/min) of the cooling zone by the area (m ^{2) of the cooling zone.}

Below the water density per unit time, when the high-temperature cast slab 5 is cooled, the nucleate boiling state cannot be stably reached, and the temperature drop is large (the corners of the cast slab, etc.) and the temperature drop is small (the width of the cast slab). The center, etc.) the time of nucleate boiling is quite different between the two, and there is a significant temperature difference in the width direction.

In addition, depending on the equipment configuration and the type of steel, the part where the cooling water of the water jet is not directly sprayed (the guide roller is immediately below and near, etc.) may reheat significantly, and the nucleate boiling state cannot be obtained stably, which may cause The cause of the significant temperature difference. Due to such a temperature difference, the cast slab 5 will be deformed, causing defects such as cracks.

On the other hand, as long as nucleate boiling can be achieved, cooling by boiling becomes dominant, so the dependence of the cooling capacity on the water density per unit time becomes smaller. Therefore, the water density per unit time greater than 2000 L/(m ² ･min) cannot be expected to increase the cooling capacity. The total amount of cooling water used has become too large, which increases the equipment investment of water treatment equipment. Therefore, it is appropriate to set the water density per unit time in the strong water cooling zone to a range of 500 L/(m ² ･min) above 2000 L/(m ² ･min).

As long as the cast slab 5 enters in the above-mentioned strong water cooling zone and the surface temperature of the cast slab is lowered by nucleate boiling, even if ^{the flow rate is not 500 L/(m 2} ･min) or more, the nucleate boiling state can be maintained stably . Therefore, when the total amount of cooling water that can be used by the continuous casting machine 1 as a whole is limited, the first to j (j: integer, 1≦j≦ni-1) section of the strong water cooling section is set per unit time In the large flow area with a water density of 500 L/(m ² ･min) or more, the remaining j+1~ni-1 interval can be set as the water density per unit time as long as it can maintain nucleate boiling. Small flow area with density above 50 L/(m ² ･min) and below 500 L/(m ^{2 ･min).} At this time, the number of sections j of the large flow area in the front stage can be arbitrarily set according to the manufacturing conditions such as the type of steel and the thickness of the cast slab.

In addition, as a result of studying the temperature range that can ensure the cutting performance of the gas cutter on the output side of the continuous casting machine, it was found that the temperature of the corner of the cast slab immediately in front of the cutter must be controlled above 350°C. Therefore, the surface temperature of the cast slab at the end of the horizontal belt 17 is preferably 350° C. or more at the position where the lowest temperature appears in the width direction of the cast slab.

As described above, in the present embodiment, the secondary cooling zone 7 of the horizontal zone 17 is divided into a plurality of sections by the strong cooling device 21, and a strong water cooling section is provided for cooling while maintaining the nucleate boiling state, and in the strong cooling device 21 A non-cooling section for stopping the injection of cooling water is provided on the downstream side of the water cooling section in the casting direction, and the range of the section is changed according to conditions such as casting speed. Therefore, the temperature at the end of casting can be controlled without significant temperature unevenness on the surface.

In this way, the surface properties of the cast slab 5 can be maintained at high quality and high-speed casting can be performed. Even if the casting conditions are changed, the cast slab 5 can be cut without any problems, and high productivity can be maintained and high-quality cast slabs can be produced stably. 5.

The non-cooling section located on the downstream side of the casting direction of the aforementioned strong water cooling section is a section where the injection of cooling water is stopped in order not to actively cool the slab, for example, the excess liquid in the piping flows down the surface of the slab State, the state where a very small amount of water is supplied to prevent the nozzle from clogging, etc., even if cooling water is not deliberately cooling the cast slab and cooling water is applied to the surface of the cast slab, as long as it is used for active cooling of the cast slab as described above The cooling water injection stop is of course included in the non-cooling zone.

In addition, in the non-cooling zone, not only the injection of cooling water is stopped, but also auxiliary means such as heat jackets and edge heaters can be used to maintain the temperature of the corners of the cast slab where the surface temperature of the cast is easy to drop. Or rise.

When the predetermined water density per unit time cannot be achieved due to some reasons such as equipment abnormality caused by the leakage of water in the piping, and the nucleate boiling state cannot be quickly achieved after the cast slab enters the strong water cooling zone, the boiling state must be carried out at the same time. While monitoring increases the amount of water, the nucleate boiling state is reliably achieved and maintained.

If the cooling water in contact with the surface of the cast slab boils, it will vaporize into water vapor, and the mist (water mist) formed by the water vapor condensed in the air can be observed. Here, in the nucleate boiling state, the cooling water in contact with the surface of the cast slab foams violently to generate a large amount of water vapor, and the amount of water mist generation increases. In contrast, in the film boiling state, the boiling cooling water has less foaming, and the amount of water vapor and water mist generated is also reduced. Therefore, cameras are installed in each section, and the amount of water mist generation is monitored by observation based on visual observation or measurement based on a transmissometer. The threshold value of the amount of water mist generated to distinguish between nucleate boiling and film boiling is obtained through experiments in advance, and by confirming whether the amount of generated water mist exceeds the threshold value, it is confirmed whether the nucleate boiling state in a predetermined interval is achieved. Moreover, when the nucleate boiling state has not been achieved, the adjustment is made by increasing the amount of cooling water. This can reliably achieve and maintain the nucleate boiling state. Example

In the following description, the continuous casting machine 1 (FIG. 1) of the above-mentioned embodiment is used to manufacture the cast slab 5, and the effect of the present invention is confirmed.

In this embodiment, the horizontal zone 17 is divided into 12 sections (n=12), and control of the presence or absence of injection and the injection flow rate are performed in each section. In addition, the machine length of the continuous casting machine 1 is 45 m, and at the end of the machine, a thermometer and a gas cutting machine 23 for measuring the temperature distribution on the surface of the cast piece are installed.

The production conditions such as the water density per unit time (L/(m ² ･min)), the casting speed, and the thickness of the flat blank in the horizontal zone are changed to produce the cast slab 5, which is aimed at the temperature unevenness during cooling and the inside of the casting machine. The estimated solidification position, the slab corner temperature during cutting, and the surface properties after casting are evaluated.

The manufacturing conditions and evaluation are summarized in Table 1 below. In the table, those falling within the scope of the present invention are Examples 1-7, and those falling outside the scope of the present invention are Comparative Examples 1-8.

In addition, the estimation of the solidification position was carried out in advance through numerical analysis. Some comparative examples, as a result of prior review, were judged to be dangerous that the solidification position could not be suppressed in the continuous casting machine 1, and actual manufacturing was not carried out. .

以下，針對表1的結果，對每個關聯的比較例及實施例進行考查。 ＜比較例1, 2、實施例1, 2＞ 比較例1, 2及實施例1, 2，分別為運用先前技術、本發明的技術所製造之厚度235mm的鑄片5。

Hereinafter, with respect to the results in Table 1, each related comparative example and embodiment will be examined. <Comparative Examples 1, 2, Examples 1, 2> Comparative Examples 1, 2 and Examples 1, 2 are cast slabs 5 with a thickness of 235 mm manufactured by using the prior art and the technique of the present invention, respectively.

In Comparative Example 1, it is an example manufactured under the previous cooling conditions (water density per unit time 10 L/(m ² ･min), no cooling stop area). In this example, since the surface always maintains film boiling stably, temperature unevenness does not occur, and it is confirmed that there is no problem as a result of inspecting the surface condition of the cast slab after manufacturing. In addition, the corner temperature of the cast piece during cutting is 580°C, which does not hinder cutting.

However, in order to keep the solidification complete position within the machine (estimated 36m position), the casting speed is limited to 1.0 mpm at most.

Therefore, in Comparative Example 2, the casting speed was increased to 2.5 mpm in order to improve productivity. Under this condition, the calculation result that the presumed solidification position is outside the machine was obtained, so no actual manufacturing was carried out. In this way, although cast slabs 5 with good surface properties can be manufactured according to the prior art, the casting speed is limited.

On the other hand, in Example 1, the technology of the present invention is used, and the water density per unit time is set to 500 L/(m ² ･min) in the first to ninth sections, and strong cooling is performed, and in the tenth to twelfth sections Stop the cooling water and adjust the surface temperature by reheating. At this time, the casting speed was increased to 2.5 mpm for casting. As a result, a uniform nucleate boiling state in the width direction was achieved by strong cooling, and temperature unevenness did not occur. In addition, it was estimated that the solidification completed position was 38m, which was sufficiently restrained in the machine, so the manufacturing was implemented. As a result, the slab corner temperature at the time of cutting was 420°C, which was lower than that of Comparative Example 1, but entered the cuttable area and could be cut without any problem. In addition, as a result of inspecting the surface condition of the cast slab after the production, no cracks were observed, and the cast slab 5 with good surface properties could be produced efficiently without any problems.

In Example 2, the technology of the present invention is used to implement strong cooling by setting the water density per unit time to 2000 L/(m ² ･min) in the first to tenth zones, and set the zone where the cooling water is stopped as the first 11~12 interval. At this time, the casting speed can be further increased to 3.5 mpm, there is no hindrance during cutting and no surface texture problems, and high-quality cast slabs 5 can be produced efficiently. <Comparative Examples 3 and 4> Comparative Examples 3 and 4 are the results of changing the cooling conditions in the strong water cooling zone with reference to the conditions of Example 1. In Comparative Example 3, the cooling stop area was not provided, and the water density per unit time was set to 500 L/(m ² ･min) in all the sections, and strong cooling was performed. At this time, the temperature unevenness caused by cooling does not occur, and the solidification completion position is also restrained in the machine. However, the strong cooling takes a long time and the machine ends cannot be fully reheated, so the corner temperature of the cast slab during cutting drops to 320°C. As a result, cutting takes time, and there is a concern that the cutting cannot be completed within the movable range of the gas cutter 23. Therefore, the casting speed must be urgently reduced. Furthermore, because the casting speed changes drastically, the surface quality and internal quality of the cast slab 5 cast at that time will be reduced.

In addition, in Comparative Example 4, the water density per unit time in the 1st to 10th sections was set to 400 L/(m ² ･min), and the cooling water was stopped in the 11th to 12th sections. As a result, at this flow rate, in the strong water cooling zone, the nucleate boiling state cannot be stably reached at the width of a part of the cast slab. The corners of the cast slab with a large temperature drop first become the nucleate boiling state, and a significant temperature occurs in the width direction. difference. Therefore, surface cracks and internal cracks of the cast slab occur, and there is a problem that the quality of the cast slab 5 is lowered. <Examples 3, 4, Comparative Examples 5, 6> In Examples 3, 4 and Comparative Examples 5, 6, for Example 1, only the first section of the strong water cooling section was set as a large flow area, and the second and subsequent sections The conditions for the reduction of the flow rate in the interval.

In Example 3, the water density per unit time in the first large flow interval is set to 500 L/(m ² ･min), and the water density per unit time in the second to 11th intervals is set to 50 L/( m ² ･min), the cooling water is stopped in the 12th section. At this time, the nucleate boiling state is reached by cooling in the first section of the strong water cooling section, and the nucleate boiling state is maintained without reheating in the subsequent sections. As a result, cooling unevenness in the width direction did not occur. In addition, the solidification completion position is 43m, which is suppressed in the machine. The corner temperature of the cast piece during cutting is 430℃, so it can be cut without any problems. Furthermore, as a result of inspecting the surface condition of the cast slab after production, no cracks were observed, and a cast slab 5 with good surface properties could be produced.

In addition, in Example 4, the water density per unit time in the strong water cooling zone is 2000 L/(m ² ･min) in the first zone and 1000 L/(m ² ･min) in the second zone. 3 The interval is 500 L/(m ² ･min), the 4th to 5th interval is 100 L/(m ² ･min), the 6th to 10th interval is 50 L/(m ² ･min), and it is set as Gradually shrink. In the 11th to 12th section, the cooling water is stopped. At this time, the nucleate boiling state is reached by cooling in the first section of the strong water cooling section, and the nucleate boiling state is maintained without reheating in the subsequent sections. As a result, cooling unevenness in the width direction did not occur. The solidification completion position is 40m, which is also restrained in the machine. The corner temperature of the cast piece during cutting is 370°C, so it can be cut without any problems. In addition, as a result of inspecting the surface condition of the cast slab after the production, no cracks were observed, and a cast slab 5 with good surface properties could be produced.

On the other hand, in Comparative Example 5, the water volume density per unit time in the small flow area in the second half of the strong water cooling section was set to 40 L/(m ² ･min). As a result, nucleate boiling cannot be maintained at the center of the width of the cast slab with large reheating, and the temperature rises, and significant temperature unevenness occurs in the width direction. Although the solidification position is restrained in the machine, the flat embryo is deformed due to temperature unevenness in the width direction and cracks are generated on the surface.

In addition, in Comparative Example 6, the water volume density per unit time in the large flow area in the first half of the strong water cooling section was set to 400 L/(m ² ･min). As a result, the nucleate boiling state cannot be quickly realized at the stage where the cast slab 5 enters in the strong water cooling zone, and the nucleate boiling state and the film boiling state are mixed in the width direction. Therefore, the unevenness of the surface temperature is large, and surface cracking occurs, and as a result of uneven cooling, the solidification completion position becomes uneven, which lowers the internal quality. <Example 5> Example 5 is an example of Example 1 where the casting speed must be significantly reduced at the start and end of casting. At this time, the casting speed was reduced to 2.0 mpm, and the time to implement strong cooling was extended, so the non-water cooling zone was expanded to the 8th to 12th zone. As a result, cooling unevenness did not occur, the solidification completion position was 35 m, and the slab corner temperature at the time of cutting was 460°C, which was able to be suppressed within the cuttable range. In addition, as a result of inspecting the surface condition of the cast slab after the production, no cracks were observed, and even if the casting speed changed significantly, the cast slab 5 with good surface properties could be produced without any problem. <Comparative Examples 7, 8, and Examples 6, 7> Comparative Examples 7 and 6, and Comparative Examples 8 and 7, were the results of the case where the thickness of the flat blank was changed to 260 mm and 200 mm, respectively. Comparative Examples 7 and 8 are the same as Comparative Example 1, and the thickness of the flat blank is changed to 260 mm and 200 mm according to the cooling conditions of the prior art.

In Comparative Example 7, the thickness of the flat blank is 260mm. Compared with Comparative Example 1, the temperature drop is reduced due to the thicker flat blank. The casting speed is reduced to 0.8 mpm, and the solidification position can be restrained in the machine. In Comparative Example 8, the thickness of the flat blank is 200mm. Compared with Comparative Example 1, the thickness of the flat blank is thinner. In order to avoid unnecessary temperature drop after the solidification of the center part, the casting speed is increased to 2.0 mpm.

In contrast, Example 6 is the case where the thickness of the flat billet is 260 mm. Compared with Example 1, the temperature drop is reduced due to the thicker flat billet, and the casting speed remains the same, but the strong water cooling zone is extended to the first ~11 interval. The water density per unit time in the strong water cooling zone is the same as in Example 1. As a result, cooling unevenness did not occur, the solidification completion position was 42 m, and the slab corner temperature at the time of cutting was 440°C, which was suppressed in the cuttable range. In addition, as a result of checking the surface condition of the cast slab after manufacturing, no cracks were observed. Even if the casting thickness is thick, the casting speed can be maintained at a high casting speed and the cast slab 5 with good surface properties can be manufactured without any problem.

Example 7 is the case where the thickness of the flat blank is 200mm. Compared with Example 1, the temperature drop becomes larger because the thickness of the flat blank becomes thinner, and the casting speed is increased to 3.0 mpm. The water density per unit time in the strong water cooling zone is the same as in Example 1, and the non-water cooling zone is expanded to the 9th to 12th zones. As a result, cooling unevenness did not occur, the solidification completion position was 37 m, and the slab corner temperature at the time of cutting was 430°C, which was suppressed in the cuttable range. In addition, as a result of inspecting the surface condition of the cast slab after manufacturing, no cracks were found. Even if the casting thickness is reduced, the casting speed does not need to be significantly reduced, and the cast slab 5 with good surface properties can be produced without any problems. .

In this way, by using the technology of the present invention, even if the thickness of the cast slab changes, the casting speed does not need to be greatly changed as in the prior art, and high-quality cast slabs 5 can be manufactured stably and efficiently.

As verified above, by setting the upstream section of the horizontal belt 17 in the casting direction as a strong water cooling section, the position of the injected cooling water in the entire width direction of the surface of the cast slab becomes the core in the strong water cooling section. The conditions of the boiling state spray cooling water to cool the cast slab 5, and the section on the downstream side of the casting direction to the end of the horizontal zone on the downstream side of the casting direction than the aforementioned strong water cooling section is set as a non-water cooling section where the injection of cooling water is stopped. In the case of changing casting conditions, the casting slab 5 can be kept at a temperature that is easy to cut and can be manufactured without the limitation of casting speed and the huge energy cost for heating.

1: Continuous casting machine 3: Mold 5: Casting 7: Secondary cooling zone 9: Vertical band 11: Bend 13: bending belt 15: Correction Department 17: Horizontal belt 19: Water supply control device 21: Strong cooling equipment 23: Gas cutting machine 25: embrittlement temperature zone

Fig. 1 is an explanatory diagram illustrating the outline of a continuous casting facility according to an embodiment of the present invention. [Fig. 2] A graph showing the temperature history of a cast slab in a continuous casting method according to an embodiment of the present invention. [Fig. 3] is an explanatory diagram explaining the outline of the conventional continuous casting equipment. [Figure 4] is a graph showing the temperature history of cast slabs in the previous general continuous casting method.

1: Continuous casting machine

3: Mold

5: Casting

7: Secondary cooling zone

9: Vertical band

11: Bend

13: bending belt

15: Correction Department

17: Horizontal belt

19: Water supply control device

21: Strong cooling equipment

23: Gas cutting machine

Claims (7)

A secondary cooling method for continuous casting cast slabs, which is a secondary cooling zone of a continuous casting machine consisting of a vertical belt, a bent part, a bent belt, a straightening part, and a horizontal belt in order from the upstream side of the casting direction. The slabs are cooled by spraying cooling water, and the solidification of the cast slabs is completed in the section up to the end of the horizontal belt. It is characterized in that: The section on the upstream side of the casting direction in the horizontal zone is set as a strong water cooling section. In this strong water cooling section, the sprayed cooling water is in a nucleate boiling state at the position of the entire width direction of the surface of the cast slab. The cooling water is sprayed to cool the cast slab, and the section on the downstream side in the casting direction to the end of the horizontal zone than the strong water cooling section is set as a non-water cooling section where the spray of the cooling water is stopped. From after the strong water cooling zone to the end of the horizontal belt, the surface temperature of the casting slab is increased in the casting direction, and the surface temperature of the casting slab at the end of the horizontal belt is within a predetermined range. The secondary cooling method for continuous casting cast slabs according to claim 1, wherein: Divide the aforementioned horizontal belt into n (n: integer, 3≦n) sections in the casting direction, set the ni~n (i: integer, 0≦i<n-1) section as the aforementioned non-water cooling section, and set the first The interval from 1 to ni-1 is set as the aforementioned strong water cooling interval, Among the aforementioned strong water-cooling intervals in the aforementioned first to ni-1 intervals, the water volume density per unit time of the aforementioned cooling water in the first to j (j: integer, 1≦j<ni-1) intervals is higher than that of j+1 The water density per unit time of the aforementioned cooling water in the interval ~ni-1 is greater. The second cooling method of continuous casting cast slab according to claim 2, wherein, in the strong water cooling section of the first to ni-1 section, the first to j (j: integer, 1≦j<ni- 1) The aforementioned water density of the aforementioned cooling water in the interval is 500 L/(m ² ･min) (where min is the time unit) above 2000 L/(m ² ･min), the first j+1~ni- The aforementioned water density of the aforementioned cooling water in section 1 is 50 L/(m ² ･min) or more and less than 500 L/(m ² ･min). The secondary cooling method of continuous casting cast slab according to any one of claims 1 to 3, wherein: The surface temperature of the cast slab at the end of the horizontal belt is 350° C. or more at the position where the lowest temperature appears in the width direction of the cast slab. A secondary cooling device for continuous casting slabs, which is a secondary cooling zone of a continuous casting machine consisting of a vertical belt, a curved belt, and a horizontal belt from the upstream side of the casting direction. The cooling water is sprayed on the casting slab. Cooling, the solidification of the cast slab is completed in the section up to the end of the horizontal belt, which is characterized by: The aforementioned horizontal band is divided into n (n: integer, 3≦n) sections in the casting direction, The secondary cooling device has a plurality of nozzles, a water supply means, and a water supply control device. The plurality of nozzles are arranged in each of the aforementioned sections of the horizontal belt. The water supply means and the water supply control device can control the control from the plurality of sections for each of the aforementioned sections. The spraying and stopping of the cooling water of each nozzle, and the water density per unit time of the cooling water, This water supply control device sprays the cooling water from the nozzles in the first to ni-1 (i: integer, 0≦i<n-1) section from the upstream side of the casting direction, and sprays the cooling water from the nozzle so as to become a strong water cooling section. In this strong water cooling zone, the sprayed cooling water is brought into a nucleate boiling state at the position of the entire width direction of the surface of the cast slab, and is in the ni~nth (i: integer, 0≦i<n-1) zone, The injection of the cooling water from the nozzle is stopped so as to become a non-water cooling zone. The continuous casting slab secondary cooling device according to claim 5, wherein: The water supply control device controls the injection of the cooling water from the nozzle so that the first to j (j: integer, 1≦j<ni-) in the strong water cooling section of the first to ni-1 section 1) The water volume density per unit time of the cooling water in the interval is greater than the water volume density per unit time of the cooling water in the j+1 to ni-1th interval. The continuous casting cast slab secondary cooling device according to claim 6, wherein the water supply control device controls the injection of the cooling water from the nozzle to the strong water cooling in the interval from 1 to ni-1 In the interval, the water density of the aforementioned cooling water in the interval 1~j (j: integer, 1≦j<ni-1) is 500 L/(m ² ･min) or more 2000 L/(m ² ･min)( Among them, min is the time unit minutes) or less, the aforementioned water density of the aforementioned cooling water in the interval j+1~ni-1 is 50 L/(m ² ･min) or more and less than 500 L/(m ² ･min) ).

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