Method for manufacturing a casting sheet

Abstract

To provide a slab production method which can suppress the generation of a quality difference in the width direction of a slab when using split type slab support rolls.SOLUTION: There is provided a slab production method of producing a slab by subjecting a slab cast with a continuous casting machine to secondary cooling in a secondary cooling zone at least having a horizontal zone. In the horizontal zone, the slab is cooled in such a manner that the boiling state of a cooling liquid in the surface of the slab is made into nuclear boiling while supporting the slab with split type slab support rolls split into two or more in a slab width direction, and gas is jetted from a gas jet nozzle to suppress the leakage of the cooling liquid of the horizontal zone from roll chock parts of the split type slab support rolls.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a slab for manufacturing a slab, a secondary cooling device, and a continuous casting machine.

Background Art

[0002] In the continuous casting of steel, the molten steel poured into the mold is cooled by the mold to form a solidified shell on the contact surface with the mold. The slab having this solidified shell as an outer shell and having unfrozen molten steel inside is continuously drawn downward from the mold while being cooled by a coolant in a secondary cooling zone provided below the mold, and eventually the solidification up to the center part is completed. The slab whose solidification up to the center part is completed is cut into a predetermined length, and a slab as a material for rolling is manufactured.

[0003] Generally, in secondary cooling, the slab is cooled in a film boiling state. Film boiling is a kind of boiling form, which is likely to occur when the surface temperature of the material to be cooled is high and the coolant is at low pressure and low flow rate. A vapor layer is formed between the coolant and the material to be cooled, which becomes an insulating layer, and it is a boiling in which the cooling rate of the material to be cooled is relatively slow. Although film boiling can stably cool the material to be cooled, as described above, since the cooling rate of the material to be cooled is slow, there is a problem of low productivity.

[0004] In continuous casting, improvement in productivity is desired together with the quality of the slab. As one measure for this, it is conceivable to increase the heat transfer coefficient between the coolant and the slab surface, that is, the heat transfer coefficient during spray cooling. Therefore, as disclosed in Patent Document 1, if the coolant is sprayed onto the slab surface at high pressure, the amount of coolant contacting the slab surface per unit time increases, the heat transfer coefficient increases, and it is considered that the productivity also improves.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in the method of Patent Document 1, since the coolant pressure is high, the coolant leaks out of the injection area from the gap between the casting and the roll chuck part of the divided type casting support roll divided into two or more in the width direction of the casting slab. When the coolant leaks, the casting slab is cooled by the leaked liquid, so there is a problem that a temperature difference occurs between the area with the roll chuck where the coolant leaks and the area without it, resulting in a quality difference in the width direction of the casting slab. The present invention has been made to solve such problems, and an object thereof is to provide a method for manufacturing a casting slab, a secondary cooling device, and a continuous casting machine that can suppress the occurrence of quality differences in the width direction of the casting slab when using a divided type casting support roll.

Means for Solving the Problems

[0007] The means for solving the above problems are as follows. [1] A method for manufacturing a casting slab in which a casting slab cast by a continuous casting machine is secondarily cooled in a secondary cooling zone having at least a horizontal zone to manufacture the casting slab. In the horizontal zone, while supporting the casting slab with a divided type casting support roll divided into two or more in the width direction of the casting slab, the boiling state of the coolant on the surface of the casting slab is made into nucleate boiling for cooling, and gas is injected from a gas injection nozzle to suppress the leakage of the coolant in the horizontal zone from the roll chuck part of the divided type casting support roll. [2] The gas injection nozzle is installed at least at one of the upstream end and the downstream end with respect to the casting direction of the horizontal zone. The gas injection nozzle at the upstream end injects gas toward the surface of the casting slab on the downstream side in the casting direction at the position of the roll chuck part of the divided type casting support roll, and the gas injection nozzle at the downstream end injects gas toward the surface of the casting slab on the upstream side in the casting direction at the position of the roll chuck part of the divided type casting support roll. The method for manufacturing a casting slab according to [1]. [3] The method for manufacturing a casting slab according to [1], wherein the injection flow rate of the gas from the gas injection nozzle is controlled based on the generation amount of steam. The method for manufacturing a slab according to [1], wherein when the amount of generated steam is greater than a preset threshold value of the amount of generated steam, the injection flow rate of the gas is increased. [5] The secondary cooling of the slab in the horizontal zone includes a pre-stage cooling process on the upstream side with respect to the casting direction and a post-stage cooling process on the downstream side with respect to the casting direction. The flow rate density of the coolant in the pre-stage cooling process is 300 L / (m 2 ·min) or more and 4000 L / (m 2 ·min) or less, and the flow rate density of the coolant in the post-stage cooling process is 100 L / (m 2 ·min) or more and 1000 L / (m 2 ·min) or less. The injection flow rate of the gas is 200 NL / min or more and 3000 NL / min or less per roll chuck part of the roll chuck part 1. The method for manufacturing a slab according to any one of [1] to [4]. [6] A secondary cooling device for secondary cooling a slab in a secondary cooling zone having at least a horizontal zone, wherein the horizontal zone is provided with a split-type slab support roll divided into two or more in the slab width direction, a cooling spray for injecting a coolant onto the surface of the slab, and a gas injection nozzle for injecting a gas onto the surface of the slab. The gas injection nozzle is installed at at least one of the upstream end and the downstream end with respect to the casting direction of the horizontal zone. The gas injection nozzle at the upstream end injects gas toward the surface of the slab on the downstream side with respect to the casting direction at the position of the roll chuck part of the split-type slab support roll, and the gas injection nozzle at the downstream end injects gas toward the surface of the slab on the upstream side with respect to the casting direction at the position of the roll chuck part of the split-type slab support roll. Secondary cooling device. [7] The horizontal zone has a pre-stage cooling part and a post-stage cooling part. The flow rate density of the coolant injected from the cooling spray in the pre-stage cooling part is 300 L / (m 2 ·min) or more and 4000 L / (m 2 ·min) or less, and the flow rate density of the coolant injected from the cooling spray in the post-stage cooling part is 100 L / (m 2 ·min) or more and 1000 L / (m 2·min) or less, and the injection flow rate of the gas is 200 NL / min or more and 3000 NL / min or less per roll chock part of the roll chock part 1, the secondary cooling device according to [6]. [8] A continuous casting machine having a mold into which molten steel is poured and cooling the molten steel, and the secondary cooling device according to [6] or [7].

Effect of the Invention

[0008] In the method for manufacturing a slab according to the present invention, a gas is injected from a gas injection nozzle to suppress leakage of the cooling liquid in the horizontal band from the roll chock part of the split slab support roll, so that the temperature difference in the width direction of the slab can be reduced. As a result, the slab can be manufactured while suppressing the occurrence of quality differences in the width direction of the slab.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Mode for Carrying Out the Invention

[0010] Hereinafter, the present invention will be described through embodiments of the present invention. FIG. 1 is a schematic diagram showing an example of a continuous casting machine 1 in which the method for manufacturing a slab according to the present embodiment can be implemented. The method for manufacturing a slab according to the present embodiment is to perform secondary cooling on the slab 3 being cast by the continuous casting machine 1 in a secondary cooling zone 11 having a vertical zone 5, a curved zone 7, and a horizontal zone 9. The secondary cooling in the horizontal zone 9 includes a pre-stage cooling process in the pre-stage cooling section 13 and a post-stage cooling process in the post-stage cooling section 15. Hereinafter, each component will be described in detail.

[0011] <Continuous casting machine 1> In the continuous casting machine 1, as shown in FIG. 1, the molten steel poured from a tundish (not shown) into the mold 17 is cooled by the mold 17, and a solidified shell is formed on the contact surface with the mold 17. The slab 3 having this solidified shell as the outer shell and having unfrozen molten steel inside is supported by the rolls 19 and is solidified to the center by being secondarily cooled by the cooling spray 21 provided between the rolls 19. The secondary cooling zone 11 for secondarily cooling the slab 3 is divided into a vertical zone 5, a curved zone 7, and a horizontal zone 9 as shown in FIG. 1. The method for manufacturing a slab according to the present embodiment relates to a method for manufacturing a slab by cooling the slab 3 in the horizontal zone 9. Also, the secondary cooling device is for secondarily cooling the slab 3 having this unfrozen molten steel. The method for manufacturing a slab according to the present embodiment will be described using a vertical bending type continuous casting machine having a vertical zone 5, a curved zone 7, and a horizontal zone 9, but it is not limited to the vertical bending type continuous casting machine. As long as it is a continuous casting machine having at least the horizontal zone 9, the method for manufacturing a slab according to the present embodiment can be implemented. That is, even a curved type continuous casting machine having only the curved zone 7 and the horizontal zone 8 and a horizontal type continuous casting machine having only the horizontal zone 9 can implement the method for manufacturing a slab according to the present embodiment.

[0012] <Pre-stage cooling process> The pre-stage cooling process is a process in which, in the pre-stage cooling section 13 in the horizontal section 9 of the secondary cooling zone 11, the boiling state of the coolant on the surface of the slab 3 in the pre-stage cooling section 13 is maintained in nucleate boiling by the cooling spray 21 for cooling. The cooling spray 21 is a device that sprays a coolant or a mixture of a coolant and a gas onto the surface of the slab 3. The coolant is, for example, water, and the gas is, for example, air. Note that the coolant may be a liquid mainly composed of pure substance (50% by mass or more) with a small amount of additives added thereto. As the additives, generally known additives such as surfactants for improving wettability may be used, and mechanical oil, metal chips, etc. that are irreversibly mixed may also be present in the coolant. In the following embodiments, an example using water as the coolant will be described.

[0013] Regarding surfactants, which are typical additives, any of anionic surfactants, cationic surfactants, and nonionic surfactants can be used without particular limitation. As the anionic surfactant, for example, sodium alkylbenzene sulfonate, alpha olefin sulfonate, and soap can be used. As the cationic surfactant, for example, alkyltrimethylammonium salt can be used. As the nonionic surfactant, for example, polyoxyethylene alkyl ether can be used.

[0014] FIG. 2 is an explanatory diagram for explaining the state of the coolant sprayed from the spray nozzle 23. FIG. 2(a) is a front view, and FIG. 2(b) is a side view. As shown in FIG. 2, the cooling spray 21 has a plurality of spray nozzles 23 provided in the width direction of the slab 3 between the respective rolls 19. The spray nozzle 23 is a so-called flat spray nozzle in which the coolant 25 sprayed from the nozzle becomes fan-shaped centered on the spray nozzle 23.

[0015] However, the type of the spray nozzle 23 is not limited to the flat spray nozzle. As a similar spray to the flat spray nozzle, an oval spray nozzle (elliptical spray, oblong spray) may be used, or a full cone spray nozzle (conical spray, round spray), which is a nozzle that sprays in a conical shape, may be used. A nozzle that sprays in a square pyramid shape like a square spray (corner spray, square spray, rectangular spray) obtained by shaping the full cone spray into a square may also be used.

[0016] FIG. 3 is an explanatory diagram for explaining the state of the coolant 25 sprayed from the spray nozzle 23 having an arrangement different from that in FIG. 2. FIG. 3(a) is a front view, and FIG. 3(b) is a side view. As shown in FIG. 3, when using a flat spray nozzle or an oval spray nozzle as the spray nozzle 23, it is preferable to arrange the nozzle so that the major axis of the rounded rectangular or elliptical cooling surface (the surface of the coolant 25 that collides with the surface of the slab 3) is inclined at an angle θ with respect to the casting direction and spray the coolant 25.

[0017] The reason is as follows. As shown in FIG. 2, between each pair of rolls 19, a plurality of spray nozzles 23 are provided in the width direction of the slab 3. When the spray nozzle 23 is a flat spray, the speed of the coolant 25 sprayed from the spray nozzle 23 and flowing on the surface of the slab 3 is fast in the major axis direction of the coolant collision surface (hereinafter referred to as the spray width direction), and relatively slow in the minor axis direction (hereinafter referred to as the spray thickness direction). Therefore, after colliding with the surface of the slab 3, the coolant 25 spreads relatively gently in the spray thickness direction, that is, in the casting direction. On the other hand, regarding the spray width direction, the coolant 25 sprayed from adjacent sprays collides with each other at their respective ends at opposite speeds, and then changes direction and spreads in the casting direction. As a result, after colliding with the surface of the slab 3, the coolant 25 flows on the surface of the slab 3 at a relatively slow speed in the casting direction.

[0018] On the contrary, as shown in FIG. 3, when the major axis of the cooling surface is inclined from the direction perpendicular to the casting direction, the interference of the coolant 25 sprayed from adjacent sprays occurs in the thickness direction of the spray with a relatively slow speed and does not occur in the width direction of the spray with a high speed. Therefore, the coolant 25 flows on the surface of the slab 3 at a high speed. According to the research of the present inventors, it has been found that when the coolant 25 moves on the surface of the slab 3, the higher the speed of the coolant 25, the higher the cooling capacity. From the above, by arranging the spray nozzle 23 so that the major axis of the cooling surface is inclined from the direction perpendicular to the casting direction, the cooling capacity is improved.

[0019] In the method for manufacturing a slab according to the present embodiment, the flow rate density per unit time of the coolant 25 in the pre-cooling step by the cooling spray 21 is 300 L / (m 2 ·min) or more and 4000 L / (m 2 ·min) or less, and the boiling state of the coolant 25 on at least a part or all of the surface of the slab 3 is made into nucleate boiling for cooling. The reason for this will be described below. Note that the flow rate density is a value calculated by dividing the total liquid volume (L / min) of the coolant used in the pre-cooling section 13 by the area (m 2 ) of the pre-cooling section 13. In the present embodiment, L means liter.

[0020] If cooling is performed with a high heat transfer coefficient before entering the horizontal zone 9 (hereinafter, performing cooling with a high heat transfer coefficient will be referred to as "intensive cooling"), the risk of cracking occurring particularly at the corner portions of the slab 3 becomes high. For this reason, it is preferable to perform intensive cooling in the horizontal zone 9. However, from the viewpoint of suppressing manufacturing costs, it is preferable to perform intensive cooling while suppressing the amount of the coolant 25 used. Therefore, a method of using a large flow rate of the coolant 25 only in the pre-cooling step and using a small flow rate of the coolant 25 in the post-cooling step was studied.

[0021] Fig. 4 is a graph showing the relationship between the flow rate of the cooling liquid, the surface temperature of the slab, and the cooling capacity. The vertical axis shows the cooling capacity, and the horizontal axis shows the surface temperature of the slab 3, with three flow rates of the cooling liquid 25 shown: large, medium, and small. In the graph of Fig. 4, the temperature range below the maximum point of the cooling capacity is the nucleate boiling region, and the temperature range above the minimum point is the film boiling region. Nucleate boiling is a boiling state in which bubbles are generated with the foaming points as nuclei, and the cooling liquid 25 can remove a very large amount of heat from the object to be cooled.

[0022] 4, when the temperature of the slab 3 is low, that is, in the nucleate boiling region, it can be seen that the flow rate of the coolant 25 has little effect on the cooling capacity. Therefore, if the surface temperature of the slab 3 is lowered by cooling with a large flow rate in the first-stage cooling process, nucleate boiling can be maintained with a small flow rate in the subsequent second-stage cooling section 15, and therefore a high cooling capacity can be achieved with a small flow rate.

[0023] The concept of the cooling method for the slab 3 in the slab manufacturing method according to this embodiment will be specifically described with reference to the graph in Fig. 4. The temperature history of the surface of the slab 3 as casting progresses from the upstream side to the downstream side of the continuous casting machine 1 is roughly from right (high temperature side) to left (low temperature side) on the graph in Fig. 4. The slab 3 in the curved zone 7 is still hot, but in order to prevent cracking of the slab 3, excessive cooling is not performed and the flow rate of the coolant 25 is reduced (to the right of point O in Fig. 4).

[0024] On the other hand, when the slab 3 passes through the curved zone 7 and enters the horizontal zone 9 (point A in Fig. 4), the risk of cracking of the slab 3 decreases, so strong cooling becomes possible, and the flow rate of the cooling liquid 25 can be significantly increased (point A' in Fig. 4). That is, in the method for manufacturing a slab according to the present embodiment, cooling is performed using a large flow rate of the cooling liquid 25 in the pre-stage cooling step. The slab 3 is strongly cooled by the large flow rate of the cooling liquid 25, and its surface temperature is greatly reduced. In the earliest case, on the downstream side in the casting direction of the surface position of the slab 3 cooled by the cooling liquid 25 sprayed from the cooling spray 21 installed between the first rolls 19 after entering the horizontal zone 9, the state of the cooling liquid 25 transitions to the nucleate boiling state (point B in Fig. 4). If the cooling is continued as it is, the surface temperature of the slab 3 further decreases and reaches point C in Fig. 4. When the surface temperature of the slab 3 decreases to point C, nucleate boiling is maintained even under the condition of a low flow rate of the cooling liquid 25, so strong cooling can be continued by nucleate boiling at a small flow rate after shifting to the post-stage cooling step (point C' in Fig. 4). Thus, in the method for manufacturing a slab according to the present embodiment, the cooling capacity is changed as indicated by the white arrow in Fig. 4.

[0025] In the cooling at a large flow rate in the pre-stage cooling step according to the present embodiment, the flow rate density is set to 300 L / (m 2 ·min) or more and 4000 L / (m 2 ·min) or less. Although the minimum value of the cooling capacity in Fig. 4 changes according to the flow rate, from the research results by the present inventors, it is known that by setting the flow rate density to 300 L / (m 2 ·min), the temperature showing the minimum value of the cooling capacity becomes about 1000°C. Generally, the surface temperature of the slab 3 in the horizontal zone 9 is 1000°C or less, which is a temperature range lower than the temperature showing the minimum value of the cooling capacity. Therefore, if the flow rate density is 300 L / (m 2 ·min) or more, the slab 3 in the horizontal zone 9 can be cooled with a cooling capacity higher than the minimum value of the cooling capacity. Also, as shown in Fig. 4, between the minimum value and the maximum value of the cooling capacity, the higher the flow rate of the cooling liquid 25, the higher the cooling capacity. Therefore, it is advantageous to increase the flow rate density in the pre-stage cooling section 13 of the horizontal zone 9.

[0026] On the one hand, according to the inventor's findings, even if the flow density is increased beyond 4000 L / (m 2 ·min), it has been found that the cooling capacity hardly changes, so the energy of the coolant injection and the coolant 25 used are wasted. For the above reasons, in the cooling at a large flow rate in the front-stage cooling section 13, the flow density is set to be 300 L / (m 2 ·min) or more and 4000 L / (m 2 ·min) or less. Note that a more suitable flow density is 300 L / (m 2 ·min) or more and 2000 L / (m 2 ·min) or less.

[0027] <Post-stage cooling process> The post-stage cooling process is to maintain the boiling state of the coolant 25 on the surface of the slab 3 in the post-stage cooling section 15 in the horizontal belt 9 in nucleate boiling. The nucleate boiling state is maintained in the front-stage cooling process, and the surface temperature of the slab 3 has decreased sufficiently. Therefore, as described above, in the post-stage cooling process, nucleate boiling can be maintained even if the coolant flow density is decreased compared to the front-stage cooling process.

[0028] The flow density at which nucleate boiling can be maintained in the post-stage cooling process is, according to the inventor's findings, 100 L / (m 2 ·min) or more and 1000 L / (m 2 ·min) or less. Note that a more suitable flow density is 100 L / (m 2 ·min) or more and 300 L / (m 2 ·min) or less. On the other hand, even if the flow density is increased beyond 1000 L / (m 2 ·min), there is no change in the cooling capacity, and the energy of the coolant injection and the coolant 25 used are wasted. Therefore, it is not preferable to increase the flow density of the coolant 25 in the post-stage cooling process beyond 1000 L / (m 2 ·min).

[0029] Also, as a result of various experiments conducted by the inventors to cool the slab 3 with water, it has been found that when the surface temperature of the slab 3 satisfies the following formula (1), the cooling capacity reaches a temperature at which it shows a maximum value.

[0030] Ts = 10 ^ [0.08×ln(W) + 2] ··· (1) Here, Ts is the surface temperature (°C) of the slab 3, and W is the flow rate density (L / (m 2 ·min), and ln is the natural logarithm.

[0031] Therefore, in the pre-stage cooling process, it is sufficient to cool at a large flow rate until the temperature is lower than the above-mentioned Ts calculated from the flow rate density in the post-stage cooling process. That is, the range of the pre-stage cooling section 13 in the horizontal zone 9 is until the surface temperature of the slab 3 drops to a temperature lower than Ts calculated from the flow rate density in the post-stage cooling process and the above (1), and thereafter is the range of the post-stage cooling section 15.

[0032] <Prevention of coolant leakage> In the pre-stage cooling process, the coolant sprayed from the cooling spray 21 collides with the surface of the slab 3, flows on the surface of the slab 3, and collides with the roll 19. When the roll 19 is a split-type slab support roll split into two or more parts in the slab width direction, a gap is generated between the slab 3 at the roll chuck part between the split rolls, so that the coolant 25 leaks. In normal secondary cooling, the cooling capacity due to the leaked coolant 25 is relatively small, but the coolant 25 in the nucleate boiling state has a very high cooling capacity. Therefore, when the coolant leaks, the temperature drop at the roll chuck part becomes large, and the uniformity of the temperature distribution of the slab 3 deteriorates.

[0033] FIG. 5 is an enlarged view of part A in FIG. 1. In the method for manufacturing a slab according to the present embodiment, a gas injection nozzle 26 is disposed immediately before a front-stage cooling section 13, which is an upstream end portion in the casting direction in the horizontal belt 9, and immediately after a roll chuck section at the downstream end portion in the casting direction in the horizontal belt 9. The gas injection nozzle 26 provided at the upstream end portion with respect to the casting direction injects gas toward the surface of the slab 3 at the position of the roll chuck section on the downstream side with respect to the casting direction. The gas injection nozzle 26 provided at the downstream end portion with respect to the casting direction injects gas toward the surface of the slab 3 at the position of the roll chuck section on the upstream side with respect to the casting direction. Thereby, leakage of the cooling liquid 25 of the horizontal belt 9 from the roll chuck section can be suppressed. As a result, a temperature drop in the roll chuck section is suppressed, and the uniformity of the temperature distribution of the slab 3 is improved.

[0034] When the cooling liquid 25 of the horizontal belt 9 leaks from the roll chuck section, a part of the slab 3 is cooled in a nucleate boiling state by the cooling liquid 25. As the slab 3 moves away from the cooling zone, the temperature of the slab 3 reheats, and thereafter, the boiling mode becomes a transition boiling. When the boiling mode becomes a transition boiling, the amount of generated water vapor, that is, the amount of generated steam, significantly increases compared to nucleate boiling. Therefore, by checking the amount of generated steam, leakage of the cooling liquid 25 of the horizontal belt 9 from the roll chuck section 28 can be detected.

[0035] The appropriate value of the gas injection flow rate changes depending on the flow rate of the coolant 25 that leaks. Therefore, it is preferable to control the gas injection flow rate from the gas injection nozzle 26 based on the flow rate of the coolant 25 leaking from the roll chuck section. Most of the coolant 25 leaked from the roll chuck section evaporates on the surface of the slab 3 to become water vapor, and a part of it becomes steam. Since the generation amount of this steam is correlated with the flow rate of the coolant 25 that leaks, the gas injection flow rate from the gas injection nozzle 26 may be controlled based on the generation amount of the steam. Specifically, the generation amount of the steam may be classified as large, medium, or small, and the gas ejection flow rate corresponding to the classification of the generation amount may be determined in advance, and gas with a flow rate corresponding to the classification of the generated steam may be ejected from the gas injection nozzle 26. Also, the flow rate of the coolant 25 leaking from the roll chuck section can be obtained by collecting the water vapor containing the steam and measuring the amount of the liquid obtained by condensing this. Therefore, the gas injection flow rate from the gas injection nozzle 26 may be controlled based on the flow rate of the coolant 25 leaking from the roll chuck section.

[0036] In addition, a threshold value of the flow rate of the coolant 25 leaking from the roll chuck section may be determined in advance, and the gas injection flow rate from the gas injection nozzle 26 may be controlled based on the threshold value. As described above, the flow rate of the coolant 25 leaking from the roll chuck section can be obtained by collecting the water vapor containing the steam and measuring the amount of the liquid obtained by condensing this. Therefore, when the amount of the liquid is larger than the threshold value, the gas injection flow rate is increased until the amount of the liquid becomes equal to or less than the threshold value. Thereby, it can be controlled so that the flow rate of the coolant 25 leaking from the roll chuck section does not become larger than a predetermined threshold value. The threshold value of the flow rate of the coolant 25 leaking from the roll chuck section can be set by obtaining the range of the leakage flow rate of the coolant 25 in which no quality defect of the slab 3 occurs.

[0037] Furthermore, the injection flow rate of the gas from the gas injection nozzle 26 may be controlled based on the leakage rate with reference to the flow rate of the coolant 25 leaking from the roll chuck portion. The leakage rate is obtained by multiplying by 100 the value obtained by dividing the flow rate of the coolant 25 leaking from the roll chuck portion by the flow rate of the coolant 25 leaking from the roll chuck portion when the gas injection nozzle 26 is not provided.

[0038] When the coolant 25 with a water volume density of 1000 L / (m 2 ·min) is injected from the cooling spray 21 without providing the gas injection nozzle 26, the flow rate of the coolant 25 leaking from the roll chuck portion becomes 13.7 L / min. The inventors conducted an experiment by changing the water volume density of the coolant 25 injected from the cooling spray 21, and investigated the relationship between the water volume density of the coolant 25 injected from the cooling spray 21 and the flow rate of the coolant 25 leaking from the roll chuck portion.

[0039] FIG. 6 is a graph showing the relationship between the water volume density of the coolant 25 injected from the cooling spray 21 and the flow rate of the leaking coolant 25. In FIG. 6, the horizontal axis represents the water volume density L / (m 2 ·min) of the coolant 25, and the vertical axis represents the flow rate (L / min) of the leaking coolant 25. As shown in FIG. 6, it was confirmed that as the water volume density of the coolant 25 injected from the cooling spray 21 increases, the flow rate of the coolant 25 leaking from the roll chuck portion also increases, and they are generally in a proportional relationship.

[0040] By using FIG. 6, the amount of leaked coolant leaking from the roll chuck portion when the gas injection nozzle 26 is not provided can be obtained. Therefore, the leakage rate can be obtained from the water volume density of the coolant 25 injected from the cooling spray 21 and the generation amounts of the steam and water vapor generated from the roll chuck portion. For example, when the water volume density of the coolant 25 injected from the cooling spray 21 is 1000 L / (m 2 ·min), the amount of leaked coolant becomes 13.7 L / min. Assuming that the generation amounts of the steam and water vapor of the coolant 25 leaked from the roll chuck portion at this time are 1.37 L / min, the leakage rate of the coolant is 10 mass%.

[0041] A threshold value of the leakage rate is determined in advance. When the leakage rate of the coolant 25 is greater than the threshold value, the injection flow rate of the gas from the gas injection nozzle 26 is increased until the leakage rate becomes equal to or less than the threshold value. Thereby, the flow rate of the coolant 25 leaking from the roll chuck portion can be controlled so that the leakage rate becomes equal to or less than the threshold value. The threshold value of the leakage rate may be determined as a leakage rate within a range where no quality defect occurs in the slab 3.

[0042] On the other hand, when the leakage amount of the coolant 25 from the roll chuck portion increases and exceeds a certain amount, the amount of the coolant 25 that does not evaporate increases. In this case, since the leaked coolant 25 can be visually confirmed, the flow rate of the leaked coolant 25 may be grasped visually, and the injection flow rate of the gas from the gas injection nozzle 26 may be controlled based on the flow rate. Further, when the leakage amount of the coolant 25 increases and the coolant 25 is present on the slab 3, the temperature becomes lower than other portions. Therefore, the roll chuck portion may be measured with a radiation thermometer, and the injection flow rate of the gas from the gas injection nozzle 26 may be controlled based on the temperature.

[0043] From the experimental results of the inventors, when the flow rate density of the coolant 25 is 100 L / (m 2 ·min), it is preferable that the injection flow rate of the gas per one roll chuck portion is 200 NL / min or more and 400 NL / min or less. The reason why there is such a preferable range is that the preferable range of the injection flow rate of the gas changes depending on the injection conditions of the coolant 25 such as the angle of the spray nozzle 23. Further, when the flow rate density of the coolant 25 is 4000 L / (m 2 ·min), it is preferable that the injection flow rate of the gas per one roll chuck portion is 2500 NL / min or more and 3000 NL / min or less. Therefore, when these are summarized, it is preferable that the injection flow rate of the gas per one roll chuck portion is 200 NL / min or more and 3000 NL / min or less. If the injection flow rate of the gas is set to the upper limit value, leakage of the coolant 25 from the roll chuck portion is prevented. Therefore, increasing the injection flow rate of the gas more than the upper limit value has no effect and the energy of the gas injection is wasted. For this reason, it is not preferable to increase the injection flow rate of the gas more than each upper limit value.

[0044] As described above, in the method for manufacturing a slab according to the present embodiment, in the horizontal zone 9 of the secondary cooling zone 11, in the former stage cooling step, the boiling state of the coolant 25 on the surface of the slab 3 is made into nucleate boiling at a large flow rate density, and in the latter stage cooling step, the boiling state of the coolant 25 on the surface of the slab 3 is made into nucleate boiling at a small flow rate density. Thereby, the slab 3 can be effectively cooled while suppressing the amount of the coolant 25 used. Further, gas injection nozzles 26 are installed at the roll chock portions at the upstream end and the downstream end with respect to the casting direction of the horizontal zone 9, and a predetermined amount of air is injected from the gas injection nozzles 26 onto the surface of the slab 3 in the roll chock portions, so that leakage of the coolant 25 from the roll chock portions of the horizontal zone 9 can be suppressed, and the uniformity of the temperature distribution of the slab 3 can be improved. As a result, it is possible to manufacture a slab 3 with stable quality. In the present embodiment, an example in which the gas injection nozzles 26 are installed at the roll chock portions immediately before the former stage cooling section 13 and immediately after the latter stage cooling section 15 is shown, but the present invention is not limited to this. The gas injection nozzles 26 may be installed at at least one of immediately before the former stage cooling section 13 and immediately after the latter stage cooling section 15. Thereby, leakage of the coolant 25 is suppressed as compared with the case where the gas injection nozzles 26 are not provided, and it is possible to manufacture a slab 3 with stable quality.

Example

[0045] To confirm the effects of the present invention, an example in which a low-carbon steel is cast using the continuous casting machine 1 shown in FIG. 1 will be described. Note that the numerical values and the like described in this example are shown for a better understanding of the present invention, and the present invention is not limited by this example at all.

[0046] The length of the continuous casting machine 1 used in this example was 45 m, and the horizontal zone 9 was composed of 15 segments each having a length of 2 m. As the casting conditions, the casting speed was 2 mpm, the thickness of the slab 3 was 250 mm, and the width of the slab 3 was 1500 mm. Water was used as the coolant 25, mixed with air, and sprayed from each cooling spray 21. The water temperature and the air temperature were 30°C.

[0047] The surface temperature of the slab 3 when it reached the horizontal zone 9 was 850°C. A radiation thermometer was used to measure the temperature. The solidification position was determined from the percussion test. Under the above conditions, the slab 3 was manufactured by variously changing the cooling conditions in the horizontal zone 9 and the conditions of the gas injection nozzles.

[0048] In the comparative example, the slab 3 was cooled without using the gas injection nozzle 26. In the inventive example, the gas injection nozzle 26 was installed at the roll chuck part 28 immediately before the front-stage cooling part 13 and immediately after the rear-stage cooling part 15 for cooling. Compressed air was injected from the gas injection nozzle 26.

[0049] Fig. 7 is a schematic diagram showing the arrangement of the gas injection nozzle 26 used in the example. Fig. 8 is a schematic diagram showing the arrangement of the gas injection nozzle 29 used in the example. In Inventive Example 1, as shown in Fig. 7, a gas injection nozzle 26 with a large gas injection port was arranged for one roll chuck part 28 of the split slab support roll 27. In Inventive Examples 2, 5 to 12, with the arrangement of the gas injection nozzle 26 shown in Fig. 7, the injection flow rate of air was controlled, and the injection flow rate of air was set to the minimum flow rate based on the amount of steam generated. In Inventive Example 3, as shown in Fig. 8, a gas injection nozzle 29 with three small gas injection ports was arranged for one roll chuck part 28 of the split slab support roll 27. In Inventive Example 4, with the arrangement of the gas injection nozzle 29 shown in Fig. 8, the flow rate of air was controlled, and the injection flow rate of air was set to the minimum flow rate based on the amount of steam generated.

[0050] The compressed air flow rate indicates the air injection flow rate per roll chuck part 28 at one location. Regarding the temperature distribution, radiation thermometers were installed before the front-stage cooling part 13 and after the rear-stage cooling part 15, and the difference between the maximum temperature and the minimum temperature of the slab 3 in the width direction was obtained. Regarding nucleate boiling, the conditions under which the nucleate boiling state was not achieved or could not be maintained were marked as "×", and those for which the nucleate boiling state was achieved and maintained were marked as "〇". Regarding the leakage rate, the amount of liquid obtained by collecting and condensing the water vapor and steam generated from the roll chuck part 28 was divided by the leakage water amount when the gas injection nozzle was not used as determined from Figure 6, and then multiplied by 100 for calculation. Also, regarding the steam, the amount of steam generated was indicated as "much", "medium", or "little". The amount of steam generated was determined according to the following criteria. Much steam: The visibility through the steam is affected, and the situation where it is difficult to see 2 m ahead. Medium steam: The visibility through the steam is affected, and it is possible to see 2 m ahead but difficult to see 5 m ahead. Little steam: The presence of steam can be confirmed, but it does not affect the visibility through the steam.

[0051] Regarding the temperature distribution, the difference between the maximum temperature and the minimum temperature in the width direction of the slab 3 was shown. Regarding the quality of the slab 3, those with a deteriorated temperature distribution and resulting quality defects were marked as "×", and those without quality defects were marked as "〇". The results of the cooling conditions and slab quality of the comparative examples and the inventive examples are shown in Table 1 below.

[0052]

Table 1

[0053] In Comparative Examples 1 to 3, the coolant 25 leaked from the front-stage cooling section 13 and the rear-stage cooling section 15, the temperature distribution of the slab 3 became non-uniform, and quality defects occurred in the slab 3. In Inventive Examples 1 and 3, there was no leakage of the coolant 25 from the front-stage cooling section 13 and the rear-stage cooling section 15, the temperature difference in the temperature distribution of the slab 3 became smaller, and no quality defects occurred in the slab 3. In Inventive Example 2, there was no leakage of the coolant 25 from the front-stage cooling section 13 and the rear-stage cooling section 15, the temperature difference in the temperature distribution of the slab 3 became smaller, and no quality defects occurred in the slab. Furthermore, in Inventive Example 2, the injection flow rate of the compressed air could be made smaller than in Inventive Examples 1 and 3. In Inventive Example 4, since the injection flow rate of the compressed air was made too small in the front-stage cooling section 13, a small amount of leakage of the coolant 25 occurred from the front-stage cooling section 13. For this reason, medium-level steam was generated, the leakage rate became 7% by mass, and the temperature difference also became slightly larger. On the other hand, there was no leakage of the coolant 25 from the rear-stage cooling section 15, and the temperature difference also became smaller. In Inventive Example 4, although the temperature difference in the front-stage cooling section 13 became slightly larger, no quality defects occurred in the slab 3.

[0054] In Inventive Examples 5, 6, 8, and 9, there was no leakage of the coolant 25 from the front-stage cooling section 13 and the rear-stage cooling section 15, the temperature difference in the temperature distribution of the slab 3 became smaller, and no quality defects occurred in the slab. Furthermore, in Inventive Examples 5, 6, 8, and 9, the injection flow rate of the compressed air could also be made smaller. In Inventive Example 7, since the injection flow rate of the compressed air was made too small in the rear-stage cooling section 15, a small amount of leakage of the coolant 25 occurred from the rear-stage cooling section 15. For this reason, medium-level steam was generated, and the temperature difference also became slightly larger. On the other hand, there was no leakage of the coolant 25 from the front-stage cooling section 13, the leakage rate became 13% by mass, and the temperature difference also became smaller. In Inventive Example 7, although the temperature difference in the rear-stage cooling section 15 became slightly larger, no quality defects occurred in the slab 3.

[0055] In Invention Example 10, since the coolant water density in the front-stage cooling section 13 was increased, a small amount of leakage of the coolant 25 occurred from the front-stage cooling section 13. For this reason, medium-level steam was generated, the leakage rate became 8% by mass, and the temperature difference also became slightly larger. On the other hand, there was no leakage of the coolant 25 from the rear-stage cooling section 15, and the temperature difference also became smaller. In Invention Example 10, although the temperature difference in the front-stage cooling section 13 became slightly larger, no quality defect of the slab 3 occurred. In Invention Examples 11 and 12, there was no leakage of the coolant 25 from the front-stage cooling section 13 and the rear-stage cooling section 15, the temperature difference in the temperature distribution of the slab 3 became smaller, and no quality defect of the slab occurred.

Explanation of Signs

[0056] 1 Continuous casting machine 3 Slab 5 Vertical zone 7 Curved zone 9 Horizontal zone 11 Secondary cooling zone 13 Front-stage cooling section 15 Rear-stage cooling section 17 Mold 19 Roll 21 Cooling spray 23 Spray nozzle 25 Coolant 26 Gas injection nozzle 27 Split-type slab support roll 28 Roll chuck section 29 Gas injection nozzle

Claims

1. A method for manufacturing a slab, comprising subjecting a slab cast by a continuous casting machine to secondary cooling in a secondary cooling zone having at least a horizontal zone to manufacture the slab, in the horizontal zone, while supporting the slab with split-type slab support rolls divided into two or more in the slab width direction, cooling by causing nucleate boiling of the boiling state of the coolant on the surface of the slab, injecting gas from a gas injection nozzle to suppress leakage of the coolant in the horizontal zone from the roll chuck portion of the split-type slab support roll, A method for manufacturing a slab, wherein the injection flow rate of the gas from the gas injection nozzle is controlled based on the amount of generated steam from the roll chuck portion.

2. The gas injection nozzle is installed at at least one of an upstream end portion and a downstream end portion with respect to the casting direction of the horizontal zone, The gas injection nozzle at the upstream end portion injects gas toward the surface of the slab on the downstream side with respect to the casting direction at the position of the roll chuck portion of the split-type slab support roll, The gas injection nozzle at the downstream end portion injects gas toward the surface of the slab on the upstream side with respect to the casting direction at the position of the roll chuck portion of the split-type slab support roll. The method for manufacturing a slab according to claim 1.

3. The method for manufacturing a slab according to claim 1, wherein the injection flow rate of the gas is increased when the amount of generated steam is greater than a predetermined threshold value of the amount of generated steam.

4. The secondary cooling of the slab in the horizontal zone includes a pre-stage cooling step on the upstream side with respect to the casting direction and a post-stage cooling step on the downstream side with respect to the casting direction, The flow rate density of the coolant in the pre-stage cooling step is 300 L / (m ２ ·min) or more and 4000 L / (m ２ ·min) or less, and the flow rate density of the coolant in the post-stage cooling step is 100 L / (m ２ ·min) or more and 1000 L / (m ２・less than (min), and the injection flow rate of the gas is 200 NL / min or more and 3000 NL / min or less per one roll chock part 1. The method for manufacturing a slab according to any one of claims 1 to 3.

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