Method for cooling continuous casting mold and continuous casting mold

Abstract

To provide a method for cooling a continuous casting mold, capable of preventing damage associated with the occurrence of burnout phenomenon in the continuous casting mold and maintaining stable cooling by nucleate boiling heat transfer, even when cooling by the nucleate boiling heat transfer is used in the cooling of the continuous casting mold, and to provide a continuous casting mold.SOLUTION: A method for cooling a continuous casting mold in which cooling water is circulated to cool the continuous casting mold, includes: a forced convection heat transfer cooling step of performing cooling based on forced convection heat transfer using cooling water, at the downstream side of the casting direction of a molten steel in the continuous casting mold; and a nucleate boiling heat transfer cooling step of performing cooling based on nucleate boiling heat transfer using the cooling water, at the upstream side of the casting direction of the molten steel in the continuous casting mold.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for cooling a continuous casting mold that contains molten steel for continuous casting, and to a continuous casting mold. [Background technology]

[0002] In recent years, there has been a demand for faster continuous casting in order to reduce costs and improve productivity. For this reason, the mold for solidifying molten steel in continuous casting is required to have a large heat extraction performance in order to rapidly form a solidified shell. In order to form a high-quality solidified shell and prevent damage to the mold due to thermal load, it is necessary to increase the amount of heat extraction by forced convection heat transfer using cooling water, in response to the heat input amount that increases proportionally with the increase in casting speed. In other words, it is necessary to improve the heat transfer coefficient by forced convection heat transfer.

[0003] Here, the inner plate provided on the wall of the mold is designed to improve the heat transfer coefficient by forced convection heat transfer by setting the flow rate of the cooling water flowing inside the wall at 7 to 10 m / sec. Therefore, the amount of cooling water to be supplied inevitably increases, leading to an increase in the size of the water supply equipment and water treatment equipment, which creates a problem in terms of cost.

[0004] For this reason, a technique has been proposed that utilizes nucleate boiling heat transfer by controlling the speed and pressure of cooling water used for forced convection heat transfer. Patent Document 1 discloses a method of cooling by nucleate boiling heat transfer by specifying the cooling water pressure etc. that generates nucleate boiling heat transfer and the cooling water pressure etc. that generates burnout from an empirical formula, and controlling the cooling water pressure etc. within an appropriate range. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 3-81049 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in the method disclosed in Patent Document 1, even if the pressure of the cooling water is controlled within an appropriate range, a slight change in the temperature or speed of the cooling water may affect the heat removal performance of the mold, which may lead to problems such as damage to the inner plate. Furthermore, when the casting speed in continuous casting is less than 1.5 m / min, the heat removal performance of the mold can be improved, but when the casting speed is 1.5 m / min or more, stable heat removal performance of the mold cannot be obtained and the thickness of the solidified shell is formed non-uniformly, making it difficult to actively utilize nucleate boiling heat transfer from the viewpoint of stable continuous casting.

[0007] The effect of heat removal from the mold based on nucleate boiling heat transfer becomes unstable due to the influence of operational changes in continuous casting. Therefore, even if the pressure of the cooling water is controlled within an appropriate range to prevent the occurrence of burnout (the phenomenon that is the starting point of the transition from nucleate boiling to film boiling), some of the water passages partitioned at multiple positions on the inner plate of the mold may reach the conditions for the burnout phenomenon, which may lead to operational problems.

[0008] The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a cooling method for a continuous casting mold, which can prevent damage associated with the occurrence of the burnout phenomenon of the continuous casting mold and can stably maintain cooling by nucleate boiling heat transfer, even when cooling of the continuous casting mold is performed by using nucleate boiling heat transfer, and a continuous casting mold. [Means for solving the problem]

[0009] [1] A method for cooling a continuous casting mold in which cooling water is circulated through the mold, the method comprising: a forced convection heat transfer cooling process in which cooling based on forced convection heat transfer is performed using the cooling water on the downstream side of the casting direction of molten steel in the continuous casting mold; and a nucleate boiling heat transfer cooling process in which cooling based on nucleate boiling heat transfer is performed using the cooling water on the upstream side of the casting direction of molten steel in the continuous casting mold. [2] The method for cooling a continuous casting mold according to [1], wherein bubbles having a diameter of 0.1 mm or less are mixed into the cooling water. [3] The method for cooling a continuous casting mold according to [1] or [2], wherein pure water is used as the cooling water. [4] A continuous casting mold having a wall portion provided with an inner plate and an outer plate, the continuous casting mold having, on a downstream side of a casting direction of molten steel in the wall portion, a first water passage configuration which is a plurality of water passages defined by the inner plate and the outer plate and extending in the casting direction, and on an upstream side of the casting direction in the wall portion, a second water passage configuration which is a flat water passage defined by the inner plate and the outer plate and extends in the casting direction, where the plurality of water passages of the first water passage configuration join together. [5] The continuous casting mold described in [4], wherein the flat water channel in the second water channel configuration has a concave portion with a depth of 1 mm or less or a convex portion with a height of 1 mm or less on the surface of the inner plate. [6] The continuous casting mold described in [4], wherein the flat water channel in the second water channel configuration has a material having a different thermal conductivity than the inner plate embedded in the surface of the inner plate. Effect of the Invention

[0010] According to the present invention, even when cooling by nucleate boiling heat transfer is used to cool a continuous casting mold, damage associated with the occurrence of the burnout phenomenon of the continuous casting mold can be prevented, and cooling by nucleate boiling heat transfer can be stably maintained. [Brief description of the drawings]

[0011] [Figure 1] FIG. 2 is a diagram showing a cooling water supply path for circulating cooling water to a continuous casting mold. [Diagram 2] FIG. 1 is a perspective schematic diagram showing a general configuration of a continuous casting mold. [Diagram 3] FIG. 1 is a schematic cross-sectional view of a conventional wall portion. [Figure 4] FIG. 1 is a schematic planar cross-sectional view of a conventional wall portion. [Diagram 5]FIG. 2 is a schematic cross-sectional view of a wall portion of the embodiment; [Figure 6] FIG. 2 is a schematic planar cross-sectional view of a wall portion of the embodiment; [Figure 7] FIG. 2 is a diagram showing a schematic diagram of the state of bubbles when nucleate boiling heat transfer is used in cooling water. [Figure 8] 4A and 4B are diagrams showing schematic views of the pattern of unevenness on the inner surface of the inner plate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0013] Fig. 1 is a diagram showing a cooling water circulation path 50 for circulating cooling water W through a continuous casting mold 10. Fig. 1 shows a cooling water circulation path 50 for supplying cooling water W to a continuous casting mold (hereinafter also simply referred to as a "mold") used in this embodiment.

[0014] As shown in FIG. 1, the cooling water circulation path 50 has a configuration for circulating the cooling water W through the continuous casting mold 10, the cooling water storage section 20, the feed water pump 30, and the regulating valve 40. The cooling water storage section 20 supplies the cooling water W to the feed water pump 30. The feed water pump 30 pressurizes the cooling water supplied from the cooling water storage section 20 and supplies the pressurized cooling water W to the regulating valve 40. The regulating valve 40 adjusts the flow rate and pressure of the cooling water W pressurized by the feed water pump 30 and supplies the cooling water W to the continuous casting mold 10. The regulating valve 40 adjusts the flow rate and pressure of the cooling water W to properly and stably cool the continuous casting mold 10. The cooling water storage section 20 recovers the cooling water W used for cooling in the continuous casting mold 10.

[0015] Next, the schematic configuration of the continuous casting mold 10 will be described with reference to FIG. 2. FIG. 2 shows a schematic perspective view of the continuous casting mold 10. As shown in FIG. 2, the continuous casting mold 10 has a wall portion 11. The wall portion 11 has a first wall portion 11a and a second wall portion 11b. The first wall portion 11a has a long configuration along the width direction B of the continuous casting mold 10. The second wall portion 11b has a configuration extending along the thickness direction C of the continuous casting mold 10. As shown in FIG. 2, the configuration of the continuous casting mold 10 is defined by a pair of opposing first wall portions 11a and a pair of opposing second wall portions 11b. The continuous casting mold 10 contains molten steel S poured through a submerged nozzle 19. The molten steel S contained in the continuous casting mold 10 is cooled, and the solidified molten steel S is continuously drawn in the casting direction A to produce a cast piece.

[0016] In the following description, when the term "wall portion 11" is used, it means that it includes both the "first wall portion 11a" and the "second wall portion 11b," and when the term "first wall portion 11a" or "second wall portion 11b" is used, it means that an explanation of each of them is provided.

[0017] Here, the configuration of the conventional wall portion in the continuous casting mold 10 will be described with reference to Figs. 3 and 4. Fig. 3 shows a schematic cross-sectional view of the conventional wall portion 110. Fig. 4 shows a schematic planar cross-sectional view of the conventional wall portion 110. Specifically, Fig. 4(a) shows the UU cross section in Fig. 3. Fig. 4(b) shows the LL cross section in Fig. 3. Hereinafter, the conventional wall portion 110 will be described as a wall portion having a long configuration along the width direction B of the mold.

[0018] 3 and 4, the wall portion 110 has bolts 120, a water passage portion 130, an inner plate 150, and an outer plate 160. The bolts 120 fix the inner plate 150 and the outer plate 160 to each other. The bolts 120 are arranged at predetermined intervals in the width direction B of the wall portion 110. The water passage portion 130 is defined by the inner plate 150 and the outer plate 160 so as to extend in the casting direction A, as shown in FIG. 4. The water passage portions 130 are provided at multiple locations between the bolts 120 on the upstream side and downstream side of the wall portion 110 in the casting direction A of the molten steel.

[0019] Cooling of the continuous casting mold 10, i.e., cooling of the wall portion 110, is performed by circulating cooling water W through water passages 130 provided at multiple locations between the multiple bolts 120 in the width direction B. The speed of the cooling water W flowing through the water passages 130 is controlled so as to be approximately constant in the casting direction A.

[0020] Specifically, the speed of the cooling water W is controlled to a target of 7 to 10 m / sec when cooling by forced convection heat transfer, to a target of 2 to 4 m / sec when cooling by nucleate boiling heat transfer, and to a target of 4 to 6 m / sec when cooling by subcooled boiling heat transfer. The total amount of cooling water W flowing through all water passages 130 provided in the continuous casting mold 10 and the pressure of the inlet cooling water W supplied to the continuous casting mold 10 are preset, and in order to maintain the set total amount of cooling water and the pressure of the inlet cooling water W, the total amount of cooling water on the inlet and outlet sides of the continuous casting mold 10, the temperature of the cooling water W, the temperature of the thermocouple provided on the inner plate 150, etc., are monitored while cooling the wall portion 110. Subcooled boiling heat transfer is a method in which non-boiled cooling water is brought into the cooling water passage when nucleate boiling heat transfer develops on the cooling surface, thereby maintaining a partial boiling state. This makes it possible to prevent nucleate boiling from developing abnormally and reaching a burnout phenomenon, etc., and is therefore industrially effective.

[0021] The water passage parts 130 are provided at 5 to 10 locations between the bolts 120 in the width direction B or thickness direction C (see FIG. 2) of the wall part 110. Therefore, the cooling water W supplied from the water supply pump 30 is distributed equally through the piping so that the cooling water W flows at an equal speed in all the water passage parts 130. In order to prevent the occurrence of a burnout phenomenon in the water passage parts 130 and to maintain cooling by nucleate boiling heat transfer, it is necessary to control the speed and pressure of the cooling water W within a predetermined range.

[0022] However, the timing at which the cooling water W in the water passing section 130 transitions to a state in which nucleate boiling heat transfer is performed (nucleate boiling state) is affected by the temperature of the cooling water W, the surface roughness and steps of the inner surface of the water passing section 130, the flow of the molten steel S discharged from the submerged nozzle 19, fluctuations in the molten steel surface, etc. For this reason, the position at which the cooling water W transitions from a state in which forced convection heat transfer is performed to the nucleate boiling state may differ among multiple water passing sections 130.

[0023] As a countermeasure to this problem, it is required to use a large number of detectors to monitor the amount and pressure of the cooling water W at the inlet and outlet of the water passage section 130 and the temperature of the inner plate 150. However, since it is difficult to prepare a large number of detectors and acquire (monitor) a large amount of data in industrial operation and production, a method is adopted in which data is acquired (monitored) from detectors provided on the upstream and downstream piping connected to the water passage section 130 regarding the temperature, amount, and pressure at the inlet and outlet of the cooling water W in the water passage section 130.

[0024] In addition, when the cooling water W in the water passage section 130 transitions from a nucleate boiling state to a burnout state and then to a film boiling state, a sudden temperature rise occurs on the inner surface of the inner plate 150. At this time, it is effective to detect the timing of the sudden temperature rise as a state in which the burnout phenomenon occurs by a thermocouple embedded in the inner surface of the inner plate 150, and to avoid operational troubles such as damage to the wall section 110 and breakouts by reducing the casting speed based on the detection. Note that, due to the configuration of the wall section 110, the thermocouple provided on the inner plate 150 is arranged in the row in which the bolts 120 are arranged, just like the bolts 120 arranged in a row along the casting direction A. Therefore, when the position in the water passage section 130 where the sudden temperature rise occurs is far from the position where the thermocouple is arranged, the detection of the temperature rise may be delayed.

[0025] In addition, the temperature detection using a thermocouple installed on the inner plate 150 is merely a mechanism for detecting abnormalities such as burnout, and although it is effective in avoiding operational problems, it is not suitable for preventing the occurrence of the problematic phenomenon (burnout) itself.

[0026] Therefore, when a problem occurred in which the wall portion 110 was damaged, the inventors conducted a detailed investigation into various operating conditions, the temperature of the thermocouple in the inner plate 150, the state of damage to the wall portion 110, etc., and came to the following conclusion.

[0027] Specifically, it was found that even if the speed of the cooling water W flowing through the water passage section 130 is set stably within a range in which cooling by nucleate boiling heat transfer can be performed, when the nucleate boiling state becomes unstable in the water passage section 130, a burnout phenomenon may occur. In addition, when the temperature of the cooling water W on the inlet side of the water passage section 130 increases, it becomes easy to transition to a nucleate boiling state and to cause the burnout phenomenon, and it becomes impossible to maintain the uniformity of the temperature of the cooling water W in the multiple water passage sections 130. Furthermore, when the total amount of heat removal in the continuous casting mold 10 is calculated based on the difference in temperature of the cooling water W on the inlet side and the outlet side of the water passage section 130, it was confirmed that the improvement in the effect based on the total amount of heat removal is not remarkable when comparing the amount of heat removal when cooling is performed only by forced convection heat transfer with the amount of heat removal when cooling is performed using nucleate boiling heat transfer.

[0028] The present inventors have conducted extensive research into the problems involved in industrially utilizing the above-mentioned nucleate boiling heat transfer in a continuous casting mold. As a result, they have come to the conclusion that the shape of the water passage portion 130 in the conventional wall portion 110 is not suitable for utilizing nucleate boiling heat transfer.

[0029] First, the shape and configuration of the water passage part 130 in the wall part 110, which has been conventionally applied assuming the use of forced convection heat transfer, is not suitable for the use of nucleate boiling heat transfer. The water passage part 130, which is a vertical hole type, is very effective in increasing the speed of the cooling water W, and has an effective shape for lengthening the cooling length forming the cooling surface, called the wetted edge length. On the other hand, when cooling by nucleate boiling heat transfer is performed, it is important to perform cooling by making the speed of the cooling water W uniform and slow, and it is effective from the viewpoint of stable cooling to make the temperature of the cooling water W on the cooling surface uniform and to make the pressure of the cooling water W equal to or higher than the boiling vapor pressure. Therefore, when cooling by nucleate boiling heat transfer is used, a configuration in which a wide planar water passage part parallel to the surface of the inner plate 150, i.e., the plating surface on the side where the molten steel S is located, is used instead of a configuration in which many vertical hole type water passage parts 130 are arranged, in order to maintain a stable boiling state.

[0030] Furthermore, when a vertical hole type water passage section 130 is used, when nucleate boiling heat transfer is utilized in the water passage section 130, bubbles fill the water passage section 130 as the nucleate boiling state develops, and surround the cooling water W flowing from bottom to top. Since the cross-sectional area of ​​the vertical hole type water passage section 130 is small, bubbles occupy the inside of the water passage section 130 as the nucleate boiling state develops. As a result, the flow of the cooling water W is impeded, and dry-out and burn-out phenomena occur on the cooling surface (surface of the inner plate 150) in the water passage section 130, resulting in a transition to a film boiling state.

[0031] In addition, when the vertical hole type water passage section 130 is filled with air bubbles, the pressure of the cooling water W at the inlet side of the water passage section 130 increases, so that the flow of the cooling water W in the water passage section 130 filled with air bubbles decreases, while the flow rate of the cooling water W in the other water passage sections 130 increases. Therefore, even if there is no change in the total flow rate of the cooling water W supplied to the wall section 110, in a configuration in which many vertical hole type water passage sections 130 are provided, the flow of the cooling water W may be locally blocked, and the burnout phenomenon is likely to occur. In particular, when the temperature of the cooling water W at the inlet side of the water passage section 130 increases, the boiling state is likely to develop further locally at the position where the film boiling state has been transitioned to, which is likely to cause problems such as damage to the wall section 110.

[0032] In contrast, when a water passage part with a large area of ​​a flat cooling surface is used, a large area contributing to cooling can be secured by providing a wide planar water passage part parallel to the surface of the inner plate, i.e., the plating surface on the side where the molten steel S is located. Therefore, nucleate boiling can be uniformly generated over a wide area of ​​the surface of the inner plate. In other words, cooling by nucleate boiling heat transfer can be stably generated at the same time over the entire range of the wall of the continuous casting mold.

[0033] Furthermore, even if a large number of bubbles are generated due to the development of nucleate boiling, the large internal space of the water passage allows the cooling water W to flow from bottom to top along the surface of the outer plate. This makes it possible to prevent a shortage of cooling water W that would lead to dryout in the water passage and a local increase in the pressure of the cooling water W on the inlet side. That is, to stabilize the nucleate boiling state in the water passage, it is necessary to configure the water passage with a large cross-sectional area.

[0034] In view of the above-mentioned results of investigation, the present inventors propose a continuous casting mold which improves the amount of heat removal in the continuous casting mold and ensures stability in industrial use.

[0035] The configuration of the wall portion 11 in the newly proposed continuous casting mold 10 will be described with reference to Figs. 5 and 6. Fig. 5 shows a schematic cross-sectional view of the wall portion 11. Fig. 6 shows a schematic planar cross-sectional view of the wall portion 11. Specifically, Fig. 6(a) shows the UU cross section in Fig. 5. Fig. 6(b) shows the LL cross section in Fig. 5. Hereinafter, the wall portion 11 in this embodiment will be described as a wall portion having a long configuration along the width direction B of the mold.

[0036] As shown in Figs. 5 and 6, the wall portion 11 has bolts 12, a first water passage arrangement 13, a second water passage arrangement 14, an inner plate 15, and an outer plate 16. The bolts 12 fix the inner plate 15 and the outer plate 16 to each other. The bolts 12 are arranged at predetermined intervals in the width direction B of the wall portion 11. The first water passage arrangement 13 and the second water passage arrangement 14 are defined by the inner plate 15 and the outer plate 16 so as to extend in the casting direction A as shown in Fig. 6. The first water passage arrangement 13 provides a plurality of water passages 13a between the plurality of bolts 12 on the downstream side of the casting direction A of the molten steel in the wall portion 11. The second water passage arrangement 14 provides a flat water passage 14a, which is defined by the inner plate 15 and the outer plate 16 on the upstream side of the casting direction A of the molten steel in the wall portion 11 and extends in the casting direction A, and where the plurality of water passages 13a of the first water passage arrangement 13 join. The water passage 13a in the first water passage structure 13 has a similar structure to the water passage section 130, which is a conventional structure described with reference to Figs.

[0037] The length of the first water passage configuration 13 (water passage 13a) in the casting direction A may be, for example, 550 mm. The length of the second water passage configuration 14 (flat water passage 14a) in the casting direction A may be, for example, 350 mm. That is, in the casting direction A, the length of the second water passage configuration 14 (flat water passage 14a) may be configured to be shorter than the length of the first water passage configuration 13 (water passage 13a).

[0038] The wall 11 of the continuous casting mold 10 is characterized in that cooling based on convection heat transfer is performed downstream of the casting direction A, and cooling based on nucleate boiling heat transfer is performed upstream of the casting direction A.

[0039] In the wall 11 of the continuous casting mold 10, a first water passage structure 13 provided on the downstream side in the casting direction A has a plurality of vertical hole type water passages 13a similar to the conventional one provided between the plurality of bolts 12. On the other hand, a second water passage structure 14 provided on the upstream side in the casting direction A has flat plate type flat water passages 14a with a large cross-sectional area provided between the plurality of bolts 12.

[0040] Here, the state of bubbles when nucleate boiling heat transfer is used in cooling water W will be described with reference to Fig. 7. Fig. 7 is a diagram that shows the state of bubbles when nucleate boiling heat transfer is used in cooling water W. Specifically, Fig. 7(a) is a diagram that shows the state of bubbles when nucleate boiling heat transfer is used in a conventional vertical hole type water passage. Fig. 7(b) is a diagram that shows the state of bubbles when nucleate boiling heat transfer is used in a flat plate type flat water passage 14a with a large cross-sectional area.

[0041] As shown in Fig. 7(a), when bubbles are generated due to nucleate boiling heat transfer in a conventional vertical hole type water passage, the bubbles occupy the water passage path and obstruct the upward flow of the cooling water W supplied from below. This slows down the speed at which the cooling water W is supplied from below, and the water pressure of the cooling water W increases. As a result, local temperature fluctuations occur in multiple water passage parts.

[0042] In contrast, when flat water passages 14a with a large cross-sectional area are used, as shown in Fig. 7(b), the flat water passages 14a are prevented from being occupied by bubbles even if the bubbles due to nucleate boiling heat transfer grow large. Therefore, the speed of the cooling water W supplied from below is kept constant, and the water pressure of the cooling water W is also kept constant. As a result, it is possible to maintain a uniform temperature in the multiple water passages 13a and the flat water passages 14a, which prevents uneven growth of bubbles due to nucleate boiling heat transfer and suppresses the occurrence of burnout.

[0043] Here, the conventional vertical hole type water passage part shown in Fig. 7(a) will be described in more detail. In the water passage part, multiple bubbles coalesce and the volume of the bubbles increases, so that the path of the water passage part becomes occupied. When this state occurs, the inner surface of the path of the water passage part becomes completely dry (dry-burning state), and the temperature of the inner surface rises sharply, causing a burnout phenomenon. Therefore, in order to suppress the local occurrence of the burnout phenomenon, it is necessary to suppress the coalescence and increase of bubbles during cooling by nucleate boiling heat transfer.

[0044] The configuration of this embodiment will be described in detail. In this embodiment, for cooling based on nucleate boiling heat transfer, fine bubbles are generated at locations of different roughness or unevenness on the surface of the inner plate 15. In view of this, in the flat water channel 14a of the flat plate type having a large cross-sectional area, in order to suppress the coalescence and increase of the generated bubbles, it is necessary to generate bubbles uniformly over the entire surface of the flat water channel 14a.

[0045] In this embodiment, it is preferable to further include "a structure in which regular uneven processing is provided over the entire surface of the flat-plate-type flat water channel" and "a method for mixing fine bubbles into the cooling water when cooling based on nucleate boiling heat transfer," which have not been noted in the prior art. These structures and methods are described in detail below.

[0046] In cooling based on nucleate boiling heat transfer, the nuclei of bubbles generated on the surface of the inner plate 15 may be affected by irregular structures such as steps, notches, and scale deposits inside the flat water channels 14a.

[0047] For this reason, it is preferable to perform a predetermined processing on the surface of the inner plate 15 inside the flat water channel 14a. Specifically, it is preferable to provide recesses with a depth of 1 mm or less or protrusions with a height of 1 mm or less in a regular arrangement. By performing the uneven processing, it becomes possible to generate bubble nuclei more uniformly on the inner surface of the flat water channel 14a.

[0048] Next, a specific example of uneven processing on the surface of the inner plate 15 inside the flat waterway 14a will be described with reference to Fig. 8. Fig. 8 is a diagram showing a typical form of uneven processing on the surface of the inner plate 15. As a predetermined processing form applied to the surface of the inner plate 15, Fig. 8(a) shows a configuration in which lattice-shaped cutouts (lattice-shaped grooves) are applied. Fig. 8(b) shows a configuration in which semi-spherical nickel members are arranged in a staggered pattern and plated. Fig. 8(c) shows a configuration in which machined holes are arranged in a staggered pattern.

[0049] The configuration shown in Fig. 8(b) may be formed by plating a material (nickel member) having a different thermal conductivity with respect to the inner plate 15 in a staggered embedded arrangement. The machined hole shown in Fig. 8(c) may be formed by drilling to have a diameter of 0.5 mm and a depth of 0.5 mm.

[0050] By carrying out the processing as shown in Fig. 8(a) to Fig. 8(c), it becomes possible to uniformly generate bubble nuclei on the inner surface of the flat water channel 14a based on nucleate boiling heat transfer. It is also known that when generating bubbles by nucleate boiling, stable generation of bubbles is easily promoted starting from the bubble nuclei. Therefore, by carrying out the processing on the surface of the inner plate 15, uniform generation of bubbles is promoted and the development of local nucleate boiling is suppressed. The processing shown in Fig. 8(a) to Fig. 8(c) may be carried out so as to be arranged regularly on the surface of the inner plate 15.

[0051] In addition, by setting the depth of the recesses and the height of the protrusions in the uneven processing to 1 mm or less, uniform generation of nuclei in nucleate boiling can be promoted. On the other hand, if recesses and protrusions having a depth or height exceeding 1 mm are formed by processing, problems arise in terms of the cost required for processing and the strength reduction of the inner plate due to processing.

[0052] Furthermore, bubbles with a diameter of 0.1 mm or less may be mixed into the cooling water W. By mixing bubbles (microbubbles) with a diameter of 0.1 mm or less into the cooling water W, these bubbles act as steam nuclei in a nucleate boiling state, eliminating the need for an interfacial energy gap required for steam nucleation, and steam is generated finely and uniformly from the surface of the inner plate 15. Specifically, in the cooling water W, 8 It is more preferable to mix bubbles (microbubbles) having a diameter of 0.1 mm or less so that the number density is 1 / L or more. That is, by adjusting the state of the bubbles (microbubbles) mixed in the cooling water W, the nucleate boiling phenomenon can be controlled with higher precision.

[0053] Furthermore, when bubbles (microbubbles) with a diameter of 0.1 mm or less are mixed into the cooling water W, the occurrence of burnout is suppressed even after nucleate boiling has occurred, increasing the probability that nucleate boiling can be maintained until a higher heat flux (high heat removal amount) is reached. The ability to achieve a high heat removal amount while suppressing the occurrence of burnout is effective in increasing the speed of continuous casting.

[0054] The method of mixing bubbles (microbubbles) with a diameter of 0.1 mm or less into the cooling water W is preferably a method in which the bubbles are mixed into the cooling water W in the form of a dissolved gas and made to appear in the liquid as bubbles at a temperature just below the boiling vapor pressure, as this is easy to handle industrially, but the method of mixing is not important.

[0055] Moreover, it is preferable to use pure water as the cooling water W. This is because the boiling phenomenon can be reproduced (controlled) with high accuracy by using pure water.

[0056] Furthermore, in the vertical hole type water passage 13a provided downstream of the casting direction A in the continuous casting mold 10, it is desirable to maintain the velocity of the cooling water W at a predetermined velocity that does not affect the nucleate boiling heat transfer caused by the pressure loss accompanying the flow of the cooling water W, and it is preferable to set the velocity to 6.0 m / sec or more.

[0057] Based on the above configuration, the cooling method for the continuous casting mold according to the present invention may be carried out. That is, the cooling method for the continuous casting mold 10 is cooled by circulating the cooling water W, and may include a forced convection heat transfer cooling step of performing cooling based on forced convection heat transfer using the cooling water W on the downstream side of the casting direction A of the molten steel S in the continuous casting mold 10, and a nucleate boiling heat transfer cooling step of performing cooling based on nucleate boiling heat transfer using the cooling water W on the upstream side of the casting direction A of the molten steel S in the continuous casting mold 10.

[0058] And, by the cooling method of the continuous casting mold and the continuous casting mold according to the present invention, even when cooling by nucleate boiling heat transfer is used for cooling the continuous casting mold 10, it is possible to prevent damage caused by the occurrence of the burnout phenomenon of the continuous casting mold 10 and to stably maintain cooling by nucleate boiling heat transfer. Furthermore, by achieving both cooling by forced convection heat transfer and cooling by nucleate boiling heat transfer, it is possible to increase the amount of heat removed from the continuous casting mold 10. That is, since the ability of heat removal by nucleate boiling heat transfer is greater than that by forced convection heat transfer, it is possible to increase the amount of heat removed from the continuous casting mold 10. By preventing damage to the continuous casting mold 10, it is also possible to extend the service life of the continuous casting mold 10.

[0059] In addition, by combining cooling by forced convection heat transfer and cooling by nucleate boiling heat transfer, the continuous casting mold can be cooled efficiently, and the supply of cooling water to the continuous casting mold can be significantly reduced, which makes it possible to reduce the size of the cooling water supply equipment (reducing the initial cost at the time of construction) and save energy (reducing the capacity of the water supply pump).

[0060] Furthermore, when performing cooling based on nucleate boiling heat transfer, the pressure of the cooling water W in the flat water passage 14a is set to 5.0 kgf / cm 2 It is preferable to maintain the pressure of the cooling water W at 5.0 kgf / cm or less. 2 This is because by maintaining the temperature of the cooling water W in the flat water passage 14a at 40°C or higher and 65°C or lower, and the speed of the cooling water W in the flat water passage 14a at 4.0 m / sec or lower, it is possible to maintain the fineness of the bubbles and further suppress the coalescence and increase of the bubbles that are generated. In addition, it is more preferable to maintain the temperature of the cooling water W in the flat water passage 14a at 40°C or higher and 65°C or lower, and to maintain the speed of the cooling water W in the flat water passage 14a at 4.0 m / sec or lower. This is because by performing cooling based on nucleate boiling heat transfer based on these conditions, it is possible to maintain the fineness of the bubbles and further suppress the coalescence and increase of the bubbles that are generated. In this way, by controlling the pressure, temperature, and speed of the cooling water W, it is possible to stabilize the cooling state by subcooled boiling heat transfer on the surface of the flat water passage 14a on the upstream side, where nucleate boiling heat transfer does not develop and bubbles are likely to be generated and disappear. Therefore, it is possible to avoid localized increase in bubbles and burnout phenomenon. EXAMPLES

[0061] The results of cooling a continuous casting mold during continuous casting using the continuous casting mold and cooling method thereof according to the present invention will be described.

[0062] First, for the continuous casting machine, the maximum casting speed was 1.0 m / min, the wall thickness of the continuous casting mold was 220 mm, and the maximum heat flux qMAX to the continuous casting mold was 3.0 × 10 6 MW / m 2 A continuous casting machine having the above structure was used. As an example of the invention, a continuous casting mold (hereinafter also referred to as "invention example configuration") was used in which a plurality of water passages were provided on the downstream side of the wall in the casting direction and a flat water passage was provided on the upstream side of the casting direction. On the other hand, as a comparative example, a continuous casting mold (hereinafter also referred to as "comparative example configuration") was used in which a plurality of water passages were provided on the upstream and downstream sides of the wall in the casting direction. Pure water was used as the cooling water, and the temperature of the cooling water was set to 45 to 50°C. In the continuous casting mold, the invention example configuration or the comparative example configuration was applied to the wall, which is the long side surface (first wall portion 11a) of the two strands, and a long-term implementation was carried out.

[0063] In the configuration of the invention example, the speed of the cooling water in the flat water channel on the upstream side is 2.5 m / sec, the speed of the cooling water in the water channel on the downstream side is 6.0 m / sec, and the pressure of the cooling water in the flat water channel and the water channel is 3.5 kgf / cm 2 In the comparative example, the speed of the cooling water in the upstream and downstream water passages was 2.5 m / sec, and the pressure of the cooling water was 2.0 kgf / cm 2 It was decided.

[0064] The total heat removal amount of the continuous casting mold was calculated using the amount of cooling water supplied to the continuous casting mold and the difference in temperature of the cooling water at the inlet and outlet of the continuous casting mold. A thermocouple was installed on the inner plate of the wall at a position 100 mm below the meniscus (below the surface of the molten steel) of the molten steel and 10 mm deep into the molten steel contained in the continuous casting mold, and the uniformity (average value) of cooling of the wall and the presence or absence of abnormalities were evaluated based on the temperature measurement results from the thermocouple.

[0065] In Example 2, a microbubble generator was installed in the middle of the cooling water circulation path that supplies cooling water from the water supply pump to the continuous casting mold as a bypass, and fine bubbles with an average bubble diameter of 0.05 mm were mixed into the cooling water from the microbubble generator. Table 1 shows the results of a long-term (approximately 3 months) continuous experiment.

[0066] [Table 1]

[0067] As shown in Table 1, in Example 1, which is an example of the invention, the average mold heat removal amount was improved by about 17% compared to the comparative example configuration (Comparative Example 1) while keeping the casting speed at 0.8 to 1.0 m / min. Furthermore, in Example 2, in which fine bubbles were mixed in the cooling water, the average mold heat removal amount was improved by about 25.5%.

[0068] In addition, the average temperature values ​​measured by a thermocouple placed 100 mm below the meniscus of the molten steel (below the surface of the molten steel) in Examples 1 and 2 were 5 to 10°C higher than the average temperature value in Comparative Example 1, improving the heat transfer efficiency in the inner plate of the wall. In Comparative Example 1, abnormal temperatures exceeding 300°C were recorded several times, and ultimately abnormalities such as cracks in the inner plate occurred, so the implementation period did not reach the target of three months.

[0069] After a long period of operation, the inner plate, water channel, and flat water channel in the wall section were checked, and it was found that in the invention example configurations of invention example 1 and invention example 2, there was almost no occurrence of fine cracks on the surface of the inner plate or a decrease in hardness, which led to an improvement in durability.

[0070] Next, the results of an embodiment in which the casting speed was increased and the continuous casting mold was cooled will be described for the configurations of the invention example and the comparative example used in the previous examples.

[0071] For the continuous casting machine, the maximum casting speed is 2.5 m / min, the wall thickness of the continuous casting mold is 220 mm, and the maximum heat flux qMAX to the continuous casting mold is 5.0 × 10 6 MW / m 2 A continuous casting machine having the above-mentioned structure was used. The structure of the example of the invention was used as an example of the invention, and the structure of the comparative example was used as a comparative example. Pure water was used as cooling water, and the temperature of the cooling water was set to 45 to 50°C. In the continuous casting mold, the structure of the example of the invention or the structure of the comparative example was applied to the wall portion, which is the long side surface (first wall portion 11a) of the two strands, and a long-term implementation was carried out.

[0072] In the configuration of the invention example, the speed of the cooling water in the flat water channel on the upstream side is 4.0 m / sec, the speed of the cooling water in the water channel on the downstream side is 7.5 m / sec, and the pressure of the cooling water in the flat water channel and the water channel is 5.5 kgf / cm 2 In the comparative example, the speed of the cooling water in the upstream and downstream water passages was 5.0 m / sec, and the pressure of the cooling water was 4.0 kgf / cm 2 It was decided.

[0073] The total heat removal amount of the continuous casting mold was calculated using the amount of cooling water supplied to the continuous casting mold and the difference in temperature of the cooling water at the inlet and outlet of the continuous casting mold. A thermocouple was installed on the inner plate of the wall at a position 50 mm below the meniscus of the molten steel (below the surface of the molten steel) and 10 mm deep into the contained molten steel, and the uniformity (average value) of the cooling of the wall and the presence or absence of abnormalities were evaluated based on the temperature measurement results from the thermocouple. Table 2 shows the results of a long-term (approximately 3 months) continuous experiment.

[0074] Furthermore, in the present embodiment, in the invention example configuration used in invention example 3, holes having a depth of 0.3 mm were machined in a regular arrangement on the surface of the inner plate of the wall portion.

[0075] [Table 2]

[0076] As shown in Table 2, in Example 3, which is an example of the invention, the casting speed was set to 2.0 to 2.5 m / min, and the average mold heat removal amount was improved by about 20.5% compared to the comparative example configuration (Comparative Example 2). It is believed that the flat water channels and the holes drilled on the surface of the inner plate inside the flat water channels allowed the nuclei of bubbles based on the nucleate boiling state to be more uniformly generated on the inner surface of the flat water channels 14a, improving the average mold heat removal amount.

[0077] After a long period of operation, the inner plate, water channel, and flat water channel in the wall were checked, and it was found that in the invention example configuration of invention example 3, there was almost no occurrence of fine cracks on the surface of the inner plate or a decrease in hardness, which led to an improvement in durability.

[0078] From the above, according to the configuration according to the present invention (example configuration) in which a plurality of water passages are provided on the downstream side of the wall in the casting direction and a flat water passage is provided on the upstream side of the casting direction, even when cooling by nucleate boiling heat transfer is used for cooling the continuous casting mold, damage due to the burnout phenomenon of the continuous casting mold can be prevented and cooling by nucleate boiling heat transfer can be stably maintained. And even when the casting speed is increased, damage to the continuous casting mold can be prevented and cooling by nucleate boiling heat transfer can be stably maintained. [Explanation of symbols]

[0079] 10 Continuous casting mold 11 Wall 11a 1st wall 11b 2nd wall section 12 Volts 13 First waterway configuration 13a Canal 14 Second waterway configuration 14a Flat waterway 15 Inner plate 16 Outer Plate 20 Cooling water storage section 30 Water Pump 40 Regulating valve 50 Cooling water circulation path A Casting direction B Width direction C Thickness direction S Molten steel W Cooling water

Claims

1. A method for cooling a continuous casting mold by circulating cooling water through the continuous casting mold, comprising the steps of: a forced convection heat transfer cooling step of performing cooling based on forced convection heat transfer using the cooling water on a downstream side of the casting direction of the molten steel in the continuous casting mold; A nucleate boiling heat transfer cooling process in which cooling based on nucleate boiling heat transfer using the cooling water is performed upstream of the casting direction of the molten steel in the continuous casting mold; A method for cooling a continuous casting mold comprising the steps of:

2. 2. The method for cooling a continuous casting mold according to claim 1, wherein air bubbles having a diameter of 0.1 mm or less are mixed in the cooling water.

3. 3. The method for cooling a continuous casting mold according to claim 1, wherein pure water is used as the cooling water.

4. 1. A continuous casting mold having a wall with an inner plate and an outer plate, a first water passage configuration, which is a plurality of water passages defined by the inner plate and the outer plate and extending in the casting direction of molten steel downstream of the wall portion in the casting direction; a second waterway arrangement, which is a flat waterway defined by the inner plate and the outer plate and extends in the casting direction upstream of the wall portion in the casting direction, where the plurality of water passages of the first waterway arrangement join together; A continuous casting mold comprising:

5. 5. The continuous casting mold according to claim 4, wherein the flat water channel in the second water channel configuration has a recess having a depth of 1 mm or less or a protrusion having a height of 1 mm or less on a surface of the inner plate.

6. 5. The continuous casting mold of claim 4, wherein the flat water passages in the second water passage configuration have a material embedded in a surface of the inner plate, the material having a different thermal conductivity than the inner plate.

Images