Cooling method for continuous casting molds and continuous casting molds

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, continuous casting has been required to increase speed in order to reduce costs and improve productivity. Therefore, molds used to solidify molten steel in continuous casting require high heat removal capacity to rapidly form a solidified shell. Furthermore, to produce a high-quality solidified shell and prevent mold damage due to heat load, it is necessary to increase the amount of heat removed by forced convection heat transfer using cooling water, in line with the heat input which increases proportionally with the casting speed. In other words, it is necessary to improve the heat transfer coefficient by forced convection heat transfer.

[0003] In this process, the inner plates installed in the mold walls are designed to improve the heat transfer coefficient through forced convection, by ensuring that the cooling water flowing within the walls has a velocity of 7-10 m / sec. Consequently, the amount of cooling water supplied inevitably increases, leading to larger water supply and treatment facilities, which poses a cost problem.

[0004] For this reason, a technology has been proposed that utilizes nucleation boiling heat transfer by controlling the velocity and pressure of the cooling water used for forced convection heat transfer. Patent Document 1 discloses a method for performing cooling by nucleation boiling heat transfer by defining the pressure of the cooling water that generates nucleation boiling heat transfer and the pressure of the cooling water that causes burnout from empirical formulas, and controlling the pressure of the cooling water within an appropriate range. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 3-81049 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] However, in the method disclosed in Patent Document 1, even when the cooling water pressure and other parameters are controlled within an appropriate range, slight changes in the temperature or velocity of the cooling water can affect the heat dissipation performance of the mold, potentially leading to problems such as damage to the inner plate. Furthermore, while the effect of improving the heat dissipation performance of the mold can be obtained when the casting speed in continuous casting is less than 1.5 m / min, when the casting speed is 1.5 m / min or higher, stable heat dissipation performance of the mold cannot be obtained, and the thickness of the solidified shell is formed unevenly. Therefore, from the viewpoint of stable continuous casting, it is difficult to actively utilize nucleation boiling heat transfer.

[0007] Furthermore, the heat dissipation effect of the mold based on nucleation boiling heat transfer becomes unstable due to the influence of changes in the operation of continuous casting. For this reason, even if the cooling water pressure and other parameters are controlled within an appropriate range to prevent burnout (a phenomenon that marks the transition from the nucleation boiling state to the film boiling state), some of the water passages partitioned at multiple locations on the inner plate of the mold may reach the conditions for burnout, potentially leading to operational problems.

[0008] The present invention has been made in view of the above circumstances, and its object is to provide a method for cooling a continuous casting mold and a continuous casting mold that can prevent damage associated with burnout of the continuous casting mold and stably maintain cooling by nucleating boiling heat transfer, even when cooling by nucleating boiling heat transfer is used in the cooling of the continuous casting mold. [Means for solving the problem]

[0009] [1] A method for cooling a continuous casting mold by circulating cooling water, comprising: a forced convection heat transfer cooling step, in which cooling is performed on the downstream side of the continuous casting mold in the casting direction of the molten steel, based on forced convection heat transfer using the cooling water; and a nucleation boiling heat transfer cooling step, in which cooling is performed on the upstream side of the continuous casting mold in the casting direction of the molten steel, based on nucleation boiling heat transfer using the cooling water. [2] The method for cooling a continuous casting mold according to [1], wherein bubbles with a diameter of 0.1 mm or less are mixed into the cooling water. [3] A 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 comprising an inner plate and an outer plate, wherein the wall portion comprises a first channel configuration, which is a plurality of water channels defined by the inner plate and the outer plate and extending in the casting direction, located downstream in the casting direction of the molten steel, and a second channel configuration, which is a flat channel defined by the inner plate and the outer plate and extending in the casting direction, and into which the plurality of water channels of the first channel configuration converge, located upstream in the casting direction of the wall portion. [5] The flat channel in the second channel configuration is provided with a recess with a depth of 1 mm or less or a protrusion with a height of 1 mm or less on the surface of the inner plate, as described in [4]. [6] The continuous casting mold according to [4], wherein in the flat channel configuration of the second channel, a material having a different thermal conductivity from that of the inner plate is embedded on the surface of the inner plate. [Effects of the Invention]

[0010] According to the present invention, even when cooling by nucleation boiling heat transfer is used in the cooling of continuous casting molds, damage associated with the burnout phenomenon of the continuous casting mold can be prevented, and cooling by nucleation boiling heat transfer can be stably maintained. [Brief explanation of the drawing]

[0011] [Figure 1] This diagram shows the cooling water supply path for circulating cooling water over a continuous casting mold. [Figure 2] This figure shows a schematic perspective view of the general configuration of a continuous casting mold. [Figure 3] This figure shows a schematic cross-sectional view of a conventional wall section. [Figure 4] This figure shows a schematic cross-sectional view of a conventional wall section. [Figure 5]This is a diagram showing a schematic cross-section of the wall portion of the present embodiment. [Figure 6] This is a diagram showing a schematic plan cross-section of the wall portion of the present embodiment. [Figure 7] This is a diagram schematically showing the state of bubbles when nucleate boiling heat transfer is used with cooling water. [Figure 8] This is a diagram schematically showing the form of the concavo-convex processing on the inner surface of the inner plate.

Modes for Carrying Out the Invention

[0012] Hereinafter, embodiments 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 with respect to the continuous casting mold 10. In FIG. 1, a cooling water circulation path 50 for supplying cooling water W to the continuous casting mold (hereinafter, also simply referred to as "mold") used in the present embodiment is shown.

[0014] As shown in FIG. 1, the cooling water circulation path 50 has a configuration for circulating cooling water W across the continuous casting mold 10, the cooling water storage portion 20, the water supply pump 30, and the regulating valve 40. The cooling water storage portion 20 supplies cooling water W to the water supply pump 30. The water supply pump 30 pressurizes the cooling water supplied from the cooling water storage portion 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 water supply 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 in order to properly and stably cool the continuous casting mold 10. The cooling water storage portion 20 recovers the cooling water W used for cooling in the continuous casting mold 10.

[0015] Next, the general configuration of the continuous casting mold 10 will be explained using Figure 2. Figure 2 shows a schematic perspective view of the general configuration of the continuous casting mold 10. As shown in Figure 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 that extends along the thickness direction C of the continuous casting mold 10. As shown in Figure 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 the molten steel S injected through the immersion nozzle 19. Then, the molten steel S contained in the continuous casting mold 10 is cooled and solidified, and the solidified molten steel S is continuously drawn out in the casting direction A to produce a cast slab.

[0016] In the following explanation, when simply referred to as "wall section 11," it means that both "first wall section 11a" and "second wall section 11b" are included, and when referred to as "first wall section 11a" or "second wall section 11b," it means that the explanation refers to each respective section.

[0017] Here, the conventional wall structure in the continuous casting mold 10 will be explained using Figures 3 and 4. Figure 3 shows a schematic cross-sectional view of the conventional wall 110. Figure 4 shows a schematic planar cross-sectional view of the conventional wall 110. Specifically, Figure 4(a) shows the UU cross-section in Figure 3. Figure 4(b) shows the LL cross-section in Figure 3. Hereafter, the conventional wall 110 will be described as a wall having a long structure along the width direction B of the mold.

[0018] As shown in Figures 3 and 4, the wall portion 110 has bolts 120, water passages 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. Multiple bolts 120 are arranged at predetermined intervals in the width direction B of the wall portion 110. As shown in Figure 4, the water passages 130 are defined by the inner plate 150 and the outer plate 160 so as to extend in the casting direction A. Multiple water passages 130 are provided between the multiple bolts 120 on the upstream and downstream sides of the wall portion 110 in the casting direction A of the molten steel.

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

[0020] Specifically, the velocity of the cooling water W is controlled to a target of 7-10 m / sec when cooling is performed by forced convection heat transfer, to a target of 2-4 m / sec when cooling is performed by nucleation boiling heat transfer, and to a target of 4-6 m / sec when cooling is performed by subcool boiling heat transfer. The total amount of cooling water W flowing through all the 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 set in advance, and the wall portion 110 is cooled while monitoring the total amount of cooling water at the inlet and outlet of the continuous casting mold 10, the temperature of the cooling water W, and the temperature of the thermocouples provided on the inner plate 150, etc., in order to maintain the set total amount of cooling water and the pressure of the inlet cooling water W. Therefore, it is industrially effective because it can prevent nuclear boiling from developing abnormally and leading to burnout.

[0021] The water passage sections 130 are provided at 5 to 10 locations between multiple bolts 120 in the width direction B or thickness direction C (see Figure 2) of the wall section 110. Therefore, the cooling water W supplied from the water supply pump 30 is distributed equally through the piping so that it reaches an equivalent velocity in all water passage sections 130. In order to maintain cooling by nuclear boiling heat transfer while preventing burnout in the water passage sections 130, it is necessary to control the velocity and pressure of the cooling water W within a predetermined range.

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

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

[0024] Furthermore, in the water passage section 130, when the cooling water W transitions from a nucleate boiling state to a burnout state and then to a film boiling state, a rapid temperature rise occurs on the inner surface of the inner plate 150. At this time, a thermocouple embedded in the inner surface of the inner plate 150 detects the timing of the rapid temperature rise as the occurrence of a burnout phenomenon, and by reducing the casting speed based on this detection, it is effective to avoid damage to the wall section 110 and operational troubles such as breakouts. Due to the configuration of the wall section 110, the thermocouples provided on the inner plate 150 are placed in the same row as the bolts 120 which are arranged in a row along the casting direction A. Therefore, if the distance between the location in the water passage section 130 where the rapid temperature rise occurs and the location where the thermocouples are placed is large, the detection of the temperature rise may be delayed.

[0025] Furthermore, the temperature detection using thermocouples installed on the inner plate 150 is merely a configuration for detecting abnormalities such as burnout. While this is effective in avoiding operational troubles, it is unsuitable 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 extent of the damage to the wall portion 110, etc., and reached the following conclusion.

[0027] Specifically, we found that even when the velocity of the cooling water W circulating in the water passage section 130 is stably set within a range where cooling by nucleation boiling heat transfer is possible, burnout may occur if the nucleation boiling state becomes unstable in the water passage section 130. Furthermore, if the temperature of the cooling water W on the inlet side of the water passage section 130 rises, it becomes easier to transition to a nucleation boiling state and for burnout to occur, making it impossible to maintain temperature uniformity of the cooling water W in multiple water passage sections 130. Moreover, when calculating the total amount of heat removed from the continuous casting mold 10 based on the temperature difference between the inlet and outlet sides of the water passage section 130, we found that the improvement in the effect based on the total amount of heat removed was not significant when comparing the amount of heat removed when cooling was performed using only forced convection heat transfer with the amount of heat removed when cooling was performed using nucleation boiling heat transfer.

[0028] The inventors diligently studied the challenges of industrially utilizing the aforementioned nucleation boiling heat transfer in continuous casting molds. As a result, they concluded that the conventional shape of the water passage 130 in the wall portion 110 is unsuitable for the use of nucleation boiling heat transfer.

[0029] First, the shape and configuration of the water passages 130 in the wall section 110, which have been conventionally applied assuming the use of forced convection heat transfer, are not suitable for the use of nucleation boiling heat transfer. The vertical-hole type water passages 130 are very effective in increasing the velocity of the cooling water W, and are an effective shape for lengthening the cooling length that forms the cooling surface, known as the wetting length. In contrast, when performing cooling by nucleation boiling heat transfer, it is important to perform cooling by making the velocity 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 uniform on the cooling surface and to make the pressure of the cooling water W above the boiling vapor pressure. Therefore, when using cooling by nucleation boiling heat transfer, instead of a configuration in which many vertical-hole type water passages 130 are arranged, it is better to provide a wide, planar water passage parallel to the surface of the inner plate 150, that is, the plated surface on the side where the molten steel S is located, in order to maintain a stable boiling state.

[0030] Furthermore, when a vertical-hole type water passage section 130 is used, when nucleation boiling heat transfer is utilized in the water passage section 130, bubbles fill up as the nucleation boiling state develops, surrounding the cooling water W flowing from bottom to top. And because 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 nucleation boiling state develops. As a result, the flow of cooling water W is obstructed, causing dry-out and burn-out phenomena on the cooling surface (surface of the inner plate 150) in the water passage section 130, and transitioning to a film boiling state.

[0031] Furthermore, in the vertical-hole type water passage section 130, when bubbles fill it, the pressure of the cooling water W at the inlet side of the water passage section 130 increases. As a result, the flow of cooling water W in the bubble-filled water passage section 130 decreases, while the flow rate of cooling water W in the other water passage sections 130 increases. Therefore, even if there is no change in the total flow rate of cooling water W supplied to the wall section 110, in a configuration with many vertical-hole type water passage sections 130, the flow of cooling water W may be obstructed locally, making burnout more likely. In particular, if the temperature of the cooling water W at the inlet side of the water passage section 130 rises, the boiling state is more likely to develop locally at the location where it has transitioned to a film boiling state, which can easily lead to problems such as damage to the wall section 110.

[0032] In contrast, when using a water passage with a large flat cooling surface area, a wide, planar water passage parallel to the surface of the inner plate, i.e., the plated surface on the side where the molten steel S is located, can be provided, thereby securing a large area that contributes to cooling. As a result, nucleation boiling can be generated uniformly over a wide area of ​​the inner plate surface. In other words, cooling by nucleation boiling heat transfer can be generated stably and at the same timing over the entire area of ​​the wall portion of the continuous casting mold.

[0033] Furthermore, even when numerous bubbles are generated due to the development of nucleated 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. Therefore, it is possible to prevent cooling water W depletion leading to dryout in the water passage and to prevent a local pressure increase in the cooling water W at the inlet. In other words, in order to stabilize the nucleated boiling state in the water passage, it is necessary to have a configuration with a large cross-sectional area for the water passage.

[0034] In view of the above findings, the present inventors propose a continuous casting mold that improves the amount of heat removed from the mold and provides stability for industrial use.

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

[0036] As shown in Figures 5 and 6, the wall portion 11 has bolts 12, a first water channel configuration 13, a second water channel configuration 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. Multiple bolts 12 are arranged at predetermined intervals in the width direction B of the wall portion 11. As shown in Figure 6, the first water channel configuration 13 and the second water channel configuration 14 are defined by the inner plate 15 and the outer plate 16 so as to extend in the casting direction A. The first water channel configuration 13 is located downstream of the molten steel in the casting direction A in the wall portion 11 and provides multiple water channels 13a between the multiple bolts 12. The second water channel configuration 14 is located upstream of the molten steel in the casting direction A in the wall portion 11 and is defined by the inner plate 15 and the outer plate 16, extends in the casting direction A, and provides a flat water channel 14a where the multiple water channels 13a of the first water channel configuration 13 merge. Furthermore, the water passage 13a in the first waterway configuration 13 has the same configuration as the conventional water passage section 130, which was explained using Figures 3 and 4.

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

[0038] The wall portion 11 of the continuous casting mold 10 is characterized by performing cooling based on convective heat transfer on the downstream side in the casting direction A, and performing cooling based on nucleation boiling heat transfer on the upstream side in the casting direction A.

[0039] Furthermore, in the wall portion 11 of the continuous casting mold 10, the first water channel configuration 13 provided on the downstream side in the casting direction A consists of multiple vertical-hole type water channels 13a provided between multiple bolts 12, similar to conventional designs. On the other hand, the second water channel configuration 14 provided on the upstream side in the casting direction A consists of flat-plate type water channels 14a with a large cross-sectional area provided between multiple bolts 12.

[0040] Here, the state of bubbles using nucleated boiling heat transfer in cooling water W will be explained with reference to Figure 7. Figure 7 is a schematic diagram showing the state of bubbles when nucleated boiling heat transfer is used in cooling water W. Specifically, Figure 7(a) is a schematic diagram showing the state of bubbles when nucleated boiling heat transfer is used in a conventional vertical hole type water passage. Figure 7(b) is a schematic diagram showing the state of bubbles when nucleated boiling heat transfer is used in a flat plate type flat water channel 14a with a large cross-sectional area.

[0041] As shown in Figure 7(a), when bubbles are generated in a conventional vertical-hole type water passage due to nucleation boiling heat transfer, the bubbles occupy the water passage path, obstructing the upward flow of cooling water W supplied from below. As a result, the rate of supply of cooling water W from below slows down, and the water pressure of the cooling water W increases. Consequently, localized temperature fluctuations occur in multiple water passages.

[0042] In contrast, when a flat channel 14a with a large cross-sectional area is used, as shown in Figure 7(b), even if bubbles based on nucleation boiling heat transfer develop significantly, it is possible to avoid the flat channel 14a being occupied by bubbles. As a result, the rate of supply of cooling water W from below is kept constant, and the water pressure of the cooling water W is also kept constant. Consequently, it becomes possible to maintain a uniform temperature in the multiple water channels 13a and flat channel 14a, thereby avoiding uneven development of bubbles based on nucleation boiling heat transfer and suppressing the occurrence of burnout.

[0043] Here, we will explain in more detail the conventional vertical-hole type water passage shown in Figure 7(a). In the water passage, multiple bubbles combine and their volume increases, causing the passage to become occupied. When this occurs, the inner surface of the passage becomes completely dry (dry-boiled state), and the temperature of the inner surface rises rapidly, causing a burnout phenomenon. Therefore, in order to suppress the localized occurrence of the burnout phenomenon, it is necessary to suppress the combination and increase of bubbles during cooling by nucleate boiling heat transfer.

[0044] Next, the configuration of this embodiment will be described in detail. In this embodiment, due to cooling based on nucleate boiling heat transfer, fine bubbles are generated in areas where the surface roughness of the inner plate 15 differs or where there are irregularities. Based on this, in the flat channel 14a of the flat plate type with a large cross-sectional area, it is necessary to generate bubbles uniformly over the entire surface of the flat channel 14a in order to suppress the coalescence and enlargement of the generated bubbles.

[0045] Furthermore, in this embodiment, it is preferable to further include a configuration that has not been given attention in the prior art, namely, "a configuration in which regular irregularities are provided across the entire surface of a flat-type flat channel," and "a method for mixing fine bubbles into the cooling water during cooling based on nucleate boiling heat transfer." These configurations and methods will be described in detail below.

[0046] In cooling based on nucleation 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 channel 14a.

[0047] Therefore, it is preferable to apply a predetermined process to the surface of the inner plate 15 inside the flat channel 14a. Specifically, it is desirable 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 applying this uneven processing, it becomes possible to generate bubble nuclei more uniformly on the inner surface of the flat channel 14a.

[0048] Next, specific examples of surface texture processing of the inner plate 15 inside the flat channel 14a will be explained using Figure 8. Figure 8 is a schematic diagram showing the form of surface texture processing of the inner plate 15. As a predetermined processing form applied to the surface of the inner plate 15, Figure 8(a) shows a configuration in which a grid-like notch (grid-like groove) is applied. Figure 8(b) shows a configuration in which hemispherical nickel members are arranged in a staggered pattern and plated. Figure 8(c) shows a configuration in which processed holes are arranged in a staggered pattern.

[0049] The configuration shown in Figure 8(b) may be plated by embedding materials with different thermal conductivity (nickel members) in a staggered pattern on the inner plate 15. The processed holes shown in Figure 8(c) may be formed by drilling with a diameter of 0.5 mm and a depth of 0.5 mm.

[0050] By applying the processing shown in Figures 8(a) to 8(c), it becomes possible to uniformly generate bubble nuclei based on nucleated boiling heat transfer on the inner surface of the flat channel 14a. Furthermore, it is known that when generating bubbles by nucleated boiling, the stable generation of bubbles is more easily promoted by using the bubble nuclei as a starting point. Therefore, by applying the processing to the surface of the inner plate 15, the generation of uniform bubbles is promoted and the development of localized nucleated boiling is suppressed. The processing shown in Figures 8(a) to 8(c) may be applied to the surface of the inner plate 15 in a regular arrangement.

[0051] Furthermore, by limiting the depth of the recesses and the height of the protrusions in the textured surface to 1 mm or less, the uniform generation of nuclei during nucleation boiling can be promoted. On the other hand, if recesses and protrusions with a depth or height exceeding 1 mm are formed by processing, it leads to problems in terms of processing costs and a decrease in the strength of the inner plate due to the processing.

[0052] Furthermore, bubbles with a diameter of 0.1 mm or less may be mixed into the cooling water W. By mixing bubbles with a diameter of 0.1 mm or less (microbubbles) into the cooling water W, these bubbles act as vapor nuclei in the nucleated boiling state, eliminating the need for the interfacial energy gap required for vapor nucleus formation, and resulting in fine and uniform vapor generation from the surface of the inner plate 15. Specifically, in the cooling water W, 1.0 × 10 8 It is more preferable to mix in bubbles (microbubbles) with a diameter of 0.1 mm or less so that the number density is 0.1 mm / L or more. In other words, by adjusting the state of the mixture of bubbles (microbubbles) in the cooling water W, the nucleation boiling phenomenon can be controlled with higher precision.

[0053] Furthermore, when 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 nucleation boiling is achieved, increasing the probability of maintaining nucleation boiling until a higher heat flux (high heat extraction rate) is reached. Achieving a high heat extraction rate while suppressing burnout is effective in increasing the speed of continuous casting.

[0054] While it is preferable for industrial purposes to mix bubbles (microbubbles) with a diameter of 0.1 mm or less into the cooling water W in the form of dissolved gas and then cause them to appear as bubbles in the liquid at a temperature just below the boiling vapor pressure, the method of mixing is not limited.

[0055] Furthermore, it is desirable to use pure water as the cooling water W. This is because using pure water allows for accurate reproduction (control) of the boiling phenomenon.

[0056] Furthermore, in the vertical-type water passage 13a provided on the downstream side of the casting direction A in the continuous casting mold 10, it is desirable to maintain a predetermined speed of cooling water W such that it does not affect the nucleation boiling heat transfer caused by pressure loss associated with the flow of cooling water W, and it is preferable that the speed be 6.0 m / sec or higher.

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

[0058] Furthermore, with the cooling method and continuous casting mold according to the present invention, even when cooling by nucleation boiling heat transfer is used to cool the continuous casting mold 10, it is possible to prevent damage associated with burnout of the continuous casting mold 10 and to stably maintain cooling by nucleation boiling heat transfer. Moreover, by combining cooling by forced convection heat transfer and cooling by nucleation boiling heat transfer, the amount of heat removed from the continuous casting mold 10 can be increased. That is, because the heat removal capacity by nucleation boiling heat transfer is greater than that of forced convection heat transfer, the amount of heat removed from the continuous casting mold 10 can be increased. 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] Furthermore, by combining cooling by forced convection heat transfer and cooling by nucleation boiling heat transfer, continuous casting molds can be cooled efficiently, significantly reducing the supply of cooling water to the molds. This allows for a reduction in the size of the cooling water supply equipment (reducing initial construction costs) and energy savings (reducing the capacity of the water supply pumps).

[0060] Furthermore, when performing cooling based on nuclear boiling heat transfer, the pressure of the cooling water W in the flat channel 14a is set to 5.0 kgf / cm². 2 It is preferable to maintain the following: Cooling water W pressure at 5.0 kgf / cm² 2 Maintaining the following conditions allows for the preservation of bubble miniaturization and further suppression of bubble aggregation and enlargement. In addition, it is preferable to maintain the temperature of the cooling water W in the flat channel 14a at 40°C to 65°C and the velocity of the cooling water W in the flat channel 14a at 4.0 m / sec or less. By performing cooling based on nucleated boiling heat transfer under these conditions, it is possible to maintain bubble miniaturization and further suppress the aggregation and enlargement of the generated bubbles. In this way, by controlling the pressure, temperature, and velocity of the cooling water W, it is possible to stabilize the cooling state at the surface of the upstream flat channel 14a in a state where bubble generation and collapse are likely to occur without the development of nucleated boiling heat transfer. Therefore, localized bubble enlargement and burnout phenomena can be avoided. [Examples]

[0061] This paper describes the cooling method for continuous casting molds according to the present invention and the results of cooling a continuous casting mold during continuous casting.

[0062] First, regarding the continuous casting machine, the maximum casting speed is 1.0 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 3.0 × 10⁻¹⁰. 6 MW / m 2 A continuous casting machine was used. As an example of the invention, a continuous casting mold (hereinafter also referred to as the "example configuration") was used, which had multiple water passages on the downstream side in the casting direction of the wall and a flat water passage on the upstream side in the casting direction. On the other hand, as a comparative example, a continuous casting mold (hereinafter also referred to as the "comparative configuration") was used, which had multiple water passages on the upstream and downstream sides in the casting direction of the wall. Pure water was used as the cooling water, and the temperature of the cooling water was set to 45-50°C. In the continuous casting mold, the example configuration or the comparative configuration was applied to the wall, which is the long side surface (first wall 11a) of the two strands, and the process was carried out over a long period of time.

[0063] In the example configuration of the invention, the velocity of the cooling water in the upstream flat channel is 2.5 m / sec, the velocity of the cooling water in the downstream channel is 6.0 m / sec, and the pressure of the cooling water in the flat channel and the channel is 3.5 kgf / cm². 2 In the comparative example configuration, the cooling water velocity in the upstream and downstream water passages was set to 2.5 m / sec, and the cooling water pressure was set to 2.0 kgf / cm². 2 That's what I decided.

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

[0065] Furthermore, in Invention Example 2, a microbubble generator was installed in a bypass configuration in the cooling water circulation path that supplies cooling water from the water supply pump to the continuous casting mold, 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 continuous implementation over a long period (approximately 3 months).

[0066] [Table 1]

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

[0068] Furthermore, the average temperature measured by a thermocouple placed 100 mm below the meniscus (below the molten steel surface) in Invention Example 1 and Invention Example 2 was 5 to 10°C higher than the average temperature in Comparative Example 1, indicating improved 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, resulting in the implementation period not reaching the target of 3 months.

[0069] After long-term testing, we inspected the inner plate, water channel, and flat channel in the wall section and found that in the inventive configurations of Invention Example 1 and Invention Example 2, there was almost no occurrence of minute cracks on the surface of the inner plate or a decrease in hardness, which can lead to an improvement in durability.

[0070] Next, we will explain the results of increasing the casting speed and cooling the continuous casting mold for the inventive example configuration and comparative example configuration used in the previous example.

[0071] Regarding the continuous casting machine, the maximum casting speed was 2.5 m / min, the thickness of the wall of the continuous casting mold was 220 mm, and the maximum heat flux qMAX to the continuous casting mold was 5.0×10 6 MW / m 2 A continuous casting machine was used. And the invention example configuration was used as the invention example, and the comparative example configuration was used as the comparative example. Pure water was used as the cooling water, and the temperature of the cooling water was 45 to 50°C. In the continuous casting mold, for the wall which is the long side surface (the first wall portion 11a) of 2 strands, the invention example configuration or the comparative example configuration was applied and implemented for a long period.

[0072] In the invention example configuration, the speed of the cooling water in the upstream parallel water channel was 4.0 m / sec, the speed of the cooling water in the downstream water passage was 7.5 m / sec, and the pressure of the cooling water in the parallel water channel and the water passage was 5.5 kgf / cm 2 In the comparative example configuration, the speed of the cooling water in the upstream and downstream water passing portions was 5.0 m / sec, and the pressure of the cooling water was 4.0 kgf / cm 2 was set.

[0073] The total heat extraction amount of the continuous casting mold was calculated using the supply water amount of the cooling water to the continuous casting mold and the temperature difference of the cooling water on the inlet and outlet sides of the continuous casting mold. Also, in the inner plate of the wall portion, a thermocouple was installed at a position 50 mm below the meniscus of the molten steel (below the molten steel surface) and 10 mm in the depth direction of the accommodated molten steel. Based on the measurement results of the temperature from the thermocouple, the cooling uniformity (average value) and the presence or absence of abnormalities of the wall portion were evaluated. Table 2 shows the results of continuous implementation for a long period (about 3 months).

[0074] Furthermore, in this embodiment, in the invention example configuration used in Invention Example 3, holes with a depth of 0.3 mm were processed on the surface of the inner plate of the wall portion so as to be regularly arranged.

[0075]

Table 2

[0076] As shown in Table 2, in Invention Example 3, which is the configuration of the invention example, the average heat removal amount from the mold was improved by approximately 20.5% compared to the comparative example configuration (Comparative Example 2), while the casting speed was set to 2.0 to 2.5 m / min. It is thought that the presence of a flat channel and the drilling of holes on the surface of the inner plate inside the flat channel allowed for more uniform generation of bubble nuclei based on nucleated boiling on the inner surface of the flat channel 14a, resulting in an improvement in the average heat removal amount from the mold.

[0077] After long-term testing, the inner plate, water channel, and flat channel in the wall section were examined, and it was found that in the inventive configuration of Invention Example 3, there was almost no occurrence of minute cracks on the surface of the inner plate or a decrease in hardness, which can lead to an improvement in durability.

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

[0079] 10 Continuous casting molds 11 Wall 11a 1st wall section 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 supply pump 40 Adjustment 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, which involves circulating cooling water to cool the continuous casting mold, A forced convection heat transfer cooling process is performed in which the cooling water is circulated through a vertical-type water channel in the continuous casting mold to perform cooling based on forced convection heat transfer, The system includes a nucleation boiling heat transfer cooling step in which cooling is performed based on nucleation boiling heat transfer by slowing the velocity of the cooling water in a flat-plate type channel with a larger cross-sectional area than the aforementioned channel, so that the velocity of the cooling water is slower than the velocity at which it flowed through the aforementioned channel. A method for cooling a continuous casting mold, wherein the pressure of the cooling water flowing through the flat channel is 5.0 kgf / cm² or less, the temperature is 40°C to 65°C, and the speed is 4.0 m / sec or less.

2. The method for cooling a continuous casting mold according to claim 1, wherein bubbles with 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 claim 1 or 2, wherein pure water is used as the cooling water.

4. A continuous casting mold having a wall portion comprising an inner plate and an outer plate, The first water channel configuration is defined by the inner plate and the outer plate and includes a plurality of vertical-hole type water channels through which cooling water flows, A second waterway configuration is defined by the inner plate and outer plate, where the multiple water channels of the first waterway configuration merge, and which includes a flat plate type water channel with a larger cross-sectional area than the water channels, and the cooling water pressure is 5.0 kgf / cm² or less, the temperature is 40°C to 65°C, and the velocity is 4.0 m / sec or less. A continuous casting mold having the following features.

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

6. The continuous casting mold according to claim 4, wherein in the flat channel of the second channel configuration, a material with a different thermal conductivity from that of the inner plate is embedded on the surface of the inner plate.

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