Continuous steel casting method and cooling water supply equipment

Abstract

To provide a method of continuous casting of steel allowing for securing the stability and safety of a cooling substance under elevated temperature and increasing the removal amount of heat from a mold.SOLUTION: A method of continuous casting of steel, using a continuous casting facility, includes feeding cooling water containing air bubbles having a diameter of 0.1 mm or smaller to a cooling-water passage of a mold of the continuous casting facility.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for continuous casting steel and a cooling water supply system for supplying cooling water to a mold in a continuous casting system. [Background technology]

[0002] In continuous casting of steel, cooling in the mold is important for removing heat from the molten steel and forming a solidified shell. In recent years, stronger heat removal in the mold has been required from the viewpoint of increasing production volume due to the high-speed casting of steel.

[0003] The thermal resistances that occur during heat removal from molten steel are the solidified shell, mold flux, air gap, and cooling water. Of these, the thermal resistance caused by the solidified shell is unavoidable, while the resistance caused by the mold flux and air gap depends on the material and mold type. However, cooling water is composed solely of water, and no improvement can be expected through changes in its properties. Patent Document 1 discloses a solution to this problem: adjusting the flow path within the mold to adjust the amount of heat removal. According to Patent Document 1, the amount of heat removal can be improved by supplying cooling water to the mold from multiple cooling water supply ports located at different heights and increasing the flow rate of the cooling water. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 5-318035 Summary of the Invention [Problem to be solved by the invention]

[0005] However, as the flow rate of cooling water increases, the improvement in the heat transfer coefficient reaches saturation due to pressure loss in the piping. In other words, there is a limit to how much heat can be removed by increasing the flow rate of cooling water. In order to achieve more intense heat removal, it is necessary to explore new methods in addition to conventional approaches to heat removal.

[0006] From the viewpoint of improving the heat transfer coefficient, a highly thermally conductive substance other than water may be used as the coolant for the continuous casting mold. However, since the continuous casting mold comes into contact with the molten steel and reaches extremely high temperatures, there is a risk that the properties of the cooling substance used as the coolant may change, making it impossible to ensure stability and safety.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a continuous steel casting method which ensures the stability and safety of the cooling substance at high temperatures and which can increase the amount of heat removed from the mold, and a cooling water supply system which supplies cooling water which can increase the amount of heat removed from the mold. [Means for solving the problem]

[0008] The means for solving the above problems are as follows. [1] A method for continuously casting steel using continuous casting equipment, comprising supplying cooling water containing bubbles having a diameter of 0.1 mm or less to a cooling water channel of a mold of the continuous casting equipment. [2] The method for continuous casting steel according to [1], wherein the number density of bubbles contained in the cooling water is 100 million bubbles / L or more. [3] A cooling water supply system that supplies cooling water to a mold of a continuous casting system, the cooling water supply system comprising: a cooling water conveying device that supplies cooling water to the mold; and a bubble generating device that injects bubbles having a diameter of 0.1 mm or less into the cooling water. [4] The cooling water supply facility according to [3], wherein the bubble generator injects bubbles into the cooling water at a rate of 100 million bubbles per liter or more. [Effects of the Invention]

[0009] According to the present invention, by incorporating air bubbles with a diameter of 0.1 mm or less into the cooling water used to cool the mold, the heat transfer coefficient of the cooling water can be improved, thereby increasing the amount of heat removed from the mold by the cooling water. This allows the required solidified shell thickness to be achieved in a shorter time than conventional methods, thereby reducing the risk of breakout during high-speed casting. Furthermore, because the cooling water containing air acts as a refrigerant, stability and safety can be ensured even at the high temperatures required for continuous steel casting. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a cross-sectional schematic view showing an example of continuous casting equipment in which the continuous steel casting method according to this embodiment can be implemented. [Figure 2] FIG. 2 is a cross-sectional schematic diagram showing a part of the mold and the cooling water supply equipment. [Figure 3] FIG. 3 is a cross-sectional schematic view showing another example of the cooling water supply facility. [Figure 4] FIG. 4 is a cross-sectional schematic view showing another example of the cooling water supply facility. [Figure 5] Figure 5 is a schematic diagram of the experimental device used to confirm the heat transfer coefficient of the cooling water. [Figure 6] FIG. 6 is a graph showing the relationship between the diameter of bubbles and the heat transfer coefficient of the cooling water for each density of bubbles contained in the cooling water. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will be described below through embodiments of the present invention. The following embodiments are preferred examples of the present invention, and the present invention is not limited to these embodiments.

[0012] FIG. 1 is a cross-sectional schematic diagram showing an example of continuous casting equipment in which the steel continuous casting method according to this embodiment can be implemented. The continuous casting equipment 10 includes a mold 12, a tundish 14 installed above the mold 12, and a plurality of strand support rolls 16 arranged in a row below the mold 12. Although not shown, a ladle containing molten steel 18 is installed above the tundish 14, and the molten steel 18 is poured into the tundish 14 from the bottom of the ladle. An immersion nozzle 20 is installed at the bottom of the tundish 14, and the molten steel 18 is poured into the mold 12 through the immersion nozzle 20. A cooling water channel is formed in the mold 12, and cooling water is supplied to the cooling water channel from a cooling water supply system 22. The molten steel 18 solidifies as heat is removed from the inner surface of the mold 12, forming a solidified shell 24. This results in a strand 28 having the solidified shell 24 as its outer shell and an unsolidified layer 26 made of the molten steel 18 inside.

[0013] In the gaps between adjacent strand support rolls 16 in the casting direction, multiple secondary cooling zones 30, each equipped with spray nozzles (not shown), are installed along the casting direction from directly below the mold 12. The strand 28 is cooled as it is withdrawn by cooling water sprayed from the spray nozzles in the secondary cooling zones 30. While the strand 28 is transported by the strand support rolls 16 and passes through the multiple secondary cooling zones 30, the solidified shell 24 is appropriately cooled, solidification of the unsolidified layer 26 progresses, and solidification of the strand 28 is completed.

[0014] Downstream in the casting direction, a plurality of transport rolls 17 are installed for continuing to transport the slab 28. A slab cutter 32 for cutting the slab 28 is disposed above the transport rolls 17. After solidification is complete, the slab 28 is cut into slabs 28a of a predetermined length by the slab cutter 32.

[0015] 2 is a cross-sectional schematic diagram showing a portion of the mold 12 and the cooling water supply equipment 22. The mold 12 has four mold copper plates 40 arranged to form a roughly rectangular tube shape according to the shape of the cast slab 28 to be cast, and four back plates 50 arranged on the outside of the mold copper plates 40. The mold copper plates 40 are made of a copper alloy or the like, and have a plurality of slits 42 on the back surface that extend in the casting direction and are aligned in the width direction.

[0016] The back plate 50 is formed with a cooling water supply port 52 and a cooling water discharge port 54. The four back plates 50 are fixed to the four mold copper plates 40, respectively, by fastening means such as bolts. By fixing the mold copper plates 40 and the back plates 50 together, a cooling water channel 56 is formed.

[0017] The cooling water supply system 22 includes a circulating water tank 60, a bubble generator 62, and a cooling water transport device 64. The cooling water used to cool the mold 12 is cooled in a cooling tower or the like (not shown) and then used again to cool the mold 12. The bubble generator 62 injects bubbles having a diameter of 0.1 mm or less into the cooling water cooled in the cooling tower or the like. The cooling water containing the bubbles is stored in the circulating water tank 60. The bubble generator 62 is, for example, a pressurized dissolution type bubble generator. The cooling water transport device 64 supplies the cooling water in the circulating water tank 60 to the cooling water channel 56 of the mold 12. As a result, the cooling water containing bubbles having a diameter of 0.1 mm or less is supplied to the cooling water channel 56. The cooling water transport device 64 is, for example, a pump.

[0018] The cooling water in the mold 12 flows through a very narrow cooling slit (cooling water channel 56) at a high speed of 7 m / s or more. Forced convection boiling occurs only near the cooling slit surface, and therefore, a faster cooling water flow rate improves cooling capacity. However, if the cooling water contains bubbles with a diameter of 0.1 mm or less, the bubbles generated by boiling at the cooling surface do not disappear with subsequent cooling by the bulk cooling water, leading to a nucleate boiling region, thereby achieving significantly greater cooling capacity. Compared to cooling in an open space, such as that typically used for steel cooling, it is difficult to achieve the cooling capacity improvement effect of nucleate boiling due to the presence of bubbles in a closed space. However, when the mold is cooled with cooling water containing bubbles with a diameter of 0.1 mm or less, the bubbles present in the cooling water channel 56 do not disappear and remain, significantly improving cooling capacity.

[0019] Figure 3 is a cross-sectional schematic diagram showing another example of cooling water supply equipment. When the bubble generator 62 is a pressure dissolution type bubble generator, the bubble generator 62 may be installed in the circulating water tank 60, as shown in Figure 3. The bubble generator 62 injects bubbles having a diameter of 0.1 mm or less into the cooling water stored in the circulating water tank 60. The cooling water transfer device 64 supplies the cooling water in the circulating water tank 60 to the cooling water channel 56 of the mold 12. As a result, cooling water containing bubbles having a diameter of 0.1 mm or less is supplied to the cooling water channel 56.

[0020] 2 and 3 show an example in which the bubble generator 62 is a pressurized dissolution type bubble generator, but the present invention is not limited to this. Various microbubble generators and ultra-file bubble generators with different bubble generation methods may be used as the bubble generator 62.

[0021] 4 is a cross-sectional schematic diagram showing another example of a cooling water supply facility. When a loop-type microbubble generator is used as the bubble generator 62, the bubble generator 62 is provided at the end of a water supply pipe through which cooling water cooled by a cooling tower or the like is transported. In this way, the bubble generator 62 may be provided at various positions in the water supply facility 22 depending on the bubble generation method of the device.

[0022] Next, an experiment to confirm the relationship between the diameter and number density of bubbles contained in cooling water and the heat transfer coefficient of the cooling water will be described. Fig. 5 is a schematic diagram of the experimental apparatus used to confirm the heat transfer coefficient of the cooling water. As shown in Fig. 5, in the experiment, the copper plate 72 of a water-cooled copper plate 70 including the copper plate 72 and the cooling water channel 74 was heated by a C gas burner 76. Then, cooling water containing bubbles with different diameters and number densities was passed through the cooling water channel 74, and the temperatures T1 and T2 at two positions d1 and d2 in the copper plate 72 and the cooling water temperature T W The heat transfer coefficient of the cooling water was calculated using the following equation (1): The diameter and number density of the bubbles injected into the cooling water were measured using the particle tracking method.

[0023]

number

[0024] Figure 6 is a graph showing the relationship between the diameter of bubbles and the heat transfer coefficient of the cooling water for each number density of bubbles contained in the cooling water. The horizontal axis of Figure 6 is the diameter of the bubbles (×10 -6 m), and the vertical axis is the heat transfer coefficient ratio (-) of the cooling water. The heat transfer coefficient ratio of the cooling water is the ratio of the heat transfer coefficient of each condition, with the heat transfer coefficient of cooling water at 18°C ​​without air bubbles set to 1.00, and was calculated by dividing the heat transfer coefficient of each condition by the heat transfer coefficient of cooling water at 18°C ​​without air bubbles. Note that (-) means that the value is dimensionless.

[0025] As shown in Figure 6, the heat transfer coefficient of the cooling water containing bubbles with a diameter of 0.1 mm or less was higher than that of the cooling water without bubbles. It is believed that bubbles with a diameter of 0.1 mm or less have the effect of stirring and reducing the thermal boundary layer formed at the boundary between the copper plate 72 and the cooling water channel 74. It is believed that this reduction in the thermal boundary layer resulted in the increase in the heat transfer coefficient of the cooling water.

[0026] On the other hand, when the heat transfer coefficient ratio of cooling water containing 1 mm diameter bubbles was measured, the heat transfer coefficient ratio was 1.00, which was the same as that of cooling water without bubbles. As shown above, large bubbles with diameters greater than 0.1 mm floated up and disappeared from the cooling water, failing to agitate the thermal boundary layer. This is thought to be why the heat transfer coefficient was the same as that of cooling water without bubbles. These results suggest that to increase the heat transfer coefficient of cooling water, it is necessary to include bubbles with diameters of 0.1 mm or less. As shown in Figure 6, the smaller the bubble diameter, the higher the heat transfer coefficient of the cooling water. Therefore, there is no need to set a lower limit on the diameter of the bubbles contained in the cooling water in order to increase the heat transfer coefficient of the cooling water.

[0027] Furthermore, when we looked at the number density of bubbles, we found that the heat transfer coefficient of the cooling water increased when the number density of bubbles contained in the cooling water was 10 million / L or more. Furthermore, the heat transfer coefficient of the cooling water increased significantly when the number density of bubbles was 100 million / L or more. These results confirmed that it is preferable for the cooling water to contain 10 million / L or more bubbles with a diameter of 0.1 mm or less, and it is even more preferable for the cooling water to contain 100 million / L or more bubbles with a diameter of 0.1 mm or less.

[0028] Referring again to FIG. 2 , cooling water from a circulating water tank 60 containing bubbles with a diameter of 0.1 mm or less is supplied to the cooling water passage 56 of the mold 12 by a cooling water conveying device 64. As described above, the inclusion of bubbles with a diameter of 0.1 mm or less in the cooling water used to cool the mold 12 increases the heat transfer coefficient of the cooling water. Therefore, cooling the mold 12 with this cooling water increases the amount of heat removed from the mold 12. Increasing the amount of heat removed from the mold 12 in this manner allows the required thickness of the solidified shell 24 to be achieved in a shorter time than conventional methods. Therefore, the implementation of the continuous steel casting method according to this embodiment reduces the risk of breakout during high-speed casting. Furthermore, because the cooling water containing air acts as a refrigerant, stability and safety comparable to conventional methods can be ensured even at the high temperatures required for continuous steel casting.

[0029] In this embodiment, an example has been described in which the cooling water supply facility 22 has the circulating water tank 60, but the cooling water supply facility 22 does not have to have the circulating water tank 60. In this case, the bubble generator 62 may inject bubbles having a diameter of 0.1 mm or less into the cooling water in the supply path of the cooling water supplied to the cooling water passage 56. [Example]

[0030] Examples of the present invention are described below. Test casting of medium-carbon steel (chemical composition: C: 0.08-0.17% by mass, Si: 0.10-0.30% by mass, Mn: 0.50-1.20% by mass, P: 0.010-0.030% by mass, S: 0.005-0.015% by mass, Al: 0.020-0.040% by mass) was carried out at a casting speed of 2.6 m / min. Test casting (Invention Examples 1-24) was carried out by varying the diameter and number density of bubbles contained in the cooling water. Thermocouple temperatures in the mold and changes in the cooling water temperature in the mold during continuous casting were measured, and the heat transfer coefficient of the cooling water was calculated using Equation (1) above. The diameter and number density of bubbles contained in the cooling water were also confirmed using a particle tracking method. The heat transfer coefficient ratios of the cooling water for Inventive Examples 1-24 are shown in Table 1 below. The heat transfer coefficient ratios shown in Table 1 are also ratios showing the heat transfer coefficients of invention examples 1 to 24, with the heat transfer coefficient of cooling water at 18°C ​​without air bubbles set to 1, and were calculated by dividing the heat transfer coefficients of invention examples 1 to 24 by the heat transfer coefficient of cooling water at 18°C ​​without air bubbles.

[0031] [Table 1]

[0032] As shown in Table 1, in all examples of the present invention in which bubbles with a diameter of 0.1 mm or less were included, the heat transfer coefficient ratio was greater than 1.00. These results confirmed that by including bubbles with a diameter of 0.1 mm or less in the cooling water, the heat transfer coefficient of the cooling water can be improved compared to conventional examples in which the cooling water did not include bubbles. In particular, by increasing the number density of the bubbles included in the cooling water to 100 million bubbles / L or more, the heat transfer coefficient ratio of the cooling water significantly increased. These results confirmed that by including bubbles with a diameter of 0.1 mm or less in the cooling water at 100 million bubbles / L or more, the heat transfer coefficient of the cooling water can be further improved.

[0033] The higher the bubble density in the cooling water, the higher the heat transfer coefficient of the cooling water. This is presumably because, as the bubble density in the cooling water increases, the ability of the bubbles to agitate the thermal boundary layer on the mold heating surface in the cooling water slit of the mold increases, and the thermal boundary layer decreases, thereby increasing the heat transfer coefficient of the cooling water. This effect is thought to be the reason why the heat transfer coefficient of the cooling water increases significantly when the bubble density is increased to 100 million bubbles / L or more. Therefore, it is clear that it is preferable to inject bubbles into the cooling water at a higher density.

[0034] On the other hand, when the same test casting was carried out using cooling water containing 1 mm diameter bubbles as a comparative example, the heat transfer coefficient ratio of the cooling water was 1.00, which was the same as the heat transfer coefficient in the conventional example where the cooling water did not contain bubbles. From this result, it was confirmed that the diameter of the bubbles contained in the cooling water needs to be 0.1 mm or less.

[0035] In this way, it was confirmed that the heat transfer coefficient of the cooling water used to cool the mold can be improved by adding bubbles with a diameter of 0.1 mm or less to the cooling water. Furthermore, by cooling the mold with this cooling water, the amount of heat removed from the mold can be increased, and the required solidified shell thickness can be achieved in a shorter time than before, which reduces the risk of breakout during high-speed casting. [Explanation of symbols]

[0036] 10 Continuous casting equipment 12 Mold 14 Tundish 16. Casting strip support roll 18 Molten Steel 20 Submerged Entry Nozzle 22 Cooling water supply equipment 24 Solidified shell 26 Unsolidified layer 28 Castings 30 Secondary cooling zone 32 Slab cutting machine 40 Cast copper plate 42 Slit 50 backplate 52 Cooling water supply port 54 Cooling water outlet 56 Cooling channel 56 60 Circulating water tank 62 Bubble Generator 64 Cooling water conveying device 70 Water-cooled copper plate 72 Copper plate 74 Cooling Channel 76 C gas burner

Claims

1. A method for continuously casting steel using continuous casting equipment, comprising: A method for continuously casting steel, comprising supplying cooling water containing bubbles having a diameter of 0.1 mm or less at a number density of 100 million bubbles / L or more to a cooling water channel of a mold of the continuous casting equipment.

2. A cooling water supply facility for supplying cooling water to a mold of a continuous casting facility, a cooling water conveying device for supplying cooling water to the mold; a bubble generator that injects bubbles having a diameter of 0.1 mm or less into the cooling water at a number density of 100 million bubbles / L or more; A cooling water supply facility having:

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