JP4337739B2 - Continuous casting method for molten steel - Google Patents

Description

  The present invention relates to a method for continuously casting molten steel while secondary cooling of a slab surface from directly under a mold to a downstream side in a casting direction.

  In order to improve the subsurface quality and internal quality of the continuous cast slab, it is necessary to optimize the cooling ability (secondary cooling ability) in the secondary cooling zone by spraying below the continuous casting mold.

  In order to improve the quality of the epidermis, it is necessary to suppress the crack in the surface of the slab (that is, the crack on the surface of the slab) that occurs immediately below the continuous casting mold. Therefore, it is necessary to set the secondary cooling capacity to a cooling capacity suitable for the steel type. In addition, in order to suppress non-uniform deformation (surface irregularities) on the surface of the slab caused by uneven cooling in the width direction of the slab, it is also necessary to perform uniform cooling in the width direction.

  As an internal defect of a slab, there is an internal crack generated near the center in the thickness direction of the slab due to solidification shrinkage or thermal stress deformation of a solidified shell of the slab. Since the frequency of occurrence of internal cracks also depends on the steel type, it is necessary to perform cooling suitable for the steel type in order to prevent internal cracks.

  In addition, there is a demand for higher casting speed, but in order to proceed with high-speed casting, it is necessary to secure the thickness of the solidified shell below the mold, and secondary cooling while improving the controllability of the cooling capability described above. It is necessary to improve performance.

  In order to change the cooling capacity of the secondary cooling zone below the continuous casting mold, conventionally, the type of water spray nozzle was changed to change the water amount and water amount distribution of the water spray. In addition, the cooling ability has been improved by mist spray using water and compressed air.

  However, the cooling characteristics of the spray are determined by the shape and size of the spray holes, and the amount of water and the amount of compressed air are also determined accordingly. For this reason, in the conventional method, the cooling capacity cannot be changed during continuous casting corresponding to the steel type, and it is difficult to change the cooling capacity in the slab width direction after setting the spray. In addition, mist spray can be uniformly cooled and can prevent nozzle clogging, so it is frequently used in secondary cooling of continuous cast slabs, but it costs high to produce compressed air and has the disadvantage of high noise. There is. For this reason, there is a practical upper limit in improving the cooling capacity, and it is difficult to obtain a high cooling capacity.

  On the other hand, below the continuous casting mold, the surface of the slab is oxidized to form a scale, and if the scale remains in the subsequent hot rolling and cold rolling, the indentation during rolling is performed. May cause wrinkles in the product. Therefore, conventionally, when the slab is completely solidified, the surface layer of the slab is melted with oxygen gas, and the scale of the slab surface is removed by high-pressure water spray. However, it is not easy to completely remove the scale, oxygen gas is required, a large amount of electric power is required to increase the pressure of the water, and there is a problem that the cost for that is high.

  By the way, when aluminum or aluminum alloy ingots are continuously cast, deformation due to rapid water cooling of the slab drawn from the mold at the start of casting (warping of the bottom of the slab), cracks due to it, breakout, etc. In order to suppress generation | occurrence | production, the method of mixing the substance which decomposes | disassembles and produces | generates a carbon dioxide gas or a carbon dioxide gas in secondary cooling water, and the method of suppressing the intensity | strength of secondary cooling is known.

  For example, Patent Document 1 discloses a continuous casting method of an ingot of aluminum or an alloy thereof using cooling water in which carbon dioxide is dissolved, and Patent Document 2 discloses decomposition on a hot steel plate surface in continuous casting of an aluminum alloy. A method for cooling a slab using cooling water in which sodium bicarbonate, ammonium bicarbonate, and the like that generate carbon dioxide gas are dissolved is described.

  Furthermore, Patent Document 3 discloses cooling water in which a predetermined amount of carbon dioxide gas is injected into cooling water having a pH of 5.0 or less by adding aluminum sulfate or the like (the amount of dissolved carbon dioxide is small, and a relatively small slow cooling effect). Cooling water in which carbon dioxide gas is bonded to an alcohol group to increase the amount of carbon dioxide dissolved and ethylene carbonate or the like is added, and a predetermined amount of carbon dioxide gas is injected into it (a relatively large slow cooling effect is obtained) ) Has been proposed for injecting into an ingot of aluminum alloy or the like at the start of casting.

Non-Patent Document 1 shows the solubility of various gases in water according to temperature. Carbon dioxide (CO ₂ ) has a very high solubility in water at room temperature, and hardly dissolves at a high temperature of 80 ° C. or higher.

  The effect of reducing the cooling capacity of cooling water containing carbon dioxide gas, etc. is that when the cooling water in which carbon dioxide gas is dissolved touches the slab, carbon dioxide is released and a gas film is formed on the surface of the slab. It is. In Non-Patent Document 2, a film that prevents contact between the water droplet and the heating plate is formed by the gas released from the lower surface of the water droplet in which the carbon dioxide gas placed on the heating plate at 300 ° C. is dissolved. Is described.

  The method of containing carbon dioxide gas or the like in the secondary cooling water has not been conventionally used for continuous casting of steel. This is because in continuous casting of steel, there is generally a strong demand for high-speed casting, and the secondary cooling capacity is improved, whereas in continuous casting of aluminum, the strength of cooling is reduced at the start of casting. This is because the property (characteristic) of the cooling water containing carbon dioxide gas that the cooling ability is reduced by dissolving carbon dioxide gas or the like in the cooling water is used.

Japanese Patent Laid-Open No. 58-212849

JP 58-93547 A Japanese Patent No. 3408222 "Science Chronology 2004," issued November 30, 2003 Maruzen Co., Ltd., page 501 [Gas solubility in water] Journal of Heat Transfer, Vol. 123 (2001), p. 719-728 “The Effect of Dissolving Gases or Solids in Water Droplets Boiling on a Hot Surface”

  The object of the present invention is to improve the water spray cooling capacity in the secondary cooling zone below the continuous casting mold, control the cooling capacity, perform uniform cooling, and the slab surface and cracks inside the slab. Provided is a continuous casting method for molten steel that can reduce non-uniform deformation in the width direction (unevenness on the slab surface) and can perform secondary cooling that is also effective in removing oxidized scales formed below the mold. There is.

  In order to increase the cooling ability of the water spray, a method of removing heat using a spray of large water droplets can be considered. As a result of repeated studies to solve the above problems, the present inventor has found that when a soluble gas (carbon dioxide) is dissolved in the spray cooling water in the secondary cooling zone, the size of the water droplets of the spray becomes that of the conventional water spray. It became larger than water droplets, improving cooling ability, and cooling water was dispersed on the slab surface to make the cooling uniform, and at the same time, it was found that it was effective in removing oxide scale generated on the slab surface .

  This invention is made | formed based on such knowledge, The summary exists in the continuous casting method of the following molten steel.

That is, a method of continuously casting molten steel while directly cooling the slab surface from directly under the mold to the downstream side in the casting direction, and soluble in the secondary cooling water in the cooling water supply pipe from the soluble gas supply pipe Gas is supplied and dissolved, or soluble gas is supplied in advance to the secondary cooling water in the cooling water supply tank and dissolved, and the secondary cooling water in which this soluble gas is dissolved is discharged from the spray nozzle to the slab. This is a continuous casting method of molten steel that is injected and collided with the surface to increase the cooling capacity of secondary cooling water .

  In this continuous casting method, as the soluble gas, a gas composed of one or two of carbon dioxide gas and nitrogen gas is industrially easy to use.

  The secondary cooling is cooling performed by directly spraying spray water onto the surface of the solidified shell grown in the mold in the secondary cooling zone below the continuous casting mold.

  According to the continuous casting method of molten steel of the present invention, the secondary cooling ability under the mold is increased to enable uniform cooling, and the cracks on the surface of the slab and uneven deformation in the width direction (unevenness on the surface of the slab) are reduced. In addition, it is possible to suppress internal cracks and remove oxidized scale on the surface of the slab to reduce the amount of scale produced.

  Hereinafter, the continuous casting method of the molten steel of this invention is demonstrated with reference to drawings.

  FIG. 1 is a diagram schematically showing a configuration of a main part of an apparatus capable of carrying out the continuous casting method of the present invention. As shown in the figure, below the continuous casting mold 2, there is disposed a roll 5 for holding a solidified shell 4 of a slab formed by solidification of molten steel 3 supplied from the immersion nozzle 1. A spray nozzle 6 is disposed between them. The spray nozzle 6 is connected to a cooling water supply pipe 7, and the cooling water supply pipe 7 is connected to a soluble gas supply pipe 8 for supplying and dissolving the soluble gas to the cooling water.

In the continuous casting method of the present invention, soluble gas is dissolved as cooling water when continuously casting molten steel while directly cooling the surface of the slab (solidified shell 4) from directly below the mold 2 to the downstream side in the casting direction. Use fresh water. The soluble gas is supplied and dissolved in the cooling water in the cooling water supply pipe from the soluble gas supply pipe, or is supplied and dissolved in advance in the cooling water in the cooling water supply tank. The cooling water in which the soluble gas is dissolved is sprayed from the spray nozzle onto the surface of the slab and collides with it.

FIG. 2 is a graph showing the data on the solubility of carbon dioxide (CO ₂ ) in water described in Non-Patent Document 1 described above with the temperature on the horizontal axis. As shown in the figure, the amount of carbon dioxide dissolved at a temperature of 25 ° C. is as large as about 0.75 by volume, and a large amount of carbon dioxide dissolves in water. The amount of dissolution decreases as the temperature rises, and hardly dissolves at a high temperature of 80 ° C. or higher.

  In this way, by dissolving the soluble gas in the cooling water, it is possible to inject (impact) a spray of water droplets larger than the water droplets of the conventional water spray onto the slab, increasing the cooling capacity and achieving uniform cooling. (That is, not only in the area where the water droplet collides, but also in the wide area including the surrounding area). In other words, when the water droplet collides with the slab (in the moment of collision), the slab is cooled in a state where carbon dioxide is dissolved and is not released, so that a high cooling capacity is ensured, and a temperature is passed after a short time after the collision. As the solubility rises rapidly, carbon dioxide gas is violently released from the cooling water (in other words, “cooling water bursts” state), and the cooling water spreads and spreads on the slab surface. Presumed to be possible. It is known that a large volume expansion is accompanied when the cooling water bursts.

The release of the carbon dioxide, heating plate in Non-Patent Document 2 cited above (300 ° C.) reports have been made about the behavior of the water droplets containing dissolved carbon dioxide placed on, due to the increase in temperature of the CO ₂ It is described that carbon dioxide gas is rapidly released from water droplets as the solubility decreases.

  Here, while using water containing carbon dioxide gas as cooling water, the effect differs between the continuous casting of steel and the continuous casting of aluminum as described above (that is, the former has improved cooling capacity). The reason why the cooling is uniformly performed while the intensity of the secondary cooling is suppressed in the latter is considered to be because the way of supplying cooling water to the slab surface is different.

  In the continuous casting of aluminum, a slit for discharging the cooling water used for cooling the mold is provided at the lower end of the water-cooled mold, and the cooling water once enters the mold and is then cast into the slab from the slit. It is directly supplied as secondary cooling water to the slab drawn from the mold. In this case, since the purpose is to suppress the strength of the secondary cooling, the cooling water is not sprayed on the slab surface vigorously, but is spilled on the slab surface. According to this supply method, when the cooling water comes into contact with the hot slab and carbon dioxide is released, the gas that prevents the cooling water and the slab from contacting with each other as described above by the gas released from the portion in contact with the slab. A film is formed, heat conduction is hindered, and the strength of secondary cooling is suppressed. In continuous casting of aluminum, the application of this cooling method is limited at the start of casting where it is necessary to suppress the strength of cooling.

  On the other hand, in the continuous casting of steel, the purpose is to increase the cooling capacity, to control this, and to perform the cooling uniformly, so that the slab is formed using the cooling water supply pipe 7 shown in FIG. The cooling water is sprayed vigorously from the lateral direction. In this case, as described above, the temperature of the cooling water (water droplets) rises after a short time after the collision with the slab surface, carbon dioxide gas is violently released, and the cooling water is dispersed and spreads on the slab surface. . Since the injected cooling water collides violently with the surface of the slab, the gas film is not formed, and it is considered that the cooling capacity is remarkably increased. In continuous casting of steel, this cooling method is applied during casting and is not limited at the start of casting.

  According to the above-described continuous casting method of the present invention, it is possible to increase the cooling capability in the secondary cooling zone below the mold, which has been difficult in the past, and to perform uniform cooling, and it is possible to cope with high-speed casting.

  In the continuous casting method of the present invention, it is possible to control the secondary cooling capacity by changing the amount of the soluble gas dissolved in the cooling water to change the degree of release of the soluble gas. Therefore, the secondary cooling ability can be optimized according to the steel type, and the surface and internal quality can be improved by suppressing cracks and internal cracks on the surface of the slab.

  In addition, if the apparatus shown in FIG. 1 is used, it is possible to control the cooling capacity by changing the amount of soluble gas supplied to the cooling water supply pipe 7, thereby suppressing cracks on the slab surface and inside, and spraying. By changing the amount of soluble gas added to the cooling water for each pipe to properly control the cooling capacity in the width direction of the slab, it is possible to reduce uneven deformation in the width direction of the slab (unevenness on the surface of the slab) Is possible. These secondary cooling capacities can be controlled even during casting.

  Furthermore, when the secondary cooling water in which the soluble gas is dissolved collides with the surface of the hot slab, the cooling water enters the gap between the slab and the scale, and the temperature of the intruding cooling water rises to increase the temperature of the cooling water. Since the soluble gas is vaporized and the pressure in the gap increases due to volume expansion, the scale can be peeled from the surface of the slab by this force. Therefore, even if the slab surface is oxidized under the mold and scale is generated, it is peeled off by the injection of secondary cooling water, so that the generation of scale is apparently suppressed, and the burden of scale removal in the subsequent process is reduced. It is reduced.

  As the soluble gas dissolved in the secondary cooling water in the continuous casting method of the present invention, it is considered that hydrogen, ammonia and the like can be applied in addition to the carbon dioxide gas and the nitrogen gas. However, carbon dioxide gas or nitrogen gas, or a mixed gas thereof, which is industrially easily used, is desirable.

  By using the apparatus having the configuration shown in FIG. 1 and applying the continuous casting method of the present invention, an extremely low carbon steel (C content: 0.002 mass%) molten steel having a width of 1500 mm and a thickness of 200 mm is used. A casting test was performed on the slab, and the effect of suppressing cracks on the surface of the slab, the effect of suppressing uneven deformation in the width direction of the slab (unevenness on the surface of the slab), and the effect of suppressing scale formation were investigated. The casting speed was 1.3 m / min.

  For comparison, the same investigation was also performed when no soluble gas was added to the secondary cooling water.

  The conditions for secondary cooling are as follows.

Number of spray nozzles: 35 in the width direction of the slab and 25 stages in the casting direction Spray water volume: 30 liters / min. Nozzle Soluble gas: Carbon dioxide gas Soluble gas addition amount: 0 in volume ratio with respect to 1 cooling water .5 addition.

  The casting test was performed as follows. That is, the molten steel 3 is supplied from the immersion nozzle 1 to the continuous casting mold 2. On the other hand, carbon dioxide gas is supplied from the soluble gas supply pipe 8 to the secondary cooling water in the cooling water supply pipe 7 to be dissolved, and this cooling water is sprayed from the spray nozzle 6. The solidified shell 4 grown in the mold 2 is held by the roll 5, and the secondary solid cooling is performed, and as the casting distance increases, the unsolidified region decreases and finally solidifies completely. In this case, since the cooling water in the cooling water supply pipe 7 is in a pressurized state, in order to dissolve the gas in the cooling water, it is necessary to increase the gas pressure higher than the pressure acting on the cooling water. .

  In this embodiment, the soluble gas supply pipe is connected to the cooling water supply pipe to dissolve the soluble gas in the cooling water. However, a cooling water supply tank is installed, and the cooling water in the tank is Alternatively, the soluble gas may be dissolved in advance and the cooling water may be sprayed onto the solidified shell. At this time, it is necessary to use a pressure vessel in which the tank itself can withstand this pressure (the pressure of the gas during dissolution).

  Table 1 shows the results of investigations on the surface crack suppression effect of the slab, the non-uniform deformation suppression effect in the slab width direction, and the scale generation suppression effect performed in the casting test.

  The surface crack of the slab is a visible crack such as a vertical crack and a horizontal crack, and the total crack length per 1 m of the slab length was measured. The total crack length when no soluble gas was added to the secondary cooling water was defined as a slab surface crack index 1, and the slab surface crack condition when a soluble gas was added was evaluated by the index.

  The unevenness on the surface of the slab was measured at one place of ½ width (width center) of the slab and two places of ¼ width (three places in total) for a slab length of 1 m, and the average value was calculated. Similarly, when the addition of the soluble gas was not performed, the slab nonuniformity index 1 was evaluated, and when the soluble gas was added, the index was evaluated.

  Moreover, the scale produced | generated per 1 m of slabs was peeled off using the wire brush etc., and the mass was measured. Similarly, the scale generation index was 1 when no soluble gas was added, and the index was evaluated when soluble gas was added.

  As apparent from the results shown in Table 1, according to the continuous casting method of the present invention in which a soluble gas is added to the secondary cooling water, cracks such as vertical cracks and transverse cracks on the surface of the slab, and the width of the slab The non-uniform deformation in the direction (unevenness on the slab surface) was remarkably reduced, and the amount of scale generation was also greatly reduced.

  According to the continuous casting method of molten steel of the present invention, the secondary cooling ability under the mold is enhanced, the surface cracks and internal cracks of the slab are suppressed, and uneven deformation in the width direction of the slab (unevenness on the surface of the slab) In addition, the oxide scale on the surface of the slab can be eliminated by injection of secondary cooling water to suppress the generation thereof. Therefore, this method can be widely used as a continuous casting method in which a slab having high quality and a small amount of scale is obtained.

It is a figure which shows typically the structure of the principal part of the apparatus which can implement the continuous casting method of this invention. Is a diagram showing the solubility versus temperature for water carbon dioxide (CO _2).

Explanation of symbols

1: Immersion nozzle 2: Continuous casting mold 3: Molten steel 4: Solidified shell 5: Roll 6: Spray nozzle 7: Piping for cooling water supply 8: Piping for soluble gas supply

Claims (2)

A method of continuously casting molten steel while directly cooling the slab surface from directly under the mold to the downstream side in the casting direction,
The soluble gas is supplied from the soluble gas supply pipe to the secondary cooling water in the cooling water supply pipe and dissolved, or the soluble gas is supplied in advance to the secondary cooling water in the cooling water supply tank. Dissolve,
A continuous casting method for molten steel, characterized in that the secondary cooling water in which the soluble gas is dissolved is sprayed from the spray nozzle onto the surface of the slab to be collided to enhance the cooling capacity of the secondary cooling water .   The method for continuous casting of molten steel according to claim 1, wherein the soluble gas is a gas composed of one or two of carbon dioxide and nitrogen.

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