JP2011200893A - Secondary cooling method in continuous casting - Google Patents

Abstract

PROBLEM TO BE SOLVED: To uniformly cool a slab during casting in the secondary cooling zone of continuous casting equipment without causing an overcooling phenomenon in the surface of the slab.SOLUTION: When a slab 10 is subjected to secondary cooling while supporting the same by split type slab supporting rolls 6 split into two or more in the width direction of the slab, a nozzle for gas blowing is arranged directly below the downstream side of the roll chock 6a of each split type slab supporting roll, in the secondary cooling zone of at least a part of a vertical part or at least a part of a curved part in the continuous casting machine, a gas is jetted from the nozzle under the jet pressure of ≥0.1 MPa or at a jet flow rate of ≥400 NL/min per nozzle, at least a part of the jetted gas is made into a gas flow going in a gap between each roll chock and the slab from the downstream side in the casting direction to the upstream side in the casting direction, and the flowing down of the remaining water of the cooling water stored in the surface of the slab during continuous casting in the gap between each roll chock and the slab is suppressed by the gas flow.

Description

  The present invention relates to a method for cooling a slab in a secondary cooling zone of a continuous casting facility, which can uniformly cool the slab even during high-speed casting.

  In continuous casting of steel, the molten steel in the ladle is once poured into the tundish, and with a predetermined amount of molten steel staying in the tundish, the molten steel in the tundish is passed through an immersion nozzle installed at the bottom of the tundish. And injected into the mold. The molten steel injected into the mold is cooled to form a solidified shell on the contact surface with the mold, and this slab with the solidified shell as the outer shell and the unsolidified molten steel inside is a secondary provided below the mold. In the cooling zone, it is continuously drawn down below the mold while being cooled by cooling water (also referred to as “secondary cooling water”) sprayed on the surface of the slab, and eventually solidification to the center is completed. A slab which is a raw material for rolling is manufactured by cutting a slab that has been solidified to the center to a predetermined length.

  If non-uniform cooling occurs in the secondary cooling zone, cracks will occur on the surface and inside of the slab, or the center segregation at the center of the slab will deteriorate, so it is uniform in the casting direction and width direction of the slab. Cooling has been proposed and implemented. In this case, since the slab slab is wide and it is necessary to arrange a plurality of spray nozzles in the width direction, non-uniform cooling tends to occur in the width direction, and in particular, uniform cooling is performed in the slab width direction. It becomes important.

  For example, in Patent Document 1, a plurality of injection holes are provided at the tip of a spray nozzle, and the surface of a slab is cooled with a plurality of parallel flat spray water sprayed from the injection holes between adjacent rolls. It is disclosed. According to Patent Document 1, since cooling is performed with a plurality of spray water, the temperature difference between cooling and recuperation is reduced, and repeated thermal stress is reduced accordingly, and surface cracks of the slab are reduced. .

  In Patent Document 2, the water distribution in the slab drawing direction of the cooling water sprayed from the spray nozzle, where A and B are points that are 20% of the maximum portion of the water distribution, the distance between A and B Then, it is disclosed that the slab is cooled by using a spray nozzle having an angle ACB of 30 degrees or more when the water amount distribution of 20% or more of the maximum portion is continuous and the spray nozzle center of the spray nozzle is C. ing. According to Patent Document 2, the cooling ability for the slab can be efficiently increased.

In Patent Document 3, cooling is performed by spraying cooling water with a water supply pressure of 25 to 100 kgf / cm ² (2.5 to 9.8 MPa) on a slab through a water supply mechanism including a booster pump in a pressurizing system. However, continuous casting is disclosed. According to Patent Document 3, the splash of cooling water that has collided with the slab is atomized, the generation of partially accumulated water on the surface of the slab is prevented, and partial overcooling is prevented, so that uniform cooling is achieved. Is supposed to be realized.

  Further, in Patent Document 4, in a vertical bending type continuous casting machine in which a secondary cooling zone is installed in a vertical portion, casting by secondary cooling water flowing along the slab surface at a curved portion below the secondary cooling zone. In order to prevent non-uniform cooling of the pieces, a method is disclosed in which compressed air is blown onto the slab at the outlet of the secondary cooling zone and the secondary cooling water staying on the slab surface is blown away.

Japanese Patent Laid-Open No. 50-103426 JP 2003-136205 A JP-A-57-91857 JP 2004-223526 A

  In continuous casting of steel, the surface temperature of the slab in the secondary cooling zone is generally controlled to 700-1000 ° C. With the recent increase in casting speed, the ability of secondary cooling has been strengthened, The slab surface temperature inside tends to decrease in general. In addition, with the increase in casting speed, a phenomenon in which a portion having a surface temperature lower than 700 ° C. locally occurs in the slab (referred to as “supercooling phenomenon”) has come to occur. As for the surface temperature of the slab where the supercooling phenomenon occurs, temperature unevenness occurs in the slab width direction.

  This supercooling phenomenon is a phenomenon in which the surface temperature of the slab during casting decreases due to supercooling in the secondary cooling zone. FIG. 1 shows the occurrence of temperature unevenness in the supercooled slab slab (FIG. 1 (A )) And the occurrence of surface cracks in the slab slab after cooling (FIG. 1B). As shown in FIG. 1, when the cast slab is observed, it can be seen that surface cracks are concentrated on the portion where the temperature unevenness occurs. In addition, the present inventors pursued the cause of the occurrence of this supercooling phenomenon, the occurrence of temperature unevenness of the slab, and the secondary cooling water staying on the surface of the slab (the remaining cooling water is referred to as “residual water”). It is known that there is a correlation with the water temperature of

  If the above prior arts are verified from the viewpoint of preventing this overcooling phenomenon, that is, temperature unevenness, none of the above prior arts is effective in preventing the overcooling phenomenon or is effective even if effective.

  That is, Patent Document 1 increases the spray water injection area, that is, the cooling area to prevent overcooling, but uses a flat spray nozzle, and as long as cooling is performed using only the flat spray nozzle, Since the collision pressure is strong and the amount of secondary cooling water is large, the generation of residual water cannot be prevented, and the occurrence of the supercooling phenomenon under high-speed casting cannot be prevented.

  Patent Document 2 is a spray nozzle with a wider injection angle in the casting direction. Compared with the flat spray nozzle of Patent Document 1, the collision pressure during cooling can be reduced, so that the supercooling phenomenon is unlikely to occur. Become. However, as long as cooling is performed by a conventional cooling method using a spray nozzle, the generation of residual water cannot be prevented. If the casting speed is increased, a supercooling phenomenon occurs.

In Patent Document 3, the generation of residual water is prevented by injecting high-pressure secondary cooling water of 25 to 100 kgf / cm ² into the slab to prevent generation of residual water and uniform cooling. It is suppressed and the effect of preventing the supercooling phenomenon is exhibited. However, in the continuous casting machine for steel, the length of the secondary cooling zone reaches 20m from the long one to 50m, and cooling with high-pressure water in all secondary cooling zones is not only an equipment cost but also an operation. The cost is high, and even if it is limited only to the secondary cooling zone in the upstream part, the operation cost is high and it is not practical.

  In Patent Document 4, residual water on the surface of the slab is removed at the outlet of the secondary cooling zone, and the supercooling phenomenon occurs in the secondary cooling zone. Even if water is removed, there is no effect in preventing the supercooling phenomenon.

  The present invention has been made in view of the above circumstances, and its object is to generate a supercooling phenomenon on the surface of the slab when cooling the slab during casting in the secondary cooling zone of the continuous casting facility. It is providing the secondary cooling method which can cool a slab uniformly.

  The secondary cooling method in continuous casting according to the present invention for solving the above problems is a split-type slab support roll in which a slab cast by a continuous casting machine is divided into two or more in the slab width direction. At the time of secondary cooling using a cooling water or a mixture of cooling water and air in a secondary cooling zone provided below the mold while supporting, at least a part of a vertical part of the continuous casting machine or at least a curved part In some secondary cooling zones, a gas blowing nozzle is arranged immediately downstream of the roll chock of the split mold slab support roll, and an injection pressure of 0.1 MPa or more from the nozzle or 400 NL / min or more per nozzle The gas is injected at an injection flow rate of at least a part of the injected gas, and the gap between the roll chock and the slab is formed as a gas flow from the casting direction downstream side to the casting direction upstream side. Collects in the slab surface during continued casting, the residual water in the cooling water, characterized in that the secondary cooling of the cast strip while suppressing the flowing down the gap between the roll chock and the slab.

  According to the present invention, in the secondary cooling zone of at least a part of the vertical part of the continuous casting machine or at least a part of the curved part, the gas blowing nozzle is disposed immediately below the roll chock of the split cast piece support roll. The gas is injected from the nozzle at an injection pressure of 0.1 MPa or more or an injection flow rate of 400 NL / min or more per nozzle, and at least a part of the injected gas is formed downstream of the gap between the roll chock and the slab in the casting direction. The gas flow is directed toward the upstream side in the casting direction, and this air flow causes secondary cooling of the slab while suppressing the residual cooling water that accumulates on the slab surface from flowing down the gap between the roll chock and the slab. Therefore, the residual water that causes the supercooling phenomenon does not flow down the slab surface, but falls from both sides of the slab support roll and is removed from the slab surface. That the slab surface even under the condition with an increased casting speed is not supercooled, without generating uneven temperature on the cast slab surface is realized that uniformly cool the slab. As a result, it is possible to cast a slab excellent in surface quality free from surface cracks with high productivity, which brings about an industrially beneficial effect.

It is a figure which shows the relationship between the generation | occurrence | production state of the temperature nonuniformity in a supercooled slab slab, and the generation | occurrence | production state of the surface crack of the slab slab after cooling. It is the schematic which shows the example of arrangement | positioning of the slab support roll in the vertical part directly under a casting_mold | template of a vertical bending type slab continuous casting machine. It is a figure which shows the width direction distribution of slab surface temperature when changing casting speed and performing continuous casting. It is a figure which shows the result of having measured the amount of water which flows down the roll chock part of a slab support roll in a full size model experiment apparatus. It is a figure which shows the result of having measured the amount of falling water when changing the injection pressure from the nozzle for gas blowing. It is a figure which shows the result of having measured the amount of falling water when changing the injection flow rate from the nozzle for gas blowing. It is the schematic of the slab continuous casting machine to which this invention is applied. It is a figure which shows the surface temperature distribution of the slab width direction in the example 1 of this invention. It is a figure which shows the surface temperature distribution of the slab width direction in the comparative example 1. FIG. It is a figure which shows the surface temperature distribution of the slab width direction in the comparative example 2. FIG.

  Hereinafter, the present invention will be described in detail. First, the background to the present invention will be described.

  As shown in FIG. 1 described above, when the casting speed is increased, the present inventors frequently experience the phenomenon of undercooling of the slab, which is a cause of temperature unevenness. It was confirmed that surface cracks frequently occurred on the piece. Therefore, we sought the cause that the supercooling phenomenon easily occurs when the casting speed is increased.

  The equipment length of the continuous casting machine is limited. Therefore, as the casting speed increases, the slab solidification must be completed within the limited equipment length, and therefore the cooling in the secondary cooling zone. Strengthen ability. Usually, the amount of cooling water or air mist (mixture of cooling water and air) injected from the water spray nozzle or air mist spray nozzle (hereinafter also referred to as “spray nozzle”) in the secondary cooling zone is increased. Therefore, the cooling capacity in the secondary cooling zone is strengthened. Generally, the secondary cooling of the continuous casting machine is controlled so that the amount of cooling water per 1 kg of molten steel to be cast is constant. In this case, when the casting speed is doubled, The amount of secondary cooling water is doubled.

  The cooling water sprayed from the spray nozzle takes heat from the slab by the sensible heat when the water temperature after colliding with the slab surface rises from the normal temperature of the initial state to the boiling temperature, and the latent heat of evaporation due to evaporation, and The cooling promoting action by the collision force of the cooling water works and the slab surface is cooled. In this case, the injected secondary cooling water cannot evaporate and remains as residual water on the slab surface or sandwiched between the slab support roll and the slab. Moreover, a part of residual water flows in the slab width direction and falls from the slab surface, and a part of the residual water is divided into two or more in the slab width direction. It flows down to the downstream side in the casting direction along the surface of the slab through a gap with the slab. When the roll pitch (distance between rolls in the casting direction) is reduced to increase the support area of the slab, the split slab support roll naturally reduces the roll diameter of the slab support roll and reduces the roll diameter. Since the rigidity of the roll is reduced and the deflection of the roll itself is increased, this is a slab support roll for reducing this deflection. Of course, there is also a slab continuous casting machine in which the split slab support roll is not arranged.

  Since the amount of secondary cooling water is small in the range where the casting speed is low, it has been confirmed that the temperature of the residual water remaining on the slab is as high as 80 ° C. or higher. In this case, it has been confirmed that the supercooling phenomenon does not occur. However, when the casting speed increases and the amount of cooling water increases, the amount of cooling water that cannot contribute to cooling increases, and the water temperature of the remaining residual water rapidly decreases and decreases to 80 ° C. to 30 ° C.

  Residual water that flows in the width direction of the slab and falls from the slab surface is not a problem, but the downstream side extends along the slab surface through the gap between the roll chock and the slab of the split slab support roll. The residual water flowing down into the tank has a higher subcooling degree as the temperature of the residual water is lowered, and the residual water exhibits an action of cooling the slab. The subcool is an effect of cooling due to a temperature difference between the saturation temperature of the cooling water and the cooling water temperature, and the subcooling degree indicates a temperature difference between the saturation temperature of the cooling water and the cooling water.

  Once the action of cooling by residual water acts on the slab, the surface temperature of the slab at that part decreases, the wettability of the slab surface improves, the cooling action further increases, and the surface temperature locally increases. Low sites are formed. It was found that this was the cause of the supercooling phenomenon.

  FIG. 2 is a schematic view showing an arrangement example of a slab support roll in a vertical portion directly under a mold of a certain vertical bending type slab continuous casting machine, and FIG. 2A is viewed from the short side of the slab. FIG. 2 and FIG. 2B are views as seen from the long side of the slab. As shown in FIG. 2, the slab support roll 6 is a split slab support roll divided into three in the width direction of the slab 10 by a roll chock 6a, and the portions of the roll chock 6a are staggered in the casting direction. 'In other words, it is arranged in a staggered manner. Two air mist spray nozzles 13 are arranged between adjacent slab support rolls 6, and the slab 10 is supported by the split slab support roll 6 while being separated from the air mist spray nozzle 13. It is cooled by the injected air mist. In FIG. 2A, the air mist spray nozzles 13 are disposed only on the long side surface of one side of the slab 10, but this is to avoid the drawing from becoming complicated. The air mist spray nozzles 13 are arranged on the long side surfaces on both sides of the slab 10. Here, among the two roll chocks of the slab support roll located at the uppermost stage in FIG. 2B, the roll chock located on the outer side in the slab width direction with respect to the center part of the slab support roll is A and on the inner side. The roll chock that is positioned is referred to as B, and in the slab support roll that is one downstream side of this slab support roll, the roll chock that is positioned outside is referred to as A ′, and the roll chock that is positioned inside is referred to as B ′.

  The width of the slab surface temperature when continuously casting steel slab slabs by using this continuous casting machine and changing the casting speed to levels 1 to 3 (casting speed: level 1 <level 2 <level 3) The direction distribution is shown in FIG. When the casting speed is low (level 1), as shown by the solid line in FIG. 3, the difference in surface temperature in the width direction of the slab is small except for the corner portion of the slab, and the supercooling phenomenon does not occur. When the casting speed is increased (level 2), as shown by the broken line in FIG. 3, a supercooling phenomenon occurs in the center portion of the slab width (position corresponding to B and B ′), and when the casting speed is further increased ( Level 3), as shown by the alternate long and short dash line in FIG. 3, the supercooling phenomenon occurs not only in the center of the width of the slab, but also on the outer sides (positions corresponding to A and A ′).

  Therefore, in order to quantify the cause of this supercooling phenomenon, the inventors made a full-scale model experimental device for five rolls of the same device as this actual device, and followed the secondary cooling conditions in the actual device. Air mist is sprayed from the air mist spray nozzle onto the pseudo slab, and at that time, the amount of water flowing down the gap between the roll chock A and the pseudo slab of the lowermost slab support roll and the gap between the roll chock B and the pseudo slab Was measured. The result is shown in FIG. As shown in FIG. 4, it was found that the roll chock B has a larger amount of falling water than the roll chock A. Moreover, it turned out that the amount of falling water increases with the raise of the amount of air mists, in other words, the raise of a casting speed, in the site | part of the roll chock A and the site | part of the roll chock B. The flow rate shown in FIG. 4 is the amount of cooling water from the air mist spray nozzle.

  That is, as shown in FIG. 4, at the level 2 where the casting speed is increased with respect to the level 1, the central part of the slab is cooled by the influence of the roll chocks B and B ′, and a supercooling phenomenon occurs, and the casting speed is increased. It was found that at level 3, which was further increased from level 2, the amount of falling water at roll chock A and A ′ increased, and the supercooling phenomenon occurred at these sites due to the influence of roll chock A and A ′.

  Here, based on the results of Level 1 and Level 2, it is understood that the supercooling phenomenon does not occur if the amount of falling water in the model experimental apparatus is 80 L / min or less by comparing FIG. 3 and FIG. It was. Therefore, the above-mentioned full-scale model experimental apparatus was used to study the amount of falling water under 80 L / min under the condition of level 3 roll chock B.

  In the test, a gas blowing nozzle is arranged in a direction perpendicular to the pseudo slab immediately below the roll chock A and roll chock B of the lowermost slab support roll, and the injection pressure and flow rate are changed from the nozzle. Then, air was sprayed, and the amount of falling water at the roll chock B at that time was measured.

  FIG. 5 is a diagram showing a result of measuring the amount of falling water when the injection pressure is changed, and FIG. 6 is a diagram showing a result of measuring the amount of falling water when changing the injection flow rate. It was confirmed that the amount of falling water was 80 L / min or less by setting the injection flow rate to 400 NL / min or more per nozzle. It is more effective to prevent overcooling by setting the injection pressure to 0.1 MPa or more and the injection flow rate to 400 NL / min or more per nozzle. Although it is not necessary to specify the upper limit of the injection pressure, since the equipment cost increases if it is increased, an upper limit of about 1.0 MPa is sufficient.

  The present invention has been made on the basis of these examination results, and supports a slab cast by a continuous casting machine while being supported by a split slab support roll divided into two or more in the slab width direction. At the time of secondary cooling using a cooling water or a mixture of cooling water and air in a secondary cooling zone provided below, at least a part of a vertical part or a part of a curved part of a continuous casting machine In the next cooling zone, a gas blowing nozzle is arranged immediately below the roll chock of the split slab support roll, and an injection pressure of 0.1 MPa or more from the nozzle or an injection flow rate of 400 NL / min or more per nozzle. A gas is injected, and at least a part of the injected gas is formed as a gas flow from the downstream side in the casting direction to the upstream side in the casting direction with a gap between the roll chock and the cast slab. Accumulates on the surface, the residual water in the cooling water, characterized in that the secondary cooling of the cast strip while suppressing the flowing down the gap between the roll chock and the slab.

  The nozzle for gas blowing to be used does not need to be defined in particular, and at least a part of the jet flow injected from the nozzle becomes a gas flow passing through the gap between the roll chock and the slab, and the roll chock and the slab are generated by the gas flow. Any type of nozzle may be used as long as the residual water flowing through the gap is forcibly pushed back to the upstream side. Specifically, a nozzle having an adjusted injection angle such as a water spray nozzle or an air mist spray nozzle used for secondary cooling of a continuous casting machine, or a single tube nozzle, The nozzle may be a Laval nozzle. In the experiment, the gas was injected from a direction perpendicular to the slab surface, but it may be injected from a direction inclined with respect to the slab surface, or may be injected in parallel to the slab surface.

  It is not necessary to perform gas injection from the gas blowing nozzle in all the secondary cooling zones of the secondary cooling zone, only in the secondary cooling zone in the upstream or middle stream where the amount of secondary cooling water is large. Even in that case, it is not necessary to carry out at all the positions of the slab support rolls adjacent to each other in the casting direction, and the gas may be injected every several slab support rolls. In other words, it is not necessary to completely remove the residual water from the slab, and even if the residual water stays at the same level as the residual water that remains when the casting speed is low, there is a problem even if it flows down the gap between the roll chock and the slab. Absent. In addition, in the horizontal part other than the vertical part and the curved part of the continuous casting machine, the flow of residual water does not occur in the roll chock, so that gas injection is unnecessary in the horizontal part.

  As the gas to be used, air is preferable because it is inexpensive, but not limited to air, nitrogen gas, carbon dioxide gas, and the like can also be used. In addition, the present invention can be applied as long as cooling water is used, whether it is a secondary cooling zone in which a water spray nozzle is disposed or a secondary cooling zone in which an air mist spray nozzle is disposed. .

  Thus, according to the present invention, since the slab is secondarily cooled while suppressing the flow of residual water that accumulates on the surface of the slab during continuous casting, the residual water that causes the supercooling phenomenon is cast. Even under casting where the casting speed is increased by removing from the surface of the slab, the slab surface is not overcooled, and the slab is cooled uniformly without causing temperature unevenness on the surface of the slab. Is done.

[Invention Example 1]
An embodiment of the present invention in the slab continuous casting machine shown in FIG. 7 will be described. In FIG. 7, reference numeral 1 is a vertical bending type slab continuous casting machine, 2 is a tundish for relaying and supplying molten steel supplied from a ladle to a mold, and 3 is a sliding for adjusting the flow rate of molten steel to the mold. Nozzle 4 is an immersion nozzle for injecting molten steel into the mold, 5 is a mold for cooling the molten steel to form the outer shell shape of the slab, and 6 is for supporting and guiding the slab. A slab support roll, 7 is a transport roll for transporting the cast slab, 8 is a gas cutter for cutting the cast slab into a predetermined length, 9 is molten steel, and 10 is cast. The cast slab 10a is a cut slab, 11 is a solidified shell, and 12 is an unsolidified phase.

  The used slab continuous casting machine 1 has an equipment length of 45 m and is capable of casting a slab slab having a width of 2000 mm. The distance from the upper end of the mold 5 to the lower end of the mold 5 is 1 m, and the range of 44 m from the position immediately below the mold to the machine end is the secondary cooling zone. Toward the end side, it was divided into four secondary cooling zones, a first cooling zone, a second cooling zone, a third cooling zone, and a fourth cooling zone, and cooling conditions were set for each secondary cooling zone. In FIG. 7, the range from the AA ′ position to the slab support roll 6 immediately above the BB ′ position is the first cooling zone, and from the BB ′ position to the slab support roll 6 just above the CC ′ position. Is the second cooling zone, and the range from the CC ′ position to the slab support roll 6 immediately above the DD ′ position is the third cooling zone, from the DD ′ position to the slab support roll 6 at the end of the machine. Is the fourth cooling zone. In the slab continuous casting machine 1, the fourth cooling zone is a horizontal portion. An air mist spray nozzle is disposed in each secondary cooling zone of the secondary cooling zone, and the slab 10 is cooled by the air mist sprayed from the air mist spray nozzle.

  Among these secondary cooling zones, a first cooling zone (vertical portion and curved portion), a second cooling zone (curved portion), and a three-part slab supporting roll shown in FIG. The present invention was applied in the third cooling zone (curved portion). That is, immediately below the roll chock 6a of all slab support rolls 6 from the first cooling zone to the third cooling zone, the injection angle in the casting direction is about 13 ° and the injection angle in the slab width direction is about 30 °. A gas blowing nozzle (not shown) was placed perpendicular to the slab surface. The distance between the tip of the gas blowing nozzle and the slab surface was about 30 mm. These gas blowing nozzles are configured such that air is jetted in the direction perpendicular to the surface of the slab from the tip thereof with an jet pressure of 0.1 MPa. In this gas blowing nozzle, the injection flow rate per nozzle at an injection pressure of 0.1 MPa was about 400 NL / min.

  Using the slab continuous casting machine 1 having this configuration, the surface temperature of the infrared camera installed on the upper surface side of the slab in the middle of the fourth cooling zone of the secondary cooling zone without injecting air from the gas blowing nozzle While measuring the slab surface temperature with a profile meter (not shown), casting of a slab slab having a thickness of 250 mm and a width of 2000 mm was started at a casting speed (Vc) of 1.5 m / min. The surface temperature distribution in the slab width direction measured by the slab surface temperature profile meter is shown in FIG. When the casting speed was 1.5 m / min, the surface temperature distribution in the slab width direction measured by the slab surface temperature profile meter had a temperature deviation of about 40 ° C. or less and was cooled substantially uniformly. .

  Thereafter, the casting speed was increased to 2.0 m / min, and the amount of secondary cooling water was increased in proportion to the casting speed. As a result, supercooling occurred at the slab surface part corresponding to the part of the roll chock 6a. As shown, the deviation of the slab surface temperature was increased to 350 ° C. or more. Therefore, in the first cooling zone, the second cooling zone, and the third cooling zone, air was injected from the gas blowing nozzle at an injection pressure of 0.1 MPa. The deviation of the slab surface temperature starts to decrease about 5 minutes after the air is injected from the gas blowing nozzle, and after about 10 minutes, the deviation of the slab surface temperature is within 50 ° C. as shown in FIG. became.

[Comparative Example 1]
In the slab continuous casting machine 1 shown in Example 1 of the present invention, for comparison, the injection pressure from the gas blowing nozzle was lowered to 0.05 MPa and air was injected. In this case, the injection flow rate per nozzle was about 250 NL / min.

  Using the slab continuous casting machine 1 having this configuration, the surface temperature of the infrared camera installed on the upper surface side of the slab in the middle of the fourth cooling zone of the secondary cooling zone without injecting air from the gas blowing nozzle While measuring the slab surface temperature with a profile meter, casting of a slab slab having a thickness of 250 mm and a width of 2000 mm was started at a casting speed of 1.5 m / min. FIG. 9 shows the surface temperature distribution in the slab width direction measured by the slab surface temperature profile meter. When the casting speed was 1.5 m / min, the surface temperature distribution in the slab width direction measured with the slab surface temperature profile meter had a temperature deviation of about 45 ° C. or less and was cooled substantially uniformly. .

  Thereafter, the casting speed was increased to 2.0 m / min, and the amount of secondary cooling water was increased in proportion to the casting speed. As a result, supercooling occurred at the slab surface part corresponding to the part of the roll chock 6a. As shown, the deviation of the slab surface temperature was increased to 350 ° C. or more. Therefore, in the first cooling zone, the second cooling zone, and the third cooling zone, air was injected from the gas blowing nozzle at an injection pressure of 0.05 MPa. The deviation of the slab surface temperature slightly decreased about 5 minutes after the air was injected from the gas blowing nozzle, but after about 10 minutes, the deviation of the slab surface temperature was 200 ° C. as shown in FIG. As described above, surface cracks were found in the supercooled part in the slab inspection after casting.

[Comparative Example 2]
In the slab continuous casting machine 1 shown in Example 1 of the present invention, the installation position of the gas blowing nozzle is changed to the fourth cooling zone in the horizontal portion instead of the first to third cooling zones, and the installation position of the gas blowing nozzle is changed. The effect of was investigated. The injection pressure from the gas blowing nozzle was 0.1 MPa, and the injection flow rate per nozzle in this case was about 400 NL / min.

  Using the slab continuous casting machine 1 having this configuration, the surface temperature of the infrared camera installed on the upper surface side of the slab in the middle of the fourth cooling zone of the secondary cooling zone without injecting air from the gas blowing nozzle While measuring the slab surface temperature with a profile meter, casting of a slab slab having a thickness of 250 mm and a width of 2000 mm was started at a casting speed of 1.5 m / min. FIG. 10 shows the surface temperature distribution in the slab width direction measured by the slab surface temperature profile meter. When the casting speed was 1.5 m / min, the surface temperature distribution in the slab width direction measured with the slab surface temperature profile meter had a temperature deviation of about 45 ° C. or less and was cooled substantially uniformly. .

  Thereafter, the casting speed was increased to 2.0 m / min, and the amount of secondary cooling water was increased in proportion to the casting speed. As a result, supercooling occurred at the slab surface portion corresponding to the portion of the roll chock 6a. As shown, the deviation of the slab surface temperature was increased to 350 ° C. or more. Therefore, in the fourth cooling zone, air was injected from the gas blowing nozzle at an injection pressure of 0.1 MPa. The deviation of the slab surface temperature does not decrease even after about 5 minutes from the injection of air from the gas blowing nozzle, and after about 10 minutes, the deviation of the slab surface temperature is 300 ° C. or more as shown in FIG. In the slab inspection after casting, surface cracks were found in the supercooled part.

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Casting piece support roll 6a Roll chock 7 Conveying roll 8 Gas cutting machine 9 Molten steel 10 Cast piece 11 Solidified shell 12 Unsolidified phase 13 Air mist spray nozzle

Claims (1)

  Cooling water or cooling water in a secondary cooling zone provided below the mold while supporting a slab cast by a continuous casting machine with a split mold slab support roll divided into two or more in the slab width direction In the secondary cooling zone of at least a part of the vertical part of the continuous casting machine or at least a part of the curved part in the secondary cooling using the mixture of air and air, downstream of the roll chock of the split slab support roll A gas blowing nozzle is arranged directly under the side, and gas is injected from the nozzle at an injection pressure of 0.1 MPa or more or an injection flow rate of 400 NL / min or more per nozzle, and at least a part of the injected gas is a roll chock. The gap between the slab and the slab is a gas flow from the downstream side in the casting direction to the upstream side in the casting direction. Characterized in that the secondary cooling billet while suppressing the flowing down the gap between the pieces, the secondary cooling method of continuous casting.

Images