JP5556073B2 - Secondary cooling method in continuous casting - Google Patents

Description

  The present invention relates to a method for cooling a slab in a secondary cooling zone of a continuous casting facility that 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 water having a water supply pressure of 25 to 100 kgf / cm ² (2.5 to 9.8 MPa) is sprayed on a slab through a water supply mechanism having a booster pump in a pressurizing system. Continuous casting with cooling 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.

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

  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 of the slab during casting is lowered by supercooling in the secondary cooling zone. FIG. 1 shows the occurrence of temperature unevenness in the supercooled slab slab (FIG. 1A). ) 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 even if effective, it is not efficient.

  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.

  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 first aspect of the present invention for solving the above problems includes a plurality of slabs cast by a continuous casting machine and provided below the mold while being supported by a slab support roll. When performing secondary cooling using cooling water or a mixture of cooling water and gas in a secondary cooling zone composed of a cooling zone, the slab is subjected to secondary cooling by setting the water temperature of the cooling water to 50 ° C. or higher. It is what.

  The secondary cooling method in continuous casting according to the second invention is characterized in that, in the first invention, the cooling water temperature is set to 50 ° C. or more in all cooling zones of the secondary cooling zone. is there.

  The secondary cooling method in continuous casting according to the third invention is the cooling zone of the secondary cooling zone in the first invention, in the cooling zone of the secondary cooling zone in the range from immediately below the mold to at least a distance of half the total length of the secondary cooling zone. The water temperature of cooling water shall be 50 degreeC or more.

A secondary cooling method in continuous casting according to a fourth aspect of the present invention is the method of the first aspect, wherein the cooling water temperature is set in the cooling zone of the secondary cooling zone where the amount of cooling water is 100 liters / (m ² · min) or more. It is characterized by being 50 ° C. or higher.

  According to the present invention, since the slab during secondary casting is secondarily cooled with cooling water having a water temperature of 50 ° C. or higher, even if residual water of secondary cooling water accumulates on the surface of the slab during continuous casting, The slab surface is not overcooled even under conditions where the temperature of the casting is high and the casting speed at which residual water increases is increased, and the slab is cooled uniformly without causing temperature unevenness on the slab surface. Is realized. 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 of the experimental apparatus which simulated secondary cooling of slab. It is a figure which shows the relationship between the water temperature of the cooling water supplied in the simulation experiment apparatus, and the water temperature of the cooling water in a slit exit position. 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 Example 1. FIG. It is a figure which shows the surface temperature distribution of the slab width direction in Example 2. FIG. It is a figure which shows the surface temperature distribution of the slab width direction in Example 3. 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 along the surface of the slab through the gap. 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, if the casting speed is increased and the amount of secondary cooling water is increased, the residual water staying on the slab increases, and the water temperature of the residual water is expected to decrease.

  Then, the relationship between the amount of secondary cooling water and the temperature of the residual water at that time was investigated using the experimental apparatus which simulated the secondary cooling of the slab shown in FIG. In the experiment, a heated flat thick steel plate 13 is disposed obliquely (inclination angle: 45 °), and slits (number of slits: 1, slit spacing: imitating the gap of the roll chock are formed at the upper end of the thick steel plate 13. 100 mm), cooling water is supplied from the two water supply pipes 15 to the inside of the shielding box 14, and the cooling water flows down the surface of the thick steel plate 13 through the slits. I went. The temperature of the cooling water flowing down is measured by a thermocouple (not shown) placed 2 mm away from the surface of the thick steel plate 13 at the exit position of the slit, and the temperature of the thick steel plate 13 is measured at the exit position of the slit. It measured with the thermocouple (not shown) embedded in the inside of the thick steel plate 13 1 mm away from the surface of the steel plate 13.

  The thick steel plate 13 and the shielding box 14 are heated to 1000 ° C. in a soaking furnace, and the thick steel plate 13 and the shielding box 14 are taken out of the soaking furnace and placed in a predetermined position, and the temperature of the thick steel plate 13 becomes 900 ° C. At that time, 35 ° C. cooling water was supplied from the water supply pipe 15 to the inside of the shielding box 14. The supply amount of cooling water was set at two levels of 10 liter / min and 15 liter / min.

  When the supply amount of cooling water is 10 liters / min, the water temperature of the cooling water at the slit exit position remains as high as 85 to 90 ° C., and the temperature of the steel plate decreases at a substantially constant rate. When 50 seconds passed from the start, the temperature was about 800 ° C., and no supercooling phenomenon occurred. However, when the cooling water supply rate is 15 liters / min, the cooling water temperature was about 80 ° C. immediately after the water supply, but the water temperature was 80 ° C. or less after about 20 seconds from the start of the water supply. It began to drop to about 60 ° C when 50 seconds had passed since the start of water supply. Moreover, after the water temperature of the cooling water at the slit exit position became less than 80 ° C., the temperature of the thick steel plate also suddenly decreased, and decreased to about 400 ° C. when 50 seconds passed from the start of water supply. That is, a supercooling phenomenon occurred when the amount of cooling water supplied was 15 liters / min.

  From these results, it was found that the lowering of the temperature of the stagnant water of the secondary cooling water is the cause of the supercooling phenomenon. That is, when the casting speed increases and the amount of secondary cooling water increases, the amount of residual water that remains on the slab increases, and the temperature of the residual water decreases to less than 80 ° C. Residual water that flows in the width direction of the slab and falls from the left and right of the slab surface is not a problem, but it flows downstream along the slab surface through the gap of the roll chock of the split slab support roll. The residual water has a higher subcooling degree as the temperature of the residual water decreases, and the residual water exhibits an action of cooling the slab. 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 becomes stronger, and the surface temperature is locally low. A site is formed. And it discovered that this was a cause of occurrence of the supercooling phenomenon. 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.

  Therefore, the influence of the cooling water temperature on the supercooling phenomenon was further investigated using the experimental apparatus shown in FIG. In this test, the amount of cooling water supplied from the water supply pipe 15 is constant at 15 liters / min, and the temperature of the cooling water supplied is 10 ° C., 20 ° C., 30 ° C., 40 ° C., 50 ° C., 60 ° C. Six levels were set.

  As a result, it was confirmed that the supercooling phenomenon occurred when the cooling water temperature was 40 ° C. or lower, but the supercooling phenomenon did not occur when the cooling water temperature was 50 ° C. or higher. FIG. 3 shows the measurement results of the coolant temperature at the slit exit position when the coolant temperature to be supplied is 30 ° C., 40 ° C., and 50 ° C. As shown in FIG. 3, when the water temperature of the cooling water was 50 ° C. or higher, the water temperature at the slit exit position was 90 ° C. or higher, and the subcooling degree was 10 ° C. or lower. In this case, the temperature of the thick steel plate decreased at a substantially constant rate, and was about 770 ° C. when 50 seconds passed from the start of water supply, and no supercooling phenomenon occurred. On the other hand, when the temperature of the cooling water is 40 ° C. or lower, the water temperature at the slit exit position was secured at about 80 ° C. at the start of water supply, but decreased with the passage of time, and 50 seconds passed from the start of water supply. At that time, the temperature became about 60 ° C., and the subcooling degree also increased to 10 degrees or more. In this case, after the temperature of the cooling water at the slit outlet becomes less than 80 ° C., the temperature of the thick steel plate also starts to drop sharply, and after about 50 seconds from the start of water supply, it drops to about 400 ° C. There has occurred.

  The present invention has been made on the basis of the above knowledge, and a secondary cooling zone comprising a plurality of cooling zones provided below a mold while supporting a slab cast by a continuous casting machine with a slab support roll. In the secondary cooling using the cooling water or the mixture of cooling water and gas, the slab is secondary cooled by setting the water temperature of the cooling water to 50 ° C. or higher.

  The secondary cooling zone of the continuous casting machine is composed of a plurality of cooling zones, and the amount of cooling water is generally controlled for each cooling zone. The cooling water sprayed on the surface of the slab is generally circulated and reused as secondary cooling water via the scale pit, sedimentation tank, cooling tower, and water supply tank, and is sprayed from the spray nozzle. The cooling water temperature depends on the water temperature at the outlet of the cooling tower. That is, the temperature of cooling water is controlled by adjusting the amount of heat removed from the cooling tower by changing the number of operating cooling fans of the cooling tower.

  In the present invention, the temperature of the cooling water supplied by adjusting the amount of heat removal in the cooling tower is basically controlled to 50 ° C. or higher. However, the temperature of the cooling water used for circulation is rarely over 50 ° C even if a bypass is installed without going through the cooling tower, and it is difficult to ensure 50 ° C especially in severely cold winter. is there. Therefore, in carrying out the present invention, in order to secure the temperature of the cooling water to be supplied at 50 ° C. or higher, a submerged heater or the like is installed in the scale pit, the sedimentation tank, the water tank, or the like, or steam is blown in. Etc. to ensure that the water temperature is 50 ° C. or higher.

Further, in the present invention, the cooling zone in which the temperature of the cooling water to be supplied is 50 ° C. or more can be integrated into one cooling water supply facility regardless of the temperature conditions. However, in a large continuous casting machine having a long equipment length, the secondary cooling zone is vast, and the capacity of a heater or the like for securing the water temperature at 50 ° C. or more may be too large. In such a case, a supercooling phenomenon is likely to occur, a cooling zone in the range from directly under the mold to half the distance of the secondary cooling zone, or the amount of cooling water is 100 liters / (m ² · min) or more The temperature of the cooling water to be supplied may be 50 ° C. or higher only in the cooling zone having a large amount of cooling water. Thus, when the cooling zone in which the coolant temperature to be supplied is 50 ° C. or higher is limited to a specific cooling zone, it is necessary to separately provide a water tank or the like in the water supply path and control the water temperature independently. Become.

  Further, the present invention can be applied to the cooling zone where the water spray nozzle is arranged or the cooling zone where the air mist spray nozzle is arranged as long as the cooling water is used.

  Thus, according to the present invention, since the slab during continuous casting is secondarily cooled with cooling water having a water temperature of 50 ° C. or higher, residual water of the secondary cooling water accumulates on the surface of the slab during continuous casting. Even if the temperature of the residual water is high and the casting speed at which the residual water is increased is increased, the slab surface is not overcooled, and the slab surface is uniformly cooled without causing temperature unevenness. Is realized.

  An embodiment of the present invention in the slab continuous casting machine shown in FIG. 4 will be described. In FIG. 4, 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. 4, 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. An air mist spray nozzle (not shown) 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.

  Using the slab continuous casting machine 1 having this configuration, the temperature of the secondary cooling water to be supplied is controlled to 30 ° C. and installed on the upper surface side of the slab at the boundary between the third cooling zone and the fourth cooling zone of the secondary cooling zone. The slab slab having a thickness of 250 mm and a width of 2000 mm was cast at a casting speed (Vc) of 1.5 m / min while measuring the slab surface temperature with a surface temperature profile meter (not shown) comprising an infrared camera. .

  FIG. 5 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 a 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 corresponding to the roll chock, and FIG. As shown, the deviation of the slab surface temperature increased to 250 ° C. or more. Therefore, the temperature of the secondary cooling water to be supplied was raised to 50 ° C. The deviation of the slab surface temperature starts to decrease after about 5 minutes after the temperature of the cooling water to be supplied is 50 ° C., and after about 10 minutes, the deviation of the slab surface temperature is within 40 ° C. as shown in FIG. It was.

  Using the slab continuous casting machine shown in FIG. 4, the water temperature of the secondary cooling water to be supplied is controlled to 30 ° C. and installed on the upper surface side of the slab at the boundary between the third cooling zone and the fourth cooling zone of the secondary cooling zone. The slab slab having a thickness of 250 mm and a width of 2000 mm was cast at a casting speed (Vc) of 1.5 m / min while measuring the slab surface temperature with a surface temperature profile meter comprising an infrared camera.

  FIG. 6 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 a 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 portion corresponding to the roll chock portion. As shown, the deviation of the slab surface temperature increased to 250 ° C. or more. Therefore, the temperature of the secondary cooling water supplied to the third cooling zone and the fourth cooling zone was raised to 50 ° C. However, the temperature difference hardly reduced even after 10 minutes had passed since the temperature of the cooling water to be supplied was 50 ° C. Thereafter, only the temperature of the secondary cooling water supplied to the first cooling zone and the second cooling zone was raised to 50 ° C. About 5 minutes after the temperature of the cooling water supplied to the first cooling zone and the second cooling zone is 50 ° C., the deviation of the slab surface temperature starts to decrease, and after about 10 minutes, the slab is slab as shown in FIG. The deviation of the surface temperature was within 40 ° C.

  Using the slab continuous casting machine shown in FIG. 4, the water temperature of the secondary cooling water to be supplied is controlled to 30 ° C. and installed on the upper surface side of the slab at the boundary between the third cooling zone and the fourth cooling zone of the secondary cooling zone. The slab slab having a thickness of 250 mm and a width of 2000 mm was cast at a casting speed (Vc) of 1.5 m / min while measuring the slab surface temperature with a surface temperature profile meter comprising an infrared camera.

The secondary cooling water amount at a casting speed of 1.5 m / min is 93 liters / (m ² · min) in the first cooling zone, 77 liters / (m ² · min) in the second cooling zone, It was 60 liters / (m ² · min), and the fourth cooling zone was 43 liters / (m ² · min).

  FIG. 7 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 a 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 accordance with the increased casting speed. The amount of secondary cooling water at a casting speed of 2.0 m / min is 127 liters / (m ² · min) in the first cooling zone, 110 liters / (m ² · min) in the second cooling zone, 93 liters / (m ² · min), the fourth cooling zone was 77 liters / (m ² · min).

When the casting speed was increased to 2.0 m / min, supercooling occurred at the slab surface part corresponding to the roll chock part, and the deviation of the slab surface temperature increased to 250 ° C. or more as shown in FIG. did. Therefore, the temperature of the secondary cooling water supplied to the first cooling zone and the second cooling zone in which the cooling water amount is 100 liters / (m ² · min) or more was raised to 50 ° C. About 5 minutes after the temperature of the cooling water supplied to the first cooling zone and the second cooling zone is 50 ° C., the deviation of the slab surface temperature starts to decrease, and after about 10 minutes, the slab is slab as shown in FIG. The deviation of the surface temperature was within 40 ° C.

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Casting piece support roll 7 Conveyance roll 8 Gas cutting machine 9 Molten steel 10 Cast piece 11 Solidified shell 12 Unsolidified phase 13 Thick steel plate 14 Shielding box 15 Water supply pipe

Claims (4)

A slab cast by a continuous casting machine provided with a split mold slab support roll is supported by the split slab support roll while being a secondary cooling zone comprising a plurality of cooling zones provided below the mold. When performing secondary cooling using cooling water or a mixture of cooling water and gas, the cooling water is cooled to a temperature of 50 ° C. or higher to secondarily cool the slab, and the gap between the roll chock of the split type slab support roll A secondary cooling method in continuous casting, wherein overcooling of a slab surface by residual water of the cooling water flowing down through the slab is prevented .   2. The secondary cooling method in continuous casting according to claim 1, wherein the temperature of the cooling water is set to 50 ° C. or higher in all cooling zones of the secondary cooling zone.   2. The water temperature of the cooling water is set to 50 ° C. or more in a cooling zone of a secondary cooling zone in a range from a position immediately below the mold to at least a half of the total length of the secondary cooling zone. Secondary cooling method in continuous casting of steel. ^2. The continuous casting according to claim 1, wherein the temperature of the cooling water is 50 ° C. or more in a cooling zone of a secondary cooling zone having a cooling water amount of 100 liters / (m ² · min) or more. Next cooling method.

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