JP2010082637A - Secondary cooling method in continuous casting - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a secondary cooling method capable of uniformly cooling a slab without generating temperature irregularity on the surface of the slab, when cooling the slab during casting in a secondary cooling zone of continuous casting equipment. <P>SOLUTION: A cast slab 10 being cast in a continuous casting machine 1 is secondarily cooled with cooling-water or a mixture of cooling-water and gas in a secondary cooling zone arranged downstream from a casting mold 5. The surface of the slab during a continuous casting is descaled all over the slab-width direction with a liquid having an impingement pressure of ≥1 MPa at two or more portions along the casting direction (shown in Figure as four portions of A-A', B-B', C-C', and D-D'), and then the slab is subjected to the secondary cooling within 10 seconds after the descaling. <P>COPYRIGHT: (C)2010,JPO&INPIT

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. While being cooled by cooling water (referred to as “secondary cooling water”) in the cooling zone, it is continuously pulled out below the mold, 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.

  In the secondary cooling zone, the surface temperature of the slab is generally controlled to 600 to 1000 ° C. However, with the recent increase in casting speed, the ability of secondary cooling has been enhanced, and the surface temperature of 600 ° C. or less. There are also operations in the area.

  In this secondary cooling zone, if uneven cooling occurs, the surface and the inside of the slab will crack or the center segregation at the center of the slab will deteriorate, so in the casting direction and width direction of the slab It has been proposed and implemented to provide uniform cooling. 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 is likely to occur in the width direction, and in particular, uniform cooling is performed in the width direction of the slab. 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 partial accumulated water on the surface of the slab is prevented, and partial overcooling is prevented. Cooling is realized.

  On the other hand, the present inventors confirmed from the investigation results so far that the scale formed on the surface of the slab and the residue of the mold powder on the surface of the slab cause supercooling in the secondary cooling. ing. A method for removing the scale and mold powder on the surface of the slab during continuous casting has not been proposed as a means for uniformly cooling the slab, but several methods have been proposed.

For example, Patent Document 4 discloses a technique in which high-pressure water or high-pressure air water of 50 kgf / cm ² (4.9 Mpa) or more is jetted onto a slab surface during casting and descaling is performed. According to Patent Document 4, the mold powder adhering to the surface of the slab is removed, and a thin steel plate having a good surface property can be produced by a direct rolling method.

Further, in Patent Document 5, spray water adjusted so that the collision surface pressure on the surface of the slab falls within the range of 0.2 to 0.8 kgf / cm ² (0.02 to 0.08 Mpa) is used. And a technique for removing the scale on the surface of the slab. According to Patent Document 5, it is said that the scale is prevented from biting between a reduction roll for reducing the slab and the slab, and a slab having no uneven impression marks is obtained.
Japanese Patent Laid-Open No. 50-103426 JP 2003-136205 A JP-A-57-91857 JP 59-229268 A JP-A-2-229653

  By the way, the present inventors have confirmed that there is a correlation between the occurrence of temperature unevenness on the slab surface in the continuous casting machine and the occurrence of slab surface cracks after casting, based on the results of previous research. . This temperature non-uniformity is a phenomenon that occurs particularly during spray cooling during high-speed casting, and is a phenomenon in which the slab surface during casting becomes black due to overcooling in the secondary cooling zone. As shown in the relationship between the occurrence state (FIG. 1 (A)) and the occurrence state of surface cracks in the slab slab after cooling (FIG. 1 (B)), observing the cast slab, Surface cracks concentrate at the site of occurrence.

  If the above prior art is verified from the viewpoint of preventing this temperature unevenness, none of the above prior arts is effective in preventing temperature unevenness. 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, The impact pressure is strong, and it is impossible to prevent temperature unevenness under high speed casting.

  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 overcooling is less likely to occur. . However, as long as cooling is performed using only the spray nozzle, the strength of the spray water is generated in the slab width direction or the casting direction, and the strength increases when the casting speed is high. Causes unevenness.

Patent Document 3 sprays high pressure secondary cooling water of 25 to 100 kgf / cm ^{2 on} the slab, and the difference in cooling strength between the portion where the cooling water is directly sprayed and the portion where the cooling water is not sprayed is large. The sprayed portion has a strong cooling strength, and thus tends to become transition boiling (details of the transition boiling will be described later), that is, is likely to be supercooled, and always has a cause of temperature unevenness. In addition, it is necessary to pressurize the secondary cooling water to a high pressure of 25 to 100 kgf / cm ² , and in a normal secondary cooling device, the flow path is narrow and the pressure loss is large, so a large number of pressurizing booster pumps are required, Not realistic.

In Patent Document 4, although the spray pressure of high-pressure water or high-pressure air water is 50 kgf / cm ² or more, the distance from the nozzle to the slab and the spray angle of the nozzle are not clear, and depending on the nozzle to be installed, it is sufficiently descaling There are cases where it is not possible. Also, it is not intended for uniform cooling of the slab, the spray position in the casting direction of high-pressure water and high-pressure air water is not clear, and even if descaling is possible, there is a possibility that uniform cooling of the slab cannot be achieved. There is.

In Patent Document 5, the impact surface pressure on the slab surface is a low pressure of 0.2 to 0.8 kgf / cm ^2, and in the case of a scale with poor peelability, there is a possibility that it cannot be sufficiently peeled and removed. Yes, uniform cooling of the slab cannot be achieved stably.

  The present invention has been made in view of the above circumstances, and its object is to generate temperature unevenness on the surface of the slab when cooling the slab during casting in the secondary cooling zone of the continuous casting facility. And providing a secondary cooling method capable of uniformly cooling the slab.

  The secondary cooling method in continuous casting according to the first aspect of the present invention for solving the above-described problem is a method in which a slab cast by a continuous casting machine is cooled or cooled in a secondary cooling zone provided below the mold. When secondary cooling is performed using a mixture of water and gas, the slab surface during continuous casting is spread over the entire slab width direction by a liquid having a collision pressure of 1 MPa or more at two or more positions in the casting direction. Descaling is performed, and the slab is secondarily cooled within 10 seconds after the descaling.

The secondary cooling method in continuous casting according to the second invention is a cooling water or a mixture of cooling water and gas in a secondary cooling zone provided below the mold of a slab cast by a continuous casting machine. When the secondary cooling is performed using the slab, the slab surface during continuous casting is subjected to a blasting process over the entire slab width direction with an abrasive having a projection density of 5 kg / cm ² or more, and 10 seconds after the blasting process. Within this, the slab is subjected to secondary cooling.

A secondary cooling method in continuous casting according to a third aspect of the present invention is the cooling water or a mixture of cooling water and gas in a secondary cooling zone provided below the mold of a slab cast by a continuous casting machine. When performing secondary cooling using a blast, the slab surface during continuous casting is blasted over the entire slab width direction with an abrasive having a projection density of 5 kg / cm ² or more, and at least two locations in the casting direction. The descaling is performed over the entire width direction of the slab with a liquid having a collision pressure of 1 MPa or more, and the slab is secondarily cooled within 10 seconds after the blasting and within 10 seconds after the descaling. It is characterized by.

  According to the present invention, slab cast on a continuous casting machine is subjected to descaling with high-pressure liquid and / or blasting with an abrasive in the secondary cooling zone to remove the scale on the slab surface. However, since the slab surface from which the scale has been removed is secondarily cooled, the MHF point, which is the point of transition from film boiling cooling to transition boiling cooling, decreases, and even under high speed casting where the casting speed is increased, it is stable. Thus, cooling maintained in the region of film boiling above the MHF point can be realized, whereby the slab surface is not overcooled, and the slab is cooled uniformly without causing temperature unevenness on the slab surface. Is achieved.

  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, the inventors of the present invention have found that when the casting speed is increased, overcooling of the slab, which is a cause of temperature unevenness, frequently occurs, and the slab is accordingly produced. It was confirmed that surface cracks frequently occurred. Therefore, we sought the cause of temperature irregularities that are likely to occur as the casting speed increases.

  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 cooling capacity in the secondary cooling zone is increased by increasing the amount of cooling water or mist injected from the water spray nozzle or air mist spray nozzle (hereinafter also referred to as “spray nozzle”) in the secondary cooling zone. is doing.

  However, as shown in FIG. 2, when the slab is cooled using a water spray nozzle or an air mist spray nozzle, the present inventors change from film boiling cooling to transition boiling cooling when the cooling capacity is increased. The knowledge that the MHF (Minmum Heat Flux) point which is a point becomes high was acquired. This is because if the amount of cooling water or mist from the water spray nozzle or air mist spray nozzle is increased to increase the cooling capacity, the collision pressure of the cooling water or mist sprayed from the spray nozzle on the slab surface will increase. The film cannot be maintained in a boiling state where a vapor film exists between the slab surface and the cooling water, and the cooling water or mist from the spray nozzle breaks the vapor film and starts to directly contact the slab surface. This is because of the transition.

  Note that FIG. 2 shows a spray attached to the upstream portion of the secondary cooling zone with a constant secondary cooling water amount per unit mass of the slab, that is, adjusting the secondary cooling water amount in proportion to the casting speed. It is a figure which shows the relationship between slab surface temperature when a slab is cooled using an air mist spray nozzle as an example of a nozzle according to casting speed according to casting speed, and low speed casting (casting speed: about 0.8-1) 0.0 m / min), the MHF point is about 620 ° C., but in the case of medium speed casting (casting speed: about 1.4 to 1.6 m / min), the MHF point is about 730 ° C. In the case of high speed casting (casting speed: about 1.8 to 2.0 m / min), it can be seen that the MHF point is 800 ° C. or higher.

Furthermore, the present inventors paid attention to the relationship between the surface state of the slab and the MHF point. FIG. 3 shows a steel slab heated at 1000 ° C., cooled with an air mist spray nozzle having a water volume of 25 L / min, an air volume of 180 Nl / min, and a distance from the slab to the nozzle tip of 190 mm, and immediately below the air mist spray nozzle. It is the result of investigating the relationship between surface temperature and heat transfer coefficient. As shown in FIG. 3, it can be seen that the MHF point increases when mold powder remains on the surface of the steel material as compared with the steel material in the normal state (scale thickness of about 800 μm). Also, when the scale thickness is thick (scale thickness of about 1400 μm), the MHF point rises compared to the normal state, especially when the steel has a high Si content (Si content = 1.6 mass%). In the case of the SiO ₂ containing scale, the MHF point was found to rise most.

  When the MHF point rises, film boiling tends to change to transition boiling despite the slab surface temperature being high. Once entering the transition boiling region during the secondary cooling, as shown in FIG. 4, the heat flux (heat transfer coefficient) increases rapidly as the slab surface temperature decreases, so the supercooling continues. . This is the cause of the occurrence of temperature unevenness. In particular, when the casting speed is 2.0 m / min or higher, the temperature unevenness increases, and when a phenomenon such as generation of a scale, such as an increase in the MHF point, occurs. It has been found that cooling is further promoted and surface cracks occur frequently. That is, the knowledge that temperature unevenness can be prevented by cooling so that the slab surface temperature does not become below the MHF point was obtained. In addition, FIG. 4 is a figure which shows typically the cooling mode when the hot slab surface is cooled with spray water, and the heat flux at that time.

  Therefore, the present inventors investigated what happens when the scale is removed and cooled. The investigation method is the same as the above-described experiment in which the relationship between the surface state of the slab and the MHF point was investigated. A steel material heated to 1000 ° C. was subjected to a water volume of 25 L / min, an air volume of 180 Nl / min, and the nozzle tip from the slab. The air mist spray nozzle with a distance of 190 mm was cooled, and the relationship between the slab surface temperature and the heat transfer rate directly under the air mist spray nozzle was investigated. The method for removing the scale of the steel is to immerse the steel in a hydrochloric acid solution at 85 ° C. and 10% by mass for 1 minute, pickle the steel, remove the scale by this pickling, and heat the steel to 1000 ° C. for investigation. did.

  FIG. 5 shows the relationship between the slab surface temperature and the heat transfer coefficient in the steel material from which the scale has been removed. FIG. 5 also shows the data of FIG. 3 described above. As is clear from FIG. 5, it was found that the film boiling state was maintained up to a low temperature range near the surface temperature of 500 ° C. in the steel material from which the scale was removed, and that the MHF point was lowered by removing the scale.

  The present invention has been made on the basis of the above knowledge, and the slab casted by a continuous casting machine is subjected to cooling water or a mixture of cooling water and gas in a secondary cooling zone provided below the mold. When the secondary cooling is performed, the slab in the slab is subjected to descaling with high-pressure liquid and / or blasting with an abrasive in the secondary cooling zone, and by descaling and / or blasting The slab surface from which the scale has been removed is subjected to secondary cooling.

  The descaling and blasting processes need not be performed over the entire secondary cooling zone, the descaling with the high-pressure liquid may be performed at two or more locations in the casting direction, and the blasting process with the abrasive may be performed at one or more locations. Also, it is not necessary to perform both descaling and blasting, and only one of them is sufficient. A combination of the two may be implemented. The descaling and blasting positions should be less effective after supercooling has occurred in the slab, and preferably before supercooling has occurred, so at least one location should be secondary. Desirably upstream of the cooling zone.

  Even if the slab surface is sprayed with liquid, if the impact pressure on the slab surface is low, the scale is not sufficiently removed. Therefore, the impact pressure required to reliably perform descaling by spraying high-pressure liquid was investigated. Here, the collision pressure of the sprayed liquid is represented by the following equation (1).

Po = 0.05757 × (Q / A) ^1.08 × P ^0.47 … (1)
In the equation (1), Po is the liquid collision pressure (kgf / cm ² ), Q is the liquid injection flow rate (L / min), A is the spray collision area, and P is the spray injection pressure (kgf / cm ² ). And A is represented by the following equation (2).

A = 2 × H × tan (θ / 2) × 0.051 × H ^0.78 × Q ^0.09 × P ^-0.0045 … (2)
In Equation (2), H is the distance (cm) between the spray nozzle and the slab, θ is the spray injection angle (deg.), Q is the liquid injection flow rate (L / min), and P is the spray injection pressure ( kgf / cm ² ).

As an example, the injection pressure Po when the injection pressure P is 150 kgf / cm ² , the injection flow rate Q is 118 L / min, the distance H is 15 cm, and the spray injection angle θ is 31 degrees is expressed by the equations (1) and (2). To about 21 kgf / cm ² .

A slab surface to be cast by disposing a spray nozzle for descaling on the exit side of a small continuous caster for testing of 5 m in casting length, adjusting the spray injection pressure and flow rate to change the collision pressure on the slab surface Descaling was performed. FIG. 6 shows the survey results. The “normal scale material” in FIG. 6 is the result when using steel containing Mn and a very small amount of Cr, and the “thick scale material” is the result when using steel with less alloy components. The “containing SiO ₂ scale material” is the result when using steel containing 1.6% by mass of Si. Immediately after descaling, the slab was water-cooled to suppress scale growth. However, due to the high temperature, some scale growth was observed after descaling.

As shown in FIG. 6, when the impact pressure is zero, that is, when descaling is not performed, the scale thickness is about 800 μm for a normal scale material, about 1400 μm for a thick scale material, and about 1200 μm for a SiO ₂ -containing scale material. It was. It was found that the thick scale material can sufficiently remove the scale even when the collision pressure is about 1 kgf / cm ² . However, it has been found that a normal scale material requires a collision pressure of 5 kgf / cm ² or more for removing the scale, and a SiO ₂ scale material requires a collision pressure of 10 kgf / cm ² or more. It was.

Therefore, in the present invention, in order to surely remove any scale, the collision pressure in descaling by high pressure liquid spray is limited to 1 MPa (10.2 kgf / cm ² ) or more.

  Also, in the blast treatment with the abrasive, conditions for reliably removing the scale were examined using the above-described small continuous casting machine for testing.

That is, a shot blasting machine using an iron ball with a diameter of 0.5 to 1.2 mm as the abrasive is installed on the exit side of the test small continuous casting machine, and the abrasive is projected onto the slab surface. The density was changed and the slab surface to be cast was blasted. In this test, since the shot blasting machine is inherently excellent in scale removal capability, the SiO ₂ scale material having poor scale peelability was tested.

FIG. 7 shows the test results. As shown in FIG. 7, it was found that the scale can be reliably removed by setting the projected density of the abrasive to 5 kg / cm ² or more. Therefore, in the present invention, the projection density of the abrasive in the blasting process is limited to 5 kg / cm ² or more.

  Even if descaling with high-pressure liquid spray and blasting with abrasives are performed to remove the scale, the slab during continuous casting is hot, the scale is generated quickly, and the boiling characteristics change. After removing the slab, it is necessary to cool the slab by cooling water or mist within 10 seconds. For example, when the casting speed is 2.0 m / min, the slab has been pulled out by 33.3 cm in 10 seconds, and if the roll pitch of the slab support roll is 33 mm or less, the slab is between certain rolls. This condition can be sufficiently satisfied by disposing a descaling spray nozzle or a shot blasting machine and cooling the slab with a secondary cooling spray nozzle disposed between the next rolls.

  As described above, according to the present invention, when the slab surface is cooled using cooling water or a mixture of cooling water and gas in the secondary cooling zone of the continuous casting machine, the casting is performed by the continuous casting machine. The slab is descaled with high-pressure liquid and / or blasted with a polishing material in the secondary cooling zone to remove the scale on the slab surface. Because secondary cooling is performed, the MHF point, which is the point of transition from film boiling cooling to transition boiling cooling, decreases, and even under high-speed casting where the casting speed is increased, the film boiling stably exceeds the MHF point. The held cooling can be realized, whereby the slab surface is not overcooled, and the slab is cooled uniformly without causing temperature unevenness on the slab surface.

  An embodiment of the present invention in the slab continuous casting machine shown in FIG. 8 will be described. In FIG. 8, reference numeral 1 is a slab continuous casting machine, 2 is a tundish for relaying and supplying molten steel supplied from a ladle to a mold, 3 is a sliding nozzle for adjusting the flow rate of molten steel to the mold, , An immersion nozzle for injecting molten steel into the mold, 5 for a mold for cooling the molten steel, 6 for a roll for supporting and guiding the slab, and 7 for conveying the cast slab 8 is a gas cutting machine for cutting a cast slab into a predetermined length, 9 is a molten steel, 10 is a slab being cast, 10a is a slab cut, and 11 is a solidified shell. , 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 zone, a second zone, a third zone, and a fourth zone, and cooling conditions were set for each secondary cooling zone.

  Further, at the most upstream position of each secondary cooling zone, that is, four positions of AA ′ position, BB ′ position, CC ′ position, and DD ′ position in FIG. Descaling nozzles (not shown) were arranged in the width direction of the slab at eight locations on the front and back surfaces of the slab.

The descaling nozzle is a commercially available high pressure descaling nozzle, the water injection pressure is 14 kgf / cm ² , the injection flow rate is about 118 L / min, the injection angle at that time is 31 degrees, and the interval between adjacent nozzles is 140 mm. The distance from the slab surface to the nozzle tip was 150 mm. From this condition, the collision pressure determined by the equations (1) and (2) is 2.1 MPa. The descaling nozzle is twisted by 5 degrees in the axial direction and attached by being tilted by 15 degrees in the vertical direction. The number of descaling nozzles is 14 per row in each secondary cooling zone, for a total of 112 (= 14 × 2 × 4).

  Except for the position where the descaling nozzle was placed, in each secondary cooling zone, the slab was cooled using an air mist spray nozzle. The amount of cooling water and the amount of cooling air in each secondary cooling zone are as shown in Table 1.

  By spraying water from the descaling nozzle and peeling the scale on the slab surface, the MHF point can be lowered and the occurrence of overcooling of the slab can be prevented, so the slab surface temperature can be lowered. It is possible to increase the casting speed.

  As a result of casting the above-mentioned “normal scale material” using the slab continuous casting machine 1, conventionally, since supercooling has occurred, the casting speed could only be increased to 2.8 m / min. In the case of 2.1 MPa, the casting speed could be increased to 4.0 m / min. Even when the impact pressure of the descaling nozzle was lowered to 1.0 MPa, supercooling did not occur and operation at a casting speed of 4.0 m / min was possible. However, when the collision pressure was reduced to 0.5 MPa, supercooling occurred and the casting speed of 3.2 m / min was the limit.

  As a result of casting the above-mentioned “thick scale material”, conventionally, since supercooling has occurred, the casting speed could only be increased to 2.4 m / min, but when the collision pressure is 2.1 MPa, The casting speed could be increased to 0.0 m / min. Even when the impact pressure of the descaling nozzle was lowered to 1.0 MPa, supercooling did not occur and operation at a casting speed of 4.0 m / min was possible. However, when the collision pressure was reduced to 0.5 MPa, supercooling occurred and the casting speed of 3.0 m / min was the limit.

As a result of casting the above-mentioned “SiO ₂ scale material”, it is a material that is likely to crack, and conventionally the casting speed could only be increased to 1.6 m / min, but the impact pressure is 2.1 MPa. The casting speed could be increased up to 3.0 m / min without cracking. Even when the impact pressure of the descaling nozzle was reduced to 1.0 MPa, operation at a casting speed of 2.6 m / min was possible. However, when the collision pressure was reduced to 0.5 MPa, supercooling occurred and the casting speed of 2.1 m / min was the limit.

  In place of the descaling nozzle of the slab continuous casting machine 1 shown in the first embodiment, a total of eight shot brass machines are arranged at the position where the descaling nozzle was installed. A 2 mm iron ball was projected onto the entire width of the slab surface as a polishing material and blasted. Other conditions were performed according to Example 1.

As a result of casting the above-mentioned “normal scale material” using this slab continuous casting machine 1, conventionally, since supercooling occurred, the casting speed could only be increased to 2.8 m / min. When the projection density was 10 kg / cm ² , the casting speed could be increased to 3.8 m / min. Even if the projection density from the shot blast machine was reduced to 5 kg / cm ² , supercooling did not occur, and operation at a casting speed of 3.8 m / min was possible. However, when the projection density was lowered to ² kg / cm ² , supercooling occurred and the casting speed of 3.0 m / min was the limit.

As a result of casting the above-mentioned “thick scale material”, since supercooling has conventionally occurred, the casting speed could only be increased to 2.4 m / min. However, the projected density of the abrasive was 10 kg / cm ² . In some cases, the casting speed could be increased to 3.6 m / min. Even if the projection density from the shot blast machine was reduced to 5 kg / cm ² , supercooling did not occur, and operation at a casting speed of 3.6 m / min was possible. However, when the projection density was reduced to ² kg / cm ² , supercooling occurred and the casting speed of 2.6 m / min was the limit.

As a result of casting the above-mentioned “SiO ₂ -containing scale material”, it is a material that is likely to crack, and conventionally, the casting speed could only be increased to 1.6 m / min, but the projection density was 10 kg / cm ² . In some cases, the casting speed could be increased to 2.4 m / min without generating cracks. Even when the projection density from the descaling nozzle was reduced to 5 kg / cm ² , operation at a casting speed of 2.4 m / min was possible. However, when the projection density was lowered to ² kg / cm ² , supercooling occurred and the casting speed of 1.8 m / min was the limit.

It is a figure which shows the relationship between the generation | occurrence | production state of the temperature nonuniformity in a slab slab, and the generation | occurrence | production state of the surface crack of the slab slab after cooling. It is a figure which shows the relationship between the slab surface temperature at the time of cooling with an air mist spray nozzle, and a heat transfer rate. It is a figure which shows the relationship between slab surface temperature and heat transfer rate when deposits, such as a scale of slab surface, are changed. It is a figure which shows typically the cooling mode when the hot slab surface is cooled with spray water, and the heat flux at that time. It is a figure which shows the relationship between slab surface temperature when a scale of a slab surface is removed, and a heat transfer rate. It is a figure which shows the relationship between the collision pressure of spray water, and peeling of a scale. It is a figure which shows the relationship between the projection density of the abrasive in a shot blast process, and peeling of a scale. It is the schematic of the slab continuous casting machine to which this invention is applied.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Roll 7 Transport roll 8 Gas cutting machine 9 Molten steel 10 Cast piece 11 Solidified shell 12 Unsolidified phase

Claims (3)

  When the slab cast by the continuous casting machine is secondarily cooled using a cooling water or a mixture of cooling water and gas in a secondary cooling zone provided below the mold, Descaling the entire surface of the slab in the width direction of the slab with liquid having a collision pressure of 1 MPa or more at two or more positions in the casting direction, and secondary cooling of the slab within 10 seconds after the descaling A secondary cooling method in continuous casting. When the slab cast by the continuous casting machine is secondarily cooled using a cooling water or a mixture of cooling water and gas in a secondary cooling zone provided below the mold, The surface of the single piece is subjected to blasting over the entire width of the slab with a polishing material having a projection density of 5 kg / cm ² or more, and the slab is secondarily cooled within 10 seconds after the blasting. Secondary cooling method in continuous casting. When the slab cast by the continuous casting machine is secondarily cooled using a cooling water or a mixture of cooling water and gas in a secondary cooling zone provided below the mold, The surface of the slab is blasted over the entire width of the slab with a polishing material having a projection density of 5 kg / cm ² or more, and the slab width is applied with a liquid having a collision pressure of 1 MPa or more at two or more positions in the casting direction. A secondary cooling method in continuous casting, wherein descaling is performed over the entire direction, and the slab is secondarily cooled within 10 seconds after the blast treatment and within 10 seconds after the descaling.

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