JP5444807B2 - Method for preventing surface cracks in continuous cast slabs - Google Patents

Description

  The present invention relates to a method for preventing surface cracks in a steel slab in continuous casting of steel.

  In continuous casting of steel, preventing surface cracks of the slab is extremely important for maintaining good surface quality of the product after rolling. Causes of slab surface cracks are due to non-uniformity of initial solidification in the mold, grain boundary proeutectoid ferrite film formed during the transformation from austenite to ferrite, and carbon nitride at the grain boundary. There are things that are caused. In any case, reducing the austenite grain size increases the surface area of the relatively fragile austenite grain boundaries, and the grain boundaries of the slabs during slab correction in the straightening zone of a continuous casting machine. The stress acting on the surface is dispersed, and surface cracks are less likely to occur. In addition, transformation of austenite grains to a fine pearlite structure without forming a proeutectoid ferrite film at the grain boundaries avoids stress concentration at the austenite grain boundaries, and surface cracks are less likely to occur. Become.

In order to prevent surface cracking of a slab, as a method of making such a slab structure that is difficult to break through an austenite structure through another structure, for example, Patent Document 1 discloses a steel casting having a carbon equivalent of less than 0.18. When casting a piece with a curved or vertical bending type continuous casting machine, the drawing time from the meniscus (molten steel surface in the mold) to the lower end of the mold should be within 1 minute, and secondary cooling is performed immediately after drawing. A casting method is disclosed in which the slab surface temperature is cooled to the A ₃ transformation temperature or less within 1 minute.

In Patent Document 2, after the slab is drawn out of the mold, the surface temperature of the slab is once cooled to the A ₃ transformation temperature or lower, and then the water density is 0.003 to 0.015 liter / cm ² · min. perform slow cooling from 0.5 to 2.0 minutes as casting method for recuperation exceeds the a ₃ transformation temperature is disclosed.

Furthermore, Patent Document 3 discloses that Q = W / (H × D × Vc × ρ) (W: secondary cooling water amount (liter / minute)) from the mold outlet to 1.5 m in the casting direction. , H: slab width (m), D: slab thickness (m), Vc: casting speed (m / min), ρ: density of molten steel (kg / m ³ )) the ratio water Q is cooled slabs under the condition that a 0.4 to 1.5 liters / kg · steel, after cooling the surface temperature of the slab once below Ar ₃ transformation point, the Ar ₃ transformation point or higher A casting method is disclosed in which reheating is performed and then the slab is straightened.

Japanese Patent Laid-Open No. 9-47854 JP 11-197809 A JP 2002-86252 A

  All of the above prior arts are techniques for controlling the surface temperature of the slab in the continuous casting machine by secondary cooling, so that the structure of the slab is aimed, thereby preventing surface cracks of the slab. It is. However, since all of the above prior arts require rapid cooling over the entire cross section of the slab, that is, the entire width of the slab, the equipment for the cooling becomes large and requires a large equipment cost. There is.

  By the way, as for the surface crack of the slab, the position in the width direction where the crack occurs and the position in the casting direction (timing at which the crack occurs) are often limited. For example, in a vertical bending type continuous casting machine, on the lower surface side of the slab near the upper straightening zone to bend the slab, and on the upper surface side of the slab near the lower straightening zone to bend the bent slab, Cracks often occur.

  The present invention has been made in view of the above circumstances. The purpose of the present invention is to prevent the occurrence of surface cracks in the prevention of surface cracks in slabs by utilizing the phase transformation from austenite to ferrite or pearlite. By controlling the secondary surface cooling and controlling the surface temperature of the slab only in the slab area, it is possible to prevent surface cracks of the slab more efficiently than the conventional technology that controls the surface temperature of the entire width of the slab. An object of the present invention is to provide a method for preventing surface cracking of continuous cast slabs.

In order to solve the above-mentioned problems, the method for preventing surface cracking of a continuous cast slab according to the present invention is a method of continuously casting molten steel using a vertical bending slab continuous casting machine or a curved slab continuous casting machine. Predict or measure the surface temperature distribution of the long side of the slab and the short side of the slab when bending strain is applied to the slab, and determine the part of the slab where the surface temperature becomes an embrittled region from the surface temperature distribution of the slab. 300 liters until the point at which the part is transformed into either a ferrite single-phase structure or a pearlite structure in the secondary cooling zone immediately below the mold for the part of the slab where the surface temperature is in the brittle temperature range It is characterized in that the part of the slab immediately after being drawn out of the mold at a cooling water density of not less than / m ² · min is continuously cooled.

According to the present invention, the surface temperature distribution of the long side surface and the short side surface of the slab at the time of applying bending strain to the slab in the continuous casting machine is predicted or measured, and the surface temperature becomes a brittle region. Only the part of the slab where the surface temperature is in the embrittlement temperature region is specified, and the part is transformed into either a ferrite single phase structure or a pearlite structure in the secondary cooling zone immediately below the mold. Because it is strongly cooled at a cooling water density of 300 liters / m ² · min or more until the time, the amount of secondary cooling water used is small, and the surface cracking of the slab can be achieved simply by increasing the secondary cooling device of the continuous casting equipment. Is effectively prevented.

It is a general-view figure of the slab which the surface crack generate | occur | produced in the short side surface. It is a side schematic diagram of a vertical bending type slab continuous casting machine suitable for carrying out the present invention. FIG. 3 is a schematic plan view of a secondary cooling device directly under a mold of the continuous casting machine shown in FIG. 2. It is a figure which shows the relationship between the area reduction rate at the time of a fracture | rupture in a hot tensile test of the steel type shown in Table 1, and test temperature.

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

  The inventors used a medium-carbon steel molten steel having chemical components shown in Table 1 and a vertical bending slab continuous casting machine having specifications shown in Table 2 to cast a slab slab having a thickness of 220 mm and a width of 1800 mm with a casting speed. Casting was performed at 0.8 to 1.1 m / min, and the molten steel temperature in the tundish was 1545 to 1560 ° C.

  As a result, cracks frequently occurred on the short side surface of the slab at a position 20 to 30 mm away from the corner portion on the upper surface side. An overview of the surface cracking of this slab is shown in FIG. On the other hand, surface cracks did not occur on the upper and lower surfaces of the long surface of the slab.

  Therefore, in order to identify this cause, we conducted temperature, thermal stress, and deformation analysis of the slab during casting. As a result, in this continuous casting machine, when the slab is bent back and straightened in the lower straightening band, (1) the vicinity of the corner of the slab becomes an embrittlement temperature range, and (2) the upper side corner of the slab. In the vicinity of the part, it was found that a tensile stress state was caused by strain due to bending back correction and thermal strain, and it was found that this caused cracks on the short side surface side.

  On the other hand, the reason why the surface crack does not occur in the corner portion of the upper side long side surface having the same temperature and stress state is not clear. Therefore, the structure of the cross section (C cross section) perpendicular to the casting direction of the slab was observed. As a result, austenite grain boundaries having a pro-eutectoid ferrite film were clearly observed in most of the C section. Since this steel type has a carbon concentration of 0.15 to 0.17% by mass, it is a steel in which austenite grains are most likely to be coarsened, and when tensile stress acts in such a state, cracks easily occur at austenite grain boundaries. It was estimated. However, the corner portion of the long side surface on the upper surface side of the slab has a fine structure.

  Considering these things, even if tensile stress is applied to the long side surface and the short side surface in the same way across the corner of the slab, the long side surface has a microstructure, so the austenite grains The straightening stress acting on the boundary is dispersed and does not lead to cracking. On the other hand, since the short side surface has a coarse austenite grain structure, it was considered that tensile stress concentrated on the grain boundary and led to cracking.

Based on this result, the present inventors have conducted various studies on secondary cooling conditions for obtaining the same structure on the short side surface side as on the long side surface side. As a result, in the secondary cooling zone directly under the mold, the slab short side surface immediately after being drawn out of the mold, until that part transforms into either a ferrite single-phase structure or a pearlite structure, It has been found that by continuing to cool at a cooling water density of not less than 300 liters / m ² · min, the same morphology can be obtained as on the long side surface side. In other words, when corrective strain or the like acts on the slab in a continuous casting machine, when the part enters the embrittlement temperature range, the part is strongly cooled in advance directly under the mold, and either the ferrite single phase structure or the pearlite structure is used. On the other hand, it was found that once transformed, the occurrence of surface cracks is suppressed even when tensile stress is applied to the slab in the embrittlement temperature range.

The present invention has been made on the basis of the above knowledge, and the method for preventing surface cracking of a continuous cast slab according to the present invention is a continuous bending of molten steel using a vertical bending slab continuous casting machine or a curved slab continuous casting machine. When casting, predict or measure the surface temperature distribution of the long side of the slab and the short side of the slab when bending strain is given to the slab in the continuous casting machine, and the surface temperature becomes brittle from the surface temperature distribution of the slab. The part of the slab that becomes the zone is specified, and the part of the slab whose surface temperature becomes the embrittlement temperature range is either the ferrite single-phase structure or the pearlite structure in the secondary cooling zone immediately below the mold. Until the time of transformation, the portion of the slab immediately after being drawn out of the mold at a cooling water density of 300 liters / m ² · min or more is continuously cooled.

  Next, exemplary embodiments of the present invention will be described with reference to the drawings. FIG. 2 is a schematic side view of a vertical bending slab continuous casting machine suitable for carrying out the present invention, and FIG. 3 is a schematic plan view of a secondary cooling device directly below the mold of the continuous casting machine shown in FIG.

  As shown in FIG. 2, the vertical bending type slab continuous casting machine 1 is provided with a mold 5 for cooling and solidifying the molten steel 9 to form the outer shell shape of the slab 10. A tundish 2 for relaying and supplying molten steel 9 supplied from a ladle (not shown) to the mold 5 is installed at a predetermined upper position. On the other hand, a plurality of pairs of slab support rolls 6 including a support roll, a guide roll, and a pinch roll are arranged below the mold 5. These slab support rolls 6 are slab support / guide devices for guiding the slab 10 pulled out from the mold 5 while guiding it downward.

  A plurality of pairs of slab support rolls 6 disposed at a position about 1 m to 4 m away from the exit of the mold 5 includes an upper correction band 14 that changes the support / guide direction of the slab 10 from a vertical direction to a bending direction. It is composed. That is, the slab 10 on the flat plate drawn out from the mold 5 in the vertical direction is gradually bent into an arc shape by the upper correction band 14, and is corrected to a curved portion having a constant radius. In the upper straightening band 14, tensile stress acts on the lower surface side of the slab 10, and compressive stress acts on the upper surface side. Therefore, in the upper correction band 14, surface cracks are likely to occur on the lower surface side of the slab 10, and surface cracks generally do not occur on the upper surface side. In this case, the upper surface side and the lower surface side are defined with the thickness center position of the slab 10 as a boundary.

  Similarly, the plurality of pairs of slab support rolls 6 disposed in the vicinity of the position where the curved portion comes into contact with the horizontal line includes the lower correction belt 15 in which the support / guide direction of the slab 10 changes from the curved direction to the horizontal direction. It is composed. That is, the arc-shaped slab 10 is gradually bent back on the flat plate by the lower correction band 15 and corrected to the horizontal portion. In the lower correction band 15, tensile stress acts on the upper surface side of the slab 10, and compressive stress acts on the lower surface side. Therefore, in the lower correction band 15, surface cracks are likely to occur on the upper surface side of the slab 10, and surface cracks generally do not occur on the lower surface side.

  In FIG. 2, both the upper correction band 14 and the lower correction band 15 are configured by a plurality of pairs of slab support rolls 6, but may be configured by only a pair of slab support rolls 6. The upper correction band 14 and the lower correction band 15 of the present invention include cases where correction is performed with a pair of guide rolls.

  A secondary cooling zone in which a spray nozzle (not shown) such as a water spray nozzle or an air mist spray nozzle is arranged is formed in the gap between the slab support rolls 6 adjacent in the casting direction. The slab 10 is cooled while being drawn out by the cooling water sprayed from the nozzle. This secondary cooling zone is divided into several cooling zones in the casting direction so that the amount of secondary cooling water can be adjusted individually in each cooling zone. FIG. 2 shows the cooling water pipe 18 on the upper surface side of the first cooling zone immediately below the mold, and the cooling water pipe 18 is provided with a flow rate adjusting valve 19, which is supplied from the control device 17 (process computer). The flow rate of the cooling water is controlled by this signal. The other cooling zones have the same configuration.

  In the slab continuous casting machine 1, the first cooling zone immediately below the mold is capable of independently controlling the amount of cooling water sprayed from the spray nozzle 20 in the width direction of the slab 10, as shown in FIG. It is configured. For example, as shown in FIG. 3, in the width direction of the slab 10, the slab 10 is divided into five ranges of zones A to E from one corner to the other corner, and on the short side surface side of the slab 10. Has a short side zone. Although it is possible to control the cooling water independently in each zone, in this slab continuous casting machine 1, the zone A and the zone E that are symmetrical with respect to the center position in the width direction of the slab 10 Are connected to the same cooling water pipe, and similarly, zone B and zone D are connected to the same cooling water pipe. The short side zone is connected to the cooling water pipe in which the left and right short side zones are the same. That is, the amount of cooling water can be controlled by dividing into four parts, zone A + zone E, zone B + zone D, zone C, and short side zone.

In addition, when necessary, the first cooling zone is cast at a cooling water density of 300 liters / m ² · min or more until the surface layer portion of the slab 10 is transformed into either a ferrite single phase structure or a pearlite structure. It is necessary to continue cooling the piece 10, that is, the hot slab 10 immediately after being drawn out from the mold 5 is rapidly lowered to A ₃ transformation temperature (ferrite precipitation temperature) or A ₁ transformation temperature (pearlite transformation temperature) or lower. It is necessary to cool, and therefore the length of the first cooling zone in the casting direction requires a considerable length. For example, depending on the casting speed and the cooling water density, a length of about 1 m or more is required. Precisely, it can be obtained by heat transfer calculation based on the casting speed and the cooling water density.

  The lower correction belt 15 is provided with a radiation thermometer 16 for measuring the surface temperature of the slab 10 when the lower correction belt 15 is corrected. Has been entered. The radiation thermometer 16 is movable in the width direction of the slab 10 and is configured to measure the surface temperature while moving, and measures the surface temperature of the entire long side surface on the upper surface side of the slab 10. . In addition, a radiation thermometer (not shown) that is movable in the thickness direction of the slab 10 for measuring the surface temperature distribution of the slab short side surface is also installed, and the surface temperature measurement of the slab short side surface is performed. The value is also input to the control device 17.

  A sliding nozzle 3 for adjusting the flow rate of the molten steel 9 injected from the tundish 2 into the mold 5 is installed at the bottom of the tundish 2, and the molten steel 9 is injected into the mold 5 at the lower surface of the sliding nozzle 3. An immersion nozzle 4 is provided for this purpose. A plurality of transport rolls 7 for transporting the cast slab 10 are installed on the downstream side of the slab support roll 6. Above the transport roll 7, the cast slab 10 to be cast is provided. A slab cutting machine 8 for cutting a slab 10a having a predetermined length is disposed.

  The vertical bending slab continuous casting machine 1 having such a configuration is used to carry out the present invention as follows.

  First, molten steel 9 is injected from the tundish 2 into the mold 5 through the immersion nozzle 4. The molten steel 9 injected into the mold 5 is cooled by the mold 5 to form a solidified shell 11 and continuously supported downward by a plurality of pairs of slab support rolls 6 as a slab 10 having an unsolidified layer 12 therein. Pulled out. Mold powder (not shown) is added on the meniscus 13 of the mold 5. The slab 10 cooled in this way increases the thickness of the solidified shell 11 and eventually completes the solidification to the center. The solidified slab 10 is cut by a slab cutting machine 8 to obtain a slab 10a.

In such a continuous casting operation, casting is started when the cooling water density in each cooling zone is in principle less than 300 liters / m ² · min. During the casting, based on the measurement results of the surface temperature of the long side surface and the short side surface of the slab 10 with the radiation thermometer arranged in the lower correction band 15, the controller 17 determines that the surface temperature in the lower correction band 15 is the embrittlement temperature. Identify the region that will be the zone. And when the site | part where surface temperature becomes an embrittlement temperature range exists, slab surface temperature is an embrittlement temperature range from the zone A to E of a secondary cooling zone 1st cooling zone, and a short side surface zone. The cooling zone corresponding to the part to be identified is specified, and a flow rate increase signal is supplied to the flow rate control valve 19 of the specified cooling zone so that the cooling water density becomes a predetermined cooling water density of 300 liters / m ² · min or more. To send. The flow control valve 19 that has received the signal for increasing the flow rate opens the opening and increases the coolant to a predetermined flow rate.

  In this casting chance, as a rule, the cooling zone in which the flow rate of the first cooling zone is increased continues casting in a changed state. However, if it is confirmed that the surface temperature of the slab 10 is clearly out of the embrittlement temperature range by the radiation thermometer arranged in the lower correction belt 15, the cooling amount is returned to the original level before the increase. You can also.

The determination as to whether or not the surface temperature of the slab 10 is in the embrittlement temperature range by the radiation thermometer can be made, for example, from a hot tensile test of the steel type. For example, FIG. 4 shows the relationship between the area reduction rate at break and the test temperature in the hot tensile test of the steel types shown in Table 1 above. In the case of this steel type, 750 to 800 ° C. is the embrittlement temperature range. Can be judged. Thus, what is necessary is just to grasp the embrittlement temperature range for every steel type beforehand. Here, the breaking area reduction ratio of, with respect to the cross-sectional area of the tensile specimen before the tensile test (D _0), the specimen of the cross-sectional area of the tensile specimen before tensile test and (D ₀₎ breaking point cross The percentage (100 × (D ₀ -D) / D ₀ ) of the difference (D ₀ -D) from the area (D), and the small area reduction rate means that the specimen breaks at a stage where the elongation is small. That means brittleness.

  In this example, a radiation thermometer is installed in the lower correction band 15 to prevent surface cracks of the slab 10 in the lower correction band 15, but the lower surface side and the short side surface side of the upper correction band 14. A radiation thermometer may be installed to prevent surface cracks of the slab 10 in the upper straightening zone 14. In addition, equipment for measuring the slab surface temperature, such as a radiation thermometer, is not always necessary, and it becomes a risk zone for surface cracking by using heat transfer and solidification calculations based on the casting conditions and secondary cooling water distribution in advance. A method of specifying a slab part and increasing the amount of cooling water in the first cooling zone for this part can also be performed. In the present invention, the time point at which bending strain is applied to the slab 10 in the continuous casting machine refers to the bending process of the slab 10 in the upper straightening strip 14 and the bending back process of the slab 10 in the lower straightening strip 15.

As described above, according to the present invention, the slab surface temperature distribution at the time of applying bending strain to the slab 10 in the continuous casting machine is predicted or measured, and the slab 10 whose surface temperature becomes an embrittlement region. Only for the slab part where the surface temperature is in the brittle temperature range, until the point at which the part transforms into either a ferrite single phase structure or a pearlite structure in the secondary cooling zone immediately below the mold. Strong cooling is performed at a cooling water density of 300 liters / m ² · min or more, so there is little increase in the amount of secondary cooling water used, and the surface cracking of the slab 10 is efficient even if the secondary cooling device is simply enhanced. It becomes possible to prevent well.

  By increasing the amount of secondary cooling water in the initial stage of solidification, the temperature at the risk of cracking is lowered, and the surface temperature of the slab at the position of the straightening zone where cracking occurs is lower than the embrittlement temperature range, thereby avoiding surface cracking. There is also a possibility to do it. However, in this method, since a large temperature difference is caused in the width direction of the slab 10, the adjacent portion that was a crack risk area newly enters the crack risk area or the temperature difference in the slab width direction. This causes a problem that the center segregation deteriorates, which is not preferable. As in the present invention, it is desirable to transform the austenite to a ferrite single-phase structure or pearlite structure only by a local rapid cooling in a short time and to strengthen the structure.

  Using a slab continuous casting machine whose specifications are shown in Table 2 and whose schematic is shown in FIG. And 0.8 m / min.

The installation position of the first cooling zone in this continuous casting is in the range from 0.7 m to 1.7 m from the meniscus. Under the present casting conditions, the length of the first cooling zone in the casting direction (= 1.0m), and confirm in advance that the long edge side and the short edge of the slab have a ferrite single-phase structure or pearlite structure by secondary cooling with a cooling water density of 300 liters / m ² · min or more. Yes. In the first cooling zone, the zone A and the zone E are the corner position and the range from the corner to the 80 mm position, and the zone B and the zone D are the remaining 80 mm from the corner and the range from the corner to the 240 mm position. The part is zone C.

  Table 3 summarizes the casting conditions, the results of the surface temperature in the lower straightening zone, the cooling conditions in the first cooling zone, and the occurrence of surface cracks.

Condition 1 is a basic cooling water amount density pattern, and immediately below the mold, cooling water of 240 liters / m ² · min was sprayed on the long side of the slab and 200 liters / m ² · min was sprayed on the short side of the slab. It is a casting test and is Comparative Example 1 for comparison with the inventive example. In condition 1, the edge of the long side surface on the upper surface side of the slab and the upper part of the short side surface become a brittle temperature region (750 to 800 ° C.), and the surface is in a range of 20 to 30 mm from the upper surface side corner of the slab short side surface. Cracking occurred.

Condition 2 is determined from the surface temperature measurement value of the slab from the end of the long side and the short side of the slab as the surface cracking risk area in the lower straightening zone, and from the condition of the amount of secondary cooling water directly under the mold, A and zone E and the short side surface cooling is judged to be insufficient, the cooling water of zone A and zone E is 300 liters / m ² · min, and the cooling water of the short side zone is doubled (= The present invention example 1 was increased to 400 liters / m ² · min). In Inventive Example 1, although the long side surface end on the upper surface side of the slab and the upper part of the short side surface are in the brittle temperature range, as a result of intense cooling of the zone A and zone E and the short side surface zone, The structure of the part was strengthened, and surface cracks did not occur in the slab, and the effects of the present invention were exhibited.

Conditions 3 and 4 are conditions in which the casting speed and the amount of secondary cooling water per slab unit volume are changed with respect to conditions 1 and 2, and condition 3 is 240 liters on the long side of the slab immediately below the mold. / m ² · min, a cast test sprayed with 200 liters / m ² · min cooling water to the slab narrow side, is a comparative example 2 for comparison with the present invention embodiment. Under condition 3, the end side away from the corner of the long side surface of the slab by about 150 mm was an embrittlement temperature region, and surface cracks occurred in the range of 100 to 200 mm from the corner of the long side surface of the slab upper surface.

Condition 4 is based on the measured surface temperature of the slab, and the end side that is about 150 mm away from the corner of the long side surface on the upper side of the slab is determined to be a surface cracking risk site in the lower straightening zone, and secondary cooling directly under the mold It is the present invention example 2 in which it is determined that the cooling of the zone B and the zone D is insufficient from the water amount condition, and the cooling water of the zone B and the zone D is increased 1.5 times (= 360 liters / m ² · min). . In Example 2 of the present invention, although the end portion side about 150 mm away from the corner of the long side surface on the upper side of the slab is an embrittlement temperature region, as a result of intense cooling of the zone B and the zone D, the structure of the part is strengthened, The surface crack did not occur in the slab, and the effect of the present invention was exhibited.

  Thus, in Invention Examples 1 and 2, as in Patent Documents 1 to 3, strong cooling was not performed on the entire surface of the slab directly under the mold, but the amount of cooling water was locally increased as necessary. It was confirmed that the present invention is more advantageous than Patent Documents 1 to 3 in that surface cracks can be avoided by simply making it, and the present invention can be handled with simple equipment.

DESCRIPTION OF SYMBOLS 1 Slab continuous casting machine 2 Tundish 3 Sliding nozzle 4 Immersion nozzle 5 Mold 6 Casting piece support roll 7 Conveying roll 8 Casting piece cutting machine 9 Molten steel 10 Cast piece 11 Solidified shell 12 Unsolidified layer 13 Meniscus 14 Upper straightening belt 15 Lower straightening Belt 16 Radiation thermometer 17 Control device 18 Cooling water piping 19 Flow control valve 20 Spray nozzle

Claims (1)

When continuously casting molten steel using a vertical bending slab continuous casting machine or a curved slab continuous casting machine, casting is performed in the continuous casting machine under casting conditions in which the cooling water density in each cooling zone is less than 300 liters / m ² · min. the slab long side surface and the billet surface temperature distribution of the short side surface at the time of giving a bending strain on one to the actual measurement, to identify sites of slab surface temperature from the template strip surface temperature distribution becomes embrittling The part of the slab where the surface temperature is in the brittle temperature range is cooled until the part transforms into either a ferrite single-phase structure or a pearlite structure in the secondary cooling zone immediately below the mold. The surface crack of a continuous cast slab, characterized in that it is increased to a cooling water density of 300 liters / m ² · min or more only in the cooling zone, and the part of the slab immediately after being drawn from the mold is continuously cooled. Prevention method.

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