JP5402790B2 - Method for cooling continuous cast bloom slab and method for manufacturing the slab - Google Patents

Description

  The present invention relates to a method for cooling a continuous cast bloom slab and a method for manufacturing the slab, and in particular, can prevent meandering and bending during cooling of the continuous cast slab, and cool the continuous cast slab. It is a cooling method that can greatly reduce cracks that sometimes occur on the surface and cracks that occur on the surface during hot rolling, and can be applied to a wide range of steel types such as low-carbon steel and highly hardened steel grades. It is related with the cooling method of a piece. Further, by appropriately cooling the slab during continuous casting, a wide range of steel types such as low carbon steel and highly hardenable steel grades can be used. The present invention relates to a method for producing a bloom slab having a clarification layer.

  Continuous casting bloom slab is a process of pouring molten steel from a ladle into a tundish, further injecting it into a mold, forming a solidified shell in the mold, further cooling the slab in a secondary cooling spray zone, and solidifying the slab. It is manufactured by drawing out the continuous casting machine.

  However, cracks may occur on the surface of the slab when the continuous cast slab is cooled or when the slab is hot-rolled. Since the occurrence of cracks is also affected by the type of cast steel, it is a method that can greatly reduce those cracks, and it is a continuous cast slab that can be applied to a wide range of steel types such as low carbon steel and high hardenability. A manufacturing method has been desired.

For example, when rolling a continuous cast bloom slab, various measures have been proposed to prevent slab surface cracks that occur after reheating and rolling the slab, and cooling outside the continuous caster. By reducing the surface temperature of the continuously cast slab from a temperature higher than the Ar ₃ transformation point to a temperature not higher than the Ar ₁ transformation point, a method called tertiary cooling that prevents the occurrence of surface cracks on the slab It has been known.

As a technique related to the tertiary cooling, Patent Document 1 discloses that a continuous cast bloom is cut into a predetermined length, and then a bloom cooler installed outside the apparatus is used to set the bloom to a temperature range immediately above the Ar ₃ transformation point. A method for preventing cracks occurring during cooling of a continuous casting bloom is described, wherein the cooling is performed by changing the water density ratio of the bloom upper surface, side surface, and lower surface when cooling.

Also, in Patent Document 2, when a bloom cooler is used to cool a bloom from a temperature range immediately above the Ar ₃ transformation point, the bloom moving speed is changed from 3 m / min to 10 m / min. There is described an invention for reducing surface defects of bloom, characterized in that the lower surface of the single piece is cooled uniformly while ensuring a predetermined cooling rate.

  In all cases, the surface of the slab is subjected to ferrite-pearlite transformation, bainite transformation, martensite transformation, etc. during cooling of the slab, and then reheated and retransformed before rolling to prevent surface flaws during rolling. ing.

However, since these methods cool the slab after cutting, there is a possibility that new problems such as bending occur during cooling. Further, there is a concern that sufficient cooling cannot be performed to avoid this problem, and even if a certain degree of cracking prevention effect is recognized, a sufficient effect may not be obtained.
Furthermore, Patent Document 3 discloses that a slab manufactured by continuous casting and cut to a predetermined length is cooled to a temperature 50 to 150 ° C. higher than the Ar ₃ transformation point, and the inside is in a red hot state. The surface defects can be sufficiently reduced even for nitrogen-added steel and lead-added steel, characterized by rapid cooling so that the surface structure becomes a bainite structure, and then hot forming by heating in the furnace. A hot working method for continuous cast slabs is described. However, this method does not solve the above-mentioned problem of bending of the slab, and the concern that sufficient cooling may not be performed to avoid the problem is the same.

  In addition, in high carbon steel and high Mn steel with high hardenability, thermal stress cracking may occur due to rapid cooling.

Japanese Patent Laid-Open No. 10-1719 JP 2005-40837 A JP-A-6-88125 JP 2002-307149 A

  The present invention can prevent the bending of the continuous cast slab and the meandering accompanying it, and greatly reduces the cracks generated on the surface during cooling of the continuous cast slab and the surface generated during hot rolling. An object of the present invention is to provide a cooling method for a continuously cast slab that can be applied to a wide range of steel types such as low carbon steel and high hardenability.

  In addition, by appropriately cooling the slab during continuous casting, austenite (hereinafter referred to as “γ” below a surface layer of the bloom slab has a thickness of 4 mm or more with respect to a wide range of steel types such as low carbon steel and high hardenability. It is an object of the present invention to provide a method for producing a bloom slab having a layer in which crystal grain boundaries are obscured.

1) Prevention of bending of cast slab As described above, the conventional tertiary cooling method is such that, as described above, the cut slab is cooled from the temperature range immediately above the Ar ₃ transformation point, and retransformation during reheating performed before the block rolling. The cooling method is aimed at refining the structure (refining γ grains), and is performed by spray water injection or water bath immersion method.

  However, since the conventional tertiary cooling method cools the slab after cutting the slab into a predetermined length, if a slab cooling device having a slab restraint function is not specially installed, In the method, there is no slab restraint (support) when the slab is cooled.

Furthermore, in the case of the water bath immersion method, the cooling control unique to the slab surface is difficult, and it is difficult to prevent bending of the slab that occurs during uneven cooling.
In this way, slab bending (meandering) may occur, and cooling before slab cutting impedes continuous casting operation (the slab moves from the slab transfer path in the continuous casting apparatus by meandering). It is conceivable that the transfer of the slab is stopped to the side and the like, and conventionally, the third cooling is carried out by the batch method outside the continuous casting machine.

  Even if the slab surface is cooled by spray cooling, if the slab is not uniformly restrained and the slab is not evenly cooled, the slab will bend after cooling and meander during subsequent slab transport. Be careful as it may interfere with operation.

  Therefore, in order to solve the problem, the present inventor performs the third cooling of the slab with a pinch roll band of a normal continuous casting apparatus, and uses the roll installed therein to restrain the slab during cooling. Inspired to do. This is because this method can cool the slab while restraining the slab, so that the slab can be strongly cooled without worrying about the bending of the slab.

However, the bending of the slab may occur not only during strong cooling but also during cooling after releasing the restraint. Therefore, it is necessary to devise a method for keeping strong cooling only in the surface layer portion of the slab and eliminating the bending residual stress in the surface portion of the slab during restraint of the slab. For this purpose, it is effective to reheat the surface temperature of the slab to the Ar ₁ transformation point or higher during slab restraint based on conventional knowledge. What is necessary is just to find the cooling conditions which can perform.

2) Prevention of slab surface cracks (1) Basic concept The cooling method according to the present invention is related to the tertiary cooling method generally employed in bloom continuous casting machines, and the surface structure of the slab is modified. Is a method in which slab meandering, which is a problem during conventional tertiary cooling, is prevented. In the present invention, the cooling of the slab is adjusted to modify the crystal structure of the surface part of the slab and refine it, thereby preventing cracks on the surface of the slab.

As described above, it is more reasonable to perform the operation at an early stage during continuous casting in order to strongly cool and reheat the slab surface while restraining the slab. For example, if there is unsolidified molten steel inside the slab, the temperature of the surface layer portion once cooled can be easily increased again by utilizing the sensible heat or latent heat.
However, if the surface of the slab is strongly cooled with an unsolidified layer inside the slab, the slab surface layer shrinks, so in the case of bloom, the slab shrinks to the center of the slab and the molten steel flows in the center. There is a risk that segregation may deteriorate or internal cracks may occur during recuperation. Moreover, when it is before bending correction of a slab, when it cools strongly like this invention, the surface temperature of a slab corner part may fall and a crack may generate | occur | produce. Therefore, in order to avoid these problems, it is necessary to start the strong cooling of the slab after the solidification to the center of the slab and after the bending of the slab is straightened.

However, the purpose of the slab strong cooling, so is to miniaturization as well as modify the crystalline structure of the slab surface portion, the slab surface temperature to start a strong cooling in Ar ₃ transformation point or more stages Thus, the Ar ₁ transformation point or less must be set to an appropriate depth of the surface layer portion. On the other hand, in order to prevent slab bending, the slab surface temperature must be returned to the Ar ₁ transformation point or higher during slab restraint.

Then, such strong cooling is after straightening the slab and after complete solidification, after cooling the surface temperature of the slab from the Ar ₃ transformation point to the Ar ₁ transformation point, the Ar ₁ transformation is performed again. It is thought that it should be done to raise it above the point.

  In order to do this, after the slab is bent and straightened, strong cooling is started as soon as possible after complete solidification, and cooling is stopped in an appropriate short time, and the slab by internal sensible heat is used. The surface layer must be reheated.

(2) Specific slab cooling conditions As described above, after the slab is bent and straightened, strong cooling is started as soon as possible after complete solidification, and cooling is stopped in an appropriate short time. To reheat the slab surface by internal sensible heat. By this, while preventing the bending of the slab, the crystal structure of the slab surface layer is modified and refined to prevent the slab surface crack after casting and the slab surface crack after partial rolling. However, this is the basic idea of the present invention.

  In order to concretely implement this concept, several suitable embodiments of slab cooling are conceivable. Depending on the cooling conditions, not only can the above-mentioned slab surface cracks be prevented, but also a part of the slab surface layer becomes martensite during rapid cooling of the slab, and cracking occurs due to expansion during reheating, This is because a crack may occur due to the thermal stress between the slab surface and the inside during the previous heating, so care must be taken.

Accordingly, the inventors have made a continuous casting in order to make the γ grain boundary unclear structure found in Patent Document 4 exist in the slab surface layer part and to modify the slab surface layer part number mm to a structure having high-temperature ductility. The requirements for cooling in-flight were investigated. Here, the “γ grain boundary unclear structure” is a mixed structure of ferrite and pearlite in which the γ grain boundary is unclear, and specifically, a slab from the high temperature side to the low temperature side from the Ar ₃ transformation point. This means a solidified structure in a state where ferrite is formed in a granular form at the γ grain boundary when is cooled. On the surface of the slab where the γ grain boundary indistinct structure is formed, the critical stress inherent to the steel against cracking increases.

  In the present invention, it is preferable that a γ grain boundary unclear structure exists in a continuous casting machine at a thickness of 4 mm or more in the slab surface layer. In order to suppress the occurrence of cracks on the surface of the slab, which is generated when such a slab having a γ grain boundary unclear structure is supplied to the block rolling process and heated and rolled in the block forming process. It is because it is preferable.

In the method described in Patent Document 4, the slab surface temperature is rapidly cooled to the Ar ₃ transformation point or less once in the secondary cooling zone immediately after the slab comes out of the mold, and the slab is further utilized by sensible heat of the internal molten steel. In this method, the structure is rapidly reheated to an Ar ₃ transformation point or more so that a γ grain boundary indistinct structure exists in the slab surface layer part thickness of several mm to form a structure having high temperature ductility.

  However, in the case of rapid cooling directly under the mold, the slab surface temperature is high (near 1100 ° C.), so the heat transfer coefficient of the spray is small, so a sufficient cooling rate cannot be secured unless super strong cooling is performed. In the case of cooling in the pinch roll zone, since the slab surface temperature is lowered to around 900 ° C., the heat transfer coefficient of the spray is large, and the slab surface cooling rate is increased even with relatively gentle cooling. There is an advantage that it is possible. For steel types that are susceptible to cracking due to rapid cooling and rapid recovery heat, it is effective to improve the structure of the slab surface layer by taking advantage of this advantage in order to suppress surface cracks in the slab.

As a result of various experimental investigations as described below for the conditions for that, as a result of slab bending during continuous casting, spray water after straightening of the slab and after complete solidification, the cooling water density at the time of strong cooling It was understood that it is preferable to set the range of 350 to 1400 L / min · m ² per surface area of the slab and to set the amount of cooling water per surface area of the slab to 650 to 1680 L / m ² in terms of water density × injection time.

  Furthermore, in the conventional tertiary cooling method, the cooling zone is generally set over several meters, but in the present invention, the length in the casting direction is 1 m or less in the pinch roll band in the continuous casting machine. It was also confirmed that it is preferable to provide a plurality of sprays in the range to shorten the cooling time.

The present invention completed based on the above knowledge is as follows.
(1) A method of cooling a slab when continuously casting molten steel to produce a bloom slab, wherein the slab is bent during straight casting and spray water is sprayed after complete solidification. Then, after cooling the surface temperature of the slab from the Ar ₃ transformation point to the Ar ₁ transformation point, the temperature is raised again to the Ar ₁ transformation point and the spray water is sprayed. the density was 350L / min · ^{m 2} or more 1400L / min · ^{m 2} or less, and characterized by performing by adjusting the value of the cooling water density × coolant injection time 650L / ^{m 2} or more 1700L / ^{m 2} or less A method for cooling a bloom slab during continuous casting.

(2) in the range inject after complete solidification of the slab in the casting direction downstream side of the inclusive 3m 10 m of the spray water, and performing only an interval within 1m in the casting direction, the (1) The method for cooling a bloom slab during continuous casting as described in 1.

( 3 ) Production of a bloom slab having an austenite grain boundary obscuring layer having a thickness of 4 mm or more below the slab surface layer, characterized by using the slab cooling method described in (1) or (2 ) above. Method.

  According to the present invention, over a wide range of steel types, both the generation of flaws during cooling of continuous cast slabs and hot rolling, and deformation of the slabs during cooling are suppressed, and the bloom has a γ grain boundary unclear structure in the surface layer. Stable production of the slab is realized.

It is a perspective view which shows notionally the structure of the test apparatus for setting the continuous casting conditions based on this invention. It is a flowchart which shows the work content of the experiment using the test apparatus shown by FIG. It is a graph which shows the temperature transition data in the cooling experiment conducted using the test apparatus shown by FIG. It is a side view which shows notionally the installation place of the thermocouple in the vicinity of the ingot surface during spray cooling in the test apparatus shown by FIG. It is a graph which shows the relationship between the structure | tissue modification depth of a slab surface layer part, and the product of the injection time in cooling water, and a water quantity density. It is a side view which shows notionally the structure of the continuous casting apparatus used in the Example, (A) is a figure which shows the whole structure, (B) is the elements on larger scale near a cooling part. It is the graph which compared the crack occurrence index which indexed the number of cracks in each part about each of the slab manufactured by the manufacturing method concerning the present invention, and the slab manufactured by the continuous casting concerning a prior art. It is a cross-sectional observation image of the steel piece obtained by the cooling experiment performed on the test conditions which apply cooling using the test apparatus shown by FIG. It is a cross-sectional observation image of the steel piece obtained by the cooling experiment performed on the test conditions which do not apply cooling using the test apparatus shown in FIG.

  The subject steels of the present invention are all steel types that undergo ferrite-pearlite transformation and bainite transformation when cooling a continuous cast slab, and in particular, by mass%, C: 0.03-0.45%, Si: 0.001-3.0%, Mn: 0.3-1.5%, P: 0.1% or less, S: 0.1% or less, Al: 0.1% or less, the balance Fe and Examples of the steel are impurities.

  In addition, each component described above is basically contained, and in place of the Fe, in mass%, Nb: 0.03 to 0.05%, V: 0.002% or less, Mo: 0.05 to 0 When the present invention is applied to a steel containing one or more of 30%, the effect of preventing surface cracking appears more remarkably.

  A normal bloom continuous casting apparatus may be used for casting. More specifically, the slab size is 220 to 370 × 220 to 600 mm, and the casting speed is 0.3 to 2.0 m / min.

  In the present invention, the inside of the slab is completely solidified, and a tertiary cooling device is provided in the pinch roll band after the bending of the slab is corrected to once cool the surface of the slab and to reheat it immediately. Perform the operation.

Such conditions were grasped as follows.
Using the test apparatus shown in FIG. 1, the relationship between the cooling conditions and the slab surface structure modification was investigated. The work contents in the experiment are shown as a flowchart in FIG.

  Each molten steel (A, B, C) having the composition shown in Table 1 is melted by 1.0 ton, and the molten steel is cast into a 315 mm × 435 mm ingot as shown in FIG. It conveyed to the cooling test apparatus shown, and the cooling experiment of the ingot surface was conducted. The cooling conditions are summarized in Table 2.

  The cooling experimental apparatus shown in FIG. 1 is provided with a driving device 1, a bloom slab 2, a cooling spray 3, and a cooling spray for simulating the movement of a slab in continuous casting, and a predetermined speed in the vertical direction by the driving device. It is comprised by the raising / lowering flame | frame 4 which can be moved by. The spray spray pitch and the slab moving speed are simulated by changing the cooling spray flow rate and the raising / lowering speed of the frame for raising and lowering the cooling spray.

  For the cooling according to the present invention, a mixed spray of water and air was used, and the ingot surface temperature at the start of spray injection was 850 to 1050 ° C. In order to simulate the temperature change during slab movement in continuous casting, the temperature change of the slab is measured by reciprocating the spray with a stroke of 250 mm at a speed of 0.7 to 2.5 m / min. The measurement was continuously performed using a thermocouple welded to the inside of the slab and a thermocouple embedded in the slab.

For steel type A, the density of cooling water per ingot surface area is 995 L / min · m ² , the spray moving speed is 2.5 m / min, one stroke is 250 mm, and the spray is sprayed for 84 seconds. The temperature transition shown in FIG. In this case, assuming that the casting speed is 0.7 m / min, this corresponds to the slab moving for 84 seconds, that is, 980 cm in the cooling region by spray injection with a spray pitch of 70 mm.

  By analyzing the temperature transition data during the cooling experiment shown in Fig. 3, the relationship between spray conditions (water volume, cooling time, casting speed) and slab surface structure modification can be organized, and optimum cooling conditions are estimated. It becomes possible to do. The heat transfer of the spray was estimated using the temperature of the thermocouple welded to the slab surface shown in FIG. 3 and the thermocouple installed at a depth of 22 mm from the slab surface. The thermocouple welded to the slab surface is for estimating the recuperated state after the end of spray injection. FIG. 4 conceptually shows the location of the thermocouple in the vicinity of the ingot surface during spray cooling.

As shown in FIG. 3, it can be seen from the thermocouple data welded to the surface of the slab that the slab surface temperature suddenly decreases due to the start of cooling, and that the heat is recovered to at least _one Ar point after the cooling spray is stopped. During spray injection, it is directly sprayed with spray water, so it has moved around 100 ° C to measure the boiling state of spray water. Can be predicted.

In addition, since the temperature of the thermocouple attached inside the slab is also affected by spray cooling, two-dimensional heat transfer analysis was performed from the temperature data during cooling, and the cooling was quantified.
A thermocouple is welded to a position on the surface of the slab facing the spray (indicated by ● in FIG. 4), and the slab is allowed to cool to 950 ° C. until the surface temperature of the slab reaches about 950 ° C. When slab was reached, cooling of the slab by spray injection was started. The reason why the spray injection start temperature is set to 950 ° C. is to match the temperature of the slab surface at the pinch roll portion to which the cooling according to the present invention is applied.

In steel type A, as a cooling condition, casting was performed when cooling was performed with a water density of 995 L / min · m ² and a spray injection time of 84 seconds (this time corresponds to a casting speed of 0.7 m / min spray pitch of 70 mm). Observation images of the single surface layer structure are shown in FIGS.

FIG. 8 shows a case in which strong cooling and recuperation are appropriately performed based on the conditions of the present invention, and a target slab surface layer modified layer is formed in a range of 4 mm from the surface layer.
On the other hand, FIG. 9 is a cross-sectional observation image of a steel slab obtained by cooling only by standing cooling without performing the above-described strong cooling, and it can be seen that the grain boundary is large and clear in the slab surface layer structure.

  Therefore, in order to be able to apply the results obtained in this experiment to general operating conditions, first the necessary surface layer reforming depth is investigated, and then cooling is performed so that such reforming depth can be stably obtained. The conditions were examined.

As a result, it was found that the structure modification depth of the slab surface layer portion should be 4 mm or more from the slab surface after continuous casting in order to prevent surface cracking of the slab.
Such slab reforming depth is related to the cooling penetration depth inside the slab, and can be obtained by calculation from the relationship between the transition of the slab surface temperature during cooling and the elapsed time. . The calculated cooling penetration depth is calculated based on the water density and the water density within a predetermined water density range per slab surface area under normal spraying conditions (specifically, mist spraying or high-pressure spraying). The product can be represented by the product of the injection time with the density, and the result shown in FIG. 5 is obtained when the range of the predetermined water density per slab surface area in the present invention is adjusted to 350 to 1400 L / min · m ² . was gotten.

From this result, when the range of the predetermined water amount density per slab surface area is adjusted to 350 to 1400 L / min · m ² , the product of the water amount density per slab surface area and the spray water injection time is 650 L / m ^2. It turned out that the slab whose modification depth of a slab surface layer part is 4 mm or more is obtained by making it above. Note that if this product exceeds 1680 L / m ² , the surface of the slab is cooled too much and recuperation becomes difficult, so 1680 L / m ² is appropriate as the upper limit.

  Moreover, the time for performing strong cooling of the slab is within a range of 3 m to 10 m in the pinch roll band of the continuous casting machine after the slab is bent and corrected, and from the complete solidification completion position to the downstream side in the casting direction. preferable. The complete solidification completion position can be calculated by solidification heat transfer analysis, but the calculated value of the solidification completion position generally has a variation (error) of about 1 m in the literature. In order to surely avoid the danger of shrinking the slab with a layer due to strong cooling, the position 3 m after the calculated value of the complete solidification completion position is set as the lower limit of the range in which the slab is strongly cooled.

Further, when a long time elapses after complete solidification, the temperature of the slab surface is lowered and becomes lower than the temperature of the Ar ₃ transformation point. For this reason, it is preferable to provide a strong cooling zone in the range of 10 m or less from complete solidification.

Note that the length of the strong cooling zone is preferably 1 m or less in the casting direction. This is because, if the cooling is over 1 m, the temperature inside the slab is excessively lowered and the surface of the slab may not rise above the Ar ₁ transformation point.

80 tons of molten steel having the composition shown in Table 3 was cast using the continuous casting machine shown in FIG. The casting conditions are summarized in Table 4.
The continuous cast bloom slab is made by injecting molten steel into the mold 8 to form a solidified shell, passes through the segment roll and the secondary cooling spray zone 5, is corrected by the pinch roll at the correction zone 6, and is conveyed to the next process. The The cooling according to the present invention was carried out by installing a cooling zone 7 in an area where solidification was completed and correction was completed.

  The slab bending correction position of this continuous casting machine is 32.5 m, and the solidification completion position when casting molten steel shown in Table 3 under the conditions of Table 4 is a position of 28.5 m. In this continuous casting machine, a cooling device is provided as a cooling zone 7 over a distance of 4.5 m from a position 4.5 m after the solidification completion position, and the cooling device in the cooling zone 7 is 0.5-1. The cooling zone length in the casting direction (the length of the spray jet zone) can be adjusted in the range of 0 m.

In this embodiment, the length of the cooling zone was adjusted to 0.7 m and the casting was cast at a casting speed of 0.7 m / min. Therefore, the spray injection time onto the slab was just 1 minute, and the water density, the injection time, The product of is 995 L / m ² .

  FIG. 7 shows the result of examining the surface cracking state after the ingot rolling of the slab cast under these conditions, in comparison with the slab cast by the conventional method. The slab of the conventional method is a slab cast by partially stopping spray injection from the cooling device in the cooling zone 7 during casting according to the present invention.

  The “top side” in FIG. 7 means the surface of the slab that faces the upper surface when the slab is moved horizontally during casting, and the “ground side” is the opposite surface of the surface that forms this “top side”. Means. The “crack occurrence index” is the number of cracks per slab and is obtained by (total number of cracks in a plurality of slabs) / (number of slabs).

  Thus, it has been confirmed that the occurrence of cracks in the slab is reduced by casting that satisfies the conditions defined in the present invention using the cooling zone 7.

1: Drive device (simulating slab moving speed)
2: Bloom slab 3: Cooling spray 4: Lifting frame 5: Segment roll and secondary cooling zone 6: Straightening zone (pinch roll zone)
7: Invention cooling zone 8: Mold

Claims (3)

A slab cooling method for producing a bloom slab by continuously casting molten steel,
By spraying spray water after straightening of the slab during continuous casting and after complete solidification,
After cooling the surface temperature of the slab from the Ar ₃ transformation point to the Ar ₁ transformation point,
Again raise it above the Ar ₁ transformation point ,
In injecting the spray water,
The cooling water amount density per slab area and 350L / min · ^{m 2} or more 1400L / min · ^{m 2} or less,
And, the cooling water density × the value of the cooling water injection time was adjusted to 650L / m ² or more 1700L / m ² or less and performing, the blooming slab cooling method in the continuous casting. Spraying the spray water,
2. The bloom slab during continuous casting according to claim 1, wherein the slab is carried out only in a section of 1 m or less in the casting direction within a range of 3 m or more and 10 m or less on the downstream side in the casting direction after complete solidification of the slab. Cooling method. A method for producing a bloom slab having an austenite grain boundary obscuring layer having a thickness of 4 mm or more under a slab surface layer, wherein the slab cooling method according to claim 1 or 2 is used.