JPH071096A - Cooling method of slab in continuous casting - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a cooling method for obtaining continuously cast slabs with low center porosity. [Structure] In the secondary cooling during continuous casting of steel, when the solid fraction of the central part of the slab becomes 0.1 to 0.3, the surface of the slab has a water density of 25 to 100 [liter / (min・
m ² )] to start controlled cooling of water cooling, and continue the above controlled cooling until the solid fraction of the central portion of the cast becomes 0.8 or more. In this method, it is possible to use a control cooling zone in which secondary cooling zones divided into a plurality of cooling blocks are appropriately combined. As a result, the effect can be ensured by avoiding the start of the controlled cooling when the solid fraction of the central portion of the cast slab is less than 0.1. In these methods, it is desirable to use an air mist spray as a cooling means. [Effect] As shown in the figure, the center porosity is small, and it is possible to manufacture a slab suitable as a material for producing a seamless pipe.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to the center of a slab in continuous casting of blooms or billets (hereinafter referred to as slabs) of various steels such as carbon steel, low alloy steel, high alloy steel and stainless steel. The present invention relates to a method for cooling a slab capable of reducing the generated center porosity.

[0002]

2. Description of the Related Art In a process for producing a seamless pipe from a continuously cast slab through a rolling or forging process by the Eugene Sejournet method, the Mannesmann method, etc., there is a center porosity in the continuously cast slab, and the extent of this is When large, pipes made from the slab are often subject to internal flaws, which are prone to quality defects.

For the purpose of reducing the center porosity of continuously cast slabs, some secondary cooling methods have been disclosed which utilize heat shrinkage during slab cooling. For example,
In 62-61764, the surface of the slab is cooled from a position 2 to 15 m before the solidification end point of the residual molten metal pool inside the slab, and solidification shrinkage is given to the slab to reduce the cross section of the slab and A continuous casting method that reduces

[0004]

The conventional secondary cooling method for cooling the surface of the slab to give the slab a solidification shrinkage to reduce the center porosity has the following problems to be further studied.

1) It is effective to reduce the number of cavities by giving solidification shrinkage before the central portion of the slab is completely solidified, as in the method disclosed in Japanese Patent Laid-Open No. 62-61764. However, if strong cooling is performed too far upstream (close to the mold) from the solidification end point, the cooling allowance will decrease at the time when center porosity really tends to occur, and the effect will be reduced. That is, for cooling for giving solidification shrinkage, the time (position of the slab) at which cooling is performed must be properly selected.

2) If the cooling is stopped in the state where the center of the cast slab is not solidified, the center porosity increases due to the heat recovery or internal cracking occurs.

3) Inadequate control of the cooling rate of the slab (for example, adjustment of the water amount density of cooling water) increases the center porosity.

An object of the present invention is to solve the above problems and provide a method for cooling a cast piece in continuous casting for producing a steel bloom or billet with low center porosity.

[0009]

The summary of the present invention is as follows.
Although there is the method for cooling the cast slab of (1), the method of the following (2) can be adopted when implementing this method, and it is desirable to perform the air mist spray of the following (3).

(1) In secondary cooling during continuous casting of steel blooms or billets, the solid fraction in the center of the slab is
At the time of 0.1 to 0.3, the surface cooling of the slab started by water cooling with a water amount density of 25 to 100 [liter / (min · m ² )], and the solid fraction in the center of the slab became 0.8 or more. A method for cooling a slab, which is characterized in that water cooling according to the above water density is continued until.

(2) The above (1)
Surface cooling.

In the secondary cooling zone of the continuous casting apparatus, at least from the position where the solid fraction of the central portion of the slab is 0.1, 0.8
The length that covers the above positions is divided into multiple blocks, and the length of one block from the first divided block to the block that covers at least the position where the solid fraction is 0.3 is , It should be shorter than the distance from the position where the solid fraction is 0.1 to the position where it becomes 0.3.

From the block next to the block covering the position where the solid fraction at the center of the cast slab is 0.1, the solid fraction is
Water density of 25 to 100 [liters / (min.
m ² )] start surface cooling of the slab by water cooling, and continue this surface cooling at least to the block covering the position where the solid fraction is 0.8.

(3) Water content density of the above (1) and (2) 25 to 10
Surface cooling of the slab by water cooling of 0 [liter / (min · m ² )] is performed with an air mist spray.

In the above method, it is sufficient to use the value obtained by the heat transfer calculation which is generally used as the solid fraction. The water amount density in the case of using an air mist spray is defined by the amount of water constituting the air mist [liter / (min · m ² )].

[0016]

1 to 5 show the method of the present invention and its effects.
It will be described based on. The above "cooling the surface of the slab with water or air mist at a water amount density of 25 to 100 [liter / (min · m ² )]" is hereinafter referred to as "cooling control".

FIG. 1 is a vertical cross-sectional view in a side direction showing a curved type continuous casting apparatus for round billet casting having a diameter of about 200 to 265 mm, which is an example of the continuous casting apparatus for carrying out the cooling method of the present invention. In this apparatus, the cast slab 1 is manufactured as follows.

The molten steel 2 injected into the mold 4 through the immersion nozzle 3 is provided with a pre-stage secondary cooling spray 5 provided immediately below the water-cooled (primary cooling) mold 4 and a final solidification stage provided before completion of solidification. It is cooled with the secondary cooling spray 6 for the second stage and is pulled out by the pinch roll 7. In FIG. 1, reference numeral 8 indicates a solidified shell (shell).

The pre-stage secondary cooling spray 5 is provided immediately below the mold 4 and is installed in a general continuous casting apparatus. Normally, the solidified shell 8 is thin immediately below the mold 4, and the static pressure of the molten steel 2 increases the bulging of the slab 1.
Cooling is applied to prevent this bulging. Further, the cooling rate by the front-stage secondary cooling spray 5 is
Although it varies depending on the roll pitch of the continuous casting apparatus and the casting speed, it is usually set to the minimum cooling rate at which bulging does not increase.

The solidified shell 8 of the slab 1 begins to be formed in the water-cooled mold 4 and becomes thicker, and the slab 1 is finally cooled by the second-stage secondary cooling spray 6 to give a desired shrinkage, It is completely solidified, and the solid fraction in the central portion of the slab 1 (hereinafter simply referred to as "solid fraction") becomes 1.0.

In the cooling method of the present invention, the cooling control for imparting the required shrinkage and solidification to the slab 1 depends on the solid fraction of the slab 1.
Start when 0.1 to 0.3 is reached. Then, the cooling control must be continued until the solid fraction exceeds 0.8. Usually, the position where the solid fraction is 0.1 to 0.8 or more is within the range covered by the latter stage secondary cooling spray 6 for the final stage of solidification shown in FIG. Hereinafter, the range in which this cooling control is performed is referred to as "cooling control zone".

The cooling medium used in the cooling control zone is preferably water or an air mist in which water and air are mixed. In either case, the water density is 25 to 100 [liter / (min ・
m ² )].

The latter-stage secondary cooling spray 6 has a total length of its cooling zone (spray zone) from the solidification completion point, that is, upstream, so that the total length of the cooling zone (spray zone) can be adapted to changes in steel type, casting speed and the like. Lengthwise toward the side (for example, 10 m for the above-mentioned round billet slabs with a diameter of 200 to 265 mm), and a plurality of small blocks of appropriate length between these (for example, 5 blocks as shown in Fig. 1). The block is divided into blocks, a cooling medium is supplied to each block according to casting conditions, and its effective length can be changed according to the solid fraction of the cast piece. It should be noted that this block is a block composed of a group of sprays, and the cooling conditions (for example, the above water amount density) can be changed for each group.

Therefore, within the range covered by the second-stage secondary cooling spray 6, cooling control is performed by adjusting the water amount density of the block after the position where the solid fraction of the cast reaches 0.1 to 0.3 as described above. Can be carried out.

The reason why the cooling conditions for the cooling control in the present invention are limited as described above will be described below.

(1) Range of solid fraction at the center of slab When the volumetric shrinkage of molten steel occurs due to solidification shrinkage, the molten steel tries to flow to fill the volumetric shrinkage, but the flow resistance of molten steel is large When the property is poor, the volume shrinkage cannot be filled up, and center porosity is likely to occur.

In particular, when the solid fraction of the central portion of the cast slab exceeds 0.3, the apparent viscosity of the molten steel rapidly increases due to the presence of the solid phase, and the fluidity thereof begins to decrease. When the solid fraction is further increased, the solid phase no longer moves, and only the molten steel moves between the dendrites of the solid phase. In addition, the size of the flow path diameter is extremely narrow, such as several to several hundreds of μm, so that the flow resistance of the molten steel is significantly increased and the fluidity is lowered, so that the molten steel cannot be supplied to the solidification shrinkage portion.

Further, when the solid fraction of the central portion of the cast slab is less than 0.1, even if the cooling is controlled to shrink the cast slab, the insufficient supply of molten steel causing the center porosity still remains at this point. Since it has not occurred, only the movement of molten steel occurs, and it does not contribute to the reduction of center porosity. Furthermore, if the excessive cooling is performed in the early stage of the secondary cooling, for example, in the cooling process by the first-stage secondary cooling spray, the surface temperature of the slab will drop more than necessary, so cooling in the range of the second-stage secondary cooling spray will occur. It becomes difficult to obtain the contraction allowance, which is indispensable for reducing the center porosity by control, or the contraction allowance is wasted.

Therefore, the cooling control based on the water amount density must be started at the time when the solid fraction of the central portion becomes 0.1 to 0.3. More desirable is completely
It is to start cooling after it exceeds 0.1.

By this cooling control, the cast slab is shrunk, and the shortage of molten steel due to solidification shrinkage inside is compensated to reduce the center porosity.

Until the center of the slab is almost completely solidified,
The central part has no strength and cracks easily occur with a small stress. If the cooling of the surface of the slab is stopped at such a time point, tensile stress acts on the central portion due to the heat recovery, and the center porosity is likely to increase.

Temperature at which strength begins to develop in the solidification phase (ZST)
Is known to have a solid fraction of 0.8 at the center of the slab (see, for example, JP-A-3-174962 by the present inventors).
(See the official gazette). To reduce center porosity,
It is necessary to continue cooling control of the surface of the cast slab until at least the temperature at which the solid fraction in the center reaches 0.8.

When the solid fraction is 0.8 or more, the residual molten steel in the slab is small, and when such a small amount of molten steel solidifies and shrinks, only extremely micro center porosity is formed. Since it does not occur, you can think that it will not be a problem.

From the above viewpoint, the solid fraction of the central portion of the slab when the cooling control is finished is set to 0.8 or more. In the area where the cooling control is not performed in the secondary cooling zone after the first-stage secondary cooling spray, that is, in the area outside the cooling control zone, cooling is performed as necessary to prevent bulging.

(2) Cooling medium for cooling control and spray conditions As the cooling medium for cooling control, water or an air mist in which water and air are mixed may be used. If the amount of cooling water is large, the heat transfer to the surface will increase due to the temperature decrease, and the temperature will decrease
Increased heat transfer rate → decreased temperature, cooling tends to proceed at an accelerating rate, and overcooling is likely to occur.

Lower limit of water density 25 [liter / (min · m ² )]
] Is the minimum water amount density for compensating for the local shortage of molten steel supply (which causes the generation of center porosity) due to the solidification shrinkage of the slab and the deterioration of the fluidity of molten steel. On the other hand, when the water amount density exceeds 100 [liter / (min · m ² )], the surface of the slab is cooled rapidly in the initial stage of cooling, so that the strength of the surface of the slab becomes large and the desired shrinkage occurs. I will not progress. In addition, the cooling rate in the low temperature portion decreases, and tensile stress acts at the center of the slab, which causes an increase in center porosity. Therefore, the range of water density is 25-100
[Liter / (min · m ² )].

When the air mist is sprayed, the temperature dependence of the heat transfer property is smaller and the temperature controllability is better than when sprayed with water.

However, when the air mist is used, if the [air flow rate (liter / min) / water amount (liter / min)] (hereinafter referred to as the air / water ratio) is less than 10, the characteristics as the air mist are reduced, Since it is no different from normal water spray, it is desirable that the air / water ratio in this case be 10 or more.

The above cooling control in the method of the present invention can be carried out as follows in actual operation.

First, as shown in FIG. 2, in the secondary cooling zone (usually the zone covered by the subsequent secondary cooling spray), 0.8 from the position where the solid fraction of at least the cast slab is 0.1.
The length L covering the above positions is Ls shorter than the distance (L _{X in} FIG. 2) from the position where the solid fraction is 0.1 to the position where the solid fraction is 0.3 1 2nd, 2nd ... ith, i + 1st, i + 2nd ...
・ It is divided into a plurality of blocks of i + nth, jth ... xth.

Among the blocks divided as described above, the block covering the position where the solid fraction of the slab is 0.1 is i
The block that also covers the position where the solid fraction is 0.3 is the i + nth block (where n is a numerical value of 1 or more), and the block that covers the position where the solid fraction is 0.8 is the xth block. And the block.

In other words, there is a position where the solid fraction is 0.1 in the area covered by the i-th block, and it is covered by a block somewhere on the downstream side (however, i + 1-th and before x-th). Each block is set so that there is a position where the solid fraction is 0.3 in the region to be covered and a position where the solid fraction is 0.8 in the region covered by the x-th block.

In the secondary cooling zone set as described above, surface cooling of the slab by water cooling with a water amount density of 25 to 100 [liter / (min · m ² )] from the i + 1th block, that is, The cooling control is started, and this surface cooling is continued until at least the x-th block.

The length Ls of one block from the 1st to the i + n-th is from the point where the solid fraction of the slab becomes 0.1 to 0.3 according to the calculation method described below under various casting conditions. The distance (Lx) may be obtained in advance and set so as to be shorter than the minimum value. The Ls may be the same value in each block, and the L may be equally divided, or the value of Ls may be changed in each block. However,
In either case, the length of one block from the 1st block to the (i + n) th block is smaller than the above Lx.

When the method shown in FIG. 2 is applied to actual casting, the above-mentioned i-th corresponding solid phase ratios, depending on the continuous casting conditions such as steel type, slab cross-sectional dimension, and casting speed, It is necessary to predict the positions of the i + n-th and x-th cooling blocks and preset the length of one cooling block and the length of the cooling control zone. This prediction calculation can be performed by heat transfer analysis using a two-dimensional unsteady heat transfer calculation model. The accuracy of the prediction of the solidification profile obtained by the calculation can be improved by comparing it with the actual measurement value of the surface temperature of the slab, which is a boundary condition, and the result of the solidification confirmation test such as tacking.

FIG. 3 is a diagram showing an example of a control system when the above-mentioned cooling control is actually applied online.
During casting, the casting conditions (steel type, slab size, casting temperature, casting speed, etc.) are input, and the solidification profile is calculated using the heat transfer calculation model to calculate the boundary solid fraction (in the central solid phase).
Check the position of 0.1, 0.3, 0.8) and the position of the cooling block containing it at any time. And the above i-th, i + n
Determine the th and xth block. According to this determination, the cooling control is performed under the predetermined condition from the (i + 1) th block to at least the xth block.

According to such a method, the cooling control is always started in the region where the solid fraction is 0.1 or more and 0.3 or less, and is continued until the solid fraction is 0.8 or more. In other words, there is no risk of starting controlled cooling when the solid fraction of the slab has not reached 0.1, and the solid fraction of 0.8
There is no fear that the cooling control will be stopped before reaching the temperature. Therefore, the effect of reducing the center porosity of the slab can be ensured.

[0048]

[Example 1] [Test 1] Center porosity is likely to occur
A 265 mm diameter round billet of 13% Cr steel was continuously cast. The equipment used is that shown in Fig. 1, the casting speed is 0.6 m / min,
ΔT in the tundish was set to 40 ° C. Water is used as the cooling medium for the cooling control performed within the range of the secondary secondary cooling spray, and the water density is 25 to 100 [liter / (min · m ² )
] Within the range. The cooling controllable range of the secondary cooling spray zone in the latter stage is set to 10 m, and this part is divided into 20 blocks with a pitch of 0.5 m, and the central solid fraction of the billet is
Blocks were determined to be 0.1, 0.3 and 0.8. And from the block covering the position where the solid fraction is less than 0.1,
Up to the block covering the position where the solid fraction exceeds 0.8,
Casting was performed by changing the start block and the stop block of the cooling control variously (that is, changing the cooling control section).

The cooling by the former secondary cooling spray was carried out under normal conditions, that is, cooling with a water amount density of about 80 [liter / (min · m ² )]. The same applies to tests 2 and 3 described later.

The effect of reducing the center porosity was evaluated by a method of investigating the generation state of the center porosity in the central longitudinal section of the round billet obtained as described above.

As a comparative example, when the water quantity density of the spray water in the controlled cooling zone is 15 [liter / (min · m ² )] and 120 [liter / (min · m ² )], the cooling control is mainly performed. We also confirmed the occurrence of center porosity when starting when the solid fraction was less than 0.1.

The results of the above test are shown in FIGS. 7 and 8.

(A) to (e) shown separately in FIG. 4 and FIG.
It is a figure which shows the change of the existence diameter of center porosity when changing the cooling area at the time of slab cooling for every water amount density, and changing the central solid fraction at the time of slab cooling start time and end time.

As shown in (a) to (c) of FIG.
It is between 25 and 100 [liter / (min · m ² )], and the central solid fraction of the slab at the start of cooling control under this condition is 0.1 to 0.
3 and the central solid fraction at the end of cooling control
It is clear that when 0.8 or more, the existence diameter of the center porosity is small and a remarkable effect of reducing the center porosity is obtained.

On the other hand, looking at (d) and (e) of FIG. 5, it can be seen that the effect of reducing the center porosity is poor if the water control density is inappropriate even when the cooling control start time is appropriate.

[Test 2] Under the same conditions as in Test 1 above, except that the water amount density was changed, and the cooling spray for cooling control was water and air mist, and the central solid fraction of the slab started cooling control. 0.1 to 0.3 at the end, at the end
Continuous casting was carried out with 0.8 to 1.0.

FIG. 6 is a diagram showing the relationship between the existing diameter of the center porosity in the central longitudinal cross section of the billet, the water density of the spray for cooling control, and the cooling medium at this time.

From this result, the air mist cooling is
It is clear that the effect of reducing the diameter of the center porosity is large, and that the effect is particularly large when the air / water ratio is large. However, the effect of the air mist spray when the air / water ratio is 8 is almost the same as the effect of the water spray.

[Test 3] Tests in which the casting speed was variously changed were conducted by the apparatus shown in FIG. Water spray and air mist spray were used as cooling control sprays, and in each case the water volume density was 50 [liters / (min · m ² )] and other conditions were the same as in Test 1 The existence diameter of the center porosity in the central longitudinal section was investigated. The result is shown in FIG. 7.

As is clear from FIG. 7, although the existing diameter of the center porosity tends to increase slightly as the casting speed increases, the value of the existing diameter itself is small and is reduced to such an extent that it does not pose a problem. .

[0061]

Example 2 Using the continuous casting apparatus having the structure shown in FIG.
Continuous casting of a 0.2% C-1% Cr steel slab (round billet) was performed. The casting temperature (ΔT in the tundish) was 30 ° C., and the slab size and casting speed were changed as follows.

Slab size: Three sizes of diameter 230 mm, 265 mm and 300 mm.

Casting speed (Vc): 2 types, 2 m / min and 2.2 m / min.

FIG. 8 and FIG. 9 are views showing the calculation results of the solidification profile by the heat transfer analysis in the billet having a diameter of 230 mm. That is, the casting speed (Vc) is 2.0m / mi
It is a figure which shows the relationship between the solidification shell thickness in the case of n (FIG. 8) and 2.2 m / min (FIG. 9), the solid fraction (fs) of a slab center part, and the distance from the molten steel meniscus in a mold. .

First, looking at the case where the casting speed in FIG. 8 is 2 m / min, the position where the solid fraction at the center of the slab is 0.1 (distance from the molten steel meniscus) is 28.8 m, and the solid fraction at the center is the same. But
The position at 0.3 is 30.6 m, and the position at which the central solid fraction is 0.8 is 32.6 m. Therefore, the distance (Lx) from the position where the solid fraction is 0.1 to the position where it is 0.3 is 30.6−28.8 = 1.6 (m), and from the position where the solid fraction is 0.1 to the position where it is 0.8 as well. The distance is 32.6−28.8 = 3.8 (m).

Next, looking at the case where the casting speed is 2.2 m / min in FIG. 9, the position where the solid fraction in the center of the slab is 0.1 (distance from the molten steel meniscus) is 31.5 m, and the solid fraction in the center is also the same. Rate is
The position at 0.3 is 33.5 m, and the position at which the central solid fraction is 0.8 is 36.5 m. Therefore, the distance (Lx) from the position where the solid fraction is 0.1 to the position where it is 0.3 is 33.5-31.5 = 2.0 (m), and from the position where the solid fraction is 0.1 to the position where it is 0.8 as well. The distance is 36.5-31.5 = 5.0 (m).

Therefore, the length of each block capable of cooling control is fixed at 1 m, and the first uppermost stream side is
Place it 28 m from the meniscus, starting from the second
Blocks up to the tenth were installed sequentially on the downstream side. The total length of the blocks that can be cooled is 10m. Therefore,
In the case of a casting speed of 2 m / min, the position where the solid fraction is 0.1 covers the first block, and the position where the solid fraction is 0.3 covers the third block, the solid fraction is 0.8. It is the fifth block that covers that position. Therefore, the cooling control was started in the second block and stopped in the fifth block.

When the casting speed is 2.2 m / min, the position where the solid fraction is 0.1 covers the above fourth block, and the position where the solid fraction is 0.3 covers the above. The sixth block of No. 6, which covers the position where the solid fraction is 0.8, is the above-mentioned No. 9 block. Therefore, the cooling control was started in the fifth block and stopped in the ninth block.

Similarly, for billets having diameters of 265 mm and 300 mm, the start and stop positions (block numbers) of cooling control were determined and casting was performed. And the center porosity (CP) defect occurrence rate of the billet after casting was investigated. The occurrence rate of CP failure was calculated by the following formula.

Occurrence rate of CP defect (%) = [(Charge number when the maximum CP existing range is 30 mm or more) / Charge number of casting] ×
100 Figure 10 shows the incidence of defective center porosity. It should be noted that what is shown in FIG. 10 as a comparative example is billet data obtained in a conventional operation in which cooling control is not performed. As shown, according to the method of the present invention, the center porosity defect occurrence rate is significantly reduced.

[0071]

According to the cooling method of the present invention in which the surface of the cast slab is appropriately secondarily cooled according to the solid fraction of the center of the cast slab, the center porosity of the cast slab can be significantly reduced. Billets etc. continuously cast by applying this method,
For example, when used as a material for producing a seamless steel pipe, a product with few internal flaws can be produced.

[Brief description of drawings]

FIG. 1 is a vertical cross-sectional view in a side direction showing an example of a continuous casting apparatus for carrying out a cooling method of the present invention.

FIG. 2 is a diagram for explaining one embodiment of a cooling method of the present invention, and is a diagram showing a correspondence relationship between a solid fraction of a slab and a spray block for cooling control.

FIG. 3 is a diagram showing an example of a control method when the method of the present invention is applied online.

FIG. 4 is a diagram showing changes in the existing diameter of center porosity in the vertical cross section of the billet center when the central solid fraction and the water amount density at the start and end of cooling control are changed.

FIG. 5 is a diagram showing changes in the existing diameter of center porosity in the vertical cross section of the billet center when the central solid fraction and the water amount density at the start and end of cooling control are changed.

FIG. 6 is a diagram showing the influence of the type and amount of cooling medium on the existing diameter of center porosity in the vertical cross section of the billet center.

FIG. 7 is a diagram showing the influence of cooling control conditions and casting speed on the existing diameter of center porosity in the vertical cross section of the billet center.

FIG. 8 is a diagram showing an example of a solidification profile obtained by a heat transfer calculation model.

FIG. 9 is a diagram showing another example of a solidification profile obtained by a heat transfer calculation model.

FIG. 10 is a view showing the center porosity defect occurrence rate in the case of billet casting by the cooling method of the present invention and the conventional cooling method in comparison.

[Explanation of symbols]

1: cast slab, 2: molten steel, 3: dipping nozzle,
4: Mold, 5: First stage secondary cooling spray, 6: Second stage secondary cooling spray, 7: Pinch roll, 8: Solidified shell (shell), 9: Cutting torch

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeru Umeda 1850 Minato Minato, Wakayama City, Wakayama Prefecture Sumitomo Metal Industries, Ltd. Wakayama Works

Claims (3)

[Claims] 1. In secondary cooling during continuous casting of steel blooms or billets, when the solid fraction in the center of the slab reaches 0.1 to 0.3, the water amount density is 25 to 100 [liter /
(min ・ m ² )] water cooling to start surface cooling of the slab,
A method for cooling a slab, characterized in that the water cooling is continued at the above water amount density until the solid fraction of the central part of the slab becomes 0.8 or more. 2. The method for cooling a cast slab according to claim 1, wherein: In the secondary cooling zone of the continuous casting device, divide the length covering at least the position where the solid fraction of the center of the slab is 0.1 to the position where it is 0.8 or more into multiple blocks,
Also, the length of one block from the first of the divided blocks to the block covering at least the position where the solid fraction is 0.3 is 0.3 from the position where the solid fraction is 0.1.
From the block next to the block covering the position where the solid fraction of the slab is 0.1 to the block covering the position where the solid fraction is 0.3. Start surface cooling of the slab by water cooling with a water amount density of 25 to 100 [liter / (min · m ² )] in either block, and cover this surface cooling at least the position where the solid fraction is 0.8. To continue until the block. 3. A water amount density of 25 to 100 [liter / (min · m ² ).
] The method for cooling a slab according to claim 1 or 2, wherein the surface cooling of the slab by water cooling is performed by an air mist spray.