JP2856068B2 - Cooling method of slab in continuous casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to the 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 that can reduce the generated center porosity.

[0002]

2. Description of the Related Art In a process of manufacturing a seamless pipe from a continuously cast slab through a rolling or forging process by the Ugine-Sejournet method, the Mannesmann method, or the like, the continuous cast slab has a center porosity. If it is large, pipes made from the slab often have internal flaws and are prone to quality defects.

For the purpose of reducing the center porosity of a continuously cast slab, several secondary cooling methods utilizing heat shrinkage during slab cooling have already been disclosed. For example,
No. 62-61764 discloses that the slab surface is cooled from a position 2 to 15 m before the solidification end point of the residual molten pool in the slab, and the slab is solidified and contracted to reduce the cross section of the slab. A continuous casting method has been shown to reduce the temperature.

[0004]

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

1) Giving solidification shrinkage before the center of the slab is completely solidified as in the method of JP-A-62-61764 is effective in reducing the cavity. However, if intense cooling is performed too much upstream (closer to the mold) than the solidification end point, the cooling allowance is reduced when the center porosity is likely to occur, and the effect is reduced. That is, it is necessary to properly select the timing (the position of the slab) for cooling to give solidification shrinkage.

[0006] 2) When cooling is stopped while the center of the slab is not solidified, the center porosity increases due to reheating, or internal cracks occur.

[0007] 3) If the control of the cooling speed of the slab (for example, the adjustment of the water density of the cooling water) is inappropriate, the center porosity increases.

An object of the present invention is to provide a method for cooling a slab in continuous casting for producing a steel bloom or billet having a small center porosity by solving the above problems.

[0009]

The gist of the present invention is as follows.
In the method of cooling the slab of (1), the method of (2) below can be adopted when implementing this method, and it is desirable to perform the air mist spraying of (3) below.

(1) In the secondary cooling during continuous casting of a steel bloom or billet, the solid fraction at the center of the slab is reduced.
When the water content reaches 0.1 to 0.3, the surface cooling of the slab is started by water cooling at a water volume density of 25 to 100 [liters / (min · m ² )], and the solid fraction in the center of the slab becomes 0.8 or more. A method for cooling a slab, wherein the water cooling is continued with the above water density until the time.

(2) The above method (1)
The surface is cooled.

In the secondary cooling zone of the continuous casting apparatus, at least the center of the slab at a position where the solid fraction is 0.1 is 0.8
The length covering the above position is divided into a plurality of blocks, and 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: The distance should be shorter than the distance from the position where the solid phase ratio is 0.1 to the position where the solid phase ratio becomes 0.3.

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

(3) The water density of 25 to 10 of the above (1) and (2)
The surface of the slab is cooled by air mist spraying with 0 [liter / (min · m ² )] water cooling.

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

[0016]

The method of the present invention and the effects thereof will be described with reference to FIGS.
It will be described based on. The “cooling of the surface of the slab with water or air mist at a water density of 25 to 100 [liter / (min · m ² )]” is hereinafter referred to as “cooling control”.

FIG. 1 is a side longitudinal sectional view showing a curved continuous casting apparatus for round billet casting having a diameter of about 200 to 265 mm, which is an example of a continuous casting apparatus for carrying out the cooling method of the present invention. In this apparatus, the 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 the solidification is completed. After cooling with the secondary cooling spray 6 for the next stage, it is pulled out with the pinch roll 7. In FIG. 1, reference numeral 8 denotes a solidified shell.

The pre-stage secondary cooling spray 5 is provided directly 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 bulging of the slab 1 increases due to the static pressure of the molten steel 2.
Cooling is performed to prevent this bulging. Also, the cooling rate by the former-stage secondary cooling spray 5 is:
Although it depends on the roll pitch of the continuous casting apparatus and the casting speed, the cooling rate is usually set to a minimum cooling rate that does not increase bulging.

The solidified shell 8 of the slab 1 starts to be formed from the inside of the water-cooled mold 4 and gradually increases in thickness, 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 at the center of the slab 1 (hereinafter simply referred to as “solid fraction”) is 1.0.

In the cooling method of the present invention, the cooling control for giving the required shrinkage and solidification to the slab 1 is performed by controlling the solid fraction of the slab 1
Start when it reaches 0.1-0.3. The cooling control must be continued until the solid fraction becomes 0.8 or more. Usually, the position where the solid fraction is 0.1 to 0.8 or more is within the range covered by the second-stage secondary cooling spray 6 for the final stage of solidification shown in FIG. Hereinafter, a range in which the cooling control is performed is referred to as a “cooling control zone”.

The cooling medium used in the cooling control zone is preferably water or an air mist obtained by mixing water and air. In each case, the water density was 25-100 [liter / (min
m ² )].

The post-stage secondary cooling spray 6 sets the total length of its cooling zone (spray zone) before the solidification completion point, that is, upstream, so that it can respond to changes in steel type, casting speed, and the like. To the side (for example, 10 m in the case of the round billet slab having a diameter of 200 to 265 mm described above) and a plurality of small blocks of an appropriate length (for example, five blocks as shown in FIG. 1). Block), a cooling medium is supplied to each block according to the casting conditions, and the effective length can be changed according to the solid phase ratio of the slab. In addition, this block is a block which consists of a group of sprays and can change cooling conditions (for example, the above-mentioned water density) for each group.

Therefore, within the range covered by the secondary cooling spray 6, cooling control is performed by adjusting the water density of the blocks after the position where the solid fraction of the slab reaches 0.1 to 0.3 as described above. Can be implemented.

The reason for limiting the cooling conditions of the cooling control as described above in the present invention will be described below.

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

In particular, when the solid phase ratio at the center of the slab exceeds 0.3, the apparent viscosity of molten steel suddenly increases due to the presence of the solid phase, and its fluidity starts to decrease. When the solid phase ratio further increases, the solid phase no longer moves, and only molten steel moves between dendrites in the solid phase. In addition, the size of the flow path diameter is very narrow, several to several hundreds of micrometers, so that the flow resistance of the molten steel is significantly increased, the flowability is reduced, and the molten steel cannot be supplied to the solidified shrinkage portion.

Even if the slab is shrunk by performing cooling control when the solid phase ratio at the center of the slab is less than 0.1, at this point, there is still insufficient supply of molten steel causing the center porosity. Since it does not occur, the movement of molten steel merely occurs and does not contribute to the reduction of center porosity. Further, if excessive cooling is performed in the early stage of the secondary cooling, for example, in the cooling process by the former secondary cooling spray, the surface temperature of the slab decreases more than necessary, so that cooling in the range of the latter secondary cooling spray is performed. It becomes difficult to obtain the contraction allowance indispensable for the reduction of the center porosity by the control, or the contraction allowance is wasted.

Therefore, the cooling control based on the water density must be started at the time when the solid fraction in the center becomes 0.1 to 0.3. Even more desirable is completely
It is to start cooling after exceeding 0.1.

The cooling control causes shrinkage of the slab to compensate for the shortage of molten steel due to internal solidification shrinkage, thereby reducing center porosity.

Until the center of the slab is almost completely solidified,
There is no strength at the center, and cracks are likely to occur with small stress. When the cooling of the slab surface is stopped at such a point, a tensile stress acts on the central portion due to reheating, and the center porosity tends to increase.

Temperature at which strength starts to develop in the solidification phase (ZST)
Has been found to have a solid fraction of 0.8 at the center of the slab (see, for example, Japanese Patent Application Laid-Open No. 3-174962 by the present inventors).
Reference). To reduce center porosity,
It is necessary to continue the cooling control of the slab surface at least to a temperature at which the solid fraction at the center becomes 0.8.

When the solid phase ratio is 0.8 or more, the residual molten steel in the slab is very small. If such a small amount of molten steel solidifies and contracts, only a very micro center porosity is formed. It can be considered that there is almost no problem because it does not happen.

From the above viewpoint, the solid phase ratio at the center of the slab at the end of the cooling control is set to 0.8 or more. In a region where cooling control is not performed in the secondary cooling zone after the secondary cooling spray in the first stage, that is, in a region outside the cooling control zone, cooling is performed as needed to prevent bulging.

(2) Cooling Medium for Cooling Control and Spray Conditions The cooling medium for cooling control may be water or an air mist obtained by mixing water and air. If the amount of cooling water is large, the heat conductivity of the surface will increase due to the temperature drop, and the temperature will drop.
Cooling tends to proceed at an accelerated rate with an increase in heat transfer → temperature drop, and overcooling is likely to occur.

Lower limit of water density 25 [liter / (min · m ² )]
] Is the lowest water density to compensate for the local shortage of molten steel supply (causing the generation of center porosity) due to the solidification shrinkage of the slab and the decrease in fluidity of the molten steel. On the other hand, when the water density exceeds 100 [liters / (min · m ² )], the slab surface is rapidly cooled in the initial stage of cooling, thereby increasing the strength of the slab surface portion and reducing the desired shrinkage. Stuck. In addition, the cooling rate in the low-temperature portion decreases, and a tensile stress acts at the central portion of the slab, resulting in an increase in center porosity. Therefore, the range of water density is 25-100
[Liter / (min · m ² )].

When air mist is sprayed, the temperature dependence of heat transfer is smaller than in the case of water spray, and the temperature controllability is excellent.

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

The above-mentioned cooling control in the method of the present invention can be performed as follows in actual operation.

First, as shown in FIG. 2, at least a part of the secondary cooling zone (usually a zone covered by the secondary cooling spray in the latter stage) is located at a position where the solid fraction of the slab is 0.1.
The length L covering the above positions is defined as Ls, which is shorter than the distance from the position where the solid fraction is 0.1 to the position where the solid fraction is 0.3 (L _{X in} FIG. 2). The second, the i-th, the i + 1st, the i + 2th, etc.
Divide into a plurality of blocks of (i + n) th, jth ... xth ...

Of the blocks divided as described above, the block covering the position where the solid fraction of the cast slab is 0.1 is i
The block that covers the position where the solid phase ratio is 0.3 is the i + n-th block (where n is a value of 1 or more), and the block that covers the position where the solid phase ratio is 0.8 is the x-th block. 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 downstream (but after the (i + 1) -th and before the x-th). Each block is set so that there is a position where the solid phase ratio is 0.3 in a region where the solid phase ratio is 0.3, and a position where the solid phase ratio is 0.8 in a region covered by the x-th block.

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

The length Ls of one block from the first to the (i + n) -th blocks is calculated from the point at which the solid fraction of the slab becomes 0.1 to 0.3 at various casting conditions by the following calculation method. The distance (Lx) may be determined in advance and set to be shorter than the minimum value. This 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 each case, the length of one block from the first to the (i + n) th is set to be smaller than Lx.

When the method shown in FIG. 2 is applied to actual casting, the solid phase ratio and the i-th, It is necessary to predict the positions of the (i + n) th and xth cooling blocks, and set the length of one cooling block and the length of the cooling control zone in advance. This prediction calculation can be performed by a heat transfer analysis using a two-dimensional unsteady heat transfer calculation model. The accuracy of the prediction result of the solidification profile obtained by the calculation can be improved by comparing it with the measured value of the slab surface temperature as the boundary condition and the solidification confirmation test result such as tacking.

FIG. 3 is a diagram showing an example of a control system when the above-described cooling control is actually applied online.
During pouring, pouring 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 each 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 including it at any time. And the i-th, i + n
And the x-th block. In accordance with this determination, cooling control is performed under predetermined conditions from the (i + 1) th block to at least the xth block.

According to such a method, the cooling control always starts in a region where the solid fraction is 0.1 or more and 0.3 or less, and continues to a region where 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 does not reach 0.1, and the solid fraction is 0.8
There is no fear that the cooling control will be stopped before the temperature reaches the limit. 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
Round billets of 13% Cr steel with a diameter of 265 mm were continuously cast. The equipment used is shown in Fig. 1 and the casting speed is 0.6m / min.
ΔT in the tundish was 40 ° C. Water is used as a cooling medium for the cooling control performed in the range of the second stage secondary cooling spray, and the water density is 25 to 100 [liter / (min · m ² )].
] Range. The cooling control range of the secondary cooling spray zone in the subsequent stage is 10 m, and this interval is divided into 20 blocks of 0.5 m pitch, and the billet central solid phase ratio is reduced.
Blocks that would be 0.1, 0.3 and 0.8 were determined. And from the block covering the position where the solid phase ratio 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 with the secondary cooling spray in the former stage was performed under normal conditions, that is, cooling with a water density of about 80 [liter / (min · m ² )]. The same applies to Tests 2 and 3 described below.

The effect of reducing the center porosity was evaluated by examining the state of occurrence of center porosity in the central longitudinal section of the round billet obtained as described above.

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

FIGS. 7 and 8 show the results of the above test.

(A) to (e) shown separately in FIG. 4 and FIG.
It is a figure which shows the change of the existing diameter of center porosity when the cooling zone at the time of slab cooling is changed for every water volume density, and the center solid phase ratio at the slab cooling start time and the end time is changed.

As shown in FIGS. 4A to 4C, the water density is
25 to 100 [liters / (min · m ² )], and the center solid fraction of the slab at the start of cooling control under this condition is 0.1 to 0.1.
3 and the center solid fraction at the end of cooling control is
At 0.8 or more, it is apparent that the diameter of the existing center porosity is small, and a remarkable effect of reducing the center porosity can be obtained.

On the other hand, from FIGS. 5D and 5E, it can be seen that the effect of reducing the center porosity is poor if the cooling control start time is appropriate and the water density is inappropriate.

[Test 2] Under the same conditions as in Test 1, except that the water density was changed, the cooling spray used for cooling control was water and air mist, and the center solid phase ratio of the slab was set to start cooling control. 0.1-0.3 at the time, at the end
Continuous casting was performed at 0.8 to 1.0.

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

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

[Test 3] A test was conducted in which the casting speed was varied with the apparatus shown in FIG. Water spray and air mist spray were used for cooling control spray. In each case, the water density was 50 [liters / (min · m ² )], and the other conditions were the same as in Test 1. The existence diameter of the center porosity in the central longitudinal section was investigated. FIG. 7 shows the result.

As is apparent from FIG. 7, although the existence diameter of the center porosity tends to slightly increase with the increase in the casting speed, the value of the existence diameter itself is small, and is reduced to a level that hardly causes a problem. .

[0061]

Embodiment 2 Using a continuous casting apparatus having the structure shown in FIG.
Continuous casting of cast slabs (round billets) of 0.2% C-1% Cr steel 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 230mm, 265mm and 300mm in diameter.

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

FIG. 8 and FIG. 9 are diagrams showing calculation results of the solidification profile of the billet having a diameter of 230 mm by the heat transfer analysis. That is, the casting speed (Vc) is 2.0 m / mi
FIG. 11 is a diagram showing the relationship between the solidified shell thickness, the solid phase ratio (fs) at the center of the slab, and the distance from the molten steel meniscus in the mold in the case of n (FIG. 8) and 2.2 m / min (FIG. 9). .

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

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

Therefore, the length of each block for which cooling control is possible is made constant at 1 m, and the first most upstream side thereof is
28 m from the meniscus, from the second
Blocks up to the 10th were installed downstream in order. The total length of the blocks that can be controlled for cooling is 10m. Therefore,
At a casting speed of 2 m / min, the first block covers the position where the solid fraction is 0.1, the third block covers the position where the solid fraction is 0.3, and the solid fraction is 0.8. The fifth block covers the position. Therefore, an operation of starting the cooling control in the second block and stopping it in the fifth block was performed.

When the casting speed is 2.2 m / min, the position where the solid fraction is 0.1 is covered by the fourth block, and the position where the solid fraction is 0.3 is covered by the above The ninth block covers the position where the solid phase ratio is 0.8 in the sixth block. Therefore, the cooling control was started in the fifth block and stopped in the ninth block.

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

CP failure rate (%) = [(number of chassis in which CP maximum existence range is 30 mm or more) / number of cast chassis] ×
100 The above-mentioned center porosity defect occurrence rate is shown in FIG. FIG. 10 shows, as a comparative example, billet data obtained by a conventional operation without cooling control. As shown in the figure, according to the method of the present invention, the incidence rate of center porosity defects is significantly reduced.

[0071]

According to the cooling method of the present invention in which the surface of a slab is appropriately secondary-cooled according to the solid phase ratio at the center of the slab, the center porosity of the slab can be significantly reduced. Billet etc. continuously cast by applying this method,
For example, when used as a material for manufacturing a seamless steel pipe, it is possible to manufacture a product with less internal flaws.

[Brief description of the drawings]

FIG. 1 is a side longitudinal sectional view showing an example of a continuous casting apparatus for performing a cooling method of the present invention.

FIG. 2 is a diagram for explaining one embodiment of the cooling method of the present invention, and is a diagram showing a correspondence relationship between a solid phase ratio of a slab and a spray block for performing 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 a change in the diameter of the center porosity existing in the center longitudinal section of the billet when the center solid phase ratio and the water density at the start and end points of the cooling control are changed.

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

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

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

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

[Explanation of symbols]

1: slab, 2: molten steel, 3: immersion nozzle,
4: Mold, 5: Front secondary cooling spray, 6: Secondary cooling spray, 7: Pinch roll, 8: Solidified shell (shell), 9: Cutting torch

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeru Umeda 1850 Minato, Wakayama City, Wakayama Prefecture Wakayama Steel Works, Sumitomo Metal Industries Co., Ltd. (56) References JP 1-27-14949 (JP, A) JP 1 -210158 (JP, A) JP-A-63-168261 (JP, A) JP-A-63-165053 (JP, A) JP-A-58-184047 (JP, A) JP-A-62-64462 (JP, A (58) Fields investigated (Int. Cl. ⁶ , DB name) B22D 11/00 B22D 11/124

Claims (3)

(57) [Claims] In the secondary cooling during continuous casting of steel blooms or billets, when the solid phase ratio at the center of the slab becomes 0.1 to 0.3, the water density is 25 to 100 [liter / liter].
(min ・ m ² ))
A method for cooling a slab, wherein the water cooling is continued with the above water density until the solid phase ratio at the center of the slab becomes 0.8 or more. 2. The method of cooling a slab according to claim 1, wherein: In the secondary cooling zone of the continuous casting device, the length covering at least the position where the solid fraction of the slab is 0.1 to 0.8 or more is divided into a plurality of blocks,
The length of one block from the first of the divided blocks to the block covering at least the position where the solid phase ratio is 0.3 is 0.3 from the position where the solid phase ratio is 0.1.
The distance from the block following the block covering the position where the solid phase ratio is 0.1 at the center of the slab to the block covering the position where the solid phase ratio is 0.3 In any of the blocks, surface cooling of the slab is started by water cooling with a water density of 25 to 100 [liters / (min · m ² )], and the surface cooling covers at least a position where the solid fraction is 0.8. Continue to block. 3. Water density 25 to 100 [liter / (min · m ² )]
3. The method for cooling a slab according to claim 1 or 2, wherein the surface of the slab is cooled by air mist spraying.