JPH09192806A - Method for continuously casting slab - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely execute the elimination of a W type crater end at better reproducibility than the conventional cooling control in the width direction of a cast slab and to stably produce the cast slab without center segregation, at the time of eliminating the center segregation in the continuously cast slab with secondary cooling control. SOLUTION: In a zone having <=60mm solidified shell thickness of the cast slab 4 in the secondary cooling zone below a mold, a secondary cooling water quantity density in the range from 100-150mm position to 250-350mm position from both edges in the width direction of the cast slab 4 is made to >=2 times of the secondary cooling water quantity density at the center side in the width direction of the cast slab. This partial cooling strengthening part A in the width direction of the cast slab is executed, and then, the solidifying progress in the width direction of the cast slab is made to the almost same degree (U type profile) or the solidifying progress near the edge parts of the cast slab is made to quicker than the solidifying progress at the center side in the width direction of the cast slab (V type profile) so as to uniformly execute the light rolling reduction in the width direction at the end stage of the solidification.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a continuously cast slab,
In particular, it relates to a continuous casting method for eliminating uneven solidification in the width direction of a slab having a rectangular cross section and preventing center segregation.

[0002]

2. Description of the Related Art In continuous casting, molten steel cast in a mold through a dipping nozzle is primarily cooled by cooling water in the mold to form a solidified shell on the outer shell. This is a method of secondary cooling in a group of guide rolls to accelerate solidification and pull out a completely solidified slab to continuously produce a slab. In such continuous casting, it is often called center segregation. Internal defects are a problem. This center segregation is a phenomenon in which molten steel components such as C, S, P, Si, and Mn are positively segregated in the central portion (final solidified portion) in the thickness direction of the cast slab. Center segregation is a particularly serious problem in thick plate materials, and is known to cause deterioration of toughness in the segregated portion and hydrogen-induced cracking.

Such center segregation is caused by residual molten steel between dendrite (dendritic) trees at the end of solidification due to solidification shrinkage of the slab or bulging of the solidification shell.
It has been known that macroscopic movement toward the crater end of the final solidification part and local accumulation of concentrated molten steel occur. Therefore, as one of the measures to prevent center segregation, a method of suppressing the movement of concentrated molten steel and the accumulation of concentrated molten steel by compensating for the solidification shrinkage at the final solidification portion by reducing the vicinity of the solidification tip by some method Is proposed.

However, the center segregation improving method by only the rolling reduction is performed in the width direction of the slab-shaped slab 4 as shown in FIG. If the final solidification position CE of W type (hereinafter referred to as W type crater end) CE is unavoidable in the longitudinal section (plan view) due to uneven solidification, it is said that the center segregation cannot be improved over the entire width direction of the slab. It has drawbacks.
As shown in FIG. 7A, this is a slab-shaped slab 4
Since the progress of solidification in the region A ′ at the positions L _{1 to} L ₂ from both edges e of the slab is slower than the progress of solidification on the center side in the width direction, the final solidification position CE is different in the width direction of the slab. This means that the uniform segregation in the width direction of the slab cannot be achieved, and the center segregation a is deteriorated at both ends in the width direction of the slab where solidification is delayed (see FIG. 7 (c)). Therefore, in order to eliminate the center segregation over the entire width in the slab-shaped slab, it is essential to eliminate the W-shaped crater end that accompanies the uneven solidification in the slab width direction.

The origin of the uneven solidification in the width direction of the slab is highly likely to occur in the mold, and is considered to be due to the method of supplying hot water from the tundish into the mold.
That is, the hot water supply method in continuous casting is based on a so-called two-hole nozzle in which two discharge ports 10 are formed outward in the width direction of the slab, as shown in FIG. 8, from a comprehensive viewpoint such as quality and operability. It is thought that the solidification is delayed in such a case where the discharge flow of the molten steel 3 from the dipping nozzle 2 hits. Further, in the mold 1, since contact between the center portion in the width direction and the mold 1 is good due to bulging, solidification progresses faster in the vicinity of the center of the width direction than in the width end portion. Further, it is considered that this non-uniform solidification is promoted by non-uniformity in the width direction of the secondary cooling after leaving the mold. In addition, the code | symbol 5 shows a solidification shell and 11 shows a continuous casting powder.

On the basis of the widthwise non-uniform solidification generating mechanism in the mold as described above, a method for improving the two-hole nozzle which causes the non-uniform solidification in the width direction, for example, a so-called straight nozzle having one discharge port at the bottom. However, in this case, the depth of inclusions increases and the internal quality deteriorates, making it difficult to apply. Many inventions have been proposed as a method for improving center segregation by eliminating W-shaped crater ends after leaving the mold, including an improvement method by secondary cooling.

As an improvement method by secondary cooling, for example, in Japanese Patent Publication No. 54-39216, a section in which the solidified thickness of the slab in the widthwise center of the slab is 5 to 90% of the thickness of the slab, A difference in strength is given to the cooling in the width direction of the slab (3/8 W from the position 1/8 W from both ends of the slab with the width W of the slab)
The secondary cooling is performed by strengthening the cooling up to the position of (1) to (2) by cooling the amount of cooling water of this part to 1.5 to 3.0 times that of other parts, and at least two pairs or more near the crater end of the cast Add the reduction with the reduction roll of (the uncoagulated reduction rate 1
%, 3%) method is disclosed.

Also, in Japanese Patent Laid-Open No. 51475/1975, in continuous casting in which molten steel is cast into a mold from a tundish using a single immersion nozzle provided in the center of the mold, solidification in the center of the slab width direction In the section where the thickness is generated to be 5 to 90% of the thickness of the slab, all or part between the position of ⅛W and the position of ⅜W from both end positions of the slab is 1 or more than the other part. It is disclosed that cooling is performed with a cooling water amount of 0.5 to 7.0 times.

However, in these inventions, although it is possible to eliminate the W-shaped crater end shape, the range of the cooling strengthening portion in the casting direction, the range of the cooling strengthening portion in the width direction of the cast piece, and the range of the amount of cooling water. Is extremely wide, and unless a more precise water amount distribution according to the solidification profile in the width direction is given, new solidification unevenness as shown in FIG. In some cases, the center segregation of No. 1 may be significantly deteriorated (B 'part in FIG. 9).

Besides the method other than the secondary cooling,
Japanese Unexamined Patent Publication (Kokai) No. 1-178355 and Material and Process Vol. 2 (1989), P1159 force a slab by gradually increasing the interval in the slab thickness direction of the guide rolls of the guide roll group. Bulging was caused, and the slab thickness was made to be 2 to 3 times the short side of the mold, and light reduction was performed with a small diameter roll of the reduction roll group near the crater end, and after eliminating the W-type crater end by bulging, it was concentrated by light reduction. A method has been proposed in which the movement / accumulation of the chemical liquid steel is prevented and the center segregation is prevented. However, in this method, since the phenomenon of bulging is left to the slab, the unsolidified thickness is not stably constant. Further, since the drum-shaped slab is rolled down, the rolling down amount in the width center portion becomes large, and it is difficult to perform uniform rolling down in the width direction.

As another method for improving center segregation,
As described in JP-A-63-157749, a method of applying electromagnetic stirring within a specific range, and JP-A-1-
As described in Japanese Patent No. 113157, there is a method of applying ultrasonic vibration to a slab, but in all cases, when there is uneven solidification in the width direction, a fundamental solution has not been reached.

The present invention has been made to solve the above-mentioned problems, and the purpose thereof is that the method by the widthwise cooling control of the secondary cooling is the most effective in eliminating the W-type crater end. Recognizing that this is the case, the elimination of the W-type crater end can be performed more reliably and reliably than the conventional widthwise cooling control of secondary cooling, and a slab without center segregation can be manufactured stably. To do so.

[0013]

In order to improve center segregation in a continuously cast slab, it is known that reduction casting at the final stage of solidification is effective. However, there is a problem in that, if non-uniform solidification occurs in the slab width direction by simply performing the reduction, the effect of improving center segregation over the entire slab width direction cannot be expected. The present invention eliminates the uneven solidification in the slab width direction by the width direction cooling control of the secondary cooling in order to effectively function the reduction for improving the center segregation.

In general, the slab width solidification morphology is shown in FIG.
As shown in, the solidification progress is delayed in a region A ′ apart from both edges of the slab by a predetermined distance, and the central segregation is remarkably deteriorated in this region. The phenomenon in which the progress of solidification is delayed in the vicinity of both edges of the slab is that the discharge flow hits the short side from the so-called two-hole immersion nozzle, and the progress of solidification is delayed in the vicinity of the width end, which is the vicinity, and the bulging causes the central portion in the width direction The solidified shell of has good contact with the mold,
On the other hand, the solidified shell at the width end portion has a poor contact with the mold, which is due to the synergistic effect of relatively delaying the solidification in the vicinity of the width end portion than in the width center portion. Such uneven solidification in the width direction in the mold remains until the final stage of solidification, and even if compensation for the solidification shrinkage is performed by light reduction or the like, segregation deteriorates at the portion where solidification is delayed. Therefore, in order to correct this unevenness of solidification behavior in the width direction of the long side surface, by strengthening the cooling in the solidification delay portion, it is possible to maintain uniform solidification progress over the entire width direction, and it is possible to achieve uniform reduction. Becomes

The inventors of the present invention investigated various correlations between the casting conditions and the solidification profile in the slab width direction, and as shown in FIG. 1, regardless of the slab width, the casting speed, and the like, as shown in FIG. It was found that the progress of solidification was delayed in the region of 100 to 300 mm, and the central segregation was significantly deteriorated in this region. It was also found that this delayed solidification portion was displaced by about 50 mm depending on the angle of the discharge port of the immersion nozzle. Japanese Unexamined Patent Publication No. 51-47527 describes that cooling is strengthened from both sides of the slab to a position of 1/8 W to 3/8 W of the slab width W (slab width 2000
The range of 250 to 750 mm with respect to mm), the inventors conducted a reproduction test based on this specification, and found that the part was significantly deviated from the solidification delay part depending on the slab width, and rather a new one as shown in FIG. It has resulted in uneven solidification. From the above, the present inventors have found that 250 to 35 mm from the position of 100 to 150 mm from both edges of the slab.
It was concluded that in the area A up to the 0 mm position, a means for correcting the coagulation lag is needed. In addition, 1
In the area of 00 mm or less, coagulation progresses from the short side of the slab, so no coagulation delay occurs.

Next, as a cooling method for correcting the unevenness of the solidification behavior in the width direction, there are primary cooling by the mold and secondary cooling by water or air after leaving the mold. It was found that it is most effective to correct the lateral non-uniform solidification in the secondary cooling zone where the shell thickness in the lag area is 60 mm or less. This is for the following reason. According to the FeS addition test in the mold, the inventors found that the difference (= d ₂ −d ₁ ) in the solidification shell thickness between the solidification delay part (the region of 100 to 300 mm from the edge) and the non-solidification delay part (the center part of the width) was the maximum. 10
It has been found that it is about mm. Therefore, this maximum 1
It is important to estimate how much 0 mm can be recovered by correcting the secondary cooling.

FIG. 2 shows the relationship between the solidification shell thickness and the solidification rate. In the figure, normal cooling and cooling enhancement (2 during normal cooling)
2 times the water content density). As a result, if the solidified shell thickness is 40 mm and the difference in shell thickness of 10 mm is to be recovered by strengthening by cooling, it is 3.9 mm / mi.
A coagulation delayed recovery rate of n will be obtained. Therefore,
2.56m to make up for 10mm difference in shell thickness
The cooling strengthening zone is required for the distance obtained by multiplying in (= 10 / 3.9) by the casting speed. FIG. 3 shows a re-arrangement of the solidification delay recovery speed obtained in FIG. 2 using the solidification shell thickness of the solidification delay portion as a parameter. The solidification delayed recovery rate decreases as the solidification shell thickness increases, in other words, the longer the cooling strengthening zone is required. As shown in FIG. 3, the solidified delayed recovery speed is saturated in the region where the solidified shell thickness is larger than 60 mm. Therefore, it was concluded that the cooling strengthening for eliminating the solidification delay should be performed in the zone where the solidification shell thickness is 60 mm or less.

Next, the amount of cooling enhancement will be described. FIG.
As shown in, the solidification delay occurring immediately below the mold is recovered by secondary cooling, and the final solidification position shape is uniform in the width direction (U type) or the solidification progress near the edge is made faster than in the width direction central part (V type). ), It is necessary that the water amount density of the solidification delay portion is 2.0 times or more the water amount density of the center portion in the width direction.

The maximum amount of cooling strengthening depends on the thickness of the cooling strengthening starting shell and has a certain degree of freedom, but if it is strengthened too much, surface defects will occur due to overcooling, so the water content density in the central portion in the width direction will occur. 10 times or less is desirable.

FIG. 5 shows the types of width direction solidification profiles. The transverse solidification profile should ideally be as shown in FIG.
As shown in FIG. 5A, not only the U-shaped profile (ideal profile) having a completely uniform shell thickness but also FIG.
As shown in (b), even in the case of the V-shaped profile in which the coagulation progresses in the width direction is faster than in the width center, the central portion of the width is likely to be rolled down, so that the center segregation improving effect can be enjoyed. However, there are limits to what can be called a V-shaped profile,
It is desirable that the difference between the final solidification position between the cooling strengthened portion and the width center portion be within 1 m. This is because, when the reduction is applied at the final solidification position, the smaller the difference between the final solidification positions, the more uniform the distribution of the solidification shrinkage compensation amount in the slab width direction, and the better the level of center segregation. .

From the above, in the present invention according to claim 1, in the continuous casting method of the slab-shaped slab for drawing molten steel supplied through the immersion nozzle into the mold while cooling, in the secondary cooling zone The solidified shell thickness of the slab is 60
In a zone of mm or less, the secondary cooling water amount density of the region from the position of 100 to 150 mm to the position of 250 to 350 mm from both edges in the width direction of the cast slab is set to Double or more, and the cooling enhancement partly in the width direction of the cast piece makes the solidification progress in the width direction of the cast piece almost the same (U-shaped profile), or the progress of solidification near the edge of the cast piece in the width direction of the cast piece. Faster than the progress of central coagulation (V-shaped profile). In the case of this V-shaped profile, it is preferable that the difference between the final solidification positions of the cooling strengthened portion and the width center portion is within 1 m. In addition, as the cooling strengthening method in the cooling strengthening section, there are a method of adjusting the water amount by using the section as a separate system, and a method of increasing the number of spray nozzles of the section.

Next, paying attention to the solidification profile of FIG. 1B, the shell thickness d of the delayed solidification portion is 15 from the edge.
The position at 0 to 200 mm is the minimum, and the position gradually increases from here toward the center in the width direction. Therefore, the present inventors have found that a better level of center segregation can be obtained by dividing the cooling-strengthened portion into two or more parts and making the water amount distribution according to the solidified shell thickness distribution.

As described above, in the present invention according to claim 2, in the invention according to claim 1, a plurality of cooling-strengthening portions partially in the width direction of the slab are formed in the width direction of the slab according to the solidification progress of the slab. The cooling water amount density distribution of each of the divided parts is set according to the progress of solidification of the slab.

As a method of dividing the cooling strengthening portion into two or more, a method of using a spray tip having different water amount characteristics, or a method of dividing the cooling system itself into two and making it possible to adjust the water amount of each system, etc. There is.

According to the present invention as described above, the shape of the final solidification position of the slab is, as shown in FIG. 6 (a), the solidification in the width direction of the slab from the W-shaped profile in which the progress of solidification near the edge is delayed from the width center. A U-shaped profile with uniform progress, or a V-shaped profile with faster progress of solidification near the slab edge than in the width center. If the reduction is performed in the state where the shape of the final solidification position is improved, the reduction is efficiently performed to the vicinity of the edge, and as shown in FIG. 6B, the level of center segregation near the edge is reduced. It will be good. This allows
Product characteristics such as HIC resistance and mechanical properties are significantly improved.

Further, according to the present invention, the actual condition of the non-uniformity in the width direction can be grasped, and the non-uniformity of the solidification can be corrected in an effective place by concentrating only the portion requiring the correction. The new solidification nonuniformity of Fig. 9 as seen in the widthwise cooling control of No. 1 does not occur, the elimination of the W-type crater end can be reliably performed with good reproducibility, and a slab without center segregation can be obtained. It can be manufactured stably.

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to an embodiment shown in the drawings. This is an example applied to continuous casting of flat slabs, and FIG. 1 shows the continuous casting equipment used in the present invention and partial strong cooling of the slab according to the present invention. In FIG. 1, molten steel 3 from a ladle and a tundish is cast into a water-cooled copper mold 1 by a dipping nozzle 2.
The solidification shell 5 is formed on the surface of the slab 4 by the primary cooling in the mold 1, and the solidification is promoted by the secondary cooling in the support roll group 6 by the spray water or the like. It is pulled out through the roll 8.

The dipping nozzle 2 arranged only one in the center of the mold 1 is a so-called two-hole nozzle, and its two side discharge ports have their discharge directions directed to both sides in the slab width direction, that is, to the short side of the slab. In addition, the discharge angle is formed to be an angle of several degrees downward with respect to the horizontal. A spray nozzle 9 is provided between each roll of the support roll group 6 and / or the reduction roll group 7 to form a secondary cooling zone.

In such continuous casting equipment, according to the present invention, (1) in the secondary cooling zone from immediately below the mold satisfying the solidified shell thickness of 60 mm or less to 10.0 m below the meniscus in the mold,
A partial cooling strengthening portion A is provided in the width direction of the slab. If the cooling strengthening portion A is provided from immediately below the mold to 5.0 m below the meniscus in the mold, the maximum difference in solidification shell thickness between the solidification delay portion and the non-solidification delay portion of 10 mm can be recovered. The cooling strengthening portion A may be provided in the secondary cooling zone from 0.0 m to 10.0 m so that the progress of solidification in the vicinity of the slab edge is faster than in the center of the slab width.

(2) Regarding the slab width direction of the cooling strengthened portion A, regardless of the slab width, the casting speed, etc., the region is at least 100 mm to 300 mm from both edges of the slab. Further, since the solidification delay portion may be displaced by about 50 mm depending on the angle of the discharge port of the immersion nozzle 2, the cooling strengthening range may be shifted in the width direction by about 50 mm (in the numerical example described later, the cooling strengthening range is 100 to 300).
mm). In the area of 100 mm or less,
Since coagulation proceeds from the short side of the slab, coagulation delay does not occur.

(3) By increasing the number of spray nozzles 9 in the cooling strengthening section A, the secondary cooling water quantity density in the cooling strengthening section A is set to the secondary cooling water quantity density of 2.
It is set to 0 times or more and 10.0 times or less so that a U-shaped or V-shaped profile can be obtained (see FIG. 4).

(4) The cooling strengthened portion A is divided into two or more parts, and the secondary cooling water amount density of each divided portion is changed according to the progress of solidification of the cast slab. For example, the secondary cooling water density from the slab edge side toward the slab center side is set to 3, 2, or 1 times the secondary cooling water amount density on the slab width direction central side.

(5) The crater end portion at the final stage of coagulation is lightly reduced by the reduction roll group 7 over a predetermined range in the front and rear. If necessary, an electromagnetic stirrer is installed upstream of the solidification completion point to electromagnetically stir the molten steel in the slab.

With the above structure, continuous casting was performed under the following conditions.

Equipment Specifications / Casting Conditions (1) Continuous casting machine: Curved continuous casting machine (curving radius: 12.5 m) (2) Slab size: 250 mm (thickness) x 2000 mm (width) (3) Steel type: C 0.15 to 0.20% 40K steel for thick plate (4) Superheated molten steel: ΔT = 20 ° C (5) Casting speed: 0.8m / min (6) Mold length: 700mm (7) Final solidification reduction : Reduction zone length 5 m, reduction gradient 1 mm / m (8) Strand electromagnetic stirring: Application under meniscus 9.0 m Table 1 shows the secondary cooling conditions of Examples of the present invention and Comparative Examples. Table 2-1 shows the widthwise distribution of final solidification positions converted from the white band generated by the electromagnetic stirring of the strands.
2 shows the widthwise distribution of the C segregation degree in the central segregation portion.

[0036]

[Table 1]

[0037]

[Table 2]

From these Tables 2-1 and 2-2, the following facts have become clear. In Comparative Example 1, since the strength of the secondary cooling is not applied in the width direction, it is clearly 15 from the edge.
At the position of 0 to 300 mm, the final solidification position was on the downstream side, and the central segregation of the solidification delay portion was greatly deteriorated. Comparative Example 2 is a reproduction test conducted by the present inventors based on the specification disclosed in Japanese Patent Application Laid-Open No. 51-47527. However, although the strength of secondary cooling is added in the width direction, Since the portion where the solidification was delayed (in this example, the position of 100 to 300 mm from the edge) was not strongly cooled, it resulted in further promoting the uneven solidification in the width direction, and the center segregation of the strongly cooled portion was significantly deteriorated.

On the other hand, according to the present invention, in any of the first to third embodiments, the solidification delay portion is accurately cooled,
The center segregation is significantly improved in the entire width direction. Among these, in Example 2, since the difference between the final solidification positions of the cooling strengthened portion near the edge and the width center portion was set to 1.0 m or less, the center segregation was further improved as compared with Example 1, and in Example 3, the cooling was performed. Since the reinforced portion was divided into two and the water amount distribution was selected in accordance with the solidification profile in that portion, the central segregation was further improved as compared with Examples 1 and 2.

Although the above description has been given of the embodiment relating to one slab having a single slab size, it goes without saying that the present invention can be applied to slabs having different slab sizes, casting speeds and the like.

[0041]

As described above, the present invention is a secondary cooling zone in which the thickness of the solidified shell of the slab is 60 mm or less, and 250 to 35 mm from the position 100 to 150 mm from both edges in the width direction of the slab.
Since the secondary cooling water amount density in the area up to the position of 0 mm is set to be twice or more the secondary cooling water amount density on the center side in the width direction of the slab, a U-shaped or V-shaped crater end is obtained. Light reduction at the end of solidification is performed uniformly in the width direction of the slab, and the center segregation at the width end is significantly improved. Furthermore, the W-type crater end can be eliminated more reliably with better reproducibility than the widthwise cooling control of the conventional secondary cooling, and the center segregation near the slab edge can be completely eliminated.
As a result, it became possible to stably produce a cast product having a uniform composition in the entire width direction of the cast product and having no center segregation.

[Brief description of the drawings]

FIG. 1A is a vertical sectional view showing an example of a continuous casting machine for carrying out a continuous casting method according to the present invention, and FIG. 1B is a transverse sectional view showing partial strong cooling according to the present invention. is there.

FIG. 2 is a graph showing the relationship between the solidification shell thickness and the solidification rate during normal cooling and during cooling strengthening.

FIG. 3 is a graph showing a relationship between a solidification shell thickness of a solidification delay portion and a solidification delay recovery speed.

FIG. 4 is a graph showing the relationship between the water content ratio between the cooling-strengthened portion and the width center portion and the difference in the solidified shell thickness near the slab edge and the width center portion in the present invention.

5A and 5B are a horizontal cross-sectional view and a vertical cross-sectional view showing a solidification profile in a slab width direction, FIG.
(B) shows a V-shaped profile, and (c) shows a W-shaped profile.

6A is a graph comparing final solidification positions of the present invention and the related art, and FIG. 6B is a graph comparing center segregation in the width direction between the present invention and the related art.

FIG. 7 is a slab produced by a conventional continuous casting method,
(A) is a cross-sectional view of the slab during casting, (b) is a vertical cross-sectional view of the slab during casting, and (c) is a cross-sectional view showing the center segregation occurrence state of the slab.

8A is a vertical cross-sectional view showing a flow state of molten steel in a mold discharged from a dipping nozzle, and FIG. 8B is a cross-sectional view showing uneven solidification of a slab in a secondary cooling zone thereof.

FIG. 9 is a cross-sectional view showing a new non-uniform solidification that occurs when the conventional cooling strengthening portion is not in the proper position.

[Explanation of symbols]

 A ... Cooling strengthening part a ... Center segregation CE ... Crater end (final solidification position) 1 ... Mold 2 ... Immersion nozzle 3 ... Molten steel 4 ... Cast piece 5 ... Solidification shell 6 ... Support roll group 7 ... Rolling roll group 8 ... Pinch roll 9 ... Spray nozzle 10 ... Discharge port 11 ... Continuous casting powder

Claims (2)

[Claims] 1. A continuous casting method of a slab-shaped slab, in which molten steel supplied through a dipping nozzle into a mold is drawn while cooling, in a zone where the solidified shell thickness of the slab in the secondary cooling zone is 60 mm or less, 100 ~ from both widthwise edges of the slab
The secondary cooling water amount density in the region from the position of 150 mm to the position of 250 to 350 mm is set to twice or more the secondary cooling water amount density of the central side in the width direction of the slab, and the cooling strengthening portion is partially in the width direction of the slab. According to the method, the solidification progress in the slab width direction is made substantially equal, or the solidification progress in the vicinity of the slab edge is made faster than the solidification progress in the slab width direction center side. 2. The continuous casting method for a slab according to claim 1, wherein a partial cooling-strengthened portion in the width direction of the cast piece is divided into a plurality of pieces in the width direction of the cast piece according to the progress of solidification of the cast piece. A continuous casting method for a slab, characterized in that the distribution of the cooling water amount density in the divided portions is a distribution according to the progress of solidification of the slab.