JP2000301304A - Continuous cast slab and continuous casting method - Google Patents

Abstract

(57) [Problem] In a continuous casting method in which bulging and a large amount of reduction corresponding to a bulging amount are reduced, a rapid change of a segregation portion in a thickness direction by solidifying while an unsolidified portion remains. To obtain a thick steel plate having good bending characteristics and draw ratio. SOLUTION: Using a pair of guide rolls 4 for a slab 10,
20 mm bulging is generated, and the solid phase ratio at the center is 0.1 to 0.
By using at least the rolling roll pair 6 until 8, the unsolidified portion 10a having the unsolidified portion rolling reduction of 0.2 to 0.5 is left,
Reduce by a reduction amount of 0.5 times or more and less than 1.0 times the bulging amount. Thus, having a high concentration segregation area d ₃ therein including a central portion of the slab thickness direction, the high concentration segregation d _3, having more than one-third of the concentration C ₁ of the concentration peak value C _max Area d
₂ over a length of 1.5 to 25% of the thickness in the slab thickness direction to produce a continuous cast slab 12.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a continuous cast slab and a continuous casting method. More specifically, the present invention relates to a continuous cast slab and a continuous casting method capable of preventing occurrence of cracks in a final product such as a thick steel plate.

[0002]

2. Description of the Related Art When a continuous cast slab made of steel is manufactured using a continuous casting method, an internal defect called center segregation often occurs in the manufactured continuous cast slab. FIG. 9 is an explanatory diagram schematically showing the continuous cast slab 1 in which the center segregation 2 has occurred. As shown in the figure, the center segregation 2 is caused by C, Mn, S and the like in the molten steel in the final solidified portion which is the center of the continuous cast slab 1 in the thickness direction (the direction of the double arrow in FIG. 9). It is generated when the component elements such as P are concentrated and positively segregated to a high concentration. The center segregation 2 of the continuous cast slab 1
Particularly when the final product is a thick steel plate, it causes toughness reduction and hydrogen-induced cracking in a high concentration segregated portion of a component element.

[0003] Specifically, the central segregation is that the molten steel in which the component elements are concentrated remains between dendrites (dendrites) at the final stage of solidification of the molten steel, and solidifies as it is at the center in the thickness direction of the slab; This occurs because the molten steel moves macroscopically toward the solidification completion point of the final solidified portion due to the flow of the molten steel due to shrinkage during solidification or bulging of the slab called bulging. Therefore, in order to prevent the occurrence of center segregation, it is important both to prevent the movement of the concentrated molten steel between dendrites at the end of solidification and to prevent the local accumulation of the concentrated molten steel. Become.

[0004] From such a viewpoint, in order to prevent the occurrence of center segregation, many continuous casting methods using a so-called "continuous casting method using bulging and large pressure reduction" have been proposed.

FIG. 10 is an explanatory view schematically showing this “continuous casting method using bulging and large pressure reduction”. As shown in the figure, below the continuous casting mold 3, from a position corresponding to a solidification completion point (liquidus crater end) 5a based on the liquidus temperature, solidification based on the solidus temperature is performed. A plurality of sets (10 in the illustrated example) in which the roll interval is gradually increased to a position corresponding to the completion point (solid line crater end) 5b
A pair of guide rolls 4 is provided with a bulging zone 5. Below the bulging zone 5, a reduction zone 7 composed of a plurality of (three in the illustrated example) reduction roll pairs 6 is provided. Further, below the rolling down zone 7,
A plurality (two in the illustrated example) of pinch roll pairs 11 are provided.

[0006] The molten steel 9 injected into the continuous casting mold 3 by the immersion nozzle 8 is cooled by the continuous casting mold 3 and spray water sprayed from a group of nozzles (not shown) provided thereunder, and solidified on the outer surface. The shell 10a is formed into the slab 10. At this stage, there is an unsolidified portion inside the slab 10.
10b exists. In the slab 10, in the bulging zone 5, the central portion in the longitudinal direction is gradually expanded in the short side direction by the ten pairs of guide rolls 4 and the molten steel static pressure, and bulging occurs. The slab 10 in which bulging has occurred is reduced stepwise in the reduction zone 7 by three pairs of reduction rolls 6. Thereby, the solidification interface of the slab 10 is pressed and solidified completely at the solidus crater end 5b. The slab 10 is continuously drawn in the direction of the outline arrow by a plurality of pairs of pinch rolls 11 through the bulging zone 5 and the reduction zone 7 to form a continuous cast slab 12.

[0007] For example, Japanese Patent Application Laid-Open No. 9-57410 discloses that in a continuous casting method using bulging and large pressure reduction, the maximum thickness of a slab 10 Bulging in the bulging zone 5 so as to be 20 to 100 mm thicker than the distance between the long sides of the continuous casting mold 3, the amount of bulging corresponding to the amount of bulging at a reduction of 20 mm or more per pair of reduction rolls 6. Has been proposed.

Japanese Patent Application Laid-Open No. 9-206903 discloses a cast slab.
The maximum thickness of 10 is 10 more than the distance between the long sides of the continuous casting mold 3.
There has been proposed an invention in which bulging is performed in the bulging zone 5 so that the bulging is about 50% thicker, and the bulging is performed in the reduction zone 7 with a reduction gradient of 80 mm / m or more per slab length.

On the other hand, as another method for preventing the occurrence of center segregation, for example, Japanese Patent Application Laid-Open No. 63-252655 discloses that the amount of cooling water injected onto the surface of the slab 10 is increased. The surface temperature of the final solidification part of the slab 10 is 700 to 800 ° C
By increasing the thickness of the solidified shell 10a, the amount of bulging generated between the plural pairs of guide rolls 4 is suppressed.
An invention has been proposed in which a rolling force having a strain rate of 2 to 0.4% is applied to a slab 10 to prevent the flow of concentrated molten steel and prevent the occurrence of center segregation.

[0010]

SUMMARY OF THE INVENTION However, Japanese Patent Application Laid-Open No. 9-57
In the invention proposed in Japanese Patent Publication No. 410 or 9-206903, since the bulging amount of the slab 10 is as large as 20 mm or more, a large reduction force is required to reduce the slab 10 and each reduction roll is required. Although the diameter of the pair 6 is inevitably increased, these proposals do not mention at all the roll diameter of the pressing roll pair 6 or the roll pitch at the pressing position. However, according to the study results of the present inventors, between the position corresponding to the liquidus crater end 5a and the position corresponding to the solidus crater end 5b, such a large diameter reduction roll pair 6 is placed. It is physically impossible to arrange the desired number, and it is difficult to actually implement the method according to these proposals.

[0011] Further, under the light reduction by the reduction roll group in the method proposed in JP-A-63-252655, the reduction can be performed only in a point-like manner in the longitudinal direction of the slab 10. Therefore, solidification shrinkage and bulging cannot be sufficiently prevented. Also, slabs with large bulging amount
If this light reduction is to be performed on 10, each reduction will locally act on the slab 10 as a concentrated load. Therefore, in the case of the slab 10 having a large bulging amount, internal cracks are easily generated at the solidification interface, and a sufficient rolling reduction cannot be secured.

As described above, the so-called "continuous casting method in which the bulging and a large amount of bulging are reduced by an amount corresponding to the bulging amount" requires an excessive rolling force. The strength is significantly reduced. For this reason, it was actually difficult to prevent the occurrence of center segregation in the continuous cast slab.

Here, an object of the present invention is to provide a continuous cast slab which does not cause a reduction in product strength due to internal cracks at the solidification interface, and a continuous casting method capable of obtaining the continuous cast slab. To provide.

[0014]

Here, the present invention has a high-concentration segregation part inside including a central part in a slab thickness direction,
This high-concentration segregation portion, the region having a concentration of 1/3 or more of the concentration peak value, the slab thickness direction in the slab thickness direction 1.
A continuous cast slab having a length of 5% or more and 25% or less.

In the present invention, the high-concentration segregation part is more than the segregation degree of the stationary part where no central segregation occurs.
It means a part where the degree of segregation is high.

The term "slab thickness" refers to the thickness of the continuous cast slab, specifically, the thickness of the slab at the center segregation generating portion during rolling. The slab thickness is a value obtained by measuring the thickness of the continuously cast slab during rolling, which is performed after bulging.

Further, from another aspect, the present invention relates to a slab having an unsolidified portion therein, from a position corresponding to a liquidus crater end of the slab to a position corresponding to a solidus crater end. Using a plurality of guide roll pairs arranged in a predetermined position of the slab thickness direction is gradually expanded in the slab thickness direction, to generate a bulging of 5mm or more and 20mm or less in total,
The slab having this bulging has a center solid phase ratio of 0.1%.
Using at least one pair of rolling rolls up to and including 0.8, the unsolidified portion where the reduction ratio of the unsolidified portion defined by the following equation (1) becomes 0.2 or more and 0.5 or less remains, and the amount of bulging is reduced. This is a continuous casting method characterized in that a reduction of 0.5 times or more and less than 1.0 times is performed.

[0018]

Lf = D1 / D2 (1) In the above equation (1), the symbol Lf indicates the unsolidified portion reduction rate, and the symbol D1 indicates the unsolidified portion reduction amount (mm). The symbol D2 indicates the thickness (mm) of the unsolidified portion having a solid fraction of 0.8 or less at the start of rolling.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a continuous cast slab and a continuous casting method according to the present invention will be described below in detail with reference to the accompanying drawings. First, the continuous casting method according to the present invention will be described.

The continuous casting method according to the present invention has a bulging step and a large rolling step as in the known continuous casting method using bulging and large rolling. Therefore, these steps will be sequentially described with reference to FIG.

[Bulging Step] The structure of the bulging zone 5, the pressing zone 7, the pinch roll pair 11, and the like in FIG. 10 has already been described, and a description thereof will be omitted.

As shown in FIG. 10, the molten steel 9 injected into the continuous casting mold 3 through the immersion nozzle 8 is supplied from the water-cooled continuous casting mold 3 and a group of nozzles (not shown) provided therebelow. Cooled by the spray water sprayed.
As a result, a solidified shell 10a is formed in the molten steel 9 to form a cast slab 10. At this point, an unsolidified portion 10b exists inside the slab 10.

In the present embodiment, the slab 10 is bulged in the bulging zone 5. The bulging zone 5 includes a plurality of (10 in this embodiment) guide roll pairs 4. 10 sets of guide roll pairs 4 are cast slabs
They are provided at predetermined positions between the position corresponding to the ten liquidus crater ends 5a and the position corresponding to the solidus crater ends 5b. 10 sets of guide roll pairs 4
In the drawing direction of the slab 10, the roll interval in the slab thickness direction is gradually increased and disposed. As a result, the slab 10 in which the unsolidified portion 2b exists is gradually bulged by the action of the molten steel static pressure.

In this embodiment, the bulging amount is a slab.
10mm thickness direction (left and right direction in Fig. 10), 5mm in total
Not less than 20 mm. If the bulging amount is less than 5 mm, the amount of reduction in the large reduction step described below will be insufficient, and the squeezing out of the concentrated molten steel for improving center segregation will be insufficient.
On the other hand, if the bulging amount exceeds 20 mm, the amount of reduction in the large reduction step described below will be excessive, and the equipment cost required for the reduction device will increase. Therefore, in the present embodiment, the bulging amount of the slab 10 in the bulging zone 5 is limited to a total of 5 mm or more and 20 mm or less in the thickness direction of the slab 10.

Thus, in the present embodiment, the slab 10
Is generated by the bulging zone 5 with a bulging amount of 5 mm or more and 20 mm or less.

[Large Reduction Step] In this embodiment, after the slab 10 is bulged by the bulging zone 5, the bulged portion is greatly reduced by the reduction zone 7. The rolling zone 7 is constituted by three rolling roll pairs 6 in this embodiment.

In the case of the large reduction by the reduction zone 7, the solid fraction at the center of the slab 10, that is, the solid fraction required for flow is 0.1 to 0.1.
It will be done within 8 or less. The center solid phase ratio of the slab 10 is 0.
A value of less than 1 is not possible due to the growth of the solidified shell 10a, while the solid fraction at the center of the slab 10 of 0.8 is a flow limit in solidification. Therefore, in the present embodiment, the slab
While the center solid phase ratio of 10 is not less than 0.1 and not more than 0.8, large reduction is performed by three pairs of reduction rolls 11.

The amount of reduction under the large pressure is 0.5 times or more and less than 1.0 times the bulging amount in the bulging zone 5. If the reduction amount is less than 0.5 times the bulging amount,
Central segregation cannot be improved, and conversely, the ratio of remaining positive segregation increases. On the other hand, the rolling amount is 1.
When it is 0 times or more, at the end of the large reduction, the unsolidified portion does not exist inside, and the high concentration segregated portion is formed with a steep distribution inside including the central portion in the slab thickness direction,
It becomes over-pressurized, and it becomes necessary to generate a large rolling force in equipment. Therefore, in the present invention, the amount of reduction under large pressure is limited to 0.5 times or more and less than 1.0 times the bulging amount in the bulging zone 5.

FIG. 1 is an explanatory diagram showing an enlarged view of the vicinity of the rolling zone 7 in FIG. FIG. 1 shows the drawing direction horizontal for convenience of explanation, and shows that one of the pressing rolls of each of the pressing roll pairs 6 moves in the thickness direction of the slab 10 to perform the pressing. It is. But the figure
As shown in FIG. 10, both the reduction rolls of each reduction roll pair 6 may be configured to move to reduce the slab 10.

As shown in the figure, large reduction is performed by each of the roll pairs 6 so that an unsolidified portion remains inside even after the reduction by each of the roll pairs 6. Due to this large pressure, the unsolidified portion 10a at the end of solidification is discharged to the upstream side at a discharge speed V. When the bulging amount is constant, the discharging speed V increases as the rolling amount increases. Here, since the solute in the unsolidified portion 10a in the final stage of solidification is concentrated, the unsolidified portion 10a in which the solute is concentrated is discharged to the upstream side at a discharge speed V by the large reduction by the reduction roll pair 6. For this reason, a gradient of the solute concentration is generated in the unsolidified portion 10a on the upstream side of the pressing position by the pressing roll pair 6.

FIG. 2 is a graph showing an example of the distribution of the solute concentration generated in the unsolidified portion 10a on the upstream side of the rolling position by the rolling roll pair 6. As shown in FIG. As shown by the graph in FIG.
Solute concentration immediately upstream of the pressing position P _r is rapidly increased,
The solute concentration gradually decreases from the rolling position _Pr toward the upstream side. Accordingly, the solidified portion of the upstream side of the pressing position P _r higher solute concentration high-concentration segregation occurs. For this reason, in the slab 10, segregation of solute concentration occurs not only in the drawing direction of the slab 10 but also in the thickness direction.

FIG. 3 is a graph showing an example of the distribution of the degree of carbon segregation in the thickness direction of the slab 10. In the graph of FIG. 3, the concentration C ₀ indicates the degree of segregation of the stationary parts d ₄ and d ₅ where no central segregation has occurred, that is, the bulk concentration.

When the slab 10 is solidified without reduction by the reduction roll pair 6 used in the present embodiment, as shown by a thin solid line in the graph of FIG. In the region d ₁ ), a high concentration segregation portion in which the degree of segregation rapidly fluctuates in a range from C ₀ to the concentration peak value C _max occurs locally and steeply. Therefore, when manufacturing, for example, the steel plate of the continuous casting slab as the raw material, a high concentration stress to segregation d ₁ is concentrated, cracks in the center of the steel plate as a final product occurs.

On the other hand, in the bulging step, bulging is performed in an amount of 5 mm or more and 20 mm or less, and at least one pair of reduction rolls is set between 0.1 and 0.8 in the center solid phase ratio. Of the bulging amount
When a large reduction is performed with a reduction amount of 0.5 times or more and less than 1.0 times, Fig. 3
As indicated by the thick solid line in the graph, the segregation of the central portion in the thickness direction of the slab 10 is left in a non-solidified state, high-concentration segregation is greatly expanded from the area d ₁ to region d ₃ .

In the high-concentration segregated portion having the region d ₃ , the region d ₂ having a concentration of not less than 1/3 of the concentration peak value C _max and the concentration C _{1 is} equal to the thickness of the slab 10 in the slab thickness direction. Sano
It is formed over a length of 1.5% or more and 25% or less. As will be described later, the thickness of the slab 10 is a value at the time of reduction performed following bulging.

[0036] Thus, in the present embodiment, a high concentration segregation region d _3, segregation is present in a wide range including a region d ₂ with a higher segregation ratio than the bulk concentration C _0. Therefore, even if there is a peak of segregation ratio in the central portion of the high-concentration segregation region d _3, intensity difference between this central portion and the central portion other than the portion, than the conventional (C ₁ -C ₀ ) is suppressed.

For this reason, when manufacturing a thick steel plate using a continuously cast slab as a raw material, stress concentration on the high concentration segregated portion in the region d ₃ is reduced. For this reason, according to the present embodiment, the occurrence of cracks that occur when the steel sheet cannot withstand the crack limit strain is suppressed. In this way, the occurrence of cracks in high concentration segregation region d ₃ is suppressed.

The bulging amount in the bulging step is 5
If the reduction amount is less than 0.5 mm or the reduction amount in the large reduction step is less than 0.5 times the bulging amount, as shown by the thin solid line in the graph of FIG. _{In 1} ), segregation occurs locally and steeply. On the other hand, the bulging amount in the bulging process is 20mm
If it is excessive or the amount of reduction in the large reduction step is more than 1.0 times the bulging amount, negative segregation occurs in the center in the thickness direction, so that a difference in strength due to segregation also occurs. Unless both the bulging amount: 5 to 20 mm and the reduction amount: 0.5 times or more and less than 1.0 times the bulging amount are satisfied, a narrow range in the thickness direction of the slab 10 (region d ₁ ).
In addition, segregation occurs locally and steeply.

Therefore, in the present embodiment, bulging is generated in an amount of 5 mm or more and 20 mm or less in the bulging step, and at least one pair of reduction roll pairs is used while the center solid phase ratio is 0.1 to 0.8. Of this bulging amount
Perform a large reduction with a reduction of 0.5 times or more and less than 1.0 times. Thus, in the central portion in the thickness direction of the slab 10, the region d ₂ having a concentration C ₁ or more of the concentration of one third of the concentration peak value C _max is 1.5% of the thickness for the slab thickness direction It is formed over a wide range over a length of at least 25%.

The unsolidified portion reduction ratio Lf of the slab 10 subjected to the large reduction in this way means the reduction amount / unsolidified thickness at the time of reduction, and the unsolidified portion reduction amount (mm) is denoted by D1. Assuming that the thickness (mm) of the unsolidified portion having a solid phase ratio of 0.8 or less at the start of rolling is D2, it is calculated by D1 / D2. When the unsolidified portion rolling reduction Lf is less than 0.2, rolling reduction is insufficient, and positive segregation remains. If the unsolidified portion reduction rate Lf exceeds 0.5, the unsolidified portion reduction rate Lf must be 1.0 or more in order to obtain complete negative segregation, and negative segregation and positive segregation may coexist. growing. Thus, in the present embodiment, large reduction is performed so that the unsolidified portion having the unsolidified portion rolling reduction Lf of 0.2 to 0.5 remains in the slab 10.

The slab 10 which has been subjected to the large pressure reduction in this way is
Is pulled out by a plurality of pinch roll pairs 11 to form a continuous cast slab 12.

[Continuous cast slab] The continuous cast slab 12 provided in this manner has, as shown by the thick solid line in the graph of FIG. Area d ₃
There is a high concentration segregation part having This high concentration segregation part, the segregation degree of the stationary part where the center segregation does not occur,
This is a part where the degree of segregation is high.

The high-concentration segregation portion having the region d ₃ includes:
A region d ₂ having a concentration C ₁ or more of ₁ / of the concentration peak value C _max is a continuous cast slab in the slab thickness direction.
It is formed over a length of 1.5% or more and 25% or less of the thickness of 12.

Here, as the thickness of the continuous cast slab 12, a value obtained by measuring the thickness of the continuous cast slab during rolling after bulging is used.

The high-concentration segregated portion having the region d ₃ is formed at the central portion in the thickness direction of the continuous cast slab 12 at the concentration peak value _Cmax .
It has a wide range of area d ₂ having a concentration C ₁ or more of the concentration of 1/3. For this reason, even if the peak value _{Cmax of the} degree of segregation exists in the central part of the high concentration segregation part having the region d ₃ , the intensity difference between the central part of the high concentration segregation part and a part other than this central part is small. , (C ₁ -C ₀ ). For this reason, in the width direction and the longitudinal direction of the continuous cast slab 12, center segregation in which the steepness of the peak value _Cmax of the segregation degree is reduced can be obtained. For this reason, in a product using the continuous cast slab 12 as a raw material, generation of cracks in the high concentration segregated portion is suppressed.

As described above, according to the present embodiment, the unsolidified portion 10a to be reduced, which has been exposed by bulging, can be effectively removed from the viewpoint of preventing central segregation without deteriorating segregation of the non-depressed portion. Can be reduced.

[0047]

EXAMPLES The present invention will be described more specifically with reference to examples. Using the continuous casting apparatus shown in FIG. 10, molten steel for thick plates (C: 0.16 to 0.18% by weight, Si: 0.30 to
A casting test of 0.40% by weight, Mn: 1.30 to 1.45% by weight, P: 0.020% or less, and S: 0.005% by weight or less) was conducted. The dimensions of the obtained continuous cast slab 12 are as follows:
In each case, the thickness was 235 mm and the width was 2300 mm.

From the obtained continuous cast slab 10, a slab cross-sectional sample having a length of 1 m in the casting direction was sampled, and the macrostructure of the slab cross-sectional sample was investigated. Was analyzed. The analysis of the carbon concentration was performed by collecting chips from a central portion of the obtained continuous cast slab 12 in the thickness direction with a drill blade having a diameter of 3 mm. Ratio C divided by C ₀
The evaluation was performed using / C ₀ .

Table 1 summarizes the test conditions and test results of this casting test. Also, the sample of the results in Table 1
FIG. 4 is a graph showing the test results of the inventive examples of No. 1 to No. 8, and FIG. 5 shows the test results of the comparative examples of No. 9 to No. 16.
Is shown in the graph. In addition, the central part segregation degree in Table 1,
The segregation zone segregation degree and the segregation zone width (%) are shown in FIG.
Of segregation C _max , C ₁ , (d ₃ / thickness of slab) × 10
Indicates 0.

[0050]

[Table 1]

As shown in Table 1 and the graph of FIG. 4, in the continuous cast slabs of Sample Nos. 1 to 8, "bulging-high pressure reduction" satisfying the conditions specified in the present invention was performed. Therefore, in each case, a high concentration segregated portion was generated in a wide range corresponding to 1.8 to 12.0% of the thickness of the slab 10.

On the other hand, as shown in Table 1 and the graph of FIG. 5, in both Sample No. 9 and Sample No. 10, since the bulging amount was less than the lower limit of the range of the present invention, high concentration segregation was observed. The length of the slab in the thickness direction could not be increased.

Sample Nos. 11 to 14 all had negative segregation at the center in the thickness direction because the unsolidified portion draft exceeded the upper limit of the range of the present invention, and the high concentration segregated portion The length of the slab in the thickness direction could not be increased, and segregation occurred locally and steeply in a narrow area (region d ₁ ) in the center in the thickness direction.

Further, Sample No. 15 and Sample No. 16
In both cases, since the bulging amount exceeds the upper limit of the range of the present invention, negative segregation occurs at the center in the thickness direction, and the length of the high-concentration segregation portion in the slab thickness direction cannot be increased. Segregation occurred locally and steeply in a narrow range (region d ₁ ) at the center in the vertical direction. Further, the continuous cast slab 12 was rolled to produce a thick plate 20 having a thickness of 30 mm.

FIG. 7 is a perspective view showing the obtained thick plate 20. From the center 21 and edge 22 of this thick plate 20, JIS
Tensile test pieces specified in Z2241 and bending test pieces specified in JIS Z2248 were cut out and subjected to a Z-direction tensile strength test and a Z-direction side bending test.

FIG. 8 is an explanatory view showing the outline of the Z-direction bending test. As shown in FIG.
In this test, both ends 24 and 25 of the bending test piece are bent around the center of the test piece, and the radius (bending radius) of the bending test piece 24 when a crack occurs is measured.

Drawdown RA (%) in Z-direction tensile strength test
And the bending radius (mmt) in the Z-direction bending test are shown in Table 1.

From Table 1, it can be seen that the thick plate 20 of the present invention of Sample No. 1 to Sample No. 8 has a smaller RA (%) and a larger bending radius than the thick plate obtained from the continuous cast slab of Comparative Example. (mm) showed good values.

On the other hand, in Comparative Examples of Sample Nos. 9 to 16, only 60.6% of the maximum aperture was obtained.
In addition, the minimum bending radius was only 1.5 mm.

[0060]

As described above in detail, according to the present invention, continuous segregation can be achieved by reducing the occurrence of center segregation in a continuous cast slab and reducing the strength of the product due to internal cracks at the solidification interface. A cast slab and a continuous casting method capable of obtaining the continuous cast slab could be provided. The significance of the present invention having such an effect is extremely remarkable.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an enlarged view of the vicinity of a reduction zone in a continuous casting method using bulging and large reduction.

FIG. 2 is a graph showing an example of a distribution of a solute concentration generated in an unsolidified portion upstream of a rolling position by a rolling roll pair.

FIG. 3 is a graph showing an example of a carbon segregation degree distribution in a thickness direction of a slab.

FIG. 4 is a graph showing test results of examples of the present invention in Examples.

FIG. 5 is a graph showing test results of Comparative Examples in Examples.

FIG. 6 is a graph showing the relationship between the degree of segregation and the distance in the thickness direction.

FIG. 7 is a perspective view showing the obtained thick plate.

FIG. 8 is an explanatory view showing an outline of a Z-direction side bending test.

FIG. 9 is an explanatory view schematically showing a continuous cast slab in which center segregation has occurred.

FIG. 10 is an explanatory view schematically showing a continuous casting method using bulging and large pressure reduction.

[Explanation of symbols]

4 guide roller pair 6 reduction roll pair 10 slab 10a unsolidified portion 12 continuously cast slab C _max polarized析濃degree peak C ₁ high concentration polarization析値d ₂ of polarized析濃degree C ₁ region d ₃ segregation

Claims (2)

[Claims] A high-concentration segregation part is provided inside the slab including a central part in a thickness direction of the slab, and the high-concentration segregation part forms a region having a concentration of 1/3 or more of the concentration peak value in the casting. A continuous cast slab having a length of 1.5 to 25% of the slab thickness in the slab thickness direction. 2. A slab having an unsolidified portion therein is provided at a predetermined position from a position corresponding to a liquidus crater end of the slab to a position corresponding to a solidus crater end in the slab thickness direction. Using a plurality of pairs of guide rolls arranged with the spacing gradually increased, a bulging of 5 to 20 mm in total is generated, and then, the slab having the bulging has a center solid phase ratio of 0.1%. To 0.8, using at least one pair of rolling rolls, leaving unsolidified portions having a non-solidified portion reduction rate of 0.2 to 0.5 defined by the following formula (1), and reducing the bulging amount to 0.5 A continuous casting method characterized in that a reduction of a reduction amount of at least twice and less than 1.0 times is performed. Lf = D1 / D2 (1) where Lf: reduction rate of unsolidified portion D1: reduction amount of unsolidified portion (mm) D2: solid phase ratio of 0.8 or less at the start of reduction Unsolidified part thickness (m
m)