JP3365338B2 - Continuous cast slab and continuous casting method - Google Patents

Description

Detailed Description of the Invention

[0001]

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

[0002]

2. Description of the Related Art When a continuously cast ingot made of steel is manufactured by a continuous casting method, an internal defect called center segregation often occurs in the continuously cast ingot produced. FIG. 9: is explanatory drawing which shows typically the continuously cast slab 1 in which the center segregation 2 generate | occur | produced. As shown in the figure, this center segregation 2 is caused by C, Mn, S in molten steel, and further C, Mn, S in the molten steel at the final solidified portion which is the center of the continuously cast slab 1 in the thickness direction (the direction of the double arrow in FIG. It occurs when component elements such as P are concentrated and positively segregated to a high concentration. The center segregation 2 of the continuously cast slab 1 is
Particularly when the final product is a thick steel plate, it causes a decrease in toughness, hydrogen-induced cracking, and the like in the high-concentration segregated portion of the component elements, and therefore it is an important problem that has been conventionally desired to be solved.

The center segregation is, specifically, that molten steel in which constituent elements are concentrated remains between dendrites at the final stage of solidification of molten steel and solidifies as it is in the central portion in the thickness direction of the slab, and This occurs because the molten steel moves macroscopically toward the solidification completion point of the final solidification portion due to the flow of the molten steel due to swelling of the slab called shrinkage during solidification or bulging. Therefore, in order to prevent the occurrence of center segregation, it is important to both prevent the movement of the concentrated molten steel between the dendrites at the final stage of solidification and to prevent the localized accumulation of the concentrated molten steel. Become.

From such a point of view, many continuous casting methods utilizing so-called "continuous casting method using bulging and large reduction" have been proposed in order to prevent the occurrence of center segregation.

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

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 nozzle group (not shown) provided below the continuous casting mold 3 and solidified on the outer surface. The shell 10a is formed into a cast piece 10. At this stage, the unsolidified portion is inside the slab 10.
There is 10b. In the bulging zone 5, the slab 10 is bulged by the central portion in the longitudinal direction being gradually expanded in the short side direction by the ten guide roll pairs 4 and the molten steel static pressure. The slab 10 having bulging is gradually reduced in the reduction zone 7 by the three pairs of reduction rolls 6. As a result, the solidification interface of the cast slab 10 is pressed and completely solidified at the solidus line crater end 5b. The slab 10 passes through the bulging zone 5 and the rolling-down zone 7 and is continuously drawn out in the direction of the white arrow by a plurality of pairs of pinch rolls 11 to form a continuously cast slab 12.

For example, in Japanese Unexamined Patent Publication No. 9-57410, in the continuous casting method using the bulging and the large reduction, the maximum thickness of the slab 10 is the slab 10 immediately after being drawn from the continuous casting mold 3. Thickness, that is, 20 to 100 mm thicker than the distance between the long sides of the continuous casting mold 3 in the bulging zone 5, and a rolling amount of 20 mm or more per rolling roll pair 6 corresponds to the bulging amount. Has been proposed.

Further, Japanese Unexamined Patent Publication No. 9-206903 discloses a cast piece.
The maximum thickness of 10 is 10 than the distance between the long sides of the continuous casting mold 3.
An invention has been proposed in which bulging is performed in the bulging zone 5 so that the thickness is increased to about 50%, and the rolling 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, in Japanese Patent Application Laid-Open No. 63-252655, by increasing the injection amount of cooling water sprayed on the surface of the slab 10, The surface temperature of the final solidified part of the slab 10 is 700 to 800 ° C.
The thickness of the solidified shell 10a is controlled to be thicker to suppress the amount of bulging generated between a plurality of pairs of guide rolls 4, and further, it can be reduced to 0 per minute by using a group of light reduction rolls.
An invention has been proposed in which a reduction force with 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]

However, Japanese Patent Application Laid-Open No. 9-57.
In the inventions proposed by No. 410 or No. 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 the roll diameter of the rolling roll pair 6 or the roll pitch at the rolling position. However, according to the examination results by the present inventors, such a large diameter rolling roll pair 6 is provided between the position corresponding to the liquidus line crater end 5a and the position corresponding to the solidus line crater end 5b. It is physically impossible to arrange the desired number, and it is difficult to actually carry out the methods according to these proposals.

Further, in the light reduction by the reduction roll group in the method proposed in Japanese Patent Laid-Open No. 63-252655, the reduction can be performed only in a dot shape in the longitudinal direction of the slab 10. Therefore, solidification shrinkage and bulging cannot be sufficiently prevented. Also, a slab with a large amount of bulging
When this light reduction is attempted on 10, the respective reductions locally act on the slab 10 as concentrated loads. Therefore, in the case of the slab 10 having a large bulging amount, internal cracking is likely to occur at the solidification interface, and it is not possible to secure a sufficient reduction amount.

[0012] As described above, the so-called "bulging and continuous casting method in which a large amount of bulging is reduced" requires an excessive amount of reduction force, so internal cracks are generated at the solidification interface and The strength is significantly reduced. Therefore, it was actually difficult to prevent the occurrence of center segregation in the continuously cast slab.

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

[0014]

Here SUMMARY OF THE INVENTION The present invention has a high concentration segregation area therein comprising a central portion of the slab thickness direction, with the high-concentration segregation comprises concentration peak value thereof Segregation, which is a region of approximately constant concentration above 1/3 of the concentration peak value
Has a strip and is 1.5% of the thickness of the slab in the thickness direction of the slab
A continuous casting slab, characterized in that it is formed over the length of 25% or less.

In the above-mentioned present invention, the high-concentration segregation portion is higher than the segregation degree of the steady portion where center segregation does not occur.
It means a portion with a high degree of segregation.

The term "thickness of slab" means the thickness of a continuously cast slab, and specifically, the thickness of the slab at the center segregation generation portion during rolling. The thickness of the cast slab is a value obtained by measuring the thickness of the continuously cast slab at the time of reduction performed subsequent to bulging.

From another aspect, the present invention provides a slab having an unsolidified portion inside, from the position corresponding to the liquidus crater end of the slab to the position corresponding to the solidus crater end. Using a plurality of pairs of guide rolls arranged at a predetermined position with gradually increasing intervals in the thickness direction of the slab, a total bulging of 5 mm or more and 20 mm or less is generated, and then,
The solid fraction of the central part is 0.1 in the slab that generated this bulging.
Using at least one set of reduction roll pairs between until 0.8 inclusive, the unsolidified portion reduction ratio defined by the following equation (1)
By setting it to 0.2 or more and 0.5 or less , the unsolidified portion remains,
It is a continuous casting method characterized by performing a reduction of 0.5 times or more and less than 1.0 times the amount of bulging.

[0018]

[Formula 2] Lf = D1 / D2 (1) However, in the above formula (1), the symbol Lf indicates the reduction rate of the unsolidified portion, and the symbol D1 indicates the reduction amount of the unsolidified portion (mm). Further, reference numeral D2 indicates the thickness (mm) of the non-solidified portion having a solid fraction of 0.8 or less at the start of rolling.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a continuously 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 reduction step, similarly to the known continuous casting method using bulging and large reduction. Therefore, these steps will be sequentially described with reference to FIG. 10 described above.

[Bulging Step] The configurations of the bulging zone 5, the rolling-down zone 7, the pinch roll pair 11, and the like in FIG. 10 have already been described, and thus the 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 nozzle group (not shown) provided below it. It is cooled by the sprayed water.
As a result, the solidified shell 10a is formed on the molten steel 9 to form the cast piece 10. At this point, the unsolidified portion 10b exists inside the cast slab 10.

In this embodiment, the slab 10 is bulged in the bulging zone 5. The bulging zone 5 is composed of a plurality of (10 in the present embodiment) guide roll pairs 4. 10 sets of guide roll pairs 4 are slabs
They are arranged at predetermined positions between the position corresponding to the liquidus line crater end 5a and the position corresponding to the solidus line crater end 5b. 10 pairs of guide rolls 4
The roll intervals in the thickness direction of the slab are gradually enlarged and arranged in the direction of drawing the slab 10. As a result, the slab 10 having the unsolidified portion 2b therein is gradually bulged by the action of the molten steel static pressure.

The bulging amount in this embodiment is the slab.
5mm in total in 10 thickness directions (left and right direction in Fig. 10)
20 mm or less. If the amount of bulging is less than 5 mm, the amount of reduction in the large reduction process described below will be insufficient, and the concentrated molten steel will not be sufficiently squeezed out to improve center segregation.
On the other hand, if the bulging amount exceeds 20 mm, the amount of reduction in the large reduction process described below becomes excessive, and the equipment cost required for the reduction device increases. Therefore, in this embodiment, the bulging amount of the cast 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 cast slab 10.

Thus, in this embodiment, the cast slab 10 is used.
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 the present embodiment, the slab 10 is bulged by the bulging zone 5 and then the bulging portion is largely reduced by the reduction zone 7. The reduction zone 7 is constituted by three reduction roll pairs 6 in this embodiment.

In the large reduction by the reduction zone 7, the solid fraction of the central portion of the cast slab 10, that is, the solid fraction required for the flow is 0.1 or more.
It is done by 8 or less. The solid fraction of the central part of the slab 10 is 0.
A value less than 1 is not possible because the solidified shell 10a grows, whereas the central solid fraction: 0.8 of the slab 10 is a solidification-wise flow limit. Therefore, in this embodiment, the slab
While the solid fraction of the central portion of 10 is from 0.1 to 0.8, large reduction by the three pairs of reduction rolls 6 is performed.

Further, the reduction amount of the large reduction 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,
The center segregation cannot be improved, and on the contrary, the proportion of the positive segregation remaining increases. On the other hand, the reduction amount is 1.
When it is 0 times or more, at the end of large reduction, the unsolidified portion does not exist inside, and the high-concentration segregated portion including the central portion in the thickness direction of the slab is formed with a sharp distribution, and
It becomes an overpressure, and it becomes necessary to generate a large rolling force in terms of equipment. Therefore, in the present invention, the amount of large reduction 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 enlarged view showing the vicinity of the reduction zone 7 in FIG. For convenience of explanation, FIG. 1 shows that the drawing direction is horizontal and that one of the reduction rolls 6 moves down in the thickness direction of the slab 10 for reduction. There is. But the figure
As shown in FIG. 10, both the reduction rolls of each reduction roll pair 6 may be moved to perform the reduction of the slab 10.

As shown in the figure, large reduction is performed by each rolling roll pair 6 such that the unsolidified portion remains inside even after rolling by each rolling roll pair 6. Due to this large pressure, the uncoagulated portion 10a at the final stage of coagulation is ejected upstream at the ejection speed V. When the bulging amount is constant, the discharge speed V increases as the reduction amount increases. Here, since the solute in the unsolidified portion 10a at the final stage of solidification is concentrated, the unsolidified portion 10a in which the solute is concentrated is discharged at the discharge speed V to the upstream side by the large reduction by the pressing roll pair 6. Therefore, a solute concentration gradient is generated in the unsolidified portion 10a on the upstream side of the rolling position by the rolling 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 the graph in Figure 2,
Immediately upstream of the rolling position P _r , the solute concentration rapidly rises,
The solute concentration gradually decreases from the rolling position P _r toward the upstream side. Therefore, a high concentration segregation portion having a high solute concentration is generated in the solidified portion on the upstream side of the rolling position P _r . Therefore, segregation of solute concentration occurs in the cast piece 10 not only in the drawing direction of the cast piece 10 but also in the thickness direction.

FIG. 3 is a graph showing an example of the distribution of the carbon segregation degree in the thickness direction of the cast slab 10. In the graph of FIG. 3, the concentration C ₀ indicates the segregation degree of the stationary parts d ₄ and d ₅ in which the center segregation does not occur, that is, the bulk concentration.

When the slab 10 is solidified without being reduced by the pair of reduction rolls 6 used in this embodiment, as shown by a thin solid line in the graph of FIG. 3, a narrow range in the thickness direction of the slab 10 ( In the region d ₁ ), a high concentration segregation portion where the degree of segregation changes rapidly in the range from C ₀ to the concentration peak value C _max is locally and sharply generated. Therefore, for example, when a thick steel plate is manufactured using this continuously cast slab as a raw material, stress concentrates on the high-concentration segregation portion d ₁ , and a crack occurs in the central portion of the thick steel plate that is the final product.

On the other hand, as in the present embodiment, bulging is generated in an amount of 5 mm or more and 20 mm or less in the bulging process, and at least one pair of rolling roll pairs is used while the central solid fraction is from 0.1 to 0.8. Of the bulging amount
If large reduction is performed with a reduction of 0.5 times or more and less than 1.0 times,
As indicated by a thick solid line in the graph of Fig. 3, the segregation of the central portion in the thickness direction of the cast slab 10 is left in an unsolidified state, and the high-concentration segregation portion is greatly expanded from the region d ₁ to the region d ₃ . .

The area d₃High concentration segregation part withTo
Is the concentration peak value C_max1/3 of the concentration C₁More than
Area d₂ (Segregation zone)ButIt is formed. Also, this Takano
Degree segregation part, About the thickness of the slab 10 in the thickness direction of the slab
It is formed over a length of from 25% to 25%. In addition,
As described below, the thickness of the slab 10 continues to bulge.
It is the value at the time of reduction performed.

As described above, in the present embodiment, the high concentration segregation portion of the region d ₃ has a wide range of segregation portions having the region d ₂ and having a higher degree of segregation than the bulk concentration C ₀ . Therefore, even if there is a peak of the degree of segregation in the central portion of the high-concentration segregated portion of the region d ₃ , the intensity difference between the central portion and the portion other than the central portion is (C ₁ -C ₀ ) is suppressed.

Therefore, when manufacturing a thick steel plate using a continuously cast slab as a raw material, stress concentration on the high-concentration segregated portion of the region d ₃ is relieved. For this reason, according to the present embodiment, the occurrence of cracking that occurs when the cracking limit strain cannot be endured is suppressed. In this way, the occurrence of cracks in the high-concentration segregated portion of the region d ₃ is suppressed.

The bulging amount in the bulging process is 5
If it is less than 0.5 mm or the amount of reduction in the large reduction step is less than 0.5 times the amount of bulging, as shown by the thin solid line in the graph of FIG. _{In 1} ), segregation occurs locally and sharply. On the other hand, the bulging amount in the bulging process is 20 mm.
If it is over, or if the amount of reduction in the large reduction step is more than 1.0 times the amount of bulging, negative segregation occurs in the central portion in the thickness direction, which also causes a strength difference due to segregation. As described above, unless the bulging amount: 5 to 20 mm and the rolling reduction: 0.5 to less than 1.0 times the bulging amount are both satisfied, the slab 10 has a narrow range in the thickness direction (region d ₁ ).
In addition, segregation occurs locally and sharply.

Therefore, in the present embodiment, a bulging amount of 5 mm or more and 20 mm or less is generated in the bulging process, and at least one rolling roll pair is used between the central solid fractions of 0.1 to 0.8. Of this bulging amount
Perform a large reduction with a reduction amount of 0.5 times or more and less than 1.0 times. As a result, in the central portion of the slab 10 in the thickness direction, a region d ₂ (segregation of concentration C ₁ of 1/3 or more of the concentration peak value C _max is obtained.
A high-concentration segregation portion (region d ₃ ) having a band is formed over a wide range over a length of 1.5% or more and 25% or less of the thickness in the thickness direction of the cast piece .

The unsolidified portion reduction ratio Lf of the cast slab 10 which has undergone large reduction in this way means the amount of reduction / unsolidified thickness at the time of reduction, and the unsolidified portion reduction amount (mm) is designated as D1. When the thickness (mm) of the unsolidified portion having a solid fraction of 0.8 or less at the start of rolling is D2, it is calculated by D1 / D2. When the rolling reduction Lf of the non-solidified portion is less than 0.2, the rolling is insufficient and the positive segregation remains. If the unsolidified portion rolling reduction Lf exceeds 0.5, the unsolidified portion rolling reduction Lf needs to be 1.0 or more in order to obtain a complete negative segregation, and negative segregation and positive segregation may coexist. growing. Therefore, in this embodiment, the slab 10, the non-solidified portions reduction ratio Lf With 0.2-0.5
A large reduction is performed while leaving the unsolidified portion .

A slab 10 that has been subjected to large reduction in this way
Is drawn out by a plurality of pairs of pinch rolls 11 to form a continuously cast slab 12.

[Continuous casting slab] The continuous casting slab 12 provided in this manner has a continuous casting slab 12 which includes a central portion in the thickness direction of the slab, as indicated by a thick solid line in the graph of FIG. There is a high concentration segregation part having a region d ₃ . The high-concentration segregation portion is a portion having a higher degree of segregation than the segregation degree of the stationary portion where center segregation does not occur. Takano
The degree of segregation is the thickness of the continuously cast slab 12 in the slab thickness direction.
It is formed over a length of 1.5% or more and 25% or less.

The high-concentration segregation portion having this region d ₃ includes
Region d ₂ having a concentration C ₁ or more of the concentration of one third of the concentration peak value C _max is (segregation zone), it is formed.

Here, as the thickness of the continuously cast slab 12, a value obtained by measuring the thickness of the continuously cast slab at the time of rolling down subsequent to bulging is used.

The high-concentration segregated portion having the region d ₃ has a concentration peak value C _max at the central portion in the thickness direction of the continuously cast slab 12.
An area d ₂ having a density of 1/3 of the density C ₁ or more is provided in a wide range. Therefore, even if the peak value C _{max of the} segregation degree exists in the central portion of the high-concentration segregation portion having the region d ₃ , the strength difference between the central portion of the high-concentration segregation portion and the portion other than this central portion is large. , (C ₁ −C ₀ ). Therefore, the center segregation in which the steepness of the peak value C _max of the segregation degree is reduced can be obtained in the width direction and the longitudinal direction of the continuously cast slab 12. Therefore, in the product using the continuously cast slab 12 as a raw material, the occurrence of cracks in the high-concentration segregation portion is suppressed.

As described above, according to the present embodiment, the unsolidified portion 10a that has been exposed by bulging and is to be reduced is effectively treated from the viewpoint of preventing center segregation without causing deterioration of segregation in the non-pressurized portion. Can be rolled down.

[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, S: 0.005% by weight or less) was performed. The dimensions of the obtained continuously cast slab 12 are:
In each case, the thickness was 235 mm and the width was 2300 mm.

From the obtained continuously cast slab 10, a slab transverse cross-section sample having a length of 1 m in the casting direction was sampled, and the macrostructure of the slab transverse cross-section sample was investigated. Was analyzed. The carbon concentration is analyzed by collecting chips from the center of the obtained continuously cast slab 12 in the thickness direction with a drill blade having a diameter of 3 mm. Ratio C divided by C ₀
/ C ₀ was used for evaluation.

Table 1 shows the test conditions and test results of this casting test. Samples from the results in Table 1
The test results of the examples of the present invention of No. 1 to sample No. 8 are shown in a graph in FIG. 4, and the test results of the comparative examples of sample No. 9 to sample No. 16 are shown in FIG.
Is shown in the graph. In addition, the center segregation degree in Table 1,
Segregation zone Segregation degree and segregation zone width (%) are shown in FIG.
Degree of segregation C _max , C ₁ , (d ₃ / thickness of cast piece) × 10
Indicates 0.

[0050]

[Table 1]

As shown in Table 1 and the graph of FIG. 4, in the continuously cast slabs of sample No. 1 to sample No. 8, "bulging-large reduction" satisfying the conditions specified in the present invention is performed. Therefore, in all cases, a high-concentration segregation portion was generated in a wide range corresponding to 1.8 to 12.0% of the thickness of the cast 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, the bulging amount is below the lower limit of the range of the present invention, so that high concentration segregation occurs. The length in the thickness direction of the cast piece could not be increased.

In each of Sample No. 11 to Sample No. 14, the rolling reduction ratio of the unsolidified portion exceeds the upper limit of the range of the present invention, so that negative segregation occurs in the central portion in the thickness direction and the high concentration segregated portion is formed. It was impossible to increase the length of the slab in the thickness direction, and segregation occurred locally and sharply in a narrow range (region d ₁ ) in the central portion in the thickness direction.

Further, sample No. 15 and sample No. 16 are
Both, since the bulging amount exceeds the upper limit of the range of the present invention, negative segregation occurs in the central portion in the thickness direction, the length in the slab thickness direction of the high concentration segregation portion cannot be increased, and the thickness Segregation locally and sharply occurred in a narrow range (region d ₁ ) in the central portion in the depth direction. Further, these continuously cast slabs 12 were rolled to manufacture 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 portion 21 and edge portion 22 of this thick plate 20, JIS
A tensile test piece specified by Z2241 and a bending test piece specified by 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 side bending test. As shown in the figure, the Z-direction side bending test is
This is a test in which 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.

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

From Table 1, it can be seen that the slabs 20 of Samples No. 1 to No. 8 of the present invention are smaller than the slabs obtained from the continuously cast slabs of Comparative Example in the reduction RA (%) and bending radius. (mm) both showed good values.

On the other hand, in each of the comparative examples of sample No. 9 to sample No. 16, only 60.6% of the maximum diaphragm was obtained,
In addition, the minimum bending radius was 1.5 mm.

[0060]

As described in detail above, according to the present invention, the occurrence of center segregation in a continuously cast slab is reduced, and continuous casting is performed without causing a reduction in product strength due to internal cracks at the solidification interface. A cast piece and a continuous casting method capable of obtaining this continuous cast piece can be provided. The significance of the present invention having such an effect is extremely remarkable.

[Brief description of drawings]

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

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

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

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

FIG. 5 is a graph showing test results of a comparative example in the 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 diagram showing an outline of a Z-direction side bending test.

FIG. 9 is an explanatory view schematically showing a continuously cast slab in which center segregation occurs.

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

[Explanation of symbols]

4 Guide roll pair 6 Reduction roll pair 10 Cast slab 10a Unsolidified portion 12 Continuous cast slab C _max Segregation concentration peak value C ₁ High concentration segregation value d ₂ Segregation concentration C ₁ area d ₃ Segregation portion

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Claims (2)

(57) [Claims] 1. A high-concentration segregation portion is provided inside including a central portion in the thickness direction of the cast slab, and the high-concentration segregation portion includes a concentration peak value.
In addition, the concentration is approximately constant at 1/3 or more of the concentration peak value.
Continuous casting slab of having a segregation zone is an area, and wherein the <br/> said slab to the thickness direction are formed over the 1.5 to 25% of the length of the slab thickness. 2. A slab having a non-solidified portion inside, at a predetermined position from the position corresponding to the liquidus crater end of the slab to the position corresponding to the solidus crater end in the slab thickness direction. Using a plurality of pairs of guide rolls arranged such that the intervals are gradually enlarged, a bulging of 5 to 20 mm in total is generated, and then, the central solid fraction of the slab in which the bulging occurs is 0.1. using at least one set of reduction roll pairs until 0.8, and the unsolidified portion reduction ratio defined by the following equation (1) is left a non-solidified portion by a 0.2-0.5, the amount of the bulging The continuous casting method is characterized by performing a reduction of 0.5 times or more and less than 1.0 times. [Formula 1] Lf = D1 / D2 (1) where Lf is the reduction rate of the non-solidified part D1: The reduction amount of the non-solidified part (mm) D2 is the solid phase rate of 0.8 or less at the start of the reduction. Thickness of unsolidified part (m
m)