JP7273307B2 - Steel continuous casting method - Google Patents

Description

The present invention relates to a continuous steel casting method for drastically reducing center segregation and center porosity of slabs.

When casting a slab, bloom, or the like by a continuous casting method, so-called center segregation, in which components such as phosphorus and manganese segregate in the center of the slab, may occur. Also, voids called center porosity are generated at the center of the slab.

At the final stage of solidification during continuous casting, the amount of steel occupying a predetermined volume in the slab becomes insufficient due to solidification shrinkage when the steel solidifies. In the slab region where unsolidified molten steel can flow, the unsolidified molten steel flows toward the solidification completion point of the final solidification zone, and the impurity-enriched molten steel at the solid-liquid interface accumulates in the final solidification zone, which causes center segregation. cause. In addition, at a position where the unsolidified molten steel cannot flow (the solid fraction at the center of the slab is 0.8 or more), voids are generated at the center of the slab, causing center porosity.

In order to reduce the center segregation, in the region where the solid-liquid coexistence region is at the center of the thickness and the unsolidified molten steel can flow, the final solidification is achieved by reducing the solidified shell by an amount corresponding to the amount of solidification shrinkage of the molten steel. It is effective to suppress the flow of molten steel near the part. Also, in order to reduce the center porosity, it is effective to reduce the center porosity by pressing down the cast slab near the solidification completion position where unsolidified molten steel cannot flow or after complete solidification. Based on this way of thinking, a light reduction technique is used in which support rolls reduce the cast slab before and after the completion of solidification in the final stage of continuous casting.

In continuous casting, center segregation can be reduced by applying an appropriate reduction to compensate for solidification shrinkage as described above. In actual machines, a light reduction technique is widely applied in which an appropriate reduction of about 0.8 to 1.2 mm/min is applied in a low solid fraction region with a central solid fraction of 0.8 or less.

Patent Document 1 discloses a slab continuous casting method characterized in that the reduction ratio is set to 0.36 to 0.72 mm/min, and the reduction is performed to a portion where the central solid fraction is equal to or higher than the flow limit solid fraction. being introduced. The reduction gradient is not changed even at the portion where the flow limit solid fraction is higher (the central solid fraction is 0.8 or higher).

Patent Document 2 describes a continuous casting method in which a steel slab continuously cast piece is drawn while being rolled down between at least one pair of opposing rolls, in which the solid phase ratio at the center of the cast piece is 0.1 to 0.4. to an arbitrary position within the range of 0.8 to 0.9, the slab is reduced so as to compensate for the amount of total solidification shrinkage, and a high solid fraction from the arbitrary position until solidification is completed The region is a range defined by the formula according to the C concentration of the steel, where the reduction gradient (%/m), which indicates the ratio (%) of the amount of reduction to the thickness of the slab per length (unit: m) in the drawing direction of the slab. A continuous casting method has been proposed to satisfy the reduction.

The solidification structure of the cross-section of a continuously cast slab may be formed entirely of columnar crystals, or may be columnar crystals on the outer peripheral side of the slab, with equiaxed grains generated at the center of the thickness of the slab. The ratio of the equiaxed grain zones in the thickness direction to the entire thickness of the slab is called the equiaxed crystal ratio.

It is known that increasing the equiaxed grain ratio of a slab is effective for reducing the center segregation and center porosity of the slab (for example, Non-Patent Document 1). As means for increasing the equiaxed grain ratio, a method of lowering the degree of superheating of the molten steel temperature in the tundish (low temperature casting) and implementation of electromagnetic stirring during casting are effective. The parts during casting where electromagnetic stirring is carried out include the inside of the mold, the secondary cooling zone, and the final stage of solidification. there is

A commonly used continuous casting apparatus initially solidifies the slab surface in the mold, then guides the slab curving with a radius of about 10 m in the subsequent secondary cooling zone, and finally leads it horizontally. To go. In the unsolidified molten steel in the curved portion, the equiaxed grains are deposited on the solidified shell on the lower surface side, so many equiaxed grains are formed from the thickness center to the lower surface side in the cross section of the cast slab after final solidification. In particular, in continuously cast slabs, the equiaxed grain zone is often formed mainly on the lower surface side of the center of the thickness. In the present invention, the ratio (%) obtained by dividing the thickness of the equiaxed zone formed on the upper surface side from the center of the thickness by 1/2 of the slab thickness is referred to as the "upper surface equiaxed crystal ratio".

In Patent Document 3, in a continuous casting method for molten metal in which a slab is pulled out while being reduced by at least one pair of rolls at the final stage of solidification, when the upper surface equiaxed grain ratio is less than 5%, the temperature at the center of the slab becomes solid. At least one pair of rolls is installed at any position in the solidification period range from the position corresponding to the phase ratio of 0.25 to the position corresponding to the flow limit solid phase ratio, preferably at the upstream side of the solidification period range, and full reduction is performed. The reduction amount is 4 to 20 mm, and the reduction amount per unit length of slab with a central solid fraction of 0.05 to 0.25 is 0.2 to 3.0 mm / m. A continuous casting method is disclosed, characterized by:

In Patent Document 4, the upper surface equiaxed grain ratio is controlled to be 5% or more, and the central solid phase ratio of the slab is controlled at an increased casting speed from 0.15 to 0.7 (flow limit solid phase ratio). is reduced so that the total reduction amount is 4 to 20 mm, and continuous casting is performed with a reduction gradient of 0.2 to 3.0 mm at a center solid fraction of 0.02 to 0.15. .

In Patent Document 5, when a B-containing austenitic stainless steel slab is produced by continuous casting, the equiaxed grain ratio of the slab is set to 10 to 50%, and the final solidification position during casting and upstream therefrom are disclosed. A method for producing a stainless steel slab is disclosed in which a taper amount of 0.1% or more relative to the thickness of the slab is imparted over a length of 1 m or more.

Patent Document 6 describes a continuous casting method in which a continuously cast slab in which many equiaxed crystal zones are formed in the axial center of the slab is drawn while being reduced between opposing rolls, in which a solid phase at the center of the slab is In the region from the position where the modulus is 0.2 to the position where the ratio is 0.8 to 0.9, the slab is reduced so as to compensate for the total solidification shrinkage amount in the region, and solidification is completed thereafter. In the area of up to, the reduction gradient (%/m), which indicates the ratio (%) of the amount of reduction to the thickness of the slab per length (unit: m) in the drawing direction of the slab, is 0.08%/m or more. A continuous casting method is disclosed which is characterized by continuous reduction at a rate such as to be 1.50%/m or less.

JP-A-06-297125 JP-A-11-77269 JP-A-4-279265 JP-A-4-309446 JP-A-11-138238 JP-A-9-285856

Iron and Steel Handbook, 5th Edition, Volume 1, Ironmaking and Steelmaking, p.430

In continuous casting, as described above, the soft reduction technique is widely applied because center segregation and center porosity can be reduced by applying an appropriate reduction that compensates for solidification shrinkage. Reduction of center segregation by forming equiaxed crystal zones in the slab is also widely used. However, drastic reduction of center segregation and center porosity has not yet been achieved.
SUMMARY OF THE INVENTION An object of the present invention is to provide a continuous casting method for steel that can drastically reduce the center segregation and center porosity of a slab in continuous casting.

That is, the gist of the present invention is as follows.
[1] The upper surface equiaxed grain ratio of the slab is 5% or more,
In the region from the central solid fraction of 0.8 to the completion of solidification (hereinafter referred to as "high solid fraction region"), one or more pairs of reduction rolls are arranged to reduce the slab,
When the upper surface equiaxed crystal ratio is 5% or more and less than 20%, the reduction gradient in the high solid fraction region is set to 4.0 mm/min or more and 10 mm/min or less , and when the top surface equiaxed crystal ratio is 20% or more. A continuous casting method for steel, characterized in that the reduction gradient in a high solid fraction region is set to 3.0 mm/min or more and 10 mm/min or less .
[2] The continuous casting method for steel according to [1], wherein two or more pairs of the reduction rolls are continuously arranged to reduce the cast slab.
[3] In a region where the central solid fraction is from 0.3 to 0.75 (hereinafter referred to as a "low solid fraction region"), a reduction gradient of 0.8 to 1.2 mm / min is performed, and the above-mentioned The continuous casting method for steel according to [1] or [2], wherein the reduction is performed with a reduction gradient of 0.8 mm/min or more in the region between the low solid phase region and the high solid phase region.

By using the continuous steel casting method of the present invention, the center segregation and center porosity of the slab can be drastically reduced.

FIG. 4 is a diagram showing the relationship between the upper surface equiaxed grain ratio, the reduction gradient in the high solid fraction region, and the maximum Mn segregation degree of the slab. FIG. 4 is a diagram showing the relationship between the top equiaxed grain ratio and the maximum degree of Mn segregation of the cast slab, and (A) and (B) are for high solid fraction area reduction gradients of 4.0 and 5.6 mm/min, respectively. be. FIG. 4 is a diagram showing the arrangement of pairs of reduction rolls in a high solid fraction region, where (A) has one pair, (B) two pairs, and (C) three pairs of reduction rolls. FIG. 4 is a diagram showing a situation in which, in addition to reduction using a pair of reduction rolls in a high solid fraction region, light reduction is performed in a low solid fraction region. FIG. 4 is a diagram showing the relationship between the logarithm of the reduction roll pair, the top equiaxed grain ratio, and the maximum Mn segregation degree, and (A) and (B) show the reduction gradients in the high solid fraction region of 4.0 and 5.6 mm / min, respectively. is the case.

A change in the central solid fraction of the slab during continuous casting will be described. From the point where the liquidus lines on the upper and lower surfaces of the slab meet at the center of the slab thickness (solidification start position), the central solid fraction becomes greater than 0, and the central solid fraction increases toward the downstream side. . The central solid fraction is 0 on the upstream side of the solidification start position. Solidification is completed at the point where the solidus lines on the upper surface side and the lower surface side of the slab touch each other at the center of the thickness of the slab, and the solid phase ratio at the center becomes 1.0. This point is also called "solidification completion position". On the downstream side of the solidification completion position, the central solid fraction remains 1.0. Hereinafter, for convenience, the solidification completion position may be referred to as "the position where the central solid fraction is 1.0". In addition, the central solid fraction is sometimes indicated as fs.

Regarding the central solid phase rate at each position in the casting direction during casting, the temperature _TC at the center in the thickness direction of the slab during continuous casting was obtained by one-dimensional heat transfer solidification calculation, and the liquidus temperature _TL , It can be calculated by the following formula (1) using the solidus temperature _TS . Enthalpy method, equivalent specific heat method, etc. can be used for heat transfer and solidification calculations. When T _C >T _L , the central solid phase ratio is 0, and when T _S >T _C , the central solid phase ratio is 1.0.
Central solid phase ratio = (T _L - T _C ) / (T _L - T _S ) (1)

As described above, in continuous casting, central segregation is reduced by applying an appropriate reduction to compensate for solidification shrinkage. In an actual machine, light reduction is performed in a low solid phase ratio region with a central solid phase ratio of 0.8 or less. 0.8 to 1.2 mm/min.

In the present invention, the region from the center solid phase ratio of 0.8 or more to the solidification completion position (center solid phase ratio=1.0) is called a "high solid phase ratio region". In addition, even in the high solid fraction region, the center segregation and center porosity of the slab can be further improved by performing appropriate reduction and forming equiaxed grains from the center of the thickness of the slab to the upper surface side. I came up with the idea. As described above, in the present invention, the ratio (%) obtained by dividing the thickness of the equiaxed zone formed on the upper surface side from the center of the thickness by 1/2 of the slab thickness is referred to as the "upper surface equiaxed crystal ratio". .

Therefore, an experiment was conducted using an actual continuous casting apparatus. In continuous casting, electromagnetic stirring is performed in the mold, and the molten steel temperature in the tundish is adjusted so that the superheating degree ΔT of the molten steel in the tundish (the difference between the molten steel temperature in the tundish and the liquidus temperature (°C)) is 20°C. The upper surface equiaxed grain ratio of the cast slab was adjusted to 0%, 5%, and 20%, respectively, by setting the temperature to any one of from below to 30° C. or higher. As the degree of superheat ΔT of molten steel decreases, the equiaxed grain ratio on the upper surface side increases. Furthermore, the slab is reduced in the high solid fraction region in the section from the solid fraction at the center of the slab to the solidification completion position, and the reduction gradient (amount of reduction per hour (mm/min)) is adjusted during the reduction. Various changes were made to evaluate the effect on the maximum Mn segregation degree at the center of the slab thickness.

The evaluation of the slab was performed on a sample of a cross section (L section) perpendicular to the width direction of the slab, which was the central portion in the width direction of the constant reduction portion.
Regarding the upper surface equiaxed crystal ratio, an etch print of the L cross section is taken, the boundary between the columnar crystal structure and the equiaxed crystal structure is determined by visual observation, and the equiaxed crystal ratio on the upper surface side from the center of the slab thickness (upper surface etc. Axial crystal ratio) was calculated.
For Mn segregation evaluation, Mn concentration mapping analysis was performed by EPMA with a beam diameter of 50 μm on the L cross section. A line with a width of 2 mm including the worst segregation part was set in the mapping data, and the maximum Mn segregation degree C/C ₀ was obtained by dividing the concentration peak value C by the average concentration C ₀ within the measurement field.

The evaluation results are shown in FIG. Comparing the same upper surface equiaxed crystal ratio data, reduction is performed in a high solid fraction region in the section from the slab center solid fraction of 0.8 to the solidification completion position. It can be seen that the maximum degree of Mn segregation at the center of the piece thickness decreases. Moreover, it can be seen that if the rolling gradient is the same, the higher the top surface equiaxed grain ratio, the lower the maximum Mn segregation degree at the center of the slab thickness. Then, the upper surface equiaxed crystal ratio is 5% or more and the reduction gradient in the high solid fraction region is 4.0 mm / min, or the upper surface equiaxed crystal ratio is 20% and the reduction in the high solid fraction region It was found that if the gradient is 3.0 mm/min, the maximum degree of Mn segregation is 1.17 or less. The maximum degree of Mn segregation decreases as the upper surface equiaxed grain ratio increases and as the reduction gradient in the high solid fraction region increases.

Next, confirmation was carried out by experiments using an actual continuous casting apparatus. In the actual continuous casting apparatus, casting was performed with an upper surface equiaxed grain ratio of 0 to 20%. A high top equiaxed grain ratio was achieved by implementing electromagnetic stirring in the mold and reducing the degree of superheat ΔT of the molten steel in the tundish. In the high solid fraction region, three pairs of reduction rolls were arranged continuously, and continuous casting was performed with two levels of reduction gradients of 4.0 mm/min and 5.6 mm/min. The quality of the obtained slab is shown in FIG. 2, with the horizontal axis representing the top equiaxed grain ratio and the vertical axis representing the maximum Mn segregation degree. It can be seen that the maximum Mn segregation degree is improved as the upper surface equiaxed grain ratio is higher and as the rolling gradient is larger.

The reason why the maximum degree of Mn segregation decreases as the upper surface equiaxed crystal ratio increases and as the reduction gradient in the high solid fraction region increases is considered as follows.
The high solid phase ratio region from the solid phase ratio of 0.8 or more to the final solidification position is a solid-liquid coexistence region exceeding the flow limit solid phase ratio, and macro molten steel flow does not occur. However, due to the generation of local negative pressure caused by bridging or the like, the molten steel locally flows and the segregation becomes concentrated. It is also known that the steep reduction in the high solid fraction region can suppress the thickening of segregation.
Local flow of molten steel in the final solidification part of the slab is less likely to occur in the equiaxed solidified structure than in the columnar solidified structure. This is because the liquid phase is evenly distributed among the crystal grains in the equiaxed crystal structure, compared to the columnar crystal structure in which the liquid phase is concentrated in the dendrite tip region. is.
Based on the above, even when the same rolling reduction is applied, the segregation is more reduced in the slab having equiaxed grains than in the slab having columnar grains as the main solidification structure at the center of the thickness of the slab. This means that the reduction gradient required to reach the target degree of segregation can be small with equiaxed grains.

Based on the above results, the steel continuous casting method of the present invention is defined as follows. Description will be made with reference to FIG.
That is, the upper surface equiaxed grain ratio of the cast slab is set to 5% or more, and one or more reduction roll pairs 1 are arranged in the region from the central solid phase ratio of 0.8 to the completion of solidification (high solid phase ratio region 61). When the upper surface equiaxed crystal ratio is 5% or more and less than 20%, the reduction gradient in the high solid fraction region 61 is set to 4.0 mm/min or more, and the upper surface equiaxed crystal ratio is 20%. In the above case, the reduction gradient in the high solid fraction region 61 is set to 3.0 mm/min or more.
When the upper surface equiaxed grain ratio is about 10%, almost all of the solidified structure of the final solidified portion of the slab becomes equiaxed grains. Furthermore, when the upper surface equiaxed grain ratio is about 20%, almost all of the solidified structure of the final solidified portion becomes equiaxed grains even if solidification non-uniformity is considered.

In determining the reduction position during continuous casting, the position where the central solid fraction is 0.8 and the solidification completion position are combined with the temperature measurement of the slab surface during continuous casting and the heat transfer solidification calculation of the slab. can be determined by

The number of pairs of rolling rolls 1 that perform rolling in the high solid fraction region 61 from the central solid fraction of 0.8 to the solidification completion position is at least one (see FIG. 3A). It is preferable to set the number of the screw roll pairs 1 in the region to two (see FIG. 3(B)). More preferably, the number of roll pairs 1 is 3 or more (see FIG. 3(C)).

The reduction gradient of the reduction performed in the area (high solid fraction area 61) from the central solid fraction of 0.8 to the solidification completion position is preferably 10 mm/min or less. This is because it has been confirmed by an experimental device that cracks do not occur if the speed is 10 mm/min or less.

A method for making the upper surface equiaxed grain ratio of the slab to be 5% or more will be described.
In-mold electromagnetic stirring is the most effective way to achieve a top equiaxed grain ratio of 5% or more. The higher the speed of the stirring flow by the electromagnetic stirring in the mold, the more the top equiaxed grain ratio can be increased.
Furthermore, the upper surface equiaxed grain ratio increases as the molten steel temperature in the tundish is lowered and the degree of superheat ΔT in the tundish molten steel (difference (°C) between the molten steel temperature in the tundish and the liquidus temperature) is decreased. can be done. It is preferable to set ΔT to 30° C. or less.
When ΔT is 30° C. or less and the flow rate of electromagnetic stirring in the mold is 0.1 m/s, the upper surface equiaxed grain ratio becomes 5% or more.

A description will be given of preferable rolling conditions for a cast slab in a low solid fraction region where the central solid fraction is 0.8 or less (see FIG. 4). As is conventionally known, center segregation of a slab is known to be reduced by reducing the slab in accordance with solidification shrinkage in a region where the solid fraction is low. In the range of the central solid phase ratio in the region where the solid phase ratio is low, the amount of light reduction for compensating for solidification shrinkage is set to about 0.8 to 1.2 mm/min. Also in the present invention, in the region where the central solid fraction is from 0.3 to 0.75 (low solid fraction region 62), the reduction gradient is 0.8 to 1.2 mm / min. It is possible to keep the central segregation of the pieces low. The lower limit of the central solid fraction was determined because it is the general lower limit of the solid fraction range in which light reduction is effective. On the other hand, when the central solid fraction exceeds 0.75, the upper limit of the rolling gradient is relaxed, so the upper limit of the central solid fraction of the low solid fraction region was set to 0.75. The range of the rolling gradient in the low solid fraction region conforms to the general light rolling gradient suitable for solidification shrinkage.

In the region between the low solid fraction region 62 and the high solid fraction region 61 (the region where the central solid fraction is between 0.75 and 0.8, hereinafter referred to as the “transition solid fraction region 63”), the reduction is It is sufficient that the reduction is performed with a gradient of 0.8 mm/min or more (see FIG. 4). The upper limit of the rolling gradient of the transition solid fraction region 63 is preferably 10 mm/min or less, like the high solid fraction region 61 . That is, the transition solid fraction region 63 may have the same rolling gradient as the low solid fraction region 62, or may have the same rolling gradient as the high solid fraction region 61. It may also be a transition region in which the rolling gradient (under high pressure) in the high solid fraction region 61 is sequentially shifted from (low rolling) to the rolling gradient (under high pressure).

Regarding the diameter of the reduction roll in the high solid fraction region from the central solid fraction of 0.8 to the solidification completion position, it has been confirmed that internal cracks do not occur if the diameter is 380 mm or more.

The present invention is preferably used in continuous casting of slabs. In continuous casting of slabs, slabs are usually cast in a state where the upper surface equiaxed grain ratio is less than 5%. The present invention significantly improves the center segregation and center porosity of the slab by setting the upper surface equiaxed grain ratio to 5% or more and combining a high reduction gradient in a high solid fraction region even in continuous casting of slabs. made it possible.

Next, an experiment was conducted using an actual continuous casting apparatus to cast a slab having a width of 2300 mm and a thickness of 230 mm using medium carbon steel with a C content of 0.17% by mass. It corresponds to a continuous casting machine with a general light reduction function. In-mold electromagnetic stirring was performed, and the conditions for in-mold electromagnetic stirring were set so that the in-mold stirring flow rate was 0.1 m/s. The casting speed was 1.0 m/min.

Regarding the roll arrangement of the continuous casting apparatus, as shown in FIG. Light reduction can be achieved by progressively narrowing the roll spacing.
In addition, in the region where the solid fraction on the downstream side 53 is 0.8 to 1.0 (high solid fraction region 61), a reduction roll pair using the reduction roll 2 on the F surface side and the reduction roll 3 on the L surface side 1 is placed and a reduction is performed. In the reduction roll pair 1, both the reduction roll 3 on the L surface side and the reduction roll 2 on the F surface side are flat rolls with a diameter of 400 mm.

FIG. 3 shows the arrangement of the reduction roll pair 1 in the high solid fraction region. FIG. 3A shows one roll pair 1, FIG. 3B shows two roll pairs 1, and FIG. 3C shows three roll pairs 1. FIG. When the number of the screw roll pairs 1 is three, the first screw roll pair 11, the second screw roll pair 12, and the third screw roll pair 13 are arranged from the upstream side 52, as shown in FIG. 3(C). Reduction conditions were set for a region where the central solid phase ratio is from 0.3 to 0.8 and a region where the central solid phase ratio is from 0.8 to 1.0. In Table 1, No. No. 1 to 4 and No. 13 to 16 have three pairs of rolls. No. 5 to 8 and No. 17 to 20 have two pairs of reduction rolls. 9 to 12 and 21 to 24 are examples in the case of one pair of pressing rolls.

Further, in any case where the number of reduction roll pairs is 1 to 3, the support roll 4U immediately before the first reduction roll pair 11 on the most upstream side has a slab center solid fraction of 0.8 or less, and each reduction roll The roll pair 1 is arranged within a solidification completion position (solid phase ratio at the slab center is 1.0) where the solid phase ratio at the slab center is 0.8 or more. Regarding the "cumulative average reduction amount (mm)" shown in Table 1, the thickness of the slab on the delivery side of the support roll 4U immediately before the high solid fraction region (the gap between the upper and lower rolls of the support roll 4U) is used as a reference, and the high solid fraction is The average rolling reduction d is calculated for each pair of rolling rolls in the region, and is used as the cumulative average rolling reduction. Further, the rolling gradient (mm/min) shown in Table 1 is a value obtained by dividing the difference in the average rolling amount between the high solid fraction region entry side and the exit side by the high solid fraction region passing time. Specifically, the average rolling reduction on the entry side of the high solid fraction region corresponds to zero, and the average rolling reduction on the exit side corresponds to the cumulative average rolling reduction _dT of the final rolling roll pair. Further, the high solid fraction region transit time is a value obtained by dividing the length L of the high solid fraction region (1.0 m in the example) by the casting speed (1.0 m/min in the example). As a result, in the example, the rolling gradient (mm/min) is the same numerical value as the cumulative average rolling reduction amount (mm) of the final rolling roll pair.

In the region where the central solid phase ratio is up to 0.8, a light reduction gradient of 0.8 to 1.2 mm/min, which is commonly used, was adopted as the light reduction condition. In this center solid fraction region, solidification shrinkage can be compensated by adopting 0.8 to 1.2 mm/min. Light rolling in this area uses the support rolls 4 that are commonly used as described above, and the roll diameter is 280 mm.

As for the rolling conditions of the region from the central solid phase ratio of 0.8 to the solidification completion position (high solid phase ratio region 61), a rolling gradient for rolling by a rolling roll pair was selected. Nos. 1 to 12 have a cumulative average rolling reduction of 4.0 mm and a rolling gradient of 4.0 mm/min. Nos. 13 to 24 have a cumulative average rolling reduction of 5.6 mm and a rolling gradient of 5.6 mm/min. As for the number of pairs of rolls, investigation was conducted on three levels: one pair, two pairs, and three pairs.

The upper surface equiaxed grain ratio of the slab was adjusted by setting the stirring flow rate of electromagnetic stirring in the mold to 0.1 m/s and then changing the molten steel superheating degree ΔT in the tundish. When the stirring flow rate is 0.1 m/s, the upper surface equiaxed grain ratio becomes 0% by setting the molten steel superheat ΔT in the tundish to 35°C or higher, and the upper surface equiaxed grain ratio becomes 5 by setting ΔT to 30°C. By setting ΔT to 25° C., the upper surface equiaxed crystal ratio is set to 10%, and by setting ΔT to 20° C. or lower, the upper surface equiaxed crystal ratio is set to 20%.

As for the slab quality, the maximum Mn segregation degree (central segregation) and center porosity at the center of the slab thickness were evaluated. The evaluation method of the maximum Mn segregation degree evaluation method was as described above.
Regarding the porosity area ratio, a color check was performed on the L cross section, the sum of the areas of the colored portions after the color check was taken as the total porosity area, and the ratio (%) of the total porosity area to the cross section of the slab was taken as the porosity area ratio.
A maximum Mn segregation degree of 1.17 or less and a center porosity area ratio of 3 or less were regarded as acceptable.
Table 1 shows the manufacturing conditions and quality evaluation results. FIG. 5 shows the relationship between the logarithm of the reduction roll pair, the top equiaxed grain ratio, and the maximum degree of Mn segregation. FIGS. 5(A) and 5(B) are the cases where the high solid fraction area reduction gradient is 4.0 and 5.6 mm/min, respectively.

In Table 1, No. In each of the groups 1 to 4, 5 to 8, 9 to 12, 13 to 16, 17 to 20, and 21 to 24, the comparative examples having a top equiaxed crystal ratio of 0% had a poor porosity area ratio. rice field. The maximum Mn segregation degree was also unsatisfactory under the condition that the upper surface equiaxed crystal ratio was 0% and the reduction gradient was 4.0 mm/min. On the other hand, when the upper surface equiaxed crystal ratio is 5% or more, both the maximum Mn segregation degree and the porosity area ratio are good, and the higher the upper surface equiaxed crystal ratio, the better the performance.
No. 1 to 12 (rolling gradient: 4.0 mm/min) and No. In each group of 13 to 24 (rolling gradient: 5.6 mm/min), as the logarithm of rolling roll pairs increased, better results were obtained for both maximum Mn segregation degree and porosity area ratio.
No. 1 to 12 (rolling gradient: 4.0 mm/min) and No. 13 to 24 (rolling gradient: 5.6 mm/min), good results were obtained for both the maximum degree of Mn segregation and the porosity area ratio as the rolling gradient increased.

1 reduction roll pair 2 reduction roll 3 reduction roll 4 support roll 5 slab 11 first reduction roll pair 12 second reduction roll pair 13 third reduction roll pair 52 upstream side 53 downstream side 61 high solid fraction region 62 low solid phase Rate region 63 transition solid phase rate region

Claims (3)

The upper surface equiaxed grain ratio of the slab is 5% or more,
In the region from the central solid fraction of 0.8 to the completion of solidification (hereinafter referred to as "high solid fraction region"), one or more pairs of reduction rolls are arranged to reduce the slab,
When the upper surface equiaxed crystal ratio is 5% or more and less than 20%, the reduction gradient in the high solid fraction region is set to 4.0 mm/min or more and 10 mm/min or less , and when the top surface equiaxed crystal ratio is 20% or more. A continuous casting method for steel, characterized in that the reduction gradient in a high solid fraction region is set to 3.0 mm/min or more and 10 mm/min or less . 2. The continuous casting method of steel according to claim 1, wherein two or more pairs of said reduction rolls are continuously arranged to reduce the cast slab. In a region where the central solid phase ratio is from 0.3 to 0.75 (hereinafter referred to as “low solid phase ratio region”), reduction is performed with a reduction gradient of 0.8 to 1.2 mm / min, and the low solid phase 3. A continuous casting method for steel according to claim 1 or 2, wherein the reduction is performed with a reduction gradient of 0.8 mm/min or more in the region between the high solid fraction region and the high solid fraction region.

Images