JP2024142642A - Continuous casting method for steel - Google Patents

Abstract

[Problem] To provide a method for continuous casting of steel, capable of suppressing internal cracks of a slab while reducing center segregation and porosity. [Solution] The continuous casting method uses a continuous casting machine (1). The continuous casting machine (1) includes a plurality of soft reduction roll pairs (6) arranged in the casting direction. At least one of the soft reduction roll pairs (6) has a diameter at a widthwise center portion larger than the diameter at both ends, and satisfies the following formula (1), where the width of the widthwise center portion is Wr (mm), the width of the slab is Ws (mm), and the thickness of the slab is D (mm). The continuous casting method includes a reduction step (#10). In the reduction step (#10), the slab (10) is soft reduced in the thickness direction at a reduction speed of 0.5 mm/min or more and 1.3 mm/min or less using the plurality of soft reduction roll pairs (6) from when the center solid fraction of the slab (10) reaches 0.3 until the center solid fraction reaches 1.0. Ws-1.15 x D ≦ Wr ≦ Ws-1.15 x D + 250 (1) [Selection diagram] Figure 1

Description

This disclosure relates to a method for continuous casting of steel.

In the continuous casting of steel, defects such as center segregation and porosity occur in the slab due to solidification and shrinkage of the slab. The slab obtained by continuous casting is rolled into a product. In recent years, products have become thicker and stronger, and as a result, there are increasing demands for the internal quality of the slab. For this reason, there is a need to further reduce center segregation and porosity in the center of the slab in its thickness direction. To improve the internal quality caused by these defects, the slab is usually lightly reduced in the thickness direction in the continuous casting machine. For light reduction, multiple reduction rolls arranged in the casting direction of the slab are usually used.

For example, Patent Document 1 discloses a technique for arranging a convex roll in the upstream portion of the soft reduction zone, in which the roll diameter in the width direction central portion is larger than the roll diameter in the both ends, in order to prevent central segregation and deterioration of porosity even if the crater end shape in both ends of the slab shifts upstream. In the continuous casting method described in Patent Document 1, the temperature distribution in the width direction is measured before the slab reaches the soft reduction zone. Patent Document 1 also describes that if the temperature in the width direction central portion of the slab is a predetermined temperature lower than the temperatures in the both ends, a convex roll is used to perform soft reduction centered on the width direction central portion of the slab.

Patent Document 2 discloses a technique for bulging the slab in order to reduce the rolling load during light reduction. In the continuous casting method described in Patent Document 2, the slab is intentionally bulged by gradually increasing the distance between the support rolls in the thickness direction, and then the slab is reduced with multiple guide rolls.

Patent Document 3 discloses a technique in which one or both of two or more consecutive reduction roll pairs are convex rolls in order to improve center porosity (center segregation) of a slab. In the continuous casting method described in Patent Document 3, the slab is reduced using a reduction roll pair including a convex roll when the central solid fraction of the slab is in the range of 0.7 or more.

JP 2012-66302 A International Publication No. 2019/167855 JP 2016-78083 A

In recent years, the thickness of cast pieces has been increased in response to the demand for thicker products, and the reduction force of the reduction rolls required to sufficiently reduce the center segregation and porosity of the cast pieces has increased. In order to increase the reduction force, in addition to increasing the size of equipment such as hydraulic devices, it is necessary to increase the rigidity of the reduction rolls and the rigidity of the segment frames so that they can withstand the increased reduction force. In order to increase the rigidity of the reduction rolls, for example, it is possible to increase the diameter of the reduction rolls. However, if the diameter of the reduction rolls is increased, the distance between adjacent reduction rolls in the casting direction inevitably increases. This makes it easier for bulging to occur in the cast pieces between the adjacent reduction rolls, and conversely, there is a possibility that center segregation will increase. In addition, in order to increase the rigidity of the reduction rolls, it is also possible to divide the reduction rolls into multiple parts in the width direction and provide bearings between each reduction roll. However, in this case, the cast pieces are not reduced at the bearing parts between the reduction rolls, so center segregation and porosity may worsen.

In the technology described in Patent Document 1, reduction is performed with a convex roll upstream of the soft reduction zone, and reduction is performed with a flat roll downstream of the convex roll. In general, the temperature of the slab is lower downstream in the casting direction than the upstream side, so the reduction force required by the reduction roll increases. Therefore, even if the slab is reduced downstream with a flat roll, the widthwise center of the slab cannot be reduced sufficiently, and central segregation and porosity may worsen. In addition, a dent is transferred to the widthwise center of the slab by the convex roll. Since this dent cannot be reduced with a flat roll, central segregation and porosity may worsen depending on the conditions. Furthermore, Patent Document 1 does not specify the specific central solid fraction or amount of soft reduction at which to start soft reduction.

The technology in Patent Document 2 intentionally bulges the slab before soft reduction. However, depending on the type of steel in the slab and the amount of bulging, internal cracks may occur, which may have a negative impact on product quality. The technology in Patent Document 3 reduces the slab using a pair of reduction rolls when the central solid fraction of the slab is in the range of 0.7 or more, but it may not be possible to fully improve central segregation that occurs before this range.

The objective of this disclosure is to provide a method for continuous casting of steel that can suppress internal cracks in the slab while reducing center segregation and porosity.

The continuous casting method according to the present disclosure is a method for continuously casting steel using a continuous casting machine. The continuous casting machine includes a plurality of pairs of soft reduction rolls arranged in the casting direction of a slab. At least one of the pairs of soft reduction rolls has a diameter at a widthwise center portion larger than the diameter at both ends. At least one of the pairs of soft reduction rolls satisfies the following formula (1), where the width at the widthwise center portion is Wr (mm), the width of the slab is Ws (mm), and the thickness of the slab is D (mm). The continuous casting method includes a reduction step. In the reduction step, the slab is soft reduced in the thickness direction at a reduction speed of 0.5 mm/min or more and 1.3 mm/min or less using the plurality of pairs of soft reduction rolls from when the central solid fraction of the slab reaches 0.3 until the central solid fraction reaches 1.0.
Ws-1.15×D≦Wr≦Ws-1.15×D+250 (1)

The continuous steel casting method disclosed herein can reduce center segregation and porosity while suppressing internal cracks in the slab.

FIG. 1 is a schematic diagram of a continuous casting machine used in a continuous casting method according to an embodiment. FIG. 2 is a cross-sectional view of the continuous casting machine taken along a plane perpendicular to the casting direction. FIG. 3 is a flow diagram showing a method for continuously casting steel according to an embodiment.

The continuous casting method according to the embodiment is a method for continuously casting steel using a continuous casting machine. The continuous casting machine includes a plurality of pairs of soft reduction rolls arranged in the casting direction of a slab. At least one of the pairs of soft reduction rolls has a diameter at a widthwise center portion larger than the diameter at both ends. At least one of the pairs of soft reduction rolls satisfies the following formula (1), where the width at the widthwise center portion is Wr (mm), the width of the slab is Ws (mm), and the thickness of the slab is D (mm). The continuous casting method includes a reduction step. In the reduction step, the slab is soft reduced in the thickness direction at a reduction speed of 0.5 mm/min or more and 1.3 mm/min or less using the plurality of pairs of soft reduction rolls from when the central solid fraction of the slab reaches 0.3 until the central solid fraction reaches 1.0 (first configuration).
Ws-1.15×D≦Wr≦Ws-1.15×D+250 (1)

In the first configuration of the continuous casting method, in the reduction step, a plurality of pairs of soft reduction rolls are used to softly reduce the slab in the thickness direction. At least one of these pairs of soft reduction rolls is a convex roll whose diameter at the widthwise center is larger than the diameter at both ends. The convex roll has a shape that satisfies the above formula (1) in relation to the slab. In short, the width of the part (widthwise center) of the convex roll whose diameter is larger than the diameter at both ends is appropriately set. In the soft reduction using this convex roll, the range in which the slab is reduced in the width direction is optimized, and both ends of the slab, which are relatively cold, are not reduced significantly. Therefore, the reduction force required for reducing the slab is reduced, and center segregation and porosity can be reduced without increasing the size of the equipment or the rigidity of the soft reduction roll.

In addition, in the continuous casting method of the first configuration, intentional bulging of the slab is not performed. Furthermore, since there is no need to increase the rigidity of the light reduction rolls, there is no need to increase the diameter of the light reduction rolls. Therefore, bulging is less likely to occur between the pair of light reduction rolls. As a result, the continuous casting method of the first configuration suppresses internal cracks in the slab.

The continuous casting method according to the first configuration preferably has the following configuration. The continuous casting machine is equipped with a mold including an electromagnetic brake. The continuous casting method further includes an application step. In the application step, a magnetic field having a magnetic flux density of 1000 Gauss or more is applied to the molten steel in the mold by the electromagnetic brake (second configuration).

In the continuous casting method according to the second configuration, in the application step, a magnetic field with a magnetic flux density of 1000 Gauss or more is applied to the molten steel in the mold by an electromagnetic brake. This makes the thickness of the solidified shell of the slab uniform in the width direction. By lightly reducing the thickness of this slab in the reduction step, it is possible to reduce center segregation and porosity.

Embodiments of the present disclosure will be described below with reference to the drawings. In each drawing, the same or equivalent components are given the same reference numerals, and the same description will not be repeated.

[Continuous casting machine]
FIG. 1 is a schematic diagram of a continuous casting machine 1 used in the continuous casting method according to the present embodiment. The continuous casting machine 1 produces a cast piece 10. In the example of the present embodiment, the cast piece 10 is a slab. In this specification, a slab means a cast piece having a width-to-thickness ratio of 3 or more. The continuous casting machine 1 includes a tundish 2, a mold 4, a plurality of support rolls 5, and a plurality of soft reduction roll pairs 6. In the example of the present embodiment, the continuous casting machine 1 is a vertical bending type. In short, the continuous casting machine 1 includes a vertical band, a curved portion, and a horizontal band. However, the continuous casting machine 1 may be, for example, a vertical type consisting of only a vertical band, or a curved type consisting of a curved portion and a horizontal band.

Molten steel M is supplied to the tundish 2 from a ladle (not shown). The molten steel M in the tundish 2 is supplied to the mold 4 through the submerged nozzle 3. The molten steel M in the mold 4 is cooled by the mold 4. The mold 4 includes an electromagnetic brake 4a. The electromagnetic brake 4a is, for example, an electromagnet. For example, the electromagnetic brake 4a is disposed on both outer sides of the mold 4 in the thickness direction of the slab 10.

A plurality of secondary cooling nozzles (not shown) and a plurality of support rolls 5 are arranged downstream of the mold 4 in the casting direction. The molten steel M is cooled in the mold 4, and then further cooled by spraying secondary cooling water from the secondary cooling nozzle. This forms a solidified shell S. The slab 10 in the process of solidifying here includes the solidified shell S (solid fraction 1.0) and unsolidified molten steel M (solid fraction less than 1.0). The slab 10 in the process of solidifying, including the unsolidified molten steel M, is guided downstream in the casting direction by the plurality of support rolls 5. In the process, the unsolidified molten steel M gradually decreases, and a completely solidified slab 10 is formed. The secondary cooling nozzles are arranged, for example, alternately with the plurality of support rolls 5 in the casting direction.

The slab 10, which contains unsolidified molten steel M, is guided downstream in the casting direction by multiple support rolls 5. During this process, the unsolidified molten steel M gradually decreases, resulting in a slab 10 in a completely solidified state. In this type of continuous casting, the slab 10 is sent in the casting direction by pinch rolls (not shown). In other words, the casting speed of the slab 10 is controlled by the pinch rolls.

Downstream of the support rolls 5 in the casting direction, multiple soft reduction roll pairs 6 are arranged. Each of the soft reduction roll pairs 6 includes two soft reduction rolls 61, 62, and softly reduces the cast piece 10. A series of soft reduction roll pairs 6 form a soft reduction zone 7. The soft reduction roll pairs 6 are arranged at approximately equal intervals between the entrance 7i and the exit 7o of the soft reduction zone 7. Here, in the soft reduction zone 7, the position of the entrance 7i coincides with the position of the soft reduction roll pair 6 arranged at the most upstream position in the casting direction, and the position of the exit 7o coincides with the position of the soft reduction roll pair 6 arranged at the most downstream position in the casting direction.

The soft reduction zone 7 is divided into a number of segments, and each segment is provided with a reduction device (not shown) and a number of soft reduction roll pairs 6. In each segment, the reduction device reduces the slab 10, for example, by hydraulic pressure. The reduction amount of the soft reduction roll pair 6 is controlled for each segment (reduction device). However, the upper limit of the force with which the reduction device can reduce the slab 10 is determined for each device. When the reaction force received from the slab 10 is equal to or greater than the upper limit of the reduction force of the reduction device, the actual reduction speed is smaller than the set reduction speed. This is because when the reaction force received from the slab 10 reaches the upper limit of the reduction force of the reduction device, the reduction of the slab 10 stops at that point, and the reduction gradient when actually reduced is less than the set reduction gradient. Here, the set reduction speed means the casting speed (m/min) set in the continuous casting machine 1 multiplied by the set reduction gradient (mm/m). Additionally, the actual reduction speed refers to the reduction speed (mm/min) when the cast piece 10 is actually reduced using the reduction device of the continuous casting machine 1.

Figure 2 is a cross-sectional view of the continuous casting machine 1 cut along a plane perpendicular to the casting direction. Figure 2 shows the appearance of the slab 10 being reduced by the pair of soft reduction rolls 6. Referring to Figure 2, the soft reduction roll 61 is arranged at a distance from the soft reduction roll 62 in the vertical direction. The vertical direction corresponds to the thickness direction of the slab 10. The soft reduction roll 61 is arranged above the slab 10, and the soft reduction roll 62 is arranged below the slab 10.

At least one of the light reduction rolls of the light reduction roll pair 6 (light reduction rolls 61, 62) is a convex roll. In the example of this embodiment, the light reduction roll 61 is a convex roll, and the light reduction roll 62 is a flat roll. However, the configuration of the light reduction roll pair 6 is not limited to this. The light reduction roll 61 may be a flat roll, and the light reduction roll 62 may be a convex roll. In addition, both the light reduction rolls 61, 62 may be convex rolls. When both the light reduction rolls 61, 62 are convex rolls, the effect of reducing center segregation and porosity is significantly exhibited.

The soft reduction roll 61 includes a large diameter portion 611 located in the center of the width direction, two small diameter portions 612, 612 located at both ends of the width direction, and two tapered portions 613, 613 connecting the large diameter portion 611 and each of the small diameter portions 612, 612. The soft reduction roll 61 is typically arranged so that the center of the large diameter portion 611 in the width direction coincides with the center of the slab 10 in the width direction. The diameter of the large diameter portion 611 is larger than the diameter of the small diameter portions 612, 612. When the slab 10 is reduced using the pair of soft reduction rolls 6, the large diameter portion 611 comes into contact with the slab 10, and the small diameter portions 612, 612 and the tapered portions 613, 613 do not come into contact with the slab 10. Each of the small diameter portions 612, 612 is rotatably supported by bearings 614, 614 arranged on the outside in the width direction.

The soft reduction roll 61 satisfies the above formula (1) when the width of the large diameter portion 611 is Wr (mm), the width of the slab 10 is Ws (mm), and the thickness of the slab 10 is D (mm). In short, the width Wr of the large diameter portion 611 is equal to or greater than (Ws-1.15 x D) and equal to or less than (Ws-1.15 x D + 250). If the width Wr of the large diameter portion 611 is smaller than (Ws-1.15 x D), the reduction near both ends of the slab 10 during soft reduction is insufficient, and central segregation and porosity cannot be reduced in that portion. Also, if the width Wr of the large diameter portion 611 is greater than (Ws-1.15 x D + 250), the both ends of the slab 10, which have a relatively low temperature, are reduced more widely by the soft reduction roll 61. Therefore, the reaction force received from the slab 10 becomes large, and there is a risk of exceeding the upper limit of the reduction force of the reduction device. As described above, when the reaction force from the slab 10 exceeds the upper limit of the reduction force, the actual reduction speed becomes smaller than the set reduction speed. Therefore, in this case, the reduction amount of the soft reduction roll pair 6 is insufficient for the solidification shrinkage of the slab 10, and center segregation and porosity cannot be reduced. For the above reasons, the width Wr of the large diameter portion 611 of the soft reduction roll 61 used in the continuous casting method according to this embodiment satisfies the above formula (1).

The shape of the light reduction roll 61 (convex roll) is not particularly limited as long as the width Wr of the large diameter portion 611 satisfies the above formula (1). For example, the large diameter portion 611 and the small diameter portions 612, 612 may be directly connected to each other without the tapered portions 613, 613. Alternatively, the large diameter portion 611 may be divided into multiple portions in the width direction, and bearings may be provided between each large diameter portion 611.

In the light reduction roll 61 (convex roll), the difference between the diameter of the large diameter portion 611 and the diameter of the small diameter portion 612, 612 is preferably 5 mm or more and 30 mm or less. If the difference between the diameter of the large diameter portion 611 and the diameter of the small diameter portion 612, 612 is 5 mm or more, the effect of reducing center segregation and porosity is sufficiently exhibited even when the light reduction roll 61 is at the end of its life. In addition, if the difference between the diameter of the large diameter portion 611 and the diameter of the small diameter portion 612, 612 is 30 mm or less, the diameter of the small diameter portion 612, 612 is prevented from becoming extremely small, and the risk of breakage of the light reduction roll 61 can be reduced. The ratio of the diameter of the large diameter portion 611 to the diameter of the small diameter portion 612, 612 is preferably 1.017 or more and 1.150 or less.

The soft reduction roll 62 includes a main body 621 with a constant diameter in the width direction. When the soft reduction roll pair 6 is used to reduce the slab 10, the main body 621 comes into contact with the slab 10. The main body 621 is rotatably supported by bearings 622, 622 arranged on the outside in the width direction.

[Continuous casting method]
Fig. 3 is a flow diagram showing the method for continuous casting of steel according to this embodiment. As shown in Fig. 3, the method for continuous casting of steel according to this embodiment includes an application step (#5) and a rolling down step (#10). A slab 10 obtained by this continuous casting method becomes a material for products such as steel plate. Each step shown in Fig. 3 will be specifically described below.

[Marking process (#5)]
In the application step (#5), a magnetic field is applied to the molten steel M in the mold 4 by the electromagnetic brake 4a. As a result, the molten steel M discharged from the discharge hole of the submerged nozzle 3 in the mold 4 flows due to the action of the magnetic field. Therefore, the flow of the molten steel M discharged from the discharge hole toward the width direction end of the solidified shell S is weakened. It can promote the floating of the particles, etc., and also plays a role in separating inclusions, etc. from the molten steel M.

The magnitude of the magnetic flux density of the magnetic field applied in the application step (#5) is preferably 1000 Gauss or more. If the magnetic flux density is 1000 Gauss or more, a sufficient braking force acts on the molten steel M. In other words, if a magnetic field of less than 1000 Gauss is applied to the molten steel M, the braking force acting on the molten steel M is insufficient. In this case, the temperature near the widthwise end of the slab 10 becomes high, and the completion of solidification is likely to be delayed, resulting in deterioration of the internal quality of the slab 10. By applying a magnetic field of 1000 Gauss or more in the application step (#5), the thickness of the solidified shell S of the slab 10 becomes uniform in the width direction. By lightly reducing the thickness of this slab 10 in the reduction step (#10) described later, it is possible to reduce center segregation and porosity.

The magnitude of the magnetic flux density of the magnetic field is not particularly limited as long as it is 1000 Gauss or more. However, it is preferable that the magnetic flux density is 3000 Gauss or less. If a magnetic field of more than 3000 Gauss is applied to the molten steel M, the temperature of the molten steel M in the vicinity of the submerged nozzle 3 will rise, and the initial solidification of the molten steel M in the mold 4 will become uneven. If the initial solidification of the molten steel M becomes uneven, there is a risk of vertical cracks occurring on the surface of the slab 10 when continuously casting steel that is prone to surface cracks (e.g., hypoperitectic steel).

[Reduction process (#10)]
In the reduction step (#10), the slab 10 is soft reduced in the thickness direction using a plurality of soft reduction roll pairs 6 in a soft reduction zone 7. The soft reduction is performed until the central solid fraction of the slab 10 becomes 0. The central solid fraction of the slab 10 is 0.3 at the entrance 7i of the soft reduction zone 7 and is then 1.0 at the entrance 7i of the soft reduction zone 7. At the outlet 7o, the central solid fraction of the slab 10 is 1.0. If the soft reduction is started after the central solid fraction of the slab 10 reaches 0.3, the central segregation and porosity can be sufficiently reduced. Similarly, if the soft reduction is terminated before the center solid fraction of the slab 10 reaches 1.0, center segregation and porosity cannot be sufficiently reduced. In the region where the solid fraction is 1.0, i.e., the region where the center of the thickness of the cast 10 is completely solidified, soft reduction has little effect on reducing center segregation and porosity. There is no need to lightly depress the

It is not necessary to soft reduce the area of the slab 10 where the central solid fraction is less than 0.3, as this does not affect the reduction of central segregation and porosity formed at the end of solidification. However, soft reduction may be performed on the area of the slab 10 where the central solid fraction is less than 0.3.

In the reduction process (#10), if the actual reduction speed in the light reduction zone 7 is less than 0.5 mm/min, the reduction amount is insufficient relative to the solidification shrinkage of the slab 10, and central segregation and porosity cannot be reduced. Furthermore, if the reduction speed is greater than 1.3 mm/min, the reduction amount becomes excessive relative to the solidification shrinkage of the slab 10, causing the molten steel M to flow back from downstream to upstream in the casting direction, and the resulting flow of concentrated molten steel worsens central segregation. Therefore, the actual reduction speed of the slab 10 in the reduction process (#10) is 0.5 mm/min or more and 1.3 mm/min or less.

In the continuous casting method of this embodiment, multiple pairs of soft reduction rolls 6 are used in the reduction step (#10). In this case, compared to when only one pair of soft reduction rolls 6 is used to reduce the slab 10, a tapered gradient can be given to the slab 10, so that the slab 10 can be gradually narrowed in response to solidification shrinkage.

[effect]
In the continuous casting method of this embodiment, in the reduction step (#10), the slab 10 is soft reduced in the thickness direction using a plurality of soft reduction roll pairs 6. In the example of this embodiment, the soft reduction roll 61 of the soft reduction roll pair 6 is a convex roll whose diameter at the center in the width direction is larger than the diameter at both ends. The soft reduction roll 61 has a shape that satisfies the above formula (1) in relation to the slab 10. In short, the width of the large diameter portion 611 of the soft reduction roll 61 is appropriately set. In the soft reduction using this soft reduction roll 61 (convex roll), the range in which the slab 10 is reduced in the width direction is optimized, and both ends of the slab 10, which have a relatively low temperature, are not reduced much. Therefore, the reduction force required for reducing the slab 10 is suppressed, and center segregation and porosity can be reduced without increasing the size of the equipment or the rigidity of the soft reduction roll.

In addition, in the continuous casting method of this embodiment, intentional bulging of the slab 10 is not performed. Furthermore, since there is no need to increase the rigidity of the light reduction rolls 61, 62, there is no need to increase the diameter of the light reduction rolls 61, 62. Therefore, bulging is less likely to occur between the pair of light reduction rolls 6. As a result, the continuous casting method of this embodiment suppresses internal cracks in the slab 10.

To confirm the effects of the continuous casting method according to the embodiment, the following tests were carried out and the results were evaluated. Specifically, the Mn segregation degree and porosity volume of the cast pieces obtained by continuous casting were evaluated. The Mn segregation degree and porosity volume will be explained later.

In this test, a cast piece was produced using the continuous casting machine shown in Figure 1. The mold was a copper water-cooled mold. The length of the mold was 800 mm. The cross section of the mold was rectangular. The soft reduction zone was provided 16 m or more from the meniscus (the molten metal surface in the mold). The cast piece was soft reduced in the thickness direction at a set reduction speed of 0.8 mm/min by a pair of soft reduction rolls from when the central solid fraction of the cast piece reached 0.3 until the central solid fraction reached 1.0. No soft reduction was performed in the region after the central solid fraction of the cast piece reached 1.0.

In the pair of light reduction rolls in the light reduction zone, only the upper light reduction roll was a convex roll. The diameter of the large diameter part of the convex roll was 15 mm larger than the diameter of the small diameter part. The specific water volume of the cooling water sprayed from the nozzle for secondary cooling was 0.7 to 1.5 L/kg-steel depending on the thickness of the cast piece and the casting speed.

The main chemical composition of the cast pieces used in this test was C: 0.15%, Si: 0.19%, Mn: 0.90%, P: 0.011%, and S: 0.003%.

The central temperature and solid fraction of the slab were calculated by two-dimensional solidification analysis in the thickness and width directions of the slab. However, the actual reduction speed during soft reduction may vary depending on the capacity of the reduction device and the reaction force from the slab. For this reason, the position of the reduction device in the thickness direction of the slab was recorded, and the actual reduction speed was calculated based on this. The upper limit of the reduction force of the reduction device was set to 600 tons, taking into account the rigidity of the segments and rolls.

The degree of Mn segregation in the slab was investigated using the following procedure. The cast slab was cut at the center in the width direction, and a sample was taken from the center in the thickness direction of the cut surface, including an area of 40 mm in the casting direction and 40 mm in the thickness direction. A surface analysis of the Mn concentration was performed on this sample using an EPMA (Electron Probe Micro Analyzer). The Mn concentration was integrated over a 2 mm width along the casting direction, centered on the position with the highest Mn concentration, and the average value was taken as the maximum Mn concentration (Cmax). The value (Cmax/C0) obtained by dividing the maximum Mn concentration (Cmax) by the Mn concentration (C0) in the bulk composition of the slab was taken as the degree of Mn segregation. The Mn concentration (C0) in the bulk composition was determined by chemical analysis of an analytical sample taken from the slab.

The porosity volume of the slab was investigated by the following procedure. Samples were taken from the center of the slab in the thickness direction, measuring 50 mm in the casting direction, 100 mm in the width direction, and 7 mm in the thickness direction. The samples were taken from 16 locations along the width direction of the slab. The density ρ of each sample was measured by the method for measuring density and specific gravity of solids specified in JIS Z 8807. Samples were also taken from the 1/4 thickness portion of the slab in the same manner as above, and the density ρ ₀ was measured in the same manner as the density ρ. The porosity volume V (cm ³ /g) per unit weight was then calculated by the following formula (2). The maximum porosity volume V value among the porosity volumes V measured for each sample was taken as the porosity volume V of the slab.

The test conditions and test results of this embodiment are shown in Table 1. In Table 1, the test conditions include the thickness of the slab produced by continuous casting, the width of the slab, the width of the large diameter part of the soft reduction roll, the magnetic flux density of the magnetic field of the electromagnetic brake, the actual reduction speed, and the central solid fraction at the entrance of the soft reduction zone. The magnetic flux density is the magnetic flux density of the magnetic field applied to the molten steel in the mold by the electromagnetic brake. In Table 1, the slab produced under the test conditions that satisfy the following condition 1 is represented as an invention example, and the slab produced under the test conditions that do not satisfy condition 1 is represented as a comparative example. In Comparative Examples 1 to 4 shown in Table 1, the test conditions that do not satisfy condition 1 are marked with a "*". In addition, the surface temperatures of the invention examples 1 to 7 and comparative examples 1 to 4 shown in Table 1 that do not satisfy the following condition 2 are marked with a "#".
Condition 1: The width of the large diameter portion of the soft reduction roll (convex roll) satisfies the above formula (1), the actual reduction rate is 0.5 mm/min or more and 1.3 mm/min or less, and the central solid fraction of the slab at the entrance of the soft reduction zone is 0.3 or less.
Condition 2: The magnetic flux density is 1000 Gauss or more.

It is known that if the Mn segregation degree of the obtained cast slab is 1.4 or less, toughness can be ensured in the product (steel plate) after rolling the cast slab. Therefore, in Table 1, the test results are shown as "Excellent" if the Mn segregation degree is 1.4 or less, and "Fail" if it is not. In addition, it is known that if the porosity volume is 4.0 cm ³ /g or less, defects are rendered harmless in the product (steel plate) after rolling the cast slab, so "Excellent" is shown if the porosity volume is 4.0 cm ³ /g or less, and "Fail" is shown if it is not.

As shown in Table 1, all of Examples 1 to 6 of the invention satisfied Condition 1 and Condition 2. Therefore, Examples 1 to 6 of the invention produced slab cast pieces with good Mn segregation and porosity volume. In other words, center segregation and porosity were reduced.

Although Example 7 of the invention satisfied condition 1, the magnetic flux density was less than 1000 Gauss, and condition 2 was not satisfied. In Example 7 of the invention, the Mn segregation degree and porosity volume were somewhat higher than those of Example 1, which had the same test conditions except for the magnetic flux density, but were below the threshold value, and a good cast piece was obtained. In other words, center segregation and porosity were reduced in Example 7 of the invention.

In Comparative Example 1, the width of the large diameter portion did not satisfy the above formula (1). Therefore, Comparative Example 1 did not satisfy Condition 1. In this case, the Mn segregation degree was greater than the threshold value, and the evaluation was "unacceptable."

In Comparative Example 2, the width of the large diameter portion did not satisfy the above formula (1), and the actual rolling reduction rate was less than 0.5 mm/min. Therefore, Comparative Example 2 did not satisfy Condition 1. In this case, both the Mn segregation degree and the porosity volume were larger than the threshold value, and the evaluation was "unacceptable."

In Comparative Example 3, like Comparative Example 2, the width of the large diameter portion did not satisfy the above formula (1), and the actual reduction rate was less than 0.5 mm/min. Therefore, Comparative Example 3 did not satisfy Condition 1. Furthermore, in Comparative Example 3, a magnetic field was not applied to the molten steel in the mold. Therefore, Comparative Example 3 did not satisfy Condition 2 either. In this case, compared to the results of Comparative Example 2, both the Mn segregation degree and the porosity volume were worse, and the evaluation was "unacceptable."

In Comparative Example 4, the central solid fraction of the slab at the entrance to the soft reduction zone was greater than 0.3, and condition 1 was not satisfied. In this case, in Comparative Example 4, the Mn segregation degree was greater than the threshold value, and the evaluation was "unacceptable."

The above describes the embodiments of the present disclosure. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by modifying the above-described embodiments as appropriate within the scope of the spirit of the present disclosure.

1: Continuous casting machine 4: Mold 4a: Electromagnetic brake 6: Pair of soft reduction rolls 61, 62: Soft reduction rolls 10: Cast strip

Claims (2)

A method for continuously casting steel using a continuous casting machine having a plurality of pairs of soft reduction rolls arranged in a casting direction of a slab, comprising the steps of:
At least one of the soft reduction rolls in the pair of soft reduction rolls has a diameter at a widthwise center portion larger than the diameters at both ends, and satisfies the following formula (1), where the width of the widthwise center portion is Wr (mm), the width of the slab is Ws (mm), and the thickness of the slab is D (mm):
The continuous casting method includes:
The continuous casting method includes a reduction step of softly reducing the slab in a thickness direction at a reduction speed of 0.5 mm/min or more and 1.3 mm/min or less using the plurality of pairs of soft reduction rolls from when the central solid fraction of the slab reaches 0.3 until the central solid fraction reaches 1.0.
Ws-1.15×D≦Wr≦Ws-1.15×D+250 (1) The continuous casting method according to claim 1,
The continuous casting machine includes a mold including an electromagnetic brake;
The continuous casting method further includes an application step of applying a magnetic field having a magnetic flux density of 1000 Gauss or more to the molten steel in the mold by the electromagnetic brake.

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