Continuous casting method for steel
The continuous casting method addresses porosity and central segregation in thicker slabs by using controlled cooling and reduction with specialized roll pairs and magnetic fields, achieving improved internal quality without equipment enlargement.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- NIPPON STEEL CORPORATION
- Filing Date
- 2025-01-16
- Publication Date
- 2026-07-29
AI Technical Summary
Continuous casting methods struggle to effectively reduce porosity and central segregation in thicker slabs due to uneven solidification and increased reduction force requirements, leading to equipment enlargement and potential bulging issues.
A continuous casting method using a continuous casting machine with secondary cooling nozzles and light reduction roll pairs, where at least one roll pair has a larger central diameter, controlled cooling and reduction steps to optimize solidification uniformity, and application of a magnetic field to manage molten steel flow.
The method reduces porosity and central segregation by optimizing solidification uniformity and reduction force, enhancing internal quality without enlarging equipment or increasing roll rigidity.
Smart Images
Figure 2026122866000001_ABST
Abstract
Description
[Technical Field]
[0001] This disclosure relates to a continuous casting method for steel. [Background technology]
[0002] In continuous casting of steel, solidification shrinkage of the slab can cause defects such as central segregation and porosity. The slabs obtained by continuous casting are rolled to become products. In recent years, there has been a trend towards thicker and stronger products, which has led to increased demands for higher internal quality of the slabs. Therefore, it is necessary to further reduce central segregation and porosity in the center of the slab in the thickness direction. To improve the internal quality caused by these defects, the slabs are often lightly reduced in the thickness direction within the continuous casting machine. For light reduction, multiple reduction rolls arranged in the casting direction of the slab are usually used.
[0003] For example, Patent Document 1 discloses a continuous casting method in which a convex roll is placed in the upstream portion of the light reduction zone. The convex roll has a diameter at its center in the width direction that is larger than the diameters at both ends. In the continuous casting method of Patent Document 1, the temperature distribution in the width direction is measured before the cast slab reaches the light reduction zone. Patent Document 1 states that if the temperature difference between the center and both ends in the width direction of the cast slab exceeds a predetermined value, light reduction is performed using the convex roll, centered on the center of the cast slab.
[0004] Patent Document 2 discloses a continuous casting method in which the cast slab is bulged in order to reduce the reduction load during light reduction. In the continuous casting method of Patent Document 2, the cast slab is intentionally bulged by gradually increasing the spacing between the support rolls in the thickness direction, and then the cast slab is reduced by multiple guide rolls.
[0005] Patent Document 3 discloses a continuous casting method using a vertical bending type continuous casting machine. In the continuous casting method of Patent Document 3, the amount of secondary cooling water injected into the central and both ends of the slab in the width direction is controlled. Patent Document 3 states that this suppresses surface cracking of the slab. [Prior art documents] [Patent Documents]
[0006] [Patent Document 1] Japanese Patent Publication No. 2012-66302 [Patent Document 2] International Publication No. 2019 / 167855 [Patent Document 3] Japanese Patent Publication No. 2008-194746 [Overview of the project] [Problems that the invention aims to solve]
[0007] In recent years, the increasing demand for thicker products has led to thicker slabs, and the reduction force required by the reduction rolls to sufficiently reduce porosity in the slabs has increased. To increase the reduction force, it is necessary to enlarge equipment such as hydraulic systems, and to increase the rigidity of the reduction rolls and segment frames so that they can withstand the increased reduction force. To increase the rigidity of the reduction rolls, one option is to increase the diameter of the reduction rolls. However, increasing the diameter of the reduction rolls inevitably increases the distance between adjacent reduction rolls in the casting direction. This can make bulging more likely to occur in the slab between adjacent reduction rolls, potentially increasing porosity.
[0008] From another perspective, continuous casting uses an immersion nozzle with 2 to 4 discharge holes, through which high-temperature molten steel is supplied within the mold. The molten steel is discharged toward the widthwise end of the solidified shell. Therefore, the area near the widthwise end of the slab tends to become hot, and solidification is delayed. Continuous casting machines also include, for example, support rolls that guide the slab downstream in the casting direction and nozzles that spray secondary cooling water onto the slab. Generally, the support rolls are divided into 2 to 4 roll body sections to distribute the load due to the static pressure or reduction of the molten steel. The roll body sections are arranged side by side in the widthwise direction of the slab. The roll body sections are connected to each other by bearing sections. The secondary cooling water sprayed from the nozzle toward the slab flows down the surface of the slab downstream in the casting direction and accumulates at the contact points between the roll body sections and the slab. The area of the slab corresponding to the roll body sections is cooled by the secondary cooling water accumulated at the contact points with the roll body sections and the support rolls in contact with the slab. On the other hand, secondary cooling water does not accumulate in the region of the slab corresponding to the support roll bearing, and this region is not in contact with the support roll. Therefore, the region of the slab corresponding to the support roll bearing is less easily cooled compared to the region of the slab corresponding to the roll body, and solidification is more likely to be delayed in this region.
[0009] Thus, due to the flow of molten steel within the mold and the uneven secondary cooling of the slab, there are regions in the solidifying slab where solidification is delayed, resulting in uneven thickness of the solidified shell across the width of the slab. In such cases, simply reducing the thickness of the slab slightly does not effectively reduce porosity in the regions where solidification is delayed, making it difficult to improve internal quality.
[0010] In the continuous casting method described in Patent Document 1, reduction is performed using convex rolls upstream of the light reduction zone, and using flat rolls downstream of the convex rolls. In this case, a depression is transferred to the central part of the slab in the width direction by the convex rolls. Since the flat rolls cannot reduce this depression, there is a risk that porosity may worsen depending on the conditions. Furthermore, Patent Document 1 does not specify the central solid fraction or the amount of light reduction at which light reduction should be initiated.
[0011] In the continuous casting method described in Patent Document 2, the cast slab is intentionally bulged before light reduction. However, depending on the type of steel used in the cast slab and the amount of bulging, internal cracks may occur, potentially negatively impacting product quality.
[0012] Furthermore, Patent Document 3 describes suppressing surface cracking of a cast slab by controlling its surface temperature. However, Patent Document 3 does not consider porosity occurring in the center of the cast slab in the thickness direction.
[0013] The object of this disclosure is to provide a continuous casting method for steel that can reduce porosity of cast slabs. [Means for solving the problem]
[0014] The continuous casting method for steel according to this disclosure uses a continuous casting machine. The continuous casting machine comprises a secondary cooling nozzle and a plurality of light reduction roll pairs. The secondary cooling nozzle sprays secondary cooling water onto the slab. The plurality of light reduction roll pairs are arranged in the casting direction of the slab. At least one of the light reduction roll pairs has a diameter at its center in the width direction that is greater than the diameters at both ends. At least one of the light reduction roll pairs satisfies the following equation (1), where Wr (mm) is the width of the center, Ws (mm) is the width of the slab, and D (mm) is the thickness of the slab. The continuous casting method comprises a cooling step and a reduction step. In the cooling step, secondary cooling water is sprayed from the secondary cooling nozzle in the range from the meniscus to an intermediate position between the meniscus and the crater end, such that the average specific water content is 0.1 L / kg-steel or more and 0.6 L / kg-steel or less. In the reduction process, the cast slab is lightly reduced in the thickness direction using multiple pairs of light reduction rolls at a reduction speed of 0.5 mm / min to 3.0 mm / min until the central solid fraction of the cast slab reaches 0.3 and then reaches 1.0. Ws - 1.15 × D ≤ Wr ≤ Ws - 50 (1) [Effects of the Invention]
[0015] According to the continuous casting method of steel according to the present disclosure, the porosity of the slab can be reduced.
Brief Description of the Drawings
[0016] [Figure 1] FIG. 1 is a schematic diagram of a continuous casting machine used in the continuous casting method according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view when the continuous casting machine is cut along a plane perpendicular to the casting direction. [Figure 3] FIG. 3 is a schematic diagram showing the relationship between the width of the large-diameter part and the thickness of the slab. [Figure 4] FIG. 4 is a schematic diagram showing the relationship between the width of the large-diameter part and the thickness of the slab. [Figure 5] FIG. 5 is a flowchart showing the continuous casting method of steel according to an embodiment.
Embodiments for Carrying Out the Invention
[0017] In the continuous casting method of steel according to the embodiment, a continuous casting machine is used. The continuous casting machine includes a secondary cooling nozzle and a plurality of soft reduction roll pairs. The secondary cooling nozzle injects secondary cooling water onto the slab. The plurality of soft reduction roll pairs are arranged in the casting direction of the slab. At least one of the soft reduction rolls in the soft reduction roll pair has a diameter at the central portion in the width direction larger than the diameters at both ends. At least one of the soft reduction rolls in the soft reduction roll pair satisfies the following formula (1) when the width of the central 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 cooling step and a reduction step. In the cooling step, secondary cooling water is injected from the secondary cooling nozzle so that the average specific water amount is 0.1 L / kg-steel or more and 0.6 L / kg-steel or less in the range from the meniscus to the intermediate position between the meniscus and the crater end position. In the reduction step, after the central solid fraction of the slab reaches 0.3 and until the central solid fraction reaches 1.0, the slab is softly reduced in the thickness direction at a reduction rate of 0.5 mm / min or more and 3.0 mm / min or less using the plurality of soft reduction roll pairs (the first configuration). Ws - 1.15×D ≤ Wr ≤ Ws - 50 (1)
[0018] In the first configuration of the continuous casting method, the slab is lightly reduced in the thickness direction using multiple pairs of light reduction rolls during the reduction process. At least one of these pairs of light reduction rolls is a convex roll in which the diameter of the central part in the width direction is larger than the diameters of the ends. The convex roll has a shape that satisfies equation (1) above in relation to the slab. In short, the width of the central part of the convex roll, which has a relatively large diameter, is appropriately set. With light reduction using this convex roll, the range in which the slab is reduced in the width direction is optimized, and the ends of the slab, which are at a relatively low temperature, are not reduced as much. Therefore, the reduction force required to reduce the slab is suppressed, and porosity can be reduced without increasing the size of the equipment or increasing the rigidity of the light reduction rolls.
[0019] In the first configuration of the continuous casting method, the average specific water content of the secondary cooling water in the cooling process, from the meniscus to the intermediate position between the meniscus and the crater end, is 0.6 L / kg-steel or less. This appropriately suppresses the amount of secondary cooling water injected into the slab, reducing the difference in solidification state in the width direction of the slab. In this case, during the reduction process, a slab with uniform solidification state in the width direction can be lightly reduced, thus efficiently reducing porosity.
[0020] In the first configuration of the continuous casting method, at least one of the light reduction rolls in the light reduction roll pair further satisfies the following equation (2) (second configuration). Wr ≤ Ws - 1.15 × D + 250 (2)
[0021] In the second configuration of the continuous casting method, the convex rolls of the light reduction roll pair used in the reduction process have a shape that satisfies equation (2) in addition to equation (1) in relation to the cast slab. In this case, the reaction force received by the convex rolls from the cast slab is reduced, so the reduction force required to reduce the cast slab can be further reduced.
[0022] In the first or second configuration of the continuous casting method, the continuous casting machine may further include a mold with an electromagnetic brake. In this case, the continuous casting method may further include an application step. 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 the electromagnetic brake (third configuration).
[0023] In the third configuration of the continuous casting method, a magnetic field with a magnetic flux density of 1000 Gauss or more is applied to the molten steel in the mold using an electromagnetic brake during the application process. This ensures that the thickness of the solidified shell of the cast slab is uniform in the width direction. By lightly reducing this cast slab in the thickness direction during the reduction process, porosity can be further reduced.
[0024] Embodiments of this disclosure will be described below with reference to the drawings. In each drawing, the same or equivalent components are denoted by the same reference numerals, and the same description will not be repeated.
[0025] <First Embodiment> Figure 1 is a schematic diagram of a continuous casting machine 1 used in the continuous casting method according to this embodiment. Cast slabs 10 are manufactured by the continuous casting machine 1. In this embodiment, the cast slabs 10 are slabs. In this specification, a slab means a cast slab in which the ratio of width to thickness is 3 or more.
[0026] In this embodiment, the continuous casting machine 1 is a vertical bending type. In short, the continuous casting machine 1 includes a vertical band A, a curved band B, and a horizontal band C. However, the continuous casting machine 1 may be a vertical type consisting of, for example, only the vertical band A, or a curved type consisting of the curved band B and the horizontal band C.
[0027] The continuous casting machine 1 comprises a tundish 2, a mold 4, a plurality of support rolls 5, a plurality of secondary cooling nozzles 6, and a plurality of light reduction roll pairs 7.
[0028] 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 via the immersion nozzle 3. The molten steel M in the mold 4 is cooled by the mold 4. This forms a solidified shell S. The mold 4 includes an electromagnetic brake 4a. The electromagnetic brake 4a is, for example, an electromagnet. The electromagnetic brake 4a is positioned, for example, on both outer sides of the mold 4 in the thickness direction of the slab 10. Here, the thickness direction is the direction perpendicular to the width direction and the casting direction of the slab 10.
[0029] The support rolls 5 and secondary cooling nozzles 6 are installed on the downstream side of the mold 4 in the casting direction. The support rolls 5 guide the solidifying slab 10 downstream in the casting direction. The secondary cooling nozzles 6 spray secondary cooling water onto the slab 10. The support rolls 5 and secondary cooling nozzles 6 are arranged along the casting direction of the slab 10. The secondary cooling nozzles 6 are arranged alternately with the support rolls 5 in the casting direction, for example. However, only some of the secondary cooling nozzles 6 are shown in Figure 1, and the other secondary cooling nozzles 6 are not shown. In the casting direction of the slab 10, the range in which the secondary cooling nozzles 6 are arranged is, for example, from the starting position of vertical band A to the starting position of horizontal band C. In addition to the above range, the secondary cooling nozzles 6 may also be arranged downstream from the starting position of horizontal band C.
[0030] The molten steel M cooled in the mold 4 is further cooled by the injection of secondary cooling water from the secondary cooling nozzle 6. This causes the solidified shell S to gradually thicken. The solidified slab 10 contains a solidified shell S (solid fraction of 1.0) and unsolidified molten steel M (solid fraction of less than 1.0). As the solidified slab 10 cools, the unsolidified molten steel M gradually decreases, and a fully solidified slab 10 is formed. The solidification of the slab 10 occurs at the crater end position C E Complete with P.
[0031] Multiple light reduction roll pairs 7 are arranged downstream of multiple support rolls 5 in the casting direction. Each of the light reduction roll pairs 7 lightly reduces the cast slab 10. The series of light reduction roll pairs 7 constitute a light reduction zone PB. Multiple light reduction roll pairs 7 are arranged at roughly equal intervals between the inlet PBi and the outlet PBo of the light reduction zone PB. Here, in the light reduction zone PB, the position of the inlet PBi coincides with the position of the light reduction roll pair 7 located at the upstream end in the casting direction, and the position of the outlet PBo coincides with the position of the light reduction roll pair 7 located at the downstream end in the casting direction.
[0032] The light reduction zone PB is divided into multiple segments, and each segment is provided with a reduction device (not shown) and multiple pairs of light reduction rolls 7. In each segment, the reduction device reduces the cast slab 10, for example, by hydraulic pressure. The amount of reduction by the pairs of light reduction rolls 7 is controlled for each segment (reduction device). However, the upper limit of the force that the reduction device can exert to reduce the cast slab 10 is determined for each device. If the reaction force received from the cast slab 10 exceeds the upper limit of the reduction force of the reduction device, the actual reduction speed will be less than the set reduction speed. This is because when the reaction force received from the cast slab 10 reaches the upper limit of the reduction force of the reduction device, the reduction of the cast slab 10 stops at that point, and the reduction gradient when it is actually reduced becomes less than the set reduction gradient. Here, the set reduction speed means the product of the casting speed (m / min) set in the continuous casting machine 1 and the set reduction gradient (mm / m). Furthermore, the actual reduction speed refers to the reduction speed (mm / min) when the cast slab 10 was actually reduced using the reduction device of the continuous casting machine 1.
[0033] Figure 2 is a cross-sectional view of the continuous casting machine 1 when cut in a plane perpendicular to the casting direction. Figure 2 shows the slab 10 being reduced by the light reduction roll pair 7. Referring to Figure 2, the light reduction roll pair 7 includes two light reduction rolls 71 and 72. Light reduction roll 71 is positioned vertically apart from light reduction roll 72. The vertical direction corresponds to the thickness direction of the slab 10. Light reduction roll 71 is positioned above the slab 10, and light reduction roll 72 is positioned below the slab 10.
[0034] At least one of the light-reduction rolls in the light-reduction roll pair 7 (light-reduction rolls 71, 72) is a convex roll. In this embodiment, the light-reduction roll 71 positioned on the upper side is a convex roll, and the light-reduction roll 72 positioned on the lower side is a flat roll. A convex roll is a light-reduction roll in which the diameter of the central part in the width direction is greater than the diameter of the ends, and a flat roll is a light-reduction roll in which the diameter in the width direction is substantially constant. However, the configuration of the light-reduction roll pair 7 is not limited to this. The light-reduction roll 71 positioned on the upper side may be a flat roll, and the light-reduction roll 72 positioned on the lower side may be a convex roll. Also, both light-reduction rolls 71 and 72 may be convex rolls. When both light-reduction rolls 71 and 72 are convex rolls, the effect of reducing porosity is significantly exhibited.
[0035] The convex roll, a light-reducing roll 71, includes a large-diameter section 711, two small-diameter sections 712, and two tapered sections 713. The large-diameter section 711 corresponds to the central part of the light-reducing roll 71 in the width direction. The two small-diameter sections 712 correspond to both ends of the light-reducing roll 71 in the width direction. The diameter of the large-diameter section 711 is larger than the diameters of the two small-diameter sections 712. The tapered sections 713 connect the large-diameter section 711 and the small-diameter sections 712, respectively. Two bearings 73 are positioned on the widthwise outer sides of the two small-diameter sections 712. Each of the small-diameter sections 712 is rotatably supported by the bearings 73.
[0036] The light reduction rolls 71 are typically positioned such that the widthwise center of the large-diameter section 711 coincides with the widthwise center of the cast slab 10. When the cast slab 10 is reduced using the light reduction roll pair 7, the large-diameter section 711 comes into contact with the cast slab 10, while the small-diameter section 712 and the tapered section 713 do not come into contact with the cast slab 10.
[0037] In the light-reducing roll 71 (convex roll), the difference between the diameter of the large-diameter section 711 and the diameter of the small-diameter section 712 is preferably 5 mm or more and 30 mm or less. If the difference between the diameter of the large-diameter section 711 and the diameter of the small-diameter section 712 is 5 mm or more, the porosity reduction effect is sufficiently exhibited even when the light-reducing roll 71 is nearing the end of its lifespan. Furthermore, if the difference between the diameter of the large-diameter section 711 and the diameter of the small-diameter section 712 is 30 mm or less, the diameter of the small-diameter section 712 is prevented from becoming extremely small, thereby reducing the risk of breakage of the light-reducing roll 71. The ratio of the diameter of the large-diameter section 711 to the diameter of the small-diameter section 712 is preferably 1.017 or more and 1.150 or less.
[0038] At least one of the light reduction rolls in the light reduction roll pair 7 (light reduction rolls 71, 72) satisfies the above formula (1), where Wr (mm) is the width of the large diameter section 711, Ws (mm) is the width of the cast slab 10, and D (mm) is the thickness of the cast slab 10. In the example of this embodiment, light reduction roll 71 satisfies the above formula (1). In short, the width Wr of the large diameter section 711 is between (Ws - 1.15 × D) and (Ws - 50) in relation to the width Ws and thickness D of the cast slab 10. If the width Wr of the large diameter section 711 is less than (Ws - 1.15 × D) in relation to the width Ws and thickness D of the cast slab 10, the reduction near both ends of the cast slab 10 will be insufficient during light reduction, and porosity cannot be sufficiently reduced at both ends of the cast slab 10. Furthermore, if the width Wr of the large-diameter section 711 is greater than (Ws-50) in relation to the width Ws of the slab 10, the light reduction rolls 71 will reduce a wider area of the slab 10, which is at a relatively low temperature. As a result, the reaction force received from the slab 10 will increase, and there is a risk that it will exceed the upper limit of the reduction force of the reduction device. As described above, if the reaction force received from the slab 10 exceeds the upper limit of the reduction force, the actual reduction speed will be lower than the set reduction speed. In this case, the amount of reduction by the light reduction rolls 7 will be insufficient for the solidification shrinkage of the slab 10, making it difficult to reduce porosity. Therefore, the width Wr of the large-diameter section 711 of the light reduction rolls 711 used in the continuous casting method according to this embodiment satisfies the above equation (1). For example, when the width Ws of the slab 10 is 2300 mm, the area represented by the above equation (1) is the shaded area in Figure 3. Figure 3 is a schematic diagram showing the relationship between the width Wr of the large-diameter section 711 and the thickness D of the slab 10.
[0039] The light reduction roll 71 preferably satisfies equation (2) in addition to equation (1) above. In short, the width Wr of the large diameter section 711 is less than or equal to (Ws - 1.15 × D + 250) in relation to the width Ws of the cast slab 10 and the thickness D of the cast slab 10. In this case, the reaction force that the light reduction roll 71 receives from the cast slab 10 can be further reduced. For example, when the width Ws of the cast slab 10 is 2300 mm, the region represented by equations (1) and (2) above is the shaded region in Figure 4. Figure 4 is a schematic diagram showing the relationship between the width Wr of the large diameter section 711 and the thickness D of the cast slab 10.
[0040] The diameter of the light reduction roll 72 is substantially constant in the width direction. When the cast slab 10 is reduced using the pair of light reduction rolls 7, the entire width of the light reduction roll 72 is in contact with the cast slab 10. Two bearings 74 are positioned on the outer side of the light reduction roll 72 in the width direction. The light reduction roll 72 is rotatably supported by the bearings 74.
[0041] [Continuous casting method] Figure 5 is a flowchart showing the continuous casting method for steel according to this embodiment. As shown in Figure 5, the continuous casting method for steel according to this embodiment comprises an application step (#5), a cooling step (#10), and a reduction step (#15). The cast slab 10 obtained by this continuous casting method becomes the material for products such as steel plates. Hereinafter, each step shown in Figure 5 will be specifically described with reference to Figures 1 and 2.
[0042] [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. The molten steel M discharged from the discharge hole of the immersion nozzle 3 in the mold 4 is subjected to a braking force in the opposite direction to the flow direction due to the action of the magnetic field. As a result, the flow of the molten steel M discharged from the discharge hole toward the widthwise end of the solidified shell S is weakened. In addition, the electromagnetic brake 4a can promote the floating of inclusions and other particles in the molten steel M, and also plays a role in separating inclusions and other particles from the molten steel M.
[0043] 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. If a magnetic field smaller than 1000 Gauss is applied to the molten steel M, the braking force acting on the molten steel M is insufficient, and the slab 10 tends to become hot near the widthwise end of the slab 10. In this case, the completion of solidification tends to be delayed, and porosity tends to accumulate and coarseen in that area.
[0044] On the other hand, if an excessive magnetic field is applied to the molten steel M, the temperature of the molten steel M near the immersion nozzle 3 rises, and the initial solidification of the molten steel M in the mold 4 becomes uneven. When the initial solidification of the molten steel M becomes uneven, there is a risk of longitudinal cracks occurring on the surface of the cast slab 10 when continuously casting steel that is prone to surface cracking (for example, subpericrystalline steel). Therefore, it is preferable that the magnitude of the magnetic flux density of the magnetic field applied in the application step (#5) be 3000 Gauss or less. In this case, longitudinal cracks on the surface of the cast slab 10 can be suppressed.
[0045] [Cooling process (#10)] In a typical continuous casting method, the thickness of the solidified shell S in the width direction of the cast slab 10 during solidification is uneven. Regions in the cast slab 10 where the thickness of the solidified shell S is relatively small in a cross-sectional view perpendicular to the casting direction are regions where solidification has been delayed. In regions where solidification has been delayed, porosity tends to accumulate, making it difficult to improve internal quality. The difference in solidification state in the width direction of the cast slab 10 becomes more pronounced as the amount of secondary cooling water injected from the secondary cooling nozzle 6 increases.
[0046] Therefore, in the cooling step (#10) of the continuous casting method of this embodiment, the amount of secondary cooling water is appropriately suppressed. Specifically, in the cooling step (#10), the meniscus (molten metal surface in the mold 4) is cooled to the meniscus and crater end position C. E In the range from P to the intermediate position MP, secondary cooling water is injected from the secondary cooling nozzle 6 so that the average specific water content is between 0.1 L / kg-steel and 0.6 L / kg-steel. Meniscus and crater end position C EThe intermediate position MP with respect to P is such that the distance in the casting direction from the meniscus is the crater end position C E and is at a position that is half of P. The intermediate position MP is, for example, the position where the curved zone B ends and the slab 10 becomes horizontal. In this case, in the cooling step (#10), the amount of secondary cooling water from the start position of the vertical zone A to the end position of the curved zone B (start position of the horizontal zone C) is controlled. Also, the intermediate position MP may be in the middle of the curved zone B as shown in FIG. 1. From another perspective, the distance of the intermediate position MP from the meniscus may be 10 m or more and 20 m or less.
[0047] The average specific water amount is the amount of secondary cooling water (L / min) injected per unit time from all the secondary cooling nozzles 6 in a certain range along the casting direction, divided by the amount of steel passing through per unit time (kg / min). When the thickness of the slab 10 is h1 (mm), the width is h2 (mm), the casting speed is v (mm / min), and the density of the steel is ρ (kg / mm 3 ), the amount of steel Q passing through per unit time can be expressed as Q = h1 × h2 × v × ρ. The density ρ of the steel is 7.8×10 -6 (kg / mm 3 ). <00002When the average specific water content of the secondary cooling water is greater than 0.6 L / kg-steel, the difference in solidification state becomes significant in the width direction of the cast slab 10, and the position where solidification is completed (crater end position C) becomes significant. E P) becomes non-uniform. Crater end position C E If P is non-uniform in the width direction of the cast slab 10, the cast slab 10 will be lightly reduced in the reduction process (#15) described later, with a mixture of unsolidified and fully solidified portions. When the cast slab 10 containing fully solidified portions is reduced, the reaction force becomes large, and the load capacity of the light reduction roll pair 7 is insufficient. In this case, there is a high possibility that the cast slab 10 cannot be sufficiently lightly reduced. Also, crater end position C E When a cast slab 10 with uneven P is lightly reduced, the reduction is inhibited by the fully solidified portion. As a result, it becomes difficult to reduce the material sufficiently to suppress porosity in the unsolidified portion where solidification is delayed, leading to a deterioration of the internal quality of the product. Therefore, in the cooling process (#10), the average specific water content of the secondary cooling water in the range from the meniscus to the intermediate position MP is controlled to 0.6 L / kg-steel or less.
[0050] However, if the average specific water content of the secondary cooling water is excessively low, the thermal load on the equipment will increase, and the likelihood of equipment trouble will rise. Therefore, in the cooling process (#10), the average specific water content of the secondary cooling water in the range from the meniscus to the intermediate position MP is controlled to 0.1 L / kg-steel or higher.
[0051] Cooling by the secondary cooling nozzle 6 extends from the meniscus to the intermediate position MP, as well as from the intermediate position MP to the crater end position C. E This may also be performed on the cast slab 10 up to P. In addition, the crater end position C in the casting direction may be performed. E This procedure may also be performed on the cast slab 10 downstream of P. However, from the viewpoint of heat removal efficiency, it is preferable that the average specific water content of the secondary cooling water downstream of the intermediate position MP in the casting direction is smaller than the average specific water content upstream of the intermediate position MP.
[0052] [Reduction process (#15)] In the reduction process (#15), the cast slab 10 is lightly reduced in the thickness direction using multiple pairs of light reduction rolls 7 in the light reduction zone PB. Light reduction is performed from when the central solid fraction of the cast slab 10 reaches 0.3 until it reaches 1.0. That is, at the inlet PBi of the light reduction zone PB, the central solid fraction of the cast slab 10 is 0.3, and at the outlet PBo of the light reduction zone PB, the central solid fraction of the cast slab 10 is 1.0. The central solid fraction of the cast slab 10 is the solid fraction at the center of its thickness. The central solid fraction is 0 when the temperature at the center of the thickness is the liquidus temperature (the temperature at which solidification begins), and the central solid fraction is 1.0 when the temperature at the center of the thickness is the solidus temperature (the temperature at which solidification is completed).
[0053] If light reduction is started after the central solid fraction of the cast slab 10 reaches 0.3, it is difficult to sufficiently reduce porosity. Similarly, if light reduction is stopped before the central solid fraction of the cast slab 10 reaches 1.0, it is difficult to sufficiently reduce porosity.
[0054] Light reduction of the central solid fraction region of the cast slab 10, i.e., the region where the center of the cast slab 10's thickness is completely solidified, has little effect on reducing porosity, so it is not necessary to lightly reduce this region. Light reduction of the central solid fraction region of the cast slab 10 that is less than 0.3 also does not affect the reduction of porosity formed at the end of solidification, so it is not necessarily necessary to lightly reduce this region. However, light reduction may be performed on the central solid fraction region of the cast slab 10 that is less than 0.3.
[0055] In the reduction process (#15), if the reduction speed in the light reduction zone PB is less than 0.5 mm / min, the reduction amount will be insufficient to adequately reduce porosity due to the solidification shrinkage of the cast slab 10. Conversely, if the reduction speed is greater than 3.0 mm / min, the reduction amount will be excessive for the solidification shrinkage of the cast slab 10, potentially causing internal cracks at the solidification interface and negatively impacting product quality. Therefore, the reduction speed of the cast slab 10 in the reduction process (#15) should be between 0.5 mm / min and 3.0 mm / min.
[0056] [effect] In the continuous casting method according to this embodiment, in the reduction process (#15), the cast slab 10 is lightly reduced in the thickness direction using a plurality of light reduction roll pairs 7. Of these light reduction roll pairs 7, the light reduction roll 71 is a convex roll. The convex light reduction roll 71 has a shape that satisfies the above equation (1) in relation to the cast slab 10. In short, the width Wr of the large diameter portion 711 of the light reduction roll 71 is appropriately set in relation to the thickness D of the cast slab 10. In light reduction using light reduction roll pairs 7 including the light reduction roll 71, the range in which the cast slab 10 is reduced in the width direction is optimized, and the ends of the cast slab 10, which are at a relatively low temperature, are not reduced as much. Therefore, the reduction force required to reduce the cast slab 10 is suppressed, and porosity can be reduced without increasing the size of the equipment or increasing the rigidity of the light reduction rolls 71.
[0057] In the continuous casting method according to this embodiment, the average specific water content of the secondary cooling water in the range from the meniscus to the intermediate position MP during the cooling step (#10) is 0.6 L / kg-steel or less. This appropriately suppresses the amount of secondary cooling water injected into the slab 10, reducing the difference in solidification state in the width direction of the slab 10. In this case, during the reduction step (#15), the slab 10, with its uniform solidification state in the width direction, can be lightly reduced, thus efficiently reducing porosity.
[0058] In the continuous casting method according to this embodiment, in the application step (#5), a magnetic field with a magnetic flux density of 1000 Gauss or more is applied to the molten steel M in the mold 4 by an electromagnetic brake 4a. As a result, the thickness of the solidified shell S of the cast slab 10 becomes uniform in the width direction. By lightly reducing this cast slab 10 in the reduction step (#15), porosity can be further reduced.
[0059] In the continuous casting method of this embodiment, multiple light reduction roll pairs 7 are used in the reduction process (#15). In this case, compared to the case where only one light reduction roll pair 7 is used to reduce the cast slab 10, a tapered slope can be created in the cast slab 10, so the cast slab 10 can be gradually reduced in response to solidification shrinkage.
[0060] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.
[0061] The shape of the light-pressure roll 71 (convex roll) is not limited to the example of the above embodiment. For example, in the example of the above embodiment, the large diameter portion 711 and the two small diameter portions 712 are connected by a tapered portion 713. However, the large diameter portion 711 and each of the small diameter portions 712 may be directly connected without going through the tapered portion 713. Also, in the example of the above embodiment, the large diameter portion 711 is provided continuously in the width direction. However, the large diameter portion 711 may be divided into multiple parts in the width direction. In this case, for example, bearings may be placed between the divided large diameter portions 711. [Examples]
[0062] To confirm the effectiveness of the continuous casting method according to the embodiment, the following tests were conducted and the results were evaluated. Specifically, multiple cast slabs were manufactured by continuous casting, and the porosity volume of each cast slab was evaluated. The explanation of porosity volume will be described later.
[0063] In this test, cast slabs were manufactured using the continuous casting machine shown in Figure 1. The mold was a water-cooled copper mold. The length of the mold was 800 mm. The cross-section of the mold was rectangular. The light reduction zone was established from a distance of 16 m from the meniscus in the casting direction. The cast slab was lightly reduced in the thickness direction by a pair of light reduction rolls from when the central solid fraction of the cast slab reached 0.3 until the central solid fraction reached 1.0. In the region after the central solid fraction of the cast slab reached 1.0, light reduction was not performed from the perspective of protecting the equipment. The temperature and solid fraction at the center of the thickness of the cast slab were calculated by two-dimensional solidification analysis in the thickness and width directions of the cast slab. The upper limit of the reduction force of the reduction device was 600 tons, taking into account the rigidity of the segments and light reduction rolls.
[0064] In the light reduction zone, the light reduction roll pair consisted of a convex roll only on the upper side, and a flat roll on the lower side. The diameter of the large-diameter section of the convex roll was 15 mm larger than the diameter of the small-diameter section.
[0065] The main chemical composition of the molten steel used in this test was C: 0.15%, Si: 0.19%, Mn: 0.90%, P: 0.011%, and S: 0.003%.
[0066] The porosity volume of the cast slab was investigated using the following procedure. Samples were taken from the center of the cast slab in the thickness direction at 50 mm in the casting direction, 100 mm in the width direction, and 7 mm in the thickness direction. Samples were taken from 16 locations along the width direction of the cast slab. For each sample, the density ρ was measured using the method for determining the density and specific gravity of solids specified in JIS Z 8807:2012. In addition, a sample was taken from the 1 / 4 thickness portion of the cast slab using the same procedure, and the density ρ0 was measured. Then, the porosity volume V (cm³) per unit weight was calculated using the following formula (3). 3 The porosity volume V was calculated ( / g). The highest porosity volume V value among those measured for each sample was taken as the porosity volume V of that slab. V = 1 / ρ - 1 / ρ0 (3)
[0067] The test conditions and test results for this embodiment are shown in Table 1. Table 1 shows the test conditions for the thickness D of the cast slab, the width Ws of the cast slab, the width Wr of the large-diameter section of the light reduction roll, the average specific water content, the magnetic flux density of the electromagnetic brake's magnetic field, and the reduction speed. 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. The average specific water content is the average specific water content of the secondary cooling water injected from the secondary cooling nozzle in the range from the meniscus to the intermediate position MP.
[0068] In Table 1, cast slabs manufactured under test conditions that satisfy condition 1 are labeled as "Example Inventions," and cast slabs manufactured under test conditions that do not satisfy condition 1 are labeled as "Comparative Examples." In Comparative Examples 1 to 3 shown in Table 1, test conditions that do not satisfy condition 1 are marked with an asterisk (*). In addition, in Example Inventions 1 to 11 shown in Table 1, test conditions that do not satisfy condition 2 are marked with a special symbol (**), and test conditions that do not satisfy condition 3 are marked with a hashtag (#). Condition 1: The width of the large-diameter section of the light-reducing roll (convex roll) satisfies the above formula (1), and the average specific water content is 0.1 L / kg-steel or more and 0.6 L / kg-steel or less. Condition 2: The width of the large-diameter section of the light-reducing roll (convex roll) satisfies equation (2) in addition to equation (1) above. Condition 3: The magnetic flux density is 1000 Gauss or more.
[0069] [Table 1]
[0070] Table 1 shows the porosity index of the cast slabs produced under each condition. The porosity index is the ratio of the porosity volume of the cast slabs produced under each condition to the porosity volume of the cast slab in Comparative Example 1. It is known that if the porosity index is 0.70 or less, defects are less likely to occur in the product (steel sheet) after the cast slab is rolled. Therefore, in the test results (evaluation) in Table 1, "Acceptable" is indicated if the porosity index is 0.70 or less, and "Unacceptable" otherwise.
[0071] As shown in Table 1, all of the invention examples 1 to 9 satisfied conditions 1 to 3. Therefore, in the invention examples 1 to 9, cast slabs with good porosity volume were obtained. In other words, porosity was reduced.
[0072] Although Invention Example 10 met conditions 1 and 2, its magnetic flux density was less than 1000 Gauss, and therefore it did not meet condition 3. Compared to Invention Example 1, which had the same test conditions except for magnetic flux density, Invention Example 10 yielded a good cast slab, although its porosity index was slightly higher.
[0073] Although Invention Example 11 satisfied conditions 1 and 3, the width Wr of the large-diameter section was relatively large, and therefore condition 2 was not satisfied. As a result, the reaction force from the cast slab during light reduction was slightly larger, and the reduction speed was reduced to 0.6 mm / min. Although the porosity index of Invention Example 11 was slightly higher compared to Invention Examples 1-9, a good cast slab was obtained.
[0074] Comparative Example 1 had a relatively high average specific water content and did not meet Condition 1. As a result, the difference in solidification state in the width direction of the cast slab was large, and the crater end position became non-uniform in the width direction of the cast slab. Consequently, the porosity became coarser in the areas of the cast slab where solidification was delayed, and the porosity index was greater than the threshold, resulting in an evaluation of "unacceptable".
[0075] In Comparative Example 2, the width Wr of the large-diameter section was relatively large, and condition 1 was not met. Naturally, Comparative Example 2 also did not meet condition 2. As a result, the reaction force from the cast slab during light reduction was excessive, and the reduction speed was insufficient. Therefore, the porosity index was greater than the threshold, and the evaluation was "unacceptable".
[0076] In Comparative Example 3, the width Wr of the large-diameter section was relatively small, and condition 1 was not met. As a result, porosity near both ends in the width direction of the cast slab could not be sufficiently reduced. Therefore, the porosity index was greater than the threshold, and the evaluation was "unacceptable". [Explanation of Symbols]
[0077] 1: Continuous casting machine 4: Mold 4a: Electromagnetic brake 6: Secondary cooling nozzle 7: Light pressure rolls 10: Cast slab C E P: Crater end location MP: intermediate position
Claims
1. A method for continuous casting of steel using a continuous casting machine, The continuous casting machine comprises a secondary cooling nozzle for spraying secondary cooling water onto the slab, and a plurality of light-reduction roll pairs arranged in the casting direction of the slab. At least one of the light reduction rolls in the pair has a diameter in the center in the width direction that is greater than the diameters at both ends, and when the width of the center is Wr (mm), the width of the cast slab is Ws (mm), and the thickness of the cast slab is D (mm), it satisfies the following equation (1): The aforementioned continuous casting method is A cooling step in which secondary cooling water is injected from the secondary cooling nozzle in a range from the meniscus to an intermediate position between the meniscus and the crater end position such that the average specific water content is 0.1 L / kg-steel or more and 0.6 L / kg-steel or less, A continuous casting method comprising: a reduction step of lightly reducing the cast slab in the thickness direction using the plurality of light reduction roll pairs at a reduction speed of 0.5 mm / min or more and 3.0 mm / min or less, from when the central solid fraction of the cast slab reaches 0.3 until the central solid fraction reaches 1.
0. Ws-1.15×D≦Wr≦Ws-50 (1)
2. A continuous casting method according to claim 1, A continuous casting method wherein at least one of the light reduction rolls in the pair of light reduction rolls further satisfies the following formula (2). Wr ≤ Ws - 1.15 × D + 250 (2)
3. A continuous casting method according to claim 1 or 2, The continuous casting machine further includes a mold that includes an electromagnetic brake, The continuous casting method further comprises an application step of applying a magnetic field with a magnetic flux density of 1000 Gauss or more to the molten steel in the mold using the electromagnetic brake.