Steel continuous casting method

Abstract

To provide a steel continuous casting method capable of suppressing surface cracking of a cast piece while reducing center segregation and porosity.SOLUTION: A continuous casting method uses a continuous casting machine (1). The continuous casting method comprises: an application step (#5); a cooling step (#10); and a depression step (#15). In the application step (#5), a magnetic field having a magnetic flux density of 1000 Gauss or higher is applied to molten steel (M). In the cooling step (#10), within a range from a meniscus to a middle position (MP) between the meniscus and a crater-end position (CEP), secondary cooling water is injected from a secondary cooling nozzle (6) so that an average specific water quantity can be 0.10 L / kg-steel to 0.49 L / kg-steel. In the depression step (#15), a cast piece (10) is lightly depressed at a depressing speed of 0.5 mm / min., to 1.3 mm / min., by a light depression roll (7) until the center solid phase rate of the cast piece (10) reaches 1.0 after reaching 0.3.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a method for continuous casting of steel. [Background technology]

[0002] In the continuous casting of steel, defects such as center segregation and porosity occur in 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, requirements for the internal quality of the slab have increased significantly. Therefore, it is necessary to further reduce center segregation and porosity in the center of the slab in the thickness direction. To improve the internal quality caused by these defects, the slab is usually soft-reduced in the thickness direction within the continuous casting machine.

[0003] When the thickness of the solidified shell of a slab is non-uniform across the width during solidification, there are regions where the completion of solidification is relatively delayed. In particular, center segregation and porosity tend to increase in these regions where the completion of solidification is delayed.

[0004] In continuous casting, a submerged entry nozzle having two to four discharge holes is typically used, and high-temperature molten steel is supplied into the mold from the discharge holes. This high-temperature molten steel is discharged toward the widthwise ends of the solidified shell. As a result, the temperatures near the widthwise ends of the slab tend to become high, which can delay the completion of solidification.

[0005] The continuous casting machine also includes 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 two to four roll bodies in the width direction of the slab to distribute the load due to the static pressure or reduction of the molten steel. The roll bodies are connected to each other by bearings. The secondary cooling water sprayed from the nozzles toward the slab flows down the surface of the slab downstream in the casting direction and accumulates at the contact points between the roll bodies and the slab. The secondary cooling water that accumulates at the contact points is discharged from the gap between the bearings of the support rolls and the slab, and also from the widthwise ends of the slab. The contact points of the slab with the support rolls are cooled by the secondary cooling water that accumulates at the contact points and the support rolls that are in contact with the slab. On the other hand, secondary cooling water does not accumulate in the areas of the slab corresponding to the bearings of the support rolls, and these areas are not in contact with the support rolls. Therefore, the area of ​​the slab corresponding to the bearing portion of the support roll is more difficult to cool than the area in contact with the support roll, and the completion of solidification is likely to be delayed.

[0006] As explained above, due to the flow of molten steel in the mold and non-uniform secondary cooling of the slab, there are regions in the slab during solidification where the completion of solidification is likely to be delayed, and the thickness of the solidified shell becomes non-uniform across the width of the slab. In such cases, even if the slab is lightly reduced in the thickness direction downstream in the casting direction, center segregation and porosity are not reduced in the regions where the completion of solidification is delayed, making it difficult to improve internal quality. Therefore, it is necessary to make the thickness of the solidified shell uniform across the width.

[0007] In response to such demands, for example, Patent Documents 1 and 2 disclose techniques for making the thickness of the solidified shell uniform by controlling the amount of secondary cooling water supplied to the slab across the width. Patent Document 1 discloses a technique in which a solidification profile (progression in the thickness of the solidified shell) of the slab across the width is determined in advance, and the amount of secondary cooling water in the slab across the width is controlled based on this profile. Patent Document 2 discloses a technique in which a device capable of detecting the position at which the slab has completed solidification online is installed in a continuous casting machine, and the amount of secondary cooling water in the slab across the width is controlled based on this information.

[0008] Furthermore, Patent Document 3 describes a continuous casting method using a vertical bending type continuous casting machine. In the continuous casting method described in Patent Document 3, the surface temperatures of the width center and corner portions of the slab in each of the bending section (curved section) and the straightening section (horizontal section) are controlled. Patent Document 3 describes that this suppresses surface cracking of the slab. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent No. 6561822 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-238256 [Patent Document 3] Japanese Patent Application Laid-Open No. 2008-194746 Summary of the Invention [Problem to be solved by the invention]

[0010] In recent continuous casting processes, there has been a trend toward adding more alloying elements to molten steel than ever before in order to improve product performance. The higher the alloying elements, the more likely cracks are to occur on the slab surface. When cracks occur on the slab surface, the cost of surface maintenance increases and the yield decreases. Therefore, there is a need to not only reduce center segregation and porosity, but also to suppress surface cracks on the slab.

[0011] The technology of Patent Document 1 is premised on the formation of negative segregation in the thickness center. To form negative segregation in the thickness center, it is necessary to heavily reduce the slab during solidification using a large pre-solidification reduction method. In the large pre-solidification reduction method, a large reduction greater than the amount of solidification shrinkage is applied to the slab during solidification, forcibly discharging the concentrated molten steel. This large pre-solidification reduction method is a different technology from the soft reduction method. In the soft reduction method, the slab during solidification is reduced by an amount equivalent to the amount of solidification shrinkage, thereby alleviating positive segregation in the thickness center. Furthermore, the technology of Patent Document 2 requires an advanced device capable of detecting the completion of solidification of the slab online, which requires high equipment development, installation, and maintenance costs, making it disadvantageous in terms of cost-effectiveness.

[0012] Furthermore, Patent Documents 1 and 2 do not give any consideration to suppressing surface cracks in the slab. If an attempt is made to make the thickness of the solidified shell uniform in the width direction of the slab only by controlling the amount of secondary cooling water across the width direction of the slab, as described in Patent Documents 1 and 2, the amount of secondary cooling water sprayed onto areas of the slab where completion of solidification is delayed increases. This results in a decrease in the surface temperature of the width direction ends of the slab, which could lead to the occurrence of cracks on the surface of the slab.

[0013] As described above, Patent Document 3 describes that surface cracks of a slab are suppressed by controlling the surface temperature of the slab. However, Patent Document 3 does not take into consideration the central segregation and porosity that occur in the center of the slab in the thickness direction.

[0014] An object of the present disclosure is to provide a method for continuous casting of steel that can suppress surface cracks in a cast slab while reducing center segregation and porosity. [Means for solving the problem]

[0015] The continuous casting method for steel according to the present disclosure uses a continuous casting machine. The continuous casting machine includes a mold, secondary cooling nozzles, support rolls, and light reduction rolls. The mold includes an electromagnetic brake. The secondary cooling nozzles are installed along the casting direction of the slab and spray secondary cooling water onto the slab. The support rolls are installed downstream of the mold in the casting direction. The light reduction rolls are installed downstream of the support rolls in the casting direction. The continuous casting method includes an applying step, a cooling step, and a reducing step. In the applying 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. In the cooling step, secondary cooling water is sprayed from the secondary cooling nozzles so that the average specific water amount is 0.10 L / kg-steel or more and 0.49 L / kg-steel or less in the range from the meniscus to a position midway between the meniscus and the crater end position. In the reduction process, the slab is lightly reduced in the thickness direction perpendicular to the width direction of the slab using light reduction rolls at a reduction rate of 0.5 mm / min to 1.3 mm / min from the time when the central solid fraction of the slab reaches 0.3 until the central solid fraction reaches 1.0. [Effects of the Invention]

[0016] According to the method for continuous casting steel according to the present disclosure, it is possible to suppress surface cracks in a slab while reducing center segregation and porosity. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram of a continuous casting machine used in a continuous casting method according to an embodiment. [Figure 2] FIG. 2 is a cross-sectional view of the continuous casting machine cut perpendicular to the casting direction. [Figure 3] FIG. 3 is a flow diagram showing a method for continuously casting steel according to an embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0018] A continuous casting method for steel according to an embodiment uses a continuous casting machine. The continuous casting machine includes a mold, secondary cooling nozzles, support rolls, and soft reduction rolls. The mold includes an electromagnetic brake. The secondary cooling nozzles are installed along the casting direction of the slab and spray secondary cooling water onto the slab. The support rolls are installed downstream of the mold in the casting direction. The soft reduction rolls are installed downstream of the support rolls in the casting direction. The continuous casting method includes an applying step, a cooling step, and a reducing step. In the applying 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. In the cooling step, secondary cooling water is sprayed from the secondary cooling nozzles so that the average specific water amount is 0.10 L / kg-steel or more and 0.49 L / kg-steel or less in the range from the meniscus to a position midway between the meniscus and the crater end position. In the reduction process, the slab is lightly reduced in the thickness direction perpendicular to the width direction of the slab with a light reduction roll at a reduction rate of 0.5 mm / min or more and 1.3 mm / min or less from the time when the central solid fraction of the slab reaches 0.3 until the central solid fraction reaches 1.0 (first configuration).

[0019] In the continuous casting method according to the first aspect, 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 an electromagnetic brake. This weakens the flow of molten steel being discharged from the discharge hole of the submerged entry nozzle toward the widthwise ends of the solidified shell in the mold. Furthermore, in the continuous casting method according to the first aspect, in the cooling step, the average specific water flow rate of the secondary cooling water in the range from the meniscus to a position midway between the meniscus and the crater end position is 0.49 L / kg-steel or less. This appropriately suppresses the amount of secondary cooling water sprayed onto the slab, preventing cooling of the slab due to local accumulation of secondary cooling water between the slab and the support rolls. This reduces the difference in solidification rate between the region of the slab where solidification completion is likely to be delayed and the other regions. In this way, by controlling both the flow of molten steel in the mold and the average specific water flow rate of the secondary cooling water, the thickness of the solidified shell becomes uniform in the widthwise direction of the slab. By lightly reducing the thickness of the cast slab in the reduction step, center segregation and porosity can be reduced.

[0020] In the continuous casting method according to the first aspect, as described above, the amount of secondary cooling water injected onto the slab is appropriately reduced. This prevents the surface temperature of the widthwise ends of the slab from decreasing excessively. This in turn prevents cracks from occurring on the surface of the slab.

[0021] In the continuous casting method of the first configuration, at the start of the reduction process, the ratio of the average specific water amount in the range from the meniscus to the intermediate position to the average specific water amount in the range beyond the intermediate position may be 10% or more and 70% or less (second configuration). Generally, the greater the difference between the surface temperature and the internal temperature of the slab at the start of soft reduction, the more likely strain will reach the center of the slab when the slab is reduced with the soft reduction rolls. Therefore, in the continuous casting method of the second configuration, the ratio of the average specific water amount in the range from the meniscus to the intermediate position to the average specific water amount in the range beyond the intermediate position to the average specific water amount in the range from the meniscus to the intermediate position is 10% or more. This allows the surface temperature of the slab at the start of soft reduction to be lowered, further reducing centerline segregation. The surface temperature of the slab refers to the surface temperature at the center of the slab in the width direction. Hereinafter, unless otherwise specified, the surface temperature of the slab refers to the surface temperature at the center of the slab in the width direction.

[0022] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In each drawing, the same or equivalent components are designated by the same reference numerals, and the same description will not be repeated.

[0023] [Continuous casting machine] FIG. 1 is a schematic diagram of a continuous casting machine 1 used in the continuous casting method according to this embodiment. The continuous casting machine 1 produces a cast slab 10. The continuous casting machine 1 includes a tundish 2, a mold 4, a plurality of support rolls 5, and a plurality of soft reduction rolls 7. In this embodiment, the continuous casting machine 1 is a vertical bending type. In short, the continuous casting machine 1 includes a vertical strip A, a curved portion B, and a horizontal strip C. However, the continuous casting machine 1 may also be a vertical type consisting of, for example, only a vertical strip.

[0024] 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 an immersion 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 brakes 4a are disposed on both outer sides of the mold 4 in the thickness direction of the slab 10. Here, the thickness direction is a direction perpendicular to the width direction of the slab 10 and the casting direction.

[0025] A plurality of secondary cooling nozzles (not shown in FIG. 1 ) and a plurality of support rolls 5 are arranged downstream in the casting direction of the mold 4. After being cooled in the mold 4, the molten steel M is further cooled by being sprayed with secondary cooling water from the secondary cooling nozzles. This forms a solidified shell S. The slab 10 in the process of solidifying here contains 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 and containing the unsolidified molten steel M is guided downstream in the casting direction by the plurality of support rolls 5. During this process, the unsolidified molten steel M gradually decreases, and a completely solidified slab 10 is formed. The solidification of the slab 10 begins at a crater end position C. E The secondary cooling nozzles are arranged, for example, alternately with a plurality of support rolls 5 in the casting direction.

[0026] Fig. 2 is a cross-sectional view of the continuous casting machine 1 cut perpendicular to the casting direction. Fig. 2 shows the state below the mold 4. Referring to Fig. 2, the support roll 5 is divided into multiple roll bodies 5a in the width direction of the slab 10. The roll bodies 5a are connected to each other by bearings 5b. The bearings 5b rotatably support the roll bodies 5a. The dimension of the bearings 5b in the width direction is, for example, 150 to 250 mm.

[0027] The slab 10 comes into contact with the roll body 5a of the support roll 5 and is guided downstream in the casting direction by the roll body 5a. The region of the slab 10 in the width direction that comes into contact with the support roll 5 (roll body 5a) is referred to as the contact portion 10a. The bearing portion 5b does not come into contact with the slab 10 and is disposed at a distance from the slab 10 in the thickness direction of the slab 10. The region of the slab 10 in the width direction that does not come into contact with the support roll 5 is referred to as the non-contact portion 10b. Here, the contact portion 10a is the region of the slab 10 that corresponds to the roll body 5a, and the non-contact portion 10b is the region of the slab 10 that corresponds to the bearing portion 5b.

[0028] The multiple secondary cooling nozzles 6 are arranged along the casting direction of the slab 10. The multiple secondary cooling nozzles 6 are arranged at intervals in the width direction of the slab 10. In a typical example, the multiple secondary cooling nozzles 6 are arranged between adjacent support rolls 5 in the casting direction of the slab 10. The amount of secondary cooling water sprayed from the multiple secondary cooling nozzles 6 is individually controlled. In the casting direction of the slab 10, the range in which the multiple secondary cooling nozzles 6 are arranged is, for example, from the start position of the vertical band A to the end position of the curved portion B (the start position of the horizontal band C). In addition to the above range, the multiple secondary cooling nozzles 6 may also be arranged downstream of the start position of the horizontal band C.

[0029] A plurality of light reduction rolls 7 are arranged downstream in the casting direction of the plurality of support rolls 5. Each light reduction roll 7 forms a pair and softly reduces the slab 10. More specifically, a series of light reduction rolls 7 form a light reduction zone 71. The light reduction zone 71 is divided into a plurality of segments, and a plurality of light reduction rolls 7 are provided in each segment. The reduction amount of the light reduction rolls 7 is controlled for each segment. The plurality of light reduction rolls 7 are arranged at approximately equal intervals between the entrance 71i and the exit 71o of the light reduction zone 71. Here, in the light reduction zone 71, the position of the entrance 71i coincides with the position of the light reduction roll 7 arranged at the most upstream position in the casting direction, and the position of the exit 71o coincides with the position of the light reduction roll 7 arranged at the most downstream position in the casting direction. In this embodiment, the position of the entrance 71i coincides with the start position of the horizontal band C. However, the position of the entrance 71i may be downstream in the casting direction of the slab 10 from the start position of the horizontal band C.

[0030] [Continuous casting method] Fig. 3 is a flow diagram showing a 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), a cooling step (#10), and a reduction step (#15). A slab 10 obtained by this continuous casting method becomes a raw material for products such as steel plate. Each step shown in Fig. 3 will be described in detail below.

[0031] [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 entry nozzle 3 in the mold 4 is subjected to a braking force in the direction opposite to the flow direction due to the action of the magnetic field. This weakens the flow of the molten steel M discharged from the discharge hole toward the width direction end of the solidified shell S. In addition, the electromagnetic brake 4a can promote the floating of inclusions, etc. in the molten steel M, and also plays a role in separating the inclusions, etc. from the molten steel M.

[0032] 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 width direction ends of the slab 10 becomes high, which tends to delay the completion of solidification, thereby deteriorating the internal quality of the slab 10.

[0033] 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, the magnetic flux density is preferably 3000 Gauss or less. This is because if a magnetic field of greater than 3000 Gauss is applied to the molten steel M, the temperature of the molten steel M in the vicinity of the submerged entry nozzle 3 will rise, causing the initial solidification of the molten steel M in the mold 4 to become non-uniform. If the initial solidification of the molten steel M becomes non-uniform, longitudinal cracks may occur on the surface of the slab 10 when continuously casting steel that is prone to surface cracks (for example, hypoperitectic steel).

[0034] [Cooling process (#10)] In a typical continuous casting method, the thickness of the solidified shell S of a slab 10 during solidification is nonuniform across the width. Regions of the 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 the completion of solidification is likely to be delayed. Generally, the non-contact portion 10b of the slab 10, which is not in contact with the support roll 5, is more difficult to cool than the contact portion 10a, which is in contact with the support roll 5, and the completion of solidification is likely to be delayed. This is because the contact portion 10a is cooled by the secondary cooling water accumulated between the contact portion 10a and the support roll 5, whereas the secondary cooling water does not accumulate in the non-contact portion 10b. The difference in the solidification state between the contact portion 10a and the non-contact portion 10b becomes more pronounced as the amount of secondary cooling water sprayed from the secondary cooling nozzle 6 increases. Therefore, in the cooling step (#10) of the continuous casting method of this embodiment, the amount of secondary cooling water is appropriately suppressed.

[0035] Specifically, in the cooling process (#10), the meniscus (the molten metal surface in the mold 4) and the crater end position C EIn the range from the meniscus P to the intermediate position MP, the average specific water flow rate WA of the secondary cooling water sprayed from the secondary cooling nozzle 6 is set to 0.10 L / kg-steel or more and 0.49 L / kg-steel or less. Here, as the thickness of the solidified shell S increases, the thermal resistance of the solidified shell S also increases. In the range beyond the intermediate position MP, the thickness of the solidified shell S of the slab 10 is relatively large. Therefore, cooling of the surface of the slab 10 in the range beyond the intermediate position MP has almost no effect on the solidified state of the interior. On the other hand, in the range from the meniscus to the intermediate position MP, the thickness of the solidified shell S of the slab 10 is relatively small. Therefore, cooling of the surface of the slab 10 in the range from the meniscus to the intermediate position MP has a significant effect on the solidified state of the interior. Therefore, in the continuous casting method according to this embodiment, the amount of secondary cooling water in the range from the meniscus to the intermediate position MP is controlled in the cooling step (#10).

[0036] Meniscus and crater end position C E The intermediate position MP between P and the meniscus is the crater end position C E The intermediate position MP is a position where the distance from the meniscus to the meniscus is halfway between the start position of the vertical band A and the end position of the meniscus to the meniscus to the end position of the horizontal band C. The intermediate position MP is, for example, a position where the bent portion B ends and the slab 10 becomes horizontal. In this case, in the cooling process (#10), the amount of secondary cooling water is controlled from the start position of the vertical band A to the end position of the bent portion B (the start position of the horizontal band C). The intermediate position MP may also be located midway through the bent portion B as shown in FIG. 1. From another perspective, the distance from the meniscus to the intermediate position MP may be 10 m or more and 20 m or less.

[0037] When the average specific water flow rate WA of the secondary cooling water is greater than 0.49 L / kg-steel, the contact portion 10a of the slab 10 is cooled more easily than the non-contact portion 10b, and the difference in the solidification state between the contact portion 10a and the non-contact portion 10b becomes large. In this case, the position where solidification is completed becomes non-uniform across the width of the slab 10. Crater end position C EIf P is non-uniform in the width direction of the slab 10, the slab 10 will be lightly reduced in a state where both unsolidified and fully solidified parts are mixed in the reduction step (#15) described below. When a slab 10 including fully solidified parts is reduced, the reaction force becomes large, and the load capacity of the light reduction rolls 7 that perform the light reduction becomes insufficient. In this case, there is a high possibility that the slab 10 will not be lightly reduced sufficiently. If the crater end position C E Even if the slab 10 with non-uniform P can be sufficiently lightly reduced, the reduction is hindered by the fully solidified portion, and therefore the reduction cannot be sufficient to suppress center segregation and porosity in the unsolidified portion where solidification is delayed, resulting in a deterioration in the internal quality of the product. Therefore, in the cooling process (#10), the average specific water flow rate WA of the secondary cooling water in the range from the meniscus to the intermediate position MP is controlled to 0.49 L / kg-steel or less.

[0038] However, if the average specific water volume of the secondary cooling water (WA) is reduced too much, the thermal load on the equipment will be greater, increasing the likelihood of equipment trouble. Therefore, in the cooling process (#10), the average specific water volume of the secondary cooling water (WA) in the range from the meniscus to the intermediate position (MP) is controlled to 0.10 L / kg-steel or more.

[0039] The solidification state of the slab 10 depends greatly on the arrangement of the support rolls 5 of the continuous casting machine 1, and the solidification state will not change unless the casting conditions are significantly changed. Therefore, a solidification heat transfer analysis was performed in advance, simulating the cooling by the support rolls 5 and the secondary cooling nozzles 6, etc., to determine the crater end position C E P may be specified, thereby making it possible to appropriately set the range for controlling the amount of secondary cooling water injected onto the slab 10.

[0040] The secondary cooling nozzle 6 may cool the slab 10 in the range beyond the intermediate position MP in addition to the range from the meniscus to the intermediate position MP. E Cooling may be performed up to P, and in addition, the crater end position C ECooling may also be performed after P. From the viewpoint of heat extraction efficiency, it is preferable that the average specific water flow rate WB of the secondary cooling water in the range after the intermediate position MP is smaller than the average specific water flow rate WA in the range from the meniscus to the intermediate position MP.

[0041] When the ratio of the average specific water volume WB to the average specific water volume WA is relatively large, the surface temperature of the slab 10 can be reduced. The larger the difference between the surface temperature and the internal temperature of the slab 10 at the start of the rolling process (#15) described below, the more likely it is that the strain introduced into the slab 10 by soft reduction will reach the center of the slab 10, thereby reducing centerline segregation. Specifically, when the ratio of the average specific water volume WB to the average specific water volume WA is 10% or more, centerline segregation can be further reduced. On the other hand, when the ratio of the average specific water volume WB to the average specific water volume WA is greater than 70%, warpage of the slab 10 is likely to occur, making operation difficult. For these reasons, in this embodiment, the ratio of the average specific water volume WB to the average specific water volume WA is 10% or more and 70% or less.

[0042] The average specific amount of secondary cooling water is calculated by dividing the amount of secondary cooling water W (L / min) sprayed per unit time from all secondary cooling nozzles 6 in a certain range along the casting direction by the amount of steel Q (kg / min) passing through per unit time. 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 passing through per unit time Q can be expressed as Q = h1 × h2 × v × ρ. The density ρ of steel is 7.8 × 10 -6 (kg / mm 3 )

[0043] [Reduction process (#15)] In the reduction step (#15), the slab 10 is soft-reduced in the thickness direction perpendicular to the width direction using multiple soft reduction rolls 7 installed in the soft reduction zone 71. The soft reduction is performed from when the center solid fraction of the slab 10 reaches 0.3 until it reaches 1.0. That is, the center solid fraction of the slab 10 is 0.3 at the entrance 71i of the soft reduction zone 71 and 1.0 at the exit 71o of the soft reduction zone 71. The center solid fraction of the slab 10 is the solid fraction at the center of the thickness direction. The center solid fraction is 0 when the temperature at the center of the thickness direction is the liquidus temperature (the temperature at which solidification begins), and the center solid fraction is 1.0 when the temperature at the center of the thickness direction is the solidus temperature (the temperature at which solidification is completed). If the soft reduction is started after the center solid fraction of the slab 10 reaches 0.3, center segregation and porosity cannot 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. Furthermore, even if soft reduction is performed on the region where the center solid fraction of the slab 10 is 1.0, i.e., the region where the center of the thickness of the slab 10 is completely solidified, it has almost no effect on reducing center segregation and porosity, so there is no need to soft reduce this region.

[0044] Soft reduction of the region of the slab 10 where the center solid fraction is less than 0.3 does not necessarily have to be performed because soft reduction does not affect the reduction of center segregation and porosity formed at the end of solidification. However, soft reduction may be performed on the region of the slab 10 where the center solid fraction is less than 0.3.

[0045] In the reduction step (#15), if the reduction rate in the soft reduction zone 71 is less than 0.5 mm / min, the reduction amount will be insufficient relative to the solidification shrinkage of the slab 10, making it impossible to reduce center segregation and porosity. Furthermore, if the reduction rate is greater than 1.3 mm / min, the reduction amount will be 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. This causes the flow of concentrated molten steel to worsen center segregation. Therefore, the reduction rate of the slab 10 in the reduction step (#15) is preferably 0.5 mm / min or more and 1.3 mm / min or less.

[0046] In this embodiment, the surface temperature of the slab 10 at the start of the reduction step (#15) is preferably 900°C or lower. If the surface temperature of the slab 10 at the start of soft reduction is higher than 900°C, the strain introduced into the slab 10 by soft reduction will concentrate in the surface layer of the slab 10 and will not fully reach the center of the slab 10. This may result in insufficient reduction of centerline segregation. On the other hand, in this embodiment, the surface temperature of the slab 10 is 900°C or lower when reduction with the soft reduction rolls 7 begins, so the strain caused by soft reduction reaches the center of the slab 10, further reducing centerline segregation.

[0047] On the other hand, if the surface temperature of the slab 10 at the start of the reduction step (#15) is lower than 600°C, the deformation resistance of the slab 10 increases, and an excessively large load capacity is required to perform the soft reduction using the soft reduction rolls 7. In this case, the cost required for performing the soft reduction increases. Therefore, the surface temperature of the slab 10 at the start of the reduction step (#15) is preferably 600°C or higher.

[0048] The surface temperature of the slab 10 is E As with P, this can be calculated by performing solidification heat transfer analysis. Alternatively, the slab 10 may be actually produced using the continuous casting machine 1, and the surface temperature may be measured using a non-contact thermometer such as a radiation thermometer. In this case, it is necessary to eliminate factors that affect the measurement of the surface temperature (such as droplets and steam sprayed from the secondary cooling nozzle 6) as much as possible.

[0049] [effect] 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 the electromagnetic brake 4a. This weakens the flow of the molten steel M that is discharged from the discharge hole of the submerged entry nozzle 3 toward the width direction end of the solidified shell S in the mold 4. Also, in the continuous casting method according to this embodiment, in the cooling step (#10), the flow of the molten steel M from the meniscus to the meniscus and crater end position C EThe average specific water flow rate (WA) of the secondary cooling water in the range from the midpoint (MP) between the slab (P) and the support roll (5) is 0.49 L / kg-steel or less. This appropriately reduces the amount of secondary cooling water sprayed onto the slab (10), preventing the slab (10) from being cooled by localized accumulation of secondary cooling water between the slab (10) and the support roll (5). This reduces the difference in solidification rate between the contact portion (10a) of the slab (10) where solidification completion is likely to be delayed and the other portion (non-contact portion (10b)). By controlling both the flow of the molten steel (M) in the mold (4) and the average specific water flow rate (WA) of the secondary cooling water, the thickness of the solidified shell (S) becomes uniform across the width of the slab (10). By softly reducing the slab (10) in the thickness direction in the reduction step (#15), centerline segregation and porosity can be reduced.

[0050] As described above, in the continuous casting method according to this embodiment, the amount of secondary cooling water sprayed onto the slab 10 is appropriately reduced. This prevents the surface temperature of the widthwise ends of the slab 10 from decreasing excessively. This prevents cracks from occurring on the surface of the slab 10. [Example]

[0051] In order 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 crater end deviation index and Mn segregation degree of the slab obtained by continuous casting were evaluated. The crater end deviation index and Mn segregation degree will be described later.

[0052] In this test, slabs were produced using the continuous casting machine shown in Figure 1. The mold was a water-cooled copper mold. The mold length was 800 mm. The mold cross section was rectangular. The soft reduction zone was located 16 m or more from the meniscus (the molten metal surface in the mold). The slab was soft reduced in the thickness direction using soft reduction rolls at a reduction rate of 0.8 mm / min from the time the central solid fraction of the slab reached 0.3 until it reached 1.0. No soft reduction was performed in the region after the central solid fraction of the slab reached 1.0. The slab was 250 mm thick and 2300 mm wide. The temperature and solid fraction at the center of the slab were calculated using two-dimensional solidification analysis in the thickness and width directions of the slab.

[0053] The main chemical composition of the slab used in this test was C: 0.15%, Si: 0.19%, Mn: 0.90%, P: 0.011%, and S: 0.003%.

[0054] The solidification profile of the slab cast using the continuous casting machine used in the test, i.e., the progression of the solidified shell thickness, was investigated using the following procedure. Electromagnetic stirring was performed on the slab 10 within a range of 5 to 20 m from the meniscus, and a cross-sectional sample of the slab was taken. The sample was then corroded with hydrochloric acid to reveal a white band. The white band is generated by stirring in the liquid portion. The distance from the surface of the slab sample to the white band across the width was measured, and this was taken as the solidified shell thickness. Note that the solidification profile is highly dependent on the roll arrangement of the continuous casting machine and will not change unless the casting conditions are significantly changed. Therefore, it is not necessary to constantly monitor the solidification profile.

[0055] At the sample location, the thickness of the solidified shell S is D (mm) and the casting time from the meniscus position is t (min). The solidification coefficient K (mm / min) is calculated from the following equation (1). 0.5 ) can be obtained. The thickness D1 (mm) of the solidified shell S at the part where solidification is slowest in the width direction of the sample is measured, and the solidification coefficient K1 (mm / min 0.5Similarly, the thickness D2 (mm) of the solidified shell S at the most solidified part in the width direction of the sample was measured, and the solidification coefficient K2 (mm / min 0.5 ) was calculated.

[0056]

number

[0057] Next, using Equation (1) and the solidification coefficient K1, the casting time t1 was calculated, which was the time required for the thickness D of the solidified shell S at the slowest solidification portion to reach half the total thickness of the slab. Assuming the casting speed was Vc (m / min), the casting time t1 was multiplied by the casting speed Vc to obtain the distance L1 (m) from the meniscus at the slowest solidification portion to the crater end position. Similarly, using Equation (1) and the solidification coefficient K2, the casting time t2 was calculated, which was the time required for the thickness D of the solidified shell S at the most advanced solidification portion to reach half the total thickness of the slab. The distance L2 (m) from the meniscus at the most advanced solidification portion to the crater end position was obtained by multiplying the casting time t2 by the casting speed Vc. The difference between the distance L2 and the distance L1 was calculated, and this was defined as the crater end deviation ΔL (m). The crater-end deviation index is an index for comparing the crater-end deviation between multiple casts. A reference cast was selected from the multiple casts, and the crater-end deviation of each cast was divided by the crater-end deviation of the reference cast to obtain the crater-end deviation index.

[0058] 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, covering an area 40 mm in the casting direction and 40 mm in the thickness direction. This sample was subjected to area analysis of the Mn concentration 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 Mn segregation degree. The Mn concentration (C0) in the bulk composition was determined by chemical analysis of an analytical sample taken from the slab.

[0059] The test conditions and test results of this example are shown in Table 1. In Table 1, the test conditions include the casting speed, the magnetic flux density of the magnetic field of the electromagnetic brake, the average specific water flow rate WA, and the ratio of the average specific water flow rate WB to the average specific water flow rate WA (WB / WA). 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 flow rate WA is the average specific water flow rate of secondary cooling water sprayed from the secondary cooling nozzle in the range from the meniscus to the intermediate position MP. The average specific water flow rate WB is the average specific water flow rate of secondary cooling water sprayed from the secondary cooling nozzle in the range beyond the intermediate position MP. In Table 1, examples of the invention satisfy the following condition 1, and comparative examples of the slabs produced under test conditions that do not satisfy condition 1 are shown. In Comparative Examples 1 to 4 shown in Table 1, test conditions that do not satisfy condition 1 are marked with a "*." Furthermore, among Examples 1 to 7 and Comparative Examples 1 to 4 shown in Table 1, those that do not satisfy condition 2 below are marked with a "#" next to the surface temperature. Condition 1: The magnetic flux density is 1000 Gauss or more, and the average specific water content WA is 0.10 L / kg-steel or more and 0.49 L / kg-steel or less. Condition 2: The ratio of the average specific water volume WB to the average specific water volume WA is 10% or more and 70% or less.

[0060] [Table 1]

[0061] The crater end deviation index of the slab produced under each condition was determined for Examples 1 to 7 and Comparative Examples 2 to 4, using the crater end deviation of the slab in Comparative Example 1 as the reference. It has been found that if the crater end deviation index of the resulting slab is 0.800 or less, the Mn segregation degree and porosity volume at the thickness center of the slab are good, and the internal quality is improved.

[0062] It is also known that if the Mn segregation degree of the obtained slab is 1.40 or less, toughness can be ensured in the product (steel plate) after rolling the slab. For these reasons, in Table 1, the test results are shown as "excellent" if the crater end deviation index is 0.800 or less and the Mn segregation degree is 1.40 or less, and "poor" if not.

[0063] As shown in Table 1, all of Examples 1 to 5 satisfied Conditions 1 and 2. Therefore, in Examples 1 to 5, the crater end deviation index and Mn segregation degree were both lower than the threshold values, and good cast slabs were obtained. In other words, centerline segregation and porosity were reduced.

[0064] Although all of Inventive Examples 6 and 7 satisfied Condition 1, the ratio of the average specific water volume WB to the average specific water volume WA was less than 10%, and therefore Condition 2 was not satisfied. In Inventive Example 6, the crater end deviation index and the degree of Mn segregation were slightly higher than those of Inventive Example 1, which had the same test conditions except for the ratio of the average specific water volume WB to the average specific water volume WA, but a good cast slab was obtained. Similarly, Inventive Example 7, compared to Inventive Example 2, which had the same test conditions except for the ratio of the average specific water volume WB to the average specific water volume WA, the degree of Mn segregation was slightly higher but below the threshold, and a good cast slab was obtained. In other words, Inventive Examples 6 and 7 also succeeded in reducing centerline segregation and porosity.

[0065] In Comparative Example 1, the average specific water amount WA was 0.75 L / kg-steel, and did not satisfy Condition 1. In this case, both the crater end deviation index and the Mn segregation degree were larger than the threshold value, and the evaluation was "Fail." In Comparative Example 2, the average specific water amount WA was 0.52 L / kg-steel, and although the average specific water amount WA was smaller than that of Comparative Example 1, it did not satisfy Condition 1. In this case, the crater end deviation index and the Mn segregation degree were smaller than those of Comparative Example 1, but the evaluation was still "Fail."

[0066] In Comparative Example 3, no magnetic field was applied to the molten steel in the mold, and therefore Comparative Example 3 did not satisfy Condition 1. In this case, both the crater end deviation index and the Mn segregation degree were greater than the threshold values, and the evaluation was "fail."

[0067] In Comparative Example 4, the average specific water content WA was 0.52 L / kg-steel, similar to Comparative Example 2, and did not satisfy Condition 1. Furthermore, in Comparative Example 4, the ratio of the average specific water content WB to the average specific water content WA was less than 10%, and did not satisfy Condition 2 either. In this case, the crater end deviation index was almost the same as the result of Comparative Example 2, but the Mn segregation degree deteriorated. In Comparative Example 4, both the crater end deviation index and the Mn segregation degree were greater than the threshold value, and the evaluation was "fail."

[0068] The embodiments of the present disclosure have been described above. 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 appropriately modifying the above-described embodiments within the scope of the present disclosure. [Explanation of symbols]

[0069] 1: Continuous casting machine 4: Mold 4a: Electromagnetic brake 5: Support roll 5a: Roll body 5b: Bearing part 6: Secondary cooling nozzle 7: Light reduction roll 10: Casting C E P: Crater end position MP: intermediate position

Claims

1. A method for continuously casting steel using a continuous casting machine including a mold including an electromagnetic brake, secondary cooling nozzles installed along the casting direction of a slab and configured to spray secondary cooling water onto the slab, support rolls installed downstream of the mold in the casting direction, and soft reduction rolls installed downstream of the support rolls in the casting direction, 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; a cooling step of injecting secondary cooling water from the secondary cooling nozzle so that an average specific water amount is 0.10 L / kg-steel or more and 0.49 L / kg-steel or less in a range from a meniscus to an intermediate position between the meniscus and a crater end position; and a reduction step of softly reducing the slab in a thickness direction perpendicular to the width direction of the slab at a reduction rate of 0.5 mm / min or more and 1.3 mm / min or less using the soft reduction rolls from when the central solid fraction of the slab reaches 0.3 until the central solid fraction reaches 1.

0.

2. The continuous casting method according to claim 1, a ratio of an average specific water amount in a range beyond the intermediate position to an average specific water amount in a range from the meniscus to the intermediate position being 10% or more and 70% or less.

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