Continuous casting method for steel

Abstract

The present invention provides a continuous casting method for steel that can suppress surface cracking of cast slabs while reducing central segregation and porosity. [Solution] The continuous casting method for steel using a continuous casting machine (1) comprises an application step, a cooling step, and a reduction step. In the application step, 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). In the cooling step, secondary cooling water is injected from a secondary cooling nozzle (6) so that the average specific water content is between 0.10 L / kg-steel and 0.49 L / kg-steel in the range from the meniscus to the intermediate position (MP). In the reduction step, the cast slab (10) is lightly reduced in the thickness direction by a light reduction roll (7). In the cooling step, from when the thickness of the solidified shell (S) of the cast slab (10) reaches 30 mm until it reaches 70 mm, the amount of water injected from the second nozzle (6B) is 1.1 times or more the amount of water injected from the first nozzle (6A).

Description

[Technical Field]

[0001] This disclosure relates to a continuous casting method for steel. [Background technology]

[0002] In continuous casting of steel, defects such as central segregation and porosity occur in the cast slabs. The cast slabs obtained by continuous casting are rolled to become products. In recent years, with the increasing thickness and strength of products, the demands on the internal quality of cast slabs have become even higher. For this reason, it is necessary to reduce central segregation and porosity in the center of the cast slab in the thickness direction. To improve the internal quality caused by these defects, the cast slab is usually lightly reduced in the thickness direction within the continuous casting machine.

[0003] In a cast slab undergoing solidification, if the thickness of the solidified shell is non-uniform across its width, there are regions where solidification is relatively delayed. In these regions of the cast slab where solidification is delayed, central segregation and porosity tend to be particularly large.

[0004] In continuous casting, an immersion nozzle with 2 to 4 discharge holes is used, and hot molten steel is supplied from the discharge holes 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 cast slab tends to become hotter, and the completion of solidification is often delayed.

[0005] Furthermore, a continuous casting machine includes, for example, a support roll that guides the slab downstream in the casting direction, and a nozzle that sprays secondary cooling water onto the slab. Generally, the support roll is 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 width 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 secondary cooling water accumulated at the contact points is discharged from the gap between the bearing section of the support roll and the slab, or from the widthwise end of the slab. The area of ​​the slab corresponding to the roll body section is cooled by the secondary cooling water accumulated at the contact points with the roll body section and by the support roll in contact with the slab. On the other hand, secondary cooling water does not accumulate in the area of ​​the slab corresponding to the bearing section of the support roll, and this area is not in contact with the support roll. Therefore, the area of ​​the cast slab corresponding to the support roll bearing is less easily cooled compared to the area of ​​the cast slab corresponding to the roll body, and the completion of solidification tends to be delayed.

[0006] As explained above, due to the flow of molten steel within the mold and the uneven secondary cooling of the slab, there are regions in the solidified slab where solidification is delayed, resulting in uneven thickness of the solidified shell across the width of the slab. In such cases, even if the slab is lightly reduced in the thickness direction, it is difficult to reduce central segregation and porosity in the regions where solidification was delayed, making it difficult to improve internal quality. Therefore, it is necessary to make the thickness of the solidified shell uniform in the width direction.

[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 its width. Patent Document 1 discloses a technique in which the solidification profile (change in the thickness of the solidified shell) in the width direction of the slab is determined in advance, and the amount of secondary cooling water in the width direction of the slab is controlled based on this profile. Patent Document 2 discloses a technique in which a device capable of detecting the solidification completion position of the slab online is installed in a continuous casting machine, and the amount of secondary cooling water in the width direction of the slab 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 temperature of the center and corners of the slab width in both the bent (curved) and straightened (horizontal) sections is controlled. Patent Document 3 states 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 Publication No. 2008-238256 [Patent Document 3] Japanese Patent Publication No. 2008-194746 [Overview of the project] [Problems that the invention aims to solve]

[0010] In recent years, continuous casting has seen a trend towards adding more alloy components to molten steel than ever before in order to improve product performance. However, a higher alloy content makes the surface of the cast slab more susceptible to cracking. When cracking occurs on the slab surface, it increases the cost of surface treatment and reduces yield. Therefore, it is necessary not only to reduce central segregation and porosity, but also to suppress surface cracking of the cast slab.

[0011] The technology described in Patent Document 1 is based on the premise of forming negative segregation at the center of the thickness. To form negative segregation at the center of the thickness, it is necessary to heavily reduce the solidified slab using the unsolidified heavy reduction method. In the unsolidified heavy reduction method, a heavy reduction greater than the solidification shrinkage amount is applied to the solidified slab, forcibly removing the concentrated molten steel. This unsolidified heavy reduction method is a different technology from the light reduction method. In the light reduction method, the solidified slab is reduced by a reduction amount equivalent to the solidification shrinkage amount, mitigating positive segregation at the center of the thickness. Furthermore, the technology described in Patent Document 2 requires advanced equipment that can detect the completion of solidification of the slab online, and the equipment development, introduction, and maintenance costs are high, making it disadvantageous in terms of cost-effectiveness.

[0012] Furthermore, Patent Documents 1 and 2 do not consider suppressing surface cracking of the cast slab. As described in Patent Documents 1 and 2, if the thickness of the solidified shell is made uniform in the width direction of the cast slab by controlling the amount of secondary cooling water across the width direction of the cast slab, the amount of secondary cooling water injected into the region of the cast slab where solidification is delayed increases. This can lead to a decrease in the surface temperature at the widthwise edges of the cast slab, potentially causing cracks to occur on the surface of the cast slab.

[0013] As mentioned above, Patent Document 3 describes how to suppress surface cracking of a cast slab by controlling its surface temperature. However, Patent Document 3 does not consider central segregation and porosity that occur in the center of the cast slab in the thickness direction.

[0014] The object of this disclosure is to provide a continuous casting method for steel that can suppress surface cracking of cast slabs while reducing central segregation and porosity. [Means for solving the problem]

[0015] In the continuous casting method of steel according to the present disclosure, a continuous casting machine is used. The continuous casting machine includes a mold, support rolls, secondary cooling nozzles, and soft reduction rolls. The mold includes an electromagnetic brake. The support rolls are installed on the downstream side in the casting direction of the mold. The support rolls include a plurality of roll body portions and bearing portions. The roll body portions are arranged in the width direction of the slab. The bearing portions connect the roll body portions to each other. The secondary cooling nozzles inject secondary cooling water onto the slab. The secondary cooling nozzles include a first nozzle and a second nozzle. The first nozzle injects secondary cooling water into the region of the slab corresponding to the roll body portion. The second nozzle injects secondary cooling water into the region of the slab corresponding to the bearing portion. The soft reduction rolls are installed on the downstream side in the casting direction of the support rolls. The continuous casting method includes an application step, a cooling step, and a reduction 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. In the cooling step, secondary cooling water is injected 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 the intermediate position between the meniscus and the crater end position. In the reduction step, from when the central solid fraction of the slab reaches 0.3 until it reaches 1.0, the slab is softly reduced in the thickness direction perpendicular to the width direction of the slab by the soft reduction rolls at a reduction rate of 0.5 mm / min or more and 1.3 mm / min or less. In the cooling step, from when the thickness of the solidification shell of the slab reaches 30 mm until it reaches 70 mm, the amount of water injected from the second nozzle is 1.1 times or more the amount of water injected from the first nozzle.

Advantages of the Invention

[0016] According to the continuous casting method of steel according to the present disclosure, it is possible to suppress surface cracking of the slab while reducing central segregation and porosity.

Brief Description of the Drawings

[0017] [Figure 1] FIG. 1 is a schematic diagram of a continuous casting machine used in the continuous casting method according to the first embodiment. [Figure 2]FIG. 2 is a cross-sectional view when the continuous casting machine shown in FIG. 1 is cut perpendicular to the casting direction. [Figure 3] FIG. 3 is a flowchart showing a continuous casting method of steel according to an embodiment. [Figure 4] FIG. 4 is a cross-sectional view when the continuous casting machine used in the continuous casting method according to the second embodiment is cut perpendicular to the casting direction.

MODE FOR CARRYING OUT THE INVENTION

[0018] In the continuous casting method of steel according to the embodiment, a continuous casting machine is used. The continuous casting machine includes a mold, a support roll, a secondary cooling nozzle, and a soft reduction roll. The mold includes an electromagnetic brake. The support roll is installed on the downstream side in the casting direction of the mold. The support roll includes a plurality of roll body parts and bearing parts. The roll body parts are arranged in the width direction of the slab. The bearing parts connect the roll body parts to each other. The secondary cooling nozzle injects secondary cooling water into the slab. The secondary cooling nozzle includes a first nozzle and a second nozzle. The first nozzle injects secondary cooling water into a region of the slab corresponding to the roll body part. The second nozzle injects secondary cooling water into a region of the slab corresponding to the bearing part. The soft reduction roll is installed on the downstream side in the casting direction of the support roll. The continuous casting method includes an application step, a cooling step, and a reduction 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 an electromagnetic brake. In the cooling step, secondary cooling water is injected from the secondary cooling nozzle 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 the intermediate position between the meniscus and the crater end position. In the reduction step, from when the central solid fraction of the slab reaches 0.3 until it reaches 1.0, the slab is softly reduced in the thickness direction perpendicular to the width direction of the slab by the soft reduction roll at a reduction speed of 0.5 mm / min or more and 1.3 mm / min or less. In the cooling step, from when the thickness of the solidification shell of the slab reaches 30 mm until it reaches 70 mm, the amount of water injected from the second nozzle is 1.1 times or more the amount of water injected from the first nozzle (the first configuration).

[0019] In the continuous casting method according to the first configuration, a magnetic field with a magnetic flux density of 1000 Gauss or more is applied to the molten steel in the mold during the application step. This weakens the flow of molten steel discharged from the discharge hole of the immersion nozzle toward the widthwise end of the solidified shell within the mold.

[0020] Furthermore, in the continuous casting method relating to the first configuration, 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 0.49 L / kg-steel or less. This appropriately suppresses the amount of secondary cooling water injected into the slab, making it less likely for the slab to cool due to localized accumulation of secondary cooling water between the slab and the support roll. Consequently, the difference between the solidification rate of the region of the slab where solidification is likely to be delayed and the solidification rate of the other regions becomes smaller. Moreover, in the cooling process, from when the thickness of the solidified shell of the slab reaches 30 mm until it reaches 70 mm, the amount of water injected from the second nozzle is 1.1 times or more the amount of water injected from the first nozzle. In other words, the amount of water injected into the region of the slab corresponding to the bearing portion of the support roll is 1.1 times or more the amount of water injected into the region of the slab corresponding to the roll body. Therefore, the region of the slab corresponding to the bearing portion of the support roll, where solidification is relatively delayed, can be sufficiently cooled.

[0021] In this way, by controlling both the flow of molten steel in the mold and the amount of secondary cooling water, the thickness of the solidified shell becomes uniform in the width direction of the slab. Therefore, in the reduction process, the slab, whose thickness is uniform in the width direction, can be lightly reduced in the thickness direction, thereby reducing central segregation and porosity.

[0022] In the continuous casting method relating to the first configuration, as described above, the amount of secondary cooling water sprayed onto the slab from the secondary cooling nozzle is appropriately suppressed. Therefore, excessive reduction in the surface temperature of the slab is prevented, and surface cracking of the slab can be suppressed.

[0023] In the first configuration of the continuous casting method, the secondary cooling nozzle may further include a third nozzle. The third nozzle sprays secondary cooling water onto the widthwise end of the cast slab. During the cooling process, from when the thickness of the solidified shell of the cast slab reaches 30 mm until it reaches 70 mm, the amount of water sprayed from the third nozzle is preferably 1.1 times or more the amount of water sprayed from the first nozzle (second configuration).

[0024] The widthwise edges of the cast slab, like the area corresponding to the support roll bearing portion, tend to have a relatively delayed completion of solidification. In the continuous casting method according to the second configuration, the amount of water injected from the third nozzle during the cooling process is 1.1 times or more the amount of water injected from the first nozzle, so the widthwise edges of the cast slab can be sufficiently cooled. This makes it possible to further reduce central segregation and porosity.

[0025] In the continuous casting method of the first or second configuration, the average surface temperature in the width direction of the cast slab is preferably 950°C or less from the time the central solid fraction of the cast slab reaches 0.3 until the central solid fraction reaches 1.0 (third configuration).

[0026] In the third configuration, during the reduction process, the average surface temperature in the width direction of the cast slab is 950°C or less within the range where light reduction is performed on the cast slab. In this case, the strain caused by light reduction is more likely to reach the center of the cast slab's thickness, and as a result, central segregation can be further reduced.

[0027] 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.

[0028] [Continuous casting machine] Figure 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 cast slabs 10. 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 section B, and a horizontal band C. However, the continuous casting machine 1 may also be a vertical type consisting of, for example, only a vertical band.

[0029] The continuous casting machine 1 comprises a tundish 2, a mold 4, multiple support rolls 5, multiple secondary cooling nozzles 6 (Figure 2, described later), and multiple light reduction rolls 7.

[0030] 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.

[0031] The support rolls 5 and secondary cooling nozzles 6 (Figure 2) 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, for example, in the casting direction. 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 of the starting position of horizontal band C.

[0032] 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.

[0033] Figure 2 is a cross-sectional view of the continuous casting machine 1 when cut perpendicular to the casting direction. Figure 2 shows the area below the mold 4. Referring to Figure 2, the support roll 5 is divided in the width direction of the cast slab 10. The support roll 5 includes a plurality of roll body sections 5a and bearing sections 5b. The roll body sections 5a are arranged in the width direction of the cast slab 10. The bearing sections 5b connect the roll body sections 5a to each other and rotatably support the roll body sections 5a. The number of roll body sections 5a is not particularly limited, but for example, it is 2 to 4. In the example in Figure 2, there are 4 roll body sections 5a, and the support roll 5 includes a plurality of bearing sections 5b. The roll body sections 5a and bearing sections 5b are arranged symmetrically, for example, in the width direction of the cast slab 10. The dimensions of the bearing sections 5b in the width direction are, for example, 150 to 250 mm.

[0034] In a cross-sectional view of the continuous casting machine 1 cut perpendicular to the casting direction, the region 10a of the cast slab 10 corresponding to the roll body portion 5a of the support roll 5 is in contact with the roll body portion 5a. Hereinafter, this region may be referred to as the contact region 10a. On the other hand, the region 10b of the cast slab 10 corresponding to the bearing portion 5b of the support roll 5 is not in contact with the bearing portion 5b. Hereinafter, this region may be referred to as the non-contact region 10b. A gap is provided between the non-contact region 10b and the bearing portion 5b. Here, regions 10a and 10b may include not only the region of the cast slab 10 in which the support roll 5 is located, but also the region between adjacent support rolls 5 in the casting direction.

[0035] Multiple secondary cooling nozzles 6 are arranged at intervals along the width direction of the cast slab 10. The secondary cooling nozzles 6 include a first nozzle 6A and a second nozzle 6B. The first nozzle 6A sprays secondary cooling water into the region 10a of the cast slab 10 corresponding to the roll body portion 5a. The second nozzle 6B sprays secondary cooling water into the region 10b of the cast slab 10 corresponding to the bearing portion 5b. The amount of cooling water sprayed from each of the first nozzle 6A and the second nozzle 6B is controlled individually.

[0036] Referring again to Figure 1, the multiple light reduction rolls 7 are installed downstream of the support rolls 5 in the casting direction. Each light reduction roll 7 is paired with another roll and lightly reduces the cast slab 10. Specifically, a series of light reduction rolls 7 constitute a light reduction zone 71. The light reduction zone 71 is divided into multiple segments, and each segment is provided with multiple light reduction rolls 7. The reduction amount of the light reduction rolls 7 is controlled for each segment. The light reduction rolls 7 are arranged, for example, at equal intervals between the inlet 71i and the outlet 71o of the light reduction zone 71. In the casting direction, the position of the inlet 71i of the light reduction zone 71 corresponds to the position of the light reduction roll 7 located at the upstream end in the casting direction, and the position of the outlet 71o of the light reduction zone 71 corresponds to the position of the light reduction roll 7 located at the downstream end in the casting direction. In this embodiment, the position of the inlet 71i coincides with the starting position of the horizontal zone C. However, the position of the inlet 71i may be downstream in the casting direction of the slab 10 from the starting position of the horizontal band C.

[0037] [Continuous casting method] Figure 3 is a flowchart showing the continuous casting method for steel according to this embodiment. As shown in Figure 3, 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 3 will be described in detail with reference to Figures 1 and 2.

[0038] [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.

[0039] The magnetic flux density of the magnetic field applied in the application process (#5) is 1000 Gauss or more. If the magnetic flux density is 1000 Gauss or more, sufficient braking force acts on the molten steel M. In other words, 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. In this case, the slab 10 becomes hot near the widthwise end of the slab 10, and the completion of solidification is likely to be delayed. As a result, the internal quality of the slab 10 deteriorates.

[0040] 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.

[0041] [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 is delayed. Generally, the non-contact region 10b of the cast slab 10, which is not in contact with the support roll 5, is less cooled and therefore solidification is delayed compared to the contact region 10a, which is in contact with the support roll 5. This is because the contact region 10a is cooled by the secondary cooling water accumulated between it and the support roll 5, whereas no secondary cooling water accumulates in the non-contact region 10b. In the cast slab 10, the difference in solidification state between the contact region 10a and the non-contact region 10b becomes more pronounced as the amount of secondary cooling water sprayed from the secondary cooling nozzle 6 increases.

[0042] 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 from the meniscus to the crater end position C. EIn 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 0.10 L / kg-steel and 0.49 L / kg-steel. Meniscus and crater end position C E The intermediate position MP with respect to P is the distance from the meniscus in the casting direction to the crater end position C. E This is the position that is half of P. The intermediate position MP is, for example, the position where the curved section B ends and the cast slab 10 becomes horizontal. In this case, the cooling process (#10) controls the amount of secondary cooling water from the starting position of the vertical band A to the ending position of the curved section B (the starting position of the horizontal band C). Alternatively, the intermediate position MP may be in the middle of the curved section B, as shown in Figure 1. From another perspective, the intermediate position MP may be at a distance of 10m or more and 20m or less from the meniscus.

[0043] The average specific water content is calculated by dividing the amount of secondary cooling water (L / min) injected per unit time from all secondary cooling nozzles 6 in a certain range along the casting direction by the amount of steel (kg / min) passing through per unit time. The thickness of the cast 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 / mm²). 3 When this is the case, the amount of steel Q passing through per unit time can be expressed as Q = h1 × h2 × v × ρ. The density ρ of steel is 7.8 × 10⁻⁶. -6 (kg / mm 3 )

[0044] If the thickness of the solidified shell S of the cast slab 10 is large, the thermal resistance of the solidified shell S will also be large. In the range from the intermediate position MP onward, the thickness of the solidified shell S of the cast slab 10 is relatively large. Therefore, cooling of the surface of the cast slab 10 in the range from the intermediate position MP onward has almost no effect on the internal solidification state. On the other hand, in the range from the meniscus to the intermediate position MP, the thickness of the solidified shell S of the cast slab 10 is relatively small. Therefore, cooling of the surface of the cast slab 10 in the range from the meniscus to the intermediate position MP has a significant effect on the internal solidification state. Accordingly, in the cooling step (#10) of this embodiment, the amount of secondary cooling water in the range from the meniscus to the intermediate position MP is controlled.

[0045] When the average specific water volume of the secondary cooling water is more than 0.49 L / kg-steel, in the slab 10, the contact region 10a is more likely to be cooled compared to the non-contact region 10b, and the difference in the solidification state between the contact region 10a and the non-contact region 10b becomes larger. In this case, in the width direction of the slab 10, the position where solidification is completed becomes non-uniform. The crater end position C E If P is non-uniform in the width direction of the slab 10, in the soft reduction process (#15) described later, the slab 10 will be lightly reduced in a state where the non-solidified part and the fully solidified part are mixed. When the slab 10 including the fully solidified part is reduced, the reaction force becomes large, and the load capacity of the soft reduction roll 7 for performing the light reduction is insufficient. In this case, there is a high possibility that the slab 10 cannot be sufficiently lightly reduced. The crater end position C E When the slab 10 with non-uniform P is lightly reduced, the reduction is inhibited by the fully solidified part. Therefore, it becomes difficult to perform a reduction sufficient to suppress center segregation and porosity in the non-solidified part where solidification is delayed, and the internal quality of the product deteriorates. Therefore, in the cooling process (#10), the average specific water volume of the secondary cooling water in the range from the meniscus to the intermediate position MP is controlled to be 0.49 L / kg-steel or less.

[0046] However, if the average specific water volume of the secondary cooling water is excessively small, the influence of the heat load on the equipment becomes large, and the possibility of equipment trouble increases. Therefore, in the cooling process (#10), the average specific water volume of the secondary cooling water in the range from the meniscus to the intermediate position MP is controlled to be 0.10 L / kg-steel or more.

[0047] The cooling by the secondary cooling nozzle 6 may be performed on the slab 10 in the range after the intermediate position MP in addition to the range from the meniscus to the intermediate position MP. Specifically, cooling may be performed in the range from the intermediate position MP to the crater end position C E P, and in addition to that, cooling may also be performed after the crater end position C E P. From the viewpoint of heat extraction efficiency, the average specific water volume of the secondary cooling water in the range after the intermediate position MP is preferably smaller than the average specific water volume in the range from the meniscus to the intermediate position MP.

[0048] Furthermore, in the cooling step (#10) of this embodiment, the cast slab 10 is divided into multiple regions 10a and 10b according to the solidification state in the width direction of the cast slab 10, and the amount of water injected from the secondary cooling nozzles 6 (first nozzle 6A and second nozzle 6B) is adjusted for each of the regions 10a and 10b. In the cooling step (#10), from when the thickness of the solidified shell S of the cast slab 10 reaches 30 mm until it reaches 70 mm, the amount of water injected from the second nozzle 6B is 1.1 times or more the amount of water injected from the first nozzle 6A.

[0049] As described above, the non-contact area 10b, where secondary cooling water is injected by the second nozzle 6B, tends to solidify more slowly than the contact area 10a, where secondary cooling water is injected by the first nozzle 6A. If the amount of water injected from the second nozzle 6B is 1.1 times or more the amount of water injected from the first nozzle 6A, a relatively large amount of water can be supplied to the non-contact area 10b. Therefore, solidification in the non-contact area 10b can be promoted.

[0050] However, if an excessive amount of secondary cooling water is supplied to a portion of the cast slab 10, the surface temperature of that portion will become very low. This will increase the surface temperature difference of the cast slab 10 in the width direction, causing thermal stress to occur in the cast slab 10. In this case, depending on the type of steel used for the cast slab 10, surface cracks may occur. Therefore, it is preferable that the amount of water injected from the second nozzle 6B be 4.0 times or less the amount of water injected from the first nozzle 6A. More preferably, the amount of water injected from the second nozzle 6B is 1.5 times or less the amount of water injected from the first nozzle 6A. In this case, the incidence of surface cracks in the cast slab 10 can be further reduced.

[0051] This water content adjustment in the width direction of the cast slab 10 is performed for cast slabs 10 within an appropriate range in the casting direction. In the region where the thickness of the solidified shell S of the cast slab 10 is less than 30 mm, that is, directly below the mold 4, the cast slab 10 is at a very high temperature. If there is a difference in the surface temperature of the cast slab 10 in this region, surface cracks caused by thermal stress may occur in the cast slab 10. Also, when cooling the unsolidified molten steel M with secondary cooling water, the solidified shell S acts as a thermal resistance, making it difficult to promote solidification in regions where the thickness of the solidified shell S is large. Therefore, the water content adjustment in the width direction of the cast slab 10 is performed for cast slabs 10 in the range from when the thickness of the solidified shell S reaches 30 mm to when it reaches 70 mm.

[0052] Increasing the number of secondary cooling nozzles 6 in the width direction of the cast slab 10 allows for more precise control of the amount of water supplied to the cast slab 10 in the width direction, but this also increases the complexity of the control and the cost required for control. The spacing between adjacent secondary cooling nozzles 6 in the width direction may be approximately the same as the width direction dimension of the bearing portion 5b of the support roll 5. For example, the spacing between adjacent secondary cooling nozzles 6 in the width direction is 150 to 250 mm.

[0053] In continuous casting of steel, if the arrangement of the roll body portion 5a and bearing portion 5b of the support roll 5 is symmetrical in the width direction of the cast slab 10, then the thickness of the solidified shell S in a cross-sectional view perpendicular to the casting direction is also symmetrical in the width direction. In this case, the arrangement of the first nozzle 6A and the second nozzle 6B (amount of secondary cooling water supplied to the cast slab 10) may also be symmetrical in the width direction.

[0054] [Reduction process (#15)] In the reduction process (#15), the cast slab 10 is lightly reduced in the thickness direction perpendicular to the width direction using multiple light reduction rolls 7 provided in the light reduction zone 71. 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 71i of the light reduction zone 71, the central solid fraction of the cast slab 10 is 0.3, and at the outlet 71o of the light reduction zone 71, 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 the 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).

[0055] If light reduction is started after the central solid fraction of the cast slab 10 reaches 0.3, it is difficult to sufficiently reduce central segregation and porosity. Furthermore, if light reduction is terminated before the central solid fraction of the cast slab 10 reaches 1.0, it is difficult to sufficiently reduce central segregation and porosity.

[0056] Light reduction of the central solid fraction region of the cast slab 10, i.e., the region where the center of the cast slab's thickness is completely solidified, has little effect on reducing central segregation and 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 central segregation and 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.

[0057] In the reduction process (#15), if the reduction rate in the light reduction zone 71 is less than 0.5 mm / min, the reduction amount will be insufficient to compensate for the solidification shrinkage of the slab 10, making it impossible to reduce central segregation and porosity. Conversely, if the reduction rate is greater than 1.3 mm / min, the reduction amount will be excessive to compensate for the solidification shrinkage of the slab 10, causing the molten steel M to flow backward from downstream to upstream in the casting direction, and the resulting flow of concentrated molten steel will worsen central segregation. Therefore, the reduction rate for lightly reducing the slab 10 in the reduction process (#15) is between 0.5 mm / min and 1.3 mm / min.

[0058] In this embodiment, the average surface temperature in the width direction of the cast slab 10 is 950°C or less from the time the central solid fraction of the cast slab 10 reaches 0.3 until it reaches 1.0. In other words, within the range where light reduction is performed on the cast slab 10 in the reduction process (#15), the average surface temperature in the width direction of the cast slab 10 is 950°C or less. If the average surface temperature in the width direction of the cast slab 10 is higher than 950°C, the strain introduced into the cast slab 10 by light reduction will concentrate on the surface layer of the cast slab 10 and will not sufficiently reach the center of the cast slab 10. Therefore, there is a risk that central segregation cannot be sufficiently reduced. In contrast, in this embodiment, within the range where light reduction is performed on the cast slab 10, the average surface temperature in the width direction of the cast slab 10 is 950°C or less. Therefore, the strain due to light reduction reaches the center of the cast slab 10, making it easier to reduce central segregation. More preferably, the average surface temperature in the width direction of the cast slab 10 is 850°C or less. In this case, the central segregation of the cast slab 10 becomes easier to reduce.

[0059] On the other hand, if the average surface temperature in the width direction of the cast slab 10 during the reduction process (#15) is lower than 600°C, the deformation resistance of the cast slab 10 increases, requiring excessive load capacity to perform light reduction. In this case, the cost required to perform light reduction increases. Therefore, the average surface temperature in the width direction of the cast slab 10 is preferably 600°C or higher.

[0060] The solidification state of the cast slab 10 largely depends on the arrangement of the support rolls 5 of the continuous casting machine 1 and the amount of secondary cooling water supplied by the secondary cooling nozzles 6. Therefore, unless the casting conditions are significantly changed, the solidification state will not change significantly. Thus, by performing a solidification heat transfer analysis that simulates cooling by the support rolls 5 and secondary cooling nozzles 6 in advance, the crater end position C can be determined. E You may specify P.

[0061] The surface temperature of the cast slab 10 is at the crater end position C. ESimilar to P, it can be calculated by performing solidification heat transfer analysis. Alternatively, the cast slab 10 may be manufactured 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 remove as many factors as possible that may affect the measurement of the surface temperature (such as droplets and steam ejected from the secondary cooling nozzle 6).

[0062] The solidification state (thickness of the solidified shell S) in the width direction of the cast slab 10 can be estimated, for example, by the following procedure. Electromagnetic stirring is performed on the cast slab 10 within a range of 5 to 20 m from the meniscus, and a sample of the cross-section of the cast slab 10 cut perpendicular to the casting direction is taken. The sample is then corroded with hydrochloric acid to reveal the white bands caused by stirring into the unsolidified areas. Subsequently, the distance from the surface of the cast slab 10 to the white bands is measured along the width direction of the cast slab 10. This allows obtaining the solidification profile of the cast slab 10, that is, the change in the thickness of the solidified shell S in the width direction of the cast slab 10.

[0063] [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 discharged from the discharge hole of the immersion nozzle 3 toward the widthwise end of the solidified shell S in the mold 4.

[0064] Furthermore, in the continuous casting method according to this embodiment, during the cooling process (#10), the meniscus is cast from the meniscus to the crater end position C. EThe average specific water content of the secondary cooling water in the range from position P to intermediate position MP is 0.49 L / kg-steel or less. This moderately suppresses the amount of secondary cooling water injected into the slab 10, making it less likely for the slab 10 to cool due to local accumulation of secondary cooling water between the slab 10 and the support roll 5. Consequently, the difference between the solidification rate of region 10a, where solidification is slower to complete, and the solidification rate of the other regions 10b of the slab 10 becomes smaller. Furthermore, in the cooling process (#10), from when the thickness of the solidified shell S of the slab 10 reaches 30 mm until it reaches 70 mm, the amount of water injected from the second nozzle 6B is 1.1 times or more the amount of water injected from the first nozzle 6A. In other words, the amount of water injected into region 10b of the slab 10 corresponding to the bearing portion 5b of the support roll 5 is 1.1 times or more the amount of water injected into region 10a of the slab 10 corresponding to the roll body portion 5a. Therefore, region 10b, where coagulation is relatively slow to complete, can be sufficiently cooled.

[0065] In this way, by controlling both the flow of molten steel M in the mold 4 and the amount of secondary cooling water, the thickness of the solidified shell S becomes uniform in the width direction of the cast slab 10. Therefore, in the reduction process (#15), the cast slab 10, whose thickness of the solidified shell S is uniform in the width direction, can be lightly reduced in the thickness direction, thereby reducing central segregation and porosity.

[0066] In the continuous casting method according to this embodiment, as described above, the amount of secondary cooling water injected into the cast slab 10 is appropriately suppressed. Therefore, an excessive drop in the surface temperature of the cast slab 10 is prevented, and surface cracking of the cast slab 10 can be suppressed.

[0067] [Second Embodiment] Figure 4 is a cross-sectional view of the continuous casting machine 1 of the second embodiment, viewed along the casting direction. Figure 4 shows the area below the mold 4. Referring to Figure 4, this embodiment differs from the first embodiment in that the secondary cooling nozzle 6 further includes a third nozzle 6C.

[0068] The third nozzle 6C sprays secondary cooling water onto the widthwise end of the cast slab 10. If the region 10a corresponding to the roll body 5a includes the widthwise end of the cast slab 10, the first nozzle 6A sprays secondary cooling water onto the region 10a excluding the widthwise end of the cast slab 10. Similarly, if the region 10b corresponding to the bearing 5b includes the widthwise end of the cast slab 10, the second nozzle 6B sprays secondary cooling water onto the region 10b excluding the widthwise end of the cast slab 10. In this embodiment, during the cooling process (#10), the amount of water sprayed from the third nozzle 6C is 1.1 times or more the amount of water sprayed from the first nozzle 6A from the time the thickness of the solidified shell S of the cast slab 10 reaches 30 mm until it reaches 70 mm.

[0069] The widthwise ends of the cast slab 10, like the region 10b corresponding to the bearing portion 5b of the support roll 5, tend to have a relatively delayed completion of solidification. In the continuous casting method according to this embodiment, in the cooling step (#10), the amount of water injected from the third nozzle 6C is 1.1 times or more the amount of water injected from the first nozzle 6A, so a relatively large amount of water can be supplied to the widthwise ends of the cast slab 10. This promotes solidification at the widthwise ends of the cast slab 10, and can further reduce central segregation and porosity. [Examples]

[0070] To confirm the effectiveness of the continuous casting method according to the embodiment, the following tests were conducted and the results were evaluated. Specifically, the crater end deviation index and Mn segregation degree of multiple cast slabs obtained by continuous casting were evaluated. The crater end deviation index and Mn segregation degree will be explained later.

[0071] In this experiment, cast slabs were manufactured using the continuous casting machine shown in Figure 1. The mold was made of copper and water-cooled, with a rectangular cross-section. The length of the mold was 800 mm. A light reduction zone was established from 16 m from the meniscus onward. The cast slab was lightly reduced in the thickness direction at a reduction rate of 0.8 mm / min using a light reduction roll from the time the central solid fraction of the cast slab reached 0.3 until the central solid fraction reached 1.0. No light reduction was performed in the region after the central solid fraction of the cast slab reached 1.0. The width of the cast slab was 2300 mm. The surface temperature, temperature at the center of the thickness, and solid fraction of the cast slab were calculated by two-dimensional solidification heat transfer analysis in the thickness and width directions of the cast slab.

[0072] The main chemical composition of the cast slab used in this test was C: 0.17%, Si: 0.25%, Mn: 1.25%, P: 0.011%, and S: 0.003%.

[0073] The solidification profile of slabs cast using the continuous casting machine used in the test, i.e., the change in the thickness of the solidified shell, was investigated using the following procedure. Electromagnetic stirring was performed on slab 10 within a range of 5 to 20 m from the meniscus, and samples of the slab's cross-section were taken. The collected samples were etched with hydrochloric acid to reveal a white band. The white band is caused by stirring into the unsolidified area. The distance from the surface of the slab sample to the white band was measured in the width direction, and this was defined as the thickness of the solidified shell. Note that the solidification profile largely depends on the arrangement of the support rolls in the continuous casting machine and will not change unless the casting conditions are significantly altered. Therefore, it is not always necessary to constantly monitor the solidification profile.

[0074] If D (mm) is the thickness of the solidified shell S at the location where the sample was taken, and t (min) is the casting time from the meniscus, then the solidification coefficient K (mm / min) can be obtained from the following equation (1). 0.5 ) can be determined.Measure the thickness D1 (mm) of the solidified shell S in the part where solidification is slowest in the width direction of the sample, and calculate the solidification coefficient K1 (mm / min) from equation (1). 0.5Similarly, the thickness D2 (mm) of the solidified shell S in the most solidified part of the sample was measured in the width direction, and the solidification coefficient K2 (mm / min) was calculated. 0.5 ) was calculated.

[0075]

number

[0076] Next, using equation (1) and the solidification coefficient K1, the casting time t1 was determined until the thickness D of the solidified shell S in the most delayed part became half the total thickness of the slab. Assuming the casting rate is Vc (m / min), multiplying the casting time t1 by the casting rate Vc yielded the distance L1 (m) in the casting direction from the meniscus in the most delayed part to the crater end. Similarly, using equation (1) and the solidification coefficient K2, the casting time t2 was determined until the thickness D of the solidified shell S in the most advanced part became half the total thickness of the slab. Multiplying the casting time t2 by the casting rate Vc yielded the distance L2 (m) in the casting direction from the meniscus in the most advanced part to the crater end. The difference between distance L2 and distance L1 was calculated and defined as the crater end deviation ΔL (m).

[0077] The degree of Mn segregation in the cast slab was investigated using the following procedure. The cast slab was cut in the center in the width direction, and a sample was taken from the center in the thickness direction of the cut surface, including a region of 40 mm in the casting direction and another region of 40 mm in the thickness direction. Surface analysis of the Mn concentration was performed on this sample using EPMA (Electron Probe Micro Analyzer). The Mn concentration was accumulated over a 2 mm width along the casting direction, centered on the location with the highest Mn concentration, and the average value was taken as the maximum Mn concentration (Cmax). The degree of Mn segregation was taken by dividing the maximum Mn concentration (Cmax) by the Mn concentration of the bulk composition of the cast slab (C0) (Cmax / C0). The Mn concentration of the bulk composition (C0) was determined by taking an analytical sample from the cast slab and performing chemical analysis.

[0078] Table 1 shows the test conditions and test results for this embodiment. In Table 1, the test conditions include the thickness of the cast slab, casting speed, magnetic flux density of the electromagnetic brake, average specific water content, water content ratio, and surface temperature. Magnetic flux density is the magnetic flux density of the magnetic field applied to the molten steel in the mold by the electromagnetic brake. 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. Water content ratio is the ratio of the amount of water injected from the second nozzle to the amount of water injected from the first nozzle in the range from when the thickness of the solidified shell of the cast slab reaches 30 mm to when it reaches 70 mm. Surface temperature is the maximum value of the average surface temperature in the width direction of the cast slab in the range from when the central solid fraction of the cast slab reaches 0.3 to when the central solid fraction reaches 1.0.

[0079] In Table 1, cast slabs manufactured under test conditions that satisfy all of the following conditions 1 to 3 are labeled as "Example Inventions," and cast slabs manufactured under test conditions that do not satisfy at least one of conditions 1 to 3 are labeled as "Comparative Examples." In Table 1, test conditions in Comparative Examples 1 to 5 that do not satisfy any of the following conditions 1 to 3 are marked with an asterisk (*). Also in Table 1, the surface temperature of Examples 1 to 10 and Comparative Examples 1 to 5 that do not satisfy condition 4 is marked with a "#." Condition 1: The magnetic flux density is 1000 Gauss or more. Condition 2: The average specific water content is between 0.10 L / kg-steel and 0.49 L / kg-steel. Condition 3: The water volume ratio is 1.1 or higher. Condition 4: The surface temperature is 950°C or lower.

[0080] [Table 1]

[0081] For Invention Examples 1-10 and Comparative Examples 2-5, the crater end deviation index of the cast slabs manufactured under each condition was determined, using the crater end deviation of the cast slab of Comparative Example 1 as the reference. The crater end deviation index is an index for comparing the crater end deviations of multiple cast slabs, and is the value obtained by dividing the crater end deviation of each cast slab by the crater end deviation of the reference cast slab (Comparative Example 1). It has been found that if the crater end deviation index of the obtained cast slab is 0.700 or less, the degree of Mn segregation and porosity volume at the center of the thickness of the cast slab are good, and the internal quality is improved. Furthermore, it has been found that if the degree of Mn segregation of the obtained cast slab is 1.40 or less, toughness can be ensured in the product (steel plate) after rolling the cast slab. Based on the above, in the test results (evaluation) in Table 1, if the crater end deviation index is 0.700 or less and the degree of Mn segregation is 1.40 or less, it is indicated as "Excellent," and otherwise it is indicated as "Unacceptable."

[0082] As shown in Table 1, all of the invention examples 1 to 8 satisfied conditions 1 to 4. Therefore, in the invention examples 1 to 8, both the crater end deviation index and the degree of Mn segregation were below the threshold, and good cast slabs were obtained. In other words, central segregation and porosity were reduced.

[0083] Although Invention Examples 9 and 10 met conditions 1 to 3, their surface temperatures were greater than 950°C, thus failing to meet condition 4. In Invention Example 9, compared to Invention Example 1, which had the same test conditions except for surface temperature, the crater end deviation index and Mn segregation degree were slightly higher, but a good cast slab was obtained. Similarly, in Invention Example 10, compared to Invention Example 6, which had the same test conditions except for surface temperature, the Mn segregation degree was slightly higher but below the threshold, and a good cast slab was obtained. In other words, central segregation and porosity were reduced in Invention Examples 9 and 10 as well.

[0084] Comparative Example 1 had an average specific water volume of 0.70 L / kg-steel, which did not meet condition 2. Furthermore, Comparative Example 1 had a water volume ratio of 1.0, which did not meet condition 3. In Comparative Example 1, the degree of Mn segregation was greater than the threshold, resulting in a "failure" evaluation.

[0085] Comparative Examples 2 and 5 had a water volume ratio of 1.0 and did not meet condition 3. In Comparative Examples 2 and 5, both the crater end deviation index and the degree of Mn segregation were greater than the threshold, and the evaluation was "unacceptable".

[0086] In Comparative Example 3, no magnetic field was applied to the molten steel in the mold. Therefore, Comparative Example 3 did not meet Condition 1. In Comparative Example 3, both the crater end deviation index and the degree of Mn segregation were greater than the threshold, and the evaluation was "unacceptable".

[0087] Comparative Example 4 had an average specific water content of 0.55 L / kg-steel and did not meet condition 2. In Comparative Example 4, both the crater end deviation index and the degree of Mn segregation were greater than the threshold, and the evaluation was "unacceptable".

[0088] 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. [Explanation of Symbols]

[0089] 1: Continuous casting machine 4: Mold 4a: Electromagnetic brake 5: Support Role 5a: Roll body 5b: Bearing section 6: Secondary cooling nozzle 6A: Nozzle 1 6B: Second nozzle 6C: Third nozzle 7: Light reduction roll 10: Cast slab 10a: Area (contact area) 10b: Area (non-contact area) 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 mold including an electromagnetic brake, a support roll installed downstream of the mold in the casting direction and including a plurality of roll bodies arranged in the width direction of the cast slab and bearings connecting the roll bodies, a secondary cooling nozzle for spraying secondary cooling water onto the cast slab, the secondary cooling nozzle including a first nozzle that sprays the secondary cooling water onto a region of the cast slab corresponding to the roll bodies, and a second nozzle that sprays the secondary cooling water onto a region of the cast slab corresponding to the bearings, and a light reduction roll installed downstream of the support roll in the casting direction. The aforementioned continuous casting method is The application step involves 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, 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, such that the average specific water content is 0.10 L / kg-steel or more and 0.49 L / kg-steel or less. The process includes a reduction step in which the cast slab is lightly reduced in the thickness direction perpendicular to the width direction of the cast slab using the light reduction roll at a reduction speed of 0.5 mm / min or more and 1.3 mm / min or less, from when the central solid fraction of the cast slab reaches 0.3 until the central solid fraction of the cast slab reaches 1.0, A continuous casting method in which, during the cooling step, the amount of water injected from the second nozzle is 1.1 times or more the amount of water injected from the first nozzle from the time the thickness of the solidified shell of the cast slab reaches 30 mm until it reaches 70 mm.

2. A continuous casting method according to claim 1, The secondary cooling nozzle further includes a third nozzle that sprays the secondary cooling water onto the widthwise end of the cast slab. A continuous casting method in which, in the cooling step, the amount of water injected from the third nozzle is 1.1 times or more the amount of water injected from the first nozzle from the time the thickness of the solidified shell of the cast slab reaches 30 mm until it reaches 70 mm.

3. A continuous casting method according to claim 1 or 2, A continuous casting method wherein the average surface temperature in the width direction of the cast slab is 950°C or less from the time the central solid fraction of the cast slab reaches 0.3 until the central solid fraction reaches 1.0.

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