Continuous casting method for steel

Abstract

To provide a continuous casting method for steal that can suppress surface cracking of a cast piece while reducing center segregation and porosity.SOLUTION: A continuous casting machine (1) is used in a continuous casting method for steal. The continuous casting method comprises an application step, a water amount adjustment step, and a pressing step. In the application step, a magnetic field with a flux density of 1000 Gauss or higher is applied to the molten steel (M) in a casting mold (4) by an electromagnetic brake (4a). In the water amount adjustment step, the amount of water injected from a second nozzle (6B) is adjusted to 1.5 times or more as much as the amount of water injected from the first nozzle (6A) from the time the thickness of the solidified shell (S) of the cast piece (10) reaches 30 mm until it reaches 70 mm. In the pressing process, a cast piece (10) is lightly pressed in the thickness direction perpendicular to the width direction with the light pressing roll (7) at a pressing rate of 0.5 mm / min or more and 1.3 mm / min or less from when the central solidus of the cast piece (10) reaches 0.1 until it reaches 1.0.SELECTED DRAWING: Figure 1

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 cast slab during solidification is non-uniform across the width, there will be regions where the completion of solidification is relatively delayed, and in particular, center segregation and porosity are likely to increase.

[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] Furthermore, continuous casting machines used in continuous casting are equipped with support rolls that guide the slab downstream in the casting direction and nozzles that spray secondary cooling water onto the slab. Generally, 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. 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. As a result, the thickness of the solidified shell of the slab during solidification becomes non-uniform across the width. 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 of the slab 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 of a slab uniform by controlling the amount of secondary cooling water supplied to the slab across its width. Patent Document 1 discloses a technique in which a solidification profile of the slab across its width (the change in the thickness of the solidified shell across its width) is determined in advance, and the amount of secondary cooling water in the slab across its 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 its width is controlled based on this information. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent No. 6561822 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-238256 Summary of the Invention [Problem to be solved by the invention]

[0009] 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.

[0010] 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 use a large pre-solidification reduction method to heavily reduce the slab during solidification. 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 to alleviate 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.

[0011] Furthermore, Patent Documents 1 and 2 give no consideration to suppressing surface cracks in the slab. If an attempt is made to make the thickness of the solidified shell of the slab uniform in the width direction 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 surface temperature of the width direction ends of the slab will decrease, and cracks may occur on the surface of the slab.

[0012] 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]

[0013] The continuous casting method for steel according to the present disclosure uses a continuous casting machine. The continuous casting machine includes a mold, support rolls, a first nozzle, a second nozzle, and a soft reduction roll. The mold includes an electromagnetic brake. The support roll is installed downstream of the mold in the casting direction and is divided into multiple roll body sections in the width direction of the slab. The roll body sections are connected to each other by bearing sections. The first nozzle injects secondary cooling water into an area of ​​the slab corresponding to the roll body section. The second nozzle injects secondary cooling water into an area of ​​the slab corresponding to the bearing section. The soft reduction roll is installed downstream of the support roll in the casting direction. The continuous casting method includes an application process, a water flow adjustment process, and a reduction process. In the application process, 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 water amount adjustment process, the amount of water injected from the second nozzle is set to at least 1.5 times the amount of water injected from the first nozzle from the time when the thickness of the solidified shell of the slab reaches 30 mm until it reaches 70 mm. In the reduction process, the slab is lightly reduced in the thickness direction perpendicular to the width direction of the slab with light reduction rolls at a reduction rate of 0.5 mm / min or more and 1.3 mm / min or less from the time when the center solid fraction of the slab reaches 0.1 until it reaches 1.0. [Effects of the Invention]

[0014] 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]

[0015] [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 of the continuous casting machine of the first embodiment as viewed along the casting direction. [Figure 3] FIG. 3 is a cross-sectional view of the continuous casting machine of the second embodiment as viewed along the casting direction. DETAILED DESCRIPTION OF THE INVENTION

[0016] The continuous casting method for steel according to this embodiment uses a continuous casting machine. The continuous casting machine includes a mold, support rolls, a first nozzle, a second nozzle, and a soft reduction roll. The mold includes an electromagnetic brake. The support rolls are installed downstream of the mold in the casting direction and are divided into multiple roll body sections in the width direction of the slab. The roll body sections are connected to each other by bearing sections. The first nozzle injects secondary cooling water into an area of ​​the slab corresponding to the roll body section. The second nozzle injects secondary cooling water into an area of ​​the slab corresponding to the bearing section. The soft reduction rolls are installed downstream of the support rolls in the casting direction. The continuous casting method includes an application process, a water flow adjustment process, and a reduction process. In the application process, 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 water amount adjustment step, the amount of water injected from the second nozzle is set to at least 1.5 times the amount of water injected from the first nozzle from the time when the thickness of the solidified shell of the slab reaches 30 mm until it reaches 70 mm. In the reduction step, the slab is lightly reduced in the thickness direction perpendicular to the width direction of the slab with light reduction rolls at a reduction rate of 0.5 mm / min or more and 1.3 mm / min or less from the time when the center solid fraction of the slab reaches 0.1 until it reaches 1.0 (first configuration).

[0017] In the continuous casting method according to the first aspect, 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 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. In addition, in the continuous casting method according to the first aspect, in the water flow adjusting step, the amount of water sprayed onto the area of ​​the slab corresponding to the roll body of the support roll is set to 1.5 times or more the amount of water sprayed onto the area of ​​the slab corresponding to the bearing portion of the support roll. This allows for sufficient cooling of the area of ​​the slab corresponding to the bearing portion of the support roll, where solidification completion is likely to be delayed. 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 of the slab can be made uniform in the width direction. By lightly reducing the slab in the thickness direction in the reduction step, centerline segregation and porosity can be reduced.

[0018] In the continuous casting method according to the first aspect, the thickness of the solidified shell of the slab is made uniform in the width direction by controlling not only the amount of secondary cooling water across the width but also the flow of molten steel in the mold using an electromagnetic brake. In this case, the surface temperature of the slab is less likely to decrease compared to when the thickness of the solidified shell of the slab is made uniform in the width direction by controlling only the amount of secondary cooling water. This makes it possible to suppress the occurrence of cracks on the surface of the slab.

[0019] In the continuous casting method of the first configuration, the continuous casting machine may further include a third nozzle for injecting secondary cooling water onto the widthwise ends of the slab. In this case, in the water amount adjusting step, the amount of water injected from the third nozzle is set to 1.5 times or more the amount of water injected from the first nozzle (second configuration).

[0020] According to the continuous casting method of the second aspect, it is possible to sufficiently cool the widthwise ends of the slab, which tend to be slow to complete solidification, as well as the regions of the slab that correspond to the bearing portions of the support rolls, thereby further reducing centerline segregation and porosity.

[0021] 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.

[0022] [First embodiment] [Continuous casting machine] 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 is equipped with a tundish 2, a mold 4, a plurality of support rolls 5, and a plurality of soft reduction rolls 7.

[0023] 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.

[0024] A plurality of secondary cooling nozzles (not shown in FIG. 1 ) and a plurality of support rolls 5 are arranged downstream of the mold 4 in the casting direction. The molten steel M is cooled in the mold 4 and then further cooled by secondary cooling water sprayed from the secondary cooling nozzles. This forms a solidified shell S. The slab 10 in the process of solidifying 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 secondary cooling nozzles are arranged, for example, alternately with the plurality of support rolls 5 in the casting direction.

[0025] FIG. 2 is a cross-sectional view of the continuous casting machine 1 of this embodiment as viewed along 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. The arrangement of the roll bodies 5a and the bearings 5b is typically symmetrical in the width direction of the slab 10.

[0026] 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 arranged 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. The contact portion 10a and the non-contact portion 10b include not only the region of the slab 10 where the support roll 5 is arranged, but also the region between the two support rolls 5 in the casting direction.

[0027] The multiple secondary cooling nozzles are arranged at intervals in the width direction. In a typical example, the multiple secondary cooling nozzles are arranged between adjacent support rolls 5. The multiple secondary cooling nozzles include a first nozzle 6A and a second nozzle 6B. The first nozzle 6A and the second nozzle 6B are arranged along the width direction of the slab 10. The first nozzle 6A sprays secondary cooling water onto the contact portion 10a of the slab 10. The second nozzle 6B sprays secondary cooling water onto the non-contact portion 10b of the slab 10. The amount of cooling water sprayed from each of the first nozzle 6A and the second nozzle 6B is controlled individually.

[0028] 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, each of which is provided with a plurality of light reduction rolls 7. 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 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 furthest upstream in the casting direction, and the position of the exit 71o coincides with the position of the light reduction roll 7 arranged furthest downstream in the casting direction.

[0029] [Continuous casting method] The continuous casting method for steel according to this embodiment includes an application step, a water amount adjustment step, and a reduction step. The slab 10 obtained by this continuous casting method is used as a raw material for products such as steel plate.

[0030] In the application step, 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.

[0031] The magnitude of the magnetic flux density of the magnetic field applied in the application step 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.

[0032] 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 upper limit of the magnetic flux density is preferably 3000 Gauss. 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. When continuously casting steel that is prone to surface cracking (e.g., hypoperitectic steel), the temperature of the molten steel M in the vicinity of the submerged entry nozzle 3 will rise. In this case, the initial solidification of the molten steel M in the mold 4 will be non-uniform, which may cause vertical cracks to form on the surface of the slab 10.

[0033] In a typical continuous casting method, the thickness of the solidified shell S of the slab 10 in the middle of solidification becomes non-uniform in the width direction. The region where the thickness of the solidified shell S is relatively small is the region of the slab 10 where the completion of solidification is likely to be delayed. Hereinafter, the region where the thickness of the solidified shell S becomes relatively small when the slab 10 is cast using a typical continuous casting method will be simply referred to as the delayed solidification portion.

[0034] In the continuous casting method of this embodiment, in the water amount adjustment step, the slab 10 is divided into a plurality of regions in the width direction, and the amount of secondary cooling water injected for each region is controlled. Specifically, the amount of water injected from the secondary cooling nozzles (in this embodiment, the first nozzle 6A and the second nozzle 6B) is individually adjusted, and a large amount of secondary cooling water is injected into the region corresponding to the delayed solidification portion of the slab 10. Hereinafter, the ratio of the largest amount of water (e.g., the amount of water injected into the delayed solidification portion) to the smallest amount of water (e.g., the amount of water injected into the portion of the slab 10 other than the delayed solidification portion) among the amounts of water injected into each region across the width direction of the slab 10 is simply referred to as the water amount ratio. In the slab 10, the region corresponding to the delayed solidification portion is the non-contact portion 10b described above.

[0035] In the water volume adjustment process, the volume of secondary cooling water sprayed from the second nozzle 6B is set to be 1.5 times or more the volume of secondary cooling water sprayed from the first nozzle 6A. In other words, the volume of water sprayed onto the non-contact portion 10b of the slab 10 is set to be 1.5 times or more the volume of water sprayed onto the contact portion 10a of the slab 10. This can also be said to be a water volume ratio of 1.5 or more. Generally, the non-contact portion 10b of the slab 10 is more difficult to cool than the contact portion 10a, and the completion of solidification is more likely to be delayed. In short, the non-contact portion 10b is a region corresponding to a delayed solidification portion. By supplying a large amount of secondary cooling water to the non-contact portion 10b, solidification in the non-contact portion 10b can be promoted.

[0036] However, if an excessive amount of secondary cooling water is supplied to a certain region of the slab 10, the surface temperature of that region will become extremely low. This will increase the difference in surface temperature of the slab 10 in the width direction, causing thermal stress in the slab 10. In this case, depending on the type of steel of the slab 10, surface cracks may occur in the slab 10. Therefore, it is preferable that the amount of water sprayed from the second nozzle 6B be 4.0 times or less the amount of water sprayed from the first nozzle 6A (water volume ratio of 4.0 or less).

[0037] Such water flow rate adjustment is performed for the slab 10 within an appropriate range in the casting direction. In the region where the thickness of the solidified shell S of the slab 10 is less than 30 mm, i.e., directly below the mold 4, the temperature of the slab 10 is extremely high. If the surface temperature of the slab 10 is varied in this region, surface cracks due to thermal stress may occur in the slab 10. Furthermore, when the unsolidified molten steel M is cooled with secondary cooling water, the solidified shell S acts as a thermal resistance, making it difficult to promote solidification in the region where the solidified shell S is thick. Therefore, the water flow rate adjustment is preferably performed for the slab 10 within the range where the thickness of the solidified shell S reaches 30 mm and 70 mm in the casting direction. A method for measuring the thickness of the solidified shell S will be described later.

[0038] The more finely the slab 10 is divided in the width direction in order to control the amount of secondary cooling water, the more precisely the amount of water supplied to the slab 10 can be controlled, but the more complex the control becomes and the higher the cost required for control. The width dimension of each divided region in the slab 10 is preferably 150 to 250 mm. This is approximately the same size as the width dimension of the bearing portions 5b of the support rolls 5.

[0039] In continuous casting of steel, when the roll bodies 5a and bearings 5b of the support rolls 5 are arranged symmetrically in the width direction of the slab 10, the delayed solidification portions are also symmetrical in the width direction. In this case, the amount of secondary cooling water supplied to the slab 10 may also be line-symmetrical in the width direction.

[0040] In the reduction process, 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. Soft reduction is performed from when the center solid fraction of the slab 10 reaches 0.1 until it reaches 1.0. That is, the center solid fraction of the slab 10 is 0.1 at the entrance 71i of the soft reduction zone 71, and 1.0 at the exit 71o of the soft reduction zone 71. Soft reduction of the region of the slab 10 with a center solid fraction less than 0.1 generally does not affect the reduction of center segregation and porosity formed at the end of solidification, so there is no need to soft-reduc this region. Furthermore, soft reduction of the region of the slab 10 with a center solid fraction of 1.0, i.e., the region where the center of the thickness of the slab 10 is completely solidified, has almost no effect on the reduction of center segregation and porosity, so there is no need to soft-reduc this region.

[0041] In the reduction step, if the reduction rate in the soft reduction zone 71 is less than 0.5 mm / min, the reduction amount is insufficient relative to the solidification shrinkage of the slab 10, resulting in increased center segregation and porosity. Furthermore, if the reduction rate is greater than 1.3 mm / min, the reduction amount becomes excessive relative to the solidification shrinkage of the slab 10, causing the molten steel M to flow back from downstream to upstream in the casting direction, resulting in increased flow of concentrated molten steel, which in turn increases center segregation. Therefore, the reduction rate of the slab 10 in the reduction step is preferably 0.5 mm / min or more and 1.3 mm / min or less.

[0042] In the continuous casting method according to this embodiment, the water flow rate adjustment step adjusts the amount of water sprayed from the secondary cooling nozzles (first nozzle 6A and second nozzle 6B) depending on the solidification state of the slab 10 in the width direction. The solidification state of the slab 10 in the width direction is highly dependent 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, by examining the cross section of the slab 10 cast using the continuous casting machine 1 once, it is possible to estimate the solidification state of the slab 10 in the width direction that will be cast using the same continuous casting machine 1. If the solidification state of the slab 10 can be estimated, the amount of water sprayed from the nozzles can be adjusted in advance so that a large amount of secondary cooling water is supplied to the delayed solidification portion. An example of this estimation method is described below.

[0043] Electromagnetic stirring is performed on the slab 10 within a range of 5 to 20 m from the meniscus (the molten metal surface in the mold 4), and a cross-sectional sample of the slab 10 is taken. The sample is corroded with hydrochloric acid to reveal a white band that occurs due to stirring in the unsolidified portion. The distance from the surface of the slab 10 to the white band is then measured across the width of the slab 10, thereby obtaining the solidification profile of the slab 10, i.e., the transition in the thickness of the solidified shell S across the width of the slab 10.

[0044] [effect] In the continuous casting method according to this embodiment, 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 the electromagnetic brake 4a. This weakens the flow of the molten steel M discharged from the discharge hole of the submerged entry nozzle 3 toward the widthwise ends of the solidified shell S in the mold 4. Furthermore, in the continuous casting method according to this embodiment, in the water flow adjustment step, the amount of water sprayed onto the non-contact portion 10b of the slab 10 is set to 1.5 times or more the amount of water sprayed onto the contact portion 10a of the slab 10. This allows for sufficient cooling of the non-contact portion 10b, which corresponds to the delayed solidification portion. In this way, by controlling both the flow of the molten steel M in the mold 4 and the amount of secondary cooling water, the thickness of the solidified shell S of the slab 10 can be made uniform in the width direction. By lightly reducing the thickness of the slab 10 in the reduction step, centerline segregation and porosity can be reduced.

[0045] In the continuous casting method according to this embodiment, not only is the amount of secondary cooling water controlled across the width, but the electromagnetic brake 4a is used to control the flow of molten steel M in the mold 4, thereby making the thickness of the solidified shell S of the slab 10 uniform across the width. In this case, the surface temperature of the slab 10 is less likely to decrease compared to when the thickness of the solidified shell S of the slab 10 is made uniform across the width by controlling the amount of secondary cooling water alone. This makes it possible to suppress the occurrence of cracks on the surface of the slab 10.

[0046] [Second embodiment] Fig. 3 is a cross-sectional view of a continuous casting machine 1 according to a second embodiment, viewed along the casting direction. Fig. 3 shows the state below the mold 4. Referring to Fig. 3, in the second embodiment, the continuous casting machine 1 is equipped with a third nozzle 6C. The third nozzle 6C injects secondary cooling water onto the width direction ends of the slab 10.

[0047] In the continuous casting method according to this embodiment, in the water volume adjustment step, the volume of water sprayed from the third nozzle 6C is set to be at least 1.5 times the volume of water sprayed from the first nozzle 6A. In other words, the volume of water sprayed onto the width direction ends of the slab 10 is set to be at least 1.5 times the volume of water sprayed onto the contact portion 10a of the slab 10. Although not particularly limited, the volume of water sprayed from the third nozzle 6C may be the same as the volume of water sprayed from the second nozzle 6B.

[0048] Generally, the widthwise ends of the slab 10 tend to reach high temperatures due to the high-temperature molten steel M being supplied from the discharge holes in the mold 4, and the completion of solidification is likely to be delayed. In other words, the widthwise ends of the slab 10, like the non-contact portion 10b, correspond to solidification delay portions. According to the continuous casting method of the second embodiment, the widthwise ends of the slab 10 can be sufficiently cooled. This makes it possible to further reduce center segregation and porosity. [Example]

[0049] 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 Mn segregation degree, porosity volume ratio, and surface cracks of the cast slab obtained by continuous casting were evaluated.

[0050] The slabs used in this test 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 entrance to the soft reduction zone was located 16 m from the meniscus (the molten metal surface in the mold). The slabs were soft reduced in the thickness direction with soft reduction rolls at a reduction rate of 0.8 mm / min from the time the central solid fraction reached 0.1 until it reached 1.0. The central temperature and solid fraction of the slab were calculated by two-dimensional solidification analysis in the thickness and width directions of the slab. The specific water flow rate of the cooling water sprayed from the secondary cooling nozzle was 0.5 to 1.5 L / kg-steel.

[0051] 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%.

[0052] Before the test, the solidification profile of the slab cast by the continuous casting machine used in the test was measured. As described above, the solidification profile was obtained by revealing a white band in a cross-sectional sample of the slab and measuring the distance from the surface of the slab to the white band across the width. This allowed the position of the delayed solidification portion in the width direction of the slab cast by the continuous casting machine to be identified in advance.

[0053] The degree of Mn segregation in the slab was investigated using the following procedure. The 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, extending 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 maximum Mn concentration (Cmax) was divided by the Mn concentration (C0) in the bulk composition of the slab, and the value (Cmax / C0) was taken as the degree of Mn segregation. The Mn concentration (C0) in the bulk composition was determined by chemical analysis of an analytical sample taken from the slab.

[0054] The porosity volume of the slab was investigated using the following procedure. Samples were taken from the center of the slab in the thickness direction, measuring 50 mm in the casting direction, 100 mm in the width direction, and 7 mm in the thickness direction. The samples were taken from 16 locations along the width direction of the slab. The density ρ of each sample was measured using the method for measuring density and specific gravity of solids specified in JIS Z 8807. The porosity volume V (cm) per unit weight was then calculated using the following formula (1): 3 The density ρ0 in equation (1) was determined by taking samples from the quarter-thickness portion of the slab in the same manner as above and measuring the density in the same manner as above. The maximum porosity volume V among the porosity volumes V measured for each sample was taken as the porosity volume V of that slab.

[0055]

number

[0056] The presence or absence of surface cracks in slabs was investigated using the following procedure. The surface of the slab was ground with a grinder to remove scale and other imperfections, after which a penetrant test was performed and the presence or absence of surface cracks in the slab was confirmed visually. If there were irremovable cracks of 5 mm or more in depth on the surface of the slab, the slab was deemed to have failed, and if not, it was deemed to have passed.

[0057] Table 1 shows the test conditions and test results.

[0058] [Table 1]

[0059] Table 1 lists the test conditions for the thickness of the slab produced by continuous casting, the casting speed, the water flow rate, the magnetic flux density of the electromagnetic brake's magnetic field, and the shell thickness deviation ratio. The shell thickness deviation ratio, an indicator of the degree of solidification nonuniformity across the width of the slab, was investigated using the following procedure. Electromagnetic stirring was performed on the slab, and the collected samples were corroded with hydrochloric acid to reveal white bands. The distance from the slab surface to the white bands was measured across the width of the slab at 100 mm intervals. The difference between the largest and smallest solidified shell thicknesses measured across the width was calculated and used as the shell thickness deviation. The shell thickness deviation ratio is an index for comparing shell thickness deviations between slabs of the same thickness. A reference slab was selected from multiple slabs, and the shell thickness deviation of each slab divided by the shell thickness deviation of the reference slab was used as the shell thickness deviation ratio.

[0060] It is known that if the Mn segregation ratio of the resulting slab is 1.4 or less, toughness can be ensured in the product (steel plate) after rolling the slab. Therefore, in Table 1, the test results are shown as "excellent" if the Mn segregation ratio is 1.4 or less, and "poor" if it is not. Furthermore, it is known that if the porosity volume ratio is 0.8 or less, defects in the product (steel plate) after rolling the slab are neutralized. Therefore, if the porosity volume ratio is 0.8 or less, it is shown as "excellent" and if it is not, it is shown as "poor." Here, the porosity volume ratio is an index for comparing the porosity volume between slabs of the same thickness. A reference slab was selected from multiple slabs, and the porosity volume of each slab divided by the porosity volume of the reference slab was used as the porosity volume ratio.

[0061] Furthermore, in Table 1, if there were irremovable cracks of 5 mm or more in depth on the surface of the slab, the result was marked as "poor," otherwise it was marked as "excellent."The Mn segregation degree, porosity volume ratio, and surface cracking were evaluated comprehensively, and if all the evaluations were satisfactory, the overall evaluation was marked as "excellent," otherwise it was marked as "poor."

[0062] For Examples 1 to 4 and Comparative Examples 1 to 5, which correspond to slabs with a thickness of 250 mm, the shell thickness deviation ratio and porosity volume ratio of each slab were determined using the slab of Comparative Example 1 as the standard.

[0063] As shown in Table 1, all of Examples 1 to 4 satisfied the conditions specified in the above embodiment. Therefore, Examples 1 to 4 produced cast slabs with good Mn segregation and porosity volume ratios. Furthermore, no irremovable cracks occurred on the surface of the cast slabs. In other words, surface cracks in the cast slabs could be suppressed while reducing center segregation and porosity.

[0064] In Comparative Example 1, the water ratio was smaller than the conditions specified in the above embodiment. In this case, the shell thickness deviation increased, and the Mn segregation degree and porosity volume ratio deteriorated. In Comparative Example 2, the water ratio was smaller than the conditions specified in the above embodiment, as in Comparative Example 1, and no magnetic field was applied to the molten steel in the mold. In this case, the shell thickness deviation increased even more, and the Mn segregation degree and porosity volume ratio deteriorated.

[0065] In Comparative Examples 3 and 4, unlike the conditions specified in the above embodiment, a magnetic field was not applied to the molten steel in the mold. However, in Comparative Examples 3 and 4, different water ratios were used within the range of the conditions specified in the above embodiment. In these cases, the shell thickness deviation became slightly larger, and the Mn segregation degree and porosity volume ratio deteriorated.

[0066] In Comparative Example 5, the water ratio was higher than the conditions specified in the above embodiment, and furthermore, a magnetic field was not applied to the molten steel in the mold. In this case, the water ratio was too high, and irremovable cracks occurred on the surface of the slab.

[0067] For Example 5 and Comparative Example 6, which correspond to slabs with a thickness of 300 mm, the shell thickness deviation ratio and porosity volume ratio of each slab were determined using the slab of Comparative Example 6 as the standard.

[0068] All of the conditions specified in the above embodiment were satisfied in Example 5. Therefore, in Example 5, a slab with a good Mn segregation degree and porosity volume ratio was obtained. Furthermore, no irremovable cracks occurred on the surface of the slab. In other words, surface cracks in the slab were suppressed while reducing center segregation and porosity. On the other hand, in Comparative Example 6, the water ratio was lower than the conditions specified in the above embodiment. In this case, the delayed solidification portion could not be sufficiently cooled, resulting in a deterioration in the Mn segregation degree and porosity volume ratio.

[0069] For Example 6 and Comparative Example 7, which correspond to slabs with a thickness of 370 mm, the shell thickness deviation ratio and porosity volume ratio of each slab were determined using the slab of Comparative Example 7 as the standard.

[0070] All of the conditions specified in the above embodiment were satisfied in Example 6. Therefore, in Example 6, a slab with a good Mn segregation degree and porosity volume ratio was obtained. Furthermore, no irremovable cracks occurred on the surface of the slab. In other words, it was possible to suppress surface cracks in the slab while reducing center segregation and porosity. On the other hand, in Comparative Example 7, the water ratio was small compared to the conditions specified in the above embodiment. In this case, the delayed solidification portion could not be sufficiently cooled, and the Mn segregation degree and porosity volume ratio deteriorated.

[0071] 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]

[0072] 1: Continuous casting machine 4: Mold 5: Support roll 5a: Roll body 5b: Bearing part 6A: First nozzle 6B: Second nozzle 6C: 3rd nozzle 7: Light reduction roll 10: Casting S: Solidified shell

Claims

1. A method for continuously casting steel using a continuous casting machine comprising: a mold including an electromagnetic brake; support rolls disposed downstream in the casting direction of the mold and divided into a plurality of roll body sections in the width direction of a slab, the roll body sections being connected to each other by bearing sections; first nozzles that spray secondary cooling water onto regions of the slab that correspond to the roll body sections; second nozzles that spray secondary cooling water onto regions of the slab that correspond to the bearing sections; and soft reduction rolls disposed downstream in the casting direction of the support rolls, 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 water amount adjusting step of adjusting the amount of water sprayed from the second nozzle to be 1.5 times or more the amount of water sprayed from the first nozzle until the thickness of the solidified shell of the slab reaches 70 mm after reaching 30 mm; a reduction step in which the slab is soft reduced in a thickness direction perpendicular to a 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 after the center solid fraction of the slab reaches 0.1 until the center solid fraction reaches 1.0; A continuous casting method comprising:

2. The continuous casting method according to claim 1, the continuous casting machine further includes a third nozzle that injects secondary cooling water onto width direction ends of the slab, In the water amount adjusting step, the amount of water sprayed from the third nozzle is set to 1.5 times or more the amount of water sprayed from the first nozzle.

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