Title of invention: continuous casting method, slab slab, and continuous casting machine
Technical field
[0001]
　The techniques disclosed in the present application relate to continuous casting methods, slab slabs, and continuous casting machines.
Background technology
[0002]
　There is a continuous casting method in which the unsolidified portion in the slab conveyed from the mold is agitated by an electromagnetic stirrer (for example, Japanese Patent Application Laid-Open No. 2010-179342, International Publication No. 2009/133739, and Japanese Patent Application Laid-Open No. 2005-305517. Issue).
Outline of the invention
Problems to be solved by the invention
[0003]
　By the way, there is a technique for suppressing the residual of molten steel (hereinafter referred to as "concentrated molten steel") having a predetermined component concentrated by segregation (solidification segregation) as macrosegregation in the slab. As this technique, there is a technique in which a slab having an unsolidified portion is reduced by a reduction roll, and the concentrated molten steel in the unsolidified portion is pushed back (discharged) from the reduction roll to the mold side.
[0004]
　However, the concentrated molten steel pushed back from the reduction roll to the mold side is difficult to mix with the molten steel (mother molten steel) transported from the mold to the reduction roll. Therefore, there is room for further improvement in order to prevent the concentrated molten steel from remaining in the slab as macrosegregation.
[0005]
　Further, when a plurality of dendrites are present in the unsolidified portion of the slab, these dendrites become a flow resistance (obstacle) of the concentrated molten steel pushed back from the reduction roll to the mold side. Therefore, it becomes difficult for the concentrated molten steel to be pushed back from the reduction roll to the mold side, and macrosegregation tends to remain on the slab.
[0006]
　Furthermore, semi-macro segregation is likely to be captured between adjacent dendrites. Therefore, if dendrites are present in the unsolidified portion of the slab, semi-macro segregation tends to remain in the slab.
[0007]
　The technique disclosed in the present application aims to reduce macrosegregation and semi-macrosegregation of slabs.
Means to solve problems
[0008]
　In the continuous casting method according to the first aspect, the unsolidified portion in the slab transported from the mold is arranged on the first electromagnetic stirrer and the downstream side of the first electromagnetic stirrer in the transport direction of the slab. This is a continuous casting method in which the slab is reduced by a reduction roll after being stirred by the second electromagnetic agitator, and the first electromagnetic agitator moves the unsolidified portion to one side in the width direction of the slab. The one-sided electromagnetic force that causes the unsolidified portion to flow to the other side in the width direction of the slab at a flow rate of 5 cm / s or more, and the other-side electromagnetic force that causes the unsolidified portion to flow to the other side in the width direction of the slab at a flow rate of 5 cm / s or more. Is given alternately to.
[0009]
　According to the continuous casting method according to the first aspect, the unsolidified portion in the slab conveyed from the mold is agitated by the first electromagnetic stirrer and the second electromagnetic stirrer, respectively.
[0010]
　Next, the slab having the unsolidified portion is reduced by the reduction roll. As a result, the concentrated molten steel in the unsolidified portion is pushed back (discharged) from the reduction roll to the mold side.
[0011]
　Further, the first electromagnetic stirrer has a one-sided electromagnetic force that causes the unsolidified portion to flow to one side in the width direction of the slab at a flow rate of 5 cm / s or more, and a first electromagnetic stirrer that causes the unsolidified portion to flow 5 cm to the other side in the width direction of the slab. The other side electromagnetic force that flows at a flow rate of / s or more is alternately applied to the slab.
[0012]
　In this way, the unsolidified portion is made to flow to one side in the width direction of the slab at a flow velocity of 5 cm / s or more by the one-side electromagnetic force, so that a shear force of a predetermined value or more is applied to the tip of the dendrite in the unsolidified portion. It works. Similarly, by causing the unsolidified portion to flow to the other side in the width direction of the slab at a flow rate of 5 cm / s or more by the electromagnetic force on the other side, the tip of the dendrite in the unsolidified portion is sheared by a predetermined value or more. Force acts. As a result, the tip of the dendrite is divided, and equiaxed crystals are easily generated.
[0013]
　Further, the first electromagnetic stirrer alternately applies one-sided electromagnetic force and the other-side electromagnetic force to the slab. As a result, in this embodiment, the tip portion of the dendrite in the unsolidified portion is more likely to be divided as compared with the case where the unsolidified portion is made to flow only to one side in the width direction of the slab by the first electromagnetic agitator.
[0014]
　Then, when the tip of the dendrite is divided, the flow resistance (obstacle) of the concentrated molten steel pushed back from the reduction roll to the mold side decreases. As a result, the concentrated molten steel is easily pushed back from the reduction roll to the mold side. Therefore, it is further suppressed that the concentrated molten steel remains as macrosegregation in the slab.
[0015]
　Further, by dividing the tip of the dendrite by the first electromagnetic stirrer, the semi-macro segregation captured between the dendrites is reduced. Therefore, semi-macro segregation is suppressed from remaining on the slab.
[0016]
　As described above, in this aspect, macrosegregation and semi-macrosegregation of slabs can be reduced.
[0017]
　The continuous casting method according to the second aspect is the continuous casting method according to the first aspect, in which the first electromagnetic stirrer intermittently applies the one-side electromagnetic force and the other-side electromagnetic force to the slab. ..
[0018]
　According to the above-mentioned continuous casting method, the first electromagnetic stirrer intermittently applies one-sided electromagnetic force and the other-side electromagnetic force to the slab. That is, the first electromagnetic stirrer applies the one-sided electromagnetic force and the other-side electromagnetic force to the slab at intervals.
[0019]
　As a result, for example, the flow velocity of the unsolidified portion decreases between the time when the application of the one-side electromagnetic force to the slab is stopped and the time when the application of the other-side electromagnetic force is started. Therefore, when the application of the electromagnetic force on the other side to the slab is started, the flow direction of the unsolidified portion is smoothly reversed, and the unsolidified portion easily flows to the other side in the width direction of the slab. Similarly, when the electromagnetic force applied to the slab is switched from the other side electromagnetic force to the one side electromagnetic force, the flow direction of the unsolidified portion is smoothly reversed, and the unsolidified portion is the slab. It becomes easy to flow to one side in the width direction of.
[0020]
　Therefore, the tip of the dendrite in the unsolidified portion can be separated while reducing the power consumption of the first electromagnetic stirrer.
[0021]
　The continuous casting method according to the third aspect is the continuous casting method according to the first aspect or the second aspect, wherein the slab has a solidified shell portion including the unsolidified portion, and the first electromagnetic stirrer An alternating current satisfying the formula (1) is applied to generate the one-sided electromagnetic force and the other-sided electromagnetic force in the first electromagnetic stirring device.
[0022]
　According to the above continuous casting method, an alternating current satisfying the formula (1) is applied to the first electromagnetic stirrer to generate one-sided electromagnetic force and the other-side electromagnetic force in the first electromagnetic stirrer.
[0023]
　Here, the position of the tip of the dendrite in the unsolidified portion varies depending on the thickness of the solidified shell portion. Specifically, as the thickness of the solidified shell portion increases, the position of the tip portion of the dendrite moves toward the center in the thickness direction of the slab. On the other hand, when the thickness of the solidified shell portion becomes thin, the position of the tip portion of the dendrite moves toward the surface side in the thickness direction of the slab.
[0024]
　Further, the depth (penetration depth) of the electromagnetic force (one-side electromagnetic force and the other-side electromagnetic force) with respect to the slab varies depending on the frequency of the alternating current applied to the first electromagnetic agitator. Specifically, as the frequency of the alternating current applied to the first electromagnetic stirrer becomes smaller, the penetration depth of the electromagnetic force into the slab becomes deeper. On the other hand, when the frequency of the alternating current applied to the electromagnetic coil of the first electromagnetic stirrer is increased, the penetration depth of the electromagnetic force into the slab becomes shallow.
[0025]
　Therefore, in this embodiment, an alternating current having a frequency satisfying the equation (1) is applied to the first electromagnetic stirrer. Specifically, as the thickness of the solidified shell portion increases, the frequency of the alternating current applied to the first electromagnetic stirrer is reduced. On the other hand, as the thickness of the solidified shell portion becomes thinner, the frequency of the alternating current applied to the first electromagnetic stirrer is increased.
[0026]
　As a result, one-sided electromagnetic force and the other-side electromagnetic force can be applied to the tip of the dendrite regardless of the thickness of the solidified shell portion. Therefore, the tip of the dendrite can be efficiently divided.
[0027]
　The continuous casting method according to the fourth aspect is the continuous casting method according to any one of the first to third aspects. The flow velocity of each is set to 5 cm / s or more.
[0028]
　According to the above-mentioned continuous casting method, the flow velocity at the solidification interface of the unsolidified portion is set to 5 cm / s or more by the one-side electromagnetic force and the other-side electromagnetic force. As a result, the tip of the dendrite can be efficiently divided.
[0029]
　The continuous casting method according to the fifth aspect is the continuous casting method according to any one of the first to fourth aspects, wherein the second electromagnetic agitator is pushed back to the mold side by the reduction roll. Stir the molten steel in the solidified part.
[0030]
　According to the above continuous casting method, the second electromagnetic agitator agitates (electromagnetically agitates) the concentrated molten steel in the unsolidified portion pushed back from the reduction roll to the mold side. As a result, the concentrated molten steel pushed back from the reduction roll to the mold side is easily mixed with the molten steel (mother molten steel) transported from the mold to the reduction roll. As a result, the concentrated molten steel is diluted. Therefore, it is suppressed that the concentrated molten steel remains as macrosegregation on the slab.
[0031]
　The continuous casting method according to the sixth aspect is the continuous casting method according to any one of the first to fifth aspects, and the second electromagnetic stirrer has the unsolidified portion on one side in the width direction of the slab. One-sided electromagnetic force that flows to the slab and the other-side electromagnetic force that causes the unsolidified portion to flow to the other side in the width direction of the slab are alternately applied to the slab.
[0032]
　According to the above-mentioned continuous casting method, the second electromagnetic stirrer causes the unsolidified portion to flow to one side in the width direction of the slab and the unsolidified portion to flow to the other side in the width direction of the slab. The electromagnetic force on the other side is alternately applied to the slab. As a result, the concentrated molten steel pushed back from the reduction roll to the mold side becomes more easily mixed with the molten steel (mother molten steel) transported from the mold to the reduction roll. As a result, the concentrated molten steel is diluted. Therefore, it is further suppressed that the concentrated molten steel remains as macrosegregation in the slab.
[0033]
　The continuous casting method according to the seventh aspect is the continuous casting method according to any one of the first to sixth aspects, wherein the thickness of the slab is within the range of 250 to 300 mm, and the transport speed of the slab is set. The first electromagnetic stirrer is arranged within a range of 0.7 to 1.1 m / min and within a range of 6 to 10 m downstream from the meniscus in the mold along the transport direction of the slab.
[0034]
　According to the above continuous casting method, the thickness of the slab is within the range of 250 to 300 mm. Further, the transport speed of the slab shall be within the range of 0.7 to 1.1 m / min. Further, the first electromagnetic stirrer is arranged within a range of 6 to 10 m downstream from the meniscus in the mold along the transport direction of the slab.
[0035]
　As a result, the first electromagnetic agitator can efficiently divide the tip of the dendrite in the unsolidified portion of the slab to generate equiaxed crystals. Therefore, macrosegregation and semi-macrosegregation of slabs can be further reduced.
[0036]
　The slab slab according to the eighth aspect is formed in the central region in the thickness direction of the slab slab, and has a central negative segregation band having a minimum Mn segregation degree in the range of 0.92 to 0.95 and the slab. A surface-side negative segregation band generated in the region L1 of the formula (3) in the slab and having a minimum Mn segregation degree in the range of 0.95 to 0.98, and the central region in the slab slab. It includes an intermediate negative segregation band generated in the region L2 of the formula (4) located between the region L1 and having a minimum value of Mn segregation degree in the range of 0.96 to 0.97.
[0037]
　The above slab slab includes a central negative segregation band, a surface side negative segregation band, and an intermediate negative segregation band. The central negative segregation band is formed in the central region in the thickness direction of the slab slab. The minimum value of the Mn segregation degree of the central negative segregation band is in the range of 0.92 to 0.95.
[0038]
　The surface-side negative segregation band is generated in the region L1 of the formula (3). The minimum value of the Mn segregation degree of the surface-side negative segregation band is in the range of 0.95 to 0.98.
[0039]
　The intermediate negative segregation band is generated in the region L2 of the equation (4) located between the central region and the region L1. The minimum value of the Mn segregation degree of the intermediate negative segregation band is in the range of 0.96 to 0.97.
[0040]
　As described above, the slab slab provided with the predetermined central negative segregation band, surface side negative segregation band, and intermediate negative segregation band is continuously cast by, for example, the continuous casting method according to any one of the first to seventh aspects. Will be done.
[0041]
　The continuous casting machine according to the ninth aspect includes a mold, a first electromagnetic agitator that agitates an unsolidified portion in a slab conveyed from the mold, and a first electromagnetic agitator that conveys the slab to the first electromagnetic agitator. A second electromagnetic agitator that is arranged on the downstream side in the direction and stirs the unsolidified portion, and a reduction roll that is arranged on the downstream side in the transport direction of the slab with respect to the second electromagnetic agitator and reduces the slab. One-sided electromagnetic force that causes the unsolidified portion to flow to one side in the width direction of the slab at a flow rate of 5 cm / s or more, and 5 cm / s or more to move the unsolidified portion to the other side in the width direction of the slab. The first electromagnetic agitator is provided with a control unit that alternately generates an electromagnetic force on the other side that flows at the flow rate of the above.
[0042]
　According to the above-mentioned continuous casting machine, the unsolidified portion in the slab conveyed from the mold is agitated by the first electromagnetic stirrer and the second electromagnetic stirrer, respectively.
[0043]
　Next, the slab having the unsolidified portion is reduced by the reduction roll. As a result, the concentrated molten steel in the unsolidified portion is pushed back (discharged) from the reduction roll to the mold side.
[0044]
　The control unit also controls the first electromagnetic stirrer. As a result, the first electromagnetic stirrer has one-sided electromagnetic force that causes the unsolidified portion to flow to one side in the width direction of the slab at a flow rate of 5 cm / s or more, and the unsolidified portion to the other side in the width direction of the slab. The other side electromagnetic force that causes the slab to flow at a flow velocity of 5 cm / s or more is alternately applied to the slab.
[0045]
　In this way, the unsolidified portion is made to flow to one side in the width direction of the slab at a flow velocity of 5 cm / s or more by the one-side electromagnetic force, so that a shear force of a predetermined value or more is applied to the tip of the dendrite in the unsolidified portion. It works. Similarly, by causing the unsolidified portion to flow to the other side in the width direction of the slab at a flow rate of 5 cm / s or more by the electromagnetic force on the other side, the tip of the dendrite in the unsolidified portion is sheared by a predetermined value or more. Force acts. As a result, the tip of the dendrite is divided, and equiaxed crystals are easily generated.
[0046]
　Further, the first electromagnetic stirrer alternately applies one-sided electromagnetic force and the other-side electromagnetic force to the slab. As a result, in this embodiment, the tip portion of the dendrite in the unsolidified portion is more likely to be divided as compared with the case where the unsolidified portion is made to flow only to one side in the width direction of the slab by the first electromagnetic agitator.
[0047]
　Then, when the tip of the dendrite is divided, the flow resistance (obstacle) of the concentrated molten steel pushed back from the reduction roll to the mold side decreases. As a result, the concentrated molten steel is easily pushed back from the reduction roll to the mold side. Therefore, it is further suppressed that the concentrated molten steel remains as macrosegregation in the slab.
[0048]
　Further, by dividing the tip of the dendrite by the first electromagnetic stirrer, the semi-macro segregation captured between the dendrites is reduced. Therefore, semi-macro segregation is suppressed from remaining on the slab.
[0049]
　As described above, in this aspect, macrosegregation and semi-macrosegregation of slabs can be reduced.
[0050]
　The continuous casting machine according to the tenth aspect is the continuous casting machine according to the ninth aspect, in which the control unit intermittently generates the one-side electromagnetic force and the other-side electromagnetic force in the first electromagnetic agitator. ..
[0051]
　According to the above-mentioned continuous casting machine, the control unit controls the first electromagnetic stirrer. As a result, the first electromagnetic stirrer intermittently applies one-sided electromagnetic force and the other-side electromagnetic force to the slab. That is, the first electromagnetic stirrer applies the one-sided electromagnetic force and the other-side electromagnetic force to the slab at intervals.
[0052]
　As a result, for example, the flow velocity of the unsolidified portion decreases between the time when the application of the one-side electromagnetic force to the slab is stopped and the time when the application of the other-side electromagnetic force is started. Therefore, when the application of the electromagnetic force on the other side to the slab is started, the flow direction of the unsolidified portion is smoothly reversed, and the unsolidified portion easily flows to the other side in the width direction of the slab. Similarly, when the electromagnetic force applied to the slab is switched from the other side electromagnetic force to the one side electromagnetic force, the flow direction of the unsolidified portion is smoothly reversed, and the unsolidified portion is the slab. It becomes easy to flow to one side in the width direction of.
[0053]
　Therefore, the tip of the dendrite in the unsolidified portion can be separated while reducing the power consumption of the first electromagnetic stirrer.
[0054]
　The continuous casting machine according to the eleventh aspect is the continuous casting machine according to the ninth aspect or the tenth aspect. An alternating current satisfying 1) is applied to the first electromagnetic stirrer to generate the one-sided electromagnetic force and the other-side electromagnetic force in the first electromagnetic stirrer.
[0055]
　According to the above-mentioned continuous casting machine, the control unit applies an alternating current satisfying the equation (1) to the first electromagnetic stirrer to generate one-sided electromagnetic force and the other-side electromagnetic force in the first electromagnetic stirrer.
[0056]
　Here, the position of the tip of the dendrite in the unsolidified portion varies depending on the thickness of the solidified shell portion. Specifically, as the thickness of the solidified shell portion increases, the position of the tip portion of the dendrite moves toward the center in the thickness direction of the slab. On the other hand, when the thickness of the solidified shell portion becomes thin, the position of the tip portion of the dendrite moves toward the surface side in the thickness direction of the slab.
[0057]
　Further, the depth (penetration depth) of the electromagnetic force (one-side electromagnetic force and the other-side electromagnetic force) with respect to the slab varies depending on the frequency of the alternating current applied to the first electromagnetic agitator. Specifically, as the frequency of the alternating current applied to the first electromagnetic stirrer becomes smaller, the penetration depth of the electromagnetic force into the slab becomes deeper. On the other hand, when the frequency of the alternating current applied to the electromagnetic coil of the first electromagnetic stirrer is increased, the penetration depth of the electromagnetic force into the slab becomes shallow.
[0058]
　Therefore, the control unit applies an alternating current having a frequency satisfying the equation (1) to the first electromagnetic stirrer. Specifically, as the thickness of the solidified shell portion increases, the frequency of the alternating current applied to the first electromagnetic stirrer is reduced. On the other hand, as the thickness of the solidified shell portion becomes thinner, the frequency of the alternating current applied to the first electromagnetic stirrer is increased.
[0059]
　As a result, one-sided electromagnetic force and the other-side electromagnetic force can be applied to the tip of the dendrite regardless of the thickness of the solidified shell portion. Therefore, the tip of the dendrite can be efficiently divided.
[0060]
　The continuous casting machine according to the twelfth aspect is the continuous casting machine according to any one of the ninth aspect to the eleventh aspect. The flow velocity of each is set to 5 cm / s or more.
[0061]
　According to the above-mentioned continuous casting machine, the flow velocity at the solidification interface of the unsolidified portion is set to 5 cm / s or more by the electromagnetic force on one side and the electromagnetic force on the other side. As a result, the tip of the dendrite can be efficiently divided.
[0062]
　The continuous casting machine according to the thirteenth aspect is the continuous casting machine according to any one of the ninth aspect to the twelfth aspect, and the second electromagnetic agitator is pushed back to the mold side by the reduction roll. Stir the molten steel in the solidified part.
[0063]
　According to the above-mentioned continuous casting machine, the second electromagnetic stirrer agitates (electromagnetically agitates) the concentrated molten steel in the unsolidified portion pushed back from the reduction roll to the mold side. As a result, the concentrated molten steel pushed back from the reduction roll to the mold side is easily mixed with the molten steel (mother molten steel) transported from the mold to the reduction roll. As a result, the concentrated molten steel is diluted. Therefore, it is suppressed that the concentrated molten steel remains as macrosegregation on the slab.
[0064]
　The continuous casting machine according to the 14th aspect is the continuous casting machine according to any one of the 9th to 13th aspects, and the second electromagnetic stirrer has the unsolidified portion on one side in the width direction of the slab. One-sided electromagnetic force that flows to the slab and the other-side electromagnetic force that causes the unsolidified portion to flow to the other side in the width direction of the slab are alternately applied to the slab.
[0065]
　According to the above-mentioned continuous casting machine, the second electromagnetic stirrer causes the unsolidified portion to flow to one side in the width direction of the slab and the unsolidified portion to flow to the other side in the width direction of the slab. The electromagnetic force on the other side is alternately applied to the slab. As a result, the concentrated molten steel pushed back from the reduction roll to the mold side becomes more easily mixed with the molten steel (mother molten steel) transported from the mold to the reduction roll. As a result, the concentrated molten steel is diluted. Therefore, it is further suppressed that the concentrated molten steel remains as macrosegregation in the slab.
The invention's effect
[0066]
　According to the technique disclosed in the present application, macrosegregation and semi-macrosegregation of slabs can be reduced.
A brief description of the drawing
[0067]
FIG. 1 is a side view of the continuous casting machine according to the embodiment as viewed from the width direction of the slab.
FIG. 2 is a graph showing the relationship between the thickness D of the solidified shell portion of a slab and the frequency F of an alternating current applied to the electromagnetic coil of the first electromagnetic stirrer.
FIG. 3 is a plan view of the slab shown in FIG. 1 as viewed from the first electromagnetic stirrer side.
FIG. 4 is a table showing the specifications of the slab used in the continuous casting test, the setting of the first electromagnetic stirrer, and the evaluation result of the slab.
FIG. 5 is the conveying speed V of the slab C is a graph showing the relationship between the distance from the slab surface.
FIG. 6 is the conveying speed V of the slab C is a graph showing the relationship between the distance from the slab surface.
FIG. 7 is a graph showing the distribution of the Mn segregation degree in the thickness direction of the slab according to Example 2 continuously cast in the continuous casting test.
Mode for carrying out the invention
[0068]
　Hereinafter, the continuous casting machine and the continuous casting method according to the embodiment will be described.
[0069]
(Continuous Casting Machine)
　First, the configuration of the continuous casting machine will be described.
[0070]
　FIG. 1 shows a continuous casting machine 10 according to the present embodiment. The continuous casting machine 10 includes a tundish 12, a mold 16, a transfer device 30, a reduction device 40, a first electromagnetic stirrer 50, and a second electromagnetic stirrer 60.
[0071]
(Tandish) The
　tundish 12 is a container for temporarily storing the molten steel W. Molten steel W is poured into the tundish 12 from a ladle (not shown). Further, a dipping nozzle 14 for discharging the molten steel W is provided at the bottom of the tundish 12. A mold 16 is arranged below the tundish 12.
[0072]
(Mold) The
　mold 16 is, for example, a water-cooled copper mold. The mold 16 cools the molten steel W poured from the immersion nozzle 14 of the tundish 12 and solidifies the surface layer of the molten steel W. As a result, the slab 20 having a predetermined shape is formed.
[0073]
　The mold 16 is formed in a tubular shape with both ends open in the axial direction. Further, the mold 16 is arranged with the axial direction as the vertical direction. An injection port 16U is formed at the upper end of the mold 16. The immersion nozzle 14 of the tundish 12 is inserted into the injection port 16U. The molten steel W is poured into the mold 16 from the immersion nozzle 14.
[0074]
　The immersion nozzle 14 is provided with an adjusting mechanism such as an adjusting valve for adjusting the discharge amount of the molten steel W. By this adjustment mechanism, the discharge amount of the molten steel W discharged from the immersion nozzle 14 to the injection port 16U is adjusted so that the liquid level of the molten steel W in the mold 16 (hereinafter referred to as “meniscus M”) becomes a predetermined height. To.
[0075]
　The molten steel W poured into the mold 16 is cooled by the mold 16 and gradually solidified from the surface layer. As a result, the molten steel W on the surface layer is solidified, and the slab 20 in which the molten steel W remains inside is formed. Further, the cross-sectional shape of the mold 16 is rectangular. As a result, the cross-sectional shape of the slab 20 is formed into a rectangular shape. In the following, the surface layer side of the slab 20 in which the molten steel W has solidified will be referred to as a solidified shell portion 20A, and the unsolidified molten steel W remaining inside the slab 20 will be referred to as an unsolidified portion 20B.
[0076]
　A discharge port 16L is formed at the lower end of the mold 16. The slab 20 formed by the mold 16 is discharged from the discharge port 16L. Further, a transfer device 30 is arranged below the mold 16.
[0077]
(Conveying device) The
　transporting device 30 transports the slab 20 discharged from the mold 16 in a predetermined direction (arrow H direction) while cooling. In the following, the arrow H direction will be the transport direction (casting direction) of the transport device 30.
[0078]
　The transport device 30 has a plurality of pairs of support rolls 32. The plurality of pairs of support rolls 32 are arranged on both sides of the slab 20 in the thickness direction (arrow t direction) at intervals in the transport direction of the slab 20. Further, both ends of each support roll 32 in the axial direction are rotatably supported by bearing portions (not shown) on both sides of the slab 20 in the width direction. These support rolls 32 form a transport path 34 that gently curves from the discharge port 16L of the mold 16 toward the reduction device 40 described later and then extends in a substantially horizontal direction.
[0079]
　The plurality of pairs of support rolls 32 transport the slab 20 in the transport direction while gripping the slab 20 from both sides in the thickness direction. As a result, bulging in which the slab 20 swells in the thickness direction is suppressed. A part of the plurality of support rolls 32 is a drive roll that is rotationally driven. The transport speed (casting speed) of the slab 20 is adjusted by this drive roll.
[0080]
　The transport speed of the slab 20 increases as the rotation speed of the drive roll increases. Further, the transport speed of the slab 20 becomes slower when the rotation speed of the drive roll is slowed down.
[0081]
　The transport device 30 has a plurality of coolers (secondary coolers) (not shown) for cooling the slab 20. The plurality of coolers have, for example, a spray nozzle that injects cooling water. These coolers are arranged at intervals in the transport direction of the slab 20, and inject cooling water onto the slab 20. As a result, the slab 20 is cooled, and the unsolidified portion 20B of the slab 20 is gradually solidified.
[0082]
　The cooling rate of the slab 20 becomes faster as the amount of cooling water injected from the cooler to the slab 20 is increased. Further, the cooling rate of the slab 20 becomes slower when the amount of cooling water injected from the cooler to the slab 20 is reduced. Further, the cooling rate of the slab 20 becomes faster when the temperature of the cooling water injected from the cooler to the slab 20 is lowered. Further, the cooling rate of the slab 20 becomes slower as the temperature of the cooling water injected from the cooler onto the slab 20 is increased.
[0083]
　The transport path 34 may be provided with an electromagnetic stirring device that electromagnetically stirs the unsolidified portion 20B of the slab 20.
[0084]
(Reducing device) The
　reducing device 40 is arranged on the downstream side of the transport path 34 extending in a substantially horizontal direction. The reduction device 40 has a pair of reduction rolls (large reduction rolls) 42. The pair of reduction rolls 42 transport the slab 20 in the transport direction while gripping the slab 20 from both sides in the thickness direction. That is, the pair of reduction rolls 42 form a transport path 34 for the slab 20.
[0085]
　Further, the pair of reduction rolls 42 reduce the slab 20 having the unsolidified portion 20B inside, so that the concentrated molten steel in the unsolidified portion 20B is conveyed from between the pair of reduction rolls 42 in the transport direction of the slab 20. Push back (discharge) to the upstream side. As a result, it is possible to prevent the concentrated molten steel from remaining as macrosegregation in the central portion of the slab 20 in the thickness direction.
[0086]
　The pair of reduction rolls 42 are formed in a columnar shape. Further, the pair of reduction rolls 42 are arranged on both sides of the slab 20 in the thickness direction. The pair of reduction rolls 42 are arranged with the axial direction (longitudinal direction) as the width direction of the slab 20. Further, both ends of the pair of reduction rolls 42 in the axial direction are rotatably supported by bearing portions (not shown) on both sides of the slab 20 in the width direction.
[0087]
　Further, the reduction roll 42 arranged on the upper side of the slab 20 is pressed (compressed) by the slab 20 by a pressing device such as a hydraulic cylinder. Specifically, the pressing device presses the bearing portions that support both ends in the axial direction of the reduction roll 42 arranged on the upper side of the slab 20 toward the center side (lower side) of the slab 20 in the thickness direction. .. As a result, the slab 20 is compressed in the thickness direction between the pair of reduction rolls 42.
[0088]
　Here, the slab 20 is conveyed while being cooled by the plurality of coolers of the conveying device 30 as described above. As a result, the unsolidified portion 20B of the slab 20 is gradually solidified toward the downstream side in the transport direction. In other words, in the slab 20, the solid phase ratio R of the slab 20 increases toward the downstream side in the transport direction.
[0089]
　The pair of reduction rolls 42 of the present embodiment has a solid phase ratio R (hereinafter, referred to as “central solid phase ratio”) of the central portion of the slab 20 in the thickness direction of the transport path 34 of the slab 20 of 0.8. It is placed at a position that is less than (R <0.8). As a result, the pair of reduction rolls 42 reduces the slab 20 having the unsolidified portion 20B having a central solid phase ratio R of less than 0.8.
[0090]
　The solid phase ratio R means the ratio (ratio) of the solidified portion to the slab 20. For example, when the solid phase ratio R is 0.8, the ratio of the solidified portion to the slab 20 is 80% (80%), and the ratio of the unsolidified portion to the slab 20 is 20% (20%). This solid phase ratio R can be obtained, for example, by solidifying and analyzing the slab 20.
[0091]
(First Electromagnetic Stirrer)
　The first electromagnetic agitator 50 applies an electromagnetic force to the unsolidified portion 20B of the slab 20 conveyed from the mold 16 by the conveying device 30, and agitates the unsolidified portion 20B (electromagnetic stirring). ) It is a non-contact type stirring device.
[0092]
　The first electromagnetic stirrer 50 is arranged on the downstream side in the transport direction of the slab 20 with respect to the mold 16. Further, the first electromagnetic agitator 50 is arranged on the upstream side in the transport direction of the slab 20 with respect to the pair of reduction rolls 42. Further, the first electromagnetic agitator 50 is arranged so as to face the solidified shell portion 20A on the upper surface side of the slab 20 that passes through the curved portion of the transport path 34. The first electromagnetic stirrer 50 may be arranged below the slab 20.
[0093]
　The first electromagnetic stirrer 50 stirs the unsolidified portion 20B on the surface layer portion of the slab 20. In other words, the first electromagnetic stirrer 50 stirs the unsolidified portion 20B at the stage where the solidified interface of the unsolidified portion 20B is on the surface layer portion of the slab 20. Further, the first electromagnetic agitator 50 is a position where the concentrated molten steel in the unsolidified portion 20B pushed back to the upstream side in the transport direction of the slab 20 by the pair of reduction rolls 42 does not reach the unsolidified portion of the slab 20. Stir 20B.
[0094]
　The first electromagnetic agitator 50 has an electromagnetic coil (derivative) (not shown) facing the solidified shell portion 20A of the slab 20. When an alternating current (three-phase alternating current) is applied to the electromagnetic coil, a magnetic field (hereinafter referred to as "moving magnetic field") that moves in the width direction of the slab 20 is generated. When this moving magnetic field acts on the unsolidified portion 20B, an electromagnetic force EP (see FIG. 3) that causes the unsolidified portion 20B to flow in the width direction of the slab 20 is generated.
[0095]
　From the viewpoint of efficiently generating equiaxed crystals, the center of the first electromagnetic agitator 50 in the transport direction of the slab 20 is downstream from the meniscus M in the mold 16 along the transport direction of the slab 20. It is preferably arranged so as to be located within a range of 6 to 10 m to the side.
[0096]
(First Control Unit)
　The first control unit 52 is electrically connected to the first electromagnetic stirrer 50. The first control unit 52 controls the electromagnetic force EP generated by the first electromagnetic agitator 50 so that the flow velocity of the unsolidified unit 20B at the solidification interface is 5 cm / s or more. The first control unit 52 is an example of a control unit.
[0097]
　Specifically, when the AC current value applied to the electromagnetic coil of the first electromagnetic stirrer 50 by the first control unit 52 increases, the electromagnetic force EP increases. On the other hand, when the AC current value applied to the electromagnetic coil by the first control unit 52 becomes small, the electromagnetic force EP becomes small.
[0098]
　Here, dendrites are generated from the solidified shell portion 20A toward the center in the thickness direction of the slab 20 in the solidification process of the unsolidified portion 20B. The position of the tip portion of the dendrite, that is, the solidification interface of the unsolidified portion 20B, varies depending on the thickness of the solidified shell portion 20A. Specifically, as the thickness of the solidified shell portion 20A increases, the position of the solidified interface of the unsolidified portion 20B moves toward the center in the thickness direction of the slab 20.
[0099]
　Further, the depth of the electromagnetic force EP penetrating the slab 20 (penetration depth) varies depending on the frequency of the alternating current applied to the electromagnetic coil of the first electromagnetic stirrer 50. Specifically, as the frequency of the alternating current applied to the electromagnetic coil of the first electromagnetic agitator 50 becomes smaller, the penetration depth of the electromagnetic force EP into the slab 20 becomes deeper. On the other hand, as the frequency of the alternating current applied to the electromagnetic coil of the first electromagnetic agitator 50 increases, the penetration depth of the electromagnetic force EP into the slab 20 becomes shallow.
[0100]
　Therefore, the first control unit 52 increases or decreases the frequency of the alternating current applied to the electromagnetic coil of the first electromagnetic agitation device 50 according to the thickness of the solidification shell unit 20A. Specifically, as the thickness of the solidification shell portion 20A increases, the frequency of the alternating current applied to the electromagnetic coil of the first electromagnetic stirrer 50 is reduced. On the other hand, as the thickness of the solidification shell portion 20A becomes thinner, the frequency of the alternating current applied to the electromagnetic coil of the first electromagnetic stirrer 50 is increased.
[0101]
　More specifically, FIG. 2 shows an analysis result showing the relationship between the thickness D of the solidification shell portion 20A and the frequency of the alternating current applied to the first electromagnetic agitator 50. The thickness D of the solidified shell portion 20A is a position facing the center of the solidified shell portion 20A on the first electromagnetic stirrer 50 side of the slab 20 in the transport direction of the slab 20 in the first electromagnetic stirrer 50. The thickness of the part). The thickness D of the solidification shell portion 20A is obtained from solidification analysis. The shaded area G shown in FIG. 2 is a region where the flow velocity of the unsolidified portion 20B at the solidifying interface is 5 cm / s or more.
[0102]
　As shown in FIG. 2, in the region G where the flow velocity at the solidification interface of the unsolidified portion 20B is 5 cm / s or more, the frequency F of the alternating current applied to the electromagnetic coil of the first electromagnetic stirrer 50 is 80 /. It is in the range of D or more and 160 / D or less.
[0103]
　Therefore, the first control unit 52 applies an alternating current having a frequency F satisfying the equation (1) to the electromagnetic coil of the first electromagnetic stirrer 50. As a result, a shear force of a predetermined value or more acts on the tip of the dendrite generated near the solidification interface in the unsolidified portion 20B. As a result, the tip of the dendrite is divided, and equiaxed crystals are easily generated.
[0104]
[Equation 1]

　However,
　F: frequency of alternating current (Hz)
　D: thickness (mm)
of the solidified shell portion on the first electromagnetic stirrer side .
[0105]
　The equation (1) is converted into the following equation (2) by using the constant A.
[0106]
[Equation 2]

　However,
　A: a constant (80 ≦ A ≦ 160)
　.
[0107]
　Further, the first control unit 52 controls the direction of the electromagnetic force EP acting on the uncoagulated unit 20B by changing the direction of the alternating current applied to the electromagnetic coil of the first electromagnetic stirrer 50.
[0108]
　Specifically, as shown in FIG. 3, when the first control unit 52 applies an alternating current in a predetermined direction to the electromagnetic coil of the first electromagnetic stirrer 50, the unsolidified portion 20B is moved to one side in the width direction of the slab 20. An electromagnetic force EP (hereinafter referred to as “one-sided electromagnetic force EP1”) that flows to the side is generated. On the other hand, when the first control unit 52 applies an alternating current in the direction opposite to the predetermined direction to the electromagnetic coil of the first electromagnetic stirrer 50, an electromagnetic force that causes the unsolidified portion 20B to flow to the other side in the width direction of the slab 20. EP (hereinafter referred to as "the other side electromagnetic force EP2") is generated.
[0109]
　Further, the first control unit 52 controls the first electromagnetic stirrer 50 so that the first electromagnetic stirrer 50 intermittently generates the one-side electromagnetic force EP1 and the other-side electromagnetic force EP2. Specifically, the first control unit 52 applies an alternating current that causes the first electromagnetic stirrer 50 to generate one-sided electromagnetic force EP1 and an alternating current that causes the first electromagnetic stirrer 50 to generate the other-side electromagnetic force EP2. It is applied to the electromagnetic coil of the first electromagnetic stirrer 50 alternately and intermittently.
[0110]
　In order to make the flow velocity of the unsolidified portion 20B at the solidification interface 5 cm / s or more, considering the acceleration, speed maintenance, deceleration, etc. of the unsolidified portion 20B, one side electromagnetic force EP1 and the other side It is preferable that the electromagnetic force EP2 is alternately applied to the slab within the range of 20 to 50 seconds. Further, it is preferable that the one-side electromagnetic force EP1 and the other-side electromagnetic force EP2 are applied to the unsolidified portion 20B of the slab 20 at intervals of 1 to 10 seconds.
[0111]
(Second Electromagnetic Stirrer)
　The second electromagnetic agitator 60 applies an electromagnetic force to the concentrated molten steel pushed back toward the mold 16 from between the pair of reduction rolls 42, and agitates the concentrated molten steel (electromagnetic stirring). It is a non-contact stirrer. The concentrated molten steel means a molten steel in which a predetermined component is concentrated by segregation (solidification segregation).
[0112]
　The second electromagnetic stirrer 60 is arranged on the downstream side in the transport direction of the slab 20 with respect to the first electromagnetic stirrer 50. Further, the first electromagnetic agitator 50 is arranged on the upstream side in the transport direction of the slab 20 with respect to the pair of reduction rolls 42. Further, the second electromagnetic agitator 60 is arranged so as to face the solidification shell portion 20A on the upper surface side of the slab 20 that passes through the horizontal portion of the transport path 34 extending in the substantially horizontal direction. The second electromagnetic stirrer 60 may be arranged below the slab 20.
[0113]
　Here, the second electromagnetic stirrer 60 has the same configuration as the first electromagnetic stirrer 50. Further, the second control unit 62 is electrically connected to the second electromagnetic stirrer 60. The second control unit 62 has the same configuration as the first control unit 52. Therefore, the second electromagnetic stirrer 60 generates the one-side electromagnetic force and the other-side electromagnetic force alternately and after a predetermined time.
[0114]
　The one-sided electromagnetic force causes the unsolidified portion 20B from which the concentrated molten steel is discharged to flow to one side in the width direction of the slab 20. Further, the electromagnetic force on the other side causes the unsolidified portion 20B from which the concentrated molten steel is discharged to flow to the other side in the width direction of the slab 20. Further, the second control unit 62 applies an alternating current having a frequency F satisfying the above equation (1) to the electromagnetic coil of the second electromagnetic stirrer 60. As a result, the flow velocity at the solidification interface of the unsolidified portion 20B becomes 5 cm / s or more.
[0115]
　As a result, the concentrated molten steel pushed back toward the mold 16 from between the pair of reduction rolls 42 is easily mixed with the molten steel (mother molten steel) transported from the mold 16 to the pair of reduction rolls 42.
[0116]
　From the viewpoint of efficiently stirring the concentrated molten steel pushed back from the pair of reduction rolls 42 to the mold 16 side, the second electromagnetic stirrer 60 has a pair of reduction rolls centered in the transport direction of the slab 20. It is preferable that the slab 20 is arranged so as to be located within a range of 4 to 8 m from the center of rotation to the upstream side along the transport direction of the slab 20.
[0117]
(Action)
　Next, the action of the present embodiment will be described while explaining the continuous casting method (slab manufacturing method) according to the present embodiment.
[0118]
　According to the continuous casting method according to the present embodiment, the unsolidified portion 20B in the slab 20 conveyed from the mold 16 is agitated by the first electromagnetic agitator 50 and the second electromagnetic agitator 60, respectively.
[0119]
　Next, the slab 20 having the unsolidified portion 20B is reduced by the reduction roll 42. As a result, the concentrated molten steel in the unsolidified portion 20B is pushed back to the mold 16 side from between the pair of reduction rolls 42.
[0120]
　Here, the concentrated molten steel pushed back toward the mold 16 from between the pair of reduction rolls 42 is agitated by the second electromagnetic agitator 60. As a result, the concentrated molten steel pushed back from between the pair of reduction rolls 42 to the mold 16 side is easily mixed with the molten steel (mother molten steel) conveyed from the mold 16 to the pair of reduction rolls 42. As a result, the concentrated molten steel is diluted. Therefore, it is possible to prevent the concentrated molten steel from remaining as macrosegregation at the center of the slab 20 in the thickness direction.
[0121]
　Further, the first electromagnetic agitator 50 is arranged on the upstream side in the transport direction of the slab 20 with respect to the pair of reduction rolls 42. The first electromagnetic stirrer 50 has a one-sided electromagnetic force EP1 that causes the unsolidified portion 20B to flow to one side in the width direction of the slab at a flow velocity of 5 cm / s or more, and the unsolidified portion 20B in the width direction of the slab 20. The other side electromagnetic force EP2, which causes the slab to flow to the other side at a flow velocity of 5 cm / s or more, is alternately applied to the slab 20.
[0122]
　In this way, the unsolidified portion is made to flow to one side in the width direction of the slab at a flow rate of 5 cm / s or more by the one-side electromagnetic force EP1, so that the tip portion of the dendrite in the unsolidified portion 20B has a predetermined value or more. Shear force acts. Similarly, the unsolidified portion 20B is made to flow to the other side in the width direction of the slab 20 at a flow velocity of 5 cm / s or more by the other side electromagnetic force EP2, so that the tip of the dendrite in the unsolidified portion 20B is reached. A shear force of a predetermined value or more acts. Therefore, the tip of the dendrite generated on the surface layer of the slab 20 is divided, and equiaxed crystals are easily generated.
[0123]
　Further, the first electromagnetic agitator 50 alternately applies one-sided electromagnetic force EP1 and the other-side electromagnetic force EP2 to the slab. As a result, in the present embodiment, the tip of the dendrite in the unsolidified portion 20B is further reduced as compared with the case where the unsolidified portion 20B is flowed only to one side in the width direction of the slab 20 by the first electromagnetic agitator 50. It becomes easy to be divided.
[0124]
　Then, when the tip of the dendrite generated on the surface layer of the slab 20 is divided, the mold 16 side is between the pair of reduction rolls 42 on the downstream side in the transport direction of the slab 20 with respect to the first electromagnetic agitator 50. The flow resistance (obstacle) of the concentrated molten steel pushed back to is reduced. As a result, the concentrated molten steel is easily pushed back from between the pair of reduction rolls 42 to the mold 16 side. Therefore, the concentrated molten steel is suppressed from remaining as macrosegregation in the central portion of the slab 20.
[0125]
　Further, by dividing the tip of the dendrite by the first electromagnetic stirrer 50, the semi-macro segregation captured between the dendrites is reduced. Therefore, it is suppressed that semi-macro segregation remains in the central portion of the slab 20.
[0126]
　As described above, in the present embodiment, first, the unsolidified portion 20B of the surface layer portion of the slab 20 is agitated by the one-side electromagnetic force EP1 and the other-side electromagnetic force EP2 of the first electromagnetic stirrer 50. Next, the concentrated molten steel in the unsolidified portion 20B pushed back to the mold 16 side by the pair of reduction rolls 42 is stirred by the second electromagnetic stirrer 60. Thereby, in the present embodiment, macro segregation and semi-macro segregation of the slab 20 can be reduced.
[0127]
　Japanese Patent Application Laid-Open No. 2010-179342 discloses a continuous casting machine that electromagnetically stirs an unsolidified portion of a slab by a first electromagnetic stirrer and a second electromagnetic stirrer. In the continuous casting machine disclosed in JP-A-2010-179342, the concentrated molten steel in the unsolidified portion pushed back to the mold side by the reduction roll pair is alternately electromagnetically agitated by the second electromagnetic agitator. However, the first electromagnetic stirrer arranged on the mold side of the second electromagnetic stirrer is not an alternating electromagnetic agitator, but a normal one-way electromagnetic agitation in which the unsolidified portion flows in one direction in the width direction of the slab. Is.
[0128]
　On the other hand, in the present embodiment, the first electromagnetic stirrer 50 arranged on the mold side of the second electromagnetic stirrer 60 does not solidify the slab 20 by the one-side electromagnetic force EP1 and the other-side electromagnetic force EP2. Part 20B is stirred alternately. Thereby, in the present embodiment, the macro segregation and the semi-macro segregation of the slab 20 can be further reduced as compared with the technique disclosed in Japanese Patent Application Laid-Open No. 2010-179342.
[0129]
　Further, the first electromagnetic agitator 50 intermittently applies the one-side electromagnetic force EP1 and the other-side electromagnetic force EP2 to the unsolidified portion 20B of the slab 20. That is, the first electromagnetic agitation device 50 stops applying the one-sided electromagnetic force EP1 to the slab 20, and then starts applying the other-side electromagnetic force EP2 to the slab 20 after a predetermined time. Similarly, the first electromagnetic agitator 50 starts applying the one-sided electromagnetic force EP1 to the slab 20 after a predetermined time after stopping the application of the other-side electromagnetic force EP2 to the slab 20.
[0130]
　As a result, for example, the unsolidified portion that flows to one side in the width direction of the slab 20 between the time when the application of the one-side electromagnetic force EP1 to the slab 20 is stopped and the time when the application of the other-side electromagnetic force EP2 is started. The drift velocity of 20B decreases. In this state, the first electromagnetic agitator 50 starts applying the other side electromagnetic force EP2 to the slab 20. As a result, the flow direction of the unsolidified portion 20B is smoothly reversed, and the unsolidified portion 20B easily flows to the other side in the width direction of the slab 20.
[0131]
　Similarly, when the electromagnetic force applied to the slab 20 is switched from the other-side electromagnetic force EP2 to the one-side electromagnetic force EP1, the flow direction of the unsolidified portion 20B is smoothly reversed and is not yet formed. The solidified portion 20B easily flows to one side in the width direction of the slab 20.
[0132]
　Therefore, the tip of the dendrite in the unsolidified portion 20B can be divided while reducing the power consumption of the first electromagnetic agitator 50.
[0133]
　Further, as described above, the position of the tip portion of the dendrite, that is, the solidification interface of the unsolidified portion 20B, varies depending on the thickness of the solidified shell portion 20A. Further, the penetration depth of the electromagnetic force EP penetrating the slab 20 varies depending on the frequency of the alternating current applied to the electromagnetic coil of the first electromagnetic stirrer 50.
[0134]
　Therefore, the first control unit 52 applies an alternating current of a predetermined frequency determined according to the thickness of the solidification shell unit 20A to the electromagnetic coil of the first electromagnetic agitation device 50. Specifically, an alternating current satisfying the equation (1) is applied to the electromagnetic coil of the first electromagnetic stirrer 50. In this formula (1), as the thickness D of the solidification shell portion 20A increases, the frequency F of the alternating current applied to the electromagnetic coil of the first electromagnetic agitator 50 decreases. On the other hand, in the formula (1), the frequency F of the alternating current applied to the electromagnetic coil of the first electromagnetic agitator 50 increases as the thickness D of the solidification shell portion 20A decreases.
[0135]
　As a result, the one-side electromagnetic force EP1 and the other-side electromagnetic force EP2 can be applied to the tip of the dendrite near the solidification interface of the unsolidified portion 20B regardless of the thickness of the solidified shell portion 20A. Therefore, the tip of the dendrite can be efficiently divided.
[0136]
　Further, similarly to the first electromagnetic agitator 50, the second electromagnetic agitator 60 alternately and intermittently applies the one-side electromagnetic force and the other-side electromagnetic force to the unsolidified portion 20B of the slab 20. As a result, the concentrated molten steel extruded from between the pair of reduction rolls 42 toward the mold 16 side and the molten steel conveyed from the mold 16 to the pair of reduction rolls 42 can be efficiently mixed. Therefore, the macrosegregation remaining in the central portion of the slab 20 is reduced.
[0137]
(Modification Example)
　Next, a modification of the above embodiment will be described.
[0138]
　In the first electromagnetic agitator 50 of the above embodiment, the one-side electromagnetic force EP1 and the other-side electromagnetic force EP2 are alternately and intermittently applied to the slab 20. However, the first electromagnetic agitator 50 may alternately and continuously apply the one-side electromagnetic force EP1 and the other-side electromagnetic force EP2 to the slab 20.
[0139]
　Further, in the second electromagnetic agitation device 60 of the above-described embodiment, similarly to the first electromagnetic agitation device 50, one-side electromagnetic force and the other-side electromagnetic force are alternately and intermittently applied to the slab 20. However, the second electromagnetic agitator 60 may alternately and continuously apply one-sided electromagnetic force and the other-side electromagnetic force to the slab 20. Further, the second electromagnetic agitator 60 may continuously or intermittently apply only one of the one-sided electromagnetic force and the other-side electromagnetic force to the slab 20.
[0140]
　Further, the first control unit 52 of the above embodiment applies an alternating current satisfying the equation (1) to the electromagnetic coil of the first electromagnetic stirrer 50. However, the frequency of the alternating current applied to the electromagnetic coil of the first electromagnetic stirrer 50 may be determined without using the equation (1).
[0141]
　Further, the arrangement of the first electromagnetic stirrer 50 and the second electromagnetic stirrer 60 with respect to the transport path 34 can be changed as appropriate. Further, the thickness and the transport speed of the slab 20 can be changed as appropriate.
[0142]
(Continuous Casting Test)
　Next, the continuous casting test will be described.
[0143]
　In this continuous casting test, a plurality of slabs according to Examples 1 to 5 were continuously cast by the continuous casting machine 10 shown in FIG. 1, and the presence or absence of semi-macro segregation and macro segregation in each slab was confirmed. In addition, a plurality of slabs according to Comparative Examples 1 to 3 were continuously cast, and the presence or absence of semi-macro segregation and macro segregation in each slab was confirmed.
[0144]
(Melted steel)
　The composition of molten iron is, in mass%, C: 0.05 to 0.15%, Si: 0.1 to 0.4%, Mn: 0.8 to 1.5%, P: 0.02. % Or less, S: 0.008% or less, and the balance was composed of Fe and impurities.
[0145]
(Mold)
　Next, as the mold 16, a water-cooled copper mold was used. The various dimensions of the mold 16 are shown in Table 1 below.
[0146]
[table 1]

[0147]
(Conveyor)
　Next, the casting speed of the slab by the transport device 30 was set to 0.7 to 1.1 m / min. The specific water content of the cooler (secondary cooler) of the transport device 30 was 0.5 to 1.2 L / kg-steel. As a result, the central solid phase ratio R at the center in the thickness direction of the slabs reduced by the pair of reduction rolls 42 was set within the range of 0.01 to 0.2 (see FIG. 4).
[0148]
(First Electromagnetic Stirrer)
　The first electromagnetic agitator 50 was arranged 9 m downstream from the meniscus M in the mold 16 along the transport direction of the slab 20.
[0149]
　Further, FIG. 4 shows the thickness of the solidified shell portion when the slab passes through the first electromagnetic agitator 50. The thickness of the solidified shell portion is the thickness of the solidified shell portion on the first electromagnetic stirrer 50 side of the slab. The thickness of this solidification shell portion was calculated by two-dimensional solidification analysis.
[0150]
　Further, FIG. 4 shows a method of stirring the unsolidified portion of the slab by the first electromagnetic stirrer 50. Here, the alternating stirring means that one-sided electromagnetic force and the other-side electromagnetic force are alternately and intermittently applied to the unsolidified portion of the slab. In this continuous casting test, one-sided electromagnetic force and the other-side electromagnetic force were alternately applied to the unsolidified portion of the slab for 30 seconds each. Further, the one-side electromagnetic force and the other-side electromagnetic force were applied to the unsolidified portion of the slab at intervals of 5 seconds.
[0151]
　Further, unidirectional stirring means that either one-sided electromagnetic force or the other-side electromagnetic force is continuously applied to the unsolidified portion of the slab.
[0152]
　Further, FIG. 4 shows the frequency of the alternating current (three-phase alternating current) applied to the electromagnetic coil of the first electromagnetic stirrer 50. The alternating current applied to the electromagnetic coil of the first electromagnetic stirrer 50 was set to 600 A. Further, FIG. 4 shows the flow velocity of the unsolidified portion of the slab at the solidified interface.
[0153]
　The flow velocity of the unsolidified portion at the solidifying interface was estimated by converting from the following formulas (a) and (b) using the Mn segregation degree C Mn . The solidification rate V was calculated by solidification calculation.
　　U = 7500 × V × Sh / (1-Sh) ・ ・ ・ (a)
　　Sh = (C Mn -1) / (K 0-1 ) ・ ・ ・ (b)
　However,
　U: Drift velocity of molten steel (cm) / S)
　V: Solidification rate (cm / s)
　K 0 : Equilibrium partition coefficient of Mn (= 0.77)
　.
[0154]
(Second Electromagnetic Stirrer)
　The second electromagnetic agitator 60 was arranged 14.6 m downstream from the meniscus M in the mold 16 along the transport direction of the slab 20.
[0155]
　Further, the method of stirring the unsolidified portion of the slab by the second electromagnetic stirrer 60 was alternating stirring as in the first electromagnetic stirrer 50. Further, in the second electromagnetic stirring device 60, similarly to the first electromagnetic stirring device 50, one-sided electromagnetic force and the other-side electromagnetic force were alternately applied to the unsolidified portion of the slab for 30 seconds each. Further, the one-side electromagnetic force and the other-side electromagnetic force were applied to the unsolidified portion of the slab at intervals of 5 seconds.
[0156]
　The alternating current (three-phase alternating current) applied to the electromagnetic coil of the second electromagnetic stirrer 60 was 900 A. The frequency of the alternating current applied to the electromagnetic coil of the second electromagnetic stirrer 60 was 1.5 Hz.
[0157]
(Reduction device) The
　pair of reduction rolls 42 were arranged 21.2 m downstream from the meniscus M in the mold 16 along the transport direction of the slab. Then, by pressing the reduction roll 42 arranged on the upper side of the slab with a hydraulic cylinder (not shown), the central solid phase ratio R at the center in the thickness direction and the width direction is in the range of 0.01 to 0.2. The piece was pressed down (see FIG. 4).
[0158]
　The maximum reduction force (maximum output) of the reduction roll 42 is 600 tonF (5.88MN). The amount of reduction of the slab by the reduction roll 42 was 25 to 35 mm (see FIG. 4). The slab thickness T shown in FIG. 4 is the thickness of the slab before being reduced by the reduction roll 42.
[0159]
(Evaluation method of
　slab ) In the evaluation of the slab, the macrostructure of the sample cut out from the cross section of the slab according to Examples 1 to 5 and Comparative Examples 1 to 3 was visually confirmed, and semi-macro segregation and macro segregation were performed. The presence or absence was confirmed respectively. Then, the case where there was at least one of semi-macro segregation and macro segregation was regarded as rejected (x), and the case where both semi-macro segregation and macro segregation were not present was regarded as passed (◯).
[0160]
　Further, mapping analysis was performed by Electron Probe Micro Analyzer (EPMA) with respect to the thickness direction of the slabs according to Examples 1 to 5 and Comparative Examples 1 to 3, and the Mn concentration distribution in the thickness direction of the slabs was created. Then, the Mn concentration distribution of each of the analyzed slabs was divided by the Mn concentration of the molten steel collected from the tundish 12, to create a distribution of Mn segregation degree C Mn in the thickness direction of the slabs .
[0161]
　Further, from the distribution of Mn segregation degree C Mn in the thickness direction of each slab after being reduced by the reduction roll 42, the lowest Mn segregation degree in the central region, region L1 and region L2 along the thickness direction of the slab. Each value was calculated (see FIG. 4).
[0162]
　The central region referred to here means a region of 10 mm on each side from the center in the thickness direction of the slab (a region of 20 mm in total). Further, the region L1 (mm) is a region agitated by the first electromagnetic stirrer 50, and means a region within the range of the following formula (3). Further, the region L2 (mm) is a region agitated by the second electromagnetic stirrer 60, and means a region within the range of the following formula (4).
[0163]
[Equation 3] 　However,

V C : conveying speed (m / min) 　is.

[0164]
　The above equations (3) and (4) are converted into the following equations (5) and (6) by using the constant B1 or the constant B2, respectively.
[0165]
[Expression 4]

　However,
　B1: constants (66 ≦ B1 ≦
　78) B2: constants (85 ≦ B2 ≦
　101) V C : conveying speed (m / min)
　is.
[0166]
　Here, the regions L1 and L2 are supplemented. 5 and 6, the conveying velocity V of the slab C and (casting Further, the regions H1 and H2 shown in FIGS. 5 and 6 are regions in which the flow velocity of the unsolidified portion is 5 cm / s or more. The graphs shown in FIGS. 5 and 6 were obtained from solidification analysis of slabs.
[0167]
　The flow velocity of the unsolidified portion of the slab is 5 cm / s or more in two regions, the region H1 shown in FIG. 5 and the region H2 shown in FIG. Of these two regions H1 and H2, the region H1 on the surface side (first electromagnetic stirrer 50 side) of the slab is estimated to be the region L1 agitated by the first electromagnetic stirrer 50, and the thickness direction of the slab 20 The region H2 on the center side of the above was estimated to be the region L2 agitated by the second electromagnetic stirrer 60.
[0168]
(Evaluation Results)
　FIG. 4 shows the evaluation results of the slabs according to Examples 1 to 5 and Comparative Examples 1 to 3.
[0169]
(Example) In
　Examples 1 to 5, neither macrosegregation nor semi-macrosegregation was confirmed. In Examples 1 to 5, the unsolidified portion of the slab was stirred by the first electromagnetic stirrer 50 by alternating stirring, and the flow velocity at the solidification interface of the unsolidified portion was set to 5.0 cm / s or more. It is considered that this is because the tip of the dendrite in the unsolidified portion was efficiently divided and equiaxed crystals were generated.
[0170]
　Further, in Examples 1 to 5, the minimum value of the Mn segregation degree in the central region of the slab was 0.92 to 0.95. Further, the minimum value of the Mn segregation degree in the slab region L1 was 0.95 to 0.98. Further, the minimum value of the Mn segregation degree in the slab region L2 was 0.96 to 0.97.
[0171]
　Further, FIG. 7 shows the distribution of the Mn segregation degree in the thickness direction of the slab according to Example 2. From the distribution of the Mn segregation degree shown in FIG. 7, the presence or absence of negative segregation bands in the central region and the regions L1 and L2 was confirmed, respectively.
[0172]
　Here, the negative segregation band means a region in which a region having a Mn segregation degree of less than 1.0 is continuous by 5 mm or more in the thickness direction of the slab. The negative segregation band in the central region is an example of the central negative segregation band. The negative segregation band in region L1 is an example of a surface-side negative segregation band. Further, the negative segregation band in the region L2 is an example of an intermediate negative segregation band.
[0173]
　The amount of reduction of the reduction roll 42 of Example 2 is 30 mm. Therefore, the center of the slab in the thickness direction is 135 mm from the surface of the slab. The central region of the slab is a region within the range of 125 mm to 145 mm from the surface of the slab. Further, the conveying speed V of the slab of Example 2 C is a 0.7 m / min. Therefore, the regions L1 and L2 of the second embodiment are as follows from the above equation (3).
　78.9 mm ≤ L1 ≤ 93.2
　mm 101.6 mm ≤ L2 ≤ 120.7 mm
[0174]
　As shown in FIG. 7, in the central region, a region having a Mn segregation degree of less than 1.0 is continuous by 17 mm in the thickness direction of the slab. Further, in the region L1, a region having a Mn segregation degree of less than 1.0 is continuous by 10 mm in the thickness direction of the slab. Further, in the region L2, a region having a Mn segregation degree of less than 1.0 is continuous by 8 mm in the thickness direction of the slab. From this, it was confirmed that negative segregation bands were generated in the central region along the thickness direction of the slab and the regions L1 and L2, respectively.
[0175]
(Comparative Example) As
　shown in FIG. 4, in Comparative Example 1, macrosegregation was not confirmed, but semi-macrosegregation was confirmed. In Comparative Example 1, the method of stirring the unsolidified portion of the slab by the first electromagnetic stirrer 50 was unidirectional stirring. Therefore, it is considered that the tip of the dendrite in the unsolidified portion was not sufficiently divided.
[0176]
　Next, in Comparative Example 2, macrosegregation and semi-macrosegregation were confirmed. In Comparative Example 2, the frequency of the alternating current applied to the electromagnetic coil of the first electromagnetic stirrer 50 was set to 1 Hz. Therefore, it is considered that the electromagnetic force (one-side electromagnetic force and the other-side electromagnetic force) of the first electromagnetic stirrer 50 acts at a position deeper than the solidification interface of the unsolidified portion. As a result, the flow velocity at the solidification interface became as slow as 3.5 cm / s, and it is probable that the tip of the dendrite in the unsolidified portion was not sufficiently divided.
[0177]
　Next, in Comparative Example 3, macrosegregation was not confirmed, but semi-macrosegregation was confirmed. In Comparative Example 3, the frequency of the alternating current applied to the electromagnetic coil of the first electromagnetic stirrer was set to 4 Hz. Therefore, it is considered that the electromagnetic force (one-side electromagnetic force and the other-side electromagnetic force) of the first electromagnetic stirrer 50 acts at a position shallower than the solidification interface of the unsolidified portion. As a result, the flow velocity at the solidification interface became as slow as 4.5 cm / s, and it is probable that the tip of the dendrite in the unsolidified portion was not sufficiently divided.
[0178]
　When the thickness of the solidified shell portion is 68 mm as in Comparative Example 2 and Comparative Example 3, the frequency is 1.2 to 2 in order to make the flow velocity of the solidified interface of the solidified portion 5 cm / s or more. It is necessary to apply an alternating current in the range of .4 Hz to the electromagnetic coil of the first electromagnetic stirrer.
[0179]
(Summary
　of Evaluation Results ) From the above evaluation results, it can be seen that in Examples 1 to 5, high-quality slabs free from macrosegregation and semi-macrosegregation were obtained.
[0180]
　Although one embodiment of the technique disclosed in the present application has been described above, the technique disclosed in the present application is not limited to such an embodiment, and one embodiment and various modifications may be appropriately combined and used. Of course, it can be carried out in various aspects without departing from the gist of the technique disclosed in the present application.
[0181]
　The disclosure of Japanese Patent Application No. 2018-042106, filed on March 8, 2018, is incorporated herein by reference in its entirety.
　All documents, patent applications, and technical standards described herein are to the same extent as if the individual documents, patent applications, and technical standards were specifically and individually stated to be incorporated by reference. Incorporated herein by reference.
The scope of the claims
[Claim 1]
　The unsolidified portion in the slab transported from the mold was stirred by the first electromagnetic stirrer and the second electromagnetic stirrer arranged downstream of the first electromagnetic stirrer in the transport direction of the slab. After that, it is a continuous casting method in which the slab is reduced by a reduction roll, and
　the first electromagnetic agitator causes the unsolidified portion to flow to one side in the width direction of the slab at a flow rate of 5 cm / s or more. A
　continuous casting method in which one-sided electromagnetic force and the other-side electromagnetic force for causing the unsolidified portion to flow to the other side in the width direction of the slab at a flow rate of 5 cm / s or more are alternately applied to the slab .
[Claim 2]

　The continuous casting method according to claim 1, 　wherein the first electromagnetic stirrer intermittently applies the one-side electromagnetic force and the other-side electromagnetic force to the slab .
[Claim 3]
　The slab has a solidified shell portion containing the unsolidified portion
　, an alternating current satisfying the formula (1) is applied to the first electromagnetic stirrer, and the one-sided electromagnetic force is applied to the first electromagnetic stirrer.
　The continuous casting method according to claim 1 or 2, wherein the other side electromagnetic force is generated .
[Equation 1]

　However,
　F: frequency of alternating current (Hz)
　D: thickness (mm)
of the solidified shell portion on the first electromagnetic stirrer side .
[Claim 4]

　The continuous casting according to any one of claims 1 to 3, 　wherein the one-side electromagnetic force and the other-side electromagnetic force make the flow velocity of the unsolidified portion at the solidification interface 5 cm / s or more, respectively. Method.
[Claim 5]

　The continuous casting method according to any one of claims 1 to 4, 　wherein the second electromagnetic stirrer stirs the molten steel in the unsolidified portion pushed back to the mold side by the reduction roll .
[Claim 6]
　The second electromagnetic stirrer has one side electromagnetic force that causes the unsolidified portion to flow to one side in the width direction of the slab and the other side electromagnetic force that causes the unsolidified portion to flow to the other side in the width direction of the slab.
　The continuous casting method according to any one of claims 1 to 5 , wherein and is alternately applied to the slab .
[Claim 7]
　The thickness of the slab is in the range of 250 to 300 mm,
　the transport speed of the slab is in the range of 0.7 to 1.1 m / min, and
　the meniscus in the mold is along the transport direction of the slab.
　The continuous casting method according to any one of claims 1 to 6 , wherein the first electromagnetic stirrer is arranged within a range of 6 to 10 m on the downstream side .
[Claim 8]
　A central negative segregation band generated in the central region of the slab slab in the thickness direction and having a minimum Mn segregation degree in the range of 0.92 to 0.95, and
　a region L1 of the formula (3) in the slab slab.
　A formula located between the surface-side negative segregation band generated in the slab and the minimum value of the Mn segregation degree being in the range of 0.95 to 0.98, and the central region and the region L1 in the slab slab. A
　slab slab comprising an intermediate negative segregation band generated in the region L2 of (4) and having a minimum value of Mn segregation degree in the range of 0.96 to 0.97 .
[Number 2]

　However,
　L1: region along the thickness direction of the slab body
　(mm) L2: region along the thickness direction of the slab body
　(mm) V C : conveying speed (m / min)
　is.
[Claim 9]
　The mold,
　the first electromagnetic stirrer that stirs the unsolidified portion in the slab conveyed from the mold, and the unsolidified portion that
　is arranged downstream of the first electromagnetic stirrer in the transport direction of the slab. A second electromagnetic stirrer that stirs the portions,
　a reduction roll that is arranged downstream of the second electromagnetic stirrer in the transport direction of the
　slab and reduces the slab, and the unsolidified portion of the slab. One-sided electromagnetic force that causes the unsolidified portion to flow to one side in the width direction at a flow rate of 5 cm / s or more, and the other-side electromagnetic force that causes the unsolidified portion to flow to the other side in the width direction of the slab at a flow rate of 5 cm / s or more. A
　continuous casting machine including a control unit that alternately generates and is generated in the first electromagnetic stirrer .
[Claim 10]

　The continuous casting machine according to claim 9 　, wherein the control unit intermittently generates the one-side electromagnetic force and the other-side electromagnetic force in the first electromagnetic stirrer .
[Claim 11]
　The slab has a solidified shell portion containing the unsolidified portion, and the
　control unit applies an alternating current satisfying the formula (1) to the first electromagnetic stirrer to the first electromagnetic stirrer.
　The continuous casting machine according to claim 9 or 10, which generates the one-side electromagnetic force and the other-side electromagnetic force .
[Equation 3]

　However,
　F: frequency of alternating current (Hz)
　D: thickness (mm)
of the solidified shell portion on the first electromagnetic stirrer side .
[Claim 12]

　The continuous casting according to any one of claims 9 to 11, 　wherein the one-side electromagnetic force and the other-side electromagnetic force make the flow velocity of the unsolidified portion at the solidification interface 5 cm / s or more, respectively. Machine.
[Claim 13]

　The continuous casting machine according to any one of claims 9 to 12, 　wherein the second electromagnetic stirrer stirs the molten steel in the unsolidified portion pushed back to the mold side by the reduction roll .
[Claim 14]
　The second electromagnetic stirrer has one side electromagnetic force that causes the unsolidified portion to flow to one side in the width direction of the slab and the other side electromagnetic force that causes the unsolidified portion to flow to the other side in the width direction of the slab.
　The continuous casting machine according to any one of claims 9 to 13 , wherein is alternately applied to the slab .