JP7218259B2 - Slab continuous casting method - Google Patents

Description

The present invention relates to a continuous slab casting method in which a magnetic field is applied to molten steel in a mold. In particular, in the continuous casting of slabs for thick plates, which are often used to cast medium-carbon steel slabs with large cross sections, the present invention relates to a continuous casting method in which an alternating current moving magnetic field is applied to the molten steel in the mold and the molten steel just below the surface of the molten steel is stirred while being swirled. .

The steel plate products of steel companies are broadly divided into thick plate products, which are shipped after being cut into flat rectangular parallelepipeds one by one, and thin plate products, which are shipped in the form of coils made by winding long steel plates. Most of the slabs that serve as rolling stock for any steel plate product are produced by continuous casting. A simple comparison of the facilities and methods of 1 often results in differences as shown in Table 1.

Of these, thin plates formed by plastic working such as press forming are used as steel types, such as ultra-low carbon steel (carbon content of 0.005 mass% or less) and low carbon steel (carbon content of 0.005 mass% or less) with excellent ductility. less than 10 mass%) is often adopted. Ultra-low carbon steel and low carbon steel have good castability, and even if they are continuously cast at a high casting speed of about 2.0 m/min, the solidified shell breaks during casting and the molten steel inside leaks out, which is called breakout. The risk of causing trouble is small. However, in ultra-low carbon steel and low carbon steel, bubbles and inclusions suspended in the molten steel are captured at the solidification interface just below the surface of the molten steel in the mold, resulting in defects such as bubbles and inclusions on the slab surface layer. is likely to occur. When a slab is rolled with bubbles and inclusion defects in the surface layer, surface defects called slivers are likely to occur in the product after rolling as well. Thin sheet products, which are often used for parts that are visible to the human eye, such as automobile outer panels, are very strict about surface quality. , is a serious problem that directly leads to an increase in the cost for cutting and maintaining the surface layer of the cast slab.

In order to reduce defects such as bubbles and inclusions in the surface layer of a cast slab, an electromagnetic stirring technique is used in which molten steel in a continuous casting mold is subjected to an alternating moving magnetic field to be swirled and stirred (see, for example, Patent Document 1).
When electromagnetic stirring is applied, air bubbles and inclusions are washed away from the solidification interface just below the surface of the molten steel in the mold, resulting in a reduction of air bubbles and inclusion defects on the slab surface layer.
The present inventor also confirmed the effect of reducing surface air bubbles and inclusion defects in ultra-low carbon steel slabs and low carbon steel slabs by electromagnetic stirring in a continuous casting facility for thin plates, and applied it to actual operation. have enjoyed the sliver defect reduction effect of

JP 2007-98398 A

On the other hand, medium-carbon steel (with a carbon content of 0.10 mass% or more and 0.18 mass% or less), which has an excellent balance of strength and weldability, is used for thick plates used in shipbuilding and construction. There are many. Compared to ultra-low carbon steel and low-carbon steel, medium carbon steel is less likely to trap bubbles and inclusion defects in the slab surface layer, and the problems of air bubbles and inclusion defects in the slab surface layer are less likely to occur. On the other hand, medium-carbon steel has a problem that vertical cracks are likely to occur on the slab surface due to deformation due to a peritectic reaction during solidification. If a slab with vertical cracks is rolled as it is, it may lead to product defects. Crack defects must be prevented from occurring. It is known that the application of electromagnetic stirring, in which the molten steel is swirled just below the surface of the molten steel in the mold, is effective in suppressing vertical cracks in medium-carbon steel slabs. The present inventor also confirmed this effect by casting a medium-carbon steel slab using a continuous caster for thin plates equipped with electromagnetic stirring equipment. However, when the same electromagnetic stirring conditions as those for ultra-low carbon steel and low-carbon steel were applied to medium carbon steel, there was a problem of frequent occurrence of breakouts.

In medium-carbon steel, in addition to the problem that vertical small cracks are likely to occur due to the peritectic reaction during solidification, the surface of the slab is easily deformed to create a gap between the mold and the slab surface. There is a problem that the solidification in the mold becomes non-uniform and breakout is likely to occur. The cause of the increase in breakout of medium carbon steel due to the application of electromagnetic stirring has not been sufficiently clarified, but the increase in deformation of the mold surface in the mold due to the application of electromagnetic stirring may cause powder entrapment and Breakouts may have increased due to running out of powder. Alternatively, it is possible that electromagnetic agitation was applied to locally accelerate the hot jet stream from the submerged nozzle, resulting in accelerated solidification retardation due to impingement of the jet stream and increased breakout.

For this reason, electromagnetic stirring has been applied to medium-carbon steel under conditions with a weaker stirring force than for ultra-low carbon steel and low-carbon steel, in order to prevent both breakouts and vertical cracks. As mentioned above, compared to ultra-low carbon steel and low-carbon steel, medium-carbon steel is less susceptible to defects such as bubbles and inclusions. Inclusion defects have been suppressed to a level at which there is no problem (a level at which a scarf for cutting the surface of the cast slab can be omitted and the steel can be rolled with black scale). (If the steel has a high sulfur content, bubbles and inclusion defects may become a problem even with medium carbon steel, but medium carbon steel for heavy plates has a sulfur content of 0.5. In many cases, it is adjusted to 0001 mass% or more and 0.005 mass% or less, and with such medium carbon steel, even under conditions where the stirring power of electromagnetic stirring is weakened, defects such as bubbles and inclusions do not pose a problem.)

However, in general, the slab cross-sectional size of a continuous caster for thick plates is larger than that for thin plates. There is a concern that breakouts will increase and it will be difficult to prevent both breakouts and vertical cracks. For this reason, in recent years, not only electromagnetic stirring that applies an alternating moving magnetic field to the molten steel in the mold, but also an electromagnetic brake in the mold that applies a static magnetic field to stabilize casting (prevention of breakout) has been installed. A method of applying it by switching with the brake and a method of applying it simultaneously have also been developed. However, the method of combining the electromagnetic stirring and the electromagnetic brake not only increases the equipment cost, but is often not applicable when modifying an existing continuous casting machine due to restrictions on the installation space.

The object of the present invention is not to limit the cross-sectional size in particular, but in the continuous casting of medium-carbon steel slabs, in which large cross-section slabs are often cast, without using both a static magnetic field and an alternating moving magnetic field. It is an object of the present invention to provide a continuous casting method capable of suppressing longitudinal cracks and also stably suppressing breakout by swirling and stirring molten steel in a mold by applying only a moving magnetic field.

The inventor of the present application further advanced the research based on the above findings. Then, in addition to the conventional electromagnetic stirring conditions for medium carbon steel, attention was paid to the number of poles of the electromagnetic stirring device. It was found that the magnetic flux density and electromagnetic force in the mold can be made uniform by casting the medium carbon steel under these conditions. In addition, it was found that non-uniform solidification was improved by uniformity of magnetic flux density, electromagnetic force, etc. in the mold. As a result, it was found that the occurrence of longitudinal cracks and breakouts can be suppressed in the casting of medium carbon steel.

The present invention uses an electromagnetic stirrer when casting medium carbon steel having a carbon content of 0.10 mass% or more and 0.18 mass% or less and a sulfur content of 0.0001 mass% or more and 0.005 mass% or less. It is a continuous casting method for slabs. The electromagnetic stirrer includes a first linear motor arranged along the longitudinal direction of one of the pair of long sides facing each other of the mold, and a and a second linear motor arranged along the longitudinal direction of the long side. By the electromagnetic stirrer, the magnetic flux density is oscillated sinusoidally with respect to the time axis in a region of 0.05 m or more and 0.25 m or less from the meniscus in the mold, and the peak position of the magnetic flux density is the length of the pair. By generating an AC moving magnetic field that moves in the longitudinal direction of the side portions independently without superimposing it on the static magnetic field, molten steel near one of the long sides of the pair of long sides and the other long side Molten steel is swirled and agitated so that adjacent molten steel is driven in directions opposite to each other in directions parallel to the longitudinal direction of the long side portion. In the mold, in the direction parallel to the longitudinal direction of the pair of long sides, the magnetic flux density By of the alternating moving magnetic field in the region excluding the regions within 100 mm from the short sides of both ends of the mold is 0.05 T or more. .14T or less. The number of poles N1 of the _first linear motor is set to 5 or more and 6 or less. The number of poles N2 of the _second linear motor is set to 5 or more and 6 or less. where N ₁ and N ₂ are natural numbers.
The phase velocity V of the AC moving magnetic field calculated by the following formula is set to 0.3 m/s or more and 1.0 m/s or less.
V=2.f.P
where f is the frequency (Hz) of the alternating moving magnetic field in the mold;
P is the pole pitch of the magnetic stirrer.
The number of poles is a natural number represented by W/P using the length W of the mold in the long side direction and the pole pitch P of the electromagnetic stirrer. The pole pitch is the distance between the north and south poles adjacent to each other in the long side direction of the mold. The length W of the mold in the long side direction of the electromagnetic stirrer is the number of coils arranged in the long side direction of the mold in the electromagnetic stirrer Nc (-) and the center interval C (m) between adjacent coils in the long side direction of the mold. is represented by the following formula.
W=Nc・C

According to the present invention, it is possible to suppress the occurrence of longitudinal cracks and breakouts in the casting of medium carbon steel.

It is a perspective view which shows a part of continuous casting machine. There is a plan view of the mold and the electromagnetic stirring device (view from the arrow II in FIG. 1). FIG. 2 is a schematic cross-sectional view (a cross-sectional view taken along line III-III in FIG. 1) showing part of the continuous casting machine. It is a figure which shows the flow-velocity distribution of molten steel in a mold. It is a figure which shows the relationship between magnetic flux density and the distance from the mold width center (the width direction center of a mold). It is a figure which shows the relationship between a time average electromagnetic force and the distance from the mold width center (the width direction center of a mold). It is a top view which shows the modification of an electromagnetic stirrer. FIG. 10 is a plan view showing another modification of the electromagnetic stirrer; FIG. 10 is a plan view showing another modification of the electromagnetic stirrer;

Preferred embodiments of the present invention are described below.

FIG. 1 shows a mold 1 , a submerged nozzle 2 and an electromagnetic stirring device 3 . The mold 1, submerged nozzle 2 and electromagnetic stirrer 3 are part of a continuous slab casting machine. The mold 1 has a pair of long sides 11 and 12 facing each other and a pair of short sides 13 and 14 facing each other. The cross section of the slab within the mold 1 is generally rectangular. The lower part of the submerged nozzle 2 is arranged near the center of the mold 1 . The electromagnetic stirring device 3 has a first linear motor 3a and a second linear motor 3b. The first linear motor 3 a is arranged along the longitudinal direction of the long side portion 11 . The second linear motor 3 b is arranged along the longitudinal direction of the long side portion 12 . Hereinafter, the direction parallel to the long side of the cross section of the slab in the mold 1 may be referred to as the long side direction or mold long side direction. The long side direction may also be referred to as the width direction. The direction parallel to the long side of the transverse cross section of the slab in the mold 1 is the direction parallel to the longitudinal direction of the long sides 11 and 12 in plan view of the mold.

The mold for continuous slab casting has short side portions 13 and 14 movable in the long side direction so that slab slabs with different slab widths (length in the long side direction of the cross section of the slab) can be cast. The space between the inner walls of the short side portion 13 and the short side portion 14 is called the width W of the mold. (Since the mold is normally tapered, the width of the mold differs between the upper part and the lower part of the mold. However, since the electromagnetic stirrer in the mold for continuous slab casting is installed at the upper part of the mold, the mold width W here is This is the upper width of the mold.Since the slab after exiting the mold undergoes further heat shrinkage, the mold width W is set larger than the target slab width by the amount of thermal contraction.)

In this embodiment, a medium carbon steel having a carbon content of 0.10 mass% or more and 0.18 mass% or less and a sulfur content of 0.0001 mass% or more and 0.005 mass% or less is cast. The casting speed is, for example, 0.7 m/min or more and 1.6 m/min or less, except for unsteady casting such as immediately after the start of casting or just before the end of casting. The long side length W of the inner dimension of the mold 1 is generally variable, but its maximum value is, for example, 1780 mm or more and 2200 mm or less. The length of the short side of the inner dimension of the mold 1 may be a fixed length or a variable length, and is, for example, 230 mm or more and 330 mm or less.

The first linear motor 3a and the second linear motor 3b are, as shown in FIG. 2, linear motors of interpolar winding type. The first linear motor 3a has an iron core 21 and a plurality of coils 22-1, 22-2, . . . , 22-18. Iron core 21 extends in a direction parallel to the longitudinal direction of long side portion 11 . The plurality of coils 22-1, 22-2, . . . , 22-18 are arranged without gaps in the long side direction. Two coils adjacent in the longitudinal direction (for example, coil 22-1 and coil 22-2) are in contact with each other. A plurality of coils 22-1, 22-2, .

The second linear motor 3b has the same configuration as the first linear motor 3a. The second linear motor 3 b has an iron core 31 and a plurality of coils 32 , 33 , 34 and 35 . The iron core 31 extends in a direction parallel to the longitudinal direction of the long side portion 12 . A plurality of coils 33-1, 33-2, . Two coils adjacent in the long side direction (for example, coil 32-1 and coil 32-2) are in contact with each other. A plurality of coils 33-1, 33-2, .

An AC power supply (not shown) is connected to each coil. The electromagnetic stirrer 3 includes, for example, a method using a two-phase alternating current or a three-phase alternating current. FIG. 2 shows an electromagnetic stirrer using three-phase alternating current. An electromagnetic stirrer using a three-phase alternating current has six current coils whose phases are different by 60° to form a set of N and S poles of a moving magnetic field. In an electromagnetic stirrer using three-phase alternating current, the position of the north pole moves by a distance corresponding to the center interval C between adjacent current coils each time the phase of the alternating current applied to each current coil changes by 60°. do. The position of the south pole also moves in the same way as the position of the north pole moves. An electromagnetic stirrer using a two-phase alternating current forms a set of N and S poles of a moving magnetic field by four current coils whose phases are different by 90°. The position of the north pole moves by a distance corresponding to the center spacing C between adjacent current coils each time the phase of the alternating current applied to each current coil changes by 90°. The position of the south pole also moves in the same way as the position of the north pole moves.

FIG. 2 illustrates, as an example, the positions of the north pole and the south pole at a certain moment when current is passed through the coils of the electromagnetic stirrer 1 using three-phase alternating current. FIG. 2 also shows the center interval C and the pole pitch P between adjacent current coils when three-phase alternating current is used in the electromagnetic stirrer 1 and the number of poles N of the electromagnetic stirrer 1 is 6. . The pole pitch P is the distance from the center position of the N pole to the center position of the S pole in the longitudinally adjacent N pole and S pole. The number of poles will be described later. The configuration of the electromagnetic stirring device 3 is not limited to that shown in FIG. Other examples of electromagnetic stirrers will be described later.

When an alternating current is passed through each coil, an alternating moving magnetic field is generated in the molten steel in the mold 1 . In this magnetic field, the magnetic flux density oscillates sinusoidally with respect to the time axis, and the peak position of the magnetic flux density moves in opposite directions in the long side direction near the long side 11 and near the long side 12. is the magnetic field. As a result, the molten steel near the long side portion 11 and the molten steel near the long side portion 12 are driven in opposite directions in the long side direction, thereby swirling and stirring the molten steel. Fig. 2 shows the position of the magnetic pole at that time, the direction in which the lines of magnetic force move (black arrows) when the phase of the current advances, and the movement of the molten steel. The directions (white arrows) in which the particles are stirred by being driven by the magnetic field are shown. When using either a two-phase AC current or a three-phase AC current, when the phase of the current changes by 180°, the positions of the N pole and the S pole are interchanged, and when the phase of the current changes by 360°, the N pole and the S pole are exchanged. The position of returns to the same position as the original state when the phase of the current was 0°. The time required for the phase of the current to change from 0° to 360° is the period T (s), and the reciprocal of the period T, f = 1/T (Hz, 1/s), is the frequency, so the magnetic field is (magnetic flux density The peak position of ) moves by a distance P, and the time required for the positions of the N pole and the S pole to switch is T/2 = 1/(2 f), and the movement speed (phase speed) V of the magnetic field is It is expressed as V=2.P.f.

In the present embodiment, an AC moving magnetic field is generated in the molten steel in the mold 1, but it is not necessary to generate a static magnetic field in the molten steel in the mold 1 by superimposing it on the AC moving magnetic field. By generating an AC moving magnetic field alone, the structure of the electromagnetic stirrer 3 is not complicated, and power consumption and equipment costs can be reduced, compared to the case where static magnetic fields are superimposed and generated.

As shown in FIG. 2, if the distance (pole pitch) between the N and S poles adjacent in the long side direction is P (m), and the distance between the current coil centers is C (m), two-phase electromagnetic stirring P=2.C for the device and P=3.C for the three-phase magnetic stirrer.

W (m) shown in FIG. It can be expressed as Nc·C. Hereinafter, W may be referred to as the length W of the mold in the long side direction of the electromagnetic stirrer. As described above, the length of the long side of the inner dimension of the mold 1 is generally variable. In order to efficiently electromagnetically stir the whole, it is desirable that the length W of the electromagnetic stirrer in the long side direction is approximately equal to the maximum length of the long side length of the inner dimension of the mold 1 . Therefore, when casting is performed with the length of the long side of the inner dimension of the mold 1 smaller than the maximum value, the length of the long side of the electromagnetic stirrer often becomes less than the length of the long side of the inner dimension of the mold 1. . When the number N of poles of the electromagnetic stirring device 1 is 6, the number Nc of current coils is 18 pieces. As shown in FIG. 2, 18 coils are arranged in each of the first linear motor 3 and the second linear motor 4 .

The number of poles N(−) is expressed by the following formula from the length W (m) of the mold in the long side direction of the electromagnetic stirrer and the pole pitch (distance between adjacent N and S poles) P (m).
N=W/P
Here, N is a natural number.
The number of poles N is also the number of poles. The pole number N1 of the first linear motor 3a and the pole number _N2 of the _second linear motor 3b are the same. In this embodiment, the number of poles of the electromagnetic stirring device 3 is set to the number of poles N1 of the _first linear motor 3a or the number of poles _N2 of the second linear motor 3b. In FIG. 2, the pole number N1 of the _first linear motor 3a is six, and the pole number N2 of the _second linear motor 3b is six. In this case, the number of poles of the electromagnetic stirrer 3 is six.

As shown in FIG. 3, the electromagnetic stirrer 3 is arranged so as to generate an AC moving magnetic field in molten steel existing in a region R of 0.05 m or more and 0.25 m or less from the meniscus. For example, when the vertical length l ₁ from the upper end to the lower end of the mold 1 is 900 mm, and the meniscus is located at a position 100 mm vertically downward from the upper end of the mold, the upper end of the iron core of the electromagnetic stirrer 3 is positioned at the meniscus. match the height of In this case, for example, an iron core having a vertical length l ₂ from the top end to the bottom end (see l ₂ in FIG. 3) of 300 mm is used. In this case, even if the meniscus level in the mold 1 changes, for example, by about ±30 mm, an AC moving magnetic field can be generated in the molten steel in the region R 0.05 m or more and 0.25 m or less from the meniscus. In addition, the method of generating an AC moving magnetic field in the molten steel in the region R whose distance from the meniscus is 0.05 m or more and 0.25 m or less is not limited to the above.

In the mold 1, as shown in FIG. 3, the flux 6 is floating on the molten steel 5. As shown in FIG. A solidified shell 7 is formed along the wall surface of the mold 1 by cooling the molten steel 5 by the mold 1 . The surface of the solidified shell 7 that contacts the molten steel 5 is called a solidified interface 8 .

FIG. 4 shows the flow velocity distribution when the molten steel in the mold 1 is stirred. The flow velocity distribution in FIG. 4 is the flow velocity distribution of molten steel present in region R within mold 1 . The vertical axis in FIG. 4 is the molten steel flow velocity in region R. In FIG. The horizontal axis of FIG. 4 is the horizontal distance y between the solidification interfaces in the region R. One end of the horizontal axis in FIG. 4 is the solidification interface A (for example, point A in FIG. 3) near the long side 11 of the mold 1 , and the other end of the horizontal axis in FIG. 4 is the long side 12 of the mold 1 . is a solidification interface B (for example, point B in FIG. 3).

In the vicinity of the wall such as the solidification interface in continuous casting, a different boundary layer flow is generated than in the main stream far from the wall. Under the condition that electromagnetic force does not act on the molten steel without applying electromagnetic stirrer or electromagnetic brake in the mold, the boundary layer consists of a viscous bottom layer in which the flow velocity is proportional to the distance from the wall surface, and the flow velocity has a linear relationship with the logarithm of the distance from the wall surface. It is known that there exists a logarithmic-law region in the viscous bottom layer and a transition layer between the logarithmic-law region and the like. It is known that when a conductive liquid such as molten steel flows near the wall and a strong static magnetic field acts perpendicular to the wall, a boundary layer called the Hartmann boundary layer is formed near the wall. The thickness δH of the Hartmann boundary layer is the position ((U−v)/ It can be expressed as the distance from the wall to the position where U=1/e). However, e is the base of the natural logarithm (e=2.71828...). In FIG. 4, as an example, U is the velocity when the velocity of the mainstream outside the boundary layer is maximum.

The Hartmann boundary layer thickness δH (m) is obtained using the magnetic flux density B (T) of the static magnetic field, the electrical conductivity σ (S/m) of the conductive fluid, and the viscosity coefficient μ (Pa s) of the conductive fluid. ,
δH=1/(B・(σ/μ) ^0.5 ) (1)
is estimated.

The above Hartmann boundary layer is based on a static magnetic field, but in the present embodiment, an AC moving magnetic field is generated in the molten steel in the mold. As a result of examining the case where an AC moving magnetic field is generated, even in the electromagnetic stirring in the mold of continuous slab casting using an AC moving magnetic field instead of a static magnetic field, at the long side mold solidification interface where the AC moving magnetic field acts perpendicularly to the wall surface, A kind of Hartmann boundary layer, which should also be called an AC moving magnetic field Hartmann boundary layer, is formed, similar to a normal Hartmann boundary layer. It was found to affect the

By analogy with the normal Hartmann boundary layer, the alternating moving magnetic field Hartmann boundary layer thickness δHa is obtained by estimating the static magnetic field Hartmann boundary layer thickness δH instead of the magnetic flux density B of the static magnetic field in the equation (1). Using the amplitude B0 of the component in the short side direction of the mold or the effective value Beff of the magnetic flux density of the alternating moving magnetic field Beff = B0 / (√2),
δHa = 1/(Beff (σ/μ) ^0.5 )
= 1/(B0 (σ/2μ) ^0.5 ) (2)
It can be estimated that Here, the amplitude B0 of the component in the short side direction of the mold is the maximum value of the magnetic flux density (amplitude of the magnetic flux density) that fluctuates with time at a specific location.

Here, when the molten steel in the mold 1 is orbitally stirred by the electromagnetic stirring device 3, the maximum flow velocity U of the mainstream molten steel in the mold 1 is approximately U=k Beff sqrt (W f P).・(A)
can be estimated. Since the magnitude of the magnetic flux density changes depending on the position in the lengthwise direction of the mold, if we adopt the geometric mean concept, the equation (A) can be further expressed as
U=k*sqrt(Bmax*Bmin*W*f*P) (A')
becomes. Here, Bmax and Bmin are maximum when the distribution of the effective value of the short-side component of the AC moving magnetic field in the long-side direction of the mold is examined at a position 10 mm away from the wall surface of the mold at the center height of the linear motor. It is the effective value at the point and the minimum point.
k is a proportionality constant. Since it is difficult to measure the U of molten steel, it is difficult to obtain a precise value for k. Estimated.
Further, the flow velocity u near the solidification interface is calculated using the distance y from one solidification interface A as follows: u=U·(1−exp(−(y/δHa)) (a)
is approximated as
where δHa(m) is the AC Hartmann boundary layer thickness. At the portion where the magnetic flux density is minimum, the AC Hartmann boundary layer thickness δHa is obtained from the above equation (2) as follows: δHa=1/(Bmin·(σ/2μ) ^0.5 ) (b)
is represented.
Since y=0 at the solidification interface A, the flow velocity gradient at the solidification interface A is obtained by differentiating equation (a) with respect to y, and when y=0, du/dy=U/δHa (B)
Calculated by
From the above formulas (A′) and (B), the flow velocity gradient at the solidification interface A is du/dy=k·sqrt(Bmax·Bmin·f·P)/δHa (C)
is estimated.

The flow velocity gradient du/dy at the solidification interface A is the flow velocity gradient (du/dy) at y=0 in FIG.
The mechanism of suppression of vertical cracks in medium carbon steel by applying electromagnetic stirring is often explained in relation to homogenization of solidification. However, it is not clear why the application of electromagnetic stirring results in uniform solidification.
Since the molten steel flow velocity at the solidification interface is zero even if electromagnetic stirring is applied, the solidification is not uniformed by the flow velocity at the solidification interface. On the other hand, it is unlikely that the flow velocity at a position far from the solidification interface contributes to uniform solidification. It is likely that electromagnetic stirring imparts a flow velocity gradient to the solidification interface over the entire width of the mold, resulting in increased heat transfer from the molten steel to the solidified shell, suppressing solidification just below the surface of the molten steel, and at the same time, homogenizing the vertical flow. It is considered that small cracks are suppressed.
Therefore, it is considered that the effect of suppressing vertical cracks by electromagnetic stirring is closely related to the flow velocity gradient du/dy at the solidification interface.

On the other hand, the mechanism by which breakout of medium carbon steel tends to occur when electromagnetic stirring is applied is not clear as described above, but it is possible that it is related to the increase in melt surface deformation and the acceleration of the discharge flow. expensive. These factors have a deeper relationship with the maximum flow velocity U stirred by electromagnetic stirring than with the flow velocity gradient du/dy at the solidification interface.

Therefore, when formula (B) regarding the flow velocity gradient at the solidification interface is adopted as an index of the merit of suppressing vertical cracks, and the maximum flow velocity U is adopted as an index of the demerit of breakout, the merit to demerit ratio M/D is
M/D = (du/dy)/U = (U/δHa)/U = 1/δHa
is represented. Further substituting the formula (2) into the above formula,
M/D = Beff (σ/2μ) ^0.5
is represented. Since .sigma. and .mu., which are physical property values of molten steel, cannot be changed, the magnetic flux density Beff should be increased in order to increase M/D. The reason for this is that while the maximum flow velocity U increases in proportion to the magnetic flux density Beff, as can be seen from the equation (A) shown above, U=k.Beff.sqrt (W.f.P), the solidification interface This is because the flow velocity gradient increases in proportion to the square of the magnetic flux density Beff. However, if Beff is increased while keeping other conditions constant in order to increase M/D, U increases and the risk of breakout increases.

The inventors of the present application have studied the effects of reducing the frequency f appearing in the equation (A) and the pole pitch P as means for suppressing the increase in U due to the increase in the magnetic flux density Beff. The in-mold electromagnetic stirrer of the thin plate continuous casting machine used by the present inventor in the casting experiment has a variable frequency f, and the effect of reducing the frequency f can be easily confirmed in actual casting. On the other hand, in order to change the pole pitch P, it is necessary to remake the linear motor, which requires a large amount of investment. However, in equation (A), the effect of reducing the frequency f and the effect of reducing the pole pitch P are equivalent. , can be estimated with sufficient accuracy. Therefore, this time, the effect of reducing the frequency f was confirmed by actual casting, and the effect of changing the magnetic flux density distribution when the pole pitch P was reduced was confirmed by numerical analysis using the three-dimensional magnetic field analysis software Infolitica Magnet 7. Regarding the accuracy of the magnetic field analysis, an existing electromagnetic stirrer for continuous thin plate casting machines was analyzed in advance, and it was verified that the magnetic field analysis had sufficient accuracy.

First, an experiment for confirming the effect of reducing the frequency f by actual casting will be described.

A medium-carbon steel was cast with a slab continuous casting machine for thin plates. The casting conditions were those shown in Table 2 below.

The quality of the slab was investigated by the following method.
In a two-strand vertical bending type continuous casting machine for thin plate, when the casting speed is in a steady state in the range of 0.9 to 1.1 m/min, three types of electromagnetic stirring conditions in the mold for both strands (Experiment 1, Experiment 2, The casting experiment switching to experiment 3) was repeated 5 times, and from both strands of each casting, slab strands under each electromagnetic stirring condition were sampled, and bubble defects and longitudinal cracks were investigated as follows. As for the deformation of the molten metal surface, representative parts were investigated. Experiment 3 is an experiment conducted under the condition that no bubble defect occurs in the ultra-low carbon steel or the low-carbon steel when the ultra-low-carbon steel or the low-carbon steel is cast. Experiment 2 is an experiment in which the frequency f was reduced from the frequency f of Experiment 3. FIG.

・Bubble defects Since it is difficult to detect air bubbles in the slab cast as it is, the entire surface of the cast slab is scarfed to a target depth of 1.5 mm, and the cast slab surface after scarfing was visually inspected to investigate the presence or absence of bubble defects with a diameter of 3 mm or more.
Past experience shows how small the number density, which is calculated by dividing the number of detected defects by the surveyed area, is necessary to omit the scarf of the slab and roll it with the black scale. Therefore, it was determined as follows.
○: The number density of bubbles with a diameter of 3 mm or more is 0.1/m ² or less ×: The number density of bubbles with a diameter of 3 mm or more is more than 0.1/m ²

・Longitudinal small cracks For slabs to be investigated for vertical small cracks, the black scale without scarfing is cooled, and a magnetic field is applied in the long side direction of the mold to detect cracks in the casting direction on the front and back surfaces. Magnetic particle testing was performed. Most of the small vertical cracks detected are removed together with the scale in the heating furnace and are shallow and harmless to the extent that they do not remain after rolling. In light of the past experience that vertical small cracks that are deep and are harmful to the product after rolling may occur, the following judgments were made.
〇: The number density of vertical small cracks is 1/m ² or less ×: The number density of vertical small cracks is greater than 1/m ²

・Mold Surface Deformation When in-mold electromagnetic stirring is applied, the stirring flow collides with the corners of the slab, and there is a tendency for the mold surface to swell at the corners. Since the deformation of the molten steel surface leaves traces in the oscillation marks of the slab, it is possible to evaluate the deformation of the molten metal surface by examining the amount of deformation of the oscillation marks. A corner sample was cut out from the black scale cast slab under each electromagnetic stirring condition, and water blasting was performed to remove the scale so that the oscillation marks could be clearly observed. In light of this, it was judged as follows.
○: The average value of the melt surface deformation amount is 6 mm or less (acceptable level). No risk of breakouts.
x: The average value of the melt surface deformation amount is greater than 6 mm. If the experiment continues for a long time, breakouts may occur.

Table 3 shows a list of experimental conditions and experimental results.

・Bubble defects Under the conditions of Experiment 1, in which electromagnetic stirring was not applied, the number density of bubble defects was about 1.0/m ² , which was approximately 10 times higher than the acceptance criteria. Under the conditions of Experiment 3, all were reduced to acceptable levels.

・Longitudinal small cracks Under the conditions of Experiment 1, in which electromagnetic stirring was not applied, the number density of vertical small cracks was as large as 4.5 pieces/m ² , whereas under the conditions of Experiments 2 and 3, in which electromagnetic stirring was applied, Both were reduced to acceptable levels.

・In Experiment 3, where the frequency of electromagnetic stirring was 3 Hz, the amount of deformation of the molten steel surface was as large as 9 mm. In Experiment 2, all of them fell within the pass level.

As mentioned above, the following things were found from this experiment in which the pole pitch P was kept at P=0.45m.
In Experiment 1, in which electromagnetic stirring was not applied, bubble defects and longitudinal cracks occurred.
In Experiment 3, in which electromagnetic stirring was applied under the electromagnetic stirring conditions for low-carbon steel, the melt surface deformation was large. Therefore, breakout may occur.
On the other hand, the frequency was reduced from 3 Hz (experiment 3) to 1 Hz (experiment 2), and the phase velocity of the alternating moving magnetic field was reduced from 2.7 m/s (experiment 3) to 0.9 m/s (experiment 2). By weakening the force, it is possible to suppress the deformation of the molten steel surface due to electromagnetic stirring while maintaining the same effect of preventing air bubbles and small vertical cracks in medium carbon steel, and suppressing the increase in the probability of breakouts. It could be confirmed.

From the above experiment, it was confirmed that the above effect can be obtained by reducing the frequency f in actual casting. The effect obtained by reducing the frequency f and the effect obtained by reducing the pole pitch P are equivalent as described above. Therefore, by reducing the pole pitch P, the above effects obtained by reducing the frequency f, specifically, the effect of preventing bubble defects and the effect of preventing vertical cracks in medium carbon steel, are maintained at the same level. It is thought that the effect of suppressing the deformation of the melt surface due to electromagnetic stirring and suppressing the increase in the risk of breakout can be obtained.

In order to confirm this, effects such as changes in magnetic flux density distribution when the pole pitch P is reduced were confirmed by numerical analysis using the three-dimensional magnetic field analysis software Infolitica Magnet 7. Regarding the accuracy of the magnetic field analysis, an existing electromagnetic stirrer for continuous thin plate casting machines was analyzed in advance, and it was verified that the magnetic field analysis had sufficient accuracy. Numerical analysis will be described below.

Here, the case of casting with a continuous slab casting machine for thick plate, which is often used for medium carbon steel, was analyzed. A continuous slab caster for thick plates sometimes has a wider mold width than a continuous slab caster for thin plates. According to formula (A) regarding the stirring force, if the mold width W increases, the stirring force increases and the maximum flow velocity U increases even if the frequency f and the pole pitch P are constant, increasing the risk of breakout.
In addition, if the linear motor width Wm is increased while keeping the number of poles N constant (general number of poles N=4) so as to cope with the increase in mold width W, the pole pitch P=Wm/N will inevitably be Therefore, an increase in the pole pitch P also increases the stirring force and the maximum flow velocity U, which further increases the risk of breakout.

In thick plate continuous casting machines, where the mold width is sometimes larger than that of thin plate continuous casting machines, vertical small cracks occur while preventing breakout risks due to electromagnetic stirring without using an electromagnetic brake that uses a static magnetic field. In order to suppress the above formula (A), it is necessary to suppress the product of W · P · f within the range proven in the thin plate continuous casting machine. It was conceived that increasing P to reduce it would be effective. However, since there is no precedent for setting the number of poles N to greater than 4 in the electromagnetic stirring in the mold of the swirl stirring method of a continuous plate caster that mainly casts medium-carbon steel, in this analysis, the number of poles N was set to be greater than 4. Further magnetic field analyzes (calculations 2-4) were performed.

Table 4 shows analysis conditions and analysis results. In this analysis, the electric conductivity σ of the molten steel was 7×10 ⁵ S/m, and the viscosity coefficient μ of the molten steel was 0.0056 Pa·S.

The "minimum value of flow velocity gradient at the solidification interface" and "maximum flow velocity by electromagnetic stirring" in Experiments 2 and 3 were used as the index of bubble defects, the index of vertical cracks, and the index of melt surface deformation. Specifically, the minimum value of "56 (1/s)" of the "minimum value of the flow velocity gradient at the solidification interface" was adopted as the index of bubble defects and the index of vertical cracks. As an index of melt surface deformation, the largest value of "maximum flow velocity by electromagnetic stirring", "0.18 (m/s)", was adopted. The determination method will be described below.
・Bubble defects and vertical small cracks ○: The flow velocity gradient at the solidification interface is 56 (1/s) or more. In this case, it was determined that no bubble defects and vertical cracks occurred.
x: The flow velocity gradient at the solidification interface is less than 56 (1/s). In this case, it was determined that bubble defects and vertical small cracks would occur.
・Mold surface deformation ◯: The maximum flow velocity due to electromagnetic stirring is 0.18 (m/s) or less. In this case, it was determined that breakout did not occur.
x: The maximum flow velocity by electromagnetic stirring exceeds 0.18 (m/s). In this case, it was determined that breakout occurred.

In calculation 1, the general pole number N=4 was used. As described above, if the linear motor width Wm is increased while keeping the number of poles N constant (generally the number of poles N = 4) so as to cope with the increase in the mold width W of the slab continuous casting machine for thick plate, An increase in the pole pitch P increases the stirring force, further increasing the risk of breakout. Therefore, in calculation 1, it is considered that the melt surface deformation was "x".

In calculations 2 to 4, the number of poles N was set to be greater than 4, and the pole pitch P was set to be smaller than when the number of poles N=4. Calculation 4 was analyzed under the condition that the current value in Calculation 3 was multiplied by 1.43. From Calculation 2 and Calculation 4, when the number of poles is N = 5 and N = 6, the effect of preventing bubble defects and vertical small cracks of medium carbon steel is maintained at the same level, and the deformation of the molten steel surface due to electromagnetic stirring is suppressed. , was found to be effective in suppressing an increase in breakout risk. In calculation 3, although the number of poles is N=5, since the magnetic flux density is small, it is considered that the effect of preventing bubble defects and preventing vertical small cracks cannot be obtained.

The results of calculations 1-3 are shown in FIGS. 5A and 5B. The vertical axis of FIG. 5A is the magnetic flux density, and the horizontal axis of FIG. 5A is the distance from the center in the width direction of the mold in the long side direction of the mold. FIG. 5B shows the relationship between the electromagnetic force and the distance from the widthwise center of the mold. The vertical axis in FIG. 5B is the electromagnetic force, and the horizontal axis in FIG. 5B is the distance from the center in the width direction of the mold in the long side direction of the mold. Electromagnetic force is proportional to frequency (f) x magnetic flux density (By) ² . Although FIG. 5A and FIG. 5B do not show the result of Calculation 4, Calculation 4 was analyzed under the condition that the current value of Calculation 3 was multiplied by 1.43. Therefore, even in calculation 4, it is considered that the distribution will be similar to that in calculation 3.

5A and 5B, in calculation 2 (number of poles N = 5) and calculation 3 (number of poles N = 6), from calculation 1 (number of poles N = 4), the magnitude of magnetic flux density and electromagnetic force is the mold length It was found to be uniform in the side direction (mold width direction). From this, it can be expected that in the casting of medium carbon steel, by setting the number of poles N to 5 to 6, the magnetic flux density, electromagnetic force, etc. in the mold become uniform, thereby improving non-uniform solidification. It was found that this can suppress the occurrence of breakout.
From the above, it was found that setting the number of poles N to 5 to 6 is effective in casting medium carbon steel.

Based on the above findings, the following findings were obtained.
In the casting of medium carbon steel, the magnetic flux density By of the alternating moving magnetic field generated in the region R in the mold 1 is 0.05 T or more and 0.14 T or less, and the phase velocity V is 0.3 m / s or more and 1.0 m / s or less. , the number of poles N1 of the _first linear motor 3a is set to 5 or more and 6 or less, and the number of poles _N2 of the second linear motor 3b is set to 5 or more and 6 or less. As a result, it is possible to suppress the occurrence of vertical cracks and breakouts in the casting of medium carbon steel.

In the casting of medium carbon steel, if the magnetic flux density By is less than 0.05 T, longitudinal cracks may occur. In casting medium carbon steel, if the magnetic flux density By exceeds 0.14 T, breakout may occur. In addition, in the casting of medium carbon steel, if the phase velocity V is less than 0.3 m/s, longitudinal cracks may occur. In the casting of medium carbon steel, if the phase velocity V exceeds 1.0 m/s, breakout may occur. Whether longitudinal cracks have occurred can be confirmed by, for example, a magnetic particle flaw detection test.

In addition, from the research of the inventors of the present application, it was found that vertical cracks are less likely to occur in the regions of up to 100 mm in the slab width direction from the short sides at both ends of the slab made of medium carbon steel. From this, when casting medium carbon _steel , as shown in _FIG . Then, even if the magnetic flux density By is not set to 0.05T or more and 0.14T or less, it can be said that longitudinal cracks hardly occur.

From the above, when casting medium carbon steel, in the mold 1, the magnetic flux density By is 0.05 T in the regions excluding the regions m ₁ and m ₂ within 100 mm in the long side direction from the short sides 13 and 14 of the mold. 0.14T or less. In the mold 1, the magnetic flux density By may be 0.05 T or more and 0.14 T or less in regions m1 and _m2 within ₁₀₀ mm in the long side direction from the short sides 13 and 14 of the mold.

Here, an example of the flow velocity gradient du/dy at the solidification interface that can obtain the effect of suppressing small vertical cracks will be described. From Experiments 1 to 3 described above, it is considered that when du/dy is 56 or more, the effect of suppressing longitudinal cracks is obtained. Although the upper limit of du/dy is not particularly limited, it is calculated by the following method, for example.
From the results of Experiments 1 to 3, assuming that the maximum value of the flow velocity U that can suppress breakout is 0.18, from the above equation (B), du/dy=0.18/δHa. From the above equation (2), δHa=1/(Beff·(σ/μ) ^0.5 ). When Beff is the maximum value of the magnetic flux density By described above, which is 0.14T, δHa=0.00064m. When δHa=0.00064m, from the above equation (B), du/dy=18/0.00064=281m/s.
From the above, the flow velocity gradient du/dy at the solidification interface at which the effect of suppressing small longitudinal cracks is obtained is, for example, 56 or more and 281 or less.
From Experiments 1 to 3, the flow velocity U that can suppress breakout is, for example, 0.18 m/s or less.

In the above, FIG. 2 shows an example of the inter-electrode winding type electromagnetic stirring device 3, but the electromagnetic stirring device may have a configuration other than that shown in FIG. For example, an inter-electrode winding type electromagnetic stirrer other than the electromagnetic stirrer 3 shown in FIG. 2 may be used. Moreover, you may use the electromagnetic stirrer of systems other than an interpolar winding system. A modification of the electromagnetic stirrer will be described below with reference to FIGS. 6 and 7. FIG. The electromagnetic stirrer shown in FIGS. 6 and 7 has three phases and six poles. Also, FIGS. 6 and 7 show the N pole and S pole at a certain moment when current is passed through the coil.

FIG. 6 shows another example of an inter-electrode winding type electromagnetic stirrer. As shown in FIG. 6, the electromagnetic stirring device 103 has a first linear motor 103a and a second linear motor 103b.
The first linear motor 103a has an iron core 121 and a plurality of coils 122-1, 122-2, . . . , 122-18. In FIG. 6, the iron core 121 is hatched so that the iron core 121 and the coil can be easily distinguished. The iron core 121 extends in the long side direction. The iron core 121 has an iron core main body 121T and a plurality of comb-tooth-like salient poles 121a and 121b protruding from the iron core main body 121T toward the mold 1 . The plurality of salient poles 121a and 121b are arranged at predetermined intervals in the long side direction. A coil is wound between two adjacent salient poles. For example, a coil 122-1 is wound between the salient poles 121a and 121b. The second linear motor 103b has the same configuration as the first linear motor 103a.

FIG. 7 shows an example of a salient pole winding type electromagnetic stirrer. As shown in FIG. 7, the electromagnetic stirrer 203 has a first linear motor 203a and a second linear motor 203b.
The first linear motor 203a has an iron core 221 and a plurality of coils 222-1, 222-2, . . . , 222-18. The iron core 221 has a main body 221T and a plurality of salient poles 221a, 221b, . The plurality of salient poles 221a, 221b, . . . , 221r are arranged at predetermined intervals in the long side direction. A plurality of coils 222-1, 222-2, . . . , 222-18 are wound around a plurality of salient poles 221a, 221b, .
The second linear motor 203b has the same configuration as the first linear motor 203a.

2, 6 and 7 show the electromagnetic stirrer having six poles, the number of poles of the electromagnetic stirrer may be five. The fact that the number of poles of the electromagnetic stirring device is 5 means that, in the pair of linear motors of the electromagnetic stirring device, one linear motor has 5 poles and the other linear motor has 5 poles. .

FIG. 8 shows an example of an electromagnetic stirrer having five poles. FIG. 8 shows a three-phase alternating current electromagnetic stirrer 303 . The magnetic stirrer 303 employs the inter-polar winding method similar to that of the electromagnetic stirrer 3 shown in FIG. FIG. 8 shows the N pole and S pole at a certain moment when current is passed through the coil.

The electromagnetic stirring device 303 has a first linear motor 303a and a second linear motor 303b. The first linear motor 303a has an iron core 321 and a plurality of coils 322-1, 322-2, . . . , 322-15. The iron core 321 extends in a direction parallel to the longitudinal direction of the long side portion 11 . The plurality of coils 322-1, 322-2, . . . , 322-15 are arranged without gaps in the longitudinal direction. Two coils adjacent in the longitudinal direction (eg, coil 322-1 and coil 322-2) are in contact with each other. A plurality of coils 322-1, 322-2, . When the number of poles of the electromagnetic stirring device 303 is 5, 15 coils are arranged in each of the first linear motor 303a and the second linear motor 303b. When the number of poles of the electromagnetic stirring device 303 is five, the number Nc of current coils is fifteen.
The second linear motor 303b of the electromagnetic stirring device 303 has the same configuration as the first linear motor 303a.

An electromagnetic stirrer having five poles may have a configuration other than that shown in FIG. Although FIG. 8 shows an example of a three-phase alternating current electromagnetic stirrer, a two-phase electromagnetic stirrer may be used. The two-phase electromagnetic stirrer may have 5 or 6 poles. For the two-phase electromagnetic stirrer, the interpolar winding system or the salient pole winding system may be adopted.

Although the embodiments of the present invention have been described above with reference to the drawings, it should be considered that the specific configuration is not limited to these embodiments. The scope of the present invention is indicated by the scope of the claims rather than the above description, and includes all modifications within the scope and meaning equivalent to the scope of the claims.

For example, although the three-phase electromagnetic stirrer 1 is illustrated in FIGS. 2, 6, 7 and 8 in the present embodiment and modifications, a two-phase electromagnetic stirrer may be used.

Reference Signs List 1 mold 2 immersion nozzle 3 electromagnetic stirrer 3a, 103a, 203a, 303a first linear motor 3b, 103b, 203b, 303b second linear motor 11, 12 long side 13, 14 short side 21, 31, 121, 131 , 221, 231, 321 Iron core 22-1, 22-2, 22-18, 122-1, 122-2, 122-18, 222-1, 222-2, 222-18, 322-1, 322- 2, 322-15 coil

Claims (1)

Continuous casting of a slab using an electromagnetic stirrer when casting medium carbon steel having a carbon content of 0.10 mass% or more and 0.18 mass% or less and a sulfur content of 0.0001 mass% or more and 0.005 mass% or less is a method,
The electromagnetic stirrer includes a first linear motor arranged along the longitudinal direction of one of the pair of long sides facing each other of the mold, and a a second linear motor arranged along the longitudinal direction of the long side;
By the electromagnetic stirrer, the magnetic flux density is oscillated sinusoidally with respect to the time axis in a region of 0.05 m or more and 0.25 m or less from the meniscus in the mold, and the peak position of the magnetic flux density is the length of the pair. By generating an AC moving magnetic field that moves in the longitudinal direction of the side portions independently without superimposing it on the static magnetic field, molten steel near one of the long sides of the pair of long sides and the other long side orbiting and stirring the molten steel so as to drive the molten steel in the vicinity in opposite directions to each other in the direction parallel to the longitudinal direction of the long side;
In the mold, in the direction parallel to the longitudinal direction of the pair of long sides, the magnetic flux density By of the alternating moving magnetic field in the region excluding the regions within 100 mm from the short sides of both ends of the mold is 0.05 T or more. .14T or less,
The number of poles N1 of the _first linear motor is set to 5 or more and 6 or less,
The number of poles _N2 of the second linear motor is 5 or more and 6 or less,
where N ₁ and N ₂ are natural numbers,
A continuous casting method for slabs, characterized in that the phase velocity V of an AC moving magnetic field calculated by the following formula is set to 0.3 m/s or more and 1.0 m/s or less.
V=2.f.P
where f is the frequency (Hz) of the alternating moving magnetic field in the mold;
P is the pole pitch of the magnetic stirrer

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