Description
Title of Invention: METHOD FOR CONTINUOUSLY CASTING STEEL
5 Technical Field
[OOOl] This invention relates to a method for optimally operating an
electromagnetic stirrer disposed in a mold, to continuously cast steel.
Background Art
10 [0002] One of the main causes of making the quality of a superficial slab
manufactured by continuous casting deteriorate is a pinhole defect. Such a pinhole
defect is generated by such Ar gas as to be brown into a submerged nozzle for
suppressing blocking up of the submerged nozzle in continuous casting, to enter molten
steel in a mold, and to be captured by a solidified shell.
15 [0003] It is effective to dispose an electromagnetic stirrer in a mold as a method for
suppressing pinhole defects. Operation factors of this electromagnetic stirrer include
the flow velocity of molten steel, a submerged nozzle, a molten steel throughput and the
Lorentz force.
[0004] For example, the following arts are disclosed as they make these operation
20 factors within proper ranges.
[0005] For example, Patent Literature 1 discloses the art of making the flow
velocity of electromagnetic stirring at a meniscus 10 to 60 c d s in order to decrease the
generation rate of defects on a surface of a slab to be obtained.
[0006] Patent Literature 2 discloses the art of making surface defects on a slab due
to attachment of air bubbles to a solidified shell, a certain count or less, using
parameters such as a distance between an immersion nozzle and a long side of a mold, a
length in a casting direction of a molten steel discharge opening of the immersion
nozzle, a throughput quantity of molten steel, and the magnetic field density at a
5 solidification interface. Patent Literature 2 describes that the distance between the
immersion nozzle and a long side of the mold is controlled by changing the shape of the
immersion nozzle and the shape of the mold.
[0007] Patent Literature 3 discloses the art of imparting electromagnetic forces so
that the average value of an electromagnetic force in a direction parallel to a major side
10 of a casting mold is 3,000 to 12,000 N/m3, the localized value of an electromagnetic
force in a direction parallel to a minor side of the casting mold is -2,000 to 2,000 N/m3,
and the localized value of an electromagnetic force in a perpendicular downward
direction is -1,000 to 1,000 N/m3 in order to accelerate float of bubbles of Ar gas and
avoid contamination of mold powder into molten steel.
15 [0008] Application of the above described arts disclosed in Patent Literatures 1 to 3
suppresses pinhole defects to some extent. However, pinhole defects do not
completely disappear. Users more and more strictly demand the quality of surfaces of
steel plates, which necessitates an art of further suppressing pinhole defects.
[0009] An electromagnetic stirrer is a device that is the most effective for
20 suppressing pinhole defects in continuously steel casting. In the above described arts
disclosed in Patent Literatures 1 to 3, electromagnetic forces generated by
electromagnetic stirrers and proper ranges of the flow velocities of molten steel
generated by the electromagnetic forces are also examined in detail.
[0010] Here, an electromagnetic stirrer is a device that generates the Lorentz force
in molten steel in a mold, to make the molten steel flow. This Lorentz force is
generated only in molten steel having conductivity, but not generated in what generally
called insulators, which have extremely low conductivity such as air bubbles of Ar gas.
[0011] Thus, air bubbles of Ar gas move relatively in the opposite direction to the
5 movement of molten steel in a mold. That is, an electromagnetic force generated by an
electromagnetic stirrer also includes a negative component that gathers air bubbles of Ar
gas on a superficial slab as shown in FIG. 8, to increase pinhole defects.
[0012] This component of an electromagnetic force, gathering air bubbles of At- gas
which are included in molten metal on a superficial slab is called "electromagnetic
10 repulsion" or "electromagnetic Archimedes force", which is described in Non Patent
Literature 1 in detail. In FIG. 8, 1 represents a wall surface of a mold, 2 represents a
solidified shell, 3 represents a solidification interface and 4 represents an air bubble of
Ar gas; the arrow pointing the top from the bottom on the page represents the Lorentz
force, and the arrow pointing the bottom from the top on the page represents
15 electromagnetic repulsion. Non Patent Literature 2 discloses thermal fluid simulation
in view of Lorentz force density acting on molten steel in continuous casting.
Citation List
Patent Literature
20 [OO 131 Patent Literature 1 : JP H6-605A
Patent Literature 2: JP 2007-2 16288A
Patent Literature 3: JP 2010-240687A
Non Patent Literature
[0014] Non Patent Literature 1 : Tetsu-to-hagang, Vol. 83 (1 997), No. 1, pp. 30-35
Non Patent Literature 2: K. Takatani: ISIJ International, Vol. 43, 2003, No. 6,
pp. 91 5-922
Summary of Invention
5 Technical Problem
COO1 51 A problem to be solved by this invention is that in conventional arts, there is
no concept of determining preferable conditions for electromagnetic stimng, focusing
on electromagnetic repulsion generated by an electromagnetic stirrer, in electromagnetic
stining of molten steel in a mold upon continuously casting steel.
10
Solution to Problem
COO1 61 An object of the present invention is to determine the best current frequency
of an electromagnetic stirrer so as to make electromagnetic repulsion generated upon
electromagnetic stimng of molten steel in a mold as low as possible, to further suppress
15 pinhole defects.
[0017] The present invention was made based on the results of the inventor's study
described below, and its primary feature is: in continuously casting steel using an
electromagnetic stirrer disposed in a mold, in a case where: an average value of Lorentz
force density components in a direction parallel to a long side of the mold within
20 existence of an iron core is Lx (N/m3), the iron core being a component of the
electromagnetic stirrer; and an average value of Lorentz force density components in a
direction parallel to a short side of the mold within existence of the iron core is Ly
(N/m3), a relationship between effective Lorentz force density F (N/m3) that is
calculated by the following formula, and current frequency (Hz) of the electromagnetic
stirrer is obtained, and current frequency of the electromagnetic stirrer within a range of
a maximum value Fmax to 0.9 Fmax of the effective Lorentz force density F is used,
where F = Lx - a.Ly, and in this formula, a is a coefficient indicating bad influence of
electromagnetic repulsion (= 3 to 7).
5 [0018] In the above described present invention, the best current frequency of the
electromagnetic stirrer is determined so as to make electromagnetic repulsion generated
upon electromagnetic stirring of the molten steel in the mold as low as possible. Thus,
it can be suppressed as far as possible to gather air bubbles of Ar gas on a superficial
slab.
10
Advantageous Effects of Invention
[0019] According to the present invention, pinhole defects can be further
suppressed compared with methods for continuously casting steel using conventional
arts because it can be suppressed as far as possible to gather air bubbles of Ar gas on the
15 superficial slab.
Brief Description of Drawings
[0020] FIG. 1 is a view to explain a mold and an electromagnetic stirrer used in the
method for continuously casting steel of the present invention, seen from the top of the
20 mold.
FIG. 2 shows the distribution of Lorentz force density at the center position of
an iron core in a slab drawing direction, obtained by numerical simulation.
FIG. 3 shows the relationship between average values Lx of Lorentz force
density components in the direction parallel to a long side of the mold within the
existence of the iron core of the electromagnetic stirrer, and current frequencies.
FIG. 4 shows the relationship between average values Ly of Lorentz force
density components in the direction parallel to a short side of the mold within the
existence of the iron core of the electromagnetic stirrer, and current frequencies.
5 FIG. 5 shows the relationship between Ly/Lx and current frequencies.
FIG. 6 shows the findings of change in the number of pinholes per unit area
(number/m2) on a solidification interface according to current frequencies, based on
numerical analysis.
FIG. 7 shows frequency dependency of effective Lorentz force density F in a
10 case where a coeficient a that indicates bad influence of electromagnetic repulsion is 5.
FIG. 8 is a view to explain electromagnetic repulsion.
Description of Embodiments
[0021] The present invention realizes the object of determining the best current
15 frequency of an electromagnetic stirrer so as to make electromagnetic repulsion
generated upon electromagnetic stirring of molten steel in a mold as low as possible, to
further suppress pinhole defects.
[0022] Upon operating a continuous casting machine where an electromagnetic
stirrer is disposed in a mold therein, the inventor found as a result of his specific study
20 on electromagnetic repulsion generated in the mold that pinhole defects can be reduced
by suppressing the electromagnetic repulsion.
[0023] Then, as a result of the inventor's further study on a method for applying an
electromagnetic force which suppresses the electromagnetic repulsion so as to keep
bubbles of Ar gas away from the vicinity of a solidification interface, it turned out that
there exists a proper current frequency upon applying the electromagnetic force.
[0024] The mold and the electromagnetic stirrer used in the above studies are same
as those described in Patent Literature 3, which have ordinary shapes and polarities as
shown in FIG. 1 when the mold is seen from the top. In FIG. 1, 11 represents a copper
5 mold (hereinafter may be referred to as a mold), 12 represents a submerged nozzle, 13
represents an electromagnetic stirrer, 13a represents an iron core constituting the
electromagnetic stirrer 13, 13aa represents a teeth part formed on the iron core 13, and
13b represents a winding that is wound around the outer circumference of the iron core
13a.
10 [0025] FIG. 2 shows the distribution of Lorentz force density at the center position
of the iron core in a slab drawing direction, obtained by numerical simulation. Here,
Lorentz force density represents an electromagnetic force per unit volume of molten
steel (PJ/m3).
[0026] The distribution of Lorentz force density shown in FIG. 2 resulted from the
15 numerical simulation under the conditions where the size of a slab was 1200 mm in
width x 250 mm in thickness, a copper plate forming the mold was 25 mm in thickness,
and the conductivity of the mold was 1 . 91~O 7 Slm.
[0027] The distribution of Lorentz force density shown in FIG. 2 is the distribution
of stirring the molten steel in the mold counterclockwise. The large Lorentz force
20 along the direction of a long side of the mold 11 is generated in the vicinity of the wall
surface of the mold 1 1.
[0028] As is clear from FIG. 2, the above described Lorentz force along the wall
surface of the mold also includes a lot of components directed toward the inside of the
mold. Such a kind of the Lorentz force directed toward the inside of the mold
functions as electromagnetic repulsion directed toward the wall surface of the mold for
bubbles of Ar gas. That is, the electromagnetic repulsion transmits bubbles of Ar gas
to the vicinity of the interface of a solidified shell, and pinhole defects are increased.
LO0291 Distribution of the Lorentz force density does not change even if an EMS
5 (Electro-Magnetic Stirrer) current value becomes larger. That is, in a case where the
current value of an electromagnetic stirrer is made to be larger, to speed up the flow
velocity, the effect of suppressing pinhole defects can be obtained by a cleaning effect
on pinholes captured by the interface of a solidified shell; on the other hand,
electromagnetic repulsion makes bubbles of Ar gas moving to the interface of the
10 solidified shell increase and thus, pinhole defects increase.
[0030] As a result of the inventor's study, it was very effective for reducing the
components of the Lorentz force directed toward the inside of the mold to change the
current frequency of the electromagnetic stirrer, as described below.
[0031] FIG. 3 shows the relationship between average values Lx (N/m3) of the
15 Lorentz force density components in the direction parallel to a long side of the mold
within the existence of the iron core of the electromagnetic stirrer, and current
frequencies (Hz). The above described values Lx in the direction parallel to a long
side of the mold were calculated assuming that the Lorentz force in the direction same
as the revolution of the molten steel due to electromagnetic stirring was positive and the
20 Lorentz force in the direction opposite thereto was negative.
LO0321 Specifically, the values Lx were calculated assuming that the Lorentz force
density in the left direction on the page of FIG. 2 was positive and the Lorentz force
density in the right direction thereon was negative in the area of the page upper than the
center of a short side of the mold; and the Lorentz force density in the right direction on
the page was positive and the Lorentz force density in the left direction on the page was
negative in the area of the page lower than the center of a short side of the mold.
[0033] According to FIG. 3, the maximum of the above described value Lx in the
direction parallel to a long side of the mold exists in the range of 2.3 to 2.5 Hz in current
5 frequency; and the current frequency should be selected out of this range of 2.3 to 2.5
Hz for making the stimng flow velocity maximum.
[0034] FIG. 4 shows the relationship between average values Ly (N/m3) of Lorentz
force density components in the direction parallel to a short side of the mold within the
existence of the above described iron core, and current frequencies (Hz). The above
10 described values Ly in the direction parallel to a short side of the mold were calculated
assuming that the Lorentz force density directed toward the inside of the mold was
positive and the Lorentz force density directed toward the outside of the mold was
negative.
[0035] Specifically, the values Ly were calculated assuming that downward Lorentz
15 force density that was leaving away from the wall surface in a long side of the mold was
positive in the area of the page of FIG. 2 upper than the center of a short side of the
mold, in and upward Lorentz force density that was leaving away from the wall surface
in a long side of the mold was positive in the area of the page lower than the center of a
short side of the mold.
20 [0036] That is, the above described value Ly in the direction parallel to a short side
of the mold represents a component of the Lorentz force density which makes the
molten steel in the mold move from the wall surface in a long side of the mold to the
center of a short side, and represents electromagnetic repulsion which makes bubbles of
Ar gas move to the wall surface of the mold. As is clear from FIG. 4, as the current
frequency of the electromagnetic stirrer is high, the above described value Ly in the
direction parallel to a short side of the mold gets large.
[0037] FIG. 5 shows the ratio LyILx of the above described value Ly in the
direction parallel to a short side of the mold to the above described value Lx in the
5 direction parallel to a long side of the mold. As is seen from FIG. 5, as the value of
LyILx is small, the electromagnetic repulsion component in the Lorentz force density
generated in the molten steel in the mold is a little.
[0038] As is seen from FIGS. 4 and 5, it is effective for reducing electromagnetic
repulsion to decrease the current frequency. As is seen from FIG. 3, it is necessary for
10 securing the stirring flow velocity originating from electromagnetic stimng to make the
above described value Lx in the direction parallel to a long side of the mold a certain
value or more. As a result of the examination on fluid simulation, which is described
later, it was confirmed that the Lorentz force was not enough in a case where the current
frequency was 0.4 Hz or less.
15 [0039] According to the above, it was assumed that there should exist the optimal
current frequency between the current frequency where the above described value Lx in
the direction parallel to a long side of the mold was maximum and the current frequency
where electromagnetic stimng was improper. Numerical simulation on
electromagnetic fields and fluid was examined to obtain this optimal current frequency.
20 [0040] Electromagnetic field simulation was carried out by calculating the
distribution of the Lorentz force density generated in the molten steel by the
electromagnetic stirrer according to the method as described above. Fluid simulation
was camed out using the obtained Lorentz force density, to evaluate the number of
bubbles of Ar gas captured by the solidified shell. Thermal fluid simulation was
camed out according to the method described in Non Patent Literature 2, to calculate a
flow of the molten steel, heat transmission, solidification and bubbles of Ar gas.
[0041] The thermal fluid simulation according to the method described in Non
Patent Literature 2 make it possible to obtain information on the flow velocity, the speed
5 of the solidification, the distribution of bubbles of Ar gas and so on in the molten steel
in the continuous casting machine. Thus, the problem was how bubbles of Ar gas
captured by the solidified shell were evaluated.
[0042] As described in Patent Literature 1, it is known that bubbles of Ar gas are
not captured by the solidified shell if the flow velocity of the molten steel on the
10 solidification interface is 10 to 60 c d s . That is, calculation may be camed out
assuming that in a case where the flow velocity of the molten steel on the solidification
interface is the flow velocity where bubbles of Ar gas are captured (hereinafter referred
to as the capture flow velocity) or below, bubbles of Ar gas existing on this location are
captured.
15 [0043] Generally speaking, a threshold of the above capture flow velocity is 20
c d s . However, the accurate value is unknown. In addition, it is considered to be
unnatural that such calculation is carried out assuming that when the flow velocity of
the molten steel is 19.9 c d s , bubbles of Ar gas are not captured by the solidified shell
and when the flow velocity thereof is 20.1 c d s , the bubbles are captured thereby.
20 [0044] Thus, the inventor invented a method of evaluating the probability that
bubbles of Ar gas were captured by the solidified shell as a continuous function as
represented by the following formula (1). Here, P,(-) is the probability that bubbles of
Ar gas are captured by the solidified shell, Co is a fixed number, and U ( d s ) is the flow
velocity of the molten steel on the solidification interface.
[0045] In a case where the fixed number Co in the following formula (1) is 100, the
capture probability P, when the flow velocity of the molten steel is 20 c d s is no more
than This is such probability that one of a million bubbles of Ar gas is captured
by the solidified shell, and this value of the probability is considered to be 0 on
5 numerical simulation. It is noted that any of 10 to 1000 is a proper value for Co used
in numerical simulations.
[0046] [Math. 11
P, = exp(- C, - U) ... Formula (1)
[0047] The speed qg (number/m3-s) where bubbles of Ar gas are captured by the
10 solidified shell is represented as the following formula (2), with the number density n,
(number/m3) of bubbles of Ar gas on the solidification interface, the solidification speed
l& (11s) and the capture probability P,(-).
[0048] [Math. 21
Vg =ng.R3'Pg . . . Formula (2)
15 [0049] The number density of bubbles of Ar gas in the solidified shell S,
(number/m3) is calculated from the following formula (3). Here, Us is the movement
speed (mls) of the solidified shell in the slab drawing direction.
[0050] [Math. 31
... Formula (3)
20 [0051] The number density S, (number/m3) of bubbles of Ar gas in the solidified
shell obtained from the above formula (3) was time-averaged, to evaluate the number of
bubbles of Ar gas. At this time, it was considered that the capture flow velocity
naturally varied according to diameters of bubbles of Ar gas, but the relationship
therebetween is unknown. Then, the examination was carried out under the condition
where each bubble of Ar gas existing mainly in the mold of the continuous casting
machine is 1 mm in diameter. The evaluation was carried out within the range of 2
mm from the superficial slab, as a range where bubbles of Ar gas of 1 mm in diameter
5 influenced on the surface of the slab.
[0052] FIG. 6 shows the results of examining the relationship between the current
frequency and the number of pinholes per unit area (number/m2) on the solidification
interface, based on numerical analysis.
[0053] It becomes clear from FIG. 6 that the number of pinholes in a case where the
10 current frequency is 1.2 Hz is less than a case where the current frequency is 2.3 Hz,
where the Lorentz force density is the maximum; and the number of pinholes largely
increases as the current frequency is 0.8 Hz and below.
[0054] The reason why the number of pinholes per unit area on the solidification
interface is the minimum, 43 (number/m2) in a case where the current frequency is 1.2
15 Hz is that while the Lorentz force density decreases due to electromagnetic stimng, the
decrease of the electromagnetic repulsion produces a large effect of decreasing bubbles
of Ar gas near the wall surface of the mold. However, pinholes increase as the current
frequency decreases to 1.2 Hz and below because the Lorentz force density for stimng
the molten steel in the mold is not enough.
20 [0055] Generally, the current frequency where the Lorentz force density is the
maximum is selected for the current frequency of an electromagnetic stirrer. In the
electromagnetic stirrer shown in FIG. 1, the current frequency where the Lorentz force
density is the maximum is 2.3 Hz, which is read in FIG. 3. The number of pinholes in
a case where the current frequency is 2.3 Hz, which is selected according to prior arts is
57 (number/m2) as shown in FIG. 6. Thus, as is seen from FIG. 6, pinhole defects can
be suppressed more than prior arts at any current frequency within the range of 0.9 Hz
to 2.3 Hz.
[0056] Therefore, the inventor obtained the knowledge under the conditions where
5 the size of the slab was 1200 mm in width x 250 rnm in thickness, the copper mold was
25 mm in thickness, and the conductivity of the copper mold was 1.9 x lo7 S/m, the
proper range of the frequency where the number of pinholes can be suppressed more
than conventional arts was 0.9 to 2.3 Hz.
[0057] It takes a relatively long time to carry out such fluid analysis for evaluating
10 pinholes compared with electromagnetic field analysis. Thus, the inventor studied a
method for selecting the optimal frequency from the result of electromagnetic field
analysis.
[OOS 81 The Lorentz force Lx (N/m3) necessary for electromagnetic stirring
functions as a positive factor for the number of pinholes, and the electromagnetic
15 repulsion Ly (N/m3) functions as a negative factor therefor. Therefore, effective
Lorentz force density F (N/m3) is defined as represented by the following formula (4).
Here, a is a coefficient that indicates bad influence of the electromagnetic repulsion.
[0059] [Math. 41
F = Lx-a-Ly . . . Formula (4)
20 [0060] Since the above described a is a coefficient that indicates bad influence in
the direction parallel to a short side of the mold, this influence varies according to the
length of a short side of the mold. The inventor examined a with which the evaluation
using the above formula (4) was equivalent to that shown in FIG. 6 concerning 200 mm
to 300 mm of a short side of the mold in length as a common continuous casting
machine. As a result, the inventor obtained the knowledge that a in the range of 3 to 7
is proper. In a case where a is less than 3, the Lorentz force parallel to a short side of
the mold is underestimated, and in a case where a is beyond 7, the Lorentz force parallel
to a short side of the mold is overestimated.
5 [0061] FIG. 7 shows frequency dependency of the effective Lorentz force density F
(N/m3) in a case where the coefficient a that indicates bad influence of electromagnetic
repulsion is 5. It is seen from FIG. 7 that the effective Lorentz force density F (N/m3)
takes the maximum value in a case where the current frequency is 1.2 Hz.
[0062] In view of FIGS. 3 to 6, pinhole defects can be suppressed more than
10 conventional arts in a case where the current frequency is within the range of 0.9 Hz to
2.3 Hz. This range corresponds to a range of the maximum value Fmax to 0.9 Fmax of
the effective Lorentz force density F (the current frequency is in the range of 0.9 to 2.0
Hz). As described above, using the above formula (4) makes it possible to determine
the best frequency of the electromagnetic stirrer only with the result of the
15 electromagnetic field analysis.
[0063] The present invention was made based on the above results of inventor's
studies, and is a method for continuously casting steel using an electromagnetic stirrer
disposed in a mold, wherein, in a case where: an average value of Lorentz force density
components in a direction parallel to a long side of the mold within existence of an iron
20 core is Lx (N/m3), the iron core being a component of the electromagnetic stirrer; and an
average value of Lorentz force density components in a direction parallel to a short side
of the mold within existence of the iron core is Ly (N/m3), a relationship between
effective Lorentz force density F (N/m3) that is calculated by the above described
Formula (4}, and current frequency (Hz) of the electromagnetic stirrer is obtained, and
current frequency of the electromagnetic stirrer within a range of a maximum value
Fmax to 0.9 Fmax of the effective Lorentz force density F is used.
[0064] According to the present invention described above, the best current
frequency of the electromagnetic stirrer, where the electromagnetic repulsion generated
5 when electromagnetic stirring is carried out on the molten steel in the mold can be made
to be as little as possible can be determined only from the result of the electromagnetic
analysis. Therefore, it can be suppressed as far as possible to gather bubbles of Ar gas
on the superficial slab, and pinhole defects can be further suppressed.
[0065] The present invention is of course not limited to the above described
10 examples, and needless to say, embodiments thereof can be properly modified as long as
such modification is within the scope of the technical concepts of the claims.
100661 While the inventor carried out fluid simulation with the method described in
Non Patent Literature 2, it is needless to say that thermal fluid simulation can be carried
out not only with the method described in Non Patent Literature 2 but also with another
15 method.
Reference Signs List
[0067] 11 . . . mold
13 . . . electromagnetic stirrer
13a . . . iron core

We claim:
[Claim 11 A method for continuously casting steel using an electromagnetic
stirrer disposed in a mold, wherein,
5 in a case where:
an average value of Lorentz force density components in a direction
parallel to a long side of the mold within existence of an iron core is Lx
(N/m3), the iron core being a component of the electromagnetic stirrer; and
an average value of Lorentz force density components in a direction
parallel to a short side of the mold within existence of the iron core is Ly
a relationship between effective Lorentz force density F (N/m3) that is
calculated by the following formula, and current frequency (Hz) of the electromagnetic
stirrer is obtained, and
15 current frequency of the electromagnetic stirrer within a range of a maximum
value Fmax to 0.9 Fmax of the effective Lorentz force density F is used,
where
F = Lx - a-Ly, and
a is a coefficient indicating bad influence of electromagnetic repulsion