DESCRIPTION
ELECTROMAGNETIC STIRRER AND CONTINUOUS CASTING METHOD
Technical Field
[00011 The present invention relates to an electromagnetic stirrer
capable of uniformly controlling flow of molten steel in one or more
of castingmold(s) in a continuous casting apparatus for billet having
round or angular cross section, and a continuous casting method using
the electromagnetic stirrer.
Background Art
[00021 Castbilletseachhavingaroundorangularcrosssection,going
through steps of tubemaking and rolling, are used as materials of
seamlesspipesandshapesteelshavingdifferentsizesincrosssection.
Sincethe seamlesspipes andshape steelshavevariouskinds ofproduct
sizes and different rolling steps, the cast billets to be their base
materials also have a variety of cross-sectional shapes. Therefore,
a casting in which the number 'of casting mold is determined depending
on production capacity is carried out. -
[00031 Here, amongcastslabsproducedbymeansofacontinuouscasting
or among rolling steel ingots after an ingot casting, a cast slab or
ingot having a regular-square cross section or round cross section
is defined as a billet, and a cast slab or ingot having a rectangle
cross section is defined as a bloom. Also, in the billet, a billet
having a regular-square cross section is defined as a square billet,
andabillethaving a round cross section is definedas a roundbillet.
[00041 A continuous casting will be described with reference to Fig.
1 that is a longitudinal cross-sectional view of a configuration
example of a continuous casting system 100 for billet to which the
presentinventioncanbeapplied, whereinthecontinuous castingsystem
100 is seen from a lateral side. In Fig. 1, 1 is a tundish, 2 is a
molten steel, 3 is,a submerged nozzle, 4'is a casting mold, 5 is an
electromagnetic stirrer, 6 is a casting roll positioned right below
the casting mold, 7 is a zone of roller aprons including a secondary
cooling spray zone, 8 is a solidifying shell, 9 is pinch rolls, k d
10 is a cast slab.
[00051 In the continuous casting, the molten steel 2 poured from a
ladletothetundishlisteemedtothe castingmold4 via the submerged
nozzle 3. While the molten steel 2 teemed to the casting mold 4 is
drawn along a group of casting rolls 6 by the rotational drive of the
pinch rolls 9, surface of the solidifying shell 8 is cooled by the
second cooling spray zone to proceed solidification, whereby the cast
slab 10 is made.
[00061 In the continuous casting, it is extremelyimportantto control
flow of molten steel in a casting mold in view of operation and quality
of cast slab, for instance inview of melt stabilizationofmoldpowder
by supplying heat to meniscus and inclusion removal at a surface of
cast slab. Asamethodforcontrollingflowofmoltensteelinacasting
mold, an electromagnetic stirring applying electromagnetic force to
the molten steel in the casting mold and stirring the molten steel
is widely known. In a case where the electromagnetic stirring is
operated with a plurality of casting molds, it is necessary to apply
the electromagnetic force to each of the plurality of casting molds
such that the casting molds have a uniform flow.
[00071 As methods for applying the electromagnetic force for
electromagnetic stirring, a rotational shifting magnetic field type
and a linear shifting magnetic field type are exemplified..
[00081 The rotational shifting magnetic field type is applied to
continuous castings of billet, bloom and the like, and the rotational
shifting magnetic field type is a method to obtain a uniform flow by
applying a rotating magnetic field to inside of casting mold by means
of a plurality of magnetic poles provided along whole circumference
of the casting mold (for example, Patent Document 1).
[0009] However, ina case where the rotationalshiftingmagnetic field
type is applied to a plurality of casting molds, since an
electromagnetic stirrer is needed for each of the casting molds, the
number of installation of the electromagnetic stirrer is increased
and the plurality of casting molds become unable to share a strand
due to increase in size of the casting molds, which causes increase
in equipment cost.
[00101 On the other hand, as the linear shifting magnetic field type,
theapplicant ofthepresent inventionhas proposed, inpatent Document
2, an electromagnetic coil in which two of tooth 12 are provided to
a core 11 of an iron core of a coil in a projecting manner to a side
of a casting mold 4, an inner winding is applied to each of the two
oftooth12, and inaddition, anouterwinding is appliedtotheoutside
ofthetwooftooth12tounifythetwooftooth12. Theelectromagnetic
coil proposed in Patent Document 2 will be described with reference
to Fig. 2A. This electromagnetic coil shifts a magnetic field in a
linear manner, by applying three-phase alternating currents A, B and
C each having a phase difference of 120° to each other to an inner
winding 13 and an outer winding 14 as shown in Fig. 2A. Hereinafter,
this electromagnetic coil is referred to as a pie-shaped
electromagnetic coil.
[00111 An electromagnetic stirrer including this pie-shaped
electromagnetic coilhasalargemagnetic flwcsince themagnetic field
inaphasewheretheouterwindingisappliedgoesinthesamedirection,
and in a case where an electromagnetic force is applied to a casting
mold having a large cross section, it is possible to obtain a favorable
electromagnetic force along whole circumference of the casting mold
(see Fig. 6A) .
[0012] However, inacasewhereapluralityofcastingmoldseachhaving
a small cross section are installed between the pie-shaped
electromagnetic coils, since the space L between the pie-shaped
electromagnetic coils becomes narrow, the magnetic flux component
going through the casting mold 4 becomes too strong, whereby shifting
magnetic fieldbecomes difficult tobemade, whichresultsinacreation
of a discontinuous region in the electromagnetic force (see the
distortion of the electromagnetic force at the non-uniform flowing
part in Fig. 6B) .
Citation List
Patent Literatures
[0013]
Patent Document 1: Japanese Patent Application Laid-Open Publication
NO. H10-230349
Patent Document 2: Japanese Patent Application Laid-Open Publication
NO. S60-44157
Summary of the Invention
Problems to be Solved by the Invention
[0014] A problem to be solved by the present invention is that, in
a case where electromagnetic stirrers of rotational shifting magnetic
field type are applied to a plurality of casting molds, since an
electromagnetic stirrer is required for each of the casting molds,
the number of installation of the electromagnetic stirrer increases,
andthepluralityof castingmolds cannotshareastrandduetoincrease
in size of the casting molds. Also, another problem to be solved by
the present invention is that, in a case where a plurality of casting
moldseachhavingasmallcross sectionareinstalled, thespacebetween
coils becomes narrow, the magnetic flux component going through the
casting molds becomes too strong, whereby shifting magnetic field
becomes difficult to be made, which results in creation of a
discontinuous region in the electromagnetic force, which can occur
at an electromagnetic stirrer including a pie-shaped electromagnetic
coil.
Means for Solving the Problems
[0015] The present invention has following configurations, for one
or more of casting mold(s), in order to stabilize slab quality by
applying a uniform electromagnetic force to straighten out flow of
the molten steel inside the casting molds using an electromagnetic
stirrer having a pair of pie-shaped electromagnetic coils.
[0016] That is, a first aspect of the present invention is an
electromagnetic stirrer 5, including electromagnetic coils C1 and C2,
wherein a casting mold 4 including a plurality of strands is disposed
betweenthe electromagnetic coilsClandC2 atpredeterminedintervals,
and three-phase alternating currents each having a phase difference
of 120° to each other are applied.
[00171 At this time, as theelectromagnetic coils ClandC2, pie-shaped
electromagnetic coils C1 and C2 are employed, the pie-shaped
electromagnetic coils C1 and C2 having a configuration in which: two
toothparts 12 are providedto a core 11of each of the electromagnetic
coils C1 and C2 in a projecting manner to a side of a casting mold
4 (two convex portions 12 projected to the side of the casting mold
4 are provided to the core 11 of each of the electromagnetic coils
C1 and C2); an inner winding 13 is applied to the outside of each of
the tooth parts 12; and an outer winding 14 is further applied to the
outside of the two tooth parts 12 with the inner winding 13 to unify
the two tooth parts 12.
[0018] For example, as shown in Figs. 2A and 2B, three-phase currents
A, B and C each having a phase difference of 120° to each other are
applied to the pie-shaped electromagnetic coils C1 and C2 having the
configurationdescribedabove. The right and left directionof plane
of paper of Figs. 2A and 2B is a casting direction. The method shown
in Fig. 2A is a method in which the currents A, B and C are applied
in a manner that the magnetic flux of the outer winding faces a same
directionbyapplyingcurrentsina same directiontothe outerwinding
14. The method shown in Fig. 2A is a method in which the currents A,
B and C are applied in the following manner: for the electromagnetic
coil C1 (lower side of plane of paper) that is one of the pair of
electromagnetic coils, the currents A, B and C are applied such that
the direction of the currents becomes, from one end side to the other
end side of the casting direction, -B, +C, -C, +A, -A, +B, in the order
mentioned; and for the electromagnetic coil C2 (upper side of plane
of paper) that is the other of the pair of electromagnetic coils, the
currents A, B and C are applied such that the direction of the currents
becomes, fromone endside totheother endside ofthe casting direction,
-B, +A, -A, +C, -C, +B in the order mentioned (hereinafter, this
configuration is referred to as "window-type wiring system") . Also,
the method shown in Fig. 2B is a method in which the currents A, B
and C are applied in the following manner: for the electromagnetic
coil C1 (lower side of plane of paper) that is one of the pair of
electromagnetic coils C1 and C2, the currents A, B and C are applied
such that the direction of the currents becomes, from one end side
to the other end side of the casting direction, -B, +C, -C, +A, -A,
+B in the order mentioned; and for the electromagnetic coil C2 (upper
sideof planeof paper) that is theotherofthepairof electromagnetic
coils C1 and C2, the currents A, B and C are applied such that the
direction of the currents becomes, from one end side to the other end
side of the casting direction, +B, -A, +A, -C, +C, -B in the order
mentioned, as the directions are symmetrical about a point centering
the center of a horizontal section of the casting mold 4 (hereinafter,
this configuration is referred to as "symmetric wiring system").
COO191 At this time, in order to unify the electromagnetic force
working in a circumferential direction at an arbitrary position in
a radius direction inside the casting mold 4, a distance L between
the electromagnetic coils C1 and C2 disposed facing to each other is
determined as no more than 500 mm when the symmetric wiring system
is applied, and 500 mm or more when the window-type wiring system is
applied.
[00201 In the present invention, the reason for setting the value 500
mm as the bases of division is to secure the distance L between the
electromagnetic coils C1 and C2, when sharing a frame of casting mold
dependingonthediameterof castingmoldtobeusedina single casting
and a twin casting.
COO211 Also, when the number of casting molds per the pair of
electromagnetic coils (the number of the casting molds 4 disposed in
the region between an end surface of one end side and an end surface
of the other end side of the casting direction of the pair of
electromagnetic coils C1 and C2) is defined as n, the external size
of each of the casting molds (in a case of round billet, the outer
diameter of mold copper plate, and in a case of angular billet, outer
width of long side of mold copper plate) is defined as cp (mm), the
width of the electromagnetic coil is defined as W (mm), the number
of the casting molds is determined so as to satisfy the following
Formula (1).
nxcpcW . . . (1)
COO221 Asecondaspectofthepresentinventionisacontinuous casting
method using an electromagnetic stirrer, the method including using
the electromagnetic stirrer 5 according to the first aspect of the
present invention as the electromagnetic stirrer, and setting the
minimum value Vmin of the flowing speed of molten steel to a
circumferential direction of casting mold in the vicinity of the
casting mold after meniscus as 10 cm/s (10 cm per second) or more.
Such a configuration makes it possible to apply the electromagnetic
force equally to each casting mold 4. Here, "the vicinity of the
casting mold" means an area where flow can be applied to the molten
steel by means of the electromagnetic stirrer 5, and as one example,
a region having a distance of 100 mm or less from the wall surface
of the casting mold having contact with the molten steel.
Effects of the Invention
COO231 In the present invention, in a continuous casting apparatus
in which one or more of casting mold(s) is/are used for casting at
the same time, it is possible to apply the electromagnetic force to
eachcastingmold4, bymeansoftheelectromagnetic stirrer5including
the pair of electromagnetic coils C1 and C2. As a result, since there
becomes no need to install an electromagnetic stirrer individually
to each casting mold, it is possible to hold down the equipment cost.
Also, since the symmetric wiring system or the window-type,wiring
system is applied depending on the distance L between the
electromagnetic coils C1 and C2, it is possible to prevent a
discontinuous region from being generated in the electromagnetic
force .
Brief Description of The Drawings
[00241
Fig. 1 is a longitudinal cross-sectional view of a configuration
example of a continuous casting system 100 for billet seen from a
lateral side;
Fig. 2A is a view showing an outline of a pie-shaped electromagnetic
coil and a window-type wiring system;
Fig. 2Bis aview showing an outline of thepie-shapedelectromagnetic
coil and.a symmetric wiring system;
Fig. 3 is a view showing a relationship between the minimum value of
the flowing speed of molten steel in a casting mold and incidence of
surface defect of cast slabs;
Fig. 4A is a view showing an outline of a case where two casting molds
are installed (in a case where n=2);
Fig. 4B is a view showing an outline of a case where three casting
molds are installed (in a case where n=3);
Fig. 5A is a view showing an electromagnetic force in a case where
thewindow-typewiring systemis employed, theview showingananalysis
result in a case where one casting mold whose outer diameter is 360
mm is installed;
Fig. 5B is a view showing an electromagnetic force in a case where
thewindow-typewiring systemis employed, theview showingananalysis
result in a case where two casting molds each having an outer diameter
of 180 mm are installed;
Fig. 6A is a view showing an electromagnetic force in a case where
the symmetric wiring system is employed, the view showing an analysis
result in a case where one casting mold whose outer diameter is 360
mm is installed;
Fig. 6B is a view showing an electromagnetic force in a case where
the symmetric wiring system is employed, the view showing an analysis
result in a case where two casting molds each having an outer diameter
of 180 mm are installed;
Fig. 7 is a view describing a flowing speed V of molten steel to a
circumferentialdirection of castingmoldinthevicinityof a casting
mold 4.
Modes for Carrying out the Invention
COO251 An object of the present invention is, for castingmoldshaving
a various sizes, to apply an electromagnetic force uniformly to inside
of one or more of the casting mold(s) by means of a shared
electromagnetic stirrer. The present invention satisfies the
following conditions.
COO261 The inventors of the present invention carried out
electromagnetic field analyses using a calculation model, regarding
thewiring systems employed.whenthe currentshavingphasedifferences
areappliedtoeachelectromagneticcoiloftheelectromagnetics tirrer
(see Figs. 5A to 6B) . Both "3.500~10~i"n Figs. 5A and 6A, and
"4.700~10~i"n Figs. 5B and 6B are Lorenzian density (~/m.~ )Ar rows
in Figs. 5A, 5B, 6A and 6B each shows a direction of a force which
the molten steels are to be received by the electromagnetic force.
COO271 As a result, the inventors have found out as follows. When
a casting mold having a small cross section is employed in which the
distance L between the electromagnetic coils C1 and C2 is no more than
500 mm, in the window-type wiring system shown in Fig. 2A, a stagnated
part is formed in the electromagnetic force. On the other hand, by
changing the system to the symmetric wiring system and applying the
currents A, B and C each having a phase difference of 120° to each
other to the inner winding 13 and the outer winding 14, an
electromagnetic forceisappliedequallyovertheentirecircumference
of the casting mold 4.
COO281 It should be noted that, when the symmetric wiring system is
applied to a case where a casting mold having a large cross section
is employed in which the distance L between the electromagnetic coils
C1 and C2 is 500 mm or more, although there is no stagnated part of
the electromagnetic force generated, the flowing speed of the molten
steel is reduced since the electromagnetic force is weak comparing
with the window-type wiring system. Therefore, in a case where a
casting mold having a large cross section is employed and the distance
L between the electromagnetic coils C1 and C2 is 500 mm or more, it
is preferred to employ the window-type wiring system shown in Fig.
2A.
COO291 Also, when the number of casting molds per the pair of
electromagnetic coils (the number of casing molds to be disposed in
a region between an end surface of one end side and an end surface
of the other end side of the casting direction of the pair of
electromagnetic coils C1 and C2) is defined as n, the outer size of
each casting mold is defined as cp (mm), and the width of the
electromagnetic coil is defined as W (mm), a reason of defining the
casting molds so as to satisfy the above Formula (1) is, to prevent
a generationof a regionwhere the electromagnetic force is not applied
as a result of installing a plurality of casting molds each having
excessive size between the pair of electromagnetic coils C1 and C2
whereby the casting mold 4 runs off from the tooth part 12 which is
a center of generation of the electromagnetic force. Another reason
is, in a case where a plurality of the casting molds 4 are installed
as well, to apply a unif o m electromagnetic force to all of the casting
molds 4, considering that the electromagnetic force by the
electromagnetic stirrer 5 is applied in a direction perpendicular to
the tooth part 12.
This is theelectromagnetic stirrer5ofthepresentinvention.
COO301 Next, the inventors of the present invention examined, using
the continuous casting system 100 shown in Fig. 1, including the
electromagnetic stirrer 5 of the present invention, the relationship
between the incidence (%) of surface defection of casting slabs and
the minimum value (cm/s) of the flowing speed of molten steel in the
vicinityofthe wall of castingmolds generatedbythe electromagnetic
stirring by means of the stirrer of the present invention.
[00311 Here, regarding the incidence of surface defection of cast
slabs, the examination was carried out targeting at powder defects.
The number of cast slabs in which the powder defect is occurred to
thetotalnumberofcastslabsof10to50 (varydependingonthediameter
of casting mold) of one charge of casting is defined as the incidence
(%) of surface defection of cast slabs for evaluation.
[00321 Regarding the flowing speed of molten steel, samples of
horizontal section were collected from the round billets of Examples
described below, and deflection angles of dendrite generated having
a distance of 10 mrn from the surface skin were measured with respect
to whole circumference of the casting moldwith intervals of 15 degrees
each (24 points in total) , and among the values obtained by converting
the measurement values, the minimum value was defined as Vmin.
[00331 As a result, the inventors have found out that, as shown in
Fig. 3, the incidence of surface defection of cast slabs increases
as the minimum value of the flowing speed of molten steel decreases.
Accordingly, finding that it is desirable to determine the wiring
system and the number of casting molds so as to secure the minimum
value of the flowing speed of molten steel by the electromagnetic
stirring in the vicinity of the casting mold after meniscus of 10 cm/s,
so that the incidence of the surface defection of cast slabs is to
be no more than 1.5 % with which the defection can be handled by trimming.
The expression 'can be handled by trimming" means that, the defective
part onthe surfaceof cast slabs canbe removedbygrindingthe surface
of cast slabs by 1 to 5 mrn by means of a grinder and the like. The
same meaning is applied hereinafter as well. Fig. 7 shows a flowing
speed V of molten steel to a circumferential direction of casting mold
in the vicinity of the casting mold 4.
[00341 In the continuous casting method of the present invention, in
view of further decreasing the incidence of the surface defection of
cast slabs, it is preferredthat theminimumvalue of the flowing speed
of molten steel in the vicinity of the wall of casting mold after
meniscus is 20 cm/s or more.
[00351 Since the stirring by means of the electromagnetic stirrer of
the present invention is an electromagnetic stirring by means of a
stirrerhavingapie-shapedironcore (core), arotatingmagnetic field
is not applied to each casting mold individually, but an
electromagnetic force is generated by the electromagnetic field
shiftingparalleltothe core andthe three-phase alternating currents
A, B and C each having phase difference of 120° to each other.
Consequently, molten steel in the vicinity of the electromagnetic
stirrer 5 (molten steel in the vicinity of the wall of the casting
mold) flows along with the shift of the magnetic field, therefore,
not only in a case where one' casting mold 4 is used as shown in Figs.
2A and 2B, but also in a case where a plurality of casting molds 4
are used as shown in Figs. 4A and 4B, the molten steel in the vicinity
of the electromagnetic stirrer 5 (molten steel in the vicinity of the
wall of the casting mold) flows uniformly. Here, the right and left
direction of the plane of paper of Figs. 4A and 4B is the casting
direction.
Examples
[00361 Hereinafter, Examples carried in order to confirm the effects
of the present invention will be described.
The present invention applies an electromagnetic force to
inside of the casting mold 4 by means of the electromagnetic stirrer
5 to uniformly flow the molten steel, thereby improving the inner
quality of cast slabs. The electromagnetic stirrer 5 is disposed to
a position where a meniscus exists, in a region between an end surface
of one end side and an end surface of the other end side of the casting
direction of the electromagnetic coils C1 and C2 each having a width .
in the casting direction of W.
roo371 As the electromagnetic stirrer 5 of the continuous casting
system100 showninFig. 1, the electromagnetic stirrer with symmetric
wiringsystemshowninFig.2Bwasused. Oneormoreofcastingmold(s)
whose diameter cp on the outer surface (outer diameter cp) is/are 180
mm, casting mold(s) whose outer diameter cp is/are 225 mm, casting
mold(s) whoseouterdiametercpis/are265mrn, andcastingmold(s) whose
outer diameter cp is/are 400 mm were used. Continuous casting was
carried out with the casting speed of 0.5 to 2.0 m/min, the applying
current value to the electromagnetic coils of 300 to 600 A, and the
intensity of magnetic field of 50 to 150 mT (millitesla). The
measurement results of flow of molten steel in the casting molds are
shown in Table 1.
[0038] Two kinds of electromagnetic stirrers having the width W of
550 mm and 400 mm, respectively, were prepared to be used. For the
electromagnetic stirrerwhosewidthwis 550 mm, the distance Lbetween
the electromagnetic coils C1 and C2 was set as two levels of 450 rnrn
and 600 mrn, and for the electromagnetic stirrer whose width W is 400
mm, the distance L between the electromagnetic coils C1 and C2 was
set as only 600 mm, then the testing was carried out.
[00391 Also, in Table 1, regarding Examples 1 to 5 that satisfy the
conditions defined in the present invention and Comparative Examples
6 to 8 that do not satisfy the conditions defined in the present
invention, each condition and the minimum value Vmin of the flowing
speed of molten steel to the casting direction in the vicinity of the
casting mold after meniscus are shown.
[00401 In the following Table 1, when the incidence A of surface
defection is Ac0.5 %, the electromagnetic stirrer was evaluated as
"very good", when 0.5 %