U.S. patent application number 10/105706 was filed with the patent office on 2002-07-25 for method for casting molten metal, apparatus for the same, and cast slab.
This patent application is currently assigned to Nippon Steel Corporation. Invention is credited to Fujisaki, Keisuke, Harada, Hiroshi, Hasegawa, Hajime, Sasai, Katsuhiro, Takeuchi, Eiichi, Toh, Takehiko.
Application Number | 20020096308 10/105706 |
Document ID | / |
Family ID | 26575703 |
Filed Date | 2002-07-25 |
United States Patent
Application |
20020096308 |
Kind Code |
A1 |
Sasai, Katsuhiro ; et
al. |
July 25, 2002 |
Method for casting molten metal, apparatus for the same, and cast
slab
Abstract
The present invention provides a continuous casting method in
which vibration is given to molten metal by a shifting magnetic
field so that the equi-axed crystal ratio can be enhanced and the
equi-axed crystals can be made fine without generating surface
defects caused by powder trapping. Further, the present invention
provides an apparatus to which the continuous casting method is
applied. Furthermore, the present invention provides a cast slab
produced by the above method and apparatus. The method of casting
molten metal comprises the steps of: pouring molten metal into a
mold and solidifying it in the mold while applying an
electromagnetic force, which is generated by an electromagnetic
coil arranged in the proximity of a molten metal pool in the mold,
upon the molten metal; and vibrating the molten metal, which has
been solidified in the mold or is being drawn out downward from the
mold while being cooled and solidified, by a shifting magnetic
field generated by the electromagnetic coil so that the molten
metal is accelerated by a high intensity and a low intensity of
acceleration in a range not exceeding a predetermined flow velocity
when the directional vectors of high acceleration and low
acceleration in the same direction or in the opposite direction are
combined with each other.
Inventors: |
Sasai, Katsuhiro; (Futtsu
City, JP) ; Takeuchi, Eiichi; (Futtsu City, JP)
; Harada, Hiroshi; (Futtsu City, JP) ; Hasegawa,
Hajime; (Futtsu City, JP) ; Toh, Takehiko;
(Futtsu City, JP) ; Fujisaki, Keisuke; (Futtsu
City, JP) |
Correspondence
Address: |
KENYON & KENYON
One Broadway
New York
NY
10004
US
|
Assignee: |
Nippon Steel Corporation
Tokyo
JP
|
Family ID: |
26575703 |
Appl. No.: |
10/105706 |
Filed: |
March 25, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10105706 |
Mar 25, 2002 |
|
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09367183 |
Aug 9, 1999 |
|
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09367183 |
Aug 9, 1999 |
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PCT/JP98/05550 |
Dec 8, 1998 |
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Current U.S.
Class: |
164/468 ;
164/504 |
Current CPC
Class: |
Y10T 428/12472 20150115;
B22D 11/20 20130101; Y10T 428/12458 20150115; B22D 11/115 20130101;
Y10T 428/12 20150115; B22D 11/041 20130101; Y10T 428/12958
20150115; Y10T 428/12576 20150115; Y10T 428/12965 20150115 |
Class at
Publication: |
164/468 ;
164/504 |
International
Class: |
B22D 027/02; B22D
011/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 8, 1997 |
JP |
9-337195 |
Dec 17, 1997 |
JP |
09-348151 |
Claims
1. A method for casting molten metal comprising the steps of:
pouring molten metal into a mold and solidifying it in the mold
while applying an electromagnetic force, which is generated by an
electromagnetic coil arranged in the proximity of a molten metal
pool in the mold, upon the molten metal; and vibrating the molten
metal, which has been solidified in the mold or is being drawn out
downward from the mold while being cooled and solidified, by a
shifting magnetic field generated by the electromagnetic coil so
that the molten metal is alternately given a high intensity and a
low intensity of acceleration.
2. A method for casting molten metal comprising the steps of:
pouring molten metal into a mold and solidifying it in the mold
while applying an electromagnetic force, which is generated by an
electromagnetic coil arranged in the proximity of a molten metal
pool in the mold, upon the molten metal; and vibrating the molten
metal periodically, which has been solidified in the mold or is
being drawn out downward from the mold while being cooled and
solidified, by a shifting magnetic field generated by the
electromagnetic coil so that the molten metal is alternately given
a high intensity and a low intensity of acceleration.
3. A method for casting molten metal comprising the steps of:
pouring molten metal into a mold and solidifying it in the mold
while applying an electromagnetic force, which is generated by an
electromagnetic coil arranged in the proximity of a molten metal
pool in the mold, upon the molten metal; and vibrating the molten
metal, which has been solidified in the mold or is being drawn out
downward from the mold while being cooled and solidified, by a
shifting magnetic field generated by the electromagnetic coil so
that the molten metal is accelerated by a high intensity and a low
intensity of acceleration in a range not exceeding a predetermined
flow velocity when the directional vectors of high acceleration and
low acceleration in the same direction or in the opposite direction
are combined with each other.
4. A method for casting molten metal comprising the steps of:
pouring molten metal into a mold and solidifying it in the mold
while applying an electromagnetic force, which is generated by an
electromagnetic coil arranged in the proximity of a molten metal
pool in the mold, upon the molten metal; and vibrating the molten
metal periodically in the one direction and the opposite direction,
which has been solidified in the mold or is being drawn out
downward from the mold while being cooled and solidified, by a
shifting magnetic field generated by the electromagnetic coil.
5. A method for casting molten metal according to any one of claims
1 to 4, wherein a process conducted in the mold is a cooling and
solidifying process, and also the process conducted in the mold is
a continuous casting process for continuously casting a slab,
bloom, slab of medium thickness, or billet.
6. A method for casting molten metal according to any one of claims
1 to 5, wherein a high intensity of acceleration of the vibrating
waves in the one direction and the opposite direction is not lower
than 10 cm/s.sup.2 and a low intensity of acceleration of the
vibrating waves in the one direction and the opposite direction is
lower than 10 cm/s.sup.2.
7. A method for casting molten metal according to claim 6, wherein
an acceleration and an acceleration time of the vibrating waves in
the one direction, or an acceleration and an acceleration time of
the vibrating waves in the opposite direction, and a coefficient of
acceleration time (acceleration.times.acceleration time) satisfy
the following expression.50 cm/s.ltoreq.coefficient of acceleration
time
8. A method for casting molten metal according to claim 6, wherein
an acceleration and an acceleration time of the vibrating waves in
the one direction, or an acceleration and an acceleration time of
the vibrating waves in the opposite direction, and a coefficient of
acceleration time (acceleration.times.acceleration time) satisfy
the following expressions.10.eta..ltoreq.coefficient of
acceleration time.eta.: viscosity cp of molten metal
9. A method for casting molten metal according to claim 6, wherein
a relation between carbon content C and acceleration satisfies the
following expressions.[C]<0.1%: 30
cm/s.sup.2.ltoreq.acceleration0.1%.- ltoreq.[C]<0.35%: -80[C]+38
cm/s.sup.2.ltoreq.acceleration0.35%.ltoreq.- [C]<0.5%:
133.3[C]-36.7 cm/s.sup.2.ltoreq.acceleration0.5%.ltoreq.[C]: 30
cm/s.sup.2.ltoreq.acceleration
10. A method for casting molten metal according to any one of
claims 1 to 5, wherein an acceleration stop time or an electric
power stop time, the period of which is not more than 0.3 sec and
not less than 0.03 sec, is provided in the process of acceleration
in the one direction and in the process of acceleration in the
opposite direction.
11. A method for casting molten metal according to claim 6, 7, 8 or
9, wherein an acceleration stop time or an electric power stop
time, the period of which is not more than 0.3 sec and not less
than 0.03 sec, is provided in the process of acceleration in the
one direction and also in the process of acceleration in the
opposite direction.
12. A method for casting molten metal according to claim 6, 7, 8 or
9, wherein acceleration is generated for t1, subsequently a
constant flow velocity is kept for t2, next acceleration is
generated in the opposite direction for t3 and thereafter a
constant flow velocity is kept for t4 in one period, and molten
metal in the mold is periodically vibrated by repeating this
period, and a vibration time t1+t2+t3+t4 in one period is
determined to be not less than 0.2 sec and less than 10 sec.
13. A method for casting molten metal according to any one of
claims 1 to 8 or claim 9, wherein the molten metal is periodically
vibrated, and a rotating flow in the one direction and the opposite
direction is given to the molten metal.
14. A method for casting molten metal according to claim 13,
characterized in that: when integration is generated for a certain
period of time, the expression of integrated value of (acceleration
time.times.acceleration) in the one direction>integrated value
of (acceleration time.times.acceleration) in the opposite direction
is satisfied; and an average rotating flow velocity caused by the
difference between the integrated values is not more than 1
m/s.
15. A method for casting molten metal according to claim 13,
wherein acceleration of the molten metal in the mold is generated
for t1, subsequently a constant flow velocity is kept for t2, next
acceleration is generated in the opposite direction for t3 and
thereafter a constant flow velocity is kept for t4 in one period,
molten metal in the mold is periodically vibrated by repeating the
period, t1a is a time until the vibrating flow velocity becomes
zero in time t1, t1b is a time after the vibrating flow velocity
becomes zero in time t1, an expression of t1b+t2>t4+t1a is
satisfied, and a rotating flow velocity in one direction caused by
the difference in time is not more than 1 m/s.
16. A method for casting molten metal according to claim 13,
wherein vibration is periodically given in a period of n cycles, a
rotating flow is generated by giving acceleration only in a
predetermined direction for the rotating time .DELTA.Tv after the
vibration, and an average rotating flow velocity, number n of
cycles and rotating time .DELTA.Tv satisfy the following
expressions.Average rotating flow velocity.ltoreq.1
m/s1.ltoreq.number n of cycles.ltoreq.200.1.ltoreq.rotating time
.DELTA.Tv.ltoreq.5 sec
17. A method for casting molten metal according to claim 13,
wherein a rotating flow is generated by increasing an acceleration
in the one direction to be larger than an acceleration in the
opposite direction, and an average rotating flow rate is not more
than 1 m/s.
18. A method for casting molten metal according to claim 13,
wherein an electric current for rotation generating a rotating flow
in one direction is further superimposed an electric current during
vibration by an electric current of the electromagnetic coil for
generating a shifting magnetic field so that an average rotating
flow velocity can be not more than 1 m/s.
19. A method for casting molten metal according to any one of
claims 1 to 9, wherein the molten metal is periodically vibrated,
and vibration of a short period is further added, and the frequency
of the vibration of this short period is not less than 100 Hz and
not more than 30 KHz.
20. A method for casting molten metal according to any one of
claims 6 to 9, wherein an electromagnetic coil is arranged in the
mold or in the proximity of the molten metal pool in the mold when
molten metal is poured into and solidified in the mold, the molten
metal in the mold is periodically vibrated in the one direction and
the opposite direction by a shifting magnetic field generated by
the electromagnetic coil, and an electromagnetic brake, which is
arranged in a range from the meniscus to a position under the mold
distant by 1 m, is applied.
21. A method for casting molten metal according to claim 11,
wherein an electromagnetic coil is arranged in the proximity to the
molten metal pool in the mold when molten metal is poured into and
solidified in the mold, the molten metal in the mold is
periodically vibrated in the one direction and the opposite
direction by a shifting magnetic field generated by the
electromagnetic coil, and an electromagnetic brake, which is
arranged at a position under the mold distant from the meniscus by
1 m, is applied being synchronized with time at which acceleration
of the electromagnetic coil is stopped in the mold or being
synchronized with time at which an electric power source is
stopped.
22. A method for casting molten metal according to any one of
claims 6 to 15, wherein the electromagnetic coil arranged in
proximity to the molten metal pool in the mold is arranged in a
range under the mold from right below the mold to a position
distant from the mold by 10 m.
23. A method for casting molten metal according to claim 22,
wherein an electromagnetic brake, which is arranged in a range from
a position above the electromagnetic coil distant by 1 m to a
position below the electromagnetic coil distant by 1 m, is
applied.
24. A method for casting molten metal according to claim 11,
wherein the electromagnetic coil arranged in proximity to the
molten metal pool in the mold is arranged in a range from a
position right below the mold to a position under the mold distant
by 10 m, and the electromagnetic brake arranged in a range from the
meniscus to a position under the mold distant by 1 m is applied
being synchronized with the time at which acceleration of the
electromagnetic coil is stopped in the mold or being synchronized
with the time at which the electric power source is stopped.
25. An electromagnetic coil device used for any one of claims 1 to
24, comprising: an electromagnetic drive device for periodically
vibrating in the one direction and the opposite direction; and a
control unit for controlling the electromagnetic drive device.
26. An electromagnetic coil device used for any one of claims 1 to
24 comprising; an electromagnetic coil; and an electric power
source for supplying an electric current to vibrate the
electromagnetic coil periodically in the one direction and the
opposite direction or a waveform generating device.
27. An electromagnetic coil device used for any one of claims 1 to
24, comprising: an electromagnetic drive device for vibrating
molten metal periodically in the one direction and the opposite
direction, the electromagnetic drive device having a function of
raising an electric current to a command value in the case of
changing a vibrating direction; and an electric current control
device for controlling the electric current.
28. An electromagnetic coil device comprising an electromagnetic
drive device, a control device for controlling an electric current,
and an electromagnetic brake used in any one of claims 1 to 24.
29. A cast slab having a negative segregation zone composed of a
multilayer structure, the pitch of which is not more than 2 mm and
the number of the layers of which is not less than three, a
dendrite or a crystalline structure zone composed of a deflection
structure of a multilayer.
30. A cast slab having a negative segregation zone composed of a
multilayer structure, the pitch of which is not more than 2 mm and
the number of the layers of which is not less than three, a
dendrite or a crystalline structure zone composed of a deflection
structure of a multilayer, wherein the thickness of the negative
segregation zone, dendrite or crystalline structure zone is not
more than 30 mm.
31. A cast slab characterized in that: a corner point (C) of a
central negative segregation line (m) of a negative segregation
zone of an average profile of the negative segregation zone of a
multilayer structure is determined, or a virtual corner point (C')
extrapolated from two adjoining sides of a central segregation line
(m) of an arcuate negative segregation zone is determined; and
parallel lines are drawn from points (E) on two adjoining sides,
which are distant from the corner point to the inside of the cast
slab by 5 mm, to the two adjoining sides, and a difference between
shell thickness D.sub.1 at a point of intersection (F) with the
central segregation line (m) and shell thickness D.sub.2 at the
center in the cast slab width direction is not more than 3 mm.
32. A cast slab characterized in that: a corner point of a center
line of dendrite or a crystalline structure zone of deflection
structure of a multilayer, which has an average profile thereof, is
determined, or a virtual corner point extrapolated from two
adjoining sides of a center line of the arcuate dendrite or
crystalline structure zone is determined; and parallel lines are
drawn from points on the two adjoining sides, which are distant
from the corner point to the inside of the cast slab by 5 mm, to
two adjoining sides, and a difference between shell thickness
D.sub.1 at a point of intersection with the central line and shell
thickness D.sub.2 at the center in the cast slab width direction is
not more than 3 mm.
33. A cast slab characterized in that: a shape of the cast slab is
circular; and fluctuation of shell thickness at a point on a
central segregation line (m) of a negative segregation zone of an
average profile of the negative segregation zone of a multilayer
structure is not more than 3 mm.
34. A cast slab characterized in that: a shape of the cast slab is
circular; and fluctuation of shell thickness at a point of a center
line of a dendrite or a crystalline structure of an average profile
of a dendrite structure or a crystalline structure zone of a
deflection structure of a multilayer is not more than 3 mm.
35. A cast slab provided when molten metal is poured into a mold
and solidified while an electromagnetic force is applied to the
molten metal by an electromagnetic coil arranged in the proximity
of the mold according to claim 31 or 33, the cast slab comprising a
negative segregation zone composed of a multilayer structure formed
in the inner circumferential direction of the mold having pitch P
defined by the following expression (2) in a range of D.sub.0.+-.15
mm in the thickness direction with respect to solidified shell
thickness D.sub.0 (mm) at the core center in the casting direction
determined by solidified shell thickness D (mm) defined by the
following expression (1).D=k(L/V).sup.n (1)D: Solidified shell
thickness L: Length from meniscus to core center of electromagnetic
coil V: Rate of casting k: Coefficient of solidification n:
ConstantP=U.times.t/2 (2)U: Rate of solidification (dD/dt (mm/s))
t: Period of vibration
36. A cast slab according to one of claims 31 to 35, the cast slab
having an equi-axed crystal ratio of not less than 50% on the
inside of a negative segregation zone composed of a multilayer
structure, on the inside of a dendrite or a crystalline structure
zone composed of a multilayer-shaped deflection structure.
37. A cast slab provided when molten metal is poured into a mold
and solidified while an electromagnetic force is given to the
molten metal by an electromagnetic coil arranged in the proximity
of the mold according to claim 32 or 34, the cast slab comprising a
dendrite or a crystalline structure zone, the growing direction of
which is regularly deflected, having pitch P defined by the
following expression (2) in a range of D.sub.0.+-.15 mm in the
thickness direction with respect to solidified shell thickness
D.sub.0 (mm) at the core center in the casting direction determined
by solidified shell thickness D (mm) defined by the following
expression (1).D=k(L/V).sup.n (1)D: Solidified shell thickness L:
Length from meniscus to core center of electromagnetic coil V: Rate
of casting k: Coefficient of solidification n:
ConstantP=U.times.t/2 (2)U: Rate of solidification (dD/dt (mm/s))
t: Period of vibration
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method of casting molten
steel when molten steel is vibrated by the action of an
electromagnetic coil. Also the present invention relates to a
continuous casting apparatus for the method of casting molten steel
and a cast slab which has been cast by the method and the
apparatus. More particularly, the present invention relates to a
method of casting molten steel, an apparatus for the method of
casting molten steel and a cast slab which has been cast by the
method and the apparatus, characterized in that: gas and powder
trapping caused in molten metal in the process of solidification of
the molten metal in a mold can be prevented; cracks on a surface of
the cast slab caused when the temperature is not uniform can be
prevented; and further the inner structure of the cast slab can be
made fine.
DESCRIPTION OF THE PRIOR ART
[0002] As a method for making a solidification structure to be
equi-axed crystal so that segregation caused in the process of
solidification can be reduced, in continuous casting of steel,
electromagnetic stirring is widely used. For example, this
technique is disclosed in Japanese Unexamined Patent Publication
(Kokai) No. 50-23338. It is possible to obtain an equi-axed
structure when molten steel in the proximity of a solidification
interface is forcibly given a fluidity by electromagnetic stirring
so that prismatic dendrites can be cut apart. In order to enhance
an equi-axed crystal ratio, various investigations have been made
into the condition of electromagnetic stirring until now and
segregation has been somewhat reduced.
[0003] However, according to the conventional electromagnetic
stirring generated in a mold, an equi-axed crystal ratio by which a
sufficiently high quality of product can be produced is not
necessarily obtained in the case of producing a type of steel (for
example, a type of steel, the carbon content of which is not more
than 0.1%) in which it is difficult to form an equi-axed crystal
structure. In order to enhance the equi-axed crystal ratio of the
above type of steel, in which it is difficult to form an equi-axed
crystal structure, it can be considered to increase the thrust of
electromagnetic stirring generated in a mold. However, when this
method is adopted, a surface velocity of molten steel in the mold
is increased, and powder trapping is caused on the surface of
molten steel. As a result, a defect is caused on the surface of the
product. In some types of steel in which the occurrence of
segregation is severely restricted, it is impossible to meet the
demand of quality only when the equi-axed crystal ratio is
enhanced. In these types of steel, the grain size of the equi-axed
crystal structure must be made further fine.
[0004] Conventionally, the following technique is reported, for
example, the following technique is disclosed in the United States
Patent Publication No. 5722480. Pulse waves, which are generated by
turning on and off an electric current, are given in an alternating
static magnetic field so that an electromagnetic force directed to
the center of a mold side wall is generated. By this
electromagnetic force, a lubricating effect and a soft contacting
effect can be provided. However, according to the above method, the
electric current is not always made to flow, and an acceleration of
the vibrating waves is not controlled. Japanese Unexamined Patent
Publication (Kokai) No. 9-182941 discloses a method in which a
stirring direction of the electromagnetic stirring is periodically
inverted so that a downward flow cannot be developed and diffusion
of inclusion to a lower portion can be prevented. However,
according to this method, vibrating waves are not given onto the
front solidified shell by a shifting magnetic field. Also, it is
not a method in which acceleration is controlled so that the
solidification structure can be made fine, inclusion can be
eliminated for purification and the meniscus can be stabilized.
[0005] Further, according to a method disclosed in Japanese
Unexamined Patent Publication (Kokai) No. 64-71557, an
electromagnetic coil for generating a magnetic field to rotate
molten metal on a horizontal surface is alternated so that it can
exist in a static condition. Therefore, a flow velocity of the
meniscus is zero in this method. According to a method disclosed in
Japanese Examined Patent Publication (Kokoku) No. 3-44858, in order
to prevent V-segregation and porosity of a cast slab, in an
electromagnetic stirring in which a circulation current is caused
on a plane perpendicular to a direction in which a cast slab is
drawn out, a stirring direction is inverted at intervals of 10 to
30 seconds. According to a method disclosed in Japanese Unexamined
Patent Publication (Kokai) No. 54-125132, the casting temperature
is prescribed for preventing ridging of stainless steel and, in
order to prevent positive and negative segregation caused in
electromagnetic stirring, a ratio of two electric currents, the
phases of which are different from each other, is prescribed, and a
direction of electric current is switched and an electric current
is made to flow in a predetermined direction for 5 to 50
seconds.
[0006] Further, according to Japanese Unexamined Patent Publication
(Kokai) No. 60-102263, in order to prevent the occurrence of
defects caused in casting steel of 9%-Ni which is used for a thick
plate at low temperatures, alternating time of electromagnetic
stirring is set at 10 to 30 seconds.
[0007] In the above techniques, alternating stirring is conducted
in a relatively long period. That is, the above techniques are
entirely different from a technique in which vibrating waves are
given onto the front solidified shell by a shifting magnetic field
and acceleration of the vibrating waves is controlled.
[0008] Therefore, it is desired to develop a new technique by which
the above problems are solved, the solidification structure is made
fine, inclusion is purified and further the meniscus is
stabilized.
SUMMARY OF THE INVENTION
[0009] An object of the present invention is to solve the above
problems caused in the conventional electromagnetic stirring
generated in a mold. That is, it is an object of the present
invention to provide a continuous casting method in which vibration
is given by a shifting magnetic field so that the equi-axed crystal
ratio can be enhanced without the occurrence of surface defect
caused by powder trapping and the equi-axed crystal structure
itself can be further made fine. Further, it is an object of the
present invention to provide a continuous casting apparatus to
which the above continuous casting method is applied, and also it
is an object of the present invention to provide a cast slab
produced by the above method and an apparatus.
[0010] It is another object of the present invention to solve
problems caused in the casting method in which an electromagnetic
force is given to molten metal so that solidification of molten
metal can be stabilized and the surface property of a cast slab can
be improved.
[0011] The summary for the present invention to accomplish the
above objects is described as follows.
[0012] (1) A method for casting molten metal comprising the steps
of: pouring molten metal into a mold and solidifying it in the mold
while applying an electromagnetic force, which is generated by an
electromagnetic coil arranged in proximity to a molten metal pool
in the mold, upon the molten metal; and vibrating the molten metal,
which has been solidified in the mold or is being drawn out
downward from the mold while being cooled and solidified, by a
shifting magnetic field generated by the electromagnetic coil so
that the molten metal is alternately given a high intensity and a
low intensity of acceleration.
[0013] (2) A method for casting molten metal comprising the steps
of: pouring molten metal into a mold and solidifying it in the mold
while applying an electromagnetic force, which is generated by an
electromagnetic coil arranged in the proximity of a molten metal
pool in the mold, upon the molten metal; and vibrating the molten
metal periodically, which has been solidified in the mold or is
being drawn out downward from the mold while being cooled and
solidified, by a shifting magnetic field generated by the
electromagnetic coil so that the molten metal is alternately given
a high intensity and a low intensity of acceleration.
[0014] (3) A method for casting molten metal comprising the steps
of: pouring molten metal into a mold and solidifying it in the mold
while applying an electromagnetic force, which is generated by an
electromagnetic coil arranged in the proximity of a molten metal
pool in the mold, upon the molten metal; and vibrating the molten
metal, which has been solidified in the mold or is being drawn out
downward from the mold while being cooled and solidified, by a
shifting magnetic field generated by the electromagnetic coil so
that the molten metal is accelerated by a high intensity and a low
intensity of acceleration in a range not exceeding a predetermined
flow velocity when the directional vectors of high acceleration and
low acceleration in the same direction or in the opposite direction
are combined with each other.
[0015] (4) A method for casting molten metal comprising the steps
of: pouring molten metal into a mold and solidifying it in the mold
while applying an electromagnetic force, which is generated by an
electromagnetic coil arranged in the proximity of a molten metal
pool in the mold, upon the molten metal; and vibrating the molten
metal periodically in the one direction and the opposite direction,
which has been solidified in the mold or is being drawn out
downward from the mold while being cooled and solidified, by a
shifting magnetic field generated by the electromagnetic coil.
[0016] (5) A method for casting molten metal according to any one
of items (1) to (4), wherein a process conducted in the mold is a
cooling and solidifying process, and also the process conducted in
the mold is a continuous casting process for continuously casting a
slab, bloom, slab of medium thickness, or billet.
[0017] (6) A method for casting molten metal according to any one
of items (1) to (5), wherein a high intensity of acceleration of
the vibrating waves in the one direction and the opposite direction
is not lower than 10 cm/s.sup.2 and a low intensity of acceleration
of the vibrating waves in the one direction and the opposite
direction is lower than 10 cm/s.sup.2.
[0018] (7) A method for casting molten metal according to item (6),
wherein an acceleration and an acceleration time of the vibrating
waves in the one direction, or an acceleration and an acceleration
time of the vibrating waves in the opposite direction, and a
coefficient of acceleration time (acceleration.times.acceleration
time) satisfy the following expression.
50 cm/s.ltoreq.coefficient of acceleration time
[0019] (8) A method for casting molten metal according to item (6),
wherein an acceleration and an acceleration time of the vibrating
waves in the one direction, or an acceleration and an acceleration
time of the vibrating waves in the opposite direction, and a
coefficient of acceleration time (acceleration.times.acceleration
time) satisfy the following expressions.
10.eta..ltoreq.coefficient of acceleration time
.eta.: viscosity cp of molten metal
[0020] (9) A method for casting molten metal according to item (6),
wherein a relation between carbon content C and acceleration
satisfies the following expressions.
[C]<0.1%: 30 cm/s.sup.2.ltoreq.acceleration
0.1%.ltoreq.[C]<0.35%: -80[C]+38
cm/s.sup.2.ltoreq.acceleration
0.35%.ltoreq.[C]<0.5%: 133.3[C]-36.7
cm/s.sup.2.ltoreq.acceleration
0.5%.ltoreq.[C]: 30 cm/s.sup.2.ltoreq.acceleration
[0021] (10) A method for casting molten metal according to any one
of items (1) to (5), wherein an acceleration stop time or an
electric power stop time, the period of which is not more than 0.3
sec and not less than 0.03 sec, is provided in the process of
acceleration in the one direction and in the process of
acceleration in the opposite direction.
[0022] (11) A method for casting molten metal according to item
(6), (7), (8) or (9), wherein an acceleration stop time or an
electric power stop time, the period of which is not more than 0.3
sec and not less than 0.03 sec, is provided in the process of
acceleration in the one direction and also in the process of
acceleration in the opposite direction.
[0023] (12) A method for casting molten metal according to item
(6), (7), (8) or (9), wherein acceleration is generated for t1,
subsequently a constant flow velocity is kept for t2, next
acceleration is generated in the opposite direction for t3 and
thereafter a constant flow velocity is kept for t4 in one period,
and molten metal in the mold is periodically vibrated by repeating
this period, and a vibration time t1+t2+t3+t4 in one period is
determined to be not less than 0.2 sec and less than 10 sec.
[0024] (13) A method for casting molten metal according to any one
of items (1) to (8) or item (9), wherein the molten metal is
periodically vibrated, and a rotating flow in the one direction and
the opposite direction is given to the molten metal.
[0025] (14) A method for casting molten metal according to item
(13), characterized in that: when integration is generated for a
certain period of time, the expression of integrated value of
(acceleration time.times.acceleration) in the one
direction>integrated value of (acceleration
time.times.acceleration) in the opposite direction is satisfied;
and an average rotating flow velocity caused by the difference
between the integrated values is not more than 1 m/s.
[0026] (15) A method for casting molten metal according to item
(13), wherein acceleration of the molten metal in the mold is
conducted for t1, subsequently a constant flow velocity is kept for
t2, next acceleration is generated in the opposite direction for t3
and thereafter a constant flow velocity is kept for t4 in one
period, molten metal in the mold is periodically vibrated by
repeating the period, t1a is a time until the vibrating flow
velocity becomes zero in time t1, t1b is a time after the vibrating
flow velocity becomes zero in time t1, an expression of
t1b+t2>t4+t1a is satisfied, and a rotating flow velocity in one
direction caused by the difference in time is not more than 1
m/s.
[0027] (16) A method for casting molten metal according to item
(13), wherein vibration is periodically given in a period of n
cycles, a rotating flow is generated by giving acceleration only in
a predetermined direction for the rotating time .DELTA.Tv after the
vibration, and an average rotating flow velocity, number n of
cycles and rotating time .DELTA.Tv satisfy the following
expressions.
Average rotating flow velocity.ltoreq.1 m/s
1.ltoreq.number n of cycles.ltoreq.20
0.1.ltoreq.rotating time .DELTA.Tv.ltoreq.5 sec
[0028] (17) A method for casting molten metal according to item
(13), wherein a rotating flow is generated by increasing an
acceleration in the one direction to be larger than an acceleration
in the opposite direction, and an average rotating flow rate is not
more than 1 m/s.
[0029] (18) A method for casting molten metal according to item
(13), wherein an electric current for rotation generating a
rotating flow in one direction is further superimposed on an
electric current during vibration by an electric current of the
electromagnetic coil for generating a shifting magnetic field so
that an average rotating flow velocity can be not more than 1
m/s.
[0030] (19) A method for casting molten metal according to any one
of items (1) to (9), wherein the molten metal is periodically
vibrated, and vibration of a short period is further added, and the
frequency of the vibration of this short period is not less than
100 Hz and not more than 30 KHz.
[0031] (20) A method for casting molten metal according to any one
of items (6) to (9), wherein an electromagnetic coil is arranged in
the mold or in the proximity of the molten metal pool in the mold
when molten metal is poured into and solidified in the mold, the
molten metal in the mold is periodically vibrated in the one
direction and the opposite direction by a shifting magnetic field
generated by the electromagnetic coil, and an electromagnetic
brake, which is arranged in a range from the meniscus to a position
under the mold distant by 1 m, is applied.
[0032] (21) A method for casting molten metal according to item
(11), wherein an electromagnetic coil is arranged in proximity to
the molten metal pool in the mold when molten metal is poured into
and solidified in the mold, the molten metal in the mold is
periodically vibrated in the one direction and the opposite
direction by a shifting magnetic field generated by the
electromagnetic coil, and an electromagnetic brake, which is
arranged at a position under the mold distant from the meniscus by
1 m, is applied being synchronized with time at which acceleration
of the electromagnetic coil is stopped in the mold or being
synchronized with time at which an electric power source is
stopped.
[0033] (22) A method for casting molten metal according to any one
of items (6) to (15), wherein the electromagnetic coil arranged in
proximity to the molten metal pool in the mold is arranged in a
range under the mold from right below the mold to a position
distant from the mold by 10 m.
[0034] (23) A method for casting molten metal according to item
(22), wherein an electromagnetic brake, which is arranged in a
range from a position above the electromagnetic coil distant by 1 m
to a position below the electromagnetic coil distant by 1 m, is
applied.
[0035] (24) A method for casting molten metal according to item
(11), wherein the electromagnetic coil arranged in proximity to the
molten metal pool in the mold is arranged in a range from a
position right below the mold to a position under the mold distant
by 10 m, and the electromagnetic brake arranged in a range from the
meniscus to a position under the mold distant by 1 m is applied
being synchronized with the time at which acceleration of the
electromagnetic coil is stopped in the mold or being synchronized
with the time at which the electric power source is stopped.
[0036] (25) An electromagnetic coil device used for any one of
items (1) to (24), comprising: an electromagnetic drive device for
periodically vibrating in the one direction and the opposite
direction; and a control unit for controlling the electromagnetic
drive device.
[0037] (26) An electromagnetic coil device used for one of items
(1) to (24) comprising; an electromagnetic coil; and an electric
power source for supplying an electric current to vibrate the
electromagnetic coil periodically in the one direction and the
opposite direction or a waveform generating device.
[0038] (27) An electromagnetic coil device used for one of items
(1) to (24), comprising: an electromagnetic drive device for
vibrating molten metal periodically in the one direction and the
opposite direction, the electromagnetic drive device having a
function of raising an electric current to a command value in the
case of changing a vibrating direction; and an electric current
control device for controlling the electric current.
[0039] (28) An electromagnetic coil device comprising an
electromagnetic drive device, a control device for controlling an
electric current, and an electromagnetic brake used in any one of
items (1) to (24).
[0040] (29) A cast slab having a negative segregation zone composed
of a multilayer structure, the pitch of which is not more than 2 mm
and the number of the layers of which is not less than three, a
dendrite or a crystalline structure zone composed of a deflection
structure of a multilayer.
[0041] (30) A cast slab having a negative segregation zone composed
of a multilayer structure, the pitch of which is not more than 2 mm
and the number of the layers of which is not less than three, a
dendrite or a crystalline structure zone composed of a deflection
structure of a multilayer, wherein the thickness of the negative
segregation zone, dendrite or crystalline structure zone is not
more than 30 mm.
[0042] (31) A cast slab characterized in that: a corner point (C)
of a central negative segregation line (m) of a negative
segregation zone of an average profile of the negative segregation
zone of a multilayer structure is determined, or a virtual corner
point (C') extrapolated from two adjoining sides of a central
segregation line (m) of an arcuate negative segregation zone is
determined; and parallel lines are drawn from points (E) on two
adjoining sides, which are distant from the corner point to the
inside of the cast slab by 5 mm, to the two adjoining sides, and a
difference between shell thickness D.sub.1 at a point of
intersection (F) with the central segregation line (m) and shell
thickness D.sub.2 at the center in the cast slab width direction is
not more than 3 mm.
[0043] (32) A cast slab characterized in that: a corner point of a
center line of dendrite or a crystalline structure zone of
deflection structure of a multilayer, which has an average profile
thereof, is determined, or a virtual corner point extrapolated from
two adjoining sides of a center line of the arcuate dendrite or
crystalline structure zone is determined; and parallel lines are
drawn from points on the two adjoining sides, which are distant
from the corner point to the inside of the cast slab by 5 mm, to
two adjoining sides, and a difference between shell thickness
D.sub.1 at a point of intersection with the central line and shell
thickness D.sub.2 at the center in the cast slab width direction is
not more than 3 mm.
[0044] (33) A cast slab characterized in that: a shape of the cast
slab is circular; and fluctuation of shell thickness at a point on
a central segregation line (m) of a negative segregation zone of an
average profile of the negative segregation zone of a multilayer
structure is not more than 3 mm.
[0045] (34) A cast slab characterized in that: a shape of the cast
slab is circular; and fluctuation of shell thickness at a point of
a center line of a dendrite or a crystalline structure of an
average profile of a dendrite structure or a crystalline structure
zone of a deflection structure of a multilayer is not more than 3
mm.
[0046] (35) A cast slab provided when molten metal is poured into a
mold and solidified while an electromagnetic force is applied to
the molten metal by an electromagnetic coil arranged in the
proximity of the mold according to item (31) or (33), the cast slab
comprising a negative segregation zone composed of a multilayer
structure formed in the inner circumferential direction of the mold
having pitch P defined by the following expression (2) in a range
of D.sub.0.+-.15 mm in the thickness direction with respect to
solidified shell thickness D.sub.0 (mm) at the core center in the
casting direction determined by solidified shell thickness D (mm)
defined by the following expression (1).
D=k(L/V).sup.n (1)
[0047] D: Solidified shell thickness
[0048] L: Length from meniscus to core center of electromagnetic
coil
[0049] V: Rate of casting
[0050] k: Coefficient of solidification
[0051] n: Constant
P=U.times.t/2 (2)
[0052] U: Rate of solidification (dD/dt (mm/s))
[0053] t: Period of vibration
[0054] (36) A cast slab according to one of items (31) to (35), the
cast slab having an equi-axed crystal ratio of not less than 50% on
the inside of a negative segregation zone composed of a multilayer
structure, on the inside of a dendrite or a crystalline structure
zone composed of a multilayer-shaped deflection structure.
[0055] (37) A cast slab provided when molten metal is poured into a
mold and solidified while an electromagnetic force is given to the
molten metal by an electromagnetic coil arranged in the proximity
of the mold according to item (32) or (34), the cast slab
comprising a dendrite or a crystalline structure zone, the growing
direction of which is regularly deflected, having pitch P defined
by the following expression (2) in a range of D.sub.0.+-.15 mm in
the thickness direction with respect to solidified shell thickness
D.sub.0 (mm) at the core center in the casting direction determined
by solidified shell thickness D (mm) defined by the following
expression (1).
D=k(L/V).sup.n (1)
[0056] D: Solidified shell thickness
[0057] L: Length from meniscus to core center of electromagnetic
coil
[0058] V: Rate of casting
[0059] k: Coefficient of solidification
[0060] n: Constant
P=U.times.t/2 (2)
[0061] U: Rate of solidification (dD/dt (mm/s))
[0062] t: Period of vibration
BRIEF DESCRIPTION OF THE DRAWINGS
[0063] FIG. 1 is a view showing an outline of an arrangement of an
electromagnetic coil in a mold according to the present
invention.
[0064] FIG. 2(a) is a diagram for explaining a pattern of an
electric current of an electromagnetic coil of the present
invention.
[0065] FIG. 2(b) is a diagram for explaining a pattern of a flow
velocity of vibration on the front face of solidification.
[0066] FIG. 3 is a graph showing a relation between a period of an
electromagnetic coil current and an equi-axed crystal ratio.
[0067] FIG. 4 is a graph showing a relation between a period of an
electromagnetic coil current and an equivalent diameter of an
equi-axed crystal circle.
[0068] FIG. 5 is a diagram showing an example in which an
acceleration stop time is provided, the period of which is not more
than 0.3 sec and not less than 0.03 sec during one direction and
the opposite direction.
[0069] FIG. 6 is a diagram showing an example in which an
acceleration in the one direction is 100 cm/s.sup.2 and an
acceleration in the opposite direction is 50 cm/s.sup.2.
[0070] FIG. 7 is a schematic illustration showing an outline of
thickness of a solidified shell at a core center in the casting
direction of an electromagnetic coil.
[0071] FIG. 8(a) is a view showing a typical example of a clear
corner of a negative segregation zone of a cast slab of the present
invention.
[0072] FIG. 8(b) is a view showing a virtual corner in the case of
an unclear negative segregation zone.
[0073] FIG. 9 is a metallograph showing a clear corner of the
negative segregation zone of FIG. 8.
BEST MODE FOR CARRYING OUT THE INVENTION
[0074] FIG. 1 is a view showing the very moment of rotation of
molten metal in a mold when an electromagnetic field is applied
upon the molten metal by an electromagnetic coil of the present
invention. In this connection, reference numeral 1 is an
electromagnetic coil, reference numeral 2 is a side wall on the
long side, reference numeral 3 is a side wall on the short side,
and reference numeral 4 is an immersion nozzle.
[0075] The first characteristic of the present invention is not
only to rotate molten metal in the mold by generating a shifting
magnetic field by the electromagnetic coil of the mold, but the
first characteristic of the present invention is also to give an
acceleration in the one direction and the opposite direction to
molten metal by a shifting magnetic field so that the molten metal
can vibrate on the front solidified shell. Further, an acceleration
of this vibrating waves is controlled. The above technique is
applied to not only a continuous casting process but also an ingot
process in which a stationary mold is used. In this embodiment, a
linear motor is used as the electromagnetic coil. However, the
present invention is not limited to the specific embodiment. As
long as a shifting magnetic field can be generated, any magnetic
filed generating device may be used, that is, a magnetic field
generating device by which a linear magnetic field is generated is
not necessarily used. For example, a magnetic field generating
device by which a rotary magnetic field is generated may be used,
and any magnetic field generating device by which vibration can be
given to molten metal in the one direction and the opposite
direction may be used.
[0076] The second characteristic of the present invention is that a
load of the linear motor is increased in the one direction and the
opposite rotation and an electric current is continuously supplied,
so that a quick rise of the electric current can be accomplished.
Due to the foregoing, an electromagnetic force can quickly reach a
predetermined value. As a result, it becomes possible to control an
acceleration given to molten metal in a wide range.
[0077] According to the above characteristics of the present
invention, it is possible to remarkably enhance the inner quality
and surface quality of a cast slab as follows. Instead of rotation
of molten metal generated by a conventional electromagnetic
stirring, vibrating waves generated by a shifting magnetic field is
given onto a front solidified shell while an acceleration is being
controlled in the present invention. Due to the foregoing, a
prismatic cutting force is increased, so that the solidified
structure can be made further finer, and at the same time, the
inner quality of slab can be much purified. Further, a change in
the meniscus can be suppressed to as small as possible, that is, an
influence given to the meniscus shape disturbance can be suppressed
to as small as possible. In this way, the inner and surface quality
of a cast slab can be remarkably improved.
[0078] In general, a flow velocity of the conventional
electromagnetic stirring conducted in continuous casting is 20 to
100 cm/s. The present inventors made investigation into a mechanism
of generation of equi-axed crystals generated by the
electromagnetic stirring in the above flow velocity range. As a
result of the investigation, the following were made clear.
Electromagnetic stirring has an effect of inclining a flow of
prismatic dendrite onto an upstream side, however, an effect of
cutting a prismatic dendrite apart, which has been conventionally
considered to be high until now, is not so high. Instead of the
effect of cutting the prismatic dendrite apart, heat transmission
between a solidified shell and molten steel is facilitated by the
electromagnetic stirring. Therefore, overheating of molten steel is
reduced, so that solidification cores can be easily formed. On the
basis of the above knowledge, the present inventors made further
investigation into a method by which an effect of cutting the
prismatic dendrite apart can be more remarkably enhanced as
compared with the conventional method without impairing an effect
of reducing overheat of molten steel when electromagnetic stirring
is carried out. As a result of the investigation, the present
inventors found the following. It is very effective that an
electric current of the electromagnetic coil is periodically
changed as shown in FIG. 2(a), so that vibrating waves, which
reciprocate on the front face of solidification, are given to
molten steel. Due to the foregoing, not only the equi-axed crystal
ratio can be enhanced, but also the grain size of equi-axed
crystals can be made fine.
[0079] When an electric current of the electromagnetic coil is
changed according to the pattern shown in FIG. 2(a), a flow
velocity of vibration on the front solidified shell follows the
change in the electric current as shown in FIG. 2(b), wherein the
curve shown in FIG. 2(b) becomes a little blunt compared with the
curve shown in FIG. 2(a). In a region of t2 or t4 in which the flow
velocity of vibration on the front solidified shell is constant,
the vibration flow provides a small effect of cutting a prismatic
dendrite apart. However, in an one direction accelerating region t1
and in an opposite direction accelerating region t3, an
acceleration is generated in a vibrating flow on the front
solidified shell. Therefore, compared with a rotational flow of a
constant flow velocity, it is possible to give a very strong force
to a prismatic dendrite. By the above effect, it is possible to
remarkably enhance an effect of cutting the prismatic dendrite
apart. Further, when the vibrating flow velocity on the front
solidified shell is made to be the same as that of the conventional
method in the region of t2, it is possible to provide an effect of
reducing overheat of molten steel by facilitating a heat
transmission between the solidification shell and molten steel.
Since a sufficiently strong force to cut a prismatic dendrite apart
is given onto the front solidified shell in the accelerating
regions t1 and t3, the present invention has an effect of cleaning
by which inclusion is prevented from being caught by the front
solidified shell.
[0080] According to the conventional method, a large quantity of
inclusion is caught by the surface layer of a cast slab, the
solidification rate of which is high, and the degree of
purification is deteriorated. However, according to the present
invention, an average oxygen concentration in a region of 20 mm
from the surface of a cast slab which was cast according to the
present invention can be made lower than that of the inner portion
of the slab. The rotating flow generated by the conventional
electromagnetic stirring causes the following problems. The
meniscus goes out of order. When the rotating flow velocity is
increased in order to enhance an equi-axed crystal ratio, powder
trapping is caused, and further the rotating flow collides with a
side wall on the short side of the mold, so that a strong
descending flow is continuously generated. However, when the
vibrating waves, which reciprocate on the front solidified shell,
are given to molten steel, it is possible to prevent the occurrence
of disturbance of the meniscus and powder trapping, and further it
is possible to suppress an influence of the descending flow.
Accordingly, casting can be stably conducted.
[0081] In addition to that, when the rotating flow is superimposed
on the vibrating waves, the purification of inclusion and the
generation of cores can be further facilitated while a shape of the
meniscus is stabilized. According to the conventional
electromagnetic stirring, a negative segregation zone of solute
elements is generated. Therefore, it is impossible to ensure the
quality of a cast slab. However, according to the present
invention, the vibrating waves reciprocate on the front solidified
shell. Therefore, very thin negative segregation zones of a
multilayer structure are generated. Accordingly, the negative
segregation zones are dispersed, and the solidified structure can
be made fine, and at the same time the negative segregation can be
prevented.
[0082] As shown in FIGS. 8(a), 8(b) and 9, thin negative
segregation zones of a multilayer structure are uniformly generated
along the outer circumference of a cast slab at the same distance
from the cast slab surface corresponding to the period of stirring.
Accordingly, cracks can be prevented from proceeding on the cast
slab surface, and further the oxidation of a grain boundary can be
suppressed. In addition to that, a growing direction of prismatic
crystals (dendrite) in a positive segregation zone located between
the negative segregation zones is alternately changed for each
positive segregation zone. Accordingly, compared with a cast slab
in which prismatic crystals (dendrite) grow in one direction, the
solidification structure is strong with respect to the occurrence
of cracks. For the above reasons, it is possible to produce a cast
slab, the surface layer of which has a highly enhanced function, by
the casting method of the present invention.
[0083] Next, a coefficient of acceleration time will be explained
below. When consideration is given to a material point in a liquid
state, concerning a material point movement, it can be said as
follows by the law of dynamics. "Concerning a momentum of a
material point in a predetermined period of time, its change is
equal to an impulse of an acting force and a period of time in
which the force acts." Therefore, it is possible to apply the law
to a change in the acting force in a vibrating condition. That is,
(acceleration.times.acceleration time), which is a coefficient of
acceleration time defined by the present invention, can be used as
a parameter of vibration, that is, (acceleration.times.acceleration
time) can express a change in the impulse or acting force which
represents a state of vibration. Due to the foregoing, it is
possible to control a state of vibration by adjusting a holding
time (t2, t4) in the melting condition and an acceleration giving
time (t1, t3) while the coefficient of acceleration time is used as
a parameter.
[0084] In order to provide an effect stably, vibration of the
present invention, which reciprocates on the front solidified
shell, has an appropriate period. An upper and a lower limit of the
appropriate period are determined as follows.
[0085] In order to give an acceleration uniformly in the
circumferential direction of a cast slab, it is necessary to invert
the accelerating direction in a period of time in which a boundary
layer on the front solidified shell is not peeled off. This period
of time is shorter than 5 seconds and was found by an experiment,
and a vibrating time of one period, which will be referred to as a
vibrating period hereinafter, is shorter than 10 seconds.
[0086] On the other hand, in order to exhibit the effect of
vibration in the casting direction of a cast slab, it is necessary
to give at least one period of vibration while the cast slab is
passing through the core portion of the electromagnetic coil. At
this time, a period of vibration is not more than a value of (core
length)/(casting speed). Therefore, the upper limit of the
vibration period is determined by a condition in which casting
operation can be stabilized in both the circumferential direction
of the cast slab and the casting direction. The shorter of the
periods becomes the upper limit of the vibration period.
[0087] The present inventors found the following. Molten steel on
the front solidified shell is accelerated in vibration when the
condition of (period of vibration).gtoreq.2/(frequency of
electromagnetic coil) is satisfied. A frequency of the
electromagnetic coil for generating a shifting magnetic field is 10
Hz at most. Therefore, a lower limit of the period of vibration is
not less than 0.2 sec.
[0088] In the present invention, a flow velocity is obtained when a
displacement of a reference point is differentiated by time, and an
acceleration is obtained when the flow velocity is differentiated
by time. The acceleration may be obtained when a flow velocity at
the point of time when the flow velocity of vibration is zero is
differentiated by time. Alternatively, the acceleration may be a
value of (maximum vibration flow velocity-minimum vibration flow
velocity)/t1 or (maximum vibration flow velocity-minimum vibration
flow velocity)/t3. The reference point is located at the center of
the long side of the mold or at a point distant from the front
solidified shell by 20 mm in front at the 1/4 width. Acceleration
time of the coefficient of acceleration time is t1 or t3 up to the
acceleration range t1, in which ti is restricted by t3. An average
rotation flow velocity is an average flow velocity obtained when
the acceleration is multiplied by the time and integrated with
respect to the total time and the thus obtained value is averaged
with respect to the time. In FIG. 2, the accelerating region (t1,
t3) is a high acceleration time, and the accelerating region (t2,
t4), the absolute value of the acceleration of which is low, is a
low accelerating region.
[0089] Next, the cast slab of the present invention will be
explained below. The first characteristic of the cast slab is that
the cast slab has a negative segregation zone composed of a
multilayer structure, the pitch of which is not more than 2 mm and
the number of the layers of which is not less than three and that
the thickness of the negative segregation zone is not more than 30
mm. Concerning the negative segregation zone, there are two cases.
One case is shown in FIGS. 8(a) and 9 in which a corner of the
negative segregation zone is clear with respect to a corner of the
cast slab, and the other case is shown in FIG. 8(b) in which a
corner of the negative segregation zone is not clear with respect
to a corner of the cast slab. First, in the case shown in FIG.
8(a), a corner point (C) of a central negative segregation line (m)
is determined in an average profile of a negative segregation zone
of a multilayer structure. Parallel lines which are parallel to the
adjoining two sides are drawn from points (E) on the adjoining two
sides distant from the corner point to the inside of the cast slab
by 5 mm. A difference between the shell thickness D.sub.1 at the
point of intersection (F) with respect to the negative segregation
line (m) and the shell thickness D.sub.2 at the center in the width
direction of the cast slab is prescribed to be not more than 3
mm.
[0090] In the case shown in FIG. 8(b), a virtual corner point (C')
is determined which is extrapolated from the adjoining two sides of
a central negative segregation line (m) of an arcuate negative
segregation zone. Parallel lines which are parallel to the
adjoining two sides are drawn from points (E) on the adjoining two
sides distant from the corner point to the inside of the cast slab
by 5 mm. A difference between the shell thickness D.sub.1 at the
point of intersection (F) with respect to the central negative
segregation line (m) and the shell thickness D.sub.2 at the center
in the width direction of the cast slab is prescribed to be not
more than 3 mm.
[0091] In the same manner, a corner point of a center line of a
dendrite or a crystalline structure zone of an average profile of
the dendrite or the crystalline structure zone of a deflection
structure is determined, or a virtual corner point extrapolated
from the adjoining two sides of the center line of the arcuate
dendrite or the crystalline structure zone is determined, and a
prescription is made in the same manner.
[0092] On the other hand, with respect to a circular cast slab,
fluctuation of the shell thickness at a point on a central
segregation line (m) of a negative segregation zone of a multilayer
structure, or fluctuation of the shell thickness at a point on a
central segregation line (m) of an average profile of a dendrite of
a segregation structure or a crystalline structure zone is
prescribed to be not more than 3 mm.
[0093] More specifically, a negative segregation zone of a
multilayer structure, a dendrite of a deflection structure or a
crystalline structure zone is prescribed. That is, concerning the
negative segregation zone, a dendrite of a deflection structure or
a crystalline structure, on the basis of a positional relation
shown in FIG. 7, the cast slab comprises a negative segregation
zone, a dendrite of a deflection structure or a crystalline
structure zone composed of a multilayer structure formed in the
inner circumferential direction of the mold having pitch P defined
by the following expression (2) in a range of D.sub.0.+-.15 mm in
the thickness direction with respect to solidified shell thickness
D.sub.0 (mm) at the core center in the casting direction determined
by solidified shell thickness D.sub.0 (mm) defined by the following
expression (1).
D=k(L/V).sup.n (1)
[0094] D: Solidified shell thickness
[0095] L: Length from meniscus to core center of electromagnetic
coil
[0096] V: Rate of casting
[0097] k: Coefficient of solidification
[0098] n: Constant (0.5 to 1.0)
P=U.times.t/2 (2)
[0099] U: Rate of solidification (dD/dt (mm/s))
[0100] t: Period of vibration
[0101] In this connection, in the present invention, the installing
position is not limited to a position inside the mold. As long as
it is a position in the continuous casting machine and molten steel
exists at the point, the present invention can be applied to any
position in principle.
[0102] In the present invention, molten metal is not limited to a
specific metal. However, the present invention will be explained
here referring to the appended drawings in which the present
invention is applied to steel.
EXAMPLES
Example 1
[0103] In this example, in order to make clear the influence, of a
vibration pattern which is generated by an electromagnetic coil, on
the equi-axed crystal ratio and the grain size of equi-axed
crystals, an experiment was made in which molten steel was poured
into a mold having an electromagnetic coil, the frequency of which
was 10 Hz. In this experiment, molten steel of 50 kg, the carbon
content of which was 0.35%, was melted in a high frequency melting
furnace and poured into a mold made of copper, wherein the width of
the mold was 200 mm, the length was 100 mm and the height was 300
mm. Immediately after the molten steel had been poured into the
mold, the molten steel was solidified while it was being vibrated
in the mold by a predetermined vibrating pattern. After the
completion of casting, the ingot was cut on a lateral section, so
that the solidified structure was revealed outside. Then, an area
ratio of an equi-axed crystal region (an equi-axed crystal area
ratio) and a diameter of an equivalent circle of the equi-axed
crystal region were evaluated. The vibrating pattern was changed as
follows. In FIG. 2, an electric current of the electromagnetic coil
was set at 100 ampere at maximum and -100 ampere at minimum. Coil
current increasing time t1 in which an one direction acceleration
is given, coil current decreasing time t3 in which an opposite
direction acceleration is given, and minimum coil current holding
time t4 were set at predetermined values. In this way, the
vibrating pattern was changed.
[0104] FIG. 3 is a view showing a relation between the period of a
change in the coil current (t1+t2+t3+t4) and the equi-axed crystal
area ratio. When the vibrating period is reduced, the equi-axed
crystal area ratio is increased. However, when the vibrating period
becomes shorter than 0.2 second, the equi-axed crystal area ratio
is suddenly decreased. The reason why is that the vibrating flow
velocity on the front solidified shell cannot follow the coil
current when the period of the coil current is decreased. FIG. 4 is
a view showing a relation between the period of the electromagnetic
coil current and the diameter of the equivalent circle of an
equi-axed crystal region. When an absolute value of acceleration on
the front solidified shell (because a value of acceleration becomes
-10 cm/s.sup.2 in the reverse side accelerating region) is lower
than 10 cm/s.sup.2, the diameter of an equivalent circle of an
equi-axed crystal region does not depend upon the vibrating period.
Therefore, it is impossible to obtain an effect of making the
equi-axed crystals fine. However, when an absolute value of
acceleration on the front solidified shell becomes a value not less
than 10 cm/s.sup.2, it can be understood that the equi-axed
crystals are made fine at a vibrating period of shorter than 10
seconds. The reason why an effect of making the crystals fine can
not be obtained except for the above operating conditions is
described as follows. When a value of acceleration of the vibrating
flow velocity on the front solidified shell is lower than 10
cm/s.sup.2, a force acting on the prismatic dendrite is weak, so
that it is impossible to obtain an effect of making the crystals
fine. When the vibrating period becomes a value not longer than 10
seconds, a boundary layer is peeled off on the front solidified
shell, so that it becomes difficult for a cutting force generated
by acceleration to act on the prismatic dendrite. From the above
viewpoint, it can be understood that the vibrating condition for
making the equi-axed crystals fine is more severe than the
condition for enhancing the equi-axed crystal ratio.
[0105] As a result, the following can be understood. In order to
enhance the equi-axed crystal ratio and make the grain size of the
equi-axed crystals fine, the period of the electromagnetic coil
current is set at a value not shorter than 0.2 sec and shorter than
10 sec, and at the same time, the absolute value of acceleration on
the front face of solidification is set at a value not less than 10
cm/s.sup.2.
[0106] In this connection, concerning the acceleration in the
present invention, the effect depends upon the carbon content of
molten steel. In the present invention, the acceleration is
restricted as follows. When C.ltoreq.0.1%, the acceleration is 30
to 300 cm/s.sup.2. When 0.1%.ltoreq.C.ltoreq.0.35%, the
acceleration is {80[C]+38} to 300 cm/s.sup.2. When
0.35%.ltoreq.C.ltoreq.0.5%, the acceleration is {133.3[C]-36.7} to
300 cm/s.sup.2. When 0.5%.ltoreq.C, the acceleration is 30 to 300
cm/s.sup.2. The reason why the upper limit is given here is that no
confirmation was made in the experiment exceeding the above
condition.
[0107] The above knowledge was obtained by the experiment made by
the present inventors when attention was paid to a relation between
the equi-axed crystal ratio and the carbon content.
Example 2
[0108] In this example, a two-strand type continuous casting
machine for continuously casting billets was used, and cast billets
of 120 mm square made of carbon steel, the carbon content of which
was 0.35%, were cast for 30 minutes at the casting speed of 1.2
m/min. Temperature in a tundish was 1530.degree. C. In one of the
strands, conventional electromagnetic stirring was generated, in
which the coil current of the electromagnetic stirring device was
set at a constant value of 200 ampere and the frequency was set at
10 Hz, for 30 minutes at the flow velocity of 60 cm/s. In the other
strand, an electromagnetic coil of the present invention capable of
giving vibration was arranged in the mold, and molten steel on the
front solidified shell was vibrated under the following conditions.
Vibration time of one period of the coil current was 2 s (the
maximum coil current was 200 ampere, the minimum coil current was
-200 ampere, the coil current increasing time was 0.8 s, the coil
current decreasing time was 0.8 s, the maximum coil current holding
time was 0.2 s, and the minimum coil current holding time was 0.2
s), and acceleration in the one direction and the opposite
direction was given under the condition of 50 cm/s.sup.2 as shown
in FIG. 2. After a lateral section of the cast billet had been cut
and the solidified structure had been revealed, the equi-axed
crystal area ratio and the diameter of the equivalent circle of an
equi-axed crystal region were evaluated. Concerning the surface
quality of the cast billets, the cast slabs were subjected to a
visual inspection line, so that each billet was visually inspected,
and the number of defects caused by powder was investigated.
[0109] Concerning the billets on which the conventional
electromagnetic stirring was conducted, the equi-axed crystal ratio
was 30%, and the diameter of the equivalent circle of an equi-axed
crystal region was 3.0 mm. The flow velocity of molten steel was 60
cm/s, which exceeded a critical flow velocity of powder trapping.
Therefore, powder on the surface of molten steel was trapped, and
the defects were caused by powder, the number of which was 5
pieces/billet. Further, there was formed a negative segregation
zone, the width of which was approximately 20 mm, on the surface
layer side of the lateral section of the cast billet. On the other
hand, when vibration was given by the electromagnetic coil of the
present invention, the equi-axed crystal area ratio of the cast
billet was 50%, and the diameter of the equivalent circle of an
equi-axed crystal region was 1.3 mm. Therefore, compared with the
conventional electromagnetic stirring, not only the equi-axed
crystal area ratio was enhanced, but also the grain size of the
equi-axed crystals was made fine. Since the molten steel on the
front face of solidification in the mold was vibrated, powder
trapping was not caused, and defects originated from powder were
not caused. On the lateral face of the cast billet, a negative
segregation zone of a multilayer, the pitch of which was 1.5 mm,
was formed on the surface layer of 15 mm, and also a dendrite of
deflection structure of a multilayer was formed.
Example 3
[0110] In this example, a two-strand type continuous casting
machine for continuously casting slabs was used, and cast pieces of
250 mm thickness.times.1500 mm width made of carbon steel, the
carbon content of which was 0.35%, were cast for 30 minutes at the
casting speed of 1.8 m/min. Temperature in a tundish was
1550.degree. C. In one of the strands, a conventional
electromagnetic stirring was generated, in which the coil current
of the electromagnetic stirring device was set at a constant value
of 500 ampere and the frequency was set at 2 Hz, for 30 minutes at
the flow velocity of 60 cm/s. In the other strand, an
electromagnetic coil of the present invention capable of giving
stirring was arranged in the mold. For 15 minutes in the first half
of casting, molten steel on the front face of solidification was
vibrated under the following conditions. Vibrating time of one
period of the coil current was 2 s (the maximum coil current was
400 ampere, the minimum coil current was -400 ampere, the coil
current increasing time was 0.8 s, the coil current decreasing time
was 0.8 s, the maximum coil current holding time was 0.2 s, and the
minimum coil current holding time was 0.2 s), and acceleration in
the one direction and the opposite direction was given under the
condition of 70 cm/s.sup.2 as shown in FIG. 2. For 15 minutes in
the second half of casting, the molten steel on the front
solidified shell was vibrated under the following conditions.
Vibrating time of one period of the coil current was 2.1 s (the
maximum coil current was 400 ampere, the minimum coil current was
-400 ampere, the coil current increasing time was 0.8 s, the coil
current decreasing time was 0.8 s, the maximum coil current holding
time was 0.2 s, and the minimum coil current holding time was 0.2
s), the acceleration stop time was 0.05 s in the acceleration in
the one direction and opposite direction, and acceleration in the
one direction and the opposite direction was given under the
condition of 50 cm/s.sup.2 as shown in FIG. 5. After a lateral
section of the cast slab had been cut and the solidified structure
had been exposed, the equi-axed crystal area ratio and the diameter
of the equivalent circle of an equi-axed crystal region were
evaluated. Concerning the surface quality of the cast slabs, the
cast slabs were subjected to a visual inspection line, so that each
slab was visually inspected, and the number of defects caused by
powder was investigated. Since vibration marks on the slab surface
correspond to a shape of the meniscus, a difference in the levels
of the vibration marks was investigated at the same time.
[0111] Concerning the slabs on which the conventional
electromagnetic vibration was generated, the equi-axed crystal
ratio was 30%, and the diameter of the equivalent circle of an
equi-axed crystal region was 3.0 mm. The flow velocity of molten
steel was 60 cm/s, which exceeded a critical flow velocity of
powder trapping. Therefore, powder on the surface of molten steel
was trapped, and the defects were caused by powder, the number of
which was 5 pieces/slab. Further, since the meniscus fell into
disorder, the difference in the levels of the vibration marks was
3.5 mm. Furthermore, there was formed a negative segregation zone,
the width of which was 20 mm, on the surface layer side of the
lateral section of the slab.
[0112] On the other hand, when vibration was given by the
electromagnetic coil of the present invention, irrespective of the
existence of the acceleration stop time, the equi-axed crystal area
ratio of the slab was 50%, and the diameter of the circle
equivalent to the equi-axed crystal region was 1.3 mm. Therefore,
the equi-axed crystal area ratio of this example was superior to
that of the conventional electromagnetic stirring, and further the
grain size of the equi-axed crystals was made fine. Further, since
the molten steel on the front face of solidification in the mold
was vibrated, no powder trapping was caused, and no defects
originated from powder were caused. On the lateral section of the
cast slab, a negative segregation zone of a multilayer, the pitch
of which was 1.5 mm corresponding to the period of vibration, was
formed on the surface layer of 15 mm, and also a dendrite of
deflection structure of a multilayer was formed. Concerning the
vibration mark, in the case of a slab in which the acceleration
stop time was not provided, the vibration mark was 5 mm, and in the
case of a slab in which the acceleration stop time was provided,
the vibration mark was 3 mm. In both cases, the shape of the
meniscus was made uniform compared with that of the conventional
electromagnetic stirring. However, when the acceleration stop time
was provided, the meniscus was made more uniform. The reason is
that a sudden acceleration was reduced when the acceleration stop
time was provided, so that the meniscus was made uniform. In the
present invention, the acceleration stop time was set to be not
more than 0.3 sec and not less than 0.03 sec. The reason is
described as follows. When the acceleration stop time is set to be
more than 0.3 sec, an effect of acceleration is deteriorated, and
when the acceleration stop time is set to be less than 0.03 sec, it
becomes impossible to make the meniscus uniform.
Example 4
[0113] In this example, a two-strand type continuous casting
machine for continuously casting slabs was used, and cast slabs of
250 mm thickness.times.1500 mm width made of carbon steel, the
carbon content of which was 0.35%, were cast for 30 minutes at the
casting speed of 1.8 m/min. Temperature in a tundish was
1550.degree. C. In one of the strands, a conventional
electromagnetic stirring was conducted, in which the coil current
of the electromagnetic stirring device was set at a constant value
of 500 ampere and the frequency was set at 2 Hz, for 30 minutes at
the flow velocity of 60 cm/s. In the other strand, an
electromagnetic coil of the present invention capable of giving
vibration was arranged in the mold. Molten steel on the front face
of solidification was vibrated under the following conditions.
Vibrating time of one period of the coil current was 2 s (the
maximum coil current was 400 ampere, the minimum coil current was
-400 ampere, the coil current increasing time was 0.4 s, the coil
current decreasing time was 0.8 s, the maximum coil current holding
time was 0.3 s, and the minimum coil current holding time was 0.5
s), and acceleration in the normal direction was set at 100
cm/s.sup.2, and acceleration in the opposite direction was set at
50 cm/s.sup.2 as shown in FIG. 6. After a lateral section of the
cast slab had been cut and the solidified structure had been
revealed, the equi-axed crystal area ratio and the diameter of the
equivalent circle of an equi-axed crystal region were evaluated.
Concerning the surface quality of the cast slabs, the cast slabs
were subjected to a visual inspection line, so that each slab was
visually inspected, and the number of defects caused by powder was
investigated. In addition to that, a microscopic examination was
made for checking the number of pieces of inclusion on the surface
layer of the slab.
[0114] Concerning the slabs on which the conventional
electromagnetic stirring was conducted, the equi-axed crystal ratio
was 28%, and the diameter of the equivalent circle of an equi-axed
crystal region was 3.1 mm. The flow velocity of molten steel was 60
cm/s, which exceeded a critical flow velocity of powder trapping.
Therefore, powder on the surface of molten steel was trapped, and
the defects were caused by powder, the number of which was 6
pieces/slab. Further, there was formed a negative segregation zone,
the width of which was approximately 20 mm, on the surface layer
side of the lateral section of the cast slab.
[0115] On the other hand, when vibration and rotation according to
a time difference in the normal and the reverse direction were
given by the electromagnetic coil of the present invention, the
equi-axed crystal area ratio of the cast slab was 55%, and the
diameter of the equivalent circle of an equi-axed crystal region
was 1.3 mm. Therefore, compared with the conventional
electromagnetic stirring, not only the equi-axed crystal area ratio
was enhanced, but also the grain size of the equi-axed crystals was
made fine. Since the molten steel on the front face of
solidification in the mold was vibrated, powder trapping was not
caused, and defects originated from powder were not caused, either.
On the lateral section of the cast slab, a negative segregation
zone of a multilayer, the pitch of which was 1.5 mm, was formed on
the surface layer of 15 mm, and also a dendrite of deflection
structure was formed. When vibration and rotation were
simultaneously given to the molten steel by the electromagnetic
coil, the prismatic dendrite was more effectively cut apart.
Therefore, compared with Example 3 in which only vibration was
given to the molten steel, the equi-axed crystal ratio was enhanced
in this example. In this connection, when rotation is added to
vibration conducted in the molten steel, powder trapping can be
suppressed by vibration, however, when a flow velocity of rotation
exceeded 1 m/s, powder trapping was caused. Therefore, the flow
velocity of rotation was restricted to be not more than 1 m/s.
Example 5
[0116] In this example, a two-strand type continuous casting
machine for continuously casting slabs was used, and cast slabs of
250 mm thickness.times.1500 mm width made of carbon steel, the
carbon content of which was 0.35%, were cast for 30 minutes at the
casting speed of 1.8 m/min. Temperature in a tundish was
1550.degree. C. In one of the strands, the conventional
electromagnetic stirring was conducted, in which the coil current
of the electromagnetic stirring device was set at a constant value
of 500 ampere and the frequency was set at 2 Hz, for 30 minutes at
the flow velocity of 60 cm/s. In the other strand, the
electromagnetic coil of the present invention capable of giving
vibration was arranged in the mold. Molten steel on the front face
of solidification was vibrated under the following conditions.
Vibrating time of one period of the coil current was 2 s (the
maximum coil current was 400 ampere, the minimum coil current was
-400 ampere, the coil current increasing time was 0.8 s, the coil
current decreasing time was 0.8 s, the maximum coil current holding
time was 0.2 s, and the minimum coil current holding time was 0.2
s), and acceleration in the one direction and the opposite
direction was set at 50 cm/s.sup.2 as shown in FIG. 2. While the
molten steel on the front face of solidification was being
vibrated, a magnetic force was applied upon the molten steel by a
static magnetic filed, the magnetic field intensity of which was
3000 gauss, by an electromagnetic brake arranged at a position
lower than the meniscus by 1 m. After a lateral section of the cast
slab had been cut and the solidified structure had been revealed,
the equi-axed crystal area ratio and the diameter of the equivalent
circle of an equi-axed crystal region were evaluated. Concerning
the surface quality of the cast slabs, the cast slabs were
subjected to a visual inspection line, so that each slab was
visually inspected, and the number of defects caused by powder was
investigated.
[0117] Concerning the slabs on which the conventional
electromagnetic stirring was generated, the equi-axed crystal ratio
was 31%, and the diameter of the equivalent circle of an equi-axed
crystal region was 2.9 mm. The flow velocity of molten steel was 60
cm/s, which exceeded a critical flow velocity of powder trapping.
Therefore, powder on the surface of molten steel was trapped, and
the defects were caused by powder, the number of which was 4
pieces/slab. Further, there was formed a negative segregation zone,
the width of which was approximately 20 mm, on the surface layer
side of the lateral section of the cast slab. On the other hand,
when vibration was given by the electromagnetic coil of the present
invention and the electromagnetic brake was applied, the equi-axed
crystal area ratio of the cast slab was 56%, and the diameter of
the equivalent circle of an equi-axed crystal region was 1.3 mm.
Therefore, compared with the conventional electromagnetic stirring,
not only the equi-axed crystal area ratio was enhanced, but also
the grain size of the equi-axed crystals was made fine. Since the
molten steel on the front solidified shell in the mold was
vibrated, powder trapping was not caused, and defects originated
from powder were not caused, either. On the lateral section of the
cast slab, a negative segregation zone of a multilayer, the pitch
of which was 1.5 mm, was formed on the surface layer of 15 mm, and
also a dendrite of deflection structure was formed. When vibration
caused by the electromagnetic coil was given together with the
electromagnetic brake, the equi-axed crystal ratio was enhanced as
compared with that in Example 3 in which only vibration was given.
The reason why the equi-axed crystal ratio was enhanced is that
permeation of molten steel of high temperature into the inside of a
cast slab was prevented by the electromagnetic brake, and the
tesseral crystal cores, which had been generated by vibration of
the electromagnetic coil, were prevented from being remelted. In
this connection, when the acceleration stop time is provided in the
vibration generated by the electromagnetic coil, it is unnecessary
to apply the electromagnetic brake continuously, that is, it is
possible to impress the electromagnetic brake synchronously with
the acceleration stop time.
[0118] Industrial Applicability
[0119] As described above, according to the method of the present
invention in which the vibration pattern is adjusted by the
electromagnetic coil so as to give vibration to molten metal, it is
possible to give a strong force onto the front solidified shell.
Accordingly, compared with the conventional method, not only the
equi-axed crystals can be increased but also the grain size of the
equi-axed crystals can be made fine. Due to the above effects, it
is unnecessary to increase the flow velocity too high for making
the solidified structure fine. Therefore, it is possible to prevent
the occurrence of surface defects caused by powder trapping.
[0120] In this connection, when the present invention is applied to
a stationary mold, the inner structure of conventional material can
be remarkably improved. Accordingly, the productivity and cost can
be improved.
* * * * *