U.S. patent application number 16/490251 was filed with the patent office on 2020-01-09 for continuous casting method and continuous casting device.
The applicant listed for this patent is NIPPON STEEL STAINLESS STEEL CORPORATION. Invention is credited to Yuki Honda, Hiroshi Morikawa, Yasuhiro Suzuki.
Application Number | 20200009650 16/490251 |
Document ID | / |
Family ID | 63370093 |
Filed Date | 2020-01-09 |
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United States Patent
Application |
20200009650 |
Kind Code |
A1 |
Honda; Yuki ; et
al. |
January 9, 2020 |
CONTINUOUS CASTING METHOD AND CONTINUOUS CASTING DEVICE
Abstract
Capture of a foreign matter into a solidified shell is
effectively suppressed. A continuous casting method is a method
which is carried out with use of a continuous casting device, and
includes: a discharging step of discharging molten steel through a
discharge hole; and a stirring step of stirring the molten steel so
that a stirring region includes a whole line segment via which the
discharge hole and a reached location are connected to each other,
the reached location being a location on a surface of the molten
steel in a mold which location is reached by the molten steel in a
case where the molten steel discharged in the discharging step
proceeds straight.
Inventors: |
Honda; Yuki; (Tokyo, JP)
; Morikawa; Hiroshi; (Yamaguchi, JP) ; Suzuki;
Yasuhiro; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL STAINLESS STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
63370093 |
Appl. No.: |
16/490251 |
Filed: |
March 2, 2018 |
PCT Filed: |
March 2, 2018 |
PCT NO: |
PCT/JP2018/008066 |
371 Date: |
August 30, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22D 41/62 20130101;
B22D 11/115 20130101; B22D 41/50 20130101 |
International
Class: |
B22D 41/62 20060101
B22D041/62; B22D 11/115 20060101 B22D011/115 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 3, 2017 |
JP |
2017-040658 |
Jul 4, 2017 |
JP |
PCT/JP2017/024528 |
Claims
1. A continuous casting method which is carried out with use of a
continuous casting device, the continuous casting device including:
a mold which includes a first surface and a second surface, which
intersects the first surface, so as to have a surrounding
structure; a submerged nozzle which has a discharge hole through
which molten steel is discharged; and a stirring device which forms
a stirring region by stirring the molten steel in the mold, the
method comprising: a discharging step of discharging, through the
discharge hole located in the mold, the molten steel upward with
respect to a horizontal direction and in a direction parallel to
the first surface; and a stirring step of stirring the molten steel
so that the stirring region includes a whole line segment via which
the discharge hole and a reached location are connected to each
other, the reached location being a location on a surface of the
molten steel in the mold or on the second surface which location is
reached by the molten steel in a case where the molten steel
discharged in the discharging step proceeds straight.
2. The method as set forth in claim 1, wherein a flow velocity of
the molten steel in the stirring region falls within a range of
0.20 m/s to 0.40 m/s.
3. The method as set forth in claim 1, wherein the reached location
is the location on the surface.
4. The method as set forth in claim 1, wherein an impulse which the
molten steel, discharged from the submerged nozzle, receives by a
time when the molten steel reaches the surface is 0.4.times.107
G.sup.2/.mu..OMEGA.-m to 2.5.times.107 G.sup.2/.mu..OMEGA.-m.
5. A continuous casting device comprising: a mold which includes a
first surface and a second surface, which intersects the first
surface, so as to have a surrounding structure; a submerged nozzle
which has a discharge hole located in the mold and from which
molten steel is discharged in a direction parallel to the first
surface through the discharge hole; and a stirring device which
forms a stirring region by stirring the molten steel in the mold,
the discharge hole being included in the stirring region and
causing the molten steel to be discharged upward with respect to a
horizontal direction, the stirring device stirring the molten steel
so that the stirring region includes a whole line segment via which
the discharge hole and a reached location are connected to each
other, the reached location being a location on a surface of the
molten steel in the mold or on the second surface which location is
reached by the molten steel in a case where the molten steel
discharged through the discharge hole proceeds straight.
6. The continuous casting device as set forth in claim 5, wherein a
flow velocity of the molten steel in the stirring region falls
within a range of 0.20 m/s to 0.40 m/s.
7. The continuous casting device as set forth in claim 5, wherein
the discharge hole causes the molten steel to be discharged upward
at an angle of 5.degree. to 30.degree. with respect to a horizontal
plane.
8. The continuous casting device as set forth in claim 5, wherein
an impulse which the molten steel, discharged from the submerged
nozzle, receives by a time when the molten steel reaches the
surface is 0.4.times.107 G.sup.2/.mu..OMEGA.-m to 2.5.times.107
G.sup.2/.mu..OMEGA.-m.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method and a device for
continuously casting steel while carrying out electromagnetic
stirring.
BACKGROUND ART
[0002] In continuous casting of steel, there is a problem that a
foreign matter, such as an air bubble and an oxide, which is
inevitably incorporated into molten steel in a mold is captured
into a solidified shell and, consequently, a defect (flaw) is
formed on a surface of a steel sheet (slab) obtained through a hot
rolling step or a cool rolling step. As a solution to the problem,
electromagnetic stirring is widely employed. By the electromagnetic
stirring, a flow of the molten steel in the mold is controlled so
that the foreign matter in the molten steel is floated and is then
captured by a mold powder applied to a surface of the molten steel.
Patent Literature 1discloses an example of such a method.
[0003] In the technique disclosed in Patent Literature 1, a
submerged nozzle is used which has two discharge holes through each
of which molten metal is discharged upward at an angle of 5.degree.
to 30.degree.. Through the two discharge holes, the molten metal is
discharged toward short sides of a mold. Then, by electromagnetic
stirring, a driving force acting in a direction perpendicular to a
casting direction is applied to the molten metal in a vicinity of a
meniscus near each of two long-side surfaces of the mold. With this
configuration, a temperature of the molten metal in a vicinity of
its surface is kept high, and a flow of the molten metal, which
flow is uniform and perpendicular to the casting direction, is
formed.
CITATION LIST
Patent Literature
[0004] [Patent Literature 1]
[0005] Japanese Patent Application Publication Tokukaihei No.
10-166120 (Published on Jun. 23, 1998)
SUMMARY OF INVENTION
Technical Problem
[0006] However, Patent Literature 1 does not clearly disclose a
method of forming a stirring flow for increasing cleaning of a
foreign matter from the molten metal. Thus, the technique disclosed
in Patent Literature 1 has a problem that an effect of suppressing
capture of a foreign matter into a solidified shell is not
sufficient.
[0007] An object of an aspect of the present invention is to
effectively suppress capture of a foreign matter into a solidified
shell.
Solution to Problem
[0008] In order to attain the above object, a continuous casting
method in accordance with an aspect of the present invention is a
continuous casting method which is carried out with use of a
continuous casting device, the continuous casting device including:
a mold which includes a first surface and a second surface, which
intersects the first surface, so as to have a surrounding
structure; a submerged nozzle which has a discharge hole through
which molten steel is discharged; and a stirring device which forms
a stirring region by stirring the molten steel in the mold, the
method including: a discharging step of discharging, through the
discharge hole located in the mold, the molten steel upward with
respect to a horizontal direction and in a direction parallel to
the first surface; and a stirring step of stirring the molten steel
so that the stirring region includes a whole line segment via which
the discharge hole and a reached location are connected to each
other, the reached location being a location on a surface of the
molten steel in the mold or on the second surface which location is
reached by the molten steel in a case where the molten steel
discharged in the discharging step proceeds straight.
[0009] The continuous casting method in accordance with an aspect
of the present invention is arranged such that a flow velocity of
the molten steel in the stirring region falls within a range of
0.20 m/s to 0.40 m/s.
[0010] The continuous casting method in accordance with an aspect
of the present invention can be arranged such that the reached
location is the location on the surface.
[0011] The continuous casting method in accordance with an aspect
of the present invention is preferably arranged such that an
impulse which the molten steel, discharged from the submerged
nozzle, receives by a time when the molten steel reaches the
surface is 0.4.times.10.sup.7 G.sup.2/.mu..OMEGA.-m to
2.5.times.10.sup.7 G.sup.2/.mu..OMEGA.-m.
[0012] A continuous casting device in accordance with an aspect of
the present invention is a continuous casting device including: a
mold which includes a first surface and a second surface, which
intersects the first surface, so as to have a surrounding
structure; a submerged nozzle which has a discharge hole located in
the mold and from which molten steel is discharged in a direction
parallel to the first surface through the discharge hole; and a
stirring device which forms a stirring region by stirring the
molten steel in the mold, the discharge hole being included in the
stirring region and causing the molten steel to be discharged
upward, the stirring device stirring the molten steel so that the
stirring region includes a whole line segment via which the
discharge hole and a reached location are connected to each other,
the reached location being a location on a surface of the molten
steel in the mold or on the second surface which location is
reached by the molten steel in a case where the molten steel
discharged through the discharge hole proceeds straight.
[0013] The continuous casting device in accordance with an aspect
of the present invention is arranged such that a flow velocity of
the molten steel in the stirring region falls within a range of
0.20 m/s to 0.40 m/s.
[0014] The continuous casting device in accordance with an aspect
of the present invention is preferably arranged such that the
discharge hole causes the molten steel to be discharged upward at
an angle of 5.degree. to 30.degree. with respect to a horizontal
plane.
[0015] The continuous casting device in accordance with an aspect
of the present invention is preferably arranged such that an
impulse which the molten steel, discharged from the submerged
nozzle, receives by a time when the molten steel reaches the
surface is 0.4.times.10.sup.7 G.sup.2/.mu..OMEGA.-m to
2.5.times.10.sup.7 G.sup.2/.mu..OMEGA.-m.
Advantageous Effects of Invention
[0016] According to an aspect of the present invention, it is
possible to effectively suppress capture of a foreign matter into a
solidified shell.
BRIEF DESCRIPTION OF DRAWINGS
[0017] FIG. 1 is a schematic view illustrating a configuration of a
continuous casting device in accordance with Embodiment 1 of the
present invention.
[0018] FIG. 2 is a cross-sectional view of the continuous casting
device, obtained by cutting the continuous casting device along a
horizontal plane at a height of a surface of molten steel in a mold
included in the continuous casting device.
[0019] FIG. 3 is a cross-sectional view of a vicinity of the
surface of the molten steel in the continuous casting device,
obtained by cutting the continuous casting device along a plane
which passes through a center of the mold and which is parallel to
long-side molds included in the mold.
[0020] FIG. 4 is a cross-sectional view of the vicinity of the
surface of the molten steel in the continuous casting device,
obtained by cutting the continuous casting device along a plane
which passes through the center of the mold and which is parallel
to short-side molds included in the mold.
[0021] FIG. 5 is a cross-sectional view of a continuous casting
device in accordance with Embodiment 2 of the present invention,
and is a cross-sectional view of a vicinity of a surface of molten
steel in the continuous casting device, obtained by cutting the
continuous casting device along a plane which passes through a
center of a mold included in the continuous casting device and
which is parallel to long-side molds included in the mold.
[0022] FIG. 6 shows the numbers of macro-streak-flaws per square
millimeter on each of an example strand of the present invention
and a comparative example strand. (a) of FIG. 6 is a graph showing
the numbers of macro-streak-flaws present at a location 2 mm below
a surface layer. (b) of FIG. 6 is a graph showing the numbers of
macro-streak-flaws present at a location 3 mm below the surface
layer.
DESCRIPTION OF EMBODIMENTS
[0023] The following description will discuss a continuous casting
device 1A and a continuous casting method in accordance with
Embodiment 1 of the present invention with reference to FIGS. 1
through 4. As used herein, an expression "A to B" means "not less
than A and not more than B".
[0024] FIG. 1 is a schematic view illustrating a configuration of
the continuous casting device 1A. As illustrated in FIG. 1, the
continuous casting device 1A includes a ladle 2, a tundish 3, a
mold 10, a submerged nozzle (discharge nozzle) 40A, and
electromagnetic stirring devices (stirring devices) 50A and 50B.
The ladle 2 receives molten steel supplied from a steel
converter.
[0025] The tundish 3 is a member for storing the molten steel
transferred from the ladle 2 and removing a foreign matter such as
an oxide. The molten steel stored in the tundish 3 is transferred
into the mold 10 through the submerged nozzle 40A (later
described).
[0026] The mold 10 is a mold for cooling the molten steel thus
transferred so that a solidified shell C is formed against an inner
surface of the mold 10. The solidified shell C is sent out from a
bottom of the mold 10. FIG. 2 is a cross-sectional view of the
continuous casting device 1A, obtained by cutting the continuous
casting device 1A along a horizontal plane at a height of a surface
S of molten steel in the mold 10. As illustrated in FIG. 2, a
contour shape of the inner surface of the mold 10 cut at the
horizontal plane is a rectangular shape. The mold 10 includes (i)
long-side molds 11A and 11B facing each other and (ii) short-side
molds 12A and 12B facing each other. The long-side molds 11A and
11B have long-side surfaces (first surface) 11Aa and 11Ba,
respectively, which form the inner surface of the mold 10. The
short-side molds 12A and 12B have short-side surfaces (second
surface) 12Aa and 12Ba, respectively, which form the inner surface
of the mold 10. In other words, the long-side surfaces 11Aa and
11Ba and the short-side surfaces 12Aa and 12Ba which intersect the
long-side surfaces 11Aa and 11Ba form a surrounding structure.
Hereinafter, a horizontal direction which is parallel to the
long-side molds 11A and 11B will be referred to as a "long-side
direction LD", and the horizontal direction which is parallel to
the short-side molds 12A and 12B will be referred to as a
"short-side direction SD" (see FIG. 2).
[0027] The submerged nozzle 40A is a member for transferring, into
the mold 10, the molten steel stored in the tundish 3. The
submerged nozzle 40A is provided so that an upper end of the
submerged nozzle 40A is connected to the tundish 3 and a lower end
of the submerged nozzle 40A is located substantially at a center of
the mold 10 (that is, in FIG. 2, substantially at a center of the
rectangular shape formed by the long-side surfaces 11Aa and 11Ba
and the short-side surfaces 12Aa and 12Ba).
[0028] FIG. 3 is a cross-sectional view of a vicinity of the
surface S in the continuous casting device 1A, obtained by cutting
the continuous casting device 1A along a plane which passes through
the center of the mold 10 and which is parallel to the long-side
molds 11A and 11B of the mold 10. Note that, since the continuous
casting device 1A has a structure symmetrical about the submerged
nozzle 40A, FIG. 3 illustrates a region which includes the
submerged nozzle 40A and the short-side mold 12B, in a state where
the region is enlarged. The submerged nozzle 40A has two discharge
holes 41A, as illustrated in FIG. 3. The two discharge holes 41A
are each located inside the mold 10, and are each a hole for
discharging the molten steel which has been supplied from tundish 3
and which has passed through an inside of the submerged nozzle 40A.
The two discharge holes 41A are formed on respective sides of the
submerged nozzle 40A which sides are located in the long-side
direction LD. Through the two discharge holes 41A, the molten steel
is discharged in directions each parallel to the long-side surfaces
11Aa and 11Ba. Each of the two discharge holes 41A is formed so
that a discharge direction 60 of a discharge flow is directed
upward with respect to the horizontal plane. Hereinafter, an angle
formed by the discharge direction 60 of each of the two discharge
holes 41A and the horizontal plane will be referred to as a
discharge angle .theta..
[0029] According to the continuous casting device 1A, the molten
steel is continuously supplied to the mold 10 through the two
discharge holes 41A of the submerged nozzle 40A, and the surface S
(also referred to as a meniscus) of the molten steel is formed at a
given height inside the mold 10. Note that, although the surface S
swings to an extent during continuous casting, an average height of
the surface S is regarded as a location of the surface S in the
present specification. Note also that a mold powder (not
illustrated) for capturing a foreign matter such as an air bubble
and an oxide is applied to the surface S.
[0030] The electromagnetic stirring devices 50A and 50B are devices
for causing, by electromagnetic forces, a stirring flow (swirling
flow) with respect to the molten steel in the vicinity of the
surface S in the mold 10. FIG. 4 is a cross-sectional view of a
vicinity of the surface S in the continuous casting device 1A,
obtained by cutting the continuous casting device 1A along a plane
which passes through the center of the mold 10 and which is
parallel to the short-side molds 12A and 12B of the mold 10. The
electromagnetic stirring devices 50A and 50B are provided on back
surfaces of the long-side molds 11A and 11B, respectively, as
illustrated in FIG. 4. The electromagnetic stirring devices 50A and
50B include stirring coil cores 51A and 51B, respectively. Each of
the stirring coil cores 51A and 51B applies an electromagnetic
force to the molten steel in the mold 10, at a height at which the
each of the stirring coil cores 51A and 51B is provided.
Specifically, the stirring coil core 51 A of the electromagnetic
stirring device 50A applies, to the molten steel in a vicinity of
the long-side mold 11A, an electromagnetic force parallel to the
long-side direction LD. Similarly, the stirring coil core 51B of
the electromagnetic stirring device 50B applies, to the molten
steel in a vicinity of the long-side mold 11B, an electromagnetic
force parallel to the long-side direction LD. Note, however, that
the continuous casting device 1A is configured such that a
direction of the electromagnetic force applied to the molten steel
by the stirring coil core 51A and a direction of the
electromagnetic force applied to the molten steel by the stirring
coil core 51B are opposite to each other. This causes a stirring
flow to be formed in the horizontal direction in the vicinity of
the surface S in the mold 10 (see an black arrows in FIG. 2). In
Embodiment 1, the electromagnetic stirring devices 50A and 50B are
provided so that respective upper ends of the stirring coil cores
51A and 51B are each located a given distance below the surface
S.
[0031] In FIG. 4, a region A1 indicates a region between an upper
end and a lower end of each of the two discharge holes 41A, which
upper end and lower end are located in a vertical direction. As
illustrated in FIG. 4, the stirring coil cores 51A and 51B are
provided so as to include, in the vertical direction, the whole two
discharge holes 41A. With this configuration, electromagnetic
forces are applied, by the stirring coil cores 51A and 51B, to the
molten steel discharged through the two discharge holes 41A from a
time point when the molten steel is discharged.
[0032] The electromagnetic stirring devices 50A and 50B cause a
stirring flow with respect to the molten steel in the mold 10.
Note, here, that the stirring flow is not formed only in a region
located at the height at which each of the stirring coil cores 51A
and 51B is provided. That is, in a case where a stirring flow is
caused in a region A2 in which the stirring coil cores 51A and 51B
are provided, the molten steel, present in regions each extending
upward or downward from the region A2 by a certain distance, is
also caused to swirl in the mold 10, so that a stirring flow is
caused. In the present specification, a region including the region
A2 and the "regions each extending upward or downward from the
region A2 by a certain distance" will be referred to as a stirring
region A3 which is formed by the electromagnetic stirring devices
50A and 50B (see FIG. 4). Specifically, the stirring region A3 in
the present specification means a region in which a flow velocity
of the molten steel falls within a range of 0.20 m/s to 0.40 m/s.
It known that, in a case where a flow velocity of molten steel in a
vicinity of a surface S falls within a range of 0.20 m/s to 0.40
m/s, it is possible to reduce both a surface defect and an internal
defect of steel to be produced. According to the continuous casting
device 1A in accordance with Embodiment 1, the stirring coil cores
51A and 51B are provided so that the stirring region A3 includes
the surface S.
[0033] Note that the continuous casting device 1A is configured
such that the electromagnetic stirring devices 50A and 50B are
provided so that the respective upper ends of the stirring coil
cores 51A and 51B are each located the given distance below the
surface S. However, the continuous casting device in accordance
with an embodiment of the present invention is not limited to such
a configuration, and can be alternatively configured such that the
respective upper ends of the stirring coil cores 51A and 51B are
each located at a height corresponding to the height of the surface
S or each located above the surface S. Also in such cases, it is
possible to configure the continuous casting device so that the
stirring region includes the surface S.
[0034] The continuous casting device 1A in accordance with
Embodiment 1 is configured as follows. That is, a substantially
whole amount of the molten steel discharged through each of the two
discharge holes 41A of the submerged nozzle 40A is caused to reach
the surface S, by appropriately setting the discharge angle
.theta., a distance L between (i) a center of each of the two
discharge holes 41A and (ii) the surface S, and a distance W
between the short-side molds 12A and 12B (that is, a length of each
of the long-side surfaces 11Aa and 11Ba in the horizontal
direction).
[0035] Here, the following description will discuss, with reference
to FIG. 3, a "reached location" on the surface S which location is
reached by the discharge flow caused by the molten steel discharged
through a discharge hole 41A. As illustrated in FIG. 3, a center of
an opening of the discharge hole 41A is regarded as a starting
point, and a point at which the surface S intersects a half line
extending in the discharge direction 60 from the starting point is
regarded as a point P. In other words, the point P is a point on
the surface S which point is reached by the molten steel,
discharged through the discharge hole 41A, in a case where the
molten steel discharged through the discharge hole 41A proceeds
straight. The discharge flow from the discharge hole 41A proceeds
through the molten steel in the mold 10 while spreading to some
extent. By setting a discharge velocity, at which the molten steel
is discharged through the discharge hole 41A, so as to be higher
than a given velocity, it is possible to cause the substantially
whole amount of the molten steel discharged through the discharge
hole 41A to directly reach the point P and the surface S in a
vicinity of the point P. In Embodiment 1, the point P and the
vicinity of the point P will be referred to as a "reached
location". In Embodiment 1, the stirring region A3 incudes a whole
line segment via which the center of the opening of the discharge
hole 41A and the point P are connected to each other.
[0036] Note that, in a case where the discharge angle .theta. is
excessively wide or in a case where the distance L is excessively
short, the surface S excessively ruffles because the discharge flow
from the discharge hole 41A directly reaches the surface S (reached
location), so that the mold powder present on the surface S is
highly likely to be entrapped in the solidified shell C as a
foreign matter. Therefore, the discharge angle .theta. is
preferably not more than 30.degree.. In a case where a velocity of
the discharge flow is 300 mm/s to 1150 mm/s, the distance L is
preferably not less than 180 mm.
[0037] In a case where the distance L is excessively long, time it
takes for the discharge flow to reach the surface S (reached
location) becomes longer. As a result, time it takes for a
high-temperature discharge flow to reach the surface S in a
vicinity of each of the short-side molds 12A and 12B becomes
longer, and a temperature of the surface S of the molten steel in
the vicinity of each of the short-side molds 12A and 12B becomes
lower. A decrease in a temperature of the molten steel causes
generation of an ununiform initial solidified shell having a
hook-like cross section. This ultimately causes an increase in
entrapment of a foreign matter into the solidified shell C.
Therefore, in a case where the velocity of the discharge flow is
300 mm/s to 1150 mm/s, the distance L is preferably not more than
230 mm.
[0038] As an example of the configuration of the continuous casting
device 1A, in a case where the distance W is longer than 520 mm, it
is possible to cause the substantially whole amount of the molten
steel discharged through the discharge hole 41A of the submerged
nozzle 40A to reach the surface S (reached location), by setting
the discharge angle .theta. to 30.degree. and setting the distance
L to 150 mm (for example, setting a width, in an up-and-down
direction, of the discharge hole 41A to 58 mm, and setting a
distance between the surface S and an upper end of the discharge
hole 41A to 121 mm).
[0039] As has been described, the continuous casting method carried
out with use of the continuous casting device 1A includes: a
discharging step of discharging, through the two discharge holes
41A located in the mold 10, the molten steel upward with respect to
the horizontal direction and in the directions parallel to the
long-side molds 11A and 11B; and a stirring step of stirring the
molten steel so that the stirring region A3 includes a whole line
segment via which a center of an opening of each of the two
discharge holes 41A and the point P are connected to each
other.
[0040] With this configuration, it is possible to cause most of
high-temperature molten steel discharged through each of the two
discharge holes 41A to reach the surface S, and accordingly
possible to delay solidification of the molten steel in the
vicinity of the surface S. This makes it possible to increase a
stirring effect of each of the electromagnetic stirring devices 50A
and 50B which stirring effect is brought about in the vicinity of
the surface S, and possible to effectively suppress capture of a
foreign matter, which is present in the molten steel, into a
solidified shell C.
[0041] Furthermore, since a distance between each of the two
discharge holes 41A and the surface S is short, the discharge flow
reaches the surface S before the velocity of the discharge flow
decreases. This makes it easy to float a foreign matter, such as an
air bubble or an inclusion in the molten steel, by the discharge
flow whose velocity is high, and makes it easy to cause the mold
powder to capture the foreign matter. Moreover, since the distance
between (i) each of the two discharge holes 41A and (ii) the
surface S is short, it is possible to suppress diffusion of the
discharge flow in a flow path through which the discharge flow
reaches the surface S, and accordingly possible to avoid blocking
the stirring flow.
[0042] The discharge flow having reached the surface S is divided
into (i) a flow toward the short-side mold 12A or 12B and (ii) a
flow toward the submerged nozzle 40A (that is, the center of the
mold 10), as illustrated in FIG. 3. As a result, it is possible to
uniform the temperature of the molten steel in the vicinity of the
surface S.
Embodiment 2
[0043] The following description will discuss another embodiment of
the present invention with reference to FIG. 5. Note that, for
convenience, an identical reference sign will be given to a member
having a function identical to that of a member described in the
foregoing embodiment.
[0044] A continuous casting device 1B in accordance with Embodiment
2 includes a submerged nozzle 40B having a structure different from
that of the submerged nozzle 40A in Embodiment 1.
[0045] FIG. 5 is a cross-sectional view of a vicinity of a surface
S in the continuous casting device 1B, obtained by cutting the
continuous casting device 1B along a plane which passes through a
center of a mold 10 and which is parallel to long-side molds 11A
and 11B of the mold 10.
[0046] As illustrated in FIG. 5, the continuous casting device 1B
in accordance with Embodiment 2 includes the submerged nozzle 40B
instead of the submerged nozzle 40A in Embodiment 1.
[0047] The submerged nozzle 40B has two discharge holes 41B (see
FIG. 5). Each of the two discharge holes 41B is formed so that a
discharge direction 60 of molten steel is directed upward with
respect to a horizontal plane. Hereinafter, an angle formed by the
discharge direction 70 of each of the two discharge holes 41B and
the horizontal plane will be referred to as a discharge angle
.phi..
[0048] The continuous casting device 1B in accordance with
Embodiment 2 is configured as follows. That is, a substantially
whole amount of the molten steel discharged through each of the two
discharge holes 41B is caused to reach a corresponding one of
short-side molds 12A and 12B (more specifically, a solidified shell
C formed against the corresponding one of the short-side molds 12A
and 12B), by appropriately setting the discharge angle .phi., a
distance L between (i) a center of each of the two discharge holes
41B and (ii) the surface S, and a distance W, on the surface S,
between short-side molds 12A and 12B.
[0049] Here, the following description will discuss, with reference
to FIG. 5, a "reached location" on each of the short-side molds 12A
and 12B which location is reached by a discharge flow caused by the
molten steel discharged through a corresponding one of the two
discharge holes 41B. As illustrated in FIG. 5, a center of an
opening of a discharge hole 41B is regarded as a starting point,
and a point at which the short-side mold 12B intersects a half line
extending in the discharge direction 70 from the starting point is
regarded as a point Q. In other words, the point Q is a point on
the short-side mold 12B which point is reached by the molten steel,
discharged through the discharge hole 41B, in a case where the
molten steel discharged through the discharge hole 41B proceeds
straight. The discharge flow caused by the molten steel discharged
through the discharge hole 41B proceeds through the molten steel in
the mold 10 while spreading to some extent. By setting a discharge
velocity at which the molten steel is discharged through the
discharge hole 41A so as to be higher than a given velocity, it is
possible to cause the substantially whole amount of the molten
steel discharged through the discharge hole 41B to directly reach
the short-side mold 12B. In Embodiment 2, the point Q and a
vicinity of the point Q will be referred to as a "reached
location". The point Q is located in the vicinity of the surface
S.
[0050] In Embodiment 2, the continuous casting device 1B is
configured such that a stirring region, formed by electromagnetic
stirring devices 50A and 50B, at least includes, in a vertical
direction, a region ranging from the "reached location" (the point
Q and the vicinity of the point Q) to a lower end of the discharge
hole 41B. This causes the stirring region, formed by the
electromagnetic stirring devices 50A and 50B, to include a whole
line segment via which the center of the opening of the discharge
hole 41B and the point Q are connected to each other.
[0051] As an example of the configuration of the continuous casting
device 1B, in a case where the distance W is 1430 mm to 1650 mm and
a velocity of the discharge flow is 300 mm/s to 1150 mm/s, it is
possible to cause the substantially whole amount of the molten
steel discharged through the discharge hole 41B of the submerged
nozzle 40B to reach the "reached location", by setting the
discharge angle .phi. to 5.degree. and setting the distance L to
125 mm (for example, setting a width, in an up-and-down direction,
of the discharge hole 41B to 50 mm, and setting a distance between
the surface S and the discharge hole 41A to 100 mm).
[0052] As has been described, a continuous casting method carried
out with use of the continuous casting device 1B includes: a
discharging step of discharging, through the two discharge holes
41B located in the mold 10, the molten steel upward with respect to
a horizontal direction and in directions parallel to long-side
molds 11A and 11B; and a stirring step of stirring the molten steel
so that the stirring region, formed by the electromagnetic stirring
devices 50A and 50B, includes a whole line segment via which a
center of an opening of each of the two discharge holes 41B and the
point Q are connected to each other.
[0053] With this configuration, it is possible to cause most of
high-temperature molten steel discharged through each of the two
discharge holes 41A to reach the vicinity of the surface S, and
accordingly possible to delay solidification of the molten steel in
the vicinity of the surface S. This makes it possible to increase a
stirring effect of each of the electromagnetic stirring devices 50A
and 50B which stirring effect is brought about in the vicinity of
the surface S, and possible to effectively suppress capture of a
foreign matter, which is present in the molten steel, into a
solidified shell C.
[0054] Each of the continuous casting devices 1A and 1B has a
structure in which the solidified shell C and part of the molten
steel are drawn out from a lower end part of the mold 10.
Therefore, the discharge flow from each of the two discharge holes
41B receives a downward force. Thus, in a case where the continuous
casting device 1B has a narrow discharge angle .phi., the discharge
flow from each of the two discharge holes 41B is likely to proceed
outside the stirring region. Accordingly, the molten steel is
preferably discharged at an angle of not less than 5.degree., as
the discharge angle .phi.. This allows the stirring region to
absolutely include the discharge flow from each of the discharge
holes 41B.
Embodiment 3
[0055] In Embodiment 3, preferred setting of an electromagnetic
stirring device in continuous casting will be described.
[0056] The following shows parameters in continuous casting in
Embodiment 3. Note that a term in square brackets indicates a
unit.
.alpha.: a discharge angle of a discharge hole 41A or a discharge
hole 41B [.degree.] A: a discharge area of the discharge hole 41A
or the discharge hole 41B [m.sup.2] W: a casting width [m]
(horizontal distance between short-side surfaces 12Aa and 12Ba) T:
a casting thickness [m] (horizontal distance between long-side
surfaces 11Aa and 11Ba) V: a discharge velocity at which molten
steel is discharged through the discharge hole 41A or the discharge
hole 41B [m/s] Vc: a casting speed [m/s] L: a submergence depth to
which a submerged nozzle 40A or a submerged nozzle 40B is submerged
[m] (distance between a center of the discharge hole 41A or the
discharge hole 41B and a surface S) B: a magnetic flux density at a
location in the molten steel, which location is 15 mm apart, in a
horizontal direction, from each of the long-side surfaces 11Aa and
11Ba [G] f: a frequency of an electromagnetic stirring device 50A
or an electromagnetic stirring device 50B [Hz] .sigma.: an electric
conductivity of a secondary conductor (the molten steel at
1500.degree. C.) [1/.mu..OMEGA.-m]
[0057] First, since a volume of the molten steel discharged through
the discharge hole 41A or 41B is identical to a volume of cast
steel, the following Expression 1 is established.
(A.times.2).times.V=W.times.T.times.Vc (Expression 1)
[0058] A discharge velocity V is determined in accordance with the
following Expression 2 on the basis of Expression 1.
V=W.times.T.times.Vc/2A (Expression 2)
[0059] Therefore, a velocity Vy, in a vertical direction, of the
molten steel discharged through the discharge hole 41A or 41A is
obtained in accordance with the following Expression 3.
Vy=V.times.sin .alpha.=W.times.T.times.Vc.times.sin .alpha./2A
(Expression 3)
[0060] Thus, time t it takes for the molten steel to reach the
surface S from the discharge hole 41A or 41A (surface reaching time
t) is obtained in accordance with the following Expression 4.
t=L/Vy=L.times.W.times.T.times.Vc.times.sin .alpha./2A (Expression
4)
[0061] Stirring hydraulic power H applied to the molten steel is
obtained in accordance with the following Expression 5.
H=B.sup.2.times.f.times..sigma.
[0062] Therefore, an impulse I which the molten steel, discharged
through the discharge hole 41A or 41A, receives by the time when
the molten steel reaches the surface S is obtained in accordance
with the following Expression 6.
I=H.times.t=B.sup.2.times.f.times..sigma.L.times.W.times.T.times.Vc.time-
s.sin.alpha./2A (Expression 6)
[0063] According to a continuous casting method in accordance with
Embodiment 3, it is preferable to set the magnetic flux density B,
the frequency f, and the casting speed Vc so that the impulse I is
0.4.times.10.sup.7 G.sup.2/.mu..OMEGA.-m to 2.5.times.10.sup.7
G.sup.2/.mu..OMEGA.-m, in a case where the discharge angle .alpha.,
the discharge area A, the casting width, the casting thickness T,
and the submergence depth L have respective given values. This
makes it possible to form a stirring flow which is highly effective
in cleaning a foreign matter from the molten steel, even in a case
where a kind of the molten steel is changed.
[0064] The present invention is not limited to the embodiments, but
can be altered by a skilled person in the art within the scope of
the claims. The present invention also encompasses, in its
technical scope, any embodiment derived by combining technical
means disclosed in differing embodiments.
EXAMPLE 1
[0065] The following description will discuss an Example of the
present invention.
[0066] In Example 1, continuous casting of SUH409L and SUS439, each
of which is a kind of ferrite-based steel, was carried out under
the following conditions.
[0067] (Continuous Casting Conditions)
A discharge angle .theta. of each discharge hole 41A: 30.degree. A
width, in a vertical direction, of each discharge hole 41A: 58 mm A
distance L, in the vertical direction, between a surface S and a
center of each discharge hole 41A: 180 mm A distance W between
short-side molds 12A and 12B: 1042 mm A casting speed: 1.30 m/min A
discharge velocity: 865 mm/s A thickness of a strand: 200 mm A
magnetic flux density at a location 15 mm apart, in a thickness
direction, from each of long-side surfaces 11Aa and 11Ba: 1150
G
[0068] The above conditions are conditions under which, in a case
where stirring is carried out with use of electromagnetic stirring
devices 50A and 50B, (i) a substantially whole amount of molten
steel discharged through each discharge hole 41A directly reaches a
surface S and (ii) the molten steel is included in a stirring
region, formed by the electromagnetic stirring devices 50A and 50B,
until the molten steel reaches the surface S from when the molten
steel is discharged through the each discharge hole 41A.
[0069] In Example 1, evaluation was carried out with respect to (i)
strands prepared while stirring was carried out with use of the
electromagnetic stirring devices 50A and 50B (a strand of SUH409L
will be referred to as an example strand 1, and a strand of SUS439
will be referred to as an example strand 2) and (ii) strands
prepared without carrying out stirring with use of the
electromagnetic stirring devices 50A and 50B (a strand of SUH409L
will be referred to as a comparative example strand 1, and a strand
of SUS439 will be referred to as a comparative example strand
2).
[0070] An X-ray transmission method was carried out with respect to
each of the example strands 1 and 2 and the comparative example
strands 1 and 2 so as to count the number of surface defects
(defects formed by capture of air bubbles or inclusions into a
solidified cell) present within 10 mm from a surface layer. In this
evaluation, the number of surface defects having a diameter of not
less than 0.4 mm was counted. The following Table 1 shows results
of counting. Note that Table 1 shows the number of defects per
cubic centimeter.
TABLE-US-00001 TABLE 1 Defect Density Kind of Steel
(number/cm.sup.3) SUH409L Comparative Example Strand 1 0.0029
SUH409L Example Strand 1 0.0000 SUS439 Comparative Example Strand 2
0.0024 SUS439 Example Strand 2 0.0007
[0071] As shown in Table 1, in the example strands 1 and 2, it was
possible to remarkably suppress formation of a surface defect, as
compared with the comparative example strands 1 and 2. This appears
to be because, since each of the strand examples 1 and 2 was cast
in a state where (i) the substantially whole amount of the molten
steel discharged through each discharge hole 41A directly reached
the surface S and (ii) the molten steel was included in the
stirring region, formed by the electromagnetic stirring devices 50A
and 50B, until the molten steel reached the surface S from when the
molten steel was discharged through the each discharge hole 41A, it
was possible to more effectively carry out stirring of the molten
steel.
[0072] Each of the example strands 1 and 2 and the comparative
example strands 1 and 2 thus prepared was subjected to steps (hot
rolling, annealing, acid pickling, cool rolling, annealing, and
acid pickling) for producing a typical ferrite-based stainless
steel sheet, so that a plurality of cold-rolled annealed steel
sheets, each having a thickness of 1 mm, were produced. A surface
of each steel sheet thus produced was inspected, and it was
determined whether the each steel sheet had a quality as a product.
As a result, although surfaces of slabs were ground, a few percent
(3.9% in a case of SUH409L, and 2.2% in a case of SUS439) in steel
sheets prepared with use of the comparative example strands 1 and 2
did not have a quality as a product. In contrast, all of steel
sheets prepared with use of the strand examples 1 and 2 had a
quality as a product, although surfaces of slabs were not
ground.
EXAMPLE 2
[0073] The following description will discuss another Example of
the present invention.
[0074] In Example 2, continuous casting of SUS304 was carried out
under the following conditions.
[0075] (Continuous casting Conditions)
A discharge angle .phi. of each discharge hole 41B: 5.degree. A
width, in a vertical direction, of each discharge hole 41B: 50 mm A
distance L, in the vertical direction, between a surface S and a
center of each discharge hole 41B: 220 mm A distance W between
short-side molds 12A and 12B: 1038 mm A casting speed: 1.40 m/min A
discharge velocity: 932 mm/s A thickness of a strand: 200 mm A
magnetic flux density at a location 15 mm apart, in a thickness
direction, from each of long-side surfaces 11Aa and 11Ba: 1150
G
[0076] The above conditions are conditions under which, in a case
where stirring is carried out with use of electromagnetic stirring
devices 50A and 50B, (i) a substantially whole amount of molten
steel discharged through each discharge hole 41B directly reaches
short-side surface 12Aa or 12Ba of a mold 10 and (ii) the molten
steel is included in a stirring region, formed by the
electromagnetic stirring devices 50A and 50B, until the molten
steel reaches the short-side surface 12Aa or 12Ba of the mold 10
from when the molten steel is discharged through the each discharge
hole 41B.
[0077] In Example 2, evaluation was carried out with respect to (i)
an example strand 3 prepared while stirring was carried out with
use of the electromagnetic stirring devices 50A and 50B and (ii) a
comparative example strand 3 prepared without carrying out stirring
with use of the electromagnetic stirring devices 50A and 50B. Note
that the strand example 3 was prepared by carrying out stirring
with use of the electromagnetic stirring devices 50A and 50B from
when 900 mm of the strand was drawn out from the mold 10.
[0078] Radiographic inspection was carried out with respect to each
of the example strand 3 and the comparative example strand 3 so as
to count the number of macro-streak-flaws (cracks caused in a case
where a mold powder is incorporated into a solidified shell)
present at locations 2 mm and 3 mm below a surface layer. The
counting was carried out with respect to a middle part of an upper
surface of each strand, at locations 800 mm, 1000 mm, 1200 mm, 1500
mm, 2000 mm, 2500 mm, and 3000 mm apart from a casting starting
location at which casting was started. In this evaluation, the
number of macro-streak-flaws having a diameter of not less than
0.15 mm was counted. FIG. 6 shows results of counting. FIG. 6 shows
the numbers of macro-streak-flaws per square millimeter on each of
the example strand 3 and the comparative example strand 3. (a) of
FIG. 6 is a graph showing the numbers of macro-streak-flaws present
at the location 2 mm below the surface layer. (b) of FIG. 6 is a
graph showing the numbers of macro-streak-flaws present at the
location 3 mm below the surface layer.
[0079] As shown in (a) and (b) of FIG. 6, the numbers of
macro-streak-flaw present at the locations 2 mm and 3 mm blow the
surface layer of the example strand 3 were smaller than those in
the comparative example strand 3. This appears to be because, since
the strand example 3 was cast in a state where (i) the
substantially whole amount of the molten steel discharged through
each discharge hole 41B directly reached the short-side surface
12Aa or 12Ba of the mold 10 and (ii) the molten steel was included
in the stirring region, formed by the electromagnetic stirring
devices 50A and 50B, until the molten steel reached the short-side
surface 12Aa or 12Ba from when the molten steel was discharged
through the each discharge hole 41B, it was possible to more
effectively carry out stirring of the molten steel. In particular,
the numbers of macro-streak-flaws were conventionally large at the
locations 1000 mm to 2000 mm apart from the casting starting
location, as in the comparative example strand 3. In contrast, in
the example strand 3, it was possible to reduce the numbers of
macro-streak-flaws also at the locations 1000 mm to 2000 mm apart
from the casting starting location. From those results, it was
found that it was possible to increase yields in grinding of a
prepared strand from 96.8% to 97.5%.
EXAMPLE 3
[0080] The following description will discuss still another Example
of the present invention.
[0081] In Example 3, continuous casting of SUS304 was carried out
under the following conditions.
A discharge angle a of each discharge hole 41: 5.degree. A
discharge area A of each discharge hole 41: 0.0026 m.sup.2 A
casting width W: 1260 mm A casting thickness T: 200 mm A discharge
velocity V: 0.70 m/min A casting speed Vc: 0.7 m/min to 1.2 m/min A
submergence depth L: 0.25 m An electric conductivity .sigma. of a
secondary conductor: 1/.rho.=1/1.3 (.rho. represents a specific
electric resistance of SUS304, see "Sutenresuko Binran" (Handbook
for Stainless Steel), unit: .mu..OMEGA.-m)
[0082] In Example 3, a magnetic flux density B was set to 1150 G
and a frequency was set to 2.7 Hz, and then an impulse I was
calculated in accordance with the foregoing Expressions 1 through
6. As a result, the following Expression 7 was obtained.
0.4.times.10.sup.7
(G.sup.2/.mu..OMEGA.-m)<I<2.5.times.10.sup.7
(G.sup.2/.mu..OMEGA.-m) (Expression 7)
[0083] In Example 3 in which the impulse falling within a range
represented by Expression 7 was applied to the molten steel, it was
possible to form a stirring flow having an excellent effect of
cleaning a foreign matter from the molten steel, and possible to
effectively suppress capture of a foreign matter, present in the
molten steel, into a solidified shell.
REFERENCE SIGNS LIST
[0084] 1A, 1B Continuous casting device [0085] 10 Mold [0086] 11Aa,
11Ba Long-side surface (first surface) [0087] 12Aa, 12Ba Short-side
surface (second surface) [0088] 40A, 40B Submerged nozzle [0089]
41A, 41B Discharge hole [0090] 50A, 50B Electromagnetic stirring
device (stirring device) [0091] A3 Stirring region [0092] S
Surface
* * * * *