U.S. patent application number 12/084068 was filed with the patent office on 2009-11-26 for method of production of ultralow carbon cast slab.
Invention is credited to Masahiro Doki, Hajime Hasegawa, Go Hirata, Yoshiaki Kimura, Akihiro Matsuzawa, Masafumi Miyazaki, Wataru Ohashi, Yasuhiko Ohtani, Katsuhiro Sasai, Katsunori Yamada.
Application Number | 20090288799 12/084068 |
Document ID | / |
Family ID | 37967923 |
Filed Date | 2009-11-26 |
United States Patent
Application |
20090288799 |
Kind Code |
A1 |
Miyazaki; Masafumi ; et
al. |
November 26, 2009 |
Method of Production of Ultralow Carbon Cast Slab
Abstract
A method of production of an ultralow carbon cast slab
characterized by adding Ti to molten steel decarburized to a carbon
concentration of 0.01 mass % or less, further adding at least one
of Ce, La, and Nd, and using an immersion nozzle to inject the
above molten steel from a tundish to a casting mold for continuous
casting while maintaining a flow rate of Ar gas blown from any
location in a range from a tundish upper nozzle to discharge ports
of said immersion nozzle at 3 Nl (normal liter)/min or less.
Inventors: |
Miyazaki; Masafumi; (Chiba,
JP) ; Matsuzawa; Akihiro; (Oita, JP) ; Sasai;
Katsuhiro; (Chiba, JP) ; Ohashi; Wataru;
(Chiba, JP) ; Ohtani; Yasuhiko; (Chiba, JP)
; Yamada; Katsunori; (Chiba, JP) ; Doki;
Masahiro; (Chiba, JP) ; Hirata; Go; (Chiba,
JP) ; Kimura; Yoshiaki; (Chiba, JP) ;
Hasegawa; Hajime; (Chiba, JP) |
Correspondence
Address: |
KENYON & KENYON LLP
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
37967923 |
Appl. No.: |
12/084068 |
Filed: |
October 27, 2006 |
PCT Filed: |
October 27, 2006 |
PCT NO: |
PCT/JP2006/322048 |
371 Date: |
October 10, 2008 |
Current U.S.
Class: |
164/473 |
Current CPC
Class: |
C21C 7/072 20130101;
B22D 41/58 20130101; Y02P 10/20 20151101; C21C 7/068 20130101; B22D
11/117 20130101; C21C 7/0006 20130101; B22D 11/108 20130101 |
Class at
Publication: |
164/473 |
International
Class: |
B22D 11/10 20060101
B22D011/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 27, 2005 |
JP |
2005-312973 |
Jul 21, 2006 |
JP |
2006-199474 |
Claims
1. A method of production of an ultralow carbon cast slab
characterized by adding Ti to molten steel decarburized to a carbon
concentration of 0.01 mass % or less, further adding at least one
of Ce, La, and Nd, and using an immersion nozzle to inject the
above molten steel from a tundish to a casting mold for continuous
casting while maintaining a flow rate of Ar gas blown from any
location in a range from a tundish upper nozzle to discharge ports
of said immersion nozzle at 3N1 (normal liter)/min or less.
2. A method of production of an ultralow carbon cast slab as set
forth in claim 1, characterized in that said immersion nozzle has
an orifice in its inner bore.
3. A method of production of an ultralow carbon cast slab as set
forth in claim 2, characterized in that said inner bore has a
circular cross-sectional shape and (i) a relation
3.ltoreq.R-r.ltoreq.30 stands between a radius R of a top end of
the inner bore [mm] and a smallest radius r of the orifice [mm] and
(ii) a length L from a top end to a bottom end of the orifice [mm]
is 50.ltoreq.L.ltoreq.150.
4. A method of production of an ultralow carbon cast slab as set
forth in claim 2, characterized in that said inner bore has an
elliptical cross-sectional shape and (i) a relation
3.ltoreq.A-a.ltoreq.30 stands between a radius A of a long-axis
direction of a top end of the inner bore [mm] and a smallest radius
a of the orifice [mm] and (ii) a length L from a top end to a
bottom end of the orifice [mm] is 50.ltoreq.L.ltoreq.150.
5. A method of production of an ultralow carbon cast slab as set
forth in claim 1, characterized in that said immersion nozzle has a
closed bottom cylindrical shape and (i) two discharge ports are
arranged at axially symmetric positions to a cylinder at a bottom
part of side walls of the cylindrical shape and (ii) a slit is
provided connecting a cylinder bottom part and bottom parts of the
two discharge ports and opening to the outside.
6. A method of production of an ultralow carbon cast slab as set
forth in claim 5, characterized in that, in said immersion nozzle,
(i) portions contiguous with the slit of the cylinder bottom part
are inclined upward toward the cylinder side walls (ii) portions
contiguous with the slit of discharge port bottom parts are
inclined upward toward discharge port side walls, and (iii) there
is substantially no step difference between the surface forming the
cylinder bottom part and the surfaces forming the discharge port
bottom parts.
7. A method of production of an ultralow carbon cast slab as set
forth in claim 6, characterized in that, in said immersion nozzle,
an inclination angle by which "portions contiguous with the slit"
of the cylinder bottom part head toward the cylinder side walls and
an inclination angle by which "portions contiguous with the slit"
of the discharge port bottom parts head toward discharge port side
walls are both at least 30.degree. upward.
8. A method of production of an ultralow carbon cast slab as set
forth in claim 5, characterized in that, in said immersion nozzle,
portions of the top parts of the discharge ports contiguous with
the side walls of the cylinder are formed by curved surfaces
smoothly contiguous with the side walls of the cylinder.
9. A method of production of an ultralow carbon cast slab as set
forth in claim 5, characterized in that said immersion nozzle is
provided with a rib connecting the two side surfaces of the
slit.
10. A method of production of an ultralow carbon cast slab as set
forth in claim 5, characterized in that, in said immersion nozzle,
the slit has an opening width of 0.15 to 0.40 of a square root of
the cross-sectional area of the discharge port openings.
11. A method of production of an ultralow carbon cast slab as set
forth in claim 1, characterized in that, in said immersion nozzle,
the side surfaces and bottom part of the cylinder and part or all
of the surfaces of the discharge ports and slit contiguous with the
melt are formed by a material of any of carbon-less spinel,
low-carbon spinel, magnesia graphite, zirconia graphite, and
silica-less alumina graphite.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method of producing an
ultralow carbon cast slab by continuous casting.
BACKGROUND ART
[0002] The dissolved oxygen in molten steel refined by a converter
or vacuum treatment container is generally removed by the
deoxidizing element Al. However, if deoxidizing ultralow carbon
steel with a large content of dissolved oxygen by Al, alumina
(Al.sub.2O.sub.3) will form. These agglomerate and merge to form
large amounts of coarse clusters of hundreds of microns size or
more.
[0003] Part of these alumina clusters enter the immersion nozzle
from the tundish at the time of continuous casting. If sticking at
the inner bore of the immersion nozzle, they cause nozzle clogging
and obstruct operation. Further, if the alumina clusters enter the
casting mold and remain at the surface layer of the cast slab, they
become causes of formation of surface flaws of thin-gauge steel
sheet and have a detrimental effect on quality.
[0004] As a countermeasure against this, in general there is the
method of blowing in Ar gas from a tundish upper nozzle, sliding
nozzle, or immersion nozzle to cause the Al.sub.2O.sub.3-based
inclusions to stick to the surfaces of bubbles, prevent them from
sticking at the inner bore of the immersion nozzle, and cause them
to float up in the casting mold for removal.
[0005] However, with this method, there were the problems that the
Ar bubbles blown in became the cause of pinhole defects in the cast
slab and that, further, the Ar bubbles floating up in the casting
mold disturbed the meniscus and caused powder entrainment, with the
entrained powder particles becoming the cause of surface flaws at
the thin-gauge steel sheet.
[0006] Further, when continuously casting molten steel, from the
viewpoint of ease of production, usually, as shown in FIG. 7, an
immersion nozzle 1 of a straight shape with a fixed inside diameter
from the top end to the bottom end of the inner bore 10 is
used.
[0007] However, in the case of an immersion nozzle with an inner
bore of a straight shape, as shown in FIG. 8, the opening part 12
of the sliding nozzle 11 is offset from the center of the immersion
nozzle 1, so when the molten steel in the tundish (not shown)
passes through the sliding nozzle 11 and flows into the immersion
nozzle 1, similarly, as shown in FIG. 8, an uneven distribution of
the molten steel flow rate inevitably occurs in the immersion
nozzle 1 (in the figure, see downward arrows at the center).
[0008] Due to this, there was the problem that uneven flows 13a,
13b with different flow rates occurred at the left and right
discharge ports, the state of fluid motion in the casting mold was
disturbed, and powder or bubbles were carried to deep positions in
the unsolidified part of the molten steel and remained in the cast
slab.
[0009] To solve these problems, two methods of solution have been
disclosed based on prior discoveries. The first is the method of
using an immersion nozzle having an orifice in the inner bore for
molten steel killed by Al.
[0010] This method has as its object the prevention of sticking of
alumina in the inner bore and suppression of uneven flow. For
example, Japanese Patent Publication (A) No. 2001-239351 discloses
an immersion nozzle having a plurality of step differences in its
inner bore. Further, Japanese Patent Publication (A) No.
2004-255407 discloses an immersion nozzle having, a plurality of
discontinuous projections in the inner bore.
[0011] Further, in these patent documents, if providing an orifice
in the inner bore (step differences or projections), the part in
the immersion nozzle where the molten steel flow rate becomes
remarkably slow is eliminated and the flow rate is made uniform. As
a result, it is disclosed, the effects of suppression of uneven
flow and prevention of sticking of alumina are obtained.
[0012] Further, Japanese Patent Publication (A) No. 2001-239351
discloses that an inert gas flow rate is suitably 1 Nl (normal
liter)/min to 40N1 (normal liter)/min. Note that below, "normal
liter" will sometimes be simply expressed as "Nl".
[0013] The second is the method of preventing the formation of
alumina clusters. For example, Japanese Patent Publication (A) No.
2002-88412, Japanese Patent Publication (A) No. 2003-49218,
Japanese Patent Publication (A) No. 2003-268435, Japanese Patent
Publication (A) No. 2005-60734, and Japanese Patent Publication (A)
No. 2005-139492 disclose the method of using Ti and rare earth
metals for deoxidation (hereinafter referred to as "deoxidation by
Ti-rare earth metals".
[0014] This method deoxidizes the molten steel by Ti to form Ti
oxides, then adds rare earth metals to change the Ti oxides to
spherical inclusions resistant to agglomeration and merger.
According to this method, it is possible to prevent sticking of
inclusions on the immersion nozzle, clogging of the immersion
nozzle, and formation of surface flaws due to alumina clusters.
[0015] Further, Japanese Patent Publication (A) No. 11-343516
discloses the method of adding one or both of Ca and a rare earth
metal after deoxidation by Ti and continuously casting without
blowing in Ar gas. This method as well is a method suppressing the
formation of cluster-like inclusions and making the inclusions
finely disperse. By this method, it is possible to obtained
titanium-killed steel excellent in surface properties.
DISCLOSURE OF THE INVENTION
[0016] First, the method disclosed in Japanese Patent Publication
(A) No. 2001-239351 and Japanese Patent Publication (A) No.
2004-255407, that is, the method of using an immersion nozzle
having an orifice in its inner bore to continuously cast Al-killed
molten steel will be explained.
[0017] In the technology disclosed in the above patent documents,
even if using an immersion nozzle having an orifice in its inner
bore, the effect of making the distribution of the flow rate
uniform is hard to obtain. This is because in the above art, there
is the problem of nozzle clogging.
[0018] This reason is that at the part below the bottom end of the
orifice, an eddy flow occurs and stirring is caused, so the alumina
will not stick, but at the part above the top end of the orifice,
no eddy flow occurs, so sticking of alumina cannot be avoided.
[0019] In particular, the top end of the orifice is the location
where sticking of alumina progresses the most. If a large amount of
alumina-based inclusions stick here, the nozzle will clog.
[0020] As disclosed in Japanese Patent Publication (A) No.
2001-239351, if blowing Ar gas into the molten steel, nozzle
clogging can be prevented. However, part of the Ar gas blown into
the molten steel fills the immersion nozzle and pushes the melt
surface in the immersion nozzle (secondary meniscus) down in
position.
[0021] The molten steel flowing in from the tundish to the
immersion nozzle drops freely from the position of the sliding
nozzle to the secondary meniscus, but if the secondary meniscus is
pushed down, the dropping distance of the molten steel will become
longer, so a strong downward flow will easily occur right below the
dropping position of the molten steel.
[0022] In some cases, as a reaction to this, as shown in FIG. 9, a
reverse rising flow 14 (in the figure, see dotted line arrows)
occurs whereby an uneven distribution of the molten steel flow rate
occurs in the immersion nozzle (in the figure, see solid line
arrows).
[0023] The uneven distribution of the molten steel flow rate
occurring due to the dropping flow of the molten steel is eased if
making the secondary meniscus close to the position of the sliding
nozzle. However, to obtain the effect of prevention of sticking of
alumina, at least a predetermined level of Ar gas flow rate is
required. Normally, the Ar gas flow rate is 5 to 20 N1/min, but
with this Ar gas flow rate, it is difficult to make the secondary
meniscus close to the position of the sliding nozzle.
[0024] If the secondary meniscus is low and the distance to the top
end of the orifice (in the figure, see "21a") is short, there is
the possibility that the molten steel will not stop once at the
part above the orifice but will pass through the orifice with the
distribution of the flow rate still not made uniform. Accordingly,
in the state of a low secondary meniscus, it is difficult to
suppress uneven flow caused by a dropping flow by just the
orifice.
[0025] Next, the method disclosed in Japanese Patent Publication
(A) No. 2002-88412, Japanese Patent Publication (A) No. 2003-49218,
Japanese Patent Publication (A) No. 2003-268435, Japanese Patent
Publication (A) No. 2005-60734, Japanese Patent Publication (A) No.
2005-139492, and Japanese Patent Publication (A) No. 11-343516,
that is, the method of continuously casting molten steel not
forming any alumina clusters will be explained.
[0026] In the art disclosed in the above patent documents, the
inclusions are resistant to agglomeration and merger, so no coarse
clusters are formed and the nozzle is resistant to clogging.
However, in the above patent documents, the shape of the inner bore
of the immersion nozzle is not defined. No technical matter
relating to the secondary meniscus is described.
[0027] In the above art, no means is devised for suppressing uneven
distribution of the flow rate and uneven flow, so there is a high
possibility of powder or bubbles being carried to deep positions of
the unsolidified part of the molten steel and of powder or bubbles
remaining in the cast slab becoming a cause of surface flaws formed
when worked to a thin-gauge steel sheet.
[0028] In this way, in the prior art, there was the problem that it
was difficult to achieve both prevention of nozzle clogging and
securing of the cast slab quality. Note that the "securing of the
cast slab quality" here means stably producing a cast slab free of
surface flaws even if worked to a thin-gauge steel sheet.
[0029] The present invention, in consideration of these problems,
has as its object the provision of a method of production of an
ultralow carbon cast slab enabling achievement of both efficiency
of continuous casting and cast slab quality.
[0030] The inventors engaged in repeated research to solve the
above-mentioned problems and as a result discovered that if
deoxidizing ultralow carbon molten steel by Ti-rare earth metals
(Ce, La, Nd) and using an immersion nozzle having an orifice in its
inner bore, it is possible to prevent nozzle clogging and
continuously cast an ultralow carbon cast steel free of surface
flaws even if worked to a thin-gauge steel sheet.
[0031] The present invention was made based on the above discovery
and has as its gist the following constitutions:
[0032] (1) A method of production of an ultralow carbon cast slab
characterized by adding Ti to molten steel decarburized to a carbon
concentration of 0.01 mass % or less, further adding at least one
of Ce, La, and Nd, and using an immersion nozzle to inject the
above molten steel from a tundish to a casting mold for continuous
casting while maintaining a flow rate of Ar gas blown from any
location in a range from a tundish upper nozzle to discharge ports
of said immersion nozzle at 3 Nl (normal liter)/min or less.
[0033] (2) A method of production of an ultralow carbon cast slab
as set forth in (1), characterized in that said immersion nozzle
has an orifice in its inner bore.
[0034] (3) A method of production of an ultralow carbon cast slab
as set forth in (2), characterized in that [0035] said inner bore
has a circular cross-sectional shape and [0036] (i) a relation
3.ltoreq.R-r.ltoreq.30 stands between a radius R of a top end of
the inner bore [mm] and a smallest radius r of the orifice [mm] and
[0037] (ii) a length L from a top end to a bottom end of the
orifice [mm] is 50.ltoreq.L.ltoreq.150.
[0038] (4) A method of production of an ultralow carbon cast slab
as set forth in (2), characterized in that [0039] said inner bore
has an elliptical cross-sectional shape and [0040] (i) a relation
3.ltoreq.A-a.ltoreq.30 stands between a radius A of a long-axis
direction of a top end of the inner bore [mm] and a smallest radius
a of the orifice [mm] and [0041] (ii) a length L from a top end to
a bottom end of the orifice [mm] is 50.ltoreq.L.ltoreq.150.
[0042] (5) A method of production of an ultralow carbon cast slab
as set forth in (1) or (2), characterized in that [0043] said
immersion nozzle has a closed bottom cylindrical shape and [0044]
(i) two discharge ports are arranged at axially symmetric positions
to a cylinder at a bottom part of side walls of the cylindrical
shape and [0045] (ii) a slit is provided connecting a cylinder
bottom part and bottom parts of the two discharge ports and opening
to the outside.
[0046] (6) A method of production of an ultralow carbon cast slab
as set forth in (5), characterized in that, in said immersion
nozzle, [0047] (i) portions contiguous with the slit of the
cylinder bottom part are inclined upward toward the cylinder side
walls [0048] (ii) portions contiguous with the slit of discharge
port bottom parts are inclined upward toward discharge port side
walls, and [0049] (iii) there is substantially no step difference
between the surface forming the cylinder bottom part and the
surfaces forming the discharge port bottom parts.
[0050] (7) A method of production of an ultralow carbon cast slab
as set forth in (6), characterized in that, in said immersion
nozzle, an inclination angle by which "portions contiguous with the
slit" of the cylinder bottom part head toward the cylinder side
walls and an inclination angle by which "portions contiguous with
the slit" of the discharge port bottom parts head toward discharge
port side walls are both at least 30.degree. upward.
[0051] (8) A method of production of an ultralow carbon cast slab
as set forth in any one of (5) to (7), characterized in that, in
said immersion nozzle, portions of the top parts of the discharge
ports contiguous with the side walls of the cylinder are formed by
curved surfaces smoothly contiguous with the side walls of the
cylinder.
[0052] (9) A method of production of an ultralow carbon cast slab
as set forth in any one of (5) to (8), characterized in that said
immersion nozzle is provided with a rib connecting the two side
surfaces of the slit.
[0053] (10) A method of production of an ultralow carbon cast slab
as set forth in any one of (5) to (9), characterized in that, in
said immersion nozzle, the slit has an opening width of 0.15 to
0.40 of a square root of the cross-sectional area of the discharge
port openings.
[0054] (11) A method of production of an ultralow carbon cast slab
as set forth in any one of (1) to (10), characterized in that, in
said immersion nozzle, the side surfaces and bottom part of the
cylinder and part or all of the surfaces of the discharge ports and
slit contiguous with the melt are formed by a material of any of
carbon-less spinel, low-carbon spinel, magnesia graphite, zirconia
graphite, and silica-less alumina graphite.
[0055] According to the present invention, in continuous casting of
ultralow carbon steel, it is possible to prevent nozzle clogging
and produce an ultralow carbon cast slab free of surface flaws even
if worked to a thin-gauge steel sheet.
BRIEF DESCRIPTION OF THE DRAWINGS
[0056] FIG. 1 is a view showing an immersion nozzle having an
orifice used in the present invention.
[0057] FIG. 2 is a view showing an immersion nozzle with a circular
cross-sectional shape and having an orifice in its inner bore.
[0058] FIG. 3 is a view showing an immersion nozzle with an
elliptical cross-sectional shape and having an orifice in its inner
bore.
[0059] FIG. 4 is a view for explaining a preferable set position of
an orifice.
[0060] FIG. 5 is a view showing an immersion nozzle with a circular
cross-sectional shape and having two orifices.
[0061] FIG. 6 is a view showing an immersion nozzle with an
elliptical cross-sectional shape and having two orifices.
[0062] FIG. 7 is a view showing a generally used immersion nozzle
with a straight shape with an inside diameter fixed from the top
end to the bottom end of the inner bore.
[0063] FIG. 8 is a view showing an uneven distribution of the
molten steel flow rate occurring when using the immersion nozzle
shown in FIG. 7.
[0064] FIG. 9 is a view showing an uneven distribution of the
molten steel flow rate and a state of a reverse rising flow
occurring due to a strong dropping flow.
[0065] FIG. 10 is a view showing the relationship between an Ar gas
flow rate and pinhole defects occurring in a cast slab.
[0066] FIG. 11 is a view showing the relationship between an Ar gas
flow rate and surface flaws occurring in a steel sheet.
[0067] FIG. 12 gives views showing an immersion nozzle, used in the
present invention, having a closed bottom cylindrical shape, having
two discharge ports at axially symmetric positions to the cylinder
at the bottom of the side walls of the cylindrical shape, and
provided with a slit connecting the bottom of the cylinder and the
bottoms of the two discharge ports and opening to the outside. (a)
is a view showing an A-A cross-section. (b) is a view showing a B-B
cross-section. (c) is a view showing a C-C cross-section.
[0068] FIG. 13 gives views showing part of an immersion nozzle. (a)
is a view showing an A-A cross-section. (b) is a view showing a B-B
cross-section. (c) is a view showing a C-C cross-section. (d) is a
view showing a D-D cross-section.
[0069] FIG. 14 gives views showing part of another immersion
nozzle. (a) is a view showing an A-A cross-section. (b) is a view
showing a B-B cross-section. (c) is a view showing a C-C
cross-section. (d) is a view showing a D-D cross-section.
[0070] FIG. 15 gives views showing part of another immersion
nozzle. (a) is a view showing an A-A cross-section. (b) is a view
showing a B-B cross-section. (c) is a view showing a C-C
cross-section.
[0071] FIG. 16 gives views showing part of another immersion
nozzle. (a) is a view showing an A-A cross-section. (b) is a view
showing a B-B cross-section. (c) is a view showing a C-C
cross-section.
[0072] FIG. 17 gives views showing part of another immersion
nozzle. (a) is a view showing an A-A cross-section. (b) is a view
showing a B-B cross-section. (c) is a view showing a C-C
cross-section. (d) is a view showing a D-D cross-section.
[0073] FIG. 18 gives views another immersion nozzle used in the
present invention. (a) is a view showing an A-A cross-section. (b)
is a view showing a B-B cross-section. (c) is a view showing a C-C
cross-section.
[0074] FIG. 19 gives views showing part of another immersion
nozzle. (a) is a view showing an A-A cross-section. (b) is a view
showing a B-B cross-section. (c) is a view showing a C-C
cross-section. (d) is a view showing a D-D cross-section.
[0075] FIG. 20 gives views showing part of another immersion
nozzle. (a) is a view showing an A-A cross-section. (b) is a view
showing a B-B cross-section. (c) is a view showing a C-C
cross-section. (d) is a view showing a D-D cross-section.
BEST MODE FOR CARRYING OUT THE INVENTION
[0076] To solve the problems in the prior art, the following three
conditions are major requirements:
[0077] Condition 1: Prevention of agglomeration and merger of
inclusions to prevent formation of clusters.
[0078] Condition 2: Making position of secondary meniscus close to
sliding nozzle as much as possible.
[0079] Condition 3: Using an immersion nozzle having an orifice in
its inner bore and/or an immersion nozzle having a slit at its
bottom.
[0080] First, as explained in the section of BACKGROUND ART, coarse
clusters become causes of nozzle clogging and surface flaws in
thin-gauge steel sheet, so Condition 1 has as its object the
prevention of formation of clusters.
[0081] Next, Condition 2 has as its object the suppression of
uneven distribution of the flow rate occurring due to a dropping
flow. The closer the position of the secondary meniscus to the
position of the sliding nozzle, the shorter the dropping distance
of the molten steel, so the more difficult the formation of a
strong downward flow. The uneven distribution of the flow rate due
to the dropping flow is substantially suppressed.
[0082] In the present invention, deoxidization by Ti-rare earth
metals (Ce, La, Nd) enables the prevention of formation of clusters
and prevention of occurrence of nozzle clogging at the casting
stage.
[0083] For this reason, it is possible to greatly reduce the Ar gas
flow rate from the Ar gas flow rate at the time of casting
Al-killed molten steel. As a result, the secondary meniscus can be
made close to the position of the sliding nozzle. Further, at the
time of casting, if using an immersion nozzle having an orifice in
its inner bore, the molten steel flow rate will be made even and
uneven flow will be suppressed.
[0084] Condition 3 relating to the immersion nozzle used has as its
object the elimination of the uneven distribution of the flow rate
occurring due to the sliding nozzle.
[0085] If setting the orifice in the inner bore of the immersion
nozzle, the molten steel will stop once at the part above the
orifice, so during this time, the uneven distribution of the flow
rate will be eliminated.
[0086] If using an immersion nozzle having an orifice in its inner
bore and having a slit at its bottom part, the uneven distribution
of the flow rate will be eliminated, so powder or bubbles will not
be carried into deep positions of the unsolidified part of the
molten steel. As a result, the occurrence of surface flaws of the
steel sheet due to powder or bubbles remaining in the cast slab
(occurring when working the cast slab into steel sheet) can be
prevented.
[0087] When Condition 2 and Condition 3 are simultaneously
satisfied, the molten steel remains for a relatively long time
between the secondary meniscus and the top end of the orifice, so
the effect of rectification of the flow by the orifice is improved
more.
[0088] Note that the presence of occurrence of an uneven flow is
usually evaluated using the difference in the coefficient of heat
transfer at the two short sides of the casting mold. The
coefficient of heat transfer is an indicator showing the ease of
transfer of heat through the wall surfaces. If the difference in
the coefficient of heat transfer of the two short sides of the
casting mold is 250 J/(m.sup.2sK) or more, "asymmetric fluid motion
of the molten steel", that is, "uneven flow", occurs as learned
from experience.
[0089] The method of calculation of the coefficient of heat
transfer is shown in formula (1) and formula (2). The coefficient
of heat transfer h [J/(m.sup.2sK)] is calculated from the amount of
heat removal q of the casting mold [J/(m.sup.2s)] and the
temperature difference (T.infin.-TMD) between the molten steel and
the casting mold surface [K]. The amount of heat removal of the
casting mold is calculated from the change in temperature
(tout-tin) before the casting mold cooling water passes over the
copper plates and after it passes over it [K], the cooling water
flow rate Qw [kg/s], the specific heat Cw of water [J/(kgK)], and
the copper plate surface area S [m.sup.2].
h=q/(T.infin.-TMD) (1)
q=QwCw(tout-tin)/S (2)
[0090] Below, the present invention will be explained in
detail.
[0091] The present invention covers ultralow carbon steel. The
upper limit value of the carbon concentration is not particularly
limited, but thin-gauge steel sheet of ultralow carbon steel is
used for steel sheet for automobiles etc. subjected to severe
working, so has to be provided with superior workability. For this
reason, the carbon concentration is preferably 0.01 mass % or less.
Note that the lower limit value of the carbon concentration is not
particularly defined.
[0092] In the present invention, in the secondary refining, the
steel is decarburized to a carbon concentration of 0.01 mass % or
less, then Ti is added to the molten steel for deoxidation. The
amount of addition of the Ti is preferably 0.04 mass % or more. If
less than 0.04 mass %, the deoxidation does not sufficiently occur
and there is a high possibility of dissolved oxygen remaining in
the molten steel.
[0093] In secondary refining, when decarburizing steel to a carbon
concentration of 0.01 mass % or less, it is also possible to
deoxidize the steel by Al preliminarily before the decarburization
in the refining apparatus, for example, at the stage of refining by
the converter. In this case, the Al concentration after deoxidation
is made 0.01 mass % or less, preferably 0.008 mass % or less, more
preferably 0.006 mass % or less.
[0094] If the Al concentration after deoxidation is 0.01 mass % or
less, the deoxidation product, that is, alumina, floats up to the
surface of the molten steel in the interval until continuously
casting the molten steel and can be removed, so the amount of
alumina remaining in the molten steel during casting becomes
smaller and the problems of nozzle clogging etc. do not occur.
[0095] Further, if the Al concentration after deoxidation is 0.008
mass % or less, the amount of alumina remaining in the molten steel
during casting becomes smaller, so this is preferable. Further, if
the Al concentration after deoxidation is 0.006 mass % or less, the
amount of alumina remaining in the molten steel during casting
becomes even smaller, so this is more preferable.
[0096] On the other hand, the upper limit of the amount of addition
of Ti is not particularly defined. The Ti oxides produced by
addition of Ti are harder to aggregate and merge than
Al.sub.2O.sub.3-based inclusions, but easily stick to the
refractory, so nozzle clogging is a concern.
[0097] Therefore, after deoxidizing the molten steel by Ti, at
least one of Ce, La, and Nd is added. By this addition, the Ti
oxides become hard to agglomerate and merge and are converted to
spherical inclusions hard to stick to the refractory.
[0098] The total amount of addition of Ce, La, and Nd is preferably
0.001 mass % to 0.01 mass %. If the above total amount of addition
is less than 0.001 mass %, the modification of the Ti oxides
becomes insufficient and spherical inclusions hard to agglomerate
and merge become hard to form. Further, if over 0.01 mass %, the
modification of the Ti oxide becomes excessive, the Ti-based
inclusions become heavier in specific gravity and become harder to
float up, and the cleanliness of the molten steel deteriorates.
[0099] Rare earth metals other than Ce, La, and Nd (for example,
Pr, Sm, etc.) do not have effects of modification equal to those of
Ce, La, and Nd, so for modification of the Ti oxides, addition of
one or more of Ce, La, and Nd is effective.
[0100] Here, FIG. 1 shows an immersion nozzle having an orifice
used in the present invention. In the present invention, the
portion of the inner bore 10 smaller in inside diameter than the
top end of the inner bore is defined as the "orifice 21", while any
portion with an inside diameter equal to the top end of the inner
bore or with an inside diameter larger than the same is defined as
a "non-orifice part 21z". At the interfaces of the orifice 21 and
the non-orifice parts 21z, the boundary at the upstream side is
referred to as the "orifice top end 21a" and the boundary at the
downstream side is referred to as the "orifice bottom end 21b".
[0101] As explained above, in the present invention, the Ar gas
flow rate can be greatly reduced from the time of the conventional
Al deoxidation. If reducing the Ar gas flow rate, the secondary
meniscus rises. When reaching 3 Nl/min or less, the secondary
meniscus rises to a position of about 100 to 120 mm from the
sliding nozzle.
[0102] If the secondary meniscus rises to the above position,
almost no strong downward flow occurs and the distance to the
orifice top end can be sufficiently secured, so it is possible to
reliably eliminate uneven distribution of the molten steel flow
rate. This fact was discovered by the inventors. Note that as the
value of the Ar gas flow rate in the present invention, it is
possible to use a value measured using a commercially available
flowmeter.
[0103] The smaller the Ar gas flow rate, the more the secondary
meniscus rises, so the Ar gas flow rate is preferably 2 Nl/min or
less, more preferably less than 1 Nl/min.
[0104] If the Ti oxides are all suitably modified, almost no
sticking of Ti oxides to the immersion nozzle will occur, so the
lower limit value of the Ar gas flow rate also includes 0
Nl/min.
[0105] Ar gas is generally blown in from one or more of a tundish
upper nozzle, sliding nozzle, or immersion nozzle, but if in the
range from the tundish upper nozzle to immersion nozzle discharge
ports, the positions of the locations where the Ar gas is blown in
and the number of those locations may be freely selected.
[0106] In the immersion nozzle used in the present invention, to
more remarkably secure the effect of suppression of an uneven flow,
there is a preferable range of orifice size.
[0107] As the cross-sectional shape of the immersion nozzle,
normally a circular or elliptical shape is used. FIG. 2 shows an
immersion nozzle with a circular cross-sectional shape, while FIG.
3 shows an immersion nozzle with an elliptical cross-sectional
shape.
[0108] When the cross-sectional shape is circular, the radius of
the top end of the inner bore is defined as R [mm] and the smallest
radius of the orifice is defined as r [mm]. On the other hand, when
the cross-sectional shape is elliptical, the radius of the top end
of the inner bore in the long axis direction is defined as A [mm]
and the smallest radius of the orifice in the long axis direction
is defined as a [mm].
[0109] Here, for the orifice, the smallest radius is used because
in the present invention, an orifice is defined as a "portion
smaller in inside diameter than the top end of the inner bore".
[0110] As the shape of the orifice, even a shape with an inside
diameter not fixed from the top end to the bottom end of the
orifice may be envisioned, so the smallest radius was used to
define it so that the invention can be applied even to such a
shape.
[0111] Next, the difference between the radius of the non-orifice
parts and the smallest radius of the orifice is defined as the
"height of the orifice". Normally, the radius of the non-orifice
parts is equal to the radius of the top end of the inner bore, so
the "height of the orifice" can be alternately referred to for the
difference between the radius of the top end of the inner bore and
the smallest radius of the orifice.
[0112] This being so, the "height of the orifice" is expressed by
"R-r" when the immersion nozzle has a circular cross-sectional
shape and "A-a" when it has an elliptical one.
[0113] Further, the distance from the top end to the bottom end of
the orifice is defined as the "length of the orifice" and labeled L
[mm].
[0114] When the immersion nozzle has a circular cross-sectional
shape, the "height of the orifice" preferably satisfies the
relation "3.ltoreq.R-r.ltoreq.30". The range of R-r<3 results in
a small effect of the orifice in making the flow rate uniform and
difficulty in suppressing the uneven flow, while the range of
R-r>30 results in a remarkably larger flow rate of the molten
steel passing through the orifice and the fluid motion in the
casting mold is easily detrimentally affected.
[0115] Next, the length L of the orifice [mm] preferably satisfies
the relation 50.ltoreq.L.ltoreq.150. With a range of L<50,
before the flow rate is made uniform, the molten steel ends up
passing through the orifice, so suppression of an uneven flow is
difficult. Further, with a range of L>150, the part of the small
inside diameter becomes longer, so the flow rate of the molten
steel becomes remarkably greater and the fluid motion in the
casting mold is easily detrimentally affected.
[0116] When the immersion nozzle has an elliptical cross-sectional
shape, the "height of the orifice" preferably satisfies the
relation "3.ltoreq.A-a.ltoreq.30", while the length L of the
orifice [mm] preferably satisfies the relation
"50.ltoreq.L.ltoreq.150". The reason is similar to the case where
the cross-sectional shape is circular.
[0117] In the immersion nozzle used in the present invention, the
position of the orifice is not particularly limited. However, as
shown in FIG. 4, if the top end U of the orifice is below the mid
point M of the top end T of the inner bore and the top ends B of
the discharge ports, the molten steel can be made to reliably stop
and the uneven distribution of the flow rate can be easily
eliminated, so this is preferred.
[0118] Further, the number of the orifices is preferably a
plurality rather than one so the flow rectification effect becomes
greater. However, if the number of orifices becomes greater, the
parts where the flow rate of the molten steel is large will
increase, so one or two is preferable.
[0119] FIG. 5 and FIG. 6 show immersion nozzles with two
orifices.
[0120] When providing a plurality of orifices in an inner bore with
a circular cross-sectional shape (see FIG. 5), the r and L of each
i-th orifice from the top end of the inner bore (respectively
labeled as "ri" and "Li") preferably satisfy the conditions
3.ltoreq.R-ri.ltoreq.30 and 50.ltoreq.Li.ltoreq.150.
[0121] Further, the top end of at least one of the orifices is
preferably below the mid point of the top end of the inner bore and
the top ends of the discharge ports.
[0122] When providing a plurality of orifices in an inner bore with
an elliptical cross-sectional shape (see FIG. 6), the a and L of
each i-th orifice from the top end of the inner bore (respectively
labeled as "ai" and "Li") preferably satisfy the conditions
3.ltoreq.A-ai.ltoreq.10 and 50.ltoreq.Li.ltoreq.150.
[0123] Further, the top end of at least one of the orifices is
preferably below the mid point of the top end of the inner bore and
the top ends of the discharge ports.
[0124] Here, other embodiments of the immersion nozzle used in the
present invention will be explained.
[0125] FIG. 12 shows an embodiment of an immersion nozzle of
another embodiment. The immersion nozzle 1 shown in FIG. 12 is a
closed bottom cylindrical shape immersion nozzle. At the bottom
part of the cylindrical side walls 5, two discharge ports 2 formed
by discharge port side walls 7 and discharge port top parts 8 are
formed symmetrically with respect to the cylindrical axis, while at
the cylinder bottom part 4 and the bottom parts 6 of the discharge
ports 2, a slit 3 formed by slit side walls 9 and opening to the
outside is provided.
[0126] If providing a slit in an immersion nozzle, the discharge
flow of the molten steel into the casting mold is more uniformly
dispersed, the uneven flow is eliminated more, and further
entrainment of powder is more stably prevented, so this is more
preferable. Here, there is a suitable relation between the opening
width Ws of the slit 3 and the square root of the cross-sectional
area Sz of the opening part 2z of a discharge port 2 due to the
reasons explained below.
[0127] First, if the slit opening width Ws/ (cross-sectional area
Sz of opening of discharge port) is over 0.4 and the slit 3 becomes
larger than the discharge port 2, the flow rate of the molten steel
passing through the slit 3 increases. Bubbles, inclusions, etc. in
the molten steel are carried to deep positions of the unsolidified
parts of the molten steel, remain in the casting mold, and become
causes of surface flaws at the time of working into a thin-gauge
steel sheet.
[0128] On the other hand, if the slit opening width Ws/
(cross-sectional area Sz of opening of discharge port) is less than
0.1, sticking of inclusions at the slit side walls 9, abrasion of
the slit side walls 9, etc. sometimes occur.
[0129] Due to the above reasons, the slit opening width Ws/
(cross-sectional area Sz of opening of discharge port) is suitably
0.15 to 0.4.
[0130] FIG. 13 shows an embodiment of a bottom part of the
immersion nozzle shown in FIG. 12. In the immersion nozzle shown in
FIG. 13, the portions of the cylinder bottom part 4 contiguous with
the slit 3 are inclined toward the cylindrical side walls 5 by an
inclination angle .theta..sub.1, while the portions of the bottom
parts 6 of the discharge ports contiguous with the slit 3 are
inclined toward the side walls 7 of the discharge ports by an
inclination angle .theta..sub.2.
[0131] The inclination angles (.theta..sub.1 and .theta..sub.2) are
preferably 30 to 60.degree.. If the inclination angles are less
than 30.degree., an eddy is formed in the immersion nozzle in some
cases. If the inclination angles are over 60.degree., the top parts
of the discharge ports approach the meniscus in the casting mold
and the discharge flow easily entrains powder. Note that the
inclination angles are preferably 30.degree. or more.
[0132] The inclination angle .theta..sub.1 of the cylinder bottom
part 4 and the inclination angle .theta..sub.2 of the discharge
port bottom parts 6 preferably match, but they do not necessarily
have to match. The difference in angle when the inclination angle
.theta..sub.1 and the inclination angle .theta..sub.2 do not match
is preferably 10.degree. or less. Further, if the surface of the
cylinder bottom part 4 and the surfaces of the discharge port
bottom parts 6 are formed to be on the same plane, the structure of
the immersion nozzle becomes simpler so this is preferable.
[0133] On the other hand, as shown in FIG. 14, the surface of the
cylinder bottom part 4 and the surfaces of the discharge port
bottom parts 6 may be connected at the required angle if providing
a step difference at the connecting surfaces. As shown in FIG. 14,
if forming the discharge port bottom parts 6 in directions
descending toward the outer circumference of the immersion nozzle,
it is possible to adjust the directions of the discharge flows of
the discharge ports 2 along with the inclinations of the discharge
port top parts 8.
[0134] Regarding the step difference between the surface of the
cylinder bottom part 4 and the surfaces of the discharge port
bottom parts 6, substantially no step difference is enough. There
is no need to completely eliminate the step differences. Here,
"substantially no step difference" means the step difference is a
step difference of an extent whereby the continuity of the downward
flow in the immersion nozzle 1 and the discharge flows from the
discharge ports 2 is not impaired. Specifically, the step
difference should be of an extent of 5 mm or less.
[0135] As shown in FIG. 15, even if there is a slight step
difference 12 between the surface of the cylinder bottom part 4 and
the surfaces of the discharge port bottom parts 6, the effect of
the present invention is not impaired.
[0136] The discharge ports 2 are preferably shaped, as shown in
FIG. 12 to FIG. 15, like baseball home plates, but as shown in FIG.
16 may also be arc shapes or curved shapes. The surface of the
cylinder bottom part 4 may also be curved. In this case, the
inclination angle .theta..sub.1 of the cylinder bottom part 4 and
the inclination angle .theta..sub.2 of the discharge port bottom
parts 6 should be made the average inclination angle of the
vicinity where the cylinder bottom part 4 or discharge port bottom
parts 6 contact the slit.
[0137] The portions of the discharge port top parts 8 contacting
the cylinder side walls 5, as shown in FIG. 17, are preferably
formed by curved surfaces 11 smoothly contiguous with the cylinder
side walls 5. By forming the above portions by curved surfaces, the
discharge flow separating from the discharge port top parts 2 can
be prevented and entrainment of powder into the discharge ports 2
can be suppressed.
[0138] The radius of curvature Rz of the curved surface 11 smoothly
connecting the discharge port top parts 8 and the cylinder side
walls 5, as shown FIG. 17, is determined by the thickness t of the
cylinder side walls 5 and the discharge angle .phi.. If making
.phi. small, a problem occurs in the strength of the material
forming the immersion nozzle, but Rz can be made larger, so this is
advantageous for prevention of divergence of the discharge flow.
For example, in the case where .phi.=45' or so, the radius of
curvature Rz is preferably 50 to 100 mm.
[0139] Further, as the immersion nozzle used in the present
invention, as shown in FIG. 18, one having an orifice 21 with an
opening cross-sectional area smaller than the opening
cross-sectional area of the cylinder between the top end of the
nozzle and the discharge ports 2 is preferable.
[0140] In an immersion nozzle having a slit at the cylinder bottom
part, if the flow rate of the melt increases, the force of the flow
of melt pushing the slit wider increases. For this reason, as shown
in FIG. 19, it is preferable to provide the slit 3 with ribs 22
connecting the slit side walls 9. By providing the ribs 22, even if
the force pushing the slit 3 wider increases, deformation or
breakage of the immersion nozzle 1 can be prevented.
[0141] As the refractory used for the immersion nozzle, alumina
graphite, alumina spinel, or another conventionally used refractory
may be used.
[0142] However, depending on the ingredients of the molten steel,
an alumina graphite immersion nozzle will sometimes be corroded and
dissolve during casting, so it is preferable to make the cylinder
side walls and cylinder bottom part of the immersion nozzle or part
or all of the surfaces of the discharge ports and slit contiguous
with the molten steel out of any of carbon-less spinel, low-carbon
spinel, magnesia graphite, zirconia graphite, or silica-less
alumina graphite (high melt loss resistance refractories).
[0143] FIG. 20 shows an immersion nozzle where the cylinder side
walls 5 and cylinder bottom part 6 and all of the surfaces of the
discharge ports 2 and slit 3 contiguous with the molten steel are
made of a high melt loss resistance refractory 23 and the other
parts are made of an ordinary refractory 24.
[0144] In the present invention, there is extremely little sticking
of nonmetallic inclusions to the immersion nozzle and no nozzle
clogging, so the surface of the cast slab is resistant to formation
of surface defects due to cluster-like inclusions.
[0145] Further, the cast slab obtained by the present invention is
free of surface flaws even if made into thin-gauge steel sheet by
hot rolling, cold rolling, or other ordinary methods since the
penetration of bubbles and powder causing surface flaws is
suppressed at the time of production.
[0146] Further, the present invention exhibits similar effects not
only when applied to continuous casting of a slab of a usual
thickness of 250 mm, but also in continuous casting of a thin slab
with a casting mold thickness thinner than this, for example, 150
mm, so a cast slab of an extremely good quality can be
obtained.
Examples 1
[0147] Below, examples and comparative examples (see Table 1) will
be given to explain the present invention using immersion nozzles
of various shapes (see FIGS. 1 to 6).
TABLE-US-00001 TABLE 1 Max. difference of coefficient of heat Ar
flow transfer at casting Pinhole Deoxidizing rate Immersion nozzle
Nozzle mold short sides uneven defects Surface flaws Item agent
(Nl/min) inner bore sticking (J/(m.sup.2 s K) flow (/m.sup.2)
(/coil) Ex. 1-1 Ti and Ce--La 0 Circular cross- None 200 None None
0.2 alloy section straight Inside range of edge trim, so no problem
Ex. 1-2 Ti and Ce--La 2.8 Circular cross- None 150 None None None
alloy section with orifice Ex. 1-3 Ti and Ce--La--Nd 0.5 Circular
cross- None 100 None None None alloy section with orifice Ex. 1-4
Ti and Ce--La 0 Circular cross- None 50 None None None alloy
section with orifice Ex. 1-5 Ti and Ce--La--Nd 0.5 Elliptical
cross- None 100 None None None alloy section with orifice Ex. 1-6
Ti and Ce--La 0.5 Circular cross- None 100 None None None alloy
section with orifice Ex. 1-7 Ti, Al, and 2.8 Circular cross- None
150 None None None Ce--La--Nd alloy section with orifice Comp. Al 7
Circular cross- Yes 300 Yes 15 10 Ex. 1-1 section straight Comp. Al
2 Circular cross- Yes 330 Yes None 10 Ex. 1-2 section with orifice
Comp. Ti and Ce--La 7 Circular cross- None 300 Yes 15 3 Ex. 1-3
alloy section straight Comp. Ti and Ce--La 4.5 Circular cross- None
280 Yes 10 3 Ex. 1-4 alloy section straight Comp. Ti and Ce--La 3.5
Circular cross- None 260 Yes 5 1 Ex. 1-5 alloy section with
orifice
Example 1-1
[0148] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in a converter and treatment
by a vacuum degassing apparatus, Ti was added for deoxidation. The
result was refluxed for 6 minutes, then Ce and La were added by a
Ce--La alloy of a mass ratio Ce/La=1.3. The result was refluxed for
3 minutes to produce molten steel having a Ti concentration of 0.03
mass %, a total concentration of Ce and La of 0.01 mass %, and a Ce
concentration/La concentration of 1.3.
[0149] This molten steel was cast by the continuous casting method
without Ar (flow rate 0 Nl/min) to obtain a cast slab of a
thickness of 250 mm and a width of 1600 mm.
[0150] The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The material of the inner bore was alumina graphite.
[0151] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, and the inner bore was
straight in shape.
[0152] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum
200 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was under the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0153] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0154] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0155] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0156] As a result, the cast slab did not have any pinhole defects.
Further, the surface of the cold rolled steel sheet had 0.2 surface
flaw/coil. However, the surface flaws occurred at positions in the
range of the edge trim, so did not become a problem in product
quality.
Example 1-2
[0157] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Ti was added for
deoxidation. The result was refluxed for 6 minutes, then Ce and La
were added by a Ce--La alloy of a mass ratio Ce/La=1.3. The result
was refluxed for 3 minutes to produce molten steel having a Ti
concentration of 0.03 mass %, a total concentration of Ce and La of
0.01 mass %, and a Ce concentration/La concentration of 1.3.
[0158] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
2.8 Nl/min to obtain a cast slab of a thickness of 250 mm and a
width of 1600 mm.
[0159] The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The material of the inner bore was alumina graphite.
[0160] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, while the inner bore had a
step difference of a height: R-r=5 mm, length: L=90 mm (orifice).
The top end of this step difference was at a position 400 mm from
the top end of the inner bore.
[0161] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum
150 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was under the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0162] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0163] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0164] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0165] As a result, there were neither pinhole defects in the cast
slab nor surface flaws on the steel sheet.
Example 1-3
[0166] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Ti was added for
deoxidation. The result was refluxed for 6 minutes; then Ce, La,
and Nd were added by a Ce--La--Nd alloy of a mass ratio Ce/La=1.3
and La/Nd=3.5. The result was refluxed for 3 minutes to produce
molten steel having a Ti concentration of 0.03 mass %, a total
concentration of Ce, La, and Nd of 0.01 mass %, and a Ld
concentration/Nd concentration of 3.5.
[0167] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
0.5 Nl/min to obtain a cast slab of a thickness of 250 mm and a
width of 1600 mm.
[0168] The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 mm and an inside diameter of
85 mm. The material of the inner bore was alumina graphite.
[0169] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, while the inner bore had a
step difference of a height: R-r=5 mm, length: L=90 mm (orifice).
The top end of this step difference was at a position 400 mm from
the top end of the inner bore.
[0170] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum
100 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was under the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0171] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0172] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0173] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0174] As a result, there were neither pinhole defects in the cast
slab nor surface flaws on the steel sheet.
Example 1-4
[0175] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Ti was added for
deoxidation. The result was refluxed for 6 minutes, then Ce and La
were added by a Ce--La alloy of a mass ratio Ce/La=1.3. The result
was refluxed for 3 minutes to produce molten steel having a Ti
concentration of 0.03 mass %, a total concentration of Ce and La of
0.01 mass %, and a Ce concentration/La concentration of 1.3.
[0176] This molten steel was cast by the continuous casting method
with no Ar (flow rate 0 Nl/min) to obtain a cast slab of a
thickness of 250 mm and a width of 1600 mm.
[0177] The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 mm and inside diameter of 85
mm. The material of the inner bore was alumina graphite.
[0178] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, while the inner bore had a
step difference of a height: R-r=5 mm, length: L=90 mm (orifice).
The top end of this step difference was at a position 400 mm from
the top end of the inner bore.
[0179] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum 50
J/(m.sup.2sK) difference of coefficient of heat transfer occurred.
This was over the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0180] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0181] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0182] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0183] As a result, there were neither pinhole defects in the cast
slab nor surface flaws on the steel sheet.
Example 1-5
[0184] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Ti was added for
deoxidation. The result was refluxed for 6 minutes, then Ce, La,
and Nd were added by a Ce--La--Nd alloy of a mass ratio Ce/La=1.3
and La/Nd=3.5. The result was refluxed for 3 minutes to produce
molten steel having a Ti concentration of 0.03 mass %, a total
concentration of Ce, La, and Nd of 0.01 mass %, a Ce
concentration/La concentration of 1.3, and a Ld concentration/Nd
concentration of 3.5.
[0185] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
0.5 Nl/min to obtain a cast slab of a thickness of 250 mm and a
width of 1600 mm.
[0186] The cross-sectional shape of the immersion nozzle was an
outer shape of an ellipse with a long axis of 170 mm and a short
axis of 120 mm and an inner shape of an ellipse with a long axis of
105 mm and a short axis of 75 mm. The material of the inner bore
was alumina graphite.
[0187] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, while the inner bore had a
step difference of a height: A-a=5 mm, length: L=90 mm (orifice).
The top end of this step difference was at a position 400 mm from
the top end of the inner bore.
[0188] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum
100 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was under the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0189] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0190] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0191] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0192] As a result, there were neither pinhole defects in the cast
slab nor surface flaws on the steel sheet.
Example 1-6
[0193] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Ti was added for
deoxidation. The result was refluxed for 6 minutes, then Ce and La
were added by a Ce--La alloy of a mass ratio Ce/La=1.3. The result
was refluxed for 3 minutes to produce molten steel having a Ti
concentration of 0.03 mass %, a total concentration of Ce and La of
0.01 mass %, and a Ce concentration/La concentration of 1.3.
[0194] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
0.5 Nl/min to obtain a cast slab of a thickness of 130 mm and a
width of 1600 mm.
[0195] Here, as the thin slab continuous casting method, a casting
mold of a thickness of 150 mm or less was used for the casting. The
cast slab was held at a temperature of 1000 to 1200.degree. C. by a
holding furnace placed at the downstream side of the machine end
and hot rolled without cooling to near ordinary temperature.
[0196] The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 70 mm. The material of the inner
bore was alumina graphite.
[0197] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, while the inner bore had a
step difference of a height: R-r=5 mm, length: L=90 mm (orifice).
The top end of this step difference was at a position 400 mm from
the top end of the inner bore.
[0198] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum
100 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was under the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0199] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0200] Next, this cast slab was hot rolled and cold rolled to
finally obtain a coil of cold rolled steel sheet of a thickness of
0.8-mm and a width of 1600 mm.
[0201] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0202] As a result, there were neither pinhole defects in the cast
slab nor surface flaws on the steel sheet.
Example 1-7
[0203] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % and a concentration of Al of 0.005 mass %
by refining in the converter and treatment by a vacuum degassing
apparatus, Ti was added for deoxidation. The result was refluxed
for 6 minutes, then Ce, La, and Nd were added by a Ce--La--Nd alloy
of a mass ratio Ce/La=1.3 and La/Nd=3.5. The result was refluxed
for 3 minutes to produce molten steel having a Ti concentration of
0.03 mass %, a total concentration of Ce, La, and Nd of 0.01 mass
%, a Ce concentration/La concentration of 1.3, and a Ld
concentration/Nd concentration of 3.5.
[0204] This molten steel was cast by the continuous casting method
into a cast slab of a thickness of 250 mm and a width of 1600 mm
with a flow rate of Ar gas blown in from a tundish upper nozzle of
2.8 Nl/min.
[0205] The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The material of the inner bore was alumina graphite.
[0206] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, while the inner bore had a
step difference of a height: R-r=5 mm, length: L=90 mm (orifice).
The top end of this step difference was at a position 400 mm from
the top end of the inner bore.
[0207] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum
150 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was under the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0208] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0209] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0210] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0211] As a result, there were neither pinhole defects in the cast
slab nor surface flaws on the steel sheet.
Comparative Example 1-1
[0212] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Al was added for
deoxidation. This was refluxed for 5 minutes to produce molten
steel with an Al concentration of 0.04 mass %.
[0213] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
7 Nl/min to obtain a cast slab of a thickness of 250 mm and a width
of 1600 mm.
[0214] The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The material of the inner bore was alumina graphite. The length
from the top end of the inner bore to the top ends of the discharge
ports was 590 mm, and the inner bore was straight in shape.
[0215] During casting, starting from when a cast mass of 150 tons
is passed, the opening degree of the sliding nozzle of the tundish
gradually became larger, so it was judged that inclusions had stuck
to the immersion nozzle. The speed was reduced to secure the supply
of molten steel to the casting mold and the casting was
completed.
[0216] At the casting mold short side copper plates, a maximum 300
J/(m.sup.2sK) difference of coefficient of heat transfer occurred.
This was over the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow occurred.
[0217] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0218] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0219] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0220] As a result, the cast slab had 15 pinhole defects/m.sup.2,
while the steel sheet had 10 surface flaws/coil.
Comparative Example 1-2
[0221] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Al was added for
deoxidation. This was refluxed for 5 minutes to produce molten
steel with an Al concentration of 0.04 mass %.
[0222] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
2 Nl/min to obtain a cast slab of a thickness of 250 mm and a width
of 1600 mm.
[0223] The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The material of the inner bore was alumina graphite. The length
from the top end of the inner bore to the top ends of the discharge
ports was 590 mm, while the inner bore had a step difference of a
height: R-r=5 mm and a length: L=90 mm (orifice). The top end of
this step difference was at a position 400 mm from the top end of
the inner bore.
[0224] Soon after the start of casting, the opening degree of the
sliding nozzle of the tundish gradually began to rise. At the point
of time of a casting mass of 150 tons, even if making the opening
degree full open, the supply of molten steel into the casting mold
became insufficient. At this point of time, it was judged that the
immersion nozzle was clogged. The casting was suspended in a state
with a total of 130 tons of molten steel left in the ladle and
tundish (cast mass 170 tons).
[0225] During casting, at the casting mold short side copper
plates, a maximum 330 J/(m.sup.2sK) difference of coefficient of
heat transfer occurred. This was over the uneven flow judgment
criteria of 250 J/(m.sup.2sK), so it was judged that uneven flow
occurred.
[0226] Along with the progression of the clogged state of the
immersion nozzle, it is believed that the effect of the step
difference (orifice) was lost, an uneven flow rate distribution
occurred, and an uneven flow resulted.
[0227] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0228] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0229] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0230] As a result, the cast slab had no pinhole defects, but the
steel sheet had 10 surface flaws/coil.
Comparative Example 1-3
[0231] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Ti was added for
deoxidation. The result was refluxed for 6 minutes, then Ce and La
were added by a Ce--La alloy of a mass ratio Ce/La=1.3. The result
was refluxed for 3 minutes to produce molten steel having a Ti
concentration of 0.03 mass %, a total concentration of Ce and La of
0.01 mass %, and a Ce concentration/La concentration of 1.3.
[0232] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
7 Nl/min to obtain a cast slab of a thickness of 250 mm and a width
of 1600 mm.
[0233] The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The material of the inner bore was alumina graphite. The length
from the top end of the inner bore to the top ends of the discharge
ports was 590 mm, and the inner bore was straight in shape.
[0234] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
However, at the casting mold short side copper plates, a maximum
300 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was over the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow occurred.
[0235] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0236] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0237] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0238] As a result, the cast slab had 15 pinhole defects/m.sup.2,
while the steel sheet had 3 surface flaws/coil.
Comparative Example 1-4
[0239] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Ti was added for
deoxidation. The result was refluxed for 6 minutes, then Ce and La
were added by a Ce--La alloy of a mass ratio Ce/La=1.2. The result
was refluxed for 3 minutes to produce molten steel having a Ti
concentration of 0.03 mass %, a total concentration of Ce and La of
0.01 mass %, and a Ce concentration/La concentration of 1.2.
[0240] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
4.5 Nl/min to obtain a cast slab of a thickness of 250 mm and a
width of 1600 mm.
[0241] The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The material of the inner bore was alumina graphite. The length
from the top end of the inner bore to the top ends of the discharge
ports was 590 mm, and the inner bore was straight in shape.
[0242] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
However, at the casting mold short side copper plates, a maximum
280 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was over the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow occurred.
[0243] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0244] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0245] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0246] As a result, the cast slab had 10 pinhole defects/m.sup.2,
while the steel sheet had 3 surface flaws/coil.
Comparative Example 1-5
[0247] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Ti was added for
deoxidation. The result was refluxed for 6 minutes, then Ce and La
were added by a Ce--La alloy of a mass ratio Ce/La=1.2. The result
was refluxed for 3 minutes to produce molten steel having a Ti
concentration of 0.03 mass %, a total concentration of Ce and La of
0.01 mass %, and a Ce concentration/La concentration of 1.2.
[0248] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
3.5 Nl/min to obtain a cast slab of a thickness of 250 mm and a
width of 1600 mm.
[0249] The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The material of the inner bore was alumina graphite. The length
from the top end of the inner bore to the top ends of the discharge
ports was 590 mm, while the inner bore had a step difference of a
height: R-r=5 mm, length: L=90 mm (orifice). The top end of this
step difference was at a position 400 mm from the top end of the
inner bore.
[0250] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
However, at the casting mold short side copper plates, a maximum
260 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was over the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow occurred.
[0251] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0252] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0253] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0254] As a result, the cast slab had 5 pinhole defects/m.sup.2,
while the steel sheet had 1 surface flaw/coil.
[0255] Here, FIG. 10 shows the relation between the Ar gas flow
rate and cast slab pinhole defects based on Examples 1-1 to 1-6 and
Comparative Examples 1-3 to 1-5 (deoxidization by Ti-rare earth
metals). In the figure, the black dots show the case of use of an
immersion nozzle with an inner bore of a straight shape, while the
black squares show the case of an immersion nozzle having an
orifice in its inner bore.
[0256] Further, FIG. 11 shows the relation between the Ar gas flow
rate and occurrence of steel sheet surface flaws based on Examples
1-2 to 1-6 and Comparative Examples 1-3 to 1-5 (deoxidization by
Ti-rare earth metals). In the figure, the black dots show the case
of use of an immersion nozzle with an inner bore of a straight
shape, while the black squares show the case of an immersion nozzle
having an orifice in its inner bore.
[0257] In the present invention, it is learned that if using an
immersion nozzle having an orifice in its inner bore or an
immersion nozzle with an inner bore of a straight shape and casting
with an Ar gas flow rate of 3 Nl/min or less (region A in the
figure), a good quality cast slab free of both cast slab pinhole
defects and steel sheet surface flaws can be obtained.
Examples 2
[0258] Below, examples and comparative example (see Table 2) will
be given to explain the present invention using an immersion nozzle
having a slit at its bottom (see FIG. 12 to FIG. 20).
TABLE-US-00002 TABLE 2 Max. difference of coefficient of heat Ar
flow transfer at casting Pinhole Deoxidizing rate Immersion nozzle
Nozzle mold short sides defect Surface flaws Item agent (Nl/min)
inner bore sticking (J/(m.sup.2 s K) Drift (/m.sup.2) (/coil) Ex.
2-1 Ti and Ce--La 2.8 Circular cross- None 100 None None None alloy
section, straight slit width 15 mm .theta..sub.1, .theta..sub.2:
45.degree. Ex. 2-2 Ti and Ce--La 0.5 Circular cross- None 80 None
None None alloy section, with orifice slit width 15 mm
.theta..sub.1, .theta..sub.2: 45.degree. Ex. 2-3 Ti and Ce--La 0
Circular cross- None 90 None None 0.2 alloy section, with orifice
Inside range of edge slit width 15 mm trim, so no problem
.theta..sub.1, .theta..sub.2: 45.degree. Ex. 2-4 Ti and Ce--La 2.8
Circular cross- None 110 None None None alloy section, with orifice
slit width 15 mm .theta..sub.1, .theta..sub.2: 45.degree. Ex. 2-5
Ti, Al and 2.8 Elliptical cross- None 120 None None None Ce--La
alloy section, straight slit width 15 mm .theta..sub.1,
.theta..sub.2: 45.degree. Ex. 2-6 Ti, Al and 0 Circular cross- None
50 None None 0.2 Ce--La alloy section, with orifice Inside range of
edge slit width 15 mm trim, so no problem .theta..sub.1,
.theta..sub.2: 45.degree. Comp. Al 2.8 Circular cross- None 280 Yes
10 3 Ex. 2-1 section, straight slit width 15 mm .theta..sub.1,
.theta..sub.2: 45.degree. Comp. Al 2.8 Circular cross- None 260 Yes
5 1 Ex. 2-2 section, straight slit width 15 mm .theta..sub.1,
.theta..sub.2: 45.degree. Comp. Al 2.8 Circular cross- None 300 Yes
15 3 Ex. 2-3 section, straight slit width 15 mm .theta..sub.1,
.theta..sub.2: 45.degree.
Example 2-1
[0259] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Ti was added for
deoxidation. The result was refluxed for 6 minutes, then Ce and La
were added by a Ce--La alloy of a mass ratio Ce/La=1.3. The result
was refluxed for 3 minutes to produce molten steel having a Ti
concentration of 0.03 mass %, a total concentration of Ce and La of
0.01 mass %, and a Ce concentration/La concentration of 1.3.
[0260] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
2.8 Nl/min to obtain a cast slab of a thickness of 250 mm and a
width of 1600 mm.
[0261] The immersion nozzle used was the immersion nozzle shown in
FIG. 13. The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The cross-sectional area of the opening of the discharge ports
was 2829 mm.sup.2, the slit width was 15 mm, and the angles
.theta..sub.1 and .theta..sub.2 were both 45.degree.. The material
of the inner bore was alumina graphite.
[0262] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, and the inner bore was
straight in shape.
[0263] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum
100 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was under the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0264] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0265] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0266] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0267] As a result, there were neither pinhole defects in the cast
slab nor surface flaws on the steel sheet.
Example 2-2
[0268] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Ti was added for
deoxidation. The result was refluxed for 6 minutes, then Ce and La
were added by a Ce--La alloy of a mass ratio Ce/La=1.3. The result
was refluxed for 3 minutes to produce molten steel having a Ti
concentration of 0.03 mass %, a total concentration of Ce and La of
0.01 mass %, and a Ce concentration/La concentration of 1.3.
[0269] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
0.5 Nl/min to obtain a cast slab of a thickness of 250 mm and a
width of 1600 mm.
[0270] The immersion nozzle used was the immersion nozzle shown in
FIG. 17 and FIG. 18. The cross-sectional shape of the immersion
nozzle was a circle with an outside diameter of 150 and an inside
diameter of 85 mm, the cross-sectional area of the opening of the
discharge ports was 2829 mm.sup.2, the slit width was 15 mm, and
the angles .theta..sub.1 and .theta..sub.2 were both 45.degree..
The radius of curvature Rz of the curved surface 11 was 60 mm. The
material of the inner bore was alumina graphite.
[0271] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, while the inner bore had a
step difference of a height: R-r=5 mm, length: L=90 mm (orifice).
The top end of this step difference was at a position 400 mm from
the top end of the inner bore.
[0272] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum 80
J/(m.sup.2sK) difference of coefficient of heat transfer occurred.
This was under the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0273] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0274] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0275] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0276] As a result, there were neither pinhole defects in the cast
slab nor surface flaws on the steel sheet.
Example 2-3
[0277] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Ti was added for
deoxidation. The result was refluxed for 6 minutes, then Ce and La
were added by a Ce--La alloy of a mass ratio Ce/La=1.3. The result
was refluxed for 3 minutes to produce molten steel having a Ti
concentration of 0.03 mass %, a total concentration of Ce and La of
0.01 mass %, and a Ce concentration/La concentration of 1.3.
[0278] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
0 Nl/min to obtain a cast slab of a thickness of 250 mm and a width
of 1600 mm.
[0279] The immersion nozzle used was the immersion nozzle shown in
FIG. 17 and FIG. 18. The cross-sectional shape of the immersion
nozzle was a circle with an outside diameter of 150 and an inside
diameter of 85 mm. The cross-sectional area of the opening of the
discharge port was 2829 mm.sup.2, the slit width was 15 mm, and the
angles .theta..sub.1 and .theta..sub.2 were both 45.degree.. The
discharge angle .phi. was 45.degree., while the radius of curvature
Rz of the curved surface 11 was 60 mm. The material of the inner
bore was alumina graphite.
[0280] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, while the inner bore had a
step difference of a height: R-r=5 mm, length: L=90 mm (orifice).
The top end of this step difference was at a position 400 mm from
the top end of the inner bore.
[0281] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum 90
J/(m.sup.2sK) difference of coefficient of heat transfer occurred.
This was under the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0282] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0283] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0284] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0285] As a result, the cast slab did not have any pinhole defects.
Further, the surface of the cold rolled steel sheet had 0.2 surface
flaw/coil. However, the surface flaws occurred at positions in the
range of the edge trim, so did not become a problem in product
quality.
Example 2-4
[0286] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Ti was added for
deoxidation. The result was refluxed for 6 minutes, then Ce and La
were added by a Ce--La alloy of a mass ratio Ce/La=1.3. The result
was refluxed for 3 minutes to produce molten steel having a Ti
concentration of 0.03 mass %, a total concentration of Ce and La of
0.01 mass %, and a Ce concentration/La concentration of 1.3.
[0287] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
2.8 Nl/min to obtain a cast slab of a thickness of 250 mm and a
width of 1600 mm.
[0288] The immersion nozzle used was the immersion nozzle shown in
FIG. 19. The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The cross-sectional area of the opening of the discharge ports
was 2829 mm.sup.2, the slit width was 15 mm, and the angles
.theta..sub.1 and .theta..sub.2 were both 45.degree.. The discharge
angle .phi. was 45.degree., while the radius of curvature Rz of the
curved surface 11 was 60 mm.
[0289] The vertex angle .theta..sub.3 of the rib was 30.degree.,
the width Wr of the bottom surface of the rib was 15 mm, and the
distance Dr between the side walls of the rib was 85 mm. The
material of the inner bore was alumina graphite.
[0290] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, while the inner bore had a
step difference of a height: R-r=5 mm, length: L=90 mm (orifice).
The top end of this step difference is at a position 400 mm from
the top end of the inner bore.
[0291] During casting, there was no sticking of inclusions on the
immersion-nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum
110 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was under the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0292] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0293] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0294] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0295] As a result, there were neither pinhole defects in the cast
slab nor surface flaws on the steel sheet.
Example 2-5
[0296] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % and a concentration of Al of 0.005 mass %
by refining in the converter and treatment by a vacuum degassing
apparatus, Ti was added for deoxidation. The result was refluxed
for 6 minutes, then Ce and La were added by a Ce--La alloy of a
mass ratio Ce/La=1.3. The result was refluxed for 3 minutes to
produce molten steel having a Ti concentration of 0.03 mass %, a
total concentration of Ce and La of 0.01 mass %, and a Ce
concentration/La concentration of 1.3.
[0297] This molten steel was cast by the continuous casting method
into a cast slab of a thickness of 250 mm and a width of 1600 mm
with a flow rate of Ar gas blown in from a tundish upper nozzle of
2.8 Nl/min.
[0298] The immersion nozzle used was the immersion nozzle shown in
FIG. 13. The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The cross-sectional area of the opening of the discharge ports
was 2829 mm.sup.2, the slit width was 15 mm, and the angles
.theta..sub.1 and .theta..sub.2 were both 45.degree.. The material
of the inner bore was alumina graphite.
[0299] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, and the inner bore was
straight in shape.
[0300] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum
120 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was under the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0301] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0302] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0303] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0304] As a result, there were neither pinhole defects in the cast
slab nor surface flaws on the steel sheet.
Example 2-6
[0305] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % and a concentration of Al of 0.005 mass %
by refining in the converter and treatment by a vacuum degassing
apparatus, Ti was added for deoxidation. The result was refluxed
for 6 minutes, then Ce and La were added by a Ce--La alloy of a
mass ratio Ce/La=1.3. The result was refluxed for 3 minutes to
produce molten steel having a Ti concentration of 0.03 mass %, a
total concentration of Ce and La of 0.01 mass %, and a Ce
concentration/La concentration of 1.3.
[0306] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
0 Nl/min to obtain a cast slab of a thickness of 250 mm and a width
of 1600 mm.
[0307] The immersion nozzle used was the immersion nozzle shown in
FIG. 17 and FIG. 18. The cross-sectional shape of the immersion
nozzle was a circle with an outside diameter of 150 and an inside
diameter of 85 mm. The cross-sectional area of the opening of the
discharge ports was 2829 mm.sup.2, the slit width was 15 mm, and
the angles .theta..sub.1 and .theta..sub.2 were both 45.degree..
The discharge angle .phi. was 20.degree., while the radius of
curvature Rz of the curved surface 11 was 51 mm. The material of
the inner bore was alumina graphite.
[0308] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, while the inner bore had a
step difference of a height: R-r=5 mm, length: L=90 mm (orifice).
The top end of this step difference was at a position 400 mm from
the top end of the inner bore.
[0309] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
Further, at the casting mold short side copper plates, a maximum 50
J/(m.sup.2sK) difference of coefficient of heat transfer occurred.
This was under the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow did not occur.
[0310] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0311] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0312] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0313] As a result, the cast slab did not have any pinhole defects.
Further, the surface of the cold rolled steel sheet had 0.2 surface
flaw/coil. However, the surface flaws occurred at positions in the
range of the edge trim, so did not become a problem in product
quality.
Comparative Example 2-1
[0314] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Al was added for
deoxidation. The result was refluxed for 5 minutes to produce
molten steel with an Al concentration of 0.04 mass %.
[0315] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
2.8 Nl/min to obtain a cast slab of a thickness of 250 mm and a
width of 1600 mm.
[0316] The immersion nozzle used was the immersion nozzle shown in
FIG. 13. The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The cross-sectional area of the opening of the discharge ports
was 2829 mm.sup.2, the slit width was 15 mm, and the angles
.theta..sub.1 and .theta..sub.2 were both 15.degree.. The material
of the inner bore was alumina graphite.
[0317] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, and the inner bore was
straight in shape.
[0318] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
However, at the casting mold short side copper plates, a maximum
280 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was over the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow occurred.
[0319] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0320] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0321] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0322] As a result, the cast slab had 10 pinhole defects/m.sup.2,
while the steel sheet had 3 surface flaws/coil.
Comparative Example 2-2
[0323] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Al was added for
deoxidation. The result was refluxed for 5 minutes to produce
molten steel with an Al concentration of 0.04 mass %.
[0324] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
2.8 Nl/min to obtain a cast slab of a thickness of 250 mm and a
width of 1600 mm.
[0325] The immersion nozzle used was the immersion nozzle shown in
FIG. 13. The cross-sectional shape of the immersion nozzle was a
circle with an outside diameter of 150 and an inside diameter of 85
mm. The cross-sectional area of the opening of the discharge ports
was 2829 mm.sup.2, the slit width was 5 mm, and the angles
.theta..sub.1 and .theta..sub.2 were both 45.degree.. The material
of the inner bore was alumina graphite.
[0326] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, and the inner bore was
straight in shape.
[0327] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
However, at the casting mold short side copper plates, a maximum
260 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was over the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow occurred.
[0328] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0329] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a-coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0330] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0331] As a result, the cast slab had 5 pinhole defects/m.sup.2,
while the steel sheet had 1 surface flaw/coil.
Comparative Example 2-3
[0332] To 300 tons of molten steel in a ladle given a concentration
of carbon of 0.002 mass % by refining in the converter and
treatment by a vacuum degassing apparatus, Al was added for
deoxidation. The result was refluxed for 5 minutes to produce
molten steel with an Al concentration of 0.04 masse.
[0333] This molten steel was cast by the continuous casting method
with a flow rate of Ar gas blown in from a tundish upper nozzle of
2.8 Nl/min to obtain a cast slab of a thickness of 250 mm and a
width of 1600 mm.
[0334] The immersion nozzle used when casting was the immersion
nozzle shown in FIG. 13. The cross-sectional shape of the immersion
nozzle was a circle with an outside diameter of 150 and an inside
diameter of 85 mm, the cross-sectional area of the opening of the
discharge ports was 2829 mm.sup.2, the slit width was 25 mm, and
the angles .theta..sub.1 and .theta..sub.2 were both 45.degree..
The material of the inner bore was alumina graphite.
[0335] The length from the top end of the inner bore to the top
ends of the discharge ports was 590 mm, and the inner bore was
straight in shape.
[0336] During casting, there was no sticking of inclusions on the
immersion nozzle and there was no problem with operability.
However, at the casting mold short side copper plates, a maximum
300 J/(m.sup.2sK) difference of coefficient of heat transfer
occurred. This was over the uneven flow judgment criteria of 250
J/(m.sup.2sK), so it was judged that uneven flow occurred.
[0337] The cast slab was cut to a length of 10000 mm to obtain 1
coil unit. The surface of this cast slab was examined by a CCD
camera. The pinhole defects were evaluated by the number present
per square meter cast slab surface area.
[0338] Next, this cast slab was hot rolled and cold rolled by
ordinary methods to finally obtain a coil of cold rolled steel
sheet of a thickness of 0.8 mm and a width of 1600 mm.
[0339] The steel sheet quality was visually inspected on the
inspection line after cold rolling and was evaluated by the number
of surface flaws occurring per coil.
[0340] As a result, the cast slab had 15 pinhole defects/m.sup.2,
while the steel sheet had 3 surface flaws/coil.
INDUSTRIAL APPLICABILITY
[0341] As explained above, according to the present invention, it
is possible to prevent nozzle clogging and possible to produce an
ultralow carbon cast slab not giving rise to surface flaws even if
processed into thin-gauge steel sheet. Accordingly, the present
invention has a high applicability in the steelmaking industry.
* * * * *