U.S. patent application number 11/314505 was filed with the patent office on 2006-05-18 for method for producing ultra low carbon steel slab.
This patent application is currently assigned to JFE Steel Corporation a corporation of Japan. Invention is credited to Toshio Fujimura, Seiji Itoyama, Takeshi Matsuzaki, Yuji Miki, Makoto Suzuki, Chikashi Tada, Hirohide Uehara, Akira Yamauchi.
Application Number | 20060102316 11/314505 |
Document ID | / |
Family ID | 34106965 |
Filed Date | 2006-05-18 |
United States Patent
Application |
20060102316 |
Kind Code |
A1 |
Itoyama; Seiji ; et
al. |
May 18, 2006 |
Method for producing ultra low carbon steel slab
Abstract
An ultra-low carbon steel slab having a carbon content of about
0.01 mass percent or less is produced by casting at a casting speed
of more than about 2.0 m/min using a mold provided with a casting
space having a short side length D of about 150 to about 240 mm and
an immersion nozzle provided with discharge spouts each having a
lateral width d, the ratio D/d being in the range of from about 1.5
to about 3.0. Accordingly, a ultra-low carbon steel slab can be
obtained having superior surface quality without performing slab
conditioning such as scarfing.
Inventors: |
Itoyama; Seiji; (Chiba,
JP) ; Fujimura; Toshio; (Miyagi, JP) ; Suzuki;
Makoto; (Chiba, JP) ; Uehara; Hirohide;
(Okayama, JP) ; Matsuzaki; Takeshi; (Okayama,
JP) ; Tada; Chikashi; (Tokyo, JP) ; Miki;
Yuji; (Okayama, JP) ; Yamauchi; Akira;
(Okayama, JP) |
Correspondence
Address: |
IP GROUP OF DLA PIPER RUDNICK GRAY CARY US LLP
1650 MARKET ST
SUITE 4900
PHILADELPHIA
PA
19103
US
|
Assignee: |
JFE Steel Corporation a corporation
of Japan
Tokyo
JP
|
Family ID: |
34106965 |
Appl. No.: |
11/314505 |
Filed: |
December 21, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10921434 |
Aug 19, 2004 |
|
|
|
11314505 |
Dec 21, 2005 |
|
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Current U.S.
Class: |
164/466 ;
164/459; 164/478 |
Current CPC
Class: |
B22D 11/115 20130101;
B22D 11/14 20130101; B22D 11/00 20130101 |
Class at
Publication: |
164/466 ;
164/459; 164/478 |
International
Class: |
B22D 11/04 20060101
B22D011/04; B22D 27/02 20060101 B22D027/02; B22D 11/051 20060101
B22D011/051 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 29, 2003 |
JP |
JP 2003-307108 |
Nov 26, 2003 |
JP |
JP 2003-395818 |
Claims
1-24. (canceled)
25. A method for producing an ultra-low carbon steel slab
comprising: providing a continuous casting apparatus comprising a
mold having a casting space with a short side length D of about 150
to about 240 mm and an immersion nozzle having at least one
discharge spout with a lateral width d, in which a ratio D/d is in
the range of from about 1.5 to about 3.0; introducing molten steel
into the mold through the immersion nozzle; casting the molten
steel at a casting speed of more than about 2.0 mm/min with the
continuous casting apparatus to produce a ultra-low carbon steel
slab having a carbon content of about 0.01 mass percent or less;
and applying a brake using an electromagnetic force to the flow of
molten steel by superimposingly applying a static magnetic field
and an AC magnetic field to the mold in a direction intersecting
the mold thickness with a magnetic field application device
provided at an upper portion of the mold including a surface level
of the molten steel in the mold, wherein the immersion nozzle is
disposed at a lower side of the magnetic field application device
and has an immersion depth of about 200 to about 350 mm.
26. The method according to claim 25, further comprising applying a
static magnetic field to the mold in a direction intersecting the
mold thickness using a lower magnetic field application device, the
lower magnetic field application device being provided at a lower
side of the upper magnetic field application device.
27. The method according to claim 25, further comprising
oscillating the mold at a frequency of about 185 cycles/min or
less.
28. The method according to claim 25, wherein the casting speed is
about 2.4 m/min or more.
29. The method according to claim 25, wherein the immersion nozzle
is a two-spout nozzle.
30. The method according to claim 25, wherein the ratio D/d is
about 2.1 to about 2.9.
31. The method according to claim 25, wherein the ultra-low carbon
steel slab is a starting material for a cold-rolled steel sheet for
forming outer plates of automobiles.
32. The method according to claim 25, wherein applying a brake
using an electromagnetic force to the flow of molten steel is
performed by superimposingly applying a static magnetic field and
an AC magnetic field to the entire mold in the direction
intersecting the mold thickness using an upper magnetic field
application device and by applying a static magnetic field to the
mold in a direction intersecting the mold thickness using a lower
magnetic field application device, the upper magnetic field
application device is provided at an upper portion of the mold
including a surface level of molten steel in the mold, and the
lower magnetic field application device is provided at a lower side
of the upper magnetic field application device, and the immersion
nozzle is disposed between the upper and the lower magnetic
application devices and has an immersion depth of about 200 to
about 350 mm.
33. The method according to claim 25, wherein the molten steel
comprises about 0.01 mass percent or less of C; about 0.01 to about
0.04 mass percent of Si, about 0.08 to about 0.20 mass percent of
Mn, about 0.008 to about 0.020 mass percent of P, about 0.003 to
about 0.008 mass percent of S, about 0.015 to about 0.060 mass
percent of Al, about 0.03 to about 0.080 mass percent of Ti, about
0.002 to about 0.017 mass percent of Nb, and 0 to about 0.0007 mass
percent of B; and the balance Fe and inevitable impurities.
34. The method according to claim 33, wherein the molten steel
comprises 0.0005 to 0.0090 mass percent of C.
35. A method of producing an ultra-low carbon steel slab
comprising: introducing molten steel into a mold having a casting
space with a short side length D of about 150 to about 240 mm
through an immersion nozzle having at least one discharge spout
with a lateral width d, in which a ratio D/d is in the range of
from about 1.5 to about 3.0; casting the molten steel at a casting
speed of more than about 2.0 mm/min with the continuous casting
apparatus to produce an ultra-low carbon steel slab having a carbon
content of about 0.01 mass percent or less; and applying a brake
using an electromagnetic force to the flow of molten steel by
superimposingly applying a static magnetic field and an AC magnetic
field to the mold in a direction intersecting the mold thickness
with a magnetic field application device provided at an upper
portion of the mold including a surface level of the molten steel
in the mold, wherein the immersion nozzle is disposed at a lower
side of the magnetic field application device and has an immersion
depth of about 200 to about 350 mm.
36. The method according to claim 35, further comprising applying a
static magnetic field to the mold in a direction intersecting the
mold thickness using a lower magnetic field application device, the
lower magnetic field application device being provided at a lower
side of the upper magnetic field application device.
37. The method according to claim 35, further comprising
oscillating the mold at a frequency of about 185 cycles/min or
less.
38. The method according to claim 35, wherein the casting speed is
about 2.4 m/min or more.
39. The method according to claim 35, wherein the immersion nozzle
is a two-spout nozzle.
40. The method according to claim 35, wherein the ratio D/d is
about 2.1 to about 2.9.
41. The method according to claim 35, wherein the ultra-low carbon
steel slab is a starting material for a cold-rolled steel sheet for
forming outer plates of automobiles.
42. The method according to claim 35, wherein applying a brake
using an electromagnetic force to the flow of molten steel is
performed by superimposingly applying a static magnetic field and
an AC magnetic field to the entire mold in the direction
intersecting the mold thickness using an upper magnetic field
application device and by applying a static magnetic field to the
mold in a direction intersecting the mold thickness using a lower
magnetic field application device, the upper magnetic field
application device is provided at an upper portion of the mold
including a surface level of molten steel in the mold, and the
lower magnetic field application device is provided at a lower side
of the upper magnetic field application device, and the immersion
nozzle is disposed between the upper and the lower magnetic
application devices and has an immersion depth of about 200 to
about 350 mm.
43. The method according to claim 35, wherein the molten steel
comprises about 0.01 mass percent or less of C; about 0.01 to about
0.04 mass percent of Si, about 0.08 to about 0.20 mass percent of
Mn, about 0.008 to about 0.020 mass percent of P, about 0.003 to
about 0.008 mass percent of S, about 0.015 to about 0.060 mass
percent of Al, about 0.03 to about 0.080 mass percent of Ti, about
0.002 to about 0.017 mass percent of Nb, and 0 to about 0.0007 mass
percent of B; and the balance Fe and inevitable impurities.
44. The method according to claim 43, wherein the molten steel
comprises 0.0005 to 0.0090 mass percent of C.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to methods for producing a steel slab
having ultra low carbon by continuous casting and, more
particularly, relates to a method for producing a steel slab
suitably used for forming outer plates of automobiles and the like
with superior surface qualities.
[0003] 2. Description of the Related Art
[0004] Steel sheets used, for example, for forming outer plates of
automobiles, which are to be processed by deep drawing and/or which
are to be formed into complicated shapes by deformation, should
have superior formability. Hence, so-called "ultra-low-carbon
steel" has been used, the carbon content of which is decreased as
low as possible. Ultra-low carbon steel generally contains a C
content of 0.01 mass percent or less. Among ultra-low-carbon steel
sheets as described above, a cold-rolled steel sheet for forming
outer plates of automobiles is particularly helpful for superior
appearance in addition to superior paintability.
[0005] A step of removing carbon in molten steel is carried out in
a refining process by oxidation using oxygen when ultra-low-carbon
steel is produced. Accordingly, a deoxidizing step for removing
oxygen dissolved in molten steel in this oxidation removing step is
further carried out using a deoxidizing agent such as aluminum,
magnesium and titanium. In this deoxidizing step, the oxygen
dissolved in molten steel is allowed to react with the deoxidizing
agent to form reaction products such as alumina, magnesia and
titania, and the reaction products thus formed remain in the molten
steel as non-metallic inclusions.
[0006] Defects such as slivers and/or blisters are unfavorably
generated on a surface of the steel sheet in forming the slab into
a thin steel sheet by hot rolling and/or cold rolling when the
non-metallic inclusions as described above are present in the
vicinity of a slab surface.
[0007] Argon gas is supplied and mold powder is added to a molten
steel surface in the mold in continuous casting to prevent an
immersion nozzle from being clogged which is used to supply molten
steel from a tundish into the mold. When being engulfed in the
molten steel, the argon gas thus supplied may simply remain in the
molten steel in the form of bubbles or may combine with the
reaction products (hereinafter referred to as "deoxidation reaction
products") formed by deoxidation described above to form bubbles
which remain in the molten steel. Surface defects are generated in
both cases described above. In addition, surface defects similar to
those of the deoxidation reaction products are also generated when
the mold powder thus added remains in the molten steel.
[0008] In the past, hot-rolling is performed in the case of an
ordinary slab prepared by continuous casting for forming a
cold-rolled steel sheet, without performing surface treatment of
the slab. However, in the case of a slab used for forming outer
plates of automobiles, a surface portion of the slab having a
thickness of approximately 1 to 4 mm is removed, for example, by
scarfing so as to remove inclusions of deoxidation reaction
products, bubbles, mold flux, and the like which may cause surface
defects of a steel sheet formed after hot-rolling and,
subsequently, hot rolling and cold rolling are performed.
[0009] Slab finishing treatment as described above decreases the
yield of the slab used as a starting material and, in addition,
disadvantageously causes delays in the process. Hence, in a step of
manufacturing a slab using a continuous casting apparatus, attempts
have been made to prevent generation of slab surface defects which
cause the above-described surface defects of steel sheets.
[0010] Fundamental ideas of the attempts described above have been
primarily based on the following (1) to (6):
[0011] (1) Slab thickness is increased so that the cross-sectional
area thereof is increased for decreasing the casting speed (m/min)
since slab width is restricted when being rolled. Accordingly, the
residence time of molten steel in a mold is increased without
degrading productivity and, as a result, the time is increased for
eliminating foreign materials such as deoxidation reaction
products, mold powder, bubbles, and the like to surface from inside
the molten steel in the mold.
[0012] (2) Casting is performed using a continuous casting
apparatus having a vertical portion to increasingly enable
deoxidation reaction products, mold powder, bubbles, and the like
to surface from inside molten steel in a mold for separation.
[0013] (3) A flow moving in the horizontal direction is generated
in the vicinity of the meniscus by an electromagnetic force so that
foreign materials floating in molten steel is prevented from being
trapped by a solidification shell (washing effect).
[0014] (4) The viscosity of a mold powder is appropriately
controlled so that the probability that the mold powder is engulfed
in molten steel is decreased.
[0015] (5) An oscillation (vertical vibration) condition of the
mold for continuous casting is appropriately controlled to reduce
generation of a nail of a solidification shell formed in the mold
(phenomenon in which part of the solidification shell leans toward
the molten steel side due to the oscillation), thereby decreasing
the amount of deoxidation reaction products, mold powder, bubbles
and the like to be trapped inside this nail.
[0016] (6) The flow of molten steel is appropriately controlled by
performing electromagnetic stirring for or applying an
electromagnetic brake to a flow of molten steel supplied into a
mold from an immersion nozzle so that the flow of molten steel
accompanied by deoxidation reaction products is prevented from
reaching a deep position in the mold.
[0017] For example, a technique has been disclosed in Japanese
Unexamined Patent Application Publication No. 5-76993 in which when
casting of molten steel containing less than 0.10 percent by weight
of carbon is performed using a continuous casting apparatus having
a vertical portion 20 m or more long at a casting speed of 1.0
m/min or more and 4 ton/min or more to form a slab having a
thickness of more than 200 mm and a width of more than 900 mm,
while the powder viscosity is set to 1.0 poise or more, and an
inert gas flow rate from an immersion nozzle is set to 1 liter/min
or more, electromagnetic stirring is performed for molten steel
present in the region from the meniscus to a depth of 1.5 m at a
flow speed of 15 to 40 cm/sec in the horizontal direction. This
technique is primarily based on the above paragraphs (1), (2), (3),
(4), and (6).
[0018] In addition, a technique has been disclosed in Japanese
Unexamined Patent Application Publication No. 7-155902 in which a
mold oscillation condition is appropriately controlled to suppress
generation of a nail portion in which inclusions are liable to be
trapped, the nail portion being formed at an initial solidification
stage of a slab surface portion. This technique is primarily based
on the above paragraph (5).
[0019] However, the above-described techniques still have
problems.
[0020] That is, as disclosed in Japanese Unexamined Patent
Application Publication No. 5-76993, when the cross-sectional area
of the slab is increased, in particular, when the thickness thereof
is increased, at a casting speed of more than 1.5 m/min, the number
of defects in the vicinity of the surface of the slab caused by
inclusions or the like did not decrease as much as expected. The
reason for this is that although a flow speed vm of molten steel at
the meniscus portion is controlled to an optimum value by applying
an electromagnetic force in the horizontal direction, throughput is
increased as the slab thickness is increased, and a discharge speed
vi from an immersion nozzle is increased when casting is performed
at the same casting speed (Vc) and the same slab width (W) as those
in the case in which the cross-sectional area is not increased.
Accordingly, although the change in average value of the flow speed
vm of molten steel is small, the change amount thereof is increased
and, as a result, mold flux is increasingly engulfed in the molten
steel. That is, it shows that the cleanness in the vicinity of the
slab surface portion is not simply determined by the flow speed of
molten steel in the vicinity of the meniscus.
[0021] In addition, the influence of a jet flow of molten steel
from the immersion nozzle becomes significant and the growth of a
shell is partly delayed along the short side of the mold. The
reason for this is that, in the case of a slab continuous casting
apparatus, when molten steel is discharged into a mold, since a
so-called "two-spout nozzle" is used to supply the molten steel
uniformly along the width direction of a casting space present in
the mold, and a width d of the discharge spout of this two-spout
nozzle is relatively small as compared to a short side length D
(corresponding to the thickness of a slab) inside the mold, the
flow speed of molten steel varies in the slab thickness direction.
Hence, molten steel having a high flow speed unevenly collides
against a part of the solidification shell along the short side
and, as a result, the growth of the part of the solidification
shell described above is delayed. In addition, the variation in
flow speed of molten steel in the slab thickness direction is also
partly responsible for variation in flow speed of molten steel in
the vicinity of the meniscus described above.
[0022] Next, in the technique described in Japanese Unexamined
Patent Application Publication No. 7-155902, in order to improve
the slab surface quality, when a negative strip time T determined
by the casting speed, the mold oscillation amplitude, and the
oscillation frequency of the mold is controlled within a specific
range by adjusting mold oscillation conditions, in particular, by
decreasing the mold oscillation amplitude and by increasing the
oscillation frequency of the mold, it was found that the following
problems occur.
[0023] That is, when ultra-low-carbon steel is formed by casting at
a casting speed of more than 2.0 m/min and a oscillation frequency
of the mold of more than 185 cycles/min, although being not so
frequently observed, an abnormal phenomenon occurs in which the
molten steel surface level is suddenly and largely varied. As a
result, mold flux may be engulfed in the molten steel or may be
trapped in a solidification shell, thereby causing surface defects
of cast steel sheets. Hence, surface defects are frequently
generated on products which are caused by the mold flux when
casting is performed at a casting speed of more than 2.0 m/min. As
a result, there has been a problem that products having superior
surface qualities are not stably obtained.
[0024] As apparent from the above descriptions, when a
ultra-low-carbon steel slab used for forming outer plates of
automobiles and the like is manufactured, in high-speed casting at
a speed of more than 2.0 m/min, stable manufacturing of a
high-quality slab cannot be performed as of today without carrying
out slab conditioning such as scarfing.
[0025] It would therefore be advantageous to provide a continuous
casting method for producing a ultra-low carbon steel slab in which
the slab having superior surface quality without any slab
conditioning such as scarfing can be stably obtained even at a high
casting speed of more than 2.0 m/min.
SUMMARY OF THE INVENTION
[0026] This invention provides a method for producing an ultra-low
carbon steel slab including:
[0027] providing a continuous casting apparatus including a mold
provided with a casting space having a short side length D of about
150 to about 240 mm and an immersion nozzle provided with at least
one discharge spout having a lateral width d, in which a ratio D/d
is in the range of from about 1.5 to about 3.0;
[0028] introducing molten steel into the mold through the immersion
nozzle; and
[0029] casting the molten steel at a casting speed of more than
about 2.0 mm/min with the continuous casting apparatus to produce a
ultra-low carbon steel slab having a carbon content of about 0.01
mass percent or less.
[0030] The slab continuous casting method preferably further
includes oscillating the mold at a frequency of about 185
cycles/min or less. The probability of occurrence of an abnormal
phenomenon is suppressed in that the molten steel surface level is
suddenly and largely varied. Hence, the number of defects caused by
flux can be decreased since the rate of occurrence of the resonance
between the oscillation of a molten steel surface and that of the
mold decreases when the mold oscillation cycle is about 185
cycles/min or less.
[0031] The casting speed is preferably about 2.4 m/min or more. The
nail depth becomes about 0.7 mm or less, that is, the thickness for
trapping foreign materials becomes not more than the nail depth
when the casting speed is about 2.4 m/min or more. Hence, the
casting speed is preferably set to about 2.4 m/min or more.
[0032] As the immersion nozzle described above, a cylindrical
nozzle (so-called "straight nozzle") or a two-spout nozzle in which
the front end is closed and two approximately circular discharge
spouts are provided toward the two short sides of the mold are
generally used.
[0033] The ratio D/d of the short side length D to the lateral
width d of the discharge spout of the immersion nozzle is
preferably about 2.1 to about 2.9 when the slab thickness,
immersion nozzle durability and the desired flow rate are taken
into consideration in addition to the product quality.
[0034] The ultra-low carbon steel slab described above is
preferably a starting material for a cold-rolled steel sheet used
for forming outer plates of automobiles.
[0035] The slab continuous casting method described above
preferably further includes applying a brake using an
electromagnetic force to the flow of the molten steel in the
casting space of the mold. The following paragraphs (A) to (C) may
be mentioned as preferred methods for applying a brake using an
electromagnetic force:
[0036] (A) Applying a brake using an electromagnetic force is
performed by applying static magnetic fields to substantially the
entire mold in the direction intersecting the mold thickness using
an upper magnetic field application device and a lower magnetic
field application device. The upper magnetic field application
device is provided at an upper portion of the mold including the
molten steel surface level in the mold and the lower magnetic field
application device is provided at a lower side of the upper
magnetic field application device. The immersion nozzle is disposed
between the upper and the lower magnetic application devices, and
the immersion depth is set to about 200 to about 350 mm.
[0037] (B) Applying a brake using an electromagnetic force is
performed by superimposingly applying a static magnetic field and
an AC magnetic field to the entire mold in the direction
intersecting the mold thickness using a magnetic field application
device provided at an upper portion of the mold including the
molten steel surface level in the mold. The immersion nozzle is
disposed below the magnetic field application device, and the
immersion depth is set to about 200 to about 350 mm.
[0038] (C) Applying a brake using an electromagnetic force is
performed by superimposingly applying a static magnetic field and
an AC magnetic field to the entire mold in the direction
intersecting the mold thickness using an upper magnetic field
application device and, in addition, by applying a static magnetic
field to the entire mold in the direction intersecting the mold
thickness using a lower magnetic field application device. The
upper magnetic field application device is provided at an upper
portion of the mold including the molten steel surface level in the
mold and the lower magnetic field application device is provided at
a lower side of the upper magnetic field application device. The
immersion nozzle is disposed between the upper and the lower
magnetic application devices, and the immersion depth is set to
about 200 to about 350 mm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0039] FIG. 1 is a graph showing the relationship between casting
speed and nail depth according to aspects of the invention;
[0040] FIG. 2 is a graph showing the relationship between the
trapping depth h from a slab surface and the number of trapped
inclusions according to aspects of the invention, the relationship
being obtained at different casting speeds;
[0041] FIG. 3 is a graph showing the relationship between distance
L from the meniscus and the number of trapped inclusions, according
to aspects of the invention, the relationship being obtained at
different casting speeds;
[0042] FIG. 4 is a graph showing the influences of slab thickness
and casting speed on a short-side bulging amount, according to
aspects of the invention;
[0043] FIG. 5 is a graph showing the influence of slab thickness on
the rate of surface defects of products according to aspects of the
invention;
[0044] FIG. 6 is a graph showing the influence of casting speed on
the rate of surface defects of products according to aspects of the
invention;
[0045] FIGS. 7A to 7C are schematic views each showing a continuous
casting mold provided with a magnetic field application device, the
mold being suitably used in accordance with aspects of the
invention;
[0046] FIG. 8 is a schematic view showing an example of application
of an AC oscillating magnetic field according to aspects of the
invention; and
[0047] FIG. 9 is a schematic view showing an example of application
of an AC travelling magnetic field according to aspects of the
invention.
DETAILED DESCRIPTION
[0048] We discovered that slabs having ultra-low carbon content can
be advantageously produced by appropriately controlling the casting
speed, the short side length D of the casting space of a continuous
casting mold, and the ratio D/d of the short side length D to a
lateral width d of a discharge spout of an immersion nozzle in
addition to, whenever necessary, appropriate control of oscillation
frequency of the mold, or effective use of an electromagnetic brake
on a molten steel flow.
[0049] A type of steel in accordance with aspects of the invention
is so-called "ultra-low-carbon steel" having a carbon content of
about 0.01 mass percent or less. Components other than C are not
particularly limited. However, a type of steel which can be
suitably processed by deep drawing for forming outer plates of
automobiles or the like is preferred. An advantage of the invention
is that, for steel used in applications with substantially no
defects caused by inclusions, inclusions are substantially not
allowed to be present in the region from the surface of a slab to a
certain depth therefrom, which region is not to be scaled off in a
subsequent step. Ultra-low-carbon steel may receive most advantages
of the invention since, in the ultra-low carbon steel, non-metallic
inclusions such as alumina are liable to be generated as
deoxidation reaction products in the refining process.
[0050] As a typical composition (not including component C) of
ultra-low-carbon steel, the following may be mentioned by way of
example: about 0.01 to about 0.04 mass percent of Si, about 0.08 to
about 0.20 mass percent of Mn, about 0.008 to about 0.020 mass
percent of P, about 0.003 to about 0.008 mass percent of S, about
0.015 to about 0.060 mass percent of Al, about 0.03 to about 0.080
mass percent of Ti, about 0.002 to about 0.017 mass percent of Nb,
and 0 to about 0.0007 mass percent of B.
[0051] The continuous casting apparatus used in accordance with the
invention is a continuous casting apparatus for forming a steel
slab and may be optionally selected from a vertical continuous
casting apparatus, a vertical bending continuous casting apparatus
and a curved continuous casting apparatus. However, among those
mentioned above, a vertical bending continuous casting apparatus is
particularly advantageous in consideration of productivity and
product quality.
[0052] The mold is a so-called "slab continuous casting mold," and
the short side length thereof is about 150 to about 240 mm. The
long side length of the mold is not particularly limited and is
preferably substantially equivalent to the length of an ordinary
cold-rolled steel sheet (in particular, a cold-rolled steel sheet
for automobiles), such as approximately 900 to 2,200 mm. The short
side length corresponds to the slab thickness when the slab is
formed and the long side length corresponds to a slab width.
[0053] The height of the mold in the vertical direction is not
particularly limited. However, since a solidification shell is
formed having a certain thickness so that a cast steel sheet
passing through the mold does not bulge even when casting is
performed at a casting speed of more than 2.0 m/min, the height is
preferably set to approximately 800 to approximately 1,000 mm.
[0054] An immersion muzzle is used as the nozzle for supplying
molten steel into the casting space of the mold from a tundish. The
material for the immersion nozzle may be a commonly used material
such as alumina-graphite. However, the material is not only limited
thereto.
[0055] In addition, as the shape of the immersion nozzle, there may
be generally mentioned a cylindrical nozzle (so-called "straight
nozzle") or a two-spout nozzle in which the front end is closed and
two approximately circular discharge spouts are provided toward the
two short sides of the mold. The cross-sectional shape of the
discharge spout may be circular, square, or rectangular (longer in
a lateral direction, or longer in a longitudinal direction) and is
not particularly limited, and any type of shape may be used as long
as the maximum width d of the discharge spout satisfies the
conditions of the invention.
[0056] Furthermore, the casting speed is set to more than about 2.0
m/min for the reasons described later. The casting speed is more
preferably set to about 2.4 m/min or more.
[0057] When a brake is applied using an electromagnetic force to
the flow of molten steel in the casting space of the mold of the
continuous casting apparatus, as a preferable method therefor, for
example, there may be mentioned a method in which a static magnetic
field is applied to the entire mold along the long-side width as
disclosed in Japanese Unexamined Patent Application Publication No.
2-284750, or a method in which a static magnetic field is applied
only to a discharge position of molten steel as disclosed in
Japanese Unexamined Patent Application Publication No. 57-17356.
The subject matter of both of JP 2-284750 and JP 57-17356 is
incorporated herein by reference.
[0058] Various phenomena occur in the mold when casting is
performed in accordance with the invention under conditions in
which the short side length (slab thickness) of the casting space
of the mold is set to about 150 to about 240 mm and the casting
speed is set to more than about 2.0 m/min. Subsequently, novel
findings relating to the phenomena mentioned above will be
described. Hereinafter, inclusions, bubbles, and the like will be
called "foreign materials."
(1) Reduction in Area of Trapping Foreign Materials
[0059] Formation of an initial solidification shell at the meniscus
portion, which is a so-called "nail", can be significantly
suppressed when the casting speed Vc is set to more than about 2.0
m/min or preferably set to about 2.4 m/min or more. We believe the
reason for this is that since the thickness of a solidification
shell formed at an optional constant depth from a molten steel
surface level is decreased as the casting speed Vc is increased,
due to the influence of a static pressure of molten steel, a force
applied toward the mold side becomes larger than a force of the
nail leaning toward the molten steel side caused by thermal
contraction of the solidification shell which depends on the
thickness thereof. In addition, when the slab thickness is
decreased, the absolute value of the amount of shell contraction in
the thickness direction represented by "slab
thickness.times.temperature difference.times.coefficient of thermal
expansion" is decreased, the leaning of the shell toward the molten
steel side is further suppressed and, as a result, the effect of
suppressing the leaning of the nail becomes more significant.
[0060] In FIG. 1, the influence of the casting speed on the nail
depth is shown. The nail depth becomes 1 mm or less when the
casting speed is more than about 2.0 m/min and the short side
length (slab thickness) of the casting space of the mold is about
240 mm or less. In addition, the nail depth becomes about 0.7 mm or
less when the casting speed is about 2.4 m/min or more.
(2) Suppression of Adsorption of Foreign Materials
[0061] Concomitant with solidification, due to segregation of a
solute concentrated at the interface of the solidification shell,
the gradient of surface tension is generated and, because of a
force caused by this gradient, a phenomenon is generated in which
foreign materials are likely to be adsorbed or trapped on the
interface of the solidification shell. Hence, an attempt has been
carried out in which the concentration of S or Ti is decreased
which has a particularly significant influence as a solute element
of enhancing a force adsorbing and trapping foreign materials.
However, in some cases, manipulation of components may
disadvantageously cause increase in cost when S is decreased and
degradation in quality when Ti is decreased.
[0062] According to the invention, the force of adsorbing and
trapping foreign materials on the interface of the solidification
shell is suppressed by increasing the casting speed Vc. That is,
when the casting speed Vc is high, such as more than about 2.0
m/min, since the solidification amount at the meniscus portion is
decreased, the segregation amount is also decreased. Hence, the
gradient of surface tension, which functions as a force of
attracting foreign materials, is also decreased. As a result, the
amount of foreign materials adsorbed and trapped at the
solidification shell side is also reduced.
(3) Reduction in Thickness of Trapping Foreign Materials
[0063] FIG. 2 shows the relationship in a surface portion of the
slab between a trapping depth h from the slab surface at which
foreign materials are trapped and the number of trapped foreign
materials. In addition, FIG. 3 shows the relationship between the
number of trapped foreign materials and a distance L from the
meniscus (the surface of molten steel) which is obtained by
converting the trapping depth h from the slab surface. The
conversion is performed in accordance with the following equation:
h=k(L/Vc).sup.1/2 In this equation, Vc indicates the casting speed,
and a solidification constant k is 20 mmmin.sup.-1/2.
[0064] Foreign materials are trapped by the shell in a region from
the molten steel surface to a depth of 20 mm as can be seen from
FIGS. 2 and 3. In addition, the trapping depth is decreased as the
casting speed is increased, and at a casting speed Vc of more than
2.0 m/min, the trapping depth h from the slab surface is 1 mm or
less.
[0065] When the trapping depth h is 1 mm or less, although foreign
materials are trapped by the shell, in a subsequent process forming
products through a hot rolling step and a cold rolling step, the
foreign materials are scraped off and removed together with oxide
scales formed on the surface of a cast steel sheet. Accordingly, a
defect-free product can be obtained without performing slab
conditioning. In addition, the nail depth becomes 0.7 mm or less,
that is, the trapping thickness h also becomes not more than the
nail depth when the casting speed is about 2.4 m/min or more.
Hence, the casting speed is more preferably set to about 2.4 m/min
or more.
(4) Reduction in Probability of Trapping Foreign Materials
[0066] The residence time of the solidification shell in the region
from the molten steel surface to a depth of 20 mm in which foreign
materials are likely to be trapped by the solidification shell
decreases as the casting speed increases. Accordingly, the
probability of trapping foreign materials by the solidification
shell decreases even when the same amount of foreign materials is
present floating in molten steel. For example, when Vc is 3.0
m/min, the trapping probability decreases to one half of that when
Vc is 1.5 m/min.
(5) Preferable Oscillation Frequency of Mold for Prevention of
Sudden Variation of Molten Steel Surface Level
[0067] When casting in preformed at a casting speed Vc of more than
about 2.0 m/min, since the thickness of the solidification shell in
the mold further decreases, although being not so apparent, a
bulging phenomenon is generated. The bulging phenomenon is a
phenomenon in which the solidification shell is pushed toward the
mold side by the influence of the static pressure of molten steel.
In this bulging phenomenon, when the temperature of the shell is
high, and when a type of steel is a ultra-low carbon steel or the
like having a small shell strength as compared to that of other
types of steel, the bulging (being pushed to the mold) speed
becomes higher than the oscillation speed of the mold. When a mold
generally having a taper to compensate for volume contraction
caused by solidification contraction and/or thermal contraction is
oscillated vertically, the solidification shell bulges by a bulging
amount .delta..sub.b concomitant with the descent of the mold. On
the contrary, concomitant with the ascent of the mold, the mold
pushes the shell thus bulged by a pushing force of .delta..sub.p
which is approximately equivalent to .delta..sub.b. When being
simply calculated, the change in molten steel surface level caused
by this change of volume is small, such as less than about 1 mm.
However, when the phenomenon described above is repeatedly
performed, the oscillation of molten steel surface level and the
oscillation of the mold may resonate with each other. As a result,
an abnormal phenomenon may occur in rare cases in which the molten
steel surface level suddenly and largely varies. It has been
difficult to detect this phenomenon using an ordinary eddy-current
type level sensor for molten steel surface since this abnormal
phenomenon occurs at the edge portion of the mold. However, we
first discovered this phenomenon by investigation of the distortion
of an oscillation mark of a cast steel slab with time. In
particular, when the casting speed is more than about 2.0 m/min and
the oscillation frequency of the mold is high, such as more than
about 185 cycles/min, this abnormal phenomenon described above is
likely to be observed. As a result, mold flux may be engulfed in
the molten steel and may be trapped in the solidification shell,
thereby causing defects in the surface portion of the cast steel
sheet. Accordingly, in the case of casting at a casting speed of
more than about 2.0 m/min, the number of surface defects in the
product caused by the mold flux is suddenly increased. As a result,
it has been difficult to decrease the surface defects.
[0068] However, from the relationship between the oscillation
frequency of the mold and the ratio of the flux-related defects to
the total defects, the ratio being used as the index showing the
rate of occurrence of the sudden abnormal phenomenon, it was found
that when the oscillation frequency of the mold is set to about 185
cycles/min or less, the abnormal phenomenon as described above can
be effectively prevented even when the casting speed Vc is more
than about 2.0 m/min.
[0069] In addition, the lower limit of the oscillation frequency of
the mold may be set in view of reduction in area of trapping
foreign materials so as not to increase the nail depth and also in
view of prevention of restraint breakout caused by the decrease in
lubricant properties (consumption amount of mold flux) in the mold.
For example, it is preferable that a negative strip time is about
0.02 seconds or more and that a negative strip length is about 0.1
mm or more. The negative strip time is one characteristic value for
defining the mold oscillation conditions and indicates a period of
time in which the descending speed of the mold is higher than that
of the cast steel slab. The negative strip length indicates the
maximum distance between the mold and the cast steel slab within
the negative strip time, the mold passing by the cast steel slab
which is being drawn. .pi.Sf/Vc>1 is satisfied when the
oscillation waveform of the mold is assumed to have a sine waveform
wherein S indicates the oscillation stroke of the mold, f indicates
the mold frequency, and Vc indicates the casting speed. For
example, when Vc is 2.0 m/min and S is 9 mm, the lower limit of the
mold frequency f is 71 cpm (cycles/minute), and when S is 5 mm, the
lower limit is 127 cpm. It is not necessary that the oscillation
waveform of the mold be limited to a sine waveform. Also, in
consideration of the specification of oscillation conditions of the
continuous casting apparatus and the controllability thereof, the
lower limit of the frequency and the waveform may be appropriately
determined.
(6) Prevention of Short-Side Bulging (Reason for Upper Limitation
of Short Side Length of Casting Space of Mold)
[0070] Although an immersion nozzle is used which satisfies the
ratio D/d of the short side length (slab thickness) D of the
casting space of the mold to the lateral width d of the discharge
spout of the immersion nozzle, when the short side length is too
large, in casting at a casting speed Vc of more than abut 2.0
m/min, problems occur. Particularly, slab shape-related defects
and/or breakout are generated by short-side bulging. On the
contrary, when the short side length is small, and when the casting
speed Vc is high, the bulging of the short side of the slab passing
through the mold, which is caused by a static pressure of molten
steel, can be suppressed, and the risk of breakout generation is
small.
[0071] However, as shown in FIG. 4, when the short side length
(that is, the slab thickness) is more than 240 mm, although the
casting speed is 2.4 m/min, by the increase in jet flow speed of
the molten steel from the discharge spout of the immersion nozzle
due to the increase in slab thickness, a secondary flow speed is
increased by application of an electromagnetic brake. It becomes
difficult to suppress the delay of growth of a shell along the
short side as a result. Accordingly, short-side bulging at the
bottom end of the mold becomes apparent and the risk (bulging
amount of 10 mm or more) of breakout generation increases.
[0072] In addition, when the short side length (that is, the slab
thickness) is more than 240 mm, by the same reason as that
described above, since the fluctuation of the molten steel surface
level is facilitated by an inversion flow and a secondary flow of
the jet flow of the molten steel, which flows are from the short
sides of the solidification shell, engulfment and trapping of mold
flux are liable to occur. In addition, due to the increase in slab
thickness, stagnation of molten steel at the meniscus portion,
particularly, in the vicinity of the immersion nozzle, is liable to
occur. As a result, as shown in FIG. 5, the number of slab surface
defects and that of the product defects increases.
(7) Reason for Lower Limitation of Short Side Length of Casting
Space of Mold
[0073] It is not preferred that the short side length (slab
thickness) of the casting space of the mold is less than about 150
mm, for the following reasons.
[0074] The above effect (1) cannot be obtained in view of
controllability of molten steel surface level when the
cross-sectional area of the slab excessively decreases. The reason
for this is that when the casting amount is changed, the
fluctuation in molten steel surface level increases as compared to
the case in which a slab having a large cross-sectional area is
formed. Also, due to the formation of molten steel ripples thereby,
the rate of generation of nails having a depth of 1 mm or more is
increased. In addition, engulfment and trapping of mold flux are
liable to occur (see FIG. 5) due to the fluctuation in molten steel
surface level. Furthermore, the outer diameter of an ordinary
immersion nozzle is determined by the sum of the wall thickness
(about 20 mm or more) determined in consideration of durability and
the inside diameter (about 70 to about 130 mm) determined to ensure
a throughput of from 5.4 ton/min (150 mm thick, 2,200 mm wide, and
Vc of 2.1 m/min or more) to 14.5 ton/min (240 mm thick, 2,200 mm
wide, and Vc of 3.5 m/min or more). In this case, when the short
side length (slab thickness) D is excessively small, the distance
between the outer wall of the immersion nozzle and the long side of
the solidification shell becomes too small (less than 20 mm), the
flow therebetween becomes non-uniform, thereby resulting in
generation of longitudinal cracks. In an extreme case, the
solidification shell is brought into contact with the nozzle and is
bonded thereto, resulting in breakout generation. Hence, the short
side length (slab thickness) D is set to not less than about 150 mm
(inside diameter of 70 mm+total outer wall thickness of 40 mm
(20.times.2)+distance between the outer wall of the immersion
nozzle and the long side of the solidification shell of 40 mm
(20.times.2)).
[0075] In addition, the long side length (slab width) of the
casting space of the mold is not particularly limited and may be
equivalent to the width of an ordinary cold-rolled steel sheet (in
particular, cold-rolled steel sheet for automobiles). A length of
approximately 900 to 2,200 mm is preferred.
[0076] The height in the vertical direction of the mold is not
particularly limited. However, the height is preferably set to
approximately 800 to 1,000 mm since a solidification shell must be
formed having a certain thickness so that a cast steel slab passing
through the mold is not bulged even when casting is performed at a
casting speed of more than about 2.0 m/min.
(8) Optimization of Ratio D/d of Short Side Length D of Casting
Space of Mold to Lateral Width d of Discharge Spout of Immersion
Nozzle
[0077] While being decelerated, the molten steel jetted out of the
discharge spout of the immersion nozzle extends its width until it
collides against the short side shell. However, the degree of
deceleration and distribution of the jet flow speed of the molten
steel which collides against the short side shell depend on the
slab width W, the casting speed Vc, and the D/d ratio. When the
width d of the discharge spout of the immersion nozzle is too small
(D/d is too large) with respect to the short side length (slab
width) D of the casting space of the mold, as D, Vc and W increase,
and the ratio of a regional width in which molten steel having a
high flow speed collides against the short side shell to the slab
thickness (short side width) decreases. Hence, growth of the
solidification shell becomes non-uniform and is liable to be
interfered with. Also, breakout may occur in some cases when the
thickness of the solidification shell is extremely decreased. On
the other hand, when the width d of the discharge spout of the
immersion nozzle is too large (D/d is too small) with respect to
the short side length (slab width) D of the casting space of the
mold, as D, Vc, and W increase, the growth of the long side of the
solidification shell is interfered with since the jet flow of the
molten steel collides against the long side of the solidification
shell before it collides against the short side thereof, thereby
resulting in generation of transversal cracks and/or oblique
cracks. In addition, breakout may also occur in some cases when the
thickness of the solidification shell is extremely decreased. In
both cases described above, the influence of the slab width is
hardly observed.
[0078] In addition, in the case in which the molten steel collides
against the short side of the solidification shell, ascends, and
then flows along the molten steel surface at the long side, when
the ratio D/d is out of the optimum range due to the variation in
flow speed of the molten steel in the slab thickness direction, the
variation of the flow speed in the vicinity of the meniscus may be
partly influenced thereby, and the amount of engulfed mold flux
increases.
[0079] The maximum width d of the discharge spout which is
determined to ensure a throughput of about 5.4 to about 14.5
ton/min is preferably equal to or smaller than the inside diameter
(70 to 130 mm) of the immersion nozzle in view of durability
thereof. Accordingly, the ratio D/d is determined in consideration
of the optimum short side length (slab thickness) D (150 to 240 mm)
of the casting space of the mold and the width d (70 to 130 mm) of
the discharge spout. In the case in which long-period casting for
300 minutes or more is carried out, the total outer wall thickness
is preferably set to 25 mm.times.2=50 mm or more. In addition, the
distance between the mold and the nozzle is preferably set to 40 mm
or more to ensure a more stable quality. That is, the required
thickness other than the inside diameter is 50+40.times.2=130 mm.
On the other hand, in the case of short-period casting, the total
outer wall thickness may be set to 20 mm.times.2=40 mm, and the
distance between the mold and the nozzle may be set to
approximately 20 mm. That is, the thickness other than the inside
diameter is 40+20.times.2=80 mm.
[0080] In Table 1, the investigation results of the influence of
the ratio D/d to the product quality are shown. The ratio D/d is
preferably in the range of from 1.5 to 3.0. However, the ratio is
more preferably in the range of from about 2.1 to about 2.9 when
the optimum slab thickness, the durability of the immersion nozzle
and the required flow rate are also taken into consideration.
TABLE-US-00001 TABLE 1 LATERAL WIDTH OF DISCHARGE MOLD CASTING
SPOUT OF STROKE S OSCILLATION SLAB SLAB SPEED IMMERSION (TOTAL
FREQUENCY THICKNESS D WIDTH W Vc NOZZLE d AMPLITUDE OF MOLD f No.
(mm) (mm) (m/mim) (mm) D/d (mm) (TIMES/min) 1 220 1100-1800 2.4 60
3.67 7 160 2 220 1100-1800 2.4 70 3.14 7 160 3 220 1100-1800 2.4 75
2.93 7 160 4 220 1100-1800 2.4 80 2.75 7 160 5 220 1100-1800 2.4
130 1.69 7 160 6 235 1100-1800 2.4 88 2.67 7 160 7 235 1100-1800
2.4 100 2.35 7 160 8 235 1100-1800 2.4 120 1.96 7 160 9 235
1100-1800 2.4 160 1.47 7 160 NUMBER OF RATE OF SLAB SURFACE SURFACE
DEFECTS OF ELECTRO- CRACKS COLD-ROLLED GENERATION tn* MAGNETIC
(.gtoreq.5 mm) STEEL SHEET OF No. (s) BRAKE (/m.sup.2) (%) BREAKOUT
REMARKS 1 0.098 TYPE 1 65 2.1 BO AT COMP. EX. SHORT SIDE 2 0.098
TYPE 1 23 0 NO COMP. EX. 3 0.098 TYPE 1 0 0 NO EXAMPLE 4 0.098 TYPE
1 0 0 NO EXAMPLE 5 0.098 TYPE 1 5 0 NO EXAMPLE 6 0.098 TYPE 2 0 0
NO EXAMPLE 7 0.098 TYPE 2 0 0 NO EXAMPLE 8 0.098 TYPE 2 1 0 NO
EXAMPLE 9 0.098 TYPE 2 .gtoreq.100 23.5 BO AT LONG COMP. EX. SIDE
TYPE 1: EMBR TYPE 2: EMLS *tn = 60/f-tp tp = 60/(.pi.Sf) .times.
acos(-1000 Vc/.pi.Sf) COMP. EX.: COMPARATIVE EXAMPLE
(9) Braking of Flow by Electromagnetic Force
[0081] When the casting speed Vc is about 2.4 m/min or more, or the
throughput is about 7 ton/min or more, although the D/d is
optimized, the increase in rate of product defects is slightly
observed.
[0082] In the case described above, it is preferred that braking
the flow with an electromagnetic force be additionally performed,
and by this braking of the flow, more stable operation and
improvement in quality can be achieved.
[0083] As a method for braking the flow using an electromagnetic
force, techniques disclosed in Japanese Unexamined Patent
Application Publication Nos. 2-284750 and 57-17356 are preferably
used as described above.
[0084] In FIGS. 7A to 7C, continuous casting molds each provided
with a magnetic field application device, which are suitably used
for this invention, are schematically shown.
[0085] FIG. 7A shows magnetic application devices 1 disposed at an
upper portion of the mold including the molten steel surface level
and at a predetermined distance thereunder for applying static
magnetic fields in two stages. FIG. 7B shows a magnetic application
device 2 is disposed only at an upper portion of the mold including
the molten steel surface level for superimposingly applying a
static magnetic field and an AC magnetic field. FIG. 7C shows the
magnetic application device 2 is disposed at an upper portion of
the mold including the molten steel surface level for
superimposingly applying a static magnetic field and an AC magnetic
field and the magnetic application device 1 is disposed at a
predetermined distance under the magnetic field application device
2 for applying a static magnetic field.
[0086] Of the various magnetic field application devices described
above, the magnitude (magnetic flux density) of a DC magnetic field
is preferably set to approximately 1,000 to approximately 7,000
gausses when the magnetic field application device for applying a
static magnetic field is used. The value mentioned above may be
applied in both cases in which two devices are provided at the
upper and the lower positions and in which only one device is
provided at the lower position.
[0087] As the AC magnetic field, there are two types, that is, an
AC oscillating magnetic field and an AC travelling magnetic field,
and in the invention, both of them are preferably used.
[0088] FIG. 8 shows the AC oscillating magnetic field is a magnetic
field in which AC currents having phases practically opposite to
each other are applied to coils adjacent to each other or a
magnetic field in which AC currents having the same phase are
applied to coils having coiling directions opposite to each other
so as to practically invert a magnetic field generated in the
adjacent coils. A local flow can be induced in molten steel in the
mold when this AC oscillating magnetic field is superimposed on the
DC magnetic field. In the figures, reference numeral 3 indicates a
DC coil, reference numeral 4 indicates an AC coil, reference
numeral 5 indicates a mold, and reference numeral 6 indicates
molten steel (portion shown by oblique lines is a slow flow
region).
[0089] In addition, the AC travelling magnetic field is a magnetic
field obtained when AC currents having phases shifted by
360.degree./N are applied to N pieces of adjacent optional coils.
In general, as shown in FIG. 9, N=3 (a phase difference of
120.degree.) is used since a high efficiency can be obtained. Also
as described above, a local flow can be induced in molten steel in
the mold when this AC travelling magnetic field is superimposed on
the DC magnetic field.
[0090] The magnetic flux density of the AC magnetic field is
preferably set to approximately 100 to approximately 1,000 gausses
when the magnetic field application device for applying an AC
magnetic field as described above is used, and the frequency of the
oscillating magnetic field is preferably set to approximately 1 to
approximately 10 Hz.
[0091] Furthermore, the magnitude of a DC magnetic field is
preferably set to approximately 1,000 to approximately 7,000
gausses, and the magnetic flux density of an AC magnetic field is
preferably set to approximately 100 to approximately 1,000 gausses
when the magnetic field application device for superimposingly
applying a static magnetic field and an AC magnetic field is
used.
[0092] Continuous casting is preformed while the molten steel flow
is braked by an electromagnetic force using the magnetic field
application device as described above. Hereinafter, novel findings
in the continuous casting as described above on the phenomena
generated in the mold will be described together with reasons for
limiting the manufacturing conditions of the invention.
(10) Nozzle Immersion Depth (Distance from Molten Steel Surface to
Upper End of Discharge Spout)
[0093] The state of a circulation flow of molten steel in the mold
is varied in accordance with the change in nozzle immersion depth.
In particular, the immersion depth is optimized since the flow
speed from the immersion nozzle is high when the casting speed is
high. That is, the flow speed of molten steel at the surface
thereof becomes too high when the immersion depth is too small.
Engulfment of flux is facilitated as a result. On the other hand,
when the depth is too large, since the flow speed of molten steel
at the surface thereof is decreased too much, the effect of washing
the interface of the solidification shell decreases. Trapping of
bubbles and inclusions is facilitated as a result.
[0094] Accordingly, in consideration of the states described above,
when the optimum value of the nozzle immersion depth was
investigated, it was found that the nozzle immersion depth is set
in the range of from about 200 mm to about 350 mm.
[0095] In addition, as a material for the immersion nozzle as
described above, for example, ordinary alumina-graphite is
preferably used. However, the material is not limited thereto.
[0096] As the immersion nozzle described above, a cylindrical
nozzle or a two-spout nozzle in which the front end is closed and
two approximately circular discharge spouts are provided toward the
two short sides of the mold may be generally used. The
cross-sectional shape of the discharge spout may be circular,
square, or rectangular (longer in a lateral direction, or longer in
a longitudinal direction) and is not particularly limited, and any
type of shape may be used as long as the maximum width d satisfies
the conditions of the invention described later.
[0097] As has thus been described, engulfment of mold flux is
prevented by the above paragraphs (5), (6), and (8) as small as
possible, trapping of foreign materials into the solidification
shell is suppressed by the above paragraphs (2) and (4) even when
flux is engulfed or inclusions are floating in the molten steel,
and even if foreign materials are trapped, the depth from the
solidification shell surface at which foreign materials are trapped
is made smaller by the above paragraphs (1) and (3) so as not to
cause defects. Accordingly, in a process for forming products, in
particular, in a slab heating step, scaling-off and removal of
foreign materials from the surface layer of the slab can be
facilitated.
[0098] Hence, the effect described above can be stably achieved by
the above paragraphs (6), (7), (8), and (9) while high productivity
is obtained.
EXAMPLE 1
[0099] By using continuous casting apparatuses having molds
provided with casting spaces having various short side lengths,
various types of slabs having slab thickness of 110 mm (by a test
continuous casting apparatus), 200, 215, 220, 235, and 260 mm (by a
vertical-bending production continuous casting apparatus), and
having slab widths of 400 mm (by the test continuous casting
apparatus), and 900 to 2,200 mm (by the vertical-bending production
continuous casting apparatus) were prepared under the conditions
shown in Table 2A and 2B by casting. In that step, the heights of
the molds were 900 mm (by the production continuous casting
apparatus) and 700 mm (by the test continuous casting apparatus),
and the immersion nozzle was a two-spout nozzle made of
alumina-graphite having a wall thickness of 25 mm, the shape of the
discharge spout being square (when the slab thickness is 220 mm or
less) or circular (when the slab thickness is more than 220 mm),
the downward discharge angle being at a constant value of
20.degree., and the nozzle immersion depth (the distance from the
molten steel surface to the upper end of the discharge spout) being
set to 200 to 250 mm. As mold flux, a material was used having a
solidification temperature of 1,000.degree. C., a viscosity of 0.05
to 0.2 Pas (0.5 to 2.0 poise) at 1,300.degree. C., and a basicity
(CaO/SiO.sub.2) of 1.0. In addition, the degree of superheat for
molten steel in a tundish was set to 10 to 30.degree. C.
Furthermore, components of molten steel, which had a ultra-low
carbon steel composition, were 0.0005 to 0.0090 mass percent of C,
less than 0.05 mass percent of Si, less than 0.50 mass percent of
Mn, less than 0.035 mass percent of P, less than 0.020 mass percent
of S, 0.005 to 0.060 mass percent of Al, less than 0.080 mass
percent of Ti, less than 0.050 mass percent of Nb, and less than
0.0030 mass percent of B. In addition, the mold oscillation
waveform was a sine waveform.
[0100] The maximum short-side bulging amount, the maximum nail
depth, the maximum number of slab surface defects and generation of
breakout were measured for the various types of slabs thus formed.
The results thereof are shown in Table 3. The maximum short-side
bulging amount is preferably 10 mm or less, more preferably 5 mm or
less. The maximum nail depth is preferably 1 mm or less, more
preferably 0.7 mm or less.
[0101] In addition, in Table 3, the results of measurement of rate
of surface defects of a cold-rolled steel sheet (sheet thickness of
0.8 mm) are also shown, the cold-rolled steel sheet being obtained
by the steps of heating each of the above slabs at a temperature of
1,100 to 1,200.degree. C. for 2 to 2.5 hours, followed by hot
rolling, cold rolling, and finish annealing in accordance with an
ordinary process.
[0102] Furthermore, investigation on the influence of the casting
speed on the slab surface defects and on the surface defects of the
cold-rolled steel sheet was summarized. The results thereof are
shown in FIG. 6.
[0103] The maximum number of slab surface defects was the number
(pieces/m.sup.2) of bubbles (a diameter of 0.2 mm or more), alumina
clusters (a diameter of 500 .mu.m or more), and slag (including
mold flux, a diameter of 0.5 mm or more) per unit area observed
after the following sequential steps of milling the slab surface by
1 mm, performing polishing using emery paper #1000, and performing
etching using a mixed solution of hydrochloric acid and hydrogen
peroxide.
[0104] In addition, the rate of surface defects of a cold-rolled
steel sheet was the ratio, on a percent basis, of the number of
defects, such as scratches and spills, caused by casting with
respect to the total defects, the number of defects being measured
on the front and the rear surfaces per 1,000 m of a cold-rolled
steel sheet.
[0105] The generation of breakout was defined as "Yes" when even at
least one breakout occurred in casting under each of the individual
conditions.
[0106] In addition, "Type 1" described as an electromagnetic brake
indicates static magnetic field application (EMBR) performed for
the entire mold at the vicinity of the bottom end of the mold,
"Type 2" described as an electromagnetic brake indicates static
magnetic field application (EMLS) performed for the entire mold at
the discharge spout of the immersion nozzle, and the "Type 1" and
"Type 2" were preformed based on the techniques disclosed in
Japanese Unexamined Patent Application Publication Nos. 2-284750
and 57-17356, respectively.
[0107] The negative strip time tn is one characteristic value for
defining the mold oscillation conditions and indicates a period of
time in which the descending speed of the mold is higher than that
of a cast steel sheet. As can be seen from Table 3 and FIG. 6, when
a slab is formed by casting in accordance with the invention, even
when the casting speed is high, such as more than about 2.0 m/min,
the degree of surface defects of the slab thus formed was slight,
and surface defects of a cold-rolled steel sheet formed therefrom
were not substantially detected, or even when the defects are
present, the number thereof was very small.
[0108] As can be seen from the example described above, in
accordance with the invention, the operation conditions are
preferably optimized so that the following states can achieved:
[0109] (1) the relative pushing force toward the mold wall by
static pressure of the molten steel increases which is applied to a
shell solidified in the vicinity of the molten steel surface in the
mold,
[0110] (2) the phenomenon of adsorbing inclusions, slag, flux and
bubbles on the interface of the solidification shell is suppressed,
and the probability of trapping foreign materials decreases,
and
[0111] (3) the depth of trapping foreign materials into the
solidification shell decreases as much as possible.
[0112] Accordingly, even when casting is performed at a high speed,
such as more than about 2.0 m/min, while high productivity and
stable operation are being maintained, a high-quality slab for a
cold-rolled steel sheet used for forming outer plates of
automobiles can be supplied without slab surface treatment.
EXAMPLE 2
[0113] Molten steel (approximately 300 tons), which was obtained by
melting in a converter followed by RH treatment, was formed into a
slab by continuous casting using a continuous casting apparatus
provided with one of the magnetic field application devices shown
in FIGS. 7A to 7C, the molten steel having a composition containing
0.0015 mass percent of C, 0.02 mass percent of Si, 0.08 mass
percent of Mn, 0.015 mass percent of P, 0.004 mass percent of S,
0.04 mass percent of Al, 0.04 mass percent of Ti, and the balance
being Fe and inevitable impurities. The manufacturing conditions in
this example are shown in Table 2. As the immersion nozzle, a
two-spout immersion nozzle was used having rectangular discharge
spouts each provided with a downward discharge angle of
15.degree..
[0114] Subsequently, surface segregation and the amount of
non-metallic inclusions of the slab thus formed and surface defects
caused by mold flux after cold rolling were measured. The results
thereof are shown in Table 3.
[0115] Surface segregation was evaluated by visual inspection from
the number of segregations per 1 m.sup.2 after the steps of slab
polishing and etching were performed. In addition, the non-metallic
inclusions were extracted by slime extraction from part of the cast
steel sheet located at a depth of one fourth of the thickness from
the surface thereof. Subsequently, the weight of the inclusions was
measured. Furthermore, the surface defects of a coil formed by cold
rolling were checked by visual inspection and then sampled,
followed by analysis. The number of defects caused by mold flux was
obtained. To reduce surface segregation, the amount of inclusions,
and the number of defects caused by mold flux to index numbers for
purposes of comparison, the worst result obtained among all the
conditions was regarded as an index number of 10. Each result was
represented by the ratio to the worst result based on the
assumption that the linear relationship was satisfied
therebetween.
[0116] As can be seen from Table 3, in accordance with the
invention, when the casting speed, the short side length D of the
casting space of the mold, the nozzle immersion depth, the ratio
D/d of the short side length D to the lateral width d of the
discharge spout of the immersion nozzle were appropriately
controlled together with appropriate application of an
electromagnetic brake to the flow of the molten steel in the mold,
the number of the surface segregations, the amount of non-metallic
inclusions, and the number of the defects caused by mold powder
could be reduced.
[0117] When the intensity of the oscillating magnetic field is too
high, engulfment of flux at the molten steel surface increased,
resulting in degradation in surface quality. In addition, when the
frequency is too high, the molten steel surface level cannot follow
the magnetic field, and the effect of washing the interface of the
solidification shell decreases, thereby resulting in increase in
number of bubbles and inclusion defects. TABLE-US-00002 TABLE 2A
LATERAL SHORT WIDTH MOLD SIDE OF STROKE LENGTH DISCHARGE S (SLAB
CASTING THROUGHPUT OF SPOUT OF (TOTAL OSCILLATION THICK- SLAB WIDTH
SPEED MOLTEN STEEL IMMERSION AMPLI- FREQUENCY NESS) W (mm) Vc
(ton/min) NOZZLE TUDE) OF MOLD Tn* No. D (mm) MINIMUM MAXIMUM
(m/min) MINIMUM MAXIMUM d (mm) D/d (mm) (TIMES/min) (s) 1 220 900
1950 1.0 1.6 3.4 80 2.75 6 120 0.177 2 220 900 1950 1.5 2.3 5.1 80
2.75 6 130 0.134 3 220 900 1950 1.8 2.3 6.1 80 2.75 6 150 0.112 4
220 900 1950 2.0 3.1 6.7 80 2.75 6 185 0.099 5 220 900 1950 2.1 3.3
7.1 80 2.75 5 170 0.075 6 220 900 1950 2.2 3.4 7.4 80 2.75 5 180
0.072 7 220 1200 1950 1.5 3.1 5.1 80 2.75 9 190 0.129 8 220 1200
1950 1.8 3.7 6.1 80 2.75 9 190 0.124 9 220 1200 1950 2.0 4.1 6.7 80
2.75 9 190 0.120 10 220 1200 2200 2.3 4.8 8.7 80 2.75 9 160 0.124
11 220 1200 2200 2.3 4.3 8.7 80 2.75 9 185 0.115 12 220 1200 1840
2.3 4.8 7.3 80 2.75 9 195 0.112 13 220 1200 1500 2.3 4.8 6.0 80
2.75 9 205 0.108 14 220 900 1950 2.1 3.3 7.1 80 2.75 6 160 0.096 15
220 900 1950 2.2 3.4 7.4 80 2.75 7 160 0.107 16 220 900 1950 2.3
3.6 7.7 80 2.75 7 160 0.102 17 220 900 2200 2.5 3.9 9.5 80 2.75 6
160 0.071 18 220 900 2200 2.7 4.2 10.3 80 2.75 8 160 0.100 19 220
900 2000 3.0 4.7 10.4 80 2.75 9 160 0.101 20 220 900 1950 3.5 5.4
11.8 80 2.75 9 180 0.086 21 110 400 400 2.5 0.9 0.9 30 3.67 6 160
0.071 22 200 900 1950 2.5 3.5 7.7 70 2.86 6 160 0.071 23 215 900
1950 2.5 3.8 8.2 88 2.44 6 160 0.071 24 235 900 1950 2.5 4.2 9.0 88
2.67 6 160 0.071 25 250 900 1950 2.5 4.4 9.6 88 2.84 6 160 0.071 26
260 900 1950 2.5 4.6 9.9 88 2.95 6 160 0.071 27 220 1200 1950 2.5
5.2 8.4 80 2.75 6 160 0.071 28 235 1200 1950 2.5 5.5 9.0 88 2.67 7
160 0.093 29 235 1200 1950 1.5 3.3 5.4 88 2.67 7 185 0.123 30 235
1200 1950 2.1 4.6 7.6 88 2.67 6 180 0.096 31 235 1200 2200 2.5 5.5
10.1 130 1.81 6 185 0.080 32 220 900 2200 2.5 3.9 9.5 80 2.75 6 185
0.080 33 220 900 2200 2.5 3.9 9.5 80 2.75 6 185 0.080 34 220 900
2200 2.5 3.9 9.5 80 2.75 6 185 0.080 35 220 900 2200 2.5 3.9 9.5 80
2.75 6 185 0.080 36 220 900 1950 2.1 3.3 7.1 80 2.75 6 160 0.096 37
220 900 2000 3.0 4.7 10.4 80 2.75 9 160 0.101 TYPE 1: OSCILLATING
MAGNETIC FIELD, TYPE 2: SHIFTING MAGNETIC FIELD
[0118] TABLE-US-00003 TABLE 2B DEPTH OF TYPE OF IMMER- AC UPPER AC
UPPER DC LOWER DC SION MAG- MAGNETIC MAGNETIC MAGNETIC NOZZLE NETIC
FIELD FIELD FIELD No. (mm) FIELD (Gauss) (Gauss) (Gauss) 1 280 NO 0
0 0 2 280 NO 0 0 0 3 280 NO 0 0 0 4 280 NO 0 0 0 5 280 NO 0 0 0 6
280 NO 0 0 0 7 280 TYPE 1 1000 1000 0 8 280 TYPE 1 700 1000 0 9 280
TYPE 1 500 1000 0 10 280 TYPE 1 300 1000 0 11 280 TYPE 1 300 1000 0
12 280 TYPE 1 300 1000 0 13 280 TYPE 1 300 1000 0 14 280 TYPE 1 300
1000 0 15 280 TYPE 1 300 1000 0 16 280 TYPE 1 300 1000 0 17 280
TYPE 1 0 1000 1500 18 280 TYPE 1 0 1500 2000 19 280 TYPE 1 0 2000
2500 20 280 TYPE 1 0 2500 3000 21 280 TYPE 1 0 0 0 22 280 TYPE 1
200 1000 0 23 280 TYPE 1 200 1000 0 24 280 TYPE 1 200 1000 0 25 280
TYPE 1 200 1000 0 26 280 TYPE 1 200 1000 0 27 280 NO 0 0 0 28 280
NO 0 0 0 29 280 TYPE 2 600 0 0 30 280 TYPE 2 600 1000 0 31 280 TYPE
2 600 1000 0 32 180 TYPE 1 200 1000 0 33 200 TYPE 1 200 1000 0 34
350 TYPE 1 200 1000 0 35 370 TYPE 1 200 1000 0 36 280 TYPE 1 300
1000 1500 37 280 TYPE 1 300 1000 1500
[0119] TABLE-US-00004 TABLE 3 MAXIMUM MAXIMUM RATIO OF SHORT-
NUMBER OF POWDER SIDE MAXIMUM SLAB RATE OF DEFECTS BULGING NAIL
SURFACE SURFACE GENERATION TO TOTAL AMOUNT DEPTH DEFECTS DEFECTS OF
DEFECTS No (mm) (mm) (/m.sup.2) (%) BREAKOUT (%) REMARKS 1 0 3.5
3.10 NO 49 COMP. EX. 1 2 1 2.7 185 2.35 NO 24 COMP. EX. 2 3 1 2.6
120 1.23 NO 20 COMP. EX. 3 4 2 1.5 90 0.30 NO 36 COMP. EX. 4 5 2
1.1 55 0.15 NO 0 EXAMPLE 1 6 1 0.7 45 0.05 NO 3 EXAMPLE 2 7 1 3.0
3.10 NO 33 COMP. EX. 5 8 1 2.9 1.54 NO 20 COMP. EX. 6 9 2 2.2 0.50
NO 16 COMP. EX. 7 10 4 0.8 0 NO 0 EXAMPLE 3 11 4 0.9 0.11 NO 5
EXAMPLE 4 12 3 1.3 2.6 NO 74 COMP. EX. 8 13 3 1.3 4.1 NO 85 COMP.
EX. 9 14 2 1.0 50 0 NO 0 EXAMPLE 5 15 3 0.6 30 0 NO 0 EXAMPLE 6 16
3 0.5 20 0 NO 0 EXAMPLE 7 17 3 0.2 10 0 NO 0 EXAMPLE 8 18 5 0.2 3 0
NO 0 EXAMPLE 9 19 5 0.1 3 0 NO 0 EXAMPLE 10 20 6 0.2 5 0 NO 0
EXAMPLE 11 21 1 1.4 70 NO COMP. EX. 10 22 1 0.1 15 0.02 NO 0
EXAMPLE 12 23 2 0.2 11 0 NO 0 EXAMPLE 13 24 5 0.3 13 0 NO 0 EXAMPLE
14 25 10 0.8 25 0.3 NO 4 COMP. EX. 11 26 15 1.1 60 0.4 NO 60 COMP.
EX. 12 27 9 0.7 0.03 YES 15 EXAMPLE 15 28 9 0.6 0.05 NO 21 EXAMPLE
16 29 0 2.5 5.90 NO 37 COMP. EX. 13 30 1 0.8 0 NO 0 EXAMPLE 17 31 2
0.4 0 NO 0 EXAMPLE 18 32 2 0.4 0.05 NO 33 EXAMPLE 19 33 2 0.4 0 NO
0 EXAMPLE 20 34 2 0.4 0 NO 0 EXAMPLE 21 35 2 0.6 1.5 NO 67 COMP.
EX. 14 36 2 1.0 20 0 NO 0 EXAMPLE 22 37 3 0.5 12 0 NO 0 EXAMPLE 23
* BLANK COLUMN: NOT MEASURED COMP. EX.: COMPARATIVE EXAMPLE
* * * * *