U.S. patent application number 14/570260 was filed with the patent office on 2015-06-25 for method of manufacturing ti-containing austenitic stainless steel sheet by twin roll strip caster.
The applicant listed for this patent is POSCO. Invention is credited to Dae Sung Lee, Sung-Jin Park, Byoung-Jun Song.
Application Number | 20150174647 14/570260 |
Document ID | / |
Family ID | 53399029 |
Filed Date | 2015-06-25 |
United States Patent
Application |
20150174647 |
Kind Code |
A1 |
Park; Sung-Jin ; et
al. |
June 25, 2015 |
Method of Manufacturing Ti-Containing Austenitic Stainless Steel
Sheet by Twin Roll Strip Caster
Abstract
There is provided a method of manufacturing a titanium
(Ti)-containing austenitic stainless steel sheet having a high
degree of surface quality by using a twin roll strip caster. The
method includes controlling a composition of molten steel such that
a TiN precipitation temperature may be higher than a temperature of
the molten steel in a tundish (T/D) by at least 50.degree. C.
(.DELTA.T.gtoreq.50.degree. C.), the TiN precipitation temperature
being defined by the following formula 2:
Log(N%)=-19,755/(T+273)+7.78+0.07[% Ti]-log [% Ti]+ 0.045[% Cr]
T(.degree.
C.)=-19,755/log(N%)-7.78-0.07(%Ti)+log(%ti)-0.045(%Cr)-273 [Formula
2]
Inventors: |
Park; Sung-Jin; (Pohang-si,
KR) ; Song; Byoung-Jun; (Pohang-si, KR) ; Lee;
Dae Sung; (Pohang-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si |
|
KR |
|
|
Family ID: |
53399029 |
Appl. No.: |
14/570260 |
Filed: |
December 15, 2014 |
Current U.S.
Class: |
164/463 |
Current CPC
Class: |
B22D 11/002 20130101;
B22D 11/0622 20130101; B22D 11/001 20130101 |
International
Class: |
B22D 11/00 20060101
B22D011/00; B22D 11/06 20060101 B22D011/06 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 20, 2013 |
KR |
10-2013-0160262 |
Claims
1. A method of manufacturing a titanium (Ti)-containing austenitic
stainless steel sheet having a high degree of surface quality by
using a twin roll strip casting process, the method comprising
controlling a composition of molten steel such that a TiN
precipitation temperature is higher than a temperature of the
molten steel in a tundish (T/D) by at least 50.degree. C.
(.DELTA.T.gtoreq.50.degree. C.), the TiN precipitation temperature
being defined by the following formula 2: Log ( N % ) = - 19 , 755
/ ( T + 273 ) + 7.78 + 0.07 [ % Ti ] - log [ % Ti ] + 0.045 [ % Cr
] T ( .degree. C . ) = - 19 , 755 log ( N % ) - 7.78 - 0.07 ( % Ti
) + log ( % Ti ) - 0.045 ( % Cr ) - 273 [ Formula 2 ]
##EQU00003##
2. The method of claim 1, wherein the molten steel comprises, by
weight %, carbon (C): 0.025% to 0.055%, silicon (Si): 0.25% to
0.55%, manganese (Mn): 1.5% to 1.8%, chromium (Cr): 17.1% to 17.7%,
nickel (Ni): 9.25% to 9.65%, titanium (Ti): 0.2% to 0.5%, nitrogen
(N): 0.025% or less, and the balance of iron (Fe) and inevitable
impurities.
3. The method of claim 1, wherein the molten steel has a Ti/C ratio
of 8 or greater.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of Korean Patent
Application No. 10-2013-0160262 filed on Dec. 20, 2013, with the
Korean Intellectual Property Office, the disclosure of which is
incorporated herein by reference.
BACKGROUND
[0002] The present disclosure relates to a method of manufacturing
a titanium (Ti)-containing austenitic stainless steel sheet using a
twin roll strip caster, and more particularly, to a method of
manufacturing a Ti-containing austenitic stainless steel sheet
having a high degree of surface quality by directly casting molten
steel into a strip in a twin roll strip caster instead of using a
conventional continuous casting process, so as to prevent a nozzle
clogging and the formation of large surface defects (caused by TiN
and oxide inclusions).
[0003] Titanium (Ti)-containing austenitic stainless steel has a
high Ti content of about 0.2% to about 0.5%, and Ti nitrides or
oxides are very likely to be formed if Ti-containing austenitic
stainless steel is cast in a general continuous casting process.
Mixtures of such Ti nitrides and oxides are known to cause clogging
of immersion nozzles of ladles or tundishes.
[0004] In addition, large clusters formed of Ti nitrides cause many
surface defects, and thus most Ti-containing steel sheets are
subjected to a post-process such as a hot-rolling process after
grinding upper/lower surfaces thereof. If such defects remain on
hot-rolled coils, final products may only be produced after
performing a coiling grinding process or a cold-rolling process on
the hot-rolled coils. Due to these reasons, additional process
costs and burdens are increased, and process yields are decreased,
to lower profits. Thus, there is a need for a method of solving
such problems.
[0005] In general, titanium (Ti) is included in highly
corrosion-resistant Ti-containing stainless steel in an amount of
0.2% to 0.5% so as to fix carbon in the form of TiC and thus to
prevent chromium (Cr), an important element guaranteeing the
corrosion resistance of stainless steel, from precipitating in the
form of Cr.sub.23C.sub.6. Like general stainless steel,
Ti-containing stainless steel may be produced through processes
using an electric furnace, a refining furnace, a ladle treatment
unit, and a continuous caster. That is, in the related art,
Ti-containing stainless steel is formed as a slab by a continuous
casting method, and the slab is hot-rolled to form a strip having a
thickness of 2 mm to 6 mm.
[0006] FIG. 1 is a schematic view illustrating a continuous casting
process of the related art. As shown in FIG. 1, a continuous
casting process may be performed using a ladle 2 containing molten
steel 1 produced through a steel making process, a tundish 4
disposed between the ladle 2 and a mold 8 as a buffer, the mold 8
for producing slabs, and a secondary cooling table 9. Molten steel
1 is formed into a slab while passing though the above-mentioned
devices, and then the slab is reheated and hot-rolled into a strip
in a region denoted by a box in FIG. 1. Then, the strip is
water-cooled and coiled.
[0007] For example, scraps and a ferro alloy are melted into molten
steel in an electric furnace, and the molten steel is refined in a
refining furnace to remove carbon, phosphorous, and sulfur
therefrom. Thereafter, the temperature and minor components of the
molten metal are adjusted in a ladle treatment unit according to
conditions for a casting process. After the temperature and minor
component adjustment, the molten steel is continuously cast and
formed as a hot-rolled coil product.
[0008] However, the addition of titanium (Ti) results in the
formation of Ti oxides (e.g. TiO.sub.2) and Ti nitrides (e.g. TiN)
because titanium (Ti) has a high affinity for oxygen and nitrogen,
and since Ti oxides and Ti nitrides have very high melting points,
Ti oxides and Ti nitrides may clog nozzles during a casting process
and degrade the surface quality of products. Particularly, if
nozzles are clogged during a casting process, molten steel may not
be supplied to a tundish or a caster, and thus it may be impossible
to perform the casting process. That is, due to such a situation,
the production of products may be interrupted.
[0009] Materials causing the clogging of nozzles may be included in
molten steel as large inclusions. In this case, the surface quality
of products may be very poor. As described above, if nozzles are
clogged during a casting process, product quality as well as
productivity may be largely affected.
[0010] In the related art, the following methods have been proposed
to prevent the clogging of nozzles.
[0011] First, a method of decreasing the amount of titanium (Ti)
has been proposed. That is, the amount of titanium (Ti) is
decreased to reduce the formation of Ti oxides and Ti nitrides. In
this case, however, the amount of carbon (C) also has to be
reduced. The basic design concept of Ti-containing stainless steel
is to increase the content of titanium (Ti) to a certain value or
greater according to the content of carbon (C) so as to obtain a
desired degree of corrosion resistance. That is, the ratio of Ti/C,
termed a Ti stabilization ratio, is set to be about 5 to about 10,
generally at least 5. Therefore, if the addition of titanium (Ti)
is reduced, the amount of carbon also has to be reduced to satisfy
the Ti stabilization ratio. In this case, a large amount of oxygen
may have to be supplied to a refining furnace to decrease the
amount of carbon. Due to this, a larger amount of oxygen may be
dissolved in molten steel, and thus the formation of Ti oxides may
be unexpectedly increased. In addition, since the period of time
necessary for a refining process is increased, the temperature of
molten steel may increase, and thus a considerable amount of
coolant may be necessary for cooling the molten steel after the
molten steel is discharged from the refining furnace. In this case,
the molten steel may easily make contact with ambient air while
being cooled by the coolant and thus may be re-oxidized to cause
the formation of large amounts of inclusions.
[0012] Secondly, in another method of reducing the amount of
titanium (Ti), a large amount of aluminum (Al), having a relatively
higher affinity for oxygen than titanium (Ti), is added before
titanium (Ti) is added so as to remove dissolved oxygen. This
method effectively prevents the formation of Ti oxides. However, a
large amount of Al.sub.2O.sub.3 that affects the surface quality of
products more negatively than Ti oxides is formed as a result of
oxygen removal using aluminum (Al), and thus a process of adding
calcium (Ca) to molten steel is required to convert Al.sub.2O.sub.3
into CaO--Al.sub.2O.sub.3 having a less negative effect. However,
it is difficult to control the content of calcium (Ca) due to a
high degree of volatility of calcium (Ca). In addition, if the
content of calcium (Ca) is excessively low or high, inclusions
having a desired shape may not be formed, and thus surface defects
may increase. Particularly, if the content of calcium (Ca) is
excessively high, calcium (Ca) included in molten steel may react
with Al.sub.2O.sub.3 included in a refractory material of a stopper
that is used to adjust the supply of molten steel from a tundish to
a caster, and as a result, a compound having a low melting point,
CaO--Al.sub.2O.sub.3, is formed to increase erosion of the stopper.
In this case, the supply of molten steel may become unstable, and
thus the quality of products may be lowered.
SUMMARY OF THE INVENTION
[0013] An aspect of the present disclosure may provide a method of
manufacturing a titanium (Ti)-containing austenitic stainless steel
sheet by using a twin roll strip caster so as to suppress the
formation of TiN and effectively prevent nozzle clogging occurring
in a general continuous casting process, thereby ensuring casting
stability and a high degree of product surface quality.
[0014] However, aspects of the present disclosure are not limited
thereto. Additional aspects will be set forth in part in the
description which follows, and will be apparent from the
description to those of ordinary skill in the related art.
[0015] According to an aspect of the present disclosure, there may
be provided a method of manufacturing a titanium (Ti)-containing
austenitic stainless steel sheet having a high degree of surface
quality by using a twin roll strip casting process, the method
including controlling a composition of molten steel such that a TiN
precipitation temperature may be higher than a temperature of the
molten steel in a tundish (T/D) by at least 50.degree. C.
(.DELTA.T.gtoreq.50.degree. C.), the TiN precipitation temperature
being defined by the following formula 2:
Log ( N % ) = - 19 , 755 / ( T + 273 ) + 7.78 + 0.07 [ % Ti ] - log
[ % Ti ] + 0.045 [ % Cr ] T ( .degree. C . ) = - 19 , 755 log ( N %
) - 7.78 - 0.07 ( % Ti ) + log ( % Ti ) - 0.045 ( % Cr ) - 273 [
Formula 2 ] ##EQU00001##
[0016] The molten steel may include, by weight %, carbon (C):
0.025% to 0.055%, silicon (Si): 0.25% to 0.55%, manganese (Mn):
1.5% to 1.8%, chromium (Cr): 17.1% to 17.7%, nickel (Ni): 9.25% to
9.65%, titanium (Ti): 0.2% to 0.5%, nitrogen (N): 0.025% or less,
and the balance of iron (Fe) and inevitable impurities.
[0017] The molten steel may have a Ti/C ratio of 8 or greater.
BRIEF DESCRIPTION OF DRAWINGS
[0018] The above and other aspects, features and other advantages
of the present disclosure will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0019] FIG. 1 is a schematic view illustrating a continuous casting
process of the related art;
[0020] FIG. 2 is a schematic view illustrating a strip caster
according to an exemplary embodiment of the present disclosure;
[0021] FIG. 3 is an electron microscope image of a TiN+TiO.sub.2
cluster of a complex oxynitride;
[0022] FIG. 4 illustrates an image of a surface of a cast material
and an image of an inside of a tundish nozzle in an comparative
example; and
[0023] FIG. 5 illustrates an image of a surface of a cast material
and an image of an inside of a tundish nozzle in an inventive
example.
DETAILED DESCRIPTION OF THE INVENTION
[0024] Exemplary embodiments of the present disclosure will now be
described in detail with reference to the accompanying
drawings.
[0025] The disclosure may, however, be exemplified in many
different forms and should not be construed as being limited to the
specific embodiments set forth herein. Rather, these embodiments
are provided so that this disclosure will be thorough and complete,
and will fully convey the scope of the disclosure to those skilled
in the art.
[0026] In the drawings, the shapes and dimensions of elements may
be exaggerated for clarity, and the same reference numerals will be
used throughout to designate the same or like elements.
[0027] FIG. 2 is a schematic view illustrating a twin roll strip
caster 100 according to an exemplary embodiment of the present
disclosure.
[0028] Referring to FIG. 2, the twin roll strip caster 100 of the
exemplary embodiment largely includes casting rolls 110, a ladle
120, a tundish 130, an immersion nozzle 140, a meniscus shield 150,
brush rolls 160, and edge dams 170. Reference numeral 180 denotes a
strip. In a twin roll strip casting method using such a twin roll
strip caster, molten steel is directly cast as a strip having a
thickness of 10 mm or less by supplying the molten steel through an
injection nozzle into a region between internally-water-cooled twin
rolls that are rapidly rotated in opposing directions.
[0029] An exemplary embodiment of the present disclosure provides a
method of manufacturing a titanium (Ti)-containing austenitic
stainless steel sheet using a twin roll strip caster. In the
method, the composition of molten steel is controlled to maintain a
TiN precipitation temperature defined by the following formula 2 at
a level higher than the temperature of the molten steel in a
tundish (T/D) by at least 50.degree. C. (.DELTA.T).
[0030] In the exemplary embodiment of the present disclosure, a
Ti-containing austenitic stainless steel sheet manufactured by the
method using a twin roll strip caster may include, by weight %,
carbon (C): 0.025% to 0.055%, silicon (Si): 0.25% to 0.55%,
manganese (Mn): 1.5% to 1.8%, chromium (Cr): 17.1% to 17.7%, nickel
(Ni): 9.25% to 9.65%, titanium (Ti): 0.2% to 0.5%, nitrogen (N):
0.025% or less, and the balance of iron (Fe) and inevitable
impurities. This composition of the Ti-containing austenitic
stainless steel sheet is a standard composition well known in the
related art.
[0031] When molten steel having the above-mentioned composition is
cast as a strip in a twin roll strip casting process, various
casting defects may be formed due to the influence of titanium
(Ti).
[0032] Generally, materials stuck in a nozzle of a tundish and
causing clogging are TiN and TiO.sub.2. TiN functions as nuclei or
seeds, and TiO.sub.2 gathers around the nuclei or seeds to form a
complex inclusion in the form of clusters. For example, FIG. 3
illustrates an electron microscope image of a TiN+TiO.sub.2 cluster
of a complex oxynitride. Therefore, it is necessary to minimize TiN
precipitation (nitride precipitation) and TiO.sub.2 formation
(oxide formation).
[0033] Titanium (Ti) included in molten steel reacts with nitrogen
to form a TiN inclusion. Since TiN has a high melting point of
about 2000.degree. C., TiN particles may agglomerate together and
grow in molten steel. In this case, the surface quality of products
is markedly degraded even though TiN has less effect on nozzle
clogging than TiO.sub.2. Therefore, it may be important to prevent
the formation of TiN in molten steel, especially, before the molten
steel is supplied to a tundish.
[0034] Therefore, thermodynamic research has been conducted into
TiN formation reaction between titanium (Ti) and nitrogen (N)
expressed by the following formula 1.
Ti+N=TiN [Formula 1]
[0035] Formula 1 is disclosed in academic publications. After
analyzing many academic materials, the following formula 2,
considered to accurately represent the formation of TiN in actual
Ti-containing austenitic stainless steel, is used in the present
disclosure.
Log ( N % ) = - 19 , 755 / ( T + 273 ) + 7.78 + 0.07 [ % Ti ] - log
[ % Ti ] + 0.045 [ % Cr ] T ( .degree. C . ) = - 19 , 755 log ( N %
) - 7.78 - 0.07 ( % Ti ) + log ( % Ti ) - 0.045 ( % Cr ) - 273 [
Formula 2 ] ##EQU00002##
[0036] Formula 2 above expresses a TiN precipitation temperature.
According to Formula 2, the TiN precipitation temperature is
determined by the contents of titanium (Ti), nitrogen (N), and
chromium (Cr), and if the contents of titanium (Ti) and nitrogen
(N) reduce or the content of chromium (Cr) increases, the TiN
precipitation temperature is reduced. That is, in a caster in which
solidification proceeds, the temperature of molten steel in a
tundish (T/D) has to be maintained to be higher than the TiN
precipitation temperature so as to minimize TiN precipitation
before the molten steel solidifies. Therefore, to minimize the
formation of oxynitrides (TiN and TiO.sub.2), it may be necessary
to minimize the addition of titanium (Ti) on the condition that the
content of nitrogen (N) in a refining furnace is minimized by
removing nitrogen (N) and preventing the introduction of nitrogen
(N).
[0037] Therefore, according to the exemplary embodiment of the
present disclosure, the composition of molten steel is controlled
to maintain the TiN precipitation temperature defined by formula 2
at a level higher than the temperature of the molten steel in a
tundish (T/D) by at least 50.degree. C. (.DELTA.T).
[0038] In a general continuous casting process, it is difficult to
perform high-temperature casting due to a break-out phenomenon (in
which non-solidified molten steel breaks out of the inside of a
slab due to rupture of the slab). In a strip casting process,
however, high-temperature casting (1550.degree. C. or higher) is
possible. Thus, a difference (.DELTA.T) between a TiN precipitation
temperature and a high-temperature casting temperature may be
maintained to be 50.degree. C. or greater, and TiN precipitation
may be suppressed. As a result, the formation of a complex
oxynitride having TiN+TiO.sub.2 clusters formed from TiN seeds may
be prevented, and thus nozzle clogging and surface defects may be
minimized.
[0039] In addition, according to the present disclosure, the
corrosion resistance of steel may be guaranteed by maintaining a Ti
stabilization ratio (a Ti/C ratio) at a level of 8 or greater. In
this case, if the addition of titanium (Ti) is reduced, the amount
of carbon (C) also has to be reduced so as to satisfy the Ti
stabilization ratio. For this, a large amount of oxygen may be
supplied to a refining furnace to decrease the amount of carbon. As
a result, a larger amount of oxygen may be dissolved in molten
steel, and thus the formation of Ti oxides may be unexpectedly
increased. In addition, since the period of time necessary for a
refining process is increased, the temperature of molten steel may
increase, and thus a considerable amount of coolant may be
necessary for cooling the molten steel after the molten steel is
discharged from the refining furnace. In this case, the molten
steel may make contact with ambient air while being cooled by the
coolant and thus may be re-oxidized to cause the formation of large
amounts of inclusions.
[0040] Although a larger amount of oxygen is dissolved in molten
steel and the formation of TiO.sub.2 oxide is increased because of
a larger amount of oxygen supplied to the refining furnace to
remove carbon, since solidification occurs rapidly owing to
characteristics of the strip casting process, the size of a
TiO.sub.2 inclusion may be small or fine, and thus the influence of
TiO.sub.2 may be small. Large clusters of TiO.sub.2 formed from TiN
seeds are the representative form of oxide that easily sticks to a
nozzle and causes surface defects.
[0041] Instead, the content of carbon (C) may be reduced to 0.3% or
lower because a large amount of carbon (C) is removed in the
refining furnace, and thus the addition of titanium (Ti) may be
reduced to satisfy the Ti stabilization ratio. Therefore, the
precipitation of TiN and the formation of TiO.sub.2 may be
fundamentally reduced.
[0042] Hereinafter, the exemplary embodiments of the present
disclosure will be described more specifically through
examples.
Examples
TABLE-US-00001 [0043] TABLE 1 Comparative Samples (general
continuous Inventive Samples casting) (strip casting) 1 2 3 1 2 3 4
5 Molten steel C 0.025 0.03 0.027 0.035 0.031 0.03 0.029 0.028
composition N 0.0147 0.015 0.013 0.011 0.015 0.012 0.01 0.0091 (wt
%) Ti 0.157 0.24 0.22 0.20 0.24 0.26 0.25 0.26 Ti/C 6.28 8.0 8.14
5.71 7.74 8.66 8.62 8.21 TiN precipitation 1487 1516 1501 1485 1519
1509 1493 1490 temperature (.degree. C.) T/D molten steel 1502 1510
1511 1545 1571 1559 1558 1562 temperature (.degree. C.) Casting
Nozzle 20 10 5 1 2 2 1 1 results clogging (mm) *Grinding 10 8 8 --
-- -- -- -- *Number of times of surface grinding (sum of counts for
upper and lower surfaces)
[0044] Ti-containing austenitic stainless steel strips were
manufactured using molten steel having compositions as shown in
Table 1 above. In Table 1, Comparative Samples 1 to 3 are strips
manufactured through a general continuous casting process, and
Inventive Samples 1 to 5 are strips manufactured using a twin roll
strip caster. The TiN precipitation temperature values shown in
Table 1 are values calculated using the above-described formula
2.
[0045] As shown in Table 1, in a general continuous casting
process, if the temperature of molten steel is increased due to
decarbonization reaction heat as the period of time of refining
increases, since high-temperature casting is impossible due to a
break-out phenomenon (strip rupture), a considerable amount of
coolant is supplied to decrease the temperature of the molten steel
after the molten steel is discharged from a furnace. In this case,
the molten steel may easily make contact with ambient air while
being cooled by the coolant, and thus may be re-oxidized to cause
the formation of large amounts of inclusions. In addition, since
casting is performed at a temperature around the TiN precipitation
temperature (1490.degree. C. to 1515.degree. C.) due to the
decreased temperature of the molten steel, a large amount of TiN
may precipitate. Therefore, the degree of nozzle clogging in
Comparative Samples 1 to 3 was high at 5 mm or greater.
[0046] In addition, since the strips produced by the general
continuous casting process had surface defects due to complex
inclusions formed of TiN+TiO.sub.2, grinding was performed four or
more times on each of the upper and lower surfaces of the strips.
FIG. 4 illustrates an image of a surface of a cast material and an
image of an inside of a tundish nozzle in a comparative
example.
[0047] On the contrary, high-temperature casting (1550.degree. C.
or higher) was possible by a twin roll strip casting method, and
thus the difference between the TiN precipitation temperature and
the temperature of casting could be maintained to be 50.degree. C.
or greater (.DELTA.T.gtoreq.50.degree. C.) as in Inventive Samples
1 to 5. Therefore, in Inventive Samples 1 to 5, TiN precipitation
could be suppressed to prevent the formation of a complex
oxynitride having TiN+TiO.sub.2 clusters formed from TiN seeds, and
thus nozzle clogging and surface defects could be minimized.
[0048] That is, it can be understood that the method of the present
disclosure suppresses TiN precipitation to maintain the degree of
tundish nozzle clogging at a level of 1 mm or lower and thus allows
a casting process to be normally completed. In addition, the degree
of surface quality of Ti-containing austenitic steel strips
manufactured by the strip casting method of the present disclosure
was high such that surface grinding was not performed.
[0049] Referring to Table 1, Inventive Samples 3 to 5 having a Ti/C
ratio of 8 or greater had a higher degree of corrosion resistance
than Inventive Samples 1 and 2 having a relatively low Ti/C
ratio.
[0050] It is necessary to confirm the purpose of Ti/C control based
on the background of development of Ti-containing stainless steel.
Ti-containing stainless steel was developed by adding titanium (Ti)
to STS304 steel as a carbon stabilizing element so as to decrease
grain boundary sensitivity for use in a grain boundary sensitivity
range (450.degree. C. to 850.degree. C.) That is, in general
stainless steel, chromium (Cr) and carbon (C) combine into
Cr.sub.23C.sub.6 carbide, and a Cr-depleted zone is formed around
the Cr.sub.23C.sub.6 carbide. Therefore, the Cr-depleted zone is
easily corroded due to a relatively low chromium (Cr) content
therein. Furthermore, in high-temperature applications (500.degree.
C. to 800.degree. C.) such as boiler heat exchangers or
high-temperature pipes, corrosion occurs more rapidly due to high
reactivity. Therefore, titanium (Ti) is generally added to
stainless steel for high-temperature applications so as to cause
reaction between carbon (C) and titanium (Ti) rather than reaction
between carbon (C) and chromium (Cr) and thus to fix chromium (Cr)
which is an element for improving corrosion resistance. In general,
a Ti/C ratio of 5 to 6 guarantees stainless steel having stable
corrosion resistance, and a higher Ti/C ratio guarantees stainless
steel having more stable corrosion resistance.
[0051] However, in the general continuous casting method of the
related art, if the Ti/C ratio is increased, Ti oxynitrides are
excessively formed because a large amount of titanium (Ti) is
added, and thus surface defects may be excessively formed. Thus it
is difficult to increase the Ti/C ratio.
[0052] However, in the strip casting process, although the addition
of titanium (Ti) is increased to obtain a Ti/C ratio of 8 or
greater, the formation of Ti oxides and TiN precipitation may be
controlled and minimized to prevent surface defects. That is, the
Ti/C ratio may be effectively maintained to be 8 or greater.
[0053] As set forth above, according to the exemplary embodiments
of the present disclosure, nozzle clogging occurring in a general
continuous casting process may be effectively prevented, and the
formation of TiN may be suppressed. Therefore, casting stability
may be guaranteed, and a Ti-containing austenitic stainless steel
sheet having a high degree of surface quality may be effectively
manufactured using a twin roll strip caster.
[0054] While exemplary embodiments have been shown and described
above, it will be apparent to those skilled in the art that
modifications and variations could be made without departing from
the scope of the present invention as defined by the appended
claims.
* * * * *