U.S. patent application number 15/843743 was filed with the patent office on 2018-06-28 for duplex stainless steel having excellent corrosion resistance and method for manufacturing the same.
The applicant listed for this patent is POSCO. Invention is credited to Suk Kyun HWANG, Sun Mi KIM, Byoung Jun SONG.
Application Number | 20180179606 15/843743 |
Document ID | / |
Family ID | 62625641 |
Filed Date | 2018-06-28 |
United States Patent
Application |
20180179606 |
Kind Code |
A1 |
SONG; Byoung Jun ; et
al. |
June 28, 2018 |
DUPLEX STAINLESS STEEL HAVING EXCELLENT CORROSION RESISTANCE AND
METHOD FOR MANUFACTURING THE SAME
Abstract
The present invention relates to a duplex stainless steel, and
to a duplex stainless steel having excellent corrosion resistance
and a method of manufacturing the same.
Inventors: |
SONG; Byoung Jun;
(Pohang-si, KR) ; KIM; Sun Mi; (Pohang-si, KR)
; HWANG; Suk Kyun; (Pohang-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
POSCO |
Pohang-si |
|
KR |
|
|
Family ID: |
62625641 |
Appl. No.: |
15/843743 |
Filed: |
December 15, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 6/004 20130101;
C21D 1/26 20130101; C22C 38/02 20130101; C21D 2211/001 20130101;
C22C 38/002 20130101; C22C 38/42 20130101; C21D 1/74 20130101; C22C
38/001 20130101; C22C 38/04 20130101; C21D 2211/005 20130101; C22C
38/44 20130101 |
International
Class: |
C21D 6/00 20060101
C21D006/00; C21D 1/26 20060101 C21D001/26; C21D 1/74 20060101
C21D001/74; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 2016 |
KR |
10-2016-0177540 |
Claims
1. A duplex stainless steel having excellent corrosion resistance,
wherein a surface portion of the steel includes an austenite
structure having an area fraction of 85% or more, and an interior
of the steel includes ferrite and austenite.
2. The duplex stainless steel of claim 1, wherein the surface
portion is up to 1/3 of a steel thickness.
3. The duplex stainless steel of claim 1, wherein the content of
nitrogen, included in the surface portion, is 0.4 wt % to 1.0 wt
%.
4. The duplex stainless steel of claim 1, wherein the interior of
the steel includes an austenite structure having an area fraction
of 40% to 60% and a ferrite structure having an area fraction of
40% to 60%.
5. The duplex stainless steel of claim 1, wherein a composition of
the steel includes carbon (C): 0.01 wt % to 0.10 wt %, silicon
(Si): 0.5 wt % to 1.5 wt %, manganese (Mn): 1.0 wt % to 5.0 wt %,
phosphorus (P): 0.03 wt % or less, sulfur (S): 0.02% wt % or less,
chromium (Cr): 19.0 wt % to 23.0 wt %, nickel (Ni): 0.5 wt % to 6.5
wt %, molybdenum (Mo): 3.5 wt % or less, copper (Cu): 0.1 wt % to
1.5 wt %, nitrogen (N): 0.1 wt % to 0.3 wt %, and a balance of iron
(Fe) and inevitable impurities.
6. A method of manufacturing a duplex stainless steel having
excellent corrosion resistance, comprising: preparing a duplex
stainless steel; and bright annealing heat treating the duplex
stainless steel in a reducing atmosphere at a temperature within a
range of 1000.degree. C. to 1200.degree. C. for 10 seconds or more
to 30 minutes or less.
7. The method of claim 6, wherein a dew point temperature of the
reducing atmosphere is -20.degree. C. to -80.degree. C.
8. The method of claim 6, wherein, in an atmosphere gas of the
reducing atmosphere, a ratio of nitrogen (N.sub.2) to hydrogen
(H.sub.2) is 1:3 or more.
9. The method of claim 8, wherein the atmosphere gas is 100%
nitrogen (N.sub.2).
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims the benefit of priority to Korean
Patent Application No. 10-2016-0177540, filed on Dec. 23, 2016 with
the Korean Intellectual Property Office, the entirety of the
disclosures of which is incorporated herein by reference.
BACKGROUND
[0002] The present disclosure relates to a duplex stainless steel,
and particularly, to a duplex stainless steel having excellent
corrosion resistance and a method of manufacturing the same.
[0003] Stainless steel is a steel material having chromium (Cr) in
an amount of 18% or greater for high corrosion resistance, and is
classified, according to chemical composition or metallurgical
structure, as an austenitic stainless steel, a ferritic stainless
steel, a precipitation hardening stainless steel, a martensitic
stainless steel, or a duplex stainless steel.
[0004] A duplex stainless steel is a stainless steel having a
structure in which austenite and ferrite phases are mixed. Such a
duplex stainless steel has the merits of austenitic stainless steel
and ferritic stainless steel as well as a high degree of strength.
In detail, since a duplex stainless steel has higher strength and
corrosion resistance than a stainless steel according to the
related art, a range of applications such as in thermal power plant
piping, a flue gas desulfurizer (FGD) duct, a seawater cooling pipe
for a nuclear power plant, a chemical tank for shipbuilding, and
the like, has been increased.
[0005] However, since such a duplex stainless steel contains a
large amount of relatively expensive elements such as nickel (Ni),
molybdenum (Mo), and the like, manufacturing costs may be
increased, so there may be disadvantages in terms of price
competitiveness, as compared to other grades of steel.
[0006] In this regard, recently there has been increasing interest
in a cost-effective lean duplex stainless steel to which relatively
inexpensive alloying elements are added, while significantly
reducing the addition of relatively expensive alloying elements
such as Ni, Mo, and the like (Patent Document 1). However, there is
a problem that corrosion resistance may be lowered in a corrosive
environment containing chlorine, due to an influence of a reduced
amount of elements improving corrosion resistance, such as an alloy
of Ni, Mo, and the like.
[0007] Therefore, in manufacturing such an economical duplex
stainless steel, it is important to secure sufficient corrosion
resistance.
RELATED ART DOCUMENT
[0008] (Patent Document 1) Korean Patent No. 10-1379079
SUMMARY
[0009] An aspect of the present disclosure provides a duplex
stainless steel having improved corrosion resistance without a
separate coating process, by transforming a surface structure of a
duplex stainless steel, and a method of manufacturing the same.
[0010] The scope of the present disclosure is not limited to the
above-mentioned aspects. Other aspects of the present disclosure
are stated in the following description, and the aspects of the
present disclosure will be clearly understood by those having
ordinary skill in the art through the following description.
[0011] According to an aspect of the present disclosure, a duplex
stainless steel having excellent corrosion resistance may be a
duplex stainless steel, wherein a surface portion of the steel
includes an austenite structure having an area fraction of 85% or
more, and an interior of the steel includes ferrite and
austenite.
[0012] According to another aspect of the present disclosure, a
method of manufacturing a duplex stainless steel having excellent
corrosion resistance may include: preparing a duplex stainless
steel; and bright annealing heat treating the duplex stainless
steel in a reducing atmosphere at a temperature within a range of
1000.degree. C. to 1200.degree. C. for 10 seconds or more to 30
minutes or less.
BRIEF DESCRIPTION OF DRAWINGS
[0013] The above and other aspects, features, and advantages of the
present disclosure will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0014] FIG. 1 is an image of a cross section of Comparative Example
1 in an example of the present disclosure;
[0015] FIG. 2 is an image in which corrosion resistance of the
duplex steel sheet of FIG. 1 is evaluated and observed;
[0016] FIGS. 3A and 3B are images of a portion in which corrosion
occurs in FIG. 2;
[0017] FIG. 4 is an image of a cross section of Inventive Example 1
in an example of the present disclosure; and
[0018] FIG. 5 is an image in which corrosion resistance of the
duplex steel sheet of FIG. 4 is evaluated and observed.
DETAILED DESCRIPTION
[0019] The inventors of the present disclosure have researched
corrosion of a duplex stainless steel having an austenite structure
having an area fraction of 40% to 60% and a ferrite structure
having an area fraction of 40% to 60%, and as a result, have
recognized that corrosion occurs in ferrite surrounded by
austenite, as a corrosion location, so the present disclosure has
been invented to address this issue.
[0020] In view of the fact that nitrogen is an austenite
stabilizing element, nitrogen was adsorbed on the duplex stainless
steel to induce transformation of a ferrite structure on a surface
into austenite, so as to improve corrosion resistance.
[0021] The method of adsorbing nitrogen on a surface may be a
nitrification treatment. Such a nitrification treatment is a method
of forming a nitride on a surface of a general steel to improve
corrosion resistance, abrasion resistance, fatigue strength and the
like. Examples of such a nitrification treatment include plasma
nitrification, gas nitrification, liquid nitrification, and the
like. By these methods, nitrogen may be adsorbed on the surface of
the stainless steel, but it is not advantageous in terms of
productivity and material properties, so a method for addressing
this problem has been devised.
[0022] Hereinafter, embodiments of the present disclosure will be
described in detail. First, a duplex stainless steel of the present
disclosure will be described in detail.
[0023] In the duplex stainless steel of the present disclosure, a
microstructure of a surface portion preferably includes austenite
having an area fraction of 85% or more, and an interior preferably
includes austenite and ferrite.
[0024] The surface portion is preferably up to 1/3 of a steel
thickness. For austenite transformation through adsorption on a
surface, a large amount of nitrogen should be solidified in steel.
In the case of an austenite phase including an excessive amount of
nitrogen, brittleness may occur due to nitrogen, so unique duplex
characteristics may be lost. Moreover, a considerable amount of
time is required for diffusion of nitrogen beyond a 1/3 point.
Thus, it is not preferable to allow the austenite phase to include
an excessive amount of nitrogen.
[0025] The content of nitrogen (N) of the surface portion is
preferably 0.4 wt % to 1.0 wt %. If the content of nitrogen of the
surface portion is less than 0.4 wt %, a heat treatment temperature
for transforming a ferrite phase in a surface into an austenite
phase should be significantly high. If the content of nitrogen is
increased to be greater than 1.0 wt %, a considerable amount of
time should be required for nitrification.
[0026] The interior of the duplex stainless steel preferably has a
duplex stainless steel structure according to the related art. For
example, the interior thereof preferably includes ferrite having an
area fraction of 40% to 60% and austenite having an area fraction
of 40% to 60%.
[0027] An example of a composition of a duplex steel to be applied
to the present disclosure is as follows. The duplex steel includes:
C: 0.01 wt % to 0.10 wt %, Si: 0.5 wt % to 1.5 wt %, Mn: 1.0 wt %
to 5.0 wt %, P: 0.03 wt % or less, S: 0.02 wt % or less, Cr: 19.0
wt % to 23.0 wt %, Ni: 0.5 wt % to 6.5 wt %, Mo: 3.5 wt % or less,
Cu: 0.1 wt % to 1.5 wt %, N: 0.1 wt % to 0.3 wt %, and a balance of
iron (Fe) and inevitable impurities. The composition of the steel
refers to the interior composition of the steel. As described
above, the surface portion has a slightly different content of
N.
[0028] Carbon (C): 0.01% to 0.1%
[0029] Carbon (C), an austenite phase forming element, is effective
in increasing the strength of a material through solid-solution
strengthening. To this end, it is required to be included in an
amount of 0.01% or more. If the content of carbon (C) is greater
than 0.10%, carbide-forming elements such as chromium (Cr),
effective in improving corrosion resistance, may easily combine
with carbon (C) along austenite-ferrite boundaries, and thus, the
content of chromium (Cr) may be decreased along grain boundaries to
cause a decrease in corrosion resistance. Therefore, preferably,
the content of carbon (C) may be 0.1% or less.
[0030] Silicon (Si): 0.5% to 1.5%
[0031] Silicon (Si) is added to obtain a deoxidizing effect to some
degree and acts as a ferrite phase forming element which
concentrates in ferrite during an annealing process. Therefore,
silicon (Si) may be added in an amount of 0.5% or greater to obtain
a proper ferrite phase faction. However, if the content of silicon
(Si) is greater than 1.5%, hardness of a ferrite phase sharply
increases to cause a decrease in elongation, and thus, it may be
difficult to form an austenite phase having an effect of
guaranteeing elongation. In addition, if silicon (Si) is added
excessively, the fluidity of slag is low in a steel making process,
and since silicon (Si) forms inclusions by combining with oxygen,
corrosion resistance decreases. Therefore, preferably, the content
of silicon (Si) may be adjusted to be within the range of 0.5% to
1.5%.
[0032] Manganese (Mn): 1.0% to 5.0%
[0033] Manganese (Mn) is an element increasing the amount of a
deoxidizer and the solid solubility of nitrogen (N), and is added
as an austenite-forming element to replace relatively expensive
nickel (Ni). If the content of manganese (Mn) is greater than 5.0%,
it is difficult to obtain corrosion resistance as high as that of
304 steel, and manganese (Mn) combines with sulfur (S) in steel to
form MnS which decreases corrosion resistance. Conversely, if the
content of manganese (Mn) is less than 1.0%, it is difficult to
guarantee a proper austenite phase fraction even though
austenite-forming elements such as nickel (Ni), copper (Cu), or
nitrogen (N) are adjusted, and the solid solubility of nitrogen (N)
to be added is too low to sufficiently dissolve nitrogen (N) at
atmospheric pressure. Therefore, it may be preferable that the
content of manganese (Mn) be within the range of 1.0% to 5.0%.
[0034] Chromium (Cr): 19.0% to 23.0%
[0035] Chromium (Cr), being an element stabilizing ferrite,
together with silicon (Si), guarantees the corrosion resistance of
a duplex stainless steel in addition to playing a major role in
forming a ferrite phase in a duplex stainless steel. If the content
of chromium (Cr) increases, corrosion resistance increases. In this
case, however, it is necessary to increase the content of
relatively expensive nickel (Ni) or the contents of other
austenite-forming elements to maintain phase fractions. Therefore,
preferably, the content of chromium (Cr) may be adjusted to be
within the range of 19.0% to 23.0% to obtain corrosion resistance
equal to or higher than that of 304 steel while maintaining phase
fractions of a duplex stainless steel.
[0036] Nickel (Ni): 0.5% to 6.5%
[0037] Nickel (Ni) functions as an austenite-stabilizing element
together with manganese (Mn), copper (Cu), and nitrogen (N) and
plays a major role in guaranteeing the formation of an austenite
phase in a duplex stainless steel. The content of relatively
expensive nickel (Ni) may be maximally reduced for cost reductions,
and in this case, the contents of manganese (Mn) and nitrogen (N)
having a function of forming an austenite phase may be increased to
maintain balance between phase fractions in spite of a decrease in
the content of nickel (Ni). However, the content of nickel (Ni) may
be adjusted to be 0.5% or greater so as to suppress the formation
of strain-induced martensite, generated during cold working, and to
sufficiently guarantee the stability of an austenite phase. If
nickel (Ni) is added in large amounts, it is difficult to maintain
a proper austenite fraction because the fraction of an austenite
phase increases, and particularly, production costs of a product
increase due to relatively expensive nickel (Ni), making it
difficult to guarantee the competitiveness to 304 steel. Therefore,
preferably, the content of nickel (Ni) may be within the range of
0.5% to 6.5%.
[0038] Nitrogen (N): 0.1% to 0.3%
[0039] Together with nickel (Ni), nitrogen (N) significantly
contributes to stabilizing an austenite phase in a duplex stainless
steel, and is an element concentrated in an austenite phase during
an annealing heat treatment process. Therefore, if the content of
nitrogen (N) is increased, corrosion resistance and strength may be
concomitantly improved. However, since the solid solubility of
nitrogen (N) may vary according to the content of manganese (Mn),
it is necessary to adjust the content of nitrogen (N). If the
content of nitrogen (N) is greater than 0.3% when the content of
manganese (Mn) is within the range proposed in the present
disclosure, the content of nitrogen (N) exceeds the solid
solubility of nitrogen (N), and thus, surface defects may be caused
because of the formation of blow holes and pin holes during a
casting process. Nitrogen (N) may be added in an amount of 0.1% or
greater to obtain corrosion resistance as high as that of 304
steel, and if the content of nitrogen (N) is excessively low, it is
difficult to maintain proper phase fractions. Therefore, it may be
preferable that the content of nitrogen (N) be within the range of
0.1% to 0.3%.
[0040] Molybdenum (Mo): 3.5% or less (excluding 0%)
[0041] Molybdenum (Mo) is a stronger element improving corrosion
resistance, as compared to chromium (Cr). A higher Mo content is
advantageous, taking into account a pitting corrosion resistance
equivalent index. However, when a large amount of Molybdenum (Mo),
as a ferrite stabilizing element, is added, it is difficult to
obtain a duplex structure. Moreover, since it is difficult to be
transformed into austenite during nitrification treatment, the
content of Molybdenum (Mo) is preferably 3.5% or less.
[0042] Copper (Cu): 0.1% to 1.5%
[0043] Copper (Cu) is an austenite stabilizing element. When an
appropriate amount of copper (Cu) is added, elongation of a duplex
stainless steel is increased, and corrosion resistance in a
sulfuric acid atmosphere may be improved. However, when an
excessive amount of copper (Cu) is added, copper (Cu) may not be
solidified in a matrix, so a problem in which a pitting potential
falls in an atmosphere containing chlorine may occur. Thus, the
content of copper (Cu) is preferably 0.1% to 1.5%.
[0044] Meanwhile, other than the composition described above,
phosphorous (P) and sulfur (S) may be included. Phosphorous (P) and
sulfur (S) are elements which easily segregate in grain boundaries
during solidification. When phosphorous (P) and sulfur (S)
segregate during solidification, a low melting point phase is
formed, so hot workability may be reduced and thus cracking may
occur. Therefore, the contents of the phosphorous (P) and sulfur
(S) are preferably adjusted to be as low as possible. In this
regard, in the present disclosure, the content of phosphorous (P)
is 0.03% or less, and the content of sulfur (S) is 0.02% or
less.
[0045] The other component of the present disclosure is iron (Fe).
However, unintended impurities of raw materials or manufacturing
environments may inevitably be included in a manufacturing process
according to the related art, and thus cannot be excluded. Such
impurities are well-known to those of ordinary skill in
manufacturing industries, and thus, specific descriptions of the
impurities will not be given in the present disclosure.
[0046] Hereinafter, a method of manufacturing a duplex stainless
steel of the present disclosure will be described in detail.
[0047] First, a duplex stainless steel having the composition
described above may be prepared. A method of manufacturing the
duplex stainless steel is not particularly limited, and may be
sufficient as long as a person skilled in the art can understand
the present disclosure. As an example, the duplex stainless steel
may be manufactured using a twin roll strip casting method.
[0048] The duplex stainless steel may be obtained by performing
cold rolling for a desired thickness, after a hot rolled steel
sheet is manufactured.
[0049] With respect to the duplex stainless steel, prepared as
described above, a bright annealing heat treatment may be
performed. The bright annealing heat treatment is preferably
performed under conditions in which a dew point of an atmosphere
gas is -20.degree. C. to -80.degree. C. During the bright annealing
heat treatment, when a dew point of the atmosphere gas increases,
due to a reaction of oxygen and a steel sheet, a Cr oxide film
becomes dense on a surface. Thus, a movement path of nitrogen,
diffused to a surface of a steel sheet in the atmosphere, is
blocked, so an effective nitrification treatment may not occur. For
an effective nitrification treatment, a dew point temperature of
the atmosphere gas is preferably -20.degree. C. to -80.degree.
C.
[0050] The atmosphere gas of the bright annealing heat treatment is
preferably a reducing atmosphere. In addition, the bright annealing
heat treatment is performed in a nitrogen (N.sub.2) atmosphere, and
is preferably performed in an atmosphere in which a volume ratio of
hydrogen (H.sub.2) to nitrogen (N.sub.2) is 3:1 or more. In
general, when ammonia (NH.sub.3) gas is heated at 500.degree. C. or
more, a nitrogen atom, generated by pyrolysis due to iron surface
catalysis, is adsorbed on a surface of a steel sheet, so the
nitrogen atom is diffused into an interior of the steel sheet and
thus a nitride layer is formed. In consideration of this, a ratio
of hydrogen to nitrogen is preferably 3:1 or more.
[0051] A temperature of the heat treatment is preferably
1000.degree. C. to 1200.degree. C. In order to manufacture a thin
plate, hot rolling and cold rolling are performed. In addition,
annealing of a sheet having been cold rolled is performed at
1000.degree. C. to 1200.degree. C., for the purpose of easy
transformation of an austenite phase during nitrification
treatment.
[0052] The heat treatment time is preferably longer than 10 seconds
and shorter than 30 minutes in a heat treatment temperature.
Considering the heat treatment temperature, when the heat treatment
time is 10 seconds or less, sufficient absorption is difficult to
occur. When the heat treatment time exceeds 30 minutes, nitrogen
penetrates into an interior of a steel sheet, so there is a risk of
losing characteristics of a duplex stainless steel.
[0053] In the present disclosure, through the operation described
above, a concentration of nitrogen in a surface portion of a duplex
stainless steel is increased, so a surface is only changed into
austenite. Thus, without additional processing and cost increases,
a duplex stainless steel with improved corrosion resistance may be
manufactured.
[0054] Hereinafter, the present disclosure will be described more
specifically through examples. However, the following examples
should be considered in a descriptive sense only, rather than for
the purposes of limitation of the scope of the present
disclosure.
Example
[0055] After a duplex stainless steel hot rolled steel sheet having
a composition of Table 1 (wt %, with a balance of iron (Fe) and
inevitable impurities) was manufactured, cold rolling was performed
thereon, and a cold rolled steel sheet was manufactured.
Thereafter, a bright annealing heat treatment was performed under
conditions of Table 2. In this case, as an atmosphere gas, a gas in
which a ratio of H.sub.2/N.sub.2 is 3:1 was used.
TABLE-US-00001 TABLE 1 Class- ification C Si Mn P S Cr Ni Mo Cu N
Composition 0.04 0.72 2.96 0.0217 0.0024 20.33 0.96 0.01 0.845
0.232
[0056] Meanwhile, a nitrogen content and a pitting corrosion
resistance index in a surface portion and an interior of the duplex
stainless steel having been heat treated were confirmed, an
austenite fraction (area %) of the surface portion was observed,
and results thereof are illustrated in Table 2.
[0057] In addition, corrosion resistance of each specimen was
evaluated, and results thereof are illustrated in Table 2. To
evaluate corrosion resistance, an accelerated spray test based on
ISO 14993 was conducted 60 times. Regarding conditions of a single
test based on ISO 14993, a 5% NaCl salt water test solution was
prepared and operations were performed as follows.
[0058] [Salt water spraying: 35.+-.1.degree. C., 2
hours].fwdarw.[Drying: 60.+-.1.degree. C., relative humidity of
less than 30%, 4 hours].fwdarw.[Wetting: 50.+-.1.degree. C.,
relative humidity of more than 95%, 2 hours]
[0059] As a result of the test described above, a corrosion
resistance evaluation standard is illustrated as `o` when a smaller
amount of rust was present than an amount of rust on a surface of
STS 316, and is illustrated as `x` when a larger amount of rust was
present than an amount of rust on a surface of STS 316.
TABLE-US-00002 TABLE 2 Pitting Nitrogen corrosion Surface Heat
concentration resistance portion Dew treatment Heat (wt %) index
austenite Class- point temperature treatment Surface Surface
fraction Corrosion ification (.degree. C.) (.degree. C.) time
portion Interior portion Interior (area %) resistance Comparative
-40 to 1050 10 0.23 0.23 24.1 24.1 50 x Example -60 seconds 1
Inventive -65 to 1050 20 0.8 0.23 33.2 24.1 90 .smallcircle.
Example -80 seconds 1 Inventive -65 to 1100 30 1 0.23 36.4 24.1 95
or .smallcircle. Example -80 seconds more 2 Inventive -20 to 1050
30 0.45 0.23 27.6 24.1 85 .smallcircle. Example -40 seconds 3
Inventive -20 to 1100 30 0.55 0.23 29.2 24.1 90 .smallcircle.
Example -40 seconds 4 Comparative -65 to 1100 10 0.3 0.23 25.2 24.1
60 x Example -80 seconds 2
[0060] Here, a pitting corrosion resistance index
(PREN)=Cr+3.3(Mo+0.5 W)+16N, and each component numeral is the
content thereof (wt %).
[0061] As illustrated in the results of Table 2, in the case of
Inventive Example satisfying conditions of the present disclosure,
a fraction of ferrite in a surface portion is reduced, sufficient
austenite is generated, and a pitting corrosion resistance index is
increased. Thus, excellent corrosion resistance may be secured.
However, in the case of Comparative Examples outside of the
conditions of the present disclosure, sufficient austenite may not
be formed in a surface portion, so it is confirmed that corrosion
resistance is inferior.
[0062] In detail, FIG. 1 is an image of a cross section of
Comparative Example 1, and it is confirmed that a large amount of a
ferrite structure is included in a surface portion. In FIG. 1, a
bright portion represents an austenite structure and a dark portion
represents a ferrite structure. After a corrosion resistance
evaluation of the duplex stainless steel of FIG. 1 as described
above was conducted, as a result of FIGS. 2, 3A, and 3B, it was
confirmed that corrosion occurred. In detail, it was confirmed that
a corrosion pit was observed in a ferrite structure surrounded by
an austenite structure.
[0063] Meanwhile, in FIGS. 4 and 5 illustrating Inventive Example
1, it is confirmed that there was almost no ferrite in a surface
portion, in a cross section of a duplex stainless steel. As a
result, it is confirmed that corrosion resistance was
excellent.
[0064] As set forth above, according to an exemplary embodiment, a
structure of a surface portion of a duplex stainless steel is
transformed into austenite, so a duplex stainless steel with
significantly improved corrosion resistance may be provided.
[0065] 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.
* * * * *