U.S. patent application number 11/722341 was filed with the patent office on 2008-05-15 for duplex stainless steel having excellent corrosion resistance with low nickel.
Invention is credited to Kwang-tae Kim, Yong-Heon Lee, Won-qeun Son.
Application Number | 20080112840 11/722341 |
Document ID | / |
Family ID | 36615116 |
Filed Date | 2008-05-15 |
United States Patent
Application |
20080112840 |
Kind Code |
A1 |
Kim; Kwang-tae ; et
al. |
May 15, 2008 |
Duplex Stainless Steel Having Excellent Corrosion Resistance with
Low Nickel
Abstract
Disclosed is duplex stainless steel that containes relatively
low content of Ni, and limits constituents of Cr--Mo--Mn--N to make
volume fraction of .alpha. and .gamma. have about 50:50, thereby
minimizing incidence of a edge crack to enhance a production yield
and decrease a processing load, in which the alloy constituents
includes Cr of 19.5.about.22.5%. Mo of 0.5-2.5%, Ni if 1.0-3.0%, Mn
of 1.5-4.5%, N of 0.15-0.25%, Fe and unavoidable elements, and a
constitution range of the alloy constituents are adjusted to make a
CPt higher than 20.degree. C. depending on the constitution range
of the alloy constituents. Thus, the contents of Cr, Mo and Ni is
decreased and the content of Mn is increased a little, so that a
production cost thereof is reduced; the corrosion resistance is
secured to be better than the STS 304 steel and the 316L steel; the
incidence of the edge cract is decreased while being hot-rolled,
thereby decreasing a load on the following process; and the surface
defective is decreased, thereby improving a production yield.
Inventors: |
Kim; Kwang-tae; (Pohang,
KR) ; Lee; Yong-Heon; (Pohang, KR) ; Son;
Won-qeun; (Pohang, KR) |
Correspondence
Address: |
CHRISTIE, PARKER & HALE, LLP
PO BOX 7068
PASADENA
CA
91109-7068
US
|
Family ID: |
36615116 |
Appl. No.: |
11/722341 |
Filed: |
December 22, 2005 |
PCT Filed: |
December 22, 2005 |
PCT NO: |
PCT/KR05/04472 |
371 Date: |
June 20, 2007 |
Current U.S.
Class: |
420/67 |
Current CPC
Class: |
C22C 38/001 20130101;
C22C 38/58 20130101; C22C 38/44 20130101 |
Class at
Publication: |
420/67 |
International
Class: |
C22C 38/44 20060101
C22C038/44; C22C 38/58 20060101 C22C038/58 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 27, 2004 |
KR |
10-2004-0113129 |
Claims
1. (canceled)
2. (canceled)
3. A duplex stainless steel having excellent corrosion resistance
with low Ni, having alloy constituents comprising Cr ranging from
19.5 wt% to 22.5 wt%, Mo ranging from 0.5 wt% to 2.5 wt%, Ni
ranging from 1.Owt% to 3.0 wt%, Mn ranging from 1.5 wt% to 4.5 wt%,
N ranging from 0.15 wt% to 0.25 wt%, Fe and unavoidable elements,
wherein a critical pitting temperature of the duplex stainless
steel is calculated as a function of constitution ranges of Cr, Mo,
Mn, and N in the alloy constituents, and the constitution ranges of
the alloy constituents are adjusted to have the critical pitting
temperature of 20.degree. C. or more.
4. The duplex stainless steel according to claim 3, further
including C of 0.03 wt% or less P of 0.03 wt% or less, and Si of 2
wt% or less.
5. The duplex stainless steel according to claim 3, wherein the
critical pitting temperature of the duplex stainless steel is
calculated by: CPT=CONST+2.65Cr+11.71Mo-1.3Mn+64.58N, and wherein
CPT is the critical pitting temperature in .degree. C and CONST is
a constant for calculating the critical pitting temperature in
C.
6. The duplex stainless steel according to claim 5, wherein the
constant, CONST, is a negative constant.
7. The duplex stainless steel according to claim 6, wherein the
negative constant is -50.47.
8. The duplex stainless steel according to claim 7, wherein the
constitution ranges of the alloy constituents are adjusted to have
the critical pitting temperature of 25.degree. C. or more.
9. The duplex stainless steel according to claim 3, wherein the
constitution ranges of the alloy constituents are adjusted to have
the critical pitting temperature of 25.degree. C. or more.
10. A method for forming duplex stainless steel having excellent
corrosion resistance with low Ni, having alloy constituents
including Cr ranging from 19.5 wt% to 22.5wt%, Mo ranging from 0.5
wt% to 2.5 wt%, Ni ranging from 1.0wt% to 3.0 wt%, Mn ranging from
1.5 wt% to 4.5 wt%, N ranging from 0.15 wt% to 0.25 wt%, Fe and
unavoidable elements, the method comprising: calculating a critical
pitting temperature of the duplex stainless steel as a function of
constitution ranges of Cr, Mo, Mn, and N in the alloy constituents,
and adjusting the constitution ranges of the alloy constituents to
have the critical pitting temperature of 20 C or more.
11. The method according to claim 10, further comprising: adding C
of 0.03 wt% or less, P of 0.03 wt% or less, and Si of 2 wt% or less
to form the duplex stainless steel.
12. The method according to claim 10, wherein the calculating the
critical pitting temperature of the duplex stainless steel
comprises calculating the critical pitting temperature of the
duplex stainless steel using:
CPT=CONST+2.65Cr+11.71Mo-1.3Mn+64.58N, and wherein CPT is the
critical pitting temperature in .degree. C and CONST is a constant
for calculating the critical pitting temperature in .degree. C.
13. The method according to claim 12, wherein the constant, CONST,
is a negative constant.
14. The method according to claim 13, wherein the constant, CONST,
is -50.47.
15. The method according to claim 14, wherein the adjusting the
constitution ranges of the alloy constituents comprises adjusting
the constitution ranges of the alloy constituents to have the
critical pitting temperature of 25 C or more.
16. The method according to claim 10, wherein the constitution
ranges of the alloy constituents are adjusted to have the critical
pitting temperature of 25 C or more.
Description
BACKGROUND ART
[0001] 1. Field of the Invention
[0002] The present invention relates to duplex stainless steel
containing Mn of high content and Cr, Mo, N and Ni of low contents
as compared with S32205 duplex stainless steel, and more
particularly, to duplex stainless steel that includes low contents
of Cr, Mo, N and Ni to thereby decrease a production cost thereof,
has excellent corrosion resistance better than STS 304 steel and
316L steel, and has a low incidence of an edge crack when it is
hot-rolled.
[0003] 2. Description of Related Art
[0004] In general, austenite stainless steel excellent in
formability and corrosion resistance uses Fe as base metal and
mainly contains Cr and Ni. Further, the austenite stainless steel
has been variously developed by adding other elements such as Mo,
Cu, etc. for various purposes.
[0005] Among the austenite stainless steel, 316L steel is excellent
in corrosion resistance, pitting resistance and high temperature
strength. However, the 316L steel is low carbon steel and contains
Ni more than lOwt% and Mo more than 2wt%, so that a cost price
thereof heavily fluctuates according to the price of Ni and Mo,
thereby decreasing competitive power.
[0006] To increase the competitive power, iron and steel business
tries to develop new steel by lowering contents of Ni and Mo and
securing corrosion resistance better than that of the 316L
steel.
[0007] As an example of duplex stainless steel that has mixed
formation of an austenite phase and a ferrite phase, S32205 duplex
stainless steel (hereinafter, referred to as "2205 steel" contains
high percentage of Cr, Mo and N to secure excellent corrosion
resistance, and contains Ni more than 5wt% to secure a volume
fraction.
[0008] Such duplex stainless steel contains a relatively low
percentage of Ni as compared with STS 316L steel containing 10% Ni,
so that its production cost is low and thus its price is
competitive, thereby increasing added value. However, the 2205
steel has poor hot-formability and thus has a very low production
yield of 80% . Further, the 2205 steel has high contents of Cr and
Mo, so that a sigma-phase deposition rate is high, thereby
deteriorating the property of steel and having a high load on
winding and cooling processes. Thus, it is hard to replace the 316L
steel by the 2205 steel.
SUMMARY OF THE INVENTION
[0009] Accordingly, it is an object of the present invention to
provide duplex stainless steel that has low contents of Cr, Mo, N
and Ni as compared with 2205 steel to thereby decrease a production
cost thereof, increases production yield by lowering an incidence
of an edge crack when it is hot-rolled, and has excellent corrosion
resistance better than STS 304 steel and 316L steel.
[0010] The present inventor develops duplex stainless steel that
contains relatively low content of Ni, and limits constituents of
Cr-Mo-Mn-N to make volume fraction of a and y have about 50:50, so
that a production cost is reduced; a CPT is secured to be higher
than 20.degree. C. of that of the STS 304 steel and the 316L steel;
and the incidence of a edge crack is minimized to enhance a
production yield and decrease a processing load.
[0011] In an exemplary embodiment of the present invention, duplex
stainless steel includes Cr of 19.5-22.5%, Mo of 0.5-2.5%, Ni of
1.0-3.0%, Mn of 1.5-4.5%, N of 0.15%-0.25%, C of 0.03% and less, P
of 0.03% and less, Si of 2% and less, Fe and un-avoidable
elements.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] These and/or other objects and advantages of the invention
will become apparent and more readily appreciated from the
following description of the preferred em- bodiments, taken in
conjunction with the accompanying drawings of which:
[0013] FIG. 1 are photographs showing an edge crack of a hot-rolled
sample according to alloy constituents;
[0014] FIG. 2 is a table showing alloy constituents and a volume
fraction of steel samples according to the present invention and
comparative steel samples;
[0015] FIG. 3 is a table showing critical pitting temperatures
(CPT) of the samples of FIG. 2;
[0016] FIG. 4 is a table showing total test results of the samples
of FIG. 2 with regard to corrosion resistance and hot formability;
and
[0017] FIG. 5 is a table showing oxidation increment of the steel
according to the present invention and the conventional steel.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0018] Hereinafter, embodiments of the present invention will be
described with reference to accompanying drawings. Below, essential
constituents are limited as follows.
[0019] Carbon (C): C is effective for strengthening a material by
solid solution strengthening. However, when the content of C is
excessive, C is easily combined with a carbide-forming element such
as Cr, which is effective for corrosion resistance in a boundary
between ferrite-austenite phases. Thus, C lowers the content of Cr
around a grain boundary, so that the corrosion resistance is
deteriorated. To maximize the corrosion resistance, the content of
C is lowered into 0.03% and below.
[0020] Nitrogen (N): N, together with Ni, is one of elements that
contribute stabilization of an austenite phase. As the content of N
increases, the corrosion resistance and high strengthening are
achieved. However, when the content of N is too high, the hot
formability of steel is deteriorated, thereby lowering the
production yield thereof. On the other hand, when the content of N
is too low, the contents of Cr and Mo should be lowered to secure
the volume fraction of steel, and it is difficult to secure the
strength of a welding part and phase stability. Therefore, the
content of N preferably ranges between 0.15% and 0.25%.
[0021] Manganese (Mn): Mn generally has a content of about 1.5% to
adjust a metal flow rate. In the meanwhile, Mn can be additionally
contained instead of Ni. In this case, the hot formability can be
secondarily improved. However, when the content of Mn is excessive,
Mn is combined with S of the steel and forms MnS, thereby
deteriorating the corrosion resistance and the hot formability.
Thus, the upper limit content of Mn is limited to 4.5%. Preferably,
the content of Mn ranges between 1.5% and 4.5%.
[0022] Chrome (Cr): Cr, together with Mo, is used as an element to
stabilize the ferrite phase. Here, Cr is essential to not only
primarily securing the ferrite phase of duplex stainless steel but
also securing excellent corrosion resistance. When the content of
Cr increases, the corrosion resistance increases, but the content
of expensive Ni should be also increased to maintain the volume
fraction. In results, the content of Cr is preferably limited
between 19.5% and 22.5% so as to maintain the volume fraction of t
he duplex stainless steel and the corrosion resistance better than
that of STS 304 and 316L steel.
[0023] Molybdenum (Mo): like Cr, Mo is used for not only
stabilizing the ferrite phase but also largely enhancing the
corrosion resistance. However, if the content of Mo is excessive,
Mo is likely to form the sigma phase when it is annealed, thereby
dete- riorating the corrosion resistance and impact resistance. In
the present embodiment, Mo just assists Cr in securing the ferrite
volume fraction and secures the proper corrosion resistance, so
that the content of Mo is preferably limited between 0.5% and
2.5%.
[0024] Nickel (Ni): Ni, together with Mn and N, is an element to
stabilize the austenite phase, and mainly used in securing the
austenite phase of the duplex stainless steel. To reduce a
production cost, if the content of expensive Ni is lowered, the
decrement of Ni can be replaced by increasing the content of Mn and
N that form the austenite phase. However, if the content of Ni is
excessively lowered, Mn and N should be ex- cessively needed so
that the corrosion resistance and the hot formability are de-
teriorated, or the contents of Cr and Mo are lowered so that it is
difficult to secure the corrosion resistance better than the 316L
steel. Thus, the content of Ni preferably ranges between 1.0% and
3.0%.
[0025] Phosphorous (P): P is seeded in the grain boundary or an
interface, and is likely to deteriorate the corrosion resistance
and toughness. Therefore, the content of P is lowered as low as
possible. Preferably, the upper limit content of P is limited to
0.03% in consideration of the efficiency of a refining process.
[0026] Sulfur (S): S deteriorates the hot formability, or forms MnS
together with Mn, thereby deteriorating the corrosion resistance.
Therefore, the content of S is lowered as low as possible.
Preferably, the content of S is lower than 0.03%.
[0027] Silicon (Si): Si is added for deoxidization, but it can act
as an element for stabilizing the ferrite phase. If the content of
Si is excessive, Si deteriorates the mechanical property such as
impact toughness of steel. Therefore, the content of Si is
preferably limited to 2% and below.
[0028] Meanwhile, samples of duplex stainless steel having
constituents according to an embodiment of the present invention
are prepared and they are tested about the volume fraction, the
corrosive resistance and the hot formability. FIG. 2 shows alloy
con- stituents of the samples and a-volume fractions after they are
annealed at a temperature of 1050.degree. C. In these alloys, the
a-volume fractions thereof range from about 40 to about 60%.
Regarding welding, phase stability, and the like, it is determined
that the duplex stainless steel has an a-volume fraction ranging
from 44 to 51% is excellent (0); the duplex stainless steel has an
a-volume fraction lower than 44% or higher than 54% is defective
(X); and the duplex stainless steel has an a-volume fraction
ranging from 51 to 54% is good (A).
[0029] In the samples of FIG. 2, the alloy constituents except Cr,
Mo, Mn and N are unified to satisfy general content rages of the
duplex stainless steel, but the content rage of Ni is limited to
2.5 wt% for experimental convenience.
[0030] FIG. 3 is a table showing critical pitting temperatures
(CPT) of the samples of FIG. 2, in which the CPT means the
corrosion resistance. Here, it is determined that the steel having
a CPT of 20.degree. C. and below is defective (X); the steel having
a CPT ranging from 20.degree. C. to 25.degree. C. and below is good
(?); and the steel having a CPT of 20.degree. C and higher is
excellent (0).
[0031] The foregoing CPTs result from annealing the hot-rolled
samples having a size of 50 mm(L).times.25 mm(W).times.3 mm(T) at a
temperature of 1050.degree. C. on the basis of an American society
for testing and materials (ASTM) G48 method, and then depositing it
in acidified ferric chloride solution for 24 hours.
[0032] According to the ASTM G48 method, a CPT measuring starting
temperature is obtained by the following equation:
CPT(.degree.
C)=(2.5.times.%Cr)+(7.6.times.%Mo)+(31.9.times.%N)-41.0.
[0033] The ASTM G48 method suggests calculating the CPTs and
selecting the closest value at intervals of 5.degree. C.
[0034] However, the CPT measuring starting temperature estimated by
the ASTM G48 has a large deviation. Why the deviation is large is
because the corrosion resistance de- teriorated by Mn is not
considered. That is, because the duplex stainless steel having
lowered contents of Ni has relatively high content of Mn, the
deviation arises in the estimated CPT obtained by the foregoing
ASTM G48 method.
[0035] To compensate the deviation, the present inventor calculates
the CPT by considering Mn as follows.
CPT(.degree. C)=-150.47+2.65Cr+11.71Mo-1.3Mn+64.58N.
[0036] According to the present invention, the estimated CPT is
approximately equal to the measured CPT.
[0037] Meanwhile, in the case of the steel having the low contents
of Cr and Mo or having the high content of Mn, the measured CPT is
relatively lower than the estimated CPT.
[0038] Therefore, it is undesirable that the content of Mn is
excessively increased or the contents of Cr and Mo is excessively
decreased in order to secure the volume fraction of duplex
stainless steel with reduced Ni.
[0039] While producing the duplex stainless steel with reduced Ni,
the production yield should be increased to decrease the incidence
of an edge crack, and the hot formability should be secured to
minimize an invariable load. The steal with constituents of FIG. 2
is produced as an ingot of 50 kg and pressed to have 20 mm(T) and
30 mm(T), and then the incidence of their edge crack is observed,
thereby getting test results as shown in FIG. 1 by selecting the
steel remarkably improved in the incidence of the edge crack as
compared with the 2205 steel.
[0040] Here, it is determined that the steel having the edge crack
like the 2205 steel is defective (X); the steel having a local edge
crack is good (A); and the steel having little edge crack is
excellent (0).
[0041] FIG. 4 shows total test results of the samples of FIG. 2
with regard to the volume fraction, the corrosion resistance, and
the hot formability. Four steels (steel Nos. 3, 4, 14, 15) satisfy
the formation properties of the duplex stainless steel, has the
corrosion resistance better than that of the 316L steel, and is
excellent in the hot formability.
[0042] Further, other steels (steel Nos. 1, 2, 5, 6, 7, 8, 9, 10,
11, 12, 13, 15, 16, 20, 21, and 22) can be selected as preferred
steel, but they are inferior to the foregoing steels. Thus, the hot
formability and the corrosion resistance are deteriorated as the
content of Mn becomes higher. Further, the hot formability is
deteriorated as the content of N becomes higher. Also, the steels
having the high content of Mn needs relatively higher content of
Mo.
[0043] FIG. 5 shows difference in high temperature oxidation
between the excellent and good steels according to the embodiment
of the present invention and comparative steels such as STS304,
STS316L and 2205 when they are reheated as slabs for hot-rolling. T
he high temperature oxidation measurement is performed by measuring
oxidation increment under the condition that the hot-rolled sample
having a size of 10 mm(L).times.10 mm(W).times.3 mm(T) is heated at
a temperature of 1250.degree. C. and remained in a heating furnace
for 180 minutes.
[0044] At this time, under gas atmosphere of the heating furnace,
the content of S is adjusted into 200 ppm. In results, the
oxidation increment of the steel according to the present invention
is 4 through 6 times lower than the convention 2205 steel, and
about 1/3 through 1/2 times higher than the 316L steel. As compared
with the conventional 2205 steel, the surface quality of the steel
according to the present invention is enhanced as a surface
defective is decreased by the surface lubrication effect due to an
oxidation layer formed on a surface of a reheating slab while being
hot-rolled.
[0045] In the duplex stainless steel according to the present
invention as compared with the 2205 duplex stainless steel, the
contents of Cr, Mo and Ni is decreased and the content of Mn is
increased a little, so that a production cost thereof is reduced;
the corrosion resistance is secured to be better than the STS 304
steel and the 316L steel; the incidence of the edge crack is
decreased while being hot-rolled, thereby decreasing a load on the
following process; and the surface defective is decreased, thereby
improving a production yield.
[0046] Although a few embodiments of the present invention have
been shown and described, it would be appreciated by those skilled
in the art that changes might be made in this embodiment without
departing from the principles and spirit of the invention, the
scope of which is defined in the claims and their equivalents.
* * * * *