PC051156PS
DUPLEX STAINLESS STEEL HAVING EXCELLENT
CORROSION RESISTANCE WITH LOW NICKEL
BACKGROUND
1. Field of the Invention
[001] 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.
2. Discussion of Related Art
[002] 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.
[003] 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 10wt% 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.
[004] 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.
[005] 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.
[006] 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
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contents of Cr and Mo, so that a sigma-phase deposition rate is high,
thereby deteriorating the properly 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
[007] 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.
[008] 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 D 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.
[009] 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 unavoidable elements
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] These and/or other objects and advantages of the invention will become
apparent and more readily appreciated from the following description of
the preferred embodiments, taken in conjunction with the accompanying
drawings of which:
[0011] FIG. 1 are photographs showing an edge crack of a hot-rolled sample
according to alloy constituents;
[0012] FIG. 2 is a table showing alloy constituents and a volume fraction of steel
samples according to the present invention and comparative steel samples;
[0013] FIG. 3 is a table showing critical pitting temperatures (CPT) of the
samples of FIG. 2;
[0014] FIG. 4 is a table showing total test results of the samples of FIG. 2 with
regard to corrosion resistance and hot formability; and
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[0015] FIG. 5 is a table showing oxidation increment of the steel according to the
present invention and the conventional steel.
DETAILED DESCRIPTION
[0016] Hereinafter, embodiments of the present invention will be described with
reference to accompanying drawings. Below, essential constituents are
limited as follows.
[0017] 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.
[0018] 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%.
[0019] 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%.
[0020] 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
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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
the duplex stainless steel and the corrosion resistance better than that of
STS304and316Lsteel.
[0021] 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 deteriorating 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%.
[0022] 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
excessively needed so that the corrosion resistance and the hot formability
are deteriorated, 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%.
[0023] 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.
[0024] 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%.
[0025] 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.
[0026] Meanwhile, samples of duplex stainless steel having constituents
according to an embodiment of the present invention are prepared and they
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are tested about the volume fraction, the corrosive resistance and the hot
formability. FIG. 2 shows alloy constituents of the samples and a-volume "
fractions after they are annealed at a temperature of 1050D. 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 (O); 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).
[0027] 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 2.5wt% for experimental convenience.
[0028] 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 20D and below is defective
(X); the steel having a CPT ranging from 20D to 25 □ and below is good
(A); and the steel having a CPT of 20D and higher is excellent (O).
[0029] The foregoing CPTs result from annealing the hot-rolled samples having a
size of 50mm(L)x25mm(W)x3mm(T) at a temperature of 1050D 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.
[0030] According to the ASTM G48 method, a CPT measuring starting
temperature is obtained by the following equation:
[0031] CPT(D) = (2.5x%Cr) + (7.6x%Mo) + (31.9x%N) -41.0.
[0032] The ASTM G48 method suggests calculating the CPTs and selecting the
closest value at intervals of 5D.
[0033] 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 deteriorated 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.
[0034] To compensate the deviation, the present inventor calculates the CPT by
considering Mn as follows.
[0035] CPT(D) = -150.47 + 2.65Cr + 11.71Mo -1.3Mn + 64.58N.
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[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. 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.
[0038] 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 50kg
and pressed to have 20mm(T) and 30mm(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.
[0039] 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 (O).
[0040] 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. 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.
[0041] 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. The high temperature
oxidation measurement is performed by measuring oxidation increment
under the condition that the hot-rolled sample having a size of 10mm(L)
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xl0mm(W)x3mm(T) is heated at a temperature of 1250D and remained in
a heating furnace for 180 minutes.
[0042] At this time, under gas atmosphere of the heating furnace, the content of S
is adjusted into 200ppm. 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.
[0043] 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.
[0044] 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.
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PC051156PS
WHAT IS CLAIMED IS:
1. Duplex stainless steel having excellent corrosion resistance with low
Ni, having alloy constituents including 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%, Fe and unavoidable elements,
wherein
a CPT is calculated by the following equation using a constitution range
of the alloy constituents as a function:
CPT(D) = -150.47 + 2.65Cr + 11.71Mo -1.3Mn + 64.58N, and
the constitution range of the alloy constituents are adjusted to make the
CPT higher than 20 D.
2. The duplex stainless steel according to claim 1, further including C of
0.03% and less, P of 0.03% and less, and Si of 2% and less.
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Disclosed is duplex stainless steel
that contains relatively low content of Ni, and limits
constituents of Cr-Mo-Mn-N to make volume frac-
tion of a and γ have about 50:50, thereby minimiz-
ing incidence of a edge crack to enhance a produc-
tion yield and decrease a processing load, in which
the alloy constituents 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%, Fe and unavoidable elements, and
a constitution range of the alloy constituents are ad-
justed to make a CPT highter than 20°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 pro-
duction cost thereof is reduced; the corrosion resis-
tance 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 decreas-
ing a load on the following process; and the surface
defective is decreased, thereby improving a produc-
tion yield.