U.S. patent application number 12/654593 was filed with the patent office on 2010-07-08 for ferritic-austenitic stainless steel.
This patent application is currently assigned to Outokumpu Stainless Aktiebolag. Invention is credited to Elisabeth Alfonsson, Per Johansson, Mats Liljas, Jun Wang.
Application Number | 20100172785 12/654593 |
Document ID | / |
Family ID | 20281174 |
Filed Date | 2010-07-08 |
United States Patent
Application |
20100172785 |
Kind Code |
A1 |
Alfonsson; Elisabeth ; et
al. |
July 8, 2010 |
Ferritic-austenitic stainless steel
Abstract
A ferritic-austenitic stainless steel having a microstructure
which essentially consists of 35-65 vol-% ferrite and 35-65 vol-%
austenite, has a chemical composition which contains in weight-%:
0.005-0.07 C, 0.1-2.0 Si, 3-8 Mn, 19-23 Cr, 0.5-1.7 Ni, optionally
Mo and/or W in a total amount of max 1.0 (Mo+W/2), optionally Cu up
to max 1.0 Cu, 0.15-0.30 N, balance iron and impurities. The
following conditions shall apply for the chromium and nickel
equivalents: 20<Cr.sub.eq<24.5, 10<Ni.sub.eq, where,
Cr.sub.eq=Cr+1.5 Si+Mo+2 Ti+0.5 Nb, Ni.sub.eq=Ni+0.5 Mn+30
(C+N)+0.5 (Cu+Co).
Inventors: |
Alfonsson; Elisabeth;
(Avesta, SE) ; Wang; Jun; (Scottsdale, AZ)
; Liljas; Mats; (Avesta, SE) ; Johansson; Per;
(Avesta, SE) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
Outokumpu Stainless
Aktiebolag
Stockholm
SE
|
Family ID: |
20281174 |
Appl. No.: |
12/654593 |
Filed: |
December 23, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10381673 |
Apr 24, 2003 |
|
|
|
PCT/SE01/01986 |
Sep 18, 2001 |
|
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12654593 |
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Current U.S.
Class: |
420/61 |
Current CPC
Class: |
C21D 2211/001 20130101;
C22C 38/48 20130101; C22C 38/46 20130101; C22C 38/002 20130101;
C22C 38/58 20130101; C22C 38/44 20130101; C22C 38/001 20130101;
C22C 38/50 20130101; C21D 2211/005 20130101; C22C 38/54 20130101;
C22C 38/42 20130101; C22C 38/06 20130101; C22C 38/02 20130101 |
Class at
Publication: |
420/61 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/54 20060101 C22C038/54; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 27, 2000 |
SE |
0003448-8 |
Claims
1. A ferritic-austenitic stainless steel having a microstructure
which essentially consists of 35-65 vol-% ferrite and 35-65 vol-%
austenite, characterised in that the steel has a chemical
composition which contains in weight-%: 0.02-0.07 C 0.1-2.0 Si 3-8
Mn 19-23 Cr 1.1-1.7 Ni optionally Mo and/or W in a total amount of
max 1.0 (Mo+W/2) optionally Cu up to max 1.0 Cu 0.15-0.30 N balance
iron and impurities, and that the following conditions shall apply
for the ferrite- and austenite formers of the alloy, respectively,
i.e. for the chromium and nickel equivalents:
20<Cr.sub.eq<24.5 10<Ni.sub.eq, where
Cr.sub.eq=Cr+1.5Si+Mo+2Ti+0.5Nb
Ni.sub.eq=Ni+0.5Mn+30(C+N)+0.5(Cu+Co)
2. A steel according to claim, characterised in that it contains
max 0.05, preferably max 0 04 C.
3. A steel according to any one of claims 1-2, characterised in
that it contains at least 0.2, preferably at least 0.35 Si.
4. A steel according to any one of claims 1-3, characterised in
that it contains max 1.5, preferably max 1.0 Si.
5. A steel according to any one of claims 3 and 4, characterised in
that it contains 0.35-0.80 Si.
6. A steel according to any one of claims 1-5, characterised in
that it contains at least 4, suitably at least 4.5 Mn.
7. A steel according to any one of claims 1-6, characterised in
that it contains max 6 Mn.
8. A steel according to any one of claims 6 and 7, characterised in
that it contains 4.5-5.5 Mn.
9. A steel according to any one of claims 1-8, characterised in
that it contains at least 20, preferably at least 20.5 Cr.
10. A steel according to any one of claims 1-9, characterised in
that it contains max 23, suitably max 22.5 Cr.
11. A steel according to any one of claims 9 and 10, characterised
in that it contains 21.0-22.0, preferably 21.2-21.8 Cr.
12. A steel according to any one of claims 1-11, characterised in
that it contains 1.35-1.70 Ni.
13. A steel according to any one of claims 1-12, characterised in
that it contains at least 0.1, preferably at least 0.15 Mo.
14. A steel according to claim 13, characterised in that it
contains max 0 8 Mo, preferably max 0.65 Mo.
15. A steel according to any one of claims 13 and 14, characterised
in that it contains 0.15-0.54 (Mo+W/2).
16. A steel according to claims 1-15, characterised in that it
contains at least 0.1, preferably at least 0.15, suitably at least
0.24 Cu.
17. A steel according to claim 16, characterised in that it
contains max 0.7 Cu.
18. A steel according to any one of claims 16 and 17, characterised
in that it contains 0.25-0.54 Cu,
19. A steel according claim 18, characterised in that it contains
at least 0.18 N.
20. A steel according to any one of claims 1-19, characterised in
that it contains max 0.26 N.
21. A steel according to any one of claims 19 and 20, characterised
in that it contains 0.20-0.24 N.
22. A steel according to any one of claims 1-21, characterised in
that it contains 0.001-0.005 B.
23. A steel according to any one of claims 1-22, characterised in
that it contains max 0.10 S.
24. A steel according to claim 23, characterised in that it
contains max 0.002 S.
25. A steel according to any one of claims 1-24, characterised in
that the coordinates of the Cr- and Ni-equivalents lie within the
frame of the area A B C D A in the Schaeffler diagram in FIG. 1,
the coordinates of said points being: Cr.sub.eq Ni.sub.eq
TABLE-US-00007 A 20.8 11.8 B 23.0 15.0 C 24.0 14.5 D 23.0 10.4
26. A steel according to claim 25, characterised in that the
coordinates of the Cr- and Ni-equivalents lie within the frame of
the area D E F G H D in the Schaeffler diagram in FIG. 1, the
coordinates of said points being: TABLE-US-00008 Cr.sub.eq
Ni.sub.eq D 23.0 10.4 E 22.0 11.0 F 22.0 13.5 G 22.3 14.0 H 23.0
14.0
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
10/381,673 filed 24 Apr. 2003, which is a 371 of PCT/SE01/01986,
filed 18 Sep. 2001, which designated the U.S. and claims priority
to Swedish Application No. 0003448-8 filed 27 Sep. 2000, the entire
contents of each of which are hereby incorporated by reference.
TECHNICAL FIELD
[0002] The invention relates to a ferritic-austenitic stainless
steel having a microstructure which essentially consists of 35-65
vol-% ferrite and 35-65 vol-% austenite.
BACKGROUND OF THE INVENTION
[0003] The ferritic-austenitic stainless steels--the duplex
steels--combine a high mechanical strength and toughness with good
corrosion resistance, particularly as far as stress corrosion is
concerned. For the corrosion resistance as well as for mechanical
features such as weldability, it is important that the essential
constituents of the steel, austenite and ferrite, are well
balanced. In modern development of duplex steels, efforts are made
to obtain a microstructure which contains 35-65% ferrite and 35-65%
austenite. The duplex steels to an increased extent compete with
traditional austenitic stainless steels within offshore, paper and
pulp industry, chemical industry, and other fields where high
strength and corrosion resistance are required. The duplex steels
which so far are commercially available are, however, too expensive
to find wider use, in spite of the fact that the duplex steels
generally contain lower contents of the expensive alloy element
nickel than comparable austenitic stainless steels.
DISCLOSURE OF THE INVENTION
[0004] It is the purpose of the invention to provide a ferritic
austenitic stainless steel of the type mentioned in the above
preamble, which steel contains a lower amount of expensive alloy
elements than today commercially available duplex steels and
austenitic stainless steels having comparable technical features,
and which can be manufactured in a way which is advantageous from a
process technical point of view. Most of the fields where duplex
steels are used today are conceivable and suitable fields of use,
i.e. for applications within offshore, paper and pulp industry,
chemical industry etc., but above all for applications where the
corrosion conditions are milder than where duplex steels are
employed today, but where high strength and/or good resistance
against stress corrosion is a benefit. The combination of
mechanical strength and corrosion resistance also makes the
material suitable for light, maintenance-free constructions within
the transportation-, building-, and construction fields.
[0005] The achievement of a plurality or all of the following
effects are other objectives of the invention [0006] A yield
strength (Rp.sub.02).gtoreq.450 MPa at room temperature and
.gtoreq.300 MPa at 150.degree. C., [0007] A microstructure which
contains 35-65% ferrite and 35-65% austenite, preferably 35-55%
ferrite and 45-65% austenite, [0008] A good structural stability,
[0009] A good general corrosion resistance and particularly a good
stress corrosion resistance, [0010] A good weldability with very
good reformation of austenite in the heat affected zone.
[0011] The above objectives can be achieved therein that the steel
has a chemical composition which contains in weight-%:
[0012] 0.005-0.07 C
[0013] 0.1-2.0 Si
[0014] 3-8 Mn
[0015] 19-23 Cr
[0016] 0.5-1.7 Ni
[0017] optionally Mo and/or W in a total amount of max 1.0
(Mo+W/2)
[0018] optionally Cu up to max 1.0 Cu
[0019] 0.15-0.30 N
[0020] balance iron and impurities, and that the following
conditions shall apply for the ferrite- and austenite formers of
the alloy, respectively, i.e. for the chromium and nickel
equivalents:
20<Cr.sub.eq<24.5
10<Ni.sub.eq, where
Cr.sub.eq=Cr+1.5Si+Mo+2Ti+0.5Nb
Ni.sub.eq=Ni+0.5Mn+30(C+N)+0.5(Cu+Co)
[0021] As far as the individual alloy elements, their importance
and interaction are concerned, the following applies. Stated alloy
contents refer to weight-% if not something else is mentioned.
[0022] Carbon contributes to the strength of the steel and it is
also a valuable austenite former and shall therefore exist in a
minimum amount of 0.005%, preferably at least 0.01%, suitably at
least 0.015%. It is, however, time consuming to bring the carbon
content down to low levels in connection with the decarburisation
of the steel, and it is also expensive because it increases the
consumption of reduction agents. I. a. from these reasons, the
carbon content should not be less than 0.02%. If the carbon content
is high, there is a risk for precipitation of carbides, which can
reduce the impact toughness of the steel and the resistance to
intercrystalline corrosion. It shall also be considered that carbon
has a very small solubility in the ferrite, which means that the
carbon content of the steel substantially is collected in the
austenitic phase. The carbon content therefore shall be restricted
to max 0.07%, preferably to max 0.05%, and suitably to max
0.04%.
[0023] Silicon can be used as a reduction agent at the
manufacturing of the steel and exists as a residue from the
manufacturing of the steel in an amount of at least 0.1%. Silicon
has favourable features in the steel to the effect that it
strengthens the high temperature strength of the ferrite, which has
a significant importance at the manufacturing. Silicon also is a
strong ferrite former and participates as such in the stabilisation
of the duplex structure and should from these reasons exist in an
amount of at least 0.2%, preferably in an amount of at least 0.35%.
Silicon, also have some unfavourable features because it
pronouncedly reduces the solubility for nitrogen) which shall exist
in high amounts, and if the content of silicon is high also the
risk of precipitation of undesired intermetallic phases is
increased. The silicon content therefore is limited to max 2.0%,
preferably to max 1.5%, and suitably to max 1.0%. An optimal
silicon content is 0.35-0.80%.
[0024] Manganese is an important austenite former and increases the
solubility for nitrogen in the steel and shall therefore exist in
an amount of at least 3%, preferably at least 4%, suitably at least
4.5%. Manganese, on the other hand, reduces the corrosion
resistance of the steel. Moreover it is difficult to decarburise
stainless steel melts having high contents of manganese, which
means that manganese need to be added after finished
decarburisation in the form of comparatively pure and consequently
expensive manganese. The steel therefore should not contain more
than 8% manganese, preferably max 6% manganese. An optimal content
is 4.5-5.5% manganese.
[0025] Chromium is the most important element for the achievement
of a desired corrosion resistance of the steel. Chromium also is
the most important ferrite former of the steel and gives in
combination with other ferrite formers and with a balanced content
of the austenite formers of the steel a desired duplex character of
the steel. If the chromium content is low, there is a risk that the
steel will contain martensite and if the chromium content is high,
there is a risk of impaired stability against precipitation of
intermetallic phases and so called 475.degree.-embrittlement, and
an unbalanced phase composition of the steel. From these reasons
the chromium content shall be at least 19%, preferably at least
20%, and suitably at least 20.5%, and max 24%, preferably max 23%,
suitably max 22.5%. A suitable chromium content is 21.0-22.0%,
nominally 21.2-21.8%.
[0026] Nickel is a strong austenite former and has a favourable
effect on the ductility of the steel and shall therefore exist in
an amount of at least 0.5%. Preferably nickel should exist in an
amount of at least 0.8%, suitably at least 1.1%. However, the raw
material price of nickel often is high and fluctuates, wherefore
nickel, according to an aspect of the invention, is substituted by
other alloy elements as far as is possible. Nor is more than 1.7%
nickel necessary for the stabilisation of the desired duplex
structure of the steel in combination with other alloy elements. An
optimal nickel content therefore is 1.35-1.70% Ni.
[0027] Molybdenum is an element which can be omitted according to a
wide aspect of the composition of the steel, i.e. molybdenum is an
optional element in the steel of the invention. Molybdenum,
however, together with nitrogen has a favourable synergy effect on
the corrosion resistance. In view of the high nitrogen content of
the steel, the steel therefore should contain at least 0.1%
molybdenum, preferably at least 0.15%. Molybdenum, however, is a
strong ferrite former, it can stabilize sigma-phase in the
microstructure of the steel, and it also has a tendency to
segregate. Further, molybdenum is an expensive alloy element. From
these reasons the molybdenum content is limited to max 1.0%,
preferably to max 0.8%, suitably to max 0.65%. An optimal
molybdenum content is 0.15-0.54%. Molybdenum can partly be replaced
by the double amount of tungsten, which has properties similar to
those of molybdenum. However, at least half of the total amount of
Mo+W/2 should consist of molybdenum. In a preferred composition the
steel, however, the steel does not contain more than max 0.3
tungsten.
[0028] Copper is also an optional element, which can be omitted
according to the widest aspect on this element. However, copper is
a valuable austenite former and can have a favourable influence on
the corrosion resistance in some environments, especially in some
acid media, and should therefore exist in an amount of at least
0.1%. On the other hand, there is a risk of precipitation of copper
in case of too high contents thereof, wherefore the copper content
should be maximized to 1.0%, preferably to max 0.7%. Optimally, the
copper content should be at least 0.15, preferably at least 0.25
and max 0.54% in order to balance the favourable and possibly
unfavourable effects of copper with reference to the features of
the steel.
[0029] Nitrogen has a fundamental importance because it is the
dominating austenite former of the steel. Nitrogen also contributes
to the strength and corrosion resistance of the steel and shall
therefore exist in a minimum amount of 0.15%, preferably at least
0.18%. The solubility of nitrogen in the steel, however, is
limited, In case of a too high nitrogen content there is a risk of
formation of flaws when the steel solidifies, and a risk of
formation of pores in connection with welding of the steel. The
steel therefore should not contain more than 0.30% nitrogen,
preferably max 0.26% nitrogen. An optimal content is
0.20-0.24%.
[0030] Boron can optionally exist in the steel as a micro alloying
addition up to max 0.005% (50 ppm) in order to improve the hot
ductility of the steel. If boron exists as an intentionally added
element, it should exist in an amount of at least 0.001% (10 ppm)
in order to provide the desired effect with reference to improved
hot ductility of the steel.
[0031] In a similar way, cerium and/or calcium optionally may exist
in the steel in amounts of max 0.03% of each of said elements in
order to improve the hot ductility of the steel.
[0032] Besides the above mentioned elements, the steel does not
essentially contain any further intentionally added elements, but
only impurities and iron. Phosphorus is, as in most steels, a
non-desired impurity and should preferably not exist in an amount
higher than max 0.035%. Sulphur also should be kept at as low as is
possible from an economically manufacturing point of view,
preferably in an amount of max 0.10%, suitably lower, e.g. max
0.002% in order not to impair the hot ductility of the steel and
hence its rollability, which can be a general problem in connection
with the duplex steels.
[0033] Within the frame of the above mentioned content ranges, the
contents of ferrite formers and austenite formers shall be balanced
according to the conditions which have been mentioned in the
foregoing, in order that the steel shall get a desired, stabile
duplex character. Preferably the nickel equivalent, Ni.sub.eq,
should be at least 10.5 and the chromium equivalent at least 21,
most advantageously at least 22, Upwards, the nickel equivalent,
Ni.sub.eq, should be limited to max 15, preferably to max 14.
Further the chromium equivalent, Cr.sub.eq, should be at least 21,
preferably at least 21.5 and most advantageously at least 22, but
can be limited to max 23.5, It is surprising that a steel with
chromium- and nickel equivalents related to one another according
to the said criteria has a balanced content of ferrite and
austenite within above mentioned content rage. Theoretically, the
steel because of its alloy composition should contain less or even
much less than 35 volume-% ferrite, but measurements carried out
through image analyses of the microstructures instead have shown
that the steel as a matter of fact contains a stabile content of at
least 35 vol-% ferrite and, for several of the tested steels
according to the invention, about 50% ferrite. On the basis of
these observations one can, according to an aspect on the relations
between the chromium- and nickel equivalents, assume that the
coordinates of the chromium- and nickel equivalents should lie
within the frame of the area ABC DA in the Schaeffler diagram in
FIG. 1, the coordinates of said points being the following:
TABLE-US-00001 Cr.sub.eq Ni.sub.eq A 20.8 11.8 B 23.0 15.0 C 24.0
14.5 D 23.0 10.4
[0034] i.e. well to the left of the region which in the Schaeffler
diagram conventional is the region of duplex steels. Nevertheless a
stabile duplex character of the steel is achieved.
[0035] Performed experiments have shown that good results are
achieved with steel alloys having compositions the chromium- and
nickel equivalents of which lie within the frame of the more
restricted area D E F G H D, the coordinates of said points
being:
TABLE-US-00002 Cr.sub.eq Ni.sub.eq D 23.0 10.4 E 22.0 11.0 F 22.0
13.5 G 22.3 14.0 H 23.0 14.0
BRIEF DESCRIPTION OF DRAWINGS
[0036] In the following description of performed experiments,
reference will be made to the accompanying drawings, in which:
[0037] FIG. 1 shows microstructures and a Schaeffler diagram,
illustrating the theoretical chromium- and nickel equivalents
according to the invention,
[0038] FIG. 2 is a bar chart which illustrates the real ferrite and
austenite contents which have been measured in examined steels
according to the invention,
[0039] FIG. 3 is a bar chart illustrating the resistance to pitting
corrosion of examined steels in the form of measured critical
pitting temperatures, CPT,
[0040] FIG. 4 is a diagram illustrating the resistance to stress
corrosion versus time to fracture at drop evaporation testing of a
number of examined alloys, and
[0041] FIG. 5 is a bar charge illustrating the weldability of a
number of examined alloys in terms of ferrite content in the heat
effected zone (HAZ) and in the welding seam itself.
DESCRIPTION OF PERFORMED EXPERIMENTS AND ACHIEVED RESULTS
[0042] The chemical compositions in weight-% of examined steels are
given in Table 1. Besides the elements stated in the table, the
steels only contained iron and other impurities than the stated
ones in normal amounts. The steels V250-V260 were manufactured in
the form of 30 kg laboratory heats. Ref A is a commercially
available steel, the composition of which has been analysed by the
applicant.
TABLE-US-00003 TABLE 1 Composition, weight-%, of examined steels
Heat/ steel C Si Mn P S Cr Ni Mo Ti Nb Cu N W V A1 B O Cr.sub.eq
Ni.sub.eq V250 0.042 0.29 4.40 0.012 0.003 21.85 1.50 0.32 0.003
0.001 0.18 0.245 <0.01 0.035 0.17 0.0004 n.a.* 22.6 12.4 V251
0.052 0.30 5.26 0.012 0.004 21.52 1.48 0.32 0.004 0.001 0.18 0.225
<0.01 0.034 0.016 0.0004 n.a.* 22.3 12.5 V252 0.032 0.30 5.16
0.012 0.004 21.80 1.49 0.32 0.002 0.001 0.22 0.285 <0.01 0.035
0.001 0.0005 0.0125 22.6 13.7 V254 0.036 0.39 5.23 0.012 0.004
21.24 1.10 0.13 0.005 0.001 0.41 0.130 <0.01 0.035 0.014
<0.001 n.a.* 21.9 8.9 V258 0.018 0.28 5.22 0.008 0.002 21.63
1.49 0.32 0.003 0.002 0.88 0.203 0 0.025 0.024 0.0004 0.0036 22.4
11.1 V260 0.038 0.31 4.71 0.008 0.003 21.77 1.50 0.32 0.004 0.001
0.24 0.227 0 0.024 0.025 0.0003 0.0017 22.6 11.9 Ref. 0.026 0.39
5.17 0.026 0.001 20.90 1.25 0.08 0.019 0.004 0.44 0.127 <0.01
0.068 <0.001 0.0041 n.a.* 21.6 8.5 A *n.a. = not analyzed
[0043] Mechanical Tests
[0044] The laboratory heats were rolled to the shape of 3 mm thick,
narrow plates, which were used for the mechanical tests. By
experience it is known that the 0.2 yield strength lies at a 80-100
MPa lower level than for materials which have been manufactured at
a full production scale. The 0.2- and 1.0 yield strengths, the
ultimate strength (Rm), the elongation in tensile test (A.sub.5)
and the Brinell hardness were examined at room temperature,
20.degree. C., and at 150.degree. C. Representative measurements
are given in Table 2.
TABLE-US-00004 TABLE 2 Mechanical strength features at 20.degree.
C. and 150.degree. C. Temp Rp.sub.0.2 Rp.sub.1.0 Rm Heat/steel
.degree. C. MPa MPa MPa A.sub.5 % HB V258 20 465 525 686 46 210 150
352 397 596 44 -- V260 20 470 526 694 46 209 150 352 399 602 42 --
V254 20 440 504 644 39 211 150 338 387 548 36 --
[0045] Microstructure Studies
[0046] In the Schaeffler diagram in FIG. 1 the coordinates of the
steels V250-V260 manufactured at a laboratory scale have been
inserted. All these coordinates lie within the ferritic-austenitic
structure area of the diagram but to the left of the line
representing the ferrite number 30, wherefore the steels should not
be duplex steels. Test measuring of the manufactured steels,
performed through image analyses of the microstructures, however,
surprisingly shows that at least the steels V25 1-V260 contain more
than 35 vol-% ferrite, as is shown by the chart diagram in FIG. 2.
The examined test specimens had been solution heat treated through
annealing at 1.050.degree. C. The structure stability was
comparable with that of the steel of the applicant having the trade
name SAF 2304.TM., which is a duplex steel corresponding UNS
S32304.
[0047] Corrosion Tests
[0048] The critical pitting temperature, CPT, was determined
according to the standardized method which is known by the
designation ASTM G 150. The results are represented by the chart
diagram in FIG. 3. The test shows that the steels V251, V258, and
V260 manufactured at a laboratory scale have a significantly better
corrosion resistance than V254 and also essentially better than the
reference steels Ref. A, ASTM 304 and ASTM 201, but the steels of
the invention manufactured at a laboratory scale do not reach the
level of ASTM 316 L or UNS S 32304, which however, have a higher
content of expensive alloy metals.
[0049] Two methods were employed for measuring the resistance to
intercrystalline corrosion. Specimens which had been sensitized for
lh at 700.degree. C. or for 8 h at 600.degree. C. and 800.degree.
C., respectively, were tested in a sulphuric acid/copper sulphate
solution according to EN-ISO 3651-2, method A (Strauss test). No
test specimen showed any signs of inter-crystalline corrosion, Nor
did testing according to the more aggressive method EN-ISO 3651-2,
method C (Streicher test) of solution heat treated tests specimens
or of specimens sensitized at 700.degree. C. for 30 min,
respectively, result in intercrystalline corrosion.
[0050] The resistance to stress corrosion was studied according to
the drop evaporation test (DET) described e.g. in MTI manual No. 3,
method MTA-5. A mono-axially loaded, resistance heated test
specimen was exposed to a dripping sodium chloride solution. The
time to fracture was determined at different load levels, defined
as a certain proportion of Rp02 at 200.degree. C. The results for
the experimental heats V260 and V254 are shown in FIG. 4 together
with data for the austenitic steel ASTM 316L. Like commercially
available duplex steels, the experimental heats exhibited an
essentially higher resistance to stress corrosion than standardized
austenitic steels, such as ASTM 316L, V260 appears to be more
resistant that V254.
[0051] In summary it can as far as the corrosion resistance is
concerned be stated that the pitting corrosion resistance is
essentially higher than for the austenitic steel ASTM 304, that no
intercrystallin corrosion could be observed, and that also the
stress corrosion resistance is essentially higher than for
conventional austenitic steels.
[0052] Weldability Tests
[0053] Weldability tests were carried out by TIG-welding of a plate
without addition of a filler metal, and by TIG-welding in a weld
joint using a filler metal of type AWS ER 2209, which is a ferritic
austenitic filler material which usually is used for welding more
highly alloyed duplex steels. The ferrite contents in the latter
case were measured in the weld and in the heat affected zone.
[0054] The weldability of the test alloys was comparable to that of
the reference material Ref. A and UNS S 31803. Non destructive
testing with x-ray controls could not detect any high porosity
levels. The material of the invention had a high degree of
austenite reformation in the heat affected zone, HAZ, and in the
weld in comparison with the reference material Ref. A and UNS S
31803. The ferrite content in the case of manual TIG welding a
steel of type UNS S 31803, the reference steel Ref. A, and the
steel V258 of the invention with a filler metal of type AWS ER2209
is shown in the bar chart in FIG. 5. When subjected to tensile
testing, all the welds were fractured in the parent material and
not in the welds.
[0055] On the basis of the experiences derived through the testing
of laboratory scale materials which have been described in the
foregoing, a 90 tons heat No. 804030 was manufactured having the
following chemical composition in weight-%, Table 3. Besides the
elements mentioned in Table 3, the steel only contained iron and
other impurities than those which are stated in the Table in normal
amounts.
TABLE-US-00005 TABLE 3 Chemical composition, weight-%, Heat No.
804030 C Si Mn P S Cr Ni Mo Ti 0.024 0.69 5.07 0.017 0.000 21.36
1.49 0.30 0.00 Nb Cu N As W V A1 B O 0.001 0.32 0.232 0.004 0.00
0.052 0.008 0.0021 0.0014
[0056] A strand was made through continuous casting of the molten
steel. The strand was cut into slabs. Some slabs were hot rolled to
the shape of plates having thicknesses of 8 mm and 15 mm
respectively, while other slabs were hot-rolled to the form of
coils having a thickness of 4 mm. Some of the hot-rolled coils were
further cold rolled to thicknesses of 3 mm, 1.5 mm and 1.0 mm,
respectively. Test specimens were taken from different parts of the
plates and coils respectively. The mechanical properties of the hot
rolled, 4 mm thick coil were tested at 20.degree. C. The results of
the tests (mean values) are given in Table 4.
TABLE-US-00006 TABLE 4 Mechanical properties at 20.degree. C.,
solution annealed condition, T = 1.050.degree. C. Rp.sub.0.2
Rp.sub.1.0 Rm MPa MPa MPa A.sub.5 % HB 558 625 775 37 230
[0057] The tests demonstrated that the steel which is produced at a
production scale is stronger than the materials which are produced
at a laboratory scale. The elongation value corresponded well with
the results from the laboratory tests, and the hardness was at a
somewhat higher level than for the laboratory scale materials,
which harmonizes with the higher yield and ultimate strength.
[0058] Test specimens of the materials that were hot rolled and hot
rolled+cold rolled, respectively, were also subjected to pitting
corrosion tests according to ASTM G 150. The plates of gauge 8 and
15 mm had a critical pitting temperature, CPT, of 17.degree. C.,
while the coils whether they were cold rolled or not had a critical
pitting temperature of 22.degree. C. The results indicate that the
production material also had an improved pitting corrosion
resistance as compared with the laboratory materials.
* * * * *