U.S. patent application number 16/371563 was filed with the patent office on 2019-07-25 for process for manufacturing hot-rolled plate, strip or coil made of duplex stainless steel.
The applicant listed for this patent is INDUSTEEL CREUSOT, UGITECH. Invention is credited to Bernard BONNEFOIS, Eric CHAUVEAU, Jean-Michel HAUSER, Jerome PEULTIER, Mickael SERRIERE.
Application Number | 20190226068 16/371563 |
Document ID | / |
Family ID | 36716663 |
Filed Date | 2019-07-25 |
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United States Patent
Application |
20190226068 |
Kind Code |
A1 |
BONNEFOIS; Bernard ; et
al. |
July 25, 2019 |
PROCESS FOR MANUFACTURING HOT-ROLLED PLATE, STRIP OR COIL MADE OF
DUPLEX STAINLESS STEEL
Abstract
The invention relates to a duplex stainless steel composition,
the composition of which consists of, in % by weight:
C.ltoreq.0.05% 21%.ltoreq.Cr.ltoreq.25% 1%.ltoreq.Ni.ltoreq.2.95%
0.16%.ltoreq.N.ltoreq.0.28% Mn.ltoreq.2.0% Mo+W/2.ltoreq.0.50%
Mo.ltoreq.0.45% W.ltoreq.0.15% Si.ltoreq.1.4% Al.ltoreq.0.05%
0.11%.ltoreq.Cu.ltoreq.0.50% S.ltoreq.0.010% P.ltoreq.0.040%
Co.ltoreq.0.5% REM.ltoreq.0.1% V.ltoreq.0.5% Ti.ltoreq.0.1%
Nb.ltoreq.0.3% Mg.ltoreq.0.1% the balance being iron and impurities
resulting from the smelting, and the microstructure consisting of
austenite and 35 to 65% ferrite by volume, the composition
furthermore satisfying the following relationships:
40.ltoreq.I.sub.F.ltoreq.70 where I.sub.F=6.times.(%
Cr+1.32.times.% Mo+1.27.times.% Si)-10.times.(% Ni+24.times.%
C+16.15.times.% N+0.5.times.% Cu+0.4.times.% Mn)-6.17 and
I.sub.LCR.gtoreq.30.5 where I.sub.LCR=% Cr+3.3.times.%
Mo+16.times.% N+2.6.times.% Ni-0.7.times.% Mn, and also to a
process for manufacturing plate, strip, coil, bar, rod, wire,
sections, forgings and castings made of this steel.
Inventors: |
BONNEFOIS; Bernard; (Le
Breuil, FR) ; PEULTIER; Jerome; (Magnien, FR)
; SERRIERE; Mickael; (Saint Sernin du Bois, FR) ;
HAUSER; Jean-Michel; (Ugine, FR) ; CHAUVEAU;
Eric; (Albertville, FR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
INDUSTEEL CREUSOT
UGITECH |
La Plaine Saint Denis
Jgine |
|
FR
FR |
|
|
Family ID: |
36716663 |
Appl. No.: |
16/371563 |
Filed: |
April 1, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14622402 |
Feb 13, 2015 |
|
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16371563 |
|
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12305014 |
May 11, 2009 |
|
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PCT/FR2007/000994 |
Jun 15, 2007 |
|
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14622402 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 6/002 20130101;
C21D 6/007 20130101; C22C 38/02 20130101; C22C 38/04 20130101; C21D
9/46 20130101; C22C 38/44 20130101; C21D 9/525 20130101; C22C
38/004 20130101; C22C 38/06 20130101; C22C 38/42 20130101; B21B
1/26 20130101; C22C 38/48 20130101; C21D 8/0263 20130101; C21D
8/021 20130101; C21D 6/004 20130101; C22C 38/001 20130101; C21D
8/0226 20130101; C22C 38/52 20130101; C22C 38/005 20130101; C21D
9/52 20130101; C21D 6/008 20130101; C21D 9/0081 20130101; C21D
6/005 20130101; C22C 38/46 20130101; C22C 38/56 20130101; C22C
38/60 20130101; C21D 2211/005 20130101; C22C 38/54 20130101; C22C
38/002 20130101; C22C 38/50 20130101 |
International
Class: |
C22C 38/60 20060101
C22C038/60; C22C 38/00 20060101 C22C038/00; C22C 38/48 20060101
C22C038/48; C21D 8/02 20060101 C21D008/02; C22C 38/50 20060101
C22C038/50; C22C 38/52 20060101 C22C038/52; C22C 38/54 20060101
C22C038/54; C22C 38/56 20060101 C22C038/56; B21B 1/26 20060101
B21B001/26; C21D 6/00 20060101 C21D006/00; C21D 9/00 20060101
C21D009/00; C21D 9/46 20060101 C21D009/46; C21D 9/52 20060101
C21D009/52; C22C 38/06 20060101 C22C038/06; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 16, 2006 |
EP |
06290991.6 |
Claims
1. A process for manufacturing a hot-rolled plate, strip or coil
made of steel comprising: providing an ingot or slab of a steel of
composition comprising in % by weight: C.ltoreq.0.05%
21%.ltoreq.Cr.ltoreq.25% 1%.ltoreq.Ni.ltoreq.2.95%
0.16%.ltoreq.N.ltoreq.0.28% Mn.ltoreq.2.0% Mo+W/2.ltoreq.0.50%
Mo.ltoreq.0.45% W.ltoreq.0.15% Si.ltoreq.1.4% Al.ltoreq.0.05%
0.11%.ltoreq.Cu.ltoreq.0.50% S.ltoreq.0.010% P.ltoreq.0.040%
Co.ltoreq.0.5% REM.ltoreq.0.1% V.ltoreq.0.5% Ti.ltoreq.0.1%
Nb.ltoreq.0.3% Mg.ltoreq.0.1% the balance being iron and impurities
resulting from the smelting, and the microstructure consisting of
austenite and 35 to 65% ferrite by volume, the composition
furthermore satisfying the following relationships:
40.ltoreq.I.sub.F.ltoreq.70 where I.sub.F=6.times.(%
Cr+1.32.times.% Mo+1.27.times.% Si)-10.times.(% Ni+24.times.%
C+16.15.times.% N+0.5.times.% Cu+0.4.times.% Mn)-6.17 and
I.sub.LCR.gtoreq.30.5 where I.sub.LCR=% Cr+3.3.times.%
Mo+16.times.% N+2.6.times.% Ni-0.7-% Mn; and hot rolling the ingot
or slab at a temperature between 1150 and 1280.degree. C. in order
to obtain plate, strip or coil.
2. The process for manufacturing hot-rolled plate made of steel
according to claim 1, in which: hot-rolling the ingot or slab at a
temperature between 1150 and 1280.degree. C. to obtain a quarto
plate; performing a heat treatment at a temperature between 900 and
1100.degree. C.; and cooling the plate by air quench.
3. The process for manufacturing hot-rolled bar or rod made of
steel comprising the steps of: providing a continuously cast ingot
or bloom of steel of composition comprising in % by weight:
C.ltoreq.0.05% 21%.ltoreq.Cr.ltoreq.25% 1%.ltoreq.Ni.ltoreq.2.95%
0.16%.ltoreq.N.ltoreq.0.28% Mn.ltoreq.2.0% Mo+W/2.ltoreq.0.50%
Mo.ltoreq.0.45% W.ltoreq.0.15% Si.ltoreq.1.4% Al.ltoreq.0.05%
0.11%.ltoreq.Cu.ltoreq.0.50% S.ltoreq.0.010% P.ltoreq.0.040%
Co.ltoreq.0.5% REM.ltoreq.0.1% V.ltoreq.0.5% Ti.ltoreq.0.1%
Nb.ltoreq.0.3% Mg.ltoreq.0.1% a balance being iron and impurities
resulting from the smelting; hot-rolling the ingot or bloom from a
temperature between 1150 and 1280.degree. C. in order to obtain a
bar, which is air-cooled, or a coil of wire stock which is
water-cooled; and then, optionally: performing a heat treatment at
a temperature between 900 and 1100.degree. C.; and quench cooling
the bar or coil of wire stock.
4. The process for manufacturing according to claim 3, further
comprising the step of: performing a cold-drawing operation carried
out on the bar or a die-drawing operation on the rod, after being
cooled.
5. The process for manufacturing a steel section according to claim
3, further comprising the step of: performing a cold-forming
operation on the hot-rolled bar.
6. The process for manufacturing a steel forging comprising the
steps of: providing a hot-rolled bar of claim 3; cutting the
hot-rolled bar into slugs; and performing a forging operation on
the slugs between 1100.degree. C. and 1280.degree. C.
7. The process for manufacturing according to claim 1, wherein
30.5.ltoreq.I.sub.LCR.ltoreq.38.6.
8. The process for manufacturing according to claim 3, wherein
40.ltoreq.I.sub.F.ltoreq.70--where I.sub.F=6.times.(%
Cr+1.32.times.% Mo+1.27.times.% Si)-10.times.(% Ni+24.times.%
C+16.15.times.% N+0.5.times.% Cu+0.4.times.% Mn)-6.17 and where
I.sub.LCR=% Cr+3.3.times.% Mo+16.times.% N+2.6.times.% Ni-0.7-%
Mn.
9. The process for manufacturing according to claim 8, wherein
30.5.ltoreq.I.sub.LCR.ltoreq.38.6.
10. A hot-rolled steel quarto plate, obtained by the process
according to claim 2 and having a thickness between 5 and 100
mm.
11. Use of hot-rolled coil obtained by the process according to
claim 1, for the manufacture of structural components for material
production or energy production installations.
12. Use according to claim 11, in which said material and energy
production installations operate between -100 and 300.degree. C.,
preferably between -50 and 300.degree. C.
13. A cold-rolled steel strip that can be obtained by cold-rolling
a hot-rolled coil obtained by the process according to claim 1.
14. A hot-rolled bar obtained by the process according to claim 3
and having a diameter of 18 mm to 250 mm or a hot-rolled rod that
can be obtained by the process according to claim 3 and having a
diameter of 4 to 30 mm.
15. A cold-drawn bar that can be obtained by the process according
to claim 4, having a diameter of 4 mm to 60 mm, or die-drawn rod or
wire that can be obtained by the process according to claim 3,
having a diameter of 0.010 mm to 20 mm.
16. A mechanical part such as pumps, valve shafts, motor and engine
shafts and couplings operating in corrosive media comprising: a bar
according to claim 14.
17. Use of a rod or wire according to claim 14 for the manufacture
of cold-formed assemblies, for the agri-foodstuff industry, for oil
and ore extraction, or for the manufacture of woven or knitted
metal fabrics for the filtration of chemicals, ores or
foodstuffs.
18. A steel section obtained by the process according to claim
5.
19. A steel forging obtained by the process according to claim
6.
20. Brackets or couplings comprising: a steel forging according to
claim 19.
Description
[0001] This is a Divisional of U.S. patent application Ser. No.
14/622,402, filed Feb. 13, 2015, which is a Continuation of U.S.
patent application Ser. No. 12/305,014, filed May 11, 2009 which is
a National Stage Application of International Patent Application
PCT/FR2007/000994, filed Jun. 15, 2007 and claims the benefit of
European Patent Application EP 06290991.6, filed Jun. 16, 2006, all
of which are hereby incorporated by reference herein.
[0002] The present invention relates to a duplex stainless steel
more particularly intended for the manufacture of structural
components for material production installations (chemical,
petrochemical, paper and offshore industries) or energy production
installations, without in any way being limited thereto, and also
to the process for manufacturing plate, strip, bar, rod, wire or
sections from this steel.
BACKGROUND
[0003] More generally, this steel may be used as a substitute for a
304L type stainless steel in many applications, for example in the
above industries or in the agri-foodstuff industry, including parts
produced from formed rod or wire (welded grids, etc.), sections
(strainers, etc.), shafts, etc. It is also possible to produce
castings and forgings.
[0004] For this purpose, stainless steel grades of 304 type and
304L type are known which, in the annealed state, have an
essentially austenitic microstructure. In the cold-worked state
they may furthermore contain a variable proportion of martensite.
However, these steels include large additions of nickel, the cost
of which is generally prohibitive. Furthermore, these grades may
pose problems from a technical standpoint in certain applications,
as they have poor tensile properties in the annealed state,
especially as regards the yield strength, and a rather low stress
corrosion resistance.
[0005] Austenitic-ferritic stainless steels are also known, which
are composed mainly of a mixture of ferrite and austenite, such as
the steels 1.4362, 1.4655, 1.4477, 1.4462, 1.4507, 1.4410, 1.4501
and 1.4424 according to the EP10088 standard, which all contain
more than 3.5% nickel. These steels are particularly resistant to
corrosion and to stress corrosion.
[0006] Ferritic or ferritic-martensitic stainless steel grades are
also known, the microstructure of which, for a defined range of
heat treatments, is composed of two constituents--ferrite and
martensite--preferably in a 50/50 ratio, such as the 1.4017 grade
according to the EN10088 standard. These grades, with a chromium
content generally less than 20%, have high tensile mechanical
properties but do not have satisfactory corrosion resistance.
[0007] Moreover, it is also desirable to simplify the process for
manufacturing steel plate, strip, bar, rod, wire or sections.
BRIEF SUMMARY
[0008] The object of the present invention is to remedy the
drawbacks of the steels and manufacturing processes of the prior
art by providing a stainless steel exhibiting good mechanical
properties and in particular a tensile yield strength greater than
400 or even 450 MPa in the annealed or solution-treated state, a
high corrosion resistance, in particular the same or better than
that of 304L, good microstructural stability and good toughness of
welded zones, without adding expensive addition elements, and also
providing a process for manufacturing plate, strip, bar, rod, wire
or sections from this steel which is simpler to implement.
[0009] For this purpose, the first subject of the invention is a
duplex stainless steel, the composition of which consists of, in %
by weight: [0010] C.ltoreq.0.05% [0011] 21% .ltoreq.Cr.ltoreq.25%
[0012] 1% .ltoreq.Ni.ltoreq.2.95% [0013]
0.16%.ltoreq.N.ltoreq.0.28% [0014] Mn.ltoreq.2.0% [0015]
Mo+W/2.ltoreq.0.50% [0016] Mo.ltoreq.0.45% [0017] W.ltoreq.0.15%
[0018] Si.ltoreq.1.4% [0019] Al.ltoreq.0.05% [0020]
0.11%.ltoreq.Cu.ltoreq.0.50% [0021] S.ltoreq.0.010% [0022]
P.ltoreq.0.040% [0023] Co.ltoreq.0.5% [0024] REM.ltoreq.0.1% [0025]
V.ltoreq.0.5% [0026] Ti.ltoreq.0.1% [0027] Nb.ltoreq.0.3% [0028]
Mg.ltoreq.0.1% the balance being iron and impurities resulting from
the smelting, and the microstructure consisting of austenite and 35
to 65% ferrite by volume, the composition furthermore satisfying
the following relationships:
[0028] 40.ltoreq.I.sub.F.ltoreq.70, preferably
40.ltoreq.I.sub.F.ltoreq.60
where
I.sub.F=6.times.(% Cr+1.32.times.% Mo+1.27.times.% Si)-10.times.(%
Ni+24.times.% C+16.15.times.% N+0.5.times.% Cu+0.4.times.%
Mn)-6.17
[0029] and
I.sub.LCR.gtoreq.30.5, preferably.gtoreq.32
where
I.sub.LCR=% Cr+3.3.times.% Mo+16.times.% N+2.6.times.%
Ni-0.7.times.% Mn.
[0030] The steel according to the invention may also include the
following optional features, taken individually or in
combination:
[0031] the proportion of ferrite is between 35 and 55% by
volume;
[0032] the chromium content is between 22 and 24% by weight;
[0033] the manganese content is less than 1.5% by weight;
[0034] the calcium content is less than 0.03% by weight; and
[0035] the molybdenum content is greater than 0.1% by weight.
[0036] A second subject of the invention consists of a process for
hot-rolled plate, strip or coil made of steel according to the
invention, in which: [0037] an ingot or slab of a steel of
composition according to the invention is provided; and [0038] said
ingot or slab is hot-rolled at a temperature between 1150 and
1280.degree. C. in order to obtain plate, strip or coil.
[0039] In one particular method of implementation, said ingot or
slab is hot-rolled at a temperature between 1150 and 1280.degree.
C. in order to obtain what is called quarto plate; a heat treatment
is then carried out at a temperature between 900 and 1100.degree.
C.; and said plate is cooled by an air quench.
[0040] A third subject of the invention consists of a process for
manufacturing hot-rolled bar or rod made of steel according to the
invention, in which: [0041] a continuously cast ingot or bloom of
steel of composition the invention is provided; [0042] said ingot
or bloom is hot-rolled from a temperature between 1150 and
1280.degree. C. in order to obtain bar, which is air-cooled, or a
coil of wire stock which is water-cooled; and then, optionally:
[0043] a heat treatment is carried out at a temperature between 900
and 1100.degree. C.; and [0044] said bar or a coil of wire stock is
quench-cooled.
[0045] In one particular method of implementation, a cold-drawing
operation is carried out on said bar or a die-drawing operation is
carried out on said rod, after being cooled.
[0046] The invention also covers a process for manufacturing a
steel section, in which a cold-forming operation is carried out on
a hot-rolled bar obtained according to the invention, and also a
process for manufacturing a steel forging, in which a hot-rolled
bar obtained according to the invention is cut up into slugs and
then a forging operation is carried out on said slugs between
1100.degree. C. and 1280.degree. C.
[0047] The invention furthermore covers various products that can
be obtained by the processes according to the invention and also
their uses, such as: [0048] hot-rolled steel quarto plate, having a
thickness between 5 and 100 mm, and strip and coil, which may be
used for the manufacture of structural components for material
production or energy production installations, in particular for
material and energy production installations operating between -100
and 300.degree. C., preferably between -50 and 300.degree. C.;
[0049] cold-rolled steel strip that can be obtained by cold-rolling
a hot-rolled coil; [0050] hot-rolled bar having a diameter of 18 mm
to 250 mm and cold-drawn bar having a diameter of 4 mm to 60 mm,
which products may be used for the manufacture of mechanical parts
such as pumps, valve shafts, motor and engine shafts and couplings
operating in corrosive media; [0051] hot-rolled rod having a
diameter of 4 to 30 mm and die-drawn rod or wire having a diameter
of 0.010 mm to 20 mm, which products can be used for the
manufacture of cold-formed assemblies, for the agri-foodstuff
industry, for oil and ore extraction, or for the manufacture of
woven or knitted metal fabrics for the filtration of chemicals,
ores or foodstuffs; [0052] sections; [0053] forgings that can be
used for the manufacture of brackets or couplings; and [0054]
castings that can be obtained by casting a steel according to the
invention.
[0055] Other features and advantages of the invention will become
apparent on reading the following description, given solely by way
of example.
[0056] The duplex stainless steel according to the invention
comprises the contents defined below.
[0057] The carbon content of the grade is equal to or less than
0.05%, preferably less than 0.03%, by weight. This is because too
high a content of this element degrades the localized corrosion
resistance by increasing the risk of chromium carbides
precipitating in the heat-affected zones of welds.
[0058] The chromium content of the grade is between 21 and 25% by
weight, preferably between 22 and 24% by weight, so as to obtain
good corrosion resistance, which is at least equivalent to that
obtained with type 304 or 304L grades.
[0059] The nickel content of the grade is between 1 and 2.95%,
preferably equal to or less than 2.7%, or even 2.5%, by weight.
This austenite-forming element is added so as to obtain good
crevice corrosion resistance. At contents of greater than 1% and
preferably greater than 1.2% by weight, it has a favourable effect
in combating the initiation of pitting corrosions. However, its
content is limited since above 2.95% by weight a degradation in the
resistance to pitting propagation is observed. Addition of nickel
also makes it possible to obtain a good toughness/ductility
compromise since it has the benefit of translating the toughness
transition curve towards low temperatures, this being particularly
advantageous for the manufacture of thick quarto plate, for which
the toughness properties are important.
[0060] Since the nickel content in the steel according to the
invention is limited, it has been found to be necessary, in order
to obtain an appropriate austenite content after heat treatment
between 900.degree. C. and 1100.degree. C., to add other
austenite-forming elements in unusually high amounts and to limit
the contents of ferrite-forming elements.
[0061] The nitrogen content of the grade is between 0.16 and 0.28%
by weight, which generally means that the nitrogen is added to the
steel during smelting. This austenite-forming element firstly makes
it possible to obtain ferrite-austenite two-phase duplex steel
containing an appropriate proportion of austenite exhibiting good
stress corrosion resistance, and also to obtain metal with high
mechanical properties. It also makes it possible to have good
microstructural stability in the heat-affected zones of welds. Its
maximum content is limited since, above 0.28%, a solubility problem
may be observed: formation of blisters during solidification of
slabs, blooms, ingots, castings or welds.
[0062] The content of manganese, which is also an austenite-forming
element below 1150.degree. C., is kept below 2.0% by weight, and
preferably below 1.5% by weight, because of the deleterious effects
of this element on many counts. Thus, it poses problems during
smelting and refining of the grade since it attacks certain
refractories used for the ladles. This requires these expensive
elements to be replaced more frequently, and therefore causes the
process to be interrupted more frequently.
[0063] Additions of ferro-manganese normally used to bring the
grade to composition also contain appreciable amounts of phosphorus
and also selenium, which elements it is not desirable to introduce
into the steel and are difficult to remove when refining the grade.
Manganese also disturbs this refining, by limiting the possibility
of decarburation. It also poses a problem further downstream in the
process, as it impairs the corrosion resistance of the grade
because of the formation of manganese sulphides (MnS) and of
oxidized inclusions. This element is conventionally added to grades
that it is desired to enrich with nitrogen, so as to increase the
solubility of that element in the grade. Without a sufficient
manganese content, it was therefore not possible to achieve such a
nitrogen level in the steel. However, the inventors have found that
it is possible to limit the addition of manganese in the steel
according to the invention, while still adding sufficient nitrogen
in order to obtain the desired effect on the ferrite-austenite
balance of the base metal and to stabilize the heat-affected zones
of welds.
[0064] Molybdenum, a ferrite-forming element, is maintained with a
content of less than 0.45% by weight, while tungsten is maintained
with a content of less than 0.15% by weight. Moreover, the contents
of these two elements are such that the sum Mo+W/2 is less than
0.50% by weight, preferably less than 0.4% by weight and even more
preferably less than 0.3% by weight. This is because the present
inventors have found that, by maintaining these two elements, and
with the sum of their contents below the values indicated, no
precipitation of embrittling intermetallics is observed, thereby
making it possible in particular to relax the process for
manufacturing steel plate or strip by permitting said plate or
strip to be air-cooled after heat treatment or hot processing.
Furthermore, they have observed that, by controlling these elements
within the claimed limits, the weldability of the grade is
improved. However, it is preferred to maintain a minimum molybdenum
content of 0.1% so as to improve the hot forgeability of the grade.
Furthermore, smelting a grade having less than 0.1% molybdenum
would imply greatly limiting the use of recycling scrap for this
grade, thereby posing processing problems, in particular requiring
a charge consisting 100% of pure ferro-alloys to be used.
[0065] Copper, an austenite-forming element, is present in an
amount of between 0.11 and 0.50% by weight, and preferably between
0.15 and 0.40% by weight. This element improves the corrosion
resistance in a reducing acid medium. However, its content is
limited to 0.50% by weight in order to prevent the formation of
epsilon-phases, which it is desirable to avoid, since these result
in hardening of the ferritic phase and embrittlement of the duplex
alloy.
[0066] The oxygen content is preferably limited to 0.010% by
weight, so as to improve its forgeability.
[0067] Boron is an optional element that may be added to the grade
according to the invention in an amount of between 0.0005% and
0.01% by weight, preferably between 0.0005% and 0.005% and even
more preferably between 0.0005% and 0.003% by weight, so as to
improve its hot conversion. However, in another embodiment, it is
preferred to limit the boron content to less than 0.0005% by weight
so as to limit the risk of cracking during welding and continuous
casting.
[0068] Silicon, a ferrite-forming element, is present with a
content of less than 1.4% by weight. Aluminium, a ferrite-forming
element, is present with a content of less than 0.05% by weight and
preferably between 0.005% and 0.040% by weight, so as to obtain
low-melting-point calcium aluminate inclusions. The maximum
aluminium content is also limited, so as to avoid excessive
formation of aluminium nitrides. The action of these two
elements--silicon and aluminium--is essentially to ensure correct
deoxidation of the steel bath during smelting.
[0069] Cobalt, an austenite-forming element, is maintained with a
content of less than 0.5% by weight, preferably less than 0.3% by
weight. This element is a residual element provided by the raw
materials. In particular, it is limited because of handling
problems that it may pose after irradiation of components in
nuclear power plants.
[0070] Rare earth metals (denoted by REM) may be added to the
composition in an amount of 0.1% by weight, preferably less than
0.06% by weight. Mention may in particular be made of cerium and
lanthanum. The contents of these two elements are limited, as they
are liable to form undesirable intermetallics.
[0071] Vanadium, a ferrite-forming element, may be added to the
grade in an amount of 0.5% by weight and preferably less than 0.2%,
so as to improve the crevice corrosion resistance of the steel.
[0072] Niobium, a ferrite-forming element, may be added to the
grade in an amount of 0.3% by weight and preferably less than
0.050% by weight. It helps to improve the tensile strength of the
grade, thanks to the formation of fine niobium nitrides. Its
content is limited, so as to limit the formation of coarse niobium
nitrides.
[0073] Titanium, a ferrite-forming element, may be added to the
grade in an amount of 0.1% by weight and preferably less than 0.02%
by weight, in order to limit the formation of titanium nitrides
formed in particular in the liquid steel.
[0074] It is also possible to add calcium to the grade according to
the invention, in order to obtain a calcium content of less than
0.03% by weight and preferably greater than 0.0002% or even greater
than 0.0005% by weight, so as to control the nature of the oxide
inclusions and improve machinability. The content of this element
is limited, as it is liable to form, with sulphur, calcium
sulphides which degrade the corrosion resistance properties. In a
preferred embodiment, the calcium content is limited to less than
0.0005%, preferably less than 0.0002%, by weight.
[0075] Sulphur is maintained with a content of less than 0.010% by
weight and preferably with a content of less than 0.003% by weight.
As mentioned above, this element forms sulphides with manganese or
calcium, the presence of sulphides being deleterious to the
corrosion resistance. Sulphur is considered as an impurity.
[0076] An addition of magnesium in an amount with a final content
of 0.1% may be made so as to modify the nature of the sulphides and
the oxides.
[0077] Selenium is preferably maintained at less than 0.005% by
weight because of its deleterious effect on the corrosion
resistance. This element is in general introduced into the grade as
impurities of ferrite and manganese ingots.
[0078] Phosphorus is maintained with a content of less than 0.040%
by weight, and is considered as an impurity.
[0079] The balance of the composition consists of iron and
impurities. Other than those already mentioned above, these
impurities may also be zirconium, tin, arsenic, lead or bismuth.
Tin may be present with a content of less than 0.100% by weight and
preferably less than 0.030% by weight in order to prevent welding
problems. Arsenic may be present with a content of less than 0.030%
by weight and preferably less than 0.020% by weight. Lead may be
present with a content of less than 0.002% by weight and preferably
less than 0.0010% by weight. Bismuth may be present with a content
of less than 0.0002% by weight and preferably less than 0.00005% by
weight. Zirconium may be present in an amount of 0.02%.
[0080] Moreover, the present inventors have found that, when the
percentages by weight of chromium, molybdenum, nitrogen, nickel and
manganese satisfy the relationship below, the grades in question
exhibit good localized corrosion resistance, that is to say
resistance to the formation of pits or crevices:
I.sub.LCR=% Cr+3.3.times.% Mo+16.times.% N+2.6.times.%
Ni-0.7.times.% Mn.gtoreq.30.5.
[0081] The microstructure of the steel according to the invention,
in the annealed state, is composed of austenite and ferrite, which
phases are preferably, after treatment for 1 h at 1000.degree. C.,
present with a ferrite proportion of 35 to 65% by volume and more
particularly preferably 35 to 55% by volume.
[0082] The present inventors have also found that the following
formula suitably describes the ferrite content at 1100.degree.
C.:
I.sub.F=6.times.(% Cr+1.32.times.% Mo+1.27.times.% Si)-10.times.(%
Ni+24.times.% C+16.15.times.% N+0.5.times.% Cu+0.4.times.%
Mn)-6.17.
[0083] Thus, to obtain a ferrite proportion between 35 and 65% at
1100.degree. C., the ferrite index I.sub.F must be between 40 and
70.
[0084] In the annealed state, the microstructure does not contain
other phases that would be deleterious to its mechanical
properties, especially such as the sigma-phase and other
intermetallic phases. In the cold-worked state, part of the
austenite may have been converted to martensite, depending on the
actual temperature of deformation and the amount of cold
deformation applied.
[0085] In general, the steel according to the invention may be
smelted and manufactured in the form of hot-rolled plate, again
called quarto plate, but also in the form of hot-rolled strip, from
slabs or ingots, and also in the form of cold-rolled strip from
hot-rolled strip. It may also be hot-rolled in the form of bar or
rod stock or sections or forgings. These products may then be
hot-converted by forging or cold-converted into bar or drawn
sections or die-drawn wire. The steel according to the invention
may also be processed by casting, whether followed by heat
treatment or not.
[0086] To obtain the best possible performance characteristic, it
will be preferential to use the process according to the invention
that firstly includes the provision of a steel ingot, slab or bloom
having a composition according to the invention.
[0087] This ingot, slab or bloom is generally obtained by melting
the raw materials in an electric furnace followed by vacuum
remelting of the AOD or VOD type with decarburization. The grade
can then be cast in the form of ingots, or in the form of slabs or
blooms by continuous casting in a bottomless mould. It is also
conceivable to cast the grade directly in the form of thin slabs,
in particular by continuous casting between counter-rotating
rolls.
[0088] After having provided the ingot, slab or bloom, this may
optionally be reheated so as to reach a temperature between 1150
and 1280.degree. C., but it is also possible to work directly on
the slab immediately on being continuously cast, while still
hot.
[0089] In the case of plate manufacture, the slab or ingot is then
hot-rolled in order to obtain a quarto plate generally having a
thickness between 5 and 100 mm. The reduction ratios generally
employed at this stage vary between 3 and 30%. This plate then
undergoes a heat treatment to put the precipitates, formed at this
stage, back into solution by reheating to a temperature between 900
and 1100.degree. C., and is then cooled.
[0090] The process according to the invention provides for
air-quench cooling, which is easier to implement than the
conventionally used cooling for this type of grade, which is more
rapid cooling, using water. However, it remains possible to carry
out a water-cooling operation, if so desired.
[0091] This slow air cooling is particular made possible thanks to
the limited nickel and molybdenum content of the composition
according to the invention, which is not subject to the
precipitation of intermetallic phases detrimental to its usage
properties. This cooling may in particular be carried out at a rate
ranging from 0.1 to 2.7.degree. C./s.
[0092] After being hot-rolled, the quarto plate may be levelled,
cut and pickled if it is desired to deliver it in this state.
[0093] It is also possible to roll this bare steel on a strip
rolling mill down to thicknesses of between 3 and 10 mm.
[0094] In the case of manufacturing long products from ingots or
blooms, it is possible to hot-roll in a single hot pass on a
multi-stand rolling mill, between fluted rolls, at a temperature
between 1150 and 1280.degree. C. in order to obtain a bar or coil
of wire stock, or rolled. The cross-section ratio between the
initial bloom and the final product is preferably greater than 3,
so as to ensure the internal soundness of the rolled product.
[0095] When a bar has been manufactured, this is cooled on leaving
the rolling mill, by simple air spreading.
[0096] When rolled rod with a diameter of greater than 13 mm is
manufactured, this may be cooled, on leaving the rolling mill, by
quenching it in coil form in a water tank.
[0097] When rod with a diameter of 13 mm or less is manufactured,
it may be cooled by a water quench in the form of turns spread out
on a conveyor after said turns have passed along a conveyor for 2
to 5 minutes through a solution treatment furnace at a temperature
between 850.degree. C. and 1100.degree. C.
[0098] A subsequent furnace heat treatment between 900.degree. C.
and 1100.degree. C. may optionally be carried out on this bar or
coil, already treated after the hot-rolling, if it is desired to
complete the recrystallization of the structure and slightly reduce
the tensile properties.
[0099] After this bar or coil of rod has cooled, various
hot-forming or cold-forming treatments may be carried out,
depending on the final usage of the product. The bar may undergo a
cold-drawing operation or the rod may undergo a die-drawing
operation, after being cooled.
[0100] The hot-rolled bar may also be cold-formed or parts may be
manufactured after the bar has been cut up into slugs and then
forged.
BRIEF DESCRIPTION OF THE DRAWINGS
[0101] To illustrate the invention, trials were made and these will
be described in particular with reference to FIGS. 1 to 5 which
show:
[0102] FIG. 1: A correlation between % ferrite after treatment at
1100.degree. C. and I.sub.F for as-processed products;
[0103] FIG. 2: A relative diametral change .DELTA.O as a function
of the deformation temperature;
[0104] FIG. 3: Pitting potentials E1 and E2, determined on forged
bars, as a function of the index I.sub.LCR;
[0105] FIG. 4: The uniform corrosion rate V, determined on forged
bars, as a function of the index I.sub.LCR; and
[0106] FIG. 5: Critical temperatures T.sub.CC and T.sub.CP,
determined on forged bars, as a function of the index
I.sub.LCR.
DETAILED DESCRIPTION
Examples
[0107] 25 kg laboratory ingots were produced by vacuum induction
melting pure ferro-alloy raw materials, followed by introducing
nitrogen by addition of ferro-alloys nitrided under a nitrogen
partial pressure and cast into a metal mould under an external
nitrogen pressure of 0.8 bar. Among these, only trials 14441 and
14604 were according to the invention.
[0108] An industrial heat according to the invention of 150 tonnes
referenced 8768 was produced. This grade was smelted by melting in
an electric furnace then vacuum-refined with decarburization in
order to achieve the intended carbon level. It was then
continuously cast into slabs measuring 220.times.1700 mm in cross
section before being hot-rolled, after reheating to 1200.degree.
C., into quarto plates with thicknesses of 7, 12 and 20 mm. The
plates thus obtained then underwent a heat treatment at around
1000.degree. so as to put the various precipitates present at this
stage back into solution. After the heat treatment, the plates were
water-cooled, then levelled, cut and pickled.
[0109] The compositions in percentages by weight of the various
grades smelted on a laboratory scale or an industrial scale are
given in Table 1, together with the compositions of the various
industrial products or semi-finished products smelted in an
electric furnace, followed by AOD refining and cast into ingots or
continuously cast, these being mentioned for comparison.
TABLE-US-00001 TABLE 1 Heat No. 14441 14604 8768 14382 14383 14439
14426 14422 14425 14424 14660 Product 25 kg 25 kg 150 t 25 kg 25 kg
25 kg 25 kg 25 kg 25 kg 25 kg 25 kg Al 0.014 0.012 0.0042 0.010
0.015 0.014 <0.002 <0.002 0.024 C 0.016 0.028 0.020 0.020
0.020 0.017 0.021 0.022 0.019 0.020 0.024 Cr 23.07 22.80 22.83
23.03 23.01 23.05 26.67 26.56 26.68 26.61 22.79 Cu 0.301 0.300 0.15
0.304 0.297 0.299 0.279 0.280 0.280 0.208 0.284 Mn 1.282 1.284 1.25
1.288 1.277 1.309 0.724 0.706 0.723 0.705 4.780 Mo 0.249 0.249 0.35
0.251 0.250 0.251 1.322 1.337 1.327 1.328 0.296 N 0.212 0.239 0.21
0.110 0.110 0.290 0.119 0.117 0.300 0.237 0.199 Ni 2.539 1.692 2.50
4.249 1.552 1.485 4.532 1.419 1.541 2.549 2.470 O 0.0049 0.0038
0.0042 0.0031 0.0039 0.0052 0.0316 0.0284 0.0205 0.0221 0.0033 P
0.023 0.023 0.024 0.024 0.024 0.022 0.025 0.022 0.025 0.022 0.025 S
0.0009 0.0010 0.0005 0.0008 0.0008 0.0009 0.0209 0.0203 0.0210
0.0203 0.0014 Si 0.430 0.358 0.44 0.399 0.455 0.403 0.424 0.391
0.407 0.408 0.494 V 0.121 0.061 0.064 0.123 0.122 0.120 0.106 0.102
0.109 0.107 0.013 W <0.010 <0.010 0.019 <0.010 <0.010
<0.010 <0.010 <0.010 <0.010 <0.010 <0.010 Ti
0.0048 0.0017 0.007 0.0027 0.0039 0.0027 0.0041 0.0059 0.0047
0.0050 0.0011 Zr 0.0048 0.0052 0.0042 0.0049 0.0055 0.0064 0.0055
0.0060 0.0058 0.0072 0.0083 Co <0.002 <0.002 0.041 <0.002
<0.002 <0.002 <0.002 <0.002 <0.002 <0.002
<0.002 Ca <0.0005 <0.0005 0.0003 <0.0005 <0.0005
<0.005 <0.0005 <0.005 <0.0005 <0.0005 <0.0002 Nb
<0.002 <0.002 0.0009 <0.002 <0.002 <0.002 <0.002
<0.002 <0.002 <0.002 <0.002 Se <0.002 <0.002
<0.002 <0.002 <0.002 <0.002 <0.002 <0.002
<0.002 <0.002 <0.002 As <0.002 <0.002 <0.002
<0.002 <0.002 <0.002 <0.002 <0.002 <0.002
<0.002 <0.002 Ce + La <0.0002 <0.0002 <0.0002
<0.0002 <0.0002 <0.0002 <0.0002 <0.0002 <0.0002
<0.0002 <0.0002 Mg <0.0005 <0.0005 0.0004 <0.0005
<0.0005 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005
<0.0005 B <0.0005 <0.0005 0.0024 <0.0005 <0.0005
<0.0005 <0.0005 <0.0005 <0.0005 <0.0005 <0.0005
Heat No. 304 304L 316L UNS32101 UNS32304 140301 436002 517077
533054 150091 Product * * * * * * * * * * Al 0.026 0.006 C 0.020
0.018 0.022 0.018 0.015 0.021 0.026 0.029 0.011 Cr 18.23 18.4 16.5
21.6 22.9 23.01 22.30 22.14 22.32 23.02 Cu 0.15 0.11 0.31 0.24
0.163 0.303 0.260 0.284 0.083 Mn 0.79 1.20 1.66 5.2 1.26 1.563
1.097 1.082 1.054 1.584 Mo 0.37 0.16 2.08 0.3 0.24 2.802 0.277
0.285 0.275 3.118 N 0.044 0.074 0.067 0.224 0.12 0.164 0.143 0.119
0.106 0.150 Ni 8.96 10.2 10.24 1.5 4.20 5.500 4.022 3.995 4.364
8.672 O 0.0037 P 0.023 0.020 0.019 0.027 0.028 0.022 0.022 0.023
0.019 S 0.0013 0.0011 0.0004 0.0008 0.0006 0.0004 0.0004 0.0006
0.0009 Si 0.37 0.50 0.71 0.40 0.206 0.414 0.464 0.400 0.390 V 0.103
0.114 0.058 0.126 W 0.028 0.017 0.013 0.022 Ti 0.0065 0.0040 0.0030
0.0033 Co 0.063 0.129 0.056 0.035 Zr Ca 0.0007 0.0026 0.0028 0.0007
Nb 0.0046 0.009 0.012 0.0063 Se <0.0020 <0.0020 <0.0020
<0.0020 <0.0020 Ce + La Mg 0.0014 <0.0005 <0.0005
<0.0005 <0.0005 B 0.0008 <0.0005 <0.0005 0.0022
<0.0005 *: Rolled plate or billet or bar.
[0110] 1. Ferrite Contents
[0111] 1.1 Ferrite Contents on As-Processed Products
[0112] Specimens ranging from 1 to 8 cm.sup.3 in volume were cut
from these laboratory heats in the as-cast state or from industrial
products in the as-cast state, and heat treatments for 30 minutes
at various temperatures were carried out on these specimens, in a
salt bath, followed by an end-of-treatment water quench, in order
to determine the proportion of ferrite at high temperature. Since
ferrite is magnetic, unlike austenite, carbides and nitrides
possibly present, an assaying method was used in which the
saturation magnetization was measured. The ferrite contents thus
determined are given in Table 2 and plotted in FIG. 1.
[0113] FIG. 1 shows that there is a good correlation between the
index I.sub.F and the ferrite contents measured on the base metal
after treatments at 1100.degree. C.
[0114] It has also been shown that heat 14441 according to the
invention has, below 1300.degree. C., a ferrite content appropriate
to hot transformation to a duplex structure. Furthermore, after
heat treatment in the 950.degree. C. to 1100.degree. C. range, it
has a ferrite content appropriate for stress corrosion
resistance.
TABLE-US-00002 TABLE 2 Heat 14382 14383 14441 14426 14422 14425
14424 140301 436002 517077 533054 150091 Product Ingot Ingot Ingot
Ingot Ingot Ingot Ingot CCB CCB CCB CCB CCB As- 55.6 50.5 52.6 50.3
25.4 processed state +900.degree. C. 45.6 89.5 54.4 71.2 98.7 100
91.9 45 51.0 47.2 20.5 +950.degree. C. 48.7 87.1 51.7 71.1 98.8
99.6 94.6 42.8 48.9 46.1 25.4 +1000.degree. C. 50.9 90.0 54.5 71.8
99.4 99.4 93.4 50.8 42.1 50.7 46.0 28.8 +1050.degree. C. 55.7 81.0
53.0 77.8 98.6 99.1 78.8 54 44.2 54.6 48.3 33.7 +1100.degree. C.
60.8 84.6 55.5 82.0 99.0 87.4 75.4 58.6 47.6 59.4 51.3 36.1
+1150.degree. C. 65.2 88.6 59.0 88.1 98.9 75.6 78.1 64.6 52.7 66.7
57.9 41.1 +1200.degree. C. 76.6 94.2 64.0 95.4 98.8 78.4 71.6 59.3
75.5 64.8 46.7 +1250.degree. C. 92.3 98.1 67.7 100 99.2 81.0 86.2
81.5 67.4 86.0 73.2 55.1 +1300.degree. C. 95.2 97.7 72.6 99.4 98.7
85.9 93.5 100 78.3 99.0 85.0 66.4 CCB = continuously cast
bloom.
[0115] 1.2. Ferrite Contents On End-Products
[0116] The ferrite content was also measured by the grid method
(according to ASTM E 562 standard) on forged bars after heat
treatment at 1030.degree. C. and in heat-affected zones of weld
beads deposited by a coated electrode with a constant energy
resulting in cooling rates at 700.degree. C. of 20.degree. C./s.
The results (ferrite contents of the base metal and of the
heat-affected zone) are given in Table 3. This shows that heats
14441 and 14604 according to the invention have a ferrite content
in the base metal and in the heat-affected zone that is favourable
to localized corrosion resistance and to stress corrosion
resistance, and also favourable to toughness (cf. Table 5).
TABLE-US-00003 TABLE 3 Ferrite contents .alpha..sub.BM
.alpha..sub.HAZ Reference Product (%) (%) 14441* forged rod 48 70
14604* forged rod 54 65 14382 forged rod 49 80 14383 forged rod 79
88 14660 forged rod 48 72 UNS S32101 HR plate 45 67 UNS S32304 HR
plate 47 75 *according to the invention; HR: hot-rolled;
.alpha..sub.BM (%): ferrite content measured on the base metal;
.alpha..sub.HAZ: ferrite content measured in the heat-affected
zone.
[0117] 2. Castability
[0118] Ingot 14439 exhibited blisters and was unusable. To avoid
this phenomenon during casting in air at atmospheric pressure, it
proved necessary to limit the nitrogen content of the heats
according to the invention to less than 0.28% by weight.
[0119] 3. Hot-Transformation Capability
[0120] The hot-deformability was evaluated using hot tensile tests
carried out on test specimens, the calibrated part of which, having
a diameter of 8 mm and a length of 5 mm, was heated by Joule
heating for 80 seconds at 1280.degree. C. and then cooled at
2.degree. C. per second down to the test temperature, which varied
between 900 and 1280.degree. C. When this temperature was reached,
the rapid tensile test was immediately started, at the rate of 73
mm/s; after fracture, the necking diameter at the break was
measured.
[0121] The relative diametral change (Table 4), as defined below,
reflects the hot-deformability:
[0122] .DELTA.O=100.times.(1-(final diameter/initial
diameter)).
TABLE-US-00004 TABLE 4 Relative diametral changes (hot tensile
test) Test temperature .DELTA.O(in %) (.degree. C.) Heat 14382 Heat
14383 Heat 14441* 1280 85.0 100.0 96.7 1250 98.3 86.7 1200 75.0
98.3 76.7 1150 70.0 95.0 61.7 1100 63.3 93.3 56.7 1050 51.7 75.0
44.2 1010 45.0 1000 65.0 40.0 980 36.7 960 58.3 950 35.8 900 35.0
51.7 36.7 *according to the invention.
[0123] On examining Table 4 and FIG. 2, which represents the data
in the form of curves, it may be seen that heat 14441 according to
the invention has a hot-deformability comparable to that of
comparative reference heat 14382.
[0124] 4. Mechanical Properties
[0125] The tensile properties R.sub.e0.2 and R.sub.m were
determined according to the NFEN 10002-1 standard. The toughness
K.sub.v was determined at various temperatures according to the NF
EN 10045 standard.
TABLE-US-00005 TABLE 5 Mechanical properties K.sub.V K.sub.V
R.sub.e0.2 R.sub.m 20.degree. C. -50.degree. C. Reference Product
(MPa) (MPa) (J) (J) 14441* forged bar 477 716 334 51 14604* forged
bar 477 691 288 18 14382 forged bar 436 664 >339 339 14383
forged bar 458 604 79 9 14660 forged bar 493 701 293 31 304L HR
plate 218 523 312 301 316L HR plate 232 537 307 298 UNS S32101 HR
plate 466 720 101 60 UNS S32304 HR plate 438 663 268 153 8768* HR
plate 519 743 *according to the invention; HR: hot-rolled;
R.sub.e0.2: at yield strength 0.2% strain; R.sub.m: tensile
strength.
[0126] The results of the laboratory heats 14441 and 14604 and of
the industrial heat 8768, all three according to the invention,
show that a yield strength of greater than 450 MPa, i.e. twice that
obtained for austenitic steels of AISI 304L type, may be
obtained.
[0127] The toughness values determined at 20.degree. C. for the
laboratory heats 14441 and 14604 and for the industrial heat 8768,
all three according to the invention, are all greater than 200 J,
this being satisfactory taking into account the yield strength
level of these grades. For heat 14383 not according to the
invention, having a low nitrogen content and a high ferrite content
in the annealed state, the toughness values at 20.degree. C. are
below 100 J. This confirms the need for a sufficient addition of
nitrogen in order to obtain a satisfactory toughness level.
[0128] 5. Corrosion Resistance
[0129] Corrosion resistance tests were carried out both on the
forged bars from laboratory heats and on coupons removed from
hot-rolled plates coming from the industrial heats.
[0130] 5.1 Localized Corrosion Resistance
[0131] The pitting corrosion resistance was evaluated by plotting
the current-potential curves and determining the pitting potential
for i=100 .mu.A/cm.sup.2. This parameter was measured in a neutral
medium (pH=6.4) with a high chloride concentration ([Cl.sup.-]=30
g/l) at 50.degree. C. (E.sub.1), representative of the brine
encountered in seawater desalination plants, and in a slightly acid
(pH=5.5) medium with a low chloride concentration ([Cl.sup.-]=250
ppm) at room temperature (E.sub.2) representative of drinking
water. The critical pitting temperature in a ferric chloride medium
(6% FeCl.sub.3) was also determined according to the ASTM G48-00
standard, method C.
[0132] In another series of trials, the pitting corrosion
resistance was determined in a deaerated neutral medium containing
0.86 mol/litre of NaCI, corresponding to 5% NaCl by weight, at
35.degree. C. A floating potential measurement for 900 seconds was
carried out. Next, a potentiodynamic curve was plotted at a rate of
100 mV/min of the floating potential up to the pitting potential.
The pitting potential (E.sub.3) was determined for i=100
.mu.A/cm.sup.2. Under these conditions, specimens according to the
invention and reference specimens of 304L grade and
austenitic-ferritic duplex grades of 1.4362 type and others were
tested.
[0133] The crevice corrosion resistance was studied by measuring
the critical crevice temperature in the neutral medium (pH=6.4)
with a high chloride concentration ([Cl.sup.-]=30 g/l). The
arrangement favouring floating crevice corrosion was in accordance
with the recommendations given in the ASTM G78-99 standard. The
critical crevice temperature is the minimum temperature for which
crevices with a depth of greater than 25 .mu.m are observed.
[0134] The values obtained are given in Table 6. Comparison between
the results obtained on the plate made of UNS S32304 and the bar
obtained from heat 14382, these two being of similar chemical
composition, indicates that the corrosion resistance of a bar is
lower than that of a hot-rolled plate of the same composition.
[0135] The present inventors have found that the localized
corrosion resistance index, that is to say resistance to the
formation of pits or crevices, abbreviated to I.sub.LCR and defined
by:
I.sub.LCR=Cr+3.3.times.Mo+16.times.N+2.6.times.Ni-0.7.times.Mn
[0136] (contents in Cr, Mo, N, Ni and Mn in % by weight) [0137]
accounts for the classification of all the compositions containing
less than 6% nickel as regards localized corrosion resistance (see
FIGS. 3, 4 and 5).
[0138] Heats 14383 and 14660 not according to the invention, having
I.sub.LCR indices equal to 28.7 and 29.8, exhibit an inferior
corrosion behaviour than a steel of AISI 304L type. Heats 14604 and
14441 according to the invention, having an I.sub.LCR of 30.9 and
33, behave at least as well as 304L type steel. To obtain a
corrosion resistance at least equal to that of AISI 304L grade, it
has been found that the steels according to the invention must
preferably have an I.sub.LCR of greater than 30.5 and preferably
greater than 32.
[0139] 5.2 Uniform Corrosion Resistance
[0140] Uniform corrosion was characterized by evaluating the
corrosion rate by loss of weight after 72 hours' immersion in a 2%
dilute sulphuric acid solution heated to 40.degree. C.
[0141] Comparing the corrosion rates for the experimental heats
containing 2.5% Ni and 0.2% N (14441 according to the invention and
14660 not according to the invention) clearly shows the negative
effect of a high Mn content on the uniform corrosion resistance in
a sulphuric medium.
TABLE-US-00006 TABLE 6 Localized and uniform corrosion resistance
data E.sub.1 E.sub.2 E.sub.3 T.sub.CP T.sub.CC V Ref. Product
I.sub.LCR (V/ECS) (V/ECS) (V/ECS) (.degree. C.) (.degree. C.)
(mm/y) 14441* forged rod 33.0 0.165 1.058 0.320 7.5 50 0.73 14604*
forged bar 30.9 0.159 0.802 5 45 1.8 14382 forged bar 35.8 0.302
1.323 0.420 15 60 0.24 14383 forged bar 28.7 0.049 0.595 0.050 0 35
4.95 14660 forged bar 29.8 0.094 0.707 7.5 45 1.11 304L HR plate NA
0.188 0.834 0.210 5 65 316L HR plate NA 0.266 0.865 7.5 75 UNS
S32101 HR plate 26.4 0.163 0.855 12.5 UNS S32304 HR plate 35.7
0.413 1.330.sup.1 17.5 95 517077 rolled bar 34.6 0.415 140301
rolled bar 47.1 1.200.sup.1 8768* HR plate 33.1 0.227 1.273.sup.1
*according to the invention; .sup.1odixation potential of the
solvent, no pitting observed; HR: hot-rolled; NA: not applicable;
E.sub.1: pitting potential in neutral medium (pH = 6.4) having a
high chloride concentration (30 g/l of Cl.sup.-) at 50.degree. C.;
E.sub.2: pitting potential in slightly acid environment (pH = 5.5)
having a low chloride concentration (250 ppm of Cl.sup.-) at
25.degree. C.; E.sub.3: pitting potential in neutral chloride
medium (5% NaCl) at 35.degree. C.; T.sub.CP: critical pitting
temperature in a ferric chloride medium; T.sub.CC: critical crevice
temperature in neutral medium (pH = 6.4) with a high chloride
concentration (30 g/l of Cl.sup.-) V: uniform corrosion rate in 2%
sulphuric acid medium at 40.degree. C.
[0142] 5.3 Repassivation Potential
[0143] The steel specimens were polished under water using SiC
paper up to 1200 and then aged for 24 hours in air.
[0144] The cyclic polarization test in a chloride medium was
carried out by starting with measurement of the floating potential
for 15 min, followed by cyclic dynamic polarization at 100 mV/min
starting from the floating potential up to the potential for which
the current reached an intensity of 300 .mu.A/cm.sup.2, followed by
return to the potential for which the current is zero.
[0145] Thus, the pitting potential (P.sub.pit) and the
repassivation potentials (P.sub.repassivation) of the previously
formed pits were determined. The results obtained are given in
Table 7.
TABLE-US-00007 TABLE 7 Repassivation as a function of the nickel
content Heat % Ni V.sub.pit-V.sub.repassivation (mV/ECS) 14382 4.5
460 14441 2.5 361 14383 1.5 227
[0146] From the repassivation potential tests in NaCl medium, the
higher the nickel content the greater the difference between the
pitting potential and the repassivation potential. This shows that
nickel is not beneficial to the repassivation of a grade according
to the invention that has previously undergone pitting
corrosion.
* * * * *