U.S. patent application number 15/214667 was filed with the patent office on 2017-03-30 for nickel-free austenitic stainless steel.
This patent application is currently assigned to The Swatch Group Research and Development Ltd. The applicant listed for this patent is The Swatch Group Research and Development Ltd. Invention is credited to Tommy CAROZZANI, Guido PLANKERT, Joel PORRET.
Application Number | 20170088923 15/214667 |
Document ID | / |
Family ID | 54238278 |
Filed Date | 2017-03-30 |
United States Patent
Application |
20170088923 |
Kind Code |
A1 |
PORRET; Joel ; et
al. |
March 30, 2017 |
NICKEL-FREE AUSTENITIC STAINLESS STEEL
Abstract
Nickel-free austenitic stainless steel comprising, in mass
percent: chromium in amounts of 10<Cr<21%; manganese in
amounts of 10<Mn<20%; molybdenum in amounts of
0<Mo<2.5%; copper in amounts of 0.5.ltoreq.Cu<4%; carbon
in amounts of 0.15<C<<%; nitrogen in amounts of
0<N.ltoreq.1, and the remainder being formed by iron and any
impurities from the melt.
Inventors: |
PORRET; Joel; (Neuchatel,
CH) ; PLANKERT; Guido; (Boppelsen, CH) ;
CAROZZANI; Tommy; (Neuchatel, CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Swatch Group Research and Development Ltd |
Marin |
|
CH |
|
|
Assignee: |
The Swatch Group Research and
Development Ltd
Marin
CH
|
Family ID: |
54238278 |
Appl. No.: |
15/214667 |
Filed: |
July 20, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 6/002 20130101;
A44C 27/003 20130101; C22C 38/06 20130101; C22C 38/22 20130101;
C22C 38/38 20130101; C22C 38/001 20130101; C22C 38/02 20130101;
C21D 6/005 20130101; C22C 38/04 20130101; C22C 38/20 20130101 |
International
Class: |
C22C 38/38 20060101
C22C038/38; A44C 27/00 20060101 A44C027/00; C22C 38/00 20060101
C22C038/00; C22C 38/22 20060101 C22C038/22; C22C 38/20 20060101
C22C038/20 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 25, 2015 |
EP |
15186980.7 |
Claims
1. A nickel-free austenitic stainless steel comprising, in mass
percent: chromium in amounts of 10<Cr<21%; manganese in
amounts of 10<Mn<20%; molybdenum in amounts of
0<Mo<2.5%; copper in amounts of 0.5.ltoreq.Cu<4% carbon in
amounts of 0.15<C<1%; nitrogen in amounts of 0<N.ltoreq.1,
and nickel in amounts of 0.ltoreq.Ni<0.5%, the nickel-free
austenitic stainless steel comprising, in mass percent, carbon in
amounts of 0.25<C<1% when the steel includes manganese in
amounts of 15.ltoreq.Mn<20%, the remainder being formed by iron
and any impurities from the melt.
2. The nickel-free austenitic stainless steel according to claim 1,
wherein the steel comprises in mass percent: chromium in amounts of
15<Cr<21%; manganese in amounts of 10<Mn<20%;
molybdenum in amounts of 0<Mo<2.5%; copper in amounts of
0.5.ltoreq.Cu<4%; carbon in amounts of 0.15%<C<1%;
nitrogen in amounts of 0<N.ltoreq.1; silicon in amounts of
0.ltoreq.Si<2%, nickel in amounts of 0.ltoreq.Ni<0.5%,
tungsten in amounts of 0.ltoreq.W<4%, aluminium in amounts of
0.ltoreq.Al<3%, and the remainder formed by iron and any
impurities from the melt.
3. The nickel-free austenitic stainless steel according to claim 1,
having a composition, expressed in mass percent, given by the
formula Fe-17Cr-11Mn-2Mo-1Cu-0.25C-0.4N.
4. The nickel-free austenitic stainless steel according to claim 2,
having a composition, expressed in mass percent, given by the
formula Fe-17Cr-11Mn-2Mo-1Cu-0.25C-0.4N.
5. The nickel-free austenitic stainless steel according to claim 1,
having a composition, expressed in mass percent, given by the
formula Fe-17Cr-12Mn-2Mo-2Cu-0.33C-0.4N.
6. The nickel-free austenitic stainless steel according to claim 2,
having a composition, expressed in mass percent, given by the
formula Fe-17Cr-12Mn-2Mo-2Cu-0.33C-0.4N.
7. The nickel-free austenitic stainless steel according to claim 1,
having a composition, expressed in mass percent, given by the
formula Fe-17Cr-14.5Mn-2Mo-2Cu-0.22C-0.35N.
8. The nickel-free austenitic stainless steel according to claim 2,
having a composition, expressed in mass percent, given by the
formula Fe-17Cr-14.5Mn-2Mo-2Cu-0.22C-0.35N.
9. The nickel-free austenitic stainless steel according to claim 1,
having a composition, expressed in mass percent, given by the
formula Fe-17Cr-17Mn-2Mo-1Cu-0.3C-0.5N.
10. The nickel-free austenitic stainless steel according to claim
2, having a composition, expressed in mass percent, given by the
formula Fe-17Cr-17Mn-2Mo-1Cu-0.3C-0.5N.
11. The nickel-free austenitic stainless steel according to claim
1, comprising mass percentages of copper in amounts of
0.5.ltoreq.Cu<4%.
12. The nickel-free austenitic stainless steel according to claim
2, comprising mass percentages of copper in amounts of
0.5.ltoreq.Cu<4%.
13. The nickel-free austenitic stainless steel according to claim
1, comprising mass percentages of carbon in amounts of
0.2.ltoreq.C<1%.
14. The nickel-free austenitic stainless steel according to claim
2, comprising mass percentages of carbon in amounts of
0.2.ltoreq.C<1%.
15. The nickel-free austenitic stainless steel according to claim
11, comprising mass percentages of carbon in amounts of
0.2.ltoreq.C<1%.
16. The nickel-free austenitic stainless steel according to claim
12, comprising mass percentages of carbon in amounts of
0.2.ltoreq.C<1%.
17. The nickel-free austenitic stainless steel according to claim
1, comprising mass percentages of molybdenum in amounts of
1.ltoreq.Mo.ltoreq.2%.
18. The nickel-free austenitic stainless steel according to claim
2, comprising mass percentages of molybdenum in amounts of
1.ltoreq.Mo.ltoreq.2%.
19. The nickel-free austenitic stainless steel according to claim
11, comprising mass percentages of molybdenum in amounts of
1.ltoreq.Mo.ltoreq.2%.
20. The nickel-free austenitic stainless steel according to claim
12, comprising mass percentages of molybdenum in amounts of
1.ltoreq.Mo.ltoreq.2%.
21. The nickel-free austenitic stainless steel according to claim
13, comprising mass percentages of molybdenum in amounts of
1.ltoreq.Mo.ltoreq.2%.
22. The nickel-free austenitic stainless steel according to claim
14, comprising mass percentages of molybdenum in amounts of
1.ltoreq.Mo.ltoreq.2%.
23. The nickel-free austenitic stainless steel according to claim
15, comprising mass percentages of molybdenum in amounts of
1.ltoreq.Mo.ltoreq.2%.
24. The nickel-free austenitic stainless steel according to claim
16, comprising mass percentages of molybdenum in amounts of
1.ltoreq.Mo.ltoreq.2%.
25. The nickel-free stainless steel according to claim 1,
containing at least one element from among S, Pb, B, Bi, P, Te, Se,
Nb, V, Ti, Zr, Hf, Ce, Ca, Co, Mg which may each be present in a
mass concentration of up to 1%.
26. The nickel-free stainless steel according to claim 2,
containing at least one element from among S, Pb, B, Bi, P, Te, Se,
Nb, V, Ti, Zr, Hf, Ce, Ca, Co, Mg which may each be present in a
mass concentration of up to 1%.
27. Timepieces and pieces of jewellery made of nickel-free
austenitic stainless steel according to claim 1.
28. Timepieces and pieces of jewellery made of nickel-free
austenitic stainless steel according to claim 2.
Description
[0001] This application claims priority from European Patent
Application No 15186980.7 filed Sep. 25, 2015, the entire
disclosure of which is hereby incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention concerns nickel-free austenitic
stainless steel compositions. More specifically, the present
invention concerns nickel-free austenitic stainless steels
particularly well-suited to utilisation in the fields of
watchmaking and jewellery.
BACKGROUND OF THE INVENTION
[0003] Nickel-free austenitic stainless steel compositions are
advantageous for applications in the fields of watchmaking and
jewellery since they are non-magnetic and hypoallergenic.
[0004] For more than 50 years, numerous nickel-free austenitic
stainless steel compositions have been proposed. Indeed, it was
quickly sought to remove nickel from austenitic stainless steel
compositions, firstly for reasons of cost and then, more recently,
for public health reasons as nickel is known to cause allergic
reactions.
[0005] These nickel-free austenitic stainless steels are
principally based on the elements Fe--Cr--Mn--Mo--C--N. Indeed, to
replace nickel, which guarantees an austenitic structure, it has
been proposed to use elements such as manganese, nitrogen and
carbon. However, these elements have the effect of increasing some
mechanical properties, such as the hardness, elastic limit and
strength of the resulting alloys, which makes it very difficult to
shape parts by machining and forging, which are the operations
usually used in the fabrication of components for watchmaking and
jewellery.
[0006] One example of a nickel-free austenitic stainless steel is
disclosed by EP Patent 1786941B1. In this document, the
compositions proposed by Berns and Gavriljuk can be obtained by
melting and solidifying alloying elements at atmospheric pressure,
but they contain high concentrations of manganese, carbon and
nitrogen, intended to maximise mechanical properties. This results
in great difficulty in shaping by machining and forging. Moreover,
the high concentration of manganese is disadvantageous from the
point of view of corrosion resistance.
[0007] Certain recently proposed compositions are intended, in
particular, for use in the production of parts that may come in
contact with the human body (wristwatches, jewellery, medical
prostheses). Examples of nickel-free austenitic stainless steels
that can be used to produce parts that come into contact with the
human body are disclosed by EP Patent 875591B1 in the name of
Bohler Edelstahl GmbH. The compositions disclosed in this document
have, in particular, high concentrations of molybdenum, to obtain
corrosion resistance allowing such alloys to be used in the field
of medicine. In order to obtain low concentrations of manganese,
carbon and nitrogen while exhibiting a high concentration of
molybdenum, these alloys must, however, undergo a step of melting
and solidification with nitrogen overpressure, i.e. a nitrogen
pressure higher than atmospheric pressure, thereby drastically
increasing the cost of the resulting alloys
[0008] To avoid the use of special installations for melting and
solidifying the alloys with nitrogen overpressure, compositions are
disclosed, in particular, in EP Patent Application 2455508A1.
Nonetheless, despite their low concentration of manganese, these
compositions exhibit high concentrations of carbon and nitrogen,
again resulting in difficulties in shaping by machining and
forging. By removing the molybdenum, it is possible to reduce the
concentration of carbon and nitrogen by producing the alloy at
atmospheric pressure, as disclosed in U.S. patent application Ser.
No. 2455508A1, but the corrosion resistance is then insufficient
for applications in the field of watchmaking and jewellery.
[0009] In the field of watchmaking and jewellery, where it is
necessary to fabricate large series of parts often having complex
shapes, a compromise must therefore be made between shapeability
(machinability and forgeability) and corrosion resistance.
Moreover, alloys obtained at atmospheric pressure must be
preferred, for reasons of cost.
[0010] To obtain an austenitic (and thus non-magnetic) stainless
steel suitable for contact with the human body, the absence of
nickel must be compensated for by other gammagenous elements which
enhance the austenitic structure The choice is limited and the most
common gammagenous elements are nitrogen, carbon and manganese.
[0011] Nitrogen and carbon are the only elements capable of
completely compensating for the absence of nickel. However, these
gammagenous elements have the effect of considerably increasing the
hardness of the resulting austenitic steels by interstitial solid
solution, making shaping operations, such as machining and
stamping, very difficult for such steels, notably in the fields of
watchmaking and jewellery. The effect of nitrogen is even more
marked than that of carbon as regards the hardness of the resulting
austenitic steel. The concentration of nitrogen must therefore be
as low as possible. However, a minimal nitrogen content is required
to obtain a completely austenitic structure, since, unlike
nitrogen, carbon alone cannot provide an austenitic structure
without precipitates. Such precipitates are harmful in terms of the
polishability and corrosion resistance of austenitic steels.
[0012] Manganese only slightly promotes the austenitic structure.
Its presence is, however, indispensable to increase the solubility
of nitrogen and thus to guarantee the creation of a nickel-free
completely austenitic structure. In fact, the more manganese is
added, the higher the solubility of nitrogen. However, manganese
impairs the corrosion resistance of austenitic steels and is also
responsible for an increase in the hardness of austenitic steels.
Manganese is thus harmful as regards the machinability and
forgeability of the resulting steels.
[0013] The presence of a small quantity of molybdenum is
indispensable, since it provides sufficient corrosion resistance as
defined by the salt spray test specified by ISO standard 9227.
Indeed, as shown with the alloys 1.3816 and 1.3815, chromium alone
produces insufficient corrosion resistance for external timepiece
components. It is therefore also necessary to have a small amount
of molybdenum, which, as proved by many studies, improves the
corrosion resistance of the resulting austenitic steels. Moreover,
corrosion resistance increases with the nitrogen content provided
that the nitrogen is in solid solution. However, the concentration
of molybdenum and chromium in the alloys must be limited, since
these elements promote a ferritic structure to the detriment of the
austenitic structure. Consequently, to compensate for the effects
of molybdenum and chromium, the concentration in the alloy of
elements such as nitrogen or carbon would have to be increased,
which would run counter to the properties of machinability and
forgeability of the alloys.
[0014] There are two possible ways of producing a nickel-free
austenitic steel.
[0015] The traditional way consists in obtaining semi-products by
casting, followed by remelting to refine the composition of the
alloy followed by various thermomechanical treatments. Since
nitrogen is introduced here into the liquid alloy, the
solidification of nickel-free austenitic stainless steels is
consequently particularly critical. Indeed, depending, in
particular, on the composition of the alloy and on the nitrogen
partial pressure, ferrite may be formed from the liquid state, and
may cause porosity in the solidified alloy. As the solubility of
nitrogen is much greater in ferrite than in austenite, the nitrogen
can be salted out of the liquid in the form of gas, thereby
creating undesired porosity.
[0016] There are two main possibilities for preventing or at least
limiting the aforementioned formation of porosity. The first
possibility consists in requiring nitrogen overpressure during
casting or remelting, for example by using techniques known as
pressurised induction melting or pressure electro slag remelting.
This allows the amount of nitrogen in the liquid alloy to be
increased beyond solubility at ambient atmospheric pressure, and
can thereby limit or prevent the formation of ferrite during
solidification. Moreover, the formation of pores is rendered more
difficult by the overpressure applied to the alloy which
solidifies. However, the use of these techniques greatly increases
the price of the alloys obtained, notably because the production
installations are expensive.
[0017] The second possibility for preventing or limiting the
formation of porosity during solidification of the alloy is the
careful selection of the elements included in the alloy
composition, for example by increasing the concentrations of
gammagenous elements (C, Mn, Cu) and/or by reducing the
concentrations of alphagenous elements (Cr, Mo) and/or by
increasing the concentrations of elements which increase the
solubility of nitrogen (Mn, Cr, Mo). Certain elements have opposite
effects, but not necessarily in the same proportions. Thus, a
completely austenitic solidification that avoids salting out of
nitrogen through ferrite formation is possible at ambient
atmospheric pressure, or lower.
[0018] The solution consisting in casting and remelting steel at
ambient atmospheric pressure is thus less expensive than the
solution consisting in working with nitrogen overpressure and is
thus to be preferred. There are, however, constraints that affect
the compositions of alloys that can be cast at ambient atmospheric
pressure.
[0019] The other technique able to be used to fabricate nickel-free
austenitic steel components utilises powder metallurgy, for example
by metal injection moulding, a technique also known as MIM. In that
case, it is not necessary to use a 100% austenitic powder, since
nitrogen can also be added during sintering, thereby transforming
the rest of the ferrite into austenite.
SUMMARY OF THE INVENTION
[0020] It is an object of the present invention to overcome the
aforementioned problems, in addition to others, by providing
compositions of a nickel-free austenitic stainless steel for which
shaping operations are facilitated, which has sufficient corrosion
resistance and which can be obtained by conventional metallurgy
(foundry) in particular at ambient atmospheric pressure or by
powder metallurgy. "Sufficient corrosion resistance" means
sufficient resistance for the field of external timepiece parts and
the field of jewellery, in particular as defined by the salt spray
test (ISO standard 9227).
[0021] To this end, the present invention concerns a nickel-free
austenitic stainless steel comprising in mass percent: [0022]
chromium in amounts of 10<Cr<21%; [0023] manganese in amounts
of 10<Mn<20%; [0024] molybdenum in amounts of
0<Mo<2.5%; [0025] copper in amounts of 0.5.ltoreq.Cu<4%;
[0026] carbon in amounts of 0.15<C<1%; [0027] nitrogen in
amounts of 0<N.ltoreq.1; [0028] nickel in amounts of
0.ltoreq.Ni<0.5%, and
[0029] the nickel-free austenitic stainless steel comprising, in
mass percent, carbon in amounts of 0.25<C<1% when the steel
includes manganese in amounts of 15.ltoreq.Mn<20%,
[0030] the remainder being formed by iron and any impurities from
the melt.
[0031] According to another feature of the invention, the
nickel-free austenitic stainless steel comprises in mass percent:
[0032] chromium in amounts of 15<Cr<21%; [0033] manganese in
amounts of 10<Mn<20%; [0034] molybdenum in amounts of
0<Mo<2.5%; [0035] copper in amounts of 0.5.ltoreq.Cu<4%;
[0036] carbon in amounts of 0.15%<C<1% [0037] nitrogen in
amounts of 0<N.ltoreq.1; [0038] silicon in amounts of
0.ltoreq.Si<2%, [0039] nickel in amounts of 0.ltoreq.Ni<0.5%,
[0040] tungsten in amounts of 0.ltoreq.W<4%, [0041] aluminium in
amounts of 0.ltoreq.Al<3%, and
[0042] the remainder being formed by iron and any impurities from
the melt.
[0043] According to yet another feature of the invention, the
nickel-free stainless steel contains at least one element from
among S, Pb, B, Bi, P, Te, Se, Nb, V, Ti, Zr, Hf, Ce, Ca, Co, Mg
which may each be present with a mass concentration of up to
1%.
[0044] Within the meaning of the invention, a "nickel-free
austenitic stainless steel" means an alloy containing no more than
0.5 mass percent of nickel.
[0045] "Any impurities" means elements not for the purpose of
modifying one (or more) properties of the alloy, but which are
inevitably present due to the melting process. In particular, in
the field of watchmaking and jewellery, it is necessary to limit
the presence of these impurities as far as possible, since such
impurities may notably form non-metallic inclusions in the alloy,
such as oxides, sulphides and silicates which may have harmful
consequences for the corrosion resistance and polishability of the
resulting alloys.
[0046] In the nickel-free austenitic stainless steel compositions
according to the invention, the mass concentration of molybdenum
must be lower than 2.5%. Indeed, the presence of molybdenum is
necessary as it enhances the corrosion resistance of the resulting
steels, in particular resistance to pitting corrosion. The
concentration of molybdenum should, however, be limited to small
amounts since molybdenum has the drawback of promoting the ferritic
structure. Consequently, the higher the concentration of
molybdenum, the greater the need to add elements such as nitrogen,
carbon and manganese, which promote the austenitic structure but
which have the drawback of making the resulting alloy harder and
thus more difficult to machine and forge.
[0047] Further, in the nickel free austenitic stainless steel
compositions according to the invention, the mass concentration of
copper must be higher than 0.5% and lower than 4%. Copper, which in
the prior art, is considered to be an impurity, is deliberately
added to the compositions of the invention, notably because copper
promotes the austenitic structure and therefore allows the
concentration of nitrogen and carbon to be limited. Moreover, the
presence of copper improves the general corrosion resistance of the
alloys and intrinsically enhances the machinability and
forgeability of the alloys of the invention. The concentration of
copper must, however, be limited to 4% since copper tends to make
steel brittle at high temperatures, which may make thermomechanical
treatments difficult.
[0048] Likewise, the concentration of manganese in the alloys of
the invention must be higher than 10% and lower than 20%. It is
known that manganese enhances the solubility of nitrogen in
nickel-free austenitic stainless steel compositions. However, the
higher the concentration of manganese, the harder the alloys will
be and the lower their machinability and forgeability. Moreover,
their resistance to corrosion decreases. Consequently, by teaching
that the concentration of manganese in nickel-free stainless steel
alloys must be limited, the present invention makes it possible to
enhance the corrosion resistance of such alloys and their
machinability and forgeability. However, a minimal concentration of
manganese is necessary to guarantee sufficient solubility of
nitrogen, in particular in order to solidify the alloy at ambient
atmospheric pressure.
[0049] According to yet another feature of the invention, the
nickel-free austenitic stainless steel comprises mass percentages
of carbon in amounts of 0.2.ltoreq.C<1%
[0050] According to yet another feature of the invention, the
nickel-free austenitic stainless steel comprises mass percentages
of molybdenum in amounts of 1.ltoreq.Mo.ltoreq.2%.
[0051] Preferred examples of compositions are given by the
following formulae: [0052] Fe-17Cr-17Mn-2Mo-1Cu-0.3C-0.5N [0053]
Fe-17Cr-12Mn-2Mo-2Cu-0.33C-0.4N [0054]
Fe-17Cr-11Mn-2Mo-1Cu-0.25C-0.4N [0055]
Fe-17Cr-14.5Mn-2Mo-2Cu-0.22C-0.35N
[0056] The first two compositions are especially advantageous when
the corresponding nickel-free austenitic steel is obtained by
conventional metallurgy (casting, remelting and thermomechanical
treatments). Indeed, at ambient atmospheric pressure, without
overpressure, solidification is completely austenitic, thereby
avoiding the formation of undesired porosity in the alloy.
Moreover, these compositions are optimised so that the temperature
at which precipitates such as carbides or nitrides appear is as low
as possible. The austenitic temperature range is thus maximal,
thereby facilitating any thermomechanical treatments.
[0057] The advantage of the first composition, containing 1%
copper, lies in the fact that the austenitic temperature range is
higher than that of the second composition, which contains 2%
copper. The second composition, containing 2% copper will, however,
be easier to shape by machining and stamping. Indeed, copper
naturally enhances the machinability and forgeability properties of
the alloys. Moreover, using more copper means that the nitrogen and
carbon content can be reduced while guaranteeing an austenitic
structure.
[0058] In addition to the fact that they can be obtained by
conventional metallurgy, the first two compositions may also be
advantageous in the case of powder metallurgy shaping. Indeed,
these compositions make it possible to obtain particularly dense
components after sintering, particularly by the technique known as
super solidus liquid-phase sintering.
[0059] The third and fourth compositions are especially suited for
powder metallurgy shaping. They offer, in particular, the
possibility of solid phase sintering in an atmosphere containing a
reduced nitrogen partial pressure. This allows the atmosphere to be
supplemented with, for example, hydrogen, known to improve
densification of stainless steels during sintering. As these alloys
also have a low content of interstitial elements after sintering,
any shaping operations after sintering, such as machining or
forging, are also facilitated. Likewise, these two compositions are
optimised so that the temperature at which precipitates such as
carbides or nitrides appear is as low as possible. It will be
noted, however, that, although the third and fourth compositions
are particularly well suited for powder metallurgy shaping, these
compositions may also be obtained by conventional means, for
example by using nitrogen overpressure during melting and
solidification.
[0060] In most cases in the prior art, the object sought was to
maximise the corrosion resistance and hardness of austenitic steels
by favouring a high content of nitrogen and molybdenum in the
alloys.
[0061] However, in the case of the present invention, the
specifications for external parts to be used in the field of
watchmaking and jewellery are different. Thus, the proposed alloys
have optimised properties which make them particularly well suited
for use in the fields of watchmaking and jewellery.
[0062] Firstly, the machinability of the alloys of the invention is
improved, mainly because the amount of nitrogen present in these
alloys is low. Indeed, by limiting the molybdenum content to a
value of less than 2.5% by weight and by adding other gammagenous
elements, such as carbon and copper, the amount of nitrogen can be
reduced while guaranteeing an austenitic structure. The addition of
a small amount of sulphur (up to 0.015% by weight) can also improve
machinability, by forming manganese sulphide, but care must be
taken since this may have an impact on the corrosion resistance of
the alloy obtained. It is specified that "machinability" means any
type of machining operation, such as piercing, milling, boring or
other operations.
[0063] Secondly, the forgeability of the alloys of the invention is
also improved.
[0064] Since nitrogen is the main element that increases mechanical
properties in this type of alloy, a limited concentration of
nitrogen makes it easier to obtain shaping by deformation.
[0065] Copper, another important element, can decrease the level of
strain hardening of the alloy, which consequently facilitates
shaping by deformation. Finally, as a result of the copper,
improved general corrosion resistance is observed.
[0066] The invention also concerns the use of a nickel-free
austenitic stainless steel, as described above, for the production
of external elements for timepieces and pieces of jewellery.
BRIEF DESCRIPTION OF THE DRAWINGS
[0067] Other features and advantages of the present invention will
appear more clearly from the following detailed description of an
embodiment of the nickel-free austenitic stainless steel according
to the invention, this example being given merely by way of
non-limiting illustration with reference to the annexed drawing, in
which:
[0068] FIG. 1 is a phase diagram illustrating the first example of
composition Fe-17Cr-17Mn-2Mo-1Cu-0.3C-0.5N of the nickel-free
austenitic stainless steel according to the invention.
[0069] FIG. 2 is a phase diagram illustrating the second example of
composition Fe-17Cr-12Mn-2Mo-2Cu-0.33C-0.4N of the nickel-free
austenitic stainless steel according to the invention.
[0070] FIG. 3 is a phase diagram illustrating the third example of
composition Fe-17Cr-11Mn-2Mo-1Cu-0.25C-0.4N of the nickel-free
austenitic stainless steel according to the invention.
[0071] FIG. 4 is a phase diagram illustrating the fourth example of
composition Fe-17Cr-14,5Mn-2Mo-2Cu-0.22C-0.35N of the nickel-free
austenitic stainless steel according to the invention.
[0072] FIG. 5 is a table setting out the compositions of
nickel-free austenitic stainless steels in mass percentages.
[0073] FIG. 6 is a Schaeffler diagram as defined by Gavriljuk and
Berns in "High Nitrogen Steels", Springer Editions 2010 which can
predict the structure of an alloy after hardening according to
composition.
DETAILED DESCRIPTION OF ONE EMBODIMENT OF THE INVENTION
[0074] The present invention proceeds from the general inventive
idea which consists in proposing nickel-free austenitic stainless
steels representing a very good compromise between machinability
and forgeability properties and corrosion resistance, taking
account of issues specific to the field of external timepiece
parts. Further, the proposed compositions can be obtained by means
of conventional metallurgy (foundry), in particular at ambient
atmospheric pressure, which is very advantageous from the point of
view of production costs, or by powder metallurgy with very high
densities after sintering. The concentrations of alphagenous
elements, such as chromium and molybdenum, are defined to obtain
sufficient corrosion resistance. The concentrations of manganese,
carbon and nitrogen are sufficiently low to enhance the
machinability and forgeability properties of the resulting alloys,
but sufficiently high to obtain the alloy by melting and
solidification at atmospheric pressure or to obtain very dense
parts by powder metallurgy. Moreover, concentrations are optimised
to obtain a maximum austenitic temperature range. Finally, the
copper makes it possible to reduce the concentration of the
aforementioned gammagenous elements, to facilitate shaping by
machining or deformation, and to improve general corrosion
resistance. The concentration of copper must, however, be limited,
since copper diminishes the austenitic temperature range and tends
to make austenitic steel brittle at high temperatures, making any
thermomechanical treatments (forging/lamination, annealing, etc.)
more difficult.
[0075] For the first example of composition, whose phase diagram is
illustrated in FIG. 1 (Fe-17Cr-17Mn-2Mo-1Cu-0.3C-0.5N), it is seen
that it is possible to obtain completely austenitic solidification
at atmospheric pressure and that for the nitrogen concentration
obtained after solidification, the temperature at which
precipitates appear is as low as possible (intersection between
line 1 and line 3). The austenitic temperature range is thus the
widest possible. This composition is also advantageous for
obtaining very dense parts by powder metallurgy. Indeed, the
existence of a broad "austenite-liquid" phase (between lines 4, 5
and 6) at 900 mbars of nitrogen allows liquid phase sintering to be
performed without losing nitrogen. The sintering temperature is
defined in that case to have approximately 30% of liquid during
sintering.
[0076] For the second example of composition illustrated in FIG. 2
(Fe-17Cr-12Mn-2Mo-2Cu-0.33C-0.4N), the increase in copper
concentration makes it possible to move the boundary of the
austenitic range (line 6) towards lower concentrations of nitrogen.
Therefore, the manganese concentration can be reduced and the alloy
obtained after solidification contains less nitrogen. As a result
of this higher concentration of copper and reduced concentrations
of nitrogen and manganese, the machinability and deformability of
the alloy are facilitated compared to the first composition.
Although the higher copper concentration reduces the austenitic
temperature range, the range is maximal for the intended
concentration of nitrogen (between 1300.degree. C. and 1050.degree.
C.).
[0077] For the third example of composition illustrated in FIG. 3
(Fe-17Cr-11Mn-2Mo-1Cu-0.25C-0.4N), ferrite is formed in the case of
solidification at atmospheric pressure, which may result in
porosity in the solidified alloy. However, this composition is
optimised for powder metallurgy shaping. Indeed, for this
composition, sintering can be performed at a high temperature
(1300.degree. C.) with a reduced nitrogen partial pressure (around
600 mbars). The sintering atmosphere can thus be supplemented with
hydrogen, which, owing to its strong reducing power, improves the
densification of the parts obtained after sintering.
[0078] The fourth example of composition illustrated in FIG. 4
(Fe-17Cr-14,5Mn-2Mo-2Cu-0.22C-0.35N) is also advantageous for
powder metallurgy shaping. Compared to the preceding example, the
sintering can be performed at a high temperature (1300.degree. C.)
with an even lower nitrogen partial pressure (approximately 400
mbars). Finally, this alloy has a very low concentration of
interstitial elements, thus facilitating any machining or forging
operations after sintering.
[0079] The table illustrated in FIG. 5 compares the MARC (Measure
of Alloying for Resistance to Corrosion) index values of the above
examples of compositions with standard austenitic stainless steels
containing nickel and the nickel-free austenitic stainless steels
available on the market. The MARC index is an excellent means of
comparing the corrosion resistance of austenitic steels,
particularly those that are nickel-free. The higher the MARC index,
the greater the resistance of the alloy to corrosion. This table
includes two standard austenitic stainless steels containing nickel
commonly used in watchmaking and jewellery, six commercial
nickel-free austenitic stainless steels, and the four
aforementioned preferred examples of compositions. Further, the
last line of the table sets out, for each alloy, the MARC index
value as defined by Speidel. M. O., in "Nitrogen containing
austenitic stainless steel", Materialwissenschaft und
Werkstofftechnik, 37(2006), pp. 875-880. This is the sum of the
concentration of elements in the composition of the austenitic
stainless steels concerned:
MARC=Cr(%)+3.3Mo(%)+20C(%)+20N(%)-0.5Mn(%)-0.25Ni(%).
[0080] The examples of compositions according to the invention have
a higher MARC index value than the austenitic stainless steel
1.4435 which is the steel most commonly used in watchmaking and
jewellery. Three of the four examples of compositions according to
the invention even have a higher MARC index value than that of
steel 1.4539, which is known for its excellent corrosion
resistance.
[0081] The present invention seeks to improve the machinability and
deformability of nickel-free austenitic stainless steels by
teaching the reduction of carbon and nitrogen content in these
alloys and the addition of copper. Thus, although the proposed
alloys have lower index values than those of the alloys 1.4456,
1.4452, UNS S29225 and UNS S29108, they have higher index values
than those of the alloys 1.3816 and 1.3815, which is sufficient to
enable them to pass the salt spray corrosion tests, Moreover,
compared to the alloys 1.4456, 1.4452, UNS S29225 and UNS S29108,
which undergo a step of melting and solidification under nitrogen
overpressure, the first, second and fourth examples of compositions
according to the invention exhibit austenitic solidification at
atmospheric pressure, thus avoiding the use of special
installations. This consequently reduces the cost of the alloys
obtained.
[0082] Finally, the position of these different alloys on the
Schaeffler diagram is illustrated in FIG. 6. The four preferred
examples of compositions, like the other alloys presented, are all
within the austenitic range of the diagram. This confirms, if
necessary, the stability of the austenitic structure for the
compositions according to the invention. It is also seen that the
examples of compositions are located between the alloys
1.3816/1.3815 (whose corrosion resistance is too low) and the
alloys 1.4456/1.4452/UNS S29225/UNS S29108 (which are very
difficult to shape by machining and forging, and whose cost price
is high as they are produced under nitrogen overpressure).
[0083] It goes without saying that the present invention is not
limited to the embodiments that have just been described and that
various simple modifications and variants can be envisaged by those
skilled in the art without departing from the scope of the
invention as defined by the annexed claims. It will be noted, in
particular, that the proposed alloys offer an excellent compromise
between the corrosion resistance, shapeability (machinability and
forgeability) and density of the parts after sintering. It is, in
fact, possible to sinter the parts under low nitrogen pressure and
to compensate with hydrogen. Moreover, in the case of composite
materials with a metal matrix, the metal matrix can be achieved
with the aid of steel compositions according to the invention. It
is also possible to treat the sintered parts at high isostatic
pressure. It is also possible to sinter at high isostatic pressure
parts shaped by pressing or by metal injection moulding. It is also
possible to produce semi-finished products at high isostatic
pressure. Finally, it is possible to forge the parts after
sintering.
* * * * *