EP3147380B1 - Nickel-free austenitic stainless steel - Google Patents

Description

Technical field of the invention

The present invention relates to nickel-free austenitic stainless steel compositions. More specifically, the present invention relates to nickel-free austenitic stainless steels particularly well suited for use in the fields of watchmaking and jewelery.

Technological background of the invention

The nickel-free austenitic stainless steel compositions are of interest for applications in the field of watchmaking and jewelery because they are non-magnetic and hypoallergenic.

For over 50 years, many nickel-free austenitic stainless steel compositions have been proposed. Indeed, in austenitic stainless steel compositions, it was very early sought to remove nickel, first of all for cost issues and, more recently, for reasons of public health because nickel is known to cause problems. allergic reactions.

These nickel-free austenitic stainless steels are mainly based on Fe-Cr-Mn-Mo-CN elements. Indeed, to replace the nickel which guarantees the austenitic structure, it has been proposed to use elements such as manganese, nitrogen and carbon. These elements, however, have the effect of increasing certain mechanical properties such as the hardness, yield strength and strength of the resulting alloys, which makes it very difficult to shape the parts by machining and forging which are usual operations in the field of manufacturing components for watchmaking and jewelery. US2011 / 226391 disclosed a nickel-free austenitic stainless steel containing: 17.97% by weight of chromium (Cr), 17.8% by weight of manganese (Mn); 0.36% by weight of nickel (Ni); 0.51% by weight of molybdenum (Mo); 0.58% by weight of nitrogen (N), 0.48% by weight of carbon.

An example of a nickel-free austenitic steel is disclosed by the European patent EP 1 786 941 B1 . In this document, the compositions proposed by Berns and Gavriljuk can be obtained by performing the melting and solidification of the atmospheric pressure alloy elements but have high concentrations of manganese, carbon and nitrogen, in order to maximize the properties. mechanical. This results in shaping by machining and forging very difficult. In addition, the high concentration of manganese is unfavorable from the point of view of corrosion resistance.

Some recently proposed compositions are particularly intended for use in the production of parts that can be in contact with the human body (wristwatches, jewelry, medical prostheses). Examples of nickel-free austenitic stainless steels that can be used to produce parts coming into contact with the human body are disclosed by the European patent EP 875 591 B1 in the name of Böhler Edelstahl GmbH. The compositions disclosed in this document exhibit in particular high concentrations of molybdenum, in order to obtain a corrosion resistance allowing the use of these alloys in the medical field. In order to be able to obtain low concentrations of manganese, carbon and nitrogen while having a high concentration of molybdenum, these alloys must however undergo a melting and solidification step with a nitrogen overpressure, that is to say a nitrogen pressure higher than the atmospheric pressure, thus drastically increasing the cost of the resulting alloys.

To avoid the use of special facilities for melting and solidifying the alloys with a nitrogen overpressure, compositions are disclosed in particular in the European patent application. EP 2 455 508 A1 . Nevertheless, despite their low concentration in these compositions have high concentrations of carbon and nitrogen, resulting again in shaping by difficult machining and forging. By removing molybdenum, it is possible to reduce the carbon and nitrogen concentration while producing the alloy at atmospheric pressure, as disclosed in US Patent Application US 2013/0149188 A1 , but the corrosion resistance is then no longer sufficient for applications in the field of watchmaking and jewelery.

In the field of watchmaking and jewelery, where it is necessary to manufacture large series of parts often having complex shapes, it is therefore necessary to obtain a compromise between aptitude for shaping (machinability and forgeability) and corrosion resistance. We must also favor the alloys obtained under atmospheric pressure, for cost issues.

To obtain a stainless steel austenitic (and therefore non-magnetic) capable of coming into contact with the human body, the absence of nickel must be compensated for by other gammagens that favor the austenitic structure. The choice is limited and the most common gammagens are nitrogen, carbon and manganese.

Nitrogen and carbon are the only elements capable of completely offsetting the absence of nickel. However, these gammagenic elements have the effect of considerably increasing the hardness of the resulting austenitic steels by solid insertion solution, making very difficult the shaping operations such as machining and stamping such steels, particularly in the fields of watchmaking and jewelery. The effect of nitrogen is even more marked than that of carbon with regard to the hardness of the resulting austenitic steel. Its concentration must therefore be as low as possible. Nevertheless, a minimum nitrogen level is necessary to obtain a totally austenitic structure because, unlike nitrogen, carbon alone does not make it possible to obtain a austenitic structure without precipitates. However, these precipitates are detrimental in terms of polishing ability and corrosion resistance of austenitic steels.

Manganese favors only the austenitic structure. Its presence is nevertheless essential in order to increase the solubility of the nitrogen and thus to guarantee the obtaining of a completely austenitic nickel-free structure. In fact, the more manganese is added, the higher the solubility of the nitrogen. However, manganese adversely affects the corrosion resistance of austenitic steels and is also responsible for increasing the hardness of austenitic steels. Manganese is therefore detrimental in terms of machinability and forgeability properties of the resulting steels.

The presence of a small quantity of molybdenum is essential because it makes it possible to achieve a sufficient corrosion resistance as defined by the salt spray test from the ISO 9227 standard. Indeed, as shown with alloys 1.3816 and 1.3815, chromium alone does not make it possible to obtain sufficient corrosion resistance of the cladding parts in watchmaking. It is therefore necessary to also have some molybdenum which many studies have proven to improve the corrosion resistance of the resulting austenitic steels. In addition, the corrosion resistance increases with the nitrogen content as long as it is in solid solution. However, it is necessary to limit the concentration of molybdenum and chromium alloys because these elements favor the ferritic structure to the detriment of the austenitic structure. Therefore, to compensate for the effects of molybdenum and chromium, it would be necessary to increase the concentration of the alloy in elements such as nitrogen or carbon, which would go against the properties of machinability and forgeability of the alloys. .

Two ways to produce nickel-free austenitic steel are possible.

The traditional way is to obtain semi-products by casting, followed by a possible redesign to refine the composition of the alloy and then different thermomechanical treatments. Since nitrogen is introduced here into the liquid alloy, the solidification of nickel-free austenitic stainless steels is therefore particularly critical. Indeed, depending in particular on the composition of the alloy and the partial pressure of nitrogen, ferrite can be formed from the liquid state, which can cause porosity in the solidified alloy. Since the solubility of nitrogen in ferrite is much less than in austenite, nitrogen can be released into the liquid in gaseous form, creating unwanted porosity.

There are two main possibilities to avoid or at least limit the formation of porosity mentioned above. The first possibility consists in imposing a nitrogen overpressure during casting or remelting, for example using techniques known under the English names Pressurized Induction Melting or Pressure ElectroSlag Remelting. This makes it possible to increase the amount of nitrogen in the liquid alloy beyond the solubility at ambient atmospheric pressure, thus being able to limit or even prevent the formation of ferrite during solidification. In addition, the formation of pores is made more difficult because of the overpressure applied to the alloy which solidifies. However, the use of these techniques greatly increases the price of the alloys obtained, especially because the production facilities are expensive.

The second possibility to avoid or limit the formation of porosity during the solidification of the alloy is to judiciously select the elements involved in the composition of the alloy, for example by increasing the concentrations of gammagens (C, Mn, Cu) and / or by reducing the concentrations of alphagenes (Cr, Mo) and / or by increasing the concentrations of elements that increase the solubility of nitrogen (Mn, Cr, Mo). Some elements have opposite effects, but not necessarily in the same proportions. Thus, completely austenitic solidification avoiding the release of nitrogen by ferrite formation is possible at ambient atmospheric pressure, or even lower.

The solution of casting and recasting steel at ambient atmospheric pressure is much cheaper than the solution of working with a nitrogen overpressure and is therefore preferred. There are, on the other hand, constraints on alloy compositions that can be flowing at ambient atmospheric pressure.

The other technique that can be used to manufacture nickel-free austenitic steel components uses powder metallurgy, for example by injection molding, a technique also known by the Anglo-Saxon name Metal Injection Molding or MIM. In this case, it is not necessary to use a 100% austenitic powder, since nitrogen can be added during sintering, thus transforming the ferrite residue into austenite.

Summary of the invention

It is an object of the present invention to overcome the above-mentioned and other problems by providing compositions of a nickel-free austenitic stainless steel whose forming operations are facilitated, which exhibit sufficient corrosion resistance, and which can be obtained by conventional metallurgy (foundry) in particular at ambient atmospheric pressure or by metallurgy of powders. By sufficient resistance to corrosion is meant sufficient strength for the fields of watchmaking and jewelery as defined in particular by the salt spray test (ISO 9227).

• du chrome en proportions 10 < Cr < 21% ;
• du manganèse en proportions 10< Mn < 20% ;
• du molybdène en proportions 0 < Mo < 2,5% ;
• du cuivre en proportions 0,5 ≤ Cu < 4% ;
• du carbone en proportions 0,15 < C < 1% ;
• de l'azote en proportions 0 < N ≤ 1 ;
• du nickel en proportions 0 ≤ Ni < 0,5% , et

l'acier inoxydable austénitique sans nickel comprenant en pourcentages massiques du carbone en proportions 0,25 < C < 1% lorsque cet acier comprend du manganèse en proportions 15 ≤ Mn < 20%,
le solde étant constitué par le fer et les impuretés éventuelles dues à la fusion.For this purpose, the present invention relates to a nickel-free austenitic stainless steel comprising in percentages by weight:

• chromium in proportions <Cr <21%;
• manganese in proportions of <Mn <20%;
• molybdenum in proportions 0 <Mo <2.5%;
• copper in proportions 0.5 ≤ Cu <4%;
• carbon in proportions of 0.15 <C <1%;
• nitrogen in proportions 0 <N ≤ 1;
• nickel in proportions 0 ≤ Ni <0.5%, and

nickel-free austenitic stainless steel comprising in percentages by weight of carbon in proportions of 0.25 <C <1% when said steel comprises manganese in proportions of ≤ Mn <20%,
the balance being constituted by the iron and the possible impurities due to the fusion.

• du chrome en proportions 15 < Cr < 21% ;
• du manganèse en proportions 10 < Mn < 20% ;
• du molybdène en proportions 0 < Mo < 2,5% ;
• du cuivre en proportions 0,5 ≤ Cu < 4% ;
• du carbone en proportions 0,15% < C < 1% ;
• de l'azote en proportions 0 < N ≤ 1 ;
• du silicium en proportions 0 ≤ Si < 2% ;
• du nickel en proportions 0 ≤ Ni < 0,5% ;
• du tungstène en proportions 0 ≤ W < 4% ;
• de l'aluminium en proportions 0 ≤ Al < 3%, et

le solde étant constitué par le fer et les impuretés éventuelles dues à la fusion.According to another characteristic of the invention, nickel-free austenitic stainless steel comprises in percentages by weight:

• chromium in proportions <Cr <21%;
• manganese in proportions of <Mn <20%;
• molybdenum in proportions 0 <Mo <2.5%;
• copper in proportions 0.5 ≤ Cu <4%;
• carbon in proportions of 0.15% <C <1%;
• nitrogen in proportions 0 <N ≤ 1;
• silicon in proportions 0 ≤ Si <2%;
• nickel in proportions 0 ≤ Ni <0.5%;
• tungsten in proportions 0 ≤ W <4%;
• aluminum in proportions 0 ≤ Al <3%, and

the balance being constituted by the iron and the possible impurities due to the fusion.

According to yet another characteristic of the invention, the nickel-free stainless steel contains at least one of S, Pb, B, Bi, P, Te, Se, Nb, V, Ti, Zr, Hf, Ce, Ca , Co, Mg which can each be present with a mass concentration of up to 1%.

For the purposes of the present invention, a nickel-free austenitic stainless steel is understood to mean an alloy containing not more than 0.5% by mass percentage of nickel.

Potential impurities means elements that are not intended to modify one (or more) properties of the alloy, but whose presence is unavoidable because of the melting process. In particular in the field of watchmaking and jewelery, it is necessary to limit the presence of these impurities to the maximum, since these impurities can in particular form in the alloy non-metallic inclusions such as oxides, sulfides and silicones. silicates which may have adverse consequences on the corrosion resistance and the polishing ability of the resulting alloys.

In nickel-free austenitic stainless steel compositions according to the invention, the mass concentration of the molybdenum must be less than 2.5%. Indeed, the presence of molybdenum is necessary because it promotes the resistance of the resulting steels to corrosion, in particular the resistance to pitting corrosion. It is, however, necessary to limit the concentration of molybdenum to small quantities because molybdenum has the disadvantage of favoring the ferritic structure. Consequently, the higher the molybdenum concentration, the more elements such as nitrogen, carbon and manganese which favor the austenitic structure must be added, but which have the disadvantage of making the resulting alloy harder and therefore less easy machinable and forgeable.

Furthermore, in the nickel-free austenitic stainless steel compositions in accordance with the invention, the mass concentration of the copper must be greater than 0.5% and less than 4%. The copper which, in the prior art, is considered as an impurity is added voluntarily in the compositions according to the invention, in particular because the copper favors the austenitic structure and thus makes it possible to limit the concentration of nitrogen and carbon. In addition, the presence of copper improves the resistance of alloys to generalized corrosion and intrinsically promotes the machinability and forging ability of the alloys according to the invention. The copper concentration must however be limited to 4% because the copper tends to weaken the steel at high temperature, which can make thermomechanical treatments difficult.

Similarly, the manganese concentration of the alloys according to the invention must be greater than 10% and less than 20%. It is known that manganese promotes the solubility of nitrogen in nickel-free austenitic stainless steel compositions. However, the higher the concentration of manganese, the harder the alloys and the poorer their ability to be machined and forged. In addition, their resistance to corrosion decreases. Therefore, by teaching to limit the manganese concentration of nickel-free stainless steel alloys, the present invention makes it possible to promote the resistance of these alloys to corrosion as well as their ability to be machined and forged. However, a minimum concentration of manganese is necessary to be able to guarantee a sufficient solubility of the nitrogen, in particular to be able to solidify the alloy at ambient atmospheric pressure.

According to yet another characteristic of the invention, nickel-free austenitic stainless steel comprises in percentages by weight of carbon in proportions of 0.2 ≤ C <1%.

According to yet another characteristic of the invention, nickel-free austenitic stainless steel comprises, in mass percentages of molybdenum, in proportions of 1 ≤ Mo ≤ 2%.

• Fe-17Cr-17Mn-2Mo-1Cu-0,3C-0,5N
• Fe-17Cr-12Mn-2Mo-2Cu-0,33C-0,4N
• Fe-17Cr-11 Mn-2Mo-1Cu-0,25C-0,4N
• Fe-17Cr-14,5Mn-2Mo-2Cu-0,22C-0,35N

Examples of preferred compositions are given by the following formulas:

• Fe-17Cr-2Mo-17mn-1Cu-0,3C 0.5N
• Fe-17Cr-12Mn-2Mo-0.4N-2Cu-0,33C
• Fe-17Cr-11Mn-2Mo-1Cu-0.25C-0.4N
• Fe-17Cr-2Mo-14,5Mn-2Cu-0,22C-0,35N

The first two compositions are especially interesting when the corresponding nickel-free austenitic steel is obtained by conventional metallurgy (casting, recasting and thermomechanical treatments). Indeed, at ambient atmospheric pressure, without overpressure, the solidification is completely austenitic, thus avoiding the formation of unwanted pores in the alloy. In addition, these compositions are optimized so that the temperature at which precipitates such as carbides or nitrides appear as low as possible. The temperature range of the austenitic domain is therefore maximal, thus facilitating all the thermomechanical treatments.

The advantage of the first composition, containing 1% copper, lies in the fact that the temperature range of the austenitic phase is higher than that of the second composition, which contains 2% copper. The second composition containing 2% copper will be easier to shape by machining and stamping. Indeed, copper naturally promotes the machinability and forgeability properties of alloys. In addition, by adding more copper, the nitrogen and carbon content can be reduced while ensuring an austenitic structure.

Besides the fact that they can be obtained by conventional metallurgy, the first two compositions can also be interesting in the case of metallurgical shaping of the powders. Indeed, these compositions make it possible to obtain particularly dense components after sintering, in particular by carrying out a sintering in the liquid phase, a technique better known by its English name "supersolidus liquid-phase sintering".

The third and fourth compositions are especially suitable for metallurgical shaping of powders. In particular, they offer the possibility of performing solid-phase sintering in an atmosphere containing a reduced nitrogen partial pressure. This thus makes it possible to complete the atmosphere with, for example, hydrogen, known to improve the densification of stainless steels during sintering. Since these alloys also have a low interstitial content after sintering, any subsequent sintering operations such as machining or forging are further facilitated. Similarly, these two compositions are optimized so that the onset temperature of the precipitates, such as carbides or nitrides, is as low as possible. It should be noted, however, that although these third and fourth compositions are particularly well suited to metallurgical shaping of the powders, these compositions can also be obtained by the traditional route using, for example, a nitrogen overpressure during melting and solidification.

In the majority of cases, in the prior art, the aim was to maximize the corrosion resistance and hardness of austenitic steels by favoring high levels of nitrogen and molybdenum in alloys.

However, in the case of the present invention, the specification for wearing parts usable in the field of watchmaking and jewelery is different. Thus, the alloys proposed have optimized properties that make them particularly well suited for use in the fields of watchmaking and jewelery.

In the first place, the machinability of the alloys according to the invention is improved, mainly because the quantity of nitrogen present in these alloys is low. Indeed, by limiting the molybdenum content to less than 2.5% by weight and by adding other gamma elements such as carbon and copper, the amount of nitrogen can be reduced while ensuring an austenitic structure. The addition of a little sulfur (up to 0.015% by weight) also improves the machinability, by manganese sulfide formation, but you have to be careful because it can have an impact on the resistance to corrosion of the alloy obtained. It is specified that machinability means any type of machining operation such as drilling, milling, boring or other.

Secondly, the forgeability of the alloys according to the invention is also improved.

Nitrogen being the main element that increases the mechanical properties in this type of alloy, a limited concentration of nitrogen makes it possible to obtain a shaping by deformation easier.

Another important element, the copper reduces the rate of hardening of the alloy, which therefore facilitates its shaping by deformation. Finally, thanks to copper, there is a better resistance to generalized corrosion.

The invention also relates to the use of a nickel-free austenitic stainless steel as described above for producing trim elements for timepieces and jewelery articles.

Brief description of the figures

• la figure 1 est un diagramme de phases illustrant le premier exemple de composition Fe-17Cr-17Mn-2Mo-1Cu-0,3C-0,5N de l'acier inoxydable austénitique sans nickel selon l'invention ;
• la figure 2 est un diagramme de phases illustrant le deuxième exemple de composition Fe-17Cr-12Mn-2Mo-2Cu-0,33C-0,4N de l'acier inoxydable austénitique sans nickel selon l'invention ;
• la figure 3 est un diagramme de phases illustrant le troisième exemple de composition Fe-17Cr-11Mn-2Mo-1Cu-0,25C-0,4N de l'acier inoxydable austénitique sans nickel selon l'invention ;
• la figure 4 est un diagramme de phases illustrant le quatrième exemple de composition Fe-17Cr-14,5Mn-2Mo-2Cu-0,22C-0,35N de l'acier inoxydable austénitique sans nickel selon l'invention ;
• la figure 5 est un tableau présentant des compositions d'aciers inoxydables austénitiques en concentrations massiques, et
• la figure 6 est un diagramme de Schaeffler tel que défini par Gavriljuk et Berns dans l'ouvrage « High Nitrogen Steels », éditions Springer 2010 qui permet de prédire la structure d'un alliage après trempe en fonction de la composition.

Other features and advantages of the present invention will emerge more clearly from the following detailed description of one embodiment of the austenitic nickel-free steel according to the invention, this example given purely by way of illustration and not only with reference to the appended drawing, in which:

• the figure 1 is a phase diagram illustrating the first example of composition Fe-17Cr-17Mn-2Mo-1Cu-0.3C-0.5N nickel-free austenitic stainless steel according to the invention;
• the figure 2 is a phase diagram illustrating the second example of composition Fe-17Cr-12Mn-2Mo-2Cu-0.33C-0.4N nickel-free austenitic stainless steel according to the invention;
• the figure 3 is a phase diagram illustrating the third example of composition Fe-17Cr-11Mn-2Mo-1Cu-0.25C-0.4N nickel-free austenitic stainless steel according to the invention;
• the figure 4 is a phase diagram illustrating the fourth example of Fe-17Cr-14.5Mn-2Mo-2Cu-0.22C-0.35N composition of nickel-free austenitic stainless steel according to the invention;
• the figure 5 is a table showing compositions of austenitic stainless steels in mass concentrations, and
• the figure 6 is a Schaeffler diagram as defined by Gavriljuk and Berns in the book "High Nitrogen Steels", Springer Editions 2010 which makes it possible to predict the structure of an alloy after quenching according to the composition.

Detailed description of an embodiment of the invention

The present invention proceeds from the general inventive idea which consists in proposing alloys of austenitic stainless steels without nickel representing a very good compromise between their ability to be machined and forged and their resistance to corrosion, taking into account the specific problems. in the field of watchmaking. In addition, the compositions proposed can be obtained by means of conventional metallurgy (foundry), in particular under pressure ambient atmospheric which is very advantageous from the point of view of production costs, or by metallurgy of powders with very high densities after sintering. The concentrations of alphagenic elements such as chromium and molybdenum are defined to obtain sufficient corrosion resistance. The concentrations of manganese, carbon and nitrogen are sufficiently low to promote the ability of the resulting alloys in machining and forging but high enough to be able to obtain the alloy by melting and solidification at atmospheric pressure or to obtain very good parts. dense by metallurgy of powders. In addition, the concentrations are optimized to obtain a maximum temperature range of the austenitic domain. Finally, the copper makes it possible to reduce the concentration of the above-mentioned gamma-elements, to facilitate shaping by machining or deformation, and to improve the resistance to generalized corrosion. The copper concentration must however be limited because the copper decreases the temperature range of the austenitic domain and tends to weaken the austenitic steel at high temperature, making it more difficult the possible thermomechanical treatments (forging / rolling, annealing, etc.). .

For the first example of composition, whose phase diagram is illustrated at figure 1 (Fe-17Cr-17Mn-2Mo-1Cu-0.3C-0.5N), we see that it is possible to obtain totally austenitic solidification at atmospheric pressure and that for the nitrogen concentration obtained after solidification, the temperature the appearance of the precipitates is as low as possible (intersection between line 1 and line 3). The temperature range of the austenitic domain is therefore the widest possible. This composition is also interesting for obtaining very dense parts by powder metallurgy. Indeed, the existence of a wide "austenite-liquid" domain (between lines 4, 5 and 6) at 900 mbar of nitrogen makes it possible to perform sintering in the liquid phase without loss of nitrogen. The sintering temperature is then defined to have about 30% of liquid during sintering.

For the second example of composition illustrated in figure 2 (Fe-17Cr-12Mn-2Mo-2Cu-0.33C-0.4N), the increase in copper concentration makes it possible to shift the boundary of the austenitic domain (line 6) to lower nitrogen concentrations. Thus, the concentration of manganese can be reduced and the alloy obtained after solidification contains less nitrogen. Due to this higher concentration of copper and reduced concentrations of nitrogen and manganese, machinability and deformability of the alloy are facilitated compared to the first composition. Although the higher copper concentration reduces the temperature range of the austenitic domain, the latter is maximum for the target nitrogen concentration (between 1300 ° C and 1050 ° C).

For the third example of composition illustrated in figure 3 (Fe-17Cr-11Mn-2Mo-1Cu-0.25C-0.4N), there is formation of ferrite in case of solidification at atmospheric pressure, this may result in porosity in the solidified alloy. However, this composition is optimized for metallurgical shaping of the powders. Indeed, for this composition, the sintering can be carried out at high temperature (1300 ° C.) with a reduced nitrogen partial pressure (about 600mbars). The sintering atmosphere can therefore be supplemented with hydrogen, which thanks to its high reducing power improves the densification of the parts obtained after sintering.

The fourth example of composition illustrated in figure 4 (Fe-17Cr-14.5Mn-2Mo-2Cu-0.22C-0.35N) is also of interest for metallurgical shaping of the powders. Compared to the previous example, sintering can be carried out at high temperature (1300 ° C) with an even lower nitrogen partial pressure (about 400 mbar). Finally, this alloy has a very low concentration of interstitial elements, thus facilitating any machining or forging operations after sintering.

The table shown at figure 5 allows to compare the MARC (Measure of Alloying for Resistance to Corrosion) indices of the above examples of compositions with standard austenitic stainless steels with nickel and nickel-free austenitic stainless steels available on the market. The MARC index is a great way to compare the corrosion resistance of austenitic steels, especially those without nickel. The higher the MARC index, the more resistant the alloy is to corrosion. This table comprises two standard austenitic stainless steels with nickel commonly used in watchmaking and jewelery, six commercial nickel-free austenitic stainless steels, as well as the four examples of preferred compositions mentioned above. In addition, the last line of the table presents, for each alloy, the MARC index as defined by Speidel, MO, "Nitrogen containing austenitic stainless steel", Materialwissenschaft und Werkstoffftechnik ", 37 (2006), pp. 875-880 . This is the sum of the concentration of the elements used in the composition of the austenitic stainless steels concerned: $$\mathrm{MARC}=\mathrm{Cr}\left(\%\right)+3.3\mathrm{MB}\left(\%\right)+20\mathrm{VS}\left(\%\right)+20\mathrm{NOT}\left(\%\right)-0.5\mathrm{mn}\left(\%\right)-0.25\mathrm{Or}\left(\%\right).$$

The examples of compositions according to the invention have in particular a higher MARC index than that of the austenitic stainless steel 1.4435 which is the steel most commonly used in watchmaking and jewelery. Three of the four examples of compositions according to the invention even have a MARC index higher than that of steel 1.4539 which is known for its excellent resistance to corrosion.

The present invention seeks to improve the machinability and deformability of nickel-free austenitic stainless steels by teaching to reduce the contents of these alloys in carbon and nitrogen and to add copper. Thus, although having lower indices than those of alloys 1.4456, 1.4452, UNS S29225 and UNS S29108, the alloys proposed have, however, indices superior to those of alloys 1.3816 and 1.3815, which is sufficient to allow them to pass with success salt spray corrosion tests. In addition, compared with the 1.4456, 1.4452, UNS S29225 and UNS S29108 alloys which undergo a melting and solidification step under a nitrogen overpressure, the first, second and fourth examples of compositions according to the invention exhibit pressure austenitic solidification. atmospheric, thus avoiding the use of special installations. This therefore reduces the cost of the alloys obtained.

Finally, the position of these different alloys on the Schaeffler diagram is illustrated on the figure 6 . The four examples of preferred compositions, like the other alloys presented, are all in the austenitic range of the diagram. This confirms if necessary the stability of the austenitic structure for the compositions according to the invention. It can be seen further that the examples of compositions are between 1.3816 / 1.3815 alloys (which have too low corrosion resistance) and 1.4456 / 1.4452 / UNS S29225 / UNS S29108 alloys (which are very difficult to shape by machining and forging, and whose cost is high because products under nitrogen overpressure).

It goes without saying that the present invention is not limited to the embodiments which 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 appended claims. It should be noted in particular that the alloys proposed have an excellent compromise between corrosion resistance, ease of shaping (machinability and forgeability) and density of the parts after sintering. It is indeed possible to sinter the parts at low nitrogen pressure and to compensate with hydrogen. On the other hand, in the case of composite materials with a metal matrix, the metal matrix can be produced using the steel compositions according to the invention. It is also possible to treat the sintered parts under high isostatic pressure, also known by its English name High Isostatic Pressure. It is It is also possible to sinter under high pressure isostatic parts shaped by pressing or injection molding. It is also possible to make semi-finished products under high isostatic pressure. Finally, it is possible to forge the pieces after sintering.

Claims (11)

1. 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 < Cu < 4%;

   - carbon in amounts of 0.15 < C < 1%;

   - nitrogen in amounts of 0 < N ≤ 1, and

   - nickel in amounts of 0 ≤ Ni < 0.5%,

   - silicon in amounts of 0 ≤ Si < 2%,

   - tungsten in amounts of 0 ≤ W < 4%,

   - aluminium in amounts of 0 ≤ Al < 3%

   the remainder being formed by iron and any impurities from the melt,
   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 ≤ Mn < 20%,
   the remainder being formed by iron and any impurities from the melt.
2. Nickel-free austenitic stainless steel according to claim 1, characterized in that 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 < Cu < 4%;

   - carbon in amounts of 0.15% < C < 1%;

   - nitrogen in amounts of 0 < N ≤ 1;

   - silicon in amounts of 0 ≤ Si < 2%,

   - nickel in amounts of 0 ≤ Ni < 0.5%,

   - tungsten in amounts of 0 ≤ W < 4%,

   - aluminium in amounts of 0 ≤ Al < 3%, and

   the remainder formed by iron and any impurities from the melt.
3. Nickel-free austenitic stainless steel according to any of claims 1 or 2, characterized in that the composition thereof, expressed in mass percent, is given by the formula Fe-17Cr-11Mn-2Mo-1Cu-0.25C-0.4N.
4. Nickel-free austenitic stainless steel according to any of claims 1 or 2, characterized in that the composition thereof, expressed in mass percent, is given by the formula Fe-17Cr-12Mn-2Mo-2Cu-0.33C-0.4N.
5. Nickel-free austenitic stainless steel according to any of claims 1 or 2, characterized in that the composition thereof, expressed in mass percent, is given by the formula Fe-17Cr-14.5Mn-2Mo-2Cu-0.22C-0.35N.
6. Nickel-free austenitic stainless steel according to any of claims 1 or 2, characterized in that the composition thereof, expressed in mass percent, is given by the formula Fe-17Cr-17Mn-2Mo-1Cu-0.3C-0.5N.
7. Nickel-free austenitic stainless steel according to any of claims 1 to 6, characterized in that said steel comprises mass percentages of copper in amounts of 0.5 ≤ Cu < 4%.
8. Nickel-free austenitic stainless steel according to any of claims 1 to 7, characterized in that said steel comprises mass percentages of carbon in amounts of 0.2 ≤ C < 1%.
9. Nickel-free austenitic stainless steel according to any of claims 1 to 8, characterized in that said steel comprises mass percentages of molybdenum in amounts of 1 ≤ Mo ≤ 2%.
10. Nickel-free stainless steel according to any of claims 1 to 9, characterized in that said 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 in a mass concentration of up to 1%.
11. Timepieces and pieces of jewellery made of nickel-free austenitic stainless steel according to any of claims 1 to 10.

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