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 0.5Cu<4%; carbon in amounts of 0.15<C<<%; nitrogen in amounts of 0<N1, and
the remainder being formed by iron and any impurities from the melt.

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 0.5Cu<4% carbon in amounts of 0.15<C<1%; nitrogen in amounts of 0<N1, and nickel in amounts of 0Ni<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 15Mn<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 0.5Cu<4%; carbon in amounts of 0.15%<C<1%; nitrogen in amounts of 0<N1; silicon in amounts of 0Si<2%, nickel in amounts of 0Ni<0.5%, tungsten in amounts of 0W<4%, aluminium in amounts of 0Al<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 0.5Cu<4%.

12. The nickel-free austenitic stainless steel according to claim 2, comprising mass percentages of copper in amounts of 0.5Cu<4%.

13. The nickel-free austenitic stainless steel according to claim 1, comprising mass percentages of carbon in amounts of 0.2C<1%.

14. The nickel-free austenitic stainless steel according to claim 2, comprising mass percentages of carbon in amounts of 0.2C<1%.

15. The nickel-free austenitic stainless steel according to claim 11, comprising mass percentages of carbon in amounts of 0.2C<1%.

16. The nickel-free austenitic stainless steel according to claim 12, comprising mass percentages of carbon in amounts of 0.2C<1%.

17. The nickel-free austenitic stainless steel according to claim 1, comprising mass percentages of molybdenum in amounts of 1Mo2%.

18. The nickel-free austenitic stainless steel according to claim 2, comprising mass percentages of molybdenum in amounts of 1Mo2%.

19. The nickel-free austenitic stainless steel according to claim 11, comprising mass percentages of molybdenum in amounts of 1Mo2%.

20. The nickel-free austenitic stainless steel according to claim 12, comprising mass percentages of molybdenum in amounts of 1Mo2%.

21. The nickel-free austenitic stainless steel according to claim 13, comprising mass percentages of molybdenum in amounts of 1Mo2%.

22. The nickel-free austenitic stainless steel according to claim 14, comprising mass percentages of molybdenum in amounts of 1Mo2%.

23. The nickel-free austenitic stainless steel according to claim 15, comprising mass percentages of molybdenum in amounts of 1Mo2%.

24. The nickel-free austenitic stainless steel according to claim 16, comprising mass percentages of molybdenum in amounts of 1Mo2%.

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

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 C. and 1050 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 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 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.