Patent Application
20170088923
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.
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.
Nickel-free austenitic stainless steel compositions are advantageous for applications in the fields of watchmaking and jewellery since they are non-magnetic and hypoallergenic.
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.
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.
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.
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 Böhler 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
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.
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.
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.
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.
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.
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.
There are two possible ways of producing a nickel-free austenitic steel.
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.
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.
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.
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.
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.
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).
To this end, the present invention concerns a nickel-free austenitic stainless steel comprising in mass percent:
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.
According to another feature of the invention, the nickel-free austenitic stainless steel comprises in mass percent:
the remainder being formed by iron and any impurities from the melt.
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%.
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.
“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.
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.
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.
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.
According to yet another feature of the invention, the nickel-free austenitic stainless steel comprises mass percentages of carbon in amounts of 0.2≦C<1%
According to yet another feature of the invention, the nickel-free austenitic stainless steel comprises mass percentages of molybdenum in amounts of 1≦Mo≦2%.
Preferred examples of compositions are given by the following formulae:
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.
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.
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.
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.
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.
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.
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.
Secondly, the forgeability of the alloys of the invention is also improved.
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.
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.
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.
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:
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.
For the first example of composition, whose phase diagram is illustrated in
For the second example of composition illustrated in
For the third example of composition illustrated in
The fourth example of composition illustrated in
The table illustrated in
MARC=Cr(%)+3.3Mo(%)+20C(%)+20N(%)−0.5Mn(%)−0.25Ni(%).
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.
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.
Finally, the position of these different alloys on the Schaeffler diagram is illustrated in
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15186980.7 | Sep 2015 | EP | regional |
