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≦Cu<4%;carbon in amounts of 0.15<C<<%;nitrogen in amounts of 0<N≦1, andthe remainder being formed by iron and any impurities from the melt.

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 re...

AI Technical Summary

Benefits of technology

The invention relates to new nickel-free austenitic stainless steels with improved properties. The compositions of the invention include specific amounts of copper, which has been added as an impurity to promote the austenitic structure and enhance the corrosion resistance of the alloys. The presence of copper also improves the machinability and forgeability of the steels. However, the concentration of copper should not exceed 4% to avoid making the steel brittle at high temperatures. Other important elements in the compositions, such as chromium, can decrease the level of strain hardening, facilitate shaping by deformation, and improve corrosion resistance.

Problems solved by technology

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.

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.

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.

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

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.

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.

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.

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.

Indeed, as shown with the alloys 1.3816 and 1.3815, chromium alone produces insufficient corrosion resistance for external timepiece components.

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.

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.

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.

There are, however, constraints that affect the compositions of alloys that can be cast at ambient atmospheric pressure.

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