CH716937A2 - Paramagnetic hard stainless steel and its manufacturing process. - Google Patents

Abstract

The present invention relates to a paramagnetic stainless steel with a chemical composition comprising by weight: 26 ≤ Cr ≤ 40%, 5 ≤ Ni ≤ 20%, 0 ≤ Mn ≤ 5%, 0 ≤ Al ≤ 5%, 0 ≤ Mo ≤ 3%, 0 ≤ Cu ≤ 2%, 0 ≤ Si ≤ 5%, 0 ≤ Ti ≤ 1%, 0 ≤ Nb ≤ 1%, 0 ≤ C ≤ 0.1%, 0 ≤ N ≤ 0.1%, 0 ≤ S ≤ 0.5 %, 0 ≤ P ≤ 0.1%, the balance being constituted by iron and any impurities each having a content less than or equal to 0.5%, said steel having an HV10 hardness of between 500 and 900. It also relates to a part in particular a watch component made from this steel and the part manufacturing process.

Description

Field of the invention

The invention relates to a paramagnetic stainless steel having a hardness between 500 and 900 HV and the part including the watch component made of this steel. It also relates to the manufacturing process of this stainless steel part.

Background of the invention

[0002] Hard and non-ferromagnetic metal alloys find applications in many fields, mainly for components subjected to strong mechanical and / or tribological stresses and having to remain insensitive to magnetic fields. This is particularly the case for many watch components, such as for example wheels, pinions, axles or even springs at the level of the movement. For the external cladding, it is also advantageous to obtain high hardnesses, for example for the caseband, the bezel, the back or even the crown. Indeed, a high hardness generally makes it possible to obtain a very good polishability for a quality aesthetic appearance, as well as a better resistance to scratches and wear, and therefore a good durability of these components exposed to the environment. outside.

In metallurgy, different mechanisms make it possible to harden the alloys, depending on their chemical compositions and their thermomechanical histories. Thus, solid solution hardening, structural hardening, work hardening, martensitic transformation in steels, spinodal decomposition, or even hardening by reduction of the grain size (Hall Petch) are known. In the most remarkable alloys, several of these hardening mechanisms are taken advantage of simultaneously. However, non-ferromagnetic alloys which have hardnesses greater than 500 HV are rare. In addition, to achieve such a level of hardness, crystalline non-ferromagnetic alloys generally require a high degree of work hardening, before a possible heat treatment aimed at obtaining the maximum hardness by precipitation of second phases. This is the case, for example, with austenitic stainless steels, which can only be hardened by work hardening, or even with a few austenitic superalloys, which can be hardened by work hardening followed by a heat treatment of precipitation. In practice, the manufacture of components in these alloys in the strain-hardened state is difficult. First of all, in the case of forming by forging, obtaining the right degree of work hardening to obtain the required hardness is not trivial, especially for parts with complex geometry. As an alternative, it is possible to machine semi-finished products with a defined and homogeneous degree of hardening, but it is not always easy to obtain the right material formats with the required degree of hardening. In addition, any machining operations are very difficult and expensive, because the alloy is already in the hardened state, at least partially. Finally, if the process used does not involve plastic deformation, such as certain powder metallurgy or additive manufacturing processes, it is simply not possible to harden these alloys. As an alternative, it would be possible to manufacture alloys intrinsically having a hardness greater than 500 HV, such as certain high entropy alloys or certain intermetallics for example, but they would again be very difficult to machine and almost impossible to deform, due to their hardness. very high and very low ductility. We can thus understand the interest of finding an alloy which can be hardened by heat treatment without the need for prior work hardening, while being non-ferromagnetic in the hardened state. The forming would thus take place in a soft and ductile state, and a hardening heat treatment would be carried out once the part is finished. This explains in particular the enormous success of carbon steels and martensitic stainless steels, but the latter are unfortunately ferromagnetic.

To obtain hardnesses greater than 500 HV in non-ferromagnetic alloys, other solutions are widely used today. Different surface hardening processes are applied in particular to austenitic stainless steels or titanium alloys, for example, after shaping of the parts. However, the thickness of the hardened layer is generally very small, of the order of a few tens of micrometers, and the surface appearance is generally modified by the treatment. For watch components, it is therefore necessary to rework the parts after hardening to obtain a clean and generally polished surface. However, these termination operations eliminate all or part of the hardened layer and this solution is therefore little used in practice, especially since the surface hardening treatments are generally expensive.

[0005] Once again, we therefore understand the need to find a non-ferromagnetic alloy which can be hardened by heat treatment at 500-900 HV, the hardness range in which carbon steels in the quenched and tempered state are generally located, martensitic stainless steels, some highly work-hardened austenitic stainless steels or some work-hardened and heat-treated austenitic superalloys.

Summary of the invention

The object of the present invention is to provide a stainless steel composition optimized to obtain paramagnetic behavior and a hardness of between 500 and 900 HV10 and this by heat treatment without prior hardening required during the manufacturing process.

The composition according to the invention is as follows by weight: - 26 ≤ Cr ≤ 40%, - 5 ≤ Ni ≤20%, - 0 ≤ Mn ≤ 5%, - 0 ≤ Al ≤ 5%, - 0 ≤ Mo ≤ 3%, - 0 ≤ Cu ≤ 2%, - 0 ≤ Si ≤ 5%, - 0 ≤ Ti ≤ 1%, - 0 ≤ Nb ≤ 1%, -0≤C≤0.1%, - 0≤ N ≤ 0.1%, - 0 ≤ S ≤ 0.5%, - 0 ≤ P ≤ 0.1%, the balance consisting of iron and any impurities each having a content less than or equal to 0.5%.

According to the invention, the method of manufacturing a stainless steel part consists in carrying out a first heat or thermomechanical treatment on a base material of the aforementioned composition in the ferritic or ferritic-austenitic field and then in quenching the material in order to keep the ferritic or ferritic-austenitic structure at room temperature. This ferritic or ferritic-austenitic microstructure is soft and therefore ductile, which allows easy shaping. Then, after possible shaping, a hardening treatment is carried out in order to transform the ferrite into an austenitic phase and into a sigma intermetallic phase rich in chromium.

The originality of the present invention comes in particular from the use of the sigma phase as a source of hardening, since this phase has always been considered to be harmful and therefore unwanted in stainless steels. Indeed, as the sigma phase is rich in chromium and that it generally forms at the grain boundaries, it drastically reduces the corrosion resistance by reducing the chromium concentration of the other phases present in the alloy. Then, it weakens stainless steels very quickly and strongly, even in very small quantities. Indeed, this phase having a complex tetragonal structure, it is intrinsically very fragile and its presence at the grain boundaries creates a privileged path for the propagation of cracks. It has therefore never been put to good use in stainless steels, despite its two particularly interesting properties which are its hardness between 900 and 1100 HV10 and its paramagnetic character.

According to the invention, the composition of the stainless steel and the process are optimized in order to obtain a fine distribution of both the sigma phase and the austenitic phase without preferred formation of the sigma phase at the grain boundaries. This particular microstructure consisting of two non-ferromagnetic phases makes it possible in particular to obtain a very good compromise between hardness and toughness, good corrosion resistance and excellent polishability.

Other characteristics and advantages of the present invention will become apparent on reading the detailed description below with reference to the following figures.

Brief description of the figures

Figures 1 and 2 show the diffractogram of a steel Fe-35% Cr-9% Ni (% by weight) according to the invention respectively before and after the hardening treatment. FIG. 3 represents an image obtained by optical microscopy of an Fe-32% Cr-9% Ni (% by weight) steel according to the invention. FIG. 4 represents the magnetic hysteresis curve for this same alloy.

Description of the invention

The invention relates to paramagnetic stainless steels having a hardness of between 500 and 900 HV10 as well as the process for manufacturing parts made with these steels. By hardness HV10 is meant a Vickers hardness measured according to the ISO 6507-1: 2018 standard. The invention also relates to a part and more precisely to a watch component made with this steel. It may be a covering component chosen from the non-exhaustive list comprising a caseband, a back, a bezel, a crown, a push-piece, a bracelet link, a bracelet, a pin buckle, a dial, a needle and a dial index. It may also be a component of the movement chosen from the non-exhaustive list comprising a toothed wheel, an axis, a pinion, a spring, a bridge, a plate, a screw and a balance.

The stainless steels according to the invention have the following composition by weight: - 26 ≤ Cr ≤ 40%, -5 ≤ Ni ≤20%, - 0 ≤ Mn ≤ 5%, - 0 ≤ Al ≤ 5%, - 0 ≤ Mo ≤ 3%, - 0 ≤ Cu ≤ 2%, - 0≤ Si ≤ 5%, - 0 ≤ Ti ≤ 1%, - 0≤ Nb ≤ 1%, - 0≤C≤0.1%, - 0≤ N≤0.1%, - 0 ≤ S ≤ 0.5%, - 0≤P≤0.1%, the balance consisting of iron and any impurities each having a content less than or equal to 0.5%.

Preferably, they have the following composition by weight: 28 ≤ Cr≤ 38%, 5 ≤ Ni ≤ 15%, 0≤ Mn≤ 3%, 0 ≤ Al ≤ 3%, 0 ≤ Mo ≤ 3%, 0 ≤ Cu ≤ 2%, 0 ≤ Si ≤ 5%, 0 ≤ Ti ≤ 1%, 0≤ Nb≤ 1%, 0 ≤ C ≤ 0.05%, 0 ≤ N ≤ 0.05%, 0 ≤ S ≤ 0.5%, 0 ≤ P ≤ 0.1%, always with a balance consisting of iron and any impurities each having a content less than or equal to 0.5%.

More preferably, they have the following composition by weight: 30 ≤ Cr ≤ 36%, 5 ≤ Ni ≤ 10%, 0 ≤ Mn ≤ 3%, 0 ≤ Al ≤ 1%, 0 ≤ Mo ≤ 1%, 0 Cu ≤ 1%, 0 ≤ Si ≤ 3%, 0≤ Ti ≤ 1%, 0 ≤ <> Nb ≤ <> 1%, 0 ≤ C ≤ 0.05%, 0 ≤ N ≤ 0.05%, 0 ≤ S ≤ 0.5%, 0 ≤P ≤0.1%, always with the same iron balance and possible impurities.

According to the invention, the method of manufacturing a stainless steel part comprises a step a) of providing or producing a blank having a composition falling within the aforementioned ranges. This blank has a predominantly ferritic or, preferably, 100% ferritic structure. The blank is obtained from a base material subjected to a thermal or thermomechanical treatment at a temperature in the range 950 ° C-1450 ° C followed by quenching. The base material can be in the form of a powder or a consolidated material. It can be carried out by casting, by pressing, by injection molding (MIM: Metal Injection Molding), by additive manufacturing, and more broadly by powder metallurgy. It is conceivable to carry out the base material and the heat treatment in a single step, for example, by an additive manufacturing technique with laser (SLM: Selective Laser Melting). These different techniques make it possible to produce a blank with a base material having dimensions substantially equal to those of the part to be produced, in which case a subsequent shaping step is not required.

The composition of the base material is optimized to obtain a predominantly or completely ferritic structure when maintaining the temperature between 950 ° C and 1450 ° C for a time between 1 minute and 24 hours. The temperature is chosen to obtain a mass fraction of austenite less than or equal to 40% and a mass fraction of ferrite greater than or equal to 60%. The presence of austenite makes it possible to obtain minimum hardness and maximum ductility to allow easy shaping, for example by forging, cutting or machining.

The thermal or thermomechanical treatment in the range 950 ° C-1450 ° C can be used to carry out homogenization, recrystallization or stress relieving treatments on base materials obtained by casting or to achieve sinterings on basic materials in powder form. The treatment in the ferritic or ferritic-austenitic field can be carried out in a single cycle or include several cycles of thermal or thermomechanical treatment. It can also be preceded or followed by other thermal or thermomechanical treatments.

After maintaining in the ferritic or ferritic-austenitic range, the blank is subjected to rapid cooling, also referred to as quenching, to a temperature below 500 ° C so as to avoid the formation of new phases during the cooling. Thus, the ferritic or ferritic-austenitic structure is preserved at room temperature. Thanks to the compositions according to the invention, the ferritic structure is sufficiently stable to be stored at room temperature after rapid cooling but sufficiently metastable to be easily and rapidly transformed into sigma phase and into austenite during a subsequent heat treatment at intermediate temperatures. between 650 ° C and 900 ° C.

At the end of step a), the alloy exhibits low hardness and high ductility, where appropriate, allowing easy shaping, for example by forging, by cutting or by machining.

After step a), the method comprises an optional step b) of shaping the blank by machining, cutting or by any operation involving a deformation such as forging. This step can be carried out in several sequences. This step is not required if the blank at the end of step a) already has the final shape of the part to be manufactured.

In addition to shaping, a plastic deformation operation can be used to increase in particular the speed of transformation of the ferrite during the subsequent step of transformation of the ferrite into austenite and in the sigma phase. In addition, since the hardening by strain-hardening is low for ferritic structures and the alloy according to the invention is predominantly or totally ferritic before the hardening treatment, this plastic deformation step does not give rise to problematic hardening for a setting. possible shape by machining or cutting. This plastic deformation in one or more sequences can be carried out at a temperature below 650 ° C.

After the possible shaping, the method comprises a step c) of heat treatment of hardening of the blank between 650 ° C and 900 ° C to obtain the final properties. The duration of the heat treatment between 650 ° C and 900 ° C is set so as to guarantee a total transformation of the ferrite and therefore to obtain a microstructure formed of a sigma phase and an austenitic phase.

The speed of transformation of the ferrite into austenite + sigma phase depends in particular on the composition of the alloy and on its thermomechanical history as mentioned above. Generally, the duration of treatment is between 30 minutes and 24 hours. After the hardening treatment, the steel has a sigma phase mass fraction of between 40% and 80% and a mass fraction of austenite between 20% and 60%, the percentages depending on the chemical composition and the heat treatments. made. The part thus obtained has a high hardness of between 500 and 900 HV10 thanks to the hardening heat treatment. As with all stainless steels, possible non-metallic inclusions can also be present in small quantities, without this affecting the mechanical and magnetic properties. In addition, inclusions making it possible to improve the machinability, such as, for example, manganese sulphides, can also be present in small quantities in the alloy.

This hardening heat treatment step can be followed by a possible step d) of surface finishing such as polishing.

Furthermore, in the presence of a blank with an austenite + ferrite structure in step a), the manufacturing process may include an additional step b ') before the hardening heat treatment, in the temperature range 950 ° C-1450 ° C to transform the austenite + ferrite structure into a 100% ferritic structure.

In summary, after the heat treatment at high temperature (950 ° C-1450 ° C) followed by quenching, the steels have the following properties in particular: • Hardness between 150 and 400 HV10. • Good ductility with plastic deformation without cracking greater than 50% in compression at room temperature. • Ferromagnetic behavior, due to the presence of ferrite.

After the hardening heat treatment, the steels according to the invention exhibit the following properties in particular: • Hardness between 500 and 900 HV10. • Paramagnetic behavior. • Excellent polishability, thanks to the very fine microstructure. • Good resistance to wear. • Good corrosion resistance.

Regarding corrosion resistance, the steel according to the invention is particularly efficient thanks to the high concentration of chromium. These steels are therefore particularly advantageous for covering components.

Finally, the invention is illustrated with the aid of the examples below.

Examples

In a first example, the steel named Fe35Cr9Ni contains in weight percentages 56% iron, 35% chromium and 9% nickel. It was made by arc fusion from high purity elements (> 99.9%), deformed at room temperature by compression with a reduction in thickness by a factor of 2 and subjected to heat treatment of homogenization in the ferritic range at 1300 ° C for 2 hours in an argon atmosphere followed by gas quenching (approx. 200K / min). After this homogenization heat treatment, the Fe35Cr9Ni alloy exhibits a single-phase ferritic microstructure with a Vickers hardness of 350 HV10. The totally ferritic structure (Im3m space group) is confirmed by X-ray diffractometry (XRD) analysis as shown in Figure 1. After homogenization, a hardening heat treatment was performed at 800 ° C for 6 hours. A fine, homogeneous and two-phase microstructure comprising the austenitic phase and the sigma phase is obtained. X-ray diffractometric analysis shown in Figure 2 confirms the presence of austenite (space group Fm3m) and a tetragonal structure corresponding to the sigma phase (space group P42 / mnm)

In this metallurgical state, the Fe35Cr9Ni alloy has a Vickers hardness of 670 HV10. Its corrosion resistance was evaluated by performing a salt spray test according to ISO 9227. After the test, the alloy shows no signs of corrosion, demonstrating its excellent resistance to corrosion in a salt environment. This is all the more remarkable given that the presence of the sigma phase, even in a small proportion, has always caused a strong reduction in corrosion resistance in stainless steels.

In a second example, the steel named Fe32Cr9Ni contains in weight percentages 59% iron, 32% chromium and 9% nickel. It was also manufactured by arc fusion from high purity elements (> 99.9%), subjected to a homogenization heat treatment at 1300 ° C for 2 hours under argon followed by gas quenching, deformed at room temperature by compression with a reduction in thickness by a factor of 2, subjected to a recrystallization heat treatment at 1200 ° C. in air for 1 minute followed by quenching in water. After this recrystallization heat treatment, the Fe32Cr9Ni alloy exhibits a single-phase ferritic microstructure with a Vickers hardness of 220 HV10. Then it was brought to 700 ° C for 6 hours under vacuum. The microstructure observed by optical microscopy under polarized light is shown in FIG. 3. A fine distribution of the two phases is observed with the austenitic phase in relief and the sigma phase in a matrix. In this metallurgical state, the Fe32Cr9Ni alloy has a Vickers hardness of 635 HV10. Regarding the magnetic properties of this steel, the hysteresis curve was measured at room temperature with a vibrating sample magnetometer (magnetization M as a function of the applied field H). Although exhibiting a relatively high volume susceptibility, this steel exhibits a linear behavior signature of the paramagnetic behavior (fig. 4).

Claims (19)

1. Paramagnetic stainless steel with a chemical composition comprising by weight: - 26 ≤ Cr ≤ 40%, - 5 ≤ Ni ≤20%, - 0 ≤ Mn ≤ 5%, - 0 ≤ Al≤ 5%, - 0 ≤ Mo ≤ 3%, - 0 ≤ Cu ≤ 2%, - 0 ≤ Si ≤ 5%, - 0 ≤ Ti ≤ 1%, - 0 ≤ Nb ≤ 1%, - 0 ≤ C ≤ 0.1%, - 0 ≤ N ≤ 0.1%, - 0 ≤ S ≤ 0.5%, - 0 ≤ P ≤ 0.1%, the balance consisting of iron and any impurities each having a content less than or equal to 0.5%, said steel having an HV10 hardness of between 500 and 900. 2. Steel according to claim 1, with a chemical composition comprising by weight: - 28 ≤ Cr ≤ 38%, - 5 ≤ Ni ≤ 15%, - 0 ≤ Mn ≤ 3%, - 0 ≤ Al ≤ 3%, - 0 ≤ Mo ≤ 3%, - 0 ≤ Cu ≤ 2%, - 0 ≤ Si ≤ 5%, - 0 ≤ <> Ti ≤ <> 1%, - 0 ≤ Nb ≤ 1%, - 0 ≤ C ≤ 0.05%, - 0 ≤ N ≤ 0.05%, - 0 ≤ S ≤ 0.5%, - 0 ≤ P ≤ 0.1%, 3. Steel according to claim 1 or 2, with a chemical composition comprising by weight: - 30 ≤ Cr ≤ 36%, - 5 ≤ Ni ≤ 10%, - 0 ≤ Mn ≤ 3%, - 0 ≤ Al ≤ 1%, - 0 ≤ Mo≤ 1%, - 0 ≤ Cu ≤ 1%, - 0 ≤ Si ≤ 3%, - 0 ≤ Ti ≤ 1%, - 0 ≤ Nb ≤ 1%, - 0 ≤ C ≤ 0.05%, - 0 ≤ N ≤ 0.05%, - 0 ≤ S ≤ 0.5%, - 0 ≤ P ≤ 0.1%, 4. Steel according to one of the preceding claims, characterized in that it has a microstructure formed of a sigma phase comprised in a percentage by mass of between 40 and 80% and of an austenitic phase comprised in a percentage by mass of between 20 and 60%. 5. Part made of steel according to one of the preceding claims. 6. Part according to claim 5, characterized in that it is a watch component covering or movement. 7. Watch comprising the horological component according to claim 6. 8. A method of manufacturing a paramagnetic stainless steel part, characterized in that it comprises the following successive steps: a) Provision or production of a blank having substantially the shape of the part to be manufactured or being of a different shape, the blank having a chemical composition according to one of claims 1 to 3 and having a predominantly or completely ferritic structure , b) Shaping of the blank if said blank from step a) has a shape different from the part to be manufactured, c) Heat treatment, called hardening treatment, of the blank to obtain the part, the hardening treatment being carried out at a temperature between 650 and 900 ° C for a time between 30 minutes and 24 hours to transform the ferrite of said structure in an austenitic phase and a sigma intermetallic phase, the hardening treatment being followed by cooling to room temperature. 9. The manufacturing method according to claim 8, characterized in that the predominantly or completely ferritic structure of the blank in step a) has been produced by carrying out a thermal or thermomechanical treatment on a base material at a temperature between 950 and 1450 ° C for a time of between 1 minute and 24 hours, the thermal or thermomechanical treatment being followed by quenching to a temperature below 500 ° C to keep the ferritic structure at room temperature. 10. Manufacturing process according to the preceding claim, characterized in that the base material is in the form of a powder or of a consolidated material. 11. The manufacturing method according to claim 9 or 10, characterized in that the base material has been obtained by casting, by pressing, by injection molding, by additive manufacturing or by powder metallurgy. 12. The manufacturing method according to claim 8, characterized in that the blank of step a) is produced by additive manufacturing with laser. 13. Manufacturing process according to one of claims 8 to 12, characterized in that the structure of the blank in step a) comprises a mass fraction of austenite less than or equal to 40% and a mass fraction of ferrite. greater than or equal to 60%. 14. The manufacturing method according to one of claims 8 to 13, characterized in that the structure of the blank in step a) comprises 100% ferrite. 15. The manufacturing method according to one of claims 8 to 14, characterized in that, at the end of step a), the blank has a hardness of between 150 and 400 HV10. 16. The manufacturing method according to one of claims 8 to 15 characterized in that the shaping step b) comprises one or more plastic deformation sequences at a temperature below 650 ° C. 17. The manufacturing method according to one of claims 8 to 16, characterized in that the shaping step b) is carried out by forging, cutting or machining. 18. The manufacturing method according to claim 9, characterized in that the heat or thermomechanical treatment is carried out in several cycles. 19. The manufacturing method according to claim 13, characterized in that, when the structure comprises austenite, the method comprises, before step c), a step b ') of heat or thermomechanical treatment on the blank to a temperature between 950 ° C and 1450 ° C for a time between 1 minute and 24 hours to obtain a completely ferritic structure, the thermal or thermomechanical treatment being followed by quenching to a temperature below 500 ° C for keep the completely ferritic structure at room temperature.