Paramagnetic hard stainless steel and its manufacturing method

Abstract

Stainless steel exhibits paramagnetic behavior and has a hardness of 500 to 900 HV10. provide. The present invention provides a composition comprising 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. The chemical composition of the material is 1% by weight, with the remainder being iron and impurities. The content of each element in the above is 0.5% or less. The steel has a hardness of 500 to 900 HV10. The present invention further provides a part made of this steel. The present invention relates to a component for a timepiece, and to a method for manufacturing the component.

Description

[Technical Field]

[0001] The present invention relates to a paramagnetic stainless steel having a hardness of 500 to 900 HV and a method for manufacturing a magnetically-insulated magnetic material using the same. The present invention further relates to a component, in particular a component for a timepiece, made of the above material. The present invention relates to a method for manufacturing a stainless steel part. [Background technology]

[0002] Hard non-magnetic metal alloys are used in many fields and are basically used for large mechanical Components that are subject to mechanical and / or tribological stress and are required to be immune to magnetic fields. This is especially true for the wheels, pinions, and shafts at the movement level. This is the case for many timepiece components such as shafts and springs. High hardness for components such as the case middle, bezel, back, and crown In fact, high hardness generally means that the material is more resistant to external environments. Excellent polishability for a high-quality aesthetic appearance of those exposed components, It also provides excellent resistance to scratches and abrasions, and therefore excellent durability. do.

[0003] In metallurgy, various methods are used to harden alloys, depending on the alloy's chemical composition and thermodynamic history. Various mechanisms are used. Thus, solid solution hardening, structural hardening, cold working, martensitic hardening of steels, Site transformation, spinodal decomposition, and even hardening due to grain size reduction (Hall Petch) are known. In the most notable alloys, several of these hardening mechanisms are used simultaneously. However, non-ferromagnetic alloys with hardness greater than 500 HV are rare. To achieve this level of hardness, crystalline non-ferromagnetic alloys generally rely on the precipitation of second phases. Therefore, a large degree of cold working is required before any optional heat treatment to obtain maximum hardness. This is the case, for example, for austenitic steels that are only suitable for hardening by cold working. Stainless steels, some austenitic superalloys suitable for hardening by precipitation heat treatment after cold working In fact, components made from these alloys in the cold-worked condition First, in the case of forging, it is difficult to obtain the required hardness. Achieving the proper degree of cold work is not a simple task, especially for parts with complex geometries. Instead, machining is performed on a semi-finished product with a defined, homogeneous degree of cold work. However, it is not always possible to obtain the appropriate material format with the required degree of cold work. Also, since the alloy is already in at least a partially hardened state, Finally, certain powder metallurgy or additive manufacturing processes are If the method used does not involve plastic deformation, such as manufacturing methods, then these alloys can simply be hardened. Instead, it is not possible to achieve this, as in certain high-entropy alloys and certain intermetallic alloys. Although it is possible to produce alloys with inherent hardness greater than 500 HV, They are also very difficult to machine and practically impossible to deform. This is due to their very high hardness and very low ductility. It is non-ferromagnetic in the hardened state, yet hardens by heat treatment without the need for prior cold working. Thus, the advantages of finding an alloy that is suitable for forming soft materials can be seen. This is done in a tough, ductile state, and the part is then hardened by a heat treatment after completion. This explains the great success of carbon steels and martensitic stainless steels in particular, Unfortunately, martensitic stainless steel is ferromagnetic.

[0004] To obtain hardness greater than 500 HV in non-ferromagnetic alloys, other solutions are currently available. After forming the parts, especially austenitic stainless steels and Various surface hardening processes are used on titanium alloys. However, the thickness of the hardened layer varies. Generally, they are very small, on the order of a few tens of microns, and the surface appearance is generally Therefore, in the case of timepiece components, a clean, whole surface is required after curing. To achieve a polished surface, the part must be reworked. The lifting operation removes all or part of the case layer, and therefore this technique is particularly useful for surface hardening. In practice, this method is rarely used due to its generally high cost.

[0005] Again, a non-ferromagnetic alloy suitable for hardening to 500-900 HV by heat treatment is required. In this hardness range, it is generally hardened and strong. Carbon steels in the hardened condition, martensitic stainless steels, and certain heavily hardened stainless steels The alloys found in the market are either stainless steels or some cold worked and heat treated austenitic superalloys. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention provides a method for producing paramagnetic alloys by heat treatment without requiring prior cold working during the manufacturing process. Behavior and Optimized Stainless Steel Compositions for Hardness of 500-900 HV10 Regarding. [Means for solving the problem]

[0007] The composition according to the present 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 remainder is made up of iron and impurities, each of which contains less than 0.5%.

[0008] According to the invention, the method for producing a stainless steel part is a ferritic or ferrite method. - a first heat treatment or thermomechanical treatment on the base material of the above composition within the austenitic range and then, at ambient temperature, ferritic or ferritic-austenitic This involves hardening the material so that it retains its structure. Austenitic microstructures are soft and therefore ductile, making them easy to form. After optional forming, the ferrite can be mixed with the austenite. The hardening treatment is carried out so as to transform the alloy into a chromium-rich intermetallic sigma phase and a chromium-rich intermetallic sigma phase.

[0009] The novelty of the present invention comes in particular from the use of the sigma phase as a hardening source, because phase has always been considered harmful and undesirable in stainless steels. In fact, since the sigma phase is chromium-rich and generally forms at grain boundaries, However, the corrosion resistance is significantly reduced by reducing the chromium concentration of other phases present in the alloy. And even a very small amount of sigma phase can cause a very rapid and substantial increase in the In fact, this complex tetragonal phase is very weak in nature. They are brittle and exist at grain boundaries, forming paths that facilitate crack propagation. It has two particularly advantageous properties: a hardness of 900-1100 HV10 and paramagnetic properties. Nevertheless, the sigma phase has never been used in stainless steel.

[0010] According to the present invention, the stainless steel composition and method favors the formation of sigma phase at grain boundaries. It should be optimized to obtain a fine distribution of both sigma and austenite phases. This particular microstructure, consisting of two non-ferromagnetic phases, provides excellent hardness, toughness, and It is possible to obtain a very good compromise between corrosion resistance and good polishability.

[0011] Further features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the drawings. It will become clear. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diffraction diagram of the Fe-35%Cr-9%Ni (wt. %) steel according to the present invention before hardening treatment. [Figure 2] FIG. 2 is a diffraction diagram of the Fe-35%Cr-9%Ni (wt. %) steel according to the present invention after hardening treatment. [Figure 3] 1 is an optical microscope image of the Fe-32%Cr-9%Ni (wt%) steel according to the present invention. [Figure 4] 1 is a magnetic hysteresis curve of the same alloy. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention relates to a paramagnetic stainless steel having a hardness of 500 to 900 HV10, and The HV10 hardness is based on the standard ISO6507-1:2018. Therefore, the measured Vickers hardness is shown. This relates to a part, especially a component for a timepiece. This part is a case middle Part, back, bezel, crown, push parts, wristlet link, wristlet, tongue A group consisting of a dial, hands, and indexes (not all inclusive) The outer part components may be selected from the following: The parts include gears, shafts, pinions, springs, bridges, plates, screws and balances. A movement component selected from the group (not all inclusive) consisting of: The system may be composed of components.

[0014] The stainless steel according to the present invention has 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 remainder is made up of iron and impurities, each of which contains less than 0.5%.

[0015] Preferably, the stainless steel according to the invention has 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% Similarly, the remainder is made up of iron and impurities, each of which contains less than 0.5%.

[0016] More preferably, the stainless steel according to the present invention has the following composition: - 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% Similarly, the remaining iron and any impurities are the same as above.

[0017] According to the invention, the method for producing a stainless steel part comprises the steps of: The method includes the step (a) of preparing or making a blank. The blank has a ferrite structure, preferably 100% ferrite structure. Substrates that have been subjected to a thermal or thermomechanical treatment at temperatures in the range of 0°C and subsequently cured The substrate can be in the form of a powder or a solid. The substrate can be cast, pre-cast, or can be made by laser, metal injection molding (MIM), additive manufacturing, and more broadly, powder metallurgy. The substrate can be prepared and heat-treated using, for example, selective laser melting (SLM) technology. These different technologies allow for: Creating a blank having a substrate having dimensions substantially equal to the dimensions of the part to be created This makes it possible to form the molded article without the need for a subsequent molding step.

[0018] The composition of the substrate is mostly or completely dissolved when held at a temperature of 950 to 1450°C for 1 minute to 24 hours. The temperature is optimized to obtain a completely ferritic structure. The weight ratio of austenite is selected to be 40% or less and ferrite to be 60% or more. The presence of pores allows for easy forming by forging, blanking, machining, etc. It is possible to obtain the minimum hardness and the maximum ductility such that

[0019] For homogenization, recrystallization or stress relief of base materials obtained by casting, or for powder To sinter the powdery substrate, a heat treatment or thermomechanical treatment is carried out in the range of 950 to 1450°C. Processes in the ferritic or ferritic-austenitic range can be used. The treatment can be carried out in a single cycle or can be carried out in multiple thermal or thermomechanical treatment cycles. It may also be preceded or followed by other thermal or thermomechanical treatments. It is also possible to do so.

[0020] After being held in the ferritic or ferritic-austenitic range, new To prevent the formation of unwanted phases, the blank is rapidly cooled to a temperature below 500°C. This is also called hardening. In this way, the ferritic or ferritic-austenitic structure The structure is kept at ambient temperature. Thanks to the composition according to the invention, the ferritic structure remains after quenching. It is stable enough to be kept at ambient temperature, and yet it can be heated to intermediate temperatures between 650 and 900°C. During subsequent heat treatment at 70°C, it readily and rapidly transforms into sigma phase and austenite. It is metastable enough to

[0021] After step (a), the alloy is hardened by forging, blanking, machining, etc. It has low hardness and high ductility, which allows it to be easily formed, where applicable.

[0022] After step (a), the method may further comprise a transformation such as machining, blanking, or forging. and an optional step (b) of forming a blank by any operation involving: The steps can be performed in multiple sequences. However, if you already have the final shape of the part you want to manufacture, this step is not necessary.

[0023] In addition to forming, a subsequent step transforms the ferrite into austenite and sigma phases During this time, plastic deformation operations can be used to enhance the transformation rate, especially of ferrite. Furthermore, in the case of ferritic structures, the hardening by cold working is low, and the hardening treatment is performed in accordance with the present invention. Since the alloys of interest are mostly or completely ferritic, this plastic deformation step is essential for the mechanical properties. Does not induce hardening which can cause problems with optional forming by machining or blanking This plastic deformation, which may be carried out in one or more sequences, may be carried out at a temperature below 650°C. This can be done.

[0024] After optional forming, the method includes subjecting the blank to a hardening heat treatment at 650°C to 900°C. and (c) a step of performing a heat treatment at 650 to 900°C to obtain the final properties. The time is fixed to ensure complete transformation of the ferrite, resulting in a sigma A microstructure containing austenite and Cr phases is obtained.

[0025] The rate of transformation of ferrite to austenite + sigma phase depends on the alloy composition, as mentioned above. The duration of the treatment generally ranges from 30 minutes to 24 hours. After hardening, the steel will have a weight fraction of 40-80% sigma phase and 0.5% austenite. 20-60%, these percentages depending on the chemical composition and the heat treatment carried out. The parts have a high hardness of 500 to 900 HV10 thanks to the hardening heat treatment. For all stainless steels, optional non-metallic additives can be added without affecting the mechanical and magnetic properties. Furthermore, it is possible to have a small amount of metal inclusions, such as manganese sulfide, which improves machinability. Small amounts of inclusions may also be present in the alloy to improve the strength of the alloy.

[0026] This hardening heat treatment step may optionally be followed by a surface finishing step (d), such as polishing. It can be done.

[0027] Furthermore, in step (a), the blank having an austenite + ferrite structure In the presence of, the manufacturing method further comprises, before the hardening heat treatment, performing a hardening heat treatment at a temperature in the range of 950 to 1450°C. Step (b) transforms the austenite + ferrite structure into a 100% ferrite structure. ') can be performed.

[0028] In summary, by hardening after heat treatment at high temperatures (950-1450°C), In particular, steel has the following properties: *Hardness of 150~400HV10. * Good plastic deformation without cracking greater than 50% under compression at ambient temperature. It has good ductility. * Exhibits ferromagnetic behavior due to the presence of ferrite.

[0029] After the hardening heat treatment, the steel according to the invention has, in particular, the following properties: *Hardness of 500~900HV10 * Exhibits paramagnetic behavior. * Due to its extremely fine structure, it exhibits excellent polishing properties. * Excellent wear resistance. *Excellent corrosion resistance.

[0030] With regard to corrosion resistance, the steel according to the invention is particularly effective thanks to its high chromium content. Such steels are therefore particularly advantageous for external part components.

[0031] Finally, the invention is illustrated by the following examples.

[0032] example In the first example, the steel designated Fe35Cr9Ni contains, by weight, 56% iron, It contains 35% chromium and 9% nickel, which are made from highly pure elements (>99.9%). Produced by arc melting and deformed by compression at ambient temperature to reduce thickness by a factor of two The alloy was then subjected to homogenization heat treatment in the ferrite region at 1300°C for 2 hours in an argon atmosphere. After this homogenization heat treatment, the specimen was gas hardened (approximately 200K / min). The alloy Fe35Cr9Ni has a single-phase ferritic structure with a Vickers hardness of 350 HV10. This perfect ferrite structure (space group Im3m) is clearly visible in X-ray diffraction as shown in Figure 1. This has been confirmed by diffraction (XRD) analysis. After homogenization, the material was heat-cured at 800°C for 6 hours. A fine and homogeneous two-phase microstructure containing austenite and sigma phases was obtained. X-ray diffraction analysis shown in Figure 2 revealed that austenite (space group Fm3m) and The presence of a tetragonal structure (space group P42 / mnm) corresponding to the sigma phase was confirmed.

[0033] In this metallic state, the alloy Fe35Cr9Ni has a Vickers hardness of 670 HV10. Its corrosion resistance was evaluated by a salt spray test according to the ISO 9227 standard. After the test, The alloy showed no signs of corrosion and exhibited excellent corrosion resistance in saltwater environments. It is noteworthy that the presence of sigma phase, even in trace amounts, can cause This is because the corrosion resistance of the steel has been significantly reduced.

[0034] In the second example, the steel designated Fe32Cr9Ni contains 59% iron and 9% chromium by weight. This steel also contains 32% chromium and 9% nickel. Manufactured by arc melting from 1300°C in argon for 2 hours after homogenization heat treatment. Then, gas hardening is carried out and the thickness is reduced by a factor of two by compressive deformation at ambient temperature. The specimen was then subjected to a recrystallization heat treatment at 1200°C in air for 1 minute, followed by water hardening. After heat treatment, the alloy Fe32Cr9Ni is a single-phase alloy with a Vickers hardness of 220HV10. The ferrite structure was then maintained at 700°C in a vacuum for 6 hours. Figure 3 shows the microstructure observed under polarized light with an optical microscope. A fine distribution of two phases was observed, with the α phase in the matrix and the sigma phase in the matrix. In this metallic state, the alloy Fe32Cr9Ni has a Vickers hardness of 635 HV. The magnetic properties of this steel were measured using a vibrating sample magnetometer (measurement of magnetization M in response to an applied magnetic field H). The hysteresis curve was measured at ambient temperature using a ferromagnetic material. Nevertheless, this steel exhibits a characteristic linear paramagnetic behavior (Figure 4). .

Claims

1. 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% A paramagnetic stainless steel having a chemical composition containing, in weight ratio: The remainder is made up of iron and impurities, each of which contains less than 0.5% of the element. The stainless steel has a hardness of 500 to 900 HV10, The stainless steel has a microstructure in which 40 to 80% by weight of sigma phase and 20 to 60% by weight of austenite phase are both finely and uniformly distributed, and the austenite phase is derived from the transformation of an alloy having a structure containing 100% ferrite.

2. 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% It has a chemical composition containing the following in a weight ratio:

2. The stainless steel according to claim 1.

3. 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% It has a chemical composition containing the following in a weight ratio:

2. The stainless steel according to claim 1.

4. Made from the paramagnetic stainless steel according to any one of claims 1 to 3. A part characterized by:

5. A component for a timepiece that constitutes the outer part or movement of the timepiece.

5. The component according to claim 4.

6. Equipped with the component according to claim 5 A portable watch characterized by the above.

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