阅读说明：本技术 无镍奥氏体不锈钢粉末组合物和由所述粉末烧结生产的零件 (Nickel-free austenitic stainless steel powder composition and parts produced by sintering said powder ) 是由 J·波雷特 L·吉勒 于 2020-05-08 设计创作，主要内容包括：本发明涉及一种奥氏体不锈钢粉末,其镍含量小于或等于0.5重量％且具体碳含量大于或等于0.05重量％且小于或等于0.11重量％。本发明还涉及通过粉末冶金制造所述粉末的方法以及由该制造方法生产的零件(3),其特征在于在零件(3)的表面上具有延伸厚度e的脱氧层(1),e大于或等于200μm。(The invention relates to an austenitic stainless steel powder having a nickel content of less than or equal to 0.5 wt.% and a specific carbon content of greater than or equal to 0.05 wt.% and less than or equal to 0.11 wt.%. The invention also relates to a method for producing said powder by powder metallurgy and to a part (3) produced by said production method, characterized in that a deoxidized layer (1) having an extended thickness e, e being greater than or equal to 200 [ mu ] m, is present on the surface of the part (3).)

1. An austenitic stainless steel powder comprising by weight:

-10<Cr<25％，

-5<Mn<20％，

-1<Mo<5％，

-0.05≤C≤0.11％，

-0≤Si<2％，

-0≤Cu<4％，

-0.5<N<1％，

-0≤O<0.3％，

-0≤Ni≤0.5％，

the balance being formed by iron and possible impurities, each having a content of 0-0.5%.

2. Powder according to claim 1, comprising by weight:

-15<Cr<20％，

-8<Mn<14％，

-2<Mo<4％，

-0.05≤C≤0.11％，

-0≤Si<1％，

-0≤Cu<0.5％，

-0.5<N<1％，

-0≤O<0.2％，

-0≤Ni≤0.5％，

the balance being formed by iron and possible impurities, each having a content of 0-0.5%.

3. Powder according to claim 1 or 2, comprising by weight:

-16.5≤Cr≤17.5％，

-10.5≤Mn≤11.5％，

-3≤Mo≤3.5％，

-0.05≤C≤0.11％，

-0≤Si≤0.6％，

-0≤Cu≤0.5％，

-0,5<N<1％，

-0≤O<0.2％，

-0≤Ni≤0.5％，

the balance being formed by iron and possible impurities, each having a content of 0-0.5%.

4. Powder according to any one of the preceding claims, characterized in that the particles forming the powder have a diameter D90 of less than or equal to 150 μm.

5. Austenitic stainless steel part with a nickel content of less than or equal to 0.5% by weight, characterized in that it contains carbon in a proportion of greater than or equal to 0.05% by weight and less than or equal to 0.11% by weight, and in that it has on the surface a so-called oxygen depleted layer (1), said oxygen depleted layer (1) containing oxides (4) in a surface fraction less than the rest (2) of the part (3), the diameter of these oxides (4) being smaller compared to the rest (2) of the part (3), said oxygen depleted layer (1) having a thickness greater than or equal to 200 μm.

6. Part according to claim 5, characterized in that the thickness of the deoxidation layer (1) is greater than or equal to 250 μm.

7. Part according to claim 5 or 6, characterized in that the thickness of the deoxidation layer (1) is greater than or equal to 300 μm.

8. Part according to any of claims 5 to 7, characterized in that the deoxidation layer (1) comprises oxides (4) with a diameter less than or equal to 2 μm and a surface fraction less than or equal to 0.1%.

9. Part according to any of claims 5 to 8, characterized in that the deoxidized layer (1) has a relative density greater than or equal to 99%.

10. Part according to any of claims 5-9, characterized in that the oxide (4) is manganese oxide and/or mixed manganese and silicon oxides.

11. A part according to any of claims 5-10, characterized in that the part (3) is a timepiece exterior part or a gemstone or jewelry item.

12. Part according to claim 11, characterized in that the external part is selected from the list comprising middle, bottom, bezel, button, bracelet link, bracelet, tongue, dial, pointer, crown and pointer index.

13. A watch comprising a part according to claim 12.

14. A method of manufacturing an austenitic stainless steel part, comprising the steps of:

-subjecting the powder according to one of claims 1 to 4 to a treatment,

-producing a blank having substantially the form of the part (3) to be manufactured from at least said powder,

-sintering the blank at a temperature of 1000-1500 ℃ for 1-10 hours in an atmosphere comprising a nitrogen carrier gas to cause carbothermal reaction between the oxide and the carbon present in the blank and to densify the blank.

15. The manufacturing method according to claim 14, characterized in that the sintering is performed in two steps, a first step at 1000-1200 ℃ for a period of 30 minutes to 5 hours, followed by a second step at 1200-1500 ℃ for a period of 1-10 hours.

16. A method according to claim 14 or 15, characterized in that the blank is produced by injection moulding, extrusion, pressing or additive manufacturing.

17. Method according to any one of claims 14 to 16, characterized by comprising a forging step after the sintering step.

18. A method according to any one of claims 14-16, characterized by comprising a hot isostatic pressing step after the sintering step.

19. Method according to any one of claims 14 to 18, characterized in that it comprises, after the sintering step, a step of transforming the austenitic stainless steel surface into a ferritic or ferritic + austenitic duplex structure, and a subsequent step of transforming said ferritic or ferritic plus austenitic duplex structure surface into an austenitic structure, so as to form on the surface of the piece (3) a so-called dense layer having a greater density than the core of the piece (3), the result being achieved by using one of the following three steps or any combination of these steps:

a fixed temperature, such that carbon and nitrogen that stabilize the austenite phase diffuse in the solid and are released into the atmosphere;

fixing the partial pressure of the nitrogen carrier gas, or even operating under a nitrogen-free atmosphere, so that the nitrogen content of the surface of the piece is reduced by denitrification, so as to form an austenitic + ferritic or fully ferritic structure on the surface;

partial pressure of fixed carbon carrier gas, e.g. CO or CH₄To reduce the amount of carbon on the surface of the part by decarburization or more simply to use a decarburising atmosphere, e.g. H₂If the alloy already contains carbon.

20. The method of claim 19 wherein the carbon carrier gas is CO or CH₄The decarburization atmosphere is H₂。

Technical Field

The invention relates to a nickel-free austenitic stainless steel powder composition. The invention also relates to parts made by sintering such powders, in particular timepiece exterior parts, and to a method of sintering manufacture.

Background

Sintered stainless steel powder is very common today. It can be carried out in particular on billets obtained by injection (metal injection moulding), extrusion, pressing or other additive manufacturing. In the most conventional manner, sintering austenitic stainless steels involves compacting and densifying powders of such steels in a high temperature furnace (1200 ℃.) (1400 ℃) under vacuum or gas shielding. For a given powder composition, the properties of the sintered part (density, mechanical and magnetic properties, corrosion resistance, etc.) are strongly dependent on the sintering cycle used. The following parameters are particularly important: heating rate, sintering temperature and time, sintering atmosphere (gas, gas flow, pressure) and cooling rate.

For the field of particular aesthetic importance of watches and the like, two characteristics of the microstructure after sintering are of particular importance, namely the density and the presence of inclusions. The presence of pores or non-metallic inclusions, in particular oxides, is in fact very detrimental to the reproduction after polishing. Therefore, to obtain a polished part with a brightness and color similar to that of welded stainless steel, a density close to 100% and a minimum of non-metallic inclusions must be targeted.

With respect to density, a known solution is to eliminate any porosity at least on the surface in parts made of nickel-free austenitic stainless steel formed by powder metallurgy. These solutions include the implementation of Hot Isostatic Pressing (HIP) on pore-closed sintered parts.

As regards the oxides, mainly the high concentration of manganese in the nickel-free austenitic stainless steel, which is problematic for powder metallurgical forming. This is because the high affinity of this element for oxygen, combined with the high specific surface area of the powder, requires a good grasp of the process carried out at high temperatures:

it is first of all necessary to use powders whose particle surface is as little oxidized as possible;

it is also desirable to minimize oxidation of the powder during heating of the part to the sintering temperature by using a highly reducing atmosphere. However, for austenitic stainless steel, the composition of which incorporates nitrogen to eliminate the use of nickel, the sintering atmosphere must include nitrogen, with the result that it is impossible to work in an atmosphere containing only a reducing agent;

finally, it is necessary to reduce the oxides which are most stable at high temperatures and to eliminate the reduction products before the pores close. This is the most critical point because during the sintering operation, at temperatures typically above 1200 ℃, oxides of manganese are reduced while the pores are closed. Thus, the parts obtained generally have a layer with good surface deoxidation and a large number of inclusions (oxides) at the core. This is because the atmosphere in the furnace is more easily renewed at the surface of the part, and the reduction product can be transported throughout the furnace chamber while the pores are still open at the start of sintering. At the core, on the other hand, the atmosphere is not renewed, the conditions not allowing the complete reduction of the oxide before the pores are closed. For nickel-free austenitic stainless steels, it is therefore difficult to have a deoxidized layer with a thickness greater than 200 μm. Since the finishing (machining, polishing) of the sintered parts requires the removal of material which may be greater than 200 μm, the surface of the finished part may present oxides which are detrimental from the point of view of aesthetics and of corrosion resistance of the polished surface.

Therefore, for parts formed by sintering nickel-free austenitic stainless steel powder, the thickness of the deoxidation layer needs to be increased.

Disclosure of Invention

The aim of the present invention is to propose a nickel-free austenitic stainless steel powder composition which enables a particularly deep deoxidation layer to be obtained after sintering, i.e. greater than or equal to 200 μm.

The deoxidation layer refers to a layer having finely dispersed small-sized oxides. Preferably, the diameter of the oxides is less than 2 μm and the surface proportion of these oxides in the layer is less than 0.1%. These finely divided oxides have no effect on the aesthetics after polishing. Outside the deoxidation layer, the diameter of the oxide can typically be as high as 5 μm, and the surface proportion of the oxide can be as high as 1%.

In order to obtain such a thicker deoxidized layer, it is necessary to select a nickel-free austenitic stainless steel powder having a carbon content concentration of 0.05% or more and 0.11% or less. In fact, carbon can reduce the most stable oxides, in particular manganese oxides and mixed manganese and silicon oxides, at temperatures lower than or equal to 1200 ℃. In view of the small densification and the open porosity below 1200 c, the presence of carbon allows a deeper deoxidation, favouring the elimination of the reduction products and the renewal of the internal atmosphere of the product. This reduction of oxides by carbon at high temperatures is commonly referred to as carbothermic reduction, for example, for manganese oxide, following the reaction:

MnO+C→Mn+CO。

it has been shown that in several sintering tests performed on nickel-free austenitic stainless steel powder, the carbon concentration has to be selected in a specific range of 0.05-0.11 wt.%. This optimum concentration makes it possible to obtain as thick a deoxidation layer as possible, while avoiding the problem of decarburisation of the part. This is because, when the mass concentration of carbon is 0.05 wt% or less, carbothermic reduction is incomplete and the deoxidized layer is reduced. On the other hand, too high carbon concentration is problematic because decarburization by reaction with hydrogen present in the sintering atmosphere cannot be controlled, and a large variation in carbon concentration among parts is observed. At carbon concentrations between 0.05% and 0.11% by mass in the nickel-free austenitic stainless steel powder, any change in carbon concentration is sufficiently small that it does not affect the microstructure and the mechanical and physical properties of the sintered part.