Use of a triple-structured steel in an acidic environment

Abstract

The invention relates to the use of a steel material in contact with an acidic medium that has a pH of less than 5, the steel material being composed of grains comprising a matrix within which precipitates are incorporated, the steel material comprising i) chromium, nickel, carbon, oxygen, manganese, molybdenum, silicon, phosphorus, sulfur, other elements and iron; ii) spherical precipitates, the size of which is between 1 nm and 150 nm; iii) equiaxed, non-columnar grains, the size of which is less than 40 µm; and iv) between 68% and 82% HAGBs, between 1% and 16% LAGBs, and between 9% and 26% twin boundaries.

Description

[0001] Description Title: Use of a triple-structured steel in an acidic environment Technical field of the invention The present invention falls within the scope of the development of a steel having improved mechanical, thermal and physicochemical properties. The invention relates more particularly to the use in an aggressive and corrosive environment of a steel material having a microstructure characterized by a triple hierarchical structuring in which a network of internal nanometric sub-cells appears capable of improving the properties and performances of the steels. Technical background The corrosion processes of an alloy depend on the composition and the metallurgical properties, in particular the microstructure, of the alloy, and the properties of the environment surrounding the alloy (pH, degree of oxidation, temperature, etc.). Low-carbon stainless steels, ieless than 0.025% by mass, are known to have good corrosion resistance in oxidizing environments, for example in the presence of nitric acid. Under these conditions, the alloy undergoes a uniform and slow dissolution thanks to the formation of a protective oxide layer on the surface. However, many stainless steels have usage limitations when the concentration of HNO3 and / or oxidizing ions and / or the temperature of the corrosive environment increases. In particular, they are then subject to intergranular corrosion, also called CIG corrosion. The increased reactivity to acidic agents of a grain boundary compared to the grains it separates can be associated with differences in local chemical composition within the alloy, for example to a segregation of elements and / or the presence of impurities, and / or to a specific crystalline structure at the grain boundary, as is known from L. Beaunier et al., “Intergranular Corrosion”, F. Dabosi, G.Béranger, B. Baroux (Eds.), Localized Corrosion, and A. Emery, “Intergranular Corrosion of Austenitic Stainless Steels in an Oxidizing Nitric Acid Environment,” PSL University (2019). Intergranular corrosion of steel is difficult to control and master. It is known to add additional chemical elements to the composition of steels to try to limit the development of intergranular corrosion. However, these modified steels exhibit a higher generalized corrosion rate than unmodified steels, which thus limits the service life of the components from which they are manufactured. New stainless steel-based materials are obtained by additive manufacturing, in particular by selective laser melting on a powder bed, known by the abbreviation SLM, an acronym for “Selective Laser Melting.”Typically, the grains of steel materials produced by SLM additive manufacturing are columnar or equiaxed along the part build direction. In addition, these materials also exhibit structural heterogeneities within the grains due to the rapid cooling induced by the manufacturing process. These heterogeneities include, for example, a cellular structure within the grains. Figure 1 illustrates a typical structure of a material formed by SLM fusion from 316L stainless steel powder. The grains of this material consist of submicrometer cells, with a diameter of less than 1 μm, which are essentially the result of rapid melting and solidification induced by the SLM process. This material also contains spherical and amorphous precipitates homogeneously distributed in the matrix and at the boundaries between the cells.These are mainly oxides rich in silicon and manganese and with sizes varying between ten and several hundred nanometers. The oxides mainly observed in the literature for 316L steel produced by SLM are composed of chromium, titanium, nickel, iron or aluminum. The quantity of oxygen in the construction chamber of the SLM manufacturing device influences the characteristics of the precipitates. A high oxygen content (1000-2000 ppm) induces a high density of small precipitates (50-100 nm). A low oxygen content (300-500 ppm) induces a lower density of precipitates but of larger size (50 nm-2 μm). The article by Shubhavardhan Ramadurga Narasimharaju et al., Journal of Manufacturing Processes, 75 (2022), 375–414 describes the different cellular structures known for 316L obtained by SLM. The articles GT Gray et al., Acta Mater., vol.138, p. 140149, (2017), A. Leicht et al., Materials Characterization, 159 (2020) 110016, and A. Chniouel, “Study of the production of 316L stainless steel by selective laser powder bed fusion: influence of process parameters, powder characteristics, and heat treatments on the microstructure and mechanical properties.” (2019) (tel.archives-ouvertes.fr / tel-02421550) describe and illustrate the different known microstructures of 316 L steel. To the inventors’ knowledge, the known materials obtained by SLM manufacturing from a 316L steel powder exhibit, in an acidic and corrosive environment, better generalized corrosion resistance than a forged 316L steel. However, they remain poorly resistant to intergranular corrosion under these conditions.Furthermore, it is well known that corrosion processes are based on electrochemical, cathodic and anodic reactions, corresponding respectively to the reduction of an oxidant present in the medium (electrolyte), in this case and mainly the nitrate ion in the case of nitric acid and to the oxidation of a material. Figure 8 represents an Evans diagram, which schematizes the theoretical behavior of stainless steels at room temperature, in a given medium by dissociating the anodic reaction (dissolution of the steel) from the cathodic reaction (reduction of the nitrate ion) as a function of the corrosion potential. The dissolution rate of the material will depend on the value of the corrosion potential (E. corr), sensitive to the chemistry of the environment. Three main potential domains can be defined on the anodic curve: If the environment is very weakly oxidizing (e.g. H2SO4), austenitic steels are in their active domain characterized by uniform corrosion, the rate of which can be high. For moderately oxidizing conditions (HNO3 alone), the steel is in its passive domain and the corrosion rate of the stainless steel considered is low. This is the domain of use of these materials, it is protected by a passive oxide layer mainly composed of chromine (Cr2O3). In this domain, the steel undergoes uniform dissolution. If the environment becomes very oxidizing (HNO3 with other oxidizing species such as V(V), Pu(VI), Cr(VI) and temperature), the dissolution of the passive film occurs, in particular via the oxidation of Cr(III) to Cr(VI) (in dissolved form Cr2O7 2- . The steel then finds itself in its transpassive domain characterized by a intergranular. In the least severe conditions, only indentations (depths less than the grain size) are observed. In the most oxidizing conditions, grain losses occur leading to major damage to the material. It therefore appears that it is particularly difficult, if not impossible, to extrapolate the behavior of a stainless steel under particularly corrosive conditions such as those claimed on the basis of observations made under very weakly oxidizing conditions (e.g. H2SO4). This is even more difficult when it is also necessary to take into account the influence of temperature. There is therefore a need to improve the resistance to intergranular corrosion and for uniform corrosion of steel parts in contact with an oxidizing and corrosive environment, in particular comprising nitric acid and oxidizing species such as Pu, Np, Cr(VI),Ce(IV) and V(V). Summary of the invention The invention relates to the use of a steel material in contact with an acidic medium having a pH of less than 5, the steel material being composed of grains comprising a matrix in which precipitates are incorporated, the steel material comprising: i) the following elements, in percentages by mass based on the mass of the steel material: 16% to 20% of chromium, 8% to 14% of nickel, 0.001% to 0.030% of carbon, 0.001% to 0.050%, preferably 0.001% to 0.030%, of oxygen, 2% at most of manganese, 3% at most of molybdenum, 0.75% at most of silicon, 0.045% at most of phosphorus, 0.03% at most of sulfur, other elements: less than 0.5%, iron: balance to 100%, ii) precipitates spherical, the size of which varies between 1 nm and 150 nm, and comprising a metallic element chosen from yttrium, titanium, iron, chromium, tungsten, silicon, zirconium, thorium, magnesium, manganese, aluminum,hafnium, molybdenum and mixtures thereof; iii) non-columnar grains of equiaxed morphology having a size of less than 40 µm, the grains comprising, or even consisting of, - a mesoscopic cellular structure made up of cells having a diameter of less than 1 µm, and microscopic cellular structures each internal to one of the cells of the mesoscopic cellular structure, the cells of the microscopic cellular structure having a diameter of less than 100 nm and being distributed regularly within the matrix of the cells of the mesoscopic cellular structure; and - spherical precipitates having a size of between 1 nm and 10 nm, called “nano-precipitates”, more than 50% by number being distributed along the walls of the microscopic cells, with an average surface density of 40 precipitates per µm²; and iv) between 68% and 82% HAGB type grain boundaries, between 1% and 16% LAGB type boundaries,and between 9% and 26% twin boundaries. The acidic medium may comprise an acid selected from sulfuric acid, hydrochloric acid, hydrofluoric acid, nitric acid and mixtures thereof. Preferably, the acidic medium comprises nitric acid. It may further optionally comprise an oxidizing species selected from Pu, Np, Cr(VI), Ce(IV) and V(V). The acidic medium may have a pH of less than 4, or even less than 3, or even less than 2, or even less than 1, or even less than 0. The acidic medium may be liquid or gaseous. The temperature of the steel material may be greater than 80°C, preferably greater than 100°C and, preferably, less than 130°C. The steel material can be used as a part selected from: - a shock absorber, - a part of an internal combustion engine or an electric motor, - a part of a turbine, - a part of a machine tool, - a part of a pump, - a cutting tool, - a striking tool,- a reactor vessel, in particular a nuclear reactor, a chemical, petrochemical or pharmaceutical reactor, - a piece of furniture, and - a piece of household electrical equipment. The material may be used in one or more of the following fields: - the field of public facilities, - the chemical, petrochemical or pharmaceutical industry, - the automotive, aeronautics, aerospace, nautical or rail transport fields, - the food industry, - construction, - the medical field, - shipbuilding, - the field of energy production, for example of nuclear, hydraulic or thermal origin, - the field of striking and cutting tools, - the field of furniture equipment, and - the field of household electrical equipment. In particular, the material may be used in a radioactive environment, for example as a component of a nuclear reactor. Definitions The “size” of a structure,for example a precipitate or a grain, is the greatest distance that separates two opposite boundaries of the structure. It is measured by image processing from images acquired by optical microscopy (OM) and / or scanning electron microscopy (SEM). The "diameter" of a cell is the greatest distance that separates two opposite boundaries of the cell. A "HAGB" type grain boundary (acronym for "High Angle Grain Boundary") is a highly misoriented grain boundary, which has a misorientation angle greater than 10°, measured by scanning electron microscopy coupled with an electron backscattered diffraction detector, called an EBSD detector. A "LAGB" type grain boundary (acronym for "Low Angle Grain Boundary") is a weakly misoriented grain boundary,which has a misorientation angle of between 2° and 10° measured by scanning electron microscopy coupled with an EBSD detector. A scanning mode of the “HAADF STEM” type (acronym for “Scanning Transmission Electron Microscopy High Angle Annular Dark Field”) is a wide-angle annular dark field scanning mode, presenting a particular contrast between the different phases observed. Brief description of the figures Other characteristics and advantages of the invention will appear during the reading of the detailed description which follows, for the understanding of which reference will be made to the appended figures in which: [Fig.1] has been described above; [Fig.2] shows a summary of the known microstructural specificities for 316L obtained by SLM: (a) diagram to indicate the different length scales of the microstructure,(b) Electron backscatter diffraction (EBSD) inverse pole figure revealing grain orientations, (c) SEM image showing melt pools, highly misoriented grain boundaries (HAGBs) and cellular solidification structures, (d) Transmission electron microscopy (TEM) image of solidification cells, (e) High angle annular dark field (HAADF) scanning TEM (STEM) image of solidification cells shown in d, (f) EBSD acquired with a 1 μm size EBSD image (g) of overlapping HAGBs and weakly misoriented grain boundaries (LAGBs). Legend representation, HAGBs (>10°) colored blue and LAGBs (2–10°) colored red. The fraction of HAGBs and LAGBs is about 59% and about 41%, (h) Average core misorientation map to demonstrate local misorientation across the individual grain,(i) HAADF STEM image showing the segregation of Mo and Cr alloying elements in the cellular structure and weakly disoriented grain boundaries, while EDS confirms the local Fe, Mo and Cr contents corresponding to this segregation. The EDS map also confirms that these particles are mainly rich in Si, O and Mn according to Shubhavardhan Ramadurga Narasimharaju et al., Journal of Manufacturing Processes, 75 (2022), 375–414. [Fig. 3] illustrates the triple structuring of an example of the inventive 316L steel material manufactured by selective laser powder bed melting additive manufacturing; [Fig.4] illustrates a nanometric cellular substructure organized within a network of larger sub-micrometer cells; [Fig.5] represents the evolution of the mass loss as a function of time of a part made of the steel material according to the invention and of two parts made of a steel of the prior art,the parts being in contact with a nitric acid solution; [Fig. 6] contains photographs of slices at the end of the experiment of the parts whose mass losses are shown in Figure 5; and [Fig. 7] contains photographs acquired by scanning electron microscopy (A, C and E) and corresponding maps acquired by interferometry (B, D, and E) of the microstructures of the steels of the examples illustrated in Figures 5 and 6. Table 1 indicates the mass content ranges of the chemical elements composing the steel powder used to manufacture an example of steel material used according to the invention, as well as for comparison,the corresponding contents as defined by ASTM A666 and RCC-MRx standards; [Table 1] Table 2 specifies the mass and atomic content of the chemical elements within the matrix and within the precipitates of a 'steel powder' used to manufacture the material used according to the invention. [Table 2] Table 3 indicates the mass content of the chemical elements in an example of a steel material used according to the invention, measured within the matrix and the precipitates. [Table 3],

[0002] Detailed Description The material may have a relative density between 70.0% and 99.9%. The relative density is used to assess the porosity of the material. It is measured, for example, by the Archimedes method. Preferably, the steel material has marks associated with the boundaries of oval melt pools with a depth of less than 100 µm. The melt pools result from the manufacturing process of the steel material. For example, during selective laser melting additive manufacturing, also known as SLM (selective laser melting), the laser beam melts particles of the powder following a predefined pattern. The powder is melted in the form of liquid melt pools that cool and solidify quickly after the laser passes through. After solidification, residual marks revealing the trace of the melt pools remain present in the microstructure of the steel material.The average depth of the marks associated with the boundaries of the weld pools is, for example, 79 µm. Preferably, the steel material has the chemical composition of a type 316L or 304L steel, for example as specified in ASTM A666 or RCC-MRx respectively. The steel material may be of 100% austenitic structure. “Other elements” are elements other than chromium, nickel, carbon, oxygen, manganese, molybdenum, silicon, and iron. The steel material may comprise, in percentage by mass, at least one of the following other elements: - 0.11% at most of nitrogen, - 0.045% at most of phosphorus, - 0.05% at most of sulfur, - 0.0300% at most of aluminum, - 0.003% at most of vanadium, - 0.75% at most of copper, - 0.10% at most of cobalt, - 0.003% at most of titanium. The other elements may be present in the matrix and / or in the precipitates, in particular in the nano-precipitates.The matrix may comprise, in proportion by mass relative to the mass of the material, at most 5000 ppm of each of the metallic elements among yttrium, titanium, tungsten, zirconium, thorium, aluminum, hafnium, silicon, manganese and molybdenum. Said metallic elements may be dissolved in the matrix. The precipitates may comprise at least one metallic oxide, at least one intermetallic compound and mixtures thereof. The metallic oxide and / or the intermetallic compound may each comprise at least one metallic element selected from titanium, iron, chromium and mixtures thereof. The steel material may comprise spherical precipitates which are oxides of manganese and silicon and whose average size varies between 10 nm and 150 nm. The steel material may comprise up to 2%, for example from 0.1% to 1.5% of at least one metal oxide, in percentages by mass based on the mass of the steel material.The steel material may comprise 0.1% to 2%, for example 0.1% to 1.5%, of silicon and manganese oxide, the percentages being expressed by mass based on the mass of the steel material. The metal oxide may more particularly be chosen from a simple oxide MO. 2-x with the index x between 0 and 1, at least one mixed oxide MM' y' O 5-x'with 0 < x' < 5 and 0 < y' ≤ 2, and at least one mixed oxide MM'y'M''y''O5-x'' with 0 < x'' < 5, 0 < y' ≤ 2 and 0 < y'' ≤ 2. M, M' and M'' are metallic elements each different from the other. M, M' and M'' are preferably each chosen from yttrium, titanium, iron, chromium, tungsten, silicon, zirconium, thorium, magnesium, manganese, aluminum, hafnium and molybdenum, preferably from titanium, iron, chromium. For example, the index "x" for different compounds is as follows: - x = 0: TiO2 - x = 1: FeO - x = 0.5: Fe2O3- x = 2 / 3: Fe3O4The index "y'" is for example equal to 1 or 2. The metallic element M of the simple oxide MO2-x, the mixed oxide MM'y'O5-x' or the mixed oxide MM' y' M'' y'' O 5-x''is more particularly chosen from yttrium, iron, chromium, titanium, aluminum, hafnium, silicon, zirconium, thorium, magnesium and manganese. The simple oxide MO2-x is for example chosen from Y2O3, Fe2O3, FeO, Fe3O4, Cr2O3, TiO2, Al2O3, HfO2, SiO2, ZrO2, ThO2, MgO, MnO, MnO2 and mixtures thereof. Preferably, the metallic element M of the simple oxide MO 2-x is chosen from titanium, iron and chromium. Preferably, the simple oxide MO 2-x is TiO2. The metallic element M of the mixed oxide MM' y' O 5-x' is for example chosen from iron and yttrium. The metallic element M' of the mixed oxide MM'y'O5-x' or of the mixed oxide MM' y' M'' y'' O 5-x'' may be more particularly chosen from titanium and yttrium. Preferably, the mixed oxide MM'y'O5-x' is chosen from FeTiO3, Y2Ti2O7, YTi2O5 and their mixtures. The mixed oxide MM' y' O 5-x'may be a pyrochlore compound, for example Y2Ti2O7, YTi2O5 and their mixture. Preferably, the mixed oxide is TiYO5-x'. The mixed oxide MM' y' M'' y'' O 5-x''is for example of general formula of the type "SiOAlMn" noted without stoichiometric index. The steel material may comprise up to 1.5% by mass, for example 0.1% to 1.5% of the intermetallic compound in percentages by mass relative to the mass of the material. The intermetallic compound may comprise two or even three metallic elements different from each other and each chosen from yttrium, titanium, iron, chromium, tungsten, silicon, zirconium, thorium, magnesium, manganese, aluminum, hafnium, and molybdenum. The intermetallic compound may comprise a metallic element chosen from iron, titanium, yttrium, chromium and tungsten. Preferably, the intermetallic compound comprises at least iron. It may comprise iron and a metallic element chosen from titanium, yttrium and their mixture, and optionally another metallic element chosen from chromium, tungsten and their mixtures.For example, the intermetallic compound is chosen from YFe3, Fe2Ti, FeCrWTi and mixtures thereof. FeCrWTi is a name known to those skilled in the art, which is not a stoichiometric formula. The precipitates may comprise at least one metal oxide and at least one intermetallic compound. The material may comprise precipitates comprising a metal oxide and precipitates comprising an intermetallic compound. In particular, the steel material may comprise precipitates comprising - at least one spherical Mn and Si oxide whose size varies between 10 and 150 nm; and, optionally - at least one metal oxide chosen from at least one simple oxide MO. 2-xwith the index x between 0 and 1, at least one mixed oxide MM'y'O5-x' with 0 < x' < 5 and 0 < y' < 2, or at least one mixed oxide MM'y'M''y''O5-x'' with 0 < x'' < 5, 0 < y' < 2 and 0 < y'' < 2, and mixtures thereof, with M, M' and M'' different from each other and each chosen from yttrium, iron, chromium, titanium, aluminum, hafnium, silicon, zirconium, thorium, magnesium and manganese; and - optionally at least one intermetallic compound chosen from YFe3, Fe2Ti, FeCrWTi and mixtures thereof. In particular, the steel material may comprise: - at least one spherical Mn and Si oxide whose size varies between 10 and 150 nm. - precipitates comprising at least one metal oxide chosen from at least one simple oxide MO2-x with the index x between 0 and 1, at least one mixed oxide MM' y' O 5-x' with 0 < x' < 5 and 0 < y' < 2, or at least one mixed oxide MM' y' M'' y'' O 5-x''with 0 < x'' < 5, 0 < y' < 2 and 0 < y'' < 2, and mixtures thereof, with M, M' and M'' different from each other and each chosen from yttrium, iron, chromium, titanium, aluminum, hafnium, silicon, zirconium, thorium, magnesium and manganese; and - optionally precipitates comprising at least one intermetallic compound chosen from YFe3, Fe2Ti, FeCrWTi and mixtures thereof. According to one embodiment of the invention, the material comprises precipitates comprising: - an oxide of Mn and Si; - optionally, a simple oxide from among Y2O3, Fe2O3, FeO, Fe3O4, Cr2O3, TiO2, Al2O3, HfO2, SiO2, ZrO2, ThO2, MgO, MnO, MnO2 and their mixtures, - a mixed oxide chosen from among FeTiO3, Y2Ti2O7, YTi2O5 and their mixtures, - the mixed oxide of general formula of the SiOAlMn type.According to one embodiment of the invention, the material comprises precipitates comprising: - an oxide of Mn and Si; - optionally, a simple oxide chosen from Y2O3, Fe2O3, FeO, Fe3O4, Cr2O3, TiO2, Al2O3, HfO2, SiO2, ZrO2, ThO2, MgO, MnO, MnO2 and mixtures thereof, - a mixed oxide chosen from FeTiO3, Y2Ti2O7, YTi2O5 and mixtures thereof, - a mixed oxide of general formula of the SiOAlMn type, and - an intermetallic compound chosen from YFe3, Fe2Ti, FeCrWTi and mixtures thereof. According to one embodiment of the invention, the material comprises: an oxide of Mn and Si; - optionally, a simple oxide chosen from Y2O3, Fe2O3, FeO, Fe3O4, Cr2O3, TiO2, Al2O3, HfO2, ZrO2, ThO2, MgO, and mixtures thereof, - a mixed oxide chosen from FeTiO3, Y2Ti2O7, YTi2O5 and mixtures thereof, - a mixed oxide of general formula of the SiOAlMn type, and - an intermetallic compound chosen from YFe3, Fe2Ti, FeCrWTi and mixtures thereof.The grain size of the material of the invention can be measured by analyzing images acquired by Scanning Electron Microscopy (SEM) coupled with an EBSD detector. It is for example calculated by averaging the measurements obtained on at least 10 grains, or even at least 50 grains analyzed on said images. The average size of the non-columnar grains of equiaxed morphology can be 25 µm. The grains can be close to the equiaxed, non-columnar morphology, in a plane parallel to the plane of the superimposed layers of the material which result from the manufacture of the material by an additive manufacturing process. They can also be equiaxed in a plane perpendicular to the plane of the superimposed layers of the material which result from the manufacture of the material by an additive manufacturing process.The interface between these superimposed layers, and therefore the direction of these layers, is generally visible by Scanning Electron Microscopy (SEM) or by optical microscopy. The mesoscopic cellular structure results from a segregation of chemical elements during the manufacturing of the steel material. A difference in chemical composition appears locally between the wall and the matrix of the cells of the mesoscopic cellular structure. The wall is for example enriched in Cr, Mo, weakly in Ni and depleted in Fe compared to the cell matrix. At the level of these cells of the mesoscopic cellular structure, a network of dislocations appears. The shape of the cells of the microscopic cellular structure can be close to the shape of the cells of the mesoscopic cellular structure. The cells of the microscopic cellular structure can form a honeycomb network within the matrix of the cells of the mesoscopic cellular structure.The microscopic cellular structure, the mesoscopic cellular structure and the arrangement of the grains within the matrix thus define a steel material exhibiting triple structuring at different scales. In particular, the microscopic cellular structure is specific to the material used according to the invention. It modifies the small-scale arrangements and impacts the properties of the material. The steel material used according to the invention is thus called "triple-structuring steel". The diameter of the cells of the mesoscopic cellular structure is preferably greater than 100 nm. The average diameter of the cells of the mesoscopic cellular structure is, for example, 385 nm. The diameter of the cells of the microscopic cellular structure is, for example, greater than 10 nm. The average diameter of the cells of the microscopic cellular structure is, for example, 30 nm.The size of the precipitates contained in the nanometric substructure of the steel material is between 1 nm and 10 nm. The average size of the spherical nanoprecipitates contained in the nanometric substructure of the steel material is, for example, 9 nm. The size of the precipitates can be determined visually from a measurement made on an image obtained with a Scanning Electron Microscope, to be then processed with image processing software such as, for example, "ImageJ" software available at the following Internet address: imagej.net / Welcome. The steel material may comprise 0.1% to 1.5% by mass of nanoprecipitates relative to the total mass of the material. This precipitate content can be measured by selective dissolution with aqua regia. The surface density of the precipitates is the number of precipitates per unit area.It can be determined by counting via imaging, such as for example imaging by Scanning Electron Microscopy (SEM) or by Transmission Electron Microscopy (TEM). Manufacturing process The steel material can be manufactured from a steel powder subjected to a consolidation process. Preferably, the steel powder has the same chemical composition as the steel material. The steel powder can be obtained conventionally by gas atomization under nitrogen or argon, or by water atomization, in particular if the powder is subsequently treated by a selective laser powder bed fusion process, for example of the SLM or L-PBF type. Unless otherwise stated, any characteristic of the precipitates or of the matrix contained in the steel powder subjected to the consolidation process is identical to the corresponding characteristic of the precipitates or of the matrix contained in the steel material.More particularly, unless otherwise stated, the size and / or distribution of the precipitates in the matrix and / or the chemical composition of the precipitates and / or the matrix are not modified by the process for manufacturing the steel material from the steel powder. The analyses carried out by the inventors on the steel powder and on the steel material obtained from said powder confirm that the size, distribution and composition of the precipitates are equivalent in the powder and in the steel material. The powder may have a median diameter d50 of between 10 µm and 200 µm. The median diameter d50 of a powder is the size for which 50% by number of the particles of the powder have a size less than d50. It can be determined by a technique such as the laser diffraction method via a granulometer described for example in standard ISO 13320 (edition 2009-12-01).The apparent density of the powder measured by ASTM B-212 can be between 3.5 g / cm. 3 and 4.5 g / cm 3 The actual density of the powder can be between 7.95 g / cm 3 and 8.05 g / cm 3. It is for example measured with a pycnometer. The steel powder preferably has a 100% austenitic structure. Preferably, the consolidation process is an additive manufacturing process. The additive manufacturing process involves the successive addition of layers of particles on top of each other on a plate, particles of each newly deposited layer being bonded to particles of the layer on which the newly deposited layer rests, prior to the deposition of another layer. Additive manufacturing is described in more detail for example in the following documents: - F. Laverne et al., "Additive manufacturing - General principles", Techniques de l'ingénieur, Fascicule BM7017 V2 (publication of February 10, 2016); - H. Fayazfara et al., "critical review of powder-based additive manufacturing of ferrous alloys: Process parameters, microstructure and mechanical properties", Materials & Design, Volume 144, 2018, Pages 98-128; - T. DebRoy et al., "Additive manufacturing of metallic components – Process, structure and properties", Progress in Materials Science, Volume 92, 2018, pages 112–224; - Ministry of Economy and Finance, French Republic, "Prospective - future of additive manufacturing - final report", January 2017 edition, ISBN: 978-2-11- 151552-9; in particular Appendix 2 (pages 205 to 220) especially when it describes additive manufacturing processes using a metal powder (Appendix 2, Manufacturing processes, paragraphs 3, 4 and 5). Preferably, the additive manufacturing process may be chosen from a selective laser powder bed melting process, a selective electron beam powder bed melting process, a selective laser powder bed sintering process, a laser spraying process and a binder spraying process.The additive manufacturing process may be a selective laser powder bed melting process, also called an SLM additive manufacturing process after the acronym in English for “Selective Laser Melting”. The selective laser powder bed melting process may be implemented by controlling one or more of the following parameters: - the laser beam scans the steel powder at a scanning speed of between 50 mm / second (dense material) and 3000 mm / second (porous material); - laser beam power: from 50 W to 1000 W; - distance between vector spaces: from 25 µm to 150 µm; - layer thickness: from 15 µm to 80 µm. Alternatively, the additive manufacturing process may be a selective powder bed electron beam melting process, referred to as an EBM additive manufacturing process after the acronym for "Electron Beam Melting".The selective powder bed electron beam melting process can be implemented by controlling one or more of the following parameters: - electron beam power: from 50 W to 4000 W; - electron beam speed: from 100 mm / s to 10000 mm / s; - distance between vector spaces: from 50 µm to 150 µm; - layer thickness: from 40 µm to 75 µm. According to another variant, the additive manufacturing process can be a laser projection process. It can be implemented by controlling one or more of the following parameters: - laser power: 400 W to 3000 W; - nozzle movement speed: 150 mm / min to 1200 mm / min; - powder flow rate: 4 g / min to 15 g / min. According to yet another variant, the additive manufacturing process may be a thermal spraying process, for example chosen from a flame thermal spraying process, an electric arc spraying process between two wires or a blown plasma spraying process.At the end of the manufacturing process of the material of the invention in which the steel powder is subjected to the consolidation process, the steel material is in massive form. At the end of the manufacturing process of the material, in particular thanks to the raw material and the applied energy volume density, the microstructure of the grains, in particular the mesoscopic cellular structure, contains the microscopic structure. The manufacturing process of the steel material may be followed by hot isostatic pressing of the steel material. The hot isostatic pressing may comprise the following successive steps, carried out in an enclosure comprising an inert gas atmosphere under a pressure of between 120 bars and 1800 bars: A. heating the material to a temperature of between 600 and 1400°C at a temperature rise rate of between 500 and 1000°C / hour; B.maintaining the temperature for a period of between 15 minutes and 5 hours; C. cooling the material at a temperature reduction rate of between 500 and 1000 °C / hour to room temperature, for example between 20 °C and 25 °C. The inert gas atmosphere may comprise a gas chosen from argon, helium or a mixture thereof. Example EXAMPLES 1. Steel powder for manufacturing the steel material A steel powder was chosen, which has a composition as shown in Table 1 corresponding to the requirements of ASTM A666 and RCC-MRx standards. The RCC-MRx standard corresponds to the Rules for the design and construction of mechanical equipment for high-temperature, experimental and fusion nuclear installations. This is a technical document for the production of components for Generation IV nuclear reactors. Characterization of the steel powder. 1.1. Chemical composition.The steel powder, type 316 L, reference FE-271-3 / TruForm 316-3 - lot n° 32-034043-10 marketed by the Praxair Company was analyzed by energy dispersive X-ray spectroscopy (or EDX according to the English acronym for "Energy Dispersive X-ray Spectroscopy") using the BRUKER Quantax XFlash analysis system. The steel powder was also analyzed by scanning electron microscope (SEM FEG Zeiss ULTRA55), by glow discharge mass spectrometry (GDMS) using the Element GD Plus system (Thermo Fisher), by inductively coupled plasma optical emission spectrometry (ICP-OES) using the Optima 8300 DV (Perkin Elmer) and by instrumental gas analysis (IGA) using the Horiba EMGA-920 chemical analyzer.The elemental composition of the matrix and precipitates of the steel powder obtained was determined by compiling these different measurements. The proportions obtained for each chemical element are expressed with a relative uncertainty of 3%: - in mass % relative to the total mass of the matrix. However, by convention, the unmeasured chemical elements are ignored for the matrix. It is then considered that the remaining mass percentage is made up of iron. - in mass % relative to the total mass of the precipitates contained in the steel powder. These proportions were normalized by relating the total mass or the total number of atoms to a value of 100. They are reproduced in Table 2 which shows that the precipitates are rich in aluminum, titanium, silicon and manganese oxides in the form of simple oxide and / or mixed oxide.The precipitates may possibly contain carbides or oxycarbides of these chemical elements which would not have been detected by SEM given their small size. 1.2. Morphology The steel powder has a 100% austenitic structure. The 100% austenitic phase was analyzed by X-ray diffraction (XRD). The apparatus used was the Brücker D8 Advance diffractometer (Bragg–193 Brentano u–2u geometry, CuKa radiation l=1.54060 Å). The particles of this powder comprise agglomerated grains and are most often essentially spherical. They have a diameter between 10 µm and 100 µm, and an average diameter of 34 µm.More specifically, the median diameters D10, D50 and D90 (for which, respectively, 10%, 50% and 90% by number of the particles composing this powder have a size smaller than the median diameter considered) measured by laser granulometry according to the ISO 13320 standard (edition 2009-12-01) are as follows: D10 = 22 µm, D50 = 32 µm, and D90 = 48 µm. The precipitates contained in the particles of the powder are most often spherical. Their maximum dimensions are such that the size measured by imaging with a Scanning Electron Microscope (SEM) is between 24 nm and 120 nm. The corresponding average size is 63 nm. The density with which the precipitates are distributed in the matrix is ​​measured by counting by SEM imaging: it is between 2 precipitates / µm³ and 100 precipitates / µm³. The corresponding average density is 6 precipitates / µm³. 1.3. Properties. The apparent density of steel powder measured by ASTM B-212 is 4 g / cm.3 ± 0.01 g / cm 3 Its actual density measured by Helium pycnometer is 7.99 g / cm 3 ± 0.03 g / cm 3. The Hall flowability (ability to flow 50 g of powder through an orifice of fixed size) measured according to ASTM B213 is 15 seconds. 2. Manufacturing process of a steel material. A part made of the steel material was manufactured by selective laser powder bed fusion with a Trumpf TruPrint Series 1000 printer. To manufacture the part, on a stainless steel platen, the laser scanned along a predefined path. At the end of this scan, a consolidated n-layer is obtained. Then, a 67° rotation of the laser scanning direction is made and a new n+1 layer is formed which is superimposed on the underlying n-layer. The process is thus repeated until the part is completely produced.The main operating parameters of the SLM process are: - Yb laser fiber with a wavelength of 1070 nm; - laser spot diameter: 30 µm; - laser power: 120 W; - laser scanning speed: 950 mm / s; - distance between two successive laser lines ("Hatching distance"): 60 µm; - powder bed thickness: 30 µm; - composition of the gaseous medium in the build chamber: argon, with an oxygen content lower than 100 ppm during consolidation. Ten pieces in the form of parallelepiped specimens (length = 30 mm, width = 20 mm, thickness = 7 mm) were obtained. After fabrication, the pieces were extracted by cutting the base of the specimens to separate them from the stainless steel substrate. No additional treatment was applied to the resulting material. The density of the steel material constituting the test pieces is 7.93 g / cm. 3(measurement by the Archimedes method), i.e. a relative density of 99.25% considering a theoretical density for 316 L steel which is 7.99 g / cm 3. The density of the material of the invention was identified by analyzing images obtained by optical microscopy and is 99.95%. By modifying at least one of the following parameters, the density could be increased without modifying the grain size of the steel material: - laser power: from 50 W to 400 W; - laser scanning speed: from 50 mm / s to 3000 mm / s. The density generally evolves parabolically with the laser power or the laser scanning speed. However, too low or too high a power or scanning speed may possibly decrease the density. The distance between two successive laser lines ("Hatching distance") was for example between 30 µm and 90 µm. 3. Characterization of the steel material obtained by the manufacturing process described in paragraph 2. 3.1. Chemical composition.The overall chemical composition of the steel material obtained by the manufacturing process described in the previous example complies with the ASTM A666 and RCC-MRx standards shown in Table 1. The elemental composition of the material was measured by EDX analysis. It is similar to the composition of the steel powder used to manufacture the steel material. However, the chemical elements are distributed differently between the matrix and the precipitates. The precipitates of the material were studied using a TEM (MET FEI Tecnai F20 FEG-TEM) coupled with an EDX detector (EDX Bruker XFlash 6T | 60). The precipitates identified correspond to Mn and Si oxides, but this does not exclude the presence of other precipitates of a different nature within the steel material. Differences in local chemical composition were also highlighted at the level of the cells of the mesoscopic elementary structure.The cells of the material of the invention were studied by TEM (MET FEI Tecnai F20 FEG-TEM) coupled with an EDX detector (EDX Bruker XFlash 6T | 60). The differences in chemical composition concern the cell wall and their matrix. The cell wall is enriched in Cr, Mo, weakly in Ni and depleted in Fe compared to the cell matrix. The chemical composition of the cells of the nanometric cellular substructure internal to the large cells of the material of the invention may present characteristics comparable to those of the large cells in terms of differences in local chemical composition. 3.2. Morphology. An X-ray diffraction analysis ("XRD") using the Brücker D8 Advance diffractometer (Bragg–193 Brentano u–2u geometry, CuKa radiation l =1.54060 Å), shows that the steel material has a 100% austenitic structure.Oxide precipitates are incorporated into the matrix of the grains that make up the steel material or into the boundaries between these grains. The average density with which these precipitates are distributed in the matrix is ​​6 precipitates / µm³. The size of the oxide precipitates is between 10 nm and 150 nm. One of the particularities of the material is a microstructure such that the grains that make up this material are non-columnar and approach an equiaxed structure. In particular, when the material is obtained by additive manufacturing, its grains are quasi-equiaxed in a plane parallel to the direction of additive manufacturing (which generally corresponds to a plane substantially perpendicular to the powder bed surfaces consolidated by the moving energy source during manufacturing).This microstructural feature of the material is such that the quasi-equiaxial structure of the grains is in a plane respectively parallel and a plane perpendicular to the z direction of additive manufacturing of the steel material. The grain size is less than 40 µm (average size of 25 µm). Furthermore, the crystallites that are the grains of the steel material have a preferential orientation. This texture of the material is reflected by the fact that the directions are preferentially oriented parallel to the z construction direction, but also by a texture intensity equal to 1.4. As illustrated in Figure 3, the grains of the steel material are themselves made up of sub-micrometric cells of nanometric size (more specifically a size less than an average diameter of 500nm).Figure 3 and Figure 4 also show the nanometric cellular substructure that is organized directly inside the larger sub-micrometric cells, while also revealing the small precipitates smaller than 10 nm and incorporated into the matrix, particularly at the level of the cell walls which appear in a lighter shade. 1. Corrosion test Corrosion tests were carried out. A test piece made of the steel material obtained by the process described in paragraph 2 and having the triple structuring illustrated in Figure 4 was immersed in a nitric acid solution with a pH between -1 and 0 for a test period of 240 hours. The temperature of the nitric acid solution was maintained at 107 °C throughout the test period, so as to keep the solution boiling and increase its oxidizing power. The nitric acid concentration of the solution was 5 M.The solution also contained the oxidizing ion V(V) in a concentration of 4.10-3 M. The oxidizing ion V(V) was added to the nitric acid solution to simulate the effects of Pu and Np. Furthermore, a comparative specimen made of a 316L steel obtained by additive manufacturing from a conventional powder and which is free of cellular structures internal to the mesoscopic cells, and a comparative specimen made of a forged 316 L steel, having the same dimensions as the specimen made of the triple-structured steel, underwent the same corrosion test. As observed in Figure 5, the specimens manufactured by additive manufacturing exhibit a lower corrosion rate than the specimen made of forged steel. After 240 hours, the mass loss is more than twice lower for the specimens obtained by additive manufacturing.The triple-structured steel specimen also exhibits a similar, although lower, mass loss than the comparative specimen manufactured by L-PBF additive manufacturing. However, these specimens exhibit different corrosion profiles, although their mass loss is similar. As observed in Figure 6, the comparative specimen made of forged steel (top) and the comparative specimen obtained by additive manufacturing (middle) exhibit numerous localized attacks at the grain boundaries that have propagated from the surface, in contact with the nitric acid solution. These grain boundary attacks, with a depth greater than 10 µm or even 20 µm, are typical of grooves formed by intergranular corrosion.The specimen of the triple-structured material (bottom) is, on the contrary, free of deep intergranular grooves, which indicates homogeneous corrosion of the material, without manifestation of excessive intergranular corrosion during the test. The development of intergranular corrosion is observed in the micrographs of Figure 7 as well as in the interferometry maps. The microstructure of the forged steel specimen (photograph A and map B) shows severe intergranular corrosion. This corrosion, localized at the grain boundaries of the material, spreads rapidly in depth and leads to loosening of the grains at the surface of the material, thus impairing durability (enlarged map B).The microstructure of the comparative specimen formed by additive manufacturing (photograph C and mapping D) is marked by moderate intergranular corrosion (not leading to the loss of surface grains) and intragranular corrosion localized in the cells of the mesoscopic structure. In the nitric acid solution, the corrosion kinetics of the cell core is greater than that of the cell wall. The microstructure of the triple-structured specimen, the cells of the microscopic structures, approximately 30 nm in diameter, limit localized corrosion by protecting the core of the mesoscopic cells. Thus, the material of the invention has improved corrosion properties compared to those of the forged steel specimen and those comparative specimen formed by additive manufacturing.More specifically, regarding the resistance to intergranular corrosion, the absence of a groove that develops in depth for the material of the invention guarantees better resistance and durability for this material. On the other hand, the presence of the triple structuring plays a beneficial role for the material of the invention because it makes it possible to prevent the phenomenon of cellular corrosion traditionally occurring in the mesoscopic cells of the comparative steel formed by additive manufacturing. The protection against corrosion of the mesoscopic cells is ensured by the presence of a nanometric cellular structure internal to the large cells.

Claims

Claims 1. Use of a steel material in contact with an acidic medium having a pH of less than 5 and comprising nitric acid and an oxidizing species selected from Pu, Np, Cr(VI), Ce(IV) and V(V), the temperature of the steel material being greater than 80°C, the steel material being composed of grains comprising a matrix in which precipitates are incorporated, the steel material comprising: i) the following elements, in percentages by mass based on the mass of the steel material: 16% to 20% of chromium, 8% to 14% of nickel, 0.001% to 0.030% of carbon, 0.001% to 0.050%, preferably 0.001% to 0.030%, of oxygen, at most 2% of manganese, at most 3% of molybdenum, at most 0.75% of silicon, 0.045% to more than phosphorus 0.03% at most of sulfur, other elements: less than 0.5% iron: complement to 100%, ii) spherical precipitates, the size of which varies between 1 nm and 150 nm, and comprising a metallic element chosen from yttrium, titanium,iron, chromium, tungsten, silicon, zirconium, thorium, magnesium, manganese, aluminum, hafnium, molybdenum and mixtures thereof; iii) non-columnar grains of equiaxed morphology having a size of less than 40 µm, the grains comprising, or even consisting of - a mesoscopic cellular structure made of cells having a diameter of less than 1 µm, and microscopic cellular structures each internal to one of the cells of the mesoscopic cellular structure, the cells of the microscopic cellular structure having a diameter of less than 100 nm and being distributed regularly within the matrix of the cells of the mesoscopic cellular structure; and, - spherical precipitates with a size between 1 nm and 10 nm, called "nano-precipitates", more than 50% in number being distributed along the walls of the microscopic cells, with an average surface density of 40 precipitates per µm²; and iv) between 68% and 82% of HAGB-type grain boundaries, between 1% and 16% of LAGB-type boundaries, and between 9% and 26% of twin boundaries.

2. Use according to claim 1, the acidic medium having a pH of less than 4, or even less than 3, or even less than 2, or even less than 1, or even less than 0.

3. Use according to any one of claims 1 and 2, the temperature of the steel material being greater than 100°C, and preferably less than 130°C.

4. Use according to any one of the preceding claims, in a radioactive environment, for example as a component of a nuclear reactor. 5.Use according to any one of claims 1 to 3 of the steel material as a part chosen from: - a shock absorber, - a part of an internal combustion engine or an electric motor, - a part of a turbine, - a part of a machine tool, - a part of a pump, - a cutting tool, - a striking tool, - a vessel of a reactor, in particular of a nuclear reactor, of a chemical, petrochemical or pharmaceutical reactor, - a part of a piece of furniture, and - a part of household appliances.

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