WO2024126739A1 - Use of a triple-structured steel in an acidic environment - Google Patents

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

intergranulaire. Dans les conditions les moins sévères, seules des indentations (profondeurs inférieures à la taille de grain) sont observées. Dans les conditions les plus oxydantes, des pertes de grains se produisent conduisant à un endommagement majeur du matériau. Il apparait donc qu’il est particulièrement difficile, voire impossible, d’extrapoler le comportement d’un acier inoxydable dans des conditions particulièrement corrosives telles que celles revendiquées sur la base d’observations réalisées dans des conditions très faiblement oxydantes (ex : H₂SO₄). Cela est encore plus difficile lorsqu’il est en outre nécessaire de prendre en compte l’influence de la température. Il existe donc un besoin pour améliorer la résistance à la corrosion intergranulaire et pour une corrosion uniforme de pièces en acier en contact avec un environnement oxydant et corrosif, en particulier comportant de l’acide nitrique et des espèces oxydantes telles que Pu, Np, Cr(VI), Ce(IV) et V(V). Résumé de l'invention L‘invention concerne l’utilisation d’un matériau en acier au contact d’un milieu acide présentant un pH inférieur à 5, le matériau en acier étant composé de grains comprenant une matrice dans laquelle sont incorporés des précipités, le matériau en acier comprenant : i) les éléments suivants, en pourcentages en masse sur la base de la masse du matériau en acier : 16 % à 20 % de chrome, 8 % à 14 % de nickel, 0,001 % à 0,030 % de carbone, 0,001 % à 0,050 %, de préférence 0,001 % à 0,030 %, d'oxygène, 2 % au plus de manganèse, 3 % au plus de molybdène, 0,75 % au plus de silicium, 0,045 % au plus de phosphore, 0,03 % au plus de soufre, autres éléments : moins de 0,5 %, fer : complément à 100 %, ii) des précipités sphériques, dont la taille varie entre 1 nm et 150 nm, et comprenant un élément métallique choisi parmi l’yttrium, le titane, le fer, le chrome, le tungstène, le silicium, le zirconium, le thorium, le magnésium, le manganèse, l’aluminium, le hafnium, le molybdène et leurs mélanges ; iii) des grains non colonnaires de morphologie équiaxe dont la taille est inférieure à 40 µm, les grains comportant, voire consistant en, - une structure cellulaire mésoscopique faite de cellules ayant un diamètre inférieur à 1 µm, et des structures cellulaires microscopiques internes chacune à une des cellules de la structure cellulaire mésoscopique, les cellules de la structure cellulaire microscopique présentant un diamètre inférieur à 100 nm et étant réparties de manière régulière au sein de la matrice des cellules de la structure cellulaire mésoscopique ; et - des précipités sphériques dont la taille est comprise entre 1 nm et 10 nm, dits « nano- précipités », plus de 50 % en nombre étant répartis le long des parois des cellules microscopiques, avec une densité surfacique moyenne de 40 précipités par µm² ; et iv) entre 68 % et 82 % de joints de grain de type HAGB, entre 1 % et 16 % de joints de type LAGB, et entre 9 % et 26 % de joints de macles. Le milieu acide peut comporter un acide choisi parmi l’acide sulfurique, l’acide chlorhydrique, l’acide fluorhydrique, l’acide nitrique et leurs mélanges. De préférence, le milieu acide comporte de l’acide nitrique. Il peut en outre comporter optionnellement une espèce oxydante choisie parmi Pu, Np, Cr(VI), Ce(IV) et V(V). Le milieu acide peut présenter un pH inférieur à 4, voire inférieur à 3, voire inférieur à 2, voire inférieur à 1, voire inférieur à 0. Le milieu acide peut être liquide ou gazeux. La température du matériau en acier peut être supérieure à 80°C, de préférence supérieure à 100°C et, de préférence, inférieure à 130 °C. Le matériau en acier peut être utilisé en tant que pièce choisie parmi : - un amortisseur de choc, - une pièce d’un moteur à combustion interne ou d’un moteur électrique, - une pièce d’une turbine, - une pièce d’une machine-outil, - une pièce d’une pompe, - un outil de coupe, - un outil de frappe, - une cuve d’un réacteur, notamment d’un réacteur nucléaire, d’un réacteur chimique, pétrochimique ou pharmaceutique, - une pièce d’un meuble, et - une pièce d’un équipement électroménager. Le matériau peut être utilisé dans un ou plusieurs des domaines suivants : - le domaine des équipements publics, - l’industrie chimique, pétrochimique ou pharmaceutique, - le domaine de l’automobile, de l’aéronautique, de l’aérospatial, du transport nautique ou du transport ferroviaire, - l’industrie alimentaire, - le bâtiment, - le domaine médical, - la construction navale, - le domaine de la production d’énergie, par exemple d’origine nucléaire, hydraulique ou thermique, - le domaine de l’outillage de frappe et de coupe, - le domaine des équipements mobiliers, et - le domaine des équipements électroménagers. Notamment, le matériau peut être utilisé dans un environnement radioactif, par exemple en tant que composant d’un réacteur nucléaire. Définitions La « taille » d’une structure, par exemple un précipité ou un grains, est la plus grande distance qui sépare deux frontières opposées de la structure. Elle est mesurée par traitement d’image à partir d’images acquises en microscopie optique (MO) et/ou en microscopie électronique à balayage (MEB). Le « diamètre » d’une cellule est la plus grande distance qui sépare deux frontières opposées de la cellule. Un joint de grain « de type HAGB » (acronyme anglais de « High Angle Grain Boundary ») est un joint de grain fortement désorienté, qui présente un angle de désorientation supérieur à 10°, mesuré par microscopie électronique à balayage couplé à un détecteur de diffraction d’électrons rétrodiffusés, dénommé détecteur EBSD. Un joint de grain « de type LAGB » (acronyme anglais de « Low Angle Grain Boundary ») est un joint de grain faiblement désorienté, qui présente un angle de désorientation compris entre 2° et 10° mesuré par microscopie électronique à balayage couplé à un détecteur EBSD. Un mode de balayage de type « HAADF STEM » (acronyme anglais de “Scanning Transmission Electron Microscopy High Angle Annular Dark Field”) est un mode de balayage à champ sombre annulaire à grand angle, présentant un contraste particulier entre les différentes phases observées. Brève description des figures D'autres caractéristiques et avantages de l'invention apparaitront au cours de la lecture de la description détaillée qui va suivre, pour la compréhension de laquelle on se reportera aux figures annexées dans lesquelles : [Fig.1] a été décrite ci-dessus ; [Fig.2] montre un récapitulatif des spécificités microstructurales connues pour le 316L obtenu par SLM : (a) schéma pour indiquer les différentes échelles de longueur de la microstructure, (b) figure de pôle inverse de diffraction par rétrodiffusion d'électrons (EBSD) révélant les orientations des grains, (c) image MEB montrant les bains de fusion, les joints fortement désorientés (HAGB) et les structures de solidification cellulaire, (d) image de microscopie électronique à transmission (MET) des cellules de solidification, (e) image MET à balayage à champ noir annulaire à angle élevé (HAADF) (STEM) des cellules de solidification illustrées en d, (f) EBSD acquise avec un Image EBSD de taille 1 μm (g) de HAGB superposés et de joints de grains faiblement désorientés (LAGB). Représentation de la légende, HAGB (>10°) colorés en bleu et LAGB (2–10°) colorés en rouge. La fraction des HAGB et des LAGB est d'environ 59% et d'environ 41%, (h) Carte de désorientation moyenne du noyau pour démontrer la désorientation locale à travers le grain individuel, (i) Image HAADF STEM montrant la ségrégation des éléments d'alliage Mo et Cr dans la structure cellulaire et les joints de grains faiblement désorientés, tandis que l'EDS confirme les teneurs locales en Fe, Mo et Cr correspondantes à cette ségrégation. La carte EDS confirme également que ces particules sont principalement riches en Si, O et Mn d’après Shubhavardhan Ramadurga Narasimharaju et al., Journal of Manufacturing Processes, 75 (2022), 375–414. [Fig. 3] illustre la triple structuration d’un exemple de matériau en acier 316L selon l’invention, fabriqué par fabrication additive par fusion sélective par laser sur lit de poudre ; [Fig.4] illustre une sous-structure cellulaire nanométrique organisée à l’intérieur d’un réseau de cellules sub-micrométriques de plus grandes taille ; [Fig.5] représente l’évolution de la perte de masse en fonction du temps d’une pièce faite du matériau en acier selon l’invention et de deux pièces faites d’un acier de l’art antérieur, les pièces étant en contact avec une solution d’acide nitrique ; [Fig.6] contient des photographies de tranches à la fin de l’expérience des pièces dont les pertes de masse sont représentées sur la figure 5 ; et [Fig. 7] contient des photographies acquises en microscopie électronique à balayage (A, C et E) et des cartographies correspondantes acquises par interférométrie (B, D, et E) des microstructures des aciers des exemples illustrés sur les figures 5 et 6. Le tableau 1 indique les gammes de teneur massique des éléments chimiques composant la poudre en acier utilisée pour fabriquer un exemple de matériau en acier utilisé selon l’invention, ainsi qu’à titre comparatif, les teneurs correspondantes telles que définies par les normes ASTM A666 et RCC-MRx ; [Table 1] Le tableau 2 précise la teneur massique et atomique des éléments chimiques au sein de la matrice et au sein des précipités d’une ’poudre en acier ’mise en œuvre pour fabriquer le matériau utilisé selon l’invention. [Table 2] Le tableau 3 indique la teneur massique des éléments chimiques dans un exemple de matériau en acier utilisé selon l’invention, mesurés au sein de la matrice et des précipités. [Table 3] Description Title: Use of a triple-structured steel in an acidic environment Technical field of the invention The present invention is part 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 appears a network of internal nanometric sub-cells capable of improving the properties and performances of the steels. Technical background The corrosion processes of an alloy depend on the composition and metallurgical properties, notably the microstructure, of the alloy, and the properties of the environment surrounding the alloy (pH, degree of oxidation, temperature, etc. ….). Stainless steels with a low carbon content, ie less than 0.025% by mass, are known to have good corrosion resistance in an oxidizing environment, 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 limits of use when the concentration of HNO ₃ and/or oxidizing ions and/or the temperature of the corrosive environment increases. In particular, they are then subject to intergranular type corrosion, also called CIG corrosion. The increased acidic reactivity of a grain boundary relative to the grains it separates may be associated with local chemical composition differences within the alloy, for example element segregation and/or presence of impurities, and/or 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 control. It is known to add additional chemical elements to the composition of steels to try to limit the development of intergranular corrosion. However, these steels Modified steels exhibit a higher generalized corrosion rate than unmodified steels, thereby limiting the lifespan of the components from which they are made. New stainless steel-based materials are obtained by additive manufacturing, in particular by selective powder bed laser melting, known by the abbreviation SLM, an acronym for “Selective Laser Melting”. Generally, the grains of steel materials produced by SLM additive manufacturing are columnar or equiaxed depending on the direction of construction of the part. Furthermore, these materials also exhibit structural heterogeneities inside the grains due to the rapid cooling induced by the manufacturing process. In these heterogeneities, we find, for example, a cellular structure inside 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 are made up of sub-micrometric cells, with a diameter of less than 1 μm, which essentially result from rapid melting and solidification induced by the SLM process. This material also contains spherical and amorphous precipitates distributed homogeneously in the matrix and at the joints between cells. These are mainly oxides rich in silicon and manganese and 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 even aluminum. The amount of oxygen in the build 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 a 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 fusion on a powder bed: influence of process parameters, powder characteristics, and heat treatments on microstructure and mechanical properties. » (2019) (tel.archives-ouvertes.fr/tel-02421550) describe and illustrate the different known microstructures of 316 L steel. To the knowledge of the inventors, the known materials obtained by SLM manufacturing from a 316L steel powder present , in acidic and corrosive environments, resistance to generalized corrosion better than forged 316L steel. However, they remain poorly resistant to intergranular corrosion in 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 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) depending on 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 areas of potential can be defined on the anodic curve: If the environment is very little oxidizing (e.g. H ₂ SO ₄ ), austenitic steels are in their active range characterized by uniform corrosion, the speed 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 area of use of these materials, it is protected by a passive oxide layer mainly made up of chromine (Cr2O3). In this area, the steel undergoes uniform dissolution. If the environment becomes very oxidizing (HNO ₃ 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. Under 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 stainless steel in particularly corrosive conditions such as those claimed on the basis of observations carried out under very weakly oxidizing conditions (e.g. H ₂ SO ₄ ). 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 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% chromium, 8% to 14% nickel, 0.001% to 0.030% carbon , 0.001% to 0.050%, preferably 0.001% to 0.030%, oxygen, not more than 2% manganese, not more than 3% molybdenum, not more than 0.75% silicon, not more than 0.045% phosphorus, 0 .03% at most sulfur, other elements: less than 0.5%, iron: 100% complement, 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 their mixtures; iii) non-columnar grains of equiaxed morphology whose size is less than 40 µm, the grains comprising, or even consisting of, - a mesoscopic cellular structure made of cells having a diameter less than 1 µm, and internal microscopic cellular structures each with 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 whose size is between 1 nm and 10 nm, called “nano-precipitates”, more than 50% in number being distributed along the walls of 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 chosen from sulfuric acid, hydrochloric acid, hydrofluoric acid, nitric acid and mixtures thereof. Preferably, the acidic medium comprises nitric acid. It may also optionally comprise an oxidizing species chosen from Pu, Np, Cr(VI), Ce(IV) and V(V). The acidic medium may have a pH 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 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 piece of furniture, and - a piece of household appliances. The material can be used in one or more of the following fields: - the field of public equipment, - the chemical, petrochemical or pharmaceutical industry, - the field of automobiles, aeronautics, aerospace, transport nautical or rail transport, - the food industry, - construction, - the medical field, - shipbuilding, - the field of energy production, for example of nuclear, hydraulic or thermal origin, - the field striking and cutting tools, - the field of furniture equipment, and - the field of household appliances. In particular, the material can 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 (English acronym for “High Angle Grain Boundary”) is a strongly disoriented grain boundary, which has a disorientation angle greater than 10°, measured by scanning electron microscopy coupled to a detector of backscattered electron diffraction, called EBSD detector. A “LAGB type” grain boundary (English acronym for “Low Angle Grain Boundary”) is a weakly disoriented grain boundary, which has a disorientation angle of between 2° and 10° measured by scanning electron microscopy coupled with a EBSD detector. A “HAADF STEM” type scanning mode (English 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 reading of the detailed description which follows, for the understanding of which we will refer 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 disoriented boundaries (HAGB) and cell solidification structures, (d) transmission electron microscopy (TEM) image of solidification cells, ( e) High angle annular dark field (HAADF) scanning TEM (STEM) image of the solidification cells shown in d, (f) EBSD acquired with a 1 μm sized EBSD image (g) of superimposed HAGBs and joints of weakly disoriented grains (LAGB). Representation of the legend, HAGB (>10°) colored in blue and LAGB (2–10°) colored in red. The fraction of HAGBs and LAGBs is ~59% and ~41%, (h) Average core misorientation map to demonstrate local misorientation across the individual grain, (i) HAADF STEM image showing segregation of alloying elements Mo and Cr 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 316L steel material according to the invention, manufactured by additive manufacturing by selective laser melting on a powder bed; [Fig.4] illustrates a nanometric cellular substructure organized inside a network of larger sub-micrometric cells; [Fig.5] represents the evolution of the loss of mass 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 nitric acid solution; [Fig.6] contains photographs of slices at the end of the experiment of the parts whose mass losses are represented 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 ranges of mass content 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 standards ASTM A666 and RCC- MRx; [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 chemical elements in an example of steel material used according to the invention, measured within the matrix and the precipitates. [Table 3]

Detailed Description The material can have a relative density between 70.0% and 99.9%. The relative density makes it possible 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 weld pools whose depth is less than 100 µm. Molten pools result from the manufacturing process of the steel material. For example, during additive manufacturing by selective laser melting, also called SLM additive manufacturing (English acronym for “selective laser melting”), the laser beam melts powder particles following a predefined scan. The powder is melted in the form of liquid weld pools which cool and solidify quickly after passing the laser. After solidification, residual marks revealing the trace of the molten baths remain present in the microstructure of the steel material. The average depth of the marks associated with the limits of the melting pools is for example 79 µm. Preferably, the steel material has the chemical composition of a 316L or 304L type steel, for example as specified respectively in standard ASTM A666 or RCC-MRx. The steel material can be 100% austenitic in structure. “Other elements” are elements other than chromium, nickel, carbon, oxygen, manganese, molybdenum, silicon and iron. The steel material may comprise, as a percentage by mass, at least one of the following other elements: - 0.11% at most nitrogen, - 0.045% at most phosphorus, - 0.05% at most sulfur, - 0.0300% at most aluminum, - 0.003% at most vanadium, - 0.75% at most copper, - 0.10% at most cobalt, - 0.003% at most titanium. The other elements may be present in the matrix and/or in the precipitates, particularly in 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 can be dissolved in the matrix. The precipitates may include at least one metal oxide, at least one intermetallic compound and mixtures thereof. The metal oxide and/or the intermetallic compound may each comprise at least one metallic element chosen from titanium, iron, chromium and their mixtures. The steel material may include spherical precipitates which are oxides of manganese and silicon and whose average size varies between 10 nm and 150 nm. The steel material may include up to 2%, for example 0.1% to 1.5% of at least one metal oxide, in mass percentages 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 material in steel. The metal oxide can be more particularly 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 each other. M, M' and M'' are each preferably chosen from yttrium, titanium, iron, chromium, tungsten, silicon, zirconium, thorium, magnesium, manganese, aluminum, hafnium and molybdenum, preferably from titanium, iron, chromium. For example, the “x” index for different compounds is as follows: - x = 0: TiO2 - x = 1: FeO - x = 0.5: Fe ₂ O ₃ - x = 2/3: Fe ₃ O ₄ The 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, Cr ₂ O ₃ , TiO ₂ , Al ₂ O ₃ , HfO ₂ , SiO ₂ , ZrO ₂ , ThO ₂ , MgO, MnO, MnO ₂ and their mixtures. 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 TiO ₂ . 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'' can 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' can be a pyrochlore compound, for example Y ₂ Ti ₂ O ₇ , YTi ₂ O ₅ and their mixture. Preferably, the mixed oxide is TiYO5-x'. The mixed oxide MM'_y'M''_y'' O _5-x'' has for example a general formula of the “SiOAlMn” type noted without a 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 YFe ₃ , Fe ₂ Ti, FeCrWTi and their mixtures. FeCrWTi is a name known to those skilled in the art, which is not a stoichiometric formula. The precipitates may contain at least one metal oxide and at least intermetallic compound. The material may include 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-x with 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 their mixtures, 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 YFe ₃ , Fe ₂ Ti, FeCrWTi and their mixtures. 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 their mixtures, 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 their mixtures. According to one embodiment of the invention, the material comprises precipitates comprising: - an oxide of Mn and Si; - optionally, a simple oxide from Y2O3, Fe2O3, FeO, Fe3O4, Cr2O3, TiO2, Al2O3, HfO2, SiO2, ZrO2, ThO2, MgO, MnO, MnO2 and their mixtures, - a mixed oxide chosen from FeTiO ₃ , Y ₂ Ti ₂ O ₇ , YTi ₂ O ₅ 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, Cr ₂ O ₃ , TiO ₂ , Al ₂ O ₃ , HfO ₂ , SiO ₂ , ZrO ₂ , ThO ₂ , MgO, MnO, MnO ₂ and their mixtures , - a mixed oxide chosen from FeTiO3, Y2Ti2O7, YTi2O5 and their mixtures, - a mixed oxide of general formula of SiOAlMn type, and - an intermetallic compound chosen from YFe ₃ , Fe ₂ Ti, FeCrWTi and their mixtures. According to one embodiment of the invention, the material comprises: an oxide of Mn and Si; - optionally, a simple oxide chosen from Y ₂ O ₃ , Fe ₂ O ₃ , FeO, Fe ₃ O ₄ , Cr2O3, TiO2, Al2O3, HfO2, ZrO2, ThO2, MgO, and their mixtures, - a mixed oxide chosen from FeTiO3 , Y2Ti2O7, YTi2O5 and their mixtures, - a mixed oxide of general formula of the SiOAlMn type, and - an intermetallic compound chosen from YFe3, Fe2Ti, FeCrWTi and their mixtures. The grain size of the material of the invention can be measured by analysis of images acquired by Scanning Electron Microscopy (SEM) coupled to 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 the said images. The average size of non-columnar grains with equiaxed morphology can be 25 µm. The grains can be close to 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 segregation into chemical elements during the manufacture of the steel material. A difference in chemical composition appears locally between the cell wall and matrix of the mesoscopic cellular structure. The wall is for example enriched in Cr, Mo, weakly in Ni and depleted in Fe compared to the cellular matrix. At the level of these cells of the mesoscopic cellular structure, a network of dislocations appears. The cell shape of the microscopic cell structure can be close to the cell shape of the mesoscopic cell structure. The cells of the microscopic cellular structure can form a honeycomb network within the cell matrix of the mesoscopic cellular structure. The microscopic cellular structure, the mesoscopic cellular structure and the arrangement of grains within the matrix thus define a steel material presenting a triple structuring at different scales. In particular, the microscopic cellular structure is specific to the material used according to the invention. It modifies arrangements on a small scale and impacts the properties of the material. The steel material used according to the invention is thus called “triple structured steel”. The cell diameter of the mesoscopic cellular structure is preferably greater than 100 nm. The average cell diameter 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 cell diameter 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 nano-precipitates 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 then be processed with image processing software such as for example "ImageJ" software available at following Internet address: imagej.net/Welcome. The steel material may include 0.1% to 1.5% by mass of nano-precipitates relative to the total mass of the material. This precipitate content can be measured by selective dissolution with aqua regia. The surface density of precipitates is the number of precipitates per unit area. It can be determined by counting via imaging, such as for example scanning electron microscopy (SEM) or transmission electron microscopy (TEM) imaging. Manufacturing process The steel material can be manufactured from 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, particularly if the powder is then treated by a selective laser fusion process on a powder bed, for example of the SLM or L-PBF type. Unless otherwise stated, any characteristic of the precipitates or matrix contained in the steel powder subjected to the consolidation process is identical to the corresponding characteristic of the precipitates or 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 of manufacturing the steel material from the steel powder. The analyzes 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 can 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 particles in the powder have a size less than d50. It can be determined by a technique such as the laser diffraction method via a particle size analyzer described for example in standard ISO 13320 (edition 2009-12-01). The apparent density of the powder measured by the ASTM B-212 standard 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 to a tray of layers of particles on top of each other, particles of each newly deposited layer being linked 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", Engineering techniques, Fascicle 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 Annex 2 (pages 205 to 220) in particular when it describes additive manufacturing processes using metal powder (Annex 2, Manufacturing processes, paragraphs 3, 4 and 5). Preferably, the additive manufacturing process can be chosen from a selective powder bed laser melting process, a selective powder bed electron beam melting process, a selective powder bed laser sintering process. , a laser projection method and a binder projection method. The additive manufacturing process can be a selective powder bed laser melting process, also called SLM additive manufacturing process after the acronym in English for “Selective Laser Melting”. The selective powder bed laser melting process can 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); - power of the laser beam: from 50 W to 1000 W; - distance between vector space: from 25 µm to 150 µm; - layer thickness: from 15 µm to 80 µm. Alternatively, the additive manufacturing process may be a selective electron beam melting process on a powder bed, called the EBM additive manufacturing process after the English acronym for “Electron Beam Melting”. The selective electron beam fusion process on a powder bed can be implemented by controlling one or more of the following parameters: - power of the electron beam: from 50 W to 4000 W; - speed of the electron beam: from 100 mm/s to 10000 mm/s; - distance between vector space: 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 can be a thermal spraying process, for example chosen from a spraying process thermal flame, an electric arc projection process between two wires or a blown plasma projection process. At the end of the process of manufacturing the material of the invention in which the steel powder is subjected to the consolidation process, the steel material is in solid form. At the end of the material manufacturing process, in particular thanks to the raw material and the volume density of energy applied, 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. Hot isostatic compression may comprise the following successive steps, carried out in an enclosure comprising an inert gaseous 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 speed temperature rise of between 500 and 1000°C/hour; B. maintaining the temperature for a period of between 15 minutes and 5 hours; C. cooling of the material at a rate of temperature drop of between 500 and 1000°C/hour to ambient temperature, for example between 20°C and 25°C. The inert gaseous atmosphere may comprise a gas chosen from argon, helium or their mixture. Example EXAMPLES 1. Steel powder for making 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 steel powder. 1.1. Chemical composition. The steel powder, type 316 L, reference FE-271-3 / TruForm 316-3 - lot no. 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 with a Scanning Electron Microscope (SEM FEG Zeiss ULTRA55), by glow discharge mass spectrometry (GDMS) using the Element GD Plus system (Thermo Fisher), by optical emission spectrometry with inductively coupled plasma (ICP-OES for “Inductively Coupled Plasma – Optical Emission Spectrometry”) using the Optima 8300 DV (Perkin Elmer) and by instrumental gas analysis (IGA for “Instrumental gas analysis”) using the Horiba EMGA-920 chemical analyzer. The elemental composition of the matrix and the 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, unmeasured chemical elements are disregarded for the matrix. It is then considered that the remaining mass percentage is made up of iron. - in % by 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 oxides of aluminum, titanium, silicon and manganese 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, however, 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 equipment used was the Brücker D8 Advance diffractometer (Bragg–193 Brentano u–2u geometry, CuKa radiation l=1.54060 Å) The particles of this powder include agglomerated grains and are most often essentially spherical. They have a diameter of between 10 µm and 100 µm, and an average diameter of 34 µm. More particularly, the median diameters D10, D50 and D90 (for which, respectively, 10%, 50% and 90% by number of particles composing this powder have a size smaller than the median diameter considered) measured by laser particle size analysis 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 powder particles are most often spherical. Their maximum dimensions are such that the size measured by scanning electron microscope (SEM) imaging 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 using 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 real density measured by a Helium pycnometer is 7.99 g/cm ³ ± 0.03 g/cm ³ . The Hall flowability (capacity to flow 50 g of powder through an orifice of fixed size) measured according to the ASTM B213 standard is 15 seconds. 2. Process for manufacturing a steel material. A part composed 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 scan followed a predefined path. At the end of this scanning, a consolidated layer n is obtained. Then, a rotation of 67° of the laser scanning direction is carried out and a new layer n+1 is formed which is superimposed on the underlying layer n. The process is thus repeated until the part is completely produced. The main operating parameters of the SLM process are as follows: - Yb laser fiber with a wavelength of 1070 nm; - diameter of the laser spot: 30 µm; - laser power: 120 W; - laser scanning speed: 950 mm/s; - distance between two successive laser lines (“Hatching distance”): 60 µm; - thickness of the powder bed: 30 µm; - composition of the gaseous medium of the construction chamber: argon, with an oxygen content less 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 manufacturing, the parts were extracted by cutting the base of the test pieces to separate them from the stainless steel substrate. No additional treatment was applied to the material thus obtained. The density of the steel material constituting the test pieces is 7.93 g/cm ³ (measured 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 analysis of images obtained by optical microscopy and is 99.95%. By modifying at least one of the following parameters, the density could be increased without however 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. Density generally scales parabolicly with laser power or laser scanning speed. However, too low or too high a power or scanning speed can potentially decrease 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 indicated in Table 1. The elemental composition of the material was measured by EDX analysis. It is similar to the composition of steel powder used to make steel material. However, the chemical elements are distributed differently between the matrix and the precipitates. The precipitates of the material were studied using TEM (MET FEI Tecnai F20 FEG-TEM) coupled to an EDX detector (EDX Bruker XFlash 6T | 60). The identified precipitates 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 have also been highlighted at the level of the cells of the mesoscopic elementary structure. The cells of the material of the invention were studied using TEM (MET FEI Tecnai F20 FEG-TEM) coupled to 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 can have characteristics comparable to those of large cells in terms of differences in local chemical composition. 3.2. Morphology. An X-ray diffraction (“XRD”) analysis 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. The oxide precipitates are incorporated into the matrix of the grains which constitute the steel material or in the joints 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 which make up this material are non-columnar and approximate 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 source energy in motion during manufacturing). This microstructural particularity of the material is such that the quasi-equiaxed 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 25 µm). Furthermore, the crystallites which are the grains of the steel material have a preferential orientation. This texture of the material results in the fact that the directions are preferably oriented parallel to the construction direction z, but also in 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 particularly a size less than an average diameter of 500nm). Figure 3 as well as Figure 4 also show the nanometric cellular substructure which is organized directly inside the sub-micrometric cells of larger sizes, while also showing the small precipitates less than 10 nm and incorporated in 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 presenting the triple structuring illustrated in Figure 4 was immersed in a solution of nitric acid having a pH between -1 and 0 for a test duration 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 concentration of nitric acid in 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 solution. nitric acid to simulate the effects of Pu and Np. Furthermore, a comparative test piece made of 316L steel obtained by additive manufacturing from a conventional powder and which is free of cellular structure internal to the mesoscopic cells, and a comparative test piece made of forged 316 L steel, presenting the same dimensions as the specimen made of triple structured steel underwent the same corrosion test. As observed in Figure 5, the specimens manufactured by additive manufacturing have a lower corrosion rate than the forged steel specimen. After 240 hours, the mass loss is more than twice as low for the specimens obtained by additive manufacturing. The triple-structured steel specimen also has a similar mass loss, although lower, than the comparative specimen manufactured by L-PBF additive manufacturing. However, these specimens present different corrosion profiles, although their mass loss is similar. As observed in Figure 6, the comparative forged steel specimen (top) and the comparative specimen obtained by additive manufacturing (middle) present numerous attacks localized to the grain boundaries which propagated from the surface, in contact with the nitric acid solution. These attacks on grain boundaries, with a depth greater than 10 µm, or even 20 µm, are typical of furrows 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 on the micrographs in Figure 7 as well as on 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 quickly propagates in depth and causes a loosening of the grains on the surface of the material, thus altering the 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 grains on the surface) 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 specimen triple structured, 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 in comparison with those of the forged steel specimen and those comparatively formed by additive manufacturing. More precisely, concerning resistance to intergranular corrosion, the absence of a groove which develops in depth for the material of the invention guarantees better strength 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. Protection against corrosion of mesoscopic cells is ensured by the presence of a nanometric cellular structure internal to large cells

Claims

Claims 1. Use of a steel material in contact with an acidic medium having a pH less than 5 and comprising nitric acid and an oxidizing species chosen 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% chromium, 8% to 14% nickel, 0.001% to 0.030% carbon, 0.001% to 0.050%, preferably 0.001% to 0.030% , oxygen, not more than 2% manganese, not more than 3% molybdenum, not more than 0.75% silicon, not more than 0.045% phosphorus not more than 0.03% sulfur, other elements: less than 0, 5% iron: 100% complement, 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 whose size is less than 40 µm, the grains comprising, or even consisting of - a mesoscopic cellular structure made of cells having a diameter less than 1 µm, and internal microscopic cellular structures each with a 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 whose size is between 1 nm and 10 nm, called “nano-precipitates”, more than 50% in number being distributed along the walls of 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. 2. Use according to claim 1, the acidic medium having a pH 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 piece of furniture, and - a piece of household appliances.