NEW IRON-BASED ALLOY STRUCTURE

Abstract

The present invention relates to the development of new iron-based alloys, in particular, a steel having improved mechanical, thermal and physicochemical (corrosion) properties. The invention relates more particularly to a steel material having a new 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 steels.

Description

Title of the invention: NEW IRON-BASED ALLOY STRUCTURE Technical field of the invention

[0001] The present invention falls within the scope of the development of new iron-based alloys, in particular, of a steel having improved mechanical, thermal and physicochemical (corrosion) properties.

[0002] The invention relates more particularly to a steel material having a new 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 steels. Technical background

[0003] The microstructures of materials depend, among other things, on their chemical composition and their manufacturing process.

[0004] Modifying the microstructure of steels (the shape and size of grains, the presence or absence of solidification cells, etc.) is an important issue and arouses great interest.

[0005] Additive manufacturing is a technology that allows parts to be designed by adding material, unlike "conventional" processes (machining, forging, rolling, etc.) which are characterized by material removal (subtractive processes). The principle of additive manufacturing (also called "3D printing") is thus that of a generative manufacturing process which can be summed up in two stages repeated until the finished product is obtained:

[0006] 1. generation of a layer of material following a fixed contour and thickness. The material is deposited only where it is needed;

[0007] 2. production of the new layer by adding material above the layer previous.

[0008] Among the additive manufacturing processes, 3D printing by selective laser melting on powder bed or SLM (Selective Laser Melting in English), is one of the most widespread and used for the production of metal parts. It is an additive manufacturing technique allowing the production of metal parts thanks to

[0009] - a high-power laser which fuses certain regions (identified upstream during of the programming step) of a metal powder deposited in the form of a bed of powders a few micrometers thick (for example between 20 pm and 70 pm) using a scraper on a plate, then

[0010] - rapid solidification / cooling.

[0011] The SLM process is capable of acting on the size and shape of the powder grains.

[0012] Generally, the grains of steels produced by selective laser powder bed fusion (SLM) are described as being columnar or equiaxed depending on the construction direction of the part. However, by modifying the shape and size of the grains, one can act on the microstructure of the grains and therefore the final properties of the produced steels.

[0013] In addition, these materials also exhibit structural heterogeneities within the grains due to the rapid cooling induced by the manufacturing process. In these heterogeneities, we find, for example, a so-called cellular structure within the grains. [Fig.1a] and [Fig.1b] summarize the structures currently known for 316L stainless steel. A recent review from 2022 (Shubhavardhan Ramadurga Narasimharaju et al., Journal of Manufacturing Processes, 75 (2022), 375-414) summarizes the different cellular structures known for 316L obtained by SLM. This is shown in [Fig.2].

[0014] The grains of 316L steel produced by SLM are made up of submicrometric cells, with diameters less than 1 μm. This intragranular substructure is essentially due to the rapid melting and solidification processes.

[0015] The 316L steel produced by SLM also contains spherical and amorphous nanoprecipitates distributed homogeneously in the matrix and cell joints. These are mainly nano-oxides rich in silicon and manganese and with sizes varying between ten and several hundred nanometers. The oxides mainly observed in the literature for the 316L steel produced by SLM are composed of chromium, titanium, nickel, iron or even aluminum. The quantity of oxygen in the construction chamber has a direct influence on the characteristics of the nanoprecipitates present in the parts. If the oxygen content is high (1000-2000 ppm) then the nanoprecipitates will be numerous but of small size (50-100 nm). If the content is on the contrary low (300-500 ppm) then there will be fewer precipitates but of larger size (50 nm-2 pm).

[0016] These nano-oxides have an impact on the properties of parts produced by SLM. Nano-precipitates have a detrimental effect on resilience and stress corrosion cracking when located in grain boundaries. However, they can harden the material by blocking the movement of dislocations during deformation or promote the "pinning" effect which inhibits grain growth. As a result, the tensile properties of the material are improved.

[0017] The presence of structural heterogeneities at the level of the microstructure of the steel grains leads to internal defects, which is generally detrimental with regard to the resistance and thermal and / or mechanical constraints of the steel.

[0018] To counter the effects of rapid solidification (the appearance of heterogeneity) of an alloy, particularly a steel, one lever is to act on the microstructure of the grains in modifying the so-called cellular structure within the grains of the alloy, particularly steel. The cellular structure itself constitutes a chemical heterogeneity within the material. By modifying the organization of this nanometric structure, the heterogeneity of the steel evolves, ultimately impacting the structure and macroscopic properties of the material.

[0019] There is therefore a real need for a new iron-based alloy, in particular a new steel, having a new microstructure which makes it possible to improve the mechanical, thermal and physicochemical (corrosion) properties of the iron-based alloy, in particular steel. Summary of the invention

[0020] The present invention aims precisely to meet this need and others, by providing a steel material with improved mechanical, thermal and physicochemical (corrosion) properties, thus being able to better resist chemical, mechanical and / or thermal stresses.

[0021] The present invention relates to a steel material whose grains comprising it comprise a matrix in which precipitates are incorporated,

[0022] the material comprises:

[0023] i) the following elements, in mass percentage: - 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%, of oxygen, - 0% to 2% manganese, - 0% to 3% molybdenum, - 0% to 1% silicon, - the rest being made up of iron;

[0024] ii) marks associated with the boundaries of oval melt pools whose depth is less than 100 pm present within the material or in the matrix;

[0025] iii) spherical precipitates with an average size ranging between 10 and 150 nm, identified as being oxides of Mn and Si, in connection with the rapid solidification of the material, the precipitates comprising at least one metallic element chosen from a metallic element M, a metallic element M', a metallic element M" or mixtures thereof; with M, M' and M” being chosen, independently of one another, from yttrium, titanium, iron, chromium, tungsten, silicon, zirconium, thorium, magnesium, manganese, aluminium, hafnium, molybdenum or mixtures thereof;

[0026] iv) non-columnar grains of equiaxed morphology whose average size is less than 40 pm;

[0027] the steel material is characterized in that

[0028] - the distribution of grain boundaries on the surface of the material is 74.2% of boundaries strongly disoriented >10° or HAGB (High-Angle Grain Boundary in English), 8.6% of weakly disoriented joints between 2° and 10° or LAGB (Low-Angle Grain Boundary in English), and 17.2% of twin joints;

[0029] - the grains that make up the material are made up - sub-micrometric cells, having an average diameter of less than 1 pm, and a cellular structure itself consisting of an internal nanometric cellular substructure whose cells have an average diameter of less than 100 nm and are organized in a regular manner uniformly covering the matrix of sub-micrometric cells; and - spherical precipitates with an average size between 1 and 10 nm, distributed mainly along the walls of said internal nanometric cellular substructure and with an average surface density of 40 precipitates per pm2.

[0030] According to one embodiment of the invention, the average size of the non-columnar grains approaching an equiaxed morphology is on average 25 μm.

[0031] According to one embodiment of the invention, the grains which make up the material are made up of sub-micrometric cells, having an average diameter of 385 nm on average.

[0032] According to one embodiment of the invention, the grains which compose the material are made up of sub-micrometric cells having an average diameter as defined below, and a cellular structure itself made up of an internal nanometric cellular sub-structure whose cells have an average diameter of 30 nm on average and are organized in a regular manner uniformly covering the matrix of sub-micrometric cells.

[0033] According to one embodiment of the invention, the grains which make up the material are also made up of spherical precipitates whose average size is on average 9 nm, distributed mainly along the walls of said internal nanometric cellular substructure and with an average surface density of 40 precipitates per pm2.

[0034] Molten pools appear during the manufacturing process. The laser melts the raw material following a predefined scan. The raw material is melted in the form of liquid melt pools which cool and solidify quickly just after the laser passes through. After solidification, marks relating to these melt pools remain present in the microstructure of the steel.

[0035] The expressions “within the material” or “in the matrix” are effectively equivalent in step ii).

[0036] In the context of the present invention, the "matrix" of the steel material has the chemical composition of a 316 L or 304 L type steel, for example as specified respectively in ASTM A666 or RCC-MRx.

[0037] The expression "the remainder being made up of" means that the sum of the chemical elements in the material or in the steel powder (raw material) totals 100%. This therefore does not exclude the presence of other minor chemical elements.

[0038] By "average size" we mean the distance which separates two opposite boundaries to the described structure (precipitates, grains) measured by image processing from observations in optical microscopy (OM) and scanning electron microscopy (SEM).

[0039] By "average diameter" is meant the distance which separates two opposite boundaries of the described structure (cells), whose morphology is spherical.

[0040] By “HAGB” is meant “High Angle Grain Boundary” which corresponds to grain boundaries which have a high misorientation angle (greater than 10°) measured by scanning electron microscopy coupled with an EBSD detector.

[0041] By “LAGB” is meant “Low Angle Grain Boundary” which corresponds to grain boundaries which have a low misorientation angle (between 2° and 10°) measured by scanning electron microscopy coupled with an EBSD detector.

[0042] By “HAADF STEM” is meant “Scanning Transmission Electron Microscopy High Angle Annular Dark Field”, corresponding to a wide-angle annular dark field scanning mode, presenting a particular contrast between the different phases observed.

[0043] The invention also relates to the use of a steel material according to the invention in the following fields:

[0044] - public facilities,

[0045] - the chemical, petrochemical and pharmaceutical industry,

[0046] - the transport sector such as automotive, aeronautics, aerospace, nautical transport, rail transport,

[0047] - the food industry,

[0048] - the building,

[0049] - the medical sector,

[0050] - shipbuilding,

[0051] - mechanical components,

[0052] - the energy sector such as nuclear, hydraulic, thermal,

[0053] - striking and cutting tools,

[0054] - the furniture,

[0055] - household appliances.

[0056] The invention further relates to a part comprising in whole or in part a steel material according to the invention. Said part may be used in the aforementioned fields. Brief description of the figures

[0057] 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:

[0058] [Fig.1a] and [Fig.1b] summarize the structures currently known for 316L stainless steel according to [1] GT Gray et al.. Acta Mater., vol. 138, p. 140 149, (2017); [2] A. Leicht et al., Materials Characterization, 159 (2020) 110016; and [3] 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).

[0059] [Fig.2] shows a summary of the known microstructural specificities for 316L obtained by SLM: (a) schematic 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 (HAGB) and cellular solidification structures, (d) transmission electron microscopy (TEM) image of the 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 pm size EBSD image (g) of superimposed HAGB and weakly misoriented grain boundaries (LAGB). Representation of the legend, HAGB (>10°) colored in blue and LAGB (2-10°) colored in red.The fraction of HAGB and LAGB is about 59% and about 41%, (h) Average core misorientation map to demonstrate local misorientation across individual grain, (i) HAADF STEM image showing segregation of alloying elements Mo and Cr in cellular structure and weakly misoriented grain boundaries, while EDS confirms Fe, Mo and Cr 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. .

[0060] [Fig.3] shows a table indicating ranges of overall mass content of the chemical elements composing the steel powder used to manufacture a material steel according to the invention, as well as for comparison, the corresponding contents as defined by the ASTM A666 and RCC-MRx standards.

[0061] [Fig.4] shows a table specifying the mass and atomic content of the chemical elements within the matrix and within the precipitates of a steel powder according to the invention.

[0062] [Fig.5] represents a table indicating the mass content of the chemical elements in the steel material of the invention, within the matrix, of the oxide precipitates.

[0063] [Fig.6] shows the triple structuring of a 316L steel material according to the invention manufactured by SLM additive manufacturing.

[0064] [Fig.7] shows a nanometric cellular substructure that organizes itself directly within a network of larger sub-micrometric cells. Detailed description of the invention

[0065] The present invention relates to a steel material whose grains comprising it comprise a matrix in which precipitates are incorporated,

[0066] the material comprises:

[0067] i) the following elements, in mass percentage: 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%, of oxygen, 0% to 2% manganese, 0% to 3% molybdenum, 0% to 1% silicon, the rest being iron;

[0068] ii) marks associated with the boundaries of oval melt pools whose depth is less than 100 pm present within the material or in the matrix;

[0069] iii) spherical precipitates with an average size varying between 10 and 150 nm, identified as being oxides of Mn and Si, in connection with the rapid solidification of the material, the precipitates comprising at least one metallic element chosen from a metallic element M, a metallic element M', a metallic element M" or mixtures thereof; with M, M' and M” being chosen, independently of one another, from yttrium, titanium, iron, chromium, tungsten, silicon, zirconium, thorium, magnesium, manganese, aluminium, hafnium, molybdenum or mixtures thereof;

[0070] iv) non-columnar grains of equiaxed morphology whose average size is less than 40 pm;

[0071] the steel material is characterized in that

[0072] - the distribution of grain boundaries on the surface of the material is 74.2% of boundaries strongly disoriented >10° or HAGB (High-Angle Grain Boundary in English), 8.6% of weakly disoriented joints between 2° and 10° or LAGB (Low-Angle Grain Boundary in English), and 17.2% of twin joints;

[0073] - the grains that make up the material are made up - sub-micrometric cells, having an average diameter of less than 1 pm, and a cellular structure itself consisting of an internal nanometric cellular substructure whose cells have an average diameter of less than 100 nm and are organized in a regular manner uniformly covering the matrix of sub-micrometric cells; and - spherical precipitates with an average size between 1 and 10 nm, distributed mainly along the walls of said internal nanometric cellular substructure and with an average surface density of 40 precipitates per pm2.

[0074] The steel material according to the invention has improved mechanical, thermal and physicochemical (corrosion) properties, thus being able to better withstand mechanical and / or thermal stresses.

[0075] Melt pool marks are present within the material. The depth and morphology of the melt pools vary depending on the parameters implemented during the additive manufacturing process. The material of the invention has an oval melt pool morphology with a depth of less than 100 μm. According to one embodiment, the depth is an average of 79 μm.

[0076] The distribution of grain boundaries on the surface of the material is 74.2% of highly disoriented boundaries >10° or HAGB, 8.6% of weakly disoriented boundaries between 2° and 10° or LAGB, and 17.2% of twin boundaries. This distribution is specific to the material of the invention.

[0077] In addition, the grains of this steel are made up of:

[0078] - of sub-micrometric cells, having an average diameter of less than 1 pm (of 385 nm on average according to one embodiment) and which result from a segregation of chemical elements. A difference in chemical composition appears locally between the wall and the matrix of these cells. The wall is enriched in Cr, Mo, weakly in Ni and depleted in Fe compared to the cell matrix. At the level of these cells, a network of dislocations appears but not only. Indeed, the cellular structure is itself made up of an internal nanometric cellular substructure, the cells of which have an average diameter of less than 100 nm (30 nm on average according to one embodiment of the invention). The shape of these internal subcells is close to the shape of large cells. The cells smaller than 100 nm are organized in a regular manner and uniformly cover the matrix large cells. The morphology of the network is similar to a "honeycomb" structure. This network is part of a triple structuring of the material (grains, cells, nanometric sub-cells). This cellular substructure internal to the large cells is specific to the material of the invention. It is also this original additional structure that modifies the small-scale arrangements and ultimately impacts the macroscopic properties of the material.

[0079] As already indicated, the spherical precipitates whose size is between 1 and 10 nm (9 nm on average according to one embodiment of the invention), are distributed mainly along the walls of said internal nanometric cellular substructure and with an average surface density of 40 precipitates per pm2. The quantity of these precipitates is moreover greater than that observed in the material having the conventional microstructure.

[0080] The average grain size of the material of the invention is measured by analysis of SEM (Scanning Electron Microscopy) maps coupled with an EBSD (Electron BackScatter Diffraction) detector.

[0081] The grains, the composition of which is equivalent to the composition of the matrix of the material of the invention, can be described as being 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. In addition to the parallel plane, the grains can further be described as 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.

[0082] 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.

[0083] The average grain size is for example calculated by averaging the measurements obtained on at least 10 grains, or even at least 50 grains by Scanning Electron Microscopy (SEM) imaging coupled with an EBSD detector.

[0084] The material may have a relative density of 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.

[0085] The inventors succeeded in developing a new microstructure for the grains by modifying the internal structure of the sub-micrometer cells by segmenting it into a smaller network in order to modify the properties and performance of the steel material. These small internal sub-cells form an ordered network. The morphology of the network is similar to a "honeycomb" structure.

[0086] The smallest spherical precipitates whose average size is between 1 and 10 nm (9 nm on average according to one embodiment of the invention) also called nano-precipitates are located most often along the walls of the nanometric cellular substructure internal to large cells, and this with an average surface density of 40 precipitates per pm2.

[0087] Concerning the composition of the precipitates, they may comprise at least one metal oxide, at least one intermetallic compound, or their mixtures. Each of this oxide or intermetallic compound comprises at least one metallic element chosen from the metallic element M, the metallic element M', the metallic element M" or their mixtures. Preferably, each of this oxide or intermetallic compound comprises the metallic element M, in particular titanium, iron, chromium or their mixtures, optionally the metallic element M' with optionally the metallic element M", or the mixture of these metallic elements.

[0088] The material of the invention may comprise 0.1% to 2% by mass, for example 0.1% to 1.5% by mass of at least one oxide of Mn and Si, relative to the total mass of the material.

[0089] The material of the invention may comprise 0% to 1.5% by mass, for example 0.1% to 1.5% by mass of at least one metal oxide, relative to the total mass of the material.

[0090] The material of the invention may comprise spherical precipitates whose average size varies between 10 and 150 nm, and which are spherical Mn and Si oxides whose size varies between 10 and 150 nm.

[0091] It should be noted that the possibility that other metal oxides consisting of other elements are present is not excluded.

[0092] The metal oxide contained in the precipitates of the steel material of the invention may be chosen from at least one simple oxide, at least one mixed oxide or their mixtures.

[0093] The metal oxide is more particularly chosen from at least one simple oxide MO2x with the index x between 0 and 1, at least one mixed oxide MM'yOw with 0 < x' < 5 and 0 < y' < 2, or at least one mixed oxide MM'yM"y with 0 < x" < 5, 0 < y' < 2 and 0 < y" < 2, or mixtures of these oxides.

[0094] For example, the index “x” for different compounds is as follows:

[0095] - x = 0 : TiO2

[0096] - x = 1 : FeO

[0097] - x = 0.5: Fe2O3

[0098] - x = 2 / 3 : Fe3O4

[0099] The index “y'” is for example equal to 0, 1 or 2.

[0100] The metallic element M contained in the simple oxide MO2 x, the mixed oxide MM'y05 x or the mixed oxide MM'y'M"y”05Y is more particularly chosen from yttrium, iron, chromium, titanium, aluminum, hafnium, silicon, zirconium, thorium, magnesium or manganese.

[0101] The simple oxide MO2 x is for example chosen from Y2O3, Fe2O3, FeO, Fe3O4, Cr2O3, TiO2, A12O3, HfO2, SiO2, ZrO2, ThO2, MgO, MnO, MnO2 or their mixtures.

[0102] According to one embodiment of the invention, the metallic element M contained in the simple oxide MO2 x is chosen from titanium, iron or chromium. More particularly, the simple oxide MO2 x is TiO2.

[0103] The metallic element M contained in the mixed oxide MM'y05 x is, for example, chosen from iron or yttrium.

[0104] The metallic element M' contained in the mixed oxide MM'yOs^ or the mixed oxide MM'y'M"y"05_X" is, more particularly, chosen from titanium or yttrium.

[0105] According to one embodiment of the invention, the mixed oxide MM'y05 x is chosen from FeTiO3, Y2Ti2O7, YTi2O5 or their mixtures.

[0106] According to one embodiment of the invention, the mixed oxide MM'y05 x is a pyrochlore compound, for example Y2Ti2O7 or YTi2O5 or their mixture.

[0107] According to one embodiment of the invention, the mixed oxide is TiYO5 x.

[0108] The mixed oxide MM'y'M"y"05_X" is, for example, of general formula of the type " SiOAlMn » noted without stoichiometric index.

[0109] The precipitates may also comprise at least one intermetallic compound comprising the metallic element M, the metallic element M' or possibly the metallic element M".

[0110] The material may optionally comprise 0% to 1.5% by mass, for example 0.1% to 1.5% by mass of the intermetallic compound relative to the total mass of the material.

[0111] The metallic element M contained in the intermetallic compound is for example iron.

[0112] The metallic element M' contained in the intermetallic compound is for example titanium or yttrium.

[0113] The metallic element M" contained in the intermetallic compound is for example chromium or tungsten.

[0114] The intermetallic compound is, for example, chosen from YFe3, Fe2Ti, FeCrWTi or their mixtures. FeCrWTi is a name known to those skilled in the art, but does not correspond to a true stoichiometric formula.

[0115] The metal oxide and the intermetallic compound may possibly coexist in the precipitates of the material.

[0116] According to one embodiment of the invention, the material comprises precipitates comprising:

[0117] - at least one spherical Mn and Si oxide whose size varies between 10 and 150 nm;

[0118] -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' / ^ with 0 < x' < 5 and 0 < y' < 2, or at least one mixed oxide MM'y'M"y"05Y with 0 < x" < 5, 0 < y' < 2 and 0 < y" < 2, or mixtures of these oxides, with M, M' and M" chosen from yttrium, iron, chromium, titanium, aluminum, hafnium, silicon, zirconium, thorium, magnesium or manganese;

[0119] - optionally at least one intermetallic compound chosen from YFe3, Fe2Ti, FeCrWTi or their mixtures;

[0120] or mixtures thereof.

[0121] According to one embodiment of the invention, the material comprises precipitates comprising:

[0122] - Mn and Si oxide;

[0123] - the simple oxide MO2 x is chosen from Y2O3, Fe2O3, FeO, Fe3O4, Cr2O3, TiO2, A12O3, HfO2, SiO2, ZrO2, ThO2, MgO, MnO, MnO2 or mixtures thereof,

[0124] - the mixed oxide MM'yOs ^ is chosen from FeTiO3, Y2Ti2O7, YTi2O5 or their mixtures,

[0125] - the mixed oxide MM'yM"y"05_X" has the general formula of the SiOAIMn type.

[0126] According to one embodiment of the invention, the material comprises precipitates comprising:

[0127] - Mn and Si oxide;

[0128] - the simple oxide MO2 x is chosen from Y2O3, Fe2O3, FeO, Fe3O4, Cr2O3, TiO2, A12O3, HfO2, SiO2, ZrO2, ThO2, MgO, MnO, MnO2 or mixtures thereof,

[0129] - the mixed oxide MM'y05 x is chosen from FeTiO3, Y2Ti2O7, YTi2O5 or their mixtures,

[0130] - the mixed oxide MM'yM"y"05_X" has the general formula of the SiOAIMn type.

[0131] - the intermetallic compound chosen from YFe3, Fe2Ti, FeCrWTi or their mixtures.

[0132] The average size of the precipitates contained in the nanometric substructure of the steel material is between 1 nm and 10 nm. In one embodiment of the invention, the spherical precipitates have an average size of 9 nm on average.

[0133] The average size of the precipitates can be determined visually from a measurement made on an image obtained with a Scanning Electron Microscope (SEM), to then be processed with image processing software such as for example "ImageJ" software available at the following Internet address: imagej.net / Welcome.

[0134] The steel material of the invention may comprise 0.1% to 1.5% by mass of spherical precipitates whose size is between 1 and 10 nm or nano-precipitates relative to the total mass of the material. This precipitate content may for example be measured by selective dissolution with aqua regia.

[0135] The surface density with which the precipitates are distributed in the joints of the nanometric cellular substructure is advantageously 40 precipitates per pm2.

[0136] The surface density of precipitates is the number of precipitates per unit area.

[0137] It can be determined by counting via imaging, such as for example imaging by Scanning Electron Microscopy (SEM) or by Transmission Electron Microscopy (TEM).

[0138] The steel material according to the invention is manufactured from a steel powder (P) subjected to a consolidation process, for example. The steel powder (P) has the same chemical composition as the material.

[0139] The steel powder (P) can be obtained conventionally by gas atomization under nitrogen or argon, but also obtained by water atomization, in particular if the powder is then treated by a selective laser powder bed fusion process (SLM or L-PBF).

[0140] Generally, the steel material obtained at the end of the manufacturing process of the invention is as defined in the present description. Unless otherwise stated, any characteristic of the precipitates or of the matrix contained in the steel powder (P) subjected to the manufacturing process of the invention is identical to the corresponding characteristic of the precipitates or of the matrix contained in the steel material of the invention, this characteristic being described in more detail in the description of the steel material. More particularly, unless otherwise stated, the size, the distribution in the matrix of the precipitates, or the chemical composition of the precipitates or of the matrix are not modified between the steel powder (P) and the steel material of the invention.Indeed, the analyses carried out on the steel powder (P) by the inventors confirm that the size, distribution and composition of the precipitates are equivalent in the powder and in the steel material.

[0141] Concerning more particularly the steel powder (P), the particles of the powder can have a median diameter (d50) between 10 pm and 200 pm. The median diameter (d50) of a powder is the size for which 50% of the population of particles composing this powder has 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 the ISO 13320 standard (edition 2009-12-01).

[0142] The apparent density of the powder (P) measured by the ASTM B-212 standard can be between 3.5 g / cm3 and 4.5 g / cm3.

[0143] The actual density of the powder can be between 7.95 g / cm3 and 8.05 g / cm3. It is for example measured with a pycnometer.

[0144] The steel powder advantageously has a 100% austenitic structure.

[0145] The consolidation process used in the manufacturing process of the invention is advantageously an additive manufacturing process.

[0146] As indicated previously, an additive manufacturing process comprises two steps repeated until the solid finished material is obtained: 1. Generation of a layer of material following a fixed contour and thickness. 2. Creation of the new layer by adding material above the previous layer.

[0147] At the end of the additive manufacturing process, the successive layers of material forming the material are stacked in a direction perpendicular to the plate of the 3D printer on which the first layer of material was deposited.

[0148] Additive manufacturing is described in more detail for example in the following documents which are incorporated by reference into this description:

[0149] - F. Laveme et al., "Additive manufacturing - General principles", Techniques de the engineer, Booklet BM7017 V2 (publication of February 10, 2016);

[0150] - H. Fayazfara et al., “critical review of powder-based additive manufacturing of ferrons alloys: Process parameters, microstructure and mechanical properties", Materials & Design, Volume 144, 2018, Pages 98-128;

[0151] - T. DebRoy et al., "Additive manufacturing of metallic components - Process, structure and properties", Progress in Materials Science, Volume 92, 2018, pages 112-224;

[0152] - 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 metal powder (Appendix 2, Manufacturing processes, paragraphs 3, 4 and 5).

[0153] More particularly, the additive manufacturing method may be chosen from a selective laser powder bed melting method, selective electron beam powder bed melting method, selective laser powder bed sintering method, laser projection method or binder projection method.

[0154] The selective laser melting (SLM) process may be performed according to one or more of the following parameters:

[0155] - the laser beam scans the steel powder at a scanning speed of between 50 mm / second (dense material) and 3000 mm / second (porous material);

[0156] - laser beam power: 50 W to 1000 W;

[0157] - distance between vector space: 25 pm to 150 pm;

[0158] - layer thickness: 15 pm to 80 pm.

[0159] The selective powder bed electron beam fusion process (EBM (from the English acronym for "Electron Beam Melting") can be carried out according to one or more of the following parameters:

[0160] - electron beam power: 50 W to 4000 W;

[0161] - electron beam speed: 100 mm / s to 10,000 mm / s;

[0162] - distance between vector space: 50 pm to 150 pm;

[0163] - layer thickness: 40 pm to 75 pm.

[0164] The laser projection process can be carried out according to one or more of the following parameters:

[0165] - laser power: 400 W to 3,000 W;

[0166] - nozzle movement speed: 150 mm / min to 1200 mm / min;

[0167] - powder flow rate: 4 g / min to 15 g / min.

[0168] The thermal spraying process is for example chosen from a flame thermal spraying process, an electric arc spraying process between two wires or a blown plasma spraying process.

[0169] At the end of the manufacturing process of the material of the invention in which the steel powder (P) is subjected to a consolidation process, the material is more particularly in massive form.

[0170] At the end of the material manufacturing process, in particular thanks to the raw material and the volume density of energy applied, the micro structure of the grains, in particular the structure of their sub-micrometric cells, is modified with the appearance of a nanometric cellular sub-structure.

[0171] In addition to the elements cited, the steel material or steel powder (P) used in the manufacturing process of the material of the invention may comprise, in percentage by mass, at least one of the following elements: 0% to 0.11% nitrogen, 0% to 0.045% phosphorus, 0% to 0.05% sulfur, 0% to 0.0300% aluminum, 0% to 2% manganese, 0% to 3% molybdenum, 0% to 0.003% vanadium.

[0172] These additional chemical elements may be present in the matrix and / or in the precipitates or nano-precipitates.

[0173] The matrix may comprise, in proportion by mass relative to the mass of the material or relative to the mass of the steel powder (P) used in the process of manufacturing the material of the invention, 0 ppm to 5000 ppm of the metallic element M, of the metallic element M' and / or of the metallic element M". This is the metallic element dissolved in the matrix, as opposed to that present in the precipitates.

[0174] The metallic element M, M' or M" contained in the matrix may be more particularly chosen from yttrium, titanium, tungsten, zirconium, thorium, aluminum, hafnium, silicon, manganese or molybdenum.

[0175] The material or steel powder (P) used in the manufacturing process of the invention may be of austenitic structure.

[0176] The matrix (and therefore by extension the steel material of the invention or the steel powder used in the manufacturing process of the invention) advantageously has the chemical composition of a steel of type 316 L or 304 L, for example, as specified respectively in the ASTM A666 or RCC-MRx standard.

[0177] The potential applications for a material according to the invention are in particular all those where a metallic object is subjected to conditions of use in an aggressive environment (corrosion, irradiation and temperature) and where there is mechanical stress (assembly elements, reactor vessel, pressure equipment, turbines, tools, shock absorbers, etc.).

[0178] A steel material according to the invention remains preferred in almost all technical application fields:

[0179] - public facilities (bridges and roads, signaling), chemical industry, petrochemical, pharmaceutical and nuclear (pressure equipment, equipment subject to the action of flames, storage capacities, various containers), agri-food (packaging and storage), construction (frames, frameworks, ironwork, hardware), mechanical and thermal industry (engines, turbines, compressors), automotive (bodywork, equipment), railway, aeronautics and aerospace, shipbuilding, medical (instruments, devices and prostheses), mechanical components (screws, springs, cables, bearings, gears), striking tools (hammers, chisels, dies) and cutting tools (milling cutters, drills, insert holders), furniture, design and household appliances, etc.

[0180] The invention thus relates to the use of a steel material according to the invention in the following fields:

[0181] - public facilities,

[0182] - the chemical, petrochemical and pharmaceutical industry,

[0183] - the transport sector such as automotive, aeronautics, aerospace, nautical transport, rail transport,

[0184] - the food industry,

[0185] - the building,

[0186] - the medical sector,

[0187] - shipbuilding,

[0188] - mechanical components,

[0189] - the energy sector such as nuclear, hydraulic, thermal,

[0190] - striking and cutting tools,

[0191] - the furniture,

[0192] - household appliances.

[0193] The invention further relates to a part comprising in whole or in part a steel material according to the invention. Said part may be used in the aforementioned fields.

[0194] The part according to the invention can be manufactured by an additive manufacturing process. More particularly, the additive manufacturing process is chosen from a selective laser powder bed melting process, selective electron beam powder bed melting process, selective laser powder bed sintering process, laser projection or binder projection process.

[0195] The method of manufacturing the part according to the invention may be followed by a treatment method comprising a hot isostatic pressing step. This hot isostatic pressing step 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. the material is brought to a constant temperature between 600 and 1400°C at a temperature rise rate between 500 and 1000°C / hour; B. the constant temperature is maintained for a period of between 15 minutes and 5 hours; C. the constant temperature is reduced according to a temperature reduction rate of between 500 and 1000°C / hour in order to reach room temperature (20 + 5°C).

[0196] The inert gaseous atmosphere may comprise a gas selected from argon, helium or a mixture thereof. EXAMPLES

[0197] 1. Steel powder used in the manufacturing process of the material of the invention.

[0198] Generally, the steel powder has a composition as shown in [Fig.3], namely a mass composition including that of a steel meeting the 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).

[0199] Characterization of steel powder.

[0200] 1.1. Chemical composition.

[0201] A steel powder (316 L steel reference FE-271-3 / TruForm 316-3 - lot no. 32-034043-10 marketed by Praxair) is analyzed by X-ray microanalysis, more precisely by energy dispersive X-ray spectroscopy (or EDX according to the English acronym for "Energy Dispersive X-ray Spectroscopy"). The analysis system used is BRUKER Quantax XFlash.

[0202] The steel powder is also analyzed by Scanning Electron Microscope (MEB FEG Zeiss ULTRA55), as well as by Glow Discharge Mass Spectrometry (GDMS) using Element GD Plus (Thermo Fisher), by Inductively Coupled Plasma - Optical Emission Spectrometry (ICP-OES) using Optima 8300 DV (Perkin Elmer) and Instrumental Gas Analysis (IGA) using Horiba EMGA-920 Chemical Analyzer.

[0203] The elemental composition of the matrix and precipitates of the steel powder obtained is 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 ignored for the matrix. It is then considered that the remaining mass percentage is made up of iron. - in mass % and atomic % relative to the total mass of precipitates contained in the steel powder.

[0204] These proportions are normalized by relating the total mass or the total number of atoms to a value of 100. They are reproduced in [Fig.4] 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.

[0205] 1.2. Morphology

[0206] The steel powder has a 100% austenitic structure. The 100% austenitic phase is analyzed by X-ray Diffraction (XRD). The apparatus used is Brucker D8 Advance diffractometer (Bragg-193 Brentano u-2u geometry, CuKa radiation 1=1.54060 Â)

[0207] The particles of this powder comprise grains agglomerated to form particles which are most often essentially spherical. They have a diameter between 10 pm and 100 pm, and an average diameter of 34 pm More particularly, the median diameters Di0, D50 and D90 (for which, respectively, 10%, 50% and 90% of the population of particles making up this powder has a size smaller than the median diameter considered) measured by laser granulometry according to standard ISO 13320 (edition 2009-12-01) are as follows: Di0 = 22 pm, D50 = 32 pm, and D90 = 48 pm.

[0208] The precipitates contained in the powder particles are most often spherical. Their maximum dimensions (which therefore most often correspond to the diameter of the spherical particle) are such that the size measured by imaging with a Scanning Electron Microscope (SEM) is generally between 24 nm and 120 nm. Their corresponding average size is 63 nm.

[0209] The density with which the precipitates are distributed in the matrix is ​​measured by counting by SEM imaging: it is between 2 precipitates / pm3 and 100 precipitates / pm3. The corresponding average density is 6 precipitates / pm3.

[0210] 1.3. Properties.

[0211] The apparent density of steel powder measured by ASTM B-212 is 4 g / cm3 ± 0.01 g / cm3. Its actual density measured by Helium pycnometer is 7.99 g / cm3 ± 0.03 g / cm3.

[0212] Hall flowability (ability to flow 50 g of powder through a fixed-size orifice) measured according to ASTM B213 is 15 seconds.

[0213] 2. Method of manufacturing a material according to the invention.

[0214] A part made of the steel material according to the invention is manufactured by additive manufacturing with the selective laser melting process on powder bed (SLM) with a Trumpf model TruPrint Series 1000 printer.

[0215] To manufacture the part, on a stainless steel substrate, the laser scan follows a path defined as lines or stripes in English. At the end of this first scan, a first consolidated n layer is obtained. Then, a new rotation of 67° of the laser scanning direction is carried out and a new n+1 layer is superimposed on the underlying n layer.

[0216] The main operating parameters of the SLM process are as follows: - Yb laser fiber with a wavelength of 1070 nm; - laser spot diameter = 30 pm; - laser power = 120 W; - laser scanning speed = 950 mm / s; - distance between two successive laser lines ("Hatching distance") = 60 pm; - powder bed thickness = 30 pm; - composition of the gaseous medium of the construction chamber = argon, with an oxygen content of less than 100 ppm during consolidation.

[0217] Ten parallelepiped specimens (length / height = 30 mm, width = 20 mm, thickness = 7 mm) are obtained. After manufacturing, the parts are extracted by cutting the base of the specimens to separate them from the stainless steel substrate.

[0218] No further treatment is applied to the raw material obtained.

[0219] The density of the steel material constituting the test pieces is 7.93 g / cm3 (measured by the Archimedes method), i.e. a relative density of 99.25% considering a theoretical density for a 316 L steel which is 7.99 g / cm3. The density of the material of the invention was identified by analysis of images obtained by optical microscopy and is 99.95%.

[0220] By modifying at least one of the following parameters, this density can be increased without modifying the grain size of the steel material:

[0221] - laser power = 50 W to 400 W;

[0222] - laser scanning speed = 50 mm / s to 3000 mm / s.

[0223] Density generally scales parabolically with laser power or laser scanning speed. However, too low or too high a power or scanning speed can possibly decrease density.

[0224] The distance between two successive laser lines ("Hatching distance") is for example between 30 pm and 90 pm.

[0225] 3. Characterization of the steel material according to the invention obtained by the method of manufacturing described in paragraph 2.

[0226] 3.1. Chemical composition.

[0227] The overall chemical composition of the steel material obtained by the manufacturing process described in the preceding example complies with the ASTM A666 and RCC-MRx standards indicated in the Table of [Fig.3].

[0228] The elemental composition of this alloy is measured by EDX analysis. It is very similar to that of the steel powder used to manufacture the steel material. However, within the steel material, the chemical elements are distributed differently between the matrix and the precipitates. The precipitates 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 I 60). The precipitates identified correspond to oxides of Mn and Si but this does not exclude the presence of other precipitates of a different nature within the material of the invention.

[0229] Differences in local chemical composition have also been highlighted at the level of sub-micrometric cells whose size is less than 1 μm. The cells of the material of the invention were studied using MET (MET FEI Tecnai F20 FEG-TEM) coupled with an EDX detector (EDX Bruker XFlash 6T I 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 nanometric sub-cells internal to the large cells of the material of the invention can present characteristics comparable to those of the large cells in terms of differences in local chemical composition.

[0230] 3.2. Morphology.

[0231] An X-ray diffraction ("XRD") analysis using Brucker D8 Advance Diffractometer (Bragg-193 Brentano u-2u geometry, CuKa radiation 1 =1.54060 Å), shows that the steel material has a 100% austenitic structure.

[0232] Oxide precipitates are incorporated into the matrix of the grains that make up the steel material or in the spaces between these grains (grain boundaries). The average density with which these precipitates are distributed in the matrix is ​​6 precipitates / pm3.

[0233] The average size of the oxide precipitates is between 10 nm and 150 nm.

[0234] One of the particularities of the material of the invention is a microstructure such that the grains which compose this material are non-columnar and approach an equiaxed structure. In particular, when the material of the invention 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).

[0235] This microstructural feature of the material of the invention 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 pm (average size of 25 pm).

[0236] 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 construction direction z, but also by a texture intensity equal to 1.4.

[0237] As illustrated by [Fig.6], 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). [Fig.6] as well as [Fig.7] also shows the nanometric cellular substructure which is organized directly inside the larger sub-micrometric cells, while also showing the small precipitates less than 10nm and incorporated in the matrix, in particularly at the level of the cell walls which appear in a lighter shade.

Claims

1. Claims Steel material whose grains comprise a matrix in which precipitates are incorporated, the material comprises: (i) the following elements, in mass percentage: - 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%, of oxygen, - 0% to 2% manganese, - 0% to 3% molybdenum, - 0% to 1% silicon, optionally, at least one of the following: - 0% to 0.11% nitrogen, - 0% to 0.045% phosphorus, - 0% to 0.05% sulfur, - 0% to 0.0300% aluminum, - 0% to 2% manganese, - 0% to 3% molybdenum, - 0% to 0.003% vanadium, - the rest being made up of iron; (ii) marks associated with the boundaries of oval melt pools with a depth of less than 100 pm present within the material or in the matrix; iii) spherical precipitates with an average size ranging between 10 and 150 nm, identified as oxides of Mn and Si, in connection with the rapid solidification of the material, the precipitates comprising at least one metallic element selected from a metallic element M, a metallic element M', a metallic element M" or mixtures thereof; with M, M' and M” being selected, independently of each other, from yttrium, titanium, iron, chromium, tungsten, silicon, zirconium, thorium, magnesium, manganese, aluminium, hafnium, molybdenum or mixtures thereof; iv) non-columnar grains of equiaxed morphology with an average size of less than 40 pm; the steel material is characterized in that - the distribution of grain boundaries on the surface of the material is 74.2% of highly disoriented boundaries >10° called HAGB, 8.6% of weakly disoriented boundaries between 2° and 10° called LAGB, and 17.2% of twin boundaries; - the grains that make up the material are made up of - sub-micrometric cells, having an average diameter of less than 1 pm, and a cellular structure itself made up of an internal nanometric cellular substructure whose cells have an average diameter of less than 100 nm and are organized in a regular manner uniformly covering the matrix of sub-micrometric cells; and - spherical precipitates whose average size is between 1 and 10 nm, distributed mainly along the walls of said internal nanometric cellular substructure and with an average surface density of 40 precipitates per pm2.

2. Material according to claim 1, characterized in that it has a relative density of between 70.0% and 99.9% measured by the Archimedes method.

3. Material according to claim 1 or 2, characterized in that the internal nanometric cellular substructure forms an ordered network whose morphology is similar to a honeycomb structure.

4. Material according to any one of claims 1 to 3, characterized in that it comprises 0.1% to 2% by mass of at least one oxide of Mn and Si, relative to the total mass of the material.

5. Material according to any one of claims 1 to 4, characterized in that it comprises 0.1% to 1.5% by mass of spherical precipitates whose size is between 1 and 10 nm or nanoprecipitates, relative to the total mass of the material.

6. Material according to any one of claims 1 to 5, characterized in that the matrix has the chemical composition of a type 316 L or 304 L steel, as specified respectively in ASTM A666 or RCC-MRx.

7. Material according to any one of claims 1 to 6, characterized in that it comprises precipitates comprising: - at least one spherical Mn and Si oxide whose size varies between 10 and 150 nm; - at least one metal oxide chosen from at least one simple oxide MO2x with the index x between 0 and 1, at least one mixed oxide MM'y O5x with 0 < x' < 5 and 0 < y' < 2, or at least one mixed oxide MM'y'M"y"05_X" with 0 < x" < 5, 0 < y' < 2 and 0 < y" < 2, or mixtures of these oxides, with M, M' and M" chosen from yttrium, iron, chromium, titanium, aluminum, hafnium, silicon, zirconium, thorium, magnesium or manganese; - optionally at least one intermetallic compound chosen from YFe3, Fe2Ti, FeCrWTi or mixtures thereof; or mixtures thereof.

8. Material according to any one of claims 1 to 7, characterized in that it comprises precipitates comprising: - the oxide of Mn and Si; - the simple oxide MO2 x is chosen from Y2O3, Fe2O3, FeO, Fe3O4, Cr2O3, TiO2, Al2O3, HfO2, SiO2, ZrO2, ThO2, MgO, MnO, MnO2 or mixtures thereof, - the mixed oxide MM'y05 x is chosen from FeTiO3, Y2Ti2O7, YTi2O5 or mixtures thereof, - the mixed oxide MM'y M"y"O5 x" is of general formula of the SiOAIMn type.

9. Use of a steel material according to any one of claims 1 to 8, in the following fields: - public equipment, - the chemical, petrochemical and pharmaceutical industry, - the transport sector such as automotive, aeronautics, aerospace, nautical transport, rail transport, - the food industry, - construction, - the medical sector, - shipbuilding, - mechanical components, - the energy sector such as nuclear, hydraulic, thermal, - striking and cutting tools, - furniture,

10. - household appliances. Part comprising in whole or in part a steel material according to any one of claims 1 to 8.

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