NEW IRON-BASED ALLOY STRUCTURE

Description

TECHNICAL SCOPE OF THE INVENTION

The present invention forms part of the development of new iron-based alloys, particularly a steel with improved mechanical, thermal and physico-chemical (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 performance of steels.

TECHNICAL BACKGROUND

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

Modifying the microstructure of steels (the shape and size of grains, the presence or absence of solidification cells, etc.) is a major challenge and is attracting a great deal of interest.

Additive manufacturing is a technology that enables parts to be designed by adding material, unlike “conventional” processes (machining, forging, laminating, etc.) which are characterized by the removal of material (subtractive processes). The principle of additive manufacturing (also known as “3D printing”) is therefore that of a generative manufacturing process consisting of two repeated stages until the finished product is obtained:

• • 1. generating a layer of material with a fixed contour and thickness. The material is deposited only where it is needed;
  • 2. creating the new layer by adding material on top of the previous layer.

Among additive manufacturing processes, 3D printing by selective laser melting (SLM) is one of the most widespread and is used to produce metal parts. It is an additive manufacturing technique that enables metal parts to be produced using

• • a high-power laser which fuses certain regions (identified upstream during the programming stage) of a metal powder deposited in the form of a bed of powders a few micrometers thick (for example between 20 μm and 70 μm) using a scraper on a plate, then
  • a rapid solidification/cooling.

The SLM process is capable of influencing the size and shape of powder grains.

Generally speaking, the grains of steels produced by selective laser melting on a powder bed (SLM) are described as columnar or equiaxed, depending on the direction of construction of the part. However, by modifying the shape and size of the grains, it is possible to influence the microstructure of the grains and therefore the final properties of the steels produced.

In addition, these materials also exhibit structural heterogeneities within the grains due to the rapid cooling induced by the manufacturing process. These heterogeneities include, for example, a so-called cellular structure within the grains. [FIG. 1a] and [FIG. 1b] summaries 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].

The grains of 316L steel produced by SLM consist of sub-micrometric cells with diameters of less than 1 μm. This intragranular substructure is essentially due to the rapid melting and solidification processes.

The 316L steel produced by SLM also contains spherical and amorphous nanoprecipitates distributed homogeneously in the matrix and cell boundaries. These are mainly nano-oxides rich in silicon and manganese and ranging in size from around ten to several hundred nanometers. The oxides most commonly observed in the literature for 316L steel produced by SLM are composed of chromium, titanium, nickel, iron and aluminum. The amount 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 small in size (50-100 nm). If, on the other hand, the oxygen content is low (300-500 ppm) then there will be fewer precipitates but they will be larger in size (50 nm-2 μm).

These nano-oxides have an impact on the properties of parts produced by SLM. Nanoprecipitates have a detrimental effect on impact strength and stress corrosion when they are located in grain boundaries. However, they can harden the material by blocking the movement of dislocations during deformation or promote the pinning effect that inhibits grain growth. As a result, the material's tensile properties are improved.

The presence of structural heterogeneities in the microstructure of steel grains leads to internal defects, which are generally detrimental to the strength and thermal and/or mechanical stresses of the steel.

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

There is therefore a real need for a new iron-based alloy, in particular a new steel, with a new microstructure that improves the mechanical, thermal and physico-chemical (corrosion) properties of iron-based alloys, in particular steel.

SUMMARY OF THE INVENTION

The aim of the present invention is precisely to meet this need and others, by providing a steel material with improved mechanical, thermal and physico-chemical (corrosion) properties, so that it can better withstand chemical, mechanical and/or thermal stresses.

This invention relates to a steel material whose constituent grains comprise a matrix in which precipitates are incorporated,

• • the material comprises:
  • i) the following elements, in percentage by mass:
    • 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,
    • 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 iron;
  • ii) marks associated with the limits of oval molten baths with a depth of less than 100 μm present within the material or in the matrix;
  • iii) spherical precipitates whose average size varies 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 metal element chosen from a metal element M, a metal element M′, a metal 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, aluminum, hafnium and molybdenum or mixtures thereof;
  • iv) non-columnar grains of equiaxed morphology with an average size less than 40 μm; 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 (High-Angle Grain Boundary), 8.6% of weakly disoriented boundaries between 2° and 100 called LAGB (Low-Angle Grain Boundary), and 17.2% of macular boundaries;
    • the grains that compose the material are composed of
      • sub-micrometric cells, having an average diameter of less than 1 μm, and a cell structure itself consisting of an internal nanometric cell sub-structure 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 of between 1 and 10 nm, distributed mainly along the walls of the said internal nanometric cell substructure and with an average areal density of 40 precipitates per μm².

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

According to one embodiment of the invention, the grains that compose the material consist of sub-micrometric cells with an average diameter of 385 nm.

According to one embodiment of the invention, the grains that compose the material are composed of sub-micrometric cells with an average diameter as defined below, and a cell structure itself composed of an internal nanometric cell 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.

According to one embodiment of the invention, the grains that compose the material also consist of spherical precipitates with an average size of 9 nm, distributed mainly along the walls of the said internal nanometric cellular substructure and with an average surface density of 40 precipitates per μm².

Molten baths are created during the manufacturing process. The laser melts the raw material following a predefined scan. The raw material is melted in the form of liquid molten baths, which cool and solidify rapidly just after the laser has passed through. After solidification, marks from these molten baths remain in the microstructure of the steel.

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

In the context of the present invention, “the matrix” of the steel material has the chemical composition of a type 316 L or 304 L steel, for example as specified respectively in the ASTM A666 (2015) or RCC-MRx (2012) standards.

The expression “the remainder consisting of” means that the sum of the chemical elements in the material or steel powder (raw material) totals 100%. This does not exclude the presence of other minority chemical elements.

By “average size” we mean the distance separating two opposite boundaries of the structure described (precipitates, grains) measured by image processing based on observations using optical microscopy (OM) and scanning electron microscopy (SEM).

By “average diameter” we mean the distance separating two opposite boundaries of the structure described (cells), whose morphology is spherical.

“HAGB” stands for “High Angle Grain Boundary”, which corresponds to grain boundaries with a high angle of disorientation (greater than 10°) measured by scanning electron microscopy coupled to an EBSD detector.

“LAGB” stands for “Low Angle Grain Boundary”, which corresponds to grain boundaries with a low angle of disorientation (between 2° and 10°) measured by scanning electron microscopy coupled to an EBSD detector.

“HAADF STEM” stands for “Scanning Transmission Electron Microscopy High Angle Annular Dark Field”, corresponding to a scanning mode with a wide angle annular dark field, presenting a particular contrast between the different phases observed.

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

• • public facilities,
  • chemical, petrochemical and pharmaceutical industries,
  • transport sector, such as the automotive, aeronautical, aerospace, marine and rail industries,
  • food industry,
  • construction,
  • medical sector,
  • shipbuilding,
  • mechanical components,
  • energy sector, such as nuclear, hydraulic and thermal power,
  • striking and cutting tooling,
  • furniture,
  • household appliances.

Another object of the invention is a part comprising all or part of a steel material according to the invention. This part can be used in the above-mentioned fields.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages of the invention will become apparent from the following detailed description, for the understanding of which reference is made to the attached figures in which:

FIG. 1a and FIG. 1b summarize the currently known structures for 316L stainless steel according to [1]G. T. Gray et al, Acta Mater, vol. 138, pp. 140 149, (2017); [2]A. Leicht et al, Materials Characterization, 159 (2020) 110016; and [3]A. Chniouel, “Etude de l'élaboration de l'acier inoxydable 316L par fusion laser selective sur lit de poudre: influence des paramètres du procédé, des caracteristiques de la poudre, et des traitements thermiques sur la microstructure et les propriétés mécaniques.” (2019) (tel.archives-ouvertes.fr/tel-02421550) (“Study of the production of 316L stainless steel by selective laser melting on a powder bed: influence of the process parameters, the characteristics of the powder and the heat treatments on the microstructure and the mechanical properties”).

FIG. 2 shows a summary of the known microstructural features of 316L obtained by SLM: (a) schematic 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 molten baths, highly disoriented boundaries (HAGB) and cell 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 μm size EBSD Image (g) of superimposed HAGBs and weakly disoriented grain boundaries (LAGB). Legend representation, HAGBs (>10°) colored blue and LAGBs (2-10°) colored red. The fraction of HAGBs and LAGBs is about 59% and about 41%, (h) Map showing average core displacement to demonstrate local disorientation through the individual grain, (i) HAADF STEM image showing the segregation of the alloying elements Mo and Cr in the cell structure and weakly disoriented grain boundaries, while the EDS confirms the 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.

FIG. 3 shows a table showing the overall mass content ranges of the chemical elements making up the steel powder used to manufacture a steel material according to the invention, and for comparison, the corresponding contents as defined by the ASTM A666 (2015) and RCC-MRx (2012) standards.

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.

FIG. 5 shows a table showing the mass content of the chemical elements in the steel material of the invention, within the matrix, of the oxide precipitates.

FIG. 6 shows the triple structuring of a 316L steel material according to the invention produced by SLM additive manufacturing.

FIG. 7 shows a nanometric cell substructure that is organised directly within a network of larger sub-micrometric cells.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a steel material whose constituent grains comprise a matrix in which precipitates are incorporated,

• • the material comprises:
  • i) the following elements, in percentage by mass:
    • 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,
    • 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 iron;
  • ii) marks associated with the limits of oval molten baths with a depth of less than 100 μm present within the material or in the matrix;
  • iii) spherical precipitates whose average size varies 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 metal element chosen from a metal element M, a metal element M′, a metal 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, aluminum, hafnium and molybdenum, or mixtures thereof;
  • iv) non-columnar grains of equiaxed morphology with an average size less than 40 μm; 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 (High-Angle Grain Boundary), 8.6% of weakly disoriented boundaries between 2° and 10° called LAGB (Low-Angle Grain Boundary), and 17.2% of macular boundaries;
    • the grains that compose the material are composed of
      • sub-micrometric cells, having an average diameter of less than 1 μm, and a cell structure itself consisting of an internal nanometric cell sub-structure 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 of between 1 and 10 nm, distributed mainly along the walls of the said internal nanometric cell substructure and with an average areal density of 40 precipitates per μm².

The steel material according to the invention has improved mechanical, thermal and physico-chemical (corrosion) properties, making it more resistant to mechanical and/or thermal stress.

Molten baths marks are present within the material. The depth and morphology of the molten baths vary according to the parameters implemented during the additive manufacturing process. The material of the invention has an oval molten bath morphology with a depth of less than 100 μm. According to one embodiment, the average depth is 79 μm.

The distribution of grain boundaries on the surface of the material is 74.2% of highly disoriented boundaries >100 called HAGB, 8.6% of weakly disoriented boundaries between 2° and 100 called LAGB, and 17.2% of macular boundaries. This distribution is specific to the material of the invention.

In addition, the grains of this steel are composed of.

• • sub-micrometric cells, with an average diameter of less than 1 μm (385 nm on average according to one embodiment) and which result from a segregation of chemical elements.

A chemical composition difference appears locally between the wall and the matrix of these cells. The wall is enriched in Cr and Mo, slightly in Ni and depleted in Fe compared to the cell matrix. A network of dislocations appears in these cells, but not exclusively. Indeed, the cell structure itself is composed of an internal nanometric cell sub-structure, the cells of which have an average diameter of less than 100 nm (of 30 nm on average according to one embodiment of the invention). The shape of these internal sub-cells is close to the shape of large cells. The cells smaller than 100 nm are regularly organized and uniformly cover the matrix of the large cells. The morphology of the network resembles a “honeycomb” structure. This network is part of a triple structuring of the material (grains, cells, nanometric sub-cells). This internal cellular substructure of the large cells is specific to the material of the invention. Moreover, it is this original additional structure that modifies the small-scale arrangements and ultimately impacts the macroscopic properties of the material.

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 the said internal nanometric cellular sub-structure and with an average surface density of 40 precipitates per μm². The quantity of these precipitates is also greater than that observed in the material with the conventional microstructure.

The average grain size of the material of the invention is measured by analyzing SEM (Scanning Electron Microscopy) mappings coupled to an EBSD (Electron BackScatter Diffraction) detector.

The grains, whose composition is equivalent to the composition of the matrix of the material of the invention, can be described as being close to equiaxed (non-columnar) morphology in a plane parallel to the plane of the superimposed layers of material that result from the manufacture of the material by an additive manufacturing process. In addition to the parallel plane, the grains may also be described as equiaxed in a plane perpendicular to the plane of the superimposed layers of material that 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 using Scanning Electron Microscopy (SEM) or optical microscopy.

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

The material can have a relative density of between 70.0% and 99.9%. The relative density is used to assess the porosity of the material. It is measured, for example, by the Archimedes method.

The inventors have succeeded in developing a new microstructure for the grains by modifying the internal structure of the sub-micrometric 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 resembles a “honeycomb” structure.

The smallest spherical precipitates, with an average size of between 1 and 10 nm (9 nm on average according to one embodiment of the invention), also known as nanoprecipitates, are most often located along the walls of the internal nanometric cellular substructure of the large cells, with an average surface density of 40 precipitates per μm².

As regards the composition of the precipitates, they may comprise at least one metal oxide, at least one intermetallic compound, or mixtures thereof. Each of this metal oxide or intermetallic compound comprises at least one metal element chosen from the metal element M, the metal element M′, the metal element M″ or mixtures thereof. Preferably, each of this oxide or intermetallic compound comprises the metal element M, in particular titanium, iron, chromium or mixtures thereof, possibly the metal element M′ with possibly the metal element M″, or the mixture of these metal elements.

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

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.

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.

It should be noted that the possibility of other metal oxides composed of other elements being present cannot be ruled out.

The metal oxide contained in the precipitates of the steel material of the invention may be chosen from at least one single oxide, at least one mixed oxide or mixtures thereof.

The metal oxide is more particularly 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′M″_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, or the mixtures of these oxides.

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

x = 0 : TiO 2 x = 1 : FeO x = 0.5 : Fe 2 ⁢ O 3 x = 2 / 3 : Fe 3 ⁢ O 4

The “y′” index is equal to 0, 1 or 2, for example.

The metal element M contained in the single oxide MO_2-x, the mixed oxide MM′_y′O_5-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 or manganese.

The MO_2-x single oxide is for example chosen from Y₂O₃, Fe₂O₃, FeO, Fe₃O₄, Cr₂O₃, TiO₂, Al₂O₃, HfO₂, SiO₂, ZrO₂, ThO₂, MgO, MnO, MnO₂ or mixtures thereof.

According to one embodiment of the invention, the metal element M contained in the single oxide MO_2-x is chosen from titanium, iron or chromium. More particularly, the single oxide MO_2-x is TiO₂.

The metal element M contained in the mixed oxide MM′_y′O_5-x′ is, for example, chosen from iron or yttrium.

The metal element M′ contained in the mixed oxide MM′_y′O_5-x′ or the mixed oxide MM′_y′M″_y″O_5-x″ is, more particularly, chosen from titanium or yttrium.

According to one embodiment of the invention, the mixed oxide MM′_y′O_5-x′ is chosen from FeTiO₃, Y₂Ti₂O₇, YTi₂O₅ or mixtures thereof.

According to one embodiment of the invention, the mixed oxide MM′_y′O_5-x′ is a pyrochlore compound, for example Y₂Ti₂O₇ or YTi₂O₅ or a mixture thereof.

According to one embodiment of the invention, the mixed oxide is TiYO_5-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 precipitates may also comprise at least one intermetallic compound comprising the metal element M, the metal element M′ and possibly the metal element M″.

The material may possibly 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.

The metal element M contained in the intermetallic compound is, for example, iron.

The metal element M′ contained in the intermetallic compound is, for example, titanium or yttrium.

The metal element M″ contained in the intermetallic compound is, for example, chromium or tungsten.

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

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

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

• • at least one spherical oxide of Mn and Si whose size varies between 10 and 150 nm;
  • at least one metal oxide chosen from at least one single 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, or at least one mixed oxide MM′_y′M″_y″O_5-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, aluminium, hafnium, silicon, zirconium, thorium, magnesium or manganese;
  • possibly at least one intermetallic compound chosen from YFe₃, Fe₂Ti, FeCrWTi or mixtures thereof;
    or mixtures thereof.

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

• • Mn and Si oxide;
  • MO_2-x single oxide is chosen from Y₂O₃, Fe₂O₃, FeO, Fe₃O₄, Cr₂O₃, TiO₂, Al₂O₃, HfO₂, SiO₂, ZrO₂, ThO₂, MgO, MnO, MnO₂ or mixtures thereof,
  • mixed oxide MM′_y′O_5-x′ is chosen from FeTiO₃, Y₂Ti₂O₇, YTi₂O₅ or mixtures thereof,
  • mixed oxide MM′_y′M″_y″O_5-x″ has the general formula of the SiOAlMn type.

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

• • Mn and Si oxide;
  • MO_2-x single oxide is chosen from Y₂O₃, Fe₂O₃, FeO, Fe₃O₄, Cr₂O₃, TiO₂, Al₂O₃, HfO₂, SiO₂, ZrO₂, ThO₂, MgO, MnO, MnO₂ or mixtures thereof,
  • mixed oxide MM′_y′O_5-x′ is chosen from FeTiO₃, Y₂Ti₂O₇, YTi₂O₅ or mixtures thereof,
  • mixed oxide MM′_y′M″_y″O_5-x″ has the general formula of the SiOAlMn type.
  • intermetallic compound chosen from YFe₃, Fe₂Ti, FeCrWTi or mixtures thereof.

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.

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 an image processing software such as the “ImageJ” software available at the following Internet address: imagej.net/Welcome.

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 nanoprecipitates relative to the total mass of the material. This precipitate content can, for example, be measured by selective dissolution with aqua regia.

The surface density with which the precipitates are distributed in the boundaries of the nanometric cell substructure is advantageously of 40 precipitates per μm².

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

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

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.

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

Generally speaking, the steel material obtained as a result 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 greater 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. In fact, 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.

With regard to the steel powder (P) in particular, the particles of 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 at which 50% of the population of particles making up this powder is smaller than d₅₀. It can be determined by a technique such as the laser diffraction method using a granulometer described, for example, in standard ISO 13320 (2009-12-01 edition).

The apparent density of the powder (P) measured by ASTM B-212 (year 2021) standards may be between 3.5 g/cm³ and 4.5 g/cm³.

The actual density of the powder may be between 7.95 g/cm³ and 8.05 g/cm³. It is measured with a pycnometer, for example.

Advantageously, the steel powder has a 100% austenitic structure.

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

As mentioned previously, an additive manufacturing process comprises two steps repeated until the solid finished material is obtained:

• • 1. Generating a layer of material with a fixed contour and thickness.
  • 2. Creating the new layer by adding material on top of the previous layer.

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

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

• F. Laverne et al, “Additive manufacturing—General principles”, Techniques de l'ingénieur, Fascicule BM7017 V2 (published on 10 Feb. 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;
• Economy and Finance Ministry, French Republic, “Prospective—futur de la fabrication additive—rapport final”, January 2017 edition, ISBN: 978-2-11-151552-9; in particular Annex 2 (pages 205 to 220) especially where it describes additive manufacturing processes using metal powder (Annex 2, Les procédés de fabrication, paragraphs 3, 4 and 5).

More specifically, the additive manufacturing process can be selected from a process of selective laser fusion on a powder bed, selective electron beam fusion on a powder bed, selective laser sintering on a powder bed, laser spraying or binder spraying.

The selective laser melting (SLM) process can be carried out according to one or more of the following parameters:

• • the laser beam scans the steel powder at a scanning speed of between 50 mm/second (dense material) and 3000 mm/second (porous material);
  • laser beam power: 50 W to 1000 W;
  • distance between vector spaces: 25 μm to 150 μm;
  • layer thickness: 15 μm to 80 μm.

The selective electron beam melting (EBM) process can be carried out using one or more of the following parameters:

• • electron beam power: 50 W to 4000 W;
  • electron beam speed: 100 mm/s to 10,000 mm/s;
  • distance between vector spaces: 50 μm to 150 μm;
  • layer thickness: 40 μm to 75 μm.

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

• • laser power: 400 W to 3,000 W;
  • Nozzle travel speed: 150 mm/min to 1200 mm/min;
  • powder flow rate: 4 g/min to 15 g/min.

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

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

At the end of the manufacturing process of the material, in particular due to the raw material and the applied energy density, the microstructure of the grains, in particular the structure of their sub-micrometric cells, is modified with the appearance of a nanometric cellular sub-structure.

In addition to the elements mentioned, the steel material or steel powder (P) used in the manufacturing process the material of the invention may comprise, in percentage by mass, at least one of the following elements:

• • 0% to 0.110% 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.

These additional chemical elements may be present in the matrix and/or in the precipitates or nanoprecipitates.

The matrix may comprise, in proportion by mass with respect to the mass of the material or with respect to the mass of the steel powder (P) used in the manufacturing process of 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 refers to the metallic element dissolved in the matrix, as opposed to the one present in the precipitates.

The metal 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.

The steel material or steel powder (P) used in the manufacturing process of the invention may have an austenitic structure.

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 type 316 L or 304 L steel, for example, as specified in standard ASTM A666 (2015) or RCC-MRx (2012) respectively.

Potential applications for a material according to the invention include all those where a metal object is subjected to conditions of use in an aggressive environment (corrosion, irradiation and temperature) and where there is mechanical stress (assembly components, reactor vessels, pressure equipment, turbines, tools, shock absorbers, etc.).

A steel material according to the invention remains the preferred choice in almost all technical applications:

• • public facilities (bridges and roadways, signaling), chemical, petrochemical, pharmaceutical and nuclear industries (pressure equipment, equipment subject to the action of flame, storage capacities, various containers), agri-food (packaging and storage), construction (frameworks, framing, ironmongery, hardware), mechanical and thermal industries (engines, turbines, compressors), automotive (bodywork, equipment), railways, aeronautics and aerospace, shipbuilding, medical (instruments, appliances and prostheses), mechanical components (screws, springs, cables, bearings, gears), striking tooling (hammers, chisels, dies) and cutting tools (milling cutters, drills, cutter heads), furniture, design and household appliances, etc.

The object of the invention is thus to use a steel material according to the invention in the following fields:

• • public facilities,
  • chemical, petrochemical and pharmaceutical industries,
  • transport sector, such as the automotive, aeronautical, aerospace, marine and rail industries,
  • food industry,
  • construction,
  • medical sector,
  • shipbuilding,
  • mechanical components,
  • energy sector, such as nuclear, hydraulic and thermal power,
  • striking and cutting tooling,
  • furniture,
  • household appliances.

Another object of the invention is a part comprising all or part of a steel material according to the invention. This part can be used in the above-mentioned fields.

The part according to the invention can be manufactured by an additive manufacturing process. More particularly, the additive manufacturing process is chosen from a process of selective laser fusion on a powder bed, selective electron beam fusion on a powder bed, selective laser sintering on a powder bed, laser spraying or binder spraying.

The manufacturing process of the part according to the invention can be followed by a treatment process comprising a hot isostatic compression step. This hot isostatic compression step may comprise the following successive steps carried out in an enclosure comprising an inert gaseous atmosphere at a pressure of between 120 bar and 1800 bar:

• • A. the material is heated to a constant temperature of 600 to 1400° C. at a rate of temperature increase of between 50° and 1000° C./hour;
  • B. the constant temperature is maintained for 15 minutes to 5 hours;
  • C. the constant temperature is reduced at a rate of temperature decrease of 500 to 1000° C./hour to reach the ambient temperature (20+5° C.).

The inert gas atmosphere may comprise a gas chosen from argon, helium or a mixture thereof.

EXAMPLES

1. Steel Powder Used in the Manufacturing Process of the Material of the Invention.

Generally, the steel powder has a composition as shown in [FIG. 3], i.e. a mass composition including that of steel complying with ASTM A666 (2003) and RCC-MRx standards (the RCC-MRx standard corresponds to the Rules for design and construction of mechanical equipment for high-temperature, experimental and melting nuclear installations. It is a technical document for the production of components for Generation IV nuclear reactors) (2015).

Characterization of the Steel Powder.

1.1. Chemical Composition.

A steel powder (316 L steel reference FE-271-3/TruForm 316-3—batch no. 32-034043-10 marketed by Praxair) is analyzed by X-ray microanalysis, more specifically by Energy Dispersive X-ray Spectroscopy (EDX). The analysis system used is BRUKER Quantax XFlash.

The steel powder is also analyzed by Scanning Electron Microscope (MEB FEG Zeiss ULTRA55) and 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.

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 % by mass of the total mass of the matrix. However, by convention, chemical elements not measured are disregarded for the matrix. It is then assumed that the remaining mass percentage is composed of iron.
  • in % by mass and in % by atom of the total mass of the precipitates contained in the steel powder.

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 have been detected by SEM due to their small size.

1.2. Morphology

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

The particles of this powder comprise grains agglomerated to form particles which are mostly essentially spherical. They have a diameter of between 10 μm and 100 μm, and an average diameter of 34 μm. More specifically, the median diameters D₁₀, D₅₀ and D₉₀ (for which, respectively, 10%, 50% and 90% of the population of particles making up this powder has a size smaller than the median diameter in question) measured by laser granulometry in accordance with standard ISO 13320 (2009-12-01 edition) are as follows: D₁₀=22 μm, D₅₀=32 μm, and D₉₀=48 μm.

The precipitates contained in the powder particles are usually spherical. Their maximum dimensions (which most often correspond to the diameter of the spherical particle) are such that the size measured by Scanning Electron Microscope (SEM) imaging is generally between 24 nm and 120 nm. Their 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 ranges from 2 precipitates/μm³ to 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 (year 2021) standard is 4 g/cm3±0.01 g/cm³. Its true density measured by Helium pycnometer is 7.99 g/cm3±0.03 g/cm³.

The Hall flow rate (ability to flow 50 g of powder through an orifice of fixed size) measured according to ASTM B213 (year 2020) standard is 15 seconds.

2. Manufacturing Process of a Material According to the Invention.

A part made of the steel material according to the invention is manufactured by additive manufacturing using the selective laser melting (SLM) process with a Trumpf TruPrint Series 1000 printer.

To manufacture the part on a stainless-steel substrate, the laser scan follows a path defined as stripes. At the end of this first scan, a first consolidated layer n is obtained. Then, a new 670 rotation of the scanning direction of the laser is operated and a new layer n+1 is superimposed on the underlying layer n.

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 ; power ⁢ of ⁢ the ⁢ laser = 120 ⁢ W ; scanning ⁢ speed ⁢ of ⁢ the ⁢ laser = 950 ⁢ mm / s ; Hatching ⁢ distance = 60 ⁢ μm ; powder ⁢ bed ⁢ thickness = 30 ⁢ μm ;

• • composition of the gaseous medium in the construction chamber=argon, with an oxygen content of less than 100 ppm during consolidation.

Ten parallelepiped test tubes (length/height=30 mm, width=20 mm, thickness=7 mm) are obtained. After manufacture, the parts are extracted by cutting the base of the test tubes to separate them from the stainless-steel substrate.

No additional treatment is applied to the raw material obtained.

The density of the steel material making up the test tubes is 7.93 g/cm³ (measured by the Archimedes method), i.e. a relative density of 99.25% based on a theoretical density for 316 L steel of 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%.

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

power ⁢ of ⁢ the ⁢ laser = 50 ⁢ W ⁢ to ⁢ 400 ⁢ W ; scanning ⁢ speed ⁢ of ⁢ the ⁢ laser = 50 ⁢ mm / s ⁢ to ⁢ 3000 ⁢ mm / s .

Density generally evolves parabolically with laser power or scanning speed. However, if the power or scanning speed is too low or too high, the density may possibly decrease.

The hatching distance is between 30 μm and 90 μm, for example.

3. Characterization of the Steel Material According to the Invention 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 (2015) and RCC-MRx (2012) standards shown in the Table in [FIG. 3].

The elemental composition of this alloy is measured by EDX analysis. It is very similar to that of the steel powder used to make 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 using a TEM (FEI Tecnai F20 FEG-TEM) coupled to an EDX detector (EDX Bruker XFlash 6T|60). The precipitates identified correspond to Mn and Si oxides, but this does not rule out the presence of other precipitates of a different nature within the material of the invention Differences in local chemical composition were also observed in sub-micrometric cells smaller than 1 μm. The cells of the material of the invention were studied using a TEM (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 its matrix. The cell wall is enriched in Cr and Mo, slightly enriched in Ni and depleted in Fe compared to the cell matrix. The chemical composition of the internal nanometric sub-cells of the large cells of the material of the invention may have characteristics comparable to those of the large cells in terms of differences in local chemical composition.

3.2. Morphology.

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 embedded in the matrix of the grains that compose 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/μm³.

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

One of the particular features of the material of the invention is a microstructure such that the grains making up 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 energy source in motion during manufacturing).

This microstructural feature of the material of the invention is such that the quasi-equiaxial structure of the grains is in a plane respectively parallel and perpendicular to the z direction of additive manufacturing of the steel material. The grain size is less than 40 μm (average size 25 μm).

In addition, the crystallites that are the grains of the steel material have a preferential orientation. This texture of the material is reflected in the fact that the directions are oriented preferentially parallel to the construction direction z, but also by a texture intensity equal to 1.4.

As illustrated in [FIG. 6], the grains of the steel material are themselves composed of sub-micrometric cells of nanometric size (more specifically a size smaller than an average diameter of 500 nm). [FIG. 6] and [FIG. 7] also show the nanometric cell substructure which is organized directly inside the larger sub-micrometric cells, while also revealing the small precipitates of less than 10 nm incorporated into the matrix, particularly in the cell walls, which appear in a lighter shade.