Method for producing ferritic fe 12 / 18 cr steel reinforced by oxide dispersion

Abstract

The invention relates to a process for producing Fe-12 / 18Cr ferritic steel reinforced by an oxide dispersion, comprising the following steps: a) supplying (100) Fe-12 / 18Cr ferritic steel powder and oxide powder; b) co-milling (200) the Fe-12 / 18Cr ferritic steel powder with the oxide powder to obtain Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion; c) sieving (300) the Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion to a predetermined initial average particle size; d) reprocessing (400) the Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion to obtain a predetermined final average particle size which is strictly less than the predetermined initial average particle size; and, e) produce (500) ferritic steel Fe - 12 / 18 Cr reinforced by a dispersion of oxides.

Description

Technical field of the invention

[0001] The invention relates to the field of metallurgy of ferritic Fe-Cr steels reinforced by a dispersion of oxides.

[0002] In particular, the invention relates to a method for producing such steel. State of the art

[0003] The conventional manufacturing process for oxide dispersion strengthened (ODS) ferritic steels is well-established and yields materials with remarkable high-temperature properties. The ferritic matrix provides several advantages (good conductivity, low thermal expansion, high stiffness, excellent resistance to radiation damage, etc.), and the oxide dispersion enhances thermal creep properties. Ferritic ODS steels are therefore being considered as structural materials for future nuclear reactors (Na-NFRs, nuclear fusion reactors, etc.) or other components requiring the qualities of these materials.

[0004] Traditionally, the dispersion of oxides, most often nano-oxides, is achieved through powder mechanosynthesis. The first step of the process involves the high-energy co-milling of atomized steel powder with an oxide powder, for example, yttrium oxide (Y₂O₃), which dissolves the oxides within the steel powder grains. After co-milling, a new powder is obtained. The coarsest powders (with an average size greater than 150-200 microns) are removed by sieving, and the remaining powder, with an average diameter of around 100 microns, is then consolidated. Consolidation can be carried out using various techniques, such as encapsulation (this step allows for the removal of moisture and gases) followed by hot isostatic pressing (HIP).This consolidation can be achieved through other means, such as sintering. One example is flash sintering (also known as SPS, for "Spark Plasma Sintering"). In all cases, the consolidation step allows for densification of the material and precipitation of oxides.

[0005] It is desirable to be able to use ferritic grades with high chromium content (between 12% and 18%), and favorable mechanical properties, particularly impact resistance and hot tensile strength. However, current ferritic ODS steels, while performing well, remain limited in these aspects. Summary of the invention

[0006] One object of the invention is to provide a process for manufacturing a ferritic Fe-12 / 18Cr steel reinforced by an oxide dispersion (ODS) that improves its impact resistance and hot tensile strength. It should be noted that a ferritic Fe-12 / 18Cr steel is a steel containing between 12% and 18% chromium.

[0007] To this end, the invention proposes a process for producing a ferritic steel Fe - 12 / 18 Cr reinforced by a dispersion of oxides, said process comprising the following steps: a) supply a Fe-12 / 18Cr ferritic steel powder and an oxide powder; b) co-mill the Fe-12 / 18Cr ferritic steel powder with the oxide powder to obtain a Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion; c) sieve the Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion thus obtained to a predetermined initial average particle size; d) reprocess the Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion from the sieved powder, so as to obtain a predetermined final average particle size which is strictly less than the predetermined initial average particle size; and e) produce the Fe-12 / 18Cr ferritic steel reinforced by an oxide dispersion from the reprocessed powder.

[0008] The process according to the invention may have at least one of the following characteristics, taken alone or in combination: Step d) comprises one or more sieving steps. Step d) includes a first sieving step to a predetermined intermediate average particle size, which is located between the initial average particle size and the final average particle size, followed by a second sieving step to the predetermined final average particle size. The initial average particle size is between 80 and 100 microns. The intermediate average particle size is between 60 and 80 microns, for example, approximately 70 microns. Step d) is a grinding step, referred to as cryogenic grinding, of the Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion from the sieved powder, said cryogenic grinding being carried out at a temperature between -40°C and -180°C for a duration of between 4 and 48 hours. cryogenic grinding takes place over a period of between 8 and 24 hours, advantageously between 10 and 18 hours.The final average particle size is between 40 and 60 microns, for example, approximately 50 microns. Step e) comprises the following substeps: e1) encapsulate and vacuum-pack the Fe-12 / 18Cr ferritic steel powder reinforced with a reprocessed oxide dispersion, which therefore has a final average particle size, then: e2) perform hot isostatic compaction. Step e) is a sintering step, for example, flash sintering. The oxide powder supplied in step a) is selected from yttrium (Y2O3) oxide powder, zirconium oxide powder, or titanium oxide powder. The oxide powder supplied in step a) has an average particle size of less than 5 microns, preferably less than 1 micron, and even more preferably less than 100 nm. Brief description of the figures

[0009] Other features and advantages of the invention will become apparent from the description, made with reference to the attached figures, for which: There figure 1 is a flowchart of a process for manufacturing a ferritic steel Fe-12 / 18Cr reinforced by an oxide dispersion according to the invention; The figure 2 is a photograph illustrating the difference in particle size obtained by a process according to the prior art (a, left) and that obtained by the process of the figure 1 (b, on the right); The figure 3 is a graph illustrating a particle size distribution of powder reprocessed by the process of figure 1 and conventional powder obtained by a prior art process; The figure 4 is a graph illustrating ductile brittle transition curves for materials obtained by CIC from unreprocessed powder (prior art) and powder reprocessed by the process of figure 1 ; and La figure 5is a graph illustrating tensile curves for materials obtained by CIC from unreprocessed powder and powder reprocessed by the process of figure 1 . Detailed description

[0010] Throughout the following description, particle size distribution is based on a dry laser particle size analysis. The principle of this measurement is as follows: a certain quantity of powder is placed on a vibrating platform. When the platform is activated, the powder falls into a vertical column (in this case, the column is filled with a gas, for example, air: dry method). A collimated laser beam passes through the vertical column, interacts with the powder, and becomes scattered as a result of this interaction. The angle at which the light is scattered then provides information about the powder size, using a physical model that relates the scattering angle to the powder size. This physical model is, for example, a Mie scattering model. Typical error margins on powder size obtained with dry laser particle size analysis are typically less than 5%.More specifically, the measurements provided in this description were obtained with a Horiba Jobin-Yvon LA-950 machine.

[0011] With reference to the figure 1 , we will describe a process for producing a ferritic steel Fe - 12 / 18 Cr reinforced by a dispersion of oxides according to the invention.

[0012] The process for producing Fe-12 / 18Cr ferritic steel reinforced by an oxide dispersion according to the invention comprises a first step 100 (step a) of supplying Fe-12 / 18Cr ferritic steel powder and an oxide powder, for example, yttrium oxide (Y₂O₃) powder. Other oxide powders may be used, such as zirconium oxide or titanium oxide powders. The oxide powders used have an average particle size of less than 5 microns, preferably less than 1 micron, and even more preferably less than 100 nm. Indeed, the smaller the particle size of the oxide powder, the easier it is to dissolve the oxide powder in the ferritic steel powder.

[0013] Next, the process for producing Fe-12 / 18Cr ferritic steel reinforced by an oxide dispersion according to the invention comprises a second step 200 (step b) of co-milling the Fe-12 / 18Cr ferritic steel powder with the oxide powder to obtain Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion. This co-milling step employs a mechanosynthesis of the powders. This allows the oxides to dissolve within the Fe-12 / 18Cr ferritic steel powder grains (matrix).

[0014] Then, the process for producing a ferritic steel Fe-12 / 18Cr reinforced by an oxide dispersion according to the invention comprises a third step 300 (step c) of sieving the ferritic steel powder Fe-12 / 18Cr reinforced by an oxide dispersion thus produced to a predetermined initial average particle size. The initial average particle size is, for example, less than or equal to 100 microns, in particular between 80 and 100 microns. The result of this third step 300 (step d) is illustrated, for example, in figure 2 a) This is an image obtained by scanning electron microscopy (SEM) for a Fe-14Cr steel powder.

[0015] Next, the process for producing Fe-12 / 18Cr ferritic steel reinforced by a sieved oxide dispersion according to the invention comprises a fourth step 400 (step d) of reprocessing the Fe-12 / 18Cr ferritic steel powder reinforced by a sieved oxide dispersion to obtain a predetermined final average particle size that is strictly less than the predetermined initial average particle size. The final average particle size is typically between 40 and 60 microns, for example, about 50 microns.

[0016] In a first embodiment of the fourth reprocessing step 400, the reprocessing is carried out by screening (sieving) the Fe-12 / 18 Cr ferritic steel powder reinforced by a sieved oxide dispersion obtained during the third step 300 of the process of producing a Fe-12 / 18 Cr ferritic steel reinforced by an oxide dispersion according to the invention.

[0017] Thus, only the finest powders are retained.

[0018] This screening is carried out here by implementing one or more sieving sub-steps 410,420, in this case a double sieving.

[0019] Thus, the fourth step 400 of the process for producing a ferritic steel Fe-12 / 18Cr strengthened by an oxide dispersion (previously sieved in step 300) comprises a first sieving substep 410 to a predetermined intermediate average particle size, which is located between the initial average particle size and the final average particle size. Typically, the intermediate average particle size is between 60 and 80 microns, for example, approximately 70 microns. Then, the fourth step 400 of the process for producing a ferritic steel Fe - 12 / 18 Cr reinforced by an oxide dispersion includes a second sieving substep 420, following the first sieving substep 410, of the ferritic steel powder Fe - 12 / 18 Cr reinforced by an oxide dispersion sieved to an intermediate particle size so as to obtain a ferritic steel powder Fe - 12 / 18 Cr reinforced by an oxide dispersion with the final predetermined average particle size.

[0020] For example, the first sieving sub-step 410 allows obtaining an intermediate particle size of 70 microns, then by taking it up again in the second sieving sub-step 420 to keep only the finest powders, less than 50 microns.

[0021] There figure 2 b) illustrates the result of this fourth step 400 of reprocessing of the process of producing a ferritic steel Fe - 12 / 18 Cr reinforced by a dispersion of oxides according to the invention.

[0022] There figure 3 shows the size distribution (particle size distribution) of Fe-12 / 18Cr ferritic steel powders reinforced by an oxide dispersion for the conventional powder (prior art) and the optimized powder, namely reprocessed during the process according to the invention.

[0023] We can therefore clearly see the effectiveness of this fourth step 400 of the process of developing a ferritic steel Fe - 12 / 18 Cr reinforced by a dispersion of oxides according to the invention on the size of the powders.

[0024] In a second embodiment of the fourth reprocessing step 400, the conventional powder, after grinding from the third step 300 of the process for producing Fe-12 / 18Cr ferritic steel reinforced by a sieved oxide dispersion, is cold-ground. Such cold grinding is typically carried out between -40°C and -180°C for a duration of 4 to 48 hours, advantageously between 8 and 24 hours, and even more advantageously between 10 and 18 hours. This allows the Fe-12 / 18Cr ferritic steel powder, reinforced by an oxide dispersion with an initial medium particle size, to be crushed, resulting in a finer Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion with a final medium particle size.

[0025] Finally, the process for producing a ferritic steel Fe - 12 / 18 Cr reinforced by an oxide dispersion according to the invention includes a fifth step 500 of producing a ferritic steel Fe - 12 / 18 Cr reinforced by an oxide dispersion from the powder thus reprocessed.

[0026] This fifth step 500 can be carried out in different ways. The following substeps can be considered: e 1) encapsulate and vacuum-pack the Fe-12 / 18Cr ferritic steel powder, reinforced with an oxide dispersion, which has been reprocessed and therefore has a final medium particle size; then e 2) perform hot isostatic compaction. During vacuum-packing, water vapor and any gases are removed.

[0027] Alternatively, the fifth step 500 of the process for producing a ferritic steel Fe - 12 / 18 Cr strengthened by an oxide dispersion which has been reprocessed according to the invention is e) a sintering step, for example a flash sintering (also known by the acronym SPS for "Spark Plasma Sintering" according to Anglo-Saxon terminology).

[0028] It appears that the use of a process for producing a ferritic Fe-12 / 18Cr steel reinforced by an oxide dispersion according to the invention, as previously described, allows the grades of the steel obtained to exhibit improved impact properties, as shown by the figure 4 .

[0029] From the same batch of Fe-12 / 18Cr ferritic steel powder reinforced with a co-ground oxide dispersion, two ODS steels were obtained: one without the 400 reprocessing step of the Fe-12 / 18Cr ferritic steel powder reinforced with an oxide dispersion (conventional powder, prior art) and the other with the 400 reprocessing step of the powder (optimized powder, according to the invention). Note on the figure 4 that the ductile energy plateau increases from approximately 3 Joules (conventional powder) to 6.8 Joules (optimized powder). The brittle-ductile transition temperature also increases from approximately -10°C to approximately -50°C, indicating a remarkable improvement in impact resistance properties. The curves of the figure 4 were obtained from mini Charpy test specimens of 3X4X27 mm.

[0030] Furthermore, the tensile properties are also improved. At room temperature, the ductility of the material with the reprocessed powder, produced using the method for manufacturing a Fe-12 / 18Cr ferritic steel reinforced by an oxide dispersion according to the invention, is significantly higher than for the same grade with conventional powders (prior art). This increase in ductility is beneficial for the material's formability at room temperature. At high temperatures (600°C / 700°C), the mechanical strength of the material obtained with the method for manufacturing a Fe-12 / 18Cr ferritic steel reinforced by an oxide dispersion according to the invention is even greater than that of the material with conventional powder, as shown in the figure 5 . This figure 5This shows tensile strength curves for materials obtained with conventional powders (dashed lines; prior art) and reprocessed / optimized powders (solid lines; invention). At room temperature, the optimized material exhibits an elongation at break of approximately 25%, compared to 15% for the conventional material. At high temperatures (from 600°C / 700°C), the mechanical strength of the optimized material is greater than that of the conventional material.

Claims

1. A process for producing a ferritic steel Fe - 12 / 18 Cr reinforced by an oxide dispersion, said process comprising the following steps: a) supplying (100) a powder of ferritic steel Fe - 12 / 18 Cr and a powder of oxides; b) carrying out a co-milling (200) of the ferritic steel powder Fe - 12 / 18 Cr with the powder of oxides to obtain a powder of ferritic steel Fe - 12 / 18 Cr reinforced by an oxide dispersion; c) sieving (300) the ferritic steel powder Fe - 12 / 18 Cr reinforced by an oxide dispersion thus obtained to a predetermined initial average particle size; d) reprocess (400) the Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion from the powder thus sieved, so as to obtain a final predetermined average particle size which is strictly less than the initial predetermined average particle size;and e) produce (500) ferritic steel Fe - 12 / 18 Cr strengthened by a dispersion of oxides from the powder thus reprocessed.; 2. Method according to claim 1, wherein step d) comprises one or more sieving steps (410, 420).

3. A method according to claim 2, wherein step d) comprises a first sieving step (410) to a predetermined intermediate average particle size, which is located between the initial average particle size and the final average particle size, and then a second sieving step (420) to the predetermined final average particle size.

4. A method according to any one of claims 1 to 3, wherein the initial average particle size, measured by dry laser particle size analysis, is between 80 and 100 microns.

5. A method according to claims 3 and 4, wherein the average intermediate particle size, measured by dry laser particle size analysis, is between 60 and 80 microns, for example about 70 microns.

6. A process according to claim 1, wherein step d) is a grinding step, referred to as cryogenic grinding, of the Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion from the powder thus sieved, said cryogenic grinding being carried out at a temperature between -40°C and -180°C for a period of between 4h and 48h.

7. A method according to claim 6, wherein said cryogenic grinding is carried out over a period of between 8h and 24h, advantageously between 10h and 18h.

8. A method according to any one of claims 1 to 7, wherein the final average particle size, measured by dry laser particle size analysis, is between 40 and 60 microns, for example about 50 microns.

9. A process according to any one of claims 1 to 8, wherein step e) comprises the following substeps: e1) encapsulating and vacuum-sealing the Fe-12 / 18 Cr ferritic steel powder reinforced by a dispersion of oxides thus reprocessed, which is therefore of final medium particle size, and then: e2) carrying out hot isostatic compaction.

10. A method according to any one of claims 1 to 8, wherein step e) is a sintering step, for example a flash sintering (SPS).

11. A method according to any one of claims 1 to 10, wherein the oxide powder supplied in step a) is selected from a powder of Yttrium (Y2O3) oxides, Zirconium oxides or Titanium oxides.

12. A method according to any one of claims 1 to 11, wherein the oxide powder supplied in step a) has an average particle size, measured by dry laser particle size analysis, of less than 5 microns, preferably less than 1 micron, even more preferably less than 100nm.

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