Process for producing a ferritic steel Fe – 12 / 18 Cr reinforced by a dispersion of oxides.
The described process for producing ferritic Fe-12/18Cr steel with oxide dispersion improves impact resistance and hot tensile properties by optimizing the particle size distribution and dispersion of oxides within the steel matrix, resulting in enhanced mechanical properties.
Patent Information
- Authority / Receiving Office
- FR · FR
- Patent Type
- Applications
- Current Assignee / Owner
- COMMISSARIAT A LENERGIE ATOMIQUE ET AUX ENERGIES ALTERNATIVES
- Filing Date
- 2024-12-12
- Publication Date
- 2026-06-19
AI Technical Summary
Conventional manufacturing processes for oxide dispersion-strengthened ferritic steels, particularly those with high chromium content, are limited in impact resistance and hot tensile strength, despite their high performance.
A process involving co-grinding, sieving, and reprocessing of ferritic steel powder with oxide powder to achieve a specific particle size distribution, followed by encapsulation and hot isostatic compaction or sintering, to enhance the dispersion of oxides within the steel matrix.
The process results in improved impact resistance and hot tensile properties, with increased ductility and mechanical strength at both room and high temperatures, enhancing the material's formability and performance.
Smart Images
Figure 00000000_0000_ABST
Abstract
Description
Title of the invention: Process for producing a ferritic steel Fe - 12 / 18 Cr reinforced by a dispersion of oxides. 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 process for producing such a steel. State of the art
[0003] The conventional manufacturing process for oxide dispersion-strengthened ferritic steels (also called ODS steels, according to Anglo-Saxon terminology) is well known and yields materials with remarkable hot-heat properties. The ferritic matrix provides a number of advantages (good conductivity, low thermal expansion, high stiffness, excellent resistance to radiation damage, etc.), and the oxide dispersion enhances the thermally refined properties. Ferritic ODS steels are therefore being considered as structural materials for future nuclear reactors (Na-FNRs, nuclear fusion reactors, etc.) or other components requiring the qualities of these materials.
[0004] Conventionally, the dispersion of oxides, most often nano-oxides, is achieved through powder mechanosynthesis. The first step of the process consists of 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 centered 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 in other ways, for example by sintering. Such as flash sintering (also known by the acronym SPS for "Spark Plasma Sintering" in English). 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 which have high chromium content (chromium content between 12% and 18%), favorable mechanical properties, in particular with regard to resistance properties to impact and hot tensile strength. However, current ferritic ODS steels, although high-performing, remain limited in these aspects. Summary of the invention
[0006] An object of the invention is to provide a process for producing a ferritic Fe-12 / 18 Cr steel reinforced by an oxide dispersion (ODS) which improves the impact resistance and hot tensile properties.
[0007] To this end, the invention proposes a process for producing a ferritic Fe-12 / 18Cr steel reinforced by an oxide dispersion, said process comprising the following steps: a) supply a ferritic steel powder Fe - 12 / 18 Cr and an oxide powder; b) carry out a co-grinding of the ferritic steel powder Fe - 12 / 18 Cr with the oxide powder to obtain a ferritic steel powder Fe - 12 / 18 Cr reinforced by an oxide dispersion; c) sieve the ferritic steel powder Fe - 12 / 18 Cr reinforced by a dispersion of oxides thus obtained to a predetermined initial average particle size; d) reprocess the Fe-12 / 18Cr ferritic steel powder reinforced with 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) to produce Fe-12 / 18 Cr ferritic steel reinforced by an oxide dispersion from the powder thus reprocessed.
[0008] The method according to the invention may have at least one of the following characteristics, taken alone or in combination: - step d) includes 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, and then a second sieving step to the predetermined final average particle size. - the initial average particle size is between 80 and 100 microns. - The average intermediate particle size is between 60 and 80 microns, for example about 70 microns. - step d) is a grinding step, called cryogenic grinding, of the Fe - 12 / 18 Cr ferritic steel powder reinforced by a dispersion of oxides 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. - cryogenic grinding is carried out over a period of between 8h and 24h, advantageously between 10h and 18h. - the final average particle size is between 40 and 60 microns, for example around 50 microns. - Step e) comprises the following sub-steps: e0 encapsulate and vacuum-pack the ferritic steel powder Fe - 12 / 18 Cr reinforced by 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 a flash sintering. - the oxide powder supplied in step a) is chosen from a powder of Yttrium (Y2O3) oxides, Zirconium oxides or Titanium oxides. - the oxide powder supplied in step a) has an average particle size of less than 5 microns, preferably less than 1 micron, even more preferably less than 100nm. Brief description of the figures
[0009] Other features and advantages of the invention will become apparent from the description, made with reference to the accompanying figures, for which:
[0010] The [Fig. 1] is a flowchart of a process for producing a ferritic steel Fe -12 / 18 Cr reinforced by a dispersion of oxides according to the invention;
[0011] Fig. 2 is a photograph illustrating the difference in particle size obtained by a prior art process (a, left) and that obtained by the process of Fig. 1 (b, right);
[0012] Fig. 3 is a graph illustrating a distribution of the particle size of the powder reprocessed by the process of Fig. 1 and the conventional powder obtained by a prior art process;
[0013] 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
[0014] Fig. 5 is a graph illustrating tensile curves for materials obtained by CIC from unreprocessed powder and powder reprocessed by the process of Fig. 1. Detailed description
[0015] With reference to [Fig.1], we will describe a process for producing a ferritic steel Fe - 12 / 18 Cr reinforced by an oxide dispersion according to the invention.
[0016] The process for producing a ferritic Fe-12 / 18Cr steel reinforced by an oxide dispersion according to the invention comprises a first step 100 (step a) of Supply of a ferritic steel powder Fe - 12 / 18 Cr and an oxide powder, for example a Yttrium (Y2O3) oxide powder. Other oxide powders may be used, such as Zirconium 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.
[0017] Next, the process for producing a ferritic steel Fe-12 / 18Cr reinforced by an oxide dispersion according to the invention comprises a second step 200 (step b) of co-milling the ferritic steel powder Fe-12 / 18Cr with the oxide powder to obtain a ferritic steel powder Fe-12 / 18Cr reinforced by an oxide dispersion. This co-milling step implements a mechanosynthesis of the powders. This allows the oxides to dissolve within the grains of ferritic steel powder Fe-12 / 18Cr (matrix).
[0018] 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 Fe-12 / 18Cr reinforced ferritic steel powder 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 [Fig. 2] a). This is a scanning electron microscopy (SEM) image of Fe-14Cr steel powder.
[0019] Next, the process for producing a 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.
[0020] 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.
[0021] Thus, only the finest powders are retained.
[0022] This screening is carried out here by implementing one or more sieving sub-steps 410,420, in this case for this example a double sieving.
[0023] Thus, the fourth step 400 of the process for producing a ferritic Fe-12 / 18Cr steel reinforced 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..
[0024] For example, the first sieving substep 410 allows obtaining an intermediate particle size of 70 microns and then taking it up again in the second sieving substep 420 to keep only the finest powders, less than 50 microns.
[0025] Fig. 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 an oxide dispersion according to the invention.
[0026] Fig. 3 shows the size distribution (particle size) of Fe-12 / 18Cr ferritic steel powders reinforced by an oxide dispersion for the conventional powder (prior art) and the optimized powder, i.e. reprocessed during the process according to the invention.
[0027] 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.
[0028] In a second embodiment of the fourth reprocessing step 400, this step is carried out by taking the conventional powder after the grinding from the third step 300 of the process for producing a Fe-12 / 18Cr ferritic steel reinforced by a sieved oxide dispersion, and performing cold grinding. Such cold grinding is typically carried out between -40°C and -180°C for a period of between 4 and 48 hours, advantageously between 8 and 24 hours, and even more advantageously between 10 and 18 hours. This makes it possible to crush the Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion with an initial medium particle size and to obtain a finer Fe-12 / 18Cr ferritic steel powder reinforced by an oxide dispersion with a final medium particle size.
[0029] 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.
[0030] This fifth step 500 can be carried out in different ways. The following substeps can be considered: e1) encapsulating and vacuum-sealing the Fe-12 / 18Cr ferritic steel powder, reinforced by an oxide dispersion, which has been reprocessed and therefore has a final medium particle size; then e2) performing hot isostatic compaction. During vacuum-sealing, water vapor and any gases are evacuated.
[0031] Alternatively, the fifth step 500 of the process for producing a ferritic steel Fe - 12 / 18 Cr reinforced 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).
[0032] It appears that the use of a process for producing a ferritic steel Fe - 12 / 18 Cr 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 in [Fig.4].
[0033] From the same batch of Fe-12 / 18Cr ferritic steel powder reinforced with a co-milled 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). Figure 4 shows 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 in [Fig.4] were obtained from mini Charpy specimens of 3X4X27 mm.
[0034] Furthermore, the tensile properties are also improved. At room temperature, the ductility of the material with the reprocessed powder, produced using the process for manufacturing a Fe-12 / 18Cr ferritic steel reinforced by an oxide dispersion according to the invention, is significantly higher than for the grade produced with conventional powders (prior art). This increase in ductility is beneficial for the formability of the material at room temperature. At high temperatures (600°C / 700°C), the mechanical strength of the material obtained with the process 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 [Fig. 5]. This [Fig. 5] shows tensile curves for the materials obtained with conventional powders (dotted 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 superior to that of the conventional material.
Claims
Demands
1. A process for producing Fe-12 / 18Cr ferritic steel reinforced by an oxide dispersion, said process 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 thus obtained to a predetermined initial average particle size; d) reprocess (400) the Fe-12 / 18Cr ferritic steel powder reinforced by a dispersion of oxides 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) Fe-12 / 18 Cr ferritic steel reinforced by an oxide dispersion from the powder thus reprocessed.;
2. A 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 is between 80 and 100 microns.
5. A method according to claims 3 and 4, wherein the intermediate average particle size is between 60 and 80 microns, for example about 70 microns.
6. A method according to claim 1, wherein step d) is a grinding step, referred to as cryogenic grinding, of 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 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 is between 40 and 60 microns, for example about 50 microns.
9. A method according to any one of claims 1 to 8, wherein step e) comprises the following substeps: e1) encapsulate and vacuum-pack the Fe-12 / 18 Cr ferritic steel powder reinforced by a dispersion of oxides thus reprocessed, which is therefore of final medium particle size, then: e2) carry 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 process according to any one of claims 1 to 11, wherein the oxide powder supplied in step a) has an average particle size of less than 5 microns, preferably less than 1 micron, even more preferably less than 100 nm.