Process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion.

Abstract

The invention relates to a process for producing Fe-Cr-Ni austenitic steel reinforced by an oxide dispersion, said process comprising the following steps: a) supplying (100) Fe-Cr-Ni austenitic steel powder; b) carrying out (200) a grinding, called cryogenic grinding, of the Fe-Cr-Ni austenitic steel powder at a temperature between -50°C and -196°C; c) supplying (300) an oxide powder; d) carrying out (400) a co-grinding of the Fe-Cr-Ni austenitic steel powder with the oxide powder thus supplied to obtain Fe-Cr-Ni austenitic steel powder reinforced by an oxide dispersion; and e) producing (500) the Fe-Cr-Ni austenitic steel reinforced by an oxide dispersion from the powder thus produced. Figure for the summary: Figure 1

Description

Title of the invention: Process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion. Technical field of the invention

[0001] The invention relates to the field of metallurgy of austenitic Fe-Cr-Ni steels strengthened by an oxide dispersion. In particular, the invention relates to a process for producing such a steel. State of the art

[0002] Currently, the production of ODS (Oxide Dispersion Strengthened) austenitic steels using powder metallurgy involves a mechanical grinding step. This step is now essential for the production of certain classes of steels, such as ODS steels. ODS steels are steels strengthened by a dispersion of oxides, most often nanometric in size. They are of interest in industry for their advanced properties in terms of corrosion resistance and high-temperature mechanical strength, and also in the nuclear sector for their resistance to radiation damage.

[0003] Today, a process for producing ODS steels involves powder metallurgy and the co-milling (also called mechanosynthesis) of an austenitic steel powder with an oxide powder, for example, yttrium oxide (Y2O3). When mechanosynthesis is carried out correctly, the oxide is dissolved in the matrix. This step is well controlled for ferritic steels. However, the high ductility of austenitic steels makes this mechanosynthesis step difficult because the powders can stick and agglomerate together or coat the grinding media or grinding chambers without the mechanosynthesis step occurring.

[0004] An attempt has been made to solve this problem by varying the chemical composition of the powder and / or the processing parameters.

[0005] There is currently no consensus on the manufacturing parameters that, to a first order, vary the kinetics or energy of the system. Parameters such as rotational speed, the mass-to-powder ratio, media filling rate, blade geometry, and many others are dependent on the technology and material being studied.

[0006] However, in order for the dissolution of the oxides in the matrix and the mechanical alloy to be effective, the intensity of the mechanosynthesis must be high enough to deform the powder.

[0007] A solution has been proposed to reduce the predominant sticking, namely the use of surfactant or PCA (for "Process Control Agent" according to the (Anglo-Saxon terminology). However, these substances induce significant contamination in the finished material. For example, it has been shown that the use of stearic acid as a proton-based compound (PCA) induces significant carbon contamination, negatively impacting the behavior of the resulting steel. The addition of carbon leads to the precipitation of coarse M23C6 carbides, known to impair corrosion resistance and weaken the material. Similarly, the precipitation of M7C3 carbides is detected at grain boundaries, potentially affecting mechanical strength. Resistance under irradiation is also reduced by oxide growth, which is accelerated by carbon contamination due to the diffusion of elements towards the carbides.

[0008] It has also been proposed to perform a two-stage co-milling process. This involves a first co-milling of ferritic Fe-Cr steel powder with an oxide powder (lasting 20 to 40 hours), followed by a second co-milling (lasting 3 to 30 hours) with the addition of pure nickel powder. This yields an austenitic Fe-Cr-Ni alloy. However, the addition of a co-milling step with pure nickel induces significant binding, thereby impacting both the yield and the chemical homogeneity.

[0009] To prevent the powders from adhering to the balls, it has also been proposed to use grinding balls made of hard materials such as zirconia (ZrO2) or tungsten carbide (WC). However, the fragments of the balls trapped in the powders, once consolidated, drastically degrade the mechanical properties, particularly the toughness and ductility, of the austenitic ODS steels thus produced.

[0010] As a final example, it has also been proposed to perform cryo-milling on austenitic steels. Unfortunately, the materials obtained once consolidated are porous and have a bimodal microstructure. Furthermore, the oxides are particularly coarse (they are visible under an optical microscope).

[0011] Due to these problems with the adhesion of the austenitic steel powder, the yield is very low. This yield is relative to the quantity of powder actually used to produce the austenitic steel compared to the quantity of powder initially supplied. Summary of the invention

[0012] An object of the invention is to provide a process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion which makes it possible to improve the aforementioned yield.

[0013] To this end, the invention proposes a process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion, said process comprising the following steps: a) supply an austenitic steel powder Fe-Cr-Ni; b) carry out a grinding, known as cryogenic grinding, of the Fe-Cr-Ni austenitic steel powder at a temperature between -50°C and -196°C; c) provide a powder of oxides; d) to co-mill the Fe-Cr-Ni austenitic steel powder with the oxide powder thus supplied to obtain an Fe-Cr-Ni austenitic steel powder reinforced by an oxide dispersion; and e) to produce Fe-Cr-Ni austenitic steel reinforced by an oxide dispersion from the powder thus produced.

[0014] The method according to the invention may have at least one of the following characteristics, taken alone or in combination: - the grinding stage lasts between 10 minutes and 20 hours, in particular between 10 minutes and 15 hours, and more particularly between 10 minutes and 100 minutes. - The process includes an additional step, between step d) and step e), consisting of sieving the austenitic steel powder reinforced by an oxide dispersion to a predetermined initial average particle size of less than 250 microns. - Step e) includes the following sub-steps: e0 encapsulate and vacuum-pack the austenitic steel powder reinforced by an oxide dispersion, then: e2) perform hot isostatic compaction. - step e) is a sintering step, for example a flash sintering. - step e) is a hot spinning step. - the oxide powder supplied in step c) is chosen from a powder of Yttrium (Y2O3) oxides, Zirconium oxides or Titanium oxides. - - the oxide powder supplied in step a) has a particle size average less than 5 microns, preferably less than 1 micron, even more preferably less than 100nm. Brief description of the figures

[0015] Fig. 1 is a flowchart of a process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention;

[0016] Fig. 2 is a graph illustrating a comparative distribution of particle size by laser granulometry;

[0017] Fig. 3 is a photograph illustrating the difference in particle size obtained by a process according to the prior art and that obtained by the process of Fig. 1;

[0018] Fig. 4 is a photograph illustrating the comparison between a new grinding ball and grinding balls after implementation of the process of Fig. 1 and the prior art process. Detailed description

[0019] With reference to [Fig.1], we will describe a process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention.

[0020] The process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention comprises a first step 100 of supplying an austenitic Fe-Cr-Ni steel powder.

[0021] Next, the process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention comprises a second step 200 of grinding, referred to as cryogenic grinding, the Fe-Cr-Ni austenitic steel powder supplied at a temperature between -50°C and -196°C. This second step 200 lasts a few tens of minutes, for example, between 10 and 100 minutes. However, it may be necessary for this step to last longer, for example, several hours. More generally, this step can be expected to last between 10 minutes and 20 hours, in particular between 10 minutes and 15 hours. An Fe-Cr-Ni austenitic steel powder is obtained with a predetermined initial average particle size.

[0022] The process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention comprises a third step 300 of supplying an oxide powder, for example, a powder of yttrium (Y2O3) oxides. Other oxide powders can be used, such as zirconium oxides or titanium oxides. The supplied oxide powder has an average particle size of less than 5 microns, preferably less than 1 micron, and even more preferably less than 100 nm. The smaller the particle size of the oxide powder, the easier it is to incorporate the oxide powder into the steel powder.

[0023] Next, the process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention comprises a fourth step 400 of co-milling the Fe-Cr-Ni austenitic steel powder obtained from cryogenic grinding with the oxide powder to obtain an austenitic Fe-Cr-Ni steel powder reinforced by an oxide dispersion. This co-milling step involves a mechanosynthesis of the powders. This allows the oxides to dissolve within the grains of Fe-Cr-Ni austenitic steel powder.

[0024] The result of this co-milling step is illustrated, for example, in [Fig. 3] b). This is a scanning electron microscopy (SEM) image of 316L steel powder reinforced with an oxide dispersion obtained by the process for producing Fe-Cr-Ni austenitic steel reinforced with an oxide dispersion according to the invention. It is compared to the same powder ([Fig. 3] a)) obtained by a prior art process.

[0025] Finally, the process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention includes a fifth step 500 of producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion from the powder thus ground and sieved. This fifth step 500 is the consolidation step. For example, it may include the following substeps: encapsulating and vacuum-sealing the ground austenitic Fe-Cr-Ni steel powder reinforced by an oxide dispersion, which therefore has a final medium particle size, and then performing hot isostatic compaction (HIC).

[0026] Alternatively, the fifth step 500 of the process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention can be carried out by sintering, for example flash sintering (better known by the acronym SPS meaning "Spark Plasma Sintering" according to Anglo-Saxon terminology).

[0027] Step 500 can also be carried out by hot spinning or other compatible consolidation techniques.

[0028] It should be noted that between steps 400 and 500, an additional and optional step may be provided in which the Fe-Cr-Ni austenitic steel powder reinforced with an oxide dispersion is sieved to a predetermined final average particle size, which is, for example, less than or equal to 250 microns. This may prove useful if hot isostatic pressing (HIP) is to be carried out for step 500. Other final average particle size values ​​may be chosen depending on the application, such as 100 microns.

[0029] It appears from the use of a process for producing Fe-Cr-Ni austenitic steel reinforced by an oxide dispersion according to the invention as previously described makes it possible to obtain a greater quantity (yield) of Fe-Cr-Ni austenitic steel powder reinforced by oxide dispersion, moreover, without deterioration of the quality of the powder as can be the case for some prior art processes.

[0030] An improvement in yield can be noted incidentally by looking at [Fig. 4]. In a), the grinding ball is new. In b), the grinding ball is simply capped and was used in the fourth grinding step 400 of the process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention. In c), the grinding ball has a surface on which the austenitic steel powders adhere during co-grinding by a prior art process.

[0031] Furthermore, as illustrated in [Fig. 2], a volume distribution of the mean diameter (particle size distribution) of Fe-Cr-Ni austenitic steel powders reinforced by an oxide dispersion is given for two batches of powders. The curve with circles represents the conventional production of 316L ODS steel by a prior art process. The curve with crosses represents the production of the same 316L steel. ODS with the process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention. Note that the co-milling in the fourth step 400 is carried out under the same conditions for both tests. A significant reduction in particle size is observed between conventional co-milling and the double milling (cryogenic milling + co-milling) of the process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention. To consolidate the material by Hot Isostatic Pressing (HIP), the powder is often sieved to less than 250 micrometers. The results of this sieving of less than 250 qm give a fraction of 1.48% for conventional grinding and 70.26% for the double co-grinding of the process of producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention.This shows the potential of the process for producing an austenitic Fe-Cr-Ni steel reinforced by an oxide dispersion according to the invention to considerably increase the yield of the process since sieving at 250 microns allows 70% of the ground powder to be retained compared to barely 1.5% in the case of the conventional process.

Claims

Demands

1. A process for producing an austenitic Fe-Cr-Ni steel strengthened by an oxide dispersion, said process comprising the following steps: a) supplying (100) an austenitic Fe-Cr-Ni steel powder; b) carrying out (200) a grinding, referred to as cryogenic grinding, of the austenitic Fe-Cr-Ni steel powder at a temperature between -50°C and -196°C; c) supplying (300) an oxide powder; d) carrying out (400) a co-grinding of the austenitic Fe-Cr-Ni steel powder with the oxide powder thus supplied to obtain an austenitic Fe-Cr-Ni steel powder strengthened by an oxide dispersion; and, e) producing (500) the austenitic Fe-Cr-Ni steel strengthened by an oxide dispersion from the powder thus produced.

2. A method according to claim 1, wherein the grinding step lasts between 10 min and 20 h, in particular between 10 min and 15 h, and more particularly between 10 min and 100 min.

3. A method according to any one of claims 1 or 2, comprising an additional step, between step d) and step e), consisting of sieving the austenitic steel powder reinforced by an oxide dispersion to a predetermined initial average particle size of less than 250 microns.

4. A method according to any one of claims 1 to 3, wherein step e) comprises the following substeps: e1 encapsulate and vacuum-pack the oxide dispersion-reinforced austenitic steel powder and then: e2) perform hot isostatic compaction.

5. A method according to any one of claims 1 to 3, wherein step e) is a sintering step, for example flash sintering.

6. A method according to any one of claims 1 to 3, wherein step e) is a hot spinning step.

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

8. A process according to any one of the preceding claims, 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.

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