A high-strength, high-toughness steel MIM powder and its gradient sintering process

Abstract

This invention discloses a high-strength, high-toughness MIM powder and its gradient sintering process, belonging to the field of powder metallurgy technology. The MIM powder, by mass fraction, comprises 80-90 parts of pre-alloyed Fe-Cr-Ni-Mo powder, 3-5 parts of nano-doped TiC powder, 0.5-1 parts of rare earth oxide composite, 0.5-1 parts of TiB2 nanoparticles, 1-1.5 parts of zinc stearate, 6-7 parts of polyoxymethylene, and 2-4 parts of polyethylene wax. The nano-doped TiC powder is prepared by magnesothermic reduction, the rare earth oxide composite is a mixture of cerium oxide and lanthanum oxide, and the TiB2 nanoparticles are synthesized through a high-temperature solid-state reaction. After ball milling, melt bonding, and extrusion granulation, the powder is processed using a gradient sintering process, including two-stage catalytic degreasing via nitric acid vapor and vacuum, pre-sintering in a hydrogen atmosphere, diffusion alloying with an argon-hydrogen mixture, grain boundary strengthening treatment, and aging treatment. By optimizing the composition and process, the strength and toughness of the material are significantly improved, making it suitable for manufacturing high-performance metal injection molded products.

Description

Technical Field

[0001] This invention belongs to the field of metal powder injection molding (MIM) technology, specifically relating to a high-strength, high-toughness steel MIM powder and its gradient sintering process. Background Technology

[0002] Metal powder injection molding (MIM) is an advanced manufacturing process that introduces modern plastic injection molding technology into the field of powder metallurgy. This technology can produce high-precision, complex-shaped metal parts, making it particularly suitable for mass production. However, traditional MIM processes struggle to simultaneously achieve ultra-high strength and high toughness in metal parts, especially in the preparation of high-strength steel, where the material's toughness is often poor. Summary of the Invention

[0003] Purpose of the invention: The purpose of this invention is to provide a high-strength, high-toughness steel MIM powder and its gradient sintering process, so as to prepare metal parts with high strength, high toughness and high hardness by optimizing the powder composition and sintering process.

[0004] Technical solution: A high-strength, high-toughness steel MIM powder, wherein the MIM powder composition, by mass parts, comprises the following components: 80-90 parts of pre-alloyed Fe-Cr-Ni-Mo powder, 3-5 parts of nano-doped TiC powder, 0.5-1 parts of rare earth oxide composite, 0.5-1 parts of TiB2 nanoparticles, 1-1.5 parts of zinc stearate, 6-7 parts of polyoxymethylene, and 2-4 parts of polyethylene wax.

[0005] Preferably, the preparation method of the nano-doped TiC powder is as follows: magnesium powder, nano-titanium dioxide, activated carbon, and sodium chloride are mixed evenly in the ratio of magnesium powder: nano-titanium dioxide: activated carbon: sodium chloride (W:W:W:W) = 0.1:1:4:10. The mixture is ball-milled using a high-energy ball mill to ensure that the raw materials are fully and evenly mixed. The ball-milled mixture is placed in a tube furnace and heated to 900°C under an argon atmosphere for 1 hour. After the reaction is completed, molten salt is slowly added dropwise to a rapidly stirred ice-water bath with a weight of 200 times that of the mixture. The product is filtered and washed, and then dried under vacuum at 70°C to obtain the final product.

[0006] Preferably, the rare earth oxide complex is a mixture of cerium oxide and lanthanum oxide. The preparation method of the rare earth oxide complex is as follows: Lanthanum nitrate and cerium nitrate (1 times the weight of lanthanum nitrate) are added to a reaction flask, and deionized water (20 times the weight of lanthanum nitrate) is added to dissolve it by ultrasonication. Ammonia water is added dropwise under rapid stirring until the pH reaches 9. The dropwise addition is stopped, and the mixture is stirred for 30 minutes. The mixture is then filtered, and the solid is dried under vacuum at 60 degrees Celsius to constant weight. Finally, it is calcined at 750 degrees Celsius for 2 hours under an argon atmosphere to obtain the final product.

[0007] Preferably, the method for preparing the TiB2 nanoparticles is as follows: using high-purity rutile TiO2 powder with a purity >99.8% and a particle size of approximately 30 μm, B2O3 powder with a purity >99.5% and a particle size of approximately 50 μm, graphite powder with a purity >99% and a particle size of approximately 10 μm, and silver oxide with a purity >99% and a particle size of 30 μm, in a molar ratio n(TiO2):n(B2O3):n(C):n(AgO) = 1:1:5:0.01 Prepare the raw materials by placing them in a stainless steel ball mill jar, evacuating the vacuum chamber, and ball milling for 6 hours to ensure thorough and uniform mixing. Transfer the ball-milled mixture into a graphite crucible, place the sample in a vacuum carbon tube furnace, evacuate the vacuum chamber to below 50 Pa, purge the furnace with argon gas 2-3 times, and hold at the set temperature (1350℃-1700℃) for 34 hours with a heating rate controlled at 10℃ / min. After the reaction is complete, cool to room temperature at a rate of 15℃ / min to obtain the final product.

[0008] A gradient sintering process for MIM powder of high-strength and high-toughness steel includes the following steps:

[0009] (1) Pre-alloyed Fe-Cr-Ni-Mo powder was ball-milled and mixed with nano-doped TiC powder, rare earth oxide composite, zinc stearate and TiB2 nanoparticles under argon protection. Polyoxymethylene and polyethylene wax were added and melt-bonded at 120℃. After extrusion granulation, the mixture was crushed into composite 1 with a particle size ≤20μm.

[0010] (2) The complex 1 obtained in step (1) was subjected to two-stage catalytic degreasing by nitric acid vapor-vacuum heating to obtain complex 2;

[0011] (3) The composite 2 obtained in step (2) is heated to 900-950℃ at 5℃ / min and kept in H2 atmosphere (dew point ≤ -40℃) for 1h to obtain composite 3;

[0012] (4) The composite 3 obtained in step (3) is heated to 1250-1280℃ at 3℃ / min, switched to Ar-5%H2, and kept at the temperature for 2h to promote the dissolution of TiC and uniform diffusion of Cr / Mo, and to carry out diffusion alloying to obtain composite 4.

[0013] (5) The composite 4 obtained in step (4) is subjected to grain boundary strengthening, rapidly cooled to 800°C, and then slowly cooled to 500°C to obtain composite 5.

[0014] (6) The composite 5 obtained in step (5) is subjected to hot isostatic pressing to eliminate micro-defects and then subjected to aging treatment to increase the density of the precipitated phase.

[0015] Preferably, in step (1), the ball milling time is 4-6 hours, the ball-to-material ratio is 10:1, and the rotation speed is 200 rpm.

[0016] Preferably, in step (2), the nitric acid vapor catalytic degreasing temperature is 120°C and the degreasing time is 4h; the vacuum heating degreasing temperature is 300°C and the degreasing time is 2h.

[0017] Preferably, in step (5), the cooling rate for rapid cooling to 800°C is 50°C / min, and the cooling rate for slow cooling to 500°C is 1°C / min.

[0018] Preferably, in step (6), the hot isostatic pressing temperature is 1200℃, the pressure is 100MPa, the static pressing time is 2h, the aging treatment temperature is 480℃, and the treatment time is 4h.

[0019] Beneficial effects:

[0020] 1. The nanoscale doped TiC powder prepared by this invention achieves controllable preparation of high-performance nanoscale doped TiC powder through an innovative magnesia-thermal reduction-molten salt assisted synthesis process and the synergistic effect of each key node: the addition of magnesium powder reduces the carbonization barrier by forming a Mg-Ti-O mesophase, which can significantly reduce the reaction activation energy, allowing the conversion to be completed at 900℃ and shortening the reaction time to 1 hour, thus reducing energy consumption; sodium chloride provides an ion conduction environment, accelerates reactant diffusion, and inhibits local overheating, ensuring reaction uniformity; ice-water bath quenching (200 times volume) achieves ultra-rapid cooling, effectively inhibiting grain growth and obtaining a particle size distribution of 50-100nm; high-energy ball milling pretreatment achieves nanoscale uniform mixing and avoids local component segregation; argon gas prevents magnesium oxidation and inhibits carbon loss from the TiC surface. Meanwhile, because the nanoscale doped TiC powder is mainly produced through surface reaction, forming a composite structure of titanium carbide (TiC) coating titanium dioxide (TiO2), TiC itself has extremely high hardness. After being coated on the TiO2 surface, it can form a protective layer similar to "armor." Although pure TiC is hard, it is also quite brittle. The flexible substrate of TiO2 can alleviate stress concentration and significantly improve the wear resistance and scratch resistance of the composite. The plastic deformation capacity of TiO2 can partially compensate for the brittleness of TiC, allowing the composite to maintain high hardness while having better impact resistance. Secondly, TiO2 forms a dense oxide layer at high temperatures (e.g., TiO2→Ti2O3), while TiC itself is resistant to high temperatures (melting point ~3067℃). The coating structure can synergistically enhance the material's oxidation resistance in high-temperature environments. Furthermore, a passivation film can be formed on the surface of TiO2 (e.g., TiO2·xH2O). Combined with the high chemical inertness of TiC, the composite exhibits better resistance to acid and alkali corrosion than pure TiC. Moreover, TiC is a good conductor (metallic conductivity), while TiO2 is an insulator or semiconductor. After coating, conductivity can be controlled through interface design.

[0021] 2. This invention uses a 1:1 mixture of lanthanum nitrate and cerium nitrate, and prepares a cerium oxide (CeO2) and lanthanum oxide (La2O3) composite oxide via co-precipitation-calcination. Compared to directly physically mixing cerium oxide (CeO2) and lanthanum oxide (La2O3), this method has the following significant advantages: First, it improves structural uniformity. The composite oxide (prepared by co-precipitation) can achieve atomic-level uniform dispersion, with Ce... 3+ and La 3+ Co-precipitation in solution forms a homogeneous mixed precursor, which, upon calcination, yields a single-phase Ce-La-O solid solution or nanoscale composite oxide particles, with La in the crystal lattice. 3+ Uniformly replace part of Ce 4+ This results in a stable Ce1-xLaxO2-δ structure (x≈0.5), with a higher oxygen vacancy concentration and stronger grain boundary diffusion. The physical mixture is merely a particle-scale mixture; CeO2 and La2O3 are simply physically mixed at the micron level, exhibiting a distinct phase interface. 3+ Unable to penetrate the CeO2 lattice, La2O3 nanoparticles remain only on the surface or at grain boundaries, potentially leading to localized segregation during sintering and inconsistent performance. Simultaneously, the composite oxide enhances grain boundary strengthening and toughness, exhibiting a uniform pinning effect. La2O3 nanoparticles are uniformly distributed at the CeO2 grain boundaries, while some La2O3 nanoparticles... 3+ Entering the CeO2 lattice, a dual mechanism inhibits grain boundary slip, improves fracture toughness, and avoids grain boundary embrittlement caused by CeO2 alone. However, physical mixtures have the risk of local segregation, and La2O3 tends to accumulate at grain boundaries, leading to increased local brittleness. Finally, the composite oxide can form a composite oxide film, with CeO2 forming a Ce2O3 protective layer and La2O3 enhancing adhesion. The two work synergistically to improve high-temperature oxidation resistance. In sulfur / chlorine-containing environments, La2O3 preferentially forms stable sulfides / chlorides to protect the CeO2 matrix, while physical mixtures have insufficient delamination protection. CeO2 and La2O3 form oxide layers independently, which may peel off due to weak interfaces.

[0022] 3. The TiB2 nanoparticles of the present invention, the preparation method of which involves high-purity raw material ratio, multi-component synergistic reaction, gradient heating and long-term holding, etc., have significant advantages over traditional TiB2 preparation methods (such as direct high-temperature synthesis, mechanical alloying, etc.): at high temperatures, TiO2 may directly react with C to form TiC, or B2O3 may not decompose completely, resulting in B2O3 residue, Ag... + It preferentially reacts with TiO2 and B2O3, blocking side reaction pathways and ensuring that Ti and B atoms preferentially combine to form TiB2. Ag2O decomposes into Ag vapor at 1350-1700℃. +Ion adsorption on the surfaces of TiO2 and B2O2 lowers the Ti-B bond formation energy and significantly reduces the reaction temperature (TiB2 can be synthesized as low as 1350℃). Simultaneously, the catalytic effect of Ag promotes uniform carbon distribution, preventing localized carbon excess and the formation of TiC impurity phases. Graphite powder with a particle size of 10μm provides a high specific surface area, ensuring sufficient contact between C atoms and TiO2 and B2O3, avoiding insufficient or excessive carbon. Compared to the traditional use of carbon black (which is prone to agglomeration), graphite powder reacts more readily with oxides, improving the purity of TiB2. Gradient heating reduces thermal stress. Directly and rapidly heating to high temperatures can easily lead to localized melting or uneven reaction of the raw materials. This method uses a heating rate of 10℃ / min to gradually bring the raw materials to the reaction temperature, avoiding particle agglomeration or cracking caused by thermal stress. Holding for 34 hours ensures that TiO2, B2O3 and C react fully. Compared with short-term holding (e.g., <10 hours), this method has a higher TiB2 yield. Vacuuming to below 50Pa prevents TiB2 from reacting with O2 and N2 in the air to form TiO2 or TiN impurities. Cooling under argon protection prevents high-temperature particle oxidation. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the main chemical reactions of the nanoscale doped TiC powder of this invention.

[0024] Figure 2 This is a schematic diagram of the main chemical reactions of the rare earth oxide composite of the present invention.

[0025] Figure 3 TEM comparison images of the rare earth oxide composite prepared in this invention and the rare earth oxide composite prepared by physical mixing.

[0026] Wherein 1 is the rare earth oxide composite prepared in this invention, and 2 is the rare earth oxide composite prepared by physical mixing.

[0027] Figure 4 The image shows a comparison of the XRD patterns of the rare earth oxide composites prepared according to this invention and those prepared by physical mixing.

[0028] Figure 5 This is a schematic diagram of the main chemical reactions of the TiB2 nanoparticles of this invention.

[0029] Figure 6 TEM comparison images of TiB2 nanoparticles prepared by the method of the present invention and TiB2 nanoparticles prepared without the addition of silver oxide;

[0030] Wherein 1 is TiB2 nanoparticles prepared by the method of the present invention, and 2 is TiB2 nanoparticles prepared without the addition of silver oxide. Detailed Implementation

[0031] To make the technical solution of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0032] Example 1

[0033] Powder composition (parts by mass):

[0034] Pre-alloyed Fe-Cr-Ni-Mo powder: 85 parts

[0035] Nanoscale doped TiC powder: 4 parts

[0036] Rare earth oxide complex (a mixture of cerium oxide and lanthanum oxide): 0.75 parts

[0037] TiB2 nanoparticles: 0.75 parts

[0038] Zinc stearate: 1.25 parts

[0039] Polyoxymethylene: 6.5 parts

[0040] Polyethylene wax: 3 parts

[0041] Preparation method:

[0042] 1. Preparation of nanoscale doped TiC powder: Magnesium powder: nano-titanium dioxide: activated carbon: sodium chloride = 0.1:1:4:10 were mixed, ball-milled at high energy, reacted at 900℃ for 1 hour under an argon atmosphere, washed in an ice-water bath, and dried under vacuum at 70℃; the main chemical reaction diagram of the nanoscale doped TiC powder is attached. Figure 1 .

[0043] 2. Preparation of rare earth oxide complex: Lanthanum nitrate and cerium nitrate were dissolved in a 1:1 ratio, ammonia was added dropwise until pH=9, dried under vacuum at 60℃, and calcined at 750℃ for 2 hours under an argon atmosphere; the main chemical reaction diagram of the rare earth oxide complex is attached. Figure 2 TEM comparison images of rare earth oxide composites and physically prepared rare earth oxide composites are attached. Figure 3 XRD comparison images of rare earth oxide composites and rare earth oxide composites prepared by physical mixing are attached. Figure 4 .

[0044] 3. Preparation of TiB2 nanoparticles: Materials were prepared by mixing in a molar ratio of n(TiO2):n(B2O3):n(C):n(AgO) = 1:1:5:0.01, ball milling for 6 hours, and then holding at 1500℃ in a vacuum carbon tube furnace for 34 hours, followed by cooling to room temperature. A schematic diagram of the main chemical reactions involved in the preparation of TiB2 nanoparticles is attached. Figure 5 TEM comparison images of TiB2 nanoparticles prepared by the method of this invention and TiB2 nanoparticles prepared without the addition of silver oxide are attached. Figure 6 .

[0045] 4. Gradient sintering process:

[0046] Step (1): Ball milling for 4 hours under argon protection, ball-to-material ratio 10:1, rotation speed 200 rpm, 120℃ melting and bonding, extrusion granulation, and crushing into composite 1 with particle size ≤20μm.

[0047] Step (2): Nitric acid vapor catalytic degreasing at 120°C for 4 hours; vacuum heating degreasing at 300°C for 2 hours.

[0048] Step (3): Increase the temperature to 925℃ at 5℃ / min and keep it in H2 atmosphere (dew point ≤ -40℃) for 1 hour.

[0049] Step (4): Increase the temperature to 1265℃ at 3℃ / min, switch to Ar-5%H2, and keep warm for 2 hours.

[0050] Step (5): Rapidly cool to 800℃ (50℃ / min), then slowly cool to 500℃ (1℃ / min).

[0051] Step (6): Hot isostatic pressing temperature 1200℃, pressure 100MPa, static pressing time 2 hours; aging treatment temperature 480℃, treatment time 4 hours.

[0052] Example 2

[0053] Powder composition (parts by mass):

[0054] Pre-alloyed Fe-Cr-Ni-Mo powder: 80 parts

[0055] Nanoscale doped TiC powder: 5 parts

[0056] Rare earth oxide complex: 1 part

[0057] TiB2 nanoparticles: 1 part

[0058] Zinc stearate: 1.5 parts

[0059] Polyoxymethylene: 6 parts

[0060] Polyethylene wax: 2 parts

[0061] Preparation method:

[0062] Same as Example 1.

[0063] Example 3

[0064] Powder composition (parts by mass):

[0065] Pre-alloyed Fe-Cr-Ni-Mo powder: 90 parts

[0066] Nanoscale doped TiC powder: 3 parts

[0067] Rare earth oxide complex: 0.5 parts

[0068] TiB2 nanoparticles: 0.5 parts

[0069] Zinc stearate: 1 part

[0070] Polyoxymethylene: 7 parts

[0071] Polyethylene wax: 4 parts

[0072] Preparation method:

[0073] Same as Example 1.

[0074] Comparative Example 1

[0075] Same as Example 1, except that TiB2 nanoparticles were prepared directly at high temperature (1600℃) without AgO catalysis.

[0076] Comparative Example 2

[0077] Same as Example 1, except that ordinary TiC powder is used instead of nano-doped TiC powder.

[0078] Comparative Example 3

[0079] Same as Example 1, except that a physically mixed rare earth oxide is used instead of a rare earth oxide composite.

[0080] Comparative Example 4

[0081] Same as Example 1, except that gradient sintering process is not performed, only conventional sintering (holding at 1250℃ for 2 hours) is performed.

[0082] Comparative Example 5

[0083] Same as Example 1, except that the hot isostatic pressing step is not performed.

[0084] The bending strength is referenced to the standard ISO 7438:2020 (Metallic materials, bending test). A specimen measuring 80 mm in length, 10 mm in width, and 4 mm in thickness is placed on a support with a span of 60 mm. A concentrated load is applied, the maximum load is recorded, and the bending strength is calculated (σ = 3FL / 2bd). 2Where F is the load, L is the span, and b and d are the sample width and thickness; the wear amount is referenced to the standard ASTM G99 (sliding wear test standard), through a pin-disc wear test, under a constant load of 50N and a rotation speed of 200rpm, with a sliding distance of 1000m, the volume loss after sliding friction of the tungsten carbide ball on the wear part is calculated, and the wear rate is calculated as (volume loss / (load × sliding distance)); the hardness (HRC) is referenced to the standard ASTM E18 (Rockwell hardness standard for metallic materials), through the Rockwell hardness test (HRC scale), using a diamond cone indenter, applying a preload of 10kgf and a main load of 150kgf to a 10mm thick sample, holding the total load for 5s, and measuring the indentation depth, converting it to an HRC value; high temperature oxidation weight gain (mg / cm³) 2 Referring to ASTM G54 (High-Temperature Oxidation Assessment), the static oxidation weight gain test involves placing the sample in a 900°C furnace, exposing it to air or oxygen for 24 hours, cooling it, weighing it, and calculating the increase in mass per unit area; impact toughness (J / cm²). 2 Referring to ASTM E23 (Impact Testing of Metallic Materials), the Charpy V-notch impact test is performed. The notched specimen is placed in a pendulum impact testing machine, and the energy absorbed at fracture is measured. The results are normalized to the notch cross-sectional area (J / cm²). 2 The resistance to 3.5% HCl corrosion (mass loss %) was calculated and measured according to the reference standard ASTM G31 (Laboratory Immersion Corrosion Test). The immersion corrosion test involves immersing the sample in a 3.5% HCl solution (temperature 25℃, time 24 hours), cleaning and drying it, weighing it, and calculating the percentage of mass loss.

[0085] Table 1. Mechanical property test results of the examples and comparative examples.

[0086]

[0087]

[0088] Examples 1, 2, and 3 demonstrate superior performance compared to Comparative Examples 1-5 in terms of mechanical properties such as hardness, wear resistance, flexural strength, and impact toughness, as well as oxidation resistance and corrosion resistance. This indicates that the powder composition and preparation method of the present invention can significantly improve the overall performance of the material. Comparative Example 1 directly prepared TiB2 nanoparticles at high temperature without adding AgO catalyst, resulting in lower TiB2 purity and larger particle size, leading to a decrease in material performance. Comparative Example 2 used ordinary TiC powder instead of nanoscale doped TiC powder. While ordinary TiC powder has high hardness, it is brittle and lacks the flexible substrate of TiO2 to alleviate stress concentration, resulting in reduced material toughness, wear resistance, and impact resistance. Comparative Example 3 used a physically mixed rare earth oxide instead of a rare earth oxide composite. The physical mixture contains a phase interface, and La... 3+The inability to enter the CeO2 lattice prevents the synergistic effect, resulting in decreased toughness and oxidation resistance of the material. Comparative Example 4, without gradient sintering and only undergoing conventional sintering, fails to achieve sufficient densification and microstructure optimization, leading to inferior material performance compared to the examples. Comparative Example 5, without hot isostatic pressing, exhibits reduced densification and increased porosity defects, thus decreasing strength and toughness. In summary, this invention, by optimizing powder composition and preparation process, effectively improves the overall performance of the material, demonstrating significant technical advantages.

[0089] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A MIM powder of high-strength high-toughness steel, characterized by, The MIM powder composition comprises, by mass fraction: 80-90 parts of pre-alloyed Fe-Cr-Ni-Mo powder, 3-5 parts of nano-doped TiC powder, 0.5-1 part of rare earth oxide compound, 0.5-1 part of TiB2 nanoparticles, 1-1.5 parts of zinc stearate, 6-7 parts of polyformaldehyde, and 2-4 parts of polyethylene wax; The nano-doped TiC powder is prepared by mixing magnesium powder, nano-titanium dioxide, activated carbon and sodium chloride in a ratio of 0.1:1:4:10 (W:W:W:W), ball milling the mixture using a high-energy ball mill to ensure that the raw materials are fully mixed, placing the ball-milled mixture in a tube furnace, heating to 900°C under an argon atmosphere for 1 hour, after the reaction is complete, slowly adding the molten salt to an ice water bath with rapid stirring at a weight of 200 times the weight of the mixture, filtering and washing the product, and drying in a vacuum at 70°C to obtain the product; The rare earth oxide compound is a mixture of cerium oxide and lanthanum oxide, and the preparation method of the rare earth oxide compound is as follows: adding lanthanum nitrate and cerous nitrate with a weight of 1 times the weight of the lanthanum nitrate into a reaction bottle, adding deionized water with a weight of 20 times the weight of the lanthanum nitrate, ultrasonic dissolution, adding ammonia water under rapid stirring until the pH is 9, stopping the addition, stirring for 30 minutes, filtering, drying the solid in a vacuum at 60°C to a constant weight, and calcining at 750°C under an argon atmosphere for 2 hours to obtain the product.

2. The MIM powder of high-strength high-toughness steel according to claim 1, characterized in that, The TiB2 nanoparticles are prepared by using rutile TiO2 powder with a purity of >99.8% and a particle size of 30μm, B2O3 powder with a purity of >99.5% and a particle size of 50μm, graphite powder with a purity of >99% and a particle size of 10μm, and silver oxide with a purity of >99% and a particle size of 30μm, and the raw materials are mixed in a molar ratio of n(TiO2):n(B2O3):n(C):n(AgO)=1:1:5:0.01, the mixture is placed in a stainless steel ball mill tank, vacuumized and ball milled for 6 hours to ensure that the raw materials are fully mixed, the ball-milled mixture is moved into a graphite crucible, the sample is placed in a vacuum carbon tube furnace, vacuumized to below 50Pa, argon is filled to wash the furnace for 2-3 times, the temperature is set to 1350-1700°C, the temperature is kept for 34 hours at a heating rate of 10°C / min, and the sample is cooled to room temperature at a rate of 15°C / min to obtain the product.

3. A process for the gradient sintering of a MIM powder of a high-strength high-ductility steel according to any one of claims 1-2, characterized in that, The method comprises the following steps: (1) ball milling the pre-alloyed Fe-Cr-Ni-Mo powder, nano-doped TiC powder, rare earth oxide compound, zinc stearate and TiB2 nanoparticles under argon protection, adding polyformaldehyde and polyethylene wax, melting and bonding at 120°C, crushing the mixture to a particle size of ≤20μm to obtain compound 1; (2) performing two-stage catalytic debinding of compound 1 by nitric acid vapor-vacuum heating to obtain compound 2; (3) heating compound 2 to 900-950°C at a rate of 5°C / min under H2 atmosphere (dew point ≤-40°C) and keeping the temperature for 1 hour to obtain compound 3; (4) The composite 3 obtained in step (3) is heated to 1250-1280℃ at a rate of 3℃ / min, switched to Ar-5% H2, and kept for 2h to promote the dissolution of TiC and the uniform diffusion of Cr / Mo, and diffusion alloying is performed to obtain a composite 4; (5) The composite 4 obtained in step (4) is subjected to grain boundary strengthening, rapidly cooled to 800℃, and then slowly cooled to 500℃ to obtain a composite 5; (6) The composite 5 obtained in step (5) is subjected to hot isostatic pressing to eliminate micro defects, and is subjected to aging treatment to improve the density of precipitates, and the composite is obtained.

4. The process for gradient sintering of MIM powder of high strength high toughness steel as claimed in claim 3 wherein, In step (1), the ball milling mixing time is 4-6h, the ball-to-material ratio is 10:1, and the rotation speed is 200rpm.

5. The process for gradient sintering of MIM powder of high strength high toughness steel as claimed in claim 3 wherein, In step (2), the catalytic degreasing temperature of nitric acid vapor is 120℃, the degreasing time is 4h; the vacuum heating degreasing temperature is 300℃, and the degreasing time is 2h.

6. The process for gradient sintering of MIM powder of high strength high toughness steel as claimed in claim 3 wherein, In step (5), the cooling rate for rapidly cooling to 800℃ is 50℃ / min, and the cooling rate for slowly cooling to 500℃ is 1℃ / min.

7. The process for gradient sintering of MIM powder of high strength high toughness steel as claimed in claim 3 wherein, In step (6), the hot isostatic pressing temperature is 1200℃, the pressure is 100MPa, the isostatic pressing time is 2h, the aging treatment temperature is 480℃, and the treatment time is 4h.

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