Adhesive and preparation method of low-alloy ultrahigh-strength steel

Abstract

The invention provides an adhesive and a preparation method of low-alloy ultrahigh-strength steel, and the adhesive comprises the following components in percentage by mass: 75-85% of polyformaldehyde, 5-11% of polypropylene carbonate, 2-5% of polylactic acid, 5-6% of stearic acid and 2-4% of ethylene-vinyl acetate copolymer. The optimized combination of the polypropylene carbonate and the polylactic acid in the adhesive has certain shape preserving performance, and can replace a skeleton component in a traditional plastic-based adhesive; the strength of the degreased brown blank can be guaranteed, the thermal decomposition temperature of framework components can be effectively reduced, introduction of impurity elements C and O of a metal matrix is reduced, and the comprehensive mechanical property of a sintered product can be improved. In addition, main decomposition products of polypropylene carbonate are carbon dioxide and cyclic propylene carbonate, and thermal decomposition products of polylactic acid are lactic acid, ethanol, carbon dioxide and water. Polypropylene carbonate and polylactic acid are used as raw materials, thermal decomposition products of polypropylene carbonate and polylactic acid are environment-friendly and are green and environment-friendly materials, polypropylene carbonate and polylactic acid are low in price, the degreasing process is simple, convenient and high in efficiency, and the cost of metal powder injection production can be reduced.

Description

Technical Field

[0001] The invention relates to the field of metal powder injection molding, and in particular to a method for preparing an adhesive and low-alloy ultra-high strength steel. Background Art

[0002] Low alloy ultra-high strength steel is widely used in aerospace and defense military fields due to its excellent properties such as high strength, high hardness, good plasticity and toughness, and low notch sensitivity, such as aircraft landing gear, solid rocket engine casing, and missile penetrator body. The preparation of 30CrMnSiNi2A steel generally adopts casting / forging process, but for some typical special-shaped components and complex components, the machining amount of casting / forging process is large and the material utilization rate is low, resulting in resource waste and cost increase. Powder injection molding technology has become a strong competitor to traditional processes such as casting, forging, and stamping due to its advantages such as near-net shaping, high material utilization rate, low labor cost, and mass production.

[0003] The biggest difference between metal powder injection molding technology and traditional powder metallurgy is that a high volume fraction of binder is introduced into the metal powder. The decomposition temperature of the skeletons in the traditional plastic-based binder system, such as polypropylene and polyethylene, is much higher than 300°C or even above 400°C. The above skeleton components are pyrolyzed at a high decomposition temperature, and their pyrolysis products are easy to react with metal elements to introduce impurities such as carbon and oxygen, thereby affecting the comprehensive mechanical properties of the sintered product.

[0004] Therefore, in order to address the problem of excessively high decomposition temperature of the skeleton components of traditional plastic-based adhesive systems, it is necessary to develop new adhesive formulas to prepare high-performance low-alloy ultra-high-strength steel products. This is a technical problem that urgently needs to be solved. Summary of the invention

[0005] The technical problem solved by the present invention is to provide a method for preparing an adhesive and low-alloy ultra-high strength steel, which on the one hand improves the performance of the low-alloy ultra-high strength steel and reduces the introduction of impurities, and on the other hand reduces the thermal decomposition temperature of the adhesive, thereby saving energy and being environmentally friendly.

[0006] The invention provides an adhesive, which comprises, by mass%, 75% to 85% of polyoxymethylene, 5% to 11% of polypropylene carbonate, 2% to 5% of polylactic acid, 5% to 6% of stearic acid and 2% to 4% of ethylene-vinyl acetate copolymer.

[0007] Correspondingly, the present invention also provides a method for preparing low-alloy ultra-high strength steel, comprising: providing metal powder and an adhesive, wherein the adhesive, in terms of mass %, comprises 75% to 85% of polyoxymethylene, 5 to 11% of polypropylene carbonate, 2 to 5% of polylactic acid, 5 to 6% of stearic acid, and 2 to 4% of ethylene-vinyl acetate copolymer; mixing the metal powder and the adhesive to form a feed; performing an injection molding process on the feed to obtain an injection blank; performing a degreasing process on the injection blank to obtain a brown blank; and sintering the brown blank to form a low-alloy ultra-high strength steel.

[0008] Optionally, the metal powder comprises alloy structural steel powder.

[0009] Optionally, the alloy structural steel powder includes 30CrMnSiNi2A metal powder.

[0010] Optionally, the volume percentage of the metal powder to the total volume of the feed is 55% to 65%, and the volume ratio of the metal powder to the binder constituting the feed is 135 to 115:75 to 90.

[0011] Optionally, the method of mixing metal powder and adhesive to form feed includes: in an inert gas atmosphere, mixing the metal powder and adhesive at 195° C. to 225° C. for 10 to 25 minutes, and then gradually cooling to 145 to 170° C. to crush and obtain feed.

[0012] Optionally, the injection molding process parameters include: injection holding time 8 to 20 seconds, holding pressure 60 to 95 MPa, injection temperature 180-200°C, and mold temperature 110-135°C.

[0013] Optionally, oxalic acid catalytic degreasing is used in the degreasing process, the degreasing temperature is 120-160°C, the degreasing is carried out under a nitrogen protective gas atmosphere, the nitrogen flow rate is 80-100L / min, and the degreasing time is t≥(60+60*h)min, where h is the maximum thickness of the injection blank, in mm.

[0014] Optionally, the sintering is performed in an argon atmosphere in a metal cavity sintering furnace, the sintering temperature is 1200-1500° C., and the holding time is 90-150 minutes.

[0015] Compared with the prior art, the technical solution of the present invention has the following advantages:

[0016] In the technical scheme of the adhesive of the present invention, the optimized combination of polypropylene carbonate and polylactic acid in the adhesive replaces the skeleton adhesive in the traditional plastic-based adhesive, reduces the thermal decomposition temperature of the skeleton component, reduces the introduction of C and O elements in the metal matrix, and helps to improve the comprehensive mechanical properties of the sintered product; polypropylene carbonate and polylactic acid have complementary properties, so the optimized combination of polypropylene carbonate and polylactic acid is used to serve as the skeleton component, so that the mixture has the strength to serve as the skeleton material and its thermal decomposition temperature is controlled within a lower temperature range; in addition, the main decomposition products of polypropylene carbonate are carbon dioxide and cyclic propylene carbonate, and the thermal decomposition products of polylactic acid are lactic acid, ethanol, carbon dioxide and water. The thermal decomposition products of the two are environmentally friendly and are green environmentally friendly materials, and polypropylene carbonate and polylactic acid are cheap, and the degreasing process is simple and efficient, which helps to reduce the cost of metal powder injection production.

[0017] In the preparation method of low-alloy ultra-high strength steel of the present invention, after metal powder and adhesive are mixed to form feed, the feed is subjected to an injection forming process to form an injection blank; the injection blank is subjected to a degreasing process to obtain a brown blank; the brown blank is sintered to form low-alloy ultra-high strength steel; the low thermal decomposition temperature of the adhesive is utilized to reduce the introduction of impurities in the low-alloy ultra-high strength steel, thereby improving the performance of the low-alloy ultra-high strength steel, and having a wider range of applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The flowchart of the preparation process of the low alloy ultra-high strength steel of the present invention is as follows;

[0019] Figure 2 This is a scanning electron microscope image of low alloy ultra-high strength steel in Example 1 of the present invention. DETAILED DESCRIPTION

[0020] As mentioned in the background technology, the decomposition temperature of the skeleton components of the traditional plastic-based adhesive system for metal injection molding is relatively high, which can easily introduce impurity elements such as C and O into the metal matrix, affecting the performance of the material to a certain extent.

[0021] On this basis, the present invention provides an adhesive, in which an optimized combination of polypropylene carbonate and polylactic acid replaces the skeleton adhesive in the traditional plastic-based adhesive, reduces the thermal decomposition temperature of the skeleton component, and helps to improve the comprehensive mechanical properties of the sintered product; at the same time, polylactic acid is added to polypropylene carbonate to improve the strength and thermal stability of polypropylene carbonate. In the adhesive, a mixture of polypropylene carbonate and polylactic acid copolymer has certain shape-keeping properties and can be used as a skeleton component of the adhesive, but excessive polylactic acid or the use of pure polylactic acid as a skeleton will also increase the decomposition temperature of the adhesive. Therefore, an optimized combination of polypropylene carbonate and polylactic acid is used as a skeleton component, so that the mixture has the strength to serve as a skeleton material and its thermal decomposition temperature is controlled within a lower temperature range; furthermore, the main decomposition products of polypropylene carbonate are carbon dioxide and cyclic propylene carbonate, and the thermal decomposition products of polylactic acid are lactic acid, ethanol, carbon dioxide and water. The thermal decomposition products of the two are environmentally friendly and are green and environmentally friendly materials. In addition, polypropylene carbonate and polylactic acid are inexpensive, and the degreasing process is simple and efficient, which helps to reduce the cost of metal powder injection production.

[0022] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, specific embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0023] An adhesive comprises, by mass%, 75% to 85% of polyoxymethylene, 5% to 11% of polypropylene carbonate, 2% to 5% of polylactic acid, 5% to 6% of stearic acid, and 2% to 4% of ethylene-vinyl acetate copolymer.

[0024] In this embodiment, in the adhesive, the mixture of polypropylene carbonate and polylactic acid has certain shape-keeping properties and can be used as the skeleton component of the adhesive. The optimized combination of polypropylene carbonate and polylactic acid ensures that the mixture has the strength to act as a skeleton structure, and the thermal decomposition temperature of the mixture is controlled in a lower range, thereby reducing the thermal decomposition temperature of the skeleton component, which helps to improve the comprehensive mechanical properties of low-alloy ultra-strength steel; at the same time, the main decomposition products of polypropylene carbonate are carbon dioxide and cyclic propylene carbonate, and the thermal decomposition products of polylactic acid are lactic acid, ethanol, carbon dioxide and water. The thermal decomposition products of the two are environmentally friendly and are green and environmentally friendly materials; and the price of polypropylene carbonate and polylactic acid is low, and the degreasing process is simple and efficient, which helps to reduce the cost of metal powder injection production.

[0025] It should be noted that polypropylene carbonate has good environmental protection, low decomposition temperature and environmentally friendly decomposition products, but its strength is insufficient; while polylactic acid has good strength. Therefore, by mixing polypropylene carbonate and polylactic acid, both the strength requirements can be met and it can be environmentally friendly.

[0026] like Figure 1 , providing metal powder and the above-mentioned adhesive to form a process diagram of low-alloy ultra-high strength steel, first, perform step S10: provide metal powder and adhesive.

[0027] In this embodiment, the metal powder adopts alloy structural steel metal powder, specifically 30CrMnSiNi2A, wherein the 30CrMnSiNi2A metal powder includes, by mass%, Cr: 0.90%-1.20%, Ni: 1.40%-1.80%, Si: 0.90%-1.20%, Mn: 1.00%-1.30%, C: 0.27%-0.34%, S: ≤0.016%, P≤0.02%, Cu≤0.02%, Fe balance; at the same time, 30CrMnSiNi2A alloy steel has excellent strength, toughness and wear resistance, and is usually used to manufacture mechanical parts and structural parts that bear high loads and impact loads. After heat treatment, the steel can obtain high strength, good plasticity and toughness, good fatigue resistance and fracture toughness, and low fatigue crack growth rate.

[0028] In some embodiments, other alloy structural steel powders may be used, such as 42CrMo or 30CrMnSiA, wherein the 42CrMo metal powder has, by mass%, a carbon content between 0.38% and 0.45%, a silicon content between 0.17% and 0.37%, a manganese content between 0.50% and 0.80%, an allowable residual content of sulfur and phosphorus of ≤0.035%, a chromium content between 0.90% and 1.20%, a molybdenum content between 0.15% and 0.25%, and the remainder is iron;

[0029] The content of 30CrMnSiA metal powder is calculated by mass % as follows: Si: 0.90~1.20%, Mn: 0.80~1.10%, S residual content allowed ≤0.025%, P%: residual content allowed ≤0.025%, Cr: 0.80~1.10%, Ni: residual content allowed ≤0.030%, Cu%: residual content allowed ≤0.025, and the rest is Fe.

[0030] In this embodiment, the 30CrMnSiNi2A metal powder is a nearly spherical powder with D50=20-50 μm. Within the diameter D range, the metal powder can have good fluidity, uniform powder spreading, large surface area, fast reaction rate, high density and mechanical properties, and help improve mixing efficiency and product quality.

[0031] In this embodiment, polyoxymethylene is the main body, a mixture of polypropylene carbonate and polylactic acid is the skeleton, stearic acid is the lubricant and dispersant, and ethylene-vinyl acetate copolymer is the surfactant.

[0032] Execute step S20: mix the metal powder and the adhesive to form a feed material.

[0033] In this embodiment, the volume percentage of the metal powder to the total volume of the feed is 55% to 65%, and the volume ratio of the metal powder to the binder constituting the feed is 135 to 115:75 to 90.

[0034] In this embodiment, in an inert gas atmosphere, the metal powder and the binder are mixed and stirred at 195° C. to 225° C. for 10 to 25 minutes, and then the temperature is gradually reduced to 145 to 170° C. for crushing to obtain feed material.

[0035] Execute step S30: perform injection molding process on the feed material to obtain an injection blank.

[0036] In this embodiment, the injection molding process parameters include: injection holding time 8-20s, holding pressure 60-95MPa, injection temperature 180-200°C, and mold temperature 110-135°C.

[0037] Execute step S40: perform a degreasing process on the injection blank to obtain a brown blank.

[0038] In this embodiment, oxalic acid catalytic degreasing is used in the degreasing process, the degreasing temperature is 120-160°C, the degreasing is carried out under a nitrogen protective gas atmosphere, the nitrogen flow rate is 80-100L / min, and the degreasing time is t≥(60+60*h)min, where h is the maximum thickness of the injection blank, in mm.

[0039] Execute step S50: sinter the brown billet to form low alloy ultra-high strength steel.

[0040] In this embodiment, the sintering is performed in an argon atmosphere in a metal cavity sintering furnace, the sintering temperature is 1200-1500° C., and the holding time is 90-150 minutes.

[0041] The preparation method of the formed low-alloy ultra-high strength steel is analyzed below in conjunction with specific embodiments.

[0042] Embodiment 1:

[0043] Step S10: 30CrMnSiNi2A nearly spherical metal powder includes the following chemical components by mass percentage: Cr: 0.90%-1.20%, Ni: 1.40%-1.80%, Si: 0.90%-1.20%, Mn: 1.00%-1.30%, C: 0.27%-0.34%, S: ≤0.016%, P≤0.02%, Cu≤0.02%, Fe balance; powder particle size D50=23.35μm; adhesive includes 80% polyoxymethylene, 9% polypropylene carbonate, 4% polylactic acid, 5% stearic acid, and 2% ethylene-vinyl acetate copolymer by mass percentage. The thermal decomposition temperature of the adhesive skeleton component is about 300°C.

[0044] Step S20: Mix the metal powder and the adhesive to form a feed. Specifically, in an argon atmosphere, mix the 30CrMnSiNi2A metal powder and the adhesive at 210° C. and stir for 20 minutes, then gradually cool down to 155° C. and crush to form a feed.

[0045] Step S30: performing injection molding process on the feed material to obtain an injection blank. Specifically, the injection holding time is 10s, the holding pressure is 90MPa, the injection temperature is 200°C, and the mold temperature is 120°C.

[0046] Step S40: Degreasing the injection blank using a degreasing furnace to obtain a brown blank. The specific technical parameters of degreasing are: degreasing using oxalic acid catalytic degreasing, degreasing temperature of 130°C, degreasing under a nitrogen protective gas atmosphere, nitrogen flow rate of 80L / min, catalytic degreasing time of 1130min, and removing part of the adhesive in the green blank to obtain a brown blank.

[0047] Step S50, high-temperature sintering the brown billet to obtain the final low-alloy ultra-high strength steel. The sintering temperature is 1300°C, the sintering time is 100 minutes, the remaining binder is removed and the brown billet is densified.

[0048] Embodiment 2:

[0049] Step S10: 30CrMnSiNi2A nearly spherical metal powder includes the following chemical components by mass percentage: Cr: 0.90%-1.20%, Ni: 1.40%-1.80%, Si: 0.90%-1.20%, Mn: 1.00%-1.30%, C: 0.27%-0.34%, S: ≤0.016%, P≤0.02%, Cu≤0.02%, Fe balance; powder particle size D50=23.35μm; adhesive includes 80% polyoxymethylene, 5% polypropylene carbonate, 5% polylactic acid, 6% stearic acid, and 4% ethylene-vinyl acetate copolymer by mass percentage. The thermal decomposition temperature of the adhesive skeleton component is about 310°C.

[0050] Step S20: Mix the metal powder and the adhesive to form a feed. Specifically, in an argon atmosphere, mix the 30CrMnSiNi2A metal powder and the adhesive at 210° C. and stir for 20 minutes, then gradually cool down to 155° C. and crush to form a feed.

[0051] Step S30: performing injection molding process on the feed material to obtain an injection blank. Specifically, the injection holding time is 10s, the holding pressure is 90MPa, the injection temperature is 200°C, and the mold temperature is 120°C.

[0052] Step S40: Degreasing the injection blank using a degreasing furnace to obtain a brown blank. The specific technical parameters of degreasing are: degreasing using oxalic acid catalytic degreasing, degreasing temperature of 130°C, degreasing under a nitrogen protective gas atmosphere, nitrogen flow rate of 80L / min, catalytic degreasing time of 1130min, and removing part of the adhesive in the green blank to obtain a brown blank.

[0053] Step S50, high-temperature sintering the brown billet to obtain the final low-alloy ultra-high strength steel. The sintering temperature is 1300°C, the sintering time is 100 minutes, the remaining binder is removed and the brown billet is densified.

[0054] Embodiment 3

[0055] Step S10: 30CrMnSiNi2A nearly spherical metal powder includes the following chemical components by mass percentage: Cr: 0.90%-1.20%, Ni: 1.40%-1.80%, Si: 0.90%-1.20%, Mn: 1.00%-1.30%, C: 0.27%-0.34%, S: ≤0.016%, P≤0.02%, Cu≤0.02%, Fe balance; powder particle size D50=23.35μm; adhesive includes 80% polyoxymethylene, 11% polypropylene carbonate, 2% polylactic acid, 5% stearic acid, and 2% ethylene-vinyl acetate copolymer by mass percentage. The thermal decomposition temperature of the adhesive skeleton component is about 290°C.

[0056] Step S20: Mix the metal powder and the binder to form a feed. Specifically, in an argon atmosphere, mix the 30CrMnSiNi2A metal powder and the binder at 210° C. and stir for 20 minutes, then gradually cool down to 155° C. and crush to form a feed.

[0057] Step S30: performing injection molding process on the feed material to obtain an injection blank. Specifically, the injection holding time is 10s, the holding pressure is 90MPa, the injection temperature is 200°C, and the mold temperature is 120°C.

[0058] Step S40: Degreasing the injection blank using a degreasing furnace to obtain a brown blank. The specific technical parameters of degreasing are: degreasing using oxalic acid catalytic degreasing, degreasing temperature of 130°C, degreasing under a nitrogen protective gas atmosphere, nitrogen flow rate of 80L / min, catalytic degreasing time of 1130min, and removing part of the adhesive in the green blank to obtain a brown blank.

[0059] Step S50, high-temperature sintering the brown billet to obtain the final low-alloy ultra-high strength steel. The sintering temperature is 1300°C, the sintering time is 100 minutes, the remaining binder is removed and the brown billet is densified.

[0060] Embodiment 4

[0061] Step S10: 30CrMnSiNi2A nearly spherical metal powder includes the following chemical components by mass percentage: Cr: 0.90%-1.20%, Ni: 1.40%-1.80%, Si: 0.90%-1.20%, Mn: 1.00%-1.30%, C: 0.27%-0.34%, S: ≤0.016%, P≤0.02%, Cu≤0.02%, Fe balance; powder particle size D50=23.35μm; adhesive includes 80% polyoxymethylene, 2% polypropylene carbonate, 11% polylactic acid, 5% stearic acid, and 2% ethylene-vinyl acetate copolymer by mass percentage. The thermal decomposition temperature of the adhesive skeleton component is about 360°C.

[0062] Step S20: Mix the metal powder and the adhesive to form a feed. Specifically, in an argon atmosphere, mix the 30CrMnSiNi2A metal powder and the adhesive at 210° C. and stir for 20 minutes, then gradually cool down to 155° C. and crush to form a feed.

[0063] Step S30: performing injection molding process on the feed material to obtain an injection blank. Specifically, the injection holding time is 10s, the holding pressure is 90MPa, the injection temperature is 200°C, and the mold temperature is 120°C.

[0064] Step S40: Degreasing the injection blank using a degreasing furnace to obtain a brown blank. The specific technical parameters of degreasing are: degreasing using oxalic acid catalytic degreasing, degreasing temperature of 130°C, degreasing under a nitrogen protective gas atmosphere, nitrogen flow rate of 80L / min, catalytic degreasing time of 1130min, and removing part of the adhesive in the green blank to obtain a brown blank.

[0065] Step S50, high-temperature sintering the brown billet to obtain the final low-alloy ultra-high strength steel. The sintering temperature is 1360°C, the sintering time is 100 minutes, the remaining binder is removed and the brown billet is densified.

[0066] Comparative Example:

[0067] Step S10: 30CrMnSiNi2A nearly spherical metal powder includes the following chemical components by mass percentage: Cr: 0.90%-1.20%, Ni: 1.40%-1.80%, Si: 0.90%-1.20%, Mn: 1.00%-1.30%, C: 0.27%-0.34%, S: ≤0.016%, P≤0.02%, Cu≤0.02%, Fe balance; powder particle size D50=23.35μm. The adhesive system includes by mass percentage: 80% polyoxymethylene, 13% high-density polyethylene, 5% stearic acid, and 2% ethylene-vinyl acetate copolymer. The thermal decomposition temperature of the adhesive skeleton component is about 420°C.

[0068] Step S20: Mix the metal powder and the adhesive to form a feed. Specifically, in an argon atmosphere, mix the 30CrMnSiNi2A metal powder and the adhesive at 210° C. and stir for 20 minutes, then gradually cool down to 155° C. and crush to form a feed.

[0069] Step S30: performing injection molding process on the feed material to obtain an injection blank. Specifically, the injection holding time is 10s, the holding pressure is 90MPa, the injection temperature is 200°C, and the mold temperature is 120°C.

[0070] Step S40: Degreasing the injection blank using a degreasing furnace to obtain a brown blank. The specific technical parameters of degreasing are: degreasing using oxalic acid catalytic degreasing, degreasing temperature of 130°C, degreasing under a nitrogen protective gas atmosphere, nitrogen flow rate of 80L / min, catalytic degreasing time of 1130min, and removing part of the adhesive in the green blank to obtain a brown blank.

[0071] Step S50, high-temperature sintering the brown billet to obtain the final low-alloy ultra-high strength steel. The sintering temperature is 1300°C, the sintering time is 100 minutes, the remaining binder is removed and the brown billet is densified.

[0072] The specific comprehensive performance of the embodiment, the hardness test adopts Vickers microhardness tester, the test force is 200N, the force holding time is 15s, and the average value is taken after ten tests; the tensile test adopts GB6397-86 metal tensile test specimen, and the average value is taken after three tests. The results are shown in Table 1.

[0073] Table 1 Comprehensive performance test results

[0074] category Hardness HV Shrinkage ratio Tensile strength MPa Yield strength MPa Embodiment 1 412 1.120 1503 1412 Embodiment 2 456 1.125 1452 1290 Embodiment 3 428 1.123 1071 913 Embodiment 4 487 1.135 1246 1078 Comparative Example 437 1.117 1203 1051

[0075] In this embodiment 1, a scanning electron microscope image of the low alloy ultra-high strength steel formed in the embodiment 1 is also provided. Figure 2 .

[0076] Although the present invention is disclosed as above, the present invention is not limited thereto. Any person skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined by the claims.

Claims

1. An adhesive, characterized in that: Calculated by mass%, it includes 75% to 85% of polyoxymethylene, 5 to 11% of polypropylene carbonate, 2 to 5% of polylactic acid, 5 to 6% of stearic acid, and 2 to 4% of ethylene-vinyl acetate copolymer.

2. A method for preparing low alloy ultra-high strength steel, characterized in that: include: Providing metal powder and the adhesive, wherein the adhesive comprises, by mass%, 75% to 85% of polyoxymethylene, 5% to 11% of polypropylene carbonate, 2% to 5% of polylactic acid, 5% to 6% of stearic acid, and 2% to 4% of ethylene-vinyl acetate copolymer; Mixing the metal powder with the adhesive to form a feed; Performing injection molding process on the feed material to obtain injection blank; Performing a degreasing process on the injection blank to obtain a brown blank; The brown billet is sintered to form the low alloy ultra-high strength steel.

3. The method for preparing low alloy ultra-high strength steel according to claim 2, characterized in that: The volume percentage of the metal powder to the total volume of the feed material is 55% to 65%, and the volume ratio of the metal powder to the adhesive constituting the feed material is 135 to 115:75 to 90.

4. The method for preparing low alloy ultra-high strength steel according to claim 2, characterized in that: The method of mixing the metal powder and the adhesive to form a feed comprises: in an inert gas atmosphere, mixing the metal powder and the adhesive at 195° C. to 225° C. for 10 to 25 minutes, and then gradually cooling to 145 to 170° C. to crush the mixture to obtain the feed.

5. The method for preparing low alloy ultra-high strength steel according to claim 2, characterized in that: The injection molding process parameters include: injection holding time 8 to 20 seconds, holding pressure 60 to 95 MPa, injection temperature 180 to 200° C., and mold temperature 110 to 135° C.

6. The method for preparing low alloy ultra-high strength steel according to claim 2, characterized in that: The degreasing process adopts oxalic acid catalytic degreasing, the degreasing temperature is 120-160°C, the degreasing is carried out in a nitrogen protective gas atmosphere, the nitrogen flow rate is 80-100L / min, the degreasing time is degreasing time t≥(60+60*h)min, the h is the maximum thickness of the injection blank, and the unit of h is mm.

7. The method for preparing low alloy ultra-high strength steel according to claim 2, characterized in that: The sintering is carried out in an argon atmosphere in a metal cavity sintering furnace, the sintering temperature is 1200-1500° C., and the heat preservation time is 90-150 minutes.

8. The method for preparing low alloy ultra-high strength steel according to claim 2, characterized in that: The metal powder includes alloy structural steel powder.

9. The method for preparing low alloy ultra-high strength steel according to claim 8, characterized in that: The alloy structural steel powder includes 30CrMnSiNi2A metal powder.

10. Low alloy ultra-high strength steel prepared by the preparation method according to any one of claims 2 to 9.

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