A super-high strength and high toughness Fe-Mn-Ni series alloy and its preparation method

Abstract

The invention relates to an ultra-high-strength and high-toughness Fe-Mn-Ni alloy and a preparation method thereof, and belongs to the technical field of alloy materials. The invention comprises the following steps: step 1, preparing ingredients according to the following chemical composition and mass ratio, wherein the mass percentages of metal powders of Fe, Mn, Ni and C elements are: Fe 91.2%, Mn 2.8%, Ni 5.7% and C 0.3%, mixing by an ultrasonic mixing method, separating a medium by a natural sedimentation method, and placing the powder in an oven for drying; step 2, placing the dried powder in a gas compressor for hot isostatic pressing treatment, and smelting the powder in a helium gas environment to obtain a Fe-Mn-Ni alloy ingot; step 3, performing a solid solution treatment on the ingot; and step 4, hot extruding the ingot after the solid solution treatment to obtain an alloy ingot; and through precise component design and optimization of heat treatment process, the hardness, toughness and structural uniformity of the Fe-Mn-Ni alloy are significantly improved, while maintaining good forming ability and mechanical stability.

Description

Technical Field

[0001] The present invention relates to a super-high strength and high toughness Fe-Mn-Ni alloy and a preparation method thereof, belonging to the technical field of alloy materials. Background Art

[0002] Low manganese steel has received extensive attention due to its low manganese content (usually less than 1.65%) and high strength and toughness. This kind of steel is based on carbon structural steel, and the performance is optimized by adding appropriate amounts of other alloying elements (such as silicon, chromium, nickel, etc.), which can meet the diverse needs of fields such as construction, automobiles, bridges, and energy. Although the reduction of manganese content in low manganese steel helps to reduce costs and improve processing performance, it may also lead to insufficient strength and reduced toughness of the material.

[0003] In recent years, high strength and toughness Fe-Mn-Ni alloys have become a research hotspot due to their potential application value in extreme environments. For example, alloys of this system can be applied to high-strength low-temperature storage tanks, wear-resistant construction machinery, and key components in corrosive environments. Compared with traditional high manganese steels and ordinary low manganese steels, such alloys can achieve a better strength and toughness match, and have excellent reliability and durability in high stress and low temperature environments.

[0004] However, the development of Fe-Mn-Ni system alloys needs to solve a series of technical problems. First is the influence of element ratio on phase transformation behavior. The changes in manganese and nickel contents not only affect the microstructure evolution of the alloy, such as the stability of austenite and martensite, but also significantly change the deformation mechanism of the material (such as twinning deformation and slip); second is the problem of precipitation strengthening and toughness balance. By reasonably designing the alloy composition and heat treatment process, stable and fine second-phase particles can be formed in the alloy, thereby effectively improving the material strength. However, if the particle distribution is uneven or excessive precipitation occurs, it may cause toughness deterioration.

[0005] Therefore, it is necessary to design a super-high strength and high toughness Fe-Mn-Ni alloy and a preparation method thereof that can achieve industrial preparation, process optimization, and long-term performance stability. Summary of the Invention

[0006] The present invention aims at the deficiencies existing in the above-mentioned prior art and provides a super-high strength and high toughness Fe-Mn-Ni alloy and a preparation method thereof.

[0007] The technical solution of the present invention to solve the above technical problems is as follows:

[0008] A super-high strength and high toughness Fe-Mn-Ni alloy and a preparation method thereof, comprising the following steps:

[0009] Step 1, the ingredients are prepared according to the following chemical composition and mass ratio, the mass percentage of the metal powders of Fe, Mn, Ni and C elements is: Fe 91.2%, Mn 2.8%, Ni 5.7%, C 0.3%, ultrasonic mixing method is used for mixing, natural sedimentation method is used for separating the medium, and the powder is placed in an oven for drying;

[0010] Step 2: placing the dried powder in a gas compressor for hot isostatic pressing, and smelting the powder in a helium gas environment to obtain a Fe-Mn-Ni alloy ingot;

[0011] Step 3, subjecting the ingot to a solution treatment;

[0012] Step 4, hot extruding the solution treated ingot to obtain an alloy ingot;

[0013] Step 5: The alloy ingot after hot extrusion treatment is kept in a muffle furnace at 750° C. for 10 minutes and then air-cooled to room temperature; it is kept in a salt bath furnace at 150° C. for 30 minutes and then air-cooled to room temperature to obtain an ultra-high strength and high toughness Fe-Mn-Ni alloy.

[0014] Furthermore, in step 1, the purity of Fe, Mn, Ni and C powders is higher than 99.95%.

[0015] Furthermore, in step 1, the Fe, Mn, Ni, and C metal powders are screened to control the average particle diameter to be between 50 and 100 μm. between.

[0016] Furthermore, in the step 1, when mixing by ultrasonic mixing, the ratio of powder mass to deionized water liquid is 10:1, the ultrasonic processor is set to 350W power and 40kHz frequency, and the pulse mode treatment is performed for 30 minutes. The liquid temperature should be controlled at 50°C. After the mixing is completed, the natural sedimentation method is used, the powder is left to stand for one day, the powder deposited at the bottom is collected, the upper clear liquid is absorbed, and finally dried in an oven at 120°C for 2 hours.

[0017] Furthermore, in the step 2, hot isostatic pressing is carried out in a temperature range of 1000°C-1100°C, a heating rate of 10°C / s, an applied pressure of 120MPa, a pressing time of 30 minutes, and a holding time of 2 hours; during the pressing process, the temperature and pressure need to be evenly transmitted to ensure the densification effect; after the treatment, the temperature is slowly lowered at a cooling rate of 10°C / min to finally obtain a Fe-Mn-Ni alloy ingot.

[0018] Furthermore, the grain size and structural uniformity of the Fe-Mn-Ni alloy ingot obtained in step 2 are detected by optical microscopy and XRD analysis to ensure that it meets the subsequent processing requirements.

[0019] Further, in step 3, the casting blank is heat-preserved at 1200 °C for 90 minutes to complete the solution treatment.

[0020] Further, in step 4, the material after solution treatment is hot-extruded within the temperature range of 900 °C to 1000 °C, the pressure applied during extrusion is 1000 Mpa, and the extrusion speed is controlled at 5 mm / s.

[0021] Further, in step 4, after extrusion is completed, cooling is carried out at a rate of 10 °C / min.

[0022] A super-high-strength and high-toughness Fe-Mn-Ni alloy system is obtained by the preparation method of the super-high-strength and high-toughness Fe-Mn-Ni alloy system described above.

[0023] The beneficial effects of adopting the above further scheme are as follows: Through precise composition design and optimization of the heat treatment process, the hardness, toughness, and tissue uniformity of the Fe-Mn-Ni alloy are significantly improved, while maintaining good formability and mechanical stability;

[0024] By heat-preserving the casting blank at 1200 °C for 90 minutes to complete the solution treatment, at this temperature, alloying elements (such as Mn, Ni, C) are fully dissolved into the matrix, and the segregation phenomenon in the as-cast structure is effectively eliminated. Element segregation often exists in the as-cast material, which may lead to non-uniform material properties and affect the quality and reliability in subsequent processing. Through the solution treatment at 1200 °C, it not only helps to dissolve the alloying elements in the raw materials, but also eliminates the coarse grain structure and makes the structure of the material more uniform;

[0025] The gas compressor can achieve densification of powder materials, pore elimination, and microstructure homogenization through uniform transmission of high-temperature and high-pressure inert gas, can maintain the geometric integrity of the material while applying isotropic pressure, avoid generating non-uniform deformation or internal stress, and effectively improve the mechanical properties and reliability of the alloy. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is the hardness diagram of the specimen of the present invention under different heat treatment methods;

[0027] Figure 2 It is the hardness diagram of the specimen of the present invention under different heat treatment methods;

[0028] Figure 3 It is the engineering stress-strain curve of the specimen of the present invention under different heat treatment methods;

[0029] Figure 4-a is the fracture morphology diagram at 2000 times magnification of the specimen at 550 °C in Example 5 of the present invention;

[0030] Figure 4 -b is the fracture morphology diagram at 2000 times magnification of the specimen at 650 °C in Example 5 of the present invention;

[0031] Figure 4 -c is the fracture morphology diagram at 2000 times magnification of the specimen at 700 °C in Example 5 of the present invention;

[0032] Figure 4 -d is the fracture morphology diagram at 2000 times magnification of the specimen at 750 °C in Example 5 of the present invention;

[0033] Figure 4 -e is the fracture morphology diagram at 2000 times magnification of the specimen at 800 °C in Example 5 of the present invention;

[0034] Figure 4 -f is the fracture morphology diagram at 2000 times magnification of the specimen at 950 °C in Example 5 of the present invention;

[0035] Figure 5 are the engineering stress-strain curves of the specimens of the present invention under different heat treatment methods;

[0036] Figure 6 are the tensile fracture morphology diagrams of the specimens of the present invention under different heat treatment methods;

[0037] Figure 7 are the engineering stress-strain curves of the specimens of the present invention under different heat treatment methods;

[0038] Figure 8 are the tensile fracture morphology diagrams of the specimens of the present invention under different heat treatment methods;

[0039] Figure 9 -a is the metallographic picture at 150 times magnification of Example 2 of the present invention;

[0040] Figure 9 -b is the metallographic picture at 250 times magnification of Example 2 of the present invention;

[0041] Figure 9 -c is the metallographic picture at 500 times magnification of Example 2 of the present invention;

[0042] Figure 9 -d is the metallographic picture at 150 times magnification of Example 3 of the present invention;

[0043] Figure 9 -e is the metallographic picture at 250 times magnification of Example 3 of the present invention;

[0044] Figure 9-f is the metallographic picture 500 times that of Example 3 of the present invention;

[0045] Figure 9 -g is the metallographic picture 150 times that of Example 4 of the present invention;

[0046] Figure 9 -h is the metallographic picture 250 times that of Example 4 of the present invention;

[0047] Figure 10 XRD pattern of Example 5 of the present invention;

[0048] Figure 11 XRD pattern of Example 6 of the present invention. Detailed implementation mode

[0049] The principles and features of the present invention are described below. The examples given are only used to explain the present invention and are not intended to limit the scope of the present invention.

[0050] A super-high strength and high toughness Fe-Mn-Ni alloy and its preparation method, comprising the following steps:

[0051] Step 1: Weigh the metal powders according to the following chemical compositions and mass ratios. The mass percentages of Fe, Mn, Ni, and C metal powders are: Fe 91.2%, Mn 2.8%, Ni 5.7%, C 0.3%. The purity of the metal powders is higher than 99.95%. The average particle diameter is controlled between 50-100 μm;

[0052] Mix the metal powders by ultrasonic mixing method. When mixing, the ratio of powder mass to deionized water liquid is 10:1. The power of the ultrasonic processor is set to 350W, the frequency is 40kHz, and the pulse mode is processed for 30 minutes to ensure that the powders are fully dispersed. During the process, the liquid temperature should be controlled at 50 °C to avoid overheating affecting the powder dispersion effect. After mixing, separate the medium by natural sedimentation method. Let the powder stand for one day, collect the powder deposited at the bottom, and suck out the supernatant. Place the powder in an oven and dry it finally in an oven at 120 °C for 2 hours to ensure that the powder is completely dry and there is no residual liquid;

[0053] Step 2: Place the dried powder in a gas compressor for hot isostatic pressing treatment, and carry out melting in a helium gas environment. The hot isostatic pressing is carried out in the temperature range of 1000°C - 1100°C, with a heating rate of 10°C / s, an applied pressure of 120 MPa, a pressurization time of 30 minutes, and a holding pressure time of 2 hours; during the pressurization process, the temperature and pressure need to be evenly transmitted to ensure the densification effect; after the treatment, cool down slowly at a cooling rate of 10°C / min to avoid generating internal stress or cracks, and obtain an Fe-Mn-Ni series alloy ingot. Its grain size and tissue uniformity are detected by optical microscopy and XRD analysis to ensure meeting the requirements of subsequent processing;

[0054] Through the uniform transmission of high-temperature and high-pressure inert gas, the gas compressor can achieve the densification of powder materials, pore elimination, and microstructure homogenization, and can maintain the geometric integrity of the material while applying isotropic pressure, avoiding uneven deformation or internal stress, and effectively improving the mechanical properties and reliability of the alloy.

[0055] Step 3: Keep the billet at 1200°C for 90 minutes. The purpose is to complete the solution treatment. At this temperature, alloying elements (such as Mn, Ni, C) are fully dissolved into the matrix, and the segregation phenomenon in the as-cast structure is effectively eliminated. Element segregation often exists in as-cast materials, which may lead to non-uniform material properties and affect the quality and reliability in subsequent processing. Through the solution treatment at 1200°C, it not only helps to dissolve the alloying elements in the raw materials, but also eliminates the coarse grain structure and makes the structure of the material more uniform;

[0056] Step 4: Carry out hot extrusion on the solution-treated ingot in the temperature range of 900°C to 1000°C to ensure that the alloy has good fluidity and plasticity. The pressure applied during extrusion is 1000 Mpa to promote densification and optimize the microstructure. The extrusion speed is controlled at 5 mm / s to ensure sufficient deformation without excessive cooling; after extrusion, cool down at a rate of 10°C / min to avoid generating internal stress or cracks, and obtain an ultra-high strength and high toughness Fe-Mn-Ni series alloy ingot. By precisely controlling these parameters, the density and mechanical properties of the alloy can be effectively improved.

[0057] An ultra-high strength and high toughness Fe-Mn-Ni series alloy is obtained by the preparation method of the ultra-high strength and high toughness Fe-Mn-Ni series alloy described above. Example

[0058] The Fe-Mn-Ni series alloy (Fe-2.8Mn-5.7Ni-0.3C) obtained according to the above preparation method in this example was held at 950 °C for 30 min, and then salt bath heat-preserved specimens were prepared for it, and the salt bath temperatures were 250 °C, 300 °C, 350 °C, and 400 °C respectively.

[0059] Comparative Example 1

[0060] In this comparative example, Ni in the preparation method of the Fe-Mn-Ni series alloy was replaced by Al, and the others remained unchanged, obtaining an Fe-Mn-Al series alloy (Fe-2.8Mn-5.7Al-0.3C), and it was held at 950 °C for 30 min, and then salt bath heat-preserved specimens were prepared for it, and the salt bath temperatures were 250 °C, 300 °C, 350 °C, and 400 °C respectively.

[0061] Through the comparative experiments of the specimens in Example 1 and Comparative Example 1, it was obtained that Figure 1 , and the results showed that: the hardness of the Fe-Mn-Ni series alloy (Fe-2.8Mn-5.7Ni-0.3C) was significantly higher than that of the Fe-Mn-Al series alloy (Fe-2.8Mn-5.7Al-0.3C); both groups of alloys showed a trend that the hardness first increased and then decreased with the increase of the salt bath temperature at different salt bath temperatures. Among them, the hardness of both reached the peak value after the salt bath treatment at 300 °C, showing the best strengthening effect. This difference may be attributed to the solid solution strengthening of the Ni element in the Fe-Mn-Ni series alloy and the stabilizing effect on the grain boundary, as well as its more favorable formation of fine martensite or bainite phases at the salt bath temperature of 300 °C. While for the Fe-Mn-Al series alloy, due to the weak solid solution effect of the Al element and its limited contribution to the high-temperature phase transformation, the overall hardness was low. Example

[0062] The prepared Fe-Mn-Ni series alloy (Fe-2.8Mn-5.7Ni-0.3C) in this example was held at 900 °C for 10 min, and salt bath heat-preserved specimens were prepared. The salt bath temperature was 200 °C, and the specimens were rolled. Each time it was held in the salt bath furnace for 20 s, the thickness was reduced by 36%.

[0063] Comparative Example 2

[0064] In this comparative example, Ni in the preparation method of the Fe-Mn-Ni series alloy was replaced by Al, and the others remained unchanged, obtaining an Fe-Mn-Al series alloy (Fe-2.8Mn-5.7Al-0.3C). It was held at 900 °C for 10 min, and salt bath heat-preserved specimens were prepared. The salt bath temperature was 200 °C, and the specimens were rolled. Each time it was held in the salt bath furnace for 20 s, the thickness was reduced by 36%. Example

[0065] In this example, the prepared Fe-Mn-Ni alloy system was held at 800 °C for 40 min and then water quenched, tempered at 550 °C for 1 h, and air cooled to room temperature. Example

[0066] In this example, the prepared Fe-Mn-Ni alloy system was air cooled after being held at 900 °C for 40 min, air cooled after being held at 790 °C for 40 min, and then water quenched after being held at 550 °C for 1 h.

[0067] Through the comparative tests of Example 2 and Comparative Example 2, it was obtained that Figure 2 , and the results showed that: the hardness of the Fe-Mn-Ni alloy system (Fe-2.8Mn-5.7Ni-0.3C) was 379.2 HV, significantly higher than the hardness of the Fe-Mn-Al alloy system (Fe-2.8Mn-5.7Al-0.3C) which was 322.1 HV. This significant difference was mainly attributed to the solid solution strengthening effect of Ni element in the Fe-Mn-Ni alloy system, as well as the promotion of Ni on the phase transformation behavior and grain refinement during rolling. Ni element could effectively stabilize austenite, form a more uniform deformed structure, and distribute fine strengthening phases at grain boundaries and in the matrix, thus significantly improving the hardness and mechanical properties of the alloy. In addition, the rolling process under the 200 °C salt bath condition further strengthened the performance of the Fe-Mn-Ni alloy system, promoted the stabilization of retained austenite and the formation of fine martensite phase, and further increased its hardness.

[0068] In contrast, the hardness performance of the Fe-Mn-Al alloy system was significantly insufficient, which was mainly due to the weak solid solution strengthening effect of Al element and its limited contribution to the phase transformation behavior. During rolling, Al did not significantly improve grain refinement or tissue stability, nor effectively promote the formation of strengthening phases, resulting in lower hardness and comprehensive performance. At the same time, the ability of Al element to regulate the phase transformation temperature was not as good as that of Ni, and it could not form equally stable and refined microstructures, which further restricted the performance improvement of the Fe-Mn-Al alloy system.

[0069] Although the Fe-Mn-Ni alloy system showed relatively high hardness in the rolling process under the 200 °C salt bath condition, its performance still had room for further optimization. By increasing the heat treatment temperature or adjusting the heat treatment time, the grain structure and phase composition of the alloy could be significantly improved, and the toughness, fatigue resistance and comprehensive mechanical properties could be further enhanced.

[0070] Through the microstructure analysis of Examples 2, 3, and 4 under different heat treatment processes (as Figure 9 shown), the following conclusions were drawn:

[0071] Through the metallographic pictures of Example 2 Figure 9 -a,Figure 9 -b, Figure 9 In figures -c, the microstructure is mainly composed of fine lath martensite, accompanied by a small amount of retained austenite distributed. The dynamic recrystallization effect significantly refines the grains, forming a uniform deformed structure. This kind of microstructure has excellent strengthening effect, with a significant increase in hardness, showing relatively high mechanical properties. However, this process aims mainly at strengthening. Although the hardness is high, the toughness and fatigue resistance are relatively insufficient, and there is room for improvement under the condition of relatively high comprehensive performance requirements.

[0072] The metallographic pictures of Example 3 Figure 9 -d, Figure 9 -e, Figure 9 In figures -e, -f, the microstructure shows tempered martensite and partially refined ferrite phases. The tempering process effectively releases the quenching residual stress, and at the same time further enhances the hardness through the precipitation of carbides. However, the softening of some strengthening phases during the tempering process results in a limited improvement in performance. In addition, the increase in the relative proportion of tempered ferrite makes the overall structure tend to be balanced, but the optimization between comprehensive strength and toughness is still inferior to more advanced heat treatment processes.

[0073] The metallographic pictures of Example 4 Figure 9 -g, Figure 9 In figures -g, -h, the microstructure is mainly composed of ferrite, with a small amount of dispersed carbides distributed locally. This process realizes the improvement of comprehensive performance through carbide precipitation and microstructure stabilization. However, the ferrite-based structure, although having good toughness and plasticity, has lower strength and hardness than the ideal effect. This indicates that this process is suitable for application scenarios with relatively high requirements for toughness but relatively low requirements for strength.

[0074] Although both Example 3 and Example 4 have improved in terms of tissue stability, compared with the strengthening microstructure dominated by fine lath martensite in Example 2, there is still a gap in their comprehensive performance. This indicates that it is still necessary to further optimize the heat treatment process (such as adjusting the heating temperature, holding time, or combining multi-step processes) to balance hardness, strength, and toughness, and fully explore the performance potential of Fe-Mn-Ni series alloys. Example

[0075] In this example, the prepared Fe-Mn-Ni series alloy was respectively held at 550 °C, 650 °C, 700 °C, 750 °C, 800 °C, 950 °C for 10 min, air-cooled to room temperature, and the ingot was held in a salt bath furnace at 150 °C for 30 min and air-cooled to room temperature. Example

[0076] In this embodiment, the prepared Fe-Mn-Ni alloy system was held at 950 °C for 10 min, air-cooled to room temperature, and then the ingots were respectively held in a salt bath furnace at 0 °C, 150 °C, 200 °C, 250 °C, and 300 °C for 30 min and air-cooled to room temperature.

[0077] Tensile tests were carried out on the specimens treated in Example 5 and Example 6. The tensile specimens were tested at a tensile rate of 0.5 mm / min. According to the tensile test results of Example 5, that is Figure 3 It can be seen that the fracture morphology shows that with the increase of the heat treatment temperature, the dimples gradually increase and deepen, showing an enhanced plastic fracture characteristic; after the 550 °C salt bath treatment, the fracture dimples are shallower and unevenly distributed, indicating that the plastic deformation ability of the material is weak. It is speculated that it may be due to the fact that the low temperature fails to effectively promote the stability of the austenite phase or the precipitated phase is not fully dissolved, resulting in insufficient matrix tissue performance; when the 750 °C salt bath treatment is carried out, the dimples are deep and large and evenly distributed, indicating that the material reaches the best tissue control effect at this temperature. The sufficient solid solution strengthening of Ni and Mn and the partial decomposition of carbides form a stable FCC austenite phase, thereby improving the toughness and plasticity; after the treatment at 800 °C and above temperatures, although the dimples further increase, the performance decreases, which is speculated to be related to grain coarsening, grain boundary weakening and possible element segregation or grain boundary precipitated phases.

[0078] Combined with Figure 3 the stress-strain curve of Figure 4 and the microscopic morphology of

[0079] it can be seen that the changes of the two are consistent. When treated at 550 °C, the material shows the lowest yield strength and elongation, and the fracture mode tends to be brittle; when treated at 750 °C, it shows the highest yield strength and elongation, which is consistent with the dimple morphology characteristics, indicating that the tissue homogenization and the stability of the FCC phase are the best at this temperature, endowing the material with excellent comprehensive mechanical properties; although the elongation is retained after the treatment at 800 °C and above, the yield strength decreases, indicating that too high a temperature weakens the grain boundary strengthening effect; generally speaking, the 750 °C salt bath 150 °C treatment can achieve the best balance between the strength and plasticity of the material, which is the preferred temperature range for the heat treatment of this alloy, and too low or too high a temperature is not conducive to the performance improvement. Figure 6As shown in Figures a, b, c, and d, the dimple morphology changes slightly under different cooling conditions. However, salt bath treatment slightly deepens the dimples and makes their distribution more uniform, and no significant cracks are observed. This indicates that there may be high residual stresses under rapid cooling conditions, while slow cooling in the salt bath effectively releases the internal stresses and improves the fracture toughness. When air-cooled to room temperature, as Figure 6 shown in Figure a, the dimples are of moderate depth and evenly distributed. The high residual stresses from rapid cooling contribute to strengthening the material strength but may reduce the toughness. After treatment at 150 °C in the salt bath, as Figure 6 shown in Figure b, the dimples are slightly deeper and evenly distributed, reflecting the stability of the austenite phase and the effective release of internal stresses; under the conditions of 250 °C and 300 °C in the salt bath, as Figure 6 shown in Figures c and d, the change in dimple depth is not significant, but the tissue uniformity is further improved, indicating that high-temperature salt bath helps to further reduce stress concentration but may cause local softening.

[0080] The stress-strain behavior under Example 6 is as Figure 5 shown. The yield strength gradually decreases with the increase of the salt bath temperature, while the strain remains relatively stable. Air-cooling to room temperature can obtain the highest yield strength. Rapid cooling promotes grain refinement but may reduce the toughness of the material due to residual stresses. Treatment at 150 °C in the salt bath shows better comprehensive performance. The yield strength is slightly lower than that of air-cooling, but the elongation rate is significantly improved, indicating that slow cooling effectively avoids the negative effects of high residual stresses and improves the plasticity while maintaining a relatively high strength; under the treatment conditions of 250 °C and 300 °C in the salt bath, the yield strength further decreases and the change in strain is not significant. It is speculated that due to further grain growth at higher temperatures, the strengthening effect weakens and the stability of austenite decreases. In summary, after austenitization treatment at 950 °C, the best balance between strength and toughness can be achieved by cooling in a 150 °C salt bath, while too high a salt bath temperature will lead to a decrease in strength. Example

[0081] In this example, the smelted Fe-Mn-Ni alloy system is held at 750 °C for 10 min and then air-cooled to room temperature. Then, the ingots are respectively held in a salt bath furnace at 150 °C, 200 °C, 250 °C, and 300 °C for 30 min and air-cooled to room temperature.

[0082] Through the tensile test on the specimens treated in Example 7, the tensile fracture morphology (as Figure 8 shown) and mechanical properties (as Figure 7Analysis (as shown) reveals a significant effect of heat treatment time on the microstructure and mechanical properties of the material. The fracture morphology shows that with the extension of the salt bath time, the dimples gradually become larger and deeper, but at the same time, the number of cracks increases and the size becomes larger, reflecting the complex influence of tissue evolution on the plasticity and toughness of the material. After salt bath treatment at 150°C, the material exhibits uniform and moderately deep dimple characteristics. Combining with the stress-strain curve, it can be seen that both the yield strength and elongation reach the best, indicating that the material structure is uniform under this condition, the FCC austenite phase is stable, and the grain boundaries do not undergo significant weakening. After 200°C salt bath treatment, the dimples deepen but the number decreases, and local cracks appear. The yield strength is close to that of the 150°C treatment, but the strain decreases significantly, indicating that local precipitation may occur at the grain boundaries or sub-grain boundaries, affecting the plasticity of the material. When salt bath treatment is carried out at 250°C, the dimples increase significantly and are accompanied by larger cracks. The fracture morphology shows brittle fracture characteristics, and both the yield strength and elongation in the stress-strain curve decrease significantly. It is speculated that this is due to the increase in grain boundary precipitates or grain coarsening, resulting in grain boundary weakening and plastic degradation. After 300°C salt bath treatment, the dimples tend to be stable and the elongation recovers somewhat, but the yield strength further decreases, which may be related to further grain growth and grain boundary precipitation in the structure. Therefore, it is once again confirmed that the moderate treatment time of the 750°C salt bath at 150°C (i.e., "heat preservation in a muffle furnace at 750°C for 10 min and then air-cooled to room temperature, and then heat preservation in a salt bath furnace at 150°C for 30 min and then air-cooled to room temperature" in step 5) effectively avoids the adverse effects of increased grain boundary precipitates and grain coarsening, and at the same time realizes the balanced optimization of strength and toughness.

[0083] As Figure 10 shown, the diffraction peaks of all samples correspond to the face-centered cubic (FCC) structure, with the main peak located at the (111) crystal plane, accompanied by (200) and (220) secondary diffraction peaks. In the samples treated at low temperature (550°C), the (111) peak is significantly broadened, indicating that the grain size is small and there are many crystal defects. At the same time, the relatively broad diffraction peak may imply a large internal micro-stress in the lattice. As the heat treatment temperature increases to 950°C, the diffraction peaks gradually become sharper, the intensity increases significantly, and there is a slight left shift in the peak position, indicating an increase in the lattice constant, which may be caused by thermal expansion and improvement of crystal quality. The clarification of the secondary diffraction peaks further verifies that the high-temperature treatment improves the orderliness of the crystal.

[0084] The treatment of the salt bath at 150°C plays a certain role in removing the crystal surface oxide layer and releasing stress of the sample. The phase composition does not show the appearance of obvious new phases, indicating the stability of the single FCC phase structure is maintained within the used heat treatment temperature range. Comprehensive analysis shows that the heat treatment temperature has a significant impact on grain size, phase structure stability, and internal stress.

[0085] According to Example 6, Figure 11XRD patterns were analyzed to study the crystal structure changes of Fe-Mn-Ni alloys after annealing at 950 °C and subsequent treatment at different salt bath temperatures (200 °C, 250 °C, 300 °C) and air cooling. The results showed that all samples retained the face-centered cubic (FCC) phase structure. The (111) peak of the air-cooled sample was broader and had lower intensity, indicating smaller grain size and larger microstress. With the increase of salt bath temperature, the (111) peak became sharper and its intensity increased, the grain size increased, the internal stress decreased, and the crystal quality improved. The secondary diffraction peaks (200) and (220) were clearer in the salt bath-cooled samples, demonstrating that the salt bath temperature contributed to improving the crystal order. Overall, salt bath cooling was more effective than air cooling in reducing crystal defects.

[0086] The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A preparation method of an ultra-high strength and high toughness Fe-Mn-Ni series alloy, characterized in that: It includes the following steps: Step 1: Weigh the ingredients according to the following chemical composition and mass ratio. The mass percentages of the metal powders of Fe, Mn, Ni, and C elements are as follows: Fe 91.2%, Mn 2.8%, Ni 5.7%, C 0.3%. Mix them using the ultrasonic mixing method, separate the medium using the natural sedimentation method, and dry the powder in an oven. Step 2: Place the dried powder in a gas compressor for hot isostatic pressing treatment. Carry out melting in a helium gas environment. The hot isostatic pressing is carried out in the temperature range of 1000°C - 1100°C. Apply pressure and hold the pressure. During the pressurization process, the temperature and pressure need to be evenly transmitted to ensure the densification effect. After completion, slowly cool down to obtain an Fe-Mn-Ni series alloy ingot. Step 3: Carry out solution treatment on the ingot. Step 4: Carry out hot extrusion on the solution-treated ingot to obtain an alloy ingot. Step 5: Keep the hot-extruded alloy ingot in a muffle furnace at 750°C for 10 minutes and then air-cool to room temperature. Keep it in a salt bath furnace at 150°C for 30 minutes and then air-cool to room temperature to obtain an ultra-high strength and high toughness Fe-Mn-Ni series alloy.

2. The preparation method of a super high-strength and high-toughness Fe-Mn-Ni alloy according to claim 1, characterized in that: In Step 1, the purities of the Fe, Mn, Ni, and C powders are all higher than 99.95%.

3. The preparation method of a super high-strength and high-toughness Fe-Mn-Ni series alloy according to claim 2, characterized in that: In Step 1, the average particle diameter of the Fe, Mn, Ni, and C metal powders is controlled between 50 - 100 through a screening process.

4. The preparation method of a super high-strength and high-toughness Fe-Mn-Ni alloy according to claim 1, characterized in that: In Step 1, when using the ultrasonic mixing method, the ratio of the powder mass to the deionized water liquid is 10:

1. The set power of the ultrasonic processor is 350W, the frequency is 40kHz, and it is processed in a pulse mode for 30 minutes. The liquid temperature should be controlled at 50°C. After completion of mixing, use the natural sedimentation method. Let the powder stand for one day, collect the deposited powder at the bottom, suck out the supernatant liquid, and finally dry it in an oven at 120°C for 2 hours.

5. The preparation method of a super high-strength and high-toughness Fe-Mn-Ni alloy according to claim 3, characterized in that: In Step 2, the heating rate of the hot isostatic pressing is 10°C / s, the applied pressure value is 120MPa, the pressurization time is 30 minutes, the pressure holding time is 2 hours, and the slow cooling rate is 10°C / min.

6. The preparation method of a super high-strength and high-toughness Fe-Mn-Ni alloy according to claim 5, characterized in that: For the Fe-Mn-Ni series alloy ingot obtained in Step 2, its grain size and tissue uniformity are detected by optical microscopy and XRD analysis to ensure meeting the requirements of subsequent processing.

7. The preparation method of a super high-strength and high-toughness Fe-Mn-Ni alloy according to claim 1, characterized in that: In Step 3, the ingot is kept at 1200°C for 90 minutes to complete the solution treatment.

8. The preparation method of a super high-strength and high-toughness Fe-Mn-Ni alloy according to claim 1, characterized in that: In Step 4, the solution-treated alloy ingot is hot-extruded in the temperature range of 900°C to 1000°C. The pressure applied during extrusion is 1000Mpa, and the extrusion speed is controlled at 5 mm / s.

9. The preparation method of a super high-strength and high-toughness Fe-Mn-Ni alloy according to claim 8, characterized in that: In Step 4, after completion of extrusion, it is cooled at a rate of 10°C / min.

10. A super-high strength and high toughness Fe-Mn-Ni alloy system, characterized in that: It is obtained by the preparation method of the ultra-high strength and high toughness Fe-Mn-Ni series alloy described in any one of claims 1 to 9.

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