NiAl nano precipitation-austenite phase double gradient coordinated regulation medium manganese steel and preparation method thereof
By adding Ni and Al elements to the medium manganese steel and forming a NiAl nano-precipitation-austeinite phase gradient distribution through cyclic torsion and low-temperature aging processes, the problem of taking into account both the yield strength and plastic toughness of the medium manganese steel is solved, and high-strength and high-plastic medium manganese steel are prepared.
Patent Information
- Application Number
- CN202510478394.X
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-04-16
- Publication Date
- 2025-07-11
AI Technical Summary
The existing medium-manganese steel has low yield strength, which is difficult to meet the high strength requirements. At the same time, the addition of NiAl precipitates affects the plastic toughness, and cannot take into account both high yield strength and high plastic toughness.
Ni and Al elements are added through microalloyation technology, and combined with cyclic torsion and low-temperature aging processes, the gradient distribution of NiAl nanoprecipitates and austenite phases is formed in the matrix, and hot rolling, two-phase annealing, cyclic torsion and low-temperature aging are adopted.
The yield strength of medium manganese steel exceeds 830MPa, the uniform elongation is greater than 25%, and the tensile toughness is higher than 260MJ/mm3, which has both high strength and excellent plastic toughness.
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Figure CN120290840A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to the technical field of advanced high-strength steel plate production, and particularly relates to a medium manganese steel with dual-gradient synergistic regulation of NiAl nano-precipitation-austenite phase and a preparation method thereof. Background Art
[0002] As a typical steel grade of advanced high-strength steel, medium manganese steel has been successfully applied in fields such as automotive steel plates, rail transit, offshore platforms, and mining machinery. Medium manganese steel added with aluminum has attracted much attention due to its weight reduction potential and excellent energy absorption capacity in various structural applications, meeting the requirements of energy conservation, emission reduction, green manufacturing, and equipment lightweight of future advanced high-strength steel. However, the yield strength of medium manganese steel added with aluminum is relatively low, generally between 500-700 MPa, and it is difficult to meet the requirements for high-quality steel in fields such as automotive steel plates, rail transit, offshore platforms, and mining machinery. To solve this problem, researchers have adopted various strategies to improve the yield strength of medium manganese steel, including martensite strengthening, precipitation strengthening, dislocation strengthening, etc. Among them, precipitation strengthening is achieved by adding nickel and aluminum elements to precipitate NiAl particles during aging, and the effect is particularly obvious, which can make its yield strength reach the 900 MPa level. Moreover, medium manganese steel added with nickel can significantly improve its corrosion resistance in acidic media and has important engineering application potential. However, a large amount of NiAl precipitation in the matrix is not beneficial to the plasticity and toughness of medium manganese steel. Therefore, developing lightweight medium manganese steel with excellent ductility and high yield strength has become a major challenge in this field. Summary of the Invention
[0003] (1) Technical Problems to be Solved
[0004] In view of the above-mentioned disadvantages and deficiencies of the prior art, the present invention provides a medium manganese steel with dual-gradient synergistic regulation of NiAl nano-precipitation-austenite phase and a preparation method thereof. Through microalloying technology and in combination with cyclic torsion and low-temperature aging processes, the medium manganese steel obtains a new microstructure feature in which the austenite content, grain size, and NiAl nano-precipitate content are gradient-distributed in the matrix, so that the medium manganese steel obtains high strength and excellent plasticity and toughness, and the yield strength of the medium manganese steel is above 830 MPa, the uniform elongation is greater than 25%, and the tensile toughness is higher than 260 MJ / mm 3 , thereby solving the technical problem that medium manganese steel cannot take into account both high yield strength and high plasticity and toughness caused by single precipitation strengthening or increasing martensite and other strong phase structures at present.
[0005] (2) Technical Solutions
[0006] To achieve the above object, the main technical solutions adopted by the present invention include:
[0007] In a first aspect, the present invention provides a method for preparing a medium manganese steel with dual-gradient synergistic regulation of NiAl nano-precipitation and austenite phase, comprising the following steps:
[0008] (a) Raw material smelting and hot rolling: Alloy melting is carried out according to a predetermined composition. After casting and forming, hot rolling is performed to obtain a hot-rolled slab.
[0009] (b) Processing of rod-shaped specimens: The hot-rolled slab is machined into cylindrical rod-shaped specimens.
[0010] (c) Critical annealing in the two-phase region: The rod-shaped specimens are placed in the two-phase region temperature range for annealing treatment, and then air-cooled to room temperature.
[0011] (d) Cyclic torsion treatment: Multi-directional torsion is performed on the annealed rod-shaped specimens at room temperature. The specific parameters are as follows: At room temperature, twist at a rate of 50 - 70° / min for 20° ± 0.5° → hold for 3 - 6 seconds → reverse twist 20° ± 0.5° to reset; Number of cycles: 20 - 100 times.
[0012] (e) Low-temperature aging treatment.
[0013] According to a preferred embodiment of the present invention, in step (b), the diameter of the cylindrical rod-shaped specimen is 2 - 8 mm.
[0014] According to a preferred embodiment of the present invention, in step (c), the two-phase region temperature range is the critical Ac1 - Ac3 temperature, the annealing temperature is 640 - 720 °C, and after holding for 0.5 - 4 h, it is air-cooled to room temperature.
[0015] According to a preferred embodiment of the present invention, in step (d), the cumulative torsional strain ε of the cyclic torsion treatment satisfies:
[0016]
[0017] Where: N is the number of cycles, 20 ≤ N ≤ 100;
[0018] θ is the single torsion angle, θ = (20° ± 0.5°) × π / 180;
[0019] r is the specimen radius, r = 1 - 4 mm;
[0020] L is the specimen gauge length, L ≥ 25 mm.
[0021] According to a preferred embodiment of the present invention, in step (c), the temperature gradient control of the two-phase region annealing is: During annealing, the temperature is raised to the target temperature at a rate of ≤ 5 °C / min, and the furnace temperature fluctuation is maintained ≤ ± 3 °C during the holding stage.
[0022] According to a preferred embodiment of the present invention, the low-temperature aging treatment in step (e) is carried out in an inert gas atmosphere (argon or nitrogen), the gas purity is ≥99.99%, and the air pressure is maintained at 0.1-0.3 MPa.
[0023] According to a preferred embodiment of the present invention, in step (e), the low-temperature aging conditions are as follows: after the specimen is treated at 500-580 °C for 1-3 h, it is air-cooled to room temperature.
[0024] In the solution of the present invention, the torsion angle is 20°±0.5°, and the number of cyclic torsions is 20-100 times. If the number of torsions is too small, it may lead to insufficient shear stress or strain in the steel bar matrix, insufficient phase transformation martensite and dislocation density in the surface area, and an effective NiAl and austenite phase gradient structure cannot be formed during the subsequent low-temperature aging process; on the contrary, if the number of torsions is too large, the shear strain is too large, and a large number of defects will be generated in the surface area, eventually resulting in crack initiation and a significant decrease in material properties. The torsion angle of 20°±0.5° can effectively activate the slip system and avoid the initiation of microcracks, achieving the best balance in the synergistic optimization of strength and toughness and defect control. When the single angle is 20°±0.5°, 20-100 cycles ensure the formation of the gradient structure. Torsion at a rate of 50-70° / min (preferably 60° / min) can ensure that the dislocation multiplication rate and the dynamic recovery rate reach equilibrium, avoiding crack initiation caused by dislocation pile-up.
[0025] Among them, processes such as raw material smelting, hot rolling, and processing of bar specimens can all be carried out with reference to traditional / conventional medium manganese steel preparation processes, or the steps of the preparation method are as follows:
[0026] (a) Raw material smelting and hot rolling: Prepare smelting raw materials according to the alloy composition of medium manganese steel, vacuum melt to make an ingot, heat the ingot to 1150 °C - 1250 °C and hold for 1-3 h, then hot roll it into a slab with a thickness of 4-15 mm through multiple passes. The final rolling temperature is not lower than 920 °C to obtain a hot-rolled slab.
[0027] (b) Processing of bar specimens: Machine the hot-rolled slab into a cylindrical bar specimen with a diameter of 2-8 mm.
[0028] (c) Critical annealing in the two-phase region: Place the bar specimen in an annealing furnace at 640-720 °C for 0.5-4 h and then air-cool it to room temperature to obtain an annealed bar specimen.
[0029] (d) Cyclic torsion treatment: Perform multi-directional torsion on the annealed bar specimen at room temperature. The specific parameters are as follows: at room temperature, torsion at a rate of 60° / min by 20°±0.5° → hold for 3-6 seconds → reverse torsion by 20°±0.5° to reset; number of cycles: 20-100 times.
[0030] (e) Low-temperature aging treatment: After aging at 500 - 580 °C for 1 - 3 h, air-cool to room temperature.
[0031] According to a preferred embodiment of the present invention, in step (a), the medium manganese steel alloy composition by mass percentage is: C: 0.1 - 0.25%, Mn: 3 - 12%, Al: 2.5 - 5%, Ni: 2.5 - 5%, Ce: 0.04 - 0.15%, and the balance is Fe.
[0032] In the medium manganese steel alloy, adding a small amount of C can ensure an increase in the austenite content in the steel and an improvement in austenite stability, and can also increase the strength of the medium manganese steel; while adding 2.5 - 5% of Al and Ni is a necessary requirement, and preferably the molar ratio of Al and Ni is close to 1:1. The increase in Al can, on the one hand, achieve a certain degree of lightweighting of the medium manganese steel, and cooperate with the Ni element to generate NiAl nano-precipitation, thereby achieving a gradient distribution of precipitates and reaching precipitation strengthening and gradient effects; Ni can increase the content ratio of austenite. In addition, adding Al and Ni can regulate the stacking fault energy of the medium manganese steel and improve the stability of austenite. Adding 0.04 - 0.15% rare earth Ce is mainly used to purify the matrix, refine the grain size of the medium manganese steel, and improve the stability of austenite. Appropriate austenite stability is beneficial to improving the gradient rate of martensite phase from the surface to the inside of the round bar during cyclic torsion. In this case, during the subsequent aging treatment, the difference in the precipitation driving force of NiAl from the surface to the inside of the round bar is significant, resulting in an obvious gradient distribution characteristic of the austenite phase and NiAl nano-precipitation particles. In other words, by regulating the stability of austenite, the different precipitation behaviors of NiAl at different depths can be promoted during the subsequent aging process, thereby realizing the gradient change of the internal structure of the material. This gradient change is of great significance for optimizing the material properties.
[0033] It should be noted that in the alloy composition of the medium manganese steel of the present invention, except for the content requirements of Ni and Al elements, there are no special limitations on other elements, as long as the austenite content in the annealed medium manganese steel matrix > 45% and the austenite has appropriate stability, and high-strength and high-toughness multi-stage gradient structure medium manganese steel can be obtained through the processes of hot rolling + critical annealing in the two-phase region + cyclic torsion + low-temperature aging. And the medium manganese steel alloy composition recorded in the above scheme is the alloy composition that can meet the requirements of "the austenite content in the steel billet matrix after annealing > 45% and the austenite has appropriate stability".
[0034] In step (a) of preparing medium manganese steel, the material needs to be thermally homogenized before hot rolling to ensure that the steel billet does not crack during subsequent hot rolling. The homogenization treatment conditions are set in the range of 1150°C to 1250°C. If the temperature is too high, it will cause the original austenite grain size to be too large, thereby reducing the material strength; conversely, if the temperature is too low, it is not sufficient to achieve the ideal compositional homogenization. In the two-phase region annealing step, the selected critical temperature range needs to be determined according to the specific medium manganese steel alloy composition. The holding time should be between 0.5 and 4 hours. If the holding time is too long, it may lead to too high austenite content and insufficient stability, making it difficult to form an effective multi-level gradient structure after subsequent cyclic torsion and low-temperature aging treatment; while if the holding time is too short, it may result in insufficient austenite content and too high stability, which will affect the formation of austenite gradient after torsion and cannot effectively trigger the TRIP effect during the deformation process, thereby affecting the plastic performance of the material.
[0035] In the above step (e), the low-temperature aging treatment conditions are aging at 500 - 580°C for 1 - 3 h. The aging temperature cannot be too low or too high. After cyclic torsion, a large amount of transformation martensite and a large number of dislocations will be generated on the surface of the medium manganese steel. During the subsequent low-temperature aging process, the above transformation martensite and dislocations become the nuclei of NiAl precipitates, promoting the precipitation of a large amount of NiAl. If the temperature is too low, the formation of the gradient structure cannot be achieved, and if the temperature is too high, the transformation martensite will reverse to austenite, and an effective NiAl gradient precipitation cannot be formed either. Similarly, the aging time cannot be too short or too long. If the time is too short, NiAl cannot precipitate. If the time is too long, the size of NiAl will increase significantly, which will have an adverse impact on the plasticity and toughness of the material.
[0036] In the second aspect, the present invention provides a medium manganese steel with dual-gradient synergistic regulation of NiAl nano-precipitation - austenite phase, which is prepared by adopting the scheme of any one of the above embodiments.
[0037] Preferably, for the medium manganese steel with dual-gradient synergistic regulation of NiAl nano-precipitation - austenite phase, the yield strength > 830 MPa, the tensile strength > 1 GPa, the uniform elongation > 25%, and the tensile toughness > 260 MJ / mm 3 。
[0038] (III) Beneficial effects
[0039] The preparation method of a medium manganese steel with dual-gradient synergistic regulation of NiAl nano-precipitation - austenite phase of the present invention has the following technical advantages:
[0040] (1) With gradient distribution characteristics: The prepared medium manganese steel exhibits a gradient distribution of austenite proportion, grain size, and NiAl nano-precipitate content from the surface to the interior. During the deformation process, this gradient structure can sequentially activate various strengthening mechanisms along the depth direction, including precipitation strengthening, dislocation strengthening, heterogeneous deformation-induced (HDI) strengthening, transformation-induced plasticity (TRIP) effect, and twinning-induced plasticity (TWIP) effect. These mechanisms work together to significantly improve the yield strength, tensile strength, uniform elongation, and tensile toughness of the material.
[0041] (2) The preparation method of the present invention is simple: This method uses conventional equipment and realizes the preparation of the above-mentioned microstructural characteristics through processes such as hot rolling, annealing in the two-phase region, followed by cyclic torsion treatment and low-temperature aging. This method is simple and easy to implement and is applicable to various fields of steel and alloy materials such as medium manganese steel, high-entropy alloy, stainless steel, and copper-magnesium alloy, providing a new idea for the preparation of materials with a multi-level gradient structure.
[0042] (3) The prepared medium manganese steel has excellent mechanical properties: The medium manganese steel prepared by the present invention exhibits excellent mechanical properties, specifically, the yield strength exceeds 830 MPa, the tensile strength reaches the GPa level, the uniform elongation is greater than 25%, and the tensile toughness is higher than 260 MJ / m 3 . These performance indicators indicate that while maintaining high strength, the material also has excellent ductility and toughness, meeting the requirements for the comprehensive performance of materials in high-end application fields.
[0043] In summary, this invention not only improves the mechanical properties of medium manganese steel but also provides a preparation technology with strong versatility and simple operation, which is of great significance for promoting the development of steel and alloy materials. Description of the Drawings
[0044] Figure 1 It is the EBSD phase diagram of the center and surface parts of the medium manganese steel co-regulated by NiAl nano-precipitation-austenite phase double gradient in Example 1 of the present invention.
[0045] Figure 2 It is the hardness distribution curve of the medium manganese steel co-regulated by NiAl nano-precipitation-austenite phase double gradient from the center to the surface in Example 1 of the present invention.
[0046] Figure 3 It is the engineering stress-strain curve of the medium manganese steel co-regulated by NiAl nano-precipitation-austenite phase double gradient in Example 1 of the present invention and the homogeneous structure medium manganese steel prepared in Comparative Example 1.
[0047] Figure 4This is the engineering stress-strain curve of the medium manganese steel with NiAl nano-precipitation-austenite phase double-gradient synergistic regulation in Example 2 of the present invention and the homogeneous structure medium manganese steel prepared in Comparative Example 1.
[0048] Figure 5 This is the engineering stress-strain curve of the medium manganese steel with NiAl nano-precipitation-austenite phase double-gradient synergistic regulation in Example 3 of the present invention and the homogeneous structure medium manganese steel prepared in Comparative Example 2. Detailed implementation manners
[0049] To better explain the present invention for easy understanding, the present invention will be described in detail below with reference to the accompanying drawings through specific implementation manners.
[0050] The design concept of the present invention is to achieve a medium manganese steel with gradient microstructure characteristics through the following measures: First, add Ni and Al elements to the medium manganese steel, and perform mechanical forming and annealing treatments to prepare for subsequent processing and phase transformation. Then, use cyclic torsion to twist the steel bar. Since there is a gradient shear strain that gradually decreases from the surface to the interior during the torsion process, this promotes a gradient distribution of austenite phase transformation from the surface to the interior of the steel bar. Specifically, this gradient shear strain results in martensite phase transformation from the surface to the interior and a gradient decrease in defect density (such as dislocation density). After cyclic torsion, the material is further subjected to low-temperature aging treatment. At this stage, in the martensite phase transformation region containing a high Ni element, especially the surface layer, due to the high density of dislocations, it promotes the nucleation and growth of NiAl nano-precipitates during the low-temperature aging process. Finally, a gradient distribution characteristic of austenite ratio, grain size, and NiAl nano-precipitate content is formed from the surface to the interior of the medium manganese steel. Such microstructure characteristics not only optimize the strength and toughness of the material but also enhance its overall mechanical properties. The multi-stage gradient structure medium manganese steel prepared by the present invention with NiAl nano-precipitation gradient and austenite phase gradient distribution can stimulate the synergistic action of various strengthening and toughening mechanisms, significantly improving its strength and plastic toughness synchronously.
[0051] The following is a detailed description in combination with specific embodiments of the present invention.
[0052] Example 1
[0053] This example provides a multi-stage gradient structure medium manganese steel with NiAl nano-precipitation gradient and austenite phase gradient distribution. The chemical composition by weight percentage is: C: 0.18%, Mn: 7.78%, Al: 2.84%, Ni: 2.81%, Ce: 0.04%, P < 0.008%, S < 0.008%, and the balance is Fe. The preparation method of the multi-stage gradient structure medium manganese steel is as follows:
[0054] (1) Smelting and hot rolling: The steel ingot is vacuum melted according to the alloy composition. After the steel ingot is kept at 1200 °C for 2 h, it is rolled into a slab with a thickness of 10 mm through 5 passes, and the final rolling temperature is not lower than 920 °C.
[0055] (2) Two-phase region critical annealing: The hot-rolled steel plate is processed into a rod-shaped specimen, annealed at 680 °C for 1 h, and then air-cooled to room temperature;
[0056] (3) Room temperature torsion + low temperature aging treatment: The annealed steel rod is twisted clockwise at a rate of 60 ° / min by 20 °, and then twisted counterclockwise by 20 ° to return to the original position, which is regarded as one cycle. The number of cycles of torsion is 60 times. Then it is kept at 500 °C for 60 min and then air-cooled to room temperature.
[0057] The microstructure (EBSD phase diagram) of medium manganese steel at different positions from the center to the surface in the multi-level gradient structure prepared in this example is as Figure 1 shown in a and d of Figure 1 . The black is austenite and the gray is ferrite. The austenite content and the average grain size at the center position ( Figure 1 d of
[0058] ) are 46% and 0.93 μm respectively, and the austenite content and the average grain size at the surface position ( Figure 1 a of
[0058] ) are 68% and 1.25 μm respectively. Figure 1 Shown in b, c, e and f of Figure 1 are the SEM morphologies of the surface area and the center area respectively. It can be seen that the volume fraction of NiAl nano-precipitates in the surface area ( Figure 1 b, c) is much higher than that in the center area ( Figure 1 e, f). After statistics, the contents of NiAl nanoparticles in the surface and center areas are 9.1% and 1.8% respectively.
[0059] As shown in Figure 2 , the hardness of medium manganese steel in the gradient structure gradually increases from the center to the surface; the abscissa is the depth Depth (mm) of medium manganese steel from the center to the surface, and the ordinate is the hardness (Hv). It can be seen from the figure that the hardness of medium manganese steel is low at the center and increases continuously from the center to the surrounding.
[0060] Comparative Example 1
[0061] The alloy composition of medium manganese steel in this comparative example is the same as that in Example 1, but the preparation method is different. Mainly on the basis of Example 1, cyclic torsion and low temperature aging treatment are not carried out, and the remaining steps including "raw material smelting and hot rolling, two-phase region critical annealing", etc. are the same as those in Example 1. Finally, a homogeneous structure of medium manganese steel is prepared.
[0062] See Figure 3As shown, the engineering stress-strain curves of the medium manganese steel with dual-gradient synergistic regulation of NiAl nano-precipitation-austenite phase in Example 1 and the homogeneous structure medium manganese steel in Comparative Example 1 are presented. The tensile strength of the medium manganese steel prepared in Example 1 is 1068 MPa, the yield strength is 924 MPa, the uniform elongation is 28.8%, the total elongation is 40.6%, and the tensile toughness is as high as 287 MJ / mm 3 . The tensile strength of the homogeneous structure medium manganese steel prepared in Comparative Example 1 is 883 MPa, the yield strength is 667 MPa, the uniform elongation is 24%, the total elongation is 39%, and the tensile toughness is 189 MJ / mm 3 , and its various mechanical property indexes are significantly lower than those of the multi-gradient structure medium manganese steel prepared in Example 1.
[0063] It can be seen from the comparison that, compared with the homogeneous structure medium manganese steel in Comparative Example 1, the medium manganese steel with dual-gradient synergistic regulation of NiAl nano-precipitation-austenite phase in Example 1 has a 38.5% and 21% increase in yield strength and tensile strength respectively, and a 20% and 52% increase in uniform elongation and tensile toughness respectively.
[0064] Example 2
[0065] This example provides a multi-gradient structure medium manganese steel with NiAl nano-precipitation gradient and austenite phase gradient distribution, and its chemical composition is the same as that in Example 1. The preparation method of the multi-gradient structure medium manganese steel is as follows:
[0066] (1) Smelting and hot rolling: A steel ingot is vacuum melted according to the alloy composition, and after being held at 1200 °C for 2 h, it is rolled into an 8-mm-thick slab in 5 passes, and the final rolling temperature is not lower than 920 °C;
[0067] (2) Critical annealing in the two-phase region: The hot-rolled steel plate is processed into a rod-shaped specimen, and then annealed at 700 °C for 0.5 h and air-cooled to room temperature;
[0068] (3) Room temperature cyclic torsion + low-temperature aging: The annealed steel rod is twisted clockwise at a rate of 60° / min by 20.5°, then twisted counterclockwise by 20.5° to return to the original position, which is regarded as one cycle. The number of cyclic torsion is 80 times, and then it is held at 550 °C for 60 min and air-cooled to room temperature.
[0069] See Figure 4 As shown, the engineering stress-strain curves of the medium manganese steel with dual-gradient synergistic regulation of NiAl nano-precipitation-austenite phase in Example 2 and the homogeneous structure medium manganese steel in Comparative Example 1 are presented. For the multi-gradient structure medium manganese steel prepared in this example, its tensile strength is 1018 MPa, the yield strength is 869 MPa, the uniform elongation is 29.1%, the total elongation is 39%, and the tensile toughness is 275 MJ / mm 3 .
[0070] Comparing, it can be seen that, compared with the medium manganese steel with a homogeneous structure in Comparative Example 1, the medium manganese steel with a dual-gradient synergistic regulation of NiAl nano-precipitation-austenite phase in Example 2 has the yield strength and tensile strength increased by 30.3% and 15.3% respectively, and the uniform elongation and tensile toughness increased by 21.25% and 45.5% respectively.
[0071] Example 3
[0072] This example provides a medium manganese steel with a multi-stage gradient structure having a NiAl nano-precipitation gradient and an austenite phase gradient distribution. The chemical composition by weight percentage is: C: 0.16%, Mn: 8.02%, Al: 3.3%, Ni: 3.05%, Ce: 0.05%, P < 0.008%, S < 0.008%, and the balance is Fe. The preparation method of the multi-stage gradient structure medium manganese steel is as follows:
[0073] (1) Smelting and hot rolling: A steel ingot is vacuum melted according to the alloy composition. After the steel ingot is held at 1220 °C for 2.5 h, it is rolled into a slab with a thickness of 10 mm through 5 passes, and the final rolling temperature is not lower than 920 °C;
[0074] (2) Critical annealing in the two-phase region: The hot-rolled slab is processed into a rod-shaped specimen, annealed at 690 °C for 1 h, and then air-cooled to room temperature;
[0075] (3) Room temperature cyclic torsion + low-temperature aging: The annealed steel rod is twisted clockwise at a rate of 60 ° / min by 19.5 °, and then twisted counterclockwise by 19.5 ° to return to the original position, which is regarded as one cycle. The number of cyclic torsion is 60 times, and then it is held at 520 °C for 75 min and then air-cooled to room temperature.
[0076] The medium manganese steel prepared in this example with a dual-gradient synergistic regulation of NiAl nano-precipitation-austenite phase has a tensile strength of 1121 MPa, a yield strength of 855 MPa, a uniform elongation of 33.2%, a total elongation of 40.8%, and a tensile toughness of 328 MJ / mm 3 .
[0077] Comparative Example 2
[0078] The medium manganese steel alloy composition in this comparative example is the same as that in Example 3, but the preparation method is different. Mainly, on the basis of Example 3, the cyclic torsion and low-temperature aging treatments are not carried out, and the remaining steps including "raw material smelting and hot rolling, critical annealing in the two-phase region", etc. are the same as those in Example 3. Finally, a medium manganese steel with a homogeneous structure is prepared.
[0079] See Figure 5As shown, the engineering stress-strain curves of the medium manganese steel with dual-gradient synergistic regulation of NiAl nano-precipitation-austenite phase in Example 3 and the homogeneous structure medium manganese steel in Comparative Example 2 are presented. For the homogeneous structure medium manganese steel in this comparative example, its tensile strength is 996 MPa, yield strength is 637 MPa, uniform elongation is 28.6%, total elongation is 32%, and tensile toughness is 234 MJ / mm 3 , and its various mechanical property indexes are significantly lower than those of the novel multi-stage gradient structure medium manganese steel in Example 3.
[0080] Through comparison, it can be seen that compared with the homogeneous structure medium manganese steel in Comparative Example 2, the yield strength and tensile strength of the multi-stage gradient structure medium manganese steel in Example 3 are increased by 34.22% and 12.5% respectively, and the uniform elongation and tensile toughness are increased by 16.1% and 40.2% respectively.
[0081] Thus, it can be seen that the medium manganese steel with dual-gradient synergistic regulation of NiAl nano-precipitation-austenite phase in the present invention, through microalloying technology and combined with cyclic torsion and low-temperature aging processes, enables the traditional medium manganese steel to obtain a novel microstructure feature with a gradient distribution of austenite content, grain size, and NiAl nano-precipitation content in the matrix, thereby enabling the medium manganese steel to obtain high strength and excellent plastic toughness, with the yield strength of the medium manganese steel above 830 MPa, uniform elongation greater than 25%, and tensile toughness higher than 260 MJ / mm 3 , thereby solving the technical problem that the medium manganese steel cannot take into account both high yield strength and high plastic toughness caused by single precipitation strengthening or increasing strong phase structures such as martensite at present.
[0082] The present invention prepares a medium manganese steel with excellent mechanical properties through a series of precisely controlled process steps. The specific methods and design concepts are as follows:
[0083] Hot rolling and annealing treatment: First, the steel billet is hot rolled, and then annealing treatment is carried out to generate a two-phase tissue structure of austenite and ferrite, laying a foundation for subsequent processing.
[0084] Cyclic torsional deformation: Then, the cyclic torsion process is used to make the round bar undergo torsional deformation. Since there is a significant gradient shear strain from the surface to the inside during the torsion process, this results in a gradient distribution characteristic of the martensite phase and dislocation density gradually decreasing from the surface to the inside.
[0085] Aging treatment: During the subsequent low-temperature aging treatment, due to the differences in martensite content and dislocation density at different depths, the formation kinetics of NiAl precipitates are also different. This process ultimately contributes to the gradient distribution of austenite ratio, grain size, and NiAl nano-precipitation content from the surface layer to the inside.
[0086] Multiple strengthening mechanisms act synergistically: During the deformation process, different gradient layers activate multiple strengthening and toughening mechanisms, including precipitation strengthening, dislocation strengthening, heterogeneous deformation induced (HDI) strengthening, and transformation-induced plasticity (TRIP) effect. These mechanisms work together to greatly improve the overall performance of the material.
[0087] Performance improvement: Compared with medium manganese steel of the same composition prepared by traditional methods, the medium manganese steel in the present invention not only has a significant increase in yield strength and tensile strength, but also the uniform elongation and tensile toughness are synchronously and greatly improved.
[0088] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements, or when the technical features in the above embodiments do not conflict with each other, can be combined in the manner recorded in the embodiments, and these modifications, replacements or combinations do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.
Claims
1. A preparation method of medium manganese steel with double-gradient synergistic regulation of NiAl nano-precipitation and austenite phase, characterized in that, It includes the following steps: (a) Raw material smelting and hot rolling: Alloy smelting is carried out according to a predetermined composition. After casting and forming, hot rolling is performed to obtain a hot-rolled slab. (b) Processing of rod-shaped specimens: The hot-rolled slab is machined into cylindrical rod-shaped specimens. (c) Two-phase region critical annealing: The rod-shaped specimens are annealed in the two-phase region temperature range and then air-cooled to room temperature. (d) Cyclic torsion treatment: Multi-directional torsion is performed on the annealed rod-shaped specimens at room temperature. The specific parameters are as follows: At room temperature, twist at a rate of 50 - 70° / min for 20° ± 0.5° → hold for 3 - 6 seconds → reverse twist 20° ± 0.5° to reset; Number of cycles: 20 - 100 times. (e) Low-temperature aging treatment.
2. The preparation method according to claim 1, characterized in that, In step (b), the diameter of the cylindrical rod-shaped specimen is 2 - 8 mm.
3. The preparation method according to claim 1, characterized in that, In step (c), the two-phase region temperature range is the critical Ac1 - Ac3 temperature. The annealing temperature is 640 - 720 °C. After holding for 0.5 - 4 h, it is air-cooled to room temperature.
4. The preparation method according to claim 1, characterized in that, In step (d), the cumulative torsional strain ε of the cyclic torsion treatment satisfies: Where: N is the number of cycles, 20 ≤ N ≤ 100; θ is the single torsion angle, θ = (20° ± 0.5°) × π / 180; r is the specimen radius, r = 1 - 4 mm; L is the specimen gauge length, L ≥ 25 mm.
5. The preparation method according to claim 1, characterized in that, In step (c), the temperature gradient control of the two-phase region annealing is: During annealing, it is heated to the target temperature at a rate of ≤ 5 °C / min, and the furnace temperature fluctuation is maintained ≤ ± 3 °C during the holding stage.
6. The preparation method according to claim 1, characterized in that, The low-temperature aging treatment in step (e) is carried out in an inert gas atmosphere with a gas purity ≥ 99.99%, and the gas pressure is maintained at 0.1 - 0.3 MPa.
7. The preparation method according to claim 1, characterized in that In step (e), the low-temperature aging conditions are: The specimen is treated at 500 - 580 °C for 1 - 3 h and then air-cooled to room temperature.
8. The preparation method according to claim 1, wherein The steps of the preparation method are as follows: (a) Raw material smelting and hot rolling: According to the medium manganese steel alloy composition, smelting raw materials are prepared, vacuum smelted into ingots, the ingots are heated to 1150 °C - 1250 °C and held for 1 - 3 h, and then hot-rolled into a slab with a thickness of 4 - 15 mm through multiple passes. The final rolling temperature is not lower than 920 °C to obtain a hot-rolled slab. (b) Processing of rod-shaped specimens: The hot-rolled slab is machined into cylindrical rod-shaped specimens with a diameter of 2 - 8 mm. (c) Two-phase region critical annealing: The rod-shaped specimens are annealed at 640 - 720 °C for 0.5 - 4 h and then air-cooled to room temperature to obtain the annealed rod-shaped specimens. (d) Cyclic torsion treatment: Multi-directional torsion is performed on the annealed rod-shaped specimens at room temperature. The specific parameters are as follows: At room temperature, twist at a rate of 60 ° / min for 20° ± 0.5° → hold for 3 - 6 seconds → reverse twist 20° ± 0.5° to reset; Number of cycles: 20 - 100 times. (e) Low-temperature aging treatment: Aging at 500 - 580 °C for 1 - 3 h and then air-cooled to room temperature.
9. The preparation method according to any one of claims 1-8, characterized in that, In step (a), the medium manganese steel alloy composition by mass percentage is: C: 0.1 - 0.25%, Mn: 3 - 12%, Al: 2.5 - 5%, Ni: 2.5 - 5%, Ce: 0.04 - 0.15%, and the balance is Fe.
10. A medium manganese steel with dual-gradient synergistic regulation of NiAl nano-precipitation and austenite phase, characterized in that, Prepared by the preparation method according to any one of claims 1-9.