A high-strength and high-plasticity product medium manganese steel and its preparation method

Abstract

The present invention provides a high-strength and high-ductility medium manganese steel and a preparation method thereof, belonging to the field of metal heat treatment. Its chemical composition by weight percentage includes: C: 0.09 - 0.11%, Mn: 3.6 - 4.2%, Al: 2 - 2.5%, Ni: 3.5 - 4.3%, S ≤ 0.006%, P ≤ 0.015%, and the balance is Fe and inevitable impurities. During the heat treatment, a stress of 20 - 70 MPa is applied to the medium manganese steel plate throughout the process, and isothermal treatment is added during the critical annealing process. Through the design of composition and process, a multiphase composite structure of acicular ferrite + martensite + austenite components is obtained. Through the good TRIP effect, the matching of high strength and high toughness can be achieved without increasing the industrial production time.

Description

Technical Field

[0001] The present invention belongs to the field of metallurgical technology, and particularly relates to a medium manganese steel with high strength and plastic product and a preparation method thereof. Background Art

[0002] At present, the automotive industry has been committed to improving fuel efficiency and passenger safety. Therefore, steel materials with excellent mechanical properties, formability, recyclability and relatively low cost have attracted much attention. Among them, steel plates with economy and safety are the first choice for modern automotive design. It reduces CO2 emissions by using advanced high-strength steels with high strength and high ductility. At present, the medium manganese steel used in automobiles has been innovated to the third generation of advanced automotive steels, which show an excellent combination of tensile strength and total elongation. It is found that the critical annealing time is proportional to the tensile strength and elongation rate, and the product of the ultimate tensile strength and elongation rate exceeds 40 GPa%. From the aspect of macroscopic mechanical properties, transformation-induced plasticity steel (TRIP steel) shows a similar performance between strength and elongation rate as twinning-induced plasticity steel (TWIP steel), but has a lower alloying cost and less manufacturing difficulty; from the microscopic structure, it retains more austenite with higher mechanical stability, which is crucial for the quantity and stability of retained austenite, thus enhancing the TRIP effect.

[0003] With the development of technology, more and more new heat treatment methods for metal materials have gradually emerged. It is now found that the spark plasma sintering (SPS) technology is a new heat treatment method, which has the advantages of fast heating rate, uniform heating, high production efficiency, energy saving, etc., and can apply an external force to the material during the heat treatment process. Therefore, it is suitable for the heat treatment of metal materials with good performance. At present, the SPS method has been widely used in the synthesis of ceramic materials, light metal materials and their related composites, and excellent comprehensive properties of the materials have been obtained. The key process in the production of medium manganese steel is the critical annealing process. Because during this process, the original martensite or ferrite transforms into austenite grains, and some grains with stronger stability can be retained at ambient temperature. When the Mn content is not high enough, in order to obtain a large amount of retained austenite with stronger stability, a relatively long critical annealing time is usually required, because the critical annealing process will transfer the Mn atoms in ferrite to austenite, thus completing the element partition and enhancing the chemical stability of austenite. However, there is no report on the heat treatment process using SPS. Therefore, it is of great significance to explore the combination of SPS technology and traditional heat treatment processes. Summary of the Invention

[0004] The purpose of the present invention is to provide a medium manganese steel with high strength and plastic product and a preparation method thereof, and a multiphase composite structure of acicular ferrite + M-A component is obtained through the design of composition and process, so as to achieve a good match of high strength, high toughness and weldability.

[0005] The present invention mainly uses the SPS technology + critical annealing process to prepare a multi-scale laminated medium manganese steel structure with coarse-grained austenite and fine-grained austenite structures, enabling the material to obtain an ultra-high strength-ductility product, which helps to improve the strength and toughness of steel for automobiles. The specific scheme is as follows:

[0006] A high-strength and high-ductility medium manganese steel, the chemical composition of the medium manganese steel includes by weight percentage: C: 0.09 - 0.11%, Mn: 3.6 - 4.2%, Al: 2 - 2.5%, Ni: 3.5 - 4.3%, S ≤ 0.006%, P ≤ 0.015%, and the balance is Fe and inevitable impurities.

[0007] The present invention also provides a preparation method for the high-strength and high-ductility medium manganese steel, which includes the following steps:

[0008] (1) Refining: Smelting molten iron, the chemical composition of the molten iron includes by mass percentage: C: 0.09 - 0.11%, Mn: 3.6 - 4.2%, Al: 2 - 2.5%, Ni: 3.5 - 4.3%, S ≤ 0.006%, P ≤ 0.015%, and the balance is Fe and inevitable impurities;

[0009] (2) Continuous casting: Using the whole-process protected casting method for the molten iron to obtain a billet;

[0010] (3) Hot rolling: Heating the billet to 1200 ± 10 °C, putting the billet into the furnace after the furnace temperature rises to 1200 °C, holding for 6 h, then rough rolling at 850 - 950 °C, with each reduction ≥ 30%, obtaining an intermediate billet after 3 - 4 passes of rolling; performing finish rolling on the intermediate billet at 800 - 850 °C, with each reduction ≥ 15%, after 10 passes of rolling, controlling the starting cooling temperature at 760 - 800 °C, the recrystallization temperature at 520 - 570 °C, and water cooling at a cooling rate of 10 - 30 °C / s to obtain medium manganese steel plates;

[0011] (4) Annealing: Placing the medium manganese steel plates in an SPS device to perform isothermal tempering and critical annealing in sequence, applying an external stress of 20 - 70 MPa throughout the process, and cooling to room temperature after completion to obtain high-strength and high-ductility medium manganese steel.

[0012] Preferably, in step (4), the isothermal tempering is: placing the medium manganese steel plates in an SPS device, heating to 350 - 450 °C, and holding for 30 - 200 min.

[0013] Preferably, in step (4), the critical annealing is: continuing to heat the medium manganese steel plates after isothermal tempering heat treatment to 600 - 800 °C, and holding for 20 - 50 min.

[0014] Preferably, in step (3), the microstructure of the obtained medium manganese steel plates includes ferrite and lath martensite.

[0015] Preferably, in step (4), the high-strength-ductility medium manganese steel prepared has a tensile strength of 1100-1500 MPa, a yield strength of 550-1000 MPa, an elongation of 27-45%, and a strength-ductility product of 30-50 GPa%.

[0016] Preferably, the microstructure of the high-strength-ductility medium manganese steel includes: fine acicular ferrite, coarse-grained austenite and fine-grained austenite with a multi-scale layered structure, and martensite.

[0017] Preferably, the volume percentage of acicular ferrite and martensite is 70-91% in total, and the volume percentage of coarse-grained austenite and fine-grained austenite is 9-30% in total.

[0018] In the composition design of medium manganese steel, the higher the C content, the higher the stability of the austenite grains and the lower the Ms temperature. This can effectively improve the material strength, but a higher C content may lead to poor weldability and severe C segregation during casting. Therefore, the C content is controlled within 0.09% ≤ C ≤ 0.11%.

[0019] Al is a ferrite-stabilizing element. Trace amounts of Al can effectively reduce inclusion content and refine grains in steel. Its addition effectively prevents carbide precipitation, allowing carbon atoms to partition into austenite during intercritical annealing. Al increases the activity of carbon in ferrite, thereby enhancing the distribution of carbon from ferrite to austenite. However, excessive Al addition leads to the formation of a delta-ferrite phase during solidification, which often results in a very rough casting and is difficult to refine during hot rolling. Furthermore, high Al content introduces additional difficulties in smelting, secondary refining, and casting. Therefore, the Al content is controlled within the range of 2% ≤ Al ≤ 2.5%.

[0020] Mn primarily acts as a solid solution strengthener. Increasing Mn can compensate for the lack of steel strength caused by reduced C content. It significantly affects the fraction and stability of retained austenite grains, which in turn influences the mechanical properties of medium-manganese steel. At room temperature, the retained austenite content is proportional to the Mn content. Therefore, the lower the Mn content, the smaller the proportion of retained austenite after intercritical annealing and the higher the proportion of martensite formed. However, higher Mn contents can exacerbate central segregation in the ingot, resulting in severe banding in the steel, which can compromise the steel's atmospheric corrosion resistance. Therefore, the Mn content is controlled within a range of 3.6% ≤ Al ≤ 4.2%.

[0021] The main function of Ni is to increase the supercooling degree of austenite, thereby refining the microstructure and achieving a strengthening effect. Additionally, it can enhance the atmospheric corrosion resistance of steel, improve the low-temperature impact toughness, and lower the cold brittleness transition temperature. It can refine the crystal grains of the rust layer, promote the transformation of γ-FeOOH into the stable α-FeOOH phase, and enhance the atmospheric corrosion resistance. Ni significantly improves the plasticity and toughness of steel. However, when the content is too high, it will increase the viscosity of the welding molten pool, which is not conducive to the removal of gases and inclusions, and is likely to increase the number of inclusions in the weld. Therefore, the Ni content is controlled within the range of 3.5% ≤ Ni ≤ 4.3%.

[0022] S easily forms low-melting eutectics, leading to hot brittleness. At the same time, it reduces the plasticity and toughness of the base metal and the weld, and also deteriorates the weathering resistance. It should be reduced as much as possible. Although P significantly improves the weathering resistance, when the content is on the high side, it is extremely likely to cause hot cracking, and phosphides are also prone to cause cold brittleness, reducing plasticity and toughness. Therefore, the S and P contents are respectively controlled within the ranges of S ≤ 0.006% and P ≤ 0.015%.

[0023] Medium manganese steel is rolled with water cooling to obtain ferrite and lath martensite. During the critical annealing process, the lath martensite provides the driving force for the nucleation of austenite, and reverse phase transformation occurs to generate lath-shaped ferrite and austenite, which can produce more austenite that is stable at room temperature. The distortion energy in the microstructure of medium manganese steel after rolling is relatively high. During the annealing process, a medium cooling rate is adopted, and grain recrystallization nucleates, realizing the partitioning of elements such as C and Mn, and finally forming a multiphase composite structure of acicular ferrite + austenite + martensite (a small amount).

[0024] The fine acicular ferrite as the matrix can provide good toughness. The dislocations inside it cooperate with the martensite component to greatly improve the strength. The austenite can significantly improve the strength and elongation of the material through TRIP. The ferrite laths have higher strength and toughness due to fine grain strengthening, with abundant dislocations inside, playing the role of dislocation strengthening. The introduction of martensite components as hard phases, and the fine martensite components can also play the role of dispersion strengthening, cooperating with dislocations to greatly improve the strength, and the austenite significantly improves the tensile properties, with higher safety. Thus, higher tensile strength and elongation are obtained.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1) Compared with other medium manganese TRIP steels, the present invention does not require the addition of extra alloying elements, and the material cost is low;

[0027] 2) Compared with traditional heat treatment processes, the present invention uses SPS sintering technology to apply an external stress to the steel plate while performing heat treatment, so as to provide driving force for austenite nucleation during the critical annealing reverse transformation process, generate more reverse transformed austenite, and thus obtain more austenite at room temperature, enhancing the TRIP effect. Mechanically, it is manifested as a significant improvement in the mechanical properties of the material, with a higher strength-ductility product, a strength ≥ 1000 MPa, an elongation ≥ 25%, and good impact performance.

[0028] 3) Compared with traditional critical annealing processes, the present invention adds an isothermal process during the critical annealing process, and C and Mn elements are precipitated from the supersaturated α-Fe in martensite, providing more C and Mn elements for austenite in the subsequent critical annealing process, for better nucleation and growth, so as to improve the mechanical properties of medium manganese steel without prolonging the critical annealing time, and it is more suitable for application in modern industrial production.

[0029] 4) The present invention rationally designs the composition and controls the content of each chemical component, and adopts a process method that is simple, easy to control, and has appropriate parameters, with high production efficiency and low cost, giving full play to the synergistic effect of each chemical component and achieving a good match between high strength and high toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The drawings here are incorporated into the specification and form a part of the specification, showing embodiments in accordance with the present invention, and are used together with the specification to explain the principles of the present invention, where:

[0031] Figure 1 is the drawing size of the micro-tensile specimen of the present invention;

[0032] Figure 2 is the (SEM) microstructure diagram of the present invention after rolling without heat treatment;

[0033] Figure 3 is the (SEM) microstructure diagram of Example 1 of the present invention;

[0034] Figure 4 is the (SEM) microstructure diagram of Example 2 of the present invention;

[0035] Figure 5 is the (SEM) microstructure diagram of Example 3 of the present invention;

[0036] Figure 6 is the stress-strain curve of the medium manganese TRIP steel plate prepared in Examples 1, 2, and 3 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0037] The following disclosure provides many different embodiments or examples for implementing different structures of the present invention. To simplify the disclosure of the present invention, components and settings of specific examples are described below. Of course, they are only examples and are not intended to limit the present invention. In addition, the present invention may repeat reference numerals and / or reference letters in different examples. Such repetition is for the purpose of simplification and clarity and does not itself indicate the relationship between the various embodiments and / or settings discussed.

[0038] Example 1:

[0039] First, smelt the hot metal. After pre-treating the hot metal for dephosphorization and desulfurization, control the electric arc furnace at 1600 °C for 25 min for smelting to obtain hot metal.

[0040] Then, perform the following operations:

[0041] A. Refining: Transfer the hot metal to a ladle furnace, stir with Ar gas at a pressure of 0.2 - 0.4 MPa and a flow rate of 130 - 150 NL / min for 40 - 50 min of refining; adjust the chemical composition mass percentage of the hot metal according to the following requirements: C: 0.1%, Mn: 3.7%, Al: 2.3%, Ni: 3.7%, S: 0.003%, P: 0.005%, and the balance is Fe and unavoidable impurities, and perform static stirring for 20 - 30 min;

[0042] B. Vacuum treatment: Transfer to a ladle refining furnace, control the vacuum degree ≤ 1 mbar, maintain the vacuum for 20 - 25 min, perform static stirring for 12 - 18 min after breaking the vacuum, adjust the chemical composition, feed Al wire, perform continuous casting electromagnetic stirring, control the stirring intensity at 0.5 - 1.0 L / (min·t), and the stirring time is not less than 8 min;

[0043] C. Protective continuous casting: Adopt the full-process protective casting method to obtain billets;

[0044] D. Thermo-mechanical rolling: Heat the billets to 1200 ± 10 °C, put the billets into the furnace after the furnace temperature rises to 1200 °C, then keep warm for 6 h, perform rough rolling at 850 - 950 °C, with each reduction ≥ 30%, obtain intermediate billets after 3 - 4 passes of rolling, perform finish rolling on the intermediate billets at 800 - 850 °C, with each reduction ≥ 15%, about 10 passes of rolling, control the starting cooling temperature at 760 - 800 °C, the recrystallization temperature at 520 - 570 °C, and cool at a cooling rate of 10 - 30 °C / s to obtain medium manganese TRIP sheets.

[0045] E. Annealing: Apply an external stress of 50 MPa to the rolled medium manganese TRIP sheets using an SPS machine for heat treatment. First, raise the temperature to 400 °C for tempering for 40 min, then heat it to 765 °C for critical annealing for 40 min, and cool to room temperature.

[0046] After this process, the room-temperature structure of medium manganese steel contains retained austenite, which provides the TRIP effect. As Figure 3 shown, its original grains are mainly martensite structure. After tempering-critical annealing, its microstructure includes: fine acicular ferrite + martensite (90.7%), coarse-grained austenite and fine-grained austenite (9.3%) with multi-scale lamination structure. Ultrafine austenite is formed in the martensite laths, effectively improving the mechanical properties of the material. Its tensile strength is about 1300 MPa, yield strength is 985 MPa, elongation is 33%, and the product of strength and plasticity is 43 GPa%.

[0047] Example 2:

[0048] First, smelt the molten iron. After pre-treatment of dephosphorization and desulfurization of the molten iron, control the temperature at 1600 °C in an electric arc furnace for 25 min for smelting to obtain molten iron.

[0049] Then, perform the following operations:

[0050] A. Refining: Transfer the molten iron to a ladle furnace, stir with Ar gas at a pressure of 0.2 - 0.4 MPa and a flow rate of 130 - 150 NL / min for 40 - 50 min; adjust the mass percentages of chemical components as follows: C: 0.1%, Mn: 3.7%, Al: 2.3%, Ni: 3.7%, S: 0.003%, P: 0.005%, and the balance is Fe and inevitable impurities, and stir statically for 20 - 30 min;

[0051] B. Vacuum treatment: Transfer to a ladle refining furnace, control the vacuum degree ≤ 1 mbar, keep the vacuum for 20 - 25 min, stir statically for 12 - 18 min after breaking the vacuum, adjust the chemical components, feed Al wire, and perform continuous casting electromagnetic stirring, control the stirring intensity at 0.5 - 1.0 L / (min·t), and the stirring time is not less than 8 min;

[0052] C. Protected continuous casting: Adopt the whole-process protected casting method to obtain billets;

[0053] D. Thermo-mechanical rolling: Heat the billets to 1200 ± 10 °C, put the ingot parts into the furnace after the temperature in the furnace rises to 1200 °C, and then keep warm for 6 h. Rough rolling is carried out at 850 - 950 °C, and the reduction per pass is ≥ 30%, and the intermediate billets are obtained after 3 - 4 passes of rolling. Finish rolling the intermediate billets at 800 - 850 °C, the reduction per pass is ≥ 15%, about 10 passes of rolling, control the starting cooling temperature at 760 - 800 °C, the recalescence temperature at 520 - 570 °C, and cool at a cooling rate of 10 - 30 °C / s to obtain medium manganese TRIP sheets.

[0054] E. Annealing: After rolling the medium manganese TRIP steel sheet, use an SPS machine to apply an external stress of 50 MPa for heat treatment. First, heat it to 400 °C for tempering for 80 min, then heat it to 765 °C for critical annealing for 40 min, and cool it to room temperature.

[0055] After this process, the room temperature structure of the medium manganese steel contains retained austenite, providing the TRIP effect for it. As can be seen from Figure 4 , its original grains are mainly martensite structure. After tempering - critical annealing, its microstructure includes: fine acicular ferrite, martensite, coarse - grained austenite and fine - grained austenite (12.4%) with multi - scale lamellar structure. Ultra - fine austenite is formed in the martensite laths, effectively improving the mechanical properties of the material. Its tensile strength is 1311 MPa, yield strength is 1285 MPa, elongation is 28%, and the strength - plasticity product is about 37 GPa%.

[0056] Example 3:

[0057] First, smelt the molten iron. After pre - treating the molten iron for dephosphorization and desulfurization, control the temperature at 1600 °C in an electric arc furnace and hold for 25 min for smelting to obtain molten iron.

[0058] Then, perform the following operations:

[0059] A. Refining: Transfer the molten iron to a ladle furnace, stir with Ar gas at a pressure of 0.2 - 0.4 MPa and a flow rate of 130 - 150 NL / min for 40 - 50 min; adjust the mass percentage of chemical components as follows: C: 0.1%, Mn: 3.7%, Al: 2.3%, Ni: 3.7%, S: 0.003%, P: 0.005%, and the balance is Fe and inevitable impurities, and stir statically for 20 - 30 min; ;

[0060] B. Vacuum treatment: Transfer to a ladle refining furnace, control the vacuum degree ≤ 1 mbar, keep the vacuum for 20 - 25 min, stir statically for 12 - 18 min after breaking the vacuum, adjust the chemical components, feed Al wire, and perform continuous casting electromagnetic stirring, control the stirring intensity at 0.5 - 1.0 L / (min·t), and the stirring time is not less than 8 min;

[0061] C. Protected continuous casting: Adopt the whole - process protected casting method to obtain billets;

[0062] D. Thermomechanical rolling: The billet is heated to 1200 ± 10 °C. After the temperature in the furnace rises to 1200 °C, the billet is put in, and then kept warm for 6 h. It is rough rolled at 850 - 950 °C, with the reduction per pass being ≥ 30%. After 3 - 4 passes of rolling, an intermediate billet is obtained. The intermediate billet is finish rolled at 800 - 850 °C, with the reduction per pass being ≥ 15%, and about 10 passes of rolling. The starting cooling temperature is controlled at 760 - 800 °C, the re - heating temperature is controlled at 520 - 570 °C, and the cooling rate is 10 - 30 °C / s for cooling to obtain medium - manganese TRIP steel sheets.

[0063] E. Annealing: The rolled medium - manganese TRIP steel sheets are heat - treated by applying an external stress of 30 MPa using an SPS machine. First, it is heated to 400 °C for tempering for 180 min, and then heated to 765 °C for critical annealing for 40 min, and then cooled to room temperature.

[0064] After this process, the room - temperature structure of medium - manganese steel contains retained austenite, which provides the TRIP effect. As Figure 5 can be seen, its original grains are mainly martensite. After tempering - critical annealing, its microstructure includes: fine acicular ferrite, martensite, coarse - grain austenite with multi - scale lamellae and fine - grain austenite (15.6%) structure. Ultra - fine austenite is formed in the martensite laths, effectively improving the mechanical properties of the material. Its tensile strength is 1273 MPa, the yield strength is 1150 MPa, the elongation is 32%, and the product of strength and plasticity is about 41 GPa%.

[0065] Those skilled in the art will readily conceive of other embodiments of the present invention after considering the specification and the practice of the invention herein. The present invention aims to cover any variations, uses, or adaptations of the present invention that follow the general principles of the present invention and include the common general knowledge or conventional technical means in the technical field of the present invention. The specification and the embodiments are only regarded as exemplary, and the true scope and spirit of the present invention are pointed out by the claims.

[0066] It should be understood that the present invention is not limited to the exact structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present invention is only limited by the appended claims.

Claims

1. A high strength and plasticity medium manganese steel, characterized in that: The chemical composition of the medium manganese steel includes, by weight percentage, C: 0.09-0.11%, Mn: 3.6-4.2%, Al: 2-2.5%, Ni: 3.5-4.3%, S≤0.006%, P≤0.015%, and the balance is Fe and unavoidable impurities; The preparation method of the high-strength and high-plasticity medium-manganese steel comprises the following steps: (1) Refining: smelting molten iron, wherein the chemical composition of the molten iron comprises, by mass percentage, C: 0.09-0.11%, Mn: 3.6-4.2%, Al: 2-2.5%, Ni: 3.5-4.3%, S≤0.006%, P≤0.015%, and the balance being Fe and unavoidable impurities; (2) continuous casting: the molten iron is cast in a fully protected manner to obtain a billet; (3) Hot rolling: the billet is heated to 1200±10℃, and the billet is placed in the furnace after the temperature rises to 1200℃. After keeping the temperature for 6 hours, the billet is rough rolled at 850-950℃, with the reduction amount ≥30% each time, and the intermediate billet is obtained after 3-4 rolling passes; the intermediate billet is finish rolled at 800-850℃, with the reduction amount ≥15% each time, and after 10 rolling passes, the medium manganese steel plate is obtained by water cooling with the cooling temperature of 760-800℃, the red return temperature of 520-570℃, and the cooling rate of 10-30℃ / s; (4) Annealing: The medium manganese steel plate is placed in an SPS device and subjected to isothermal tempering and critical annealing in sequence, with an external stress of 20-70 MPa applied throughout the process. After the annealing, the plate is cooled to room temperature to obtain the high-strength and high-ductility medium manganese steel.

2. The method for preparing high-strength and high-ductility medium-manganese steel according to claim 1, characterized in that: The following steps are involved: (1) Refining: smelting molten iron, wherein the chemical composition of the molten iron comprises, by mass percentage, C: 0.09-0.11%, Mn: 3.6-4.2%, Al: 2-2.5%, Ni: 3.5-4.3%, S≤0.006%, P≤0.015%, and the balance being Fe and unavoidable impurities; (2) continuous casting: the molten iron is cast in a fully protected manner to obtain a billet; (3) Hot rolling: the billet is heated to 1200±10℃, and the billet is placed in the furnace after the temperature rises to 1200℃. After keeping the temperature for 6 hours, the billet is rough rolled at 850-950℃, with the reduction amount ≥30% each time, and the intermediate billet is obtained after 3-4 rolling passes; the intermediate billet is finish rolled at 800-850℃, with the reduction amount ≥15% each time, and after 10 rolling passes, the medium manganese steel plate is obtained by water cooling with the cooling temperature of 760-800℃, the red return temperature of 520-570℃, and the cooling rate of 10-30℃ / s; (4) Annealing: The medium manganese steel plate is placed in an SPS device and subjected to isothermal tempering and critical annealing in sequence, with an external stress of 20-70 MPa applied throughout the process. After the annealing, the plate is cooled to room temperature to obtain the high-strength and high-ductility medium manganese steel.

3. The method for preparing high-strength and high-ductility medium manganese steel according to claim 2, characterized in that: In step (4), the isothermal tempering is as follows: placing the medium manganese steel plate in an SPS device, heating it to 350-450° C., and keeping it warm for 30-200 min.

4. The method for preparing high-strength and high-ductility medium manganese steel according to claim 2, wherein: In step (4), the critical annealing is: continuing to heat the medium manganese steel plate after isothermal tempering heat treatment to 600-800°C and keeping the temperature for 20-50 minutes.

5. The method for preparing high-strength and high-ductility medium manganese steel according to claim 2, characterized in that: In step (3), the microstructure of the medium manganese steel plate prepared includes ferrite and lath martensite.

6. The method for preparing high-strength and high-ductility medium-manganese steel according to claim 2, wherein: In step (4), the high-strength-ductility medium-manganese steel prepared has a tensile strength of 1100-1500 MPa, a yield strength of 550-1000 MPa, an elongation of 27-45%, and a strength-ductility product of 30-50 GPa%.

7. The method for preparing high-strength and high-plasticity medium-manganese steel according to claim 6, characterized in that: The microstructure of the high-strength and high-ductility medium manganese steel includes fine acicular ferrite, coarse-grained austenite and fine-grained austenite with a multi-scale layered structure, and martensite.

8. The method for preparing high-strength and high-plasticity medium manganese steel according to claim 7, characterized in that: The volume percentage of the acicular ferrite and the martensite is 70-91% in total, and the volume percentage of the coarse-grained austenite and the fine-grained austenite is 9-30% in total.

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