High strength and plastic product multi-gradient light cold rolled medium manganese steel and preparation method thereof

Abstract

This invention discloses a high-strength, multi-gradient lightweight cold-rolled medium-manganese steel and its preparation method, relating to the field of advanced high-strength steel plate preparation technology. The method includes: smelting, casting, and hot rolling according to a designed composition range to obtain a hot-rolled slab with lightweight characteristics; annealing the hot-rolled slab in a two-phase region to obtain an annealed plate; pickling the annealed plate to remove the surface oxide layer followed by cold rolling, with a total cold rolling reduction of 50%-70%, to obtain a cold-rolled slab; annealing the cold-rolled slab in a two-phase region to obtain a cold-rolled annealed plate; and subjecting the obtained cold-rolled annealed plate to friction stir processing, with a stirring needle rotation speed of 60-150 rpm / min, a processing speed of 100-500 mm / min, and a shoulder reduction of 0.1-0.35 mm. The prepared multi-gradient cold-rolled lightweight medium manganese steel has a yield strength of over 800 MPa, while its plasticity is also slightly improved, giving the medium manganese steel a better strength-plasticity combination. The strength-plasticity product can reach over 50 GPa·% level, which significantly improves the application and service performance of medium manganese steel in the automotive manufacturing field.

Description

Technical Field

[0001] This invention relates to the field of advanced high-strength steel plate preparation technology, specifically to a high-strength, multi-gradient lightweight cold-rolled medium-manganese steel and its preparation method. Background Technology

[0002] In recent years, with the continuous increase in car ownership, environmental pollution and vehicle exhaust emissions have become increasingly serious, making energy conservation and emission reduction in automobiles an urgent priority. To address this challenge, the global automotive industry is committed to achieving lightweighting of vehicles to reduce carbon footprint and improve fuel efficiency. Increasing the strength and reducing the density of automotive steel is a key approach to achieving lightweighting. Against this backdrop, the research and application of advanced high-strength steel (AHSS) has become a trend, especially medium-manganese steel, which, due to its superior strength and ductility, has become a research focus for advanced automotive high-strength steel. Currently, research on lightweighting of automotive steel focuses on adding lightweight alloying elements to medium-manganese steel to reduce density and improve mechanical properties, thereby reducing steel sheet thickness.

[0003] Adding Al to medium-manganese steel can reduce the material's density and significantly improve the alloy's specific strength, thus enhancing strength while maintaining lightweight properties. Furthermore, the addition of Al helps to expand the temperature range of the two-phase region, thereby promoting the inverse transformation of austenite and the distribution process of Mn. Therefore, medium-manganese Fe-Mn-Al steel exhibits significant potential and competitive advantages, laying the foundation for its widespread application in the automotive manufacturing industry. However, research on further improving the performance of medium-manganese steel has found that traditional rolling and heat treatment techniques struggle to achieve an excellent strength-plasticity combination, especially since the yield strength remains relatively low, limiting its application in high-strength structural components. In recent years, gradient structure materials, with their unique internal structure, have been considered an advanced material solution due to their ability to improve mechanical properties at multiple scales. Research shows that introducing gradient structures into the material can overcome the strength-plasticity balance problem in the material system; therefore, introducing gradient structures into medium-manganese steel can, to some extent, achieve an excellent strength-plasticity combination.

[0004] Prior art 1 discloses a gradient microstructure high-aluminum medium-manganese steel welded part for automobiles and its preparation process. A gradient microstructure is prepared in the medium-manganese steel through multi-pass hot rolling, critical annealing, laser welding, and low-temperature laser shot peening, improving tensile properties and resistance to hydrogen embrittlement. However, the prepared gradient structure is singular, and the strength-ductility product is only 20-25 GPa·%. Prior art 2 discloses a multi-gradient high-strength-ductility product medium-manganese steel based on friction stir processing and its preparation method. This method prepares austenite, grain size, and KAM gradients in the thickness direction of the medium-manganese steel through hot rolling, critical annealing, and friction stir processing, improving tensile strength. However, the elongation of the medium-manganese steel described in the patent decreases. Prior art 3 discloses a composite gradient structure medium-manganese steel and its preparation method. This method introduces a gradient structure in microstructure, grain size, and dislocation density into the rod-shaped medium-manganese steel through torsion treatment, improving the overall performance of the material. However, rods are not suitable for the commonly used automotive sheet metal of medium-manganese steel, greatly limiting the application fields of medium-manganese steel. Summary of the Invention

[0005] The technical problem this invention aims to solve is to address the shortcomings and deficiencies of existing technologies by providing a high-strength-ductility-product multi-gradient lightweight cold-rolled medium-manganese steel and its preparation method. This method, through adjusting the Al and Si element content in the medium-manganese steel and combining rolling, annealing processes, and friction stir processing, enables the medium-manganese steel to maintain a novel microstructure with a multi-gradient structure along its thickness. These multi-gradient structures include a gradient distribution of grain size, austenite volume, and dislocation density, thereby achieving a combination of lightweight and excellent strength and toughness. The prepared multi-gradient medium-manganese steel exhibits a yield strength exceeding 800 MPa, while also showing a slight improvement in ductility, resulting in a superior strength-ductility combination. The strength-ductility product can reach levels exceeding 50 GPa·%, significantly improving the application and service performance of medium-manganese steel in the automotive manufacturing field.

[0006] According to a first aspect of the present invention, a method for preparing high-strength, multi-gradient lightweight cold-rolled medium-manganese steel is provided. By precisely controlling the Mn and C content in the medium-manganese steel, the volume fraction and stability of austenite are controlled, which provides favorable conditions for the subsequent formation of a gradient structure. Simultaneously, the introduction of Al not only reduces the influence of ultrafine grain structure on work hardening ability but also lowers the material density, ensuring lightweight. By combining rolling, annealing processes, and friction stir processing technology, the medium-manganese steel exhibits a multi-gradient microstructure in the thickness direction, thereby achieving a material performance that combines lightweight and excellent strength and toughness.

[0007] The method includes the following steps:

[0008] S1. Smelting, casting and hot rolling are carried out according to the designed medium manganese steel alloy composition to obtain a hot-rolled slab with lightweight characteristics.

[0009] S2. The hot-rolled slab is annealed in the two-phase region to obtain an annealed plate;

[0010] S3. After pickling to remove the surface oxide layer, the annealed plate is cold rolled to obtain a cold-rolled slab; the cold-rolled slab is then annealed in a two-phase region to obtain a cold-rolled annealed plate.

[0011] S4. The obtained cold-rolled annealed plate is clamped and fixed on the processing platform. The initial parameters of friction stir processing and the process control parameters of friction stir processing are set to process the high-strength, high-density, multi-gradient lightweight cold-rolled medium-manganese steel, which has a multi-gradient structure in the thickness direction.

[0012] Further, in step S1, the chemical composition of the medium-manganese steel, by mass percentage, is C: 0.1-0.3%, Mn: 6.2-7.9%, Si: 1.3-2%, Al: 2.1-5.2%, with the balance being Fe. Here, the provided medium-manganese steel alloy composition, while ensuring lightweighting, facilitates the fabrication of a gradient structure within the material.

[0013] Furthermore, the chemical composition of the medium manganese steel, by mass percentage, is: C: 0.19%, Mn: 6.78%, Si: 1.44%, Al: 2.99%, with the balance being Fe.

[0014] Further, in step S1, steel ingots are obtained by smelting and casting. The steel ingots are coated with high-temperature anti-oxidation coating and then heated for homogenization treatment, and then air-cooled to room temperature. The homogenized steel ingots are heated to 1100-1200℃ and held for 1-2 hours, and then subjected to multiple hot rolling passes with a final rolling temperature of not less than 900℃ to obtain a hot-rolled slab with lightweight characteristics.

[0015] Further, in step S1, the homogenization process parameters are: holding temperature 1200℃, holding time 10h. The heat treatment process parameters for the homogenized steel ingot are: heating to 1200℃ and holding for 1h, hot rolling into a 6mm thick slab, and then air cooling to room temperature.

[0016] Further, in step S2, the hot-rolled slab is annealed at 750-850℃ for 1-2 hours and then water-cooled to room temperature. In step S3, after pickling to remove the surface oxide layer, the annealed slab undergoes multiple cold rolling passes with a total cold rolling reduction of 50%-70% to obtain a cold-rolled slab. The cold-rolled slab is then annealed at 750-850℃ for 1-2 hours and then water-cooled to room temperature.

[0017] Furthermore, in steps S2 and S3, the annealing process parameters are: annealing temperature 800℃, holding time 1h.

[0018] Further, in step S4, the initial parameters for friction stir machining include the diameter of the stirring head, the diameter of the stirring pin, the length of the stirring pin, and the tilt angle. The process control parameters for friction stir machining are the stirring head rotation speed and the machining speed. Specifically, the stirring head diameter ranges from 10-15 mm, the stirring pin diameter ranges from 4-6 mm, the stirring pin length ranges from 1-1.2 mm, the tilt angle ranges from 1-3°, and the shoulder reduction is 0.1-0.35 mm. The stirring pin rotation speed range in the friction stir machining process control parameters is 60-150 rpm / min, and the machining speed is 100-500 mm / min.

[0019] Further, in step S4, the parameters for friction stir machining are as follows: stirring pin rotation speed is 80 rpm / min, machining speed is 100 mm / min, stirring head diameter is 15 mm, stirring pin root diameter and head diameter are 6 mm and 4 mm respectively, stirring pin length is 1 mm, stirring head tilt angle is 2.5°, and shoulder pressing amount is 0.2 mm.

[0020] According to a second aspect of the technical solution of the present invention, a high-strength, high-ductility, multi-gradient lightweight cold-rolled medium-manganese steel is provided, wherein the medium-manganese steel is prepared by the preparation method according to any one of the above aspects;

[0021] The medium manganese steel has a grain size gradient, a dislocation density gradient, and a phase gradient in the thickness direction. Due to the combined effect of the temperature field and the stress field, the surface of the plate undergoes strong plastic deformation, forming a gradient structure along the thickness direction.

[0022] The beneficial effects of this invention are as follows:

[0023] (1) The medium manganese steel prepared by this invention has strong deformation resistance and low material density, which meets the requirements of lightweight and high safety of automobiles.

[0024] (2) The preparation method is simple and easy to implement, the process is simple, the cycle is short, the required equipment is conventional, the operation is simple, and the applicability is strong.

[0025] (3) The performance improvement method of the present invention is applicable to most traditional medium manganese steels and most advanced high-strength steels containing austenite with high stability.

[0026] (4) The medium manganese steel prepared by this method introduces a multi-gradient structure with grain size gradient, dislocation density gradient and phase fraction gradient. Through grain size refinement, synergistic strengthening and back stress-induced additional strengthening, an excellent balance between strength and plasticity is obtained.

[0027] (5) The medium manganese steel prepared by the present invention has a 15%-25% higher tensile strength, a slightly higher elongation, and a 4%-10% higher strength-ductility product compared with homogeneous medium manganese steel with the same alloy composition but only annealed in two-phase region. The strength-ductility product can reach up to about 55 GPa. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a schematic flowchart illustrating the preparation method of high-strength, high-ductility, multi-gradient lightweight cold-rolled medium-manganese steel according to an embodiment of the technical solution of the present invention.

[0030] Figure 2 This is a physical image of a manganese steel (gradient structure) according to an embodiment of the technical solution of the present invention.

[0031] Figure 3 The hardness distribution curves from the upper surface to the bottom surface of the manganese steel in the gradient structure and the comparative example (manganese steel in the conventional homogeneous structure) of the embodiment of the technical solution of the present invention are shown.

[0032] Figure 4 The figures show the stress-strain curves of manganese steel in a gradient structure and a comparative example (manganese steel in a traditional homogeneous structure) according to an embodiment of the technical solution of the present invention.

[0033] Figure 5 This is a schematic diagram illustrating the actual measurement results of grain size, dislocation density, and austenite content changes obtained by the method according to the present invention (in manganese steel with a gradient structure) according to an embodiment of the technical solution of the present invention. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0035] The technical solution of this invention first provides a method for preparing high-strength, high-ductility, multi-gradient lightweight cold-rolled medium-manganese steel, such as... Figure 1 As shown, the method includes the following specific steps:

[0036] S1: Smelting, casting, and hot rolling are carried out according to the designed medium manganese steel composition range to obtain hot-rolled slabs.

[0037] The chemical composition of the medium-manganese steel mentioned in step S1, by mass percentage, is: C: 0.1-0.3%, Mn: 6.2-7.9%, Si: 1.3-2%, Al: 2.1-5.2%, with the balance being Fe. In a preferred embodiment, the composition is: C: 0.19%, Mn: 6.78%, Si: 1.44%, Al: 2.99%, with the balance being Fe.

[0038] The medium-manganese steel alloy composition provided by this invention facilitates the fabrication of gradient structures within the material while ensuring lightweight construction. Specifically, carbon (C) enhances the stability of austenite, which is beneficial for the subsequent preparation of gradient structures; however, excessive C content degrades the mechanical properties of medium-manganese steel. Si (Si) significantly regulates the distribution of C in austenite, thereby improving its stability and also benefiting the preparation of gradient structures; however, excessive Si content may lead to overly stable austenite, affecting the TRIP effect. Al (Al) not only significantly reduces the density of steel, resulting in lighter automotive steel, but also increases the stacking fault energy in austenite, further delaying fracture. However, excessive Al content leads to the formation of more δ-ferrite, reducing mechanical properties; therefore, the optimal composition is: C 0.1-0.3%, Al ≤ 5.2%, Si ≤ 2%.

[0039] In step S1, steel ingots are obtained by smelting and casting according to the designed medium manganese steel alloy composition. The steel ingots are coated with high-temperature anti-oxidation coating and then heated for homogenization treatment, and then air-cooled to room temperature. The homogenized steel ingots are heated to 1100-1200℃ and held for 1-2 hours, and then hot-rolled in multiple passes with a final rolling temperature of not less than 900℃ to obtain hot-rolled slabs.

[0040] In a preferred embodiment, the homogenization process parameters in step S1 are preferably: a holding temperature of 1200℃ and a holding time of 10 hours, to eliminate dendritic segregation and elemental segregation generated during casting, thereby homogenizing Mn and C elements in the ingot. If the homogenization temperature is too high, the austenite grain size will be coarse and the strength insufficient; if the temperature is too low, the homogenization degree will be insufficient. The preferred heat treatment parameters for the homogenized steel ingot are heating to 1200℃ and holding for 1 hour, hot rolling into a 6mm thick slab, and then air cooling to room temperature.

[0041] S2: The hot-rolled slab is annealed in the two-phase region to obtain an annealed plate.

[0042] S3: After pickling to remove the surface oxide layer, the annealed sheet is cold-rolled to obtain a cold-rolled slab. The cold-rolled slab is then annealed in a two-phase region to obtain a cold-rolled annealed sheet.

[0043] In step S2, the hot-rolled slab prepared in S1 is annealed at 750-850℃ for 1-2 hours and then water-cooled to room temperature. In step S3, the annealed slab from S2 is pickled to remove the surface oxide layer, and then subjected to multiple cold rolling passes with a total cold rolling reduction of 50%-70% to obtain a cold-rolled slab. The cold-rolled slab is then annealed at 750-850℃ for 1-2 hours and then water-cooled to room temperature to obtain a cold-rolled annealed slab.

[0044] In a preferred embodiment, the annealing process parameters in steps S2 and S3 are preferably: annealing temperature 800℃, holding time 1h. The annealing temperature is between the two-phase region temperature. The annealing temperature and the holding time are related. The higher the annealing temperature, the shorter the holding time required. If the holding time is too long, it may result in coarse grain size, incomplete recrystallization, and insufficient strength. If the holding time is too short, the austenite content will be insufficient, and the plasticity will be affected to a certain extent.

[0045] S4: The cold-rolled annealed plate is clamped and fixed on the processing platform. The initial parameters of friction stir processing and the process control parameters of friction stir processing are set to make the obtained medium manganese steel have a multi-gradient structure in the thickness direction.

[0046] The initial parameters for friction stir machining in S4 include the diameter of the stirring head, the diameter of the stirring pin, the length of the stirring pin, and the tilt angle. The process control parameters for friction stir machining are the stirring head rotation speed and the machining speed. Specifically, the stirring head diameter ranges from 10-15 mm, the stirring pin diameter ranges from 4-6 mm, the stirring pin length ranges from 1-1.2 mm, the tilt angle ranges from 1-3°, and the shoulder reduction is 0.1-0.35 mm. The stirring pin rotation speed range in the friction stir machining process control parameters is 60-150 rpm / min, and the machining speed is 100-500 mm / min.

[0047] In a preferred embodiment, the friction stir machining parameters in step S4 are as follows: stirring pin rotation speed is 80 rpm / min, machining speed is 100 mm / min, stirring head diameter is 15 mm, stirring pin root diameter and head diameter are 6 mm and 4 mm respectively, stirring pin length is 1 mm, stirring head tilt angle is 2.5°, and shoulder pressing amount is 0.2 mm.

[0048] The multi-gradient structure in the thickness direction of S4 refers to the grain size gradient, dislocation density gradient, and phase gradient. Due to the combined effect of temperature and stress fields, the surface of the plate undergoes strong plastic deformation, forming a gradient structure along the thickness direction.

[0049] Mechanical properties were determined by tensile testing, while grain size, dislocation density, and austenite volume fraction were determined by backscatter diffraction (EBSD) and X-ray diffraction (XRD).

[0050] The mechanical properties of the manganese steel in the multi-gradient structure prepared by this invention meet the requirements of tensile strength greater than 1100MPa, yield strength greater than 800MPa, and elongation as high as 48%.

[0051] The present invention also provides a high-strength, high-ductility, multi-gradient lightweight cold-rolled medium-manganese steel, which is prepared using the above method.

[0052] Example

[0053] The chemical composition of the medium-manganese steel, by mass percentage, is: C: 0.1-0.3%, Mn: 6.2-7.9%, Si: 1.3-2%, Al: 2.1-5.2%, with the balance being Fe. Preferably, the composition is: C: 0.19%, Mn: 6.78%, Si: 1.44%, Al: 2.99%, with the balance being Fe.

[0054] S1, according to the above-mentioned medium manganese steel alloy composition, is smelted and forged into a 90×60×30mm steel ingot to eliminate defects such as shrinkage cavities and porosity. After coating the steel ingot with an anti-oxidation coating, it is heated to 1200℃ and held for 10 hours for homogenization treatment, and then air-cooled to room temperature. The homogenized steel ingot is heated to 1200℃ and held for 1 hour, and then rolled in 4 passes to a 6mm thick slab, with a final rolling temperature of not less than 900℃, to obtain a hot-rolled slab;

[0055] S2, after annealing the hot-rolled slab at 800℃ for 1 hour, water-cool it to room temperature to obtain the annealed slab;

[0056] S3. The annealed sheet is immersed in a concentrated hydrochloric acid solution of approximately 40% at a temperature of 60-100℃ to remove the oxide layer on the surface. After multiple cold rolling passes to 2mm, the total cold rolling reduction rate is 67%, resulting in a cold-rolled slab. The cold-rolled slab is then annealed at 800℃ for 1 hour and water-cooled to room temperature to obtain a cold-rolled annealed sheet.

[0057] S4. The cold-rolled annealed sheet is clamped and fixed on the processing platform. A W-25Re (wt%) alloy stirring head with a 15mm diameter stirring head, a 6mm root diameter, a 4mm head diameter, and a 1mm length, threaded, is used for processing at a stirring pin speed of 80rpm / min and a processing speed of 100mm / min. The stirring pin is processed counterclockwise, the stirring head tilt angle is 2.5°, and the shoulder reduction is 0.2mm. The resulting medium manganese steel sheet is shown below. Figure 2 As shown.

[0058] Samples were taken along the thickness direction of the sheet metal after friction stir processing, and the hardness of the manganese steel at different locations along the thickness direction in the gradient structure was analyzed. Figure 3As shown, the hardness gradually decreases along the thickness direction from the upper surface after friction stir processing. Dog-bone shaped tensile specimens were taken perpendicular to the processing direction, and the engineering stress-strain curves of the multi-gradient cold-rolled lightweight medium-manganese steel prepared in this embodiment were measured as follows: Figure 4 As shown, its tensile strength is 1135 MPa, its yield strength is 853 MPa, its elongation after fracture is 48.5%, and its strength-ductility product is as high as 55 GPa·s.

[0059] Samples were taken along the thickness direction of the sheet metal after friction stir processing, and EBSD and XRD analyses were performed on the manganese steel at different locations along the thickness direction in the gradient structure. Figure 5 The figure shows the variations in average grain size, KAM, and austenite volume fraction at different locations along the thickness direction. At a position 0.3 mm from the upper surface of the friction stir machine, the austenite content is 25.1%, the average grain size is 0.4 μm, and the KAM value is 0.49. At a position 1.2 mm from the upper surface of the friction stir machine, the austenite content is 37.7%, the average grain size is 0.53 μm, and the KAM value is 0.43. At the bottom surface (1.8 mm from the upper surface of the friction stir machine), the austenite content is 29.8%, the average grain size is 0.46 μm, and the KAM value is 0.38. An austenite content gradient is formed along the thickness direction, increasing and then decreasing again. The dislocation density (KAM value) gradually decreases along the thickness direction from the upper surface of the friction stir machine, and the average grain size also forms a gradient along the thickness direction, increasing and then decreasing again.

[0060] Comparative Example

[0061] The homogeneous structure of the manganese steel in this comparative example has the same alloy composition as the example, except for the preparation method. Specifically, the preparation method follows the smelting, forging, homogenization, hot rolling, two-phase annealing, and cold rolling processes of Examples S1-S3, but excludes the friction stir processing of S4, thus preparing a homogeneous medium-manganese steel.

[0062] In this comparative homogeneous structure, the hardness distribution of manganese steel at different locations along the thickness direction is as follows: Figure 3 As shown, the hardness is uniformly distributed along the thickness direction. The engineering stress-strain curve of the manganese steel in the homogeneous structure prepared in this comparative example is as follows. Figure 4 As shown, its tensile strength is 911 MPa, yield strength is 574 MPa, elongation after fracture is 47%, and strength-ductility product is approximately 42.8 GPa·%, which is significantly lower than that of the multi-gradient lightweight cold-rolled medium manganese steel prepared in the embodiments of the present invention.

[0063] A comparison of the multi-gradient lightweight cold-rolled manganese steel in the embodiment with the manganese steel in the comparative homogeneous structure shows that the tensile strength of the multi-gradient lightweight cold-rolled manganese steel in the embodiment is increased by 24.6%, the yield strength is increased by 48.6%, the elongation after fracture is slightly increased, and the strength-ductility product is increased by about 28%.

[0064] In summary, this invention, by controlling the Mn, C, and Al content in medium-manganese steel and combining this with rolling, two-phase annealing, and friction stir processing, achieves a gradient distribution of grain size, austenite volume, and dislocation density along the thickness direction of the medium-manganese steel. Controlling the Mn and C content in the medium-manganese steel to manage the austenite volume fraction and stability provides favorable conditions for the formation of the gradient structure. The introduction of Al not only reduces the impact of ultrafine grain structure on work hardening ability but also lowers the material density, ensuring lightweight construction. Simultaneously, the introduction of the gradient structure can significantly improve the material's strength without sacrificing plasticity, thereby achieving an excellent balance between strength and toughness, significantly enhancing the application and serviceability of medium-manganese steel in the automotive manufacturing field.

[0065] The above are merely specific embodiments of the present invention, but the protection of the present invention is not limited thereto. Any equivalent variations or substitutions of the features of the present technical solution that can be conceived by those skilled in the art are covered within the protection scope of the present invention. The protection scope of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing high-strength, multi-gradient, lightweight cold-rolled medium-manganese steel, characterized in that, The method includes the following steps: S1. Smelting, casting, and hot rolling are carried out according to the designed medium manganese steel alloy composition to obtain hot-rolled slabs; S2. The hot-rolled slab is annealed in a two-phase region to obtain an annealed plate; S3. After pickling to remove the surface oxide layer, the annealed plate is cold rolled to obtain a cold-rolled slab; the cold-rolled slab is then subjected to two-phase annealing to obtain a cold-rolled annealed plate. S4. The cold-rolled annealed sheet is clamped and fixed on the processing platform. The initial parameters and process control parameters of friction stir processing are set to obtain the high-strength, multi-gradient, lightweight cold-rolled medium-manganese steel. The medium-manganese steel has a multi-gradient structure in the thickness direction. Among them, the prepared multi-gradient structure forms an austenite content gradient along the thickness direction, from low to high and then decreasing; the dislocation density gradually decreases along the thickness direction from the upper surface processed by friction stir; the average grain size forms a gradient along the thickness direction, from low to high and then decreasing; its yield strength is above 800 MPa; and its strength-ductility product reaches 50 GPa·%.

2. The method for preparing high-strength, multi-gradient, lightweight cold-rolled medium-manganese steel according to claim 1, characterized in that, In step S1, the chemical composition of the medium manganese steel by mass percentage is: C: 0.1-0.3%, Mn: 6.2-7.9%, Si: 1.3-2%, Al: 2.1-5.2%, with the balance being Fe.

3. The method for preparing high-strength, multi-gradient, lightweight cold-rolled medium-manganese steel according to claim 2, characterized in that, The chemical composition of the medium manganese steel, by mass percentage, is: C: 0.19%, Mn: 6.78%, Si: 1.44%, Al: 2.99%, with the balance being Fe.

4. The method for preparing high-strength, multi-gradient, lightweight cold-rolled medium-manganese steel according to claim 1, characterized in that, In step S1, steel ingots are obtained by smelting and casting. The steel ingots are coated with high-temperature anti-oxidation coating and then heated for homogenization treatment, and then air-cooled to room temperature. The homogenized steel ingots are heated to 1100-1200℃ and held for 1-2 hours, and then subjected to multiple hot rolling passes with a final rolling temperature of not less than 900℃ to obtain hot-rolled slabs.

5. The method for preparing high-strength, multi-gradient, lightweight cold-rolled medium-manganese steel according to claim 4, characterized in that, In step S1, the homogenization process parameters are: holding temperature 1200℃, holding time 10h; the heat treatment process parameters of the homogenized steel ingot are: heating to 1200℃ and holding for 1h, hot rolling into a 6mm thick slab, and then air cooling to room temperature.

6. The method for preparing high-strength, multi-gradient, lightweight cold-rolled medium-manganese steel according to claim 1, characterized in that, In step S2, the hot-rolled slab is annealed at 750-850℃ for 1-2 hours and then water-cooled to room temperature; in step S3, the annealed slab is pickled to remove the surface oxide layer, and then subjected to multiple cold rolling passes with a total cold rolling reduction of 50%-70% to obtain a cold-rolled slab; the cold-rolled slab is annealed at 750-850℃ for 1-2 hours and then water-cooled to room temperature.

7. The method for preparing high-strength, multi-gradient, lightweight cold-rolled medium-manganese steel according to claim 6, characterized in that, In steps S2 and S3, the annealing process parameters are: annealing temperature 800℃, holding time 1h.

8. The method for preparing high-strength, multi-gradient lightweight cold-rolled medium-manganese steel according to claim 1, characterized in that, In step S4, the initial parameters of friction stir machining include the diameter of the stirring head, the diameter of the stirring pin, the length of the stirring pin, and the tilt angle. The control parameters of friction stir machining process are the stirring head rotation speed and the machining speed. The stirring head diameter ranges from 10 to 15 mm, the stirring pin diameter ranges from 4 to 6 mm, the stirring pin length ranges from 1 to 1.2 mm, the tilt angle ranges from 1 to 3°, the shoulder pressing amount is 0.1 to 0.35 mm, and the stirring pin rotation speed ranges from 60 to 150 rpm and the processing speed is 100 to 500 mm / min in the friction stir machining process control parameters.

9. The method for preparing high-strength, multi-gradient, lightweight cold-rolled medium-manganese steel according to claim 8, characterized in that, In step S4, the parameters for friction stir machining are as follows: stirring needle rotation speed is 80 rpm, machining speed is 100 mm / min, stirring head diameter is 15 mm, stirring needle root diameter and head diameter are 6 mm and 4 mm respectively, stirring needle length is 1 mm, stirring head tilt angle is 2.5°, and shoulder pressing amount is 0.2 mm.

10. A high-strength, multi-gradient, lightweight cold-rolled medium-manganese steel, characterized in that, The medium-manganese steel is prepared using the method for preparing high-strength, multi-gradient, lightweight cold-rolled medium-manganese steel according to any one of claims 1-9. The medium manganese steel has a grain size gradient, a dislocation density gradient, and a phase gradient in the thickness direction. Due to the combined effect of the temperature field and the stress field, the surface of the plate undergoes strong plastic deformation, forming a gradient structure along the thickness direction.

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