Multi-gradient light cold-rolled medium manganese steel with high product of strength and elongation and preparation method thereof

Abstract

The invention discloses high-strength-and-elongation-product multi-gradient light cold-rolled medium manganese steel and a preparation method thereof, and relates to the technical field of advanced high-strength steel plate preparation, and the preparation method comprises the steps that smelting, casting and hot rolling are conducted according to the designed component range, and a hot-rolled plate blank with the light characteristic is obtained; the hot-rolled plate blank is subjected to two-phase region annealing, and an annealed plate is obtained; and the annealed plate is subjected to acid pickling to remove a surface oxide layer and then is subjected to cold rolling, the total reduction rate of cold rolling is 50-70%, and a cold-rolled plate blank is obtained. The cold-rolled plate blank is subjected to two-phase region annealing, and a cold-rolled annealed plate is obtained; and the obtained cold-rolled annealed plate is subjected to stirring friction machining treatment, the rotating speed of a stirring needle ranges from 60 rpm / min to 150 rpm / min, the machining speed ranges from 100 mm / min to 500 mm / min, and the shaft shoulder rolling reduction ranges from 0.1 mm to 0.35 mm. The yield strength of the prepared multi-gradient cold-rolled light medium manganese steel is 800 MPa or above, meanwhile, the plasticity is slightly improved, the medium manganese steel obtains more excellent strength-plasticity combination, the product of strength and ductility can reach 50 GPa.% or above, and the application and service performance of the medium manganese steel in the field of automobile manufacturing is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of advanced high-strength steel plate preparation, and in particular to a high-strength-plastic multi-gradient lightweight cold-rolled medium-manganese steel and a preparation method thereof. Background Art

[0002] In recent years, with the continuous increase in the number of cars, environmental pollution and automobile exhaust emissions have become increasingly serious, making automobile energy conservation and emission reduction urgent. In order to meet this challenge, the global automotive industry is committed to achieving lightweight vehicles to reduce carbon footprint and improve fuel efficiency. Improving the strength of automotive steel and reducing its density are key ways to achieve lightweight vehicles. In this context, the research and application of advanced high-strength steel (AHSS) has become a trend, especially medium manganese steel, which has become the research focus of advanced automotive high-strength steel due to its excellent strength and plasticity. At present, the research on lightweight automotive steel focuses on adding light alloy elements to medium manganese steel to reduce density and improve mechanical properties, thereby reducing the thickness of steel plates.

[0003] Adding Al elements to medium manganese steel can reduce the density of the material and significantly improve the specific strength of the alloy, that is, enhance the strength of the material while maintaining lightweight. In addition, the addition of Al elements also helps to expand the temperature range of the two-phase region, thereby promoting the reverse phase transformation of austenite and the distribution process of Mn. Therefore, medium manganese Fe-Mn-Al steel has shown significant potential and competitive advantages, laying the foundation for its wide application in the automotive manufacturing industry. However, in the study on further improving the performance of medium manganese steel, it was found that traditional rolling and heat treatment technologies are difficult to achieve an excellent strength-plasticity combination, especially the yield strength is still low, which limits the application of medium manganese steel in high-strength structural parts. In recent years, gradient structural materials have been regarded as an advanced material solution because they can improve mechanical properties at multiple scales due to their unique internal structure. Studies have shown that introducing a gradient structure into the material can overcome the problem of strong-plastic balance in the material system, so the introduction of a gradient structure into medium manganese steel can achieve an excellent combination of strength-plasticity to a certain extent.

[0004] Prior art 1 discloses a medium manganese steel welded part with high aluminum content and a preparation process thereof for automobiles. A gradient structure is prepared in the medium manganese steel by multiple hot rolling, critical annealing, laser welding and laser shot peening in the medium and low temperature zone, which improves the tensile properties and the sensitivity to hydrogen embrittlement. However, the prepared gradient structure is single and the strength-plasticity product is only 20-25GPa·%. Prior art 2 discloses a medium manganese steel with multiple gradients and high strength-plasticity product based on stir friction processing and a preparation method thereof. The method prepares austenite, grain size and KAM gradient in the thickness direction of the medium manganese steel by hot rolling, critical annealing and stir friction processing technology, and the tensile strength is improved, but the elongation of the medium manganese steel described in the patent decreases. Prior art 3 discloses a medium manganese steel with a composite gradient structure and a preparation method thereof. This method introduces a gradient structure of organization, grain size and dislocation density into the rod-shaped medium manganese steel by means of torsion treatment, which improves the comprehensive performance of the material. However, the rod is not suitable for the automobile sheet commonly used for medium manganese steel, which greatly limits the application field of medium manganese steel. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a high-strength-plasticity multi-gradient light-weight cold-rolled medium-manganese steel and a preparation method thereof in view of the shortcomings and deficiencies of the prior art. The method adjusts the Al and Si element contents in the medium-manganese steel and cooperates with the rolling, annealing process and stir friction processing technology to make the medium-manganese steel maintain a new microstructure feature of a multi-gradient structure in the thickness direction. These multi-gradient structures include a gradient distribution of grain size, austenite volume and dislocation density, thereby achieving a combination of lightness and excellent strength and toughness. The yield strength of the prepared multi-gradient structure medium-manganese steel is above 800MPa, and the plasticity is also slightly improved, so that the medium-manganese steel obtains a more excellent strength-plasticity combination, and the strength-plasticity product can reach 50GPa·% or more, which significantly improves the application and service performance of the medium-manganese steel in the field of automobile manufacturing.

[0006] According to the first aspect of the technical solution of the present invention, a method for preparing a high-strength, high-plasticity, multi-gradient, lightweight cold-rolled medium-manganese steel is provided. The volume fraction and stability of austenite are controlled by finely adjusting the content of Mn and C elements in the medium-manganese steel, which provides favorable conditions for the subsequent formation of a gradient structure. At the same time, the introduction of the Al element not only reduces the influence of the ultrafine grain structure on the work hardening ability, but also reduces the density of the material, ensuring lightweight. By combining rolling, annealing processes and stir friction processing technology, the medium-manganese steel presents the microstructural characteristics of a multi-gradient structure in the thickness direction, thereby achieving material properties that combine lightweight with excellent strength and toughness.

[0007] Wherein, the method comprises the following steps:

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

[0009] S2, performing two-phase zone annealing on the hot-rolled slab to obtain an annealed sheet;

[0010] S3, pickling the annealed sheet to remove the surface oxide layer and then cold rolling to obtain a cold-rolled slab; performing two-phase zone annealing on the cold-rolled slab to obtain a cold-rolled annealed sheet;

[0011] S4. The obtained cold-rolled annealed plate is clamped and fixed on the processing platform, and the initial parameters of the stir friction processing and the control parameters of the stir friction processing are set to obtain the high-strength and plastic 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 is C: 0.1-0.3%, Mn: 6.2-7.9%, Si: 1.3-2%, Al: 2.1-5.2%, and the balance is Fe. Here, the provided medium manganese steel alloy composition helps to prepare a gradient structure inside the material while ensuring lightweight.

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

[0014] Furthermore, in step S1, a steel ingot is obtained by smelting and casting, and the steel ingot is coated with a high-temperature anti-oxidation coating, heated and homogenized, and then air-cooled to room temperature; the homogenized steel ingot is heated to 1100-1200°C and kept warm for 1-2h, and then subjected to multiple hot rolling passes with a final rolling temperature of not less than 900°C to obtain a hot-rolled slab with lightweight characteristics.

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

[0016] Further, in step S2, the hot-rolled slab is annealed at 750-850°C 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 with a total cold rolling reduction of 50%-70% to obtain a cold-rolled slab. The cold-rolled slab is annealed at 750-850°C 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° C., and holding time 1 h.

[0018] Further, in step S4, the initial parameters of the friction stir processing include the diameter of the stirring head, the diameter of the stirring needle, the length of the stirring needle and the tilt angle, and the control parameters of the friction stir processing process are the stirring head speed and the processing speed. Among them, the stirring head diameter range is 10-15mm, the stirring needle diameter range is 4-6mm, the stirring needle length range is 1-1.2mm, the tilt angle range is 1-3°, and the shoulder pressure is 0.1-0.35mm. The stirring needle speed range in the control parameters of the friction stir processing process is 60-150rpm / min, and the processing speed is 100-500mm / min.

[0019] Furthermore, in step S4, the friction stir processing parameters are: the stirring needle speed is 80 rpm / min, the processing speed is 100 mm / min, the stirring head diameter is 15 mm, the root diameter and the head diameter of the stirring needle are 6 mm and 4 mm respectively, the stirring needle length is 1 mm, the stirring head inclination angle is 2.5°, and the shoulder pressure is 0.2 mm.

[0020] According to a second aspect of the technical solution of the present invention, there is provided a high-strength-ductility-multi-gradient lightweight cold-rolled medium manganese steel, 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 effects of the temperature field and the stress field, the plate surface undergoes intense plastic deformation, forming a gradient structure along the thickness direction.

[0022] The beneficial effects of the present invention are:

[0023] (1) The medium manganese steel prepared by the present invention has strong deformation bearing capacity and low material density, meeting the requirements of lightweight and high safety of automobiles;

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

[0025] (3) The method for improving performance of the present invention is applicable to most conventional medium manganese steels and most advanced high-strength steels containing austenite with relatively high stability;

[0026] (4) The medium manganese steel prepared by this method introduces a multi-gradient structure of grain size gradient, dislocation density gradient, and phase fraction gradient, and obtains an excellent trade-off between strength and plasticity through grain size refinement, synergistic strengthening, and additional strengthening caused by back stress;

[0027] (5) Compared with the homogeneous structure medium manganese steel with the same alloy composition only subjected to two-phase annealing, the tensile strength of the medium manganese steel prepared by the present invention is increased by 15%-25%, the elongation is slightly improved, and the strength-ductility product is increased by 4%-10%. The maximum strength-ductility product can reach about 55 GPa·%. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions in the embodiments or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0029] Figure 1 The present invention is a schematic flow chart of a method for preparing a 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 picture of (manganese steel in gradient structure) according to an embodiment of the technical solution of the present invention.

[0031] Figure 3 It is a hardness distribution curve from the upper surface to the bottom surface of the embodiment (manganese steel in gradient structure) and the comparative example (manganese steel in traditional homogeneous structure) according to the technical solution of the present invention.

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

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

[0034] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below through specific embodiments in conjunction with the accompanying drawings. The specific embodiments described here are only used to explain the present invention and are not used to limit the present invention.

[0035] The technical solution of the present invention first provides a method for preparing a high-strength and high-ductility multi-gradient lightweight cold-rolled medium manganese steel. 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] Wherein, the chemical composition of the medium manganese steel in step S1 is calculated by mass percentage as follows: C: 0.1-0.3%, Mn: 6.2-7.9%, Si: 1.3-2%, Al: 2.1-5.2%, and the balance is Fe. In a preferred embodiment, C: 0.19%, Mn: 6.78%, Si: 1.44%, Al: 2.99%, and the balance is Fe.

[0038] The medium manganese steel alloy composition provided by the present invention helps to prepare a gradient structure inside the material while ensuring lightweight. Among them, the C element can improve the stability of austenite, which is beneficial to the preparation of subsequent gradient structures, but the mechanical properties of high-middle manganese steel deteriorate when the C content is too high. The Si element can significantly adjust the distribution of C in austenite, thereby improving the stability of austenite, which is beneficial to the preparation of subsequent gradient structures, but too high a Si content may cause austenite to be too stable, thereby affecting the TRIP effect. The Al element can not only significantly reduce the density of steel, thereby reducing the weight of automotive steel, but also increase the stacking fault energy in austenite, further delaying fracture, but too much Al content will lead to the generation of more δ-ferrite and reduce mechanical properties, so C0.1-0.3%, Al≤5.2%, Si≤2%.

[0039] Among them, in step S1, smelting and casting are carried out according to the designed medium manganese steel alloy composition to obtain a steel ingot, the steel ingot is coated with a high-temperature anti-oxidation coating, heated and homogenized, and then air-cooled to room temperature; the homogenized steel ingot is heated to 1100-1200°C and kept warm for 1-2h, and then subjected to multiple hot rolling passes, with the final rolling temperature not lower than 900°C, to obtain a hot-rolled slab.

[0040] In a preferred embodiment, the homogenization process parameters in step S1 are preferably: a holding temperature of 1200°C and a holding time of 10 hours, so as to eliminate the dendrite segregation and element segregation generated during the casting process and homogenize the Mn and C elements in the ingot. If the temperature of the homogenization treatment is too high, the austenite grain size will be coarse and the strength will be insufficient, and if the temperature is too low, the degree of homogenization will be insufficient. The heat treatment process parameters of the steel ingot after homogenization treatment are preferably heated to 1200°C for 1 hour, hot rolled into a 6mm thick slab, and then air cooled to room temperature.

[0041] S2: performing two-phase zone annealing on the hot-rolled slab to obtain an annealed sheet.

[0042] S3: The annealed sheet is pickled to remove the surface oxide layer and then cold rolled to obtain a cold-rolled slab. The cold-rolled slab is subjected to two-phase zone annealing to obtain a cold-rolled annealed sheet.

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

[0044] In a preferred embodiment, the annealing process parameters in steps S2 and S3 are preferably: annealing temperature 800°C, holding time 1h. The annealing temperature is between the two-phase region temperature, and there is a certain relationship between the annealing temperature and the holding time. The higher the annealing temperature, the shorter the holding time required. If the holding time is too long, coarse grain size, complete recrystallization, insufficient strength and the like may occur. If the holding time is too short, the austenite content is insufficient, and the plasticity is affected to a certain extent.

[0045] S4: The cold-rolled annealed sheet is clamped and fixed on the processing platform, and the initial parameters of the friction stir processing and the control parameters of the friction stir processing are set so that the obtained medium manganese steel has a multi-gradient structure in the thickness direction.

[0046] The initial parameters of the friction stir processing in S4 include the diameter of the stirring head, the diameter of the stirring needle, the length of the stirring needle and the tilt angle, and the control parameters of the friction stir processing process are the stirring head speed and the processing speed. Among them, the stirring head diameter range is 10-15mm, the stirring needle diameter range is 4-6mm, the stirring needle length range is 1-1.2mm, the tilt angle range is 1-3°, and the shoulder pressure is 0.1-0.35mm. The stirring needle speed range in the control parameters of the friction stir processing process is 60-150rpm / min, and the processing speed is 100-500mm / min.

[0047] In a preferred embodiment, the friction stir processing parameters in step S4 are: the stirring needle speed is 80 rpm / min, the processing speed is 100 mm / min, the stirring head diameter is 15 mm, the root diameter and the head diameter of the stirring needle are 6 mm and 4 mm respectively, the stirring needle length is 1 mm, the stirring head inclination angle is 2.5°, and the shoulder pressure 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 field and stress field, strong plastic deformation occurs on the surface of the plate, forming a gradient structure along the thickness direction.

[0049] The mechanical properties were determined by tensile test method, and the grain size, dislocation density and austenite volume fraction were determined by backscatter diffraction (EBSD) and X-ray diffraction (XRD) methods.

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

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

[0052] Example

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

[0054] S1, smelt and forge the above-mentioned medium manganese steel alloy composition into a 90×60×30mm steel ingot to eliminate defects such as shrinkage cavities and looseness. After coating the steel ingot with anti-oxidation coating, heat it to 1200℃ and keep it for 10h, homogenize it, and then air cool it to room temperature. After the homogenized steel ingot is heated to 1200℃ and kept for 1h, it is rolled into a 6mm thick slab through 4 passes, and the final rolling temperature is not less than 900℃ to obtain a hot-rolled slab;

[0055] S2, annealing the hot-rolled slab at 800°C for 1 hour and then cooling it to room temperature with water to obtain an annealed sheet;

[0056] S3, placing the annealed sheet in a concentrated hydrochloric acid solution with a concentration of about 40%, soaking and cleaning at a temperature of 60-100°C to remove the surface oxide layer, and then cold rolling to 2mm through multiple passes, with a total cold rolling reduction of 67%, to obtain a cold-rolled slab. After annealing the cold-rolled slab at 800°C for 1h, water-cooling it 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, and a W-25Re (wt%) alloy stirring head with a stirring head diameter of 15 mm, a stirring needle root diameter and a stirring needle head diameter of 6 mm and 4 mm respectively, and a stirring needle length of 1 mm is used for processing at a stirring needle speed of 80 rpm / min and a processing speed of 100 mm / min. The stirring needle is processed in a counterclockwise direction, the stirring head inclination angle is 2.5°, and the shoulder pressure is 0.2 mm. The processed medium manganese steel sheet is obtained as shown in Figure 2 shown.

[0058] Samples were taken along the thickness direction of the plate after friction stir processing, and the hardness of the manganese steel in the gradient structure at different positions along the thickness direction was analyzed, such as Figure 3As shown, the hardness gradually decreases along the thickness direction from the upper surface during the friction stir processing. A dog-bone tensile specimen was taken perpendicular to the processing direction, and the engineering stress-strain curve of the multi-gradient cold-rolled light medium manganese steel prepared in this embodiment was measured as follows: Figure 4 As shown, its tensile strength is 1135MPa, yield strength is 853MPa, elongation after fracture is 48.5%, and the strength-plasticity product is as high as 55GPa·%.

[0059] Samples were taken along the thickness direction of the plate after friction stir processing, and EBSD and XRD analysis were performed on different positions of the manganese steel in the gradient structure along the thickness direction. Figure 5 The figure shows the changes in the average grain size, KAM and austenite volume fraction measured at different positions along the thickness direction. The austenite content at a position 0.3mm from the upper surface of the stir friction is 25.1%, the average grain size is 0.4μm, and the KAM value is 0.49. The austenite content at a position 1.2mm from the upper surface of the stir friction is 37.7%, the average grain size is 0.53μm, and the KAM value is 0.43. The austenite content at the bottom position (1.8mm from the upper surface of the stir friction) is 29.8%, the average grain size is 0.46μm, and the KAM value is 0.38. An austenite content gradient is formed from low to high and then decreases along the thickness direction, the dislocation density (KAM value) gradually decreases from the upper surface of the stir friction processing along the thickness direction, and the average grain size forms a gradient from low to high and then decreases along the thickness direction.

[0060] Comparative Example

[0061] The alloy composition of the homogeneous structure medium manganese steel in this comparative example is the same as that in the embodiment, and only the preparation method is different. That is, the preparation method is processed according to the smelting, forging, homogenization, hot rolling, two-phase zone annealing, and cold rolling of the embodiments S1-S3, but the stir friction processing of S4 is not performed to prepare the homogeneous structure medium manganese steel.

[0062] The hardness distribution of manganese steel at different positions along the thickness direction in the homogeneous structure of this comparative example is as follows: Figure 3 As shown, the hardness is evenly distributed along the thickness direction. The engineering stress-strain curve of the manganese steel in the homogeneous structure prepared in this comparative example is measured 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 about 42.8 GPa·%, which is significantly lower than the multi-gradient lightweight cold-rolled medium manganese steel prepared in the embodiment of the present invention.

[0063] Comparing the multi-gradient lightweight cold-rolled medium manganese steel in the embodiment with the homogeneous structure medium manganese steel in the comparison example, it can be seen that the multi-gradient lightweight cold-rolled medium manganese steel in the embodiment has a 24.6% increase in tensile strength, a 48.6% increase in yield strength, a slightly increased elongation after fracture, and a 28% increase in strength-ductility product.

[0064] In summary, the present invention adjusts the content of Mn, C, and Al elements in medium manganese steel and cooperates with rolling, two-phase annealing process, and stir friction processing technology to make the medium manganese steel present a gradient distribution including grain size, austenite volume, and dislocation density in the thickness direction. The volume fraction and stability of austenite are controlled by adjusting the content of Mn and C elements in the medium manganese steel, which provides favorable conditions for the formation of a gradient structure. The introduction of the Al element not only reduces the influence of the ultrafine grain structure on the work hardening ability, but also reduces the density of the material, ensuring lightweight. At the same time, the introduction of the gradient structure can significantly improve the strength of the material without sacrificing plasticity, thereby achieving an excellent balance of strength and toughness, and significantly enhancing the application and serviceability of medium manganese steel in the field of automobile manufacturing.

[0065] The above are only specific embodiments of the present invention, but the protection of the present invention is not limited thereto. Any equivalent changes or substitutions of the features of the present technical solution that can be thought of by a person skilled in the art are within the protection scope of the present invention. The protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A method for preparing high-strength-plastic multi-gradient light cold-rolled medium manganese steel, characterized in that: The method comprises the following steps: S1. Smelting, casting and hot rolling are performed according to the designed medium manganese steel alloy composition to obtain a hot-rolled slab; S2, performing two-phase zone annealing on the hot-rolled slab to obtain an annealed sheet; S3, pickling the annealed sheet to remove the surface oxide layer and then cold rolling to obtain a cold-rolled slab; performing two-phase zone annealing on the cold-rolled slab to obtain a cold-rolled annealed sheet; S4, clamping and fixing the cold-rolled annealed plate on a processing platform, setting initial parameters of the stir friction processing and control parameters of the stir friction processing, and processing to obtain the high-strength and plastic multi-gradient lightweight cold-rolled medium manganese steel, wherein the medium manganese steel has a multi-gradient structure in the thickness direction.

2. The method for preparing a high-strength-plasticity 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 is as follows by mass percentage: C: 0.1-0.3%, Mn: 6.2-7.9%, Si: 1.3-2%, Al: 2.1-5.2%, and the remainder is Fe.

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

4. The method for preparing a high-strength-ductility-multi-gradient lightweight cold-rolled medium manganese steel according to claim 1, characterized in that: In step S1, a steel ingot is obtained by smelting and casting, and the steel ingot is coated with a high-temperature anti-oxidation coating, heated and homogenized, and then air-cooled to room temperature; the homogenized steel ingot is heated to 1100-1200°C and kept warm for 1-2h, and then subjected to multiple hot rolling passes with a final rolling temperature of not less than 900°C to obtain a hot-rolled slab.

5. The method for preparing a high-strength-ductility-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°C, holding time 10h; the heat treatment process parameters of the steel ingot after homogenization treatment are heating to 1200°C for 1h, hot rolling to a 6mm thick slab, and then air cooling to room temperature.

6. The method for preparing a high-strength-plasticity multi-gradient light cold-rolled medium manganese steel according to claim 1, characterized in that: In step S2, the hot-rolled slab is annealed at 750-850°C for 1-2h 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 with a total cold rolling reduction of 50%-70% to obtain a cold-rolled slab; the cold-rolled slab is annealed at 750-850°C for 1-2h and then water-cooled to room temperature.

7. The method for preparing a high-strength-ductility-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° C., and holding time 1 h.

8. The method for preparing a high-strength-ductility-multi-gradient light cold-rolled medium manganese steel according to claim 1, characterized in that: In step S4, the initial parameters of the friction stir processing include the diameter of the stirring head, the diameter of the stirring needle, the length of the stirring needle and the tilt angle, and the control parameters of the friction stir processing process are the stirring head rotation speed and the processing speed; Among them, the diameter range of the stirring head is 10-15mm, the diameter range of the stirring needle is 4-6mm, the length range of the stirring needle is 1-1.2mm, the inclination angle range is 1-3°, the shoulder pressure is 0.1-0.35mm, the stirring needle speed range in the control parameters of the friction stir processing is 60-150rpm / min, and the processing speed is 100-500mm / min.

9. The method for preparing a high-strength-ductility-multi-gradient light cold-rolled medium manganese steel according to claim 8, characterized in that: In step S4, the friction stir processing parameters are: the stirring needle speed is 80 rpm / min, the processing speed is 100 mm / min, the stirring head diameter is 15 mm, the root diameter and the head diameter of the stirring needle are 6 mm and 4 mm respectively, the stirring needle length is 1 mm, the stirring head inclination angle is 2.5°, and the shoulder pressure is 0.2 mm.

10. A high strength and plasticity multi-gradient light cold-rolled medium manganese steel, characterized in that: The medium manganese steel is prepared by the method for preparing high-strength and high-ductility multi-gradient lightweight cold-rolled medium manganese steel according to any one of claims 1 to 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 effects of the temperature field and the stress field, the plate surface undergoes intense plastic deformation, forming a gradient structure along the thickness direction.

Images