An ultra-high strength high-plasticity ultra-fine microstructure low-density steel and a preparation method thereof

Abstract

The application discloses a kind of ultra-high strength high plasticity ultra-fine structure low-density steel (less than 6.8 g / cm 3 ), the steel is composed of ultra-fine recrystallized austenite grain, intergranular B2 phase and nanometer B2 phase particle precipitated in unrecrystallized matrix;Among them, nanometer B2 phase particle precipitated in unrecrystallized matrix is dispersedly distributed in unrecrystallized matrix;According to volume percentage, the volume fraction of unrecrystallized matrix is 15%-25%, the average grain size of ultra-fine recrystallized austenite grain is not more than 0.5 μm, the average size of intergranular B2 phase is not more than 0.4 μm, and the average size of dispersedly distributed nanometer B2 phase particle is not more than 50 nm.

Description

Technical Field

[0001] This invention relates to the field of heat treatment technology for deformation of metallic materials, specifically to a low-density steel with ultra-high strength, high plasticity, and ultra-fine microstructure, and its preparation method. Background Technology

[0002] The large amounts of greenhouse gases such as CO2, CH4, and SO2 carried in automobile exhaust are one of the primary factors contributing to the increasingly severe global environmental problems. Reducing vehicle weight can be linearly translated into reduced fuel consumption; therefore, achieving lightweighting of automobiles while ensuring vehicle safety is an inevitable trend for the future. Fe-Mn-Al-C austenitic low-density steel has attracted widespread attention in the automotive industry due to its excellent comprehensive mechanical properties and weight reduction effect. However, because Fe-Mn-Al-C austenitic steel has a high stacking fault energy, deformation mainly occurs through dislocation plane slip, without dislocation pile-up or dislocation cell structure. This makes the precipitated κ′ carbides easily sheared by the slipping dislocations, resulting in slip surface softening and poor work hardening ability, which limits its application in practical industrial fields.

[0003] To address the low work hardening rate in Fe-Mn-Al-C steel, Ni was added to form a new hard intermetallic compound, B2 phase. Unlike κ′ carbides, the B2 phase is less susceptible to dislocation shearing, ensuring good work hardening capability. However, given the competition between strength and plasticity, few studies have achieved a good strength-plasticity balance in Fe-Mn-Al-C-Ni austenitic low-density steel, preventing it from simultaneously possessing high strength and high plasticity. Summary of the Invention

[0004] In view of the above-mentioned shortcomings of the existing technology, the purpose of this invention is to provide an ultra-high strength, high plasticity, ultra-fine microstructure low-density steel and its preparation method, so as to solve the problem that the Fe-Mn-Al-C-Ni austenitic low-density steel in the existing technology cannot have both high strength and high plasticity.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] A low-density steel with ultra-high strength, high plasticity, and ultra-fine microstructure is disclosed. The steel comprises ultra-fine recrystallized austenite grains, intergranular B2 phase, and nano-B2 phase particles precipitated within a non-recrystallized matrix. The nano-B2 phase particles precipitated within the non-recrystallized matrix are dispersed within it. The volume fraction of the non-recrystallized matrix is ​​15%–25%. The average grain size of the ultra-fine recrystallized austenite grains does not exceed 0.5 μm, the average size of the intergranular B2 phase does not exceed 0.4 μm, and the average size of the dispersed nano-B2 phase particles does not exceed 50 nm. The low-density steel of this invention undergoes solution treatment, warm rolling, and annealing sequentially. During this process, partial recrystallization occurs during annealing. The non-recrystallized matrix refers to the matrix that did not undergo recrystallization during annealing.

[0007] Preferably, at room temperature, the steel has a yield strength of at least 1500 MPa, a tensile strength of at least 1700 MPa, and a total elongation of at least 10%; the steel has a density of < 6.8 g / cm³. 3 .

[0008] Preferably, the steel is obtained by sequentially undergoing solution treatment, warm rolling treatment, and annealing treatment.

[0009] Preferably, the microstructure of the steel after solution treatment consists of an austenitic matrix and a B2 phase. The B2 phase is distributed between the austenitic grains, and the volume fraction of the B2 phase distributed between the austenitic grains is 5% to 20% by volume percentage.

[0010] Preferably, the warm-rolled steel consists of an austenitic matrix and a B2 phase, wherein the dislocation density of the steel obtained after treatment is higher than 1.0 × 10⁻⁶. 16 m -2 The average size of dislocation cells is less than 0.35 μm.

[0011] Preferably, the steel comprises the following components by mass percentage: 20-30% Mn, 8-12% Al, 0.8-1.2% C, 4-10% Ni, and unavoidable impurity elements, with the balance being Fe.

[0012] This invention also provides a method for preparing ultra-high strength, high plasticity, and ultra-fine microstructure low-density steel. The method for preparing the above-mentioned ultra-high strength, high plasticity, and ultra-fine microstructure low-density steel specifically includes the following steps:

[0013] Step 1: Perform solution treatment on the steel plate;

[0014] Step 2: After heat preservation, the steel plate treated in Step 1 is subjected to multiple passes of warm rolling deformation. After the warm rolling is completed, it is cooled to room temperature with water. The deformation amount of each pass is 3 to 6%, and the heat preservation between each pass is 3 to 5 minutes. The total deformation amount is controlled at 85% to 95%.

[0015] Step 3: Perform partial recrystallization annealing on the steel plate treated in Step 2.

[0016] Preferably, the solution treatment temperature is 1100℃~1200℃, and the solution is kept at that temperature for 1~2 hours before being water-cooled to room temperature.

[0017] Preferably, in step 2, the steel plate is kept at 200℃ to 400℃ and subjected to multiple passes of warm rolling deformation at this temperature.

[0018] Preferably, in step 3, the annealing temperature is 800℃~950℃ and the annealing time is 3~30min.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] First, the method described in this invention is applied to Fe-Mn-Al-C-Ni austenitic low-density steel, resulting in ultra-high dislocation density. Simultaneously, the cross-slip of dislocations is activated by warm deformation, leading to an ultra-fine dislocation cell structure. Second, the heat treatment method described above allows the numerous dislocations and dislocation substructures obtained during warm rolling to act as non-uniform nucleation sites during partial recrystallization, resulting in an ultra-fine structure composed of ultra-fine recrystallized austenite grains, intergranular B2 phase, and nano-B2 particles in the unrecrystallized region. This achieves an excellent combination of ultra-high strength and high plasticity. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the process flow of the present invention.

[0022] Figure 2 The stress-strain curves for Example 1 and Comparative Examples 1 and 2 are shown.

[0023] Figure 3 The image shows the BSE contrast diagram of Example 1, which reveals its partially recrystallized structure. The red area in the diagram represents the non-recrystallized area in Example 1, and the green arrows represent nano-sized B2 particles in the non-recrystallized area.

[0024] Figure 4 The image shows a bright-field TEM image of the recrystallization region in the microstructure of Example 1, which shows an ultrafine grain distribution with an average austenite size of 0.36 μm and an average B2 phase size of 0.24 μm. Detailed Implementation

[0025] The technical solutions of the present invention will be clearly and completely described in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the present invention are within the scope of protection of the present invention.

[0026] Unless otherwise specified in the specific circumstances, the numerical ranges listed herein include upper and lower limits, as well as all integers and fractions within that range, but are not limited to the specific values ​​listed when the range is defined.

[0027] I. A type of ultra-high strength, high plasticity, ultra-fine microstructure, low-density steel

[0028] The steel of this invention is composed of ultrafine recrystallized austenite grains, intergranular B2 phase, and nano-B2 phase particles precipitated in the unrecrystallized matrix. The nano-B2 phase particles precipitated in the unrecrystallized matrix are dispersed within it. By volume percentage, the volume of the unrecrystallized matrix accounts for 15% to 25% of the total volume of the steel. The average grain size of the ultrafine recrystallized austenite grains does not exceed 0.5 μm, the average size of the intergranular B2 phase does not exceed 0.4 μm, and the average size of the dispersed nano-B2 phase particles does not exceed 50 nm. The low-density steel of this invention undergoes solution treatment, warm rolling, and annealing sequentially. During this process, recrystallization occurs in part of the matrix during annealing. The unrecrystallized matrix refers to the matrix that did not undergo recrystallization during annealing.

[0029] This invention, based on in-depth research on austenitic low-density steel, aims to improve the competitive relationship between high strength and high plasticity in Fe-Mn-Al-C-Ni austenitic low-density steel, and conceives a method to simultaneously improve the strength and plasticity of low-density steel. Existing technologies generally believe that, under the same deformation amount, cold rolling will achieve more intense local strain than warm rolling, resulting in higher dislocation density and higher strength. This invention discovers that a deformation heat treatment method combining large deformation warm rolling with high-temperature annealing can bring unexpected technical effects. This invention uses warm rolling to allow the austenite in Fe-Mn-Al-C-Ni austenitic low-density steel to bear more stress. Simultaneously, this treatment method ensures that insufficient stress is applied to the hard B2 phase for effective deformation. Ultimately, under the same deformation conditions, compared to cold pressing, the material treated by warm rolling forms a coarser B2 phase and a finer austenite structure, achieving a higher dislocation density within the austenite and ultimately realizing superior comprehensive mechanical properties. Meanwhile, warm rolling facilitates thermally activated cross-slip, promoting the transformation of dislocations from planar slip to cross-slip, thus obtaining a finer dislocation cell structure. The large number of dislocations and dislocation substructures obtained during warm rolling provide numerous heterogeneous nucleation sites for recrystallization during subsequent annealing, resulting in an ultrafine microstructure after recrystallization, which significantly improves the strength and plasticity of Fe-Mn-Al-C-Ni steel.

[0030] In some embodiments, the steel is obtained by sequentially undergoing solution treatment, warm rolling, and annealing. Specifically, the steps include:

[0031] Step 1: Perform solution treatment on the steel plate;

[0032] Step 2: After heat preservation, the steel plate treated in Step 1 is subjected to multiple passes of warm rolling deformation. After the warm rolling is completed, it is cooled to room temperature with water. The deformation amount of each pass is 3 to 6%, and the heat preservation between each pass is 3 to 5 minutes. The total deformation amount is controlled at 85% to 95%.

[0033] Step 3: Perform partial recrystallization annealing on the steel plate treated in Step 2.

[0034] In some embodiments, the solution treatment temperature is 1100℃~1200℃, the holding time is 1~2h, and then water-cooled to room temperature. After solution treatment, the initial microstructure of the steel plate changes. The microstructure of the solution-treated steel plate consists of an austenite matrix and a B2 phase. The B2 phase is distributed at the austenite grain boundaries, and the volume percentage of the B2 phase in the austenite is 5%~20%. Too low a solution treatment temperature will result in a high B2 phase content; too high a solution treatment temperature will result in coarse grains. Both of these situations are detrimental to simultaneously improving the strength and elongation of the steel. Therefore, the solution treatment temperature can be 1100℃, 1150℃, 1200℃, etc., as well as all ranges and sub-ranges between these values; the solution treatment holding time can be 1h, 1.5h, 2h, etc., as well as all ranges and sub-ranges between these numbers. It should be understood that, in the embodiments, any of the above ranges can be combined with any other range.

[0035] In some embodiments, in step 2, the steel sheet is held at 200°C to 400°C and subjected to multi-pass warm rolling deformation at this temperature. The warm rolling process forms a large number of dislocations and small-sized dislocation cells, providing numerous heterogeneous nucleation sites for recrystallization. The warm-rolled steel sheet consists of an austenitic matrix and a B2 phase, wherein the dislocation density is higher than 1.0 × 10⁻⁶. 16 m -2 The average size of dislocation cells is less than 0.35 μm. Compared with existing room temperature rolling, warm rolling makes austenite easier to deform, especially achieving a higher dislocation density within the austenite. Too low a temperature during warm rolling makes austenite deformation difficult, while too high a temperature reduces dislocation density; both negatively impact the final steel's strength and elongation. Therefore, the warm rolling temperature can be 200℃, 250℃, 300℃, 350℃, 400℃, etc., and all ranges and sub-ranges between these values. In warm rolling, too low a deformation per pass leads to uneven deformation, while too high a deformation makes deformation difficult; both prevent the steel from simultaneously improving strength and elongation. Therefore, the deformation per pass can be 3%, 4%, 5%, 6%, etc., and all ranges and sub-ranges between these values; the total deformation can be 85%, 90%, 95%, etc., and all ranges and sub-ranges between these values; it should be understood that, in the implementation scheme, any of the above ranges can be combined with any other range.

[0036] In some embodiments, in step 3, the annealing temperature is 800℃~950℃, and the annealing time is 3~30min. Annealing is performed to induce recrystallization within the warm-rolled steel. Based on the numerous heterogeneous nucleation sites provided by warm rolling, recrystallization occurs within the steel, resulting in ultra-fine recrystallized grains in the recrystallized region. This ultra-fine grain strengthening, combined with the precipitation strengthening of the nano-B2 phase and dislocation strengthening in the non-recrystallized region, allows the low-density steel to achieve a perfect combination of ultra-high strength (1.5-2.2GPa) and excellent plasticity (total elongation over 10%), significantly improving the safety of the steel in use. Too low an annealing temperature leads to an excessively high volume fraction of non-recrystallized portions, while too high an annealing temperature leads to an excessively high volume fraction of recrystallized portions; neither of these conditions is conducive to simultaneously improving the material's strength and plasticity. The annealing temperature can be 800℃, 850℃, 900℃, 950℃, etc., as well as all ranges and sub-ranges between these values; the annealing time can be 3 min, 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, etc., as well as all ranges and sub-ranges between these values. It should be understood that, in the implementation scheme, any of the above ranges can be combined with any other range.

[0037] In some embodiments, at room temperature, the steel of the present invention has a yield strength of at least 1500 MPa, a tensile strength of at least 1700 MPa, and a total elongation of at least 10%.

[0038] In some embodiments, the steel comprises, by mass percentage, 20-30% Mn, 8-12% Al, 0.8-1.2% C, 4-10% Ni, and unavoidable impurity elements, with the balance being Fe.

[0039] II. Examples and Comparative Examples

[0040] Example 1

[0041] Using austenitic low-density steel with a chemical composition of Fe-28Mn-11Al-1C-5Ni (wt.%), according to... Figure 1 The deformation heat treatment process shown involves first solution treating a 10mm thick steel plate at 1200℃ for 1 hour, followed by water cooling to room temperature. The resulting solution-treated material is then subjected to multi-pass large deformation rolling at 300℃, with each pass involving approximately 5% deformation and holding between passes for 3-5 minutes. The total rolling reduction is 90%. After rolling, the material is water-cooled to room temperature. Subsequently, the rolled plate is annealed at 850℃ for 30 minutes, and then water-cooled to room temperature after annealing.

[0042] The steel obtained in this embodiment was subjected to room temperature mechanical property testing, and the results are as follows: Figure 2As shown in Table 1, the yield strength is 1573 MPa, the tensile strength is 1733 MPa, and the total elongation is 23.3%.

[0043] Comparative Example 1

[0044] The adjustment is based on Example 1, the difference being that after the steel plate is warm rolled, it is directly water-cooled to room temperature without annealing.

[0045] The room temperature mechanical properties of the steel obtained in this comparative example were tested, and the results are as follows: Figure 2 As shown in Table 1, the yield strength is 1999 MPa, the tensile strength is 2080 MPa, and the total elongation is 5.6%.

[0046] Comparative Example 2

[0047] The comparison example is based on Example 1, with the following difference: the annealing process of the steel plate is different. Specifically, the annealing process of the steel plate obtained after rolling is as follows: the plate is annealed at 1000℃ for 30 minutes, and after annealing, it is water-cooled to room temperature.

[0048] The room temperature mechanical properties of the steel obtained in this comparative example were tested, and the results are as follows: Figure 2 As shown in Table 1, the yield strength is 1510 MPa, the tensile strength is 1551 MPa, and the total elongation is 10.9%.

[0049] As can be seen from the above examples and comparative examples, the austenitic low-density steel Fe-28Mn-11Al-1C-5Ni prepared by the deformation heat treatment process proposed in this invention has an elongation rate increased by 216% compared with Comparative Example 1, without losing too much strength; compared with Comparative Example 2, the strength is increased by 182 MPa, and the elongation rate is also 12.4% higher, achieving a perfect combination of ultra-high strength and high plasticity. This is because no annealing treatment was performed in Comparative Example 1, so no recrystallization occurred in the material obtained in Comparative Example 1. This resulted in a high strength in Comparative Example 1, but a very low elongation rate compared with Example 1; while in Comparative Example 2, although annealing treatment was performed, the annealing temperature was much higher than that in Example 1. This resulted in an excessive volume fraction of recrystallized part in the material obtained in Comparative Example 2, with a large grain size and a very low volume fraction of non-recrystallized matrix, far below 15%. This led to a decrease in both the strength and elongation of the material in Comparative Example 2. The warm rolling process used in this invention enables Fe-Mn-Al-C-Ni austenitic low-density steel to achieve an ultra-high dislocation density. At the same time, it promotes the cross-slip of dislocations during deformation, forming a large number of uniform and fine dislocation cells. The numerous dislocations and dislocation substructures act as non-uniform nucleation sites during the subsequent partial recrystallization annealing process, promoting the refinement of recrystallized grains and achieving ultra-fine recrystallized grains. The ultra-fine grain strengthening, combined with the precipitation strengthening of the B2 phase and the dislocation strengthening in the non-recrystallized region, enables this austenitic low-density steel to achieve an excellent balance between strength and plasticity.

[0050] Example 2

[0051] The method was modified from Example 1, with the following differences: a 20mm steel plate was subjected to multi-pass large deformation rolling at 200°C, with each pass involving approximately 5% deformation, and a holding time of 3-5 minutes between passes. The total rolling reduction was 95%. After rolling, the plate was water-cooled to room temperature. Subsequently, the resulting rolled plate was annealed at 850°C for 10 minutes, and then water-cooled to room temperature after annealing.

[0052] The room temperature mechanical properties of the steel obtained in this embodiment were tested, and the results are shown in Table 1. The yield strength was 1775 MPa, the tensile strength was 1824 MPa, and the total elongation was 11.3%.

[0053] Example 3

[0054] The example is an adjustment based on Example 2, the difference being that the temperature of the multi-pass large deformation rolling is different. In this example, the temperature of the multi-pass large deformation rolling is 300°C.

[0055] The room temperature mechanical properties of the steel obtained in this embodiment were tested, and the results are shown in Table 1. The yield strength was 1581 MPa, the tensile strength was 1769 MPa, and the total elongation was 32.6%.

[0056] In this embodiment, the steel sheet obtained after warm rolling is tested, and the dislocation density of the steel is 7.91 × 10⁻⁶. 16 m -2 The average size of the dislocation cell is 0.20 μm, and the dislocation density is higher than 1.0 × 10⁻⁶. 16 m -2 The average size of dislocation cells is less than 0.35 μm, which has a beneficial effect on the simultaneous improvement of the final strength and plasticity of the steel. Based on this embodiment, the present invention conducted further comparative experiments, changing only the warm rolling temperature: the temperature during multi-pass large deformation rolling of the steel was 450°C. The steel plate obtained after warm rolling was tested, and the dislocation density of the steel was found to be 3.2 × 10⁻⁶. 15 m -2 The average size of dislocation cells was 0.41 μm. This indicates a significant decrease in dislocation density and a significant increase in the average size of dislocation cells in the comparative experiment. This resulted in a significant decrease in the steel's performance even when treated with the same annealing process as in Example 3. The steel obtained in this comparative example, after room temperature mechanical property testing, showed a yield strength of 1326 MPa, a tensile strength of 1451 MPa, and a total elongation of 8.9%. This same situation occurred in the comparative experiment with a warm rolling temperature below 200°C, further demonstrating the influence of warm rolling temperature on the microstructure of steel. It also proves that the method described in this invention can simultaneously improve the strength and elongation of steel.

[0057] Comparative Example 3

[0058] The comparison example is based on Example 3, but differs in that the total rolling reduction is different; the total rolling reduction in this example is 80%.

[0059] The room temperature mechanical properties of the steel obtained in this embodiment were tested, and the results are shown in Table 1. The yield strength was 1452 MPa, the tensile strength was 1521 MPa, and the total elongation was 5.3%.

[0060] Example 4

[0061] The method is based on Example 2, but differs in that the annealing temperature and time are different. In this example, the annealing temperature is 800°C and the annealing time is 30 minutes.

[0062] The room temperature mechanical properties of the steel obtained in this embodiment were tested, and the results are shown in Table 1. The yield strength was 1697 MPa, the tensile strength was 1786 MPa, and the total elongation was 18.5%.

[0063] In conjunction with Examples 2-4 and Comparative Example 3, the total rolling reduction in Comparative Example 3 was insufficient, resulting in a lack of heterogeneous nucleation sites within the rolled material. This led to a significant reduction in the number of ultrafine microstructures formed during recrystallization, ultimately causing a substantial decrease in both the strength and elongation of the final material. In contrast, the material prepared in the embodiments of this invention exhibits a finer dislocation cell structure formed through warm rolling. Furthermore, the numerous dislocations and dislocation substructures obtained during warm rolling provide a large number of heterogeneous nucleation sites for recrystallization during subsequent annealing, resulting in an ultrafine microstructure after recrystallization. Ultimately, this significantly improves both the strength and plasticity of the low-density steel.

[0064] Table 1 Mechanical properties of the embodiments and comparative examples

[0065] Example 1 1573 1733 23.3 Comparative Example 1 1999 2080 5.6 Comparative Example 2 1510 1551 10.9 Example 2 1775 1824 11.3 Example 3 1581 1769 32.6 Comparative Example 3 1452 1521 5.3 Example 4 1697 1786 18.5

[0066] As shown in Table 1, the deformation heat treatment process of large deformation warm rolling followed by high-temperature annealing proposed in this invention enables Fe-28Mn-11Al-1C-5Ni (wt.%) austenitic low-density steel to achieve both ultra-high yield strength and excellent plasticity, significantly improving the strength-plasticity competition relationship in Fe-Mn-Al-C-Ni materials, while also possessing an ultra-high specific strength value. It should be noted that the process principle provided by this invention is applicable to all Fe-Mn-Al-C-Ni austenitic low-density steels.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit the technical solutions. Those skilled in the art should understand that any modifications or equivalent substitutions to the technical solutions of the present invention without departing from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A low-density steel with ultra-high strength, high plasticity, and ultra-fine microstructure, characterized in that, The steel is composed of ultrafine recrystallized austenite grains, intergranular B2 phase, and nano-B2 phase particles precipitated in the unrecrystallized matrix; wherein, the nano-B2 phase particles precipitated in the unrecrystallized matrix are dispersedly distributed in the unrecrystallized matrix; calculated by volume percentage, the volume fraction of the unrecrystallized matrix is ​​15%~25%, the average grain size of the ultrafine recrystallized austenite grains does not exceed 0.5μm, the average size of the intergranular B2 phase does not exceed 0.4μm, and the average size of the dispersed nano-B2 phase particles does not exceed 50nm; The steel comprises the following components by mass percentage: 20-30% Mn, 8-12% Al, 0.8-1.2% C, 4-10% Ni, and unavoidable impurity elements, with the balance being Fe.

2. The ultra-high strength, high plasticity, ultra-fine microstructure, low-density steel according to claim 1, characterized in that, At room temperature, the steel has a yield strength of at least 1500 MPa, a tensile strength of at least 1700 MPa, and a total elongation of at least 10%; the density of the steel is < 6.8 g / cm³. 3 .

3. The ultra-high strength, high plasticity, ultra-fine microstructure, low-density steel according to claim 1, characterized in that, The steel is obtained by sequentially undergoing solution treatment, warm rolling treatment, and annealing treatment.

4. The ultra-high strength, high plasticity, ultra-fine microstructure, low-density steel according to claim 3, characterized in that, The microstructure of steel after solution treatment consists of an austenitic matrix and a B2 phase. The B2 phase is distributed between the austenitic grains, and the volume fraction of the B2 phase distributed between the austenitic grains is 5% to 20% by volume percentage.

5. The ultra-high strength, high plasticity, ultra-fine microstructure, low-density steel according to claim 3, characterized in that, The warm-rolled steel consists of an austenitic matrix and a B2 phase, wherein the dislocation density of the steel is higher than 1.0 × 10¹⁶ m. -2 The average size of dislocation cells is less than 0.35 μm.

6. A method for preparing ultra-high strength, high plasticity, ultra-fine microstructure, low-density steel, characterized in that, The preparation of the ultra-high strength, high plasticity, ultra-fine microstructure, low-density steel according to any one of claims 1 to 5 specifically includes the following steps: Step 1: Perform solution treatment on the steel plate; Step 2: After heat preservation, the steel plate treated in Step 1 is subjected to multiple passes of warm rolling deformation. After the warm rolling is completed, it is water-cooled to room temperature. The deformation amount of each pass is 3~6%, and the heat preservation between each pass is 3~5 minutes. The total deformation amount is controlled at 85%~95%. Step 3: Perform partial recrystallization annealing on the steel plate treated in Step 2.

7. The preparation method according to claim 6, characterized in that, In step 1, the solution temperature is 1100℃~1200℃, and the solution is kept at that temperature for 1~2 hours before being cooled to room temperature by water.

8. The preparation method according to claim 6, characterized in that, In step 2, the steel plate is kept at 200℃~400℃ and subjected to multiple passes of warm rolling deformation at this temperature.

9. The preparation method according to claim 6, characterized in that, In step 3, the annealing temperature is 800℃~950℃, and the annealing time is 3~30min.

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