A 2400MPa grade ultra-high strength steel and its preparation method

Abstract

The present application relates to the field of alloy technology and discloses a 2400MPa grade ultra-high strength steel and a preparation method thereof. The chemical composition of the 2400MPa grade ultra-high strength steel, calculated by mass percentage, includes: C 0.3%~0.5%, Si 1.0%~2.5%, Mn 3.5%~5%, V 0.2%~0.3%, Ni 1%~3%, Nb 0.02%~0.05%, Mo 0.3%~0.5%, Cr 0.5%~2%, and the rest is Fe and unavoidable impurities. The microstructure of the 2400MPa grade ultra-high strength steel includes martensite and austenite, the average grain size of the martensite is 0.4μm to 2.0μm, and the major-minor axis ratio of the martensite grains is less than or equal to 1.6. The present application improves the strength, plasticity and toughness of the 2400MPa grade ultra-high strength steel, making it have high strength, high plasticity and high toughness.

Description

Technical Field

[0001] The present application belongs to the field of alloy technology, and specifically relates to a 2400 MPa grade ultra-high strength steel and a preparation method thereof. Background Art

[0002] Ultra-high-strength steel with a tensile strength exceeding 2400 MPa is a crucial structural material in applications such as aircraft landing gear, engine casings, protective armor, and civilian vehicles. 2400 MPa-grade ultra-high-strength steel ensures both safety during service and lightweighting. Typical high-strength steels include quenching and partitioning steel, dual-phase steel, twin-induced plasticity steel, and phase transformation-induced plasticity steel. However, the inverse relationship between strength and plasticity in metal materials is a major obstacle in the development of ultra-high-strength steel.

[0003] Currently, the best performance of 300M grade ultra-high-strength steel for hot stamping is a tensile strength of 1920 MPa and an elongation of only 9.2%, which is insufficient for aircraft landing gear. Maraging steel, achieved by precipitation strengthening with the addition of more than 10% of expensive alloying elements such as cobalt (Co), tungsten (W), and molybdenum (Mo), achieves tensile strengths as high as 2000 MPa. However, the high density of dislocations within the maraging steel causes rapid necking after yielding, resulting in a uniform plasticity of less than 5%, and long aging times lead to high energy consumption. Low-alloy medium-high carbon steel, through complex manufacturing processes, achieves tensile strengths of 2383 MPa and elongation of 8.8%, but the complex and difficult-to-control preparation process is inefficient, making it difficult to meet industrial production requirements.

[0004] With the development of nano-manufacturing technology, based on the dual-phase structure design, a low volume fraction (10-30%) of metastable austenite is introduced into the ultrafine-grained lath martensite through annealing in the critical zone, thereby obtaining a martensite-austenite dual-phase structure, improving the toughness of martensitic steel, and forming a high-strength steel with a martensite-austenite dual-phase structure with high strength and toughness. However, excessive austenite content or large grain size leads to reduced strength.

[0005] In the related art, a high-yield 2000MPa grade ultra-high strength steel and its preparation method, an ultra-high strength steel with a yield strength greater than 2000MPa and its preparation method, and a low-temperature 2500MPa grade ultra-high strength and high-toughness steel and its preparation method are provided. However, toughness is not taken into consideration and the effect of balancing strength, plasticity and toughness is not achieved.

[0006] In summary, it is difficult for the 2400MPa grade ultra-high strength steel in related technologies to achieve high strength, high plasticity and high toughness. Summary of the Invention

[0007] This application aims to solve at least one of the technical problems existing in the related art.

[0008] Therefore, a first aspect of the present application provides a 2400 MPa grade ultra-high strength steel.

[0009] A second aspect of the present application provides a method for preparing 2400 MPa grade ultra-high strength steel.

[0010] In view of this, according to the first aspect of the embodiment of the present application, a 2400MPa grade ultra-high strength steel is proposed, the chemical composition of which, by mass percentage, comprises: C 0.3%~0.5%, Si 1.0%~2.5%, Mn 3.5%~5%, V 0.2%~0.3%, Ni1%~3%, Nb 0.02%~0.05%, Mo 0.3%~0.5%, Cr 0.5%~2%, and the rest are Fe and unavoidable impurities; the microstructure of the 2400MPa grade ultra-high strength steel includes martensite and austenite, the average grain size of the martensite is 0.4μm to 2.0μm, and the aspect ratio of the martensite grains is less than or equal to 1.6; the volume fraction of the austenite is 5% to 20%; the original austenite of the 2400MPa grade ultra-high strength steel is a heterogeneous structure of banded and equiaxed structures, the grain width of the banded original austenite is 5μm to 10μm, and the grain equivalent circle size of the equiaxed original austenite is 2μm to 10μm.

[0011] In a possible implementation, the mass ratio of the Mo content to the Cr content is 0.4 to 1.

[0012] In one possible implementation, the 2400MPa grade ultra-high strength steel has a yield strength greater than 1800MPa, a tensile strength greater than 2400MPa, a uniform elongation greater than 7%, an elongation after fracture greater than 15%, and a toughness greater than 25J / cm at room temperature. 2 , the toughness at low temperature is greater than 25J / cm 2 ; Wherein, the room temperature includes 25℃, and the low temperature range is -20℃ to -60℃.

[0013] According to the second aspect of the embodiment of the present application, a preparation method of 2400MPa grade ultra-high strength steel is proposed, which is used to prepare the 2400MPa grade ultra-high strength steel as mentioned above, comprising the following steps: preparing cast metal; hot rolling the cast metal and cooling it to obtain hot rolled metal; tempering the hot rolled metal and cooling it to obtain tempered metal; cryogenically treating the tempered metal and returning it to room temperature to obtain cryogenically tempered metal; tempering and cooling the cryogenically tempered metal to obtain the 2400MPa grade Ultra-high strength steel, wherein the microstructure of the 2400MPa grade ultra-high strength steel includes martensite and austenite, the average grain size of the martensite is 0.4μm to 2.0μm, and the aspect ratio of the martensite grains is less than or equal to 1.6; the volume fraction of the austenite is 5% to 20%; the original austenite of the 2400MPa grade ultra-high strength steel is a heterogeneous structure of banded and equiaxed structures, the grain width of the banded original austenite is 5μm to 10μm, and the equivalent circle size of the grains of the equiaxed original austenite is 2μm to 10μm.

[0014] In one possible implementation, the step of preparing the as-cast metal includes: preparing raw materials according to the chemical composition of the aforementioned 2400 MPa grade ultra-high strength steel; smelting and casting the raw materials to form billets or ingots to obtain the as-cast metal.

[0015] In one possible implementation, the cast metal is hot rolled and cooled to obtain a hot-rolled metal, comprising the steps of: heating the cast metal to a fully austenitizing temperature and keeping it warm for 2 to 3 hours; subjecting the cast metal after keeping warm to a first-stage rolling process, wherein the total reduction in the first-stage rolling process is greater than 66%, and the final rolling temperature is greater than 950°C; subjecting the metal after the first-stage rolling to a warming process, wherein the warming process is performed at a temperature of 800 to 950°C; subjecting the metal after the warming process to a second-stage rolling process, wherein the total reduction in the second-stage rolling process is greater than 80%, and the final rolling temperature is greater than 800°C, and the original austenite of the metal after the second-stage rolling process is strip-shaped and equiaxed; cooling the metal after the second-stage rolling process to room temperature in air to obtain the hot-rolled metal, wherein the volume fraction of austenite in the microstructure of the hot-rolled metal is 20% to 40%.

[0016] In a possible implementation, the complete austenitization temperature is 1050°C to 1200°C.

[0017] In one possible implementation, the hot-rolled metal is subjected to a tempering treatment and cooled to obtain the tempered metal, comprising: austenitizing the hot-rolled metal at a temperature of 750°C to 900°C for 60 min to 100 min, so that the band-shaped and equiaxed original austenite is refined so that the grain width of the band-shaped original austenite is 5 μm to 10 μm, and the grain equivalent circle size of the equiaxed original austenite is 2 μm to 10 μm; cooling the austenitized metal to room temperature in air to obtain the tempered metal, wherein the volume fraction of austenite in the microstructure of the tempered metal is 10% to 30%.

[0018] In one possible implementation, the step of subjecting the quenched and tempered metal to cryogenic treatment and returning it to room temperature to obtain the cryogenic quenched and tempered metal includes: placing the quenched and tempered metal in a cryogenic environment at -196°C and cryogenically treating it for 10 to 60 minutes; and returning the cryogenically treated metal to room temperature in air to obtain the cryogenic quenched and tempered metal, wherein the volume fraction of austenite in the microstructure of the cryogenic quenched and tempered metal is 5% to 20%.

[0019] In one possible implementation, the step of subjecting the cryogenically quenched and tempered metal to tempering and partitioning treatment and cooling to obtain 2400 MPa grade ultra-high strength steel includes: subjecting the cryogenically quenched and tempered metal to tempering and partitioning treatment at a temperature of 150°C to 450°C for 10 min to 120 min, and cooling it to room temperature in air to obtain the 2400 MPa grade ultra-high strength steel; wherein the austenite of the 2400 MPa grade ultra-high strength steel is in the form of a film.

[0020] The 2400MPa grade ultra-high strength steel and its preparation method provided in this application can achieve at least the following technical effects:

[0021] In the present application, a 2400 MPa grade ultra-high strength steel has a chemical composition, calculated by mass percentage, comprising C 0.3%-0.5%, Si 1.0%-2.5%, Mn 3.5%-5%, V 0.2%-0.3%, Ni 1%-3%, Nb 0.02%-0.05%, Mo 0.3%-0.5%, Cr 0.5%-2%, and the remainder being Fe and unavoidable impurities. The strength, plasticity, and toughness of the 2400 MPa grade ultra-high strength steel are improved through the synergistic effect of the various chemical components (including their proportions). The original austenite of 2400MPa grade ultra-high strength steel is a heterogeneous structure of banded and equiaxed structures, the grain width of the banded original austenite is 5μm to 10μm, the equivalent circle size of the grain of the equiaxed original austenite is 2μm to 10μm, the volume fraction of austenite is 5% to 20%, and the average grain size of martensite is 0.4 μm to 2.0 μm, and the major-minor axis ratio of the martensite grain is less than or equal to 1.6, thereby activating more slip systems, improving the strength, plasticity and toughness of 2400MPa grade ultra-high strength steel, and making 2400MPa grade ultra-high strength steel have ultra-high strength, high plasticity and high toughness.

[0022] The above general description and the following description are exemplary and explanatory only and are not intended to limit the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] One or more embodiments are exemplarily described by corresponding drawings. These exemplary descriptions and drawings do not limit the embodiments. Elements with the same reference numerals in the drawings are shown as similar elements. The drawings do not constitute a scale limitation. In addition,

[0024] Figure 1 A flow chart of the preparation method provided in the embodiment of the present disclosure;

[0025] Figure 2 This is a tensile engineering stress-strain curve of the ultra-high strength steel obtained in Example 1 of the present disclosure;

[0026] Figure 3 This is a bar graph of the toughness of the ultra-high strength steel obtained in Example 1 of the present disclosure at room temperature and low temperature;

[0027] Figure 4 This is the electron backscatter diffraction (EBSD) phase diagram of the ultra-high strength steel obtained in Example 1 of the present disclosure;

[0028] Figure 5 This is the original austenite reconstruction diagram of the ultra-high strength steel obtained in Example 1 of the present disclosure;

[0029] Figure 6 1 is a weighted distribution histogram of the major-minor axis ratio of the martensite grains of the ultra-high strength steel obtained in Example 1 of the present disclosure. DETAILED DESCRIPTION

[0030] In order to be able to understand the features and technical content of the embodiments of the present disclosure in more detail, the implementation of the embodiments of the present disclosure is described in detail below in conjunction with the accompanying drawings. The accompanying drawings are for reference only and are not used to limit the embodiments of the present disclosure. In the following technical description, for the sake of convenience of explanation, a full understanding of the disclosed embodiments is provided through multiple details. However, one or more embodiments can still be implemented without these details. In other cases, to simplify the drawings, well-known structures and devices can be simplified for display.

[0031] It should be noted that, unless there is any conflict, the embodiments and features in the embodiments of the present disclosure can be combined with each other.

[0032] It should be noted that in the description of the embodiments of the present disclosure, "ultra-high-strength steel" is an abbreviation of "2400 MPa grade ultra-high-strength steel", which will not be repeated below.

[0033] According to a first aspect of an embodiment of the present application, a 2400 MPa-grade ultra-high-strength steel is provided, the chemical composition of which, by mass percentage, comprises: C 0.3% to 0.5%, Si 1.0% to 2.5%, Mn 3.5% to 5%, V 0.2% to 0.3%, Ni 1% to 3%, Nb 0.02% to 0.05%, Mo 0.3% to 0.5%, Cr 0.5% to 2%, with the remainder being Fe and unavoidable impurities. The microstructure of the ultra-high-strength steel comprises martensite and austenite, the average grain size of the martensite being 0.4 μm to 2.0 μm, and the aspect ratio of the martensite grains being less than or equal to 1.6. The volume fraction of austenite is 5% to 20%. The original austenite of ultra-high strength steel is a heterogeneous structure of banded and equiaxed structures. The grain width of the banded original austenite is 5μm to 10μm, and the equivalent circle size of the grain of the equiaxed original austenite is 2μm to 10μm.

[0034] In this embodiment, the synergistic effects of various chemical components and their proportions enhance the strength, ductility, and toughness of the ultra-high-strength steel, resulting in ultra-high-strength steel that combines high strength, high ductility, and high toughness. This also reduces the use of expensive metal elements (for example, Mo 0.3%-0.5%), effectively lowering costs. Compared to maraging steel and nano-bainite steel, ultra-high-strength steel does not contain expensive alloying elements such as Co and W, and its carbon content is relatively low, resulting in excellent weldability.

[0035] Specifically, C (0.3%-0.5%) provides solid solution strengthening and sufficient carbon (C) for tempering and partitioning, stabilizing austenite. Si (1.0%-2.5%) inhibits carbide formation during tempering and partitioning, preventing or reducing austenite decomposition and achieving an appropriate volume fraction of stable austenite. Mn (3.5%-5%) stabilizes austenite and expands the austenite phase, increasing the material's hardenability. After hot rolling and cooling of the cast metal, an appropriate amount of austenite remains at room temperature. V (0.2%-0.3%) refines the original austenite grains and forms dispersed nano-precipitates, increasing the material's strength. Niobium (Nb) (0.02%-0.05%) refines the grains and enhances toughness through precipitation strengthening. Nickel (Ni) (1%-3%) stabilizes the austenite structure and reduces the ductile-brittle transition temperature. Niobium (Nb) 0.02%-0.05% and nickel (Ni) 1%-3% jointly enhance the toughness of 2400MPa grade ultra-high strength steel at room temperature, low temperature, and high temperature environments. Mo 0.3%-0.5% and Cr 0.5%-2% optimize strength and plasticity.

[0036] In this embodiment, the original austenite of the ultra-high strength steel is a heterogeneous structure of banded and equiaxed structures, the grain width of the banded original austenite is 5μm to 10μm, the grain equivalent circle size of the equiaxed original austenite is 2μm to 10μm, the volume fraction of austenite is 5% to 20%, and the average grain size of the martensite is 0.4μm to 2.0μm, and the major-minor axis ratio of the martensite grains is less than or equal to 1.6, thereby activating more slip systems, improving the strength, plasticity and toughness of the 2400MPa grade ultra-high strength steel, and making the 2400MPa grade ultra-high strength steel have ultra-high strength, high plasticity and high toughness. In the description of the embodiment of the present disclosure, the average grain size refers to the average value of the equivalent circle size of the grains, which will not be repeated below.

[0037] Specifically, the ultra-high-strength steel's prior austenite is structured into a heterogeneous structure of banded and equiaxed structures, with the banded prior austenite grain width ranging from 5 to 10 μm and the equiaxed prior austenite grain equivalent circle size ranging from 2 to 10 μm. This allows the volume fraction of austenite in the quenched and tempered metal microstructure to be 10 to 30%. Furthermore, the quenched and tempered metal is cryogenically treated, allowing the volume fraction of austenite in the cryogenically tempered metal microstructure to be 5 to 20%. Finally, the cryogenically quenched and tempered metal is tempered and partitioned to achieve a volume fraction of austenite in the microstructure of the ultra-high-strength steel of 5% to 20%, thereby achieving an average grain size of martensite of 0.4 μm to 2.0 μm, and a major-minor axis ratio of the martensite grains of less than or equal to 1.6, thereby increasing the number of slip systems and enabling the ultra-high-strength steel to exhibit good coordinated deformation capability and sufficient transformation-induced plasticity (TRIP) effect, ultimately enabling the ultra-high-strength steel to obtain the mechanical properties of ultra-high strength, high plasticity and high toughness.

[0038] Specifically, the ultra-high strength steel has a yield strength greater than 1800 MPa, a tensile strength greater than 2400 MPa, a uniform elongation greater than 7%, an elongation after fracture greater than 15%, and a toughness (V-shaped impact notch) greater than 25 J / cm at room temperature. 2 , the toughness at low temperature (V-shaped impact mouth) is greater than 25J / cm 2 .

[0039] This embodiment can be applied to improve the composition of 300M steel in related art. By using Mn+ microalloying alloying elements and low-cost manganese (Mn), the austenitization temperature is lowered and the austenite phase is broadened. Furthermore, the precipitation of V and Cr during hot rolling and quenching and tempering processes plays a role in precipitation and grain refinement, thereby improving the strength, plasticity, and toughness of ultra-high-strength steel.

[0040] In one possible implementation, in the microstructure of ultra-high strength steel, austenite is in the form of a film (e.g. Figure 4 shown).

[0041] In one possible implementation, a 2400 MPa-grade ultra-high-strength steel is provided. Its chemical composition, measured by mass percentage, includes: C 0.3%–0.5%, Si 1.0%–2.5%, Mn 3.5%–5%, V 0.2%–0.3%, Ni 1%–3%, Nb 0.02%–0.05%, Mo 0.3%–0.5%, Cr 0.5%–2%, P ≤ 0.015%, S ≤ 0.008%, N ≤ 0.004%, with the remainder being Fe and unavoidable impurities. The ultra-high-strength steel has a microstructure consisting of martensite and austenite. The average martensite grain size is 0.4–2.0 μm, and the martensite grain aspect ratio is less than or equal to 1.6. The volume fraction of austenite is 5–20%. The original austenite in ultra-high-strength steel is a heterogeneous structure of banded and equiaxed structures. The grain width of the banded original austenite is 5μm to 10μm, and the equivalent circle size of the equiaxed original austenite grain is 2μm to 10μm. Optimizing the composition of ultra-high-strength steel can improve its strength, plasticity, and toughness.

[0042] In some embodiments, the mass ratio of the Mo content to the Cr content, Mo / Cr, is 0.4 to 1.

[0043] Based on Mo 0.3%~0.5% and Cr 0.5%~2%, the mass ratio of Mo content to Cr content Mo / Cr is 0.4 to 1, which promotes precipitation while adding more precipitated phases, optimizing strength and plasticity.

[0044] In a possible implementation manner, the mass ratio of the Mo content to the Cr content, Mo / Cr, is 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, or other values ​​between 0.4 and 1.

[0045] In some embodiments, the yield strength of 2400 MPa grade ultra-high strength steel is greater than 1800 MPa, the tensile strength is greater than 2400 MPa, the uniform elongation is greater than 7%, the elongation after fracture is greater than 15%, and the toughness at room temperature is greater than 25 J / cm 2 , the toughness at low temperature is greater than 25J / cm 2 ; Among them, room temperature includes 25℃, and low temperature ranges from -20℃ to -60℃.

[0046] In this embodiment, the ultra-high strength steel has good mechanical properties, which makes the material have good forming performance and the toughness does not decrease significantly in low temperature environment. It can be used in shield machines, aircraft landing gear, armored steel, engineering machinery and military high-strength fields with high strength and toughness requirements.

[0047] Combine Figure 1As shown, according to the second aspect of the embodiment of the present application, a method for preparing 2400 MPa grade ultra-high strength steel is provided, which is used to prepare the aforementioned 2400 MPa grade ultra-high strength steel, comprising the following steps.

[0048] S11. Prepare cast metal.

[0049] In some embodiments, the step of preparing the as-cast metal includes: preparing raw materials according to the chemical composition (including proportion) of the ultra-high strength steel as described above, smelting the raw materials, and casting them into billets or ingots to obtain the as-cast metal.

[0050] The chemical composition (including proportion) of the ultra-high strength steel is as described in the above embodiments and will not be repeated here. The raw materials are smelted and cast to form billets or ingots to obtain cast metal, which provides a basis for subsequent hot rolling.

[0051] Compared to maraging steel and nano-bainite steel, ultra-high-strength steel does not contain expensive alloying elements such as Co and W, and has a low carbon content, resulting in excellent weldability. Furthermore, the material has low raw material costs, a simple preparation process, low energy consumption, and is environmentally friendly.

[0052] S12, hot rolling the cast metal and cooling it to obtain hot-rolled metal.

[0053] The cast metal is hot-rolled to form a strip-shaped and equiaxed austenite. The hot-rolled metal is then cooled to obtain a hot-rolled metal having a microstructure comprising austenite and martensite, wherein the volume fraction of austenite is 20% to 40% and the volume fraction of martensite is 60% to 80%.

[0054] In some embodiments, the cast metal is hot rolled and cooled to obtain the hot-rolled metal, which comprises the following steps: heating the cast metal to a fully austenitizing temperature and keeping it warm for 2 to 3 hours. The cast metal after keeping warm is subjected to a first-stage rolling, wherein the total reduction of the first-stage rolling is greater than 66%, and the final rolling temperature is greater than 950°C. The metal after the first-stage rolling is subjected to a warming temperature of 800°C to 950°C. The metal after the warming temperature is subjected to a second-stage rolling, wherein the total reduction of the second-stage rolling is greater than 80%, and the final rolling temperature is greater than 800°C. The original austenite of the metal after the second-stage rolling is strip-shaped and equiaxed. The metal after the second-stage rolling is cooled to room temperature in air to obtain a hot-rolled metal, wherein the microstructure of the hot-rolled metal includes austenite and martensite, wherein the volume fraction of austenite is 20% to 40%, and the volume fraction of martensite is 60% to 80%.

[0055] The as-cast metal is heated to the complete austenitizing temperature and kept at that temperature for 2 to 3 hours to prepare for the first stage rolling, the warming and the second stage rolling.

[0056] The total reduction of the first stage rolling is greater than 66%, the final rolling temperature is greater than 950℃, and the waiting temperature is 800℃ to 950℃. The total reduction of the second stage rolling is greater than 80%, and the final rolling temperature is greater than 800℃. This makes the original austenite of the metal after the second stage rolling into a banded and equiaxed shape. In other words, the original austenite in the microstructure of the metal after the second stage rolling is a small banded and equiaxed heterogeneous structure, which prepares the structure for the subsequent formation of martensite and austenite, and improves the strength, plasticity and toughness of the 2400MPa grade ultra-high strength steel. Among them, the temperature during the first stage rolling, waiting for warming and second stage rolling is maintained above 800℃.

[0057] The hot-rolled metal is obtained by cooling the metal after the second stage rolling to room temperature in air. The microstructure of the hot-rolled metal includes austenite and martensite, wherein the volume fraction of austenite is 20% to 40% and the volume fraction of martensite is 60% to 80%, preparing for subsequent modulation treatment.

[0058] In some embodiments, the full austenitizing temperature is 1050°C to 1200°C.

[0059] S13, subjecting the hot rolled metal to a tempering treatment and cooling the hot rolled metal to obtain a tempered metal.

[0060] The hot rolled metal is subjected to a tempering treatment, that is, after rolling, the tempering treatment is performed in the fully austenitized zone, so that the grain size of the original austenite is refined, and then the martensite grains are refined, so as to obtain a tempered metal after cooling, and the tempered metal includes austenite and martensite, wherein the volume fraction of austenite is 10% to 30%, and the volume fraction of martensite is 70% to 90%.

[0061] In some embodiments, the step of subjecting the hot-rolled metal to a tempering treatment and cooling to obtain the tempered metal includes: austenitizing the hot-rolled metal at a temperature of 750° C. to 900° C. for 60 to 100 minutes to refine the banded and equiaxed prior austenite, such that the grain width of the banded prior austenite is 5 μm to 10 μm, and the grain equivalent circle size of the equiaxed prior austenite is 2 μm to 10 μm. The austenitized metal is cooled in air to room temperature to obtain the tempered metal, wherein the tempered metal includes austenite and martensite, wherein the volume fraction of the austenite is 10% to 30%, and the volume fraction of the martensite is 70% to 90%.

[0062] The hot-rolled metal is subjected to an austenitizing treatment at a temperature of 750°C to 900°C for 60 to 100 minutes to refine the original austenite grains, reduce the grain width of the banded original austenite to 5 to 10 μm, and reduce the equivalent circle size of the equiaxed original austenite grains to 2 to 10 μm, so as to obtain a quenched and tempered metal after cooling. The volume fraction of austenite in the quenched and tempered metal is 10 to 30%, and the volume fraction of martensite is 70 to 90%.

[0063] S14, subjecting the quenched and tempered metal to cryogenic treatment and returning it to room temperature to obtain cryogenic quenched and tempered metal.

[0064] The structure of the original austenite is regulated by subjecting the quenched and tempered metal to deep cryogenic treatment, so that the content of austenite can be regulated after it returns to room temperature.

[0065] In some embodiments, the step of subjecting the quenched and tempered metal to cryogenic treatment and returning it to room temperature to obtain the cryogenically tempered metal includes placing the quenched and tempered metal in a cryogenic environment at -196°C for 10 to 60 minutes. The cryogenically treated metal is returned to room temperature in air to obtain the cryogenically tempered metal. The cryogenically tempered metal has a microstructure comprising austenite and martensite, wherein the volume fraction of austenite is 5% to 20%, and the volume fraction of martensite is 80% to 95%.

[0066] By placing the tempered metal in a cryogenic environment of -196°C and performing cryogenic treatment for 10 to 60 minutes, the structure of the original austenite can be regulated, so that the content of austenite can be regulated after the cryogenically treated metal is restored to room temperature in the air.

[0067] S15. The deep-cold quenched and tempered metal is subjected to tempering and partitioning treatment and cooled to obtain 2400 MPa grade ultra-high strength steel.

[0068] The microstructure of the ultra-high-strength steel consists of martensite and austenite. The average grain size of the martensite is 0.4 to 2.0 μm, and the aspect ratio of the martensite grains is less than or equal to 1.6. The volume fraction of the austenite is 5 to 20%. The original austenite of the ultra-high-strength steel is a heterogeneous structure of banded and equiaxed grains. The grain width of the banded original austenite is 5 to 10 μm, and the equivalent circle size of the grains of the equiaxed original austenite is 2 to 10 μm.

[0069] By combining cryogenic treatment of the quenched and tempered metal with tempering and partitioning treatment, the stability and content of austenite in the 2400MPa grade ultra-high strength steel are regulated to achieve a film-like austenite with a volume fraction of 5% to 20%, and a volume fraction of martensite of 80% to 95%. The average grain size of the martensite is 0.4 μm to 2.0 μm, and the major-minor axis ratio of the martensite is less than or equal to 1.6, so that there are more slip systems and the characteristics of the metastable retained austenite, including the volume fraction and stability, are controlled, so that it exhibits good coordinated deformation ability and sufficient phase transformation induced plasticity (TRIP) effect, ultimately enabling the 2400MPa grade ultra-high strength steel to obtain ultra-high strength, high plasticity and high toughness mechanical properties.

[0070] In some embodiments, the step of subjecting the cryogenically quenched and tempered metal to a tempering and partitioning treatment and cooling to obtain a 2400 MPa grade ultra-high strength steel comprises: subjecting the cryogenically quenched and tempered metal to a tempering and partitioning treatment at a temperature of 150° C. to 450° C. for 10 to 120 minutes, and cooling in air to room temperature to obtain the 2400 MPa grade ultra-high strength steel. The austenite of the ultra-high strength steel is in the form of a film.

[0071] The 2400MPa-grade ultra-high-strength steel is obtained by subjecting the deep-cryogenically quenched and tempered metal to a temperature of 150°C to 450°C, followed by a tempering and partitioning treatment for 10 to 120 minutes, and cooling to room temperature in air. The stability and content of austenite in the 2400MPa-grade ultra-high-strength steel are controlled. By ensuring that the austenite in the 2400MPa-grade ultra-high-strength steel is film-like with a volume fraction of 5% to 20%, the volume fraction of martensite is 80% to 95%, the average grain size of the martensite is 0.4 μm to 2.0 μm, and the major-minor axis ratio of the martensite is less than or equal to 1.6, increasing the number of slip systems and controlling the characteristics of the metastable retained austenite, including its volume fraction and stability, resulting in good coordinated deformation capability and sufficient transformation-induced plasticity (TRIP) effect. Ultimately, the 2400MPa-grade ultra-high-strength steel achieves the mechanical properties of ultra-high strength, high plasticity, and high toughness.

[0072] Since the deformation capacity of martensite in ultra-high strength steel is limited, the plasticity of ultra-high strength steel is related to martensite grains and austenite. Therefore, this embodiment improves the plasticity of the material by regulating austenite and martensite grains.

[0073] In this embodiment, the chemical composition (including proportions) of the 2400 MPa-grade ultra-high-strength steel is configured to reduce costs, simplify processing, and enhance the mechanical properties of the 2400 MPa-grade ultra-high-strength steel. Specifically, the chemical composition (including proportions) is adjusted to achieve a volume fraction of austenite (metastable austenite) of 20% to 40% in the microstructure of the hot-rolled metal after hot rolling and cooling. Furthermore, the original austenite grain size is refined during the tempering process, with the grain width of the banded original austenite being 5μm to 10μm and the equivalent circle size of the equiaxed original austenite grain being 2μm to 10μm. Furthermore, after cryogenic treatment (with liquid nitrogen), unstable austenite is screened, and after tempering and partitioning, 5% to 20% of retained austenite with good stability is obtained.

[0074] This embodiment also takes into account that in order to improve the strength of 2400MPa-grade ultra-high-strength steel, the chemical composition C is set to 0.3%-0.5% and Mn is set to 3.5%-5%. This easily leads to a large volume fraction of retained austenite in the room-temperature structure after hot working. Excessive volume fraction of retained austenite will reduce the yield strength. At the same time, if the stability is not properly controlled, a large amount of metastable austenite will transform into ultra-hard martensite, which will lead to brittle fracture behavior of the material and fail to achieve high strength and high plasticity. Therefore, this embodiment refines the grain size through tempering treatment, so that the grain width of the banded original austenite is 5μm to 10μm, and the equivalent circle size of the grain of the equiaxed original austenite is 2μm to 10μm. Then, through deep cryogenic treatment and tempering and partitioning treatment processes, the characteristics of the metastable retained austenite, including volume fraction and stability, are controlled, so that it exhibits good coordinated deformation ability and sufficient phase transformation induced plasticity (TRIP) effect, ultimately enabling the material to obtain ultra-high strength, high plasticity and high toughness mechanical properties.

[0075] Furthermore, this embodiment hot-rolls the cast metal, controlling the deformation temperature and deformation amount, so that the original austenite has a heterogeneous structure of banded and equiaxed structures. After hot rolling, the hot-rolled metal is tempered in the fully austenitized range to refine the grain size of the original austenite, thereby refining the martensite grains, so that the grain width of the banded original austenite is 5μm to 10μm, and the equivalent circle size of the grain of the equiaxed original austenite is 2μm to 10μm. By subjecting the tempered metal to cryogenic treatment and tempering and partitioning the cryogenically tempered metal, the stability and content of the austenite are regulated, so that the austenite of the 2400MPa grade ultra-high strength steel is thin-film with a volume fraction of 5% to 20%. Through the coordination of the above steps, the average grain size of the martensite in the 2400MPa-grade ultra-high-strength steel is made from 0.4 μm to 2.0 μm, and the aspect ratio of the martensite grains is less than or equal to 1.6, which increases the number of slip systems. This is the key to improving the strength, plasticity and toughness of the 2400MPa-grade ultra-high-strength steel, and enables the 2400MPa-grade ultra-high-strength steel to exhibit good coordinated deformation ability and sufficient phase transformation induced plasticity (TRIP) effect, ultimately giving the material the mechanical properties of ultra-high strength, high plasticity and high toughness.

[0076] That is to say, by hot rolling the as-cast metal and controlling the deformation temperature and deformation amount, the original austenite is made into a heterogeneous structure of banded and equiaxed structures. After hot rolling, the hot-rolled metal is tempered in the fully austenitized range to refine the grain size of the original austenite, thereby refining the martensite grains, so that the grain width of the banded original austenite is 5μm to 10μm, and the equivalent circle size of the grains of the equiaxed original austenite is 2μm to 10μm. By subjecting the tempered metal to deep cryogenic treatment, the less stable retained austenite is transformed into martensite, which not only avoids premature yielding of the austenite but also increases the strength of the hard phase martensite. The deep cryogenic tempered metal is then subjected to tempering and partitioning treatment, so that the C and Mn elements in the martensite are partitioned into the austenite, and the microstructure is a fine lamellar martensite matrix and a film-like austenite structure. Among them, the fine lath-shaped martensite has a high dislocation density, and the precipitation of V can effectively improve the strength of the material. The thin film-like austenite has a small grain size and a low dislocation density within the grains, which can effectively improve strength and plasticity. During the deformation process, the strength can be further improved while retaining the original plasticity and toughness, thereby achieving the purpose of high strength and toughness. Therefore, compared with maraging steel, the 2400MPa-grade ultra-high-strength steel in this embodiment, which combines ultra-high strength, high plasticity, and high toughness, can further improve strength and elongation while achieving low cost, thereby realizing the manufacturing and engineering application of a new generation of ultra-high-strength and ultra-high-strength steel.

[0077] The preparation method of 2400MPa-grade ultra-high strength steel in this embodiment has a simple process and low cost, and can effectively improve the strength, plasticity and toughness of 2400MPa-grade ultra-high strength steel, so that 2400MPa-grade ultra-high strength steel has ultra-high strength, high plasticity and high toughness.

[0078] Example 1

[0079] A 2400 MPa grade ultra-high strength steel, whose chemical composition, calculated by mass percentage, includes: C 0.39%, Si 1.61%, Mn 4.0%, V 0.2%, Ni 1.92%, Nb 0.04%, Mo 0.37%, Cr 0.8%, P ≤ 0.015%, S ≤ 0.008%, N ≤ 0.004%, and the remainder is Fe and unavoidable impurities.

[0080] Combine Figure 1 As shown, a method for preparing 2400MPa grade ultra-high strength steel comprises the following steps:

[0081] Raw materials are prepared according to the chemical composition (including the proportion) of the aforementioned 2400 MPa grade ultra-high strength steel. The raw materials are smelted and cast into billets or ingots to obtain as-cast metal.

[0082] The cast metal is heated to 1100°C to 1200°C and kept warm for 2 to 3 hours. Then the first stage of rolling is carried out, with the total reduction of the first stage rolling being greater than 66%, the final rolling thickness being 20mm, and the final rolling temperature being greater than 950°C. The metal after the first stage of rolling is allowed to warm at a temperature of 950°C. The warmed metal is subjected to the second stage of rolling, with the total reduction of the second stage rolling being greater than 80%, the final rolling thickness being 4mm, and the final rolling temperature being maintained above 900°C. The metal after the second stage of rolling is cooled to room temperature in air to obtain hot-rolled metal. In the microstructure of the hot-rolled metal, the volume fraction of austenite is 24%.

[0083] The hot rolled metal was austenitized at 820°C for 80 minutes to refine the original austenite grains. The austenitized metal was cooled to room temperature in air to obtain a quenched and tempered metal.

[0084] The quenched and tempered metal is placed in liquid nitrogen (-196°C cryogenic environment) for 10 minutes. The cryogenically treated metal is then returned to room temperature in air to obtain cryogenically tempered metal.

[0085] The cryogenically quenched and tempered metal was subjected to tempering and partitioning treatment at 200°C for 60 minutes and then cooled to room temperature in air to obtain 2400MPa grade ultra-high strength steel. The volume fraction of austenite (metastable austenite) in 2400MPa grade ultra-high strength steel is 5.4%. It is understood that there is no limitation on the method of obtaining the volume fraction of austenite, as long as the volume fraction of austenite can be obtained. For example, it can be based on Figure 4 The electron backscatter diffraction (EBSD) phase diagram is shown, and the volume fraction of austenite is obtained using XRD energy spectrum.

[0086] Combine Figure 4 As shown, Figure 4 The green color in the figure represents the (101) grain orientation, the red color represents the (001) grain orientation, and the blue color represents the (111) grain orientation. Figure 4 It can be seen that the fine lath-like martensite and film-like austenite exhibit good coordinated deformation ability and sufficient transformation induced plasticity (TRIP) effect, which enables the 2400MPa grade ultra-high strength steel to obtain ultra-high strength, high plasticity and high toughness mechanical properties.

[0087] Combine Figure 5 As shown in the original austenite reconstruction diagram of ultra-high strength steel, the original austenite is a band-shaped and equiaxed heterogeneous structure, wherein the grain width of the band-shaped original austenite is 6.6μm to 8.0μm ( Figure 5 The figure shows the original austenite with a grain width of 6.6 μm. The equivalent circle size of the grains of the equiaxed original austenite is 2.9 μm to 8.1 μm ( Figure 5 The figure shows the equiaxed original austenite with equivalent circle grain sizes of 2.9μm, 4.2μm and 8.1μm respectively). Figure 5 It can be used to illustrate the structure of the hot-rolled metal after austenitizing for 80 minutes in this embodiment to achieve grain refinement.

[0088] In this embodiment, the average grain size of the martensite in the 2400MPa grade ultra-high strength steel is 0.76μm, and the aspect ratio of the martensite grains is 1.53, which activates more slip systems, making the 2400MPa grade ultra-high strength steel have the mechanical properties of ultra-high strength, high plasticity and high toughness. It is understood that the method of obtaining the average grain size of the martensite and the aspect ratio of the martensite grains is not limited. For example, it can be combined with Figure 6 Based on the area-weighted distribution histogram of the martensite grain length-short axis ratio, the average grain size of martensite and the martensite grain length-short axis ratio are obtained.

[0089] That is, in this embodiment, the microstructure of the ultra-high-strength steel comprises martensite and austenite. The average grain size of the martensite is 0.76 μm, and the aspect ratio of the martensite grains is less than or equal to 1.53. The volume fraction of the austenite is 5.4%. The prior austenite of the ultra-high-strength steel is a heterogeneous structure consisting of banded and equiaxed structures. The grain width of the banded prior austenite ranges from 6.6 μm to 8.0 μm, and the equivalent circle size of the grains of the equiaxed prior austenite ranges from 2.9 μm to 8.1 μm.

[0090] The mechanical properties of the 2400MPa ultra-high strength steel prepared in this embodiment are as follows: yield strength of 1960MPa, tensile strength of 2402MPa, elongation after fracture of 16.84%, and uniform elongation of 7.7%. Figure 3 The V-shaped impact toughness at room temperature of 25°C is 29.13 J / cm 2 The toughness at -20℃ (V-shaped impact mouth) is 28.98 J / cm 2 The toughness at -40℃ (V-shaped impact mouth) is 28.24 J / cm 2 The toughness at -60℃ (V-shaped impact mouth) is 26.71J / cm 2 It is understood that there is no limitation on the method of obtaining the yield strength, tensile strength, elongation after fracture and uniform elongation. For example, Figure 2 The tensile engineering stress-strain curve is obtained.

[0091] Example 2

[0092] A 2400 MPa grade ultra-high strength steel, whose chemical composition, calculated by mass percentage, includes: C 0.35%, Si 1.11%, Mn 3.6%, V 0.25%, Ni 2.2%, Nb 0.03%, Mo 0.41%, Cr 0.8%, P ≤ 0.015%, S ≤ 0.008%, N ≤ 0.004%, and the remainder is Fe and unavoidable impurities.

[0093] A method for preparing 2400 MPa grade ultra-high strength steel comprises the following steps:

[0094] According to the chemical composition of the above-mentioned 2400 MPa grade ultra-high strength steel, raw materials are prepared, smelted and cast into billets or ingots to obtain as-cast metal.

[0095] The cast metal is heated to 1100°C to 1200°C and kept warm for 2 to 3 hours. Then the first stage of rolling is carried out, with the total reduction of the first stage rolling being greater than 66%, the final rolling thickness being 20mm, and the final rolling temperature being greater than 950°C. The metal after the first stage of rolling is held at a temperature of 850°C. The held-at-temperature metal is subjected to the second stage of rolling, with the total reduction of the second stage rolling being greater than 80%, the final rolling thickness being 4mm, and the final rolling temperature being maintained above 800°C. The metal after the second stage of rolling is cooled to room temperature in air to obtain hot-rolled metal. In the microstructure of the hot-rolled metal, the volume fraction of austenite is 20% to 40%.

[0096] The hot-rolled metal is austenitized at a temperature of 750°C to 900°C for 60 to 100 minutes to refine the original austenite grains. The austenitized metal is then cooled in air to room temperature to obtain a quenched and tempered metal.

[0097] The quenched and tempered metal is placed in liquid nitrogen (cryogenic environment of -196°C) for 30 minutes. The cryogenically treated metal is returned to room temperature in air to obtain a cryogenically tempered metal. The volume fraction of austenite in the cryogenically tempered metal microstructure is 5% to 20%.

[0098] The deep-cold quenched and tempered metal was subjected to tempering and partitioning treatment at 350°C for 20 minutes and then cooled to room temperature in air to obtain 2400MPa grade ultra-high strength steel.

[0099] In this embodiment, the microstructure of the ultra-high-strength steel includes martensite and austenite. The average grain size of the martensite is 0.4 to 2.0 μm, and the aspect ratio of the martensite grains is less than or equal to 1.6. The volume fraction of the austenite is 5% to 20%. The prior austenite of the ultra-high-strength steel is a heterogeneous structure consisting of banded and equiaxed grains. The grain width of the banded prior austenite is 5 to 10 μm, and the equivalent circle size of the grains of the equiaxed prior austenite is 2 to 10 μm.

[0100] The mechanical properties of the 2400 MPa ultra-high strength steel prepared in this embodiment are as follows: yield strength of 1860 MPa, tensile strength of 2411 MPa, elongation after fracture of 16.01%, uniform elongation of 8.10%, and toughness (V-type impact notch) of 28.57 J / cm at room temperature of 25°C. 2 The toughness at -60℃ (V-shaped impact mouth) is 25.87 J / cm 2 .

[0101] Example 3

[0102] A 2400 MPa grade ultra-high strength steel, whose chemical composition, calculated by mass percentage, includes: C 0.5%, Si 2.49%, Mn 4.8%, V 0.3%, Ni 3.0%, Nb 0.05%, Mo 0.3%, Cr 0.75%, P ≤ 0.015%, S ≤ 0.008%, N ≤ 0.004%, and the remainder is Fe and unavoidable impurities.

[0103] A method for preparing 2400 MPa grade ultra-high strength steel comprises the following steps:

[0104] According to the chemical composition of the above-mentioned 2400 MPa grade ultra-high strength steel, raw materials are prepared, smelted and cast into billets or ingots to obtain as-cast metal.

[0105] The cast metal is heated to 1100°C to 1200°C and kept warm for 2 to 3 hours. Then the first stage of rolling is carried out, with the total reduction of the first stage rolling being greater than 66%, the final rolling thickness being 20mm, and the final rolling temperature being greater than 950°C. The metal after the first stage of rolling is held at a temperature of 900°C. The held-at-temperature metal is subjected to the second stage of rolling, with the total reduction of the second stage rolling being greater than 80%, the final rolling thickness being 4mm, and the final rolling temperature being maintained above 850°C. The metal after the second stage of rolling is cooled to room temperature in air to obtain hot-rolled metal. In the microstructure of the hot-rolled metal, the volume fraction of austenite is 20% to 40%.

[0106] The hot-rolled metal is austenitized at a temperature of 750°C to 900°C for 60 to 100 minutes to refine the original austenite grains. The austenitized metal is then cooled in air to room temperature to obtain a quenched and tempered metal.

[0107] The quenched and tempered metal is placed in liquid nitrogen (cryogenic environment of -196°C) for 60 minutes. The cryogenically treated metal is returned to room temperature in air to obtain a cryogenically tempered metal. The volume fraction of austenite in the cryogenically tempered metal microstructure is 5% to 20%.

[0108] The deep-cold quenched and tempered metal was subjected to tempering and partitioning treatment at 150°C for 120 minutes and then cooled to room temperature in air to obtain 2400MPa grade ultra-high strength steel.

[0109] In this embodiment, the microstructure of the ultra-high-strength steel includes martensite and austenite. The average grain size of the martensite is 0.4 to 2.0 μm, and the aspect ratio of the martensite grains is less than or equal to 1.6. The volume fraction of the austenite is 5% to 20%. The prior austenite of the ultra-high-strength steel is a heterogeneous structure consisting of banded and equiaxed grains. The grain width of the banded prior austenite is 5 to 10 μm, and the equivalent circle size of the grains of the equiaxed prior austenite is 2 to 10 μm.

[0110] The mechanical properties of the 2400 MPa grade ultra-high strength steel prepared in this embodiment are as follows: yield strength of 1842 MPa, tensile strength of 2446 MPa, elongation after fracture of 16.87%, uniform elongation of 8.20%, and toughness of 28.69 J / cm at room temperature of 25°C. 2 , the toughness at -60℃ is 25.52 J / cm 2 .

[0111] The above are merely preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, and improvements made within the spirit and principles of the present application shall be included within the scope of protection of the present application. The above are merely preferred embodiments of the present application. It should be noted that those skilled in the art may make various improvements and variations without departing from the technical principles of the present application, and such improvements and variations shall also be considered within the scope of protection of the present application.

Claims

1. A 2400MPa grade ultra-high strength steel, characterized in that: The chemical composition is expressed in percentage by mass and includes: C 0.3%~0.5%, Si 1.0%~2.5%, Mn 3.5%~5%, V 0.2%~0.3%, Ni 1%~3%, Nb 0.02%~0.05%, Mo 0.3%~0.5%, Cr 0.5%~2%, the rest is Fe and unavoidable impurities; The microstructure of the 2400 MPa grade ultra-high strength steel includes martensite and austenite, the average grain size of the martensite is 0.4 μm to 2.0 μm, and the aspect ratio of the martensite grains is less than or equal to 1.6; the volume fraction of the austenite is 5% to 20%; The original austenite of the 2400 MPa grade ultra-high strength steel is a heterogeneous structure of banded and equiaxed structures. The grain width of the banded original austenite is 5 μm to 10 μm, and the grain equivalent circle size of the equiaxed original austenite is 2 μm to 10 μm.

2. The 2400 MPa grade ultra-high strength steel according to claim 1, characterized in that: The mass ratio of the Mo content to the Cr content is 0.4 to 1.

3. The 2400 MPa grade ultra-high strength steel according to claim 1, characterized in that: The 2400MPa grade ultra-high strength steel has a yield strength greater than 1800MPa, a tensile strength greater than 2400MPa, a uniform elongation greater than 7%, an elongation after fracture greater than 15%, and a toughness greater than 25J / cm at room temperature. 2 , the toughness at low temperature is greater than 25J / cm 2 ; The room temperature includes 25°C, and the low temperature ranges from -20°C to -60°C.

4. A method for preparing 2400 MPa grade ultra-high strength steel, for preparing the 2400 MPa grade ultra-high strength steel according to any one of claims 1 to 3, characterized in that: The steps include: preparing the as-cast metal; hot-rolling the cast metal and cooling it to obtain a hot-rolled metal; subjecting the hot-rolled metal to a tempering treatment and cooling the hot-rolled metal to obtain a tempered metal; Submerging the quenched and tempered metal and returning it to room temperature to obtain submerged quenched and tempered metal; The deep-cold quenched and tempered metal is subjected to tempering and partitioning treatment and cooled to obtain the 2400 MPa grade ultra-high strength steel; The microstructure of the 2400 MPa grade ultra-high strength steel includes martensite and austenite, the average grain size of the martensite is 0.4 μm to 2.0 μm, and the aspect ratio of the martensite grains is less than or equal to 1.6; the volume fraction of the austenite is 5% to 20%; The original austenite of the 2400 MPa grade ultra-high strength steel is a heterogeneous structure of banded and equiaxed structures. The grain width of the banded original austenite is 5 μm to 10 μm, and the grain equivalent circle size of the equiaxed original austenite is 2 μm to 10 μm.

5. The preparation method according to claim 4, characterized in that Steps in preparing the metal for casting include: Prepare raw materials according to the chemical composition of the 2400 MPa grade ultra-high strength steel according to claim 1; The raw materials are smelted and cast to form billets or ingots to obtain the as-cast metal.

6. The preparation method according to claim 4, characterized in that The step of hot-rolling and cooling the cast metal to obtain a hot-rolled metal comprises: Heating the as-cast metal to a complete austenitizing temperature and keeping the temperature for 2 to 3 hours; The cast metal is subjected to a first-stage rolling after being kept warm, wherein the total reduction of the first-stage rolling is greater than 66% and the final rolling temperature is greater than 950° C.; The metal after the first stage of rolling is subjected to a temperature treatment ranging from 800° C. to 950° C.; The heated metal is subjected to a second-stage rolling process, wherein the total reduction in the second-stage rolling process is greater than 80%, the final rolling temperature is greater than 800° C., and the original austenite of the metal after the second-stage rolling process is in a strip-like and equiaxed shape; The metal after the second stage rolling is cooled to room temperature in air to obtain the hot-rolled metal. In the microstructure of the hot-rolled metal, the volume fraction of austenite is 20% to 40%.

7. The preparation method according to claim 6, characterized in that The complete austenitizing temperature is 1050°C to 1200°C.

8. The preparation method according to claim 6, characterized in that The step of subjecting the hot-rolled metal to a tempering treatment and cooling the hot-rolled metal to obtain the tempered metal comprises: austenitizing the hot-rolled metal at a temperature of 750° C. to 900° C. for 60 to 100 minutes to refine the original austenite in the form of strips and equiaxes; The austenitized metal is cooled to room temperature in air to obtain the quenched and tempered metal. In the microstructure of the quenched and tempered metal, the volume fraction of austenite is 10% to 30%.

9. The preparation method according to claim 4, characterized in that The step of subjecting the quenched and tempered metal to cryogenic treatment and returning it to room temperature to obtain the cryogenic quenched and tempered metal comprises: placing the quenched and tempered metal in a cryogenic environment at -196°C for 10 to 60 minutes; The metal after the cryogenic treatment is restored to room temperature in air to obtain the cryogenically tempered metal. In the microstructure of the cryogenically tempered metal, the volume fraction of austenite is 5% to 20%.

10. The preparation method according to claim 4, characterized in that The step of subjecting the cryogenically quenched and tempered metal to tempering and partitioning treatment and cooling to obtain 2400 MPa grade ultra-high strength steel comprises: The deep-cold quenched and tempered metal is subjected to a tempering and partitioning treatment at a temperature of 150° C. to 450° C. for 10 to 120 minutes, and then cooled to room temperature in air to obtain the 2400 MPa grade ultra-high strength steel; Wherein, the austenite of the 2400MPa grade ultra-high strength steel is in the form of a film.

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