2000MPa ultrahigh-strength low-density medium-manganese steel plate and preparation method thereof

Abstract

The invention discloses a 2000MPa ultrahigh-strength low-density medium-manganese steel plate and a preparation method thereof, and relates to the technical field of steel production. The preparation method comprises the following steps: smelting the following components in percentage by weight: 0.3 to 0.4 percent of C, 6.0 to 12.5 percent of Mn, 4.5 to 8.0 percent of Al, less than 0.001 percent of S, less than 0.001 percent of P and the balance of Fe and inevitable impurities, and casting into a steel ingot; wherein the mass ratio of the Mn element to the Al element is (1.33-2.78): 1; the steel ingot is forged into a rectangular rod through conventional forging and multiple transverse forging processes; the forged rectangular rod is subjected to one-pass hot rolling treatment, a hot rolled plate is obtained, and the finish rolling temperature is not lower than 30 DEG C below AC3; performing subzero treatment; and performing low-temperature tempering treatment to obtain a target product. According to the medium-manganese steel plate and the preparation method thereof, a martensite / delta-ferrite / austenite multi-layer microscopic structure in topological double ordered arrangement is realized by combining component design and microscopic structure topological optimization design, and the medium-manganese steel plate with ultrahigh strength, high plasticity, high toughness and low density is obtained.

Description

Technical Field

[0001] The present invention relates to the technical field of steel production, and in particular to a 2000MPa ultra-high strength and low density medium manganese steel plate and a preparation method thereof. Background Art

[0002] Ultra-high strength steel is a key material in the fields of transportation, deep-sea engineering, aerospace, and national defense. It can meet the country's major needs for green, lightweight, safety, extreme service environment, and special performance of large-scale equipment. However, improving strength, plasticity, toughness, and reducing density at the same time has long been a major theoretical problem in the field of steel materials, and it is also a bottleneck from basic research to technological innovation and application practice. Especially when the strength of steel reaches the 2000 MPa level, its plasticity will drop drastically (elongation <8%). The fundamental reason is that the initial high-density dislocations of traditional martensite are difficult to continue to proliferate, and the micro-plastic deformation of the disordered geometric orientation structure is extremely uneven, which is easy to produce local stress / strain concentration. At the same time, there are problems such as complex process and high alloy cost, which seriously restrict its forming and application. Although recent progress has been made in solving the trade-offs between strength and plasticity, strength and toughness, and strength and density, there is still a lack of solutions to improve the mutually exclusive properties of strength, plasticity, toughness, and density.

[0003] In view of the "inverted" relationship between strength and plasticity of ultra-high strength steel, researchers have proposed a series of solutions, such as the use of TRIP / TWIP effect, gradient structure, heterogeneous structure, dislocation control, grain structure, solid solution, nano-scale dispersion and control of the second phase. However, due to the specific processes required by the above mechanisms and the complexity of the phase transformation of steel materials, it is difficult to organically combine the above mechanisms and mechanisms to develop high-plasticity 2000 MPa ultra-high strength steel. In addition, the introduction of metastable phases is also a common means to improve strength and plasticity. However, this method still has limited enhancement effect of metastable phases at ultra-high strength of 2000 MPa. At the same time, in most metal materials, the increase in strength is at the expense of reduced plasticity and machinability, and the disadvantages of the above methods are that they use more expensive and strategically limited alloying elements, such as Co, Ni, V, Mo or Ti, which is not conducive to the sustainability of these alloys.

[0004] Based on the current status of research on ultra-high-strength metal materials at the 2000 MPa level, it is known that the common strategy to improve the strength / toughness synergy is to reduce the average grain size, which is actually limited by the strength / plasticity trade-off. In order to improve the plasticity of the material without sacrificing strength, a microstructure design with grain refinement is usually adopted, such as building a heterogeneous structure. However, overcoming the trade-off between strength and density is even more difficult, because the design of low-density medium-manganese steel requires the addition of a sufficiently high Al content, and the resulting δ-ferrite usually deteriorates the mechanical properties of the steel. This may lead to the precipitation of an ordered B2 phase and brittle fracture, a low strain hardening rate due to the precipitation of a large number of shearable nanocrystalline coherent κ-carbides, or a reduction in yield strength due to the formation of more soft δ-ferrite grains. Therefore, it is urgent to explore new common mechanisms, break through the "inverted" bottleneck of ultra-high-strength steel performance, solve the dependence on expensive alloys and complex processes, expand the performance boundary of strength-plasticity-toughness of 2000 MPa ultra-high-strength steel, and develop a 2000 MPa-level ultra-high-strength, low-density medium manganese steel preparation method that balances strength, plasticity, toughness and density. Summary of the invention

[0005] One of the purposes of the present invention is to provide a method for preparing a 2000MPa ultra-high strength and low-density medium manganese steel plate, which can enable the medium manganese steel to obtain a topologically double ordered arrangement of martensite / δ-ferrite / austenite multi-level microstructure, thereby greatly improving the mechanical properties of the medium manganese steel such as strength, plasticity and toughness while reducing the density of the steel plate.

[0006] In order to achieve the above object, the present invention adopts the following technical scheme: A method for preparing a 2000MPa ultra-high strength and low-density medium manganese steel plate comprises the following steps: (1) Smelt and cast into steel ingots according to the following components and their weight percentages: C: 0.3%~0.4%, Mn: 6.0~12.5%, Al: 4.5~8.0%, S: <0.001%, P: <0.001%, the balance is Fe and unavoidable impurities; the mass ratio of Mn and Al elements is (1.33~2.78):1; (2) Forging the steel ingot into a rectangular bar through conventional forging and multiple transverse forging processes; (3) subjecting the forged rectangular bar to one hot rolling process to obtain a hot rolled plate, wherein the final rolling temperature is not less than 30°C below AC3; (4) Cryogenic treatment of hot-rolled plates; (5) The plate after deep cryogenic treatment is subjected to low temperature tempering treatment to obtain the target product (i.e., 2000 MPa ultra-high strength and low density medium manganese steel plate).

[0007] Furthermore, in step (2), the specific steps of the conventional forging process include: heating the steel ingot to 1200°C ± 25°C and keeping it at that temperature for 6 hours, then forging it at 1000°C to 1200°C into an ingot with a square cross-section (such as an ingot with a cross-sectional size of 80*80mm), and air cooling it to room temperature.

[0008] Furthermore, in step (2), the specific steps of the multiple transverse forging process include: heating the ingot after conventional forging to 170-190°C above AC3 temperature and keeping it warm for 3 hours, and then performing multiple transverse forging at 30°C below AC3 to 140°C above AC3.

[0009] Furthermore, in step (2), the ingot is forged into a rectangular bar with a square cross section after multiple transverse forgings, and the cross-sectional side length of the rectangular bar is 1 / 8 (i.e., 10 / 80) to 3 / 16 (i.e., 15 / 80) of the cross-sectional side length of the ingot. Specifically, in step (2), the ingot is forged into a rectangular bar with a cross-sectional size of 10*10mm to 15*15mm after multiple transverse forgings.

[0010] Furthermore, in step (3), the thickness of the hot-rolled plate is 1 / 16 (i.e. 5 / 80) to 3 / 40 (i.e. 6 / 80) of the side length of the cross section of the ingot. Specifically, in step (3), the thickness of the hot-rolled plate is 5 to 6 mm.

[0011] Furthermore, in step (3), the rectangular bar forged in step (2) is directly subjected to one pass of hot rolling without cooling, wherein the total pressure reduction ratio is 50% to 60%, and is air-cooled to room temperature after the hot rolling treatment.

[0012] Furthermore, in step (4), the medium for cryogenic treatment is liquid nitrogen.

[0013] Furthermore, in step (4), the cryogenic treatment time is 20 minutes.

[0014] Furthermore, in step (5), the specific step of low temperature tempering treatment includes: heating the plate after deep cryogenic treatment to a temperature below AC3 of 250 to 500°C at a rate of 5 to 10°C / s and keeping the temperature for 10 to 75 min.

[0015] Another object of the present invention is to provide a 2000MPa ultra-high strength and low density medium manganese steel plate, which is prepared by the above-mentioned preparation method of the 2000MPa ultra-high strength and low density medium manganese steel plate.

[0016] Furthermore, the medium manganese steel plate has a topologically double ordered martensite / δ-ferrite / austenite multi-level microstructure, wherein thin strips of δ-ferrite and discretely distributed austenite are embedded in a martensite matrix having two topological arrangements, 45° to the rolling direction and along the rolling direction.

[0017] Furthermore, the tensile strength of the medium manganese steel plate is 2.0-2.4 GPa, the uniform elongation is 18-25%, the total elongation is 24-30%, and the fracture toughness is 100-120 MPa m 1 / 2 , density is 6.8g / cm 3 ~7.1g / cm 3 .

[0018] The present invention realizes the hierarchical distribution of martensite / δ-ferrite / austenite through highly innovative microstructure topology optimization design, fundamentally changing the problem of low plasticity / toughness of martensite and δ-ferrite, so that the layered structure organization cooperates in deformation enhancement, toughness and plasticity to obtain 2000MPa ultra-high strength and low density medium manganese steel plate.

[0019] The key to the present invention lies in a special forging and rolling process. Conventional forging is used in the first stage to obtain a steel ingot with uniform structure. Rapid transverse forging is used in the second stage to obtain elongated original austenite. In the third stage, one hot rolling air cooling with a final rolling temperature of not less than 30°C below AC3 is used, followed by deep freezing treatment. Finally, a martensite / delta-ferrite / austenite multi-level microstructure with topological double ordered arrangement is obtained, wherein thin strip-shaped delta-ferrite and discretely distributed austenite are embedded in a martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction.

[0020] The present invention realizes a solution for ultra-high-strength steel with component saving, green process, high strength, high plasticity, high toughness and reduced density. According to the test, the tensile strength of the experimental steel prepared by the present invention reaches 2.0-2.4 GPa, the uniform elongation reaches 18-25%, the total elongation reaches 24-30%, and the fracture toughness reaches 100-120 MPa m 1 / 2 , density reaches 6.8g / cm 3 ~7.1g / cm 3 , while improving strength, plasticity, toughness and reducing density. Moreover, the present invention solves the problem of dependence on expensive alloys and complex processes, expands the performance boundary of strength-plasticity-toughness of 2000 MPa-level ultra-high strength steel, and opens up a new way to solve the trade-off problem between strength, plasticity, toughness and density. Its mechanical properties far exceed those of similar medium manganese steels, greatly improving its competitiveness in the application of high-performance lightweight steel materials for automobiles. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a multi-level 3D microstructure diagram of topologically double ordered martensite / δ-ferrite / austenite in Example 2; Figure 2This is a SEM image of the thin strip-shaped δ-ferrite and discretely distributed austenite embedded in the martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction in Example 1; Figure 3 This is a SEM image of the thin strip-shaped δ-ferrite and discretely distributed austenite embedded in the martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction in Example 2; Figure 4 This is a SEM image of the thin strip-shaped δ-ferrite and discretely distributed austenite embedded in the martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction in Example 3; Figure 5 This is a SEM image of the thin strip-shaped δ-ferrite and discretely distributed austenite embedded in the martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction in Example 4; Figure 6 This is a SEM image of the thin strip-shaped δ-ferrite and discretely distributed austenite embedded in the martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction in Example 5; Figure 7 This is a SEM image of the thin strip-shaped δ-ferrite and discretely distributed austenite embedded in the martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction in Example 6; Figure 8 This is a SEM image of the thin strip-shaped δ-ferrite and discretely distributed austenite embedded in the martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction in Example 7; Fig. 9 TEM image of the thin strip-shaped δ-ferrite and discretely distributed austenite embedded in the martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction in Example 2; Fig.10 This is the OA diagram of Example 2 in which thin strip-shaped δ-ferrite and discretely distributed austenite are embedded in a martensite matrix with two topological arrangements, one at 45° to the rolling direction and the other along the rolling direction. DETAILED DESCRIPTION

[0022] In order to facilitate the understanding of those skilled in the art, the present invention is further described below in conjunction with the examples, and the contents mentioned in the examples are not limitations of the present invention. It should be noted in advance that the following examples were completed in the laboratory, and those skilled in the art should understand that the amounts of the components given in the examples only represent the ratio relationship between the components, rather than specific limitations.

[0023] In order to break through the bottleneck of existing steel manufacturing (how to simultaneously improve strength, plasticity, toughness and reduce density), the present invention proposes a preparation method of 2000MPa ultra-high strength and low-density medium manganese steel, comprising the following steps: (1) Smelting and casting to obtain a steel ingot according to the following components and their weight percentages; C: 0.3% to 0.4%, Mn: 6.0 to 12.5%, Al: 4.5 to 8.0%, S: <0.001%, P: <0.001%, the remainder is Fe and unavoidable impurities; the mass ratio of Mn to Al is between 1.33 and 2.78; (2) Conventional forging + multiple rapid transverse forging to forge the steel ingot into a rectangular bar; (3) Hot rolling: hot rolled plate is obtained by a large reduction in one pass; (4) Cryogenic treatment for 20 minutes; (5) Ultra-fast heating and low-temperature tempering treatment: first, heat the temperature to 250-500°C below AC3 at a rate of 5-10°C / s and keep it at this temperature for 10-75 min to obtain the product.

[0024] The conventional forging conditions in step (2) are as follows: the steel ingot is heated to 1200°C ± 25°C and kept at this temperature for 6 hours, then forged at 1200°C to 1000°C into an ingot with a cross section of 80*80mm and air-cooled to room temperature.

[0025] The conditions for the multiple rapid transverse forging in step (2) are as follows: the ingot is reheated to 170-190°C above the AC3 temperature and kept at that temperature for 3 hours, and then multiple rapid transverse forging is performed at 30°C below AC3 to 140°C above AC3.

[0026] The conditions for the multiple rapid transverse forging in step (2) are as follows: after multiple transverse forgings, a rectangular bar with a cross-sectional size of 10*10mm to 15*15mm is formed, wherein the blank is continuously rotated 90° during the continuous forging process.

[0027] The hot rolling treatment conditions in step (3) are as follows: the rectangular bar forged in step (2) is not cooled, but is subjected to one hot rolling process, the final rolling temperature is not less than 30°C below AC3, the total pressure reduction rate is 50% to 60%, and then air-cooled to room temperature.

[0028] Wherein, the thickness of the hot-rolled plate in step (3) is 5 to 6 mm.

[0029] Wherein, the cryogenic medium in step (4) is liquid nitrogen.

[0030] The annealed structure of the product obtained in step (5) has a topologically double ordered martensite / δ-ferrite / austenite multi-level microstructure, wherein thin strips of δ-ferrite and discretely distributed austenite are embedded in a martensite matrix having two topological arrangements, one at 45° to the rolling direction and the other along the rolling direction.

[0031] The product obtained in step (5) has a tensile strength of 2.0 to 2.4 GPa, a uniform elongation of 18 to 25%, a total elongation of 24 to 30%, and a fracture toughness of 100 to 120 MPa m 1 / 2 , density is 6.8g / cm 3 ~7.1g / cm 3 .

[0032] In general, the present invention realizes a topologically double ordered martensite / δ-ferrite / austenite multi-level microstructure by combining composition design with highly innovative microstructure topology optimization design, thereby obtaining a medium manganese steel plate with ultra-high strength, high plasticity, high toughness and low density.

[0033] The following is an explanation using specific examples.

[0034] In each embodiment, the rolling surface temperature is measured using a Japanese ICON portable infrared thermometer, the high temperature instrument has a temperature range of 500 to 3000°C, and the low temperature instrument has a temperature range of -50 to 1100°C.

[0035] In each embodiment, the post-rolling critical annealing is carried out in a box-type resistance furnace with a rated power of 8.5 kW, a rated voltage of 380 V, and a rated temperature of 1400°C.

[0036] The AC1 and AC3 temperatures in each embodiment were calculated using Thermo-Calc (2023a) software.

[0037] The tensile tests of the embodiments were carried out on an MTS CMT5303 series electronic universal testing machine.

[0038] The microstructure of the samples in each embodiment was observed by secondary electron imaging and electron backscatter diffraction (EBSD) using a focused ion beam (FIB) combined with a field emission scanning electron microscope.

[0039] The fracture toughness of each example was measured by the elastic-plastic unloading compliance method using small side-notch compact tension (CT) specimens. Example 1

[0040] In this embodiment, 2000MPa ultra-high strength and low density medium manganese steel is prepared in the following manner.

[0041] (1) A steel ingot is obtained by smelting and casting according to the following components and their weight percentages: C: 0.3%, Mn: 6.0%, Al: 4.5%, S: <0.001%, P: <0.001%, the balance is Fe and inevitable impurities; the mass ratio of Mn and Al elements Mn / Al is 1.33.

[0042] (2) Conventional forging + multiple rapid transverse forging to forge the steel ingot into a rectangular bar: The steel ingot is heated to 1200℃±25℃ and kept at this temperature for 6h, then forged at 1200℃~1000℃ into an ingot with a cross section of 80*80mm and air-cooled to room temperature. The ingot is heated to 170℃ above AC3 temperature and kept at this temperature for 3 hours, then subjected to multiple rapid transverse forging at 30℃ below AC3 and 140℃ above AC3. After multiple transverse forgings, a rectangular bar with a cross section of 10*10mm is formed, in which the billet is continuously rotated 90° during the continuous forging process.

[0043] (3) hot rolling to obtain a hot-rolled plate: the rectangular bar forged in step (2) is not cooled, but is subjected to one pass of hot rolling, with the final rolling temperature being not less than 30°C below AC3 and the total pressure reduction ratio being 50%, and is rolled into a hot-rolled plate with a thickness of 5 mm, and then air-cooled to room temperature.

[0044] (4) Cryogenic treatment under liquid nitrogen for 20 min.

[0045] (5) Ultra-fast heating and low-temperature tempering treatment: The temperature is raised to 500°C below AC3 at a rate of 5°C / s and kept at this temperature for 75 minutes to obtain 2000MPa ultra-high strength and low-density medium manganese steel.

[0046] The 2000MPa ultra-high strength and low density medium manganese steel prepared in this embodiment has a topologically double ordered martensite / δ-ferrite / austenite multi-level microstructure, in which thin strips of δ-ferrite and discretely distributed austenite are embedded in a martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction (see Figure 2 ).

[0047] The test results show that the tensile strength of the 2000MPa ultra-high strength low-density medium manganese steel prepared in this embodiment is 2.0GPa, the uniform elongation is 18%, the total elongation is 24%, and the fracture toughness is 100MPa m 1 / 2 , density is 7.1g / cm 3 . Example 2

[0048] In this embodiment, 2000MPa ultra-high strength and low density medium manganese steel is prepared in the following manner.

[0049] (1) A steel ingot is obtained by smelting and casting according to the following components and their weight percentages: C: 0.3%, Mn: 12.5%, Al: 8.0%, S: <0.001%, P: <0.001%, the balance is Fe and inevitable impurities; the mass ratio of Mn and Al elements Mn / Al is 1.56.

[0050] (2) Conventional forging + multiple rapid transverse forging to forge the steel ingot into a rectangular bar: The steel ingot is heated to 1200℃±25℃ and kept at this temperature for 6h, then forged at 1200℃~1000℃ into an ingot with a cross section of 80*80mm and air-cooled to room temperature. The ingot is heated to 190℃ above AC3 temperature and kept at this temperature for 3 hours, then subjected to multiple rapid transverse forging at 30℃ below AC3 and 140℃ above AC3. After multiple transverse forging, a rectangular bar with a cross section of 15*15mm is formed, in which the billet is continuously rotated 90° during the continuous forging process.

[0051] (3) hot rolling to obtain hot-rolled plates: the rectangular bars forged in step (2) are subjected to one pass of hot rolling without cooling, with the final rolling temperature being not less than 30°C below AC3 and the total pressure reduction ratio being 60%, and are rolled into hot-rolled plates with a thickness of 6 mm, and then air-cooled to room temperature.

[0052] (4) Cryogenic treatment under liquid nitrogen for 20 min.

[0053] (5) Ultra-fast heating and low-temperature tempering treatment: The temperature is raised to 450°C below AC3 at a rate of 5°C / s and kept at this temperature for 40 min to obtain 2000MPa ultra-high strength and low-density medium manganese steel.

[0054] The 2000MPa ultra-high strength and low density medium manganese steel prepared in this embodiment has a topologically double ordered martensite / δ-ferrite / austenite multi-level microstructure (see Figure 1 ), in which thin strips of δ-ferrite and discretely distributed austenite are embedded in a martensitic matrix with two topological arrangements, 45° to the rolling direction and along the rolling direction (see Figure 3 , 9 , 10).

[0055] After testing, the tensile strength of the 2000MPa ultra-high strength and low-density medium manganese steel prepared in this embodiment is 2.3GPa, the uniform elongation is 24%, the total elongation is 26%, and the fracture toughness is 110 MPa m 1 / 2 , density is 6.8g / cm 3 . Example 3

[0056] In this embodiment, 2000MPa ultra-high strength and low density medium manganese steel is prepared in the following manner.

[0057] (1) A steel ingot is obtained by smelting and casting according to the following components and their weight percentages: C: 0.35%, Mn: 8.0%, Al: 6.0%, S: <0.001%, P: <0.001%, the balance is Fe and inevitable impurities; the mass ratio of Mn and Al elements Mn / Al is 1.33.

[0058] (2) Conventional forging + multiple rapid transverse forging to forge the steel ingot into a rectangular bar: The steel ingot is heated to 1200℃±25℃ and kept at this temperature for 6h, then forged at 1200℃~1000℃ into an ingot with a cross section of 80*80mm and air-cooled to room temperature. The ingot is heated to 170℃ above AC3 temperature and kept at this temperature for 3 hours, then subjected to multiple rapid transverse forging at 30℃ below AC3 and 140℃ above AC3. After multiple transverse forging, a rectangular bar with a cross section of 10*10mm is formed, in which the ingot is continuously rotated 90° during the continuous forging process.

[0059] (3) hot rolling to obtain a hot-rolled plate: the rectangular bar forged in step (2) is not cooled, but is subjected to one pass of hot rolling, with the final rolling temperature being not less than 30°C below AC3 and the total pressure reduction ratio being 50%, and is rolled into a hot-rolled plate with a thickness of 5 mm, and then air-cooled to room temperature.

[0060] (4) Cryogenic treatment under liquid nitrogen for 20 min.

[0061] (5) Ultra-fast heating and low-temperature tempering treatment: The temperature is raised to 400°C below AC3 at a rate of 7°C / s and kept at this temperature for 30 min to obtain 2000MPa ultra-high strength and low-density medium manganese steel.

[0062] The 2000MPa ultra-high strength and low density medium manganese steel prepared in this embodiment has a topologically double ordered martensite / δ-ferrite / austenite multi-level microstructure, in which thin strips of δ-ferrite and discretely distributed austenite are embedded in a martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction (see Figure 4 ).

[0063] After testing, the tensile strength of the 2000MPa ultra-high strength and low-density medium manganese steel prepared in this embodiment is 2.13GPa, the uniform elongation is 20%, the total elongation is 27%, and the fracture toughness is 108MPa m 1 / 2 , density is 6.93g / cm 3 . Example 4

[0064] In this embodiment, 2000MPa ultra-high strength and low density medium manganese steel is prepared in the following manner.

[0065] (1) A steel ingot is obtained by smelting and casting according to the following components and their weight percentages: C: 0.35%, Mn: 10.5%, Al: 7.0%, S<0.001%, P<0.001%, the balance is Fe and inevitable impurities; the mass ratio of Mn and Al elements Mn / Al is 1.5.

[0066] (2) Conventional forging + multiple rapid transverse forging to forge the steel ingot into a rectangular bar: The steel ingot is heated to 1200℃±25℃ and kept at this temperature for 6h, then forged at 1200℃~1000℃ into an ingot with a cross section of 80*80mm and air-cooled to room temperature. The ingot is heated to 190℃ above AC3 temperature and kept at this temperature for 3 hours, then subjected to multiple rapid transverse forging at 30℃ below AC3 and 140℃ above AC3. After multiple transverse forging, a rectangular bar with a cross section of 15*15mm is formed, in which the billet is continuously rotated 90° during the continuous forging process.

[0067] (3) hot rolling to obtain hot-rolled plates: the rectangular bars forged in step (2) are subjected to one pass of hot rolling without cooling, with the final rolling temperature being not less than 30°C below AC3 and the total pressure reduction ratio being 60%, and are rolled into hot-rolled plates with a thickness of 6 mm, and then air-cooled to room temperature.

[0068] (4) Cryogenic treatment under liquid nitrogen for 20 min.

[0069] (5) Ultra-fast heating and low-temperature tempering treatment: The temperature is raised to 350°C below AC3 at a rate of 7.5°C / s and kept at this temperature for 25 minutes to obtain 2000MPa ultra-high strength and low-density medium manganese steel.

[0070] The 2000MPa ultra-high strength and low density medium manganese steel prepared in this embodiment has a topologically double ordered martensite / δ-ferrite / austenite multi-level microstructure, in which thin strips of δ-ferrite and discretely distributed austenite are embedded in a martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction (see Figure 5 ).

[0071] After testing, the tensile strength of the 2000MPa ultra-high strength and low-density medium manganese steel prepared in this embodiment is 2.21GPa, the uniform elongation is 25%, the total elongation is 30%, and the fracture toughness is 116MPa m 1 / 2 , density is 6.85g / cm 3 . Example 5

[0072] In this embodiment, 2000MPa ultra-high strength and low density medium manganese steel is prepared in the following manner.

[0073] (1) A steel ingot is obtained by smelting and casting according to the following components and their weight percentages: C: 0.4%, Mn: 8.0%, Al: 4.5%, S<0.001%, P<0.001%, the balance is Fe and inevitable impurities; the mass ratio of Mn and Al elements Mn / Al is 1.78.

[0074] (2) Conventional forging + multiple rapid transverse forging to forge the steel ingot into a rectangular bar: The steel ingot is heated to 1200℃±25℃ and kept at this temperature for 6h, then forged at 1200℃~1000℃ into an ingot with a cross section of 80*80mm and air-cooled to room temperature. The ingot is heated to 170℃ above AC3 temperature and kept at this temperature for 3 hours, then subjected to multiple rapid transverse forging at 30℃ below AC3 and 140℃ above AC3. After multiple transverse forging, a rectangular bar with a cross section of 10*10mm is formed, in which the ingot is continuously rotated 90° during the continuous forging process.

[0075] (3) hot rolling to obtain a hot-rolled plate: the rectangular bar forged in step (2) is not cooled, but is subjected to one pass of hot rolling, with the final rolling temperature being not less than 30°C below AC3 and the total pressure reduction ratio being 50%, and is rolled into a hot-rolled plate with a thickness of 5 mm, and then air-cooled to room temperature.

[0076] (4) Cryogenic treatment under liquid nitrogen for 20 min.

[0077] (5) Ultra-fast heating and low-temperature tempering treatment: The temperature is raised to 300°C below AC3 at a rate of 8.5°C / s and kept at this temperature for 20 min to obtain 2000MPa ultra-high strength and low-density medium manganese steel.

[0078] The 2000MPa ultra-high strength and low density medium manganese steel prepared in this embodiment has a topologically double ordered martensite / δ-ferrite / austenite multi-level microstructure, in which thin strips of δ-ferrite and discretely distributed austenite are embedded in a martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction (see Figure 6 ).

[0079] After testing, the tensile strength of the 2000MPa ultra-high strength and low-density medium manganese steel prepared in this embodiment is 2.4GPa, the uniform elongation is 18%, the total elongation is 26%, and the fracture toughness is 120MPa m 1 / 2 , density is 6.99g / cm 3 . Example 6

[0080] In this embodiment, 2000MPa ultra-high strength and low density medium manganese steel is prepared in the following manner.

[0081] (1) A steel ingot is obtained by smelting and casting according to the following components and their weight percentages: C: 0.4%, Mn: 9.5%, Al: 5.5%, S<0.001%, P<0.001%, the balance is Fe and inevitable impurities; the mass ratio of Mn and Al elements Mn / Al is 1.72.

[0082] (2) Conventional forging + multiple rapid transverse forging to forge the steel ingot into a rectangular bar: The steel ingot is heated to 1200℃±25℃ and kept at this temperature for 6h, then forged at 1200℃~1000℃ into an ingot with a cross section of 80*80mm and air-cooled to room temperature. The ingot is heated to 180℃ above AC3 temperature and kept at this temperature for 3 hours, then subjected to multiple rapid transverse forging at 30℃ below AC3 and 140℃ above AC3. After multiple transverse forging, a rectangular bar with a cross section of 12.5*12.5mm is formed, in which the ingot is continuously rotated 90° during the continuous forging process.

[0083] (3) hot rolling to obtain hot-rolled plates: the rectangular bars forged in step (2) are subjected to one pass of hot rolling without cooling, with the final rolling temperature being not less than 30°C below AC3 and the total pressure reduction ratio being 60%, and are rolled into hot-rolled plates with a thickness of 5 mm, and then air-cooled to room temperature.

[0084] (4) Cryogenic treatment under liquid nitrogen for 20 min.

[0085] (5) Ultra-fast heating and low-temperature tempering treatment: The temperature is raised to 250°C below AC3 at a rate of 10°C / s and kept at this temperature for 15 minutes to obtain 2000MPa ultra-high strength and low-density medium manganese steel.

[0086] The 2000MPa ultra-high strength and low density medium manganese steel prepared in this embodiment has a topologically double ordered martensite / δ-ferrite / austenite multi-level microstructure, in which thin strips of δ-ferrite and discretely distributed austenite are embedded in a martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction (see Figure 7 ).

[0087] After testing, the tensile strength of the 2000MPa ultra-high strength and low-density medium manganese steel prepared in this embodiment is 2.33GPa, the uniform elongation is 23%, the total elongation is 28%, and the fracture toughness is 113MPa m 1 / 2 , density is 6.89g / cm 3 . Example 7

[0088] In this embodiment, 2000MPa ultra-high strength and low density medium manganese steel is prepared in the following manner.

[0089] (1) A steel ingot is obtained by smelting and casting according to the following components and their weight percentages: C: 0.4%, Mn: 12.5%, Al: 4.5%, S<0.001%, P<0.001%, the balance is Fe and inevitable impurities; the mass ratio of Mn and Al elements Mn / Al is 2.78.

[0090] (2) Conventional forging + multiple rapid transverse forging to forge the steel ingot into a rectangular bar: The steel ingot is heated to 1200℃±25℃ and kept at this temperature for 6h, then forged at 1200℃~1000℃ into an ingot with a cross section of 80*80mm and air-cooled to room temperature. The ingot is heated to 190℃ above AC3 temperature and kept at this temperature for 3 hours, then subjected to multiple rapid transverse forging at 30℃ below AC3 and 140℃ above AC3. After multiple transverse forging, a rectangular bar with a cross section of 15*15mm is formed, in which the billet is continuously rotated 90° during the continuous forging process.

[0091] (3) hot rolling to obtain a hot-rolled plate: the rectangular bar forged in step (2) is not cooled, but is subjected to one hot rolling process, with the final rolling temperature being not less than 30°C below AC3 and the total pressure reduction ratio being 60%, and is rolled into a hot-rolled plate with a thickness of 6 mm, and then air-cooled to room temperature.

[0092] (4) Cryogenic treatment under liquid nitrogen for 20 min.

[0093] (5) Ultra-fast heating and low-temperature tempering treatment: The temperature is raised to 250°C below AC3 at a rate of 10°C / s and kept at this temperature for 10 min to obtain 2000MPa ultra-high strength and low-density medium manganese steel.

[0094] The 2000MPa ultra-high strength and low density medium manganese steel prepared in this embodiment has a topologically double ordered martensite / δ-ferrite / austenite multi-level microstructure, in which thin strips of δ-ferrite and discretely distributed austenite are embedded in a martensite matrix with two topological arrangements of 45° to the rolling direction and along the rolling direction (see Figure 8 ).

[0095] After testing, the tensile strength of the 2000MPa ultra-high strength and low-density medium manganese steel prepared in this embodiment is 2.36GPa, the uniform elongation is 23%, the total elongation is 25%, and the fracture toughness is 118MPa m 1 / 2 , density is 6.86g / cm 3 .

[0096] In addition, the chemical composition and performance test results of the above embodiments can be found in Table 1 below.

[0097] Table 1 Chemical composition and performance test results

[0098] The above embodiments are preferred implementation schemes of the present invention. In addition, the present invention may also be implemented in other ways. Any obvious replacement without departing from the concept of the present technical solution is within the protection scope of the present invention.

Claims

1. A method for preparing a 2000MPa ultra-high strength and low density medium manganese steel plate, characterized in that: The steps include: (1) Smelt and cast into steel ingots according to the following components and their weight percentages: C: 0.3%~0.4%, Mn: 6.0~12.5%, Al: 4.5~8.0%, S: <0.001%, P: <0.001%, the balance is Fe and unavoidable impurities; the mass ratio of Mn and Al elements is (1.33~2.78):1; (2) Forging the steel ingot into a rectangular bar through conventional forging and multiple transverse forging processes; (3) subjecting the forged rectangular bar to one hot rolling process to obtain a hot rolled plate, wherein the final rolling temperature is not less than 30°C below AC3; (4) Cryogenic treatment of hot-rolled plates; (5) The plate after deep cryogenic treatment is subjected to low temperature tempering treatment to obtain the target product.

2. The method for preparing the 2000MPa ultra-high strength and low density medium manganese steel plate according to claim 1, characterized in that: In step (2), the specific steps of the conventional forging process include: heating the steel ingot to 1200°C ± 25°C and keeping it at that temperature for 6 hours, then forging it into an ingot with a square cross-section at 1000°C to 1200°C, and air cooling it to room temperature.

3. The method for preparing the 2000MPa ultra-high strength and low density medium manganese steel plate according to claim 2, characterized in that: In step (2), the specific steps of the multiple transverse forging process include: heating the ingot after conventional forging to 170-190°C above AC3 temperature and keeping it warm for 3 hours, and then performing multiple transverse forging at 30°C below AC3 to 140°C above AC3.

4. The method for preparing the 2000MPa ultra-high strength and low density medium manganese steel plate according to claim 3, characterized in that: In step (2), the ingot is made into a rectangular bar with a square cross-section after multiple transverse forgings, and the side length of the cross-section of the rectangular bar is 1 / 8 to 3 / 16 of the side length of the cross-section of the ingot.

5. The method for preparing the 2000MPa ultra-high strength and low density medium manganese steel plate according to claim 4, characterized in that: In step (3), the thickness of the hot-rolled plate is 1 / 16 to 3 / 40 of the side length of the cross section of the ingot.

6. The method for preparing the 2000MPa ultra-high strength and low density medium manganese steel plate according to claim 1, characterized in that: In step (3), the forged rectangular bar is directly subjected to one pass of hot rolling without cooling, wherein the total pressure reduction rate is 50% to 60%, and is air-cooled to room temperature after the hot rolling treatment.

7. The method for preparing the 2000MPa ultra-high strength and low density medium manganese steel plate according to claim 1, characterized in that: In step (4), the medium for cryogenic treatment is liquid nitrogen, and the time for cryogenic treatment is 20 minutes.

8. The method for preparing the 2000MPa ultra-high strength and low density medium manganese steel plate according to claim 1, characterized in that: In step (5), the specific steps of low temperature tempering treatment include: heating the plate after deep cryogenic treatment to a temperature below AC3 of 250 to 500°C at a rate of 5 to 10°C / s and keeping the temperature for 10 to 75 min.

9. A 2000MPa ultra-high strength low density medium manganese steel plate, characterized by: The steel plate is prepared by the method for preparing a 2000MPa ultra-high strength and low density medium manganese steel plate as described in any one of claims 1 to 8.

10. The 2000MPa ultra-high strength and low density medium manganese steel plate according to claim 9, characterized in that: The medium manganese steel plate has a topologically double ordered martensite / δ-ferrite / austenite multi-level microstructure, wherein thin strips of δ-ferrite and discretely distributed austenite are embedded in a martensite matrix having two topological arrangements, 45° to the rolling direction and along the rolling direction.

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