High strength, high specific strength, low density steel and method of making

Abstract

This invention discloses a high-strength, high-ductility, low-density steel and its preparation method. The composition is: C: 0.85–1.15%, Mn: 9.5–12%, Al: 8.5–11.5%, Ni: 2.5–3.5%, Cr: 2–3%, V: 0.3–0.5%, S≤0.005%, P≤0.008%, iron, and unavoidable impurities. The preparation method includes smelting, casting, forging, hot rolling, cold rolling, and recrystallization annealing processes. Through a high-C, medium-Mn, and high-Al synergistic design, Cr suppresses grain boundary carbide segregation, Ni promotes the formation of the B2 ordered phase, and V microalloying promotes the precipitation of the VC precipitate, forming a multi-scale layered structure composed of banded δ-ferrite, equiaxed (Fe,Ni)Al-type B2 phase, austenite, and nano-VC precipitates. The recrystallization annealing process achieves multi-scale layered structure control and synergistic strengthening of equiaxed (Fe,Ni)Al-type B2 phase and VC nanoprecipitates through rapid heating-short holding-rapid cooling. This invention breaks through the bottleneck of balancing strength, plasticity, and density in low-density steel, providing a high-performance material solution for automotive lightweighting.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a high-strength, high-ductility, low-density steel and its preparation method. Background Technology

[0002] As a core strategy for addressing the energy crisis and environmental challenges, automotive lightweighting has always faced the challenge of balancing high strength and low density in its technological evolution. Traditional lightweighting methods mainly focus on two types of materials: one is third-generation advanced high-strength steel, represented by QP steel and MS steel. Although it can increase tensile strength to 1200MPa, its elongation after fracture is usually less than 15%, which makes it prone to springback cracking during cold stamping. In addition, its density is usually higher than 7.8g / cm³, which limits the potential for lightweighting; the other is the use of lightweight materials such as aluminum alloys and magnesium alloys. Although they can significantly reduce density, they have problems such as poor formability, insufficient collision energy absorption, and high cost.

[0003] Against this backdrop, developing novel metallic materials that combine high strength, low density, and excellent formability has become a key path to overcome the bottlenecks in the automotive lightweighting industry. By increasing the proportion of Mn and Al in steel, and constructing a composition system with Fe, Mn, Al, and C as the core elements, the density of steel can be effectively reduced to below 7.3 g / cm³. Simultaneously, introducing Al-containing precipitates into the material provides additional precipitation strengthening and solid solution strengthening, helping to improve the strength-ductility balance, and has become an important development direction for lightweighting technology. However, existing technologies generally suffer from the following drawbacks: increasing the Al content in steel easily leads to the precipitation of brittle κ carbides, deteriorating the material's ductility and increasing the risk of processing cracks; furthermore, the technological evolution of existing low-density steels has always faced the challenge of balancing strength, ductility, and density, making it difficult to simultaneously achieve high strength-ductility product and high specific strength.

[0004] Patent CN108396244A discloses a cold-rolled medium-manganese, high-aluminum, low-density steel and its preparation method. The alloy composition by weight percentage is: C: 1.20–1.30%, Mn: 10.7–11.3%, Al: 9.5–10%, P ≤ 0.005%, S ≤ 0.003%, with the balance being Fe and unavoidable impurities. The microstructure is austenite + ferrite + κ carbides. The precipitation strengthening effect of κ carbides enables the low-density steel to achieve a high tensile strength of 1300 MPa. However, the elongation is less than 20%, and the strength-ductility product is relatively low, increasing the difficulty of stamping automotive parts and raising manufacturing costs.

[0005] Patent CN106756567A discloses a method for preparing hot-rolled low-density steel with a strength-ductility product ≥40 GPa•%. The alloy composition, by weight percentage, is: 0.6%–1.0% C, 8%–12% Mn, 7%–11% Al, 0.01%–0.2% V, P≤0.003%, S≤0.002%, with the balance being Fe and unavoidable impurities. The microstructure is austenite + ferrite. This simple preparation process achieves a strength-ductility product of 40 GPa•%. However, the tensile strength is less than 1 GPa, and the specific strength is low, making it difficult to meet the safety requirements of automotive body collision energy-absorbing components. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a high-strength, high-density, low-density steel and its preparation method.

[0007] To achieve this technical objective, the present invention adopts the following solution: In a first aspect, the present invention provides a high-strength, high-ductility, low-density steel. The chemical composition and mass percentage of the high-strength, high-ductility, low-density steel are as follows: C: 0.85-1.15%, Mn: 9.5-12.0%, Al: 8.5-11.5%, Ni: 2.5-3.5%, Cr: 2.0-3.0%, V: 0.3-0.5%, S≤0.005%, P≤0.008%, with the remainder being Fe and unavoidable impurities.

[0008] Furthermore, high-strength-ductility-product, high specific strength, and low-density steel has a yield strength ≥1060MPa, tensile strength ≥1300MPa, elongation ≥38.0%, density of 6.63~6.78g / cm³, strength-ductility-product ≥50GPa•%, and specific strength ≥195MPa / (g / cm³).

[0009] Furthermore, the microstructure of the high-strength, high-ductility, high-specific-strength, low-density steel product consists of banded δ-ferrite, equiaxed (Fe,Ni)Al-type B2 phase, austenite, and VC nano-precipitates dispersed in the matrix.

[0010] Furthermore, in high-strength, high-ductility, low-density steel, banded δ-ferrite accounts for 15-20%, with an average band width of 1.5-2.5 μm; equiaxed (Fe,Ni)Al-type B2 phase accounts for 5-10%, with a grain size ≤1.5 μm; austenite accounts for 70-80%, with a grain size of 6-8 μm; and nanoscale VC precipitates have an average particle size ≤80 nm.

[0011] Secondly, the present invention provides a method for preparing high-strength, high-ductility, low-density steel, comprising smelting, casting, forging, hot rolling, cold rolling, and recrystallization annealing processes, specifically: According to the chemical composition of the high-strength, high-ductility, high-specific-strength, low-density steel of the present invention, the steel is smelted and cast into ingots, and the ingots are forged into rectangular billets. The hot rolling process parameters are controlled as follows: the rectangular billet is heated to a temperature of 1180-1220℃, held for 1.5-2.5 hours, the initial rolling temperature is 1140-1180℃, the final rolling temperature is 880-920℃, the hot rolling reduction rate is 85-93%, and the billet is water-cooled to room temperature after hot rolling. The thickness of the hot-rolled plate is 4.0-5.5 mm. The cold rolling process parameters are controlled as follows: the cold rolling reduction rate is 60-75%, and the cold rolled plate thickness is 1.0-2.0 mm; The parameters for the recrystallization annealing process are controlled as follows: heating rate is 80-100℃ / s, annealing temperature is 950-1050℃, holding time is 60-300s, and the cooling method after recrystallization annealing is low-temperature water cooling with a water temperature of 0-5℃.

[0012] The roles of the main elements in the chemical composition of the high-strength, high-ductility, low-density steel of this invention are as follows: C: As a core strengthening element, it enhances the strength of the matrix through interstitial solid solution mechanism; C element also regulates the stability of austenite, raising its stacking fault energy to the critical threshold for inhibiting deformation-induced martensitic phase transformation, ensuring that the material does not undergo brittle transformation during plastic deformation, and achieving synergistic optimization of strength and toughness. The C content of this invention ranges from 0.85% to 1.15%.

[0013] Mn: As an austenite-forming element, it can increase the stability of austenite at room temperature and improve the plasticity and processing properties of steel. Mn also reduces the tendency of carbon segregation at the ferrite-austenite phase boundary, alleviating the risk of interface embrittlement. The Mn content in this invention ranges from 9.5% to 12.0%.

[0014] Al primarily functions to reduce material density, lowering the density of steel to below 6.8 g / cm³ through atomic substitution. It also acts as a strong ferrite-forming element, promoting the formation of δ-ferrite. δ-ferrite precipitates at high temperatures, distributing in the matrix as bands with an average width of 1.5–2.5 μm. Its high orientation continuity allows for controlled strain distribution, improving the material's strength-ductility product and energy absorption efficiency. The Al content in this invention ranges from 8.5% to 11.5%.

[0015] Ni helps suppress the abnormal growth of δ-ferrite grains during high-temperature annealing, improving the uniformity and refinement of the microstructure. Furthermore, Ni and Al tend to form an ordered B2 phase at high temperatures. The resulting equiaxed (Fe,Ni)Al-type B2 structure has a regular and ordered arrangement, increasing the difficulty of dislocation cross-slip and enhancing the dispersion of intragranular stress, thereby improving the yield strength. The equiaxed (Fe,Ni)Al-type B2 phase exists as a strengthening phase in the multiphase microstructure of this invention, synergistically improving the strength-ductility coordination and deformation uniformity of the steel with austenite and δ-ferrite. The Ni content in this invention ranges from 2.5% to 3.5%.

[0016] Cr: It can increase the solubility of C in austenite, thereby reducing the driving force for carbide precipitation, inhibiting the precipitation of κ carbides at austenite grain boundaries, and preventing a decrease in the plasticity of the material. In addition, Cr can improve the corrosion resistance of low-density steel. The Cr content in this invention ranges from 2.0% to 3.0%.

[0017] V: V combines with C atoms to form VC precipitates with a particle size ≤80nm. VC has excellent thermal stability and high interfacial bonding strength, and can be dispersed in the matrix to enhance the precipitation strengthening effect of the material. The VC precipitates can effectively pin grain boundaries and phase boundaries, inhibit grain growth and microstructure coarsening, while also hindering the preferential growth of brittle κ carbides, thus optimizing the plasticity and ductility of steel. The V content in this invention is controlled within the range of 0.3% to 0.5%.

[0018] Compared with existing technologies, the beneficial effects of adopting the above technical solution are as follows: This invention achieves synergistic optimization of composition through a matching design of high C, medium Mn, and high Al, combined with Ni, Cr, and V microalloying. This results in a multi-phase microstructure consisting of banded δ-ferrite, equiaxed (Fe,Ni)Al-type B2 phase, austenite, and VC nano-precipitates. Austenite can dissolve a large number of C atoms, enhancing matrix strength through interstitial solid solution. The banded δ-ferrite forms a layered framework at the microscale, exhibiting a periodic distribution within the austenite. During deformation, it can withstand large strains without easily fracturing, improving the overall toughness of the material. The equiaxed (Fe,Ni)Al-type B2 phase precipitates at the submicron scale, increasing yield strength through a grain refinement strengthening mechanism. Simultaneously, its uniform distribution along austenite grain boundaries promotes uniform stress dispersion, enhancing the material's stable plastic deformation capability. The VC nano-precipitates further hinder dislocation movement and achieve precipitation strengthening. This multi-scale layered structure establishes a synergistic distribution and interface regulation mechanism between the reinforcing phase and the matrix at the micro-, submicro-, and nano-scale levels, enhancing the material's adaptability to localized stress concentration and uneven plastic deformation, resulting in excellent strength and plasticity. High-reduction hot rolling refines the δ-ferrite bands and austenite grains, while water cooling to room temperature after final rolling suppresses the precipitation of brittle phases and prevents grain coarsening, improving microstructure uniformity. Cold rolling ensures increased dislocation density within the low-density steel, providing sufficient driving force for recrystallization during annealing. The recrystallization annealing process employs rapid heating-short holding-rapid cooling, enabling higher heating and cooling rates. While promoting the formation of equiaxed (Fe,Ni)Al-type B2 phase and VC nanoprecipitates, it inhibits the coarsening of VC precipitates and the growth of B2 phase grains, enhancing the refinement and microstructure matching of the multi-scale layered structure.

[0019] In summary, this invention, through synergistic innovation in composition design and preparation process, constructs a multi-scale layered structure, achieving multiple objectives such as multiphase distribution, microstructure refinement, precipitate optimization, and brittleness suppression. It breaks through the process bottleneck of traditional low-density steel, which struggles to balance strength, plasticity, and density, and provides an efficient, stable, and high-performance material preparation solution for automotive lightweighting. Attached Figure Description

[0020] Figure 1 This is a microstructure diagram of the high-strength, high-specific-strength, low-density steel in Example 1 of the present invention.

[0021] Figure 2 This refers to the VC nano-precipitated phase in the high-strength, high-specific-strength, low-density steel of Example 1 of the present invention.

[0022] Figure 3 This is a microstructure diagram of the steel in Comparative Example 1 of the present invention.

[0023] Figure 4This is a microstructure diagram of the steel in Comparative Example 2 of the present invention.

[0024] Figure 5 This is a microstructure diagram of the steel in Comparative Example 3 of the present invention. Detailed Implementation

[0025] To fully understand the purpose, features and effects of the present invention, the present invention will be described in detail through the following specific embodiments, but the present invention is not limited thereto.

[0026] This invention provides a high-strength, high-ductility, low-density steel and its preparation method, including smelting, casting, forging, hot rolling, cold rolling, and recrystallization annealing processes, specifically carried out according to the following steps. Example 1

[0027] According to the present invention, the high-strength, high-ductility, low-density steel is smelted, cast, and forged into rectangular billets. The chemical composition of the billet is as follows by mass percentage: C: 0.96%, Mn: 11.0%, Al: 11.0%, Ni: 3.2%, Cr: 2.3%, V: 0.35%, S: 0.003%, P: 0.006%, with the remainder being Fe and unavoidable impurities.

[0028] Hot rolling process parameter control: heating temperature is 1190℃, holding time is 2h, initial rolling temperature is 1150℃, final rolling temperature is 880℃, hot rolling reduction rate is 88%, water cooling to room temperature after hot rolling, and hot rolled plate thickness is 5.1mm. Cold rolling process parameter control: cold rolling reduction rate is 65%, and cold rolled plate thickness is 1.8mm; Recrystallization annealing process parameter control: heating rate is 95℃ / s, annealing temperature is 1000℃, holding time is 180s, and the cooling method after recrystallization annealing is low-temperature water cooling with a water temperature of 0~5℃.

[0029] The high-strength, high-specific-strength, low-density steel prepared in this embodiment has a multi-scale layered structure. (See also...) Figure 1 and Figure 2 The microstructure consists of 17% banded δ-ferrite (average band width 2.5 μm), 10% equiaxed (Fe,Ni)Al-type B2 phase (average grain size 1.1 μm), and 73% austenite (average grain size 7.0 μm), with an average VC precipitate size of 80 nm.

[0030] The high-strength-ductility-product, high specific strength, and low-density steel prepared in this embodiment has a yield strength of 1068 MPa, a tensile strength of 1300 MPa, an elongation of 38.7%, a density of 6.67 g / cm³, a strength-ductility-product of 50.3 GPa·%, and a specific strength of 195 MPa / (g / cm³). Example 2

[0031] According to the present invention, the high-strength, high-ductility, low-density steel is smelted, cast, and forged into rectangular billets. The chemical composition is as follows by mass percentage: C: 1.05%, Mn: 10.0%, Al: 8.5%, Ni: 3.5%, Cr: 3.0%, V: 0.35%, S: 0.003%, P: 0.007%, with the remainder being Fe and unavoidable impurities.

[0032] Hot rolling process parameter control: heating temperature is 1180℃, holding time is 2.5h, initial rolling temperature is 1140℃, final rolling temperature is 900℃, hot rolling reduction rate is 85%, water cooling to room temperature after hot rolling, and hot rolled plate thickness is 5.5mm.

[0033] Cold rolling process parameter control: cold rolling reduction rate is 64%, and cold rolled plate thickness is 2.0 mm.

[0034] Recrystallization annealing process parameter control: heating rate is 90℃ / s, annealing temperature is 970℃, holding time is 90s, and the cooling method after recrystallization annealing is low-temperature water cooling with a water temperature of 0~5℃.

[0035] The high-strength, high-specific-strength, low-density steel prepared in this embodiment has a multi-scale layered structure. The microstructure consists of 16% banded δ-ferrite (average band width 2.3 μm), 5% equiaxed (Fe,Ni)Al-type B2 phase (average grain size 1.5 μm), and 79% austenite (average grain size 8.0 μm). The average grain size of the VC precipitate is 77 nm.

[0036] The high-strength-ductility-product, high specific strength, and low-density steel prepared in this embodiment has a yield strength of 1073 MPa, a tensile strength of 1337 MPa, an elongation of 38.4%, a density of 6.78 g / cm³, a strength-ductility-product of 51.3 GPa·%, and a specific strength of 197 MPa / (g / cm³). Example 3

[0037] According to the present invention, the high-strength, high-ductility, low-density steel is smelted, cast, and forged into rectangular billets. The chemical composition of the billet is as follows by mass percentage: C: 0.88%, Mn: 12.0%, Al: 11.5%, Ni: 2.6%, Cr: 2.2%, V: 0.50%, S: 0.005%, P: 0.008%, with the remainder being Fe and unavoidable impurities.

[0038] Hot rolling process parameter control: heating temperature is 1180℃, holding time is 1.5h, initial rolling temperature is 1140℃, final rolling temperature is 880℃, hot rolling reduction rate is 90%, water cooling to room temperature after hot rolling, and hot rolled plate thickness is 4.5mm.

[0039] Cold rolling process parameter control: cold rolling reduction rate is 72%, and cold rolled plate thickness is 1.3mm.

[0040] The parameters for recrystallization annealing are controlled as follows: heating rate is 80℃ / s, annealing temperature is 990℃, holding time is 300s, and the cooling method after recrystallization annealing is low-temperature water cooling with a water temperature of 0~5℃.

[0041] The high-strength, high-specific-strength, low-density steel prepared in this embodiment has a multi-scale layered structure. The microstructure consists of 18% banded δ-ferrite (average band width 1.5 μm), 7% equiaxed (Fe,Ni)Al-type B2 phase (average grain size 1.3 μm), and 75% austenite (average grain size 6.8 μm). The average grain size of the VC precipitate is 68 nm.

[0042] The high-strength-ductility-product, high specific strength, and low-density steel prepared in this embodiment has a yield strength of 1064 MPa, a tensile strength of 1328 MPa, an elongation of 39.3%, a density of 6.63 g / cm³, a strength-ductility-product of 52.2 GPa·%, and a specific strength of 200 MPa / (g / cm³). Example 4

[0043] According to the present invention, the high-strength, high-ductility, low-density steel is smelted, cast, and forged into rectangular billets. The chemical composition is as follows by mass percentage: C: 0.85%, Mn: 9.5%, Al: 8.5%, Ni: 2.5%, Cr: 2.0%, V: 0.33%, S: 0.004%, P: 0.008%, with the remainder being Fe and unavoidable impurities.

[0044] Hot rolling process parameter control: heating temperature is 1210℃, holding time is 2.5h, initial rolling temperature is 1170℃, final rolling temperature is 880℃, hot rolling reduction rate is 85%, water cooling to room temperature after hot rolling, and hot rolled plate thickness is 4.8mm.

[0045] Cold rolling process parameter control: cold rolling reduction rate is 60%, and cold rolled plate thickness is 1.9mm.

[0046] The parameters for recrystallization annealing are controlled as follows: heating rate is 100℃ / s, annealing temperature is 950℃, holding time is 60s, and the cooling method after recrystallization annealing is low-temperature water cooling with a water temperature of 0~5℃.

[0047] The high-strength, high-specific-strength, low-density steel prepared in this embodiment has a multi-scale layered structure. The microstructure consists of 15% banded δ-ferrite (average band width 2.5 μm), 5% equiaxed (Fe,Ni)Al-type B2 phase (average grain size 1.2 μm), and 80% austenite (average grain size 6.0 μm). The average grain size of the VC precipitate is 79 nm.

[0048] The high-strength-ductility-product, high specific strength, and low-density steel prepared in this embodiment has a yield strength of 1060 MPa, a tensile strength of 1317 MPa, an elongation of 40.1%, a density of 6.75 g / cm³, a strength-ductility-product of 52.8 GPa·%, and a specific strength of 195 MPa / (g / cm³). Example 5

[0049] According to the present invention, the high-strength, high-ductility, low-density steel is smelted, cast, and forged into rectangular billets. The chemical composition is as follows by mass percentage: C: 1.15%, Mn: 12.0%, Al: 10.0%, Ni: 2.7%, Cr: 2.5%, V: 0.40%, S: 0.005%, P: 0.005%, with the remainder being Fe and unavoidable impurities.

[0050] Hot rolling process parameter control: heating temperature is 1200℃, holding time is 2h, initial rolling temperature is 1160℃, final rolling temperature is 920℃, hot rolling reduction rate is 90%, water cooling after hot rolling, and hot rolled plate thickness is 4.5mm. Cold rolling process parameter control: cold rolling reduction rate is 70%, and cold rolled plate thickness is 1.4mm; The parameters for recrystallization annealing are controlled as follows: heating rate is 85℃ / s, annealing temperature is 980℃, holding time is 200s, and the cooling method after recrystallization annealing is low-temperature water cooling with a water temperature of 0~5℃.

[0051] The high-strength, high-specific-strength, low-density steel prepared in this embodiment has a multi-scale layered structure. The microstructure consists of 18% banded δ-ferrite (average band width 2.0 μm), 8% equiaxed (Fe,Ni)Al-type B2 phase (average grain size 1.1 μm), and 74% austenite (average grain size 7.5 μm). The average grain size of the VC precipitate is 75 nm.

[0052] The high-strength-ductility-product, high specific strength, and low-density steel prepared in this embodiment has a yield strength of 1078 MPa, a tensile strength of 1345 MPa, an elongation of 38.0%, a density of 6.68 g / cm³, a strength-ductility-product of 51.1 GPa·%, and a specific strength of 201 MPa / (g / cm³). Example 6

[0053] According to the present invention, the high-strength, high-ductility, low-density steel is smelted, cast, and forged into rectangular billets. The chemical composition of the billet is as follows by mass percentage: C: 1.10%, Mn: 10.5%, Al: 9.5%, Ni: 3.4%, Cr: 2.7%, V: 0.3%, S: 0.002%, P: 0.007%, with the remainder being Fe and unavoidable impurities.

[0054] Hot rolling process parameter control: heating temperature is 1220℃, holding time is 2h, initial rolling temperature is 1180℃, final rolling temperature is 920℃, hot rolling reduction rate is 93%, water cooling to room temperature after hot rolling, and hot rolled plate thickness is 4.2mm.

[0055] Cold rolling process parameter control: cold rolling reduction rate is 67%, and cold rolled plate thickness is 1.4mm.

[0056] Recrystallization annealing process parameter control: heating rate is 100℃ / s, annealing temperature is 1050℃, holding time is 180s, and the cooling method after recrystallization annealing is low-temperature water cooling with a water temperature of 0~5℃.

[0057] The high-strength, high-specific-strength, low-density steel prepared in this embodiment has a multi-scale layered structure. The microstructure consists of 19% banded δ-ferrite (average band width 2.1 μm), 7% equiaxed (Fe,Ni)Al-type B2 phase (average grain size 1.0 μm), and 74% austenite (average grain size 8.0 μm). The average grain size of the VC precipitate is 76 nm.

[0058] The high-strength-ductility-product, high specific strength, and low-density steel prepared in this embodiment has a yield strength of 1080 MPa, a tensile strength of 1349 MPa, an elongation of 38.2%, a density of 6.72 g / cm³, a strength-ductility-product of 51.5 GPa·%, and a specific strength of 201 MPa / (g / cm³). Example 7

[0059] According to the present invention, the high-strength, high-ductility, low-density steel is smelted, cast, and forged into rectangular billets. The chemical composition is as follows by mass percentage: C: 1.0%, Mn: 11.5%, Al: 10.0%, Ni: 3.1%, Cr: 2.9%, V: 0.45%, S: 0.002%, P: 0.005%, with the remainder being Fe and unavoidable impurities.

[0060] Hot rolling process parameter control: heating temperature is 1220℃, holding time is 1.5h, initial rolling temperature is 1180℃, final rolling temperature is 880℃, hot rolling reduction rate is 93%, water cooling to room temperature after hot rolling, and hot rolled plate thickness is 4.0mm.

[0061] Cold rolling process parameter control: cold rolling reduction rate is 75%, and cold rolled plate thickness is 1.0 mm.

[0062] The parameters for recrystallization annealing are controlled as follows: heating rate is 85℃ / s, annealing temperature is 1020℃, holding time is 100s, and the cooling method after recrystallization annealing is low-temperature water cooling with a water temperature of 0~5℃.

[0063] The high-strength, high-specific-strength, low-density steel prepared in this embodiment has a multi-scale layered structure. The microstructure consists of 20% banded δ-ferrite (average band width 1.9 μm), 10% equiaxed (Fe,Ni)Al-type B2 phase (average grain size 1.1 μm), and 70% austenite (average grain size 6.0 μm). The average grain size of the VC precipitate is 68 nm.

[0064] The high-strength-ductility-product, high specific strength, and low-density steel prepared in this embodiment has a yield strength of 1064 MPa, a tensile strength of 1308 MPa, an elongation of 39.3%, a density of 6.70 g / cm³, a strength-ductility-product of 51.4 GPa·%, and a specific strength of 195 MPa / (g / cm³).

[0065] Comparative Example 1 Compared with Example 1, the difference is that the heating rate of the recrystallization annealing process is 5°C / s, while the other components and processes remain the same.

[0066] See Figure 3 The grain size range of the B2 phase in the obtained steel microstructure increased to 3-5 μm.

[0067] The steel has a yield strength of 956 MPa, a tensile strength of 1230 MPa, an elongation of 31.4%, a density of 6.67 g / cm³, a strength-ductility product of 38.6 GPa·%, and a specific strength of 184 MPa / (g / cm³).

[0068] Comparative Example 2 Compared with Example 1, the difference is that the annealing temperature of the recrystallization annealing process is 850°C, while the other components and processes remain the same.

[0069] See Figure 4 The austenite in the prepared steel microstructure decomposes, the δ-ferrite boundary becomes blurred, and the multi-scale layered structure disappears.

[0070] The steel has a yield strength of 1403 MPa, a tensile strength of 1527 MPa, an elongation of 5.2%, a density of 6.67 g / cm³, a strength-ductility product of 7.9 GPa·%, and a specific strength of 229 MPa / (g / cm³).

[0071] Comparative Example 3 Compared with Example 1, the difference is that the cooling method of the recrystallization annealing process is air cooling, while the other components and processes remain the same.

[0072] See Figure 5 In the resulting steel microstructure, austenite grains grow, and κ carbides precipitate along the austenite grain boundaries.

[0073] The steel has a yield strength of 1237 MPa, a tensile strength of 1389 MPa, an elongation of 22.3%, a density of 6.67 g / cm³, a strength-ductility product of 31.0 GPa·%, and a specific strength of 208 MPa / (g / cm³).

[0074] The results of the three comparative examples show that, under the condition of consistent composition, the parameters of the recrystallization annealing process play a crucial role in microstructure control and mechanical properties. Reducing the heating rate to 5℃ / s leads to coarsening of the B2 phase grains, reducing the strengthening effect of fine grains; reducing the annealing temperature to 850℃ causes austenite decomposition, disappearance of the multi-scale layered structure, and severe deterioration of plasticity; changing the cooling method from low-temperature water cooling to air cooling causes austenite grain growth and precipitation of κ carbides at grain boundaries, which disrupts the microstructure stability and deformation coordination.

[0075] Finally, it should be noted that the above-listed embodiments are merely preferred embodiments of the present invention. Of course, those skilled in the art can make modifications and variations to the present invention. If such modifications and variations fall within the scope of the claims of the present invention and their equivalents, they should be considered as being within the protection scope of the present invention.

Claims

1. A high-strength, high-ductility, low-density steel, characterized in that, The chemical composition and mass percentage of high strength-ductility, high specific strength, and low density steel are as follows: C: 0.85–1.15%, Mn: 9.5–12.0%, Al: 8.5–11.5%, Ni: 2.5–3.5%, Cr: 2.0–3.0%, V: 0.3–0.5%, S≤0.005%, P≤0.008%, with the remainder being Fe and unavoidable impurities; The microstructure of high strength-ductility, high specific strength, and low density steel products consists of banded δ ferrite, equiaxed (Fe,Ni)Al type B2 phase, austenite, and VC nano-precipitates dispersed in the matrix. The proportion of banded δ-ferrite is 15-20%, with an average band width of 1.5-2.5 μm; the proportion of equiaxed (Fe,Ni)Al-type B2 phase is 5-10%, with a grain size ≤1.5 μm; the proportion of austenite is 70-80%, with a grain size of 6-8 μm; and the average particle size of nanoscale VC precipitate is ≤80 nm.

2. The high-strength, high-specific-strength, low-density steel according to claim 1, characterized in that, High-strength, high-ductility, low-density steel products have a yield strength ≥1060MPa, tensile strength ≥1300MPa, elongation ≥38.0%, and density of 6.63~6.78g / cm³. 3 Strength-ductility volume ≥ 50 GPa•%, specific strength ≥ 195 MPa / (g / cm²) 3 ).

3. A method for preparing high-strength, high-ductility, low-density steel according to any one of claims 1-2, comprising smelting, casting, forging, hot rolling, cold rolling, and recrystallization annealing steps, characterized in that, The hot rolling process parameters are controlled as follows: the rectangular billet is heated to a temperature of 1180-1220℃, held for 1.5-2.5 hours, the initial rolling temperature is 1140-1180℃, the final rolling temperature is 880-920℃, the hot rolling reduction rate is 85-93%, and the billet is water-cooled to room temperature after hot rolling. The thickness of the hot-rolled plate is 4.0-5.5 mm. The cold rolling process parameters are controlled as follows: the cold rolling reduction rate is 60-75%, and the cold rolled plate thickness is 1.0-2.0 mm; The parameters for the recrystallization annealing process are controlled as follows: heating rate is 80-100℃ / s, annealing temperature is 950-1050℃, holding time is 60-300s, and the cooling method after recrystallization annealing is low-temperature water cooling with a water temperature of 0-5℃.

Images