Nb-Mo-V-Si-B gold high-strength plastic hot-rolled low-density steel and preparation method thereof

Abstract

The invention relates to Nb-Mo-V-Si-B alloyed high-strength plasticity hot-rolled low-density steel and a preparation method thereof, and belongs to the technical field of ferrous metallurgy and metal material machining, the low-density steel is composed of the following chemical components of, by mass, 0.72%-0.85% of C, 7.50%-9.30% of Al, 13.00%-17.00% of Mn, 0.50%-0.65% of Si, 0.15%-0.35% of Nb, 0.30%-0.50% of Mo, 0.30%-0.40% of V, 0.001%-0.003% of B, smaller than or equal to 0.008% of P, smaller than or equal to 0.0012% of S and the balance Fe and inevitable impurities. According to the method, the Fe-Mn-Al-C series low-density steel serves as a base body, technological conditions are optimized, the steel which is excellent and stable in mechanical property and has high strength and high plasticity is obtained, the preparation process is simple, consumed time is short, the production efficiency of the high-performance low-density steel can be greatly improved, and the method is suitable for structural application scenes with the requirements for weight reduction and high strength and plasticity.

Description

Technical Field

[0001] This invention relates to a Nb-Mo-V-Si-B alloyed high-strength, high-ductility hot-rolled low-density steel and its preparation method, belonging to the field of iron and steel metallurgy and metal material processing technology. Background Technology

[0002] With the rapid development of modern industry and continuous technological progress, key sectors such as automobiles, aerospace, rail transportation, shipbuilding, and energy equipment have placed higher demands on the safety, reliability, and operational efficiency of structural components, and have correspondingly imposed more stringent standards on the comprehensive performance of high-performance structural materials. Lightweighting, high strength, and high ductility are considered key indicators in material design and application. Although traditional high-strength steels (such as ultra-high-strength low-alloy steel and martensitic steel) exhibit excellent strength performance, they cannot fundamentally solve the weight reduction requirement, and their plasticity often decreases significantly with increasing strength, leading to problems such as poor formability, limited weldability, and reduced service safety. Therefore, it is difficult to simultaneously achieve both lightweighting and a synergistic improvement in overall performance.

[0003] In recent years, new types of steel, represented by Fe-Mn-Al-C series high-strength low-density steel, have gradually become ideal choices for lightweight materials due to their ultra-high strength (1000MPa level) and low density. However, maintaining a certain level of ductility while increasing strength is a major challenge in steel research and development. Ultra-high strength steel usually faces a trade-off between strength and ductility: increasing strength often leads to a significant reduction in material ductility, making it prone to cracking or brittle fracture during forming. Therefore, how to effectively maintain a certain level of ductility while ensuring ultra-high strength has become a key technical challenge in material development.

[0004] Invention patent CN 111235484 A discloses a high-strength, high-hardness, low-density steel and its preparation method. The alloy composition by mass percentage is: C 0.7%~1.8%, Mn 25%~34%, Al 8%~12%, Si 0.3%~0.9%, Cr 0.3%~1.2%, V 0.1%~0.7%, Ti 0.1%~0.8%, Mo 0.7%~1.3%, with the remainder being Fe and unavoidable impurities. After smelting, die casting, hot rolling, water quenching, hot rolling, solution treatment, cold rolling, aging treatment, nitriding, and denitrification treatment, a low-density steel with a yield strength of 870.21~1077.36 MPa and a tensile strength of 950.35~1027.70 MPa is obtained. However, the invention does not mention elongation as an indicator, making it difficult to assess the plasticity of the material. Furthermore, the high Mn content in the steel described in this invention easily leads to metallurgical defects such as segregation, porosity, and inclusions during smelting and casting, resulting in difficulties in composition control, uneven microstructure, and large performance fluctuations. In addition, its preparation process is lengthy and complex, especially involving demanding nitriding and denitrification treatments, leading to high energy consumption, long cycles, and high costs, severely restricting its feasibility and economic viability for industrial application. Invention patent CN 116065081 B discloses a 1000 MPa grade low-density steel bar and its preparation method. The alloy composition by mass percentage is as follows: C: 0.31%~0.34%; Mn: 12.1%~12.5%; Al: 8.51%~8.55%; Ni: 10.0%~15.0%; P: 0.005%~0.008%; S: 0.001%~0.002%; H: 0.0004%~0.0005%; O: 0.001%~0.008%; N: 0.003%~0.005%, with the remainder being Fe and unavoidable impurity elements. After smelting and casting, high-temperature homogenization followed by finishing, forging, solution treatment, quenching, and aging treatment, a low-density steel with a tensile strength ≥1000 MPa, yield strength ≥900 MPa, and elongation after fracture ≥30% is obtained. However, the preparation method of the low-density steel described in this invention also suffers from problems such as numerous steps and long cycles. Furthermore, the steel contains a high concentration of the high-cost metal Ni and various trace impurity elements (H, O, N), which places higher demands on the purity of the smelting material and the control of its composition, increasing the uncertainty and technical difficulty of the metallurgical process. Invention patent CN 107674955 A discloses a Fe-Mn-Al-C series high-strength low-density steel with a strength-ductility product greater than 50 GPa·%, and its alloy composition by mass percentage is: C: 0.98%~1.00%, Mn: 19.4%~20.00%, Al: 9.82%~10.00%, P: ≤0.003%, S: ≤0.003%, with the balance being Fe and unavoidable impurities.Although this steel has certain performance advantages, its manufacturing process is complex, requiring multiple steps including smelting, solidification casting, hot rolling, solution treatment, at least two cold rolling processes, and annealing. The entire process is cumbersome, time-consuming, energy-intensive, and requires sophisticated equipment, making it unsuitable for large-scale industrial production. Furthermore, the high Mn and Al content in this alloy system further exacerbates elemental segregation, inclusion formation, and microstructure control, posing significant challenges to alloy homogeneity and compositional stability.

[0005] In summary, current technologies for producing low-density steel with tensile strengths of 1000 MPa while maintaining high ductility typically require complex manufacturing processes. Furthermore, most alloy systems rely on high-cost or high-content alloying elements, generally resulting in difficulties in composition control and poor industrial adaptability. Therefore, developing new types of steel that combine low density, high strength, and good ductility to overcome the application bottlenecks of traditional high-density steel in lightweight applications is a crucial direction for current research in advanced metallic materials. Summary of the Invention

[0006] To address the aforementioned problems in existing technologies, this invention provides a Nb-Mo-V-Si-B alloyed high-strength, high-ductility hot-rolled low-density steel and its preparation method. This invention, through the control of alloying elements such as Nb, Mo, V, Si, and B in Fe-Mn-Al-C system high-strength, low-density steel, as well as processing technology and quenching parameters, produces a low-density steel with both high strength and high elongation. The production cycle is short and efficient, and the prepared steel possesses high strength, low density, good ductility, and excellent strength-ductility product, demonstrating significant potential for industrial application.

[0007] A Nb-Mo-V-Si-B alloyed high-strength, high-ductility, hot-rolled, low-density steel is composed of the following chemical composition by mass percentage: C 0.72%~0.85%, Al 7.50%~9.30%, Mn 13.00%~17.00%, Si 0.50%~0.65%, Nb 0.15%~0.35%, Mo 0.30%~0.50%, V 0.30%~0.40%, B 0.001~0.003%, P≤0.008%, S≤0.0012%, with the balance being Fe and unavoidable impurities.

[0008] Furthermore, the microstructure of the low-density steel is a multiphase structure composed of austenite, δ-ferrite, and κ-carbides, wherein austenite is the main matrix structure, accounting for 70% to 80% of the total structure.

[0009] Furthermore, the low-density steel has a tensile strength of 770-785 MPa, a tensile strength of 1041-1051 MPa, an elongation after fracture of 38.0%-40.1%, a strength-ductility product of 39.5-42.2 GPa·s, and a density of 6.86-7.09 g / cm³. 3 .

[0010] In this invention, each alloying element plays a synergistic and important role in microstructure regulation and performance improvement. The main functions of each alloying element are as follows: C: As an austenite stabilizing element, C exists in the form of interstitial solid solution or carbides, and is the core element for achieving structural strengthening and high strength. Mn: Mn is a key austenite stabilizing element. It significantly improves the strength of steel through solid solution strengthening and enhances the stability of austenite, which helps to obtain excellent ductility and plasticity. Furthermore, increasing its content is beneficial to further improving the plasticity of the material. Al: Al exists in the form of substitutional solid solution, which can effectively reduce the density of steel, refine grains, improve plasticity, and at the same time improve high-temperature stability and corrosion resistance. Si: Si can promote the formation of ferrite, enhance the antioxidant capacity and thermal stability, and at the same time facilitate the uniform precipitation of κ-carbides; B: The introduction of trace amounts of B can significantly improve the hardenability of steel, thereby optimizing the quenching process and further improving the overall mechanical properties; Nb can form dispersed carbonitrides (such as NbC and Nb(C,N)) with C and N at high temperatures, which can suppress grain growth by pinning austenite grain boundaries and achieve significant grain refinement, thereby improving the strength and toughness of steel. On the other hand, carbonitrides precipitated during cooling provide precipitation strengthening through the Orowan mechanism, further enhancing the strength of steel. In addition, Nb can delay the recrystallization process and increase the recrystallization temperature, which is beneficial to the stable control of the microstructure during hot working and lays the foundation for excellent comprehensive mechanical properties. Mo: Mo enhances strength through a combination of solid solution strengthening and carbide precipitation strengthening. The high-melting-point carbide Mo2C formed by Mo can effectively suppress dislocation movement at high temperatures, enhancing high-temperature strength and thermal stability. At the same time, the solid solution of Mo atoms in the matrix induces lattice distortion, increasing the resistance to dislocation movement and further enhancing strength. In addition, Mo can improve hardenability and promote microstructure homogenization, thereby improving the plasticity and toughness of the material. V: V inhibits austenite grain growth and achieves grain refinement by forming stable carbides such as VC and V4C3 at high temperatures; during cooling, these carbides precipitate diffusely and enhance yield strength through the Orowan mechanism; at the same time, V can also form VN with N, which has higher thermal stability. Compared with other carbides, its precipitation strengthening effect is more significant and has less impact on plasticity, which is conducive to achieving a good synergy between strength and toughness. P and S are unfavorable elements in steel, which seriously affect the strength, toughness and corrosion resistance of steel. Their content needs to be controlled at the lowest possible level. Therefore, this invention controls the content of P to below 0.008% and the content of S to below 0.0012%.

[0011] Another object of the present invention is to provide a method for preparing the above-mentioned Nb-Mo-V-Si-B alloyed high-strength and ductile hot-rolled low-density steel, including batching, vacuum smelting and casting, controlled rolling and cooling, and quenching processes. The controlled rolling and cooling process includes heat treatment, rough rolling, finish rolling and cooling steps. The quenching process involves treating the rolled steel billet at 820~860℃ for 20 min and then rapidly water cooling it to room temperature.

[0012] In the above technical solution, the vacuum smelting and casting process involves heating and melting the prepared raw materials, pouring them into a casting mold, and cooling them to obtain a low-density steel ingot with a thickness of 100~150 mm.

[0013] In the above technical solution, the heat treatment step in the controlled rolling and cooling process involves heating the ingot in a homogenizing furnace for a homogenization time of ≥60 min, a furnace time of 2 h, and a tapping temperature of 1150~1180℃.

[0014] In the above technical solution, the rough rolling step in the controlled rolling and cooling process involves rough rolling the heat-treated ingot using a large reduction method. The initial rolling temperature is 1080~1100℃, and the ingot is rolled in 4 passes. The reduction rate of the last three passes is ≥20%, and the cumulative compression ratio is 40%~55%.

[0015] Furthermore, the thickness of the ingot obtained after rough rolling is 40~55 mm.

[0016] In the above technical solution, the finishing rolling step in the controlled rolling and cooling process involves finishing rolling the ingot obtained after rough rolling. The initial rolling temperature is 880~950℃, the final rolling temperature is 790~820℃, and the rolling is performed in 10 passes with a reduction rate of 13%~24% per pass, resulting in a cumulative reduction rate of 88%~94%. The initial cooling temperature after finishing rolling is 780~800℃.

[0017] In the above technical solution, the cooling step in the controlled rolling and cooling process involves cooling the ingot obtained after precision rolling to room temperature using laminar flow cooling to obtain a rolled steel billet with a thickness of 4~8 mm.

[0018] This invention optimizes the alloying elements such as Nb, Mo, V, Si, and B in Fe-Mn-Al-C high-strength, low-density steel, combined with a simplified hot-working process (controlled rolling and cooling with one-step quenching). This achieves a tensile strength exceeding 1040 MPa while effectively maintaining the material's ductility, resulting in a good synergy between strength and plasticity. The preparation method of this invention has a short process flow, significantly simplifies heat treatment steps, shortens the production cycle, reduces energy consumption, and improves production efficiency. The resulting steel possesses high strength, low density, good plasticity, and excellent strength-ductility product, exhibiting outstanding potential for industrial applications, especially suitable for critical fields with stringent comprehensive performance requirements, such as automobile manufacturing.

[0019] The beneficial effects of this invention are: In terms of alloy composition design, this invention is based on Fe-Mn-Al-C low-density steel, and introduces alloying elements Si, Nb, Mo, V and microalloying element B to construct a multi-element alloy system with synergistic strengthening effect. While maintaining the low-density characteristics of steel, this design fully utilizes the synergistic effect between various strengthening mechanisms (such as solid solution strengthening, precipitation strengthening, grain refinement strengthening and dislocation strengthening), effectively improving the comprehensive mechanical properties of the material and achieving a high degree of synergistic matching between high strength and high plasticity.

[0020] The low-density steel prepared by this invention exhibits a stable austenite + δ-ferrite + κ-carbide multiphase structure, in which austenite is the main matrix structure, accounting for 70-80% of the total structure, providing an excellent plasticity basis for the material; δ-ferrite improves the thermal stability and processing stability of the structure, and helps to control the evolution of the structure and the forming performance during hot working; κ-carbide is dispersed in the grain boundaries of austenite and δ-ferrite, which can effectively pin dislocations and inhibit grain boundary slip, significantly improving the strength and high-temperature performance of the material.

[0021] The preparation process employed in this invention is simple and efficient, not relying on complex cold rolling and multi-step heat treatment processes. Stable microstructure and excellent properties can be obtained through conventional smelting-casting, controlled rolling and cooling, and a single-step quenching treatment. This short-process technology not only simplifies the industrial manufacturing path and significantly reduces energy consumption and production costs, but also improves preparation efficiency and engineering adaptability, demonstrating good scalability and promising prospects for industrial application.

[0022] The density of the low-density steel prepared by this invention is controlled between 6.86 and 7.09 g / cm³. 3It is 9.5% to 12.4% lower than that of traditional high-strength steel. While achieving lightweighting, it also exhibits excellent mechanical properties, with tensile strength ≥1040 MPa, elongation ≥38%, and strength-ductility product ≥39 GPa • % , fully demonstrating the optimal synergistic characteristics of this material in terms of strength, ductility and strength. Attached Figure Description

[0023] Figure 1 This is a stress-strain curve of the steel in Embodiment 4 of the present invention; Figure 2 This is a microstructure morphology diagram of the steel in Example 4 of the present invention; Figure 3 This is the XRD pattern of the steel in Embodiment 4 of the present invention. Detailed Implementation

[0024] The following non-limiting embodiments are intended to enable those skilled in the art to more fully understand the invention, but do not limit the invention in any way.

[0025] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0026] One of the specific implementation methods: A method for preparing Nb-Mo-V-Si-B alloyed high-strength, high-ductility, hot-rolled, low-density steel specifically includes the following steps: (1) Ingredients: Weigh the raw materials according to the following mass percentages: C 0.72%~0.85%, Al 7.50%~9.30%, Mn 13.00%~17.00%, Si 0.50%~0.65%, Nb 0.15%~0.35%, Mo 0.30%~0.50%, V 0.30%~0.40%, B 0.001~0.003%, P: ≤0.008%, S: ≤0.0012%, with the balance being Fe and unavoidable impurities; (2) Vacuum smelting and casting: The raw materials prepared in step (1) are placed into a vacuum smelting furnace for heating and melting. The molten metal obtained by smelting is poured into a casting mold and cooled to obtain a low-density steel ingot with a thickness of 100~150 mm. (3) Controlled rolling and cooling: The ingot obtained in step (2) is heated in a soaking furnace for a soaking time of ≥60 min and a furnace time of 2 h. The tapping temperature is 1150~1180℃. Then, a two-stage rolling process of roughing and finishing is adopted. The roughing start temperature is 1080~1100℃. After 4 passes of rolling, the reduction rate of the last three passes is ≥20%. Large reduction rolling is adopted. The cumulative reduction ratio of roughing is 40%~45%. The finishing stage start temperature is 880~950℃. The finishing temperature is 790~820℃. After 10 passes of rolling, the reduction rate of each pass is 13%~24%. The cumulative reduction rate is about 88%~94%. The initial cooling temperature after finishing is 780~800℃. After rolling, laminar flow cooling is adopted to cool to room temperature to obtain a rolled steel billet with a thickness of 4~8 mm. (4) Quenching treatment: The rolled steel billet is heated in a high-temperature furnace at a temperature of 820~860℃ for 20 min, and then rapidly water-cooled to room temperature to obtain Nb-Mo-V-Si-B alloyed high-strength and plastic hot-rolled low-density steel with a thickness of 4~8 mm.

[0027] In the method of the present invention, in step (3), the thickness of the ingot obtained after rough rolling is 40~55 mm.

[0028] In the method described in this invention, the Nb-Mo-V-Si-B alloyed high-strength, high-ductility hot-rolled low-density steel has a tensile strength of 1041-1051 MPa, an elongation after fracture of 38.0%-40.1%, a strength-ductility product of 39.5-42.2 GPa·%, and a density of 6.86-7.09 g / cm³. 3 .

[0029] Examples 1-3 The preparation method of Nb-Mo-V-Si-B alloyed high-strength and ductile hot-rolled low-density steel with thicknesses of 4 mm, 6 mm and 8 mm respectively specifically includes the following steps: (1) Ingredients: Ingredients were prepared according to the chemical composition of Examples 1-3 given in Table 1; (2) Smelting and casting: The prepared raw materials are placed in a vacuum smelting furnace for heating and melting. The molten metal obtained from smelting is poured into a casting mold and cooled to obtain low-density steel ingots with thicknesses of 100 mm, 130 mm and 150 mm respectively. (3) Controlled rolling and cooling: The low-density steel ingots of different thicknesses obtained above were heated in a soaking furnace. The holding time in the furnace was 60 min, 75 min and 90 min respectively, and the holding time in the furnace was 2 h. The tapping temperatures were 1150℃, 1165℃ and 1180℃ respectively. Then rough rolling and finish rolling were carried out. The rough rolling start temperature was 1080℃, 1090℃ and 1100℃ respectively. After 4 passes of rolling, the reduction rate and intermediate billet thickness are shown in Table 2. The finish rolling start temperature was 880℃, 900℃ and 950℃ respectively. After 12 passes of rolling, the reduction rate is shown in Table 2. The final rolling temperature was 790℃, 800℃ and 820℃ respectively. The initial cooling temperature after finish rolling was 780℃, 790℃ and 800℃ respectively. After rolling, laminar flow cooling was used to cool to room temperature to obtain rolled steel billets with thicknesses of 4 mm, 6 mm and 8 mm respectively. (4) Quenching treatment: The rolled steel billets with thicknesses of 4 mm, 6 mm and 8 mm obtained above are heated in a high-temperature furnace at temperatures of 820℃, 840℃ and 860℃ respectively, for a duration of 20 min. Then they are rapidly water-cooled to room temperature to obtain Nb-Mo-V-Si-B alloyed high-strength and ductile hot-rolled low-density steel with thicknesses of 4 mm, 6 mm and 8 mm respectively.

[0030] Examples 4-6 Compared with Example 3 (thickness 8 mm), Examples 4-6 differ in that the mass percentage of chemical components in step (1) and the process parameters in steps (2)-(4) are different. The specific mass percentage is shown in Table 1 and the process parameters are shown in Table 2.

[0031] Comparative Example 1 The difference between this comparative example and Example 4 is that the quenching temperature in step (4) of this comparative example is 800°C.

[0032] Comparative Example 2 The difference between this comparative example and Example 4 is that the quenching temperature in step (4) of this comparative example is 900°C.

[0033] Comparative Example 3 The difference between this comparative example and Example 4 is that the C content in step (1) of this comparative example is 0.5 wt.%, which is relatively low.

[0034] Comparative Example 4 The difference between this comparative example and Example 4 is that the Mn content in step (1) of this comparative example is only 11.34 wt.%, and it does not contain alloying elements Nb, Mo and V.

[0035] Comparative Example 5 The difference between this comparative example and Example 4 is that step (1) in this comparative example does not contain Si element.

[0036] The chemical composition of the steels obtained in Examples 1-6 and Comparative Examples 1-5 of this invention is shown in Table 1, the main process parameters are shown in Table 2, and the performance tests are conducted in accordance with GB / T 228.1-2021. The yield strength, tensile strength, elongation after fracture, and strength-ductility product of the steels obtained above were tested and calculated. The results are shown in Table 3.

[0037] Table 1 Chemical composition (mass percentage) of the examples and comparative examples

[0038] Table 2. Process parameters for the examples and comparative examples

[0039] Table 3 Performance parameters of the steel obtained in the examples and comparative examples

[0040] To verify the rationality of the alloy composition design and heat treatment system of this invention, Example 4 and Comparative Examples 1-5 were selected for systematic comparison. The main variables in each experimental group included quenching temperature and the content of key alloying elements (C, Mn, Nb, Mo, V, Si), etc., to explore their influence on the microstructure and mechanical properties of the experimental steel. The results are as follows: Figure 1 The stress-strain curve of the steel in Example 4 of this invention shows that the stress-strain curve of the steel obtained in Example 4 exhibits excellent strength-plasticity matching characteristics. Its yield strength is 773 MPa, its tensile strength reaches 1041 MPa, and its elongation after fracture is as high as 38.0%, which combines high strength and high ductility. Figure 2 This is a microstructure image of the steel used in Example 4 of the present invention. Figure 3 The XRD pattern of the steel shows that the microstructure of the steel obtained in Example 4 is mainly composed of austenite, accounting for approximately 73%. Figure 3 As shown in the figure, rod-shaped and granular δ-ferrites are distributed therein, as well as finely dispersed κ-carbides.

[0041] The characteristics of this structure endow the material with excellent comprehensive properties, which are mainly reflected in the following aspects: (1) As a face-centered cubic structure, austenite has a lot of slip systems, which is conducive to deformation coordination and energy absorption. It is the key phase to achieve high elongation. At the same time, the high stability of the austenite matrix and the potential deformation-induced martensitic transformation (TRIP effect) help to continuously improve the strain hardening rate during the tensile process, delay necking and improve the uniform plastic deformation capacity of the material; (2) δ ferrite is dispersed in austenite, effectively refining the austenite grains and enhancing the rigidity of the matrix, providing support for the external load, thereby improving the yield strength and improving the stress distribution. The introduction of δ ferrite also enhances the ability to hinder crack propagation under multiaxial stress, which helps to improve the overall fracture toughness. (3) κ-carbides are dispersed along grain boundaries and dislocation-intensive regions, which achieve second-phase strengthening by hindering dislocation movement, while promoting the improvement of strain hardening capacity, thereby maintaining good plasticity while ensuring high strength.

[0042] In summary, the synergistic regulation of austenite, δ-ferrite, and κ-carbides in the steel obtained in Example 4 significantly improved the stability of the microstructure and stress bearing capacity, achieving a good match between strength and plasticity.

[0043] The main difference between Comparative Example 1 and Example 4 is that the quenching temperature was reduced to 800°C. This temperature is lower than the critical heating temperature required for full austenitization, resulting in incomplete austenite transformation and a large amount of residual ferrite and undissolved large-sized carbide particles in the microstructure. The coexistence of residual ferrite and austenite forms a multiphase mixed microstructure, which disrupts the uniformity of the matrix, weakens deformation compatibility and crack propagation resistance, and significantly affects the overall plasticity and toughness of the material. At the same time, undissolved carbides cannot participate in dispersed precipitation strengthening during subsequent cooling, resulting in a reduction in the number, size, and uneven distribution of strengthening phases, significantly reducing the precipitation strengthening effectiveness. In addition, ferrite itself has low strength and hardness, and its uneven distribution can become a stress concentration source during plastic deformation, further exacerbating the risk of local yielding and early cracking. In summary, excessively low quenching temperatures are not only detrimental to the formation of a uniform and stable austenitic matrix structure, but also hinder the effective precipitation of strengthening phases, ultimately leading to a significant decrease in the comprehensive mechanical properties of the material, such as yield strength, tensile strength, and elongation.

[0044] The main difference between Comparative Example 2 and Example 4 is that the quenching temperature is increased to 900°C. Although this temperature ensures complete austenitization, excessively high heat treatment temperatures significantly promote austenite grain growth, leading to reduced grain boundary area, grain coarsening, and weakening the grain boundary strengthening mechanism. Furthermore, large grain sizes also reduce the dislocation density and the number of substructures in the material, resulting in insufficient precipitation or large-sized precipitation phases (such as κ-carbides), further weakening precipitation strengthening. In the presence of alloying elements (such as Nb, Mo, and V), excessively high temperatures can also cause some alloy carbides to dissolve excessively, losing their ability to disperse and strengthen within grain boundaries and pin grain boundaries, leading to disordered grain growth and affecting microstructure stability and mechanical properties, manifested as a slight decrease in plasticity and a significant decrease in tensile strength and yield strength. The main difference between Comparative Example 3 and Example 4 is that the carbon content is reduced to 0.5 wt.%. Carbon is a key element in the formation of strengthening phases such as κ-carbides and is also an important means of achieving solid solution strengthening. The decrease in its content directly affects the effectiveness of the two major strengthening mechanisms: firstly, the reduced solubility of carbon in austenite weakens lattice distortion and dislocation density formation, thus weakening the solid solution strengthening effect; secondly, the reduced amount and coarsening of κ-carbides significantly decrease precipitation strengthening efficiency. Furthermore, the reduction in carbon content also reduces the TRIP or TWIP mechanism, resulting in insufficient phase transformation ability and loss of strain-induced strengthening effect. Ultimately, this leads to a decrease in the overall strength of the experimental steel, poor strength-toughness synergy, and ineffective elongation.

[0045] The main difference between Comparative Example 4 and Example 4 is that the Mn content is reduced to 11.34 wt.%, and the alloying elements Nb, Mo, and V are absent. As a key element for stabilizing austenite, a decrease in Mn content leads to austenite shrinkage, increases the tendency for ferrite or pearlite formation, reduces the thermal stability of the microstructure at high temperatures, and affects the uniformity of the microstructure after final cooling. Furthermore, Mn has strong diffusion capabilities, promoting carbon diffusion and the precipitation of strengthening phases; its reduction weakens the ability to regulate carbide precipitation behavior. More critically, the absence of microalloying elements such as Nb, Mo, and V prevents the formation of fine, dispersed second-phase particles, such as NbC, Mo2C, and VC, in the steel. These particles could normally inhibit grain growth through the Zener drag effect while providing precipitation strengthening. Their absence directly leads to grain coarsening, matrix softening, and a significant reduction in the number of strengthening phases, thus comprehensively disrupting multiple mechanisms such as grain refinement, precipitation strengthening, and grain boundary strengthening, ultimately resulting in a significant decrease in the material's strength and plasticity.

[0046] The main difference between Comparative Example 5 and Example 4 lies in the reduced Si content in the steel. Si is a typical ferrite-forming element, and at medium to high temperatures, it plays multiple roles, including improving deoxidation efficiency, stabilizing grain boundary structure, enhancing thermal stability, and promoting the precipitation of κ-carbides. The decrease in Si content weakens the stability of ferrite, leading to changes in the behavior of the austenite-ferrite phase region, making it easier to form multiphase structures and reducing microstructure uniformity. Furthermore, Si significantly promotes the precipitation kinetics of κ-carbides; its reduced content will delay the nucleation and growth of the strengthening phase, resulting in insufficient precipitation, increased size, uneven distribution, and reduced precipitation strengthening effect. Ultimately, this leads to a decrease in the strength and plasticity of the experimental steel.

Claims

1. A Nb-Mo-V-Si-B alloyed high-strength, high-ductility, hot-rolled, low-density steel, characterized in that: The low-density steel is composed of the following chemical composition by mass percentage: C 0.72%~0.85%, Al 7.50%~9.30%, Mn 13.00%~17.00%, Si 0.50%~0.65%, Nb 0.15%~0.35%, Mo 0.30%~0.50%, V 0.30%~0.40%, B 0.001~0.003%, P≤0.008%, S≤0.0012%, with the balance being Fe and unavoidable impurities.

2. The Nb-Mo-V-Si-B alloyed high-strength, high-ductility, hot-rolled, low-density steel according to claim 1, characterized in that: The microstructure of the low-density steel is a multiphase structure composed of austenite, δ-ferrite, and κ-carbides, with austenite being the main matrix structure, accounting for 70% to 80% of the total microstructure.

3. The Nb-Mo-V-Si-B alloyed high-strength, high-ductility, hot-rolled, low-density steel according to claim 1, characterized in that: The low-density steel has a yield strength of 770-785 MPa, a tensile strength of 1041-1051 MPa, an elongation after fracture of 38.0%-40.1%, a strength-ductility product of 39.5-42.2 GPa·s, and a density of 6.86-7.09 g / cm³. 3 .

4. The method for preparing Nb-Mo-V-Si-B alloyed high-strength, high-ductility, hot-rolled, low-density steel according to any one of claims 1 to 3, characterized in that: The preparation method includes batching, vacuum smelting and casting, controlled rolling and cooling, and quenching processes. The controlled rolling and cooling process includes heat treatment, rough rolling, finish rolling and cooling steps. The quenching process involves treating the rolled steel billet at 820~860℃ for 20 min and then rapidly water cooling it to room temperature.

5. The preparation method according to claim 4, characterized in that: The vacuum smelting and casting process involves heating and melting the prepared raw materials, pouring them into a casting mold, and cooling them to obtain a low-density steel ingot with a thickness of 100-150 mm.

6. The preparation method according to claim 4, characterized in that: In the heat treatment step of the controlled rolling and cooling process, the ingot is heated in a soaking furnace for a soaking time of ≥60 min, the furnace time is 2 h, and the tapping temperature is 1150~1180℃.

7. The preparation method according to claim 4, characterized in that: In the controlled rolling and cooling process, the rough rolling step involves rough rolling the heat-treated ingot using a large reduction method. The initial rolling temperature is 1080~1100℃, and the ingot is rolled in 4 passes. The reduction rate of the last three passes is ≥20%, and the cumulative compression ratio is 40%~55%.

8. The preparation method according to claim 7, characterized in that: The thickness of the ingot obtained after rough rolling is 40~55mm.

9. The preparation method according to claim 4, characterized in that: In the controlled rolling and cooling process, the ingot obtained after rough rolling is subjected to finish rolling. The initial rolling temperature is 880~950℃, the final rolling temperature is 790~820℃, and it is rolled in 10 passes with a reduction rate of 13%~24% per pass, resulting in a cumulative reduction rate of 88%~94%. The initial cooling temperature after finish rolling is 780~800℃.

10. The preparation method according to claim 4, characterized in that: In the controlled rolling and controlled cooling process, the ingot obtained after precision rolling is cooled to room temperature using laminar flow cooling to obtain a rolled steel billet with a thickness of 4~8 mm.

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