Nb-mo-v-si-b alloyed high strength-to-plasticity hot-rolled low-density steel and method for manufacturing the same

Abstract

The application relates to an Nb-Mo-V-Si-B alloyed high-strength and high-plasticity hot-rolled low-density steel and a preparation method thereof, and belongs to the technical field of steel metallurgy and metal material processing. The low-density steel is composed of the following chemical components in percentage by mass: 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%, and the balance of Fe and inevitable impurities. The application takes Fe-Mn-Al-C low-density steel as a matrix, optimizes process conditions, obtains steel with excellent and stable mechanical properties, high strength and high plasticity, has a simple preparation flow, short time consumption, can greatly improve the production efficiency of high-performance low-density steel, and is suitable for structural application scenarios with requirements of weight reduction and high strength and plasticity.

Description

TECHNICAL FIELD

[0001] The present application relates to a Nb-Mo-V-Si-B alloyed high strength and ductility hot-rolled low-density steel and a preparation method thereof, and belongs to the technical field of steel metallurgy and metal material processing. BACKGROUND

[0002] With the rapid development of modern industry and the continuous progress of technology, higher requirements for the safety, reliability and operating efficiency of structural parts in key fields such as automobiles, aerospace, rail transportation, shipbuilding and energy equipment are put forward, and the comprehensive performance of high-performance structural materials is also correspondingly put forward. Among them, lightweight, high strength and high ductility are regarded as key indicators in material design and application. Although traditional high-strength steels (such as ultra-high-strength low-alloy steels, martensitic steels, etc.) perform excellently in strength performance, they cannot fundamentally solve the weight reduction demand, and their plasticity often decreases significantly with the increase of strength, resulting in poor formability, limited weldability, and decreased service safety, etc., so as to be difficult to balance the lightweight and comprehensive performance of the structure.

[0003] In recent years, new types of steels represented by Fe-Mn-Al-C high-strength low-density steels have gradually become the ideal choice for lightweight materials due to their ultra-high strength (1000 MPa level) and relatively low density. However, it is a major challenge in steel research and development to improve strength while maintaining a certain plasticity. Ultra-high-strength steels usually face the trade-off problem between strength and ductility: improving strength often leads to a significant decrease in material plasticity, which is prone to cracking or brittle fracture during forming. Therefore, on the basis of ensuring ultra-high strength, how to effectively maintain a certain plasticity has become a key technical problem 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 is as follows: 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%, and the rest is Fe and unavoidable impurities. After smelting, mold casting, hot short, water toughening, hot rolling, solid solution treatment, cold rolling, aging treatment, and nitriding and denitriding 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 the elongation index, making it difficult to evaluate the plasticity level of the material. At the same time, the Mn content in the steel described in the invention is too high, which can easily cause segregation, porosity and inclusions and other metallurgical defects during smelting and casting, making it difficult to control the composition, resulting in uneven structure and large performance fluctuations. In addition, the preparation process is long and complex, especially the high requirement of nitriding and denitriding treatment, resulting in high energy consumption, long cycle and high cost, which seriously restricts the feasibility and economy of its industrial application. Invention patent CN 116065081 B discloses a 1000 MPa grade low-density steel bar and its preparation method. The alloy composition 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%, and the rest is Fe and unavoidable impurity elements. After refining and casting, high-temperature homogenization, finishing, forging, solid solution, quenching and aging treatment, a low-density steel with a tensile strength of ≥1000 MPa, a yield strength of ≥900 MPa and an elongation of ≥30% is obtained. However, the preparation method of the low-density steel described in the invention also has the problems of multiple steps and long cycle, and the steel contains a high amount of high-cost metal Ni and a variety of trace impurity elements (H, O, N), which puts higher requirements on the smelting purity and composition control of the material, increasing the uncertainty and technical difficulty of the metallurgical process. Invention patent CN 107674955 A discloses a Fe-Mn-Al-C system high-strength low-density steel with a strength-plasticity product of greater than 50 GPa · %. The alloy composition is as follows: C: 0.98%~1.00%, Mn: 19.4%~20.00%, Al: 9.82%~10.00%, P: ≤0.003%, S: ≤0.003%, and the balance is Fe and unavoidable impurities.Although the steel material has certain advantages in performance, its preparation process is complex, and needs to go through multiple steps such as smelting, solidification casting, hot rolling, solid solution treatment, at least twice cold rolling and annealing, the whole process is complicated, long cycle, high energy consumption, and high equipment requirement, which is not conducive to industrialized production. In addition, the content of Mn and Al in the alloy system is high, which further aggravates the element segregation, inclusion generation and organization control difficulty, and poses a great challenge to the uniformity and composition stability of the alloy.

[0005] As can be seen from the above, when the existing technology prepares a low-density steel with a tensile strength of 1000 MPa level while taking into account the higher plasticity, it usually needs to rely on a relatively complex preparation process, and most alloy systems rely on high-cost or high-content alloy elements, and there are generally problems such as great difficulty in composition control, poor industrial adaptability and the like. Therefore, developing a new type of steel material with low density, high strength and good ductility, and breaking through the application bottleneck of traditional high-density steel in the field of lightweight is an important direction of current advanced metal material research. SUMMARY

[0006] In view of the above problems existing in the prior art, the present application provides a Nb-Mo-V-Si-B alloyed high strength and plasticity hot-rolled low-density steel and a preparation method thereof. The present application prepares a low-density steel with high strength and high elongation by controlling the alloying elements such as Nb, Mo, V, Si and B in the Fe-Mn-Al-C system high-strength low-density steel, the processing technology and the quenching parameters, the production cycle is short and the efficiency is high, the prepared steel material has high strength, low density, good plasticity and excellent strength and plasticity product, and has outstanding industrial application potential.

[0007] A Nb-Mo-V-Si-B alloyed high strength and plasticity hot-rolled low-density steel, which is composed of the following mass percentage chemical components: 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 %, the balance being Fe and unavoidable impurities.

[0008] Further, the microstructure of the low-density steel is a complex phase structure composed of austenite, δ-ferrite and κ-carbide, wherein the austenite is the main matrix structure and accounts for 70%~80% of the total structure.

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

[0010] The alloying elements in the present application play an important role in microstructure control and performance improvement, and the main roles of the alloying elements are as follows:

[0011] C: C, as an austenite stabilizing element, exists in the form of interstitial solid solution or carbide, is a core element for realizing microstructure strengthening and high strength;

[0012] Mn: Mn is a key austenite stabilizing element, significantly improves the strength of the steel through solid solution strengthening, and enhances the stability of austenite, which is helpful to obtain excellent ductility and plasticity, and the increase of the content of Mn is beneficial to the further improvement of the plasticity of the material;

[0013] Al: Al exists in the form of substitutional solid solution, can effectively reduce the density of the steel, refine the grains, improve the plasticity, and at the same time improve the high-temperature stability and the corrosion resistance;

[0014] Si: Si can promote the formation of ferrite, enhance the oxidation resistance and thermal stability, and at the same time is beneficial to the uniform precipitation of kappa-carbide;

[0015] B: the introduction of trace B can significantly improve the hardenability of the steel, so as to optimize the quenching process and further improve the overall mechanical properties;

[0016] Nb: on the one hand, Nb can form carbonitride (such as NbC, Nb(C,N)) with C and N at high temperature, which is dispersedly distributed, can inhibit grain growth by pinning austenite grain boundaries, realize significant fine-grain strengthening, and thus improve the strength and toughness of the steel; on the other hand, the carbonitride precipitated during cooling provides precipitation strengthening effect through Orowan mechanism, further enhances the strength of the 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 a foundation for excellent comprehensive mechanical properties;

[0017] Mo: Mo improves the strength through solid solution strengthening and carbide precipitation strengthening, the high-melting-point carbide Mo2C formed by Mo can effectively inhibit dislocation movement at high temperature, enhance high-temperature strength and thermal stability; at the same time, Mo atoms dissolved in the matrix cause lattice distortion, increase the resistance of dislocation movement, and further improve the strength; in addition, Mo can improve the hardenability and promote microstructure homogenization, thereby improving the plasticity and toughness of the material;

[0018] V: V can inhibit the growth of austenite grains by forming stable carbides such as VC and V4C3 at high temperatures, so as to realize grain refinement; during the cooling process, the carbides are dispersedly precipitated, and the yield strength is enhanced by the Orowan mechanism; meanwhile, V can also form VN with higher thermal stability, compared with other carbides, the precipitation strengthening effect of VN is more significant, and the influence on plasticity is smaller, which is beneficial to realize the good coordination of strength and toughness;

[0019] P and S are adverse elements in the steel, which seriously affect the strength, toughness and corrosion resistance of the steel, and therefore, the content of P and S needs to be controlled at the lowest possible level, so the content of P is controlled to be below 0.008%, and the content of S is controlled to be below 0.0012%.

[0020] Another purpose of the present application is to provide a preparation method of the Nb-Mo-V-Si-B alloyed high-strength and high-plasticity hot-rolled low-density steel, which comprises the steps of batching, vacuum smelting and casting, controlled rolling and controlled cooling, and quenching treatment, the controlled rolling and controlled cooling process comprises the steps of heat treatment, rough rolling, finish rolling and cooling, and the quenching treatment process is that the rolled steel billet is treated at 820-860 DEG C for 20 min and then rapidly water-cooled to room temperature.

[0021] In the above technical solution, the vacuum smelting and casting process is that the prepared raw materials are heated and melted, then poured into a casting mold, and cooled to obtain a low-density steel ingot with a thickness of 100-150 mm.

[0022] In the above technical solution, the heat treatment step in the controlled rolling and controlled cooling process is that the ingot is heated in a soaking furnace, the soaking time is greater than or equal to 60 min, the in-furnace time is 2 h, and the steel tapping temperature is 1150-1180 DEG C.

[0023] In the above technical solution, the rough rolling step in the controlled rolling and controlled cooling process is that the ingot after heat treatment is rough-rolled by using a large reduction method, the rough rolling temperature is 1080-1100 DEG C, the ingot is rolled by 4 passes, the reduction rate of the last three passes is greater than or equal to 20%, and the cumulative compression ratio is 40%-55%.

[0024] Further, the thickness of the ingot obtained after the rough rolling is 40-55 mm.

[0025] In the above technical solution, the finish rolling step in the controlled rolling and controlled cooling process is that the ingot obtained after the rough rolling is finish-rolled, the rough rolling temperature is 880-950 DEG C, the finish rolling temperature is 790-820 DEG C, the ingot is rolled by 10 passes, the reduction rate of each pass is 13%-24%, the cumulative reduction rate is 88%-94%, and the starting cooling temperature after the finish rolling is 780-800 DEG C.

[0026] In the above technical solution, the cooling step in the controlled rolling and controlled cooling process adopts laminar flow cooling to cool the cast ingot obtained after finish rolling to room temperature to obtain a rolled steel billet with a thickness of 4-8 mm.

[0027] The present application realizes the tensile strength exceeding 1040MPa while effectively guaranteeing the plasticity of the material, and achieves the good synergy of strength and plasticity by optimizing the design of alloying elements such as Nb, Mo, V, Si and B in the Fe-Mn-Al-C high-strength low-density steel, and combining the simplified hot working process route (controlled rolling and controlled cooling and one-step quenching process).

[0028] The present application has the following beneficial effects:

[0029] The present application is based on the Fe-Mn-Al-C low-density steel, introduces alloying elements Si, Nb, Mo and V and micro-alloying element B, and constructs a multi-alloy system with synergistic strengthening effect. Mn Al The present application realizes the tensile strength exceeding 1040MPa while effectively guaranteeing the plasticity of the material, and achieves the good synergy of strength and plasticity by optimizing the design of alloying elements such as Nb, Mo, V, Si and B in the Fe-Mn-Al-C high-strength low-density steel, and combining the simplified hot working process route (controlled rolling and controlled cooling and one-step quenching process).

[0030] The present application realizes the tensile strength exceeding 1040MPa while effectively guaranteeing the plasticity of the material, and achieves the good synergy of strength and plasticity by optimizing the design of alloying elements such as Nb, Mo, V, Si and B in the Fe-Mn-Al-C high-strength low-density steel, and combining the simplified hot working process route (controlled rolling and controlled cooling and one-step quenching process).

[0031] The present application realizes the tensile strength exceeding 1040MPa while effectively guaranteeing the plasticity of the material, and achieves the good synergy of strength and plasticity by optimizing the design of alloying elements such as Nb, Mo, V, Si and B in the Fe-Mn-Al-C high-strength low-density steel, and combining the simplified hot working process route (controlled rolling and controlled cooling and one-step quenching process).

[0032] The density of the prepared low-density steel is controlled in 6.86~7.09 g / cm 3 , which is lower than 9.5%~12.4% of traditional high-strength steel, and the mechanical performance is excellent, the tensile strength is greater than or equal to 1040 MPa, the elongation is greater than or equal to 38%, and the strength-plasticity product is greater than or equal to 39 GPa·%, which fully reflects the synergistic optimal characteristics of the material in strength, plasticity and strength. BRIEF DESCRIPTION OF DRAWINGS

[0033] Figure 1 is a stress-strain curve diagram of the steel material in embodiment 4 of the present application;

[0034] Figure 2 is a microstructure morphology diagram of the steel material in embodiment 4 of the present application;

[0035] Figure 3 is an XRD diagram of the steel material in embodiment 4 of the present application. DETAILED DESCRIPTION

[0036] The following non-limiting examples can make those skilled in the art more fully understand the present application, but do not limit the present application in any way.

[0037] The test methods in the following examples are conventional methods unless otherwise specified; the reagents and materials can be obtained by commercial means unless otherwise specified.

[0038] One of the specific embodiments:

[0039] A preparation method of a Nb-Mo-V-Si-B alloyed high-strength-ductility hot-rolled low-density steel, specifically comprising the following steps:

[0040] (1) batching: the raw materials are weighed according to the following 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 %, the balance being Fe and inevitable impurities;

[0041] (2) vacuum smelting and casting: the raw materials prepared in step (1) are placed into a vacuum smelting furnace for heating and melting, the obtained metal liquid is poured into a casting mold, and then cooled to obtain a low-density steel ingot with a thickness of 100~150 mm;

[0042] (3) Controlling rolling and cooling: the ingot obtained in step (2) is heated in a soaking furnace, the soaking time is greater than or equal to 60 min, the furnace time is 2 h, and the tapping temperature is 1150-1180℃; then a two-stage rolling process of rough rolling and finish rolling is adopted, the rough rolling starting temperature is 1080-1100℃, the ingot is rolled by 4 passes, the reduction of the last three passes is greater than or equal to 20%, large reduction is adopted, and the cumulative compression ratio of rough rolling is 40%-45%; the finish rolling starting temperature is 880-950℃, the finish rolling temperature is 790-820℃, the ingot is rolled by 10 passes, the reduction of each pass is 13%-24%, and the cumulative reduction is about 88%-94%; the starting cooling temperature after finish rolling is 780-800℃, and the ingot is cooled to room temperature by laminar flow cooling to obtain a rolled steel billet with a thickness of 4-8 mm;

[0043] (4) Quenching treatment: the rolled steel billet is heated in a high-temperature furnace, the temperature of the heat treatment is 820-860℃, the time of the heat treatment is 20 min, and then the rolled steel billet is rapidly water-cooled to room temperature to obtain a Nb-Mo-V-Si-B alloyed high-strength and high-plasticity hot-rolled low-density steel product with a thickness of 4-8 mm.

[0044] In the method, the thickness of the ingot obtained after rough rolling in step (3) is 40-55 mm.

[0045] In the method, the Nb-Mo-V-Si-B alloyed high-strength and high-plasticity hot-rolled low-density steel product 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 and plasticity product of 39.5-42.2 GPa·%, and a density of 6.86-7.09 g / cm 3 .

[0046] Examples 1-3

[0047] A method for preparing a Nb-Mo-V-Si-B alloyed high-strength and high-plasticity hot-rolled low-density steel with thicknesses of 4 mm, 6 mm and 8 mm, respectively, specifically includes the following steps:

[0048] (1) Blending: the chemical components of examples 1-3 given in table 1 are blended;

[0049] (2) Smelting and casting: the prepared raw materials are placed in a vacuum smelting furnace for heating and melting, the obtained molten metal is poured into a casting mold, and after cooling treatment, low-density steel ingots with thicknesses of 100 mm, 130 mm and 150 mm are obtained;

[0050] (3) Controlled rolling and controlled cooling: the low-density steel ingots with different thicknesses obtained above were heated in a soaking furnace, the soaking time in the furnace was 60 min, 75 min and 90 min respectively, the time in the furnace was 2 h, and the casting temperature was 1150℃, 1165℃ and 1180℃ respectively; then rough rolling and finish rolling were carried out, the rough rolling temperature in the rough rolling stage was 1080℃, 1090℃ and 1100℃ respectively, 4 passes of rolling were carried out, the reduction and the intermediate blank thickness are shown in Table 2, the rough rolling temperature in the finish rolling stage was 880℃, 900℃ and 950℃ respectively, 12 passes of rolling were carried out, the reduction is shown in Table 2, and the finish rolling temperature was 790℃, 800℃ and 820℃ respectively; the starting cooling temperature after finish rolling was 780℃, 790℃ and 800℃ respectively, and the steel blank in a rolled state with thicknesses of 4 mm, 6 mm and 8 mm respectively was obtained by using laminar cooling after rolling to room temperature;

[0051] (4) Quenching treatment: the steel blanks in a rolled state with thicknesses of 4 mm, 6 mm and 8 mm obtained above were heated in a high-temperature furnace, the temperature of the heat treatment was 820℃, 840℃ and 860℃ respectively, the heat treatment time was 20 min, and then rapid water cooling to room temperature was carried out, thereby obtaining Nb-Mo-V-Si-B alloyed high-strength and high-plasticity hot-rolled low-density steel with thicknesses of 4 mm, 6 mm and 8 mm respectively.

[0052] Examples 4-6

[0053] Examples 4-6 and Example 3 (thickness of 8 mm) differ in that the mass percentage content of the chemical composition in step (1) and the process parameters in steps (2)-(4) are different, and the specific mass percentage content is shown in Table 1 and the process parameters are shown in Table 2.

[0054] Comparative Example 1

[0055] This comparative example and Example 4 differ in that the quenching temperature in step (4) of this comparative example is 800℃.

[0056] Comparative Example 2

[0057] This comparative example and Example 4 differ in that the quenching temperature in step (4) of this comparative example is 900℃.

[0058] Comparative Example 3

[0059] This comparative example and Example 4 differ in that the content of C element in step (1) of this comparative example is 0.5wt.%, which is relatively low.

[0060] Comparative Example 4

[0061] The comparative example is compared with example 4, and the difference lies in that the content of Mn element in step (1) in the comparative example is only 11.34 wt.%, and does not contain alloy elements Nb, Mo and V.

[0062] Comparative example 5

[0063] The comparative example is compared with example 4, and the difference lies in that the comparative example does not contain Si element in step (1).

[0064] The chemical composition of the steel materials obtained in examples 1-6 and comparative examples 1-5 is shown in table 1, the main process parameters are shown in table 2, and the performance test refers to GB / T 228.1-2021. The yield strength, tensile strength, elongation after fracture of the steel materials prepared above are detected, and the product of strength and plasticity is calculated. The results are shown in table 3.

[0065] Table 1 Chemical composition (mass percentage) of examples and comparative examples

[0066]

[0067] Table 2 Process parameters of examples and comparative examples

[0068]

[0069] Table 3 Performance parameters of steel materials obtained in examples and comparative examples

[0070]

[0071] In order to verify the rationality of the alloy composition design and heat treatment system of the application, examples 4 and comparative examples 1-5 are selected for systematic comparison. The main variables of each experimental group include quenching temperature, content of key alloy elements (C, Mn, Nb, Mo, V, Si), etc. The influence law of the above on the microstructure and mechanical properties of the experimental steel is discussed, and the results are as follows:

[0072] Figure 1 The stress-strain curve of the steel material in example 4 of the application is shown in the figure. It can be seen that the stress-strain curve of the steel material obtained in example 4 shows excellent matching characteristics of strength and plasticity. The yield strength is 773 MPa, the tensile strength reaches 1041 MPa, and the elongation after fracture is as high as 38.0%, which has high strength and high ductility. Figure 2 The microstructure morphology of the steel material in example 4 of the application is shown in the figure. Figure 3 The XRD diagram of the steel material is shown in the figure. It can be seen that the microstructure of the steel material obtained in example 4 is mainly based on austenite, and the phase ratio is about 73% (as shown in the figure). Figure 3 Among them, there are rod-shaped and granular δ-ferrite, and fine and dispersed κ-carbide.

[0073] The organization characteristics endow the material with excellent comprehensive performance, mainly reflected in the following aspects: (1) Austenite as a face-centered cubic structure, austenite has more slip systems, which is conducive to deformation coordination and energy absorption, and is the key phase to realize high elongation. At the same time, the higher stability of austenite matrix and the potential deformation-induced martensite transformation (TRIP effect) help to continuously improve the strain hardening rate during stretching, delay necking and improve the uniform plastic deformation ability of the material; (2) The dispersion of δ-ferrite in austenite effectively refines the austenite grains and enhances the rigidity of the matrix, providing support for external load, thereby improving the yield strength and stress distribution. The introduction of δ-ferrite also enhances the resistance to crack propagation under multi-axial stress state, which helps to improve the overall fracture toughness. (3) κ-carbide is dispersed along the grain boundaries and dislocation dense areas, realizing second phase strengthening by hindering dislocation movement, and at the same time promoting the improvement of strain hardening capacity, so as to maintain good plasticity while ensuring high strength.

[0074] In summary, the synergistic regulation of austenite, δ-ferrite and κ-carbide in the steel material obtained in Example 4 significantly improves the stability and stress bearing capacity of the organization, and realizes the good matching of strength and plasticity.

[0075] The main difference between Comparative Example 1 and Example 4 is that the quenching temperature is reduced to 800°C. This temperature is lower than the critical heating temperature required to achieve complete austenitization, resulting in incomplete austenite transformation, with a large amount of residual ferrite and unsolved large-size carbide particles in the organization. The coexistence of residual ferrite and austenite forms a multi-phase mixed organization, which destroys the uniformity of the matrix organization, weakens the deformation coordination and crack propagation resistance, and significantly affects the overall plasticity and toughness of the material. At the same time, the unsolved carbide cannot participate in dispersion strengthening during the subsequent cooling process, resulting in a decrease in the number of strengthening phases, a large size, and an uneven distribution, which significantly reduces the effectiveness of precipitation strengthening. In addition, ferrite itself has low strength and hardness, and its uneven distribution will become a stress concentration source during plastic deformation, further exacerbating the risk of local yield and early cracking. In summary, the excessively low quenching temperature is not only not conducive to the formation of a uniform and stable austenite matrix structure, but also hinders 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.

[0076] The main difference between Comparative Example 2 and Example 4 is that the quenching temperature is increased to 900 °C. Although this temperature can ensure complete austenitization, excessively high heat treatment temperature can significantly promote the growth of austenite grains, leading to a decrease in grain boundary area, grain coarsening, and weakening of the grain boundary strengthening mechanism. In addition, large grain size will also reduce the dislocation density and substructure number of the material, leading to insufficient precipitation of strengthening phases (such as κ-carbides) or large size of the precipitates, further weakening the precipitation strengthening effect. In the presence of alloying elements (such as Nb, Mo, V), if the temperature is too high, it will also promote the excessive dissolution of part of the alloy carbides, losing the ability of dispersion strengthening and pinning the grain boundaries in the grain boundaries and the grain, thus making the grain grow disorderly, and further affecting the microstructure stability and mechanical properties, showing a slight decrease in plasticity, and a significant decrease in tensile strength and yield strength.

[0077] The main difference between Comparative Example 3 and Example 4 is that the content of C element is reduced to 0.5 wt.%. Carbon is a key element for the formation of strengthening phases such as κ-carbides, and is also an important means to achieve solid solution strengthening. A decrease in its content will directly affect the effectiveness of the two major strengthening mechanisms: first, the solubility of carbon in austenite decreases, weakening the formation of lattice distortion and dislocation density, thus weakening the solid solution strengthening effect; second, the amount of κ-carbide precipitation decreases and the size coarsens, significantly reducing the efficiency of precipitation strengthening. In addition, the reduction of carbon content will also reduce the TRIP or TWIP mechanism, resulting in insufficient phase transformation ability and loss of strain-induced strengthening effect. Ultimately, the comprehensive strength of the experimental steel decreases, the strength and toughness synergistic performance is not good, and the elongation is not effectively guaranteed.

[0078] The main difference between Comparative Example 4 and Example 4 is that the content of Mn is reduced to 11.34 wt.% and does not contain alloying elements Nb, Mo and V. As a key element for stabilizing austenite, a decrease in its content will cause the austenite region to shrink, increase the tendency of ferrite or pearlite formation, reduce the thermal stability of the microstructure at high temperatures, and affect the uniformity of the microstructure after final cooling. In addition, Mn has strong diffusion ability and can promote carbon diffusion and precipitation of strengthening phases, and its reduction will weaken the ability to control carbide precipitation behavior. More critically, the absence of micro-alloying elements such as Nb, Mo, and V means that the steel cannot form fine and dispersed second phase particles such as NbC, Mo2C, and VC, which can inhibit grain growth through Zener drag effect and provide precipitation strengthening. Their absence directly leads to grain coarsening, matrix softening, and a significant decrease in the number of strengthening phases, thereby comprehensively destroying the mechanisms of grain refinement, precipitation strengthening, and grain boundary strengthening, ultimately leading to a significant decrease in the strength and plasticity of the material.

[0079] The main difference between Comparative Example 5 and Example 4 is the decrease of Si content in the steel. Si is a typical ferrite-forming element, and has multiple effects of improving deoxidation efficiency, stabilizing grain structure, enhancing thermal stability and promoting the precipitation of kappa-carbides at medium-high temperature. The decrease of Si content weakens the stability of ferrite, changes the austenite-ferrite phase region behavior, and easily forms a multi-phase structure, which reduces the consistency of the structure. On the other hand, Si has a significant promoting effect on the precipitation kinetics of kappa-carbides, and the decrease of Si content will delay the nucleation and growth process of the strengthening phase, resulting in insufficient precipitation, increased size, uneven distribution, and reduced precipitation strengthening effect. Ultimately, the strength and plasticity of the experimental steel decrease.

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; 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. 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·%, and a density of 6.86-7.09 g / cm³. 3 ; The method for preparing the low-density steel 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 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%. The finishing rolling step involves finishing rolling the ingot obtained after rough rolling at an initial rolling temperature of 880~950℃ and a final rolling temperature of 790~820℃. The ingot undergoes 10 rolling 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℃. The quenching process involves treating the rolled steel billet at 820~860℃ for 20 minutes and then rapidly water-cooling it to room temperature.

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 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.

3. The Nb-Mo-V-Si-B alloyed high-strength, high-ductility, hot-rolled, low-density steel according to claim 1, 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℃.

4. The Nb-Mo-V-Si-B alloyed high-strength, high-ductility, hot-rolled, low-density steel according to claim 1, characterized in that: The thickness of the ingot obtained after rough rolling is 40~55 mm.

5. The Nb-Mo-V-Si-B alloyed high-strength, high-ductility, hot-rolled, low-density steel according to claim 1, 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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