Low alloy steel with high fracture toughness and strength and elongation product higher than 50 GPa%, manufacturing method and application of low alloy steel

Abstract

The invention provides low-cost low-alloy steel with the fracture toughness higher than 140 MPa.m < 1 / 2 > and the product of strength and elongation higher than 50 GPa% and a manufacturing method. The microstructure of the steel is as follows: 78-88% of martensite matrix, 10-20% of retained austenite, 1-3% of transition carbide and less than 1% of stable nano precipitated carbide. The steel comprises the following element components in percentage by mass: 0.60 to 0.70 percent of C, 1.25 to 1.75 percent of Mn, 1.25 to 1.75 percent of Si, 0.02 to 0.05 percent of Nb + V and the balance of iron. According to the manufacturing process, at least one time of normalizing treatment is carried out, and then a quenching-distribution-tempering (Q-P-T) heat treatment process is carried out. The mechanical property of the low-alloy steel is improved by designing a proper microstructure fraction, optimizing steel components and adjusting a heat treatment process, so that the fracture toughness of the low-alloy steel is higher than 140 MPa.m < 1 / 2 >, and the product of strength and elongation is higher than 50 GPa%.

Description

Technical Field

[0001] This invention relates to the composition and heat treatment process of steel, specifically, to a steel with a strength-ductility product of 50 GPa% or more and a fracture toughness higher than 140 MPa·m. 1 / 2 Low-cost low-alloy steel, manufacturing methods and applications. Background Technology

[0002] Strength, ductility, and toughness are key mechanical properties of metallic structural materials, impacting lightweighting and safety in automotive and aerospace structural components. However, increasing strength leads to decreasing ductility, a strength-ductility conflict (or inversion). Similarly, increasing strength leads to decreasing toughness, another strength-toughness conflict. Grain refinement, high dislocation density, and carbide precipitation are three fundamental pathways to improving the strength of metallic materials, but grain boundaries, numerous dislocation entanglements, and carbides all hinder dislocation movement, thus reducing ductility. The strength-toughness conflict arises because, from an atomic structure and bonding perspective, high strength is characterized by strong directional bonds, high Peierls-Nabarro forces, and limited dislocation migration, all of which contribute to the brittle behavior of metallic materials, i.e., reduced toughness. Therefore, resolving the strength-ductility and strength-toughness inversions in materials remains a research pursuit and a challenge.

[0003] Since strength and plasticity are usually mutually exclusive, the strength-plasticity product, i.e., the product of material strength and plasticity, is used as a comprehensive performance index of strength and plasticity. Patent CN105274436B (hereinafter referred to as Patent Document 1) provides a high-carbon microalloyed steel with a strength-plasticity product of over 50 GPa% and a heat treatment process. Although the steel in Patent Document 1 has extremely high strength and plasticity, its toughness is very low (V-type Charpy impact toughness is only 7.3 J / cm). 2 The empirical formula, when converted to fracture toughness, is only 36 MPa·m. 1 / 2 This is attributed to the fact that a large amount of retained austenite (volume fraction greater than 25%) strain induces the sensitivity of martensite to notches, while in practical engineering applications, toughness is an important indicator to ensure structural safety.

[0004] Patent CN102016083B (hereinafter referred to as Patent Document 2) discloses a non-stainless steel alloy that, through long-term solution treatment and tempering, achieves a fracture toughness higher than 110 MPa·m. 1 / 2 It is a high-strength steel, but its strength-ductility product is less than 30 GPa%, and it uses more high-cost alloying elements such as Co, Cr, Ni, Mo, W, etc., resulting in higher costs.

[0005] Patent CN115667570B (hereinafter referred to as Patent Document 3) discloses a precipitation-hardening martensitic stainless steel, which obtains high-strength stainless steel through long-term aging hardening. However, it uses high content of alloying elements such as Ni, Co, Mo, Ti, and Cr, resulting in high cost. Furthermore, the strength-ductility product of this invention is less than 30 GPa%, and the fracture toughness is less than 100 MPa·m. 1 / 2 .

[0006] Patent CN103502498B (hereinafter referred to as Patent Document 4) discloses a high-strength steel with a strength-ductility product of less than 30 GPa% and a fracture toughness of less than 80 MPa·m. 1 / 2 .

[0007] Patent CN111172467B (hereinafter referred to as Patent Document 5) provides a medium-high carbon steel. This invention does not involve high plasticity, but its fracture toughness is less than 110 MPa·m. 1 / 2 .

[0008] Reference 1, "Kobayashi J, Ina D, Futamura A, et al. Fracture toughness of an advanced ultrahigh-strength TRIP-aided steel[J]. ISIJ international, 2014, 54(4): 955-962," describes a series of TRIP steels obtained through isothermal heat treatment, although their fracture toughness exceeds 140 MPa·m. 1 / 2 However, the strong plasticity is less than 30 GPa%.

[0009] Reference 2, "He Y, Yang K, Qu W, et al. Strength and toughening of a 2800-MPa grade maraging steel[J]. Materials Letters, 2002, 56(5): 763-769," describes the aging treatment of a high-Ni steel to obtain a maraging steel with a strength exceeding 2 GPa, but an elongation of less than 14%, and a fracture toughness of 50 MPa·m. 1 / 2 The high alloying element content results in high raw material costs and requires a long aging process, leading to higher manufacturing costs.

[0010] Reference 3, "Liu L, Yu Q, Wang Z, et al. Making ultrastrong steel tough by grain-boundary delamination[J]. Science, 2020, 368(6497):1347-1352," describes how D&P steel was obtained through multi-pass rolling and mechanical heat treatment, achieving a strength-ductility product of 46 GPa% and a fracture toughness of 101.5 MPa·m. 1 / 2 Although it has high mechanical properties, its production process involves multiple steps such as warm rolling, cold rolling and annealing, which is very time-consuming and not conducive to industrial production. Moreover, its high V (0.7wt.%) makes its raw material cost higher than other low alloy steels (such as TRIP steel, Q&P steel, HSLA steel, etc.). Summary of the Invention

[0011] Therefore, it is necessary to develop a low-cost, high-strength, high-ductility, and low-alloy steel and its manufacturing process. This invention provides the following solution:

[0012] A low-alloy steel with high strength-ductility product is provided, characterized in that: the volume fraction of its microstructure observed along its cross-section is: martensite matrix: 65-88%, retained austenite: 10-30%, transition carbides: 1-5%, stable nano-precipitated carbides: less than 2%; and the mass percentage of element C is not less than 0.45%; the strength-ductility product is not less than 50 GPa%.

[0013] Preferably, the volume fraction of the microstructure is: 78-88% martensite matrix, 10-20% retained austenite, 1-3% transition carbides, and less than 1% stable nano-precipitated carbides;

[0014] A high-strength-ductility low-alloy steel is provided, characterized in that: the martensitic matrix is ​​primary lath martensite and fine twinned secondary martensite, the retained austenite is fine lamellar or granular austenite, and the martensitic matrix is ​​coherent with KS or NW, the transition carbides are η or ε carbides, and the stable nano-precipitated carbides are niobium carbide and / or vanadium carbide.

[0015] A high-strength, high-ductility low-alloy steel is provided, characterized in that: the steel contains the following elements by mass percentage: C: 0.45~0.75, Mn: 0.5~2.5, Si: 0.5~2.5, Nb+V: 0.01~0.09, with the remainder being iron;

[0016] Preferably, the steel contains the following elemental mass percentages: C: 0.50–0.75, Mn: 1.0–2.0, Si: 1.0–2.0, Nb+V: 0.01–0.06, with the remainder being iron;

[0017] Preferably, the steel contains the following elemental mass percentages: C: 0.60–0.70, Mn: 1.25–1.75, Si: 1.25–1.75, Nb+V: 0.02–0.05, with the remainder being iron;

[0018] A high-strength-ductility-product low-alloy steel is provided, characterized in that: the steel obtained after heat treatment has the following microstructure volume fractions: martensite matrix: 80-90%, retained austenite: 10-20%, transition carbides: 1-3%, stable nano-precipitated carbides: less than 1%; strength 1400-1900 MPa, elongation 25-35%, strength-ductility-product 50-55 GPa%, fracture toughness 140-150 MPa·m. 1 / 2 .

[0019] A method for manufacturing a high-strength, high-ductility low-alloy steel is provided, characterized by comprising at least the following steps:

[0020] Step 1: Provide the raw material;

[0021] Step 2: The billet is subjected to at least one normalizing treatment and at least one austenitizing treatment to obtain a fully austenitic structure;

[0022] Step 3: The austenitic structure is subjected to quenching and distribution / tempering treatments in sequence to obtain low alloy steel with a strength-ductility product of not less than 50 GPa%.

[0023] A method for manufacturing a high-strength, high-ductility low-alloy steel is provided, characterized in that: the billet contains the following element mass percentages: C: 0.45-0.75, Mn: 0.5-2.5, Si: 0.5-2.5, Nb+V: 0.01-0.09, with the remainder being iron;

[0024] A method for manufacturing high-strength, high-ductility low-alloy steel is provided, characterized in that: the normalizing treatment is to hold the billet at a temperature of T0 (higher than Ac3) for a period of time t0 to fully austenitize the billet, and then cool it to below 150°C at a cooling rate of not less than 2°C / s to obtain uniformly distributed martensite and a small amount of ferrite, and perform the austenitization process.

[0025] The austenitizing process is as follows: the billet is held at a temperature T1 higher than Ac3 for a period of time t1 to achieve complete austenitization, and then a quenching process is performed.

[0026] The quenching process involves rapidly cooling the billet at a rate exceeding the critical cooling rate to below M. s If the temperature of the point is below T2, the quenching time is t2, and the distribution / tempering process is performed.

[0027] The distribution / tempering process is as follows: the billet is heated at a temperature higher than M. sThe temperature T3 at the point is held for a period of time t3, and then rapid cooling is performed at a cooling rate exceeding the critical cooling rate to obtain the final steel workpiece;

[0028] A method for manufacturing high-strength, high-ductility low-alloy steel is provided, characterized in that: the normalizing process temperature T0 (°C) satisfies: T0 ≥ 830 - 195 × C - 26 × Mn + 12 × Si 2 +65×Nb×V, where C, Mn, Si, Nb, and V are the carbon, manganese, silicon, niobium, and vanadium contents of the billet expressed as weight percentages (wt.%); time t0 (min) satisfies: t0 ≥ 5 + 0.6×h 2 Where h (mm) is the shape feature dimension, such as plate thickness, diameter, wall thickness, etc.

[0029] A method for manufacturing high-strength, high-ductility low-alloy steel is provided, characterized in that: the austenitizing process temperature T1 (°C) satisfies: T1 ≥ 790 - 195 × C - 26 × Mn + 12 × Si 2 +65×Nb×V, where C, Mn, Si, Nb, and V are the carbon, manganese, silicon, niobium, and vanadium contents of the billet expressed as weight percentages, respectively; the time t1 (min) satisfies: t1≥1+0.2×h, where h (mm) is the shape feature dimension, such as plate thickness, diameter, and wall thickness;

[0030] A method for manufacturing high-strength, high-ductility low-alloy steel is provided, characterized in that: the quenching temperature T2 (°C) satisfies: T2≤590-595×[1-exp(-0.95×C)]-30×Mn-10×Si-5×Nb-3×V, where C, Mn, Si, Nb, and V are the carbon, manganese, silicon, niobium, and vanadium contents of the billet expressed as weight percentages; the time t2 (s) satisfies: t2≥0.5+2×h, where h (mm) is the shape characteristic dimension, such as plate thickness, diameter, and wall thickness;

[0031] A method for manufacturing high-strength, high-ductility low-alloy steel is provided, characterized in that: the distribution / tempering temperature T3 (°C) satisfies: T3≥530-610×[1-exp(-0.96×C)]-35×Mn-12×Si-9×Nb-9×V, where C, Mn, Si, Nb, and V are the carbon, manganese, silicon, niobium, and vanadium contents of the billet expressed as weight percentages; the time t2 (s) satisfies: t2≥10+5×h, where h (mm) is the shape characteristic dimension, such as plate thickness, diameter, and wall thickness;

[0032] A high-strength, high-ductility low-alloy steel is provided, characterized by the following microstructure volume fraction observed along the final steel cross-section: martensite matrix content 78-88%, retained austenite content 10-20%, transition carbides 1-3%, stable nano-precipitated carbides less than 1%; and the mass percentage of element C is not less than 0.45. It is obtained through the following process: firstly, at least one normalizing pretreatment is performed, with the process being: holding at 820℃-870℃ for 0.5-5 hours, followed by cooling to below 150℃; then, a quench-partition-temper (QPT) process is performed, with an austenitizing temperature of 800-850℃, followed by quenching to M... s Hold at a temperature below the set temperature for 10 to 60 seconds, then distribute / temper at a temperature of 350 to 425°C for 300 to 900 seconds, and finally quench to room temperature.

[0033] A low-alloy steel with high fracture toughness and a strength-ductility product greater than 50 GPa% is provided for the application of components made of said steel in structural or safety components of mechanized land vehicles.

[0034] The principle behind the chemical composition design and heat treatment process selection of this invention is based on the sensitivity of high-volume-fraction retained austenite to notches induced by strain in martensite. In terms of composition design, this invention removes austenite-stabilizing elements such as Ni and Cr, which are typically added to alloy steels, to avoid the formation of excessive retained austenite. It only contains alloying elements C, Mn, Si, Nb, and V. This low alloy content also reduces costs. Among the alloying elements, the high C content can lower the martensitic transformation initiation temperature (M). s It retains an appropriate amount of retained austenite; Si inhibits the precipitation of brittle cementite on the one hand, and promotes the distribution of carbon from martensite to retained austenite on the other hand, increasing the stability of austenite; Mn can improve the hardenability of steel and stabilize austenite; Nb forms stable and fine NbC on the one hand, and can refine the grain on the other hand; V can form fine nano carbides, providing precipitation strengthening.

[0035] In this invention, normalizing is a pretreatment process that, on the one hand, makes the alloying elements more uniform and avoids element segregation. The microstructure obtained by normalizing pretreatment is martensite with a small amount of ferrite and pearlite, providing more austenite nucleation sites for subsequent austenitization, refining the austenite microstructure, and thus enabling the subsequent QPT process to obtain a refined martensitic matrix, fine and dispersed austenite, and dispersed carbides, thereby improving performance. Multiple normalizing pretreatments can further refine the microstructure. In addition, for some samples with larger cross-sectional areas, the austenitization process during normalizing pretreatment may take a long time, leading to grain growth. However, the martensite, bainite, ferrite, and small amount of pearlite microstructure obtained after air cooling can also provide more austenite nucleation sites during the subsequent austenitization process, refining the austenite microstructure and the matrix microstructure obtained in the subsequent QPT process.

[0036] Furthermore, in the subsequent QPT process, the austenitizing temperature is selected based on the alloy's Ac3 temperature, and the holding time is controlled to ensure complete austenitization while preventing grain growth. A suitable quenching temperature (T...) q This process can avoid the formation of excessive residual austenite. The carbon partitioning / tempering step allows carbon to be fully partitioned from carbon supersaturated martensite into austenite, which on the one hand improves the stability of austenite and on the other hand reduces the dislocation density in martensite, thereby improving its deformation capacity. In addition, the addition of Si can suppress the formation of brittle cementite, and transition carbides can precipitate during the partitioning / tempering process, thereby playing a role in precipitation strengthening.

[0037] The beneficial effects of this invention are as follows:

[0038] Based on the sensitivity of martensite to notches induced by strain from a large amount of retained austenite, the microstructure design and heat treatment process of this invention can achieve a strength-ductility product of over 50 GPa% and a fracture toughness higher than 140 MPa·m. 1 / 2 This invention provides a low-cost, low-alloy steel. Compared to austenitic steel, maraging steel, and high-manganese TWIP steel, the alloy steel of this invention has fewer alloying elements and significantly lower content, resulting in a substantial reduction in cost. Simultaneously, its strength-ductility product and fracture toughness are far superior to existing low-alloy martensitic steels (such as the steel in Patent Document 1 with a fracture toughness of only 36 MPa·m). 1 / 2 This invention achieves excellent comprehensive strength, plasticity, and toughness properties. The toughening strategy and high-carbon, low-alloying of this invention result in a strength-plasticity product exceeding 50 GPa% and a fracture toughness higher than 140 MPa·m. 1 / 2 There are currently no reports of low-cost, high-carbon, low-alloy steels, either domestically or internationally. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0040] Figure 1 This is a schematic diagram of a heat treatment process involving quenching, distribution, and tempering after a normalizing pretreatment.

[0041] Figure 2 This is a schematic diagram of the processing steps involving two normalizing processes, two austenitizing processes, and two quenching processes.

[0042] Figure 3 This is a scanning electron microscope image of the high-strength-plastic-toughness steel (hot-rolled plate) prepared in Example 1.

[0043] Figure 4 This is a scanning electron microscope image of the high-strength-plastic-tough steel (after normalizing pretreatment) prepared in Example 1.

[0044] Figure 5 This is a scanning electron microscope image of the high-strength-plastic-tough steel (after QPT heat treatment) prepared in Example 1. Detailed Implementation

[0045] The present invention will now be described in detail with reference to specific embodiments, and preferred embodiments of the present application are shown in the accompanying drawings. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any way. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention. These all fall within the protection scope of the present invention.

[0046] This invention provides a low-alloy steel with high fracture toughness and a strength-ductility product higher than 50 GPa% and a manufacturing process thereof. Specifically, a steel billet is first provided, wherein the steel billet has the following elemental mass percentages: C: 0.45-0.75, Mn: 0.5-2.5, Si: 0.5-2.5, Nb+V: 0.01-0.09, with the remainder being iron; preferably, C: 0.50-0.75, Mn: 1.0-2.0, Si: 1.0-2.0, Nb+V: 0.01-0.06, with the remainder being iron; more narrowly, C: 0.60-0.70, Mn: 1.25-1.75, Si: 1.25-1.75, Nb+V: 0.02-0.05, with the remainder being iron.

[0047] The steel billet mentioned herein can be obtained through one or more processes such as smelting, casting, forging, hot rolling, cold rolling, and additive manufacturing. Its thickness is generally 0.15-100 mm, preferably not exceeding 20 mm, and its form can be various plates, formed parts, etc.; this is well known in the art. (Reference) Figure 1 The steel billet is prepared according to Figure 1The heat treatment process shown includes at least the following steps: Step 1, performing at least one normalizing treatment on the billet, and proceeding to the next step; Step 2, performing at least one austenitizing treatment on the billet, and proceeding to the next step; It is worth noting that the microstructure can be completely austenitized by one or more steps 1 and 2, and then proceeding to step 3; Step 3, performing at least one quenching treatment on the fully austenitized billet to partially transform the billet into a martensitic microstructure, preferably at least 10%, and then proceeding to the next step; Step 4, performing at least one distribution / tempering treatment on the billet. The final microstructure of the billet cross section is as follows: martensite matrix: 65-88%, retained austenite: 10-30%, transition carbides: 1-5%, and stable nano-precipitated carbides: less than 2%; preferably: martensite matrix content is 78-88%, retained austenite content is 10-20%, transition carbides: 1-3%, and stable nano-precipitated carbides: less than 1%; the martensite matrix is ​​primary lath martensite and fine twinned secondary martensite, the retained austenite is fine lamellar or granular austenite, and it is coherent with the martensite matrix in a KS or NW relationship, the transition carbides are η or ε carbides, and the stable nano-precipitated carbides are niobium carbide and / or vanadium carbide.

[0048] In the normalizing process, the normalizing process involves holding the billet at a temperature T0 (above Ac3) for a period of time t0 to fully austenitize it, and then cooling it to below 150°C at a cooling rate of not less than 2°C / s to obtain uniformly distributed martensite and a small amount of ferrite; the austenitizing process involves holding the billet at a temperature T1 above Ac3 for a period of time t1 to fully austenitize it; the quenching process involves rapidly cooling the billet at a cooling rate exceeding the critical cooling rate to below M... s The quenching time is t2, and the temperature is below T2. The distribution / tempering process is as follows: the billet is heated to a temperature above M. s The temperature T3 at the point is maintained for a period of time t3, and then rapid cooling is performed at a cooling rate exceeding the critical cooling rate to obtain the final steel workpiece.

[0049] Wherein, the normalizing process temperature T0 (°C) satisfies:

[0050] T0≥830-195×C-26×Mn+12×Si 2 +65×Nb×V (1)

[0051] Wherein C, Mn, Si, Nb, and V represent the carbon, manganese, silicon, niobium, and vanadium contents of the billet, expressed as weight percentages (wt.%).

[0052] Time t0 (min) satisfies:

[0053] t0≥5+0.6×h 2 (2)

[0054] Where h (mm) is the shape feature dimension, such as plate thickness, diameter, wall thickness, etc.

[0055] The austenitizing process temperature T1 (°C) satisfies:

[0056] T1≥790-195×C-26×Mn+12×Si 2 +65×Nb×V (3)

[0057] Wherein C, Mn, Si, Nb, and V represent the carbon, manganese, silicon, niobium, and vanadium contents of the billet, expressed as weight percentages.

[0058] Time t1 (min) satisfies:

[0059] t1≥1+0.2×h (4)

[0060] Where h (mm) is the shape feature dimension, such as plate thickness, diameter, wall thickness, etc.

[0061] The quenching temperature T2 (°C) satisfies:

[0062] T2≤590-595×[1-exp(-0.95×C)]-30×Mn-10×Si-5×Nb-3×V (5)

[0063] Wherein C, Mn, Si, Nb, and V represent the carbon, manganese, silicon, niobium, and vanadium contents of the billet, expressed as weight percentages.

[0064] Time t2(s) satisfies:

[0065] t2≥0.5+2×h (6)

[0066] Where h (mm) is the shape feature dimension, such as plate thickness, diameter, wall thickness, etc.

[0067] The distribution / tempering temperature T3 (°C) satisfies:

[0068] T3≥530-610×[1-exp(-0.96×C)]-35×Mn-12×Si-9×Nb-9×V (7)

[0069] Wherein C, Mn, Si, Nb, and V represent the carbon, manganese, silicon, niobium, and vanadium contents of the billet, expressed as weight percentages.

[0070] Time t3(s) satisfies:

[0071] t3≥10+5×h (8)

[0072] Where h (mm) is the shape feature dimension, such as plate thickness, diameter, wall thickness, etc.

[0073] Furthermore, in the manufacturing process, the heating equipment used can be various devices with heating and heat preservation functions, including resistance heating furnaces, induction heating furnaces, salt bath furnaces, etc. During cooling, various media (water, aqueous solutions, oil, molten salt, etc.) can be used as long as the cooling rate is met. Additionally, the heat treatment manufacturing process can include multiple processes such as normalizing, austenitizing, quenching, and tempering / distribution. Figure 2 The diagram shows a processing procedure with two normalizing processes and two quenching processes.

[0074] Table 1 shows the specific components and process parameters of the embodiments and comparative examples of the present invention.

[0075]

[0076]

[0077]

[0078]

[0079] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A high-strength, high-ductility low-alloy steel, characterized in that: Observed along the cross section, the volume fraction of the microstructure of this low alloy steel includes: martensite matrix: 65-88%, retained austenite: 10-30%, transition carbides: 1-5%, and stable nano-precipitated carbides: less than 2%; and the mass percentage of element C in this low alloy steel is not less than 0.45, and the strength-ductility product is not less than 50 GPa.

2. The high-strength, high-ductility low-alloy steel as described in claim 1, characterized in that: The microstructure volume fraction is: 78-88% martensite matrix, 10-20% retained austenite, 1-3% transition carbides, and less than 1% stable nano-precipitated carbides.

3. The high-strength, high-ductility low-alloy steel as described in claim 1 or 2, characterized in that: The martensitic matrix comprises primary lath martensite and fine twinned secondary martensite; the retained austenite is fine lamellar or granular austenite, and forms a KS or NW coherent relationship with the martensitic matrix; the transition carbide is η or ε carbide; the stable nano-precipitated carbide is niobium carbide and / or vanadium carbide.

4. The high strength-ductility low-alloy steel as described in any one of claims 1-3, characterized in that: The elemental mass percentages of the low alloy steel are: C: 0.45–0.75, Mn: 0.5–2.5, Si: 0.5–2.5, Nb+V: 0.01–0.09, with the remainder being iron.

5. The high-strength, high-ductility low-alloy steel as described in claim 4, characterized in that: The elemental mass percentages of the low alloy steel are: C: 0.50-0.75, Mn: 1.0-2.0, Si: 1.0-2.0, Nb+V: 0.01-0.06, with the remainder being iron.

6. The high-strength, high-ductility low-alloy steel as described in claim 4, characterized in that: The elemental mass percentages of the low alloy steel are: C: 0.60-0.70, Mn: 1.25-1.75, Si: 1.25-1.75, Nb+V: 0.02-0.05, with the remainder being iron.

7. The high-strength, high-ductility low-alloy steel as described in any one of claims 1-6, characterized in that: The low-alloy steel has a strength of 1400–1900 MPa, an elongation of 25–35%, a strength-ductility product of 50–55 GPa%, and a fracture toughness of 140–150 MPa·m. 1 / 2 .

8. A method for manufacturing a low-alloy steel with a high strength-ductility product according to any one of claims 1-7, characterized in that, Includes the following steps: Step 1: Provide a billet containing 0.45-0.75% C, 0.5-2.5% Mn, 0.5-2.5% Si, 0.01-0.09% Nb+V, with the remainder being iron; Step 2: The billet is subjected to at least one normalizing treatment and at least one austenitizing treatment to obtain a fully austenitic structure; Step 3: The austenitic structure is subjected to quenching and distribution / tempering treatments in sequence to obtain low alloy steel with a strength-ductility product of not less than 50 GPa%.

9. The method for manufacturing high-strength, high-ductility low-alloy steel according to claim 8, characterized in that: The normalizing process involves holding the billet at a temperature of T0 (higher than Ac3) for a period of time t0 to fully austenitize it, and then cooling it to below 150°C at a cooling rate of not less than 2°C / s to obtain uniformly distributed martensite and a small amount of ferrite, thus performing the austenitizing process. The austenitizing process is as follows: the billet is held at a temperature T1 higher than Ac3 for a period of time t1 to achieve complete austenitization, and then a quenching process is performed. The quenching process involves rapidly cooling the billet at a rate exceeding the critical cooling rate to below M. s If the temperature of the point is below T2, the quenching time is t2, and the distribution / tempering process is performed. The distribution / tempering process is as follows: the billet is heated at a temperature higher than M. s The temperature T3 at the point is maintained for a period of time t3, and then rapid cooling is performed at a cooling rate exceeding the critical cooling rate to obtain the final steel workpiece.

10. The method for manufacturing high-strength, high-ductility low-alloy steel as described in claim 9, characterized in that: The normalizing process temperature T0 (°C) satisfies: T0≥830-195×C-26×Mn+12×Si 2 +65×Nb×V, where C, Mn, Si, Nb and V represent the carbon, manganese, silicon, niobium, and vanadium contents of the billet, expressed as weight percentages (wt.%). Time t0 (min) satisfies: t0 ≥ 5 + 0.6 × h 2 Where h (mm) is the shape feature dimension, including plate thickness, diameter, and wall thickness; The austenitizing process temperature T1 (°C) satisfies: T1≥790-195×C-26×Mn+12×Si 2 +65×Nb×V, where C, Mn, Si, Nb and V represent the carbon, manganese, silicon, niobium, and vanadium content of the billet, expressed as weight percentages, respectively. Time t1 (min) satisfies: t1≥1+0.2×h, where h (mm) is the shape feature dimension, plate thickness, diameter, and wall thickness. The quenching temperature T2 (°C) satisfies: T2≤590-595×[1-exp(-0.95×C)]-30×Mn-10×Si-5×Nb-3×V, C, Mn, Si, Nb, and V represent the carbon, manganese, silicon, niobium, and vanadium contents of the billet, expressed as weight percentages. Time t2(s) satisfies: t2≥0.5+2×h, where h(mm) is the shape feature dimension, plate thickness, diameter, and wall thickness; The distribution / tempering temperature T3 (°C) satisfies: T3≥530-610×[1-exp(-0.96×C)]-35×Mn-12×Si-9×Nb-9×V, C, Mn, Si, Nb, and V represent the carbon, manganese, silicon, niobium, and vanadium contents of the billet, expressed as weight percentages. Time t3(s) satisfies: t3≥10+5×h, where h(mm) is the shape feature dimension, plate thickness, diameter, and wall thickness.

11. A high-strength, high-ductility low-alloy steel, characterized in that: Observation along the final steel cross-section reveals that the microstructure of this low-alloy steel comprises 78-88% martensite matrix, 10-20% retained austenite, 1-3% transition carbides, and less than 1% stable nano-precipitated carbides; with element C accounting for no less than 0.45% by mass. It is obtained through the following process: firstly, at least one normalizing pretreatment is performed, with the process involving holding at 820℃-870℃ for 0.5-5 hours, followed by cooling to below 150℃; then, a quench-partition-temper (QPT) process is performed, with an austenitizing temperature of 800-850℃, followed by quenching to M… s Hold at a temperature below the set temperature for 10 to 60 seconds, then distribute / temper at a temperature of 350 to 425°C for 300 to 900 seconds, and finally quench to room temperature.

12. A component made of low-alloy steel with high fracture toughness and a strength-ductility product greater than 50 GPa%, characterized in that: The component is made of low alloy steel as described in any one of claims 1 to 10 and is suitable for structural or safety components of mechanized land vehicles.

Images