A 2.0gpa-grade low-carbon low-alloy martensitic steel and a preparation method thereof

Abstract

This invention relates to a 2.0 GPa grade low-carbon low-alloy martensitic steel and its preparation method, belonging to the field of martensitic steel technology. The 2.0 GPa grade low-carbon low-alloy martensitic steel has the following chemical composition by weight percentage: C: 0.15~0.25%, Mn: 1.50~2.50%, Si: 0.80~1.50%, Nb: 0~0.03%, Ti: 0~0.03%, with the remainder being Fe and unavoidable impurities. The preparation method mainly includes the following steps: (1) alloy melting; (2) homogenization treatment and billet preparation; (3) high-temperature rolling; (4) rapid heating; (5) rapid cooling quenching; and (6) cyclic heat treatment. The rapid heating and cyclic heat treatment methods achieve microstructure refinement. The alloy steel obtained by this invention has a yield strength of not less than 1450 MPa; a tensile strength of not less than 1950 MPa, preferably not less than 2000 MPa; a uniform elongation of not less than 3.5%; and a Vickers hardness of not less than 500 HV1.0. The alloy composition used in this invention is inexpensive, and the preparation process adopted in this invention is simple and can be used for the industrial preparation of high-performance steel.

Description

Technical Field

[0001] This invention relates to the field of metal materials and processing technology, and in particular to an ultra-high strength low-carbon low-alloy martensitic steel and its preparation method. Technical Background

[0002] Advanced high-strength steels, due to their high strength and good overall service performance, are widely used in engineering machinery, building structures, wear-resistant components, and other high-load-bearing parts. While existing ultra-high-strength steels such as maraging steel, TRIP steel, and TWIP steel can achieve high strength levels, they typically require the addition of large amounts of alloying elements such as Ni, Cr, Mo, or high levels of Mn, resulting in higher material costs and more complex manufacturing processes. Therefore, developing a steel with low alloy content, lower cost, and still achieving ultra-high strength has significant engineering application value.

[0003] Low-carbon low-alloy steel generally refers to a class of steel with low carbon content and controlled total alloying element content. This type of steel has advantages such as low cost, good weldability, and relatively simple manufacturing processes, and has a wide range of applications in industry. However, traditional low-carbon low-alloy steels are mostly composed of ferrite, pearlite, bainite, or relatively coarse quenched structures, and their strength levels, especially yield strength and tensile strength, are still insufficient to meet the requirements of higher-grade load-bearing components for lightweighting and high strength. Therefore, how to further improve its strength level while maintaining the advantages of low alloying and low cost has become an important research direction in this field.

[0004] In existing technologies, refining the original austenite grains and obtaining a fine martensite structure after quenching is one of the effective ways to improve the strength of low-carbon low-alloy steel. Based on this idea, existing technologies can be broadly divided into two categories. The first category is the deformation refinement route, represented by controlled rolling, cold rolling, or large plastic deformation. For example, patent document CN101713046B discloses a method for preparing ultrafine-grained martensitic steel that achieves nano-precipitate strengthening and grain refinement through alloy composition design combined with rolling, cooling, and heat treatment processes; patent document CN110306127B further employs large plastic deformation combined with subsequent heat treatment to introduce high-density defects and refine the microstructure. While the above methods can obtain a finer microstructure and higher strength, they generally rely on large cumulative deformation, numerous processing passes, or high alloying levels, resulting in complex processes, high energy consumption, heavy equipment load, and limitations on product thickness, which are not conducive to the industrial manufacturing of low-cost, large-size components.

[0005] The second type is the cyclic heat treatment refining route. In the mid-1960s, R.A. Grange et al., while studying the microstructure refinement of high-hardenability Cr-Ni-Mo steel, discovered that by subjecting plate-shaped samples to multiple cyclic austenitizing treatments, an ultrafine austenite grain size of 3–5 μm could be obtained, and applied for a US patent for Method of Producing Ultrafine-Grained Steel (US Patent No. 3,178,324). Subsequently, cyclic heat treatment technology was gradually applied to the microstructure refinement research and production practice of steel grades such as spring steel. Published literature has reported that cyclic grain refinement heat treatment of steel grades such as 50CrVA and 60Si2CrVA under salt bath heating conditions can obtain ultrafine austenite grains of about 3–6 μm. However, the above-mentioned cyclic heat treatment technology is mainly aimed at high alloy content steel systems, and its austenitizing temperature ( The value is relatively low. For low-carbon, low-alloy steel, its... The high temperature inevitably leads to rapid growth of the parent phase austenite grains during the cyclic heat treatment process.

[0006] Therefore, existing technologies still lack a preparation method for low-carbon, low-alloy steel that balances low cost and industrial feasibility. This method should be able to further refine the original austenite grains and the final martensite structure by rationally controlling the heating rate, austenitizing holding time, and quenching rate, without excessive reliance on high alloying and large plastic deformation, thereby obtaining high-strength low-carbon, low-alloy martensitic steel with a strength of 2.0 GPa. Summary of the Invention

[0007] The purpose of this invention is to provide a 2.0 GPa grade low-carbon low-alloy martensitic steel and its preparation method, to solve the problem that existing steels rely heavily on high alloying or large plastic deformation processes for strength and toughness control. Through the synergistic effect of low-cost composition design, Nb and Ti microalloying, and multiple short-time cycle heat treatment processes, this invention achieves significant refinement of the austenite grains and martensite structure in low-carbon low-alloy steel without excessive reliance on severe plastic deformation, obtaining a single-phase fine-grained martensite structure, enabling the material to possess both 2.0 GPa grade tensile strength and a certain degree of plasticity.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] This invention provides a 2.0 GPa grade low-carbon low-alloy martensitic steel, wherein the mass percentage of each component in the alloy steel is as follows: C: 0.15~0.25%, Mn: 1.50~2.50%, Si: 0.80~1.50%, Nb: 0~0.03%, Ti: 0~0.03%, with the remainder being Fe and unavoidable impurities; wherein the total mass percentage of alloying elements is less than 4%, and the mass percentage of Nb+Ti is not greater than 0.05%.

[0010] Furthermore, the preferred mass percentages of each component in a 2.0 GPa grade low-carbon low-alloy martensitic steel are: C: 0.20%, Mn: 1.80%, Si: 1.20%, Nb: 0.02%, Ti: 0.02%, with the remainder being Fe and unavoidable impurities; wherein, the total mass percentage of alloying elements is 3.24%, and the mass percentage of Nb+Ti is 0.04%.

[0011] The 2.0 GPa grade low-carbon low-alloy martensitic steel of the present invention has a single-phase martensitic structure, and no obvious ferrite and retained austenite are observed in its microstructure; the average effective grain size of martensite is ≤2.0 μm, and the average grain size of the original austenite parent phase is ≤3.0 μm.

[0012] This invention also provides a method for preparing ultrafine-grained low-carbon low-alloy martensitic steel, comprising the following steps: alloy melting, homogenization treatment and billet preparation, high-temperature rolling, rapid heating, rapid cooling quenching, and cyclic heat treatment. The specific steps are as follows:

[0013] Step (1) Prepare the ingredients according to the ingredients, stir and melt them evenly in a vacuum induction furnace, and cast them into ingots.

[0014] Step (2) The ingot obtained in step (1) is homogenized at 1150~1300 ℃ for a holding time of not less than 4.0 h, and then furnace cooled to room temperature to obtain a billet with uniform structure and properties. The obtained billet is heated at room temperature and forged at a temperature not lower than 1200 ℃, and multiple upsetting and drawing are performed to finally obtain a slab.

[0015] Step (3) The slab obtained in step (2) is heated to 1000~1200 ℃ in a vacuum or argon-protected hot furnace, held for 0.5~1.0 h and then hot rolled. The final rolling temperature is not lower than 900 ℃ and the cumulative deformation is not lower than 50%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 5.0~10.0 mm.

[0016] Step (4) Immerse the plate in a molten salt bath at a temperature of 930~980 ℃, with a heating rate of ≥150 ℃ / s and a heating time of 5~10 s, without heat preservation.

[0017] Step (5): After reaching the target temperature, the material is immediately quenched. The quenching medium is room temperature water or mineral oil, and the cooling rate is ≥200 ℃ / s.

[0018] Step (6): Using steps (4) to (5) as a loop, repeat the process 2 to 3 times.

[0019] Furthermore, the alloy smelting method used in step (1) is a combination of vacuum induction and electroslag remelting.

[0020] Furthermore, the salt bath described in step (4) is a NaCl molten salt bath, and the heating rate is not less than 180 ℃ / s. The target temperature is 950 ℃.

[0021] Furthermore, the quenching process described in step (5) uses water quenching with a cooling rate of 300 ℃ / s.

[0022] Furthermore, the heat treatment cycle in step (6) is 3 times.

[0023] Furthermore, the material prepared in step (6) exhibits a single-phase martensitic structure, in which no obvious ferrite or retained austenite was observed in the microstructure; the average effective grain size of the martensite is ≤1.8 μm. Using the martensite as a matrix, the microstructure contains ultrafine martensitic lath substructures with a width ≤50 nm and nanotwins surrounded by high-density dislocations. Ti-Nb composite precipitates are precipitated in the microstructure, and these precipitates have a face-centered cubic structure. The average grain size of the original austenite parent phase is ≤2.5 μm.

[0024] Furthermore, the steel prepared through the above steps has a yield strength of not less than 1450 MPa; a tensile strength of not less than 1950 MPa, preferably exceeding 2000 MPa; a uniform elongation of not less than 3.5%; and a Vickers hardness of not less than 500HV1.0.

[0025] The alloy composition of this invention is controlled within the field of low-carbon low-alloy steel, with the carbon content controlled within the range of 0.15–0.25 wt.%. If the carbon content is too low, it is difficult to form a sufficiently strong martensitic structure after quenching; if the carbon content is too high, it will increase the tendency for embrittlement and is not conducive to the overall performance of the material. Mn is controlled within the range of 1.50–2.50 wt.% to improve the hardenability of the steel and promote martensite formation; Si is controlled within the range of 0.80–1.50 wt.% to exert solid solution strengthening and further enhance strength. Nb and Ti, as key microalloying elements, form thermally stable (Nb,Ti)(C,N) composite precipitates, continuously inhibiting austenite grain growth during hot rolling and subsequent rapid heating, thereby synergistically achieving grain refinement with the rapid heating process. This alloy system maintains both low cost and low alloying characteristics, and is well-matched with the rapid heating-rapid quenching process, enabling the acquisition of a 2.0 GPa level single-phase martensite structure in a low-carbon low-alloy steel system.

[0026] This invention utilizes the control of hot rolling and subsequent rapid heat treatment process parameters to rapidly dissolve ε-carbides in the industrial hot-rolled microstructure during rapid heating, thereby shortening the time required to achieve full austenitization. Simultaneously, multiple rapid heating cycles significantly increase the austenite nucleation rate, and the continuous pinning of grain boundaries by (Nb,Ti)(C,N) precipitates suppresses grain coarsening. Thus, without relying on large plastic deformation processes, low-carbon low-alloy steel can be transformed into an ultrafine-grained lath martensite microstructure through a synergistic process of "rapid carbide dissolution + rapid austenite nucleation + suppressed grain growth + rapid quenching."

[0027] Compared with the prior art, the present invention:

[0028] (1) Excellent performance. The 2.0 GPa grade low-carbon low-alloy martensitic steel prepared by this invention can obtain an ultrafine single-phase martensite structure, with an average effective martensite grain size of no more than 2.0 μm, preferably no more than 1.8 μm, and an average original austenite grain size of no more than 3.0 μm, preferably no more than 2.5 μm. Based on the above-mentioned ultrafine structure characteristics, the prepared steel has a yield strength of no less than 1450 MPa, a tensile strength of no less than 1950 MPa, preferably above 2000 MPa, and maintains a uniform elongation of no less than 3.5%, exhibiting excellent strength-plasticity matching and comprehensive mechanical properties, which can meet the requirements of high load-bearing components for ultra-high strength materials.

[0029] (2) The process is simple and easy to implement. The process route adopted in this invention does not rely on extreme plastic deformation and complex multi-pass thermomechanical processing, and can achieve significant refinement of the original austenite grains and the final martensite structure of low-carbon low-alloy steel. The smelting, hot rolling, rapid salt bath heating and quenching processes used all have a good industrial implementation basis, the process flow is relatively simple, and it has good potential for engineering scale-up and industrial application.

[0030] (3) Low cost and high cost-effectiveness. The steel used in this invention belongs to a low-carbon, low-alloy system with a low total amount of alloying elements. It can achieve a strength level of 2.0 GPa without adding a large amount of high-cost alloying elements, thus resulting in lower raw material costs. At the same time, this invention avoids the high requirements of strong plastic deformation processes on equipment, energy consumption, and processing passes. It also features rapid heating time, high cyclic heat treatment efficiency, which helps to reduce manufacturing costs and improve production efficiency. Therefore, it has a high overall cost-effectiveness. Attached Figure Description

[0031] Figure 1 This is a process route diagram used in this invention;

[0032] Figure 2 The microstructure of the 2.0 GPa grade low-carbon low-alloy martensitic steel of Example 1;

[0033] Figure 3 The engineering stress-strain curve of the 2.0 GPa grade low-carbon low-alloy martensitic steel in Example 1 is shown.

[0034] Figure 4 The heating rate curve is for the 2.0 GPa grade low-carbon low-alloy martensitic steel of Example 1. Detailed Implementation

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] Example 1

[0037] A 2.0 GPa grade low-carbon low-alloy martensitic steel has the following chemical composition by weight percentage: C: 0.20%, Mn: 1.80%, Si: 1.20%, Nb: 0.02%, Ti: 0.02%, with the remainder being Fe and unavoidable impurities; its preparation method is as follows:

[0038] Step (1) Alloy smelting: Prepare the ingredients according to the composition, smelt them evenly in a vacuum induction furnace, and cast them into ingots.

[0039] Step (2) Homogenization and billet preparation: The ingot obtained in step (1) is homogenized at 1300 ℃ for 4.0 h, and then furnace cooled to room temperature to obtain a billet with uniform structure and properties. The billet is heated at room temperature and forged at a temperature not lower than 1200 ℃. Multiple upsetting and drawing processes are performed to finally obtain a slab.

[0040] Step (3) High-temperature rolling: The slab obtained in step (2) is heated to 1200℃ in a vacuum or argon-protected hot furnace, held for 1.0 h and then hot rolled. The final rolling temperature is 900℃ and the cumulative deformation is 50%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 10.0 mm.

[0041] Step (4) Rapid heating: Immerse the plate in a molten salt bath maintained at 950°C, with a heating rate of 181°C / s, without heat preservation.

[0042] Step (5) Rapid quenching: After reaching the target temperature, the material is immediately quenched. The quenching medium is room temperature water, and the cooling rate is 301℃ / s.

[0043] Step (6) Cyclic heat treatment: Repeat steps (4) to (5) as one cycle, and process 3 times.

[0044] The prepared steel has a yield strength of 1538 MPa, a tensile strength of 2030 MPa, a uniform elongation of 3.6%, and a Vickers hardness of 511 HV1.0. Its average effective martensite grain size is 1.6 μm, and the average size of the original austenite parent phase grains is 2.4 μm.

[0045] Example 2

[0046] A 2.0 GPa grade low-carbon low-alloy martensitic steel has the following chemical composition by weight percentage: C: 0.20%, Mn: 1.80%, Si: 1.10%, Nb: 0.015%, Ti: 0.015%, with the remainder being Fe and unavoidable impurities; its preparation method is as follows:

[0047] Step (1) Alloy smelting: Prepare the ingredients according to the composition, smelt them evenly in a vacuum induction furnace, and cast them into ingots.

[0048] Step (2) Homogenization and billet preparation: The ingot obtained in step (1) is homogenized at 1300 ℃ for 4.0 h, and then furnace cooled to room temperature to obtain a billet with uniform structure and properties. The billet is heated at room temperature and forged at a temperature not lower than 1200 ℃. Multiple upsetting and drawing processes are performed to finally obtain a slab.

[0049] Step (3) High-temperature rolling: The slab obtained in step (2) is heated to 1200℃ in a vacuum or argon-protected hot furnace, held for 1.0 h and then hot rolled. The final rolling temperature is 900℃ and the cumulative deformation is 50%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 10.0 mm.

[0050] Step (4) Rapid heating: Immerse the plate in a molten salt bath maintained at 960°C, with a heating rate of 185°C / s, without heat preservation.

[0051] Step (5) Rapid quenching: After reaching the target temperature, the material is immediately quenched. The quenching medium is mineral oil, and the cooling rate is 212 ℃ / s.

[0052] Step (6) Cyclic heat treatment: Repeat steps (4) to (5) as one cycle, and process 3 times.

[0053] The prepared steel has a yield strength of 1461 MPa, a tensile strength of 2022 MPa, a uniform elongation of 3.5%, and a Vickers hardness of 505 HV1.0. Its average effective martensite grain size is 1.9 μm, and the average size of the original austenite parent phase grains is 2.6 μm.

[0054] Example 3

[0055] A 2.0 GPa grade low-carbon low-alloy martensitic steel has the following chemical composition by weight percentage: C: 0.18%, Mn: 1.60%, Si: 1.20%, Nb: 0.02%, Ti: 0.02%, with the remainder being Fe and unavoidable impurities; its preparation method is as follows:

[0056] Step (1) Alloy smelting: Prepare the ingredients according to the composition, smelt them evenly in a vacuum induction furnace, and cast them into ingots.

[0057] Step (2) Homogenization and billet preparation: The ingot obtained in step (1) is homogenized at 1300 ℃ for 4.0 h, and then furnace cooled to room temperature to obtain a billet with uniform structure and properties. The billet is heated at room temperature and forged at a temperature not lower than 1200 ℃. Multiple upsetting and drawing processes are performed to finally obtain a slab.

[0058] Step (3) High-temperature rolling: The slab obtained in step (2) is heated to 1200℃ in a vacuum or argon-protected hot furnace, held for 1.0 h and then hot rolled. The final rolling temperature is 900℃ and the cumulative deformation is 50%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 10.0 mm.

[0059] Step (4) Rapid heating: Immerse the plate in a molten salt bath maintained at 980℃, with a heating rate of 190℃ / s, without heat preservation.

[0060] Step (5) Rapid quenching: After reaching the target temperature, the material is immediately quenched. The quenching medium is room temperature water, and the cooling rate is 284 ℃ / s.

[0061] Step (6) Cyclic heat treatment: Repeat steps (4) to (5) twice as one cycle.

[0062] The prepared steel has a yield strength of 1472 MPa, a tensile strength of 1983 MPa, a uniform elongation of 3.7%, and a Vickers hardness of 508 HV1.0. Its average effective martensite grain size is 1.9 μm, and the average size of the original austenite parent phase grains is 2.7 μm.

[0063] Example 4

[0064] A 2.0 GPa grade low-carbon low-alloy martensitic steel has the following chemical composition by weight percentage: C: 0.16%, Mn: 1.80%, Si: 1.20%, Nb: 0.02%, Ti: 0.02%, with the remainder being Fe and unavoidable impurities; its preparation method is as follows:

[0065] Step (1) Alloy smelting: Prepare the ingredients according to the composition, smelt them evenly in a vacuum induction furnace, and cast them into ingots.

[0066] Step (2) Homogenization and billet preparation: The ingot obtained in step (1) is homogenized at 1150 ℃ for 4.5 h, and then furnace cooled to room temperature to obtain a billet with uniform structure and properties. The billet is heated at room temperature to a temperature not lower than 1200 ℃ to start forging, and multiple upsetting and drawing processes are carried out to finally obtain a slab.

[0067] Step (3) High-temperature rolling: The slab obtained in step (2) is heated to 1200℃ in a vacuum or argon-protected hot furnace, held for 1.0 h and then hot rolled. The final rolling temperature is 900℃ and the cumulative deformation is 50%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 5.0 mm.

[0068] Step (4) Rapid heating: Immerse the plate in a molten salt bath maintained at 950°C, with a heating rate of 184°C / s, without heat preservation.

[0069] Step (5) Rapid quenching: After reaching the target temperature, the material is immediately quenched. The quenching medium is room temperature water, and the cooling rate is 272 ℃ / s.

[0070] Step (6) Cyclic heat treatment: Repeat steps (4) to (5) as one cycle, and process 3 times.

[0071] The prepared steel has a yield strength of 1504 MPa, a tensile strength of 2011 MPa, a uniform elongation of 3.6%, and a Vickers hardness of 502 HV1.0. Its average effective martensite grain size is 1.8 μm, and the average size of the original austenite parent phase grain is 2.4 μm.

[0072] Example 5

[0073] A 2.0 GPa grade low-carbon low-alloy martensitic steel has the following chemical composition by weight percentage: C: 0.20%, Mn: 1.80%, Si: 1.00%, Nb: 0.02%, Ti: 0.02%, with the remainder being Fe and unavoidable impurities; its preparation method is as follows:

[0074] Step (1) Alloy smelting: Prepare the ingredients according to the composition, smelt them evenly in a vacuum induction furnace, and cast them into ingots.

[0075] Step (2) Homogenization and billet preparation: The ingot obtained in step (1) is homogenized at 1300 ℃ for 4.0 h, and then furnace cooled to room temperature to obtain a billet with uniform structure and properties. The billet is heated at room temperature and forged at a temperature not lower than 1200 ℃. Multiple upsetting and drawing processes are performed to finally obtain a slab.

[0076] Step (3) High-temperature rolling: The slab obtained in step (2) is heated to 1200℃ in a vacuum or argon-protected hot furnace, held for 1.0 h and then hot rolled. The final rolling temperature is 900℃ and the cumulative deformation is 50%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 10.0 mm.

[0077] Step (4) Rapid heating: Immerse the plate in a molten salt bath maintained at 950°C, with a heating rate of 182°C / s, without heat preservation.

[0078] Step (5) Rapid quenching: After reaching the target temperature, the material is immediately quenched. The quenching medium is mineral oil, and the cooling rate is 213 ℃ / s.

[0079] Step (6) Cyclic heat treatment: Repeat steps (4) to (5) twice as one cycle.

[0080] The prepared steel has a yield strength of 1471 MPa, a tensile strength of 1986 MPa, a uniform elongation of 3.8%, and a Vickers hardness of 501 HV1.0. Its average effective martensite grain size is 1.7 μm, and the average size of the original austenite parent phase grains is 2.5 μm.

[0081] Example 6

[0082] A 2.0 GPa grade low-carbon low-alloy martensitic steel has the following chemical composition by weight percentage: C: 0.20%, Mn: 1.90%, Si: 1.20%, Nb: 0.02%, Ti: 0.02%, with the remainder being Fe and unavoidable impurities; its preparation method is as follows:

[0083] Step (1) Alloy smelting: Prepare the ingredients according to the composition, smelt them evenly in a vacuum induction furnace, and cast them into ingots.

[0084] Step (2) Homogenization and billet preparation: The ingot obtained in step (1) is homogenized at 1300 ℃ for 4.0 h, and then furnace cooled to room temperature to obtain a billet with uniform structure and properties. The billet is heated at room temperature and forged at a temperature not lower than 1200 ℃. Multiple upsetting and drawing processes are performed to finally obtain a slab.

[0085] Step (3) High-temperature rolling: The slab obtained in step (2) is heated to 1200℃ in a vacuum or argon-protected hot furnace, held for 1.0 h and then hot rolled. The final rolling temperature is 900℃ and the cumulative deformation is 50%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 10.0 mm.

[0086] Step (4) Rapid heating: Immerse the plate in a molten salt bath maintained at 950°C, with a heating rate of 188°C / s, without heat preservation.

[0087] Step (5) Rapid quenching: After reaching the target temperature, the material is immediately quenched. The quenching medium is room temperature water, and the cooling rate is 291 ℃ / s.

[0088] Step (6) Cyclic heat treatment: Repeat steps (4) to (5) as one cycle, and process 3 times.

[0089] The prepared steel has a yield strength of 1511 MPa, a tensile strength of 2025 MPa, a uniform elongation of 3.5%, and a Vickers hardness of 507 HV1.0. Its average effective martensite grain size is 1.8 μm, and the average size of the original austenite parent phase grains is 2.5 μm.

[0090] Example 7

[0091] A 2.0 GPa grade low-carbon low-alloy martensitic steel has the following chemical composition by weight percentage: C: 0.18%, Mn: 1.80%, Si: 1.20%, Nb: 0.02%, Ti: 0.02%, with the remainder being Fe and unavoidable impurities; its preparation method is as follows:

[0092] Step (1) Alloy smelting: Prepare the ingredients according to the composition, smelt them evenly in a vacuum induction furnace, and cast them into ingots.

[0093] Step (2) Homogenization and billet preparation: The ingot obtained in step (1) is homogenized at 1300 ℃ for 4.0 h, and then furnace cooled to room temperature to obtain a billet with uniform structure and properties. The billet is heated at room temperature and forged at a temperature not lower than 1200 ℃. Multiple upsetting and drawing processes are performed to finally obtain a slab.

[0094] Step (3) High-temperature rolling: The slab obtained in step (2) is heated to 1200℃ in a vacuum or argon-protected hot furnace, held for 1.0 h and then hot rolled. The final rolling temperature is 900℃ and the cumulative deformation is 50%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 10.0 mm.

[0095] Step (4) Rapid heating: Immerse the plate in a molten salt bath maintained at 950°C, with a heating rate of 192°C / s, without heat preservation.

[0096] Step (5) Rapid quenching: After reaching the target temperature, the material is immediately quenched. The quenching medium is room temperature water, and the cooling rate is 282 ℃ / s.

[0097] Step (6) Cyclic heat treatment: Repeat steps (4) to (5) twice as one cycle.

[0098] The prepared steel has a yield strength of 1515 MPa, a tensile strength of 2011 MPa, a uniform elongation of 3.5%, and a Vickers hardness of 510 HV1.0. Its average effective martensite grain size is 1.7 μm, and the average size of the original austenite parent phase grains is 2.4 μm.

[0099] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A 2.0 GPa grade low-carbon low-alloy martensitic steel, characterized in that... , Its chemical composition by weight percentage is as follows: C: 0.15~0.25%, Mn: 1.50~2.50%, Si: 0.80~1.50%, Nb: 0~0.03%, Ti: 0~0.03%, with the remainder being Fe and unavoidable impurities.

2. The 2.0 GPa grade low-carbon low-alloy martensitic steel according to claim 1, characterized in that: Its chemical composition by weight percentage is as follows: C: 0.15~0.20%, Mn: 1.50~2.00%, Si: 1.00~1.50%, Nb: 0.02~0.03%, Ti: 0.02~0.03%, with the remainder being Fe and unavoidable impurities; among which, the total mass percentage of alloying elements is less than 4%, and the mass percentage of Nb+Ti is not greater than 0.05%.

3. The 2.0 GPa grade low-carbon low-alloy martensitic steel according to claim 2, characterized in that, Its chemical composition by weight percentage is: C: 0.20%, Mn: 1.80%, Si: 1.20%, Nb: 0.02%, Ti: 0.02%, with the remainder being Fe and unavoidable impurities; the total mass percentage of alloying elements is 3.24%, and the mass percentage of Nb+Ti is 0.04%; the microstructure of the alloy steel is a single-phase martensitic structure, and no obvious ferrite or retained austenite is observed in its microstructure; the average effective grain size is ≤2.0 μm, and the average grain size of the original austenite parent phase is ≤3.0 μm.

4. A method for preparing 2.0 GPa grade low-carbon low-alloy martensitic steel as described in any one of claims 1 to 3, characterized in that, Includes the following steps: Step (1) Alloy smelting: The ingredients are prepared according to any one of the components described in claims 1-3, and the mixture is smelted by uniform stirring in a vacuum induction furnace and cast into ingots; Step (2) Homogenization and billet making: The ingot obtained in step (1) is homogenized at 1150~1300 ℃ for a holding time of not less than 4.0 h, and then furnace cooled to room temperature to obtain a billet with uniform structure and properties; the billet is heated at room temperature and forged when the temperature is not lower than 1200 ℃, and multiple upsetting and drawing are performed to finally obtain a slab. Step (3) High-temperature rolling: The slab obtained in step (2) is heated to 1000~1200 ℃ in a vacuum or argon-protected hot furnace, held for 0.5~1.0 h and then hot rolled. The final rolling temperature is not lower than 900 ℃ and the cumulative deformation is not lower than 50%. After rolling, it is air-cooled to room temperature to finally obtain a plate with a thickness of 5.0~10.0 mm. Step (4) Rapid heating: Immerse the plate in a molten salt bath at a temperature of 930~980 ℃, with a heating rate of ≥150 ℃ / s and a heating time of 5~10 s, without heat preservation; Step (5) Rapid quenching: After reaching the target temperature, the material is immediately quenched. The quenching medium is room temperature water or mineral oil, and the cooling rate is ≥200 ℃ / s. Step (6) Cyclic heat treatment: Using steps (4) to (5) as one cycle, perform the cyclic treatment 2 to 3 times.

5. The method for preparing a 2.0 GPa grade low-carbon low-alloy martensitic steel according to claim 4, characterized in that: The alloy smelting method described in step (1) is a combination of vacuum induction and electroslag remelting.

6. The method for preparing a 2.0 GPa grade low-carbon low-alloy martensitic steel according to claim 4, characterized in that: The salt bath mentioned in step (4) is a NaCl molten salt bath with a heating rate of not less than 180 ℃ / s and a target temperature of 950 ℃.

7. The method for preparing a 2.0 GPa grade low-carbon low-alloy martensitic steel according to claim 4, characterized in that: The quenching process described in step (5) uses water quenching with a cooling rate of 300 ℃ / s.

8. The method for preparing a 2.0 GPa grade low-carbon low-alloy martensitic steel according to claim 4, characterized in that: The heat treatment cycle described in step (6) is 3 times.

9. The method for preparing a 2.0 GPa grade low-carbon low-alloy martensitic steel according to claim 4, characterized in that: The material prepared in step (6) has a martensitic single-phase structure, and no obvious ferrite or retained austenite was observed in the microstructure.

10. The method for preparing a 2.0 GPa grade low-carbon low-alloy martensitic steel according to claim 4, characterized in that: In the preferred embodiment, the material prepared in step (6) has an average effective grain size of ≤1.8 μm and an average grain size of ≤2.5 μm.

11. The method for preparing a 2.0 GPa grade low-carbon low-alloy martensitic steel according to claim 4, characterized in that: The prepared steel has a yield strength ≥1450 MPa; tensile strength ≥1950 MPa, preferably ≥2000 MPa; uniform elongation ≥3.5%; and Vickers hardness ≥500 HV1.0.

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