High-reliability anti-brittle fracture 1.9gpa grade medium-low alloy steel and preparation method thereof

Abstract

The application discloses a kind of high reliability anti brittle fracture 1.9GPa grade medium-low alloy steel and preparation method thereof, belong to metal material field.The chemical composition of the steel is as follows in percentage by mass:C 0.32~0.35%, Cr 3.2~3.6%, Ni 0.9~1.2%, Mo 0.2~0.5%, W 0.5~1.0%, Si 1.2~1.6%, Mn 0.5~1.0%, Nb 0.01~0.02%, V 0.05~0.10%, the balance is Fe and impurity.Preparation method includes double vacuum smelting, ladder high temperature homogenization treatment, forging breakdown, two fire vertical reversal rolling and composite heat treatment.The application keeps 1.9GPa grade ultrahigh strength, and fracture toughness reaches 105MPa·m 1 / 2 The above realizes good balance of high strength and high toughness, and the cost is relatively low, and is suitable for aerospace and other fields.

Description

Technical Field

[0001] This invention belongs to the field of metallic materials, specifically relating to a high-reliability, 1.9GPa-resistant medium-low alloy steel and its preparation method. Background Technology

[0002] With the increasing demand for lightweight and safety in aerospace, deep-sea engineering and other fields, 1.9GPa-grade ultra-high-strength steel has emerged as a key structural material for achieving this goal. Fracture toughness is its "lifeline" for safe service in extreme environments, directly determining the material's ability to resist crack propagation and sudden brittle fracture when defects are present.

[0003] To meet this demand, the industry has conducted extensive research. For example, patent application CN118957420A discloses an ultra-high strength alloy structural steel for aerospace applications. This steel achieves high strength through a high carbon content (0.39~0.43%) and a Cr-Ni-Mo-WV alloying system, and is smelted using an electric arc furnace and electroslag remelting method. However, while high carbon content increases strength, it significantly increases the brittleness of martensite, significantly deteriorating the material's fracture toughness. Even with AlN grain refinement, the toughness loss caused by high-carbon martensite cannot be compensated, making it difficult to apply in scenarios requiring high reliability and fracture resistance.

[0004] Patent application CN117626102A discloses a high-strength and high-toughness ultra-high-strength steel and its preparation method. This method adds rare earth elements to the composition, introduces a special conditioning agent during the smelting process, and uses a special quenching oil in the heat treatment stage to obtain excellent comprehensive mechanical properties. Although this method improves toughness, its complex auxiliary material addition and non-standardized process route not only bring challenges to cost control and quality stability, but also limit its industrial application.

[0005] Furthermore, patent application CN121592948A provides a low-alloy steel based on the QPT (quench-distribution-tempering) heat treatment process, which achieves extremely high strength-ductility product and fracture toughness by controlling the retained austenite content. However, although this method has extremely high toughness, its lower limit of strength is only 1400 MPa, which cannot stably meet the ultra-high strength load-bearing requirements of 1.9 GPa, and its process window is quite sensitive, placing extremely high demands on production control.

[0006] In summary, existing technologies, when addressing the balance between strength and toughness in 1.9 GPa-level ultra-high-strength steel, are either limited by the intrinsic brittleness caused by high carbon content, constrained by the increased costs resulting from high alloys and complex processes, or struggle to simultaneously guarantee ultra-high strength and excellent fracture resistance. Therefore, developing a new generation of ultra-high-strength steel that can achieve an excellent match between 1.9 GPa-level ultra-high strength and high fracture toughness at a reasonable cost has become an urgent technical problem to be solved in this field. Summary of the Invention

[0007] The technical problem to be solved by this invention is that existing 1.9GPa grade ultra-high strength steel cannot simultaneously meet the requirements of high strength, high fracture toughness and cost control.

[0008] The purpose of this application is to provide a medium-low alloy steel that can achieve both high strength and excellent fracture toughness at an ultra-high strength of 1.9 GPa and its preparation method. Through reasonable alloy design and coordinated control of the entire process, a balance between high strength and high toughness can be achieved, and production costs can be reduced.

[0009] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows.

[0010] In a first aspect, the present invention provides a method for preparing a high-reliability, brittle fracture-resistant, 1.9 GPa grade medium-low alloy steel, comprising the following steps: S1. Smelting: Ingots are obtained by a dual vacuum smelting method of vacuum induction and vacuum self-consumption; The chemical composition of the ingot, by mass percentage, is: C 0.32~0.35%, Cr 3.2~3.6%, Ni 0.9~1.2%, Mo 0.2~0.5%, W 0.5~1.0%, Si 1.2~1.6%, Mn 0.5~1.0%, Nb 0.01~0.02%, V 0.05~0.10%, with the balance being Fe and unavoidable impurities; S2. Stepped high-temperature homogenization treatment: First stage: Heat the ingot to 600~800℃ and hold for 1~2 hours; Second stage: After the first stage of heat preservation is completed, continue to raise the temperature to 1100~1200℃ and keep it warm for 3~4 hours; S3. Forging blanking: The homogenized ingot is upsetting and drawing forging. The initial forging temperature is 1000~1050℃, and the final forging temperature is not lower than 850℃. S4. Two-stage vertical reversing rolling: First rolling: The forged steel billet is rolled at 900~950℃ for 3~5 passes to obtain an intermediate slab, with the reduction per pass controlled at 15%~25%; Second rolling: The intermediate slab is rolled to the target thickness at 900~950℃, with the rolling direction perpendicular to the first rolling, for 3~5 passes, and the reduction per pass is controlled at 15%~25%; S5. Composite heat treatment: The rolled steel plate is subjected to annealing, high-temperature quenching and two low-temperature tempering processes in sequence; The annealing treatment is performed at a temperature of 600~700℃, held for 1~2 hours, and then cooled in the furnace. The high-temperature quenching temperature is 850~900℃, and the holding time is 0.5~1 hour, followed by oil quenching; The temperature for both low-temperature tempering processes was 250~300℃, and the holding time was 1.5~2.5 hours.

[0011] In step S2 of the above preparation method, the heating rate in the first stage is 50~80℃ / h.

[0012] In step S2 of the above preparation method, the heating rate in the second stage is 30~50℃ / h.

[0013] In step S3 of the above preparation method, during the first upsetting-drawing forging, the height of the ingot is compressed to 0.4 to 0.6 times the original height, and then drawn into shape to obtain a rough block with a height-to-diameter ratio of 2.0 to 2.5.

[0014] In step S3 of the above preparation method, after the first upsetting-drawing forging, the upsetting-drawing cycle is repeated 2 to 3 times.

[0015] In step S4 of the above preparation method, the first rolling process reduces the thickness of the forged billet from 240-300 mm to 100-120 mm.

[0016] In step S4 of the above preparation method, the second rolling process reduces the thickness of the intermediate slab from 100-120 mm to 30-50 mm.

[0017] Secondly, the present invention provides a high-reliability brittle fracture resistance 1.9GPa grade medium-low alloy steel prepared by the above preparation method, the chemical composition of which, by mass percentage, is: C 0.32~0.35%, Cr 3.2~3.6%, Ni 0.9~1.2%, Mo 0.2~0.5%, W 0.5~1.0%, Si 1.2~1.6%, Mn 0.5~1.0%, Nb 0.01~0.02%, V 0.05~0.10%, with the balance being Fe and unavoidable impurities.

[0018] Furthermore, the microstructure of the aforementioned high-reliability brittle fracture resistant 1.9GPa grade medium-low alloy steel is mainly lath martensite, and granular carbides are dispersed in the lath martensite matrix.

[0019] Furthermore, the aforementioned high-reliability, brittle fracture-resistant 1.9 GPa grade medium-low alloy steel has a tensile strength of over 1900 MPa and a fracture toughness of 105 MPa·m. 1 / 2 above.

[0020] The beneficial effects of this invention are as follows: This invention employs a low-cost, medium-low alloy system. Through alloy composition design and coordinated control of the entire process, the steel maintains an ultra-high strength of 1.9 GPa while achieving a fracture toughness of 105 MPa·m. 1 / 2 The above achieves a good balance between high strength and high toughness. Double vacuum smelting ensures the purity of the material, while the combination of stepped high-temperature homogenization treatment, two-stage vertical reversing rolling, and composite heat treatment results in a microstructure dominated by high-strength and high-toughness lath martensite, with granular carbides dispersed in the matrix, significantly improving the material's resistance to brittle fracture.

[0021] This invention employs a medium-low alloy system, effectively reducing raw material costs and eliminating the need for complex heat treatment processes, thus demonstrating excellent economic efficiency and industrial feasibility. Its superior strength and toughness enable it to meet the stringent reliability requirements of high-tech fields such as aerospace, and it possesses broad application prospects against the backdrop of continuously growing demand for lightweight equipment. Attached Figure Description

[0022] Figure 1 The image shows the morphology of the material prepared in Example 1 after tempering treatment using a scanning electron microscope. Figure 2 The image shows the morphology of the material prepared for Comparative Example 1 after tempering treatment using a scanning electron microscope. Detailed Implementation

[0023] To make the technical problems, solutions, and beneficial effects of this application clearer, the following detailed description is provided in conjunction with the embodiments. Unless otherwise defined, all technical terms used herein have the same meaning as understood by one of ordinary skill in the art.

[0024] This invention provides a highly reliable 1.9 GPa-resistant medium-low alloy steel and its preparation method by combining rational alloy composition design with synergistic control of the entire process. The technical solution is described in detail below with reference to specific process steps.

[0025] Step S1, Smelting: Ingots are prepared using a dual-vacuum smelting method combining vacuum induction melting and vacuum arc remelting. The chemical composition of the ingots, by mass percentage, is: C 0.32~0.35%, Cr 3.2~3.6%, Ni 0.9~1.2%, Mo 0.2~0.5%, W 0.5~1.0%, Si 1.2~1.6%, Mn 0.5~1.0%, Nb 0.01~0.02%, V 0.05~0.10%, with the balance being Fe and unavoidable impurities. Dual-vacuum smelting effectively reduces the gas content and non-metallic inclusions in the steel, improving the purity of the material and laying the foundation for obtaining high fracture toughness.

[0026] The roles of each alloying element are as follows: C is the core strengthening element, and its content is controlled below 0.35%, so that the matrix structure is mainly composed of high-strength and tough lath martensite. As a non-limiting example, the C content can be 0.32%, 0.33%, 0.34%, 0.35%, or within any two of the above values.

[0027] Cr can significantly improve hardenability and corrosion resistance, while also providing some solid solution strengthening, and at a relatively low cost. As a non-limiting example, the Cr content can be 3.2%, 3.3%, 3.4%, 3.5%, 3.6%, or within any two of these values.

[0028] Ni stabilizes austenite and promotes the formation of thin-film retained austenite in the microstructure, thereby significantly improving toughness and lowering the ductile-brittle transition temperature. As a non-limiting example, the Ni content may be 0.9%, 1.0%, 1.1%, 1.2%, or within any two of the above values.

[0029] W and Mo work together to improve tempering stability and high-temperature strength, with Mo also effectively suppressing high-temperature tempering brittleness. As a non-limiting example, the Mo content can be 0.2%, 0.3%, 0.4%, 0.5%, or within any two of the above values; the W content can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, or within any two of the above values.

[0030] Si can inhibit the precipitation of cementite during tempering, delay softening, and improve tempering resistance. As a non-limiting example, the Si content can be 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, or within any two of the above values.

[0031] In addition to improving hardenability, Mn can also fix sulfur, preventing the formation of sulfides that easily lead to brittleness. As a non-limiting example, the Mn content can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, or within any two of the above values.

[0032] Nb and V work synergistically to refine grains through microalloying, while the addition of V can also mitigate the potential segregation tendency of Nb. As a non-limiting example, the Nb content can be 0.01%, 0.012%, 0.014%, 0.016%, 0.018%, 0.02%, or any combination of two of these values; the V content can be 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.10%, or any combination of two of these values.

[0033] Step S2, Stepped high-temperature homogenization treatment: The first stage involves homogenization, where the ingot is heated to 600-800℃ at a rate of 50-80℃ / h and held for 1-2 hours. This stage aims to promote the decomposition of martensite in the as-cast microstructure into ferrite and fine carbides, providing a large number of nucleation sites for the subsequent austenitization process. This increases the nucleation density and effectively suppresses abnormal austenite grain growth.

[0034] As a non-limiting example, the first-stage heating rate can be 50℃ / h, 55℃ / h, 60℃ / h, 65℃ / h, 70℃ / h, 75℃ / h, 80℃ / h, or within any two of the above values; the first-stage temperature can be 600℃, 620℃, 640℃, 660℃, 680℃, 700℃, 720℃, 740℃, 760℃, 780℃, 800℃, or within any two of the above values; the first-stage holding time can be 1.0 hour, 1.2 hours, 1.4 hours, 1.6 hours, 1.8 hours, 2.0 hours, or within any two of the above values.

[0035] After the first stage of heat preservation, a second stage of homogenization treatment is carried out, involving further heating to 1100-1200℃ at a heating rate of 30-50℃ / h, and holding for 3-4 hours. The core purpose of this stage is to allow alloying elements with slower diffusion rates, such as Mo, Nb, and V, to fully diffuse, thereby eliminating micro-segregation and achieving macro-homogenization of the composition. Using different heating rates in the two stages avoids structural stress or grain coarsening problems caused by excessively rapid heating.

[0036] As a non-limiting example, the second-stage heating rate can be 30℃ / h, 35℃ / h, 40℃ / h, 45℃ / h, 50℃ / h, or within any two of the above values; the second-stage temperature can be 1100℃, 1120℃, 1140℃, 1150℃, 1160℃, 1180℃, 1200℃, or within any two of the above values; the second-stage holding time can be 3.0 hours, 3.2 hours, 3.4 hours, 3.6 hours, 3.8 hours, 4.0 hours, or within any two of the above values.

[0037] Step S3, Forging: The homogenized ingot is upsetting and drawing forging.

[0038] First, the ingot is heated to a forging temperature of 1000-1050℃, followed by initial upsetting to compress its height to 0.4-0.6 times its original height. Then, it is drawn to obtain a rough block with a height-to-diameter ratio controlled at 2.0-2.5. This upsetting-drawing cycle is repeated 2-3 times to fully break down the as-cast microstructure and promote dynamic recrystallization and grain refinement. Throughout the entire forging process, the final forging temperature should not be lower than 850℃ to ensure deformation is completed in the austenitic region and avoid mixed grain formation in the two-phase region. Multiple cycles of upsetting and drawing help improve microstructure uniformity, providing a uniformly structured billet for subsequent rolling. The process should aim for single-fire forming as much as possible to reduce the number of firings, improve efficiency, and avoid grain coarsening caused by repeated heating.

[0039] As a non-limiting example, the initial forging temperature can be 1000℃, 1010℃, 1020℃, 1030℃, 1040℃, 1050℃, or any combination of two of the above values; the initial upsetting compression ratio can be 0.4, 0.45, 0.5, 0.55, 0.6, or any combination of two of the above values; the rough block height-to-diameter ratio can be 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, or any combination of two of the above values; the number of upsetting-drawing cycles can be 2 or 3; and the final forging temperature can be 850℃, 860℃, 870℃, 880℃, 890℃, 900℃, 910℃, 920℃, or any combination of two of the above values.

[0040] Step S4, Two-pass vertical reversing rolling: A two-pass vertical reversing rolling process is adopted to achieve grain refinement and microstructure homogenization.

[0041] The first rolling process involves rolling the forged steel billet at a temperature of 900-950℃ over 3-5 passes, reducing its thickness from 240-300mm to 100-120mm, with each pass reducing the thickness by 15%-25%. After rolling, the billet is cut to obtain multiple first intermediate slabs of uniform thickness. This first rolling process initially breaks down the forging structure and refines the grains through dynamic recrystallization.

[0042] As a non-limiting example, the rolling temperature of the first pass can be 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, or within any two of the above values; the number of rolling passes in the first pass can be 3, 4, or 5; the reduction per pass in the first pass can be 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, or 25%, or within any two of the above values; the thickness before the first pass can be 240mm, 250mm, 260mm, 270mm, 280mm, 290mm, or 300mm, or within any two of the above values; the thickness after the first pass can be 100mm, 105mm, 110mm, 115mm, or 120mm, or within any two of the above values.

[0043] Second rolling pass: Adjust the rolling direction to be perpendicular to the first rolling pass. This reversing rolling process breaks the texture formed in the first rolling pass, improves the isotropy of the microstructure, and avoids the directional differences in properties caused by unidirectional rolling. The initial rolling temperature is still controlled at 900~950℃, which is within the austenite recrystallization temperature range, facilitating sufficient dynamic recrystallization. Similarly, through 3~5 passes, the thickness of the intermediate slab is further reduced from 100~120mm to 30~50mm, with the reduction per pass maintained at 15%~25%. This ensures sufficient deformation penetration while avoiding the risk of cracking due to excessive deformation in a single pass.

[0044] As a non-limiting example, the second rolling temperature can be 900℃, 910℃, 920℃, 930℃, 940℃, 950℃, or any combination of two of the above values; the number of rolling passes in the second rolling can be 3, 4, or 5; the reduction per pass in the second rolling can be 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, or 25%, or any combination of two of the above values; the thickness before the second rolling can be 100mm, 105mm, 110mm, 115mm, or 120mm, or any combination of two of the above values; the thickness after the second rolling can be 30mm, 35mm, 40mm, 45mm, or 50mm, or any combination of two of the above values.

[0045] Step S5, Composite Heat Treatment: First, annealing is performed. The rolled steel plate is heated to 600-700℃ and held at that temperature for 1-2 hours before being cooled in the furnace. This allows the processed microstructure to return to equilibrium, eliminates internal stress, and prepares the steel for subsequent quenching. As a non-limiting example, the annealing temperature can be 600℃, 620℃, 640℃, 660℃, 680℃, 700℃, or any combination of two of these values; the annealing holding time can be 1.0 hour, 1.2 hours, 1.4 hours, 1.6 hours, 1.8 hours, 2.0 hours, or any combination of two of these values.

[0046] The annealed steel plate is then subjected to high-temperature quenching. It is heated to 850-900℃ and held for 0.5-1 hour before oil quenching, forming a microstructure primarily composed of lath martensite, providing a strengthening basis for subsequent tempering. As a non-limiting example, the quenching temperature can be 850℃, 860℃, 870℃, 880℃, 890℃, 900℃, or any combination of two of these values; the quenching holding time can be 0.5 hours, 0.6 hours, 0.7 hours, 0.8 hours, 0.9 hours, 1.0 hour, or any combination of two of these values.

[0047] Finally, two low-temperature tempering processes were performed, each held at 250–300°C for 1.5–2.5 hours. Through the dispersed precipitation of carbides and the transformation of retained austenite, the fracture toughness was increased to 105 MPa·m while maintaining ultra-high strength above 1.9 GPa. 1 / 2 The above achieves a synergistic optimization of strength and toughness. Two tempering processes are more effective than single tempering in fully eliminating stress and promoting uniform carbide precipitation. As a non-limiting example, the first tempering temperature can be 250℃, 260℃, 270℃, 280℃, 290℃, 300℃, or any combination of two of these values; the first tempering holding time can be 1.5 hours, 1.8 hours, 2.0 hours, 2.2 hours, 2.5 hours, or any combination of two of these values; the second tempering temperature can be 250℃, 260℃, 270℃, 280℃, 290℃, 300℃, or any combination of two of these values; the second tempering holding time can be 1.5 hours, 1.8 hours, 2.0 hours, 2.2 hours, 2.5 hours, or any combination of two of these values.

[0048] The medium-low alloy steel prepared through the above steps has the chemical composition described previously, and its microstructure is mainly lath martensite with granular carbides dispersed in the matrix. The resulting steel has a tensile strength exceeding 1900 MPa and a fracture toughness reaching 105 MPa·m. 1 / 2above.

[0049] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0050] The following specific embodiments will be provided to explain the solution of the present invention. Those skilled in the art will understand that the following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0051] Example 1 S1. Smelting. Based on the chemical composition ratio, ingots were prepared by vacuum induction + vacuum consumable double vacuum smelting. The chemical composition was: C 0.33%, Cr 3.47%, Ni 1.04%, Mo 0.44%, W 0.75%, Si 1.40%, Mn 0.76%, Nb 0.01%, V 0.16%, with the balance being Fe and unavoidable impurities.

[0052] S2. Stepped high-temperature homogenization treatment. First, the temperature is increased to 680℃ at a rate of 60℃ / h and held for 1.5 hours. Then, the temperature is increased to the homogenization temperature of 1150℃ at a rate of 35℃ / h and held for 3.5 hours.

[0053] S3. Forging and billet preparation. First, the steel ingot is slowly cooled to a forging temperature of 1020℃, followed by the first upsetting to compress its height to 0.5 times the original height; then, it is drawn into shape to obtain a rough block with a height-to-diameter ratio controlled at 2.2; then, the upsetting-drawing cycle is repeated twice; throughout the entire billet preparation process, the final forging temperature is 900℃, and one-fire forging is attempted as much as possible.

[0054] S4. Two-pass vertical reversal rolling. First pass: The billet is rolled at 920℃, reducing its thickness from 290mm to 110mm in four passes, with each pass reduction controlled at 20%. After rolling, the billet is cut to obtain multiple first intermediate slabs of uniform thickness. Second pass: The rolling direction is adjusted to be perpendicular to the first pass direction, while the initial rolling temperature remains at 920℃. Similarly, the billet thickness is further reduced from 110mm to 42mm in four passes, with the pass reduction still maintained at 20%.

[0055] S5. Composite heat treatment. First, annealing is performed, in which the steel is heated to 680℃ and held for 2 hours before being cooled in the furnace; then, high-temperature quenching is performed, in which the steel is held at 880℃ for 1 hour before being oil quenched; finally, two low-temperature tempering processes are performed, each time held at 260℃ for 2 hours, and after cooling, a high-reliability brittle fracture resistance grade of 1.9GPa medium-low alloy steel is obtained.

[0056] Testing showed that the medium-low alloy steel prepared in Example 1 had a tensile strength of 1941 MPa and a fracture toughness of 113 MPa·m. 1 / 2 . Figure 1 The scanning electron microscope (SEM) image of the material prepared in Example 1 after tempering shows that the matrix is ​​mainly composed of lath martensite, exhibiting a typical parallel bundle arrangement. The lath bundles are interwoven and tightly arranged, forming a dense microstructure. Simultaneously, a large number of white granular precipitates (carbides) are uniformly dispersed within the lath martensite matrix and at the lath boundaries. These precipitates are small in size and show no agglomeration. The lath martensite matrix itself possesses a high-density dislocation and subgrain boundary structure, which, through dislocation strengthening and grain refinement mechanisms, endows the material with high strength and high toughness. The lath boundaries effectively hinder crack propagation and suppress the risk of brittle fracture. During tempering, supersaturated carbon atoms dissolved in the martensite matrix precipitate to form dispersed carbides, significantly hindering dislocation movement and proliferation, further improving yield strength and tensile strength. Simultaneously, carbide precipitation reduces the supersaturation of the martensite matrix, effectively releasing residual quenching stress, further improving the toughness and plasticity of the material, achieving synergistic optimization of strength and toughness.

[0057] Example 2 S1. Smelting. Based on the chemical composition ratio, ingots were prepared by vacuum induction + vacuum consumable double vacuum smelting. The chemical composition was: C 0.34%, Cr 3.50%, Ni 1.00%, Mo 0.35%, W 0.72%, Si 1.36%, Mn 0.75%, Nb 0.02%, V 0.08%, with the balance being Fe and unavoidable impurities.

[0058] S2. Stepped high-temperature homogenization treatment. First, the temperature is increased to 700℃ at a rate of 70℃ / h and held for 2 hours. Then, the temperature is increased to the homogenization temperature of 1150℃ at a rate of 35℃ / h and held for 3 hours.

[0059] S3. Forging and billet preparation. First, the steel ingot is slowly cooled to a forging temperature of 1020℃, followed by the first upsetting to compress its height to 0.6 times the original height; then, it is drawn into shape to obtain a rough block with a height-to-diameter ratio controlled at 2.5; then, the upsetting-drawing cycle is repeated twice; throughout the entire billet preparation process, the final forging temperature is 850℃, and the goal is to achieve one-fire forging as much as possible.

[0060] S4. Two-pass vertical reversal rolling. First pass: The billet is rolled at 910℃ for three passes, reducing its thickness from 250mm to 105mm, with each pass reduction controlled at 25%. After rolling, the billet is cut to obtain multiple first intermediate slabs of uniform thickness. Second pass: The rolling direction is adjusted to be perpendicular to the first pass direction, while the initial rolling temperature remains at 910℃. Similarly, the billet thickness is further reduced from 105mm to 44mm in three passes, with the pass reduction still maintained at 25%.

[0061] S5. Composite heat treatment. First, annealing is performed by heating the steel to 650℃ and holding it at that temperature for 1.5 hours before furnace cooling. Then, high-temperature quenching is performed by holding the steel at 890℃ for 1 hour before oil quenching. Finally, two low-temperature tempering processes are performed, each time holding the steel at 280℃ for 2 hours. After cooling, a high-reliability, 1.9GPa grade medium-low alloy steel with resistance to brittle fracture is obtained.

[0062] Testing showed that the medium-low alloy steel prepared in Example 2 had a tensile strength of 1945 MPa and a fracture toughness of 108 MPa·m. 1 / 2 .

[0063] Example 3 S1. Smelting. Based on the chemical composition ratio, ingots were prepared by vacuum induction + vacuum consumable double vacuum smelting. The chemical composition was: C 0.35%, Cr 3.60%, Ni 1.20%, Mo 0.50%, W 1.00%, Si 1.56%, Mn 0.95%, Nb 0.015%, V 0.08%, with the balance being Fe and unavoidable impurities.

[0064] S2. Stepped high-temperature homogenization treatment. First, the temperature is increased to 800℃ at a rate of 80℃ / h and held for 2 hours. Then, the temperature is increased to a homogenization temperature of 1200℃ at a rate of 50℃ / h and held for 4 hours.

[0065] S3. Forging and billet preparation. First, the steel ingot is slowly cooled to a forging temperature of 1050℃, followed by the first upsetting to compress its height to 0.6 times the original height; then, it is drawn into shape to obtain a rough block with a height-to-diameter ratio controlled at 2.5; then the upsetting-drawing cycle is repeated 3 times; throughout the billet preparation process, the final forging temperature is 920℃, and one-fire forging is attempted as much as possible.

[0066] S4. Two-pass vertical reversal rolling. First pass: The billet is rolled at 900℃ for 5 passes, reducing its thickness from 260mm to 115mm, with each pass reduction controlled at 15%. After rolling, the billet is cut to obtain multiple first intermediate slabs of uniform thickness. Second pass: The rolling direction is adjusted to be perpendicular to the first pass direction, while the initial rolling temperature remains at 900℃. Similarly, the billet thickness is further reduced from 115mm to 50mm in 5 passes, with the pass reduction still maintained at 15%.

[0067] S5. Composite heat treatment. First, annealing is performed, in which the steel is heated to 700℃ and held for 2 hours before being cooled in the furnace; then, high-temperature quenching is performed, in which the steel is held at 900℃ for 1 hour before being oil quenched; finally, two low-temperature tempering processes are performed, each time at 300℃ for 2.5 hours, and after cooling, a high-reliability, brittle fracture-resistant 1.9GPa grade medium-low alloy steel is obtained.

[0068] Testing showed that the medium-low alloy steel prepared in Example 3 had a tensile strength of 1930 MPa and a fracture toughness of 120 MPa·m. 1 / 2 .

[0069] Comparative Example 1 The preparation steps for Comparative Example 1 were the same as those for Example 1, except that the carbon content in the ingot's chemical composition was increased to 0.40% and no W was added. Its tensile strength was 1912 MPa and its fracture toughness was 82 MPa·m. 1 / 2 Although its tensile strength can reach 1.9 GPa, its fracture toughness is much lower than that of Example 1.

[0070] Figure 2 The image shows the scanning electron microscope (SEM) morphology of the material prepared in Comparative Example 1 after tempering. It can be seen that the matrix is ​​a mixture of lamellar and plate martensite, exhibiting a disordered and interwoven strip-like arrangement. Compared to the uniform lamellar bundle structure of Example 1, the regularity of the microstructure is significantly reduced. The areas marked by red arrows in the image represent coarse martensite plates; these types of martensite are larger, have a greater aspect ratio, and exhibit more disordered lath boundaries.

[0071] Due to the increased carbon content, the proportion of lamellar martensite in the mixed microstructure significantly increased. Lamellar martensite has a higher carbon content, a more concentrated internal dislocation structure, and the large martensite plates easily become channels for crack propagation. Compared to the single lamellar martensite microstructure of Example 1, the matrix of Comparative Example 1 cannot effectively hinder crack propagation through the dense lamellar boundaries, resulting in a significant reduction in the overall toughness of the material. Simultaneously, the lack of W element affects the precipitation behavior of carbides, reducing the number of carbide precipitates and coarsening their size, directly weakening the precipitation strengthening effect and limiting the increase in yield strength and tensile strength. Insufficient carbide precipitation fails to effectively reduce the supersaturation of the martensite matrix, and the residual stress from quenching cannot be fully released, further exacerbating stress concentration within the material and causing additional losses in plasticity and toughness.

[0072] In summary, Comparative Example 1, due to its carbon content exceeding the limits of this invention and lack of W element, resulted in an excessively high proportion of lamellar martensite in the microstructure and insufficient carbide precipitation strengthening, failing to achieve synergistic optimization of strength and toughness, and exhibiting significant defects in its overall mechanical properties.

[0073] Comparative Example 2 The preparation steps of Comparative Example 2 are the same as those of Example 2, except that the homogenization treatment and rolling method are different, specifically: In S2, the ingot is directly heated to 1150℃ at a rate of 70℃ / h and held at that temperature for 3 hours, without using a stepped heating method. In S4, a two-pass unidirectional rolling process is adopted, and the rolling direction of the second pass is the same as that of the first pass.

[0074] Testing showed that the alloy steel prepared in Comparative Example 2 had a tensile strength of 1902 MPa and a fracture toughness of 92 MPa·m. 1 / 2 Both indicators were lower than in Example 3. The results show that omitting the first-stage low-temperature homogenization treatment and using a direct high-temperature homogenization method failed to effectively suppress the abnormal growth of austenite grains. Simultaneously, unidirectional rolling could not break the texture formed in the first heat treatment, resulting in significant anisotropy in the microstructure and making cracks more prone to propagate along specific directions. These two process differences jointly led to a decrease in the synergistic effect of material strength and toughness.

[0075] Comparative Example 3 The preparation steps of Comparative Example 3 are the same as those of Example 3, except that the quenching temperature in the composite heat treatment is different, specifically: In S5, the high-temperature quenching temperature is increased to 950℃, and after holding at that temperature for 1 hour, it is oil quenched.

[0076] Testing showed that the alloy steel prepared in Comparative Example 3 had a tensile strength of 1830 MPa and a fracture toughness of 102 MPa·m. 1 / 2The overall mechanical properties were significantly lower than those of Example 3. The results show that excessively high quenching temperature leads to coarsening of austenite grains, an increase in the size of martensite lath bundles obtained after quenching, and a weakening of the lath boundary's resistance to cracking. At the same time, excessively high quenching temperature may cause changes in the stability of some austenite, affecting the dispersed precipitation of carbides during subsequent tempering, ultimately resulting in a decrease in both strength and toughness.

Claims

1. A method for preparing high-reliability, brittle fracture-resistant 1.9 GPa grade medium-low alloy steel, characterized in that, Includes the following steps: S1. Smelting: Ingots are obtained by a dual vacuum smelting method of vacuum induction and vacuum self-consumption; The chemical composition of the ingot, by mass percentage, is: C 0.32~0.35%, Cr 3.2~3.6%, Ni 0.9~1.2%, Mo 0.2~0.5%, W 0.5~1.0%, Si 1.2~1.6%, Mn 0.5~1.0%, Nb 0.01~0.02%, V 0.05~0.10%, with the balance being Fe and unavoidable impurities; S2. Stepped high-temperature homogenization treatment: First stage: Heat the ingot to 600~800℃ and hold for 1~2 hours; Second stage: After the first stage of heat preservation is completed, continue to raise the temperature to 1100~1200℃ and keep it warm for 3~4 hours; S3. Forging blanking: The homogenized ingot is upsetting and drawing forging. The initial forging temperature is 1000~1050℃, and the final forging temperature is not lower than 850℃. S4. Two-stage vertical reversing rolling: First rolling: The forged steel billet is rolled at 900~950℃ for 3~5 passes to obtain an intermediate slab, with the reduction per pass controlled at 15%~25%; Second rolling: The intermediate slab is rolled to the target thickness at 900~950℃, with the rolling direction perpendicular to the first rolling, for 3~5 passes, and the reduction per pass is controlled at 15%~25%; S5. Composite heat treatment: The rolled steel plate is subjected to annealing, high-temperature quenching and two low-temperature tempering processes in sequence; The annealing treatment is performed at a temperature of 600~700℃, held for 1~2 hours, and then cooled in the furnace. The high-temperature quenching temperature is 850~900℃, and the holding time is 0.5~1 hour, followed by oil quenching; The temperature for both low-temperature tempering processes was 250~300℃, and the holding time was 1.5~2.5 hours.

2. The preparation method according to claim 1, characterized in that: In step S2, the heating rate in the first stage is 50~80℃ / h.

3. The preparation method according to claim 1, characterized in that: In step S2, the heating rate in the second stage is 30~50℃ / h.

4. The preparation method according to claim 1, characterized in that: In step S3, during the initial upsetting-drawing forging, the height of the ingot is compressed to 0.4 to 0.6 times its original height, and then drawn into shape to obtain a rough block with a height-to-diameter ratio of 2.0 to 2.

5.

5. The preparation method according to claim 1, characterized in that: In step S3, after the initial upsetting-drawing forging, the upsetting-drawing cycle is repeated 2 to 3 times.

6. The preparation method according to claim 1, characterized in that: In step S4, the first rolling process reduces the thickness of the forged billet from 240-300 mm to 100-120 mm.

7. The preparation method according to claim 1, characterized in that: In step S4, the second rolling process reduces the thickness of the intermediate slab from 100-120 mm to 30-50 mm.

8. A high-reliability, brittle fracture-resistant, 1.9 GPa grade medium-low alloy steel prepared by the preparation method according to any one of claims 1 to 7, characterized in that, Its chemical composition by mass percentage is as follows: C 0.32~0.35%, Cr 3.2~3.6%, Ni 0.9~1.2%, Mo 0.2~0.5%, W 0.5~1.0%, Si 1.2~1.6%, Mn 0.5~1.0%, Nb 0.01~0.02%, V 0.05~0.10%, with the balance being Fe and unavoidable impurities.

9. The high-reliability brittle fracture resistance 1.9GPa grade medium-low alloy steel according to claim 8, characterized in that: Its microstructure is mainly lath martensite, and granular carbides are dispersed in the lath martensite matrix.

10. The high-reliability brittle fracture resistance 1.9GPa grade medium-low alloy steel according to claim 8, characterized in that: Its tensile strength is above 1900 MPa, and its fracture toughness is 105 MPa·m. 1 / 2 above.

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