Ultrahigh-strength high-toughness martensite / bainite multiphase aging steel and preparation method thereof
By introducing martensitic/bainitic dual-phase structure and alloy carbide precipitation into high Co-Ni secondary hardening steel, the bottleneck of strength and toughness matching in traditional processes is solved, and a combination of high strength and high toughness is achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- BEIJING INST OF TECH
- Filing Date
- 2026-03-17
- Publication Date
- 2026-05-15
AI Technical Summary
Traditional heat treatment processes are insufficient to further improve the balance between strength and toughness in high Co-Ni secondary hardened steels, and over-aging leads to a decrease in both strength and toughness.
After austenitization, isothermal quenching and cryogenic treatment are performed below Ms point to form a martensite/bainite multiphase structure. Alloy carbides are precipitated by high-temperature tempering to control the microstructure transformation process.
While maintaining strength, the toughness of the material is significantly improved, with yield strength ≥1650MPa, tensile strength ≥2000MPa, and U-notch impact energy ≥70J, overcoming the shortcomings of traditional processes.
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Figure CN122038698A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of alloy materials technology, specifically relating to an ultra-high strength and high toughness martensitic / bainitic multiphase aging steel and its preparation method. Technical Background
[0002] Traditional heat treatment processes for high-Co-Ni secondary hardening steels mainly include quenching, deep cryogenic treatment, and aging, resulting in ultra-high strength and excellent toughness. The strength primarily comes from the phase transformation strengthening caused by martensitic transformation and the precipitation strengthening of alloy carbides, while the toughness mainly comes from the dissolution of cementite and the reversal of the austenite film (3nm-5nm). Currently, Aermet100 secondary hardening steel possesses excellent combined strength and toughness mechanical properties and is widely used in aircraft landing gear, high-strength bolts, etc. After slight over-aging (holding at 482℃ for 5 hours), it achieves a good balance of strength and toughness. Similarly, M54 secondary hardening steel, with performance comparable to Aermet100, is also used after slight over-aging (holding at 510℃ for 10 hours). Both steels achieve optimal toughness with a slight loss of strength. If the aging temperature is further increased or the aging time extended, the coarsening of carbides and the thickening of the reversal austenite film will lead to a significant decrease in both strength and toughness. The formation of reverse-transformed austenite lags behind that of alloy carbides. During the slight over-aging stage, a thin film of reverse-transformed austenite forms, while the carbides become slightly coarsened. Continued over-aging treatment will cause both to become severely coarsened. Therefore, it is difficult to further break through the bottleneck of strength and toughness matching by controlling the synergistic effect of carbides and austenite through traditional heat treatment processes. A new strength and toughness matching strategy is urgently needed.
[0003] In recent years, it has been found that martensite / bainite multiphase microstructures in high-carbon bearing steel exhibit better strength-toughness balance than single martensite microstructures. The paper "Tempering response and improved mechanical properties in secondary hardened steel by introducing an optimized austempering process" describes the introduction of 10% bainite into M50 high-carbon bearing steel (Fe-0.85C-4Cr-4Mo) using an isothermal quenching process. The fine-sized acicular bainite can segment the austenite grains, resulting in a refined microstructure. Furthermore, the harder martensite in the multiphase microstructure can plastically constrain the softer bainite, thereby improving the material's toughness without reducing strength. Regarding martensitic / bainitic complex phase microstructures, the literature "Influence of chronological control of transformation on the microstructure and mechanical properties of complexphase steels" studied the effect of the transformation sequence of martensite and bainite on the microstructure and properties. It was found that the formation of bainite first followed by the formation of martensite leads to significant stress concentration in the microstructure, which is detrimental to the toughness of the material. However, the formation of martensite first can reduce the adverse effect on toughness.
[0004] In summary, the martensitic / bainitic dual-phase microstructure provides a novel approach to achieving a balance between strength and toughness in high Co-Ni secondary hardening steels. Summary of the Invention
[0005] To address the challenge of achieving a balance between strength and toughness in high-Co-Ni secondary hardening steels, the first technical solution of this application discloses a method for preparing ultra-high strength and high toughness martensitic / bainitic multiphase aging steel, comprising the following steps:
[0006] (1) The steel ingot is subjected to austenitizing treatment;
[0007] (2) The sample after step (1) is directly quenched from the austenitizing temperature to below Ms point for isothermal holding for 1h to 24h. After holding, it is oil quenched to room temperature.
[0008] (3) The sample after step (2) is subjected to cryogenic treatment and then returned to room temperature;
[0009] (4) The sample after step (3) is aged and then cooled to room temperature.
[0010] The steel ingot contains the following components by mass percentage: C 0.25%~0.35%, Cr 0.8%~1.5%, Ni 7%~12%, Mo 1%~3%, Co 7%~12%, W 1%~2%, V 0.05%~0.15%, with the balance being Fe and unavoidable impurities.
[0011] Furthermore, the austenitizing treatment temperature is 1050℃~1080℃, and the holding time is 1h~2h.
[0012] Furthermore, in step (2), the insulation temperature below point Ms is 190℃~220℃, and the temperature control accuracy during the isothermal insulation process is ±5℃.
[0013] Furthermore, the heat-insulating medium in step (2) is molten salt.
[0014] Furthermore, the cryogenic treatment temperature in step (3) is -193℃ to -73℃, and the holding time is 1h to 3h.
[0015] Furthermore, the insulation medium in step (3) is dry ice.
[0016] Furthermore, in step (4), the aging treatment temperature is 450℃~550℃ and the treatment time is 8h~12h.
[0017] And, ultra-high strength and high toughness martensitic / bainitic multiphase aging steel prepared according to any of the above preparation methods.
[0018] Beneficial effects
[0019] This application quenches the steel ingot after austenitization treatment in step (1) to below Ms point (the temperature at which martensite transformation begins) and then performs isothermal quenching treatment (step (2)) to obtain a primary martensite + bainite + retained austenite structure. After isothermal treatment, the sample is oil quenched to room temperature and then subjected to deep cryogenic treatment (step (3)) to transform some of the retained austenite into martensite. Afterward, the sample is warmed back to room temperature. The sample after cryogenic treatment is subjected to high-temperature tempering (step (4)) to precipitate alloy carbides in the martensite + bainite multiphase matrix, and then air-cooled to room temperature; compared with the prior art, after the treatment of steps (1) to (3), the yield strength of the material obtained in this application is ≥1000MPa, the tensile strength is ≥1700MPa, and the U-notch impact energy is ≥65J; the matrix structure of the material obtained after the treatment of steps (1) to (4) includes martensite and bainite, and alloy carbides are precipitated in the matrix, and the yield strength of the material is ≥1650MPa, the tensile strength is ≥2000MPa, and the U-notch impact energy is ≥70J. This invention demonstrates that by utilizing the accelerating effect of primary martensite on the transformation of bainite, bainite is successfully introduced into high Co-Ni secondary hardening steel through low-temperature transformation to obtain a martensite / bainite multiphase structure. While slightly reducing the strength, the toughness of the material is improved, overcoming the shortcomings of traditional over-aging processes that cause a simultaneous decrease in strength and toughness. This provides a new approach to the strength-toughness matching of high Co-Ni secondary hardening steel. Attached Figure Description
[0020] Figure 1 The metallographic structure of the workpiece in Comparative Example 1 is shown below.
[0021] Figure 2 The metallographic structure of the workpiece in Example 1;
[0022] Figure 3 This is the metallographic structure of the workpiece in Example 2;
[0023] Figure 4 This is the metallographic structure of the workpiece in Example 3. Detailed Implementation
[0024] The present invention will be further described below with reference to the accompanying drawings and embodiments. These embodiments are implemented based on the technical solution of the present invention, but the scope of protection of the present invention is not limited to the embodiments described below. Any non-substantial changes and substitutions made based on the present invention are within the scope of protection claimed by the present invention.
[0025] Example 1
[0026] The alloy was smelted using a dual process of vacuum induction melting and electroslag remelting. The alloy's chemical composition (mass percentage) was: 0.3% C, 1% Cr, 10% Ni, 2% Mo, 7% Co, 1.3% W, 0.1% V, with the balance being Fe. The forged and annealed workpiece was held at 1060℃ for 1.5 hours and then immediately quenched in a 190℃ salt bath furnace (isothermal quenching) for 1 hour, followed by oil quenching to room temperature (its microstructure is as shown). Figure 2 (As shown), the workpiece is then held at -73℃ for 2 hours and then warmed to room temperature. The cryogenically treated workpiece is aged at 510℃ for 10 hours, and then air-cooled to room temperature.
[0027] Example 2
[0028] The alloy was smelted using a dual process of vacuum induction melting and electroslag remelting. The alloy's chemical composition (mass percentage) was: 0.3% C, 1% Cr, 10% Ni, 2% Mo, 7% Co, 1.3% W, 0.1% V, with the balance being Fe. The forged and annealed workpiece was held at 1060℃ for 1.5 hours and then immediately quenched in a 220℃ salt bath furnace (isothermal quenching) for 1 hour, followed by oil quenching to room temperature (its microstructure is as shown). Figure 3 (As shown). The workpiece was then kept at -73℃ for 2 hours and then warmed to room temperature. The cryogenically treated workpiece was aged at 510℃ for 10 hours and then air-cooled to room temperature.
[0029] Example 3
[0030] The alloy was smelted using a dual process of vacuum induction melting and electroslag remelting. The alloy's chemical composition (mass percentage) was: 0.3% C, 1% Cr, 10% Ni, 2% Mo, 7% Co, 1.3% W, 0.1% V, with the balance being Fe. The forged and annealed workpiece was held at 1060℃ for 1.5 hours and then immediately quenched in a 220℃ salt bath furnace (isothermal quenching) for 8 hours, followed by oil quenching to room temperature (its microstructure is as shown). Figure 4 (As shown), the workpiece is then held at -73℃ for 2 hours and then warmed to room temperature. The cryogenically treated workpiece is aged at 510℃ for 10 hours, and then air-cooled to room temperature.
[0031] Example 4
[0032] The alloy was smelted using a dual process of vacuum induction melting and electroslag remelting. The alloy's chemical composition (mass percentage) was: 0.3% C, 1% Cr, 10% Ni, 2% Mo, 7% Co, 1.3% W, 0.1% V, with the balance being Fe. After forging and annealing, the workpiece was held at 1060℃ for 1.5 hours and then immediately quenched in a 220℃ salt bath furnace (isothermal quenching) for 24 hours. Following this, it was oil quenched to room temperature, and then held at -73℃ for 2 hours before being warmed back to room temperature.
[0033] Comparative Example 1
[0034] The alloy was smelted using a dual process of vacuum induction melting and electroslag remelting. The alloy's chemical composition (mass percentage) was: 0.3% C, 1% Cr, 10% Ni, 2% Mo, 7% Co, 1.3% W, 0.1% V, with the balance being Fe. The forged and annealed workpiece was held at 1060℃ for 1.5 hours and then oil-quenched (continuous quenching) to room temperature (its microstructure is as follows). Figure 1 (As shown) After that, it was kept at ~73℃ for 2 hours and then brought back to room temperature. The cryogenically treated workpiece was aged at 510℃ for 10 hours and then brought back to room temperature.
[0035] Comparative Example 2
[0036] The alloy was smelted using a dual process of vacuum induction melting and electroslag remelting. The alloy's chemical composition (mass percentage) was: 0.3% C, 1% Cr, 10% Ni, 2% Mo, 7% Co, 1.3% W, 0.1% V, with the balance being Fe. The forged and annealed workpiece was held at 1060℃ for 1.5 hours, then oil-quenched (continuous quenching) to room temperature, followed by holding at -73℃ for 2 hours and then warming to room temperature. The cryogenically treated workpiece was aged at 510℃ for 12 hours, and then warmed to room temperature.
[0037] like Figures 1-4 As shown, in Comparative Example 1, the forged and annealed workpiece was held at 1060℃ for 1.5h and then oil-quenched to room temperature (i.e., continuous quenching process), resulting in a matrix ( Figure 1 The microstructure consists entirely of martensite. Examples 1, 2, and 3 were prepared using different isothermal processes (190℃×1h, 220℃×1h, and 220℃×8h), and their metallographic structures are as follows: Figure 2 , 3 As shown in Figure 4, the white part is the Mao island or retained austenite; the light gray part is bainitic ferrite or fresh martensite. When the isothermal time is short, the two are difficult to distinguish. As the isothermal time increases, the acicular morphology of bainitic ferrite gradually becomes obvious; the coarse black laths are primary martensite.
[0038] The mechanical properties of Examples 1-4 and Comparative Examples 1-2 were compared, and the results are shown in Table 1.
[0039] Table 1. Experimental Results of Mechanical Properties
[0040] steel grades Yield strength (MPa) Tensile strength (MPa) Reduction of area (%) Elongation after fracture (%) U-shaped impact energy (J) Comparative Example 1 1747 2011 67 13.4 77 Comparative Example 2 1698 1964 65 13 76 Example 1 1695 2008 68 14.7 84 Example 2 1718 2015 66 14.5 78 Example 3 1728 2017 66 13.5 73 Example 4 1044 1777 46 12 67
[0041] As shown in Comparative Examples 1 and 2 in the table above, the traditional quenching-tempering process, which uses over-aging, leads to a simultaneous decrease in both strength and toughness. The process of this invention allows the steel grade to achieve a U-notch impact energy of approximately 85J without a significant decrease in strength. Therefore, this invention provides a novel approach to achieving a balance between strength and toughness for high-Co-Ni secondary hardening steel.
Claims
1. A method for preparing ultra-high strength and high toughness martensitic / bainitic multiphase aged steel, characterized in that, Includes the following steps: (1) The steel ingot is subjected to austenitizing treatment; (2) The sample after step (1) is directly quenched from the austenitizing temperature to below Ms point for isothermal holding for 1h to 24h. After holding, it is oil quenched to room temperature. (3) The sample after step (2) is subjected to cryogenic treatment and then returned to room temperature; (4) The sample after step (3) is aged and then cooled to room temperature. The steel ingot contains the following components by mass percentage: C 0.25%~0.35%, Cr 0.8%~1.5%, Ni 7%~12%, Mo 1%~3%, Co 7%~12%, W 1%~2%, V 0.05%~0.15%, with the balance being Fe and unavoidable impurities.
2. The preparation method according to claim 1, characterized in that, The austenitizing treatment temperature is 1050℃~1080℃, and the holding time is 1h~2h.
3. The preparation method according to claim 1, characterized in that, In step (2), the insulation temperature below point Ms is 190℃~220℃, and the temperature control accuracy during the isothermal insulation process is ±5℃.
4. The preparation method according to claim 1, characterized in that, The insulation medium in step (2) is molten salt.
5. The preparation method according to claim 1, characterized in that, The cryogenic treatment temperature in step (3) is -193℃ to -73℃, and the holding time is 1h to 3h.
6. The preparation method according to claim 1, characterized in that, The insulation medium in step (3) is dry ice.
7. The preparation method according to claim 1, characterized in that, The aging treatment temperature in step (4) is 450℃~550℃, and the treatment time is 8h~12h.
8. Ultra-high strength and high toughness martensitic / bainitic multiphase aging steel prepared by any of the preparation methods described in claims 1 to 7.