Heat treatment method for improving obdurability of low-cobalt high-nickel secondary hardening steel

Abstract

The invention provides a heat treatment method for improving the obdurability of low-cobalt high-nickel secondary hardening steel, which comprises the following steps: S1, carrying out solution treatment on the low-cobalt high-nickel secondary hardening steel, carrying out air cooling and heat preservation to form intermediate low-cobalt high-nickel secondary hardening steel comprising a bainite structure, and then carrying out oil quenching to room temperature; s2, the low-cobalt high-nickel secondary hardening steel obtained in the step S1 is subjected to subzero treatment, and then the temperature is returned to the room temperature; and S3, the low-cobalt high-nickel secondary hardening steel obtained in the step S2 is subjected to secondary hardening tempering and cooling. According to the heat treatment method provided by the invention, the intermediate cooling heat preservation is introduced after the solution treatment, so that the fracture toughness of the low-cobalt high-nickel secondary hardening steel is remarkably improved, the strength and the toughness are more balanced, the matching of the strength and the toughness is better, the low-cobalt high-nickel secondary hardening steel is suitable for being applied under the requirement condition of relatively high toughness, and the application range of the low-cobalt high-nickel secondary hardening steel is expanded.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary hardening steel, and in particular to a heat treatment method for improving the strength and toughness of low-cobalt high-nickel secondary hardening steel. Background Art

[0002] In many high-end manufacturing fields, extremely high requirements are placed on the strength and toughness of materials. High-cobalt-nickel secondary hardening ultra-high strength steel has attracted much attention due to its excellent mechanical properties. High-cobalt-nickel secondary hardening ultra-high strength steel obtains a martensitic structure with a high dislocation density through solid solution quenching, laying the foundation for high strength. Deep cryogenic treatment can eliminate or greatly reduce residual austenite, further improving strength. During tempering, nanoscale M2C alloy carbides are precipitated that maintain a coherent or semi-coherent relationship with the martensitic matrix. These fine carbides hinder dislocation movement without significantly reducing the toughness of the material, thereby achieving ultra-high strength. In addition, the reversed austenite film produced during tempering can effectively absorb crack energy and significantly improve the fracture toughness of the material. Therefore, the strength and toughness of high-cobalt-nickel secondary hardening ultra-high strength steel are achieved through the combined action of a unique microstructure and multiple strengthening mechanisms.

[0003] Both Aer Met100 and Ferrium M54 are secondary hardening ultra-high strength steels. Ferrium M54, a W-containing, low-cobalt, high-nickel secondary hardening ultra-high strength steel designed and developed by QuesTek Innovations LLC, has a significantly lower Co content than Aer Met100, reducing its production cost. Furthermore, Ferrium M54 offers significant advantages in stress corrosion resistance, offering greater application prospects.

[0004] Ferrium M54 steel's strength and toughness are comparable to Aer Met100 steel, with a slightly higher ultimate tensile strength and slightly lower fracture toughness. This is because Ferrium M54 has a slightly higher carbon content than AerMet100. Furthermore, Ferrium M54 contains alloying elements such as vanadium and tungsten, which contribute to the formation of more carbides, increasing the material's strength and hardness. However, this has a limited effect on fracture toughness and may even slightly reduce it. AerMet100, on the other hand, has a higher cobalt content, which contributes to the formation of more stable retained austenite, which absorbs energy at the crack tip and improves fracture toughness.

[0005] Therefore, it is necessary to provide a heat treatment method for improving the strength and toughness of low-cobalt, high-nickel secondary hardening steel without changing the alloy composition and preparation process. Summary of the Invention

[0006] The technical problem solved by the present invention is to provide a heat treatment method for improving the strength and toughness of low-cobalt, high-nickel secondary hardening steel. The heat treatment method provided in this application can significantly improve the fracture toughness of low-cobalt, high-nickel secondary hardening steel, ensuring both strength and fracture toughness.

[0007] In view of this, the present application provides a heat treatment method for improving the strength and toughness of low-cobalt, high-nickel secondary hardening steel, comprising the following steps:

[0008] S1. Solution treating the low-cobalt, high-nickel secondary hardening steel, air cooling it to 290-310°C, then holding the temperature to form an intermediate low-cobalt, high-nickel secondary hardening steel including a bainite structure, and then oil quenching it to room temperature;

[0009] S2, subjecting the low-cobalt, high-nickel secondary hardened steel obtained in step S1 to cryogenic treatment, and then returning the temperature to room temperature;

[0010] S3. Perform secondary hardening and tempering on the low-cobalt, high-nickel secondary hardening steel obtained in step S2, and cool it.

[0011] In some specific embodiments, in step S1, the temperature of the solution treatment is 1050-1070° C., and the holding time is 80-90 min.

[0012] In some specific embodiments, in step S1, the time of keeping warm after air cooling to 290-310°C is 50-70 minutes.

[0013] In some specific embodiments, in step S1 , the content of bainite structure in the metallographic structure of the intermediate low-cobalt high-nickel secondary hardening steel is less than 5%.

[0014] In some specific embodiments, the time for transferring the low-cobalt, high-nickel secondary hardening steel obtained in step S1 to cryogenic treatment is ≤120 min.

[0015] In some specific embodiments, in step S2, the temperature of the cryogenic treatment is -70 to -80°C, and the time is 90 to 110 minutes.

[0016] In some specific embodiments, in step S3, the temperature of the secondary hardening and tempering is 510-525° C., and the temperature is kept at this temperature for 8-12 hours.

[0017] In some specific embodiments, in step S2, the warming method is warming to room temperature, and / or, in step S3, the cooling method is cooling to room temperature.

[0018] In some specific embodiments, the metallographic structure of the low-cobalt, high-nickel secondary hardening steel after the solid solution treatment includes full austenite and MC type carbides, the metallographic structure of the low-cobalt, high-nickel secondary hardening steel after oil quenching to room temperature includes retained austenite, MC type carbides, bainite and martensite, the metallographic structure of the low-cobalt, high-nickel secondary hardening steel obtained in step S2 includes retained austenite, MC type carbides, bainite and martensite, and the metallographic structure of the low-cobalt, high-nickel secondary hardening steel obtained in step S3 includes retained austenite, MC type carbides, bainite, tempered martensite, nano-scale M2C type carbides and nano-scale inverted residual austenite.

[0019] In some specific embodiments, the composition of the low-cobalt, high-nickel secondary hardening steel includes, by mass percentage, C: 0.26-0.34%, Co: 6.0-8.0%, Ni: 8.0-12%, Cr: 0.6-1.40%, Mo: 1.6-2.4%, W: 1.0-1.6%, V: 0.04-0.16%, Ti≤0.015%, T[O]≤0.0015%, T[N]≤0.0015%, S≤0.0015%, P≤0.0020%, Al≤0.015%, and the rest is Fe and unavoidable impurities.

[0020] The present application provides a heat treatment method for improving the strength and toughness of low-cobalt, high-nickel secondary hardening steel, which first subject the low-cobalt, high-nickel secondary hardening steel to solid solution treatment, then air-cool it to 290-310°C and then keep it warm to form an intermediate low-cobalt, high-nickel secondary hardening steel including a bainite structure, then oil quench it to room temperature, and then subject the obtained low-cobalt, high-nickel secondary hardening steel to deep cold treatment, and finally subject the obtained low-cobalt, high-nickel secondary hardening steel to secondary hardening and tempering; in the heat treatment method provided in the present application, the low-cobalt, high-nickel secondary hardening steel is heat treated using a "solid solution-intermediate cooling and insulation-quenching-deep cooling-tempering" system, so that a very small amount of bainite is formed in the secondary hardening steel after intermediate cooling and insulation, and a mixed structure including martensite and bainite is formed after quenching and deep cooling, thereby significantly improving the fracture toughness of the low-cobalt, high-nickel secondary hardening steel after the above series of heat treatments, thereby ensuring the matching of strength and fracture toughness. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 A schematic diagram of the heat treatment process for improving the strength and toughness of low-cobalt, high-nickel secondary hardening steel provided by the present invention. DETAILED DESCRIPTION

[0022] In order to further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0023] In view of the requirements of low-cobalt high-nickel secondary hardening steel for fracture toughness performance in the prior art, the present application provides a heat treatment method for low-cobalt high-nickel secondary hardening steel, which can significantly improve the fracture toughness of low-cobalt high-nickel secondary hardening steel through the heat treatment method of "solid solution-intermediate cooling and heat preservation (subcritical isothermal)-quenching-deep cooling-tempering", and its comprehensive performance (strength and toughness) is more balanced, and the strength and toughness match is better. Specifically, the present application provides a preparation method for low-cobalt high-nickel secondary hardening steel, and its processing flow is as follows: Figure 1 As shown, the specific steps include:

[0024] S1. Solution treating the low-cobalt, high-nickel secondary hardening steel, air cooling it to 290-310°C, then holding the temperature to form an intermediate low-cobalt, high-nickel secondary hardening steel including a bainite structure, and then oil quenching it to room temperature;

[0025] S2, subjecting the low-cobalt, high-nickel secondary hardened steel obtained in step S1 to cryogenic treatment, and then returning the temperature to room temperature;

[0026] S3. Perform secondary hardening and tempering on the low-cobalt, high-nickel secondary hardening steel obtained in step S2, and cool it.

[0027] In the heat treatment method provided in the present application, the present application first performs a solution treatment on the low-cobalt, high-nickel secondary hardening steel. The solution treatment temperature is 1050-1070°C and the holding time is 80-90 minutes. Specifically, the solution treatment temperature is 1050°C, 1055°C, 1060°C, 1065°C, and 1070°C, and the holding time is 80 minutes, 85 minutes, and 90 minutes. The solution treatment dissolves the carbon and alloying elements (such as Co, Ni, Cr, and Mo) in the low-cobalt, high-nickel secondary hardening steel to form a uniform supersaturated single-phase austenite, in which a small amount of stable carbides (such as MC type) may remain, but austenite is the main component. Therefore, the microstructure of the low-cobalt, high-nickel secondary hardening steel after the solution treatment is: full austenite + a small amount of undissolved MC type carbides. The solution treatment provides a uniform matrix for subsequent subcritical isothermal and quenching.

[0028] After the above-mentioned solution treatment, the obtained low-cobalt, high-nickel secondary hardening steel is cooled by air cooling to 290-310°C and kept warm to form an intermediate low-cobalt, high-nickel secondary hardening steel including a bainite structure; the above-mentioned method is intermediate cooling and heat preservation, which can also be called "subcritical heat preservation". In the above process, the heat preservation time is 50-70 minutes, specifically, the heat preservation time is 55-65 minutes, more specifically, the heat preservation time is 55-60 minutes. The cooling temperature is specifically 290°C, 295°C, 300°C, 305°C, and 310°C. In this process, a small part of the austenite in the low-cobalt, high-nickel secondary hardening steel is transformed into bainite, and its structure includes austenite + a small amount of undissolved MC-type carbide + a small amount of bainite; due to the small amount of bainite obtained in the hardened steel after subcritical heat preservation, it has good toughness and certain strength. The intermediate low-cobalt, high-nickel secondary hardening steel is then oil-quenched to room temperature. The rapid cooling method described above allows martensite to form in the secondary hardening steel. The hardened steel after oil quenching obtains a high-strength martensitic matrix, in which the austenite is transformed into lath martensite with high hardness and high defect density. The alloying elements (Co, Ni, etc.) are solid-dissolved in the matrix. At the same time, part of the austenite is not transformed due to its low Ms point and is distributed in the form of a thin film between the martensite laths. Therefore, the microstructure of the hardened steel after oil quenching includes a small amount of retained austenite + a small amount of undissolved MC-type carbide + a small amount of bainite + martensite.

[0029] The present application then performs cryogenic treatment on the low-cobalt, high-nickel secondary hardening steel that has been oil-quenched to room temperature, and then returns the temperature to room temperature; during this process, the temperature of the cryogenic treatment is -70 to -80°C, and the time is 90 to 110 minutes; specifically, the temperature of the cryogenic treatment is -73 to -78°C, and the time is 95 to 105 minutes; more specifically, the temperature of the cryogenic treatment is -74 to -77°C, and the time is 98 to 103 minutes; in the present application, the time for transferring the oil-quenched low-cobalt, high-nickel secondary hardening steel to cryogenic treatment is ≤120 minutes. The method of returning the temperature is room temperature. The cryogenic treatment is to further reduce the retained austenite. This process is cryogenic treatment. At low temperatures, the stability of the retained austenite is reduced, and part of it is transformed into martensite. The local dislocation density in the martensite is slightly reduced, and the internal stress is partially released. Therefore, the microstructure of the low-cobalt, high-nickel secondary hardening steel after cryogenic treatment includes a smaller amount of retained austenite + a small amount of undissolved MC-type carbide + a small amount of bainite + martensite. Low-cobalt, high-nickel secondary hardening steel can be cryogenically treated to increase hardness and dimensional stability and reduce the adverse effects of retained austenite during subsequent tempering.

[0030] Finally, the low-cobalt, high-nickel secondary hardening steel is subjected to secondary hardening and tempering, followed by cooling, thereby completing the heat treatment of the low-cobalt, high-nickel secondary hardening steel. The secondary hardening and tempering temperature is 510-525°C for 8-12 hours, more specifically, the secondary hardening and tempering temperature is 516-523°C for 9-11 hours, and more specifically, the secondary hardening and tempering temperature is 518-520°C for 10 hours. The cooling step is to naturally cool the steel in air to room temperature. The secondary hardening and tempering is to promote the precipitation of alloy carbides; after the secondary hardening and tempering, nano-scale M2C-type carbides (mainly Mo and Cr) precipitate in the supersaturated martensite of the low-cobalt, high-nickel secondary hardening steel, producing a strong precipitation strengthening effect. The martensite decomposes into tempered martensite, further eliminating internal stress, improving toughness, and forming reversed residual austenite around the retained austenite or Ni-rich area. Its organizational composition includes: a smaller amount of retained austenite + a small amount of undissolved MC-type carbides + a small amount of bainite + tempered martensite + nano-scale M2C-type carbides + nano-scale reversed retained austenite. The low-cobalt, high-nickel secondary hardening steel after the secondary hardening and tempering achieves "secondary hardening" while optimizing the strength-toughness match.

[0031] In the present application, the composition of the low cobalt and high nickel secondary hardening steel, in terms of mass percentage, includes: C: 0.26-0.34%, Co: 6.0-8.0%, Ni: 8.0-12%, Cr: 0.6-1.40%, Mo: 1.6-2.4%, W: 1.0-1.6%, V: 0.04-0.16%, Ti≤0.015%, T[O]≤0.0015%, T[N]≤0.0015%, S≤0.0015%, P≤0.0020%, Al≤0.015%, and the rest is Fe and unavoidable Free of impurities; specifically, the composition of the low-cobalt, high-nickel secondary hardening steel includes: C: 0.30-0.33%, Co: 6.2-7.0%, Ni: 9.0-10.0%, Cr: 1.00-1.20%, Mo: 1.8-2.0%, W: 1.1-1.3%, V: 0.05-0.10%, Ti≤0.015%, T[O]≤0.0015%, T[N]≤0.0015%, S≤0.0015%, P≤0.0020%, Al≤0.015%, and the rest are Fe and unavoidable impurities.

[0032] The present invention provides a heat treatment method for improving the low-cobalt, high-nickel secondary hardening steel. The method adopts the system of "solid solution-subcritical isothermal-quenching-deep cooling-tempering" to heat treat the low-cobalt, high-nickel secondary hardening ultra-high strength steel. This heat treatment method forms a very small amount of bainite in the steel after subcritical isothermal treatment. After quenching and deep cooling, a mixed structure of martensite and bainite is formed. Bainite is a structure between martensite and pearlite, with good toughness and certain strength. Therefore, the fracture toughness and strength of the secondary hardening steel after heat treatment in this application have a good match. Experimental results show that the ultimate tensile strength is at least 1980MPa, K IC Fracture toughness of at least 130 MPa·m 1 / 2 , suitable for use in applications requiring higher toughness.

[0033] In order to further understand the present invention, the heat treatment method for improving the secondary hardening of low-cobalt and high-nickel steel provided by the present invention is described in detail below in conjunction with the examples. The protection scope of the present invention is not limited by the following examples.

[0034] The present application provides a heat treatment method for improving the strength and toughness of low-cobalt, high-nickel secondary hardening steel, which utilizes the subcritical isothermal method after solid solution to introduce a very small amount of bainite into the steel, thereby significantly improving the fracture toughness under the condition of slightly reducing the strength.

[0035] In the following examples and comparative examples, the hot forging annealing state of low cobalt high nickel secondary hardening steel was used. 90. 300, The bar specification is 400mm, and the grain size of the bar structure is 8.5, 8, and 8 respectively. The components and weight percentage are: C: 0.30%, Co: 7.0%, Ni: 10%, Cr: 1.0%, Mo: 2.0%, W: 1.3%, V: 0.1%, Ti≤0.015%, T[O]≤0.0015%, T[N]≤0.0015%, S≤0.0015%, P≤0.0020%, Al≤0.015%, and the rest are Fe and unavoidable impurities.

[0036] The high-temperature heating furnace, controlled cooling cabinet and tempering furnace with the same temperature control accuracy were used in the embodiment and the comparative example.

[0037] Example 1

[0038] from 90mm bar material is used as heat treatment sample blank. A 90mm bar is placed in a high-temperature heating furnace with a temperature of 1060°C and a temperature control accuracy of ±10°C. The timer starts when the furnace temperature returns to 1060°C ±10°C and the temperature is kept at this temperature for 90 minutes. The bar temperature is then lowered to 300°C by air cooling and quickly transferred to a temperature-equalizing furnace with a temperature of 300°C and a temperature control accuracy of ±10°C for subcritical isothermal treatment. The timer starts when the furnace temperature returns to 300°C ±10°C and the temperature is kept at this temperature for 60 minutes. The bar is then quickly cooled to room temperature by oil quenching.

[0039] After the quenching treatment is completed, the bar is placed in a -73℃ controlled freezer within 120 minutes. When the temperature drops to -73℃, the timing starts and the temperature is kept in this range for 95 minutes. After the treatment is completed, the bar is taken out and allowed to return to room temperature naturally in the air.

[0040] Place the cryogenically treated bar into a tempering furnace with a temperature of 516°C and a temperature control accuracy of ±5°C. Start timing after the furnace temperature returns to 516°C and keep it warm for 10 hours. Then take out the bar and let it cool naturally to room temperature in the air.

[0041] The samples that have undergone the above heat treatment are finely processed and prepared into standard tensile specimens and fracture toughness specimens, and their tensile properties and fracture toughness are tested respectively. The test results are shown in Table 1.

[0042] Example 2

[0043] from 300mm bar material sample blank is taken as heat treatment sample, and the selected The 300mm bar was placed in a high-temperature heating furnace at 1060°C and held at that temperature for 80 minutes. The bar temperature was then lowered to 300°C by air cooling and then quickly transferred to a 300°C equalizing furnace for subcritical isothermal treatment for 60 minutes. The bar was then quickly cooled to room temperature by oil quenching.

[0044] After the quenching treatment is completed, the quenched sample is placed in a -80℃ controlled freezer within 120 minutes, the temperature is reduced to -80℃ and maintained for 100 minutes, and then taken out and allowed to return to room temperature in the air;

[0045] The cryogenically treated bar was placed in a tempering furnace at 516°C, kept warm for 8 hours, and then taken out and air-cooled to room temperature;

[0046] The samples that have undergone the above heat treatment are finely processed and prepared into standard tensile specimens and fracture toughness specimens, and their tensile properties and fracture toughness are tested respectively. The test results are shown in Table 1.

[0047] Example 3

[0048] from 400mm bar material is used as heat treatment sample blank. The 400mm bar was placed in a high-temperature heating furnace at 1060°C and held at that temperature for 80 minutes. The bar temperature was then lowered to 300°C by air cooling and then quickly transferred to a 300°C equalizing furnace for subcritical isothermal treatment for 60 minutes. The bar was then quickly cooled to room temperature by oil quenching.

[0049] After the quenching treatment is completed, the quenched sample is placed in a -73℃ controlled freezer within 120 minutes, kept at -73℃ for 105 minutes, and then taken out and allowed to return to room temperature in the air;

[0050] The cryogenically treated bar was placed in a tempering furnace at 516°C for 12 hours, then taken out and air-cooled to room temperature.

[0051] The samples that have undergone the above heat treatment are finely processed and prepared into standard tensile specimens and fracture toughness specimens, and their tensile properties and fracture toughness are tested respectively. The test results are shown in Table 1.

[0052] Comparative Example 1

[0053] right The sample blank of 90mm specification bar is taken as heat treatment sample and treated by conventional heat treatment process (solution hardening - quenching - deep cooling - tempering). The specific steps are as follows: The 90mm bar is placed in a heating furnace and heated to 1060℃ for 90 minutes for solution treatment, then oil quenched, followed by deep cooling treatment at -73℃×95min, returned to room temperature in air, and finally subjected to secondary hardening and tempering at 516℃×10h.

[0054] The samples that have undergone the above heat treatment are finely processed and prepared into standard tensile specimens and fracture toughness specimens, and their tensile properties and fracture toughness are tested respectively. The test results are shown in Table 1.

[0055] Comparative Example 2

[0056] right The sample blank of 300mm specification bar material is taken as heat treatment sample and treated by conventional heat treatment process (solution hardening-quenching-cold cooling-tempering). The specific steps are as follows: The 300mm bar is placed in a heating furnace and heated to 1060℃ for 80 minutes for solution treatment, then oil quenched, followed by deep cooling treatment at -80℃×100min, returned to room temperature in air, and finally subjected to secondary hardening and tempering at 516℃×8h.

[0057] The heat-treated specimens were then processed into standard tensile and fracture toughness specimens, and their tensile properties and fracture toughness were tested, respectively. The test results are shown in Table 1.

[0058] Comparative Example 3

[0059] right The sample blank of 400mm specification bar material is taken as heat treatment sample and treated by conventional heat treatment process (solution hardening-quenching-cryogenic-tempering). The specific steps are as follows: The 400mm bar is placed in a heating furnace and heated to 1060℃, kept warm for 80min for solution treatment, then oil quenched, followed by deep cooling treatment at -73℃×105min, returned to room temperature in air, and finally subjected to secondary hardening and tempering at 516℃×12h.

[0060] The heat-treated specimens were then processed into standard tensile and fracture toughness specimens, and their tensile properties and fracture toughness were tested, respectively. The test results are shown in Table 1.

[0061] Comparative Example 4

[0062] from 300mm bar material sample blank is taken as heat treatment sample, and the selected The 300mm bar was placed in a high-temperature heating furnace at 1060℃ and kept at this temperature for 80 minutes. It was then air-cooled to 300℃ and quickly transferred to a 300℃ equalizing furnace for subcritical isothermal treatment for 30 minutes. It was then oil-quenched to room temperature.

[0063] After the quenching treatment is completed, the quenched sample blank is placed in a -80℃ controlled freezer within 120 minutes, the temperature is reduced to -80℃ and kept for 100 minutes, and then taken out and allowed to return to room temperature in the air;

[0064] The cryogenically treated bar was placed in a tempering furnace at 516°C, kept warm for 8 hours, and then taken out and air-cooled to room temperature;

[0065] The samples that have undergone the above heat treatment are finely processed and prepared into standard tensile specimens and fracture toughness specimens, and their tensile properties and fracture toughness are tested respectively. The test results are shown in Table 1.

[0066] Comparative Example 5

[0067] from 300mm bar material sample blank is taken as heat treatment sample, and the selected The 300mm bar was placed in a high-temperature heating furnace at 1060°C and held at that temperature for 80 minutes. The bar temperature was then lowered to 350°C by air cooling and then quickly transferred to a 350°C equalizing furnace for subcritical isothermal treatment for 60 minutes. The bar was then quickly cooled to room temperature by oil quenching.

[0068] After the quenching treatment is completed, the quenched sample is placed in a -80℃ controlled freezer within 120 minutes, the temperature is reduced to -80℃ and maintained for 100 minutes, and then taken out and allowed to return to room temperature in the air;

[0069] The cryogenically treated bar was placed in a tempering furnace at 516°C, kept warm for 8 hours, and then taken out and air-cooled to room temperature;

[0070] The samples after the above heat treatment were processed into standard tensile specimens and fracture toughness specimens, and their tensile properties and fracture toughness were tested respectively. The test results are shown in Table 1. Table 1 Strength and toughness data of low cobalt and high nickel secondary hardening steel bars of Examples 1 to 3 and Comparative Examples 1 to 5

[0071]

[0072] In Comparative Example 4, the bainite content formed after subcritical isothermal treatment at 300° C. for 30 min is insufficient compared to Example 2; the performance of Comparative Example 4 is relatively close to that of Comparative Example 2.

[0073] In Comparative Example 5, compared with Example 2, the subcritical isothermal treatment was performed at 350°C for 60 minutes. Due to the high temperature, the bainite content formed was too much, the secondary precipitated carbides were also more, and the martensite content was relatively less. Compared with Comparative Example 2 and Example 2, the strength of Comparative Example 5 decreased too much.

[0074] The above embodiments are only intended to help understand the method and core concept of the present invention. It should be noted that, without departing from the principles of the present invention, a number of improvements and modifications may be made to the present invention by those skilled in the art, and such improvements and modifications also fall within the scope of protection of the claims of the present invention.

[0075] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A heat treatment method for improving the strength and toughness of low-cobalt, high-nickel secondary hardening steel, comprising the following steps: S1. Solution treating the low-cobalt, high-nickel secondary hardening steel, air cooling it to 290-310°C, then holding the temperature to form an intermediate low-cobalt, high-nickel secondary hardening steel including a bainite structure, and then oil quenching it to room temperature; S2, subjecting the low-cobalt, high-nickel secondary hardened steel obtained in step S1 to cryogenic treatment, and then returning the temperature to room temperature; S3. Perform secondary hardening and tempering on the low-cobalt, high-nickel secondary hardening steel obtained in step S2, and cool it.

2. The heat treatment method according to claim 1, characterized in that In step S1, the temperature of the solution treatment is 1050-1070° C., and the holding time is 80-90 minutes.

3. The heat treatment method according to claim 1 or 2, characterized in that In step S1, the time of keeping the temperature after air cooling to 290-310°C is 50-70 minutes.

4. The heat treatment method according to claim 3, characterized in that In step S1, the content of bainite structure in the metallographic structure of the intermediate low-cobalt high-nickel secondary hardening steel is less than 5%.

5. The heat treatment method according to claim 4, characterized in that The time for transferring the low-cobalt, high-nickel secondary hardening steel obtained in step S1 to deep cryogenic treatment is ≤120 min.

6. The heat treatment method according to claim 4, characterized in that In step S2, the temperature of the cryogenic treatment is -70 to -80°C, and the time is 90 to 110 minutes.

7. The heat treatment method according to claim 1 or 4, characterized in that In step S3, the temperature of the secondary hardening and tempering is 510-525° C., and the temperature is kept for 8-12 hours.

8. The heat treatment method according to claim 1 or 4, characterized in that In step S2, the method of returning to temperature is returning to room temperature, and / or, in step S3, the method of cooling is cooling to room temperature.

9. The heat treatment method according to claim 8, characterized in that The metallographic structure of the low-cobalt, high-nickel secondary hardening steel after the solid solution treatment includes full austenite and MC type carbides. The metallographic structure of the low-cobalt, high-nickel secondary hardening steel after oil quenching to room temperature includes retained austenite, MC type carbides, bainite and martensite. The metallographic structure of the low-cobalt, high-nickel secondary hardening steel obtained in step S2 includes retained austenite, MC type carbides, bainite and martensite. The metallographic structure of the low-cobalt, high-nickel secondary hardening steel obtained in step S3 includes retained austenite, MC type carbides, bainite, tempered martensite, nano-scale M2C type carbides and nano-scale inverted residual austenite.

10. The heat treatment method according to any one of claims 1 to 9, characterized in that The composition of the low-cobalt, high-nickel secondary hardening steel, in terms of mass percentage, includes: C: 0.26-0.34%, Co: 6.0-8.0%, Ni: 8.0-12%, Cr: 0.6-1.40%, Mo: 1.6-2.4%, W: 1.0-1.6%, V: 0.04-0.16%, Ti≤0.015%, T[O]≤0.0015%, T[N]≤0.0015%, S≤0.0015%, P≤0.0020%, Al≤0.015%, and the rest is Fe and unavoidable impurities.

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