Heat treatment method for ultrahigh-strength and high-toughness steel

Abstract

The present invention relates to the technical field of heat treatment methods for special metal materials, and provides a heat treatment method for ultrahigh-strength and high-toughness steel. The heat treatment method comprises a quenching treatment, a subzero treatment and a tempering treatment. By accurately controlling the treatment temperature, the treatment time, the furnace temperature uniformity, the charging amount, etc. during each stage of the heat treatment, the formation of a structure having good strength and toughness is synergistically promoted, such that ultrahigh-strength and high-toughness steel having well matched strength and toughness can be obtained, thereby stably realizing the technical indexes of a room-temperature tensile strength of greater than or equal to 1930 MPa, a room-temperature yield strength of greater than or equal to 1620 MPa and a fracture toughness of greater than or equal to 110 MPa·m1 / 2. In addition, the variation coefficient of the mechanical properties of the prepared ultrahigh-strength and high-toughness steel reaches an ultrahigh standard of the variation coefficient of tensile strength being less than or equal to 0.18 and the variation coefficient of fracture toughness being less than or equal to 2.0, such that the product has mechanical properties that are up to standard with small fluctuations and has strong stability, which can satisfy the requirements for production.

Description

Heat treatment method for ultra-high strength and high toughness steel Technical Field

[0001] The present invention relates to the technical field of heat treatment methods for special metal materials, and in particular to a heat treatment method for ultra-high strength and high toughness steel. Background Art

[0002] With the rapid development of modern science and technology, the requirements for metal materials are becoming increasingly stringent, especially in specialized environments such as aviation and aerospace, which often require ultra-high strength and high toughness. Ultra-high-strength steel not only possesses exceptional strength, but also excellent ductility, fatigue resistance, fracture toughness, and stress corrosion resistance, meeting these requirements.

[0003] 3.1Cr-11.5Ni-13.5Co-1.2Mo (0.21-0.25C) steel is a new type of ultra-high strength steel. The current international and domestically used high-strength and high-toughness 3.1Cr-11.5Ni-13.5Co-1.2Mo (0.210.25C) steel (chemical composition see Table 1) has a room temperature tensile strength of ≥1930MPa, a room temperature yield strength of ≥1620MPa, and a fracture toughness of ≥110MPa·m. 1 / 2 This alloy uses C, Cr, and Mo as the main strengthening elements, and has ultra-high strength and high toughness. It also has excellent resistance to stress corrosion cracking and fatigue fracture. It is an ideal material for aerospace landing gear and is currently widely used in aerospace fields such as aircraft landing gear.

[0004] Table 1 Chemical composition

[0005] 3.1Cr-11.5Ni-13.5Co-1.2Mo (0.210.25C) steel is a secondary hardening ultra-high-strength steel. It is typically treated by quenching, deep cooling, and tempering to form a lath martensite matrix with a high dislocation density. Fine carbides are dispersed and precipitated that are coherent or semi-coherent with the matrix, producing a strengthening effect. Simultaneously, the martensitic substructure and thin film-like reversed austenite provide toughening. However, due to the steel's narrow manufacturing process window, the three-step heat treatment of quenching, deep cooling, and tempering is closely linked and interconnected. Fluctuations in the heat treatment process can easily lead to instability in the mechanical properties testing process, resulting in substandard mechanical properties. This poses a significant challenge to material manufacturers. Therefore, there is an urgent need to achieve a stable balance between strength and toughness in this ultra-high-strength, high-toughness steel through the stable regulation of heat treatment processes and parameters. Summary of the Invention

[0006] The purpose of the present invention is to provide a heat treatment method for ultra-high strength and high toughness steel, which can stably improve the strength and toughness of ultra-high strength and high toughness steel by regulating the heat treatment process and parameters of quenching + controlled deep cryogenic treatment + tempering treatment.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] The present invention provides a heat treatment method for ultra-high strength and high toughness steel, comprising the following steps:

[0009] (1) Quenching treatment:

[0010] The heating furnace with a furnace temperature uniformity of less than or equal to ±14°C is heated to 885°C. After the temperature of the temperature controller is stably controlled at 885°C, the mechanical property specimens are placed in the effective area of ​​the heating furnace using a tray for heating. The timing starts after the furnace temperature rises to 885°C within 5 to 10 minutes, and the temperature is kept at this temperature for 60 to 70 minutes. The temperature of the cooling oil in the oil tank is controlled to be less than or equal to 60°C before quenching. The oil tank is stirred. The mechanical property specimens are quickly taken out of the furnace and dispersed and poured into the oil tank for oil cooling.

[0011] (2) Cryogenic treatment:

[0012] Cool down the deep freezer with a furnace temperature uniformity of less than or equal to ±8°C to -73°C. After the temperature of the deep freezer temperature controller is stably controlled at -73°C, place the quenched mechanical property specimens on a tray into the central effective area of ​​the deep freezer for deep freezing treatment. After closing the door, start timing when the instrument temperature reaches -73°C again. Keep warm for 60-70 minutes, turn off the power, take the specimens out of the oven, and allow them to return to room temperature in the air.

[0013] (3) Tempering treatment:

[0014] The tempering furnace with a temperature uniformity of less than or equal to ±3°C is heated to 482°C. After the temperature of the temperature controller is stably controlled at 482°C, the mechanical property specimens after cryogenic treatment are placed in the effective area of ​​the tempering furnace for heating. After the instrument returns to 482°C, the timing is started. The temperature is kept for 370 to 390 minutes. After being taken out of the furnace, it is air-cooled for 2 hours to room temperature to obtain ultra-high strength and high toughness steel.

[0015] Furthermore, based on the above technical solution of the present invention, in step (1), the quenching furnace is a box-type furnace, and its loading capacity is ≤ 4 tensile specimens + 4 impact specimens + 4 fracture toughness specimens.

[0016] Furthermore, based on the above technical solution of the present invention, in step (2), the single-layer loading capacity of the deep freezer is ≤12 tensile specimens + 12 impact specimens + 12 fracture toughness specimens.

[0017] Furthermore, based on the above technical solution of the present invention, in step (3), the tempering furnace is a pit furnace; its single-layer furnace capacity is ≤8 tensile specimens + 8 impact specimens + 8 fracture toughness specimens.

[0018] Furthermore, based on the above technical solution of the present invention, in step (1), the sample is quickly taken out of the furnace and transferred to the cooling oil for quenching for less than or equal to 6 seconds;

[0019] And / or, the cooling time of the sample in the cooling oil is 20 to 60 minutes, and sufficient and uninterrupted stirring is performed to promote uniformity of quenching of the material.

[0020] Furthermore, based on the above technical solution of the present invention, the cooling oil is B244 vacuum quenching oil;

[0021] And / or, the cooling oil has an oil cooling rate of 60-90° C. / s.

[0022] Furthermore, based on the above technical solution of the present invention, the time interval between the quenching treatment and the cryogenic treatment does not exceed 1 to 2 hours;

[0023] And / or, the time interval between the cryogenic treatment and the tempering treatment is 2 to 8 hours.

[0024] The present invention also provides a steel material prepared by the heat treatment method for ultra-high strength and high toughness steel as described above.

[0025] Furthermore, based on the above technical solution of the present invention, the steel material is 3.1Cr-11.5Ni-13.5Co-1.2Mo steel, wherein the mass percentage of C is 0.21-0.25%.

[0026] Furthermore, based on the above technical solution of the present invention, the coefficient of variation of the mechanical properties of the steel material includes:

[0027] The coefficient of variation of tensile strength is ≤0.18; the coefficient of variation of fracture toughness is ≤2.0.

[0028] The present invention provides a heat treatment method for ultra-high strength and high toughness steel, which has the following beneficial effects:

[0029] Through the precise control of quenching, deep cooling and tempering during the heat treatment of the mechanical properties of the material, including the precise control of the treatment temperature, treatment time, furnace temperature uniformity and furnace loading at each stage of heat treatment, the formation of beneficial strong and tough structures is synergistically promoted, and ultra-high strength and high toughness steel with a good match of strength and toughness can be obtained, stably achieving room temperature tensile strength ≥1930MPa, room temperature yield strength ≥1620MPa, and fracture toughness ≥110MPa·m 1 / 2The technical indicators of the ultra-high strength and high toughness steel are achieved, and the coefficient of variation of the mechanical properties of the prepared ultra-high strength and high toughness steel reaches the ultra-high standard of tensile strength coefficient of variation ≤ 0.18 and fracture toughness coefficient of variation ≤ 2.0, so that the mechanical properties of the product are qualified with small fluctuations and strong stability, which can meet the production needs and make the sample test results more stable and reliable. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. It should be understood by those skilled in the art that the embodiments are merely to help understand the present invention and should not be regarded as specific limitations of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present invention. The process parameters for which specific conditions are not specified in the following examples are generally in accordance with conventional conditions.

[0031] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to form one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed in the present invention.

[0032] According to a first aspect of the present invention, there is provided a method for heat treatment of ultra-high strength and high toughness steel, comprising the following steps:

[0033] (1) Quenching treatment:

[0034] The heating furnace with a furnace temperature uniformity of less than or equal to ±14°C (such as ±13°C, ±12°C, ±11°C, ±10°C, ±9°C, ±8°C, ±7°C, ±6°C, ±5°C, ±4°C, ±3°C, ±2°C, ±1°C, etc.) is heated to 885°C. After the temperature of the temperature controller is stably controlled at 885°C, the mechanical properties sample is placed in the effective area of ​​the heating furnace with a tray for heating. The furnace temperature is raised to 885°C within 5-10 minutes (such as 6 minutes, 7 minutes, 8 minutes, 9 minutes, etc.) and the timing is started. The temperature is kept for 60-70 minutes (such as 62 minutes, 64 minutes, 66 minutes, 68 minutes, etc.); the oil temperature of the cooling oil in the oil tank before quenching is controlled to be less than or equal to 60°C (such as 40°C, 50°C, 52°C, 54°C, 56°C, 58°C, etc.), the oil tank is started for stirring, and the mechanical properties sample is quickly taken out of the furnace and dispersed and poured into the oil tank for oil cooling;

[0035] (2) Cryogenic treatment:

[0036] Cool the deep freezer with a furnace temperature uniformity of less than or equal to ±8°C (such as ±7°C, ±6°C, ±5°C, ±4°C, ±3°C, ±2°C, ±1°C, etc.) to -73°C. After the temperature of the temperature controller of the deep freezer is stably controlled at -73°C, use a tray to load the mechanical properties specimens after quenching treatment into the central effective area of ​​the deep freezer for deep freezing treatment. After closing the door, start timing when the instrument temperature reaches -73°C again, keep warm for 60 to 70 minutes (such as 62 minutes, 64 minutes, 66 minutes, 68 minutes, etc.), turn off the power and take the specimens out of the furnace, and restore them to room temperature in the air;

[0037] (3) Tempering treatment:

[0038] Heat the tempering furnace with a furnace temperature uniformity less than or equal to ±3°C (such as ±2.5°C, ±2°C, ±1.5°C, ±1°C, etc.) to 482°C. After the temperature of the temperature controller is stably controlled at 482°C, place the mechanical property specimens after cryogenic treatment into the effective area of ​​the tempering furnace for heating. Start timing after the instrument returns to 482°C, and keep warm for 370 to 390 minutes (such as 372 minutes, 374 minutes, 376 minutes, 378 minutes, 380 minutes, 382 minutes, 384 minutes, 386 minutes, 388 minutes, etc.). After taking out of the furnace, air cool for 2 hours to room temperature to obtain ultra-high strength and high toughness steel.

[0039] Specifically, the furnace temperature uniformity in step (1) is preferably ±5°C to ±10°C. If the furnace temperature uniformity is too small, the controllable heat treatment temperature range of the material will be narrowed, the treatment conditions will be harsh, and the operation difficulty will be increased in the actual quenching treatment. The furnace body of the quenching furnace provided by the present invention can meet the process requirements by maintaining the furnace temperature uniformity at ±14°C.

[0040] Furthermore, in step (1), the mechanical properties of the material are optimized when quenching at 885°C. This is because when quenching at a temperature below 885°C, harmful carbides such as M6C and M23C6 in the material cannot be effectively dissolved, and a full solid solution cannot be achieved. When quenching at a temperature above 885°C, the grain size of the material increases, reducing the overall performance of the material.

[0041] In step (1), it is necessary to ensure that the furnace temperature uniformity, furnace loading and holding time work together within a limited range so that the alloy elements in the steel can be more evenly dissolved in the steel, making the steel structure more uniform, thereby obtaining a more uniform structure in the subsequent cooling process.

[0042] Specifically, the main purpose of the deep cooling process in step (2) is to reduce the residual austenite in the material and eliminate the influence of the residual austenite on the performance during the subsequent tempering process. The deep freezer provided by the present invention can meet the requirements of deep cooling treatment of materials to eliminate residual austenite under the furnace temperature uniformity of ±8°C.

[0043] In step (2), it is necessary to ensure that the furnace temperature uniformity, furnace load, and holding time work together within the specified range to effectively reduce the retained austenite in the ultra-high strength, high toughness steel and achieve a good plasticity-toughness match. After completion, the sample needs to be completely restored to room temperature in air.

[0044] Specifically, in step (3), the strengthening and toughening mechanism of the ultra-high strength and high toughness steel provided by the present invention is mainly to precipitate fine and dispersed M2C carbides and reversely transform austenite on the lath martensite matrix with high dislocation density to achieve strengthening, and the best strength and toughness matching of the material can be achieved at 482°C. If tempered above 482°C, the precipitated M2C carbides will coarsen and gradually transform from a close-packed hexagonal structure (HCP) to an orthorhombic structure of M2C carbides, thereby reducing the performance of the material; if tempered below 482°C, the material cannot be strengthened. In order to achieve strengthening by completely precipitating needle-shaped M2C carbides, the uniformity of furnace temperature is extremely important. The furnace temperature uniformity of the material is required to be high, and the tempering furnace temperature uniformity is required to be within ±3°C. Furthermore, it is necessary to strictly control the tempering time to 370-390 minutes and the single-layer furnace loading capacity of the pit furnace to ≤8 tensile specimens + 8 impact specimens + 8 fracture toughness specimens to ensure that the strength and toughness of the material meet the requirements. A short tempering time means high strength and low toughness; a long tempering time means low strength and high toughness.

[0045] As an optional embodiment of the present invention, in step (1), the quenching furnace is a box-type furnace, and its loading capacity is ≤ 4 tensile specimens + 4 impact specimens + 4 fracture toughness specimens.

[0046] As an optional embodiment of the present invention, in step (2), the single-layer loading capacity of the deep freezer is ≤12 tensile specimens + 12 impact specimens + 12 fracture toughness specimens.

[0047] As an optional embodiment of the present invention, in step (3), the tempering furnace is a pit furnace; its single-layer furnace capacity is ≤8 tensile specimens + 8 impact specimens + 8 fracture toughness specimens.

[0048] As an optional embodiment of the present invention, in step (1), the sample is quickly removed from the furnace and transferred to the cooling oil for quenching for less than or equal to 6 seconds (e.g., 1 second, 2 seconds, 3 seconds, 4 seconds, 5 seconds, etc.);

[0049] And / or, the cooling time of the sample in the cooling oil is 20 to 60 minutes (such as 30 minutes, 40 minutes, 50 minutes, etc.), and sufficient and uninterrupted stirring is performed to promote uniformity of quenching of the material.

[0050] Specifically, once the sample is removed from the furnace, the sample's microstructure begins to transform because the room temperature is lower than the steel's austenitizing temperature. If the transfer time exceeds 6 seconds, the slow cooling rate of the sample surface can cause transformations to microstructures such as bainite, affecting subsequent material properties. Therefore, the sample's transfer time in air should be minimized, allowing the sample to be quickly transferred to the cooling oil for rapid cooling. This prevents the formation of undesirable microstructures such as bainite during air cooling, which can affect material properties.

[0051] Specifically, strict control of the cooling time can enable the austenite structure to be fully and thoroughly transformed into lath martensite, providing the prerequisite for achieving a good strength-toughness match. If the cooling time is less than 20 minutes, the lath martensite with high dislocation density generated after quenching will not be fully transformed, thereby affecting the strength during the subsequent tempering process.

[0052] When testing the mechanical properties of the quenched specimens, it is necessary to ensure that the specimens are completely immersed in oil during oil cooling. This is to ensure that the austenite is fully transformed into martensite and to improve the uniformity of the matrix structure.

[0053] As an optional embodiment of the present invention, the cooling oil is B244 vacuum quenching oil;

[0054] And / or, the cooling oil has an oil cooling rate of 60 to 90°C / s (e.g., 65°C / s, 70°C / s, 75°C / s, 80°C / s, 85°C / s, etc.).

[0055] Quenching media typically used are 25# transformer oil and B244 vacuum quenching oil. B244 vacuum quenching oil was chosen for this application because of its exceptionally high cooling rate, making it particularly suitable for quenching carbon and alloy steels. Furthermore, after quenching, it produces a lath martensite matrix with a high dislocation density, providing the prerequisite for achieving a good balance of strength and toughness.

[0056] As an optional embodiment of the present invention, the time interval between the quenching treatment and the cryogenic treatment does not exceed 1 to 2 hours (e.g., 1.2 hours, 1.4 hours, 1.6 hours, 1.8 hours, etc.);

[0057] And / or, the time interval between the cryogenic treatment and the tempering treatment is 2 to 8 hours (e.g., 3 hours, 4 hours, 5 hours, 6 hours, 7 hours, etc.).

[0058] After the quenching treatment is completed, the sample needs to be transferred for deep cryogenic treatment within 1 to 2 hours to ensure sufficient transformation of austenite to lath martensite and achieve good plasticity-toughness matching.

[0059] After deep freezing treatment, tempering treatment should be carried out at least 2 hours apart from May to October, and at least 3 hours apart from November to April to ensure that the sample is fully restored to room temperature after deep freezing.

[0060] According to a second aspect of the present invention, there is provided a steel material prepared by the above-mentioned heat treatment method for ultra-high strength and high toughness steel.

[0061] As an optional embodiment of the present invention, the steel material is 3.1Cr-11.5Ni-13.5Co-1.2Mo steel, wherein the mass percentage of C is 0.21-0.25% (such as 0.22%, 0.23%, 0.24%, etc.).

[0062] As an optional embodiment of the present invention, the coefficient of variation of the mechanical properties of the steel material includes:

[0063] The coefficient of variation of tensile strength is ≤0.18; the coefficient of variation of fracture toughness is ≤2.0.

[0064] The present invention will be further described in detail below with reference to specific examples and comparative examples.

[0065] Example 1

[0066] Step (1) Quenching treatment: Heat the heating furnace with a temperature uniformity of ±10°C to 885°C. After the temperature of the temperature controller is stabilized at 885°C, place the mechanical properties specimen (the specimen includes Standard tensile specimens, 10mm*10mm*55mm impact specimens, and fracture toughness specimens (compact or three-point bend specimens) with a B=25 rating were placed in the effective zone of the heating furnace and heated. The furnace temperature was raised to 885°C within 10 minutes, and the timer began to hold the temperature for 60 minutes. B244 vacuum quenching oil with a cooling rate of 86°C / s was used. The oil temperature was controlled at 30°C before quenching. After holding, the specimens were quickly placed in the cooling oil within 5 seconds. The cooling oil was stirred and allowed to cool in the oil for 25 minutes before cooling to room temperature.

[0067] Step (2) cryogenic treatment: cryogenic treatment is carried out 1 hour after quenching is completed, and the deep freezer with a furnace temperature uniformity of ±1°C is cooled to -73°C. After the temperature of the deep freezer temperature controller is stably controlled at -73°C, the mechanical property specimen is placed in the central effective area of ​​the deep freezer with a tray for cryogenic treatment. After the deep freezer reaches -73°C again 30 minutes after closing the cabinet door, the deep freezer is cryogenically treated for 60 minutes, the power is turned off and the specimen is taken out of the furnace and restored to room temperature in the air.

[0068] Step (3) tempering treatment: heat the pit-type tempering furnace with a furnace temperature uniformity of ±3°C to 482°C. After the temperature of the temperature controller is stably controlled at 482°C, place the mechanical property sample that has been cryogenically treated for 4 hours into the effective area of ​​the tempering furnace for heating. After 20 minutes, the instrument returns to 482°C and starts timing. Keep warm for 370 minutes.

[0069] Example 2

[0070] Step (1) Quenching treatment: Heat the heating furnace with a temperature uniformity of ±14°C to 885°C. After the temperature of the temperature controller is stabilized at 885°C, place the mechanical properties specimen (the specimen includes Standard tensile specimens, 10mm*10mm*55mm impact specimens, and fracture toughness specimens (compact or three-point bend specimens) with a B=25 rating were placed in the active zone of the heating furnace and heated. The furnace temperature was raised to 885°C within 7 minutes, and the timer began to hold the temperature for 70 minutes. B244 vacuum quenching oil with a cooling rate of 86°C / s was used. The oil temperature was controlled at 60°C before quenching. After holding, the specimens were quickly placed in the cooling oil within 6 seconds. The cooling oil was stirred and allowed to cool for 60 minutes before cooling to room temperature.

[0071] Step (2) cryogenic treatment: cryogenic treatment is carried out 2 hours after quenching is completed, and the deep freezer with a furnace temperature uniformity of ±8°C is cooled to -73°C. After the temperature of the deep freezer temperature controller is stably controlled at -73°C, the mechanical property specimen is placed in the central effective area of ​​the deep freezer with a tray for cryogenic treatment. After the deep freezer reaches -73°C again 30 minutes after closing the cabinet door, the deep freezer is cryogenically treated for 70 minutes, the power is turned off and the specimen is taken out of the furnace and restored to room temperature in the air.

[0072] Step (3) tempering treatment: heat the pit-type tempering furnace with a furnace temperature uniformity of ±2°C to 482°C. After the temperature of the temperature controller is stably controlled at 482°C, place the mechanical property sample that has been cryogenically treated for 4 hours into the effective area of ​​the tempering furnace for heating. After 20 minutes, the instrument returns to 482°C and starts timing. Keep warm for 390 minutes.

[0073] Comparative Example 1

[0074] The operating steps of this comparative example are the same as those of Example 1, except that in step (1), the sample is quickly taken out of the furnace and transferred to cooling oil for quenching for 8 seconds.

[0075] Comparative Example 2

[0076] The operating steps of this comparative example are the same as those of Example 1, except that in step (1), the cooling time of the sample in the cooling oil is 10 minutes.

[0077] Comparative Example 3

[0078] The operating steps of this comparative example are the same as those of Example 1, except that in step (1), the cooling oil used is 25# transformer oil.

[0079] Comparative Example 4

[0080] The operating steps of this comparative example are the same as those of Example 1, except that, in step (2), an alcohol cryogenic tank is used for cryogenic treatment, and the furnace load is ≤ 4 tensile specimens + 4 impact specimens + 4 fracture toughness specimens.

[0081] Comparative Example 5

[0082] The operating steps of this comparative example are the same as those of Example 1, except that, in step (3), a box-type tempering furnace is used for tempering treatment, and the furnace capacity is ≤ 4 tensile specimens + 4 impact specimens + 4 fracture toughness specimens.

[0083] Comparative Example 6

[0084] The operating steps of this comparative example are the same as those of Example 1, except that in step (1), the cooling oil used is 25# transformer oil; in step (2), an alcohol cryogenic tank is used for cryogenic treatment; and in step (3), a box-type tempering furnace is used for tempering treatment.

[0085] Comparative Example 7

[0086] The operating steps of this comparative example are the same as those of Example 1, except that in step (3), the tempering time is 350 min.

[0087] Comparative Example 8

[0088] The operating steps of this comparative example are the same as those of Example 1, except that in step (3), the furnace temperature uniformity is ±4°C.

[0089] Examples 1-2 and Comparative Examples 1-8 were tested using GB / T228 and GB / T4161 standards to obtain the yield strength, tensile strength, elongation after fracture, reduction of area, and fracture toughness of the samples. Specifically, three samples of each type were tested in each example and comparative example, and the coefficient of variation (Cv (tensile strength) and Cv (fracture toughness) values) of the mechanical properties of the samples obtained in each example and comparative example were statistically analyzed. The mechanical property data in Table 2 below are the average values ​​of the three samples.

[0090] Table 2 Mechanical properties

[0091] According to Table 2, compared with Example 1, in Comparative Example 1, since in step (1), the sample is quickly taken out of the furnace and transferred to the cooling oil for quenching for 8 seconds, the surface of the sample in Comparative Example 1 is in contact with the air to form bainite and other structures, resulting in the appearance of excess impurity structures other than lath martensite in the sample, which affects the performance of the material and thus makes the coefficient of variation of the tensile strength and fracture toughness of the sample larger.

[0092] It can be seen from Table 2 that compared with Example 1, in Comparative Example 2, since the cooling time of the sample in the cooling oil in step (1) is 10 minutes, the cooling time of the sample in Comparative Example 2 is shorter, resulting in the lath martensite with high dislocation density generated after quenching not being fully transformed, which in turn affects the strength of the material during the subsequent tempering process, resulting in a larger coefficient of variation of the tensile strength and fracture toughness of the sample.

[0093] According to Table 2, compared with Example 1, in Comparative Example 3, since the cooling oil used in step (1) is 25# transformer oil, and the cooling rate is lower than that of B244 vacuum quenching oil, the sample in Comparative Example 3 is not sufficiently cooled under the same cooling time, so that the austenite structure in the sample is not fully converted into lath martensite, which affects the material properties and makes its fracture toughness variation coefficient larger.

[0094] As can be seen from Table 2, compared with Example 1, in Comparative Example 4, since in step (2), an alcohol cryogenic tank is used for cryogenic treatment, and its furnace capacity is smaller than that of a cryogenic cabinet, the furnace capacity, furnace temperature uniformity and holding time in Comparative Example 4 do not achieve the best synergistic effect, resulting in a larger coefficient of variation of the tensile strength and fracture toughness of the material.

[0095] As can be seen from Table 2, compared with Example 1, in Comparative Example 5, since in step (3), a box-type tempering furnace is used for tempering treatment, and its furnace capacity is smaller than that of a pit-type tempering furnace, the furnace capacity, furnace temperature uniformity and holding time in Comparative Example 5 do not achieve the best synergistic effect, resulting in a larger coefficient of variation of the tensile strength and fracture toughness of the material.

[0096] It can be seen from Table 2 that compared with Example 1, according to the above description, the quenching medium, cryogenic equipment and tempering equipment selected in Comparative Example 6 are not as effective as those in Example 1, resulting in a larger coefficient of variation of the tensile strength and fracture toughness of the material.

[0097] According to Table 2, compared with Example 1, in Comparative Example 7, the tempering time is 350 min in step (3), which is short, resulting in high material strength but reduced toughness, thereby making the coefficient of variation of the tensile strength and fracture toughness of the sample larger.

[0098] According to Table 2, compared with Examples 1-2, in Comparative Example 8, since the furnace temperature uniformity in step (3) is ±4°C, the furnace temperature uniformity of Comparative Example 8 is greater, resulting in the inability to fully precipitate needle-shaped, fine, and dispersed M2C carbides on the lath martensite matrix with high dislocation density, thereby affecting the strengthening effect of the material and making the coefficient of variation of the tensile strength and fracture toughness of the sample larger.

[0099] In summary, the mechanical properties of the material obtained by using Examples 1-2 have the best match between strength and toughness. At the same time, the corresponding tensile strength and fracture toughness have the smallest coefficient of variation, the smallest fluctuation, and the strongest stability, which can meet the requirements of material production.

[0100] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A heat treatment method for a super high-strength and high-toughness steel, characterized in that, It includes the following steps: (1) Quenching treatment: Heat the heating furnace with a furnace temperature uniformity less than or equal to ±14°C to 885°C. After the temperature of the temperature controller stabilizes at 885°C, use a tray to load the mechanical property specimens into the effective area of the heating furnace for heating. Start timing after the furnace temperature rises to 885°C within 5 - 10 minutes, and keep the temperature for 60 - 70 minutes; before quenching, control the oil temperature in the cooling oil tank to be less than or equal to 60°C, turn on the agitation of the oil tank, quickly take out the mechanical property specimens from the furnace and disperse them into the oil tank for oil cooling; (2) Cryogenic treatment: Cool the cryogenic cabinet with a furnace temperature uniformity less than or equal to ±8°C to -73°C. After the temperature of the temperature controller of the cryogenic cabinet stabilizes at -73°C, use a tray to load the mechanical property specimens after quenching treatment into the central effective area of the cryogenic cabinet for cryogenic treatment. After closing the cabinet door, start timing when the instrument temperature reaches -73°C again, keep the temperature for 60 - 70 minutes, turn off the power and take out the specimens, and restore them to room temperature in the air; (3) Tempering treatment: Heat the tempering furnace with a furnace temperature uniformity less than or equal to ±3°C to 482°C. After the temperature of the temperature controller stabilizes at 482°C, load the mechanical property specimens after cryogenic treatment into the effective area of the tempering furnace for heating. Start timing after the instrument temperature returns to 482°C again, keep the temperature for 370 - 390 minutes, air cool for 2 hours to room temperature after taking out the specimens to obtain ultra-high strength and high-toughness steel.

2. The heat treatment method of the ultra-high strength and high toughness steel according to claim 1, characterized in that, In step (1), the quenching furnace is a box-type furnace, and its loading capacity ≤ 4 tensile specimens + 4 impact specimens + 4 fracture toughness specimens.

3. The heat treatment method of the ultra-high strength and high toughness steel according to claim 1, characterized in that, In step (2), the single-layer loading capacity of the cryogenic cabinet ≤ 12 tensile specimens + 12 impact specimens + 12 fracture toughness specimens.

4. The heat treatment method of the ultra-high strength and high toughness steel according to claim 1, characterized in that, In step (3), the tempering furnace is a pit-type furnace; its single-layer loading capacity ≤ 8 tensile specimens + 8 impact specimens + 8 fracture toughness specimens.

5. The heat treatment method of the ultra-high strength and high toughness steel according to claim 1, characterized in that, In step (1), the quenching time for quickly taking out the specimens from the furnace and transferring them to the cooling oil is less than or equal to 6 s; and / or, the cooling time of the specimens in the cooling oil is 20 - 60 minutes, and full and uninterrupted agitation is carried out to promote the uniformity of material quenching.

6. The heat treatment method of the ultra-high strength and high toughness steel according to claim 1, characterized in that, The cooling oil is B244 vacuum quenching oil; and / or, the oil cooling rate of the cooling oil is 60 - 90°C / s.

7. The heat treatment method of the ultra-high strength and high toughness steel according to claim 1, characterized in that, The time interval between quenching treatment and cryogenic treatment does not exceed 1 - 2 hours; and / or, the time interval between cryogenic treatment and tempering treatment is 2 - 8 hours.

8. A steel material prepared by the heat treatment method of ultra-high strength and high-toughness steel according to any one of claims 1 - 7.

9. The steel material according to claim 8, characterized in that, The steel material is 3.1Cr - 11.5Ni - 13.5Co - 1.2Mo steel, and the mass percentage of C is 0.21 - 0.25%.

10. The steel material according to claim 8, characterized in that, The coefficient of variation of the mechanical properties of the steel material includes: Coefficient of variation of tensile strength ≤ 0.18; coefficient of variation of fracture toughness ≤ 2.0.

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