A heat treatment method for improving the strength and toughness of an ultra-high strength maraging stainless steel

Abstract

The application discloses a heat treatment method for improving the strength and toughness of ultrahigh-strength martensite aging stainless steel, and belongs to the technical field of metal heat treatment.The method is characterized in that, after homogenizing forging of the martensite aging hardening stainless steel, a pre-aging process and a deep cooling process are added between a solid solution treatment process and a long-time aging treatment process, and a deep cooling treatment is additionally added after the long-time aging treatment process.The method specifically comprises the following steps: (1) solid solution treatment; (2) first deep cooling treatment; (3) pre-aging treatment; (4) second deep cooling treatment; (5) long-time aging treatment; and (6) third deep cooling treatment.The heat treatment method of double-aging process+three deep cooling processes solves the problem of the compromise between the precipitation degree of precipitated phases and the content and morphology of austenite under the existing heat treatment process technology, and finally, the martensite stainless steel with ultrahigh strength and ultrahigh toughness is obtained.

Description

Technical Field

[0001] This invention belongs to the field of metal heat treatment technology, and particularly relates to a heat treatment process for ultra-high strength martensitic aging stainless steel. Background Technology

[0002] Precipitation-hardening stainless steel, as a high-strength stainless steel, possesses high strength and excellent corrosion resistance, making it an important candidate material for many applications such as aerospace, nuclear industry, shipbuilding, military machinery, and chemical industry. During solution treatment, precipitation-hardening stainless steel forms a supersaturated matrix microstructure. This is followed by aging treatment to produce precipitates. Depending on the steel grade, these precipitated strengthening phases include Cu-rich phases, NiAl phases, Laves phases, Ni3Ti phases, and carbides. To obtain better strength and toughness, the size, shape, and quantity of these precipitates should be adjusted to improve the overall mechanical properties of the steel.

[0003] Martensitic stainless steels possess ultra-high strength, good ductility, and corrosion resistance, making them crucial in the automotive, aerospace, nuclear, gear, bearing, and other industries. They are key materials for future lightweight engineering designs and corresponding CO2 emission reduction strategies. Various alloying elements, such as C, Cr, Ni, Al, Ti, Mo, V, Mn, Nb, Co, Cu, W, Si, B, and N, are combined in such alloy systems to achieve desired microstructures and properties. To date, experimental research on ultra-high strength steels has been largely limited to exploring ways to improve strength, with few reports of breakthroughs in the combined optimization of strength and toughness. Furthermore, a fundamental understanding of certain key microstructural features in steel and the stability of reverse-transformed austenite has not yet been achieved.

[0004] From a materials science perspective, achieving excellent toughness while maintaining relatively high strength is of great significance in ultra-high strength steel. Therefore, understanding the influence of the volume fraction, carbon content, distribution, morphology, and stability of reverse-transformed austenite on mechanical properties such as impact fatigue life, dynamic fracture toughness, and ductility is crucial. However, current heat treatment methods alone cannot simultaneously improve the strength (especially yield strength) and notched impact energy of ultra-high strength stainless steel. Summary of the Invention

[0005] In order to overcome the problems existing in the prior art, the present invention provides a heat treatment process for strengthening and toughening ultra-high strength martensitic aging stainless steel.

[0006] The specific technical solution is as follows:

[0007] An ultra-high strength martensitic aging stainless steel, characterized in that the chemical composition of the martensitic aging stainless steel by mass percentage is: C: 0.1-0.3%, Cr: 11-14%, Ni: 5.0-8.0%, Mo: 1.0-4.0%, Co: 6.5-8.5%, V: 0.2-0.45%, Nb: 0.012-0.04%, Cu: 0-2%, Al: 0-1.5%, P: ≤0.02%, S: ≤0.02%, with the balance being Fe.

[0008] The ultra-high strength martensitic stainless steel is smelted according to the composition of the present invention, then homogenized at 1050-1250℃ for 8-15h to eliminate segregation, and then hot rolled and forged to obtain a slab.

[0009] The heat treatment process for the steel plate after the above forging treatment includes the following steps:

[0010] (1) Solution treatment;

[0011] (2) First cryogenic treatment;

[0012] (3) Pre-aging treatment;

[0013] (4) Second cryogenic treatment;

[0014] (5) Long-term treatment;

[0015] (6) Third cryogenic treatment;

[0016] The solution treatment temperature is 1025-1175℃, the solution treatment holding time is 0.5-1.5h, and the cooling method is water cooling to room temperature to obtain a martensitic structure of the whole lath.

[0017] The first cryogenic treatment involves holding the liquid nitrogen at -196°C for 1-8 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0018] The pre-aging treatment temperature is 510-610℃, the pre-aging treatment holding time is 20min-4h, and the pre-aging cooling method is oil cooling to room temperature;

[0019] The second cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 1-8 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0020] The long-term aging treatment temperature is 450-500℃, the long-term aging treatment holding time is 8-40h, and the long-term aging cooling method is oil cooling to room temperature;

[0021] The third cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 1-8 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0022] The specific concept behind the above solution is to employ a double-aging process combined with a triple cryogenic heat treatment method, which solves the problem of balancing the degree of precipitate precipitation with the content and morphology of austenite under existing heat treatment technologies. This ultimately yields martensitic stainless steel with both ultra-high strength and ultra-high toughness. Strengthening through both cutting and bypassing mechanisms gives the steel excellent comprehensive properties to meet the service requirements of ultra-high strength stainless steel in harsh operating environments.

[0023] After solution treatment, the alloying elements can be completely dissolved into the matrix. Solid solution strengthening is expressed as: Where τ ss It is the strength contributed by solid solution strengthening, k ss,i It is the strengthening coefficient of element i in martensite, c i It represents the atomic fraction of element i in martensite. Strengthening elements i in martensite include C, Co, Ni, Cu, Al, Cr, and Mo.

[0024] After high-temperature pre-aging treatment, the high temperature promotes the diffusion of interstitial C atoms to the martensite lath boundaries, increasing the content of thin-film reverse-transformed austenite. Furthermore, the increased pre-aging temperature enriches Co and Ni atoms at the martensite lath boundaries, improving the stability of the film austenite and enhancing toughness. In addition, the high temperature promotes significant precipitation of precipitates (mainly M2C carbides). Fine, dispersed M2C carbides are considered hard particles, and through bypass strengthening, they can effectively improve the yield strength of the steel. The yield strength contributed by the Orowan mechanism is: The calculation is performed, where M is the Taylor coefficient of 3, b is the Burgers vector of 0.25 nm, G is the shear modulus of 80 GPa, f is the volume fraction of M2C type carbide, R is the average radius of the carbide, and π is pi.

[0025] Multiple deep cooling processes can maintain the austenite content in steel at a low level. Besides rationally designing the alloy composition of ultra-high strength steel and strictly controlling its content, the yield strength and fracture toughness of steel can be significantly extended by optimizing the aging process without a significant reduction in strength. One of the most important reasons is that the toughness and stability of the reverse-transformed austenite are significantly improved during the high-temperature holding process of the double-aging process. Furthermore, during the low-temperature holding process of the double-aging process, carbon separated from the martensite / ferrite in the steel will definitely diffuse into the reverse-transformed austenite. S As the temperature decreases, the barriers to approach and bonding between carbon and iron in austenite will change, resulting in reduced brittleness of martensite and increased stability of austenite.

[0026] The heat treatment method for improving the strength and toughness of ultra-high strength martensitic aging stainless steel according to the present invention has the following advantages compared with the prior art:

[0027] This invention improves the diffusion coefficient of each element in steel through pre-aging treatment, which can effectively promote the diffusion of each element in the steel, resulting in the precipitation of more fine and dispersed precipitates (ε-Cu, NiAl, M2C). In addition, the high-temperature pre-aging treatment provides favorable thermodynamic and kinetic conditions for the formation of reverse-transformed austenite.

[0028] Through multiple cryogenic treatments, the retained austenite in the steel is kept at a low level. Subsequent aging treatment causes the retained austenite to first decompose and then precipitate and transform from the matrix at the aging temperature, ultimately forming a thin film of reverse-transformed austenite. With the diffusion of various elements in the steel, the alloy content of the reverse-transformed austenite increases, thereby increasing the stability of the reverse-transformed austenite.

[0029] The precipitates in the steel have a finer and more dispersed distribution, and the austenite content is suitable for improved stability, thus giving ultra-high strength stainless steel its ultra-high strength and toughness.

[0030] The heat treatment process for ultra-high strength martensitic stainless steel of this invention is simple, has low manufacturing cost, and is easy to industrialize. The final product is an ultra-high strength martensitic stainless steel with excellent mechanical properties, including a tensile strength ≥2000MPa, a yield strength ≥1600MPa, a hardness ≥53HRC, and a notched impact energy ≥25J. Attached Figure Description

[0031] Figure 1 The heat treatment process is as described in Example 1;

[0032] Figure 2 The image shows the phase structure of EBSD after heat treatment in Example 1;

[0033] Figure 3 The image shows the metallographic microstructure of Example 1 after heat treatment;

[0034] Figure 4 The image shows the metallographic microstructure of Example 2 after heat treatment;

[0035] Figure 5 The stress-strain curve of Example 1 after heat treatment;

[0036] Figure 6 The stress-strain curve of Example 2 after heat treatment;

[0037] Figure 7 The stress-strain curves of Comparative Example 1 after heat treatment are shown.

[0038] Figure 8 The stress-strain curves of Comparative Example 2 after heat treatment are shown.

[0039] Figure 9 Here are SEM images of the notched impact fracture morphology of Example 1 after heat treatment;

[0040] Figure 10 Here are SEM images of the notched impact fracture morphology of Example 2 after heat treatment;

[0041] Figure 11 Here are SEM images of the notched impact fracture morphology of Comparative Example 1 after heat treatment.

[0042] Figure 12 The image shows the SEM image of the notched impact fracture morphology of Comparative Example 2 after heat treatment. Detailed Implementation

[0043] The present invention will be further described in detail below with reference to embodiments:

[0044] A mixture of 0.21C-13.2Cr-5.9Ni-2.5Mo-7.5Co-0.42V-0.012Nb-0.9Cu-0.52Al by mass percentage was melted in a 30kg vacuum induction furnace to form an ingot with a diameter of 100mm × the balance. After homogenization at 1250℃ for 15 hours, the ingot was forged into a slab measuring 25mm × 50mm × the balance. The following are metallographic micrographs, stress-strain curves, and SEM images of the notched impact fracture morphology after heat treatment in Examples 1-6 and Comparative Examples 1-2, as shown below. Figures 3-12 As shown.

[0045] Example 1

[0046] like Figure 1 As shown, a heat treatment process for ultra-high strength martensitic aging stainless steel includes the following specific steps:

[0047] The steel plates subjected to the above forging process shall undergo the following heat treatment operations:

[0048] (1) Solution treatment;

[0049] The solution treatment temperature is 1080℃, and the solution treatment holding time is 0.6h;

[0050] The cooling method is water cooling to room temperature to obtain a martensitic structure of all laths;

[0051] (2) First cryogenic treatment;

[0052] The first cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 6 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0053] (3) Pre-aging treatment;

[0054] The pre-aging treatment temperature is 575℃;

[0055] The pre-aging treatment and heat preservation time is 0.5 hours;

[0056] The pre-aging cooling method is oil cooling to room temperature.

[0057] (4) Second cryogenic treatment;

[0058] The second cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 4 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0059] (5) Long-term treatment;

[0060] The long-term aging treatment temperature is 485℃;

[0061] The long-term aging treatment and heat preservation time is 15 hours;

[0062] The long-term cooling method is oil cooling to room temperature;

[0063] (6) Third cryogenic treatment;

[0064] The third cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 4 hours, then removing it and allowing it to stand in the air until it reaches room temperature. Figure 2 The EBSD phase diagram of the steel microstructure shown in Example 1 is illustrated, where the white area represents martensite and the black area represents austenite. After two aging cycles and three deep cryogenic treatments, the austenite content in the microstructure is 2.23%, uniformly distributed between the martensite laths. Under mechanical tension or impact, the austenite layer alters the crack propagation direction, reduces the crack propagation rate, and increases crack propagation energy consumption, effectively improving its strength and toughness. The yield strength of this material also shows a certain improvement under the double aging and three deep cryogenic heat treatment process. Figure 9 The image shown is a SEM image of the notched impact fracture morphology of Example 1 after heat treatment. It can be seen that some dimples are large and deep, forming obvious pits and the tear ridges are clearly visible.

[0065] In Example 1, the mechanical properties of the steel were tested at room temperature using a Shimadzu AG-X100kN universal testing machine for tensile testing. According to GB / T228-2002 and GB / 4338-84, the tensile specimens were rod-shaped with a parallel section diameter of 5mm, a parallel section length of 30mm, and a gauge length of 25mm, with threads attached to the clamping end. The tensile rate was 1mm / min. Impact performance testing was conducted using a PSW-750 impact testing machine. According to GB / T229-2007, the impact specimens were standard V-notch impact specimens (2mm depth) with dimensions of 55mm × 10mm × 10mm. The microhardness of the experimental steel was measured using a Rockwell hardness tester RDS-150D with an indenter load of 1470kN and a holding time of 5s. The final tensile strength is 2015.58 MPa, the yield strength is 1621.96 MPa, the notched impact energy is 29.7 J, the elongation is 11.34%, the cross-sectional expansion rate is 49.47%, and the hardness is 53.9 HRC.

[0066] Example 2

[0067] A heat treatment process for ultra-high strength martensitic aging stainless steel, the specific operation steps of which are as follows:

[0068] The steel plates subjected to the above forging process shall undergo the following heat treatment operations:

[0069] (1) Solution treatment;

[0070] The solution treatment temperature is 1100℃, and the solution treatment holding time is 0.5h;

[0071] The cooling method is water cooling to room temperature to obtain a martensitic structure of all laths;

[0072] (2) First cryogenic treatment;

[0073] The first cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 5 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0074] (3) Pre-aging treatment;

[0075] The pre-aging treatment temperature is 560℃;

[0076] The pre-aging treatment and heat preservation time is 1 hour;

[0077] The pre-aging cooling method is oil cooling to room temperature.

[0078] (4) Second cryogenic treatment;

[0079] The second cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 6 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0080] (5) Long-term treatment;

[0081] The long-term aging treatment temperature is 490℃;

[0082] The long-term aging treatment and heat preservation time is 18 hours;

[0083] The long-term cooling method is oil cooling to room temperature;

[0084] (6) Third cryogenic treatment;

[0085] The third cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 2 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0086] The final tensile strength of steel in Example 2 is 2008.04 MPa, the yield strength is 1678 MPa, the notched impact energy is 27.1 J, the elongation is 12.26%, the section shrinkage rate is 48.93%, and the hardness is 53.1 HRC.

[0087] Example 3

[0088] A heat treatment process for ultra-high strength martensitic aging stainless steel, the specific operation steps of which are as follows:

[0089] The steel plates subjected to the above forging process shall undergo the following heat treatment operations:

[0090] (1) Solution treatment;

[0091] The solution treatment temperature is 1050℃, and the solution treatment holding time is 55min;

[0092] The cooling method is water cooling to room temperature to obtain a martensitic structure of all laths;

[0093] (2) First cryogenic treatment;

[0094] The first cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 6 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0095] (3) Pre-aging treatment;

[0096] The pre-aging treatment temperature is 550℃;

[0097] The pre-aging treatment and heat preservation time is 1.5 hours;

[0098] The pre-aging cooling method is oil cooling to room temperature.

[0099] (4) Second cryogenic treatment;

[0100] The second cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 4.5 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0101] (5) Long-term treatment;

[0102] The long-term aging treatment temperature is 470℃;

[0103] The long-term aging treatment and heat preservation time is 20 hours;

[0104] The long-term cooling method is oil cooling to room temperature;

[0105] (6) Third cryogenic treatment;

[0106] The third cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 5 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0107] The final tensile strength of steel in Example 3 is 2010.06 MPa, the yield strength is 1655.71 MPa, the notched impact energy is 28.3 J, the elongation is 12.31%, the section shrinkage rate is 49.53%, and the hardness is 53.8 HRC.

[0108] Example 4

[0109] A heat treatment process for ultra-high strength martensitic aging stainless steel, the specific operation steps of which are as follows:

[0110] The steel plates subjected to the above forging process shall undergo the following heat treatment operations:

[0111] (1) Solution treatment;

[0112] The solution treatment temperature is 1090℃, and the solution treatment holding time is 0.6h;

[0113] The cooling method is water cooling to room temperature to obtain a martensitic structure of all laths;

[0114] (2) First cryogenic treatment;

[0115] The first cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 6 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0116] (3) Pre-aging treatment;

[0117] The pre-aging treatment temperature is 595℃;

[0118] The pre-aging treatment and heat preservation time is 25 minutes;

[0119] The pre-aging cooling method is oil cooling to room temperature.

[0120] (4) Second cryogenic treatment;

[0121] The second cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 8 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0122] (5) Long-term treatment;

[0123] The long-term aging treatment temperature is 480℃;

[0124] The long-term aging treatment and heat preservation time is 16 hours;

[0125] The long-term cooling method is oil cooling to room temperature;

[0126] (6) Third cryogenic treatment;

[0127] The third cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 5 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0128] The final tensile strength of steel in Example 4 is 2012.52 MPa, the yield strength is 1623.63 MPa, the notched impact energy is 29.3 J, the elongation is 12.52%, the section shrinkage rate is 49.23%, and the hardness is 53.6 HRC.

[0129] Example 5

[0130] A heat treatment process for ultra-high strength martensitic aging stainless steel, the specific operation steps of which are as follows:

[0131] The steel plates subjected to the above forging process shall undergo the following heat treatment operations:

[0132] (1) Solution treatment;

[0133] The solution treatment temperature is 1100℃, and the solution treatment holding time is 0.5h;

[0134] The cooling method is water cooling to room temperature to obtain a martensitic structure of all laths;

[0135] (2) First cryogenic treatment;

[0136] The first cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 2 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0137] (3) Pre-aging treatment;

[0138] The pre-aging treatment temperature is 530℃;

[0139] The pre-aging treatment and heat preservation time is 3 hours;

[0140] The pre-aging cooling method is oil cooling to room temperature.

[0141] (4) Second cryogenic treatment;

[0142] The second cryogenic treatment involves holding the liquid nitrogen at -196°C for 3 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0143] (5) Long-term treatment;

[0144] The long-term aging treatment temperature is 475℃;

[0145] The long-term aging treatment and heat preservation time is 16 hours;

[0146] The long-term cooling method is oil cooling to room temperature;

[0147] (6) Third cryogenic treatment;

[0148] The third cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 2 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0149] The final tensile strength of steel in this embodiment 5 is 2004.01 MPa, the yield strength is 1635.22 MPa, the notched impact energy is 28.2 J, the elongation is 12.13%, the section shrinkage rate is 48.55%, and the hardness is 53.2 HRC.

[0150] Example 6

[0151] A heat treatment process for ultra-high strength martensitic aging stainless steel, the specific operation steps of which are as follows:

[0152] The steel plates subjected to the above forging process shall undergo the following heat treatment operations:

[0153] (1) Solution treatment;

[0154] The solution treatment temperature is 1060℃, and the solution treatment holding time is 40min;

[0155] The cooling method is water cooling to room temperature to obtain a martensitic structure of all laths;

[0156] (2) First cryogenic treatment;

[0157] The first cryogenic treatment was carried out by holding the liquid nitrogen at -196°C for 2.5 hours, and then allowing it to stand in the air until it reached room temperature.

[0158] (3) Pre-aging treatment;

[0159] The pre-aging treatment temperature is 550℃;

[0160] The pre-aging treatment and heat preservation time is 1.5 hours;

[0161] The pre-aging cooling method is oil cooling to room temperature.

[0162] (4) Second cryogenic treatment;

[0163] The second cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 3.5 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0164] (5) Long-term treatment;

[0165] The long-term aging treatment temperature is 500℃;

[0166] The long-term aging treatment and heat preservation time is 12 hours;

[0167] The long-term cooling method is oil cooling to room temperature;

[0168] (6) Third cryogenic treatment;

[0169] The third cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 3.5 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0170] The final tensile strength of steel in this embodiment is 2011.24 MPa, the yield strength is 1681.4 MPa, the notched impact energy is 28.8 J, the elongation is 12.28%, the section shrinkage rate is 48.96%, and the hardness is 53.9 HRC.

[0171] Comparative Example 1

[0172] A heat treatment process for ultra-high strength martensitic aging stainless steel, the specific operation steps of which are as follows:

[0173] The steel plates subjected to the above forging process shall undergo the following heat treatment operations:

[0174] (1) Solution treatment;

[0175] The solution treatment temperature is 1080℃, and the solution treatment holding time is 0.6h;

[0176] The cooling method is water cooling to room temperature to obtain a martensitic structure of all laths;

[0177] (2) First cryogenic treatment;

[0178] The first cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 6 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0179] (3) Long-term treatment;

[0180] The long-term aging treatment temperature is 482℃;

[0181] The long-term aging treatment and heat preservation time is 22 hours;

[0182] The long-term cooling method is oil cooling to room temperature.

[0183] The final tensile strength of the steel in Comparative Example 1 was 1868.08 MPa, the yield strength was 1143.48 MPa, the notched impact energy was 22.5 MPa, the elongation was 12%, the section shrinkage was 47.37%, and the hardness was 51.90 HRC.

[0184] Comparative Example 2

[0185] A heat treatment process for ultra-high strength martensitic aging stainless steel, the specific operation steps of which are as follows:

[0186] The steel plates subjected to the above forging process shall undergo the following heat treatment operations:

[0187] (1) Solution treatment;

[0188] The solution treatment temperature is 1100℃, and the solution treatment holding time is 0.5h;

[0189] The cooling method is water cooling to room temperature to obtain a martensitic structure of all laths;

[0190] (2) First cryogenic treatment;

[0191] The first cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 5 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0192] (3) Long-term treatment;

[0193] The long-term aging treatment temperature is 490℃;

[0194] The long-term aging treatment and heat preservation time is 18 hours;

[0195] The long-term cooling method is oil cooling to room temperature;

[0196] (4) Second cryogenic treatment;

[0197] The third cryogenic treatment involves holding the sample in liquid nitrogen at -196°C for 2 hours, then removing it and allowing it to stand in the air until it reaches room temperature.

[0198] The final tensile strength of the steel in Comparative Example 2 was 1980.47 MPa, the yield strength was 1258.75 MPa, the notched impact energy was 17 J, the elongation was 13.45%, the section shrinkage was 45.91%, and the hardness was 53.43 HRC.

[0199] Table 1 shows the mechanical properties of Examples 1-6 after the improved heat treatment process and the mechanical properties of Comparative Examples 1-2 after the conventional heat treatment process. It can be seen that the yield strength is significantly improved, and the tensile strength is also somewhat improved. Furthermore, the notched impact energy is also significantly improved. This indicates that the ultra-high strength stainless steel achieves a certain improvement in both strength and toughness simply by adjusting the heat treatment process.

[0200] Table 1 Mechanical properties of Examples 1-6 and Comparative Examples 1-2

[0201]

[0202] The above are merely preferred embodiments of the present invention. Any modifications or substitutions made within the principles of the present invention are within the protection scope of the present invention.

Claims

1. A heat treatment method for improving the strength and toughness of ultra-high strength martensitic aging stainless steel, characterized in that, The chemical composition of the martensitic aging stainless steel, by mass percentage, is: C: 0.1–0.3%, Cr: 11–14%, Ni: 5.0–8.0%, Mo: 1.0–4.0%, Co: 6.5–8.5%, V: 0.2–0.45%, Nb: 0.012–0.04%, Cu: 0–2%, Al: 0–1.5%, P: ≤0.02%, S: ≤0.02%, with the balance being Fe. The process includes the following steps: (1) Solution treatment; (2) First cryogenic treatment; The first cryogenic treatment is to keep the liquid nitrogen at -196°C for 1-8 hours, and then let it stand in the air until it reaches room temperature. (3) Pre-aging treatment; the pre-aging treatment temperature is 510-610℃, the pre-aging treatment holding time is 20 min-4h, and the pre-aging cooling method is oil cooling to room temperature; (4) Second cryogenic treatment; The second cryogenic treatment is carried out by keeping the liquid nitrogen at -196°C for 1-8 hours, and then letting it stand in the air until it reaches room temperature. (5) Long-term aging treatment; the long-term aging treatment temperature is 450-500℃, the long-term aging treatment holding time is 8-40 h, and the long-term aging cooling method is oil cooling to room temperature; (6) Third cryogenic treatment; the third cryogenic treatment is to keep the liquid nitrogen at -196°C for 1-8 hours, and then let it stand in the air until the room temperature.

2. The heat treatment method for improving the strength and toughness of ultra-high strength martensitic aging stainless steel according to claim 1, characterized in that, The solution treatment temperature is 1025-1175℃, the solution treatment holding time is 0.5-1.5 h, and the cooling method is water cooling to room temperature to obtain a martensitic structure of the whole lath.

3. The heat treatment method for improving the strength and toughness of ultra-high strength martensitic aging stainless steel according to claim 1, characterized in that, The final steel has a tensile strength ≥2000 MPa, a yield strength ≥1600 MPa, a hardness ≥53 HRC, and a notched impact energy ≥25 J.

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