A W-containing ultra-high strength steel and its preparation method, and surface carbonitriding composite strengthening process

Abstract

This application relates to the technical field of surface chemical heat treatment of metallic materials, specifically to a W-containing ultra-high strength steel and its preparation method, and a surface carbonitriding composite strengthening process. The preparation method is as follows: steel smelting, forging and billet forming, post-forging annealing heat treatment of the bar, carburizing, cryogenic treatment, and surface pressure nitriding. The ultra-high strength steel comprises the following chemical composition: C 0.27-0.34 wt.%; Cr 0.4-1.4 wt.%; Ni 9-10 wt.%; Co 6-8 wt.%; Mo 1.5-2.5 wt.%; W 1.1-1.6 wt.%; the remainder being Fe and impurity elements. The maximum hardness of the carbonitriding layer of the W-containing ultra-high strength steel of this application is not less than 900 HV, the tensile strength of the steel is not less than 2000 MPa, the yield strength is not less than 1700 MPa, and the impact absorption capacity is not less than 60 J, meeting the service condition requirements for gears used in aero-engines.

Description

Technical Field

[0001] This application relates to the technical field of chemical heat treatment of metallic materials, and in particular to a W-containing ultra-high strength steel and its preparation method, and a surface carbonitriding composite strengthening process. Background Technology

[0002] Aviation propulsion systems generate power output, and the aviation power transmission system, through mechanical torque distribution devices, effectively transmits power to various related components. As a core component of the aviation power transmission system, aviation gears play a crucial role in ensuring the stability of power transmission under complex operating conditions such as specific dynamic loads, high-temperature environments, high-speed operation, and heavy loads. Therefore, the reliability of aviation gear components is a key factor in ensuring aircraft operational efficiency and flight safety. Consequently, aviation gears are typically made of medium-to-low carbon alloy steel, requiring high strength, heat resistance, and high surface hardness. Therefore, surface modification methods such as carburizing or surface nitriding are often used to strengthen the tooth surface.

[0003] Surface chemical heat treatment of steel is a heat treatment process in which the workpiece is placed in a certain active medium and heated to allow the atoms of the active substance to diffuse into the surface layer of the workpiece, thereby changing its surface chemical composition. Based on the type of diffusion element, it can be divided into diffusion of non-metals and diffusion of metals. Among them, carbonitriding technology is a non-metallic surface diffusion diffusion process. It involves placing the workpiece in a medium that can generate active carbon and nitrogen atoms, and through heating and holding at that temperature, the workpiece surface absorbs carbon and nitrogen atoms and diffuses them inward to form carbonitrides, thereby achieving surface hardening and improving the fatigue performance of the material.

[0004] Surface carbonitriding processes can be classified according to process temperature as follows: high-temperature low-pressure vacuum carburizing (900-1100℃), which is dominated by carbon atom diffusion due to the high temperature; and low-temperature gas vacuum nitriding (450-550℃), which is dominated by nitrogen atom diffusion, also known as "nitrogen softening".

[0005] Currently, the steels used for carbonitriding treatment both domestically and internationally are mainly 42CrMo, GCr15, AerMet100, and CSS-42L.

[0006] 42CrMo steel is an alloy structural steel with high strength and toughness, good hardenability, no obvious temper brittleness, and minimal deformation during quenching. The chemical composition (mass fraction) of the steel is as follows: C 0.38~0.45%, Cr 0.9~1.2%, Mn 0.5~0.8%, Si 0.17~0.37%, Mo 0.15~0.25%, P≤0.035%, Ni≤0.3%, S≤0.035%, Cu≤0.3%. After carbonitriding, the surface hardness can reach over 850 HV, the tensile strength is 1120 MPa, the yield strength is 930 MPa, the elongation after fracture is 12%, the reduction of area is 45%, and the impact energy is 63 J. This steel is mainly used in engine cylinders.

[0007] GCr15 steel is a bearing steel with good hardenability. Various alloying elements are added to improve its corrosion resistance and mechanical properties. The chemical composition (mass fraction) of the steel is as follows: C 0.93%, Cr 1.87%, Mo 0.05%, Mn 0.35%, Si 0.25%. After carbonitriding, the surface hardness can reach 885HV, the tensile strength is 2000MPa, the yield strength is 1800MPa, and the impact absorption energy is 16J. This steel is mainly used in various bearing rings; its relatively low toughness limits its application in gears.

[0008] AerMet100 steel is an ultra-high strength, secondary hardening steel with the following chemical composition (mass fraction): C 0.23%, Co 13.4%, Ni 11.1%, Cr 3.1%, Mo 1.2%. After carbonitriding, its surface hardness can reach approximately 850 HV, tensile strength 1965 MPa, yield strength 1724 MPa, and impact energy 56 J. This steel can be used for gear applications, but its operating temperature is relatively low, and because it is a high-Co-Ni steel, the high cost of Co and Ni limits its manufacturing cost and thus its use in certain applications.

[0009] CSS-42L steel is a bearing steel with the following chemical composition (mass fraction): C 0.1~0.25%, Cr 13~19%, Co 5~14%, Mo 3~5%, Ni 1.75~5.25%, V 0.25~1.25%, Nb 0.01~0.1%. After carburizing, the surface hardness can reach over 850 HV, the tensile strength is 1764 MPa, the yield strength is 1200 MPa, the elongation is 21%, and the reduction of area is 60%. This steel is mainly used in aero-engine bearings. However, due to the formation of a large number of network carbides on the surface after carburizing, the bearing steel's resistance to stress corrosion cracking is increased, limiting its application in gears.

[0010] To date, surface carbonitriding technology has not been widely adopted in gear applications. This is because the material properties are not suitable for gear applications. For example, carbonitriding of materials such as 42CrMo, GCr15, and CSS-42L reduces the tensile strength of the material itself. In particular, GCr15 and CSS-42L have low impact absorption capacity and cannot withstand severe alternating and impact loads, increasing the risk of gear fracture and tooth surface wear, thus failing to meet gear requirements. Although AerMet100 has been used in gears, its high Co-Ni content, with expensive Co and Ni elements, results in high manufacturing costs. Summary of the Invention

[0011] This application provides a W-containing ultra-high strength steel, its preparation method, and a surface carbonitridium composite strengthening process.

[0012] The ultra-high strength steel of this application has ultra-high strength, high toughness and good heat resistance. Compared with other bearing steels and gear steels, the ultra-high strength steel of this application has a low Co content and superior mechanical properties. In addition, it has good resistance to SCC cracking and low cost, so it has broad application prospects.

[0013] The ultra-high strength steel of this application, after low-pressure vacuum carburizing treatment, has a maximum hardness of 700-780 HV in the carburized layer, a tensile strength of not less than 2000 MPa, a yield strength of not less than 1700 MPa, and an impact absorption energy of not less than 60 J.

[0014] The ultra-high strength steel of this application, after surface carbonitriding treatment, has a maximum hardness of not less than 900 HV in the carbonitriding layer, a tensile strength of not less than 2000 MPa, a yield strength of not less than 1700 MPa, and an impact absorption capacity of not less than 60 J, which meets the service condition requirements of gears for aero-engines.

[0015] Furthermore, under high loads, the working tooth surface temperature of gears increases significantly, which requires gear steel to have excellent high-temperature thermal stability. Therefore, the W element in the matrix can significantly improve the high-temperature thermal stability of gears in high-temperature environments, so as to ensure that gears still exhibit good high-temperature red hardness in oil-free dry operation.

[0016] In a first aspect, this application provides a method for preparing ultra-high strength steel, employing the following technical solution:

[0017] A method for preparing ultra-high strength steel specifically includes the following steps:

[0018] (1) Steel smelting and preparation

[0019] The smelting process employs a combination of vacuum induction and vacuum self-consumption methods.

[0020] Vacuum induction smelting uses industrial pure iron as raw material and is produced using an electric furnace + LF + VD process. By rationally controlling the electromagnetic stirring time in the vacuum induction furnace, the flow of molten steel is increased, non-metallic inclusions are aggregated, floated and removed, and the gas content is further reduced, thereby improving the purity of the molten steel.

[0021] The smelting temperature is 1500-1530℃, the refining temperature is 1520-1560℃, and the casting temperature is 1550-1570℃.

[0022] Vacuum arc remelting mainly ensures the production of high-quality steel ingots by controlling process parameters such as melting vacuum degree and melting speed. The melting vacuum degree is ≤0.5Pa, the melting speed is 3.0-7.0Kg / min, the melting current is 5000-9500A, the ingot is pit-cooled for ≥60h after demolding, the annealing temperature is ≤650℃, and the holding time is ≥20h.

[0023] (2) Forging and billet forming

[0024] The steel ingots undergo high-temperature homogenization treatment, with the furnace heating temperature ≥1210℃ and holding time ≥40h. Forging is then performed, with the initial forging temperature ≥1000℃, using multiple upsetting and drawing processes, and a final forging temperature ≥850℃. At least three upsetting and drawing processes are used, with the intermediate billets reheated at 1050-1160℃ for 1-3h. The finished bars are then finished with a final heat treatment to avoid reheating.

[0025] (3) Post-forging annealing heat treatment of bar stock

[0026] The bars are annealed after forging. The annealing temperature is 600-700℃, and the holding time is 20-40 hours.

[0027] (4) Carburizing: Low-pressure carburizing + high-pressure gas quenching

[0028] The low-pressure carburizing heating temperature is 950-1100℃, the acetylene flow rate during the strong carburizing period is 80-100L / h, the acetylene flow rate during the diffusion period is 50-65L / h, the carbon potential is 0.8-1.5%, and the target carburized layer depth is 0.8-1.5mm.

[0029] High-pressure gas quenching is performed using helium for cooling. The quenching pressure is 10-20 bar, and the quenching time is 10-20 min.

[0030] (5) Cryogenic treatment

[0031] The cryogenic temperature is -180℃ to -70℃, the holding time is 1-2 hours, and it is air-cooled.

[0032] After vacuum low-pressure carburizing, high-pressure gas quenching and deep cryogenic treatment, a certain carbon concentration gradient is formed on the sample surface, with an effective carbonized layer of 0.8-1.5 mm, a surface hardness of 750-780 HV, and a diffusion layer hardness of 650-750 HV, thus simultaneously achieving toughening of the surface and the core.

[0033] (6) Surface nitriding: pressurized gas nitriding

[0034] Surface nitriding is performed on the surface after carburizing to further improve surface hardness and the depth of the carburized layer, thereby enhancing the surface wear resistance of the test specimen. The heating temperature for pressurized gas nitriding is 500-550℃, the nitriding pressure is 0.3-0.5MPa, and the ammonia decomposition rate is 20-30%. The holding time is 10-12 hours.

[0035] This application combines the final tempering treatment and surface nitriding treatment of the test steel into one process. By using pressurized gas nitriding treatment, the matrix strength of the test steel can be improved and good mechanical properties can be obtained. At the same time, an excellent surface nitriding layer can be obtained to form a carbonitriding layer.

[0036] The heating temperature for pressurized gas nitriding is 500-550℃, consistent with the tempering temperature required to achieve high strength and toughness in the test steel. The nitriding pressure is 0.1-0.5MPa, and the nitriding medium used is ammonia gas with a decomposition rate of 10-30%. First, the furnace is evacuated, then ammonia gas is introduced to pressurize the furnace and raise the temperature to the set value. Ammonia gas is then continuously introduced to maintain the furnace pressure for a period of time, with a holding time of 8-12 hours.

[0037] After carbonitriding treatment, the surface of the test steel developed an ultra-high hardness of over 900 HV, which is higher than that of the untreated and carburized samples. The ability to resist plastic deformation was significantly enhanced, and the wear resistance was greatly improved.

[0038] Secondly, this application provides an ultra-high strength steel prepared using the above-mentioned method, employing the following technical solution:

[0039] An ultra-high strength steel, by mass percentage, comprises the following chemical composition: C 0.27-0.34 wt.%; Cr 0.4-1.4 wt.%; Ni 9-10 wt.%; Co 6-8 wt.%; Mo 1.5-2.5 wt.%; W 1.1-1.6 wt.%; V 0.05-0.15 wt.%; Nb 0.02-0.04 wt.%; the remainder being Fe and impurity elements.

[0040] Optionally, the content of the impurity elements is controlled as follows: Ti≤0.02wt.%; Si≤0.1wt.%; P≤0.005wt.%; S≤0.003wt.%; Al≤0.015wt.%; O≤0.001wt.%; N≤0.002wt.%.

[0041] The following issues should be considered in the alloy element design of the ultra-high strength steel of this application: promoting the precipitation of secondary phases to achieve the strongest secondary hardening, reducing the damage of over-aging of steel, and improving the hardenability of steel.

[0042] The ultra-high strength steel of this application contains a significant amount of elements such as Cr, Mo, and W. These elements readily combine with C to form carbides, and the carbides generated during tempering, such as MC, M2C, and M7C3, cause the material's hardness to peak again after quenching. Ni promotes the dissolution of Fe3C, which is beneficial for improving the steel's toughness and making austenite more stable. Co prevents the adverse effects of Fe3C re-dissolution on the steel's toughness, thereby improving the steel's strength and toughness. At the same time, Co effectively improves the hardenability of martensitic steel. V pins grain boundaries, preventing grain coarsening at high temperatures. When the V content is low, it can combine with Mo2C to form (Mo,V)2C, making the carbides less prone to over-aging and thus improving the steel's tempering resistance.

[0043] In summary, this application includes at least one of the following beneficial technical effects:

[0044] Compared with traditional bearing steel, the ultra-high strength steel of this application has higher tensile strength and yield strength. It contains W element, which can form a synergistic precipitation effect with Mo element during aging, so that carbides have more nucleation sites in the matrix, which increases the precipitation motive force of M2C carbides. At the same time, Co element can hinder dislocation recovery, delay dislocation annihilation and rearrangement at high temperature, maintain dislocation strengthening effect, and help improve strength.

[0045] Compared to traditional AerMet100 and CSS-42L, the ultra-high strength steel of this application has a higher carbon content, resulting in higher strength, a lower cobalt content, and added w (w). W, by increasing the tempering temperature or extending the tempering time, can delay over-aging, improve the resistance of precipitated phases to coarsening, promote the dispersed distribution of alloy carbides with mild coarsening, and facilitate the decomposition of Fe3C. Because the ultra-high strength steel of this application has a low cobalt content, superior mechanical properties, good resistance to SCC cracking, and low cost, it is a superior product.

[0046] Through carbonitriding technology, the surface hardness of the ultra-high strength steel in this application (up to 900HV) is much higher than that of the untreated sample (600HV), the ability to resist plastic deformation is significantly enhanced, and the wear resistance is greatly improved.

[0047] In the post-carburizing treatment, the ultra-high strength steel of this application uses high-pressure gas quenching. Compared with traditional oil quenching, the residual stress of the parts after gas quenching is smaller, resulting in less deformation of the workpiece. At the same time, high-pressure gas quenching can precisely control the cooling rate, effectively reducing the deformation generated during the carburizing process.

[0048] The ultra-high strength steel of this application has a tensile strength ≥2000MPa, a yield strength ≥1700MPa, an impact absorption energy ≥60J, and a carbonitriding hardness ≥900HV. Attached Figure Description

[0049] Figure 1 The metallographic structure (200 μm) of the carbonitrided layer of the ultra-high strength steel provided in this application.

[0050] Figure 2 The metallographic structure (20 μm) of the ultra-high strength steel matrix provided in this application after carbonitriding. Detailed Implementation

[0051] Before describing the embodiments of this application in detail, it should be understood that the terminology used herein is for the purpose of describing a particular embodiment only. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the term pertains.

[0052] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. Furthermore, in the description of this application, unless otherwise stated, "multiple" means two or more.

[0053] The endpoints and any values ​​of the ranges disclosed in this application are not limited to the precise ranges or values, and such ranges or values ​​should be understood to include values ​​close to such ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0054] In this application, the terms "comprising" or "including" are open-ended expressions, meaning they include the content specified in this application but do not exclude other aspects.

[0055] This application provides an ultra-high strength steel. The ultra-high strength steel comprises the following chemical composition: C 0.27-0.34 wt.%; Cr 0.4-1.4 wt.%; Ni 9-10 wt.%; Co 6-8 wt.%; Mo 1.5-2.5 wt.%; W 1.1-1.6 wt.%; V 0.05-0.15 wt.%; Nb 0.02-0.04 wt.%; the remainder being Fe and impurity elements. The content of impurity elements is controlled as follows: Ti ≤ 0.02 wt.%; Si ≤ 0.1 wt.%; P ≤ 0.005 wt.%; S ≤ 0.003 wt.%; Al ≤ 0.015 wt.%; O ≤ 0.001 wt.%; N ≤ 0.002 wt.%.

[0056] This application also provides a method for preparing the above-mentioned ultra-high strength steel, which specifically includes the following steps:

[0057] (1) Steel smelting preparation: The smelting preparation adopts vacuum induction smelting + vacuum consumable remelting to make steel ingots with chemical composition that meet the requirements.

[0058] Vacuum induction smelting: melting temperature 1500-1530℃, refining temperature 1520-1560℃, casting temperature 1550-1570℃.

[0059] Vacuum self-consuming remelting: melting vacuum degree ≤0.5Pa, melting speed 3.0-7.0Kg / min, melting current 5000-9500A, pit cooling after ingot demolding ≥60h, annealing temperature ≤650℃, holding time ≥20h.

[0060] (2) Forging and billet forming: The steel ingot is subjected to high temperature homogenization treatment, with a heating temperature ≥1210℃ and a holding time ≥40h.

[0061] The steel ingots after high-temperature homogenization are forged and formed. The initial forging temperature of the steel ingots is 1000-1150℃, and the final forging temperature is ≥850℃. The forging and forming process is 3 upsetting + 3 drawing.

[0062] (3) Post-forging annealing heat treatment of bar stock: annealing temperature is 600-700℃, holding time is 20-40h, air cooling or furnace cooling.

[0063] (4) Carburizing: The prepared bar is surface machined by low-pressure carburizing + high-pressure gas quenching.

[0064] The heating temperature for low-pressure carburizing is 950-1100℃, the acetylene flow rate during the strong carburizing period is 80-100L / h, the acetylene flow rate during the diffusion period is 50-65L / h, and the carbon potential is 0.8-1.5%.

[0065] High-pressure gas quenching is performed using helium for cooling. The quenching pressure is 10-20 bar, and the quenching time is 10-20 min.

[0066] (5) Cryogenic treatment: The cryogenic temperature is -180℃ to -70℃, the heat preservation time is 1-2h, and the air is cooled.

[0067] (6) Surface pressurized nitriding: The carburized sample after cryogenic treatment is subjected to pressurized gas nitriding treatment.

[0068] The heating temperature for pressurized gas nitriding is 500-550℃, the nitriding pressure is 0.3-0.5MPa, and the ammonia decomposition rate is 20-30%. The holding time is 10-12 hours.

[0069] Furthermore, this application also provides a surface carbonitriding composite strengthening process for ultra-high strength steel. Specifically, it includes surface carburizing treatment of the bar surface after post-forging heat treatment and surface pressure nitriding treatment of the bar surface after cryogenic treatment.

[0070] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0071] Where specific techniques or conditions are not specified in the examples, they shall be performed in accordance with the techniques or conditions described in the literature in this field or in accordance with the product instructions. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0072] The present application will be further described in detail below with reference to the embodiments and test results.

[0073] The ultra-high strength steel of this application was prepared in the laboratory. Three heats of experimental steel were prepared by vacuum induction + vacuum self-consumption process, using 150kg ingots.

[0074] Example 1

[0075] This embodiment provides an ultra-high strength steel. It is the first batch of experimental steel.

[0076] The chemical composition of this ultra-high strength steel, by mass percentage, is shown in Table 1.

[0077] The preparation method of the above-mentioned ultra-high strength steel specifically includes the following steps:

[0078] (1) Steel smelting and preparation

[0079] The smelting process employs a combination of vacuum induction smelting and vacuum consumable remelting to produce steel ingots with the required chemical composition.

[0080] Vacuum induction smelting: melting temperature 1520℃, refining temperature 1540℃, casting temperature 1560℃.

[0081] Vacuum self-consuming remelting: the melting vacuum degree is 0.4Pa, the melting speed is 5.0Kg / min, the melting current is 7500A, the ingot is pit-cooled for ≥60h after demolding, the annealing temperature is 650℃, and the holding time is ≥20h.

[0082] (2) Forging and billet forming

[0083] The steel ingot is subjected to high-temperature homogenization treatment. After being heated to 1210℃ and held at that temperature for 40 hours, the steel ingot is forged into a billet.

[0084] The steel ingots after high-temperature homogenization treatment are forged and formed. The initial forging temperature of the steel ingots is 1100℃ and the final forging temperature is 850℃. The forging and forming process is 3 upsetting + 3 drawing.

[0085] (3) Post-forging annealing heat treatment of bar stock

[0086] After forging, the bar stock is annealed at a holding temperature of 640℃ for 30 hours, followed by air cooling or furnace cooling.

[0087] (4) Carburizing

[0088] The prepared rods were cut and sampled, and the surfaces were machined, showing that the surface finish reached Ra0.8.

[0089] Surface carburizing was carried out using low-pressure carburizing. The heating temperature for low-pressure carburizing was 1050℃, the acetylene flow rate during the strong carburizing period was 90L / h, the acetylene flow rate during the diffusion period was 60L / h, and the carbon potential was 1.01%.

[0090] High-pressure gas quenching was performed using helium for cooling. The quenching pressure was 16 bar and the quenching time was 15 minutes.

[0091] (5) Cryogenic treatment

[0092] The samples treated with low-pressure carburizing were subjected to cryogenic treatment at a temperature of -73℃ for 2 hours to promote the transformation of more residual austenite into martensite and improve the strength of the matrix. The samples were then air-cooled.

[0093] (6) Surface nitriding: pressurized gas nitriding

[0094] The carburized samples after cryogenic treatment were subjected to final pressurized gas nitriding treatment.

[0095] The heating temperature for pressurized gas nitriding was 516℃, the nitriding pressure was 0.5MPa, and the ammonia decomposition rate was 30%. The holding time was 10h.

[0096] Example 2-3

[0097] Examples 2 and 3 each provide an ultra-high strength steel. These are the second and third heats of the test steel, respectively. The chemical composition is shown in Table 1. All other operating procedures are consistent with those in Example 1.

[0098] Comparative Examples 1-4

[0099] Comparative Example 1 provides 42CrMo steel after carbonitriding in the prior art. The chemical composition is shown in Table 1.

[0100] Comparative Example 2 provides GCr15 steel after carbonitriding in the prior art. The chemical composition is shown in Table 1.

[0101] Comparative Example 3 provides AerMet100 steel after carbonitriding in the prior art. The chemical composition is shown in Table 1.

[0102] Comparative Example 4 provides CSS-42L steel after carbonitriding in the prior art. The chemical composition is shown in Table 1.

[0103] Table 1 Chemical composition (mass fraction, wt.%) of the ultra-high strength steel of this application and the steel in the comparative example.

[0104]

[0105] Performance testing

[0106] The mechanical properties and post-distillation hardness of the ultra-high strength steel prepared in the above embodiments and the steel in the comparative examples were tested. The test results are shown in Table 2. The metallographic examination of the ultra-high strength steel in Example 1 is as follows. Figure 1 and Figure 2 As shown.

[0107] The testing methods for tensile strength, yield strength, elongation after fracture, and reduction of area are in accordance with GB / T228.1-2010, and the testing method for impact toughness is in accordance with GB / T 229-2020. The testing methods for carburizing hardness and carbonitriding hardness are in accordance with GB / T4340.1-2024.

[0108] Table 2 Mechanical properties and post-distillation hardness of the ultra-high strength steel of this application and the steel in the comparative example.

[0109]

[0110] From Table 1 and Table 2, and combined with Figure 1-2It is evident that the ultra-high strength steel of this application differs significantly from the steel in the comparative examples in terms of chemical composition and mechanical properties. The ultra-high strength steel of this application is significantly higher than the 42CrMo steel, GCr15 steel, and CSS-42L steel in the comparative examples in terms of tensile strength and yield strength, and significantly higher than the AerMet100 steel in the comparative steel in terms of impact absorption energy.

[0111] Furthermore, the ultra-high strength steel of this application has high surface hardness after carbonitriding, which can meet the requirements for gear use, and simultaneously achieves surface and core strengthening and toughening.

[0112] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0113] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A method for producing a W-containing ultra-high strength steel, characterized by, The preparation method of the ultra-high strength steel specifically includes the following steps: (1) Steel smelting and preparation: The smelting and preparation adopts vacuum induction smelting + vacuum consumable remelting to produce steel ingots with chemical composition that meet the requirements. (2) Forging and billet forming: The steel ingot is subjected to high-temperature homogenization treatment, and the steel ingot after high-temperature homogenization treatment is forged and formed; (3) Post-forging annealing heat treatment of bar stock: annealing holding temperature is 600-700℃, holding time is 20-40h, air cooling or furnace cooling; (4) Carburizing: The prepared bar is surface machined by low-pressure carburizing + high-pressure gas quenching. In the surface carburizing process, the heating temperature for low-pressure carburizing is 950-1100℃, the acetylene flow rate during the strong carburizing period is 80-100L / h, the acetylene flow rate during the diffusion period is 50-65L / h, and the carbon potential is 0.8-1.5%. In the high-pressure gas quenching, helium is used for cooling, the pressure during the gas quenching period is 10-20 bar, and the gas quenching time is 10-20 min. (5) Cryogenic treatment: The cryogenic temperature is -180℃ to -70℃, the holding time is 1-2 hours, and the air is cooled. (6) Surface pressurized nitriding: The carburized sample after cryogenic treatment is subjected to pressurized gas nitriding treatment; the pressurized gas nitriding treatment is also used as the final tempering treatment; The heating temperature for pressurized gas nitriding is 500-550℃, the nitriding pressure is 0.3-0.5MPa, the ammonia decomposition rate is 20-30%, and the holding time is 10-12h. The ultra-high strength steel comprises the following chemical composition by weight percentage: C 0.27-0.34 wt.%; Cr 0.4-1.4 wt.%; Ni 9-10 wt.%; Co 6-8 wt.%; Mo 1.5-2.5 wt.%; W 1.1-1.6 wt.%; V 0.05-0.15 wt.%; Nb 0.02-0.04 wt.%; the remainder being Fe and impurity elements; The ultra-high strength steel has a tensile strength ≥2000MPa, a yield strength ≥1700MPa, an impact absorption energy ≥60J, and a carbonitriding hardness ≥900HV.

2. The method of producing ultra-high-strength steel according to claim 1, characterized by, The conditions for vacuum induction smelting are: smelting temperature 1500-1530℃, refining temperature 1520-1560℃, and casting temperature 1550-1570℃.

3. The method of producing ultra-high-strength steel according to claim 1, characterized by, The conditions for vacuum self-consumable remelting are: melting vacuum degree ≤ 0.5 Pa, melting speed 3.0-7.0 Kg / min, melting current 5000-9500 A, pit cooling of steel ingot after demolding ≥ 60 h, annealing temperature ≤ 650 ℃, and holding time ≥ 20 h.

4. The method for preparing ultra-high strength steel according to claim 1, characterized in that, In the forging and billet forming process, the conditions for the high-temperature homogenization treatment are: heating temperature ≥1210℃, holding time ≥40h; The forging process is as follows: the initial forging temperature of the steel ingot is 1000-1150℃, the final forging temperature is ≥850℃, and a forging process of 3 upsetting + 3 drawing is adopted.

5. A W-containing ultra-high strength steel prepared by any one of claims 1-4.

6. The ultra-high strength steel according to claim 5, characterized in that, The content of the impurity elements is controlled as follows: Ti≤0.02wt.%; Si≤0.1wt.%; P≤0.005wt.%; S≤0.003wt.%; Al≤0.015wt.%; O≤0.001wt.%; N≤0.002wt.% 7. A surface carbonitriding composite strengthening process for ultra-high strength steel, characterized in that, This includes surface carburizing treatment using low-pressure carburizing + high-pressure gas quenching after forging and annealing heat treatment of bar stock; and surface pressurized nitriding treatment after cryogenic treatment. In the surface carburizing process, the heating temperature for low-pressure carburizing is 950-1100℃, the acetylene flow rate during the strong carburizing period is 80-100L / h, the acetylene flow rate during the diffusion period is 50-65L / h, and the carbon potential is 0.8-1.5%. In the high-pressure gas quenching process, helium is used for cooling, the pressure during the gas quenching period is 10-20 bar, and the gas quenching time is 10-20 min. The surface pressurized nitriding treatment is as follows: the carburized sample after cryogenic treatment is subjected to pressurized gas nitriding treatment; the pressurized gas nitriding treatment is also used as the final tempering treatment; the heating temperature of pressurized gas nitriding is 500-550℃, the nitriding pressure is 0.3-0.5MPa, the ammonia decomposition rate is 20-30%, and the holding time is 10h-12h. The ultra-high strength steel comprises the following chemical composition by weight percentage: C 0.27-0.34 wt.%; Cr 0.4-1.4 wt.%; Ni 9-10 wt.%; Co 6-8 wt.%; Mo 1.5-2.5 wt.%; W 1.1-1.6 wt.%; V 0.05-0.15 wt.%; Nb 0.02-0.04 wt.%; with the remainder being Fe and impurity elements.

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