A Co-free low-alloy high-strength and tough martensitic ultra-high strength steel and its preparation method

Abstract

The present application relates to the technical field of ultra-high strength steel, and specifically to a Co-free low-alloy high-strength and tough martensitic ultra-high strength steel and its preparation method. The present application obtains a tempered martensitic matrix structure and a high-density nanoscale ε precipitation phase by precisely controlling the content of elements such as C, Si, Mn, Cr, Mo, and Ni in the ultra-high strength steel through the process of quenching + deep cooling + low-temperature tempering. While using Ni to toughen the martensitic matrix, Si and Mn are used to improve the yield strength and tempering resistance, thus avoiding the addition of the precious metal Co and improving the economic efficiency of the material. The Co-free low-alloy high-strength and tough martensitic ultra-high strength steel provided by the present application has excellent performance, with a tensile strength ≥ 2200MPa, a yield strength ≥ 1700MPa, an elongation ≥ 8%, a cross-sectional shrinkage ≥ 30%, and a fracture toughness ≥ 55MPa·m 1 / 2 .

Description

Technical Field

[0001] The present application relates to the technical field of ultra-high strength steel, and in particular to a Co-free low-alloy high-strength and tough martensitic ultra-high strength steel and a preparation method thereof. Background Art

[0002] As the core load-bearing component of the aircraft, the landing gear bears the huge loads during takeoff, landing and taxiing. Therefore, the performance and reliability of the landing gear are crucial.

[0003] As a typical low-alloy ultra-high-strength steel, 300M steel combines low alloying costs with high strength and toughness, making it the primary material for landing gear, particularly in civil aircraft. As aircraft takeoff weights continue to increase, the strength requirements for landing gear materials are also increasing. While secondary hardening ultra-high-strength steels with superior overall strength and toughness compared to 300M steel exist, the high alloying costs associated with their high Ni and Co content limit their application.

[0004] Therefore, it is crucial to develop low-alloy ultra-high strength steels with higher strength grades and economical performance. Summary of the Invention

[0005] The present application provides a Co-free low-alloy high-strength and tough martensitic ultra-high strength steel and a preparation method thereof.

[0006] This application aims to develop a low-cost, Co-free, ultra-high-strength steel with a tensile strength of 2200 MPa to address the problem that the strength grade of the current landing gear trunk material cannot meet the requirements of the development of future large passenger aircraft.

[0007] This application precisely controls the content of elements such as C, Si, Mn, Cr, Mo, and Ni in ultra-high strength steel, and obtains a tempered martensitic matrix structure and a high-density nanoscale ε precipitation phase through a process of quenching + deep cooling + low-temperature tempering. Ni is used to toughen the martensitic matrix, while Si and Mn are used to improve the yield strength and tempering resistance, avoiding the addition of precious metal Co and improving the economy of the material.

[0008] The Co-free low-alloy high-strength and tough martensitic ultra-high strength steel provided in this application has excellent performance, with a tensile strength of ≥2200 MPa, a yield strength of ≥1700 MPa, an elongation of ≥8%, a cross-sectional shrinkage of ≥30%, and a fracture toughness of ≥55 MPa·m 1 / 2 Compared with the existing secondary hardening ultra-high strength steel of the same strength grade, the alloying cost is greatly reduced.

[0009] In a first aspect, the present application provides a Co-free low-alloy high-strength and high-toughness martensitic ultra-high-strength steel, which adopts the following technical solution:

[0010] A Co-free low-alloy high-strength and tough martensitic ultra-high-strength steel has a chemical composition, in percentage by mass, of C: 0.45-0.55%, Si: 0.5-2%, Mn: 0.3-2%, Cr: 0.8-2%, Mo: 0.3-1.5%, Ni: 1.5-5%, and the balance is Fe and unavoidable impurities.

[0011] Optionally, the ultra-high strength steel contains fine-grained elements, and in terms of mass percentage, the fine-grained elements include V: 0.01-0.2%, and Nb: 0.01-0.10%.

[0012] Optionally, the alloy system of the ultra-high strength steel does not include the precious metal element Co.

[0013] The ultra-high strength steel provided in this application does not include the precious metal element Co, thereby improving the economy of the material.

[0014] Optionally, in the alloy system of the ultra-high strength steel, the content of Cr+Mo+Ni is controlled to be ≤10%.

[0015] Optionally, in the alloy system of the ultra-high strength steel, the content of Cr+Mo+Ni is controlled to be ≤5%.

[0016] The present application can further improve the economic efficiency of the material by further controlling the Cr+Mo+Ni content in the ultra-high strength steel alloy system.

[0017] Optionally, the ultra-high strength steel has a tensile strength of ≥2200 MPa, a yield strength of ≥1700 MPa, an elongation of ≥8%, a cross-sectional shrinkage of ≥30%, and a fracture toughness of ≥55 MPa·m 1 / 2 .

[0018] In a second aspect, the present application provides a method for preparing a Co-free low-alloy high-strength and high-toughness martensitic ultra-high strength steel, which adopts the following technical solution:

[0019] A method for preparing Co-free low-alloy high-strength and tough martensitic ultra-high strength steel, the preparation method specifically comprising: smelting, normalizing treatment, annealing treatment, quenching treatment, deep cooling treatment, and tempering treatment.

[0020] Optionally, the smelting method is selected from any one or more of a converter, an electric furnace, a vacuum induction furnace, a LF furnace, and a vacuum consumable furnace.

[0021] Optionally, the normalizing treatment includes heating the rod to 910-960° C., keeping the temperature for 1-3 hours, and air cooling to room temperature.

[0022] Optionally, the annealing treatment includes heating the rod to 620-680° C., keeping the temperature for 6-15 hours, and air cooling to room temperature.

[0023] Optionally, the quenching treatment includes heating the bar to 870-960° C., keeping the temperature for 1-2 hours, and oil cooling.

[0024] Optionally, the cryogenic treatment includes keeping the rod at -73 to 0°C for 0.5 to 4 hours and air cooling it to room temperature.

[0025] Optionally, the tempering treatment includes heating the rod to 150-300° C., keeping the temperature for 1-6 hours, and air-cooling to room temperature.

[0026] In summary, this application includes at least one of the following beneficial technical effects:

[0027] This application precisely controls the content of elements such as C, Si, Mn, Cr, Mo, and Ni in ultra-high strength steel, and obtains a tempered martensitic matrix structure and a high-density nanoscale ε precipitation phase through a process of quenching + deep cooling + low-temperature tempering. Ni is used to toughen the martensitic matrix, while Si and Mn are used to improve the yield strength and tempering resistance, avoiding the addition of precious metal Co and improving the economy of the material.

[0028] The Co-free low-alloy high-strength and tough martensitic ultra-high strength steel provided in this application has excellent performance, with a tensile strength of ≥2200 MPa, a yield strength of ≥1700 MPa, an elongation of ≥8%, a cross-sectional shrinkage of ≥30%, and a fracture toughness of ≥55 MPa·m 1 / 2 Compared with the existing secondary hardening ultra-high strength steel of the same strength grade, the alloying cost is greatly reduced. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is the microstructure of the ultra-high strength steel after tempering in Example 2 of the present application. DETAILED DESCRIPTION

[0030] Before describing the embodiments of the present application in detail, it should be understood that the terminology used herein is only for the purpose of describing particular embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the term belongs.

[0031] It should be noted that the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of the technical features indicated. Therefore, features defined as "first" or "second" may explicitly or implicitly include one or more of such features. Furthermore, in the description of this application, unless otherwise specified, "plurality" means two or more.

[0032] The endpoints of the ranges and any values disclosed in this application 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 obtain one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed herein.

[0033] In this application, the term "comprise" or "include" is an open expression, that is, it includes the content specified in this application, but does not exclude other aspects of the content.

[0034] The present application provides a Co-free low-alloy high-strength and tough martensitic ultra-high strength steel, whose chemical composition, in mass percentage, includes: C: 0.45-0.55%, Si: 0.5-2%, Mn: 0.3-2%, Cr: 0.8-2%, Mo: 0.3-1.5%, Ni: 1.5-5%, V: 0.01-0.2%, Nb: 0.01-0.10%, and the balance is Fe and unavoidable impurities.

[0035] The following is a detailed description of the functions and dosage of the components contained in this application:

[0036] Calculated by mass percentage, the ultra-high strength steel of the present application includes C: 0.45-0.55%.

[0037] In this application, C is the most important strengthening element in low alloy martensitic steel. Controlling the C content above 0.45% is the key to ensuring its strength. However, the higher the C content, the worse the toughness and plasticity of the material. Therefore, the C content is controlled to be 0.45-0.55%.

[0038] Calculated by mass percentage, the ultra-high strength steel of the present application includes Si: 0.5-2%, and Mn: 0.3-2%.

[0039] In this application, Si can inhibit the decomposition and transformation of retained austenite into Fe3C, improve the tempering resistance of martensite, and significantly increase the tempering temperature. However, high Si content can reduce decarburization performance. Mn can improve the hardenability and strength of the steel, but excessive Mn content can sharply reduce the Ms point, leading to excessive retained austenite. The appropriate combination of Si and Mn can achieve an excellent balance of strength and toughness at an appropriate tempering temperature. Therefore, the Si content is controlled at 0.5-2% and the Mn content at 0.3-2%.

[0040] Calculated by mass percentage, the ultra-high strength steel of the present application includes Cr: 0.8-2%, and Mo: 0.3-1.5%.

[0041] In this application, Cr can improve the strength of steel through solid solution strengthening and precipitation strengthening, while also improving the steel's hardenability and tempering resistance. However, excessive Cr content can significantly reduce the steel's Ms, leading to excessive retained austenite. Furthermore, Cr-containing carbides with excessively high precipitation temperatures are likely to form before solution treatment, raising the solution temperature and resulting in coarsening of grains and microstructure. Mo can increase the steel's strength, improve hardenability and tempering resistance, and inhibit temper brittleness. However, excessive Mo content increases the quenching temperature. Therefore, the Cr content is controlled at 0.8-2% and the Mo content at 0.3-1.5%.

[0042] Calculated by mass percentage, the ultra-high strength steel of the present application includes Ni: 1.5-5%.

[0043] In the present application, Ni can significantly improve the cleavage fracture resistance of the martensite matrix, but too high a Ni content will form too much retained austenite, which will damage the toughness of the steel and significantly reduce the yield strength of the material. Therefore, the Ni content is controlled to 1.5-5%.

[0044] The ultra-high strength steel of the present application includes one or more fine-grained elements such as V and Nb, with a V content of 0.01-0.2% and a Nb content of 0.01-0.1%, respectively. The carbides and nitrides formed by these elements can effectively inhibit grain growth during quenching and holding.

[0045] In order to improve the economy of the material, the ultra-high strength steel of the present application does not contain the precious alloy element Co, and the Cr+Mo+Ni content is ≤10%. Preferably, the Cr+Mo+Ni content can be ≤5%.

[0046] The ultra-high-strength steel provided herein also contains unavoidable impurity elements, primarily including S, P, O, N, Al, and Ti, with S ≤ 0.003%, P ≤ 0.005%, O ≤ 0.002%, N ≤ 0.003%, Al ≤ 0.05%, and Ti ≤ 0.01%. Effective control of these impurity elements can improve the toughness and fatigue life of the material.

[0047] This application also provides a method for preparing the above-mentioned ultra-high strength steel, which specifically includes: smelting, normalizing treatment, annealing treatment, quenching treatment, cryogenic treatment, and tempering treatment.

[0048] The above-mentioned smelting step refers to the use of one or more methods including but not limited to converters, electric furnaces, vacuum induction furnaces, LF furnaces, vacuum consumable furnaces, etc. to prepare ultra-high strength steel whose chemical composition meets the required chemical composition range.

[0049] The normalizing treatment step includes heating the bar to 910-960°C, keeping the temperature for 1-3 hours, and then air cooling to room temperature.

[0050] The annealing step includes heating the rod to 620-680° C., keeping the temperature for 6-15 hours, and air cooling to room temperature.

[0051] The purpose of normalizing and annealing is to eliminate the stress of the bar and improve the machinability of the bar.

[0052] The quenching treatment includes heating the bar to 870-960° C. (preferably 900-950° C.), keeping the temperature for 1-2 hours, and oil cooling.

[0053] The cryogenic treatment comprises cryogenically treating the rod in the range of -73 to 0°C (preferably -40 to 0°C), keeping the temperature for 0.5 to 4 hours (preferably 1 to 2 hours), and air cooling to room temperature.

[0054] The tempering treatment step includes heating the rod to 150-300° C. (preferably 200-300° C.), keeping the temperature for 1-6 hours (preferably 1-3 hours), and air cooling to room temperature.

[0055] The purpose of the above-mentioned quenching treatment is to completely dissolve the main alloying elements in the steel into the matrix. The purpose of the deep cryogenic treatment is to promote the transformation of retained austenite to martensite and reasonably control the volume fraction of retained austenite. The purpose of the tempering treatment is to promote the precipitation of nano-ε carbides and soften the matrix, thereby obtaining an excellent combination of strength and toughness.

[0056] To make the purpose, technical solutions, and advantages of this application more clear, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without making any creative work shall fall 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 understood as limiting this application.

[0057] If no specific techniques or conditions are specified in the examples, the experiments were carried out according to the techniques or conditions described in the literature in the field or according to the product instructions. All reagents or instruments used without specifying the manufacturer are commercially available conventional products.

[0058] The present application is further described in detail below with reference to the examples, drawings and test results.

[0059] Example 1

[0060] This embodiment provides a Co-free low-alloy high-strength and high-toughness martensitic ultra-high strength steel.

[0061] The chemical composition of the above ultra-high strength steel is shown in Table 1 in terms of mass percentage.

[0062] The preparation method of the ultra-high strength steel specifically comprises the following steps:

[0063] (1) Melting: A vacuum induction furnace is used to prepare rods whose chemical composition meets the required chemical composition range.

[0064] (2) Normalizing treatment: Heat the bar to 930℃, keep it warm for 2h, and then air cool it to room temperature.

[0065] (3) Annealing step: heat the bar to 650 °C, keep it at this temperature for 12 h, and air cool it to room temperature.

[0066] (4) Quenching treatment: heat the bar to 940℃, keep warm for 1h, and oil cool.

[0067] (5) Cryogenic treatment: cryogenically treat the bar at -30°C, keep warm for 1 hour, and air cool to room temperature.

[0068] (6) Tempering treatment: Heat the bar to 260℃, keep it warm for 2h, and air cool it to room temperature.

[0069] Example 2-3

[0070] Examples 2-3 each provide a Co-free low-alloy high-strength and high-toughness martensitic ultra-high strength steel.

[0071] The above embodiment differs from embodiment 1 in that the chemical composition of the ultra-high strength steel and the parameter control of the preparation process are specifically shown in Tables 1 and 2. They are consistent with embodiment 1.

[0072] Table 1 Chemical composition of ultra-high strength steel of Examples and Comparative Examples

[0073]

[0074]

[0075] Table 2 Parameter control of ultra-high strength steel preparation process of embodiment and comparative example

[0076]

[0077] Comparative Example

[0078] Comparative Examples 1-3

[0079] Comparative Examples 1-3 each provide a Co-free low-alloy high-strength and high-toughness martensitic ultra-high strength steel.

[0080] The difference between the comparative example and Example 1 lies in the chemical composition of the ultra-high strength steel and the parameter control during the preparation process, which are specifically shown in Tables 1 and 2. They are consistent with those in Example 1.

[0081] Performance test results

[0082] The following tests were performed on the ultra-high strength steels of the above examples and comparative examples.

[0083] Among them, tensile strength (Rm / MPa), yield strength (R p0.2 / MPa), elongation (A / %), and area reduction (Z / %) are tested in accordance with GB / T 228.1-2010, and fracture toughness (KIC / MPa·m 1 / 2 ) The detection method of ) refers to GB / T 4161-2007.

[0084] The test results are shown in Table 3 and Figure 1 shown. Figure 1 This is the microstructure of the ultra-high strength steel of Example 2 after tempering.

[0085] Table 3 Performance test results of ultra-high strength steel of Examples and Comparative Examples

[0086]

[0087]

[0088] Depend on Figure 1 It can be seen that the microstructure of the ultra-high strength steel of Example 2 of the present application after tempering is lath-shaped tempered martensite and ε carbide. As shown in Table 3, the tensile strength of the ultra-high strength steel provided by the present application is ≥2200MPa, the yield strength is ≥1700MPa, the elongation is ≥8%, the cross-sectional shrinkage is ≥30%, and the fracture toughness is ≥55MPa·m 1 / 2 Compared with existing secondary hardening ultra-high strength steels of the same strength grade, the alloying cost is significantly reduced.

[0089] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example" or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in this specification and features of different embodiments or examples without contradiction.

[0090] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A Co-free low alloy high strength and toughness martensitic ultra-high strength steel, characterized in that: The chemical composition of the ultra-high strength steel includes, in mass percentage, C: 0.45-0.55%, Si: 0.5-2%, Mn: 0.3-2%, Cr: 0.8-2%, Mo: 0.3-1.5%, Ni: 1.5-5%, V: 0.01-0.2%, Nb: 0.01-0.10%, the balance is Fe and inevitable impurities; In the alloy system of the ultra-high strength steel, the content of Cr+Mo+Ni is controlled to be ≤5%; The preparation method of the low-alloy high-strength and high-toughness martensitic ultra-high strength steel specifically includes: smelting, normalizing treatment, annealing treatment, quenching treatment, cryogenic treatment, and tempering treatment; The normalizing treatment includes heating the bar to 910-960°C, keeping the temperature for 1-3 hours, and air cooling to room temperature; The annealing treatment includes heating the bar to 620-680°C, keeping the temperature for 6-15 hours, and air cooling to room temperature; The quenching treatment includes heating the bar to 870-960°C, keeping the temperature for 1-2 hours, and oil cooling; The cryogenic treatment includes keeping the bar at -73~0℃ for 0.5-4h and air cooling to room temperature; The tempering treatment includes heating the bar to 150-300°C, keeping the temperature for 1-6 hours, and air cooling to room temperature; The ultra-high strength steel has a cross-sectional shrinkage of ≥30% and a fracture toughness of ≥55 MPa·m 1 / 2 .

2. The ultra-high strength steel according to claim 1, characterized in that The ultra-high strength steel has a tensile strength of ≥2200 MPa, a yield strength of ≥1700 MPa, and an elongation of ≥8%.

3. The ultra-high strength steel according to claim 1, characterized in that The smelting method is selected from any one or more of a converter, an electric furnace, a vacuum induction furnace, a LF furnace, and a vacuum consumable furnace.

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