An anti-cleavage martensitic ultra-high strength steel and its preparation method

Abstract

This application relates to the technical field of metal smelting, and specifically discloses a cleavage-resistant martensitic ultra-high strength steel and a preparation method thereof. The cleavage-resistant martensitic ultra-high strength steel. In terms of weight percentage, its chemical composition includes: C 0.35 - 0.50%; Cr 1 - 5%; Mo 0.5 - 3%; Ni 2 - 9%; Co 1 - 8%; the balance is Fe and unavoidable impurities; the Ni content is 1 - 5% higher than the Co content. In view of the problems of excessively high cost and too strong resource dependence of existing secondary hardening ultra-high strength steels above 2000 MPa level, this application has developed a 2200 MPa level ultra-high strength steel with low cost and excellent strength and toughness.

Description

Technical Field

[0001] The present application relates to the technical field of metal smelting, and more specifically, to a cleavage-resistant martensitic ultra-high strength steel and a preparation method thereof. Background Art

[0002] Ultra-high strength steel is widely used in key load-bearing components such as aerospace, energy and transportation, and weapons and equipment. It is often subjected to cyclic loads during service. Therefore, the fatigue performance of ultra-high strength steel is particularly important for its service safety and reliability. Studies have shown that when the strength grade of steel materials is low, its fatigue performance is positively correlated with tensile strength, while when the strength grade of steel materials is above 1800-2000MPa, its fatigue performance decreases with the increase of strength. Therefore, the fatigue resistance of steel materials under ultra-high strength needs to comprehensively consider the matching relationship between strength and toughness, especially strength and fracture toughness.

[0003] Due to its excellent strength and toughness, secondary hardening ultra-high strength steel has been widely used. Representative products are the American Aermet series of secondary hardening ultra-high strength steels, such as Aermet100, which has a tensile strength of 1960MPa and a plane strain fracture toughness (K IC ) greater than 110MPa·m 1 / 2 ; Aermet310 has a tensile strength of 2170MPa, K IC Up to 70MPa·m 1 / 2 The chemical composition of typical secondary hardening ultra-high strength steel is shown in Table 1. However, the addition of more than 25% of alloying elements greatly increases the production cost of this type of secondary hardening ultra-high strength steel, restricting its large-scale industrial application.

[0004] Table 1 Chemical composition of typical secondary hardening ultra-high strength steel

[0005]

[0006] Therefore, there is an urgent need to develop an ultra-high strength steel with low cost and a tensile strength of more than 2200 MPa, while having both excellent strength and toughness. Summary of the invention

[0007] In view of the high cost and resource dependence of the existing secondary hardening ultra-high strength steel of 2000MPa or above, the present application develops an ultra-high strength steel of 2200MPa or above with low cost and excellent strength and toughness. The present application provides a cleavage-resistant martensitic ultra-high strength steel and a preparation method thereof.

[0008] In a first aspect, the present application provides a cleavage-resistant martensitic ultra-high strength steel, which adopts the following technical solution:

[0009] A cleavage-resistant martensitic ultra-high strength steel, in terms of weight percentage, the chemical composition of the cleavage-resistant martensitic ultra-high strength steel includes: C 0.35-0.50%; Cr 1-5%; Mo 0.5-3%; Ni 2-9%; Co 1-8%; the balance is Fe and inevitable impurities.

[0010] Optionally, in the chemical composition of the cleavage-resistant martensitic ultra-high strength steel, the Ni content is 1-5% higher than the Co content.

[0011] Optionally, the tensile strength of the ultra-high strength steel ≥2200 MPa, and the fracture toughness ≥65 MPa·m 1 / 2 。

[0012] In a second aspect, the present application provides a method for preparing a cleavage-resistant martensitic ultra-high strength steel, adopting the following technical solution:

[0013] A method for preparing a cleavage-resistant martensitic ultra-high strength steel, the preparation method includes melting, normalizing, annealing, quenching, cryogenic treatment, and low-temperature tempering treatment.

[0014] Further, the melting is to prepare an ultra-high strength steel with a chemical composition meeting the required range by using including but not limited to a converter, an electric furnace, a vacuum induction furnace, an LF furnace, a vacuum consumable method or a combination of two or more of the above methods.

[0015] Further, the normalizing treatment includes heating the bar to 910-960 °C, holding for 1-3 h, and air-cooling to room temperature.

[0016] Further, the annealing treatment includes heating the bar to 620-680 °C, holding for 6-15 h, and air-cooling to room temperature.

[0017] Further, the quenching treatment includes heating the bar to 900-960 °C, holding for 1-2 h, and oil-cooling.

[0018] Further, the cryogenic treatment includes cryogenic treating the bar in the range of (-196)-(-73) °C, holding for 0.5-3 h, and air-cooling to room temperature.

[0019] Further, the low-temperature tempering treatment includes heating the bar to 120-200 °C, holding for 1-6 h, and air-cooling to room temperature.

[0020] In a specific embodiment, a method for preparing a cleavage-resistant martensitic ultra-high strength steel specifically includes the following steps:

[0021] (1) Melting: Prepare ultra-high strength steel with chemical composition within the required range by using methods including but not limited to converters, electric furnaces, vacuum induction furnaces, LF furnaces, vacuum consumable methods, or combinations of two or more of the above methods.

[0022] (2) Normalizing treatment: Heat the bar to 910 - 960 °C, hold for 1 - 3 h, and air cool to room temperature.

[0023] (3) Annealing treatment: Heat the bar to 620 - 680 °C, hold for 6 - 15 h, and air cool to room temperature.

[0024] (4) Quenching treatment: Heat the bar to 900 - 960 °C, hold for 1 - 2 h, and oil cool.

[0025] (5) Cryogenic treatment: Deep cool the bar in the range of (-196) - (-73) °C, hold for 0.5 - 3 h, and air cool to room temperature.

[0026] (6) Low-temperature tempering treatment: Heat the bar to 120 - 200 °C, hold for 1 - 6 h, and air cool to room temperature.

[0027] In summary, the present application has the following beneficial effects:

[0028] Aiming at the problems of excessively high cost and strong resource dependence of existing secondary hardening ultra-high strength steel above 2000 MPa grade, the present application has developed an ultra-high strength steel above 2200 MPa grade with low cost and excellent strength and toughness.

[0029] The anti-cleavage martensite ultra-high strength steel provided by the present application obtains a tempered martensite matrix structure and a high-density nanoscale ε precipitation phase through precise control of the contents of C, Cr, Mo, Ni, and Co elements in the ultra-high strength steel and through the process of quenching + cryogenic treatment + low-temperature tempering. The Ni element of 2 - 9% is used to toughen the martensite matrix, the Co element of 1 - 8% is used to regulate the Ms point and improve the strength and temper resistance, and the Ni content is controlled to be 2 - 5% higher than the Co content to synergistically regulate the material properties, and at the same time, it has good tensile strength and ductility.

[0030] The anti-cleavage martensite ultra-high strength steel provided by the present application has excellent performance, with a tensile strength of up to 2200 MPa or more, an elongation rate of more than 11%, a reduction of area of more than 40%, and a fracture toughness K IC greater than 65 MPa·m 1 / 2 . Compared with existing secondary hardening ultra-high strength steel of the same strength grade, when the tensile strength and fracture toughness are basically the same, the alloy element content is significantly reduced, and the alloying cost is greatly reduced. Specific embodiments

[0031] Before describing the embodiments of the present application in detail, it should be understood that the terms used herein are for the purpose of describing specific embodiments 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 this term belongs.

[0032] The present application provides a cleavage-resistant martensitic ultra-high strength steel. In terms of weight percentage, its chemical composition includes: C 0.35 - 0.50%; Cr 1 - 5%; Mo 0.5 - 3%; Ni 2 - 9%; Co 1 - 8%; the balance is Fe and unavoidable impurities.

[0033] Further, in terms of weight percentage, in its chemical composition, the Ni content is 1 - 5% higher than the Co content.

[0034] The following specifically describes each chemical component and the selection of its dosage in the cleavage-resistant martensitic ultra-high strength steel of the present application.

[0035] In terms of weight percentage, the ultra-high strength steel of the present application includes C 0.35 - 0.50%. In the present application, the C element is the most important strengthening element, and the C element content directly determines the strength of martensite. Controlling the C content above 0.35% is the key to ensuring the strength of the material. However, if the C content is too high (>0.50%), the toughness and plasticity of the material will be significantly deteriorated, especially the fracture toughness will be damaged. Therefore, the C content in the ultra-high strength steel of the present application is controlled to be 0.35 - 0.50%.

[0036] In terms of weight percentage, the ultra-high strength steel of the present application includes Cr 1 - 5%. In the present application, the Cr element can improve the strength of the steel through solid solution strengthening and precipitation strengthening, while improving the hardenability and tempering resistance of the steel. However, if the Cr content is too high, the Ms point (the starting temperature of martensite transformation) of the steel will be significantly reduced, resulting in too much retained austenite. At the same time, before solution treatment, there are easily Cr carbides with too high precipitation temperature, thus increasing the solution temperature and resulting in coarse grains and tissues. Therefore, the Cr content in the ultra-high strength steel of the present application is controlled to be 1 - 5%.

[0037] In terms of weight percentage, the ultra-high strength steel of the present application includes Mo 0.5 - 3%. In the present application, the Mo element can improve the strength of the steel, improve the hardenability and tempering resistance, and inhibit the effect of temper brittleness. However, if the Mo content is too high, the quenching temperature will be increased. Therefore, the Mo content in the ultra-high strength steel of the present application is controlled to be 0.5 - 3%.

[0038] By weight percentage, the ultra-high strength steel of the present application includes 2-9% Ni. In the present application, the Ni element is the main toughening element, and the Ni element can improve the cleavage fracture resistance of the martensite matrix to ensure sufficient toughness. However, too high Ni content will significantly reduce the Ms point of the steel, resulting in excessive and massive retained austenite, damaging the toughness of the steel. Therefore, the Ni content in the ultra-high strength steel of the present application is controlled to be 2-9%.

[0039] By weight percentage, the ultra-high strength steel of the present application includes 1-8% Co. In the present application, the Co element is the main element to increase the Ms point. By adding Co, the Ms point of the steel is adjusted to a reasonable range, thus avoiding the formation of excessive retained austenite. Co can also delay the dislocation recovery in the martensite matrix during tempering and improve the tempering resistance. However, too high Co content will damage the toughness of the steel. Therefore, the Co content in the ultra-high strength steel of the present application is controlled to be 1-8%.

[0040] By weight percentage, it is required in the present application that the Ni content is 1-5% higher than the Co content. The combination of Ni and Co contents can adjust the Ms point to a reasonable range, avoid the formation of excessive retained austenite. The addition of Co can also make up for the reduction of strength caused by Ni. At the same time, Co can significantly improve the tempering resistance and avoid too low tempering temperature of the steel. Therefore, it is required in the ultra-high strength steel of the present application that the Ni content is 1-5% higher than the Co content.

[0041] The ultra-high strength steel of the present application also includes inevitable impurity elements. The impurity elements mainly include S, P, O, N, H, Al, Ti, etc. Further, by weight percentage, S≤0.003%, P≤0.005%, S≤0.003%, O≤0.003%, N≤0.003%, H≤0.00002%, Al≤0.03%, Ti≤0.01%. The present application controls the impurity elements within the above ranges to ensure the strength, toughness and fatigue life of the material.

[0042] To make the purpose, technical solutions and advantages of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below. Based on the embodiments of the present application, all other embodiments obtained by those skilled in the art without creative efforts shall fall within the protection scope of the present application.

[0043] The present application will be further described in detail below with reference to examples, comparative examples and test results.

[0044] Examples

[0045] Example 1

[0046] This example provides a cleavage-resistant martensitic ultra-high strength steel. The chemical composition of the ultra-high strength steel is specifically shown in Table 2.

[0047] The preparation method of the above-mentioned cleavage-resistant martensitic ultra-high strength steel is as follows:

[0048] (1) Melting: Use the vacuum consumable method to prepare an ultra-high strength steel with chemical composition within the required range.

[0049] (2) Normalizing treatment: Heat the bar to 950 °C, hold for 1 h, and air cool to room temperature.

[0050] (3) Annealing treatment: Heat the bar to 660 °C, hold for 8 h, and air cool to room temperature.

[0051] (4) Quenching treatment: Heat the bar to 950 °C, hold for 1 h, and oil cool.

[0052] (5) Cryogenic treatment: Deep cool the bar in the range of (-73) °C, hold for 2 h, and air cool to room temperature.

[0053] (6) Low-temperature tempering treatment: Heat the bar to 170 °C, hold for 1 h, and air cool to room temperature.

[0054] Table 2 Chemical compositions and mechanical properties of the ultra-high strength steels provided by the examples and comparative examples

[0055]

[0056] Examples 2 - 3

[0057] Examples 2 - 3 respectively provide a kind of cleavage-resistant martensitic ultra-high strength steel. The differences between the above ultra-high strength steel and that of Example 1 are: the contents of Ni and Co and the low-temperature tempering temperature, as shown in Table 2 specifically. The remaining operation steps are the same as those of Example 1.

[0058] Comparative examples

[0059] Comparative example 1

[0060] Comparative example 1 provides a kind of cleavage-resistant martensitic ultra-high strength steel. The differences between the above ultra-high strength steel and that of Example 1 are: the Ni content is about 6% higher than the Co content, as shown in Table 2 specifically. The remaining operation steps are the same as those of Example 1.

[0061] Comparative examples 2 - 3

[0062] Comparative examples 2 - 3 respectively provide a kind of cleavage-resistant martensitic ultra-high strength steel. The differences between the above ultra-high strength steel and that of Example 1 are: the Co content is 0 and the low-temperature tempering temperature, as shown in Table 2 specifically. The remaining operation steps are the same as those of Example 1.

[0063] Performance test results

[0064] The following mechanical property tests were respectively carried out on the ultra-high strength steels provided in the above examples and comparative examples, as well as AF1410 (aged at 510 °C for 5 h) and Aermet100 (aged at 482 °C for 5 h). The test results are shown in Table 2.

[0065] Among them, the test methods for tensile strength, yield strength, elongation, and reduction of area refer to "GB / T 228.1-2010", and the test method for fracture toughness refers to "GB / T 4161-2007".

[0066] As can be seen from Table 2, according to the test results of Examples 1-3, the tensile strength of the ultra-high strength steel provided by this application is ≥2200 MPa, and the fracture toughness is ≥65 MPa·m 1 / 2 . In Comparative Example 1, the Ni content is about 6% higher than the Co content. At this time, the tensile strength of the ultra-high strength steel provided by Comparative Example 1 is close to 2200 MPa, while the fracture toughness is only 59 MPa·m 1 / 2 . As can be seen from Comparative Examples 2-3, in the case of no Co, after tempering at 120 °C and 150 °C, although the tensile strength of the ultra-high strength steel meets ≥2200 MPa, the fracture toughness is only 50.01 MPa·m 1 / 2 and 60.7 MPa·m 1 / 2 . The tensile strengths of AF1410 (aged at 510 °C for 5 h) and Aermet100 (aged at 482 °C for 5 h) are both lower than 2000 MPa.

[0067] Finally, it should be noted that the above examples 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 examples, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing examples, or perform equivalent replacements on some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of each example of this application.

Claims

1. A cleavage-resistant martensitic ultra-high strength steel, characterized in that: The chemical composition of the cleavage-resistant martensitic ultra-high strength steel includes, by weight percentage: C 0.35-0.50%; Cr 1-5%; Mo 0.5-3%; Ni 2-9%; Co 1-5.05%; the remainder is Fe and unavoidable impurities; in the chemical composition of the cleavage-resistant martensitic ultra-high strength steel, the Ni content is 1-5% higher than the Co content; The preparation method of the cleavage-resistant martensitic ultra-high strength steel comprises smelting, normalizing treatment, annealing treatment, quenching treatment, cryogenic treatment, Low temperature tempering treatment, the low temperature tempering treatment includes heating the rod to 120-200°C, keeping the temperature for 1-6 hours, and air cooling to room temperature.

2. The cleavage-resistant martensitic ultra-high strength steel according to claim 1, characterized in that: The tensile strength of the ultra-high strength steel is ≥2200MPa, and the fracture toughness is ≥65MPa·m 1 / 2 .

3. The cleavage-resistant martensitic ultra-high strength steel according to claim 1, characterized in that: The smelting is to prepare ultra-high strength steel with chemical composition within the required range by using a converter, an electric furnace, a vacuum induction furnace, an LF furnace, a vacuum consumable method or a combination of two or more of the above methods.

4. The cleavage-resistant martensitic ultra-high strength steel according to claim 1, characterized in that: The normalizing treatment includes heating the rod to 910-960° C., keeping the temperature for 1-3 hours, and air cooling to room temperature.

5. The cleavage-resistant martensitic ultra-high strength steel according to claim 1, characterized in that: The annealing treatment includes heating the rod to 620-680° C., keeping the temperature for 6-15 hours, and air cooling to room temperature.

6. The cleavage-resistant martensitic ultra-high strength steel according to claim 1, characterized in that: The quenching treatment includes heating the rod to 900-960° C., keeping the temperature for 1-2 hours, and oil cooling.

7. The cleavage-resistant martensitic ultra-high strength steel according to claim 1, characterized in that: The cryogenic treatment includes subjecting the rod to cryogenic treatment in the range of (-196)-(-73)°C, keeping the temperature for 0.5-3h, and air cooling to room temperature.

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