Heat treatment method for refining maraging steel structure

Abstract

The invention relates to the technical field of maraging steel heat treatment, in particular to a heat treatment method for refining a maraging steel structure. The method comprises the following steps: firstly, preserving heat of maraging steel at 830-880 DEG C for 1-2 hours, and performing water cooling to room temperature; secondly, keeping the temperature in liquid nitrogen for 3-5 hours, and then returning the temperature to room temperature; then the temperature is increased to 790-850 DEG C from the room temperature at the speed of 0.05-0.2 DEG C / s, and water cooling is directly conducted to the room temperature without heat preservation; and finally, keeping the temperature at 450-550 DEG C for 2-3 hours, and air-cooling to room temperature. According to the heat treatment method, the structure of the maraging steel can be fully refined in one step, the problems that in the traditional technology, the process is complex, and obdurability and structure refining are difficult to consider are solved, and a new method is provided for strengthening and toughening the maraging steel.

Description

Technical Field

[0001] The present invention relates to the technical field of heat treatment of maraging steel, and in particular to a heat treatment method for refining the structure of maraging steel, which is suitable for manufacturing high-strength materials in aviation, aerospace and other fields. Background Art

[0002] Maraging steel is based on Ni, Co, Mo, and Ti as the main alloying elements. The main element Ni in the alloy forms a flexible Fe-Ni martensite matrix. During the aging process, the Ni, Mo, and Ti elements in the alloy precipitate Ni3(Ti,Mo) and molybdenum-rich phases from the supersaturated solid solution to further strengthen the material. With its ultra-high strength advantage, it plays an important role in aviation, aerospace, military and other fields.

[0003] Research has found that high-angle grain boundaries in maraging steel significantly influence its mechanical properties. Therefore, refining the original austenite grains or martensitic microstructure (blocks and packets) has become a primary method for improving the strength, ductility, and toughness of maraging steel. Traditional microstructure refinement methods primarily involve cyclic quenching and deformation treatment. Cyclic quenching is complex, energy-intensive, costly, and time-consuming. Deformation treatment, on the other hand, can easily produce texture, making the steel's properties directional and limiting its application in products.

[0004] Patent publication number CN119287120A proposes a heat treatment process for grain refinement of C250 maraging steel. This process involves multi-stage high-temperature solution treatment (945-955°C, 925-935°C, 895-905°C, and 815-825°C) to refine the grains of the C250 maraging steel to a grain grade of 8-9. However, this process is complex and energy-intensive, requiring multiple high-temperature treatments, resulting in long treatment times and significantly increased costs. Furthermore, the degree of grain refinement is limited, with grain sizes remaining at the micron level (10-20 μm). Summary of the Invention

[0005] To improve the microstructure refinement of maraging steel while simultaneously reducing costs and simplifying the process, the present invention aims to provide a heat treatment method for refining the microstructure of maraging steel. This method is simple, low-cost, and significantly refines the microstructure. It addresses the challenges of conventional techniques, such as complex processes and the difficulty in achieving both strength, toughness, and microstructure refinement. This method is of great significance for the widespread application of maraging steel.

[0006] The technical solution of the present invention is:

[0007] A heat treatment method for refining the microstructure of maraging steel comprises heat treating the maraging steel according to the following steps:

[0008] S1 solution treatment: keep at 830-880℃ for 1-2 hours, then cool to room temperature with water;

[0009] S2 cryogenic treatment: Immediately keep the maraging steel in S1 in liquid nitrogen for 3 to 5 hours, then return to room temperature;

[0010] S3 austenite reversal treatment: heating from room temperature to 790-850℃ at a heating rate of 0.05-0.2℃ / s, and then directly cooling to room temperature without heat preservation;

[0011] S4 aging treatment: keep warm at 450-550℃ for 2-3 hours, then air cool to room temperature.

[0012] The heat treatment method for refining the structure of maraging steel is applicable to maraging steel having the following chemical composition by weight percentage: C ≤ 0.01%, S < 0.01%, P < 0.01%, Ni 17.0-19.0%, Co 0-11.0%, Mo 0-5.0%, Ti 0.8-1.2%, N < 0.1%, O < 0.01%, and the balance being Fe.

[0013] The heat treatment method for refining the structure of maraging steel has the effect of refining the original austenite grains or refining the martensite structure.

[0014] The heat treatment method for refining the structure of maraging steel has an average effective grain size of less than 2.2 μm, a tensile strength of ≥1600 MPa, and an elongation of ≥10.0%.

[0015] The design concept of the present invention is:

[0016] The core of the present invention is to control the heating rate and temperature during the continuous heating process to regulate the austenite reverse transformation process, thereby obtaining a certain proportion of acicular austenite and spherical austenite. The combination of liquid nitrogen cryogenics with ultra-slow heating rate and control of the final temperature has the effect of refining the original austenite grains or refining the martensite structure. It can achieve Ni partitioning and reconstruction phase transformation to fully refine the structure (average effective grain size <2.2μm), further improving the tensile strength and elongation of maraging steel, and breaking through the limitations of traditional cyclic quenching / deformation treatment.

[0017] The solution treatment, cryogenic treatment, austenite reversal treatment and aging treatment of the present invention have synergistic effects as follows:

[0018] S1 solution treatment (830-880°C for 1-2 hours, water cooling): This temperature range is higher than the austenitization temperature (Ac3 line) of maraging steel. The purpose is to fully dissolve the alloying elements (such as Ni, Co, Mo, Ti, etc.) in the steel into the austenite matrix to form a uniform supersaturated solid solution, providing a structural foundation for subsequent deep cooling, reverse transformation, and aging. At the same time, it eliminates precipitates or segregation in the original structure to ensure the uniformity of subsequent deep cooling and aging treatments.

[0019] S2 liquid nitrogen deep cooling (3 to 5 hours): As the key bridge connecting solid solution (S1) and reverse transformation (S3), it induces martensitic phase transformation through ultra-low temperature, increases dislocation density, and coordinates ultra-slow heating to control Ni distribution, thereby promoting subsequent reverse transformation refinement.

[0020] S3 ultra-slow heating (0.05-0.2°C / s to 790-850°C followed by water cooling): Controls the austenite reverse transformation rate, achieving Ni partitioning and reconfiguration-type phase transformation, forming a mixed acicular / spherical austenite structure, avoiding the textural issues associated with traditional deformation treatments (such as rolling and forging). Furthermore, combined with direct water cooling without insulation, this limits the austenite grain growth time and preserves the ultrafine-grained structure.

[0021] S4 aging treatment (450-550℃ for 2-3h, air cooling): precipitation of nano-scale strengthening phase significantly improves strength, stabilizes ultrafine grain structure to maintain high plasticity, and achieves strength-plasticity matching.

[0022] The advantages and beneficial effects of the present invention are:

[0023] 1. Compared with the multiple-cycle treatment in the related art, the heat treatment method of the present invention has the advantages of simplicity and low energy consumption. The average effective grain size of the obtained maraging steel can reach or even exceed the level of multiple-cycle heat treatment, so it has great application and promotion value.

[0024] 2. The present invention has a wide composition window for maraging steel and good universality.

[0025] 3. The present invention can achieve strength-plasticity matching of maraging steel and has good application value.

[0026] 4. The heat treatment method provided by the present invention, through the synergistic effect of liquid nitrogen deep cooling + ultra-slow heating, can fully refine the microstructure of maraging steel in one step, and achieve ultra-fine grains and synergistic improvement of strength and plasticity while simplifying the process, proposing a new method for strengthening and toughening maraging steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is an electron backscatter diffraction (EBSD) grain boundary image of the maraging steel obtained by the heat treatment method of Example 1.

[0028] Figure 2 This is the EBSD grain boundary map of the maraging steel obtained by the heat treatment method of Example 4.

[0029] Figure 3 This is the EBSD grain boundary map of the maraging steel obtained by the heat treatment method of Example 7.

[0030] Figure 4 This is the EBSD grain boundary map of the maraging steel obtained by the heat treatment method of Comparative Example 1.

[0031] Figure 5 This is the EBSD grain boundary map of the maraging steel obtained by the heat treatment method of Comparative Example 2.

[0032] Figure 6 This is the EBSD grain boundary map of the maraging steel obtained by the heat treatment method of Comparative Example 3.

[0033] Figure 7 The room temperature tensile curve is obtained by the heat treatment method of Example 7. DETAILED DESCRIPTION

[0034] In a specific implementation process, the present invention provides a heat treatment method for refining the structure of maraging steel, comprising the following steps:

[0035] (1) Heating the maraging steel to 830-880°C, holding the temperature for 1-2 hours, and then water-quenching to room temperature;

[0036] (2) Immediately keep the maraging steel after treatment in step (1) in liquid nitrogen for 3 to 5 hours, and then return the temperature to room temperature.

[0037] (3) heating the maraging steel after treatment in step (2) from room temperature to 790-850° C. at a heating rate of 0.05-0.2° C. / s, and directly cooling the steel to room temperature with water without heat preservation.

[0038] (4) The maraging steel treated in step (3) is aged at 450-550° C. for 2-3 hours and then air-cooled to room temperature.

[0039] The maraging steel forgings used in the embodiment of the present invention have a specification of Φ30mm×500mm.

[0040] The maraging steel used in the embodiment of the present invention can be obtained by smelting, forging or commercial purchase.

[0041] The present invention will be further described in detail below with reference to the embodiments, performance test and accompanying drawings.

[0042] Examples 1 to 3

[0043] Examples 1 to 3 respectively provide a heat treatment process for refining the structure of molybdenum-free maraging steel. The chemical composition of the forgings is shown in Table 1.

[0044] Table 1 Chemical composition of materials of Examples 1 to 3 (wt%, Fe balance)

[0045] C S P Ni Co Mo Ti O N 0.01 0.008 0.008 18.0 10.6 0 0.99 0.004 0.005

[0046] The heat treatment process of the maraging steel in the first embodiment includes the following steps:

[0047] (1) Maraging steel was heated to 850°C, kept at this temperature for 1 hour, and then water-quenched to room temperature;

[0048] (2) The maraging steel treated in step (1) was immediately kept in liquid nitrogen for 4 hours and then returned to room temperature.

[0049] (3) The maraging steel treated in step (2) was heated from room temperature to 790° C. at a heating rate of 0.1° C. / s, and then directly cooled to room temperature with water without insulation.

[0050] (4) The maraging steel treated in step (3) was aged at 500° C. for 3 hours and then air-cooled to room temperature.

[0051] The heat-treated maraging steel was processed into specimens and the room temperature mechanical properties were tested.

[0052] The difference between the above-mentioned Examples 2 to 3 and Example 1 is that the heating rate and the heat treatment temperature in step (3) of the heat treatment method are specifically shown in Table 2 below.

[0053] Table 2 Heating rate and heat treatment temperature in step 3 of the heat treatment method of Examples 1 to 3

[0054] serial number Heating rate (℃ / s) Temperature (℃) Example 1 0.1 790 Example 2 0.05 800 Example 3 0.2 830

[0055] Examples 4 to 6

[0056] Examples 4 to 6 respectively provide a heat treatment process for refining the microstructure of cobalt-free maraging steel. The chemical composition of the forgings is shown in Table 3.

[0057] Table 3 Chemical composition of materials of Examples 4 to 6 (wt%, Fe balance)

[0058] C S P Ni Co Mo Ti O N 0.01 0.008 0.008 18.0 0 5.0 0.99 0.004 0.005

[0059] The heat treatment process of the maraging steel in Example 4 comprises the following steps:

[0060] (1) Maraging steel was heated to 850°C, kept at this temperature for 1 hour, and then water-quenched to room temperature;

[0061] (2) The maraging steel treated in step (1) was immediately kept in liquid nitrogen for 4 hours and then returned to room temperature.

[0062] (3) The maraging steel treated in step (2) was heated from room temperature to 850° C. at a heating rate of 0.1° C. / s, and then directly cooled to room temperature by water without insulation.

[0063] (4) The maraging steel treated in step (3) was aged at 500° C. for 3 hours and then air-cooled to room temperature.

[0064] The heat-treated maraging steel was processed into specimens and the room temperature mechanical properties were tested.

[0065] The difference between the above-mentioned Examples 5 to 6 and Example 4 lies in the heating rate and the heat treatment temperature in step 3 of the heat treatment method, as shown in Table 4 below.

[0066] Table 4 Heating rate and heat treatment temperature in step 3 of the heat treatment method of Examples 5 to 6

[0067] serial number Heating rate (℃ / s) Temperature (℃) Example 4 0.1 850 Example 5 0.05 820 Example 6 0.2 830

[0068] Examples 7 to 9

[0069] Examples 7 to 9 respectively provide a heat treatment process for refining the microstructure of cobalt- and molybdenum-containing maraging steel. The chemical composition of the forgings is shown in Table 5.

[0070] Table 5 Chemical composition of materials of Examples 7 to 9 (wt%, Fe balance)

[0071] C S P Ni Co Mo Ti O N 0.01 0.008 0.008 18.0 10.8 4.9 0.99 0.004 0.005

[0072] The heat treatment process of the maraging steel in Example 7 comprises the following steps:

[0073] (1) Maraging steel was heated to 850°C, kept at this temperature for 1 hour, and then water-quenched to room temperature;

[0074] (2) The maraging steel treated in step (1) was immediately kept in liquid nitrogen for 4 hours and then returned to room temperature.

[0075] (3) The maraging steel treated in step (2) was heated from room temperature to 840° C. at a heating rate of 0.1° C. / s, and then directly cooled to room temperature with water without insulation.

[0076] (4) The maraging steel treated in step (3) was aged at 500° C. for 3 hours and then air-cooled to room temperature.

[0077] The heat-treated maraging steel was processed into specimens and the room temperature mechanical properties were tested.

[0078] The difference between the above-mentioned Examples 8 to 9 and Example 7 lies in the heating rate and the heat treatment temperature in step 3 of the heat treatment method, as shown in Table 6 below.

[0079] Table 6 Heating rate and heat treatment temperature in step 3 of the heat treatment method of Examples 8 to 9

[0080] serial number Heating rate (℃ / s) Temperature (℃) Example 7 0.1 840 Example 8 0.05 800 Example 9 0.2 850

[0081] Comparative Examples 1 to 3

[0082] Comparative Examples 1 to 3 respectively provide a heat treatment method for refining the structure of maraging steel.

[0083] The differences between Comparative Example 1 and Example 1, Comparative Example 2 and Example 4, and Comparative Example 3 and Example 7 are as follows: the heating rate and treatment temperature in step 3 of the heat treatment method are specifically shown in Table 7 below.

[0084] Table 7 Heating rate and treatment temperature in step 3 of the heat treatment method of comparative examples 1 to 3

[0085] serial number Heating rate (℃ / s) Temperature (℃) Comparative Example 1 0.1 750 Comparative Example 2 10 900 Comparative Example 3 10 750

[0086] Comparative Examples 4 to 6

[0087] Comparative Examples 4 to 6 provide a conventional heat treatment method for maraging steel. The difference between Comparative Example 4 and Example 1, Comparative Example 5 and Example 4, and Comparative Example 6 and Example 7 is that there is no heat treatment method step (3).

[0088] The heat treatment method of the maraging steel of Comparative Examples 4 to 6 comprises the following steps:

[0089] (1) Maraging steel was heated to 850°C, kept at this temperature for 1 hour, and then water-quenched to room temperature;

[0090] (2) The maraging steel treated in step (1) was immediately kept in liquid nitrogen for 4 hours and then returned to room temperature.

[0091] (3) The maraging steel treated in step (2) is directly aged at 500° C. for 3 hours and then air-cooled to room temperature.

[0092] Detection test

[0093] The effective grain size statistics and mechanical property tests of the maraging steels obtained in Examples 1 to 9 and Comparative Examples 1 to 6 were performed, and the results are shown in Table 8 below.

[0094] 1. Microstructure Observation: The microstructures of the maraging steels of the embodiment and the comparative example after heat treatment were observed by EBSD.

[0095] 2. Mechanical properties testing: Refer to GB / T 228.1-2021 to test the mechanical properties of maraging steel.

[0096] like Figure 1 As shown in FIG. 1 , the grain boundary GB diagram of the maraging steel obtained by the heat treatment method of Example 1 shows that the structure of the molybdenum-free maraging steel is uniformly refined, and the average effective grain size is only 1.96 μm.

[0097] like Figure 2 As shown in FIG. 1 , the grain boundary GB diagram of the maraging steel obtained by the heat treatment method of Example 4 shows that the microstructure of the cobalt-free maraging steel is uniformly refined, and the average effective grain size is only 1.66 μm.

[0098] like Figure 3 As shown in FIG. 1 , the grain boundary GB diagram of the maraging steel obtained by the heat treatment method of Example 7 shows that the structure of the maraging steel containing cobalt and molybdenum is uniformly refined, and the average effective grain size is only 1.82 μm.

[0099] like Figure 4 As shown, the GB map of the maraging steel grain boundary obtained by the heat treatment method of Comparative Example 1 shows that the structure of the molybdenum-free maraging steel is not completely refined, there is a large area of ​​coarse grain area, and the average effective grain size is only 3.51 μm.

[0100] like Figure 5 As shown, the grain boundary GB diagram of the maraging steel obtained by the heat treatment method of Comparative Example 2 shows that the microstructure of the cobalt-free maraging steel has not been refined, and the average effective grain size is only 5.21 μm.

[0101] like Figure 6 As shown, the grain boundary GB diagram of the maraging steel obtained by the heat treatment method of Comparative Example 3 shows that the structure of the cobalt- and molybdenum-containing maraging steel has not been refined, and the average effective grain size is only 5.42 μm.

[0102] like Figure 7 As shown, the room temperature tensile curve obtained by the heat treatment method of Example 7 shows that the cobalt-containing and molybdenum-containing maraging steel has excellent mechanical properties after microstructure refinement: the tensile strength reaches 2.3 GPa, and the elongation after fracture reaches 11.5%.

[0103] Table 8 Mechanical properties test results of maraging steel obtained in Examples 1 to 9 and Comparative Examples 1 to 6

[0104]

[0105]

[0106] The test results in Table 8 show that, by controlling the heat treatment heating rate within a range of 0.05-0.2°C / s and the temperature within a range of 790-850°C, Examples 1-9 of the present invention achieve maraging steels with tensile strengths greater than 1600 MPa (1601-2310 MPa), elongation ≥10.0% (10.0-12.0%), and effective grain sizes less than 2.2 μm (1.66-2.10 μm). This demonstrates that the heat treatment methods provided by Examples 1-9 of the present invention can refine the microstructure of maraging steels, thereby enabling the aged maraging steels to achieve excellent mechanical properties.

[0107] In Comparative Example 1, when the heat treatment heating rate was controlled at 0.1°C / s and the temperature was controlled at 750°C, the resulting maraging steel exhibited a tensile strength of only 1566 MPa, an elongation of only 10.0%, and an effective grain size of 3.51 μm. This indicates that the low heat treatment temperature resulted in incomplete reconstructive transformation during the austenite reversal, failing to refine the microstructure of the molybdenum-free maraging steel.

[0108] In Comparative Example 2, when the heat treatment heating rate was controlled at 10°C / s and the temperature was controlled at 900°C, the resulting maraging steel had a tensile strength of 1661 MPa, an elongation of only 9.5%, and an effective grain size of 4.21 μm. This indicates that the faster heating rate and higher peak temperature make it difficult to maintain Ni partitioning during the phase transformation, and the microstructure of the cobalt-free maraging steel fails to be effectively refined.

[0109] In Comparative Example 3, when the heat treatment heating rate was controlled at 10°C / s and the temperature was controlled at 750°C, the resulting maraging steel had a tensile strength of 2275 MPa, an elongation of only 8.5%, and an effective grain size of 4.02 μm. This indicates that the lower heat treatment temperature and faster heating rate result in shear-dominated austenite reverse transformation, making it difficult to achieve microstructure refinement in cobalt- and molybdenum-containing maraging steel.

[0110] Comparative Examples 4-6, in which solution-treated maraging steels were directly aged, all exhibited lower strength and toughness than the corresponding examples. This suggests that continuous temperature-raising heat treatment can induce Ni partitioning or reconstruction-type transformations during the austenite transformation, leading to superior mechanical properties in the maraging steels.

[0111] In summary, the present invention provides a heat treatment method for refining the microstructure of maraging steel. This heat treatment method utilizes a continuous heating process. By controlling the heating rate and temperature within the following ranges, the maraging steel, after solution cryogenic treatment, is heated from room temperature to 790-850°C at a heating rate of 0.05-0.2°C / s, and then directly water-cooled to room temperature without insulation. This method achieves a fully refined microstructure of the maraging steel, achieving an effective grain size of less than 2.2 μm. After aging treatment, the cobalt- and molybdenum-containing maraging steel can achieve a tensile strength of 2310 MPa and an elongation of 11.5%.

[0112] Although the present invention has been described in detail above using general descriptions and specific embodiments, it will be apparent to those skilled in the art that modifications and improvements may be made based on the present invention. Therefore, such modifications and improvements, which do not depart from the spirit of the present invention, are intended to be within the scope of protection claimed herein.

Claims

1. A heat treatment method for refining the structure of maraging steel, characterized in that: Maraging steel is heat treated as follows: S1 solution treatment: keep at 830-880℃ for 1-2 hours, then cool to room temperature with water; S2 cryogenic treatment: Immediately keep the maraging steel in S1 in liquid nitrogen for 3 to 5 hours, then return to room temperature; S3 austenite reversal treatment: heating from room temperature to 790-850℃ at a heating rate of 0.05-0.2℃ / s, and then directly cooling to room temperature without heat preservation; S4 aging treatment: keep warm at 450-550℃ for 2-3 hours, then air cool to room temperature.

2. The heat treatment method for refining the structure of maraging steel according to claim 1, characterized in that: Calculated by weight percentage, the heat treatment method is suitable for maraging steel having the chemical composition of: C≤0.01%, S<0.01%, P<0.01%, Ni17.0-19.0%, Co0-11.0%, Mo0-5.0%, Ti0.8-1.2%, N<0.1%, O<0.01%, and the balance being Fe.

3. The heat treatment method for refining the structure of maraging steel according to claim 1, characterized in that: This method has the effect of refining the original austenite grains or refining the martensite structure.

4. A heat treatment method for refining the structure of maraging steel according to claim 1, characterized in that: The average effective grain size of the maraging steel is <2.2μm, the tensile strength is ≥1600MPa, and the elongation is ≥10.0%.

5. The heat treatment method for refining the structure of maraging steel according to claim 1, characterized in that: In step S3, the heating rate is preferably 0.1°C / s.

6. The heat treatment method for refining the structure of maraging steel according to claim 1, characterized in that: In step S4, the aging temperature is preferably 500° C., and the holding time is preferably 3 hours.

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