Multi-step aging treatment method for improving toughness of ultrahigh-strength steel ingot

Abstract

The invention provides a multi-step aging treatment method for improving the toughness of an ultrahigh-strength steel ingot, and belongs to the technical field of steel heat treatment. The treatment method comprises the following steps: carrying out solution treatment on the ultrahigh-strength steel at the temperature above AC3, carrying out water cooling or oil cooling to the room temperature, then carrying out subzero treatment, taking out and recovering to the room temperature; the multi-step aging treatment comprises the first step of single-time or multi-time low-temperature aging at the temperature of 400-550 DEG C and the second step of high-temperature aging at the temperature of 550-650 DEG C, and the ultrahigh-strength steel ingot is obtained. According to the method, the content of reversed austenite is increased, especially film-shaped austenite with stable mechanics is increased, meanwhile, it is guaranteed that a precipitated phase is not obviously coarsened, and good obdurability matching is achieved.

Description

Technical Field

[0001] The invention relates to the technical field of steel heat treatment, and in particular to a multi-step aging treatment method for improving the toughness of ultra-high strength steel ingots. Background Art

[0002] Ultra-high strength steels such as secondary hardening steel and maraging steel, such as AF1410, AerMet 100, M54 and C300 steel, are widely used in important load-bearing parts such as aerospace and automobiles due to their ultra-high strength and good toughness. After solution aging, these ultra-high strength steels form a lath martensite matrix and a small amount of austenite, with a large amount of M distributed on the martensite. 2 In order to meet the more extreme environmental applications of ultra-high strength steel, the content and strengthening ability of nano-precipitated phases are further increased, and higher strength steels such as AerMet310, AerMet340, NiAl and M 2 C composite strengthened secondary hardened ultra-high strength steel, etc. However, due to the increase in the content and density of the nano-precipitated phase, the risk factors in crack nucleation and expansion are further increased, resulting in a sharp decrease in the toughness of ultra-high strength steel, such as the impact toughness Akv of AerMet340 secondary hardened steel. 2 It is only 15J, which limits its application scenarios.

[0003] Reverse transformation of austenite is an important factor affecting the toughness of ultra-high strength steels such as secondary hardening steel and maraging steel. A large number of studies have found that the toughness of martensitic steel can be adjusted by introducing an appropriate proportion of metastable austenite soft phase into a strong martensitic matrix. The toughening mechanism of metastable austenite is largely affected by its mechanical stability. For mechanically stable austenite, it remains unchanged before the crack tip arrives, and can act as a soft phase to hinder crack propagation. In contrast, unstable austenite undergoes deformation-induced martensite before crack initiation, producing a transformation-induced plasticity (TRIP) effect, which causes energy absorption, volume expansion and local stress release, which is beneficial to the improvement of toughness. However, the formed martensite is hard and brittle, which is harmful to toughness. In general, film austenite nucleates between martensitic laths, has high mechanical stability, can deflect crack propagation, and is beneficial to improving toughness. The block austenite formed at the boundary of martensite bundle (block) and martensite group (packet) has poor stability and is transformed into brittle fresh martensite during the early deformation process. The brittle new martensite formed may introduce new positions for crack nucleation, accelerate crack propagation, and weaken the effect of metastable austenite in enhancing toughness. Therefore, it is crucial to increase the content of reversed austenite and control the morphology during the entire aging process. However, reversed austenite is usually formed during overaging, and the greater the degree of overaging, the higher the content of reversed austenite. This is usually accompanied by a significant coarsening of the precipitate phase, which is not conducive to the strength of ultra-high strength steel.

[0004] Due to the kinetic mismatch between precipitation and austenite reversal, aging treatment has a great influence on the reversal of austenite and NiAl, M 2 The influence of C precipitation is extremely complex, and it is difficult to ensure strength while improving the toughness of steel. In order to solve this dilemma, it is urgent to develop an aging treatment method to improve the toughness of ultra-high strength steel. Summary of the invention

[0005] The object of the present invention is to provide a multi-step aging treatment method for improving the toughness of ultra-high strength steel ingots, so as to solve the technical problem of poor toughness of existing ultra-high strength steels.

[0006] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0007] The present invention provides a multi-step aging treatment method for improving the toughness of an ultra-high strength steel ingot, comprising the following steps:

[0008] 1) Solution process: The steel ingot is C3 Solution treatment is carried out at temperatures above

[0009] 2) Quenching and deep freezing process: the steel ingot after solution treatment is cooled and deep-cryogenically treated in sequence to obtain a cooled steel ingot;

[0010] 3) Multi-step aging process: The cooled steel ingot is subjected to one or more steps of low-temperature aging treatment at 400-550°C, and then subjected to high-temperature aging treatment at 550-650°C, and an ultra-high strength steel ingot is obtained after cooling.

[0011] Further, the steel ingot comprises secondary hardening steel or maraging steel;

[0012] The chemical composition of the steel ingot includes: C: 0.08-0.30%, Co: 0-12.0%, Ni: 5.0-18.0%, Mo: 0.50-2.0%, Cr: 0.5-4.0%, Al: 0-3.0%, Ti: 0-3.0%, Nb: 0-0.50%, Si≤0.05%, Mn≤0.05%, S≤0.005%, P≤0.005%, O≤0.005%, N≤0.005%, and the rest is Fe.

[0013] Furthermore, the preparation method of the secondary hardened steel is:

[0014] After the raw materials are melted by vacuum induction melting and vacuum consumable remelting or electroslag remelting smelting process, they are kept at 1150-1250°C for 6-20 hours for homogenization treatment; then they are successively subjected to three-pier and three-draw forging, normalizing treatment and annealing treatment to obtain secondary hardened steel.

[0015] Furthermore, the temperature of the normalizing treatment is 1000-1050° C., and the time of the normalizing treatment is 1-2 hours; the temperature of the annealing treatment is 600-680° C., and the time of the annealing treatment is 8-20 hours.

[0016] Furthermore, the temperature of the solution treatment is 850-1100°C.

[0017] Furthermore, in the step 2), the cooling is water cooling or oil cooling;

[0018] The temperature of the deep freezing treatment is -150 to -73°C, and the time of the deep freezing treatment is 1 to 5 hours.

[0019] Furthermore, the low-temperature aging treatment time is 5 to 40 hours, and the high-temperature aging treatment time is 20 minutes to 3 hours.

[0020] Furthermore, after the low-temperature aging treatment, a deep cooling treatment is performed at -150 to -73°C, and then a high-temperature aging treatment is performed.

[0021] Furthermore, in step 3), the multi-step low-temperature aging treatment is two or more steps of low-temperature aging treatment.

[0022] Beneficial effects of the present invention:

[0023] (1) The present application provides a multi-step aging treatment method for improving the toughness of ultra-high strength steel. The aging process can obtain a higher content of film-like austenite and ensure that various precipitated phases such as carbides are maintained at a lower size and higher density.

[0024] (2) Compared with the conventional single aging process, the multi-step aging treatment method for improving the toughness of ultra-high strength steel provided in the present application significantly increases the toughness of the ultra-high strength steel while obtaining higher strength.

[0025] (3) The multi-step aging treatment method for improving the toughness of ultra-high strength steel provided in this application has the advantages of simple equipment and process flow, and is suitable for engineering applications such as medium and large structural parts. DETAILED DESCRIPTION

[0026] The present invention provides a multi-step aging treatment method for improving the toughness of an ultra-high strength steel ingot, comprising the following steps:

[0027] 1) Solution process: The steel ingot is C3 Solution treatment is carried out at temperatures above

[0028] 2) Quenching and deep freezing process: the steel ingot after solution treatment is cooled and deep-cryogenically treated in sequence to obtain a cooled steel ingot;

[0029] 3) Multi-step aging process: The cooled steel ingot is subjected to one or more steps of low-temperature aging treatment at 400-550°C, and then subjected to high-temperature aging treatment at 550-650°C, and an ultra-high strength steel ingot is obtained after cooling.

[0030] In the present invention, the temperature of the low-temperature aging treatment is preferably 450-510°C, more preferably 480-500°C; the temperature of the high-temperature aging treatment is preferably 580-620°C, more preferably 590-610°C.

[0031] In the present invention, the steel ingot comprises secondary hardening steel or maraging steel, preferably secondary hardening steel.

[0032] In the present invention, the chemical composition of the steel ingot comprises: C: 0.08-0.30%, Co: 0-12.0%, Ni: 5.0-18.0%, Mo: 0.50-2.0%, Cr: 0.5-4.0%, Al: 0-3.0%, Ti: 0-3.0%, Nb: 0-0.50%, Si≤0.05%, Mn≤0.05%, S≤0.005%, P≤0.005%, O≤0.005%, N≤0.005%, and the rest is Fe; preferably The chemical composition of the steel ingot includes: C: 0.1-0.25%, Co: 5.0-12.0%, Ni: 6.0-15.0%, Mo: 1.0-1.5%, Cr: 1.0-3.0%, Al: 1.0-3.0%, Ti: 0.015-3.0%, Nb: 0.05-0.50%, Si≤0.05%, Mn≤0.05%, S≤0.005%, P≤0.005%, O≤0.005%, N≤0.005%, and the rest is Fe.

[0033] In the present invention, the preparation method of the secondary hardened steel is:

[0034] After the raw materials are melted by vacuum induction melting and vacuum consumable remelting or electroslag remelting smelting process, they are kept at 1150-1250°C for 6-20 hours for homogenization treatment; then they are successively subjected to three-pier and three-draw forging, normalizing treatment and annealing treatment to obtain secondary hardened steel.

[0035] In the present invention, the temperature of the homogenization treatment is preferably 1200-1230°C, more preferably 1200°C.

[0036] In the present invention, the temperature of the normalizing treatment is 1000-1050°C, preferably 1010-1040°C, and more preferably 1020-1030°C; the time of the normalizing treatment is 1-2h, and preferably 1.5h; the temperature of the annealing treatment is 600-680°C, preferably 620-660°C, and more preferably 640-650°C; the time of the annealing treatment is 8-20h, preferably 10-18h, and more preferably 12-16h.

[0037] In the present invention, the A C3 The temperature is 780-850°C, and the temperature of the solution treatment is 850-1100°C, preferably 900-1050°C, and more preferably 950-1000°C.

[0038] In the present invention, the solution treatment substantially completely dissolves M 23 C 6 、M 7 C 3 、M 6 C carbides and maintain a smaller original austenite grain size.

[0039] In the present invention, in step 2), cooling is water cooling or oil cooling, preferably water cooling;

[0040] The cryogenic treatment temperature is -150 to -73°C, preferably -120 to -80°C, more preferably -100 to -90°C; the cryogenic treatment time is 1 to 5 hours, preferably 2 to 4 hours, more preferably 3 hours. In the present invention, the cryogenic treatment is to reduce the content of retained austenite as much as possible.

[0041] In the present invention, the low-temperature aging treatment time is 5 to 40 hours, preferably 6 to 25 hours, and more preferably 10 to 20 hours; the high-temperature aging treatment time is 20 minutes to 3 hours, preferably 40 minutes to 2 hours, and more preferably 1 to 2 hours.

[0042] In the present invention, the purpose of the first step of low temperature aging treatment is to form fine and high density M 2 C carbide precipitation phase and other precipitation phases such as NiAl; the purpose of the second step of high temperature aging treatment is to form reverse transformed austenite.

[0043] In the present invention, after the low-temperature aging treatment, a cryogenic treatment is performed at -150 to -73°C, and then a high-temperature aging treatment is performed. The temperature of the cryogenic treatment is preferably -120 to -80°C, and more preferably -100 to -90°C.

[0044] In the step 3), the multi-step low-temperature aging treatment is two or more steps of low-temperature aging treatment.

[0045] The present invention aims to increase the content of reverse transformed austenite, especially mechanically stable film austenite, so as to improve toughness; at the same time, ensure that the precipitated phase does not undergo significant coarsening, so as to ensure good strength. Finally, a good match between strength and toughness is achieved. The present invention provides a multi-step aging treatment method for improving the toughness of ultra-high strength steel, which is particularly suitable for secondary hardening ultra-high strength steel, maraging steel, etc.

[0046] The design concept of the present invention is as follows: after the secondary hardening steel or maraging steel is solution treated, it is oil quenched or water quenched to room temperature, and then cryogenically treated to reduce the content of residual austenite as much as possible to obtain a lath martensite matrix. During aging, the precipitate phase is fully precipitated through single or multiple low-temperature aging, and cryogenic treatment can be performed after each low-temperature aging to remove unstable austenite. After low-temperature aging, the content of residual austenite is relatively small. In the subsequent high-temperature aging, the diffusion of elements such as nickel that promote the reverse transformation of austenite is accelerated to promote the formation of film-like austenite. Compared with the traditional single aging treatment, the content of reverse transformation is significantly increased, especially the film-like austenite, and the size of the precipitate phase is not significantly coarsened.

[0047] The technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0048] Example 1

[0049] The chemical composition mass fraction of ultra-high strength steel is: C: 0.25%, Co: 5.0%, Ni: 11.0%, Mo: 1.5%, Cr: 3.0%, Al: 1.0%, Ti: 0.015%, Si≤0.05%, Mn≤0.05%, S≤0.005%, P≤0.005%, O≤0.005%, N≤0.005%, and the rest is Fe.

[0050] The prepared raw materials are subjected to vacuum induction melting + vacuum electroslag remelting smelting process. After melting, the riser is cut off the ingot, and it is kept at 1200℃ for 10 hours for homogenization treatment, and then three-pier and three-draw forging processing is carried out; the forged steel ingot is normalized and annealed, wherein the normalizing temperature is 1000℃, the normalizing time is 1h; the annealing temperature is 600℃, and the annealing time is 20h. Then the aging heat treatment is carried out:

[0051] (1) Solution treatment: The annealed ultra-high strength steel ingot is kept at 900°C for 1 hour for solution treatment;

[0052] (2) Quenching and deep cooling process: The steel ingot in step (1) is then oil-quenched to room temperature, then deep-cooled at -73°C for 1 h, and air-cooled to room temperature;

[0053] (3) Multi-step aging process: The steel ingot treated in step (2) is subjected to a first step of low-temperature aging at 482°C for 5 h, oil quenched to room temperature, then deep-cooled at -73°C for 1 h, subjected to a second step of high-temperature aging at 550°C for 3 h, and then air-cooled to room temperature to obtain an ultra-high strength steel ingot.

[0054] Comparison with the traditional single aging process: The steel ingot treated in step (2) was aged at 482°C for 5h and then air-cooled to room temperature to obtain an ultra-high strength steel ingot.

[0055] Three groups of samples were cut from the steel ingots prepared by multi-step aging heat treatment and the traditional single aging samples, and the mechanical properties were tested. The results are shown in Table 1.

[0056] Example 2

[0057] The chemical composition mass fraction of ultra-high strength steel is: C: 0.2%, Co: 5.0%, Ni: 11.0%, Mo: 1.5%, Cr: 3.0%, Al: 2.0%, Ti: 0.015%, Si≤0.05%, Mn≤0.05%, S≤0.005%, P≤0.005%, O≤0.005%, N≤0.005%, and the rest is Fe.

[0058] The prepared raw materials are subjected to vacuum induction melting + vacuum consumable remelting smelting process. After melting, the riser is cut off the ingot, and it is kept at 1200℃ for 10 hours for homogenization treatment, and then three-pier and three-draw forging is carried out; the forged steel ingot is normalized and annealed, wherein the normalizing temperature is 1000℃, the normalizing time is 1h; the annealing temperature is 600℃, and the annealing time is 20h. Then the aging heat treatment is carried out:

[0059] (1) Solution treatment: The annealed ultra-high strength steel ingot is kept at 920°C for 1 hour for solution treatment;

[0060] (2) Quenching and deep freezing process: The steel ingot in step (1) is then oil-quenched to room temperature, then deep-cooled at -73°C for 1 hour, and air-cooled to room temperature;

[0061] (3) Multi-step aging process: The steel ingot treated in step (2) is subjected to the first low-temperature aging at 465°C for 4 h, and then subjected to the second low-temperature aging at 465°C for 4 h, and oil quenched to room temperature. After each low-temperature aging, it is deep-cooled at -73°C for 1.5 h, and subjected to the second high-temperature aging at 580°C for 2 h, and then air-cooled to room temperature to obtain an ultra-high strength steel ingot.

[0062] Comparison with the traditional single aging process: The steel ingot treated in step (2) was aged at 465°C for 8h and then air-cooled to room temperature to obtain an ultra-high strength steel ingot.

[0063] Three groups of samples were cut from the steel ingots prepared by multi-step aging heat treatment and the single aging samples, respectively, and mechanical properties were tested. The results are shown in Table 1.

[0064] Example 3

[0065] The chemical composition mass fraction of ultra-high strength steel is: C: 0.25%, Co: 14.0%, Ni: 11.0%, Mo: 1.5%, Cr: 2.5%, Ti: 0.015%, Si≤0.05%, Mn≤0.05%, S≤0.005%, P≤0.005%, O≤0.005%, N≤0.005%, and the rest is Fe.

[0066] The prepared raw materials are subjected to vacuum induction melting + vacuum electroslag remelting smelting process. After melting, the riser is cut off the ingot, and it is kept at 1200℃ for 10 hours for homogenization treatment, and then three-pier and three-draw forging processing is carried out; the forged steel ingot is normalized and annealed, wherein the normalizing temperature is 1000℃, the normalizing time is 1h; the annealing temperature is 600℃, and the annealing time is 20h. Then the aging heat treatment is carried out:

[0067] (1) Solution treatment: The annealed ultra-high strength steel ingot is kept at 1000°C for 1 hour for solution treatment;

[0068] (2) Quenching and deep freezing process: The steel ingot in step (1) is then oil-quenched to room temperature, then deep-cooled at -73°C for 1 hour, and air-cooled to room temperature;

[0069] (3) Multi-step aging process: The steel ingot treated in step (2) is subjected to the first low-temperature aging at 450°C for 8 h, and then subjected to the second low-temperature aging at 450°C for 8 h, and oil quenched to room temperature. After each low-temperature aging, it is deep-cooled at -73°C for 1 h, and subjected to the second high-temperature aging at 600°C for 1 h, and then air-cooled to room temperature to obtain an ultra-high strength steel ingot.

[0070] Comparison with the traditional single aging process: The steel ingot treated in step (2) was aged at 450°C for 16 h, and then air-cooled to room temperature to obtain an ultra-high strength steel ingot.

[0071] Three groups of samples were cut from the steel ingots prepared by multi-step aging heat treatment and the single aging samples, respectively, and mechanical properties were tested. The results are shown in Table 1.

[0072] Table 1 Comparison of mechanical properties between the embodiments of the present invention and traditional single aging

[0073]

[0074]

[0075] It can be seen from the above embodiments that the present invention provides a multi-step aging treatment method for improving the toughness of ultra-high strength steel ingots. The aging process of the present invention can obtain a higher content of film-like austenite and ensure that various precipitation phases such as carbides are maintained at a lower size and a higher density. The treatment method of the present invention increases the content of reverse-transformed austenite, especially mechanically stable film-like austenite, and at the same time ensures that the precipitation phase does not undergo significant coarsening, thereby achieving a good strength-toughness match.

[0076] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A multi-step aging treatment method for improving the toughness of ultra-high strength steel ingots, characterized in that: The following steps are involved: 1) Solution process: The steel ingot is C3 Solution treatment is carried out at temperatures above 2) Quenching and deep freezing process: the steel ingot after solution treatment is cooled and deep-cryogenically treated in sequence to obtain a cooled steel ingot; 3) Multi-step aging process: The cooled steel ingot is subjected to one or more steps of low-temperature aging treatment at 400-550°C, and then subjected to high-temperature aging treatment at 550-650°C, and an ultra-high strength steel ingot is obtained after cooling.

2. The multi-step aging treatment method for improving the toughness of ultra-high strength steel ingots according to claim 1, characterized in that: The steel ingot comprises secondary hardening steel or maraging steel; The chemical composition of the steel ingot includes: C: 0.08-0.30%, Co: 0-12.0%, Ni: 5.0-18.0%, Mo: 0.50-2.0%, Cr: 0.5-4.0%, Al: 0-3.0%, Ti: 0-3.0%, Nb: 0-0.50%, Si≤0.05%, Mn≤0.05%, S≤0.005%, P≤0.005%, O≤0.005%, N≤0.005%, and the rest is Fe.

3. The multi-step aging treatment method for improving the toughness of ultra-high strength steel ingots according to claim 2, characterized in that: The preparation method of the secondary hardened steel is: After the raw materials are melted by vacuum induction melting and vacuum consumable remelting or electroslag remelting smelting process, they are kept at 1150-1250°C for 6-20 hours for homogenization treatment; then they are successively subjected to three-pier and three-draw forging, normalizing treatment and annealing treatment to obtain secondary hardened steel.

4. The multi-step aging treatment method for improving the toughness of ultra-high strength steel ingots according to claim 3, characterized in that: The temperature of the normalizing treatment is 1000-1050° C., and the time of the normalizing treatment is 1-2 hours; the temperature of the annealing treatment is 600-680° C., and the time of the annealing treatment is 8-20 hours.

5. The multi-step aging treatment method for improving the toughness of ultra-high strength steel ingots according to claim 1, characterized in that: The temperature of the solution treatment is 850-1100°C.

6. The multi-step aging treatment method for improving the toughness of ultra-high strength steel ingots according to claim 1 or 5, characterized in that: In the step 2), cooling is water cooling or oil cooling; The temperature of the deep freezing treatment is -150 to -73°C, and the time of the deep freezing treatment is 1 to 5 hours.

7. The multi-step aging treatment method for improving the toughness of ultra-high strength steel ingots according to any one of claims 1 to 5, characterized in that: The time of the low-temperature aging treatment is 5 to 40 hours, and the time of the high-temperature aging treatment is 20 minutes to 3 hours.

8. The multi-step aging treatment method for improving the toughness of ultra-high strength steel ingots according to claim 7, characterized in that: After the low-temperature aging treatment, a deep cooling treatment is performed at -150 to -73°C, and then a high-temperature aging treatment is performed.

9. The multi-step aging treatment method for improving the toughness of ultra-high strength steel ingots according to claim 1, characterized in that: In the step 3), the multi-step low-temperature aging treatment is two or more steps of low-temperature aging treatment.

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