Maraging steel and preparation method thereof

Abstract

The invention relates to maraging steel and a preparation method thereof, relates to the technical field of material preparation, and is mainly used for solving the technical problem that existing large-size maraging steel is insufficient in strength and toughness. According to the main technical scheme, the microstructure of the maraging steel has an ultra-fine grain complex phase structure; wherein the ultra-fine grain complex phase structure comprises an ultra-fine grain martensite phase and an ultra-fine grain austenite phase; wherein in the ultra-fine grain complex phase structure, the ultra-fine grain martensite phase and the ultra-fine grain austenite phase are distributed in an interlaced manner; the average grain size of the ultra-fine grain austenite phase ranges from 0.3 micrometer to 1.0 micrometer. The effective grain size of the ultra-fine grain martensite phase ranges from 0.4 micrometer to 1.2 micrometers. The method is mainly used for improving the strength and toughness of the large-size maraging steel.

Description

Technical Field

[0001] This invention relates to the field of materials technology, and in particular to a martensitic aging steel and its preparation method. Background Technology

[0002] Large-size ultra-high-strength steel components used in high-end equipment often operate under extreme stress environments such as high fatigue and high impact, and their comprehensive performance level determines the safety and effectiveness limits of the equipment during service. Taking typical components such as high-speed / hyper-speed wind tunnel balances, landing gear and shafts of advanced aircraft as examples, transonic airflow and strong impact landings place higher demands on the strength and toughness of ultra-high-strength steel.

[0003] Although there are currently steels with ultra-high strength of 2.2 GPa and high toughness of 15 J (AKV value) or above, they are still in the laboratory-level small-scale state. However, due to the "size effect", as the effective cross-sectional size of the component increases, the characteristic microstructure is difficult to fully develop, resulting in a continuous decline in toughness and plasticity. For components with large cross-sectional characteristics, the intrinsic performance will be difficult to meet the requirements of service conditions, which seriously restricts the transformation and upgrading of equipment.

[0004] Therefore, there is an urgent need to develop large-size steel with ultra-high strength and high toughness to improve the manufacturing capability of large components for high-end equipment. Summary of the Invention

[0005] In view of this, the present invention provides a martensitic aging steel and a method for preparing the same, the main purpose of which is to improve the strength and toughness of large-sized steel.

[0006] To achieve the above objectives, the present invention mainly provides the following technical solutions:

[0007] On one hand, embodiments of the present invention provide a maraging steel, wherein the microstructure of the maraging steel has an ultrafine-grained multiphase structure; wherein the ultrafine-grained multiphase structure includes an ultrafine-grained martensite phase and an ultrafine-grained austenite phase; wherein,

[0008] In the ultrafine-grained multiphase microstructure: the ultrafine-grained martensite phase and the ultrafine-grained austenite phase are interwoven and distributed;

[0009] The average grain size of the ultrafine-grained austenite phase is 0.3 μm-1.0 μm; the effective grain size of the ultrafine-grained martensite phase is 0.4 μm-1.2 μm.

[0010] Preferably, in the martensitic aging steel, the volume content of the ultrafine-grained austenite phase is 5-50%.

[0011] Preferably, in the martensitic aging steel, the volume content of the ultrafine-grained austenite phase is 5-30%.

[0012] Preferably, the average grain size of the ultrafine austenitic phase is 0.3 μm-0.8 μm.

[0013] Preferably, the effective grain size of the ultrafine-grained martensite phase is 0.4 μm-1.0 μm.

[0014] Preferably, the matrix structure of the martensitic aging steel includes a fine-grained martensite phase and the ultrafine-grained multiphase structure; wherein the effective grain size of the fine-grained martensite phase is 1.5μm-3μm.

[0015] Preferably, at any location between the center and 1 / 4 of the equivalent thickness of the martensitic aging steel, the area distribution ratio of the ultrafine-grained multiphase structure is 40% or more.

[0016] Preferably, at any location between the center and 1 / 4 of the equivalent thickness of the martensitic aging steel, the area distribution ratio of the ultrafine-grained multiphase structure is 50% or more.

[0017] Preferably, the martensitic aging steel has a single weight of over 120 kg and an effective cross-sectional size of 40 mm-150 mm.

[0018] Preferably, the martensitic aging steel comprises the following chemical composition by weight percentage:

[0019] C ≤ 0.01%; Ni: 17.0-18.5 wt%; Co: 11.0-12.5 wt%; Mo: 4.5-5.5 wt%; Ti: 0.9-1.1 wt%; S ≤ 0.005%; P ≤ 0.01%; O ≤ 0.0020%; N ≤ 0.0015%; balance is Fe and unavoidable impurities.

[0020] Preferably, the room temperature mechanical properties of the martensitic aging steel are as follows: yield strength ≥ 2.1 GPa, tensile strength ≥ 2.2 GPa, elongation after fracture ≥ 7%, reduction of area ≥ 50%, and V-notch impact energy 15 J-25 J.

[0021] In another aspect, embodiments of the present invention provide a method for preparing martensitic aging steel as described in any of the above claims, characterized in that it includes the following steps:

[0022] Solution heat treatment steps: The steel is subjected to solution heat treatment to obtain steel after solution heat treatment;

[0023] Cryogenic treatment step: The steel after solution heat treatment is subjected to cryogenic treatment to obtain cryogenically treated steel;

[0024] Repeated solution treatment step: The deep cryogenically treated steel is subjected to repeated solution treatment to obtain steel with repeated solution treatment;

[0025] Aging treatment step: The steel that has undergone repeated solution treatment is subjected to aging treatment to obtain martensitic aged steel.

[0026] Preferably, the steel is a forged material.

[0027] Preferably, in the solution heat treatment step: the steel is heated to the solution heat treatment temperature of 800℃-900℃ at a heating rate of ≥50℃ / h, held at that temperature for ts, and then cooled to obtain the solution heat-treated steel; preferably, ts> 1.5Db / 100; where Db is the effective cross-sectional dimension of the steel in mm; and ts is in h.

[0028] Preferably, in the solution heat treatment step, the cooling method is water cooling to room temperature after the product is removed from the furnace.

[0029] Preferably, in the cryogenic treatment step: the steel after solution heat treatment is cooled to ≤-160℃ at a cooling rate of 1-5℃ / min for cryogenic treatment; after cryogenic treatment, it is warmed back to room temperature to obtain the cryogenically treated steel; wherein, the cryogenic treatment time tc>1.5Db / 100; Db is the effective cross-sectional dimension of the steel in mm; tc is in h.

[0030] Preferably, in the repeated solution treatment step: the cryogenically treated steel is subjected to 2-5 solution treatments; wherein each solution treatment step is as follows: the steel is placed in a heat treatment furnace at a temperature below 200℃, heated to the solution treatment temperature of 850℃±20℃ at a heating rate of ≥5℃ / min, and held at that temperature for 40min-60min; after holding, the steel is removed from the furnace and cooled to room temperature at a cooling rate of ≥50℃ / min. Preferably, in the aging treatment step: the steel after repeated solution treatment is heated to 500℃±20℃ at a heating rate of ≥50℃ / h, held at that temperature for 60min-180min, and then cooled to room temperature to obtain martensitic aging steel.

[0031] Preferably, in the aging process step, water cooling is selected as the cooling method.

[0032] Compared with the prior art, the martensitic aging steel and its preparation method according to the embodiments of the present invention have at least the following beneficial effects:

[0033] On one hand, embodiments of the present invention provide a martensitic aging steel, wherein the microstructure of the martensitic aging steel has an ultrafine-grained multiphase structure; wherein the ultrafine-grained multiphase structure includes an ultrafine-grained martensite phase and an ultrafine-grained austenite phase; wherein in the ultrafine-grained multiphase structure: the ultrafine-grained martensite phase and the ultrafine-grained austenite phase are interwoven and distributed; the average grain size of the ultrafine-grained austenite phase is 0.3μm-1.0μm; and the effective grain size of the ultrafine-grained martensite phase is 0.4μm-1.2μm. The following explanation is needed regarding the above scheme: By customizing a specific proportion of multi-level multiphase structure in martensitic aging steel, the present invention can achieve the following effects: 1) When impact cracks propagate between adjacent grains, the direction will be deflected, and the ultrafine grain or fine grain structure will significantly increase the deflection frequency, thereby improving the crack propagation resistance; 2) The ultrafine grain austenite phase is relatively soft and interwoven with the ultrafine grain martensite phase. This ultrafine grain austenite phase can act as a "soft barrier" to passivate cracks; 3) Under impact, the austenite phase will transform into the martensite phase, and the latter will expand in volume. The resulting compressive stress can offset the tensile stress generated during crack propagation, thereby improving its propagation resistance. Therefore, the martensitic aging steel provided by this invention can increase the V-notch impact absorption energy of 2.2GPa grade large cross-section ultra-high strength steel to 20J-25J and above without damaging precipitation strengthening, through multiple toughening mechanisms such as reducing stress concentration and inhibiting crack nucleation and propagation. This is 2-3 times the impact toughness of existing steel of the same strength grade, and can expand the high-performance preparation route of large-size ultra-high strength steel components.

[0034] On the other hand, embodiments of the present invention provide a method for preparing martensitic aging steel, which includes the following steps: performing solution heat treatment on steel to obtain solution heat-treated steel; performing deep cryogenic treatment on the solution heat-treated steel to obtain deep cryogenically treated steel; performing repeated solution treatment on the deep cryogenically treated steel to obtain repeatedly solution-treated steel; and performing aging treatment on the repeatedly solution-treated steel to obtain martensitic aging steel. The following points should be noted regarding the above scheme: 1) The microstructure is homogenized through solution heat treatment, allowing the alloying elements to fully dissolve into the matrix; 2) Deep cryogenic treatment further promotes the transformation of residual austenite to martensite after solution cooling, resulting in a fully martensitic microstructure; 3) This invention further uses repeated solution treatment above the critical heating (austenitization) rate and critical cooling (martenitization) rate to prevent austenite grain growth and promote the subsequent martensite nucleation rate, thereby refining the martensite grains and rapidly martensite, which can improve the stability of supercooled austenite and obtain an ultrafine residual austenitic microstructure; 4) Finally, through aging treatment, nanoscale precipitates (Ni3Ti and Mo-rich phases) are formed in the matrix, which can improve the strength of the matrix and ensure high toughness without affecting the distribution of ultrafine grains and fine grain structure. Furthermore, by designing a solution treatment and aging process and strictly controlling parameters such as heating and cooling rates and holding time, this invention customizes a multi-level multiphase microstructure with a specific ratio, resulting in large-section ultra-high-strength and high-toughness steel with better strength and toughness matching. Its room temperature mechanical properties are: yield strength ≥ 2.1 GPa, tensile strength ≥ 2.2 GPa, elongation after fracture ≥ 7%, reduction of area ≥ 50%, and V-notch impact energy ≥ 15 J-25 J.

[0035] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it in accordance with the contents of the specification, the preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings. Attached Figure Description

[0036] Figure 1 This is a metallographic microstructure image of a large-sized martensitic aging steel (bar) sample prepared in Example 1; wherein, Figure 1 The multiples of (a) and (b) are different; the sample was taken from the position between the center of the heat-treated bar and 1 / 2 radius.

[0037] Figure 2 This is an electron backscattering diffraction pattern of the ultrafine-grained multiphase microstructure (ultrafine-grained martensite phase + ultrafine-grained austenite phase) in a large-size martensitic aging steel (bar) sample prepared in Example 1; wherein, Figure 2 Figure (a) is a grain structure diagram, and (b) is a phase distribution diagram; the sample was taken from the center of the heat-treated bar to a position between 1 / 2 radius.

[0038] Figure 3 This is a metallographic microstructure image of a large-sized martensitic aging steel (bar) sample prepared in Example 2; wherein, Figure 3 The multiples of (a) and (b) are different; the sample was taken from the position between the center of the heat-treated bar and 1 / 2 radius.

[0039] Figure 4 This is an electron backscattering diffraction pattern of the ultrafine-grained multiphase microstructure (ultrafine-grained martensite phase + ultrafine-grained austenite phase) in a large-size martensitic aging steel (bar) sample prepared in Example 2; wherein, Figure 4 Figure (a) is a grain structure diagram, and (b) is a phase distribution diagram; the sample was taken from the center of the heat-treated bar to a position between 1 / 2 radius.

[0040] Figure 5 This is a metallographic microstructure image of a large-size martensitic aging steel (plate) sample prepared in Example 3; wherein, Figure 5 The magnifications of Figures (a) and (b) are different; the sample was taken from the center of the heat-treated bar to a position between 1 / 4 of the equivalent thickness.

[0041] Figure 6 This is an electron backscattering diffraction pattern of the ultrafine-grained multiphase microstructure (ultrafine-grained martensite phase + ultrafine-grained austenite phase) in a large-size martensitic aging steel (plate) sample prepared in Example 3; wherein, Figure 6 Figure (a) is a grain structure diagram, and (b) is a phase distribution diagram; the sample was taken from the center of the heat-treated bar to a position between 1 / 4 of the equivalent thickness.

[0042] Figure 7 This is a metallographic microstructure image of a large-size martensitic aging steel (plate) sample prepared in Example 4; wherein, Figure 7 The magnifications of Figures (a) and (b) are different; the sample was taken from the center of the heat-treated bar to a position between 1 / 4 of the equivalent thickness.

[0043] Figure 8 This is an electron backscattering diffraction pattern of the ultrafine-grained multiphase microstructure (ultrafine-grained martensite phase + ultrafine-grained austenite phase) in a large-size martensitic aging steel (plate) sample prepared in Example 4; wherein, Figure 8 Figure (a) is a grain structure diagram, and (b) is a phase distribution diagram; the sample was taken from the center of the heat-treated bar to a position between 1 / 4 of the equivalent thickness.

[0044] Figure 9 This is a metallographic microstructure image of a large-sized martensitic aging steel (bar) sample prepared in Example 5; wherein, Figure 9The multiples of (a) and (b) are different; the sample was taken from the position between the center of the heat-treated bar and 1 / 2 radius.

[0045] Figure 10 This is an electron backscattering diffraction pattern of the ultrafine-grained multiphase microstructure (ultrafine-grained martensite phase + ultrafine-grained austenite phase) in a large-size martensitic aging steel (bar) sample prepared in Example 5; wherein, Figure 10 Figure (a) is a grain structure diagram, and (b) is a phase distribution diagram; the sample was taken from the center of the heat-treated bar to a position between 1 / 2 radius.

[0046] Figure 11 This is a metallographic microstructure image of a large-sized martensitic aging steel (plate) sample prepared in Comparative Example 1; among which, Figure 11 The magnifications of Figures (a) and (b) are different; the sample was taken from the center of the heat-treated bar to a position between 1 / 4 of the equivalent thickness.

[0047] Figure 12 This is the electron backscattering diffraction pattern of the fine-grained martensite matrix in a large-sized martensitic aging steel (plate) sample prepared in Comparative Example 1; where, Figure 12 Figure (a) is a grain structure diagram, and (b) is a phase distribution diagram; the sample was taken from the center of the heat-treated bar to a position between 1 / 4 of the equivalent thickness.

[0048] Figure 13 This is a schematic diagram showing the sampling locations (grid areas) of the steel samples in all embodiments and comparative examples; wherein, Figure 13 Figure (a) shows the sampling location for the bar stock, and Figure (b) shows the sampling location for the plate stock. Detailed Implementation

[0049] To further illustrate the technical means and effects adopted by the present invention to achieve the intended purpose, the specific embodiments, structures, features, and effects according to the present invention will be described in detail below with reference to the accompanying drawings and preferred embodiments. In the following description, different "embodiments" or "embodiments" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form.

[0050] On one hand, embodiments of the present invention provide a martensitic aging steel, wherein the microstructure of the martensitic aging steel has an ultrafine-grained multiphase structure; wherein the ultrafine-grained multiphase structure includes an ultrafine-grained martensite phase and an ultrafine-grained austenite phase; wherein in the ultrafine-grained multiphase structure: the ultrafine-grained martensite phase and the ultrafine-grained austenite phase are interwoven and distributed (the ultrafine-grained martensite phase and the ultrafine-grained austenite phase are synergistically associated and have an interactive texture); the average grain size of the ultrafine-grained austenite phase is 0.3μm-1.0μm; and the effective grain size of the ultrafine-grained martensite phase is 0.4μm-1.2μm.

[0051] Preferably, in the martensitic aging steel, the content of the ultrafine-grained austenite phase is 5%-50%, more preferably 5%-30%. Preferably, the average grain size of the ultrafine-grained austenite phase is 0.3μm-0.8μm. The effective grain size of the ultrafine-grained martensite phase is 0.4μm-1.0μm.

[0052] The matrix structure of the heat-treated steel includes a fine-grained martensite phase and an ultrafine-grained multiphase structure; wherein the effective grain size of the fine-grained martensite phase is 1.5μm-3μm.

[0053] Specifically, at any location between the center and 1 / 4 of the equivalent thickness of the heat-treated steel, the distribution ratio of the ultrafine-grained multiphase structure is 40% or more, preferably 50% or more. It should be noted that: See [link to relevant documentation]. Figure 13 As shown, in cylindrical steel, the "center to 1 / 4 equivalent thickness" can be understood as the annular portion from the center of the circular cross-section to half the radius; in plate-shaped steel, the center to 1 / 4 equivalent thickness is the portion from one-quarter to three-quarters of the thickness.

[0054] The martensitic aging steel has a single weight of over 120 kg and an effective cross-sectional size of 40 mm to 150 mm.

[0055] The martensitic aging steel comprises the following chemical components by mass percentage:

[0056] C ≤ 0.01%; Ni: 17.0-18.5 wt%; Co: 11.0-12.5 wt%; Mo: 4.5-5.5 wt%; Ti: 0.9-1.1 wt%; S ≤ 0.005%; P ≤ 0.01%; O ≤ 0.0020%; N ≤ 0.0015%; balance is Fe and unavoidable impurities.

[0057] On the other hand, embodiments of the present invention provide a method for preparing the martensitic aging steel, comprising the following steps:

[0058] Solution heat treatment steps: The steel is subjected to solution heat treatment to obtain the steel after solution heat treatment.

[0059] The steel is heated to a solution heat treatment temperature of 800℃-900℃ and held at that temperature for ts. After cooling, the steel is obtained after solution heat treatment; ts>1.5Db / 100; where Db is the effective cross-sectional dimension of the steel in mm; and ts is in h. The cooling method is water cooling from the furnace to room temperature.

[0060] The steel is a forged material, such as bar or plate.

[0061] Cryogenic treatment step: The steel after solution heat treatment is subjected to cryogenic treatment to obtain cryogenically treated steel.

[0062] In this step: the steel after solution heat treatment is cooled to ≤-160℃ at a cooling rate of 1℃ / min-5℃ / min for cryogenic treatment; wherein, the cryogenic treatment time tc is determined to be >1.5Db / 100; Db is the effective cross-sectional dimension of the steel in mm; tc is in h.

[0063] Repeated solution treatment step: The deep-cryogenically treated steel is subjected to repeated solution treatment to obtain steel with repeated solution treatment.

[0064] In this step, the cryogenically treated steel is subjected to 2 to 5 solution treatments. The steps for each solution treatment are as follows: the steel is placed in a heat treatment furnace at a temperature below 200°C, and heated to the solution treatment temperature of 850±20°C at a heating rate of ≥5°C / min, and held for 40-60 minutes (the holding time is calculated after the furnace temperature is uniform); after the holding time is completed, the steel is removed from the furnace and cooled to room temperature at a cooling rate of ≥50°C / min.

[0065] Aging treatment step: The steel that has undergone repeated solution treatment is subjected to aging treatment to obtain martensitic aged steel.

[0066] In this step, the steel that has undergone repeated solution treatment is heated to 500℃±20℃, held at that temperature for 60min-180min, and then cooled to room temperature to obtain martensitic aging steel; preferably, water cooling is used as the cooling method.

[0067] Hereinafter, the above-mentioned solution of the present invention is described as follows:

[0068] 1) This invention uses a method of "directional construction" of specific structures to develop large-size steel with ultra-high strength and high toughness.

[0069] 2) By customizing a multi-level complex structure with a specific ratio, this invention can increase the V-notch impact absorption energy of 2.2GPa grade large cross-section ultra-high strength steel to 20J-25J and above without damaging precipitation strengthening, through multiple toughening mechanisms such as reducing stress concentration and inhibiting crack nucleation and propagation. This is 2 to 3 times the impact toughness of existing steel of the same strength grade, and can expand the high-performance preparation route of large-size ultra-high strength steel components.

[0070] 3) This invention, through optimized design of the solution treatment and aging process, and strict control of parameters such as heating and cooling rates and holding time, customized a multi-stage multiphase microstructure with a specific ratio, resulting in large-section ultra-high-strength and high-toughness steel with better strength and toughness matching. Its room temperature mechanical properties are: yield strength ≥ 2.1 GPa, tensile strength ≥ 2.2 GPa, elongation after fracture ≥ 7%, reduction of area ≥ 50%, and V-notch impact energy ≥ 15 J-25 J.

[0071] The present invention will be further illustrated below with specific embodiments:

[0072] Example 1

[0073] This embodiment prepares a large-size maraging steel (80mm diameter bar, 130kg weight), wherein, by mass percentage, the maraging steel comprises the following chemical composition: C: 0.008wt%; Ni: 17.0wt%; Co: 12.0wt%; Mo: 4.5wt%; Ti: 0.9wt%; S: 0.004wt%; P: 0.008wt%; O: 0.0020wt%; N: 0.0015wt%; the balance being Fe and unavoidable impurities;

[0074] The method for preparing martensitic aging steel in this embodiment includes the following steps:

[0075] Solution heat treatment: A large steel bar weighing 130 kg and with a specification of Φ80 mm is loaded into a pit-type heating furnace and heated from room temperature to 850°C at a heating rate of 3°C / min (i.e., 180°C / h). The temperature is held at this temperature for 90 min. After the holding period, the steel bar is transferred to a water-cooling tank for water cooling to room temperature to obtain the solution heat-treated steel bar.

[0076] Cryogenic treatment: The solution heat-treated bars are transferred to an ultra-low temperature cryogenic furnace and cooled to -177°C at a cooling rate of 4°C / min. The temperature is then held at this temperature for 2 hours. After the holding period, the bars are lifted out and warmed to room temperature to obtain cryogenically treated steel.

[0077] Repeated solution treatment: The cryogenically treated steel is transferred into a heating furnace at a temperature of 90°C. The temperature is then increased to 850°C at a rate of 15°C / min and held at this temperature for 40 minutes. After holding, the steel is water-cooled to room temperature. During the water cooling process, the cooling rate must be controlled at ≥50°C / min. This completes the first solution treatment step.

[0078] After the furnace temperature drops to 150°C, the steel is put back into the furnace and the solution treatment is repeated twice with the same procedures and parameters as the first solution treatment to obtain steel that has undergone repeated solution treatment.

[0079] Aging treatment: The steel after repeated solution treatment is transferred to an aging heat treatment furnace, heated from room temperature to 500℃ at a rate of 100℃ / h, and held at this temperature for 60 minutes. After the holding period, it is cooled to room temperature by water to obtain martensitic aging steel.

[0080] In Example 1, after repeated solution treatment and aging treatment, a dual-mode composite matrix structure composed of an ultrafine-grained multiphase structure (dark area) and a fine-grained martensitic phase (α′) (bright area) can be observed at any position between the center and half the radius of the martensitic aged steel (see Example 1). Figure 1 (As shown). The proportion of the ultrafine-grained multiphase structure (dark area) and the fine-grained martensite phase was determined by the grayscale difference between them. The ultrafine-grained multiphase structure accounted for 52.5%, while the fine-grained martensite phase accounted for 47.5%. The effective grain size of the fine-grained martensite phase was 2.36 μm. The ultrafine-grained multiphase structure consisted of ultrafine-grained martensite phase + ultrafine-grained austenite phase (α′+γ).

[0081] Figure 2 The internal structure of the ultrafine-grained multiphase microstructure (yellow dashed area) is further shown, which consists of an interwoven distribution of ultrafine austenite (green) and ultrafine martensite (red). The average grain size of the ultrafine austenite phase was determined to be 0.55 μm and the effective grain size of the ultrafine martensite phase was determined to be 0.98 μm using Channel 5 software.

[0082] X-ray diffraction analysis revealed that the volume content of ultrafine-grained austenite phase in the martensitic aging steel was 5.7%.

[0083] The properties of the martensitic aging steel prepared in this embodiment are shown in Table 1.

[0084] Table 1 Mechanical properties of martensitic aging steel in Example 1

[0085]

[0086] Example 2

[0087] This embodiment prepares a large-size maraging steel (bar stock with a diameter of 120 mm and a weight of 230 kg), wherein, by mass percentage, the maraging steel comprises the following chemical composition: C: 0.01%; Ni: 18.5 wt%; Co: 12.5 wt%; Mo: 5.0 wt%; Ti: 0.9 wt%; S: 0.005%; P: 0.009%; O: 0.0020%; N: 0.0014%; the balance being Fe and unavoidable impurities.

[0088] The method for preparing martensitic aging steel in this embodiment includes the following steps:

[0089] Solution heat treatment: A large steel bar weighing 230 kg and with a specification of Φ120 mm is loaded into a pit-type heating furnace and heated from room temperature to 860°C at a rate of 2°C / min. The temperature is held at this temperature for 120 min. After the holding period, the bar is transferred to a water-cooling tank for water cooling to room temperature to obtain the solution heat-treated steel.

[0090] Cryogenic treatment: The solution heat-treated bars are transferred to an ultra-low temperature cryogenic furnace and cooled to -177°C at a cooling rate of 2°C / min. The temperature is then maintained at this temperature for 2.5 hours. After the holding period, the bars are lifted out and warmed to room temperature to obtain cryogenically treated steel.

[0091] Repeated solution treatment: The cryogenically treated steel is transferred into a heating furnace at a temperature of 86°C. The temperature is then increased to 860°C at a rate of 10°C / min and held at this temperature for 45 minutes. After holding, the steel is water-cooled to room temperature. During the water-cooling process, the cooling rate must be controlled at ≥50°C / min. This completes the first solution treatment step.

[0092] After the furnace temperature drops to 120°C, the steel is put back into the furnace and the solution treatment is repeated three times using the same procedures and parameters as the first solution treatment, resulting in steel that has undergone repeated solution treatment.

[0093] Aging treatment: The steel after repeated solution treatment is transferred to an aging heat treatment furnace, heated from room temperature to 500℃ at a heating rate of 100℃ / h, and held at this temperature for 150min. After the holding period, it is cooled to room temperature by water to obtain martensitic aging steel.

[0094] In Example 2, after repeated solution treatment and aging treatment, a dual-mode composite matrix structure composed of an ultrafine-grained multiphase structure (dark area) and a fine-grained martensitic phase (α′) (bright area) can be observed at any location between the center and half the radius of the martensitic aged steel (see Example 2). Figure 3(As shown). The proportion of the ultrafine-grained multiphase structure (dark area) and the fine-grained martensite phase was determined by the grayscale difference between them. The ultrafine-grained multiphase structure accounted for 74.8%, while the fine-grained martensite phase accounted for 25.2%. The effective grain size of the fine-grained martensite phase was 2.21 μm. The ultrafine-grained multiphase structure consisted of ultrafine-grained martensite phase + ultrafine-grained austenite phase (α′ + γ).

[0095] Figure 4 The internal structure of the ultrafine-grained multiphase microstructure (yellow dashed area) is further shown, which is composed of an interwoven distribution of ultrafine austenite phase (green) and ultrafine martensite phase (red). The average grain size of the ultrafine austenite phase was determined to be 0.75 μm and the effective grain size of the ultrafine martensite was 0.54 μm using Channel 5 software.

[0096] X-ray diffraction analysis revealed that the volume content of ultrafine-grained austenite phase in the martensitic aging steel was 21.4%.

[0097] The properties of the martensitic aging steel prepared in this embodiment are shown in Table 2.

[0098] Table 2 Mechanical properties of martensitic aging steel in Example 2

[0099]

[0100] Example 3

[0101] This embodiment prepares a large-size maraging steel (plate with a diameter of 80 mm and a weight of 810 kg), wherein, by mass percentage, the maraging steel comprises the following chemical composition: C: 0.009 wt%; Ni: 18.1 wt%; Co: 11.0 wt%; Mo: 5.5 wt%; Ti: 1.1 wt%; S: 0.005 wt%; P: 0.008 wt%; O: 0.0020 wt%; N: 0.0015 wt%; the balance being Fe and unavoidable impurities.

[0102] The method for preparing martensitic aging steel in this embodiment includes the following steps:

[0103] Solution heat treatment: A large steel plate weighing 810 kg and with a thickness of 80 mm is loaded into a pit-type heating furnace and heated from room temperature to 900°C at a heating rate of 2°C / min. The temperature is held at this temperature for 180 min. After the holding period, the plate is transferred to a water-cooling tank for water cooling to room temperature to obtain the steel after solution heat treatment.

[0104] Cryogenic treatment: The solution heat-treated bars are transferred to an ultra-low temperature cryogenic furnace and cooled to -177°C at a cooling rate of 1.5°C / min. The temperature is then held at this temperature for 3 hours. After the holding period, the bars are lifted out and warmed to room temperature to obtain cryogenically treated steel.

[0105] Repeated solution treatment: The cryogenically treated steel is transferred into a heating furnace at a temperature of 80°C. The temperature is then increased to 870°C at a rate of 10°C / min and held at this temperature for 60 minutes. After holding, the steel is water-cooled to room temperature. During the water-cooling process, the cooling rate must be controlled at ≥50°C / min. This completes the first solution treatment step.

[0106] After the furnace temperature drops to 130°C, the steel is put back into the furnace and the solution treatment is repeated three times using the same procedures and parameters as the first solution treatment, resulting in steel that has undergone repeated solution treatment.

[0107] Aging treatment: The steel after repeated solution treatment is transferred to an aging heat treatment furnace, heated from room temperature to 520℃ at a heating rate of 100℃ / h, and held at this temperature for 90 minutes. After the holding period, it is cooled to room temperature by water to obtain martensitic aging steel.

[0108] In Example 3, after repeated solution treatment and aging treatment, a dual-mode composite matrix structure composed of an ultrafine-grained multiphase structure (dark area) and a fine-grained martensitic phase (α′) (bright area) can be observed at any location between the center and 1 / 4 of the equivalent thickness of the martensitic aged steel (see Example 3). Figure 5 As shown in the figure. The proportion of the ultrafine-grained multiphase structure (dark area) and the fine-grained martensite phase was determined by the grayscale difference between them. The ultrafine-grained multiphase structure accounted for 57.7%, while the fine-grained martensite phase accounted for 42.3%. The effective grain size of the fine-grained martensite phase was 1.98 μm. The ultrafine-grained multiphase structure consisted of ultrafine-grained martensite phase + ultrafine-grained austenite phase (α′+γ).

[0109] Figure 6 The internal structure of the ultrafine-grained multiphase microstructure (yellow dashed area) is further shown, which is composed of an interwoven distribution of ultrafine austenite phase (green) and ultrafine martensite phase (red). The average grain size of the ultrafine austenite phase was determined to be 0.69 μm and the effective grain size of the ultrafine martensite phase was 0.89 μm using Channel 5 software.

[0110] X-ray diffraction analysis revealed that the volume content of ultrafine-grained austenite phase in the martensitic aging steel was 6.2%.

[0111] The properties of the martensitic aging steel prepared in this embodiment are shown in Table 3.

[0112] Table 3 Mechanical properties of martensitic aging steel in Example 3

[0113]

[0114] Example 4

[0115] This embodiment prepares a large-size maraging steel (plate with a diameter of 40 mm and a weight of 450 kg), wherein, by mass percentage, the maraging steel comprises the following chemical composition: C: 0.01 wt%; Ni: 17.8 wt%; Co: 11.9 wt%; Mo: 4.9 wt%; Ti: 0.9 wt%; S: 0.004 wt%; P: 0.009 wt%; O: 0.0015 wt%; N: 0.0015 wt%; the balance being Fe and unavoidable impurities.

[0116] The method for preparing martensitic aging steel in this embodiment includes the following steps:

[0117] Solution heat treatment: A large steel plate weighing 450 kg and 40 mm thick is loaded into a pit-type heating furnace and heated from room temperature to 800°C at a heating rate of 3°C / min. The temperature is held at this temperature for 150 min. After the holding period, the plate is transferred to a water cooling tank for water cooling to room temperature to obtain the steel after solution heat treatment.

[0118] Cryogenic treatment: The solution heat-treated bars are transferred to an ultra-low temperature cryogenic furnace and cooled to -177°C at a cooling rate of 2°C / min. The temperature is then held at this temperature for 2 hours. After the holding period, the bars are lifted out and warmed to room temperature to obtain cryogenically treated steel.

[0119] Repeated solution treatment: The cryogenically treated steel is transferred into a heating furnace at a temperature of 83°C. The temperature is then increased to 830°C at a rate of 12°C / min and held at this temperature for 50 minutes. After holding, the steel is water-cooled to room temperature. During the water-cooling process, the cooling rate must be controlled to be ≥50°C / min. This completes the first solution treatment step.

[0120] After the furnace temperature drops to 110°C, the steel is put back into the furnace and the solution treatment is repeated twice with the same procedures and parameters as the first solution treatment to obtain steel that has undergone repeated solution treatment.

[0121] Aging treatment: The steel after repeated solution treatment is transferred to an aging heat treatment furnace, heated from room temperature to 500℃ at a heating rate of 100℃ / h, and held at this temperature for 90 minutes. After the holding period, it is cooled to room temperature by water to obtain martensitic aging steel.

[0122] In Example 4, after repeated solution treatment and aging treatment, a dual-mode composite matrix structure composed of an ultrafine-grained multiphase structure (dark area) and a fine-grained martensitic phase (α′) (bright area) can be observed at any location between the center and 1 / 4 of the equivalent thickness of the martensitic aged steel (see Example 4). Figure 7 As shown in the figure. The proportion of the ultrafine-grained multiphase structure (dark area) and the fine-grained martensite phase was determined by the grayscale difference between them. The ultrafine-grained multiphase structure accounted for 61.6%, while the fine-grained martensite phase accounted for 38.4%. The effective grain size of the fine-grained martensite phase was 2.17 μm. The ultrafine-grained multiphase structure consisted of ultrafine-grained martensite phase + ultrafine-grained austenite phase (α′ + γ).

[0123] Figure 8 The internal structure of the ultrafine-grained multiphase microstructure (yellow dashed area) is further shown, which is composed of an interwoven distribution of ultrafine austenite phase (green) and ultrafine martensite phase (red). The average grain size of the ultrafine austenite phase was determined to be 0.61 μm and the effective grain size of the ultrafine martensite phase was 0.80 μm using Channel5 software.

[0124] X-ray diffraction tests were used to determine that the volume content of austenite phase in the martensitic aging steel was 5.3%.

[0125] The properties of the martensitic aging steel prepared in this embodiment are shown in Table 4.

[0126] Table 4 Mechanical properties of martensitic aging steel in Example 4

[0127]

[0128] Example 5

[0129] This embodiment prepares a large-size maraging steel (bar stock with a diameter of 150 mm and a weight of 520 kg), wherein, by mass percentage, the maraging steel comprises the following chemical composition: C: 0.01 wt%; Ni: 17.0 wt%; Co: 11.0 wt%; Mo: 5.5 wt%; Ti: 1.0 wt%; S: 0.003 wt%; P: 0.01 wt%; O: 0.0020 wt%; N: 0.0015 wt%; the balance being Fe and unavoidable impurities.

[0130] The method for preparing martensitic aging steel in this embodiment includes the following steps:

[0131] Solution heat treatment: A large steel bar weighing 520 kg and with a specification of Φ150 mm is loaded into a pit-type heating furnace and heated from room temperature to 850°C at a heating rate of 1.5°C / min. The temperature is held at this temperature for 150 min. After the holding period, the bar is transferred to a water cooling tank for water cooling to room temperature to obtain the steel after solution heat treatment.

[0132] Cryogenic treatment: The solution heat-treated bars are transferred to an ultra-low temperature cryogenic furnace and cooled to -177°C at a cooling rate of 2°C / min. The temperature is then maintained at this temperature for 3 hours. After the holding period, the bars are lifted out and warmed to room temperature to obtain cryogenically treated steel.

[0133] Repeated solution treatment: The cryogenically treated steel is transferred into a heating furnace at a temperature of 92°C. The temperature is then increased to 860°C at a rate of 12°C / min and held at this temperature for 60 minutes. After holding, the steel is water-cooled to room temperature. During the water-cooling process, the cooling rate must be controlled at ≥50°C / min. This completes the first solution treatment step.

[0134] After the furnace temperature drops to 135°C, the steel is put back into the furnace and the solution treatment is repeated three times using the same procedures and parameters as the first solution treatment, resulting in steel that has undergone repeated solution treatment.

[0135] Aging treatment: The steel after repeated solution treatment is transferred to an aging heat treatment furnace, heated from room temperature to 480℃ at a heating rate of 100℃ / h, and held at this temperature for 180min. After the holding period, it is cooled to room temperature by water to obtain martensitic aging steel.

[0136] In Example 1, after repeated solution treatment and aging treatment, a dual-mode composite matrix structure composed of an ultrafine-grained multiphase structure (dark area) and a fine-grained martensitic phase (α′) (bright area) can be observed at any position between the center and half the radius of the martensitic aged steel (see Example 1). Figure 9 As shown in the figure. The proportion of the ultrafine-grained multiphase structure (dark area) and the fine-grained martensite phase was determined by the grayscale difference between them. The ultrafine-grained multiphase structure accounted for 53.7%, while the fine-grained martensite phase accounted for 46.3%. The effective grain size of the fine-grained martensite phase was 2.27 μm. The ultrafine-grained multiphase structure consisted of ultrafine-grained martensite phase + ultrafine-grained austenite phase (α′+γ).

[0137] Figure 10The internal structure of the ultrafine-grained multiphase microstructure (yellow dashed area) is further shown, which consists of an interwoven distribution of ultrafine austenite (green) and ultrafine martensite (red). The average grain size of the ultrafine austenite phase was determined to be 0.57 μm and the effective grain size of the ultrafine martensite phase was 0.86 μm using Channel 5 software.

[0138] X-ray diffraction tests were used to determine that the volume content of austenite phase in the martensitic aging steel was 13.2%.

[0139] The properties of the martensitic aging steel prepared in this embodiment are shown in Table 5.

[0140] Table 5 Mechanical property results of Example 5

[0141]

[0142] Comparative Example 1

[0143] Comparative Example 1 prepared a large-size maraging steel (plate with a diameter of 80 mm and a weight of 810 kg), wherein, by mass percentage, the maraging steel comprises the following chemical composition: C: 0.009 wt%; Ni: 18.1 wt%; Co: 11.0 wt%; Mo: 5.5 wt%; Ti: 1.1 wt%; S: 0.005 wt%; P: 0.008 wt%; O: 0.0020 wt%; N: 0.0015 wt%; the balance being Fe and unavoidable impurities.

[0144] The preparation method of the martensitic aging steel in Comparative Example 1 includes the following steps:

[0145] Solution heat treatment: A large steel plate weighing 810 kg and with a thickness of 80 mm is loaded into a pit-type heating furnace and heated from room temperature to 870°C at a heating rate of 2°C / min. The temperature is held at this temperature for 180 min. After the holding period, the plate is transferred to a water-cooling tank for water cooling to room temperature to obtain the steel after solution heat treatment.

[0146] Cryogenic treatment: The solution heat-treated bars are transferred to an ultra-low temperature cryogenic furnace and cooled to -177°C at a cooling rate of 1.5°C / min. The temperature is then held at this temperature for 3 hours. After the holding period, the bars are lifted out and warmed to room temperature to obtain cryogenically treated steel.

[0147] Solution treatment: The cryogenically treated steel is transferred into a heating furnace at a temperature of 80°C. The temperature is then increased to 850°C at a rate of 10°C / min and held at this temperature for 60 minutes. After the holding period, the steel is cooled to room temperature at a rate of 50°C / min to obtain the solution-treated steel.

[0148] Aging treatment: The solution-treated steel is transferred to an aging heat treatment furnace and heated from room temperature to 500℃ at a heating rate of 100℃ / h. The temperature is then held at this temperature for 90 minutes. After the holding period, the steel is water-cooled to room temperature to obtain martensitic aged steel.

[0149] In Comparative Example 1, no ultrafine-grained multiphase structure (dark areas) could be observed at any location between the center and 1 / 4 of the equivalent thickness of the martensitic aging steel (see [reference]). Figure 11 (As shown).

[0150] Figure 12 It can be further seen that the martensitic aging steel sample of Comparative Example 1 is basically composed of fine-grained martensite.

[0151] X-ray diffraction tests were used to determine that the volume content of ultrafine-grained austenite phase in martensitic aging steel was 0%.

[0152] The properties of the martensitic aging steel prepared in Comparative Example 1 are shown in Table 6. Although its strength is similar to that of Examples 1-5, its impact energy is 50% of that of the above examples.

[0153] Table 6 Mechanical property results of Comparative Example 1

[0154]

[0155] The reason why the martensitic aging steel prepared in Comparative Example 1 has poor impact energy compared with the embodiments is that, since Comparative Example 1 only undergoes one solution treatment, the driving force for the transformation of austenite to martensite during cooling is higher, the nucleation rate is faster, the growth region is larger, and the final martensite grain size is larger. At the same time, the martensite transformation is complete, and no residual austenite is formed. Therefore, it is impossible to construct an ultrafine-grained multiphase structure, and thus the toughening mechanism brought about by the ultrafine-grained multiphase structure in the embodiments of the present invention cannot be achieved.

[0156] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A martensitic aging steel, characterized in that, The microstructure of the martensitic aging steel exhibits an ultrafine-grained multiphase structure; wherein the ultrafine-grained multiphase structure comprises an ultrafine-grained martensite phase and an ultrafine-grained austenite phase; wherein... In the ultrafine-grained multiphase microstructure: the ultrafine-grained martensite phase and the ultrafine-grained austenite phase are interwoven and distributed; The average grain size of the ultrafine-grained austenite phase is 0.3 μm-1.0 μm; the effective grain size of the ultrafine-grained martensite phase is 0.4 μm-1.2 μm.

2. The martensitic aging steel according to claim 1, characterized in that, In the martensitic aging steel, the volume content of the ultrafine-grained austenite phase is 5%-50%.

3. The martensitic aging steel according to claim 2, characterized in that, In the martensitic aging steel, the volume content of the ultrafine-grained austenite phase is 5%-30%.

4. The martensitic aging steel according to claim 1, characterized in that, The average grain size of the ultrafine austenitic phase is 0.3 μm-0.8 μm.

5. The martensitic aging steel according to claim 1, characterized in that, The effective grain size of the ultrafine-grained martensite phase is 0.4μm-1.0μm.

6. The martensitic aging steel according to claim 1, characterized in that, The matrix structure of the martensitic aging steel includes a fine-grained martensite phase and an ultrafine-grained multiphase structure; wherein... The effective grain size of the fine-grained martensite phase is 1.5μm-3μm.

7. The martensitic aging steel according to claim 1, characterized in that, At any location between the center and 1 / 4 of the equivalent thickness of the martensitic aging steel, the area distribution ratio of the ultrafine-grained multiphase structure is more than 40%.

8. The martensitic aging steel according to claim 7, characterized in that, At any location between the center and 1 / 4 of the equivalent thickness of the martensitic aging steel, the area distribution ratio of the ultrafine-grained multiphase structure is more than 50%.

9. The martensitic aging steel according to claim 1, characterized in that, The martensitic aging steel has a unit weight of over 120 kg and an effective cross-sectional size of 40 mm-150 mm.

10. The martensitic aging steel according to claim 1, characterized in that, The martensitic aging steel comprises the following chemical composition by weight percentage: C≤0.01%; Ni: 17.0-18.5wt%; Co: 11.0-12.5wt%; Mo: 4.5-5.5wt%; Ti: 0.9-1.1wt%; S≤0.005%; P≤0.01%; O≤0.0020%; N≤0.0015%; balance is Fe and unavoidable impurities.

11. The martensitic aging steel according to claim 1, characterized in that, The room temperature mechanical properties of the martensitic aging steel are as follows: yield strength ≥ 2.1 GPa, tensile strength ≥ 2.2 GPa, elongation after fracture ≥ 7%, reduction of area ≥ 50%, and V-notch impact energy 15 J-25 J.

12. The method for preparing martensitic aging steel according to any one of claims 1-10, characterized in that, It includes the following steps: Solution heat treatment steps: The steel is subjected to solution heat treatment to obtain steel after solution heat treatment; Cryogenic treatment step: The steel after solution heat treatment is subjected to cryogenic treatment to obtain cryogenically treated steel; Repeated solution treatment step: The deep cryogenically treated steel is subjected to repeated solution treatment to obtain steel with repeated solution treatment; Aging treatment step: The steel that has undergone repeated solution treatment is subjected to aging treatment to obtain martensitic aged steel.

13. The method for preparing martensitic aging steel according to claim 12, characterized in that, The steel is a forged material.

14. The method for preparing martensitic aging steel according to claim 12, characterized in that, In the solution heat treatment step: The steel is heated to a solution heat treatment temperature of 800℃-900℃ at a heating rate of ≥50℃ / h, held at that temperature for ts, and then cooled to obtain the solution-treated steel; preferably, ts 1.5Db / 100; where Db is the effective cross-sectional dimension of the steel in mm; ts is in h.

15. The method for preparing martensitic aging steel according to claim 14, characterized in that, In the solution heat treatment step, the cooling method selected is water cooling to room temperature after the product is taken out of the furnace.

16. The method for preparing martensitic aging steel according to claim 12, characterized in that, In the cryogenic treatment step: the steel after solution heat treatment is cooled to ≤-160℃ at a cooling rate of 1℃ / min-5℃ / min for cryogenic treatment; after cryogenic treatment, it is warmed back to room temperature to obtain the cryogenically treated steel; wherein, the cryogenic treatment time tc 1.5Db / 100; Db is the effective cross-sectional dimension of the steel, in mm; tc is in h.

17. The method for preparing martensitic aging steel according to claim 12, characterized in that, In the repeated solution treatment steps: The cryogenically treated steel is subjected to 2 to 5 solution treatments. The steps for each solution treatment are as follows: the steel is placed in a heat treatment furnace at a temperature below 200°C and heated to the solution treatment temperature of 850°C±20°C at a heating rate of ≥5°C / min, and held at that temperature for 40-60 minutes. After the holding time is completed, the steel is removed from the furnace and cooled to room temperature at a cooling rate of ≥50°C / min.

18. The method for preparing martensitic aging steel according to claim 12, characterized in that, In the aging process: the steel after repeated solution treatment is heated to 500℃±20℃ at a heating rate of ≥50℃ / h, held for 60min-180min, and then cooled to room temperature to obtain martensitic aging steel.

19. The method for preparing martensitic aging steel according to claim 18, characterized in that, In the aging process, water cooling is selected as the cooling method.

Images