Preparation method of high-strength and high-plasticity duplex stainless steel with multiple heterostructures

Abstract

The invention belongs to the technical field of metal material modification, and particularly relates to a preparation method of high-strength and high-plasticity duplex stainless steel with multiple heterostructures. According to the method, through multi-stage thermal-mechanical composite treatment, a multi-heterostructure is constructed in duplex stainless steel, and combination of high strength and high plasticity is achieved. The method specifically comprises the following steps: regulating austenite proportion through high-temperature solution treatment; fine island-shaped austenite is separated out from a ferrite matrix through high-temperature aging treatment, and phase composition heterogeneity is formed; high-density dislocation and deformation energy storage are introduced in multi-pass deformation treatment; the rolled material is partially or completely recrystallized through short-time high-temperature annealing, and structure heterogeneity with heterogeneous grain sizes is formed; through low-temperature long-time aging treatment in a miscible gap temperature range, a nano-scale Fe-rich region and a nano-scale Cr-rich region are formed in ferrite, namely, a nano-scale structure is separated, and nano-scale chemical heterogeneity is introduced. The tensile strength of the prepared duplex stainless steel is improved by 50% or above compared with that of a commercial hot rolling state, and the total elongation is kept to be 22% or above.

Description

Technical Field

[0001] This invention belongs to the field of metal material modification technology, specifically relating to a method for preparing high-strength, high-plasticity, multi-heterogeneous duplex stainless steel. Background Technology

[0002] In fields such as petrochemicals, nuclear power, and marine engineering, materials are used for extended periods in harsh environments involving high temperatures, high pressures, and strong corrosion. As the requirements for equipment reliability and safety in these fields continue to increase, the demand for structural materials possessing high strength, high ductility, and excellent corrosion resistance is becoming increasingly urgent. 2507 duplex stainless steel, composed of nearly equal volume fractions of ferrite and austenite phases, combines the high strength and good resistance to stress corrosion cracking of ferritic stainless steel with the excellent toughness and corrosion resistance of austenitic stainless steel. It is considered one of the ideal materials for meeting the requirements of extreme service environments and is widely used in marine engineering, petrochemical storage and transportation equipment, pressure vessels, and nuclear power pipelines. However, with increasingly demanding service conditions, traditional duplex stainless steel still has limitations in terms of overall performance. Typically, cold deformation can effectively improve the strength of 2507 duplex stainless steel, but it significantly reduces its ductility, work hardening rate, and ductility. Although commercially available hot-rolled 2507 duplex stainless steel has good plasticity, its strength is lower than that of cold-rolled duplex stainless steel, making it difficult to meet the needs of higher-strength industrial applications.

[0003] In recent years, the concept of heterogeneous structure design has become an important research direction in materials science. Compared with homogeneous materials, heterogeneous structures can significantly improve the strength of materials through heterogeneous deformation-induced strengthening effects, while alleviating local stress concentration during deformation, thus balancing strength and ductility. It has been reported that Ming Chen et al. developed a method to introduce a bimodal heterogeneous structure into 2507 duplex stainless steel to improve its strength-ductility balance. After treatment with this method, the tensile strength of the stainless steel reached 894 MPa, and the total elongation was 36.4%. However, the strength of the treated duplex stainless steel is still significantly lower than that of cold-struck 2507 duplex stainless steel (1100 MPa–1350 MPa), which cannot meet the needs of industrial applications requiring higher strength (Chen M, Li J, Liu H, et al. Achievement of high strength-ductility combination inaustenitic and ferritic duplex stainless steel by heterogeneous deformation[J]. Journal of Materials Research and Technology, 2022, 21: 943-950). Meanwhile, Chinese invention patent CN111944973A discloses a method for preparing heterogeneous layered duplex stainless steel. This method first involves high-temperature homogenization annealing of hot-rolled duplex stainless steel to obtain initial duplex stainless steel, followed by deep cold rolling to form a lamellar ultrafine-grained structure, effectively enhancing its strength. Finally, it undergoes incomplete recrystallization annealing at 950–1100℃ for 1–3 minutes to prepare the heterogeneous layered duplex stainless steel. While this method significantly improves the strength of duplex stainless steel, the resulting duplex stainless steel exhibits significantly lower plasticity than commercially available hot-rolled stainless steel. Therefore, developing a method that can significantly improve the strength of duplex stainless steel while maintaining good plasticity is an important problem that urgently needs to be solved. Summary of the Invention

[0004] To overcome the shortcomings of the prior art, the present invention provides a method for preparing high-strength, high-plasticity, multi-heterogeneous duplex stainless steel. This method uses multi-stage thermo-mechanical composite treatment to simultaneously construct phase composition heterogeneity, grain size heterogeneity, and chemical heterogeneity in the material, enabling duplex stainless steel to achieve a better combination of strength and plasticity, and meeting the requirements of high strength, high plasticity, and corrosion resistance in extreme service environments.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] This invention provides a method for preparing high-strength, high-plasticity, multi-heterostructure duplex stainless steel, the method comprising the following steps:

[0007] S1. High-temperature solution treatment: Place duplex stainless steel at 1300–1380℃ for 20–60 min and then water-cool it to obtain duplex stainless steel with a low austenite ratio.

[0008] S2. High-temperature aging treatment: The duplex stainless steel in S1 is placed at 1050–1200℃ and held for 10–30 minutes. After water cooling, fine-sized island-shaped austenite is further precipitated on the ferrite matrix.

[0009] S3, High Deformation Rolling Treatment: The duplex stainless steel in S2 is subjected to multiple deformation treatments until the total reduction rate is 70-95%.

[0010] S4. High-temperature short-time annealing treatment: The duplex stainless steel in S3 is subjected to a heat treatment of 0.5–5 min at 1050–1200℃. After water cooling, the austenite ratio and the grain size of austenite and ferrite can be adjusted.

[0011] S5. Low-temperature long-term aging treatment: The duplex stainless steel in S4 is placed at 350–500℃ for 10–1000h and then water-cooled to induce the formation of phase-separated nanostructures inside the ferrite, resulting in high-strength, high-plasticity duplex stainless steel with multiple heterogeneous structures.

[0012] Preferably, the duplex stainless steel is industrial grade 2507 duplex stainless steel.

[0013] In the preparation method provided by this invention, the austenite ratio is controlled through a first-stage high-temperature solution treatment, providing the necessary conditions for the subsequent precipitation of small-sized island austenite. A second-stage high-temperature aging treatment induces the precipitation of fine-sized island austenite, forming phase composition heterogeneity. A third-stage high-deformation rolling introduces high-density dislocations, substructures, and deformation energy storage, providing the necessary conditions for material recrystallization. A fourth-stage short-time high-temperature annealing treatment and the difference in recrystallization kinetics between austenite and ferrite introduce grain size heterogeneity within the duplex stainless steel. The final stage, aging treatment within the miscibility gap temperature range, introduces phase-separated nanostructures into the ferrite, thereby forming a nanoscale chemical heterogeneous structure. These multiple heterogeneous structures can achieve a tensile strength increase of over 50% in 2507 duplex stainless steel while maintaining a total elongation of over 22% through the synergistic effect of different mechanisms such as heterogeneous structure-induced strengthening, solution strengthening, dislocation strengthening, grain boundary strengthening, phase boundary strengthening, and phase separation strengthening.

[0014] Preferably, the high deformation rolling process described in S3 is carried out using a room temperature synchronous cold rolling method, with a reduction of 0.5%–20% per pass, without cracking.

[0015] Preferably, the water cooling described in S1, S2, S4, and S5 is water cooling to room temperature.

[0016] Preferably, the phase-separated nanostructure formed inside the ferrite in S5 has a characteristic wavelength between 2 and 20 nm and a composition amplitude between 10 and 60 at.%.

[0017] Preferably, the austenite content in the duplex stainless steel obtained by S1 is 10%–35%.

[0018] Preferably, the duplex stainless steel obtained by S2 contains 30%–45% austenite, and the average austenite grain size is 1–20 μm.

[0019] Preferably, the duplex stainless steel obtained by S4 contains 30%–65% austenite and the average grain size of ferrite is 2–50 times that of austenite.

[0020] Compared with the prior art, the beneficial effects of the present invention are:

[0021] This invention discloses a method for preparing high-strength, high-plasticity, multi-heterogeneous duplex stainless steel. The method includes: controlling the austenite ratio of duplex stainless steel through high-temperature solution treatment; precipitating fine island-like austenite within the ferrite matrix through high-temperature aging treatment to form microstructural heterogeneity; introducing high-density dislocations and deformation energy storage through intense plastic deformation via multi-pass deformation treatment; partially or completely recrystallizing the rolled material through short-time high-temperature annealing treatment to produce significant differences in grain size between austenite and ferrite, forming grain size heterogeneity; and promoting the formation of nanoscale Fe-rich and Cr-rich regions within the ferrite matrix through low-temperature long-term aging treatment within the miscibility gap temperature range, i.e., phase-separated nanostructure, introducing nanoscale chemical heterogeneity. This invention employs a multi-stage thermo-mechanical composite treatment to simultaneously construct multiple heterogeneous structures within the material, including phase composition heterogeneity (austenite and ferrite phases), grain size heterogeneity (difference in average grain size between austenite and ferrite), and chemical heterogeneity (phase-separated nanostructure), enabling duplex stainless steel to achieve an excellent combination of high strength and high ductility. Compared to commercially available hot-rolled duplex stainless steel, the multi-heterogeneous duplex stainless steel prepared using this method exhibits a tensile strength increase of over 50% and a total elongation maintained above 22%. Therefore, the proposed method for preparing multi-heterogeneous duplex stainless steel significantly enhances its strength while retaining its excellent ductility, effectively improving the strength-ductility matching performance of duplex stainless steel and positively promoting its further industrial applications. Furthermore, the method of this invention offers strong process controllability; the heat treatment and rolling processes used are mature industrial techniques, suitable for mass production and easily scalable for industrialization. Attached Figure Description

[0022] Figure 1 A schematic diagram of the preparation process for high-strength, high-plasticity, multi-heterogeneous duplex stainless steel.

[0023] Figure 2 The diagram shows the microstructure of the duplex stainless steel after the high-temperature solution treatment in step (1) of Examples 1-4 (in the left figure, red represents austenite and blue represents ferrite. The proportion of austenite phase is determined by evaluating the area of ​​the red part in the whole figure; in the right figure, different colored blocks represent different grains, and the color represents the grain orientation. The grain size of austenite and ferrite is obtained by measuring the average value of all austenite and ferrite grains in the figure).

[0024] Figure 3 The diagram shows the microstructure of the duplex stainless steel after the high-temperature aging treatment in step (2) of Examples 1-4 (in the left figure, red represents austenite and blue represents ferrite. The proportion of austenite phase is determined by evaluating the area of ​​the red part in the whole figure; in the right figure, different colored blocks represent different grains, and the color represents the grain orientation. The grain size of austenite and ferrite is obtained by measuring the average value of all austenite and ferrite grains in the figure).

[0025] Figure 4 The diagram shows the microstructure of the initial hot-rolled duplex stainless steel in Comparative Example 3 (in the left image, red represents austenite and blue represents ferrite; the austenite phase ratio is determined by evaluating the area occupied by the red portion of the entire image; in the right image, different colored blocks represent different grains, and the color represents the grain orientation; the grain size of austenite and ferrite is obtained by measuring the average value of all austenite and ferrite grains in the image).

[0026] Figure 5 The diagram shows the microstructure of the duplex stainless steel corresponding to Comparative Example 2 (in the left image, red represents austenite and blue represents ferrite; the proportion of austenite phase is determined by evaluating the area occupied by the red portion of the entire image; in the right image, different colored blocks represent different grains, and the color represents the grain orientation; the grain size of austenite and ferrite is obtained by measuring the average value of all austenite and ferrite grains in the image).

[0027] Figure 6 This is a schematic diagram of the microstructure of the multi-heterogeneous duplex stainless steel corresponding to Example 1 (in the left image, red represents austenite and blue represents ferrite, and the proportion of austenite phase is determined by evaluating the area occupied by the red part in the whole image; in the right image, different colored blocks represent different grains, and the color represents the grain orientation. The grain size of austenite and ferrite is obtained by measuring the average value of all austenite and ferrite grains in the image).

[0028] Figure 7This is a schematic diagram of the microstructure of the multi-heterogeneous duplex stainless steel corresponding to Example 2 (in the left image, red represents austenite and blue represents ferrite, and the proportion of austenite phase is determined by evaluating the area occupied by the red part of the whole image; in the right image, different colored blocks represent different grains, and the color represents the grain orientation. The grain size of austenite and ferrite is obtained by measuring the average value of all austenite and ferrite grains in the image).

[0029] Figure 8 This is a schematic diagram of the microstructure of the multi-heterogeneous duplex stainless steel corresponding to Example 3 (in the left figure, red represents austenite and blue represents ferrite, and the proportion of austenite phase is determined by evaluating the area of ​​the red part in the whole figure; in the right figure, different colored blocks represent different grains, and the color represents the grain orientation. The grain size of austenite and ferrite is obtained by measuring the average value of all austenite and ferrite grains in the figure).

[0030] Figure 9 This is a schematic diagram of the microstructure of the multi-heterogeneous duplex stainless steel corresponding to Example 4 (in the left image, red represents austenite and blue represents ferrite, and the proportion of austenite phase is determined by evaluating the area of ​​the red part in the whole image; in the right image, different colored blocks represent different grains, and the color represents the grain orientation. The grain size of austenite and ferrite is obtained by measuring the average value of all austenite and ferrite grains in the image).

[0031] Figure 10 This is a schematic diagram of the phase separation nanostructure in duplex stainless steel corresponding to Comparative Example 2.

[0032] Figure 11 The figures show the engineering stress-strain mechanical property curves of the multi-heterogeneous duplex stainless steels corresponding to Examples 1-4 and Comparative Examples 1-4. Detailed Implementation

[0033] The specific embodiments of the present invention will be further described below. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0034] Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods, and the experimental materials used in the following embodiments are all available through conventional commercial channels.

[0035] like Figure 1 As shown, this invention, taking industrial-grade 2507 duplex stainless steel as an example, provides a method for preparing high-strength, high-ductility, multi-heterogeneous duplex stainless steel. The method includes the following steps:

[0036] (1) High temperature solution treatment: The duplex stainless steel is placed at 1300–1380℃ for 20–60 min and then cooled with water to obtain duplex stainless steel with a low austenite ratio.

[0037] (2) High temperature aging treatment: The duplex stainless steel in S1 is placed at 1050–1200℃ for 10–30 min and then water-cooled to further precipitate small island-shaped austenite on the ferrite matrix.

[0038] (3) High deformation rolling treatment: The duplex stainless steel in S2 is subjected to multiple deformation treatments until the total reduction rate is 70-95%.

[0039] (4) High temperature short time annealing treatment: The duplex stainless steel in S3 is subjected to a heat treatment of 0.5–5 min at 1050–1200℃. After water cooling, the austenite ratio and the grain size of austenite and ferrite can be adjusted.

[0040] (5) Low-temperature long-term aging treatment: The duplex stainless steel in S4 is kept at 350–500℃ for 10–1000h and then cooled with water to induce the formation of phase-separated nanostructures inside the ferrite, thus obtaining high-strength and high-plasticity multi-heterogeneous duplex stainless steel.

[0041] To fully and clearly present the technical solution and significant advantages of the present invention, the present invention will be described and characterized in detail below with reference to specific embodiments and comparative examples.

[0042] Example 1:

[0043] This embodiment provides a method for preparing multi-heterogeneous duplex stainless steel, specifically including the following steps:

[0044] (1) High-temperature solution treatment: Industrial-grade 2507 duplex stainless steel hot-rolled plate was selected and solution treated at 1330℃ for 30 min, followed by water cooling to room temperature to obtain duplex stainless steel with a low austenite ratio. The microstructure of the obtained duplex stainless steel was analyzed using backscattered electron diffraction (EBSD), and its austenite content was approximately 12.7%. Figure 2 The content is relatively low.

[0045] (2) High-temperature aging treatment: The solution-treated duplex stainless steel was heated to 1050℃, held for 20 min, and then water-cooled to room temperature, further precipitating fine-sized island-like austenite on the ferrite matrix. The microstructure of the obtained duplex stainless steel was analyzed using EBSD, revealing a significantly increased austenite content of approximately 39.2%, with an average austenite grain size of approximately 6.0 μm. Figure 3 );

[0046] (3) Multi-pass high deformation rolling treatment: multi-pass rolling is carried out by room temperature synchronous cold rolling, with a total reduction rate of 90% and a reduction of 5% per pass;

[0047] (4) Short-time high-temperature annealing: The rolled duplex stainless steel was heated to 1050℃, held for 2 minutes, and then water-cooled to room temperature. This step further adjusted the austenite ratio and the grain size of austenite and ferrite. The resulting duplex stainless steel had an austenite content of approximately 43.5%, an average austenite grain size of approximately 4.1 μm, and an average ferrite grain size of approximately 21.9 μm. The average ferrite grain size was approximately 5.3 times the average austenite grain size. Figure 6 );

[0048] (5) Low-temperature long-term aging treatment (i.e., aging treatment within the temperature range of the miscibility gap): After holding at 450℃ for 100h and water cooling to room temperature, phase separation nanostructure is induced to form inside the ferrite, resulting in a multi-heterogeneous dual-phase stainless steel sample. The characteristic wavelength of the phase separation nanostructure is 8.24nm and the composition amplitude is 19.6at.% (Table 1).

[0049] Example 2:

[0050] This embodiment provides a method for preparing multi-heterogeneous duplex stainless steel, specifically including the following steps:

[0051] (1) High-temperature solution treatment: Industrial-grade 2507 duplex stainless steel hot-rolled plate was selected and solution treated at 1330℃ for 30 min, followed by water cooling to room temperature to obtain duplex stainless steel with a low austenite ratio. The microstructure of the obtained duplex stainless steel was analyzed using EBSD, and its austenite content was approximately 12.7%. Figure 2 The content is relatively low.

[0052] (2) High-temperature aging treatment: The solution-treated duplex stainless steel was heated to 1050℃, held for 20 min, and then water-cooled to room temperature, further precipitating fine-sized island-like austenite on the ferrite matrix. The microstructure of the obtained duplex stainless steel was analyzed using EBSD, revealing a significantly increased austenite content of approximately 39.2%, with an average austenite grain size of approximately 6.0 μm. Figure 3 );

[0053] (3) Multi-pass high deformation rolling treatment: multi-pass rolling is carried out by room temperature synchronous cold rolling, with a total reduction rate of 90% and a reduction of 2% per pass;

[0054] (4) Short-time high-temperature annealing: The rolled duplex stainless steel was heated to 1050℃, held for 5 minutes, and then water-cooled to room temperature. This step further adjusted the austenite ratio and the grain size of austenite and ferrite. The resulting duplex stainless steel had an austenite content of approximately 61.9%, an average austenite grain size of approximately 4.4 μm, an average ferrite grain size of approximately 10.9 μm, and an average ferrite grain size approximately 2.5 times the average austenite grain size. Figure 7 );

[0055] (5) Low-temperature long-term aging treatment: After holding at 450℃ for 100h and water cooling to room temperature, phase-separated nanostructures were induced to form inside the ferrite, resulting in a multi-heterogeneous duplex stainless steel sample. The characteristic wavelength of the phase-separated nanostructure was 8.39nm, and the composition amplitude was 22.92at.% (Table 1).

[0056] Example 3:

[0057] This embodiment provides a method for preparing multi-heterogeneous duplex stainless steel, specifically including the following steps:

[0058] (1) High-temperature solution treatment: Industrial-grade 2507 duplex stainless steel hot-rolled plate was selected and solution treated at 1330℃ for 30 min, followed by water cooling to room temperature to obtain duplex stainless steel with a low austenite ratio. The microstructure of the obtained duplex stainless steel was analyzed using EBSD, and its austenite content was approximately 12.7%. Figure 2 The content is relatively low.

[0059] (2) High-temperature aging treatment: The solution-treated duplex stainless steel was heated to 1050℃, held for 20 min, and then water-cooled to room temperature, further precipitating fine-sized island-like austenite on the ferrite matrix. The microstructure of the obtained duplex stainless steel was analyzed using EBSD, revealing a significantly increased austenite content of approximately 39.2%, with an average austenite grain size of approximately 6.0 μm. Figure 3 );

[0060] (3) Multi-pass high deformation rolling treatment: multi-pass rolling is carried out by room temperature synchronous cold rolling, with a total reduction rate of 90% and a reduction of 5% per pass;

[0061] (4) Short-time high-temperature annealing: The rolled duplex stainless steel was heated to 1100℃, held for 1 min, and then water-cooled to room temperature. This step further adjusted the austenite ratio and the grain size of austenite and ferrite. The duplex stainless steel obtained in this step had an austenite content of approximately 41.7%, an average austenite grain size of approximately 3.8 μm, an average ferrite grain size of approximately 28.6 μm, and an average ferrite grain size of approximately 7.5 times the average austenite grain size. Figure 8 );

[0062] (5) Low-temperature long-term aging treatment: After holding at 450℃ for 100h and water cooling to room temperature, phase-separated nanostructures were induced to form inside the ferrite, resulting in a multi-heterogeneous duplex stainless steel sample. The characteristic wavelength of the phase-separated nanostructure was 7.91nm, and the composition amplitude was 23.16at.% (Table 1).

[0063] Example 4:

[0064] This embodiment provides a method for preparing multi-heterogeneous duplex stainless steel, specifically including the following steps:

[0065] (1) High-temperature solution treatment: Industrial-grade 2507 duplex stainless steel hot-rolled plate was selected and solution treated at 1330℃ for 30 min, followed by water cooling to room temperature to obtain duplex stainless steel with a low austenite ratio. The microstructure of the obtained duplex stainless steel was analyzed using EBSD, and its austenite content was approximately 12.7%. Figure 2 The content is relatively low.

[0066] (2) High-temperature aging treatment: The solution-treated duplex stainless steel was heated to 1050℃, held for 20 min, and then water-cooled to room temperature, further precipitating fine-sized island-like austenite on the ferrite matrix. The microstructure of the obtained duplex stainless steel was analyzed using EBSD, revealing a significantly increased austenite content of approximately 39.2%, with an average austenite grain size of approximately 6.0 μm. Figure 3 );

[0067] (3) Multi-pass high deformation rolling treatment: multi-pass rolling is carried out by room temperature synchronous cold rolling, with a total reduction rate of 90% and a reduction of 5% per pass;

[0068] (4) Short-time high-temperature annealing: The rolled duplex stainless steel was heated to 1100℃, held for 2 minutes, and then water-cooled to room temperature. This step further adjusted the austenite ratio and the grain size of austenite and ferrite. The resulting duplex stainless steel had an austenite content of approximately 46.0%, an average austenite grain size of approximately 4.5 μm, an average ferrite grain size of approximately 30.5 μm, and an average ferrite grain size approximately 6.8 times the average austenite grain size. Figure 9 );

[0069] (5) Low-temperature long-term aging treatment: After holding at 450℃ for 100h and water cooling to room temperature, phase-separated nanostructures were induced to form inside the ferrite, resulting in a multi-heterogeneous duplex stainless steel sample. The characteristic wavelength of the phase-separated nanostructure was 8.00nm, and the composition amplitude was 22.81at.% (Table 1).

[0070] Comparative Example 1:

[0071] This comparative example provides a method for preparing duplex stainless steel with multiple heterogeneous structures, which, compared with Example 3, does not involve low-temperature long-term aging treatment. Specifically, it includes the following steps:

[0072] (1) High-temperature solution treatment: Industrial grade 2507 duplex stainless steel hot-rolled plate was selected and solution treated at 1330℃ for 30 min, then water-cooled to room temperature to obtain duplex stainless steel with a low austenite ratio, of which austenite accounted for approximately 12.7%. Figure 2 );

[0073] (2) High-temperature aging treatment: The solution-treated duplex stainless steel was heated to 1050℃, held for 20 min, and then water-cooled to room temperature. Fine-sized island-like austenite further precipitated on the ferrite matrix, with austenite accounting for approximately 39.2% and an average austenite grain size of approximately 6.0 μm. Figure 3 );

[0074] (3) Multi-pass high deformation rolling treatment: multi-pass rolling is carried out by room temperature synchronous cold rolling, with a total reduction rate of 90% and a reduction of 5% per pass;

[0075] (4) Short-time high-temperature annealing treatment: Heat the rolled duplex stainless steel to 1100℃, hold for 1 minute, and water cool to room temperature. Figure 8 The austenite phase ratio and ferrite and austenite grain size of the duplex stainless steel obtained in this step are the same as those obtained in Example 3.

[0076] Comparative Example 2:

[0077] Industrial 2507 duplex stainless steel hot-rolled plate was selected, held at 450℃ for 100h, and water-cooled to room temperature. The characteristic wavelength of the phase separation nanostructure in the duplex stainless steel was 6.42nm, and the composition amplitude was 29.57at.% (Table 1).

[0078] Comparative Example 3:

[0079] Industrial-grade 2507 duplex stainless steel hot-rolled plate was selected and its performance was tested directly without any heat treatment or aging treatment.

[0080] Comparative Example 4:

[0081] This comparative example provides a method for preparing a multi-heterogeneous duplex stainless steel, which, compared to Example 1, does not involve short-time high-temperature annealing or low-temperature long-time aging treatment. Specifically, it includes the following steps:

[0082] (1) High-temperature solution treatment: Industrial grade 2507 duplex stainless steel hot-rolled plate was selected and solution treated at 1330℃ for 30 min, then water-cooled to room temperature to obtain duplex stainless steel with a low austenite ratio, of which austenite accounted for approximately 12.7%. Figure 2);

[0083] (2) High-temperature aging treatment: The solution-treated duplex stainless steel was heated to 1050℃, held for 20 min, and then water-cooled to room temperature. Fine-sized island-like austenite further precipitated on the ferrite matrix, with austenite accounting for approximately 39.2% and an average austenite grain size of approximately 6.0 μm. Figure 3 );

[0084] (3) Multi-pass high deformation rolling treatment: multi-pass rolling is carried out by room temperature synchronous cold rolling, with a total reduction rate of 90% and a reduction of 5% per pass.

[0085] Experimental Example: Characterization and Performance Testing of Duplex Stainless Steel

[0086] (1) Microscopic tissue detection

[0087] The microstructure of the duplex stainless steels corresponding to Examples 1–4 and Comparative Examples 2–3 was analyzed using EBSD. The microstructures of the duplex stainless steels corresponding to Comparative Examples 2 and 3 are as follows: Figure 4 , 5 As shown, the microstructure diagrams of the multi-heterogeneous duplex stainless steels corresponding to Examples 1–4 are as follows: Figure 6-9 As shown. Simultaneously, the ferrite phase-separated nanostructure in Comparative Example 2 was characterized using TEM, and the characterization results are as follows. Figure 10 As shown.

[0088] Compared to Comparative Example 3, Comparative Example 2 underwent low-temperature aging at 450℃. Although Comparative Example 2 showed obvious Fe-rich regions (represented by red) and Cr-rich regions (represented by green) in its ferrite, constituting significant nanoscale chemical heterogeneity, Figure 10 However, the austenite and ferrite content and the average grain size of austenite and ferrite in the duplex stainless steels corresponding to Comparative Examples 2 and 3 did not change. Figure 4 , 5 This indicates that aging at 450℃ only introduces nanoscale chemical heterogeneity within the ferrite, without altering the microstructure (phase content and average grain size) at the material's microscale. Further quantitative characterization of the phase-separated nanostructure was performed on Examples 1-4 and Comparative Examples 1-4 using small-angle neutron scattering (SANS), and the results are shown in Table 1. The samples treated with the miscibility gap temperature range (i.e., low-temperature aging at 450℃), namely Examples 1-4 and Comparative Example 2, all formed nanoscale phase-separated structures, indicating the formation of nanoscale chemical heterogeneity. The untreated samples, however, did not form phase-separated nanostructures. Figure 6-9(The microstructure test results of steps S3 and S4 are consistent) which further verifies that the low-temperature aging treatment only introduces nanoscale chemical heterogeneity within the ferrite and does not change the microstructure of the material. The above results indicate that the duplex stainless steel (Examples 1-4) prepared by the method of this invention forms chemical heterogeneity (phase-separated nanostructure).

[0089] Meanwhile, Example 3 only underwent a longer aging treatment at 450℃ compared to Comparative Example 1. Therefore, the microstructure of Comparative Example 1 is the same as that of Example 3, and no further characterization of the microstructure of Comparative Example 1 is required. Compared to the original hot-rolled duplex stainless steel of Comparative Example 3, the duplex stainless steel prepared by the method of this invention (Examples 1-4) not only exhibits phase composition heterogeneity (austenite and ferrite phases), but also shows significant grain size refinement. Furthermore, there is a significant difference in grain size between austenite and ferrite (the grain size of ferrite is 2.5-7.5 times that of austenite), forming obvious grain size heterogeneity. This indicates that the duplex stainless steel prepared by the method of this invention (Examples 1-4) also exhibits phase composition heterogeneity (austenite and ferrite phases) and grain size heterogeneity (difference in average grain size between austenite and ferrite).

[0090] It is evident that the method of this invention can be used to prepare duplex stainless steel, thereby simultaneously constructing multiple heterogeneous structures in the material, including heterogeneous phase composition, heterogeneous grain size, and heterogeneous chemical composition.

[0091] Table 1 Comparison of characteristic parameters of phase-separated nanostructures in Examples 1–4 and Comparative Examples 1–4

[0092]

[0093] (2) Alloy performance testing

[0094] All specimens were fabricated into standard tensile specimens according to GB / T 228.1-2021, and their yield strength, tensile strength, and total elongation were tested at room temperature. The engineering stress-strain curves for each specimen are shown below. Figure 11 As shown in the figure, the tensile property results are compared in Table 2.

[0095] From Table 2 and Figure 11 It can be seen that the 2507 multi-heterogeneous duplex stainless steel (Examples 1–4) prepared by the method of the present invention all have a strength of over 1250 MPa and an elongation of over 22%. The strength is significantly better than that of samples with only microstructural heterogeneity (Comparative Example 1) or chemical heterogeneity (Comparative Example 2) and the original commercial hot-rolled sample (Comparative Example 3). The elongation and plasticity are significantly better than those of the cold-rolled sample (Comparative Example 4), achieving an excellent combination of strength and plasticity.

[0096] Table 2 Comparison of alloy properties between Examples 1–4 and Comparative Examples 1–4

[0097]

[0098] As can be seen from the above embodiments and comparative examples, the present invention provides a method for preparing high-strength, high-ductility duplex stainless steel with multiple heterogeneous structures. Through multi-stage thermo-mechanical composite treatment, structural heterogeneity and chemical heterogeneity (phase-separated nanostructure) can be effectively introduced into the duplex stainless steel. Compared with samples containing only a single type of heterostructure and the original hot-rolled sample, the interaction of different types of heterostructures can significantly improve the strength of 2507 duplex stainless steel while maintaining its ductility and malleability.

[0099] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and these variations still fall within the protection scope of the present invention.

Claims

1. A method for preparing high-strength, high-ductility, multi-heterogeneous duplex stainless steel, characterized in that, Includes the following steps: S1. High-temperature solution treatment: Place duplex stainless steel at 1300–1380℃ for 20–60 min and then water-cool it to obtain duplex stainless steel with a low austenite ratio. S2. High-temperature aging treatment: The duplex stainless steel in S1 is placed at 1050–1200℃ and held for 10–30 minutes. After water cooling, fine-sized island-shaped austenite is further precipitated on the ferrite matrix. S3, High Deformation Rolling Treatment: The duplex stainless steel in S2 is subjected to multiple deformation treatments until the total reduction rate is 70-95%. S4. High-temperature short-time annealing treatment: The duplex stainless steel in S3 is subjected to a heat treatment of 0.5–5 min at 1050–1200℃. After water cooling, the austenite ratio and the grain size of austenite and ferrite can be adjusted. S5. Low-temperature long-term aging treatment: The duplex stainless steel in S4 is placed at 350–500℃ for 10–1000h and then water-cooled to induce the formation of phase-separated nanostructures inside the ferrite, resulting in high-strength, high-plasticity duplex stainless steel with multiple heterogeneous structures.

2. The method for preparing a high-strength, high-ductility, multi-heterogeneous duplex stainless steel according to claim 1, characterized in that, The duplex stainless steel is industrial grade 2507 duplex stainless steel.

3. The method for preparing a high-strength, high-ductility, multi-heterogeneous duplex stainless steel according to claim 1, characterized in that, The high deformation rolling process described in S3 is carried out using a room temperature synchronous cold rolling method, with a reduction of 0.5%–20% per pass, without cracking.

4. The method for preparing a high-strength, high-ductility, multi-heterogeneous duplex stainless steel according to claim 1, characterized in that, The water cooling mentioned in S1, S2, S4, and S5 refers to water cooling to room temperature.

5. The method for preparing a high-strength, high-ductility, multi-heterogeneous duplex stainless steel according to claim 1, characterized in that, The phase-separated nanostructure formed inside the ferrite in S5 has a characteristic wavelength between 2 and 20 nm and a composition amplitude between 10 and 60 at.%.

6. The method for preparing a high-strength, high-ductility, multi-heterogeneous duplex stainless steel according to claim 1, characterized in that, The duplex stainless steel obtained from S1 contains 10%–35% austenite.

7. The method for preparing a high-strength, high-ductility, multi-heterogeneous duplex stainless steel according to claim 1, characterized in that, The duplex stainless steel obtained from S2 contains 30%–45% austenite, with an average austenite grain size of 1–20 μm.

8. The method for preparing a high-strength, high-ductility, multi-heterogeneous duplex stainless steel according to claim 1, characterized in that, The duplex stainless steel obtained from S4 contains 30%–65% austenite, and the average grain size of ferrite is 2–50 times that of austenite.

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