A method for preparing a reverse gradient nanotwinned high strength and toughness metal material by wire rolling

Abstract

The present invention discloses a method for preparing a reverse gradient nano-twinned high-strength and tough metal material by wire rolling, which includes the following steps: annealing and polishing the metal material, performing rolling by means of a multi-pass rolling process, controlling the total deformation amount to be 10-48%, and preparing a reverse gradient nano-structure from central nano-twins to surface coarse grains. The present invention constructs a reverse gradient nano-structure from surface coarse grains to core nano-twins by regulating the pass reduction and rolling temperature. The reverse gradient nano-twinned material prepared by the present invention has excellent mechanical properties compared with the annealed state. Among them, for 316L with a gradient deformation of 20%, the tensile strength is 600-800 MPa, the elongation is 40-50%, and the uniform elongation is increased by 50-100%. The process of the present invention is compatible with the existing rolling production line, and has advantages such as low cost, high efficiency, and scalable production, and is applicable to fields with strict requirements for strength and toughness such as aviation fasteners and automobile anti-collision components.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing technology, specifically relating to a method for preparing high-strength and high-toughness metal materials with reverse gradient nanotwins by wire rolling. Background Technology

[0002] Traditional metallic materials generally exhibit a strength-ductility trade-off, a phenomenon stemming from the interplay between dislocation movement and hindering mechanisms during material deformation. Specifically, when strength is increased through grain refinement, solid solution strengthening, or precipitation, dislocation movement is hindered, leading to a significant decrease in plastic deformation capacity; conversely, improving ductility often comes at the cost of strength. Gradient nanomaterials have become a research hotspot in materials science in recent years. Compared to traditional homogeneous nanomaterials, gradient structures can effectively alleviate the strength-ductility trade-off while providing better fatigue resistance and wear resistance.

[0003] Currently, gradient nanomaterials are mainly obtained through surface mechanical treatment, rolling and extrusion, deposition and coating technologies, and 3D printing and additive manufacturing. However, these methods still have several prominent bottlenecks: the effective reinforcing layer thickness is usually <500μm, which has limited contribution to the reinforcement of thick cross-section components (such as plates >10mm); the processing efficiency is extremely low (0.2~0.5m² / h), and the surface roughness Ra>3.2μm requires further processing; the equipment investment cost is high, which restricts large-scale application. In addition, existing gradient structures mostly exhibit a single pattern of monotonically increasing grain size from the surface to the core, which easily leads to strain mismatch at the soft and hard phase interface and causes early cracks; at the same time, the grain size gradient in the transition region is often not gentle enough (only spanning 3~5 grains), which limits the synergistic optimization of stress redistribution and dislocation storage capacity.

[0004] In the field of gradient nanotwinned materials, related preparation technologies are currently still in the laboratory exploration stage and face a series of challenges: For commonly used engineering materials such as high-strength steel and magnesium alloys, it is difficult to achieve controllable twinning effects; some processes require liquid nitrogen cryogenic (-196℃) environments, resulting in complex equipment and high energy consumption; the size of the obtained samples is limited (diameter typically not exceeding 10mm), failing to meet the dimensional requirements of actual structural components. Taking typical austenitic stainless steel AISI 316L as an example, the upper limit of performance achievable by existing technologies is: tensile strength σ... b When the tensile strength is approximately 800 MPa, the elongation δ ≤ 10%; or when the elongation δ ≈ 40%, the tensile strength σ b ≤ 400MPa. This combination of performance characteristics is difficult to meet for components such as aerospace structures (requiring σ...). bThis meets the demands of high-end applications such as those requiring ≥ 600MPa and δ ≥ 30%. However, the cost per ton of this process is 3-5 times higher than traditional rolling, becoming a major obstacle to its large-scale adoption. Therefore, developing a novel strengthening method that combines excellent mechanical properties, good process economy, and suitability for engineering production has become a key technical problem urgently needing breakthroughs in the materials field. Summary of the Invention

[0005] To address the aforementioned technical problems, the present invention aims to provide a method for preparing reverse gradient nanostructured metallic materials by multi-pass rolling at room temperature (-50~200 ℃). By controlling the rolling process parameters and cross-sectional shape design, a multi-scale structure characterized by nanotwins and strain-induced α' martensite is constructed in steel wire, achieving a synergistic breakthrough in strength and ductility.

[0006] To achieve the above objectives, the present invention first provides a method for preparing high-strength and tough metal materials with reverse gradient nanotwins by rolling wire, comprising the following steps: annealing and polishing the metal material, rolling it using a multi-pass rolling process, controlling the total deformation to be 10-48%, and preparing a reverse gradient nanostructure from the central nanotwin to the surface coarse grains.

[0007] In one embodiment of the present invention, the total deformation is preferably 10-48%, more preferably 12-40%, and can be specifically selected from 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, etc.

[0008] In one embodiment of the present invention, the reduction amount per pass is controlled between 0.5% and 20%, preferably 1% to 10%. The reduction amount per pass is related to the total deformation; when the total deformation is large, the reduction amount per pass can be increased accordingly. For example, when the total deformation is 40% to 60%, the reduction amount per pass can be controlled to be 15% to 20%.

[0009] In one embodiment of the present invention, the metal material is steel, preferably austenitic stainless steel or TWIP steel.

[0010] In one embodiment of the present invention, the metal material is preferably wire with a diameter Φ of 1~30mm, an initial microstructure of equiaxed austenite, and a grain size of 1~100μm.

[0011] In one embodiment of the present invention, the annealing process involves raising the temperature of the annealing environment to 900-1200°C, holding it at this temperature for 1-30 minutes, and then rapidly transferring it to water for quenching after annealing, with a dwell time of <3 seconds. The purpose of annealing is to eliminate internal stress and defects generated during the previous processing of the metal material and to improve the uniformity of the metal's microstructure.

[0012] In one embodiment of the present invention, the polishing is carried out by using common polishing methods such as mechanical polishing, chemical polishing, and electrochemical polishing to reduce the surface roughness of the annealed metal material to Ra<0.8μm, so as to reduce rolling defects.

[0013] In one embodiment of the present invention, the multi-pass rolling process is carried out under conditions near room temperature, which refers to conditions of -50 to 200 °C.

[0014] In one embodiment of the present invention, the specific steps of the multi-pass rolling process are as follows:

[0015] (1) Adjust the initial rolling gap of the twin-roll mill to the original diameter of the metal material to be processed;

[0016] (2) Control the amount of reduction in each pass, and continuously roll the metal material to be processed in each pass. Repeat this step until the total deformation reaches the required amount.

[0017] In one embodiment of the present invention, the rolling rate during rolling is 0.1~100 m / min. In the present invention, the rolling rate, i.e., the linear velocity of the workpiece passing through the rolls, has little effect on the rolling effect.

[0018] In one embodiment of the present invention, the cumulative reduction in the previous pass accounts for 50-90% of the total reduction.

[0019] In one embodiment of the present invention, the rolling process employs a circular, elliptical, or other cross-sectional design with a circular arc structure, and the microstructure gradient distribution is achieved by controlling the strain gradient distribution.

[0020] The present invention also provides a high-strength and high-toughness metallic material with reverse gradient nanotwins prepared according to the above preparation method.

[0021] In one embodiment of the present invention, the equivalent strain ε>1.5 in the central region of the reverse gradient nanotwin high-strength and tough metal material, the equivalent strain ε=0.3 ~ 1.5 in the transition region, and the equivalent strain ε<0.3 in the edge region.

[0022] In one embodiment of the present invention, the grain size of the central region is 10~100nm, which belongs to nanotwins. The dislocation density in this region is high, the strain concentration region is α' phase, the transition region is nanotwins and microtwins, the grain size is 100~500nm, and there is a gradient distribution of α' phase; the edge region is coarse grains and a small amount of microtwins, the grain size is 1~100μm.

[0023] In one embodiment of the present invention, the thickness of the nanotwins decreases continuously from the edge to the center.

[0024] In one embodiment of the present invention, when the material is 316L stainless steel, its mechanical properties meet the following requirements: tensile strength ≥ 600 MPa; elongation ≥ 20%, preferably elongation ≥ 30%; and fracture toughness is 50-100% higher than that of traditional materials.

[0025] In one embodiment of the present invention, when the material is 304 stainless steel, its properties meet the following requirements: tensile strength ≥ 800 MPa; elongation ≥ 20%, preferably elongation ≥ 40%.

[0026] The present invention also discloses the application of the above-mentioned reverse gradient nanotwinned high-strength and tough metal material in the manufacture of cable armor, aerospace fasteners, automotive anti-collision components or other structural components with stringent requirements for strength and toughness.

[0027] This invention also provides a method for optimizing the performance of gradient nanostructured metallic materials, characterized by the following steps: annealing and polishing the metallic material; rolling it using a multi-pass rolling process at -50~200 ℃, controlling the total reduction deformation to be 10~48%, and the reduction per pass to be 0.5~20%, thereby improving plasticity at low deformation rates and increasing strength at high deformation rates by controlling the deformation rate and deformation amount. The low deformation rate refers to a reduction of 0.5~5% per pass, and the high deformation rate refers to a reduction of 5~20% per pass. The metallic material is steel wire.

[0028] Beneficial effects:

[0029] This invention targets annealed, solution-treated stainless steel and other metallic materials. It innovatively employs multi-pass controlled rolling combined with gradient strain design. By precisely controlling the deformation rate and amount in each pass, a nanotwin structure with reverse gradient characteristics is constructed within the flat wire. This breakthrough overcomes existing technological bottlenecks from both microstructure design and fabrication process perspectives, achieving synergistic optimization of material strength and plasticity while also possessing industrial-scale production feasibility. Compared to traditional flat wire fabrication techniques, it produces the following significant advantages:

[0030] (1) The construction of a reverse gradient nanotwin structure breaks through the strength-ductility trade-off limit of materials: traditional gradient structures mostly exhibit a monotonous change of "surface nanocrystals → internal coarse grains", while this invention forms a reverse gradient structure of "central nanotwins (10~100nm) → transition region (coexistence of nanotwins and microtwins) → external coarse grains (1~100μm) and a small number of twins" through strain path control. This structure can achieve the synergistic effect of stress redistribution and strain hardening, effectively avoid interfacial stress concentration, and improve the overall mechanical coordination of materials. Especially in austenitic stainless steel (such as 316L), nanotwins (reinforcement and toughening) and α'-martensite (phase transformation toughening) form a multi-scale coupled strengthening mechanism, breaking through the strength-ductility trade-off of traditional materials. The size of twins in the gradient transition region changes continuously and gently, avoiding the problem of interfacial stress concentration in traditional gradient materials.

[0031] (2) Simple, efficient, and low-cost manufacturing process: This invention uses a conventional twin-roll mill (no special molds or equipment required), and gradient structure control can be achieved through multiple deformation passes. The process is simple and easy to promote industrially. The processing time per batch is less than 30 minutes, which is far more efficient than equal-diameter angular extrusion (4~8 hours / batch) or surface mechanical grinding (0.5m² / h). The raw material utilization rate is >95% (no cutting waste), and the cost per ton is reduced by more than 70% compared to ECAP. It can process Φ1~30mm wire / bar materials, meeting the structural component needs of the automotive, aerospace, and other fields.

[0032] (3) Significant improvement in the comprehensive mechanical properties of materials: This invention first anneales stainless steel and other metal materials to reduce internal defects in the metal materials before rolling, and then combines multi-pass rolling process. By selecting a structure with a circular, elliptical or other interface containing arc shape, the reduction amount is controlled to be 1~10% each time and the total deformation amount is 10~50%, which realizes the synergistic improvement of the strength and ductility of stainless steel and other metal materials. Among them, when the reduction amount is low, the tensile strength of stainless steel can reach more than 600MPa and the elongation is more than 40%; when the reduction amount is high, although the elongation is lower, the tensile strength can reach as high as 1340MPa.

[0033] (4) Flat steel wire is a metallic material made by rolling or drawing round steel wire into a flat cross-section. Due to its unique shape and excellent mechanical properties, it has wide applications in transportation and machinery manufacturing, textiles and industrial equipment, construction and infrastructure, electrical and electronic equipment, and furniture and daily consumer goods. It includes, but is not limited to, umbrella frames, heating elements, flat springs, steel sheet heddles, metal needle cloth, flat steel wire reinforcement for pressure vessel shells, and armoring for ultra-high voltage power transmission network cables and submarine optical cables. Currently, the main products are flat wires or strips with large deformation (total deformation under compression > 50%), and the preparation process usually involves complex heat treatment processes. Domestic and foreign researchers and industry have not yet realized that the flat wire rolling process can introduce a gradient nanotwin microstructure, and that this gradient nanostructure helps to optimize the comprehensive mechanical properties and even other physical properties of the rolled flat wire. The purpose of rolling at room temperature (-50~200 ℃) in this invention is to introduce and control the gradient nanotwin microstructure in the flat wire and obtain the best strengthening and toughening effect. Studies have shown that when the total compression deformation is between 10% and 50%, the strengthening and toughening effect is significant. Attached Figure Description

[0034] Figure 1 The solid solution grain morphology of 316L stainless steel after annealing in Example 1;

[0035] Figure 2 The XRD results are for the solution-treated 316L stainless steel in Example 1.

[0036] Figure 3 The results are the hardness gradient characterization results of 316L after low strain rate gradient deformation in Examples 1, 2 and 3.

[0037] Figure 4 The following are the low strain rate gradient deformation tensile curves of 316L in Examples 1, 2 and 3;

[0038] Figure 5 This is a representative microstructure of the gradient nanostructure in the central region of 316L in Example 1;

[0039] Figure 6 This is a representative microstructure of the 316L transition region gradient nanostructure in Example 1;

[0040] Figure 7 This is a representative microstructure of the 316L edge region gradient nanostructure in Example 1;

[0041] Figure 8 The TEM characterization results are for the α' phase in the central region of 316L in Example 1;

[0042] Figure 9 The TEM characterization results are for the 316L central region nanotwins in Example 1.

[0043] Figure 10 The tensile curves of 316L under low strain rate uniform deformation are shown in Comparative Examples 1, 2 and 3.

[0044] Figure 11 The strong-plasticity inverse relationship diagrams of yield strength and uniform elongation of 316L were prepared for Examples 1, 2 and 3 and Comparative Examples 1, 2 and 3.

[0045] Figure 12 The results are for characterizing the high strain rate hardness gradient of 316L prepared in Examples 4, 5 and 6.

[0046] Figure 13 The high strain rate gradient deformation tensile curves of 316L prepared in Examples 4, 5 and 6;

[0047] Figure 14 High strain rate uniform deformation tensile curves of 316L prepared for comparative examples 4, 5 and 6;

[0048] Figure 15 The results are for characterizing the low strain rate hardness gradient of 304 stainless steel prepared in Examples 7, 8 and 9.

[0049] Figure 16 The low strain rate gradient deformation tensile curves of 304 stainless steel prepared in Examples 7, 8 and 9;

[0050] Figure 17 The low strain rate uniform deformation tensile curves of 304 stainless steel prepared for comparative examples 7, 8 and 9.

[0051] Figure 18 The results are for characterizing the high strain rate hardness gradient of 304 stainless steel prepared in Examples 10, 11 and 12.

[0052] Figure 19 The high strain rate gradient deformation tensile curves of 304 stainless steel prepared in Examples 10, 11 and 12 are shown.

[0053] Figure 20 High strain rate uniform deformation tensile curves of 304 stainless steel prepared for comparative examples 10, 11 and 12. Detailed Implementation

[0054] To better understand the above-mentioned objectives, features, and advantages of the embodiments of this application, the application will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the features in the embodiments of this application can be combined with each other.

[0055] The following description sets forth numerous specific details to provide a thorough understanding of the embodiments of this application. The described implementations are only a portion, not all, of the embodiments described herein. All other implementations obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of the embodiments of this application.

[0056] Terminology Explanation:

[0057] The "multi-pass rolling" involved in this invention refers to a cold rolling process in which the plastic deformation of metal materials is completed in multiple consecutive passes through rolls because the deformation amount of a single rolling pass is limited by factors such as material plasticity and equipment load, and the target thickness and performance cannot be achieved in one pass.

[0058] The "reduction per pass" involved in this invention refers to the relative change in thickness of a metal workpiece after being rolled by the rolls in a single rolling pass, and its calculation formula is ε. i =Δh i / h i-1 It represents the degree of deformation in a single pass.

[0059] The "total deformation" involved in this invention refers to the relative change in the cumulative thickness of a metal rolled piece from the initial cold-rolled billet to the final finished product, and its calculation formula is ε. 总 =ΔH / H0.

[0060] The "rolling rate" involved in this invention refers to the linear velocity of the workpiece (the metal material being rolled) as it passes through the rolls. It is one of the core parameters of the cold rolling process and directly affects the rolling efficiency, product accuracy, and rolling force.

[0061] The "strain rate" involved in this invention refers to the rate at which the rolled piece undergoes plastic strain per unit time during each deformation process by precisely controlling the amount of reduction in each pass.

[0062] The "preliminary passes" involved in this invention refer to the rolling passes in the preceding stage of a complete multi-pass rolling sequence, accounting for 1 / 3 to 2 / 3 of the total number of passes. The process characteristics of these passes are the use of a large reduction rate, which lays the organizational foundation for the finishing rolling process of the middle and later passes.

[0063] The "gradient nanostructure" involved in this invention refers to a nanoscale structure in metallic or alloy materials where microscopic features such as grain size, dislocation density, phase composition, or twinning structure exhibit a continuous gradient change in space. Unlike traditional uniform nanomaterials (with uniform grain size), gradient nanostructures achieve optimized combinations of material properties, such as high strength, high toughness, fatigue resistance, and wear resistance, through a controllable gradient distribution of microstructure. "Reverse gradient nanostructure" refers to a novel nanocomposite structure where microstructural units such as grain size, dislocation density, phase composition, or twinning structure exhibit a reverse gradient distribution along the thickness / depth direction of the material. Its core characteristic is the opposite of traditional forward gradient nanostructures: the core of the material consists of nanoscale structural units (size 1~100 nm), while the surface layer consists of coarse-grained / micrometer-scale structural units (size > 1 μm), and the size of the microstructural units continuously increases from the core to the surface.

[0064] The "central region" in this invention refers to the area in the cross-section of a rolled metal workpiece where the vertical distance from the upper and lower surfaces is not less than 1 / 3 of the total thickness of the cross-section. This region is the concentrated area of ​​cold-rolled plastic deformation. The "edge region" refers to the outermost region of the cross-section of a rolled metal workpiece, the area in contact with the rolling tool or environmental medium. Its location can be quantitatively defined as: the area where the vertical distance from the outer surface of the metal material's cross-section does not exceed 30% of the total thickness of the cross-section. The core characteristic of this region is dominated by interfacial friction or continuum constraint stress during processing, specifically manifested as: significant orientation deformation of grains under shear stress, forming a fibrous structure arranged along the rolling direction. The "transition region" refers to the intermediate region in the cross-section of a rolled metal workpiece between the central region and the edge region, serving as a structural-performance buffer region connecting the core high-strength region and the surface tough-plastic region. Its location can be quantitatively defined as: the area where the vertical distance from the outer surface of the metal material's cross-section is 20-40% of the total thickness of the cross-section.

[0065] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which embodiments of this application belong. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the embodiments of this application.

[0066] Example 1: Low-rate gradient deformation multi-pass controlled rolling of 316L stainless steel

[0067] A method for preparing stainless steel with a reverse gradient nanostructure includes the following steps:

[0068] (1) Raw material selection: AISI 316L austenitic stainless steel wire conforming to ASTM A276 standard, with a diameter of Φ2.4mm, and the following chemical composition (wt.%): C≤0.03, Cr 16.5~17.5, Ni 10.0~12.0, Mo 2.0~2.5, Mn≤2.0, Si≤1.0, P≤0.045, S≤0.03, Fe balance;

[0069] (2) Pretreatment: The 316L wire was annealed and then polished. The annealing process involved placing the 316L wire at a temperature of 1000~1100 ℃ for 10 min and then quickly transferring it to water for quenching to obtain a solution-treated state (ST state). To eliminate surface scratches and avoid stress concentration during rolling to cause cracks, reduce the friction coefficient, and ensure controllable rolling strain gradient, the solution-treated wire was polished using an electrolytic polishing process. During polishing, the solution-treated wire was used as the anode, the 304 stainless steel sheet was used as the cathode, and a phosphoric acid-glycerol-water mixture (volume ratio 7:2:1) was used as the electrolyte. Mechanical polishing was used as an auxiliary process, and alumina suspension was used for final polishing to reduce the surface roughness (Ra<0.8μm).

[0070] (3) Multi-pass rolling: The roller spacing was adjusted to 2.4 mm, the reduction per pass was controlled at 3%, and the rolling speed was 5 m / min. The sample was fed into the mill from one end while maintaining the same orientation for axial cold rolling. The evolution of the cross-sectional shape was monitored during the rolling process, and the cross-sectional dimensions (major axis and minor axis) of the sample were detected online. This process was repeated until the total deformation was 15%, forming a reverse gradient nanostructure from the central nanotwins / subgrains to the surface coarse grains. The strain rate was gradually reduced in subsequent passes until the total deformation reached 20%. The sample was named 316L-GCR20-low strain rate. In subsequent passes, the purpose of reducing the strain rate was to ensure precise control of the interface dimensions. The strain rate could be reduced by decreasing the reduction per pass by 2%, 1.5%, 1%, etc., until the total deformation reached 20%.

[0071] Example 2

[0072] The difference between Example 2 and Example 1 is that the total deformation in step (3) is 40%. During rolling, the rolling is first carried out with a reduction of 3% until the total deformation is 30%, and then the reduction of each pass is gradually reduced until the total deformation is 40%, which is named 316L-GCR40-low strain rate.

[0073] Example 3

[0074] The difference between Example 3 and Example 1 is that the total deformation in step (3) is 60%. During rolling, the rolling is first carried out with a reduction of 3% until the total deformation is 45%, and then the reduction of each pass is gradually reduced until the total deformation is 60%, which is named 316L-GCR60-low strain rate.

[0075] The grain morphology of the steel in Example 1 after solution treatment (ST state) after annealing is as follows: Figure 1 As shown, Figure 1 After annealing, the grain size of 316L wire is 20~80μm.

[0076] Figure 2 The XRD pattern of 316L after annealing in Example 1 shows that the microstructure is a fully austenitic phase (γ phase) with no α' martensite or α ferrite peaks.

[0077] Hardness gradient characterization:

[0078] Rolled 316L stainless steel wire was selected and hardness was tested using a microhardness tester (load 200g, holding pressure 15s); the test direction was horizontal and vertical of the sample cross-section; the test path was to take points every 150μm from the surface to the core (≥12 data points per group).

[0079] Figure 3 The hardness distribution results of Examples 1, 2, and 3, in the solution-treated (ST) state, showed a clear reverse gradient distribution after rolling, meaning the hardness increased from the edge to the center of the sample cross-section. The gradient slope in the horizontal direction was significantly greater than that in the vertical direction; for example, in GCR20, the horizontal hardness difference could reach over 100 Hv, while the vertical difference was approximately 40 Hv. The hardness increased significantly with increasing deformation (20% → 60%). These results verify the effect of different strain distributions during rolling deformation on hardness regulation, providing a quantitative basis for gradient structure design.

[0080] Mechanical property testing:

[0081] By comparing the tensile stress-strain curves under 20%, 40%, and 60% gradient rolling deformation in Examples 1, 2, and 3, the influence of different deformation amounts on the mechanical properties of the material is analyzed. Figure 4The results show a yield strength of approximately 580 MPa at 20% deformation, exhibiting typical ductile fracture characteristics. The strain hardening stage lasts until approximately 40% elongation, with a peak stress of 680 MPa. At 40% deformation, the yield strength of stainless steel increases significantly to approximately 900 MPa, still exhibiting typical ductile fracture characteristics. The strain hardening stage lasts until approximately 20% elongation, with a peak stress of 920 MPa. Stainless steel at 60% deformation exhibits a high yield strength of approximately 1000 MPa, fractures immediately after reaching its ultimate tensile strength of 1060 MPa, but with an elongation of only 2-3%. This demonstrates that the rolling amount has a decisive influence on the tensile behavior of 316L stainless steel.

[0082] Characterization of reverse gradient nanostructures:

[0083] The microstructure of the sample with 20% deformation in Example 1 was characterized using electronic channel contrast imaging. The sample was taken from the cross-section of the rolled flat wire, and the test area covered the complete gradient structure from the edge to the center. The nanostructural unit was determined to be nanotwins, with the twin spacing gradually decreasing from the edge to the center, and the twin size (width) decreasing to the nanoscale.

[0084] Figure 5 It exhibits gradient nanostructure characteristics, with the central region (equivalent strain ε>1.5) showing a representative microstructure of high-density nanotwins (thickness 20~70 nm); the characteristic phase transformation is strain-induced formation of α' martensite phase (volume fraction ~35%).

[0085] Figure 6 The transition region (equivalent strain ε=0.3~1.5) forms a multi-scale twinning system, with nanotwins (50~100nm) accounting for about 40% and submicron twins (100~500nm) accounting for about 30%; the α' phase gradient distribution changes continuously from 15% (near the edge) to 25% (near the center).

[0086] Figure 7 The matrix structure of the edge region (equivalent ε<0.3) is coarse-grained (20~80μm) and retains the original austenite structure. The deformation characteristics are a small amount of annealed twins (average spacing >2μm); the α' phase content is low.

[0087] This invention achieves the construction of three-dimensional reverse gradient nanostructures through an innovative rolling process, with feature sizes continuously transitioning from the micrometer scale (edge) to the nanometer scale (center); multi-scale microstructure control forms a nanotwin / dislocation / phase transformation composite reinforcement system; and structure-performance correlation design provides a new approach for developing next-generation gradient functional materials.

[0088] TEM characterization and phase composition analysis of nanotwin structures:

[0089] The central region of the sample with 20% deformation in Example 1 was characterized by fine tissue analysis using transmission electron microscopy (TEM).

[0090] Figure 8 For TEM characterization, bright-field images show lath-like α' martensite (scale 20–50 nm) distributed at twin boundaries or grain boundaries with large deformation. Selected area diffraction (...) Figure 8 The illustration confirms the strain-induced martensitic phase transformation; the volume fraction of the α' phase was calculated to be 32%.

[0091] High-resolution TEM Figure 9 The study revealed nanotwin structure characteristics, with parallel nanotwins averaging approximately 50 nm in thickness and exhibiting a high density of dislocations within the twins. TEM images showed α' martensite in the core of the sample, and the proportion of α' martensite phase and twin size were determined, further confirming the reverse gradient nanotwin structure.

[0092] Comparative Example 1: 316L - A Comparative Study of Low-Rate Uniform Deformation and Gradient Deformation

[0093] The difference between Comparative Example 1 and Example 1 is that the aluminum alloy used in Example 1 is a wire with a circular cross-section, while Comparative Example 1 uses a plate with a rectangular cross-section (thickness of 2 mm). The rest is the same as Example 1, and it is named 316L-CR20-low strain rate.

[0094] Comparative Example 2

[0095] The difference between Comparative Example 2 and Comparative Example 1 is that the total deformation is 40%, and the multi-pass rolling process is the same as in Example 2. It is named 316L-CR40-Low Strain Rate.

[0096] Comparative Example 3

[0097] The difference between Comparative Example 3 and Comparative Example 1 is that the total deformation is 60%, and the multi-pass rolling process is the same as in Example 3. It is named 316L-CR60-Low Strain Rate.

[0098] Comparative Examples 1 to 3 involve rolling rectangular plates. The overall shape of the plates did not change significantly before and after the rolling deformation, and the deformation can be judged to be uniform based on the microstructure distribution.

[0099] The microstructure and properties of uniform deformation and gradient deformation are compared. The tensile curve of uniform deformation at low strain rate is shown in the figure. Figure 10 As shown.

[0100] Table 1 Tensile properties of samples prepared in Example 1 and Comparative Example 1

[0101]

[0102] Figure 11 The figure shows the inverse relationship between the yield strength and uniform elongation of the stainless steel flat wires rolled in Example 1 and Comparative Example 1 at low strain rates and the mechanical properties of deformed 316L stainless steel as described in the literature (Yan FK, et. Acta Materialia 60 (2012) 1059; Li JS, et. Journal of Materials Science 53 (2018) 10442; Li J, et. Vacuum 157 (2018) 128; LiuXB, et. Materials Characterization 192 (2022) 112182). The black dashed line in the figure represents the baseline of the inverse relationship between the strength and plasticity of 316L stainless steel under conventional processes. As shown in the figure, in this invention, when the total rolling deformation of 316 stainless steel is 20%, its mechanical properties are located in the upper left region of the black dashed line. This indicates that, at the same yield strength, its uniform elongation (plasticity) is significantly higher than the benchmark level of conventional materials, suggesting that 20% deformation is suitable for applications requiring high plasticity and exhibits optimal strength-plasticity matching. When the total rolling deformation of 316 stainless steel is 40%, its mechanical properties are located above or slightly above the black dashed line. Its yield strength has reached a high strength level (800-1000 MPa), but its uniform elongation is still significantly better than the benchmark of conventional materials at the same strength. This means that in scenarios requiring high strength while retaining high plasticity, its strength-plasticity matching remains superior. When the total deformation of 316 stainless steel during rolling is 60%, its mechanical properties are at or near the black line. At this point, its strength-plasticity relationship perfectly matches the conventional rule that the higher the strength, the lower the plasticity. The high strength-plasticity synergistic breakthrough effect brought by the reverse gradient nanostructure has disappeared, and it has returned to the ordinary strength-plasticity matching level.

[0103] In summary, the above results demonstrate that the mechanical properties of 316L treated by the gradient deformation method of this invention are significantly improved, showing obvious advantages.

[0104] Compared with the traditional uniform deformation process, the gradient rolling process of the present invention (Examples 1 and 2) significantly improves the elongation while maintaining a comparable strength level; the reverse gradient structure achieves a synergistic improvement in strength and plasticity through strain distribution optimization; although the uniform deformation process has high hardness and uniform distribution, it sacrifices the plasticity of the material.

[0105] Example 4: High strain rate gradient deformation multi-pass controlled rolling of 316L

[0106] The difference between Example 4 and Example 1 is that the reduction per pass is 10%. First, the rolling is carried out with a fixed reduction until the total deformation is 15%, and then the strain rate is gradually reduced until the total deformation is 20%. It is named 316L-GCR20-High Strain Rate.

[0107] Example 5

[0108] The difference between Example 5 and Example 2 is that the reduction per pass is 10%, and it is named 316L-GCR40-High Strain Rate.

[0109] Example 6

[0110] The difference between Example 6 and Example 3 is that the reduction amount per pass is 10%, and it is named 316L-GCR60-High Strain Rate.

[0111] In Examples 4, 5, and 6, the reduction per pass was 10%, which falls within the high strain rate range. All other rolling parameters were the same as in Example 1.

[0112] Hardness gradient characterization and mechanical property testing:

[0113] 316L stainless steel wires processed by high-rate rolling in Examples 4, 5, and 6 were selected, and hardness tests were conducted using a microhardness tester (load 200g, holding pressure 15s). The test directions were the horizontal and vertical directions of the sample cross-section, and the test path was from the surface to the core, with points taken every 150μm (≥12 data points per group). Tensile property tests were also performed simultaneously to obtain engineering stress-strain curves.

[0114] Figure 12 Hardness gradient characterization results show that all samples exhibit a clear reverse gradient distribution, meaning that hardness increases from the surface to the core. The gradient characteristic is positively correlated with strain rate, with higher strain rates resulting in a larger gradient.

[0115] By comparing the tensile stress-strain curves of 20%, 40%, and 60% rolling deformation in Examples 4, 5, and 6, the influence of different deformation amounts on the mechanical properties of the material at high strain rates is analyzed. Figure 13 The results showed that 20% of the samples had a yield strength of approximately 650 MPa, exhibiting typical ductile fracture characteristics, with the strain hardening stage lasting until approximately 40% elongation and a peak stress of 850 MPa. Another 40% of the samples showed a significantly increased yield strength of approximately 1000 MPa, still exhibiting some ductile fracture characteristics, with the strain hardening stage lasting until approximately 8% elongation and a peak stress of 1050 MPa. The remaining 60% showed an ultra-high yield strength of approximately 1160 MPa, fractured after reaching a tensile strength of 1190 MPa, with an elongation of only 2%.

[0116] Comparative Example 4

[0117] The difference between Comparative Example 4 and Comparative Example 1 is that the reduction per pass is 10%, and it is named 316L-CR20-High Strain Rate.

[0118] Comparative Example 5

[0119] The difference between Comparative Example 5 and Comparative Example 2 is that the reduction per pass is 10%, and it is named 316L-CR40-High Strain Rate.

[0120] Comparative Example 6

[0121] The difference between Comparative Example 6 and Comparative Example 3 is that the reduction per pass is 10%, and it is named 316L-CR60-High Strain Rate.

[0122] The microstructure and properties of uniform deformation and gradient deformation are compared. The tensile curve of uniform deformation at high strain rate is shown in the figure. Figure 14 As shown.

[0123] Table 2 Tensile properties of samples from Example 4 and Comparative Example 4

[0124]

[0125] Example 7

[0126] The difference between Example 7 and Example 1 is that the raw material used is 304 stainless steel with the following chemical composition (wt.%): C≤0.08, Cr 18~20, Ni 8~10.5, Mo ≤0.1, Mn≤2.0, V≤0.01, Si≤1.0, P≤0.045, S≤0.03, Fe balance; named 304-GCR20-low strain rate.

[0127] Example 8

[0128] The difference between Example 8 and Example 2 is that the raw material used is 304 stainless steel, named 304-GCR40-low strain rate.

[0129] Example 9

[0130] The difference between Example 9 and Example 3 is that the raw material used is 304 stainless steel, named 304-GCR60-low strain rate.

[0131] Comparative Example 7

[0132] The difference between Comparative Example 7 and Comparative Example 1 is that the raw material used is 304 stainless steel, named 304-CR20-low strain rate.

[0133] Comparative Example 8

[0134] The difference between Comparative Example 8 and Comparative Example 2 is that the material used is 304 stainless steel, named 304-CR40-low strain rate.

[0135] Comparative Example 9

[0136] The difference between Comparative Example 9 and Comparative Example 3 is that the material used is 304 stainless steel, named 304-CR60-low strain rate.

[0137] Hardness characterization analysis and mechanical property testing:

[0138] Gradient and comparative examples 7, 8 and 9 uniformly deformed 304 stainless steel wires were selected and subjected to hardness testing using a microhardness tester (load 200g, holding pressure 15s). The test directions were the horizontal and vertical directions of the sample cross-section. The test path was from the surface to the core, with points taken every 150μm (≥12 data points per group).

[0139] like Figure 15 As shown, for gradient deformation samples, the hardness distribution results at 20%, 40%, and 60% all exhibit a clear reverse gradient distribution characteristic, meaning the hardness increases from the edge of the sample cross-section to the center. The gradient slope in the horizontal direction is significantly greater than that in the vertical direction; for example, in GCR20, the horizontal hardness difference can reach over 150 Hv, while the vertical difference is approximately 100 Hv.

[0140] The results verify the effect of different strain distributions of rolling deformation on hardness regulation, providing a quantitative basis for gradient structure design.

[0141] By comparing the tensile stress-strain curves of 304 stainless steel under low-rate gradient deformation and uniform deformation at 20%, 40%, and 60% rolling deformation in Examples 7, 8, and 9, the influence of gradient deformation and uniform deformation on the mechanical properties of the material is analyzed. Figure 16 In the low-to-medium rate gradient deformation, the 20% yield strength is approximately 520 MPa, exhibiting typical ductile fracture characteristics. The strain hardening stage lasts until approximately 42% elongation, with a peak stress of 860 MPa. The 40% yield strength is approximately 970 MPa, also exhibiting typical ductile fracture characteristics. The strain hardening stage lasts until approximately 20% elongation, with a peak stress of 1020 MPa. The 60% yield strength exhibits an ultra-high yield strength of approximately 1190 MPa, but fractures immediately after reaching the ultimate tensile strength of 1240 MPa, with an elongation of only 2%.

[0142] The microstructure and properties of uniform deformation and gradient deformation are compared. The tensile curve of uniform deformation at low strain rate is shown in the figure. Figure 17 As shown. Comparison Figure 16 and 17It can be seen that the mechanical properties of the sample with gradient deformation are significantly better than those of the sample with uniform deformation.

[0143] This embodiment demonstrates that the deformation mode has a decisive influence on the tensile behavior of 304 stainless steel. 20% is suitable for applications requiring high plasticity and performs best in terms of strength-plasticity matching; 40% is suitable for applications requiring both high strength and relatively high plasticity, and still performs well in terms of strength-plasticity matching; 60% deformation returns to a normal strength-plasticity matching, and the toughening breakthrough effect of the reverse gradient nanostructure disappears.

[0144] In the mechanical property test, the hardness test showed a clear reverse gradient distribution characteristic, and the gradient change trend was similar to that of the 316L sample.

[0145] Tensile properties show that the strength is significantly improved compared to the original material. While achieving high strength, it maintains good plasticity, and the performance improvement is comparable to that of the 316L sample.

[0146] The patented gradient rolling process can be successfully applied to 304 stainless steel, achieving a gradient nanotwin structure similar to that of 316L in 304. This method exhibits good universality and is applicable to strain-induced twinning metal material systems, providing an effective way to expand the types of gradient nanostructure materials.

[0147] Example 10

[0148] The difference between Example 10 and Example 4 is that the raw material used is 304 stainless steel, named 304-GCR20-high strain rate.

[0149] Example 11

[0150] The difference between Example 11 and Example 5 is that the raw material used is 304 stainless steel, named 304-GCR40-high strain rate.

[0151] Example 12

[0152] The difference between Example 12 and Example 6 is that the raw material used is 304 stainless steel, named 304-GCR60-high strain rate.

[0153] Comparative Example 10

[0154] The difference between Comparative Example 10 and Comparative Example 4 is that the raw material used is 304 stainless steel, named 304-CR20-high strain rate.

[0155] Comparative Example 11

[0156] The difference between Comparative Example 11 and Comparative Example 5 is that the raw material used is 304 stainless steel, named 304-CR40-high strain rate.

[0157] Comparative Example 12

[0158] The difference between Comparative Example 12 and Comparative Example 6 is that the raw material used is 304 stainless steel, named 304-CR60-high strain rate.

[0159] Gradient and uniformly deformed 304 stainless steel wires processed by Examples 10, 11 and 12 were selected, and hardness tests were performed using a microhardness tester (load 200g, holding pressure 15s). The test directions were the horizontal and vertical directions of the sample cross-section. The test path was to take points every 150μm from the surface to the core (≥12 data points per group).

[0160] For gradient deformation samples Figure 18 The hardness distribution results at 20%, 40%, and 60% all exhibited a clear reverse gradient distribution characteristic, meaning the hardness increased from the edge of the sample cross-section to the center. The gradient slope in the horizontal direction was significantly greater than that in the vertical direction; for example, in GCR20, the horizontal hardness difference could reach over 150 Hv, while the vertical difference was approximately 70 Hv.

[0161] The results verify the effect of different strain distributions of rolling deformation on hardness regulation, providing a quantitative basis for gradient structure design.

[0162] This embodiment analyzes the influence of gradient deformation and uniform deformation on the mechanical properties of 304 stainless steel under high strain rate by comparing the tensile stress-strain curves of 20%, 40%, and 60% rolling deformation under high-rate gradient deformation and uniform deformation in Examples 10, 11, and 12. Figure 19 During medium-to-high rate gradient deformation, 20% exhibits a yield strength of approximately 600 MPa, displaying typical ductile fracture characteristics. The strain hardening stage lasts until approximately 45% elongation, with a peak stress of 980 MPa. 60% exhibits an ultra-high yield strength of approximately 1260 MPa, but fractures immediately after reaching an ultimate tensile strength of 1340 MPa, with an elongation of only 2%.

[0163] The microstructure and properties of uniform deformation and gradient deformation are compared. The tensile curve of uniform deformation at high strain rate is shown in the figure. Figure 20 As shown. Comparison Figure 19 and 20 It can be seen that the mechanical properties of the sample with gradient deformation are significantly better than those of the sample with uniform deformation.

[0164] The above results demonstrate that the deformation mode has a decisive influence on the tensile behavior of 304 stainless steel. For the gradient deformation samples, 20% and 40% are suitable for applications requiring high plasticity and show the best performance in terms of strength-plasticity matching; 60% can meet the requirements of ultra-high strength applications.

[0165] Comparative Example 13

[0166] The difference between Comparative Example 13 and Example 1 is that the pretreatment step is omitted, that is, the annealing process is omitted.

[0167] The method used in Comparative Example 13 for rolling stainless steel wire yielded unsatisfactory results. This is because, in the unannealed state, the wire had too many crystallographic defects, making it impossible to form a sufficient microstructural gradient after gradient rolling. The effect is analogous to the excessive deformation in Example 3, where a large number of defects were introduced into the edge region of the flat wire, negating its role as a buffer for toughness and ductility. Annealing significantly eliminates the original wire defects, and subsequent moderate gradient rolling (10-50%) can produce high-strength, high-toughness metallic materials with excellent reverse gradient nanotwins.

[0168] Compared with the traditional uniform deformation process, the gradient rolling process of this invention increases the elongation by 50-100% while maintaining a comparable strength level. The reverse gradient structure achieves a synergistic improvement in strength and plasticity through strain distribution optimization. Although the uniform deformation process has a uniform hardness distribution, it sacrifices the overall plasticity of the material.

[0169] The embodiments provided above are not intended to limit the scope of the invention, nor are the described steps intended to limit the order of execution. Any obvious modifications made to the invention by those skilled in the art based on existing common knowledge also fall within the scope of protection defined by the claims.

Claims

1. A method for preparing high-strength and high-toughness metallic materials with reverse gradient nanotwins by wire rolling, characterized in that, Includes the following steps: After annealing and polishing, the metal material is rolled using a multi-pass rolling process, controlling the total deformation to be 10-48%, the reduction per pass to be 0.5-20%, the rolling speed to be 0.1-100 m / min, and the rolling temperature to be -50-200°C. A reverse gradient nanostructure is prepared at ℃, forming a structure from central nanotwins to surface coarse grains. The metal material is selected from austenitic stainless steel or TWIP steel, and is a wire with a diameter Φ of 1~30 mm. The initial microstructure is equiaxed austenite with a grain size of 1~100 μm. The central region of the reverse gradient nanotwin high-strength and tough metal material consists of nanotwins with an average thickness of 10~100 nm. The strain concentration region is composed of nano-α' phase, and the transition region consists of nanotwins and microtwins with a grain size of 100~500 nm. The edge region consists of coarse grains and microtwins. In this reverse gradient nanotwin high-strength and tough metal material, the thickness of the twins continuously decreases from the edge to the center.

2. The method according to claim 1, characterized in that, The total deformation should be controlled at 12-40%, and the reduction in each pass should be controlled at 1-10%.

3. The method according to claim 1, characterized in that, The annealing process involves raising the temperature of the annealing environment to 900-1200℃, holding it at this temperature for 1-30 minutes, and then quickly transferring it to water for quenching after annealing, with a dwell time of <3 seconds. The polishing process involves reducing the surface roughness of the annealed metal material to Ra<0.8μm through mechanical polishing, chemical polishing, or electrochemical polishing.

4. The method according to claim 1, characterized in that, The specific steps of the multi-pass rolling process are as follows: (1) Adjust the initial rolling gap of the twin-roll mill to the original diameter of the metal material to be processed; (2) Control the amount of reduction in each pass, and continuously roll the metal material to be processed in each pass. Repeat this step until the total deformation reaches the required amount.

5. The method according to claim 4, characterized in that, During rolling, the cumulative reduction in the previous passes accounts for 50-90% of the total reduction.

6. The high-strength and high-toughness metallic material with reverse gradient nanotwins prepared by the method according to any one of claims 1 to 5.

7. The high-strength and high-toughness metallic material with reverse gradient nanotwins according to claim 6, characterized in that, The grain size of the edge region of the reverse gradient nanotwin high-strength and tough metallic material is 1~100μm.

8. The high-strength and high-toughness metallic material with reverse gradient nanotwins according to claim 6, characterized in that, When the metal material is 316L stainless steel, its mechanical properties meet the following requirements: tensile strength ≥ 600 MPa; elongation ≥ 20%; when the metal material is 304 stainless steel, its properties meet the following requirements: tensile strength ≥ 800 MPa; elongation ≥ 20%.

9. The application of the reverse gradient nanotwin high-strength and tough metallic material according to any one of claims 6 to 8 in the manufacture of cable armor, aerospace fasteners, automotive anti-collision components or other structural components with stringent requirements for strength and toughness.

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