A method for reducing the rigidity of a metal plate by rolling strengthening

Abstract

The present application relates to the technical field of metal plate rolling processing, and discloses a rolling strengthening method capable of reducing the rigidity of a metal plate, comprising: applying asymmetric rolling conditions to an initial plate to generate a velocity gradient in the thickness direction of the plate; performing one-pass large-reduction rolling on the plate to form discontinuously distributed shear bands; heating the rolled plate to its dynamic recrystallization temperature interval and performing multi-pass small-reduction warm rolling on the plate; in the process of warm rolling, promoting the dynamic recrystallization of the shear bands to form a fine-grained continuous core layer, and inhibiting the complete recrystallization of the surface layer structure of the plate to maintain its work-hardened state, thereby obtaining a metal plate with lower rigidity than the initial plate. The present application constructs a heterogeneous structure of soft core-hard shell in the plate, and cooperatively realizes high strength and low macro-rigidity, and has strong controllability and is suitable for the preparation of high-performance structural materials.

Description

Technical Field

[0001] This invention relates to the field of metal sheet rolling technology, and in particular to a rolling strengthening method that can reduce the stiffness of metal sheets. Background Technology

[0002] With the rapid development of high-end equipment manufacturing, higher requirements are being placed on the performance of metal structural materials. In fields such as aerospace and new energy vehicles, components are not only required to have high strength to ensure load-bearing safety, but also are expected to have lower macroscopic stiffness or higher elastic deformation capacity to achieve functions such as energy absorption, buffering, vibration reduction, and noise reduction.

[0003] However, the stiffness of metallic materials is mainly determined by the interatomic bonding force, which is an intrinsic property. Although traditional strengthening methods (such as cold rolling, alloying, and heat treatment) can improve strength, they have limited impact on the elastic modulus and are difficult to achieve a synergistic improvement of "high strength and low macroscopic stiffness".

[0004] Existing rolling processes, such as cryogenic rolling, asynchronous rolling, and cumulative rolling, can improve material properties, but none of them systematically solve the technical problem of "actively reducing macroscopic stiffness while significantly improving strength." Therefore, a new rolling strengthening method is urgently needed to construct a heterogeneous structure within a single sheet metal to achieve synergistic control of high strength and low macroscopic stiffness. Summary of the Invention

[0005] The purpose of this invention is to solve the problem that high strength and low macroscopic stiffness cannot coexist in the prior art by proposing a rolling strengthening method that can reduce the stiffness of metal sheets.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: a rolling strengthening method for reducing the stiffness of metal sheets, comprising the following steps:

[0007] Step S1: Apply asymmetric rolling conditions to the initial sheet material to generate a velocity gradient in the thickness direction of the sheet material.

[0008] Step S2: Under asymmetric rolling conditions, the sheet metal is subjected to a single large reduction rolling process, causing shear instability in the internal metal of the sheet metal and forming discontinuous shear bands.

[0009] Step S3: Heat the rolled sheet to its dynamic recrystallization temperature range, and perform multiple passes of low-reduction temperature rolling on the sheet within the dynamic recrystallization temperature range.

[0010] Step S4: During the warm rolling process, by differentially controlling the process parameters, the shear band is dynamically recrystallized to form a fine-grained continuous core layer, and the surface structure of the sheet is completely recrystallized to maintain its work-hardened state, thereby obtaining a metal sheet with a stiffness lower than that of the initial sheet.

[0011] Furthermore, step S1 also includes the following sub-steps:

[0012] S1-1 uses a dual-drive reversible rolling mill, configuring the main drive motors of the upper and lower work rolls of the rolling mill to an independent speed regulation mode. According to the material and thickness of the plate, the speed commands of the upper and lower work rolls are set so that the ratio of the linear speeds of the upper and lower work rolls is 1.2 to 2.0, establishing a basic linear speed difference.

[0013] S1-2, an alumina-titanium carbide composite coating is prepared on the surface of the upper working roll by plasma spraying process, with a friction coefficient of 0.45 to 0.60. At the same time, a graphite-based lubricant is uniformly applied to the surface of the lower working roll, with a friction coefficient of 0.15 to 0.25, to establish differentiated friction interface conditions and enhance the shear effect caused by the difference in basic linear velocity.

[0014] S1-3, start the main drive motor, adjust the roll gap to the initial preset value through the hydraulic pressing system of the rolling mill, and feed the metal sheet to be processed into the rolling mill so that the work rolls with different surface conditions and linear speeds bite into the sheet at the same time, forming a stable rolling deformation zone, ensuring that the sheet is subjected to the synergistic effect of shear stress and compressive stress in the deformation zone.

[0015] Furthermore, step S2 also includes the following sub-steps:

[0016] S2-1, calculate the reduction rate based on the inlet thickness and target outlet thickness of the plate to be processed, set the roll gap value of the mill, ensure that the reduction rate is 30% to 50%, and after the plate is rolled in this pass, the cumulative equivalent plastic strain in the continuous region including the geometric center in the thickness direction exceeds the critical strain value that induces shear instability. The thickness of the continuous region accounts for 40% to 60%, and the critical strain value is calibrated through pre-experiment.

[0017] S2-2, while keeping the linear velocity difference and friction interface conditions set in step S1 unchanged, drive the work roll to rotate at a linear velocity of 1.5 to 3.0 m / s, so that the plate passes through the roll gap continuously to complete one large reduction rolling.

[0018] S2-3 During the rolling process, the bending roll system and the shifting roll system of the rolling mill are started. By detecting the plate shape deviation in real time, a laser plate shape meter is used to dynamically adjust the bending roll force and the shifting roll amount. The bending roll force is 50 to 200 kN and the shifting roll amount is 50 to 150 mm to compensate for the thinning of the plate edge or the wavy defects in the middle caused by asymmetrical deformation.

[0019] S2-4, Deploy an online ultrasonic flaw detection system on the mill exit side. The probe array is evenly distributed along the width of the plate, with a probe spacing of 50 to 100 mm and a detection frequency set to 5 to 10 MHz.

[0020] S2-5. The ultrasonic flaw detection system collects and analyzes ultrasonic echo signals. When a discontinuous high-reflection area with a specific frequency and attenuation characteristics appears in the signal, it is determined that a discontinuous shear band has been formed inside the plate. The specific frequency is 2 to 4 MHz and the attenuation coefficient is ≥3 dB / cm. If this feature is not detected, the reduction rate is adjusted and the plate is rolled again.

[0021] The critical strain value for shear instability is determined as follows: Under the same asynchronous rolling conditions, single-pass rolling tests with reduction rates of 20%, 30%, 40%, and 50% are conducted on samples of the same material; the rolled samples are cut open along the thickness direction, and after grinding, polishing, and etching, metallographic samples including the geometric center of the thickness are prepared, and the microstructure is observed using an optical microscope or a scanning electron microscope; the minimum equivalent plastic strain value corresponding to the first observation of the discontinuous shear band near the geometric center is determined as the critical strain value for shear instability of the material under the current process conditions, which is usually 0.8 to 1.2.

[0022] The steps for determining discontinuous shear bands include: performing time-frequency analysis on the acquired ultrasonic echo signals using Fourier transform to extract their energy distribution within a specific frequency band determined by the material's acoustic properties and the size of the shear band; calculating the ratio of the local peak value of the signal energy within this specific frequency band to the average background energy within an adjacent 50mm range along the width direction of the plate; when the ratio exceeds 3.0, and the width positions exhibiting this high ratio feature are symmetrically distributed along the rolling centerline, then it is determined that a discontinuous shear band has been formed.

[0023] Furthermore, step S3 also includes the following sub-steps:

[0024] S3-1, Immediately place the rolled sheet from step S2 into a continuous heating furnace, embed the temperature control thermocouple of the heating furnace into the middle area of ​​the sheet thickness direction, set the heating temperature according to the material of the sheet, so that the thermocouple reading reaches the dynamic recrystallization temperature range of the material: 350 to 450°C for aluminum alloy, 300 to 400°C for magnesium alloy, and 800 to 900°C for stainless steel, and hold for 10 to 20 minutes to ensure uniform sheet temperature.

[0025] S3-2, the heated plate is fed into the first warm rolling mill at a speed of 0.8 to 1.5 m / s for rolling. The roll gap of the rolling mill is set so that the reduction in this pass is 5% to 10%, and the resulting strain energy density is lower than the preset upper limit value. The preset upper limit value is 80% of the critical unit strain energy increment that causes the increase in dislocation density of the surface structure.

[0026] S3-3, the sheet after the first warm rolling is conveyed to the heat-insulating roller table or tunnel furnace. The roller table temperature is maintained at the lower limit of the dynamic recrystallization temperature range ±20℃. The conveying speed is controlled so that the dwell time of the sheet before entering the next warm rolling mill is limited to a preset time window.

[0027] S3-4, Repeat steps S3-2 and S3-3 to allow the sheet to pass through 3 to 5 warm rolling mills and corresponding insulated conveyor sections in sequence to complete multiple warm rolling passes, with the total reduction rate controlled between 20% and 35%.

[0028] The preset upper limit and preset time window are determined as follows: A differential scanning calorimeter is used to measure the minimum unit strain energy increment required for dynamic recrystallization nucleation in the core shear band, and the critical unit strain energy increment that triggers an increase in dislocation density in the surface structure. The preset upper limit is set to be 80% lower than the critical unit strain energy increment. A metallographic microscope is used to observe the growth of dynamically recrystallized grains in the core and the changes in dislocation density in the surface structure in real time. The shortest time required for the dynamically recrystallized grains in the core to complete one round of growth within the warm rolling interval is measured (grain size increase of 50%), and the starting time for significant static recovery of the surface structure is measured (dislocation density decrease of 20%). The preset time window is set to be longer than the shortest time and shorter than the starting time, typically 3 to 8 minutes.

[0029] Furthermore, step S4 also includes the following sub-steps:

[0030] S4-1, in the final pass of multi-pass warm rolling, the reduction is controlled to be 8% to 12% so that the deformation heat generated in this pass can maintain the temperature of the shear band region within the range where dynamic recrystallization continues, within ±30℃ in the middle of the dynamic recrystallization temperature range, in order to promote the formation of a fine-grained continuous core layer.

[0031] S4-2 controls the cumulative strain of at least the last two passes in multi-pass warm rolling to be lower than the critical strain threshold that can induce dynamic recrystallization of the surface structure. The critical strain threshold is calibrated through pre-experimentation.

[0032] In S4-3, between each pass of the warm rolling process, a flowing cooling medium is sprayed onto the surface of the sheet at a spray angle of 30 to 60° and a flow rate of 50 to 100 L / min through an atomizing spray device installed above the roller table. The cooling medium is a water-based atomizing coolant containing 5% to 10% mineral oil additives. This reduces the temperature rise between passes, lowers the surface temperature by 20 to 50°C, and inhibits static recrystallization of the surface structure, thereby maintaining its work-hardened state.

[0033] The critical strain threshold is determined as follows: In a warm rolling simulation experiment with the same process conditions, a microhardness tester is used to monitor the microhardness change of the surface structure, with a load of 500g and a holding time of 10s, or the dislocation density change is observed by transmission electron microscopy; the cumulative strain value corresponding to the inflection point of microhardness or the significant decrease in dislocation density is calibrated as the critical strain threshold, where a 10% decrease in hardness corresponds to a 20% decrease in dislocation density, typically ranging from 0.5 to 0.8.

[0034] The flowing cooling medium is an atomized coolant. Its spray angle, flow rate and atomization degree are configured to reduce the surface temperature without causing uneven warping of the board as a whole. The spray parameters are optimized through numerical simulation to ensure that the temperature difference between the surface and core layers of the board is controlled between 30 and 80°C and the board warping is ≤3mm / m.

[0035] The beneficial effects of the technical solution provided by this invention include at least the following:

[0036] This invention first employs asynchronous rolling with a large reduction to pre-place a discontinuous shear band in the core region of the sheet as a structural precursor, and then performs multi-pass low-reduction warm rolling in the dynamic recrystallization temperature zone. This enables differentiated and competitive control of the core and surface structures of the sheet, thereby actively constructing a stable heterogeneous structure of "soft core-hard shell" within a single sheet.

[0037] This invention, through the constructed "soft core-hard shell" heterogeneous structure, can significantly improve the overall yield strength and tensile strength of the plate while reducing its macroscopic flexural elastic modulus compared to the original plate, effectively solving the technical contradiction that high strength and low macroscopic stiffness are difficult to coexist.

[0038] This invention combines an online ultrasonic flaw detection system with quantitative process design, enabling real-time monitoring and determination of the formation state of shear bands and the recrystallization process of the core layer, thereby significantly improving process stability and product performance consistency.

[0039] This invention introduces calibration methods for key parameters such as critical strain threshold, dynamic recrystallization window, and upper limit of strain energy density, enabling precise reverse setting of process parameters based on target performance, thus achieving on-demand customization and flexible control of product stiffness and strength indicators. Attached Figure Description

[0040] To more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0041] Figure 1 This is a schematic diagram of the method flow provided in an embodiment of the present invention. Detailed Implementation

[0042] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the following, in conjunction with the accompanying drawings and preferred embodiments, details the specific implementation, structure, features, and effects of a rolling strengthening method for reducing the stiffness of metal sheets according to the present invention. In the following description, different "one embodiment" or "another embodiment" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form.

[0043] 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 this invention pertains.

[0044] The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0045] The following describes in detail, with reference to the accompanying drawings, a specific scheme of the rolling strengthening method for reducing the stiffness of metal sheets provided by the present invention.

[0046] Please see Figure 1 The diagram illustrates a rolling strengthening method for reducing the stiffness of sheet metal according to an embodiment of the present invention, comprising the following steps:

[0047] Step S1: Apply asymmetric rolling conditions to the initial sheet material to generate a velocity gradient in the thickness direction of the sheet material.

[0048] Step S2: Under asymmetric rolling conditions, the sheet metal is subjected to a single large reduction rolling process, causing shear instability in the internal metal of the sheet metal and forming discontinuous shear bands.

[0049] Step S3: Heat the rolled sheet to its dynamic recrystallization temperature range, and perform multiple passes of low-reduction temperature rolling on the sheet within the dynamic recrystallization temperature range.

[0050] Step S4: During the warm rolling process, by differentially controlling the process parameters, the shear band is dynamically recrystallized to form a fine-grained continuous core layer, and the surface structure of the sheet is completely recrystallized to maintain its work-hardened state, thereby obtaining a metal sheet with a stiffness lower than that of the initial sheet.

[0051] It should be noted that asymmetric rolling conditions refer to the process of creating uneven metal flow along the thickness direction of the plate during rolling deformation by applying different linear velocities to the upper and lower work rolls of the rolling mill (e.g., using an asynchronous rolling mill or rolls with different diameters) or applying differentiated friction conditions to the upper and lower surfaces of the plate. This results in a velocity gradient and additional shear stress.

[0052] A single large reduction rolling process refers to applying a large thickness compression to the sheet metal in one rolling stroke, with a reduction rate usually not less than 50%. The aim is to make the strain energy accumulated in the core area of ​​the sheet metal quickly reach and exceed the critical value that induces "shear instability" in the metal.

[0053] Shear instability is an instability mode in the process of plastic deformation of metals. When the shear strain rate in a local area exceeds a certain critical value, the deformation will concentrate in a narrow band-shaped region, resulting in the formation of a "shear band" with high-density dislocation entanglement.

[0054] Discontinuous shear bands refer to a series of spatially discontinuous but banded microstructure regions with extremely high distortion energy, which serve as "high-energy precursors" for subsequent recrystallization processes.

[0055] The dynamic recrystallization temperature range refers to the temperature range within which a metal can undergo dynamic recrystallization simultaneously during plastic deformation. This means that new grains nucleate and grow during the deformation process. This temperature range is usually between the material's recrystallization temperature and melting point, and can be calibrated through thermal simulation tests.

[0056] Multi-pass low-reduction warm rolling refers to continuous rolling in multiple passes (e.g., 3 to 6 passes) within a temperature range with a single-pass reduction rate (e.g., 5% to 20%) that is significantly lower than the aforementioned "large reduction". Its main purpose is to continuously provide a mild mechanical driving force to promote specific microstructure transformation rather than to achieve large deformation.

[0057] Differentiated control of process parameters refers to the differentiated design and control of parameters such as pass reduction, rolling temperature, pass interval time and cooling conditions during the warm rolling process in order to achieve different microstructure evolution targets for the core and the surface.

[0058] A fine-grained continuous core layer refers to an equiaxed grain layer formed by dynamic recrystallization, with a significantly refined average grain size (e.g., reaching submicron or nanometer scale) and continuously distributed in the central region of the plate thickness. This layer has relatively low rheological stress.

[0059] Maintaining its work-hardened state means controlling the process to prevent the deformation structure on the surface of the sheet from undergoing complete recrystallization and softening, thereby preserving the strengthening characteristics such as high dislocation density introduced by the large deformation in the early stage and maintaining a high yield strength.

[0060] The core innovation of this application lies in proposing and implementing a reverse material design path of "pre-setting a high-energy defect structure and then selectively reconstructing the structure through thermo-mechanical coupling". This path aims to actively construct a "soft core-hard shell" heterostructure to synergistically achieve high strength and low macroscopic stiffness.

[0061] Its general working principle is as follows: First, steps S1 and S2 do not pursue the uniform densification of traditional rolling, but actively and controllably utilize large strain asynchronous rolling to create a "high-energy structural precursor" in the core of the plate in the form of discontinuous shear bands. These shear bands are regions with extremely high distortion energy and structural defect density, providing the necessary driving force and nucleation sites for subsequent microstructure reconstruction.

[0062] Subsequently, steps S3 and S4 constitute the "competitive structural control" stage. The sheet containing shear bands is placed in its dynamic recrystallization temperature range for warm rolling, cleverly utilizing the intrinsic differences in the response of the core shear bands (high energy storage, high defect density region) and the surface matrix (relatively low energy storage region) to the same thermo-mechanical coupling conditions. By "differentiatedly controlling" the warm rolling process parameters (such as pass reduction, interval time, and cooling conditions), the input energy can be "selectively" utilized: the core shear bands, due to their high driving force, preferentially undergo sufficient dynamic recrystallization, thus "transforming" into a uniform and refined soft continuous core layer; simultaneously, the energy input to the surface layer and the thermal interaction time are precisely controlled to prevent them from triggering complete static or dynamic recrystallization, thereby "preserving" and stabilizing the work-hardened state imparted by the preceding large deformation.

[0063] Ultimately, the synergistic effect of these steps constructs a stable layered composite structure with distinct properties within a single sheet material. Under macroscopic bending loads, the "soft core" readily undergoes significant elastic deformation, thereby substantially reducing the overall macroscopic bending stiffness of the component; while the "hard shell" provides the primary load-bearing strength and buckling stability. This method thus breaks the traditional "inverse relationship" between material strength and stiffness.

[0064] In one specific implementation, a 304 austenitic stainless steel sheet with an initial thickness of 3 mm was used as the processing object. First, steps S1 and S2 were performed on a four-roll asynchronous rolling mill. The speed ratio of the upper and lower work rolls was set to 1:1.2 to establish a speed gradient, and a friction-increasing coating was sprayed onto the upper roll surface, while the lower roll surface was kept lubricated to enhance the shear effect. Under these conditions, the sheet was rolled to 0.9 mm with a 70% reduction in one pass. Online ultrasonic testing and subsequent cross-sectional metallographic verification confirmed that a diffusely distributed discontinuous shear band had formed in the core of the sheet.

[0065] Next, step S3 is executed: the rolled plate is sent into a continuous gas-fired radiant tube heating furnace, the furnace temperature is set to 850℃ (this temperature is in the active range of dynamic recrystallization of this steel grade), and the thermocouple reading set in the middle of the plate thickness reaches the set value to ensure that the core area enters the target temperature zone.

[0066] Then, step S4, "differentiated control," is executed: the heated sheet material is fed into a three-stand warm rolling mill for multi-pass, low-reduction warm rolling. The roll gap of the first stand is set to roll the sheet from 0.9mm to 0.83mm (reduction rate of approximately 7.8%). After rolling, the sheet enters the holding roller table connected to the heating furnace, stays for approximately 30 seconds, and then enters the second stand, rolling to 0.77mm (reduction rate of approximately 7.2%). After another holding period, it enters the third stand, rolling to a final thickness of 0.72mm (reduction rate of approximately 6.5%). The total cumulative reduction during the entire warm rolling process is 20%, far lower than the initial 70%, which meets the "low reduction" characteristic. During this process, forced cooling of the surface is achieved by adjusting the cooling water spray between the finishing mill stands (spraying only on the upper and lower surfaces of the sheet), suppressing static recovery and recrystallization between passes.

[0067] After the above-described process, a three-point bending test was performed on the resulting 0.72 mm thick sheet. Its flexural modulus of elasticity was approximately 145 GPa, a decrease of about 25.6% compared to the original fully annealed 304 stainless steel sheet (approximately 195 GPa). Simultaneously, its room temperature yield strength increased from approximately 205 MPa to over 650 MPa. This example fully verifies that the method described in this application can successfully achieve the intended technical effect of "reducing macroscopic stiffness while simultaneously increasing strength."

[0068] Step S1 further includes the following sub-steps:

[0069] S1-1, configure the main drive motors of the upper and lower work rolls of the rolling mill to independent speed regulation mode, and set different speed commands to establish a basic linear speed difference. The setting value of the speed command makes the ratio of the linear speeds of the upper and lower work rolls not equal to 1.

[0070] S1-2, spray or embed high friction coefficient material on the surface of the upper working roll, and apply lubricant or cover the surface of the lower working roll with low friction coefficient coating to establish differentiated friction interface conditions to enhance the shear effect caused by the difference in basic linear velocity.

[0071] S1-3, start the main drive motor and adjust the roll gap so that the work rolls with different surface conditions and linear speeds bite into the plate at the same time, forming a stable rolling deformation zone.

[0072] It should be noted that the independent speed control mode refers to the independent control of the drive motors of the upper and lower work rolls, enabling them to operate according to their respective set speed commands, thereby allowing a continuous and controllable speed difference between the two.

[0073] The difference in basic linear velocity refers to the difference in surface linear velocity caused by the different rotational speeds of the upper and lower working rollers. This difference is the root cause of asymmetric shear flow in the metal inside the sheet.

[0074] The ratio of linear speeds not equal to 1 is the core quantitative definition of "asynchronous rolling". It excludes the conventional rolling state where the upper and lower rolls have the same speed, and ensures the inevitable introduction of shear deformation.

[0075] High friction coefficient materials refer to materials such as sandblasted roller surfaces, embedded hard particles such as tungsten carbide, or coated with special friction coatings, which aim to increase the interlocking and dragging effect between the roller surface and the upper surface of the plate.

[0076] Low-friction coefficient coatings, such as Teflon coatings, graphite lubricants, or high-efficiency rolling oil films, are designed to reduce the frictional resistance between the roll surface and the underside of the sheet, thus promoting sliding.

[0077] Differentiated friction interface conditions refer to the asymmetric friction state actively created on the upper and lower surfaces of the plate through the above-mentioned means. This condition, together with the "basic linear velocity difference", jointly determines the velocity gradient shape and intensity in the thickness direction of the plate.

[0078] A stable rolling deformation zone refers to the area where the sheet metal is continuously and smoothly bitten into, compressed, and delivered out of the rolls under set roll gap, speed difference, and friction conditions. The establishment of stability in this zone is a prerequisite for the controllability of all subsequent deformation processes.

[0079] This application systematically constructs a highly controllable "non-uniform deformation environment" through step S1 and its sub-steps. This is the primary and crucial foundational step for the subsequent manufacturing of "gradient structures". Specifically, this application does not regard "asymmetric rolling" as a single, general process option, but decomposes and strengthens it into a dual synergistic control of "speed difference drive" and "friction difference modulation".

[0080] Step S2 further includes the following sub-steps:

[0081] S2-1, Set the target exit thickness, and calculate and set the roll gap value of the rolling mill based on the target exit thickness and the plate entry thickness. The target exit thickness must ensure that after the plate is rolled in this pass, the cumulative equivalent plastic strain in a continuous region including the geometric center in the thickness direction exceeds the critical strain value that induces shear instability.

[0082] S2-2, under the condition of maintaining the difference in linear velocity and the friction interface, drive the work roll to rotate so that the plate passes through the roll gap in a continuous manner to complete one large reduction rolling.

[0083] S2-3, Start the rolling mill's bending and shifting roll system to dynamically adjust the roll shape to compensate for edge thinning or center wavy defects in the sheet metal caused by asymmetrical deformation.

[0084] S2-4, Deploy an online ultrasonic flaw detection system on the mill exit side, so that its probe array scans along the width direction of the plate;

[0085] S2-5: Collect and analyze ultrasonic echo signals. When a discontinuous high-reflection region with specific frequency and attenuation characteristics appears in the signal, it is determined that a discontinuous shear band has been formed.

[0086] Furthermore, in sub-step S2-1, the cumulative equivalent plastic strain exceeding the critical strain value for inducing shear instability within a continuous region of the geometric center is determined as follows:

[0087] Under the same asynchronous rolling conditions, a series of single-pass rolling tests with different reduction rates were conducted on samples of the same material.

[0088] The rolled sample was cut open along the thickness direction to prepare a metallographic sample including the geometric center of the thickness for observation.

[0089] The minimum equivalent plastic strain value corresponding to the discontinuous shear band structure will be observed for the first time near the geometric center, and will be calibrated as the critical strain value for shear instability of the material under the processing conditions.

[0090] Furthermore, in sub-steps S2-5, the steps for determining the formed discontinuous shear band include:

[0091] The collected ultrasonic echo signals are subjected to time-frequency analysis to extract their energy distribution in a specific frequency band determined by the acoustic properties of the material and the shear band size.

[0092] Along the width of the plate, calculate the ratio of the local peak value of the signal energy in this specific frequency band to the average background energy of the adjacent region;

[0093] When the ratio exceeds a preset threshold, and the width of the high ratio feature is symmetrically distributed along the rolling centerline, it is determined that a discontinuous shear band has been formed.

[0094] It should be noted that the target exit thickness refers to the final thickness that the sheet should reach after step S2, which is preset for single-pass large reduction rolling. This thickness is not arbitrarily set, but rather the starting point for all subsequent process calculations.

[0095] A continuous region containing a geometric center in the thickness direction specifically refers to a volumetric region that extends continuously in the thickness direction, with the theoretical geometric center plane of the plate as its core, extending upwards and downwards to both sides. This region is the target space for "shear instability" and the subsequent formation of the "core layer," and its extent can be defined by metallographic observation, for example, the central 1 / 3 region in the thickness direction.

[0096] Cumulative equivalent plastic strain is a scalar quantity that integrates multiaxial strain components and is used to quantify the degree of plastic deformation. Its value is distributed in a gradient along the thickness direction of the plate.

[0097] The critical strain value for inducing shear instability is a key material-process coupling parameter in this invention. It refers to the minimum equivalent plastic strain required to transform the material core from uniform plastic flow to localized shear band deformation under specific asynchronous rolling conditions. This value is not a material constant, but a state function related to the initial microstructure, temperature, and strain rate.

[0098] The linear velocity difference and friction interface conditions specifically refer to the combination of process states established and maintained by step S1 for generating a strong velocity gradient.

[0099] The bending roll and shifting roll systems are the core shape control actuators of modern strip mills. The bending roll system instantly changes the deflection of the work roll by applying hydraulic bending force, thereby adjusting the roll gap shape. The shifting roll system uniformizes roll wear and optimizes contact pressure distribution by periodically moving the work roll along its axis. The two systems work together to dynamically compensate for uneven load-bearing roll gaps caused by asymmetrical deformation.

[0100] An online ultrasonic flaw detection system is a device system integrated at the exit side of a rolling production line to perform real-time non-destructive testing on moving sheet metal. Its "probe array" consists of multiple ultrasonic probes arranged at certain intervals, enabling comprehensive scanning of the entire width of the sheet metal.

[0101] The discontinuous high-reflection region with specific frequency and attenuation characteristics describes the typical ultrasonic response of the shear band: the specific frequency is determined by the characteristic thickness and acoustic impedance of the shear band; the high attenuation characteristics originate from the strong scattering of sound waves by high-density defects within the shear band; discontinuity refers to the spatially discontinuous distribution of the reflected signal, corresponding to the discrete shape of the shear band.

[0102] Step S3 further includes the following sub-steps:

[0103] S3-1, The rolled sheet is placed into the heating furnace, and the temperature control thermocouple of the heating furnace is set in the middle area of ​​the sheet thickness direction. The sheet is determined to have been heated to the dynamic recrystallization temperature range when the thermocouple reading reaches the preset value.

[0104] S3-2, The heated plate is fed into the first warm rolling mill for rolling. The roll gap of the rolling mill is set so that the strain energy density generated by the reduction in this pass is lower than the preset upper limit value.

[0105] S3-3, the first rolled plate is conveyed to the heat-insulating roller table or tunnel furnace, and its conveying speed is controlled so that the dwell time of the plate before entering the next warm rolling mill is limited to a preset time window;

[0106] S3-4, repeat S3-2 and S3-3, so that the plate passes through the subsequent warm rolling mill and the heat preservation conveyor section in sequence to complete multiple warm rolling passes.

[0107] Furthermore, the preset upper limit and preset time window are determined in the following way:

[0108] The minimum unit strain energy increment required to induce dynamic recrystallization nucleation in the core shear band, and the critical unit strain energy increment that triggers an increase in dislocation density in the surface structure, were measured.

[0109] Set the preset upper limit value to be lower than the critical unit strain energy increment;

[0110] The shortest time required for the core dynamic recrystallized grains to complete one round of growth during the warm rolling interval was measured, as well as the start time for the surface microstructure to undergo significant static recovery.

[0111] Set the preset time window to be longer than the shortest time but shorter than the start time.

[0112] It should be noted that the central region in the thickness direction of the sheet material specifically refers to a specific area extending along the thickness direction with the geometric center plane of the sheet material as the core. The temperature control thermocouple is set here to directly monitor and control the real-time temperature of the core target area, ensuring the accuracy of heat treatment.

[0113] The dynamic recrystallization temperature range refers to the process window within which a material can undergo dynamic recrystallization simultaneously during plastic deformation. Its lower limit is the temperature at which dynamic recrystallization begins, and its upper limit is usually close to but lower than the abnormal grain coarsening temperature of the material.

[0114] Strain energy density refers to the elastoplastic strain energy stored per unit volume of material during a single rolling deformation. Its value is related to the reduction, rheological stress, and deformation heat effect, and is a key indicator for measuring the energy input intensity of a single deformation.

[0115] The preset upper limit is a process safety threshold set for the "strain energy density". Its core function is to limit the "impact" intensity of a single deformation on the surface tissue.

[0116] Insulated roller conveyors or tunnel furnaces refer to conveying equipment with heat preservation functions, used to maintain the temperature of the sheet material between warm rolling passes and prevent the temperature from dropping below the dynamic recrystallization window due to heat loss.

[0117] The preset time window refers to the allowable range set for the heat preservation dwell time between passes. Its lower limit must ensure that the core recrystallization process can continue, and its upper limit must prevent significant static softening of the surface structure.

[0118] The minimum unit strain energy increment refers to the minimum energy input threshold required to trigger dynamic recrystallization nucleation in the shear band region of the core. If the value is lower than this, the nucleation driving force is insufficient.

[0119] The critical unit strain energy increment is the minimum energy input required for a net increase in the dislocation density of the surface layer (i.e., the work hardening effect is enhanced). Exceeding this value may drive undesirable dynamic recrystallization in the surface layer.

[0120] The shortest time required to complete one growth cycle refers to the shortest process time required for a dynamically recrystallized grain in the core to grow from nucleation to a stable size at a given temperature. If the time is shorter than this, the recrystallization process is insufficient.

[0121] The onset time of obvious static recovery refers to the point at which the dislocation structure of the surface tissue begins to undergo significant reorganization during the heat preservation process, resulting in a measurable decrease in hardness. Beyond this time, the work-hardened state begins to degrade.

[0122] In one specific implementation, taking 6061 aluminum alloy sheet as an example, the determination process is explained in detail:

[0123] 1. Determine the preset upper limit value

[0124] Under the same asynchronous rolling conditions (upper and lower roll speed ratio 1.2, friction coefficient difference 0.3), a set of 6061 aluminum alloy samples with a thickness of 2 mm were prepared.

[0125] Single-pass warm rolling tests were conducted at different reduction rates (5%, 8%, 10%, and 12%) within the dynamic recrystallization temperature range (approximately 350°C).

[0126] Immediately after rolling, the sample was water-quenched and taken along the thickness direction. The presence of dynamic recrystallization nuclei in the shear zone region of the core was observed using a transmission electron microscope. The nucleation enthalpy change was measured by differential scanning calorimetry, and the minimum unit strain energy increment was calculated to be approximately 15 MJ / m³.

[0127] Meanwhile, the change in dislocation density in the surface tissue was observed, and it was found that when the reduction rate was ≥10%, the surface dislocation density increased significantly, and the critical unit strain energy increment was calculated to be approximately 20 MJ / m³.

[0128] Therefore, the "preset upper limit" is set to 16 MJ / m³, which is below the critical value (20 MJ / m³), to ensure that core recrystallization is promoted without causing excessive hardening of the surface.

[0129] 2. Determine the preset time window

[0130] Under a heat treatment condition of 350℃, the samples that have formed shear bands were subjected to isothermal treatment for 1, 3, 5, 8 and 10 minutes respectively.

[0131] Real-time observation of the dynamic recrystallization grain size changes in the core using a metallographic microscope revealed that the grain size increased by about 50% after 3 minutes of heat preservation, completing one round of growth.

[0132] By measuring the surface hardness change with a microhardness tester, it was found that the hardness began to decrease after 8 minutes of heat treatment, indicating that static recovery had begun.

[0133] Therefore, the "preset time window" is set to 3 to 7 minutes to ensure sufficient recrystallization of the core while avoiding softening of the surface.

[0134] 3. Verification Results

[0135] After multiple warm rolling processes using the above parameters, the core of the prepared sheet has a uniform fine-grained structure (average grain size ≈ 2 μm), while the surface layer maintains a high dislocation density.

[0136] Three-point bending tests showed that its flexural modulus of elasticity decreased by about 22% compared with the original plate, and its yield strength increased by about 180%, verifying the feasibility and effectiveness of the parameter settings in this method.

[0137] Step S4 further includes the following sub-steps:

[0138] S4-1, in the final pass of multi-pass warm rolling, controls the amount of reduction so that the deformation heat generated in this pass can maintain the temperature of the shear band region within the range where dynamic recrystallization continues, thereby promoting the formation of a fine-grained continuous core layer.

[0139] S4-2 controls the cumulative strain in at least the last two passes of multi-pass warm rolling to be below the critical strain threshold that can induce dynamic recrystallization of the surface structure.

[0140] S4-3 exposes the surface of the sheet to a flowing cooling medium between each pass of the warm rolling process, reducing the temperature rise between passes and inhibiting static recrystallization of the surface structure, thereby maintaining its work-hardened state.

[0141] Furthermore, in sub-step S4-2, the critical strain threshold is determined in the following manner:

[0142] In a warm rolling simulation with the same process conditions, the changes in microhardness or dislocation density of the surface microstructure were monitored.

[0143] The cumulative strain value corresponding to the inflection point of microhardness or the decrease in dislocation density is calibrated as the critical strain threshold.

[0144] Furthermore, in sub-step S4-3, the flowing cooling medium is an atomized coolant, and its spray angle, flow rate and atomization degree are configured to reduce the surface temperature without causing uneven warping of the entire board.

[0145] It should be noted that the final pass refers to the last rolling operation in a multi-pass warm rolling process sequence. Its reduction control plays both a conclusive and supplementary role.

[0146] Deformation heat refers to the thermal effect in which most of the mechanical work is converted into heat energy during the plastic deformation of a metal, leading to an increase in the material's temperature. This effect can be used to achieve temperature control of specific areas without relying on external heating.

[0147] The range within which dynamic recrystallization continues refers to the temperature range that ensures the continuous and sufficient nucleation and growth of dynamic recrystallization until a complete fine-grained structure is formed. Maintaining this range is a necessary condition for obtaining a continuous and uniform core layer.

[0148] Cumulative strain refers to the total equivalent plastic strain applied to the surface of a material during multiple warm rolling processes. This parameter is the cumulative driving force that induces microstructural changes (such as dynamic recrystallization).

[0149] The critical strain threshold is the minimum cumulative strain required for dynamic recrystallization of the surface layer microstructure. Below this threshold, the strain-driving force is insufficient to overcome the nucleation energy barrier; upon reaching or exceeding this threshold, dynamic recrystallization will be triggered. This threshold is closely related to material composition, initial microstructure, and deformation temperature / rate.

[0150] The interval between passes specifically refers to the process time between the end of one warm rolling pass and the start of the next. This stage is a critical risk window for the material to undergo static recovery and recrystallization.

[0151] Flowing cooling medium refers to gaseous or liquid coolant that acts on the surface of a plate in a forced convection manner. Its flow characteristics ensure the efficiency and uniformity of heat exchange.

[0152] Atomized coolant refers to a mixed fluid in which liquid coolant (such as water-based emulsion) is broken into tiny droplets through a nozzle. This greatly increases the heat exchange surface area and can quickly absorb heat through latent heat of vaporization.

[0153] The spray angle, flow rate, and atomization degree are the three core control dimensions of the cooling process: the spray angle determines the contact method and coverage area between the cooling medium and the plate surface; the flow rate determines the cooling capacity per unit time; and the atomization degree determines the droplet size and evaporation efficiency. The coordinated configuration of these three elements aims to achieve efficient, uniform cooling without additional deformation.

[0154] The above-described embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application, and should all be included within the protection scope of this application.

Claims

1. A rolling strengthening method for reducing the stiffness of sheet metal, characterized in that, Includes the following steps: Step S1: Apply asymmetric rolling conditions to the initial sheet material to generate a velocity gradient in the thickness direction of the sheet material. Step S2: Under asymmetric rolling conditions, the sheet metal is subjected to a single large reduction rolling process, causing shear instability in the internal metal of the sheet metal and forming discontinuous shear bands. Step S3: Heat the rolled sheet to its dynamic recrystallization temperature range, and perform multiple passes of low-reduction temperature rolling on the sheet within the dynamic recrystallization temperature range. Step S4: During the warm rolling process, by differentially controlling the process parameters, the shear band is dynamically recrystallized to form a fine-grained continuous core layer, and the surface structure of the sheet is completely recrystallized to maintain its work-hardened state, thereby obtaining a metal sheet with a stiffness lower than that of the initial sheet. Step S2 further includes the following sub-steps: S2-1, Set the target exit thickness, and calculate and set the roll gap value of the rolling mill based on the target exit thickness and the plate entry thickness. The target exit thickness must ensure that after the plate is rolled in this pass, the cumulative equivalent plastic strain in a continuous region including the geometric center in the thickness direction exceeds the critical strain value that induces shear instability. S2-2, under the condition of maintaining the difference in linear velocity and the friction interface, drive the work roll to rotate so that the plate passes through the roll gap in a continuous manner to complete one large reduction rolling. S2-3, Start the rolling mill's bending and shifting roll system to dynamically adjust the roll shape to compensate for edge thinning or center wavy defects in the sheet metal caused by asymmetrical deformation. S2-4, Deploy an online ultrasonic flaw detection system on the mill exit side, so that its probe array scans along the width direction of the plate; S2-5: Collect and analyze ultrasonic echo signals. When a discontinuous high-reflection region with a specific frequency and attenuation characteristics appears in the signal, it is determined that a discontinuous shear band has been formed. Step S4 further includes the following sub-steps: S4-1, in the final pass of multi-pass warm rolling, controls the amount of reduction so that the deformation heat generated in this pass can maintain the temperature of the shear band region within the range where dynamic recrystallization continues, thereby promoting the formation of a fine-grained continuous core layer. S4-2 controls the cumulative strain in at least the last two passes of multi-pass warm rolling to be below the critical strain threshold that can induce dynamic recrystallization of the surface structure. S4-3 exposes the surface of the sheet to a flowing cooling medium between each pass of the warm rolling process, reducing the temperature rise between passes and inhibiting static recrystallization of the surface structure, thereby maintaining its work-hardened state. In sub-step S4-2, the critical strain threshold is determined in the following manner: In a warm rolling simulation with the same process conditions, the changes in microhardness or dislocation density of the surface microstructure were monitored. The cumulative strain value corresponding to the inflection point of microhardness or the decrease in dislocation density is calibrated as the critical strain threshold.

2. The rolling strengthening method for reducing the stiffness of metal sheets according to claim 1, characterized in that: Step S1 further includes the following sub-steps: S1-1, Configure the main drive motors of the upper and lower work rolls of the rolling mill to independent speed regulation mode and set different speed commands to establish a basic linear speed difference. The setting value of the speed command makes the ratio of the linear speeds of the upper and lower work rolls not equal to 1. S1-2, spray or embed high friction coefficient material on the surface of the upper working roll, and apply lubricant or cover the surface of the lower working roll with low friction coefficient coating to establish differentiated friction interface conditions to enhance the shear effect caused by the difference in basic linear velocity. S1-3, start the main drive motor and adjust the roll gap so that the work rolls with different surface conditions and linear speeds bite into the plate at the same time, forming a stable rolling deformation zone.

3. The rolling strengthening method for reducing the stiffness of metal sheets according to claim 1, characterized in that: In sub-step S2-1, the cumulative equivalent plastic strain exceeding the critical strain value for inducing shear instability within a continuous region of the geometric center is determined as follows: Under the same asymmetric rolling conditions, a series of single-pass rolling tests with different reduction rates were conducted on samples of the same material. The rolled sample was cut open along the thickness direction to prepare a metallographic sample including the geometric center of the thickness for observation. The minimum equivalent plastic strain value corresponding to the discontinuous shear band structure will be observed for the first time near the geometric center, and will be calibrated as the critical strain value for shear instability of the material under the processing conditions.

4. The rolling strengthening method for reducing the stiffness of metal sheets according to claim 1, characterized in that: In sub-steps S2-5, the step of determining that a discontinuous shear band has been formed includes: The collected ultrasonic echo signals are subjected to time-frequency analysis to extract their energy distribution in a specific frequency band determined by the acoustic properties of the material and the shear band size. Along the width of the plate, calculate the ratio of the local peak value of the signal energy in this specific frequency band to the average background energy of the adjacent region; When the ratio exceeds a preset threshold and the width of the ratio feature is symmetrically distributed along the rolling centerline, it is determined that a discontinuous shear band has been formed.

5. The rolling strengthening method for reducing the stiffness of metal sheets according to claim 1, characterized in that: Step S3 further includes the following sub-steps: S3-1, The rolled sheet is placed into the heating furnace, and the temperature control thermocouple of the heating furnace is set in the middle area of ​​the sheet thickness direction. The sheet is determined to have been heated to the dynamic recrystallization temperature range when the thermocouple reading reaches the preset value. S3-2, The heated plate is fed into the first warm rolling mill for rolling. The roll gap of the warm rolling mill is set so that the strain energy density generated by the reduction in this pass is lower than the preset upper limit value. S3-3, the first rolled plate is conveyed to the heat-insulating roller table or tunnel furnace, and its conveying speed is controlled so that the dwell time of the plate before entering the next warm rolling mill is limited to a preset time window; S3-4, repeat S3-2 and S3-3, so that the plate passes through the subsequent warm rolling mill and the heat preservation conveyor section in sequence to complete multiple warm rolling passes.

6. The rolling strengthening method for reducing the stiffness of metal sheets according to claim 5, characterized in that, The preset upper limit and preset time window are determined in the following way: The minimum unit strain energy increment required to induce dynamic recrystallization nucleation in the core shear band, and the critical unit strain energy increment that triggers an increase in dislocation density in the surface structure, were measured. Set the preset upper limit value to be lower than the critical unit strain energy increment; The shortest time required for the core dynamic recrystallized grains to complete one round of growth during the warm rolling interval was measured, as well as the start time for the surface microstructure to undergo significant static recovery. Set the preset time window to be longer than the shortest time but shorter than the start time.

7. The rolling strengthening method for reducing the stiffness of metal sheets according to claim 1, characterized in that: In sub-step S4-3, the flowing cooling medium is an atomized coolant, and its spray angle, flow rate and atomization degree are configured to reduce the surface temperature without causing uneven warping of the entire board.

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