A corrosion-resistant nickel-plated steel strip and its preparation method

Abstract

This invention discloses a high-performance, corrosion-resistant nickel-plated steel strip with excellent deep-drawing properties and its preparation method, belonging to the field of surface engineering of metallic materials. The method includes: pre-treating the steel strip, followed by electrodeposition under the synergistic effect of pulsed current, pulsed ultrasound, and alternating magnetic field to form a fine-grained coating; subsequently, a gradient-enhanced magnetic field is applied during diffusion annealing to form an Fe-Ni gradient diffusion layer. This invention achieves a synergistic effect of coating grain refinement and interface gradient transition through a non-obvious combination of three-field synergistic electrodeposition and gradient magnetic field annealing. This results in nickel-plated steel strips exhibiting no microcracks after 62% deep-drawing deformation and a salt spray test life exceeding 200 hours, solving the problem of easy cracking of the coating during extreme deep drawing. It can be widely used in battery casings and precision electronic components.

Description

Technical Field

[0001] This invention relates to the field of surface engineering of metallic materials, specifically to a corrosion-resistant nickel-plated steel strip and its preparation method. Background Technology

[0002] Nickel-plated steel strip is widely used in battery casings, precision electronic components, and automotive parts due to its excellent corrosion resistance and good processing performance. Especially with the rapid development of the electric vehicle industry, battery casings place extremely stringent requirements on the deep-drawing performance of steel strips, often requiring a deformation of 30% or more. However, existing technologies mainly focus on improving coating adhesion by increasing the coating thickness, adding an intermediate layer, or performing conventional thermal diffusion annealing. For example, a columnar nickel plating is obtained by DC electrodeposition followed by annealing at 600-700℃ to form an Fe-Ni diffusion layer. However, those skilled in the art have gradually discovered in practice that even if the coating adhesion meets conventional testing requirements, under extreme deep-drawing conditions… Microcracks, invisible to the naked eye, will inevitably appear on the surface of the underlying coating. These microcracks become preferential channels for the penetration of corrosive media, leading to microleakage or a sudden drop in pressure resistance in the battery casing during long-term use. The reason for this is that existing technologies have failed to recognize that the columnar crystal structure formed by DC electrodeposition has severe dislocation pile-up at the grain boundaries during deep drawing deformation. While conventional annealing can form a diffusion layer, it often leads to grain coarsening, which in turn reduces the plastic deformation coordination ability of the coating itself. Therefore, how to simultaneously solve the contradiction between the coating's grain boundary coordination deformation ability and the interfacial stress transition at the microstructural level, so that the coating remains intact and dense after extreme deep drawing deformation of more than 60%, has become an urgent technical problem to be solved in this field. Summary of the Invention

[0003] The technical problem to be solved by this invention is: how to simultaneously resolve the contradiction between the grain boundary coordination deformation capability and the interface stress transition of the coating from the microstructure level, so that the coating remains intact and dense after extreme deep drawing deformation of more than 60%.

[0004] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: A method for preparing corrosion-resistant nickel-plated steel strip includes the following steps: S1: Substrate pretreatment: The cold-rolled low-carbon steel strip is degreased and activated to obtain a clean substrate surface; S2: Pulsed ultrasound-alternating magnetic field synergistic electrodeposition: The pretreated steel strip substrate is placed in the nickel plating solution as the cathode. While applying pulsed current, pulsed ultrasound and an alternating magnetic field perpendicular to the steel strip surface are introduced into the plating solution to perform composite field synergistic electrodeposition, forming a fine-grained initial nickel plating layer on the steel strip surface. S3: Post-plating heat treatment and gradient magnetic field-assisted diffusion: The nickel-plated steel strip obtained in S2 is placed in an annealing furnace, first subjected to stress-relief annealing, and then heated to the diffusion annealing temperature; during the heating and holding process of diffusion annealing, a magnetic field with a gradient increasing in strength over time is applied. S4: Cooling: After diffusion annealing, cool to room temperature to obtain the finished product.

[0005] Furthermore, in the above-mentioned method for preparing a corrosion-resistant nickel-plated steel strip, the parameters of the pulse current in step S2 are: average current density 2-6 A / dm³. 2 The duty cycle is 20%-50%, and the frequency is 500-2000 Hz; the power density of the pulsed ultrasound is 0.3-0.8 W / cm². 2 The frequency of the pulse is the same as the frequency of the pulse current; the magnetic field strength of the alternating magnetic field is 0.1-0.5 T, and its frequency is the same as the frequency of the pulse current.

[0006] Furthermore, in the above-mentioned method for preparing a corrosion-resistant nickel-plated steel strip, the phase difference between the pulsed ultrasonic wave and the pulsed current is 0° or 180°, so as to achieve synchronous control of the cavitation effect and the electrocrystallization process.

[0007] Furthermore, in the above-mentioned method for preparing a corrosion-resistant nickel-plated steel strip, the gradient enhancement magnetic field in step S3 is controlled as follows: the initial magnetic field strength is 0.1-0.3 T, and it is linearly or stepwise enhanced to 0.8-1.5 T within a heating or holding time of 30-60 min.

[0008] Furthermore, in the above-mentioned method for preparing a corrosion-resistant nickel-plated steel strip, the intensity change rate dH / dt of the gradient-enhanced magnetic field is controlled within 0.01-0.03 T / min.

[0009] Furthermore, in the above-mentioned method for preparing a corrosion-resistant nickel-plated steel strip, the diffusion annealing temperature in step S3 is 600-750℃, and the direction of the gradient enhancement magnetic field is parallel to the rolling direction of the steel strip.

[0010] Furthermore, in the above-mentioned method for preparing a corrosion-resistant nickel-plated steel strip, the stress-relief annealing temperature in step S3 is 300-450℃, and the holding time is 10-20 min.

[0011] Furthermore, in the above-mentioned method for preparing a corrosion-resistant nickel-plated steel strip, the nickel plating solution in S2 is a Watt-type nickel plating solution or an aminosulfonate nickel plating solution.

[0012] Furthermore, in the above-mentioned method for preparing a corrosion-resistant nickel-plated steel strip, during electrodeposition in S2, the coating thickness is monitored by an online thickness gauge, and the conveyor speed is adjusted within the range of 1.4-1.6 m / min.

[0013] The present invention also relates to a corrosion-resistant nickel-plated steel strip prepared by the above-mentioned method for preparing corrosion-resistant nickel-plated steel strip.

[0014] The beneficial effects of this invention are as follows: By organically integrating pulsed ultrasound-alternating magnetic field synergistic electrodeposition with gradient magnetic field-assisted diffusion annealing, a non-obvious synergistic effect is produced. On the one hand, the pulsed current, pulsed ultrasound, and alternating magnetic field are synchronized in frequency and adjustable in phase. Utilizing the superposition of ultrasonic cavitation effect and magnetohydrodynamic convection, the coating grains are refined from micron-sized columnar grains in traditional processes to equiaxed grains of 200-500 nm, and the crystal texture is optimized, significantly improving the plastic deformation capability of the coating itself. On the other hand, the introduction of a gradient-enhanced magnetic field during diffusion annealing utilizes the difference in magnetic properties between Fe and Ni to differentially control atomic diffusion. This not only forms a compositional gradient transition layer that effectively alleviates interfacial stress concentration but also suppresses high-temperature grain coarsening through the pinning effect of the magnetic field on dislocation movement, thus solving the problem of interfacial stress transition while ensuring fine grain strengthening. The above two features are interdependent and indispensable. When there is only a fine-grained coating without a gradient interface, deep drawing deformation will still induce microcracks due to interface stress concentration. When there is only a gradient interface but the grains are coarse, the coating itself is not plastic enough, which will also lead to cracking. Only when the two work together can the nickel-plated steel strip remain intact and dense after 62% extreme deep drawing deformation. The salt spray resistance time has been increased from less than 50 hours in conventional processes to more than 220 hours, achieving a simultaneous leapfrog improvement in deep drawing performance and corrosion resistance. Detailed Implementation

[0015] To explain in detail the technical content, objectives, and effects of the present invention, the following description is provided in conjunction with the embodiments.

[0016] Example 1 This embodiment relates to a corrosion-resistant nickel-plated steel strip and its preparation method, including the following steps; S1: Substrate pretreatment; In this embodiment, a cold-rolled low-carbon steel strip with a thickness of 0.25 mm and a width of 300 mm is selected as the base material. Its chemical composition by mass percentage is: C 0.045%, Mn 0.22%, Si 0.015%, P 0.008%, S 0.004%, with the balance being Fe and unavoidable impurities.

[0017] The steel strip is continuously wound through the following pretreatment processes, with the belt speed set at 1.5 m / min and a speed control accuracy of ±0.05 m / min (achieved through a closed-loop speed regulation system with encoder feedback): A mixed alkaline solution with a concentration of 60 g / L NaOH, 30 g / L Na2CO3, and 20 g / L Na3PO4 was used. The temperature was controlled at 70±2℃, the spraying pressure was 0.2 MPa, and the treatment time was 30 seconds to remove surface rolling oil and organic contaminants.

[0018] The steel strip was immersed in an electrolytic degreasing tank with a solution composition of 40 g / L NaOH, 25 g / L Na₂CO₃, and 0.5 g / L sodium dodecyl sulfate. The temperature was 60±2℃, and cathodic electrolysis was used with a current density of 5 A / dm³. 2 The processing time is 10 seconds, which further removes the fine oil film on the surface.

[0019] After each degreasing process, the surface is rinsed with deionized water at a pressure of 0.15 MPa for 10 seconds, with a conductivity ≤10 μS / cm. After rinsing, the surface is dried with an air knife to prevent moisture from being carried into the electroplating tank.

[0020] The steel strip was immersed in a 10% H2SO4 solution and treated at room temperature for 10 seconds to remove the surface oxide layer and activate the base metal surface.

[0021] Deionized water is used for spray cleaning, with a conductivity of ≤5 μS / cm, to ensure no acid residue remains.

[0022] S2: Pulsed ultrasound-alternating magnetic field synergistic electrodeposition; The pretreated steel strip is continuously introduced into the electrodeposition section at a conveyor speed of 1.5 m / min. The electroplating tank is a horizontal structure with dimensions of 2.0 m (length) × 0.5 m (width) × 0.6 m (height) and an effective volume of 500 L.

[0023] Prepare the plating solution according to the formula: Nickel sulfate (NiSO4·6H2O): 280 g / L, Nickel chloride (NiCl2·6H2O): 45 g / L, Boric acid (H3BO3): 40 g / L, Sodium saccharin (C7H4NNaO3S·2H2O): 0.8 g / L, Sodium dodecyl sulfate (C 12 H 25 NaO4S): 0.1 g / L; prepared with deionized water, pH adjusted to 4.0±0.1 using 10% dilute sulfuric acid or 10% sodium hydroxide solution, and temperature controlled at 55±1℃ via titanium heating tubes and heat exchangers. The plating solution is circulated and filtered using a 5 μm PP filter cartridge at a flow rate of 10 L / min. The plating solution composition is tested weekly, and replenished periodically to the initial concentration.

[0024] A high-frequency pulse rectifier power supply is used. The anode is a titanium basket loaded with nickel corners (sulfur content ≤0.01%), and the cathode is the steel strip to be plated. The distance between the anode and the cathode is 80±5 mm, and the distance is kept constant by guide rollers and insulating pads.

[0025] Two sets of electromagnet coils are symmetrically arranged on both sides of the cathode area of ​​the electroplating tank. The coil cores are made of stacked silicon steel sheets, and the magnetic field direction is perpendicular to the surface of the steel strip. The coils are powered by an independent intermediate frequency power supply, which receives a synchronization signal from the pulse power supply. The magnetic field frequency and the pulse current frequency are strictly synchronized through a phase-locked loop circuit.

[0026] The rising edge of the pulse current is time-aligned with the positive zero-crossing point of the alternating magnetic field sine wave (i.e., the magnetic field strength crosses zero from negative to positive). The phase difference accuracy is controlled within ±5°, and calibration is performed by monitoring the pulse current waveform and the magnetic field induction coil signal using an oscilloscope (model: Tektronix MDO3024).

[0027] The magnetic field strength is expressed as the effective spatial value (RMS). During equipment installation and commissioning, a gaussmeter (probe model: LakeShore HMMT-6J04-VF) is calibrated under static conditions (no steel strip movement, no plating solution flow). The probe is placed 5 mm directly above the center point of the cathode surface, and the gaussmeter readings (unit: T) are recorded under different excitation currents (unit: A). The calibration points should avoid the core saturation region (in this embodiment, the magnetic field strength is in the linear region below 1.5 T). Ten calibration points are selected at equal intervals within the range of 0-1.5 T, and the least squares method is used for linear fitting (R²≥0.995) to obtain the "excitation current-magnetic field strength" calibration curve. During production, operators control the magnetic field strength by setting the excitation current according to this curve. In this embodiment, to achieve a magnetic field strength of 0.3 T (RMS), the corresponding excitation current value is obtained by referring to the calibration curve (this value varies depending on the equipment; in this embodiment it is 12.5 A, for reference only, and the actual value shall be determined by the on-site calibration).

[0028] Six sets of immersion ultrasonic transducers (made of 316L stainless steel) are arranged below the cathode area. Each transducer has an emitting surface dimension of 300 mm long × 200 mm wide, and a single transducer has an emitting area of ​​0.06 m². 2 The total launch area of ​​the six groups is 0.36 m². 2 The vibrating plates are arranged side-by-side along the running direction of the steel belt, with the emitting surface of the vibrating plate parallel to the surface of the cathode steel belt, and the vertical spacing is 50±5 mm. This spacing is adjusted and locked by a precision lead screw mechanism. Each group of vibrating plates is controlled by an independent ultrasonic generator, which receives the synchronization signal from the pulse power supply to achieve synchronous triggering of ultrasonic pulses and current pulses.

[0029] Ultrasonic output power is measured using the power meter reading (unit: W). During equipment installation and commissioning, a hydrophone (model: Onda HGL-0200) is used to measure sound pressure on the cathode surface (where the steel strip is located). Measurement points are set up every 100 mm along the width of the steel strip and every 200 mm along its length, for a total of 15 points. The effective sound pressure value at each point is recorded. The sound pressure fluctuation coefficient is calculated as (maximum sound pressure - minimum sound pressure) / average sound pressure × 100%. The output power of each vibrating plate is adjusted to ensure the total power meter reading is 1800 W, while simultaneously ensuring the sound pressure fluctuation coefficient is ≤10%. After calibration, the total power meter reading of 1800 W is used as the control target during production; sound pressure measurements are no longer repeated.

[0030] Using the main frequency clock of the pulse power supply as a reference, the signal is simultaneously output to the alternating magnetic field power supply and the ultrasonic generator through the signal distributor, ensuring that the three fields have completely consistent frequencies and adjustable phases.

[0031] Pulse current: Average current density 4.0 A / dm 2 Peak current density 13.3 A / dm 2 The duty cycle is 30%, the pulse frequency is 1000 Hz, and the pulse waveform is a rectangular wave (positive pulse).

[0032] Alternating magnetic field: The excitation current is set according to the calibration curve so that the magnetic field strength is 0.3 T (RMS), the magnetic field frequency is 1000Hz, and the phase difference is 0°.

[0033] Pulsed ultrasound: Total power 1800 W (power meter reading), pulse frequency 1000 Hz, pulse width corresponding to a duty cycle of 30% (i.e., the ultrasound emission time in each pulse cycle is 0.3 ms), and the ultrasound pulse and the current pulse are triggered in phase.

[0034] Electrodeposition time and coating thickness control: The effective deposition zone length of the electroplating tank is 1.5 m, the conveyor belt speed is 1.5 m / min, and the theoretical deposition time is 60 seconds. In actual production, the coating thickness is monitored online every 10 minutes using an X-ray fluorescence thickness gauge (model: Fischer XDV-SDD, measurement point located 1 m after the electroplating tank outlet). If the measured thickness deviates from the range of 3.8-4.2 μm, closed-loop correction is performed by fine-tuning the conveyor belt speed, with an adjustment range of 1.4-1.6 m / min. When the conveyor belt speed is adjusted, the speed of the subsequent heat treatment section is adjusted synchronously (achieved through the same drive system). Based on prior verification through finite element simulation of thermal conduction and actual temperature measurement, when the speed adjustment is ≤ ±0.1 m / min (i.e., the conveyor speed is within the range of 1.4-1.6 m / min), the temperature change of the steel strip in each temperature zone is ≤ ±15℃. This temperature change has an acceptable impact on the thickness of the diffusion layer and the grain size of the coating (diffusion layer thickness change ≤ 0.5 μm, grain size change ≤ 50 nm), and does not affect the deep-drawing performance and corrosion resistance of the final product. If the measured thickness continuously deviates and requires adjustment exceeding ±0.1 m / min, production should be suspended and the heat treatment temperature field recalibrated. In this embodiment, the measured thickness is stable at 3.9-4.1 μm, and no adjustment of the conveyor speed is required.

[0035] S3: Gradient magnetic field assisted diffusion annealing; The electrodeposited nickel-plated steel strip is continuously fed into a protective atmosphere annealing furnace at a conveyor speed of 1.5 m / min. The annealing furnace is a multi-stage continuous annealing furnace with a total effective length of 30 m, divided into a preheating section (3 m), a heating section (9 m), a holding section (6 m), a slow cooling section (6 m), and a rapid cooling section (6 m). A mixed protective gas with a volume fraction of 95% N2 + 5% H2 is introduced into the furnace, with a dew point ≤ -40℃ and a slight positive pressure (10-20 Pa). Air curtains are installed at the furnace inlet and outlet to prevent air from entering.

[0036] During the equipment commissioning phase, establish the "position-strip temperature" correspondence required for process control according to the following steps: A type K thermocouple (Φ0.2 mm, fiberglass insulation) was bonded to the steel strip surface using high-temperature adhesive (OMEGABOND 700, temperature resistance 1100℃). The steel strip surface was cleaned with alcohol before bonding, and the adhesive layer thickness was ≤0.3 mm. To avoid affecting the operation of the steel strip, the thermocouples were bonded to the edge of the steel strip (10 mm from the edge), and only 3-5 thermocouples (at different locations) were bonded to each roll of steel strip. Temperature was measured at different locations sequentially, and data acquisition for all measuring points was completed through multiple roll runs. This bonding method did not damage the 0.25 mm steel strip.

[0037] While the steel belt was running at 1.5 m / min, the temperature values ​​of each thermocouple measuring point at different locations inside the furnace (the location was determined by an encoder) were recorded. At the same time, an infrared thermal imager (model: FLIR A655sc) was used to measure the temperature at the same location, with the emissivity initially set to 0.25.

[0038] By comparing the readings of the infrared thermal imager with those of the thermocouple, the emissivity setting of the infrared thermal imager was adjusted so that the deviation between the two was ≤ ±5℃ at all measuring points. In this embodiment, the calibrated emissivity setting was 0.28 (the actual value after slight oxidation of the steel strip surface).

[0039] Because the surface condition of the steel strip may change during continuous production, emissivity verification is performed at the following two times: ① Every 8 hours of operation or every time a roll of steel strip is changed, a handheld infrared thermometer (model: Fluke 62 MAX+, emissivity temporarily set to 0.28) is used at the furnace inlet for comparative verification with a thermocouple. Production is not stopped during verification, and a non-contact method is used: the handheld infrared thermometer is aimed at the surface of the steel strip (approximately 200 mm away), and simultaneously a contact surface thermocouple (model: OMEGA HHT13, probe is a K-type thermocouple, response time <1 second) is lightly touched to the same position on the edge of the steel strip, and the temperatures of both are read. Readings are recorded at 5 different time points, and the average deviation is calculated. If the average deviation is ≤ ±5℃, the emissivity setting is considered correct; if the deviation exceeds ±5℃, the emissivity setting of the infrared thermometer is adjusted until the average deviation is ≤ ±5℃, and the new emissivity value is recorded. ② During each shutdown maintenance (once a month), a more precise calibration can be performed using the thermocouple patch method (using high-temperature adhesive) as a verification of the online validation method. If the emissivity needs to be adjusted beyond ±0.03, check the surface condition of the steel strip (e.g., whether there is oil or oxide scale) and adjust the pretreatment process.

[0040] After emissivity calibration, an infrared thermal imager is used to continuously monitor the surface temperature of the steel strip and record the temperature distribution of the steel strip at various locations within the furnace, forming the "location-steel strip temperature" comparison table in this embodiment. This comparison table needs to be recalibrated as the furnace structure, cooling conditions, and conveyor speed change, and when different equipment or processes are modified.

[0041] The position-temperature correspondence obtained after calibration in this embodiment is shown in Table 1: Table 1 Multiple sets of electromagnets are arranged along the steel strip's running direction (i.e., the rolling direction) outside the heating, holding, and slow cooling sections of the annealing furnace. The electromagnets are continuously arranged along the length of the steel strip, with each set being 0.5 m long, totaling 48 sets. The effective magnetic field area is 24 m long (covering 9 m of the heating section, 6 m of the holding section, 6 m of the slow cooling section, and the first 3 m of the rapid cooling section). Each set of electromagnets is powered by an independent programmable excitation power supply, and the excitation current can be independently adjusted according to a preset curve. A water-cooled jacket is installed between the electromagnets and the furnace body, with the cooling water temperature controlled at 25±2℃.

[0042] The electromagnets were calibrated using a gaussmeter under static conditions (no steel belt, room temperature). The gaussmeter probe was placed at the center of the steel belt's running path (i.e., the spatial trajectory of the steel belt's centerline during normal operation). Five calibration points were selected at equal intervals within the range of 0-1.5 T (e.g., 0, 0.4, 0.8, 1.2, 1.5 T), and the corresponding excitation current values ​​were recorded. The least squares method was used to linearly fit the "excitation current-magnetic field strength" calibration curve for each group of electromagnets. In this embodiment, all magnetic field strengths were below 1.5 T, making linear fitting applicable.

[0043] Each electromagnet coil has a pre-embedded type K thermocouple (embedded inside the coil winding, 5 mm from the surface) to monitor the coil temperature in real time, with a sampling period of 1 second. The temperature data is uploaded to the PLC control system.

[0044] The temperature coefficient of resistance of the copper coil is α = 0.004 / ℃ (in degrees Celsius). Using the coil temperature T0 (usually 20℃) as a reference during calibration, the actual excitation current is corrected according to the following formula: I_actual = I_target × [1 + α × (T_current - T0)] in: I_target: The target excitation current (unit: A) obtained from the calibration curve, corresponding to the required magnetic field strength; T_current: Current coil temperature (unit: °C), monitored in real time by the embedded thermocouple; T0: Coil temperature during calibration (unit: °C), recorded in the calibration curve appendix; I_actual: The actual excitation current output to the electromagnet coil (unit: A).

[0045] When the coil temperature exceeds 60℃, the system will issue an alarm, and the operator will check the operating status of the water-cooled jacket.

[0046] If the water-cooling jacket fails, causing the coil temperature to exceed 80°C, the PLC system will automatically cut off the electromagnet power supply and trigger an alarm to prevent the coil from burning out. The magnetic field strength must be recalibrated before resuming production (as high temperatures may cause changes in the core's permeability).

[0047] This embodiment employs a "preset segmented control based on conveyor belt time" method. The control system triggers timing using an inlet photoelectric sensor, calculates the position of the steel strip in the furnace in real time based on the set conveyor belt speed (1.5 m / min), and adjusts the excitation current of each set of electromagnets with a control cycle of 50 ms. When the actual conveyor belt speed deviates from the set value (monitored in real time by an encoder), the control system automatically compensates for the timing speed to ensure the accuracy of the position calculation. The speed deviation alarm threshold is ±0.1 m / min; if this range is exceeded, the system issues an audible and visual alarm, and the operator needs to check the speed control loop.

[0048] The magnetic field strength is set according to the following position-strength correspondence table. Magnetic field strength is expressed in Tesla (T), and the excitation current is calculated based on the calibration curves and temperature compensation formulas of each electromagnet group. Refer to Table 2.

[0049] Table 2 At the boundary of the sections (6 m, 9 m, 21 m), the magnetic field strength needs to complete a linear transition within a 0.2 m travel distance. Specifically, the control system calculates the target magnetic field strength using a linear interpolation formula based on the distance between the current position and the boundary starting point. H_target = H_start + (H_end - H_start) × (d / L) in: d: Distance from the current position to the transition point (0 ≤ d ≤ L); L: Length of the transition zone (0.2 m); H_start: Magnetic field strength at the transition starting point; H_end: Magnetic field strength at the transition endpoint.

[0050] The control cycle is 50 ms. After each calculation, the target magnetic field strength is converted into an excitation current (based on the calibration curve and temperature compensation) and output to the electromagnet power supply. The position accuracy of the start and end points of the transition zone is guaranteed by an encoder, with a positioning error ≤ ±10 mm.

[0051] Radiation cooling is employed, with the furnace wall lined with an insulation layer (100 mm thick, aluminum silicate fiber), and an external water jacket for auxiliary temperature control. A K-type thermocouple (6 in total) is installed every 1 m along the inner wall of the furnace in the slow cooling section to monitor the furnace temperature. The cooling rate is controlled by adjusting the circulating water flow rate of the water jacket (control range 0-5 m³ / h), and a PID controller stabilizes the cooling rate at 15±3℃ / min. The cooling rate is calculated by dividing the difference between the infrared thermometer readings at the inlet (18 m) and outlet (24 m) by the residence time (4 min). The PID parameters are set as follows: proportional gain 0.8, integral time 50 seconds, and derivative time 10 seconds.

[0052] Rapid cooling section (24-30 m): Forced water jacket cooling is adopted, with the water jacket circulation flow rate fixed at 9 m³ / h and the cooling rate at approximately 40±5℃ / min. An infrared thermometer (emissivity 0.28, verified every 8 hours with thermocouple patch) is installed at the outlet of the rapid cooling section (30 m) to monitor the temperature of the steel strip.

[0053] Temperature over-limit handling logic: If the temperature of the steel strip at the outlet of the rapid cooling section exceeds 150℃, the system shall perform the following operations: (1) First check the water jacket circulating water flow rate and inlet water temperature. If the flow rate is <8 m 3 / h or inlet water temperature >30℃, prioritize adjusting the water system to restore normal operation; (2) if the water system is normal but the temperature still exceeds the standard, the conveyor belt speed is automatically reduced by 0.05 m / min (speed reduction step) through the PLC system, and the speed change value is recorded at the same time; (3) after the speed is reduced, the dwell time of each heat treatment section is recalculated, but the temperature field is not adjusted. According to the prior verification by finite element simulation of heat conduction and actual temperature measurement, when the conveyor belt speed changes within the range of 1.4-1.6 m / min (corresponding adjustment amount ≤ ±0.1 m / min), the temperature change of the steel belt in each temperature zone is ≤ ±15℃. The effect of this temperature fluctuation on the thickness of the diffusion layer and the grain size is within acceptable range and does not affect the final product performance. In this embodiment, the speed adjustment range is set to be consistent with the electrodeposition section (1.4-1.6 m / min). If the speed needs to be reduced to below 1.4 m / min and the outlet temperature still cannot be reduced to below 150℃, production is stopped and the water jacket of the fast cooling section is checked for blockage or water cooling capacity reduction. In this embodiment, the outlet temperature is stable at 120-140℃ and requires no adjustment.

[0054] The finite element simulation of heat conduction explains that the above conclusions regarding the temperature variation range are based on the following simulation parameters: steel strip thickness 0.25 mm, density 7.85 g / cm³. 3 Specific heat capacity 500 J / (kg·K), thermal conductivity 45 W / (m·K); the heat transfer coefficients of each section inside the furnace are set according to the actual furnace structure (the natural convection heat transfer coefficient of the preheating section is 10 W / (m·K)). 2·K), the forced convection heat transfer coefficient of the heating section is 30 W / (m²). 2 ·K), the heat transfer coefficient of the water jacket in the cooling section is 200 W / (m²). 2 The boundary conditions were determined using the measured furnace temperature (K). The simulation employed the ANSYS steady-state thermal analysis module with a mesh size of 0.5 mm. The simulation results deviated from actual thermocouple temperature measurements (measured at speeds of 1.4, 1.5, and 1.6 m / min) by ≤±8℃, demonstrating sufficient engineering accuracy.

[0055] Specific operations: Preheating section (0-3 m): The steel strip enters the preheating section and is heated from room temperature to 400℃ within 2 minutes by the electric heating radiation tube. It is then kept at 400±10℃ for 2 minutes. No magnetic field is applied during this stage.

[0056] Heating Section (3-12 m): The steel strip enters the heating section and is heated from 400℃ to 680℃ within 6 minutes. The heating section is divided into 9 independent temperature control zones along its length, each zone being 1 m long. Temperature control is achieved through thermocouple feedback. Magnetic field control is set in segments according to the table above, with a linear transition at 6 m and 9 m.

[0057] Insulation section (12-18 m): The steel strip enters the insulation section, where the temperature is maintained at 680±5℃ for 4 minutes. The electromagnets corresponding to the 6 temperature zones in the insulation section are all set with a constant magnetic field strength of 1.2 T.

[0058] Slow cooling section (18-24 m): The steel strip enters the slow cooling section, with a cooling rate of 15±3℃ / min, and the steel strip temperature drops from 680℃ to 550℃. The magnetic field control is set in segments according to the table above, and a linear transition is performed at 21 m.

[0059] Rapid cooling section (24-30 m): The steel strip enters the rapid cooling section, where the cooling rate is approximately 40℃ / min, and the temperature of the steel strip drops from 550℃ to below 150℃. No magnetic field is applied during this stage.

[0060] Cooling after exiting the furnace: After the steel strip leaves the furnace, it is allowed to cool naturally to room temperature in the air.

[0061] S4: Cooling; After annealing and cooling, the steel strip is leveled using a tension straightener to ensure a straight shape (flatness ≤ 0.5 mm / m). Finally, after being inspected for penetrating defects by an online pinhole detector, it is coiled to obtain the finished nickel-plated steel strip. The total thickness of the finished steel strip is 0.258-0.262 mm, of which the total coating thickness (including the diffusion layer) is 4-6 μm.

[0062] Comparative Example 1 The solution in Example 1 differs from the one in that pulsed current, alternating magnetic field, and pulsed ultrasound are not applied (conventional DC electroplating with a current density of 4.0 A / dm² is used). 2 Furthermore, no gradient magnetic field is applied during annealing (conventional annealing at 680℃ for 20 minutes is used, without magnetic field).

[0063] Comparative Example 2 The scheme in Example 1 differs in that only a pulsed current (average current density 4.0 A / dm²) is applied. 2 The annealing process was carried out at a duty cycle of 30% and a frequency of 1000 Hz. No alternating magnetic field or pulsed ultrasound was applied, and no gradient magnetic field was applied during annealing (conventional annealing at 680℃ for 20 min was used without a magnetic field).

[0064] Comparative Example 3 The scheme of Example 1 is different in that pulsed current and pulsed ultrasound are applied (parameters are the same as in Example 1), but no alternating magnetic field is applied, and no gradient magnetic field is applied during annealing (conventional annealing at 680℃, holding for 20 min, without magnetic field).

[0065] Comparative Example 4 The scheme of Example 1 is different in that pulsed current and alternating magnetic field are applied (parameters are the same as in Example 1), but pulsed ultrasound is not applied, and no gradient magnetic field is applied during annealing (conventional annealing at 680℃ for 20 min is used, without magnetic field).

[0066] Comparative Example 5 The scheme of Example 1 is different in that a complete pulsed ultrasonic-alternating magnetic field synergy is applied during electrodeposition (same as Example 1), but no gradient magnetic field is applied during annealing, and only conventional annealing at 680℃ (holding for 20 min, without magnetic field) is used.

[0067] Comparative Example 6 The scheme of Example 1 differs in that a complete pulsed ultrasound-alternating magnetic field synergy is applied during electrodeposition (same as Example 1), and a constant magnetic field (1.2 T, constant, no gradient enhancement) is applied during annealing, that is, gradient enhancement is not performed (the initial magnetic field strength is directly set to 1.2 T and kept constant in the heating and holding sections).

[0068] Comparative Example 7 The scheme of Example 1 differs in that a complete pulsed ultrasonic-alternating magnetic field synergy is applied during electrodeposition (same as Example 1), and a gradient magnetic field is applied during annealing, but the temperature of the steel strip is higher than the Curie point when the magnetic field is turned off. The Curie point of the Fe-Ni alloy is about 450-500℃ (that is, the magnetic field is turned off when the temperature of the steel strip is 600℃, instead of 550℃).

[0069] Comparative Example 8 The difference from the scheme in Example 1 is that the coating thickness is not controlled in a closed loop during electrodeposition (it only runs according to the theoretical deposition time of 60 seconds, without adjustment based on the thickness gauge feedback), and no gradient magnetic field is applied during annealing (conventional annealing at 680℃ for 20 min is used, without a magnetic field).

[0070] Experimental methods: Nickel-plated steel strip samples were prepared according to the process conditions of Example 1 and the various comparative examples. Three rolls were prepared for each condition, each roll being 500 m in length. The following performance tests were performed: 1. Ultimate Deep Drawing Deformation: Using a cylindrical deep drawing tester, the steel strip is stamped into a battery steel shell with a diameter of 10 mm. The stamping depth is gradually increased (0.5 mm per step) until the first visible micro-crack appears on the coating surface (observed using a 10x magnifying glass). The ultimate stamping depth at this point is recorded and converted into deformation (%). Five specimens are tested for each sample, and the average value is taken.

[0071] 2. Salt spray test after stamping: The stamped steel shell under the above-mentioned extreme deep drawing deformation is subjected to a neutral salt spray test (according to ASTM B117, 5% NaCl solution, 35℃, continuous spraying), and the time (in hours) when the first red rust spot appears on the surface is recorded. Three specimens are tested for each sample, and the average value is taken.

[0072] 3. Corrosion current of plate samples: Tafel polarization curves were performed on unstamped plate samples (electrochemical workstation, three-electrode system, 3.5% NaCl solution, scan rate 0.5 mV / s). The corrosion current density (μA / cm²) was calculated using the Tafel extrapolation method. 2 Three samples were tested for each sample, and the average value was taken.

[0073] 4. Average grain size of coating: The cross-section of the coating was analyzed using EBSD (electron backscatter diffraction), and at least 200 grains were counted to calculate the average grain size (nm).

[0074] Diffusion layer gradient characteristics: GDOES (Glow Discharge Emission Spectroscopy) was used to analyze the distribution of Fe and Ni elements at the interface to determine whether there is a significant compositional gradient transition layer (Fe content continuously decreases from the substrate side to the coating side, without a plateau region). Judgment criteria: significant gradient (present), insignificant gradient (absent).

[0075] The experimental results are shown in Table 3: Table 3 Results analysis: Compared to Example 1, Comparative Example 1 showed that the ultimate deep-drawing deformation decreased from 62% to 28% (a reduction of 54.8%), the salt spray test time after stamping decreased from 226 h to 38 h (a reduction of 83.2%), and the plate corrosion current decreased from 0.08 μA / cm. 2 Increased to 0.82 μA / cm 2 (Increased by 9.3 times), the coating grain size coarsens from 350 nm to 5200 nm (increased by 13.9 times), and the diffusion layer shows no gradient characteristics. This indicates that without the application of three-field synergistic electrodeposition and gradient magnetic field annealing, the coating grains are coarse and lack a gradient transition layer, making it extremely prone to microcracks during deep drawing and severely reducing corrosion resistance. This comparative example demonstrates the crucial role of pulsed ultrasound-alternating magnetic field synergistic electrodeposition and gradient magnetic field annealing.

[0076] Compared to Example 1, Comparative Example 2 showed that the ultimate deep-drawing deformation decreased from 62% to 41% (a reduction of 33.9%), the salt spray test time after stamping decreased from 226 h to 74 h (a reduction of 67.3%), and the plate corrosion current decreased from 0.08 μA / cm. 2 Increased to 0.44 μA / cm 2 (4.5 times increase), the coating grain size coarsens from 350 nm to 2100 nm (5.0 times increase), and the diffusion layer shows no gradient characteristics. Compared to Comparative Example 1, the coating grains in Comparative Example 2 are somewhat refined (2100 nm vs 5200 nm), but far from reaching the nanometer / submicron scale of Example 1. This indicates that while applying pulsed current alone can refine the grains to some extent, it cannot achieve the refining effect of three fields working together, and the lack of gradient magnetic field annealing results in no gradient transition layer at the interface.

[0077] Compared to Example 1, Comparative Example 3 showed that the ultimate deep-drawing deformation decreased from 62% to 51% (a reduction of 17.7%), the salt spray test time after stamping decreased from 226 h to 128 h (a reduction of 43.4%), and the plate corrosion current decreased from 0.08 μA / cm. 2 Increased to 0.21 μA / cm 2 (Increased by 1.6 times), the grain size of the coating increased from 350 nm to 580 nm (an increase of 65.7%), and the diffusion layer showed no gradient characteristics. Compared with Comparative Example 2, the grains were further refined after adding ultrasound (580 nm vs 2100 nm), but still did not reach the level of Example 1. This indicates that pulsed ultrasound makes a significant contribution to grain refinement, but in the absence of an alternating magnetic field, the mass transfer of the plating solution is still limited, and the degree of grain refinement is insufficient.

[0078] Compared to Example 1, Comparative Example 4 showed that the ultimate deep-drawing deformation decreased from 62% to 35% (a reduction of 43.5%), the salt spray test time after stamping decreased from 226 h to 52 h (a reduction of 77.0%), and the plate corrosion current decreased from 0.08 μA / cm.2 Increased to 0.58 μA / cm 2 (Increased by 6.3 times), the coating grain size increased from 350 nm to 4200 nm (an increase of 11.0 times), and the diffusion layer showed no gradient characteristics. Compared with Comparative Example 2, the grain size actually coarsened after increasing the alternating magnetic field (4200 nm vs 2100 nm). This is because without the synergistic effect of ultrasound, the alternating magnetic field alone may exacerbate concentration polarization, leading to abnormal grain growth. This indicates that the alternating magnetic field must work synergistically with ultrasound to achieve a positive effect.

[0079] Compared to Example 1, Comparative Example 5 showed that the ultimate deep-drawing deformation decreased from 62% to 56% (a reduction of 9.7%), the salt spray test time after stamping decreased from 226 h to 168 h (a reduction of 25.7%), and the plate corrosion current decreased from 0.08 μA / cm. 2 Increased to 0.15 μA / cm 2 (Increase of 87.5%), the coating grain size increased from 350 nm to 450 nm (increase of 28.6%), and the diffusion layer showed gradient characteristics but were not significant (none). This indicates that when only three-field synergistic electrodeposition is used without gradient magnetic field annealing, although the coating grains are significantly refined, the lack of an Fe-Ni gradient diffusion layer means that interfacial stress concentration during deep drawing can still lead to microcrack initiation and decreased corrosion resistance. This comparative example demonstrates the crucial role of gradient magnetic field annealing in forming a gradient transition layer and eliminating interfacial stress.

[0080] Compared to Example 1, Comparative Example 6 showed that the ultimate deep-drawing deformation decreased from 62% to 48% (a reduction of 22.6%), the salt spray test time after stamping decreased from 226 h to 102 h (a reduction of 54.9%), and the plate corrosion current decreased from 0.08 μA / cm. 2 Increased to 0.32 μA / cm 2 (Increased by 3.0 times), the coating grain size increased from 350 nm to 820 nm (an increase of 134%), and the diffusion layer showed gradient characteristics but were not significant (none) (the Fe content decreased sharply). This indicates that although a constant magnetic field without gradient enhancement can promote diffusion, it cannot form an ideal gradient transition layer, and the lack of a gradient magnetic field to inhibit grain growth leads to grain coarsening. This comparative example demonstrates the crucial role of a gradient-enhanced magnetic field in forming an ideal gradient transition layer and inhibiting grain coarsening.

[0081] Compared to Example 1, Comparative Example 7 showed that the ultimate deep-drawing deformation decreased from 62% to 58% (a reduction of 6.5%), the salt spray test time after stamping decreased from 226 h to 195 h (a reduction of 13.7%), and the plate corrosion current decreased from 0.08 μA / cm. 2 Increased to 0.11 μA / cm 2(Increase of 37.5%), the coating grain size increased from 350 nm to 380 nm (increase of 8.6%), and the diffusion layer showed gradient characteristics but was slightly inferior to Example 1. This indicates that prematurely shutting off the magnetic field (above the Curie point) resulted in a lack of magnetic field effect in the high-temperature section, insufficient gradient formation in the diffusion layer, and slight grain coarsening. This comparative example demonstrates the importance of maintaining the magnetic field until diffusion is complete at temperatures below the Curie point.

[0082] Compared to Example 1, Comparative Example 8 showed that the ultimate deep-drawing deformation decreased from 62% to 44% (a reduction of 29.0%), the salt spray test time after stamping decreased from 226 h to 86 h (a reduction of 62.0%), and the plate corrosion current decreased from 0.08 μA / cm. 2 Increased to 0.38 μA / cm 2 (Increased by 3.8 times), the coating grain size increased from 350 nm to 960 nm (an increase of 174%), and the diffusion layer showed no gradient characteristics. This comparative example lacked both closed-loop thickness control and gradient magnetic field annealing, but the thickness deviation (measured at 4.5-5.2 μm, excessively thick and uneven) led to increased internal stress in the coating. Combined with the lack of a gradient transition layer, deep-drawing performance significantly decreased. This comparative example demonstrates the synergistic effect of thickness uniformity control and gradient magnetic field annealing—thickness deviation exacerbates interfacial stress, which cannot be alleviated without a gradient transition layer.

[0083] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent modifications made based on the content of the present invention specification, or direct or indirect applications in related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing corrosion-resistant nickel-plated steel strip, characterized in that, Includes the following steps: S1: Substrate pretreatment: The cold-rolled low-carbon steel strip is degreased and activated to obtain a clean substrate surface; S2: Pulsed ultrasound-alternating magnetic field synergistic electrodeposition: The pretreated steel strip substrate is placed in the nickel plating solution as the cathode. While applying pulsed current, pulsed ultrasound and an alternating magnetic field perpendicular to the steel strip surface are introduced into the plating solution to perform composite field synergistic electrodeposition, forming a fine-grained initial nickel plating layer on the steel strip surface. The parameters of the pulse current are: average current density 2-6 A / dm³ 2 The duty cycle is 20%-50%, and the frequency is 500-2000 Hz; the power density of the pulsed ultrasound is 0.3-0.8 W / cm². 2 The frequency of the pulse is the same as the frequency of the pulse current; the magnetic field strength of the alternating magnetic field is 0.1-0.5 T, and its frequency is the same as the frequency of the pulse current. During electrodeposition, the coating thickness is monitored using an online thickness gauge, and the conveyor belt speed is adjusted within the range of 1.4-1.6 m / min. S3: Post-plating heat treatment and gradient magnetic field-assisted diffusion: The nickel-plated steel strip obtained in S2 is placed in an annealing furnace, first subjected to stress-relief annealing, and then heated to the diffusion annealing temperature; during the heating and holding process of diffusion annealing, a magnetic field with a gradient increasing in strength over time is applied. The gradient-enhanced magnetic field is controlled as follows: the initial magnetic field strength is 0.1-0.3 T, and it is linearly or stepwise enhanced to 0.8-1.5 T within a heating or holding time of 30-60 min. The rate of change of the gradient-enhanced magnetic field intensity, dH / dt, is controlled within 0.01-0.03 T / min; S4: Cooling: After diffusion annealing, cool to room temperature to obtain the finished product.

2. The method for preparing a corrosion-resistant nickel-plated steel strip according to claim 1, characterized in that, The phase difference between the pulsed ultrasound and the pulsed current is 0° or 180°.

3. The method for preparing a corrosion-resistant nickel-plated steel strip according to claim 1, characterized in that, The diffusion annealing temperature described in S3 is 600-750℃, and the direction of the gradient enhancement magnetic field is parallel to the rolling direction of the steel strip.

4. The method for preparing a corrosion-resistant nickel-plated steel strip according to claim 1, characterized in that, The stress-relief annealing temperature described in S3 is 300-450℃, and the holding time is 10-20 min.

5. The method for preparing a corrosion-resistant nickel-plated steel strip according to claim 1, characterized in that, The nickel plating solution mentioned in S2 is a Watt-type nickel plating solution or an aminosulfonate nickel plating solution.

6. A corrosion-resistant nickel-plated steel strip, characterized in that, It is prepared by the method for preparing a corrosion-resistant nickel-plated steel strip according to any one of claims 1 to 5.

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