Nickel-plated steel strip and manufacturing method therefor, and battery housing formed therefrom

Abstract

The present invention provides a nickel-plated steel strip, comprising a substrate, a semi-bright nickel layer, and a nickel-iron alloy layer located between the substrate and the semi-bright nickel layer. The roughness Ra' of the surface of the nickel-plated steel strip satisfies 22 nm < Ra' ≤ 50 nm, and is measured by means of an atomic force microscope within the range of 2.5 μm × 2.5 μm on the surface of the nickel-plated steel strip. The nickel-plated steel strip of the present invention has good welding performance and corrosion resistance. The present invention further provides a method for manufacturing the nickel-plated steel strip and a battery casing or casing assembly made of the nickel-plated steel strip.

Description

A nickel-plated steel strip, its manufacturing method, and a battery casing formed therefrom. Technical Field

[0001] This invention relates to the field of metallurgical technology, and more particularly to a nickel-plated steel strip and its manufacturing method. The nickel-plated steel strip possesses excellent corrosion resistance and is suitable for manufacturing battery casings. Battery casings made from this nickel-plated steel strip exhibit good battery performance. Background Technology

[0002] The battery casing not only serves as a container to withstand physical impacts and prevent potential risks such as electrolyte leakage, but also, as part of the battery's conductivity, the internal environment formed by the battery's internal components, such as the positive and negative electrodes and the electrolyte, is closely related to the material type and performance of the battery casing. Nickel-plated steel strips not only possess good conductivity stability but also require good corrosion resistance, while simultaneously avoiding or minimizing the leaching of iron and nickel ions that could affect battery performance.

[0003] To improve corrosion resistance, nickel-plated steel strips need to undergo alloying treatment after nickel plating to form a nickel-iron alloy layer. Nickel-plated steel strips with both a nickel plating layer and a nickel alloy layer are suitable for use as battery casings due to their excellent processing performance and corrosion resistance.

[0004] Generally, the production process of nickel-plated battery steel shells is divided into two modes: one is to first stamp the steel strip into a battery shell, and then electroplate the formed battery shell, which is called post-nickel-plated steel shell; the other is to first electroplate the surface of the steel strip before stamping, and then stamp the pre-nickel-plated steel strip with a protective layer into a battery shell, which is called pre-nickel-plated steel shell.

[0005] Applying nickel plating to the formed battery casing makes it difficult to ensure uniformity of the plating, especially given the significant thickness differences within the inner wall of the casing, which compromises yield. In contrast, pre-plated nickel steel casings offer a more uniform and complete coating, providing comprehensive protection of the substrate without any blind spots. This effectively prevents electrolyte corrosion during rapid charging and discharging, avoiding electrolyte leakage and improving the safety and reliability of new energy batteries. Therefore, in recent years, pre-plating nickel technology has gradually replaced post-plating nickel technology in battery casing manufacturing.

[0006] In the material design of lithium-ion batteries, the positive and negative electrode tabs are generally aluminum foil and copper foil, respectively. Due to the design requirements of lithium-ion batteries on the chemical stability, conductivity and bonding of the positive and negative electrode tabs, the positive and negative electrode tabs need to be welded to the corresponding parts of the casing in the battery structure to form a circuit inside the battery cell.

[0007] Furthermore, in battery modules for new energy vehicles, electric two-wheelers, and portable power tools, multiple battery cells are often connected in series or parallel via current collector connectors. When connecting battery cells to external terminals, spot welding is necessary because nickel has stable contact resistance. This is important for increasing voltage by connecting battery cells in series or for obtaining large current by connecting them in parallel. Nickel also meets the requirements for spot welding. The weldability of the battery casing material is particularly crucial for cylindrical lithium-ion battery cases.

[0008] CN105506689A discloses a technology and production method for barrel plating nickel on battery casings. By plating a sulfur-free semi-bright nickel plating layer on the battery steel casing, and then electroplating a sulfur-containing bright plating layer, a potential difference is generated between the bright nickel and the semi-bright nickel due to the more negative potential of the sulfur-containing bright nickel, forming a corrosion cell. The bright nickel with higher sulfur content becomes the anode, and the underlying semi-bright nickel becomes the cathode. The bright nickel layer becomes a sacrificial plating layer and is corroded, thus delaying the corrosion rate of the corrosive medium to the iron substrate and significantly improving the corrosion resistance of the plating layer.

[0009] Because the battery casing is a deep blind hole component, this solution is prone to problems such as missed plating or uneven nickel plating at the bottom of the battery casing during the barrel plating process. This leads to unstable welding performance of the positive and negative electrode tabs inside the battery casing, resulting in phenomena such as poor soldering and desoldering.

[0010] CN102473862B discloses a Ni-plated steel plate for battery casing with excellent pressure resistance. The average roughness Ra of the semi-bright Ni-plated layer, measured by a stylus-type roughness meter, is above 1.0 μm and below 2.0 μm. The surface roughness Ra′ of the semi-bright Ni-plated layer with a surface area of ​​2.5 μm × 2.5 μm is calculated to be between 5 and 22 nm using an atomic force microscope.

[0011] CN102763237A discloses a method for manufacturing a Ni-plated steel sheet for a battery can with excellent pressurization properties. The surface roughness Ra1 of the semi-gloss Ni-plated layer in the range of 2.5μm×2.5μm, determined by atomic force microscopy, is between 3-11nm, and the surface roughness Ra2, determined by stylus roughness measuring instrument, is between 0.3μm and 2.0μm.

[0012] These two schemes primarily consider the machinability of nickel-plated steel strips, aiming to prevent scratches during the processing of nickel-plated steel strips into steel shells, thereby improving product quality and production efficiency. However, research has found that if Ra′ is too low, the scratch performance of stamped steel shells can be improved to some extent, but the welding performance of the steel shell or shell assembly and the tab connecting piece will be greatly affected. Summary of the Invention

[0013] In view of the above technical problems existing in the prior art, the first aspect of the present invention provides a nickel-plated steel strip, comprising a base substrate, a semi-bright nickel layer and a nickel-iron alloy layer located between the base substrate and the semi-bright nickel layer, wherein the surface roughness Ra' of the nickel-plated steel strip satisfies 22nm < Ra' ≤ 50nm, and the roughness Ra' is measured by an atomic force microscope within a range of 2.5μm×2.5μm on the surface of the nickel-plated steel strip.

[0014] There is a common technical prejudice in the prior art: it is considered that the smaller the nanoscale roughness Ra' on the surface of the nickel-plated steel strip, the better. The profound influence of Ra' on the subsequent key process of battery casings—welding performance has not been fully foreseen or valued. The battery casing needs to be resistance-welded with the tab or connecting sheet, and the welding quality directly determines the reliability and safety of the battery connection. The nano-scale protrusions on the surface of the nickel-plated steel strip (represented by the Ra' value) play the key role of microscopic current conduction "contacts" during the resistance welding process. It ensures the stable establishment of current channels during welding, avoids instantaneous fluctuation of contact area caused by absolutely flat contact surface (too small Ra'), thereby preventing cold welding. Appropriate protrusions help welding heat to disperse more uniformly, avoiding coating burning or weak welding caused by local overheating. The present invention eliminates the technical prejudice that it is not the case that the smaller Ra' is, the better. The present invention proves that when Ra' is in the range of 22nm < Ra' ≤ 50nm, it can not only ensure excellent welding performance through appropriate nano-protrusions, but also ensure that the coating has sufficient density and hardness through process control (such as specific wetting agent and leveling process), so as to take into account stamping processability.

[0015] When a nickel-plated steel strip is stamped into a battery casing, the nickel-plated steel strip retains its original thickness and coating morphology. To increase battery capacity, it is necessary to reduce the thickness of the nickel-plited steel strip and improve the strength of the nickel-plated steel strip. In addition, the nickel-plated steel strip undergoes multiple passes of stretching and thinning during the process of being stamped into a battery casing, during which both the thickness and coating morphology of the nickel-plated steel strip change. To ensure the corrosion resistance of the battery casing, especially the corrosion resistance of the inner surface of the battery casing in contact with the electrolyte, and to improve production efficiency, it is necessary to avoid scratching of the nickel-plated steel strip during the stamping process. From the perspective of scratching control, it is necessary to control the surface roughness Ra' of the nickel-plated steel strip within a suitable range. Theoretically, the larger Ra' is, the more likely scratching is to occur, which affects the corrosion resistance of the casing. However, if Ra' is too small, it is difficult for stamping oil to be retained during the stamping process, resulting in loss of lubrication. Since the nickel-plated steel strip needs to be assembled into a battery cell after being formed into the battery casing, the battery casing or casing assembly formed from the nickel-plated steel strip needs to be welded to tabs for conductive connection, and the battery pack is welded to the battery casing or casing assembly via connecting sheets. Welding quality is evaluated by welding bonding force. If the welding bonding force is too low, the tabs or connecting sheets are prone to separate from the casing, or poor contact occurs, which affects the stability of battery performance. Studies have shown that the micromorphology of the nickel-plated steel strip is related to welding performance, and the fluctuation of the surface micromorphology can be evaluated by Ra'. Surface protrusions can serve as contact points for conducting current and improve welding performance. If the protrusions of the coating surface micromorphology are too large, that is, Ra' is relatively large, the contact gap between the tab / connecting sheet and the battery casing or casing assembly is too large or uneven contact is prone to occur, leading to increased contact impedance during welding, increased heat generation, endangering battery performance and battery safety, and the welding quality cannot meet application requirements. In addition, a relatively large Ra' will cause local stress concentration at the welding point, which will also reduce the welding strength. If Ra' is relatively small, the contact gap between the tab / connecting sheet and the battery casing is too small, the contact impedance decreases during welding, and insufficient welding is prone to occur, resulting in insufficient welding bonding force.

[0016] Atomic force microscopy (AFM) can accurately measure the three-dimensional micromorphology of the nickel-plated steel strip surface and obtain surface roughness parameters. Studies have found that controlling Ra' within the range of 22 nm < Ra' ≤ 50 nm can effectively improve the weldability, corrosion resistance and stamping performance of the nickel-plated steel strip. Preferably, Ra' is 30 nm or more.

[0017] Preferably, the average width RSm of roughness profile elements on the surface of the nickel-plated steel strip is 80-200 μm, more preferably 100-180 μm, which is measured by a stylus roughness tester.

[0018] The nickel-plated steel strip of this invention exhibits a periodic undulating surface profile with appropriate peak-to-trough spacing. This facilitates the storage of stamping oil (lubrication) and ensures weldability. In this invention, the average width RSm of the roughness profile unit on the surface of the nickel-plated steel strip is 80-200 μm, preferably 100-180 μm. If RSm is too small (<80 μm), it means there are too many peaks, which are easily scratched by the die during stamping, leading to plating damage and an increased risk of iron leakage. If RSm is too large (>200 μm), the surface is too "flat," resulting in reduced macroscopic oil storage capacity, insufficient stamping lubrication, and a tendency to scratch. Therefore, controlling RSm within a suitable range ensures the integrity and lubrication of the plating during stamping from a macroscopic morphological perspective, thereby further protecting and synergistically achieving the excellent weldability guaranteed by Ra′.

[0019] The nickel-plated steel strip of the present invention has an appropriate degree of protrusion on its surface microstructure, which can ensure that the welding current passes through the contacts and ensure good conductivity inside the battery.

[0020] Preferably, the average surface roughness Ra2 of the nickel-plated steel strip is 0.10-0.50 μm, more preferably 0.20-0.50 μm, and even more preferably 0.20-0.40 μm, as measured by a stylus roughness tester.

[0021] Preferably, the average surface roughness peak-valley profile depth Rz of the nickel-plated steel strip is less than 3.5 μm, more preferably greater than 2.0 μm, and is measured by a stylus roughness tester.

[0022] Preferably, the substrate is aluminum killed steel or interstitial steel, wherein, by weight percentage, the carbon content of aluminum killed steel is 0.02%-0.08%, and the carbon content of interstitial steel is less than 0.005%.

[0023] Preferably, the aluminum-killed steel contains the following chemical elements by weight percentage: C: 0.02-0.08%, Mn: 0.15-0.35%, Si: 0.010-0.035%, P≤0.015%, S≤0.015%, Al: 0.03-0.05%, with the balance being Fe and unavoidable impurities; the interstitial atom-free steel contains the following chemical elements by weight percentage: C≤0.005%, Mn: 0.15-0.30%, Si: 0.01-0.05%, P≤0.012%, S≤0.010%, with the balance being Fe and unavoidable impurities.

[0024] Preferably, the weld bond strength of the nickel-plated steel strip is ≥90N, more preferably ≥95N.

[0025] Preferably, the surface corrosion resistance of the nickel-plated steel strip tested according to GB / T 6461-2002 is above grade 9; and / or, the Vickers hardness HV on the surface of the nickel-plated steel strip is above 170. The present invention does not particularly limit the upper limit of the Vickers hardness on the surface of the nickel-plated steel strip, but it is usually below 240.

[0026] Preferably, the thickness of the nickel-plated steel strip is 0.10-1.00 mm.

[0027] Preferably, the single-side thickness of the nickel-iron alloy layer in the nickel-plated steel strip is 0.10-0.60 μm, preferably 0.20-0.50 μm.

[0028] The nickel-plated steel strip of the present invention has good corrosion resistance, processing performance and welding performance, and excellent comprehensive performance.

[0029] A second aspect of the present invention provides a battery casing or casing assembly, which is formed from the above-mentioned nickel-plated steel strip.

[0030] A third aspect of the present invention provides a method for manufacturing a nickel-plated steel strip, the method comprising the following steps carried out in sequence:

[0031] Rolling: cold rolling is performed on a base plate, and the cold rolling reduction rate is 70%-95%;

[0032] Nickel electroplating: electroplating the rolled base plate in a plating solution to form a semi-bright nickel layer on the surface of the rolled base plate, so as to obtain a nickel-plated steel strip precursor;

[0033] Heat treatment: continuous annealing is performed on the nickel-plated steel strip precursor to form a nickel-iron alloy layer between the rolled base plate and the semi-bright nickel layer, so as to obtain an annealed steel strip;

[0034] Skin pass: skin pass is performed on the annealed steel strip to obtain the nickel-plated steel strip, wherein the skin pass elongation is 0.5-1.5%, the average roughness Ra2 of the nickel-plated steel strip measured by a stylus roughness tester is 0.10-0.50 μm, preferably 0.20-0.40 μm; the average roughness peak-valley profile depth Rz is less than 3.5 μm, preferably 2.0 μm or more;

[0035] The obtained nickel-plated steel strip comprises a base plate, a semi-bright nickel layer and a nickel-iron alloy layer located between the base plate and the semi-bright nickel layer, the surface roughness Ra' of the nickel-plated steel strip satisfies 22 nm < Ra' ≤ 50 nm, preferably 30 nm ≤ Ra' ≤ 50 nm, and the roughness Ra' is measured by an atomic force microscope within a range of 2.5 μm × 2.5 μm on the surface of the nickel-plated steel strip.

[0036] Generally, the surface roughness Ra1 of the base plate after cold rolling is 0.6-1.4 μm.

[0037] Preferably, the thickness of the obtained nickel-plated steel strip is 0.10-1.00 mm, and the single-sided thickness of the nickel-iron alloy layer in the nickel-plated steel strip is 0.10-0.60 μm, preferably 0.20-0.50 μm.

[0038] Preferably, the nickel electroplating step satisfies one or more of the following:

[0039] The plating solution contains 220-320 g / L nickel sulfate, 20-50 g / L nickel chloride, 20-50 g / L boric acid, and 2-5 g / L wetting agent. Preferably, the wetting agent is a sulfonated product of fatty alcohol / alkylphenol polyoxyethylene propylene ether, and the balance is water.

[0040] The surface tension of the plating solution is less than 45 mN / m;

[0041] Current density 5-50A / dm 2 Preferred 10-30A / dm 2 The electroplating temperature is 45-65℃, and the pH value of the plating solution is 3-5.

[0042] Preferably, the sulfonated product is obtained by condensing and sulfonating a C4-C12 fatty alcohol or a C4-C12 alkylphenol with ethylene oxide or propylene oxide.

[0043] Preferably, the above method further includes a pretreatment step of the substrate before nickel electroplating, the pretreatment step including electrochemical alkaline washing and acid washing; preferably, the alkaline washing solution comprises: 45-65 g / L sodium hydroxide, 35-45 g / L anhydrous sodium carbonate, 35-45 g / L sodium phosphate, and the balance being water; preferably, the electrochemical alkaline washing is carried out under the following conditions: temperature 55-65℃, current density 10-20 A / dm³ 2 Alkaline washing time is 1-5 min; and / or, the pickling solution is a hydrochloric acid solution with a mass concentration of 2-8%, the pickling temperature is 15-30℃, and the pickling time is 1-3 min.

[0044] Preferably, in the heat treatment step, the continuous annealing temperature is 700-900℃, more preferably 780-900℃, and the annealing time is 15-120 seconds.

[0045] Preferably, in the nickel electroplating step, a semi-bright nickel layer is formed on at least one surface of the rolled substrate by electroplating, wherein the single-sided thickness of the semi-bright nickel layer is 0.50-1.00 μm, more preferably 0.60-0.80 μm.

[0046] Preferably, the nickel-plated steel sheet manufactured by the above method satisfies one or more of the following characteristics:

[0047] The welding bond strength of the nickel-plated steel strip is ≥90N, more preferably ≥95N;

[0048] The surface corrosion resistance of nickel-plated steel strip tested according to GB / T 6461-2002 is level 9 or above;

[0049] The Vickers hardness (HV) of the nickel-plated steel strip surface is above 170. Attached Figure Description

[0050] Figure 1 shows the morphology of the nickel-plated steel strip surface obtained by atomic force microscopy in Embodiment 2 of the present invention;

[0051] Figure 2 shows the test curve of the welding bond strength of the nickel-plated steel strip in Embodiment 1 of the present invention;

[0052] Figure 3 shows the morphology of the semi-bright nickel layer on the surface of the nickel-plated steel strip in Embodiment 2 of the present invention. Detailed Implementation

[0053] The implementation of the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0054] The following describes the process for manufacturing the nickel-plated steel strip of the present invention. In an embodiment of the present invention, the nickel-plated steel strip is manufactured according to the following process.

[0055] (1) Selecting a substrate

[0056] In this invention, the substrate undergoes processes such as hot rolling, cold rolling, and annealing. The substrate used in this invention needs to have high purity and good processability, which imposes certain restrictions on the chemical composition of the steel strip. Specifically, it can be low-carbon aluminum-killed steel with a carbon content between 0.02% and 0.08%, or interstitial-free steel with a carbon content below 0.005%. The steel may also contain appropriate amounts of elements such as silicon and manganese to ensure that the subsequently formed battery casing or casing assembly has good strength and overall performance.

[0057] For example, when using low-carbon aluminum-killed steel, the aluminum-killed steel contains the following chemical elements by weight percentage: C: 0.02-0.08%, Mn: 0.15-0.35%, Si: 0.010-0.035%, P≤0.015%, S≤0.015%, Al: 0.03-0.05%, with the balance being Fe and unavoidable impurities.

[0058] When using interstitial steel, the interstitial steel contains the following chemical elements by weight percentage: C≤0.005%, Mn:0.15-0.30%, Si:0.01-0.05%, P≤0.012%, S≤0.010%, with the balance being Fe and unavoidable impurities.

[0059] (2) Rolling

[0060] The substrate surface is rolled to obtain a surface roughness Ra1 of 0.6-1.4 μm, preferably 0.6-1.2 μm.

[0061] Specifically, the rolling process employed is as follows: the substrate is rolled to the target thickness using a cold rolling process while controlling the surface roughness of the substrate, with a cold rolling reduction rate of 70%-95%. By increasing the cold rolling reduction rate, the deformation energy in the steel is increased, and the recrystallization driving force is enhanced, thereby lowering the recrystallization temperature and improving the formability of the steel strip. Roughness exhibits heritability; that is, controlling Ra1 affects controlling Ra2. In the rolling process, there are no particular restrictions on the surface roughness of the work rolls, as long as the desired roughness range is achieved. In this invention, the surface roughness of the substrate after rolling can be adjusted by modifying the work roll roughness, rolling load, etc.

[0062] The surface of the work rolls used to form Ra1 can be treated by electrical discharge machining (EDM) or shot peening. There are no particular restrictions on the rolling equipment.

[0063] (3) Preprocessing

[0064] Pretreatment includes electrochemical alkaline washing and acid washing. Electrochemical alkaline washing mainly removes oil, iron powder, oxides, etc. from the substrate surface. The alkaline washing solution includes sodium hydroxide, sodium phosphate, and sodium carbonate, etc. The concentration of the alkaline washing solution and the electrochemical alkaline washing process parameters are reasonably set according to the degree of residual contaminants on the steel strip surface to improve the surface cleanliness of the steel strip and obtain good adhesion between the coating and the steel strip surface.

[0065] In this invention, the main components of the electrochemical alkaline cleaning solution include: sodium hydroxide 45-65 g / L, anhydrous sodium carbonate 35-45 g / L, and sodium phosphate 35-45 g / L. The cathode electrolytic degreasing process parameters are: temperature 55-65℃, current density 10-20 A / dm³. 2 Alkali washing time: 1-5 minutes.

[0066] The pickling solution uses a concentration of 2-8%, preferably 5wt% hydrochloric acid. The pickling time is 1-3 minutes, and the pickling temperature is 15-30℃, for example, room temperature (25℃). The main purpose is to remove oxides and other impurities from the steel strip surface, further improving surface cleanliness and enhancing surface activity. Immediately after pickling, the steel strip surface is rinsed with deionized water at room temperature for 1-3 minutes to prevent the pickling solution from corroding the steel strip and to prevent hydrogen ions from being carried into the plating solution and affecting its pH value.

[0067] (4) Nickel plating

[0068] Plating solution:

[0069] The base plating bath is a standard Watt's nickel plating bath, consisting of nickel sulfate, nickel chloride, and boric acid. The main function of nickel sulfate is to provide Ni... 2+ In this invention, the concentration of nickel chloride is controlled at 220-320 g / L; nickel chloride is used to improve the conductivity and dispersibility of the plating solution, and its concentration is controlled at 20-50 g / L, preferably 30-50 g / L; boric acid is used as a slow-release agent, and its concentration is controlled at 20-50 g / L, preferably 30-50 g / L.

[0070] During electroplating, hydrogen evolution side reactions often occur at the cathode. Hydrogen evolution not only reduces cathode current efficiency but also causes pinholes in the plating layer due to the retention of hydrogen bubbles on the electrode surface. To prevent and reduce pinhole formation and lower the surface tension of the plating solution, a small amount of wetting agent can be added. In this invention, a sulfonated product of fatty alcohol / alkylphenol polyoxyethylene propylene ether is mainly added as a wetting agent. This agent is obtained by condensing and sulfonating C4 to C12 fatty alcohols or C4 to C12 alkylphenols with ethylene oxide or propylene oxide. This wetting agent, due to its longer hydrophobic groups, has greater surface activity than general wetting agents and a stable structure, exhibiting excellent surface tension reduction and penetration effects. Furthermore, as a sulfonated product, it is not easily decomposed in nickel plating solutions and is very stable, ensuring that no organic decomposition products are generated on the plating surface, maintaining the stability of the plating layer's physicochemical properties. Its concentration in the plating solution is 2-5 g / L, which can maintain the surface tension of the plating solution below 45 mN / m. The use of nickel plating additives can affect the uniformity of the coating, which not only affects corrosion resistance but also the microstructure of the coating surface (such as the degree and distribution of protrusions and depressions on the coating surface), thereby affecting the performance of the resulting battery casing or casing assembly.

[0071] electrode:

[0072] The anode can be either soluble or insoluble. Nickel blocks or balls are selected as soluble anodes and placed in a titanium basket (which can be made of TA2 industrial pure titanium). When using insoluble anodes, sulfate formation must be controlled.

[0073] Electroplating process:

[0074] Temperature 45-65℃, pH of plating solution 3-5, current density 5-50A / dm³ 2 Preferably 10-30 A / dm 2The electroplating time is adjusted according to the thickness and structural requirements of the nickel plating layer to obtain an ideal semi-bright nickel plating layer. After electroplating, a semi-bright nickel layer is formed on at least one surface of the rolled substrate. The single-sided thickness of the semi-bright nickel plating layer is 0.50-1.00 μm, preferably 0.60-1.00 μm, and more preferably 0.60-0.80 μm, to obtain the precursor of the nickel-plated steel strip. The plating layer within this thickness range provides both wear resistance and corrosion protection, and also enables the finished nickel-plated steel strip to have good processing performance. A smaller plating layer thickness can save manufacturing costs while ensuring the performance of the plating layer.

[0075] (5) Heat treatment

[0076] The nickel-plated steel strip precursor is subjected to continuous annealing to form a nickel-iron alloy layer between the substrate and the semi-bright nickel plating layer. The annealing temperature is controlled at 700-900°C, preferably 780-900°C, more preferably 780-830°C, and the annealing time is 15-120 seconds, preferably 30-60 seconds. The heat treatment can be carried out in a non-oxidizing atmosphere or a reducing protective gas atmosphere such as nitrogen or argon.

[0077] (6) Leveling

[0078] After annealing, the steel strip is leveled. The specific process is as follows: the leveling rolling force is controlled at 300-1000 tons to optimize the surface roughness, surface morphology, plate shape and mechanical properties of the finished nickel-plated steel strip.

[0079] The elongation at flattening of nickel-plated steel strip refers to the ratio of the increase in length of the nickel-plated steel strip under tensile force during the flattening process to its original length, usually expressed as a percentage. It is an important indicator for measuring the plastic deformation capacity of nickel-plated steel strip.

[0080] The calculation formula is: Elongation = (Length of steel strip after leveling – Initial length of steel strip) / Initial length of steel strip × 100%;

[0081] In this invention, the flattening elongation rate is controlled at 0.5-1.5%. The flattening elongation rate is measured online using a laser rangefinder: measuring devices are installed before and after the steel strip enters the flattening machine to monitor the length change of the steel strip in real time. The system calculates the flattening elongation rate according to a preset program and automatically generates a report. After flattening by the flattening work rollers, the average surface roughness Ra2 of the nickel-plated steel strip is 0.10-0.50 μm, and Rz is less than 3.5 μm, preferably above 2.0 μm.

[0082] The thickness of the nickel-plated steel strip after leveling is 0.10-1.00 mm. In this invention, the thickness of the nickel-plated steel strip refers to the total thickness including the substrate, the semi-bright nickel layer, and the nickel-iron alloy layer between them. During the leveling process, there are no particular restrictions on the surface roughness of the work rolls, as long as the required roughness range is met. In this invention, the surface roughness Ra2 and Rz of the leveled nickel-plated steel strip can be adjusted by adjusting the work roll roughness, rolling load, or the elongation of the steel sheet. The work roll surfaces that form Ra2 and Rz are treated with grinding wheels. Furthermore, there are no particular restrictions on the leveling equipment.

[0083] The characteristics and testing methods of the nickel-plated steel strip of the present invention are described below.

[0084] (1) Roughness

[0085] a) The surface roughness Ra′ of the nickel-plated steel strip of the present invention is greater than 22 nm, preferably greater than 30 nm and less than 50 nm, and is measured in the range of 2.5 μm × 2.5 μm using an atomic force microscope (Bruker Dimension series). When Ra′ is less than 22 nm, the scratch performance of the nickel-plated steel strip after stamping the shell is improved to a certain extent, but the weldability of the shell and the tab is greatly affected. When Ra′ is greater than 50 nm, more shell surfaces are scratched during the stamping of the nickel-plated steel strip. It should be noted that in this study, Ra′ is increased to ensure weldability, but a balance is achieved with scratch resistance by improving the coating performance through nickel plating additives.

[0086] Atomic force microscopy (AFM) is used to detect the extremely weak interatomic interactions between the sample surface and the micro-force-sensitive element, allowing for the probing of the physical properties, including morphology, in the nanoscale region of the sample. This enables the analysis of its surface structure and properties. AFM offers high resolution, enabling the measurement of surface morphology within extremely small areas of the specimen. The roughness Ra′ within the measured range is defined identically to the arithmetic mean deviation of the profile Ra. During testing, points are randomly selected on the surface of the steel strip, and the morphology within a 2.5 μm × 2.5 μm area is measured. Six measurement lines are taken within this range to record the Ra′ values. The average of the six test results is used as the surface roughness Ra′ of the nickel-plated steel strip.

[0087] b) In the process of manufacturing nickel-plated steel strip, the present invention also measures the average surface roughness Ra1 of the substrate after rolling, the average surface roughness Ra2 of the steel strip after leveling, and the average roughness peak and valley profile depth Rz.

[0088] The average surface roughness Ra1 of the rolled substrate is 0.6-1.4 μm, preferably 0.6-1.2 μm. The average surface roughness Ra2 of the leveled steel strip is 0.10-0.50 μm, and the average depth of the roughness peak-valley profile Rz is less than 3.5 μm, preferably above 2.0 μm.

[0089] Ra1 and Ra2 were measured using a TR201 stylus-type surface roughness tester. Specifically, the measuring cross-section of the detector (stylus) was aligned with the measuring direction of the maximum values ​​of the roughness amplitude parameters Ra1 and Ra2. This direction was perpendicular to the machining texture of the surface being measured. The internal drive mechanism of the instrument propelled the sensor to slide at a constant speed along the surface being measured. The sensor obtained the values ​​by sensing the peak value changes of the surface being measured through its built-in sharp stylus. Roughness was measured according to GB / T 10610-2009, with a roughness sampling length lr of 0.25 mm and a roughness evaluation length ln of 1.25 mm. For each nickel-plated steel strip, Ra was measured at three different locations on the surface of the steel strip, and the average value was recorded as Ra1 and Ra2 for that nickel-plated steel strip.

[0090] Rz was measured using the same measurement standard (GB / T 10610-2009) and conditions as Ra1 and Ra2, and was also measured using a TR201 stylus-type surface roughness tester. The calculation used the sum of the maximum profile peak height Rp and the maximum profile valley depth Rv within a sampling length, i.e., the deviation between the highest and lowest peak (average roughness peak-valley profile depth). For each nickel-plated steel strip, Rz was measured at three different locations on the steel strip surface, and the average value was taken.

[0091] RSm was measured using the same measurement standard (GB / T 10610-2009) and conditions as Ra1, Ra2, and Rz, and was also measured using a TR201 stylus surface roughness tester. The average width of the contour unit within a sampling length was used in the calculation. For each nickel-plated steel strip, RSm was measured at three different locations on the strip surface, and the average value was taken.

[0092] If the surface roughness is too high, excessively sharp surface peaks can cause wear on the nickel-plated steel strip, leading to localized iron leakage during stamping and subsequently pitting corrosion. If the surface roughness is too low, the space for lubricant to enter between the steel plate and the die during stamping is reduced, resulting in insufficient lubrication and scratches. Both of these conditions pose significant safety risks to the subsequently assembled batteries. The inventors discovered that by controlling the surface roughness Ra2 and further controlling the surface roughness Rz and RSm within appropriate ranges, pitting corrosion and scratches can be effectively avoided.

[0093] (2) Welding bond strength

[0094] The weld bond strength of the nickel-plated steel strip of this invention is ≥90N. After the nickel-plated battery steel strip forms the nickel-plated battery casing, it needs to be connected to the electrode tabs for conductivity by welding. To verify the bond strength between the two, the minimum force required to break after welding is determined using the following test. The minimum force required to break after welding is the weld bond strength. The higher the value, the better the bond strength. The specific method for determining the weld bond strength is as follows:

[0095] To determine the weld bond strength, nickel-plated steel strips were first stamped into shells or shell assemblies. Then, using a Miyaki MDB-400B resistance welding machine with a 2.5mm welding needle, a copper-nickel composite strip was welded to the bottom of the shell. The copper-nickel composite strip was 9mm wide, 0.1mm thick, and 13cm long. Welding process parameters: energy C=35, preload T=21N, T=30ms. The welded shell or shell assembly was then subjected to a tensile test on a universal testing machine. Tensile (or peel) was applied at a set rate until the weld point broke to obtain the weld bond strength of the nickel-plated steel strip.

[0096] (3) Corrosion resistance

[0097] The surface corrosion resistance of the nickel-plated steel strip of the present invention, as tested according to GB / T 6461-2002, is above level 9.

[0098] Specifically, corrosion resistance testing was conducted according to GB / T 6461-2002. A 5% sodium chloride aqueous solution was used as the spray solution for neutral salt spray testing of nickel-plated annealed steel strips. The time was set according to the coating and performance requirements. After corrosion, the sample surface was cleaned and kept dry. The surface of the corroded steel strip was evaluated according to the evaluation criteria described in GB / T 6461-2002, including the appearance of the coating after the test, the appearance after removing surface corrosion products, and the distribution and number of corrosion defects such as pitting, cracks, and bubbles. The protection rating R of the coating was determined according to the percentage of the steel strip area with corrosion to the tested steel strip area. P R P A higher value indicates better corrosion resistance provided by the coating.

[0099] (4) Surface hardness

[0100] The Vickers hardness of the steel strip surface was tested according to GB / T 4340.1-2024. The Vickers hardness (HV) of the nickel-plated steel strip surface of the present invention is above 170.

[0101] Since bending of the sample during the shearing process is unavoidable, bending will occur when the hardness tester is loaded, leading to a large deviation in the test results. Therefore, the cut sample is cold-mounted. The testing equipment for hardness testing in this invention is a Wolpert-401MVD micro Vickers hardness tester, the test load is 0.01 kg, the loading time is 15 s, and each sample is tested 8 times, and the average value is taken.

[0102] (5) Coating thickness and alloy layer thickness

[0103] In this invention, the thickness of the initial semi-bright nickel layer is measured according to GB / T 4955-2005 "Metallic Coatings - Coating Thickness Measurement - Anodic Dissolution Coulometric Method". A precisely defined area of ​​the coating is anoly dissolved using a suitable electrolyte, and the complete dissolution of the coating is determined by the change in the electrolytic cell voltage. The thickness of the coating is calculated using the amount of electricity consumed in the electrolysis (in coulombs).

[0104] The thickness of the nickel-iron alloy layer was determined by observing the morphology using SEM and combining it with EDS analysis to detect the distribution of elements along the thickness direction of the steel strip. The cross-section of the nickel-plated steel sheet perpendicular to the substrate was ground, and the composition was continuously analyzed from the surface of the nickel-plated steel strip towards the substrate. Specifically, the location where the Ni content in the steel strip cross-section reached 90% was designated as the first boundary, and the location where the Fe content in the steel strip cross-section reached 90% towards the substrate was designated as the second boundary. The distance between the first and second boundaries is the thickness of the nickel-iron alloy layer. To ensure the representativeness and accuracy of the measurement results, measurements were taken at more than 10 different locations on the steel strip, and the average value was calculated as the final thickness of the nickel-iron alloy layer in the steel strip. Unless otherwise specified, the thickness of the coating in this invention refers to its single-sided thickness.

[0105] Examples and Comparative Examples

[0106] Example 1:

[0107] The substrate is made by annealing a cold-rolled sheet (0.80 mm thick) of low-carbon aluminum-killed steel with the chemical composition shown below.

[0108] C: 0.042 wt%, Mn: 0.18 wt%, Si: 0.010 wt%, P: 0.015 wt%, S: 0.010 wt%, Al: 0.043 wt%, balance being Fe and unavoidable impurities.

[0109] Rolling: The surface of the steel strip is adjusted by cold rolling to obtain suitable surface roughness and mechanical properties. The rolling reduction rate is 87%.

[0110] Pretreatment: The rolled substrate undergoes electrochemical alkaline washing, acid washing, and water washing according to the aforementioned process. The main components of the alkaline washing solution include: sodium hydroxide 55 g / L, anhydrous sodium carbonate 40 g / L, and sodium phosphate 40 g / L. Cathode electrolytic degreasing process parameters: temperature 55℃, current density 10 A / dm³. 2 Alkali washing time: 3 minutes.

[0111] Semi-bright nickel plating: Nickel sulfate 280g / L, nickel chloride 30g / L, boric acid 30g / L. Additionally, 2-5g / L of sodium octylphenol polyoxypropylene ether sulfonate is added, resulting in a plating bath surface tension below 45mN / m. pH: 3.5, bath temperature: 50℃, current density: 10A / dm³. 2 A semi-bright nickel layer is formed on at least one surface of the substrate, and the single-sided thickness of the semi-bright nickel layer is 0.62 μm.

[0112] Heat treatment: Continuous annealing temperature is 780-800℃, annealing time is 60 seconds, and a non-oxidizing atmosphere is selected. After continuous annealing heat treatment, a nickel-iron diffusion alloy layer with a thickness of 0.13μm is obtained at the interface between the semi-bright nickel layer and the substrate.

[0113] Leveling: The nickel-plated steel strip that has completed the above nickel plating and annealing alloying heat treatment processes is leveled. The leveling elongation is 0.6%, and the average surface roughness Ra2 of the leveled nickel-plated steel strip is 0.38 μm, Rz is 3.0 μm, and RSm is 183 μm.

[0114] The nickel-plated steel strip, as measured by atomic force microscopy, has a surface roughness Ra′ of 38.068 nm and a Vickers hardness HV of 180. No nickel layer peeled off during the stamping process, its corrosion resistance was rated at level 9, and its weld strength was 107.2 N.

[0115] Example 2:

[0116] A steel strip with a thickness of 0.30 mm was used, the carbon content of the substrate was 0.03% by weight, the single-sided thickness of the semi-bright nickel layer formed by electroplating was controlled to be 0.6 μm, the rolling reduction rate was 80%, the flattening elongation rate was 0.5%, and the remaining parameters were the same as in Example 1.

[0117] After continuous annealing heat treatment, a nickel-iron diffusion alloy layer with a thickness of 0.10 μm was obtained at the interface between the semi-bright nickel layer and the substrate. The average surface roughness Ra2 of the leveled nickel-plated steel strip was 0.23 μm, Rz was 2.5 μm, and RSm was 159 μm. Atomic force microscopy analysis of the nickel-plated steel strip showed a surface roughness Ra′ of 31.989 nm and a Vickers hardness HV of 187. No semi-bright nickel layer peeled off during the stamping process, the corrosion resistance was rated at level 9, and the weld bonding strength was 95.1 N.

[0118] Example 3:

[0119] The steel strip with a thickness of 0.65 mm was used, the carbon content of the substrate was 0.051% by weight, the single-sided thickness of the semi-bright nickel layer formed by electroplating was controlled to be 1.00 μm, the rolling reduction rate was 70%, the flattening elongation rate was 0.8%, and the other parameters were the same as those in Example 1.

[0120] After continuous annealing heat treatment, a nickel-iron diffusion alloy layer with a thickness of 0.60 μm was obtained at the interface between the semi-bright nickel layer and the substrate. The average surface roughness Ra2 of the leveled nickel-plated steel sheet was 0.10 μm, Rz was 2.0 μm, and RSm was 200 μm. Atomic force microscopy analysis of the nickel-plated steel strip showed a surface roughness Ra′ of 22.553 nm and a Vickers hardness HV of 179. No semi-bright nickel layer peeled off during the stamping process, the corrosion resistance was rated at level 9, and the weld bond strength was 90.3 N.

[0121] Example 4:

[0122] The steel strip with a thickness of 0.35 mm and a carbon content of 0.048% by weight was used. The single-sided thickness of the semi-bright nickel layer formed by electroplating was controlled to be 0.75 μm. The rolling reduction rate was 83%, the leveling elongation rate was 1.0%, and the remaining parameters were the same as in Example 1.

[0123] After continuous annealing heat treatment, a nickel-iron diffusion alloy layer with a thickness of 0.32 μm was obtained at the interface between the semi-bright nickel layer and the substrate. The average surface roughness Ra2 of the leveled nickel-plated steel strip was 0.32 μm, Rz was 2.8 μm, and RSm was 92 μm. Atomic force microscopy analysis of the nickel-plated steel strip showed a surface roughness Ra′ of 37.752 nm and a Vickers hardness HV of 206. No semi-bright nickel layer peeled off during the stamping process, the corrosion resistance was rated at level 10, and the weld bonding strength was 97.4 N.

[0124] Example 5:

[0125] The steel strip with a thickness of 0.40 mm was used, the carbon content of the substrate was 0.036% by weight, the single-sided thickness of the semi-bright nickel layer formed by electroplating was controlled to be 0.68 μm, the rolling reduction rate was 90%, the flattening elongation rate was 1.2%, and the remaining parameters were the same as in Example 1.

[0126] After continuous annealing heat treatment, a nickel-iron diffusion alloy layer with a thickness of 0.21 μm was obtained at the interface between the semi-bright nickel layer and the substrate. The average surface roughness Ra2 of the leveled nickel-plated steel strip was 0.43 μm, Rz was 3.1 μm, and RSm was 126 μm. Atomic force microscopy analysis of the nickel-plated steel strip showed a surface roughness Ra′ of 45.810 nm and a Vickers hardness HV of 195. No semi-bright nickel layer peeled off during the stamping process, the corrosion resistance was rated at level 9, and the weld bonding strength was 124.2 N.

[0127] Example 6:

[0128] A 0.25 mm thick steel strip was used. The substrate contained 0.003 wt% carbon (interstitial atomic steel), Mn: 0.19%, Si: 0.02%, P≤0.012%, S≤0.010%, with the balance being Fe and unavoidable impurities. The single-sided thickness of the semi-bright nickel layer formed by electroplating was controlled to be 0.92 μm, the rolling reduction rate was 95%, and the leveling elongation rate was 1.5%. All other parameters were the same as in Example 1.

[0129] After continuous annealing heat treatment, a nickel-iron diffusion alloy layer with a thickness of 0.45 μm was obtained at the interface between the semi-bright nickel layer and the substrate. The average surface roughness Ra2 of the leveled nickel-plated steel strip was 0.50 μm, Rz was 3.4 μm, and RSm was 80 μm. Atomic force microscopy analysis of the nickel-plated steel strip showed a surface roughness Ra′ of 49.967 nm and a Vickers hardness HV of 210. No semi-bright nickel layer peeled off during the stamping process, the corrosion resistance was rated at level 10, and the weld bonding strength was 140.1 N.

[0130] Comparative Example 1

[0131] A steel strip with a thickness of 0.60 mm was used. The carbon content of the substrate was 0.040% by weight. The single-sided thickness of the semi-bright nickel layer formed by electroplating was controlled to be 1.50 μm. The rolling reduction rate was 65%, the flattening elongation rate was 1.8%, and the remaining parameters were the same as in Example 1.

[0132] After continuous annealing heat treatment, a nickel-iron diffusion alloy layer with a thickness of 0.73 μm was obtained at the interface between the semi-bright nickel layer and the substrate. The average surface roughness Ra2 of the leveled nickel-plated steel strip was 0.12 μm, Rz was 1.7 μm, and RSm was 153 μm. Atomic force microscopy analysis of the nickel-plated steel strip showed a surface roughness Ra′ of 17.235 nm and a Vickers hardness HV of 195. No semi-bright nickel layer peeled off during the stamping process, the corrosion resistance was rated at level 8, and the weld bonding strength was 50.10 N.

[0133] The nickel-plated steel strip in Comparative Example 1 had poor quality semi-bright nickel layer due to excessive thickness of the nickel plating layer formed by electroplating and low surface roughness after rolling and leveling, resulting in poor corrosion resistance and weldability.

[0134] Comparative Example 2

[0135] A steel strip with a thickness of 0.80 mm was used. The carbon content of the substrate was 0.060% by weight, and the single-sided thickness of the semi-bright nickel layer formed by electroplating was controlled to be 0.40 μm. The rolling reduction rate was 63%, the flattening elongation rate was 1.9%, no wetting agent was added, and all other parameters were the same as in Example 1.

[0136] After continuous annealing heat treatment, a nickel-iron diffusion alloy layer with a thickness of 0.09 μm was obtained at the interface between the semi-bright nickel layer and the substrate. The average surface roughness Ra2 of the leveled nickel-plated steel strip was 0.55 μm, Rz was 3.9 μm, and RSm was 241 μm. Atomic force microscopy analysis of the nickel-plated steel strip showed a surface roughness Ra′ of 56.658 nm and a Vickers hardness HV of 160. During the stamping process, some of the semi-bright nickel layer peeled off. The corrosion resistance was rated at level 8, and the weld bonding strength was 68.5 N.

[0137] In Comparative Example 2, the nickel-plated steel strip did not have a wetting agent added, resulting in a thin nickel plating layer and poor quality of the nickel-iron alloy layer formed during annealing. Furthermore, the high surface roughness of the nickel-plated steel strip after flattening led to poor quality of the semi-bright nickel layer. Consequently, the nickel-plated steel strip suffered from plating peeling, poor corrosion resistance, and poor weldability during the stamping process.

[0138] Comparative Example 3

[0139] A steel strip with a thickness of 0.30 mm was used. The carbon content was 0.035% by weight. The single-sided thickness of the semi-bright nickel layer formed by electroplating was controlled to be 1.10 μm. The rolling reduction rate was 68%, the leveling elongation rate was 2.0%, and the remaining parameters were the same as in Example 1.

[0140] After continuous annealing heat treatment, a nickel-iron diffusion alloy layer with a thickness of 0.62 μm was obtained at the interface between the semi-bright nickel layer and the substrate. The average surface roughness Ra2 of the leveled nickel-plated steel strip was 0.42 μm, Rz was 4.1 μm, and RSm was 158 μm. Atomic force microscopy analysis showed that the surface roughness Ra′ of the plating layer was 19.531 nm, and the Vickers hardness HV was 190. During the stamping process, some of the semi-bright nickel layer peeled off. The corrosion resistance was rated at level 7, and the weld bonding strength was 62.6 N.

[0141] The nickel-plated steel strip in Comparative Example 3 had poor quality semi-bright nickel layer due to the high content of wetting agent in the plating solution and the excessive thickness of the nickel plating layer. This resulted in the plating layer peeling off during the stamping process, as well as poor corrosion resistance and weldability.

[0142] Comparative Example 4

[0143] A steel strip with a thickness of 0.40 mm was used. The carbon content was 0.030% by weight. The single-sided thickness of the semi-bright nickel layer formed by electroplating was controlled to be 1.20 μm. The rolling reduction rate was 66%, the leveling elongation rate was 1.7%, and no wetting agent was added. All other parameters were the same as in Example 1.

[0144] After continuous annealing heat treatment, a nickel-iron diffusion alloy layer with a thickness of 0.67 μm was obtained at the interface between the semi-bright nickel layer and the substrate. The average surface roughness Ra2 of the leveled nickel-plated steel strip was 1.20 μm, Rz was 5.8 μm, and RSm was 166 μm. Atomic force microscopy analysis showed that the surface roughness Ra′ of the plating layer was 71.872 nm, and the Vickers hardness HV was 185. During the stamping process, some of the semi-bright nickel layer peeled off. The corrosion resistance was rated at level 8, and the weld bonding strength was 81.3 N.

[0145] The nickel-plated steel strip in Comparative Example 4 had a poor quality semi-bright nickel layer due to excessive nickel plating thickness, high surface roughness after rolling and leveling, and the absence of wetting agent. Consequently, the nickel-plated steel strip suffered from plating peeling off during stamping, as well as poor corrosion resistance and weldability.

[0146] Comparative Example 5

[0147] When electroplating a semi-bright nickel layer, 0.2 g / L of sodium dodecyl sulfate, a conventional ionic wetting agent, was added to the plating solution, and all other parameters were the same as in Example 1.

[0148] The average surface roughness Ra2 of the leveled nickel-plated steel strip is 0.10 μm, Rz is 1.5 μm, and RSm is 135 μm. The surface roughness Ra′ of the plating layer, measured by atomic force microscopy, is 8.937 nm, and the Vickers hardness HV is 196. No semi-bright nickel layer peeled off during the stamping process. The corrosion resistance is rated at level 8, and the weld bond strength is 63.7 N.

[0149] The nickel-plated steel strip in Comparative Example 5 uses sodium dodecyl sulfate, a conventional ionic wetting agent. Compared with the sulfonated product of fatty alcohol / alkylphenol polyoxyethylene propylene ether, which has better stability, sodium dodecyl sulfate cannot be completely dissolved at the plating bath temperature of 50°C. It also produces foam, causing the plating layer to become mottled, hazy, or pitted, affecting the roughness. Ultimately, this results in poor quality of the semi-bright nickel layer, which in turn leads to poor corrosion resistance and weldability of the nickel-plated steel strip.

[0150] Table 1 shows the substrate performance parameters of Examples 1-6 (S1-S6) and Comparative Examples 1-5 (B1-B5), as well as the relevant performance parameters of the nickel-plated steel strip.

[0151] Figure 1 shows the atomic force microstructure of the upper surface of the nickel-plated steel strip in Embodiment 2 of the present invention. As can be seen from the scale values, the surface undulation is relatively large, and the roughness Ra′ fluctuates between about 30-40 nm. The surface morphology within a range of 2.5 μm × 2.5 μm was measured, and the Ra′ measurement values ​​within the range were read from 6 measurement lines for comparison. The average roughness was 31.989 nm.

[0152] Figure 2 shows the weld bond strength test curve of the nickel-plated steel strip in Embodiment 1 of the present invention. The five curves of different colors represent the weld bond strength curves of the five samples, and the average weld bond strength is 107.2 N.

[0153] Figure 3 shows the morphology of the semi-bright nickel layer on the surface of the nickel-plated steel strip in Embodiment 2 of the present invention. As can be seen from the figure, after the nickel-plated steel strip is leveled, irregular protrusions and pits are formed on the surface. By controlling the leveling process, the surface roughness of the plating layer can be controlled.

Claims

1. A nickel-plated steel strip, comprising a substrate, a semi-bright nickel layer, and a nickel-iron alloy layer located between the substrate and the semi-bright nickel layer, characterized in that, The roughness Ra' of the surface of the nickel-plated steel strip satisfies 22nm < Ra' ≤ 50nm, preferably 30nm ≤ Ra' ≤ 50nm, and the roughness Ra' is measured by an atomic force microscope within a range of 2.5μm×2.5μm on the surface of the nickel-plated steel strip.

2. The nickel-plated steel strip according to claim 1, characterized in that, The average width RSm of the roughness profile units on the surface of the nickel-plated steel strip is 80-200μm, preferably 100-180μm.

3. The nickel-plated steel strip according to claim 1, characterized in that, The average roughness Ra2 of the surface of the nickel-plated steel strip is 0.10-0.50μm, preferably 0.20-0.40μm; and / or the average peak-to-valley profile depth Rz of the surface roughness of the nickel-plated steel strip is less than 3.5μm, preferably 2.0μm or more.

4. The nickel-plated steel strip according to claim 1, characterized in that, The substrate is aluminum killed steel or interstitial-free steel, wherein, in terms of weight percentage, the carbon content of the aluminum killed steel is 0.02%-0.08%, and the carbon content of the interstitial-free steel is 0.005% or less.

5. The nickel-plated steel strip according to claim 4, characterized in that, The aluminum killed steel comprises the following chemical elements in weight percentage: C: 0.02-0.08%, Mn: 0.15-0.35%, Si: 0.010-0.035%, P≤0.015%, S≤0.015%, Al: 0.03-0.05%, with the balance being Fe and unavoidable impurities; the interstitial-free steel comprises the following chemical elements in weight percentage: C≤0.005%, Mn: 0.15-0.30%, Si: 0.01-0.05%, P≤0.012%, S≤0.010%, with the balance being Fe and unavoidable impurities.

6. The nickel-plated steel strip according to any one of claims 1-5, characterized in that, The welding bonding force of the nickel-plated steel strip is ≥90N, preferably ≥95N.

7. The nickel-plated steel strip according to any one of claims 1-5, characterized in that, The surface corrosion resistance of the nickel-plated steel strip tested according to GB / T 6461-2002 is above grade 9; the Vickers hardness HV of the surface of the nickel-plated steel strip is above 170.

8. The nickel-plated steel strip according to any one of claims 1-5, characterized in that, The thickness of the nickel-plated steel strip is 0.10-1.00mm; and / or the single-sided thickness of the nickel-iron alloy layer is 0.10-0.60μm, preferably 0.20-0.50μm.

9. A battery case or a case assembly, formed from the nickel-plated steel strip according to any one of claims 1-8.

10. A method for manufacturing nickel-plated steel strip, characterized in that, The method comprises the following steps carried out in sequence: Rolling: cold rolling the substrate, with a cold rolling reduction rate of 70%-95%; Nickel electroplating: electroplating the rolled substrate in a plating solution to form a semi-bright nickel layer on the surface of the rolled substrate, and obtaining a nickel-plated steel strip precursor; Heat treatment: subjecting the nickel-plated steel strip precursor to continuous annealing to form a nickel-iron alloy layer between the rolled substrate and the semi-bright nickel layer, and obtaining an annealed steel strip; Temper rolling: temper rolling the annealed steel strip to obtain a nickel-plated steel strip, wherein the temper elongation is 0.5-1.5%, the average surface roughness Ra2 of the nickel-plated steel strip measured by a stylus roughness tester is 0.10-0.50μm, preferably 0.20-0.40μm, and the average peak-to-valley profile depth Rz of roughness is less than 3.5μm, preferably 2.0μm or more; The roughness Ra' on the surface of the nickel-plated steel strip satisfies 22nm < Ra' ≤ 50nm, preferably 30nm ≤ Ra' ≤ 50nm, and the roughness Ra' is measured by an atomic force microscope within a 2.5μm × 2.5μm area on the surface of the nickel-plated steel strip.

11. The method according to claim 10, characterized in that, The electroplating nickel step satisfies one or more of the following: The plating solution contains 220-320g / L of nickel sulfate, 20-50g / L of nickel chloride, 20-50g / L of boric acid and 2-5g / L of a wetting agent; preferably, the wetting agent is a sulfonated product of fatty alcohol / alkylphenol polyoxyethylene propylene ether, with the balance being water; The surface tension of the plating solution is lower than 45mN / m; Current density 5-50A / dm 2 Preferred 10-30A / dm 2 The electroplating temperature is 45-65℃, and the pH value of the plating solution is 3-5.

12. The method according to claim 11, characterized in that, The sulfonated product is obtained by condensation and sulfonation of C4-C12 fatty alcohols or C4-C12 alkylphenols with ethylene oxide or propylene oxide.

13. The method according to any one of claims 10-12, characterized in that, The method further includes a pretreatment step of the substrate before nickel electroplating, the pretreatment step including electrochemical alkaline washing and acid washing; preferably, the alkaline washing solution comprises: sodium hydroxide 45-65 g / L, anhydrous sodium carbonate 35-45 g / L, sodium phosphate 35-45 g / L, and the balance being water; preferably, the electrochemical alkaline washing is carried out under the following conditions: temperature 55-65℃, current density 10-20 A / dm³. 2 Alkaline washing time is 1-5 min; and / or, the pickling solution is a hydrochloric acid solution with a mass concentration of 2-8%, the pickling temperature is 15-30℃, and the pickling time is 1-3 min.

14. The method according to any one of claims 10-12, characterized in that, In the heat treatment step, the temperature of continuous annealing is 700-900°C, preferably 780-900°C, and the annealing time is 15-120 seconds.

15. The method according to any one of claims 10-12, characterized in that, In the nickel electroplating step, a semi-bright nickel layer is formed by electroplating on at least one surface of the rolled substrate, and the single-sided thickness of the semi-bright nickel layer is 0.50-1.0 μm, preferably 0.60-0.80 μm.

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