Steel for pre-nickel-plated battery shell and preparation method of steel

Abstract

The invention relates to the technical field of metal materials, and discloses steel for a pre-nickel-plated battery shell and a preparation method of the steel. The method comprises the steps of hot rolling, coiling, acid pickling, cold rolling, nickel electroplating and continuous annealing. The nickel electroplating process comprises the steps that nickel sulfate, nickel chloride, cobaltous sulfate, boric acid, lauryl sodium sulfate, citric acid, pretreatment filler, nanometer titanium dioxide particles and cerium salt are prepared into electroplating liquid, then the steel matrix obtained after cold rolling is placed in the electroplating liquid to be electroplated, and the concentration of the pretreatment filler is 1.2-1.5 g / L; the concentration of the nano titanium dioxide particles is 0.5 to 1.0 g / L, and the concentration of the cerium salt is 0.1 to 0.2 g / L. The steel for the pre-nickel-plated battery shell prepared by the method has excellent corrosion resistance, relatively high hardness and good stamping performance, the binding force between a nickel-plated layer and a steel substrate is strong, and the surface iron exposure rate is low.

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and particularly relates to a steel for pre-plated nickel battery cases and a preparation method thereof. Background Art

[0002] With the booming development of the new energy vehicle industry, the demand for power lithium batteries has increased sharply. As a key component for protecting the internal components of the battery, the performance of the battery case directly affects the safety and service life of the battery. The steel for pre-plated nickel battery cases is widely used in the manufacture of battery cases, but there are many problems with the current market products. On the one hand, the bonding force between the nickel plating layer and the steel substrate is insufficient. During the battery production and use process, the nickel plating layer is prone to peeling off, resulting in a decrease in the corrosion resistance of the battery case and even potential safety hazards such as battery short circuits. On the other hand, the hardness of the existing steel for pre-plated nickel battery cases needs to be improved. When the low-hardness battery case is under short-circuit risk, the internal pressure increases and it is extremely easy to crack, increasing the risk of failure during service.

[0003] Chinese Patent Application CN116516429A discloses a pre-plated nickel steel strip for power lithium batteries and a preparation method thereof. This patent application aims to improve the hardness, wear resistance and corrosion resistance of the steel strip through a specific electroplating solution formula containing various components such as nickel sulfate, nickel chloride, cobalt sulfate, etc., and combined with subsequent heat treatment processes. However, while improving the comprehensive performance of the steel strip, this patent does not fully consider the high-speed stamping requirements during the battery case production process. Its preparation process mainly focuses on electroplating and heat treatment parameters, and there are no specific measures to improve the stamping performance of the steel strip itself. When facing high-speed stamping, problems such as stamping cracking of the steel shell and a decrease in the bonding force between the nickel plating layer and the substrate will occur, affecting the production efficiency and quality of the battery case.

[0004] Chinese Patent Application CN118099554A discloses a nickel-plated steel, a manufacturing method thereof and a battery case. This patent application focuses on the welding performance and corrosion resistance of the nickel-plated steel, and realizes performance improvement by controlling relevant parameters of the Fe-Ni diffusion alloy layer, such as the distance between the position containing 80% Fe element and the position containing 80% Ni element, etc. However, this patent has deficiencies in high-speed stamping. Its treatment process for the steel substrate mainly includes cold rolling, continuous electroplating nickel and continuous furnace annealing heat treatment, etc. When optimizing the welding and corrosion resistance performance, these processes do not fully consider the special requirements of high-speed stamping for the strength, plasticity and surface quality of the steel strip. During high-speed stamping, due to the poor combination of the strength and plasticity of the steel strip, defects such as stamping cracking occur, and at the same time, the surface quality cannot meet the strict requirements of high-speed stamping due to different process focuses.

[0005] Chinese Patent Application CN117448673A discloses a kind of ultra-low carbon pre-plated nickel steel strip and its preparation method. By designing the chemical composition of the ultra-low carbon steel matrix, such as strictly controlling the contents of elements such as C, Si, Mn, etc., and combining specific rolling and annealing processes, the comprehensive mechanical properties of the steel strip are improved. However, due to the use of ultra-low carbon composition in this patent, the hardness of the product is relatively low and cannot meet the requirements of the battery steel shell for pressure resistance performance.

[0006] Chinese Patent Application CN115787004A discloses a high-speed pre-plated nickel steel strip for new energy vehicle battery shells and its manufacturing method. This patent application to a certain extent considers the requirements of high-speed stamping. By optimizing the process route, controlling raw material inclusions and other measures, the deep drawing property of the steel strip is improved. But there are still deficiencies in the face of higher requirements of high-speed stamping scenarios. Although its nickel plating process adopts a three-layer coating structure, during the high-speed stamping process, the bonding force between the coatings and the bonding force between the coating and the steel strip matrix cannot meet the requirements under complex stress states, and phenomena such as coating delamination and peeling are likely to occur. In addition, this patent also adopts an ultra-low carbon composition, and the pressure resistance performance of the product is relatively low. Summary of the Invention

[0007] The purpose of the present invention is to overcome the problems existing in the prior art, such as poor bonding force of the nickel plating layer, relatively low hardness, and poor corrosion resistance of the steel for pre-plated nickel battery shells, and to provide a kind of steel for pre-plated nickel battery shells and its preparation method. The steel for pre-plated nickel battery shells prepared by the method described in the present invention has a firmly bonded nickel plating layer with the steel matrix and is not easy to fall off during subsequent processing and use; it has high hardness and can meet the requirements of the pressure resistance performance when the battery has a short circuit; in various environments, especially in media such as electrolytes contacted during the use of the battery, it has excellent corrosion resistance, effectively prolongs the service life of the battery shell, and improves the safety and stability of the battery; at the same time, it has a low exposed iron rate and excellent corrosion resistance in the natural environment, effectively improving the surface quality of the product.

[0008] To achieve the above purpose, in the first aspect of the present invention, a method for preparing a steel for pre-plated nickel battery shells is provided. The method includes hot rolling, coiling, pickling, cold rolling, electroplating nickel, and continuous annealing; The process of electroplating nickel includes: preparing an electroplating solution by mixing nickel sulfate, nickel chloride, cobalt sulfate, boric acid, sodium dodecyl sulfate, citric acid, a pretreatment filler, nano-titanium dioxide particles, and a cerium salt, and then placing the steel matrix obtained after cold rolling into the electroplating solution for electroplating; The concentration of the pretreatment filler is 1.2 - 1.5 g / L, the concentration of the nano-titanium dioxide particles is 0.5 - 1.0 g / L, and the concentration of the cerium salt is 0.1 - 0.2 g / L; The pre-nickel-plated battery shell steel has a hardness HV0.3 of 140-160, a yield strength of 240MPa-270MPa, a tensile strength of 360MPa-390MPa, and an elongation of ≥35%.

[0009] Preferably, the preparation process of the pretreated filler comprises the following steps: (1) subjecting a hydrogen-containing double-capped, epoxy-terminated allyl polyether, concentrated sulfuric acid, and a catalyst to a first reaction under a first heating condition to obtain an epoxy monomer; (2) subjecting the epoxy monomer and KH-550 to a second reaction under a second heating condition, cooling, and purifying to obtain coupling agent A; (3) mixing hydrogen peroxide and concentrated sulfuric acid to obtain a pretreatment solution, and then placing a filler in the pretreatment solution, stirring, and centrifuging, wherein the filler includes graphene oxide and diamond; (4) The centrifuged product is dispersed in toluene by ultrasonication, and then a complex coupling agent is added and stirred under an inert atmosphere, and the mixture is centrifuged, washed and dried to obtain a pretreated filler, wherein the complex coupling agent includes a coupling agent A and an aminosilane coupling agent.

[0010] Preferably, the preparation process of the pretreated filler comprises: Mixing a hydrogen-containing double-capped head, an epoxy-terminated allyl polyether and concentrated sulfuric acid, heating to 65° C.-75° C., adding a catalyst, and keeping the temperature for 4 h-5 h to obtain an epoxy monomer, wherein the catalyst is chloroplatinic acid, the amount of the catalyst is 0.2-0.4 wt% of the total amount of the hydrogen-containing double-capped head and the epoxy-terminated allyl polyether, and the molar ratio of the hydrogen-containing double-capped head to the epoxy-terminated allyl polyether is 1:2-2.2; The epoxy monomer and KH-550 are mixed, heated to 65°C-75°C, stirred for reaction for 2h-3h, cooled after the reaction, purified, and coupling agent A is obtained, wherein the mass ratio of the epoxy monomer to KH-550 is 8-10:1; Mixing hydrogen peroxide and concentrated sulfuric acid to obtain a pretreatment solution, then placing a filler in the pretreatment solution, stirring at 60°C-65°C for 3h-4h, collecting the product by centrifugation, washing and drying, wherein the filler includes graphene oxide and diamond, and the mass ratio of graphene oxide to diamond is 1:2, and the amount ratio of hydrogen peroxide to concentrated sulfuric acid is 1:2; The centrifugal product is placed in anhydrous toluene for ultrasonic dispersion for 1h-2h, and then a complex coupling agent is added, stirred for 20h-24h under a nitrogen environment, centrifuged and washed, and vacuum dried to obtain a pretreated filler, wherein the complex coupling agent includes coupling agent A and aminosilane coupling agent, the amount of coupling agent A is 15-18wt% of the complex coupling agent, and the amount of the complex coupling agent is 0.2-0.5wt% of the filler.

[0011] Preferably, pulse electroplating is adopted for the electroplating, with the forward pulse current density being 6 - 8 A / dm², the pulse width being 2 - 4 ms; the reverse pulse current density being 1 - 2 A / dm², the pulse width being 0.5 - 1 ms, and the duty cycle being 30 - 50%; the temperature of the electroplating solution for the electroplating is 55 - 60 °C, and the electroplating time is 40 - 50 min.

[0012] Preferably, the continuous annealing is carried out using a continuous annealing furnace with a full hydrogen protection atmosphere, the annealing temperature is 720 - 740 °C, the annealing time is 120 - 180 s, the hydrogen flow rate is controlled at 20 - 30 m³ / h, and the pressure is maintained at 50 - 80 kPa.

[0013] Preferably, the heating temperature for the hot rolling is 1200 - 1240 °C, and the final rolling temperature is 860 - 920 °C.

[0014] Preferably, the coiling temperature is 560 - 600 °C.

[0015] Preferably, the cold rolling adopts five - stand continuous rolling, the total reduction ratio is 82 - 87%, and bright roll rolling is adopted on the last stand, with the roughness controlled at ≤0.4 μm and RPc ≥ 230.

[0016] Preferably, in the electroplating solution, the concentration of nickel sulfate is 220 - 250 g / L, the concentration of nickel chloride is 40 - 45 g / L, the concentration of cobalt sulfate is 2 - 3 g / L, the concentration of boric acid is 30 - 40 g / L, the concentration of sodium dodecyl sulfate is 0.25 - 0.35 g / L, and the concentration of citric acid is 5 - 7 g / L.

[0017] Preferably, during the electroplating process, ultrasonic stirring is carried out on the electroplating solution, wherein the ultrasonic frequency is 20 - 40 kHz and the ultrasonic power is 100 - 200 W.

[0018] Preferably, the preparation process further includes skin - passing.

[0019] The conditions for the skin - passing include: the roughness Ra of the skin - passing roll is 1.4 - 1.6 μm, and the skin - passing elongation is 0.7 - 1.2%.

[0020] In the second aspect of the present invention, a steel for pre - plated nickel battery cases prepared by the method described above is provided. The steel for pre - plated nickel battery cases includes a steel substrate and a nickel layer covering the surface of the steel substrate; The steel substrate contains the following chemical components by weight percentage: C: 0.03% - 0.06%, Si: 0.01% - 0.03%, Mn: 0.15% - 0.30%, P: 0.008% - 0.015%, S: ≤0.010%, Ti: 0.035% - 0.055%, Cr: 0.04% - 0.08%, Ni: 0.02% - 0.05%, the balance being Fe and unavoidable impurities.

[0021] Preferably, the thickness of the nickel layer covering the surface of the steel substrate is 1 - 10 μm.

[0022] Compared with the prior art, the present invention has at least the following beneficial effects: The method of the present invention can obtain pre - plated nickel steel for battery cases with strong binding force of the nickel - plated layer, excellent corrosion resistance, low surface iron exposure rate, high hardness and good stamping performance by electroplating a steel substrate in an electroplating solution containing specific components and controlling the concentrations of pretreatment fillers, nano - titanium dioxide particles and cerium salts in the electroplating solution within a specific range.

[0023] Specifically, the pre - plated nickel steel for battery cases prepared according to the preparation method of the present invention has the following excellent properties after testing: The binding force between the nickel - plated layer and the substrate is strong. Using the cross - cut test and tape adhesion test, the nickel layer shows no peeling phenomenon. In the accelerated corrosion test simulating the battery electrolyte environment, after 72 hours of testing, there are no obvious corrosion signs on the surface of the steel strip, showing excellent corrosion resistance. The hardness reaches HV0.3: 140 - 160, with a relatively high surface hardness and strong compressive capacity. It has good stamping performance, with a yield strength of 240 MPa - 270 MPa, a tensile strength of 360 MPa - 390 MPa, and an elongation rate of ≥35%. It can meet the stamping forming requirements of battery cases with complex shapes, effectively reducing the rejection rate during the production process and improving the quality and production efficiency of the battery cases. Particularly, the surface iron exposure rate is low. After testing, the surface iron exposure rate is ≤3%. Compared with the products without using the continuous annealing process of this patent, the surface iron exposure rate can be reduced by 2 - 4 percentage points, greatly improving the surface quality and corrosion resistance of the products. Detailed Description of the Invention

[0024] The following provides a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not used to limit the present invention.

[0025] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0026] The method for preparing steel for pre-plated nickel battery cases provided by the present invention includes hot rolling, coiling, pickling, cold rolling, nickel electroplating, and continuous annealing.

[0027] In a more specific embodiment, the method includes hot metal pretreatment, converter smelting, secondary refining, continuous casting, hot rolling, coiling, pickling, cold rolling, preparation of pretreatment filler, nickel electroplating, and continuous annealing.

[0028] In a preferred embodiment, the steel substrate obtained after cold rolling contains the following chemical components in weight percentages: C: 0.03% - 0.06%, Si: 0.01% - 0.03%, Mn: 0.15% - 0.30%, P: 0.008% - 0.015%, S: ≤0.010%, Ti: 0.035% - 0.055%, Cr: 0.04% - 0.08%, Ni: 0.02% - 0.05%, and the balance is Fe and unavoidable impurities.

[0029] The following details the functions of the steel substrate containing the above chemical components and limiting each component within the above ranges.

[0030] Carbon (C): Controlling the carbon content within 0.03% - 0.06% improves the strength of the steel to a certain extent. Meanwhile, in combination with the production process of the present invention, it reduces the adverse effects of interstitial solid solution carbon on the plasticity and deep drawing performance of the steel, improves the purity of the steel, and ensures good bonding between the nickel plating layer and the steel substrate.

[0031] Silicon (Si): Limiting the silicon content to 0.01% - 0.03% prevents the formation of an oxide film that is difficult to remove on the steel surface, avoids affecting the quality of the nickel plating layer and surface performance, and ensures surface stability during stamping and use.

[0032] Manganese (Mn): An appropriate amount of manganese can improve the strength and toughness of the steel, make up for the strength loss caused by low carbon, optimize the processing performance of the steel, and help improve the deformation ability of the steel strip during stamping. Therefore, the manganese content is controlled within 0.15% - 0.30%.

[0033] Phosphorus (P): Within a certain range, phosphorus can improve the corrosion resistance of the steel, but its upper limit needs to be strictly controlled to prevent the negative impact of phosphorus segregation on the stamping performance and forming quality of the steel strip. Therefore, the phosphorus content is controlled within 0.008% - 0.015%.

[0034] Sulfur (S): The sulfur content is minimized to reduce hot brittleness, improve the ductility and toughness of the steel, avoid cracks during rolling and stamping, and at the same time enhance the corrosion resistance of the steel. Therefore, the sulfur content is controlled ≤0.010%.

[0035] Titanium (Ti): As a strong carbide-forming element, controlling its content within 0.035% - 0.055% can hinder the growth of austenite grains, refine the grains, improve the deep drawing performance and strength of the steel, and at the same time improve the bonding force between the nickel plating layer and the substrate.

[0036] Chromium (Cr) and Nickel (Ni): Chromium and nickel have a synergistic effect. Controlling the chromium content within 0.04% - 0.08% and the nickel content within 0.02% - 0.05% can significantly improve the corrosion resistance of the steel, form a dense oxide film on the steel surface, and enhance the resistance to corrosive media such as electrolytes. At the same time, chromium also helps to improve the adhesion of the nickel layer, making the nickel plating layer more firmly bonded to the steel substrate. Traditional battery case steel usually does not contain Cr and Ni, while in the present invention, through the synergistic effect of trace amounts of Cr and Ni, the corrosion resistance of the substrate (Cr forms a passive film) and the bonding force between the substrate and the nickel plating layer (Ni reduces the interfacial potential difference) can be improved.

[0037] In the present invention, operations such as hot metal pretreatment, converter smelting, secondary refining, and continuous casting can be carried out according to common processes in the art, as long as the object of the present invention can be achieved.

[0038] In one embodiment, the hot metal pretreatment adopts processes such as slag skimming and injection to deeply remove harmful impurities such as sulfur and phosphorus in the hot metal, reduce the sulfur content to a low level, reduce the influence of impurities on the steel properties during subsequent smelting, and improve the purity of the molten steel.

[0039] In one embodiment, the converter smelting precisely controls the oxygen blowing intensity, slag-making system, and end-point carbon content to ensure uniform composition of the molten steel, effectively remove harmful gases and inclusions in the molten steel, and create good conditions for subsequent refining.

[0040] In one embodiment, the secondary refining adopts LF refining + RH refining. In the LF refining stage, through white slag refining, further desulfurization and deoxidation are carried out, and the composition of the molten steel is precisely adjusted. During RH refining, vacuum degassing treatment is adopted to remove gases such as hydrogen and nitrogen in the molten steel, reduce the content of harmful elements in the steel, and improve the purity and quality stability of the molten steel.

[0041] In one embodiment, the continuous casting adopts protective casting technology to prevent secondary oxidation of the molten steel, optimize the crystallizer cooling system and casting speed, ensure good surface quality of the cast slab, dense and uniform internal structure, reduce internal defects, and provide high-quality billets for subsequent rolling.

[0042] In the present invention, the heating temperature of the hot rolling is controlled at 1200 - 1240 °C to fully dissolve alloying elements, which can improve the plasticity and deformation ability of the steel. In the present invention, the finishing rolling temperature of the hot rolling is controlled at 860 - 920 °C, which can ensure rolling in the non-recrystallization zone of austenite, refine the grains, and improve the strength and toughness of the steel strip.

[0043] In the present invention, the coiling temperature may be 560-600° C. Low-temperature coiling can improve the surface quality of the steel strip, refine the grains, and improve the comprehensive performance of the steel strip.

[0044] In one embodiment, the pickling adopts a continuous pickling process to completely remove the iron oxide scale on the surface of the hot-rolled steel strip through pickling to ensure that the surface of the steel strip is clean.

[0045] In some embodiments, the cold rolling adopts five-stand continuous rolling, and the total reduction rate can be 82%-87%. A large reduction rate can increase the grain distortion energy in the steel, reduce the recrystallization temperature, refine the grains, and significantly improve the deep drawing performance of the steel strip. Furthermore, smooth roller rolling is adopted in the last stand, and the roughness is controlled at ≤0.4μm and RPc≥230, which effectively improves the surface quality of the finished plate and improves the adhesion of the nickel plating layer.

[0046] In the present invention, the purpose of preparing the pre-treated filler is to use it as a chemical component of the electroplating solution for preparing the electroplating solution.

[0047] In one embodiment, the process of preparing the pretreated filler comprises the following steps: (1) subjecting a hydrogen-containing double-capped, epoxy-terminated allyl polyether, concentrated sulfuric acid, and a catalyst to a first reaction under a first heating condition to obtain an epoxy monomer; (2) subjecting the epoxy monomer and KH-550 to a second reaction under a second heating condition, cooling, and purifying to obtain coupling agent A; (3) mixing hydrogen peroxide and concentrated sulfuric acid to obtain a pretreatment solution, and then placing a filler in the pretreatment solution, stirring, and centrifuging, wherein the filler includes graphene oxide and diamond; (4) The centrifuged product is dispersed in toluene by ultrasonication, and then a complex coupling agent is added and stirred under an inert atmosphere, and the mixture is centrifuged, washed and dried to obtain a pretreated filler, wherein the complex coupling agent includes a coupling agent A and an aminosilane coupling agent.

[0048] In a more specific embodiment, the process of preparing the pretreated filler comprises the following steps: Mixing a hydrogen-containing double-capped head, an epoxy-terminated allyl polyether and concentrated sulfuric acid, heating to 65° C.-75° C., adding a catalyst, and keeping the temperature for 4 h-5 h to obtain an epoxy monomer, wherein the catalyst may be chloroplatinic acid, and the amount of the catalyst is 0.2-0.4 wt % of the total amount of the hydrogen-containing double-capped head and the epoxy-terminated allyl polyether, and the molar ratio of the hydrogen-containing double-capped head to the epoxy-terminated allyl polyether is 1:2-2.2; Mix the epoxy monomer and KH-550, heat up to 65°C - 75°C, stir and react for 2h - 3h. After the reaction, cool and purify to obtain coupling agent A. Among them, the mass ratio of the epoxy monomer to KH-550 is 8 - 10:1; Mix hydrogen peroxide and concentrated sulfuric acid to obtain a pretreatment solution. Then place the filler in the pretreatment solution, stir at 60°C - 65°C for 3h - 4h, centrifuge to collect the product, wash and dry. Among them, the filler includes graphene oxide and diamond, and the mass ratio of graphene oxide to diamond is 1:2, and the dosage ratio of hydrogen peroxide to concentrated sulfuric acid is 1:2; Ultrasonically disperse the centrifuged product in anhydrous toluene for 1h - 2h, then add the complex coupling agent, stir in a nitrogen environment for 20h - 24h, centrifuge, wash and vacuum dry to obtain the pretreated filler. Among them, the complex coupling agent includes coupling agent A and amino-silane coupling agent. The dosage of coupling agent A is 15 - 18wt% of the complex coupling agent, and the dosage of the complex coupling agent is 0.2% - 0.5wt% of the filler.

[0049] In the present invention, the concentration of the concentrated sulfuric acid used in the process of preparing the pretreated filler is 98wt%.

[0050] In a preferred embodiment, the electroplating nickel process includes: formulating an electroplating solution with nickel sulfate, nickel chloride, cobalt sulfate, boric acid, sodium dodecyl sulfate, citric acid, pretreated filler, nano-titanium dioxide particles and cerium salt, and then placing the steel substrate obtained after cold rolling in the electroplating solution for electroplating. In the electroplating process of the present invention, only nickel covers the surface of the steel substrate; the inventors found that the presence of the pretreated filler can increase the hardness and wear resistance of the formed coating, thus ensuring that the coating is not easily scratched during the stamping process; trace amounts of nano-titanium dioxide particles and cerium salt can effectively refine the grains of the nickel plating layer.

[0051] In a more preferred embodiment, the concentration of nickel sulfate in the electroplating solution is 220 - 250 g / L, the concentration of nickel chloride is 40 - 45 g / L, the concentration of cobalt sulfate is 2 - 3 g / L, the concentration of boric acid is 30 - 40 g / L, the concentration of sodium dodecyl sulfate is 0.25 - 0.35 g / L, the concentration of citric acid is 5 - 7 g / L, the concentration of the pretreatment filler is 1.2 - 1.5 g / L, the concentration of nano-titanium dioxide particles is 0.5 - 1.0 g / L, and the concentration of cerium salt is 0.1 - 0.2 g / L. Limiting the components and contents in the electroplating solution within this range, especially controlling the concentrations of the pretreatment filler, nano-titanium dioxide particles, and cerium salt within a suitable range, can precisely control the concentrations of the components in the electroplating solution, enabling nickel sulfate and nickel chloride to synergistically ensure the stable supply of nickel ions and promote anode activation. Cobalt sulfate and boric acid refine the crystal grains and maintain the pH stability of the plating solution. Citric acid complexes impurities to improve the purity of the coating. Combining the wetting and dispersing effects of sodium dodecyl sulfate and the strengthening effects of fillers such as graphene oxide and nano-titanium dioxide, ultimately achieving high-quality performance with significantly enhanced bonding strength between the nickel plating layer and the steel substrate, excellent corrosion resistance, qualified hardness, and suitability for high-speed stamping forming, effectively solving problems such as coating peeling, corrosion failure, and processing defects of the existing pre-plated nickel battery case steel.

[0052] In the present invention, pulse electroplating is used for the electroplating, wherein the forward pulse current density is 6 - 8 A / dm², the pulse width is 2 - 4 ms, the reverse pulse current density is 1 - 2 A / dm², the pulse width is 0.5 - 1 ms, and the duty cycle is 30 - 50%. Using this electroplating method can achieve grain refinement, surface leveling, enhanced bonding strength, and improved uniformity of the nickel plating layer through the periodic action of "deposition - dissolution - activation", solving common problems in traditional direct current electroplating such as rough coatings, poor bonding strength, and insufficient corrosion resistance, and especially meeting the requirements of power lithium battery cases for high-speed stamping forming and harsh corrosion-resistant environments.

[0053] In some embodiments, the conditions for the electroplating include: the temperature of the electroplating solution is 55 - 60 °C, and the electroplating time is 40 - 50 min. In a more specific embodiment, the electroplating process can be carried out under the condition of ultrasonic stirring, wherein the ultrasonic frequency can be 20 - 40 kHz, and the ultrasonic power can be 100 - 200 W.

[0054] In the present invention, continuous annealing is carried out using a continuous annealing furnace with a full hydrogen protective atmosphere. The annealing temperature is 720 - 740 °C, the annealing time is 120 - 180 s, the hydrogen flow rate is controlled at 20 - 30 m³ / h, and the pressure is maintained at 50 - 80 kPa. Among them, during the annealing process, by controlling the annealing temperature, the bonding strength between the nickel plating layer and the steel substrate can be improved. By precisely controlling the flow rate and pressure of hydrogen in the furnace within the aforementioned range, hydrogen can uniformly contact the surface of the steel strip. Further, the running speed of the steel strip is strictly controlled to keep it in a stable running state in the furnace, where the running speed can be 180 - 220 m / min. In summary, under the aforementioned continuous annealing conditions, the atomic diffusion between the nickel plating layer and the steel substrate can be effectively promoted, a closer bonding interface can be formed between the nickel plating layer and the steel substrate, and the defects and pores at the interface can be reduced. Moreover, hydrogen has a reducing effect at high temperatures and can reduce the extremely small amount of iron oxides that may exist on the surface of the steel strip to iron, further reducing the possibility of exposed iron on the surface, thereby effectively reducing the surface exposed iron rate.

[0055] In the present invention, the preparation process further includes tempering the continuously annealed steel strip. Among them, the tempering can be carried out using a laser texturing roll. In some embodiments, the conditions for tempering include: the tempering elongation is 0.7 - 1.2%, the roughness Ra of the tempering roll is 1.4 - 1.6 μm, ensuring that the surface roughness of the steel plate is 0.5 μm ≤ Ra ≤ 0.9 μm, and RPc ≥ 130.

[0056] The pre - nickel - plated steel for battery cases prepared by the method of the present invention has excellent corrosion resistance, high hardness, good stamping performance, significantly reduced surface exposed iron rate. At the same time, the bonding strength between the nickel plating layer and the steel substrate is strong, and there is no phenomenon of nickel layer peeling off, and the comprehensive performance is excellent.

[0057] The present invention also provides a pre - nickel - plated steel for battery cases prepared by the method described above. The pre - nickel - plated steel for battery cases includes a steel substrate and a nickel layer covering the surface of the steel substrate.

[0058] In some preferred embodiments, the steel substrate contains the following chemical components by weight percentage: C: 0.03% - 0.06%, Si: 0.01% - 0.03%, Mn: 0.15% - 0.30%, P: 0.008% - 0.015%, S: ≤0.010%, Ti: 0.035% - 0.055%, Cr: 0.04% - 0.08%, Ni: 0.02% - 0.05%, and the balance is Fe and unavoidable impurities.

[0059] In some more preferred embodiments, the steel substrate contains the following chemical components by weight percentage: C: 0.03% - 0.05%, Si: 0.015% - 0.025%, Mn: 0.15% - 0.25%, P: 0.008% - 0.015%, S: ≤0.010%, Ti: 0.035% - 0.045%, Cr: 0.05% - 0.075%, Ni: 0.025% - 0.04%, with the balance being Fe and unavoidable impurities. Limiting the chemical composition of the steel substrate within this range can further improve the comprehensive performance of the steel for pre-plated nickel battery cases.

[0060] In one embodiment, the thickness of the nickel layer covering the surface of the steel substrate can be 1 - 10 μm. Limiting the thickness of the nickel layer within this range can ensure that the steel for pre-plated nickel battery cases has excellent comprehensive performance.

[0061] The steel for pre-plated nickel battery cases prepared by the method of the present invention and having the structure and chemical composition of the present invention has excellent corrosion resistance, high hardness, good stamping performance, significantly reduced surface iron exposure rate, and at the same time, the nickel plating layer has a strong bonding force with the steel substrate and no nickel layer peeling phenomenon.

[0062] The present invention will be described in detail below through examples, but the protection scope of the present invention is not limited thereto. In the following examples, unless otherwise specified, the raw materials used are common commercially available products.

[0063] In the following examples, the hydrogen-containing double-ended cap was purchased from Shandong Huachen New Materials Co., Ltd.; the terminal epoxy allyl polyether was purchased from Hangzhou Danwei Technology Co., Ltd.; the structural formula of the amino silane coupling agent is NH2(CH2)3Si(OC2H5)3 and was purchased from Hangzhou Danwei Technology Co., Ltd. Example 1

[0064] The steel substrate of the steel for pre-plated nickel battery cases provided in this example contains the following chemical components in weight percentage: C: 0.03%, Si: 0.02%, Mn: 0.20%, P: 0.010%, S: 0.008%, Ti: 0.040%, Cr: 0.06%, Ni: 0.03%, with the balance being Fe and unavoidable impurities.

[0065] The preparation process of the steel for pre-plated nickel battery cases includes: hot metal pretreatment, converter smelting, secondary refining, continuous casting, hot rolling, coiling, pickling, cold rolling, preparation of pretreatment filler, electroplating nickel, continuous annealing and skin pass; The heating temperature of the hot rolling is 1230 °C, and the finishing rolling temperature is 880 °C; The coiling temperature is 580 °C; The cold rolling adopts five-stand continuous rolling, and the total reduction ratio is 85%; The steel substrate obtained after cold rolling contains the following chemical components in weight percentage: C: 0.03%, Si: 0.02%, Mn: 0.20%, P: 0.010%, S: 0.008%, Ti: 0.040%, Cr: 0.06%, Ni: 0.03%, and the balance is Fe and inevitable impurities.

[0066] The process of preparing the pretreated filler comprises the following steps: The hydrogen-containing double-capped head, the epoxy-terminated allyl polyether and the concentrated sulfuric acid (concentration of 98wt%) were mixed, heated to 70°C (pretreatment temperature), a catalyst was added, and the reaction was kept warm for 4.5h to obtain an epoxy monomer, wherein the catalyst was chloroplatinic acid, the amount of the catalyst was 0.3wt% of the total amount of the hydrogen-containing double-capped head and the epoxy-terminated allyl polyether, the molar ratio of the hydrogen-containing double-capped head to the epoxy-terminated allyl polyether was 1:2, and the mass ratio of the concentrated sulfuric acid to the hydrogen-containing double-capped head was 2:1; The epoxy monomer and KH-550 were mixed, heated to 70°C, stirred for reaction for 2.5 hours, cooled after the reaction, and purified to obtain coupling agent A, wherein the mass ratio of the epoxy monomer to KH-550 was 9:1; Mixing hydrogen peroxide and concentrated sulfuric acid (with a concentration of 98 wt%) to obtain a pretreatment solution, then placing a filler in the pretreatment solution, stirring at 60° C. for 3.5 hours, collecting the product by centrifugation, washing and drying, wherein the filler includes graphene oxide and diamond, and the mass ratio of graphene oxide to diamond is 1:2, and the amount ratio of hydrogen peroxide to concentrated sulfuric acid is 1:2; The centrifuged product was placed in anhydrous toluene for ultrasonic dispersion for 1.5 hours, and then a complex coupling agent was added, stirred for 22 hours under a nitrogen environment, centrifuged and washed, and vacuum dried to obtain a pretreated filler, wherein the complex coupling agent included coupling agent A and aminosilane coupling agent, the amount of coupling agent A was 16wt% of the complex coupling agent, and the amount of the complex coupling agent was 0.3wt% of the filler.

[0067] The nickel electroplating process includes: the electroplating solution is composed of 230g / L nickel sulfate, 40g / L nickel chloride, 2g / L cobalt sulfate, 35g / L boric acid, 0.3g / L sodium dodecyl sulfate, 6g / L citric acid, 1.5g / L pre-treated filler, 0.8g / L nano titanium dioxide particles and 0.1g / L cerium salt (cerium nitrate), and the steel substrate obtained after cold rolling is placed in the electroplating solution for electroplating; the electroplating adopts pulse electroplating technology, the forward pulse current density is set to 6A / dm², the pulse width is 3ms, the reverse pulse current density is 1.5A / dm², the pulse width is 0.8ms, and the duty cycle is 40%; the electroplating solution temperature is 55°C, and the electroplating time is 45min. During the electroplating process, the electroplating solution is ultrasonically stirred, the ultrasonic frequency is 30kHz, and the power is 150W; the thickness of the nickel layer covering the surface of the steel substrate is 3μm.

[0068] Continuous annealing is carried out in a continuous annealing furnace with a full hydrogen protective atmosphere. The annealing temperature is 730 °C, the annealing time is 150 s, the hydrogen flow rate is controlled at 25 m³ / h, and the pressure is maintained at 60 kPa. Further, the running speed of the steel strip is strictly controlled to keep a stable running state in the furnace, and the running speed is 200 m / min; Skin pass rolling is carried out using a laser-textured roll. The surface roughness Ra of the skin pass roll is 1.5 μm, and the skin pass elongation is 0.9%.

[0069] Examples 2-5 and Comparative Examples 1-7 are implemented according to the method of Example 1. The difference is that the chemical compositions and process conditions are different. The final finished product thickness specifications are all 0.30 mm. See Tables 1, 2 and 3 for details.

[0070] Table 1 Chemical composition of steel substrate, wt% Number C Si Mn P S Ti Cr Ni Nb Example 1 0.03% 0.02% 0.20% 0.010% 0.008% 0.040% 0.06% 0.032% - Example 2 0.05% 0.025% 0.25% 0.012% 0.007% 0.045% 0.07% 0.040% - Example 3 0.035% 0.015% 0.18% 0.009% 0.006% 0.038% 0.05% 0.025% - Example 4 0.045% 0.022% 0.22% 0.011% 0.007% 0.042% 0.065% 0.035% - Example 5 0.048% 0.023% 0.23% 0.013% 0.008% 0.044% 0.075% 0.040% - Comparative Example 1 0.021% 0.02% 0.15% 0.012% 0.008% 0.035% 0.05% 0.03% - Comparative Example 2 0.03% 0.02% 0.20% 0.010% 0.008% 0.040% 0.06% 0.032% - Comparative Example 3 0.03% 0.02% 0.20% 0.010% 0.008% 0.040% 0.06% 0.032% - Comparative Example 4 0.03% 0.02% 0.20% 0.010% 0.008% 0.040% 0.06% 0.032% - Comparative Example 5 0.004% 0.03% 0.14% 0.018% 0.012% 0.055% - - 0.015% Comparative Example 6 0.006% 0.020% 0.32% 0.012% 0.007% 0.045% - - 0.015% Comparative Example 7 0.03% 0.02% 0.20% 0.010% 0.008% 0.040% 0.06% 0.032% - Table 2 Hot rolling and electroplating processes Number Heating Temperature / °C Final Rolling Temperature / °C Coiling Temperature / °C Total Reduction Rate of Pickling and Cold Rolling / % Pretreatment Temperature / °C Electroplating Time / min Concentration of Pretreatment Filler g / L, Concentration of Nano-Titanium Dioxide g / L, Concentration of Cerium Salt g / L Example 1 1230 880 580 85 70 45 1.5、0.8、0.1 Example 2 1225 875 575 84 68 42 1.2、1、0.2 Example 3 1235 885 585 86 72 48 1.3、0.5、0.2 Example 4 1228 878 578 84.5 71 44 1.4、0.6、0.15 Example 5 1232 882 582 85.5 69 46 1.4、0.9.1.2 Comparative Example 1 1230 880 580 85 70 45 1.5、0.8、0.1 Comparative Example 2 1230 880 580 85 70 45 1.5、0.3、0.1 Comparative Example 3 1230 880 580 85 70 45 1.5、1.5、0.1 Comparative Example 4 1230 880 580 85 70 45 1.0、0.8、0.1 Comparative Example 5 1230 880 580 85 70 45 1.8、0.8、0.1 Comparative Example 6 1230 880 580 85 70 45 1.5、0.8、0.05 Comparative Example 7 1230 880 580 85 70 45 1.5、0.8、0.3 Table 3 Annealing processes

[0071] Test Examples Comprehensive performance tests are carried out on the final finished products of the examples and comparative examples. The test items and results are shown in Table 4. The adhesion of the nickel plating layer is tested by the cross-cut test and the tape adhesion test to evaluate the bonding strength between the nickel layer and the substrate; the corrosion resistance is detected by the accelerated corrosion test of 5% NaCl salt spray, and the corrosion situation of the steel strip surface after 48 hours is recorded; the hardness test is carried out using a Vickers hardness tester to measure the surface HV0.3 hardness value; the stamping performance is evaluated by the stamping test on a steel shell stamping equipment at 150 pieces / min to evaluate the yield strength, tensile strength, elongation and the quality after stamping, and observe whether there are defects such as cracking and lugs; the surface iron exposure rate is observed on the nickel-plated surface using a scanning electron microscope (SEM). In the unit detection area, the area containing iron elements is identified and counted by energy dispersive spectroscopy (EDS) technology, and the area ratio is the surface iron exposure rate.

[0072] Table 4 Number Adhesion of Nickel Plating Layer Surface Iron Exposure Rate Results of Accelerated Corrosion Test Hardness HV0.3 Yield Strength Rp0.2 / MPa Tensile Strength Rm / MPa Elongation A50 / % Stamping Forming Evaluation Nickel Layer Thickness (μm) Example 1 No peeling 2.32% No obvious corrosion signs on the surface 146 245 370 36 No obvious defects, good forming 3.1 Example 2 No peeling 2.15% No obvious corrosion signs on the surface 151 262 380 35 No obvious defects, good forming 3.6 Example 3 No peeling 1.46% No obvious corrosion signs on the surface 148 257 375 37 No obvious defects, good forming 4.5 Example 4 No peeling 2.23% No obvious corrosion signs on the surface 152 255 385 36 No obvious defects, good forming 3.8 Example 5 No peeling 2.69% No obvious corrosion signs on the surface 155 263 390 35 No obvious defects, good forming 3.7 Comparative Example 1 No peeling 2.38% No obvious corrosion signs on the surface 123 224 340 36 Low strength and hardness 3.4 Comparative Example 2 No peeling 5.73% Obvious corrosion appears 135 240 366 36 Low hardness and high iron exposure rate 2.8 Comparative Example 3 Slight peeling 5.94% A small number of corrosion points appear 162 263 390 35 Stamping coating peeling, high iron exposure rate 2.5 Comparative Example 4 Slight peeling 3.12% A small amount of corrosion appears 138 243 376 38 Stamping coating has peeling 3.2 Comparative Example 5 No peeling 5.69% A small number of corrosion points appear 132 240 360 34 Local mixed crystals appear, low elongation, serious stamping lugs 4.2 Comparative Example 6 No peeling 5.36% A small number of corrosion points appear 125 225 365 37 Low hardness and strength 3.2 Comparative Example 7 No peeling, but the coating thickness is uneven 5.42% A small number of corrosion points appear 143 248 372 36 Stamping has coating peeling 3.2 As can be seen from Table 4, the performances of Examples 1-5 can all meet the expected requirements of the present invention. The nickel plating layer has strong adhesion, excellent corrosion resistance, low surface iron exposure rate, high hardness and good stamping performance, and can meet the high-quality production requirements of power lithium battery cases.

[0073] The content of C element in Comparative Example 1 is on the low side, resulting in insufficient hardness and strength of the material.

[0074] In Comparative Example 2, the concentration of nano-titanium dioxide was too low, the grains of the coating were coarse, the surface hardness was low, the iron exposure rate increased, and the corrosion resistance was weak.

[0075] In Comparative Example 3, the nano-titanium dioxide exceeded the upper limit requirement, resulting in a rough coating surface, a high surface hardness, a significant increase in the iron exposure rate, and weak corrosion resistance.

[0076] In Comparative Example 4, the concentration of the pretreatment filler was lower than the lower limit requirement, resulting in a weak bonding force between the coating and the steel substrate, coating peeling, and weak corrosion resistance.

[0077] In Comparative Example 5, a super-low-carbon composition design was adopted, resulting in a low hardness. And it was extremely easy to occur mixed crystals, resulting in a low elongation rate and defects such as stamping lugs. The concentration of the pretreatment filler exceeded the upper limit requirement, the filler aggregated during electroplating, large grains formed on the coating surface, the adhesion was poor, and cracks appeared on the coating surface of the shell after stamping.

[0078] In Comparative Example 6, a super-low-carbon composition design was also adopted, and Cr and Ni elements were not added, resulting in low hardness and yield strength and insufficient pressure resistance. The concentration of the cerium salt was too low, the grains on the coating surface were coarse, the porosity increased, resulting in an increase in the iron exposure rate and poor corrosion resistance.

[0079] In Comparative Example 7, due to the high concentration of cerium salt in the electroplating solution, the coating surface was coarse, the iron exposure rate was high, and the coating surface was uneven in thickness.

[0080] It should be understood that the parts not described in detail in this specification all belong to the prior art.

[0081] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solution of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for preparing steel for pre-plated nickel battery cases, characterized in that, The method includes hot rolling, coiling, pickling, cold rolling, nickel electroplating and continuous annealing; The nickel electroplating process comprises: preparing an electroplating solution with nickel sulfate, nickel chloride, cobalt sulfate, boric acid, sodium dodecyl sulfate, citric acid, pre-treated filler, nano titanium dioxide particles and cerium salt, and then placing the steel substrate obtained after cold rolling in the electroplating solution for electroplating; The concentration of the pre-treated filler is 1.2-1.5 g / L, the concentration of the nano titanium dioxide particles is 0.5-1.0 g / L, and the concentration of the cerium salt is 0.1-0.2 g / L; The pre-nickel-plated battery shell steel has a hardness HV0.3 of 140-160, a yield strength of 240MPa-270MPa, a tensile strength of 360MPa-390MPa, and an elongation of ≥35%.

2. The method according to claim 1, characterized in that, The preparation process of the pretreated filler comprises the following steps: (1) subjecting a hydrogen-containing double-capped, epoxy-terminated allyl polyether, concentrated sulfuric acid, and a catalyst to a first reaction under a first heating condition to obtain an epoxy monomer; (2) subjecting the epoxy monomer and KH-550 to a second reaction under a second heating condition, cooling, and purifying to obtain coupling agent A; (3) mixing hydrogen peroxide and concentrated sulfuric acid to obtain a pretreatment solution, and then placing a filler in the pretreatment solution, stirring, and centrifuging, wherein the filler includes graphene oxide and diamond; (4) The centrifuged product is dispersed in toluene by ultrasonication, and then a complex coupling agent is added and stirred under an inert atmosphere, and the mixture is centrifuged, washed and dried to obtain a pretreated filler, wherein the complex coupling agent includes a coupling agent A and an aminosilane coupling agent.

3. The method according to claim 1 or 2, characterized in that, The preparation process of the pretreated filler comprises: Mixing a hydrogen-containing double-capped head, an epoxy-terminated allyl polyether and concentrated sulfuric acid, heating to 65° C.-75° C., adding a catalyst, and keeping the temperature for 4 h-5 h to obtain an epoxy monomer, wherein the catalyst is chloroplatinic acid, the amount of the catalyst is 0.2-0.4 wt% of the total amount of the hydrogen-containing double-capped head and the epoxy-terminated allyl polyether, and the molar ratio of the hydrogen-containing double-capped head to the epoxy-terminated allyl polyether is 1:2-2.2; The epoxy monomer and KH-550 are mixed, heated to 65°C-75°C, stirred for reaction for 2h-3h, cooled after the reaction, purified, and coupling agent A is obtained, wherein the mass ratio of the epoxy monomer to KH-550 is 8-10:1; Mixing hydrogen peroxide and concentrated sulfuric acid to obtain a pretreatment solution, then placing a filler in the pretreatment solution, stirring at 60° C.-65° C. for 3 h-4 h, collecting the product by centrifugation, washing and drying, wherein the filler comprises graphene oxide and diamond, and the mass ratio of graphene oxide to diamond is 1:2, and the amount ratio of hydrogen peroxide to concentrated sulfuric acid is 1:2; The centrifugal product is placed in anhydrous toluene for ultrasonic dispersion for 1h-2h, and then a complex coupling agent is added, stirred for 20h-24h under a nitrogen environment, centrifuged and washed, and vacuum dried to obtain a pretreated filler, wherein the complex coupling agent includes coupling agent A and aminosilane coupling agent, the amount of coupling agent A is 15-18wt% of the complex coupling agent, and the amount of the complex coupling agent is 0.2-0.5wt% of the filler.

4. The method according to claim 1, wherein The electroplating is carried out by pulse electroplating. The forward pulse current density is 6 - 8 A / dm², and the pulse width is 2 - 4 ms; the reverse pulse current density is 1 - 2 A / dm², the pulse width is 0.5 - 1 ms, and the duty cycle is 30 - 50%; the temperature of the electroplating solution for electroplating is 55 - 60 °C, and the electroplating time is 40 - 50 min.

5. The method according to claim 1, characterized in that, The continuous annealing is carried out by a fully hydrogen - protected atmosphere continuous annealing furnace. The annealing temperature is 720 - 740 °C, the annealing time is 120 - 180 s, the hydrogen flow rate is controlled at 20 - 30 m³ / h, and the pressure is maintained at 50 - 80 kPa.

6. The method according to claim 1, characterized in that, The heating temperature of the hot rolling is 1200 - 1240 °C, and the finishing rolling temperature is 860 - 920 °C; and / or, the coiling temperature is 560 - 600 °C; and / or, the cold rolling adopts five - stand tandem rolling, the total reduction ratio is 82 - 87%, the last stand uses bright roll rolling, the surface roughness is controlled at ≤0.4 μm, and RPc≥230.

7. The method according to claim 1, characterized in that, In the electroplating solution, the concentration of nickel sulfate is 220 - 250 g / L, the concentration of nickel chloride is 40 - 45 g / L, the concentration of cobalt sulfate is 2 - 3 g / L, the concentration of boric acid is 30 - 40 g / L, the concentration of sodium dodecyl sulfate is 0.25 - 0.35 g / L, and the concentration of citric acid is 5 - 7 g / L; and / or, during the electroplating process, ultrasonic stirring is carried out on the electroplating solution, wherein the ultrasonic frequency is 20 - 40 kHz, and the ultrasonic power is 100 - 200 W.

8. The method according to claim 1, characterized in that, This method further includes skin pass rolling; The conditions of the skin pass rolling include: the surface roughness Ra of the skin pass roll is 1.4 - 1.6 μm, and the skin pass elongation is 0.7 - 1.2%.

9. The steel for pre-plated nickel battery cases prepared by the method according to any one of claims 1-8, characterized in that, This pre - nickel - plated battery case steel includes a steel substrate and a nickel layer covering the surface of the steel substrate; The steel substrate contains the following chemical components by weight percentage: C: 0.03% - 0.06%, Si: 0.01% - 0.03%, Mn: 0.15% - 0.30%, P: 0.008% - 0.015%, S: ≤0.010%, Ti: 0.035% - 0.055%, Cr: 0.04% - 0.08%, Ni: 0.02% - 0.05%, and the balance is Fe and inevitable impurities.

10. The steel for pre-plated nickel battery cases according to claim 9, wherein, The thickness of the nickel layer covering the surface of the steel substrate is 1 - 10 μm.

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