Nickel-plated steel strip and method for producing same

Abstract

The invention provides a nickel-plated steel strip and a manufacturing method thereof. The nickel-plated steel strip comprises a base plate formed by cold-rolled steel and a Fe-Ni alloy layer on the surface of the base plate. The micro Vickers hardness Hv 10 g of the surface of the Fe-Ni alloy layer ranges from 200 to 240. In addition to 95% or above of Fe and inevitable impurities, the base plate further comprises the following chemical elements in percentage by mass: 0.03%-0.07% of C, 0%-0.03% of Si, 0.1%-0.35% of Mn, 0%-0.015% of P, 0.005%-0.035% of Cu, 0.005%-0.015% of Ni, 0.02%-0.06% of Cr and 0%-0.065% of Als. According to the nickel-plated steel strip, through optimized component design and a rapid heat treatment process, the nickel-plated steel strip has excellent stamping forming performance and high-strength mechanical performance, and is suitable for being used as power battery shell steel CTC assembly.

Description

Technical Field

[0001] This invention relates to steel and its manufacturing method, and more particularly to a nickel-plated steel strip and its manufacturing method. Background Technology

[0002] In recent years, the rapid development of the new energy vehicle industry has placed higher demands on the range and safety of lithium-ion batteries used in new energy vehicles. Currently, the higher energy density (ternary lithium battery system), larger cell size (46 series battery cells), and the development of CTC (Continuous Tolerance) battery assembly technology in new energy vehicles have placed higher demands on the strength, rigidity, and safety controllability of the battery casing. The high precision and high conductivity requirements inside the cell have placed more stringent demands on the control of foreign particles such as metal shavings. Any abnormal metal shavings inside the cell can easily cause short circuits or even explosions, posing a greater challenge to the safety and controllability of the power battery. Furthermore, the change in the CTC assembly mode of the battery cell has also placed higher demands on the structural strength of the battery casing.

[0003] Patent application (CN114411213A, April 9, 2022) discloses a method for producing nickel-plated steel strip using a bell-type furnace annealing process, specifically specifying an annealing temperature of 400-500℃ and a holding time of 2-12 hours. As a known technology for bell-type annealing of low-carbon steel, this method cannot effectively achieve the performance tempering of the nickel-plated steel strip base material to meet the high elongation and high r-value requirements of cylindrical lithium-ion battery stamping. The base material first undergoes appropriate annealing and tempering to achieve the required mechanical properties for stamping, followed by nickel plating and bell-type annealing alloying heat treatment (400-500℃, holding time 2-12 hours) to achieve Ni coating alloying. In other words, the nickel-plated steel strip described in this patent application cannot effectively achieve the performance tempering of the nickel-plated steel strip to meet the high strength requirements of the steel shell for CTC assembly of battery cells.

[0004] Patent application (CN117488181A, October 8, 2023) discloses a pre-nickel-plated steel sheet for new energy battery casings and its preparation method. This invention achieves surface treatment of the steel strip by pre-plating nickel and alloying annealing. Specifically, during the alloying diffusion heat treatment process, the continuous annealing alloying temperature is 550-650℃, and the continuous annealing time is 20-200s. This method improves the corrosion resistance of the steel strip when used as a battery cell; however, it still cannot solve the problem of excessive metal shavings during the stamping process of the pre-nickel-plated battery casing steel product, which seriously affects the safety of the individual battery cells.

[0005] The patent application (CN109136444A, January 4, 2019) discloses a steel for new energy vehicle battery cases suitable for rapid and thinning stamping processing and its production method. This product is cold-rolled strip steel, without pre-nickel plating treatment on the surface. The corrosion resistance protection of the steel case is mainly achieved by barrel plating nickel after shell stamping. However, as a deep blind-hole plating part for new energy batteries, there are inevitably phenomena such as missed plating at the bottom of the steel case or uneven plating layer during barrel plating, which seriously affects the corrosion resistance of the steel case. In addition, due to the need for new energy vehicle lithium-ion batteries to meet stringent heat resistance requirements, this product is prone to abnormal grain growth under high-temperature heating conditions, unable to ensure the stability of the structure, and prone to structural failure of the steel case shell, not meeting the technical requirements of CTC assembly for new energy vehicle batteries. Summary of the Invention

[0006] In order to solve the above technical problems and meet the requirements of new energy vehicle batteries for excellent formability, high strength, and safety and controllability, on the one hand, the present invention provides a nickel-plated steel strip, which includes a substrate formed by cold-rolled steel and a Fe-Ni alloy layer on the surface of the substrate; the micro Vickers hardness Hv10g of the surface of the Fe-Ni alloy layer is 200 - 240; in addition to containing more than 95% of Fe and inevitable impurities, the substrate further contains the following chemical elements in mass percentage:

[0007] C: 0.03 - 0.07%, Si: 0 < Si ≤ 0.03%, Mn: 0.1 - 0.35%, Cu: 0.005 - 0.035%, Ni: 0.005 - 0.015%, Cr: 0.02 - 0.06%, Als: 0 < Als ≤ 0.065%.

[0008] Preferably, the above substrate contains the following chemical elements in mass percentage: C: 0.03 - 0.07%, Si: 0 < Si ≤ 0.03%, Mn: 0.1 - 0.35%, Cu: 0.005 - 0.035%, Ni: 0.005 - 0.015%, Cr: 0.02 - 0.06%, Als: 0 < Als ≤ 0.065%, and the balance is Fe and inevitable impurities.

[0009] Preferably, the content of impurity elements in mass percentage meets: S: 0 - 0.012%, P: 0 - 0.015%, N: 0 - 0.004%.

[0010] The design idea of the above element components is as follows:

[0011] Carbon (C) is the most economical element for increasing the strength of steel strip. Adding C to steel increases its strength but decreases its plasticity. Furthermore, an appropriate amount of C can form fine, dispersed carbides with microalloying elements such as Cr in the steel, effectively pinning grain boundaries and preventing abnormal grain growth under heat. This improves the structural stability of the steel at high temperatures and enhances the heat resistance of battery casings made from this steel. However, if the carbon content is too high, carbides will precipitate excessively in the matrix, often becoming nucleation sites for cracks during the stamping process, thus deteriorating the stamping performance of the steel strip. Therefore, in the substrate formed from cold-rolled steel of this invention, to ensure the stamping performance of the product and appropriately improve the strength of the battery casing to enhance its pressure resistance, the mass percentage of C is controlled at 0.03-0.07%, preferably 0.035-0.065%.

[0012] Si: Si can improve the strength of steel plates through solid solution strengthening, thereby appropriately increasing the strength of the stamped battery casing and improving the pressure resistance of the casing. However, since Si is an element with a high oxygen potential, when the Si content is too high, an oxide film is easily formed on the substrate surface, hindering the interdiffusion of Fe and Ni elements during subsequent nickel plating and alloying processes, and reducing the adhesion of the Ni coating. Therefore, the Si content needs to be controlled within a reasonable range to improve the corrosion resistance of the material while ensuring its processing performance. Based on this, in the substrate formed from cold-rolled steel of the present invention, the mass percentage of Si is controlled to Si ≤ 0.03%, preferably 0.005-0.03%, and more preferably 0.005-0.02%.

[0013] Mn: Mn mainly exists in steel in a solid solution state. It can combine with carbon to improve the strength of the steel plate. However, when the Mn content is too high, it will cause a decrease in the plastic strain ratio of the steel plate after annealing, impairing the isotropy of the deep-drawing battery shell steel and hindering the formability of the deep-drawing battery shell. Based on this, in the substrate formed of cold-rolled steel in this invention, the mass percentage content of Mn is controlled at 0.1-0.35%, preferably 0.15-0.3%.

[0014] Excessive phosphorus (P) content can cause segregation at the center of the cast billet, affecting the hot working properties of the steel. Furthermore, since the battery casing is a deeply thinned and stretched component, the presence of P weakens grain boundary bonding, exacerbating the secondary processing brittleness of cold-rolled strip steel. This secondary processing brittleness severely affects the thinning and stretching capacity of the strip steel, deteriorating its formability. Where technically feasible, to obtain steel with better performance and higher quality, the content of impurity element P in the substrate should be reduced as much as possible. Therefore, in the substrate formed from cold-rolled steel of this invention, the mass percentage content of impurity element P is controlled at 0-0.015%.

[0015] S: S is a harmful impurity in steel. When the S content in steel is too high, it can easily cause segregation in the center of the cast billet, leading to hot brittleness and hindering the formability of the steel. Therefore, in the substrate formed from cold-rolled steel of this invention, the mass percentage of S is controlled to S≤0.012%, preferably ≤0.011%.

[0016] Cu: Cu is an important element for improving the corrosion resistance of steel. Firstly, in continuous electroplating nickel, the strip surface generally needs acid activation treatment to improve its plating suitability. An appropriate amount of Cu in the strip promotes the formation of a suitable surface morphology for electroplating during acid activation, thereby improving the plating suitability and adhesion of the coating. Secondly, for battery casings, many tiny microcracks form on the Ni plating surface during stamping. Adding an appropriate amount of Cu to the nickel-plated strip effectively reduces the coupling current between the nickel plating and the steel substrate, significantly improving the corrosion resistance of the nickel-plated steel strip. However, excessive Cu will deteriorate the material's hot working properties. Therefore, in the substrate formed from cold-rolled steel of this invention, the mass percentage of Cu is controlled at 0.005-0.035%, preferably 0.006-0.032%.

[0017] Ni: In the substrate formed from cold-rolled steel of the present invention, adding an appropriate amount of Ni element can not only improve the corrosion resistance of the steel, but also improve its hot working performance. Since an appropriate amount of Cu element needs to be added to the steel of the present invention to improve its corrosion resistance, the addition of Cu element is prone to liquidation on the surface of the steel during hot rolling, leading to surface cracks. These surface cracks are highly susceptible to microstructural inheritance during subsequent cold rolling annealing, ultimately deteriorating the deep drawing performance of the steel. Adding an appropriate amount of Ni element to the steel can effectively reduce the liquidation of Cu element on the steel surface during hot rolling, improving the surface quality of the hot-rolled steel. Furthermore, adding an appropriate amount of Ni element can also improve the hot strength of the steel, maintaining the structural stability of the battery casing stamped using this steel under heated conditions. However, excessive addition of Ni element will increase the raw material cost. Therefore, in the substrate formed from cold-rolled steel of the present invention, the mass percentage content of Ni element is controlled at 0.005-0.015%, preferably 0.005-0.012%.

[0018] Cr: The Cr element can not only dissolve in ferrite, inhibit anodic reactions, and improve the corrosion resistance of steel, but also precipitate in large amounts at grain boundaries to form dispersed Cr-containing carbides, improving the tissue stability of steel under heated conditions, enhancing the heat resistance of the steel for stamping battery cases while also improving the stress corrosion resistance of the battery case. However, when the content of Cr in the steel is too high, it will deteriorate the hot working properties of the steel. Based on this, in the substrate formed from cold-rolled steel in the present invention, the mass percentage content of the Cr element is controlled to be 0.02 - 0.06%, preferably 0.025 - 0.055%, and more preferably 0.033 - 0.045%.

[0019] Als: Al is a commonly used deoxidizing element. During steelmaking, aluminum is used for deoxidation to remove the oxygen contained in the molten steel. The generated alumina inclusions are easy to remove, ensuring the purity of the molten steel and optimizing the internal quality of the steel, and avoiding sand hole defects during the stamping process of the battery case. In addition, adding aluminum to the steel will form acid-soluble aluminum Als, and Als will react with N atoms to form AlN, which helps to improve the aging resistance of the battery case steel. However, like Si, as an element with a relatively high oxygen potential, the content of Al in the steel should not be too high, otherwise it will lead to deterioration of the surface properties and affect the bonding strength of the Ni coating. Based on this, in the substrate formed from cold-rolled steel in the present invention, the mass percentage content of the Als element is controlled to be below 0.065%, preferably 0 < Als ≤ 0.065%, and preferably 0.03 - 0.06%.

[0020] N: In the substrate formed from cold-rolled steel in the present invention, the N element is an inevitable impurity element in the steel. Excessive N dissolved in the steel will cause the aging phenomenon of the steel, generally increasing the strength and hardness of the steel, reducing the plasticity of the steel, and affecting the stamping forming performance. Therefore, in the substrate formed from cold-rolled steel in the present invention, the content of the N element is controlled to be below 0.004%.

[0021] Preferably, the microstructure of the substrate is ferrite + cementite.

[0022] Preferably, the grain size of the substrate is grade 11 - 13, preferably grade 11.6 - 12.8.

[0023] Preferably, on any cross-section of the substrate, the area percentage of the cementite ≤ 20%.

[0024] Preferably, the nickel-plated steel strip satisfies at least one of the following properties: yield strength is 300 - 370 MPa, preferably 316 - 364 MPa; tensile strength is 370 - 450 MPa, preferably 385 - 443 MPa; elongation after fracture is 27.0 - 32.0%, preferably 28.3 - 31.8%; r value is 0.6 - 1.2, preferably 0.72 - 1.15.

[0025] Preferably, the nickel-plated steel strip has a corrosion resistance rating of 9-10 according to GB / T 6461-2002 "Rating of specimens and test pieces after corrosion tests of metals and other inorganic coatings on metal substrates" and GB / T10125-2012 "Civilization tests in artificial atmospheres - Salt spray test".

[0026] Preferably, the nickel-plated steel strip is evaluated as A based on the metal scrap detection method described in the performance testing method section after punching.

[0027] The inventors discovered that the problem of metal shavings falling off during the stamping process of nickel-plated steel strip is highly correlated with the diffusion state of the plating layer. If the diffusion state of the nickel plating layer is poor and the hardness of the nickel plating layer is low, it is very easy for the die to stick during subsequent stamping processes. As metal shavings gradually accumulate on the die, the number of metal shavings increases during the stamping process and the forming process becomes unsmooth, ultimately affecting the safety of the battery cell. Therefore, appropriately increasing the hardness of the plating layer on the nickel-plated steel surface is a suitable choice to solve the problem of excessive metal shavings during the stamping process of the steel shell. Therefore, the micro Vickers hardness Hv10g of the Fe-Ni alloy layer surface is 200-240, preferably 210-235. When the micro Vickers hardness Hv10g of the Fe-Ni alloy layer surface exceeds 240, it will cause severe wear of the stamping die, thereby leading to an increase in the number of metal shavings during the stamping process.

[0028] Preferably, the Fe element content on the surface of the Fe-Ni alloy layer is 4%-12% by mass, more preferably 5%-10%. If the Fe element content on the surface of the Fe-Ni alloy layer is less than 4%, it indicates that the heat treatment of the Fe-Ni alloy layer is insufficient, the diffusion state is poor, and it is easy to cause problems such as insufficient bonding force due to excessive internal stress in the Fe-Ni alloy layer and excessive nickel scrap during the stamping process. If the Fe element content on the surface of the Fe-Ni alloy layer is higher than 12%, it is easy for Fe element to directly contact the battery electrolyte, thereby deteriorating the corrosion resistance of the steel. Therefore, the Fe element content on the surface of the Fe-Ni alloy layer is controlled at 4%-12% by mass, preferably 5%-12%, more preferably 5%-10%.

[0029] In this paper, the term "Fe content on the surface of the Fe-Ni alloy layer" refers to the Fe content from the outer surface of the Fe-Ni alloy layer to a depth of 2 μm inside it.

[0030] Preferably, the thickness of the Fe-Ni alloy layer is 2-5 μm, more preferably 2.2-4.8 μm. When the thickness of the Fe-Ni alloy layer is less than 2 μm, the nickel-plated steel strip will have poor corrosion resistance and will not meet application requirements. When the thickness of the Fe-Ni alloy layer is greater than 5 μm, the cost of raw materials will increase.

[0031] Preferably, the Ni coating is a semi-bright Ni coating or a dark Ni coating without sulfur (S) additives. Bright Ni coatings containing sulfur are prone to hot brittleness after alloying heat treatment, leading to severe cracking of the coating on steel battery casings after stamping and deterioration of corrosion resistance. Therefore, semi-bright additives without sulfur can be added to the electroplating solution to form the semi-bright Ni coating.

[0032] In this document, the Fe-Ni alloy layer can be located on any one side surface of the substrate, or on both sides of the substrate. Preferably, the Fe-Ni alloy layer is located on the outer surface of the battery casing made of the nickel-plated steel strip of the present invention.

[0033] On the other hand, the present invention also provides a method for manufacturing the above-mentioned nickel-plated steel strip, the method comprising the following steps performed sequentially:

[0034] (1) Smelting and casting molten steel to obtain slabs;

[0035] (2) Hot rolling of the slab;

[0036] (3) Winding;

[0037] (4) Pickling;

[0038] (5) Cold rolling to obtain cold-rolled steel;

[0039] (6) Continuous nickel plating;

[0040] (7) Heat treatment; wherein the heat treatment heating rate is 50-100℃ / s, the heat treatment temperature is 650-750℃, and the holding time is 40-100s;

[0041] (8) Flat.

[0042] In existing technologies, promoting Fe-Ni interdiffusion is generally achieved by increasing the homogenization temperature of heat treatment. However, this method easily leads to coarsening of the microstructure and reduction of strength in the nickel-plated steel base plate, which cannot meet the requirements of CTC assembly of power battery cells. To overcome this problem, this invention employs rapid heat treatment to form the Fe-Ni alloy layer, wherein the heat treatment heating rate is 50-100℃ / s, the heat treatment temperature is 650-750℃, and the holding time is 40-100s. Preferably, the rapid heat treatment is a continuous annealing rapid heat treatment. The holding time refers to the period of time during which the temperature is maintained at the heat treatment temperature after it has been raised.

[0043] Rapid heat treatment allows the steel strip substrate and its Ni coating to undergo recrystallization and Fe-Ni interdiffusion, effectively reducing pinhole defects in the Ni coating and preventing direct contact between the steel strip substrate and the battery electrolyte, thus improving the material's corrosion resistance. Therefore, in the manufacturing method of this invention, rapid heat treatment annealing for recrystallization effectively reduces the activation energy required for recrystallization of the steel substrate and Ni coating, increases the recrystallization nucleation sites, and refines the grain structure, thereby improving the strip strength and meeting the high-strength requirements of nickel-plated steel strips for new energy vehicle power batteries. Furthermore, the refinement of the Ni coating grains helps improve the surface hardness of the steel strip, preventing soft Ni coatings that could cause sticking to the die, and improving the steel strip's stamping performance.

[0044] The inventors discovered that at heat treatment temperatures exceeding 450°C, the grains of the nickel plating layer transform from the nanocrystalline state of the electroplated state to the recrystallized crystalline state. However, further increasing the treatment temperature has little effect on the grain size of the nickel plating layer. The influence on the microhardness of the nickel plating layer is mainly affected by the interdiffusion state of Fe and Ni elements between the nickel plating layer and the substrate. The more complete the Fe-Ni interdiffusion, the greater the microhardness of the Fe-Ni alloy layer. However, to ensure that the battery cell casing obtained after forming the nickel-plated steel strip meets the high-strength requirements of CTC assembly in new energy vehicles, improving the strength performance of carbon steel usually involves lowering the annealing temperature. However, lowering the annealing temperature can easily lead to insufficient diffusion of the Fe-Ni alloy layer, resulting in lower hardness of the Fe-Ni alloy layer. This ultimately causes the nickel plating layer to stick to the die during the stamping process, deteriorating the stamping performance of the steel strip. Therefore, the method of this invention performs a rapid heat treatment with continuous annealing on Ni-plated steel strips (heating rate of 50-100℃ / s, soaking temperature of 650-750℃, soaking time of 40-100s), enabling the Ni plating on the steel strip substrate and its surface to achieve a full interdiffusion process of Fe-Ni elements while completing the recrystallization and tempering process. This ultimately achieves a full synergy between the strip steel performance and the diffusion characteristics of the nickel plating layer, meeting the forming and service conditions required for power batteries. The rapid heat treatment process, under the same soaking temperature conditions, effectively reduces the activation energy required for recrystallization of the steel strip substrate by increasing the recrystallization nucleation core, thereby refining the grain structure and improving the strip steel strength to meet the high-strength performance requirements of nickel-plated steel strips for new energy vehicle power batteries. Meanwhile, the homogenization temperature and time conditions of the rapid heat treatment of the present invention ensure a suitable interdiffusion state of Fe-Ni elements between the nickel plating layer and the base steel plate, which helps to improve the surface hardness of the nickel-plated steel strip, avoid the Ni plating layer being too soft and thus causing phenomena such as sticking to the knife, improve the stamping and forming performance of the steel strip, and enhance the corrosion resistance of the nickel-plated steel strip.

[0045] Preferably, in step (2), the hot rolling temperature is 1200-1270℃. During the heating process of the slab, carbonitride second-phase particles dissolve, and these particles re-precipitate during the hot rolling coiling process. However, these particles are smaller and more dispersed in size compared to those in the continuously cast slab, which is beneficial to improving the corrosion resistance of the steel plate. When the hot rolling temperature is too low, the degree of dissolution of the second-phase particles decreases, affecting the precipitation in subsequent processing and negatively impacting the corrosion resistance of the steel. When the heating temperature is too high, it leads to grain coarsening and the formation of a thicker oxide scale, which is difficult to remove in subsequent processes. Therefore, in the manufacturing method of the present invention, the hot rolling temperature is controlled at 1200-1270℃.

[0046] Preferably, in step (2), the final rolling temperature is 850-950℃. When the final rolling temperature is too low, it cannot be guaranteed that the steel is rolled in the austenitic single-phase region, resulting in mixed grains in the steel, affecting the microstructure and performance stability; when the final rolling temperature is too high, the grains are prone to coarsening. Therefore, in the manufacturing method of the present invention, the final rolling temperature is controlled at 850-950℃.

[0047] Preferably, in step (3), the winding temperature is 600-710℃. Controlling the winding temperature within a suitable temperature range can achieve the precipitation of fine, dispersed cementite, which is easily dissolved during subsequent continuous annealing, reducing the impact of carbide particles on the corrosion resistance of the material.

[0048] Preferably, in step (4), conventional pickling can be used to remove the oxide scale from the surface of the hot-rolled strip.

[0049] Preferably, in step (5), the cold rolling reduction rate is 70%-95%. The plastic strain ratio r of cold-rolled steel sheet generally increases with the increase of the cold rolling reduction rate; increasing the cold rolling reduction rate increases the deformation energy in the steel, improves the recrystallization driving force, thereby reducing the recrystallization temperature and facilitating the formation of {111} texture after annealing. However, when the cold rolling reduction rate exceeds 95%, the mill load increases significantly, reducing the production efficiency of the cold rolling mill and increasing manufacturing costs; therefore, in the manufacturing method of the present invention, the cold rolling reduction rate is controlled at 70%-95%. Preferably, the cold rolling reduction rate is controlled at 80-90%.

[0050] Preferably, step (6) includes pretreatment and continuous electroplating of the cold-rolled steel: the pretreated cold-rolled steel is placed in a plating bath solution at 40-65°C and pH 3.0-5.0 for continuous electroplating, with a current density of 3-20 A / dm³. 2 Preferably 5-15 A / dm 2Excessive current density can easily lead to surface defects caused by overheating in electroplating, and can also cause weakened adhesion of the nickel plating layer due to excessive stress. Conversely, insufficient current density will reduce production efficiency.

[0051] Preferably, in step (7), the heating rate is 67-87℃ / s.

[0052] Preferably, the pretreatment includes, but is not limited to, degreasing, electrolytic degreasing, and acid activation. The pretreatment can degrease and activate the cold-rolled steel so that it remains in an active state before starting nickel electroplating.

[0053] Preferably, step (6) may include: cleaning, continuous nickel plating, cleaning, and drying. After acid activation, the cold-rolled steel can be cleaned and then added to the plating bath for continuous nickel plating.

[0054] Preferably, the plating bath used for continuous nickel electroplating can be a commonly used plating bath for nickel plating, such as a Watt's bath, an aminosulfonic acid bath, a borofluoride bath, or a chloride bath.

[0055] Preferably, a Watt bath is used as the plating bath, and the Watt bath solution formula is as follows: nickel sulfate: 250-380 g / L, nickel chloride: 30-60 g / L, boric acid: 20-60 g / L, pH: 3.0-5.0, bath temperature: 40-65℃; the current density in the electroplating process is controlled at 3-20 A / dm³. 2 Preferably 5-15 A / dm 2 To form a Ni coating on the surface of a steel substrate.

[0056] Preferably, the additive used in the nickel plating process is a semi-bright nickel additive that does not contain sulfur. Bright Ni plating with sulfur-containing additives is prone to hot brittleness after alloying heat treatment, which leads to severe cracking of the plating in steel battery cases after stamping and deteriorates corrosion resistance. Therefore, bright nickel additives containing sulfur are not selected.

[0057] Preferably, the micro Vickers hardness Hv10g of the Fe-Ni alloy layer surface formed after continuous annealing heat treatment is 200-240, and more preferably 210-235.

[0058] Preferably, the surface Fe element weight percentage of the Fe-Ni alloy layer formed after continuous annealing heat treatment is 4%-12%, more preferably 5-12%.

[0059] Preferably, in step (8), leveling can optimize the surface roughness, surface morphology, plate shape, and mechanical properties of the product. Considering the optimization effect of leveling, the leveling rate is controlled at 0.5-2.0%, preferably 1.0-2.0%. Through leveling, the surface morphology of the nickel-plated layer of the strip steel is mainly optimized, so that the roughness Ra is between 0.5-1.6μm, thereby meeting the usage requirements of nickel-plated steel strip products.

[0060] The nickel-plated steel strip and its manufacturing method of the present invention have the following advantages and beneficial effects compared with the prior art:

[0061] 1. The cold-rolled steel in the nickel-plated steel strip of this invention employs a rational chemical element composition design, improving the steel's strength and stamping performance. Through research, the inventors discovered that the addition of appropriate amounts of alloying elements such as C, Cu, and Cr to the substrate formed from cold-rolled steel, compared to conventional battery casing steel compositions, allows the nickel-plated steel strip of this invention to improve the corrosion resistance of the stamped steel while maintaining its strength and excellent stamping performance. This meets the demands for high corrosion resistance, heat resistance, high strength, and safety required for future capacity expansion of cylindrical lithium-ion power batteries in new energy vehicles and the development of CTC assembly technology.

[0062] 2. This invention optimizes processes such as nickel plating and continuous annealing, ensuring excellent stamping performance and strength of the steel while improving its corrosion resistance and safety after stamping. Furthermore, the preparation method of this invention has clearly defined control points and is easy to implement. The production of nickel-plated steel strip is completed through a single continuous annealing process, meeting the requirements of green and low-carbon process development. The nickel-plated steel strip obtained by this method is suitable for manufacturing battery casings for power batteries.

[0063] 3. The nickel-plated steel strip of this invention exhibits excellent performance, with a yield strength of 300-370 MPa, a tensile strength of 370-450 MPa, an elongation after fracture of 26.0-32.0%, an r-value of 0.6-1.2, a grain size of 11-13, a surface corrosion resistance rating of 9-10, and an A-grade evaluation for metal scrap after stamping. This nickel-plated steel strip can be used to manufacture cylindrical battery casings, meeting the demands for high corrosion resistance, heat resistance, high strength, and safety in the future capacity expansion of cylindrical lithium-ion power batteries for new energy vehicles and the development of CTC assembly technology. Furthermore, its preparation method aligns with the requirements of green and low-carbon process development, demonstrating promising prospects and application value. Attached Figure Description

[0064] Figure 1 The diagram shows the layered structure of the nickel-plated steel strip of the present invention; wherein, 10 is a substrate formed of cold-rolled steel and 20 is an Fe-Ni alloy layer.

[0065] Figure 2The image shown is a metallographic diagram of the nickel-plated steel strip of Embodiment 1 of the present invention.

[0066] Figure 3 The diagram shown is a schematic diagram of the manufacturing process of the nickel-plated steel strip of the present invention. Detailed Implementation

[0067] The present invention will be described below through specific embodiments, and those skilled in the art can understand the advantages and effects of the present invention from the disclosed content. Although the present invention will be described in conjunction with preferred embodiments, this does not mean that the features of the present invention are limited to these embodiments. On the contrary, the purpose of describing the invention in conjunction with embodiments is to cover other options or modifications that may be extended based on the claims of the present invention. In order to provide a deep understanding of the present invention, many specific details will be included in the following description. The implementation of the present invention may also be carried out without using these details. In addition, in order to avoid obscuring or blurring the focus of the present invention, some specific details will be omitted in the description. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of the present invention can be combined with each other.

[0068] Performance testing methods

[0069] Grain size rating: Samples of nickel-plated steel strips from the examples and comparative steel strips from the comparative examples were taken, and the cross-sections of the samples were ground and polished. Then, 4% nitric acid alcohol solution was used for etching to expose the substrate microstructure of the nickel-plated steel strips. Subsequently, the grain size of the finished substrate microstructure was rated by the intercept point method according to GB / T 6394-2017 "Method for Determination of Average Grain Size of Metals".

[0070] Tensile testing: The tensile strength, yield strength, and elongation after fracture of the steels in each example and comparative example were tested at room temperature (25°C) according to GB / T 228.1-2010 "Metallic materials – Tensile testing – Part 1: Test method at room temperature". Furthermore, the plastic strain ratio (r value) of the steels in each example and comparative example was tested according to GB / T 5027-2016 "Metallic materials – Determination of plastic strain ratio (r value) in sheet and strip".

[0071] Corrosion resistance test: According to GB / T 6461-2002 "Rating of specimens and test pieces of metal and other inorganic coatings on metal substrates after corrosion test" and GB / T10125-2012 "Salt spray test for corrosion test in artificial atmosphere", neutral salt spray corrosion test was conducted on the nickel-plated steel strips of each embodiment and the comparative steel strips of the comparative example. The salt spray corrosion conditions were controlled as follows: 35°C, 5% NaCl solution, and standing for 24 hours after edge wrapping. After the salt spray test, the strips were left to stand for 1 hour, cleaned, and dried. Then, the surface corrosion status of the nickel-plated steel strips of each embodiment and the comparative steel strips of the comparative example was rated.

[0072] Fe-Ni alloy layer thickness: A test sample is taken at approximately half the width of the strip, typically a square with sides of 100 mm. The thickness of the Fe-Ni alloy layer on the steel surface is measured using an energy-dispersive X-ray fluorescence spectrometer (EDXRF) THICK 800A, based on elemental measurements at different locations. The specific principle is as follows: When a nickel-plated sample is irradiated with X-rays, the X-rays interact with the Ni atoms in the sample, exciting characteristic X-rays. These characteristic X-rays are related to the Ni element and its content in the sample. The X-ray fluorescence spectrometer determines the thickness of the nickel plating layer (i.e., the Fe-Ni alloy layer) by detecting the energy of these characteristic X-rays. The plating thickness is measured seven times at different locations on each sample. After removing the maximum and minimum values, the arithmetic mean of the remaining five Ni plating thickness measurements is used.

[0073] Fe-Ni alloy layer iron content detection: In order to characterize the Fe element content distribution on the surface of the Fe-Ni alloy layer, the surface of the nickel-plated steel strip of each embodiment and the comparative steel strip of the comparative example was scanned by scanning electron microscopy energy dispersive spectroscopy (OXFORD Instruments X-Max) (the detection beam spot covers the volume range from the outer surface of the Fe-Ni alloy layer to its interior of about 2 μm), with a scanning area ≥500*200μm, so as to obtain the Fe content in the detection volume of the Fe-Ni alloy layer coating.

[0074] Micro Vickers hardness testing of the Fe-Ni alloy layer surface: A test sample is taken at approximately half the width of the strip, typically a square with sides of 100 mm. The nickel plating surface is tested using a Vickers microhardness tester (model: ZHVS-1000AT / EOS100B2.0 fully automatic micro Vickers hardness tester, manufacturer: Laizhou Zhijin Testing Instruments Co., Ltd.). A diamond indenter is used, the test load is 10 gf, and the pressure is maintained for 10 seconds. Five different locations are tested on each sample. After removing the maximum and minimum values, the arithmetic mean of the remaining three Vickers hardness measurements is calculated.

[0075] Metal shavings detection: Nickel-plated steel strips from the examples and comparative steel strips from the comparative examples were used to stamp 21700-type battery cases using a battery case stamping machine. Ten steel cases stamped from either the nickel-plated steel strips of each example or the comparative steel strips of the comparative examples were then tested for the number and size of internal metal particles. With the steel case opening downwards and aligned with the tape, the bottom and body of the case were tapped three times each using a tapping rod. Ten consecutive steel cases were grouped together. The number and size of metal shavings falling onto the tape were counted using a microscope. The metal shavings had to meet the following criteria: a. 0-30μm particles were not counted; b. 30-100μm particles did not exceed 25. If both a and b were met, the case was counted as A; otherwise, it was counted as B.

[0076] Example

[0077] The nickel-plated steel strips of Examples 1-6 were obtained through the following steps:

[0078] (1) Smelting and casting molten steel to obtain slabs;

[0079] (2) Hot rolling: The hot rolling heating temperature is 1200-1270℃, and the hot rolling finishing temperature is 850-950℃;

[0080] (3) Winding: The winding temperature is 600-710℃;

[0081] (4) Pickling: Routine pickling to remove phosphorus;

[0082] (5) Cold rolling: The cold rolling reduction rate is 80%-90% to obtain cold-rolled steel;

[0083] (6) Continuous nickel plating: including pretreatment and continuous electroplating: the cold-rolled steel is degreased, electrolytically degreased, washed with water, acid activated, and cleaned in sequence; then the pretreated cold-rolled steel is placed in a plating bath solution at 40-65℃ and pH 3.0-5.0 for continuous electroplating. The plating bath is a Watt bath with the following formula: nickel sulfate: 250-380g / L, nickel chloride: 30-60g / L, boric acid: 20-60g / L, and the current density is controlled at 3-20A / dm³. 2 The Ni coating thickness is controlled to be 2-5 μm.

[0084] (7) Continuous annealing: Control the annealing temperature to 650-750℃ and the soaking time to 40-100s. The heating rate is controlled at 50-100℃ / s.

[0085] (8) Leveling: The leveling rate should be controlled at 0.5-2.0%.

[0086] The nickel-plated steel strips in Examples 1-6 were all prepared using the method including the above steps, and their chemical composition and related process parameters met the design specifications and control requirements of this invention.

[0087] The comparative steel strips of Comparative Examples 1-7 were also prepared by the same method including the above steps, except that at least one of the chemical composition and / or process parameters of Comparative Examples 1-7 did not meet the requirements of the present invention.

[0088] The chemical elemental composition of the substrates of the nickel-plated steel strip and the comparative steel strip prepared by the above method is shown in Table 1 below.

[0089] Table 1 lists the mass percentage of each chemical element in the substrates of the nickel-plated steel strips of Examples 1-6 and the comparative steel strips of Comparative Examples 1-7.

[0090] Table 1. (wt.%, balance Fe and other unavoidable impurities besides P)

[0091] C Si Mn P S Cu Ni Cr Als N Example 1 0.056 0.008 0.29 0.012 0.0074 0.025 0.012 0.022 0.032 0.0019 Example 2 0.043 0.027 0.13 0.006 0.011 0.015 0.006 0.055 0.052 0.0023 Example 3 0.031 0.023 0.21 0.015 0.0062 0.006 0.003 0.033 0.049 0.0026 Example 4 0.035 0.012 0.26 0.014 0.004 0.01 0.005 0.029 0.064 0.0038 Example 5 0.05 0.01 0.32 0.011 0.006 0.032 0.015 0.035 0.050 0.0025 Example 6 0.067 0.006 0.11 0.009 0.009 0.016 0.008 0.045 0.042 0.0032 Comparative Example 1 0.023 0.025 0.25 0.011 0.007 0.01 0.01 0.025 0.053 0.0036 Comparative Example 2 0.045 0.028 0.12 0.012 0.006 0.023 0.006 0.045 0.057 0.0029 Comparative Example 3 0.034 0.015 0.020 0.007 0.008 0.012 0.006 0.021 0.036 0.0022 Comparative Example 4 0.03 0.027 0.015 0.006 0.008 0.031 0.008 0.013 0.048 0.0028 Comparative Example 5 0.073 0.023 0.24 0.013 0.008 0.014 0.012 0.032 0.043 0.0023 Comparative Example 6 0.027 0.023 0.24 0.008 0.006 0.003 0.005 0.01 0.038 0.0027 Comparative Example 7 0.036 0.021 0.17 0.009 0.007 0.006 0.010 0.054 0.047 0.0034

[0092] Table 2 lists the specific process parameters for the nickel-plated steel strips of Examples 1-6 and the comparative steel strips of Comparative Examples 1-7.

[0093] Table 2

[0094]

[0095]

[0096] Samples were taken from the nickel-plated steel strips of Examples 1-6 and the comparative steel strips of Comparative Examples 1-7, and the surfaces of the samples from each example and comparative example were observed and analyzed. The thickness of the Ni coating on the steel surface was measured using an energy-dispersive X-ray fluorescence spectrometer (THICK 800A). After measurement, the Fe content and microhardness of the Fe-Ni alloy layer on the surface were detected using an energy-dispersive X-ray spectroscopy (OXFORD Instruments X-Max) scanner and a Vickers hardness tester, respectively. The relevant test results are listed in Table 3 below.

[0097] Table 3

[0098]

[0099] As shown in Table 3, the Ni plating type of the nickel-plated steel strips in Examples 1-6 is a semi-bright nickel plating without sulfur; the mass percentage of Fe element in the Fe-Ni alloy layer is between 4% and 12%, and the micro Vickers hardness Hv10g of the Fe-Ni alloy layer is between 200 and 240. Comparative Example 2 has a bright Ni plating.

[0100] Samples were taken again from the nickel-plated steel strips of Examples 1-6 and the comparative steel strips of Comparative Examples 1-5, and the grain size of each example and comparative sample was rated. At the same time, mechanical property tests, corrosion resistance tests, and metal chip characteristics after punching were evaluated. The test results are listed in Table 4.

[0101] Table 4 lists the grain size grades and performance test results of the nickel-plated steel strips of Examples 1-6 and the comparative steel strips of Comparative Examples 1-7.

[0102] Table 4

[0103]

[0104]

[0105] As shown in Table 4, the yield strength of the nickel-plated steel strips in Examples 1-6 is between 300-370 MPa, the tensile strength is between 370-450 MPa, the elongation after fracture is between 27.0-32.0%, the plastic strain ratio r value is between 0.6-1.2, and the substrate grain size is 11-13. The surface corrosion resistance rating of the nickel-plated steel strips in Examples 1-6 is all 9 or 10, and the metal scrap evaluation after stamping is all A. The comprehensive performance of the nickel-plated steel strips in each example meets the requirements of new energy vehicle batteries for excellent formability, high strength, and safety controllability. However, the surface corrosion resistance, metal scrap evaluation after stamping, steel strip strength, and formability of the comparative examples do not meet the requirements.

[0106] In Comparative Example 1, the content of element C was lower than that required by the present invention, and the annealing temperature was higher than that required by the present invention, resulting in the grain size, yield strength, and tensile strength all being lower than the expected values. In addition, because the annealing temperature was higher than that required by the present invention, the Fe content on the surface of the Fe-Ni alloy layer was too high, resulting in poor corrosion resistance.

[0107] In Comparative Example 2, due to the use of bright nickel plating, sulfur (S) element was present in the plating layer. The sulfur element in the nickel plating layer is prone to causing hot brittleness of the plating layer after heat treatment, resulting in a higher hardness value of the nickel plating surface layer. Ultimately, this leads to an increase in metal chips during the stamping process of the nickel plating layer and poor corrosion resistance.

[0108] In Comparative Example 3, the annealing heating rate was higher than the requirements of the present invention, resulting in excessively high strip strength and Fe-Ni alloy layer surface hardness. This ultimately led to poor performance in metal chip evaluation and mechanical properties that did not meet the requirements of the present invention. The grain size, yield strength, and tensile strength were too high, while the elongation after fracture and r-value were too low, resulting in poor product formability.

[0109] In Comparative Example 4, although the annealing temperature was lower than the requirements of this invention, the surface hardness of the Fe-Ni alloy layer still met the requirements of this invention due to its small thickness. The metal chip assessment was normal. However, the small thickness of the Fe-Ni alloy layer resulted in poor corrosion resistance, failing to meet product usage requirements. Regarding the mechanical properties of the strip, although the lower annealing temperature helped improve the strip strength, the precipitation strengthening effect of Cr was reduced due to the lower Cr content in the substrate. Therefore, the mechanical properties of this comparative example strip still met the requirements of this invention. However, the lower annealing temperature resulted in a lower r-value for the strip than required by this invention, leading to poor product formability.

[0110] In Comparative Example 5, because the C content was higher than that required by the present invention, the strengthening effect of C ensured that the strip strength still met the requirements of the present invention, even when the annealing heating rate was lower than that required by the present invention. However, because the annealing heating rate was lower than that required by the present invention and the Fe-Ni alloy layer thickness was greater than that required by the present invention, Fe-Ni diffusion was insufficient, resulting in the surface hardness of the Fe-Ni alloy layer being lower than that required by the present invention, and the metal chip evaluation performance was poor.

[0111] In Comparative Example 6, the contents of C, Cu and Cr were all lower than those required by the present invention, resulting in grain size, yield strength and tensile strength being lower than the expected values ​​of the present invention.

[0112] In Comparative Example 7, the annealing heating rate was lower than the requirements of the present invention, which caused coarsening of the microstructure and reduction of strength of the nickel-plated steel substrate, resulting in lower than expected values ​​for the grain size, yield strength and tensile strength of the steel strip; at the same time, the low annealing heating rate also caused the surface hardness of the alloy layer to be lower than the requirements of the present invention, and the metal chip evaluation performance was poor.

[0113] In summary, the above results demonstrate that the nickel-plated steel strip of this invention employs a rational chemical composition design and optimizes processes such as continuous electroplating and continuous annealing. This approach ensures excellent stamping performance while enhancing the corrosion resistance, safety, and heat resistance of the stamped steel. Compared to conventional battery casing steel, the nickel-plated steel strip of this invention is more suitable for the demanding service environment requirements of new energy power batteries, demanding high corrosion resistance and heat resistance.

[0114] The nickel-plated steel strip of this invention exhibits excellent performance, with a grain size of 11-13, yield strength between 300-370 MPa, tensile strength between 370-450 MPa, elongation after fracture between 27.0-32.0%, plastic strain ratio r between 0.6-1.2, surface corrosion resistance rating of 9 or 10, and metal scrap evaluation after stamping of the casing of A grade. It meets the stamping and forming requirements of power batteries and can meet the stringent testing conditions and service environment of power batteries, thus possessing excellent prospects for promotion and application value.

[0115] Meanwhile, the manufacturing process of the nickel-plated steel strip of the present invention is convenient to implement and the key control points are clear. The nickel-plated steel strip obtained by this manufacturing method is suitable for manufacturing battery casings for power batteries.

[0116] It should be noted that all technical features described in this invention can be freely combined or integrated in any manner, unless they contradict each other. Various modifications and variations can be made to this invention without departing from its scope, as will be apparent to those skilled in the art. For example, features shown or described as part of one embodiment can be used with another embodiment to produce yet another embodiment. Therefore, this invention is intended to cover these modifications that fall within the scope of the appended claims and their equivalents.

Claims

1. A nickel-plated steel strip, wherein, The nickel-plated steel strip includes a substrate formed of cold-rolled steel and an Fe-Ni alloy layer on the surface of the substrate; the micro-Vickers hardness Hv10g on the surface of the Fe-Ni alloy layer is 200-240; in addition to containing more than 95% of Fe and inevitable impurities, the substrate also contains the following chemical elements in mass percentage: C: 0.03-0.07%, Si: 0 < Si ≤ 0.03%, Mn: 0.1-0.35%, Cu: 0.005-0.035%, Ni: 0.005-0.015%, Cr: 0.02-0.06%, Als: 0 < Als ≤ 0.065%; Preferably, the Fe element content on the surface of the Fe-Ni alloy layer is 4%-12%; preferably, the thickness of the Fe-Ni alloy layer is 2-5 μm.

2. The nickel-plated steel strip as described in claim 1 or 2, wherein, The substrate contains the following chemical elements in mass percentage: C: 0.03-0.07%, Si: 0 < Si ≤ 0.03%, Mn: 0.1-0.35%, Cu: 0.005-0.035%, Ni: 0.005-0.015%, Cr: 0.02-0.06%, Als: 0 < Als ≤ 0.065%, and the balance is Fe and inevitable impurities.

3. The nickel-plated steel strip as described in any one of claims 1-3, wherein, The content of impurity elements in mass percentage satisfies: P: 0-0.015%, S: 0-0.012%, N: 0-0.004%.

4. The nickel-plated steel strip as described in any one of claims 1-3, wherein, The microstructure of the substrate is ferrite + cementite, preferably, the grain size of the substrate is 11-13 grades.

5. The nickel-plated steel strip as described in any one of claims 1-4, wherein, The nickel-plated steel strip satisfies at least one of the following properties: yield strength is 300-370 MPa, tensile strength is 370-450 MPa, elongation after fracture is 27.0-32.0%, and r value is 0.6-1.

2.

6. The nickel-plated steel strip as described in any one of claims 1-5, wherein, The Ni coating of the nickel-plated steel strip is a semi-bright Ni coating or a dark Ni coating without adding S element additives.

7. A method for manufacturing the nickel-plated steel strip according to any one of claims 1-6, wherein, The method includes the following steps carried out in sequence: 1) Smelting and casting molten steel to obtain a slab; 2) Hot rolling the slab; 3) Coiling; 4) Pickling; 5) Cold rolling to obtain cold-rolled steel; 6) Continuous nickel plating; 7) Heat treatment, where the heating rate is: 50-100 °C / s, the heat treatment temperature is: 650-750 °C, and the holding time is: 40-100 s; and 8) Skin pass.

8. The method for manufacturing nickel-plated steel strip as described in claim 7, wherein, Step 6) includes pretreatment and continuous electroplating: The pretreated cold-rolled steel is placed in a plating bath solution at 40-65℃ and pH 3.0-5.0 for continuous electroplating, with a current density of 3-20 A / dm³. 2 Preferably 5-15 A / dm 2 .

9. The method for manufacturing nickel-plated steel strip as described in claim 8, wherein, The plating bath solution is a Watts bath solution, and the Watts bath solution contains: nickel sulfate: 250-380 g / L, nickel chloride: 30-60 g / L, boric acid 20-60 g / L.

10. The method for manufacturing nickel-plated steel strip according to any one of claims 7-9, wherein, The method satisfies at least one of the following: In step 2), the hot rolling heating temperature is 1200-1270 °C, and / or, the final rolling temperature is 850-950 °C; In step 3), the coiling temperature is 600-710 °C; In step 5), the cold rolling reduction rate is 70%-95%; In step 8), the skin pass rate of the skin pass is 0.5-2.0%.

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