Nickel-plated steel sheet for can and method for manufacturing the same

Abstract

A nickel-plated steel sheet for a can according to one embodiment of the present application includes a base steel sheet, a nickel-plated layer on one side or both sides of the base steel sheet, and a Fe-Ni alloy layer between the base steel sheet and the nickel-plated layer, the base steel sheet contains, in terms of % by weight, C: 0.02 to 0.07%, Mn: 0.1 to 0.4%, Al: 0.01 to 0.06%, and Ti: 0.02 to 0.06%, with the balance containing Fe and other inevitable impurities, has an ASTM grain size of 11.3 or more, a tensile strength of 420 MPa or more, an elongation of 20% or more, an in-plane anisotropy Δr of 0.4 or less, and the Fe-Ni alloy layer has a thickness of 0.5 to 2.5 µm.

Description

Technical Field

[0001] One embodiment of the present invention relates to a nickel-plated steel sheet for cans and a method for manufacturing the same. Specifically, one embodiment of the present invention relates to a nickel-plated steel sheet with excellent strength and processability for use in cylindrical battery casings for electric vehicles, and a method for manufacturing the same. Background Technology

[0002] Cylindrical battery cans used as casings are typically treated with nickel plating (Ni) on the steel sheet to resist corrosion caused by the electrolyte containing the battery. Recently, with the increasing demand for electric vehicles, the demand for cylindrical battery casing materials used in electric vehicles is increasing significantly.

[0003] On the other hand, to ensure battery safety, the strength requirements for battery can materials are increasingly increasing. For electric vehicle batteries, various factors such as overcurrent and external impacts during driving can trigger abnormal internal chemical reactions, producing large amounts of gas. This gas not only increases internal pressure but can also trigger a chain reaction explosion, thus requiring battery can materials with high strength to improve the battery's pressure resistance. Furthermore, using high-strength materials, by reducing the thickness of the battery can and increasing the internal space, can further improve battery performance. Battery cans are manufactured through machining processes that reduce thickness by more than 30%. Even with the same yield strength, materials with higher tensile strength have a higher work hardening rate, resulting in greater durability after processing. Therefore, to achieve high strength after forming, battery can materials are particularly required to have high tensile strength.

[0004] The materials used for battery cans also require specific mechanical properties for machinability. Since cylindrical battery cans undergo drawing and ironing processes during forming, in addition to strength, a certain level of elongation is required. Furthermore, to minimize earing during cylindrical forming, a smaller in-plane anisotropy Δr is more advantageous. Larger in-plane anisotropy not only increases the size of the earing portion that needs to be removed after processing but also causes thickness variations in different areas, making it difficult to fully utilize the internal space. Besides mechanical properties, microstructure also affects machinability; smaller grains facilitate uniform deformation, resulting in superior processed shapes. Coarse grains not only cause shape distortion due to uneven deformation but also lead to an uneven, orange-peel-like surface.

[0005] For cylindrical battery casings, nickel plating is typically performed to prevent corrosion from the internal electrolyte or atmospheric conditions during processing. The plating also has certain performance requirements. Nickel plating is applied to the parts in contact with the processing mold. To prevent plating peeling during processing, heat treatment is used to improve adhesion. This heat treatment involves the diffusion of Fe from the steel plate into the Ni of the plating, forming an Fe-Ni alloy layer at the interface. If the alloy layer is too thin, it is difficult to ensure adhesion between the steel plate and the plating; if the alloy layer is too thick, the Fe component will be exposed to the plating surface, potentially leading to rust due to Fe oxidation. Therefore, an alloy layer of moderate thickness is required.

[0006] To manufacture high-strength can steel plates, a method of high-reduction secondary rolling of low-carbon steel has been proposed. Since the secondary reduction occurs after recrystallization annealing, it offers the advantage of significantly increased strength through work hardening. However, such high-level secondary reduction results in a substantial decrease in elongation, thus compromising the can-making processability.

[0007] Furthermore, to ensure strength and workability, it is known that an appropriate amount of P and Nb are added to an extremely low carbon steel and then hardened by baking. However, since it is an extremely low carbon steel, it has the following disadvantages: in order to induce baking hardening with a low C content, the C and Nb contents must be very strictly controlled at the same time, so that some C does not precipitate as NbC, but remains in a solid solution state.

[0008] In addition, a method has been proposed: adding a large amount of nitrogen (N) (above 130 ppm) to low-carbon steel to improve strength through solid solution strengthening, and using a smaller secondary reduction rate to increase elongation. However, when a large amount of N is added as an interstitial element, compositional deviation is prone to occur, and once compositional deviation occurs, the possibility of material deviation is also high. Therefore, in order to control compositional deviation to a low level, the steelmaking process requires extra effort, which is also a drawback.

[0009] Furthermore, a method was described: increasing strength through precipitation strengthening by adding Ti, and further reducing elongation loss due to work hardening by using a secondary reduction rate of less than 15%, thereby ensuring a balance between strength and ductility. However, the increase in strength caused by work hardening leads to a decrease in elongation, thus posing a challenge to ensuring processability. Summary of the Invention

[0010] (a) Technical problems to be solved One embodiment of the present invention provides a nickel-plated steel sheet for cans and a method for manufacturing the same. Specifically, one embodiment of the present invention provides a nickel-plated steel sheet with excellent strength and processability for use in cylindrical battery casings for electric vehicles, and a method for manufacturing the same.

[0011] (II) Technical Solution According to an embodiment of the present invention, a nickel-plated steel sheet for tanks comprises a base steel sheet, a nickel plating layer on one or both sides of the surface of the base steel sheet, and an Fe-Ni alloy layer between the base steel sheet and the nickel plating layer. The base steel sheet, by weight percent, comprises C: 0.02 to 0.07%, Mn: 0.1 to 0.4%, Al: 0.01 to 0.06%, and Ti: 0.02 to 0.06%, with the balance being Fe and other unavoidable impurities. It has an ASTM grain size grade of 11.3 or higher, a tensile strength of 420 MPa or higher, an elongation of 20% or higher, and an in-plane anisotropy Δr of 0.4 or lower. The thickness of the Fe-Ni alloy layer is 0.5 to 2.5 μm.

[0012] The base steel plate may also contain one or more of the following: Si: less than 0.05 wt%, P: less than 0.015 wt%, S: less than 0.015 wt%, and N: less than 0.006 wt%.

[0013] The base steel plate may also contain one or more of the following: Nb: less than 0.01 wt%, Ni: less than 0.1 wt%, Cr: less than 0.1 wt%, Cu: less than 0.1 wt%, Mo: less than 0.01 wt%, and V: less than 0.01 wt%.

[0014] The base steel plate contains TiC precipitates, and the average particle size of the TiC precipitates can be above 1 nm and below 1 μm.

[0015] A method for manufacturing a nickel-plated steel sheet for cans according to an embodiment of the present invention comprises: hot rolling a slab to manufacture a hot-rolled steel sheet, wherein the slab comprises, by weight %, 0.02 to 0.07% C, 0.1 to 0.4% Mn, 0.01 to 0.06% Al and 0.02 to 0.06% Ti, with the balance being Fe and other unavoidable impurities; cold rolling the hot-rolled steel sheet at a reduction rate of 70 to 90% to manufacture a cold-rolled steel sheet; recrystallizing the cold-rolled steel sheet at a homogenization temperature of 720 to 850°C; leveling the recrystallized steel sheet at a reduction rate of 0.5 to 1.8%; nickel plating one or both sides of the leveled steel sheet to manufacture a nickel-plated steel sheet; and alloying annealing the nickel-plated steel sheet at a homogenization temperature of 600 to 740°C for 5 to 60 seconds.

[0016] The slab may also contain one or more of the following: Si: less than 0.05 wt%, P: less than 0.015 wt%, S: less than 0.015 wt%, and N: less than 0.006 wt%.

[0017] The slab may also contain one or more of the following: Nb: less than 0.01 wt%, Ni: less than 0.1 wt%, Cr: less than 0.1 wt%, Cu: less than 0.1 wt%, Mo: less than 0.01 wt%, and V: less than 0.01 wt%.

[0018] Before the process of manufacturing hot-rolled steel sheets, a step of heating the slab at a temperature above 1200°C may also be included.

[0019] The process of manufacturing hot-rolled steel sheets can be hot finishing at a temperature above Ar3, followed by coiling at a temperature of 580 to 720°C.

[0020] (III) Beneficial Effects According to one embodiment of the present invention, a nickel-plated steel sheet for cans has excellent durability and processability, and can be effectively used for cylindrical battery casings. Attached Figure Description

[0021] Figure 1 This is a schematic cross-section of a nickel-plated steel sheet for tanks according to an embodiment of the present invention.

[0022] Figure 2 The result is the GDS analysis of the nickel-plated steel sheet manufactured in Example 2 of the invention. Detailed Implementation

[0023] The terms "first," "second," "third," etc., are used to describe parts, components, regions, layers, and / or segments, but these parts, components, regions, layers, and / or segments should not be limited by these terms. These terms are only used to distinguish one part, component, region, layer, or segment from another. Therefore, without departing from the scope of the invention, the first part, component, region, layer, or segment described below can also be described as a second part, component, region, layer, or segment.

[0024] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the invention. Unless the context clearly indicates otherwise, the singular forms used herein are intended to include the plural forms as well. The word "comprising" as used in the specification can specifically refer to a particular feature, domain, integer, step, action, element, and / or component, but does not exclude the presence or addition of other features, domains, integers, steps, actions, elements, components, and / or groups.

[0025] In addition, unless otherwise specified, % means weight, 1 ppm is 0.0001 weight.

[0026] In one embodiment of the present invention, the inclusion of additional elements refers to the replacement of a portion of the remaining iron (Fe) by additional elements, the replacement amount being equivalent to the amount of additional elements added.

[0027] Although not otherwise defined, all terms used herein (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Terms defined in dictionaries should be interpreted as having the same meaning as disclosed in relevant technical literature and herein, and should not be interpreted in an idealized or overly formal sense.

[0028] The embodiments of the present invention will be described in detail below to enable those skilled in the art to implement the invention. However, the present invention can be implemented in various different ways and is not limited to the embodiments described herein.

[0029] One embodiment of the present invention relates to nickel-plated steel sheets used in electric vehicle battery casings, etc., after stamping. The material for this application requires a strength at an appropriate level or higher to improve the battery's pressure resistance. To ensure processability, an elongation at an appropriate level or higher is required, and to obtain a proper shape, in-plane anisotropy and grain size at an appropriate level or lower are required. Furthermore, for the nickel plating layer, its corrosion resistance for preventing corrosion and its adhesion for preventing detachment during processing should be ensured to be at a certain level or higher.

[0030] Figure 1 A cross-section of a nickel-plated steel sheet 100 for tanks according to an embodiment of the present invention is shown schematically. Figure 1 As shown, the nickel-plated steel sheet 100 for tanks includes a base steel sheet 10, a nickel plating layer 30 on one or both sides of the surface of the base steel sheet 10, and an Fe-Ni alloy layer 20 between the base steel sheet 10 and the nickel plating layer 30. Figure 1 The diagram shows that the Fe-Ni alloy layer 20 and the nickel plating layer 30 are located on one side of the base steel plate 10, but the Fe-Ni alloy layer 20 and the nickel plating layer 30 can also be located on both sides of the base steel plate 10.

[0031] According to an embodiment of the present invention, the base steel plate 10 of the nickel-plated steel sheet 100 for cans contains, by weight %, C: 0.02 to 0.07%, Mn: 0.1 to 0.4%, Al: 0.01 to 0.06% and Ti: 0.02 to 0.06%, with the balance containing Fe and other unavoidable impurities.

[0032] The following is a detailed description of each component.

[0033] Carbon (C): 0.020 to 0.070% by weight Carbon (C) is an element added to increase the strength of steel plates. If the C content is too low, the strength will be low, making it difficult to use as a structural material. Furthermore, if the C content is too low, the load on the steelmaking process will increase, potentially reducing productivity. On the other hand, if the C content is higher than necessary, the elongation will decrease, and formability may decline. More specifically, it may contain 0.030 to 0.065% by weight of C. More specifically, it may contain 0.040 to 0.063% by weight of C.

[0034] In one embodiment of the present invention, a portion of C combines with a small amount of added Ti to exist in the form of fine TiC precipitates. The fine TiC prevents excessive grain growth, thereby helping to effectively improve strength.

[0035] Manganese (Mn): 0.10 to 0.40% by weight Mn combines with dissolved sulfur (S) in steel to precipitate as MnS, which is an element that prevents hot shortness caused by dissolved S. Furthermore, Mn dissolved in steel, along with carbon (C), has the effect of increasing the strength of the steel. However, compared with titanium (Ti), the strength-increasing effect is weaker, and if the Mn content is too high, the workability of the steel may decrease. More specifically, it may contain 0.13 to 0.30 wt% Mn. More specifically, it may contain 0.14 to 0.25 wt% Mn.

[0036] Aluminum (Al): 0.010 to 0.060% by weight Al is a highly effective deoxidizing element, reacting with nitrogen in steel to precipitate AlN, thus preventing the decrease in formability caused by dissolved nitrogen. However, if added in large quantities, the effect of further addition may be limited. More specifically, it may contain 0.015 to 0.055% by weight of Al. More specifically, it may contain 0.020 to 0.050% by weight of Al.

[0037] Titanium (Ti): 0.020 to 0.060 wt% Ti combines with C to precipitate as stable and fine TiC. These fine TiC precipitates inhibit grain growth and effectively impede dislocation movement, thus contributing to increased strength. If too little Ti is added, the sufficient strength increase from TiC cannot be expected. If too much Ti is added, it not only impairs machinability due to reduced ductility but also easily leads to gate blockage during continuous casting. More specifically, it may contain 0.022 to 0.050 wt% Ti. More specifically, it may contain 0.023 to 0.045 wt% Ti.

[0038] The base steel plate 10 may also contain one or more of the following: Si: less than 0.05 wt%, P: less than 0.015 wt%, S: less than 0.015 wt%, and N: less than 0.006 wt%.

[0039] Silicon (Si): less than 0.050% by weight Si is an element that can be used as a decarburizing agent and helps improve strength through solid solution strengthening, so it is difficult to completely eliminate it. However, if in excess, Si-based oxides will form on the surface during annealing, which may induce coating defects and reduce coatability. Therefore, taking this into consideration, less than 0.05% by weight of Si may be included. More specifically, 0.001 to 0.050% by weight of Si may be further included. More specifically, 0.005 to 0.035% by weight of Si may be further included.

[0040] Phosphorus (P): less than 0.015% by weight While adding a certain amount of phosphorus (P) can increase strength without significantly reducing the ductility of steel, excessive P can cause segregation at grain boundaries, leading to over-hardening of the steel and potentially reducing elongation. Therefore, it is permissible to further include less than 0.015 wt% of P. More specifically, it is permissible to further include 0.001 to 0.015 wt% of P. More specifically, it is permissible to further include 0.003 to 0.013 wt% of P.

[0041] Sulfur (S): less than 0.015% by weight When sulfur (S) exists in a solid solution state, it causes hot brittleness during hot rolling. Therefore, it is necessary to add manganese (Mn) to induce the precipitation of MnS. As the amount of S increases, a corresponding level of Mn must be added, so it is not advisable to have it in large quantities. Therefore, the upper limit of S can be limited to below 0.015 wt%. More specifically, it can further contain 0.001 to 0.015 wt% S. More specifically, it can further contain 0.003 to 0.013 wt% S.

[0042] Nitrogen (N): less than 0.0060% by weight Nitrogen (N) is an unavoidable residual element in steel, but N present in a solid solution state causes aging, thereby significantly reducing workability. To minimize the reduction in ductility caused by excessive aging, its upper limit can be limited to below 0.006 wt%. More specifically, it can further contain 0.0001 to 0.0060 wt% N. More specifically, it can further contain 0.0005 to 0.0055 wt% N.

[0043] In addition to the aforementioned alloy components, the balance includes Fe and unavoidable impurities. However, in one embodiment of the invention, the addition of other components is not excluded. The unavoidable impurities are those that may be unintentionally introduced from raw materials or the surrounding environment during conventional steel manufacturing processes, and their introduction cannot be ruled out. These unavoidable impurities are as understood by those skilled in the art of conventional steel manufacturing. For example, it may further include one or more of Nb: less than 0.01 wt%, Ni: less than 0.1 wt%, Cr: less than 0.1 wt%, Cu: less than 0.1 wt%, Mo: less than 0.01 wt%, and V: less than 0.01 wt%.

[0044] For the base steel plate 10, the ASTM grain size number is 11.3 or higher. The ASTM grain size number is an indicator related to grain size, determined by observing an image of the optical microstructure and calculating it according to ASTM E112 (Standard Test Methods for Determining Average Grain Size). The ASTM grain size number can be measured on a cross-section including the thickness direction of the base steel plate 10. More specifically, it can be measured based on the TD plane.

[0045] A higher ASTM grain size rating indicates a smaller average grain size. A lower ASTM grain size rating can lead to excessively increased strength and decreased elongation, resulting in difficulties in forming. More specifically, ASTM grain size ratings range from 11.3 to 13.0. More specifically, ASTM grain size ratings range from 11.3 to 12.0.

[0046] Returning to the description of the nickel-plated steel sheet 100 for cans, the Fe-Ni alloy layer 20 is located between the base steel sheet 10 and the nickel plating layer 30. If only the nickel plating layer 30 exists without the Fe-Ni alloy layer 20, the adhesion to the base steel sheet 10 will be poor, and it will easily detach during processing. The Fe-Ni alloy layer 20 is formed by the diffusion of Ni into the base steel sheet 10 and the diffusion of Fe into the nickel plating layer 30. In one embodiment of the invention, the Fe-Ni alloy layer 20 refers to the region between the location where Fe is 5% by weight and the location where Ni is 5% in the thickness direction. The thickness of the Fe-Ni alloy layer 20 can be the distance between the aforementioned locations. The thickness of the Fe-Ni alloy layer 20 can be measured by measuring the cross-section of the nickel-plated steel sheet 100 for cans using GDS (glow discharge spectrometer) or EDS (energy dispersive X-ray spectrometer). The Fe-Ni alloy layer 20 has a concentration gradient of Fe and Ni, where Fe can have a concentration gradient that increases from the outside to the inside of the steel sheet, and Ni can have a concentration gradient that decreases from the outside to the inside of the steel sheet. On average in the thickness direction, the Fe-Ni alloy layer 20 may contain 35 to 65 wt% Fe and 35 to 65 wt% Ni.

[0047] The thickness of the Fe-Ni alloy layer 20 can be from 0.5 to 2.5 μm. If the thickness of the Fe-Ni alloy layer 20 is too thin, adhesion is difficult to ensure. If the thickness of the Fe-Ni alloy layer 20 is too thick, the Fe component present in the base steel plate 10 will be exposed to the surface, which may lead to poor corrosion resistance. More specifically, the thickness of the Fe-Ni alloy layer 20 can be from 0.7 to 2.3 μm. More specifically, the thickness of the Fe-Ni alloy layer 20 can be from 1.0 to 2.0 μm. In one embodiment of the present invention, the thickness of the Fe-Ni alloy layer 20 and the nickel plating layer 30 is relative to the thickness of one side of the steel plate. When the Fe-Ni alloy layer 20 and the nickel plating layer 30 are present on both sides, at least one of the two sides can meet the aforementioned thickness requirements.

[0048] The nickel plating 30 helps ensure resistance to corrosion from battery electrolytes and the atmosphere. The plating thickness varies depending on the forming quantity and electrolyte type, but on at least one surface where wear primarily occurs during forming, the plating thickness can be set to 2.0 μm or more. In the case of hot-dip plating, it is difficult to control the plating thickness below a certain level, and there is a tendency for large thickness deviations; therefore, electroplating can be used.

[0049] According to an embodiment of the present invention, the nickel-plated steel sheet 100 for tanks can simultaneously ensure excellent strength, elongation, and in-plane anisotropy. Specifically, the tensile strength is 420.0 MPa or higher. More specifically, the tensile strength can be from 420.0 to 600.0 MPa. More specifically, the tensile strength can be from 425.0 to 550.0 MPa. More specifically, the tensile strength can be from 430.0 to 500.0 MPa.

[0050] Specifically, the elongation rate is 20.0% or higher. More specifically, the elongation rate can be 20.0% to 30.0%. More specifically, the elongation rate can be 20.5% to 28.0%. More specifically, the elongation rate can be 21.0% to 25.0%.

[0051] Specifically, the in-plane anisotropy Δr is less than 0.40. More specifically, the in-plane anisotropy Δr can be from 0.01 to 0.40. More specifically, the in-plane anisotropy Δr can be from 0.10 to 0.35.

[0052] Tensile strength, elongation, and in-plane anisotropy Δr can be measured by conventional tensile testing at room temperature. In-plane anisotropy can be obtained using the following equation (1).

[0053] (Relation 1) Δr = (r0 + r 90 - 2×r 45 ) / 2 Among them, r0, r 45 r 90 These are the plastic anisotropy coefficients (Lankford values) when stretched in directions forming angles of 0°, 45°, and 90° with the rolling direction, respectively.

[0054] A method for manufacturing nickel-plated steel sheet for cans according to an embodiment of the present invention includes: hot rolling a slab to manufacture a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet; recrystallizing annealing the cold-rolled steel sheet; leveling the recrystallized annealed steel sheet; nickel plating one or both sides of the leveled rolled steel sheet to manufacture a nickel-plated steel sheet; and alloying annealing the nickel-plated steel sheet.

[0055] The steps will be described in detail below.

[0056] First, the slab is hot-rolled to produce hot-rolled steel sheets.

[0057] The alloy composition of the slab has already been described in the aforementioned base steel plate 10 for nickel-plated steel sheets used in cans, and therefore will not be repeated. During the manufacturing process of the nickel-plated steel sheet for cans, the alloy composition does not actually change; therefore, the alloy composition of the base steel plate 10 is essentially the same as that of the slab.

[0058] Before the process of manufacturing hot-rolled steel sheets, a step of heating the slab at a temperature above 1200°C may be included. During slab manufacturing, various precipitates formed in the steel must be dissolved again, thus requiring a temperature above 1200°C. More specifically, the temperature can be heated to 1200 to 1350°C.

[0059] Hot-rolled steel sheets can be manufactured by hot finishing the reheated slab at a temperature above Ar3 and then coiling it at 580 to 720°C. The reason for limiting the hot rolling termination temperature to above Ar3 is to ensure rolling occurs in the austenitic single-phase region. If rolling occurs in the two-phase region, rolling stability may decrease due to material inhomogeneity. Ar3 temperature is well-known, and in one embodiment of the invention, Ar3 temperature can be calculated from 910 - (310 × [C]) - (80 × [Mn]) - (0.35 × (25.4–8)). More specifically, the hot rolling termination temperature can be from 850°C to 1000°C.

[0060] During the coiling process after finishing rolling, the precipitation behavior of TiC changes depending on the coiling temperature. If the temperature is too low or too high, TiC cannot precipitate normally. Therefore, the coiling temperature can be controlled at 580 to 720℃, which is conducive to precipitation. The thickness of hot-rolled steel plates can be 2 to 6 mm.

[0061] Next, the hot-rolled steel sheet is cold-rolled to produce cold-rolled steel sheet. An appropriate level of cold-rolling reduction is important for both strength and workability. A higher reduction facilitates the formation of recrystallization nuclei during annealing, resulting in finer grains and increased strength. Furthermore, a higher reduction tends to reduce in-plane anisotropy. However, if the reduction is too high, elongation decreases, negatively impacting workability, and deformation resistance increases, leading to reduced productivity. Considering this, the cold-rolling reduction is set in the range of 70% to 90%. More specifically, the reduction can be 73% to 85%. A pickling process can be added before cold rolling to remove the oxide scale generated during hot rolling. The thickness of the cold-rolled steel sheet can be 0.3 to 1 mm.

[0062] Next, the cold-rolled steel sheet undergoes recrystallization annealing. The main purpose of recrystallization annealing is to eliminate the internal stress formed during cold rolling and ensure processability. For this purpose, annealing needs to be carried out at a sufficiently high temperature to allow recrystallization to occur completely. For cold-rolled steel sheets with a steel composition system according to an embodiment of the present invention, considering the increase in recrystallization temperature caused by TiC, a temperature of 720°C or higher is required to induce recrystallization. If the temperature is too low, recrystallization is insufficient, and some deformed grains are present, which may lead to a significant reduction in the ductility of the steel sheet and may cause cracking during forming due to increased strength. However, if the annealing temperature is too high, it is difficult to ensure strength due to grain growth, and fracture or poor shape may occur due to the reduction in strength during annealing. Therefore, recrystallization annealing can be carried out on the cold-rolled steel sheet at a soaking temperature of 720 to 850°C. More specifically, annealing can be carried out at a temperature of 725 to 835°C. The soaking time can be 10 to 120 seconds.

[0063] Next, the recrystallized annealed steel plate is leveled and rolled.

[0064] Leveling rolling not only corrects the shape but also helps create a suitable density of dislocations. The dislocations formed during leveling rolling become sites where carbides easily precipitate during the alloying annealing process following electroplating, thus contributing to further strength improvement. To achieve this effect, a reduction rate of 0.5% or higher is required. However, if the reduction rate is too high, it provides a driving force for recrystallization and coarsening of surface grains during the alloying annealing process. Furthermore, locally coarse grains can cause poor processing performance due to uneven elongation and localized fractures. Therefore, it needs to be implemented at a level below 1.8%. More specifically, the reduction rate can be between 0.6% and 1.7%.

[0065] Next, nickel plating is performed on one or both sides of the flattened and rolled steel sheet to manufacture nickel-plated steel sheet. Nickel plating is necessary to ensure corrosion resistance to battery electrolytes and the atmosphere. The plating thickness varies depending on the forming volume and electrolyte type, but on at least one side where wear mainly occurs during forming, the plating thickness can be set to 2.0 μm or more. More specifically, the plating thickness can be 2.0 to 5.0 μm. In the case of hot-dip plating, it is difficult to control the plating thickness below a certain level, and there is a tendency for large thickness deviations; therefore, electroplating can be used. General conditions such as the nickel plating bath and the current density during electroplating can be used, and specific descriptions are omitted.

[0066] Next, the nickel-plated steel sheet undergoes alloying annealing.

[0067] The nickel plating 30 has poor adhesion to the steel plate in the electroplated state and may easily peel off during processing. To prevent this, alloying annealing is required at high temperature to form an Fe-Ni alloy layer 20 between the nickel plating and the steel plate through diffusion.

[0068] At this point, the temperature is maintained at a homogenization temperature of 600 to 740°C for 5 to 60 seconds. If the alloying annealing temperature is too low or the time is too short, adhesion will be difficult to ensure due to the thin Fe-Ni alloy layer. If the annealing temperature is too high or the time is too long, the Fe component in the steel sheet will be exposed to the coating surface due to the thick alloy layer, making it difficult to ensure corrosion resistance. However, the correlation between the alloying annealing temperature and time and the alloy layer thickness is limited to steel sheets manufactured under the composition and manufacturing conditions described in this invention, and may not apply to steel sheets with different compositions and manufacturing processes. More specifically, the temperature can be maintained at a homogenization temperature of 610 to 730°C for 7 to 58 seconds.

[0069] After alloying annealing, cold rolling can be further carried out within a range of less than 2.0% in order to correct the shape of the steel plate.

[0070] The present invention will now be described in more detail through embodiments. However, the following embodiments are for illustrative purposes only, and the present invention is not limited to the embodiments described below.

[0071] Example 1 Steel sheets with the compositions shown in Table 1 and the manufacturing conditions shown in Table 2 were manufactured. The compositions represent actual values, and slabs with the corresponding compositions were manufactured. The slabs were reheated to 1220°C, hot-rolled to a uniform thickness of 4 mm at a temperature above 900°C, and then coiled at 640°C to manufacture hot-rolled steel sheets. The coiled hot-rolled steel sheets were cold-rolled at the cold-rolling reduction rates shown in Table 2, then recrystallized and annealed for 30 seconds at the temperatures shown in Table 2, and then leveled and rolled at the leveling reduction rates shown in Table 2 to manufacture recrystallized and annealed steel sheets. The recrystallized and annealed steel sheets were also nickel-plated to a thickness of 3.0 μm, and then alloyed and annealed for 20 seconds at the temperatures and times shown in Table 2 to manufacture the final nickel-plated steel sheets.

[0072] For each of the steel plates manufactured, the ASTM grain size grade, tensile strength, elongation, in-plane anisotropy, and alloy layer thickness were measured, and the results are shown in Table 3.

[0073] The ASTM grain size number is determined by observing images of optical microstructure and calculating according to the ASTM E112 standard (Standard Test Methods for Determining Average Grain Size).

[0074] Tensile strength, elongation, and in-plane anisotropy Δr were measured by conventional tensile testing at room temperature. The in-plane anisotropy Δr can be obtained using the following equation (1).

[0075] (Relation 1) Δr = (r0 + r 90 - 2×r 45 ) / 2 Among them, r0, r 45 r 90 These are the plastic anisotropy coefficients (Lankford values) when stretched in directions forming angles of 0°, 45°, and 90° with the rolling direction, respectively.

[0076] The thickness of the alloy layer was measured using a GDS (Glow Discharge Spectrometer) on the cross-section of the electroplated steel sheet. The lengths to the locations where Fe and Ni accounted for 5% by weight were measured respectively.

[0077] Regarding coating adhesion, if the coating does not peel off from the steel plate when the material is stretched by 20%, it is considered good. Regarding corrosion resistance, if no surface discoloration occurs after immersion in pure water for 20 minutes and drying after forming, it is considered good.

[0078] Table 1 Table 2 Table 3 As shown in Tables 1 to 3, Examples 1 to 14 of the invention simultaneously meet the requirements for steel composition and manufacturing conditions, confirming excellent tensile strength, elongation, and in-plane anisotropy. Furthermore, it can be confirmed that the Fe-Ni alloy layer thickness is moderate, thus exhibiting excellent adhesion and corrosion resistance.

[0079] Comparative Example 1 represents a case with low carbon content. Due to the low carbon content, sufficient strength was not ensured. Furthermore, the low carbon content resulted in coarse grains, with an ASTM grain size grade of 11.1, leading to lower tensile strength. On the other hand, Comparative Example 2 represents a case with excessive carbon content. Although the grains were finer and sufficiently high tensile strength was achieved, the elongation was lower, resulting in poor processability.

[0080] Comparative Example 3 represents a case with low Ti content. Because the Ti content is insufficient to form a sufficient amount of TiC, the grains are coarse, resulting in lower tensile strength. Comparative Example 4 represents a case with high Ti content. Although the grain size is fine and the tensile strength is excellent, the elongation is lower.

[0081] Comparative Example 5 shows lower tensile strength due to low cold rolling reduction, resulting in coarsened grains. Furthermore, the in-plane anisotropy Δr is significantly worsened. As shown in Comparative Example 6, if the cold rolling reduction is too high, the elongation decreases, which also reduces processability.

[0082] Comparative Example 7 failed to achieve proper recrystallization due to a low recrystallization annealing temperature. Consequently, the elongation was low, and processability was significantly reduced. This was because the TiC precipitates were extremely fine. As shown in Comparative Example 8, if the recrystallization annealing temperature is too high, coarse grains and low tensile strength occur. Furthermore, as shown in Comparative Example 8, a high recrystallization annealing temperature results in low strength at high temperatures, increasing the risk of fracture and necessitating a relatively lower throughput, thus reducing productivity.

[0083] Comparative Example 9 represents a case with a very low leveling reduction rate, resulting in low tensile strength. As shown in Comparative Example 10, if the leveling reduction rate is too high, problems such as reduced elongation and coarsening of surface grains occur.

[0084] Comparative Example 11 represents a case of low alloying annealing temperature, and Comparative Example 13 represents a case of short alloying annealing time; both resulted in the formation of relatively thin Fe-Ni alloy layers. In both cases, the adhesion between the steel sheet and the coating was weak, leading to coating peeling during battery can forming. Comparative Example 12 represents a case of high alloying annealing temperature, and Comparative Example 14 represents a case of long alloying annealing time; both resulted in the formation of thicker Fe-Ni alloy layers. This resulted in Fe being exposed on the surface, making it prone to rusting after washing and drying.

[0085] This invention is not limited to the embodiments described above and can be manufactured in various different ways. Those skilled in the art should understand that other specific methods can be used without altering the technical concept or essential features of the invention. Therefore, it should be understood that the above embodiments are exemplary in all respects and not restrictive.

[0086] [Explanation of reference numerals in the attached figures] 100: Nickel-plated steel sheet 10: Base material steel plate 20: Fe-Ni alloy layer 30: Nickel plating

Claims

1. A nickel-plated steel sheet for tanks, wherein, The nickel-plated steel sheet for the tank comprises a base steel sheet, a nickel plating layer on one or both sides of the surface of the base steel sheet, and an Fe-Ni alloy layer between the base steel sheet and the nickel plating layer. The base steel plate, by weight percent, comprises C: 0.02 to 0.07%, Mn: 0.1 to 0.4%, Al: 0.01 to 0.06%, and Ti: 0.02 to 0.06%, with the balance including Fe and other unavoidable impurities. The ASTM grain size grade is 11.3 or higher. Tensile strength above 420MPa The elongation rate is over 20%. The in-plane anisotropy Δr is less than 0.

4. The thickness of the Fe-Ni alloy layer is 0.5 to 2.5 μm.

2. The nickel-plated steel sheet for tanks according to claim 1, wherein, The base steel plate further comprises one or more of the following: Si: less than 0.05 wt%, P: less than 0.015 wt%, S: less than 0.015 wt%, and N: less than 0.006 wt%.

3. The nickel-plated steel sheet for tanks according to claim 1, wherein, The base steel plate further comprises one or more of the following: Nb: less than 0.01 wt%, Ni: less than 0.1 wt%, Cr: less than 0.1 wt%, Cu: less than 0.1 wt%, Mo: less than 0.01 wt%, and V: less than 0.01 wt%.

4. The nickel-plated steel sheet for tanks according to claim 1, wherein, The base steel plate contains TiC precipitates, and the average particle size of the TiC precipitates is greater than 1 nm and less than 1 μm.

5. A method for manufacturing a nickel-plated steel sheet for cans, comprising: The step of hot rolling a slab to produce a hot-rolled steel sheet, wherein the slab comprises, by weight %, 0.02 to 0.07% C, 0.1 to 0.4% Mn, 0.01 to 0.06% Al and 0.02 to 0.06% Ti, with the balance comprising Fe and other unavoidable impurities; The step of cold rolling the hot-rolled steel sheet with a reduction rate of 70 to 90% to manufacture a cold-rolled steel sheet; The cold-rolled steel sheet is subjected to recrystallization annealing at a homogenization temperature of 720 to 850°C. The step of leveling and rolling the recrystallized annealed steel sheet with a reduction rate of 0.5% to 1.8%; The steps of nickel plating one or both sides of a flat-rolled steel sheet to manufacture nickel-plated steel sheet; as well as The nickel-plated steel sheet is subjected to alloying annealing at a homogenization temperature of 600 to 740°C for 5 to 60 seconds.

6. The method for manufacturing nickel-plated steel sheet for tanks according to claim 5, wherein, The slab further comprises one or more of the following: Si: less than 0.05 wt%, P: less than 0.015 wt%, S: less than 0.015 wt%, and N: less than 0.006 wt%.

7. The method for manufacturing nickel-plated steel sheet for tanks according to claim 5, wherein, The slab further comprises one or more of the following: Nb: less than 0.01 wt%, Ni: less than 0.1 wt%, Cr: less than 0.1 wt%, Cu: less than 0.1 wt%, Mo: less than 0.01 wt%, and V: less than 0.01 wt%.

8. The method for manufacturing nickel-plated steel sheet for tanks according to claim 5, wherein, Before the process of manufacturing hot-rolled steel sheets. It also includes the step of heating the slab at a temperature above 1200°C.

9. The method for manufacturing nickel-plated steel sheet for tanks according to claim 5, wherein, The process of manufacturing hot-rolled steel sheets involves hot finishing at a temperature above Ar3, followed by coiling at a temperature of 580 to 720°C.

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