Nickel-plated steel sheet and manufacturing method therefor

Abstract

A Ni-plated steel sheet according to one embodiment of the present invention comprises: a base steel sheet; a Ni-plated layer positioned on one surface or both surfaces of the base steel sheet; and an Fe-Ni alloy layer positioned between the base steel sheet and the Ni-plated layer, wherein the base steel sheet comprises, by wt%, 0.02-0.07% of C, 0.9-1.6% of Mn, 0.01-0.06% of Al, 0.01-0.05% of Nb, and the balance of Fe and other inevitable impurities.

Description

Nickel-plated steel sheet and method of manufacturing the same

[0001] One embodiment of the present invention relates to a nickel-plated steel sheet and a method for manufacturing the same. Specifically, one embodiment of the present invention relates to a nickel-plated steel sheet for cans with excellent strength and processability, used for electric vehicle cylindrical battery cases, and a method for manufacturing the same.

[0002] In the case of cylindrical battery cans used in cylindrical battery cases, it is common practice to use steel plates coated with nickel (Ni) to withstand corrosion caused by the electrolyte entering the battery contents. Recently, with the increasing demand for electric vehicles, the demand for materials for cylindrical battery cases for electric vehicles has been rising significantly.

[0003] Meanwhile, to ensure battery safety, there is an increasing demand for strength among the characteristics of battery can materials. During driving, electric vehicle batteries can generate large amounts of gas due to abnormal chemical reactions caused by various factors, such as overcurrent and external impact. Since this generated gas increases internal pressure and can even trigger a chain reaction of explosions, high strength is required for battery can materials to enhance the battery's pressure resistance. Furthermore, using high-strength materials can further improve battery performance by reducing the thickness of the battery can and expanding the internal space. Battery cans are manufactured through mechanical processing that reduces thickness by more than 30%, and even with the same yield strength, materials with higher tensile strength exhibit a higher work hardening rate, allowing for greater durability after processing. Therefore, high tensile strength is particularly required for battery can materials to achieve high strength after forming.

[0004] Materials for battery cans require mechanical properties in terms of processability. Since cylindrical battery cans undergo processing steps such as drawing and ironing during forming, they require a certain level of elongation in addition to strength. Furthermore, to minimize earing during cylindrical forming, a smaller in-plane anisotropy Δr is advantageous. If the in-plane anisotropy is large, not only does the earing area that must be cut off after processing become larger, but thickness variations also occur, making it difficult to fully utilize the internal space. In addition to mechanical properties, the microstructure also affects processability; smaller grain sizes allow for superior processed shapes resulting from uniform deformation. Conversely, coarse grains can induce not only shape distortion due to non-uniform deformation but also an "orange peel" phenomenon where the surface becomes uneven.

[0005] Cylindrical battery cases are generally nickel-plated to prevent corrosion caused by the internal electrolyte or the atmosphere during processing, and specific characteristics are required for the plating layer. Ni plating is applied to the parts in contact with the processing mold; to prevent the plating layer from peeling off during processing, adhesion is improved through a heat treatment that forms an Fe-Ni alloy layer at the interface by facilitating diffusion between the Fe in the steel plate and the Ni in the plating layer. If the alloy layer is too thin, it is difficult to ensure adhesion between the steel plate and the plating layer; if it is too thick, Fe components may be exposed to the surface of the plating layer, potentially leading to rust formation due to Fe oxidation. Therefore, it is necessary to form an alloy layer of appropriate thickness.

[0006] A method has been proposed to perform secondary rolling with a high reduction rate on low-carbon steel to manufacture high-strength steel sheets for cans. Since secondary rolling is performed after recrystallization annealing, the advantage of significantly improving strength due to work hardening can be obtained. However, there is a disadvantage that it is difficult to secure can workability because the elongation decreases significantly when performing secondary rolling at such a high level.

[0007] In addition, it is known that baking hardening is utilized by adding appropriate amounts of P and Nb to ultra-low carbon steel to ensure strength and machinability. However, this method has the disadvantage that, despite the low C content of ultra-low carbon steel, the content of C and Nb must be simultaneously and very strictly controlled so that some C remains dissolved without precipitating as NbC, thereby inducing baking hardening.

[0008] In addition, a method has been proposed to improve strength through solid solution strengthening by adding a large amount of N to low-carbon steel and to increase elongation by applying a relatively small secondary reduction ratio. However, when a large amount of N, an interstitial element, is added, compositional deviation can easily occur, and if compositional deviation occurs, material deviation is also highly likely to occur. Therefore, there is a disadvantage that additional effort is required during the steelmaking process to control compositional deviation to a low level.

[0009] In addition, it is known that by adding Ti, strength is increased through precipitation strengthening, and a lower secondary reduction ratio is applied to reduce the decrease in elongation caused by work hardening and to secure a balance between strength and ductility. However, the addition of Ti has the characteristic of forming a large number of inclusions during the steelmaking process due to its high oxygen affinity, which reduces cleanliness. Since a large amount of inclusions in the steel can become a starting point for cracks during the forming process, there is a disadvantage in that additional effort is required to remove the inclusions.

[0010] In one embodiment of the present invention, a nickel-plated steel sheet and a method for manufacturing the same are provided. Specifically, in one embodiment of the present invention, a nickel-plated steel sheet for cans with excellent strength and processability, used for electric vehicle cylindrical battery cases, and a method for manufacturing the same are provided.

[0011] A Ni-plated steel sheet according to one embodiment of the present invention comprises a base steel sheet, a Ni plating layer located on one or both sides of the surface of the base steel sheet, and an Fe-Ni alloy layer located between the base steel sheet and the Ni plating layer, wherein the base steel sheet comprises, in weight%, C: 0.02 to 0.07%, Mn: 0.9 to 1.6%, Al: 0.01 to 0.06%, and Nb: 0.01 to 0.05%, and the remainder being Fe and other unavoidable impurities.

[0012] The base steel sheet may further include one or more of Si: 0.05 wt% or less, P: 0.015 wt% or less, S: 0.015 wt% or less, and N: 0.006 wt% or less.

[0013] The base steel sheet may further include one or more of Ti: 0.01 wt% or less, Ni: 0.1 wt% or less, Cr: 0.1 wt% or less, Cu: 0.1 wt% or less, and V: 0.01 wt% or less.

[0014] The base steel sheet contains NbC precipitates, and the average particle size of the NbC precipitates may be 10 nm or less.

[0015] A Ni-plated steel sheet according to one embodiment of the present invention may have a tensile strength of 495 MPa or more at 25°C, an elongation of 19.5% or more at 25°C, an absolute value of in-plane anisotropy Δr of 0.40 or less, and a tensile strength of 400 MPa or more at 300°C.

[0016] A method for manufacturing a Ni-plated steel sheet according to one embodiment of the present invention comprises the steps of: manufacturing a hot-rolled steel sheet by hot-rolling a slab containing, in weight percent, C: 0.02 to 0.07%, Mn: 0.9 to 1.6%, Al: 0.01 to 0.06%, and Nb: 0.01 to 0.05%, and the remainder being Fe and other unavoidable impurities; manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; manufacturing a Ni-plated steel sheet by plating Ni on one or both sides of the cold-rolled steel sheet; and alloying annealing the Ni-plated steel sheet.

[0017] The slab may further include one or more of Si: 0.05 wt% or less, P: 0.015 wt% or less, S: 0.015 wt% or less, and N: 0.006 wt% or less.

[0018] The slab may further include one or more of Ti: 0.01 wt% or less, Ni: 0.1 wt% or less, Cr: 0.1 wt% or less, Cu: 0.1 wt% or less, and V: 0.01 wt% or less.

[0019] Prior to the step of manufacturing hot-rolled steel sheets, the step of heating the slab to 1200°C or higher may be further included.

[0020] The step of manufacturing hot-rolled steel sheets can be performed by hot finishing rolling at Ar3 or higher, followed by coiling at a temperature of 580 to 720°C.

[0021] The step of manufacturing cold-rolled steel sheets can be cold-rolled with a reduction rate of 75 to 90%.

[0022] After the step of manufacturing the cold-rolled steel sheet, the method may further include a step of recrystallizing the cold-rolled steel sheet at a cracking temperature of 680 to 850°C.

[0023] In the recrystallization annealing stage, the cooling rate from the cracking temperature to 500℃ can be 10℃ / s or less.

[0024] In the recrystallization annealing step, the cooling rate at 500°C to 300°C may be 10°C / s or more.

[0025] A Ni-plated steel sheet according to one embodiment of the present invention has excellent durability and processability, so it can be usefully used in cylindrical battery cases.

[0026] FIG. 1 is a schematic cross-section of a Ni-plated steel sheet according to one embodiment of the present invention.

[0027] Terms such as first, second, and third are used to describe various parts, components, regions, layers, and / or sections, but are not limited thereto. These terms are used solely to distinguish one part, component, region, layer, or section from another part, component, region, layer, or section. Accordingly, the first part, component, region, layer, or section described below may be referred to as the second part, component, region, layer, or section without departing from the scope of the present invention.

[0028] The technical terms used herein are for the reference of specific embodiments only and are not intended to limit the invention. The singular forms used herein include plural forms unless phrases clearly indicate otherwise. As used in the specification, the meaning of "comprising" specifies certain characteristics, areas, integers, steps, actions, elements, and / or components, and does not exclude the presence or addition of other characteristics, areas, integers, steps, actions, elements, and / or components.

[0029] Also, unless otherwise specified, % means weight %, and 1 ppm is 0.0001 weight %.

[0030] In one embodiment of the present invention, the meaning of including additional elements is that the remainder of iron (Fe) is replaced by an amount of the additional element.

[0031] Unless otherwise defined, all terms used herein, including technical and scientific terms, have the same meaning as generally understood by those skilled in the art to which this invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with relevant technical literature and the present disclosure, and are not interpreted in an ideal or highly formal sense unless otherwise defined.

[0032] Hereinafter, embodiments of the present invention are described in detail so that those skilled in the art can easily implement the invention. However, the present invention may be embodied in various different forms and is not limited to the embodiments described herein.

[0033]

[0034] One embodiment of the present invention relates to a Ni-plated steel sheet used for electric vehicle battery cases, etc., after can forming. For such applications, the material requires strength above an appropriate level to improve the pressure resistance of the battery. To ensure processability, elongation above an appropriate level is required, and to obtain a normal shape, in-plane anisotropy and grain size below an appropriate level are required. Furthermore, regarding the Ni plating layer, corrosion resistance to prevent corrosion and adhesion to prevent detachment during processing must be ensured above a certain level.

[0035] FIG. 1 schematically shows a cross-section of a Ni-plated steel sheet (100) according to one embodiment of the present invention. As shown in FIG. 1, the Ni-plated steel sheet (100) comprises a base steel sheet (10), a Ni plating layer (30) located on one or both sides of the surface of the base steel sheet (10), and an Fe-Ni alloy layer (20) located between the base steel sheet (10) and the Ni plating layer (30). Although FIG. 1 shows the Fe-Ni alloy layer (20) and the Ni plating layer (30) located on one side of the base steel sheet (10), it is also possible for the Fe-Ni alloy layer (20) and the Ni plating layer (30) to be located on both sides of the base steel sheet (10).

[0036] A base steel plate (10) of a Ni-plated steel plate (100) according to one embodiment of the present invention comprises, in weight%, C: 0.02 to 0.07%, Mn: 0.9 to 1.6%, Al: 0.01 to 0.06%, and Nb: 0.01 to 0.05%, and the remainder is Fe and other unavoidable impurities.

[0037] Below, each component is explained in detail.

[0038] Carbon (C): 0.020 to 0.070 wt%

[0039] C is an element added to improve the strength of steel plates; if the content is low, the strength is low, making it difficult to use as a structural material. In addition, if the content is too low, productivity may decrease because the load on the steelmaking process increases. Conversely, if the C content is higher than necessary, it may reduce formability by lowering the elongation. More specifically, C may be included in an amount of 0.025 to 0.065 weight%.

[0040] In one embodiment of the present invention, some of the C combines with a small amount of added Nb to exist in the form of fine NbC precipitates, and the fine NbC can contribute to effective strength improvement by preventing excessive growth of crystal grains.

[0041] Manganese (Mn): 0.9 to 1.6 wt%

[0042] Mn is an element that prevents hot shortness caused by dissolved sulfur by combining with dissolved sulfur in steel to precipitate as MnS. It also has the effect of increasing the strength of steel when dissolved in steel, along with carbon. However, compared to Nb, the strength-enhancing effect is lower, and if too much Mn is included, the workability of the steel may decrease. More specifically, Mn may be included in an amount of 1.0 to 1.5 weight percent.

[0043] Aluminum (Al): 0.010 to 0.060 wt%

[0044] Al is an element with a very large deoxidizing effect and prevents the deterioration of formability caused by dissolved N by reacting with N in steel to precipitate AlN. However, if added in large amounts, the effect of additional addition may be reduced. More specifically, Al may be included in an amount of 0.015 to 0.055 weight%.

[0045] Niobium (Nb): 0.010 to 0.050 weight%

[0046] Nb can combine with C to precipitate in the form of stable, fine NbC. Fine NbC precipitates inhibit grain growth and contribute to strength improvement. If Nb is too low, it is difficult to expect a sufficient increase in strength due to NbC. If too much Nb is added, it can significantly increase deformation resistance during hot rolling, thereby impairing hot rolling performance. More specifically, Nb may be included in an amount of 0.015 to 0.040 weight%.

[0047]

[0048] The base steel plate (10) may further include one or more of Si: 0.05 wt% or less, P: 0.015 wt% or less, S: 0.015 wt% or less, and N: 0.006 wt% or less.

[0049] Silicon (Si): 0.050 wt% or less

[0050] Si is an element that can be used as a decarburizing agent and contributes to the improvement of strength through solid solution strengthening, so it is difficult to completely exclude it. However, if present in excessive amounts, Si-based oxides may form on the surface during annealing, causing defects during plating and potentially reducing plating performance. Therefore, considering this, Si may be included in an amount of 0.05 weight% or less. More specifically, it may be further included in an amount of 0.001 to 0.050 weight%. Even more specifically, it may be further included in an amount of 0.005 to 0.035 weight%.

[0051] Phosphorus (P): 0.015 wt% or less

[0052] Although the addition of P below a certain amount is an element that can increase strength without significantly reducing the ductility of steel, if too much P is added, it may segregate at grain boundaries, causing the steel to become excessively hardened and the elongation to decrease. Therefore, P may be further included in an amount of 0.015 weight% or less. More specifically, 0.001 to 0.015 weight% may be further included. Even more specifically, 0.003 to 0.013 weight% may be further included.

[0053] Sulfur (S): 0.015 wt% or less

[0054] Since S is an element that causes red hot brittleness during hot rolling when present in a solid solution state, the precipitation of MnS must be induced through the addition of Mn. It is undesirable for S to be present in large quantities, as a corresponding level of Mn must be added as the amount of S increases. Therefore, the upper limit of S can be restricted to 0.015 weight% or less. More specifically, 0.001 to 0.015 weight% may be further included. More specifically, 0.003 to 0.013 weight% may be further included.

[0055] Nitrogen (N): 0.0060 wt% or less

[0056] Although N is contained as an element that is inevitably retained in steel, N existing in a solid solution state causes aging, which significantly reduces workability. To minimize the reduction in ductility caused by the occurrence of unnecessary levels of aging, the upper limit may be restricted to 0.006 weight% or less. More specifically, 0.0001 to 0.0060 weight% may be further included. More specifically, 0.0005 to 0.0055 weight% may be further included.

[0057] In addition to the alloy composition described above, the remainder comprises Fe and unavoidable impurities. However, the addition of other compositions in one embodiment of the present invention is not excluded. The unavoidable impurities may be unintentionally incorporated from raw materials or the surrounding environment during the ordinary steel manufacturing process and cannot be excluded. The unavoidable impurities are understandable to a person skilled in the ordinary steel manufacturing field. For example, one or more of Ti: 0.01 wt% or less, Mo: 0.01 wt% or less, V: 0.01 wt% or less, Ni: 0.1 wt% or less, Cr: 0.1 wt% or less, and Cu: 0.1 wt% or less may be further included.

[0058] The base steel plate (10) contains NbC precipitates, and the average particle size of the NbC precipitates may be 10 nm or less. By including NbC of an appropriate particle size, the strength can be further improved. More specifically, the average particle size of the NbC precipitates may be 1 to 10 nm.

[0059]

[0060] Returning to the description of the Ni-plated steel plate (100), the Fe-Ni alloy layer (20) is located between the base steel plate (10) and the Ni plating layer (30). If only the Ni plating layer (30) exists without the Fe-Ni alloy layer (20), the adhesion to the base steel plate (10) is not excellent, and it may easily detach during processing. The Fe-Ni alloy layer (20) is formed through the diffusion of Ni into the base steel plate (10) and the diffusion of Fe into the Ni plating layer (30). In one embodiment of the present invention, the Fe-Ni alloy layer (20) refers to the region between the point where Fe is 5 weight% and the point where Ni is 5% in the thickness direction. The thickness of the Fe-Ni alloy layer (20) can be the distance between the aforementioned points. The thickness of the Fe-Ni alloy layer (20) can be measured by GDS (Glow Discharge Spectrometer) or EDS (Energy Disperse X-ray Spectrometer) measurement of the cross-section of the Ni-plated steel plate (100). The Fe-Ni alloy layer (20) has a concentration gradient of Fe and Ni, and may have a concentration gradient in which the concentration of Fe increases from the surface of the steel plate to the center, and a concentration gradient in which the concentration of Ni decreases from the surface of the steel plate to the center. The Fe-Ni alloy layer (20) may contain 35 to 65 weight% of Fe and 35 to 65 weight% of Ni as an average in the thickness direction.

[0061] The thickness of the Fe-Ni alloy layer (20) may be 0.5 to 2.5 μm. If the thickness of the Fe-Ni alloy layer (20) is too thin, it is difficult to ensure adhesion. If the thickness of the Fe-Ni alloy layer (20) is too thick, the Fe component present in the base steel plate (10) may be exposed to the surface, and corrosion resistance may be compromised. More specifically, the thickness of the Fe-Ni alloy layer (20) may be 0.6 to 2.4 μm. More specifically, the thickness of the Fe-Ni alloy layer (20) may be 0.7 to 2.3 μm.

[0062] The Ni plating layer (30) helps ensure corrosion resistance against the battery electrolyte and the atmosphere. The plating thickness may vary depending on the molding amount and the type of electrolyte, and at least one surface where wear mainly occurs during molding can be plated with a thickness of 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 the thickness variation to be large, so plating can be done through electroplating.

[0063] A Ni-plated steel sheet (100) according to one embodiment of the present invention can simultaneously secure excellent strength, elongation, and in-plane anisotropy. Specifically, the tensile strength at 25°C is 495 MPa or higher. More specifically, the tensile strength at 25°C may be 500 to 600 MPa. More specifically, the tensile strength at 25°C may be 500 to 550 MPa.

[0064] Specifically, the elongation at 25°C is 19.5% or more. More specifically, the elongation at 25°C may be 20.0% to 30.0%. More specifically, the elongation at 25°C may be 20.5% to 28.0%.

[0065] Specifically, the absolute value of the in-plane anisotropy Δr is 0.40 or less. More specifically, the absolute value of the in-plane anisotropy Δr may be 0.01 to 0.40. More specifically, the absolute value of the in-plane anisotropy Δr may be 0.10 to 0.35.

[0066] Tensile strength, elongation, and in-plane anisotropy Δr can be measured through a conventional tensile test at room temperature. In-plane anisotropy can be obtained through the following Equation 1.

[0067] (Relationship 1)

[0068] Δr = (r0 + r 90 - 2×r 45 ) / 2

[0069] (where r0, r 45 , r 90The plastic anisotropy modulus (Lankford value) when tensile in directions forming angles of 0°, 45°, and 90° with the rolling direction, respectively.

[0070] The tensile strength at 300°C is 400 MPa or more. More specifically, the tensile strength at 300°C may be 405 to 450 MPa. More specifically, the tensile strength at 300°C may be 406 to 440 MPa.

[0071]

[0072] A method for manufacturing a Ni-plated steel sheet according to one embodiment of the present invention comprises the steps of: hot-rolling a slab to produce a hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; recrystallizing and annealing the cold-rolled steel sheet; straight-rolling the recrystallized and annealed steel sheet; producing a Ni-plated steel sheet by plating Ni on one or both sides of the straight-rolled steel sheet; and alloying and annealing the Ni-plated steel sheet.

[0073] Below, each step is explained in detail.

[0074] First, hot-rolled steel sheets are manufactured by hot-rolling a slab.

[0075] As the alloy composition of the slab has been explained in the base steel plate (10) of the aforementioned Ni-plated steel plate, a redundant explanation is omitted. Since the alloy components do not substantially change during the manufacturing process of the Ni-plated steel plate, the alloy composition of the base steel plate (10) and the alloy composition of the slab are substantially the same.

[0076] Prior to the step of manufacturing hot-rolled steel sheets, a step of heating the slab at 1200°C or higher may be further included. A temperature of 1200°C or higher is required because various precipitates formed in the steel during slab manufacturing must be re-dissolved. More specifically, it may be heated to 1200 to 1350°C.

[0077] A hot-rolled steel sheet can be manufactured by hot-finish rolling a reheated slab at a temperature of Ar3 or higher and then coiling it at 580 to 720°C. The reason for limiting the hot-rolling finish temperature to Ar3 or higher is to perform rolling in the austenite single-phase region. If rolling is performed in the double-phase region, rolling stability may decrease due to non-uniform material properties. The Ar3 temperature is widely known, and in one embodiment of the present invention, the Ar3 temperature can be calculated as 910 - (310 × [C]) - (80 × [Mn]) - (0.35 × (25.4 - 8)). The above [C] and [Mn] represent the content (weight%) of C and Mn in the slab. More specifically, the hot-rolling finish temperature may be 850°C to 1000°C.

[0078] When coiling after finish rolling, the precipitation behavior of NbC changes depending on the coiling temperature; since NbC does not precipitate properly if the temperature is too low or too high, the coiling temperature can be controlled to 580 to 720°C, which facilitates precipitation. The thickness of the hot-rolled steel sheet can be 2 to 6 mm.

[0079] Next, a cold-rolled steel sheet is manufactured by cold-rolling a hot-rolled steel sheet. An appropriate level of cold reduction ratio is important in terms of both strength and workability. The higher the reduction ratio, the more smoothly recrystallization nucleation occurs during annealing, resulting in grain refinement and increased strength. Additionally, in-plane anisotropy tends to decrease as the reduction ratio increases. However, if it is too high, elongation decreases, which is detrimental to workability, and resistance to deformation increases, leading to reduced productivity. Considering this, the cold reduction ratio is set within the range of 70 to 90%. More specifically, the reduction ratio can be 73 to 85%. A pickling process can be added prior to cold rolling to remove scale generated during hot rolling. The thickness of the cold-rolled steel sheet can be 0.3 to 1 mm.

[0080] The process may further include a step of recrystallizing annealing the cold-rolled steel sheet. The primary purpose of recrystallizing annealing is to remove internal stress formed during cold rolling and to ensure workability. To achieve this, an annealing process at a sufficiently high temperature is required to ensure that recrystallization occurs completely. To induce recrystallization in a cold-rolled steel sheet having a steel composition system according to one embodiment of the present invention, a temperature of 680°C or higher is required, taking into account the increase in the recrystallization temperature caused by NbC. If the temperature is too low, recrystallization is not completely finished and some deformed grains remain, which may lead to a significant decrease in the ductility of the steel sheet and cause cracks during forming due to increased strength. However, if the annealing temperature is too high, it is difficult to secure strength through grain growth, and fracture or shape defects may occur due to a decrease in strength during annealing. Therefore, the cold-rolled steel sheet may be recrystallized annealed at a cracking temperature of 680 to 850°C. More specifically, it may be annealed at a temperature of 720 to 820°C. The cracking time may be 10 to 120 seconds.

[0081] In the recrystallization annealing stage, the cooling rate from the cracking temperature to 500°C may be 10°C / s or less. If the cooling rate is high in this temperature range, the temperature variation in the width direction of the steel sheet increases, which may result in shape defects and variations in material properties of the steel sheet. More specifically, the cooling rate from the cracking temperature to 500°C may be 2 to 9°C / s.

[0082] In the recrystallization annealing stage, the cooling rate at 500°C to 300°C may be 10°C / s or higher. If the cooling rate is low in this temperature range, Fe3C carbide precipitation occurs actively, resulting in less residual solid solution carbon and thus failing to obtain the strength increase effect due to age hardening, which may result in low strength. More specifically, the cooling rate at the cracking temperature at 500°C to 300°C may be 13 to 40°C / s.

[0083] Next, the recrystallized annealed steel sheet is straight-rolled.

[0084] Straight rolling not only corrects the shape but also serves to form dislocations of appropriate density. Dislocations formed during the straight rolling process become locations where carbides are likely to precipitate during the alloying annealing process performed after plating, thereby contributing to additional strength improvement. To achieve this effect, a reduction ratio of 0.5% or more is required. However, if the reduction ratio is too high, it provides a driving force for the recrystallization and coarsening of surface grains during the alloying annealing process, and since locally coarse grains cause workability defects such as local fracture due to uneven elongation, it is necessary to perform it at a level of 1.8% or less. More specifically, the reduction ratio may be 0.6 to 1.7%.

[0085] Next, a Ni-plated steel sheet is manufactured by plating Ni on one or both sides of a straight-rolled steel sheet. Ni plating is required to ensure corrosion resistance against the battery electrolyte and the atmosphere. The plating thickness may vary depending on the forming volume and the type of electrolyte, and at least one side where wear mainly occurs during forming can be plated to a thickness of 2.0 μm or more. More specifically, it can be plated to a thickness of 2.0 to 5.0 μm. In the case of hot-dip galvanizing, it is difficult to control the plating thickness below a certain level and there is a tendency for large thickness variations, so electroplating may be used. General conditions such as the Ni plating bath and current density during electroplating can be used, and a detailed description is omitted.

[0086] Next, the Ni-plated steel sheet is alloyed and annealed.

[0087] The Ni plating layer (30) does not adhere well to the steel plate immediately after plating and may easily detach during processing. To prevent this, it is necessary to form an Fe-Ni alloy layer (20) between the Ni plating layer and the steel plate by diffusion through alloying annealing at a high temperature.

[0088] At this time, the process is maintained at a cracking temperature of 600 to 850°C for 5 to 60 seconds. If the alloying annealing temperature is too low or the time is too short, the Fe-Ni alloy layer becomes thin, making it difficult to ensure adhesion. If the annealing temperature is too high or the time is too long, the alloy layer becomes too thick, causing the Fe component contained in the steel plate to be exposed to the surface of the plating 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 plates manufactured according to the composition and manufacturing conditions described in the present invention, and may not apply to steel plates with different compositions and manufacturing processes. More specifically, the process can be maintained at a cracking temperature of 620 to 750°C for 5 to 60 seconds.

[0089] After alloying annealing, additional cold rolling may be performed in a range of 2.0% or less to correct the shape of the steel sheet.

[0090]

[0091] The present invention will be explained in more detail below through examples. However, these examples are merely for illustrating the invention and the invention is not limited thereto.

[0092]

[0093] Example 1

[0094] A steel grade containing Si: 0.02%, Al: 0.035%, P: 0.01%, S: 0.01%, N: 0.003%, and the remainder being Fe and other unavoidable impurities, with the composition and weight% of Table 1 below, and a steel sheet having the manufacturing conditions of Table 1 below were manufactured. The compositions indicated are actual values, and each slab having the corresponding composition was manufactured. After reheating the slab to 1220°C, it was hot-rolled to a uniform thickness of 4 mm at 900°C or higher, and then coiled at 640°C to manufacture a hot-rolled steel sheet.

[0095] After cold rolling the coiled hot-rolled steel sheet with the cold reduction rate of Table 1, recrystallization annealing was performed for 30 seconds at the annealing temperature of Table 1, and after cooling at the cooling rate of Table 1, upright rolling was performed with an upright reduction rate of 1% to produce a recrystallized annealed steel sheet. After electroplating Ni uniformly to a thickness of 3.0 μm on the recrystallized annealed steel sheet, alloying annealing was performed at a temperature of 750°C for 20 seconds to produce a final Ni-plated steel sheet.

[0096] For each manufactured steel plate, the change in tensile strength, tensile strength at 25°C, elongation at 25°C, absolute value of in-plane anisotropy Δr, and tensile strength at 300°C were measured, and the results are shown in Table 2.

[0097] Tensile strength at 25°C and elongation at 25°C can be obtained through tensile testing according to ASTM E8 / E8M standards at room temperature. Tensile strength at 300°C can be obtained through tensile testing with the tensile specimen heated to the corresponding temperature.

[0098] The absolute value |Δr| of the in-plane anisotropy Δr can be obtained by preparing specimens to form angles of 0°, 45°, and 90° with respect to the rolling direction and performing the above tensile test, and calculating |Δr| = |(r0 + r90 - 2×r45) / 2|. r0, r45, and r90 are the plastic anisotropy coefficients (Lankford values) when tension is applied in directions forming angles of 0°, 45°, and 90° with respect to the rolling direction, respectively.

[0099] Classification C(wt%) Mn(wt%) Nb(wt%) Cold Reduction Rate (%) Annealing Temperature (°C) Cracking Temperature ~ 500°C Cooling Rate (°C / sec) 500 ~ 300°C Cooling Rate (°C / sec) Invention Example 1 0.025 1.00.03080750520 Invention Example 2 0.050 1.00.03080750520 Invention Example 3 0.065 1.00.03080750520 Invention Example 4 0.050 1.00.03080750520 Invention Example 5 0.050 1.30.03080750520 Invention Example 6 0.050 1.50.03080750520 Invention Example 70.0501.00.01580750520Invention Example 80.0501.00.04080750520Invention Example 90.0501.00.03075750520Invention Example 100.0501.00.03085750520Invention Example 110.0501.00.03080700520Invention Example 120.0501.00.03080800520Invention Example 130.050 1.00.03080830520Invention Example 140.0501.00.03080750320Invention Example 150.0501.00.03080750820Invention Example 160.0501.00.03080750515Invention Example 170.0501.00.03080750525Invention Example 180.0501.00.03080750535Comparative Example 10.0151.00. 03080750520Comparative Example 20.075 1.00.03080750520Comparative Example 30.050 0.80.03080750520Comparative Example 40.050 1.70.03080750520Comparative Example 50.050 1.00.00580750520Comparative Example 60.050 1.50.00580750520Comparative Example 70.050 1.00.06080750520

[0100] Classification 25℃ Tensile Strength (MPa) 25℃ Elongation (%) Plastic Anisotropy Absolute Value (|Δr|) 300℃ Tensile Strength (MPa) Invention Example 1505 23.00.22436 Invention Example 2520 23.20.23432 Invention Example 3538 21.20.27423 Invention Example 4518 23.20.28420 Invention Example 5530 22.50.28425 Invention Example 6545 20.60.29430 Invention Example 7538 22.70.30416 Invention Example 8540 23.30.22436 Invention Example 9505 24.60.29410 Invention Example 10519 21.20.18427 Invention Example 11541 22.00.11435 Invention Example 12536 22.40.21422 Invention Example 13512 23.60.26406 Invention Example 14519 23.40.24415 Invention Example 15532 22.40.16439 Invention Example 16502 26.20.26412 Invention Example 17536 23.30.25438 Invention Example 18550 21.20.28452 Comparative Example 1482 24.60.30416 Comparative Example 2542 18.50.23429 Comparative Example 3490 24.60.21416 Comparative Example 4560 17.90.23432 Comparative Example 5480 26.10.42382 Comparative Example 6529 23.00.37386 Comparative Example 7545 19.20.16452

[0101] As shown in Tables 1 and 2, Invention Examples 1 to 16 satisfied all steel composition and manufacturing conditions, and it was confirmed that the tensile strength at 25°C, elongation, in-plane anisotropy, and tensile strength at 300°C were all excellent.

[0102] Comparative Example 1 is a case where the C content is low, and sufficient strength is not secured due to the low C content. On the other hand, Comparative Example 2 is a case where the C content is excessive, and the elongation is low, resulting in poor processability.

[0103] Comparative Example 3 is a case where the Mn content is low, and sufficient strength was not secured. Comparative Example 4 is a case where the Mn content is too high, and the elongation is low, resulting in poor processability.

[0104] Comparative Example 5 is a case where the Nb content is low, and the Nb content is insufficient to form a sufficient amount of NbC. As a result, the tensile strength is low. In Comparative Example 6, the Mn compensates for the low Nb content, so the strength is adequate, but because NbC is not sufficiently formed, the tensile strength at 300°C is low and the in-plane anisotropy Δr is significantly inferior.

[0105] Comparative Example 7 is a case with a high Nb content, in which the tensile strength is excellent but the elongation is low. In addition, there is a problem of poor hot rolling productivity due to the excessive precipitation of NbC, which increases deformation resistance during hot rolling.

[0106]

[0107] The present invention is not limited to the embodiments described above but can be manufactured in various different forms, and those skilled in the art will understand that the invention can be implemented in other specific forms without altering the technical concept or essential features of the invention. Therefore, the embodiments described above should be understood as illustrative in all respects and not restrictive.

[0108] [Explanation of the symbol]

[0109] 100: Ni-plated steel sheet

[0110] 10: Base steel plate

[0111] 20: Fe-Ni alloy layer

[0112] 30: Ni plating layer

Claims

1. A base steel plate, a Ni plating layer located on one or both sides of the surface of the base steel plate, and an Fe-Ni alloy layer located between the base steel plate and the Ni plating layer, The above base steel sheet comprises, in weight percent, C: 0.02 to 0.07%, Mn: 0.9 to 1.6%, Al: 0.01 to 0.06%, and Nb: 0.01 to 0.05%, and the remainder being Fe and other unavoidable impurities, a Ni-plated steel sheet.

2. In Paragraph 1, The above base steel sheet is a Ni-plated steel sheet further comprising one or more of Si: 0.05 wt% or less, P: 0.015 wt% or less, S: 0.015 wt% or less, and N: 0.006 wt% or less.

3. In Paragraph 1, The above base steel sheet is a Ni-plated steel sheet further comprising one or more of Ti: 0.01 wt% or less, Ni: 0.1 wt% or less, Cr: 0.1 wt% or less, Cu: 0.1 wt% or less, and V: 0.01 wt% or less.

4. In Paragraph 1, The above base steel sheet contains NbC precipitates, and is a Ni-plated steel sheet having an average particle size of NbC precipitates of 10 nm or less.

5. In Paragraph 1, Ni-plated steel sheet having a tensile strength of 495 MPa or more at 25°C, an elongation of 19.5% or more at 25°C, an absolute value of in-plane anisotropy Δr of 0.40 or less, and a tensile strength of 400 MPa or more at 300°C.

6. A step of manufacturing a hot-rolled steel sheet by hot-rolling a slab containing, in weight percent, C: 0.02 to 0.07%, Mn: 0.9 to 1.6%, Al: 0.01 to 0.06%, and Nb: 0.01 to 0.05%, and the remainder being Fe and other unavoidable impurities; A step of manufacturing a cold-rolled steel sheet by cold-rolling the above hot-rolled steel sheet; A step of manufacturing a Ni-plated steel sheet by plating Ni on one or both sides of the above cold-rolled steel sheet; and A method for manufacturing a Ni-plated steel sheet comprising the step of alloying and annealing the above Ni-plated steel sheet.

7. In Paragraph 6, A method for manufacturing a Ni-plated steel sheet in which the above slab further comprises one or more of Si: 0.05 wt% or less, P: 0.015 wt% or less, S: 0.015 wt% or less, and N: 0.006 wt% or less.

8. In Paragraph 6, A method for manufacturing a Ni-plated steel sheet, wherein the above slab further comprises one or more of Ti: 0.01 wt% or less, Ni: 0.1 wt% or less, Cr: 0.1 wt% or less, Cu: 0.1 wt% or less, and V: 0.01 wt% or less.

9. In Paragraph 6, Prior to the step of manufacturing the above hot-rolled steel sheet, A method for manufacturing a Ni-plated steel sheet, further comprising the step of heating the above slab at 1200°C or higher.

10. In Paragraph 6, The step of manufacturing the above hot-rolled steel sheet is a method for manufacturing a Ni-plated steel sheet by coiling at a temperature of 520 to 720°C after hot finish rolling at Ar3 or higher.

11. In Paragraph 6, The step of manufacturing the above cold-rolled steel sheet A method for manufacturing a Ni-plated steel sheet by cold rolling with a reduction rate of 75 to 90%.

12. In Paragraph 6, After the step of manufacturing the above cold-rolled steel sheet A method for manufacturing a Ni-plated steel sheet, further comprising the step of recrystallizing the above cold-rolled steel sheet at a cracking temperature of 680 to 850°C.

13. In Paragraph 12, A method for manufacturing a Ni-plated steel sheet in which the cooling rate from the cracking temperature to 500℃ in the above recrystallization annealing step is 10℃ / s or less.

14. In Paragraph 12, A method for manufacturing a Ni-plated steel sheet in which the cooling rate at 500°C to 300°C during the above recrystallization annealing step is 10°C / s or higher.

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