Nickel-plated steel sheet and method for manufacturing same

Abstract

A Ni-plated steel sheet, according to one embodiment of the present invention, comprises: a base steel sheet; a Ni-plated layer positioned over 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, the base steel sheet comprising, by weight %, 0.02 to 0.07% of C, 0.1 to 0.5% of Mn, 0.01 to 0.06% of Al, 0.02 to 0.15% of Mo, and the remainder being Fe and other unavoidable 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: 0.1 to 0.5%, Al: 0.01 to 0.06%, and Mo: 0.02 to 0.15%, 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: 0.020 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 MoC precipitates, and the average particle size of the MoC precipitates may be 20 nm or less.

[0015] The base steel sheet may have a bainite structure area ratio of 1.0% or less.

[0016] The thickness of the Fe-Ni alloy layer on one side may be 0.5㎛ or more.

[0017] The tensile strength at 25℃ may be 360MPa or more, the tensile strength at 500℃ may be 180MPa or more, and the tensile strength at 600℃ may be 120MPa or more.

[0018] The elongation at 25℃ can be 25% or more.

[0019] 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: 0.1 to 0.5%, Al: 0.01 to 0.06%, and Mo: 0.02 to 0.15%, with 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.

[0020] The slab may further include one or more of Si: 0.05 wt% or less, P: 0.020 wt% or less, S: 0.015 wt% or less, and N: 0.006 wt% or less.

[0021] 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.

[0022] 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.

[0023] The step of manufacturing hot-rolled steel sheets can be performed by rolling at a temperature of 520 to 720°C after hot finish rolling at Ar3 or higher.

[0024] The step of manufacturing cold-rolled steel sheets can be cold-rolled with a reduction rate of 60 to 90%.

[0025] After the step of manufacturing a cold-rolled steel sheet, the method may further include a step of recrystallizing the cold-rolled steel sheet at a cracking temperature of 650 to 850°C; and a step of straightening the recrystallized annealed steel sheet with a reduction rate of 0.5 to 1.8%.

[0026] The alloying annealing step can be performed by annealing the Ni-plated steel sheet at a cracking temperature of 600 to 800°C.

[0027] 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.

[0028] FIG. 1 is a schematic cross-section of a Ni-plated steel sheet according to one embodiment of the present invention.

[0029]

[0030] 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.

[0031] 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.

[0032] Also, unless otherwise specified, % means weight %, and 1 ppm is 0.0001 weight %.

[0033] 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.

[0034] 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.

[0035] 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.

[0036]

[0037] 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.

[0038] 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).

[0039] 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: 0.1 to 0.5%, Al: 0.01 to 0.06%, and Mo: 0.02 to 0.15%, and the remainder is Fe and other unavoidable impurities.

[0040] Below, each component is explained in detail.

[0041] Carbon (C): 0.020 to 0.070 wt%

[0042] 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. Additionally, 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, a bainite structure may be formed, which significantly reduces elongation during cooling. More specifically, C may be included in an amount of 0.023 to 0.060 weight%.

[0043] In one embodiment of the present invention, some of the C combines with a small amount of added Mo to exist in the form of fine MoC precipitates, and the fine MoC can contribute to effective strength improvement by preventing excessive growth of crystal grains.

[0044] Manganese (Mn): 0.1 to 0.50 wt%

[0045] 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 0.150 to 0.300 weight%.

[0046] Aluminum (Al): 0.010 to 0.060 wt%

[0047] 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%.

[0048] Molybdenum (Mo): 0.020 to 0.150 wt%

[0049] Mo can combine with C to precipitate in the form of stable, fine MoC. Fine MoC precipitates, which are stable at high temperatures, inhibit grain growth and effectively hinder dislocation movement, thereby significantly contributing to the improvement of strength at high temperatures. If the amount of Mo is too small, it is difficult to expect a sufficient increase in strength due to MoC. Mo is a representative element that increases the hardenability of steel, but if added in excessive amounts, a bainite structure is easily formed. Since the bainite structure reduces ductility and impedes machinability, it needs to be suppressed. More specifically, Mo may be included in an amount of 0.022 to 0.140 weight%.

[0050] The base steel plate (10) may further include one or more of Si: 0.05 wt% or less, P: 0.020 wt% or less, S: 0.015 wt% or less, and N: 0.006 wt% or less.

[0051] Silicon (Si): 0.050 wt% or less

[0052] 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%.

[0053] Phosphorus (P): 0.020 wt% or less

[0054] 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 0.020 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.

[0055] Sulfur (S): 0.015 wt% or less

[0056] 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.

[0057] Nitrogen (N): 0.0060 wt% or less

[0058] 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.

[0059] 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.

[0060]

[0061] The base steel plate (10) contains MoC precipitates, and the average particle size of the MoC precipitates may be 20 nm or less. By including MoC with an appropriate particle size, strength can be further improved. If the MoC is present in too large a size, the number of MoCs is small, so the movement of dislocations during plastic deformation cannot be effectively suppressed, which may result in inferior strength. More specifically, the average particle size of the MoC precipitates may be 1.0 to 20.0 nm. More specifically, the average particle size of the MoC precipitates may be 5.0 to 18.0 nm. The MoC precipitates can be observed through a cross-section including the thickness direction of the steel plate, specifically the TD plane. By observing the cross-section through high-resolution TEM or SEM, the particle size can be measured by assuming a circle with the same area as the MoC and determining the diameter of that circle. The average particle size refers to the average of the number of precipitates.

[0062] The base steel plate (10) may have a bainite structure area ratio of 1.0% or less. The bainite structure area ratio can be obtained by observing the cross-section of the steel plate through an optical microscope after polishing. The bainite structure has a layered structure in which pearlite phases and cementite phases are repeated, and as a phase contrasting with the pearlite structure, the structure is very dense. The cementite phase constituting the bainite has an average interlayer spacing of 100 nm or less, which is very dense compared to the pearlite structure.

[0063]

[0064] 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.

[0065] The thickness of the Fe-Ni alloy layer (20) may be 0.5 μm or more. 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 2.0 μm. More specifically, the thickness of the Fe-Ni alloy layer (20) may be 0.7 to 1.5 μm.

[0066] 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.

[0067] A Ni-plated steel sheet (100) according to one embodiment of the present invention can simultaneously secure excellent strength, elongation, and tensile strength at room temperature and high temperature. Specifically, the tensile strength at 25°C may be 365.0 to 450.0 MPa, the tensile strength at 500°C may be 180.0 to 230.0 MPa, and the tensile strength at 600°C may be 120.0 to 135.0 MPa.

[0068] More specifically, the tensile strength at 25°C may be 360 ​​MPa or more, the tensile strength at 500°C may be 180 MPa or more, and the tensile strength at 600°C may be 120 MPa or more.

[0069] Specifically, the elongation rate is 25.0% or more. More specifically, the elongation rate may be 26.0% to 35.0%. More specifically, the elongation rate may be 27.0% to 30.0%.

[0070]

[0071] 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.

[0072] Below, each step is explained in detail.

[0073] First, hot-rolled steel sheets are manufactured by hot-rolling a slab.

[0074] 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.

[0075] 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.

[0076] 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 520 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.

[0077] 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 520 to 720°C, which facilitates precipitation. The thickness of the hot-rolled steel sheet may be 2 to 6 mm. More specifically, the coiling temperature may be 540 to 700°C.

[0078] Next, hot-rolled steel sheets are cold-rolled to produce cold-rolled steel sheets. 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, leading to grain refinement and increased strength. Additionally, in-plane anisotropy tends to decrease with higher reduction ratios. However, if the ratio is too high, elongation decreases, which is detrimental to workability, and resistance to deformation increases, resulting in reduced productivity. Considering this, the cold reduction ratio is set within the range of 60 to 90%. More specifically, the reduction ratio can be 70 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.

[0079] Next, the cold-rolled steel sheet is subjected to recrystallization annealing. The primary purpose of recrystallization 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 650°C or higher is required, taking into account the increase in recrystallization temperature caused by MoC. 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 can be recrystallized annealed at a cracking temperature of 660 to 830°C. The cracking time may be 10 to 120 seconds.

[0080] Next, the recrystallized annealed steel sheet is straight-rolled.

[0081] 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%.

[0082] 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 battery electrolytes 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 1.0 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 can be used. General conditions such as the Ni plating bath and current density during electroplating can be used, and a detailed description is omitted.

[0083] Next, the Ni-plated steel sheet is alloyed and annealed.

[0084] 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.

[0085] At this time, if the alloying annealing temperature is low or the time is too short at a cracking temperature of 600 to 800°C, 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 thick, and the Fe component contained in the steel plate is 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, annealing can be performed at a cracking temperature of 620 to 750°C. The annealing time can be maintained for 5 to 60 seconds. More specifically, it can be maintained for 10 to 40 seconds.

[0086] 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.

[0087]

[0088] 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.

[0089]

[0090] Example 1

[0091] A steel grade containing Si: 0.02%, Mn: 0.25%, 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. The coiled hot-rolled steel sheet was cold-rolled at the cold reduction rate of Table 1, then recrystallization annealed for 30 seconds at the temperature of Table 1, and then straight-point rolling was performed at the straight-point reduction rate of Table 1 to manufacture a recrystallization annealed steel sheet. A final Ni-plated steel sheet was manufactured by electroplating Ni to a thickness of 3.0 μm on a recrystallized annealed steel sheet and then performing alloying annealing at the temperature of Table 1 for 20 seconds.

[0092] For each manufactured steel plate, the average MoC grain size, bainite structure area ratio, tensile strength at 25°C, elongation at 25°C, tensile strength at 500°C, tensile strength at 600°C, and Fe-Ni alloy layer thickness were measured, and the results are shown in Table 2.

[0093] The average MoC grain size can be calculated from the sizes of various MoCs found by observing the cross-section of the steel plate using high-resolution TEM or SEM. The bainite structure area ratio was measured by observing the cross-section of the steel plate under an optical microscope after polishing.

[0094] 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 500°C and 600°C can be obtained through tensile testing with the tensile specimen heated to the corresponding temperature.

[0095] The alloy layer thickness refers to the thickness of the compositional change layer located between the steel plate and the Ni plating layer, where Fe and Ni components coexist due to diffusion, and can be measured by GDS (Glow Discharge Spectrometer) or EDS (Energy Disperse X-ray Spectrometer) measurements of the cross-section of the plated steel plate. Generally, the composition of the alloy layer is formed such that the Fe content is high in the interior closer to the steel plate, and the Ni content is high in the exterior closer to the plating layer. In the present invention, the alloy layer thickness is defined as the length from the point of Fe 5% to the point of Ni 5% by weight.

[0096] CMo Coiling Temperature (°C) Reduction Rate (%) Recrystallization Annealing Temperature (°C) Alloying Annealing Temperature (°C) Invention Example 1 0.0 25 0.0 50 6 20 80 750 680 Invention Example 2 0.0 50 0.0 50 6 20 80 750 680 Invention Example 3 0.0 6 5 0.0 50 6 20 80 750 680 Invention Example 4 0.0 50 0.0 25 6 20 80 750 680 Invention Example 5 0.0 50 0.0 50 6 20 80 750 680 Invention Example 6 0.0 50 0.1 0.0 6 20 80 750 680 Invention Example 7 0.0 50 0.1 3 062080750680Invention Example 80.0500.05054080750680Invention Example 90.0500.05070080750680Invention Example 100.0500.05062065750680Invention Example 110.0500.05062085750680Invention Example 120.0500.05062080660680Invention Example 130.0500.05062080750680Invention Example 140.0500.05062080800680Invention Example 150.0 500.05062080830680Invention Example 160.0500.05062080750620Invention Example 170.0500.05062080750680Invention Example 180.0500.05062080750740Comparative Example 10.0150.05062080750680Comparative Example 20.0750.05062080750680Comparative Example 30.0500.01562080750680Comparative Example 40.0500.16062080750680Comparative Example 50.0500.05050080750680Comparative Example 60.0500.05074080750680Comparative Example 70.0500.05062055750680Comparative Example 80.0500.05062095750680Comparative Example 90.0500.05062080630680Comparative Example 100.0500.05062080870680Comparative Example 110.0500.05062080750580Comparative Example 120.0500.05062080750820

[0097] Average MoC Grain Size (nm) Bainite Structure Area Ratio (%) 25℃ Tensile Strength (MPa) 25℃ Elongation (%) 500℃ Tensile Strength (MPa) 600℃ Tensile Strength (MPa) Alloy Layer Thickness (㎛) Invention Example 19.5 0.0 36 9.3 28.0 18 6.5 126.0 0.9 Invention Example 29.5 0.0 38 1.3 27.5 19 5.4 127.8 1.1 Invention Example 39.6 0.0 40 6.8 27.9 20 3.6 129.9 1.0 Invention Example 48.0 0.0 37 0.5 27.8 19 8.7 124.0 1.0 Invention Example 57.5 0.0 38 9.9 28.3 19 7.6 126.8 1.0 Invention Example 69.4 0.1 39 5.5 27.6 20 0.1 125.2 0.9 Invention Example 78. 40.24 16.0 25.6 194.1 28.21.0 Invention Example 85.80.0 380.5 28.2 196.8 123.21.0 Invention Example 9 15.30.0 391.2 28.5 204.5 126.01.0 Invention Example 10 11.20.0 365.1 27.8 180.6 121.61.0 Invention Example 11 11.0 0.0 405.1 25.6 206.9 127.31.0 Invention Example 128.50.0 373.5 27.3 199.7 125.81.1 Invention Example 13 12.50.0 374.5 28.5 205.5 124.61.0 Invention Example 14 13.60.03 87.327.6194.7121.91.0 Invention Example 15 17.10.0371.927.6204.6124.30.9 Invention Example 16 11.20.0380.527.8194.3121.50.6 Invention Example 179.70.0375.127.5199.4124.01.0 Invention Example 188.40.0381.228.1200.5128.41.0 Comparative Example 1 10.80.0349.527.4181.2121.60.9 Comparative Example 2 12.61.8407.622.0208.6128.61.1 Comparative Example 3 6.20.0344.428. 8169.5113.90.9 Comparative Example 47.53.6421.519.5199.6135.61.0 Comparative Example 54.61.2422.123.2185.6122.00.9 Comparative Example 628.40.0355.436.2171.286.61.1 Comparative Example 78.70.0342.630.6165.1110.11.0 Comparative Example 88.40.0436.223.1216.6132.51.0 Comparative Example 97.10.0720.510.5305.2126.51.0 Comparative Example 1023.30.0342.531.6172.5112.61.0 Comparative Example 1 10.00.0388.528.0201.0126.70.3 Comparative Example 1 211.40.0356.628.4175.8121.21.8.

[0098] As shown in Tables 1 to 2, Invention Examples 1 to 18 satisfied both the steel composition and manufacturing conditions, and it was confirmed that both tensile strength and elongation were excellent. In addition, it was confirmed that the adhesion and corrosion resistance were also excellent because the thickness of the Fe-Ni alloy layer was formed appropriately.

[0099] Comparative Example 1 is a case where the C content is low, and because the C content is low, the tensile strength is less than 350 MPa, so sufficient tensile strength was not secured. On the other hand, Comparative Example 2 is a case where the C content is excessive, a large amount of bainite structure is formed, and the elongation at 25°C is inferior.

[0100] Comparative Example 3 is a case where the Mo content is low, and MoC is not sufficiently formed, resulting in inferior tensile strength at 25°C, 500°C, and 600°C. Comparative Example 4 is a case where an excessive amount of Mo is added, resulting in high hardenability and the formation of a large amount of bainite structure, and inferior elongation at 25°C.

[0101] Comparative Example 5 is a case where the coiling temperature is low, and the cooling rate of the hot-rolled plate is fast, resulting in the formation of a large amount of batenitite phase, which led to an inferior elongation at 25°C. Comparative Example 6 is a case where the coiling temperature is high, and coarse MoC was formed. Consequently, the movement of dislocations could not be effectively suppressed, making it difficult to contribute to the improvement of deformation resistance. As a result, the tensile strength at 25°C, 500°C, and 600°C could not be obtained at the desired level.

[0102] Comparative Example 7 is a case where the cold reduction ratio is low, and the tensile strength at 25°C, 500°C, and 600°C was inferior. This result is understood to be the result of grain coarsening caused by a slow recrystallization nucleation rate. Comparative Example 8 is a case where the cold reduction ratio is too high, and the elongation at 25°C was inferior. In addition, due to the high cold reduction ratio, the rolling load during cold rolling is high, resulting in low productivity.

[0103] Comparative Example 9 has a low recrystallization annealing temperature, so recrystallization did not occur, resulting in high tensile strength but very low elongation at 25°C. Comparative Example 10 is a case where the recrystallization annealing temperature was too high at 870°C; although sufficient recrystallization occurred, MoC grew coarsely, so sufficient tensile strength at 25°C, 500°C, and 600°C was not secured.

[0104] Comparative Example 11 is a case where the alloying annealing temperature is low at 580°C, and the Fe-Ni alloy layer thickness is thin because sufficient diffusion does not occur at the Fe-Ni interface. When the alloy layer thickness is low in this way, a problem occurs in which the Ni plating layer easily peels off during forming. Comparative Example 12 is a case where the alloying annealing temperature is excessively high; although a sufficient alloy layer thickness was secured, the tensile strength at 25°C and 500°C was inferior.

[0105]

[0106] 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.

[0107] [Explanation of the symbol]

[0108] 100: Ni-plated steel sheet

[0109] 10: Base steel plate

[0110] 20: Fe-Ni alloy layer

[0111] 30: Ni plating layer

Claims

It comprises 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: 0.1 to 0.5%, Al: 0.01 to 0.06%, and Mo: 0.02 to 0.15%, and the remainder being Fe and other unavoidable impurities, a Ni-plated steel sheet. 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: 0.020 wt% or less, S: 0.015 wt% or less, and N: 0.006 wt% or less. 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. In paragraph 1, The above base steel sheet is a Ni-plated steel sheet containing MoC precipitates, wherein the average particle size of the MoC precipitates is 20 nm or less. In paragraph 1, The above base steel sheet is a Ni-plated steel sheet having a bainite structure area ratio of 1.0% or less. In paragraph 1, Ni-plated steel sheet having an Fe-Ni alloy layer thickness of 0.5㎛ or more on one side. In paragraph 1, Ni-plated steel sheet having a tensile strength of 360 MPa or more at 25°C, a tensile strength of 180 MPa or more at 500°C, and a tensile strength of 120 MPa or more at 600°C. In paragraph 1, Ni-plated steel sheet with an elongation of 25% or more at 25℃. A step of manufacturing a hot-rolled steel sheet by hot-rolling a slab comprising, in weight percent, C: 0.02 to 0.07%, Mn: 0.1 to 0.5%, Al: 0.01 to 0.06%, and Mo: 0.02 to 0.15%, 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. In Paragraph 9, A method for manufacturing a Ni-plated steel sheet, wherein the above slab further comprises one or more of Si: 0.05 wt% or less, P: 0.020 wt% or less, S: 0.015 wt% or less, and N: 0.006 wt% or less. In Paragraph 9, 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. In Paragraph 9, 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. In Paragraph 9, The step of manufacturing the above hot-rolled steel sheet is A method for manufacturing a Ni-plated steel sheet by hot finishing rolling at Ar3 or higher and then coiling at a temperature of 520 to 720℃. In Paragraph 9, 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 60 to 90%. In Paragraph 9, After the step of manufacturing the above cold-rolled steel sheet A step of recrystallizing the above cold-rolled steel sheet at a cracking temperature of 650 to 850℃; A step of straight-rolling a recrystallized annealed steel sheet with a reduction rate of 0.5 to 1.8%; A method for manufacturing a Ni-plated steel sheet including further In Paragraph 9, The above alloying annealing step is a method for manufacturing a Ni-plated steel sheet by annealing the Ni-plated steel sheet at a cracking temperature of 600 to 800°C.

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