Ni-plated steel sheet for cans and its manufacturing method

Abstract

A Ni-plated steel sheet for cans and a method for manufacturing the same are provided. The Ni-plated steel sheet of the present invention contains, by weight, 0.020 to 0.060% C, 0.050% or less Si, 0.1 to 0.6% Mn, 0.010 to 0.060% Al, 0.015% or less P, 0.015% or less S, 0.006% or less N, and less than 0.030% Cr, with the remainder being Fe and other unavoidable impurities, and has a composition index represented by Relational Formula 1 of 0.85 or less, an ASTM grain size number of 11.0 to 11.7, and a steel microstructure containing carbides with an equivalent circle average size of 1.0 to 2.0 μm, and has an Fe—Ni alloy layer 0.6 to 1.4 μm thick formed on the surface.

Description

[Technical Field]

[0001] The present invention relates to a Ni-plated steel sheet having excellent strength and formability and a method for manufacturing the same, and more particularly to a Ni-plated steel sheet having excellent strength and formability and usable for battery cases of electric vehicles and the like, and a method for manufacturing the same. [Background technology]

[0002] In the case of cylindrical cans used for cylindrical battery cases, steel sheets are generally plated with nickel (Ni) to prevent corrosion caused by the electrolyte injected as the battery contents. Recently, with the increasing demand for electric vehicles, the demand for cylindrical battery case materials for electric vehicles has increased significantly.

[0003] Meanwhile, to ensure battery safety at high temperatures, battery case materials are increasingly required to have strength at high temperatures. When a battery generates heat due to factors such as overcurrent, it can trigger an abnormal chemical reaction, causing the temperature to rise to approximately 600°C. Therefore, battery case materials must have heat resistance so that they can maintain their shape even at high temperatures, as they may be exposed to temperatures of several hundred degrees Celsius for a short period of time. Battery cases can deform due to temperature increases while a vehicle is running, which can affect the vehicle's operation, so deformation at high temperatures must also be strictly controlled to prevent this.

[0004] The heat resistance characteristics of electric vehicle batteries can be evaluated in various ways. For example, electric vehicle batteries are heated to temperatures of around 600°C to evaluate the stability of the cells. To prevent battery deformation, the battery case material must have a yield strength of 80 MPa or more at 600°C.

[0005] In addition to the physical properties at high temperatures, the physical properties at room temperature are also important for battery case materials. If the strength at room temperature is low, loosening may occur after the battery contents are filled, and to prevent this, a certain level of yield strength is required.

[0006] Furthermore, from the perspective of material processability, further room-temperature physical properties are required. Cylindrical battery cases require multi-stage processing steps, such as drawing and ironing, during molding, so in addition to the high-temperature properties, room-temperature processability is also required. Yield strengths that are higher than a certain level should be avoided because they cause severe mold wear during processing, necessitating frequent mold replacement. In addition, a certain level of high elongation is also required to mold the can into the desired shape without cracking.

[0007] Cylindrical battery cases are typically nickel-plated to prevent corrosion from the internal electrolyte and the atmosphere during processing, but the plating layer also has specific requirements. Ni plating is typically applied to a thickness of 1 μm or more, and during processing, to prevent the plating layer from falling off, heat treatment is used to form an Fe-Ni alloy layer at the interface through diffusion between the Fe in the steel sheet and the Ni in the plating layer, improving adhesion. If the alloy layer is too thin, it is difficult to ensure adhesion between the steel sheet and the plating layer. If it is too thick, the Fe component is exposed to the surface of the plating layer, which can lead to Fe oxidation and rust. Therefore, it is important to form the alloy layer to the appropriate thickness.

[0008] Patent Document 1 describes a method of utilizing fine precipitates by adding Nb, Cr, W, etc. to ultra-low carbon steel with C: 0.003% or less to ensure room temperature and high temperature properties. While this method can improve properties through fine precipitates, it has drawbacks in that it requires the addition of expensive precipitate-forming elements and increases the load on the steelmaking process to produce the ultra-low carbon steel.

[0009] Patent Document 2 describes a method of performing secondary rolling on ordinary low-carbon steel with a C content of 0.04 to 0.06% in order to produce high-strength steel sheets for cans. The most common method for increasing the strength of can materials is to perform a secondary reduction of 20 to 30% after recrystallization annealing, which is characterized by the advantage of significantly improving strength through work hardening. However, such a high level of reduction significantly reduces elongation, which has the disadvantage of making it difficult to ensure the workability of cans.

[0010] Patent Document 3 describes a method in which adding a large amount of N (130 ppm or more) improves strength through solid solution strengthening, and applying a low secondary rolling reduction of 20% or less can increase elongation. However, adding a large amount of N, an interstitial element, makes it more likely that compositional variations will occur, and when compositional variations occur, there is a high possibility that material quality variations will also occur. Therefore, there is a disadvantage in that additional efforts are required in the steelmaking process to keep compositional variations low.

[0011] Patent Document 4 describes a method for achieving a balance between strength and ductility by utilizing precipitation strengthening through the addition of Ti and applying a relatively low secondary rolling reduction of 15% or less to reduce the decrease in elongation due to work hardening. 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, reducing cleanliness. A disadvantage of this method is that an increased number of inclusions in steel can serve as the initiation point for cracks during the forming process, requiring additional efforts to remove the inclusions. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Korean Patent Publication No. 2019-0078406 [Patent Document 2] Korean Patent Publication No. 1999-0053991 [Patent Document 3] Korean Patent Publication No. 2018-0109964 [Patent Document 4] Korean Patent Publication No. 2021-0091795 Summary of the Invention [Problem to be solved by the invention]

[0013] An object of the present invention is to produce a Ni-plated steel sheet that is excellent in both strength and formability and can be used as a material for battery cases of electric vehicles.

[0014] Furthermore, the technical problems to be achieved by the present invention are not limited to the above-mentioned technical problems, and other technical problems not mentioned can be clearly understood by a person having ordinary skill in the technical field to which the present invention pertains from the following description. [Means for solving the problem]

[0015] One aspect of the present invention is In weight percent, C: 0.020 to 0.060%, Si: 0.050% or less, Mn: 0.1 to 0.6%, Al: 0.010 to 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.006% or less, Cr: less than 0.030%, with the remainder being Fe and other unavoidable impurities, and the component index represented by the following relational formula 1 is 0.85 or less, ASTM grain size number meets 11.00-11.70, The steel has a microstructure containing carbides with an average circle equivalent size of 1.0 to 2.0 μm, and The present invention relates to a Ni-plated steel sheet for cans, on the surface of which an Fe—Ni alloy layer having a thickness of 0.6 to 1.4 μm is formed.

[0016] [Equation 1] Component index=C×10+Mn Here, C and Mn mean the weight percentages of C and Mn, respectively.

[0017] The Ni-plated steel sheet for cans has a room-temperature yield strength of 320 to 400 MPa, a room-temperature elongation of 30% or more, and a yield strength at 600°C of 80 MPa or more.

[0018] Another aspect of the present invention is a step of reheating the steel slab satisfying the above-mentioned composition and Relation 1 at 1150°C or higher; a step of hot-finish rolling the reheated steel slab at Ar3 or more to produce a hot-rolled steel sheet; cooling the hot-rolled steel sheet and then coiling it at a temperature of 580 to 720°C; a step of performing a first cold rolling on the coiled hot-rolled steel sheet at a rolling reduction of 78 to 90% to produce a first cold-rolled steel sheet; annealing the first cold-rolled steel sheet at a temperature of 630 to 810°C; a step of subjecting the annealed steel sheet to a second cold rolling at a reduction rate of 0.6 to 1.2% to produce a second cold rolled steel sheet; Electroplating the secondary cold-rolled steel sheet with Ni to produce a Ni-plated steel sheet; and and a step of forming an Fe-Ni alloy layer having a thickness of 0.6 to 1.4 μm on the surface of the cold-rolled steel sheet by alloying annealing the Ni-plated steel sheet at a temperature of 650 to 700°C for a time period in which an alloying index defined by the following relational expression 2 satisfies 2.10 to 2.60.

[0019] [Equation 1] Component index=C×10+Mn Here, C and Mn mean the weight percentages of C and Mn, respectively.

[0020] [Equation 2] Alloying index=(2-(700-T) / 100)×t 0.1 Here, T means temperature (°C) and t means time (seconds).

[0021] In the above Ni electroplating, it is preferable to perform Ni electroplating to a thickness in the range of 0.6 to 5.0 μm. [Effects of the Invention]

[0022] According to the present invention, it is possible to provide a Ni-plated steel sheet that has a room temperature yield strength of 320 to 400 MPa, a room temperature elongation of 30% or more, a yield strength at 600°C of 80 MPa or more, and an Fe-Ni alloy layer thickness of 0.6 to 1.4 μm, and that can be used for cylindrical battery cases. DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention will be described below.

[0024] The present invention relates to a Ni-plated steel sheet used for battery cases of electric vehicles after can forming. Materials for such applications require strength at or above an appropriate level to maintain shape at room and high temperatures, and must also optimize the balance between strength and elongation, taking into account the decrease in elongation that occurs when strength is increased to ensure formability. Furthermore, the Ni-plated layer must have a certain level of corrosion resistance to prevent corrosion and adhesion to prevent it from falling off during processing.

[0025] Therefore, the present inventors have conducted extensive research and experiments to achieve the above object, and have found that a plated steel sheet having the above target physical properties can be manufactured by optimizing the types and contents of alloying elements and manufacturing conditions, and have thus presented the present invention.

[0026] Hereinafter, the chemical composition of the cold-rolled steel sheet provided by the present invention will be described in detail. Hereinafter, the content of each component means wt% unless otherwise specified.

[0027] Carbon (C):0.020~0.060% Carbon (C) is an element added to improve the strength of steel sheets. If the carbon content is low, the strength will be low, making it difficult to use as a structural material. Therefore, a carbon content of 0.020% or more is preferred. Furthermore, if the carbon content is reduced to less than 0.020%, the load on the steelmaking process will increase, resulting in reduced productivity. Conversely, if the carbon (C) content is higher than necessary, the strength will be excessively high, which will increase the wear rate of the mold during forming and reduce elongation, thereby reducing formability. Therefore, it is preferred to control the carbon content to 0.060% or less. More preferably, it is controlled to a range of 0.030 to 0.050%.

[0028] Silicon (Si): 0.050% or less Si is an element that can be used as a deoxidizer and contributes to improving strength through solid solution strengthening, so it is difficult to completely eliminate it. However, if it is excessive, Si-based oxides are formed on the surface during annealing, which may induce defects during plating and reduce platability. Therefore, taking this into consideration, the upper limit is preferably set to 0.050% or less. More preferably, it is controlled to 0.030% or less.

[0029] Manganese (Mn): 0.1 to 0.6% Mn is an element that combines with solute S in steel and precipitates as MnS, thereby preventing hot shortness caused by solute S. To achieve this effect, Mn is preferably contained in an amount of 0.1% or more. Mn also dissolves in steel and, together with C, increases the strength of the steel. However, excessive Mn content reduces the workability of the steel, so it is preferable that Mn be contained in an amount of 0.6% or less.

[0030] Aluminum (Al): 0.010 to 0.060% Al is an element with a very strong deoxidizing effect, and by reacting with N in the steel to precipitate AlN, it prevents the deterioration of formability due to solute N. To achieve this effect, the addition of 0.010% or more is necessary. However, if a large amount is added, the effect of further addition is minimal, so the content is limited to 0.060% or less.

[0031] Phosphorus (P): 0.015% or less The addition of P up to a certain amount can increase the strength of steel without significantly reducing its ductility. However, if added in excess of 0.015%, P segregates at grain boundaries, causing excessive hardening of the steel and reducing elongation. Therefore, it is preferable to limit the P content to 0.015% or less.

[0032] Sulfur (S): 0.015% or less Since S is an element that induces red shortness when dissolved, the addition of Mn is necessary to induce the precipitation of MnS. The more S there is, the more Mn must be added, so a large amount is not desirable. Therefore, the upper limit of S is set at 0.015%.

[0033] Nitrogen (N): 0.006% or less N is contained in steel as an element that is unavoidable to remain in the steel, but N present in a solid solution state causes aging and significantly reduces workability. In order to minimize the reduction in ductility due to the occurrence of unnecessary aging, it is preferable to limit the upper limit to 0.006%.

[0034] Chromium (Cr): Less than 0.030% Cr is an element that can improve workability when added in small amounts, and is therefore often added to materials for processing. However, when added, it can concentrate on the surface and form Cr-based oxides, which can reduce conductivity and cause surface defects due to unplated surfaces during electroplating. In particular, when plating thinly to a thickness of a few microns or less, its effect on the surface is significant, so it is preferable to limit its content to less than 0.030%. More preferably, it is controlled to 0.020% or less.

[0035] The component index expressed by the following relational expression 1 is 0.85 or less The inventors have defined and introduced a composition index as shown in the following relational expression 1 as an index having a correlation with the strength increased by C and Mn. If the composition index defined by the following relational expression 1 exceeds 0.85, the yield strength becomes excessively high, which may make processing difficult.

[0036] [Equation 1] Component index = C x 10 + Mn (where C and Mn in the formula mean the weight percentages of C and Mn, respectively)

[0037] In addition to the above composition, the remainder preferably contains Fe and inevitable impurities, and the steel material of the present invention does not exclude the addition of other components. The inevitable impurities mentioned above may be unintentionally mixed in from raw materials or the surrounding environment during the normal steel manufacturing process, and therefore cannot be excluded. The meaning of the inevitable impurities mentioned above is well understood by engineers in the field of normal steel manufacturing.

[0038] Next, the microstructure of the cold-rolled steel sheet of the present invention will be described.

[0039] The cold-rolled steel sheet of the present invention has a mixed structure of ferrite and cementite, and the structure preferably contains, in area %, 90% or more of ferrite and 10% or less of cementite. If the cementite is excessive, cracks are easily induced during working, and workability is reduced.

[0040] The microstructure of the cold-rolled steel sheet of the present invention satisfies the ASTM grain size number of 11.00 to 11.70.

[0041] Furthermore, the microstructure of the cold-rolled steel sheet of the present invention may contain carbides having an average circle-equivalent size of 1.0 to 2.0 μm. Generally, carbon (C) may be combined with iron to exist in the form of carbides. However, if the size of the carbides is small, burrs tend to be generated during shearing during processing, potentially resulting in poor shear surface shape. If the size of the carbides is large, cracks may be induced during forming. In consideration of this, in the present invention, it is preferable to limit the average circle-equivalent size of the carbides to the range of 1.0 to 2.0 μm.

[0042] On the other hand, a 0.6 to 1.4 μm thick Fe—Ni alloy layer is formed on the surface of the cold-rolled steel sheet of the present invention. If the thickness of the Fe—Ni alloy layer is too thin, it becomes difficult to ensure plating adhesion, and if it is too thick, the Fe component contained in the steel sheet may be exposed to the surface of the plating layer, making it difficult to ensure corrosion resistance.

[0043] The Ni-plated steel sheet of the present invention having the alloy composition and microstructure described above satisfies the room-temperature yield strength of 320 to 400 MPa, the room-temperature elongation of 30% or more, and the yield strength at 600°C of 80 MPa or more, and can be effectively used as a Ni-plated steel sheet for cans.

[0044] Next, a method for producing a Ni-plated steel sheet according to one embodiment of the present invention will be described in detail.

[0045] The method for producing a Ni-plated steel sheet for cans of the present invention includes the steps of: reheating a steel slab that satisfies the above-described chemical composition and Relational Formula 1 at 1150°C or higher; hot finish rolling the reheated steel slab at Ar3 or higher to produce a hot-rolled steel sheet; cooling the hot-rolled steel sheet and then coiling it at a temperature of 580 to 720°C; primary cold-rolling the coiled hot-rolled steel sheet at a reduction of 78 to 90% to produce a primary cold-rolled steel sheet; annealing the annealed steel sheet at a temperature of 650 to 700°C for a time period such that an alloying index defined by Relation 2 is 2.1 to 2.6, thereby forming an Fe-Ni alloy layer having a thickness of 0.6 to 1.4 μm on the surface of the cold-rolled steel sheet; secondly cold-rolling the annealed steel sheet at a rolling reduction of 0.6 to 1.2%, to produce a second cold-rolled steel sheet; electroplating the second cold-rolled steel sheet with Ni to produce a Ni-plated steel sheet; and alloying annealing the Ni-plated steel sheet at a temperature of 650 to 700°C for a time period such that an alloying index defined by Relation 2 is 2.1 to 2.6, thereby forming an Fe-Ni alloy layer having a thickness of 0.6 to 1.4 μm on the surface of the cold-rolled steel sheet.

[0046] Slab Reheat First, in the present invention, the steel slab is reheated to a temperature of 1150°C or higher. Since various precipitates formed in the steel during the production of the slab must be redissolved, a temperature of 1150°C or higher is preferred. Preferably, the reheating temperature can be in the range of 1150 to 1300°C.

[0047] hot rolling Next, in the present invention, the reheated slab is hot-finish rolled at a temperature of Ar3 or higher to produce a hot-rolled steel sheet. The reason for limiting the hot-rolling finish temperature to Ar3 or higher is to perform rolling in the austenite single-phase region. When performing dual-phase rolling, there is a possibility that rolling stability will decrease due to inhomogeneous material.

[0048] Hot-rolled steel sheet coiling In the present invention, the hot-rolled steel sheet is cooled and then coiled at 580 to 720°C. The size of the crystal grains in the hot-rolled steel sheet changes depending on the coiling temperature; when the temperature is low, the crystal grains are formed finely, and when the temperature is high, the crystal grains are formed coarsely. Furthermore, if the coiling temperature is excessively high, coarse carbides are formed, which may induce cracks during can processing. In order to obtain crystal grains and carbide sizes suitable for strength and processability, it is preferable to control the coiling temperature of the hot-rolled steel sheet to 580 to 720°C.

[0049] Cold rolling of hot-rolled sheets Next, in the present invention, the coiled hot-rolled steel sheet is subjected to primary cold rolling at a reduction of 78 to 90% to produce a primary cold-rolled steel sheet. The primary cold rolling reduction is important for increasing strength at high temperatures. As the reduction increases, recrystallization nucleation during annealing becomes smoother, resulting in finer crystal grains and increased high-temperature strength. To obtain a sufficient level of high-temperature strength, it is preferable to apply a reduction of 78% or more. However, if it exceeds 90%, the deformation resistance during rolling increases excessively, making rolling difficult and potentially resulting in a poor shape after rolling. In consideration of this, it is preferable to set the cold rolling reduction in the range of 78 to 90%. A pickling process can be added before cold rolling to remove scale formed during hot rolling.

[0050] Annealing of primary cold-rolled steel sheet In the present invention, the above-mentioned primary cold-rolled steel sheet is annealed at a temperature of 630 to 810°C to produce an annealed steel sheet. The main purpose of annealing the cold-rolled steel sheet is to remove internal stress formed during cold rolling and ensure workability. For this purpose, a process of annealing at a sufficiently high temperature is required to ensure complete recrystallization. A temperature of 630°C or higher is required to induce recrystallization in the cold-rolled steel sheet having the composition of the present invention. However, if the temperature exceeds 810°C, it becomes difficult to ensure strength at high temperatures due to grain growth, and there is a possibility that defects such as sheet fracture will be induced due to a decrease in strength during annealing. Therefore, it is preferable to control the temperature below this temperature.

[0051] Secondary rolling of annealed steel sheets Next, the annealed steel sheet is cold-rolled at a reduction of 0.8 to 1.8% to produce a second cold-rolled steel sheet. Since the annealed steel sheet is exposed to high temperatures, it is generally likely to have a poor shape. Therefore, it is preferable to perform second cold rolling at a reduction of 0.8 to 1.8% to improve the shape and impart the desired roughness. If the cold rolling reduction is low, such as less than 0.8%, it is difficult to obtain the effects of improving the shape and imparting roughness, while if it exceeds 1.8%, the strength tends to increase above the desired level.

[0052] Ni plating of secondary cold-rolled steel sheet In the present invention, the secondary cold-rolled steel sheet is electroplated with Ni to produce a plated steel sheet. Ni plating is preferably performed to ensure corrosion resistance to battery electrolytes and the atmosphere. The plating thickness may vary depending on factors such as the amount of forming and the type of electrolyte, and considering the roughness, plating is preferably performed in a thickness range of 0.6 to 5.0 μm. Hot-dip plating is not suitable as a cylindrical battery case material because it is difficult to control the plating thickness below a certain level and tends to result in large thickness variations.

[0053] Alloying annealing of plated steel sheets Next, in the present invention, the above-mentioned plated steel sheet is annealed to produce an alloyed annealed steel sheet. The Ni plating layer does not have good adhesion to the steel sheet immediately after electroplating, and may easily fall off during processing. To prevent this, annealing must be performed at a high temperature to form an Fe-Ni alloy layer between the Ni plating layer and the steel sheet through diffusion. If the alloyed annealing temperature is low, the Fe-Ni alloy layer becomes thin, making it difficult to ensure adhesion. Therefore, annealing at a temperature above a certain level is preferred. On the other hand, if the annealing temperature is high, the alloy layer becomes thick, and the Fe component contained in the steel sheet is exposed at the surface of the plating layer, making it difficult to ensure corrosion resistance. Therefore, annealing at a temperature below a certain level is preferred.

[0054] The present inventors have found that the thickness of the alloy layer that can simultaneously ensure both adhesion and corrosion resistance is 0.6 to 1.4 μm, and that for this purpose, alloying annealing is preferably performed at a temperature of 650 to 700°C for a time that satisfies an alloying index of 2.1 to 2.6, as defined by the following relational expression 2. The present inventors have found that the alloying index, which is linearly proportional to temperature and exponentially proportional to time, correlates with the thickness of the alloy layer, and have formulated the alloying index. They have been able to form an alloy layer of the desired thickness when the alloying index is in the range of 2.10 to 2.60.

[0055] [Equation 2] Alloying index=(2-(700-T) / 100)×t 0.1 Here, T means temperature (°C) and t means time (seconds).

[0056] However, the correlation between the alloying temperature and time and the thickness of the alloy layer is limited to steel sheets manufactured according to the composition and manufacturing conditions described in the present invention, and may not hold for steel sheets with different compositions and manufacturing processes. For example, when alloying annealing is performed without recrystallization annealing after the first cold rolling, if recrystallization and alloying occur together, the diffusion rate at the interface between Fe and Ni becomes high, and the alloy layer tends to be formed with an excessively large thickness. [Example]

[0057] The present invention will be described in detail below with reference to examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in detail, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0058] (Example) Steel sheets were manufactured having the composition shown in Table 1 below and the manufacturing conditions shown in Table 2 below. The components are shown as actual values, and slabs having the corresponding components were manufactured. The slabs were reheated to 1,220°C, hot-rolled to a uniform thickness of 4 mm at 900°C or higher, and then coiled at the temperature shown in Table 2 below. The coiled hot-rolled steel sheets were cold-rolled at the reduction shown in Table 2 below and annealed for 30 seconds at the temperature shown in Table 2 below to manufacture annealed steel sheets. The annealed steel sheets were subjected to a second cold-rolling at the reduction shown in Table 2 below to manufacture secondarily cold-rolled steel sheets. The secondarily cold-rolled steel sheets were similarly Ni-electroplated to a thickness of 1.5 μm and then subjected to alloying annealing for 30 seconds at the temperature shown in Table 2 below to manufacture final plated steel sheets.

[0059] [Table 1]

[0060] [Table 2]

[0061] At this time, the composition index, ASTM grain size number, average carbide size, alloy layer thickness, tensile properties at room temperature and 600°C, coating layer adhesion, and shape fixability of each of the produced steel sheets were calculated or evaluated, and the results are shown in Tables 3 and 4 below.

[0062] The ASTM grain size number is an index correlated with grain size and is calculated and measured from an image obtained by observing the optical microstructure according to ASTM E112 (Standard Test Methods for Determining Average Grain Size). A larger ASTM grain number indicates a smaller average grain size.

[0063] The circle-equivalent average carbide size refers to the average carbide size observed during structural observation of the final manufactured material. Here, carbides refer to all carbides observed through general structural observation, and do not distinguish between phases that include all carbides, such as cementite and pearlite.

[0064] Furthermore, the thickness of the alloy layer refers to the thickness of the composition-change layer located between the steel sheet and the Ni plating layer, where Fe and Ni components coexist due to diffusion, and can be measured by measuring the cross section of the plated steel sheet using a Glow Discharge Spectrometer (GDS) or an Energy Disperse X-ray Spectrometer (EDS). Generally, the composition of the alloy layer is such that the Fe content is higher in the inner part closer to the steel sheet and the Ni content is higher in the outer part closer to the plating layer. In the present invention, the thickness of the alloy layer is defined as the distance between the point where Fe is 5% and the point where Ni is 5% by weight. This is because if each point is defined as a point where the composition is 0%, it is difficult to accurately determine the points, and measurement errors are likely to occur.

[0065] The yield strength and elongation at room temperature were measured by a tensile test at room temperature. The room temperature tensile test specimens were prepared according to JIS-5 standard, and the physical properties were measured by pulling at a rate of 10 mm per minute. The yield strength at 600°C was measured by a high-temperature tensile test in which the test specimen was maintained at 600°C for 10 minutes and then pulled at a rate of 0.04 mm / s. The test specimens for the high-temperature tensile test were standard ASTM E8 plate-type specimens with a width of 12.5 mm. The specimens were heated to 600°C at a heating rate of 5 to 10°C / s, and then held for 10 minutes to ensure uniformity of the test specimen temperature and atmosphere before being pulled.

[0066] The plating adhesion indicates the degree of bonding between the plating layer and the steel sheet, and was evaluated by observing the surface of the cylindrical battery case after molding using an optical microscope. If cracks of 10 μm or more were found on the surface after molding, it was determined that the corrosion resistance of the plating layer had been lost, and the plating adhesion was deemed poor.

[0067] Shape fixability is an index that indicates the degree to which the shape remains unchanged after being molded into a structure at room temperature. It was evaluated by measuring the difference in shape between immediately after molding of a cylindrical battery case and after battery manufacturing and one or more charge / discharge cycles. If there was a difference of 0.3 mm or more in shape between immediately after molding and after manufacturing and charge / discharge, it was determined that the shape fixability was poor.

[0068] In addition, the following cases are noted in the notes: poor steel sheet shape during manufacturing, cracks in the steel sheet during battery case formation, poor shear surface shape during shearing, and insufficient corrosion resistance after plating. Regarding corrosion resistance, if rust spots are observed in a salt spray test (SST), in which a 5 wt% NaCl solution is sprayed at a temperature of 35°C and a relative humidity of 95% for six hours, the product is deemed to not meet the standards. Furthermore, if burrs of 0.3 μm or more are generated during shearing, the sheared shape is deemed to be poor.

[0069] [Table 3]

[0070] [Table 4] JPEG2025537618000005.jpg35167

[0071] As shown in Tables 1-4 above, invention steels 1 to 21 satisfy both the element content and manufacturing conditions proposed in the present invention, and satisfy the element index of 0.85 or less, carbide size of 1.0 to 2.0 μm, and alloy layer thickness of 0.6 to 1.4 μm proposed by the inventors. They also have room temperature yield strengths of 320 to 400 MPa, room temperature elongation rates of 30% or more, and yield strengths at 600°C of 80 MPa or more. They have good mechanical properties, and good plating adhesion and shape fixability. Therefore, it can be confirmed that they meet the required properties as a cylindrical battery case material. Specifically, invention steels 1 to 3 are examples showing the effects of changes in C content, invention steels 4 and 5 are examples showing the effects of changes in Mn content, invention steels 6 and 7 are examples showing the effects of changes in coiling temperature, invention steels 8 to 10 are examples showing the effects of changes in primary cold reduction, invention steels 11 to 13 are examples showing the effects of changes in annealing temperature, invention steels 14 to 16 are examples showing the effects of changes in secondary cold reduction, and invention steels 17 to 19 are examples showing the effects of changes in alloying temperature, and it can be seen that all required properties are met within the range of manufacturing conditions proposed by the inventors.

[0072] In contrast, Comparative Steel 1 had a low carbon content of less than 0.02%, and the grains were large (ASTM grain size less than 11) and the carbides were small (less than 1.0). The large grains resulted in a low room-temperature yield strength of less than 320 MPa and a low yield strength at 600°C of less than 80 MPa, resulting in small carbides and a poor shear shape. Comparative Steel 2 had a carbon content of more than 0.06%, resulting in a composition index of more than 0.85 and a room-temperature strength of more than 400 MPa. Not only was the elongation low (less than 30%), but the carbides were large (more than 2.0 μm), resulting in cracks during processing.

[0073] Comparative Steel 3 had a low Mn content of less than 0.1%, and cracks occurred at the edges of the hot-rolled steel sheet during hot rolling. Brittleness at high temperatures is induced by solute S, and if it combines with Mn and precipitates in the form of MnS, brittleness can be suppressed. However, in this case, the Mn content was low and sufficient precipitation was not possible, which led to cracks.

[0074] Comparative steels 4 to 6 had a composition index exceeding 0.85, and had a yield strength exceeding 400 MPa at room temperature, an elongation rate less than 30%, and cracks occurred during processing.

[0075] In comparative steels 7 and 8, the Cr content exceeded 0.03%, and unplated areas occurred on the surface, resulting in poor plating surfaces. This could lead to rust spots on the exposed steel sheet when unplated areas occurred, making it difficult to ensure corrosion resistance.

[0076] Comparative Steel 9 was produced at a low coiling temperature of less than 580°C. The grains were small, exceeding ASTM grain size number 11.8, and the yield strength at 600°C exceeded 90 MPa, which is advantageous for ensuring safety. However, the average carbide size was small, less than 1.0 μm, resulting in poor shear shape and room-temperature strength exceeding 400 MPa, which may cause severe die wear, making this undesirable. Comparative Steel 10, on the other hand, was produced at a coiling temperature exceeding 720°C. The grains were coarse, less than ASTM grain size number 11.0, and the average carbide size exceeded 2.0 μm. As a result, the yield strength at 600°C was insufficient, less than 80 MPa, the elongation was less than 30%, and cracks occurred during battery case molding.

[0077] Comparative steel 11 had a low primary cold reduction of less than 78%, which resulted in poor grain nucleation and coarse grains with an ASTM grain number of less than 11.0. As a result, the room-temperature yield strength was low at less than 320 MPa, the shape fixability was poor, and the yield strength at 600°C was low, making it difficult to ensure safety at high temperatures. Comparative steel 12 had a primary cold reduction of more than 90%, and the shape of the steel sheet after cold rolling was poor, making it difficult to pass through the subsequent annealing process.

[0078] In Comparative Steel 13, the annealing temperature of the primary cold-rolled steel sheet was low, less than 630°C. Recrystallization did not occur during the annealing process of the primary cold-rolled steel sheet, but did occur during the alloying annealing process. The present invention aimed to achieve the high-temperature accelerated aging effect during alloying annealing, in which solute carbon pins dislocations at high temperatures, increasing strength. However, if recrystallization did not occur during the annealing of the primary cold-rolled steel sheet, it was difficult to achieve this effect. As a result, the room-temperature yield strength was significantly low at less than 250 MPa, and shape fixability was poor. Furthermore, recrystallization occurred during alloying annealing, increasing the diffusion rate and resulting in a thick alloy layer exceeding 1.4 μm. As a result, Fe was exposed on the surface of the coating layer, resulting in poor corrosion resistance. On the other hand, in Comparative Steel 14, the annealing temperature of the primary cold-rolled steel sheet exceeded 810°C, resulting in a large grain size of less than 11.0 ASTM grain size number. As a result, the yield strength at 600°C was low at less than 80 MPa, reducing safety at high temperatures.

[0079] Comparative steels 15 to 18 did not satisfy the alloying index range of 2.1 to 2.6 defined by Relational Formula 2. When the alloying index was less than 2.1, the alloy layer was too thin, resulting in poor adhesion of the plating layer, while when it exceeded 2.6, the alloy layer was too thick, resulting in poor corrosion resistance. Specifically, in Comparative Example 15, even when the temperature was low (less than 650°C) and the annealing time was long (30 seconds), the alloy layer was formed to be excessively thin. However, annealing for a long time (30 seconds or more) required a long manufacturing time, resulting in low productivity. In Comparative Example 18, even when the temperature was above 700°C and the annealing time was short (10 seconds), the alloy layer was formed to be excessively thick. However, when the annealing time was short, it was difficult to control the thickness at high temperatures, resulting in large variations.

[0080] Comparative steel 19 had a secondary cold rolling reduction of less than 0.8%, and the shape after secondary cold rolling was poor. This was because the secondary cold rolling reduction was too low to sufficiently improve the shape of the steel sheet that had been partially deformed during high-temperature annealing. Comparative steel 20 had a secondary cold rolling reduction of more than 1.8%, and the room-temperature yield strength exceeded 400 MPa due to the effect of work hardening during cold rolling, which significantly induced die wear, making it undesirable.

[0081] As described above, the detailed description of the present invention has been given with reference to the preferred embodiment of the present invention, but it goes without saying that a person skilled in the art to which the present invention pertains can make various modifications without departing from the scope of the present invention. Therefore, the scope of the present invention should not be limited to the described embodiment, but should be determined by the claims below as well as equivalents thereof.

Claims

1. In weight percent, C: 0.020 to 0.060%, Si: 0.050% or less, Mn: 0.1 to 0.6%, Al: 0.010 to 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.006% or less, Cr: less than 0.030%, with the remainder being Fe and other unavoidable impurities, and the component index represented by the following relational formula 1 is 0.85 or less, ASTM grain size number is 11.00 to 11.70; The steel has a microstructure containing carbides with an average circle equivalent size of 1.0 to 2.0 μm, and The Ni-plated steel sheet for cans has an Fe-Ni alloy layer of 0.6 to 1.4 μm thickness formed on its surface. [Relationship 1] Component index=C×10+Mn Here, C and Mn mean the weight percentages of C and Mn, respectively.

2. 2. The Ni-plated steel sheet for cans according to claim 1, wherein the plated steel sheet has a microstructure containing, by area %, 90% or more of ferrite and 10% or less of cementite.

3. 2. The Ni-plated steel sheet for cans according to claim 1, wherein the Ni-plated steel sheet for cans satisfies a room-temperature yield strength of 320 to 400 MPa, a room-temperature elongation of 30% or more, and a yield strength at 600°C of 80 MPa or more.

4. a step of reheating a steel slab at 1150°C or higher, the steel slab containing, in weight percent, C: 0.020 to 0.060%, Si: 0.050% or less, Mn: 0.1 to 0.6%, Al: 0.010 to 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.006% or less, Cr: less than 0.030%, and the remainder being Fe and other unavoidable impurities, and the steel slab having a composition index of 0.85 or less represented by the following relational expression 1: The reheated steel slab was ３ a step of performing hot finish rolling to produce a hot-rolled steel sheet; cooling the hot-rolled steel sheet and then coiling it at a temperature of 580 to 720°C; a step of first cold rolling the coiled hot-rolled steel sheet at a reduction ratio of 78 to 90% to produce a first cold-rolled steel sheet; Annealing the first cold-rolled steel sheet at a temperature of 630 to 810°C; a step of subjecting the annealed steel sheet to a second cold rolling at a reduction rate of 0.6 to 1.2% to prepare a second cold rolled steel sheet; Electroplating the secondary cold-rolled steel sheet with Ni to produce a Ni-plated steel sheet; and and forming an Fe—Ni alloy layer having a thickness of 0.6 to 1.4 μm on the surface of the cold-rolled steel sheet by alloying annealing the Ni-plated steel sheet at a temperature of 650 to 700° C. for a time period such that an alloying index defined by the following Relation 2 satisfies 2.10 to 2.60: [Relationship 1] Component index=C×10+Mn Here, C and Mn mean the weight percentages of C and Mn, respectively. [Relationship 2] Alloying index = (2 - (700 - T) / 100) × t ０．１ Here, T means temperature (° C.) and t means time (seconds).

5. 5. The method for producing a Ni-plated steel sheet for cans according to claim 4, wherein the Ni electroplating is performed to a thickness in the range of 0.6 to 5.0 μm.

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