Plated steel sheet and method for manufacturing same

Abstract

The present invention relates to a plated steel sheet and a method for manufacturing the same, and more particularly, to a Ni-plated steel sheet having excellent strength and workability, and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a plated steel sheet and a method for manufacturing the same, and more particularly, to a Ni-plated steel sheet having excellent strength and workability and a method for manufacturing the same. Background Art

[0002] In the case of a cylindrical can used for a cylindrical battery case, a method of nickel-plating (Ni) a steel sheet is generally used to resist corrosion caused by electrolyte entering the battery contents. In recent years, with the increasing demand for electric vehicles, the demand for materials for cylindrical battery cases for electric vehicles has increased significantly.

[0003] In addition, in order to ensure battery safety at high temperatures, the requirement for the strength of the battery case material at high temperatures is getting higher and higher. When the battery generates heat due to factors such as overcurrent, an abnormal chemical reaction is caused, and the temperature may rise to about 600°C. Therefore, the battery case material may be instantaneously exposed to a high temperature of several hundred degrees Celsius, so it is necessary to ensure heat resistance characteristics to maintain the shape at high temperatures. When the vehicle is running, the battery case may be deformed due to temperature rise, which may affect the running of the vehicle. Therefore, in order to prevent this, deformation at high temperatures must be strictly suppressed.

[0004] The heat resistance characteristics of an electric vehicle battery can be evaluated by various methods. As an example, the stability of a battery (cell) is evaluated by heating the electric vehicle battery to a temperature of about 600°C. In order to prevent deformation of the battery, it is preferable for the battery case material to ensure a yield strength above a certain level at 600°C. At this time, when the battery case is formed, it is also necessary to consider a reduction in material thickness of about 30 - 50%.

[0005] In addition, in addition to the above physical properties at high temperatures, the physical properties of the battery case material at room temperature are also important. When the strength at room temperature is low, sagging may occur after filling the battery contents, so a yield strength above a certain level is required to prevent this.

[0006] Furthermore, room temperature physical properties are also required in terms of the workability of the material. When forming a cylindrical battery case, multi-stage processing techniques such as drawing and ironing are required. Therefore, in addition to the above high temperature characteristics, processing characteristics at room temperature are also required. When the yield strength is above a certain level, there is a problem that serious die wear occurs during processing and the die needs to be frequently replaced. Therefore, this situation needs to be avoided.

[0007] Patent Document 1 describes a solution of adding Nb, Cr, W, etc. to ultra-low carbon steel with C below 0.003% and using fine precipitates to ensure room temperature and high temperature properties. The properties can be improved by fine precipitates, but there is a drawback that a large amount of various expensive precipitate-forming elements must be added.

[0008] Patent Document 2 describes a method of manufacturing a high-strength steel sheet for cans by subjecting ordinary low-carbon steel with C at the level of 0.04 - 0.06% to secondary rolling. As the most common method for increasing the strength of can materials, it is characterized in that secondary rolling is carried out at a reduction rate of 20 - 30% after recrystallization annealing, so the advantage of significantly increasing the strength by work hardening can be obtained. However, when subjecting to the above-mentioned high level of reduction, the elongation rate is significantly reduced, so there is a drawback that it is difficult to ensure can processability.

[0009] Patent Document 3 describes a method that can improve the strength by solid solution strengthening by adding a large amount of N of 130 ppm or more, and applying a low secondary reduction rate of 20% or less to increase the elongation rate. However, when adding a large amount of N as an interstitial element, composition deviation may easily occur, and when composition deviation occurs, the possibility of material deviation is high. Therefore, in order to control the composition deviation at a low level, there is a drawback that additional efforts are required in the steelmaking process.

[0010] Patent Document 4 describes the following method: ensuring the strength by precipitation strengthening through adding Ti, reducing the reduction of the elongation rate caused by work hardening by further applying a relatively lower secondary reduction rate of 15% or less, and ensuring the balance between strength and ductility. However, the addition of Ti has the following characteristics: due to the high oxygen affinity of Ti, a large amount of inclusions are formed in the steelmaking process, so the cleanliness is reduced. When there are more inclusions in the steel, it may become the starting point of cracks during the forming process, so there is a drawback that additional efforts are required to remove the inclusions.

[0011] [Prior Art Documents]

[0012] [Patent Documents]

[0013] (Patent Document 1) Korean Patent Publication No. 2019-0078406

[0014] (Patent Document 2) Korean Patent Publication No. 1999-0053991

[0015] (Patent Document 3) Korean Patent Publication No. 2018-0109964

[0016] (Patent Document 4) Korean Patent Publication No. 2021-0091795 Summary of the Invention

[0017] (1) Technical problems to be solved

[0018] According to an embodiment of the present invention, an object is to provide a plated steel sheet and a method for manufacturing the same.

[0019] According to an embodiment of the present invention, an object is to provide a Ni-plated steel sheet having excellent strength and workability and a method for manufacturing the same.

[0020] The technical problems of the present invention are not limited to the above. Those skilled in the art to which the present invention pertains can easily understand additional technical problems of the present invention based on the entire text of the specification of the present invention.

[0021] (2) Technical solutions

[0022] According to an embodiment of the present invention, a plated steel sheet can be provided, the plated steel sheet including: a base steel sheet which, in weight %, contains: C: 0.0010 - 0.0050%, Si: 0.050% or less, Mn: 0.10 - 0.60%, Al: 0.010 - 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.0060% or less, Nb: 0.0100 - 0.0400%, the balance being Fe and inevitable impurities; an Fe-Ni alloy layer which is formed on the base steel sheet and has a thickness of 0.6 - 1.8 μm; and a Ni plating layer which is formed on the Fe-Ni alloy layer, the R value defined in the following relational expression 1 being 0 or less, and the plated steel sheet having a room temperature yield strength of 220 - 280 MPa.

[0023] [Relational expression 1]

[0024] R = ([C] / 12.011 - [Nb] / 92.906) × 10 6

[0025] (In the formula, [C] and [Nb] are the weight % of each element.)

[0026] The R value defined in the relational expression 1 can be -120.0 or more.

[0027] The sum of the solid solution C content and the solid solution N content of the base steel sheet can be 4.0 ppm or less.

[0028] The ASTM-based crystal grain size of the base steel sheet can be 11.0 or more.

[0029] After the plated steel sheet is rolled with a reduction rate of 30%, the yield strength at 600°C can be 100 MPa or more.

[0030] According to an embodiment of the present invention, a method for manufacturing a plated steel sheet can be provided. The manufacturing method includes the following steps: reheating a steel slab, which, by weight percentage, contains: C: 0.0010 - 0.0050%, Si: 0.050% or less, Mn: 0.10 - 0.60%, Al: 0.010 - 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.0060% or less, Nb: 0.0100 - 0.0400%, the balance being Fe and inevitable impurities, and the R value defined in the following relational expression 1 is 0 or less; performing finish rolling on the reheated steel slab; cooling and coiling the finish-rolled steel sheet; cold-rolling the coiled steel sheet at a reduction ratio of 78.0 - 90.0%; and performing recrystallization annealing on the cold-rolled steel sheet within a temperature range of 730 - 800°C; performing Ni electroplating on the cold-rolled steel sheet; and performing alloying annealing on the Ni-electroplated steel sheet within a temperature range of 650 - 750°C.

[0031] [Relational Expression 1]

[0032] R = ([C] / 12.011 - [Nb] / 92.906) × 10 6

[0033] (In the formula, [C] and [Nb] are the weight percentages of the respective elements.)

[0034] The reheating step can be performed within a temperature range of 1180°C or higher, the finish rolling step can be performed within a temperature range above Ar3, and the cooling and coiling step can be performed within a temperature range of 580 - 720°C.

[0035] Before the cold-rolling step, a step of pickling the coiled steel sheet can be further included.

[0036] During the Ni electroplating, the plating thickness can be 0.6 μm or more.

[0037] After the recrystallization annealing step, a step of rolling at a reduction ratio of 3.0% or less can be further included.

[0038] After the alloying annealing step, a step of rolling at a reduction ratio of 2.0% or less can be further included.

[0039] (III) Beneficial Effects

[0040] According to an embodiment of the present invention, a plated steel sheet and a method for manufacturing the same can be provided.

[0041] According to an embodiment of the present invention, a Ni-plated steel sheet with excellent strength and workability and a method for manufacturing the same can be provided.

[0042] According to an embodiment of the present invention, a Ni-plated steel sheet excellent in strength and workability and usable as a battery case of an electric vehicle or the like, and a method for manufacturing the same can be provided. Best Mode for Carrying Out the Invention

[0043] Preferred specific embodiments of the present invention will be described below. The specific embodiments of the present invention can be modified into various forms, and the scope of the present invention should not be construed as being limited to the specific embodiments described below. These specific embodiments are provided to more specifically explain the present invention to those skilled in the art.

[0044] Hereinafter, the present invention will be described in detail.

[0045] The plated steel sheet according to an embodiment of the present invention may include a base steel sheet, an Fe—Al alloy layer, and a Ni plating layer.

[0046] Base Steel Sheet

[0047] In terms of weight %, the base steel sheet according to an embodiment of the present invention may contain: C: 0.0010 - 0.0050%, Si: 0.050% or less, Mn: 0.10 - 0.60%, Al: 0.010 - 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.0060% or less, Nb: 0.0100 - 0.0400%, the balance being Fe and inevitable impurities.

[0048] Hereinafter, the composition of the base steel sheet of the present invention will be described in detail.

[0049] In the present invention, unless otherwise specified, % representing the content of each element is based on weight.

[0050] Carbon (C): 0.0010 - 0.0050%

[0051] Carbon (C) is an element added to improve the strength of the steel plate. When the content is low, the strength is low and it is difficult to be used as a structural member. Therefore, the content of the carbon (C) can be 0.0010% or more. In addition, when the content of the carbon (C) is less than 0.0010%, in order to reduce the content, the load of the steelmaking process increases significantly, so the productivity decreases. According to an embodiment of the present invention, the content of the carbon (C) can be 0.0015% or more. On the other hand, when the content of the carbon (C) is too high, due to the too high strength, during forming, the mold wear rate increases and the elongation rate decreases, and the formability may decrease. Therefore, the upper limit of the content of the carbon (C) can be limited to 0.0050%. In the present invention, the carbon (C) combines with the added Nb and mostly exists in the form of fine NbC precipitates. The fine NbC is also stable at high temperatures and can help improve the strength at high temperatures by preventing excessive grain growth. According to an embodiment of the present invention, the content of the carbon (C) can be 0.0040% or less.

[0052] Silicon (Si): 0.050% or less

[0053] Silicon (Si) is an element that can be used as a decarburizer. Since it can help improve the strength through solid solution strengthening, the silicon (Si) can be included in the steel. However, when the content of the silicon (Si) is too high, Si-based oxides are generated on the surface during annealing, and defects are caused during plating, and the plating property may decrease. Therefore, in the present invention, considering this, the upper limit of the content of the silicon (Si) can be limited to 0.050%. According to an embodiment of the present invention, the content of the silicon (Si) can be 0.03% or less. In addition, considering the inevitable inclusion during the manufacturing process, 0% is excluded.

[0054] Manganese (Mn): 0.10 - 0.60%

[0055] Manganese (Mn) is an element that combines with the dissolved S in the steel and precipitates in the form of MnS, thereby preventing the hot shortness caused by the dissolved S. In order to achieve this effect, the content of the manganese (Mn) can be 0.1% or more. In addition, the manganese (Mn) also has the effect of dissolving in the steel and improving the strength of the steel together with the C. According to an embodiment of the present invention, the content of the manganese (Mn) can be 0.15% or more. However, when the content of the manganese (Mn) is too high, the workability of the steel may decrease. Therefore, the upper limit of the content of the manganese (Mn) can be limited to 0.60%. According to an embodiment of the present invention, the upper limit of the content of the manganese (Mn) can be limited to 0.35%.

[0056] Aluminum (Al): 0.010 - 0.060%

[0057] Aluminum (Al) is an element with a very high deoxidation effect. It reacts with N in steel to precipitate AlN, thereby preventing the reduction of formability caused by dissolved N. To obtain the above effect, more than 0.010% of aluminum (Al) can be added. As an embodiment of the present invention, the content of the aluminum (Al) can be 0.015% or more. However, when added in large amounts, the effect of further addition is very small, so the content of the aluminum (Al) can be limited to 0.060% or less. As an embodiment of the present invention, the upper limit of the content of aluminum (Al) can be limited to 0.045%.

[0058] Phosphorus (P): 0.015% or less

[0059] Adding a certain amount of phosphorus (P) can increase strength without significantly reducing the ductility of the steel. However, when the content of the phosphorus (P) exceeds 0.015%, the phosphorus (P) segregates at the grain boundaries, causing the steel to overharden, and the elongation rate may decrease. Additionally, considering the inevitable inclusion during the manufacturing process, 0% is excluded.

[0060] Sulfur (S): 0.015% or less

[0061] Sulfur (S) is an element that causes red hot brittleness during solid solution, so the precipitation of MnS must be induced by adding Mn. As the content of sulfur (S) increases, a corresponding level of Mn must be further added, so it is not preferable to have a large amount of sulfur (S). Therefore, in the present invention, the upper limit of the content of sulfur (S) can be limited to 0.015%. Additionally, considering the inevitable inclusion during the manufacturing process, 0% is excluded.

[0062] Nitrogen (N): 0.0060% or less

[0063] Nitrogen (N) present in a solid solution state causes aging, which may significantly reduce workability. To minimize the reduction of ductility caused by the occurrence of aging at an unnecessary level, the upper limit of the nitrogen (N) can be limited to 0.0060%. Additionally, nitrogen (N) is an inevitable element remaining in the steel, so 0% is excluded.

[0064] Niobium (Nb): 0.0100 - 0.0400%

[0065] Niobium (Nb) can combine with C and precipitate in the form of fine and stable NbC at high temperatures. The fine NbC precipitates can help improve the high-temperature strength by inhibiting grain growth. When the content of niobium (Nb) is less than 0.0100%, it may be difficult to expect a sufficient effect of increasing the high-temperature strength due to NbC. As an embodiment of the present invention, the content of the niobium (Nb) can be 0.0150% or more. Additionally, when the content of niobium (Nb) exceeds 0.0400%, the deformation resistance increases significantly during hot rolling, and it may hinder the hot rollability. As an embodiment of the present invention, the upper limit of the content of the niobium (Nb) can be limited to 0.0350%.

[0066] In addition to the above components, the steel of the present invention may contain the balance of iron (Fe) and inevitable impurities. During the normal manufacturing process, it may be inevitable to mix in unwanted impurities, so these impurities cannot be completely excluded. These impurities are well known to those skilled in the art, and thus all of their details are not particularly mentioned in this specification.

[0067] The R value defined in the following relational expression 1 of the base steel plate can be 0 or less.

[0068] [Relational Expression 1]

[0069] R = ([C] / 12.011 - [Nb] / 92.906) × 10 6

[0070] (In the formula, [C] and [Nb] are the weight percentages of the respective elements.)

[0071] In the present invention, relational expression 1 is introduced as an index of the tendency for C and Nb to combine and precipitate in the form of NbC. When the content of Nb is sufficient, by allowing C to precipitate sufficiently in the form of NbC, the solid-solution C can be minimized, thereby improving the cold formability. When the addition amount of Nb is insufficient, C cannot exist in the form of NbC and remains in a state of being partially dissolved in the steel. When the solid-solution C undergoes accelerated aging at high temperatures during alloying annealing at high temperatures, there may be a problem of a significant increase in the strength of the steel. For the strength increase caused by aging, the friction with the die increases during processing and hinders the workability. Therefore, it is necessary to control the content relationship between C and Nb to minimize the solid-solution C as much as possible. Thus, in the present invention, if the relationship between these contents is to be controlled by the following relational expression 1, its value can be limited to 0 or less.

[0072] When the R value defined in Equation 1 exceeds 0, there is a shortage of Nb, and C may exist in a solid-solved state. When there is solid-solved C, the strength increases significantly during alloying annealing, which may hinder formability at room temperature. In addition, when NbC is insufficient, the grains grow significantly, and the strength at high temperatures also decreases, and the stability may decrease when manufacturing products. As an embodiment of the present invention, the R value can be -10.0 or less. Additionally, in order to prevent a reduction in economy due to an excessive addition amount of Nb, the lower limit of the R value can be limited to -120.0. According to an embodiment of the present invention, the lower limit of the R value can be -100.0.

[0073] According to an embodiment of the present invention, the sum of the solid-solved C content and the solid-solved N content of the base steel plate can be 4.0 ppm or less.

[0074] When the sum of the solid-solved C content and the solid-solved N content exceeds 4.0 ppm, during alloying, aging occurs at high temperatures to increase strength, and there may be a problem of poor formability. In the present invention, the solid-solved C content and the solid-solved N content can be measured by an Internal friction test. According to an embodiment of the present invention, the sum of the solid-solved C content and the solid-solved N content can be 3.8 ppm or less.

[0075] Hereinafter, the fine structure of the steel of the present invention will be described in detail.

[0076] In the present invention, unless otherwise specifically stated, the % representing the fraction of the fine structure is based on the area.

[0077] According to an embodiment of the present invention, the ASTM-based crystal grain size of the base steel plate can be 11.0 or more.

[0078] In the present invention, by measuring the grains at the 1 / 2 position in the thickness direction of the steel plate, its size can be restricted to obtain the effect of improving strength by hindering the dislocation movement at grain boundaries at high temperatures.

[0079] The ASTM grain size number is an index related to the grain size, which is calculated and measured according to the ASTM E112 standard (Standard Test Methods for Determining Average Grain Size) through an image obtained by optical fine structure observation. The larger the ASTM grain size, the smaller the average grain size.

[0080] When the crystal grain size is less than 11.0, due to fewer grain boundaries, dislocation movement at high temperatures cannot be effectively hindered, and there may be a problem of reduced high-temperature strength. When the high-temperature strength decreases, the battery may explode at high temperatures, and safety may be reduced.

[0081] Fe-Ni alloy layer

[0082] It may include an Fe-Ni alloy layer formed on the base steel plate with a thickness of 0.6 - 1.8 μm.

[0083] The thickness of the Fe-Ni alloy layer refers to the thickness of the compositional change layer where Fe and Ni components coexist by diffusion between the base steel plate and the Ni coating. The cross-section of the coated steel plate can be measured using a Glow Discharge Spectrometer (GDS) or an Energy Disperse X-ray Spectrometer (EDS). Generally, for the composition of the Fe-Ni alloy layer, the Fe content is high in the inner part close to the base steel plate, and the Ni content is high in the outer part close to the coating. In the present invention, in terms of weight%, the thickness of the alloy layer is defined as the length from the position where Fe is 5% to the position where Ni is 5%. When the position where the composition is defined as 0% at each position, it is difficult to determine the accurate position, and measurement errors are likely to occur.

[0084] When the thickness of the Fe-Ni alloy layer is less than 0.6 μm, the bonding force between the coating and the steel plate is weak, and there is a problem of coating peeling during processing. According to an embodiment of the present invention, the thickness of the Fe-Ni alloy layer can be 0.8 μm or more. On the other hand, when the thickness of the Fe-Ni alloy layer exceeds 1.8 μm, the Fe component of the steel plate is exposed on the surface, and rusting may occur. To inhibit rusting, the thickness of the Ni coating can be plated thicker, but the economy is reduced, so it is not preferred. According to an embodiment of the present invention, the thickness of the Fe-Ni alloy layer can be 1.4 μm or less.

[0085] Ni coating

[0086] It may include a Ni coating formed on the Fe-Ni alloy layer.

[0087] According to an embodiment of the present invention, the Ni coating can be a pure Ni coating, or a Ni-based coating with a Ni content of 50% or more and containing the remaining other elements.

[0088] Hereinafter, the manufacturing method of the coated steel plate of the present invention will be described in detail.

[0089] The plated steel sheet according to an embodiment of the present invention can be manufactured by reheating, finish rolling, cooling, coiling, cold rolling, recrystallization annealing, Ni plating, and alloying annealing of a steel slab satisfying the above alloy composition.

[0090] Reheating

[0091] The steel slab satisfying the alloy composition of the present invention can be reheated to a temperature range of 1180 °C or higher.

[0092] In order to redissolve various precipitates generated in the steel during slab manufacturing, a reheating process can be performed. For this effect, the reheating temperature can be limited to 1180 °C or higher. Additionally, considering the aspects of manufacturing equipment, the upper limit of the reheating temperature can be limited to 1280 °C.

[0093] Finish rolling

[0094] The reheated steel slab can be finish rolled within a temperature range above Ar3.

[0095] In order to suppress rolling shape defects caused by stress imbalance at each part generated in the austenite and ferrite two-phase region during finish rolling, and in order to perform rolling in the austenite single-phase region, the finish rolling temperature can be limited to above the Ar3 temperature. On the other hand, when the finish rolling temperature is lower than Ar3, two-phase region rolling is performed, and due to non-uniform material quality, the rolling stability may decrease.

[0096] [Formula]

[0097] Ar3 = 910 - 310[C] - 80[Mn] - 20[Cu] - 15[Cr] - 55[Ni] - 80[Mo] - 0.35(t - 8)

[0098] (In the formula, t is the thickness of the steel sheet (mm), and [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are the weight percentages of each element.)

[0099] Cooling and coiling

[0100] The finish-rolled steel sheet can be cooled and coiled within a temperature range of 580 - 720 °C.

[0101] The grain size of the steel sheet can be controlled by the coiling temperature. In order to ensure strength and workability, and in order to ensure grains and precipitates of appropriate size, its temperature can be limited.

[0102] When the coiling temperature is lower than 580 °C, the grains will not be overly refined. When the coiling temperature exceeds 720 °C, overly large grains may not be formed.

[0103] In addition, in the present invention, the cooling conditions from finish rolling to coiling temperature are not particularly limited, but air cooling can be carried out.

[0104] Cold rolling

[0105] The steel sheet after the coiling can be cold rolled at a reduction ratio of 78.0 - 90.0%.

[0106] During cold rolling, the reduction ratio is important for improving the strength at high temperatures. As the reduction ratio increases, recrystallization nucleation during annealing proceeds smoothly, the grains become fine, and the high-temperature strength tends to increase. In order to obtain a sufficient level of high-temperature strength, the reduction ratio can be limited to 78.0% or more. However, when the reduction ratio exceeds 90.0%, the deformation resistance caused by rolling increases excessively, so it is difficult to roll, and the shape after rolling may deteriorate. In the present invention, the reduction ratio may refer to the cumulative reduction ratio.

[0107] According to an embodiment of the present invention, by adding a pickling process before cold rolling, the scale generated during hot rolling can be removed.

[0108] Recrystallization annealing

[0109] The steel sheet after the cold rolling can be subjected to recrystallization annealing in the temperature range of 730 - 800°C.

[0110] The recrystallization annealing can be carried out to remove the internal stress formed during cold rolling and ensure workability. For this purpose, an annealing process at a sufficiently high temperature is required for complete recrystallization to occur.

[0111] In addition, as a subsequent process, when alloying annealing is carried out, recrystallization and alloying occur simultaneously, and the diffusion rate at the interface between Fe and Ni becomes faster. In order to prevent the thickness of the desired Fe-Ni alloy layer from becoming too thick, the recrystallization annealing process can be carried out separately.

[0112] In the present invention, considering the increase in the recrystallization temperature caused by NbC, the temperature can be limited to 730°C or more. When annealing at a temperature lower than 730°C, since recrystallization is not completed, there are partially deformed grains, the ductility of the steel sheet is significantly reduced, and cracks may occur during forming due to the increase in strength. However, during recrystallization annealing, when the temperature exceeds 800°C, it is difficult to ensure the strength at high temperatures, and fracture or shape defects may occur during the annealing process due to the decrease in strength.

[0113] According to an embodiment of the present invention, in order to correct the shape of the steel sheet, it can be further rolled at a reduction ratio of 3.0% or less after recrystallization annealing.

[0114] Ni plating

[0115] The steel sheet after the recrystallization annealing can be Ni electroplated.

[0116] When manufacturing the product, in order to ensure the corrosion resistance to the electrolyte and the atmosphere, Ni plating can be carried out.

[0117] In the case of hot dip plating, it is difficult to control the plating thickness below a certain thickness, and the thickness deviation tends to become large, which is not suitable for use as a cylindrical battery case material. Therefore, in the present invention, Ni plating can be carried out by electroplating. In the present invention, the electroplating conditions are not particularly limited, and Ni plating can be carried out under the conventional conditions applicable in the same technical field.

[0118] According to an embodiment of the present invention, when Ni plating, a pure Ni plating bath with a Ni content of 50% or more can be used, and a Ni-based plating bath containing the balance of other elements can also be used.

[0119] When Ni plating, the plating thickness can vary according to the forming amount and the type of electrolyte, etc. In the present invention, considering the roughness, the plating can be carried out with a thickness of 0.6 μm or more. According to an embodiment of the present invention, the upper limit of the plating thickness can be 5.0 μm.

[0120] Alloying annealing

[0121] The steel sheet after the Ni electroplating can be subjected to alloying annealing in the temperature range of 650 - 750 °C.

[0122] The adhesion of the Ni coating to the electroplated steel sheet is not excellent, and it may be easily peeled off during processing. To prevent this, by high-temperature annealing, an Fe-Ni alloy layer can be formed by diffusion between the Ni coating and the base steel sheet.

[0123] When the alloying annealing temperature is lower than 650 °C, the thickness of the Fe-Ni alloy layer is insufficient, and it may be difficult to ensure the adhesion. On the other hand, when the alloying annealing temperature exceeds 750 °C, the Fe-Ni alloy layer is too thick, and the Fe component of the base steel sheet is exposed to the surface of the Ni coating, so the corrosion resistance may be reduced.

[0124] According to an embodiment of the present invention, in order to correct the shape of the steel sheet after alloying annealing, it can be further rolled with a reduction rate of 2.0% or less.

[0125] The room temperature yield strength of the plated steel sheet of the present invention manufactured as described above can be 220 - 280 MPa, the yield strength at 600 °C after 30% deformation is 100 MPa or more, and it can have excellent characteristics of strength and workability.

[0126] The room temperature yield strength is measured by a tensile test in which a conventional JIS-5 standard plate specimen is stretched at a rate of 10 mm / min at room temperature, and the high temperature yield strength is measured by a tensile test at a rate of 0.04 mm / sec after maintaining at 600 °C for 10 minutes after 30% deformation. Considering the deep drawing and ironing processes, when forming the battery case, 30% deformation is applied to the specimen before the high temperature tensile test to reduce the thickness by 30-50%. In order to ensure the consistency of measurement, the inventor of the present invention performs rolling at a reduction rate of 30% to apply the deformation amount instead of the deep drawing and ironing processes. Although the actual deformation amount varies depending on the size of the battery case, in order to compare the advantages and disadvantages of materials under the same conditions, the inventor of the present invention designed a method for evaluating the high temperature yield strength after deformation by unifying the strain corresponding to a reduction rate of about 30%. Detailed Description of the Invention

[0127] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention for more detailed description and do not limit the scope of the rights of the present invention.

[0128] (Example)

[0129] A steel slab having the composition shown in Table 1 below was used to manufacture a plated steel sheet under the conditions shown in Table 2 below. The steel slab was reheated to 1220 °C and then hot rolled at a constant thickness of 4 mm at a temperature of 900 °C or higher, which is above Ar3, and then coiled at 640 °C. Among them, the finish rolling temperature was above the Ar3 temperature. The coiled steel sheet was cold rolled at the reduction rate shown in Table 2 and then annealed for 30 seconds at the recrystallization annealing temperature. Thereafter, Ni electroplating was performed to a thickness of 2.0 μm, and annealing was performed for 20 seconds at the alloying annealing temperature shown in Table 2.

[0130] [Table 1]

[0131]

[0132] [Relationship 1]

[0133] R = ([C] / 12.011 - [Nb] / 92.906) × 10 6

[0134] (In the formula, [C] and [Nb] are the weight percentages of the respective elements.)

[0135] [Table 2]

[0136]

[0137] For the manufactured steel plates, the ASTM crystal grain size, the sum of the solid-solution C content and the solid-solution N content, and the thickness of the Fe-Ni alloy layer were observed and shown in Table 3 below. The room-temperature yield strength and the high-temperature yield strength were measured and shown. In addition, the formability, the coating adhesion, the shape freezing property, the corrosion resistance, and the productivity of the steel plate shape were evaluated and shown.

[0138] First, in the present invention, the solid-solution C content and the solid-solution N content can be measured by an internal friction test.

[0139] The ASTM crystal grain size is an index related to the grain size, which is calculated and measured according to the ASTM E112 standard (Standard Test Method for Measuring Average Grain Size) based on the image obtained by optical microstructure observation. The larger the ASTM crystal grain size, the smaller the average grain size.

[0140] The thickness of the Fe-Ni alloy layer refers to the thickness of the compositional change layer in which Fe and Ni components coexist by diffusion between the base steel plate and the Ni coating. The cross-section of the coated steel plate can be measured using a glow discharge spectrometer (GDS) or an energy dispersive X-ray spectrometer (EDS). In terms of weight %, the Fe-Ni alloy layer thickness is defined as the length from the position where Fe is 5% to the position where Ni is 5%, and the Fe-Ni alloy layer thickness was measured and shown.

[0141] The room-temperature yield strength is measured by a tensile test in which a plate-shaped specimen conforming to the conventional JIS-5 specimen standard is stretched at a speed of 10 mm / min at room temperature, and the high-temperature yield strength is measured by a tensile test at a speed of 0.04 mm / s after rolling at a reduction ratio of 30% and holding at 600 °C for 10 minutes.

[0142] The formability is judged by measuring the degree of die wear when forming a battery case. Compared with the material of the inventive example having a room-temperature yield strength of 230 - 270 MPa, when the die wear increases by more than 20%, it is judged to be poor. The die wear can be measured by dividing the wear thickness of the die by the number of processing times. The wear part can be any part that comes into contact and causes wear during forming, but in order to reduce the measurement deviation, the processing is carried out more than 10,000 times and the average value is calculated.

[0143] The plating adhesion indicates the degree of bonding between the plating layer and the steel sheet, which is observed and evaluated by an optical microscope after the cylindrical battery case is formed. When observing after forming, if cracks of 10 μm or more exist in the surface part, the corrosion resistance of the plating layer is judged to be poor, and the plating adhesion is judged to be poor. The forming can be carried out under the condition of deep-drawing a material with a thickness of 0.3 - 0.8 mm into a diameter of 20 - 50 mm and a height 3.0 - 4.0 times the diameter. The researchers in this study processed it into a diameter of 21 mm and a height of 70 mm for forming, so as to evaluate the plating adhesion.

[0144] The shape freeze resistance is an index indicating the degree to which the shape remains unchanged and is maintained after being formed into a structure at room temperature, and can be evaluated by measuring the shape difference after the cylindrical battery case is formed and after the battery is manufactured and charged and discharged more than once. A battery with a diameter of 21 mm and a height of 70 mm is made using an NCM811 cathode material containing Ni, and 100 charge and discharge cycles are carried out in the way of charging to more than 95% and discharging to less than 5%. At this time, if the shape difference is 0.3 mm or more after forming and after manufacturing and charging and discharging, the shape freeze resistance is judged to be poor.

[0145] In the case of corrosion resistance, when rust is observed through a salt spray test (SST, Salt Spray Test) of spraying a 5 wt% NaCl aqueous solution at a temperature of 35 °C and a relative humidity of 95% for 6 hours, it is judged not to meet the standard. The corrosion resistance is related to the thickness of the alloy layer. When the thickness of the Fe-Ni alloy layer is relatively thick, the internal Fe may be exposed on the surface, so rust may occur on the surface.

[0146] In addition, in the case of the hot-rolled sheet in each process step, when the difference between the highest point and the lowest point within a length of 500 mm in the rolling vertical direction is 1 mm or more, the shape is judged to be poor. In the case of the cold-rolled sheet and the annealed sheet, when the difference between the highest point and the lowest point within a length of 500 mm in the rolling vertical direction is 0.5 mm or more, the steel sheet shape is judged to be poor.

[0147] [Table 3]

[0148]

[0149] As shown in Table 3, in the case of the inventive examples that meet the alloy composition and manufacturing conditions of the present invention, the features proposed by the present invention are met, and the physical properties desired by the present invention can also be ensured.

[0150] On the other hand, in Comparative Example 1 where the C content is low and less than 0.0010%, sufficient NbC cannot be formed. As a result, larger grains are formed, the crystal grain size based on ASTM is less than 11.0, and the yield strength at normal temperature and high temperature fails to meet the standard, resulting in poor shape freezing property. Since there is less NbC, during alloying annealing, the diffusion rate of Ni is fast, and the thickness of the alloy layer exceeds 1.8 μm. Therefore, the plating adhesion is good, but the corrosion resistance is poor.

[0151] In Comparative Examples 2 and 3 where the C content exceeds 0.0050% and the C content is too high, even if a large amount of Nb is added, the precipitation index will exceed 0. Therefore, during alloying annealing by solid-solution C, aging occurs, and the yield strength at normal temperature exceeds 300 MPa, resulting in poor formability.

[0152] In Comparative Example 4 where the Nb content is low and less than 0.0100%, although the C content is low at the level of 0.0014%, the precipitation index exceeds 0. Therefore, due to solid-solution C, the yield strength at normal temperature exceeds 280 MPa, and the formability is poor. In addition, since the precipitation amount of NbC is small, the diffusion of Ni is too fast, resulting in the thickness of the alloy layer exceeding 1.8 μm, and the corrosion resistance is also poor.

[0153] In Comparative Example 5 where the Nb content is excessive and exceeds 0.0400%, the shape of the hot-rolled sheet is poor and it cannot enter the next process. Nb is not only an expensive element, but also the deformation resistance during hot rolling increases significantly, thereby reducing the hot rollability.

[0154] In Comparative Examples 6 to 8, although all components meet the proposed range, the value of Relational Expression 1 exceeds 0. Therefore, when the sum of the solid-solution C or solid-solution N content is too high and alloying annealing is carried out, high-temperature aging occurs, the yield strength increases significantly and exceeds 280 MPa, resulting in poor formability.

[0155] In Comparative Example 9 where the Mn content is low and less than 0.10%, the shape of the hot-rolled sheet is poor. Mn combines with S to form MnS at high temperature. When Mn is less, due to the un-precipitated S, brittleness occurs during hot rolling, resulting in a poor shape of the hot-rolled sheet. For a hot-rolled sheet with a poor shape, it is difficult to cold roll and cannot enter the next process.

[0156] In Comparative Examples 10 and 11 where the Mn content exceeds 0.60%, due to solid-solution strengthening, the strength at normal temperature increases excessively, resulting in poor formability.

[0157] In Comparative Example 12 where the primary reduction rate is low and less than 78.0%, large grains are formed, the strength standards at normal temperature and high temperature are not reached, and the shape freezing property is poor. Due to the low reduction rate, during recrystallization, the grain nucleation is not active, so the grains become coarser after recrystallization is completed.

[0158] Comparative Example 13 is a case where the single-pass reduction rate is too high and exceeds 90.0%. The grain size is small, the room temperature strength exceeds 280 MPa, and thus the formability is poor. In addition, due to the high cold rolling reduction rate, there is also a problem of poor shape of the cold rolled sheet.

[0159] Comparative Example 14 is a case where the recrystallization annealing temperature is low and lower than 730 °C. The room temperature yield strength exceeds the range proposed by the present invention, and the formability is very poor. For the manufactured steel sheet, the residual rolling structure was confirmed by microstructure observation. Due to the rolling structure, the grain size could not be measured. It can be confirmed therefrom that the steel of the present invention cannot be recrystallized due to the too low annealing temperature, and the ductility of the rolled tensile grains is very low, thus greatly hindering the formability.

[0160] In Comparative Example 15, since the recrystallization annealing temperature exceeds 800 °C, coarse grains are formed, the room temperature strength is low, and the shape freezing property is poor. In addition, during the annealing process at a high temperature, the strength of the steel sheet decreases, and there is also a problem of poor shape of the annealed sheet.

[0161] In Comparative Example 16, the alloying annealing temperature is low and lower than 650 °C, the formed alloy layer is very thin, and the thickness of the alloy layer is less than 0.6 μm, so the plating adhesion is poor.

[0162] In Comparative Example 17, it was manufactured with an alloying annealing temperature higher than 750 °C. The thickness of the alloy layer exceeds 1.8 μm, and the plating adhesion is good, but the corrosion resistance is poor.

[0163] As described above, the present invention has been described in detail by way of examples, but other forms of examples may also be included. Therefore, the technical idea and scope of the claims are not limited to the examples.

Claims

1. A plated steel sheet, comprising: A base steel sheet, by weight %, the base steel sheet contains: C: 0.0010 - 0.0050%, Si: 0.050% or less, Mn: 0.10 - 0.60%, Al: 0.010 - 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.0060% or less, Nb: 0.0100 - 0.0400%, the balance being Fe and unavoidable impurities; An Fe-Ni alloy layer, the Fe-Ni alloy layer is formed on the base steel sheet and has a thickness of 0.6 - 1.8 μm; and A Ni plating layer, the Ni plating layer is formed on the Fe-Ni alloy layer, The R value defined in the following relational expression 1 is 0 or less, The normal temperature yield strength of the plated steel sheet is 220 - 280 MPa, [Relationship 1] R = ([C] / 12.011 - [Nb] / 92.906) × 10 6 In the formula, [C] and [Nb] are the weight % of each element.

2. The coated steel sheet according to claim 1, wherein The R value defined in the relational expression 1 is -120.0 or more.

3. The plated steel sheet according to claim 1, wherein, The sum of the solid solution C content and the solid solution N content of the base steel sheet is 4.0 ppm or less.

4. The plated steel sheet according to claim 1, wherein, The ASTM-based crystal grain size of the base steel sheet is 11.0 or more.

5. The plated steel sheet according to claim 1, wherein, After the plated steel sheet is rolled with a reduction ratio of 30%, the yield strength at 600 °C is 100 MPa or more.

6. A method for manufacturing a plated steel sheet, comprising the following steps: Reheating a steel billet, by weight %, the steel billet contains: C: 0.0010 - 0.0050%, Si: 0.050% or less, Mn: 0.10 - 0.60%, Al: 0.010 - 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.0060% or less, Nb: 0.0100 - 0.0400%, the balance being Fe and unavoidable impurities, the R value defined in the following relational expression 1 is 0 or less; Rough rolling the reheated steel billet; Cooling and coiling the rolled steel sheet; Cold rolling the coiled steel sheet with a reduction ratio of 78.0 - 90.0%; And Performing recrystallization annealing on the cold-rolled steel sheet in the temperature range of 730 - 800 °C; Performing Ni electroplating on the cold-rolled steel sheet; and Performing alloying annealing on the Ni-electroplated steel sheet in the temperature range of 650 - 750 °C, [Relationship 1] R = ([C] / 12.011 - [Nb] / 92.906) × 10 6 In the formula, [C] and [Nb] are the weight % of each element.

7. The manufacturing method of the plated steel sheet according to claim 6, wherein, The reheating step is performed in a temperature range of 1180 °C or more, the rough rolling step is performed in a temperature range above Ar3, and the cooling and coiling step is performed in a temperature range of 580 - 720 °C.

8. The manufacturing method of the plated steel sheet according to claim 6, wherein, Before the cold rolling step, it further includes a step of pickling the coiled steel sheet.

9. The manufacturing method of the plated steel sheet according to claim 6, wherein, During the Ni electroplating, the plating thickness is 0.6 μm or more.

10. The manufacturing method of the plated steel sheet according to claim 6, wherein, After the recrystallization annealing step, it further includes a step of rolling with a reduction ratio of 3.0% or less.

11. The manufacturing method of the plated steel sheet according to claim 6, wherein, After the alloying annealing step, it further includes a step of rolling with a reduction ratio of 2.0% or less.

Images