Coated steel sheet and its manufacturing method

Abstract

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

Description

[Technical Field]

[0001] The present invention relates to a plated steel sheet and a method for producing the same, and more particularly to a Ni-plated steel sheet having excellent strength and formability and a method for producing 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 resist corrosion caused by the electrolyte in 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 high-temperature strength. 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 to maintain their shape at high temperatures, as they may be exposed to high temperatures of several hundred degrees for a short period of time. When a vehicle is running, the battery case may deform due to temperature increases, which could affect the vehicle's operation. To prevent this, deformation at high temperatures must be strictly controlled.

[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 cell stability. To prevent battery deformation, it is preferable for the battery case material to have a certain level of yield strength at 600°C or higher. At this time, it is also important to consider that the material thickness decreases by approximately 30 to 50% when the battery case is formed.

[0005] In addition to the high-temperature properties, the room-temperature properties of the battery case material are also important. If the strength at room temperature is low, sagging may occur after the battery contents are filled. To prevent this, a certain level of yield strength is required.

[0006] Furthermore, room-temperature physical properties are also required from the perspective of material processability. 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. If the yield strength is above a certain level, severe mold wear occurs during processing, necessitating frequent mold replacement, which is undesirable.

[0007] 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 the disadvantage of requiring the addition of large amounts of elements that form multiple types of expensive precipitates.

[0008] 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% to produce high-strength steel sheets for cans. The most common method for increasing the strength of can materials involves performing secondary rolling at a reduction rate of 20 to 30% after recrystallization annealing, which offers the advantage of significantly improving strength through work hardening. However, such high levels of reduction have the disadvantage of making it difficult to ensure can formability because the elongation rate decreases significantly.

[0009] Patent Document 3 describes a method for improving strength through solid solution strengthening by adding a large amount of N (130 ppm or more) and increasing elongation by applying a low secondary rolling reduction of 20% or less. However, when adding a large amount of N, which is an interstitial element, compositional deviations can easily occur, and when compositional deviations occur, there is a high possibility that material deviations will also occur. Therefore, there is a disadvantage in that additional efforts are required in the steelmaking process to keep compositional deviations low.

[0010] Patent Document 4 describes a method for ensuring strength through precipitation strengthening by adding Ti, and further reducing the loss of elongation due to work hardening by applying a relatively low secondary reduction rate of 15% or less, thereby ensuring a balance between strength and ductility. However, the addition of Ti has the characteristic of forming many inclusions during the steelmaking process due to its high oxygen affinity, reducing cleanliness. When there are many inclusions in steel, they can become the starting point for cracks during the forming process, which has the disadvantage of requiring additional efforts to remove the inclusions. [Prior art documents] [Patent documents]

[0011] [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]

[0012] According to one embodiment of the present invention, a plated steel sheet and a method for manufacturing the same are provided.

[0013] According to one embodiment of the present invention, there is provided a Ni-plated steel sheet having excellent strength and formability, and a method for producing the same.

[0014] The object of the present invention is not limited to the above-mentioned content, and a person skilled in the art will have no difficulty in understanding further object of the present invention from the entire content of this specification. [Means for solving the problem]

[0015] According to one embodiment of the present invention, a base steel sheet containing, by weight%, C: 0.0010 to 0.0050%, Si: 0.050% or less, Mn: 0.10 to 0.60%, Al: 0.010 to 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.0060% or less, Nb: 0.0100 to 0.0400%, the balance being Fe and unavoidable impurities; an Fe-Ni alloy layer having a thickness of 0.6 to 1.8 μm formed on the base steel sheet; and a Ni plating layer formed on the Fe-Ni alloy layer; The R value defined by the following relational expression 1 is 0 or less, It is possible to provide a plated steel sheet having a room temperature yield strength of 220 to 280 MPa. [Equation 1] R=([C] / 12.011-[Nb] / 92.906)×10 6 (In the formula, [C] and [Nb] are the weight percentages of each element.)

[0016] The R value defined in the above relational expression 1 may be −120.0 or more.

[0017] The base steel sheet may have a total content of solute C and solute N of 4.0 ppm or less.

[0018] The base steel sheet may have an ASTM grain size of 11.0 or more.

[0019] The plated steel sheet may have a yield strength of 100 MPa or more at 600°C after being rolled at a reduction ratio of 30%.

[0020] According to one embodiment of the present invention, a method for manufacturing a steel slab includes, by weight percent, C: 0.0010 to 0.0050%, Si: 0.050% or less, Mn: 0.10 to 0.60%, Al: 0.010 to 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.0060% or less, Nb: 0.0100 to 0.0400%, the balance being Fe and unavoidable impurities, and the steel slab has an R value defined by the following Relation 1 of 0 or less; finish rolling the reheated steel slab; cooling and coiling the finish-rolled steel sheet; cold rolling the coiled steel sheet at a reduction of 78.0 to 90.0%; and recrystallization annealing the cold-rolled steel sheet at a temperature range of 730 to 800°C; Ni electroplating the cold-rolled steel sheet; and A method for producing a plated steel sheet can be provided, which includes the step of: alloying annealing the Ni-electroplated steel sheet at a temperature in the range of 650 to 750°C. [Equation 1] R=([C] / 12.011-[Nb] / 92.906)×10 6 (In the formula, [C] and [Nb] are the weight percentages of each element.)

[0021] The reheating step is carried out in a temperature range of 1180°C or higher, The above finish rolling stage is carried out in a temperature range of Ar3 or higher, The cooling and winding steps can be performed at a temperature range of 580 to 720°C.

[0022] The method may further include pickling the coiled steel sheet before the cold rolling.

[0023] During the Ni electroplating, the plating thickness may be 0.6 μm or more.

[0024] After the recrystallization annealing step, the method may further include a step of rolling at a reduction of 3.0% or less.

[0025] After the alloying annealing step, the method may further include a step of rolling at a reduction rate of 2.0% or less. [Effects of the Invention]

[0026] According to one embodiment of the present invention, a plated steel sheet and a manufacturing method thereof can be provided.

[0027] According to one embodiment of the present invention, it is possible to provide a Ni-plated steel sheet having excellent strength and formability, and a method for manufacturing the same.

[0028] According to one embodiment of the present invention, it is possible to provide a Ni-plated steel sheet having excellent strength and workability and suitable for use as a battery case for an electric vehicle, and a method for producing the same. DETAILED DESCRIPTION OF THE INVENTION

[0029] Hereinafter, preferred embodiments of the present invention will be described. The embodiments of the present invention can be modified in various forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to further explain the present invention in detail to those skilled in the art to which the invention pertains.

[0030] The present invention will be described in detail below.

[0031] A plated steel sheet according to one embodiment of the present invention may include a base steel sheet, an Fe—Al alloy layer, and a Ni plated layer.

[0032] Base steel sheet A base steel sheet according to one embodiment of the present invention can contain, in weight percent, C: 0.0010 to 0.0050%, Si: 0.050% or less, Mn: 0.10 to 0.60%, Al: 0.010 to 0.060%, P: 0.015% or less, S: 0.015 or less, N: 0.0060% or less, Nb: 0.0100 to 0.0400%, with the balance being Fe and unavoidable impurities.

[0033] The composition of the base steel sheet of the present invention will be described in detail below.

[0034] In the present invention, unless otherwise specified, the percentage indicating the content of each element is based on weight.

[0035] Carbon (C):0.0010~0.0050% Carbon (C) is an element added to improve the strength of steel sheets. Low carbon content results in low strength, making it difficult to use as a structural material. Therefore, carbon (C) content can be 0.0010% or more. Furthermore, reducing the carbon (C) content below 0.0010% significantly increases the load on the steelmaking process, reducing productivity. According to one embodiment of the present invention, carbon (C) can be 0.0015% or more. On the other hand, excessively high carbon (C) content can result in excessively high strength, which can increase the die wear rate during forming and reduce elongation, thereby reducing formability. Therefore, the upper limit of carbon (C) can be limited to 0.0050%. In the present invention, carbon (C) combines with co-added Nb to exist mostly in the form of fine NbC precipitates. Fine NbC is stable even at high temperatures and prevents excessive grain growth, thereby contributing to improved strength at high temperatures. According to one embodiment of the present invention, carbon (C) can be 0.0040% or less.

[0036] Silicon (Si): 0.050% or less Silicon (Si) can be used as a decarburizing agent and contributes to improving strength through solid solution strengthening, and therefore can be included in steel. However, excessive Si content can cause Si-based oxides to form on the surface during annealing, which can lead to defects during plating and reduce platability. Therefore, in the present invention, the upper limit is set to 0.050% in consideration of this. According to one embodiment of the present invention, silicon (Si) can be included in an amount of 0.03% or less. Note that 0% is excluded to account for cases where silicon is unavoidably included during the manufacturing process.

[0037] Manganese (Mn): 0.10-0.60% Manganese (Mn) is an element that combines with solute S in steel to precipitate as MnS, preventing hot shortness caused by solute S. To achieve this effect, manganese (Mn) can be contained in an amount of 0.1% or more. It also dissolves in steel and, together with C, increases the strength of the steel. According to one embodiment of the present invention, manganese (Mn) can be contained in an amount of 0.15% or more. However, since an excessive Mn content can reduce the workability of the steel, the upper limit can be limited to 0.60%. According to one embodiment of the present invention, the upper limit can be limited to 0.35%.

[0038] Aluminum (Al): 0.010-0.060% Aluminum (Al) is an element with a very strong deoxidizing effect, and by reacting with N in the steel to precipitate AlN, it is possible to prevent the deterioration of formability due to solute N. To achieve this effect, 0.010% or more of Al can be added. In one embodiment of the present invention, 0.015% or more of Al can be included. However, when added in large amounts, the effect of further addition is minimal, so the content can be limited to 0.060% or less. In one embodiment of the present invention, the upper limit of the aluminum (Al) content can be limited to 0.045%.

[0039] Phosphorus (P): 0.015% or less The addition of a certain amount of phosphorus (P) is an element that can increase the strength of steel without significantly reducing its ductility. However, if the content exceeds 0.015%, it can segregate at grain boundaries, causing the steel to become excessively hard and reducing its elongation. Note that 0% is excluded to take into account cases where P is unavoidably present during the manufacturing process.

[0040] Sulfur (S): 0.015% or less Sulfur (S) is an element that induces red shortness when dissolved, so Mn must be added to induce the precipitation of MnS. The more sulfur (S) there is, the more Mn must be added, so a large amount is undesirable. Therefore, in the present invention, the upper limit of the sulfur (S) content can be limited to 0.015%. Note that 0% is excluded to take into account the possibility that sulfur may be unavoidably contained during the manufacturing process.

[0041] Nitrogen (N): 0.0060% or less Nitrogen (N) present in a solid solution state can cause aging, significantly reducing workability. To minimize the reduction in ductility due to unnecessary aging, its upper limit can be limited to 0.0060%. Note that 0% is excluded because it is an unavoidable element that remains in steel.

[0042] Niobium (Nb): 0.0100~0.0400% Niobium (Nb) can be combined with carbon to precipitate in the form of fine NbC, which is stable at high temperatures. Fine NbC precipitates can suppress grain growth and contribute to improving high-temperature strength. If the niobium (Nb) content is less than 0.0100%, it may be difficult to expect a sufficient increase in high-temperature strength due to NbC. In one embodiment of the present invention, the niobium (Nb) content may be 0.0150% or more. On the other hand, if the niobium (Nb) content exceeds 0.0400%, the deformation resistance during hot rolling may increase significantly, thereby impairing hot rollability. In one embodiment of the present invention, the upper limit may be limited to 0.0350%.

[0043] In addition to the above-mentioned composition, the steel material of the present invention may contain the remaining iron (Fe) and inevitable impurities. The inevitable impurities cannot be excluded because they may be unintentionally mixed in during the normal manufacturing process. Since such impurities are known to anyone skilled in the field of normal steel manufacturing, the details of all of them will not be specifically mentioned in this specification.

[0044] The base steel sheet may have an R value defined by the following relational expression 1 of 0 or less. [Equation 1] R=([C] / 12.011-[Nb] / 92.906)×10 6 (In the formula, [C] and [Nb] are the weight percentages of each element.)

[0045] In the present invention, Relational Formula 1 was introduced as an index for the tendency of C to combine with Nb and precipitate as NbC. When the Nb content is sufficient, C precipitates sufficiently into NbC, minimizing solute C and improving room-temperature workability. When the amount of Nb added is insufficient, C does not exist in the form of NbC and remains partially dissolved in the steel. However, the solute C may cause accelerated aging during high-temperature alloying annealing, significantly increasing the strength of the steel. Since the increase in strength due to aging increases friction with the die during processing and impairs workability, it is necessary to control the relationship between the C and Nb contents so as to minimize solute C. Therefore, in the present invention, when attempting to control the relationship between these contents using the following Relational Formula 1, the value can be limited to 0 or less.

[0046] When the R value defined by Relational Formula 1 exceeds 0, Nb may be insufficient and C may exist in a dissolved state. When solute C is present, the strength during alloying annealing increases significantly, which may impair formability at room temperature. Furthermore, when NbC is insufficient, the grains grow large, which reduces the strength at high temperatures and may reduce stability during product manufacturing. In one embodiment of the present invention, the R value may be -10.0 or less. Meanwhile, in order to prevent a decrease in economic efficiency due to the addition of more Nb than necessary, the lower limit of the R value may be limited to -120.0. According to one embodiment of the present invention, the lower limit of the R value may be -100.0.

[0047] In a base steel sheet according to one embodiment of the present invention, the total content of solute C and solute N may be 4.0 ppm or less.

[0048] If the total content of solute C and solute N exceeds 4.0 ppm, the strength increases due to aging at high temperatures during alloying, which can lead to poor formability. In the present invention, the total content of solute C and solute N can be measured by an internal friction test. According to one embodiment of the present invention, the total content of solute C and solute N can be 3.8 ppm or less.

[0049] The steel microstructure of the present invention will now be described in detail.

[0050] In the present invention, unless otherwise specified, the percentage representing the fraction of the microstructure is based on the area.

[0051] The base steel sheet according to one embodiment of the present invention may have an ASTM grain size of 11.0 or more.

[0052] In the present invention, the size of the grains can be limited by measuring the grains at a point halfway through the steel sheet thickness direction in order to obtain the effect of improving strength by preventing potential movement at high temperatures due to the grain boundaries.

[0053] ASTM grain size number is an index that correlates with grain size and is calculated and measured from images obtained through optical microstructure observation according to ASTM E112 (Standard Test Methods for Determining Average Grain Size). A larger ASTM grain number means a smaller average grain size.

[0054] If the grain size is less than 11.0, there are few grain boundaries, which makes it difficult to effectively block dislocation movement at high temperatures, resulting in reduced high-temperature strength. This reduced high-temperature strength can lead to battery explosions at high temperatures, reducing safety.

[0055] Fe-Ni alloy layer The base steel sheet may include an Fe—Ni alloy layer having a thickness of 0.6 to 1.8 μm formed on the base steel sheet.

[0056] The Fe-Ni alloy layer thickness refers to the thickness of the compositionally changed layer located between the base steel sheet and the Ni coating layer, where Fe and Ni elements coexist due to diffusion. It can be measured by using a Glow Discharge Spectrometer (GDS) or an Energy Disperse X-ray Spectrometer (EDS) on a cross section of the coated steel sheet. Generally, the composition of the Fe-Ni alloy layer is such that the Fe content is higher in the inner part closer to the base steel sheet and the Ni content is higher in the outer part closer to the coating layer. In the present invention, the alloy layer thickness is defined as the distance between the point where the Fe content is 5% by weight and the point where the Ni content is 5% by weight. 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.

[0057] If the thickness of the Fe-Ni alloy layer is less than 0.6 μm, the bonding strength between the plating layer and the steel sheet is weak, which can lead to the plating layer peeling off during processing. According to one embodiment of the present invention, the thickness can be 0.8 μm or more. On the other hand, if the thickness exceeds 1.8 μm, the Fe component of the steel sheet is exposed to the surface, which can lead to rust. To prevent rust, the Ni plating layer can be made thicker, but this is not economical and therefore not preferred. According to one embodiment of the present invention, the thickness can be 1.4 μm or less.

[0058] Ni plating layer The Fe—Ni alloy layer may include a Ni plating layer formed on the Fe—Ni alloy layer.

[0059] The Ni plating layer according to one embodiment of the present invention may be a pure Ni plating layer, or may be a Ni-based plating layer containing 50% or more Ni with the remainder containing other elements.

[0060] The method for producing a plated steel sheet of the present invention will be described in detail below.

[0061] A plated steel sheet according to one embodiment of the present invention can be produced by reheating a steel slab satisfying the alloy composition described above, followed by finish rolling, cooling, coiling, cold rolling, recrystallization annealing, Ni plating, and alloying annealing.

[0062] reheating A steel slab satisfying the alloy composition of the present invention can be reheated to a temperature range of 1180°C or higher.

[0063] The reheating process can be performed to redissolve various precipitates formed in the steel during slab production. For this reason, the reheating temperature can be limited to 1180°C or higher. However, taking into account the manufacturing equipment, the upper limit of the reheating temperature can be limited to 1280°C.

[0064] Finishing rolling The reheated steel slab can be finish rolled in a temperature range of Ar3 or higher.

[0065] During finish rolling, in order to prevent defects in the rolled shape due to the local stress imbalance that occurs in the austenite-ferrite two-phase region, the temperature can be limited to Ar3 or higher to perform rolling in the austenite single-phase region. On the other hand, if the temperature is lower than Ar3, rolling in the two-phase region will be performed, and rolling stability may decrease due to non-uniform material. [formula] Ar3=910-310[C]-80[Mn]-20[Cu]-15[Cr]-55[Ni]-80[Mo]-0.35(t-8) (In the formula, t is the thickness of the steel plate (mm), and [C], [Mn], [Cu], [Cr], [Ni], and [Mo] are the weight percentages of each element.)

[0066] Cooling and winding The finish-rolled steel sheet can be cooled to a temperature range of 580 to 720°C and coiled.

[0067] The grain size of the steel sheet can be controlled by the coiling temperature, and the temperature can be limited to ensure grains and precipitates of appropriate sizes to ensure strength and workability.

[0068] If the coiling temperature is less than 580°C, the crystal grains may be excessively refined, and if the temperature exceeds 720°C, the crystal grains may be excessively coarse.

[0069] On the other hand, in the present invention, the cooling conditions up to the coiling temperature after finish rolling are not particularly limited, but air cooling can be performed.

[0070] cold rolling The coiled steel sheet can be cold rolled at a reduction ratio of 78.0 to 90.0%.

[0071] The reduction ratio during cold rolling is important for increasing strength at high temperatures. A higher reduction ratio facilitates recrystallization nucleation during annealing, resulting in finer crystal grains and increased high-temperature strength. To obtain a sufficient level of high-temperature strength, the reduction ratio can be limited to 78.0% or more. However, if the reduction ratio exceeds 90.0%, the deformation resistance during rolling increases excessively, making rolling difficult and resulting in poor shape after rolling. In the present invention, the reduction ratio may refer to the cumulative reduction ratio.

[0072] According to one embodiment of the present invention, a pickling process may be added before cold rolling to remove scale generated during hot rolling.

[0073] Recrystallization annealing The cold-rolled steel sheet can be recrystallized annealed in a temperature range of 730 to 800°C.

[0074] The recrystallization annealing can be performed to remove internal stress formed during cold rolling to ensure workability, and for this purpose, a process of annealing at a temperature high enough to cause complete recrystallization is required.

[0075] In addition, in the subsequent alloying annealing process, recrystallization and alloying occur simultaneously, which increases the diffusion rate at the interface between Fe and Ni, thereby preventing the desired Fe-Ni alloy layer from being formed too thick. To prevent this, a recrystallization annealing process may be performed separately.

[0076] In the present invention, the recrystallization temperature can be limited to 730°C or higher, taking into account the increase in recrystallization temperature due to NbC. When annealing at temperatures below 730°C, recrystallization is not completely completed, resulting in the presence of some deformed grains, which can significantly reduce the ductility of the steel sheet and increase its strength, resulting in cracks during forming. However, when the temperature during recrystallization annealing exceeds 800°C, it becomes difficult to ensure strength at high temperatures due to grain growth, and the decrease in strength during annealing can lead to fracture or shape defects.

[0077] According to an embodiment of the present invention, after recrystallization annealing, rolling may be further performed at a reduction rate of 3.0% or less in order to correct the shape of the steel sheet.

[0078] Ni plating The recrystallization annealed steel sheet can be Ni electroplated.

[0079] During product manufacturing, Ni plating can be performed to ensure corrosion resistance to electrolytes and the atmosphere.

[0080] In the case of hot-dip plating, it is difficult to control the plating thickness below a certain level, and thickness deviation tends to be large, making it unsuitable for cylindrical battery case materials. Therefore, in the present invention, Ni plating can be performed by electroplating. In the present invention, the electroplating conditions are not particularly limited, and Ni plating can be performed under normal conditions applicable in the same technical field.

[0081] According to one embodiment of the present invention, a pure Ni plating bath can be used for Ni plating, and a Ni-based plating bath containing 50% or more Ni with the remainder containing other elements can be used.

[0082] The plating thickness during Ni plating can vary depending on the amount of molding, the type of electrolyte, etc. In the present invention, considering the roughness, plating can be performed to a thickness of 0.6 μm or more. According to one embodiment of the present invention, the upper limit of the thickness can be 5.0 μm.

[0083] alloying annealing The Ni electroplated steel sheet can be subjected to alloying annealing in the temperature range of 650 to 750°C.

[0084] The Ni-plated layer does not adhere well to the steel sheet immediately after electroplating, and can easily fall off during processing. To prevent this, an Fe-Ni alloy layer can be formed between the Ni-plated layer and the base steel sheet through diffusion by annealing at high temperature.

[0085] If the alloying annealing temperature is less than 650°C, the thickness of the Fe-Ni alloy layer will be insufficient, making it difficult to ensure adhesion. On the other hand, if the annealing temperature exceeds 750°C, the Fe-Ni alloy layer will become excessively thick, exposing the Fe component of the base steel sheet to the surface of the Ni plating layer, which may result in a decrease in corrosion resistance.

[0086] According to an embodiment of the present invention, in order to correct the shape of the steel sheet after alloying annealing, rolling may be further carried out at a reduction rate of 2.0% or less.

[0087] The plated steel sheet of the present invention produced in this manner has a room temperature yield strength of 220 to 280 MPa and a yield strength of 100 MPa or more at 600°C after 30% deformation, and can have excellent strength and workability.

[0088] Room-temperature yield strength was measured by a tensile test using a standard JIS-5 plate specimen at room temperature, stretched at 10 mm / min. High-temperature yield strength was measured by a tensile test at 0.04 mm / s after 30% deformation and holding at 600°C for 10 minutes. The 30% deformation was applied to the specimen before the high-temperature tensile test, taking into account the drawing and ironing processes that reduce the thickness by 30-50% during battery case formation. For consistency in measurements, the inventors applied deformation by rolling at a 30% reduction instead of drawing and ironing. Although the actual deformation amount varies depending on the size of the battery case, the inventors devised a method for evaluating high-temperature yield strength after deformation by standardizing the deformation rate to approximately 30% reduction in order to compare the merits and demerits of materials under the same conditions. [Example]

[0089] The present invention will be described in more 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 more detail, and are not intended to limit the scope of the present invention.

[0090] (Example) Plated steel sheets were produced from steel slabs having the composition shown in Table 1 under the conditions shown in Table 2. The steel slabs were reheated to 1220°C and then hot-rolled to a thickness of 4 mm at temperatures above 900°C, which is a temperature above Ar3, and then coiled at 640°C. All finish rolling was performed at temperatures above Ar3. The coiled steel sheets were cold-rolled with the reduction shown in Table 2 and then annealed for 30 seconds at the recrystallization annealing temperature. Subsequently, Ni electroplating was performed to a thickness of 2.0 μm, and then annealed for 20 seconds at the alloying annealing temperature shown in Table 2.

[0091] [Table 1] [Equation 1] R=([C] / 12.011-[Nb] / 92.906)×10 6 (In the formula, [C] and [Nb] are the weight percentages of each element.)

[0092] [Table 2]

[0093] The ASTM grain size, total content of solute C and solute N, and thickness of the Fe-Ni alloy layer of the produced steel sheets were observed and are shown in Table 3 below. The room temperature yield strength and high temperature yield strength were also measured and shown. In addition, formability, coating layer adhesion, shape fixability, corrosion resistance, and productivity for the steel sheet shape were evaluated and shown.

[0094] First, the contents of solute C and solute N in the present invention were measured by an internal friction test.

[0095] The ASTM grain size number is an index that correlates with grain size and is calculated and measured from images obtained by optical microstructure observation according to ASTM E112 (Standard Test Methods for Determining Average Grain Size). The larger the ASTM grain size number, the smaller the average grain size.

[0096] The Fe-Ni alloy layer thickness refers to the thickness of the composition-change layer located between the base steel sheet and the Ni coating layer, where Fe and Ni elements coexist due to diffusion. The cross section of the coated steel sheet was measured using a Glow Discharge Spectrometer (GDS) or an Energy Disperse X-Ray Spectrometer (EDS). The Fe-Ni alloy layer thickness was defined as the length from the point where Fe was 5% to the point where Ni was 5% by weight, and the thickness relative to this was measured and shown.

[0097] The room temperature yield strength was measured by a tensile test in which a standard JIS-5 test piece was pulled at room temperature at a rate of 10 mm / min, and the high temperature yield strength was measured by a tensile test in which the specimen was rolled at a reduction ratio of 30% and then held at 600°C for 10 minutes, at a rate of 0.04 mm / s.

[0098] Formability was determined by measuring the degree of mold wear during molding of the battery case, and was judged to be poor when the mold wear increased by 20% or more compared to the material of the invention examples, which has a room temperature yield strength of 230 to 270 MPa. The mold wear can be measured by dividing the mold wear thickness by the number of times it has been processed. The worn area can be any area that comes into contact with the material during molding and causes wear, but to reduce measurement deviation, the number of times it has been processed was more than 10,000 times and the average value was calculated.

[0099] Plating adhesion indicates the degree of bonding between the plating layer and the steel sheet, and was evaluated by observing the surface through an optical microscope after forming the cylindrical battery case. If cracks of 10 μm or larger were found on the surface after forming, it was determined that the corrosion resistance of the plating layer had deteriorated, and the plating adhesion was poor. The forming process can be carried out by drawing a material with a thickness of 0.3 to 0.8 mm to a diameter of 20 to 50 mm and a height of 3.0 to 4.0 times the diameter. The researchers formed a cylindrical battery case with a diameter of 21 mm and a height of 70 mm and evaluated the plating adhesion.

[0100] Shape fixability is an index that indicates the degree to which a structure maintains its shape after molding 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 at least one charge / discharge. A battery with a diameter of 21 mm and a height of 70 mm was manufactured using Ni-containing NCM811 anode material, and charged to 95% or more and discharged to 5% or less 100 times. If there was a difference of 0.3 mm or more in shape immediately after molding and after manufacturing and charging / discharging, the shape fixability was deemed poor.

[0101] In the case of corrosion resistance, if rust spots are observed during 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.Corrosion resistance is related to the thickness of the alloy layer, and if the thickness of the Fe-Ni alloy layer is thick, the internal Fe may be exposed to the surface, causing rust to form on the surface.

[0102] In addition, for each process step, in the case of hot-rolled sheets, if the difference between the highest point and the lowest point within a length of 500 mm in the direction perpendicular to the rolling direction is 1 mm or more, the shape is judged to be poor, and in the case of cold-rolled sheets and annealed sheets, if the difference between the highest point and the lowest point within a length of 500 mm in the direction perpendicular to the rolling direction is 0.5 mm or more, the steel sheet shape is judged to be poor.

[0103] [Table 3]

[0104] As shown in Table 3, in the case of the invention examples that satisfy the alloy composition and manufacturing conditions of the present invention, the characteristics proposed in the present invention are satisfied and the physical properties aimed at in the present invention are also secured.

[0105] On the other hand, in Comparative Example 1, the C content was low at less than 0.0010%, and a sufficient amount of NbC could not be formed. As a result, the crystal grains were formed large, with a crystal grain size of less than 11.0 according to the ASTM standard, and the room temperature and high temperature yield strengths did not meet the standards, resulting in poor shape fixability. Because the amount of NbC was small, the Ni diffusion rate was fast during alloying annealing, and the alloy layer thickness exceeded 1.8 μm, resulting in good plating adhesion but poor corrosion resistance.

[0106] In Comparative Examples 2 and 3, the C content was excessive, exceeding 0.0050%, and even when a large amount of Nb was added, the precipitation index exceeded 0. As a result, aging occurred during alloying annealing due to the solute C, and the room temperature yield strength exceeded 300 MPa, resulting in poor formability.

[0107] In Comparative Example 4, the Nb content was low at less than 0.0100%, and the C content was low at 0.0014%, but the precipitation index exceeded 0. As a result, the room temperature yield strength exceeded 280 MPa due to the solute C, resulting in poor formability. In addition, the low NbC precipitation amount caused Ni to diffuse too quickly, resulting in the alloy layer thickness exceeding 1.8 μm, and poor corrosion resistance.

[0108] In Comparative Example 5, the Nb content was excessive, exceeding 0.0400%, and the hot-rolled sheet had a poor shape and could not be subjected to the next process. Nb is not only an expensive element, but also significantly increases deformation resistance during hot rolling, thereby reducing hot-rollability.

[0109] In Comparative Examples 6 to 8, although all the components were within the suggested ranges, the value of Relational Formula 1 exceeded 0. As a result, the total content of solute C or solute N exceeded the limit, and high-temperature aging occurred during alloying annealing, causing the yield strength to rise significantly to over 280 MPa, resulting in poor formability.

[0110] In Comparative Example 9, the Mn content was low at less than 0.10%, and the hot-rolled sheet had a poor shape. Mn combines with S at high temperatures to form MnS, but when the Mn content was low, the unprecipitated S caused embrittlement during hot rolling, resulting in a poor shape of the hot-rolled sheet. For hot-rolled sheets with poor shape, cold rolling was difficult, and the next step could not be carried out.

[0111] In Comparative Examples 10 and 11, the Mn content exceeded 0.60%, and the strength at room temperature was excessively increased due to solid solution strengthening, resulting in poor formability.

[0112] In Comparative Example 12, the primary reduction was low at less than 78.0%, and the crystal grains were large, did not meet the room temperature and high temperature strength standards, and the shape fixability was poor. Because the reduction was low, crystal grain nucleation was not active during recrystallization, and the crystal grains became coarse after recrystallization was completed.

[0113] In Comparative Example 13, the primary reduction was excessively high at over 90.0%, resulting in a small grain size and poor formability due to room temperature strength exceeding 280 MPa. In addition, there was also a problem of poor cold-rolled sheet shape due to the high cold reduction.

[0114] In Comparative Example 14, the recrystallization annealing temperature was low, less than 730°C, and the room temperature yield strength exceeded the range proposed in the present invention, resulting in very poor formability. Microstructural observation of the manufactured steel sheet confirmed that a rolling structure remained, but the grain size could not be measured from the rolling structure. This confirms that the annealing temperature was too low for the steel of the present invention to recrystallize, and that the rolled elongated grains had very low ductility, significantly impairing formability.

[0115] In Comparative Example 15, the recrystallization annealing temperature exceeded 800°C, resulting in coarse grains, low room temperature strength, and poor shape fixability. In addition, the strength of the steel sheet was reduced during the high temperature annealing process, resulting in poor shape of the annealed sheet.

[0116] In Comparative Example 16, the alloying annealing temperature was low at less than 650° C., and the alloy layer was formed to a thickness of less than 0.6 μm, resulting in poor plating adhesion.

[0117] In Comparative Example 17, the alloying annealing temperature was high, exceeding 750°C, and the alloy layer thickness exceeded 1.8µm, so the coating adhesion was good but the corrosion resistance was poor.

[0118] Although the present invention has been described in detail with reference to the above examples, other embodiments are possible, and the spirit and scope of the following claims should not be limited to the examples.

Claims

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

2. The plated steel sheet according to claim 1, wherein the R value defined by the relational expression 1 is −120.0 or more.

3. The plated steel sheet according to claim 1, wherein the base steel sheet has a total content of solute C and solute N of 4.0 ppm or less.

4. The plated steel sheet according to claim 1 , wherein the base steel sheet has an ASTM grain size of 11.0 or more.

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

6. a step of reheating a steel slab containing, by weight %, C: 0.0010 to 0.0050%, Si: 0.050% or less, Mn: 0.10 to 0.60%, Al: 0.010 to 0.060%, P: 0.015% or less, S: 0.015% or less, N: 0.0060% or less, Nb: 0.0100 to 0.0400%, the balance being Fe and unavoidable impurities, and having an R value defined by the following relational expression 1 of 0 or less; finish rolling the reheated steel slab; cooling and coiling the finish-rolled steel sheet; cold rolling the coiled steel sheet at a reduction of 78.0 to 90.0%; and annealing the cold-rolled steel sheet at a temperature of 730 to 800°C; Ni electroplating the cold-rolled steel sheet; and annealing the Ni-electroplated steel sheet at a temperature in the range of 650 to 750°C for alloying. [Relationship 1] R=([C] / 12.011-[Nb] / 92.906)×10 ６ (In the formula, [C] and [Nb] are the weight percentages of each element.)

7. The reheating step is carried out at a temperature range of 1180°C or higher, The finish rolling step is carried out at a temperature range of Ar3 or higher, The method for manufacturing a plated steel sheet according to claim 6, wherein the cooling and winding step is performed at a temperature in the range of 580 to 720°C.

8. The method for producing a plated steel sheet according to claim 6, further comprising the step of pickling the coiled steel sheet before the cold rolling step.

9. The method for producing a plated steel sheet according to claim 6, wherein the Ni electroplating has a plating thickness of 0.6 μm or more.

10. The method for producing a plated steel sheet according to claim 6, further comprising the step of rolling the steel sheet at a reduction of 3.0% or less after the recrystallization annealing step.

11. The method for producing a plated steel sheet according to claim 6, further comprising the step of rolling the steel sheet at a reduction of 2.0% or less after the alloying annealing step.

Images