Cold-rolled steel sheet and manufacturing method thereof

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof and, more particularly, to: a cold-rolled steel sheet suitable for use as a steel material for automobile reinforcements, such as bumper beams, sill side beams, and pillars, or as a steel material for side frames and cross members; and a manufacturing method thereof.

Description

Cold-rolled steel sheet and method of manufacturing the same

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more specifically, to a cold-rolled steel sheet suitable for use as steel for automotive reinforcing materials such as bumper beams, sill side beams, and pillars, or as steel for side frames, cross members, etc., and a method for manufacturing the same.

[0002] For steel materials primarily used as reinforcement components related to the crash safety of automobile passengers, it is required to possess high processing characteristics, particularly excellent bending characteristics, and ultra-high strength. Accordingly, ultra-high strength steel with a tensile strength of 1470 MPa or higher, manufactured using a single martensite phase, is already being mass-produced and applied as automotive reinforcement materials. However, there is a need to develop ultra-high strength steel with a tensile strength of 1700 MPa or higher to protect passengers and electric vehicle batteries.

[0003] Recently, the Hot Press Forming (HPF) method has been developed, in which a material is formed using a die at a high temperature—an environment conducive to forming—and then water-cooled to secure the required strength. Since the HPF method can secure high strength relative to the same thickness, it is widely used in the manufacture of parts; however, the HPF method has the disadvantage of requiring excessive equipment investment and increased process costs. Therefore, there is a need to develop materials for cold stamping and roll forming. Specifically, there is a need to develop cold-rolled steel sheets that are suitable for use in cold stamping and roll forming, possess ultra-high strength of 1700 MPa or higher and a high yield ratio to secure crash performance for the protection of passengers and electric vehicle batteries, and have excellent bending characteristics and shape for forming parts.

[0004] Patent Document 1 is a representative prior art of this method. Patent Document 1 relates to a steel having a single-phase martensitic structure containing C: 0.25~0.4%, Si: 1.0% or less, Mn: 1.5~2.5%, P: 0.02% or less, S: 0.003% or less, Al: 0.01~0.1%, N: 0.005% or less, B: 0.0005~0.005%, and also containing Ti: 0.005~0.1%, Nb: 0.005~0.1%, and a total of 0.005~0.1%. It discloses that the steel can be obtained by heating and holding the steel in a temperature range above the Ae3 transformation point and below 900°C, then rapidly cooling it to below 200°C at an average cooling rate of 300°C / s, and subsequently tempering it at below 250°C. However, in the case of Patent Document 1, there is a problem in that defects occur during molding because the shape (flatness) is inferior due to rapid cooling (water cooling).

[0005] Patent Document 2 relates to a thin steel sheet having a high-strength structure comprising C: 0.05% or more and 0.35% or less, Si: 0.01% or more and 2.0% or less, Mn: 0.8% or more and 3.0% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.005% or more and 0.10% or less, and N: 0.0060% or less, with a ferrite area ratio of 0% or more and 90% or less, a bainite area ratio of 5% or less (including 0%), a martensite and tempered martensite area ratio of 10% or more (including 100%), and a retained austenite area ratio of 2.0% or less (including 0%), a standard deviation of yield strength in the width direction of 30 MPa or less, and a maximum bending amount of the steel sheet when sheared at a length of 1 m of 10 mm or less. However, in the case of Patent Document 2, there is a problem that shape defects occur due to rapid cooling after annealing.

[0006] Patent Document 3 contains, in weight percent, C: 0.2~0.4%, Si: 0.5% or less (excluding 0%), Mn: 1.0~2.0%, P: 0.03% or less (excluding 0%), S: 0.015% or less (excluding 0%), Al: 0.1% or less (excluding 0%), Cr: 0.5% or less (excluding 0%), Mo: less than 0.2% (excluding 0%), Ti: 0.1% or less (excluding 0%), Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; the microstructure consists of a tempered martensite single-phase structure or a mixed structure of martensite + tempered martensite, and the microstructure contains FHAGB per unit area of ​​45㎛ × 45㎛ This relates to an ultra-high strength cold-rolled steel sheet with excellent hole expansion properties, having an area of ​​60% or more and an LHAGB of 8mm or more. However, in the case of Patent Document 3, there is no specification regarding the bending characteristics that are essential for improving the part forming, impact characteristics, and hydrogen embrittlement of ultra-high strength steel.

[0007] Meanwhile, the introduction of martensite is essential for manufacturing ultra-high-strength steel with a tensile strength of 1,700 MPa or higher. Such steel is prone to brittle fracture caused by hydrogen remaining within or introduced from the outside; this phenomenon is referred to as hydrogen embrittlement. Hydrogen embrittlement causes material failure at a strength lower than the fracture threshold, meaning the material can fracture due to hydrogen embrittlement even when a very small stress is applied compared to the actual fracture strength. In particular, this hydrogen embrittlement becomes more sensitive as the strength of the steel increases. Furthermore, since resistance to hydrogen embrittlement improves with superior bending characteristics even with the same initial hydrogen content, it is necessary to improve the bending properties of the steel.

[0008] [Prior Art Literature]

[0009] (Patent Document 1) Japanese Patent Publication No. JP 2010-248565

[0010] (Patent Document 2) Japanese Patent Publication No. JP 2020-019992

[0011] (Patent Document 3) Korean Published Patent Application No. 2023-0043267

[0012] One aspect of the present invention is to provide a cold-rolled steel sheet and a method for manufacturing the same.

[0013] An advantageous aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet with excellent bending characteristics and a method for manufacturing the same.

[0014] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0015] One embodiment of the present invention comprises, in weight%, carbon (C): 0.320~0.450%, silicon (Si): 0.020~0.60%, manganese (Mn): 0.30~2.30%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.0050% or less (excluding 0%), aluminum (Al): 0.0050~0.080%, chromium (Cr): 0.0010~0.50%, molybdenum (Mo): 0.0010~0.350%, niobium (Nb): 0.0010~0.050%, titanium (Ti): 0.0050~0.250%, boron (B): 0.00050~0.0050%, nitrogen (N): 0.010% or less (excluding 0%), It is divided into a core containing copper (Cu): 0.0010~0.30%, nickel (Ni): 0.0010~0.30%, and the remainder being Fe and other unavoidable impurities; and a surface layer formed on the outer side of the core in the thickness direction; wherein, when the core is measured by EBSD, the number fraction of the region with a KAM value of 0~5.0° is set to 100%, and the number fraction of the region with a KAM value of 0~1.0° is set to 100%. 01A cold-rolled steel sheet is provided having a surface layer of 15~30%, and the surface layer includes a decarburized layer, with an average thickness (Dave) of the decarburized layer of 20~80㎛.

[0016] (However, the above surface layer is the region up to 80㎛ in the thickness direction from the surface of the cold-rolled steel sheet.)

[0017] The above cold-rolled steel sheet can satisfy the following equations 1 to 3.

[0018] [Relation 1] 60.0 ≤ H = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb + 10000B + 14.5Ni + 9.6Cu ≤ 180.0

[0019] [Equation 2] 0.30 ≤ Cs = C + 0.03Mn + 0.02Si ≤ 0.550

[0020] [Equation 3] 0.0020 ≤ Cs / H ≤ 0.00750

[0021] (However, the content of the alloying elements described in the above equations 1 to 3 is in weight%.)

[0022] The microstructure of the above-mentioned core may include, in area %, a total of one or more of ferrite and bainite: 5% or less (including 0%), and the remainder being one or more of fresh martensite and tempered martensite.

[0023] The above center has an average GND value (GNDave) of 280×10 12 ~325×10 12 m -2 It could be.

[0024] When measuring the above center with EBSD, the number fraction of the region where the KAM value is 0 to 5.0° is set to 100%, and the average value (KAMave) of the above KAM value may be 1.50 to 1.90°.

[0025] The above center can satisfy the following relationship 4.

[0026] [Relationship 4] 6.0×10 12 % / ㎡ ≤ X = (Cs×KAMave×GNDave) / K 01 ≤ 18.0×10 12 % / ㎡

[0027] The above center can satisfy the following relationship 5.

[0028] [Relationship 5] 0.090×10 18 % / ㎥ ≤ Y = ((Cs×KAMave×GNDave) / K 01 ) / Dave ≤ 0.480×10 18 % / ㎥

[0029] The above cold-rolled steel sheet has a yield strength (YS): 1450–2000 MPa, tensile strength (TS): 1900–2300 MPa, yield ratio (YR): 0.68–0.99, total elongation (T_EL): 4.0–10.0%, uniform elongation (U_EL): 2.0–7.0%, maximum 3-point bending angle (3P): 45–85°, U_EL × 3P: 180–300%°, and U_EL × 3P / YS: 0.11–0.27%° MPa -1 , U_EL×3P / TS: 0.08~0.19%°MPa -1 It could be.

[0030] The above cold-rolled steel sheet may have a plating layer formed on at least one surface.

[0031] Another embodiment of the present invention comprises, in weight%, carbon (C): 0.320~0.450%, silicon (Si): 0.020~0.60%, manganese (Mn): 0.30~2.30%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.0050% or less (excluding 0%), aluminum (Al): 0.0050~0.080%, chromium (Cr): 0.0010~0.50%, molybdenum (Mo): 0.0010~0.350%, niobium (Nb): 0.0010~0.050%, titanium (Ti): 0.0050~0.250%, boron (B): 0.00050~0.0050%, nitrogen (N): 0.010% or less (excluding 0%), A step of heating a slab containing copper (Cu): 0.0010~0.30%, nickel (Ni): 0.0010~0.30%, and the remainder being Fe and other unavoidable impurities; a step of finishing hot rolling the heated slab to obtain a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet; a step of cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; a step of continuously annealing the cold-rolled steel sheet at Ac3~Ac3+150℃ with a dew point temperature of -30~20℃; a step of first cooling the continuously annealed cold-rolled steel sheet; a step of secondarily cooling the first-cooled cold-rolled steel sheet; a step of reheating the secondarily cooled cold-rolled steel sheet and then overaging it at an overaging treatment temperature (HT) of 90~270℃ for an overaging treatment time (ht) of 180~900 seconds; The present invention provides a method for manufacturing a cold-rolled steel sheet that satisfies the following equation 6 during the first cooling, second cooling, and over-aging treatment, comprising the step of tension leveling the cold-rolled steel sheet that has undergone the over-aging treatment.

[0032] [Equation 6] 5.0 ≤ Z = ((HT-T2) / ht)×CR2 ≤ 27.0

[0033] (However, in Equation 6 above, T2 represents the cooling termination temperature during secondary cooling, and CR2 represents the average cooling rate (°C / s) during secondary cooling.)

[0034] The heating of the above slab can be carried out at 1100~1300℃.

[0035] The above finishing hot rolling can be performed at Ar3 to Ar3+120℃.

[0036] The above winding can be performed at Ms~700℃.

[0037] The above cold rolling can be performed with a cold reduction rate of 30 to 70%.

[0038] The above annealing can be performed for 50 to 200 seconds.

[0039] The above first cooling can be carried out at a first average cooling rate (CR1) of 0.5 to 6.0°C / s up to a first cooling end temperature (T1) of 600°C to Ac3.

[0040] The above secondary cooling can be performed at a secondary average cooling rate (CR2) of 30 to 1000℃ / s up to a secondary cooling end temperature (T2) of 40 to 200℃.

[0041] During the above secondary cooling, the Mf-secondary cooling end temperature (T2) can be controlled to be 5°C or higher.

[0042] During the above over-aging treatment, the over-aging treatment temperature (HT) - secondary cooling end temperature (T2) can be controlled to be 15℃ or higher.

[0043] The above tension leveling can be performed with an elongation rate of 0.05 to 2.0%.

[0044] After the tension leveling above, the step of forming a plating layer on at least one surface of the cold-rolled steel sheet may be additionally included.

[0045] According to one aspect of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided.

[0046] According to an advantageous aspect of the present invention, an ultra-high strength cold-rolled steel sheet with excellent bending characteristics and a method for manufacturing the same can be provided.

[0047] FIG. 1 shows the distribution of relational expressions 1 and 2 of Inventive Examples 1 to 5 and Comparative Examples 1 to 4 according to one embodiment of the present invention.

[0048] FIG. 2 shows the distribution of the average thickness (Dave) of the decarburized layer of Invention Examples 1 to 5 and Comparative Examples 1 to 13 according to one embodiment of the present invention and the relationship Equation 4.

[0049] FIG. 3 shows the distribution of yield strength (YS) and U_EL×3P of Invention Examples 1 to 5 and Comparative Examples 4 to 13 according to one embodiment of the present invention.

[0050] Advantageous embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0051] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0052] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0053] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0054] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0055] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0056] Hereinafter, a cold-rolled steel sheet according to one embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. Unless otherwise specified, the alloy composition described below refers to weight percent.

[0057] Carbon (C): 0.320~0.450%

[0058] C is an interstitial solid solution element and is the most effective and important element for improving the strength of steel. Furthermore, it is an element that must be added to ensure the strength of steel. If the content of C is less than 0.320%, it may be difficult to obtain the strength targeted in the present invention. If the content of C exceeds 0.450%, the strength increases rapidly, and the elongation may become inferior. Additionally, hydrogen embrittlement resistance may decrease, and weldability may become inferior. Therefore, it is advantageous for the content of C to have a range of 0.320% to 0.450%. The lower limit of the C content is more advantageous at 0.330%, more advantageous at 0.340%, and most advantageous at 0.350%. The upper limit of the C content is more advantageous at 0.440%, more advantageous at 0.430%, and most advantageous at 0.420%.

[0059] Silicon (Si): 0.020~0.60%

[0060] Si is an element effective for improving resistance to tempering softening and is also effective for improving strength through solid solution strengthening. If the Si content is less than 0.020%, it may be difficult to sufficiently obtain the aforementioned effects. If the Si content exceeds 0.60%, there is a risk that excessive ferrite will be generated after continuous annealing and cooling, thereby weakening the strength of the steel. Furthermore, as Si is an element that increases resistivity, resistance spot weldability may be inferior. Therefore, it is advantageous for the Si content to be in the range of 0.020 to 0.60%. The lower limit of the Si content is more advantageous at 0.030%, more advantageous at 0.040%, and most advantageous at 0.050%. The upper limit of the Si content is more advantageous at 0.50%, more advantageous at 0.40%, and most advantageous at 0.30%.

[0061] Manganese (Mn): 0.30~2.30%

[0062] Mn is an element added to ensure strength. If the Mn content is less than 0.30%, the hardenability is low; consequently, if the cooling rate after continuous annealing is not sufficiently fast, martensite is not formed, making it difficult to secure the strength level targeted by the present invention. If the Mn content exceeds 2.30%, the Ms temperature decreases during cooling after continuous annealing, and as the temperature at which cooling must end decreases, the shape of the steel sheet becomes defective. Furthermore, it is difficult to secure a martensite structure. In addition, Mn-based segregation zones occur along the longitudinal direction of the slab during steelmaking / continuous casting operations, degrading bending characteristics. That is, as manganese bands (Mn bands) are formed within the slab, cracks occur during continuous casting, and there is a problem of increased defect occurrence during the rolling process. Therefore, it is advantageous for the Mn content to be in the range of 0.30 to 2.30%. The lower limit of the above Mn content is more advantageous at 0.40%, more advantageous at 0.50%, and most advantageous at 0.60%. The upper limit of the above Mn content is more advantageous at 2.20%, more advantageous at 2.10%, and most advantageous at 2.0%.

[0063] Phosphorus (P): 0.030% or less (excluding 0%)

[0064] P is an impurity element contained in steel. If the content of P exceeds 0.030%, weldability deteriorates, and it is prone to segregation at grain boundaries, leading to intergranular embrittlement. Furthermore, the grain boundaries are prone to fracture due to hydrogen in the steel, raising concerns about brittleness in the steel. Meanwhile, while it is advantageous to exclude P from the steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is advantageous for the content of P to be 0.030% or less (excluding 0%). It is even more advantageous for the content of P to be 0.020% or less.

[0065] Sulfur (S): 0.0050% or less (excluding 0%)

[0066] S is an impurity element included in steel, similar to P. If the content of S exceeds 0.0050%, it can impair ductility and weldability, and a large amount of MnS precipitates may be formed, which can lead to inferior bending properties. Meanwhile, it is advantageous for S not to be included in steel as much as possible, but 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is advantageous for the content of S to be 0.0050% or less (excluding 0%). It is more advantageous for the content of S to be 0.0030% or less, and even more advantageous for it to be 0.0020% or less.

[0067] Aluminum (Al): 0.0050~0.080%

[0068] Al can be added to remove oxygen from the molten steel. If the Al content is less than 0.0050%, deoxidation is not sufficiently achieved, which impairs the cleanliness of the steel. If the Al content exceeds 0.080%, not only does the castability of the slab deteriorate, but the temperature required for single-phase heating during continuous annealing also increases, which may cause production and equipment problems. Therefore, it is advantageous for the Al content to be in the range of 0.0050% to 0.080%. The lower limit of the Al content is more advantageous at 0.010%, and more advantageous at 0.020%. The upper limit of the Al content is more advantageous at 0.070%, and more advantageous at 0.060%.

[0069] Chrome (Cr): 0.0010~0.50%

[0070] Cr is an element that facilitates securing a low-temperature transformation structure by suppressing ferrite transformation. Additionally, when utilizing a continuous annealing process involving slow cooling, as in the present invention, there is an advantage in suppressing ferrite formation. If the Cr content is less than 0.0010%, the hardenability is low; consequently, if the cooling rate after continuous annealing is not sufficiently fast, martensite is not formed, making it difficult to secure the strength level targeted in the present invention. If the Cr content exceeds 0.50%, resistance to delayed fracture may deteriorate, carbides such as CrC may form to reduce bending properties, and manufacturing costs may increase due to an excessive amount of alloy input. Therefore, it is advantageous for the Cr content to be in the range of 0.0010% to 0.50%. Regarding the lower limit of the Cr content, 0.0050% is more advantageous, 0.010% is more advantageous, and 0.020% is the most advantageous. The upper limit of the above Cr content is more advantageous at 0.40%, more advantageous at 0.30%, and most advantageous at 0.20%.

[0071] Molybdenum (Mo): 0.0010~0.350%

[0072] Mo is an element that exhibits effects such as improving the hardenability of steel, generating Mo-based fine carbides that serve as hydrogen trap sites, and improving resistance to delayed fracture through martensite refinement. If the Mo content is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the Mo content exceeds 0.350%, the aforementioned effects do not increase significantly compared to the cost increase resulting from the addition of expensive alloying elements. Therefore, it is advantageous for the Mo content to be in the range of 0.0010% to 0.350%. The lower limit of the Mo content is more advantageous at 0.0020%, more advantageous at 0.0030%, and most advantageous at 0.0040%. The upper limit of the Mo content is more advantageous at 0.250%, more advantageous at 0.20%, and most advantageous at 0.150%.

[0073] Niobium (Nb): 0.0010~0.050%

[0074] Nb is an element that segregates at austenite grain boundaries, suppresses the coarsening of austenite grains during continuous annealing, and contributes to strength improvement by forming fine precipitates. If the Nb content is less than 0.0010%, sufficient austenite grain refinement and precipitation strengthening effects cannot be obtained. If the Nb content exceeds 0.050%, the precipitation of coarse carbonitrides increases, and there is a risk that strength and elongation will decrease due to the reduction in carbon content in the steel. In addition, there are problems such as reduced workability of the base material and increased manufacturing costs. Therefore, it is advantageous for the Nb content to be in the range of 0.0010 to 0.050%. Regarding the lower limit of the Nb content, it is more advantageous for it to be 0.0020%, more advantageous for it to be 0.0030%, and most advantageous for it to be 0.0040%. The upper limit of the above Nb content is more advantageous at 0.040%, more advantageous at 0.030%, and most advantageous at 0.020%.

[0075] Titanium (Ti): 0.0050~0.250%

[0076] Ti is a nitride-forming element that scavenges dissolved N by precipitating it as TiN. If the Ti content is less than 0.0050%, it is difficult to obtain a strength-increasing effect, and the scavenging effect of dissolved N is reduced, leading to the formation of a large amount of AlN, which may cause cracks during continuous casting. If the Ti content exceeds 0.250%, the strength of the martensite may decrease due to the precipitation of additional carbides in addition to the removal of dissolved N, and the hole expansion and bending characteristics may be impaired due to the excessive formation of carbonitrides such as TiC and TiN. Therefore, it is advantageous for the Ti content to be in the range of 0.0050% to 0.250%. The lower limit of the Ti content is more advantageous at 0.010%, and even more advantageous at 0.0150%. The upper limit of the above Ti content is more advantageous at 0.150%, and more advantageous at 0.10%.

[0077] Boron (B): 0.00050~0.0050%

[0078] B is an element that inhibits ferrite formation. Accordingly, the present invention has the advantage of increasing resistance to hydrogen embrittlement by inhibiting the formation of ferrite upon cooling after continuous annealing and by strengthening the austenite grain boundaries to suppress hydrogen intrusion. If the content of B is less than 0.00050%, it is difficult to obtain the aforementioned effects sufficiently, and there is no hardenability effect at all, making it difficult to secure the strength targeted by the present invention. If the content of B exceeds 0.0050%, ductility may be significantly reduced. Therefore, it is advantageous for the content of B to have a range of 0.00050% to 0.0050%. The lower limit of the B content is more advantageous at 0.00070%, and more advantageous at 0.0010%. The upper limit of the B content is more advantageous at 0.0040%, and more advantageous at 0.0030%.

[0079] Nitrogen (N): 0.010% or less (excluding 0%)

[0080] N is an impurity element, and if its content exceeds 0.010%, it significantly increases the risk of cracking during continuous casting due to the formation of AlN, etc. Although it is advantageous for the above N not to be included in the steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is advantageous for the above N content to have a range of 0.010% or less (excluding 0%). It is more advantageous for the above N content to be 0.0080% or less, and even more advantageous for it to be 0.0060% or less.

[0081] Copper (Cu): 0.0010~0.30%

[0082] Cu improves corrosion resistance in the operating environment of automobiles and also has the effect of suppressing hydrogen intrusion into the steel plate by coating the surface of the steel plate with corrosion products. In addition, as an element incorporated when utilizing scrap as a raw material, recycled materials can be utilized as raw materials, thereby reducing manufacturing costs. If the content of Cu is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the content of Cu exceeds 0.30%, it may lead to the occurrence of surface defects. Therefore, it is advantageous for the content of Cu to have a range of 0.0010% to 0.30%. The lower limit of the Cu content is more advantageous at 0.0050%, and even more advantageous at 0.0070%. The upper limit of the Cu content is more advantageous at 0.20%, and even more advantageous at 0.10%.

[0083] Nickel (Ni): 0.0010~0.30%

[0084] Ni is an element that, like Cu, acts to improve corrosion resistance. Additionally, as an element incorporated when utilizing scrap as a raw material, recycled materials can be utilized as raw materials, thereby reducing manufacturing costs. If the Ni content is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the Ni content exceeds 0.30%, it may lead to the occurrence of surface defects. Therefore, it is advantageous for the Ni content to be in the range of 0.0010% to 0.30%. The lower limit of the Ni content is more advantageous at 0.0050%, and more advantageous at 0.0070%. The upper limit of the Ni content is more advantageous at 0.20%, and more advantageous at 0.10%.

[0085] The remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0086] The cold-rolled steel sheet of the present invention satisfies the aforementioned alloy composition and can simultaneously satisfy the following equations 1 to 3.

[0087] [Relation 1] 60.0 ≤ H = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb + 10000B + 14.5Ni + 9.6Cu ≤ 180.0

[0088]

[0089] The above Equation 1 is a compositional equation designed to improve hardenability by securing a target microstructure. If the value of H is less than 60.0, a large amount of soft ferrite and bainite structures are formed during cooling, making it difficult to secure the target strength. If the value of H exceeds 180.0, the strength becomes excessively high, making it difficult to secure the target elongation. This can lead to processing cracks during forming and cause an increase in costs due to the input of a large amount of ferroalloy. Therefore, it is advantageous for the value of H to have a range of 60.0 to 180.0. It is more advantageous for the lower limit of the value of H to be 65.0. It is more advantageous for the upper limit of the value of H to be 175.0.

[0090] [Equation 2] 0.30 ≤ Cs = C + 0.03Mn + 0.02Si ≤ 0.550

[0091] The above Equation 2 is a component relationship related to strength. If the value of Cs is less than 0.30, the strength of the martensite structure is low, making it difficult to secure the target strength. If the value of Cs exceeds 0.550, the strength becomes excessively high, making it difficult to secure the target elongation and bendability, which may cause processing cracks during forming and lead to increased costs due to the input of a large amount of ferroalloy. Therefore, it is advantageous for the value of Cs to have a range of 0.30 to 0.550. It is more advantageous for the lower limit of the H value to be 0.330. It is more advantageous for the upper limit of the H value to be 0.530.

[0092] [Equation 3] 0.0020 ≤ Cs / H ≤ 0.00750

[0093] The above Equation 3 is a component relationship formula for simultaneously securing the target microstructure and strength. If the value of Cs / H is less than 0.0020, it is a region where the H value is high and the Cs value is low; this indicates high hardenability, making it easy to form hard structures (martensite, bainite, etc.); however, the inherent strength of the martensite is low, making it difficult to secure the target strength. If the value of Cs / H exceeds 0.00750, it is a region where the H value is low and the Cs value is high; this indicates that a large amount of C, Mn, and Si components are added, causing the inherent strength of the martensite to become too high, which may make it difficult to secure the target elongation and bending characteristics. Therefore, it is advantageous for the value of Cs / H to have a range of 0.0020 to 0.00750. It is more advantageous for the lower limit of the Cs / H value to be 0.00220. It is more advantageous for the upper limit of the Cs / H value to be 0.0050.

[0094] Meanwhile, the cold-rolled steel sheet of the present invention may be divided into a central part; and a surface layer formed on the outer side of the central part in the thickness direction. Meanwhile, since the depth of the surface layer may change depending on the thickness of the steel sheet, it is not specifically limited thereto, but as an example, the surface layer may be an area up to 80 μm in the thickness direction from the surface of the steel sheet.

[0095] In the present invention, the center is the number fraction of the region with a KAM value of 0 to 1.0° (K) when the number fraction of the region with a KAM value of 0 to 5.0° is set to 100% during EBSD measurement. 01 ) can be 15~30%. The dislocation density within the grain can be evaluated by the Kernel Average Misorientation (KAM) value. The KAM value is the average value of the amount of crystal rotation (crystal orientation difference) between a target measurement point and surrounding measurement points; the larger this value, the more deformation exists within the crystal. The inventors investigated the relationship between the KAM value and the microstructure and confirmed that differences in physical properties occur depending on the moisture fraction in the region where the KAM value is 0~1.0°. In the present invention, to secure the target physical properties, it is advantageous to control the moisture fraction in the region where the KAM value is 0~1.0° to a range of 15~30%. The moisture fraction (K in the region where the KAM value is 0~1.0° 01 When ) is less than 15%, it consists solely of a fresh martensite structure with low carbide content and high mobile dislocation density, making it difficult to secure high yield strength and yield ratio. The moisture fraction (K in the region where the above KAM value is 0~1.0° 01 If ) exceeds 30%, it contains a large amount of excessively tempered martensitic structure, which may make it difficult to secure the target tensile strength. Therefore, during EBSD measurement, when the number fraction of the region with a KAM value of 0–5.0° is set to 100%, the number fraction of the region with a KAM value of 0–1.0° (K01 It is advantageous for ) to have a range of 15–30%. The moisture fraction (K in the region where the above KAM value is 0–1.0° 01 It is more advantageous for the lower limit of ) to be 16%. The moisture fraction (K in the region where the above KAM value is 0~1.0° 01 It is more advantageous for the upper limit of ) to be 0.29%. Meanwhile, in the present invention, since the region outside the range where the KAM value is 0 to 5.0°, i.e., the region exceeding 5.0°, hardly affects the characteristics of the present invention, it is advantageous to control the moisture fraction in the 0 to 1.0° region by considering the moisture fraction in the 0 to 5.0° region as 100%.

[0096] The above-mentioned core microstructure may comprise, in area %, a total of one or more types of ferrite and bainite: 5% or less (including 0%), and the remainder being one or more types of fresh martensite and tempered martensite. That is, the total of one or more types of fresh martensite and tempered martensite may be 95% or more (including 100%). The above-mentioned fresh martensite and tempered martensite are structures that are highly advantageous for securing the strength and bending characteristics targeted by the present invention. However, one or more types of ferrite and bainite may inevitably be formed during the manufacturing process, and if the total fraction of one or more types of ferrite and bainite exceeds 5%, it may be difficult to secure the physical properties intended by the present invention. Therefore, it is advantageous for the total fraction of one or more types of ferrite and bainite to be 5% or less, and more advantageous for it to be 3% or less. The above tempered martensite may include auto-tempered martensite formed during the cooling process. Meanwhile, the surface microstructure may include one or more types of fresh martensite and tempered martensite, and may include one or more types of ferrite and bainite that are inevitably formed during the manufacturing process. The total fraction of one or more types of ferrite and bainite may be 10% or less (excluding 0%).

[0097] When measuring the above-mentioned core using EBSD, if the number fraction of the region where the KAM value is 0 to 5.0° is set to 100%, the average KAM value (KAMave) may be 1.50 to 1.90°. If the average KAM value (KAMave) is 1.50° or less, it may contain a large amount of excessively tempered martensite structure, making it difficult to secure the target tensile strength. If the average KAM value (KAMave) exceeds 1.70°, the strength may become excessively high due to the structure being composed of a high dislocation density, which may result in inferior elongation. Therefore, it is advantageous for the average KAM value (KAMave) to be 1.50 to 1.90°. It is more advantageous for the lower limit of the average KAM value (KAMave) to be 1.52°. It is more advantageous for the upper limit of the average value (KAMave) of the above KAM values ​​to be 1.88°.

[0098] The above center has an average GND value (GNDave) of 280×10 12 ~325×10 12 It can be / m². The above GND (Geometrically Necessary Dislocations) is a value that can be calculated from the KAM value, which quantifies the dislocation density; a higher value indicates a higher dislocation density. The above average GND value (GNDave) is 280×10 12 If it is less than / ㎡, it contains a large amount of excessively tempered martensite structure, making it difficult to secure the target tensile strength. The above average GND value (GNDave) is 325×10 12 If it exceeds / ㎡, the strength becomes excessively high due to the composition of a structure with high dislocation density, which may result in inferior elongation. Therefore, the above average GND value (GNDave) is 280×10 12 ~325×10 12 It is advantageous to have / ㎡. The lower limit of the above average GND value (GNDave) is 285×10 12It is more advantageous to have / ㎡. The upper limit of the above average GND value (GNDave) is 320×10 12 It is more advantageous to have / ㎡.

[0099] The above surface layer may include a decarburized layer. The decarburized layer refers to a layer containing C with a lower content than the C content contained in the steel plate. As an example, the decarburized layer may contain a C content of 0.85 times or less than the average value of the C content contained in the steel plate.

[0100] The average thickness (Dave) of the decarburized layer may be 20 to 80 µm. If the average thickness (Dave) of the decarburized layer is less than 20 µm, it may be difficult to secure a sufficient soft phase, making it difficult to secure the target bending characteristics. If the average thickness (Dave) of the decarburized layer exceeds 80 µm, an excessive soft phase is formed, making it difficult to secure the target strength and potentially resulting in inferior fatigue characteristics. The lower limit of the average thickness of the decarburized layer may be 25 µm. The upper limit of the average thickness of the decarburized layer may be 75 µm.

[0101] The above center may satisfy the following Equation 4. The following Equation 4 is Equation 2 (Cs), the moisture fraction (K) of the region where the KAM value is 0~1.0°. 01 It is a relational equation related to ), the average KAM value (KAMave), and the average GND value (GNDave). The value of X below is 6.0×10 12 If it is less than % / ㎡, it is difficult to secure the target strength, and 18.0×10 12 If it exceeds % / m², the strength becomes too high, making it difficult to secure the target elongation and bending characteristics. Therefore, the following Equation 4 is 6.0×10 12 ~18.0×10 12 It is advantageous to have a range of % / ㎡. The lower limit of the X value below is 6.50×10 12 It is more advantageous to have % / ㎡. The upper limit of the X value below is 17.50×10 12 It is more advantageous to have % / ㎡.

[0102] [Relationship 4] 6.0×10 12 % / ㎡ ≤ X = (Cs×KAMave×GNDave) / K 01 ≤ 18.0×10 12 % / ㎡

[0103] The above-mentioned center may satisfy the following Equation 5. The following Equation 5 is Equation 2 (Cs) for simultaneously securing strength and bending characteristics, and the moisture fraction (K) in the region where the KAM value is 0~1.0°. 01 It is a relationship equation related to ), the average KAM value (KAMave), the average GND value (GNDave), and the average thickness of the decarburization layer (Dave). The value of Y below is 0.090×10 8 If it is less than % / ㎥, it is difficult to secure the target strength, and 0.480×10 8 If it exceeds % / ㎥, the depth of the decarburization layer is insufficient, making it difficult to secure the target bending characteristics. Therefore, the value of Y below is 0.090×10 8 ~0.480×10 8 It is advantageous to have a range of % / ㎥. The lower limit of the above Y value is 0.10×10 8 It is more advantageous to have % / ㎥. The upper limit of the above Y value is 0.470×10 8 It is more advantageous to have % / ㎥.

[0104] [Relationship 5] 0.090×10 18 % / ㎥ ≤ Y = ((Cs×KAMave×GNDave) / K 01 ) / Dave ≤ 0.480×10 18 % / ㎥

[0105] The cold-rolled steel sheet of the present invention has a yield strength (YS): 1450~2000 MPa, tensile strength (TS): 1900~2300 MPa, yield ratio (YR): 0.68~0.99, total elongation (T_EL): 4.0~10.0%, uniform elongation (U_EL): 2.0~7.0%, maximum 3-point bending angle (3P): 45~85°, U_EL×3P: 180~300%°, and U_EL×3P / YS: 0.11~0.27%°MPa -1, U_EL×3P / TS: 0.08~0.19%°MPa -1 It may be. It is more advantageous for the above yield strength to be 1500–1950 MPa. It is more advantageous for the above tensile strength to be 1950–2250 MPa. It is more advantageous for the above yield ratio to be 0.70–0.98. It is more advantageous for the above total elongation to be 4.5–9.5%. It is more advantageous for the above uniform elongation to be 2.5–6.5%. It is more advantageous for the above 3-point bending maximum angle (3P) to be 50–80°. It is more advantageous for the above U_EL×3P to be 200–290%°. The above U_EL×3P / YS is 0.12–0.26° MPa -1 It is more advantageous to have. The above U_EL×3P / TS is 0.09~0.18%°MPa -1 It is more advantageous to be.

[0106] The thickness of the cold-rolled steel sheet of the present invention may be 0.6 to 2.5 mm. The lower limit of the thickness of the cold-rolled steel sheet is more advantageously 0.7 mm, and 0.8 mm. The upper limit of the thickness of the cold-rolled steel sheet is more advantageously 2.4 mm, and 2.3 mm.

[0107] The cold-rolled steel sheet of the present invention may have a plating layer formed on at least one surface. The present invention does not specifically limit the type of plating layer, and any type of plating layer commonly used in the relevant technical field may be formed. However, as an example, the plating layer may be an electro-galvanized layer.

[0108] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0109] First, a slab satisfying the aforementioned alloy composition is heated. The slab may satisfy the above-described equations 1 to 3. The slab heating process is performed to facilitate the subsequent hot rolling process and to sufficiently obtain the target physical properties of the steel sheet. The heating of the slab may be performed at 1100 to 1300°C. If the slab heating temperature is below 1100°C, a problem arises in which the hot rolling load increases rapidly. If the slab heating temperature exceeds 1300°C, the amount of surface scale increases, and the yield of the material decreases. The lower limit of the slab heating temperature is more advantageous at 1110°C, more advantageous at 1120°C, and most advantageous at 1130°C. The upper limit of the above slab heating temperature is more advantageous at 1290℃, more advantageous at 1280℃, and most advantageous at 1270℃.

[0110] Subsequently, the heated slab is finished hot-rolled to obtain a hot-rolled steel sheet. The finish hot-rolling can be performed at a temperature of Ar3 to Ar3+120℃. If the finish hot-rolling temperature is below Ar3, rolling occurs in a two-phase region of ferrite + austenite or in a ferrite region, resulting in a mixed grain structure, and plate breakage may occur due to fluctuations in the hot-rolling load. If the finish hot-rolling temperature exceeds Ar3+120℃, a large amount of surface scale may form, which may degrade the surface quality. The lower limit of the finish hot-rolling temperature is more advantageous at Ar3+10℃, more advantageous at Ar3+20℃, and most advantageous at Ar3+30℃. The upper limit of the finish hot-rolling temperature is more advantageous at Ar3+110℃, more advantageous at Ar3+100℃, and most advantageous at Ar3+90℃. Meanwhile, the above Ar3 refers to the temperature at which austenite begins to transform into ferrite upon cooling, and can be calculated using the following Equation 1.

[0111] [Equation 1] Ar3(°C) = 910 - 203√C + 44.7Si + 31.5Mo

[0112] Subsequently, the hot-rolled steel sheet is coiled. The coiling may be performed at Ms to 700°C. If the coiling temperature exceeds 700°C, internal oxidation occurs on the surface of the steel sheet, causing the microstructure formed in the surface layer to become non-uniform, and consequently, the bending characteristics may deteriorate. Meanwhile, it is advantageous to manage the coiling temperature low to ensure material uniformity across the entire length and width by forming the microstructure of the hot-rolled steel sheet into a single-phase structure rather than a composite structure as much as possible. However, if the coiling temperature is below Ms, the strength of the hot-rolled steel sheet becomes excessively high, which may make actual production impossible due to the increased rolling load during the subsequent cold rolling process. It is more advantageous for the lower limit of the coiling temperature to be Ms + 10°C. It is more advantageous for the upper limit of the coiling temperature to be 600°C. Ms refers to the temperature at which austenite begins to transform into martensite upon cooling, and can be calculated using Equation 2 below.

[0113] [Equation 2] Ms(℃) = 521 - 379C - 15.1Si - 43.9Mn - 19.5Cr - 14.7Mo + 43.7Nb + 91.9Ti + 169B

[0114] Meanwhile, after the above coiling, it can be cooled by air cooling or water cooling. In addition, after the above cooling, a pickling process can be performed to remove the oxide layer formed on the surface of the hot-rolled steel sheet.

[0115] Subsequently, the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold rolling may be performed with a cold reduction rate of 30 to 70%. If the cold reduction rate is less than 30%, it is difficult to secure the thickness desired in the present invention, and there is a concern that austenite may be formed during annealing heat treatment due to the persistence of crystal grains formed during hot rolling, which may affect the final physical properties. If the cold reduction rate exceeds 70%, the reduction amount in the length and width directions may become uneven due to work hardening occurring during cold rolling, which may lead to material variation in the steel sheet. In addition, it may be difficult to secure the thickness desired in the present invention due to the rolling load. The lower limit of the cold reduction rate is more advantageous at 31%, more advantageous at 32%, and most advantageous at 33%. The upper limit of the above cold rolling rate is more advantageous at 69%, more advantageous at 68%, and most advantageous at 67%.

[0116] Subsequently, the above cold-rolled steel sheet is continuously annealed at Ac3 to Ac3+150°C with a dew point temperature of -30 to 20°C. By controlling the dew point temperature in this way, a decarburization layer can be formed on the surface of the steel sheet during the continuous annealing process. The dew point temperature in a typical continuous annealing furnace is at the level of -40 to -50°C. However, as in the present invention, if the oxygen partial pressure is increased by raising the dew point temperature above -30°C, the carbon in the steel sheet meets the oxygen in the annealing furnace and is released as CO gas, causing decarburization to occur on the surface layer of the steel sheet. If the dew point temperature is below -30°C, a sufficient decarburization layer may not be formed on the surface of the steel sheet. If the dew point temperature exceeds 20°C, the equipment lifespan and productivity may decrease. Therefore, it is advantageous for the dew point temperature to have a range of -30 to 20°C. It is more advantageous for the lower limit of the dew point temperature to be -25°C. It is more advantageous for the upper limit of the above dew point temperature to be 15℃. Meanwhile, although the present invention does not specifically limit the method of controlling the above dew point temperature, as an example, the above dew point temperature can be controlled using humid nitrogen (N2+H2O). If the above continuous annealing temperature is less than Ac3, a mixed grain structure may be formed as two-phase annealing occurs over the entire length of the steel sheet instead of a single-phase annealing. Consequently, it is difficult to secure the physical properties targeted by the present invention, and in particular, the difference in hardness between phases becomes large, which may significantly reduce hole expandability. If the above continuous annealing temperature exceeds Ac3+150℃, equipment trouble may occur due to overloading of the annealing furnace. It is more advantageous for the lower limit of the above continuous annealing temperature to be Ac3+5℃, and it is even more advantageous for it to be Ac3+10℃. The upper limit of the above continuous annealing temperature is more advantageous as Ac3+130℃, and more advantageous as Ac3+110℃. Meanwhile, the above Ac3 refers to the temperature at which 100% transformation of ferrite into austenite is completed upon heating, and can be calculated using Equation 3 below.The above continuous annealing can be performed for 50 to 200 seconds. If the above continuous annealing time is less than 50 seconds, it may be difficult to secure a single-phase austenite structure, and as undissolved carbides remain and coarsen, bending characteristics and hydrogen embrittlement resistance may be reduced, and it may be difficult to sufficiently form a decarburized layer. If the above continuous annealing time exceeds 200 seconds, the austenite size coarsen, which has the disadvantage of making it difficult to secure strength. The lower limit of the above continuous annealing time is more advantageous at 60 seconds, and 70 seconds is more advantageous. The upper limit of the above continuous annealing time is more advantageous at 190 seconds, and 180 seconds is more advantageous.

[0117] [Equation 3] Ac3(℃) = 900 - 206C + 26.2Si - 25Mn - 12.3Cr + 9.12Mo + 50.2Nb + 148Ti - 131B

[0118] Subsequently, the continuously annealed cold-rolled steel sheet is cooled first. The first cooling may be performed at a first average cooling rate (CR1) of 0.5 to 6.0°C / s up to a first cooling end temperature (T1) of 600°C to Ac3. If the first cooling end temperature (T1) is less than 600°C, the bending characteristics may deteriorate as a large amount of soft ferrite and bainite, in addition to martensite, is formed during the cooling process. If the first cooling end temperature (T1) exceeds Ac3, the temperature difference between the first cooling end temperature and the second cooling end temperature (T2) becomes severe, causing a rapid phase transformation and resulting in a defective product shape. It is more advantageous for the lower limit of the first cooling end temperature to be 620°C, and more advantageous for it to be 640°C. The upper limit of the above first cooling end temperature is more advantageously Ac3-10℃, and the upper limit of Ac3-20℃ is more advantageous. If the above first average cooling rate is less than 0.5℃ / s, ferrite is formed during cooling, making it impossible to secure the level of strength targeted by the present invention. If the above first average cooling rate exceeds 6.0℃ / s, the average cooling rate during the subsequent second cooling decreases, increasing the fraction of low-temperature transformation phases other than martensite, making it impossible to secure the level of strength targeted by the present invention. The lower limit of the above first average cooling rate is more advantageously 1℃ / s. The upper limit of the above first average cooling rate is more advantageously 5℃ / s.

[0119] Subsequently, the first-cooled cold-rolled steel sheet is cooled a second time. The second cooling is intended to secure one or more of the main phases of the present invention, namely martensite and tempered martensite. The second cooling can be performed at a second average cooling rate (CR2) of 30 to 1000°C / s until a second cooling end temperature (T2) of 40 to 200°C. If the second cooling end temperature (T2) is less than 40°C, shape defects are caused by rapid phase transformation, and there is a disadvantage that continuous production is difficult due to strip meandering problems. If the second cooling end temperature (T2) exceeds 200°C, it may be difficult to secure the strength targeted by the present invention. The lower limit of the second cooling end temperature is more advantageous at 50°C, more advantageous at 55°C, and most advantageous at 60°C. The upper limit of the above secondary cooling end temperature is more advantageous at 195°C, more advantageous at 190°C, and most advantageous at 185°C. If the above secondary average cooling rate is less than 30°C / s, soft ferrite transformation occurs during cooling, making it difficult to secure the target strength. If the above secondary average cooling rate exceeds 1000°C / s, the product shape may become defective due to rapid phase transformation. The lower limit of the above secondary average cooling rate is more advantageous at 40°C / s, more advantageous at 50°C / s, and most advantageous at 60°C / s. The upper limit of the above secondary average cooling rate is more advantageous at 800°C / s, more advantageous at 600°C / s, and most advantageous at 400°C / s.

[0120] During the above secondary cooling, the Mf-secondary cooling termination temperature (T2) can be controlled to be 5°C or higher. If Mf-T2 is less than 5°C, the martensite transformation may not occur sufficiently, making it difficult to secure the target strength. It is more advantageous for Mf-T2 to be 10°C or higher. Meanwhile, Mf represents the temperature at which the martensite transformation is terminated during cooling, and can be calculated using the following Equation 4.

[0121] [Equation 4] Mf(°C) = 371 - 412C - 17.4Si - 47.4Mn - 20.9Cr - 17Mo + 49.2Nb + 95Ti + 202B

[0122] Subsequently, the above-mentioned secondary cooled cold-rolled steel sheet is reheated and then overaged at an overaging treatment temperature (HT) of 90 to 270°C for an overaging treatment time (ht) of 180 to 900 seconds. Through the above reheating and overaging treatment, the martensite obtained by the aforementioned rapid cooling process is transformed into tempered martensite, thereby increasing the yield strength. If the above-mentioned overaging treatment temperature (HT) is less than 90°C, there is a disadvantage in that tempering is not sufficiently performed, resulting in low yield strength and inability to secure sufficient toughness. If the above-mentioned overaging treatment temperature (HT) exceeds 270°C, there is a disadvantage in that bendability becomes inferior due to the precipitation and coarsening of a large amount of carbides. The lower limit of the above-mentioned overaging treatment temperature is more advantageous at 100°C, more advantageous at 110°C, and most advantageous at 120°C. The upper limit of the above overaging treatment temperature is more advantageous at 265°C, more advantageous at 260°C, and most advantageous at 255°C. If the above overaging treatment time (ht) is less than 180 seconds, tempering is not sufficiently performed, and the yield strength may be lowered. If the above overaging treatment time (ht) exceeds 900 seconds, carbides may coarsen due to excessive tempering, and bending characteristics may deteriorate. The lower limit of the above overaging treatment time is more advantageous at 185 seconds, more advantageous at 190 seconds, and most advantageous at 195 seconds. The upper limit of the above overaging treatment time is more advantageous at 895 seconds, more advantageous at 890 seconds, and most advantageous at 885 seconds.

[0123] When the overaging treatment is performed, the overaging treatment temperature (HT) - secondary cooling end temperature (T2) can be controlled to be 15°C or higher. If the overaging treatment temperature (HT) - secondary cooling end temperature (T2) is less than 15°C, sufficient tempering is not achieved, making it difficult to secure the target yield strength. It is more advantageous for the overaging treatment temperature (HT) - secondary cooling end temperature (T2) to be 20°C or higher.

[0124] Meanwhile, during the above first cooling, second cooling, and overaging treatments, the following Equation 6 can be satisfied. Equation 6 below is a relationship related to the process for securing the target microstructure and satisfying the required physical properties. If the value of Z below is less than 5.0, it may be difficult to secure the target tensile strength because it contains a large amount of martensite structure with excessive tempering; if it exceeds 27.0, the strength is high due to the microstructure composition with high dislocation density, but the elongation and bending characteristics may be inferior. The lower limit of the Z value below is more advantageous at 5.50, and more advantageous at 6.0. The upper limit of the Z value below is more advantageous at 26.0, and more advantageous at 25.0.

[0125] [Equation 6] 5.0 ≤ Z = ((HT-T2) / ht)×CR2 ≤ 27.0

[0126] (However, in Equation 6 above, T2 represents the cooling termination temperature during secondary cooling, and CR2 represents the average cooling rate (°C / s) during secondary cooling.)

[0127] Subsequently, the temper-rolled cold-rolled steel sheet is subjected to tension leveling (T / L). The tension leveling is intended to correct the shape of the steel sheet. The tension leveling can be performed with an elongation of 0.05 to 2.0%. If the elongation is less than 0.05% during tension leveling, it may be difficult to correct the shape. If the elongation exceeds 2.0% during tension leveling, work hardening may become severe, resulting in poor bending characteristics and hydrogen embrittlement resistance, and the difference in yield strength between the vertical and horizontal directions relative to the rolling direction may become significant, which may adversely affect dimensional accuracy during part processing. It is more advantageous for the lower limit of the elongation during tension leveling to be 0.075%, and more advantageous for it to be 0.10%. When leveling the tension above, it is more advantageous for the upper limit of the elongation rate to be 1.8%, and more advantageous for it to be 1.5%.

[0128] Meanwhile, after the tension leveling above, a step of forming a plating layer on at least one surface of the cold-rolled steel sheet may be additionally included. The present invention does not specifically limit the method of forming the plating layer, and any method commonly used in the relevant technical field may be used. However, as an example, the plating layer may be an electro-galvanized layer.

[0129] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0130] (Example)

[0131] A slab having the alloy compositions listed in Tables 1 and 2 below was heated to 1200°C, and then the heated slab was finished hot-rolled at 900°C to obtain a hot-rolled steel sheet. Subsequently, the hot-rolled steel sheet was coiled at 500°C and then cold-rolled under the conditions listed in Table 3 below to obtain a cold-rolled steel sheet. Subsequently, a cold-rolled steel sheet with a thickness of 1.0 to 1.6 mm was manufactured by continuous annealing, primary cooling, secondary cooling, reheating / overaging treatment, and tension leveling under the conditions listed in Tables 3 and 4 below. Meanwhile, the conditions listed in Tables 3 and 4 below were based on the surface temperature of the steel sheet.

[0132] The microstructure and mechanical properties of the cold-rolled steel sheets manufactured in this manner were measured, and the results are shown in Tables 5 and 6 below.

[0133] The average thickness (Dave) of the decarburization layer was measured using a GDS (Glow discharge spectrometer).

[0134] The types and fractions of the microstructure in the center of the steel plate were observed using a scanning electron microscope (SEM) and an optical microscope (OM) at a position t / 4 (t: thickness of the steel plate) in the thickness direction of the steel plate, and the fractions of each phase were analyzed three times through image analysis to calculate the average value.

[0135] Water fraction in the region where the KAM value is 0~1.0° (K 01The average KAM value (KAMave) and average GND value (GNDave) were measured three times at 2000x magnification (Confidence Index (CI)≥0.3, measurement area: 45×45㎛, Step size: 80nm) at the 1 / 4 position in the thickness direction of the steel plate using EBSD (Backscattered Electron Diffraction Pattern Analyzer), and calculated by quantifying using OIM (Orientation Imaging Microscopy) Analysis software. In particular, for the calculation of KAM, the Nearest 3rd and Maximum 5 were used, and for the calculation of GND, Phase Iron (Alpha), Preset Slip Systemps BCC Slip, Nearest 1st and Maximum 5 were used. At this time, during the above EBSD measurement, the number fraction of the region where the KAM value was 0~5.0° was considered as 100%.

[0136] Yield strength (YS), tensile strength (TS), and elongation (EL) were measured by processing cold-rolled steel sheets according to JIS standards in a direction perpendicular to the rolling direction and then performing a tensile test under conditions of a test speed of 28 mm / min.

[0137] The maximum angle of 3-point bending was measured 5 times under VDA conditions (0.4R) and the average value was calculated.

[0138] Steel Grade No. Alloy Composition (Wet%) CSI Mn PS Al Cr Mo Nb Invention Steel 10.4000.08 1.850.0100.00100.0310.0500.0300.010 Invention Steel 20.3600.111.900.0110.00090.0250.0300.0300.005 Invention Steel 30.4200.151.500.0080.00150.0320.1000.0200.004 Invention Steel 40.3800.21 1.640.0100.00100.0250.0500.0500.014 Invention Steel 50.390 0.211.800.0120.00120.0350.1200.0200.014Comparison 10.2400.051.200.0100.00110.0250.1000.0500.035Comparison 20.2850.050.150.0110.00180.0210.0300.0200.007Comparison 30.2700.050.210.0110.00090.0350.0010.0050.015Comparison 40.5000.212.600.0010.00120.0350.0030.0100.025

[0139] Steel Grade No. Alloy Composition (Wt%) TiBNNiCuHCsCs / H Invention Steel 10.0200.00210.00400.0130.0101380.460.0034 Invention Steel 20.0310.00180.00320.0300.0201270.420.0033 Invention Steel 30.0200.00210.00350.0500.0801240.470.0038 Invention Steel 40.0390.00210.00410.0800.0901420.440.0031 Invention Steel 5 0.0200.00210.00410.0200.0311490.450.0030Comparison 10.0350.00210.00250.050.0301430.280.0020Comparison 20.0250.00100.00240.0570.037540.290.0054Comparison 30.01500.00510.0320.020510.280.0055Comparison 40.0250.00200.00400.0210.0521860.590.0032H = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb + 10000B + 14.5Ni + 9.6CuCs = C + 0.03Mn + 0.02Si

[0140] Classification Steel Grade No. Thickness Ac3(°C) Annealing Temperature(°C) Dew Point Temperature(°C) Annealing Time(s) Primary Cooling End Temperature(T1)(°C) Primary Average Cooling Rate(CR1)(°C / s) Mf(°C) Invention Example 1 Invention Steel 1 1.477685681377353118 Invention Example 2 Invention Steel 2 1.478684561107153133 Invention Example 3 Invention Steel 3 1.4782842 71257543124 Invention Example 4 Invention Lecture 41.4792856101307813136 Invention Example 5 Invention Lecture 51.478284071257643122 Comparative Example 1 Comparative Lecture 11.482883531157853217 Comparative Example 2 Comparative Lecture 21.4843845-21258013248 Comparative Example 3 Comparative Lecture 31.4843865-510580 53251 Comparative Example 4 Comparative Example 41.4742842-15115739342 Comparative Example 5 Invention 11.4776851-501207543118 Comparative Example 6 Invention 11.0776863271327353118 Comparative Example 7 Invention 11.2776849101257053118 Comparative Example 8 Invention 11.2776859-10126761 3118 Comparative Example 9 Invention Steel 11.677684931327383118 Comparative Example 10 Invention Steel 41.479284221257453136 Comparative Example 11 Invention Steel 41.679284951307613136 Comparative Example 12 Invention Steel 41.479285231257523136 Comparative Example 13 Invention Steel 41.2792861251207323136

[0141] Classification Steel Grade No. Secondary Cooling Termination Temperature (T2) (°C) Secondary Average Cooling Rate (CR2) (°C / s) Mf-T2 (°C / s) Reheating / Overaging Treatment Temperature (HT) (°C) Overaging Treatment Time (ht) (s) HT-T2 (°C) Z Tension Leveling Elongation (%) Invention Example 1 Invention Steel 194852419548610117.70.25 Invention Example 2 Invention Steel 29585381745227912.90.20 Honorary 3 Invention Lecture 394913019551010118.00.25 Invention Example 4 Invention Lecture 49189451844869317.00.30 Invention Example 5 Invention Lecture 59578271754808013.00.25 Comparative Example 1 Comparative Lecture 1100611171854868510.70.25 Comparative Example 2 Comparative Lecture 212062128192504728.90.25 Comparative Example 3 Comparative Lecture 3110 631412014869111.80.35 Comparative Example 4 Comparative Example 43589719752216227.60.50 Comparative Example 5 Invention Example 19585231876129212.80.45 Comparative Example 6 Invention Example 117056-52187486172.00.25 Comparative Example 7 Invention Example 124065-122275480354.70.25 Comparative Example 8 Invention Example 116515-471856 06200.50.35 Comparative Example 9 Invention Steel 192160269357010.30.25 Comparative Example 10 Invention Steel 4958541105510101.70.25 Comparative Example 11 Invention Steel 410578311351503015.60.08 Comparative Example 12 Invention Steel 497873925096015313.90.08 Comparative Example 13 Invention Steel 414550-9201516565.40.01Z = ((HT-T2) / ht)×CR2

[0142] Microstructure of the center (Area %) Average thickness of the decarburized layer (Dave) (㎛) Moisture fraction of the region with KAM values ​​of 0~1.0° (K 01 )(%) Average value of KAM (KAMave)(°) Average value of GND (GNDave) ( / ㎡)X(% / ㎡)Y(% / ㎥)F+BFM+TM Invention Example 10 10054191.72311×10 12 13.0×10 12 0.24×10 18 Invention Example 2010052191.65302×10 12 11.0×10 12 0.21×10 18Invention Example 3010058201.73312×10 12 12.7×10 12 0.22×10 18 Invention Example 4010062181.69307×10 12 12.4×10 12 0.20×10 18 Invention Example 5010060191.70310×10 12 12.5×10 12 0.21×10 18 Comparative Example 169456241.46274×10 12 4.7×10 12 0.08×10 18 Comparative Example 2138758261.48278×10 12 4.6×10 12 0.08×10 18 Comparative Example 3118954241.49276×10 12 4.8×10 12 0.09×10 18 Comparative Example 4010036171.91328×10 12 21.4×10 12 0.59×10 18 Comparative Example 501000.2181.72312×10 12 13.7×10 12 68.57×10 18 Comparative Example 6010086211.59294×10 12 10.2×10 12 0.12×10 18 Comparative Example 7217964321.49275×10 12 5.9×10 12 0.09×10 18 Comparative Example 8178345311.45274×10 12 5.9×10 12 0.13×10 18 Comparative Example 9010056131.85315×10 12 20.6×10 12 0.37×10 18 Comparative Example 10010054141.87314×10 12 18.0×10 12 0.33×10 18 Comparative Example 11010056141.86312×1012 17.8×10 12 0.32×10 18 Comparative Example 1 2010055311.52275×10 12 5.8×10 12 0.11×10 18 Comparative Example 13010083261.62306×10 12 8.2×10 12 0.10×10 18 F: Ferrite, B: Bainite, FM: Fresh Martensite, TM: Tempered Martensite X = (Cs×KAMave×GNDave) / K 01 Y = ((Cs×KAMave×GNDave) / K 01 ) / Dave

[0143] Yield Strength (YS) (MPa) Tensile Strength (YS) (MPa) Yield Ratio (YR) Total Elongation (T_EL) (%) Uniform Elongation (U_EL) (%) 3-Point Bending Maximum Angle (3P) (°) U_EL × 3P (%°) U_EL × 3P / YS (%° MPa -1 )U_EL×3P / TS(%°MPa -1Invention Example 1 1596 20740.776.84.6592710.170.13 Invention Example 2 1624 20680.796.74.3622670.160.13 Invention Example 3 16012 1020.767.04.3602580.160.12 Invention Example 4 157720980.756.54.4622730.170.13 Invention Example 5 1584 20750.766 .53.9582260.140.11 Comparative Example 1128015800.816.93.9953710.290.23 Comparative Example 2130216850.776.83.8863270.250.19 Comparative Example 3131516950.786.53.9893470.260.20 Comparative Example 4185223050.803.51.941780.040.03 Comparative Example 5159720590.786.54.2421760.110.09 Comparative Example 6145618900.776.94.2753150.220.17 Comparative Example 7144918750.776.23.0692070.140.11 Comparative Example 8143018670.776.83.9562180.150.12 Comparative Example 9141521050.676.23 .8652470.170.12 Comparative Example 10142520950.686.54.0662640.190.13 Comparative Example 11143220850.696.23.8532010.140.10 Comparative Example 12168518950.895.83.0461380.080.07 Comparative Example 13144519650.746.43.1752330.160.12

[0144] Figure 1 shows the distribution of Equations 1 and 2 for Inventive Examples 1 to 5 and Comparative Examples 1 to 4. Figure 2 shows the distribution of the average thickness (Dave) of the decarburized layer and Equation 4 for Inventive Examples 1 to 5 and Comparative Examples 1 to 13. Figure 3 shows the distribution of the yield strength (YS) and U_EL×3P for Inventive Examples 1 to 5 and Comparative Examples 4 to 13. As can be seen from Tables 1 to 6 and Figures 1 to 3, Inventive Examples 1 to 5, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, satisfied both the tensile properties and bending properties desired by the present invention by securing the characteristics (microstructure, decarburized layer, KAM, GND, etc.).

[0145] In the case of Comparative Example 1, which does not satisfy C content, Cs, and Cs / H, it can be seen that the yield strength, tensile strength, 3-point bending maximum angle (3P), U_EL×3P, U_EL×3P / YS, and U_EL×3P / TS of the present invention are not satisfied because the microstructure, average value of KAM value, average value of GND, X value, and Y value are not secured.

[0146] In the case of Comparative Example 2, which does not satisfy C content, Mn content, H, and Cs, it can be seen that the microstructure, average value of KAM value, average value of GND, X value, and Y value are not secured, and thus the yield strength, tensile strength, maximum 3-point bending angle (3P), and U_EL×3P that the present invention aims to obtain are not satisfied.

[0147] In the case of Comparative Example 3, which does not satisfy C content, Mn content, B content, H, and Cs, it can be seen that the yield strength, tensile strength, 3-point bending maximum angle (3P), U_EL×3P, and U_EL×3P / TS of the present invention are not satisfied because the microstructure, average value of KAM value, average value of GND, and X value are not secured.

[0148] In the case of Comparative Example 4, which does not satisfy C content, Mn content, H, Cs, secondary cooling end temperature, and Z, it can be seen that the tensile strength, total elongation, uniform elongation, 3-point bending maximum angle (3P), U_EL×3P, U_EL×3P / YS, and U_EL×3P / TS of the present invention are not satisfied because the average value of KAM, the average value of GND, the X value, and the Y value are not secured.

[0149] In the case of Comparative Example 5, which does not satisfy the dew point temperature, it can be seen that the average thickness of the decarburized layer, the Y value, is not secured, and thus the maximum 3-point bending angle (3P), U_EL×3P, which the present invention aims to obtain is not satisfied.

[0150] In the case of Comparative Example 6, which does not satisfy the dew point temperature, Mf-second cooling end temperature, and Z, it can be seen that the tensile strength, U_EL×3P, which the present invention aims to obtain is not satisfied because the average thickness of the decarburization layer is not secured.

[0151] In the case of Comparative Example 7, which does not satisfy the second cooling end temperature, Mf-second cooling end temperature, reheat / overaging treatment temperature, and Z, it can be seen that the yield strength and tensile strength intended by the present invention are not satisfied because the microstructure, the moisture fraction in the region where the KAM value is 0~1.0°, the average value of the KAM value, the average value of the GND, and the X value are not secured.

[0152] In the case of Comparative Example 8, which does not satisfy the secondary swelling cooling rate, Mf secondary cooling end temperature, and Z, it can be seen that the yield strength and tensile strength intended by the present invention are not satisfied because the microstructure, the moisture fraction in the region where the KAM value is 0~1.0°, the average value of the KAM value, the average value of the GND, and the X value are not secured.

[0153] In the case of Comparative Example 9, which does not satisfy the over-aging treatment temperature - secondary cooling end temperature Z, it can be seen that the yield strength and yield ratio intended by the present invention are not satisfied because the moisture fraction X value in the region where the KAM value is 0~1.0° is not secured.

[0154] In the case of Comparative Example 10, which does not satisfy the over-aging treatment temperature - secondary cooling end temperature, Z, it can be seen that the yield strength intended by the present invention is not satisfied because the moisture fraction, X value, in the region where the KAM value is 0~1.0° is not secured.

[0155] In the case of Comparative Example 11, which does not satisfy the over-aging treatment time, it can be seen that the yield strength intended by the present invention is not satisfied because the moisture fraction and X value in the region where the KAM value is 0~1.0° are not secured.

[0156] In the case of Comparative Example 12, which does not satisfy the over-aging treatment time, it can be seen that the moisture fraction, GND average value, and X value in the region where the KAM value is 0~1.0° are not secured, and thus the tensile strength, U_EL×3P, U_EL×3P / YS, and U_EL×3P / TS that the present invention aims to obtain are not satisfied.

[0157] In the case of Comparative Example 13, which does not satisfy the dew point temperature, Mf-second cooling end temperature, and tension leveling elongation, it can be seen that the yield strength intended by the present invention is not satisfied because the average thickness of the decarburized layer is not secured.

Claims

In wt%, Carbon (C): 0.320–0.450%, Silicon (Si): 0.020–0.60%, Manganese (Mn): 0.30–2.30%, Phosphorus (P): ≤0.030% (excluding 0%), Sulfur (S): ≤0.0050% (excluding 0%), Aluminum (Al): 0.0050–0.080%, Chromium (Cr): 0.0010–0.50%, Molybdenum (Mo): 0.0010–0.350%, Niobium (Nb): 0.0010–0.050%, Titanium (Ti): 0.0050–0.250%, Boron (B): 0.00050–0.0050%, Nitrogen (N): ≤0.010% (excluding 0%), Copper (Cu): 0.0010~0.30%, Nickel (Ni): 0.0010~0.30%, containing the remainder Fe and other unavoidable impurities, It is divided into a central part; and a surface layer formed on the outer side of the thickness direction of the central part. The above central part is the number fraction of the region with a KAM value of 0–1.0° (K) when the number fraction of the region with a KAM value of 0–5.0° is set to 100% during EBSD measurement. 01 ) is 15~30%, and A cold-rolled steel sheet having a surface layer that includes a decarburized layer, and an average thickness (Dave) of the decarburized layer of 20 to 80 μm. (However, the above surface layer is an area up to 80㎛ in the thickness direction from the surface of the cold-rolled steel sheet.) ) In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet satisfying the following equations 1 to 3. [Relationship 1] 60.0 ≤ H = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb + 10000B + 14.5Ni + 9.6Cu ≤ 180.0 [Equation 2] 0.30 ≤ Cs = C + 0.03Mn + 0.02Si ≤ 0.550 [Equation 3] 0.0020 ≤ Cs / H ≤ 0.00750 (However, the content of the alloying elements described in the above equations 1 to 3 is in weight%.) In paragraph 1, A cold-rolled steel sheet in which the microstructure of the above-mentioned center comprises, in area %, a total of one or more of ferrite and bainite: 5% or less (including 0%), and the remainder being one or more of fresh martensite and tempered martensite. In paragraph 1, The above center has an average GND value (GNDave) of 280×10 12 ~325×10 12 Cold-rolled steel sheet in / ㎡. In paragraph 1, The above-mentioned center is a cold-rolled steel sheet in which, when measured by EBSD, the average value (KAMave) of the KAM value is 1.50~1.90° when the number fraction of the region with a KAM value of 0~5.0° is set to 100%. In paragraph 1, The above-mentioned center is a cold-rolled steel sheet satisfying the following relationship 4. [Relationship 4] 6.0×10 12 % / ㎡ ≤ X = (Cs×KAMave×GNDave) / K 01 ≤ 18.0×10 12 % / ㎡ In paragraph 1, The above-mentioned center is a cold-rolled steel sheet satisfying the following relationship 5. [Relationship 5] 0.090×10 18 % / ㎥ ≤ Y = ((Cs×KAMave×GNDave) / K 01 ) / Dave ≤ 0.480×10 18 % / ㎥ In paragraph 1, The above cold-rolled steel sheet has a yield strength (YS): 1450–2000 MPa, tensile strength (TS): 1900–2300 MPa, yield ratio (YR): 0.68–0.99, total elongation (T_EL): 4.0–10.0%, uniform elongation (U_EL): 2.0–7.0%, maximum 3-point bending angle (3P): 45–85°, U_EL × 3P: 180–300%°, and U_EL × 3P / YS: 0.11–0.27%° MPa -1 , U_EL×3P / TS: 0.08~0.19%°MPa -1 Cold-rolled steel sheet. In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet having a plating layer formed on at least one surface. In wt%, Carbon (C): 0.320–0.450%, Silicon (Si): 0.020–0.60%, Manganese (Mn): 0.30–2.30%, Phosphorus (P): ≤0.030% (excluding 0%), Sulfur (S): ≤0.0050% (excluding 0%), Aluminum (Al): 0.0050–0.080%, Chromium (Cr): 0.0010–0.50%, Molybdenum (Mo): 0.0010–0.350%, Niobium (Nb): 0.0010–0.050%, Titanium (Ti): 0.0050–0.250%, Boron (B): 0.00050–0.0050%, Nitrogen (N): ≤0.010% (excluding 0%), Copper (Cu): A step of heating a slab containing 0.0010~0.30% nickel (Ni): 0.0010~0.30%, the remainder being Fe and other unavoidable impurities; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above heated slab; Step of winding the above hot-rolled steel sheet; A step of obtaining a cold-rolled steel sheet by cold-rolling the above-mentioned coiled hot-rolled steel sheet; A step of continuously annealing the above cold-rolled steel sheet at a dew point temperature of -30 to 20℃ at Ac3 to Ac3+150℃; A step of first cooling the above continuously annealed cold-rolled steel sheet; A step of secondarily cooling the above first-cooled cold-rolled steel sheet; A step of reheating the above secondary cooled cold-rolled steel sheet and then overaging it at an overaging treatment temperature (HT) of 90 to 270°C for an overaging treatment time (ht) of 180 to 900 seconds; and The method includes the step of tension leveling the above-mentioned over-aged cold-rolled steel sheet; A method for manufacturing a cold-rolled steel sheet that satisfies the following relationship 6 during the above first cooling, second cooling, and overaging treatment. [Equation 6] 5.0 ≤ Z = ((HT-T2) / ht)×CR2 ≤ 27.0 (However, in Equation 6 above, T2 represents the cooling termination temperature during secondary cooling, and CR2 represents the average cooling rate (°C / s) during secondary cooling.) In Paragraph 10, A method for manufacturing cold-rolled steel sheets in which the heating of the above slab is performed at 1100~1300℃. In Paragraph 10, The above finishing hot rolling is a method for manufacturing cold-rolled steel sheets performed at Ar3 to Ar3+120℃. In Paragraph 10, The above coiling is a method for manufacturing cold-rolled steel sheets performed at Ms~700℃. In Paragraph 10, The above cold rolling is a method for manufacturing a cold-rolled steel sheet, performed with a cold reduction rate of 30 to 70%. In Paragraph 10, A method for manufacturing cold-rolled steel sheets in which the above annealing is performed for 50 to 200 seconds. In Paragraph 10, A method for manufacturing a cold-rolled steel sheet, wherein the above first cooling is performed at a first average cooling rate (CR1) of 0.5 to 6.0℃ / s until a first cooling end temperature (T1) of 600℃ to Ac3. In Paragraph 10, A method for manufacturing a cold-rolled steel sheet, wherein the above secondary cooling is performed at a secondary average cooling rate (CR2) of 30 to 1000℃ / s until a secondary cooling end temperature (T2) of 40 to 200℃. In Paragraph 10, A method for manufacturing a cold-rolled steel sheet in which, during the above secondary cooling, the Mf-secondary cooling end temperature (T2) is controlled to be 5℃ or higher. In Paragraph 10, A method for manufacturing a cold-rolled steel sheet in which, during the above overaging treatment, the overaging treatment temperature (HT) - secondary cooling end temperature (T2) is controlled to be 15℃ or higher. In Paragraph 10, A method for manufacturing a cold-rolled steel sheet in which the above tension leveling is performed with an elongation of 0.05 to 2.0%. In Paragraph 10, A method for manufacturing a cold-rolled steel sheet, further comprising the step of forming a plating layer on at least one surface of the cold-rolled steel sheet after the above tension leveling.

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