Cold-rolled steel sheet and manufacturing method therefor

Abstract

The present invention relates to a cold-rolled steel sheet and a manufacturing method therefor and, more specifically, to: a cold-rolled steel sheet suitable for use as a steel material for automotive reinforcements such as bumper beams, sill side beams, and the like, or a steel material for protection of electric vehicle battery cases such as side frames, cross members, and the like; and a manufacturing method therefor.

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 reinforcement materials such as bumper beams and sill side beams, or steel for protecting electric vehicle battery cases such as side frames and cross members, 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 have high processing characteristics, particularly excellent bendability, and ultra-high strength. To this end, research is actively being conducted on ultra-high strength steel with a tensile strength of 1470 MPa or higher and methods for manufacturing the same using a single martensite phase.

[0003] Recently, the Hot Press Forming (HPF) method has been developed, in which materials are formed using dies at high temperatures—an environment conducive to forming—and then water-cooled to achieve the required strength. Since the HPF method allows for securing high strength relative to the same thickness, it is widely used in parts manufacturing; however, this method has the disadvantage of requiring excessive equipment investment and increased process costs. Consequently, there is a need to develop materials for cold stamping and roll forming. Specifically, there is a demand for the development of ultra-high-strength cold-rolled steel sheets that are suitable for cold stamping and roll forming, possess high strength and high yield ratios to ensure impact performance, and exhibit excellent bending characteristics for part forming, spot weldability for part assembly, and corrosion resistance for long-term part lifespan.

[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 the shape (flatness) deteriorates due to rapid cooling (water cooling), causing defects during molding.

[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] Meanwhile, to manufacture ultra-high-strength steel with a tensile strength of 1470 MPa or higher, it is essential to introduce martensite or some bainite. In this case, brittle fracture is prone to occur due to hydrogen remaining within the steel or introduced from the outside, a phenomenon referred to as hydrogen embrittlement. Hydrogen embrittlement manifests as fracture at a strength lower than the fracture strength; the material can fracture due to hydrogen embrittlement even at applied stresses that are very small compared to the actual fracture strength of the material. In particular, this hydrogen embrittlement becomes more sensitive as the strength of the steel increases. Therefore, in ultra-high-strength steel, it is necessary to control the initial amount of hydrogen remaining in the material to prevent such hydrogen embrittlement.

[0007] Therefore, in order to solve the aforementioned problems, it is necessary to develop an ultra-high strength cold-rolled steel sheet with a tensile strength of 1470 MPa or higher that has excellent shape, bending characteristics, and resistance to hydrogen embrittlement.

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

[0012] A preferred aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet with a tensile strength of 1470 MPa or higher, having excellent shape, bending characteristics, and resistance to hydrogen embrittlement, and a method for manufacturing the same.

[0013] 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.

[0014] One embodiment of the present invention comprises, in weight percent, carbon (C): 0.240–0.350%, silicon (Si): 0.030–0.50%, manganese (Mn): 0.40–2.50%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.00350% or less (excluding 0%), boron (B): 0.00050–0.0050%, and the remainder being Fe and other unavoidable impurities; a non-surface layer including a center; and a surface layer formed on the outer side of the non-surface layer in the thickness direction; wherein the fraction of low-angle grain boundaries having an orientation difference of 2° or more and less than 15° in the non-surface layer is 30 area percent or more, and the Mn segregation ratio (Mn max / Mn ave A cold-rolled steel sheet is provided having a value of 1.6 or less and a surface roughness (Rsk) of the surface layer of -0.70㎛ or more.

[0015] (However, the above-mentioned center refers to the region from 1 / 2t (t: steel thickness) to ±20㎛ in the thickness direction of the steel, and the above-mentioned Mn max represents the highest Mn content in the center, and the above Mn ave represents the average Mn content of the steel, and the surface layer refers to the region extending up to 20㎛ in the thickness direction from the surface of the steel.)

[0016] The above cold-rolled steel sheet may additionally contain one or more of the following: aluminum (Al): 0.010~0.10%, niobium (Nb): 0.0030~0.050%, titanium (Ti): 0.0050~0.150%, and nitrogen (N): 0.010% or less (excluding 0%).

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

[0018] [Equation 1] 0.250 ≤ X = C + 0.03Si + 0.02Mn ≤ 0.40

[0019] [Equation 2] 0.20 ≤ Y = B / S ≤ 3.50

[0020] [Equation 3] 1.0 ≤ Z = Y / X ≤ 11.40

[0021] (However, the content of the alloying elements listed in the above equations 1 and 2 is in weight%.)

[0022] The microstructure of the above-mentioned non-surface layer 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 tempered martensite may contain carbides with an average size of 150 nm or less (excluding 0 nm) within the lath.

[0024] The above cold-rolled steel sheet may contain MnS inclusions with an average size of 10㎛ or less (excluding 0㎛).

[0025] The above cold-rolled steel sheet may have a diffusible hydrogen content of 0.1 ppm or less.

[0026] The above cold-rolled steel sheet may have a product of Rsk and Pc of -130㎛·ks or more.

[0027] The above cold-rolled steel sheet may have a yield strength (YS1) in the vertical direction of the rolling direction of 1300~1750 MPa, a tensile strength (TS) in the vertical direction of the rolling direction of 1670~1950 MPa or more, a yield ratio (YS1 / TS) of 0.95 or less, an elongation (El) of 2~10%, a yield strength (YS2) in the horizontal direction of the rolling direction of 1330~1780 MPa, and a △YS(YS2-YS1) of 150 MPa or less.

[0028] The above cold-rolled steel sheet may have a bendability (R / t) of 4.0 or less.

[0029] After resistance spot welding, the above cold-rolled steel sheet may have a weld hardness of 650 Hv or less and a cross tensile strength (CTS) of 3.0 kN or more.

[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 the steps of: obtaining a slab by continuously casting molten steel containing, in weight percent, carbon (C): 0.240~0.350%, silicon (Si): 0.030~0.50%, manganese (Mn): 0.40~2.50%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.00350% or less (excluding 0%), boron (B): 0.00050~0.0050%, and the remainder being Fe and other unavoidable impurities, such that the specific water content during secondary cooling is 0.5~3.0 L / kg·s; heating the slab; finishing hot rolling the heated slab to obtain a hot-rolled steel sheet; and coiling the hot-rolled steel sheet. A step of obtaining a cold-rolled steel sheet by cold-rolling the coiled hot-rolled steel sheet using a roll having a roughness (Ra) of 2.5 μm or more; a step of continuously annealing the cold-rolled steel sheet at Ac3+20℃ to Ac3+110℃; a step of first cooling the continuously annealed cold-rolled steel sheet to a first cooling end temperature (T1); a step of secondarily cooling the first-cooled cold-rolled steel sheet to a second cooling end temperature (T2); a step of reheating the secondarily cooled cold-rolled steel sheet to the overaging treatment temperature (H) and then performing overaging treatment; and a step of temper-rolling the overaged cold-rolled steel sheet with a rolling force of 500 to 1000 tons. The present invention provides a method for manufacturing a cold-rolled steel sheet comprising the step of tension leveling the temper-rolled cold-rolled steel sheet, wherein, during the secondary cooling, the martensite transformation end temperature (Mf) - secondary cooling end temperature (T2) is controlled to a range of 20 to 150°C.

[0032] The above slab may additionally include one or more of the following: aluminum (Al): 0.010~0.10%, niobium (Nb): 0.0030~0.050%, titanium (Ti): 0.0050~0.150%, and nitrogen (N): 0.010% or less (excluding 0%).

[0033] The above slab can satisfy the following equations 1 to 3.

[0034] [Equation 1] 0.250 ≤ X = C + 0.03Si + 0.02Mn ≤ 0.40

[0035] [Equation 2] 0.20 ≤ Y = B / S ≤ 3.50

[0036] [Equation 3] 1.0 ≤ Z = Y / X ≤ 11.40

[0037] (However, the content of the alloying elements listed in the above equations 1 and 2 is in weight%.)

[0038] The heating of the above slab can be carried out at 1100~1300℃.

[0039] The above finishing hot rolling can be performed at Ar3 to Ar3+120℃.

[0040] The above winding can be performed at Ms~600℃.

[0041] The above cold rolling can be performed with a cold reduction rate of 45 to 70%.

[0042] The above continuous annealing can be performed for 50 to 200 seconds.

[0043] During the first cooling above, the first cooling end temperature (T1) is controlled to 670~750℃ and can be carried out at an average cooling rate of 1~6℃ / s.

[0044] During the above second cooling, the second cooling end temperature (T2) is controlled to 40~250℃ and can be carried out at an average cooling rate of 30~600℃ / s.

[0045] When the above overaging treatment is performed, the overaging treatment temperature (H) is controlled to 130~300℃ and can be performed for 5~12 minutes.

[0046] During the above secondary cooling and overaging treatment, the overaging treatment temperature (H) - secondary cooling end temperature (T2) can be controlled to be in the range of 30 to 150°C.

[0047] The above tension leveling can be performed with an elongation rate of 0.05 to 0.65%.

[0048] 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.

[0049] According to one aspect of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided.

[0050] According to a preferred aspect of the present invention, an ultra-high strength cold-rolled steel sheet with a tensile strength of 1470 MPa or higher, having excellent shape, bending characteristics, and resistance to hydrogen embrittlement, and a method for manufacturing the same can be provided.

[0051] Figure 1 is a profile of the Mn content in the center of Invention Example 1 measured using EPMA (Electron Probe Micro Analyzer).

[0052] Preferred 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.

[0053] 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.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] Hereinafter, a cold-rolled steel sheet according to an 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.

[0058] Carbon (C): 0.240~0.350%

[0059] 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 martensitic steel. If the content of C is less than 0.240%, it may be difficult to secure the strength targeted in the present invention. If the content of C exceeds 0.350%, the strength increases rapidly, which may result in inferior elongation. Additionally, coarse carbides may form, leading to reduced hydrogen embrittlement resistance and inferior weldability. Therefore, it is desirable for the content of C to be in the range of 0.240 to 0.350%. The lower limit of the C content is more advantageous at 0.250%, more advantageous at 0.260%, and most advantageous at 0.270%. The upper limit of the C content is more advantageous at 0.340%, more advantageous at 0.330%, and most advantageous at 0.320%.

[0060] Silicon (Si): 0.030~0.50%

[0061] Si plays a role in suppressing the formation of carbides and controlling the size of carbides during the reheating and overaging treatment steps performed after continuous annealing and cooling. If the Si content is less than 0.030%, it may be difficult to sufficiently obtain the aforementioned effects. If the Si content exceeds 0.50%, there is a risk that excessive ferrite will be formed 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 compromised. Therefore, it is desirable for the Si content to be in the range of 0.030 to 0.50%. The lower limit of the Si content is more advantageous at 0.040%, more advantageous at 0.050%, and most advantageous at 0.060%. The upper limit of the Si content is more advantageous at 0.40%, more advantageous at 0.30%, and most advantageous at 0.20%.

[0062] Manganese (Mn): 0.40~2.50%

[0063] Mn is an element added to ensure strength. If the Mn content is less than 0.40%, 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 2.50%, the Ms temperature decreases during cooling after continuous annealing, resulting in a lower final cooling temperature and consequently poor shape of the steel sheet. Furthermore, it is difficult to secure an initial martensite structure. In addition, Mn-based segregation zones occur along the longitudinal direction of the slab during steelmaking / continuous casting operations, degrading bendability and hydrogen embrittlement resistance. Specifically, as Mn is segregated along the thickness direction and manganese bands (Mn bands) are formed within the slab, there is a problem where cracks occur during continuous casting and the occurrence of defects increases during the rolling process. Therefore, it is desirable for the Mn content to be in the range of 0.40 to 2.50%. The lower limit of the above Mn content is more advantageous at 0.50%, more advantageous at 0.60%, and most advantageous at 0.70%. The upper limit of the above Mn content is more advantageous at 2.40%, more advantageous at 2.30%, and most advantageous at 2.20%.

[0064] Phosphorus (P): 0.030% or less (excluding 0%)

[0065] P is an impurity element contained in steel, and weldability deteriorates if the content of P exceeds 0.030%. Furthermore, P tends to segregate at grain boundaries, causing intergranular embrittlement, and the grain boundaries become susceptible to fracture by hydrogen, adversely affecting hydrogen embrittlement resistance. Meanwhile, while it is advantageous to have a smaller amount of P added to the steel, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is desirable that the content of P be 0.030% or less (excluding 0%). It is more advantageous for the content of P to be 0.020% or less, more advantageous for 0.0150% or less, and most advantageous for 0.010% or less.

[0066] Sulfur (S): 0.00350% or less (excluding 0%)

[0067] S is an impurity element included in steel, similar to P. If the content of S exceeds 0.00350%, it can impair ductility and weldability, and a large amount of MnS precipitates are formed, which reduces bendability and hydrogen embrittlement resistance. Meanwhile, although it is advantageous to have a smaller amount of S added to the steel, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is desirable that the content of S be 0.00350% or less (excluding 0%). It is more advantageous for the content of S to be 0.0030% or less, more advantageous for 0.0020% or less, and most advantageous for 0.0010% or less.

[0068] Boron (B): 0.00050~0.0050%

[0069]

[0070] B is an element that inhibits the formation of ferrite. Accordingly, the present invention has the advantage of inhibiting the formation of ferrite during cooling after continuous annealing, and strengthens the austenite grain boundaries to inhibit hydrogen intrusion, thereby increasing resistance to hydrogen embrittlement. If the content of B is less than 0.00050%, there is no hardenability effect, making it difficult to secure the strength targeted in the present invention. If the content of B exceeds 0.0050%, ductility may be significantly reduced. Therefore, it is preferable 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 0.00060%, more advantageous at 0.00070%, and most advantageous at 0.00080%. The upper limit of the above B content is more advantageous at 0.0040%, more advantageous at 0.0030%, and most advantageous at 0.0020%.

[0071] 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.

[0072] The cold-rolled steel sheet of the present invention may additionally include one or more of the following: aluminum (Al): 0.010~0.10%, niobium (Nb): 0.0030~0.050%, titanium (Ti): 0.0050~0.150%, and nitrogen (N): 0.010% or less (excluding 0%).

[0073] Aluminum (Al): 0.010~0.10%

[0074] Al may be added to remove oxygen from the molten steel. If the Al content is less than 0.010%, deoxidation is not sufficiently achieved, which impairs the cleanliness of the steel. If the Al content exceeds 0.10%, 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 desirable for the Al content to be in the range of 0.010 to 0.10%. The lower limit of the Al content is more advantageous at 0.020%, more advantageous at 0.030%, and most advantageous at 0.040%. The upper limit of the Al content is more advantageous at 0.090%, more advantageous at 0.080%, and most advantageous at 0.0750%.

[0075] Niobium (Nb): 0.0030~0.050%

[0076] 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 0.0030%, 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 desirable for the Nb content to be in the range of 0.0030% to 0.050%. The lower limit of the Nb content is more advantageous at 0.0040%, more advantageous at 0.0050%, and most advantageous at 0.0060%. 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%.

[0077] Titanium (Ti): 0.0050~0.150%

[0078] 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 0.150%, the strength of the martensite may decrease as additional carbides are precipitated in addition to the removal of dissolved N, and the hole expansion and bendability may be impaired due to the excessive formation of carbonitrides such as TiC and TiN. Therefore, it is desirable for the Ti content to be in the range of 0.0050% to 0.150%. The lower limit of the Ti content is more advantageous at 0.010%, more advantageous at 0.020%, and most advantageous at 0.030%. The upper limit of the above Ti content is more advantageous at 0.140%, more advantageous at 0.130%, and most advantageous at 0.120%.

[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. The above N content is excluded from 0% to account for cases where it is unavoidably included in the manufacturing process. Therefore, it is desirable 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, more advantageous for 0.0060% or less, and most advantageous for 0.0040% or less.

[0081] The cold-rolled steel sheet of the present invention satisfies the aforementioned alloy composition and can simultaneously satisfy the following equations 1 to 3.

[0082] [Equation 1] 0.250 ≤ X = C + 0.03Si + 0.02Mn ≤ 0.40

[0083] [Equation 2] 0.20 ≤ Y = B / S ≤ 3.50

[0084] [Equation 3] 1.0 ≤ Z = Y / X ≤ 11.40

[0085] [Equation 1] 0.250 ≤ X = C + 0.03Si + 0.02Mn ≤ 0.40

[0086] The above Equation 1 is a component relationship closely related to weld hardness and spot weld cross tensile strength (CTS). If the value of X is less than 0.250, the hardenability is low, making it difficult to sufficiently secure the target strength. If the value of X exceeds 0.40, the weld hardness becomes excessively high, increasing the risk of brittle fracture, which in turn lowers the cross tensile strength and may result in inferior impact stability. Therefore, it is desirable for the value of X to have a range of 0.250 to 0.40. The lower limit of the value of X is more advantageous at 0.260, more advantageous at 0.270, and most advantageous at 0.280. The upper limit of the value of X is more advantageous at 0.390, more advantageous at 0.380, and most advantageous at 0.370.

[0087] [Equation 2] 0.20 ≤ Y = B / S ≤ 3.50

[0088] The above Equation 2 is a component relationship related to hydrogen embrittlement resistance. If the value of Y is less than 0.20, the grain boundary strengthening effect of B decreases, and MnS inclusions are excessively formed, which may result in inferior bending characteristics and hydrogen embrittlement resistance. If the value of Y exceeds 3.50, the B content increases, raising the cost of the ferroalloy, and manufacturing costs may increase because desulfurization must be strengthened to control S to a low level. Therefore, it is desirable for the value of Y to have a range of 0.20 to 3.50. The lower limit of the value of Y is more advantageous at 0.30, more advantageous at 0.40, and most advantageous at 0.50. The upper limit of the value of Y is more advantageous at 3.40, more advantageous at 3.30, and most advantageous at 3.20.

[0089] [Equation 3] 1.0 ≤ Z = Y / X ≤ 11.40

[0090] The above Equation 3 is a component relationship formula for simultaneously securing strength, spot weld cross tensile strength (CTS), and hydrogen embrittlement resistance. If the value of Z is less than 1.0, the Y value is relatively low, which may result in inferior hydrogen embrittlement resistance. If the value of Z exceeds 11.40, the X value is relatively low, which may make it difficult to secure strength due to insufficient hardenability. Therefore, it is desirable for the value of Z to have a range of 1.0 to 11.40. The lower limit of the Z value is more advantageous at 2.0, more advantageous at 3.0, and most advantageous at 4.0. The upper limit of the Z value is more advantageous at 11.0, more advantageous at 10.0, and most advantageous at 9.0.

[0091] The cold-rolled steel sheet of the present invention may be divided into a non-surface layer in terms of microstructure; and a surface layer formed outside the non-surface layer 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; however, as an example, the surface layer may be an area up to 20㎛ in the thickness direction from the surface of the steel sheet. The non-surface layer refers to the area outside the surface layer described below. The non-surface layer may include a center, and the center refers to an area from 1 / 2t (t: steel thickness) to ±20㎛ in the thickness direction of the steel.

[0092] The microstructure of the above-mentioned non-surface layer may include one or more types of fresh martensite and tempered martensite. The above-mentioned martensite and tempered martensite are structures that are highly advantageous for securing the strength 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 area%, it may be difficult to secure the physical properties intended to be obtained by the present invention. Therefore, the total fraction of one or more types of ferrite and bainite may be 5 area% or less (including 0%). It is more advantageous for the total fraction of one or more types of ferrite and bainite to be 4 area% or less (including 0%), more advantageous to be 3 area% or less (including 0%), and most advantageous to be 2 area% or less (including 0%). Meanwhile, the present invention does not specifically limit the microstructure of the surface layer, but as an example, the microstructure of the surface layer may include, in area %, a total fraction of one or more of ferrite and bainite: 10% or less (including 0%), and the remainder being one or more of fresh martensite and tempered martensite.

[0093] The above-mentioned tempered martensite may contain carbides with an average size of 150 nm or less (excluding 0 nm) within the lath. If the average size of the carbides exceeds 150 nm, the bending characteristics and hydrogen embrittlement resistance may be inferior. It is more advantageous for the average size of the carbides to be 130 nm or less, more advantageous for 110 nm or less, and most advantageous for 100 nm or less. In the present invention, since it is advantageous for the average size of the carbides to be smaller, the lower limit thereof is not specifically limited. However, the lower limit of the average size of the carbides may be 10 nm as an example. Meanwhile, the carbides may be carbides containing Fe and Mn as an example, and the carbides may additionally contain one or more of Si, Cr, Mo, Nb, and Ti.

[0094] Meanwhile, when measuring the microstructure of a steel plate using EBSD, it is classified into low-angle grain boundaries, which are grain boundaries with an orientation difference of 2° or more and less than 15°, and high-angle grain boundaries, which are grain boundaries with an orientation difference of 15° or more. In the present invention, the area fraction of low-angle grain boundaries with an orientation difference of 2° or more and less than 15° in the non-surface layer may be 30% or more. If the area fraction of the low-angle grain boundaries is less than 30%, the martensite laths become coarse, making it difficult to secure the target strength. It is more advantageous for the area fraction of the low-angle grain boundaries to be 31% or more, more advantageous for 32% or more, and most advantageous for 33% or more. Meanwhile, in the present invention, it is advantageous for the area fraction of the low-angle grain boundary to be higher, so there is no specific limit regarding the upper limit, but for example, the upper limit of the area fraction of the low-angle grain boundary may be 50%.

[0095] The cold-rolled steel sheet of the present invention has a Mn segregation ratio (Mn) at the center. max / Mn ave) may be 1.6 or less. If the Mn segregation ratio in the center exceeds 1.6, MnS inclusions increase, and hydrogen accumulates in these MnS inclusions, which may reduce hydrogen embrittlement. It is more advantageous for the Mn segregation ratio in the center to be 1.5 or less, more advantageous for it to be 1.4 or less, and most advantageous for it to be 1.3 or less. Meanwhile, the present invention does not specifically limit the lower limit of the Mn segregation ratio in the center, but as an example, the lower limit may be 1.01.

[0096] Meanwhile, the above Mn max represents the highest Mn content in the center, and the above Mn ave represents the average Mn content of the steel.

[0097] The cold-rolled steel sheet of the present invention may include MnS inclusions having a maximum size of 10 μm or less (excluding 0 μm). If the maximum size of the MnS inclusions exceeds 10 μm, the bending characteristics and hydrogen embrittlement resistance may be significantly reduced. It is more advantageous for the maximum size of the MnS inclusions to be 9 μm or less, more advantageous for 7 μm or less, and most advantageous for 5 μm or less. Meanwhile, the present invention does not specifically limit the lower limit of the maximum size of the MnS inclusions. However, the lower limit of the maximum size of the MnS inclusions may be 0.001 μm as an example. Meanwhile, the MnS inclusions may have a plate-like shape.

[0098] The cold-rolled steel sheet of the present invention may have a surface roughness (Rsk) of -0.70㎛ or higher in the surface layer. The surface roughness (Rsk (Skewness)) is one of several factors of surface roughness related to the asymmetry of sharp protrusions. The closer the value of the surface roughness (Rsk) is to 0 or the more positive the value becomes, the more advantageous it is for securing bending characteristics. As the negative value of the surface roughness (Rsk) increases, valleys deepen on the flat surface, causing stress concentration in this area, which increases sensitivity to crack formation, and consequently, the bending characteristics become inferior. If the surface roughness (Rsk) is less than -0.70㎛, the bending characteristics may be inferior. It is more advantageous for the surface roughness (Rsk) to be -0.60㎛ or higher, more advantageous for -0.50㎛ or higher, and most advantageous for -0.40㎛ or higher. Meanwhile, in the present invention, since a higher surface roughness (Rsk) is advantageous, there is no specific limitation on its upper limit. However, as an example, the upper limit of the surface roughness (Rsk) may be +0.5㎛.

[0099] The cold-rolled steel sheet of the present invention may have a product of Rsk and Pc of -130 μm·ks or more. Pc is a factor representing the number of peaks that completely deviate from the bandwidth within a unit length. The closer the product of Rsk and Pc is to zero or the more positive the value, the more favorable it is for bending characteristics. That is, in the present invention, bending characteristics can be improved by controlling the product of Rsk and Pc to a level of -130 μm·ks or more. A product of Rsk and Pc of -120 μm·ks or more is more favorable, a product of -110 μm·ks or more is more favorable, and a product of -100 μm·ks or more is most favorable. Meanwhile, in the present invention, since a higher product of Rsk and Pc is more favorable, there is no specific limitation on its upper limit. However, as an example, the upper limit of the product of Rsk and Pc may be +60 μm·ks.

[0100] The cold-rolled steel sheet of the present invention may have a diffusible hydrogen content of 0.1 ppm or less. Hydrogen (H) is an element that directly affects hydrogen embrittlement occurring in ultra-high-strength steel, so it is advantageous to minimize it. Hydrogen can be contained in steel during the steel manufacturing process, such as the steelmaking process, cold rolling / annealing process, and electroplating process. Hydrogen contained in the steel in this way can move to stress concentration areas at room temperature and cause cracks at stress concentration areas. Diffusible hydrogen refers to hydrogen that moves to stress concentration areas at room temperature, and generally refers to hydrogen released from the steel at temperatures of 300°C or lower when the temperature is raised at a rate of 100°C / Hr. If the above diffusible hydrogen content exceeds 0.1 ppm, the resistance to hydrogen embrittlement may decrease. It is more advantageous for the above diffusible hydrogen content to be 0.09 ppm or less, more advantageous for 0.08 ppm or less, and most advantageous for 0.07 ppm or less. Meanwhile, in the present invention, since a lower diffusible hydrogen content is advantageous, the lower limit thereof is not specifically limited. However, as an example, the lower limit of the diffusible hydrogen content may be 0.005 ppm.

[0101] As described above, the cold-rolled steel sheet of the present invention provided may have a yield strength (YS1) in the vertical direction of the rolling direction of 1300 to 1750 MPa, a tensile strength (TS) in the vertical direction of the rolling direction of 1670 to 1950 MPa, a yield ratio (YS1 / TS) of 0.95 or less, an elongation (El) of 2 to 10%, a yield strength (YS2) in the horizontal direction of the rolling direction of 1330 to 1780 MPa, and a △YS(YS2-YS1) of 150 MPa or less. It is more advantageous for the yield strength (YS1) in the vertical direction of the rolling direction of 1350 to 1700 MPa. It is more advantageous for the tensile strength (TS) in the vertical direction of the rolling direction of 1700 to 1900 MPa. It is more advantageous for the yield ratio (YS1 / TS) in the vertical direction of the rolling direction of 0.90 or less. In the present invention, since a lower yield ratio is advantageous, its lower limit is not specifically limited. However, as an example, the lower limit of the yield ratio may be 0.82. It is more advantageous for the elongation to be 3 to 9%. It is more advantageous for the horizontal yield strength (YS2) in the rolling direction to be 1350 to 1750 MPa. It is more advantageous for △YS(YS2-YS1) to be 130 MPa or less. In the present invention, since a lower △YS(YS2-YS1) is advantageous, its lower limit is not specifically limited. However, as an example, the lower limit of △YS(YS2-YS1) may be 5 MPa.

[0102] The cold-rolled steel sheet of the present invention may have a bendability (R / t) of 4.0 or less. It is more advantageous for the bendability (R / t) to be 3.9 or less. In the present invention, since a lower bendability (R / t) is more advantageous, there is no specific limitation on its lower limit. However, as an example, the bendability (R / t) may be 0.5.

[0103] The cold-rolled steel sheet of the present invention may not develop cracks with an average size of 3 mm or more when subjected to a 90° V-bending test under conditions where R / t is 4.0 and immersed in a 0.1N HCl solution for 120 hours under conditions of room temperature and atmospheric pressure.

[0104] The cold-rolled steel sheet of the present invention may have a weld hardness of 650 Hv or less and a cross tensile strength (CTS) of 3.0 kN or more after resistance spot welding. It is more advantageous for the weld hardness to be 640 Hv or less. In the present invention, since a lower weld hardness is more advantageous, there is no specific limitation on its lower limit. However, as an example, the weld hardness may be 350 Hv. It is more advantageous for the weld cross tensile strength (CTS) to be 4.0 kN or more. In the present invention, since a higher weld cross tensile strength (CTS) is more advantageous, there is no specific limitation on its upper limit. However, as an example, the upper limit of the weld cross tensile strength (CTS) may be 15 kN.

[0105] The thickness of the cold-rolled steel sheet of the present invention may be 0.6 to 2.3 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 2.2 mm, and 2.1 mm.

[0106] 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.

[0107] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0108] First, a slab is obtained by continuously casting molten steel satisfying the aforementioned alloy composition such that the specific water content during secondary cooling is 0.50 to 3.0 L / kg·s. The molten steel may satisfy the aforementioned Equations 1 to 3. Controlling the specific water content during secondary cooling affects continuous casting operability and slab quality, but it may also be effective in improving hydrogen embrittlement resistance by reducing component segregation through the reduction of internal cracks. If the specific water content during secondary cooling is less than 0.5 L / kg·s, the solidification layer formed on the surface of the slab is thin, and repetitive stress exceeding the critical strain is applied to the interface between the solidification layer and the liquid phase layer by the guide rolls of the continuous casting machine, which may cause internal cracks to occur. Consequently, Mn, P, S, etc., may segregate between the internal cracks, which may reduce hydrogen embrittlement resistance. If the non-water content during the above secondary cooling exceeds 3.0 L / kg·s, cracks may occur due to overcooling in the slab surface layer and edge, resulting in deterioration of quality. Therefore, it is desirable for the non-water content during the above secondary cooling to be in the range of 0.50 to 3.0 L / kg·s. The lower limit of the non-water content during the above secondary cooling is more advantageous at 0.75 L / kg·s, and 1.0 L / kg·s is more advantageous. The upper limit of the non-water content during the above secondary cooling is more advantageous at 2.75 L / kg·s, and 2.5 L / kg·s is more advantageous. The above secondary cooling may be performed in the segments of a continuous casting machine during continuous casting, and the primary cooling performed before the above secondary cooling may be performed within the mold of the continuous casting machine.

[0109] Subsequently, the slab is heated. The slab heating process is performed to facilitate the subsequent hot rolling process and to sufficiently obtain the target physical properties of the steel plate. The heating of the slab may be performed at 1100 to 1300°C. If the slab heating temperature is below 1100°C, a problem may occur in which the hot rolling load increases rapidly. If the slab heating temperature exceeds 1300°C, the amount of surface scale increases, which may lower the yield of the material. 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 slab heating temperature is more advantageous at 1290°C, more advantageous at 1280°C, and most advantageous at 1270°C.

[0110] Subsequently, the heated slab is subjected to finish hot rolling 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 600°C. If the coiling temperature exceeds 600°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 desirable to manage the coiling temperature at a low level 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 Mf, the strength of the hot-rolled steel sheet becomes excessively high, which may increase the rolling load during the subsequent cold rolling process, making actual production impossible. It is more advantageous for the lower limit of the coiling temperature to be Ms + 50°C. It is more advantageous for the upper limit of the coiling temperature to be 550°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(℃) = 539 - 423C - 30.4Mn - 7.5Si + 30Al - 17.7Ni - 12.1Cr - 7.5Mo

[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 wound hot-rolled steel sheet is cold-rolled using a roll having a surface roughness (Ra) of 2.5 μm or more to obtain a cold-rolled steel sheet. If the surface roughness (Ra) of the cold-rolling roll is less than 2.5 μm, the negative value of the surface roughness (Rsk) of the steel sheet becomes excessively large, which may degrade the bending characteristics. In the present invention, since a larger surface roughness (Ra) of the cold-rolling roll results in fewer deep grooves in the rolling direction that are inferior to the bending characteristics, the upper limit is not specifically limited. The surface roughness (Ra) of the cold-rolling roll may more advantageously be 3.0 μm or more. In the present invention, since a larger surface roughness (Ra) of the cold-rolling roll is advantageous, the upper limit is not specifically limited; however, as an example, the upper limit of the surface roughness (Ra) of the cold-rolling roll may be 6.5 μm. The above cold rolling can be performed with a cold reduction rate of 45 to 70%. If the above cold reduction rate is less than 45%, it is difficult to secure the thickness desired in the present invention, and there is a concern that the residual crystal grains formed during hot rolling may cause austenite to form during annealing heat treatment, affecting the final physical properties. In addition, the negative value of surface roughness (Rsk) may become excessively large, leading to inferior bending characteristics. If the above cold reduction rate exceeds 70%, the amount of reduction in the length and width directions may become uneven due to work hardening occurring during cold rolling, which may result in material variation of the steel sheet. Furthermore, it may be difficult to secure the thickness desired in the present invention due to the rolling load. The lower limit of the above cold reduction rate is more advantageous at 46%, more advantageous at 47%, and most advantageous at 48%. The upper limit of the above cold rolling rate is more advantageous at 68%, more advantageous at 66%, and most advantageous at 64%.

[0116] Subsequently, the above cold-rolled steel sheet is continuously annealed at Ac3+20℃ to Ac3+110℃. If the continuous annealing temperature is less than Ac3+20℃, 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 expansion properties. Furthermore, it may be difficult to sufficiently secure the low-angle grain boundary fraction intended by the present invention. If the above continuous annealing temperature exceeds Ac3+110℃, equipment trouble may occur due to overloading of the annealing furnace. The lower limit of the above continuous annealing temperature is more advantageous at Ac3+30℃, more advantageous at Ac3+40℃, and most advantageous at Ac3+50℃. The upper limit of the above continuous annealing temperature is more advantageous at Ac3+100℃, more advantageous at Ac3+90℃, and most advantageous at Ac3+80℃. 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 30 to 230 seconds. If the above continuous annealing time is less than 30 seconds, there is a disadvantage in that it is difficult to secure a single-phase austenite structure. If the above continuous annealing time exceeds 230 seconds, the austenite size becomes coarse, resulting in a disadvantage in that it is difficult to secure strength and bending characteristics. The lower limit of the above continuous annealing time is more advantageous at 40 seconds, more advantageous at 50 seconds, and most advantageous at 60 seconds. The upper limit of the above continuous annealing time is more advantageous at 220 seconds, more advantageous at 210 seconds, and most advantageous at 200 seconds.

[0117] [Equation 3] Ac3(℃) = 875 - 206C + 26.2Si - 25.0Mn - 12.3Cr + 9.12Mo + 50.2Nb + 148Ti - 131B

[0118] Subsequently, the continuously annealed cold-rolled steel sheet is first cooled to a first cooling end temperature (T1). During the first cooling, the first cooling end temperature is controlled to 670 to 750°C and can be carried out at an average cooling rate of 1 to 6°C / s. If the first cooling end temperature is less than 670°C, a large amount of soft ferrite and bainite other than martensite is formed during the cooling process, causing the surface layer structure to become uneven and the bending characteristics may deteriorate. If the first cooling end temperature exceeds 750°C, 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 680°C, and more advantageous for it to be 690°C. The upper limit of the above first cooling end temperature is more advantageously 740°C, and 730°C is more advantageous. If the above first average cooling rate is less than 1°C / s, ferrite is formed during cooling, making it difficult to secure the level of strength targeted by the present invention. If the above first average cooling rate exceeds 6°C / s, the average cooling rate during the subsequent second cooling decreases, increasing the fraction of low-temperature transformation phases other than martensite, making it difficult to secure the level of strength targeted by the present invention. The lower limit of the above first average cooling rate is more advantageously 2°C / s. The upper limit of the above first average cooling rate is more advantageously 5°C / s.

[0119] Subsequently, the firstly cooled cold-rolled steel sheet is secondarily cooled to a second cooling end temperature (T2). The second cooling is intended to secure one or more of the main phases of the present invention, namely martensite and tempered martensite. During the second cooling, the second cooling end temperature (T2) is controlled to 40 to 250°C and can be carried out at an average cooling rate of 30 to 600°C / s. 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 250°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 240°C, more advantageous at 230°C, and most advantageous at 220°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 600°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 35°C / s, more advantageous at 40°C / s, and most advantageous at 45°C / s. The upper limit of the above secondary average cooling rate is more advantageous at 500°C / s, more advantageous at 400°C / s, and most advantageous at 300°C / s.

[0120] During the above secondary cooling, it is preferable that the martensite transformation end temperature (Mf) - secondary cooling end temperature (T2) be controlled within the range of 20 to 150°C. If the martensite transformation end temperature (Mf) - secondary cooling end temperature (T2) is less than 20°C, the martensite transformation may not occur sufficiently, making it difficult to secure the target strength. Furthermore, it may be difficult to sufficiently secure the low-angle grain boundary fraction intended by the present invention. If the martensite transformation end temperature (Mf) - secondary cooling end temperature (T2) exceeds 150°C, the product shape may deteriorate. It is more advantageous for the lower limit of the martensite transformation end temperature (Mf) - secondary cooling end temperature (T2) to be 30°C, and more advantageous for it to be 40°C. The upper limit of the martensite transformation end temperature (Mf) - secondary cooling end temperature (T2) is more advantageous at 140°C and more advantageous at 130°C. Meanwhile, the above Mf represents the temperature at which the martensite transformation ends during cooling, and can be calculated using the following Equation 4.

[0121] [Equation 4] Mf(°C) = 371 - 412C - 17.4Si - 47.4Mn - 20.9Cr - 17.0Mo + 49.2Nb + 95.0Ti + 202B

[0122] Subsequently, the second-cooled cold-rolled steel sheet is reheated to the overaging treatment temperature (H) and then subjected to overaging treatment. Through the reheating and overaging treatment, the martensite obtained by the aforementioned rapid cooling process is transformed into tempered martensite, thereby increasing the yield strength. During the overaging treatment, the overaging treatment temperature (H) is controlled to 130 to 300°C and can be performed for 5 to 12 minutes. If the overaging treatment temperature is below 130°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 overaging treatment temperature exceeds 300°C, there is a disadvantage in that bendability and hydrogen embrittlement resistance are inferior due to the precipitation and coarsening of a large amount of carbides. The lower limit of the above overaging treatment temperature is more advantageous at 140°C, more advantageous at 150°C, and most advantageous at 160°C. The upper limit of the above overaging treatment temperature is more advantageous at 280°C, more advantageous at 260°C, and most advantageous at 240°C. If the above overaging treatment time is less than 5 minutes, tempering is not sufficiently performed, and the yield strength may decrease. If the above overaging treatment time exceeds 12 minutes, carbides may coarsen due to excessive tempering, and bending characteristics may deteriorate. The lower limit of the above overaging treatment time is more preferable at 5.5 minutes, more preferable at 6.0 minutes, and most preferable at 6.5 minutes. The upper limit of the above overaging treatment time is more preferable at 11.5 minutes, more preferable at 11 minutes, and most preferable at 10.5 minutes.

[0123] During the above secondary cooling and overaging treatment, the overaging treatment temperature (H) - secondary cooling end temperature (T2) can be controlled to be in the range of 30 to 150°C. If the overaging treatment temperature (H) - secondary cooling end temperature (T2) is less than 30°C, sufficient tempering may not occur, making it difficult to secure the target yield strength. If the overaging treatment temperature (H) - secondary cooling end temperature (T2) exceeds 150°C, the strength may decrease or the carbides may become coarse due to excessive tempering, resulting in inferior bending characteristics. The lower limit of the overaging treatment temperature (H) - secondary cooling end temperature (T2) is more advantageous at 40°C, more advantageous at 50°C, and most advantageous at 60°C. The upper limit of the above over-aging treatment temperature (H) - secondary cooling end temperature (T2) is more advantageous at 140°C, more advantageous at 130°C, and most advantageous at 120°C.

[0124] Subsequently, the above-mentioned over-aged cold-rolled steel sheet is subjected to temper rolling (SPM (Skin Pass Mill)) with a rolling force of 500 to 1000 tons. The above temper rolling enables control of surface roughness (Rsk). When the rolling force during the above temper rolling is less than 500 tons, the load is low, making it difficult to control the surface roughness (Rsk); when it exceeds 1000 tons, the surface undergoes severe work hardening, which may result in inferior bending characteristics. When the above temper rolling is performed, it is more advantageous for the lower limit of the rolling force to be 550 tons, and more advantageous for it to be 600 tons. When the above temper rolling is performed, it is more advantageous for the upper limit of the rolling force to be 950 tons, and more advantageous for it to be 900 tons.

[0125] 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 0.050% to 0.65%. If the elongation is less than 0.050% during the tension leveling, it may be difficult to correct the shape. If the elongation exceeds 0.50% during the tension leveling, work hardening may become severe, and bending characteristics may deteriorate. It is more advantageous for the lower limit of the elongation during the tension leveling to be 0.080%, and more advantageous for it to be 0.10%. It is more advantageous for the upper limit of the elongation during the tension leveling to be 0.75%, and more advantageous for it to be 0.70%.

[0126] 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.

[0127] 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.

[0128] (Example)

[0129] A slab was obtained by continuously casting molten steel having the alloy composition listed in Table 1 below with the non-water content during secondary cooling listed in Table 2 below. The slab was heated to 1200°C, and then the heated slab was finished hot-rolled at 900°C to obtain a hot-rolled steel sheet, which was coiled at 450°C. Subsequently, the coiled hot-rolled steel sheet was cold-rolled under the conditions listed in Table 2 below to obtain a cold-rolled steel sheet. Subsequently, a cold-rolled steel sheet with a thickness of 1.4 mm was manufactured by continuous annealing, primary cooling, secondary cooling, reheating / overaging treatment, temper rolling, and tension leveling under the conditions listed in Tables 2 to 4 below. Meanwhile, the conditions listed in Tables 2 to 4 below were based on the surface temperature of the steel sheet.

[0130] For the cold-rolled steel sheets manufactured in this manner, the microstructure, average size of carbides in the laths of tempered martensite, maximum size of MnS inclusions, Mn segregation ratio in the center, surface roughness, diffusible hydrogen content, and mechanical properties were measured, and the results are shown in Tables 5 and 6 below.

[0131] The types and fractions of the microstructure were observed at the non-surface layer (1 / 4 t (t: thickness of the steel)) of the steel plate using a scanning electron microscope (SEM) and an optical microscope (OM), and the fractions of each phase were analyzed 20 times through image analysis to calculate the average values. In addition, the area fraction of low-angle grain boundaries with an orientation difference of 2° or more and less than 15° was calculated by using an EBSD (Backscattered Electron Diffraction Pattern Analyzer) to measure the non-surface layer (1 / 4 t (t: thickness of the steel)) of the steel plate at ×2000 magnification (45×45㎛) 10 times (Confidence Index (CI)≥0.3, Step Size: 80nm) and obtaining the average values.

[0132] The average size of carbides in the lath of tempered martensite was calculated by preparing a specimen as a thin film, photographing it with a transmission electron microscope (TEM), and then calculating the average value.

[0133] The maximum size of MnS inclusions was measured 20 times at 500x magnification using an Electron Probe Micro Analyzer (EPMA), and the average value was calculated.

[0134] Mn segregation ratio in the center (Mn max / Mn ave ) was measured using EPMA (Electron Probe Micro Analyzer) and OES (Optical Emission Spectrometry). Mn max For the region from 1 / 2t (t: steel thickness) to ±20㎛, the Mn concentration was measured at 0.1㎛ intervals in the thickness direction of the steel, and the highest Mn content was calculated, and Mn ave It was measured using OES (Optical Emission Spectrometry).

[0135] Surface roughness (Rsk, Pc) was measured 5 times using a contact-type 2D roughness meter, and the average value was calculated excluding the maximum value (Max) and minimum value (Min).

[0136] The diffusible hydrogen content was measured using Thermal Desorption Spectroscopy (TDS) equipment, which quantitatively measures the gaseous emission of hydrogen released from a specimen according to temperature changes. At this time, the diffusible hydrogen content was measured by determining the amount of hydrogen released from the steel plate at a temperature of 300°C or lower when the steel plate was heated from room temperature at a rate of 100°C / Hr.

[0137] Yield strength, tensile strength, yield ratio, and total elongation were measured by processing cold-rolled steel sheets into specimens of JIS No. 5 and then performing a tensile test under conditions of a test speed of 28 mm / min.

[0138] The bending workability (R / t) was determined by processing a cold-rolled steel sheet into a specimen with a width of 30 mm × a length of 100 mm, performing a 90° bending test at a test speed of 800 mm / min, checking for the occurrence of cracks in the bending section using a stereoscopic microscope, and calculating the minimum bending radius (R value of the die) at which no cracks occurred by dividing it by the thickness (mm) of the specimen.

[0139] Hydrogen embrittlement resistance was measured by extracting three specimens with a width of 30 mm × a length of 100 mm from a cold-rolled steel sheet, performing a 90° V-bending test under conditions of R / t of 4, and visually checking how many cracks of 3 mm or more occurred when immersed in a 0.1 N HCl solution for 120 hours under conditions of room temperature and atmospheric pressure.

[0140] Spot weldability was determined by performing welding with a welding current in the range of 5.0 to 9.0 kA under conditions of Force: 4.5 KN, Welding time: 340 ms, and Holding time: 260 ms, and then deriving the lower limit welding current that satisfies the Plug diameter (4.25√t: steel thickness), and measuring the hardness and cross tensile strength (CTS) of the spot weld welded under the above conditions. In addition, the hardness of the spot weld was measured 10 times using Vickers hardness (load: 500 gf) on the spot weld and the average value was calculated.

[0141] Steel Grade No. Alloy Composition (Weight%) CSI Mn PS Al Nb Ti BN XY Z 10.30 0.1 11.9 7 0.01 00.00 100.03 00.03 5 0.02 00.00 25 0.00 400.34 2.5 0 7.35 20.27 0.20 2.1 00.01 10.00 9 0.02 5 0.02 4 0.02 5 0.00 200.00 41 0.32 2.22 6.9 43 0.30 0.1 11.950.0110.00070.0310.0210.0310.00150.00370.342.146.2940.280.091.990.0150.00150.0340.0370.0410.00270.00350.321.805.6350.220.150.150.0090.00110.0250.0310.0250.00210.0 0370.231.918.3060.300.152.150.0070.00410.0400.0210.0450.00100.00310.350.240.6970.310.201.500.0070.00290.0410.0340.0410.00010.00410.350.030.0980.230.030.050.0140.00150 .0340.0420.0470.00210.00370.231.406.0990.410.152.100.0100.00110.0310.0350.0250.00200.00310.461.823.96100.330.203.800.0100.00120.0350.0250.0190.00210.00350.411.754.27X = C + 0.03Si + 0.02MnY = B / SZ = Y / X, where X and Y values ​​are calculated to 2 decimal places.

[0142] Classification Steel Grade No. Secondary Cooling Water Amount (ℓ / kg·s) Ar3 (°C) Ms (°C) Cold Reduction Rate (%) Cold Rolling Roll Roughness (㎛) Ac3 (°C) Annealing Temperature (°C) Annealing Time (s) Invention Example 1 1.580 435 256 4.579 6857 141 Invention Example 2 2.081 3360 56 4.580 286 1151 Invention Example 3 3.080 435 356 4.579 8859 139 Invention Example 4 4.580 7360 56 4.580 285 4145 Comparative Example 1 51.5821441564.5860850142 Comparative Example 2 61.5806347564.5796861140 Comparative Example 3 71.5806362564.3812859139 Comparative Example 481.5814441564.5861861142 Comparative Example 5 91.5787302564. 5772864151 Comparative Example 6 101.5802283564.5746852142 Comparative Example 7 10.3804352482.5796857142 Comparative Example 8 11.5804352351.5796861139 Comparative Example 9 11.5804352564.5796780142 Comparative Example 10 12.080 4352564.3796861145 Comparative Example 1111.5804352564.3796859139 Comparative Example 1212.0804352482.7796864142 Comparative Example 1312.0804352381.5796869137 Comparative Example 1410.4804352564.5796864141

[0143] Classification Steel Grade No. 1st Cooling End Temperature (T1) (°C) 1st Average Cooling Rate (°C / s) Mf (°C) 2nd Cooling End Temperature (T2) (°C) 2nd Average Cooling Rate (°C / s) Mf - T2 (°C) Invention Example 1 17 15 315 68 97 267 Invention Example 2 27 05 316 195 74 66 Invention Example 3 37 123 15 71 02 76 55 Invention Example 4 47 16 316 69 77 569 Comparative Example 1 57 25 32 75 125 74 150 Comparative Example 2 67 313 14 810 57 74 3 Comparative Example 3 77 36 317 49 77 377 Comparative Example 4 872532809476186 Comparative Example 59715310415278-48 Comparative Example 610705354.952723 Comparative Example 7171531561017555 Comparative Example 817213156977359 Comparative Example 9171231561057851 Comparative Example 101716315627056-114 Comparative Example 111719315623059-74 Comparative Example 12172131561077249 Comparative Example 1317143156977559 Comparative Example 1417153156927764

[0144] Classification Steel Grade No. Reheating / Overaging Treatment Temperature (H) (°C) Overaging Treatment Time (min) H-T2 (°C) Temper Rolling Reduction Force (ton) Tension Leveling Elongation (%) Invention Example 1 11858.49665000.35 Invention Example 2 21708.7756500.35 Invention Example 3 31858.4836500.35 Invention Example 4 41768.5796500.35 Comparative Example 1 51757.9506500.35 Comparative Example 2 61958.1906500.35 Comparative Example 3 71758.4785500.35 Comparative Example 4 81818.1 876500.35 Comparative Example 593058.77536500.55 Comparative Example 6101658.11136500.35 Comparative Example 711788.5776800.55 Comparative Example 811858.1886800.35 Comparative Example 911878.0826500.35 Comparative Example 1012759.156500.35 Comparative Example 1113208.7906500.35 Comparative Example 1211948.9872500.35 Comparative Example 1311947.9973500.02 Comparative Example 1411858.5936800.95

[0145] Classification Steel Grade No. Microstructure Low-angle grain boundary fraction (Area %) Average carbide size (nm) MnS Average inclusion size (㎛) Mn maxMn Segregation Ratio Rsk (㎛) Pc (㎛) Rsk × Pc (㎛·ks) Diffusible Hydrogen Amount (ppm) At least one of F and B At least one of FM and TM Invention Example 1 1 298 40 595 2.44 1.24 -0.42 172 -72 0.035 Invention Example 2 2 298 395 042.44 1.16 -0.10 183 -18 0.024 Invention Example 3 3 298 395 242.24 1.15 -0.20 178 -36 0.035 Invention Example 4 4 ​​298 385 66 2.47 1.24 -0.32 178 -57 0.042 Comparative Example 1 5 158 537 425 0.16 1.07 -0.42187 -790.038 Comparative Example 26 29839105252.411.12 -0.37175 -650.110 Comparative Example 37793294751.641.09 -0.38169 -640.068 Comparative Example 481585285760.061.20 -0.42174 -770.051 Comparative Example 598923518952.631.25 -0.38175 -670.089 Comparative Example 61 029841108248.172.15-0.41169-690.108 Comparative Example 7129838112183.621.84-0.35167-580.102 Comparative Example 81298395652.461.25-0.75204-1530.042 Comparative Example 912575293752.071.05-0.41189-770.021 Comparative Example 1018922816752.271.15 -0.38 1877 10.052 Comparative Example 11 179 329 201 52.66 1.35 -0.45 175 -79 0.048 Comparative Example 12 1298 3869 52.80 1.42 -0.72 201 -145 0.074 Comparative Example 13 1298 3874 52.64 1.34 -0.71 205 -146 0.085 Comparative Example 14 1298 4079 123.43 1.74 -0.42 175 -74 0.105 F: Ferrite, B: Bainite, FM: Fresh Martensite, FM: Tempered Martensite

[0146] Classification Steel Grade No. Width Direction Rolling Direction △ YS(YS2-YS1)(MPa) Bending Workability (R / t) Number of Cracks with Average Size of 3mm or More (pieces) Weld Hardness (Hv) Weld Cross Tensile Strength (CTS) (kN) Yield Strength (YS1)(MPa) Tensile Strength (MPa) YR Elongation (%) Yield Strength (YS2)(MPa) Invention Example 1 1 14 21 17 56 0.8 15 14 69 48 3.6 06 15 5.1 Invention Example 2 2 14 35 17 87 0.8 05 14 68 33 3.6 05 84 5.7 Invention Example 3 3 14 7 41 77 40.8 35 15 11 373.606145.3 Invention Example 44145817580.8351501433.606015.6 Comparative Example 15128716740.7761352653.604616.4 Comparative Example 26143217360.8251465334.636273.5 Comparative Example 37136016940.8051432723.916204.6 Comparative Example 48129716840.7761387903.60 4658.9 Comparative Example 59138717050.8151436494.637012.8 Comparative Example 610147017950.8241521515.036243.2 Comparative Example 71143517540.8251514795.036174.3 Comparative Example 81141617390.8161476604.626164.8 Comparative Example 91125616250.7741354985.036213. 5 Comparative Example 10 113 14 160 50.8 26 135 23 84.3 16 184.9 Comparative Example 11 13 78 15 94 0.8 65 145 274.6 36 174.1 Comparative Example 12 113 25 17 260.7 76 135 63 14.3 16 21 4.3 Comparative Example 13 112 94 17 360.7 55 130 511 4.3 16 184.4 Comparative Example 14 114 87 17 680.8 44 165 817 15.4 36 204.2

[0147] As can be seen from Tables 1 to 6 above, in the case of Inventive Examples 1 to 4, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, it can be seen that the microstructure, average size of carbides in the laths of tempered martensite, maximum size of MnS inclusions, Mn segregation ratio in the center, surface roughness, diffusible hydrogen content, and mechanical properties proposed by the present invention are satisfied. On the other hand, in the case of Comparative Examples 2 and 3, which satisfy the manufacturing conditions proposed by the present invention but do not satisfy the alloy composition, it can be seen that the mechanical properties are inferior as one or more of the microstructure, average size of carbides in the laths of tempered martensite, maximum size of MnS inclusions, Mn segregation ratio in the center, surface roughness, and diffusible hydrogen content are not satisfied.

[0148] In the case of Comparative Examples 1, 4, 5, and 6, which do not satisfy the alloy composition and manufacturing conditions proposed by the present invention, it can be seen that the mechanical properties are inferior as they do not satisfy one or more of the following: microstructure, average size of carbides in the lath of tempered martensite, maximum size of MnS inclusions, Mn segregation ratio in the center, surface roughness, and diffusible hydrogen content.

[0149] In the case of Comparative Examples 7 to 14, which satisfy the alloy composition proposed by the present invention but do not satisfy the manufacturing conditions, it can be seen that the mechanical properties are inferior as one or more of the following are not satisfied: microstructure, average size of carbides in the lath of tempered martensite, maximum size of MnS inclusions, Mn segregation ratio in the center, surface roughness, and diffusible hydrogen content.

[0150] Figure 1 is a profile of the Mn content in the center of Invention Example 1 measured using an EPMA (Electron Probe Micro Analyzer). As can be seen from Figure 1, in the case of Invention Example 1, the Mn in the center of the steel maxIt can be seen that α is 2.44. Meanwhile, in the thickness direction of the steel, from the center (1 / 2t), + means the surface direction of the steel and - means the center direction of the steel.

Claims

1. In weight%, carbon (C): 0.240~0.350%, silicon (Si): 0.030~0.50%, manganese (Mn): 0.40~2.50%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.00350% or less (excluding 0%), boron (B): 0.00050~0.0050%, and the remainder being Fe and other unavoidable impurities, It is divided into a non-surface layer including a central part; and a surface layer formed on the outer side of the non-surface layer based on the thickness direction. In the above non-surface layer, the fraction of low-angle grain boundaries having an orientation difference of 2° or more and less than 15° is 30% or more of area, and Mn segregation ratio (Mn) in the above central part max / Mn ave ) is 1.6 or less, and Cold-rolled steel sheet having a surface roughness (Rsk) of -0.70㎛ or higher in the above surface layer. (However, the above-mentioned center refers to the region from 1 / 2t (t: steel thickness) to ±20㎛ in the thickness direction of the steel, and the above-mentioned Mn max represents the highest Mn content in the center, and the above Mn ave represents the average Mn content of the steel, and the surface layer refers to the region extending up to 20㎛ in the thickness direction from the surface of the steel.) 2. In Claim 1, The above cold-rolled steel sheet is a cold-rolled steel sheet that additionally contains one or more of the following: aluminum (Al): 0.010~0.10%, niobium (Nb): 0.0030~0.050%, titanium (Ti): 0.0050~0.150%, and nitrogen (N): 0.010% or less (excluding 0%).

3. In Claim 1, The above cold-rolled steel sheet is a cold-rolled steel sheet satisfying the following equations 1 to 3. [Equation 1] 0.250 ≤ X = C + 0.03Si + 0.02Mn ≤ 0.40 [Equation 2] 0.20 ≤ Y = B / S ≤ 3.50 [Equation 3] 1.0 ≤ Z = Y / X ≤ 11.40 (However, the content of the alloying elements listed in the above equations 1 and 2 is in weight%.) 4. In Claim 1, A cold-rolled steel sheet in which the microstructure of the above-mentioned non-surface layer 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.

5. In Claim 4, The above tempered martensite is a cold-rolled steel sheet containing carbides with an average size of 150 nm or less (excluding 0 nm) within the lath.

6. In Claim 1, The above cold-rolled steel sheet is a cold-rolled steel sheet containing MnS inclusions with an average size of 10㎛ or less (excluding 0㎛).

7. In Claim 1, The above cold-rolled steel sheet is a cold-rolled steel sheet having a diffusible hydrogen content of 0.1 ppm or less.

8. In Claim 1, The above cold-rolled steel sheet is a cold-rolled steel sheet in which the product of Rsk and Pc is -130㎛·ks or more.

9. In Claim 1, The above cold-rolled steel sheet has a yield strength (YS1) in the vertical direction of the rolling direction of 1300~1750MPa, a tensile strength (TS) in the vertical direction of the rolling direction of 1670~1950MPa or higher, a yield ratio (YS1 / TS) of 0.95 or lower, an elongation (El) of 2~10%, a yield strength (YS2) in the horizontal direction of the rolling direction of 1330~1780MPa, and a △YS(YS2-YS1) of 150MPa or lower.

10. In Claim 1, The above cold-rolled steel sheet is a cold-rolled steel sheet having a bendability (R / t) of 4.0 or less.

11. In Claim 1, The above cold-rolled steel sheet is a cold-rolled steel sheet having a weld hardness of 650 Hv or less and a weld cross tensile strength (CTS) of 3.0 kN or more after resistance spot welding.

12. In Claim 1, The above cold-rolled steel sheet is a cold-rolled steel sheet having a plating layer formed on at least one surface.

13. A step of obtaining a slab by continuously casting molten steel containing, in wt%, carbon (C): 0.240~0.350%, silicon (Si): 0.030~0.50%, manganese (Mn): 0.40~2.50%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.00350% or less (excluding 0%), boron (B): 0.00050~0.0050%, and the remainder being Fe and other unavoidable impurities, such that the specific water content becomes 0.5~3.0ℓ / kg·s during secondary cooling; Step of heating the above slab; 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 using a roll having a roughness (Ra) of 2.5 μm or more; A step of continuously annealing the above cold-rolled steel sheet at Ac3+20℃ to Ac3+110℃; A step of first cooling the above continuously annealed cold-rolled steel sheet to a first cooling end temperature (T1); A step of secondarily cooling the above first cooled cold-rolled steel sheet to a second cooling end temperature (T2); A step of reheating the above secondary cooled cold-rolled steel sheet to the above overaging treatment temperature (H) and then overaging treatment; A step of temper rolling the above-mentioned over-aged cold-rolled steel sheet with a rolling force of 500 to 1000 tons; and The method includes the step of tension leveling the above temper-rolled cold-rolled steel sheet; and A method for manufacturing a cold-rolled steel sheet in which, during the second cooling above, the martensite transformation end temperature (Mf) - second cooling end temperature (T2) is controlled to a range of 20 to 150°C.

14. In Claim 11, A method for manufacturing a cold-rolled steel sheet in which the above slab additionally comprises one or more of the following: aluminum (Al): 0.010~0.10%, niobium (Nb): 0.0030~0.050%, titanium (Ti): 0.0050~0.150%, and nitrogen (N): 0.010% or less (excluding 0%).

15. In Claim 11, The above slab is a method for manufacturing a cold-rolled steel sheet satisfying the following equations 1 to 3. [Equation 1] 0.250 ≤ X = C + 0.03Si + 0.02Mn ≤ 0.40 [Equation 2] 0.20 ≤ Y = B / S ≤ 3.50 [Equation 3] 1.0 ≤ Z = Y / X ≤ 11.40 (However, the content of the alloying elements listed in the above equations 1 and 2 is in weight%.) 16. In Claim 11, A method for manufacturing cold-rolled steel sheets in which the heating of the above slab is performed at 1100~1300℃.

17. In Claim 11, The above finishing hot rolling is a method for manufacturing cold-rolled steel sheets performed at Ar3 to Ar3+120℃.

18. In Claim 11, The above coiling is a method for manufacturing cold-rolled steel sheets performed at Ms~600℃.

19. In Claim 11, The above cold rolling is a method for manufacturing a cold-rolled steel sheet, performed with a cold reduction rate of 45 to 70%.

20. In Claim 11, The above continuous annealing is a method for manufacturing cold-rolled steel sheets, performed for 50 to 200 seconds.

21. In Claim 11, A method for manufacturing a cold-rolled steel sheet, wherein, during the first cooling, the first cooling end temperature (T1) is controlled to 670~750℃ and the cooling is performed at an average cooling rate of 1~6℃ / s.

22. In Claim 11, A method for manufacturing a cold-rolled steel sheet, wherein, during the second cooling, the second cooling end temperature (T2) is controlled to 40~250℃ and the cooling is performed at an average cooling rate of 30~600℃ / s.

23. In Claim 11, A method for manufacturing a cold-rolled steel sheet in which, during the above overaging treatment, the overaging treatment temperature (H) is controlled to 130~300℃ and is performed for 5~12 minutes.

24. In Claim 11, A method for manufacturing a cold-rolled steel sheet in which, during the above-mentioned secondary cooling and overaging treatment, the overaging treatment temperature (H) - secondary cooling end temperature (T2) is controlled to be in the range of 30 to 150°C.

25. In Claim 11, A method for manufacturing a cold-rolled steel sheet in which the above tension leveling is performed with an elongation of 0.05 to 0.65%.

26. In Claim 25, 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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