Cold rolled steel sheet and method for manufacturing same

Abstract

One aspect of the present invention is that a cold rolled steel sheet and a method for manufacturing same are to be provided. One advantageous aspect of the present invention is that an ultra-high strength cold rolled steel sheet and a method for manufacturing same are to be provided, the cold rolled steel sheet having a tensile strength of 1470 MPa or higher and excellent bending properties, spot weldability, and corrosion resistance while having excellent formability.

Description

Cold rolled steel sheet and its manufacturing method

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more particularly, to a cold-rolled steel sheet suitable for use as a steel material for reinforcing automobile parts such as bumper beams and sill side beams, or a steel material for protecting electric vehicle battery cases such as side frames and cross members, and a method for manufacturing the same.

[0002] Steels primarily used in reinforcing parts related to the crash safety of automobile occupants require high processing properties, particularly excellent bending properties, and ultra-high strength. To this end, active research is being conducted on ultra-high-strength steels with a tensile strength of 1470 MPa or higher, utilizing a single martensitic phase, and on methods for manufacturing such steels.

[0003] Recently, the Hot Press Forming (HPF) method has been developed, which forms a material using a die at high temperatures, which is an environment conducive to forming, and then secures the required strength through water cooling. Because it can secure high strength for the same thickness, the HPF method is widely used in the manufacturing of parts. However, the HPF method has the disadvantage of requiring excessive facility investment and increased process costs. Therefore, the development of materials for cold stamping and roll forming is necessary. In other words, the development of ultra-high-strength cold-rolled steel sheets is required that are suitable for use as materials for cold stamping and roll forming, have high strength and a high yield ratio to ensure crashworthiness, and have excellent bending properties for part forming, spot weldability for part assembly, and corrosion resistance to ensure a long part life.

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

[0005] Patent Document 2 relates to a sheet steel sheet having a high strength composition including 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, and having 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%), and having a yield strength standard deviation in the width direction of 30 MPa or less, and a maximum bending amount of the sheet 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] Therefore, in order to solve the above-mentioned problems, it is necessary to develop an ultra-high strength cold-rolled steel sheet having a tensile strength of 1470 MPa or more, which has excellent shape, bending characteristics, spot weldability, and corrosion resistance.

[0007] [Prior Art Literature]

[0008] (Patent Document 1) Japanese Patent Application Laid-Open No. 2010-248565

[0009] (Patent Document 2) Japanese Patent Application Laid-Open No. 2020-019992

[0010] One aspect of the present invention is to provide a cold rolled steel sheet and a method for manufacturing the same.

[0011] An advantageous aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet having a tensile strength of 1470 MPa or more, which has an excellent shape, as well as excellent bending properties, spot weldability, and corrosion resistance, and a method for manufacturing the same.

[0012] One embodiment of the present invention provides a cold-rolled steel sheet comprising, in wt%, carbon (C): 0.150 to 0.350%, silicon (Si): 0.0050 to 0.70%, manganese (Mn): 0.30 to 2.50%, chromium (Cr): 0.0050 to 0.60%, molybdenum (Mo): 0.0030 to 0.50%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.0070% or less (excluding 0%), the remainder being iron and other unavoidable impurities, and divided into a central portion; and a surface layer portion formed on the outer side based on the thickness direction of the central portion; and a surface roughness (Rsk) of the surface layer portion is -0.65 to 0.65.

[0013] (However, the above surface layer refers to the area from the surface of the steel material to 20㎛ in the thickness direction).

[0014] The above cold rolled steel sheet may have a product of the surface roughness (Rsk) of the surface layer and Pc of -150 or more.

[0015] The above cold rolled steel sheet may have an absolute value of the product of the surface roughness (Rsk) of the surface layer and the bending workability (R / t) of 2.4 or less.

[0016] Another embodiment of the present invention comprises, in wt%, carbon (C): 0.150 to 0.350%, silicon (Si): 0.0050 to 0.70%, manganese (Mn): 0.30 to 2.50%, chromium (Cr): 0.0050 to 0.60%, molybdenum (Mo): 0.0030 to 0.50%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.0070% or less (excluding 0%), the remainder being iron and other unavoidable impurities, and comprises a central portion; And a surface layer formed on the outer side based on the thickness direction of the center; and when the number fraction (Number Fraction) of the area where the KAM value is 0 to 5.0° is measured by EBSD and is set to 100%, the number fraction (m) of the area where the KAM value is 0 to 1.0° in the center is 30% or less (excluding 0%), the number fraction (n) of the area where the KAM value is 0 to 1.0° in the surface layer is 55% or less (excluding 0%), and the number fraction (m) of the area where the KAM value is 0 to 1.0° in the center and the number fraction (n) of the area where the KAM value is 0 to 1.0° in the surface layer have a relationship of m < n.

[0017] (However, the above surface layer refers to an area from the surface of the steel to 20㎛ in the thickness direction, and the above KAM (Kernel Average Misorientation) refers to the orientation difference.)

[0018] The above cold rolled steel sheet may additionally contain one or more of boron (B): 0.00050 to 0.0050%, aluminum (Al): 0.010 to 0.10%, niobium (Nb): 0.010 to 0.010%, titanium (Ti): 0.0050 to 0.080%, and nitrogen (N): 0.010% or less (excluding 0%).

[0019] The above cold rolled steel sheet can satisfy the following relationships 1 to 4.

[0020] [Relationship 1] X = C + 0.03Si + 0.02Mn: 0.24~0.40

[0021] [Relationship 2] Y = (Cr + 0.8C + 0.07(Si+Mn) - 0.1Mo - 50B - 70P - 30S)X100 + 100 : 30~120

[0022] [Relationship 3] Y / X = 125~375

[0023] [Relationship 4] Z = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb+ 10000B : 55~250

[0024] (However, in the above equations 1 to 4, the content of each alloy element is in weight%.)

[0025] The water fraction (m) of the area where the KAM value is 0 to 1.0° in the center and the water fraction (n) of the area where the KAM value is 0 to 1.0° in the surface layer may have a relationship of m < n.

[0026] The microstructure of the central portion may include, in area %, a total fraction of at least one type of ferrite and bainite (a): 5% or less (excluding 0%), and at least one type of residual martensite and tempered martensite, and the microstructure of the surface portion may include, in area %, a total fraction of at least one type of ferrite and bainite (b): 10% or less (excluding 0%), and at least one type of residual martensite and tempered martensite.

[0027] The above a and b can have a relationship of 0.1 < a / b < 0.8.

[0028] The average size of carbides in the lath of the above tempered martensite may be 200 nm or less (excluding 0 nm).

[0029] The above cold rolled steel sheet may have a maximum size of MnS inclusions of 50㎛ or less (excluding 0㎛).

[0030] The residual stress of the above surface layer may be -30 MPa or less.

[0031] The above cold rolled steel sheet may have a yield strength of 1050 to 1700 MPa, a tensile strength of 1470 to 1900 MPa, a yield ratio of 0.65 to 0.95, an elongation of 4 to 10%, a bending workability (R / t) of 4 or less, and a maximum three-point bending angle of 40º or more.

[0032] In another embodiment of the present invention, the product of the moisture content (m) and the elongation in the area where the KAM value is 0 to 1.0° in the center of the cold-rolled steel sheet is 80 to 210%. 2 And, the ratio of the water content (m) and the yield strength (YS) (YS / m) in the area where the KAM value is 0 to 1.0° in the center may be 45 MPa / % or more.

[0033] The above cold-rolled steel sheet may have a hardness of 650 Hv or less at the weld after resistance spot welding, and a cross tensile strength (CTS) of 4.5 kN or more at the weld.

[0034] When the above cold rolled steel plate is subjected to 30 cycles of corrosion resistance test, there may be 0 holes with a diameter of 3 mm or more.

[0035] Another embodiment of the present invention comprises the steps of: heating a slab comprising, in wt%, carbon (C): 0.150 to 0.350%, silicon (Si): 0.0050 to 0.70%, manganese (Mn): 0.30 to 2.50%, chromium (Cr): 0.0050 to 0.60%, molybdenum (Mo): 0.0030 to 0.50%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.0070% or less (excluding 0%), with the remainder being iron and other unavoidable impurities; finishing hot-rolling the heated slab to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet; A step of obtaining a cold rolled steel sheet by cold rolling the coiled hot rolled steel sheet at a cold reduction ratio of 35 to 70% using a roll having a surface roughness (Ra) of 2.5 ㎛ or more; a step of continuously annealing the cold rolled steel sheet; a step of first cooling the continuously annealed cold rolled steel sheet to a first cooling end temperature (T1) of 660°C to Ac3; a step of second cooling the first-cooled cold rolled steel sheet to a second cooling end temperature (T2) of 50 to 300°C; a step of reheating the second-cooled cold rolled steel sheet and then subjecting it to an overaging treatment at an overaging treatment temperature (H) of 100 to 250°C; a step of temper rolling the overaged cold rolled steel sheet with a pressing force of 500 to 1000 tons; And the method for manufacturing a cold rolled steel sheet includes a step of tension leveling the cold rolled steel sheet subjected to temper rolling at an elongation of 0.05 to 0.65%.

[0036] The above slab may additionally contain one or more of boron (B): 0.00050 to 0.0050%, aluminum (Al): 0.010 to 0.10%, niobium (Nb): 0.0010 to 0.10%, titanium (Ti): 0.0050 to 0.080%, and nitrogen (N): 0.010% or less (excluding 0%).

[0037] The above slab can satisfy the following relationships 1 to 4.

[0038] [Relationship 1] X = C + 0.03Si + 0.02Mn: 0.24~0.40

[0039] [Relationship 2] Y = (Cr + 0.8C + 0.07(Si+Mn) - 0.1Mo - 50B - 70P - 30S)X100 + 100 : 30~120

[0040] [Relationship 3] Y / X = 125~375

[0041] [Relationship 4] Z = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb+ 10000B : 55~250

[0042] (However, in the above equations 1 to 4, the content of each alloy element is in weight%.)

[0043] The above slab heating can be performed at 1100 to 1300°C.

[0044] The above finishing hot rolling can be performed at Ar3~Ar3+120℃.

[0045] The above winding can be performed at Ms~700℃.

[0046] The above continuous annealing can be performed at Ac3 to Ac3+100°C for 30 to 230 seconds.

[0047] The above primary cooling can be performed at an average cooling rate of 1 to 6°C.

[0048] The above secondary cooling can be performed at an average cooling rate of 30 to 300°C.

[0049] During the first and second cooling, the first cooling end temperature (T1) - second cooling end temperature (T2) can be controlled to be 650°C or lower.

[0050] The above over-treatment can be performed for 5 to 12 minutes.

[0051] During the above secondary cooling and over-aging treatment, the over-aging treatment temperature (H) - secondary cooling end temperature (T2) can be controlled to be 30°C or higher.

[0052] According to one aspect of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0053] According to an advantageous aspect of the present invention, it is possible to provide an ultra-high strength cold-rolled steel sheet having a tensile strength of 1470 MPa or more, which has an excellent shape and excellent bending properties, spot weldability, and corrosion resistance, and a method for manufacturing the same.

[0054] Figure 1 is a photograph of invention example 6 observed using TEM.

[0055] Figure 2 is a photograph of Comparative Example 12 observed using TEM.

[0056] Figure 3 is a photograph observed after evaluating the corrosion resistance of the plate of Invention Example 6.

[0057] Figure 4 is a photograph observed after evaluating the corrosion resistance of the plate in Comparative Example 3.

[0058] The terminology used herein is for the purpose of describing specific embodiments only and is not intended to limit the present invention. As used herein, the singular forms "a," "an," and "the" include plural forms as well, unless the context clearly dictates otherwise. The word "comprising," as used herein, specifies a particular feature, region, integer, step, operation, element, and / or component, but does not exclude the presence or addition of other features, regions, integers, steps, operations, elements, components, and / or groups.

[0059] Although not defined otherwise, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which the present invention pertains. Terms defined in commonly used dictionaries are further interpreted to have meanings consistent with the relevant technical literature and the present disclosure, and are not to be construed as ideal or overly formal unless otherwise defined.

[0060] 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. The alloy composition described below refers to weight percent unless otherwise specified.

[0061] Carbon (C): 0.150~0.350%

[0062] C is an interstitial solid solution element and is the most effective and important element for improving the strength of steel. In addition, it is an element that must be added to secure the strength of martensitic steel. If the C content is less than 0.150%, it may be difficult to obtain the yield ratio and tensile strength targeted in the present invention. If the C content exceeds 0.350%, the strength may increase rapidly and the elongation may be poor. In addition, the weldability may be poor. Therefore, the C content is advantageously in the range of 0.150 to 0.350%. The lower limit of the C content is more advantageously 0.170%, 0.190% is even more advantageous, and 0.20% is most advantageous. The upper limit of the C content is more advantageously 0.330%, 0.30% is even more advantageous, and 0.270% is most advantageous.

[0063] Silicon (Si): 0.0050~0.70%

[0064] Si suppresses the formation of carbides and controls the size of carbides during the reheating and overaging treatment steps performed after continuous annealing and cooling. If the Si content is less than 0.0050%, it may be difficult to sufficiently obtain the above-described effect. If the Si content exceeds 0.70%, there is a concern that excessive ferrite may be formed after continuous annealing and cooling, which may weaken the strength of the steel. In addition, Si is an element that increases resistivity, which may result in poor resistance spot weldability. Therefore, the Si content is advantageously in the range of 0.0050 to 0.70%. The lower limit of the Si content is more advantageously 0.010%, more advantageously 0.030%, and most advantageously 0.050%. The upper limit of the Si content is more advantageously 0.60%, and most advantageously 0.50%.

[0065] Manganese (Mn): 0.30~2.50%

[0066] Mn is an element added to secure strength. When the Mn content is less than 0.30%, the hardenability is low, so if the cooling rate is not sufficiently fast during cooling after continuous annealing, martensite is not formed, making it difficult to secure the strength targeted by the present invention. When the Mn content exceeds 2.50%, the Ms temperature decreases during cooling after continuous annealing, which lowers the temperature at which cooling must be completed, resulting in poor shape of the steel sheet. In addition, it is difficult to secure martensite structure. In addition, during steelmaking / continuous casting, a Mn-based segregation zone occurs in the longitudinal direction of the slab, which deteriorates bendability. In other words, manganese segregates in the thickness direction, forming a manganese band within the slab, which causes cracks to occur during continuous casting and increases the occurrence of defects during the rolling process. Therefore, it is advantageous for the Mn content to be in the range of 0.30 to 2.50%. The lower limit of the above Mn content is more advantageously 0.50%, more advantageously 1.0%, and most advantageously 1.50%. The upper limit of the above Mn content is more advantageously 2.30%.

[0067] Chromium (Cr): 0.0050~0.60%

[0068] Cr is an element that facilitates securing a low-temperature transformed structure by suppressing ferrite transformation. In addition, when utilizing a continuous annealing process with slow cooling as in the present invention, there is an advantage of suppressing ferrite formation. When the Cr content is less than 0.0050%, since hardenability is low, martensite is not formed if the cooling rate is not sufficiently fast during cooling after continuous annealing, making it difficult to secure the strength targeted by the present invention. When the Cr content exceeds 0.60%, delayed fracture resistance may deteriorate, carbides such as CrC may be formed, which may lower bending workability, and manufacturing costs may increase due to excessive alloy input. Therefore, the Cr content is advantageously in the range of 0.0050 to 0.60%. The lower limit of the Cr content is more advantageously 0.010%. The upper limit of the Cr content is more advantageously 0.50%.

[0069] Molybdenum (Mo): 0.0030~0.50%

[0070] Mo is an element that improves the hardenability of steel, creates Mo-based fine carbides that serve as hydrogen trap sites, and improves delayed fracture resistance by refining martensite. If the content of Mo is less than 0.0030%, it may be difficult to sufficiently obtain the above-mentioned effects. If the content of Mo exceeds 0.50%, the above-mentioned effects do not increase significantly compared to the increase in cost due to the addition of expensive alloying elements. Therefore, it is advantageous that the content of Mo is in the range of 0.0030 to 0.50%. It is more advantageous that the lower limit of the Mo content is 0.0050%. It is more advantageous that the upper limit of the Mo content is 0.40%.

[0071] Phosphorus (P): 0.030% or less (excluding 0%)

[0072] P is an impurity element contained in steel, and if the content of P exceeds 0.030%, weldability deteriorates and there is a risk of steel becoming brittle. Meanwhile, it is advantageous to not contain P in steel as much as possible, but considering cases where it is unavoidably contained during the manufacturing process, 0% is excluded. Therefore, it is advantageous that the content of P is 0.030% or less (excluding 0%). It is more advantageous that the content of P is 0.0250% or less.

[0073] Sulfur (S): 0.0070% or less (excluding 0%)

[0074] S, like P, is an impurity element contained in steel. If the content of S exceeds 0.0070%, ductility and weldability may be impaired, and a large amount of MnS precipitates may be formed, resulting in poor bending workability. Meanwhile, it is advantageous that S is not contained in steel as much as possible, but 0% is excluded in consideration of cases where it is inevitably contained during the manufacturing process. Therefore, it is advantageous that the content of S is 0.0070% (excluding 0%) or less. It is more advantageous that the content of S is 0.0050% or less, more advantageously 0.0030% or less, and most advantageously 0.0020% or less.

[0075] The remaining component is iron (Fe). However, during the normal manufacturing process, unintended impurities from raw materials or the surrounding environment may inevitably be mixed in, and thus cannot be excluded. Since these impurities are readily apparent to anyone skilled in the normal manufacturing process, their full details are not specifically mentioned in this specification.

[0076] A cold rolled steel sheet according to one embodiment of the present invention may additionally contain at least one of boron (B): 0.00050 to 0.0050%, aluminum (Al): 0.010 to 0.10%, niobium (Nb): 0.0010 to 0.10%, titanium (Ti): 0.0050 to 0.080%, and nitrogen (N): 0.010% or less (excluding 0%).

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

[0078] B is an element that suppresses ferrite formation, and therefore, the present invention has the advantage of suppressing the formation of ferrite during cooling after continuous annealing. If the content of B is less than 0.00050%, there is no hardenability effect at all, so not only cannot the strength targeted by the present invention be secured, but there is also a problem that excessive ferrite is formed in the surface layer, resulting in poor bending workability. If the content of B exceeds 0.0050%, ductility may be significantly reduced. Therefore, it is advantageous that the content of B is in the range of 0.00050 to 0.0050%. It is more advantageous that the lower limit of the B content is 0.00070%. It is more advantageous that the upper limit of the B content is 0.0040%.

[0079] Aluminum (Al): 0.010~0.10%

[0080] Al can be added to remove oxygen from molten steel. If the Al content is less than 0.010%, deoxidation is not sufficient, which harms the cleanliness of the steel. If the Al content exceeds 0.10%, not only will the castability of the slab deteriorate, but the temperature required for single-phase heating during continuous annealing will also increase, which may cause production and equipment problems. Therefore, it is advantageous for the Al content to be in the range of 0.010 to 0.10%. It is more advantageous for the upper limit of the Al content to be 0.0750%.

[0081] Niobium (Nb): 0.0010~0.10%

[0082] Nitrogen is an element that segregates at austenite grain boundaries, suppresses the coarsening of austenite grains during the continuous annealing process, and contributes to the improvement of strength by forming fine precipitates. When the content of Nb is less than 0.0010%, the austenite grain refinement and precipitation strengthening effects cannot be sufficiently obtained. When the content of Nb exceeds 0.10%, there is a concern that the precipitation of coarse carbonitrides increases, and the strength and elongation may decrease due to the reduction in the carbon content in the steel. In addition, there is a problem that the workability of the base material deteriorates and the manufacturing cost increases. Therefore, the content of Nb is advantageously in the range of 0.0010 to 0.10%. The lower limit of the Nb content is more advantageously 0.0030%. The upper limit of the Nb content is more advantageously 0.090%.

[0083] Titanium (Ti): 0.0050~0.080%

[0084] Ti is a nitride-forming element that scavenges dissolved nitrogen by precipitating it as TiN. If the Ti content is less than 0.0050%, it is difficult to obtain the effect of increasing strength, and since the effect of scavenging dissolved nitrogen is reduced, cracks may occur during continuous casting as a large amount of AlN is formed. If the Ti content exceeds 0.080%, the strength of martensite may decrease due to the precipitation of additional carbides in addition to the removal of dissolved nitrogen, and hole expandability and bending workability may be hindered due to the formation of excessive carbon and nitrides such as TiC and TiN. Therefore, it is advantageous that the Ti content is in the range of 0.0050 to 0.080%. It is more advantageous that the lower limit of the Ti content is 0.010%. The upper limit of the above Ti content is more advantageously 0.0060%, more advantageously 0.0050%, and more advantageously 0.0040%.

[0085] Nitrogen (N): 0.010% or less (excluding 0%)

[0086] N is an impurity element, and when its content exceeds 0.010%, it significantly increases the risk of cracks occurring during continuous casting due to AlN formation, etc. It is advantageous to not include the above N content in the steel as much as possible, but considering cases where it is unavoidably included in the manufacturing process, 0% is excluded. Therefore, it is advantageous for the above N content to be in the 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 advantageously 0.0060% or less.

[0087] A cold-rolled steel sheet according to one embodiment of the present invention can satisfy the above-described alloy composition and the following relationship equations 1 to 4.

[0088] [Relationship 1] X = C + 0.03Si + 0.02Mn: 0.24~0.40

[0089] The above equation 1 is a component equation closely related to the weld hardness and spot weld cross tensile strength (CTS). When the value of X is less than 0.24, the hardenability is low, making it difficult to sufficiently secure the target strength. When the value of X exceeds 0.40, the hardness of the weld becomes excessively high, increasing the risk of brittle fracture, which may lower the CTS and deteriorate the crash safety. Therefore, it is advantageous that the value of X is in the range of 0.24 to 0.40. It is more advantageous that the lower limit of the X value is 0.25. It is more advantageous that the upper limit of the X value is 0.39.

[0090] [Relationship 2] Y = (Cr + 0.8C + 0.07(Si+Mn) - 0.1Mo - 50B - 70P - 30S)X100 + 100 : 30~120

[0091] The above equation 2 is a component relationship related to strength and corrosion resistance. When the value of Y is less than 30, the hardenability is insufficient, making it difficult to obtain the microstructure targeted by the present invention, making it difficult to secure sufficient strength. When the value of Y exceeds 120, not only does the manufacturing cost increase, but there is also a problem of reduced corrosion resistance such as penetration corrosion. Therefore, it is advantageous for the value of Y to be in the range of 30 to 120. It is more advantageous for the lower limit of the Y value to be 40. It is more advantageous for the upper limit of the Y value to be 110.

[0092] [Relationship 3] Y / X = 125~375

[0093] The above equation 3 is a component equation for simultaneously securing spot weldability and corrosion resistance. When the value of Y / X is less than 125, spot weldability may deteriorate. When the value of Y / X exceeds 375, corrosion resistance such as penetration corrosion may deteriorate. Therefore, it is advantageous for the value of Y / X to be in the range of 125 to 375. It is more advantageous for the lower limit of the Y / X value to be 135. It is more advantageous for the upper limit of the Y / X value to be 350.

[0094] [Relationship 4] Z = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb+ 10000B : 55~250

[0095] The above equation 4 is a component relationship for securing hardenability. When the value of Z is less than 55, soft ferrite and bainite structures are transformed during cooling, making it difficult to secure the target strength. When the value of X exceeds 250, the strength becomes excessively high, making it difficult to secure the target elongation, which may cause processing cracks during forming. Therefore, it is advantageous for the value of X to be in the range of 55 to 250. It is more advantageous for the lower limit of the X value to be 60. It is more advantageous for the upper limit of the X value to be 240.

[0096] A cold rolled steel sheet according to one embodiment of the present invention can be divided into a central portion; and a surface portion formed on the outer side of the central portion in the thickness direction. Meanwhile, the depth of the surface portion may vary depending on the thickness of the steel sheet, and thus is not particularly limited thereto. However, as an example, the surface portion may be a region extending from the surface of the steel sheet to a thickness direction of up to 20 μm.

[0097] According to one embodiment of the present invention, the cold-rolled steel sheet preferably has a surface roughness (Rsk) of the surface portion of -0.65 to 0.65. The surface roughness (Rsk (Skewness)) is one of several factors of surface roughness related to the asymmetry of the sharp protrusion portion. The closer the value of the surface roughness (Rsk) is to 0 or to a positive value, the more advantageous it is for securing bending characteristics. As the negative value of the surface roughness (Rsk) increases, the deeper the valley on the flat surface, which causes stress concentration in this portion, increasing the susceptibility to crack occurrence, thereby deteriorating the bending characteristics. When the surface roughness (Rsk) is less than -0.65 or exceeds 0.65, the bending characteristics may deteriorate. Therefore, the surface roughness (Rsk) is preferably in the range of -0.65 to 0.65. The lower limit of the surface roughness (Rsk) is more advantageously -0.6, more advantageously -0.5, and most advantageously -0.4. The upper limit of the surface roughness (Rsk) is more advantageously 0.6, more advantageously 0.4, and most advantageously 0.2.

[0098] According to one embodiment of the present invention, the cold-rolled steel sheet may have a product of the surface roughness (Rsk) of the surface portion and Pc of -150 or more. The Pc is a factor indicating the number of peaks that completely deviate from the bandwidth within a unit length. The product of Rsk and Pc is more advantageous for bending characteristics as its value increases. That is, in the present invention, the bending characteristics can be improved by controlling the product of the surface roughness (Rsk) of the surface portion and Pc to a level of -150 or more. Therefore, it is advantageous for the product of the surface roughness (Rsk) of the surface portion and Pc to be -150 or more. It is more advantageous for the product of the surface roughness (Rsk) of the surface portion and Pc to be -140 or more, more advantageously -130 or more, and most advantageously -120 or more. Meanwhile, in the present invention, since a higher product of Rsk and Pc is advantageous, there is no particular limitation on the upper limit thereof. However, as an example, the upper limit of the product of the surface roughness (Rsk) of the surface layer and Pc may be +60.

[0099] According to one embodiment of the present invention, the cold-rolled steel sheet may have an absolute value of the product of the surface roughness (Rsk) of the surface layer and the bending workability (R / t) of 2.4 or less.

[0100] Hereinafter, a cold-rolled steel sheet according to another embodiment of the present invention will be described.

[0101] A cold-rolled steel sheet according to another embodiment of the present invention can satisfy the same alloy composition as the cold-rolled steel sheet according to the aforementioned embodiment. In addition, it can satisfy the aforementioned relationships 1 to 4.

[0102] A cold-rolled steel sheet according to another embodiment of the present invention may also be divided into a central portion; and a surface portion formed on the outer side of the central portion in the thickness direction. Meanwhile, the depth of the surface portion may vary depending on the thickness of the steel sheet, and thus is not particularly limited thereto. However, as an example, the surface portion may be a region extending from the surface of the steel sheet to a thickness direction of up to 20 μm.

[0103] According to another embodiment of the present invention, in the cold-rolled steel sheet, when the number fraction (Number Fraction) of the region where the KAM value is 0 to 5.0° is set to 100% during EBSD measurement, it is advantageous that the number fraction (m) of the region where the KAM value is 0 to 1.0° in the center is 30% or less (excluding 0%). The dislocation density in the crystal grain can be evaluated by the KAM (Kernel Average Misorientation) value. KAM refers to the orientation difference, and more specifically, the KAM value is the average value of the crystal rotation amount (crystal orientation difference) between the target measurement point and the surrounding measurement points, and the larger this value is, the more strain exists in the crystal. The present inventors investigated the relationship between the KAM value and the microstructure and confirmed that a difference in the physical properties occurs depending on the number fraction of the region where the KAM value is 0 to 1.0° in the center. In the present invention, in order to secure the target properties, it is advantageous to control the water content (m) in the region where the KAM value is 0 to 1.0° in the center to a range of 30% or less (excluding 0%). If the water content (m) in the region where the KAM value is 0 to 1.0° in the center exceeds 30%, a large amount of tissue with little deformation will be included, making it difficult to secure the target tensile strength. It is more advantageous for the water content in the region where the KAM value is 0 to 1.0° in the center to be 29% or less.

[0104] According to another embodiment of the present invention, in the cold-rolled steel sheet, when the number fraction in the region where the KAM value is 0 to 5.0° is taken as 100% during EBSD measurement, the number fraction (n) in the region where the KAM value is 0 to 1.0° in the surface layer may be 55% or less (excluding 0%). If the number fraction (n) in the region where the KAM value is 0 to 1.0° in the surface layer exceeds 55%, a large amount of a structure with little deformation may be included, making it difficult to secure the target tensile strength. It is more advantageous that the number fraction (n) in the region where the KAM value is 0 to 1.0° in the surface layer is 50% or less.

[0105] In another embodiment of the present invention, the cold rolled steel sheet may have a relationship in which the water content (m) in the region where the KAM value is 0 to 1.0° in the center and the water content (n) in the region where the KAM value is 0 to 1.0° in the surface layer have a relationship of m < n. When m is greater than or equal to n, it may be difficult to secure the target strength.

[0106] Meanwhile, the cold-rolled steel sheet according to the above-described embodiment and other embodiments may include a microstructure of the central portion, in area %, a sum of at least one type of ferrite and bainite (a): 5% or less (excluding 0%), and at least one type of residual martensite and tempered martensite, and a microstructure of the surface portion may include a sum of at least one type of ferrite and bainite (b): 10% or less (excluding 0%), and at least one type of residual martensite and tempered martensite, in area %.

[0107] It is advantageous that the main phase of the above-mentioned central microstructure includes at least one of martensite and tempered martensite. The martensite and tempered martensite are very advantageous structures for securing the strength, hole expandability, bending properties, and weldability targeted by the present invention. However, at least one of ferrite and bainite may inevitably be formed during the manufacturing process, and if the total fraction (a) of at least one of the ferrite and bainite exceeds 5%, it may be difficult to secure the physical properties targeted by the present invention. Therefore, it is advantageous that the total fraction (a) of at least one of the ferrite and bainite is 5% or less, and more advantageously 3% or less. In the present invention, there is no particular limitation on the lower limit of the total fraction (a) of at least one of the ferrite and bainite, but as an example, it may be 0.5%.

[0108] It is advantageous that the main phase of the surface microstructure includes at least one of martensite and tempered martensite. In addition, it is advantageous that the total fraction (b) of at least one of ferrite and bainite is 10% or less (excluding 0%). Since the ferrite and bainite are softer than martensite and tempered martensite, the bending properties can be further improved by forming an appropriate level of ferrite and bainite in the surface. If the total fraction of at least one of ferrite and bainite exceeds 10%, the bending properties may be deteriorated. Therefore, it is advantageous that the total fraction (b) of at least one of ferrite and bainite is 10% or less, and more advantageously 7% or less. In the present invention, there is no particular limitation on the lower limit of the total fraction (b) of at least one of ferrite and bainite, but as an example, it may be 0.5%.

[0109] The above a and b may have a relationship of 0.1 < a / b < 0.8. When the above a / b is 0.1 or less, the bending properties may be inferior. When the above a / b is 0.8 or more, it may be difficult to secure the target strength.

[0110] The average size of carbides in the lath of the above tempered martensite may be 200 nm or less (excluding 0 nm). If the average size of the carbide exceeds 200 nm, the bending properties may be deteriorated. The average size of the carbide is more advantageously 150 nm or less. In the present invention, since a smaller average size of the carbide is advantageous, there is no particular limitation on the lower limit thereof. However, the lower limit of the average size of the carbide may be, for example, 10 nm. Meanwhile, the carbide may be, for example, at least one of a carbide containing Fe and Mn, a carbide containing Mn and Cr, and a carbide containing Fe, Mn, Cr, and Mo.

[0111] The cold rolled steel sheet according to the above embodiment and other embodiments may have a maximum size of MnS inclusions of 50 μm or less (excluding 0 μm). If the maximum size of the MnS inclusions exceeds 50 μm, the bending properties and hydrogen embrittlement resistance may be significantly deteriorated. It is more advantageous that the maximum size of the MnS inclusions is 40 μm or less, and it is even more advantageous that it is 30 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 20 μm, for example.

[0112] The cold-rolled steel sheet according to the above-described embodiment and other embodiments may have a residual stress of -30 MPa or less in the surface portion. If the residual stress of the surface portion exceeds -30 MPa, bending characteristics and hydrogen embrittlement resistance may deteriorate. Therefore, it is advantageous that the residual stress of the surface portion is -30 MPa or less. It is more advantageous that the residual stress of the surface portion is -40 MPa or less, and it is even more advantageous that it is -50 MPa or less. Meanwhile, in the present invention, since the lower the residual stress of the surface portion is, the more advantageous it is, the lower limit thereof is not particularly limited. However, as an example, the lower limit of the residual stress of the surface portion may be -300 MPa.

[0113] The cold-rolled steel sheet according to the above-described embodiment and other embodiments may have a yield strength of 1050 to 1700 MPa, a tensile strength of 1470 to 1900 MPa, a yield ratio of 0.65 to 0.95, an elongation of 4 to 10%, bending workability (R / t): 4 or less, a maximum three-point bending angle of 40° or more, and a surface residual stress of -30 MPa or less. The lower limit of the yield ratio is more advantageously 0.70, and is more advantageously 0.75. The lower limit of the elongation is more advantageously 5%, and is more advantageously 6%. The bending workability is more advantageously 3.5 or less, and is more advantageously 3.0 or less. In the present invention, since the lower the bending workability, the more advantageous it is, the lower limit thereof is not particularly limited. However, as an example, the lower limit of the bending workability may be 2.5. The higher the residual stress (MPa) of the surface layer, the more tensile stress is applied, which may be detrimental to securing bending characteristics and hydrogen embrittlement resistance.

[0114] The cold rolled steel sheet according to the above embodiment and other embodiments tends to have a lower elongation when the water content (m) in the region where the KAM value is 0 to 1.0° in the center decreases, but the reduction in elongation is prevented through control of the manufacturing process, especially the cooling rate, elongation during tension leveling, etc. Accordingly, the product of the elongation and the water content (m) in the region where the KAM value is 0 to 1.0° in the center is 80 to 210%. 2may be. In addition, the cold-rolled steel sheet according to the above-described embodiment and other embodiments tends to have a lower yield strength when the water content (m) in the region where the KAM value is 0 to 1.0° in the center increases, but a high yield ratio can be secured by controlling the alloy composition, the cooling rate during the manufacturing process, and the reheating / overaging process. Accordingly, the ratio (YS / m) of the yield strength (YS) and the water content (m) in the region where the KAM value is 0 to 1.0° in the center may be 45 MPa / % or more. More advantageously, the ratio (YS / m) of the yield strength (YS) and the water content (m) in the region where the KAM value is 0 to 1.0° in the center may be 45 to 68 MPa / %.

[0115] The cold-rolled steel sheet according to the above-described embodiment and other embodiments may have a hardness of the welded portion of 650 Hv or less and a cross tensile strength (CTS) of the welded portion of 4.5 kN or more after resistance spot welding. The present invention does not specifically limit the hardness of the welded portion, but as an example, the lower limit of the hardness of the welded portion after the resistance spot welding may be 440 Hv. In addition, the present invention does not specifically limit the upper limit of the cross tensile strength (CTS) of the welded portion after the resistance spot welding, but as an example, the upper limit of the cross tensile strength (CTS) of the welded portion may be 12 kN.

[0116] The cold rolled steel sheet according to the above embodiment and other embodiments may have 0 holes having a diameter of 3 mm or more when the plate corrosion resistance test is performed 30 cycles. One cycle of the plate corrosion resistance evaluation may be performed, for example, as follows: Slat Spray (5% NaCl, neutral, 35°C, salt spray amount: 1 to 2 mL / hr, 6 hr) → Drying (50°C, 20 to 40% RH, 3 hr) → Wetting (50°C, 95% RH, 14 hr) → Cold air blasting (50% RH, 1 hr).

[0117] The cold-rolled steel sheet according to the above-described embodiment and other embodiments may have a plating layer formed on at least one surface. The present invention is not particularly limited to the type of the plating layer, but as an example, the plating layer may be an electrogalvanized layer.

[0118] The cold rolled steel sheet according to the above embodiment and other embodiments may have a thickness of 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 more advantageously 0.8 mm. The upper limit of the thickness of the cold rolled steel sheet is more advantageously 2.4 mm, and more advantageously 2.3 mm.

[0119] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0120] First, a slab satisfying the above-described alloy composition is heated. The slab can satisfy the above-described relationships 1 to 4. The slab heating process is performed to smoothly perform the subsequent hot rolling process and to sufficiently obtain the target physical properties of the steel plate. The slab may be heated at 1100 to 1300°C. If the slab heating temperature is lower than 1100°C, a problem occurs in which the hot rolling load rapidly increases. If the slab heating temperature exceeds 1300°C, the amount of surface scale increases, thereby reducing the material yield. The lower limit of the slab heating temperature is more advantageously 1110°C, more advantageously 1120°C, and most advantageously 1130°C. The upper limit of the above slab heating temperature is more advantageously 1290°C, more advantageously 1280°C, and most advantageously 1270°C.

[0121] Thereafter, the heated slab is subjected to a finish hot rolling to obtain a hot-rolled steel sheet. The finish hot rolling can be performed at Ar3 to Ar3+120℃. When the finish hot rolling temperature is lower than Ar3, a dual-phase region of ferrite + austenite or a ferrite region rolling occurs, resulting in a mixed grain structure, and sheet breakage may occur due to fluctuations in the hot rolling load. When the finish hot rolling temperature exceeds Ar3+120℃, a large amount of surface scale may occur, resulting in poor surface quality. The lower limit of the finish hot rolling temperature is more advantageously Ar3+10℃, more advantageously Ar3+20℃, and most advantageously Ar3+30℃. The upper limit of the finish hot rolling temperature is more advantageously Ar3+110℃, more advantageously Ar3+100℃, and most advantageously Ar3+90℃. Meanwhile, the above Ar3 refers to the temperature at which austenite begins to transform into ferrite during cooling, and can be obtained through the following equation 1.

[0122] [Formula 1] Ar3(℃) = 910 - 203√C + 44.7Si + 31.5Mo

[0123] Thereafter, the hot-rolled steel sheet is coiled. The coiling can be performed at Ms~700℃. If the coiling temperature exceeds 700℃, internal oxidation occurs on the surface of the steel sheet, causing the microstructure formed in the surface layer to become non-uniform, which may result in poor bending properties. On the other hand, it is advantageous to control the coiling temperature low in order to form the microstructure of the hot-rolled steel sheet as a single-phase structure rather than a composite structure as much as possible, thereby ensuring material uniformity across the entire length and width. However, if the coiling temperature is lower than Ms, the strength of the hot-rolled steel sheet may increase excessively, which may increase the rolling load during the cold rolling process, which is a subsequent process, making actual production impossible. It is more advantageous that the lower limit of the coiling temperature be Ms+50℃. It is more advantageous that the upper limit of the coiling temperature be 690℃. The Ms refers to the temperature at which austenite begins to transform into martensite upon cooling, and can be obtained through Equation 2 below.

[0124] [Equation 2] Ms(℃) = 539 - 423C - 30.4Mn - 7.5Si + 30Al - 17.7Ni - 12.1Cr - 7.5Mo

[0125] Meanwhile, after the above-described coiling, cooling can be performed through air cooling or water cooling. In addition, after the above-described cooling, a pickling process can be performed to remove the oxide layer formed on the surface of the hot-rolled steel sheet.

[0126] Thereafter, the coiled hot-rolled steel sheet is cold-rolled at a cold reduction ratio of 35 to 70% using a roll having a surface roughness (Ra) of 2.5 ㎛ or more to obtain a cold-rolled steel sheet. If the surface roughness (Ra) of the cold-rolled roll is less than 2.5 ㎛, the -value of the surface roughness (Rsk) of the steel sheet may become excessively large, thereby deteriorating the bending characteristics. In the present invention, the larger the surface roughness (Ra) of the cold-rolled roll, the fewer deep grooves are formed in the rolling direction, which is detrimental to the bending characteristics, so there is no particular limitation on the upper limit thereof. The surface roughness (Ra) of the cold-rolled roll may be more advantageously 3.0 ㎛ or more, and in the present invention, since the larger the surface roughness (Ra) of the cold-rolled roll, the more advantageous it is, so there is no particular limitation on the upper limit thereof. However, as an example, the upper limit of the surface roughness (Ra) of the cold-rolled roll may be 6.5 ㎛. When the cold reduction ratio is less than 35%, it is not only difficult to secure the thickness desired in the present invention, but also there is a concern that austenite may be generated during annealing heat treatment due to the residual crystal grains formed during hot rolling, which may affect the final physical properties. In addition, the -value of the surface roughness (Rsk) may become excessively large, resulting in poor bending properties. When the cold reduction ratio exceeds 70%, the amount of reduction in the length and width directions may become uneven due to the work hardening that occurs during cold rolling, which may cause material deviations 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 ratio is more advantageously 36%, more advantageously 37%, and most advantageously 38%. The upper limit of the cold reduction ratio is more advantageously 68%, more advantageously 66%, and most advantageously 64%.

[0127] Thereafter, the cold rolled steel sheet is continuously annealed. The continuous annealing can be performed at Ac3 to Ac3+100°C for 30 to 230 seconds. If the continuous annealing temperature is lower than Ac3, a two-phase annealing, not a single-phase annealing, may occur over the entire length of the steel sheet, thereby forming a mixed grain structure. As a result, it is difficult to secure the properties targeted by the present invention, and in particular, the hardness difference between the phases may become large, so that hole expandability may be significantly reduced. If the continuous annealing temperature exceeds Ac3+100°C, equipment trouble may occur due to overload of the annealing furnace. The lower limit of the continuous annealing temperature is more advantageously Ac3+5°C, more advantageously Ac3+10°C, and most advantageously Ac3+15°C. The upper limit of the above continuous annealing temperature is more advantageously Ac3+95℃, more advantageously Ac3+90℃, and most advantageously Ac3+85℃. Meanwhile, Ac3 refers to the temperature at which 100% ferrite is transformed into austenite upon heating, and can be obtained through the following equation 3.

[0128] [Equation 3] Ac3(℃) = 875 - 206C + 26.2Si - 25.0Mn - 12.3Cr + 9.12Mo + 50.2Nb + 148Ti - 131B

[0129] 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, there is a disadvantage in that the austenite size becomes coarser, making it difficult to secure strength and bending properties. The lower limit of the above continuous annealing time is more advantageously 40 seconds, more advantageously 50 seconds, and most advantageously 60 seconds. The upper limit of the above continuous annealing time is more advantageously 220 seconds, more advantageously 210 seconds, and most advantageously 200 seconds.

[0130] Thereafter, the continuously annealed cold-rolled steel sheet is first cooled to a first cooling end temperature (T1) of 660 to Ac3°C. If the first cooling end temperature (T1) is less than 660°C, a large amount of soft ferrite and bainite other than martensite may be formed during the cooling process, which may cause the surface structure to become uneven and deteriorate the bending properties. If the first cooling end temperature (T1) exceeds Ac3, the temperature difference between the first cooling end temperature (T1) and the second cooling end temperature (T2) may become severe, causing a rapid phase transformation, which may result in a poor product shape. It is more advantageous that the lower limit of the first cooling end temperature is 665°C. It is more advantageous that the upper limit of the first cooling end temperature is Ac3-10°C. The first cooling may be performed at an average cooling rate of 1 to 6°C. If the above first average cooling rate is less than 1°C / s, ferrite is formed during cooling, making it impossible to secure the 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 impossible to secure the strength targeted by the present invention. It is more advantageous that the lower limit of the above first average cooling rate is 2°C / s. It is more advantageous that the upper limit of the above first average cooling rate is 5°C / s.

[0131] Thereafter, the first-cooled cold-rolled steel sheet is secondarily cooled to a second cooling end temperature (T2) of 50 to 300°C. The second cooling is to secure at least one of martensite and tempered martensite, which are the main phases of the present invention. If the second cooling end temperature (T2) is less than 110°C, there is a disadvantage in that shape defects are caused by rapid phase transformation and continuous production is difficult due to meandering of the strip. If the second cooling end temperature (Tf) exceeds 300°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 advantageously 115°C, more advantageously 120°C, and most advantageously 130°C. The upper limit of the secondary cooling end temperature is more advantageously 290°C, more advantageously 280°C, and most advantageously 270°C. The secondary cooling can be performed at an average cooling rate of 30 to 300°C. If the 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 secondary average cooling rate exceeds 300°C / s, the product shape may become poor due to rapid phase transformation. The lower limit of the secondary average cooling rate is more advantageously 35°C / s, more advantageously 40°C / s, and most advantageously 45°C / s. The upper limit of the secondary average cooling rate is more advantageously 290°C / s, more advantageously 280°C / s, and most advantageously 270°C / s.

[0132] During the first and second cooling, the first cooling end temperature (T1) - the second cooling end temperature (T2) may be controlled to be 650°C or lower. If the first cooling end temperature (T1) - the second cooling end temperature (T2) exceeds 650°C, shape defects may occur. It is more advantageous for the first cooling end temperature (T1) - the second cooling end temperature (T2) to be 600°C or lower.

[0133] Afterwards, the second-cooled cold-rolled steel sheet is reheated and then subjected to an overaging treatment at an overaging treatment temperature (H) of 100 to 250°C. 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. If the reheating temperature and overaging temperature are less than 100°C, tempering is not sufficiently performed, which results in a low yield strength and insufficient toughness. If the reheating temperature and overaging temperature exceed 250°C, which results in a deterioration in bending workability due to the precipitation and coarsening of a large amount of carbides. The lower limit of the reheating temperature and overaging temperature is more advantageously 110°C, more advantageously 120°C, and most advantageously 130°C. The upper limit of the reheating temperature and overaging temperature is more advantageously 245°C, more advantageously 240°C, and most advantageously 235°C. The overaging can be performed for 5 to 12 minutes. If the overaging time is less than 5 minutes, tempering may not be sufficiently performed, resulting in a decrease in yield strength. If the overaging time exceeds 12 minutes, excessive tempering may cause coarsening of carbides, resulting in poor bending properties. The lower limit of the overaging time is more advantageously 5.5 minutes, more advantageously 6.0 minutes, and most advantageously 6.5 minutes. The upper limit of the overaging time is more advantageously 11.5 minutes, more advantageously 11 minutes, and most advantageously 10.5 minutes.

[0134] During the above secondary cooling and overaging treatment, the overaging treatment temperature (H) - secondary cooling end temperature (T2) can be controlled to be 30°C or higher. If the overaging treatment temperature (H) - secondary cooling end temperature (T2) is less than 30°C, tempering may not be sufficiently achieved, making it difficult to secure the target yield strength. It is more advantageous for the overaging treatment temperature (H) - secondary cooling end temperature (T2) to be 50°C or higher.

[0135] Afterwards, the cold-rolled steel sheet subjected to the above-mentioned overaging is subjected to skin pass mill (SPM) rolling with a rolling force of 500 to 1000 tons. The skin pass mill rolling enables control of the surface roughness (Rsk). If the rolling force is less than 500 tons during the skin pass rolling, the load is low, making it difficult to control the surface roughness (Rsk), and if it exceeds 1000 tons, the work hardening of the surface may be severe, resulting in poor bending characteristics. The lower limit of the rolling force during the skin pass rolling is more advantageously 550 tons, and the upper limit of the rolling force during the skin pass rolling is more advantageously 950 tons, and the upper limit of the rolling force during the skin pass rolling is more advantageously 900 tons.

[0136] Thereafter, the cold rolled steel sheet subjected to temper rolling is subjected to tension leveling (T / L) at an elongation of 0.05 to 0.65%. The tension leveling is intended to correct the shape of the steel sheet. If the elongation is less than 0.05% during the tension leveling, shape correction may be difficult. If the elongation exceeds 0.65% during the tension leveling, work hardening may become severe and bending characteristics may deteriorate. The lower limit of the elongation during the tension leveling is more advantageously 0.10%, and 0.15% is even more advantageous. The upper limit of the elongation during the tension leveling is more advantageously 0.60%, and 0.55% is even more advantageous.

[0137] Meanwhile, after the tension leveling, 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 for forming the plating layer, but as an example, an electrogalvanizing method may be used.

[0138] Hereinafter, the present invention will be described in more detail through examples. However, the description of these examples is merely intended to illustrate the implementation of the present invention and is not intended to limit the present invention. This is because the scope of the present invention is determined by the matters set forth in the claims and matters reasonably inferred therefrom.

[0139] (Example 1)

[0140] A slab having the alloy composition described in Table 1 below was heated at 1200°C, and then the heated slab was final hot-rolled at 900°C to obtain a hot-rolled steel sheet, which was then coiled at 450°C. Thereafter, cold-rolled at the cold reduction ratio described in Table 2 below to obtain a cold-rolled steel sheet. Thereafter, a cold-rolled steel sheet was manufactured by continuous annealing, primary cooling, secondary cooling, reheating / overaging treatment, temper rolling, and tension leveling under the conditions described in Tables 2 and 3 below. Meanwhile, the conditions described in Tables 2 and 3 below were based on the surface temperature of the steel sheet.

[0141] For the cold-rolled steel sheet manufactured in this manner, the microstructure, the water content in the region where the KAM value is 0 to 1.0°, the average size of carbides in the lath of tempered martensite, the maximum size of MnS inclusions, surface roughness, surface residual stress, and mechanical properties were measured, and the results are shown in Tables 4 to 7 below.

[0142] The types and fractions of microstructures were observed at the surface (20 ㎛ in the thickness direction from the surface) and the center (1 / 4t (t: thickness of the steel)) of the steel plate using a scanning electron microscope (SEM) and an optical microscope (OM). Then, the fractions of each phase were analyzed 20 times through image analysis and the average value was calculated.

[0143] The water fraction in the area where the KAM (Kernel Average Misorientation) value is 0 to 1.0° was measured three times (Confidence Index (CI) ≥ 0.3) at 2000x magnification using EBSD (Backscatter Electron Diffraction Pattern Analyzer) at the surface (20 ㎛ in the thickness direction from the surface) and the center (1 / 4t (t: thickness of the steel)) of the steel plate, and the average value was calculated. At this time, the water fraction (Number Fraction) in the area where the KAM value is 0 to 5.0° during the EBSD measurement was regarded as 100%.

[0144] The average size of carbides in the lath of tempered martensite was calculated by preparing thin film specimens, photographing them with a transmission electron microscope (TEM), and calculating the average value.

[0145] 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.

[0146] Surface roughness (Rsk, Pc) was measured 10 times using a contact-type 2D roughness meter, and the average value was calculated excluding the maximum (Max) and minimum (Min) values.

[0147] The surface residual stress was measured 10 times using XRD (X-ray generator: 40 kV, 30 mA, Scan step: 0.0194°, Scan speed: 1.247° / min, Scan mode: 0D (continuous), Scan axis: 2θ / θ, Wavelength Kα₁: 0.154 nm), and the average value excluding the maximum (Max) and minimum (Min) values ​​was calculated.

[0148] Yield strength, tensile strength, yield ratio, and total elongation were measured by processing cold-rolled steel sheets into JIS No. 5 standard specimens and then performing a tensile test at a test speed of 28 mm / min.

[0149] Bending workability (R / t) was determined by processing a cold-rolled steel sheet into a specimen with a width of 100 mm and a length of 30 mm, and then performing a 90° bending test at a test speed of 800 mm / min. The presence or absence of cracks in the bent portion was confirmed using a stereoscopic microscope, and the minimum bending radius (R value of the mold) at which no cracks occurred was calculated by dividing it by the thickness (mm) of the specimen.

[0150] The maximum angle of three-point bending was measured five times under VDA conditions (0.4R) and the average value was calculated.

[0151] The corrosion resistance of the plate was measured by producing a specimen measuring 75 mm in width × 150 mm in length, and performing a corrosion test of 30 cycles in which 1 cycle is Slat Spray (5% NaCl, neutral, 35℃, salt spray amount: 1-2 mL / hr, 6 hr) → Drying (50℃, 20-40% RH, 3 hr) → Wetting (50℃, 95% RH, 14 hr) → Cold air blasting (50% RH, 1 hr). Then, visually checking the number of holes with a diameter of 3 mm or more was performed. If the number of holes with a diameter of 3 mm or more was 0, it was evaluated as not occurring, and if there were 1 or more, it was evaluated as occurring.

[0152] Spot weldability was performed under the conditions of Force: 4.95KN, Welding time: 35 cycles, Holding time: 1 cycle, and Welding current in the range of 4.0 to 7.0kA. Then, the welding current satisfying the minimum nugget diameter (4.25√t, t: steel thickness) was derived, and the hardness and cross tensile strength (CTS) of the spot welded joint welded under the above conditions were measured. In addition, the hardness of the spot welded joint was measured 10 times with Vickers hardness (load: 500gf) for the spot welded joint, and the average value was calculated.

[0153] Steel grade No. Alloy composition (weight %) CSiMnPSAlCrMoNbTiBNXYY / XZ10.230.091.950.0100.00100.0300.1100.1100.0350.0200.00250.00400.275721118020.290.151.750.0110.00090.0240.0500.0400.0290.0210.00250.00360.334914815930.240.201.650.0110.00140.0340.0900.1000.025 0.0150.00190.00370.284917515140.260.052.100.0080.00150.0250.1200.2000.0150.0310.00210.00410.307525016650.220.351.950.0110.00080.0150.0500.0600.0050.0140.00150.00350.275118912660.460.152.100.0100.00100.0350.1700.0500.0250.0250.001 90.00420.518717118070.350.752.300.0120.00120.0350.3500.0900.0250.0250.00210.00410.428520221780.240.252.200.0100.00110.0250.7000.0500.0500.0250.00350.00250.2911539726290.350.752.000.0110.00180.0210.0100.0060.0250.0210.00350.00240. 4148117186100.140.010.400.0120.00100.0350.0300.0100.0050.0250.00200.00310.152013351110.200.120.400.0110.00090.0350. 0010.0010.0150.0150.00010.00510.214019053120.240.121.710.0100.00800.0300.1200.1500.030.0250.00200.00400.2839140166X = C + 0.03Si + 0.02MnY = (Cr + 0.8C + 0.07(Si+Mn) - 0.1Mo - 50B - 70P - 30S)X100 + 100Z = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb+ 10000BHowever, the above Y / X is calculated with the significant figures of X to 2 decimal places and the significant figures of Y to 0 decimal places.

[0154] Classification Steel grade No. Ar3 (℃) Ms (℃) Cold rolling reduction ratio (%) Cold rolling roll roughness (㎛) Ac3 (℃) Annealing temperature (℃) First cooling end temperature (T1) (℃) First average cooling speed (℃ / s) Invention example 1 1820 380 564.580 786 17203 Invention example 2 280 936 2564.579 48357453 Invention example 3 382 338 5564.580 986 77153 Invention example 4481 5363564.58018527313Invention Example 55832383564.58208607323Comparative Example 16781278564.57628347303Comparative Example 27826312564.58108457253Comparative Example 38823361564.58108567363Comparative Example 498243 25564.58088477153Comparative Example 5 10834468564.58248497113Comparative Example 6 11825442564.58138427153Comparative Example 7 12821383564.58078537053Comparative Example 8 1820380302.78078456403Comparative Example 9 1820 380561.58078557213Comparative Example 101820380564.58078646503Comparative Example 111820380604.58078617103Comparative Example 121820380564.58078517213Comparative Example 131820380462.08078567163Ar3(℃) = 910 - 203√C + 44.7Si + 31.5MoMs(℃) = 539 - 423C - 30.4Mn - 7.5Si + 30Al - 17.7Ni - 12.1Cr - 7.5MoAc3(℃) = 875 - 206C + 26.2Si - 25.0Mn - 12.3Cr + 9.12Mo + 50.2Nb + 148Ti - 131B

[0155] Classification Steel grade No. Second cooling end temperature (T2) (℃) Second average cooling rate (℃ / s) T1-T2 (℃) Reheating / overaging treatment temperature (H) (℃) Overaging treatment time (min) H-T2 (℃) Temper rolling pressure (ton) Tension leveling elongation (%) Invention example 1198656221758.6776500.15 Invention example 22951206501758.5806500 .15 Invention Example 33105616101859.1806500.15 Invention Example 44103626281768.9736500.15 Invention Example 55105626271758.6706500.15 Comparative Example 1697756331738.3766500.70 Comparative Example 27102856231817.9796500.70 Comparative Example 3895906 411918.1966500.85Comparison Example 49105876101878.3825500.90Comparison Example 51097786141858.9886500.15Comparison Example 61197636181898.5926500.15Comparison Example 71297646081918.9946500.15Comparison Example 8198845421788.980250 0.15Comparison Example 9 1300674211678.7-1332500.15Comparison Example 10 1121655291798.5582500.15Comparison Example 111320673903618.7412500.15Comparison Example 12 1125615963208.51952500.15Comparison Example 13 1405606761848.51442500.75

[0156] Fraction of the area where the microstructure (area %)a / bKAM value is 0~1.0° (%)m <n만족여부표층부중심부F 및 B 중하나 이상(b)M 및 TM 중 하나 이상F 및 B 중하나 이상(a)M 및 TM 중 하나 이상 표층부(n)중심부(m)발명예13971990.333922○발명예22981990.503519○발명예33971990.333823○발명예42981990.503920○발명예52981990.504121○비교예12981990.503919○비교예22981990.503720○비교예33973971.003720○비교예43972980.674921○비교예5138711890.855632○비교예6208015850.756035○비교예72981990.503620○비교예8158518821.205638○비교예9118917831.554235○비교예10198117830.896523○비교예11178315850.886038○비교예1213878920.623735○비교예132984962.002021×F: 페라이트, B: 베이나이트, M: 마르텐사이트, TM: 템퍼트 마르텐사이트

[0157] Classification Carbide Average size (nm) MnS Inclusion Maximum size (㎛) Surface roughness Rsk Pc Rsk × Pc Invention example 15910-0.41 164-67 Invention example 2659-0.38 179-68 Invention example 3529-0.31 190-59 Invention example 45611-0.35 178-62 Invention example 5469-0.40 151-60 Comparative example 1848-0.39 136-53 Comparative example 27510-0.42 157-66 Comparative example 3899-0.41 149-61 Comparative example 4 10118-0.61159-97Comparative Example 58711-0.55169-93Comparative Example 6628-0.45167-75Comparative Example 78765-0.41171-70Comparative Example 85811-0.79197-156Comparative Example 95610-0.71201-143Comparative Example 105810-0.71185-76Comparative Example 112219-0.70189-76Comparative Example 122158-0.78189-91Comparative Example 135310-0.72215-155

[0158] Yield strength (YS) (MPa) Yield strength (YS) / Moisture content (m) in the area where the central KAM value is 0 to 1.0° (MPa / %) Tensile strength (TS) (MPa) Yield elongation (EL) (%) Elongation (EL) × Moisture content (m) in the area where the central KAM value is 0 to 1.0° (%) 2 ) Invention Example 1 12945915740.827154 Invention Example 2 14257517560.817133 Invention Example 3 12745515510.827161 Invention Example 4 12976515890.826120 Invention Example 5 12616015670.807147 Comparative Example 1 2 12511222050.96357 Comparative Example 2 18529319250.96360 Comparative Example 3 14787415350.96480 Comparative Example 4 14997115670.96363 Comparative Example 5 110 535 1415 0.788 256 Comparison Example 6 109 531 139 50.788 280 Comparison Example 7 127 564 1565 0.8 15100 Comparison Example 8 110 129 1465 0.757 163 Comparison Example 9 1267 36 1589 0.80 7 151 Comparison Example 10 115 050 1455 0.7679 Comparison Example 11 1020 27 139 50.739 163 Comparison Example 12 138 039 1467 0.944 175 Comparison Example 13 147 77 01536 0.963 105

[0159] ClassificationBending workability(R / t)Rsk×(R / t)3-point bending angle(º)Hardness of weld(Hv)CTS(kN)Surface residual stress(MPa)Penetrating corrosionInvention example12.91.19625206.5-53Non-occurringInvention example23.51.33596025.2-56Non-occurringInvention example32.90.90605316.2-49Non-occurringInvention example42.91.02585655.3-57Non-occurringInvention example52.91.16615176.5-46Non-occurringComparative example15.01.95328361.02Non-occurringComparative example24.61.93367261.9-1Non-occurringComparative example34.61.8940548 5.78 Occurrence Comparison Example 44.32.62466583.925 Non-occurrence Comparison Example 52.51.38613477.5-45 Non-occurrence Comparison Example 62.51.13624367.0-56 Non-occurrence Comparison Example 75.02.05375290.6-61 Non-occurrence Comparison Example 84.33.40395216.7-51 Non-occurrence Comparison Example 94.33.53395126.6-57 Non-occurrence Comparison Example 104.33.35385225.9-61 Non-occurrence Comparison Example 114.33.53475206.7-61 Non-occurrence Comparison Example 125.03.95385196.5-51 Non-occurrence Comparison Example 1354.05355186.26 Non-occurrence

[0160] As can be seen from Tables 1 to 7 above, in the case of Invention Examples 1 to 5 that satisfy the alloy composition and manufacturing conditions of the present invention, the microstructure to be obtained by the present invention, the water content in the region where the KAM value is 0 to 1.0°, a / b, the average size of carbides in the lath of tempered martensite, the size of MnS inclusions, the surface roughness (Rsk) of the surface layer, and the residual stress of the surface layer are secured, so that not only the mechanical properties but also the shape are excellent, and the spot weldability and corrosion resistance are excellent.

[0161] In the case of Comparative Example 1, which does not satisfy the C content, X value, and tension leveling elongation, it can be seen that the yield strength, tensile strength, yield ratio, elongation, EL×m, bending workability, 3-point bending angle, weld strength, CTS, and surface residual stress that the present invention seeks to obtain are not secured.

[0162] In the case of Comparative Example 2, which does not satisfy the Si content, X value, and tension leveling elongation, it can be seen that the yield strength, tensile strength, yield ratio, elongation, EL×m, bending workability, 3-point bending angle, weld strength, CTS, and surface residual stress that the present invention seeks to obtain are not secured.

[0163] In the case of Comparative Example 3, which did not satisfy the Cr content, Y / X value, Z value, and tension leveling elongation, a / b was not satisfied, and therefore the yield ratio, bending workability, three-point bending angle, and surface residual stress that the present invention seeks to obtain were not secured, and it can be seen that penetrating corrosion occurred.

[0164] In the case of Comparative Example 4, which does not satisfy the Si content, X value, Y / X value, and tension leveling elongation, it can be seen that the yield ratio, elongation, EL×m, bending workability, Rsk×bending workability, weld strength, CTS, and surface residual stress that the present invention seeks to obtain are not secured.

[0165] In the case of Comparative Example 5, which does not satisfy the C content, X value, Y value, Z value, and coiling temperature, it can be seen that the microstructure, a / b, and KAM value do not satisfy the water content in the region of 0 to 1.0°, and thus the YS / m, tensile strength, and EL×m that the present invention seeks to obtain are not secured.

[0166] In the case of Comparative Example 6, which does not satisfy the Cr content, Mo content, B content, X value, and Z value, it can be seen that the microstructure and the moisture content in the area where the KAM value is 0 to 1.0° are not satisfied, and therefore the YS / m, tensile strength, and EL×m that the present invention seeks to obtain are not secured.

[0167] In the case of Comparative Example 7, which does not satisfy the S content, it can be seen that the bending processability, three-point bending angle, and CTS that the present invention seeks to obtain are not secured because the maximum size of MnS is not satisfied.

[0168] In the case of Comparative Example 8, which does not satisfy the cold rolling reduction ratio, the first cooling end temperature, and the temper rolling reduction force, the microstructure, a / b, Rsk, and KAM values ​​do not satisfy the moisture content, Rsk, and Rsk×Pc in the region of 0 to 1.0°, and thus it can be seen that the YS / m, tensile strength, and Rsk×bending workability that the present invention seeks to obtain are not secured.

[0169] In the case of Comparative Example 9, which does not satisfy the surface roughness of the cold-rolled roll, H-T2, and the temper rolling reduction force, the microstructure, a / b, and the moisture content in the area where the central KAM value is 0 to 1.0°, Rsk, are not satisfied, and thus it can be seen that the YS / m and Rsk×bending workability that the present invention seeks to obtain are not secured.

[0170] In the case of Comparative Example 10, which does not satisfy the first cooling end temperature and the temper rolling reduction force, it can be seen that the tensile strength that the present invention seeks to obtain is not secured because the microstructure, a / b, and the surface layer KAM value do not satisfy the moisture content in the region of 0 to 1.0° and Rsk.

[0171] In the case of Comparative Example 11, which does not satisfy the first average cooling rate, reheating / overaging treatment temperature, and temper rolling reduction force, the microstructure, a / b, KAM value, water content in the region of 0 to 1.0°, carbide average size, and Rsk are not satisfied, and thus it can be seen that the yield strength, YS / m, tensile strength, and Rsk×bending workability that the present invention seeks to obtain are not secured.

[0172] In the case of Comparative Example 12, which does not satisfy the reheating / overaging treatment temperature and the temper rolling reduction force, the microstructure, the water content in the area where the central KAM value is 0 to 1.0°, the average carbide size, and Rsk are not satisfied, and thus it can be seen that the YS / m, tensile strength, Rsk×bending workability, and 3-point bending angle that the present invention seeks to obtain are not secured.

[0173] In the case of Comparative Example 13, which does not satisfy the surface roughness of the cold rolled roll, the secondary cooling end temperature, the secondary average cooling rate, T1-T2, the temper rolling reduction force, and the tension leveling elongation, a / b, m <n, Rsk, Rsk×Pc를 만족하지 않음에 따라 본 발명이 얻고자 하는 항복비, 굽힘가공성, Rsk×굽힘가공성, 3점 굽힘각도, 표층부 잔류응력을 확보하지 못하고 있음을 알 수 있다.

[0174] Fig. 1 is a TEM photograph of Invention Example 1. Fig. 2 is a TEM photograph of Comparative Example 12. As can be seen from Figs. 1 and 2, while Invention Example 1 has the desired average carbide size, Comparative Example 13 does not have the desired average carbide size of the decarburized layer proposed by the present invention.

[0175] Fig. 3 is a photograph taken after evaluating the corrosion resistance of a plate of Invention Example 1. Fig. 4 is a photograph taken after evaluating the corrosion resistance of a plate of Comparative Example 3. As can be seen from Figs. 3 and 4, no penetration corrosion occurred in the case of Invention Example 1, but penetration corrosion occurred in the case of Comparative Example 3.

Claims

1. Contains, by weight%, carbon (C): 0.150 to 0.350%, silicon (Si): 0.0050 to 0.70%, manganese (Mn): 0.30 to 2.50%, chromium (Cr): 0.0050 to 0.60%, molybdenum (Mo): 0.0030 to 0.50%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.0070% or less (excluding 0%), and the remainder is composed of iron and other unavoidable impurities. It is divided into a central part; and a surface part formed on the outer side based on the thickness direction of the central part; Cold rolled steel sheet having a surface roughness (Rsk) of -0.65 to 0.65 of the surface layer. (However, the above surface layer refers to the area from the surface of the steel to 20㎛ in the thickness direction).

2. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet in which the product of the surface roughness (Rsk) of the surface layer and Pc is -150 or more.

3. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet in which the absolute value of the product of the surface roughness (Rsk) of the surface layer and the bending workability (R / t) is 2.4 or less.

4. Contains, by weight%, carbon (C): 0.150 to 0.350%, silicon (Si): 0.0050 to 0.70%, manganese (Mn): 0.30 to 2.50%, chromium (Cr): 0.0050 to 0.60%, molybdenum (Mo): 0.0030 to 0.50%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.0070% or less (excluding 0%), and the remainder is composed of iron and other unavoidable impurities. It is divided into a central part; and a surface part formed on the outer side based on the thickness direction of the central part; When measuring EBSD, the number fraction in the area where the KAM value is 0 to 5.0° is set to 100%, The water content (m) in the area where the KAM value is 0 to 1.0° in the center is 30% or less (excluding 0%), The water fraction (n) in the area where the KAM value is 0 to 1.0° in the above surface layer is 55% or less (excluding 0%), A cold rolled steel sheet in which the water content ratio (m) in the region where the KAM value is 0 to 1.0° in the center and the water content ratio (n) in the region where the KAM value is 0 to 1.0° in the surface layer have a relationship of m < n. (However, the above surface layer refers to the area from the surface of the steel to 20㎛ in the thickness direction.) 5. In claim 1 or 4, The above cold rolled steel sheet is a cold rolled steel sheet additionally containing at least one of boron (B): 0.00050 to 0.0050%, aluminum (Al): 0.010 to 0.10%, niobium (Nb): 0.0010 to 0.10%, titanium (Ti): 0.0050 to 0.080%, and nitrogen (N): 0.010% or less (excluding 0%).

6. In claim 1 or 4, The above cold rolled steel sheet is a cold rolled steel sheet satisfying the following relationships 1 to 4. [Relationship 1] X = C + 0.03Si + 0.02Mn : 0.24~0.40 [Relationship 2] Y = (Cr + 0.8C + 0.07(Si+Mn) - 0.1Mo - 50B - 70P - 30S)X100 + 100 : 30~120 [Relationship 3] Y / X = 125~375 [Relationship 4] Z = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb+ 10000B : 55~250 (However, in the above equations 1 to 4, the content of each alloy element is in weight%.) 7. In claim 1 or 4, The microstructure of the above central part includes, in area %, a total fraction (a) of at least one of ferrite and bainite: 5% or less (excluding 0%), and at least one of the remainder martensite and tempered martensite, A cold rolled steel sheet having a microstructure of the surface layer, in area %, a total fraction (b) of at least one of ferrite and bainite: 10% or less (excluding 0%), and at least one of the remainder martensite and tempered martensite.

8. In claim 7, The above cold rolled steel sheet has a relationship of 0.1 < a / b < 0.

8.

9. In claim 7, A cold rolled steel sheet having an average size of carbides in a lath of the above tempered martensite of 200 nm or less (excluding 0 nm).

10. In claim 1 or 4, The above cold rolled steel sheet is a cold rolled steel sheet with a maximum size of MnS inclusions of 50㎛ or less (excluding 0㎛).

11. In claim 1 or 4, Cold rolled steel sheet having a residual stress of -30 MPa or less in the surface layer.

12. In claim 1 or 4, The above cold rolled steel sheet is a cold rolled steel sheet having a yield strength of 1050 to 1700 MPa, a tensile strength of 1470 to 1900 MPa, a yield ratio of 0.65 to 0.95, an elongation of 4 to 10%, a bending workability (R / t): 4 or less, and a maximum three-point bending angle of 40º or more.

13. In claim 1 or 4, The product of the moisture content (m) and elongation in the area where the KAM value is 0 to 1.0° in the center of the above cold rolled steel sheet is 80 to 210%. 2 A cold rolled steel sheet having a ratio (YS / m) of the water content (m) and the yield strength (YS) in the area where the KAM value is 0 to 1.0° in the center of 45 MPa / % or more.

14. In claim 1 or 4, The above cold rolled steel sheet is a cold rolled steel sheet having, after resistance spot welding, a hardness of the weld: 650 Hv or less, and a cross tensile strength (CTS) of the weld: 4.5 kN or more.

15. In claim 1 or 4, The above cold rolled steel sheet is a cold rolled steel sheet with 0 holes with a diameter of 3 mm or more when subjected to 30 cycles of a plate corrosion resistance test. A step of heating a slab comprising, by weight%, carbon (C): 0.150 to 0.350%, silicon (Si): 0.0050 to 0.70%, manganese (Mn): 0.30 to 2.50%, chromium (Cr): 0.0050 to 0.60%, molybdenum (Mo): 0.0030 to 0.50%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.0070% or less (excluding 0%), and the remainder being iron (Fe) and other unavoidable impurities; A step of obtaining a hot-rolled steel sheet by final hot-rolling the above heated slab; A step of coiling the above hot-rolled steel plate; A step of obtaining a cold rolled steel sheet by cold rolling the above-mentioned hot rolled steel sheet using a roll having a surface roughness (Ra) of 2.5㎛ or more at a cold rolling reduction ratio of 35 to 70%; A step of continuously annealing the above cold rolled steel sheet; A step of first cooling the continuously annealed cold rolled steel sheet to a first cooling end temperature (T1) of 660℃~Ac3; A step of secondarily cooling the first-cooled cold-rolled steel sheet to a second cooling end temperature (T2) of 50 to 300°C; A step of reheating the second-cooled cold-rolled steel sheet and then subjecting it to an overaging treatment at an overaging treatment temperature (H) of 100 to 250°C; A step of temper rolling the above-mentioned cold rolled steel sheet with a pressure of 500 to 1,000 tons; and A method for manufacturing a cold rolled steel sheet, comprising: a step of tension leveling the cold rolled steel sheet subjected to the above-mentioned temper rolling at an elongation of 0.05 to 0.65%.

17. In claim 16, A method for manufacturing a cold rolled steel sheet, wherein the above slab additionally contains at least one of boron (B): 0.00050 to 0.0050%, aluminum (Al): 0.010 to 0.10%, niobium (Nb): 0.0010 to 0.10%, titanium (Ti): 0.0050 to 0.080%, and nitrogen (N): 0.010% or less (excluding 0%).

18. In claim 16, The above slab is a method for manufacturing a cold rolled steel sheet satisfying the following relationships 1 to 4. [Relationship 1] X = C + 0.03Si + 0.02Mn : 0.24~0.40 [Relationship 2] Y = (Cr + 0.8C + 0.07(Si+Mn) - 0.1Mo - 50B - 70P - 30S)X100 + 100 : 30~120 [Relationship 3] Y / X = 125~375 [Relationship 4] Z = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb+ 10000B : 55~250 (However, in the above equations 1 to 4, the content of each alloy element is in weight%.) 19. In claim 16, The above slab heating is a method for manufacturing cold rolled steel sheets, which is performed at 1100 to 1300°C.

20. In claim 16, The above finishing hot rolling is a method for manufacturing cold rolled steel sheets, which is performed at Ar3 to Ar3+120℃.

21. In claim 16, The above method for manufacturing cold rolled steel sheets is performed at Ms~700℃.

22. In claim 16, The above continuous annealing is a method for manufacturing a cold rolled steel sheet, wherein the continuous annealing is performed at Ac3 to Ac3+100℃ for 30 to 230 seconds.

23. In claim 16, A method for manufacturing a cold rolled steel sheet, wherein the above primary cooling is performed at an average cooling rate of 1 to 6°C.

24. In claim 16, A method for manufacturing a cold rolled steel sheet, wherein the above secondary cooling is performed at an average cooling rate of 30 to 300°C.

25. In claim 16, A method for manufacturing a cold rolled steel sheet, wherein, during the first and second cooling, the first cooling completion temperature (T1) - the second cooling completion temperature (T2) are controlled to be 650°C or lower.

26. In claim 16, A method for manufacturing cold rolled steel sheets in which the above over-aging treatment is performed for 5 to 12 minutes.

27. In claim 16, A method for manufacturing a cold rolled steel sheet, wherein, during the secondary cooling and over-aging treatment, the over-aging treatment temperature (H) - secondary cooling end temperature (T2) is controlled to be 30°C or higher.

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