Cold-rolled steel sheet and manufacturing method thereof

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, and for the protection of electric vehicle battery cases such as side frames, cross members, and the like, and to a manufacturing method therefor.

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-described 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 an excellent shape, a low yield ratio (YR), and excellent bending characteristics and hole expansion rate.

[0007] [Prior Art Literature]

[0008] [Patent Document]

[0009] (Patent Document 1) Japanese Patent Application Laid-Open No. 2010-248565

[0010] (Patent Document 2) Japanese Patent Application Laid-Open No. 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] One embodiment of the present invention comprises, in wt%, carbon (C): 0.180 to 0.280%, silicon (Si): 0.030 to 0.70%, manganese (Mn): 1.50 to 2.40%, chromium (Cr): 0.0050 to 0.60%, molybdenum (Mo): 0.0030 to 0.50%, boron (B): 0.00050 to 0.0050%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.00350% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), aluminum (Al): 0.010 to 0.10%, niobium (Nb): 0.0030 to 0.050%, titanium (Ti): 0.0050 to 0.050%, and the remainder being Fe and A cold-rolled steel sheet is provided, which is composed of other unavoidable impurities, satisfies the following relationships 1 to 3, includes a center portion; and a surface portion formed on the outer side of the center portion in the thickness direction; and has a residual stress of -30 MPa or less and a surface roughness (Rsk) of -0.7 or more.

[0013] [Relationship 1] 0.31 ≤ X = C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.40

[0014] [Relation 2] 100 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 195

[0015] [Relationship 3] 270 ≤ Y / X ≤ 600

[0016] (However, in the above equations 1 to 3, the content of each alloy element is in weight%.)

[0017] The above surface layer may be a region of up to 20㎛ in the thickness direction from the surface of the steel material.

[0018] The microstructure of the above central portion may include, in area %, a total of at least one type of ferrite and bainite: 5% or less (including 0%), and at least one type of residual martensite and tempered martensite.

[0019] The microstructure of the above surface layer may include, in area %, a total of at least one type of ferrite and bainite: 10% or less (excluding 0%), and at least one type of residual martensite and tempered martensite.

[0020] When the above cold-rolled steel sheet is measured by EBSD, the number fraction in the region where the KAM value is 0 to 5.0° is set to 100%, and the number fraction in the region where the KAM value is 0 to 1.0° at a position 1 / 4 in the thickness direction may be 22 to 30%.

[0021] The above cold rolled steel sheet may have a product of Rsk and Pc of -130 or more.

[0022] The above cold rolled steel sheet may have a yield strength (YS): 1000 to 1300 MPa, a tensile strength (TS): 1470 to 1670 MPa, a yield ratio (YS / TS): 0.85 or less, and an elongation (El): 4 to 10%.

[0023] The above cold rolled steel sheet may have a yield strength (YS) × elongation (El) of 4000 MPa% or more.

[0024] The above cold rolled steel sheet may have a bending workability (R / t): 3.5 or less.

[0025] The above cold rolled steel sheet may have a hole expandability (HER): 30% or more.

[0026] The above cold rolled steel sheet may have a hardness deviation of 250 Hv or less at a 1 / 4 position in the thickness direction.

[0027] The above cold rolled steel sheet can have a plating layer formed on at least one surface.

[0028] Another embodiment of the present invention comprises, in wt%, carbon (C): 0.180 to 0.280%, silicon (Si): 0.030 to 0.70%, manganese (Mn): 1.50 to 2.40%, chromium (Cr): 0.0050 to 0.60%, molybdenum (Mo): 0.0030 to 0.50%, boron (B): 0.00050 to 0.0050%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.00350% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), aluminum (Al): 0.010 to 0.10%, niobium (Nb): 0.0030 to 0.050%, titanium (Ti): 0.0050 to 0.050%, and the remainder being Fe and The present invention provides a method for manufacturing a cold rolled steel sheet, comprising: a step of heating a slab composed of other unavoidable impurities and satisfying the following relationships 1 to 3; a step of finishing hot rolling the heated slab to obtain a hot rolled steel sheet; a step of coiling the hot rolled steel sheet; a step of cold rolling the coiled hot rolled steel sheet using a roll having a surface roughness (Ra) of 2.5 ㎛ or more to obtain a cold rolled steel sheet; a step of continuously annealing the cold rolled steel sheet; a step of first cooling the continuously annealed cold rolled steel sheet; a step of second cooling the first-cooled cold rolled steel sheet; a step of reheating the second-cooled cold rolled steel sheet and then subjecting it to an overaging treatment; a step of temper rolling the overaged cold rolled steel sheet with a rolling force of 500 to 1,000 tons; and a step of tension leveling the temper rolled cold rolled steel sheet to an elongation of 0.030 to 0.50%.

[0029] [Relationship 1] 0.31 ≤ X = C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.40

[0030] [Relation 2] 100 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 200

[0031] [Relationship 3] 270 ≤ Y / X ≤ 600

[0032] (However, in the above equations 1 to 3, the content of each alloy element is in weight%.)

[0033] Heating of the above slab can be performed at 1100 to 1300°C.

[0034] The above finishing hot rolling can be performed at Ar3~Ar3+120℃.

[0035] The above winding can be performed at Ms~600℃.

[0036] The above cold rolling can be performed at a cold rolling reduction ratio of 45 to 70%.

[0037] The above continuous annealing can be performed at Ac3+20℃ to Ac3+80℃.

[0038] The above continuous annealing can be performed for 30 to 230 seconds.

[0039] In the above first cooling, the first cooling end temperature is controlled to 670 to 750°C, and can be performed at an average cooling rate of 1 to 6°C / s.

[0040] In the above secondary cooling, the secondary cooling end temperature (Tf) is controlled to 110 to 300°C, and can be performed at an average cooling rate of 30 to 300°C / s.

[0041] During the above secondary cooling, the Mf-secondary cooling end temperature (Tf) can be controlled to be 130°C or lower.

[0042] In the above over-aging treatment, the over-aging treatment temperature (H) is controlled to 100 to 250°C and can be performed for 5 to 12 minutes.

[0043] In the above overaging treatment, the overaging treatment temperature (H) - secondary cooling end temperature (Tf) can be controlled to be 100°C or less.

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

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

[0046] Figure 1 is an example of a photograph of a steel plate to which an ultra-fine hardness tester is applied to measure hardness deviation according to an embodiment of the present invention.

[0047] Figure 2 is a photograph of invention example 1 according to an embodiment of the present invention observed using a scanning electron microscope (SEM).

[0048] Figure 3 is a photograph of Comparative Example 9 according to an embodiment of the present invention observed using a scanning electron microscope (SEM).

[0049] 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.

[0050] 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.

[0051] 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.

[0052] Carbon (C): 0.180~0.280%

[0053] 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 martensite steel. If the C content is less than 0.180%, it may be difficult to obtain the yield ratio and tensile strength targeted in the present invention. If the C content exceeds 0.280%, the strength may increase rapidly and the elongation may be poor. In addition, the weldability may be poor. Therefore, the C content is preferably in the range of 0.180 to 0.280%. The lower limit of the C content is more preferably 0.190%, and the upper limit of the C content is more preferably 0.20%. The upper limit of the C content is more preferably 0.270%.

[0054] Silicon (Si): 0.030~0.70%

[0055] Si is an effective element for improving temper softening resistance, and is also an effective element for improving strength through solid solution strengthening. If the Si content is less than 0.030%, 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 generated 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 preferably in the range of 0.030 to 0.70%. The lower limit of the Si content is more preferably 0.050%. The upper limit of the Si content is more preferably 0.60%.

[0056] Manganese (Mn): 1.50~2.40%

[0057] Mn is an element added to secure strength. When the Mn content is less than 1.50%, 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 2.40%, 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, the Mn content is preferably in the range of 1.50 to 2.40%. It is more preferable that the lower limit of the Mn content is 1.60%. It is more preferable that the upper limit of the Mn content is 2.30%.

[0058] Chromium (Cr): 0.0050~0.60%

[0059] 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 preferably in the range of 0.0050 to 0.60%. The lower limit of the Cr content is more preferably 0.010%. The upper limit of the Cr content is more preferably 0.50%.

[0060] Molybdenum (Mo): 0.0030~0.50%

[0061] 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-described effects. If the content of Mo exceeds 0.50%, the above-described effects do not increase significantly compared to the increase in cost due to the addition of expensive alloying elements. Therefore, the content of Mo is preferably in the range of 0.0030 to 0.50%. The lower limit of the Mo content is more preferably 0.0050%. The upper limit of the Mo content is more preferably 0.40%.

[0062] Boron (B): 0.00050~0.0050%

[0063] 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, the content of B is preferably in the range of 0.00050 to 0.0050%. The lower limit of the content of B is more preferably 0.00070%. The upper limit of the content of B is more preferably 0.0040%.

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

[0065] 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, the content of P is preferably 0.030% or less (excluding 0%). The content of P is more preferably 0.0250% or less.

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

[0067] S, like P, is an impurity element contained in steel. If the content of S exceeds 0.00350%, 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, the content of S is preferably 0.00350% (excluding 0%) or less. The content of S is more preferably 0.0030% or less, and even more preferably 0.0020% or less.

[0068] Nitrogen (N): 0.010% or less (excluding 0%)

[0069] 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 preferable that the above N content has a range of 0.010% or less (excluding 0%). It is more preferable that the above N content is 0.0080% or less, and it is even more preferable that it is 0.0060% or less.

[0070] Aluminum (Al): 0.010~0.10%

[0071] Al can be added to remove oxygen from molten steel. If the Al content is less than 0.010%, deoxidation will not be sufficient, thereby compromising 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, the Al content is preferably in the range of 0.010 to 0.10%. The upper limit of the Al content is more preferably 0.0750%.

[0072] Niobium (Nb): 0.0030~0.050%

[0073] Niobium 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 0.0030%, the austenite grain refinement and precipitation strengthening effects cannot be sufficiently obtained. When the content of Nb exceeds 0.050%, the precipitation of coarse carbonitrides increases, and there is a concern that 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 preferably in the range of 0.0030 to 0.050%. The lower limit of the Nb content is more preferably 0.0040%. The upper limit of the Nb content is more preferably 0.040%.

[0074] Titanium (Ti): 0.0050~0.050%

[0075] Ti is a nitride-forming element that scavenges dissolved nitrogen by precipitating it as TiN. When the content of Ti 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. When the content of Ti exceeds 0.050%, in addition to removing dissolved nitrogen, the strength of martensite may be reduced due to the precipitation of additional carbides, and hole expandability and bending workability may be deteriorated due to the formation of excessive carbon and nitrides such as TiC and TiN. Therefore, the content of Ti is preferably in the range of 0.0050 to 0.050%. The lower limit of the Ti content is more preferably 0.010%. The upper limit of the Ti content is more preferably 0.040%.

[0076] 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.

[0077] It is preferable that the cold-rolled steel sheet of the present invention satisfies the above-described alloy composition and the following relationship equations 1 to 3.

[0078] [Relationship 1] 0.310 ≤ X = C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.40

[0079] The above equation 1 is a component equation closely related to the hardness of the weld. When the value of X is less than 0.310, it is difficult to sufficiently secure the hardness and strength of the weld. When the value of X exceeds 0.40, the hardness of the weld becomes excessively high, which increases the risk of brittle fracture, and thus the crash stability may be deteriorated. Therefore, the value of X is preferably in the range of 0.310 to 0.40. The lower limit of the X value is more preferably 0.320. The upper limit of the X value is more preferably 0.390.

[0080] [Relation 2] 100 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 190

[0081] The above equation 2 is a component relationship related to hardenability for securing the microstructure and strength targeted by the present invention. When the value of Y is less than 100, the hardenability is insufficient, making it difficult to obtain the microstructure targeted by the present invention and securing sufficient strength. When the value of Y exceeds 195, not only does the manufacturing cost increase, but the strength becomes excessively high, which causes a problem of reduced elongation. Therefore, the value of Y is preferably in the range of 100 to 195. The lower limit of the Y value is more preferably 105. The upper limit of the Y value is more preferably 190.

[0082] [Relationship 3] 270 ≤ Y / X ≤ 600

[0083] The above equation 3 is a component relationship for securing an appropriate level of weld hardness and hardenability. When the value of Y / X is less than 270, Ceq is satisfied, but it may be difficult to secure strength because the target microstructure cannot be obtained due to insufficient hardenability. When the value of Y / X exceeds 600, hardenability is sufficiently secured, but it may be difficult to secure weld strength due to low Ceq. Therefore, the value of Y / X is preferably in the range of 270 to 600. It is more preferable that the lower limit of the Y / X value is 310. It is more preferable that the upper limit of the Y / X value is 580.

[0084] The cold rolled steel sheet according to the present invention can be divided into a central portion in terms of microstructure; 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.

[0085] The above central microstructure may include, in area %, the sum of at least one type of ferrite and bainite: 5% or less (including 0%), and at least one type of residual martensite and tempered martensite, and the above surface microstructure may include, in area %, the sum of at least one type of ferrite and bainite: 10% or less (excluding 0%), and at least one type of residual martensite and tempered martensite.

[0086] It is preferable 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 of at least one of the ferrite and bainite exceeds 5%, it may be difficult to secure the properties sought by the present invention. Therefore, the total fraction of at least one of the ferrite and bainite is preferably 5% or less, and more preferably 3% or less.

[0087] It is preferable that the main phase of the surface microstructure includes at least one of martensite and tempered martensite. In addition, it is preferable that the total fraction 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%, it is difficult to secure sufficient strength, and the bending properties may be deteriorated. Therefore, the total fraction of at least one of ferrite and bainite is preferably 10% or less, and more preferably 7% or less. In the present invention, there is no particular limitation on the lower limit of the total fraction of at least one of ferrite and bainite, but may be 1% as an example.

[0088] In the cold rolled steel sheet of the present invention, when the number fraction of the region where the KAM value is 0 to 5.0° during EBSD measurement is set to 100%, the number fraction of the region where the KAM value is 0 to 1.0° at a position 1 / 4 in the thickness direction can be 22 to 30%. The dislocation density within the crystal grain can be evaluated by the KAM (Kernel Average Misorientation) value. The KAM value is an 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 inventors of the present invention investigated the relationship between the KAM value and the microstructure, and confirmed that a difference in physical properties occurs depending on the number fraction of the region where the KAM value is 0 to 1.0° at a position 1 / 4 in the thickness direction. In the present invention, in order to secure the target properties, it is preferable to control the water content in the region where the KAM value at the 1 / 4 position in the thickness direction is 0 to 1.0° in the range of 22 to 30%. If the water content is less than 22%, a large amount of highly deformable tissues will be included, making it difficult to secure the target yield strength. On the other hand, if it exceeds 30%, a large amount of less deformable tissues will be included, making it difficult to secure the target tensile strength. Therefore, it is preferable that the water content in the region where the KAM value at the 1 / 4 position in the thickness direction is 0 to 1.0° be in the range of 22 to 30%. It is more preferable that the lower limit of the water content is 23%, and it is more preferable that the upper limit is 29%. Meanwhile, in the present invention, since the area outside the area where the KAM value is 0 to 5.0°, i.e., the area exceeding 5.0°, has little effect on the characteristics of the present invention, it is preferable to control the water content in the area where the KAM value is 0 to 1.0° by considering the water content in the area where the KAM value is 0 to 5.0° as 100%.

[0089] The cold rolled steel sheet of the present invention preferably has a residual stress of -30 MPa or less in the surface portion. As the residual stress (MPa) of the surface portion approaches 0 or has a positive value, tensile stress is applied, which may be detrimental to securing bending characteristics and hydrogen embrittlement resistance. That is, in the present invention, bending characteristics and hydrogen embrittlement resistance can be improved by controlling the residual stress of the surface portion to a level of -30 MPa or less. Therefore, the residual stress of the surface portion is preferably -30 MPa or less, and more preferably -50 MPa or less. Meanwhile, in the present invention, since a lower residual stress of the surface portion is advantageous, there is no particular limitation on the lower limit. However, as an example, the lower limit of the residual stress of the surface portion may be -300 MPa.

[0090] The cold rolled steel sheet of the present invention preferably has a surface roughness (Rsk) of -0.7 or more. 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 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 area, increasing the susceptibility to crack occurrence, resulting in poor bending characteristics. When the value of the surface roughness (Rsk) is less than -0.7, the bending characteristics may be poor. Therefore, the surface roughness (Rsk) is preferably in the range of -0.7 or more. The surface roughness (Rsk) is more preferably -0.65 or more, and even more preferably -0.6 or more. Meanwhile, in the present invention, since a higher surface roughness (Rsk) is advantageous, there is no particular limitation on its upper limit. However, as an example, the upper limit of the surface roughness (Rsk) may be +0.5.

[0091] The cold rolled steel sheet of the present invention may have a product of Rsk and Pc of -130 or more. Pc is a factor indicating the number of peaks that completely deviate from the bandwidth within a unit length. The closer the product of Rsk and Pc is to 0 or a positive value, the more advantageous it is for bending characteristics. That is, in the present invention, the bending characteristics can be improved by controlling the product of Rsk and Pc to a level of -130 or more. Therefore, the product of Rsk and Pc is preferably -130 or more, and more preferably -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 Rsk and Pc may be +60.

[0092] As described above, the cold rolled steel sheet of the present invention may have a yield strength of 1000 to 1300 MPa, a tensile strength of 1470 to 1670 MPa, a yield ratio of 0.85 or less, an elongation of 4 to 10%, a bending workability (R / t): 3.5 or less, and a hole expandability (HER): 30% or more. The yield ratio is more preferably 0.80 or less, and in the present invention, since a lower yield ratio is more advantageous, the lower limit thereof is not particularly limited, but as an example, the lower limit of the yield ratio may be 0.7. The bending workability is more preferably 3.0 or less, and in the present invention, since a lower bending workability is more advantageous, the lower limit thereof is not particularly limited, but as an example, the lower limit of the bending workability may be 2.5. The above hole expandability is more preferably 35% or more, and in the present invention, the higher the hole expandability, the more advantageous it is, so there is no particular limitation on the upper limit thereof, but as an example, the upper limit of the hole expandability may be 20%. In addition, the cold-rolled steel sheet of the present invention may have a yield strength (YS) × elongation (El) of 4000 MPa% or more.

[0093] The cold-rolled steel sheet of the present invention may have a hardness deviation of 250 Hv or less at a 1 / 4 position in the thickness direction. By controlling the hardness deviation to a low level in this way, excellent hole expandability can be secured. The hardness deviation may be more advantageously 240 Hv or less, and since a lower hardness deviation is advantageous in the present invention, the lower limit thereof is not particularly limited. However, as an example, the lower limit of the hardness deviation may be 230 Hv.

[0094] 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 preferably 0.7 mm, and even more preferably 0.8 mm. The upper limit of the thickness of the cold-rolled steel sheet is more preferably 2.2 mm, and even more preferably 2.1 mm.

[0095] 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 electrogalvanized layer.

[0096] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0097] First, a slab satisfying the aforementioned alloy composition and equations 1 to 3 is heated. 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 sheet. 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 increases rapidly. 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 preferably 1110°C, more preferably 1120°C, and most preferably 1130°C. The upper limit of the slab heating temperature is more preferably 1290°C, more preferably 1280°C, and most preferably 1270°C.

[0098] 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°C. When the finish hot rolling temperature is lower than Ar3, a dual-phase region of ferrite + austenite or a ferrite region is rolled, resulting in a mixed grain structure, and sheet breakage may occur due to variations in the hot rolling load. When the finish hot rolling temperature exceeds Ar3+120°C, a large amount of surface scale may occur, resulting in poor surface quality. The lower limit of the finish hot rolling temperature is more preferably Ar3+10°C, more preferably Ar3+20°C, and most preferably Ar3+30°C. The upper limit of the finish hot rolling temperature is more preferably Ar3+110°C, more preferably Ar3+100°C, and most preferably Ar3+90°C. 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.

[0099] [Formula 1] Ar3(℃) = 910 - 203√C + 44.7Si + 31.5Mo

[0100] Thereafter, the hot-rolled steel sheet is coiled. The coiling can be performed at Ms~600℃. If the coiling temperature exceeds 600℃, 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. Meanwhile, it is preferable 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 Mf, the strength of the hot-rolled steel sheet may become excessively high, increasing the rolling load during cold rolling, which is a subsequent process, and making actual production impossible. The lower limit of the coiling temperature is more preferably Ms+50℃. The upper limit of the coiling temperature is more preferably 550℃. The Ms refers to the temperature at which austenite begins to transform into martensite upon cooling, and can be obtained through Equation 2 below.

[0101] [Equation 2] Ms(℃) = 539 - 423C - 30.4Mn - 7.5Si + 30Al - 17.7Ni - 12.1Cr - 7.5Mo

[0102] 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.

[0103] Thereafter, the coiled hot-rolled steel sheet is cold-rolled using a roll having a surface roughness (Ra) of 2.5 ㎛ or more to obtain a cold-rolled steel sheet. When 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, the higher the upper limit thereof is not particularly limited. However, as an example, the upper limit of the surface roughness (Ra) of the cold-rolled roll may be 6.5 ㎛. The above cold rolling can be performed at a cold reduction ratio of 45 to 70%. If the cold reduction ratio is less than 45%, it is not only difficult to secure the thickness desired in the present invention, but there is also 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. If the cold reduction ratio exceeds 70%, the work hardening that occurs during cold rolling may cause uneven reduction in the length and width directions, which may result in 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. Therefore, the cold reduction ratio is preferably in the range of 45 to 70%. The lower limit of the cold rolling reduction ratio is more preferably 46%, more preferably 47%, and most preferably 48%. The upper limit of the cold rolling reduction ratio is more preferably 68%, more preferably 66%, and most preferably 64%.

[0104] Thereafter, the cold rolled steel sheet is continuously annealed. The continuous annealing can be performed at Ac3+20℃ to Ac3+80℃. If the continuous annealing temperature is lower than Ac3+20℃, a two-phase annealing, not a single-phase annealing, occurs over the entire length of the steel sheet, which may form 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, which may significantly reduce hole expandability. If the continuous annealing temperature exceeds Ac3+80℃, equipment trouble may occur due to overload of the annealing furnace. The lower limit of the continuous annealing temperature is more preferably Ac3+21℃, even more preferably Ac3+24℃, and most preferably Ac3+25℃. The upper limit of the continuous annealing temperature is more preferably Ac3+70°C, more preferably Ac3+60°C, and most preferably Ac3+50°C. Meanwhile, Ac3 refers to the temperature at which austenite begins to appear upon heating, and can be obtained through the following equation 3.

[0105] [Equation 3] Ac3(℃) = 875 - 206C + 26.2Si - 25.0Mn - 12.3Cr + 9.12Mo + 50.2Nb + 148Ti - 131B

[0106] The continuous annealing can be performed for 30 to 230 seconds. If the 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 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 continuous annealing time is more preferably 40 seconds, more preferably 50 seconds, and most preferably 60 seconds. The upper limit of the continuous annealing time is more preferably 220 seconds, more preferably 210 seconds, and most preferably 200 seconds.

[0107] Thereafter, the continuously annealed cold rolled steel sheet is first cooled. During the first cooling, the first cooling end temperature is controlled to 670 to 750°C, and can be performed 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, which may cause the surface structure to become uneven and deteriorate the bending properties. If the first cooling end temperature exceeds 750°C, the temperature difference between the first cooling end temperature and the second cooling end temperature (Tf) becomes severe, causing a rapid phase transformation, which may result in a poor product shape. The lower limit of the first cooling end temperature is more preferably 680°C. The upper limit of the first cooling end temperature is more preferably 740°C. If the above first average cooling rate is less than 1℃ / s, ferrite is formed during cooling, making it impossible to secure the strength level targeted by the present invention. If the above first average cooling rate exceeds 6℃ / 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 level targeted by the present invention. The lower limit of the above first average cooling rate is more preferably 2℃ / s. The upper limit of the above first average cooling rate is more preferably 5℃ / s.

[0108] Thereafter, the first-cooled cold-rolled steel sheet is cooled a second time. The second cooling is to secure at least one of martensite and tempered martensite, which are the main phases of the present invention. During the second cooling, the second cooling end temperature (Tf) is controlled to 110 to 300°C, and can be performed at an average cooling rate of 30 to 300°C / s. When the second cooling end temperature (Tf) 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 strip meandering. When 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 preferably 115°C, more preferably 120°C, and most preferably 130°C. The upper limit of the secondary cooling end temperature is more preferably 290°C, more preferably 280°C, and most preferably 270°C. When 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. When 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 preferably 35°C / s, more preferably 40°C / s, and most preferably 45°C / s. The upper limit of the secondary average cooling rate is more preferably 290°C / s, more preferably 280°C / s, and most preferably 270°C / s.

[0109] During the secondary cooling, the Mf-second cooling end temperature (Tf) can be controlled to be 130°C or lower. If the Mf-Tf exceeds 130°C, the spacing of the martensite laths becomes excessively dense, and the structure is formed with a lot of deformation, so that the yield strength increases, making it difficult to secure the target yield ratio. It is more preferable that the Mf-Tf is 120°C or lower. Meanwhile, the Ms refers to the temperature at which martensite transformation is completed during cooling, and can be obtained through the following equation 4.

[0110] [Formula 4] Mf(℃) = 431 - 412C - 17.4Si - 47.4Mn - 20.9Cr - 17.0Mo + 49.2Nb + 95.0Ti + 202B

[0111] Afterwards, the second-cooled cold-rolled steel sheet is reheated and then subjected to an 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 100 to 250°C and can be performed for 5 to 12 minutes. If the overaging treatment temperature is less than 100°C, tempering is not sufficiently performed, which has the disadvantage of low yield strength and inability to secure sufficient toughness. If the overaging treatment temperature exceeds 250°C, there is the disadvantage of poor bending workability due to the precipitation and coarsening of a large amount of carbides. The lower limit of the overaging treatment temperature is more preferably 110°C, even more preferably 120°C, and most preferably 130°C. The upper limit of the overaging temperature is more preferably 245°C, more preferably 240°C, and most preferably 235°C. If the overaging time is less than 5 minutes, tempering may not be sufficient, 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 preferably 5.5 minutes, more preferably 6.0 minutes, and most preferably 6.5 minutes. The upper limit of the overaging time is more preferably 11.5 minutes, more preferably 11 minutes, and most preferably 10.5 minutes.

[0112] In the above overaging treatment, the overaging treatment temperature (H) - secondary cooling end temperature (Tf) can be controlled to be 100°C or lower. If the overaging treatment temperature (H) - secondary cooling end temperature (Tf) exceeds 100°C, tempering occurs excessively, increasing the yield strength and making it difficult to secure the target yield ratio. It is more preferable that the overaging treatment temperature (H) - secondary cooling end temperature (Tf) be 90°C or lower.

[0113] Thereafter, the over-aged cold-rolled steel sheet is subjected to skin pass mill (SPM) rolling with a rolling force of 500 to 1,000 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 1,000 tons, the work hardening of the surface may be severe, resulting in poor bending properties. The lower limit of the rolling force during the skin pass rolling is more preferably 550 tons, and more preferably 600 tons. The upper limit of the rolling force during the skin pass rolling is more preferably 950 tons, and more preferably 900 tons.

[0114] Thereafter, the cold rolled steel sheet subjected to temper rolling is tension leveled (T / L) at an elongation of 0.030 to 0.50%. The tension leveling is for correcting the shape of the steel sheet. If the elongation is less than 0.030% during the tension leveling, shape correction may be difficult. If the elongation exceeds 0.50% 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 preferably 0.050%, and more preferably 0.10%. The upper limit of the elongation during the tension leveling is more preferably 0.40%, and more preferably 0.35%.

[0115] Meanwhile, after the tension leveling, an additional step of forming a plating layer on at least one surface of the cold-rolled steel sheet may be included. The present invention does not specifically limit the method for 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 electrogalvanized layer.

[0116] 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.

[0117] (Example)

[0118] 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, the coiled hot-rolled steel sheet was cold-rolled under the conditions described in Table 2 below to obtain a cold-rolled steel sheet. Thereafter, continuous annealing, primary cooling, secondary cooling, reheating / overaging treatment, temper rolling, and tension leveling were performed under the conditions described in Tables 2 and 3 below to manufacture a cold-rolled steel sheet having a thickness of 1.4 mm. Meanwhile, the conditions described in Tables 2 and 3 below were based on the surface temperature of the steel sheet.

[0119] For the cold rolled steel sheet manufactured in this manner, the microstructure, moisture content in the region where the KAM value is 0 to 1.0°, surface residual stress, surface roughness, and mechanical properties were measured, and the results are shown in Tables 4 and 5 below.

[0120] 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 three times through image analysis and the average value was calculated.

[0121] The number fraction of the region where the above 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 a position of 1 / 4t (t: thickness of steel) of the steel plate, and the average value was calculated. At this time, the number fraction of the region where the KAM value is 0 to 5.0° during the EBSD measurement was regarded as 100%.

[0122] The surface residual stress was measured five 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 was calculated excluding the maximum (Max) and minimum (Min) values.

[0123] Surface roughness (Rsk, Pc) was measured five times using a contact-type 2D roughness meter, and the average value was calculated excluding the maximum (Max) and minimum (Min) values.

[0124] 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.

[0125] 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.

[0126] Hardness deviation was measured at a position of 1 / 4t (t: thickness of steel) of the steel plate using an ultra-micro hardness tester at a load of 1g, as shown in Figure 1, at 5 vertical points × 10 horizontal points, a total of 50 points, and the difference between the maximum (Max) and minimum (Min) values ​​was calculated.

[0127] The hole expansion ratio (HER) was measured according to the ISO 16330 standard, and the holes were sheared with a clearance of 12% using a 10 mm diameter punch.

[0128] Steel grade No. Alloy composition (weight %) CSiMnPSAlCrMoNbTiBNXYY / X10.240.111.810.0100.00090.0250.060.070.0310.0250.00200.00350.3615844120.230.211.910.0110.00110.0310.140.080.0250.0300.00190.00410.3616144930.250.061.750.0140.0008 0.0300.100.060.0270.0230.00210.00420.3715341340.240.151.700.0120.00150.0310.110.100.0250.0340.00190.00360.3615342550.240.211.860.0100.00070.0300.210.060.0410.0400.00150.00480.3618149960.170.251.950.01 10.00140.0390.250.150.0310.0210.00250.00350.3018761770.250.152.800.0120.00120.0350.150.090.0250.0250.00210.00250.219746480.250.752.000.0150.00100.0300.100.090.0250.0200.00200.00310.4117943890.290.112. 000.0100.00100.0350.150.080.0290.0270.00240.00380.42178426100.210.151.200.0100.00110.0410.070.040.03 00.0250.00250.00410.30136454110.200.111.550.0070.00100.0310.0010.0010.0210.0200.00200.00510.30120400X = C+Mn / 20+Si / 30+2P+4SY = 48.8+49logC+35.1Mn+25.9Si+14.5Ni+9.6Cu+76.5Cr+105.9Mo+1325Nb+10000B

[0129] Classification Steel grade No. Ar3 (℃) Ms (℃) Cold rolling reduction ratio (%) Cold rolling roll roughness (㎛) Ac3 (℃) Annealing temperature (℃) First cooling end temperature (℃) First average cooling speed (℃ / s) Invention example 11818374564.57888567153 Invention example 22825367564.57908597053 Invention example 33813375564.57858467123 Invention example 44820373564.579385271 63 Invention Example 55822363564.57908607253 Comparative Example 16842386564.58008427313 Comparative Example 27818335564.57618567363 Comparative Example 38845315564.57978507253 Comparative Example 49808346564.57728617153 Comparative Example 510825403 564.58108557013Comparison Example 6 11824400564.58028757093Comparison Example 7 1818374352.07888567123Comparison Example 8 1818374421.57888547163Comparison Example 9 1818374564.57887807253Comparison Example 10 1818374564.57888506 453Comparative Example 111818374604.57888617163Comparative Example 121818374564.57888597203Comparative Example 131818374564.57888647183Comparative Example 143813375564.57858547213Comparative Example 153813375564.57858657153Ar3(℃) = 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

[0130] Classification Steel grade No. Mf (℃) Second cooling end temperature (Tf) (℃) Second average cooling rate (℃ / s) Mf-Tf (℃) Reheating / overaging treatment temperature (H) (℃) Overaging treatment time (min) H-Tf (℃) Temper rolling pressure (ton) Tension leveling elongation (%) Invention example 1 1246 1756 171 1858.4 106 500.05 Invention example 2 22242 1806 062 1708.7-106 500.05 Invention example 3 3245 1955 850 1858.7-106500.05 Invention Example 4425020059501768.5-246500.05 Invention Example 5524119460471758.6-196500.05 Comparative Example 1626021061501958.3-156500.05 Comparative Example 2719215562372108.1556500.15 Comparative Example 382201663521918.2236500.05 Comparative Example 49215152626 32158.6636500.25Comparison Example 510287184651031959.1116500.05Comparison Example 61127719563821898.9-66500.05Comparison Example 7124617563711918.7166500.05Comparison Example 8124619564511898.6-66500.05Comparison Example 9124618062662008.5206500.05Comparison Example 101246165 61812018.7366500.05Comparative Example 111246105761411858.5206500.25Comparative Example 12124685701612308.61456500.25Comparative Example 13124618559613008.71156500.05Comparative Example 14324517460611958.7112500.55Comparative Example 15324518961562028.6136500.65Mf(℃) = 431-412C-17.4Si-47.4Mn-20.9Cr-17.0Mo+49.2Nb+95.0Ti+202B

[0131] Classification Microstructure (area %) Moisture content of areas where KAM value is 0 to 1.0° (%) Surface residual stress (MPa) Surface roughness Surface center Rsk Pc Rsk × Pc At least one of F and B At least one of M and TM At least one of F and B At least one of M and TM Invention example 139719925-78-0.35170-60 Invention example 229819926-69-0.32181-58 Invention example 339719927-72-0.36191-69 Invention example 429819925-81-0.36184-66 Invention example 529819926-92-0.37165-61 Comparative example 1 118959532-75-0.39175-68Comparative Example 2138719921-62-0.42168-71Comparative Example 3128819925-79-0.38181-69Comparative Example 429819920-62-0.39172-67Comparative Example 51585109033-75-0.45178-80Comparative Example 61 38799132-82-0.44181-80Comparative Example 729829820-91-0.73189-138Comparative Example 819919923-89-0.72190-137Comparative Example 91585109040-79-0.42201-84Comparative Example 10188239723-84-0.41185-76Comparative Example 1 11783158520-69-0.42189-79Comparative Example 1249639720-69-0.48187-90Comparative Example 13298010032-87-0.39175-68Comparative Example 1439729826-25-0.41178-73Comparative Example 1529819925-15-0.38182-69F: Ferrite, B: Bainite, M: Martensite, TM: Tempered Martensite

[0132] Classification Yield strength (MPa) Tensile strength (MPa) Yield elongation (%) Bending workability (R / t) Strength deviation (MPa) Hole expandability (%) Invention example 1 1 1 0 6 1 5 3 2 0.7 2 6 2.5 1 8 9 4 5 Invention example 2 1 2 0 0 1 5 4 2 0.7 8 6 2.5 1 6 5 1 Invention example 3 1 0 9 5 1 5 2 6 0.7 2 6 2.5 1 4 5 5 0 Invention example 4 1 1 6 8 15210.7762.513650Invention Example 5126115670.8062.515949Comparative Example 1102514250.7272.52536Comparative Example 2136015670.8753.218934Comparative Example 3110515360.7263.915941Comparative Example 4140115810.8943.91864 0Comparison Example 5 998 140 50.7 182 526 522Comparison Example 6 1025 1425 0.7 29 2.5 25 925Comparison Example 7 1125 1535 0.7 36 4.3 145 45Comparison Example 8 115 015 42 0.7 5 64 313 942Comparison Example 9 945 1545 0.6 15 4.6 35 618Comparison Example 10 108 9 15 0 10.7 364.318928Comparison Example 11132515890.8353.217545Comparison Example 12135015590.8753.216951Comparison Example 13129514680.8864.312558Comparison Example 14110515250.7264.315745Comparison Example 15110115360.7264.316843

[0133] As can be seen from Tables 1 to 5 above, it can be seen that invention examples 1 to 5 that satisfy the conditions proposed by the present invention secure the mechanical properties targeted by the present invention.

[0134] On the other hand, it can be seen that the mechanical properties of Comparative Examples 1 to 15, which do not satisfy the alloy composition or manufacturing conditions proposed by the present invention, are inferior.

[0135] Figures 2 and 3 are scanning electron microscope (SEM) photographs of Inventive Example 1 and Comparative Example 9, respectively. Inventive Example 1 is composed mostly of martensite and tempered martensite and has a uniform microstructure, whereas Comparative Example 9 does not satisfy the annealing temperature proposed in the present invention, and thus has martensite and tempered martensite as its main structures, but it can be confirmed that a large amount of soft ferrite is mixed in.

Claims

1. Contains, in wt%, carbon (C): 0.180 to 0.280%, silicon (Si): 0.030 to 0.70%, manganese (Mn): 1.50 to 2.40%, chromium (Cr): 0.0050 to 0.60%, molybdenum (Mo): 0.0030 to 0.50%, boron (B): 0.00050 to 0.0050%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.00350% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), aluminum (Al): 0.010 to 0.10%, niobium (Nb): 0.0030 to 0.050%, titanium (Ti): 0.0050 to 0.050%, and the remainder is Fe and others. It is made up of unavoidable impurities, Satisfying the following relations 1 to 3, center; and Includes a surface layer formed on the outer side based on the thickness direction of the center; The residual stress of the above surface layer is -30 MPa or less, Cold rolled steel sheet having a surface roughness (Rsk) of -0.7 or higher. [Relationship 1] 0.31 ≤ X = C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.40 [Relation 2] 100 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 195 [Relationship 3] 270 ≤ Y / X ≤ 600 (However, in the above equations 1 to 3, the content of each alloy element is in weight%.) 2. In claim 1, The above surface layer is a cold rolled steel plate that is an area of ​​up to 20㎛ in the thickness direction from the surface of the steel.

3. In claim 1, A cold rolled steel sheet comprising the microstructure of the central portion of the above, in area %, the sum of at least one of ferrite and bainite: 5% or less (including 0%), and the remainder including at least one of martensite and tempered martensite.

4. In claim 1, A cold rolled steel sheet comprising the microstructure of the surface layer in terms of area %, the sum of at least one of ferrite and bainite: 10% or less (excluding 0%), and the remainder including at least one of martensite and tempered martensite.

5. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet in which, when the number fraction in the area where the KAM value is 0 to 5.0° is measured by EBSD, is 100%, and the number fraction in the area where the KAM value is 0 to 1.0° at a position 1 / 4 in the thickness direction is 22 to 30%.

6. 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 or more.

7. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a yield strength (YS): 1000 to 1300 MPa, a tensile strength (TS): 1470 to 1670 MPa, a yield ratio (YS / TS): 0.85 or less, and an elongation (El): 4 to 10%.

8. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a yield strength (YS) × elongation (El) of 4000 MPa% or more.

9. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a bending workability (R / t): 3.5 or less.

10. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a hole expandability (HER) of 30% or more.

11. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a hardness deviation of 250 Hv or less at 1 / 4 of the thickness direction.

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. Contains, by weight%, carbon (C): 0.180 to 0.280%, silicon (Si): 0.030 to 0.70%, manganese (Mn): 1.50 to 2.40%, chromium (Cr): 0.0050 to 0.60%, molybdenum (Mo): 0.0030 to 0.50%, boron (B): 0.00050 to 0.0050%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.00350% or less (excluding 0%), nitrogen (N): 0.010% or less (excluding 0%), aluminum (Al): 0.010 to 0.10%, niobium (Nb): 0.0030 to 0.050%, titanium (Ti): 0.0050 to 0.050%, and the remainder is Fe and others. A step of heating a slab comprising inevitable impurities and satisfying the following relationships 1 to 3; 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; A step of continuously annealing the above cold rolled steel sheet; A step of first cooling the continuously annealed cold rolled steel sheet; A step of second cooling the first-cooled cold rolled steel sheet; A step of reheating the second-cooled cold-rolled steel sheet and then performing an over-aging treatment; 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 0.030 to 0.50%. [Relationship 1] 0.31 ≤ X = C + Mn / 20 + Si / 30 + 2P + 4S ≤ 0.40 [Relation 2] 100 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 200 [Relationship 3] 270 ≤ Y / X ≤ 600 (However, in the above equations 1 to 3, the content of each alloy element is in weight%.) 14. In claim 13, A method for manufacturing cold rolled steel sheets in which the above slab is heated at 1100 to 1300°C.

15. In claim 13, The above finishing hot rolling is a method for manufacturing cold rolled steel sheets, which is performed at Ar3 to Ar3+120℃.

16. In claim 13, The above-mentioned method for manufacturing cold rolled steel sheets is performed at Ms~600℃.

17. In claim 13, The above cold rolling is a method for manufacturing cold rolled steel sheets, which is performed at a cold rolling reduction ratio of 45 to 70%.

18. In claim 13, The above continuous annealing is a method for manufacturing cold rolled steel sheets, which is performed at Ac3+20℃ to Ac3+80℃.

19. In claim 13, A method for manufacturing cold rolled steel sheets in which the above continuous annealing is performed for 30 to 230 seconds.

20. In claim 13, A method for manufacturing a cold rolled steel sheet, wherein the first cooling is performed at an average cooling speed of 1 to 6°C / s, and the first cooling completion temperature is controlled to 670 to 750°C during the first cooling.

21. In claim 13, A method for manufacturing a cold rolled steel sheet, wherein the secondary cooling is performed at an average cooling speed of 30 to 300°C / s, and the secondary cooling end temperature (Tf) is controlled to 110 to 300°C during the secondary cooling.

22. In claim 13, A method for manufacturing a cold rolled steel sheet, wherein the Mf-second cooling end temperature (Tf) is controlled to be 130°C or lower during the above secondary cooling.

23. In claim 13, A method for manufacturing a cold rolled steel sheet, wherein the over-aging treatment temperature (H) is controlled to 100 to 250°C and performed for 5 to 12 minutes during the above over-aging treatment.

24. In claim 13, A method for manufacturing a cold rolled steel sheet, wherein, during the above overaging treatment, the overaging treatment temperature (H) - the second cooling end temperature (Tf) is controlled to be 100°C or less.

25. In claim 13, 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 tension leveling.

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