Cold-rolled steel sheet and method of manufacturing same

Abstract

One aspect of the present invention is to provide a cold-rolled steel sheet and a method of manufacturing same. A preferred aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet and a method of manufacturing same, the ultra-high strength cold-rolled steel sheet having a tensile strength of 1470 MPa or higher and also having an excellent shape and excellent phosphate treatability.

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] Meanwhile, automotive steel sheets are typically phosphate-treated prior to electrodeposition to ensure coating adhesion during the painting process. The phosphate crystals formed during phosphate treatment significantly impact corrosion resistance and paint adhesion after electrodeposition. Because phosphate crystals must be small and dense to ensure excellent adhesion to the coating, automakers have established standards for phosphate crystal size and phosphate adhesion content, which must be met before commercialization. Meanwhile, to increase the strength of automotive steel sheets, it's common to add elements such as Si, Mn, and Cr. However, pre-treatment of steel sheets containing these elements before painting poses a problem: these elements form oxides on the surface of the steel sheet, reducing the reactivity with phosphate during phosphate treatment. This reduced reactivity between the steel sheet and phosphate can lead to coarse phosphate crystals that may not fully cover the steel sheet. This can deteriorate paint adhesion and corrosion resistance after electrodeposition. Therefore, in order to achieve ultra-high strength of tensile strength of 1470 MPa or higher and excellent phosphate treatment properties, optimization of components and process conditions is necessary.

[0005] 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.

[0006] 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.

[0007] Patent Document 3 discloses a method for forming an iron coating layer of 20 to 1,500 mg / m2 on a steel plate using electroplating. However, Patent Document 3 has the problem of requiring separate electroplating equipment, which increases costs as the process length increases.

[0008] Patent Document 4 describes a method of forming an oxide film on the surface of a steel sheet by raising the temperature of the steel sheet to 350 to 650°C in an oxidizing atmosphere, and then heating and cooling the steel sheet to the recrystallization temperature in a reducing atmosphere. However, in this method, the thickness of the oxide film formed on the surface of the steel sheet varies depending on the oxidation method, and in some cases, sufficient oxidation does not occur, or the oxide film becomes too thick, causing the oxide film to remain or peel off during subsequent annealing in a reducing atmosphere, thereby deteriorating the surface appearance. In the examples, a technique for oxidizing in air is described. However, oxidation in air has problems such as forming a thick oxide film, making subsequent reduction difficult, or requiring a reducing atmosphere with a high hydrogen concentration.

[0009] Therefore, in order to solve the above-described problems, it is necessary to develop ultra-high strength cold-rolled steel sheets and plated steel sheets having a tensile strength of 1470 MPa or more, which have excellent shape, bending processability, and phosphate treatment properties.

[0010] [Prior Art Literature]

[0011] (Patent Document 1) Japanese Patent Application Laid-Open No. 2010-248565

[0012] (Patent Document 2) Japanese Patent Application Laid-Open No. 2020-019992

[0013] (Patent Document 3) Japanese Patent Publication No. 5-320952

[0014] (Patent Document 4) Japanese Patent Publication No. 55-145122

[0015] One aspect of the present invention is to provide a cold rolled steel sheet and a method for manufacturing the same.

[0016] A preferred 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 and excellent bending workability and phosphate treatment properties, and a method for manufacturing the same.

[0017] One embodiment of the present invention comprises: a steel plate; a Ni coating layer formed on the steel plate; A cold-rolled steel sheet including a C-enriched layer formed on the Ni coating layer, wherein the base steel sheet contains, in wt%, carbon (C): 0.190 to 0.270%, silicon (Si): 0.030 to 0.60%, manganese (Mn): 1.50 to 2.40%, chromium (Cr): 0.0050 to 0.50%, 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%, A cold-rolled steel sheet is provided, which contains 0.0050 to 0.050% of titanium (Ti), the remainder being iron (Fe) and other unavoidable impurities, the Ni coating layer having an average thickness of 0.005 to 0.065 μm, and the C-enriched layer having a C content of 0.5 to 10 wt%.

[0018] The above steel plate can satisfy the following relationships 1 to 3.

[0019] [Relationship 1] X= -10Si + 40Mn - 80Cr ≥ 50

[0020] [Relationship 2] 110 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 200

[0021] [Relationship 3] 1.2 ≤ Y / X ≤ 3.7

[0022] (However, in the above equations 1 to 3, the content of each alloy element is in weight%.)

[0023] The above steel sheet includes a central portion; and a surface portion formed on the outer side of the central portion in the thickness direction, and the microstructure of the central portion includes, in area %, a 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 microstructure of the surface portion may include, in area %, a 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.

[0024] In the above 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 in the region where the KAM value is 0 to 1.0° at a 1 / 4 position in the thickness direction may be 22 to 30%.

[0025] The above Ni coating layer may have a surface roughness (Rsk) of -0.65 or more.

[0026] An oxide layer including oxide having a major diameter of 1.5 ㎛ or less can be formed between the above-mentioned steel plate and the Ni coating layer.

[0027] The above surface layer may be a region of up to 20㎛ in the thickness direction from the surface of the steel material.

[0028] The above cold rolled steel sheet 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, and an elongation of 4 to 10%.

[0029] The above cold rolled steel sheet may have a bending workability (R / t): 3.5 or less.

[0030] The above cold rolled steel sheet may have a degreasing rate of 90% or more.

[0031] The above cold rolled steel sheet may have a phosphate coverage of 95% or more.

[0032] The above cold rolled steel sheet may additionally include a plating layer between the Ni coating layer and the C concentration layer.

[0033] Another embodiment of the present invention comprises, in wt%, carbon (C): 0.190 to 0.270%, silicon (Si): 0.030 to 0.60%, manganese (Mn): 1.50 to 2.40%, chromium (Cr): 0.0050 to 0.50%, 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%, A step of heating a slab consisting of residual Fe and other unavoidable impurities; a step of finishing hot rolling the heated slab to obtain a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet; a step of cold rolling the coiled hot-rolled steel sheet at a cold reduction ratio of 45 to 70% 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 performing an overaging treatment; a step of adding 1 to 15 mg / m of Ni to the overaged cold-rolled steel sheet. 2 A step of forming a Ni coating layer by plating with an adhesion amount of; a step of subjecting the cold rolled steel sheet on which the Ni coating layer is formed to temper rolling with a pressing force of 500 to 1000 tons; a step of tension leveling the temper rolled cold rolled steel sheet; and a step of applying a total amount of 500 to 1500 mg / m on both sides of the tension leveled cold rolled steel sheet. 2 A method for manufacturing a cold rolled steel sheet is provided, including a step of applying oil to the steel sheet.

[0034] The above slab can satisfy the following relationships 1 to 3.

[0035] [Relationship 1] X= -10Si + 40Mn - 80Cr ≥ 50

[0036] [Relationship 2] 110 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 200

[0037] [Relationship 3] 1.2 ≤ Y / X ≤ 3.7

[0038] (However, in the above equations 1 to 3, the content of each alloy element is in weight%.)

[0039] Heating of the above slab can be performed at 1100 to 1300°C.

[0040] The above finishing hot rolling can be performed at Ar3~Ar3+120℃.

[0041] The above winding can be performed at Ms~600℃.

[0042] The above continuous annealing can be performed at Ac3+20℃ to Ac3+80℃.

[0043] The above continuous annealing can be performed for 30 to 230 seconds.

[0044] 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.

[0045] 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.

[0046] During the above secondary cooling, the Mf-secondary cooling end temperature (Tf) can be controlled to be 130°C or lower.

[0047] 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.

[0048] In the above overaging treatment, the overaging treatment temperature (H) - secondary cooling end temperature (Tf) can be controlled to be 100°C or less.

[0049] The above tension leveling can be performed at an elongation of 0.01 to 0.50%.

[0050] 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.

[0051] According to one aspect of the present invention, it is an object to provide a cold rolled steel sheet and a method for manufacturing the same.

[0052] According to a preferred aspect of the present invention, it is an object 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 workability and phosphate treatment properties, and a method for manufacturing the same.

[0053] Figure 1 is a schematic diagram of a cold rolled steel sheet according to one embodiment of the present invention.

[0054] Figure 2 is a photograph after degreasing of Invention Example 1 according to one embodiment of the present invention.

[0055] Figure 3 is a photograph after degreasing of Comparative Example 10 according to one embodiment of the present invention.

[0056] Figure 4 is a scanning electron microscope (SEM) photograph of Invention Example 1 after phosphate treatment according to one embodiment of the present invention.

[0057] Figure 5 is a scanning electron microscope (SEM) photograph of Comparative Example 11 after phosphate treatment according to one embodiment of the present invention.

[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] Fig. 1 is a schematic diagram of a cold rolled steel sheet according to one embodiment of the present invention. Hereinafter, a cold rolled steel sheet according to one embodiment of the present invention will be described with reference to Fig. 1. The cold rolled steel sheet (100) of the present invention preferably includes a base steel sheet (10); a Ni coating layer (20) formed on the base steel sheet (10); and a C concentrated layer (30) formed on the Ni coating layer (20).

[0061] First, the alloy composition of the steel plate of the present invention will be described. The alloy composition described below refers to weight percent unless otherwise specified.

[0062] Carbon (C): 0.190~0.270%

[0063] 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 content of C is less than 0.190%, it may be difficult to obtain the yield ratio and tensile strength targeted in the present invention. If the content of C exceeds 0.270%, the strength may increase rapidly and the elongation may be poor. In addition, the weldability may be poor. Therefore, the content of C is preferably in the range of 0.190 to 0.270%. The lower limit of the C content is more preferably 0.20%. The upper limit of the C content is more preferably 0.260%.

[0064] Silicon (Si): 0.030~0.60%

[0065] Si is an effective element for increasing 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 0.60%. The lower limit of the Si content is more preferably 0.050%. The upper limit of the Si content is more preferably 0.50%.

[0066] Manganese (Mn): 1.50~2.40%

[0067] 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%.

[0068] Chromium (Cr): 0.0050~0.50%

[0069] 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 the 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 0.40%.

[0070] Molybdenum (Mo): 0.0030~0.50%

[0071] 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%.

[0072] Boron (B): 0.00050~0.0050%

[0073] 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%.

[0074] Phosphorus (P): 0.030% or less (excluding 0%)

[0075] 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.

[0076] Sulfur (S): 0.00350% or less (excluding 0%)

[0077] 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.

[0078] Nitrogen (N): 0.010% or less (excluding 0%)

[0079] 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.

[0080] Aluminum (Al): 0.010~0.10%

[0081] 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%.

[0082] Niobium (Nb): 0.0030~0.050%

[0083] 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%.

[0084] Titanium (Ti): 0.0050~0.050%

[0085] 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%.

[0086] 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.

[0087] The steel plate of the present invention can satisfy the alloy composition described above and the following equations 1 to 3.

[0088] [Relationship 1] X= -10Si + 40Mn - 80Cr ≥ 50

[0089] The above equation 1 is a component relationship closely related to phosphate treatability. When the value of X is less than 50, surface oxides containing Si and Cr components may be excessively formed, resulting in poor phosphate treatability. Therefore, the value of X is preferably 50 or greater. The value of X is more preferably 55 or greater.

[0090] [Relationship 2] 110 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 200

[0091] 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 110, 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 200, 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 110 to 200. The lower limit of the Y value is more preferably 120. The upper limit of the Y value is more preferably 190.

[0092] [Relationship 3] 1.2 ≤ Y / X ≤ 3.7

[0093] The above equation 3 is a component equation for securing an appropriate level of phosphate treatability and hardenability. When the value of Y / X is less than 1.2, the phosphate treatability is excellent, but due to insufficient hardenability, the target microstructure cannot be obtained, which may make it difficult to secure strength. When the value of Y / X exceeds 3.7, the hardenability is sufficiently secured, but surface oxides containing Si and Cr components are excessively formed, which may make it difficult to secure phosphate. Therefore, the value of Y / X is preferably in the range of 1.2 to 3.7. It is more preferable that the lower limit of the Y / X value is 1.5. It is more preferable that the upper limit of the Y / X value is 3.1.

[0094] Meanwhile, the steel plate of 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 plate, and thus is not particularly limited thereto. However, as an example, the surface portion may be a region of up to 20 μm in the thickness direction from the surface of the steel plate.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] 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%.

[0099] The cold-rolled steel sheet of the present invention may have an oxide layer formed on the base steel sheet. That is, the oxide layer may be formed between the base steel sheet and the Ni coating layer below. The oxide layer formed on the base steel sheet may deteriorate interfacial performance, such as degreasing and phosphating, and therefore it is preferable not to form it if possible. However, since the oxide layer may inevitably be formed during the manufacturing process, it is preferable to reduce its area as much as possible. The oxide layer may include an oxide having a major diameter of 1.5 μm or less. When the major diameter of the oxide exceeds 1.5 μm, interfacial performance, such as degreasing and phosphating, may deteriorate. Therefore, the major diameter of the oxide is preferably 1.5 μm or less. The major diameter of the oxide is more preferably 1.4 μm or less, and even more preferably 1.3 μm or less. Meanwhile, in the present invention, a smaller oxide size is advantageous, and therefore, there is no particular limitation on the lower limit thereof. However, considering the alloy composition and manufacturing conditions, it is difficult to control the major diameter of the oxide to less than 0.1 μm. That is, as an example, the lower limit of the major diameter of the oxide may be 0.1 μm. In addition, the present invention does not specifically limit the type of the oxide, and as an example, it may be a composite oxide in which Si, Mn, Al, etc. are combined with oxygen.

[0100] The cold-rolled steel sheet of the present invention preferably has a nickel coating layer formed on the base steel sheet. The nickel coating layer promotes nucleation and growth of phosphate crystals. More specifically, when the steel sheet is immersed in a zinc phosphate solution, a potential difference occurs between a region where nickel exists and a region where nickel does not exist, forming a micro cell. Accordingly, electrochemically precious nickel acts as a cathode, and relatively inexpensive iron acts as an anode, promoting the dissolution of iron. At this time, hydrogen, which is a cathode reaction, is reduced at the nickel nucleus region, thereby increasing the pH, thereby promoting nucleation and growth of phosphate crystals at the nickel nucleus. The nickel coating layer may have an average thickness of 0.005 to 0.065 μm. When the average thickness of the nickel coating layer is less than 0.005 μm, nucleation and growth of phosphate crystals may not occur. When the average thickness of the Ni coating layer exceeds 0.065㎛, hydroxyl groups (-Ni-OH) are formed on the surface, which may react excessively with the polar portion of the anti-rust additive (surfactant) included in the anti-rust oil, preventing degreasing. Therefore, the average thickness of the Ni coating layer is preferably in the range of 0.005 to 0.065㎛. The lower limit of the average thickness of the Ni coating layer is more preferably 0.01㎛, and even more preferably 0.015㎛. The upper limit of the average thickness of the Ni coating layer is more preferably 0.06㎛, and even more preferably 0.055㎛. The Ni coating layer may have a surface roughness (Rsk) of -0.65 or more. The surface roughness (Rsk (Skewness)) is one of several factors of surface roughness related to the asymmetry of the sharp leading portion. The closer the value of the above surface roughness (Rsk) is to 0 or to a positive value, the more advantageous it is for securing bending characteristics.As the -value of the surface roughness (Rsk) increases, the valley on the flat surface becomes deeper, causing stress concentration in this area, increasing the sensitivity to crack occurrence, which in turn deteriorates the bending characteristics. When the value of the surface roughness (Rsk) is less than -0.65, the bending characteristics may deteriorate. Therefore, it is preferable that the surface roughness (Rsk) has a range of -0.65 or more. It is more preferable that the surface roughness (Rsk) is -0.6 or more, and it is even more preferable that it is -0.55 or more. Meanwhile, in the present invention, since a higher surface roughness (Rsk) is advantageous, there is no particular limitation on the upper limit thereof. However, as an example, the upper limit of the surface roughness (Rsk) may be +0.5.

[0101] It is preferable that the cold-rolled steel sheet of the present invention has a C-enriched layer formed on the Ni coating layer. The C-enriched layer may be formed by C, which is a main component contained in oil or anti-rust oil applied to prevent corrosion such as rust from occurring on the steel sheet. The C-enriched layer serves to prevent corrosion such as rust. The C-enriched layer preferably has a C content of 0.05 to 10 wt%. If the C content of the C-enriched layer is less than 0.05 wt%, corrosion such as rust may occur on the steel sheet. If the C content of the C-enriched layer exceeds 10 wt%, degreasing may not occur. Therefore, the C content of the C-enriched layer is preferably in the range of 0.05 to 10 wt%. The lower limit of the C content of the C-enriched layer is more preferably 1%, and even more preferably 2%. The upper limit of the C content of the C-enriched layer is more preferably 9%, and even more preferably 8%.

[0102] 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%, and a bending workability (R / t) of 3.5 or less. The yield ratio is more preferably 0.83 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 0.80. The bending workability is more preferably 3.3 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 3.0.

[0103] In addition, the cold-rolled steel sheet of the present invention may have a degreasing rate of 90% or more. The degreasing rate refers to the degree of removal of anti-rust oil, etc. applied to the surface of the steel sheet to form phosphate on the surface of the steel sheet. In the present invention, a higher degreasing rate is advantageous, and therefore, the upper limit thereof is not particularly limited. However, as an example, the upper limit of the degreasing rate may be 95%.

[0104] In addition, the cold-rolled steel sheet of the present invention may have a phosphate coverage of 95 area% or more. The phosphate coverage means (phosphate formation area on the surface of the steel sheet / total area of ​​the surface of the steel sheet) × 100. In the present invention, since a higher phosphate coverage is advantageous, there is no particular limitation on the upper limit thereof, but as an example, the upper limit of the phosphate coverage may be 96%.

[0105] The thickness of the cold-rolled steel sheet of the present invention may be 0.6 to 2.3 mm. The lower limit of the thickness of the cold-rolled steel sheet is more 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.

[0106] Meanwhile, the cold-rolled steel sheet of the present invention may additionally include a plating layer between the Ni coating layer and the C concentration layer. The present invention does not specifically limit the type of the 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.

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

[0108] First, a slab 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.

[0109] 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.

[0110] [Formula 1] Ar3(℃) = 910 - 203√C + 44.7Si + 31.5Mo

[0111] 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.

[0112] [Equation 2] Ms(℃) = 539 - 423C - 30.4Mn - 7.5Si + 30Al - 17.7Ni - 12.1Cr - 7.5Mo

[0113] 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.

[0114] Thereafter, the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling may be performed using a roll having a surface roughness (Ra) of 2.5 ㎛ or more. 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, and therefore, the upper limit thereof is not particularly limited. 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 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%.

[0115] 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 above 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 100% transformation of ferrite into austenite is completed upon heating, and can be obtained through the following equation 3.

[0116] [Equation 3] Ac3(℃) = 875 - 206C + 26.2Si - 25.0Mn - 12.3Cr + 9.12Mo + 50.2Nb + 148Ti - 131B

[0117] 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.

[0118] 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°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. The lower limit of the above first average cooling rate is more preferably 2°C / s. The upper limit of the above first average cooling rate is more preferably 5°C / s.

[0119] 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 27°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 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.

[0120] 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.

[0121] [Formula 4] Mf(℃) = 431 - 412C - 17.4Si - 47.4Mn - 20.9Cr - 17.0Mo + 49.2Nb + 95.0Ti + 202B

[0122] 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.

[0123] In the above overaging treatment, the overaging treatment temperature (H) - secondary cooling end temperature (Tf) can be controlled to be 100°C or less. 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 less.

[0124] Afterwards, a Ni coating layer is formed on the cold-rolled steel sheet subjected to the over-aging treatment. The formation of the Ni coating layer is performed by adding Ni at 1 to 15 mg / m. 2 It may include plating with an adhesion amount of . Through this, a Ni coating layer having an average thickness of 0.005 to 0.065 ㎛ can be formed. The lower limit of the Ni adhesion amount is 2 mg / m 2 It is more desirable to be 3mg / m 2 It is more preferable. The upper limit of the above Ni attachment amount is 14 mg / m 2 It is more desirable to be 13mg / m 2 It is more preferable. In the present invention, there is no particular limitation, but as an example, the plating may be electroplating.

[0125] Thereafter, the cold-rolled steel sheet having the Ni coating layer formed thereon is subjected to skin pass rolling (SPM (Skin Pass Mill)). The skin pass rolling enables control of the surface roughness (Rsk). The skin pass rolling can be performed with a pressing force of 500 to 1000 tons. If the pressing force during the skin pass rolling is less than 500 tons, 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 properties. The lower limit of the pressing force during the skin pass rolling is more preferably 550 tons, and more preferably 600 tons. The upper limit of the pressing force during the skin pass rolling is more preferably 950 tons, and more preferably 900 tons.

[0126] Thereafter, the cold rolled steel sheet subjected to temper rolling is subjected to tension leveling (T / L). The tension leveling is intended to correct the shape of the steel sheet. The tension leveling may be performed at an elongation of 0.01 to 0.50%. If the elongation during the tension leveling is less than 0.01%, shape correction may be difficult. If the elongation during the tension leveling exceeds 0.50%, work hardening may become severe and bending characteristics may deteriorate. The lower limit of the elongation during the tension leveling is more preferably 0.03%, and more preferably 0.05%. The upper limit of the elongation during the tension leveling is more preferably 0.40%, and more preferably 0.35%.

[0127] 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.

[0128] Afterwards, the total amount of 500 to 1500 mg / m is applied to both sides of the above tension-leveled cold-rolled steel sheet. 2 Oil is applied so that the amount of oil applied is 500mg / m 2 If it is less than 1500mg / m, corrosion such as rust may occur on the steel plate. 2 If it exceeds , degreasing may not occur and phosphate treatment may be reduced. The lower limit of the oil amount is 600 mg / m 2 It is more desirable to have 700mg / m 2 It is more preferable that the upper limit of the oil content is 1400 mg / m 2 It is more desirable to have 1300mg / m 2It is more preferable. Meanwhile, the present invention does not specifically limit the type of the oil, but for example, 550HN, N-6130, BW-90EG, etc. can be used.

[0129] 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.

[0130] (Example)

[0131] 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, Ni plating, temper rolling, tension leveling, and oil (550HN) application 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.

[0132] For the cold-rolled steel sheet manufactured in this manner, the microstructure, the moisture content in the region where the KAM value is 0 to 1.0°, the major diameter of the oxide in the oxide layer, the Ni coating layer thickness and surface roughness, the C content in the C concentrated layer, and the mechanical properties were measured, and the results are shown in Tables 4 and 5 below.

[0133] 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.

[0134] 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%.

[0135] The major diameter of the oxide in the oxide layer was determined by photographing the surface of the cold-rolled steel sheet at a magnification of 20,000 times using a scanning electron microscope (SEM), and then distinguishing five oxides in descending order of major diameter, and calculating the average value.

[0136] The thickness of the Ni coating layer was measured using a GDS (Glow discharge spectrometer) after washing the cold-rolled steel sheet with methanol.

[0137] The surface roughness (Rsk) of the Ni coating layer was measured five times using a contact-type 2D roughness tester, and the average value was calculated excluding the maximum (Max) and minimum (Min) values.

[0138] The C content in the C-concentrated layer was measured using a GDS (Glow discharge spectrometer) after washing the cold-rolled steel sheet with methanol.

[0139] Meanwhile, the degreasing rate and phosphate coverage of the cold-rolled steel sheet were measured. To this end, the following processes were applied to the manufactured cold-rolled steel sheet: degreasing → washing → surface conditioning → phosphate treatment. At this time, the free acidity (FAC) of the phosphate solution used for the phosphate treatment was 0.75 pt, the total acidity (TOAC) was 23 pt, and the accelerator was 2.8 pt.

[0140] The degreasing rate was measured by washing under the conditions of herbal medicine: EC-90, pH: 10.9, temperature: 45℃, and time: 120s, then taking a picture after 10 seconds and checking it with the naked eye first, and then using image analysis software secondarily.

[0141] Phosphate coverage was measured by taking five BSE images at 1500x magnification using a scanning electron microscope (SEM) and using image analysis software.

[0142] Steel grade No. Alloy composition (weight %) CSiMnPSAlCrMoNbTiBNXYY / X10.250.101.810.0100.00090.0250.060.070.0310.0250.00200.0035671582.420.230.151.910.0110.00110.0310.140.080.0250.0300.00190.0 041641602.530.240.061.750.0140.00080.0300.100.060.0270.0230.00210.0042611522.540.240.561.250.0110.00140.0390.250.030.0250.0210.00250.0035241576.450.240.651.650.0 120.00120.0350.210.150.0150.0250.00210.0025431663.960.250.751.560.0150.00100.0300.250.050.0250.0200.00200.0031351714.970.150.151.750.0100.00100.0350.0010.0010.01 50.0270.00100.0038681041.580.220.251.680.0100.00110.0410.700.040.0300.0250.00250. 0041920523.590.200.111.550.0070.00100.0310.550.250.0210.0200.00400.00511720812.3X = C+Mn / 20+Si / 30+2P+4SY = 48.8+49logC+35.1Mn+25.9Si+14.5Ni+9.6Cu+76.5Cr+105.9Mo+1325Nb+10000B

[0143] Classification Steel grade No. Ar3 (℃) Ms (℃) Cold rolling reduction ratio (%) Cold rolling roll roughness (㎛) Ac3 (℃) Annealing temperature (℃) First cooling end temperature (℃) First average cooling rate (℃ / s) Mf (℃) Second cooling end temperature (Tf) (℃) Second average cooling rate (℃ / s) Mf-Tf (℃) Invention example 11815370564.578686171532421746268 Invention example 22822371564.578885170532431 796164Invention Example 33815379564.578785971232491955954Comparative Example 14837355564.581085470132612016060Comparative Example 25844336564.580484973632391655974Comparative Example 36844327564.58058527253238175616 3Comparison Example 4 78 38 4 12 5 6 4.5 8 0 9 8 5 9 7 15 3 2 8 7 18 4 6 5 10 3Comparison Example 5 8 8 2 7 3 6 9 5 6 4.5 7 9 18 6 5 7 0 13 2 4 5 19 2 6 2 5 3Comparison Example 6 9 8 3 2 3 9 15 6 4.5 7 9 7 8 6 7 7 0 9 3 2 6 1 1 9 0 6 1 7 1Comparison Example 7 18 15 3 7 0 4.5 7 8 6 8 5 3 7 16 3 2 4 2 1 8 9 6 3 5 3Comparison Example 81815370561.578685172532421846158Comparative Example 91815370564.578685071532421756067Comparative Example 101815370564.578686171632421816161Comparative Example 111815370564.578685972132421856757Ar3(℃) = 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 - 131BMf(℃) = 431-412C-17.4Si-47.4Mn-20.9Cr-17.0Mo+49.2Nb+95.0Ti+202B

[0144] Classification Steel grade No. Reheating / overaging treatment temperature (H) (℃) Overaging treatment time (min) H-Tf (℃) Ni adhesion (mg / m2) Temper rolling pressure (ton) Tension leveling elongation (%) Oil flow rate (mg / m2) Invention example 1 1 758.6 1 3 6 500.05 8 59 Invention example 2 2 1 748.5-5 3 6 500.05 9 01 Invention example 3 3 1 958.50 3 6 500.05 9 32 Comparative example 1 4 1 8 4 1 73 6 500.05 9 21 Comparative example 2 5 2 0 1 8 3 6 3 6 500.05 9 10 Comparative example 3 6 1 948.41936500.05897Comparative Example 472018.51736500.05900Comparative Example 582008.9826500.05901Comparative Example 691878.4-336500.05897Comparative Example 711878.2-236500.05854Comparative Example 811898.4-536500.05934Comparative Example 912008.62531500.05856Comparative Example 1011978.316236500.05935Comparative Example 1111968.41136500.051670

[0145] Microstructure (area %) Moisture content (%) in the area where the KAM value is 0 to 1.0° Oxide diameter (㎛) Ni coating layer C content (weight %) in the concentrated layer Average thickness of the center of the surface layer (㎛) Surface roughness (Rsk) 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 1298199260.50.034-0.414.5 Invention Example 2397199250.50.032-0.424.6 Invention Example 3397199240.50.029-0.384.5 Comparative Example 11288496330.950.031-0.424.3 Comparative Example 21387397310.840.028-0.424.6 Comparative Example 31288397320.890.028-0.414.7 Comparative Example 41585595330.50.031-0.435.1 Comparative Example 5 397 199 2 6 0.95 0.034-0.45 5.0 Comparative Example 6 397 397 2 5 0.87 0.020-0.42 4.7 Comparative Example 7 298 199 2 4 0.50 030-0.69 11.1 Comparative Example 8 397 298 2 4 0.50 031-0.66 10.9 Comparative Example 9 298 397 2 5 0.50 030-0.67 10.3 Comparative Example 10 298 298 2 6 0.50 080-0.42 4.3 Comparative Example 11 397 397 2 4 0.50 032-0.48 13.1 F: Ferrite, B: Bainite, M: Martensite, TM: Tempered Martensite

[0146] Classification Yield strength (MPa) Tensile strength (MPa) Yield elongation (%) Bending workability (R / t) Degreasing rate (area%) Phosphate Coverage (area%) Invention example 1 1098 15250.726 2.59798 Invention example 2 1105 15150.737 2.59898 Invention example 3 1135 15310.746 2.59797 Comparative example 1 102 114 240.727 4.39876 Comparative example 2 1032 146 50.70 53.99979 Comparative example 3 1052 145 90.726 4.39774Comparative Example 4 101414010.7243.99897Comparative Example 5 120115470.7862.59987Comparative Example 6 120915420.7862.59789Comparative Example 7 110515320.7263.95686Comparative Example 8 112415240.7464.35385Comparative Example 9 110115390.7263.65183Comparative Example 10 109815240.7263.23274Comparative Example 11115115310.7563.24581

[0147] As can be seen from Tables 1 to 5 above, it can be seen that invention examples 1 to 3 that satisfy the alloy composition and manufacturing conditions proposed by the present invention secure the mechanical properties and phosphate treatability targeted by the present invention.

[0148] In the case of Comparative Example 1, which does not satisfy the Mn content, X value, and Y / X value, it can be seen that the tensile strength, bending workability, and phosphate treatability are at insufficient levels because the microstructure and the moisture content in the region where the KAM value is 0 to 1.0° are not secured.

[0149] In the case of comparative examples 2 and 3 that do not satisfy the Si content, X value, and Y / X value, it can be seen that the tensile strength, bending workability, and phosphate treatability are at insufficient levels because the microstructure and the moisture content in the region where the KAM value is 0 to 1.0° are not secured.

[0150] In the case of comparative example 4, which does not satisfy the C content, Cr content, Mo content, and Y value, it can be seen that the tensile strength and bending workability are at an insufficient level because the microstructure and the moisture content in the region where the KAM value is 0 to 1.0° are not secured.

[0151] In the case of comparative examples 5 and 6, which do not satisfy the Cr content, X value, Y value, and Y / X value, it can be seen that the phosphate treatability is at an insufficient level.

[0152] In the case of Comparative Example 7, which does not satisfy the cold reduction ratio, it can be seen that the bending workability, degreasing rate, and phosphate treatment property are at an inadequate level because the surface roughness of the Ni coating layer and the C content of the C concentrated layer are not secured.

[0153] In the case of Comparative Example 8, which does not satisfy the surface roughness of the cold rolled roll, it can be seen that the bending workability, degreasing rate, and phosphate treatment property are at an inadequate level because the surface roughness of the Ni coating layer and the C content of the C concentrated layer are not secured.

[0154] In the case of Comparative Example 9, which does not satisfy the temper rolling pressure, it can be seen that the bending workability, degreasing rate, and phosphate treatment property are at an inadequate level because the surface roughness of the Ni coating layer and the C content of the C concentrated layer are not secured.

[0155] In the case of Comparative Example 10, which does not satisfy the Ni adhesion amount, it can be seen that the degreasing rate and phosphate treatment property are insufficient because the average thickness of the Ni coating layer is not secured.

[0156] In the case of Comparative Example 11, which does not satisfy the oil content, it can be seen that the degreasing rate and phosphate treatment property are insufficient because the C content of the C concentrated layer is not secured.

[0157] Fig. 2 is a photograph of Invention Example 1 after degreasing, and Fig. 3 is a photograph of Comparative Example 10 after degreasing. As can be seen from Figs. 2 and 3, Invention Example 1 has excellent degreasing properties, while Comparative Example 10 has poor degreasing properties.

[0158] Fig. 4 is a scanning electron microscope (SEM) photograph of Invention Example 1 after phosphate treatment, and Fig. 5 is a scanning electron microscope (SEM) photograph of Comparative Example 1 after phosphate treatment. As can be seen from Figs. 4 and 5, Invention Example 1 has excellent phosphate treatment properties, whereas Comparative Example 1 has poor phosphate treatment properties.

[0159] [Explanation of symbols]

[0160] 100: Cold rolled steel sheet

[0161] 10: Steel plate

[0162] 20: Ni coating layer

[0163] 30: C concentrated layer

Claims

1. Steel plate; A Ni coating layer formed on the above steel plate; A cold rolled steel sheet comprising a C concentrated layer formed on the Ni coating layer; The above steel plate is, Contains, in wt%, carbon (C): 0.190 to 0.270%, silicon (Si): 0.030 to 0.60%, manganese (Mn): 1.50 to 2.40%, chromium (Cr): 0.0050 to 0.50%, 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, The above Ni coating layer has an average thickness of 0.005 to 0.065 μm, The above C-concentrated layer is a cold-rolled steel sheet having a C content of 0.5 to 10 wt%.

2. In claim 1, The above-mentioned cold rolled steel plate is a cold rolled steel plate satisfying the following relationships 1 to 3. [Relationship 1] X = -10Si + 40Mn - 80Cr ≥ 50 [Relationship 2] 110 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 200 [Relationship 3] 1.2 ≤ Y / X ≤ 3.7 (However, in the above equations 1 to 3, the content of each alloy element is in weight%.) 3. In claim 1, The above steel plate is, A central portion; and a surface portion formed on the outer side based on the thickness direction of the central portion; The microstructure of the above central part includes, in area %, a total of at least one of ferrite and bainite: 5% or less (including 0%), and at least one of the remainder martensite and tempered martensite. 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.

4. In claim 1, The above steel plate is, Cold rolled steel sheet in which the number fraction in the area where the KAM value is 0 to 5.0° at a position 1 / 4 in the thickness direction is 22 to 30%, when the number fraction in the area where the KAM value is 0 to 1.0° is set to 100% during EBSD measurement.

5. In claim 1, The above Ni coating layer is a cold rolled steel sheet having a surface roughness (Rsk) of -0.65 or higher.

6. In claim 1, A cold rolled steel sheet having an oxide layer including oxide having a major diameter of 1.5 ㎛ or less formed between the above-mentioned steel sheet and the Ni coating layer.

7. 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.

8. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a yield strength of 1000 to 1300 MPa, a tensile strength of 1470 to 1670 MPa, a yield ratio of 0.85 or less, and an elongation of 4 to 10%.

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 with a degreasing rate of 90% or more.

11. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet with a phosphate coverage of 95% or more.

12. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet additionally including a plating layer between the Ni coating layer and the C concentrated layer.

13. Contains carbon (C): 0.190 to 0.270%, silicon (Si): 0.030 to 0.60%, manganese (Mn): 1.50 to 2.40%, chromium (Cr): 0.0050 to 0.50%, 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 A step of heating a slab consisting of iron 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 45 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; 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; The cold rolled steel sheet subjected to the above-mentioned over-aging treatment is added with Ni at 1 to 15 mg / m 2 A step of forming a Ni coating layer by plating with an amount of adhesion; A step of subjecting a cold rolled steel sheet having the above Ni coating layer formed thereon to temper rolling with a pressure of 500 to 1000 tons; A step of tension leveling the above cold rolled steel sheet; and The total amount on both sides of the above tension-leveled cold-rolled steel sheet is 500 to 1500 mg / m 2 A method for manufacturing a cold rolled steel sheet, comprising: a step of applying oil thereto; 14. In claim 13, The above slab is a method for manufacturing a cold rolled steel sheet satisfying the following relationships 1 to 3. [Relationship 1] X = -10Si + 40Mn - 80Cr ≥ 50 [Relationship 2] 110 ≤ Y = 48.8 + 49logC + 35.1Mn + 25.9Si + 14.5Ni + 9.6Cu + 76.5Cr + 105.9Mo + 1325Nb + 10000B ≤ 200 [Relationship 3] 1.2 ≤ Y / X ≤ 3.7 (However, in the above equations 1 to 3, the content of each alloy element is in weight%.) 15. In claim 13, A method for manufacturing cold rolled steel sheets in which the above slab is heated at 1100 to 1300°C.

16. 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℃.

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

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 cold rolled steel sheets, wherein the above tension leveling is performed at an elongation of 0.01 to 0.50%.

26. 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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