Cold-rolled steel sheet and method of manufacturing same

Abstract

One aspect of the present invention provides a cold-rolled steel sheet and a method of manufacturing same. A preferred aspect of the present invention provides: an ultra-high strength cold-rolled steel sheet having a tensile strength of at least 1670 MPa and excellent bending properties, hydrogen embrittlement resistance, and corrosion resistance; and a method of manufacturing same.

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] Meanwhile, in order to manufacture ultra-high strength steel with a tensile strength of 1470 MPa or higher, it is essential to introduce martensite or some bainite. In this case, brittle fracture easily occurs due to hydrogen remaining within the steel or introduced from the outside, which is called hydrogen embrittlement. Hydrogen embrittlement occurs at a strength lower than the strength at which fracture occurs, and the material can be destroyed by hydrogen embrittlement even at an applied stress that is very small compared to the actual fracture strength of the material. In particular, this hydrogen embrittlement becomes more sensitive as the strength of the steel increases. Meanwhile, since the resistance to hydrogen embrittlement improves as the bending properties improve when the initial hydrogen content in the steel is the same, it is necessary to improve the bending properties as well.

[0007] Therefore, to solve the above-described problems, it is necessary to develop ultra-high-strength cold-rolled steel sheets with excellent bending properties, hydrogen embrittlement resistance, and corrosion resistance.

[0008] [Prior Art Literature]

[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] A preferred aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet having a tensile strength of 1670 MPa or more and excellent bending properties, hydrogen embrittlement resistance, and corrosion resistance, and a method for manufacturing the same.

[0013] One embodiment of the present invention comprises, in wt%, carbon (C): 0.250 to 0.350%, silicon (Si): 0.030 to 0.50%, manganese (Mn): 0.40 to 2.50%, chromium (Cr): 0.0050 to 0.30%, molybdenum (Mo): 0.0030 to 0.30%, 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 0.150%, and the remainder is Fe and A cold-rolled steel sheet is provided, which is made of other unavoidable impurities, and includes a center portion; and a surface portion formed on the outer side of the center portion in the thickness direction; wherein the surface portion includes a decarburized layer having an average thickness of 15 to 65 ㎛ in the thickness direction from the surface, and the surface roughness (Rsk) of the surface portion is -0.55 ㎛ or more.

[0014] (However, the above surface layer refers to the area from the surface of the steel material to 65㎛ in the thickness direction.)

[0015] The above cold rolled steel sheet can satisfy the following relationships 1 to 3.

[0016] [Relation 1] 40 ≤

[0017] [Relationship 2] Y = 100[Cr + 0.8C + 0.07(Si+Mn) - 0.1Mo - 50B - 70P - 30S] + 100 ≤ 120

[0018] [Relationship 3] 0.1 ≤ Y / X ≤ 1.4

[0019] (However, the content of the alloy elements described in the above equations 1 to 3 is in weight%.)

[0020] 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.

[0021] The area fraction of low angle grain boundaries having an orientation difference of 2° or more and less than 15° from the center may be 28% or more.

[0022] The above decarburized layer includes a section (section A) in which the C content ratio is 0.3 or less within a region 0.1 to 10 ㎛ away from the surface in the thickness direction, and the section A may have an average thickness of 0.5 ㎛ or more.

[0023] The microstructure of the above section A may include, in area %, ferrite: 80% or more, and the remainder bainite, martensite, and at least one of tempered martensite.

[0024] The above cold rolled steel sheet may have a yield strength of 1300 to 1750 MPa, a tensile strength of 1670 MPa or more, a yield ratio of 0.95 or less, and an elongation of 2 to 10%.

[0025] The above cold rolled steel sheet may have a bending workability (R / t): 3.9 or less.

[0026] The above cold-rolled steel sheet may be subjected to a 90° V-bending test under the condition of R / t of 4, and when immersed in a 0.1 N HCl solution for 120 hours, there may be no cracks larger than 3 mm.

[0027] When the above cold rolled steel plate is subjected to 30 cycles of corrosion resistance evaluation, there may be 0 holes with a diameter of 3 mm or more.

[0028] The above cold rolled steel sheet can have a plating layer formed on at least one surface.

[0029] Another embodiment of the present invention comprises, in wt%, carbon (C): 0.250 to 0.350%, silicon (Si): 0.030 to 0.50%, manganese (Mn): 0.40 to 2.50%, chromium (Cr): 0.0050 to 0.30%, molybdenum (Mo): 0.0030 to 0.30%, 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 0.150%, and the remainder being Fe and A step of heating a slab composed of 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 so as to satisfy the following relationship 4; 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 at Ac3+20℃ to Ac3+90℃ for 50 to 200 seconds at a dew point temperature of -25 to 20℃; 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 1000 tons; And a step of tension leveling the cold rolled steel sheet subjected to the above-mentioned temper rolling is provided.

[0030] [Relationship 4] C_HR_S = C + 0.1(Si+Mn+Cr+Ti) + 0.25Mo + 1.6Nb + 41B - 0.001CT ≤ 0.4

[0031] (However, the content of the alloy element described in the above relational expression 4 is in weight%, and CT means the coiling temperature.)

[0032] The above slab can satisfy the following relationships 1 to 3.

[0033] [Relation 1] 40 ≤

[0034] [Relationship 2] Y = 100[Cr + 0.8C + 0.07(Si+Mn) - 0.1Mo - 50B - 70P - 30S] + 100 ≤ 120

[0035] [Relationship 3] 0.1 ≤ Y / X ≤ 1.4

[0036] (However, the content of the alloy elements described in the above relational expressions 1 to 3 is in weight%.)

[0037] Heating of the above slab can be performed at 1100 to 1300°C.

[0038] The above finishing hot rolling can be performed at Ar3~Ar3+120℃.

[0039] The above winding can be performed at Ms~650℃.

[0040] The above cold rolling can be performed at a cold rolling reduction ratio of 45 to 70%.

[0041] 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.

[0042] In the above secondary cooling, the secondary cooling end temperature (Tf) is controlled to 40 to 250°C, and can be performed at an average cooling rate of 30 to 600°C / s.

[0043] During the above secondary cooling, the Mf-secondary cooling end temperature (Tf) can be controlled to be 20°C or higher.

[0044] In the above over-aging treatment, the over-aging treatment temperature (H) is controlled to 130 to 300°C and can be performed for 5 to 12 minutes.

[0045] In the above overaging treatment, the overaging treatment temperature (H) - secondary cooling end temperature (Tf) can be controlled to be 50°C or higher.

[0046] The above tension leveling can be performed at an elongation of 0.05 to 0.80%.

[0047] 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.

[0048] According to one aspect of the present invention, a cold rolled steel sheet and a method for manufacturing the same can be provided.

[0049] According to a preferred aspect of the present invention, an ultra-high strength cold-rolled steel sheet having a tensile strength of 1670 MPa or more and excellent bending properties, hydrogen embrittlement resistance and corrosion resistance, and a method for manufacturing the same can be provided.

[0050] Figure 1 is a photograph of a cross-section of Invention Example 1 observed using SEM.

[0051] Figure 2 is a photograph of the surface cross-section of Comparative Example 10 observed using SEM.

[0052] Figure 3 shows the C content distribution from the surface to the thickness direction for Invention Example 1 and Comparative Example 10.

[0053] Figure 4 shows the change in C content ratio from the surface to the thickness direction for Invention Example 1.

[0054] 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.

[0055] 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.

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

[0057] Carbon (C): 0.250~0.350%

[0058] 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 0.250%, it may be difficult to obtain the strength targeted in the present invention. If the content of C exceeds 0.350%, the strength may increase rapidly and the elongation may be poor. In addition, hydrogen embrittlement resistance may deteriorate and weldability may be poor. Therefore, the content of C is preferably in the range of 0.250 to 0.350%. The lower limit of the C content is more preferably 0.260%. The upper limit of the C content is more preferably 0.340%.

[0059] Silicon (Si): 0.020~0.50%

[0060] 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 0.020%, it may be difficult to sufficiently obtain the above-described effect. If the Si content exceeds 0.50%, 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 0.50%. The lower limit of the Si content is more preferably 0.030%. The upper limit of the Si content is more preferably 0.40%.

[0061] Manganese (Mn): 0.40~2.50%

[0062] Mn is an element added to secure strength. When the Mn content is less than 0.40%, the hardenability is low, so if the cooling rate is not sufficiently fast during cooling after continuous annealing, martensite is not formed, making it difficult to secure the strength targeted by the present invention. When the Mn content exceeds 2.50%, the Ms temperature decreases during cooling after continuous annealing, and the temperature at which cooling must be completed decreases, 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 0.40 to 2.50%. It is more preferable that the lower limit of the Mn content is 0.50%. It is more preferable that the upper limit of the Mn content is 2.40%.

[0063] Chromium (Cr): 0.0050~0.30%

[0064] 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 0.30%, delayed fracture resistance may deteriorate, carbides such as CrC may be formed, reducing bending workability, and manufacturing costs may increase due to excessive alloy input. In addition, crevice / penetration corrosion may be induced by Cr-rich dense corrosion products, which may lower corrosion resistance. Therefore, the Cr content is preferably in the range of 0.0050 to 0.30%. More preferably, the lower limit of the Cr content is 0.010%. It is more preferable that the upper limit of the above Cr content be 0.20%.

[0065] Molybdenum (Mo): 0.0030~0.30%

[0066] 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 0.30%, 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 0.30%. The lower limit of the Mo content is more preferably 0.0050%. The upper limit of the Mo content is more preferably 0.20%.

[0067] Boron (B): 0.00050~0.0050%

[0068] B is an element that suppresses ferrite formation, and accordingly, the present invention has the advantage of suppressing the formation of ferrite during cooling after continuous annealing, strengthening austenite grain boundaries, suppressing hydrogen intrusion, and increasing hydrogen embrittlement resistance. When the content of B is less than 0.00050%, there is no hardenability effect at all, making it difficult to secure the strength targeted by the present invention. When 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 B content is more preferably 0.00070%. The upper limit of the B content is more preferably 0.0040%.

[0069] Phosphorus (P): 0.030% or less (excluding 0%)

[0070] 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 0.020% or less.

[0071] Sulfur (S): 0.00350% or less (excluding 0%)

[0072] 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.

[0073] Nitrogen (N): 0.010% or less (excluding 0%)

[0074] 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.

[0075] Aluminum (Al): 0.010~0.10%

[0076] 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%.

[0077] Niobium (Nb): 0.0030~0.050%

[0078] 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%.

[0079] Titanium (Ti): 0.0050~0.150%

[0080] 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 0.150%, the strength of martensite may decrease due to the precipitation of additional carbides in addition to the removal of dissolved nitrogen, and hole expandability and bending workability may be 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 0.150%. The lower limit of the Ti content is more preferably 0.010%. The upper limit of the Ti content is more preferably 0.120%.

[0081] 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.

[0082] The cold rolled steel sheet of the present invention can satisfy the above-described alloy composition and the following relationship equations 1 to 3.

[0083] [Relation 1] 40 ≤

[0084] The above equation 1 is closely related to strength and is a component relationship equation for securing hardenability. When the value of X is less than 40, a large amount of soft ferrite and bainite structures are formed during cooling, making it difficult to secure the target strength. When the value of X exceeds 220, the strength becomes excessively high, making it difficult to secure the target elongation, which may cause processing cracks during forming and increase the cost of the alloy steel. Therefore, the value of X is preferably in the range of 40 to 220. It is more preferable that the lower limit of the X value is 100. It is more preferable that the upper limit of the X value is 210.

[0085] [Relationship 2] Y = 100[Cr + 0.8C + 0.07(Si+Mn) - 0.1Mo - 50B - 70P - 30S] + 100 ≤ 120

[0086] The above equation 2 is a component relationship related to corrosion resistance (penetrating corrosion). If the value of Y exceeds 120, not only does the manufacturing cost increase, but there is also a problem of reduced corrosion resistance such as penetration corrosion. Therefore, the value of Y is preferably in the range of 120 or less. It is more preferable that the value of Y is 90 or less. Meanwhile, the present invention does not specifically limit the lower limit of the Y value, but as an example, the lower limit may be 20.

[0087] [Relationship 3] 0.1 ≤ Y / X ≤ 1.4

[0088] The above equation 3 is a component relationship equation for simultaneously securing strength and corrosion resistance. When the value of Y / X is less than 0.1, there is a problem that the manufacturing cost increases because a large amount of expensive alloying components (Mo, Nb, etc.) must be added to simultaneously secure strength and corrosion resistance. When the value of Y / X exceeds 1.4, corrosion resistance deteriorates and it may be difficult to secure strength due to insufficient hardenability. Therefore, the value of Y / X is preferably in the range of 0.1 to 1.4. It is more preferable that the lower limit of the Y / X value is 0.2. It is more preferable that the upper limit of the Y / X value is 1.2.

[0089] The cold-rolled steel sheet 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 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 65 μm.

[0090] It is preferable that the surface layer includes a decarburized layer having an average thickness of 15 to 65 μm in the thickness direction from the surface of the steel plate. Through this, bending characteristics and hydrogen embrittlement resistance can be improved. If the average thickness of the decarburized layer is less than 15 μm, it is difficult to secure a sufficiently soft phase, making it difficult to secure the target bendability (R / t). If the average thickness of the decarburized layer exceeds 65 μm, it is difficult to secure the target strength due to the excessively thick decarburized layer, and fatigue characteristics may deteriorate. Therefore, the average thickness of the decarburized layer is preferably in the range of 15 to 65 μm. The lower limit of the average thickness of the decarburized layer is more preferably 17 μm, more preferably 19 μm, and most preferably 21 μm. The upper limit of the average thickness of the decarburized layer is more preferably 60 μm, more preferably 55 μm, and most preferably 50 μm.

[0091] The above decarburized layer includes a section (section A) in which the C content ratio is 0.3 or less within a region 0.1 to 10 ㎛ away from the surface in the thickness direction, and the A section may have an average thickness of 0.5 ㎛ or more. If the average thickness of the A section is less than 0.5 ㎛, the ferrite fraction, which is a soft phase in the surface layer of the steel plate, may decrease, resulting in a deterioration in bending characteristics. Therefore, the average thickness of the A section may be 0.5 ㎛ or more. It is more advantageous for the average thickness of the A section to be 1 ㎛ or more. In the present invention, there is no particular limitation on the upper limit of the average thickness of the A section, but as an example, it may be 6 ㎛ or less. Meanwhile, the content ratio refers to a fraction compared to the average C content of the steel plate.

[0092] The microstructure of the above section A may include, in area %, ferrite: 80% or more, and the remainder may include at least one of bainite, martensite, and tempered martensite. If the fraction of the ferrite is less than 80%, the bending properties may deteriorate due to insufficient ductility. Therefore, the fraction of the ferrite is preferably 80% or more. It is more preferable that the fraction of the ferrite is 85% or more.

[0093] The above central microstructure may include, in area %, at least one of martensite and tempered martensite. The martensite and tempered martensite are very advantageous structures for securing the strength and bending properties 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 ferrite and bainite exceeds 3%, it may be difficult to secure the properties targeted by the present invention. It is more preferable that the total fraction of at least one of ferrite and bainite is 2% or less. The above central structure refers to a region outside the surface layer below.

[0094] Meanwhile, when measuring the microstructure of a steel sheet using EBSD, it is divided into a low-angle grain boundary, which is a grain boundary with an orientation difference of 2° or more and less than 15°, and a high-angle grain boundary, which is a grain boundary with an orientation difference of 15° or more. In the present invention, the area fraction of the low-angle grain boundary with an orientation difference of 2° or more and less than 15° from the center may be 28% or more. If the area fraction of the low-angle grain boundary is less than 28%, it may be difficult to secure the target strength because the martensite lath in the microstructure is coarse. Therefore, the area fraction of the low-angle grain boundary may be 28% or more. It is more advantageous if the area fraction of the low-angle grain boundary is 30% or more.

[0095] The cold rolled steel sheet of the present invention preferably has a surface roughness (Rsk) of the surface layer of -0.55㎛ or more. The surface roughness (Rsk (Skewness)) is one of several factors of surface roughness related to the asymmetry of the sharp protrusion part. 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 part, increasing the susceptibility to crack occurrence, thereby deteriorating the bending characteristics. When the surface roughness (Rsk) is less than -0.55㎛, the bending characteristics may be deteriorated. Therefore, the surface roughness (Rsk) may have a range of -0.55㎛ or more. It is more advantageous for the surface roughness (Rsk) to be -0.50㎛ 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 μm.

[0096] As described above, the cold rolled steel sheet of the present invention may have a yield strength of 1300 to 1750 MPa, a tensile strength of 1670 MPa or more, a yield ratio of 0.95 or less, an elongation of 2 to 10%, and a bending workability (R / t) of 3.9 or less. The yield strength is more preferably 1350 to 1700 MPa. The tensile strength is more preferably 1700 MPa or more, and in the present invention, since a higher tensile strength is advantageous, the upper limit thereof is not particularly limited. However, as an example, the upper limit of the tensile strength may be 1870 MPa. Meanwhile, the yield strength and tensile strength may be values ​​measured in a direction perpendicular to the rolling direction. The yield ratio is more preferably 0.90 or less, and in the present invention, since a lower yield ratio is more advantageous, there is no particular limitation on the lower limit thereof. However, as an example, the lower limit of the yield ratio may be 0.88. The elongation is more preferably 3 to 9%. The bending workability is more preferably 3.6 or less, and in the present invention, since a lower bending workability is more advantageous, there is no particular limitation on the lower limit thereof. However, as an example, the lower limit of the bending workability may be 3.5.

[0097] The above cold-rolled steel sheet may be subjected to a 90° V-bending test under the condition of R / t of 4, and when immersed in a 0.1 N HCl solution for 120 hours, there may be no cracks larger than 3 mm.

[0098] The cold rolled steel sheet above may have 0 holes with a diameter of 3 mm or more when the plate corrosion resistance evaluation is performed 30 cycles. One cycle of the plate corrosion resistance evaluation may be performed, for example, as follows: Slat Spray (5% NaCl, neutral, 35°C, salt spray amount: 1-2 mL / hr, 6 hr) → Drying (50°C, 20-40% RH, 3 hr) → Wetting (50°C, 95% RH, 14 hr) → Cold air blasting (50% RH, 1 hr).

[0099] 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.

[0100] 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.

[0101] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0102] 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.

[0103] 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.

[0104] [Formula 1] Ar3(℃) = 910 - 203√C + 44.7Si + 31.5Mo

[0105] Thereafter, the hot-rolled steel sheet is coiled so as to satisfy the following relationship 4. If the value of C_HR_S below exceeds 0.4, the strength of the hot-rolled steel sheet becomes excessively high, making it difficult to control the surface roughness (Rsk) during cold rolling, which may result in a deterioration in the bending properties. Therefore, the value of C_HR_S below is preferably 0.4 or less. It is more preferable that the value of C_HR_S above is 0.35 or less. The coiling may be performed at Ms to 650°C. If the coiling temperature (CT) above exceeds 650°C, internal oxidation may occur on the surface of the steel sheet, causing the microstructure formed in the surface layer to become non-uniform, thereby deteriorating the bending properties. Meanwhile, it is preferable to manage the coiling temperature low in order to form the microstructure of the hot-rolled steel sheet into a single-phase structure rather than a composite structure, if possible, to secure material uniformity over 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, which may increase the rolling load during the subsequent cold rolling process, making actual production impossible. It is more preferable that the lower limit of the coiling temperature be Ms+10℃. It is more preferable that the upper limit of the coiling temperature be 600℃. The Ms refers to the temperature at which austenite begins to transform into martensite upon cooling, and can be obtained using the following equation 2.

[0106] [Relationship 4] C_HR_S = C + 0.1(Si+Mn+Cr+Ti) + 0.25Mo + 1.6Nb + 41B - 0.001CT ≤ 0.4

[0107] [Equation 2] Ms(℃) = 521 - 379C - 15.1Si - 43.9Mn - 19.5Cr - 14.7Mo + 43.7Nb + 91.9Ti + 169B

[0108] 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.

[0109] 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, the larger the surface roughness (Ra) of the cold-rolled roll, the more advantageous it is, so there is no particular limitation on the upper limit thereof. However, as an example, the upper limit of the surface roughness (Ra) of the cold-rolled roll may be 6.5 ㎛. 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%.

[0110] Thereafter, the cold-rolled steel sheet is continuously annealed at Ac3+20℃~Ac3+90℃ for 50~200 seconds with a dew point temperature of -25~20℃. By controlling the dew point temperature in this way, a decarburized layer can be formed on the surface of the steel sheet during the continuous annealing process. The dew point temperature in a conventional continuous annealing furnace is at the level of -40~-50℃. However, when the oxygen partial pressure is increased by raising the dew point temperature to -25℃ or higher as in the present invention, the C in the steel sheet and the O in the annealing furnace meet and are released as CO gas, causing decarburization on the surface of the steel sheet. When the dew point temperature is lower than -25℃, the decarburized layer may not be sufficiently formed on the surface of the steel sheet. When the dew point temperature exceeds 20℃, the equipment life and productivity may be reduced. Therefore, the dew point temperature is preferably in the range of -20~20℃. It is more preferable that the lower limit of the dew point temperature is -20℃. It is more preferable that the upper limit of the above dew point temperature is 15℃. Meanwhile, the present invention does not specifically limit the method for controlling the dew point temperature, but as an example, the dew point temperature can be controlled using moist nitrogen (N2+H2O). If the continuous annealing temperature is less 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, making it 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+90℃, equipment trouble may occur due to overload of the annealing furnace. The lower limit of the continuous annealing temperature is more preferably Ac3+25℃, and even more preferably Ac3+30℃. The upper limit of the above continuous annealing temperature is more preferably Ac3+80℃, and even more preferably Ac3+70℃. Meanwhile, Ac3 refers to the temperature at which austenite begins to appear upon heating, and can be obtained through the following equation 3.The continuous annealing is preferably performed for 50 to 200 seconds. If the continuous annealing time is less than 50 seconds, it is difficult to secure a single-phase austenite structure, and as undissolved carbides remain and coarsen, bending properties and hydrogen embrittlement resistance may deteriorate, and it may be difficult to sufficiently form a decarburized layer. If the continuous annealing time exceeds 200 seconds, there is a disadvantage in that the austenite size coarsens, making it difficult to secure strength. The lower limit of the continuous annealing time is more preferably 60 seconds, and even more preferably 70 seconds. The upper limit of the continuous annealing time is more preferably 190 seconds, and even more preferably 180 seconds.

[0111] [Equation 3] Ac3(℃) = 900 - 206C + 26.2Si - 25Mn - 12.3Cr + 9.12Mo + 50.2Nb + 148Ti - 131B

[0112] Thereafter, the continuously annealed cold-rolled steel sheet is first cooled. 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 may be formed during the cooling process, which may result in poor 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 rapid phase transformation, which may result in 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 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-mentioned first average cooling rate exceeds 6℃ / s, the average cooling rate during the subsequent second cooling decreases, and the fraction of low-temperature transformation phases other than martensite increases, making it impossible to secure the strength level targeted by the present invention. The lower limit of the above-mentioned first average cooling rate is more preferably 2℃ / s. The upper limit of the above-mentioned first average cooling rate is more preferably 5℃ / s.

[0113] 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. The second cooling end temperature (Tf) is controlled to 40 to 250°C, and can be performed at an average cooling rate of 30 to 600°C / s. When the second cooling end temperature (Tf) is less than 40°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 250°C, it may be difficult to secure the strength targeted by the present invention. The lower limit of the second cooling end temperature is more preferably 50°C, even more preferably 55°C, and most preferably 60°C. The upper limit of the secondary cooling end temperature is more preferably 240°C, more preferably 230°C, and most preferably 220°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 600°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 500°C / s, more preferably 400°C / s, and most preferably 300°C / s.

[0114] During the secondary cooling, the Mf-secondary cooling end temperature (Tf) can be controlled to be 20°C or higher. If the Mf-Tf is less than 20°C, martensitic transformation may not occur sufficiently, making it difficult to secure the target strength. It is more preferable that the Mf-Tf be 30°C or higher. Meanwhile, the Ms refers to the temperature at which martensitic transformation ends during cooling, and can be obtained through the following equation 4.

[0115] [Formula 4] Mf(℃) = 371 - 412C - 17.4Si - 47.4Mn - 20.9Cr - 17Mo + 49.2Nb + 95Ti + 202B

[0116] 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) can be controlled to 130 to 300°C. If the overaging treatment temperature is less than 130°C, tempering is not sufficiently performed, resulting in a low yield strength and inability to secure sufficient toughness. If the overaging treatment temperature exceeds 300°C, there is a disadvantage in that the bending workability deteriorates due to the precipitation and coarsening of a large amount of carbides. The lower limit of the overaging treatment temperature is more preferably 140°C, more preferably 150°C, and most preferably 160°C. The upper limit of the above overaging treatment temperature is more preferably 280°C, more preferably 260°C, and most preferably 240°C. Meanwhile, the overaging treatment can be performed for 5 to 12 minutes. If the overaging treatment time is less than 5 minutes, tempering may not be sufficiently performed, which may lower the yield strength. If the overaging treatment time exceeds 12 minutes, excessive tempering may cause coarsening of carbides, which may deteriorate the bending properties. The lower limit of the above overaging treatment time is more preferably 5.5 minutes, more preferably 6.0 minutes, and most preferably 6.5 minutes. The upper limit of the above overaging treatment time is more preferably 11.5 minutes, more preferably 11 minutes, and most preferably 10.5 minutes.

[0117] In the above overaging treatment, the overaging treatment temperature (H) - secondary cooling end temperature (Tf) can be controlled to be 50°C or higher. If the overaging treatment temperature (H) - secondary cooling end temperature (Tf) is less than 50°C, tempering is not sufficiently achieved, making it difficult to secure the target yield strength. It is more preferable that the overaging treatment temperature (H) - secondary cooling end temperature (Tf) be 60°C or higher.

[0118] 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.

[0119] 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 0.05 to 0.80%. If the elongation is less than 0.05% during the tension leveling, shape correction may be difficult. If the elongation exceeds 0.80% during the tension leveling, work hardening may become severe, resulting in poor bending properties and hydrogen embrittlement resistance, and the difference in yield strength between the vertical and horizontal directions of the rolling direction may become severe, which may adversely affect dimensional accuracy during part processing. The lower limit of the elongation during the tension leveling is more preferably 0.07%, and even more preferably 0.09%. In the above tension leveling, the upper limit of elongation is more preferably 0.75%, and even more preferably 0.50%.

[0120] 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.

[0121] 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.

[0122] (Example)

[0123] A slab having an alloy composition as 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. Thereafter, the hot-rolled steel sheet was coiled under the conditions as described in Table 2 below, and then cold-rolled 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 as described in Tables 2 and 3 below to manufacture a cold-rolled steel sheet having a thickness of 1.4 mm. Meanwhile, the conditions as described in Tables 2 and 3 below were based on the surface temperature of the steel sheet.

[0124] The microstructure, decarburization layer-related composition, surface roughness, and mechanical properties of the cold-rolled steel sheet manufactured in this manner were measured, and the results are shown in Tables 4 and 5 below.

[0125] The types and fractions of the microstructures of the central portion and decarburized layer of the steel plate were observed at the position t / 4 (t: steel thickness) in the thickness direction of the steel using a scanning electron microscope (SEM) and an optical microscope (OM), and the fractions of each phase were analyzed three times through image analysis to calculate the average value. In addition, the area fraction of low-angle grain boundaries with an orientation difference of 2° to 15° in the central portion was measured three times (Confidence Index (CI) ≥ 0.3) using an EBSD (backscatter electron diffraction pattern analyzer) at 2000x magnification, and the average value was calculated.

[0126] The composition of the decarbonization layer was measured using a GDS (Glow discharge spectrometer) device.

[0127] 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.

[0128] 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.

[0129] Bending workability (R / t) was determined by processing a cold-rolled steel sheet into a specimen measuring 30 mm in width × 100 mm in length, performing a 90° bending test at a test speed of 800 mm / min, and then using a stereoscopic microscope to check for cracks in the bending area. The minimum bending radius at which no cracks occurred (the R value of the mold) was divided by the thickness of the specimen (mm).

[0130] Hydrogen embrittlement resistance was measured by extracting three specimens measuring 30 mm in width × 100 mm in length from a cold-rolled steel sheet, performing a 90° V-bending test under the condition of R / t of 4, and visually checking how many cracks larger than 3 mm occurred when immersed in a 0.1 N HCl solution for 120 hours.

[0131] The corrosion resistance of the plate was measured by producing a specimen measuring 75 mm in width × 150 mm in length, and performing a corrosion test of 30 cycles in which 1 cycle is Slat Spray (5% NaCl, neutral, 35℃, salt spray amount: 1-2 mL / hr, 6 hr) → Drying (50℃, 20-40% RH, 3 hr) → Wetting (50℃, 95% RH, 14 hr) → Cold air blasting (50% RH, 1 hr). Then, visually checking the number of holes with a diameter of 3 mm or more was performed. If the number of holes with a diameter of 3 mm or more was 0, it was evaluated as not occurring, and if there were 1 or more, it was evaluated as occurring.

[0132] Steel grade No. Alloy composition (weight %) CSiMnPSAlCrMoNbTiBNXYY / X10.300.112.200.0120.00100.0300.0900.0040.0350.0200.00250.0040182500.2720.270.202.100.0110.00090.0250.0210.0030.0240.0250.00200.00 41154500.3330.300.111.950.0110.00070.0310.0500.0050.0310.0310.00150.0037155570.3740.320.112.100.0090.00120.0340.0350.0040.0340.0410.00210.0035170670.4050.310.151.870.00 90.00110.0250.0210.0030.0250.0250.00210.0037149640.4360.320.112.150.0070.00150.0400.4500.0020.0150.0450.00100.00311671280.7670.310.201.500.0070.00110.0410.5110.0030.034 0.0410.00210.00411871250.6780.180.023.900.0110.00110.0310.6500.0020.0110.0250.00200. 00312341170.5090.180.050.250.0090.00110.0350.0240.0030.0010.0190.00110.003537471.28X = 48.8+49logC+35.1Mn+25.9Si+14.5Ni+9.6Cu+76.5Cr+105.9Mo+1325Nb+10000BY = 100[Cr + 0.8C + 0.07(Si+Mn) - 0.1Mo - 50B - 70P - 30S] + 100

[0133] Classification Steel grade No. Ar3 (℃) Ms (℃) Coiling temperature (℃) C_HR_S Cold rolling reduction ratio (%) Cold rolling roll roughness (㎛) Ac3 (℃) Annealing temperature (℃) Annealing time (sec) Dew point temperature (℃) First cooling end temperature (℃) First average cooling speed (℃ / s) Invention example 1 180 43 114 6 5 0.24 564.57 89 85 7 14 2 7 7 24 3 Invention example 2 28 14 3 2 7 4 7 0 0.16 564.58 0 28 6 11 4 7 10 7 15 3 Invention example 3 38 0 43 2 3 4 6 5 0.16 564.57 9 88 59 13 9 5 7 21 3 Invention example 4 4 ​​8 0 03114710.22564.579285414267093Invention Example 558043224810.16564.579885013937143Comparative Example 168003004780.28564.578586110577253Comparative Example 278063304950.18564.580585912737353Comparative Example 388252724200.32564.5762834134-57 253Comparative Example 49826443559-0.30564.586085414237153Comparative Example 518043112340.47482.7789847142-207153Comparative Example 618043114680.23351.5789854139-57213Comparative Example 718043114740.23564.578978114237123Comparative Example 818043114850.22564 .57898513557163Comparative Example 9 18043114680.23564.578989139-307193Comparative Example 10 18043114550.25564.5789854142-457213Comparative Example 11 18043114550.25564.5789851165257253Comparative Example 12 18043114760.23461.578965113717143Ar3(℃) = 910 - 203√C + 44.7Si + 31.5MoMs(℃) = 521 - 379C - 15.1Si - 43.9Mn - 19.5Cr - 14.7Mo + 43.7Nb + 91.9Ti + 169BAc3(℃) = 900 - 206C + 26.2Si - 25Mn - 12.3Cr + 9.12Mo + 50.2Nb + 148Ti - 131BC_HR_S = C + 0.1(Si+Mn+Cr+Ti) + 0.25Mo + 1.6Nb + 41B - 0.001CT

[0134] 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 111439472491858.4916500.20 Invention example 221609974611708. 7746500.20 Invention Example 3315710176561858.4846500.20 Invention Example 4414310575381768.5716500.20 Invention Example 551569774591757.9786500.20 Comparative Example 1613110177301958.1946500.20 Comparative Example 271649773671758.478550 0.20Comparison Example 38101947671818.1876500.20Comparison Example 49286125781611878.7626500.20Comparison Example 5114310175421788.5775500.20Comparison Example 611439773461858.1886800.20Comparison Example 7114311578281878.0726500.20Comparison Example 8114310256411919.1896500.20Comparative Example 911439959441958.7966500.20Comparative Example 10114310772361918.9847000.20Comparative Example 11114310772361918.9847000.20Comparative Example 1211439775461847.9873500.20Mf(℃) = 371 - 412C - 17.4Si - 47.4Mn - 20.9Cr - 17Mo + 49.2Nb + 95Ti + 202B

[0135] Average thickness of decarburized layer (㎛) Area where C content ratio is 0.3 or less within the area 0.1 to 10 ㎛ away from the surface in the thickness direction (A section) (㎛) Microstructure of section A (area %) Central microstructure Surface roughness (Rsk) (㎛) At least one of FB, FM and TM At least one of F and B At least one of M and TM Ideal low angle grain boundary area fraction (%) Invention example 1 422.09 46 29835 - 0.39 Invention example 2 451.89 37 29836 - 0.36 Invention example 3 491.79 46 29835 - 0.35 Invention example 4 431.69 19 29836 - 0.29 Invention example 5 451.69 28 29833 - 0.42 Comparative example 1 361.28 9 11 29836 - 0.41 Comparative example 2 391.38 8 12 29834 - 0.38 Comparative example 3 220.36 535 29832 - 0.39 Comparative example 4 411.693789227-0.41Comparative Example 5310.6831729834-0.59Comparative Example 6341.0871329835-0.64Comparative Example 7140.22575158525-0.42Comparative Example 8140.21585109024-0.38Comparative Example 91019929829-0.45Comparative Example 101019929834-0.39Comparative Example 11706.598229835-0.41Comparative Example 12280.8871329834-0.66

[0136] Classification Yield strength (MPa) Tensile strength (MPa) Yield elongation (%) Bending workability (R / t) Hydrogen embrittlement Number of cracks (cracks) Occurrence of penetration corrosion Invention example 1 14 15 17 49 0.8 153.20 Not occurred Invention example 2 14 29 17 52 0.8 253.20 Not occurred Invention example 3 14 36 17 62 0.8 153.20 Not occurred Invention example 4 14 52 17 59 0.8 353.20 Not occurred Invention example 5 14 30 17 70 0.8 163.60 Not occurred Comparative example 1 14 32 17 36 0.8 253.60 Occurrence Comparative example 2 14 42 17 42 0.8 353.60 Occurrence Comparative example 3 13 751 7250.8064.31 Occurrence Comparison Example 4 12 15 14 8 50.8272.90 Non-occurrence Comparison Example 5 14 25 17 5 6 0.8154.32 Non-occurrence Comparison Example 6 14 3 11 7 6 4 0.8154.32 Non-occurrence Comparison Example 7 12 9 5 16 5 8 0.7844.32 Non-occurrence Comparison Example 8 12 5 6 16 5 0.7644.63 Non-occurrence Comparison Example 9 14 25 17 6 ​​5 0.8154.32 Non-occurrence Comparison Example 10 14 5 6 17 7 5 0.8254.32 Non-occurrence Comparison Example 11 13 5 6 16 5 9 0.8252.90 Non-occurrence Comparison Example 12 14 2 11 7 4 10.8254.32 Non-occurrence

[0137] As can be seen from Tables 1 to 5 above, in the case of Invention Examples 1 to 5, the alloy composition and manufacturing conditions of the present invention are satisfied, and thus the microstructure-related composition, decarburization layer-related composition, and surface roughness-related composition proposed by the present invention are satisfied, and thus, it can be seen that not only the mechanical properties but also the hydrogen embrittlement resistance and corrosion resistance are at an excellent level.

[0138] In the case of Comparative Example 1, it can be seen that the corrosion resistance is insufficient because the Cr and Mo contents and Y value are not satisfied.

[0139] In the case of Comparative Example 2, it can be seen that the corrosion resistance is insufficient because the Cr content and Y value are not satisfied.

[0140] In the case of Comparative Example 3, the C, Si, Mn, Cr and Mo contents, X value, and Mf-Tf are not satisfied, so the decarburization layer-related composition proposed by the present invention is not satisfied, and it can be seen that the bendability, hydrogen embrittlement resistance and corrosion resistance are at an inadequate level.

[0141] In the case of Comparative Example 4, the C, Mn and Nb contents, X value, and annealing temperature were not satisfied, so the microstructure-related composition proposed by the present invention was not satisfied, and it can be seen that the yield strength and tensile strength were at insufficient levels.

[0142] In the case of Comparative Example 5, the coiling temperature and C_HR_S were not satisfied, so the decarburization layer-related configuration and surface roughness-related configuration proposed by the present invention were not satisfied, and it can be seen that the bendability and hydrogen embrittlement resistance were at an insufficient level.

[0143] In the case of Comparative Example 6, the cold rolling reduction ratio and the cold rolling roll surface roughness were not satisfied, so the surface roughness-related configuration proposed by the present invention was not satisfied, and it can be seen that the bendability and hydrogen embrittlement resistance were at an insufficient level.

[0144] In the case of Comparative Example 7, the annealing temperature was not satisfied, so the decarburization layer-related composition and microstructure-related composition were not satisfied, and thus the yield strength, tensile strength, bending strength, and hydrogen embrittlement resistance were found to be inadequate.

[0145] In the case of Comparative Example 8, the annealing time was not satisfied, so the decarburization layer-related composition and microstructure-related composition were not satisfied, and it can be seen that the yield strength, tensile strength, bending strength, and hydrogen embrittlement resistance were at an inadequate level.

[0146] In the case of Comparative Examples 9 and 10, the dew point temperature is low, so the decarburization layer-related configuration proposed by the present invention is not satisfied, and thus the bendability and hydrogen embrittlement resistance are at an insufficient level.

[0147] In the case of Comparative Example 11, it can be seen that the tensile strength is insufficient because the dew point temperature is high and the decarburization layer-related configuration proposed by the present invention is not satisfied.

[0148] In the case of Comparative Example 12, the cold rolling roll surface roughness, annealing temperature, and temper rolling pressure were not satisfied, so the surface roughness-related configuration was not satisfied, and it can be seen that the bendability and hydrogen embrittlement resistance were at an insufficient level.

[0149] Fig. 1 is a photograph of a cross-section of Invention Example 1 observed using an SEM. Fig. 2 is a photograph of a cross-section of a surface layer of Comparative Example 10 observed using an SEM. As can be seen from Figs. 1 and 2, Invention Example 1 has the decarburized layer microstructure proposed by the present invention, whereas Comparative Example 1 does not have the decarburized layer microstructure proposed by the present invention.

[0150] Fig. 3 shows the C content distribution from the surface to the thickness direction for Inventive Example 1 and Comparative Example 10. Fig. 4 shows the change in C content ratio from the surface to the thickness direction for Inventive Example 1. As can be seen from Figs. 3 and 4, in the case of Inventive Example 1, the decarburization layer proposed by the present invention was formed, whereas in the case of Comparative Example 10, the decarburization layer proposed by the present invention was not formed.

Claims

1. Contains, by weight%, carbon (C): 0.250 to 0.350%, silicon (Si): 0.030 to 0.50%, manganese (Mn): 0.40 to 2.50%, chromium (Cr): 0.0050 to 0.30%, molybdenum (Mo): 0.0030 to 0.30%, 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 0.150%, and the remainder is Fe and others. It is made up of unavoidable impurities, A central portion; and a surface portion formed on the outer side based on the thickness direction of the central portion; The above surface layer includes a decarburized layer having an average thickness of 15 to 65 μm in the thickness direction from the surface, Cold rolled steel sheet having a surface roughness (Rsk) of -0.55㎛ or more of the surface layer. (However, the above surface layer refers to the area from the surface of the steel to 65㎛ in the thickness direction.) 2. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet satisfying the following relationships 1 to 3. [Relation 1] 40 ≤ [Relationship 2] Y = 100[Cr + 0.8C + 0.07(Si+Mn) - 0.1Mo - 50B - 70P - 30S] + 100 ≤ 120 [Relationship 3] 0.1 ≤ Y / X ≤ 1.4 (However, the contents of alloy elements described in the above equations 1 to 3 are in weight%.) 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 having an area fraction of low angle grain boundaries having an orientation difference of 2° or more and less than 15° from the center of the steel sheet of 28% or more.

5. In claim 1, The above decarburized layer includes a section (section A) in which the C content ratio is 0.3 or less within a region 0.1 to 10 ㎛ away from the surface in the thickness direction. The above section A is a cold rolled steel plate with an average thickness of 0.5㎛ or more.

6. In claim 5, The microstructure of the above section A is a cold rolled steel sheet including, in area %, ferrite: 80% or more, and the remainder bainite, martensite, and at least one of tempered martensite.

7. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a yield strength of 1300 to 1750 MPa, a tensile strength of 1670 MPa or more, a yield ratio of 0.95 or less, and an elongation of 2 to 10%.

8. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet having a bending workability (R / t): 3.9 or less.

9. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet that is subjected to a 90° V-bending test under the condition of R / t of 4, and has 0 cracks larger than 3 mm when immersed in a 0.1 N HCl solution for 120 hours.

10. In claim 1, The above cold rolled steel sheet is a cold rolled steel sheet with 0 holes with a diameter of 3 mm or more when the corrosion resistance of the plate is evaluated for 30 cycles.

11. 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.

12. Contains, by weight%, carbon (C): 0.250 to 0.350%, silicon (Si): 0.030 to 0.50%, manganese (Mn): 0.40 to 2.50%, chromium (Cr): 0.0050 to 0.30%, molybdenum (Mo): 0.0030 to 0.30%, 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 0.150%, and the remainder is Fe and others. A step of heating the slab, which consists of inevitable impurities; A step of obtaining a hot-rolled steel sheet by final hot-rolling the above heated slab; A step of coiling the hot-rolled steel plate so as to satisfy the following relational expression 4; 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 at a dew point temperature of -25 to 20°C at Ac3+20°C to Ac3+90°C for 50 to 200 seconds; 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. [Relationship 4] C_HR_S = C + 0.1(Si+Mn+Cr+Ti) + 0.25Mo + 1.6Nb + 41B - 0.001CT ≤ 0.4 (However, the content of the alloy element described in the above relational expression 4 is in weight%, and CT means the coiling temperature.) 13. In claim 12, The above slab is a method for manufacturing a cold rolled steel sheet satisfying the following relationships 1 to 3. [Relation 1] 40 ≤ [Relationship 2] Y = 100[Cr + 0.8C + 0.07(Si+Mn) - 0.1Mo - 50B - 70P - 30S] + 100 ≤ 120 [Relationship 3] 0.1 ≤ Y / X ≤ 1.4 (However, the contents of the alloy elements described in the above formulas 1 to 3 are in weight%.) 14. In claim 12, A method for manufacturing cold rolled steel sheets in which the above slab is heated at 1100 to 1300°C.

15. In claim 12, 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 12, The above method for manufacturing cold rolled steel sheets is performed at Ms~650℃.

17. In claim 12, 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 12, 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.

19. In claim 12, A method for manufacturing a cold rolled steel sheet, wherein the secondary cooling is performed at an average cooling speed of 30 to 600°C / s, and the secondary cooling end temperature (Tf) is controlled to 40 to 250°C during the secondary cooling.

20. In claim 12, A method for manufacturing a cold rolled steel sheet, wherein the Mf-second cooling end temperature (Tf) is controlled to be 20°C or higher during the above secondary cooling.

21. In claim 12, A method for manufacturing a cold rolled steel sheet, wherein the over-aging treatment temperature (H) is controlled to 130 to 300°C and performed for 5 to 12 minutes during the above-mentioned over-aging treatment.

22. In claim 12, 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 50°C or higher.

23. In claim 12, A method for manufacturing cold rolled steel sheets, wherein the above tension leveling is performed at an elongation of 0.05 to 0.80%.

24. In claim 12, 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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