Cold-rolled steel sheet and method of manufacturing same

Abstract

The present invention relates to a cold-rolled steel sheet and a method for manufacturing same and, more specifically, to a cold-rolled steel sheet and a method for manufacturing same, the cold-rolled steel sheet being suitable for use as a steel material for automobile reinforcements, such as bumper beams, side sill beams, and pillars, or as a steel material for side frames, cross members, and the like.

Description

Cold-rolled steel sheet and method of manufacturing the same

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more specifically, to a cold-rolled steel sheet suitable for use as steel for automotive reinforcing materials such as bumper beams, sill side beams, and pillars, or as steel for side frames, cross members, etc., and a method for manufacturing the same.

[0002] For steel materials primarily used in reinforcing components related to the crash safety of automobile passengers, there is a requirement to develop ultra-high-strength steel with a tensile strength of 1,470 MPa or higher that exhibits excellent processing characteristics (hole expansion and bendability) when manufactured using cold forming techniques. To manufacture ultra-high-strength steel with a tensile strength of 1,470 MPa or higher, the introduction of a martensitic structure is essential. This martensitic structure is formed during the rapid cooling of high-temperature austenite. More specifically, due to the rapid cooling, there is insufficient time for carbon atoms within the austenite to diffuse, resulting in the formation of a martensitic structure with a Body-Centered Tetragonal (BCT) structure rather than a Body-Centered Cubic (BCC) ferrite structure; steel utilizing this structure is called martensitic steel. Typically, while martensitic steel possesses very high strength and hardness, it has low impact energy, leading to a high risk of brittle fracture. Therefore, it is necessary to develop a technology that can secure impact characteristics for ultra-high strength steel with a tensile strength of 1470 MPa or higher and a martensitic structure.

[0003] Representative prior art includes Patent Documents 1 to 3.

[0004] Patent Document 1 describes a material containing, in weight percent, C: 0.08% or more and 0.35% or less, Si: 0.50% or more and 2.50% or less, Mn: 2.00% or more and 3.50% or less, P: 0.001% or more and 0.100% or less, S: 0.0200% or less, Al: 0.010% or more and 1.000% or less, and N: 0.0005% or more and 0.0100% or less, with the remainder consisting of Fe and unavoidable impurities; the steel structure comprises tempering martensite in an area percentage of 75.0% or more, quenching martensite in an area percentage of 1.0% or more and 20.0% or less, and retained austenite in an area percentage of 5.0% or more and 20.0% or less, and the hardness ratio of quenching martensite to tempering martensite is 1.5 or more and 3.0 A cold-rolled steel sheet with excellent ductility as well as elongation flangeability is disclosed, wherein the ratio of the maximum KAM value on the tempering martensite side near the ideal interface between tempering martensite and quenching martensite to the average KAM value in tempering martensite is 1.5 or more and 30.0 or less, and the average value of the ratio of the grain size in the rolling direction to the grain size in the thickness direction of the original austenite grain is 2.0 or less.

[0005] Patent Document 2 contains, in weight percent, C: 0.2~0.4%, Si: 0.5% or less (excluding 0%), Mn: 1.0~2.0%, P: 0.03% or less (excluding 0%), S: 0.015% or less (excluding 0%), Al: 0.1% or less (excluding 0%), Cr: 0.5% or less (excluding 0%), Mo: less than 0.2% (excluding 0%), Ti: 0.1% or less (excluding 0%), Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; the microstructure consists of a tempered martensite single-phase structure or a mixed structure of martensite + tempered martensite, and the microstructure contains FHAGB per unit area of ​​45㎛ × 45㎛ An ultra-high strength cold-rolled steel sheet with excellent hole expansion properties, having an area of ​​60% or more and an LHAGB of 8mm or more, is disclosed.

[0006] Patent Document 3 describes, in weight percent, carbon (C): 0.24–0.30%, silicon (Si): 0.05–0.3%, manganese (Mn): 1.0–2.5%, aluminum (Al): 0.01–0.05%, phosphorus (P): greater than 0% and less than or equal to 0.02%, sulfur (S): greater than 0% and less than or equal to 0.01%, chromium (Cr): 0.2–0.5%, molybdenum (Mo): 0.1–0.3%, at least one element selected from the group consisting of titanium (Ti), niobium (Nb), and vanadium (V): 0.01–0.1%, boron (B): greater than 0% and less than or equal to 0.003%, and the final microstructure consists of tempered martensite of 70% or more and less than 100%, bainite of 0% or more and less than or equal to 20%, and 0% or more A high-strength cold-rolled galvanized steel sheet with excellent yield ratio and bending characteristics is disclosed, which is composed of 10% or less of ferrite and carbides with an average diameter size in the range of 10 nm to 50 nm.

[0007] However, patent documents 1 to 3 do not specify improvements in hole expansion and impact characteristics.

[0008] [Prior Art Literature]

[0009] (Patent Document 1) Korean Published Patent Application No. 10-2019-0107089

[0010] (Patent Document 2) Korean Published Patent Application No. 10-2023-0043267

[0011] (Patent Document 3) Korean Published Patent Application No. 10-2022-0133842

[0012] One aspect of the present invention is to provide a cold-rolled steel sheet and a method for manufacturing the same.

[0013] A preferred aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet with excellent hole expansion properties and a method for manufacturing the same.

[0014] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0015] One embodiment of the present invention comprises, in weight%, carbon (C): 0.160~0.350%, silicon (Si): 0.010~0.80%, manganese (Mn): 0.30~2.60%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.0050% or less (excluding 0%), aluminum (Al): 0.0030~0.080%, chromium (Cr): 0.0010~0.60%, molybdenum (Mo): 0.0010~0.40%, niobium (Nb): 0.0010~0.10%, titanium (Ti): 0.0050~0.20%, boron (B): 0.00050~0.0070%, nitrogen (N): 0.010% or less (excluding 0%), copper (Cu): Contains 0.0010~0.30%, Nickel (Ni): 0.0010~0.30%, the remainder being Fe and other unavoidable impurities; contains 0.10~5.90 Ti-based carbonitrides / mm² with an equivalent average size of 3.0~8.0㎛ and 100~2000 Nb-based carbonitrides / mm² with an equivalent average size of 0.20~9.0㎛; and has a boundary density of 1.90~4.0mm -1 Provides cold-rolled steel sheets.

[0016] The above cold-rolled steel sheet can satisfy the following equations 1 to 3.

[0017] [Relation 1] 50 ≤

[0018] [Equation 2] 5 ≤ Y = -26C + 1150Ti - 100Nb + 1290N ≤ 260

[0019] [Equation 3] 0.02 ≤ Y / X ≤ 4.0

[0020] The above cold-rolled steel sheet may have a microstructure in area % of: a total of one or more of ferrite and bainite: 5% or less (including 0%), and the remainder being one or more of fresh martensite and tempered martensite.

[0021] The above Ti-based carbonitride may have an aspect ratio of 3.0 or less.

[0022] The above Ti-based carbonitride may have a fraction of Ti-based carbonitrides with an aspect ratio of 2.5 or higher relative to the total Ti-based carbonitrides of 1 to 18%.

[0023] The above cold-rolled steel sheet has a yield strength (TS): 1200 MPa or higher, tensile strength (TS): 1470 MPa or higher, total elongation (T_EL): 4.0% or higher, uniform elongation (U_EL): 2.5% or higher, impact value: 21~100 J / ㎠, hole expansion (HER): 20~100%, and hole expansion (HER) / tensile strength (TS): 0.0140~0.0540% MPa -1 It could be.

[0024] The above cold-rolled steel sheet may have a plating layer formed on at least one surface.

[0025] Another embodiment of the present invention comprises, in weight%, carbon (C): 0.160~0.350%, silicon (Si): 0.010~0.80%, manganese (Mn): 0.30~2.60%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.0050% or less (excluding 0%), aluminum (Al): 0.0030~0.080%, chromium (Cr): 0.0010~0.60%, molybdenum (Mo): 0.0010~0.40%, niobium (Nb): 0.0010~0.10%, titanium (Ti): 0.0050~0.20%, boron (B): 0.00050~0.0070%, nitrogen (N): 0.010% or less (excluding 0%), copper (Cu): A step of preparing molten steel containing 0.0010~0.30%, nickel (Ni): 0.0010~0.30%, and the remainder being Fe and other unavoidable impurities; a step of pouring the molten steel into a tundish at a temperature of 1545℃ or lower; a step of pouring the molten steel into the tundish into a mold and then continuously casting it so that the secondary cooling ratio is 0.50~3.0ℓ / kg·s to obtain a slab; a step of heating the slab at 1100~1300℃; 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 at 300~700℃; a step of cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; a step of continuously annealing the cold-rolled steel sheet at Ac3+5℃~Ac3+65℃ for 50~250 seconds; The present invention provides a method for manufacturing a cold-rolled steel sheet comprising: a step of first cooling the continuously annealed cold-rolled steel sheet to a first cooling end temperature (T1) of Bs+50℃ to Ac3; a step of second cooling the first-cooled cold-rolled steel sheet to a second cooling end temperature (T2) of 40 to 250℃; a step of reheating the second-cooled cold-rolled steel sheet and then overaging it; and a step of tension leveling the reheated and overaged cold-rolled steel sheet.

[0026] The above molten steel can satisfy the following equations 1 to 3.

[0027] [Relation 1] 50 ≤

[0028] [Equation 2] 5 ≤ Y = -26C + 1150Ti - 100Nb + 1290N ≤ 260

[0029] [Equation 3] 0.02 ≤ Y / X ≤ 4.0

[0030] The above finishing hot rolling can be performed at Ar3 to Ar3+120℃.

[0031] The above cold rolling can be performed with a cold reduction rate of 35 to 70%.

[0032] The above first cooling can be performed at a first average cooling rate (CR1) of 0.5 to 6.0℃ / s.

[0033] The above secondary cooling can be performed at a secondary average cooling rate (CR2) of 40 to 300℃ / s.

[0034] During the above secondary cooling, the Mf-secondary cooling end temperature (T2) can be controlled to be 20℃ or higher.

[0035] The above overaging treatment can be performed at 140 to 260°C for 3 to 14 minutes.

[0036] The above tension leveling can be performed with an elongation rate of 0.05 to 0.55%.

[0037] After the tension leveling above, the step of forming a plating layer on at least one surface of the cold-rolled steel sheet may be additionally included.

[0038] According to one aspect of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided.

[0039] According to a preferred aspect of the present invention, an ultra-high strength cold-rolled steel sheet with excellent hole expansion properties and a method for manufacturing the same can be provided.

[0040] Figure 1 shows the distribution of tensile strength and hole expansion of Invention Examples 1 to 5 and Comparative Examples 1 to 13.

[0041] Preferred embodiments of the present invention are described below. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0042] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0043] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0044] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0045] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0046] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0047] Hereinafter, a cold-rolled steel sheet according to an embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. Unless otherwise specified, the alloy composition described below refers to weight percent.

[0048] Carbon (C): 0.160~0.350%

[0049] C is an interstitial solid solution element and is the most effective and important element for improving the strength of steel. If the content of C is less than 0.160%, it may be difficult to obtain the strength targeted in the present invention. If the content of C exceeds 0.350%, the strength increases rapidly, and the hole expansion and impact characteristics may deteriorate. Therefore, it is advantageous for the content of C to have a range of 0.160 to 0.350%. It is more advantageous for the lower limit of the C content to be 0.170%, and more advantageous for it to be 0.180%. It is more advantageous for the upper limit of the C content to be 0.340%, and more advantageous for it to be 0.330%.

[0050] Silicon (Si): 0.010~0.80%

[0051] Si is an element effective for improving resistance to tempering softening, and is also an element effective for improving strength through solid solution strengthening. If the Si content is less than 0.010%, it may be difficult to sufficiently obtain the aforementioned effects. If the Si content exceeds 0.80%, there is a risk that excessive ferrite will be generated after continuous annealing and cooling, thereby weakening the strength of the steel. Furthermore, as Si is an element that increases resistivity, resistance spot weldability may be inferior. Therefore, it is advantageous for the Si content to be in the range of 0.010 to 0.80%. The lower limit of the Si content is more advantageous at 0.020%, and more advantageous at 0.030%. The upper limit of the Si content is more advantageous at 0.70%, and more advantageous at 0.60%.

[0052] Manganese (Mn): 0.30~2.60%

[0053] Mn is an element added to ensure strength. If the Mn content is less than 0.30%, the hardenability is low; consequently, if the cooling rate after continuous annealing is not sufficiently fast, martensite is not formed, making it difficult to secure the level of strength targeted in the present invention. If the Mn content exceeds 2.60%, the Ms temperature decreases during cooling after continuous annealing, and as the temperature at which cooling must end decreases, the shape of the steel sheet becomes defective. Furthermore, it is difficult to secure a martensite structure. In addition, during steelmaking / continuous casting operations, Mn-based segregation zones occur along the longitudinal direction of the slab, degrading hole expansion, impact characteristics, and bending characteristics. That is, as manganese bands (Mn bands) are formed within the slab, cracks occur during continuous casting, and there is a problem of increased defect occurrence during the rolling process. Therefore, it is advantageous for the Mn content to be in the range of 0.30 to 2.60%. The lower limit of the Mn content is more advantageous for being 0.40%, and 0.50% is more advantageous. The upper limit of the Mn content is more advantageous for being 2.50%, and 2.40% is more advantageous.

[0054] Phosphorus (P): 0.030% or less (excluding 0%)

[0055] P is an impurity element contained in steel. If the content of P exceeds 0.030%, weldability deteriorates, and it is prone to segregation at grain boundaries, leading to intergranular embrittlement. Furthermore, there is a risk that the grain boundaries will be prone to fracture by hydrogen within the steel, potentially causing brittleness in the steel. Meanwhile, while it is advantageous to exclude P from the steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is advantageous for the content of P to be 0.030% or less (excluding 0%). It is even more advantageous for the content of P to be 0.020% or less.

[0056] Sulfur (S): 0.0050% or less (excluding 0%)

[0057] S is an impurity element included in steel, similar to P. If the content of S exceeds 0.0050%, it can impair ductility and weldability, and a large amount of MnS precipitates may be formed, which can lead to inferior bending properties. Meanwhile, it is advantageous for S not to be included in steel as much as possible, but 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is advantageous for the content of S to be 0.0050% or less (excluding 0%). It is more advantageous for the content of S to be 0.0030% or less, and even more advantageous for it to be 0.0020% or less.

[0058] Aluminum (Al): 0.0030~0.080%

[0059] Al can be added to remove oxygen from the molten steel. If the Al content is less than 0.0030%, deoxidation is not sufficiently achieved, which impairs the cleanliness of the steel. If the Al content exceeds 0.080%, not only does the castability of the slab deteriorate, but the temperature required for single-phase heating during continuous annealing also increases, which may cause production and equipment problems. Therefore, it is advantageous for the Al content to be in the range of 0.0030% to 0.080%. The lower limit of the Al content is more advantageous at 0.0050%, and more advantageous at 0.010%. The upper limit of the Al content is more advantageous at 0.070%, and more advantageous at 0.060%.

[0060] Chrome (Cr): 0.0010~0.60%

[0061] Cr is an element that facilitates securing a low-temperature transformation structure by suppressing ferrite transformation. Additionally, when utilizing a continuous annealing process involving slow cooling, as in the present invention, there is an advantage in suppressing ferrite formation. If the Cr content is less than 0.0010%, the hardenability is low; consequently, if the cooling rate after continuous annealing is not sufficiently fast, martensite is not formed, making it difficult to secure the strength level targeted in the present invention. If the Cr content exceeds 0.60%, resistance to delayed fracture may deteriorate, carbides such as CrC may form to reduce bending characteristics, manufacturing costs may increase due to excessive alloy input, and through-corrosion may be accelerated. Therefore, it is advantageous for the Cr content to be in the range of 0.0010% to 0.60%. The lower limit of the Cr content is more advantageous at 0.0020%, and even more advantageous at 0.0030%. The upper limit of the above Cr content is more advantageous at 0.50%, and more advantageous at 0.40%.

[0062] Molybdenum (Mo): 0.0010~0.40%

[0063] Mo is an element that exhibits effects such as improving the hardenability of steel, generating Mo-based fine carbides that serve as hydrogen trap sites, and improving resistance to delayed fracture through martensite refinement. If the content of Mo is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the content of Mo exceeds 0.40%, the aforementioned effects do not increase significantly compared to the cost increase resulting from the addition of expensive alloying elements. Therefore, it is advantageous for the content of Mo to have a range of 0.0010% to 0.40%. The lower limit of the Mo content is more advantageous at 0.0020%, and more advantageous at 0.0030%. The upper limit of the Mo content is more advantageous at 0.35%, and more advantageous at 0.30%.

[0064] Niobium (Nb): 0.0010~0.10%

[0065] Nb is an element that segregates at austenite grain boundaries, suppresses the coarsening of austenite grains during continuous annealing, and contributes to strength improvement by forming fine precipitates. If the Nb content is less than 0.0010%, sufficient austenite grain refinement and precipitation strengthening effects cannot be obtained. If the Nb content exceeds 0.10%, the precipitation of coarse carbonitrides increases, hole expansion and impact properties may be inferior, and there is a risk that strength may decrease due to the reduction of carbon content in the steel. In addition, there are problems such as reduced machinability of the base material and increased manufacturing costs. Therefore, it is advantageous for the Nb content to be in the range of 0.0010 to 0.10%. It is more advantageous for the lower limit of the Nb content to be 0.0020%, and even more advantageous for it to be 0.0030%. The upper limit of the above Nb content is more advantageous at 0.080%, and more advantageous at 0.050%.

[0066] Titanium (Ti): 0.0050~0.20%

[0067] Ti is a nitride-forming element that scavenges dissolved N by precipitating it as TiN. If the Ti content is less than 0.0050%, it is difficult to obtain the effect of increasing strength, and the scavenging effect of dissolved N is reduced, leading to the formation of a large amount of AlN, which may cause cracks during continuous casting. If the Ti content exceeds 0.20%, the strength of the martensite may decrease as additional carbides are precipitated in addition to the removal of dissolved N. Furthermore, the hole expansion and impact properties may be inferior due to the excessive formation of carbonitrides such as TiC and TiN, and hydrogen embrittlement may be inferior due to the deterioration of bending properties. Therefore, it is advantageous for the Ti content to be in the range of 0.0050% to 0.20%. The lower limit of the Ti content is more advantageous at 0.0070%, and even more advantageous at 0.010%. The upper limit of the above Ti content is more advantageous at 0.170%, and more advantageous at 0.150%.

[0068] Boron (B): 0.00050~0.0070%

[0069] B is an element that inhibits ferrite formation. Accordingly, the present invention has the advantage of increasing resistance to hydrogen embrittlement by inhibiting the formation of ferrite upon cooling after continuous annealing and by strengthening the austenite grain boundaries to suppress hydrogen intrusion. If the content of B is less than 0.00050%, it is difficult to obtain the aforementioned effects sufficiently, and there is no hardenability effect at all, making it difficult to secure the strength targeted by the present invention. If the content of B exceeds 0.0070%, ductility may be significantly reduced. Therefore, it is advantageous for the content of B to have a range of 0.00050% to 0.0070%. The lower limit of the B content is more advantageous at 0.00070%, and more advantageous at 0.0010%. The upper limit of the B content is more advantageous at 0.0060%, and more advantageous at 0.0055%.

[0070] Nitrogen (N): 0.010% or less (excluding 0%)

[0071] N is an impurity element, and if its content exceeds 0.010%, it significantly increases the risk of cracking during continuous casting due to the formation of AlN, etc. Although it is advantageous for the above N content to be excluded from the steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is advantageous for the above N content to have a range of 0.010% or less (excluding 0%). It is more advantageous for the above N content to be 0.0080% or less, and even more advantageous for it to be 0.0060% or less.

[0072] Copper (Cu): 0.0010~0.30%

[0073] Cu improves corrosion resistance and also has the effect of suppressing hydrogen intrusion into the steel plate by coating the surface of the steel plate with corrosion products. In addition, as an element incorporated when utilizing scrap as a raw material, recycled materials can be utilized as raw materials, thereby reducing manufacturing costs. If the content of Cu is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the content of Cu exceeds 0.30%, it may result in surface defects. Therefore, it is advantageous for the content of Cu to have a range of 0.0010% to 0.30%. The lower limit of the Cu content is more advantageous at 0.0050%, and more advantageous at 0.0070%. The upper limit of the Cu content is more advantageous at 0.25%, and more advantageous at 0.20%.

[0074] Nickel (Ni): 0.0010~0.30%

[0075] Ni is an element that, like Cu, acts to improve corrosion resistance. Additionally, as an element incorporated when utilizing scrap as a raw material, it allows for the use of recycled materials, thereby reducing manufacturing costs. If the Ni content is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the Ni content exceeds 0.30%, it may lead to the occurrence of surface defects. Therefore, it is advantageous for the Ni content to be in the range of 0.0010% to 0.30%. The lower limit of the Ni content is more advantageous at 0.0050%, and more advantageous at 0.0070%. The upper limit of the Ni content is more advantageous at 0.25%, and more advantageous at 0.20%.

[0076] The remaining component is iron (Fe). However, since unintended impurities (such as Zr, W, Ca, Ce, La, Mg, Sb, and Sn) from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0077] It is advantageous for the cold-rolled steel sheet of the present invention to satisfy the aforementioned alloy composition and simultaneously satisfy the following equations 1 to 3.

[0078] [Relation 1] 50 ≤

[0079] The above Equation 1 is a compositional relationship related to hardenability for securing a target microstructure. If the value of X is less than 50, the soft ferrite and bainite structures transform during cooling, making it difficult to secure the target strength. If the value of X exceeds 250, the strength becomes excessively high, which may result in inferior hole expansion and impact characteristics; furthermore, it may be difficult to secure the target elongation, which may cause processing cracks during forming and increase the cost of the ferroalloy. Therefore, it is advantageous for the value of X to have a range of 50 to 250. The lower limit of the value of X is more advantageous at 55, and more advantageous at 60. The upper limit of the value of X is more advantageous at 240, and more advantageous at 230.

[0080] [Equation 2] 5 ≤ Y = -26C + 1150Ti - 100Nb + 1290N ≤ 260

[0081] The above Equation 2 is a compositional relationship related to the precipitation temperature of TiN, etc. If the value of Y is less than 5, the effect of Ti scavenging dissolved N is reduced, and as a large amount of AlN is formed, there is a possibility that cracks may occur during continuous casting. If the value of Y exceeds 260, TiN is formed at a temperature above the melting point, so coarsening adversely affects hole expansion and impact properties. Therefore, it is advantageous for the value of Y to have a range of 5 to 260. The lower limit of the value of Y is more advantageous for 8, and more advantageous for 10. The upper limit of the value of Y is more advantageous for 250, and more advantageous for 240.

[0082] [Equation 3] 0.02 ≤ Y / X ≤ 4.0

[0083] The above Equation 3 is a compositional equation designed to simultaneously secure excellent strength, hole expandability, and impact characteristics. When the value of Y / X is less than 0.02, the high X value indicates excellent hardenability, which is advantageous for securing strength; however, the low Y value reduces the scavenging effect of Ti on dissolved N, leading to the formation of a large amount of AlN and potentially causing cracks during continuous casting. When the value of Y / X exceeds 4.0, the low X value results in insufficient hardenability, causing the soft ferrite and bainite structures to transform upon cooling, making it difficult to secure the target strength. Additionally, the high Y value leads to the formation of TiN at temperatures above the melting point, which can result in coarsening and potentially inferior hole expandability and impact characteristics. Therefore, it is advantageous for the value of Y / X to have a range of 0.02 to 4.0. The lower limit of the Y / X value is more advantageous at 0.03, and even more advantageous at 0.04. The upper limit of the above Y value is more advantageous when it is 3.8, and more advantageous when it is 3.6.

[0084] The microstructure of the cold-rolled steel sheet of the present invention may comprise, in area %, a total of one or more types of ferrite and bainite: 5% or less (including 0%), and the remainder being one or more types of fresh martensite and tempered martensite. That is, the total of one or more types of fresh martensite and tempered martensite may be 95% or more (including 100%). The fresh martensite and tempered martensite are structures that are highly advantageous for securing excellent strength, bending characteristics, and impact characteristics targeted by the present invention. However, one or more types of ferrite and bainite may inevitably be formed during the manufacturing process, and if the total fraction of one or more types of ferrite and bainite exceeds 5%, it may be difficult to secure the physical properties intended by the present invention. Therefore, it is advantageous for the total fraction of one or more types of ferrite and bainite to be 5% or less, and more advantageous for it to be 3% or less. The above tempered martensite may include auto-tempered martensite formed during the cooling process.

[0085] The cold-rolled steel sheet of the present invention may contain 0.10 to 5.90 particles / mm² of Ti-based carbonitrides having an equivalent average size of 3.0 to 8.0 μm. If the equivalent average size of the Ti-based carbonitrides is less than 3.0 μm or the fraction is less than 0.10 particles / mm², the grain refinement effect is low, causing the martensite substructure to become coarse, which may result in inferior hole expansion and impact properties. If the equivalent average size of the Ti-based carbonitrides exceeds 8.0 μm or the fraction exceeds 5.90 particles / mm², the hole expansion and impact properties may be inferior due to the coarse and non-uniform Ti-based carbonitrides. It is more advantageous for the lower limit of the equivalent average size of the Ti-based carbonitrides to be 3.1 μm, and more advantageous for it to be 3.2 μm. The upper limit of the original average size of the above Ti-based carbonitride is more advantageous for being 7.9 μm, and 7.8 μm is more advantageous. The lower limit of the fraction of the above Ti-based carbonitride is more advantageous for being 0.20 pieces / mm², and 0.30 pieces / mm² is more advantageous. The upper limit of the fraction of the above Ti-based carbonitride is more advantageous for being 5.80 pieces / mm², and 5.70 pieces / mm² is more advantageous. Meanwhile, the above Ti-based carbonitride mentioned in the present invention may exist in various forms such as Ti carbide, Ti nitride, Ti composite carbide, Ti composite nitride, and Ti composite carbonitride.

[0086] The above Ti-based carbonitride may have an aspect ratio of 3.0 or less. If the aspect ratio of the above Ti-based carbonitride exceeds 3.0, the hole expansion and impact characteristics may be inferior due to the non-uniform Ti-based carbonitride. An aspect ratio of 2.9 for the above Ti-based carbonitride is more advantageous, and 2.8 is more advantageous. Since a smaller aspect ratio of the above Ti-based carbonitride is more advantageous, the present invention does not specifically limit the lower limit thereof, but as an example, the lower limit thereof may be 1.0. In terms of the manufacturing process or manufacturing cost, a lower limit of 1.1 for the aspect ratio of the above Ti-based carbonitride is more advantageous, and 1.2 is more advantageous.

[0087] The above Ti-based carbonitride may have a fraction of Ti-based carbonitrides with an aspect ratio of 2.5 or higher relative to the total Ti-based carbonitride, ranging from 1% to 18%. If the fraction of Ti-based carbonitrides with an aspect ratio of 2.5 or higher relative to the total Ti-based carbonitride is less than 1%, additional processing is required, which may increase manufacturing costs. If the fraction of Ti-based carbonitrides with an aspect ratio of 2.5 or higher relative to the total Ti-based carbonitride exceeds 18%, the hole expansion and impact characteristics may be inferior due to coarse and non-uniform Ti-based carbonitrides. It is more advantageous for the lower limit of the fraction of Ti-based carbonitrides with an aspect ratio of 2.5 or higher relative to the total Ti-based carbonitride to be 2%, and more advantageous for it to be 3%. It is more advantageous for the upper limit of the fraction of Ti-based carbonitrides with an aspect ratio of 2.5 or higher relative to the total Ti-based carbonitride to be 17%, and more advantageous for it to be 16%.

[0088] The cold-rolled steel sheet of the present invention may contain 100 to 2,000 Nb-based carbonitrides / mm² with an equivalent average size of 0.20 to 9.0 μm. If the equivalent average size of the Nb-based carbonitrides is less than 0.20 μm, additional processing is required, which may increase manufacturing costs. If the equivalent average size of the Nb-based carbonitrides exceeds 9.0 μm, the grain refinement effect is small, which may cause the martensite substructure to coarsen, and it may be difficult to secure strength. It is more advantageous for the lower limit of the equivalent average size of the Nb-based carbonitrides to be 0.50 μm, and more advantageous for it to be 1.0 μm. It is more advantageous for the upper limit of the equivalent average size of the Nb-based carbonitrides to be 8.0 μm, and more advantageous for it to be 7.0 μm. If the fraction of the Nb-based carbonitrides is less than 100 / mm², the grain refinement effect may be small. If the fraction of the above Nb-based carbonitride exceeds 2000 pieces / mm², the elongation may be inferior. The lower limit of the fraction of the above Nb-based carbonitride is more advantageous at 110 pieces / mm², and 120 pieces / mm² is more advantageous. The upper limit of the fraction of the above Nb-based carbonitride is more advantageous at 1800 pieces / mm², and 1600 pieces / mm² is more advantageous. Meanwhile, the above Nb-based carbonitride mentioned in the present invention may exist in various forms such as Nb carbide, Nb nitride, Nb composite carbide, Nb composite nitride, Nb composite carbonitride, etc.

[0089] The cold-rolled steel sheet of the present invention has a boundary density of 1.90~4.0mm -1 It may be. The above boundary density is 1.95mm -1 If it is less than, the substructure of the martensite (packets, blocks, prior austenite) becomes coarse, making it difficult to secure the target strength, and hole expandability and impact characteristics may be low. The above boundary density is 4.0 mm -1If it exceeds , the fine grain size may be advantageous for strength, hole expandability, and impact properties; however, since large amounts of expensive elements such as Nb, Ti, and Mo must be added to refine the grain, economic feasibility may be inferior due to increased manufacturing costs. The lower limit of the above boundary density is 2.0 mm -1 It is more advantageous to be 2.1mm -1 It is more advantageous to have this. The upper limit of the above boundary density is 3.90 mm -1 It is more advantageous to be 3.80mm -1 It is more advantageous to be.

[0090] Meanwhile, the above boundary density can be obtained through Electron Backscatter Diffraction (EBSD) analysis. In the above EBSD analysis, boundary density is a concept representing the amount of grain boundaries per unit area, and it can be used to quantitatively express how many grain boundaries exist in the microstructure of a material. The above boundary density is an important factor in analyzing the characteristics of the microstructure and evaluating its influence on the mechanical properties and hydrogen embrittlement of the material. That is, the higher the boundary density, the more grain boundaries exist; this contributes to suppressing dislocation movement and thereby increasing strength, and contributes to improving resistance to hydrogen embrittlement by acting as hydrogen trap sites. Meanwhile, in the present invention, the above boundary density may refer to grain boundaries having high-angle grain boundaries of 15° or more.

[0091] The cold-rolled steel sheet of the present invention provided as described above has a yield strength (TS): 1200 MPa or more, a tensile strength (TS): 1470 MPa or more, a total elongation (T_EL): 4.0% or more, a uniform elongation (U_EL): 2.5% or more, an impact value: 21~100 J / ㎠, a hole expandability (HER): 20~100%, and a hole expandability (HER) / tensile strength (TS): 0.0140~0.0540% MPa -1 The yield strength may be more advantageously 1210 MPa or higher, and more advantageously 1220 MPa or higher. In the present invention, the upper limit of the yield strength is not specifically limited, but as an example, it may be 1900 MPa. The tensile strength may be more advantageously 1490 MPa or higher, and more advantageously 1500 MPa or higher. In the present invention, the upper limit of the tensile strength is not specifically limited, but as an example, it may be 2150 MPa. The total elongation may be more advantageously 4.5% or higher, and more advantageously 5.0% or higher. In the present invention, the upper limit of the total elongation is not specifically limited, but as an example, it may be 10%. The uniform elongation may be more advantageously 2.6% or higher, and more advantageously 2.7% or higher. In the present invention, the upper limit of the uniform elongation is not specifically limited, but as an example, it may be 5%. The impact value may more advantageously be 22 to 95 J / cm², and more advantageously be 23 to 90 J / cm². The hole expansion capacity may more advantageously be 22 to 95%, and more advantageously be 23 to 90%. The hole expansion capacity / tensile strength may more advantageously be 0.0160 to 0.0520% MPa -1 and, more favorably, 0.0180~0.050%MPa -1 It may be. Meanwhile, the above yield strength and tensile strength may be measured in a direction perpendicular to the rolling direction.

[0092] The thickness of the cold-rolled steel sheet of the present invention may be 0.6 to 2.5 mm. The lower limit of the thickness of the cold-rolled steel sheet is more advantageously 0.7 mm, and 0.8 mm. The upper limit of the thickness of the cold-rolled steel sheet is more advantageously 2.4 mm, and 2.3 mm.

[0093] The cold-rolled steel sheet of the present invention may have a plating layer formed on at least one surface. The present invention does not specifically limit the type of plating layer, and any type of plating layer commonly used in the relevant technical field may be formed. However, as an example, the plating layer may be an electro-galvanized layer.

[0094] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0095] First, molten steel satisfying the aforementioned alloy composition is prepared. The molten steel may satisfy the above-described equations 1 to 3. In the present invention, the molten steel preparation process is not specifically limited, and any process commonly used in the relevant technical field may be used.

[0096] Subsequently, the molten steel is poured into a tundish at a temperature of 1545°C or lower. This implies that the temperature of the molten steel in the tundish is 1545°C or lower. If the temperature of the molten steel in the tundish exceeds 1545°C, it affects the casting structure of the slab during continuous casting, the formation of internal cracks, and the degree of center segregation and porosity. Additionally, the cooling rate of the slab slows down, leading to the formation of coarse Ti-based carbonitrides, which adversely affects hole expansion and impact properties. It is more advantageous for the temperature of the molten steel in the tundish to be 1540°C, and even more advantageous for it to be 1535°C. In the present invention, the lower limit of the temperature of the molten steel in the tundish is not specifically limited, but as an example, the lower limit of the temperature of the molten steel in the tundish may be 1400°C. The lower limit of the molten steel temperature in the above tundish is more advantageous at 1410℃, and more advantageous at 1420℃.

[0097] Subsequently, the molten steel injected into the tundish is poured into a mold, and a slab is obtained by continuous casting with a secondary cooling ratio of 0.50 to 3.0 L / kg·s. Controlling the secondary cooling ratio affects the operability of the continuous casting and the quality of the cast slab, but it is effective in improving hydrogen embrittlement resistance by reducing component segregation through the reduction of internal cracks. Additionally, it can affect hole expansion and impact characteristics by influencing the precipitation and growth of Ti-based and Nb-based carbonitrides. If the secondary cooling ratio is less than 0.50 L / kg·s, the solidification layer on the extreme surface of the slab is thin, and repetitive stress exceeding the critical strain is applied to the interface between the solidification layer and the liquid phase layer by the guide rolls of the continuous casting machine, causing internal cracks to occur. Consequently, Mn, P, S, etc., may segregate between the internal cracks, which can lead to a decrease in hydrogen embrittlement resistance. In addition, as the slab cooling rate slows down, Ti-based and Nb-based carbonitrides may coarsen, which may result in inferior pore expansion and impact properties. If the above secondary cooling water content exceeds 3.0 L / kg·s, cracks may occur due to overcooling of the slab surface and edge, leading to inferior quality. It is more advantageous for the lower limit of the water content during secondary cooling to be 0.60 L / kg·s, and more advantageous for it to be 0.70 L / kg·s. It is more advantageous for the upper limit of the above secondary cooling water content to be 2.9 L / kg·s, and more advantageous for it to be 2.8 L / kg·s.

[0098] Subsequently, the above slab is heated at 1100 to 1300°C. The above slab heating process is performed to facilitate the subsequent hot rolling process and to sufficiently obtain the target physical properties of the steel plate. If the above slab heating temperature is below 1100°C, a problem arises in which the hot rolling load increases rapidly. If the above slab heating temperature exceeds 1300°C, the amount of surface scale increases, and the yield of the material decreases. In addition, it may be difficult to secure the size and fraction of Nb-based carbonitrides that the present invention aims to obtain. The lower limit of the above slab heating temperature is more advantageous at 1110°C, more advantageous at 1120°C, and most advantageous at 1130°C. The upper limit of the above slab heating temperature is more advantageous at 1290°C, more advantageous at 1280°C, and most advantageous at 1270°C.

[0099] Subsequently, the heated slab is finished hot-rolled to obtain a hot-rolled steel sheet. The finish hot-rolling can be performed at a temperature of Ar3 to Ar3+120℃. If the finish hot-rolling temperature is below Ar3, rolling occurs in a two-phase region of ferrite + austenite or in a ferrite region, resulting in a mixed grain structure, and plate breakage may occur due to fluctuations in the hot-rolling load. If the finish hot-rolling temperature exceeds Ar3+120℃, a large amount of surface scale may form, which may degrade the surface quality. The lower limit of the finish hot-rolling temperature is more advantageous at Ar3+10℃, more advantageous at Ar3+20℃, and most advantageous at Ar3+30℃. The upper limit of the finish hot-rolling temperature is more advantageous at Ar3+110℃, more advantageous at Ar3+100℃, and most advantageous at Ar3+90℃. Meanwhile, the above Ar3 refers to the temperature at which austenite begins to transform into ferrite upon cooling, and can be calculated using the following Equation 1.

[0100] [Equation 1] Ar3(°C) = 910-203√C+44.7Si+31.5Mo

[0101] Subsequently, the hot-rolled steel sheet is coiled at 300 to 700°C. If the coiling temperature (CT) exceeds 700°C, internal oxidation occurs on the surface of the steel sheet, causing the microstructure formed in the surface layer to become non-uniform and precipitates to coarsen. These precipitates may remain after annealing, potentially degrading hole expansion and impact properties. Meanwhile, it is advantageous to manage the coiling temperature at a low level to ensure material uniformity across the entire length and width by forming the microstructure of the hot-rolled steel sheet into a single-phase structure rather than a complex structure as much as possible. However, if the coiling temperature is below 300°C, the strength of the hot-rolled steel sheet becomes excessively high, which may result in a high rolling load during the subsequent cold rolling process, making actual production impossible. A lower limit of 320°C is more advantageous, and 350°C is even more advantageous. The upper limit of the above coiling temperature is more advantageous at 690℃, and more advantageous at 680℃.

[0102] Meanwhile, after the above coiling, it can be cooled by air cooling or water cooling. In addition, after the above cooling, a pickling process can be performed to remove the oxide layer formed on the surface of the hot-rolled steel sheet.

[0103] Subsequently, the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold rolling can be performed with a cold reduction rate of 35 to 70%. If the cold reduction rate is less than 35%, it is difficult to secure the thickness desired in the present invention, and there is a concern that non-uniform austenite may be generated during annealing heat treatment due to the persistence of crystal grains formed during hot rolling, which may affect the final physical properties. If the cold reduction rate exceeds 70%, the reduction amount in the length and width directions may become non-uniform due to work hardening occurring during cold rolling, and this may lead to material variation in the steel sheet. In addition, it may be difficult to secure the thickness desired in the present invention due to the rolling load. The lower limit of the cold reduction rate is more advantageous at 36%, more advantageous at 37%, and more advantageous at 48%. The upper limit of the above cold rolling rate is more advantageous at 69%, more advantageous at 68%, and more advantageous at 67%.

[0104] Subsequently, the above cold-rolled steel sheet is continuously annealed at Ac3+5℃ to Ac3+65℃ for 50 to 250 seconds. If the continuous annealing temperature is less than Ac3+5℃, two-phase annealing occurs over the entire length of the steel sheet instead of a single-phase annealing, resulting in the formation of a mixed-grain structure, which may make it difficult to secure the physical properties targeted by the present invention. If the above continuous annealing temperature exceeds Ac3+65℃, equipment trouble may occur due to overloading of the annealing furnace, and since austenite grows excessively and the martensite substructure becomes coarsened even after cooling, it is difficult to secure the target strength, and hole expansion and impact characteristics may be inferior. The lower limit of the above continuous annealing temperature is more advantageous at Ac3+10℃, and more advantageous at Ac3+15℃. The upper limit of the above continuous annealing temperature is more advantageous at Ac3+60℃, and more advantageous at Ac3+55℃. Meanwhile, the above Ac3 refers to the temperature at which 100% transformation of ferrite into austenite is completed upon heating, and can be calculated using Equation 2 below. If the above continuous annealing time is less than 50 seconds, it may be difficult to secure a single-phase austenite structure, and as undissolved carbides remain and coarsen, it is difficult to secure the target mechanical properties. If the above continuous annealing time exceeds 250 seconds, the austenite size coarsens and the martensite substructure is not fine even after cooling, which has the disadvantage of making it difficult to secure strength, and hole expansion and impact characteristics may be inferior. The lower limit of the above continuous annealing time is more advantageous at 60 seconds, and 70 seconds is more advantageous. The upper limit of the above continuous annealing time is more advantageous at 240 seconds, and 230 seconds is more advantageous.

[0105] [Formula 2] Ac3(℃) = 900-206C+26.2Si-25Mn-12.3Cr+9.12Mo+50.2Nb+148Ti-131B

[0106] Subsequently, the continuously annealed cold-rolled steel sheet is first cooled to a first cooling end temperature (T1) of Bs+50℃ to Ac3. If the first cooling end temperature (T1) is less than Bs+50℃, it is difficult to secure the target strength as a large amount of soft ferrite and bainite is formed during the cooling process. If the first cooling end temperature (T1) exceeds Ac3℃, the temperature difference between the first cooling end temperature (T1) and the second cooling end temperature (T2) becomes severe, causing a rapid phase transformation, which may result in defective product shape. It is more advantageous for the lower limit of the first cooling end temperature to be Bs+60℃, and more advantageous for it to be Bs+70℃. It is more advantageous for the upper limit of the first cooling end temperature to be Ac3-10℃, and more advantageous for it to be Ac3-20℃. Meanwhile, the above Bs refers to the temperature at which the bainite transformation begins during cooling, and can be calculated using Equation 3 below. The above first cooling may be performed at a first average cooling rate (CR1) of 0.5 to 6.0°C / s. If the above first average cooling rate (CR1) is less than 0.5°C / s, ferrite is formed during cooling, making it impossible to secure the level of strength targeted by the present invention. If the above average cooling rate (CR1) exceeds 6.0°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 level of strength targeted by the present invention. It is more advantageous for the lower limit of the above average cooling rate (CR1) to be 1°C / s. It is more advantageous for the upper limit of the above average cooling rate (CR1) to be 5°C / s.

[0107] [Formula 3] Bs(℃) = 830-270C-90Mn-37Ni-70Cr-83Mo

[0108] Subsequently, the firstly cooled cold-rolled steel sheet is secondarily cooled to a second cooling end temperature (T2) of 40 to 250°C. The second cooling is intended to secure one or more of the main phases of the present invention, namely martensite and tempered martensite. If the second cooling end temperature (T2) is less than 40°C, shape defects are caused by rapid phase transformation, and there is a disadvantage that continuous production is difficult due to strip meandering problems. If the second cooling end temperature (T2) exceeds 250°C, it may be difficult to secure the strength targeted by the present invention. The lower limit of the second cooling end temperature is more advantageous at 50°C, more advantageous at 55°C, and most advantageous at 60°C. The upper limit of the second cooling end temperature is more advantageous at 240°C, more advantageous at 230°C, and most advantageous at 220°C. The above secondary cooling can be performed at a secondary average cooling rate (CR2) of 40 to 300°C / s. If the above secondary average cooling rate is less than 40°C / s, a soft ferrite transformation occurs during cooling, making it difficult to secure the target strength. If the above secondary average cooling rate exceeds 300°C / s, the product shape may become defective due to rapid phase transformation. The lower limit of the above secondary average cooling rate is more advantageous at 45°C / s, more advantageous at 50°C / s, and most advantageous at 55°C / s. The upper limit of the above secondary average cooling rate is more advantageous at 280°C / s, more advantageous at 260°C / s, and most advantageous at 240°C / s.

[0109] During the above secondary cooling, the Mf-secondary cooling end temperature (T2) can be controlled to be 20°C or higher. If the Mf-T2 is less than 20°C, the martensite transformation may not occur sufficiently, making it difficult to secure the target strength. It is more advantageous for the Mf-T2 to be 30°C or higher. The Mf represents the temperature at which the transformation of austenite into martensite is completed during cooling, and can be calculated using Equation 4 below. Meanwhile, the present invention does not specifically limit the upper limit of the Mf-T2, but as an example, it may be 200°C.

[0110] [Equation 4] Mf(°C) = 371 - 412C - 17.4Si - 47.4Mn - 20.9Cr - 17.0Mo + 49.2Nb + 95.0Ti + 202B

[0111] Subsequently, the above-mentioned secondary cooled cold-rolled steel sheet is reheated and then subjected to overaging treatment. The overaging treatment may be performed at 140 to 260°C for 3 to 14 minutes. Through the reheating and overaging treatment, the martensite obtained by the aforementioned rapid cooling process is transformed into tempered martensite, thereby increasing the yield strength. If the reheating temperature and overaging treatment temperature are below 140°C, there is a disadvantage that tempering is insufficient, resulting in low yield strength and inability to secure sufficient toughness. If the reheating temperature and overaging treatment temperature exceed 260°C, there is a disadvantage that bendability and hydrogen embrittlement resistance are inferior due to the precipitation and coarsening of large amounts of carbides. The lower limit of the reheating and overaging treatment temperature is more advantageous at 145°C, more advantageous at 150°C, and most advantageous at 155°C. The upper limit of the above reheating and overaging treatment temperature is more advantageous at 255°C, more advantageous at 250°C, and most advantageous at 245°C. If the above overaging treatment time is less than 3 minutes, tempering is not sufficiently performed, and the yield strength may be lowered. If the above overaging treatment time exceeds 14 minutes, carbides may coarsen due to excessive tempering, and bending characteristics may deteriorate. The lower limit of the above overaging treatment time is more advantageous at 3.5 minutes, more advantageous at 4 minutes, and most advantageous at 4.5 minutes. The upper limit of the above overaging treatment time is more advantageous at 13.5 minutes, more advantageous at 12 minutes, and most advantageous at 11.5 minutes.

[0112] Subsequently, the cold-rolled steel sheet that has undergone the reheating and overaging treatment is tension leveled (T / L). The tension leveling can be performed with an elongation of 0.05 to 0.55%. If the elongation is less than 0.05% during the tension leveling, shape correction may be difficult. If the elongation exceeds 0.55% during the tension leveling, work hardening may become severe, leading to poor bending characteristics, and the difference in yield strength between the vertical and horizontal directions relative to the rolling direction may become significant, which may adversely affect dimensional accuracy during part processing. The lower limit of the elongation during the tension leveling is more advantageous at 0.075%, and 0.10% is more advantageous. The upper limit of the elongation during the tension leveling is more advantageous at 0.50%, and 0.45% is more advantageous.

[0113] Meanwhile, after the tension leveling above, a step of forming a plating layer on at least one surface of the cold-rolled steel sheet may be additionally included. The present invention does not specifically limit the method of forming the plating layer, and any method commonly used in the relevant technical field may be used. However, as an example, the plating layer may be an electro-galvanized layer.

[0114] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0115] (Example)

[0116] After preparing molten steel having the alloy compositions listed in Tables 1 and 2 below, the molten steel was poured into a tundish at the temperature listed in Table 3 below. Subsequently, the molten steel poured into the tundish was poured into a mold, and a slab was obtained by continuous casting with a secondary cooling ratio under the conditions listed in Table 3 below. Subsequently, a cold-rolled steel sheet was manufactured by heating the slab, hot rolling, coiling, cold rolling, continuous annealing, primary cooling, secondary cooling, reheating / overaging treatment, and tension leveling under the conditions listed in Tables 3 to 5 below. The microstructure, precipitates, and mechanical properties of the cold-rolled steel sheet manufactured in this manner were measured, and the results are shown in Tables 6 and 7 below.

[0117] The types and fractions of the microstructure were observed using a scanning electron microscope (SEM) and an optical microscope (OM) at a position t / 4 (t: thickness of the steel plate) in the thickness direction of the steel plate, and the fractions of each phase were analyzed three times through image analysis to calculate the average value.

[0118] Ti-based carbonitrides were prepared using Auto-SEM (EDS) (EDS, ≥3㎛) equipment on 40mm of the above cold-rolled steel sheet. 2 of The area was analyzed three times to obtain the average value. Nb-based carbonitrides were prepared using the replica method, measured five times at a magnification of 100,000x using a transmission electron microscope (TEM), and then quantified using Image Pro Plus software. Here, the replica method refers to a method in which the surface of a sample is polished and etched, a cellulose tape is attached to the surface, and then the tape is removed to extract only the precipitate for observation using TEM.

[0119] Boundary density was calculated by measuring three times at 2000x magnification (Confidence Index (CI)≥0.3, measurement area: 45×45㎛, Step size: 80nm) at a 1 / 4 position in the thickness direction of the steel plate using EBSD (Backscattered Electron Diffraction Pattern Analyzer) and quantifying it using OIM (Orientation Imaging Microscopy) Analysis software. The boundary density was measured for high-angle grain boundaries of 15° or more.

[0120] Yield strength, tensile strength, and elongation were measured by processing cold-rolled steel sheets according to JIS standards perpendicular to the rolling direction and then performing a tensile test under conditions of a test speed of 28 mm / min.

[0121] Hole expandability (HER) was measured according to the ISO 16330 standard, and the hole was sheared with a 10 mm diameter punch to a clearance of 12%.

[0122] The impact value was calculated by taking a specimen with a width of 10 mm and a length of 55 mm from a cold-rolled steel sheet, processing it with a V-notch (0.2 mm), and then measuring it three times at 20°C in compliance with ASTM E23 standards using a Zwick / Roell HIT50P (50J) impact tester to obtain the average value.

[0123] Steel Grade No. Alloy Composition (Wt%) CSI Mn PS Al Cr Mo Nb 10.29 0.15 1.7 10.00 90.00 100.02 40.01 20.00 30.03 20 20.18 0.12 2.00.01 10.00 90.03 50.01 50.00 50.02 40 30.26 0.05 2.100.01 00.00 130.02 10.12 00.08 00.03 50 40.28 0.11 1.95 0.00 80.00 140.03 10.01 50.05 00.02 10 50.3 10.08 2.100.01 00.00 90.02 90. 2100.0250.031060.190.050.400.0090.00110.0270.0010.0050.000570.150.050.350.0130.00140.0320.0050.0050.011080.290.152.450.0110.00090.0310.0500.0050.120090.190.050.150.0120.00110.0320.0310.0050.0120100.320.503.800.0110.00110.0350.2500.0100.0260

[0124] Steel Grade No. Alloy Composition (Wt%) TiBNCuNiXYY / X10.0310.00310.00410.100.05163300.1820.0260.00210.00360.090.03152270.1830.0310.00210.00410.050.05181310.1740.0250.00250.00390.110.03154240.1650.0310.00250.00390.10 0.05186300.1660.2500.00090.00450.080.03412887.2070.0250.00060.00240.050.0244270.6180.1300.0015 0.00510.120.052931370.4790.0280.00070.00550.080.0347330.70100.0380.00400.00250.090.04267360.13X = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb + 10000B + 14.5Ni + 9.6CuY = -26C + 1150Ti - 100Nb + 1290N The above Y / X is a value calculated by rounding the significant figures of the X and Y values ​​to 0 decimal places.

[0125] Classification Steel Grade No. Turndash Steel Content Temperature (°C) Secondary Cooling Water Amount (ℓ / kg·s) Slab Heating Temperature (°C) Finishing Hot Rolling Temperature (°C) Coiling Temperature (°C) Cold Reduction Rate (%) Invention Example 1 1 15 1 31.25 1 22 59 10 47 556 Invention Example 2 2 15 15 1.35 1 23 09 1 24 56 56 Invention Example 3 3 15 11 1.55 1 20 59 2545656 Invention Example 4415151.50121590947556 Invention Example 5515161.35120591449556 Comparative Example 1615261.25120191552556 Comparative Example 2715311.35123592549556 Comparative Example 3815171.55122591446 956 Comparative Example 4915261.55122590351556 Comparative Example 51015011.25120691152156 Comparative Example 6115481.35122590551556 Comparative Example 7115150.45120093150556 Comparative Example 8115211.35102092152556 Comparative Example 9 11525 1.55 12059 1875056 Comparative Example 10 21516 1.45 12529 065 1558 Comparative Example 11 21517 1.55 12509 0149558 Comparative Example 12 21521 1.65 12259 1350258 Comparative Example 13 21526 1.55 12369 155 1056

[0126] Classification Steel Grade No. Thickness (mm) Ac3 (°C) Annealing Temperature (°C) Annealing Time (sec) Bs (°C) Primary Cooling End Temperature (°C) Primary Average Cooling Rate (°C / s) Invention Example 1 1.480 78351 1059 57253 Invention Example 2 2.1.481 38301 155727313 Invention Example 3 3.1.480 18261 295547263 Invention Example 4 4.1.480 18451 425737452 Invention Example 5 5.1.478 98321 105397652 Comparative Example 1 6.1.488 98951 52741 8053 Comparative Example 2 7.1.486 68801 457 568253 Comparative Example 3 8.1.480 78251 6252 56253 Comparative Example 491.48638691247628253 Comparative Example 5101.47567951243826253 Comparative Example 611.48078251165957412 Comparative Example 711.48078361255957192 Comparative Example 811.4807835135595721 3 Comparative Example 9 11.48078351355957213 3 Comparative Example 10 21.28138951255727353 3 Comparative Example 11 21.28138603555727403 3 Comparative Example 12 21.28137851355727253 3 Comparative Example 13 21.48138252505726153

[0127] Classification Steel Grade No. Mf (°C) Secondary Cooling Termination Temperature (T2) (°C) Mf-T2 (°C) Secondary Average Cooling Rate (°C / s) Reheating / Overaging Treatment Temperature (°C) Overaging Treatment Time (Min) Tension Leveling Elongation (%) Invention Example 1 1 1 7 3 9 5 7 8 1 1 5 2 0 4 8.7 0.15 Invention Example 2 2 1 8 9 8 6 1 0 3 1 2 5 1 9 5 8.5 0.20 Invention Example 3316595701301938.60.20 Invention Example 44164105591251858.90.15 Invention Example 5514395481301869.00.20 Comparative Example 162971051921251958.60.25 Comparative Example 27295952001351988.40.20 Comparative Example 3815090 601452018.60.15 Comparative Example 492871051821352058.50.15 Comparative Example 510514561451959.70.20 Comparative Example 6117394791422069.20.20 Comparative Example 71173115581351898.90.20 Comparative Example 81173105681251958 .50.20 Comparative Example 91173105681152038.40.20 Comparative Example 10218995941351978.90.20 Comparative Example 11218998911252079.60.20 Comparative Example 122189106831351859.70.20 Comparative Example 13218994951251799.50.20

[0128] Classification Microstructure Ti-based carbonitrides Nb-based carbonitrides Boundary density (mm²) -1)FM+TM(Area %) At least one of F and B (Area %) Equivalent Average Size (㎛) Fraction (pieces / mm²) Aspect Ratio Fraction of Ti-based carbonitrides with an aspect ratio of 2.5 or higher relative to total Ti-based carbonitrides (%) Equivalent Average Size (㎛) Fraction (pieces / mm²) Invention Example 1 1000 4.6 1.9 7 1.6 5 6.7 4.5 7 2 2 3.25 Invention Example 2 99 14.3 1.8 5 1.5 2 8.5 4.1 8 2 5 2.65 Invention Example 3 1000 4.1 1.7 9 1.7 2 7.8 4.7 7 10 3.15 Invention Example 4 1000 3.9 2.1 11.5 6 6.5 3.9 7 5 9 2.95 Invention Example 5 1000 3.5 1.9 12.0 17.5 4.6 8 15 3.29 Comparative Example 19 8 28.5 3.2 5 3.2 5 2 21.2 1 2 2.1 5 Comparative Example 27 4 26 4.2 2.0 11.5 25.6 3.9 6 9 5 1.7 5 Comparative Example 3 10 0 8.2 6.1 5 3.0 6 20 12.5 5 6 5 4.0 5 Comparative Example 47 3 27 3.8 2.0 5 1.6 5 6.9 4.5 6 8 5 1. 62 Comparative Example 5 1000 5.2 2.3 6 2.2 5 7.6 4.9 75 6 1.0 3 Comparative Example 6 1000 8.1 6.0 6 3.0 2 18.9 5 0 6 3.2 6 Comparative Example 7 1000 7.9 5.9 6 3.0 1 1 9 10.5 8 5 6 3.1 5 Comparative Example 8 1000 4.5 1.9 5 1.8 5 8.5 9.5 9 5 2.9 5 Comparative Example 9 75 2 5 5.9 1.5 6 2.1 2 8.9 9.7 305 1.85 Comparative Example 107 426 5.4 1.8 42.5 6 8.7 8.7 405 1.87 Comparative Example 117 327 4.5 1.7 6 2.0 35.5 3.8 786 1.79 Comparative Example 127 228 3.8 2.5 6 1.8 95.6 5.2 69 01.56 Comparative Example 137 327 4.6 1.9 5 1.8 56 44.5 8 51 1.79 FM: Fresh Martensite, TM: Tempered Martensite, F: Ferrite, B: Bainite

[0129] Yield Strength (YS) (MPa) Tensile Strength (TS) (MPa) Total Elongation (T_EL) (%) Uniform Elongation (U_EL) (%) Impact Value (J / ㎠) Hole Expansion (HER) (%) HER / TS (%MPa -1Invention Example 1 145217696.93.841340.0192 Invention Example 2 128915267.84.256620.0406 Invention Example 3 135216897.54.152580.0343 Invention Example 4 142517157.23.945390.0227 Invention Example 5 152117926. 73.738320.0179 Comparative Example 1 120414738.24.520190.0129 Comparative Example 2 108513848.94.655750.0542 Comparative Example 3 155618355.62.619170.0093 Comparative Example 4 105213658.74.765750.0549 Comparative Example 5165219255.42.312150.0078 Comparative Example 6142517465.93.519160.0092 Comparative Example 7143217526.13.418160.0091 Comparative Example 8139517436.53.420190.0109 Comparative Example 9125615867.23 .520200.0126Comparative Example 10119514688.14.319350.0238Comparative Example 11118514568.24.519340.0234Comparative Example 12105913985.63.118190.0136Comparative Example 13112514425.73.818260.0180

[0130] Figure 1 shows the distribution of tensile strength and hole expansion of Inventive Examples 1 to 5 and Comparative Examples 1 to 13. As can be seen from Tables 1 to 7 and Figure 1, Inventive Examples 1 to 5, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, have excellent mechanical properties by securing the microstructure, Ti-based carbonitrides, Nb-based carbonitrides, and boundary density targeted by the present invention.

[0131] In the case of Comparative Example 1, which does not satisfy the Nb and Ti content, X value, Y value, and Y / X value proposed by the present invention, it can be seen that the impact value, hole expansion ability, and HER / TS are insufficient because the Ti-based carbonitride and Nb-based carbonitride targeted by the present invention were not secured.

[0132] In the case of Comparative Example 2, which does not satisfy the C content, X value, and Y value proposed by the present invention, it can be seen that the yield strength, tensile strength, and HER / TS are insufficient because the microstructure and boundary density targeted by the present invention are not secured.

[0133] In the case of Comparative Example 3, which does not satisfy the Nb content, X value, and Y value proposed by the present invention, it can be seen that the impact value, hole expansion ability, and HER / TS are insufficient because the Ti-based carbonitride, Nb-based carbonitride, and boundary density targeted by the present invention are not secured.

[0134] In the case of Comparative Example 4, which does not satisfy the Mn content, X value, and Y value proposed by the present invention, it can be seen that the yield strength, tensile strength, and HER / TS are insufficient because the microstructure and boundary density targeted by the present invention are not secured.

[0135] In the case of Comparative Example 5, which does not satisfy the Mn content, X value, Y value, and Mf-T2 proposed by the present invention, it can be seen that the uniform elongation, impact value, hole expansion, and HER / TS are at an insufficient level because the boundary density targeted by the present invention is not secured.

[0136] In the case of Comparative Example 6, which satisfies the alloy composition proposed by the present invention but does not satisfy the molten steel temperature in the tundish, it can be seen that the impact value, hole expansion ability, and HER / TS are insufficient because the Ti-based carbonitride targeted by the present invention is not secured.

[0137] In the case of Comparative Example 7, which satisfies the alloy composition proposed by the present invention but does not satisfy the secondary cooling ratio and finishing hot rolling temperature, it can be seen that the impact value, hole expansion ability, and HER / TS are insufficient because the Ti-based carbonitride and Nb-based carbonitride targeted by the present invention are not secured.

[0138] In the case of Comparative Example 8, which satisfies the alloy composition proposed by the present invention but does not satisfy the slab heating temperature, it can be seen that the impact value, hole expansion ability, and HER / TS are insufficient because the Nb-based carbonitride targeted by the present invention is not secured.

[0139] In the case of Comparative Example 9, which satisfies the alloy composition proposed by the present invention but does not satisfy the coiling temperature, it can be seen that the impact value and HER / TS are insufficient because the microstructure, Nb-based carbonitride, and boundary density targeted by the present invention are not secured.

[0140] It can be seen that in the case of Comparative Example 10, which satisfies the alloy composition proposed by the present invention but has a high annealing temperature, the microstructure and boundary density targeted by the present invention are not secured, and thus the yield strength, tensile strength, and impact value are insufficient.

[0141] In the case of Comparative Example 11, which satisfies the alloy composition proposed by the present invention but does not satisfy the annealing time, it can be seen that the yield strength, tensile strength, and impact value are insufficient because the microstructure and boundary density targeted by the present invention are not secured.

[0142] It can be seen that in the case of Comparative Example 12, which satisfies the alloy composition proposed by the present invention but has a low annealing temperature, the microstructure and boundary density targeted by the present invention are not secured, and thus the yield strength, tensile strength, impact value, hole expansion, and HER / TS are at an insufficient level.

[0143] In the case of Comparative Example 13, which satisfies the alloy composition proposed by the present invention but does not satisfy the first cooling termination temperature, it can be seen that the yield strength, tensile strength, and impact value are insufficient because the microstructure and boundary density targeted by the present invention are not secured.

Claims

1. In wt%, Carbon (C): 0.160~0.350%, Silicon (Si): 0.010~0.80%, Manganese (Mn): 0.30~2.60%, Phosphorus (P): 0.030% or less (excluding 0%), Sulfur (S): 0.0050% or less (excluding 0%), Aluminum (Al): 0.0030~0.080%, Chromium (Cr): 0.0010~0.60%, Molybdenum (Mo): 0.0010~0.40%, Niobium (Nb): 0.0010~0.10%, Titanium (Ti): 0.0050~0.20%, Boron (B): 0.00050~0.0070%, Nitrogen (N): 0.010% or less (excluding 0%), Copper (Cu): 0.0010~0.30%, Nickel (Ni): 0.0010~0.30%, containing the remainder Fe and other unavoidable impurities, It contains 0.10 to 5.90 Ti-based carbonitrides / mm² with an average original size of 3.0 to 8.0 µm, and It contains 100 to 2000 Nb-based carbonitrides / mm² with an average original size of 0.20 to 9.0 µm, and Boundary density is 1.90~4.0mm -1 Cold-rolled steel sheet.

2. In Paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet satisfying the following equations 1 to 3. [Relation 1] 50 ≤ [Equation 2] 5 ≤ Y = -26C + 1150Ti - 100Nb + 1290N ≤ 260 [Equation 3] 0.02 ≤ Y / X ≤ 4.0 3. In Paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet in which, in terms of area %, the total of one or more types of ferrite and bainite is 5% or less (including 0%), and the remainder is one or more types of fresh martensite and tempered martensite.

4. In Paragraph 1, The above Ti-based carbonitride is a cold-rolled steel sheet with an aspect ratio of 3.0 or less.

5. In Paragraph 1, The above Ti-based carbonitride is a cold-rolled steel sheet in which the fraction of Ti-based carbonitrides having an aspect ratio of 2.5 or more relative to the total Ti-based carbonitrides is 1 to 18%.

6. In Paragraph 1, The above cold-rolled steel sheet has a yield strength (TS): 1200 MPa or higher, tensile strength (TS): 1470 MPa or higher, total elongation (T_EL): 4.0% or higher, uniform elongation (U_EL): 2.5% or higher, impact value: 21~100 J / ㎠, hole expansion (HER): 20~100%, and hole expansion (HER) / tensile strength (TS): 0.0140~0.0540% MPa -1 Cold-rolled steel sheet.

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

8. In wt%, Carbon (C): 0.160–0.350%, Silicon (Si): 0.010–0.80%, Manganese (Mn): 0.30–2.60%, Phosphorus (P): ≤0.030% (excluding 0%), Sulfur (S): ≤0.0050% (excluding 0%), Aluminum (Al): 0.0030–0.080%, Chromium (Cr): 0.0010–0.60%, Molybdenum (Mo): 0.0010–0.40%, Niobium (Nb): 0.0010–0.10%, Titanium (Ti): 0.0050–0.20%, Boron (B): 0.00050–0.0070%, Nitrogen (N): ≤0.010% (excluding 0%), Copper (Cu): A step of preparing molten steel containing 0.0010~0.30% nickel (Ni), 0.0010~0.30% nickel, and the remainder being Fe and other unavoidable impurities; A step of pouring the above molten steel into a tundish at a temperature of 1545℃ or lower; A step of obtaining a slab by pouring the molten steel injected into the above tundish into a mold, and then continuously casting such that the secondary cooling ratio is 0.50 to 3.0 L / kg·s; A step of heating the above slab at 1100~1300℃; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above heated slab; A step of coiling the above hot-rolled steel sheet at 300~700℃; A step of obtaining a cold-rolled steel sheet by cold-rolling the above-mentioned coiled hot-rolled steel sheet; A step of continuously annealing the above cold-rolled steel sheet at Ac3+5℃ to Ac3+65℃ for 50 to 250 seconds; A step of first cooling the above continuously annealed cold-rolled steel sheet to a first cooling end temperature (T1) of Bs+50℃~Ac3; A step of secondarily cooling the above first cooled cold-rolled steel sheet to a second cooling end temperature (T2) of 40 to 250℃; A step of reheating the above secondary cooled cold-rolled steel sheet and then over-aging it; and A method for manufacturing a cold-rolled steel sheet comprising the step of tension-leveling the cold-rolled steel sheet that has been reheated and over-aged.

9. In Paragraph 8, The above molten steel is a method for manufacturing a cold-rolled steel sheet satisfying the following equations 1 to 3. [Relation 1] 50 ≤ [Equation 2] 5 ≤ Y = -26C + 1150Ti - 100Nb + 1290N ≤ 260 [Equation 3] 0.02 ≤ Y / X ≤ 4.0 10. In Paragraph 8, The above finishing hot rolling is a method for manufacturing cold-rolled steel sheets performed at Ar3 to Ar3+120℃.

11. In Paragraph 8, The above cold rolling is a method for manufacturing a cold-rolled steel sheet, performed with a cold reduction rate of 35 to 70%.

12. In Paragraph 8, A method for manufacturing a cold-rolled steel sheet in which the above first cooling is performed at a first average cooling rate (CR1) of 0.5 to 6.0℃ / s.

13. In Paragraph 8, A method for manufacturing cold-rolled steel sheets in which the above secondary cooling is performed at a secondary average cooling rate (CR2) of 40 to 300℃ / s.

14. In Paragraph 8, A method for manufacturing a cold-rolled steel sheet in which, during the above secondary cooling, the Mf-secondary cooling end temperature (T2) is controlled to be 20℃ or higher.

15. In Paragraph 8, The above overaging treatment is a method for manufacturing cold-rolled steel sheets, performed at 140 to 260°C for 3 to 14 minutes.

16. In Paragraph 8, A method for manufacturing a cold-rolled steel sheet in which the above tension leveling is performed with an elongation of 0.05 to 0.55%.

17. In Paragraph 8, A method for manufacturing a cold-rolled steel sheet, further comprising the step of forming a plating layer on at least one surface of the cold-rolled steel sheet after the above tension leveling.

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