Ultra-high strength cold-rolled steel sheet and method for manufacturing same

Abstract

Provided is a cold-rolled steel sheet consisting of carbon (C): 0.23 wt% to 0.35 wt%, silicon (Si): 0.05 wt% to 0.5 wt%, manganese (Mn): 0.3 wt% to 2.3 wt%, phosphorus (P): more than 0 wt% and not more than 0.02 wt%, sulfur (S): more than 0 wt% and not more than 0.005 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): more than 0 wt% and not more than 0.8 wt%, molybdenum (Mo): more than 0 wt% and not more than 0.4 wt%, titanium (Ti): 0.01 wt% to 0.1 wt%, vanadium (V): more than 0 wt% and not more than 0.3 wt%, boron (B): 0.001 wt% to 0.005 wt%, and a balance of iron (Fe) and other unavoidable impurities, wherein a final microstructure of the cold-rolled steel sheet includes cementite, a transition carbide, and a fine precipitate, the transition carbide including ε-carbide having an atomic ratio of a substitutional element selected from Fe, Mn, Cr, and Mo to C of 2.5:1, or η-carbide having an atomic ratio of the substitutional element to C of 2:1.

Description

TECHNICAL FIELD

[0001] The present invention relates to a cold-rolled steel sheet and a method of manufacturing the same, and more particularly, to an ultra-high-strength cold-rolled steel sheet with high yield ratio and excellent bendability and a method of manufacturing the same.BACKGROUND ART

[0002] The demand for vehicle body crashworthiness has continued to grow within the automobile industry. Recently, while the number of vehicle components has decreased with the rise of electric vehicles, the overall weight of vehicles has increased due to the incorporation of batteries. As a result, the demand for crashworthiness is expanding further. Efforts are therefore being made to achieve ultra-high strength in collision-related components, such as front bumper beams, side sills, and door impact beams, which play key roles in enhancing crashworthiness. Particularly, the increased use of roll forming techniques has broadened the application of martensitic steels, which possess the highest strength among various types of cold-rolled steels. The bendability of steel sheets serves as a critical factor due to the characteristics of roll forming.

[0003] The related art includes Japanese Patent Publication No. 2005-105367.DETAILED DESCRIPTION OF THE INVENTIONTECHNICAL PROBLEM

[0004] The present invention provides an ultra-high-strength cold-rolled steel sheet with high yield ratio and excellent bendability and a method of manufacturing the same, and more particularly, a cold-rolled steel sheet capable of implementing martensite steels with a tensile strength of 1400 MPa or more, and a method of manufacturing the same.TECHNICAL SOLUTION

[0005] According to an aspect of the present invention, there is provided a cold-rolled steel sheet consisting of carbon (C): 0.23 wt% to 0.35 wt%, silicon (Si): 0.05 wt% to 0.5 wt%, manganese (Mn): 0.3 wt% to 2.3 wt%, phosphorus (P): more than 0 wt% and not more than 0.02 wt%, sulfur (S): more than 0 wt% and not more than 0.005 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): more than 0 wt% and not more than 0.8 wt%, molybdenum (Mo): more than 0 wt% and not more than 0.4 wt%, titanium (Ti): 0.01 wt% to 0.1 wt%, vanadium (V): more than 0 wt% and not more than 0.3 wt%, boron (B): 0.001 wt% to 0.005 wt%, and a balance of iron (Fe) and other unavoidable impurities, wherein a final microstructure of the cold-rolled steel sheet includes cementite, a transition carbide, and a fine precipitate, the transition carbide including ε-carbide having an atomic ratio of a substitutional element selected from Fe, Mn, Cr, and Mo to C of 2.5:1, or η-carbide having an atomic ratio of the substitutional element to C of 2:1, and the fine precipitate having an atomic ratio of an alloying element selected from Mo, V, and Ti to C of 1:1, and wherein the cold-rolled steel sheet has a yield strength (YP) of 1170 MPa or more, a tensile strength (TS) of 1400 MPa or more, an elongation (EI) of 3.0% or more, a yield ratio of 70% or more, and a bendability (R / t) of 4.0 or less.

[0006] The cementite, the transition carbide, and the fine precipitate may each have an average size of 50 nm or less and an average aspect ratio of 4.0 or less.

[0007] The cementite, the transition carbide, and the fine precipitate may each have an area fraction of more than 0% and not more than 5%.

[0008] The final microstructure may consist of only tempered martensite.

[0009] The final microstructure may consist of tempered martensite, ferrite, and bainite, the tempered martensite having an area fraction of 70% or more and less than 100%, and the ferrite and bainite having an area fraction of more than 0% and not more than 20%.

[0010] According to another aspect of the present invention, there is provided a method of manufacturing a cold-rolled steel sheet, the method including (a) hot rolling a steel material consisting of carbon (C): 0.23 wt% to 0.35 wt%, silicon (Si): 0.05 wt% to 0.5 wt%, manganese (Mn): 0.3 wt% to 2.3 wt%, phosphorus (P): more than 0 wt% and not more than 0.02 wt%, sulfur (S): more than 0 wt% and not more than 0.005 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): more than 0 wt% and not more than 0.8 wt%, molybdenum (Mo): more than 0 wt% and not more than 0.4 wt%, titanium (Ti): 0.01 wt% to 0.1 wt%, vanadium (V): more than 0 wt% and not more than 0.3 wt%, boron (B): 0.001 wt% to 0.005 wt%, and a balance of iron (Fe); (b) cold rolling the hot-rolled steel material; and (c) sequentially performing annealing, first heat treatment, and second heat treatment on the cold-rolled steel material, wherein a final microstructure of the cold-rolled steel sheet obtained by performing steps (a) to (c) includes cementite, a transition carbide, and a fine precipitate, the transition carbide including ε-carbide having an atomic ratio of a substitutional element selected from Fe, Mn, Cr, and Mo to C of 2.5:1, or η-carbide having an atomic ratio of the substitutional element to C of 2:1, and the fine precipitate having an atomic ratio of an alloying element selected from Mo, V, and Ti to C of 1:1, and wherein the cementite is formed during the first heat treatment, the transition carbide is formed during the second heat treatment, and the fine precipitate is formed during the hot rolling.

[0011] Step (a) may be performed under conditions of a reheating temperature of 1150 °C to 1300 °C, a finishing delivery temperature of 800 °C to 1000 °C, and a coiling temperature of 500 °C to 650 °C, and step (c) may be performed under conditions of an annealing temperature of 800 °C to 900 °C and a first heat treatment temperature of 100°C to 300 °C, and the second heat treatment may include maintaining a second heat treatment temperature T satisfying Inequality 1 for a second heat treatment holding time t. 3800 ≤ T + 300 × 10 + log t ≤ 5650 (where a unit of T is °C and a unit of t is hours).

[0012] Step (a) may be performed under conditions of a reheating temperature of 1150 °C to 1300 °C, a finishing delivery temperature of 800 °C to 1000 °C, and a coiling temperature of 500 °C to 650 °C, and step (c) may include performing coating and be performed under conditions of an annealing temperature of 800 °C to 900 °C and a first heat treatment temperature of 450 °C to 600 °C, and the second heat treatment may include maintaining a second heat treatment temperature T satisfying Inequality 1 for a second heat treatment holding time t. 3800 ≤ T + 300 × 10 + log t ≤ 5650 (where a unit of T is °C and a unit of t is hours).

[0013] In step (c), the first heat treatment, following the annealing, may be performed after cooling to a first heat treatment temperature.

[0014] In step (c), the second heat treatment, following the first heat treatment, may be performed after cooling to room temperature and then raising the temperature.ADVANTAGEOUS EFFECTS

[0015] According to an embodiment of the present invention, an ultra-high-strength cold-rolled steel sheet with high yield ratio and excellent bendability and a method of manufacturing the same may be implemented. For example, according to the present invention, a high-strength cold-rolled steel sheet with a high tensile strength, a high yield ratio (YP / TS) of more than 70%, and an excellent bendability (R / t) of 4.0 or less may be implemented. As such, the use of a material with excellent impact absorbability and formability for creating complex-shaped components is expected to enhance passenger safety and improve fuel efficiency through vehicle body lightweighting.

[0016] However, the scope of the present invention is not limited to the above effect.DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1 includes microscopic images showing a result of analyzing cementite (Fe 3 C) among carbides in a final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention. FIG. 2 includes microscopic images showing a result of analyzing ε-carbide (Fe 2.5 C) among carbides in a final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention. FIG. 3 is a schematic view for describing a method of measuring the size of carbides in a final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention. FIG. 4 is a graph showing the distribution of carbide sizes in a final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention. FIG. 5 is a graph showing the distribution of carbide aspect ratios in a final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention. FIG. 6 is a graph showing a step of sequentially performing annealing, first heat treatment, and second heat treatment in a method of manufacturing a cold-rolled steel sheet, according to an embodiment of the present invention. FIG. 7 is a microscopic image of a final microstructure according to Test Example 20 of the present invention. FIG. 8 is a microscopic image of a final microstructure according to Test Example 21 of the present invention. FIG. 9 is a microscopic image of a final microstructure according to Test Example 22 of the present invention. FIG. 10 is a microscopic image of a final microstructure according to Test Example 23 of the present invention. FIG. 11 is a graph showing a second heat treatment condition in a method of manufacturing a cold-rolled steel sheet, according to test examples of the present invention. MODE OF THE INVENTION

[0018] A cold-rolled steel sheet and a method of manufacturing the same, according to an embodiment of the present invention, will now be described in detail. The terms used herein are selected based on their functions in the present invention, and their definitions should be made in the context of the entire specification. A detailed description of an ultra-high-strength cold-rolled steel sheet with high yield ratio and excellent bendability and a method of manufacturing the same will be provided below.

[0019] A cold-rolled steel sheet according to an embodiment of the present invention consists of carbon (C): 0.23 wt% to 0.35 wt%, silicon (Si): 0.05 wt% to 0.5 wt%, manganese (Mn): 0.3 wt% to 2.3 wt%, phosphorus (P): more than 0 wt% and not more than 0.02 wt%, sulfur (S): more than 0 wt% and not more than 0.005 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): more than 0 wt% and not more than 0.8 wt%, molybdenum (Mo): more than 0 wt% and not more than 0.4 wt%, titanium (Ti): 0.01 wt% to 0.1 wt%, vanadium (V): more than 0 wt% and not more than 0.3 wt%, boron (B): 0.001 wt% to 0.005 wt%, and a balance of iron (Fe) and other unavoidable impurities.

[0020] The functions and contents of the components included in the cold-rolled steel sheet will now be described.Carbon (C)

[0021] C is the most effective and important element for increasing the strength of steel. C is added and dissolved in austenite to form a martensite structure when quenched. Furthermore, C combines with elements such as Fe, Cr, and Mo to form carbides and enhances strength and hardness. C may be added at a content ratio of 0.23 wt% to 0.35 wt% of a total weight in a base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of C is less than 0.23 wt% of the total weight, the above-described effect may not be achieved and a sufficient strength may not be ensured. On the other hand, when the content of C is greater than 0.35 wt% of the total weight, weldability and workability may be reduced.Silicon (Si)

[0022] Si is an element added to ensure bendability and hydrogen embrittlement resistance by suppressing the formation of cementite. Si is also an element added to increase strength and suppress the formation of cementite due to the solid solution strengthening effect in ferrite. Si is well known as a ferrite stabilizing element and thus may improve ductility by increasing the fraction of ferrite during cooling. Si is also known as an element capable of ensuring strength by promoting the formation of martensite through carbon enrichment in austenite. Meanwhile, Si may be added together with Al as a deoxidizer for removing oxygen from steel during a steelmaking process, and have a solid solution strengthening effect. Si may be added at a content ratio of 0.05 wt% to 0.5 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of Si is less than 0.05 wt% of the total weight, ductility may not be ensured and the above-described effects of Si addition may not be properly realized. On the other hand, when the content of Si is greater than 0.5 wt% of the total weight due to excessive addition, ferrite may be excessively formed to reduce strength, oxide may be formed on the surface of the steel sheet to reduce the coatability of the steel sheet, red scale may be formed during reheating and hot rolling to degrade the surface quality, toughness and plasticity may be reduced, and the weldability of steel may also be reduced.Manganese (Mn)

[0023] Mn is an element that contributes to strength enhancement by improving solid solution strengthening and quenchability. For example, Mn is an element that facilitates the formation of low-temperature transformation phases and provides the effect of increasing strength through solid solution strengthening. Some of Mn is dissolved in steel and the other combines with S contained in the steel to form non-metallic inclusions such as MnS. MnS has ductility and thus elongates in the direction of plastic working. However, due to the formation of MnS, the content of S in the steel is reduced to make the grains susceptible and suppress the formation of FeS, a low-melting-point compound. Mn reduces the acid resistance and oxidation resistance of steel, but increases yield strength by refining pearlite and solid-solution-strengthening ferrite. Mn may be added at a content ratio of 0.3 wt% to 2.3 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of Mn is less than 0.3 wt%, the above-described effect of strength enhancement may not be sufficiently realized. When the content of Mn is greater than 2.3 wt%, bendability and hydrogen embrittlement resistance may be reduced due to the formation of Mn bands and MnS. For example, segregation zones may be formed inside and outside the continuously casted slab and the steel sheet and the formation and propagation of cracks may be caused to reduce bendability. That is, slab quality and weldability may be reduced, and center segregation may occur to reduce the ductility and workability of the base steel sheet.Phosphorus (P)

[0024] P may serve to increase the strength of steel through solid solution strengthening and suppress the formation of carbides. P may be added at a content ratio of more than 0 wt% and not more than 0.02 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of P is greater than 0.02 wt%, welded joints may become embrittled, brittleness may be caused by grain boundary segregation, press formability may be reduced, and impact resistance may be lowered.Sulfur (S)

[0025] S is an element that combines with Mn or Ti to improve the machinability of steel and forms fine MnS precipitates to enhance workability, but generally hinders ductility and weldability. S may be added at a content ratio of more than 0 wt% and not more than 0.005 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of S is greater than 0.005 wt%, the number of MnS inclusions may be increased to reduce bendability and hydrogen embrittlement resistance, and segregation may occur during continuous casting solidification to cause high-temperature cracks.Aluminum (Al)

[0026] Al is an element commonly used as a deoxidizer, and prevents slab cracks during the formation of nitrides, promotes the formation of ferrite to enhance elongation, suppresses the formation of carbides, and stabilizes austenite by increasing the concentration of C in austenite. Al serves as a layer between Fe and zinc (Zn) coating to enhance coatability, and effectively suppresses the formation of Mn bands in a hot-rolled coil. Al may be added at a content ratio of 0.01 wt% to 0.05 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of Al is less than 0.01 wt%, the above-described effects of Al addition may not be properly realized. On the other hand, when the content of Al is greater than 0.05 wt% due to excessive addition, strength may be reduced due to the formation of ferrite, Al inclusions may be increased to reduce continuous castability, the enrichment of Al may occur on the surface of the steel sheet to deteriorate coatability, and AIN may be formed in the slab to cause hot rolling cracks.Chromium (Cr)

[0027] Cr is an element capable of enhancing hardenability and ensuring high strength, and may improve quenchability as an austenite stabilizing element. Cr increases elongation by forming Cr-based precipitates in the grains during annealing. Cr may be added at a content ratio of more than 0 wt% and not more than 0.8 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of Cr is greater than 0.8 wt% due to excessive addition, the saturation effect may occur, laser weldability and ductility may be reduced, and coatability may be hindered.Molybdenum (Mo)

[0028] Mo is an element added to improve quenchability and ensure strength and toughness, and is also an element capable of enhancing hydrogen embrittlement resistance due to the grain refinement and precipitation effect. Mo may be added at a content ratio of more than 0 wt% and not more than 0.4 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of Mo is greater than 0.4 wt%, production costs may increase and weldability may decrease.Titanium (Ti)

[0029] Ti contributes to grain refinement and BN formation suppression. Ti may be added at a content ratio of 0.01 wt% to 0.1 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of Ti is less than 0.01 wt%, a reduction in ductility of the casting slab due to excessive formation of BN precipitates may reduce slab quality and strength. Meanwhile, when the content of Ti is greater than 0.1 wt%, bendability and hydrogen embrittlement resistance may be reduced due to the coarsening of TiN precipitates, and recrystallization temperature may be excessively increased to cause a non-uniform structure.Boron (B)

[0030] B is an element added to increase the hardenability of steel by suppressing the formation of ferrite. B is also a strong quenching element and serves to enhance strength by preventing the segregation of P. Because secondary work embrittlement may be caused when the segregation of P occurs, B may be added to prevent the segregation of P and increase resistance to work embrittlement. B may be added at a content ratio of 0.001 wt% to 0.005 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of B is less than 0.001 wt%, strength may not be ensured due to low quenchability. When the content of B is greater than 0.005 wt% due to excessive addition, grain boundary embrittlement may be increased due to the formation of BN, weldability may be reduced, and the surface quality of the steel may be hindered due to the formation of B oxides.

[0031] Meanwhile, the ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention may optionally contain V. The addition of a microalloying element such as V contributes to strength enhancement through the formation of fine precipitates (VC or (Ti,V)C) which are different from cementite and transition carbides. When the ultra-high-strength cold-rolled steel sheet according to an embodiment of the present invention optionally includes V, V may be added at a content ratio of more than 0 wt% and not more than 0.3 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of V is greater than 0.3 wt% due to excessive addition, the production costs of steel may be significantly increased, the rolling load may also be significantly increased due to excessive precipitation during rolling, and elongation may be reduced.

[0032] The remainder of the composition of the ultra-high-strength cold-rolled steel sheet is Fe. However, unintended impurities may be inevitably introduced from raw materials or the ambient environment during a general production process and, as a result, the addition of these impurities is not avoidable. Such impurities are known to one of ordinary skill in the art and thus are not particularly described in this specification.

[0033] FIG. 1 includes microscopic images showing a result of analyzing cementite (Fe 3 C) among carbides in a final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention, FIG. 2 includes microscopic images showing a result of analyzing ε-carbide (Fe 2.5 C) among carbides in a final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention, FIG. 3 is a schematic view for describing a method of measuring the size of carbides in a final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention, FIG. 4 is a graph showing the distribution of carbide sizes in a final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention, and FIG. 5 is a graph showing the distribution of carbide aspect ratios in a final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention.

[0034] Referring to FIGS. 1 to 5, the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention includes cementite, a transition carbide, and a fine precipitate. The cementite (Fe 3 C) has an atomic ratio of Fe to C of 3:1. The transition carbide includes ε-carbide having an atomic ratio of a substitutional element selected from Fe, Mn, Cr, and Mo to C of 2.5:1, or η-carbide having an atomic ratio of the substitutional element to C of 2:1. The fine precipitate has an atomic ratio of an alloying element selected from Mo, V, and Ti to C of 1:1. The carbides and the fine precipitate may partially include nitrogen (N).

[0035] In the cold-rolled steel sheet according to an embodiment of the present invention, to ensure bendability and hydrogen embrittlement resistance, the cementite, the transition carbide, and the fine precipitate may each have an average size of 50 nm or less and an average aspect ratio of 4.0 or less. Referring to FIG. 3, the average size represents an average size including the major and minor axes of oval or acicular carbide particles 10, and more specifically, to an average size including a minor axis length a and a major axis length b. The average aspect ratio represents a ratio of the major axis length to the minor axis length, b / a. In the cold-rolled steel sheet according to an embodiment of the present invention, the cementite, the transition carbide, and the fine precipitate may each have an area fraction of more than 0% and not more than 5%. The area fractions of the cementite, the transition carbide, and the fine precipitate were measured using at least five microscopic images of the microstructures through replica analysis with a scanning electron microscope.

[0036] The final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention may consist of only tempered martensite. Alternatively, the final microstructure of a cold-rolled steel sheet according to another embodiment of the present invention may consist of tempered martensite, ferrite, and bainite, the tempered martensite having an area fraction of 70% or more and less than 100%, and the ferrite and / or bainite having an area fraction of more than 0% and not more than 20%. For example, the final microstructure may consist of tempered martensite, ferrite, and bainite, the tempered martensite having an area fraction of 70% or more and less than 100%, the bainite having an area fraction of more than 0% and not more than 20%, and the ferrite having an area fraction of more than 0% and not more than 10%.

[0037] Meanwhile, the final microstructure of a cold-rolled steel sheet according to another modified embodiment of the present invention may consist of only tempered martensite. In this case, bainite and ferrite are not present.

[0038] The above-described microstructure is based on the result of analyzing a 1 / 4 point of a thickness direction from a direction perpendicular to the rolling direction with the scanning electron microscope. In the present invention, when the area fraction of the tempered martensite is less than 70%, the desired strength may not be achieved. Furthermore, in the present invention, because the ferrite and bainite are inevitably formed due to an insufficient cooling rate and serve as a main factor for reducing strength, the smaller the area fractions thereof, the better. The sum of area fractions of the two phases of ferrite and bainite is required not to exceed 20%.

[0039] The cold-rolled steel sheet with the above-described alloying element composition and microstructure according to an embodiment of the present invention includes a cementite-type carbide but may achieve the properties of a yield strength (YP) of 1170 MPa or more, a tensile strength (TS) of 1400 MPa or more, an elongation (EI) of 3.0% or more, a yield ratio of 70% or more, and a bendability (R / t) of 4.0 or less. For example, the cold-rolled steel sheet according to an embodiment of the present invention may have a YP of 1170 MPa to 1400 MPa, a TS of 1400 MPa to 1700 MPa, an El of 3.0% to 9.0%, a yield ratio of 70% to 90%, and a bendability (R / t) of 2.0 to 4.0. In the bendability (R / t), R represents the minimum bending radius and t represents the thickness.

[0040] A method of manufacturing the cold-rolled steel sheet with the above-described composition and microstructure, according to an embodiment of the present invention, will now be described.

[0041] The steel sheet manufacturing method according to an embodiment of the present invention includes (a) hot rolling a steel material consisting of C: 0.23 wt% to 0.35 wt%, Si: 0.05 wt% to 0.5 wt%, Mn: 0.3 wt% to 2.3 wt%, P: more than 0 wt% and not more than 0.02 wt%, S: more than 0 wt% and not more than 0.005 wt%, Al: 0.01 wt% to 0.05 wt%, Cr: more than 0 wt% and not more than 0.8 wt%, Mo: more than 0 wt% and not more than 0.4 wt%, Ti: 0.01 wt% to 0.1 wt%, V: more than 0 wt% and not more than 0.3 wt%, B: 0.001 wt% to 0.005 wt%, and a balance of Fe; (b) cold rolling the hot-rolled steel material; and (c) sequentially performing annealing, first heat treatment, and second heat treatment on the cold-rolled steel material.

[0042] The hot rolling step (a) may be performed under conditions of a reheating temperature of 1150 °C to 1300 °C, a finishing delivery temperature of 800 °C to 1000 °C, and a coiling temperature of 500 °C to 650 °C.

[0043] When the steel material is reheated at the above-mentioned temperature of 1150 °C to 1300 °C, the components segregated during continuous casting may be redissolved. To enhance strength through precipitation and solid solution strengthening, strengthening elements need to be sufficiently dissolved in austenite before hot rolling and thus the steel material needs to be heated to 1150 °C or above. When the reheating temperature is lower than 1150 °C, various carbides may not be dissolved sufficiently, and the components segregated during continuous casting may not be uniformly distributed. However, a reheating temperature higher than 1300 °C may cause adverse effects such as austenite coarsening and decarburization and prevent strength from being ensured. That is, when the reheating temperature is higher than 1300 °C, very coarse austenite grains may be formed and thus strength may not be easily ensured. In addition, when the reheating temperature is higher than 1300 °C, heating costs and process time may be increased to increase production costs and reduce productivity.

[0044] The finishing delivery temperature (FDT) is an important factor affecting the final material properties, and rolling at 800 °C to 1000 °C may refine austenite. However, when the hot rolling temperature is lower than 800 °C, the rolling load may increase and a mixed grain structure may occur at the edge. Rolling at a temperature higher than 1000 °C may cause grain coarsening and prevent the desired mechanical properties from being achieved. After hot rolling, cooling is performed at a cooling rate of 1 °C / s to 100 °C / s. The higher the cooling rate, the smaller the average grain size.

[0045] Meanwhile, when the coiling temperature is lower than 500 °C, the hot-rolled coil may have a non-uniform shape and the cold rolling load may be increased. When the coiling temperature is higher than 650 °C, a non-uniform microstructure may occur due to the difference in cooling rate between the center and edge of the steel sheet, and the inside of the grain boundaries may be oxidized.

[0046] Meanwhile, the hot rolling may be performed under a condition of a reduction ratio of 35% to 65%. The microstructure of the steel material after the hot rolling may include bainite, martensite, and ferrite.

[0047] The cold rolling step (b) may include performing pickling and then performing cold rolling at a reduction ratio of 35% to 65%. The higher the reduction ratio, the greater the increase in formability due to the microstructural refinement effect. When the cold rolling is performed at a reduction ratio less than 35%, a uniform microstructure may not be easily obtained, and when the cold rolling is performed at a reduction ratio greater than 65%, the roll force may increase and thus the process load may also increase.

[0048] FIG. 6 is a graph showing a step of sequentially performing annealing, first heat treatment, and second heat treatment in a method of manufacturing a cold-rolled steel sheet, according to an embodiment of the present invention.

[0049] Referring to FIG. 6, the cold-rolled steel material is heated to a temperature of Ac3 or above at a heating rate of 1 °C / s to 10 °C / s. The temperature Ac3 may be calculated as shown below. Ac 3 C <mprescripts / > <none / > ∘ = 910 − 203 C 0.5 − 30 Mn + 44.7 Si + 31.5 Mo − 15.2 Ni , where [C], [Mn], [Si], [Mo], and [Ni] represent wt% values of C, Mn, Si, Mo, and nickel (Ni) in the steel material.

[0050] Based on the cold-rolled steel sheet manufacturing method according to an embodiment of the present invention, an annealing process for maintaining a temperature of Ac3 or above, and more specifically, an annealing temperature between 800 °C to 900 °C, for 60 sec. to 600 sec. is performed.

[0051] Subsequently, cooling is performed to 500 °C to 700 °C at a cooling rate of 1 °C / s to 20 °C / s, and then to a martensite finish temperature (or cooling end temperature) at a cooling rate of 5 °C / s to 50 °C / s. Herein, the martensite finish temperature is 100 °C to 350 °C.

[0052] After that, for a non-coated steel sheet, a first heat treatment process for maintaining a first heat treatment temperature of 100 °C to 300 °C for 10 sec. to 100 sec. and then performing cooling to room temperature at a cooling rate of 20 °C / s or less is performed.

[0053] When the first heat treatment temperature is lower than 100 °C, cementite is not formed at all regardless of the holding time. When the temperature of 100 °C to 300 °C is maintained for a short holding time of 10 sec. or less, cementite may not be formed. When the first heat treatment temperature is higher than 300 °C or when the temperature of 100 °C to 300 °C is maintained for 100 sec. or longer, the desired strength may not be ensured due to the formation of bainite.

[0054] Meanwhile, for a coated steel sheet, a first heat treatment process for maintaining a first heat treatment temperature of 450 °C to 600 °C for 5 sec. to 60 sec. and then performing cooling to room temperature at a cooling rate of 20 °C / s or less is performed. When the cooling ends at a temperature of 300 °C or below and then the primary heat treatment is maintained, transformation heat due to the formation of bainite may cause degradation in material properties. Meanwhile, when the cooling ends at a temperature of 450 °C or above, martensite transformation may occur during the cooling due to bainite transformation delay (up to 60 sec.) and thus the material properties may be ensured.

[0055] When the first heat treatment temperature is lower than 450 °C, the temperature of a coating bath may be reduced to deteriorate the coating and alloying quality and thus a coated steel sheet may not be manufactured. When the temperature of 450 °C to 600 °C is maintained for 5 sec. or less, a coating solution may not be sufficiently coated on the steel sheet and thus the coating quality may not be ensured. When the first heat treatment temperature is higher than 600 °C, dross and ash occur while passing through the coating bath and thus the surface quality may not be ensured. When the first heat treatment is performed at the temperature of 450 °C to 600 °C for longer than 60 sec., the desired tensile strength may not be ensured due to the formation and increased fraction of bainite.

[0056] When the first heat treatment temperature is maintained in the range between 300 °C and 450 °C, strength degradation is caused by transformation heat due to the formation of bainite.

[0057] To completely end the transformation of martensite, the lower the first heat treatment temperature in the above-mentioned temperature range, the better.

[0058] The cold-rolled steel sheet manufacturing method according to an embodiment of the present invention is characterized in that the cooling after the annealing and before the first heat treatment is performed only to the first heat treatment temperature without applying a rapid cooling process to room temperature. When the rapid cooling process to room temperature is applied after the annealing and before the first heat treatment, even after the first heat treatment is performed, the final microstructure of the cold-rolled steel sheet does not include cementite. However, as in the present invention, when the cooling after the annealing and before the first heat treatment is performed only to the first heat treatment temperature without applying the rapid cooling process to room temperature, cementite may be formed during the first heat treatment. In general, a steel material including cementite has poor workability.

[0059] However, the rapid cooling process to room temperature after the annealing and before the first heat treatment to fundamentally prevent the formation of cementite increases production costs due to the addition of equipment or the like.

[0060] In the cold-rolled steel sheet according to an embodiment of the present invention, because the rapid cooling process to room temperature is not applied after the annealing and before the first heat treatment, although the final microstructure includes cementite, the properties of a YP of 1170 MPa or more, a TS of 1400 MPa or more, an El of 3.0% or more, a yield ratio of 70% or more, and a bendability (R / t) of 4.0 or less may be ensured by precisely controlling subsequent processes. Thus, an ultra-high-strength cold-rolled steel sheet with high yield ratio and excellent bendability may be implemented.

[0061] The second heat treatment, following the first heat treatment, may be performed after cooling to room temperature and then raising the temperature. The second heat treatment may include a process of maintaining a second heat treatment temperature T satisfying Inequality 1 for a second heat treatment holding time t. 3800 ≤ T + 300 × 10 + log t ≤ 5650 (where the unit of T is °C and the unit of t is hours).

[0062] In the ultra-high-strength cold-rolled steel sheet according to the present invention, when the value of Inequality 1 is lower than 3800, the yield strength of the cold-rolled steel sheet may be insufficient, and when the value of Inequality 1 is higher than 5650, the desired material properties may not be ensured due to bendability deterioration.

[0063] The second heat treatment satisfies Inequality 1 and may include, for example, heating to a temperature of 100°C to 210°C at a heating rate of 10 °C / s or less and then maintaining the second heat treatment temperature T of 100 to 210 °C for 3 hours to 20 hours.

[0064] The final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention, which is formed by applying the above-described process conditions, includes cementite, a transition carbide, and a fine precipitate, the transition carbide including ε-carbide having an atomic ratio of a substitutional element selected from Fe, Mn, Cr, and Mo to C of 2.5:1, or η-carbide having an atomic ratio of the substitutional element to C of 2:1, and the fine precipitate having an atomic ratio of an alloying element selected from Mo, V, and Ti to C of 1:1.

[0065] The cementite is formed during the first heat treatment after the annealing and cooling. When the first heat treatment temperature is 100 °C to 300 °C, the cementite is present in martensite. When the first heat treatment temperature is 450 °C to 600 °C, the cementite is not formed in martensite but is present in bainite when the bainite is present. The cementite is present at a ratio of 0% to 5% of a total area fraction, and the lower the ratio, the better.

[0066] The transition carbide is formed during the second heat treatment. In the cold-rolled steel sheet manufacturing method according to an embodiment of the present invention, when the second heat treatment is not performed, the transition carbide is not present. The transition carbide is required to increase yield strength, and may be present at a ratio of 0% to 5% of the total area fraction.

[0067] The fine precipitate is formed during the hot rolling or during the coiling after the hot rolling, and does not contain Fe unlike the cementite and transition carbide. The fine precipitate may be present at a ratio of 0% to 5% of the total area fraction.

[0068] During the second heat treatment, the cementite undergoes growth, and the transition carbide undergoes both formation and growth. However, the fine precipitate does not experience growth when maintained at the low second heat treatment temperature of the present invention. Because the amount of the microalloying element configuring steel is very small, i.e., 0.5 wt% or less, in the present invention, the fine precipitate, which grows depending on the diffusion rate of the microalloying element, may not grow easily when the low temperature of 300 °C or below is maintained for about 100 hours or less. However, the Fe-based cementite and transition carbide form and grow depending on the diffusion rate of C, which is oversaturated in the martensite matrix. Therefore, the formation and growth of the cementite and the transition carbide proceed under the conditions proposed by the present invention. As described above, the formation of transition carbide is a very important factor for ensuring yield strength. As such, the desired yield strength may not be easily ensured under the second heat treatment condition of an excessively low temperature or short time. In addition, the growth of acicular cementite is a factor that deteriorates bendability. Because a relatively high temperature or long time of the second heat treatment accelerates the growth of acicular cementite, an appropriate second heat treatment condition needs to be set to ensure bendability. In the present invention, an ultra-high-strength cold-rolled steel sheet with high yield ratio and excellent bendability may be implemented by controlling the range of Inequality 1.Test Examples

[0069] Test examples will now be described for better understanding of the present invention. However, the following test examples are merely to promote understanding of the present invention, and the present invention is not limited to thereto.1. Composition of Samples

[0070] The present test examples provide samples with the alloying element composition (unit: wt%) of Table 1. [Table 1]Component SystemCSiMnCrMoTiVBFeA0.250.12.00.40.20.030.10.0025Bal.B0.240.11.90.30.20.060.10.0025Bal.C0.220.22.20.30.20.030.10.0022Bal.D0.260.71.80.40.20.030.10.0020Bal.

[0071] In Table 1, Component Systems A and B satisfy the composition of the cold-rolled steel sheet according to an embodiment of the present invention, i.e., C: 0.23 wt% to 0.35 wt%, Si: 0.05 wt% to 0.5 wt%, Mn: 0.3 wt% to 2.3 wt%, P: more than 0 wt% and not more than 0.02 wt%, S: more than 0 wt% and not more than 0.005 wt%, Al: 0.01 wt% to 0.05 wt%, Cr: more than 0 wt% and not more than 0.8 wt%, Mo: more than 0 wt% and not more than 0.4 wt%, Ti: 0.01 wt% to 0.1 wt%, V: more than 0 wt% and not more than 0.3 wt%, B: 0.001 wt% to 0.005 wt%, and a balance of Fe. However, Component System C falls below and does not satisfy the composition range of C: 0.23 wt% to 0.35 wt%, and Component System D exceeds and does not satisfy the composition range of Si: 0.05 wt% to 0.5 wt%.2. Process Conditions and Property Evaluation

[0072] Table 2 shows various heat treatment conditions for samples with the compositions shown in Table 1, and Table 3 shows a result of evaluating the properties after the compositions and heat treatment conditions shown in Tables 1 and 2 are applied.

[0073] In Table 2, 'Component System' represents the composition shown in Table 1, and 'Inequality 1' represents the value obtained by calculating Inequality 1 [(T + 300) × (10 + log(t))]. In Table 3, 'YP (MPa)', 'TS (MPa)', and 'EL (%)' represent a yield strength, a tensile strength, and an elongation of the samples, respectively. [Table 2]Test ExampleComponent SystemAnnealing Temperature (°C)Cooling End Temperature (°C)First Heat Treatment Temperature (°C)Second Heat Treatment Condition (Inequality 1)1B80025025048502B82025025048503A84015015048504A84020020048505A84030030048506A84045045048507A84025025035038A84025025037729A840250250431110A840250250463511A840250250485012A840250250517413A840250250538914A840250250592815A840250250625116A840250250646717A840250250569018A840250250508619A840250250474620A840250250485021B840250250485022C840250250485023D840250250485024A8403503504850 [Table 3] Test ExampleYP (MPa)TS (MPa)EL (%)Yield Ratio (%)Benda bility (R / t)Fine Precipitate Fraction (%)Cementite Fraction (%)Transition Carbide Fraction (%)Carbide Size (nm)Carbide Average Aspect Ratio1119615767.675.93.60.070.492.3341.83.692120515807.976.33.60.070.553.7629.43.883132016558.079.83.10.080.303.0124.02.384119616288.573.53.00.080.382.9733.53.125122615277.080.33.80.081.182.6642.33.616121816516.873.82.80.080.022.2229.42.447112116356.868.63.80.080.44---8114016267.470.13.40.080.42---9120016257.373.82.80.080.612.1322.93.6710123616266.776.02.80.080.662.3121.13.0111124716247.276.82.60.070.513.3530.53.4412127916167.279.12.60.080.433.2233.82.8313134816096.083.82.80.080.753.3832.92.9614133615486.986.34.10.081.912.2834.64.0215132015047.087.84.20.072.391.0534.84.0616136114257.195.54.20.073.391.6635.04.1217127115737.480.84.10.081.013.8862.85.3318122915738.178.13.30.080.902.8933.03.5419120615747.676.63.20.070.602.1129.23.1220124716247.276.82.60.080.912.9130.53.4421118415336.977.22.40.120.511.1226.12.5222109314176.591.03.40.080.670.9921.02.3323107715437.969.82.70.080.310.2825.62.8224113013608.083.12.60.072.222.9857.95.87

[0074] Referring to Tables 1 to 3, Test Examples 1 and 2 show the difference in properties depending on the annealing temperature. Test Examples 1 and 2, which are cold-rolled steel sheets implemented according to embodiments of the present invention and satisfy the annealing temperature range of 800 °C to 900 °C, achieve the properties of a YP of 1170 MPa or more, a TS of 1400 MPa or more, an El of 3.0% or more, a yield ratio of 70% or more, and a bendability (R / t) of 4.0 or less. Furthermore, in the final microstructure of the cold-rolled steel sheets, the cementite, the transition carbide, and the fine precipitate each has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of more than 0% and not more than 5%. Referring to Tables 1 to 3, Test Examples 3 to 6 show the difference in properties depending on the first heat treatment temperature. Test Examples 3 to 5, which are non-coated cold-rolled steel sheets implemented according to embodiments of the present invention and satisfy the first heat treatment temperature range of 100 °C to 300 °C, achieve the properties of a YP of 1170 MPa or more, a TS of 1400 MPa or more, an El of 3.0% or more, a yield ratio of 70% or more, and a bendability (R / t) of 4.0 or less. Furthermore, in the final microstructure of the cold-rolled steel sheets, the cementite, the transition carbide, and the fine precipitate each has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of more than 0% and not more than 5%.

[0075] Test Example 6, which is a coated cold-rolled steel sheet implemented according to an embodiment of the present invention and satisfies the first heat treatment temperature range of 450 °C to 600 °C, achieves the properties of a YP of 1170 MPa or more, a TS of 1400 MPa or more, an El of 3.0% or more, a yield ratio of 70% or more, and a bendability (R / t) of 4.0 or less. Furthermore, in the final microstructure of the cold-rolled steel sheet, the cementite, the transition carbide, and the fine precipitate each has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of more than 0% and not more than 5%.

[0076] On the other hand, Test Example 24, which is annealed at 350 °C and exceeds and does not satisfy the first heat treatment temperature range of 100 °C to 300 °C, does not achieve the desired properties of a YP of 1170 MPa or more and a TS of 1400 MPa or more, and does not satisfy a carbide average size range of 50 nm or less and a carbide average aspect ratio range of 4.0 or less. When the first heat treatment temperature is maintained in the range between 300 °C and 450 °C, strength degradation is caused by transformation heat. However, when the first heat treatment temperature satisfies the range of 450 °C to 600 °C as in Test Example 6, transformation may be suppressed and thus the material properties may be ensured.

[0077] Referring to Tables 1 to 3, the difference in properties depending on the second heat treatment condition is shown. The value of Inequality 1 shown in Table 2 represents the value of [(T + 300) × (10 + log(t))] based on a second heat treatment temperature T and a second heat treatment holding time t. In Inequality 1, the unit of the second heat treatment temperature T is °C, and the unit of the second heat treatment holding time t is hours.

[0078] For example, Test Examples 1 to 6, 11, and 20 to 24 are applied with conditions of a second heat treatment temperature T of 150 °C and a second heat treatment holding time t of 6 hours, Test Example 7 is applied with conditions of a second heat treatment temperature T of 25 °C and a second heat treatment holding time t of 6 hours, Test Example 8 is applied with conditions of a second heat treatment temperature T of 50 °C and a second heat treatment holding time t of 6 hours, Test Example 9 is applied with conditions of a second heat treatment temperature T of 100 °C and a second heat treatment holding time t of 6 hours, Test Example 10 is applied with conditions of a second heat treatment temperature T of 130 °C and a second heat treatment holding time t of 6 hours, Test Example 12 is applied with conditions of a second heat treatment temperature T of 180 °C and a second heat treatment holding time t of 6 hours, Test Example 13 is applied with conditions of a second heat treatment temperature T of 200 °C and a second heat treatment holding time t of 6 hours, Test Example 14 is applied with conditions of a second heat treatment temperature T of 250 °C and a second heat treatment holding time t of 6 hours, Test Example 15 is applied with conditions of a second heat treatment temperature T of 280 °C and a second heat treatment holding time t of 6 hours, Test Example 16 is applied with conditions of a second heat treatment temperature T of 300 °C and a second heat treatment holding time t of 6 hours, Test Example 17 is applied with conditions of a second heat treatment temperature T of 200 °C and a second heat treatment holding time t of 24 hours, Test Example 18 is applied with conditions of a second heat treatment temperature T of 150 °C and a second heat treatment holding time t of 20 hours, and Test Example 19 is applied with conditions of a second heat treatment temperature T of 120 °C and a second heat treatment holding time t of 20 hours.

[0079] Initially, Test Examples 1 to 6 show that, although the annealing temperature and the first heat treatment conditions are changed within the ranges proposed by the present invention, when the second heat treatment temperature T and the second heat treatment holding time t are applied in such a manner that the value of Inequality 1 [(T + 300) × (10 + log(t))] given as the second heat treatment condition satisfies the range between 3800 and 5650, the desired properties are ensured.

[0080] Test Examples 7 and 8 (see group C of FIG. 11) show that, when the value of Inequality 1 given as the second heat treatment condition is lower than 3800, the transition carbide is not formed in the final microstructure and thus the YP fails to reach the desired value (1170 MPa or more).

[0081] Test Examples 9 to 13, 18, and 19 (see group A of FIG. 11) show that, when the value of Inequality 1 given as the second heat treatment condition is between 3800 and 5650, a YP of 1170 MPa or more, a TS of 1400 MPa or more, an El of 3.0% or more, a yield ratio of 70% or more, and a bendability (R / t) of 4.0 or less are satisfied, and the cementite, the transition carbide, and the fine precipitate each has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of more than 0% and not more than 5%.

[0082] Test Examples 14 to 17 (see group B of FIG. 11) show that, when the value of Inequality 1 given as the second heat treatment condition is higher than 5650, deterioration in bendability occurs due to the increase in size and aspect ratio of carbides. That is, the desired property of a bendability (R / t) of 4.0 or less is not achieved and the average aspect ratio of carbides does not satisfy the range of 4.0 or less. In other words, the desired bendability (R / t) of 4.0 or less is not satisfied due to carbide shape defects.

[0083] Meanwhile, referring to Tables 1 to 3 and FIGS. 7 to 10, Test Examples 20 to 23 show the difference in properties depending on the alloy composition. Specifically, Test Examples 20 and 21, which are cold-rolled steel sheets implemented according to embodiments of the present invention and satisfy the composition of C: 0.23 wt% to 0.35 wt%, Si: 0.05 wt% to 0.5 wt%, Mn: 0.3 wt% to 2.3 wt%, P: more than 0 wt% and not more than 0.02 wt%, S: more than 0 wt% and not more than 0.005 wt%, Al: 0.01 wt% to 0.05 wt%, Cr: more than 0 wt% and not more than 0.8 wt%, Mo: more than 0 wt% and not more than 0.4 wt%, Ti: 0.01 wt% to 0.1 wt%, V: more than 0 wt% and not more than 0.3 wt%, B: 0.001 wt% to 0.005 wt%, and a balance of Fe, achieve the properties of a YP of 1170 MPa or more, a TS of 1400 MPa or more, an El of 3.0% or more, a yield ratio of 70% or more, and a bendability (R / t) of 4.0 or less. Furthermore, in the final microstructure of the cold-rolled steel sheets, the cementite, the transition carbide, and the fine precipitate each has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of more than 0% and not more than 5%.

[0084] On the other hand, Test Example 22, which satisfies the annealing, first heat treatment, and second heat treatment conditions of the present invention but falls below and does not satisfy the composition range of C: 0.23 wt% to 0.35 wt%, shows that the desired properties of a YP of 1170 MPa or more and a TS of 1400 MPa or more are not achieved. Test Example 23, which satisfies the annealing, first heat treatment, and second heat treatment conditions of the present invention but exceeds and does not satisfy the composition range of Si: 0.05 wt% to 0.5 wt%, the desired property of a YP of 1170 MPa or more is not achieved due to the transformation of intermediate phases such as ferrite and bainite. Particularly, Test Example 23, which satisfies the carbide size and carbide aspect ratio characteristics, ensures bendability but does not achieve the desired YP because ferrite is formed by more than 10%.

[0085] A cold-rolled steel sheet and a method of manufacturing the same, according to embodiments of the present invention, have been described above. According to the present invention, a high-strength cold-rolled steel sheet with a high tensile strength, a high yield ratio (YP / TS) of more than 70%, and an excellent bendability (R / t) of 4.0 or less may be implemented. As such, the use of a material with excellent impact absorbability and formability for creating complex-shaped components is expected to enhance passenger safety and improve fuel efficiency through vehicle body lightweighting.

[0086] While the present invention has been particularly shown and described with reference to embodiments thereof, it will be understood by one of ordinary skill in the art that various changes in form and details may be made therein without departing from the scope of the present invention as defined by the following claims.

Claims

1. A cold-rolled steel sheet consisting of carbon (C): 0.23 wt% to 0.35 wt%, silicon (Si): 0.05 wt% to 0.5 wt%, manganese (Mn): 0.3 wt% to 2.3 wt%, phosphorus (P): more than 0 wt% and not more than 0.02 wt%, sulfur (S): more than 0 wt% and not more than 0.005 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): more than 0 wt% and not more than 0.8 wt%, molybdenum (Mo): more than 0 wt% and not more than 0.4 wt%, titanium (Ti): 0.01 wt% to 0.1 wt%, vanadium (V): more than 0 wt% and not more than 0.3 wt%, boron (B): 0.001 wt% to 0.005 wt%, and a balance of iron (Fe) and other unavoidable impurities, wherein a final microstructure of the cold-rolled steel sheet comprises cementite, a transition carbide, and a fine precipitate, the transition carbide comprising ε-carbide having an atomic ratio of a substitutional element selected from Fe, Mn, Cr, and Mo to C of 2.5:1, or η-carbide having an atomic ratio of the substitutional element to C of 2:1, and the fine precipitate having an atomic ratio of an alloying element selected from Mo, V, and Ti to C of 1:1, and wherein the cold-rolled steel sheet has a yield strength (YP) of 1170 MPa or more, a tensile strength (TS) of 1400 MPa or more, an elongation (EI) of 3.0% or more, a yield ratio of 70% or more, and a bendability (R / t) of 4.0 or less.

2. The cold-rolled steel sheet of claim 1, wherein the cementite, the transition carbide, and the fine precipitate each has an average size of 50 nm or less and an average aspect ratio of 4.0 or less.

3. The cold-rolled steel sheet of claim 1, wherein the cementite, the transition carbide, and the fine precipitate each has an area fraction of more than 0% and not more than 5%.

4. The cold-rolled steel sheet of claim 1, wherein the final microstructure consists of only tempered martensite.

5. The cold-rolled steel sheet of claim 1, wherein the final microstructure consists of tempered martensite, ferrite, and bainite, the tempered martensite having an area fraction of 70% or more and less than 100%, and the ferrite and bainite having an area fraction of more than 0% and not more than 20%.

6. A method of manufacturing a cold-rolled steel sheet, the method comprising (a) hot rolling a steel material consisting of carbon (C): 0.23 wt% to 0.35 wt%, silicon (Si): 0.05 wt% to 0.5 wt%, manganese (Mn): 0.3 wt% to 2.3 wt%, phosphorus (P): more than 0 wt% and not more than 0.02 wt%, sulfur (S): more than 0 wt% and not more than 0.005 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): more than 0 wt% and not more than 0.8 wt%, molybdenum (Mo): more than 0 wt% and not more than 0.4 wt%, titanium (Ti): 0.01 wt% to 0.1 wt%, vanadium (V): more than 0 wt% and not more than 0.3 wt%, boron (B): 0.001 wt% to 0.005 wt%, and a balance of iron (Fe); (b) cold rolling the hot-rolled steel material; and (c) sequentially performing annealing, first heat treatment, and second heat treatment on the cold-rolled steel material, wherein a final microstructure of the cold-rolled steel sheet obtained by performing steps (a) to (c) comprises cementite, a transition carbide, and a fine precipitate, the transition carbide comprising ε-carbide having an atomic ratio of a substitutional element selected from Fe, Mn, Cr, and Mo to C of 2.5:1, or η-carbide having an atomic ratio of the substitutional element to C of 2:1, and the fine precipitate having an atomic ratio of an alloying element selected from Mo, V, and Ti to C of 1:1, and wherein the cementite is formed during the first heat treatment, the transition carbide is formed during the second heat treatment, and the fine precipitate is formed during the hot rolling.

7. The method of claim 6, wherein step (a) is performed under conditions of a reheating temperature of 1150 °C to 1300 °C, a finishing delivery temperature of 800 °C to 1000 °C, and a coiling temperature of 500 °C to 650 °C, and wherein step (c) is performed under conditions of an annealing temperature of 800 °C to 900 °C and a first heat treatment temperature of 100 °C to 300 °C, and the second heat treatment comprises maintaining a second heat treatment temperature T satisfying Inequality 1 for a second heat treatment holding time t. 3800 ≤ T + 300 × 10 + log t ≤ 5650 (where a unit of T is °C and a unit of t is hours).

8. The method of claim 6, wherein step (a) is performed under conditions of a reheating temperature of 1150 °C to 1300 °C, a finishing delivery temperature of 800 °C to 1000 °C, and a coiling temperature of 500 °C to 650 °C, and wherein step (c) comprises performing coating and is performed under conditions of an annealing temperature of 800 °C to 900 °C and a first heat treatment temperature of 450 °C to 600 °C, and the second heat treatment comprises maintaining a second heat treatment temperature T satisfying Inequality 1 for a second heat treatment holding time t. 3800 ≤ T + 300 × 10 + log t ≤ 5650 (where a unit of T is °C and a unit of t is hours).

9. The method of claim 6, wherein, in step (c), the first heat treatment, following the annealing, is performed after cooling to a first heat treatment temperature.

10. The method of claim 6, wherein, in step (c), the second heat treatment, following the first heat treatment, is performed after cooling to room temperature and then raising the temperature.

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