Ultra-high strength cold-rolled steel sheet and its manufacturing method

Abstract

The present invention relates to a composition comprising carbon (C): 0.23% by weight to 0.35% by weight, silicon (Si): 0.05% by weight to 0.5% by weight, manganese (Mn): 0.3% by weight to 2.3% by weight, phosphorus (P): more than 0% by weight and not more than 0.02% by weight, sulfur (S): more than 0% by weight and not more than 0.005% by weight, aluminum (Al): 0.01% by weight to 0.05% by weight, chromium (Cr): more than 0% by weight and not more than 0.8% by weight, molybdenum (Mo): more than 0% by weight and not more than 0.4% by weight, titanium (Ti): 0.01% by weight to 0.1% by weight, vanadium (V): The present invention provides a cold-rolled steel sheet comprising more than 0 wt% and not more than 0.3 wt%, boron (B): 0.001 wt% to 0.005 wt%, and the remainder being iron (Fe) and other unavoidable impurities, wherein the final microstructure includes cementite, transition carbides, and fine precipitates, and the transition carbides include ε-carbide in which the atomic ratio of a substitutional element, which is any one of iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo), to carbon is 2.5:1, or η-carbide in which the atomic ratio is 2:1.

Description

[Technical Field]

[0001] The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof, and more particularly to an ultra-high strength cold-rolled steel sheet excellent in yield ratio and bendability and a manufacturing method thereof. [Background technology]

[0002] The automotive industry has seen a continuous increase in the demand for vehicle crashworthiness. While the number of auto parts has decreased in recent years as electric vehicles have become more common, the introduction of batteries has increased vehicle weight, further increasing the need for crashworthiness. This has led to ongoing efforts to develop ultra-high strength crash components, such as front bumper beams, side seals, and door impact beams, which contribute to crashworthiness. In particular, martensitic steel, which boasts the highest strength among cold-rolled steel sheets, has seen its application expand with the increased use of roll forming techniques. Due to the characteristics of this manufacturing method, the bendability of the steel sheet is a very important factor.

[0003] Related prior art includes Japanese Patent Application Laid-Open No. 2005-105367. Summary of the Invention [Problem to be solved by the invention]

[0004] The technical problem to be solved by the present invention is to provide an ultra-high strength cold rolled steel sheet having a high yield ratio and excellent bendability, and a manufacturing method thereof, and in particular, to provide a cold rolled steel sheet capable of realizing martensitic steel having a tensile strength of 1400 MPa or more, and a manufacturing method thereof. [Means for solving the problem]

[0005] In order to solve the above problems, a cold-rolled steel sheet according to one embodiment of the present invention comprises 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 the remainder being iron (Fe) and other unavoidable impurities. The cold-rolled steel sheet is made of pure steel, and the final microstructure of the cold-rolled steel sheet includes cementite, transition carbides, and fine precipitates, the transition carbides including ε-carbides in which an atomic ratio of a substitutional element, which is any one of iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo), to carbon is 2.5:1, or η-carbides in which the atomic ratio is 2:1, and the fine precipitates include an atomic ratio of an alloying element, which is any one of molybdenum (Mo), vanadium (V), and titanium (Ti), to carbon is 1:1, and the steel sheet has a yield strength (YP) of 1170 MPa or more, a tensile strength (TS) of 1400 MPa or more, an elongation (El) of 3.0% or more, a yield ratio of 70% or more, and a bending workability (R / t) of 4.0 or less.

[0006] In the cold-rolled steel sheet, the cementite, the transition carbides, and the fine precipitates may each have an average size of 50 nm or less and an average aspect ratio of 4.0 or less.

[0007] In the cold-rolled steel sheet, the cementite, the transition carbides, and the fine precipitates may each have an area fraction of more than 0% and not more than 5%.

[0008] In the cold rolled steel sheet, the final microstructure may consist solely of tempered martensite.

[0009] In the cold-rolled steel sheet, the final microstructure may be composed of tempered martensite, ferrite, and bainite, with the area fractions of tempered martensite being 70% or more and less than 100%, and ferrite and bainite being more than 0% and 20% or less.

[0010] In order to solve the above problems, a method for producing a cold-rolled steel sheet according to one embodiment of the present invention is to provide a steel sheet having a composition including: (a) carbon (C): 0.23% by weight to 0.35% by weight, silicon (Si): 0.05% by weight to 0.5% by weight, manganese (Mn): 0.3% by weight to 2.3% by weight, phosphorus (P): more than 0% by weight and not more than 0.02% by weight, sulfur (S): more than 0% by weight and not more than 0.005% by weight, aluminum (Al): 0.01% by weight to 0.05% by weight, chromium (Cr): 0.01% by weight to 0.05% by weight, and manganese (Mn): 0.3% by weight to 2.3% by weight. (Cr): more than 0 wt% and 0.8 wt% or less, molybdenum (Mo): more than 0 wt% and 0.4 wt% or less, titanium (Ti): 0.01 wt% to 0.1 wt%, vanadium (V): more than 0 wt% and 0.3 wt% or less, boron (B): 0.001 wt% to 0.005 wt%, and the remainder iron (Fe); (b) cold-rolling the hot-rolled steel; and (c) a method for manufacturing a cold-rolled steel sheet, the method comprising the steps of: sequentially performing annealing, a first heat treatment, and a second heat treatment on a cold-rolled steel material; wherein the final microstructure of the cold-rolled steel sheet realized by performing steps (a) to (c) comprises cementite, transition carbides, and fine precipitates, the transition carbides comprising ε-carbide having an atomic ratio of a substitutional element, which is any one of iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo), to carbon of 2.5:1, or η-carbide having the same atomic ratio of 2:1, the fine precipitates comprising an alloying element, which is any one of molybdenum (Mo), vanadium (V), and titanium (Ti), to carbon of 1:1, the cementite being formed during the first heat treatment, the transition carbides being formed during the second heat treatment, and the fine precipitates being formed during the hot-rolling step.

[0011] In the method for manufacturing the cold-rolled steel sheet, the step (a) includes a hot rolling step under conditions of a reheating temperature of 1150°C to 1300°C, a finish rolling temperature of 800°C to 1000°C, and a coiling temperature of 500°C to 650°C, the step (c) is carried out 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 step may include a step of maintaining a second heat treatment temperature (T) that satisfies the following Equation 1 for a second heat treatment maintenance time (t).

[0012]

number

[0013] (Note that the unit of T is ° C., and the unit of t is hour.)

[0014] In the method for manufacturing the cold-rolled steel sheet, the step (a) includes a hot rolling step under conditions of a reheating temperature of 1150°C to 1300°C, a finish rolling temperature of 800°C to 1000°C, and a coiling temperature of 500°C to 650°C; the step (c) includes a plating step, which 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 step includes a step of maintaining a second heat treatment temperature (T) that satisfies the following Equation 1 for a second heat treatment maintenance time (t).

[0015]

number

[0016] (Note that the unit of T is ° C. and the unit of t is hours.)

[0017] In the method for manufacturing the cold-rolled steel sheet, the step (c) is characterized in that after the annealing process, the steel sheet is cooled to the first heat treatment temperature, and then the first heat treatment process is performed.

[0018] In the method for manufacturing the cold-rolled steel sheet, the step (c) is characterized in that after the first heat treatment process, the steel sheet is cooled to room temperature, and then heated, and then the second heat treatment process is performed. [Effects of the Invention]

[0019] According to embodiments of the present invention, it is possible to realize an ultra-high strength cold-rolled steel sheet having both a high yield ratio and excellent bendability, and a manufacturing method thereof. For example, according to the present invention, it is possible to realize a high strength cold-rolled steel sheet having excellent bendability, with a yield ratio (YP / TS) of over 70% and high tensile strength, and a bending property (R / t) of 4.0 or less. As a result, it is expected that the application of a material with excellent formability for forming into parts with complex shapes and excellent crash absorption capabilities will contribute to improving passenger stability in automobiles and fuel efficiency through vehicle weight reduction.

[0020] Of course, the scope of the present invention is not limited by such effects. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 2 is a diagram showing an analysis of cementite (Fe3C) in carbides appearing in the final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention. [Figure 2] FIG. 1 is an analysis diagram of ε-carbide (Fe2.5C) among carbides appearing in the final microstructure of a cold-rolled steel sheet according to an example of the present invention. [Figure 3] FIG. 1 is a schematic diagram illustrating a method for measuring carbides appearing in the final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention. [Figure 4] FIG. 2 is a distribution diagram showing the size of carbides appearing in the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention. [Figure 5] FIG. 2 is a distribution diagram showing the aspect ratio of carbides appearing in the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention. [Figure 6] 1 is a diagram showing an outline of heat treatment illustrating steps of sequentially performing annealing, first heat treatment, and second heat treatment in a method for producing a cold-rolled steel sheet according to an embodiment of the present invention. [Figure 7] 10 is a photograph of the final microstructure according to Experimental Example 20 of the present invention. [Figure 8] 10 is a photograph of the final microstructure according to Experimental Example 21 of the present invention. [Figure 9] 10 is a photograph of the final microstructure according to Experimental Example 22 of the present invention. [Figure 10] 10 is a photograph of the final microstructure according to Experimental Example 23 of the present invention. [Figure 11] 3 is a graph illustrating the conditions of the second heat treatment step in the method for producing a cold-rolled steel sheet according to an experimental example of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0022] A cold-rolled steel sheet and a manufacturing method thereof according to an embodiment of the present invention will now be described in detail. The terms used below are appropriately selected in consideration of the functions of the present invention, and these terms should be defined based on the overall content of this specification. Hereinafter, we will provide specific details of an ultra-high strength cold-rolled steel sheet having a high yield ratio and excellent bendability, and a manufacturing method thereof.

[0023] A cold-rolled steel sheet according to an embodiment of the present invention contains carbon (C): 0.23% to 0.35% by weight, silicon (Si): 0.05% to 0.5% by weight, manganese (Mn): 0.3% to 2.3% by weight, phosphorus (P): more than 0% by weight and not more than 0.02% by weight, sulfur (S): more than 0% by weight and not more than 0.005% by weight, aluminum (Al): 0.01% to 0.05% by weight, chromium (Cr): more than 0% by weight and not more than 0.8% by weight, molybdenum (Mo): more than 0% by weight and not more than 0.4% by weight, titanium (Ti): 0.01% to 0.1% by weight, vanadium (V): more than 0% by weight and not more than 0.3% by weight, boron (B): 0.001% to 0.005% by weight, and the remainder being iron (Fe) and other unavoidable impurities.

[0024] Hereinafter, the role and content of each component contained in the cold-rolled steel sheet will be described.

[0025] Carbon (C)

[0026] Carbon (C) is the most effective and important element for increasing the strength of steel. Furthermore, carbon dissolves in austenite and forms a martensite structure during quenching. Furthermore, carbon combines with elements such as iron, chromium, and molybdenum to form carbides, improving strength and hardness. Carbon (C) may be added to a base steel sheet constituting a cold-rolled steel sheet according to an embodiment of the present invention in an amount of 0.23 wt% to 0.35 wt% of the total weight. If the carbon content is less than 0.23 wt% of the total weight, the above-mentioned effects cannot be achieved and sufficient strength cannot be ensured. On the other hand, if the carbon content exceeds 0.35 wt% of the total weight, problems arise such as reduced weldability and workability.

[0027] Silicon (Si)

[0028] Silicon (Si) is an element added to ensure bendability and hydrogen embrittlement resistance by suppressing cementite formation. Silicon is also added to increase strength through ferrite solid solution strengthening and suppress carbide formation. Silicon is well known as a ferrite stabilizing element, increasing the ferrite fraction during cooling and increasing softness. It is also known to promote martensite formation through austenite carbon enrichment, thereby ensuring strength. Silicon, along with aluminum, is added as a deoxidizer to remove oxygen from steel during the steelmaking process and can also have a solid solution strengthening effect. Silicon may be added in a content ratio of 0.05 wt% to 0.5 wt% of the total weight of the base steel sheet constituting the cold-rolled steel sheet according to one embodiment of the present invention. If the silicon content is less than 0.05 wt% of the total weight, softness cannot be ensured and the above-mentioned effects of silicon addition cannot be reliably achieved. On the other hand, if the silicon content is too high and exceeds 0.5 wt % of the total weight, the strength will decrease due to the excessive formation of ferrite, oxides will form on the surface of the steel sheet, which will reduce the galvanizability of the steel sheet, red scale will be formed during reheating and hot rolling, which may cause problems with surface quality, and there will be problems such as reduced toughness and plastic workability, and the weldability of the steel may be reduced.

[0029] Manganese (Mn)

[0030] Manganese (Mn) is an element that contributes to improving strength through solid-solution strengthening and increased hardenability. For example, manganese facilitates the formation of low-temperature transformation phases, thereby increasing strength through solid-solution strengthening. Some manganese dissolves in steel, while others combine with sulfur contained in the steel to form MnS, a non-metallic inclusion. Because MnS is soft, it elongates in the direction of processing during plastic working. However, the formation of MnS reduces the sulfur content in the steel, weakening the grains and suppressing the formation of FeS, a low-melting-point compound, which impairs the acid and oxidation resistance of the steel. However, it also refines pearlite and solid-solution strengthens ferrite, thereby improving yield strength. Manganese may be added in a content ratio of 0.3 to 2.3 wt% of the total weight of the base steel sheet constituting the cold-rolled steel sheet according to one embodiment of the present invention. If the manganese content is less than 0.3 wt%, the aforementioned effect of ensuring strength cannot be fully achieved. Furthermore, if the manganese content is 2.3 wt% or more, problems such as reduced bendability and hydrogen embrittlement resistance may occur due to the formation of manganese bands and MnS. For example, problems may arise in that internal and external segregation bands are formed in continuously cast slabs and steel sheets, which induce crack initiation and propagation, thereby reducing bendability. That is, slab quality and weldability may be reduced, and center segregation may occur, reducing the softness of the base steel sheet and reducing formability.

[0031] Rin (P)

[0032] Phosphorus (P) can increase strength through solid solution strengthening and suppress the formation of carbides. Phosphorus may be added to the base steel sheet constituting the cold-rolled steel sheet according to an embodiment of the present invention in an amount of more than 0 wt % and not more than 0.02 wt % of the total weight. If the phosphorus content exceeds 0.02 wt %, problems may arise, such as embrittlement of the weld, induction of embrittlement through grain boundary segregation, reduced press formability, and reduced impact resistance.

[0033] Sulfur (S)

[0034] Sulfur (S) improves the machinability of steel by bonding with manganese, titanium, etc., and improves workability by forming fine MnS precipitates. However, it generally impairs softening and weldability. The sulfur may be added to the base steel sheet constituting the cold-rolled steel sheet according to one embodiment of the present invention in an amount of more than 0 wt. % and not more than 0.005 wt. % of the total weight. If the sulfur content exceeds 0.005 wt. %, the number of MnS inclusions increases, resulting in poor bendability and hydrogen embrittlement resistance. Furthermore, segregation during continuous casting solidification can cause high-temperature cracking.

[0035] Aluminum (Al)

[0036] Aluminum (Al) is an element primarily used as a deoxidizer, preventing slab cracking during nitride formation, promoting ferrite formation, improving elongation, suppressing carbide formation, and stabilizing austenite by increasing the carbon concentration in austenite. Furthermore, aluminum acts as a layer between the iron and the galvanized layer to improve galvanizability and is effective in suppressing the formation of manganese bands in hot-rolled coils. The aluminum (Al) content is preferably 0.01 wt% to 0.05 wt% of the total weight of the base steel sheet constituting the cold-rolled steel sheet according to one embodiment of the present invention. When the aluminum (Al) content is less than 0.01 wt%, the above-mentioned effects of adding aluminum can be reliably achieved. On the other hand, if the aluminum (Al) content is excessive, exceeding 0.05 wt%, there are problems such as a decrease in strength due to the formation of ferrite, a decrease in continuous castability due to an increase in aluminum inclusions, a decrease in galvanizability due to concentration on the surface of the steel sheet, and the formation of AlN in the slab, which can induce hot rolling cracks.

[0037] Chromium (Cr)

[0038] Chromium (Cr) is an element that improves hardenability and ensures high strength, and also has the effect of improving hardenability as an austenite stabilizing element. Furthermore, chromium increases the elongation rate by precipitating Cr-based precipitates intragranularly during annealing heat treatment. Preferably, chromium (Cr) is added to the base steel sheet constituting the cold-rolled steel sheet according to one embodiment of the present invention in an amount of more than 0 wt% and not more than 0.8 wt% of the total weight. If the chromium (Cr) content is excessive, exceeding 0.8 wt%, a saturation effect occurs, resulting in problems such as reduced laser weldability and softness, and impaired platability.

[0039] Molybdenum (Mo)

[0040] Molybdenum (Mo) is an element added to improve hardenability and ensure strength and toughness, and can improve hydrogen embrittlement resistance through grain refinement and precipitation effects. Molybdenum (Mo) is preferably added in an amount of more than 0 wt% and not more than 0.4 wt% of the total weight of the base steel sheet constituting the cold-rolled steel sheet according to one embodiment of the present invention. If the Mo content exceeds 0.4 wt%, problems arise, such as increased manufacturing costs and reduced weldability.

[0041] Titanium (Ti)

[0042] Titanium (Ti) contributes to grain refinement and suppression of BN formation. Titanium (Ti) is preferably added in a content ratio of 0.01 wt% to 0.1 wt% of the total weight of the base steel sheet constituting the cold-rolled steel sheet according to one embodiment of the present invention. If the titanium (Ti) content is less than 0.01 wt%, excessive BN precipitates may cause a decrease in the softness of the cast slab, resulting in a decrease in slab quality and strength. On the other hand, if the titanium (Ti) content exceeds 0.1 wt%, coarsening of TiN precipitates may cause a decrease in bendability and hydrogen embrittlement resistance, and an excessive increase in recrystallization temperature may cause a non-uniform structure.

[0043] Boron (B)

[0044] Boron (B) is an element added to suppress ferrite formation and increase the hardenability of steel. Boron is also a strong hardenable element, preventing phosphorus (P) segregation and improving strength. Since phosphorus (P) segregation can cause secondary work embrittlement, adding boron prevents phosphorus (P) segregation and increases resistance to work embrittlement. Preferably, boron is added in a content ratio of 0.001 wt% to 0.005 wt% of the total weight of the base steel sheet constituting the cold-rolled steel sheet according to one embodiment of the present invention. If the boron content is less than 0.001 wt%, strength cannot be ensured due to poor hardenability. However, if the boron content is excessive and exceeds 0.005 wt%, problems such as increased intergranular embrittlement due to BN formation, reduced weldability, and impaired surface quality of the steel due to the formation of boron oxides may occur.

[0045] Meanwhile, the ultra-high strength cold-rolled steel sheet according to the embodiment of the present invention may optionally contain vanadium (V). The addition of vanadium (V), a fine alloying element, forms fine precipitates (VC or (Ti,V)C) different from cementite and transition carbides, contributing to improved strength. When the ultra-high strength cold-rolled steel sheet according to the embodiment of the present invention optionally contains vanadium, the vanadium (V) is preferably added in an amount of more than 0 wt% and not more than 0.3 wt% of the total weight of the base steel sheet constituting the cold-rolled steel sheet according to the embodiment of the present invention. If the vanadium (V) content is excessive, exceeding 0.3 wt%, the manufacturing cost of the steel may increase significantly, and the rolling load may increase significantly due to the large amount of precipitation during rolling, resulting in a reduced elongation rate.

[0046] The remaining component of the ultra-high strength cold-rolled steel sheet is iron (Fe). However, since unintended impurities may be inevitably mixed in from raw materials or the surrounding environment during a normal manufacturing process, it is not possible to eliminate these impurities. These impurities are known to anyone skilled in normal manufacturing processes, and therefore, the entire contents of these impurities will not be specifically mentioned in this specification.

[0047] FIG. 1 shows the results of analyzing cementite (Fe3C) in carbides that appeared in the final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention, and FIG. 2 shows the results of analyzing ε-carbide (Fe 2.5 Fig. 3 is a schematic diagram illustrating a method for measuring carbides appearing in the final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention, Fig. 4 is a distribution diagram showing the size of carbides appearing in the final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention, and Fig. 5 is a distribution diagram showing the aspect ratio of carbides appearing in the final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention.

[0048] 1 to 5, the final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention includes cementite, transition carbides, and fine precipitates. The cementite (FeC) has an atomic ratio of iron (Fe) to carbon of 3:1. The transition carbides include ε-carbides in which the atomic ratio of a substitutional element selected from iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo) to carbon is 2.5:1, or η-carbides in which the atomic ratio is 2:1. The fine precipitates are characterized by an atomic ratio of an alloying element selected from molybdenum (Mo), vanadium (V), and titanium (Ti) to carbon of 1:1. The carbides and fine precipitates may contain a portion of nitrogen.

[0049] In order to ensure bendability and hydrogen embrittlement resistance in the cold-rolled steel sheet according to the embodiment of the present invention, the cementite, the transition carbides, and the fine precipitates 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 refers to the average size including the major and minor axes of the elliptical or acicular carbides 10, specifically, the average size including the minor axis size (a) and the major axis size (b). The average aspect ratio refers to the ratio of the major axis length to the minor axis length (b / a). In the cold-rolled steel sheet according to the embodiment of the present invention, the cementite, the transition carbides, and the fine precipitates may each have an area fraction of more than 0% and less than 5%. The area fractions of the cementite, the transition carbides, and the fine precipitates were analyzed using a minimum of five microstructure photographs via replica analysis of a transmission electron microscope.

[0050] The final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention may consist solely of tempered martensite. Alternatively, the final microstructure of the cold-rolled steel sheet according to another embodiment of the present invention may consist of tempered martensite, ferrite, and bainite, with the area fractions of tempered martensite being 70% or more and less than 100%, and ferrite and / or bainite being more than 0% and 20% or less. For example, the final microstructure may consist of tempered martensite, ferrite, and bainite, with the area fractions of tempered martensite being 70% or more and less than 100%, bainite being more than 0% and 20%, and ferrite being more than 0% and 10% or less.

[0051] On the other hand, the final microstructure of the cold rolled steel sheet according to other modified embodiments of the present invention may consist of only tempered martensite, in which case bainite and ferrite are absent.

[0052] The above-mentioned microstructure is based on the results of analyzing a quarter point in the thickness direction using a scanning electron microscope in a direction perpendicular to the rolling direction. In the present invention, if the area fraction of tempered martensite is less than 70%, the target strength cannot be achieved. Furthermore, in the present invention, ferrite and bainite are microstructures that are inevitably formed due to an insufficient cooling rate and are the main factors that reduce strength, so the smaller the area fraction, the better. The sum of the area fractions of the two phases, ferrite and bainite, should not exceed 20%.

[0053] A cold-rolled steel sheet according to an embodiment of the present invention having the above-described alloying element composition and microstructure may exhibit properties such as a yield strength (YP) of 1170 MPa or more, a tensile strength (TS) of 1400 MPa or more, an elongation (El) of 3.0% or more, a yield ratio of 70% or more, and a bending workability (R / t) of 4.0 or less, despite containing cementite-type carbides. For example, a cold-rolled steel sheet according to an embodiment of the present invention may have a yield strength (YP) of 1170 MPa to 1400 MPa, a tensile strength (TS) of 1400 MPa to 1700 MPa, an elongation (El) of 3.0% to 9.0%, a yield ratio of 70% to 90%, and a bending workability (R / t) of 2.0 to 4.0. In the bending workability (R / t), R is the minimum bending radius ratio, and t is the unit thickness.

[0054] Hereinafter, a method for manufacturing a cold-rolled steel sheet having the above-mentioned composition and microstructure according to one embodiment of the present invention will be described.

[0055] A method for manufacturing a steel sheet according to an embodiment of the present invention includes the steps of: (a) forming a steel sheet with a composition of carbon (C): 0.23% to 0.35% by weight, silicon (Si): 0.05% to 0.5% by weight, manganese (Mn): 0.3% to 2.3% by weight, phosphorus (P): more than 0% by weight and not more than 0.02% by weight, sulfur (S): more than 0% by weight and not more than 0.005% by weight, aluminum (Al): 0.01% to 0.05% by weight, chromium (Cr): more than 0% by weight and not more than 0.8% by weight, molybdenum (Mo % or less by weight, titanium (Ti): 0.01 to 0.1% by weight, vanadium (V): 0 to 0.3% by weight, boron (B): 0.001 to 0.005% by weight, and the remainder iron (Fe); (b) hot-rolling the hot-rolled steel; and (c) sequentially subjecting the cold-rolled steel to annealing, a first heat treatment, and a second heat treatment.

[0056] The step (a) of hot rolling can be carried out under the conditions of a reheating temperature of 1150°C to 1300°C, a finish rolling temperature of 800°C to 1000°C, and a coiling temperature of 500°C to 650°C.

[0057] When the steel is reheated at the aforementioned temperatures (1150°C to 1300°C), elements segregated during the continuous casting process can be redissolved. To improve strength through precipitation and solution strengthening, strengthening elements must be fully dissolved in austenite before hot rolling, and for this reason, the steel must be heated at 1150°C or higher. Reheating temperatures lower than 1150°C may result in insufficient dissolution of various carbides, resulting in insufficient uniform distribution of the elements segregated during the continuous casting process. However, reheating temperatures higher than 1300°C may cause adverse effects such as austenite coarsening and decarburization, making it difficult to achieve the desired strength. In other words, reheating temperatures higher than 1300°C may result in the formation of very coarse austenite grains, making it difficult to ensure sufficient strength. Furthermore, reheating temperatures higher than 1300°C may increase heating costs and process time, resulting in increased manufacturing costs and reduced productivity.

[0058] The finish rolling temperature (FDT) is a very important factor affecting the final material properties, and rolling at 800°C to 1000°C can refine the austenite. However, if the hot rolling temperature is lower than 800°C, the rolling load increases during rolling, and a duplex structure may occur at the edge. Furthermore, rolling at a high temperature above 1000°C results in coarsening of the grains, making it impossible to obtain the desired mechanical properties. After hot rolling, cooling is carried out at a rate of 1 to 100°C / s, and a faster cooling rate is more advantageous for reducing the average grain size.

[0059] On the other hand, if the coiling temperature is lower than 500°C, the shape of the hot-rolled coil becomes non-uniform and the cold rolling load increases.If the coiling temperature is higher than 650°C, the difference in cooling rate between the center and edge of the steel sheet can cause a non-uniform microstructure, which can lead to oxidation inside the grain boundaries.

[0060] Meanwhile, the hot rolling may be performed under the condition of a rolling reduction of 35% to 65%. The microstructure of the steel material after hot rolling may include bainite, martensite, and ferrite.

[0061] The step (b) of cold rolling may include a step of cold rolling at a reduction of 35% to 65% after a pickling process. A higher reduction ratio improves formability by refining the structure. If the reduction ratio is less than 35% during cold rolling, it is difficult to obtain a uniformly fine structure, and if it is designed to exceed 65%, the roll force becomes high, increasing the process load.

[0062] FIG. 6 is a diagram showing an outline of heat treatment illustrating steps of sequentially performing annealing, first heat treatment, and second heat treatment in a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention.

[0063] 6, the cold-rolled steel material is heated to a temperature of Ac3 or higher at a heating rate of 1 to 10° C. / s. The Ac3 temperature can be calculated using the following formula:

[0064] Ac3(℃)=910-203[C] 0.5 -30[Mn]+44.7[Si]+31.5[Mo]-15.2[Ni]

[0065] Here, [C], [Mn], [Si], [Mo] and [Ni] are the weight percentage values ​​of carbon, manganese, silicon, molybdenum and nickel in the steel.

[0066] According to the method for producing a cold-rolled steel sheet according to the embodiment of the present invention, an annealing step is performed in which the annealing temperature is maintained at Ac3 or higher, preferably 800°C to 900°C, for 60 seconds to 600 seconds.

[0067] Thereafter, the material is cooled to 500 to 700°C at a cooling rate of 1 to 20°C / s, and then cooled to the martensitic transformation finish temperature at a cooling rate of 5 to 50°C / s (cooling finish temperature). Here, the martensitic transformation finish temperature is 100 to 350°C.

[0068] Thereafter, in the case of non-plated materials, a first heat treatment step is carried out in which the material is maintained at a first heat treatment temperature of 100°C to 300°C for 10 to 100 seconds, and then cooled to room temperature at a cooling rate of 20°C / s or less.

[0069] If the first heat treatment temperature is less than 100°C, no cementite is formed regardless of the maintenance time. Furthermore, cementite cannot be formed even when the temperature is maintained at 100°C to 300°C for a short time of 10 seconds or less. If the first heat treatment temperature exceeds 300°C or is maintained in the temperature range of 100°C to 300°C for 100 seconds or more, the formation of bainite prevents the desired strength from being achieved.

[0070] Meanwhile, in the case of plated materials, the first heat treatment process involves maintaining the first heat treatment temperature at 450°C to 600°C for 5 to 60 seconds, followed by cooling to room temperature at a cooling rate of 20°C / s or less. If the first heat treatment is maintained after cooling is completed at a temperature of 300°C or less, the material may deteriorate due to transformation heat generated by the formation of bainite. On the other hand, if cooling is completed at a temperature of 450°C or more, martensitic transformation occurs during cooling due to the bainite transformation delay (up to 60 seconds), ensuring the material quality.

[0071] If the first heat treatment temperature is less than 450°C, the temperature of the coating bath is reduced, impairing the coating and alloying quality, making it impossible to produce a coated steel sheet. Furthermore, if the temperature is maintained within the 450°C to 600°C temperature range for a short time of 5 seconds or less, the coating solution cannot be sufficiently applied to the steel sheet, making it impossible to ensure coating quality. If the first heat treatment temperature exceeds 600°C, dross and ash are generated during passage through the coating bath, making it impossible to ensure surface quality. If the first heat treatment time exceeds 60 seconds within the 450°C to 600°C temperature range, the formation and increase in the fraction of bainite makes it impossible to ensure the target tensile strength.

[0072] If the first heat treatment temperature is maintained in the range of more than 300°C and less than 450°C, a decrease in strength occurs due to heat generated by transformation caused by the formation of bainite.

[0073] In the case of the first heat treatment temperature mentioned above, it is advantageous to set the temperature as low as possible within the above temperature range in order to completely complete the transformation of martensite.

[0074] In a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention, the cooling step after the annealing step and before the first heat treatment step involves cooling only to the first heat treatment temperature without performing a cooling step of quenching to room temperature. When a cooling step of quenching to room temperature is performed after the annealing step and before the first heat treatment step, the final microstructure of the cold-rolled steel sheet does not contain cementite even when the first heat treatment step is performed. However, when the annealing step is performed and the cooling step is performed only to the first heat treatment temperature without performing a step of quenching to room temperature before the first heat treatment step, as in the present invention, cementite may be formed during the first heat treatment step. Generally, steels containing cementite have a problem of degraded workability.

[0075] However, in order to fundamentally prevent the formation of cementite, the process of rapidly cooling to room temperature after the annealing process before the first heat treatment process increases manufacturing costs due to the addition of additional equipment.

[0076] In the cold-rolled steel sheet according to the embodiment of the present invention, the process of quenching to room temperature after the annealing process and before the first heat treatment process is not applied, so the final microstructure contains cementite. However, by precisely controlling the subsequent processes, it is possible to secure physical properties such as yield strength (YP): 1170 MPa or more, tensile strength (TS): 1400 MPa or more, elongation (El): 3.0% or more, yield ratio: 70% or more, and bendability (R / t): 4.0 or less, thereby realizing an ultra-high strength cold-rolled steel sheet with excellent yield ratio and bendability.

[0077] After the first heat treatment process, the substrate may be cooled to room temperature and then heated to perform a second heat treatment process, which may include maintaining a second heat treatment temperature (T) that satisfies the following Equation 1 for a second heat treatment maintenance time (t):

[0078]

number

[0079] (Note that the unit of T is ° C. and the unit of t is hours.)

[0080] In the ultra-high strength cold-rolled steel sheet according to the present invention, if the value of the above formula 1 is less than 3800, the yield strength of the cold-rolled steel sheet will not be achieved, and if the value of the above formula 1 is more than 5650, the target material properties will not be ensured due to inferior bendability.

[0081] The second heat treatment process satisfies Equation 1, and may include, for example, a step of raising the temperature to 100°C to 210°C at a temperature increase rate of 10°C / s or less, and then maintaining the second heat treatment temperature (T) at 100°C to 210°C for 3 hours to 20 hours (t).

[0082] The final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention, which is realized by applying the above-mentioned process conditions, includes cementite, transition carbides, and fine precipitates, and the transition carbides include ε-carbides in which the atomic ratio of a substitutional element, which is any one of iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo), to carbon is 2.5:1, or η-carbides in which the atomic ratio is 2:1, and the fine precipitates are characterized in that the atomic ratio of an alloying element, which is any one of molybdenum (Mo), vanadium (V), and titanium (Ti), to carbon is 1:1.

[0083] The cementite is formed during the first heat treatment step after the annealing and cooling. When the first heat treatment temperature is 100°C to 300°C, the cementite exists within the martensite. When the first heat treatment temperature is 450°C to 600°C, the cementite does not form within the martensite, but exists within the bainite if present. Such cementite exists at a ratio of 0% to 5% of the total area fraction, and preferably exists at a low ratio.

[0084] The transition carbides are formed during the second heat treatment process. In the method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention, if the second heat treatment process is not performed, no transition carbides will be present. Transition carbides must be present to increase the yield strength, and may be present at a ratio of 0% to 5% of the total area fraction.

[0085] The fine precipitates are formed during the hot rolling or during coiling after the hot rolling, and unlike the cementite and the transition carbides, do not contain iron (Fe) as a constituent element. The fine precipitates may be present at a ratio of 0% to 5% of the total area fraction.

[0086] During the second heat treatment, cementite grows, while transition carbides simultaneously form and grow. However, fine precipitates do not grow when maintained at the low second heat treatment temperature of the present invention. This is because, in the present invention, the amount of fine alloying elements added to the steel is very small, less than 0.5 wt %, and fine precipitates grow depending on the diffusion rate of the fine alloying elements. Growth is extremely difficult at temperatures below 300°C for less than about 100 hours. However, Fe-based cementite and transition carbides form and grow depending on the diffusion rate of supersaturated carbon in the martensite matrix. Therefore, cementite and transition carbides form and grow under the conditions proposed in the present invention. As previously explained, the formation of transition carbides is a very important factor in ensuring yield strength. Therefore, it is difficult to achieve the desired yield strength under conditions of excessively low temperatures or short second heat treatment times. Furthermore, the growth of acicular cementite can deteriorate bendability. Since a relatively high second heat treatment temperature or a long period of time accelerates the growth of acicular cementite, it is necessary to set appropriate second heat treatment conditions to ensure bendability. In the present invention, by adjusting the range of the value of Equation 1 above, it is possible to realize an ultra-high strength cold rolled steel sheet having both a high yield ratio and excellent bendability.

[0087] Experimental example

[0088] In the following, preferred experimental examples are presented to facilitate understanding of the present invention, but the following experimental examples are merely for the purpose of facilitating understanding of the present invention and are not intended to limit the present invention.

[0089] 1. Composition of test specimen

[0090] In this experimental example, test specimens having the alloy element compositions (unit: wt %) shown in Table 1 were provided.

[0091] [Table 1]

[0092] The component systems A and B in Table 1 satisfy the composition of the cold-rolled steel sheet according to one embodiment of the present invention, which is 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 the remainder being iron (Fe). However, component system C is unsatisfactory because it falls below the composition range of carbon (C): 0.23 wt% to 0.35 wt%, and component system D is unsatisfactory because it exceeds the composition range of silicon (Si): 0.05 wt% to 0.5 wt%.

[0093] 2. Process conditions and physical property evaluation

[0094] Table 2 shows various heat treatment process conditions for each test piece having the composition disclosed in Table 1, and Table 3 shows the evaluation results of physical properties after applying the compositions and heat treatment processes disclosed in Tables 1 and 2.

[0095] In Table 2, the "Component System" column indicates the composition disclosed in Table 1, and "Formula 1" indicates the calculated value of the previously explained Formula 1 [(T + 300) × (10 + log(t))]. In Table 3, the "YP (MPa)", "TS (MPa)", and "EL (%)" columns indicate the yield strength, tensile strength, and elongation rate of the test specimen, respectively.

[0096] [Table 2]

[0097] [Table 3]

[0098] Tables 1 to 3 show the difference in physical properties depending on the annealing temperature in Experimental Examples 1 and 2. Experimental Examples 1 and 2 are cold-rolled steel sheets embodied according to embodiments of the present invention, and therefore satisfy the annealing temperature range of 800°C to 900°C. Therefore, they satisfy the physical properties of yield strength (YP) of 1170 MPa or more, tensile strength (TS) of 1400 MPa or more, elongation (El) of 3.0% or more, yield ratio of 70% or more, and bending workability (R / t) of 4.0 or less. In the final microstructure of the cold-rolled steel sheet, cementite, transition carbides, and fine precipitates each have 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% to 5% or less. Tables 1 to 3 show the difference in physical properties depending on the first heat treatment temperature in Experimental Examples 3 to 6. Experimental Examples 3 to 5 are unplated cold-rolled steel sheets embodied according to embodiments of the present invention. The first heat treatment temperature range is 100°C to 300°C, and the steel sheets satisfy the physical properties of yield strength (YP) of 1170 MPa or more, tensile strength (TS) of 1400 MPa or more, elongation (El) of 3.0% or more, yield ratio of 70% or more, and bending workability (R / t) of 4.0 or less. It can be confirmed that in the final microstructure of the cold-rolled steel sheets, cementite, transition carbides, and fine precipitates each have an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of each of more than 0% and 5% or less.

[0099] Furthermore, Experimental Example 6 is a plated cold-rolled steel sheet embodied according to an embodiment of the present invention, and satisfies the first heat treatment temperature range of 450°C to 600°C, thereby satisfying the physical properties of yield strength (YP) of 1170 MPa or more, tensile strength (TS) of 1400 MPa or more, elongation (El) of 3.0% or more, yield ratio of 70% or more, and bending workability (R / t) of 4.0 or less. It was also confirmed that in the final microstructure of the cold-rolled steel sheet, cementite, transition carbides, and fine precipitates each have an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of each of more than 0% and 5% or less.

[0100] In contrast, according to Experimental Example 24, when the annealing temperature is 350°C, the first heat treatment temperature exceeds the range of 100°C to 300°C, which is unsatisfactory. Therefore, it is confirmed that the target physical properties of yield strength (YP) of 1170 MPa or more and tensile strength (TS) of 1400 MPa or more are not achieved. It is also confirmed that the range of the average carbide size of 50 nm or less is not satisfied, and the range of the average carbide aspect ratio of 4.0 or less is not satisfied. When the first heat treatment temperature is maintained in the range of more than 300°C but less than 450°C, as in Experimental Example 24, strength is reduced due to transformation heat generation. However, when the first heat treatment temperature is in the range of 450°C to 600°C, as in Experimental Example 6, the material quality can be ensured by suppressing transformation.

[0101] Referring to Tables 1 to 3, the difference in physical properties depending on the second heat treatment process conditions can be seen. The value of Equation 1 shown in Table 2 represents the relationship between the second heat treatment temperature (T) and the second heat treatment maintenance time (t), which is [(T + 300) × (10 + log(t))]. In Equation 1, the second heat treatment temperature (T) is in °C, and the second heat treatment maintenance time (t) is in hours.

[0102] For example, Experimental Examples 1 to 6, 11, and 20 to 24 were subjected to the following conditions: second heat treatment temperature (T): 150°C, second heat treatment maintenance time (t): 6 hours; Experimental Example 7 was subjected to the following conditions: second heat treatment temperature (T): 25°C, second heat treatment maintenance time (t): 6 hours; Experimental Example 8 was subjected to the following conditions: second heat treatment temperature (T): 50°C, second heat treatment maintenance time (t): 6 hours; Experimental Example 9 was subjected to the following conditions: second heat treatment temperature (T): 100°C, second heat treatment maintenance time (t): 6 hours; Experimental Example 10 was subjected to the following conditions: second heat treatment temperature (T): 130°C, second heat treatment maintenance time (t): 6 hours; Experimental Example 12 was subjected to the following conditions: second heat treatment temperature (T): 180°C, second heat treatment maintenance time (t): 6 hours; In Experimental Example 11, the second heat treatment temperature (T): 200°C, second heat treatment maintenance time (t): 6 hours was applied, in Experimental Example 14, the second heat treatment temperature (T): 250°C, second heat treatment maintenance time (t): 6 hours was applied, in Experimental Example 15, the second heat treatment temperature (T): 280°C, second heat treatment maintenance time (t): 6 hours was applied, in Experimental Example 16, the second heat treatment temperature (T): 300°C, second heat treatment maintenance time (t): 6 hours was applied, in Experimental Example 17, the second heat treatment temperature (T): 200°C, second heat treatment maintenance time (t): 24 hours was applied, in Experimental Example 18, the second heat treatment temperature (T): 150°C, second heat treatment maintenance time (t): 20 hours was applied, and in Experimental Example 19, the second heat treatment temperature (T): 120°C, second heat treatment maintenance time (t): 20 hours was applied.

[0103] First, by examining Experimental Examples 1 to 6, it can be confirmed that even when the annealing temperature and the conditions of the first heat treatment step are changed within the ranges proposed by the present invention, the target physical properties can be ensured when the second heat treatment step conditions are set to a second heat treatment temperature (T) and a second heat treatment duration (t) that satisfy the range of the calculated value of Equation 1 [(T + 300) × (10 + log(t))] of 3800 to 5650.

[0104] Considering Experimental Examples 7 and 8 (Group C in FIG. 11), it can be seen that when the value of Equation 1 is lower than 3800 as a condition of the second heat treatment process, transition carbides are not formed in the final microstructure, and therefore the yield strength does not reach the target value (1170 MPa or more).

[0105] When Experimental Examples 9 to 13 and Experimental Examples 18 to 19 (Group A in FIG. 11 ) are examined, it can be seen that when the value of Equation 1 is 3800 or more and 5650 or less as the second heat treatment process condition, the following conditions are satisfied: yield strength (YP): 1170 MPa or more, tensile strength (TS): 1400 MPa or more, elongation (El): 3.0% or more, yield ratio: 70% or more, and bending workability (R / t): 4.0 or less. The cementite, the transition carbides, and the fine precipitates each have an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of the cementite, the transition carbides, and the fine precipitates is greater than 0% and less than 5%.

[0106] When considering Experimental Examples 14 to 17 (Group B in FIG. 11 ), it was confirmed that when the value of Equation 1 exceeds 5650 as a condition of the second heat treatment process, the bendability deteriorates due to an increase in the size and aspect ratio of the carbides. That is, it was confirmed that the target property of bendability (R / t) of 4.0 or less was not achieved, and the range in which the average aspect ratio of the carbides is 4.0 or less was not satisfied. That is, it was confirmed that the target property of bendability (R / t) of 4.0 or less was not satisfied due to the poor shape of the carbides.

[0107] 7 to 10, differences in physical properties due to alloy composition can be seen in Experimental Examples 20 to 23. Specifically, Experimental Examples 20 and 21 are cold-rolled steel sheets embodied according to embodiments of the present invention, and contain 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 up to 0.02 wt%, sulfur (S): more than 0 wt% and up to 0.005 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): more than 0 wt% and up to 0.8 wt%, molybdenum (Mo): more than 0 wt% and up to 0.4 wt%, and titanium (Ti): 0.01 wt% to 0.1 wt%. , boron (B): 0.001 wt% to 0.005 wt%, and the remainder iron (Fe), the cold-rolled steel sheet satisfies the following physical properties: yield strength (YP): 1170 MPa or more, tensile strength (TS): 1400 MPa or more, elongation (El): 3.0% or more, yield ratio: 70% or more, and bending workability (R / t): 4.0 or less. In the final microstructure of the cold-rolled steel sheet, it can be confirmed that the cementite, transition carbides, and fine precipitates each have an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of each of them is greater than 0% and less than 5%.

[0108] In contrast, Experimental Example 22 confirmed that even when the conditions for the annealing process, first heat treatment process, and second heat treatment process of the present invention were satisfied, the carbon (C) content fell below the composition range of 0.23 wt% to 0.35 wt% and the target properties of yield strength (YP) of 1170 MPa or more and tensile strength (TS) of 1400 MPa or more were not achieved. Furthermore, Experimental Example 23 confirmed that even when the conditions for the annealing process, first heat treatment process, and second heat treatment process of the present invention were satisfied, the silicon (Si) content fell above the composition range of 0.05 wt% to 0.5 wt% and the target properties of yield strength (YP) of 1170 MPa or more were not achieved due to the transformation of the intermediate phases ferrite and bainite. In particular, in Experimental Example 23, the carbide size and aspect ratio characteristics were satisfied and bendability was ensured, but ferrite was formed at more than 10%, resulting in an unsatisfactory yield strength.

[0109] So far, we have described a cold-rolled steel sheet and its manufacturing method according to the technical concept of the present invention. According to the present invention, it is possible to realize a high-strength cold-rolled steel sheet with high tensile strength and high yield characteristics with a yield ratio (YP / TS) of over 70% and excellent bendability with a bending performance (R / t) of 4.0 or less. This is expected to contribute to improved passenger stability and fuel efficiency through vehicle weight reduction through the application of a material with excellent formability and crash absorption capabilities for forming complex-shaped parts.

[0110] Although the present invention has been described above with reference to exemplary embodiments, various modifications and variations may be made by those skilled in the art. Such modifications and variations are within the scope of the present invention. Therefore, the scope of the present invention should be determined by the following claims.

Claims

1. A cold-rolled steel sheet consisting of carbon (C): 0.23% to 0.35% by weight, silicon (Si): 0.05% to 0.5% by weight, manganese (Mn): 0.3% to 2.3% by weight, phosphorus (P): more than 0% by weight and not more than 0.02% by weight, sulfur (S): more than 0% by weight and not more than 0.005% by weight, aluminum (Al): 0.01% to 0.05% by weight, chromium (Cr): more than 0% by weight and not more than 0.8% by weight, molybdenum (Mo): more than 0% by weight and not more than 0.4% by weight, titanium (Ti): 0.01% to 0.1% by weight, vanadium (V): more than 0% by weight and not more than 0.3% by weight, boron (B): 0.001% to 0.005% by weight, and the balance being iron (Fe) and other unavoidable impurities, The final microstructure of the cold-rolled steel sheet includes cementite, transition carbides, and fine precipitates, the transition carbides include ε-carbides in which an atomic ratio of a substitutional element, which is any one of iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo), to carbon is 2.5:1, or η-carbides in which the atomic ratio is 2:1, the fine precipitates include alloying elements, which are any one of molybdenum (Mo), vanadium (V), and titanium (Ti), to carbon is 1:1, A cold-rolled steel sheet characterized by having a yield strength (YP): 1170 MPa or more, a tensile strength (TS): 1400 MPa or more, an elongation (El): 3.0% or more, a yield ratio: 70% or more, and a bending workability (R / t): 4.0 or less.

2. The cold-rolled steel sheet according to claim 1, wherein the cementite, the transition carbides, and the fine precipitates each have an average size of 50 nm or less and an average aspect ratio of 4.0 or less.

3. The cold-rolled steel sheet according to claim 1, wherein the area fraction of each of the cementite, the transition carbides, and the fine precipitates is more than 0% and not more than 5%.

4. 2. The cold rolled steel sheet according to claim 1, characterized in that the final microstructure consists solely of tempered martensite.

5. 2. The cold-rolled steel sheet according to claim 1, wherein the final microstructure is composed of tempered martensite, ferrite, and bainite, and the area fractions thereof are as follows: tempered martensite: 70% or more but less than 100%, and ferrite and bainite: more than 0% but not more than 20%.

6. (a) Carbon (C): 0.23% to 0.35% by weight, silicon (Si): 0.05% to 0.5% by weight, manganese (Mn): 0.3% to 2.3% by weight, phosphorus (P): more than 0% by weight and up to 0.02% by weight, sulfur (S): more than 0% by weight and up to 0.005% by weight, aluminum (Al): 0.01% to 0.05% by weight, chromium (Cr): more than 0% by weight and up to 0.8% by weight, molybdenum (Mo): more than 0% by weight and up to 0.4% by weight, 1. A method for manufacturing a cold-rolled steel sheet, comprising: (a) hot-rolling a steel material consisting of tungsten (Ti): 0.01 wt% to 0.1 wt%, vanadium (V): more than 0 wt% and 0.3 wt% or less, boron (B): 0.001 wt% to 0.005 wt%, and the remainder iron (Fe); (b) cold-rolling the hot-rolled steel material; and (c) sequentially performing annealing, a first heat treatment, and a second heat treatment on the cold-rolled steel material, The final microstructure of the cold-rolled steel sheet realized by performing steps (a) to (c) includes cementite, transition carbides, and fine precipitates, the transition carbides include ε-carbides having an atomic ratio of a substitutional element, which is any one of iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo), to carbon of 2.5:1, or η-carbides having the atomic ratio of 2:1, the fine precipitates include alloying elements, which are any one of molybdenum (Mo), vanadium (V), and titanium (Ti), to carbon of 1:1, 10. A method for manufacturing a cold-rolled steel sheet, wherein the cementite is formed during the first heat treatment, the transition carbides are formed during the second heat treatment, and the fine precipitates are formed during the hot rolling step.

7. Step (a) includes hot rolling under the conditions of a reheating temperature of 1150°C to 1300°C, a finish rolling temperature of 800°C to 1000°C, and a coiling temperature of 500°C to 650°C; 7. The method of claim 6, 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 process includes maintaining a second heat treatment temperature (T) that satisfies Equation 1 below for a second heat treatment maintenance time (t): [Equation 1] (Note that the unit of T is ° C. and the unit of t is hours.)

8. Step (a) includes hot rolling under the conditions of a reheating temperature of 1150°C to 1300°C, a finish rolling temperature of 800°C to 1000°C, and a coiling temperature of 500°C to 650°C; 7. The method of manufacturing a cold-rolled steel sheet according to claim 6, wherein step (c) includes a step of performing a plating process, 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 step includes a step of maintaining a second heat treatment temperature (T) that satisfies Equation 1 below for a second heat treatment maintenance time (t): [Equation 2] (Note that the unit of T is ° C. and the unit of t is hours.)

9. 7. The method of manufacturing a cold-rolled steel sheet according to claim 6, wherein in step (c), the first heat treatment is performed after cooling to the first heat treatment temperature after the annealing process.

10. 7. The method of claim 6, wherein in step (c), the steel sheet is cooled to room temperature after the first heat treatment, and then heated to perform the second heat treatment.

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