Ultrahigh-strength cold-rolled steel sheet and method for producing same

Abstract

The invention provides a cold-rolled steel sheet. The cold-rolled steel sheet contains 0.23 wt% to 0.35 wt% of carbon (C), 0.05 wt% to 0.5 wt% of silicon (Si), 0.3 wt% to 2.3 wt% of manganese (Mn), 0 wt% to 0.02 wt% (excluding 0) of phosphorus (P), 0 wt% to 0.005 wt% (excluding 0) of sulfur (S), 0.01 wt% to 0.05 wt% of aluminum (Al), 0 wt% to 0.8 wt% (excluding 0) of chromium (Cr), 0 wt% to 0.4 wt% (excluding 0) of molybdenum (Mo), and the balance being steel. A cold-rolled steel sheet having a final microstructure containing cementite, transition carbides and fine precipitates, the transition carbides containing [epsilon]-carbides, the fine precipitates containing [epsilon]-carbides, the transition carbides containing [epsilon]-carbides, the fine precipitates containing [epsilon]-carbides, the fine precipitates containing [epsilon]-carbides, the fine precipitates containing [epsilon]-carbides, the fine precipitates containing [epsilon]-carbides, the fine precipitates containing [epsilon]-carbides, the fine precipitates containing [epsilon]-carbides, and the fine precipitates containing [epsilon]-carbides, the fine precipitates containing [epsilon]-carbides, the fine precipitates containing [epsilon]-carbides, the fine wherein the atomic ratio of the substitutional element, which is one of Fe, Mn, Cr or Mo, to carbon is 2.5: 1, or comprises [eta]-carbides wherein the atomic ratio is 2: 1.

Description

Technical Field

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more particularly to an ultra-high strength cold-rolled steel sheet having a high yield ratio and excellent bendability and a method for manufacturing the same. Background Art

[0002] The automotive industry has an increasing demand for vehicle body crash resistance. Recently, although the number of vehicle components has decreased with the rise of electric vehicles, the overall weight of the vehicle has increased due to the addition of batteries. Therefore, the demand for crash resistance has further expanded. Therefore, efforts are being made to achieve ultra-high strength of collision-related components such as front bumper beams, side beams, and door anti-collision beams, which play a key role in improving crash resistance. In particular, the increased use of roll forming technology has expanded the application range of martensitic steel, which has the highest strength among various types of cold-rolled steel. Due to the characteristics of roll forming, the bendability of the steel sheet is a crucial factor.

[0003] Related art includes Japanese Patent Laid-Open No. 2005-105367. Summary of the invention

[0004] Technical issues

[0005] The present invention provides an ultra-high strength cold-rolled steel sheet having a high yield ratio and excellent bendability and a method for manufacturing the same, and more specifically provides a cold-rolled steel sheet of martensitic steel capable of achieving a tensile strength of 1400 MPa or more and a method for manufacturing the same.

[0006] Technical Solution

[0007] According to one aspect of the present invention, there is provided a cold-rolled steel sheet, the 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): greater than 0 wt % and not greater than 0.02 wt %, sulfur (S): greater than 0 wt % and not greater than 0.005 wt %, aluminum (Al): 0.01 wt % to 0.05 wt %, chromium (Cr): greater than 0 wt % and not greater than 0.8 wt %, molybdenum (Mo): greater than 0 wt % and not greater than 0.4 wt %, titanium (Ti): 0.01 wt % to 0.1 wt %, vanadium (V): greater than 0 wt % and not greater than 0.3 wt %, boron (B): 0.0 0.01 wt % to 0.005 wt %, and the balance is iron (Fe) and other inevitable impurities, wherein the final microstructure of the cold-rolled steel sheet comprises cementite, transition carbides and fine precipitates, the transition carbides comprising ε-carbides in which the atomic ratio of substitutional elements selected from Fe, Mn, Cr and Mo to C is 2.5:1, or η-carbides in which the atomic ratio of substitutional elements to C is 2:1, and the atomic ratio of alloying elements selected from Mo, V and Ti to C in the fine precipitates is 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 (El) of 3.0% or more, a yield ratio of 70% or more, and a bendability (R / t) of 4.0 or less.

[0008] The cementite, transition carbide, and fine precipitates may each have an average size of 50 nm or less and an average aspect ratio of 4.0 or less.

[0009] Cementite, transition carbide, and fine precipitates may each have an area fraction greater than 0% and not greater than 5%.

[0010] The final microstructure may consist solely of tempered martensite.

[0011] The final microstructure may be composed of tempered martensite, ferrite, and bainite, the area fraction of tempered martensite being 70% or more and less than 100%, and the area fractions of ferrite and bainite being greater than 0% and not greater than 20%.

[0012] According to another aspect of the present invention, there is provided a method for manufacturing a cold-rolled steel sheet, the method comprising (a) hot rolling a steel material, the 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): greater than 0 wt% and not greater than 0.02 wt%, sulfur (S): greater than 0 wt% and not greater than 0.005 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): greater than 0 wt% and not greater than 0.8 wt%, molybdenum (Mo): greater than 0 wt% and not greater than 0.4 wt%, titanium (Ti): 0.01 wt% to 0.1 wt%, vanadium (V): greater than 0 wt% and not greater than 0.3 wt% , boron (B): 0.001 wt% to 0.005 wt% and the balance is 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, wherein the final microstructure of the cold rolled steel sheet obtained by performing steps (a) to (c) comprises cementite, transition carbides and fine precipitates, the transition carbides comprising ε-carbides in which the atomic ratio of substitutional elements selected from Fe, Mn, Cr and Mo to C is 2.5:1, or η-carbides in which the atomic ratio of substitutional elements to C is 2:1, and the atomic ratio of alloying elements selected from Mo, V and Ti to C in the fine precipitates is 1:1, and wherein cementite is formed in the first heat treatment, transition carbides are formed in the second heat treatment, and fine precipitates are formed during hot rolling.

[0013] Step (a) can be carried out under the conditions of a reheating temperature of 1150°C to 1300°C, a finishing rolling temperature of 800°C to 1000°C, and a coiling temperature of 500°C to 650°C, step (c) can be carried out under the 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 can include maintaining the second heat treatment temperature T satisfying Inequality 1 for the second heat treatment holding time t.

[0014] Inequality 1: 3800 ≤ (T + 300) × (10 + log (t)) ≤ 5650

[0015] (Wherein, the unit of T is °C and the unit of t is hour).

[0016] Step (a) may be performed under the conditions of a reheating temperature of 1150°C to 1300°C, a finishing rolling temperature of 800°C to 1000°C, and a coiling temperature of 500°C to 650°C, step (c) may include coating, and may be performed under the 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.

[0017] Inequality 1: 3800 ≤ (T + 300) × (10 + log (t)) ≤ 5650

[0018] (Wherein, the unit of T is °C and the unit of t is hour).

[0019] In step (c), the first heat treatment after annealing may be performed after cooling to the first heat treatment temperature.

[0020] In step (c), the second heat treatment after the first heat treatment may be performed after cooling to room temperature and then raising the temperature.

[0021] Beneficial effects

[0022] According to an embodiment of the present invention, an ultra-high strength cold-rolled steel sheet having a high yield ratio and excellent bendability and a method for manufacturing the same can be achieved. For example, according to the present invention, a high-strength cold-rolled steel sheet having high tensile strength, a high yield ratio (YP / TS) greater than 70%, and excellent bendability (R / t) of 4.0 or less can be achieved. Therefore, it is expected that a material having excellent impact absorbency and formability is used to form a component of a complex shape to improve passenger safety and improve fuel efficiency by lightweighting the vehicle body.

[0023] However, the scope of the present invention is not limited to the above-mentioned effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 Included are microscopic images showing analysis results of cementite (Fe3C) in carbides in the final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention.

[0025] Figure 2 The ε-carbide (Fe 2.5 C) Microscopic image of the analysis results.

[0026] Figure 3 is a schematic diagram for describing a method of measuring the size of carbides in the final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention.

[0027] Figure 4 is an image showing the distribution of carbide sizes in the final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention.

[0028] Figure 5 : is an image showing the distribution of the aspect ratio of carbides in the final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention.

[0029] Figure 6 1 and 2 are images showing steps of sequentially performing annealing, a first heat treatment, and a second heat treatment in the method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention.

[0030] Figure 7 is a microscopic image of the final microstructure of Test Example 20 according to the present invention.

[0031] Figure 8 is a microscopic image of the final microstructure of Test Example 21 according to the present invention.

[0032] Fig. 9 is a microscopic image of the final microstructure of Test Example 22 according to the present invention.

[0033] Fig.10 is a microscopic image of the final microstructure of Test Example 23 according to the present invention.

[0034] Fig.11 are images showing the second heat treatment conditions in the method of manufacturing a cold-rolled steel sheet according to a test example of the present invention. DETAILED DESCRIPTION

[0035] A cold rolled steel sheet and a method for 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 and a method for manufacturing the same having a high yield ratio and excellent bendability will be provided below.

[0036] A cold rolled steel sheet according to an embodiment of the present invention is composed 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): greater than 0 wt% and not greater than 0.02 wt%, sulfur (S): greater than 0 wt% and not greater than 0.005 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): greater than 0 wt% and not greater than 0.8 wt%, molybdenum (Mo): greater than 0 wt% and not greater than 0.4 wt%, titanium (Ti): 0.01 wt% to 0.1 wt%, vanadium (V): greater than 0 wt% and not greater than 0.3 wt%, boron (B): 0.001 wt% to 0.005 wt%, and the balance of iron (Fe) and other inevitable impurities,

[0037] The functions and contents of components contained in the cold-rolled steel sheet will now be described.

[0038] Carbon (C)

[0039] C is the most effective and important element for improving the strength of steel. C is added and dissolved in austenite to form a martensitic structure during quenching. In addition, C combines with elements such as Fe, Cr and Mo to form carbides and improve strength and hardness. C can be added in a content ratio of 0.23% by weight to 0.35% by weight of the total weight of the base steel sheet used to form the cold-rolled steel sheet according to an embodiment of the present invention. When the content of C is less than 0.23% by weight of the total weight, the above-mentioned effects may not be achieved, and sufficient strength may not be ensured. On the other hand, when the content of C is greater than 0.35% by weight of the total weight, weldability and processability may be reduced.

[0040] Silicon (Si)

[0041] 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 improve strength and suppress the formation of cementite due to the solid solution strengthening effect in ferrite. Si is a well-known ferrite stabilizing element, so ductility can be improved by increasing the fraction of ferrite during cooling. Si is also a well-known element that can promote the formation of martensite by carbon enrichment in austenite to ensure strength. At the same time, Si can be added together with Al as a deoxidizer to remove oxygen from steel in the steelmaking process and has a solid solution strengthening effect. Si can be added in a content ratio of 0.05% by weight to 0.5% by weight of the total weight of the base steel sheet used to form the cold-rolled steel sheet according to an embodiment of the present invention. When the content of Si is less than 0.05% by weight of the total weight, ductility may not be ensured, and the above-mentioned effect of Si addition may not be properly achieved. On the other hand, when the Si content is greater than 0.5 wt. % of the total weight due to excessive addition, ferrite may be excessively formed to reduce the strength, oxides may be formed on the surface of the steel plate to reduce the coating properties of the steel plate, red oxide scale may be formed during reheating and hot rolling to reduce the surface quality, toughness and plasticity may be reduced, and the weldability of the steel may also be reduced.

[0042] Manganese (Mn)

[0043] Mn is an element that contributes to strength improvement by improving solid solution strengthening and quenching ability. For example, Mn is an element that promotes the formation of low temperature transformation phase and provides the effect of improving strength by solid solution strengthening. A part of Mn is dissolved in steel, and another part is combined with S contained in steel to form non-metallic inclusions (such as MnS). MnS has ductility, so it is elongated in the direction of plastic processing. However, due to the formation of MnS, the content of S in steel is reduced, so that grains become susceptible, and the formation of FeS (low melting point compound) is suppressed. Mn reduces the acid resistance and oxidation resistance of steel, but improves yield strength by refining pearlite and solid solution strengthening ferrite. Mn can be added in a content ratio of 0.3 weight % to 2.3 weight % of the total weight of the base steel plate for forming the cold-rolled steel plate according to the embodiment of the present invention. When the content of Mn is less than 0.3 weight %, it may not be possible to fully achieve the above-mentioned strength improvement effect. 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 regions may be formed inside and outside of the continuously cast slab and steel plate, and the formation and propagation of cracks may cause reduced bendability. That is, slab quality and weldability may be reduced, and center segregation may occur, thereby reducing ductility and workability of the base steel plate.

[0044] Phosphorus (P)

[0045] P can improve the strength of steel by solid solution strengthening and can inhibit the formation of carbides. P can be added in a content ratio of more than 0 wt% and not more than 0.02 wt% of the total weight of the base steel sheet used to form the cold-rolled steel sheet according to the embodiment of the present invention. When the content of P is more than 0.02 wt%, the weld joint may become brittle, embrittlement may be caused by grain boundary segregation, press formability may be reduced, and impact resistance may be reduced.

[0046] Sulfur (S)

[0047] S is an element that combines with Mn or Ti to improve the machinability of steel and forms fine MnS precipitates to improve machinability, but generally inhibits ductility and weldability. S may be added in a content ratio of greater than 0 wt% and not greater than 0.005 wt% of the total weight of the base steel sheet used to form 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 increase, thereby reducing bendability and hydrogen embrittlement resistance, and segregation may occur during continuous casting solidification, resulting in high temperature cracks.

[0048] Aluminum (Al)

[0049] Al is an element generally used as a deoxidizer, and prevents slab cracks during nitride formation, promotes the formation of ferrite, thereby improving elongation, inhibits the formation of carbides, and stabilizes austenite by increasing the C concentration in austenite. Al acts as a layer between the Fe and zinc (Zn) coatings, thereby improving coating performance, and effectively inhibits the formation of Mn bands in hot-rolled coils. Al can be added in a content ratio of 0.01 wt % to 0.05 wt % of the total weight of the base steel sheet used to form 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-mentioned effect of Al addition may not be properly achieved. On the other hand, due to excessive addition, when the content of Al is greater than 0.05 wt %, due to the formation of ferrite, the strength may be reduced, Al inclusions may increase, thereby reducing continuous casting, Al enrichment may occur on the surface of the steel sheet, thereby deteriorating coating performance, and AlN may be formed in the slab, resulting in hot rolling cracks.

[0050] Chromium (Cr)

[0051] Cr is an element that can improve hardenability and ensure high strength, and can improve quenching ability as an austenite stabilizing element. Cr improves elongation by forming Cr-based precipitates in grains during annealing. Cr can be added in a content ratio of greater than 0 wt % and not greater than 0.8 wt % of the total weight of the base steel sheet used to form 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, a saturation effect may occur, laser weldability and ductility may be reduced, and coating performance may be hindered.

[0052] Molybdenum (Mo)

[0053] Mo is an element added to improve quenching ability and ensure strength and toughness, and is also an element capable of improving hydrogen embrittlement resistance due to grain refinement and precipitation effects. Mo may be added in a content ratio of greater than 0 wt % and not greater than 0.4 wt % of the total weight of the base steel sheet used to form 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.

[0054] Titanium (Ti)

[0055] Ti contributes to grain refinement and BN formation inhibition. Ti can be added in a content ratio of 0.01 wt % to 0.1 wt % of the total weight of the base steel sheet used to form the cold-rolled steel sheet according to an embodiment of the present invention. When the content of Ti is less than 0.01 wt %, the reduction in ductility of the cast slab caused by excessive formation of BN precipitates may reduce the quality and strength of the slab. At the same time, when the content of Ti is greater than 0.1 wt %, due to the coarsening of TiN precipitates, bendability and hydrogen embrittlement resistance may be reduced, and the recrystallization temperature may be excessively increased, resulting in an uneven structure.

[0056] Boron (B)

[0057] B is an element added to improve the hardenability of steel by inhibiting the formation of ferrite. B is also a strong quenching element and is used to improve strength by preventing segregation of P. Since secondary processing embrittlement may occur when segregation of P occurs, B may be added to prevent segregation of P and improve resistance to processing embrittlement. B may be added in a content ratio of 0.001 wt % to 0.005 wt % of the total weight of the base steel sheet used to form 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 quenching ability. When the content of B is greater than 0.005 wt % due to excessive addition, grain boundary embrittlement may increase, weldability may decrease, and the surface quality of the steel may be hindered due to the formation of BN.

[0058] Meanwhile, the ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention may optionally contain V. The addition of microalloying elements (e.g., V) contributes to strength improvement by forming fine precipitates (VC or (Ti, V) C) (which are different from cementite and transition carbide). When the ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention optionally contains V, V may be added at a content ratio of greater than 0 wt % and not greater than 0.3 wt % of the total weight of the base steel sheet used to form 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 cost of the steel may increase significantly, the rolling load may also increase significantly due to excessive precipitation during rolling, and the elongation may decrease.

[0059] The remainder of the composition of the ultra-high strength cold-rolled steel sheet is Fe. However, unexpected impurities may inevitably be introduced from the raw materials or the surrounding environment during the general production process, and therefore, the addition of these impurities is inevitable. These impurities are known to those skilled in the art, and therefore are not particularly described in this specification.

[0060] Figure 1 including a microscopic image showing the analysis results of cementite (Fe3C) in carbides in the final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention, Figure 2 The ε-carbide (Fe 2.5 C) is a microscopic image of the analysis results. Figure 3 is a schematic diagram for describing a method for measuring the size of carbides in the final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention, Figure 4 is an image showing the distribution of carbide sizes in the final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention, and Figure 5 : is an image showing the distribution of the aspect ratio of carbides in the final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention.

[0061] refer to Figures 1 to 5 , the final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention comprises cementite, transition carbide and fine precipitates. The atomic ratio of Fe to C of cementite (Fe3C) is 3:1. The transition carbide comprises an ε-carbide in which the atomic ratio of a substitution element selected from Fe, Mn, Cr and Mo to C is 2.5:1, or an η-carbide in which the atomic ratio of a substitution element to C is 2:1. The atomic ratio of an alloying element selected from Mo, V and Ti to C of the fine precipitates is 1:1. Carbides and fine precipitates may partially contain nitrogen (N).

[0062] In the cold-rolled steel sheet according to the embodiment of the present invention, in order to ensure bendability and hydrogen embrittlement resistance, cementite, transition carbide and fine precipitates may each have an average size of 50 nm or less and an average aspect ratio of 4.0 or less. Figure 3 , the average size represents the average size including the major axis and the minor axis of the elliptical or needle-shaped carbide particles 10, and more specifically represents the average size including the minor axis length a and the major axis length b. The average aspect ratio represents the 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, cementite, transition carbide and fine precipitates may each have an area fraction greater than 0% and not greater than 5%. The area fractions of cementite, transition carbide and fine precipitates are measured by repeated analysis using a scanning electron microscope using at least five microscopic images of the microstructure.

[0063] The final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention may consist only 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, the area fraction of tempered martensite being 70% or more and less than 100%, and the area fraction of ferrite and / or bainite being greater than 0% and not greater than 20%. For example, the final microstructure may consist of tempered martensite, ferrite and bainite, the area fraction of tempered martensite being 70% or more and less than 100%, the area fraction of bainite being greater than 0% and not greater than 20%, and the area fraction of ferrite being greater than 0% and not greater than 10%.

[0064] Meanwhile, the final microstructure of the 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 do not exist.

[0065] The above microstructure is based on the results of analyzing the 1 / 4 point in the thickness direction from the direction perpendicular to the rolling direction using a scanning electron microscope. In the present invention, when the area fraction of tempered martensite is less than 70%, the desired strength may not be achieved. In addition, in the present invention, ferrite and bainite are inevitably formed due to insufficient cooling rate, and as the main factor that reduces the strength, the smaller the area fraction, the better. It is required that the sum of the area fractions of the two phases of ferrite and bainite does not exceed 20%.

[0066] The cold-rolled steel sheet with the above alloying element composition and microstructure according to the embodiment of the present invention contains cementite-type carbides, but can achieve the properties of yield strength (YP) of 1170MPa or more, tensile strength (TS) of 1400MPa or more, elongation (El) of 3.0% or more, yield ratio of 70% or more, and bendability (R / t) of 4.0 or less. For example, the cold-rolled steel sheet according to the embodiment of the present invention can have YP of 1170MPa to 1400MPa, TS of 1400MPa to 1700MPa, El of 3.0% to 9.0%, yield ratio of 70% to 90%, and bendability (R / t) of 2.0 to 4.0. In bendability (R / t), R represents the minimum bending radius and t represents thickness.

[0067] A method of manufacturing a cold-rolled steel sheet having the above-described composition and microstructure according to an embodiment of the present invention will now be described.

[0068] A method for manufacturing a steel plate according to an embodiment of the present invention includes (a) hot rolling a steel material composed 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: greater than 0 wt% and not greater than 0.02 wt%, S: greater than 0 wt% and not greater than 0.005 wt%, Al: 0.01 wt% to 0.05 wt%, Cr: greater than 0 wt% and not greater than 0.8 wt%, Mo: greater than 0 wt% and not greater than 0.4 wt%, Ti: 0.01 wt% to 0.1 wt%, V: greater than 0 wt% and not greater than 0.3 wt%, B: 0.001 wt% to 0.005 wt% and the balance of 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.

[0069] The hot rolling step (a) may be performed under the conditions of a reheating temperature of 1150 to 1300°C, a finishing temperature of 800 to 1000°C, and a coiling temperature of 500 to 650°C.

[0070] When the steel material is reheated at the above-mentioned temperature of 1150°C to 1300°C, the components segregated during the continuous casting process can be redissolved. In order to improve the strength by precipitation and solid solution strengthening, the strengthening elements need to be fully dissolved in austenite before hot rolling, so the steel material needs to be heated to 1150°C or higher. When the reheating temperature is lower than 1150°C, various carbides may not be fully dissolved, and the segregated components may not be evenly dispersed during the continuous casting process. However, a reheating temperature higher than 1300°C may cause adverse effects such as austenite coarsening and decarburization, and strength cannot be ensured. That is, when the reheating temperature is higher than 1300°C, very coarse austenite grains may be formed, so strength may not be easily ensured. In addition, when the reheating temperature is higher than 1300°C, the heating cost and process time may increase, thereby increasing production costs and reducing productivity.

[0071] The finishing temperature (FDT) of hot rolling is an important factor affecting the final material properties, and rolling at 800°C to 1000°C can refine austenite. However, when the hot rolling temperature is lower than 800°C, the rolling load may increase and a mixed grain structure may appear at the edge. Rolling at a temperature higher than 1000°C may cause grain coarsening and fail to achieve the desired mechanical properties. After hot rolling, cooling is performed at a cooling rate of 1°C / s to 100°C / s. The greater the cooling rate, the smaller the average grain size.

[0072] Meanwhile, when the coiling temperature is lower than 500°C, the hot rolled coil may have an uneven shape and the cold rolling load may increase. When the coiling temperature is higher than 650°C, an uneven microstructure may occur due to the difference in cooling rate between the center and edge of the steel plate, and the inside of the grain boundary may be oxidized.

[0073] Meanwhile, the hot rolling may be performed under a reduction ratio of 35% to 65%. The microstructure of the steel material after the hot rolling may include bainite, martensite, and ferrite.

[0074] The cold rolling step (b) may include pickling and then cold rolling at a reduction ratio of 35% to 65%. The higher the reduction ratio, the more the formability is improved due to the microstructure refinement effect. When cold rolling is performed at a reduction ratio of less than 35%, it may not be easy to obtain a uniform microstructure, and when cold rolling is performed at a reduction ratio of more than 65%, the rolling force may increase, and thus the process load may also increase.

[0075] Figure 6 1 and 2 are images showing steps of sequentially performing annealing, a first heat treatment, and a second heat treatment in the method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention.

[0076] refer to Figure 6, the cold rolled steel material is heated to a temperature of Ac3 or higher at a heating rate of 1°C / s to 10°C / s. The temperature Ac3 can be calculated as shown below.

[0077] Ac3(℃)=910-203[C] 0.5 -30[Mn]+44.7[Si]+31.5[Mo]-15.2[Ni],

[0078] Here, [C], [Mn], [Si], [Mo] and [Ni] represent the weight % values ​​of C, Mn, Si, Mo and nickel (Ni) in the steel material.

[0079] Based on the cold rolled steel sheet manufacturing method according to the embodiment of the present invention, the annealing process is performed to maintain the temperature of Ac3 or higher (more specifically, the annealing temperature between 800° C. and 900° C.) for 60 seconds to 600 seconds.

[0080] Subsequently, cooling is performed to 500 to 700°C at a cooling rate of 1 to 20°C / s, and then to a martensite final temperature (or cooling end temperature) at a cooling rate of 5 to 50°C / s. Here, the martensite final temperature is 100 to 350°C.

[0081] Afterwards, for the uncoated steel sheet, a first heat treatment process is performed, the first heat treatment temperature of 100° C. to 300° C. is maintained for 10 seconds to 100 seconds, and then cooled to room temperature at a cooling rate of 20° C. / s or less.

[0082] When the first heat treatment temperature is lower than 100°C, cementite is not formed regardless of the holding time. When the temperature of 100°C to 300°C is held for a short holding time of 10 seconds 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 held for 100 seconds or more, the desired strength may not be ensured due to the formation of bainite.

[0083] Meanwhile, for the coated steel sheet, a first heat treatment process is performed, the first heat treatment temperature of 450°C to 600°C is maintained for 5 seconds to 60 seconds, and then cooled to room temperature at a cooling rate of 20°C / s or less. When cooling is completed at a temperature of 300°C or lower and then maintained for a single heat treatment, the transformation heat caused by the formation of bainite may cause the material properties to deteriorate. Meanwhile, when cooling is completed at a temperature of 450°C or higher, martensitic transformation may occur during cooling due to the delayed bainite transformation (up to 60 seconds), so the material properties can be ensured.

[0084] When the first heat treatment temperature is lower than 450°C, the temperature of the coating bath may decrease, thereby deteriorating the coating and alloy quality, and thus the coated steel sheet may not be manufactured. When the temperature of 450°C to 600°C is maintained for 5 seconds or less, the 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, scum and dust are generated when passing through the coating bath, and thus the surface quality may not be ensured. When the first heat treatment is performed at a temperature of 450°C to 600°C for more than 60 seconds, the desired tensile strength may not be ensured due to the formation and increase in the fraction of bainite.

[0085] When the first heat treatment temperature is maintained within the range between 300° C. and 450° C., the strength is deteriorated due to transformation heat generated by the formation of bainite.

[0086] In order to completely complete the transformation to martensite, the lower the first heat treatment temperature within the above temperature range, the better.

[0087] The cold rolled steel sheet manufacturing method according to an embodiment of the present invention is characterized in that the cooling after annealing and before the first heat treatment is only carried out to the first heat treatment temperature, and the rapid cooling process is not applied to room temperature. When the rapid cooling process is applied to room temperature after annealing and before the first heat treatment, even after the first heat treatment, the final microstructure of the cold rolled steel sheet does not include cementite. However, as in the present invention, when the cooling after annealing and before the first heat treatment is only carried out to the first heat treatment temperature and the rapid cooling process is not applied to room temperature, cementite can be formed in the first heat treatment process. Generally, the steel material comprising cementite has poor processability.

[0088] However, a rapid cooling process to room temperature after annealing and before the first heat treatment in order to fundamentally prevent the formation of cementite increases production costs due to increased equipment and the like.

[0089] In the cold rolled steel sheet according to the embodiment of the present invention, since a rapid cooling process to room temperature is not applied after annealing and before the first heat treatment, although the final microstructure contains cementite, by accurately controlling the subsequent processes, the properties of YP of 1170 MPa or more, TS of 1400 MPa or more, El of 3.0% or more, yield ratio of 70% or more, and bendability (R / t) of 4.0 or less can be ensured. Therefore, an ultra-high strength cold rolled steel sheet having a high yield ratio and excellent bendability can be achieved.

[0090] The second heat treatment after 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.

[0091] Inequality 1: 3800 ≤ (T + 300) × (10 + log (t)) ≤ 5650

[0092] (Wherein, the unit of T is °C and the unit of t is hour).

[0093] In the ultra-high strength cold-rolled steel sheet according to the present invention, when the value of Inequality 1 is less than 3800, the yield strength of the cold-rolled steel sheet may be insufficient, and when the value of Inequality 1 is greater than 5650, desired material properties may not be ensured due to poor bendability.

[0094] 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° C. to 210° C. for 3 to 20 hours.

[0095] The final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention formed by applying the above-mentioned process conditions includes cementite, transition carbides, and fine precipitates, wherein the transition carbides include ε-carbides in which the atomic ratio of substitution elements selected from Fe, Mn, Cr and Mo to C is 2.5:1, or η-carbides in which the atomic ratio of substitution elements to C is 2:1, and the atomic ratio of alloying elements selected from Mo, V and Ti to C of the fine precipitates is 1:1.

[0096] Cementite is formed during the first heat treatment after annealing and cooling. When the first heat treatment temperature is 100°C to 300°C, cementite exists in martensite. When the first heat treatment temperature is 450°C to 600°C, cementite is not formed in martensite, but when bainite is present, it exists in bainite. Cementite exists in a proportion of 0% to 5% of the total area fraction, and the smaller the proportion, the better.

[0097] Transition carbides are formed during the second heat treatment. In the cold rolled steel sheet manufacturing method according to the embodiment of the present invention, when the second heat treatment is not performed, transition carbides do not exist. Transition carbides are required to increase the yield strength, and transition carbides may exist in a proportion of 0% to 5% of the total area fraction.

[0098] The fine precipitates are formed during hot rolling or during coiling after hot rolling, and unlike cementite and transition carbide, the fine precipitates do not contain Fe. The fine precipitates may be present in a proportion of 0% to 5% of the total area fraction.

[0099] During the second heat treatment, cementite is subjected to growth, and transition carbides are subjected to formation and growth. However, when maintained at the low second heat treatment temperature of the present invention, fine precipitates will not grow. Since the amount of microalloying elements constituting steel in the present invention is very small (i.e. 0.5 weight % or less), when 300°C or lower low temperatures are maintained for about 100 hours or shorter, fine precipitates (whose growth depends on the diffusion rate of microalloying elements) may not grow easily. However, the formation and growth of Fe-based cementite and transition carbides depend on the diffusion rate of C (which is supersaturated in the martensitic matrix). Therefore, the formation and growth of cementite and transition carbides are carried out under the conditions proposed by the present invention. As mentioned above, the formation of transition carbides is a very important factor to ensure yield strength. Therefore, under the conditions of the second heat treatment at too low a temperature or too short a time, it may not be easy to ensure the desired yield strength. In addition, the growth of needle-shaped cementite is a factor that makes the bending property worse. Since the relatively high temperature or relatively long time of the second heat treatment accelerates the growth of acicular cementite, it is necessary to set appropriate second heat treatment conditions to ensure bendability. In the present invention, by controlling the range of Inequality 1, an ultra-high strength cold-rolled steel sheet with a high yield ratio and excellent bendability can be achieved.

[0100] Test Example

[0101] Test examples will now be described to better understand the present invention. However, the following test examples are only for facilitating understanding of the present invention, and the present invention is not limited thereto.

[0102] 1. Sample composition

[0103] This test example provides samples having the alloy element compositions (unit: weight %) shown in Table 1.

[0104] [Table 1]

[0105]

[0106]

[0107] In Table 1, component systems A and B satisfy the composition of the cold-rolled steel sheet according to the embodiment of the present invention, namely, 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: greater than 0 wt% and not greater than 0.02 wt%, S: greater than 0 wt% and not greater than 0.005 wt%, Al: 0.01 wt% to 0.05 wt%, Cr: greater than 0 wt% and not greater than 0.8 wt%, Mo: greater than 0 wt% and not greater than 0.4 wt%, Ti: 0.01 wt% to 0.1 wt%, V: greater than 0 wt% and not greater than 0.3 wt%, B: 0.001 wt% to 0.005 wt% and the balance of Fe. However, component system C is lower than and does not satisfy the composition range of C: 0.23 wt% to 0.35 wt%, and component system D is greater than and does not satisfy the composition range of Si: 0.05 wt% to 0.5 wt%.

[0108] 2. Process conditions and property evaluation

[0109] Table 2 shows various heat treatment conditions of the samples having the compositions shown in Table 1, and Table 3 shows the results of evaluating properties after applying the compositions and heat treatment conditions shown in Tables 1 and 2.

[0110] 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 the yield strength, tensile strength and elongation of the sample, respectively.

[0111] [Table 2]

[0112]

[0113]

[0114] [Table 3]

[0115]

[0116]

[0117] With reference to Tables 1 to 3, Test Examples 1 and 2 show differences in properties according to the annealing temperature. Test Examples 1 and 2 (which are cold-rolled steel sheets obtained according to an embodiment of the present invention and satisfying an annealing temperature range of 800°C to 900°C) achieve the following properties: YP of 1170MPa or greater, TS of 1400MPa or greater, El of 3.0% or greater, yield ratio of 70% or greater, and bendability (R / t) of 4.0 or less. In addition, in the final microstructure of the cold-rolled steel sheet, cementite, transition carbide, and fine precipitates each have an average size of 50nm or less, an average aspect ratio of 4.0 or less, and an area fraction greater than 0% and not greater than 5%. With reference to Tables 1 to 3, Test Examples 3 to 6 show differences in properties according to the first heat treatment temperature. Test Examples 3 to 5, which are uncoated cold-rolled steel sheets obtained according to an embodiment of the present invention and satisfying a first heat treatment temperature range of 100° C. to 300° C., achieve the following properties: YP of 1170 MPa or more, TS of 1400 MPa or more, El of 3.0% or more, yield ratio of 70% or more, and bendability (R / t) of 4.0 or less. In addition, in the final microstructure of the cold-rolled steel sheet, cementite, transition carbide, 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% and not more than 5%.

[0118] Test Example 6, which is a coated cold-rolled steel sheet obtained according to an embodiment of the present invention and satisfies the first heat treatment temperature range of 450°C to 600°C, achieves the following properties: YP of 1170 MPa or more, TS of 1400 MPa or more, El of 3.0% or more, yield ratio of 70% or more, and bendability (R / t) of 4.0 or less. In addition, in the final microstructure of the cold-rolled steel sheet, cementite, transition carbide, 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% and not more than 5%.

[0119] On the other hand, Test Example 24, which was annealed at 350°C and was higher than and did not meet the first heat treatment temperature range of 100°C to 300°C, did not achieve the desired properties of YP of 1170MPa or more and TS of 1400MPa or more, and did not meet the carbide average size range of 50nm or less and the carbide average aspect ratio range of 4.0 or less. When the first heat treatment temperature was maintained in the range between 300°C and 450°C, the strength deteriorated due to the transformation heat. However, when the first heat treatment temperature met the range of 450°C to 600°C as in Test Example 6, the transformation can be suppressed, and thus the material properties can be ensured.

[0120] Referring to Tables 1 to 3, the property differences according to the second heat treatment conditions are shown. The value of Inequality 1 shown in Table 2 represents the value of [(T+300)×(10+log(t))] based on the second heat treatment temperature T and the second heat treatment holding time t. In Inequality 1, the unit of the second heat treatment temperature T is ℃, and the unit of the second heat treatment holding time t is hours.

[0121] For example, Test Examples 1 to 6, 11 and 20 to 24 applied the 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 applied the 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 applied the 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 applied the 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 applied the 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 applied the 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 applied the 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 applied the 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 applied the 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 applied the 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 applied the 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 applied the 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 applied the conditions of a second heat treatment temperature T of 120°C and a second heat treatment holding time t of 20 hours.

[0122] Initially, Test Examples 1 to 6 showed that, although the annealing temperature and the first heat treatment conditions varied within the range proposed in the present invention, the desired properties were ensured when the second heat treatment temperature T and the second heat treatment holding time t were applied in such a manner that the values ​​of Inequality 1 [(T+300)×(10+log(t))] given as the second heat treatment conditions satisfied the range between 3800 and 5650.

[0123] Test Examples 7 and 8 (see Fig.11Group C) shows that when the value of Inequality 1 given as the second heat treatment condition is less than 3800, transition carbides are not formed in the final microstructure, and thus YP fails to reach the desired value (1170 MPa or more).

[0124] Test Examples 9 to 13, 18 and 19 (see Fig.11 Group A) shows that when the value of Inequality 1 given as the second heat treatment condition is between 3800 and 5650, YP of 1170 MPa or more, TS of 1400 MPa or more, El of 3.0% or more, yield ratio of 70% or more and bendability (R / t) of 4.0 or less are satisfied, and cementite, transition carbide 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 greater than 0% and not greater than 5%.

[0125] Test Examples 14 to 17 (see Fig.11 Group B) shows that when the value of Inequality 1 given as the second heat treatment condition is greater than 5650, the bendability deteriorates due to the increase in the size and aspect ratio of the carbides. That is, the desired property of bendability (R / t) of 4.0 or less is not achieved, and the average aspect ratio of the carbides does not satisfy the range of 4.0 or less. In other words, due to carbide shape defects, the desired bendability (R / t) of 4.0 or less is not satisfied.

[0126] Also, refer to Tables 1 to 3 and Figures 7 to 10 , Test Examples 20 to 23 show differences in properties according to the alloy composition. Specifically, Test Examples 20 and 21 (which are cold-rolled steel sheets obtained according to an embodiment of the present invention and satisfy the following composition: 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: greater than 0 wt% and not greater than 0.02 wt%, S: greater than 0 wt% and not greater than 0.005 wt%, Al: 0.01 wt% to 0.05 wt%, Cr: greater than 0 wt% and % 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 the balance Fe) achieves the following properties: YP of 1170 MPa or more, TS of 1400 MPa or more, El of 3.0% or more, yield ratio of 70% or more and bendability (R / t) of 4.0 or less. In addition, in the final microstructure of the cold-rolled steel sheet, cementite, transition carbide 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% and not more than 5%.

[0127] On the other hand, Test Example 22 (which satisfies the annealing, first heat treatment, and second heat treatment conditions of the present invention, but is less than and does not satisfy the composition range of C: 0.23 wt % to 0.35 wt %) shows that the desired properties of YP of 1170 MPa or more and 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 is greater than and does not satisfy the composition range of Si: 0.05 wt % to 0.5 wt %) does not achieve the desired properties of YP of 1170 MPa or more due to the transformation of intermediate phases such as ferrite and bainite. In particular, 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 to be greater than 10%.

[0128] The cold-rolled steel sheet and the manufacturing method thereof according to the embodiment of the present invention have been described above. According to the present invention, a high-strength cold-rolled steel sheet having high tensile strength, a high yield ratio (YP / TS) greater than 70%, and excellent bendability (R / t) of 4.0 or less can be achieved. Therefore, it is expected to use a material having excellent impact absorbency and formability to form a component of a complex shape to improve passenger safety and improve fuel efficiency by lightweighting the vehicle body.

[0129] While the invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined in the following claims.

Claims

1. A cold-rolled steel sheet, the cold-rolled steel sheet comprising: 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): greater than 0 wt% and not more than 0.02 wt%, Sulfur (S): greater than 0 wt% and not more than 0.005 wt%, Aluminum (Al): 0.01 wt% to 0.05 wt%, Chromium (Cr): greater than 0 wt% and not more than 0.8 wt%, Molybdenum (Mo): greater than 0 wt% and not more than 0.4 wt%, Titanium (Ti): 0.01 wt% to 0.1 wt%, Vanadium (V): greater than 0 wt% and not more than 0.3 wt%, Boron (B): 0.001 wt% to 0.005 wt%, and the remainder of Iron (Fe) and other inevitable impurities, The final microstructure of the cold-rolled steel sheet comprises cementite, transition carbide and fine precipitates, wherein the transition carbide comprises ε-carbide in which the atomic ratio of substitution elements selected from Fe, Mn, Cr and Mo to C is 2.5:1, or η-carbide in which the atomic ratio of substitution elements to C is 2:1, and the atomic ratio of alloying elements selected from Mo, V and Ti to C in the fine precipitates is 1:1, and 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 (El) 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 according to claim 1, wherein: Cementite, transition carbide and 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: Cementite, transition carbide, and fine precipitates each have an area fraction greater than 0% and not greater than 5%.

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

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

6. A method for manufacturing a cold-rolled steel sheet, the method comprising: (a) hot rolling a steel material, the 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): greater than 0 wt% and not greater than 0.02 wt%, Sulfur (S): greater than 0 wt% and not greater than 0.005 wt%, Aluminum (Al): 0.01 wt% to 0.05 wt%, Chromium (Cr): greater than 0 wt% and not greater than 0.8 wt%, Molybdenum (Mo): greater than 0 wt% and not greater than 0.4 wt%, Titanium (Ti): 0.01 wt% to 0.1 wt%, Vanadium (V): greater than 0 wt% and not greater than 0.3 wt%, Boron (B): 0.001 wt% to 0.005 wt% and the balance of 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, wherein the final microstructure of the cold rolled steel sheet obtained by carrying out steps (a) to (c) comprises cementite, transition carbides and fine precipitates, the transition carbides comprising ε-carbides in which the atomic ratio of substitution elements selected from Fe, Mn, Cr and Mo to C is 2.5:1, or η-carbides in which the atomic ratio of substitution elements to C is 2:1, and the atomic ratio of alloying elements selected from Mo, V and Ti to C in the fine precipitates is 1:1, and Cementite is formed during the first heat treatment, transition carbides are formed during the second heat treatment, and fine precipitates are formed during hot rolling.

7. The method according to claim 6, wherein: Step (a) is carried out under the conditions of a reheating temperature of 1150°C to 1300°C, a finishing rolling temperature of 800°C to 1000°C, and a coiling temperature of 500°C to 650°C, and Step (c) is performed under the 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 includes maintaining the second heat treatment temperature T satisfying Inequality 1 for a second heat treatment holding time t, Inequality 1: 3800 ≤ (T + 300) × (10 + log (t)) ≤ 5650 (Wherein, the unit of T is °C and the unit of t is hour).

8. The method according to claim 6, wherein: Step (a) is carried out under the conditions of a reheating temperature of 1150°C to 1300°C, a finishing rolling temperature of 800°C to 1000°C, and a coiling temperature of 500°C to 650°C, and Step (c) includes coating, and is performed under the 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 includes maintaining the second heat treatment temperature T satisfying Inequality 1 for a second heat treatment holding time t, Inequality 1: 3800 ≤ (T + 300) × (10 + log (t)) ≤ 5650 (Wherein, the unit of T is °C and the unit of t is hour).

9. The method according to claim 6, wherein: In step (c), the first heat treatment after annealing is performed after cooling to the first heat treatment temperature.

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

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