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), greater than 0 wt% and less than or equal to 0.02 wt% of phosphorus (P), greater than 0 wt% and less than or equal to 0.005 wt% of sulfur (S), 0.01 wt% to 0.05 wt% of aluminum (Al), greater than 0 wt% and less than or equal to 0.8 wt% of chromium (Cr), and the balance of steel. The present invention relates to a cold-rolled steel sheet having a final microstructure containing cementite, transition carbides, and fine precipitates, the final microstructure being characterized in that: the final microstructure of the cold-rolled steel sheet contains, by weight, more than 0% and less than or equal to 0.4% of molybdenum (Mo), 0.01% to 0.1% of titanium (Ti), 0.001% to 0.005% of boron (B), and the remainder being iron (Fe) and unavoidable impurities; the transition carbide comprises epsilon-carbide in which the atomic ratio of a displacement element to carbon is 2.5: 1 or eta-carbide in which the atomic ratio of the displacement element to carbon is 2: 1, the displacement element is any one of Fe, Mn, Cr or Mo, the atomic ratio of the alloy element of the fine precipitate to carbon is 1: 1, and the alloy element is any one of Mo or Ti.

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 collision resistance. Recently, although the number of vehicle parts has decreased due to the popularity of electric vehicles, the weight of the vehicle has increased due to the use of batteries, so the demand for collision resistance has further expanded. Therefore, efforts are being made to achieve ultra-high strength of collision-related parts such as front bumper beams, side beams, and door anti-collision beams, which contributes to collision 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 steels), and due to the characteristics of the above-mentioned technology, the bendability of the steel sheet is a very important 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 capable of forming martensitic steel having 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, a cold-rolled steel sheet is provided, 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 %, boron (B): 0.001 wt % to 0.005 wt. %, and the balance is iron (Fe) and inevitable impurities, wherein the final microstructure of the cold-rolled steel sheet comprises cementite, transition carbide and fine precipitates, the transition carbide comprises 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 η-carbide in which the atomic ratio of the substitution element to C is 2:1, and the atomic ratio of an alloying element selected from Mo and Ti to C in the fine precipitates is 1:1, and the cold-rolled steel sheet has a yield point (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 80% 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 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%, 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 subjecting the cold rolled steel material to annealing, a first heat treatment and a second heat treatment process, 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 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 and Ti to C in the fine precipitates is 1:1, the cementite is formed during the first heat treatment process, the transition carbides are formed during the second heat treatment process, and the fine precipitates are formed during the hot rolling process.

[0013] Step (a) may 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, and step (c) may be carried out under the conditions of an annealing temperature of 800°C to 900°C, a first heat treatment temperature of 100°C to 300°C, and a second heat treatment temperature of 100°C to 210°C.

[0014] Step (a) may 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, and step (c) may include plating and may be carried out under the conditions of an annealing temperature of 800°C to 900°C, a first heat treatment temperature of 450°C to 600°C, and a second heat treatment temperature of 100°C to 210°C.

[0015] In step (c), cooling to a first heat treatment temperature may be performed after the annealing process, and then the first heat treatment process may be performed.

[0016] In step (c), cooling to room temperature may be performed after the first heat treatment process, and then a second heat treatment process may be performed by heating.

[0017] The second heat treatment process may include maintaining the second heat treatment temperature for 3 hours to 20 hours.

[0018] Beneficial effects

[0019] 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 can be expected that the application of materials having excellent impact absorbency and excellent formability to form parts of complex shapes can contribute to improving vehicle passenger safety and improving fuel efficiency by reducing vehicle body weight. However, the scope of the present invention is not limited by the above-mentioned effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] 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.

[0021] Figure 2 The final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention includes ε-carbide (Fe 2.5 C) Microscopic image of the analysis results.

[0022] 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.

[0023] Figure 4 : is an image showing the size distribution of carbides in the final microstructure of a cold-rolled steel sheet according to an embodiment of the present invention.

[0024] Figure 5 : is an image showing the aspect ratio distribution of carbides in the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention.

[0025] Figure 6 1 and 2 are images showing steps of sequentially performing annealing, a first heat treatment, and a second heat treatment processes in a method of manufacturing a cold-rolled steel sheet according to an embodiment of the present invention.

[0026] Figure 7 is a microscopic image of the final microstructure of Test Example 1 according to the present invention.

[0027] Figure 8 is a microscopic image of the final microstructure of Test Example 2 according to the present invention.

[0028] Fig. 9 is a microscopic image of the final microstructure of Test Example 3 according to the present invention.

[0029] Fig.10 is a microscopic image of the final microstructure of Test Example 4 according to the present invention. DETAILED DESCRIPTION

[0030] 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 appropriately selected in consideration of their functions in the present invention, and the definitions of these terms should be based on the entire content of this specification. A detailed description of an ultra-high strength cold rolled steel sheet having a high yield ratio and excellent bendability and a method for manufacturing the same will be provided below.

[0031] The 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 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%, boron (B): 0.001 wt% to 0.005 wt%, and the balance of iron (Fe) and inevitable impurities.

[0032] The functions and contents of components contained in the cold-rolled steel sheet will now be described.

[0033] Carbon (C)

[0034] C is the most effective and important element for improving the strength of steel. In addition, 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 for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of C is less than 0.23% by weight of the total weight, the above-mentioned effect 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.

[0035] Silicon (Si)

[0036] 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 carbides 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 increasing the concentration of C 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 also 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 adding Si may not be properly achieved. On the other hand, when the Si content is greater than 0.5 wt. % of the total weight, ferrite may be excessively formed to reduce strength, oxides may be formed on the surface of the steel sheet to reduce the galvanizability of the steel sheet, red scale may be formed during reheating and hot rolling to cause problems in surface quality, toughness and plasticity may be reduced, and the weldability of the steel may also be reduced.

[0037] Manganese (Mn)

[0038] Mn is an element that helps to improve strength 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 MnS inclusions. MnS has ductility and is therefore elongated in the direction of plastic processing. However, due to the formation of MnS, the content of S in steel is reduced, so that the grains become susceptible and the formation of FeS (low melting point compound) is suppressed. Although the acid resistance and oxidation resistance of steel are hindered, the yield point can be improved by refining pearlite and solid solution strengthening ferrite. Mn can be added in a content ratio of 0.3 wt % to 2.3 wt % 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 wt %, the above-mentioned strength ensuring effect may not be fully achieved. 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, bendability may be reduced by forming inner and outer segregation regions of the continuously cast slab and steel plate and causing the formation and propagation of cracks. That is, slab quality and weldability may be reduced, and center segregation may occur, thereby reducing ductility and workability of the base steel plate.

[0039] Phosphorus (P)

[0040] P can improve the strength of steel by solid solution strengthening and 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 also be reduced.

[0041] Sulfur (S)

[0042] S is an element that combines with Mn, Ti, etc. to improve the machinability of steel and forms fine MnS precipitates to improve machinability, but generally inhibits ductility and weldability. S can 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 the 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.

[0043] Aluminum (Al)

[0044] Al is an element mainly used as a deoxidizer, and prevents slab cracks during nitride formation, promotes the formation of ferrite, improves elongation, inhibits the formation of carbides, and stabilizes austenite by increasing the C concentration in austenite. Al is also an element used as a layer between Fe and zinc (Zn) plating layers to improve platability, and can effectively inhibit the formation of Mn bands in hot-rolled coils. Al can preferably be added at 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 adding Al may not be properly achieved. On the other hand, 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 castability, Al may be concentrated on the surface of the steel sheet, thereby reducing platability, and AlN may be formed in the slab, resulting in hot rolling cracks.

[0045] Chromium (Cr)

[0046] Cr is an element that can improve hardenability and ensure high strength, and can improve quenching ability as an austenite stabilizing element. In addition, Cr improves elongation by forming Cr-based precipitates in grains during annealing. Cr may preferably be added in a content ratio of greater than 0 wt % and not greater than 0.8 wt % of the total weight of a base steel sheet for forming a cold-rolled steel sheet according to an embodiment of the present invention. When the content of Cr is greater than 0.8 wt %, a saturation effect may occur, laser weldability and ductility may be reduced, and plating performance may be hindered.

[0047] Molybdenum (Mo)

[0048] 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 preferably 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 %, manufacturing costs may increase, and weldability may decrease.

[0049] Titanium (Ti)

[0050] Ti contributes to grain refinement and BN formation suppression. Ti may preferably 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 the excessive formation of BN precipitates may reduce the slab quality and strength. 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.

[0051] Boron (B)

[0052] 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 preferably 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 %, grain boundary embrittlement may increase, weldability may decrease, and the surface quality of the steel may be hindered due to the formation of BN.

[0053] 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, and Figure 2 The final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention includes ε-carbide (Fe 2.5 C) 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 size distribution of carbides 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 aspect ratio distribution of carbides in the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention.

[0054] refer to Figures 1 to 5 , the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention comprises cementite, transition carbide and fine precipitates. In cementite (Fe3C), the atomic ratio of Fe to C 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 fine precipitates are characterized in that the atomic ratio of an alloying element selected from Mo and Ti to C is 1:1. Carbides and fine precipitates may partially contain nitrogen.

[0055] 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 of at least five microscopic images using a scanning electron microscope.

[0056] The final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention may be composed only of tempered martensite. Alternatively, the final microstructure of the cold-rolled steel sheet according to another embodiment of the present invention may be composed of tempered martensite, ferrite and bainite, and the tempered martensite may have an area fraction of 80% or more and less than 100%, while the ferrite and bainite may have an area fraction greater than 0% and not greater than 20%. The above microstructure is based on the results of analyzing the 1 / 4 point in the thickness direction from a 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 80%, the target strength may not be achieved. In addition, in the present invention, due to insufficient cooling rate, ferrite and bainite are inevitably formed, and as the main factor in reducing strength, its smaller area fraction is preferred, and the sum of the area fractions of the two phases of ferrite and bainite is required not to exceed 20%.

[0057] The cold-rolled steel sheet according to the embodiment of the present invention (which has the above-mentioned alloy element composition and microstructure) includes cementite-type carbides, but can achieve a yield point (YP) of 1170MPa or more, a tensile strength (TS) of 1400MPa 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. For example, the cold-rolled steel sheet according to the embodiment of the present invention can have a yield point (YP) of 1170MPa to 1400MPa, a tensile strength (TS) of 1400MPa to 1700MPa, an elongation (El) of 3.0% to 9.0%, a yield ratio of 70% to 90%, and a bendability (R / t) of 2.0 to 4.0. In bendability (R / t), R represents the minimum bending radius and t represents thickness.

[0058] 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.

[0059] According to an embodiment of the present invention, a method for manufacturing a steel plate 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%; 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 process on the cold rolled steel material.

[0060] 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.

[0061] When the steel material is reheated to 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 fully and evenly dispersed during the continuous casting process. However, at a reheating temperature exceeding 1300°C, adverse effects such as austenite coarsening or decarburization may occur, and the target strength may not be obtained. That is, when the reheating temperature is higher than 1300°C, very coarse austenite grains may be formed, so the 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, so it may lead to increased manufacturing costs and reduced productivity.

[0062] The finishing temperature (FDT) of hot rolling is a very important factor affecting the quality of the final material, and hot rolling at 800°C to 1000°C can refine austenite. However, when the finishing temperature is lower than 800°C, the hot rolling load may increase and a mixed grain structure may appear at the edge. In addition, when hot rolling is performed at a temperature higher than 1000°C, the target mechanical properties may not be obtained due to coarse grains. After hot rolling, cooling may be performed at a cooling rate of 1°C / s to 100°C / s, and as the cooling rate increases, the average grain size may decrease.

[0063] 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, the difference in cooling rate between the center and edge of the steel plate may cause an uneven microstructure and the inside of the grain boundary may be oxidized.

[0064] 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.

[0065] The cold rolling step (b) may include performing pickling and then performing cold rolling at a reduction ratio of 35% to 65%. The higher the reduction ratio, the more the formability is improved due to the microstructure refinement effect. When cold rolling is performed at a reduction ratio of less than 35%, a uniform microstructure may not be easily obtained, 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.

[0066] Figure 6 1 and 2 are images showing steps of sequentially performing annealing, a first heat treatment, and a second heat treatment processes in a method of manufacturing a cold-rolled steel sheet according to an embodiment of the present invention.

[0067] 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.

[0068] Ac3(℃)=910-203[C] 0.5 -30[Mn]+44.7[Si]+31.5[Mo]-15.2[Ni],

[0069] Here, [C], [Mn], [Si], [Mo] and [Ni] represent the weight % values ​​of C, Mn, Si, Mo and Ni in the steel material.

[0070] Based on the method of manufacturing a cold-rolled steel sheet, according to an embodiment of the present invention, an annealing process is performed to maintain a temperature of Ac3 or higher (more specifically, an annealing temperature of 800° C. to 900° C.) for 60 seconds to 600 seconds.

[0071] Subsequently, cooling is performed to 500 to 700°C at a cooling rate of 1 to 20°C / s, and then to the martensitic transformation finish temperature (or cooling end temperature) at a cooling rate of 5 to 50°C / s. Here, the martensitic transformation finish temperature is 100 to 350°C.

[0072] Afterwards, the unplated cold-rolled steel sheet is subjected to a first heat treatment process, wherein 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.

[0073] Meanwhile, for the plated cold-rolled 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 a heat treatment is performed, the transformation heat caused by the formation of bainite may cause material degradation. 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 material quality can be ensured.

[0074] In order to completely complete the transformation to martensite, the lower the first heat treatment temperature within the above temperature range, the more preferable it is.

[0075] According to an embodiment of the present invention, the method for manufacturing cold rolled steel sheet is characterized in that the cooling after annealing process and before the first heat treatment process is only carried out to the first heat treatment temperature, rather than being carried out quickly to room temperature. When being cooled to room temperature quickly after annealing process and before the first heat treatment process, although the first heat treatment process is carried out, cementite is not included in the final microstructure of cold rolled steel sheet. However, as in the present invention, when being cooled to the first heat treatment temperature only after annealing process and before the first heat treatment process rather than being carried out quickly to room temperature, cementite can be formed during the first heat treatment process. Generally, the steel material comprising cementite has poor processability.

[0076] However, rapidly performing cooling to room temperature after the annealing process and before the first heat treatment process to fundamentally prevent the formation of cementite increases manufacturing costs because an additional system or the like is required.

[0077] In the cold-rolled steel sheet according to an embodiment of the present invention, although the final microstructure contains cementite since cooling to room temperature is not quickly performed after the annealing process and before the first heat treatment process, the properties of a yield point (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 can be ensured by accurately controlling the subsequent processes, and thus an ultra-high strength cold-rolled steel sheet with a high yield ratio and excellent bendability can be achieved.

[0078] After the first heat treatment process, cooling to room temperature can be performed, and then a second heat treatment process can be performed by heating. The second heat treatment process includes a process of heating to a second heat treatment 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 for 3 to 20 hours. When the second heat treatment temperature is lower than 100°C, the target yield point may not be easily achieved during the heat treatment process, and when the second heat treatment temperature is higher than 210°C, the bendability may be reduced. In addition, when the heat treatment time is too long, since a decrease in bendability will occur (even within the range of the above-mentioned second heat treatment temperature), the time for maintaining the second heat treatment temperature is controlled to be 3 to 20 hours.

[0079] 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 is characterized by containing cementite, transition carbides and fine precipitates, the transition carbides containing ε-carbides having an atomic ratio of substitution elements selected from Fe, Mn, Cr and Mo to C of 2.5:1, or η-carbides having an atomic ratio of substitution elements to C of 2:1, and the fine precipitates having an atomic ratio of alloying elements selected from Mo and Ti to C of 1:1.

[0080] Cementite is formed during the first heat treatment process 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, and 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 more preferred.

[0081] Transition carbides are formed during the second heat treatment process. According to an embodiment of the present invention, in the method for manufacturing a cold-rolled steel sheet, when the second heat treatment process is not performed, transition carbides do not exist. Transition carbides are required to increase the yield point, and transition carbides may exist in a ratio of 0% to 5% of the total area fraction.

[0082] 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.

[0083] Test Example

[0084] 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.

[0085] 1. Composition of the sample

[0086] In the test examples, samples having the alloy element compositions (unit: weight %) shown in Table 1 were provided.

[0087] [Table 1]

[0088] Steel Type C Si Mn Cr Mo Ti B Fe A 0.25 0.1 2.0 0.4 0.2 0.03 0.0025 margin B 0.24 0.1 1.9 0.3 0.2 0.06 0.0025 margin C 0.22 0.2 2.2 0.3 0.2 0.03 0.0022 margin D 0.26 0.7 1.8 0.4 0.2 0.03 0.0020 margin

[0089] In Table 1, steel types 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%, B: 0.001 wt% to 0.005 wt% and the balance of Fe. However, steel type C is lower than and does not satisfy the composition range of C of 0.23 wt% to 0.35 wt%, and steel type D exceeds and does not satisfy the composition range of Si of 0.05 wt% to 0.5 wt%.

[0090] 2. Process conditions and property evaluation

[0091] Table 2 shows various heat treatment process 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 process conditions shown in Tables 1 and 2.

[0092] In Table 2, "Steel Type" indicates the composition shown in Table 1, and in Table 3, "YP (MPa)", "TS (MPa)" and "EL (%)" respectively indicate the yield point, tensile strength and elongation of the specimen.

[0093] [Table 2]

[0094]

[0095]

[0096] [Table 3]

[0097]

[0098]

[0099] Referring to Tables 1 to 3, Test Examples 1 to 4 show differences in properties according to alloy compositions. Specifically, Test Examples 1 and 2 (which are cold-rolled steel sheets obtained according to an embodiment of the present invention and satisfying 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 %, B: 0.001 wt % to 0.005 wt % and the balance of Fe) can satisfy the properties of a yield point (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, and contains cementite, transition carbide, and fine precipitates in its final microstructure, each of which has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of greater than 0% and not greater than 5%. In contrast, Test Example 3, which is lower than and does not satisfy the composition range of 0.23 wt % to 0.35 wt % of C, may not achieve the target property of a yield point (YP) of 1170 MPa or more. Test Example 4, which exceeds and does not satisfy the composition range of Si of 0.05 wt % to 0.5 wt %, may not achieve the target property of a yield point (YP) of 1170 MPa or more due to the formation of intermediate phases such as ferrite and bainite. In particular, Test Example 4 may ensure bendability because the carbide size and carbide aspect ratio are satisfied, but may not achieve the yield point because ferrite is formed to be greater than 10%.

[0100] Referring to Tables 1 to 3, Test Examples 5 to 7 show differences in properties according to annealing temperatures. Test Examples 6 and 7, 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, can satisfy the properties of a yield point (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, and contain cementite, transition carbide, and fine precipitates in their final microstructure, each of which has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of greater than 0% and not greater than 5%.

[0101] In contrast, Test Example 5, which is below and does not meet the annealing temperature range of 800° C. to 900° C., may not achieve the target property of a yield point (YP) of 1170 MPa or more.

[0102] Referring to Tables 1 to 3, Test Examples 8 to 12 show differences in properties according to the first heat treatment temperature. Test Examples 8 to 10, which are unplated 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, can satisfy the properties of a yield point (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, and contain cementite, transition carbide, and fine precipitates in their final microstructure, each of which has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of greater than 0% and not greater than 5%.

[0103] Test Example 12 (which is a plated 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) can satisfy the properties of a yield point (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, and contains cementite, transition carbide and fine precipitates in its final microstructure, each of which has 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%.

[0104] In contrast, Test Example 11, which was annealed at an annealing temperature of 350°C, may not achieve the target properties of a yield point (YP) of 1170 MPa or more and a tensile strength (TS) of 1400 MPa or more, and does not satisfy the average carbide size range of 50 nm or less and the average carbide aspect ratio range of 4.0 or less. When the first heat treatment temperature is maintained in a range of more than 300°C and less than 450°C, a decrease in strength occurs due to transformation heat. However, when the first heat treatment temperature of 450°C to 600°C is satisfied as in Test Example 12, the transformation can be suppressed, and thus the material quality can be ensured.

[0105] Referring to Tables 1 to 3, Test Examples 13 to 22 show differences in properties according to the second heat treatment temperature. Test Examples 15 to 19, which are cold-rolled steel sheets obtained according to an embodiment of the present invention and satisfying a second heat treatment temperature range of 100°C to 210°C, can satisfy the properties of a yield point (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, and contain cementite, transition carbide, and fine precipitates in their final microstructure, each of which has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of greater than 0% and not greater than 5%.

[0106] In contrast, Test Examples 13 and 14, which are lower than and do not meet the second heat treatment temperature range of 100° C. to 210° C., may fail to achieve the target property of a yield point (YP) of 1170 MPa or more because transition carbides are not formed.

[0107] Test Examples 20 to 22, which exceed and do not satisfy the second heat treatment temperature range of 100° C. to 210° C., may fail to achieve the target property of bendability (R / t) of 4.0 or less and fail to satisfy the carbide average aspect ratio range of 4.0 or less. That is, the target property of bendability (R / t) of 4.0 or less may not be satisfied due to carbide shape defects.

[0108] Referring to Tables 1 to 3, Test Examples 23 to 25 show differences in properties according to the second heat treatment time. Test Examples 24 and 25, which are cold-rolled steel sheets obtained according to an embodiment of the present invention and satisfying a second heat treatment temperature range of 100°C to 210°C and a second heat treatment time of 3 hours to 20 hours, can satisfy the properties of a yield point (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, and contain cementite, transition carbide, and fine precipitates in their final microstructure, each of which has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of greater than 0% and not greater than 5%.

[0109] In contrast, Test Example 23, which exceeds and does not satisfy the second heat treatment time of 3 hours to 20 hours, may not achieve the target property of bendability (R / t) of 4.0 or less, and does not satisfy the carbide average size range of 50 nm or less and the carbide average aspect ratio range of 4.0 or less. That is, when an excessively long second heat treatment time of 24 hours is applied, the aspect ratio increases due to the growth of carbides, and the target property of bendability (R / t) of 4.0 or less may not be satisfied.

[0110] 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 realized. Therefore, it is expected that the application of materials having excellent impact absorbency and excellent formability to form parts of complex shapes can contribute to improving vehicle passenger safety and improving fuel efficiency through vehicle body weight reduction.

[0111] 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%, Boron (B): 0.001 wt% to 0.005 wt%, the remainder is Iron (Fe) and unavoidable impurities, The final microstructure of the cold-rolled steel sheet comprises cementite, transition carbide and fine precipitates, 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 and Ti to C in the fine precipitates is 1:1, and The cold-rolled steel sheet has a yield point (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 80% or more and less than 100%, and the area fractions of ferrite and bainite are greater than 0% and not greater 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%, 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 process 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 and Ti to C in the fine precipitates is 1:1, and Cementite is formed during the first heat treatment process, transition carbides are formed during the second heat treatment process, 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 Wherein step (c) is carried out under the conditions of an annealing temperature of 800°C to 900°C, a first heat treatment temperature of 100°C to 300°C and a second heat treatment temperature of 100°C to 210°C.

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 Wherein step (c) comprises plating, and is carried out under the conditions of an annealing temperature of 800°C to 900°C, a first heat treatment temperature of 450°C to 600°C, and a second heat treatment temperature of 100°C to 210°C.

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

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

11. The method according to claim 7 or 8, wherein: The second heat treatment process includes maintaining the second heat treatment temperature for 3 hours to 20 hours.

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