Cold-rolled steel sheet and method for producing the same

Abstract

The present invention provides a cold-rolled steel sheet composed of carbon (C): 0.23 to 0.35% by weight, silicon (Si): 0.05 to 0.5% by weight, manganese (Mn): 0.3 to 2.3% by weight, phosphorus (P): more than 0 and 0.02% by weight or less, sulfur (S): more than 0 and 0.005% by weight or less, aluminum (Al): 0.01 to 0.05% by weight, chromium (Cr): more than 0 and 0.8% by weight or less, molybdenum (Mo): more than 0 and 0.4% by weight or less, titanium (Ti): 0.01 to 0.1% by weight, boron (B): 0.001 to 0.005% by weight, and the balance of iron (Fe) and other inevitable impurities. The final microstructure of the cold-rolled steel sheet includes cementite, transition carbides, and fine precipitates. The transition carbides include ε-carbide with an atomic ratio of 2.5:1 between a substitutional element which is any one of iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo) and carbon, or η-carbide with an atomic ratio of 2:1. The fine precipitates include an alloy element which is any one of molybdenum (Mo) and titanium (Ti) and carbon with an atomic ratio of 1:1.

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 a super high-strength cold-rolled steel sheet having excellent yield ratio and bendability and a method for manufacturing the same.

Background Art

[0002] In the automotive industry, the requirements for the collision stability of vehicle bodies have been continuously increasing. Recently, as the spread of electric vehicles has expanded, the number of automotive parts has decreased, but the weight of vehicles due to the introduction of batteries has increased, and the requirements for collision stability have further expanded. As a result, the super high-strengthening of collision members such as front bumper beams, side seals, and door impact beams that contribute to collision stability has been continuously promoted. In particular, in the case of martensitic steel having the highest strength among cold-rolled steel sheets, its application has been expanded due to the increasing utilization of roll forming techniques. Due to the characteristics of the above-mentioned method, the bendability of the steel sheet acts as a very important factor.

[0003] As a related prior art, there is Japanese Patent Laid-Open No. 2005-105367.

Summary of the Invention

Problems to be Solved by the Invention

[0004] The technical problem to be achieved by the present invention is to provide a super high-strength cold-rolled steel sheet having a high yield ratio and excellent bendability and a method for manufacturing the same, and in particular, to provide a cold-rolled steel sheet capable of embodying martensitic steel having a tensile strength of 1400 MPa or more and a method for manufacturing the same.

Means for Solving the Problems

[0005] The cold-rolled steel sheet according to an embodiment of the present invention for solving the above problems consists of carbon (C): 0.23 to 0.35% by weight, silicon (Si): 0.05 to 0.5% by weight, manganese (Mn): 0.3 to 2.3% by weight, phosphorus (P): more than 0 and 0.02% by weight or less, sulfur (S): more than 0 and 0.005% by weight or less, aluminum (Al): 0.01 to 0.05% by weight, chromium (Cr): more than 0 and 0.8% by weight or less, molybdenum (Mo): more than 0 and 0.4% by weight or less, titanium (Ti): 0.01 to 0.1% by weight, boron (B): 0.001 to 0.005% by weight, and the balance iron (Fe) and other inevitable impurities. The final microstructure of the cold-rolled steel sheet includes cementite, transition carbide, and fine precipitate. The transition carbide includes ε-carbide with an atomic ratio of a substitutional element, which is any one of iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo), and carbon of 2.5:1, or η-carbide with an atomic ratio of 2:1. The fine precipitate has an atomic ratio of an alloy element, which is any one of molybdenum (Mo) and titanium (Ti), and carbon of 1:1, and has a yield strength (YP): 1170 MPa or more, a tensile strength (TS): 1400 MPa or more, an elongation (El): 3.0% or more, a yield ratio: 70% or more, and a bend formability (R / t): 4.0 or less.

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

[0007] In the cold-rolled steel sheet, the cementite, the transition carbide, and the fine precipitate may each have an area fraction of more than 0% and 5% or less.

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

[0009] In the cold-rolled steel sheet, the final microstructure consists of tempered martensite, ferrite, and bainite, and in terms of area fraction, tempered martensite: 80% or more and less than 100%, ferrite and bainite: more than 0% and 20% or less.

[0010] A method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention for solving the above problems includes: (a) hot-rolling a steel material composed of carbon (C): 0.23 to 0.35% by weight, silicon (Si): 0.05 to 0.5% by weight, manganese (Mn): 0.3 to 2.3% by weight, phosphorus (P): more than 0 and 0.02% by weight or less, sulfur (S): more than 0 and 0.005% by weight or less, aluminum (Al): 0.01 to 0.05% by weight, chromium (Cr): more than 0 and 0.8% by weight or less, molybdenum (Mo): more than 0 and 0.4% by weight or less, titanium (Ti): 0.01 to 0.1% by weight, boron (B): 0.001 to 0.005% by weight, and the balance 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. In the method for manufacturing a cold-rolled steel sheet, the final microstructure of the cold-rolled steel sheet realized by performing the steps (a) to (c) includes cementite, transitional carbide, and fine precipitates. The transitional carbide includes ε-carbide having an atomic ratio of 2.5:1 between a substitutional element which is any one of iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo) and carbon, or η-carbide having an atomic ratio of 2:1. The fine precipitate has an atomic ratio of 1:1 between an alloy element which is any one of molybdenum (Mo) and titanium (Ti) and carbon. The cementite is formed during the first heat treatment, the transitional carbide is formed during the second heat treatment, and the fine precipitate is formed during the hot-rolling step.

[0011] In the method for manufacturing a cold-rolled steel sheet, the step (a) may include hot-rolling under the conditions of a reheating temperature of 1150 to 1300°C, a finish rolling temperature of 800 to 1000°C, and a coiling temperature of 500 to 650°C. The step (c) may be performed under the conditions of an annealing temperature of 800 to 900°C, a first heat treatment temperature of 100 to 300°C, and a second heat treatment temperature of 100 to 210°C.

[0012] In the method for manufacturing the cold-rolled steel sheet, the step (a) includes a hot rolling step under the conditions of reheating temperature: 1150 to 1300 °C, finish rolling temperature: 800 to 1000 °C, and coiling temperature: 500 to 650 °C. The step (c) includes a plating step, and it may be carried out under the conditions of annealing temperature: 800 to 900 °C, first heat treatment temperature: 450 to 600 °C, and second heat treatment temperature: 100 to 210 °C.

[0013] In the method for manufacturing the cold-rolled steel sheet, the step (c) is characterized in that after the annealing step, after cooling to the first heat treatment temperature, the first heat treatment step is performed.

[0014] In the method for manufacturing the cold-rolled steel sheet, the step (c) is characterized in that after the first heat treatment step, after cooling to room temperature, the temperature is raised and the second heat treatment step is performed.

[0015] In the method for manufacturing the cold-rolled steel sheet, the second heat treatment step may include a step of maintaining at the second heat treatment temperature for 3 to 20 hours.

Advantages of the Invention

[0016] According to an embodiment of the present invention, it is possible to embody an ultra-high strength cold-rolled steel sheet having a high yield ratio and excellent bendability and a method for manufacturing the same. For example, according to the present invention, it is possible to embody a high yield characteristic with a yield ratio (YP / TS) exceeding 70% and a high strength cold-rolled steel sheet excellent in bendability with a bend performance (R / t) of 4.0 or less, together with a high tensile strength. As a result, it is expected to contribute to the improvement of the stability of automobile passengers through excellent formability for forming parts with complex shapes and the application of a material excellent in impact absorption ability, and the improvement of fuel efficiency through the weight reduction of the vehicle body. Of course, the scope of the present invention is not limited by such effects.

Brief Description of the Drawings

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MODE FOR CARRYING OUT THE INVENTION

[0018] A cold-rolled steel sheet and a manufacturing method thereof according to an embodiment of the present invention will be described in detail. The terms described later are terms appropriately selected in consideration of the functions in the present invention, and the definitions of such terms should be made based on the content throughout this specification. Hereinafter, specific contents of an ultra-high strength cold-rolled steel sheet having excellent bendability together with a high yield ratio and a manufacturing method thereof will be provided.

[0019] The cold-rolled steel sheet according to an embodiment of the present invention consists of carbon (C): 0.23 to 0.35% by weight, silicon (Si): 0.05 to 0.5% by weight, manganese (Mn): 0.3 to 2.3% by weight, phosphorus (P): more than 0 and 0.02% by weight or less, sulfur (S): more than 0 and 0.005% by weight or less, aluminum (Al): 0.01 to 0.05% by weight, chromium (Cr): more than 0 and 0.8% by weight or less, molybdenum (Mo): more than 0 and 0.4% by weight or less, titanium (Ti): 0.01 to 0.1% by weight, boron (B): 0.001 to 0.005% by weight, and the balance of iron (Fe) and other inevitable impurities.

[0020] Hereinafter, the roles and contents of the respective components contained in the cold-rolled steel sheet will be described.

[0021] Carbon (C)

[0022] Carbon (C) is the most effective and important element for increasing the strength of steel. Further, by adding carbon, it dissolves in austenite and forms a martensite structure during quenching. Furthermore, it combines with elements such as iron, chromium, and molybdenum to form carbides, improving strength and hardness. Carbon (C) may be added at a content ratio of 0.23 to 0.35% by weight of the total weight in the base steel sheet constituting the cold-rolled steel sheet according to an embodiment of the present invention. When the carbon content is less than 0.23% by weight of the total weight, the above-described effects cannot be realized, and there is a problem that sufficient strength cannot be ensured. On the contrary, when the carbon content exceeds 0.35% by weight of the total weight, there is a problem that weldability and workability deteriorate.

[0023] Silicon (Si)

[0024] Silicon (Si) is an element added to ensure bending properties and hydrogen embrittlement resistance characteristics through suppression of cementite formation. Also, silicon is an element added for strength increase and suppression of carbide formation due to the solid solution strengthening effect of ferrite. Silicon is well known as a ferrite stabilizing element, and can increase the fraction of ferrite and ductility during cooling. It is also known as an element that can promote the formation of martensite by carbon enrichment of austenite and ensure strength. On the other hand, silicon, together with aluminum, is added as a deoxidizer for removing oxygen in steel in the steelmaking process and can also have a solid solution strengthening effect. The above-mentioned silicon may be added in a content ratio of 0.05 to 0.5% by weight based on the total weight of the base steel plate constituting the cold-rolled steel sheet according to an embodiment of the present invention. When the content of silicon is less than 0.05% by weight based on the total weight, ductility cannot be ensured and the above-described addition effects of silicon cannot be fully exerted. Conversely, when the content of silicon is added in a large amount exceeding 0.5% by weight based on the total weight, ferrite is excessively formed and the strength decreases, oxides are formed on the surface of the steel sheet, the plating property of the steel sheet decreases, and there is a risk of giving problems to the surface quality by generating red scale during reheating and hot rolling, there are problems of decreased toughness and plastic workability, and there is a risk of decreasing the weldability of the steel.

[0025] Manganese (Mn)

[0026] Manganese (Mn) is an element that contributes to the improvement of strength through solid solution strengthening and an increase in hardenability. For example, manganese is an element that facilitates the formation of a low-temperature transformation phase and provides an effect of increasing strength through solid solution strengthening. A part of the manganese dissolves in the steel, and a part combines with sulfur contained in the steel to form MnS, which is a non-metallic inclusion. Since this MnS is ductile, it elongates significantly in the processing direction during plastic processing. However, due to the formation of MnS, while the sulfur component in the steel decreases, the crystal grains become brittle, and the formation of FeS, which is a low-melting-point compound, is suppressed. Although it inhibits the acid resistance and oxidation resistance of the steel, it makes the pearlite finer and solid solution strengthens the ferrite, thereby improving the yield strength. Manganese may be added in a content ratio of 0.3 to 2.3% by weight of the total weight in the base steel plate constituting the cold-rolled steel sheet according to an embodiment of the present invention. When the content of manganese is less than 0.3% by weight, the above-described effect of ensuring strength cannot be fully exerted. Also, when the content of manganese is 2.3% by weight or more, problems such as a decrease in bendability and hydrogen embrittlement resistance occur due to the formation of manganese bands and MnS. For example, segregation bands are formed inside and outside the continuous casting slab and the steel plate, inducing the generation and propagation of cracks, resulting in a problem of decreased bendability. That is, the quality and weldability of the slab may decrease, center segregation may occur, the ductility of the base steel plate may decrease, and the workability may decrease.

[0027] Phosphorus (P)

[0028] Phosphorus (P) can increase strength through solid solution strengthening and perform a function of suppressing the formation of carbides. The phosphorus may be added in a content ratio of more than 0 and 0.02% by weight or less of the total weight in the base steel plate constituting the cold-rolled steel sheet according to an embodiment of the present invention. When the content of phosphorus exceeds 0.02% by weight, the welded part may become brittle, brittleness may be induced through grain boundary segregation, the press formability may decrease, and the impact resistance may decrease.

[0029] Sulfur (S)

[0030] Sulfur (S) can combine with manganese, titanium, etc. to improve the machinability of steel, forming precipitates of fine MnS to enhance workability. However, it is generally an element that inhibits ductility and weldability. The sulfur may be added in a content ratio of more than 0 and not more than 0.005% by weight based on the total weight in the base steel plate constituting the cold-rolled steel sheet according to an embodiment of the present invention. When the sulfur content exceeds 0.005% by weight, the number of MnS inclusions increases, resulting in inferior bendability and hydrogen embrittlement resistance, and there may be a problem of segregation during continuous casting solidification and generation of hot cracks.

[0031] Aluminum (Al)

[0032] Aluminum (Al) is an element mainly used as a deoxidizer. It prevents slab cracking during nitride formation, promotes ferrite formation, improves elongation, suppresses carbide formation, and enhances the amount of carbon enrichment in austenite to stabilize austenite. Also, aluminum acts as a layer between iron and the zinc plating layer to improve plating properties and is an effective element for suppressing the formation of manganese bands in hot-rolled coils. The aluminum (Al) is preferably added in a content ratio of 0.01 to 0.05% by weight based on the total weight in the base steel plate constituting the cold-rolled steel sheet according to an embodiment of the present invention. When the aluminum (Al) content is less than 0.01% by weight, the above-described addition effects of aluminum cannot be fully exerted. Conversely, when the aluminum (Al) content exceeds 0.05% by weight and is added excessively, the strength decreases through ferrite formation, the aluminum inclusions increase to reduce continuous castability, it concentrates on the surface of the steel plate to reduce plating properties, and there is a problem of forming AlN in the slab to induce hot rolling cracks.

[0033] Chromium (Cr)

[0034] Chromium (Cr) is an element that can improve hardenability and ensure high strength. It is an austenite stabilizing element and has the effect of improving hardenability. Also, chromium precipitates Cr-based precipitates in the grains during annealing heat treatment to increase elongation. The chromium (Cr) is preferably added in a content ratio of more than 0 and not more than 0.8% by weight based on the total weight in the base steel sheet constituting the cold-rolled steel sheet according to an embodiment of the present invention. When the content of chromium (Cr) exceeds 0.8% by weight and is added excessively, a saturation effect appears, resulting in problems such as a decrease in laser weldability and ductility and an inhibition of plating properties.

[0035] Molybdenum (Mo)

[0036] Molybdenum (Mo) is an element added to improve hardenability and ensure strength and toughness. It is an element that can improve hydrogen embrittlement resistance by grain refinement and precipitation effects. The molybdenum (Mo) is preferably added in a content ratio of more than 0 and not more than 0.4% by weight based on the total weight in the base steel sheet constituting the cold-rolled steel sheet according to an embodiment of the present invention. When the content of molybdenum (Mo) exceeds 0.4% by weight, there are problems such as an increase in manufacturing cost and a decrease in weldability.

[0037] Titanium (Ti)

[0038] Titanium (Ti) contributes to suppressing grain refinement and the formation of BN. The titanium (Ti) is preferably added in a content ratio of 0.01 to 0.1% by weight based on the total weight in the base steel sheet constituting the cold-rolled steel sheet according to an embodiment of the present invention. When the content of the titanium (Ti) is less than 0.01% by weight, the quality of the slab deteriorates and the strength decreases due to a decrease in the ductility of the casting slab caused by excessive precipitation of BN precipitates. On the other hand, when the content of the titanium (Ti) exceeds 0.1% by weight, there are problems such as a decrease in bendability and hydrogen embrittlement resistance due to coarsening of TiN precipitation and an excessive increase in the recrystallization temperature, inducing a non-uniform structure.

[0039] Boron (B)

[0040] Boron (B) is an element added to suppress the formation of ferrite and increase the hardening ability of steel. Also, boron is a powerful hardening element and plays a role in preventing segregation of phosphorus (P) and improving strength. If segregation of phosphorus (P) occurs, secondary processing brittleness may occur. Therefore, by adding boron to prevent segregation of phosphorus (P), the resistance to processing brittleness is increased. The boron is preferably added in a content ratio of 0.001 to 0.005% by weight based on the total weight of the base steel plate constituting the cold-rolled steel sheet according to an embodiment of the present invention. When the content of boron is less than 0.001% by weight, strength cannot be ensured due to low hardenability. When the content of boron exceeds 0.005% by weight and is added excessively, grain boundary brittleness due to the formation of BN increases, weldability decreases, and problems may be induced that inhibit the surface quality of the steel due to the formation of boron oxides.

[0041] Figure 1 shows the result of analyzing cementite (Fe3C) among the carbides appearing in the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention, and Figure 2 shows ε-carbide (Fe 2.5 C) among the carbides appearing in the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention. Figure 3 is a diagram schematically showing the measurement method of the carbides appearing in the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention, Figure 4 is a distribution diagram showing the size of the carbides appearing in the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention, and Figure 5 is a distribution diagram showing the aspect ratio of the carbides appearing in the final microstructure of the cold-rolled steel sheet according to an embodiment of the present invention.

[0042] Referring to FIGS. 1 to 5, the final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention includes cementite, transition carbide, and fine precipitates. The cementite (Fe3C) has an atomic ratio of iron (Fe) to carbon of 3:1. The transition carbide includes ε-carbide with an atomic ratio of a substitutional element, which is any one of iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo), to carbon of 2.5:1, or η-carbide with an atomic ratio of 2:1. The fine precipitate is characterized in that the atomic ratio of an alloy element, which is any one of molybdenum (Mo) and titanium (Ti), to carbon is 1:1. The carbide and the fine precipitate may contain a part of nitrogen.

[0043] In the cold-rolled steel sheet according to the embodiment of the present invention, in order to ensure bendability and hydrogen embrittlement resistance, the cementite, the transition carbide, and the fine precipitate may each have an average size of 50 nm or less and an average aspect ratio of 4.0 or less. Referring to FIG. 3, the average size is the average size including the major and minor axes of the elliptical or acicular carbide 10, specifically, the average size including the size a of the minor axis and the size b of the major axis. Further, the average aspect ratio means the ratio (b / a) of the length of the major axis to the minor axis. In the cold-rolled steel sheet according to the embodiment of the present invention, the cementite, the transition carbide, and the fine precipitate may each have an area fraction of more than 0% and 5% or less. The analysis of the area fractions of the cementite, the transition carbide, and the fine precipitate was measured by utilizing replica analysis of a transmission electron microscope and using at least five or more microstructural photographs.

[0044] The final microstructure of the cold-rolled steel sheet according to the 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 consists of tempered martensite, ferrite and bainite, and in terms of area fraction, tempered martensite: 80% or more and less than 100%, ferrite and bainite: more than 0% and 20% or less. The above-described microstructure is based on the result of analyzing the 1 / 4 point in the thickness direction in a direction perpendicular to the rolling direction by a scanning electron microscope. In the present invention, when the area fraction of tempered martensite is less than 80%, the target strength cannot be achieved. Also, in the present invention, ferrite and bainite are microstructures inevitably generated by an insufficient cooling rate and are the main factors for reducing the strength. Therefore, the smaller the area fraction, the more preferable, and the total area fraction of the two phases of ferrite and bainite must not exceed 20%.

[0045] The cold-rolled steel sheet according to an embodiment of the present invention having the above-described composition and microstructure of alloying elements, despite containing cementite-type carbides, can achieve physical properties of yield strength (YP): 1170 MPa or more, tensile strength (TS): 1400 MPa or more, elongation (El): 3.0% or more, yield ratio: 70% or more, and bend formability (R / t): 4.0 or less. For example, the cold-rolled steel sheet according to an embodiment of the present invention may have a yield strength (YP): 1170 to 1400 MPa, a tensile strength (TS): 1400 to 1700 MPa, an elongation (El): 3.0 to 9.0%, a yield ratio: 70 to 90%, and a bend formability (R / t): 2.0 to 4.0. In the above bend formability (R / t), R is the minimum bend radius ratio and t is the unit thickness.

[0046] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention having the above-described composition and microstructure will be described.

[0047] The manufacturing method of a steel sheet according to an embodiment of the present invention includes: (a) a step of hot-rolling a steel material composed of carbon (C): 0.23 to 0.35 wt%, silicon (Si): 0.05 to 0.5 wt%, manganese (Mn): 0.3 to 2.3 wt%, phosphorus (P): more than 0 and 0.02 wt% or less, sulfur (S): more than 0 and 0.005 wt% or less, aluminum (Al): 0.01 to 0.05 wt%, chromium (Cr): more than 0 and 0.8 wt% or less, molybdenum (Mo): more than 0 and 0.4 wt% or less, titanium (Ti): 0.01 to 0.1 wt%, boron (B): 0.001 to 0.005 wt%, and the remaining iron (Fe); (b) a step of cold-rolling the hot-rolled steel material; and (c) a step of sequentially performing annealing, a first heat treatment, and a second heat treatment on the cold-rolled steel material.

[0048] The step (a) of hot-rolling can be carried out under the conditions of a reheating temperature of 1150 to 1300 °C, a finish rolling temperature of 800 to 1000 °C, and a coiling temperature of 500 to 650 °C.

[0049] When the steel material is reheated at the above-mentioned temperature (1150 to 1300 °C), the components segregated during the continuous casting process can be redissolved. When attempting to improve the strength through precipitation and solid solution strengthening, the strengthening elements must be sufficiently dissolved in austenite before hot rolling. Therefore, it is necessary to heat the steel material to 1150 °C or higher. When the reheating temperature is lower than 1150 °C, the solution of various carbides may not be sufficient, and there may be a problem that the components segregated during the continuous casting process do not distribute uniformly enough. However, when the reheating temperature exceeds 1300 °C, there are adverse effects such as coarsening of austenite and decarburization, and the desired strength cannot be obtained. That is, when the reheating temperature exceeds 1300 °C, very coarse austenite crystal grains are formed, and it may become difficult to ensure the strength. Also, when the reheating temperature exceeds 1300 °C, the heating cost increases, the process time is added, which may lead to an increase in manufacturing cost and a decrease in productivity.

[0050] The finish rolling temperature (FDT) is a very important factor affecting the final material properties. Rolling at 800 - 1000°C is a temperature at which austenite can be refined. However, if the finish rolling temperature is lower than 800°C, the rolling load increases during rolling, and a mixed grain structure may occur in the EDGE part. Also, rolling in a high temperature range above 1000°C cannot obtain the targeted mechanical properties due to the coarsening of crystal grains. After hot rolling, cooling is carried out at a cooling rate of 1 - 100°C / s. The faster the cooling rate, the more beneficial it is for the reduction of the average crystal grain size.

[0051] On the other hand, when the coiling temperature is lower than 500°C, there are problems of making the shape of the hot rolled coil non-uniform and increasing the cold rolling load. When the coiling temperature is higher than 650°C, non-uniform fine structures may be generated due to the difference in the cooling rate between the center and the edge of the steel plate, and problems such as oxidation of the interior of the grain boundaries may occur.

[0052] On the other hand, the hot rolling may be carried out under the condition that the reduction ratio is 35 - 65%. The fine structure of the steel material after hot rolling can include bainite, martensite, and ferrite.

[0053] The step of (b) cold rolling can include the step of cold rolling at a reduction ratio of 35 - 65% after performing a pickling process. The higher the reduction ratio, the more the formability is improved due to the effect of microstructure refinement. When the reduction in cold rolling is less than 35%, it is difficult to obtain a uniform fine structure. When designed to exceed 65%, the roll force becomes high and the process load becomes high.

[0054] Figure 6 is a diagram showing an overview of the heat treatment that illustrates the steps of sequentially performing an annealing step, a first heat treatment step, and a second heat treatment step in the method for manufacturing a cold rolled steel sheet according to an embodiment of the present invention.

[0055] Referring to Figure 6, the temperature of the cold rolled steel material is raised to a temperature above Ac3 at a heating rate of 1 - 10°C / s. The Ac3 temperature can be calculated by the following formula.

[0056] Ac3 (°C) = 910 - 203[C] 0.5 - 30[Mn] + 44.7[Si] + 31.5[Mo] - 15.2[Ni]

[0057] Here, [C], [Mn], [Si], [Mo], and [Ni] are the weight % values of carbon, manganese, silicon, molybdenum, and nickel in the steel material.

[0058] According to the method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention, an annealing step of maintaining for 60 to 600 seconds at a temperature above Ac3, preferably an annealing temperature between 800 and 900 °C, is performed.

[0059] Thereafter, it is cooled to 500 to 700 °C at a cooling rate of 1 to 20 °C / s, and then cooled to the martensite transformation finish temperature (cooling finish temperature) at a cooling rate of 5 to 50 °C / s. Here, the martensite transformation finish temperature is 100 to 350 °C.

[0060] Thereafter, in the case of a non-plated material, a first heat treatment step of maintaining at 100 to 300 °C, which is the first heat treatment temperature, for 10 to 100 seconds and then cooling to room temperature at a cooling rate of 20 °C / s or less is performed.

[0061] On the other hand, in the case of a plated material, a first heat treatment step of maintaining at 450 to 600 °C, which is the first heat treatment temperature, for 5 to 60 seconds and then cooling to room temperature at a cooling rate of 20 °C / s or less is performed. When maintaining the primary heat treatment after cooling ends at a temperature of 300 °C or lower, the material may deteriorate due to the transformation heat generation caused by the formation of bainite. On the other hand, when cooling ends at a temperature of 450 °C or higher, the martensite transformation proceeds during cooling due to the delay of bainite transformation (~60 seconds), and the material properties can be ensured.

[0062] In the case of the first heat treatment temperature described above, in order to completely finish the transformation of martensite, within the above temperature range, preferably, the lower the better.

[0063] In the method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention, after performing the annealing step and before performing the first heat treatment step, the cooling step is characterized by cooling to the first heat treatment temperature without applying a cooling step of rapidly cooling to room temperature. If a cooling step of rapidly cooling to room temperature is applied after performing the annealing step and before performing the first heat treatment step, even if the first heat treatment step is performed, the final microstructure of the cold-rolled steel sheet does not contain cementite. However, when, as in the present invention, cooling to the first heat treatment temperature is performed without applying the step of rapidly cooling to room temperature after performing the annealing step and before performing the first heat treatment step, cementite can be formed during the process of performing the first heat treatment step. Generally, steel materials containing cementite have a problem that their workability deteriorates.

[0064] However, in order to fundamentally prevent the formation of cementite, the step of rapidly cooling to room temperature after performing the annealing step and before performing the first heat treatment step increases the manufacturing cost due to the addition of equipment and the like.

[0065] In the cold-rolled steel sheet according to an embodiment of the present invention, since the step of rapidly cooling to room temperature is not applied after performing the annealing step and before performing the first heat treatment step, the final microstructure contains cementite. However, by precisely controlling the subsequent steps, physical properties such as yield strength (YP): 1170 MPa or more, tensile strength (TS): 1400 MPa or more, elongation (El): 3.0% or more, yield ratio: 70% or more, and bend formability (R / t): 4.0 or less can be ensured, and an ultra-high strength cold-rolled steel sheet excellent in yield ratio and bendability can be realized.

[0066] After the first heat treatment step and after cooling to room temperature, the temperature can be raised to perform the second heat treatment step. The second heat treatment step includes a step of raising the temperature to 100 to 210°C at a heating rate of 10°C / s or less and then maintaining it at 100 to 210°C, which is the second heat treatment temperature, for 3 to 20 hours. When the second heat treatment temperature is less than 100°C, it is difficult to achieve the target yield strength during heat treatment. When it exceeds 210°C, the bendability decreases. Furthermore, even within the range of the second heat treatment temperature, if the heat treatment time is too long, a decrease in bendability occurs. Therefore, the maintenance time of the second heat treatment is adjusted to 3 to 20 hours.

[0067] The final microstructure of the cold-rolled steel sheet according to the embodiment of the present invention implemented by applying the above-described process conditions includes cementite, transitional carbide, and fine precipitates. The transitional carbide includes ε-carbide with an atomic ratio of 2.5:1 between a substitutional element, which is any one of iron (Fe), manganese (Mn), chromium (Cr), and molybdenum (Mo), and carbon, or η-carbide with an atomic ratio of 2:1. The fine precipitate is characterized in that the atomic ratio between an alloy element, which is any one of molybdenum (Mo) and titanium (Ti), and carbon is 1:1.

[0068] The cementite is formed during the first heat treatment process after the annealing and cooling. When the first heat treatment temperature is 100 to 300°C, the cementite exists in martensite. When the first heat treatment temperature is 450 to 600°C, the cementite is not formed in martensite, and when bainite exists, it exists in bainite. Such cementite exists at a ratio of 0 to 5% with respect to the total area fraction, and preferably, it is good to exist at a low ratio.

[0069] The transitional carbide is formed during the second heat treatment process. In the manufacturing method of the cold-rolled steel sheet according to the embodiment of the present invention, when the second heat treatment process is not performed, no transitional carbide exists. Transitional carbide must exist in order to increase the yield strength, and it can exist at a ratio of 0 to 5% with respect to the total area fraction.

[0070] The fine precipitate is formed during hot rolling or during coiling after hot rolling. Different from the cementite and the transitional carbide, iron (Fe) is not contained in the constituent elements. The fine precipitate can exist at a ratio of 0 to 5% with respect to the total area fraction.

[0071] Experimental Example

[0072] Hereinafter, preferred experimental examples are presented to assist in understanding the present invention. However, the following experimental examples are merely for assisting in understanding the present invention, and the present invention is not limited by the following experimental examples.

[0073] 1. Composition of Test Specimens

[0074] In this experimental example, test pieces having the composition of alloy elements in Table 1 (unit: wt%) are provided.

[0075]

Table 1

[0076] Component systems A and B in Table 1 have a composition that constitutes a cold-rolled steel sheet according to an embodiment of the present invention: carbon (C): 0.23 to 0.35 wt%, silicon (Si): 0.05 to 0.5 wt%, manganese (Mn): 0.3 to 2.3 wt%, phosphorus (P): more than 0 and 0.02 wt% or less, sulfur (S): more than 0 and 0.005 wt% or less, aluminum (Al): 0.01 to 0.05 wt%, chromium (Cr): more than 0 and 0.8 wt% or less, molybdenum (Mo): more than 0 and 0.4 wt% or less, titanium (Ti): 0.01 to 0.1 wt%, boron (B): 0.001 to 0.005 wt%, and the composition of the remaining iron (Fe) is satisfied. However, component system C is below the composition range of carbon (C): 0.23 to 0.35 wt% and is not satisfactory, and component system D exceeds the composition range of silicon (Si): 0.05 to 0.5 wt% and is not satisfactory.

[0077] 2. Process Conditions and Evaluation of Physical Properties

[0078] Table 2 shows the conditions of various heat treatment processes for the test pieces having the compositions disclosed in Table 1, and Table 3 shows the results of evaluating the physical properties after applying the compositions and heat treatment processes disclosed in Tables 1 and 2.

[0079] In Table 2, the "Component System" item indicates the composition disclosed in Table 1, and in Table 3, the "YP (MPa)", "TS (MPa)", and "EL (%)" items indicate the yield strength, tensile strength, and elongation of the test piece, respectively.

[0080]

Table 2

[0081]

Table 3

[0082] Referring to Tables 1 to 3, in Experimental Examples 1 to 4, differences in physical properties due to the composition of the alloy can be confirmed. Specifically, Experimental Examples 1 and 2 are cold-rolled steel sheets embodied according to the examples of the present invention, having a composition range of carbon (C): 0.23 to 0.35 wt%, silicon (Si): 0.05 to 0.5 wt%, manganese (Mn): 0.3 to 2.3 wt%, phosphorus (P): more than 0 and 0.02 wt% or less, sulfur (S): more than 0 and 0.005 wt% or less, aluminum (Al): 0.01 to 0.05 wt%, chromium (Cr): more than 0 and 0.8 wt% or less, molybdenum (Mo): more than 0 and 0.4 wt% or less, titanium (Ti): 0.01 to 0.1 wt%, boron (B): 0.001 to 0.005 wt%, and the balance iron (Fe). They satisfy the physical properties of yield strength (YP): 1170 MPa or more, tensile strength (TS): 1400 MPa or more, elongation (El): 3.0% or more, yield ratio: 70% or more, bend formability (R / t): 4.0 or less. 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 5% or less, respectively, which can be confirmed. On the contrary, according to Experimental Example 3, it is below the composition range of carbon (C): 0.23 to 0.35 wt% and unsatisfactory, and it can be confirmed that the target physical property of yield strength (YP): 1170 MPa or more is not achieved. Also, according to Experimental Example 4, it exceeds the composition range of silicon (Si): 0.05 to 0.5 wt% and is unsatisfactory, and it can be confirmed that the target physical property of yield strength (YP): 1170 MPa or more is not achieved due to the transformation of ferrite and bainite, which are intermediate phases. In particular, in Experimental Example 4, although the bendability characteristics were ensured by satisfying the characteristics of the carbide size and the carbide aspect ratio, ferrite was generated in an amount exceeding 10%, and the yield strength was not reached.

[0083] Referring to Tables 1 to 3, in Experimental Examples 5 to 7, the differences in physical properties due to the annealing temperature can be confirmed. Experimental Examples 6 and 7 are cold-rolled steel sheets embodied according to the embodiments of the present invention. When the annealing temperature satisfies the range of 800 to 900°C, the yield strength (YP) is 1170 MPa or more, the tensile strength (TS) is 1400 MPa or more, the elongation (El) is 3.0% or more, the yield ratio is 70% or more, and the bend formability (R / t) is 4.0 or less. It can be confirmed that in the final microstructure of the cold-rolled steel sheet, cementite, transitional carbide, and fine precipitate 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 5% or less, respectively.

[0084] On the contrary, according to Experimental Example 5, it can be confirmed that the annealing temperature is below the range of 800 to 900°C and fails to reach the target physical property where the yield strength (YP) is 1170 MPa or more.

[0085] Referring to Tables 1 to 3, in Experimental Examples 8 to 12, the differences in physical properties due to the first heat treatment temperature can be confirmed. Experimental Examples 8 to 10 are non-plated cold-rolled steel sheets embodied according to the embodiments of the present invention. When the first heat treatment temperature satisfies the range of 100 to 300°C, the yield strength (YP) is 1170 MPa or more, the tensile strength (TS) is 1400 MPa or more, the elongation (El) is 3.0% or more, the yield ratio is 70% or more, and the bend formability (R / t) is 4.0 or less. It can be confirmed that in the final microstructure of the cold-rolled steel sheet, cementite, transitional carbide, and fine precipitate 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 5% or less, respectively.

[0086] In addition, Experimental Example 12 is a plated cold-rolled steel sheet embodied according to an embodiment of the present invention. The first heat treatment temperature satisfies the range of 450 to 600 °C, and the yield strength (YP) is 1170 MPa or more, the tensile strength (TS) is 1400 MPa or more, the elongation (El) is 3.0% or more, the yield ratio is 70% or more, and the bend formability (R / t) is 4.0 or less. In the final microstructure of the cold-rolled steel sheet, it can be confirmed that the cementite, transition carbide, and fine precipitate 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 5% or less, respectively.

[0087] On the contrary, according to Experimental Example 11, when the annealing temperature is 350 °C, it can be confirmed that the target physical properties of the yield strength (YP) of 1170 MPa or more and the tensile strength (TS) of 1400 MPa or more are not achieved. It does not satisfy the range where the average size of the carbide is 50 nm or less, and it can be confirmed that it does not satisfy the range where the average aspect ratio of the carbide is 4.0 or less. When the first heat treatment temperature is maintained in the range of more than 300 °C and less than 450 °C as in Experimental Example 11, a decrease in strength due to transformation heat generation occurs. However, when the first heat treatment temperature satisfies the range of 450 to 600 °C as in Experimental Example 12, the material can be ensured by suppressing transformation.

[0088] Referring to Tables 1 to 3, in Experimental Examples 13 to 22, the difference in physical properties due to the second heat treatment temperature can be confirmed. Experimental Examples 15 to 19 are cold-rolled steel sheets embodied according to an embodiment of the present invention. The second heat treatment temperature satisfies the range of 100 to 210 °C, and the yield strength (YP) is 1170 MPa or more, the tensile strength (TS) is 1400 MPa or more, the elongation (El) is 3.0% or more, the yield ratio is 70% or more, and the bend formability (R / t) is 4.0 or less. In the final microstructure of the cold-rolled steel sheet, it can be confirmed that the cementite, transition carbide, and fine precipitate 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 5% or less, respectively.

[0089] On the contrary, according to Experimental Examples 13 and 14, since it is below the range of the second heat treatment temperature: 100 to 210°C and unsatisfactory, transition carbides are not formed, it can be confirmed that the target physical properties with a yield strength (YP) of 1170 MPa or more are not achieved.

[0090] Also, according to Experimental Examples 20 to 22, since it exceeds the range of the second heat treatment temperature: 100 to 210°C and is unsatisfactory, the target physical property of bend formability (R / t): 4.0 or less is not achieved, and it can be confirmed that the range where the average aspect ratio of carbides is 4.0 or less is not satisfied. That is, it can be confirmed that due to the poor shape of carbides, the target physical property of bend formability (R / t): 4.0 or less is not satisfied.

[0091] Referring to Tables 1 to 3, in Experimental Examples 23 to 25, the difference in physical properties due to the holding time of the second heat treatment can be confirmed. Experimental Examples 24 to 25 are cold-rolled steel sheets embodied according to the examples of the present invention, satisfying the range of the second heat treatment temperature: 100 to 210°C and the holding time of the second heat treatment: 3 to 20 hours, and having a yield strength (YP): 1170 MPa or more, a tensile strength (TS): 1400 MPa or more, an elongation (El): 3.0% or more, a yield ratio: 70% or more, and a bend formability (R / t): 4.0 or less. In the final microstructure of the cold-rolled steel sheet, it can be confirmed that cementite, transition carbides, and fine precipitates each have an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of more than 0% and 5% or less, respectively.

[0092] On the contrary, according to Experimental Example 23, since it exceeds the range of the holding time of the second heat treatment: 3 to 20 hours and is unsatisfactory, the target physical property of bend formability (R / t): 4.0 or less is not achieved, the range where the average size of carbides is 50 nm or less is not satisfied, and it can be confirmed that the range where the average aspect ratio of carbides is 4.0 or less is not satisfied. That is, when the holding time of the second heat treatment is set to 24 hours and applied in an excessive manner, it can be confirmed that the aspect ratio increases due to the growth of carbides, and the target physical property of bend formability (R / t): 4.0 or less is not satisfied.

[0093] The above describes the cold-rolled steel sheet and its manufacturing method according to the technical idea of the present invention. According to the present invention, it is possible to embody a high yield characteristic with a yield ratio (YP / TS) exceeding 70% together with a high tensile strength, and a high-strength cold-rolled steel sheet excellent in bendability with a bend performance (R / t) of 4.0 or less can be embodied. As a result, it is expected to contribute to the improvement of the stability of automobile passengers through excellent formability for forming parts with complex shapes and the application of materials excellent in impact absorption ability, and the improvement of fuel efficiency through the weight reduction of the vehicle body.

[0094] In the above, the embodiments of the present invention have been mainly described, but various changes and modifications can be made at the level of those skilled in the art. As long as such changes and modifications do not depart from the scope of the present invention, it can be said that they belong to the present invention. Therefore, the scope of the rights of the present invention must be determined by the appended claims.

Claims

1. A cold-rolled steel sheet consisting of carbon (C): 0.23 to 0.35% by weight, silicon (Si): 0.05 to 0.5% by weight, manganese (Mn): 0.3 to 2.3% by weight, phosphorus (P): more than 0 and 0.02% by weight or less, sulfur (S): more than 0 and 0.005% by weight or less, aluminum (Al): 0.01 to 0.05% by weight, chromium (Cr): more than 0 and 0.8% by weight or less, molybdenum (Mo): more than 0 and 0.4% by weight or less, titanium (Ti): 0.01 to 0.1% by weight, boron (B): 0.001 to 0.005% by weight, and the balance being iron (Fe) and other inevitable impurities, wherein the final microstructure of the cold-rolled steel sheet contains cementite, transitional carbide and fine precipitate, the transitional carbide contains ε-carbide with an atomic ratio of 2.5:1 between a substitutional element which is any one of iron (Fe), manganese (Mn), chromium (Cr) and molybdenum (Mo) and carbon, or η-carbide with an atomic ratio of 2:1, and the fine precipitate has an atomic ratio of 1:1 between an alloy element which is any one of molybdenum (Mo) and titanium (Ti) and carbon, and has a yield strength (YP): 1170 MPa or more, a tensile strength (TS): 1400 MPa or more, an elongation (El): 3.0% or more, a yield ratio: 70% or more, and a bend formability (R / t): 4.0 or less. A cold-rolled steel sheet characterized by the above.

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

3. The cold-rolled steel sheet according to claim 1, wherein the cementite, the transitional carbide and the fine precipitate each have an area fraction of more than 0% and 5% or less.

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, and in terms of area fraction, tempered martensite: 80% or more and less than 100%, ferrite and bainite: more than 0% and 20% or less.

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

1. The fine precipitate is an alloy element that is any one of molybdenum (Mo) and titanium (Ti), and carbon. The atomic ratio is 1:

1. The cementite is formed during the first heat treatment, the transition carbide is formed during the second heat treatment, and the fine precipitate is formed during the hot rolling step. A method for manufacturing a cold-rolled steel sheet, characterized in that.

7. The step (a) includes hot rolling under the conditions of reheating temperature: 1150 to 1300 °C, finish rolling temperature: 800 to 1000 °C, and coiling temperature: 500 to 650 °C. The step (c) is performed under the conditions of annealing temperature: 800 to 900 °C, first heat treatment temperature: 100 to 300 °C, and second heat treatment temperature: 100 to 210 °C. The method for manufacturing a cold-rolled steel sheet according to claim 6, characterized in that.

8. The step (a) includes hot rolling under the conditions of reheating temperature: 1150 to 1300 °C, finish rolling temperature: 800 to 1000 °C, and coiling temperature: 500 to 650 °C. The step (c) includes a plating step, and is characterized in that it is carried out under the conditions of annealing temperature: 800 to 900 °C, first heat treatment temperature: 450 to 600 °C, and second heat treatment temperature: 100 to 210 °C. The method for manufacturing a cold-rolled steel sheet according to claim 6.

9. The step (c) is characterized in that, after the annealing step and after cooling to the first heat treatment temperature, the first heat treatment step is carried out. The method for manufacturing a cold-rolled steel sheet according to claim 6.

10. The step (c) is characterized in that, after the first heat treatment step and after cooling to room temperature, the temperature is raised and the second heat treatment step is carried out. The method for manufacturing a cold-rolled steel sheet according to claim 6.

11. The second heat treatment step includes a step of maintaining at the second heat treatment temperature for 3 to 20 hours. The method for manufacturing a cold-rolled steel sheet according to claim 7 or 8.

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