Ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties and its manufacturing method

Abstract

The present invention relates to an ultra-high strength cold rolled steel sheet having excellent yield strength and bending properties and a manufacturing method thereof, and more particularly to an ultra-high strength cold rolled steel sheet having excellent yield strength and bending properties that can be used as automotive structural members such as members, seat rails, pillars, etc., and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to an ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties and a manufacturing method thereof, and more particularly to an ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties that can be used for automotive structural components such as members, seat rails, pillars, etc., and a manufacturing method thereof. [Background technology]

[0002] Recently, stricter safety regulations for automobile passengers and pedestrians have mandated the construction of safety devices, countering the need for weight reduction to improve fuel efficiency and resulting in increased vehicle weight. Consumers are increasingly interested in environmentally friendly, fuel-efficient hybrid and electric vehicles. To produce these environmentally friendly and safe vehicles, however, it is necessary to reduce the weight of the vehicle body structure and ensure the stability of the body materials. However, hybrid vehicles incorporate various devices, such as an electric engine, electric battery, and secondary fuel storage tank, in addition to the conventional gasoline engine. Furthermore, the continuous addition of driver convenience facilities increases the vehicle body weight. Therefore, to achieve vehicle weight reduction, it is essential to develop a thin material with excellent strength, ductility, and bending properties. Therefore, to solve this problem, it is necessary to develop giga-grade steel sheets that can ensure high strength and ductility, with a tensile strength of over 980 MPa.

[0003] Meanwhile, in recent years, with the expansion of regulations regarding the crashworthiness of automobiles, high-strength steels with excellent yield strength have been adopted for structural components such as members, seat rails, and pillars to improve the crashworthiness of vehicle bodies. The higher the yield strength relative to tensile strength, i.e., the higher the yield ratio (yield strength / tensile strength), the better the structural components absorb impact energy. However, as the strength of steel sheet increases, the elongation decreases, resulting in poor formability. Therefore, there is a need for the development of materials that simultaneously offer high yield ratios, improved formability, and improved bending properties, which are key physical properties for part processing.

[0004] A typical manufacturing method for increasing yield strength is to use water cooling during continuous annealing. Specifically, by soaking in water after the annealing process and then tempering, a steel sheet can be manufactured in which the microstructure is transformed from martensite to tempered martensite. Patent Document 1 is a representative prior art example of this method. Patent Document 1 describes a technique for continuously annealing a steel material containing 0.18 to 0.3% carbon, water cooling it to room temperature, and then overaging it at a temperature of 120 to 300°C for 1 to 15 minutes to produce a steel material with a martensite volume fraction of 80 to 97% and the remainder ferrite. When ultra-high-strength steel is manufactured using this water-cooling followed by tempering, although the yield ratio is very high, there is a problem of deterioration in the shape quality of the coil due to temperature deviations in the width and length directions. This can lead to problems during roll forming, such as poor material quality in certain locations and reduced workability.

[0005] Patent Document 2 is a conventional technique for improving the workability of the above-mentioned high-tensile steel sheet. Patent Document 2 relates to a steel sheet having a composite structure mainly composed of tempered martensite, and is characterized by dispersing fine precipitated Cu particles with a particle size of 1 to 100 nm inside the structure in order to improve workability. However, Patent Document 1 has problems in that excessive Cu content of 2 to 5% is added in order to precipitate good fine Cu particles, which may cause red shortness due to Cu and excessively increase production costs.

[0006] Meanwhile, Patent Document 3 proposes a steel sheet with a ferrite matrix and a microstructure containing 2-10 area% pearlite, with improved strength achieved through precipitation strengthening and grain refinement, primarily through the addition of carbonitride-forming elements such as Ti. Patent Document 3 has the advantage of easily achieving high strength at low manufacturing costs, but the disadvantage is that the fine precipitates cause a rapid rise in recrystallization temperature, necessitating high-temperature annealing to induce sufficient recrystallization and ensure ductility. Furthermore, existing precipitation-strengthened steels, which are strengthened by precipitating carbonitrides in a ferrite matrix, have the drawback of being difficult to obtain high-strength steels of 600 MPa or higher.

[0007] Therefore, there is a need to develop a steel material that can solve the above-mentioned problems, has high yield strength and bending properties, and has ultra-high strength that allows cold forming. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Registration No. 2528387 [Patent Document 2] Japanese Patent Publication No. 2005-264176 [Patent Document 3] Korean Patent Publication No. 2015-0073844 Summary of the Invention [Problem to be solved by the invention]

[0009] One aspect of the present invention is to provide an ultra-high strength cold rolled steel sheet having excellent yield strength and bending properties, and a method for manufacturing the same. [Means for solving the problem]

[0010] One embodiment of the present invention is, in weight percent, C: 0.03 to 0.12%, Si: 0.03 to 0.30%, Mn: 2.1 to 2.9%, Al: 0.005 to 0.07%, Nb: 0.01 to 0.08%, Ti: 0.005 to 0.08%, B: 0.0005 to 0.005%, Cr: 0.7 to 1.4%, Mo: 0.005 to 0.10%, N: 0.008% or less (excluding 0%), and the balance Fe and other unavoidable impurities, and satisfies the following relational expressions 1 to 3, the microstructure contains, in area %, 4 to 19% fresh martensite, 78 to 95% total of tempered martensite and bainite, and 0.2 to 2.0% retained austenite, and the average crystal grain size of the microstructure is 0.5 to 6 μm, and the ultrahigh strength cold-rolled steel sheet has excellent yield strength and bending properties.

[0011] [Equation 1] 0.18≦C+Si / 30+Mn / 20+2P+4S≦0.30 [Equation 2] 180≦48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb≦270 [Equation 3] 700≦ ( 48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb ) / ( C+Si / 30+Mn / 20+2P+4S ) ≦1200 (However, the contents of the alloying elements shown in the above Relational Formulas 1 to 3 are expressed in weight percent.)

[0012] Another embodiment of the present invention is a method for producing a steel sheet comprising the steps of: heating a slab containing, by weight %, C: 0.03 to 0.12%, Si: 0.03 to 0.30%, Mn: 2.1 to 2.9%, Al: 0.005 to 0.07%, Nb: 0.01 to 0.08%, Ti: 0.005 to 0.08%, B: 0.0005 to 0.005%, Cr: 0.7 to 1.4%, Mo: 0.005 to 0.10%, N: 0.008% or less (excluding 0%), the balance being Fe and other inevitable impurities, and satisfying the following relations 1 to 3; finish rolling the heated slab so that the delivery temperature of finish rolling is Ar3+50°C to Ar3+150°C to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet to Ms+50°C to Ms+300°C and then coiling it; and The present invention provides a method for producing an ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties, the method comprising the steps of cold-rolling a coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet in a temperature range of Ar3+10°C to Ar3+70°C; soaking the continuously annealed cold-rolled steel sheet for 50 to 200 seconds; primarily cooling the soaked cold-rolled steel sheet to 620 to 700°C at a cooling rate of 1 to 10°C / s; secondarily cooling the primarily cooled cold-rolled steel sheet to 360 to 420°C at a cooling rate of 5 to 50°C / s; and overaging the secondarily cooled cold-rolled steel sheet for 250 to 650 seconds, followed by a cooling treatment at 320 to 400°C, wherein the following Relational Expressions 4 to 6 are satisfied during the secondary cooling and overaging treatment:

[0013] [Equation 1] 0.18≦C+Si / 30+Mn / 20+2P+4S≦0.30 [Equation 2] 180≦48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb≦270 [Equation 3] 700≦ ( 48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb ) / ( C+Si / 30+Mn / 20+2P+4S ) ≦1200 [Equation 4] 10≦A≦70 [Equation 5] 30≦B≦100 [Equation 6] 2.5≦overaging time / B≦14 (Note that the contents of the alloying elements in the above Relational Formulas 1 to 3 are expressed in weight percent, and in the above Relational Formulas 4 to 6, A is the Ms-secondary cooling end temperature (°C), and B is the Ms-overaging treatment end temperature (°C).) [Effects of the Invention]

[0014] According to one aspect of the present invention, it is possible to provide an ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties, and a method for manufacturing the same. [Brief explanation of the drawings]

[0015] [Figure 1] 10 is a photograph of the microstructure of Example 6 according to an embodiment of the present invention, observed by SEM. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an ultra-high strength cold rolled steel sheet having excellent yield strength and bending properties according to one embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. The contents of the alloy compositions described below are in weight percent unless otherwise specified.

[0017] C: 0.03 to 0.12% Carbon (C) is a very important element added for solid solution strengthening. Carbon also contributes to improving strength by combining with precipitated elements to form fine carbides. If the C content is less than 0.03%, it is very difficult to achieve the desired strength. On the other hand, if the C content exceeds 0.12%, the increased hardenability leads to excessive martensite formation during cooling, resulting in a rapid increase in strength and a deterioration in bending properties. This may make it difficult to achieve the HER, R / t, and maximum three-point bending angle desired by the present invention. Furthermore, weldability may be reduced, increasing the likelihood of welding defects occurring during component processing at the customer's facility. Therefore, the C content is preferably in the range of 0.03 to 0.12%. The lower limit of the C content is more preferably 0.04%, and even more preferably 0.05%. The upper limit of the C content is more preferably 0.10%, and even more preferably 0.09%.

[0018] Si: 0.03 to 0.30% Silicon (Si) is one of the five major elements of steel, and a small amount is naturally added during the manufacturing process. Si contributes to increased strength and inhibits carbide formation, preventing carbon from forming as carbides during annealing, soaking, and cooling. Furthermore, this carbon distributes and accumulates in retained austenite, allowing the austenite phase to remain at room temperature, which is beneficial for ensuring elongation. If the Si content is less than 0.03%, it may be difficult to fully achieve the above-mentioned effects. On the other hand, if the Si content exceeds 0.30%, the effect of solid solution strengthening may be enhanced, resulting in reduced elongation and the occurrence of surface scale defects, which may degrade the plating surface quality and reduce chemical conversion treatability. Therefore, the Si content is preferably in the range of 0.03 to 0.30%. The lower limit of the Si content is more preferably 0.04%, and even more preferably 0.05%. The upper limit of the Si content is more preferably 0.25%, and even more preferably 0.20%.

[0019] Mn: 2.1 to 2.9% Manganese (Mn) is an element that completely precipitates sulfur in steel as MnS, prevents hot embrittlement due to the formation of FeS, and solid-solution strengthens the steel. If the Mn content is less than 2.1%, it is difficult to achieve the strength targeted by the present invention. On the other hand, if the Mn content exceeds 2.9%, problems such as weldability and hot rolling are likely to occur. At the same time, the hardenability may be increased, causing excessive martensite formation, resulting in reduced elongation. Furthermore, there are problems with the formation of Mn-bands (Mn oxide bands) in the microstructure, increasing the risk of processing cracks and sheet breakage. Furthermore, there are problems with Mn oxides dissolving on the surface during annealing, significantly impairing platability. Therefore, the Mn content is preferably in the range of 2.1 to 2.9%. The lower limit of the Mn content is more preferably 2.2%, and even more preferably 2.3%. The upper limit of the Mn content is more preferably 2.8%, and even more preferably 2.7%.

[0020] Al: 0.005 to 0.07% Aluminum (Al) is an element added for deoxidation during steelmaking. If the Al content is less than 0.005%, it is difficult to obtain a sufficient deoxidizing effect. If it exceeds 0.07%, it reacts with oxygen (O) in the molten steel to form high-melting-point oxides (inclusions), which may cause nozzle clogging. Furthermore, the inclusions formed in this way have sharp shapes, which may result in reduced bending properties. Therefore, the Al content is preferably 0.005 to 0.07%. The lower limit of the Al content is more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Al content is more preferably 0.06%, and even more preferably 0.05%.

[0021] Nb: 0.01 to 0.08% Niobium (Nb) is an element that segregates at austenite grain boundaries to prevent coarsening of austenite grains during annealing and forms fine carbides, thereby contributing to increased strength. If the Nb content is less than 0.01%, the above-mentioned effects are insufficient. On the other hand, if the Nb content exceeds 0.08%, coarse carbides precipitate, reducing the amount of solute carbon in the steel, which can decrease strength and elongation and increase manufacturing costs. Therefore, the Nb content is preferably in the range of 0.01 to 0.08%. The lower limit of the Nb content is more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Nb content is more preferably 0.07%, and even more preferably 0.06%.

[0022] Ti: 0.005 to 0.08% Titanium (Ti) is a fine carbide-forming element that contributes to ensuring yield strength and tensile strength. Furthermore, Ti is a nitride-forming element that precipitates N in steel as TiN, suppressing AlN precipitation and reducing the risk of cracks during continuous casting. If the Ti content is less than 0.005%, it may be difficult to achieve the above-mentioned effects. On the other hand, if the Ti content exceeds 0.08%, coarse carbides precipitate, reducing the amount of solute carbon in the steel, which can decrease strength and elongation and cause nozzle clogging during continuous casting. Therefore, the Ti content is preferably in the range of 0.005 to 0.08%. The lower limit of the Ti content is more preferably 0.007%, and even more preferably 0.01%. The upper limit of the Ti content is more preferably 0.07%, and even more preferably 0.06%.

[0023] B: 0.0005 to 0.005% Boron (B) is an element that significantly contributes to ensuring the hardenability of steel, and to achieve this effect, it is preferable to add 0.0005% or more. However, if the B content exceeds 0.005%, boron carbides are formed at grain boundaries, providing sites for ferrite nucleation, which may actually deteriorate the hardenability. Therefore, the B content is preferably in the range of 0.0005 to 0.005%. The lower limit of the B content is more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the B content is more preferably 0.0045%, and even more preferably 0.004%.

[0024] Cr: 0.7 to 1.4% Chromium (Cr) is an element that improves hardenability and increases the strength of steel. If the Cr content is less than 0.7%, it may be difficult to achieve the target strength. On the other hand, if the Cr content exceeds 1.4%, the ductility of the steel sheet may decrease. Therefore, the Cr content is preferably in the range of 0.7 to 1.4%. The lower limit of the Cr content is more preferably 0.75%, and even more preferably 0.8%. The upper limit of the Cr content is more preferably 1.3%, and even more preferably 1.2%.

[0025] Mo: 0.005 to 0.10% Molybdenum (Mo) is a carbide-forming element, and when added in combination with carbonitride-forming elements such as Ti, Nb, and V, it plays a role in maintaining the size of precipitates finely, thereby improving yield strength and tensile strength. In addition, Mo has the advantage of improving the hardening ability of steel and forming fine martensite at grain boundaries, making it possible to control the yield ratio. To achieve the above effects, the Mo content must be 0.0 05It is preferable that Mo be added in an amount of 0.005% or more. However, since Mo is an expensive element, the higher its content, the more disadvantageous it becomes in manufacturing, so it is preferable to appropriately control its content. If the Mo content exceeds 0.10%, not only will the manufacturing cost rise sharply, reducing economic efficiency, but the excessive grain refinement and solid solution strengthening effects will actually reduce the ductility of the steel. Therefore, the Mo content is preferably in the range of 0.005% to 0.10%. The lower limit of the Mo content is more preferably 0.007%, and even more preferably 0.01%. The upper limit of the Mo content is more preferably 0.08%, and even more preferably 0.06%.

[0026] N: 0.008% or less (excluding 0%) Nitrogen (N) is an element that is inevitably contained in steel during the manufacturing process, but it also contributes to improving the strength of steel by forming carbonitrides. However, if the N content exceeds 0.008%, not only does the risk of brittleness increase significantly, but the excess N that remains after forming TiN can cause B, which contributes to hardening, to be consumed in the form of BN. Therefore, the N content is preferably 0.008% or less. The N content is more preferably 0.007% or less, and even more preferably 0.006% or less.

[0027] On the other hand, the cold-rolled steel sheet of the present invention preferably satisfies the above-mentioned alloying elements and also satisfies the following relations 1 to 3. This makes it possible to produce an ultra-high strength steel sheet having a tensile strength of 980 MPa or more and excellent bending workability, which is the target of the present invention.

[0028] [Equation 1] 0.18≦C+Si / 30+Mn / 20+2P+4S≦0.30

[0029] The above-mentioned relational expression 1 is a chemical relational expression for ensuring strength and weldability. If the value of the above-mentioned relational expression 1 is less than 0.18, it is difficult to ensure the strength targeted by the present invention, and if it exceeds 0.30, there is a possibility that the weldability will deteriorate. Therefore, the value of the above-mentioned relational expression 1 is preferably in the range of 0.18 to 0.30. The lower limit of the value of the above-mentioned relational expression 1 is more preferably 0.19, and even more preferably 0.20. The upper limit of the value of the above-mentioned relational expression 1 is more preferably 0.28, and even more preferably 0.26.

[0030] [Equation 2] 180≦48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb≦270

[0031] The above-mentioned relational expression 2 is a relational expression relating to the hardenability index for ensuring hardenability. If the value of the above-mentioned relational expression 2 is less than 180, it will be difficult to ensure the strength targeted in the present invention due to insufficient hardenability, and if it exceeds 270, the hardenability will be excessively high, which may result in deterioration of bending properties and formability. Therefore, the value of the above-mentioned relational expression 2 is preferably in the range of 180 to 270. The lower limit of the value of the above-mentioned relational expression 2 is more preferably 190, and even more preferably 200. The upper limit of the value of the above-mentioned relational expression 2 is more preferably 260, and even more preferably 250.

[0032] [Equation 3] 700≦ ( 48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb ) / ( C+Si / 30+Mn / 20+2P+4S ) ≦1200

[0033] The above-mentioned relational expression 3 is a component relational expression for simultaneously ensuring the strength, hardenability, and weldability targeted by the present invention. If the value of the above-mentioned relational expression 3 is less than 700, not only may the weldability be reduced, but also the hardenability may be insufficient, making it difficult to ensure the strength targeted by the present invention. If the value of the above-mentioned relational expression 3 is more than 1200, the hardenability may be excessively high, resulting in reduced bending properties and formability. Therefore, the value of the above-mentioned relational expression 3 is preferably in the range of 700 to 1200. The lower limit of the value of the above-mentioned relational expression 3 is more preferably 700, and even more preferably 800. The upper limit of the value of the above-mentioned relational expression 3 is more preferably 1150, and even more preferably 1100.

[0034] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, there is a possibility that unintended impurities may be inevitably mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, the entire contents of these impurities will not be specifically mentioned in this specification.

[0035] However, among these, phosphorus and sulfur are the most commonly mentioned impurities, and therefore, a brief explanation of these will be given below.

[0036] P: 0.04% or less (excluding 0%) Phosphorus (P) is an element that can segregate at grain boundaries and / or interphase boundaries, causing embrittlement. Therefore, its content must be controlled as low as possible, preferably to 0.04% or less. The P content is more preferably limited to 0.03% or less, and even more preferably to 0.02% or less.

[0037] S: 0.005% or less (excluding 0%) Sulfur (S) is an impurity that can cause high-temperature cracks by forming MnS nonmetallic inclusions in steel and by segregating during continuous casting solidification. Therefore, its content must be controlled as low as possible, preferably to 0.005% or less. The S content is more preferably limited to 0.004% or less, and even more preferably to 0.003% or less.

[0038] The impurities may include one or more of Sb, Mg, Sn, Sb, Zn, and Pb as tramp elements, the total of which may be 0.1 wt% or less. Tramp elements are impurity elements derived from scrap and the like used as raw materials in the steelmaking process, and if the total exceeds 0.1%, it may cause surface cracks in the slab and reduce the surface quality of the steel sheet.

[0039] Hereinafter, the microstructure and the like of an ultra-high strength cold rolled steel sheet having excellent yield strength and bending properties according to one embodiment of the present invention will be described.

[0040] The microstructure of the cold-rolled steel sheet of the present invention preferably contains, in area percentages, 4 to 19% fresh martensite, 78 to 95% total of tempered martensite and bainite, and 0.2 to 2.0% retained austenite. The microstructure of the cold-rolled steel sheet of the present invention contains tempered martensite (hereinafter also referred to as "TM") and bainite (hereinafter also referred to as "B") as main structures. Since it is difficult to distinguish between the tempered martensite and bainite in terms of the microstructure, the present invention controls the total fraction of the tempered martensite and bainite. If the total fraction of the tempered martensite and bainite is less than 78%, it is difficult to ensure the target strength, and if it exceeds 95%, the bending properties and elongation may decrease. The fresh martensite (hereinafter also referred to as "FM") is a structure advantageous for ensuring strength. If the fraction of the fresh martensite is less than 4%, it is difficult to ensure the target strength, and if it exceeds 19%, the bending properties and elongation may decrease. The retained austenite (hereinafter also referred to as "RA") is a structure advantageous for ensuring elongation. If the fraction of the retained austenite is less than 0.2%, it is difficult to fully obtain the above effect, and if it exceeds 2.0%, it may transform into martensite during processing, resulting in a deterioration in HER or bending properties. On the other hand, the microstructure may further contain 10% or less of ferrite. Although the ferrite structure is a structure that is inevitably formed in the manufacturing process, it may also perform positive functions. For example, the ferrite can contribute to ensuring elongation. However, if the fraction exceeds 10%, it may be difficult to ensure the strength that the present invention aims to achieve. The fraction of ferrite is more preferably 7% or less, and even more preferably 5% or less.

[0041] On the other hand, the average crystal grain size of the microstructure is preferably 0.5 to 6 μm. A finer average crystal grain size is advantageous for ensuring physical properties such as strength and HER. However, controlling the average crystal grain size to less than 0.5 μm may require excessively large amounts of Nb, Ti, Mo, V, and other elements effective for grain refinement, resulting in increased manufacturing costs. If the average crystal grain size exceeds 6 μm, it may be difficult to achieve the strength targeted by the present invention, and HER and bending properties may be significantly reduced. Therefore, the average grain size is preferably in the range of 0.5 to 6.0 μm. The lower limit of the average crystal grain size is more preferably 1.0 μm, and even more preferably 1.5 μm. The upper limit of the average crystal grain size is more preferably 5.5 μm, and even more preferably 5.0 μm.

[0042] The cold-rolled steel sheet of the present invention provided as described above may have a yield strength (YS): 800 to 980 MPa, a tensile strength (TS): 980 to 1180 MPa, an elongation (EL): 4 to 12%, a yield ratio (YS / TS): 0.70 to 0.95, a hole expansion ratio (HER): 35 to 80%, an R / t: 0.8 or less, and a maximum three-point bending angle: 90 to 140°. The yield strength is more preferably 820 to 960 MPa, and even more preferably 850 to 950 MPa. The tensile strength is more preferably 1000 to 1170 MPa, and even more preferably 1020 to 1160 MPa. The elongation is more preferably 5 to 11%, and even more preferably 6 to 10%. The yield ratio is more preferably 0.72 to 0.92, and even more preferably 0.75 to 0.90. The hole expansion ratio is more preferably 40 to 75%, and even more preferably 45 to 70%. The R / t is more preferably 0.15 to 0.70, and even more preferably 0.20 to 0.60. The maximum angle of the three-point bending is more preferably 95 to 135°, and even more preferably 100 to 130°.

[0043] The cold-rolled steel sheet of the present invention may have a hardness (HvBM) of 300 to 400 Hv. The base metal hardness is more preferably 310 to 390 Hv, and even more preferably 320 to 380 Hv. Furthermore, the fusion zone (HvFZ) hardness of the weld formed after welding may be 350 to 450 Hv. If the fusion zone hardness of the weld is less than 350 Hv, sufficient fusion zone hardness cannot be ensured, and the strength of the weld may be low. On the other hand, if it exceeds 450 Hv, the fusion zone hardness may be too high, increasing the sensitivity to crack generation and reducing the strength and impact absorption energy of the weld. The hardness of the cold-rolled steel sheet, i.e., the hardness (HvBM) corresponding to the base metal after welding, is preferably as similar as possible to the fusion zone hardness (HvFZ), and therefore the ratio (HvFZ / HvBM) is preferably 1.30 or less. The above HvFZ / HvBM is more preferably 1.25 or less, and even more preferably 1.20 or less.

[0044] Hereinafter, an ultra-high strength cold rolled steel sheet having excellent yield strength and bending properties according to one embodiment of the present invention will be described.

[0045] First, a slab satisfying the above-described alloy composition is heated. In the present invention, the slab heating temperature is not particularly limited, but for example, the slab can be heated at 1100 to 1300°C. If the slab heating temperature is less than 1100°C, the slab temperature is low, and a rolling load may be generated during rough rolling. If the slab heating temperature is more than 1300°C, the structure may become coarse, resulting in disadvantages such as increased power costs. The lower limit of the slab heating temperature is more preferably 1125°C, and even more preferably 1150°C. The upper limit of the slab heating temperature is more preferably 1275°C, and even more preferably 1250°C. Meanwhile, the slab can have a thickness of 230 to 270 mm.

[0046] The heated slab is then finish-rolled so that the exit temperature of the finish rolling is Ar3+50°C to Ar3+150°C to obtain a hot-rolled steel sheet. If the exit temperature of the finish rolling is less than Ar3+50°C, there is a high possibility that the hot deformation resistance will increase sharply. If the exit temperature of the finish rolling is more than Ar3+150°C, there is a high possibility that not only will an excessively thick oxide scale be formed but also the microstructure of the steel sheet will become coarse. Therefore, the exit temperature of the finish rolling is preferably in the range of Ar3+50°C to Ar3+150°C. The lower limit of the exit temperature of the finish rolling is more preferably Ar3+60°C, and even more preferably Ar3+70°C. The upper limit of the exit temperature of the finish rolling is more preferably Ar3+140°C, and even more preferably Ar3+130°C. Meanwhile, Ar3 refers to the temperature at which the steel transforms to austenite during heating, and is, for example, 910-203°C. 1 / 2 The value can be calculated using a formula such as +44.7Si+31.5Mo-30Mn-11Cr+700P+400Al+400Ti.

[0047] Thereafter, the hot-rolled steel sheet is cooled to Ms + 50°C to Ms + 300°C and then coiled. If the coiling temperature is less than Ms + 50°C, excess martensite or bainite is generated, resulting in an excessive increase in the strength of the hot-rolled steel sheet, which may cause problems such as defective shape due to the load during cold rolling. On the other hand, if the coiling temperature exceeds Ms + 300°C, an increase in surface scale may occur, which may deteriorate the pickling properties. Therefore, the coiling temperature is preferably in the range of Ms + 50°C to Ms + 300°C. The lower limit of the coiling temperature is more preferably Ms + 60°C, and even more preferably Ms + 70°C. The upper limit of the coiling temperature is more preferably Ms + 290°C, and even more preferably Ms + 270°C. Meanwhile, after the coiling, the coiled hot-rolled steel sheet may be cooled to room temperature at a cooling rate of 0.1°C / s or less. The above Ms means the temperature at which martensite begins to transform upon cooling, and its value can be calculated using an equation such as 539-423C-30.4Mn-7.5Si+30Al.

[0048] The coiled and cooled hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. The cold rolling can be performed at a reduction of 40 to 70%. If the cold rolling reduction is less than 40%, the driving force for recrystallization is weakened, which may cause problems in obtaining good recrystallized grains and makes shape control very difficult. If the cold rolling reduction exceeds 70%, cracks may occur at the edges of the steel sheet and the rolling load may increase rapidly. Therefore, it is preferable that the cold rolling be performed at a reduction of 40 to 70%. Meanwhile, pickling may be performed before the cold rolling to remove scale and impurities adhering to the surface.

[0049] The cold-rolled steel sheet is then continuously annealed in a temperature range of Ar3+10°C to Ar3+70°C. If the continuous annealing temperature is less than Ar3+10°C, the steel sheet will not be sufficiently transformed into austenite, making it difficult to obtain the martensite or bainite fraction desired by the present invention in subsequent processes, and it may be difficult to ensure strength. On the other hand, if the temperature exceeds Ar3+70°C, the austenite grain size will become coarse, making it difficult to ensure the target bending properties. Therefore, the continuous annealing temperature is preferably in the range of Ar3+10°C to Ar3+70°C. The lower limit of the continuous annealing temperature is more preferably Ar3+20°C, and even more preferably Ar3+30°C. The upper limit of the continuous annealing temperature is more preferably Ar3+60°C, and even more preferably Ar3+50°C.

[0050] The continuously annealed cold-rolled steel sheet is then soaked for 50 to 200 seconds. This is to ensure a sufficient austenite fraction at the annealing temperature proposed by the present invention, along with recrystallization and grain growth of the cold-rolled structure. If the soaking time is less than 50 seconds, reverse transformation to austenite does not occur sufficiently, resulting in an increase in the ferrite fraction in the final structure, which may make it difficult to achieve the target strength. On the other hand, if the soaking time exceeds 200 seconds, the austenite grain size may become coarse, potentially reducing the bending properties of the final product. The lower limit of the soaking time is more preferably 55 seconds, and even more preferably 60 seconds. The upper limit of the soaking time is more preferably 190 seconds, and even more preferably 180 seconds.

[0051] The soaked cold-rolled steel sheet is then subjected to primary cooling at a cooling rate of 1 to 10°C / s to 620 to 700°C. The primary cooling step is intended to ensure the equilibrium carbon concentration of ferrite and austenite to increase the ductility and strength of the steel sheet. If the primary cooling end temperature is less than 630°C or exceeds 700°C, it becomes difficult to ensure the ductility and strength targeted in the present invention. If the cooling rate is less than 1°C / s, the ferrite transformation is accelerated, making it difficult to ensure the targeted microstructure fraction. If the cooling rate exceeds 10°C / s, excessive martensite transformation makes it difficult to ensure the elongation percentage.

[0052] The primarily cooled cold-rolled steel sheet is then secondarily cooled to 360 to 420°C at a cooling rate of 5 to 50°C / s. The secondary cooling is one of the important control factors in the present invention, and the secondary cooling end temperature is an extremely important condition for simultaneously ensuring strength, ductility, and bending properties. If the secondary cooling end temperature is less than 360°C, it becomes difficult to ensure ductility due to an excessive increase in martensite fraction. If it exceeds 420°C, it becomes difficult to ensure sufficient martensite, making it difficult to ensure the target strength. Therefore, the secondary cooling end temperature, which is one of the important control factors for ensuring the target physical properties in the present invention, is preferably in the range of 360 to 420°C. The lower limit of the secondary cooling end temperature is more preferably 365°C, and even more preferably 370°C. The upper limit of the secondary cooling end temperature is more preferably 410°C, and even more preferably 405°C. If the secondary cooling rate is less than 5°C / s, ferrite transformation occurs preferentially before martensite and bainite transformation, resulting in the failure to obtain the appropriate amount of fine structure fraction desired by the present invention. If the rate exceeds 50°C / s, the excessive cooling rate can cause shape deterioration, resulting in poor sheet threadability and sheet breakage. The lower limit of the secondary cooling rate is more preferably 7.5°C / s, and even more preferably 10°C / s. The upper limit of the secondary cooling rate is more preferably 47.5°C / s, and even more preferably 45°C / s.

[0053] On the other hand, in order to ensure the target levels of tempered martensite and bainite fractions, which are important microstructures in the present invention, it is important to precisely control the difference between the Ms temperature and the end temperature of secondary cooling. More specifically, it is preferable to satisfy the following relational expression 4. If the difference between Ms and the end temperature of secondary cooling, i.e., the value of A, is less than 10°C, martensite or bainite transformation may be insufficient, making it difficult to achieve the target strength. If the value of A exceeds 70°C, the time spent in the martensite region increases excessively, making it difficult to ensure ductility due to an excessive increase in the martensite fraction. Therefore, it is preferable that the difference between Ms and the end temperature of secondary cooling, i.e., the value of A, be 10 to 70°C. The lower limit of the value of A is more preferably 15°C, and even more preferably 20°C. The upper limit of the value of A is more preferably 65°C, and even more preferably 60°C. Meanwhile, Ms refers to the temperature at which martensitic transformation begins, and its value can be calculated using the following formula 1.

[0054] [Equation 4] 10≦A≦70 (In the above relational expression 4, A is the Ms-secondary cooling end temperature (°C).)

[0055] The secondarily cooled cold-rolled steel sheet is then overaged for 250 to 650 seconds, and the temperature is then terminated at 320 to 400°C. The overaging is preferably performed at a temperature equal to or higher than the temperature at the end of the second cooling. The overaging promotes the transformation of fresh martensite formed at the end of the second cooling into tempered martensite, thereby stably ensuring high yield strength and bending properties. Therefore, the overaging is a very important factor in ensuring the high bending workability desired in the present invention. In the present invention, the overaging time is precisely controlled within the range of 250 to 650 seconds. If the overaging time is less than 250 seconds, slight transformation from fresh martensite to tempered martensite may occur, potentially reducing bending workability. On the other hand, if the overaging time exceeds 650 seconds, productivity may decrease and excessive tempered martensite transformation may make it difficult to achieve the tensile strength desired in the present invention. The lower limit of the overaging treatment time is more preferably 260 seconds, and even more preferably 270 seconds. The upper limit of the overaging treatment time is more preferably 600 seconds, and even more preferably 550 seconds. If the overaging treatment end temperature is less than 320°C, it may be difficult to ensure elongation due to excessive fresh martensite transformation, and bending properties may be reduced. If the overaging treatment end temperature is more than 400°C, a slight transformation from fresh martensite to tempered martensite may occur, and bending properties may be reduced. The lower limit of the overaging treatment end temperature is more preferably 325°C, and even more preferably 330°C. The upper limit of the overaging treatment end temperature is more preferably 395°C, and even more preferably 380°C. On the other hand, to further improve HER and bending properties, after the overaging treatment, the steel may be cooled to the secondary cooling end temperature and then reheated to perform further overaging treatment.

[0056] On the other hand, in order to ensure the tempered martensite fraction, which is an important microstructure in the present invention, at a target level, it is important to precisely control the difference between the Ms temperature and the overaging treatment end temperature. More specifically, it is preferable to satisfy the following relational expression 5. If the difference between the Ms temperature and the overaging treatment end temperature, i.e., the value of B, is less than 30°C, martensitic transformation may be insufficient, making it difficult to ensure the target strength. If the value of B exceeds 100°C, excessive fresh martensite transformation may occur, making it difficult to ensure the target elongation and bending properties. Therefore, it is preferable that the difference between the Ms temperature and the overaging treatment end temperature, i.e., the value of B, be 30 to 100°C. The lower limit of the value of B is more preferably 35°C, and even more preferably 40°C. The upper limit of the value of B is more preferably 95°C, and even more preferably 90°C.

[0057] [Equation 5] 30≦B≦100 (In the above relational expression 5, B is the Ms-overaging treatment finish temperature (°C).)

[0058] In the present invention, in order to achieve the target fine structure fraction and mechanical properties, it is preferable that the following relational expression 6 is satisfied during the secondary cooling and overaging treatment.

[0059] [Equation 6] 2.5≦overaging time / B≦14

[0060] The above-mentioned relational expression 6 is used in the present invention to precisely control the target microstructure and ensure the target physical properties. If the value of the above-mentioned relational expression 6 is less than 2.5, the overaging holding time is short or the overaging treatment end temperature is low, which may result in excessive fresh martensitic transformation, making it difficult to achieve the target elongation and bending properties. On the other hand, if the value of the above-mentioned relational expression 6 exceeds 14, the overaging holding time is long or the overaging treatment end temperature is high, making it difficult to achieve the target microstructure fraction and the target physical properties. Therefore, the value of the above-mentioned relational expression 6 is preferably in the range of 2.5 to 14. The lower limit of the value of the above-mentioned relational expression 6 is more preferably 3.0, and even more preferably 3.5. The upper limit of the value of the above-mentioned relational expression 6 is more preferably 12, and even more preferably 10.

[0061] Meanwhile, the present invention may further include a step of temper rolling the overaged cold-rolled steel sheet at an elongation of 0.1 to 2.0% after the overaging treatment. Typically, temper rolling results in little increase in tensile strength, but an increase in yield strength of at least 50 MPa or more. If the elongation is less than 0.1%, it may be difficult to control the shape, and if it exceeds 2.0%, the high elongation process may significantly destabilize operability. [Example]

[0062] The present invention will be described in more detail through the following examples. However, it should be noted that the following examples are intended to illustrate and explain the present invention in more detail, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0063] Example 1 Molten steel having the alloy composition shown in Table 1 below was prepared, then formed into a 250 mm thick slab, heated at 1200°C for 12 hours, and finish-rolled under the conditions shown in Table 2 below, followed by coiling to produce a hot-rolled steel sheet. The hot-rolled steel sheet produced in this manner was pickled, and then cold-rolled at a cold reduction of 50% to produce a cold-rolled steel sheet. This cold-rolled steel sheet was subjected to continuous annealing, soaking, primary and secondary cooling, and overaging under the conditions shown in Table 3 below, to produce a final cold-rolled steel sheet.

[0064] The microstructure, average grain size, and mechanical properties of the final cold-rolled steel sheets thus produced were measured, and the results are shown in Table 4 below.

[0065] The microstructure and average grain size were measured using an electron backscatter diffraction (EBSD) instrument.

[0066] Among the mechanical properties, tensile strength (TS), yield strength (YS), and elongation (EL) were measured by tensile tests after tensile test specimens were taken in the horizontal direction of rolling. At this time, the gauge length was 80 mm and the width of the tensile test specimen was 20 mm.

[0067] The hardness of the fusion zone (HvFZ) and the base material (HvBS) was measured five times at 1 / 4 t (t = thickness point) using a Vickers hardness tester under a load of 500 gf after BOP (Bead On Plate) welding of cold-rolled steel sheets using a CO2 laser welding machine at 6 kW for 3 min, and then averaged.

[0068] [Table 1]

[0069] [Table 2]

[0070] [Table 3]

[0071] [Table 4]

[0072] As can be seen from Tables 1 to 4 above, in the case of Examples 1 to 5, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, the microstructure that the present invention aims to obtain can be secured and excellent mechanical properties can be obtained.

[0073] On the other hand, Comparative Examples 1 to 8 do not satisfy the alloy composition proposed by the present invention and do not satisfy some of the manufacturing conditions, and therefore the microstructure that the present invention aims to obtain cannot be secured, and the mechanical properties are reduced.

[0074] Example 2 A molten steel having the alloy composition of Invention Steel 1 described in Example 1 was prepared, then formed into a 250 mm thick slab, heated at 1200°C for 12 hours, finish-rolled under the conditions shown in Table 5 below, and coiled to produce a hot-rolled steel sheet. The hot-rolled steel sheet produced in this manner was pickled, and then cold-rolled at a cold reduction of 50% to produce a cold-rolled steel sheet. This cold-rolled steel sheet was subjected to continuous annealing, soaking, primary and secondary cooling, and overaging under the conditions shown in Table 6 below to produce a final cold-rolled steel sheet.

[0075] The final cold-rolled steel sheets thus produced were subjected to measurement of microstructure, average grain size, and mechanical properties, and the results are shown in Table 7 below.

[0076] The microstructure and average grain size were measured using an electron backscatter diffraction (EBSD) instrument.

[0077] Among the mechanical properties, tensile strength (TS), yield strength (YS), and elongation (EL) were measured by tensile tests after tensile test specimens were taken in the horizontal direction of rolling. At this time, the gauge length was 80 mm and the width of the tensile test specimen was 20 mm.

[0078] Among the mechanical properties, HER was measured according to the ISO 16330 standard, and the holes were sheared using a 10 mm diameter punch with a 12% clearance.

[0079] Among the mechanical properties, R / t is the value obtained by dividing R (limit bending radius) by the thickness of the steel sheet. Here, the above R was calculated by taking a test piece 30 mm wide x 35 mm long in the direction horizontal to the rolling direction (long axis), grinding one side by 0.2 mm, and conducting a bending test using the V-block method in accordance with JIS Z 2248 so that the ground surface did not come into contact with the punch, and then varying the bending radius from 0 to 5 mm to determine the minimum bending radius at which the material could be bent without breaking.

[0080] Among the mechanical properties, the maximum angle of three-point bending was measured three times for each test piece based on the VDA (Verband Der Automobilindustrie) standard, and the average value was calculated.

[0081] [Table 5]

[0082] [Table 6]

[0083] [Table 7]

[0084] As can be seen from Tables 5 to 7 above, in the case of Examples 6 to 10, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, the type and fraction of microstructure and average grain size targeted by the present invention are secured, thereby ensuring the mechanical properties (tensile properties, HER, bending properties) that the present invention aims to achieve.

[0085] On the other hand, Comparative Examples 9 to 16 satisfy the alloy composition proposed by the present invention, but do not satisfy the manufacturing conditions, and therefore the microstructure or average grain size targeted by the present invention cannot be ensured, and mechanical properties are deteriorated. In particular, in the case of Comparative Examples 9 and 10, the finish rolling temperature and coiling temperature did not satisfy the conditions of the present invention, and sheet breakage occurred.

[0086] Figure 1 is a photograph of the microstructure observed by SEM in Example 6. As can be seen from Figure 1, it can be confirmed that Example 6 has the microstructure that the present invention aims for properly formed.

[0087] Example 3 A molten steel having the alloy composition of Invention Steel 2 described in Example 1 was prepared, then formed into a 250 mm thick slab, heated at 1200°C for 12 hours, finish-rolled under the conditions shown in Table 8 below, and coiled to produce a hot-rolled steel sheet. The hot-rolled steel sheet produced in this manner was pickled, and then cold-rolled at a cold reduction of 50% to produce a cold-rolled steel sheet. This cold-rolled steel sheet was subjected to continuous annealing, soaking, primary and secondary cooling, and overaging under the conditions shown in Table 9 below to produce a final cold-rolled steel sheet.

[0088] The final cold-rolled steel sheets thus produced were subjected to measurement of microstructure, average grain size, and mechanical properties, and the results are shown in Table 10 below.

[0089] The microstructure and average grain size were measured using an electron backscatter diffraction (EBSD) instrument.

[0090] Among the mechanical properties, the maximum angle of three-point bending was measured three times for each test piece based on the VDA (Verband Der Automobilindustrie) standard, and the average value was calculated.

[0091] [Table 8]

[0092] [Table 9]

[0093] [Table 10]

[0094] As can be seen from Tables 8 to 10 above, in the case of Examples 11 to 15, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, the microstructure targeted by the present invention is secured, thereby ensuring the bending properties that the present invention aims to achieve.

[0095] On the other hand, although Comparative Examples 17 to 20 satisfy the alloy composition proposed by the present invention, they do not satisfy the overaging treatment conditions and relational expressions 5 and 6 among the manufacturing conditions, and therefore the microstructure targeted by the present invention cannot be secured, and the bending properties are reduced.

Claims

1. The alloy contains, by weight, C: 0.03 to 0.12%, Si: 0.03 to 0.30%, Mn: 2.1 to 2.9%, Al: 0.005 to 0.07%, Nb: 0.01 to 0.08%, Ti: 0.005 to 0.08%, B: 0.0005 to 0.005%, Cr: 0.7 to 1.4%, Mo: 0.005 to 0.10%, N: 0.008% or less (excluding 0%), with the balance being Fe and other unavoidable impurities; The following relations 1 to 3 are satisfied: The microstructure contains, in area percentage, fresh martensite: 4 to 19%, the sum of tempered martensite and bainite: 78 to 95%, and retained austenite: 0.2 to 2.0%. The average crystal grain size of the microstructure is 0.5 to 6 μm, and the ultra-high strength cold-rolled steel sheet has excellent yield strength and bending properties. [Relationship 1] 0.18≦C+Si / 30+Mn / 20+2P+4S≦0.30 [Relationship 2] 180≦48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb≦270 [Relationship 3] 700≦(48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb) / (C+Si / 30+Mn / 20+2P+4S)≦1200 (However, the contents of the alloying elements in the above Relational Formulas 1 to 3 are expressed in weight percent.)

2. 2. The ultra-high strength cold rolled steel sheet according to claim 1, wherein the impurities further include P: 0.04% or less (excluding 0%) and S: 0.005% or less (excluding 0%).

3. 2. The ultra-high strength cold rolled steel sheet according to claim 1, wherein the impurities include one or more of Sb, Mg, Sn, Sb, Zn, and Pb, and the total amount thereof is 0.1 wt% or less.

4. 2. The ultra-high strength cold rolled steel sheet according to claim 1, wherein the microstructure further contains 10% or less of ferrite.

5. The cold-rolled steel sheet has a yield strength (YS): 800 to 980 MPa, a tensile strength (TS): 980 to 1180 MPa, an elongation (EL): 4 to 12%, a yield ratio (YS / TS): 0.70 to 0.95, a hole expansion ratio (HER): 35 to 80%, R / t: 0.8 or less, and a maximum three-point bending angle: 90 to 140°.

1. An ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties according to claim 1.

6. 2. The ultra-high strength cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet has a hardness (HvBM) of 300 to 400 Hv, a fusion zone hardness (HvFZ) of a weld formed after welding of 350 to 450 Hv, and HvFZ / HvBM is 1.30 or less.

7. A method for producing an ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties according to claim 1, comprising: heating a slab containing, by weight, C: 0.03 to 0.12%, Si: 0.03 to 0.30%, Mn: 2.1 to 2.9%, Al: 0.005 to 0.07%, Nb: 0.01 to 0.08%, Ti: 0.005 to 0.08%, B: 0.0005 to 0.005%, Cr: 0.7 to 1.4%, Mo: 0.005 to 0.10%, N: 0.008% or less (excluding 0%), with the balance being Fe and other unavoidable impurities, and satisfying the following relations 1 to 3; Finish rolling the heated slab so that the delivery temperature of the finish rolling is Ar3+50°C to Ar3+150°C to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet to Ms+50°C to Ms+300°C and then coiling it; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuous annealing the cold-rolled steel sheet in a temperature range of Ar3+10°C to Ar3+70°C; soaking the continuously annealed cold-rolled steel sheet for 50 to 200 seconds; subjecting the soaked cold-rolled steel sheet to primary cooling at a cooling rate of 1 to 10°C / s to 620 to 700°C; Secondarily cooling the primarily cooled cold-rolled steel sheet to 360 to 420°C at a cooling rate of 5 to 50°C / s; and overaging the second-cooled cold-rolled steel sheet for 250 to 650 seconds, and then finishing at 320 to 400°C. A method for producing an ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties, which satisfies the following Relational Expressions 4 to 6 during the secondary cooling and overaging treatment: [Relationship 1] 0.18≦C+Si / 30+Mn / 20+2P+4S≦0.30 [Relationship 2] 180≦48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb≦270 [Relationship 3] 700≦(48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb) / (C+Si / 30+Mn / 20+2P+4S)≦1200 [Relationship 4] 10≦A≦70 [Relationship 5] 30≦B≦100 [Relationship 6] 2.5≦overaging treatment time / B≦14 (Note that the contents of the alloy elements in the above Relational Formulas 1 to 3 represent weight percent, and in the above Relational Formulas 4 to 6, A is Ms-secondary cooling end temperature (°C), and B is Ms-overaging treatment end temperature (°C).)

8. The method for producing an ultra-high strength cold rolled steel sheet excellent in yield strength and bending properties according to claim 7, wherein the slab heating is performed at 1100 to 1300°C.

9. The method for producing an ultra-high strength cold rolled steel sheet having excellent yield strength and bending properties according to claim 7, wherein the slab has a thickness of 230 to 270 mm.

10. 8. The method for manufacturing an ultra-high strength cold rolled steel sheet having excellent yield strength and bending properties according to claim 7, further comprising the step of cooling the coiled hot rolled steel sheet to room temperature at a cooling rate of 0.1° C. / s or less after the coiling.

11. The method for producing an ultra-high strength cold rolled steel sheet having excellent yield strength and bending properties according to claim 7, wherein the cold rolling is performed at a reduction ratio of 40 to 70%.

12. 8. The method for manufacturing an ultra-high strength cold rolled steel sheet having excellent yield strength and bending properties according to claim 7, further comprising, after the overaging treatment, temper rolling the overaged cold rolled steel sheet at an elongation of 0.1 to 2.0%.

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