Ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties and method for manufacturing the same

Abstract

This invention relates to an ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties, and a method for manufacturing the same. More specifically, it relates to an ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties, and a method for manufacturing the same. The cold-rolled steel sheet can be used as a structural component for automobiles, such as a component, seat rail, and pillar.

Description

Technical Field

[0001] This invention relates to an ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties, and a method for manufacturing the same. More specifically, it relates to an ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties, and a method for manufacturing the same. The ultra-high strength cold-rolled steel sheet can be used as a structural component for automobiles, such as a member, seat rail, or pillar. Background Technology

[0002] In recent years, stricter safety regulations for car passengers and pedestrians have necessitated the development of safety features, which, in contrast to weight reduction aimed at improving fuel efficiency, has increased vehicle weight. Consumers are increasingly interested in environmentally friendly and fuel-efficient hybrid or electric vehicles. To produce such environmentally friendly and safe vehicles, it is essential to ensure lightweight body structures and stable body materials. However, hybrid vehicles add various components such as electric motors, batteries, and secondary fuel tanks in addition to traditional gasoline engines. Furthermore, the weight of the vehicle body continues to increase with the continuous improvement of driver convenience features. Therefore, to achieve lightweight body construction, it is necessary to develop a thin material with excellent strength, ductility, and bending properties. Thus, to address this issue, it is necessary to develop a gigabit-class steel sheet that can ensure a tensile strength of over 980 MPa and high ductility.

[0003] On the other hand, with the expansion of automotive impact stability regulations in recent years, high-strength steel with excellent yield strength is being used in structural components such as structural members, seat rails, and pillars to improve the impact resistance of the vehicle body. Structural components are characterized by a higher yield strength relative to tensile strength, i.e., a yield-to-tensile strength ratio (yield strength / tensile strength), which is more conducive to absorbing impact energy. However, generally, as the strength of the steel sheet increases, the elongation decreases and the formability decreases. Therefore, there is a need to develop a material that simultaneously improves a high yield-to-tensile strength ratio, formability, and bending properties, which are key physical properties during component processing.

[0004] A representative manufacturing method for improving yield strength involves water cooling during continuous annealing. Specifically, after homogenization during the annealing process, the steel is immersed in water and tempered, thereby producing a steel sheet with a microstructure that transforms from martensite to tempered martensite. Patent Document 1 is a representative prior art example of this method. Patent Document 1 relates to a technique in which steel with a carbon content of 0.18-0.3% is continuously annealed, water-cooled to room temperature, and then subjected to an over-aging treatment at 120-300°C for 1-15 minutes to produce steel with a martensite volume fraction of 80-97% and the balance being ferrite. As mentioned above, when manufacturing ultra-high strength steel by water cooling followed by tempering, the yield strength ratio is very high, but due to temperature deviations in the width and length directions, the shape quality of the coil deteriorates. Therefore, during roll forming, problems such as poor material properties and poor machinability occur at certain locations.

[0005] Patent Document 2 is a prior art for improving the workability of the high-tensile steel sheet. Patent Document 2 relates to a steel sheet composed of a composite microstructure primarily composed of tempered martensite, characterized by the dispersion of finely precipitated Cu particles with a particle size of 1-100 nm within the microstructure to improve workability. However, in Patent Document 1, Cu is added excessively at a content of 2-5% to precipitate good fine Cu particles, which may lead to Cu-induced red-hot brittleness and excessively increased manufacturing costs.

[0006] On the other hand, Patent Document 3 proposes a steel sheet with a ferrite matrix and a fine microstructure containing 2-10% by area pearlite. Its strength is primarily enhanced through precipitation strengthening and grain refinement via the addition of carbonitride-forming elements such as Ti. Patent Document 3 offers the advantage of readily achieving high strength at low manufacturing costs, but its disadvantage is a sharp increase in recrystallization temperature due to the fine precipitates. High-temperature annealing is necessary to ensure sufficient recrystallization and maintain ductility. Furthermore, existing precipitation-strengthened steels, which use carbonitride precipitation on a ferrite matrix for strengthening, struggle to achieve strengths exceeding 600 MPa.

[0007] Therefore, to solve the above problems, it is necessary to develop a steel that exhibits high yield strength and bending properties while also possessing ultra-high strength that can be cold-formed.

[0008] [Existing Technical Documents]

[0009] (Patent Document 1) Japanese Patent Publication No. 2528387

[0010] (Patent Document 2) Japanese Patent Publication No. 2005-264176

[0011] (Patent Document 3) Korean Patent Publication No. 2015-0073844 Summary of the Invention

[0012] Technical problems to be solved

[0013] One aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties, and a method for manufacturing the same.

[0014] Technical solution

[0015] One embodiment of the present invention provides an ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties. The ultra-high strength cold-rolled steel sheet comprises, by weight percent: C: 0.03-0.12%, Si: 0.03-0.30%, Mn: 2.1-2.9%, Al: 0.005-0.07%, Nb: 0.01-0.08%, Ti: 0.005-0.08%, B: 0.0005-0.005%, Cr: 0.7%. -1.4%, Mo: 0.005-0.10%, N: less than 0.008% (excluding 0%), balance Fe and other unavoidable impurities, the ultra-high strength cold-rolled steel sheet satisfies the following relationships 1 to 3, in area %, the microstructure comprises: newly formed martensite: 4-19%, the sum of tempered martensite and bainite: 78-95%, and retained austenite: 0.2-2.0%, the average grain size of the microstructure is 0.5-6μm.

[0016] [Relationship 1] 0.18≤C+Si / 30+Mn / 20+2P+4S≤0.30

[0017] [Relationship 2] 180≤48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb≤270

[0018] [Relationship 3] 700≤(48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb) / (C+Si / 30+Mn / 20+2P+4S)≤1200

[0019] (Whereinafter, the content of alloy components recorded in Equations 1 to 3 is expressed as weight %.)

[0020] Another embodiment of the present invention provides a method for manufacturing ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties, the method comprising the following steps: heating a slab containing, by weight percent: C: 0.03-0.12%, Si: 0.03-0.30%, Mn: 2.1-2.9%, Al: 0.005-0.07%, Nb: 0.01-0.08%, Ti: 0.005-0.08%, B: 0.0005-0.005%, Cr: 0.7-1.4%, Mo: 0.005-0.10%, N: less than 0.008% (excluding 0%), with the balance being Fe and other unavoidable impurities, and satisfying the following relationships 1 to 3; and finishing rolling the heated slab to a finishing exit temperature of Ar3+50°C to Ar3+150°C. The hot-rolled steel sheet is obtained; the hot-rolled steel sheet is cooled to Ms+50℃ to Ms+300℃ and then coiled; the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; the cold-rolled steel sheet is continuously annealed in a temperature range of Ar3+10℃ to Ar3+70℃; the continuously annealed cold-rolled steel sheet is subjected to homogenization heat treatment for 50-200 seconds; the homogenized cold-rolled steel sheet is cooled once at a cooling rate of 1-10℃ / second to 620-700℃; the once-cooled cold-rolled steel sheet is cooled a second time at a cooling rate of 5-50℃ / second to 360-420℃; and the second-cooled cold-rolled steel sheet is over-aged for 250-650 seconds, and then the process is terminated at 320-400℃, wherein the second cooling and the over-aging treatment satisfy the following relationships 4 to 6.

[0021] [Relationship 1] 0.18≤C+Si / 30+Mn / 20+2P+4S≤0.30

[0022] [Relationship 2] 180≤48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb≤270

[0023] [Relationship 3] 700≤(48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb) / (C+Si / 30+Mn / 20+2P+4S)≤1200

[0024] [Relation 4] 10≤A≤70

[0025] [Relationship 5] 30≤B≤100

[0026] [Relationship 6] 2.5 ≤ Overdue processing time / B ≤ 14

[0027] (Whereinafter, the content of alloy components recorded in Equations 1 to 3 is expressed as % by weight, and in Equations 4 to 6, A represents Ms - the secondary cooling termination temperature (°C), and B represents Ms - the over-aging treatment termination temperature (°C).)

[0028] Beneficial effects

[0029] According to one aspect of the present invention, an ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties and a method thereof can be provided. Attached Figure Description

[0030] Figure 1 These are photographs of the fine tissue of Example 6 of the present invention observed using SEM according to an embodiment of the present invention.

[0031] Best practice

[0032] The following describes an ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties according to an embodiment of the present invention. First, the alloy composition of the present invention will be described. Unless otherwise stated, the alloy composition content described below is expressed in weight percent.

[0033] C: 0.03-0.12%

[0034] Carbon (C) is a very important element added for solid solution strengthening. Furthermore, carbon combines with precipitating elements to form fine carbides, thereby contributing to increased strength. When the C content is less than 0.03%, it is very difficult to ensure the desired strength. On the other hand, when the C content exceeds 0.12%, due to increased hardenability, excessive martensite is formed during cooling, resulting in a sharp increase in strength, but bending characteristics may deteriorate, making it difficult to obtain the HER, R / t, and maximum three-point bending angle desired by the present invention. Furthermore, due to decreased weldability, the possibility of welding defects increases when processing parts in customer companies. Therefore, the C content preferably has a range of 0.03-0.12%. The lower limit of the C content is more preferably 0.04%, and further preferably 0.05%. The upper limit of the C content is more preferably 0.10%, and further preferably 0.09%.

[0035] Si: 0.03-0.30%

[0036] Silicon (Si) is one of the five elements in steel, and a small amount of Si is naturally added during the manufacturing process. This Si helps increase strength and suppresses carbide formation, preventing carbon from forming carbides during annealing and cooling. Furthermore, carbon is distributed and accumulated in the retained austenite, allowing the austenite phase to remain at room temperature, thus helping to ensure elongation. When the Si content is less than 0.03%, it may be difficult to fully ensure the above effects. On the other hand, when the Si content exceeds 0.30%, the elongation may decrease due to the increased solid solution strengthening effect, leading to surface oxide scale defects, resulting in a decrease in the quality of the plated surface, and potentially reduced chemical processing properties. Therefore, the Si content is preferably in the range of 0.03-0.30%. 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%.

[0037] Mn: 2.1-2.9%

[0038] Manganese (Mn) is an element that causes sulfur in steel to completely precipitate as MnS, thereby preventing hot brittleness caused by FeS formation and strengthening the steel through solid solution. When the Mn content is less than 2.1%, it is difficult to ensure the strength desired in this invention. On the other hand, when the Mn content exceeds 2.9%, the possibility of problems such as weldability and hot rollability increases, and hardenability increases, which may lead to excessive martensite formation and thus potentially reduced elongation. Furthermore, the formation of manganese bands (Mn-bands) (bands of Mn oxides) in the microstructure increases the risk of processing cracks and plate breakage, and the dissolution of Mn oxides on the surface during annealing greatly hinders plating. Therefore, the Mn content is preferably in the range of 2.1-2.9%. 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%.

[0039] Al: 0.005-0.07%

[0040] Aluminum (Al) is an element added during steelmaking for deoxidation. When the Al content is less than 0.005%, it is difficult to achieve a sufficient deoxidation effect. When the Al content exceeds 0.07%, the Al reacts with oxygen (O) in the molten steel to form high-melting-point oxides (inclusions), which may cause nozzle clogging. Furthermore, as described above, the inclusions formed have sharp shapes, which may worsen their bending properties. Therefore, the Al content is preferably 0.005-0.07%. 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%.

[0041] Nb: 0.01-0.08%

[0042] Niobium (Nb) is an element that contributes to increased strength by segregating at austenite grain boundaries to suppress coarsening of austenite grains during annealing heat treatment and forming fine carbides. When the Nb content is less than 0.01%, the above effect is insufficient. On the other hand, when the Nb content exceeds 0.08%, coarse carbides precipitate, and as the dissolved carbon content in the steel decreases, strength and elongation may decrease, and manufacturing costs may increase. Therefore, the Nb content is preferably in the range of 0.01-0.08%. 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%.

[0043] Ti: 0.005-0.08%

[0044] Titanium (Ti), as a fine carbide-forming element, helps ensure yield strength and tensile strength. Furthermore, as a nitride-forming element, Ti suppresses AlN precipitation by causing N in steel to precipitate as TiN, thus reducing the risk of cracking during continuous casting. When the Ti content is less than 0.005%, these effects may be difficult to achieve. On the other hand, when the Ti content exceeds 0.08%, coarse carbides precipitate, and as the dissolved carbon content in the steel decreases, strength and elongation may decrease, and nozzle clogging may occur during continuous casting. Therefore, the Ti content is preferably in the range of 0.005-0.08%. 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%.

[0045] B: 0.0005-0.005%

[0046] Boron (B) is an element that greatly contributes to ensuring the hardenability of steel, and to achieve this effect, it is preferable to add more than 0.0005% B. However, when the B content exceeds 0.005%, boron carbide forms at grain boundaries, providing nucleation sites for ferrite, which may actually worsen hardenability. Therefore, the B content is preferably in the range of 0.0005-0.005%. 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%.

[0047] Cr: 0.7-1.4%

[0048] Chromium (Cr) is an element that improves hardenability and increases the strength of steel. When the Cr content is less than 0.7%, it may be difficult to ensure the desired strength. On the other hand, when the Cr content exceeds 1.4%, the ductility of the steel sheet may decrease. Therefore, the Cr content is preferably in the range of 0.7-1.4%. 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%.

[0049] Mo: 0.005-0.10%

[0050] Molybdenum (Mo) is a carbide-forming element. When added in combination with carbonitride-forming elements such as Ti, Nb, and V, it finely maintains the size of the precipitates, thereby improving yield strength and tensile strength. Furthermore, Mo improves the hardenability of steel by finely forming martensite at grain boundaries, thus offering the advantage of controllable yield strength ratio. For these effects, Mo is preferably added at 0.0005% or more. However, Mo is a high-valence element, so increasing its content is detrimental to manufacturing; therefore, it is preferable to appropriately control the Mo content. When the Mo content exceeds 0.10%, it causes a sharp increase in manufacturing costs, reducing economic efficiency and, due to excessive grain refinement and solid solution strengthening effects, actually reducing the ductility of the steel. Therefore, the Mo content is preferably in the range of 0.005-0.10%. 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%.

[0051] N: less than 0.008% (except 0%)

[0052] Nitrogen (N) is an element that is unavoidably present during the manufacturing process, but it contributes to improving the strength of steel by forming carbonitrides. However, when the N content exceeds 0.008%, not only does the risk of brittleness increase significantly, but the excess N remaining after TiN formation may consume B, which contributes to hardenability, in the form of BN. Therefore, the N content is preferably 0.008% or less. More preferably, the N content is 0.007% or less, and even more preferably 0.006% or less.

[0053] On the other hand, the cold-rolled steel sheet of the present invention satisfies the above-mentioned alloy composition and preferably satisfies the following equations 1 to 3. Therefore, it is possible to manufacture ultra-high strength steel sheets with excellent bending workability and tensile strength of 980 MPa or more, as desired by the present invention.

[0054] [Relationship 1] 0.18≤C+Si / 30+Mn / 20+2P+4S≤0.30

[0055] Equation 1 is a compositional relationship used to ensure strength and weldability. When the value of Equation 1 is less than 0.18, it is difficult to ensure the strength desired by the present invention, and when the value of Equation 1 exceeds 0.30, weldability may be poor. Therefore, the value of Equation 1 preferably has a range of 0.18 to 0.30. The lower limit of the value of Equation 1 is more preferably 0.19, and even more preferably 0.20. The upper limit of the value of Equation 1 is more preferably 0.28, and even more preferably 0.26.

[0056] [Relationship 2] 180≤48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb≤270

[0057] Equation 2 is a relation related to the hardenability index used to ensure hardenability. When the value of Equation 2 is less than 180, the hardenability is insufficient, making it difficult to ensure the strength desired by the present invention. When the value of Equation 2 exceeds 270, the hardenability becomes too high, and therefore the bending properties and formability may be poor. Therefore, the value of Equation 2 preferably has a range of 180 to 270. The lower limit of the value of Equation 2 is more preferably 190, and even more preferably 200. The upper limit of the value of Equation 2 is more preferably 260, and even more preferably 250.

[0058] [Relationship 3] 700≤(48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb) / (C+Si / 30+Mn / 20+2P+4S)≤1200

[0059] Formula 3 is a compositional formula used to simultaneously ensure the desired strength, hardenability, and weldability of the present invention. When the value of Formula 3 is less than 700, not only may the weldability deteriorate, but the hardenability is also insufficient, making it difficult to ensure the desired strength of the present invention. When the value of Formula 3 exceeds 1200, the hardenability becomes too high, and therefore the bending properties and formability may be poor. Therefore, the value of Formula 3 preferably has a range of 700 to 1200. The lower limit of the value of Formula 3 is more preferably 700, and even more preferably 800. The upper limit of the value of Formula 3 is more preferably 1150, and even more preferably 1100.

[0060] The remaining component of this invention is iron (Fe). However, undesirable impurities may inevitably be introduced from raw materials or the surrounding environment during the ordinary manufacturing process, and therefore these impurities cannot be completely eliminated. These impurities are well known to those skilled in the art, and therefore their contents are not specifically mentioned in this specification.

[0061] However, phosphorus and sulfur are the most frequently mentioned impurities, so they will be briefly explained below.

[0062] P: Below 0.04% (except 0%)

[0063] Phosphorus (P) is an element that can induce brittleness through segregation at grain boundaries and / or phase boundaries. Therefore, the phosphorus (P) content should be controlled at the lowest possible level, and preferably limited to 0.04% or less. More preferably, the P content is limited to 0.03% or less, and even more preferably to 0.02% or less.

[0064] S: Less than 0.005% (except 0%)

[0065] Sulfur (S), as an impurity, can cause high-temperature cracking due to its segregation as a non-metallic inclusion of MnS in steel and during continuous casting solidification. Therefore, the sulfur (S) content should be controlled at a low level, preferably below 0.005%. More preferably, the S content should be limited to below 0.004%, and even more preferably below 0.003%.

[0066] Furthermore, the impurities may include one or more of Sb, Mg, Sn, Zn, and Pb as inclusion elements, and the total amount of the inclusion elements may be less than 0.1% by weight. Inclusion elements originate from scrap steel or other materials used as raw materials in the steelmaking process. When the total amount of inclusion elements exceeds 0.1%, it may cause surface cracks in the slab, and the surface quality of the steel plate may be reduced.

[0067] The following describes the microstructure of an ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties according to one embodiment of the present invention.

[0068] The microstructure of the cold-rolled steel sheet of the present invention, in area percent, preferably comprises: 4-19% newly formed martensite, 78-95% of the sum of tempered martensite and bainite, and 0.2-2.0% retained austenite. The microstructure of the cold-rolled steel sheet of the present invention comprises tempered martensite (hereinafter also referred to as "TM") and bainite (hereinafter also referred to as "B") as the main microstructure. Since the tempered martensite and bainite are not easily distinguishable in the microstructure, the fraction of the sum of tempered martensite and bainite is controlled in the present invention. When the fraction of the sum of tempered martensite and bainite is less than 78%, it is difficult to ensure the desired strength; when the fraction of the sum of tempered martensite and bainite exceeds 95%, bending characteristics and elongation may be poor. The newly formed martensite (hereinafter also referred to as "FM") is a microstructure that is beneficial for ensuring strength. When the fraction of newly formed martensite is less than 4%, it is difficult to ensure the desired strength; when the fraction of newly formed martensite exceeds 19%, bending properties and elongation may be poor. The retained austenite (hereinafter also referred to as "RA") is a microstructure that is beneficial for ensuring elongation. When the fraction of retained austenite is less than 0.2%, it may be difficult to fully obtain the above-mentioned effect; when the fraction of retained austenite exceeds 2.0%, it transforms into martensite during processing, thus HER or bending properties may be poor. On the other hand, the fine microstructure may further contain less than 10% ferrite. The ferrite microstructure is a microstructure that inevitably forms during the manufacturing process, but it can also have a positive effect. For example, the ferrite can help ensure elongation. However, when the fraction of ferrite exceeds 10%, it may be difficult to ensure the strength desired by the present invention. The fraction of ferrite is more preferably less than 7%, and more preferably less than 5%.

[0069] On the other hand, the average grain size of the microstructure is preferably 0.5-6 μm. A finer average grain size is more beneficial for ensuring physical properties such as strength and HER (heavy morphology). However, in order to control the average grain size of the microstructure to less than 0.5 μm, the amount of Nb, Ti, Mo, and V, which are effective in grain refinement, is excessively increased, thus potentially increasing manufacturing costs. When the average grain size exceeds 6 μm, it is difficult to ensure the strength desired by the present invention, and HER and bending characteristics may be quite poor. Therefore, the average grain size preferably has a range of 0.5-6.0 μm. The lower limit of the average grain size is more preferably 1.0 μm, and even more preferably 1.5 μm. The upper limit of the average grain size is more preferably 5.5 μm, and even more preferably 5.0 μm.

[0070] The cold-rolled steel sheet of the present invention, as described above, has a yield strength (YS) of 800-980 MPa, a tensile strength (TS) of 980-1180 MPa, an elongation (EL) of 4-12%, a yield-to-tensile strength ratio (YS / TS) of 0.70 to 0.95, a hole expansion ratio (HER) of 35-80%, a radius of curvature (R / t) of less than 0.8, and a maximum three-point bending angle of 90-140°. The yield strength is more preferably 820-960 MPa, and even more preferably 850-950 MPa. The tensile strength is more preferably 1000-1170 MPa, and even more preferably 1020-1160 MPa. The elongation is more preferably 5-11%, and even more preferably 6-10%. The yield-to-tensile strength 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 40-75%, and even more preferably 45-70%. 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 95-135°, and even more preferably 100-130°.

[0071] Furthermore, the hardness (HvBM) of the cold-rolled steel sheet of the present invention can be 300-400 Hv. The hardness of the base material is more preferably 310-390 Hv, and even more preferably 320-380 Hv. Furthermore, the hardness (HvFZ) of the fusion zone of the welded portion formed after welding can be 350-450 Hv. When the hardness of the fusion zone of the welded portion is less than 350 Hv, sufficient hardness of the fusion zone cannot be ensured, and therefore the strength of the welded portion may decrease. On the other hand, when the hardness of the fusion zone of the welded portion exceeds 450 Hv, the hardness of the fusion zone is too high, increasing the sensitivity to cracking, and therefore the strength and impact energy absorption of the welded portion will decrease. The more similar the hardness of the cold-rolled steel sheet, i.e., the hardness (HvBM) corresponding to the base material after welding, is to the hardness (HvFZ) of the fusion zone, the better. Therefore, the ratio of HvFZ to HvBM (HvFZ / HvBM) is preferably 1.30 or less. The HvFZ / HvBM ratio is more preferably 1.25 or less, and even more preferably 1.20 or less.

[0072] The following describes an ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties according to one embodiment of the present invention.

[0073] First, a slab satisfying the above alloy composition is heated. In this invention, the heating temperature of the slab is not particularly limited; for example, the slab can be heated at 1100-1300°C. When the heating temperature of the slab is below 1100°C, the slab temperature is low, which may generate rolling load during rough rolling. When the heating temperature of the slab exceeds 1300°C, the microstructure may become coarser, and there may be disadvantages such as increased electricity 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. Furthermore, the slab can have a thickness of 230-270 mm.

[0074] The heated slab is then finished rolled to an exit temperature of Ar3+50°C to Ar3+150°C to obtain a hot-rolled steel sheet. When the exit temperature is below Ar3+50°C, the resistance to hot deformation may increase sharply. When the exit temperature exceeds Ar3+150°C, not only is an excessively thick oxide scale formed, but the fine microstructure of the steel sheet is also likely to coarsen. Therefore, the exit temperature preferably falls within the range of Ar3+50°C to Ar3+150°C. The lower limit of the exit temperature is more preferably Ar3+60°C, and even more preferably Ar3+70°C. The upper limit of the exit temperature is more preferably Ar3+140°C, and even more preferably Ar3+130°C. Furthermore, Ar3 represents the temperature at which the steel transforms into austenite during heating, for example, as indicated by Formula 910-203C. 1 / 2 The Ar3 value is obtained by using the following formulas: +44.7Si+31.5Mo-30Mn-11Cr+700P+400Al+400Ti, etc.

[0075] The hot-rolled steel sheet is then cooled to Ms+50°C to Ms+300°C before being coiled. When the coiling temperature is below Ms+50°C, excessive martensite or bainite forms, leading to an excessive increase in the strength of the hot-rolled steel sheet. This can cause problems such as shape defects due to load during cold rolling. On the other hand, when the coiling temperature exceeds Ms+300°C, the surface oxide scale increases, which may worsen pickling properties. Therefore, the coiling temperature preferably falls within 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. Furthermore, after coiling, the coiled hot-rolled steel sheet can be cooled to room temperature at a cooling rate of 0.1°C / second or less. Ms represents the temperature at which martensite begins to transform upon cooling. For example, the value of Ms can be obtained using formula 539-423C-30.4Mn-7.5Si+30Al.

[0076] The hot-rolled steel sheet, after being coiled and cooled, is then cold-rolled to obtain a cold-rolled steel sheet. The cold rolling can be performed with a reduction rate of 40-70%. When the cold rolling reduction rate is less than 40%, the recrystallization driving force deteriorates, making it difficult to obtain good recrystallized grains, and shape correction becomes very difficult. When the cold rolling reduction rate exceeds 70%, the likelihood of cracks forming at the edge of the steel sheet is high, and the rolling load may increase sharply. Therefore, the cold rolling is preferably performed with a reduction rate of 40-70%. Alternatively, pickling can be performed before cold rolling to remove oxide scale or impurities adhering to the surface.

[0077] Subsequently, the cold-rolled steel sheet is continuously annealed within a temperature range of Ar3+10°C to Ar3+70°C. When the continuous annealing temperature is below Ar3+10°C, it cannot fully transform into austenite, making it difficult to obtain the desired martensite or bainite fraction in subsequent processes, thus potentially compromising strength. On the other hand, when the continuous annealing temperature exceeds Ar3+70°C, the austenite grain size coarsens, making it difficult to ensure the desired bending properties. Therefore, the continuous annealing temperature preferably falls within 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.

[0078] The continuously annealed cold-rolled steel sheet is then subjected to a homogenization heat treatment for 50-200 seconds. This is to ensure recrystallization and grain growth in the cold-rolled structure, as well as a sufficient austenite fraction at the annealing temperature proposed in this invention. When the homogenization heat treatment time is less than 50 seconds, the reverse transformation to austenite is insufficient, and the ferrite fraction in the final structure increases, thus making it difficult to ensure the desired strength. On the other hand, when the homogenization heat treatment time exceeds 200 seconds, the austenite grain size becomes coarser, thus potentially resulting in poor bending properties in the final product. The lower limit of the homogenization heat treatment time is more preferably 55 seconds, and more preferably 60 seconds. The upper limit of the homogenization heat treatment time is more preferably 190 seconds, and more preferably 180 seconds.

[0079] Subsequently, the heat-treated cold-rolled steel sheet is cooled once at a cooling rate of 1-10°C / second to 620-700°C. This first cooling step is used to ensure a balanced carbon concentration of ferrite and austenite to increase the ductility and strength of the steel sheet. When the first cooling termination temperature is below 630°C or above 700°C, it is difficult to ensure the ductility and strength desired by the present invention. When the cooling rate is less than 1°C / second, the ferrite transformation is accelerated, making it difficult to ensure the desired fraction of fine microstructure. When the cooling rate exceeds 10°C / second, due to excessive martensitic transformation, it is difficult to ensure elongation.

[0080] Subsequently, the cold-rolled steel sheet, after initial cooling, undergoes a secondary cooling process at a rate of 5-50°C / second, cooling it to 360-420°C. This secondary cooling is one of the key control factors in this invention, and the final temperature of the secondary cooling is a crucial condition for simultaneously ensuring strength, ductility, and bending properties. When the final temperature of the secondary cooling is below 360°C, the martensite fraction increases excessively, making it difficult to ensure ductility; when the final temperature of the secondary cooling exceeds 420°C, it is difficult to ensure sufficient martensite, thus making it difficult to ensure the desired strength. Therefore, the final temperature of the secondary cooling, one of the key control factors for ensuring the desired physical properties of this invention, preferably has a range of 360-420°C. The lower limit of the final temperature of the secondary cooling is more preferably 365°C, and even more preferably 370°C. The upper limit of the final temperature of the secondary cooling is more preferably 410°C, and even more preferably 405°C. When the secondary cooling rate is less than 5°C / second, due to the slow cooling rate, ferrite transformation preferentially occurs before the martensite and bainite transformations, making it impossible to obtain the appropriate amount of fine microstructure desired by the present invention. When the secondary cooling rate exceeds 50°C / second, the sheet throughput decreases due to shape defects caused by excessive cooling, and sheet breakage may occur. The lower limit of the secondary cooling rate is more preferably 7.5°C / second, and even more preferably 10°C / second. The upper limit of the secondary cooling rate is more preferably 47.5°C / second, and even more preferably 45°C / second.

[0081] On the other hand, in order to ensure that the fractions of tempered martensite and bainite, which are important microstructures in this invention, are at the target level, it is important to precisely control the difference between the Ms temperature and the secondary cooling termination temperature. More specifically, it is preferable to satisfy the following relationship 4. When the difference between Ms and the secondary cooling termination temperature, i.e., the value of A, is less than 10°C, the transformation of martensite or bainite is less, and it may be difficult to ensure the desired strength. When the value of A exceeds 70°C, the time spent in the martensite region is longer, and the fraction of martensite increases excessively, thus making it difficult to ensure ductility. Therefore, the difference between Ms and the secondary cooling termination temperature, i.e., the value of A, is preferably 10-70°C. The lower limit of the value of A is more preferably 15°C, and further preferably 20°C. The upper limit of the value of A is more preferably 65°C, and further preferably 60°C. On the other hand, Ms represents the temperature at which the martensite begins to transform, and its value can be obtained by the following formula 1.

[0082] [Relation 4] 10≤A≤70

[0083] (Where, A in Equation 4 is Ms - the secondary cooling termination temperature (°C).)

[0084] Subsequently, the cold-rolled steel sheet, after secondary cooling, undergoes an over-aging treatment for 250-650 seconds, which is then terminated at 320-400°C. The over-aging treatment is preferably performed at the same temperature as the termination of secondary cooling or at a higher temperature. This over-aging treatment is a process used to promote the transformation of newly formed martensite at the termination of secondary cooling into tempered martensite, thereby consistently ensuring high yield strength and high bending properties. Therefore, the over-aging treatment is a crucial factor in ensuring the high bending workability desired by this invention. In this invention, the over-aging treatment time is precisely controlled within the range of 250-650 seconds. When the over-aging treatment time is less than 250 seconds, a small amount of transformation from newly formed martensite to tempered martensite occurs, which may result in poor bending workability. On the other hand, when the over-aging treatment time exceeds 650 seconds, it may be difficult to ensure the tensile strength desired by this invention due to reduced productivity and excessive tempered martensite transformation. The lower limit of the over-aging treatment time is more preferably 260 seconds, and even more preferably 270 seconds. The upper limit of the over-aging treatment time is more preferably 600 seconds, and more preferably 550 seconds. When the over-aging treatment termination temperature is below 320°C, it is difficult to ensure elongation due to excessive transformation of newly formed martensite, and the bending characteristics may be poor. When the over-aging treatment termination temperature exceeds 400°C, a small amount of transformation from newly formed martensite to tempered martensite occurs, so the bending characteristics may be poor. The lower limit of the over-aging treatment termination temperature is more preferably 325°C, and more preferably 330°C. The upper limit of the over-aging treatment termination temperature is more preferably 395°C, and more preferably 380°C. On the other hand, when it is desired to further improve HER and bending characteristics, reheating can be performed after cooling to the secondary cooling termination temperature after the over-aging treatment, so that further over-aging treatment can be performed.

[0085] On the other hand, in order to ensure that the fraction of tempered martensite, which is an important microstructure in this invention, is at the target level, it is important to precisely control the difference between the Ms temperature and the over-aging treatment termination temperature. More specifically, it is preferable to satisfy the following relationship 5. When the difference between the Ms temperature and the over-aging treatment termination temperature, i.e., the value of B, is less than 30°C, the martensitic transformation is insufficient, and therefore it may be difficult to ensure the desired strength. When the value of B exceeds 100°C, it may be difficult to ensure the desired elongation and flexural properties due to excessive nascent martensite transformation. Therefore, the difference between the Ms temperature and the over-aging treatment termination temperature, i.e., the value of B, is preferably 30-100°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.

[0086] [Relationship 5] 30≤B≤100

[0087] (Where, B in Equation 5 is Ms - the end temperature of the over-aging treatment (°C).)

[0088] Furthermore, in this invention, in order to achieve the desired fraction of fine microstructures and mechanical and physical properties, the secondary cooling and the over-aging treatment preferably satisfy the following relationship 6.

[0089] [Relationship 6] 2.5 ≤ Overdue processing time / B ≤ 14

[0090] The relation 6 is used to ensure the desired physical properties by precisely controlling the microstructure desired by the present invention. When the value of relation 6 is less than 2.5, the over-aging holding time is short or the over-aging treatment termination temperature is low, and due to excessive nascent martensite transformation, it may be difficult to ensure the desired elongation or flexural properties. On the other hand, when the value of relation 6 exceeds 14, the over-aging holding time is long or the over-aging treatment termination temperature is high, and it may be difficult to ensure the desired fraction of microstructure, and therefore it may be difficult to ensure the desired physical properties. Therefore, the value of relation 6 preferably has a range of 2.5 to 14. The lower limit of the value of relation 6 is more preferably 3.0, and even more preferably 3.5. The upper limit of the value of relation 6 is more preferably 12, and even more preferably 10.

[0091] On the other hand, in this invention, after the over-aging treatment, a further step may be included: leveling the over-aged cold-rolled steel sheet with an elongation of 0.1-2.0%. Typically, in the case of leveling rolling, there is almost no increase in tensile strength, and an increase in yield strength of at least 50 MPa occurs. When the elongation is less than 0.1%, shape control may be difficult, and when the elongation exceeds 2.0%, workability becomes very unstable due to the high-tension operation. Detailed Implementation

[0092] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative of the invention for more detailed explanation and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the contents recorded in the claims and the contents reasonably deduced therefrom.

[0093] (Example 1)

[0094] After preparing molten steel with the alloy composition described in Table 1, a slab with a thickness of 250 mm is manufactured. After heating at 1200°C for 12 hours, it is finished rolled and coiled under the conditions described in Table 2 to produce a hot-rolled steel sheet. The hot-rolled steel sheet manufactured as described above is pickled and then cold-rolled with a 50% cold-rolling reduction rate to produce a cold-rolled steel sheet. Under the conditions described in Table 3, the cold-rolled steel sheet is subjected to continuous annealing, homogenization, primary and secondary cooling, and over-aging treatment to produce the final cold-rolled steel sheet.

[0095] The results of measuring the microstructure, average grain size and mechanical and physical properties of the final cold-rolled steel sheet manufactured as described above are recorded in Table 4 below.

[0096] The microstructure and average grain size were measured using electron backscatter diffraction (EBSD) equipment.

[0097] For the mechanical and physical properties of tensile strength (TS), yield strength (YS), and elongation (EL), tensile specimens were collected along the horizontal direction of rolling and measured by tensile testing. A standard specimen with a gauge length of 80 mm and a width of 20 mm was used.

[0098] Regarding the hardness of the molten part (HvFZ) and the hardness of the base material (HvBS), for cold-rolled steel sheets, a bead on plate (BOP) weld is performed using a CO2 laser welding machine at 6kW for 3 minutes. Then, the Vickers hardness tester is used to measure the hardness five times at 1 / 4t (t = thickness point) with a load of 500gf, and the average value is taken.

[0099] [Table 1]

[0100]

[0101] [Table 2]

[0102]

[0103] [Table 3]

[0104]

[0105] [Table 4]

[0106]

[0107] As can be seen from Tables 1 to 4, under the conditions of Invention Examples 1 to 5 that meet the alloy composition and manufacturing conditions proposed in this invention, the fine microstructure desired by this invention can be ensured, thus resulting in excellent mechanical and physical properties.

[0108] On the other hand, it can be seen that since Comparative Examples 1 to 8 do not meet the alloy composition proposed in this invention and do not meet some of the manufacturing conditions, the fine microstructure expected to be obtained in this invention cannot be ensured, and therefore the mechanical and physical properties are poor.

[0109] (Example 2)

[0110] After preparing molten steel with the alloy composition of the steel 1 described in Example 1, a slab with a thickness of 250 mm was manufactured. After heating at 1200°C for 12 hours, it was finished rolled and coiled under the conditions described in Table 5 below to manufacture a hot-rolled steel sheet. The hot-rolled steel sheet manufactured as described above was pickled and then cold-rolled at a cold-rolling reduction rate of 50% to manufacture a cold-rolled steel sheet. Under the conditions described in Table 6 below, the cold-rolled steel sheet was subjected to continuous annealing, homogenization treatment, primary and secondary cooling, and over-aging treatment to manufacture the final cold-rolled steel sheet.

[0111] The results of measuring the microstructure, average grain size and mechanical and physical properties of the final cold-rolled steel sheet manufactured as described above are recorded in Table 7 below.

[0112] The microstructure and average grain size were measured using electron backscatter diffraction (EBSD) equipment.

[0113] For the mechanical and physical properties of tensile strength (TS), yield strength (YS), and elongation (EL), tensile specimens were collected along the horizontal direction of rolling and measured by tensile testing. A standard specimen with a gauge length of 80 mm and a width of 20 mm was used.

[0114] The HER in the mechanical and physical properties is measured according to the ISO 16330 standard, and the hole is machined using a 10mm diameter punch with a clearance of 12%.

[0115] In mechanical and physical properties, R / t is the value of R (limit bending radius) divided by the thickness of the steel plate. To obtain R, a test piece with a width of 30 mm and a length of 35 mm is taken in both the rolling direction and the horizontal direction (major axis). One side is then ground 0.2 mm, ensuring the ground surface does not contact the punch. A bending test is performed using the V-block method conforming to JIS Z 2248. The bending radius is varied from 0 to 5 mm to obtain the minimum bending radius that the material can be bent without fracture, thus calculating R.

[0116] The maximum angle of three-point bending in the mechanical and physical properties is the average value measured after each test piece is measured three times, according to the standard of the German Association of the Automotive Industry (VDA).

[0117] [Table 5]

[0118]

[0119] [Table 6]

[0120]

[0121] [Table 7]

[0122]

[0123] As can be seen from Tables 5 to 7, under the conditions of Invention Examples 6 to 10 that meet the alloy composition and manufacturing conditions proposed in this invention, the type and fraction of fine microstructure and the average grain size desired by this invention can be ensured, thereby ensuring the mechanical and physical properties (tensile properties, HER, bending properties) desired by this invention.

[0124] On the other hand, it can be seen that since Comparative Examples 9 to 16 meet the alloy composition proposed in this invention but do not meet the manufacturing conditions, the fine microstructure or average grain size desired by this invention cannot be ensured, resulting in poor mechanical and physical properties. In particular, in the cases of Comparative Examples 9 and 10, the finishing rolling temperature and winding temperature do not meet the conditions of this invention, resulting in plate breakage.

[0125] Figure 1 These are photographs of the fine structures observed using SEM in Invention Example 6. (The text repeats itself here.) Figure 1 It can be seen that the fine structure desired by the present invention was appropriately formed in Example 6.

[0126] (Example 3)

[0127] After preparing molten steel with the alloy composition of the inventive steel 2 described in Example 1, a slab with a thickness of 250 mm was manufactured. After heating at 1200°C for 12 hours, it was finished rolled and coiled under the conditions described in Table 8 below to manufacture a hot-rolled steel sheet. The hot-rolled steel sheet manufactured as described above was pickled and then cold-rolled at a cold-rolling reduction rate of 50% to manufacture a cold-rolled steel sheet. Under the conditions described in Table 9 below, the cold-rolled steel sheet was subjected to continuous annealing, homogenization treatment, primary and secondary cooling, and over-aging treatment to manufacture the final cold-rolled steel sheet.

[0128] The results of measuring the microstructure, average grain size and mechanical and physical properties of the final cold-rolled steel sheet manufactured as described above are recorded in Table 10 below.

[0129] The microstructure and average grain size were measured using electron backscatter diffraction (EBSD) equipment.

[0130] The maximum angle of three-point bending in mechanical and physical properties is the average value measured after each test piece is measured three times, according to the standard of the German Association of the Automotive Industry (VDA).

[0131] [Table 8]

[0132]

[0133] [Table 9]

[0134]

[0135] [Table 10]

[0136]

[0137] As can be seen from Tables 8 to 10, in the case of Invention Examples 11 to 15, which meet the alloy composition and manufacturing conditions proposed in this invention, the fine microstructure desired by this invention can be ensured, and thus the bending properties desired by this invention can be ensured.

[0138] On the other hand, it can be seen that since Comparative Examples 17 to 20 meet the alloy composition proposed in this invention but do not meet the over-aging treatment conditions in the manufacturing conditions and Relationships 5 and 6, the fine microstructure desired by this invention cannot be ensured, and therefore the bending characteristics are poor.

Claims

1. An ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties, comprising, by weight%,: C: 0.03-0.12%, Si: 0.03-0.30%, Mn: 2.1-2.9%, Al: 0.005-0.07%, Nb: 0.01-0.08%, Ti: 0.005-0.08%, B: 0.0005-0.005%, Cr: 0.7-1.4%, Mo: 0.005-0.10%, N: less than 0.008% and excluding 0%, with the balance being Fe and other unavoidable impurities. The ultra-high strength cold-rolled steel sheet satisfies the following relationships 1 to 3. By area percentage, the fine microstructure comprises: newly formed martensite: 4-19%, tempered martensite and bainite combined: 78-95%, and retained austenite: 0.2-2.0%. The average grain size of the fine microstructure is 0.5-6 μm. [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 wherein The content of alloy components recorded in Equations 1 to 3 is expressed by weight.

2. The ultra-high-strength cold-rolled steel sheet with excellent yield strength and bending properties according to claim 1, wherein, The impurities further comprise: P: less than 0.04% and excluding 0% and S: less than 0.005% and excluding 0%.

3. The ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties according to claim 1, wherein, The impurities include one or more of Sb, Mg, Sn, Sb, Zn and Pb, and the total amount of the impurities is less than 0.1% by weight.

4. The ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties according to claim 1, wherein, The fine structure further comprises less than 10% ferrite.

5. The ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties according to claim 1, wherein, The cold-rolled steel sheet has a yield strength (YS) of 800-980MPa, a tensile strength (TS) of 980-1180MPa, an elongation (EL) of 4-12%, a yield-to-tensile ratio (YS / TS) of 0.70 to 0.95, a porosity (HER) of 35-80%, a radius of curvature (R / t) of less than 0.8, and a maximum three-point bending angle of 90-140°.

6. The ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties according to claim 1, wherein, The hardness (HvBM) of the cold-rolled steel sheet is 300-400Hv, the hardness (HvFZ) of the molten part of the welded part formed after welding is 350-450Hv, and the HvFZ / HvBM ratio is below 1.

30.

7. A method for manufacturing ultra-high strength cold-rolled steel sheets with excellent yield strength and bending properties, comprising the following steps: The slab is heated, and the slab, by weight%, contains: C: 0.03-0.12%, Si: 0.03-0.30%, Mn: 2.1-2.9%, Al: 0.005-0.07%, Nb: 0.01-0.08%, Ti: 0.005-0.08%, B: 0.0005-0.005%, Cr: 0.7-1.4%, Mo: 0.005-0.10%, N: less than 0.008% and excluding 0%, with the balance being Fe and other unavoidable impurities, and satisfies the following relationships 1 to 3; The heated slab is precision rolled to a precision roll exit temperature of Ar3+50°C to Ar3+150°C to obtain a hot-rolled steel plate. The hot-rolled steel sheet is cooled to Ms+50°C to Ms+300°C and then coiled. The hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; The cold-rolled steel sheet is continuously annealed within a temperature range of Ar3+10℃ to Ar3+70℃; The continuously annealed cold-rolled steel sheet is subjected to homogenization heat treatment for 50-200 seconds; The cold-rolled steel sheet that has undergone homogenization heat treatment is cooled once at a cooling rate of 1-10℃ / second to 620-700℃; The cold-rolled steel sheet, after its initial cooling, is subjected to a secondary cooling at a rate of 5-50°C / second, cooling it to 360-420°C; and After subjecting the secondary cooled cold-rolled steel sheet to an aging treatment for 355-650 seconds, the process is terminated at 320-400°C. The secondary cooling and the over-aging treatment satisfy the following relationships 4 to 6. [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 [Relation 4] 10≤A≤70 [Relationship 5] 30≤B≤100 [Relationship 6] 2.5 ≤ Overdue processing time / B ≤ 14 In the above, the content of alloy components recorded in Equations 1 to 3 is expressed as weight %, and in Equations 4 to 6, A represents Ms - the secondary cooling termination temperature, where the temperature unit is ℃, and B represents Ms - the over-aging treatment termination temperature, where the temperature unit is ℃.

8. The method for manufacturing ultra-high strength cold-rolled steel sheets with excellent yield strength and bending properties according to claim 7, wherein, The slab is heated at 1100-1300℃.

9. The method for manufacturing ultra-high strength cold-rolled steel sheets with excellent yield strength and bending properties according to claim 7, wherein, The slab has a thickness of 230-270 mm.

10. The method for manufacturing ultra-high strength cold-rolled steel sheet with excellent yield strength and bending properties according to claim 7, wherein, After the coiling, the process further includes the step of cooling the coiled hot-rolled steel sheet to room temperature at a cooling rate of less than 0.1°C / second.

11. The method for manufacturing ultra-high strength cold-rolled steel sheets with excellent yield strength and bending properties according to claim 7, wherein, The cold rolling is carried out with a reduction rate of 40-70%.

12. The method for manufacturing ultra-high strength cold-rolled steel sheets with excellent yield strength and bending properties according to claim 7, wherein, Following the over-aging treatment, the process further includes a step of leveling and rolling the over-aged cold-rolled steel sheet with an elongation of 0.1-2.0%.

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