Ultrahigh-strength cold-rolled steel sheet having excellent yield strength and bending characteristics, and method for manufacturing same

Abstract

The present invention relates to an ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending characteristics and a method for manufacturing the same, and more particularly, to an ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending characteristics, which can be used as a structural member for an automobile, such as a member, a seat rail, a column, and the like, and a method for manufacturing the same.

Description

[0001] This application is a divisional application of Chinese Patent Application No. 202180081861.5, filed on November 30, 2021, entitled "Ultra-high Strength Cold Rolled Steel Sheet Having Excellent Yield Strength and Bending Characteristics and Method for Manufacturing the Same", and claims priority to Korean Application No. 10-2020-0176410. TECHNICAL FIELD

[0002] The present application relates to an ultra-high strength cold rolled steel sheet having excellent yield strength and bending characteristics and a method for manufacturing the same, and more particularly, to an ultra-high strength cold rolled steel sheet having excellent yield strength and bending characteristics and a method for manufacturing the same, which can be used as an automotive structural member such as a member, a seat rail, and a pillar. BACKGROUND

[0003] In recent years, due to the strengthening of safety regulations for automobile passengers and pedestrians, safety devices must be established, which is contrary to lightening for improving fuel efficiency of automobiles, increasing the weight of the vehicle body. Consumers are increasingly interested in environmentally friendly and fuel-efficient hybrid or electric vehicles, and in order to produce such environmentally friendly and safe vehicles, it is necessary to ensure the lightening of the vehicle body structure and the stability of the vehicle body material. However, hybrid vehicles add various devices such as electric engines, batteries, and secondary fuel tanks, in addition to conventional gasoline engines. In addition, as convenience facilities for drivers and the like continue to increase, the weight of the vehicle body is also increasing. Therefore, in order to achieve lightening of the vehicle body, a material that is thin and has excellent strength, ductility, and bending characteristics must be developed. Therefore, in order to solve this problem, it is necessary to develop a gigapascal steel sheet that can ensure high strength and high ductility, etc., with a tensile strength of 980 MPa or more.

[0004] On the other hand, in recent years, as the impact stability regulations of automobiles are expanded, high-strength steel having excellent yield strength is used in structural members such as members, seat rails, and pillars, in order to improve the impact resistance of the vehicle body. The structural member is characterized in that the higher the yield strength, i.e., the yield strength / tensile strength ratio (yield strength / tensile strength), the more advantageous it is in terms of absorbing impact energy, with respect to the tensile strength. However, in general, as the strength of the steel sheet increases, the elongation decreases and the formability decreases, and thus a material that simultaneously improves the high yield strength ratio, formability, and bending characteristics, which are the main physical properties when the member is processed, is required to be developed.

[0005] A representative manufacturing method for increasing the yield strength is to use water cooling at continuous annealing. That is, after soaking in the annealing process, immersion in water is performed, and tempering is performed, whereby a steel sheet in which the microstructure is changed from martensite to tempered martensite can be manufactured. Patent Document 1 is a representative prior art of this method. Patent Document 1 relates to a technology in which, after continuous annealing of a steel material having carbon of 0.18-0.3% and water cooling to normal temperature, overaging treatment is performed at a temperature of 120-300°C for 1-15 minutes, thereby manufacturing a steel material having a volume fraction of 80-97% of martensite and the balance of ferrite. As described above, when an ultra-high strength steel is manufactured by the method of tempering after water cooling, the yield strength ratio is very high, but due to temperature deviation in the width direction and the length direction, there is a problem of deterioration in the shape quality of the coil. Therefore, in the roll forming process, there are problems such as quality defects of the material depending on the site, poor workability, and the like.

[0006] Patent Document 2 is a prior art for improving the workability of the high-tension steel sheet. Patent Document 2 relates to a steel sheet composed of a complex structure having tempered martensite as the main body, characterized in that fine Cu particles having a particle size of 1-100 nm are dispersed in the structure to improve workability. However, in Patent Document 1, Cu is excessively added in a content of 2-5% in order to precipitate fine Cu particles, and thus red brittleness due to Cu can occur, and there is a problem of excessive increase in manufacturing cost.

[0007] On the other hand, Patent Document 3 proposes a steel sheet having a fine structure in which ferrite is the base structure and containing 2-10% by area of pearlite, and the strength is mainly increased by precipitation strengthening and grain refinement according to the addition of a carbonitride forming element such as Ti. Patent Document 3 has the advantage of easily obtaining high strength at low manufacturing cost, but has the disadvantage that the recrystallization temperature sharply rises due to the fine precipitates, and in order to cause sufficient recrystallization to ensure ductility, high-temperature annealing must be performed. In addition, the existing precipitation strengthened steel in which carbonitride is precipitated on the ferrite base to strengthen is difficult to obtain high-strength steel of 600 MPa or more.

[0008] Therefore, by solving the above problems, there is a need to develop a steel material that exhibits high yield strength and bending properties while having ultra-high strength that can be cold formed.

[0009] [Related Art Documents] (Patent Document 1) Japanese Patent No. 2528387 (Patent Document 2) Japanese Patent Application Laid-Open No. 2005-264176 (Patent Document 3) Korean Patent Application Laid-Open No. 2015-0073844 SUMMARY

[0010] Technical problem to be solved An aspect of the present application aims to provide an ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties and a manufacturing method thereof.

[0011] Technical solution One embodiment of the present application provides an ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties, comprising, in 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: 0.008% or less (except 0%), the balance of Fe and other inevitable impurities, the ultra-high strength cold-rolled steel sheet satisfying the following relation expression 1 to relation expression 3, a microstructure comprising, in area%, newly formed martensite: 4-19%, the sum of tempered martensite and bainite: 78-95%, and residual austenite: 0.2-2.0%, the average grain size of the microstructure being 0.5-6 μm.

[0012] [relation expression 1] 0.18 ≤ C+Si / 30+Mn / 20+2P+4S ≤ 0.30 [relation expression 2] 180 ≤ 48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb ≤ 270 [relation expression 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 the alloying components described in the relation expression 1 to the relation expression 3 represents weight%.) Another embodiment of the present invention provides a method of manufacturing an ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties, the method including the steps of: heating a slab including, in 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: 0.008 % or less (except 0 %), the balance of Fe and other inevitable impurities, and satisfying the following relational expression 1 to relational expression 3; finish-rolling the heated slab so that the finish-rolling exit temperature is Ar3+50 ℃ to Ar3+150 ℃, thereby obtaining a hot-rolled steel sheet; coiling the hot-rolled steel sheet after cooling to Ms+50 ℃ to Ms+300 ℃; cold-rolling the 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 ℃ to Ar3+70 ℃; homogenizing the continuously annealed cold-rolled steel sheet for 50-200 seconds; primary cooling the homogenized cold-rolled steel sheet at a cooling rate of 1-10 ℃ / sec to 620-700 ℃; secondary cooling the primary cooled cold-rolled steel sheet at a cooling rate of 5-50 ℃ / sec to 360-420 ℃; and terminating at 320-400 ℃ after overaging the secondary cooled cold-rolled steel sheet for 250-650 seconds, wherein the secondary cooling and the overaging satisfy the following relational expression 4 to relational expression 6.

[0013] [Relational expression 1] 0.18 ≤ C+Si / 30+Mn / 20+2P+4S ≤ 0.30 [Relational expression 2] 180 ≤ 48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb ≤ 270 [Relational expression 3] 700 ≤ 48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb / C+Si / 30+Mn / 20+2P+4S ≤ 1200 [Relational expression 4] 10 ≤ A ≤ 70 [Relational expression 5] 30 ≤ B ≤ 100 [Relational expression 6] 2.5 ≤ overaging time / B ≤ 14 (wherein the content of the alloying components described in the relational expression 1 to the relational expression 3 represents weight %, A in the relational expression 4 to relational expression 6 represents Ms-secondary cooling termination temperature (℃), and B represents Ms-overaging termination temperature (℃).). Advantages According to one aspect of the present application, an ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties and a method of manufacturing the same can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 is a photograph of a microstructure of Inventive Example 6 according to one embodiment of the present application, observed with a SEM. BEST MODE FOR CARRYING OUT THE INVENTION

[0015] Hereinafter, an ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties according to one embodiment of the present application will be described. First, the alloy composition of the present application will be described. Unless otherwise specified, the content of the alloy composition described below represents weight %.

[0016] C: 0.03-0.12% Carbon (C) is a very important element added for solid solution strengthening. In addition, carbon combines with a precipitated element to generate fine carbides, thereby contributing to an increase in strength. When the content of the C is less than 0.03%, it is very difficult to secure the desired strength. On the other hand, when the content of the C exceeds 0.12%, since the hardenability increases, excessive martensite is formed during cooling, so the strength sharply increases, the bending properties can be deteriorated, and thus it can be difficult to obtain the HER, R / t, and three-point bending maximum angle desired to be obtained by the present application. In addition, since the weldability is deteriorated, the possibility of occurrence of welding defects when processing parts in a client company increases. Therefore, the content of the C preferably has a range of 0.03-0.12%. The lower limit of the content of the C is more preferably 0.04%, and further preferably 0.05%. The upper limit of the content of the C is more preferably 0.10%, and further preferably 0.09%.

[0017] Si: 0.03-0.30% Silicon (Si) is one of the five major elements of steel, and a small amount of Si is naturally added in the manufacturing process. The Si helps to increase strength, suppress the formation of carbides, so that carbon does not generate carbides during annealing soaking and cooling. In addition, carbon is distributed and accumulated in residual austenite, so that the austenite phase remains at room temperature, thereby favorably ensuring elongation. When the content of the Si is less than 0.03%, it can be difficult to sufficiently ensure the above-mentioned effects. On the other hand, when the content of the Si exceeds 0.30%, the elongation can decrease due to an increase in solid solution strengthening effect, surface scale defects can occur, leading to a decrease in plating surface quality, and chemical conversion treatment properties can decrease. Therefore, the content of the Si preferably has a range of 0.03-0.30%. The lower limit of the Si content is more preferably 0.04%, and further preferably 0.05%. The upper limit of the Si content is more preferably 0.25%, and further preferably 0.20%.

[0018] Mn: 2.1-2.9% Manganese (Mn) is an element that causes sulfur in steel to completely precipitate as MnS, thereby preventing hot shortness caused by the generation of FeS, and solid solution strengthening the steel. When the content of the Mn is less than 2.1%, it is difficult to ensure the strength desired in the present application. On the other hand, when the content of the Mn exceeds 2.9%, there is a high possibility that problems such as weldability, hot rolling properties, and the like will occur, and the hardenability increases, martensite can be excessively formed, and thus elongation can decrease. In addition, a manganese band (Mn-Band) (a band of Mn oxides) is formed in the fine structure, the risk of processing cracks and sheet breaks increases, and the Mn oxides dissolve out on the surface at the time of annealing, greatly hindering plating properties. Therefore, the content of the Mn preferably has a range of 2.1-2.9%. The lower limit of the Mn content is more preferably 2.2%, and further preferably 2.3%. The upper limit of the Mn content is more preferably 2.8%, and further preferably 2.7%.

[0019] Al: 0.005-0.07% Aluminum (Al) is an element that is added for deoxidization at the time of steelmaking. When the content of the Al is less than 0.005%, it is difficult to sufficiently obtain a deoxidization effect, and when the content of the Al exceeds 0.07%, the Al reacts with oxygen (O) in the molten steel to form an oxide (an inclusion) having a high melting point, and thus nozzle clogging can occur, and in addition, the inclusions formed as described above have a sharp shape, and thus bending properties can deteriorate. Therefore, the content of the Al is preferably 0.005-0.07%. The lower limit of the Al content is more preferably 0.010%, and further preferably 0.020%. The upper limit of the Al content is more preferably 0.06%, and further preferably 0.05%.

[0020] Nb: 0.01-0.08% Niobium (Nb) is an element that segregates at the austenite grain boundaries to inhibit coarsening of the austenite grains at the time of annealing heat treatment, and forms fine carbides to contribute to an increase in strength. When the content of the Nb is less than 0.01%, the above effect is insufficient. On the other hand, when the content of the Nb exceeds 0.08%, coarse carbides are precipitated, the strength and elongation can be reduced as the amount of carbon in solid solution in the steel decreases, and there is a problem of an increase in manufacturing cost. Therefore, the content of the Nb preferably has a range of 0.01-0.08%. The lower limit of the content of the Nb is more preferably 0.02%, and further preferably 0.03%. The upper limit of the content of the Nb is more preferably 0.07%, and further preferably 0.06%.

[0021] Ti: 0.005-0.08% Titanium (Ti) is an element that contributes to ensuring the yield strength and tensile strength as a fine carbide-forming element. In addition, Ti is a nitride-forming element that has an effect of suppressing the precipitation of AlN by precipitating N in the steel as TiN, and thus has the advantage of reducing the risk of cracks occurring at the time of continuous casting. When the content of the Ti is less than 0.005%, it can be difficult to obtain the above effect. On the other hand, when the content of the Ti exceeds 0.08%, coarse carbides are precipitated, the strength and elongation can be reduced as the amount of carbon in solid solution in the steel decreases, and nozzle clogging can occur at the time of continuous casting. Therefore, the content of the Ti preferably has a range of 0.005-0.08%. The lower limit of the content of the Ti is more preferably 0.007%, and further preferably 0.01%. The upper limit of the content of the Ti is more preferably 0.07%, and further preferably 0.06%.

[0022] B: 0.0005-0.005% Boron (B) is an element that greatly contributes to ensuring the hardenability of the steel material, and in order to obtain this effect, it is preferable to add B of 0.0005% or more. However, when the content of the B exceeds 0.005%, boron carbide is formed at the grain boundaries, providing nucleation sites for ferrite, and thus the hardenability can be deteriorated. Therefore, the content of the B preferably has a range of 0.0005-0.005%. The lower limit of the content of the B is more preferably 0.0010%, and further preferably 0.0015%. The upper limit of the content of the B is more preferably 0.0045%, and further preferably 0.004%.

[0023] Cr: 0.7-1.4% Chromium (Cr) is an element that improves hardenability and increases the strength of the steel. When the content of the Cr is less than 0.7%, it can be difficult to ensure the desired strength. On the other hand, when the content of the Cr exceeds 1.4%, the ductility of the steel sheet can be reduced. Therefore, the content of the Cr preferably has a range of 0.7-1.4%. The lower limit of the content of the Cr is more preferably 0.75%, further preferably 0.8%. The upper limit of the content of the Cr is more preferably 1.3%, further preferably 1.2%.

[0024] Mo: 0.005-0.10% Molybdenum (Mo) is an element that forms carbides, and when it is added in combination with carbonitride forming elements such as Ti, Nb, V, etc., it finely maintains the size of precipitates, thereby playing a role in increasing the yield strength and tensile strength. In addition, the Mo improves the hardenability of the steel, finely forms martensite on the grain boundaries, and thus has the advantage that the yield strength ratio can be controlled. In order to achieve the above effects, the Mo is preferably added in an amount of 0.0005% or more. However, the Mo is an expensive element, and thus the more the content of the Mo increases, the more the manufacturing becomes disadvantageous, and thus it is preferable to appropriately control the content of the Mo. When the content of the Mo exceeds 0.10%, a sharp increase in manufacturing cost occurs, not only the economic efficiency is reduced, but also the ductility of the steel is reduced due to excessive grain refinement effects and solid solution strengthening effects. Therefore, the content of the Mo preferably has a range of 0.005-0.10%. The lower limit of the content of the Mo is more preferably 0.007%, further preferably 0.01%. The upper limit of the content of the Mo is more preferably 0.08%, further preferably 0.06%.

[0025] N: 0.008% or less (excluding 0%) The nitrogen (N) is an element that is inevitably contained during the manufacturing process, but the nitrogen (N) is an element that contributes to increasing the strength of the steel by forming carbonitride. However, when the content of the N exceeds 0.008%, not only the risk of embrittlement greatly increases, but also the excess N remaining after the formation of TiN can consume B, which contributes to hardenability, in the form of BN. Therefore, the content of the N is preferably 0.008% or less. The content of the N is more preferably 0.007% or less, further preferably 0.006% or less.

[0026] On the other hand, the cold-rolled steel sheet of the present application satisfies the above alloying components while preferably satisfying the following relationship equation 1 to relationship equation 3. Thereby, it is possible to manufacture the ultra-high strength steel sheet of the present application, which has very excellent bendability and a tensile strength of 980 MPa or more.

[0027] [Relationship equation 1] 0.18 ≤ C + Si / 30 + Mn / 20 + 2P + 4S ≤ 0.30 The relationship 1 is a component relationship for ensuring strength and weldability. When the value of the relationship 1 is less than 0.18, it is difficult to ensure the strength desired in the present application, and when the value of the relationship 1 exceeds 0.30, the weldability can be poor. Therefore, the value of the relationship 1 preferably has a range of 0.18 to 0.30. The lower limit of the value of the relationship 1 is more preferably 0.19, and further preferably 0.20. The upper limit of the value of the relationship 1 is more preferably 0.28, and further preferably 0.26.

[0028] [Relationship 2] 180 ≤ 48.8 + 49 log C + 35.1 Mn + 25.9 Si + 76.5 Cr + 105.9 Mo + 132.5 Nb ≤ 270 The relationship 2 is a relationship related to a hardenability index for ensuring hardenability. When the value of the relationship 2 is less than 180, the hardenability is insufficient, and it is difficult to ensure the strength desired in the present application, and when the value of the relationship 2 exceeds 270, the hardenability becomes too high, and thus the bend properties and formability can be poor. Therefore, the value of the relationship 2 preferably has a range of 180 to 270. The lower limit of the value of the relationship 2 is more preferably 190, and further preferably 200. The upper limit of the value of the relationship 2 is more preferably 260, and further preferably 250.

[0029] [Relationship 3] 700 ≤ 48.8 + 49 log C + 35.1 Mn + 25.9 Si + 76.5 Cr + 105.9 Mo + 132.5 Nb / C + Si / 30 + Mn / 20 + 2P + 4S ≤ 1200 The relationship 3 is a component relationship for simultaneously ensuring the strength, hardenability, and weldability desired in the present application. When the value of the relationship 3 is less than 700, not only the weldability can be poor, but also the hardenability is insufficient, and it is difficult to ensure the strength desired in the present application, and when the value of the relationship 3 exceeds 1200, the hardenability becomes too high, and thus the bend properties and formability can be poor. Therefore, the value of the relationship 3 preferably has a range of 700 to 1200. The lower limit of the value of the relationship 3 is more preferably 700, and further preferably 800. The upper limit of the value of the relationship 3 is more preferably 1150, and further preferably 1100.

[0030] The remaining component of the present application is iron (Fe). However, in the usual manufacturing process, unavoidable impurities can be mixed from raw materials or the surrounding environment, and thus these impurities cannot be completely excluded. These impurities are well known to those skilled in the art, and thus all of them are not particularly mentioned in the present specification.

[0031] However, among these, phosphorus and sulfur are often mentioned as impurities, and therefore, a brief description thereof will be given below.

[0032] P: 0.04% or less (excluding 0%) Phosphorus (P) is an element that can cause embrittlement by segregating at grain boundaries and / or phase boundaries. Therefore, the content of phosphorus (P) should be controlled as low as possible, and the content of phosphorus (P) is preferably limited to 0.04% or less. The content of P is more preferably limited to 0.03% or less, and further preferably limited to 0.02% or less.

[0033] S: 0.005% or less (excluding 0%) Sulfur (S) as an impurity can cause high-temperature cracks by MnS non-metallic inclusions in steel and segregation during continuous casting solidification. Therefore, the content of sulfur (S) should be controlled as low as possible, and is preferably limited to 0.005% or less. The content of S is more preferably limited to 0.004% or less, and further preferably limited to 0.003% or less.

[0034] Further, the impurities can contain one or more of Sb, Mg, Sn, Sb, Zn, and Pb as inclusion elements, and the total amount of the inclusion elements can be 0.1% by weight or less. Inclusion elements are inclusion elements derived from scrap steel and the like used as raw materials in the steelmaking process, and when the total amount of the inclusion elements exceeds 0.1%, surface cracks of the slab can occur, and the surface quality of the steel sheet can be reduced.

[0035] Hereinafter, the microstructure and the like of the ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties according to one embodiment of the present application will be described.

[0036] The fine structure of the cold-rolled steel sheet of the present application preferably contains, in area%, newly formed martensite: 4 to 19%, the sum of tempered martensite and bainite: 78 to 95%, and residual austenite: 0.2 to 2.0%. The fine structure of the cold-rolled steel sheet of the present application contains tempered martensite (hereinafter, also referred to as "TM") and bainite (hereinafter, also referred to as "B") as main structures. The tempered martensite and bainite are not easily distinguished on the fine structure, and therefore, in the present application, the fraction of the sum of the tempered martensite and bainite is controlled. When the fraction of the sum of the tempered martensite and bainite is less than 78%, it is difficult to ensure the desired strength, and when the fraction of the sum of the tempered martensite and bainite exceeds 95%, the bend property and elongation can be poor. The newly formed martensite (hereinafter, also referred to as "FM") is a structure that is advantageous for ensuring strength. When the fraction of the newly formed martensite is less than 4%, it is difficult to ensure the desired strength, and when the fraction of the newly formed martensite exceeds 19%, the bend property and elongation can be poor. The residual austenite (hereinafter, also referred to as "RA") is a structure that is advantageous for ensuring elongation. When the fraction of the residual austenite is less than 0.2%, it can be difficult to sufficiently obtain the above-mentioned effects, and when the fraction of the residual austenite exceeds 2.0%, it is converted to martensite at the time of working, and therefore, the HER or bend property can be poor. On the other hand, the fine structure can further contain ferrite of 10% or less. The ferrite structure is a structure that is inevitably formed during the manufacturing process, but can also have a positive effect. For example, the ferrite can contribute to ensuring elongation. However, when the fraction of the ferrite exceeds 10%, it can be difficult to ensure the strength that is desired to be obtained by the present application. The fraction of the ferrite is more preferably 7% or less, and further preferably 5% or less.

[0037] On the other hand, the average grain size of the fine structure is preferably 0.5 to 6 μm. The finer the average grain size, the more advantageous it is for ensuring physical properties such as strength and HER, but in order to control the average grain size of the fine structure to be less than 0.5 μm, the addition amount of Nb, Ti, Mo, V, and the like, which are effective for grain refinement, is excessively increased, and therefore, the manufacturing cost can increase. When the average grain size exceeds 6 μm, it is difficult to ensure the strength that is desired to be obtained by the present application, and the HER and bend property can be considerably poor. Therefore, the average grain size preferably has a range of 0.5 to 6.0 μm. The lower limit of the average grain size is more preferably 1.0 μm, and further preferably 1.5 μm. The upper limit of the average grain size is more preferably 5.5 μm, and further preferably 5.0 μm.

[0038] The cold-rolled steel sheet of the present application provided as described above can have a yield strength (YS) of 800 to 980 MPa, a tensile strength (TS) of 980 to 1180 MPa, an elongation (EL) of 4 to 12%, a yield ratio (YS / TS) of 0.70 to 0.95, a hole expansion ratio (HER) of 35 to 80%, an R / t of 0.8 or less, and a maximum angle of three-point bending of 90 to 140°. The yield strength is more preferably 820 to 960 MPa, and further preferably 850 to 950 MPa. The tensile strength is more preferably 1000 to 1170 MPa, and further preferably 1020 to 1160 MPa. The elongation is more preferably 5 to 11%, and further preferably 6 to 10%. The yield ratio is more preferably 0.72 to 0.92, and further preferably 0.75 to 0.90. The hole expansion ratio is more preferably 40 to 75%, and further preferably 45 to 70%. The R / t is more preferably 0.15 to 0.70, and further preferably 0.20 to 0.60. The maximum angle of three-point bending is more preferably 95 to 135°, and further preferably 100 to 130°.

[0039] Further, the cold-rolled steel sheet of the present application can have a hardness (HvBM) of 300 to 400 Hv. The base material has a hardness more preferably of 310 to 390 Hv, and further preferably of 320 to 380 Hv. Further, the hardness (HvFZ) of the fusion zone of the weld formed after welding can be 350 to 450 Hv. When the hardness of the fusion zone of the weld is less than 350 Hv, sufficient hardness of the fusion zone cannot be ensured, and thus the strength of the weld can be reduced. On the other hand, when the hardness of the fusion zone of the weld exceeds 450 Hv, the hardness of the fusion zone is too high, the sensitivity to cracking increases, and thus the strength of the weld and the impact absorption energy are reduced. The hardness of the cold-rolled steel sheet, i.e., the hardness of the base material (HvBM) after welding is more excellent the more similar it is to the hardness of the fusion zone (HvFZ), and thus the ratio (HvFZ / HvBM) of HvFZ to HvBM is preferably 1.30 or less. The ratio of HvFZ to HvBM is more preferably 1.25 or less, and further preferably 1.20 or less.

[0040] Hereinafter, an ultra-high strength cold-rolled steel sheet having excellent yield strength and bending properties according to one embodiment of the present application will be described.

[0041] First, a slab satisfying the above alloy composition is heated. In the present application, the slab heating temperature is not particularly limited, and for example, the slab can be heated at 1100 to 1300°C. When the slab heating temperature is lower than 1100°C, the slab temperature is low, and rolling load can be generated during rough rolling, and when the slab heating temperature exceeds 1300°C, the structure can be coarsened, and there can be disadvantages such as an increase in power cost. The lower limit of the slab heating temperature is more preferably 1125°C, and further preferably 1150°C. The upper limit of the slab heating temperature is more preferably 1275°C, and further preferably 1250°C. In addition, the slab can have a thickness of 230 to 270 mm.

[0042] After that, the heated slab is finish-rolled so that the finish-rolling exit temperature is Ar3+50°C to Ar3+150°C, thereby obtaining a hot-rolled steel sheet. When the finish-rolling exit temperature is lower than Ar3+50°C, the hot deformation resistance can be sharply increased. When the finish-rolling exit temperature exceeds Ar3+150°C, not only an excessively thick oxide scale is generated, but also the possibility of coarsening of the fine structure of the steel sheet is high. Therefore, the finish-rolling exit temperature preferably has a range of Ar3+50°C to Ar3+150°C. The lower limit of the finish-rolling exit temperature is more preferably Ar3+60°C, and further preferably Ar3+70°C. The upper limit of the finish-rolling exit temperature is more preferably Ar3+140°C, and further preferably Ar3+130°C. In addition, Ar3 indicates the temperature at which transformation into austenite occurs upon heating, and for example, the value of Ar3 can be obtained by the formula 910-203C 1 / 2 +44.7Si +31.5Mo -30Mn -11Cr +700P +400Al +400Ti, etc.

[0043] After that, the hot-rolled steel sheet is cooled to Ms+50°C to Ms+300°C and then coiled. When the coiling temperature is lower than Ms+50°C, too much martensite or bainite is formed, resulting in excessive increase in the strength of the hot-rolled steel sheet, and thus problems such as load-induced shape defects can occur during cold rolling. On the other hand, when the coiling temperature exceeds Ms+300°C, the surface oxide scale increases, and thus pickling properties can be deteriorated. Therefore, the coiling temperature preferably has a range of Ms+50°C to Ms+300°C. The lower limit of the coiling temperature is more preferably Ms+60°C, and further preferably Ms+70°C. The upper limit of the coiling temperature is more preferably Ms+290°C, and further preferably Ms+270°C. On the other hand, after the coiling, the coiled hot-rolled steel sheet can be cooled to room temperature at a cooling rate of 0.1°C / sec or less. Ms indicates the temperature at which martensite starts to transform upon cooling, and for example, the value of Ms can be obtained by the formula 539-423C -30.4Mn -7.5Si +30Al, etc.

[0044] After that, the hot-rolled steel sheet which is coiled and cooled is cold-rolled to obtain a cold-rolled steel sheet. The cold-rolling can be performed at a reduction ratio of 40-70%. When the cold-rolling reduction ratio is less than 40%, the recrystallization driving force deteriorates, it is difficult to obtain good recrystallized grains, and it has a disadvantage that shape correction is very difficult. When the cold-rolling reduction ratio exceeds 70%, the possibility of generating cracks in the edge portion of the steel sheet is high, and the rolling load can sharply increase. Therefore, the cold-rolling is preferably performed at a reduction ratio of 40-70%. On the other hand, before the cold-rolling, pickling can also be performed to remove scale or impurities adhered to the surface, etc.

[0045] After that, the cold-rolled steel sheet is continuously annealed in a temperature range of Ar3+10°C to Ar3+70°C. When the continuous annealing temperature is lower than Ar3+10°C, it is not possible to sufficiently transform into austenite, it is difficult to obtain the fraction of martensite or bainite which is desired to be obtained in the present invention in the subsequent process, and thus it can be difficult to secure the strength. On the other hand, when the continuous annealing temperature exceeds Ar3+70°C, the austenite grain size is coarsened, and thus it can be difficult to secure the desired bending property. Therefore, the continuous annealing temperature preferably has a range of Ar3+10°C to Ar3+70°C. The lower limit of the continuous annealing temperature is more preferably Ar3+20°C, and further preferably Ar3+30°C. The upper limit of the continuous annealing temperature is more preferably Ar3+60°C, and further preferably Ar3+50°C.

[0046] After that, the cold-rolled steel sheet which is continuously annealed is subjected to a soaking treatment for 50-200 seconds. This is to secure recrystallization and grain growth of the cold-rolled structure and a sufficient fraction of austenite at the annealing temperature proposed in the present invention. When the soaking treatment time is less than 50 seconds, the reverse transformation to austenite is not sufficient, the fraction of ferrite in the final structure increases, and thus it can be difficult to secure the desired strength. On the other hand, when the soaking treatment time exceeds 200 seconds, the austenite grain size is coarsened, and thus the bending property in the final product can be poor. The lower limit of the soaking treatment time is more preferably 55 seconds, and further preferably 60 seconds. The upper limit of the soaking treatment time is more preferably 190 seconds, and further preferably 180 seconds.

[0047] After that, the cold-rolled steel sheet which has been homogenized is once cooled at a cooling rate of 1-10°C / sec to 620-700°C. The once cooling step is used to secure the equilibrium carbon concentration of ferrite and austenite to increase the ductility and strength of the steel sheet. When the once cooling end temperature is lower than 630°C or exceeds 700°C, it is difficult to secure the desired ductility and strength of the present application. When the cooling rate is less than 1°C / sec, the ferrite transformation is accelerated, and it is difficult to secure the desired fraction of fine structure, and when the cooling rate exceeds 10°C / sec, it is difficult to secure the elongation due to excessive martensite transformation.

[0048] After that, the once-cooled cold-rolled steel sheet is twice cooled at a cooling rate of 5-50°C / sec to 360-420°C. The twice cooling is one of the important control factors in the present application, and the twice cooling end temperature is a very important condition to simultaneously secure the strength, ductility and bending properties. When the twice cooling end temperature is lower than 360°C, the fraction of martensite is excessively increased, and thus it is difficult to secure the ductility, and when the twice cooling end temperature exceeds 420°C, it is difficult to secure sufficient martensite, and thus it is difficult to secure the desired strength. Therefore, the twice cooling end temperature, which is one of the important control factors to secure the desired physical properties of the present application, preferably has a range of 360-420°C. The lower limit of the twice cooling end temperature is more preferably 365°C, and further preferably 370°C. The upper limit of the twice cooling end temperature is more preferably 410°C, and further preferably 405°C. When the twice cooling rate is less than 5°C / sec, due to the slow cooling rate, the ferrite transformation occurs preferentially before the martensite and bainite transformations, and the desired fraction of fine structure to be obtained in the present application cannot be obtained, and when the twice cooling rate exceeds 50°C / sec, due to the shape difference problem caused by the excessive cooling rate, the sheet passing property is deteriorated, and sheet breakage can occur. The lower limit of the twice cooling rate is more preferably 7.5°C / sec, and further preferably 10°C / sec. The upper limit of the twice cooling rate is more preferably 47.5°C / sec, and further preferably 45°C / sec.

[0049] On the other hand, in order to ensure the fraction of the tempered martensite and the bainite, which are important microstructures in the present application, at a target level, it is important to accurately control the difference between the Ms temperature and the secondary cooling termination temperature. In more detail, it is preferable to satisfy the following relation 4. When the difference between the Ms and the secondary cooling termination temperature, i.e., the value of A is less than 10°C, the transformation of the martensite or the bainite is less, and it can be difficult to ensure the desired strength, and when the value of A exceeds 70°C, the time spent in the martensite region is long, the fraction of the martensite is excessively increased, and thus it is difficult to ensure the ductility. Therefore, the difference between the 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, the Ms represents the temperature at which the martensite starts to transform, and the value thereof can be obtained by the following formula 1.

[0050] [Relation 4] 10 ≤ A ≤ 70 (wherein A in the relation 4 is Ms - secondary cooling termination temperature (°C).) After the secondary cooling of the cold-rolled steel sheet, overaging treatment is performed for 250 to 650 seconds, and then terminated at 320 to 400°C. The overaging treatment is preferably performed at the same temperature as the temperature at which the secondary cooling is terminated or at a higher temperature. The overaging treatment is a process for promoting the transformation of the newly formed martensite at the time of termination of the secondary cooling into tempered martensite, and thus high yield strength and high bending properties can be stably ensured. Therefore, in order to ensure the high bending workability that is desired to be obtained in the present application, the overaging treatment is a very important factor, and in the present application, the overaging treatment time is precisely controlled to be in the range of 250 to 650 seconds. When the overaging treatment time is less than 250 seconds, a small amount of transformation of the newly formed martensite into tempered martensite occurs, and thus the bending workability can be poor. On the other hand, when the overaging treatment time exceeds 650 seconds, it can be difficult to ensure the tensile strength that is desired to be obtained in the present application due to a decrease in productivity and excessive transformation into tempered martensite. The lower limit of the overaging treatment time is more preferably 260 seconds, and further preferably 270 seconds. The upper limit of the overaging treatment time is more preferably 600 seconds, and further preferably 550 seconds. When the overaging treatment termination temperature is lower than 320°C, it is difficult to ensure the elongation due to excessive transformation of the newly formed martensite, and the bending properties can be poor. When the overaging treatment termination temperature exceeds 400°C, a small amount of transformation from the newly formed martensite into tempered martensite occurs, and thus the bending properties can be poor. The lower limit of the overaging treatment termination temperature is more preferably 325°C, and further preferably 330°C. The upper limit of the overaging treatment termination temperature is more preferably 395°C, and further preferably 380°C. On the other hand, when it is desired to further improve the HER and the bending properties, reheating is performed after cooling to the secondary cooling termination temperature after the overaging treatment, and thus further overaging treatment can be performed.

[0051] On the other hand, in order to ensure the fraction of the tempered martensite, which is a fine structure important in the present application, at a target level, it is important to precisely control the difference between the Ms temperature and the overaging treatment termination temperature. In more detail, it is preferable to satisfy the following relation 5. When the difference between the Ms temperature and the overaging treatment termination temperature, i.e., the value of B is less than 30°C, the martensite transformation is insufficient, and thus it can be difficult to ensure the desired strength. When the value of B exceeds 100°C, it can be difficult to ensure the desired elongation and bending properties due to excessive transformation of the newly formed martensite. Therefore, the difference between the Ms temperature and the overaging treatment termination temperature, i.e., the value of B is preferably 30 to 100°C. The lower limit of the value of B is more preferably 35°C, and further preferably 40°C. The upper limit of the value of B is more preferably 95°C, and further preferably 90°C.

[0052] [Relation 5] 30 ≤ B ≤ 100 (wherein B in the relation 5 is Ms - overaging treatment termination temperature (°C).) Also, in the present application, in order to secure the fraction of the desired microstructure and the mechanical physical properties, the following relation 6 is preferably satisfied at the time of the secondary cooling and the overaging treatment.

[0053] [Relation 6] 2.5 ≤ overaging treatment time / B ≤ 14 The relation 6 is used to secure the desired physical properties by precisely controlling the desired microstructure of the present application. When the value of the relation 6 is less than 2.5, the overaging holding time is short or the overaging treatment termination temperature is low, and it can be difficult to secure the desired elongation or bending characteristics due to excessive newly formed martensite transformation. On the other hand, when the value of the relation 6 exceeds 14, the overaging holding time is long or the overaging treatment termination temperature is high, and it can be difficult to secure the fraction of the desired microstructure, and thus it can be difficult to secure the desired physical properties. Therefore, the value of the relation 6 preferably has a range of 2.5 to 14. The lower limit of the value of the relation 6 is more preferably 3.0, and further preferably 3.5. The upper limit of the value of the relation 6 is more preferably 12, and further preferably 10.

[0054] On the other hand, in the present application, after the overaging treatment, a step of performing temper rolling of the overaging-treated cold-rolled steel sheet at an elongation of 0.1 to 2.0% can be further included. Generally, in the case of temper rolling, there is almost no increase in tensile strength, and at least an increase in yield strength of 50 MPa or more occurs. When the elongation is less than 0.1%, it can be difficult to control the shape, and when the elongation exceeds 2.0%, the operability can become very unstable due to high stretching operation. DETAILED DESCRIPTION

[0055] Hereinafter, the present application will be described in more detail by examples. However, it should be noted that the following examples are merely for illustrating the present application in more detail and are not intended to limit the scope of the rights of the present application. This is because the scope of the rights of the present application is determined by the content recited in the claims and the content reasonably deduced therefrom.

[0056] (Example 1) After preparing molten steel having the alloy composition recited in Table 1 below, a slab having a thickness of 250 mm was manufactured, and after heating at 1200°C for 12 hours, finish rolling was performed under the conditions recited in Table 2 below, and then coiling was performed to manufacture a hot-rolled steel sheet. The hot-rolled steel sheet manufactured as described above was subjected to pickling, and then cold-rolled at a cold-rolling reduction rate of 50% to manufacture a cold-rolled steel sheet. The cold-rolled steel sheet was subjected to continuous annealing, soaking treatment, primary cooling and secondary cooling, overaging treatment under the conditions recited in Table 3 below, thereby manufacturing a final cold-rolled steel sheet.

[0057] After measuring the microstructure, average grain size, and mechanical physical properties of the final cold-rolled steel sheet manufactured as described above, the results thereof are recorded in Table 4 below.

[0058] The microstructure and average grain size were measured using an Electron Back Scatter Diffraction (EBSD) apparatus.

[0059] For the tensile strength (TS), yield strength (YS), and elongation (EL) among the mechanical physical properties, after collecting a tensile test piece in the rolling horizontal direction, the measurement was performed by a tensile test. At this time, a test piece standard having a gauge length of 80 mm and a width of the tensile test piece of 20 mm was used.

[0060] In terms of the hardness of the fusion zone (HvFZ) and the hardness of the base material (HvBS), for the cold-rolled steel sheet, a bead on plate (BOP) was performed using a CO2 laser welding machine under the condition of 6 kW-3 minutes (min), and then using a Vickers hardness tester, the measurement was performed 5 times at 1 / 4 t (t = thickness point) with a load of 500 gf, and the average was taken.

[0061] [Table 1] [Table 2] [Table 3] [Table 4] As can be seen from Tables 1 to 4, in the case of Inventive Examples 1 to 5 satisfying the alloy composition and manufacturing conditions proposed in the present application, the microstructure desired to be obtained in the present application can be ensured, and thus excellent mechanical physical properties are obtained.

[0062] On the other hand, it can be seen that since Comparative Examples 1 to 8 do not satisfy the alloy composition proposed in the present application and do not satisfy some of the manufacturing conditions, the microstructure desired to be obtained in the present application cannot be ensured, and thus the mechanical physical properties are poor.

[0063] (Example 2) After preparing a molten steel having the alloy composition of the inventive steel 1 described in Example 1, a slab having a thickness of 250 mm was manufactured, and after heating at 1200°C for 12 hours, the hot-rolled steel plate was manufactured by coiling after finish rolling under the conditions described in Table 5 below. The hot-rolled steel plate manufactured as described above was pickled, and then cold-rolled at a cold-rolling reduction of 50% to manufacture a cold-rolled steel plate. The cold-rolled steel plate was subjected to continuous annealing, soaking treatment, primary cooling and secondary cooling, overaging treatment under the conditions described in Table 6 below, thereby manufacturing a final cold-rolled steel plate.

[0064] After measuring the microstructure, average grain size and mechanical physical properties of the final cold-rolled steel plate manufactured as described above, the results thereof were described in Table 7 below.

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

[0066] For the tensile strength (TS), yield strength (YS), elongation (EL) among the mechanical physical properties, after collecting a tensile test piece in the rolling horizontal direction, the measurement was performed by a tensile test. At this time, a test piece standard having a gauge length of 80 mm and a width of the tensile test piece of 20 mm was used.

[0067] The HER among the mechanical physical properties was measured according to the ISO 16330 standard, and the hole was sheared using a punch having a diameter of 10 mm at a clearance of 12%.

[0068] The R / t among the mechanical physical properties is a value obtained by dividing R (limit bending radius) by the thickness of the steel plate. At this time, for the R, a test piece having a width of 30 mm x a length of 35 mm was collected in the rolling direction and the horizontal direction (major axis), and then one side was ground by 0.2 mm so that the ground surface does not come into contact with the punch, and a bending test was performed by a V-block method according to JIS Z 2248, and the bending radius at this time was variously changed from 0-5 mm, and the minimum bending radius at which the material can be bent without being broken was obtained to calculate the R.

[0069] The three-point bending maximum angle among the mechanical physical properties was measured according to the standard of the Verband Der Automobilindustrie (VDA), and the average value was measured after measuring 3 times for each test piece.

[0070] [Table 5] [Table 6] [Table 7] As can be seen from Tables 5 to 7, in the case of Inventive Examples 6 to 10 satisfying the alloy composition and the manufacturing conditions according to the present application, the kinds and fractions of the fine structures and the average grain size desired in the present application can be ensured, and thus the mechanical physical properties (tensile properties, HER, bending properties) desired to be obtained in the present application can be ensured.

[0071] On the other hand, it can be seen that since Comparative Examples 9 to 16 satisfy the alloy composition according to the present application but do not satisfy the manufacturing conditions, the fine structures or the average grain size desired in the present application cannot be ensured, and thus the mechanical physical properties are poor, and in particular, in the case of Comparative Examples 9 and 10, the finish rolling temperature and the coiling temperature, respectively, do not satisfy the conditions according to the present application, and thus plate cracking occurs.

[0072] Figure 1 is a photograph of the fine structure of Inventive Example 6 observed with an SEM. As can be seen from Figure 1 It can be seen that the fine structure desired in the present application is appropriately formed in Inventive Example 6.

[0073] (Example 3) After preparing a molten steel having the alloy composition of Inventive Steel 2 described in Example 1, a slab having a thickness of 250 mm was manufactured, and after heating at 1200°C for 12 hours, finish rolling was performed under the conditions described in Table 8 below, and coiling was performed to manufacture a hot-rolled steel sheet. The hot-rolled steel sheet manufactured as described above was subjected to pickling, and then cold-rolled at a cold-rolling reduction rate of 50% to manufacture a cold-rolled steel sheet. The cold-rolled steel sheet was subjected to continuous annealing, soaking treatment, primary cooling and secondary cooling, and overaging treatment under the conditions described in Table 9 below, thereby manufacturing a final cold-rolled steel sheet.

[0074] After measuring the fine structure, the average grain size, and the mechanical physical properties of the final cold-rolled steel sheet manufactured as described above, the results thereof were described in Table 10 below.

[0075] The fine structure and the average grain size were measured using an electron backscatter diffraction (EBSD) apparatus.

[0076] The three-point bending maximum angle in the mechanical physical properties was measured according to the standard of the German automobile industry association (VDA), and the average value was measured after measuring 3 times for each test piece.

[0077] [Table 8] [Table 9] [Table 10] As is apparent from Tables 8 to 10, in the case of Inventive Examples 11 to 15 satisfying the alloy composition and the manufacturing conditions proposed in the present application, the fine structure desired in the present application can be ensured, and thus the bending properties desired to be obtained in the present application can be ensured.

[0078] On the other hand, it is apparent that since Comparative Examples 17 to 20 satisfy the alloy composition proposed in the present application but do not satisfy the overaging treatment condition in the manufacturing conditions and the relational expressions 5 and 6, the fine structure desired in the present application cannot be ensured, and thus the bending properties 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 in, 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 250-650 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 sheets 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, After 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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