Ultra-high strength cold-rolled steel sheet with excellent bendability and method for manufacturing the same

Abstract

The present invention relates to a super high strength cold rolled steel sheet with excellent bendability and a manufacturing method thereof, and more particularly to a super high strength cold rolled steel sheet with excellent bendability that can be used for automobiles and a manufacturing method thereof.

Description

Technical Field

[0001] The present invention relates to an ultra-high strength cold-rolled steel sheet having excellent bendability and a method for manufacturing the same, and more particularly to an ultra-high strength cold-rolled steel sheet having excellent bendability that can be used for automobiles and a method for manufacturing the same. Background Art

[0002] In recent years, due to the strengthening of safety regulations for automobile passengers and pedestrians, it has become necessary to establish safety devices. This is contrary to the weight reduction for improving the fuel efficiency of automobiles, and there is a problem of an increase in the body weight. Consumers are becoming increasingly interested in environmentally friendly and fuel-efficient hybrid or electric vehicles. In order to produce such environmentally friendly and safe automobiles, it is necessary to ensure the weight reduction of the body structure and the stability of the body material. However, in addition to the conventional gasoline engine, various devices such as an electric motor, a battery, and a secondary fuel tank are added to hybrid vehicles. In addition, as the convenience facilities for drivers and the like continue to increase, the weight of the body is also increasing. Therefore, in order to achieve the weight reduction of the body, it is necessary to develop a thin material having excellent strength, ductility, and bending characteristics. Therefore, in order to solve such problems, it is necessary to develop a gigapascal steel sheet having a high strength of 980 MPa or more and high ductility.

[0003] In addition, structural materials or reinforcing materials absorb collision energy during a collision, thereby playing a role in protecting passengers. If the strength of the spot weld portion is insufficient, it breaks during a collision, and thus sufficient collision absorption energy cannot be obtained. In addition, most of the components mainly using such ultra-high strength steel need to be bent, such as side sills. Therefore, even if the elongation is excellent, if the bendability is poor, it cannot be used as a component. Bendability represents the minimum bend radius ratio (R / t) per unit thickness, where the minimum bend radius ratio (R) represents the minimum radius at which no crack occurs in the outer rolled portion of the steel sheet after a bending test. Each automobile company has slightly different requirements for bendability. Taking the most stringent Japanese automobile companies as an example, based on a cold-rolled steel sheet with a tensile strength of 980 MPa grade, it is required to satisfy the condition of R / t ≤ 1. However, some customer companies not only require R / t, but also require 180° full compression bending physical properties for reducing the risk of processing cracks and excellent bendability. However, in ultra-high strength steel sheets with a tensile strength of 980 MPa or more, it is quite difficult to ensure the physical properties. Therefore, in ultra-high strength steel sheets with a tensile strength of 980 MPa or more, there is an urgent need to develop a steel sheet having a high yield strength and excellent bendability.

[0004] In order to improve bend formability, the phase transformation composition and fraction existing in the steel should be appropriately controlled. Generally, it is known that the lower the strength ratio between soft phases such as ferrite (F) and hard phases such as bainite (B) or martensite (M), the more excellent the bend formability. For this reason, bainite or tempered martensite should be formed instead of martensite, but this phase transformation has the problem of significantly reducing the elongation rate. Therefore, it is most important to appropriately ensure the composition ratio of the phase transformation.

[0005] Patent Document 1 is the prior art for improving the workability of the high-tensile steel plate. Patent Document 1 relates to a steel plate composed of a composite structure with tempered martensite as the main body, characterized in that fine precipitated Cu particles with a particle size of 1 - 100 nm are dispersed inside the structure to improve workability. However, in Patent Document 1, in order to precipitate good fine Cu particles, Cu is excessively added in an amount of 2 - 5%, so red-hot brittleness caused by Cu may occur, and there is a problem of excessive increase in manufacturing cost.

[0006] A representative manufacturing method for increasing the yield strength is the method of using water cooling during continuous annealing. That is, after soaking in the annealing process, it is immersed in water and tempered, thereby a steel plate in which the fine structure is transformed from martensite to tempered martensite can be manufactured. Patent Document 2 is the representative prior art of this method. Patent Document 2 relates to a technology in which a steel material with a carbon content of 0.18 - 0.3% is continuously annealed and then water-cooled to room temperature, and then over-aged at a temperature of 120 - 300°C for 1 - 15 minutes to manufacture a steel material with a martensite volume fraction of 80 - 97% and the balance being ferrite. As described above, when manufacturing ultra-high strength steel by the tempering method after water cooling, the yield ratio is very high, but due to the temperature deviation in the width direction and length direction, there is a problem that the shape quality of the coil deteriorates. Therefore, in order to solve this problem and ensure an appropriate fine structure, it is necessary to precisely control the temperature and cooling conditions during continuous annealing.

[0007] In addition, Patent Document 3 proposes a steel plate having a fine structure with ferrite as the matrix structure and containing 2 - 10 area% of pearlite, and mainly strengthening precipitation by adding carbide-forming elements such as Ti and improving the strength by grain refinement. Compared with the low manufacturing cost, Patent Document 3 has the advantage of easily obtaining high strength, but the disadvantage is that the recrystallization temperature rises sharply due to the fine precipitates, and in order to ensure ductility by causing sufficient recrystallization, high-temperature annealing must be carried out. In addition, the existing precipitation-strengthened steel that is strengthened by precipitating carbide in the ferrite matrix has the problem of being difficult to obtain a high-strength steel of 600 MPa or more.

[0008] Therefore, it is necessary to develop a steel material that solves the above problems, does not crack even in a 180° complete compression bending test, has a high yield ratio that can be cold formed, and has an ultra-high strength with a tensile strength of 980 MPa or more.

[0009] [Prior art documents]

[0010] (Patent Document 1) Japanese Patent Publication Gazette No. 2005-264176

[0011] (Patent Document 2) Japanese Patent Gazette No. 2528387

[0012] (Patent Document 3) Korean Patent Publication Gazette No. 2015-0073844 Summary of the invention

[0013] Technical problems to be solved

[0014] An object of one aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet with excellent bendability and a manufacturing method thereof.

[0015] Technical solution

[0016] An embodiment of the present invention provides an ultra-high strength cold-rolled steel sheet with excellent bendability. In terms of weight %, the cold-rolled steel sheet contains: C: 0.06 - 0.17%, Si: 0.1 - 0.8%, Mn: 1.9 - 2.9%, Nb: 0.005 - 0.07%, Ti: 0.004 - 0.05%, B: 0.0004 - 0.005%, Cr: 0.20% or less (except 0%), Mo: 0.04 - 0.45%, and the balance of Fe and other inevitable impurities. The cold-rolled steel sheet satisfies the following relational expressions 1 to 3. In terms of area %, the fine structure contains: 80 - 98% tempered martensite, the balance of fresh martensite, bainite, ferrite, and retained austenite, and the average length of the short axis of the laths of the tempered martensite is 500 nm or less.

[0017] [Relational expression 1] 0.40 ≤ C + Mn / 6 + (Cr + Mo + V) / 5 + (Si + Ni + Cu) / 15 ≤ 0.70

[0018] [Relational expression 2] 110 ≤ 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb ≤ 210

[0019] [Relational expression 3] 0.20 ≤ Mo + 200B ≤ 0.70

[0020] (Wherein, the contents of the alloy components described in the relational expressions 1 to 3 represent weight %. )

[0021] Another embodiment of the present invention provides a method for manufacturing an ultra-high strength cold-rolled steel sheet with excellent bendability, which comprises the following steps: heating a slab, which, by weight%, contains: C: 0.06 - 0.17%, Si: 0.1 - 0.8%, Mn: 1.9 - 2.9%, Nb: 0.005 - 0.07%, Ti: 0.004 - 0.05%, B: 0.0004 - 0.005%, Cr: not more than 0.20% (except 0%), Mo: 0.04 - 0.45%, and the balance of Fe and other inevitable impurities, and the slab satisfies the following relational expressions 1 to 3; finish rolling the heated slab so that the finish rolling exit temperature is Ar3 + 50°C to Ar3 + 150°C to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet to Ms + 50°C to Ms + 300°C and then coiling it; cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet in the temperature range of 820 - 860°C; performing homogenization heat treatment on the continuously annealed cold-rolled steel sheet for 50 - 200 seconds; cooling the homogenization heat-treated cold-rolled steel sheet at a cooling rate of 1 - 10°C / second for the first cooling to 620 - 700°C; cooling the first-cooled cold-rolled steel sheet at a cooling rate of 5 - 50°C / second for the second cooling to 360 - 420°C; performing overaging treatment on the second-cooled cold-rolled steel sheet at 370 - 420°C or performing overaging treatment after reheating, wherein, during the second cooling and the overaging treatment, the cold-rolled steel sheet satisfies the following relational expressions 4 to 8.

[0022] [Relational expression 1] 0.40 ≤ C + Mn / 6 + (Cr + Mo + V) / 5 + (Si + Ni + Cu) / 15 ≤ 0.70

[0023] [Relational expression 2] 110 ≤ 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb ≤ 210

[0024] [Relational expression 3] 0.20 ≤ Mo + 200B ≤ 0.70

[0025] [Relational expression 4] 0 ≤ A ≤ 50

[0026] [Relational expression 5] 0 ≤ B ≤ 40

[0027] [Relational expression 6] 0 ≤ 2.8A + 0.5B ≤ 100

[0028] [Relational expression 7] 0 ≤ 3.1A + 2.3B ≤ 200

[0029] [Relational expression 8] 0.25 ≤ (3.1A + 2.3B) / (2.8A + 0.5B) ≤ 3.5

[0030] (In the relationships 1 to 3, the contents of the alloy components are expressed in wt%, and in the relationships 4 to 8, A represents the Ms-secondary cooling termination temperature (°C), and B represents the overaging treatment temperature - secondary cooling termination temperature (°C).)

[0031] Advantageous effects

[0032] According to one aspect of the present invention, an ultra-high strength cold-rolled steel sheet excellent in bendability and a method for manufacturing the same can be provided. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 It is a photograph of the microstructure of Invention Example 1 according to an embodiment of the present invention observed by SEM.

[0034] Figure 2 It is a photograph of the microstructure of Invention Example 1 according to an embodiment of the present invention observed by TEM.

[0035] BEST MODE FOR CARRYING OUT THE INVENTION

[0036] Hereinafter, an ultra-high strength cold-rolled steel sheet excellent in bendability according to an embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. Unless otherwise specified, the contents of the alloy compositions described below are expressed in wt%.

[0037] C: 0.06 - 0.17%

[0038] Carbon (C) is a very important element added for solid solution strengthening. In addition, carbon combines with precipitation elements to form fine carbides, thereby contributing to an increase in strength. When the content of C is less than 0.06%, it is very difficult to ensure the desired strength. On the other hand, when the content of C exceeds 0.17%, due to an increase in hardenability, too much martensite is formed during cooling, so the strength increases sharply and the bendability deteriorates. In addition, due to poor weldability, the possibility of welding defects occurring during processing of parts in the customer company increases. Therefore, the content of C preferably has a range of 0.06 - 0.17%. The lower limit of the C content is more preferably 0.08%, further preferably 0.10%. The upper limit of the C content is more preferably 0.165%, further preferably 0.16%, and most preferably 0.145%.

[0039] Si: 0.1 - 0.8%

[0040] Silicon (Si) is one of the five major elements of steel, and a small amount of Si is naturally added during the manufacturing process. The Si helps to increase strength and inhibit the formation of carbides, so that carbon does not form carbides during annealing, homogenization heat treatment, and cooling. In addition, carbon is distributed and accumulated in retained austenite, so that the retained austenite phase remains at room temperature, which is beneficial to ensuring elongation. When the content of the Si is less than 0.1%, it may be difficult to fully ensure the above effects. On the other hand, when the content of the Si exceeds 0.80%, surface scale defects may be caused, resulting in a reduction in the quality of the plating surface and a reduction in the phosphatizing processability. Therefore, the content of the Si preferably has a range of 0.1 - 0.8%. The lower limit of the Si content is more preferably 0.2%, and further preferably 0.3%. The upper limit of the Si content is more preferably 0.7%, and further preferably 0.6%.

[0041] Mn: 1.9 - 2.9%

[0042] Manganese (Mn) is an element that causes sulfur in steel to be completely precipitated as MnS, thereby preventing the formation of FeS that causes hot brittleness, and solid-solution strengthening the steel. When the content of the Mn is less than 1.9%, it is difficult to ensure the strength desired in the present invention. On the other hand, when the content of the Mn exceeds 2.9%, the possibility of problems such as weldability and hot rolling property is high, and the hardenability may be increased to excessively form martensite, so the elongation may be reduced. In addition, manganese bands (bands of Mn oxides) are formed in the fine structure, so there is a problem of an increased risk of generating processing cracks and plate fracture, and Mn oxides are dissolved out on the surface during annealing, so there is a problem of significantly reducing the plating property. Therefore, the content of the Mn preferably has a range of 1.9 - 2.9%. The lower limit of the Mn content is more preferably 2.0%, and further preferably 2.1%. The upper limit of the Mn content is more preferably 2.8%, and further preferably 2.7%.

[0043] Nb: 0.005 - 0.07%

[0044] Niobium (Nb) is an element that segregates at the austenite grain boundaries to inhibit the coarsening of austenite grains during annealing heat treatment and forms fine carbides to help increase strength. When the content of the Nb is less than 0.005%, the above effects are insufficient. On the other hand, when the content of the Nb exceeds 0.07%, coarse carbides are precipitated, and as the amount of solid-solution carbon in the steel decreases, the strength and elongation may be reduced, and there is a problem of increased manufacturing cost. Therefore, the content of the Nb preferably has a range of 0.005 - 0.07%. The lower limit of the Nb content is more preferably 0.01%, and further preferably 0.015%. The upper limit of the Nb content is more preferably 0.06%, and further preferably 0.05%.

[0045] Ti: 0.004 - 0.05%

[0046] Titanium (Ti), as a fine carbide - forming element, helps to ensure the yield strength and tensile strength. In addition, as a nitride - forming element, Ti has the effect of precipitating N in the steel in the form of TiN to inhibit the precipitation of AlN, and thus has the advantage of reducing the risk of crack generation during continuous casting. When the content of the said Ti is less than 0.004%, it may be difficult to obtain the above - mentioned effects. On the other hand, when the content of the said Ti exceeds 0.05%, coarse carbides precipitate, and as the amount of dissolved carbon in the steel decreases, the strength and elongation may decrease, and nozzle blockage may occur during continuous casting. Therefore, the content of the said Ti preferably has a range of 0.004 - 0.05%. The lower limit of the Ti content is more preferably 0.008%, and further preferably 0.012%. The upper limit of the Ti content is more preferably 0.04%, and further preferably 0.03%.

[0047] B: 0.0004 - 0.005%

[0048] Boron (B) is an element that contributes greatly to ensuring the hardenability of steel. To obtain this effect, it is preferable to add B in an amount of 0.0004% or more. However, when the content of the said B exceeds 0.005%, boron carbide is formed at the grain boundaries to provide nucleation sites for ferrite, and thus the hardenability may deteriorate instead. Therefore, the content of the said B preferably has a range of 0.0004 - 0.005%. The lower limit of the B content is more preferably 0.0006%, and further preferably 0.0008%. The upper limit of the B content is more preferably 0.004%, and further preferably 0.003%.

[0049] Cr: 0.20% or less (except 0%)

[0050] Chromium (Cr) is an element that improves the hardenability and increases the strength of steel. However, when the content of the said Cr exceeds 0.2%, due to the uneven formation of Cr oxides in a brine atmosphere, pitting corrosion problems may occur. Therefore, the Cr content preferably has a range of 0.20% or less. The content of the Cr is more preferably 0.15% or less, and further preferably 0.10% or less. In addition, in the present invention, the effect of improving the hardenability and strength can also be obtained by a small amount of Cr, so the lower limit of the said Cr is not particularly limited.

[0051] Mo: 0.04 - 0.45%

[0052] Molybdenum (Mo) is an element that forms carbides. When it is added in combination with carbide and nitride forming elements such as Ti, Nb, and V, it maintains the fine size of the precipitates, thereby enhancing the yield strength and tensile strength. In addition, the said Mo improves the hardenability of the steel, forming martensite finely in the grain boundary, so it has the advantage of being able to control the yield ratio. For the above effects, it is preferred to add more than 0.04% of Mo. However, the said Mo is a high-cost element, and the more the content of Mo, the more unfavorable it is for manufacturing. Therefore, it is preferred to appropriately control the content of Mo. When the content of Mo exceeds 0.45%, it will cause a sharp increase in manufacturing costs, so not only the economy is reduced, but also due to the excessive grain refinement effect and solid solution strengthening effect, there is a problem of reduced ductility of the steel. Therefore, the content of the said Mo preferably has a range of 0.04 - 0.45%. The lower limit of the content of Mo is more preferably 0.06%, and further preferably 0.08%. The upper limit of the content of Mo is more preferably 0.40%, and further preferably 0.35%.

[0053] In addition, the cold-rolled steel sheet of the present invention preferably satisfies the following relational expressions 1 to 3 while satisfying the above alloy components. Thus, it is possible to manufacture a super high-strength steel sheet with a tensile strength of 980 MPa or more and extremely excellent bend formability as desired by the present invention.

[0054] [Relational expression 1] 0.40 ≤ C + Mn / 6 + (Cr + Mo + V) / 5 + (Si + Ni + Cu) / 15 ≤ 0.70

[0055] The said relational expression 1 is a component relational expression for ensuring strength and weldability. When the value of the said relational expression 1 is less than 0.40, it is difficult to ensure the strength of the material and the welded part as desired by the present invention. When the value of the said relational expression 1 exceeds 0.70, the weldability may be poor. Therefore, the value of the said relational expression 1 preferably has a range of 0.40 - 0.70. The lower limit of the value of the relational expression 1 is more preferably 0.45, and further preferably 0.50. The upper limit of the value of the relational expression 1 is more preferably 0.68, and further preferably 0.65.

[0056] [Relational expression 2] 110 ≤ 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb ≤ 210

[0057] The relational expression 2 is a compositional relational expression related to the hardenability index for ensuring hardenability. When the value of the relational expression 2 is less than 110, it is difficult to ensure the strength desired by the present invention due to insufficient hardenability. When the value of the relational expression 2 exceeds 210, the hardenability is too high, and the bend formability may deteriorate. Therefore, the value of the relational expression 2 preferably has a range of 100 - 200. The lower limit of the value of the relational expression 2 is more preferably 120, and further preferably 130. The upper limit of the value of the relational expression 2 is more preferably 200, and further preferably 190.

[0058] [Relational expression 3] 0.20 ≤ Mo + 200B ≤ 0.70

[0059] The relational expression 3 is a compositional relational expression for more stably ensuring the strength desired by the present invention. When the value of the relational expression 3 is less than 0.20, it is difficult to ensure the strength desired by the present invention due to insufficient hardenability. When the value of the relational expression 3 exceeds 0.70, the hardenability is too high, and not only may the bend formability deteriorate, but there is also a disadvantage of an increase in manufacturing cost. Therefore, the value of the relational expression 3 preferably has a range of 0.20 to 0.70. The lower limit of the value of the relational expression 3 is more preferably 0.25, and further preferably 0.30. The upper limit of the value of the relational expression 3 is more preferably 0.65, and further preferably 0.60.

[0060] The remaining component of the present invention is iron (Fe). However, in the normal manufacturing process, undesirable impurities are inevitably mixed in from raw materials or the surrounding environment, so these impurities cannot be completely excluded. These impurities are well known to those skilled in the art, and therefore all of their contents are not particularly mentioned in this specification.

[0061] In addition, the impurities may include one or more of P, S, Al, Sb, N, Mg, Sn, Sb, Zn, and Pb as inclusion elements, and the total amount of the inclusion elements may be 0.1% by weight or less. The inclusion elements are inclusion elements derived from scrap steel and the like used as raw materials in the steelmaking process. When the total amount of the inclusion elements exceeds 0.1%, surface cracks of the slab may occur, and the surface quality of the steel plate may be reduced.

[0062] Hereinafter, the microstructure and the like of the ultra-high strength cold-rolled steel sheet with excellent bend formability according to an embodiment of the present invention will be described.

[0063] In terms of area percentage, the microstructure of the cold-rolled steel sheet of the present invention preferably contains 80-98% tempered martensite and the balance of fresh martensite, bainite, ferrite and retained austenite. The microstructure of the cold-rolled steel sheet of the present invention contains tempered martensite (hereinafter, also referred to as "TM") as the main structure. However, when the fraction of the tempered martensite is less than 80%, it is difficult to ensure the desired strength. When the fraction of the tempered martensite exceeds 98%, the bendability and elongation may deteriorate. Therefore, the fraction of the martensite preferably has a range of 80-98%. The lower limit of the martensite fraction is more preferably 82%, and further preferably 84%. The upper limit of the martensite fraction is more preferably 97%, and further preferably 96%. The fresh martensite (hereinafter, also referred to as "FM"), bainite (hereinafter, also referred to as "B"), ferrite (hereinafter, also referred to as "F") and retained austenite (hereinafter, also referred to as "RA") as the balance structure are microstructures inevitably formed in the manufacturing process. However, the balance structure also has a positive function in the present invention. The fresh martensite is a structure conducive to ensuring strength. Therefore, the higher the fraction of the fresh martensite, the more conducive to ensuring strength. However, when the fraction of the fresh martensite exceeds 11%, the elongation and bendability may be poor. Therefore, the fraction of the fresh martensite is preferably 11% or less. The fraction of the fresh martensite is more preferably 10% or less, further preferably 9% or less, and most preferably 8% or less.

[0064] The bainite helps to reduce the hardness difference between phases, and thus can play an important role in improving the bending characteristics. However, when the fraction of the bainite exceeds 3%, the fraction of the martensite relatively decreases, making it difficult to ensure the desired strength. The ferrite is a structure conducive to ensuring elongation. However, when the fraction of the ferrite exceeds 3%, the fraction of the martensite will relatively decrease, so it may be difficult to ensure the desired strength. The retained austenite is a structure conducive to ensuring elongation. However, when the fraction of the retained austenite exceeds 3%, the fraction of the martensite will relatively decrease, making it may be difficult to ensure the desired strength. Therefore, the fractions of the bainite, ferrite and retained austenite are preferably 3% or less respectively.

[0065] In addition, the average length of the short axis of the lath of the tempered martensite is preferably 500 nm or less. The narrower the lath spacing of the tempered martensite, the more advantageous it is in ensuring strength and bendability. However, when the average length of the short axis of the lath of the tempered martensite exceeds 500 nm, it is difficult to obtain the above effects. The average length of the short axis of the lath is more preferably 400 nm or less, and further preferably 300 nm or less.

[0066] The yield strength (YS) of the cold-rolled steel sheet of the present invention provided as described above can be 780 - 920 MPa, the tensile strength (TS) can be 980 - 1200 MPa, the elongation (EL) can be 8% or more, the yield ratio (YS / TS) can be 0.75 or more, the hole expansion ratio (HER) can be 40% or more, the bend formability (YS×EL×HER) can be 300 GPa%% or more, and it has the advantage of not generating cracks during the 180° full compression bending test. The yield strength is more preferably 790 - 910 MPa, and further preferably 800 - 900 MPa. The tensile strength is more preferably 990 - 1180 MPa, and further preferably 1000 - 1160 MPa. The elongation is more preferably 9% or more, and further preferably 10% or more. The yield ratio is more preferably 0.76 or more, and further preferably 0.77 or more. The hole expansion ratio is more preferably 45% or more, and further preferably 50% or more. The bend formability is more preferably 350 GPa%% or more, and further preferably 400 GPa%% or more. In addition, the 180° full compression bending test can be carried out by the following method, that is, first bend the steel sheet to be measured by 90°, then insert another steel sheet with a thickness twice that of the steel sheet to be measured therebetween, and then bend the steel sheet to be measured by 180° again to completely compress it.

[0067] Hereinafter, a method for manufacturing an ultra-high strength cold-rolled steel sheet with excellent bend formability according to an embodiment of the present invention will be described.

[0068] First, a slab satisfying the above alloy composition is heated. In the present 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 lower than 1100 °C, the slab temperature is low, and rolling load may be generated during rough rolling. When the heating temperature of the slab exceeds 1300 °C, the structure may coarsen, and there may be disadvantages such as an increase in power cost. The lower limit of the heating temperature of the slab is more preferably 1125 °C, and further preferably 1150 °C. The upper limit of the heating temperature of the slab is more preferably 1275 °C, and further preferably 1250 °C. In addition, the slab can have a thickness of 230 - 270 mm.

[0069] Thereafter, 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 may increase sharply. When the finish-rolling exit temperature exceeds Ar3 + 150°C, not only will excessive scale be 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.

[0070] Thereafter, 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, excessive martensite or bainite is formed, resulting in an excessive increase in the strength of the hot-rolled steel sheet, and thus problems such as poor shape caused by load may occur during cold rolling. On the other hand, when the coiling temperature exceeds Ms + 300°C, the surface scale increases, and thus the pickling property may deteriorate. 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. In addition, after the coiling, the coiled hot-rolled steel sheet can be cooled to room temperature at a cooling rate of 0.1°C / second or less.

[0071] Thereafter, the coiled and cooled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold rolling can be carried out at a reduction ratio of 40 - 70%. When the cold rolling reduction ratio is less than 40%, the recrystallization driving force deteriorates, and problems are likely to occur in obtaining good recrystallized grains, and there is a disadvantage that shape correction is very difficult. When the cold rolling reduction ratio exceeds 70%, the possibility of cracks occurring in the edge portion of the steel sheet is high, and the rolling load may increase sharply. Therefore, the cold rolling is preferably carried out at a reduction ratio of 40 - 70%. In addition, before the cold rolling, pickling can also be carried out to remove scale or impurities attached to the surface.

[0072] Thereafter, the cold-rolled steel sheet is continuously annealed within a temperature range of 820 - 860°C. When the continuous annealing temperature is lower than 820°C, it is difficult to form sufficient austenite, and thus it is difficult to ensure the strength desired in the present invention. On the other hand, when the temperature of the continuous annealing exceeds 860°C, the austenite grain size coarsens, and the bend formability in the final product may deteriorate. Therefore, the continuous annealing temperature preferably has a range of 820 - 860°C. The lower limit of the continuous annealing temperature is more preferably 825°C, and further preferably 830°C. The upper limit of the continuous annealing temperature is more preferably 855°C, and further preferably 850°C.

[0073] Thereafter, the cold-rolled steel sheet that has been continuously annealed is solution heat-treated for 50 - 200 seconds. This is to ensure the recrystallization and grain growth of the cold-rolled structure and to ensure a sufficient austenite fraction at the annealing temperature proposed in the present invention. When the solution heat-treatment time is less than 50 seconds, the reverse phase transformation to austenite is insufficient, and the fraction of ferrite in the final structure increases, so it may be difficult to ensure the desired strength. On the other hand, when the solution heat-treatment time exceeds 200 seconds, the austenite grain size coarsens, and the bend formability in the final product may deteriorate. The lower limit of the solution heat-treatment time is more preferably 55 seconds, and further preferably 60 seconds. The upper limit of the solution heat-treatment time is more preferably 190 seconds, and further preferably 180 seconds.

[0074] Thereafter, the cold-rolled steel sheet that has been solution heat-treated is first cooled at a cooling rate of 1 - 10°C / second to 620 - 700°C. The first cooling step is used to ensure the equilibrium carbon concentration of ferrite and austenite to increase the ductility and strength of the steel sheet. When the termination temperature of the first cooling is lower than 630°C or exceeds 700°C, it is difficult to ensure the ductility and strength desired in the present invention. When the cooling rate is less than 1°C / second, the ferrite phase transformation is accelerated, and thus there is a drawback that it is difficult to ensure the fraction of the desired fine structure. When the cooling rate exceeds 10°C / second, there is a drawback that it is difficult to ensure the elongation due to excessive martensite phase transformation.

[0075] Thereafter, the cold-rolled steel sheet that has been cooled once is subjected to secondary cooling at a cooling rate of 5-50 °C / second and cooled to 360-420 °C. The secondary cooling is one of the important control factors in the present invention, and the secondary cooling termination temperature is a very important condition for ensuring strength, ductility, and bend formability simultaneously. When the secondary cooling termination temperature is lower than 360 °C, the martensite fraction increases excessively, so it is difficult to ensure ductility. When the secondary cooling termination temperature exceeds 420 °C, it is difficult to ensure sufficient martensite, so it is difficult to ensure the desired strength. Therefore, the secondary cooling termination temperature, which is one of the important control factors for ensuring the desired physical properties of the present invention, preferably has a range of 360-420 °C. The lower limit of the secondary cooling termination temperature is more preferably 365 °C, and further preferably 370 °C. The upper limit of the secondary cooling termination temperature is more preferably 405 °C, and further preferably 400 °C. When the secondary cooling rate is less than 5 °C / second, due to the slow cooling rate, prior to the martensite and bainite phase transformations, the ferrite phase transformation preferentially occurs, so there is a disadvantage that the fraction of the fine microstructure desired to be obtained in the present invention cannot be obtained. When the secondary cooling rate exceeds 50 °C / second, due to the shape difference problem caused by the excessive cooling rate, the sheet passing property decreases, and sheet breakage may occur. The lower limit of the secondary cooling rate is more preferably 7.5 °C / second, and further preferably 10 °C / second. The upper limit of the secondary cooling rate is more preferably 47.5 °C / second, and further preferably 45 °C / second.

[0076] In addition, in order to ensure the fraction of tempered martensite, which is an important fine microstructure in the present invention, at the target level, it is important to precisely control the difference between Ms and the secondary cooling termination temperature. More specifically, it is preferable to satisfy the following relational expression 4. When the difference between Ms and the secondary cooling termination temperature, that is, the value of A, is less than 0, the amount of martensite is small, and it may be difficult to ensure the desired strength. When the value of A exceeds 50 °C, the residence time in the martensite region is long, and the fraction of martensite increases excessively, so it is difficult to ensure ductility. Therefore, the difference between Ms and the secondary cooling termination temperature, that is, the value of A, is preferably 0-50 °C. The lower limit of the value of A is more preferably 1 °C, and further preferably 2 °C. The upper limit of the value of A is more preferably 45 °C, and further preferably 40 °C. In addition, Ms represents the temperature at which the martensite phase transformation starts, and its value can be obtained by the following formula 1.

[0077] [Relational expression 4] 0 ≤ A ≤ 50

[0078] (Wherein, A in the relational expression 4 is Ms - secondary cooling termination temperature (°C).)

[0079] [Formula 1] Ms = 539 - 423C - 30.4Mn - 7.5Si + 30Al

[0080] Thereafter, at 370 - 420 °C, the cold-rolled steel sheet that has been secondarily cooled is subjected to overaging treatment or overaging treatment after reheating. The overaging treatment is preferably carried out at the same temperature as the temperature at the end of secondary cooling or at a temperature higher than the temperature at the end of secondary cooling. The overaging treatment is a process for promoting the transformation of the newly formed martensite phase at the end of secondary cooling into tempered martensite, whereby high yield strength and high bend formability can be stably ensured. Therefore, in order to ensure the high bend formability desired in the present invention, the overaging treatment temperature is a very important factor. In the present invention, the overaging treatment temperature is precisely controlled within the range of 370 - 420 °C. When the overaging treatment temperature is lower than 370 °C, the transformation from newly formed martensite to tempered martensite occurs in small amounts, and thus the bend formability may deteriorate. On the other hand, when the overaging treatment temperature exceeds 420 °C, it may be difficult to ensure the tensile strength due to excessive tempered martensite transformation. Therefore, the overaging treatment temperature preferably has a range of 370 - 420 °C. The lower limit of the overaging treatment temperature is more preferably 375 °C, and further preferably 380 °C. The upper limit of the overaging treatment temperature is more preferably 415 °C, and further preferably 410 °C.

[0081] In addition, in order to ensure the fraction of tempered martensite, which is an important microstructure in the present invention, at a target level, it is important to precisely control the overaging treatment temperature and the temperature at the end of secondary cooling. More specifically, it is preferable to satisfy the following relational expression 5. When the difference between the overaging treatment temperature and the temperature at the end of secondary cooling, that is, the value of B is less than 0, it is difficult to obtain the overaging treatment effect. When the value of B exceeds 40 °C, it may be difficult to ensure the desired tensile strength due to excessive tempered martensite transformation. Therefore, the difference between the overaging treatment temperature and the temperature at the end of secondary cooling, that is, the value of B, is preferably 0 - 40 °C. The lower limit of the value of B is more preferably 2.5 °C, and further preferably 5 °C. The upper limit of the value of B is more preferably 35 °C, and further preferably 30 °C.

[0082] [Relational expression 5] 0 ≤ B ≤ 40

[0083] (wherein B in the relational expression 5 is the overaging treatment temperature - the temperature at the end of secondary cooling (°C).)

[0084] Moreover, in the present invention, in order to obtain the desired fraction and strength level of the microstructure, during the secondary cooling and the overaging treatment, it is preferable to satisfy the following relational expressions 6 to 8.

[0085] [Relational expression 6] 0 ≤ 2.8A + 0.5B ≤ 100

[0086] The relational expression 6 is used to ensure the desired yield strength of the present invention. When the value of the relational expression 6 is less than 0, it is difficult to ensure sufficient martensite, so it is difficult to obtain a high yield strength. When the value of the relational expression 6 exceeds 100, due to ensuring too much tempered martensite, there may be a problem of excessive increase in yield strength. Therefore, the value of the relational expression 6 preferably has a range of 0 to 100. The lower limit of the value of the relational expression 6 is more preferably 2, and further preferably 4. The upper limit of the value of the relational expression 6 is more preferably 90, and further preferably 80.

[0087] [Relational expression 7] 0 ≤ 3.1A + 2.3B ≤ 200

[0088] The relational expression 7 is used to ensure the desired tensile strength of the present invention. When the value of the relational expression 7 is less than 0, it is difficult to ensure sufficient fresh martensite, so it is difficult to ensure the desired tensile strength. When the value of the relational expression 7 exceeds 200, the phase transformation to tempered martensite occurs excessively, so it is difficult to ensure the tensile strength. Therefore, the value of the relational expression 7 preferably has a range of 0 to 200. The lower limit of the value of the relational expression 7 is more preferably 2, and further preferably 4. The upper limit of the value of the relational expression 7 is more preferably 190, and further preferably 180.

[0089] [Relational expression 8] 0.25 ≤ (3.1A + 2.3B) / (2.8A + 0.5B) ≤ 3.5

[0090] The relational expression 8 is used to simultaneously ensure the desired yield strength and tensile strength of the present invention. When the value of the relational expression 8 is less than 0.25 or exceeds 3.5, it is difficult to ensure the desired tissue fraction, so there is a disadvantage that it is difficult to simultaneously ensure the desired yield strength and tensile strength. Therefore, the value of the relational expression 8 preferably has a range of 0.25 to 3.5. The lower limit of the value of the relational expression 8 is more preferably 0.50, and further preferably 0.75. The upper limit of the value of the relational expression 8 is more preferably 3.25, and further preferably 3.0.

[0091] In addition, in the present invention, after the overaging treatment, it may further include a step of skin pass rolling the cold-rolled steel sheet subjected to the overaging treatment at an elongation rate of 0.1 - 2.0%. Generally, during skin pass rolling, the yield strength can be increased by at least 50 MPa or more with little increase in tensile strength. When the elongation rate is less than 0.1%, it may be difficult to control the shape. When the elongation rate exceeds 2.0%, due to the high tensile operation, the operability becomes very unstable. Detailed implementation mode

[0092] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention in more detail and are not used to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the content described in the claims and the content reasonably deduced therefrom.

[0093] (Example)

[0094] After preparing molten steel having the alloy composition described in Table 1 below, continuous casting is carried out to manufacture a slab with a thickness of 250 mm. The slab is heated at 1200 °C for 12 hours and then hot-rolled and coiled. At this time, the finish rolling exit temperature during hot rolling is controlled in the range of Ar3 + 50 °C to Ar3 + 150 °C, and the coiling temperature is controlled in the range of Ms + 50 °C to Ms + 300 °C. Thereafter, the hot-rolled steel sheet having a thickness of 3.2 mm obtained by the hot rolling is pickled and then cold-rolled at a cold rolling reduction rate of 50% to obtain a cold-rolled steel sheet having a thickness of 1.6 mm. Using the conditions described in Tables 2 and 3 below, the cold-rolled steel sheet is manufactured into a final product. After measuring the microstructure and mechanical physical properties of the cold-rolled steel sheet manufactured as described above, the results are shown in Table 4 below.

[0095] The fraction of the microstructure is measured using an Electron Back Scatter Diffraction (EBSD) device. For the average length of the short axis of the tempered martensite laths, five random photographs are taken at a magnification of 40000 times with a transmission electron microscope (TEM), and then the average value is measured and calculated using Image-Plus Pro software. In addition, the measured microstructure is composed of a structure in which tempered martensite, the remaining fresh martensite, bainite, ferrite, and retained austenite coexist.

[0096] The tensile strength (TS), yield strength (YS), and elongation (EL) are measured by a tensile test in the rolling horizontal direction, and a specimen standard with a gauge length of 50 mm and a width of 25 mm of the tensile specimen is used.

[0097] The hole expansion ratio (HER) is measured according to the ISO 16330 standard, and the hole is sheared and processed with a clearance of 12% using a punch with a diameter of 10 mm.

[0098] In the 180° full compression bending test, first, the steel sheet to be measured is bent 90°, then another steel sheet with a thickness twice that of the steel sheet to be measured is inserted therein, and then the steel sheet to be measured is bent 180° again for full compression, and then it is visually judged whether cracks are generated. The case where no cracks are generated is indicated by ○, and the case where cracks are generated is indicated by ×.

[0099] [Table 1]

[0100]

[0101] [Table 2]

[0102]

[0103]

[0104] [Table 3]

[0105]

[0106]

[0107]

[0108] [Table 4]

[0109]

[0110] As shown in the above Tables 1 to 4, it can be seen that in the case of Invention Examples 1 to 12 that satisfy the alloy composition and manufacturing conditions proposed by the present invention, the fine microstructure expected to be obtained by the present invention is ensured, and thus excellent mechanical and physical properties are achieved.

[0111] On the other hand, in the case of Comparative Examples 1 to 17 that do not satisfy the alloy composition or manufacturing conditions proposed by the present invention, it can be confirmed that due to the inability to ensure the fine microstructure expected to be obtained by the present invention, the mechanical and physical properties are poor.

[0112] Figure 1 It is a photograph of the fine microstructure of Invention Example 1 observed by SEM. Figure 2 It is a photograph of the fine microstructure of Invention Example 1 observed by TEM. Through Figure 1 and Figure 2 it can be seen that tempered martensite, which is the main microstructure of the present invention, is uniformly distributed in Invention Example 1.

Claims

1. A ultra-high strength cold-rolled steel sheet with excellent bendability. By weight%, the cold-rolled steel sheet contains: C: 0.06 - 0.17%, Si: 0.1 - 0.8%, Mn: 1.9 - 2.9%, Nb: 0.005 - 0.07%, Ti: 0.004 - 0.05%, B: 0.0004 - 0.005%, Cr: below 0.20% and excluding 0%, Mo: 0.04 - 0.45%, and the balance of Fe and other inevitable impurities. The cold-rolled steel sheet satisfies the following relational expressions 1 to 3. By area%, the fine microstructure contains 80 - 98% tempered martensite, and the balance of fresh martensite, bainite, ferrite, and retained austenite. The average length of the short axis of the laths of the tempered martensite is 500 nm or less. [Relational expression 1] 0.40 ≤ C + Mn / 6 + (Cr + Mo + V) / 5 + (Si + Ni + Cu) / 15 ≤ 0.70 [Relational expression 2] 110 ≤ 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb ≤ 210 [Relational expression 3] 0.20 ≤ Mo + 200B ≤ 0.70 Among them, The contents of the alloying elements described in the above relational expressions 1 to 3 are expressed in weight%.

2. The ultra-high strength cold-rolled steel sheet excellent in bendability according to claim 1, wherein, The impurities include one or more of P, S, Al, Sb, N, Mg, Sn, Sb, Zn, and Pb as inclusion elements, and the total amount of the inclusion elements is 0.1 weight% or less.

3. The ultra-high strength cold-rolled steel sheet with excellent bendability according to claim 1, wherein, The fresh martensite is 11% or less, the bainite is 3% or less, the ferrite is 3% or less, and the retained austenite is 3% or less.

4. The ultra-high strength cold-rolled steel sheet excellent in bendability according to claim 1, wherein, The yield strength (YS) of the cold-rolled steel sheet is 780 - 920 MPa, the tensile strength (TS) is 980 - 1200 MPa, the elongation (EL) is 8% or more, the yield ratio (YS / TS) is 0.75 or more, the hole expansion ratio (HER) is 40% or more, the bendability (YS × EL × HER) is 300 GPa%% or more, and no cracks occur during the 180° full compression bending test.

5. A method for manufacturing a ultra-high strength cold-rolled steel sheet with excellent bendability, which includes the following steps: Heating a slab. By weight%, the slab contains: C: 0.06 - 0.17%, Si: 0.1 - 0.8%, Mn: 1.9 - 2.9%, Nb: 0.005 - 0.07%, Ti: 0.004 - 0.05%, B: 0.0004 - 0.005%, Cr: below 0.20% and excluding 0%, Mo: 0.04 - 0.45%, and the balance of Fe and other inevitable impurities. The slab satisfies the following relational expressions 1 to 3; Rough rolling the heated slab so that the rough rolling exit temperature is Ar3 + 50 °C to Ar3 + 150 °C to obtain a hot-rolled steel sheet; Cooling the hot-rolled steel sheet to Ms + 50 °C to Ms + 300 °C and then coiling it; Cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; The cold-rolled steel sheet is continuously annealed within a temperature range of 820 - 860 °C; The continuously annealed cold-rolled steel sheet is subjected to soaking heat treatment for 50 - 200 seconds; The cold-rolled steel sheet after soaking heat treatment is subjected to primary cooling at a cooling rate of 1 - 10 °C / second until it is cooled to 620 - 700 °C; The cold-rolled steel sheet after primary cooling is subjected to secondary cooling at a cooling rate of 5 - 50 °C / second until it is cooled to 360 - 420 °C; At 370 - 420 °C, the cold-rolled steel sheet after secondary cooling is subjected to overaging treatment or overaging treatment after reheating, wherein, during the secondary cooling and the overaging treatment, the cold-rolled steel sheet satisfies the following relational expressions 4 to relational expression 8, [Relational expression 1] 0.40 ≤ C + Mn / 6 + (Cr + Mo + V) / 5 + (Si + Ni + Cu) / 15 ≤ 0.70 [Relational expression 2] 110 ≤ 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb ≤ 210 [Relational expression 3] 0.20 ≤ Mo + 200B ≤ 0.70 [Relational expression 4] 0 ≤ A ≤ 50 [Relational expression 5] 0 ≤ B ≤ 40 [Relational expression 6] 0 ≤ 2.8A + 0.5B ≤ 100 [Relational expression 7] 0 ≤ 3.1A + 2.3B ≤ 200 [Relational expression 8] 0.25 ≤ (3.1A + 2.3B) / (2.8A + 0.5B) ≤ 3.5 wherein, the contents of the alloying elements described in the relational expressions 1 to relational expression 3 are expressed in wt%, in the relational expressions 4 to relational expression 8, A is the Ms - secondary cooling termination temperature, where the temperature unit is °C, and B is the overaging treatment temperature - secondary cooling termination temperature, where the temperature unit is °C.

6. The method for manufacturing an ultra-high strength cold-rolled steel sheet excellent in bend formability according to claim 5, wherein, The heating of the slab is carried out at 1100 - 1300 °C.

7. The method for manufacturing an ultra-high strength cold-rolled steel sheet having excellent bend formability according to claim 5, wherein, The slab has a thickness of 230 - 270 mm.

8. The method for manufacturing an ultra-high strength cold-rolled steel sheet excellent in bend formability according to claim 5, wherein, After the coiling, it further includes the step of cooling the coiled hot-rolled steel sheet to room temperature at a cooling rate of 0.1 °C / second or less.

9. The method for manufacturing an ultra-high strength cold-rolled steel sheet excellent in bendability according to claim 5, wherein, The cold rolling is carried out at a reduction rate of 40 - 70%.

10. The method for manufacturing an ultra-high strength cold-rolled steel sheet having excellent bend formability according to claim 5, wherein, After the overaging treatment, it further includes the step of skin pass rolling the cold-rolled steel sheet after overaging treatment at an elongation rate of 0.1 - 2.0%.

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