Cold-rolled steel sheet and method for producing same

Abstract

The invention relates to a cold-rolled steel sheet and a manufacturing method thereof. An object of one aspect of the present invention is to provide a cold-rolled steel sheet having excellent strength and bending formability, and a method for manufacturing the same.

Description

Technical Field

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same. Background Art

[0002] In order to achieve the lightweight and enhanced safety of automobiles, the development of high-strength steel sheets has been continuously promoted. In recent years, in order to increase the driving range of electric vehicles and protect the battery, the importance of ultra-high-strength steels with a tensile strength of 1500 MPa or more has been increasing. In addition, since existing MART steels are insufficient in formability, if a cold-forming ultra-high-strength steel sheet with both formability can be developed, its economic value is expected to be very high. As a method for improving the elongation rate to improve the formability of steel, a method of utilizing the Transformation Induced Plasticity (TRIP) phenomenon by introducing retained austenite is widely used. However, in the case of such TRIP steel sheets, Si and Al need to be added to introduce retained austenite, and a relatively large amount of retained austenite can be obtained when accompanied by bainite transformation. However, bainite transforms at a relatively high temperature, so the tensile strength (TS, Tensile Strength) is low, and the yield strength (YS, Yield Strength) is also low, making it unsuitable for use as ultra-high-strength steel.

[0003] Therefore, the recent trend is to increase the strength of the steel sheet while adopting the quenching (Quenching) and partitioning (Partitioning) processes to utilize the TRIP phenomenon. In the case of so-called Q&P steels, the main structure of the matrix is tempered martensite, which has excellent yield strength and hole expansion property (hole expansion ratio (HER; Hole Expansion Ratio)). When retained austenite is actively formed, an appropriate level of elongation can also be obtained.

[0004] In addition, from the perspective of passenger protection, the importance of automobile safety has become increasingly prominent. It is believed that by improving autonomous driving technology, the risk of car accidents can be fundamentally solved. However, during the transitional period before the technology matures, due to the passengers' ignorance of the accident situation, the severity of the accident may instead increase. Therefore, in recent years, automobile collision test regulations have been further strengthened. It is known that the bending characteristics of steel are crucial for reducing the risk of cracks in automobile structural components during a car collision. If the bending characteristics of the steel are excellent, the material will not crack, but will absorb more collision energy through folding, and the remaining parts can withstand the collision and guide the structure to collapse stably.

[0005] However, simply applying the Q&P process is not sufficient to obtain excellent bend formability, and further ideas are needed to ensure excellent bend formability and a high strength of 1500 MPa or more. Summary of the Invention

[0006] (1) Technical problem to be solved

[0007] An object of one aspect of the present invention is to provide a cold-rolled steel sheet and a method for manufacturing the same.

[0008] An object of a preferred aspect of the present invention is to provide a cold-rolled steel sheet having excellent strength and bend formability and a method for manufacturing the same.

[0009] (2) Technical solution

[0010] An embodiment of the present invention provides a cold-rolled steel sheet which, by weight%, comprises: C: not less than 0.15% and less than 0.30%, Si: 0.8 - 2.5%, Al: 0.005 - 0.3%, Mn: 2.0 - 3.0%, Cr: 0.001 - 0.5%, Mo: 0.001 - 0.2%, B: 0.0005 - 0.0050%, Nb: 0.001 - 0.05%, Ti: 0.001 - 0.05%, P: not more than 0.04% (except 0%), S: not more than 0.01% (except 0%), N: not more than 0.01% (except 0%), the balance being Fe and other inevitable impurities; and, by area%, the fine structure consists of not more than 15% (except 0%) of ferrite, not more than 5% (except 0%) of fresh martensite, not less than 75% and less than 95% of tempered martensite, not more than 12% (except 0%) of retained austenite, and the balance of bainite, and the cold-rolled steel sheet satisfies the following relational expression 1.

[0011] [Relational expression 1] (RA 不稳定(unstable) + RA 稳定(stable ) / RA 稳定 ≤2.0

[0012] (In the relational expression 1, RA 不稳定 represents the fraction of unstable retained austenite, and RA 稳定 represents the fraction of stable retained austenite.)

[0013] The cold-rolled steel sheet may further contain one or more of Cu not more than 0.1% (except 0%) and Ni not more than 0.1% (except 0%).

[0014] The cold-rolled steel sheet may further contain V not more than 0.05% (except 0%).

[0015] The average grain size of the retained austenite may be not more than 0.5 μm.

[0016] The cold-rolled steel sheet may satisfy the following relational expression 2.

[0017] [Relationship 2] 4×P.El - U.El ≥ 13.0

[0018] (Wherein, in the said Relationship 2, P.El represents the local elongation rate, and U.El represents the uniform elongation rate.)

[0019] The cold-rolled steel sheet can have a tensile strength of 1470 MPa or more, a yield strength of 1000 MPa or more, a total elongation rate (T.El) of 8.5% or more, and a bend formability (R 最小(min) / t) of 2.5 or less.

[0020] One embodiment of the present invention provides a method for manufacturing a cold-rolled steel sheet, which includes the following steps: heating a slab, by weight%, the slab contains: C: more than 0.15% and less than 0.30%, Si: 0.8 - 2.5%, Al: 0.005 - 0.3%, Mn: 2.0 - 3.0%, Cr: 0.001 - 0.5%, Mo: 0.001 - 0.2%, B: 0.0005 - 0.0050%, Nb: 0.001 - 0.05%, Ti: 0.001 - 0.05%, P: 0.04% or less (except 0%), S: 0.01% or less (except 0%), N: 0.01% or less (except 0%), the balance of Fe and other inevitable impurities; hot finish rolling the heated slab at 830 - 950 °C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 400 - 650 °C; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet at 800 - 950 °C; cooling the continuously annealed cold-rolled steel sheet at an average cooling rate of 1 °C / second or more and less than 10 °C / second for the first cooling until the first cooling termination temperature of 500 - 700 °C; cooling the cold-rolled steel sheet after the first cooling at an average cooling rate of 10 °C / second or more for the second cooling until the second cooling termination temperature of 25 - 300 °C; and reheating the cold-rolled steel sheet after the second cooling at 150 - 450 °C and holding for more than 500 seconds and 1500 seconds or less.

[0021] The slab may further contain one or more of Cu of 0.1% or less (except 0%) and Ni of 0.1% or less (except 0%).

[0022] The slab may further contain V of 0.05% or less (except 0%).

[0023] The heating temperature of the slab may be 1150 - 1250 °C.

[0024] The cold rolling reduction rate during cold rolling may be 30 - 60%.

[0025] (III) Beneficial effects

[0026] According to one aspect of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided.

[0027] According to a preferred aspect of the present invention, a cold-rolled steel sheet having excellent strength and bend formability and a method for manufacturing the same can be provided. Best Mode for Carrying Out the Invention

[0028] Hereinafter, a cold-rolled steel sheet according to an embodiment of the present invention will be described. First, the alloy composition will be described. Unless otherwise specified, the contents of the alloy compositions described below are expressed in weight %.

[0029] C: 0.15% or more and less than 0.30%

[0030] Carbon (C) is an element that ensures the strength of steel through solid solution strengthening and precipitation strengthening. When the content of C is less than 0.15%, it is difficult to ensure a tensile strength (TS) of 1.5 GPa or more. On the other hand, when the content of C is 0.30% or more, the arc weldability and laser weldability deteriorate, and the formation of coarse carbides increases the risk of crack generation. Therefore, the content of C preferably has a range of 0.15% or more and less than 0.30%. In terms of ensuring strength, the lower limit of the content of C is more preferably 0.17%. In addition, in terms of possible brittleness due to weldability and coarse carbides, the upper limit of the content of C is more preferably 0.28%.

[0031] Si: 0.8 - 2.5%

[0032] Silicon (Si) is a core element of transformation-induced plasticity (TRIP) steel, and its role is to increase the fraction and elongation of retained austenite by hindering the precipitation of cementite. When the content of Si is less than 0.8%, almost no retained austenite remains, resulting in an excessive reduction in elongation. On the other hand, when the content of Si exceeds 2.5%, it is impossible to prevent the deterioration of the physical properties of the welded part caused by the formation of LME cracks, and the surface characteristics and plating properties of the steel deteriorate. Therefore, the content of Si preferably has a range of 0.8 - 2.5%. Considering elongation, the lower limit of the content of Si is more preferably 0.9%, further preferably 1.0%, and depending on the elongation required for the application product, it can also be controlled at 1.1%. The upper limit of the content of Si is more preferably 2.2%, further preferably 2.0%, and considering the application object of the application product or the plating method, etc., it can also be controlled at 1.7%.

[0033] Al: 0.005 - 0.3%

[0034] Aluminum (Al) is not only an element included for deoxidizing steel, but also an element effective in stabilizing retained austenite by suppressing the precipitation of cementite. When the content of said Al is less than 0.005%, the deoxidation of the steel is insufficient, and the cleanliness of the steel is impaired. On the other hand, when the content of said Al exceeds 0.3%, the castability of the steel is impaired. Therefore, the content of said Al preferably has a range of 0.005 - 0.3%. In terms of the cleanliness of the steel, the lower limit of the content of said Al is more preferably 0.01%, further preferably 0.02%, and can also be controlled at 0.03% considering the products used or the requirements of users. Considering the above castability of the steel, the upper limit of the content of said Al is more preferably 0.27%, further preferably 0.22%, and can also be controlled at 0.17% considering the processing environment of the products used or users.

[0035] Mn: 2.0 - 3.0%

[0036] Manganese (Mn) is an element added to ensure strength. When the content of said Mn is less than 2.0%, it is difficult to ensure strength. On the other hand, when the content of said Mn exceeds 3.0%, the phase transformation rate becomes slow, forming an excessive amount of fresh martensite, making it difficult to obtain excellent formability. In addition, a banded structure is formed due to the segregation of Mn, thereby impairing the material uniformity and formability of the material. Therefore, the content of said Mn preferably has a range of 2.0 - 3.0%. Considering the target strength of the steel, the lower limit of the content of said Mn is more preferably 2.2%. Considering formability, the upper limit of the content of said Mn is more preferably 2.8%.

[0037] Cr: 0.001 - 0.5%

[0038] Chromium (Cr) is an element added to ensure strength and hardenability. In the case of adding only Mn, a very large amount of Mn exceeding the Mn content range of the present invention needs to be added, and this problem can be eliminated by adding 0.001% or more of said Cr. When the content of said Cr exceeds 0.5%, the local corrosion resistance becomes poor, and oxides are formed on the surface, thereby impairing the phosphate treatment property. Therefore, the content of said Cr preferably has a range of 0.001 - 0.5%. According to the characteristics of the products used, considering corrosion resistance and phosphate treatment property, the upper limit of the content of said Cr can also be controlled at 0.3%.

[0039] Mo: 0.001 - 0.2%

[0040] Molybdenum (Mo) is added to ensure strength and hardenability. To achieve this effect of improving strength and hardenability, the content of the added Mo should be 0.001% or more. However, as an expensive element, the economic efficiency of the steel plate deteriorates, and the phase transformation is excessively delayed, which may lead to the formation of fresh martensite. Therefore, the content of Mo is preferably not more than 0.2%. In terms of the above costs, the upper limit of the content of Mo is more preferably 0.15%, and can also be controlled at 0.12%.

[0041] B: 0.0005 - 0.0050%

[0042] Boron (B) is an element added to ensure hardenability. In the case of adding Mn alone, a very large amount of Mn exceeding the Mn content range of the present invention needs to be added, and this problem can be eliminated by adding 0.0005% or more of the B. However, when the content of B exceeds 0.0050%, boron-based carbides are formed at the grain boundaries, which instead impairs the hardenability. Therefore, the content of B preferably has a range of 0.0005 - 0.0050%. In terms of hardenability, the upper limit of the content of B is more preferably 0.0040%, more preferably 0.0030%, and can also be controlled at 0.025%.

[0043] Nb: 0.001 - 0.05%

[0044] Niobium (Nb) is an element added to ensure the strength of the steel plate and achieve the micro-refinement of the structure. When the addition amount of Nb is less than 0.001%, it is difficult to obtain the effects of improving strength and achieving the micro-refinement of the structure. When the content of Nb exceeds 0.05%, the recrystallization is delayed due to local grain fixation, thereby impairing the uniformity of the structure. Therefore, the content of Nb preferably has a range of 0.001 - 0.05%. In terms of uniformity, the upper limit of the content of Nb is more preferably 0.03%.

[0045] Ti: 0.001 - 0.05%

[0046] Titanium (Ti) is an element added to ensure the strength of the steel plate and make the structure micro-refined. When the added Ti is less than 0.001%, it is difficult to obtain the effects of improving strength and making the structure micro-refined. On the other hand, when the content of Ti exceeds 0.05%, due to the formation of excessive TiN, the castability is impaired, and due to local grain fixation, the recrystallization is delayed, thereby impairing the uniformity of the structure. Therefore, the content of Ti preferably has a range of 0.001 - 0.05%. In terms of improving the impact characteristics, the upper limit of the content of Ti is more preferably 0.03%.

[0047] P: 0.04% or less (except 0%)

[0048] Phosphorus (P) exists as an impurity in steel. Therefore, it is beneficial to control the content of said P at as low a level as possible. However, in order to improve the strength of steel, P is also deliberately added. However, when too much of said P is added, the toughness of the steel deteriorates. Therefore, in the present invention, in order to prevent such problems, it is preferable to limit the upper limit of the content of said P to 0.04%. In terms of improving toughness, the content of said P is more preferably 0.015% or less, and further preferably 0.010% or less. When toughness above the required standard is demanded for the steel, it can also be controlled to 0.007% or less.

[0049] S: 0.01% or less (except 0%)

[0050] Similar to said P, sulfur (S) exists as an impurity in steel. Therefore, it is beneficial to control the content of said S at as low a level as possible. In addition, said S deteriorates the ductility and impact properties of the steel. Therefore, it is preferable to limit the upper limit of the content of said S to 0.01%. The content of said S is more preferably 0.005% or less, further preferably 0.003% or less, and most preferably 0.0015% or less.

[0051] N: 0.01% or less (except 0%)

[0052] In the present invention, nitrogen (N) is unavoidably included as an impurity in the steel during the process, and its upper limit is preferably limited to 0.01%. The content of said N is more preferably 0.007% or less, further preferably 0.005% or less, and most preferably 0.003% or less.

[0053] In addition to the above composition of the steel, the rest may contain Fe and unavoidable impurities. Unavoidable impurities are impurities that may inadvertently mix in during the normal steel manufacturing process. Therefore, it is impossible to completely exclude these impurities, and those skilled in the art of normal steel manufacturing can easily understand their meaning. In addition, the present invention does not completely exclude the addition of components other than the above composition of the steel.

[0054] In addition, according to an embodiment, the cold-rolled steel sheet according to the present invention may further contain one or more of Cu at 0.1% or less (except 0%) and Ni at 0.1% or less (except 0%). The copper (Cu) and nickel (Ni) are elements that improve the strength of the steel. However, these elements are elements that improve the strength and hardenability of the steel. However, when too much of these elements is added, it may exceed the target strength grade. And since Cu and Ni are high-priced elements, from an economic perspective, it is preferable to limit the upper limits of the copper (Cu) and nickel (Ni) to 0.1% respectively. In addition, Cu and Ni act as solid solution strengthening elements. Therefore, when less than 0.03% of Cu and Ni are added, the solid solution strengthening effect may be negligible. Therefore, Cu and Ni can be added at 0.03% or more respectively.

[0055] In addition, according to an embodiment, the cold-rolled steel sheet according to the present invention may further contain V of 0.05% or less (excluding 0%). Even a trace amount of vanadium (V) can improve the strength of the steel, but the effect on improving the elongation is not significant. Therefore, it is preferable to control the content of V to 0.05% or less. The content of the V is more preferably 0.04% or less, and still more preferably 0.03% or less.

[0056] By area%, the microstructure of the cold-rolled steel sheet according to the present invention may be composed of ferrite of 15% or less (excluding 0%), fresh martensite of 5% or less (excluding 0%), tempered martensite of more than 75% and less than 95%, retained austenite of 12% or less (excluding 0%), and the balance of bainite.

[0057] The purpose of the cold-rolled steel sheet according to the present invention is to ensure bend formability at a tensile strength of 1470 MPa or more. In particular, in order to obtain high local formability, it is necessary to reduce the hardness difference between the microstructures constituting the steel sheet. When the fraction of the softest ferrite exceeds 15%, the yield strength decreases, and the hole expansion property and bend formability deteriorate. In addition, when the fraction of fresh martensite, which is brittle and has high strength without tempering, exceeds 5%, similarly, the formability deteriorates. Tempered Martensite is a structure obtained by tempering the martensite formed when the steel sheet is cooled below the Ms temperature through reheating or temperature holding process. It has high strength. When the fraction of tempered martensite is 75% or more, it is impossible to obtain a high strength of 1470 MPa or more and excellent bend formability. However, when the fraction of the tempered martensite is 95% or more, the entire microstructure has high strength, but the tempered martensite becomes a full martensite structure with insufficient ductility and the required formability cannot be obtained. Retained Austenite refers to the austenite remaining at room temperature after the entire manufacturing process of the steel sheet is completed. When it has sufficient stability, when additional deformation is applied to the steel, the retained austenite does not immediately transform into martensite, but gradually transforms into martensite according to the deformation level and contributes to the elongation and bend characteristics. If the retained austenite is sufficiently stable, the higher its content, the better. However, when the fraction of the retained austenite exceeds 12%, the stability decreases and the bend characteristics are significantly reduced.

[0058] The average grain size of the retained austenite may be 0.5 μm or less. When the average grain size of the retained austenite exceeds 0.5 μm, it may be difficult to ensure excellent bend characteristics. In addition, the retained austenite can be measured by EBSD. When analyzed by EBSD, the phase classified as FCC can be regarded as the retained austenite.

[0059] The cold-rolled steel sheet according to the present invention preferably satisfies the following relational expression 1.

[0060] [Relational expression 1] (RA 不稳定 + RA 稳定 ) / RA 稳定 ≤ 2.0

[0061] (wherein, in the relational expression 1, RA 不稳定 represents the fraction of unstable retained austenite, and RA 稳定 represents the fraction of stable retained austenite.)

[0062] The unstable retained austenite can be defined as the difference between the fraction of retained austenite before the tensile test and the fraction of retained austenite after the tensile test after measuring the fraction of retained austenite in the gripping portion of the tensile test piece before and after the tensile test. The stable retained austenite can be defined as the fraction of retained austenite after the tensile test. The pressure applied to the gripping portion during the tensile test can be 250 N. The fraction of retained austenite can be measured by XRD peak analysis. In order to obtain good bending properties, even when a small deformation is applied, compared with the unstable retained austenite that is easily transformed into martensite, even with a small deformation, the proportion of stable retained austenite that remains austenite must be relatively high.

[0063] The cold-rolled steel sheet according to the present invention provided as described above can satisfy the following relational expression 2.

[0064] [Relational expression 2] 4 × P.El - U.El ≥ 13.0

[0065] (wherein, in the relational expression 2, P.El represents the local elongation rate, and U.El represents the uniform elongation rate.)

[0066] In the tensile test of a known steel plate, the deformation is relatively uniform in the interval from the start of the tensile test to when the flow stress reaches the tensile strength (TS). However, after reaching the tensile strength (TS) corresponding to the maximum flow stress that the steel plate can withstand, the deformation non-uniformity between the fine microstructures inside the material increases, and finally, a process of void generation, aggregation, and macroscopic fracture occurs at the phase interface. The final fracture process of the material during the tensile test can be represented by the local elongation value, and the extreme surface area where fracture occurs during the bending test is also a process in which the material fractures due to high strain, which is highly correlated with the local elongation. Therefore, the local elongation in the tensile test and the bending characteristics are closely related. From this, the present inventor knows that when the phase fraction of the fine microstructure is controlled to reduce the hardness difference between phases and the alloy composition range is controlled by suppressing excessive alloy addition, the post-elongation will increase. In addition, it is known that when the fraction of retained austenite is high, the uniform elongation of the material will increase, but when the deformation reaches the tensile strength, the bending characteristics will deteriorate because part of the retained austenite transforms into high-carbon martensite. Based on these results, it is known that by restricting the local elongation to be relatively high and the uniform elongation to be relatively low as shown in the relational expression 2, the bending characteristics of the steel plate can be improved. When the value of the relational expression 2 is less than 13.0, the bending characteristics may deteriorate, and it may be difficult to obtain a bending formability (R 最小 / t) of less than 2.5. In addition, the total elongation (T.El) can be measured through a tensile test, and the total nominal strain until the test piece fractures is called the total elongation (T.El). The total elongation (T.El) can be divided into the uniform elongation (U.El) until the tensile strength is reached and the post-elongation (P.El) until final fracture thereafter.

[0067] The cold-rolled steel plate may have a tensile strength of 1470 MPa or more, a yield strength of 1000 MPa or more, a total elongation (T.El) of 8.5% or more, and a bending formability (R 最小 / t) of 2.5 or less. In the present invention, the higher the tensile strength, yield strength, and total elongation, the more preferable, and there is no limit to their upper limits, but it is difficult for the tensile strength to exceed 1650 MPa, the yield strength to exceed 1300 MPa, and the total elongation to exceed 16%. In addition, in the present invention, the lower the bending formability (R 最小 / t), the more preferable, and thus there is no limit to its lower limit. However, as an example, the lower limit of the bending formability (R 最小 / t) can be 0.8. In addition, in the bending formability (R 最小 / t), R 最小It represents the minimum punch curvature radius that does not produce cracks when a 90-degree V-bending test is performed using punches with various radii of curvature (R), and t represents the thickness (mm) of the steel plate. When performing bending forming using a 90-degree V-bending die, if an acute punch with a smaller R value can be used for bending, it can be considered that the bending characteristics of the material are better. Therefore, the smaller the minimum R value (R 最小 ) that does not produce cracks, the more excellent the bending characteristics of the material. Additionally, the R 最小 value is affected by the thickness of the material being tested itself. As the thickness increases, it is difficult for the R 最小 value to have a smaller value. Therefore, the bending characteristics are usually expressed by the ratio of the R 最小 value to the thickness t.

[0068] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to an embodiment of the present invention will be described.

[0069] First, a slab having the above alloy composition is heated. The slab heating temperature can be 1150 - 1250 °C. When the slab heating temperature is lower than 1150 °C, it may not be possible to perform the next hot rolling. On the other hand, when the reheating temperature of the slab exceeds 1250 °C, a large amount of energy is unnecessarily consumed to increase the temperature of the slab. Therefore, the slab heating temperature preferably has a range of 1150 - 1250 °C. Considering the temperature conditions of the hot rolling process, the lower limit of the slab heating temperature is more preferably 1170 °C, and further preferably 1180 °C. In terms of reducing costs and energy consumption, the upper limit of the slab heating temperature is more preferably 1230 °C, and further preferably 1220 °C.

[0070] After that, the heated slab is hot finish rolled at 830 - 950 °C to obtain a hot-rolled steel sheet. When the hot finish rolling temperature (hereinafter also referred to as "FDT") is lower than 830 °C, the rolling load is large and the shape defects increase, resulting in poor productivity. On the other hand, when the hot finish rolling temperature exceeds 950 °C, the surface quality deteriorates due to an increase in oxides caused by excessive high-temperature operation. Therefore, the hot finish rolling temperature preferably has a range of 830 - 950 °C. In terms of the above productivity, the lower limit of the hot finish rolling temperature is more preferably 850 °C, and further preferably 880 °C. In terms of ensuring the surface quality, the upper limit of the hot finish rolling temperature is more preferably 930 °C, and further preferably 910 °C.

[0071] Thereafter, the hot-rolled steel sheet is coiled at 400 - 650°C. When the coiling temperature (hereinafter also referred to as "CT") exceeds 650°C, there are drawbacks such as causing coarse internal oxidation of hot rolling and deteriorating surface characteristics. When the coiling temperature is lower than 400°C, it corresponds to transitional boiling, the controllability of the coiling temperature deteriorates, and there are drawbacks such as the shape of the steel sheet deteriorating. To prevent the above problems, the lower limit of the coiling temperature is more preferably 440°C, and further preferably 480°C. In addition, the upper limit of the coiling temperature is more preferably 610°C, and further preferably 570°C.

[0072] Thereafter, the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. During the cold rolling, the cold reduction rate can be 30 - 60%. When the cold reduction rate is less than 30%, it is not only difficult to ensure the accuracy of the desired thickness, but also may be difficult to correct the shape of the steel sheet. On the other hand, when the cold reduction rate exceeds 60%, the possibility of cracks occurring in the edge part of the steel sheet increases, and the cold rolling load may become too large. Therefore, the cold reduction rate can have a range of 30 - 60%.

[0073] Thereafter, the cold-rolled steel sheet is continuously annealed at 800 - 950°C. When the continuous annealing temperature (hereinafter also referred to as "SS") is lower than 800°C, sufficient austenite reverse transformation cannot occur, and a ferrite phase with a level exceeding 15% may be formed after annealing. On the other hand, when the continuous annealing temperature exceeds 950°C, the surface quality and productivity decrease, and coarse austenite is formed, so the material properties may deteriorate. The lower limit of the continuous annealing temperature is more preferably 820°C, and further preferably 840°C. The upper limit of the continuous annealing temperature is more preferably 930°C, and further preferably 900°C.

[0074] Thereafter, the continuously annealed cold-rolled steel sheet is cooled once at an average cooling rate of 1°C / second or more and less than 10°C / second to a first cooling termination temperature of 500 - 700°C (hereinafter also referred to as "SCS"). The first cooling termination temperature can be defined as the time point at which secondary cooling (rapid cooling) is started by further applying a rapid cooling device not applied during the first cooling. When the cooling process is divided into first and second coolings and carried out in stages, the temperature distribution of the steel sheet is uniform during the slow cooling step, so that the final temperature and material deviation can be reduced. When the first cooling termination temperature is lower than 500°C, there is a risk of inducing soft bainite transformation, and it is difficult to cool to below 500°C at a cooling rate of less than 10°C / second due to the limitation of the actual equipment length. When the first cooling termination temperature exceeds 700°C, the cooling amount until the second cooling termination temperature increases, and the shape of the steel sheet becomes poor. In addition, when the first cooling rate is less than 1°C / second, a ferrite phase is formed during the cooling process, and it is difficult to obtain high-strength steel. When the first cooling rate is 10°C / second or more, the cooling amount during the second cooling increases, and the final temperature deviation and material deviation increase. Therefore, the first cooling rate preferably has a range of 1°C / second or more and less than 10°C / second. The lower limit of the first cooling rate is more preferably 2°C / second, and even more preferably 3°C / second. The upper limit of the first cooling rate is more preferably 7°C / second, and even more preferably 5°C / second.

[0075] Thereafter, the cold-rolled steel sheet cooled in the first cooling is cooled secondarily at an average cooling rate of 10°C / second or more to a second cooling termination temperature of 25 - 300°C (hereinafter also referred to as "RCS"). The second cooling termination temperature is to make the temperature of the steel sheet below Ms so that martensite transformation occurs during the cooling process, and the martensite finally becomes tempered martensite while passing through a re-heating step as a subsequent process. It is difficult to cool the second cooling termination temperature to below 25°C, which is below normal temperature. In addition, when the second cooling termination temperature exceeds 300°C, martensite cannot be sufficiently generated during the cooling process, and it is difficult to obtain sufficient yield strength, tensile strength, and bend formability. When the second cooling rate is less than 10°C / second, even if the target second cooling termination temperature is reached, high-temperature phases such as upper bainite are mixed during the cooling process, so that the target tempered martensite fraction and high strength cannot be obtained. In addition, in the present invention, the faster the second cooling rate, the more advantageous it is, so the upper limit thereof is not particularly limited, but due to equipment limitations, it is difficult to exceed 100°C / second.

[0076] Thereafter, the cold-rolled steel sheet after the secondary cooling is heated to 150 - 450°C and held for a time exceeding 500 seconds and less than 1500 seconds. Through the above process, the interphase carbon distribution required for the stabilization of retained austenite and additional phase transformations are obtained. In the present invention, the end temperature of the heating range is conveniently referred to as the reheat temperature (hereinafter also referred to as "RHS"), and the end temperature of the holding range is conveniently referred to as the overaging temperature (hereinafter also referred to as "OAS"). When the RHS or OAS temperature is lower than 150°C, the strength is too high and the formability deteriorates. On the other hand, when the RHS or OAS temperature exceeds 450°C, it is difficult to obtain the high strength expected in the present invention. When the holding time in the holding range is less than 500 seconds, the total amount of phase transformation at the end of the holding step is insufficient, the fraction of retained austenite increases, and a large amount of fresh martensite is generated, so the bending characteristics may deteriorate. In addition, the value of the relational expression 1 may exceed 2.0. The lower limit of the holding time is more preferably 600 seconds, and further preferably 700 seconds. The upper limit of the holding time is more preferably 1300 seconds, and further preferably 1000 seconds. In addition, the holding time in the holding range refers to the time required from the end of the heating range to the end of the holding range. Detailed Description of the Invention

[0077] 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 for more specific description 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 recorded in the claims and the content reasonably deduced therefrom.

[0078] (Examples)

[0079] Slabs having the alloy compositions described in Tables 1 and 2 below were subjected to slab heating, hot rolling, coiling, cold rolling, annealing, primary cooling, secondary cooling, reheat, and holding processes under the conditions described in Tables 3 and 4 below to manufacture cold-rolled steel sheets. In addition, the conditions described in Tables 3 and 4 below are based on the surface temperature of the steel sheet.

[0080] The microstructure and mechanical and physical properties of the cold-rolled steel sheets manufactured as described above were measured, and the results are shown in Tables 5 and 6 below.

[0081] The microstructure was measured from the photographs observed by a scanning electron microscope (SEM) by the point counting method. In particular, the fraction of retained austenite was measured by XRD. The average grain size of retained austenite was measured by EBSD analysis.

[0082] The tensile strength (TS), yield strength (YS), and total elongation (T.El) are measured by conducting a tensile test on a test piece cut in a direction perpendicular to the rolling direction of the cold-rolled steel sheet. In addition, the total elongation (T.El) is divided into the uniform elongation (U.El) until the tensile strength is reached and the local elongation (P.El) until final fracture thereafter. During the tensile test, a No. 5 tensile test piece conforming to KS B0801 standard is used, the gauge length is 50 mm, and the width of the tensile test section is 25 mm. During the tensile test, the elongation is measured by the so-called nominal strain obtained by dividing the elongation amount of the test piece by the initial gauge length (50 mm), and the total nominal strain until the fracture of the test piece occurs is called the total elongation (T.El). The strength is measured by the so-called nominal stress obtained by dividing the load measured in the tensile test by the initial cross-section of the test piece, and the stress value when this nominal stress reaches the maximum is called the tensile strength (TS). Among them, the uniform elongation (U.El) is the nominal strain value when the nominal stress reaches the tensile strength (TS), and the local elongation (P.El) is the nominal strain from when the tensile strength is reached until final fracture occurs. That is, the total elongation is the sum of the uniform elongation and the local elongation.

[0083] The bending property is obtained by observing the surface of the cold-rolled steel sheet with a magnifying glass of 1000 times magnification after conducting a 90-degree V-bending test on the cold-rolled steel sheet to obtain the minimum R 最小 / t value. At this time, the 90-degree V-bending test method is used, in which, after preparing punches with various different radii (R, Radius) of 90 degrees, a steel sheet with a certain thickness (t) is placed between V-shaped dies (Die) with an angle of 90 degrees, and the punch is brought into close contact with the die.

[0084] [Table 1]

[0085]

[0086] [Table 2]

[0087]

[0088] [Table 3]

[0089]

[0090] [Table 4]

[0091]

[0092] [Table 5]

[0093]

[0094] [Table 6]

[0095]

[0096] As can be seen from Table 1 to Table 6 above, among Invention Examples 1 to 8 that satisfy the alloy composition and manufacturing conditions proposed by the present invention, excellent mechanical and physical properties can be ensured by ensuring the fine microstructure expected to be obtained by the present invention.

[0097] In the cases of Comparative Example 1, Comparative Example 2, and Comparative Example 5, it can be seen that the manufacturing conditions proposed by the present invention are satisfied, but since the alloy composition is not satisfied, the fine microstructure expected to be obtained by the present invention cannot be ensured, and thus the mechanical and physical properties are poor.

[0098] In the cases of Comparative Example 3 and Comparative Example 4, it can be seen that the alloy composition proposed by the present invention is satisfied, but since the manufacturing conditions are not satisfied, the fine microstructure expected to be obtained by the present invention cannot be ensured, and thus the mechanical and physical properties are poor.

Claims

1. A cold-rolled steel sheet, by weight %, the cold-rolled steel sheet comprises: C: not less than 0.15% and less than 0.30%, Si: 0.8 - 2.5%, Al: 0.005 - 0.3%, Mn: 2.0 - 3.0%, Cr: 0.001 - 0.5%, Mo: 0.001 - 0.2%, B: 0.0005 - 0.0050%, Nb: 0.001 - 0.05%, Ti: 0.001 - 0.05%, P: not more than 0.04% and excluding 0%, S: not more than 0.01% and excluding 0%, N: not more than 0.01% and excluding 0%, the balance being Fe and other inevitable impurities, by area %, the microstructure consists of ferrite of not more than 15% and excluding 0%, fresh martensite of not more than 5% and excluding 0%, tempered martensite of not less than 75% and less than 95%, retained austenite of not more than 12% and excluding 0%, and the balance being bainite, the cold-rolled steel sheet satisfies the following relational expression 1, [Relationship 1](RA 不稳定 +RA 稳定 ) / RA 稳定 ≤2.0 Among them, In the relation 1, RA 不稳定 represents the fraction of unstable retained austenite, and RA 稳定 represents the fraction of stable retained austenite.

2. The cold-rolled steel sheet according to claim 1, wherein, the cold-rolled steel sheet further comprises one or more of Cu of not more than 0.1% and excluding 0% and Ni of not more than 0.1% and excluding 0%.

3. The cold-rolled steel sheet according to claim 1, wherein, the cold-rolled steel sheet further comprises V of not more than 0.05% and excluding 0%.

4. The cold-rolled steel sheet according to claim 1, wherein, the average grain size of the retained austenite is not more than 0.5 μm.

5. The cold-rolled steel sheet according to claim 1, wherein, the cold-rolled steel sheet satisfies the following relational expression 2, [Relational expression 2] 4×P.El - U.El ≥ 13.0 wherein, in the relational expression 2, P.El represents the local elongation and U.El represents the uniform elongation.

6. The cold-rolled steel sheet according to claim 1, wherein, The cold-rolled steel sheet has a tensile strength of 1470 MPa or more, a yield strength of 1000 MPa or more, a total elongation (T.El) of 8.5% or more, and a bend formability (R 最小 / t) of 2.5 or less.

7. A method for manufacturing a cold-rolled steel sheet, which comprises the following steps: heating a slab, by weight %, the slab comprises: C: not less than 0.15% and less than 0.30%, Si: 0.8 - 2.5%, Al: 0.005 - 0.3%, Mn: 2.0 - 3.0%, Cr: 0.001 - 0.5%, Mo: 0.001 - 0.2%, B: 0.0005 - 0.0050%, Nb: 0.001 - 0.05%, Ti: 0.001 - 0.05%, P: not more than 0.04% and excluding 0%, S: not more than 0.01% and excluding 0%, N: not more than 0.01% and excluding 0%, the balance being Fe and other inevitable impurities; hot finish rolling the heated slab at 830 - 950 °C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 400 - 650 °C; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet at 800 - 950 °C; performing a first cooling on the continuously annealed cold-rolled steel sheet at an average cooling rate of not less than 1 °C / second and less than 10 °C / second until a first cooling termination temperature of 500 - 700 °C; performing a second cooling on the cold-rolled steel sheet after the first cooling at an average cooling rate of not less than 10 °C / second until a second cooling termination temperature of 25 - 300 °C; and reheating the cold-rolled steel sheet after the second cooling at 150 - 450 °C and holding for more than 500 seconds and not more than 1500 seconds.

8. The manufacturing method of the cold-rolled steel sheet according to claim 7, wherein, The slab further contains one or more of Cu of 0.1% or less, excluding 0%, and Ni of 0.1% or less, excluding 0%.

9. The manufacturing method of the cold-rolled steel sheet according to claim 7, wherein, The slab further contains V of 0.05% or less, excluding 0%.

10. The manufacturing method of the cold-rolled steel sheet according to claim 7, wherein, The heating temperature of the slab is 1150 - 1250 °C.

11. The manufacturing method of the cold-rolled steel sheet according to claim 7, wherein, The cold rolling reduction ratio during cold rolling is 30 - 60%.

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