Cold-rolled steel sheet and its manufacturing method

Abstract

The present invention relates to a cold-rolled steel sheet and a method for producing the same. An object of one embodiment 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.

Description

[Technical Field]

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

[0002] The development of high-strength steel sheets continues to advance in order to reduce the weight and improve safety of automobiles. Recently, ultra-high-strength steel sheets with tensile strengths of 1500 MPa or higher have become increasingly important for improving the driving range and battery protection of electric vehicles. However, existing MART steels lack formability, so the development of ultra-high-strength steel sheets for cold forming that also possess formability is expected to have significant economic value. To improve the formability of steel, a widely used method is to introduce retained austenite and utilize the TRIPS (transformation induced plasticity) phenomenon to increase elongation. However, in the case of TRIP steel sheets, the addition of Si and Al is required to introduce retained austenite, and when bainite transformation occurs, even more retained austenite can be obtained. However, because bainite transforms at a relatively high temperature, its tensile strength (TS) is low and its yield strength (YS) is also low for use as ultra-high-strength steel.

[0003] Therefore, the recent trend is to use the Quenching and Partitioning process to utilize the TRIP phenomenon while increasing the strength of steel sheets. In the case of so-called Q&P steel, the main structure of the matrix is ​​tempered martensite, which has excellent yield strength and hole expansion ratio (HER). When retained austenite is actively formed, an appropriate level of elongation can also be achieved.

[0004] Meanwhile, the importance of automobile safety is increasing daily from the perspective of passenger protection. While improvements in autonomous driving technology are expected to fundamentally eliminate the risk of automobile collisions, regulations on automobile crash tests have recently been tightened due to concerns that passengers may not be able to recognize situations that lead to an accident during the transitional period until the technology matures, which could actually increase the severity of an accident if one does occur. It is known that the bending properties of steel are important for reducing the risk of cracks occurring in automobile structural components during a collision. When steel has excellent bending properties, it can absorb more of the collision energy by folding instead of cracking, allowing the remaining parts to withstand the impact and induce the collapse of a stable structure.

[0005] However, simply applying the Q&P process is not enough to obtain excellent bending formability, and further ideas are needed to ensure excellent bending formability along with high strength of 1500 MPa or more. Summary of the Invention [Problem to be solved by the invention]

[0006] An object of one embodiment of the present invention is to provide a cold-rolled steel sheet and a manufacturing method thereof.

[0007] An object of a preferred embodiment 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. [Means for solving the problem]

[0008] One embodiment of the present invention is a ferroelectric material containing, by weight, C: 0.15% or more and less than 0.30%, Si: 0.8 to 2.5%, Al: 0.005 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.2%, B: 0.0005 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), %, excluding ferrite), N: 0.01% or less (excluding 0%), the balance being Fe and other unavoidable impurities, and the microstructure is composed, in area %, of ferrite: 15% or less (excluding 0%), fresh martensite: 5% or less (excluding 0%), tempered martensite: 75% or more but less than 95%, retained austenite: 12% or less (excluding 0%), the balance being bainite, and the cold-rolled steel sheet satisfies the following relational expression 1:

[0009] [Equation 1] (RA unstable +RA stable ) / RA stabl e≦2.0 (However, in the above relational expression 1, RA unstable means the fraction of unstable retained austenite, and RA stable means the fraction of stable retained austenite.)

[0010] The cold-rolled steel sheet may further contain one or more of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%).

[0011] The cold-rolled steel sheet may further contain V: 0.05% or less (excluding 0%).

[0012] The retained austenite may have an average grain size of 0.5 μm or less.

[0013] The cold-rolled steel sheet can satisfy the following relational expression 2.

[0014] [Equation 2] 4×P.El-U.El≧13.0 (However, in the above relational expression 2, P.El means local elongation, and U.El means uniform elongation.)

[0015] 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 bending formability (R min / t).

[0016] One embodiment of the present invention relates to a method for producing a hot-rolled steel sheet, comprising the steps of: heating a slab containing, by weight, C: 0.15% or more but less than 0.30%, Si: 0.8 to 2.5%, Al: 0.005 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.2%, B: 0.0005 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance being Fe and other inevitable impurities; and finish hot-rolling the heated slab at 830 to 950°C to obtain a hot-rolled steel sheet. The present invention provides a method for manufacturing a cold-rolled steel sheet, the method including: coiling the hot-rolled steel sheet at 400 to 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 to 950°C; primarily cooling the continuously annealed cold-rolled steel sheet to a primary cooling end temperature of 500 to 700°C at an average cooling rate of 1°C / s or more and less than 10°C / s; secondarily cooling the primarily cooled cold-rolled steel sheet to a secondary cooling end temperature of 25 to 300°C at an average cooling rate of 10°C / s or more; and reheating the secondarily cooled cold-rolled steel sheet to 150 to 450°C and holding the reheated cold-rolled steel sheet for more than 500 seconds and not more than 1,500 seconds.

[0017] The slab may further contain one or more of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%).

[0018] The slab may further contain V: 0.05% or less (excluding 0%).

[0019] The heating temperature of the slab may be 1150 to 1250°C.

[0020] The cold rolling reduction rate during the cold rolling may be 30 to 60%. [Effects of the Invention]

[0021] According to one embodiment of the present invention, a cold-rolled steel sheet and a manufacturing method thereof can be provided.

[0022] According to a preferred embodiment of the present invention, it is possible to provide a cold-rolled steel sheet having excellent strength and bend formability, and a method for manufacturing the same. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, a cold-rolled steel sheet according to one embodiment of the present invention will be described. First, the alloy composition will be described. The contents of the alloy compositions described below are in terms of weight percent unless otherwise specified.

[0024] C: 0.15% or more and less than 0.30% Carbon (C) is an element that ensures the strength of steel materials through solid solution strengthening and precipitation strengthening. If the C content is less than 0.15%, it is difficult to ensure a tensile strength (TS) of 1.5 GPa. On the other hand, if the C content is 0.30% or more, arc weldability and laser weldability deteriorate, and the risk of cracking due to the formation of coarse carbides increases. Therefore, the C content is preferably in the range of 0.15% or more and less than 0.30%. From the viewpoint of ensuring strength, the lower limit of the C content is more preferably 0.17%. Furthermore, from the viewpoints of weldability and the possibility of embrittlement due to coarse carbides, the upper limit of the C content is more preferably 0.28%.

[0025] Si: 0.8 to 2.5% Silicon (Si) is a key element in TRIP (Transformation Induced Plasticity) steel, inhibiting cementite precipitation and thereby increasing the fraction of retained austenite and elongation. If the Si content is less than 0.8%, little retained austenite remains, resulting in excessively low elongation. On the other hand, if the Si content exceeds 2.5%, deterioration of the physical properties of the weld due to LME cracking cannot be prevented, and the surface characteristics and platability of the steel deteriorate. Therefore, the Si content is preferably in the range of 0.8 to 2.5%. Taking elongation into consideration, the lower limit of the Si content is more preferably 0.9%, even more preferably 1.0%, and may be controlled to 1.1% depending on the elongation required for the applicable product. The upper limit of the Si content is more preferably 2.2%, even more preferably 2.0%, and may be controlled to 1.7% taking into account the intended use of the applicable product, the plating method, etc.

[0026] Al: 0.005 to 0.3% Aluminum (Al) is an element contained not only for the deoxidation of steel but also for the suppression of cementite precipitation and the stabilization of retained austenite. If the Al content is less than 0.005%, the deoxidation of the steel is insufficient, impairing the cleanliness of the steel. On the other hand, if the Al content exceeds 0.3%, the castability of the steel is impaired. Therefore, the Al content is preferably in the range of 0.005 to 0.3%. From the viewpoint of the cleanliness of the steel, the lower limit of the Al content is more preferably 0.01%, and even more preferably 0.02%, and may be controlled to 0.03% in consideration of the requirements of the product or the user. In consideration of the castability of the steel, the upper limit of the Al content is more preferably 0.27%, and even more preferably 0.22%, and may be controlled to 0.17% in consideration of the product or the processing environment of the user.

[0027] Mn: 2.0 to 3.0% Manganese (Mn) is an element added to ensure strength. If the Mn content is less than 2.0%, it becomes difficult to ensure strength. On the other hand, if the Mn content exceeds 3.0%, the phase transformation rate slows down and excessive fresh martensite is formed, making it difficult to obtain excellent formability. Furthermore, Mn segregation forms a band structure, impairing the material uniformity and formability of the material. Therefore, the Mn content is preferably in the range of 2.0 to 3.0%. In consideration of the target strength of the steel material, the lower limit of the Mn content is more preferably 2.2%. In consideration of formability, the upper limit of the Mn content is more preferably 2.8%.

[0028] Cr: 0.001 to 0.5% Chromium (Cr) is an element added to ensure strength and hardenability. Adding Mn alone would require adding a very large amount of Mn, exceeding the Mn content range of the present invention. However, adding 0.001% or more of Cr can solve this problem. If the Cr content exceeds 0.5%, local corrosion resistance deteriorates and oxides form on the surface, impairing phosphate treatability. Therefore, the Cr content is preferably in the range of 0.001 to 0.5%. Depending on the properties of the product to be used, taking into account corrosion resistance and phosphate treatability, the upper limit of the Cr content may be controlled to 0.3%.

[0029] Mo: 0.001 to 0.2% Molybdenum (Mo) is added to ensure strength and hardenability. To achieve these strength and hardenability improving effects, the Mo content must be 0.001% or more. However, Mo is an expensive element, which can reduce the economic viability of the steel sheet and can excessively delay phase transformation, potentially inducing the formation of fresh martensite. Therefore, the Mo content is preferably not more than 0.2%. From the above-mentioned cost perspective, the upper limit of the Mo content is more preferably 0.15%, and may be controlled to 0.12%.

[0030] B: 0.0005 to 0.0050% Boron (B) is an element added to ensure hardenability. When Mn is added alone, a very large amount of Mn must be added, exceeding the Mn content range of the present invention. However, adding 0.0005% or more of B can solve this problem. However, if the B content exceeds 0.0050%, boron-based carbides are formed at the grain boundaries, which actually impairs hardenability. Therefore, the B content is preferably in the range of 0.0005 to 0.0050%. From the viewpoint of hardenability, the upper limit of the B content is more preferably 0.0040%, more preferably 0.0030%, and may be controlled to 0.0025%.

[0031] Nb: 0.001 to 0.05% Niobium (Nb) is an element added to ensure the strength of steel sheets and refine the structure. If the Nb content is less than 0.001%, it is difficult to obtain the effects of improving strength and refining the structure. If the Nb content exceeds 0.05%, localized grain fixation will delay recrystallization and impair the uniformity of the structure. Therefore, the Nb content is preferably in the range of 0.001 to 0.05%. From the viewpoint of uniformity, the upper limit of the Nb content is more preferably 0.03%.

[0032] Ti: 0.001 to 0.05% Titanium (Ti) is an element added to ensure the strength of steel sheets and refine the structure. If Ti is added in an amount less than 0.001%, it is difficult to obtain the effects of improving strength and refining the structure. If the Ti content exceeds 0.05%, castability is impaired due to the excessive formation of TiN, and the impact properties of the steel material are impaired due to the excessive formation of carbides. Therefore, the Ti content is preferably in the range of 0.001 to 0.05%. From the viewpoint of improving impact properties, the upper limit of the Ti content is more preferably 0.03%.

[0033] P: 0.04% or less (excluding 0%) Phosphorus (P) exists as an impurity in steel, and while it is advantageous to control its content as low as possible, it is sometimes intentionally added to increase the strength of steel. However, if excessive P is added, the toughness of the steel deteriorates, so in the present invention, to prevent this, it is preferable to limit the upper limit to 0.04%. From the viewpoint of improving toughness, the P content is more preferably 0.015% or less, and even more preferably 0.010% or less. When higher toughness than the standard is required for the steel, the P content may be controlled to 0.007% or less.

[0034] S: 0.01% or less (excluding 0%) Like P, sulfur (S) is present in steel as an impurity, and it is advantageous to control its content as low as possible. Furthermore, because S deteriorates the ductility and impact properties of steel, it is preferable to limit its upper limit to 0.01%. The S content is more preferably 0.005% or less, even more preferably 0.003% or less, and most preferably 0.0015% or less.

[0035] N: 0.01% or less (excluding 0%) In the present invention, nitrogen (N) is an impurity that is inevitably contained in the steel material during the manufacturing process, and its upper limit is preferably limited to 0.01%. The N content is more preferably 0.007% or less, even more preferably 0.005% or less, and most preferably 0.003% or less.

[0036] In addition to the above-mentioned steel composition, the remainder may include Fe and inevitable impurities. Since inevitable impurities may be unintentionally mixed in during the normal steel manufacturing process, they cannot be completely eliminated, and this meaning is easily understood by engineers in the field of normal steel manufacturing. However, the present invention does not completely exclude the addition of other components than the above-mentioned steel composition.

[0037] Meanwhile, the cold-rolled steel sheet according to the present invention may further contain one or more of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%), depending on the embodiment. Copper (Cu) and nickel (Ni) are elements that increase the strength of the steel. However, although these elements increase the strength and hardenability of the steel, if added in excessive amounts, the target strength grade may be exceeded. Since these elements are expensive elements, it is preferable to limit the upper limit of each to 0.1% from an economical perspective. Meanwhile, since Cu and Ni act as solid-solution strengthening elements, if added in amounts less than 0.03%, the solid-solution strengthening effect may be insignificant. Therefore, each may be added in an amount of 0.03% or more.

[0038] Furthermore, the cold-rolled steel sheet according to the present invention may further contain V: 0.05% or less (excluding 0%) depending on the embodiment. Although vanadium (V) can increase the strength of steel material even with a small amount of addition, its effect on improving elongation is not significant, so its content is preferably controlled to 0.05% or less. The V content is more preferably 0.04% or less, and even more preferably 0.03% or less.

[0039] The cold-rolled steel sheet according to the present invention may have a microstructure consisting of, in area percentages, ferrite: 15% or less (excluding 0%), fresh martensite: 5% or less (excluding 0%), tempered martensite: 75% or more but less than 95%, retained austenite: 12% or less (excluding 0%), and the remainder bainite.

[0040] The cold-rolled steel sheet according to the present invention aims to ensure excellent bending formability with a tensile strength of 1470 MPa or more. To achieve particularly high local formability, it is necessary to reduce the difference in hardness between the microstructural phases constituting the steel sheet. If the fraction of ferrite, the softest phase, exceeds 15%, the yield strength decreases, resulting in poor hole expandability and bend formability. Similarly, if tempering is not performed and the proportion of brittle, high-strength fresh martensite exceeds 5%, formability also deteriorates. Tempered martensite is a structure obtained by tempering martensite formed by cooling a steel sheet below its Ms temperature through a reheating or temperature holding process. It has high strength, and when its fraction is 75% or more, high strength of 1470 MPa or more and excellent bending formability can be achieved. However, if the fraction of tempered martensite is 95% or more, the entire microstructure becomes a full martensite structure, which has high strength but poor ductility, and the required formability cannot be obtained. Retained austenite refers to the austenite that remains at room temperature after all manufacturing processes for steel sheets are completed. If it has sufficient stability, it does not immediately transform into martensite when further deformation is applied to the steel, but gradually transforms into martensite depending on the level of deformation, contributing to elongation and bending properties. If retained austenite is sufficiently stable, a higher fraction is better, but if its fraction exceeds 12%, stability may decrease and bending properties may be significantly reduced.

[0041] The retained austenite may have an average grain size of 0.5 μm or less. If the average grain size of the retained austenite exceeds 0.5 μm, it may be difficult to ensure excellent bending properties. Meanwhile, the retained austenite can be measured using EBSD, and a phase classified as FCC when analyzed using EBSD can be considered to be the retained austenite.

[0042] The cold-rolled steel sheet according to the present invention preferably satisfies the following relational expression 1.

[0043] [Equation 1] (RA unstable +RA stable ) / RA stable ≦2.0 (However, in the above relational expression 1, RA unstable means the fraction of unstable retained austenite, and RA stable means the fraction of stable retained austenite.)

[0044] The unstable retained austenite can be defined as the difference between the fraction of retained austenite before and after the tensile test, measured by measuring the fraction of retained austenite in the grip portion of the tensile test specimen before and after the tensile test. The stable retained austenite can also be defined as the fraction of retained austenite after the tensile test. The pressure applied to the grip portion during the tensile test may be 250 N. The fraction of retained austenite can be measured by XRD peak analysis. To achieve good bending properties, the proportion of stable retained austenite, which remains in austenite even during small deformations, must be higher than the unstable retained austenite, which easily transforms into martensite when small deformations are applied.

[0045] The cold rolled steel sheet according to the present invention provided as described above can satisfy the following relational expression 2.

[0046] [Equation 2] 4×P.El-U.El≧13.0 (However, in the above relational expression 2, P.El means local elongation, and U.El means uniform elongation.)

[0047] In tensile tests of steel sheets, the material deforms relatively uniformly from the start of the test until the flow stress reaches the tensile strength (TS). However, once the tensile strength (TS), which corresponds to the maximum flow stress the steel sheet can withstand, increases, the deformation heterogeneity between the microstructures within the material increases, ultimately resulting in the formation and coalescence of voids at phase interfaces and macroscopic fracture. In tensile tests, the final fracture process of a material can be represented by the local elongation value. However, the extreme surface where fracture occurs during bending tests also fractures due to high deformation rates, and this is closely related to the local elongation. Therefore, the local elongation in tensile tests is closely related to bending properties. Based on this, the inventors discovered that the local elongation (post-elongation) increases when the hardness difference between phases is reduced by controlling the phase fraction of the microstructure and the alloy composition range is controlled by suppressing excessive alloy addition. On the other hand, when the fraction of retained austenite is high, the uniform elongation of the material increases, but when the material is deformed to the extent that it reaches tensile strength, some of the retained austenite transforms into high-carbon martensite, resulting in a deterioration in bending properties. From these results, it was confirmed that the bending properties of steel sheets can be improved by limiting the local elongation to a high value and the uniform elongation to a relatively low value, as in the above relational expression 2. When the value of the above relational expression 2 is smaller than 13.0, bending properties may deteriorate, and a bending formability (R) of 2.5 or less may be observed. min / t) may be difficult to achieve. On the other hand, total elongation (T.El) can be measured by tensile testing, and the total nominal deformation rate until the test piece breaks is called total elongation (T.El). The total elongation (T.El) can be divided into uniform elongation (U.El) until the tensile strength is reached and local elongation (P.El) thereafter until final break.

[0048] 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 bending formability (R minIn the present invention, the higher the tensile strength, yield strength and total elongation, the better, and therefore there are no upper limits to these values. However, the tensile strength is unlikely to exceed 1650 MPa, the yield strength is unlikely to exceed 1300 MPa, and the total elongation is unlikely to exceed 16%. In addition, in the present invention, the bending formability (R min The lower the bending formability (R min The lower limit of the bending formability (R / t) may be 0.8. min / t), R min means the minimum punch radius of curvature at which cracks do not occur when a 90-degree V-bending test is performed using punches with various radiuses of curvature (R), and t means the thickness of the steel plate (mm). When bending is performed using a 90-degree V-bending die, if the bending is possible with an acute-angled punch with a small R value, the bending properties of the material can be considered to be better, and therefore, the minimum R value at which cracks do not occur (R min The smaller the R, the better the bending properties of the material. min The value is affected by the thickness of the material being tested, with the thicker the material, the smaller the R min Because it is difficult to have a value for R min The bending properties are represented by the ratio of the value and thickness t.

[0049] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0050] First, a slab having the aforementioned alloy composition is heated. The slab heating temperature may be 1150 to 1250°C. If the slab heating temperature is less than 1150°C, it may be impossible to perform the next step, hot rolling. On the other hand, if the slab heating temperature exceeds 1250°C, more energy than necessary is consumed to increase the slab temperature. Therefore, the slab heating temperature is preferably in the range of 1150 to 1250°C. In consideration of the temperature conditions of the hot rolling process, the lower limit of the slab heating temperature is more preferably 1170°C, and even more preferably 1180°C. From the viewpoints of reducing costs and energy consumption, the upper limit of the slab heating temperature is more preferably 1230°C, and even more preferably 1220°C.

[0051] The heated slab is then finish hot rolled at 830 to 950°C to obtain a hot-rolled steel sheet. If the finish hot rolling temperature (hereinafter also referred to as "FDT") is less than 830°C, the rolling load is large, resulting in an increase in shape defects and reduced productivity. On the other hand, if the finish hot rolling temperature exceeds 950°C, the surface quality deteriorates due to an increase in oxides caused by excessively high-temperature operation. Therefore, the finish hot rolling temperature is preferably in the range of 830 to 950°C. From the viewpoint of the productivity described above, the lower limit of the finish hot rolling temperature is more preferably 850°C, and even more preferably 880°C. From the viewpoint of ensuring surface quality, the upper limit of the finish hot rolling temperature is more preferably 930°C, and even more preferably 910°C.

[0052] Thereafter, the hot-rolled steel sheet is coiled at 400 to 650°C. If the coiling temperature (hereinafter also referred to as "CT") exceeds 650°C, there is a drawback in that coarse internal oxidation occurs during hot rolling, resulting in poor surface properties, and if the coiling temperature is less than 400°C, there is a drawback in that the coiling temperature falls within the transition boiling region, making it difficult to control the coiling temperature and causing deterioration in the shape of the steel sheet. In order to prevent the above-mentioned problems, the lower limit of the coiling temperature is more preferably 440°C, and even more preferably 480°C. Furthermore, the upper limit of the coiling temperature is more preferably 610°C, and even more preferably 570°C.

[0053] The coiled hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. The cold rolling reduction may be 30 to 60%. If the cold rolling reduction is less than 30%, it may be difficult to ensure the target thickness accuracy and it may also be difficult to correct the shape of the steel sheet. On the other hand, if the cold rolling reduction exceeds 60%, there is a high possibility that cracks may occur at the edge of the steel sheet, and the cold rolling load may become excessively large. Therefore, the cold rolling reduction may be in the range of 30 to 60%.

[0054] The cold-rolled steel sheet is then continuously annealed at 800 to 950°C. If the continuous annealing temperature (hereinafter also referred to as "SS") is less than 800°C, sufficient austenite reverse transformation may not occur, and the ferrite phase after annealing may be formed at a level exceeding 15%. On the other hand, if the continuous annealing temperature exceeds 950°C, the surface quality and productivity may decrease, and coarse austenite may be formed, resulting in deterioration of the material. The lower limit of the continuous annealing temperature is more preferably 820°C, and even more preferably 840°C. The upper limit of the continuous annealing temperature is more preferably 930°C, and even more preferably 900°C.

[0055] The continuously annealed cold-rolled steel sheet is then subjected to primary cooling at an average cooling rate of 1°C / s or more but less than 10°C / s to a primary cooling end temperature (hereinafter also referred to as "SCS") of 500 to 700°C. The primary cooling end temperature can be defined as the point at which secondary cooling (quenching) begins, when a quenching device not used in the primary cooling is added. When the cooling process is performed in stages, dividing it into primary and secondary cooling, the temperature distribution of the steel sheet can be uniform during the slow cooling stage, thereby reducing variations in final temperature and material properties. If the primary cooling end temperature is less than 500°C, soft bainite transformation may be induced, and it is difficult to cool the steel sheet to below 500°C at a cooling rate of less than 10°C / s due to the actual equipment length. If the primary cooling end temperature exceeds 700°C, the cooling amount until the secondary cooling end temperature becomes large, resulting in poor steel sheet shape. On the other hand, if the primary cooling rate is less than 1°C / s, a ferrite phase will form during cooling, making it difficult to obtain high-strength steel, and if it is 10°C / s or more, the cooling amount in the secondary cooling will be large, resulting in increased final temperature deviation and material variation. Therefore, the primary cooling rate is preferably in the range of 1°C / s or more and less than 10°C / s. The lower limit of the primary cooling rate is more preferably 2°C / s, and more preferably 3°C / s. The upper limit of the primary cooling rate is more preferably 7°C / s, and more preferably 5°C / s.

[0056] The cold-rolled steel sheet that has undergone primary cooling is then subjected to secondary cooling at an average cooling rate of 10°C / s or more to a secondary cooling finish temperature (hereinafter also referred to as "RCS") of 25 to 300°C. The secondary cooling finish temperature is set to Ms or lower to allow martensitic transformation to occur during cooling. This martensite eventually becomes tempered martensite after a subsequent reheating step. It is difficult to cool the secondary cooling finish temperature to 25°C or lower, which is lower than room temperature. On the other hand, if the secondary cooling finish temperature exceeds 300°C, sufficient martensite is not formed during cooling, making it difficult to obtain sufficient yield strength, tensile strength, and bend formability. If the secondary cooling rate is less than 10°C / s, even if the target secondary cooling finish temperature is reached, high-temperature phases such as upper bainite are mixed in during cooling, making it difficult to obtain the target tempered martensite fraction and high strength. On the other hand, in the present invention, the faster the secondary cooling rate, the more advantageous it is, and therefore there is no particular upper limit to the rate, but it is difficult to exceed 100°C / s due to limitations in equipment.

[0057] The secondarily cooled cold-rolled steel sheet is then heated to 150 to 450°C and held for more than 500 seconds but not more than 1,500 seconds. This process achieves interphase carbon partitioning and further phase transformation, necessary for stabilizing the retained austenite. In the present invention, the end temperature of the heating section is conveniently referred to as the reheating temperature (hereinafter also referred to as "RHS"), and the end temperature of the holding section is conveniently referred to as the overaging temperature (hereinafter also referred to as "OAS"). If the RHS or OAS temperature is less than 150°C, the strength becomes excessively high, resulting in poor formability. On the other hand, if the RHS or OAS temperature exceeds 450°C, it is difficult to achieve the high strength desired by the present invention. If the holding time in the holding section is 500 seconds or less, the total phase transformation amount is insufficient at the end of the holding stage, resulting in a large fraction of retained austenite and the formation of a large amount of fresh martensite, which may result in poor bending properties. Furthermore, the value of the above relational expression 1 may exceed 2.0. The lower limit of the holding time is more preferably 600 seconds, and even more preferably 700 seconds. The upper limit of the holding time is more preferably 1300 seconds, and even more preferably 1000 seconds. Meanwhile, the holding time in the holding section refers to the time required from the end point of the heating section to the end point of the holding section. [Example]

[0058] The present invention will be described in more detail below through examples. However, it should be noted that the following examples are intended to illustrate and embody the present invention, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0059] (Example) Cold-rolled steel sheets were produced from slabs having the alloy compositions shown in Tables 1 and 2 below by performing slab heating, hot rolling, coiling, cold rolling, annealing, primary cooling, secondary cooling, reheating, and holding processes under the conditions shown in Tables 3 and 4 below. The conditions shown in Tables 3 and 4 below were based on the surface temperature of the steel sheet.

[0060] The microstructure and mechanical properties of the cold rolled steel sheets thus produced were measured, and the results are shown in Tables 5 and 6 below.

[0061] The microstructure was measured by the point counting method from photographs taken using a scanning electron microscope (SEM), and the fraction of retained austenite was measured by XRD. The average grain size of the retained austenite was measured using EBSD analysis.

[0062] Tensile strength (TS), yield strength (YS), and total elongation (T.El) were measured by cutting test specimens perpendicular to the rolling direction from cold-rolled steel sheets and then conducting tensile tests on the specimens. Total elongation (T.El) was divided into uniform elongation (U.El) until tensile strength was reached and local elongation (P.El) until final fracture. For the tensile tests, KS B0801 No. 5 tensile test specimens were used, with a gauge length of 50 mm and a width of 25 mm. The elongation during the tensile tests was measured at the nominal strain, calculated by dividing the amount of elongation of the test specimen by the initial gauge length (50 mm). The total nominal strain until fracture of the test specimen was called the total elongation (T.El). Strength is measured at the nominal stress, which is calculated by dividing the load measured during a tensile test by the initial cross section of the test piece. The stress value when this nominal stress reaches its maximum value is called the tensile strength (TS). Here, uniform elongation (U.El) refers to the nominal deformation value when the nominal stress reaches the tensile strength (TS), and local elongation (P.El) refers to the nominal deformation amount from when the tensile strength is reached to when final fracture occurs. In other words, total elongation is the sum of the uniform elongation and local elongation.

[0063] The bending properties are the minimum radius at which cracks do not occur when the surface of the cold-rolled steel sheet is observed under a 1000x magnifying glass after a 90° V-bending test. minThe / t value was determined by using a 90-degree V-bending bending test method in which a steel sheet of a certain thickness (t) was placed between 90-degree V-shaped dies, with punches of various different radii (R) prepared, and the punch and die were brought into close contact with each other.

[0064] [Table 1]

[0065] [Table 2]

[0066] [Table 3]

[0067] [Table 4]

[0068] [Table 5]

[0069] [Table 6]

[0070] As can be seen from Tables 1 to 6 above, in the case of Examples 1 to 8, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, the microstructure that the present invention aims to obtain is secured, and therefore excellent mechanical properties are secured.

[0071] In the case of Comparative Examples 1, 2, and 5, although the manufacturing conditions proposed by the present invention were met, the alloy composition was not met, and therefore the microstructure that the present invention aims to obtain could not be secured, and as a result, the mechanical properties were deteriorated.

[0072] In the case of Comparative Examples 3 and 4, although the alloy composition proposed by the present invention was satisfied, the manufacturing conditions were not satisfied, and therefore the microstructure that the present invention aims to obtain could not be secured, resulting in deterioration of the mechanical properties.

Claims

1. In weight percent, C: 0.15% or more and less than 0.30%, Si: 0.8 to 2.5%, Al: 0.005 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.2%, B: 0.0005% to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance being Fe and other unavoidable impurities, The microstructure is composed of, in area %, ferrite: 15% or less (excluding 0%), fresh martensite: 5% or less (excluding 0%), tempered martensite: 75% or more but less than 95%, retained austenite: 12% or less (excluding 0%), and the remainder bainite. A cold-rolled steel sheet that satisfies the following relational expression 1. [Relationship 1] (RA ｕｎｓｔａｂｌｅ ++RA ｓｔａｂｌｅ ) / RA ｓｔａｂｌｅ ≦2.0 (However, in the above-mentioned relational expression 1, RA ｕｎｓｔａｂｌｅ means the fraction of unstable retained austenite, and RA ｓｔａｂｌｅ means the fraction of stable retained austenite.)

2. The cold-rolled steel sheet according to claim 1, further comprising one or more of Cu: 0.1% or less (excluding 0%) and Ni: 0.1% or less (excluding 0%).

3. The cold-rolled steel sheet according to claim 1, further containing V: 0.05% or less (excluding 0%).

4. The cold-rolled steel sheet according to claim 1, wherein the retained austenite has an average grain size of 0.5 μm or less.

5. The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet satisfies the following relational expression 2: [Relationship 2] 4xP. El-U. El≧13.0 (However, in the above-mentioned relational expression 2, P.El means local elongation, and U.El means uniform elongation.)

6. 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 bending formability (R) of 2.5 or less. ｍｉｎ 2. The cold rolled steel sheet according to claim 1, having a tensile strength of 1.0 ...

7. a step of heating a slab containing, by weight, C: 0.15% or more and less than 0.30%, Si: 0.8 to 2.5%, Al: 0.005 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.001 to 0.2%, B: 0.0005 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.001 to 0.05%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), the balance being Fe and other unavoidable impurities; Finish hot rolling the heated slab at 830 to 950°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 400 to 650°C; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuous annealing the cold-rolled steel sheet at 800 to 950°C; subjecting the continuously annealed cold-rolled steel sheet to a primary cooling end temperature of 500 to 700°C at an average cooling rate of 1°C / s or more and less than 10°C / s; Secondarily cooling the primarily cooled cold-rolled steel sheet to a secondary cooling end temperature of 25 to 300°C at an average cooling rate of 10°C / s or more; and reheating the second-cooled cold-rolled steel sheet at 150 to 450°C and then holding the reheated cold-rolled steel sheet for more than 500 seconds and not more than 1,500 seconds.

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

9. The method for producing a cold-rolled steel sheet according to claim 7, wherein the slab further contains V: 0.05% or less (excluding 0%).

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

11. The method for producing a cold-rolled steel sheet according to claim 7, wherein the cold rolling reduction rate during the cold rolling is 30 to 60%.

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