Cold-rolled steel sheet and method for producing same

Abstract

The present invention relates to a cold-rolled steel sheet having excellent strength and formability and a method for manufacturing the same.

Description

Technical Field

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

[0002] In order to enhance the lightweight and safety of automobiles, the development of steel plates with high strength is being continuously promoted. In recent years, in order to improve the mileage of electric vehicles and protect batteries, the importance of ultra-high strength steel with a tensile strength of 1500MPa or above is increasing. In addition, due to the insufficient elongation and formability of existing MART steels, if an ultra-high strength steel plate for cold forming that overcomes these problems and has both formability can be developed, its economic value is expected to be higher. In addition, as a method of improving the elongation 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 plates, Si and Al need to be added to introduce retained austenite, and a larger amount of retained austenite can be obtained when accompanied by bainite transformation. However, the bainite phase transforms at relatively high temperatures, so the tensile strength (TS, Tensile Strength) is low, and the yield strength (YS, Yield Strength) is also low, which is not suitable for use as ultra-high strength steel.

[0003] Therefore, the recent trend is to improve the strength of steel sheets while using quenching and partitioning processes to utilize the TRIP phenomenon. In the case of so-called Q&P steel, the main structure of the matrix is ​​a tempered martensite phase, the yield strength and hole expansion (Hole Expansion Ratio (HER)) are excellent, retained austenite can be formed, and the elongation can also be improved. However, in order to further improve the elongation, it is difficult to obtain strength when the Q&P process temperature is increased, so it is still difficult to simultaneously obtain high strength and sufficient elongation in 1.5GPa ultra-high strength steel. Summary of the invention

[0004] 1. Technical issues to be resolved

[0005] According to one aspect of the present invention, an object is to provide a cold-rolled steel sheet having excellent strength and formability and a method for manufacturing the same.

[0006] The technical problems of the present invention are not limited to the above contents. It is not difficult for those skilled in the art to understand the additional technical problems of the present invention from the full text of the specification of the present invention.

[0007] (II) Technical solution

[0008] One aspect of the present invention provides a cold-rolled steel sheet, which comprises, by weight, C: 0.20% or more and less than 0.30%, Si: 1.0-3.0%, Al: 0.01-0.3%, Mn: 2.0-3.0%, Cr: 0.001-0.5%, Mo: 0.08-0.32%, B: 0.0001-0.0050%, Nb: 0.001-0.05%, Ti: 0.08-0.25%, P: 0.04% or less ( 0%), S: 0.01% or less (except 0%), N: 0.01% or less (except 0%), the balance of Fe and other inevitable impurities, in terms of area%, the microstructure comprises: ferrite: 10% or less (except 0%), retained austenite: more than 2% and less than 15%, fresh martensite: less than 5%, the balance of tempered martensite and bainite, the number of carbides per average unit area where the total proportion of Ti, Mo and C atoms relative to all atoms of carbides exceeds 75% is 10 13 pcs / m 2 above.

[0009] The size of the carbides in the cold-rolled steel sheet, in which the total ratio of Ti, Mo and C atoms relative to all atoms of the carbides exceeds 75%, may be 10-200 nm.

[0010] The yield strength of the cold-rolled steel sheet may be greater than 1000 MPa.

[0011] The tensile strength of the cold-rolled steel sheet may be greater than 1470 MPa.

[0012] The elongation of the cold-rolled steel sheet may be greater than 12%.

[0013] The hole expandability (HER) of the cold-rolled steel sheet may be greater than 25%.

[0014] Another aspect of the present invention provides a method for manufacturing a cold-rolled steel sheet, the method comprising the following steps: reheating a slab, wherein the slab comprises, by weight%, C: 0.20% or more and less than 0.30%, Si: 1.0-3.0%, Al: 0.01-0.3%, Mn: 2.0-3.0%, Cr: 0.001-0.5%, Mo: 0.08-0.32%, B: 0.0001-0.0050%, Nb: 0.001-0.05%, Ti: 0.08-0.25%, P: 0.04% or less (except 0%), S: 0.01% or less (except 0%), N: 0.01% or less (except 0%), and the balance of Fe and other inevitable impurities; hot finishing rolling the reheated slab at 830-950°C to obtain a hot-rolled steel sheet; The hot rolled steel sheet is coiled at 400-550°C; a heat treatment step is performed, wherein the coiled hot rolled steel sheet is kept at a temperature in the range of 550-650°C for 5-15 hours; the heat treated hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet; the cold rolled steel sheet is continuously annealed at a temperature in the range of 830-900°C; the cold rolled steel sheet after the continuous annealing is once cooled at an average cooling rate of less than 10°C / second to a first cooling termination temperature in the range of 500-700°C; the cold rolled steel sheet after the primary cooling is secondarily cooled at an average cooling rate of more than 10°C / second to a second cooling termination temperature in the range of 150-350°C; and the cold rolled steel sheet after the secondary cooling is reheated to a temperature in the range of 200-400°C and then kept at a temperature in the range of 200-400°C for 300-1000 seconds.

[0015] When the slab is reheated, the reheating temperature may be 1150-1250°C.

[0016] During the cold rolling, the cold rolling reduction rate can be 30-60%.

[0017] After the hot finish rolling, the steel sheet may be cooled to the coiling temperature at an average cooling rate of 10-100° C. / sec.

[0018] In the holding step, the terminal temperature of the reheating zone after the secondary cooling may be 285-348°C.

[0019] In the holding step, the terminal temperature of the holding interval of the reheating after the secondary cooling may be 273-342°C.

[0020] (III) Beneficial effects

[0021] According to one aspect of the present invention, a cold-rolled steel sheet having excellent strength and formability and a method for manufacturing the same can be provided.

[0022] Various beneficial advantages and effects of the present invention are not limited to the above contents, and can be more easily understood in the process of describing specific embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a photograph of Ti—Mo-based precipitates observed on the test piece obtained in Inventive Example 1, taken using a SEM. Best Mode for Carrying Out the Invention

[0024] Below, the preferred embodiment of the present invention is described. However, the embodiment of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiment described below. In addition, the embodiment of the present invention is provided to more completely describe the present invention to those skilled in the art.

[0025] In addition, the terms used in this specification are used to illustrate specific embodiments and are not intended to limit the present invention. For example, unless the relevant definition indicates a clear opposite meaning, the singular form used in this specification also includes the plural form. In addition, the meaning of "comprising or including" used in the specification is used to specify the composition and does not exclude the existence or addition of other compositions.

[0026] First, the alloy composition of the cold-rolled steel sheet of the present invention will be described. The content of the alloy composition mentioned below is expressed in weight %.

[0027] C: 0.20% or more and less than 0.30%

[0028] Carbon (C) is an element that ensures the strength of steel by solid solution strengthening and precipitation strengthening. When the content of C is less than 0.20%, it is difficult to ensure a tensile strength (TS) of 1.5 GPa. On the other hand, when the content of C is more than 0.30%, arc weldability and laser weldability deteriorate, and the risk of forming coarse carbides leading to cracks increases. Therefore, the content of C preferably has a range of more than 0.20% and less than 0.30%. The lower limit of the content of C is more preferably 0.22%. The upper limit of the content of C is more preferably 0.28%, and more preferably 0.26%.

[0029] Si: 1.0-3.0%

[0030] Silicon (Si) is the core element of transformation induced plasticity (TRIP) steel, and its role is to improve the fraction and elongation of retained austenite by hindering the precipitation of cementite. When the content of the Si is less than 1.0%, retained austenite hardly remains, causing excessive reduction in elongation, on the other hand, when the content of the Si exceeds 3.0%, it is impossible to prevent the deterioration of the physical properties of the weld caused by the formation of LME cracks, and the surface characteristics and coating properties of steel material deteriorate. Therefore, the content of the Si preferably has a scope of 1.0-3.0%. Alternatively, the lower limit of the content of the Si can be 1.48%, or the upper limit of the content of the Si can be 2.23%.

[0031] Al: 0.01-0.3%

[0032] Aluminum (Al) is not only an element included for deoxidation of steel, but also an element effective in stabilizing retained austenite by suppressing the precipitation of cementite. When the content of Al is less than 0.01%, the deoxidation of steel is insufficient and the cleanliness of steel is impaired. On the other hand, when the content of Al exceeds 0.3%, the castability of steel is impaired. Therefore, the content of Al preferably has a range of 0.01-0.3%. Alternatively, the lower limit of the content of Al can be 0.02%, or the upper limit of the content of Al can be 0.092%.

[0033] Mn: 2.0-3.0%

[0034] Manganese (Mn) is an element added to ensure strength. When the content of Mn is less than 2.6%, it is difficult to ensure strength. On the other hand, when the content of Mn exceeds 3.0%, the phase transformation rate slows down, forming too much new martensite, making it difficult to obtain excellent formability. In addition, due to the segregation of Mn, a banded structure is formed, which impairs the material uniformity and formability of the material. Therefore, the content of Mn preferably has a range of 2.0-3.0%. The lower limit of the content of Mn is more preferably 2.2%, and more preferably 2.3%. The upper limit of the content of Mn is more preferably 2.8%, and more preferably 2.7%.

[0035] Cr: 0.001-0.5%

[0036] Chromium (Cr) is an element added to ensure strength and hardenability. In the case of adding Mn alone, it is necessary to add a very large amount of Mn exceeding the Mn content range of the present invention, and this problem can be eliminated by adding 0.001% or more of the Cr. When the Cr content exceeds 0.5%, local corrosion deteriorates and oxides are formed on the surface, thereby impairing phosphate treatment. Therefore, the Cr content preferably has a range of 0.001-0.5%. Alternatively, the lower limit of the Cr content may be 0.01%, or the upper limit of the Cr content is more preferably 0.3%.

[0037] Mo: 0.08-0.32%

[0038] Molybdenum (Mo) is an element added to ensure strength and hardenability. When Mo (Mo) is added together with Ti, it forms carbides together with Ti. In order to obtain the structural strengthening effect brought about by the formation of such carbides, the Mo content must be added to be 0.08% or more. However, since Mo (Mo) is an expensive element, the economic efficiency of the steel plate deteriorates, the phase transformation is excessively delayed, and the formation of new martensite may be caused. Therefore, the Mo content is preferably not more than 0.32%. Alternatively, the lower limit of the Mo content may be 0.132%, or the upper limit of the Mo content is more preferably 0.27%, and further preferably 0.22%.

[0039] B: 0.0001-0.0050%

[0040] Boron (B) is an element added to ensure hardenability. In the case of adding Mn alone, it is necessary to add a very large amount of Mn exceeding the Mn content range of the present invention, and this problem can be eliminated by adding more than 0.0001% of the B. However, when the content of the B exceeds 0.0050%, boron carbides are formed at the grain boundaries, which damage hardenability. Therefore, the content of the B preferably has a range of 0.0001-0.0050%. Alternatively, the lower limit of the content of the B can be 0.0005%, or the upper limit of the content of the B is more preferably 0.0025%.

[0041] Nb: 0.001-0.05%

[0042] Niobium (Nb) is an element added to ensure the strength of the steel plate and achieve microstructure refinement. When the amount of Nb added is less than 0.001%, it is difficult to obtain the effect of improving strength and achieving microstructure refinement. When the content of Nb exceeds 0.05%, the recrystallization is delayed due to local grain fixation, thereby damaging the uniformity of the structure. Therefore, the content of Nb is preferably in the range of 0.001-0.05%. Alternatively, the lower limit of the content of Nb may be 0.002%, or the upper limit of the content of Nb is more preferably 0.03%.

[0043] Ti: 0.08-0.25%

[0044] Titanium (Ti) is an element added to ensure the strength of the steel plate and to achieve microstructure refinement. In addition, in the present invention, titanium (Ti) is a main element that forms carbides by adding 0.08% or more. When the content of Ti exceeds 0.25%, the castability is impaired due to the formation of too much TiN, and the impact properties of the steel are impaired due to the formation of too much carbide. Therefore, the content of Ti preferably has a range of 0.08-0.25%. Alternatively, the lower limit of the content of Ti may be 0.096%, or the upper limit of the content of Ti is more preferably 0.22%.

[0045] P: 0.04% or less (except 0%)

[0046] Phosphorus (P) exists in steel as an impurity, so it is advantageous to control the content of P to the lowest possible level, but P is also intentionally added to improve the strength of steel. However, when too much P is added, the toughness of the steel deteriorates, so in the present invention, in order to prevent this problem, it is preferred to limit the upper limit of the P content to 0.04%. The P content is more preferably 0.015% or less, further preferably 0.010% or less, and most preferably 0.007% or less.

[0047] S: 0.01% or less (except 0%)

[0048] Similar to P, sulfur (S) exists in steel as an impurity, so it is advantageous to control the S content to the lowest possible level. In addition, S deteriorates the ductility and impact properties of steel, so it is preferred to limit the upper limit of the S content to 0.01%. Alternatively, the S content is more preferably 0.005% or less, further preferably 0.003% or less, and most preferably 0.0015% or less.

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

[0050] In the present invention, nitrogen (N) is contained in the steel as an impurity, and the upper limit of nitrogen (N) is preferably limited to 0.01%. Alternatively, the N content is more preferably 0.007% or less, further preferably 0.005% or less, and most preferably 0.003% or less.

[0051] In addition to the above steel composition, the rest may include Fe and inevitable impurities. Inevitable impurities are impurities that may be accidentally mixed in the usual steel manufacturing process, so these impurities cannot be completely eliminated, and the meaning can be easily understood by those skilled in the art of ordinary steel manufacturing. In addition, the present invention does not completely exclude the addition of components other than the above steel composition.

[0052] In addition, the cold-rolled steel sheet according to one embodiment of the present invention may further include, in terms of weight %, one or more selected from Cu: 0.1% or less and Ni: 0.1% or less.

[0053] Cu: 0.1% or less, Ni: 0.1% or less

[0054] The copper (Cu) and nickel (Ni) are elements that improve the intensity of steel. However, the element is an element that improves the intensity and hardenability of steel, but in the case of adding excessive elements, it is possible to exceed the target strength grade, and because the Cu and Ni are expensive elements, economically, the upper limit of the copper (Cu) and nickel (Ni) is preferably limited to the above-mentioned level respectively. In addition, the Cu and Ni play the role of solid solution strengthening elements, and therefore, when adding less than 0.03% Cu and Ni, the solid solution strengthening effect may be very small, so the Cu and Ni preferably add more than 0.03% respectively.

[0055] In addition, the cold-rolled steel sheet according to one embodiment of the present invention may further include V: 0.05% or less (except 0%) in terms of weight %.

[0056] V: 0.05% or less (except 0%)

[0057] Even if a trace amount of vanadium (V) is added, it can improve the strength of the steel material, but V has little effect on improving the elongation, so it is preferred to control the V content to 0.05% or less. The V content is more preferably 0.04% or less, and further preferably 0.03% or less.

[0058] The fine structure of the cold-rolled steel sheet according to one embodiment of the present invention comprises, by area %, ferrite: 10% or less (except 0%), retained austenite: more than 2% and 15% or less, fresh martensite: less than 5%, and the remainder tempered martensite and bainite.

[0059] The purpose of the cold rolled steel sheet of the present invention is to ensure excellent formability with a tensile strength (TS) of 1470 MPa or more. In particular, in order to obtain high local formability, it is necessary to reduce the hardness difference between the fine structural phases constituting the steel sheet. When the fraction of the ferrite exceeds 10%, the yield strength decreases and the formability such as hole expansion deteriorates.

[0060] Retained austenite is a structure that improves the elongation of steel through the TRIP effect. The higher the fraction of retained austenite, the higher the elongation can be obtained. In order to obtain the desired level of elongation, the fraction of retained austenite is preferably more than 2%. However, in order to obtain more than 15% of retained austenite, phase transformation is required at high temperature, and high strength of more than 1470MPa cannot be obtained.

[0061] The newly formed martensite phase is formed during the final cooling process and contributes greatly to the strength, but since it is in an untempered state, the formability is greatly impaired. Therefore, in order to obtain the high elongation of the present invention, the fraction of the newly formed martensite phase must be controlled so that it does not exceed 5%.

[0062] The remaining fine structure of the steel sheet of the present invention obtained in addition to the above-mentioned structure is tempered martensite and bainite.

[0063] In addition, according to one embodiment of the present invention, the fine structure may include, in terms of area%, ferrite: 1.7-10%, retained austenite: 3-15%, fresh martensite: 1.0-4.5% and the remainder of tempered martensite and bainite.

[0064] Furthermore, in the cold-rolled steel sheet according to the present invention, the number per average unit area of ​​carbides in which the total ratio of Ti, Mo, and C atoms relative to all atoms of carbides exceeds 75% (e.g., exceeds 75% and is 100% or less or is 75.1% or more and is 100% or less) may be 10 13 pcs / m 2 More than 0.08% of Ti and Mo are added to the cold-rolled steel sheet according to the present invention, respectively. Since the carbon content is as high as 0.2% or more and less than 0.3%, Ti-Mo carbides are formed during the annealing process. This carbide is a mixture of small particles with a size of 10-25nm and large particles with a size of 100-200nm. The large particles are first formed while undergoing hot rolling and heat treatment of the hot-rolled material, and the small particles are mainly formed during the annealing process. By comparing the results of the tensile material and the microstructure observation, it can be seen that when the total proportion of Ti, Mo and C atoms exceeds 75% compared to the entire carbide atoms with a size of more than 10nm (or, in the range of 10-200nm), the number of carbides per average unit area is at least 10 13 pcs / m2 When the TS-El product of the material is increased, the carbide particles are formed by adding Ti. When the components are analyzed by EDS in TEM, it can be clearly confirmed by atomic ratio that carbon (C), Ti and Mo are the main components. At this time, in this specification, the size of the carbide refers to the equivalent circle diameter.

[0065] In addition, although not particularly limited, in order to further improve the above effect, the upper limit of the number per average unit area of ​​carbides in which the total ratio of Ti, Mo and C atoms exceeds 75% compared to the entire atoms of the carbide may be 5×10 14 pcs / m 2 .

[0066] According to one embodiment of the present invention, a cold-rolled steel sheet has a tensile strength (TS) of more than 1470 MPa, a yield strength (YS) of more than 1000 MPa, an elongation (El) of more than 12%, and a hole expandability (HER) of more than 25%, thereby ensuring excellent strength and formability at the same time.

[0067] Furthermore, according to one embodiment of the present invention, in addition to the above-mentioned effects, a uniform elongation of 7.5% or more can be further ensured.

[0068] Hereinafter, a method for producing a cold-rolled steel sheet having excellent strength and formability according to one embodiment of the present invention will be described.

[0069] First, the slab having the above alloy composition is reheated. When the slab is reheated, the reheating temperature is preferably 1150-1250°C. When the reheating temperature of the slab is lower than 1150°C, the next hot rolling may not be performed. On the other hand, when the reheating temperature of the slab exceeds 1250°C, a large amount of energy is unnecessarily consumed in order to increase the temperature of the slab. Therefore, the reheating temperature of the slab preferably has a range of 1150-1250°C. The lower limit of the reheating temperature of the slab is more preferably 1170°C, and more preferably 1180°C. The upper limit of the reheating temperature of the slab is more preferably 1230°C, and more preferably 1220°C.

[0070] Afterwards, the reheated slab is hot finished rolled at 830-950°C to obtain a hot rolled steel plate. 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, thereby deteriorating productivity. On the other hand, when the hot finish rolling temperature exceeds 950°C, the surface quality deteriorates due to the increase of oxides caused by excessive high temperature operation. Therefore, the hot finish rolling temperature preferably has a range of 830-950°C. The lower limit of the hot finish rolling temperature is more preferably 880°C. The upper limit of the hot finish rolling temperature is more preferably 930°C, and further preferably 910°C.

[0071] In addition, according to one embodiment of the present invention, after the hot finish rolling, it is preferably cooled to the following coiling temperature at an average cooling rate of 10-100°C / second. When the average cooling rate is less than 10°C / second, there may be disadvantages that the hot rolling productivity is reduced and a cooling medium with reduced cooling capacity must be used in actual production. In addition, when the average cooling rate exceeds 100°C / second, the temperature deviation inside the steel plate is uneven, the shape deteriorates, and the strength of the steel plate may become too high, so the average cooling rate preferably has a range of 10-100°C / second. Alternatively, the lower limit of the average cooling rate after hot rolling may be 30°C / second, or the upper limit of the average cooling rate after hot rolling may be 80°C / second.

[0072] Afterwards, the hot-rolled steel sheet is coiled at 400-550°C. When the coiling temperature (hereinafter also referred to as "CT") exceeds 550°C, coarse hot-rolled internal oxidation is caused, and there is a disadvantage of poor surface properties. Ti-Mo-based precipitates are coarsely precipitated during cooling from the FDT temperature, and the number of cases in which the final size is greater than 250nm increases, and the precipitation solidification effect is reduced. When the coiling temperature is lower than 400°C, corresponding to the transition boiling zone, there are disadvantages that the controllability of the coiling temperature is poor and the shape of the steel sheet is poor. The lower limit of the coiling temperature is more preferably 440°C, and more preferably 480°C. The upper limit of the coiling temperature is more preferably 530°C, and more preferably 520°C.

[0073] After that, the coiled hot-rolled steel sheet is heat treated in a temperature range of 550-650°C for 5-15 hours. The heat treatment uses a batch furnace that can load 1-3 coils at a time, and the temperature is slowly increased. The average temperature increase rate from room temperature is less than 1°C / second. The cooling can be carried out by air cooling and furnace cooling until the average cooling rate at room temperature is less than 1°C / second, and the cooling is slow.

[0074] When the heat treatment temperature of the hot-rolled steel sheet is lower than 550°C, the strength of the hot-rolled steel sheet after heat treatment is too high, and the precipitation amount of Ti-Mo carbides may become insufficient. When the heat treatment temperature exceeds 650°C, the surface properties of the material deteriorate, and the size of the Ti-Mo precipitates becomes coarse to more than 250nm. Alternatively, the lower limit of the heat treatment temperature of the hot-rolled steel sheet may be 580°C, or the upper limit of the heat treatment temperature of the hot-rolled steel sheet may be 620°C.

[0075] In addition, when the holding time in the range of 550-650°C is shorter than 5 hours, the strength of the hot-rolled steel plate after heat treatment is too high, the final precipitation amount of Ti-Mo carbides may be insufficient, the temperature deviation in the coil is too large, and the material deviation will increase. On the other hand, when the holding time exceeds 15 hours, the surface properties of the material deteriorate and the Ti-Mo precipitates may become coarse. Alternatively, the lower limit of the holding time in the range of 550-650°C may be 12 hours, or the upper limit of the holding time in the range of 550-650°C may be 14 hours.

[0076] Afterwards, the coiled hot-rolled steel sheet is cold rolled to obtain a cold-rolled steel sheet. During the cold rolling, the cold rolling reduction rate can be 30-60%. When the cold rolling reduction rate is less than 30%, it is not only difficult to ensure the target thickness accuracy, but also difficult to correct the shape of the steel sheet. On the other hand, when the cold rolling reduction rate exceeds 60%, the possibility of cracks on the edge of the steel sheet increases, and the cold rolling load may become too large. Therefore, the cold rolling reduction rate preferably has a range of 30-60%. Alternatively, the lower limit of the cold rolling reduction rate can be 33%, or the upper limit of the cold rolling reduction rate can be 55%.

[0077] Afterwards, the cold-rolled steel sheet is continuously annealed in the range of 830-900°C. The continuous annealing step is to heat the steel sheet to the austenite single phase region to form close to 100% austenite for subsequent phase transformation. When the continuous annealing temperature (hereinafter also referred to as "SS") is lower than 830°C, sufficient austenite reverse transformation may not be achieved, and there is a risk of forming more than 10% ferrite phase after annealing. On the other hand, when the continuous annealing temperature exceeds 900°C, the surface quality and productivity are reduced, and coarse austenite is formed, so that the material may deteriorate. Alternatively, the lower limit of the continuous annealing temperature may be 841°C, or the upper limit of the continuous annealing temperature may be 877°C.

[0078] Afterwards, the cold-rolled steel sheet that has undergone the continuous annealing is cooled once at an average cooling rate of less than 10°C / second, and cooled to a primary cooling termination temperature of 500-700°C (hereinafter also referred to as "SCS"). The primary 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 that was not applied in the primary cooling. When the cooling process is divided into primary and secondary cooling and carried out in stages, the temperature distribution of the steel plate is uniform in the slow cooling step, thereby reducing the final temperature and material deviation. When the primary cooling termination temperature is lower than 500°C, there is a risk of inducing soft bainite transformation, and due to the length of the actual equipment, it is difficult to cool to below 500°C at a cooling rate of less than 10°C / second. When the primary cooling termination temperature exceeds 700°C, the cooling amount until the secondary cooling termination temperature increases, and the shape of the steel plate becomes poor. In addition, when the primary 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 primary cooling rate exceeds 10°C / second, the cooling amount during the secondary cooling process increases, and the final temperature deviation and material deviation increase. Alternatively, the lower limit of the primary cooling termination temperature may be 580°C, or the upper limit of the primary cooling termination temperature may be 620°C.

[0079] Afterwards, the cold-rolled steel sheet that has been cooled once is subjected to secondary cooling at an average cooling rate of more than 10°C / second, and is cooled to a secondary cooling termination temperature (hereinafter also referred to as "RCS") of 150-350°C. The secondary cooling termination temperature is set below the Ms temperature of the steel sheet, so that martensitic transformation occurs during the cooling process, and the martensite eventually becomes a tempered martensite phase while undergoing a reheating step as a subsequent process. When the secondary cooling termination temperature is lower than 150°C, the amount of martensitic transformation is too much, the tensile strength is too high, and the elongation becomes insufficient, and the yield strength also increases, so forming becomes difficult. However, when most of the structure is composed of tempered martensite, high hole expansion can be maintained. On the other hand, when the secondary cooling termination temperature exceeds 350°C, martensite is not fully generated during the cooling process, and it is difficult to obtain sufficient yield strength, tensile strength and hole expansion. In addition, when the final fraction of newly formed martensite increases, the elongation and hole expansion are greatly impaired. When the secondary cooling rate is less than 10°C / sec, even if the target secondary cooling end temperature is reached, a high temperature phase such as upper bainite is mixed in during the cooling process, so the target tempered martensite fraction and high strength cannot be obtained. Alternatively, the lower limit of the secondary cooling end temperature may be 200°C, or the upper limit of the secondary cooling end temperature may be 325°C.

[0080] After that, the cold-rolled steel sheet after the secondary cooling is heated to 200-400°C and then maintained in the temperature range of 200-400°C for a time in the range of 300-1000 seconds. Through the process, the interphase carbon distribution and additional phase transformation required for the stabilization of the retained austenite are obtained. In the present invention, the end temperature of the heating interval is conveniently referred to as the reheating temperature (hereinafter also referred to as "RHS"), and the end temperature of the holding interval is conveniently referred to as the overaging temperature (hereinafter also referred to as "OAS"). When the RHS or OAS temperature is lower than 200°C, the strength is too high and the elongation is deteriorated. On the other hand, when the RHS and OAS temperatures exceed 400°C, it is difficult to obtain the high strength corresponding to the steel of the present invention. The elongation of the steel of the present invention is related to the retained austenite. Since the secondary cooling termination temperature is too low, when the tempering martensite transformation is too much, the austenite residual space may be absolutely insufficient, and when the carbon is not fully distributed in the austenite in the reheating step, the stability of the austenite is reduced, and it is difficult to obtain an elongation of more than 12%.

[0081] According to one aspect of the present invention, the end point temperature (RHS) of the reheating zone after the secondary cooling may be 285-348° C. At this time, it should be noted that the end point temperature of the reheating zone after the secondary cooling is higher than the secondary cooling termination temperature.

[0082] In addition, according to one aspect of the present invention, the end point temperature (OAS) of the holding interval of the reheating after the secondary cooling can be 273-342° C. It should be noted that the end point temperature of the holding interval of the reheating after the secondary cooling is lower than the end point temperature of the reheating interval after the secondary cooling. DETAILED DESCRIPTION

[0083] The present invention is described in more detail below by way of examples. However, it should be noted that the following examples are only used to illustrate the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the contents described in the claims and the contents reasonably derived therefrom.

[0084] (Example)

[0085] A slab having an alloy composition (weight %) as described in Table 1 below is prepared, and then reheated at 1180-1220°C, hot rolled, coiled, annealed, cooled once, cooled twice, reheated and maintained under the conditions described in Table 2 below, thereby manufacturing a cold rolled steel sheet. At this time, the average cooling rate after hot finishing rolling is 30-50°C / second, the cold rolling reduction rate is 33-55%, the average cooling rate of the primary cooling is 4.5-6.5°C / second, and the average cooling rate of the secondary cooling is 20-40°C / second. In addition, the holding time in the holding process is 450-700 seconds (s).

[0086] The tensile properties evaluation results of the steel plate manufactured as described above are shown in Table 3 below. Tensile strength (TS), yield strength (YS), and total elongation (t-El) are measured by a tensile test perpendicular to the rolling direction. The total elongation (t-El) is further divided into the uniform elongation (u-El) until the tensile strength is reached and the local elongation (p-El) thereafter until the final fracture. In the tensile test, a No. 5 tensile test piece of the KS B0801 standard is used, the gauge length (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 dividing the elongation of the test piece by the initial gauge length (50 mm) to obtain the so-called nominal strain, and the total nominal strain until the fracture of the test piece occurs is called the total elongation (t-El). Strength (Strength) is measured by dividing the load measured in the tensile test by the initial cross-section of the test piece to obtain the so-called nominal stress, and the stress value when this nominal stress reaches the maximum value is called the tensile strength (TS). Here, again, the uniform elongation (u-El) refers to the nominal strain value when the nominal stress reaches the tensile strength (TS), and the local elongation (p-El) refers to the nominal strain from the tensile strength to the final fracture. That is, the total elongation is the sum of the uniform elongation and the local elongation.

[0087] Hole expansion (HER) is measured based on ISO 16630. First, prepare a square test piece with a side length of 120 mm. Then, punch a hole with a size of 10 mm (Do) in the center, and set the clearance to 12%. Then, use an angle of 60 o The conical punch pushes the punch upward to expand the hole until a crack that penetrates the entire thickness is formed. The diameter of the expanded hole when a crack that penetrates the thickness is generated is set as Df, and the hole expansion (hole expansion rate) value can be calculated by the following formula.

[0088] HER(%)=

[0089] In addition, the results of measuring the fine structure fraction are shown together in Table 3. The fine structure fraction was measured from a scanning electron microscope (SEM) photograph by a point counting method, and the fraction of retained austenite was measured by XRD and shown in Table 3 below.

[0090] In addition, the cold-rolled steel sheet produced by the above method was made into 5 test pieces, and then the number per unit area (pieces / m2) of carbides in which the total ratio of Ti, Mo and C atoms exceeded 75% compared with the entire atoms of carbides with a size range of 10 to 200 nm in each test piece was measured by TEM observation. 2 ), and then calculate the average value, which is represented by D in Table 3 below.

[0091] [Table 1]

[0092]

[0093] [Table 2]

[0094]

[0095] [Table 3]

[0096]

[0097] (F: ferrite, TM: tempered martensite, B: bainite, FM: fresh martensite, retained γ: retained austenite)

[0098] (D : The number per average unit area of ​​carbides in which the total ratio of Ti, Mo and C atoms relative to all atoms of the carbide exceeds 75%)

[0099] It can be seen from Tables 1 to 4 that Inventive Examples 1 to Inventive Examples 4 satisfy the addition range of Ti and Mo of the present invention and also satisfy the phase fraction, and therefore exhibit a yield strength (YS) of more than 1000 MPa, a tensile strength (TS) of more than 1470 MPa, an elongation (El) of more than 12%, and a hole expandability (HER) of more than 25%.

[0100] In particular, Figure 1 A photograph of Ti—Mo-based precipitates observed in the test piece obtained in Inventive Example 1, taken with a SEM, is shown.

[0101] On the other hand, in Comparative Examples 1 to 9 manufactured using Steel Types A to I, since the lower limit of Si and the lower limits of the addition amounts of Ti and Mo proposed in the present invention cannot be satisfied, even though all other manufacturing conditions and phase fraction conditions are satisfied and a yield strength of 1000 MPa or more and a tensile strength of 1470 MPa or more are achieved, it can be seen that the elongation does not reach 12% or the hole expandability (HER) cannot satisfy 25% or more.

[0102] In addition, in the case of Comparative Example 10 in which the ferrite phase fraction exceeded 10%, it was found that the yield strength did not reach 1000 MPa.

[0103] In addition, in the case of Comparative Example 12, although the composition range of the present invention is met, due to the high coiling temperature and the high heat treatment temperature, Ti and Mo are consumed in large quantities in the form of coarse Ti-Mo carbides of more than 250 nm, and the desired material is not obtained.

Claims

1. A cold-rolled steel sheet, comprising, by weight%, C: 0.20% or more and less than 0.30%, Si: 1.0-3.0%, Al: 0.01-0.3%, Mn: 2.0-3.0%, Cr: 0.001-0.5%, Mo: 0.08-0.32%, B: 0.0001-0.0050%, Nb: 0.001-0.05%, Ti: 0.08-0.25%, P: 0.04% or less and excluding 0%, S: 0.01% or less and excluding 0%, N: 0.01% or less and excluding 0%, and the balance of Fe and other inevitable impurities, In terms of area%, the microstructure includes: ferrite: less than 10% and excluding 0%; retained austenite: more than 2% and less than 15%; new martensite: less than 5%; the rest is tempered martensite and bainite. The number per average unit area of ​​carbides in which the total proportion of Ti, Mo and C atoms relative to all atoms of the carbides exceeds 75% is 10 13 pcs / m 2 above.

2. The cold-rolled steel sheet according to claim 1, wherein: The size of the carbide in which the total proportion of Ti, Mo and C atoms exceeds 75% compared to the entire atoms of the carbide is 10-200 nm.

3. The cold-rolled steel sheet according to claim 1, wherein: The yield strength is 1000 MPa or more.

4. The cold-rolled steel sheet according to claim 1, wherein: The tensile strength is 1470 MPa or more.

5. The cold-rolled steel sheet according to claim 1, wherein: The elongation is more than 12%.

6. The cold-rolled steel sheet according to claim 1, wherein: The hole expansion is more than 25%.

7. A method for manufacturing a cold-rolled steel sheet, comprising the following steps: The slab is reheated, and the slab comprises, in terms of weight %, C: 0.20% or more and less than 0.30%, Si: 1.0-3.0%, Al: 0.01-0.3%, Mn: 2.0-3.0%, Cr: 0.001-0.5%, Mo: 0.08-0.32%, B: 0.0001-0.0050%, Nb: 0.001-0.05%, Ti: 0.08-0.25%, P: 0.04% or less and excluding 0%, S: 0.01% or less and excluding 0%, N: 0.01% or less and excluding 0%, and the balance is Fe and other inevitable impurities; hot finishing rolling the reheated slab at 830-950° C. to obtain a hot rolled steel plate; The hot rolled steel sheet is coiled at 400-550° C. A heat treatment step, maintaining the coiled hot-rolled steel sheet at a temperature in the range of 550-650° C. for 5-15 hours; cold rolling the hot-rolled steel sheet subjected to the heat treatment to obtain a cold-rolled steel sheet; Continuously annealing the cold-rolled steel sheet at a temperature of 830-900° C.; The cold-rolled steel sheet after the continuous annealing is subjected to primary cooling at an average cooling rate of less than 10°C / second to a primary cooling termination temperature of 500-700°C; The cold-rolled steel sheet subjected to the primary cooling is subjected to secondary cooling at an average cooling rate of more than 10° C. / s to a secondary cooling termination temperature of 150-350° C.; as well as The secondary cooled cold rolled steel sheet is then heated to a temperature in the range of 200-400° C. and then maintained at the temperature in the range of 200-400° C. for 300-1000 seconds.

8. The method for manufacturing a cold-rolled steel sheet according to claim 7, wherein: When the slab is reheated, the reheating temperature is 1150-1250°C.

9. The method for manufacturing a cold-rolled steel sheet according to claim 7, wherein: During the cold rolling, the cold rolling reduction rate is 30-60%.

10. The method for manufacturing a cold-rolled steel sheet according to claim 7, wherein: After the hot finish rolling, the steel is cooled to the coiling temperature at an average cooling rate of 10-100° C. / sec.

11. The method for manufacturing a cold-rolled steel sheet according to claim 7, wherein: In the holding step, the terminal temperature of the reheating zone after the secondary cooling is 285-348°C.

12. The method for manufacturing a cold-rolled steel sheet according to claim 7, wherein: In the holding step, the terminal temperature of the holding interval of the reheating after the secondary cooling is 273-342°C.

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