Cold-rolled steel sheet and its manufacturing method

Abstract

The present invention relates to a cold-rolled steel sheet having excellent strength and formability, and a method for producing 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 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. In recent years, ultra-high-strength steel sheets with tensile strengths of 1,500 MPa or higher have become increasingly important for improving the driving range and battery protection of electric vehicles. However, conventional MART steels lack sufficient elongation and formability. The development of ultra-high-strength cold-forming steel sheets that overcome these limitations and combine formability is expected to further enhance their economic value. To improve the formability of steels, the introduction of retained austenite and the use of the TRIP (Transformation Induced Plasticity) phenomenon are widely used to increase elongation. However, the introduction of retained austenite requires the addition of Si and Al, and the addition of bainite transformation can produce even more retained austenite. However, because the bainite phase transforms at a relatively high temperature, its tensile strength (TS) is low and its yield strength (YS) is also too low for use as ultra-high-strength steel.

[0003] Therefore, the recent trend is to use quenching and partitioning processes to increase the strength of steel sheets while utilizing the TRIP phenomenon. In the case of so-called Q&P steel, the main structure of the matrix is ​​tempered martensite, which provides excellent yield strength and hole expansion ratio (HER), allows the formation of retained austenite, and increases elongation. However, if the temperature in the Q&P process is increased to further increase elongation, it becomes difficult to obtain sufficient strength. Therefore, it remains difficult to achieve both high strength and sufficient elongation in 1.5 GPa ultra-high strength steel. Summary of the Invention [Problem to be solved by the invention]

[0004] According to one aspect of the present invention, there is provided a cold-rolled steel sheet having excellent strength and formability, and a method for manufacturing the same.

[0005] The object of the present invention is not limited to the above content. Anyone having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the present invention from the entire content of the present specification. [Means for solving the problem]

[0006] One aspect of the present invention is In weight percent, C: 0.20% or more but less than 0.30%, Si: 1.0 to 3.0%, Al: 0.01 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.08 to 0.32%, B: 0.0001 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.08 to 0.25%, 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, in area percentage, ferrite: 10% or less (excluding 0%), retained austenite: more than 2% and 15% or less, fresh martensite: less than 5%, and the remainder includes tempered martensite and bainite. The number of carbides per unit area where the total ratio of Ti, Mo and C atoms to all atoms of the carbide exceeds 75% is 10 13 / m 2 The present invention provides a cold-rolled steel sheet having at least one of the following characteristics:

[0007] In the cold-rolled steel sheet, the size of the carbides may be 10 to 200 nm, and the total ratio of Ti, Mo, and C atoms to all atoms in the carbides may exceed 75%.

[0008] The cold-rolled steel sheet may have a yield strength of 1000 MPa or more.

[0009] The cold-rolled steel sheet may have a tensile strength of 1470 MPa or more.

[0010] The cold-rolled steel sheet may have an elongation of 12% or more.

[0011] The cold-rolled steel sheet may have a hole expandability (HER) of 25% or more.

[0012] Yet another aspect of the present invention is reheating a slab containing, by weight, C: 0.20% or more but less than 0.30%, Si: 1.0 to 3.0%, Al: 0.01 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.08 to 0.32%, B: 0.0001 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.08 to 0.25%, 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 reheated slab at 830 to 950 ° C to obtain a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet at 400 to 550 ° C; a step of performing a heat treatment on the coiled hot-rolled steel sheet at a temperature in the range of 550 to 650°C for 5 to 15 hours; cold-rolling the heat-treated hot-rolled steel sheet to obtain a cold-rolled steel sheet; Continuously annealing the cold-rolled steel sheet at a temperature of 830 to 900 ° C.; Primary 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 less than 10 ° C. / s; Secondarily cooling the primary-cooled cold-rolled steel sheet to a secondary cooling end temperature of 150 to 350 ° C. at an average cooling rate of 10 ° C. / s or more; and reheating the secondary-cooled cold-rolled steel sheet to a temperature range of 200 to 400°C, and then holding the temperature range of 200 to 400°C for 300 to 1,000 seconds.

[0013] When the slab is reheated, the reheating temperature may be 1150 to 1250°C.

[0014] During the cold rolling, the cold rolling reduction may be 30 to 60%.

[0015] After the above finish hot rolling, the sheet can be cooled to the coiling temperature at an average cooling rate of 10 to 100°C / s.

[0016] In the holding step, the end temperature of the reheating section after the secondary cooling may be 285 to 348°C.

[0017] In the holding step, the end point temperature of the holding section of the reheating after the secondary cooling may be 273 to 342°C. [Effects of the Invention]

[0018] According to one aspect of the present invention, it is possible to provide a cold-rolled steel sheet having excellent strength and formability, and a method for manufacturing the same.

[0019] The various beneficial advantages and effects of the present invention are not limited to the above-mentioned contents, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a photograph taken by SEM of Ti—Mo-based precipitates observed in a test piece obtained from Invention Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0022] Meanwhile, the terms used in this specification are intended to describe specific embodiments and are not intended to limit the present invention. For example, the singular forms "a," "an," and "the" as used in this specification include the plural forms unless the related definition clearly indicates otherwise. Furthermore, the meaning of "comprises" as used in this specification embodies a configuration and does not exclude the presence or addition of other configurations.

[0023] First, the alloy composition of the cold-rolled steel sheet of the present invention will be described. The contents of the alloy composition mentioned below are in wt %.

[0024] C: 0.20% 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 0.20%, 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 0.20% or more and less than 0.30%. The lower limit of the C content is more preferably 0.22%. The upper limit of the C content is more preferably 0.28%, and even more preferably 0.26%.

[0025] Si: 1.0 to 3.0% Silicon (Si) is a key element in TRIP (Transformation Induced Plasticity) steel, which inhibits cementite precipitation, thereby increasing the retained austenite fraction and elongation. If the Si content is less than 1.0%, almost no retained austenite remains, resulting in excessively low elongation. On the other hand, if the Si content exceeds 3.0%, it becomes impossible to prevent deterioration of the physical properties of the weld due to the formation of LME cracks, and the surface properties and platability of the steel material deteriorate. Therefore, the Si content is preferably in the range of 1.0 to 3.0%. Alternatively, the lower limit of the Si content may be 1.48%, or the upper limit of the Si content may be 2.23%.

[0026] Al: 0.01 to 0.3% Aluminum (Al) is an element contained not only for deoxidizing steel but also for stabilizing retained austenite by suppressing cementite precipitation. If the Al content is less than 0.01%, the steel is not sufficiently deoxidized, 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 0.01 to 0.3%. Alternatively, the lower limit of the Al content may be 0.02%, or the upper limit of the Al content may be 0.092%.

[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, excessive fresh martensite is formed, and it becomes difficult to obtain excellent formability. In addition, band structures are formed due to Mn segregation, which impairs the material uniformity and formability of the material. Therefore, the Mn content is preferably in the range of 2.0 to 3.0%. The lower limit of the Mn content is more preferably 2.2%, and even more preferably 2.3%. The upper limit of the Mn content is more preferably 2.8%, and even more preferably 2.7%.

[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%. Alternatively, the lower limit of the Cr content may be 0.01%, and more preferably the upper limit of the Cr content is 0.3%.

[0029] Mo: 0.08 to 0.32% Molybdenum (Mo) is an element added to ensure strength and hardenability, and when added together with Ti, it forms carbides with Ti. To obtain the microstructure strengthening effect through the formation of such carbides, the Mo content must be 0.08% or more. However, since Mo is an expensive element, it 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 0.32%. Alternatively, the lower limit of the Mo content may be 0.132%, and the upper limit of the Mo content is more preferably 0.27%, and even more preferably 0.22%.

[0030] B: 0.0001 to 0.0050% Boron (B) is an element added to ensure hardenability. Adding Mn alone would require adding a very large amount of Mn, exceeding the Mn content range of the present invention. However, adding 0.0001% or more of B can solve this problem. However, if the B content exceeds 0.0050%, boron-based carbides are formed at grain boundaries, impairing hardenability. Therefore, the B content is preferably in the range of 0.0001 to 0.0050%. Alternatively, the lower limit of the B content may be 0.0005%, and more preferably, the upper limit of the B content is 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%. Alternatively, the lower limit of the Nb content may be 0.002%, and more preferably, the upper limit of the Nb content is 0.03%.

[0032] Ti: 0.08 to 0.25% Titanium (Ti) is an element added to ensure the strength of steel sheet and refine the structure. In the present invention, titanium is a major element that forms carbides when added in an amount of 0.08% or more. If the Ti content exceeds 0.25%, 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 0.08 to 0.25%. Alternatively, the lower limit of the Ti content may be 0.096%, and more preferably, the upper limit of the Ti content is 0.22%.

[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, excessive P addition deteriorates the toughness of the steel, so in the present invention, to prevent this, the upper limit is preferably set to 0.04%. The P content is more preferably 0.015% or less, even more preferably 0.010% or less, and most preferably 0.007% or less.

[0034] S: 0.01% or less (excluding 0%) Like P, sulfur (S) exists as an impurity in steel, 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%. Alternatively, 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 contained in the steel as an impurity, and its upper limit is preferably limited to 0.01%. Alternatively, 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 engineers in the field of normal steel manufacturing can easily understand the meaning of this. However, the present invention does not completely exclude the addition of compositions other than the above-mentioned steel composition.

[0037] Meanwhile, the cold rolled steel sheet according to an embodiment of the present invention may further contain, by weight %, at least one selected from the group consisting of Cu: 0.1% or less and Ni: 0.1% or less.

[0038] Cu: 0.1% or less, Ni: 0.1% or less Copper (Cu) and nickel (Ni) are elements that increase the strength of steel. However, although these elements increase the strength and hardenability of steel, if added in excessive amounts, the target strength grade may be exceeded. Since these elements are expensive elements, it is preferable to limit their upper limits to the levels mentioned above from an economical standpoint. On the other hand, because Cu and Ni act as solid solution strengthening elements, if added in amounts less than 0.03%, the solid solution strengthening effect may be slight. Therefore, it is preferable to add 0.03% or more of each.

[0039] Furthermore, the cold rolled steel sheet according to one embodiment of the present invention may further contain, by weight %, V: 0.05% or less (excluding 0%).

[0040] V: 0.05% or less (excluding 0%) Although vanadium (V) can increase the strength of steel materials 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, more preferably 0.04% or less, and even more preferably 0.03% or less.

[0041] The microstructure of the cold-rolled steel sheet according to one embodiment of the present invention comprises, in area percentages, ferrite: 10% or less (excluding 0%), retained austenite: more than 2% and 15% or less, fresh martensite: less than 5%, and the remainder being tempered martensite and bainite.

[0042] The cold-rolled steel sheet of the present invention is intended to ensure excellent formability at a tensile strength (TS) of 1470 MPa or more, and in order to obtain particularly high local formability, the difference in hardness between the microstructural phases constituting the steel sheet must be reduced. If the ferrite fraction exceeds 10%, the yield strength decreases, and formability such as hole expandability deteriorates.

[0043] Retained austenite is a structure that increases the elongation of steel materials due to the TRIP effect, and the higher the retained austenite fraction, the higher the elongation that can be obtained. To obtain the required level of elongation, the retained austenite fraction should preferably exceed 2%. However, to obtain retained austenite of more than 15%, a phase transformation must be performed at a high temperature, which makes it impossible to obtain high strength of 1470 MPa or more.

[0044] The fresh martensite phase is formed during final cooling and contributes greatly to strength, but is in an untempered state and significantly impairs formability. Therefore, in order to obtain the high elongation of the present invention, it is necessary to control the fraction of fresh martensite so that it does not exceed 5%.

[0045] The remaining microstructure of the steel sheet of the present invention, excluding the above structures, is obtained as tempered martensite and bainite.

[0046] On the other hand, according to one embodiment of the present invention, the microstructure can include, in area percentages, ferrite: 1.7 to 10%, retained austenite: 3 to 15%, fresh martensite: 1.0 to 4.5%, and the remainder being tempered martensite and bainite.

[0047] In addition, the cold-rolled steel sheet according to the present invention has a total ratio of Ti, Mo and C atoms to all atoms of carbides exceeding 75% (for example, more than 75% and 100% or less, or 75.1% or more and 100% or less), and the number of carbides per average unit area is 10 13 / m 2 The cold-rolled steel sheet according to the present invention may contain 0.08% or more Ti and Mo, respectively, and a high carbon content of 0.2% or more but less than 0.3%, which allows Ti-Mo carbides to be formed during annealing. These carbides are a mixture of small particles with a size of 10 to 25 nm and large particles with a size of 100 to 200 nm, with the large particles being formed earlier during the hot rolling and heat treatment processes of the hot-rolled material, and the small particles being formed mainly during annealing. Comparison of the results of observation of the tensile properties and the microstructure shows that the number of carbides per average unit area in which the total ratio of Ti, Mo, and C atoms to all atoms in carbides with a size of 10 nm or more (or in the range of 10 to 200 nm) exceeds 75% is at least 10 13 / m 2 It has been found that when the number of carbides is 1 or more, the TS-El product of the material is improved. Such carbide particles are formed by adding Ti, and when the components are analyzed using EDS in a TEM, it can be confirmed that the atomic ratio is mainly carbon (C), Ti, and Mo. In this specification, the size of the carbide refers to the circle equivalent diameter.

[0048] On the other hand, although not particularly limited, from the viewpoint of further improving the above-mentioned effect, the upper limit of the number of carbides per average unit area in which the total ratio of Ti, Mo and C atoms to all atoms of the carbides exceeds 75% is 5 × 10 14 / m 2 It may be one.

[0049] The cold-rolled steel sheet provided by one embodiment of the present invention has a tensile strength (TS) of 1470 MPa or more, a yield strength (YS) of 1000 MPa or more, an elongation (El) of 12% or more, and a hole expandability (HER) of 25% or more, thereby ensuring excellent strength and formability at the same time.

[0050] Furthermore, according to one embodiment of the present invention, in addition to the above-mentioned effects, a uniform elongation rate of 7.5% or more can be further ensured.

[0051] 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.

[0052] First, a slab having the above-described alloy composition is reheated. The reheating temperature during the reheating of the slab is preferably 1150 to 1250°C. If the reheating temperature of the slab is less than 1150°C, it may be impossible to perform the next step, hot rolling. On the other hand, if the reheating temperature exceeds 1250°C, a large amount of energy is unnecessarily consumed to increase the slab temperature. Therefore, the slab reheating temperature is preferably in the range of 1150 to 1250°C. The lower limit of the slab reheating temperature is more preferably 1170°C, and even more preferably 1180°C. The upper limit of the slab reheating temperature is more preferably 1230°C, and even more preferably 1220°C.

[0053] The reheated 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. The lower limit of the finish hot rolling temperature is more preferably 880°C. The upper limit of the finish hot rolling temperature is more preferably 930°C, and even more preferably 910°C.

[0054] On the other hand, according to one embodiment of the present invention, after the finish hot rolling, it is preferable to cool the steel sheet to the coiling temperature described below at an average cooling rate of 10 to 100°C / s. If the average cooling rate is less than 10°C / s, the productivity of the hot rolling decreases, and there is a possibility that a cooling medium with a low cooling capacity must be intentionally used during actual production. Furthermore, if the average cooling rate exceeds 100°C / s, the temperature deviation inside the steel sheet will not be uniform, which may result in poor shape and excessively high strength of the steel sheet. Therefore, it is preferable that the average cooling rate be in the range of 10 to 100°C / s. Alternatively, the lower limit of the average cooling rate after hot rolling may be 30°C / s, or the upper limit of the average cooling rate after hot rolling may be 80°C / s.

[0055] The hot-rolled steel sheet is then coiled at 400 to 550°C. If the coiling temperature (hereinafter also referred to as "CT") exceeds 550°C, there are drawbacks in that coarse internal oxidation of the hot rolled steel sheet occurs, resulting in poor surface properties, and Ti-Mo-based precipitates often precipitate coarsely during cooling from the FDT temperature, ultimately reaching a size of 250 nm or more, thereby reducing the effect of precipitation hardening. If the coiling temperature is less than 400°C, there are drawbacks in that the coiling temperature is in a transition boiling region, making it difficult to control, and the shape of the steel sheet deteriorates. The lower limit of the coiling temperature is more preferably 440°C, and even more preferably 480°C. The upper limit of the coiling temperature is more preferably 530°C, and even more preferably 520°C.

[0056] The coiled hot-rolled steel sheet is then heat-treated at a temperature of 550 to 650°C for 5 to 15 hours. The heat treatment is carried out using a batch furnace that can load 1 to 3 coils at a time, and the temperature is increased slowly from room temperature at an average rate of 1°C / sec or less. Cooling can be performed by air cooling or furnace cooling, and the average cooling rate to room temperature is slow at less than 1°C / sec.

[0057] If the heat treatment temperature of the hot-rolled steel sheet is less than 550°C, the strength of the heat-treated hot-rolled steel sheet may be too high, resulting in an insufficient amount of final precipitated Ti-Mo carbides, whereas if the heat treatment temperature exceeds 650°C, the surface properties of the material will deteriorate and the size of the Ti-Mo precipitates will become coarse to 250 nm or more. 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.

[0058] Furthermore, if the holding time in the range of 550 to 650°C is shorter than 5 hours, the strength of the hot-rolled steel sheet after heat treatment may be too high, resulting in an insufficient amount of final Ti-Mo carbide precipitation, and the temperature deviation within the coil may become too large, increasing material deviation. Conversely, if the holding time exceeds 15 hours, the surface properties of the material may deteriorate and the Ti-Mo precipitates may become coarse. Alternatively, the lower limit of the holding time in the range of 550 to 650°C may be 12 hours, or the upper limit of the holding time in the range of 550 to 650°C may be 14 hours.

[0059] The coiled hot-rolled steel sheet is then cold-rolled to obtain a cold-rolled steel sheet. During the cold rolling, the cold reduction may be 30 to 60%. If the cold reduction is less than 30%, not only is it difficult to ensure the target thickness accuracy, but shape correction of the steel sheet may also be difficult. On the other hand, if the cold reduction exceeds 60%, cracks are more likely to occur at the edges of the steel sheet, and the cold rolling load may become excessively large. Therefore, the cold reduction is preferably in the range of 30 to 60%. Alternatively, the lower limit of the cold reduction may be 33%, or the upper limit of the cold reduction may be 55%.

[0060] The cold-rolled steel sheet is then subjected to continuous annealing in the range of 830 to 900°C. The continuous annealing step heats the steel sheet to the austenite single-phase region to form nearly 100% austenite for use in subsequent phase transformation. If the continuous annealing temperature (hereinafter also referred to as "SS") is less than 830°C, sufficient austenite reverse transformation may not occur, and ferrite phase may be formed in 10% or more after annealing. On the other hand, if the continuous annealing temperature exceeds 900°C, surface quality and productivity may be reduced, and coarse austenite may be formed, resulting in deterioration of material properties. 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.

[0061] The continuously annealed cold-rolled steel sheet is then primarily cooled to a primary cooling end temperature (SCS) of 500 to 700°C at an average cooling rate of less than 10°C / s. The primary cooling end temperature can be defined as the point at which secondary cooling (quenching) begins when quenching equipment 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 the final temperature and material deviation. If the primary cooling end temperature is less than 500°C, soft bainite transformation may be induced, and due to the length of the actual equipment, it is difficult to cool the steel sheet to below 500°C at a cooling rate of less than 10°C / s. 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 be formed during cooling, making it difficult to obtain high-strength steel, and if it exceeds 10°C / s, the cooling amount in the secondary cooling will be large, resulting in increased final temperature deviation and material deviation.Alternatively, the lower limit of the primary cooling end temperature may be 580°C, or the upper limit of the primary cooling end temperature may be 620°C.

[0062] The cold-rolled steel sheet 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 150 to 350°C. The secondary cooling finish temperature is set to be equal to or lower than the Ms temperature of the steel sheet, allowing martensitic transformation to occur during cooling. This martensite eventually becomes tempered martensite after a subsequent reheating step. If the secondary cooling finish temperature is less than 150°C, the amount of martensite transformation is too great, resulting in excessively high tensile strength, insufficient elongation, and high yield strength, making forming difficult. However, if most of the structure is composed of tempered martensite, high hole expandability can be maintained. On the other hand, if the secondary cooling finish temperature exceeds 350°C, martensite is not sufficiently generated during cooling, making it difficult to obtain sufficient yield strength, tensile strength, and hole expandability. Furthermore, if the final fresh martensite fraction becomes high, elongation and hole expandability will be significantly impaired. If the secondary cooling rate is less than 10°C / s, even if the target secondary cooling end temperature is reached, high-temperature phases such as upper bainite will be mixed in during cooling, making it impossible to obtain the target tempered martensite fraction and high strength. 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.

[0063] The secondary-cooled cold-rolled steel sheet is then heated to 200 to 400°C and held at this temperature for 300 to 1,000 seconds. This process achieves interphase carbon partitioning and further phase transformation, which are 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 200°C, the strength becomes too high, resulting in poor elongation. On the other hand, if the RHS or OAS temperature exceeds 400°C, it becomes difficult to obtain high strength equivalent to that of the steel of the present invention. The elongation of the steel material of the present invention is related to the retained austenite. If the secondary cooling end temperature is too low and the tempered martensite transformation occurs excessively, there may be an absolute shortage of space for the austenite to remain. Furthermore, if carbon is not sufficiently distributed into the austenite during the reheating stage, the stability of the austenite decreases, making it difficult to obtain an elongation of 12% or more.

[0064] According to one aspect of the present invention, the end temperature (RHS) of the reheating section after the secondary cooling may be 285 to 348° C. In this case, it should be noted that the end temperature of the reheating section after the secondary cooling is higher than the above-mentioned secondary cooling end temperature.

[0065] According to one aspect of the present invention, the end temperature (OAS) of the holding section of the reheating after the secondary cooling may be 273 to 342° C. Here, it should be noted that the end temperature of the holding section of the reheating after the secondary cooling is lower than the end temperature of the reheating section after the secondary cooling described above. [Example]

[0066] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are for illustrative purposes only 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.

[0067] (Example) A slab having the alloy composition (wt%) shown in Table 1 below was prepared, then reheated at 1180 to 1220°C, and subjected to hot rolling, coiling, annealing, primary cooling, secondary cooling, reheating, and holding processes under the conditions shown in Table 2 below to produce a cold-rolled steel sheet. The average cooling rate after finish hot rolling was 30 to 50°C / s, the cold reduction was 33 to 55%, the average cooling rate for the primary cooling was 4.5 to 6.5°C / s, and the average cooling rate for the secondary cooling was 20 to 40°C / s. The holding time in the holding process was 450 to 700 seconds (s).

[0068] The evaluation results of the tensile properties of the steel sheets manufactured in this manner are shown in Table 3 below. Tensile strength (TS), yield strength (YS), and total elongation (t-El) were measured by tensile tests in the direction perpendicular to the rolling direction. Total elongation (t-El) was divided into the uniform elongation (u-El) until the tensile strength was again reached and the 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 test was measured at the so-called 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 using 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 uniform elongation and local elongation.

[0069] Hole expansion ratio (HER) was measured according to the ISO 16630 standard. First, a square test specimen with a side length of 120 mm was prepared. Then, a 10 mm (Do) hole was punched in the center with a 12% clearance. The punched hole was then pushed up with a cone-shaped punch at a 60° angle, and the hole was expanded until a crack penetrating the entire thickness was formed. If the diameter of the expanded hole when a through-thickness crack occurred is Df, the hole expansion ratio (HOR) can be calculated using the following formula:

[0070]

number

[0071] The results of measuring the fraction of microstructure are also shown in Table 3. The fraction of microstructure was measured using the point counting method from scanning electron microscope (SEM) photographs, and the fraction of retained austenite was measured using XRD and is shown in Table 3 below.

[0072] In addition, five test pieces were prepared from the cold-rolled steel sheet produced by the above-mentioned method, and the number of carbides per unit area (pieces / m) in which the total ratio of Ti, Mo, and C atoms to all atoms in the carbides with a size range of 10 to 200 nm exceeded 75% was measured. 2 ) was measured by TEM observation, and the average value was calculated and shown as D in Table 3 below.

[0073] [Table 1]

[0074] [Table 2]

[0075] [Table 3]

[0076] As can be seen from Tables 1 to 4 above, Invention Examples 1 to 4 satisfied the Ti and Mo addition ranges of the present invention and also satisfied the phase fractions, and thereby exhibited a yield strength (YS) of 1000 MPa or more, a tensile strength (TS) of 1470 MPa or more, an elongation (El) of 12% or more, and a hole expandability (HER) of 25% or more.

[0077] In particular, a photograph of Ti-Mo based precipitates observed in the test piece obtained from Inventive Example 1, taken with an SEM, is shown in FIG.

[0078] On the other hand, Comparative Examples 1 to 9, which were manufactured using steel types A to I, do not satisfy the lower limit of Si and the lower limits of the added amounts of Ti and Mo proposed by the present invention. Therefore, even though they satisfied both other manufacturing conditions and phase fraction conditions, and achieved a yield strength of 1000 MPa or more and a tensile strength of 1470 MPa or more, it can be seen that the elongation did not reach 12% or the hole expandability (HER) did not satisfy 25% or more.

[0079] Furthermore, 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.

[0080] Furthermore, in the case of Comparative Example 12, although the composition range of the present invention was satisfied, the coiling temperature and the heat treatment temperature were high, and therefore large amounts of Ti and Mo were consumed by coarse Ti-Mo carbides of 250 nm or more, and the required material could not be obtained.

Claims

1. In weight percent, C: 0.20% or more but less than 0.30%, Si: 1.0 to 3.0%, Al: 0.01 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.08 to 0.32%, B: 0.0001 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.08 to 0.25%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), the remainder being Fe and other unavoidable impurities, The microstructure includes, by area percentage, ferrite: 10% or less (excluding 0%), retained austenite: more than 2% and 15% or less, fresh martensite: less than 5%, and the balance including tempered martensite and bainite. The number of carbides per unit area in which the total ratio of Ti, Mo and C atoms to all atoms of the carbides exceeds 75% is 10 １３ / m ２ 1 or more, cold-rolled steel sheets.

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

3. The cold-rolled steel sheet according to claim 1, having a yield strength of 1000 MPa or more.

4. The cold-rolled steel sheet according to claim 1, having a tensile strength of 1470 MPa or more.

5. The cold-rolled steel sheet according to claim 1, having an elongation of 12% or more.

6. The cold-rolled steel sheet according to claim 1, having a hole expandability (HER) of 25% or more.

7. reheating a slab containing, by weight, C: 0.20% or more but less than 0.30%, Si: 1.0 to 3.0%, Al: 0.01 to 0.3%, Mn: 2.0 to 3.0%, Cr: 0.001 to 0.5%, Mo: 0.08 to 0.32%, B: 0.0001 to 0.0050%, Nb: 0.001 to 0.05%, Ti: 0.08 to 0.25%, P: 0.04% or less (excluding 0%), S: 0.01% or less (excluding 0%), N: 0.01% or less (excluding 0%), the remainder being Fe and other unavoidable impurities; Finish hot rolling the reheated slab at 830 to 950°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 400 to 550°C; heat treating the coiled hot-rolled steel sheet at a temperature in the range of 550 to 650°C for 5 to 15 hours; cold-rolling the heat-treated hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuous annealing the cold-rolled steel sheet at a temperature of 830 to 900°C; Primary 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 less than 10 ° C. / s; Secondarily cooling the primarily cooled cold-rolled steel sheet to a secondary cooling end temperature of 150 to 350°C at an average cooling rate of 10°C / s or more; and reheating the secondary-cooled cold-rolled steel sheet to a temperature range of 200 to 400°C, and then holding the temperature range of 200 to 400°C for 300 to 1,000 seconds.

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

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

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

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

12. The method for manufacturing a cold-rolled steel sheet according to claim 7, wherein in the holding step, an end point temperature of a holding section of the reheating after the secondary cooling is 273 to 342°C.

Images