Cold-rolled steel sheet having excellent strength and formability and method for manufacturing the same

Abstract

The present invention relates to a cold-rolled steel sheet having excellent strength and formability and a manufacturing method thereof, and more specifically to a cold-rolled steel sheet having excellent strength and formability that can be preferably applied to structural members such as vehicle body members, seat rails, and pillars, and a manufacturing method thereof.

Description

[Technical Field]

[0001] The present invention relates to a cold-rolled steel sheet having excellent strength and formability and a manufacturing method thereof, and more specifically to a cold-rolled steel sheet having excellent strength and formability that can be preferably applied to structural members such as vehicle body members, seat rails, and pillars, and a manufacturing method thereof. [Background technology]

[0002] Recently, as safety regulations for automobile passengers and pedestrians have become stricter, the construction of safety devices has become mandatory, which has led to an increase in the weight of automobile bodies, contrary to efforts to reduce weight for improved fuel efficiency.Consumers are increasingly interested in hybrid and electric vehicles, which are environmentally friendly and highly fuel-efficient, but in order to produce such environmentally friendly and safe vehicles, it is necessary to reduce the weight of the body structure and ensure the stability of the body materials.

[0003] However, unlike conventional gasoline engines, eco-friendly vehicles are equipped with various devices such as electric motors, batteries, and secondary fuel storage tanks, and as driver convenience facilities continue to be added, the weight of the vehicle body is increasing. Therefore, to achieve vehicle weight reduction, it is essential to develop a material that has ultra-high strength and is thin yet has excellent strength, ductility, hole expandability, and bending properties. To solve this problem, it is necessary to develop cold-rolled steel sheets with ultra-high strength of over 1180 MPa, excellent hole expandability, and bending properties.

[0004] Meanwhile, as regulations on automobile crash stability have expanded recently, ultra-high strength steels with excellent yield strength are being used for structural members such as members, seat rails, and pillars to improve the crashworthiness of the vehicle body.

[0005] The higher the yield strength relative to the tensile strength, i.e., the higher the yield ratio (yield strength / tensile strength), the better the structural components absorb impact energy. However, as the strength of steel plate increases, the elongation rate generally decreases, resulting in poor formability. Therefore, there is a need to develop materials that simultaneously have a high yield ratio, formability, and improved hole expandability and bending properties, which are important physical properties when processing parts.

[0006] A typical manufacturing method for increasing yield strength is to use water cooling during continuous annealing. For example, after soaking in the annealing process, the steel sheet is immersed in water and tempered, thereby transforming the microstructure from martensite to tempered martensite.

[0007] A representative prior art example of the above-mentioned method is Patent Document 1. Patent Document 1 relates to a technique for producing a steel material having a martensite volume fraction of 80 to 97% and the remainder being ferrite, by continuously annealing a steel material containing 0.18 to 0.3% carbon, water-cooling it to room temperature, and then overaging it at a temperature of 120 to 300°C for 1 to 15 minutes. When ultra-high strength steel is produced using this water-cooling followed by tempering, the yield ratio is very high, but temperature deviations in the width and length directions can cause problems such as poor quality of the coil shape. This can lead to problems during roll forming, such as poor material quality in some areas and reduced workability.

[0008] Patent Document 2 is a conventional technique related to improving the workability of the above-mentioned high-tensile steel sheet. Patent Document 2 relates to a steel sheet having a composite structure mainly composed of tempered martensite, and is characterized by dispersing fine precipitated Cu particles with a particle size of 1 to 100 nm within the structure to improve workability. However, Patent Document 2 has problems in that excessive addition of Cu to the Cu content of 2 to 5% in order to precipitate good fine Cu particles may cause red shortness due to Cu, and also excessively increases manufacturing costs.

[0009] Therefore, there is a need to solve the above-mentioned problems and develop a steel material that has an ultra-high strength of 1180 MPa or more, a high yield ratio, and a tensile strength of 1180 MPa or more, which does not crack even in a 180° full crimp bending test and can be cold formed. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Patent No. 2528387 [Patent Document 2] Japanese Patent Application Laid-Open No. 2005-264176 Summary of the Invention [Problem to be solved by the invention]

[0011] One aspect of the present invention is to provide a cold-rolled steel sheet having excellent strength and formability, and a method for producing the same.

[0012] The object of the present invention is not limited to the above-mentioned content, and 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 specification of the present invention. [Means for solving the problem]

[0013] One aspect of the present invention is In weight percent, it contains C: 0.10 to 0.20%, Si: 0.05 to 0.5%, Al: 0.01 to 0.18%, Mn: 2.4 to 3.5%, Cr: 0.05 to 0.8%, Mo: 0.05 to 0.4%, B: 0.0001 to 0.003%, Nb: 0.005 to 0.07%, Ti: 0.005 to 0.07%, the balance being Fe and other unavoidable impurities, At a position 5 μm from the surface in the thickness direction, the average number of MC and M(C, N) precipitates per unit area with a circle equivalent diameter of less than 100 nm (excluding 0 nm) was 10 5 ~10 7 pieces / m 2wherein M represents one or more elements selected from the group consisting of Nb, Ti, Si, Cr, Mo, and Fe.

[0014] Furthermore, still another aspect of the present invention is a step of heating a slab containing, by weight, 0.10 to 0.20% C, 0.05 to 0.5% Si, 0.01 to 0.18% Al, 2.4 to 3.5% Mn, 0.05 to 0.8% Cr, 0.05 to 0.4% Mo, 0.0001 to 0.003% B, 0.005 to 0.07% Nb, 0.005 to 0.07% Ti, the remainder being Fe and other unavoidable impurities; a step of finish-rolling the heated slab so that the finish-rolling delivery temperature is A3+50°C to A3+160°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 600 to 700°C; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; continuous annealing the cold-rolled steel sheet at a continuous annealing temperature (SS) range of A3 to 900°C with a dew point temperature of -35 to +20°C; a step of primarily cooling the continuously annealed cold-rolled steel sheet to a primary cooling finish temperature (SCS) of 550 to 650°C at an average cooling rate of less than 7°C / s (excluding 0°C / s); Secondary cooling the primarily cooled cold-rolled steel sheet to a secondary cooling finish temperature (RCS) of 400 to 450°C at an average cooling rate of 8°C / s or more; Tertiary cooling the second-cooled cold-rolled steel sheet to a tertiary cooling finish temperature (TCS) of 300 to 380°C at an average cooling rate of less than 8°C / s (excluding 0°C / s); and maintaining the tertiarily cooled cold-rolled steel sheet at a maintaining temperature (RHS) equal to the tertiary cooling finish temperature (TCS) or reheating the tertiarily cooled cold-rolled steel sheet at a reheating temperature (RHS) range of 300 to 500°C, which is higher than the tertiary cooling finish temperature (TCS). [Effects of the Invention]

[0015] 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.

[0016] The various yet significant advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0017] [Figure 1] 1 shows a photograph of a cross-sectional test piece of the cold-rolled steel sheet obtained in Example 1 taken with a scanning electron microscope (SEM). [Figure 2] 1 shows a photograph of a cross-sectional test piece for the cold-rolled steel sheet obtained in Comparative Example 1 taken with a scanning electron microscope (SEM). DETAILED DESCRIPTION OF THE INVENTION

[0018] The following describes preferred embodiments of the present invention. However, the embodiments of the present invention can be modified in various ways, 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.

[0019] 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 form used in this specification includes the plural form unless a related definition clearly indicates otherwise. Furthermore, the meaning of "comprises" used in this specification embodies a configuration and does not exclude the presence or addition of other configurations.

[0020] Hereinafter, a cold-rolled steel sheet according to one embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. The contents of the alloy composition described below are in wt %.

[0021] C: 0.10 to 0.20% Carbon (C) is a very important element added for solid solution strengthening. Furthermore, C contributes to improving strength by combining with precipitation strengthening elements to form fine carbides. If the C content is less than 0.10%, it is very difficult to achieve the desired strength. On the other hand, if the C content exceeds 0.20%, the hardenability increases, causing excessive martensite formation during cooling, resulting in a rapid increase in strength and a deterioration in bending workability. Furthermore, the weldability deteriorates, increasing the likelihood of welding defects occurring during component processing at customer companies. Therefore, the C content is preferably in the range of 0.10 to 0.20%. To further improve the above-mentioned effects, the lower limit of the C content may be 0.11% or 0.12%, or the upper limit of the C content may be 0.19% or 0.18%.

[0022] Si: 0.05 to 0.5% Silicon (Si) not only contributes to increased strength but also inhibits carbide formation, distributing carbon during annealing, soaking, and cooling without forming carbides and accumulating in the retained austenite, thereby ensuring the presence of the austenite phase at room temperature. This contributes to maintaining elongation. If the Si content is less than 0.05%, it may be difficult to fully achieve the above-mentioned effects. On the other hand, if the Si content exceeds 0.5%, it becomes impossible to prevent deterioration of weld physical properties due to the formation of LME cracks, and surface defects such as red scale are induced, resulting in poor surface properties and platability of the steel. To further improve the above-mentioned effects, the lower limit of the Si content may be 0.10% or 0.15%, and the upper limit of the Si content may be 0.49% or 0.45%.

[0023] Al: 0.01 to 0.18% Aluminum (Al) is an element contained not only for deoxidizing steel but also for suppressing cementite precipitation and stabilizing retained austenite. If the Al content is less than 0.01%, it may be difficult to fully ensure the above-mentioned effects. On the other hand, if the Al content exceeds 0.18%, the castability of the steel may be impaired. On the other hand, to further improve the above-mentioned effects, the lower limit of the Al content may be 0.02% or 0.03%, or the upper limit of the Al content may be 0.17% or 0.16%.

[0024] Mn: 2.4 to 3.5% Manganese (Mn) is an element added to ensure strength. If the Mn content is less than 2.4%, it becomes difficult to ensure strength. On the other hand, if the Mn content exceeds 3.5%, the bainite transformation rate slows and excessive fresh martensite is formed, making it difficult to obtain high hole expandability. In addition, band structures are formed due to Mn segregation, impairing the material uniformity and formability of the material. On the other hand, to further improve the above-mentioned effects, the lower limit of the Mn content may be 2.5% or 2.6%, and the upper limit of the Mn content may be 3.4% or 3.2%.

[0025] Cr: 0.05 to 0.8% Chromium (Cr) is an element added to ensure strength and hardenability. When Mn is added alone, a significantly larger amount of Mn must be added than the Mn content range of the present invention to ensure strength and hardenability. However, adding 0.05% or more of Cr can solve this problem. On the other hand, if the Cr content exceeds 0.8%, local corrosion resistance deteriorates and oxides form on the surface, impairing phosphating properties. To further improve the above-mentioned effects, the lower limit of the Cr content may be 0.06% or 0.07%, and the upper limit of the Cr content may be 0.7% or 0.6%.

[0026] Mo: 0.05 to 0.4% Molybdenum (Mo) is an element added to ensure strength and hardenability. When Mn is added alone, a very large amount of Mn must be added, exceeding the Mn content range of the present invention. Adding 0.05% or more of Mo can solve this problem. On the other hand, if the Mo content exceeds 0.4%, phase transformation is suppressed, making it difficult to obtain a bainite structure, and as an expensive element, the economic viability of the steel sheet is reduced. To further improve the above-mentioned effects, the lower limit of the Mo content may be 0.06% or 0.07%, and the upper limit of the Mo content may be 0.35% or 0.3%.

[0027] B: 0.0001 to 0.003% 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. Adding 0.0001% or more of B can solve this problem. On the other hand, if the B content exceeds 0.0030%, B accumulates excessively on the surface, impairing plating adhesion. To further improve the above-mentioned effects, the lower limit of the B content may be 0.0002% or 0.0003%, and the upper limit of the B content may be 0.0025% or 0.0023%.

[0028] Nb: 0.005 to 0.07% Niobium (Nb) is an element added to ensure strength and refine the microstructure. If the Nb content is less than 0.005%, it is difficult to obtain the effects of improving strength and refining the microstructure. On the other hand, if the Nb content exceeds 0.07%, local grain fixation delays recrystallization, impairing the uniformity of the microstructure. On the other hand, to further improve the above-mentioned effects, the lower limit of the Nb content may be 0.010% or 0.015%, and the upper limit of the Nb content may be 0.06% or 0.05%.

[0029] Ti: 0.005 to 0.07% Titanium (Ti) is an element added to ensure strength and refine the microstructure. If the Ti content is less than 0.005%, the effects of improving strength and refining the microstructure are difficult to obtain. On the other hand, if the Ti content exceeds 0.07%, castability is impaired due to excessive TiN formation, and recrystallization is delayed due to local grain fixation, resulting in a loss of microstructure uniformity. To further improve the above-mentioned effects, the lower limit of the Ti content may be 0.010% or 0.015%, and the upper limit of the Ti content may be 0.06% or 0.05%.

[0030] The remaining component of the present invention is iron (Fe). However, in a normal manufacturing process, unintentional impurities may be inevitably mixed in from raw materials or the surrounding environment, and this cannot be excluded. Since these impurities are known to anyone skilled in the normal manufacturing process, the contents of all of them will not be specifically mentioned in this specification.

[0031] Meanwhile, although not particularly limited, according to one embodiment of the present invention, the impurities may include one or more of P, S, Sb, N, Mg, Sn, Sb, Zn, and Pb as tramp elements, and the total amount thereof may be 0.1 wt% or less. The tramp elements are impurity elements, including those contained in scrap used as raw materials in the steelmaking process, and if the total amount thereof exceeds 0.1%, it may cause surface cracks in the slab and degrade the surface quality of the steel sheet.

[0032] Although not particularly limited, according to one embodiment of the present invention, the cold-rolled steel sheet may have a microstructure, in area percentages, of 1 to 10% fresh martensite (hereinafter also referred to as "FM"), 80 to 98% of one or more selected from the group consisting of tempered martensite (hereinafter also referred to as "TM") and bainite (hereinafter also referred to as "B"), less than 2% (including 0%) retained austenite (hereinafter also referred to as "RA"), and 5% or less (including 0%) ferrite (hereinafter also referred to as "F"). If the FM fraction is less than 1% or the fraction of one or more selected from the group consisting of TM and B is less than 80%, it may be difficult to ensure the strength targeted by the present invention. On the other hand, if the FM fraction exceeds 10% or the fraction of one or more selected from the group consisting of TM and B exceeds 98%, the elongation and bending properties may be deteriorated. Furthermore, if the F fraction exceeds 5%, it may be difficult to ensure the strength targeted by the present invention.

[0033] According to one embodiment of the present invention, the cold-rolled steel sheet has an average number (D5) per unit area of ​​MC and M(C,N)-based precipitates having a circle equivalent diameter of less than 100 nm (excluding 0 nm) (or 1 nm or more and 99 nm or less) at a position 5 μm from the surface in the thickness direction of the steel sheet. 5 ~10 7 pieces / m 2 The average number of particles per unit area (D5) can be in the range of 10 5 pieces / m 2 Less than or equal to 10 7 pieces / m 2If the content exceeds , hole expandability and bending workability may be deteriorated. Here, the M represents one or more elements selected from the group consisting of Nb, Ti, Si, Cr, Mo, and Fe. In this specification, the precipitates refer to MC and M(C,N)-based precipitates, and the M may be a single precipitate containing only one element selected from the group consisting of Nb, Ti, Si, Cr, Mo, and Fe, or a composite precipitate containing two or more elements selected from the group consisting of Nb, Ti, Si, Cr, Mo, and Fe.

[0034] Meanwhile, the above-mentioned average number per unit area (D5) can be measured by a conventional method known in the art, and the measurement method is not particularly limited. For example, the average number per unit area (D5) can be measured by polishing and etching a cross section within a range of 5 μm from the surface of the steel sheet in the thickness direction. Five fields of view of the cross-sectional test piece of the corroded surface obtained after such etching are photographed using a transmission electron microscope (TEM) at 5,000 magnifications, the number of each precipitate satisfying the above-mentioned circle equivalent diameter is counted, and the average number of these photographs is calculated.

[0035] Although not particularly limited, according to one embodiment of the present invention, the cold-rolled steel sheet has an average number per unit area (D 70 ) is 10 9 ~10 12 pieces / m 2 The average number of particles per unit area (D 70 ) is 10 9 pieces / m 2 If the average number of particles per unit area (D 70 ) is 10 12 pieces / m 2 If it exceeds this value, the hole expandability may deteriorate.

[0036] In addition, the average number of particles per unit area (D 70 The method for measuring the average number of particles per unit area (D) can be measured by a conventional method known in the art, and the measuring method is not particularly limited. 70 ) can be measured by polishing and etching a cross section of the steel plate within a range of 70 μm from the surface in the thickness direction. Five fields of view of the test piece of the corroded surface obtained after such etching are photographed using a transmission electron microscope (TEM) at 5,000x magnification, the number of precipitates satisfying the above-mentioned circle equivalent diameter is counted, and the average of these numbers is calculated.

[0037] Although not particularly limited, according to one embodiment of the present invention, the cold-rolled steel sheet has an average C content (W C70 ) relative to the average C content at a position 5 μm from the steel sheet surface (W C5 ) ratio (W C5 / W C70 ) may be 0.2 to 0.8. In order to ensure hydrogen embrittlement resistance by utilizing stress relaxation near the surface layer of the steel sheet through dew point control in the continuous annealing process, the average C content (W C70 ) relative to the average C content at a position 5 μm from the steel sheet surface (W C5 ) ratio (W C5 / W C70 ) can be set to the above range. In particular, the above ratio (W C5 / W C70 If the ratio (W) is less than 0.2, the hydrogen embrittlement resistance may be poor. C5 / W C70 ) exceeds 0.8, excessive decarburization can cause a problem of reduced strength.

[0038] On the other hand, the above ratio (W C5 / W C70 The method for measuring the ratio (W) can be performed by a conventional method known in the art, and the measuring method is not particularly limited. For example, C5 / W C70When measuring the C content of the steel sheet, in order to eliminate the influence of oxides, the steel sheet is etched from the surface to 5 nm in the thickness direction, and then a cross-sectional test piece is obtained by cutting the cold-rolled steel sheet in the thickness direction (meaning the direction perpendicular to the rolling direction). Next, the C content of each of the five regions corresponding to positions 70 μm from the surface in the thickness direction of the cross-sectional test piece is measured using GDS, and the average value (W C70 Similarly, for the cross-sectional test piece, the C content of each of the five regions corresponding to positions 5 μm from the surface in the thickness direction is measured, and then the average value (W C5 ) is calculated using the above method. C5 / W C70 ) can be obtained.

[0039] Although not particularly limited, according to one embodiment of the present invention, a plating layer may be provided on at least one surface of the cold-rolled steel sheet.

[0040] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to still another embodiment of the present invention will be described.

[0041] First, a slab satisfying the above-described alloy composition is heated. In the present invention, the slab heating temperature is not particularly limited, but may be, for example, 1100 to 1300°C. If the slab heating temperature is less than 1100°C, drawbacks such as rolling load during rough rolling may occur. Furthermore, if the slab heating temperature exceeds 1300°C, drawbacks such as coarsening of the microstructure and increased power costs may occur. The lower limit of the slab heating temperature is more preferably 1125°C, and even more preferably 1150°C. The upper limit of the slab heating temperature is more preferably 1275°C, and even more preferably 1250°C. Meanwhile, the slab may have a thickness of 230 to 270 mm.

[0042] The heated slab is then finish-rolled to a finish rolling outlet temperature (hereinafter also referred to as "FDT") of A3 + 50°C to A3 + 160°C to obtain a hot-rolled steel sheet. If the finish rolling outlet temperature is less than A3 + 50°C, there is a high possibility that the hot deformation resistance will increase sharply. If the finish rolling outlet temperature exceeds A3 + 160°C, there is a high possibility that not only will an excessively thick oxide scale be formed but also that the microstructure of the steel sheet will become coarse. Therefore, the finish rolling outlet temperature is preferably in the range of A3 + 50°C to A3 + 160°C. The lower limit of the finish rolling outlet temperature is more preferably A3 + 60°C, and even more preferably A3 + 70°C. The upper limit of the finish rolling outlet temperature is more preferably A3 + 150°C, and even more preferably A3 + 140°C. Meanwhile, the A3 temperature can be calculated using the following formula 1. [Formula 1] A3=910-203×C 1 / 2 +44.7×Si+31.5×Mo-30×Mn-11×Cr+400×Al+400×Ti

[0043] The hot-rolled steel sheet is then coiled at 600 to 700°C. If the coiling temperature (hereinafter also referred to as "CT") is less than 600°C, martensite or bainite is excessively generated, resulting in an excessive increase in the strength of the hot-rolled steel sheet, which may cause problems such as shape defects due to load during cold rolling. On the other hand, if the temperature exceeds 700°C, the pickling properties may deteriorate due to an increase in surface scale. Therefore, the coiling temperature is preferably in the range of 600 to 700°C.

[0044] On the other hand, in order to further improve the above-mentioned effect, the lower limit of the winding temperature may be 633°C, and the upper limit of the winding temperature may be 658°C.

[0045] Meanwhile, according to one embodiment of the present invention, the cooling process after the coiling is not particularly limited. For example, the coiled hot-rolled steel sheet may be cooled to room temperature at a cooling rate of 0.1°C / s or less (excluding 0°C / s).

[0046] Thereafter, the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. In the present invention, the reduction ratio during the cold rolling is not particularly limited. For example, the cold rolling may be performed at a reduction ratio of 20% or more (or 20% to 70%; i.e., in the range of 20 to 70%). If the cold rolling reduction ratio is less than 20%, the driving force for recrystallization may be weakened, which may make it difficult to obtain good recrystallized grains and may make shape correction very difficult. On the other hand, if the cold rolling reduction ratio exceeds 70%, cracks may be likely to occur at the edge of the steel sheet and the rolling load may increase rapidly. Therefore, it is preferable that the cold rolling be performed at a reduction ratio of 20% or more. Meanwhile, before the cold rolling, pickling may be performed to remove scale and impurities adhering to the surface.

[0047] Next, the cold-rolled steel sheet is subjected to continuous annealing at a continuous annealing temperature (hereinafter also referred to as "SS") in the range of A3 to 900°C. The continuous annealing is performed by heating the steel sheet to the austenite single-phase region to form nearly 100% austenite, which is then used for phase transformation. If the continuous annealing temperature is lower than A3, sufficient recrystallization and austenite transformation do not occur, and the martensite and bainite fractions that the present invention aims to achieve after annealing cannot be ensured. On the other hand, if the continuous annealing temperature exceeds 900°C, productivity decreases, coarse austenite is formed, which may deteriorate the material quality, and surface quality such as peeling of the plated material may also deteriorate.

[0048] According to one embodiment of the present invention, in order to further improve the above-mentioned effects, the lower limit of the continuous annealing temperature SS may be 830°C, or the upper limit of the continuous annealing temperature SS may be 840°C.

[0049] Furthermore, by controlling the dew point temperature during annealing heat treatment of the above cold-rolled steel sheet within a range of -35 to +20°C at a continuous annealing temperature (hereinafter also referred to as "SS") of A3 to 900°C, stress in the surface layer of the steel sheet can be alleviated, thereby ensuring resistance to hydrogen embrittlement.

[0050] According to one embodiment of the present invention, in order to further improve the above-mentioned effects, the lower limit of the dew-point temperature in the continuous annealing temperature range may be −20° C., or the upper limit of the dew-point temperature in the continuous annealing temperature range may be +10° C.

[0051] In the present invention, the atmosphere during the continuous annealing is not particularly limited. For example, the continuous annealing can be performed in a gas atmosphere consisting of, by volume, 95% or more (including 100%) nitrogen and the remainder hydrogen. If the nitrogen fraction is less than 95%, an oxidizing atmosphere is formed in the furnace unless the hydrogen fraction is correspondingly increased, which may result in the formation of oxides on the steel sheet surface, thereby deteriorating the surface quality. Furthermore, if the hydrogen fraction is increased, further process difficulties, such as explosion prevention, may arise. Meanwhile, the upper limit of the nitrogen fraction in the atmosphere during the continuous annealing is not particularly limited.

[0052] The continuously annealed cold-rolled steel sheet is then subjected to primary cooling at an average cooling rate of less than 7°C / s (excluding 0°C / s) to a primary cooling finish temperature (SCS) of 550 to 650°C. The primary cooling finish temperature (SCS) can be defined as the point at which secondary cooling (quenching) begins, when a quenching device not used in the primary cooling is added. In the present invention, the cooling process is divided into primary and secondary stages and performed in stages, thereby uniforming the temperature distribution of the steel sheet during the slow cooling stage, reducing the final temperature and material deviation, and obtaining the desired phase. If the primary cooling finish temperature exceeds 650°C, the cooling amount to the secondary cooling finish temperature increases, resulting in poor steel sheet shape and a lower bainite fraction than the target level. Meanwhile, to ensure the desired physical properties, the primary cooling is preferably controlled to a cooling rate of less than 7°C / s, and the lower limit of the primary cooling finish temperature is preferably 550°C. If the average cooling rate during the primary cooling is 7°C / s or more, the amount of cooling during the secondary cooling will be large, which may cause problems such as increased final temperature deviation and material deviation. On the other hand, there is no particular limit to the lower limit of the average cooling rate during the primary cooling.

[0053] According to one embodiment of the present invention, in order to further improve the above-mentioned effects, the lower limit of the primary cooling finish temperature (SCS) may be 560°C, or the upper limit of the primary cooling finish temperature (SCS) may be 600°C.

[0054] According to one embodiment of the present invention, in order to further improve the above-mentioned effects, the lower limit of the average cooling rate during the primary cooling may be 1.0°C / s, or the upper limit of the average cooling rate during the primary cooling may be 6.9°C / s.

[0055] The cold-rolled steel sheet that has undergone primary cooling is then secondarily cooled to a secondary cooling finish temperature (RCS) of 400 to 450°C at an average cooling rate of 8°C / s or more (or 8°C / s or more and 20°C / s or less; i.e., 8 to 20°C / s). The secondary cooling finish temperature is set at a cooling rate of 8°C / s or more before the steel sheet cools below the Ms temperature to prevent the introduction of bainite during cooling and to prevent shape defects due to a rapid cooling rate and martensitic transformation. If the secondary cooling finish temperature is less than 400°C, shape defects may occur due to a rapid temperature difference and martensitic transformation. On the other hand, if the secondary cooling finish temperature exceeds 450°C, sufficient martensitic transformation may not be induced during tertiary cooling, resulting in failure to achieve the target strength. If the secondary cooling rate is less than 8°C / s, high-temperature phase transformation may occur during cooling, even if the target secondary cooling finish temperature is reached, and the target martensite fraction and high strength may not be achieved.

[0056] Meanwhile, according to one embodiment of the present invention, the upper limit of the average cooling rate during the secondary cooling is not particularly limited and may be, for example, 20°C / s. In order to further improve the above-mentioned effects, the upper limit of the average cooling rate during the secondary cooling may be 15°C / s.

[0057] According to one embodiment of the present invention, the secondary cooling may further employ a quenching equipment not employed in the primary cooling. The present invention does not particularly limit the type of quenching equipment, but a preferred example is a hydrogen quenching equipment. More specifically, the hydrogen quenching equipment may have a gas atmosphere consisting of 50 to 80% hydrogen by volume and the remainder nitrogen. If the hydrogen fraction exceeds 80%, it may be difficult to manage the equipment, such as explosion control, while if it is less than 50%, it may be difficult to utilize the efficient heat transfer properties of hydrogen, a light element.

[0058] The secondarily cooled cold-rolled steel sheet can then be tertiary cooled to a tertiary cooling finish temperature (TCS) of 300 to 380°C at an average cooling rate of less than 8°C / s (excluding 0°C / s). By cooling the steel sheet below its Ms temperature, martensite transformation occurs during cooling, and this martensite eventually transforms into a tempered martensite phase through a subsequent reheating step. Because the Ms temperature of most 1180 MPa-class ultra-high strength steel sheets is below 400°C, the present invention controls the tertiary cooling finish temperature to within the range of 300 to 380°C. If the tertiary cooling finish temperature is below 300°C, the amount of martensite transformation may be too high, resulting in high strength, insufficient elongation, and excessively high yield strength, which may result in poor formability. On the other hand, if the tertiary cooling finish temperature exceeds 380°C, sufficient martensite transformation may not occur, making it difficult to achieve the desired strength. On the other hand, if the average cooling rate during the tertiary cooling is 8°C / s or more, a rapid phase transformation below Ms may cause defective shape.

[0059] Meanwhile, according to one embodiment of the present invention, in order to further improve the above-mentioned effects, the lower limit of the average cooling rate during the tertiary cooling may be 1.0°C / s, or the upper limit of the average cooling rate during the tertiary cooling may be 7.9°C / s.

[0060] Furthermore, according to one embodiment of the present invention, the tertiary cooling may be performed so that Ms-TCS (°C) is in the range of 30 to 60°C. By satisfying this range, a martensite phase capable of ensuring the target strength can be obtained, and rapid transformation can be suppressed to ensure shape quality.

[0061] On the other hand, the Ms temperature can be calculated from the following formula 2. [Formula 2] Ms=539-423×[C]-30.4×[Mn]-7.5×[Si]+30×[Al] (In the above formula 2, [C], [Mn], [Si], and [Al] represent the weight percent content of each element in parentheses.)

[0062] Furthermore, according to one embodiment of the present invention, the reheating can be performed so that the RHS-TCS (°C) is in the range of 10 to 150°C. By satisfying this range, it is possible to suppress deterioration of hole expandability and bending workability while ensuring the target strength.

[0063] Thereafter, the tertiarily cooled cold-rolled steel sheet may be maintained at a maintaining temperature (RHS) equal to the tertiary cooling finish temperature (TCS), or may be reheated at a reheating temperature (RHS) range of 300 to 500°C, which is higher than the tertiary cooling finish temperature (TCS).

[0064] The above-mentioned holding or reheating process results in interphase carbon partitioning and further bainite phase transformation necessary for stabilizing the retained austenite. In the present invention, the holding temperature of the holding section or the reheating temperature, which is the end temperature of the reheating section, is referred to as the "RHS" for convenience. If the above-mentioned holding or reheating temperature is less than 300°C, the martensite formed during the primary cooling is not tempered, resulting in excessively high strength and poor elongation. On the other hand, if the above-mentioned holding or reheating temperature exceeds 500°C, excessive tempering or bainite transformation occurs during reheating, making it difficult to achieve the target strength.

[0065] Furthermore, according to one embodiment of the present invention, the above-mentioned maintaining or reheating can be performed so that the RHS-TCS (°C) is in the range of 10 to 150°C. By satisfying this range, it is possible to suppress deterioration of hole expandability and bending workability while ensuring the target strength.

[0066] Meanwhile, the present invention may further include a step of hot-dip galvanizing the cold-rolled steel sheet in a coating bath at 430 to 490°C after the reheating step. The type of hot-dip galvanizing is not particularly limited in the present invention, and the hot-dip galvanizing may be, for example, hot-dip galvanizing. Note that, in the present invention, the cold-rolled steel sheet after the reheating may be hot-dip galvanized as is, or may be cooled to room temperature and then reheated and hot-dip galvanized.

[0067] In addition, the present invention may further include a step of temper rolling the cold-rolled steel sheet at a reduction of less than 2% (excluding 0%) after the hot-dip coating. The temper rolling is for correcting the shape of the steel sheet and adjusting its yield strength. The temper rolling may be performed after cooling to room temperature after the hot-dip coating. [Example]

[0068] The present invention will be described in more detail below with reference to examples. However, it should be noted that the following examples are merely for illustrative purposes 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 matters described in the claims and matters that can be reasonably inferred therefrom.

[0069] (Example) Molten steel having the alloy composition shown in Table 1 below was prepared and then continuously cast to produce a 250 mm thick slab. The slab was heated at 1200°C for 12 hours and then hot-rolled and coiled under the conditions shown in Table 2 below to obtain a 3.0 mm thick hot-rolled steel sheet. The slab was then pickled and cold-rolled at a 50% cold reduction to obtain a 1.5 mm thick cold-rolled steel sheet. The cold-rolled steel sheet was then subjected to continuous annealing, primary cooling, secondary cooling, tertiary cooling, and reheating under the conditions shown in Tables 2 and 3 below, followed by hot-dip galvanization. The gas used during the continuous annealing was 95 vol% N-5 vol% H, and the gas used during the secondary cooling was 75 vol% H-25 vol% N. The microstructure, precipitates, and mechanical properties of the cold-rolled steel sheet produced in this manner were measured, and the results are shown in Tables 4 and 5 below.

[0070] The microstructure fraction was measured using Electron Backscatter Diffraction (EBSD) and XRD. 70 ) and the surface C ratio (W C5 / W C70 ) were observed using a transmission electron microscope (TEM) after preparing samples using the replica method, similar to the method described above. However, in the present invention, it is difficult to distinguish between TM and B, so the total fraction was shown.

[0071] Tensile strength (TS), yield strength (YS), yield ratio (YR), total elongation (T-EL), and uniform elongation (U-EL) were measured by tensile tests in the horizontal rolling direction. The gauge length was 50 mm and the width of the tensile test specimen was 15 mm. Hole expandability (HER) was measured according to the ISO 16330 standard, and the hole was sheared using a 10 mm diameter punch with 12% clearance.

[0072] To evaluate hydrogen embrittlement resistance, test pieces 30 mm wide and 110 mm long were taken from the steel sheet in the width direction, parallel to the rolling direction, and bent at 90° with a bending radius of 7 mm. The pieces were then fastened with bolts equal to the amount of expansion due to springback, immersed in hydrochloric acid of pH 3, and examined for the occurrence of cracks after 96 hours. In this example, pieces with no cracks were judged to have excellent hydrogen embrittlement resistance, and are marked with "○" in the hydrogen embrittlement resistance column in Table 5, while pieces with cracks are marked with "×".

[0073] In addition, to evaluate weldability, welding was performed under the conditions of Force 4.5KN, Welding time 170ms, and Holding time 250ms. The welding current range was then defined as the range between the lower limit current that satisfies 4√t (t = thickness) and the upper limit current obtained by subtracting 0.2kA from the welding current at which Explosion occurred. A welding current range of 1.5kA or more was judged to be "good," and a current range of less than 1.5kA was judged to be "poor."

[0074] [Table 1]

[0075] [Table 2]

[0076] [Table 3]

[0077] [Table 4]

[0078] [Table 5]

[0079] As can be seen from Tables 1 to 5 above, in the case of Examples 1 to 5, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, the microstructure, precipitates, mechanical properties, etc. that the present invention aims to obtain are ensured.

[0080] On the other hand, in the case of comparative examples that do not satisfy the alloy composition or manufacturing conditions proposed by the present invention, it can be confirmed that one or more of the characteristics of the microstructure, precipitates, and mechanical properties that the present invention aims to achieve cannot be secured.

[0081] Meanwhile, FIG. 1 shows a photograph of the microstructure of Inventive Example 1 observed by SEM, and FIG. 2 shows a photograph of the microstructure of Comparative Example 1 observed by SEM.

Claims

1. The alloy contains, by weight, C: 0.10 to 0.20%, Si: 0.05 to 0.5%, Al: 0.01 to 0.18%, Mn: 2.4 to 3.5%, Cr: 0.05 to 0.8%, Mo: 0.05 to 0.4%, B: 0.0001 to 0.003%, Nb: 0.005 to 0.07%, Ti: 0.005 to 0.07%, the balance being Fe and other unavoidable impurities, At a position 5 μm from the surface in the thickness direction, the average number of MC and M(C,N)-based precipitates per unit area with a circle equivalent diameter of less than 100 nm (excluding 0 nm) was 10 ５ ~10 ７ pieces / m ２ wherein M represents one or more elements selected from the group consisting of Nb, Ti, Si, Cr, Mo, and Fe.

2. The cold-rolled steel sheet according to claim 1, wherein the microstructure comprises, in area %, 1 to 10% fresh martensite, 80 to 98% of one or more selected from the group consisting of tempered martensite and bainite, less than 2% (including 0%) retained austenite, and 5% or less (including 0%) ferrite.

3. At a position 70 μm from the surface in the thickness direction, the average number of MC and M(C,N)-based precipitates per unit area with a circle equivalent diameter of less than 100 nm (excluding 0 nm) was 10 ９ ~10 １２ pieces / m ２ The cold-rolled steel sheet according to claim 1,

4. The average C content (W Ｃ７０ ) the average C content (W Ｃ５ ) ratio (W Ｃ５ / W Ｃ７０ 2. The cold-rolled steel sheet according to claim 1, wherein σ is 0.2 to 0.

8.

5. The cold-rolled steel sheet according to claim 1 , further comprising a plating layer provided on at least one surface of the cold-rolled steel sheet.

6. a step of heating a slab containing, in weight percent, 0.10 to 0.20% C, 0.05 to 0.5% Si, 0.01 to 0.18% Al, 2.4 to 3.5% Mn, 0.05 to 0.8% Cr, 0.05 to 0.4% Mo, 0.0001 to 0.003% B, 0.005 to 0.07% Nb, 0.005 to 0.07% Ti, the balance being Fe and other unavoidable impurities; The heated slab is subjected to finishing rolling at a temperature of A ３ +50℃~A ３ A step of finish rolling to +160°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at 600 to 700°C; cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; The cold-rolled steel sheet is A ３ a step of performing continuous annealing in a continuous annealing temperature range of 0°C to 900°C with a dew point temperature of -35°C to +20°C; primary cooling the continuously annealed cold-rolled steel sheet to a primary cooling end temperature of 550 to 650°C at an average cooling rate of less than 7°C / s (excluding 0°C / s); secondarily cooling the primarily cooled cold-rolled steel sheet to a secondary cooling end temperature of 400 to 450°C at an average cooling rate of 8°C / s or more; Tertiarily cooling the secondarily cooled cold-rolled steel sheet to a tertiary cooling end temperature of 300 to 380°C at an average cooling rate of less than 8°C / s (excluding 0°C / s); and maintaining the tertiarily cooled cold-rolled steel sheet at a temperature equal to the end temperature of the tertiary cooling or reheating the cold-rolled steel sheet at a reheating temperature range of 300 to 500°C, which is higher than the end temperature of the tertiary cooling.

7. The method for producing a cold-rolled steel sheet according to claim 6, wherein the slab is heated to 1100 to 1300°C.

8. The method for producing a cold-rolled steel sheet according to claim 6, wherein the cold rolling is performed at a reduction ratio of 20 to 70%.

9. The method for producing a cold-rolled steel sheet according to claim 6, wherein the continuous annealing is performed in a gas atmosphere containing, by volume %, 95% or more (including 100%) of nitrogen and the remainder being hydrogen.

10. The method for producing a cold-rolled steel sheet according to claim 6, wherein the secondary cooling is carried out in a hydrogen quenching facility in a gas atmosphere consisting of 50 to 80% by volume of hydrogen and the remainder being nitrogen.

11. The method for manufacturing a cold-rolled steel sheet according to claim 6, further comprising the step of hot-dip galvanizing the cold-rolled steel sheet in a coating bath at 430 to 490°C after the maintaining or reheating step.

12. The method for producing a cold-rolled steel sheet according to claim 11, further comprising the step of temper rolling the cold-rolled steel sheet at a reduction of less than 2% (excluding 0%) after the hot-dip coating.

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