Cold-rolled steel sheet and method for manufacturing the same

Abstract

The present invention relates to a cold-rolled steel sheet and a method of manufacturing the same, and more particularly, to a high-strength cold-rolled steel sheet having excellent formability and a method of manufacturing the same.

Description

Technical Field

[0001] This invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more specifically, to a high-strength cold-rolled steel sheet with excellent formability and a method for manufacturing the same. Background Technology

[0002] In the automotive industry, with regulations on greenhouse gas emissions due to global warming, there is a growing focus on lightweight vehicle bodies and ensuring crash stability to improve fuel efficiency and vehicle reliability. Consequently, the requirements for manufacturing technologies that ensure the use of high-strength steel are increasing.

[0003] In the case of high-strength steel sheets for cold forming that also possess formability, productivity can be improved, resulting in superior economic efficiency and greater safety for the final components. Accordingly, automotive components using high-strength steel sheets not only require strength, but also formability and weldability for component forming, demanding excellent physical properties such as elongation and LME resistance as described above.

[0004] To improve the formability of steel sheets, the method of introducing retained austenite to utilize the TRIP (transformation-induced plasticity) phenomenon as a way to increase elongation is widely used.

[0005] However, in the case of TRIP steel sheets, large amounts of elements such as C, Si, Mn, and Al are typically added to the steel to introduce retained austenite. However, when the content of these added elements is excessive or insufficient relative to the physical properties of the target steel grade, the material may be over- or under-resourced. Furthermore, steel sheets containing these elements generate oxides on their surface during annealing heat treatment, which degrades the plating properties when the steel sheets are immersed in a molten zinc plating bath.

[0006] Therefore, in order to meet the target material requirements, it is necessary to appropriately control the addition of elements such as C, Si, Mn, and Al. Summary of the Invention

[0007] (a) Technical problems to be solved According to one embodiment of the present invention, a cold-rolled steel sheet and a method for manufacturing the same are intended to be provided.

[0008] According to one embodiment of the present invention, it is intended to provide a high-strength cold-rolled steel sheet with excellent formability and a method for manufacturing the same.

[0009] The technical problems of this invention are not limited to those described above. Those skilled in the art will have no difficulty understanding the additional technical problems of this invention from the overall content of this specification.

[0010] (II) Technical Solution According to one embodiment of the present invention, a cold-rolled steel sheet, by weight%, comprises: C: 0.050-0.250%, Si: 0.10-3.00%, Al: 0.005-3.000%, Mn: 1.00-3.00%, P: less than 0.0400%, S: less than 0.0100%, N: less than 0.0100%, with the balance being Fe and other unavoidable impurities. The R1 value defined in Equation 1 below is 0.35 or more, and the R2 value defined in Equation 2 below is 0.59 or less. The fine microstructure, by area%, comprises 50.0-85.0% ferrite, 0.5-10.0% retained austenite, 3.0-30.0% bainite, and less than 30.0% newly formed martensite.

[0011] [Relation 1] R1 = [C] + 0.15 × [Mn] (In the formula, [C] and [Mn] are the weight percentages of each element.) [Relationship 2] R² = [C] + 0.2333 × [Mn] (In the formula, [C] and [Mn] are the weight percentages of each element.) The cold-rolled steel sheet may further comprise, by weight percent, one or more of the following: Cu: less than 0.1%, Ni: less than 0.1%, Mo: less than 0.300%, and Cr: less than 0.200%.

[0012] The cold-rolled steel sheet may further contain one or more of Nb, Ti and V in a total of less than 0.100% by weight.

[0013] The tensile strength of the cold-rolled steel sheet can be above 690 MPa, and the elongation can be above 25%.

[0014] The cold-rolled steel sheet may further include a galvanized layer on at least one side.

[0015] According to one embodiment of the present invention, a method for manufacturing cold-rolled steel sheet may include the following steps: heating a steel billet, wherein the steel billet, by weight percent, comprises: C: 0.050-0.250%, Si: 0.10-3.00%, Al: 0.005-3.000%, Mn: 1.00-3.00%, P: less than 0.0400%, S: less than 0.0100%, N: less than 0.0100%, the balance being Fe and other unavoidable impurities, and the R1 value defined by the following relation 1 is 0.35 or more, and the R2 value defined by the following relation 2 is 0.59 or less; heating the steel billet... The process includes: hot finishing rolling of the steel billet; coiling the hot-finished steel sheet; cold rolling of the coiled steel sheet; continuous annealing of the cold-rolled steel sheet at a temperature range of 800-900°C; primary cooling of the continuously annealed steel sheet at an average cooling rate of 1.0°C / second or higher to a temperature range of 550-720°C; secondary cooling of the primary cooled steel sheet at an average cooling rate of 10.0°C / second or higher to a temperature range of 150-480°C; and reheating the secondary cooled steel sheet to a temperature range of 350-480°C.

[0016] [Relation 1] R1 = [C] + 0.15 × [Mn] (In the formula, [C] and [Mn] are the weight percentages of each element.) [Relationship 2] R² = [C] + 0.2333 × [Mn] (In the formula, [C] and [Mn] are the weight percentages of each element.) The billet may further comprise, by weight percent, one or more of the following: Cu: less than 0.1%, Ni: less than 0.1%, Mo: less than 0.300%, and Cr: less than 0.200%.

[0017] The billet may further contain one or more of Nb, Ti and V in total less than 0.100% by weight.

[0018] The heating step is performed in a temperature range of 1150-1250℃. The hot finishing rolling step is carried out in a temperature range of 830-980℃. The winding step is performed at an average cooling rate of 10-100℃ / second, followed by a temperature range of 450-700℃. The cold rolling step is carried out with a cold rolling reduction rate of 30-60%.

[0019] It may further include: the step of plating the reheated steel sheet in a zinc plating bath at 450-470°C; and the step of alloying the plating steel sheet in a temperature range of 470-550°C.

[0020] The continuous annealing step can be carried out in an atmosphere with a dew point temperature ranging from -15.0°C to 30.0°C.

[0021] (III) Beneficial Effects According to one embodiment of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided.

[0022] According to one embodiment of the present invention, a high-strength cold-rolled steel sheet with excellent formability and a method for manufacturing the same can be provided.

[0023] According to one embodiment of the present invention, a cold-rolled steel sheet with excellent strength and ductility, which can be used in the automotive industry, and a method thereof can be provided.

[0024] According to one embodiment of the present invention, a cold-rolled steel sheet with excellent strength and ductility, and excellent adhesion of the coating after plating, and a method thereof can be provided.

[0025] The various and beneficial advantages and effects of the present invention are not limited to those described above, and can be more easily understood in the process of describing specific embodiments of the present invention. Best practice

[0026] While not strictly necessary, it should be noted that the technical solutions according to various aspects of the present invention can also be usefully used in technical solutions of other aspects. Furthermore, the components and various useful parameters of the various aspects of the present invention can be appropriately combined with other aspects to obtain advantageous effects.

[0027] Preferred embodiments of the present invention will be described below. The specific embodiments of the present invention can be modified in various forms, and the scope of the present invention should not be construed as limited to the specific embodiments described below. These specific embodiments are provided to illustrate the present invention in more detail to those skilled in the art.

[0028] The present invention will now be described in detail.

[0029] The steel composition of the present invention will be described in detail below.

[0030] In this invention, unless otherwise specified, the percentage of each element content is based on weight.

[0031] According to one embodiment of the present invention, the cold-rolled steel sheet may contain, by weight percent: C: 0.050-0.250%, Si: 0.10-3.00%, Al: 0.005-3.000%, Mn: 1.00-3.00%, P: less than 0.0400%, S: less than 0.0100%, N: less than 0.0100%.

[0032] Carbon (C): 0.050-0.250% Carbon (C) is an element that ensures the strength of steel through solid solution strengthening and precipitation strengthening, and it is also an effective element for stabilizing retained austenite to ensure high elongation. When the carbon (C) content is less than 0.050%, the target level of tensile strength may not be guaranteed. According to one embodiment of the invention, it can be 0.060% or more. According to another embodiment of the invention, it can be 0.070% or more. On the other hand, when the C content exceeds 0.250%, the target level of tensile strength may be exceeded, or weldability may deteriorate as the carbon (C) content increases. According to one embodiment of the invention, the C content can be 0.230% or less. According to another embodiment of the invention, the C content can be 0.220% or less.

[0033] Silicon (Si): 0.10-3.00% Silicon (Si) is a useful element for improving the strength of steel plates through solid solution strengthening and precipitation hardening. Because it inhibits cementite formation, it promotes the enrichment of carbon in austenite, making it an essential element for improving the strength and elongation of steel by forming retained austenite after annealing. When the silicon (Si) content is less than 0.10%, retained austenite will not form, making it difficult to obtain uniform elongation. According to one embodiment of the invention, the content can be 0.30% or more. According to another embodiment of the invention, the content can be 0.50% or more. On the other hand, when the silicon (Si) content exceeds 3.00%, the physical properties of the welded part may deteriorate due to LME cracking, and the surface properties and plating properties of the steel may worsen. According to one embodiment of the invention, the silicon (Si) content can be 2.50% or less. According to another embodiment of the invention, the silicon (Si) content can be 2.00% or less.

[0034] Aluminum (Al): 0.005-3.000% Aluminum (Al) is an element that deoxidizes molten steel and, similar to silicon, improves the stability of austenite, thus increasing elongation. When the aluminum (Al) content is less than 0.005%, deoxidation of the steel cannot be fully achieved, potentially impairing the cleanliness of the steel. According to one embodiment of the invention, the aluminum (Al) content can be 0.015% or more. According to another embodiment of the invention, the aluminum (Al) content can be 0.020% or more. On the other hand, when the aluminum (Al) content is excessive, the phase transformation temperature rises significantly, the ferrite fraction increases, and there may be a problem of not being able to achieve ultra-high strength. Therefore, in this invention, the upper limit of the aluminum (Al) content can be limited to 3.000%. According to one embodiment of the invention, the aluminum (Al) content can be 2.500% or less. According to another embodiment of the invention, the aluminum (Al) content can be 2.000% or less.

[0035] Manganese (Mn): 1.00-3.00% Manganese (Mn) is an element added to ensure strength. When the manganese (Mn) content is less than 1.00%, it may be difficult to ensure strength. According to one embodiment of the invention, the manganese (Mn) content can be 1.10% or more. According to another embodiment of the invention, the manganese (Mn) content can be 1.20% or more. On the other hand, when the manganese (Mn) content exceeds 3.00%, the bainitic phase transformation rate slows down, and too much new martensite is formed, making it difficult to obtain high elongation. In addition, the segregation of manganese (Mn) forms banded structures, which may impair the material's uniformity and formability. According to one embodiment of the invention, the manganese (Mn) content can be 2.70% or less. According to another embodiment of the invention, the manganese (Mn) content can be 2.50% or less.

[0036] Phosphorus (P): below 0.0400% Phosphorus (P) is included as an impurity, and its segregation at grain boundaries reduces toughness. Therefore, it is advantageous to keep its content as low as possible. Excessive addition of phosphorus (P) deteriorates the toughness of the steel, thus its upper limit can be limited to 0.0400%. According to one embodiment of the invention, it can be below 0.0300%. Furthermore, the lower limit of the phosphorus (P) content is not particularly limited and can be 0.0020%.

[0037] Sulfur (S): below 0.0100% Sulfur (S), like P, is contained in the steel as an impurity. Sulfur (S) combines with Mn to form inclusions, reducing porosity and potentially weldability and hot rollability; therefore, it is advantageous to keep its content as low as possible. According to one embodiment of the invention, considering the unavoidable inclusion, the upper limit of the sulfur (S) content can be limited to 0.0100%. According to another embodiment of the invention, the sulfur (S) content can be 0.0021% or less. Furthermore, the lower limit of the sulfur (S) content is not particularly limited, and the sulfur (S) content can be 0.0009%.

[0038] Nitrogen (N): below 0.0100% Nitrogen (N) is contained as an impurity in steel, and it is advantageous to keep its content as low as possible. According to one embodiment of the invention, considering the unavoidable inclusion, the upper limit of the nitrogen (N) content can be limited to 0.0100%.

[0039] The steel of this invention, in addition to the above-described composition, may contain remaining iron (Fe) and unavoidable impurities. These unavoidable impurities may be unintentionally introduced during ordinary manufacturing processes and therefore cannot be excluded. These impurities are well known to those skilled in the art of ordinary steel manufacturing, and therefore, not all of them are specifically mentioned in this specification.

[0040] According to one embodiment of the invention, the cold-rolled steel sheet may further comprise, by weight percent, one or more of the following: Cu: less than 0.1%, Ni: less than 0.1%, Mo: less than 0.300%, and Cr: less than 0.200%.

[0041] One or more of the following: copper (Cu): less than 0.1%, nickel (Ni): less than 0.1%, molybdenum (Mo): less than 0.300%, and chromium (Cr): less than 0.200%. Copper (Cu), nickel (Ni), molybdenum (Mo), and chromium (Cr) are elements that improve the strength of steel. These elements enhance both the strength and hardening energy of the steel. However, adding excessive amounts may exceed the target strength grade, and since they are high-priced elements, their upper limits can be limited to 0.1%, 0.1%, 0.300%, and 0.200%, respectively, for economic reasons. Furthermore, copper (Cu), nickel (Ni), molybdenum (Mo), and chromium (Cr) act as solid solution strengthening elements. Therefore, when adding more than one of these elements, adding less than 0.03% may result in negligible solid solution strengthening effects; thus, the addition amount can be greater than 0.03% for each.

[0042] According to one embodiment of the invention, the cold-rolled steel sheet may further contain, by weight percent, one or more of Nb, Ti and V totaling less than 0.100%.

[0043] The total content of one or more of niobium (Nb), titanium (Ti), and vanadium (V) is less than 0.100%. Niobium (Nb), titanium (Ti), and vanadium (V) are elements that improve the strength of steel. However, excessive amounts of these elements may lead to delayed recrystallization due to localized grain fixation, potentially compromising the uniformity of the microstructure. Therefore, the total content of these elements can be limited to below 0.100%. When adding one or more of niobium (Nb), titanium (Ti), and vanadium (V), the strength-enhancing effect may be negligible if the addition is less than 0.030%. Therefore, when adding them, their total content can be set to 0.030% or more.

[0044] According to one embodiment of the present invention, the R1 value defined in Equation 1 below can be 0.35 or higher.

[0045] [Relation 1] R1 = [C] + 0.15 × [Mn] (In the formula, [C] and [Mn] are the weight percentages of each element.) According to one embodiment of the present invention, by controlling the content relationship between C and Mn through Equation 1, both strength and ductility can be ensured simultaneously.

[0046] When the R1 value defined in Equation 1 is less than 0.35, there may be a problem that the target tensile strength (TS) cannot be adequately guaranteed. According to one embodiment of the present invention, it can be 0.36 or higher. In addition, there is no particular upper limit to the R1 value; according to one embodiment of the present invention, it can be 0.45.

[0047] According to one embodiment of the present invention, the R2 value defined in the following Equation 2 can be 0.59 or less.

[0048] [Relationship 2] R² = [C] + 0.2333 × [Mn] (In the formula, [C] and [Mn] are the weight percentages of each element.) In this invention, by additionally controlling the content relationship between C and Mn through Equation 2, both strength and ductility can be ensured simultaneously.

[0049] When the R² value defined in Equation 2 exceeds 0.59, it exceeds the range of the target tensile strength, potentially making it difficult to ensure the target elongation. According to one embodiment of the invention, the R² value can be 0.57 or lower. Furthermore, the lower limit of the R² value is not particularly limited; according to one embodiment of the invention, the lower limit of the R² value can be 0.40. According to another embodiment of the invention, it can be 0.45 or higher.

[0050] The steel microstructure of the present invention will be described in detail below.

[0051] In this invention, unless otherwise specified, the percentage of fine tissue fraction is expressed on an area basis.

[0052] The microstructure of the cold-rolled steel sheet according to one embodiment of the present invention, in terms of area%, may include 50.0-85.0 area% ferrite, 0.5-10.0 area% retained austenite, 3.0-30.0 area% bainite, and less than 30.0 area% newly formed martensite.

[0053] When the ferrite content is less than 50.0%, the amount of soft ferrite is insufficient, making it difficult to ensure elongation. The increased content of bainite, retained austenite, and newly formed martensite may lead to excessively high strength. On the other hand, when the ferrite content exceeds 85.0%, it may be difficult to adequately ensure the target strength.

[0054] When the retained austenite fraction is less than 0.5%, it may be difficult to ensure elongation. According to one embodiment of the invention, the retained austenite fraction is 1.0% or more. On the other hand, when the retained austenite fraction exceeds 10.0%, there is a problem that elongation cannot be obtained due to insufficient ferrite fraction.

[0055] When the bainite fraction is less than 3.0%, there may be concerns that the target strength and elongation cannot be adequately guaranteed. On the other hand, when the bainite fraction exceeds 30.0%, the bainite phase transformation in the initial austenite increases, and the tensile strength may decrease due to insufficient transformation of newly formed martensite.

[0056] When the newly formed martensite fraction exceeds 30.0%, excessive hard martensite may occur, potentially exceeding the target tensile strength. According to one embodiment of the invention, the lower limit for the newly formed martensite fraction can be 4.5%.

[0057] According to one embodiment of the present invention, the tensile strength of the cold-rolled steel sheet can be 690 MPa or more, and the elongation can be 25% or more.

[0058] According to one embodiment of the present invention, the tensile strength can be below 780 MPa.

[0059] According to one embodiment of the present invention, the cold-rolled steel sheet may include a coating on at least one side. In this invention, the type of coating is not particularly limited and may be a galvanized layer. According to one embodiment of the present invention, the galvanized layer may be a hot-dip galvanized layer. Furthermore, according to one embodiment of the present invention, it may be an alloyed hot-dip galvanized layer.

[0060] The steel manufacturing method of the present invention will be described in detail below.

[0061] According to one embodiment of the present invention, cold-rolled steel sheet can be manufactured by heating, hot rolling, coiling, cold rolling, continuous annealing, primary cooling, secondary cooling and reheating a steel billet that satisfies the above alloy composition.

[0062] heating A steel billet with an alloy composition conforming to one embodiment of the present invention can be heated to a temperature range of 1150-1250°C.

[0063] The method of manufacturing the steel billet is not particularly limited; it can be a continuously cast billet or a billet manufactured using a thin slab continuous casting machine, etc. Furthermore, hot rolling can be performed immediately after continuous casting.

[0064] When heating steel billets, if the temperature is below 1150℃, the hot finishing temperature is prone to becoming excessively low, and the rolling load may become high. Furthermore, from a manufacturing cost perspective, the upper limit of the heating temperature can be limited to below 1250℃.

[0065] Hot precision rolling The heated steel billet can be hot-rolled in a temperature range of 830-980℃.

[0066] During hot finishing rolling, if the temperature is below 830°C, the rolling load is high, resulting in poor shape and potentially reduced productivity. According to one embodiment of the invention, the temperature can be above 880°C. On the other hand, if the hot finishing rolling temperature exceeds 980°C, excessive high-temperature operation leads to increased oxides, potentially deteriorating surface quality. According to one embodiment of the invention, the temperature can be below 950°C. According to another embodiment of the invention, the temperature can be below 930°C.

[0067] Collect The hot-rolled steel sheet can be cooled at an average cooling rate of 10-100℃ / second and then coiled at a temperature range of 450-700℃.

[0068] When the temperature exceeds 700°C during coiling, thick hot-rolled internal oxidation occurs on the surface of the steel plate, potentially reducing its pickling properties. According to one embodiment of the invention, the coiling temperature can be below 670°C. According to another embodiment of the invention, the coiling temperature can be below 640°C. Furthermore, to refine the effective grain size and improve toughness, the lower limit of the coiling temperature can be limited to 450°C. According to one embodiment of the invention, the coiling temperature can be above 480°C. According to another embodiment of the invention, the coiling temperature can be above 500°C.

[0069] When the hot-rolled steel is cooled to the coiling temperature, if the average cooling rate is less than 10°C / second, the productivity of hot rolling decreases. In actual production, there may be a problem where a cooling medium with low cooling capacity must be deliberately used. On the other hand, when the average cooling rate exceeds 100°C / second, the temperature deviation inside the steel plate becomes uneven, the shape deteriorates, and the strength of the steel plate may become excessively high.

[0070] Cold rolling The coiled steel sheet can be cold rolled at a cold rolling reduction rate of 30-60%.

[0071] During cold rolling, if the reduction rate is less than 30%, it is not only difficult to ensure the target thickness accuracy, but the shape correction of the steel sheet may also become difficult. On the other hand, when the reduction rate exceeds 60%, the possibility of cracks forming at the edges of the steel sheet increases, and the load during cold rolling may be too large.

[0072] Continuous annealing The cold-rolled steel sheet can be continuously annealed in a temperature range of 800-900℃.

[0073] During continuous annealing, if the temperature is below 800°C, sufficient recrystallization and austenitic phase transformation cannot occur, and it may be difficult to ensure the target martensite and bainite fractions after annealing. On the other hand, when the continuous annealing temperature exceeds 900°C, productivity decreases, coarse austenite is formed, and the material may deteriorate. In addition, surface quality issues such as peeling of the plating material may worsen.

[0074] According to one embodiment of the present invention, continuous annealing can be carried out in an atmosphere with a dew point temperature ranging from -15.0°C to 30.0°C.

[0075] During continuous annealing, internal oxidation of the steel sheet can be achieved by controlling the dew point temperature. This internal oxidation ensures plating performance and resistance to LME (Low Metal Electrode Spectroscopy).

[0076] When the dew point temperature is below -15.0°C, the amount of oxygen flowing into the steel decreases, only exacerbating surface oxidation without causing internal oxidation. Therefore, a large amount of oxides will be present on the surface, potentially preventing the achievement of the aforementioned beneficial effects. According to one embodiment of the invention, the dew point temperature can be above -10.0°C. Furthermore, when the dew point temperature exceeds 30.0°C, internal oxidation increases, and the effect of inhibiting the diffusion of alloying elements to suppress surface oxidation increases. However, the water vapor supply increases dramatically, thus requiring an unnecessarily large humidification equipment capacity, posing a problem. Additionally, the condensation of cooled water vapor may cause equipment problems if used in continuous annealing over a long period. According to one embodiment of the invention, the dew point temperature can be below 20.0°C.

[0077] According to one embodiment of the present invention, when the dew point temperature exceeds the proposed range, although the target material can be met when manufacturing cold-rolled steel sheets, there may be a problem of plating peeling after plating.

[0078] Furthermore, according to one embodiment of the present invention, in order to obtain a meaningful effect during internal oxidation, the hydrogen concentration in the atmosphere gas during annealing can be 1% or more, in volume percentage.

[0079] When the hydrogen concentration is less than 1%, it is impossible to effectively oxidize and remove the trace amounts of oxygen inevitably contained in H2 and N2 gases. The increased oxygen partial pressure may induce surface oxidation of the steel plate. On the other hand, when the hydrogen concentration exceeds 70%, there is an explosion hazard when the gas flows out, and the cost of high-hydrogen operations increases. Therefore, the hydrogen concentration can be limited to below 70%. Besides hydrogen, and apart from the unavoidable impurity gases, it can essentially be nitrogen.

[0080] According to one embodiment of the present invention, the continuous annealing can be carried out in a continuous alloying hot-dip cladding furnace.

[0081] One cooling The continuously annealed steel sheet can be cooled to a temperature range of 550-720°C at an average cooling rate of 1.0°C / second or higher.

[0082] During primary cooling, the cooling termination temperature can be defined as the temperature at which the quenching equipment not used in the primary cooling is applied again and the secondary cooling begins.

[0083] According to one embodiment of the present invention, when the cooling process is divided into primary and secondary cooling stages, the temperature distribution of the steel plate can be made uniform during the slow cooling stage, reducing the final temperature and material deviation and ensuring the target microstructure.

[0084] During the first cooling cycle, if the termination temperature is below 550°C, the bainite fraction may become excessively high. In practical equipment conditions, it may be difficult to cool to a temperature range below 550°C at the target cooling rate. On the other hand, when the termination temperature of the first cooling cycle exceeds 720°C, the amount of cooling to the termination temperature of the second cooling cycle becomes large, which may result in poor steel plate shape and a bainite fraction that is lower than the target level.

[0085] When the average cooling rate is less than 1.0 °C / s during a single cooling cycle, the ferrite fraction of phase transformation increases. Due to insufficient martensite and retained austenite fractions, which affect strength and work hardening energy, it may be difficult to ensure the target strength and work hardening energy. Furthermore, there is no particular upper limit to the average cooling rate; for workability, it can be limited to 100.0 °C / s or less. According to one embodiment of the invention, it can be limited to 10.0 °C / s or less.

[0086] Secondary cooling The steel plate that has undergone the first cooling process can be cooled to a temperature range of 150-480°C at an average cooling rate of 10.0°C / second or higher.

[0087] As previously stated, according to one embodiment of the present invention, secondary cooling can be supplemented by a quenching device not used in the primary cooling. In this invention, the type of quenching device is not particularly limited; according to one embodiment, a hydrogen quenching device can be used. According to one embodiment of the present invention, the hydrogen quenching device can use a gas consisting of 50-80% hydrogen and the balance nitrogen by volume. When the hydrogen volume fraction exceeds 80%, management of the device, such as explosion control, may become difficult, presenting a disadvantage. On the other hand, when the hydrogen volume fraction is less than 50%, it may be difficult to utilize the efficient heat transfer characteristics of the light element hydrogen, presenting a disadvantage.

[0088] During secondary cooling, if the termination temperature is below 150°C, the initial martensitic phase transformation is excessive, resulting in excessively high yield strength and tensile strength, potentially leading to poor formability. According to one embodiment of the invention, the termination temperature during secondary cooling can be above the Ms temperature to prevent martensitic phase transformation during cooling. According to one embodiment of the invention, the termination temperature during secondary cooling can be above 200°C. According to another embodiment of the invention, it can be above 250°C. On the other hand, when the secondary cooling termination temperature exceeds 450°C, the cooling temperature is above the bainitic phase transformation initiation point, which may prevent the formation of sufficient bainite, increasing the probability of excessive formation of newly formed martensite, potentially exceeding the appropriate tensile strength range.

[0089] During secondary cooling, if the average cooling rate is less than 10.0°C / second, workability may deteriorate. According to one embodiment of the present invention, the average cooling rate during secondary cooling can be 20.0°C / second or higher. Furthermore, there is no particular upper limit to the average cooling rate during secondary cooling; according to one embodiment of the present invention, the upper limit can be 60.0°C / second.

[0090] Reheating The steel plate that has undergone the secondary cooling can be reheated to a temperature range of 350-480℃.

[0091] According to one embodiment of the present invention, a bainitic phase transformation can be obtained through the reheating process. According to one embodiment of the present invention, the endpoint temperature during reheating can be conveniently referred to as the reheating temperature.

[0092] During reheating, if the temperature is below 350°C, the strength may become excessively high, and the elongation may deteriorate. On the other hand, if the temperature exceeds 450°C, the austenite remains without phase transformation and becomes newly formed martensite during final cooling, which may adversely affect the elongation. Furthermore, the so-called nose temperature, where bainite phase transformation is most active, is approximately 400-450°C. According to one embodiment of the invention, the lower limit of the reheating temperature can be 400°C. According to one embodiment of the invention, the upper limit of the reheating temperature can be 450°C.

[0093] Plating The reheated steel sheet can be plated in a zinc plating bath at 450-470°C.

[0094] According to one embodiment of the invention, the reheated steel sheet can be galvanized as needed. Furthermore, leveling rolling can be performed as an additional step.

[0095] According to one embodiment of the present invention, the temperature of the zinc plating bath is not particularly limited and can be a common condition applicable in the same technical field.

[0096] Alloying heat treatment The coated steel sheet can be subjected to alloying heat treatment in a temperature range of 470-550℃.

[0097] According to one embodiment of the present invention, the galvanized steel sheet can be subjected to alloying heat treatment as needed. The alloying heat treatment can be performed to obtain an appropriate level of alloying, and its temperature can be determined based on the surface condition of the steel sheet. By controlling the surface condition of the steel and limiting the upper limit of the alloying heat treatment temperature to 550°C, softening of the steel sheet and loss of retained austenite due to excessive tempering can be prevented. Furthermore, to facilitate rapid alloying, the alloying heat treatment temperature can be higher than the hot-dip galvanizing temperature, and its lower limit can be limited to 470°C.

[0098] According to one embodiment of the present invention, after the alloying heat treatment, in order to correct the shape of the steel plate and adjust the yield strength, the alloying heat-treated steel plate can be cooled to room temperature and then subjected to leveling rolling with a reduction rate of less than 1%. Detailed Implementation

[0099] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative of the invention and are not intended to limit the scope of the invention.

[0100] (Example) After preparing a steel billet with the alloy composition described in Table 1 below, it is heated in a temperature range of 1180-1220°C and manufactured into a steel sheet under the conditions described in Table 2 below. At this time, hot rolling is carried out in a temperature range of 880-930°C, and coiling is carried out in a temperature range of 480-640°C after hot rolling, with an average cooling rate of more than 10°C / second.

[0101] [Table 1] [Relationship 1] R1 = [C] + 0.15 × [Mn] (In the formula, [C] and [Mn] are the weight percentages of each element.) [Relationship 2] R² = [C] + 0.2333 × [Mn] (In the formula, [C] and [Mn] are the weight percentages of each element.) [Table 2] The microstructure and mechanical properties of the manufactured steel plates were measured and are shown in Table 3 below.

[0102] The fine microstructure was observed at the 1 / 4 position from the surface of the steel plate towards the center of thickness using electron backscatter diffraction (EBSD). Its area fraction was calculated using phase diagrams, and the residual austenite fraction was measured using X-ray diffraction (XRD).

[0103] Furthermore, yield strength (YS), tensile strength (TS), and total elongation (T-El) were measured and shown through a tensile test conducted at the right angle of the rolling mill. A specimen specification with a gauge length of 50 mm and a width of 25 mm was used.

[0104] In addition, the plating properties of the manufactured steel sheets were visually evaluated after plating, as shown in Table 3 below. To evaluate plating properties, after hot-dip galvanizing, an automotive structural sealant was applied to a thickness of approximately 5 mm and cured at 150-170°C. Afterward, the plating steel sheet, cooled to room temperature, was bent 90° to peel off the sealant. Poor plating adhesion was indicated when the coating adhered to the sealant but the entire interface between the galvanized steel sheet and the base steel sheet was peeled off. After the plating properties evaluation, no peeling occurred, indicated by “○”; peeling occurred indicating poor plating adhesion, indicated by “×”.

[0105] [Table 3] F: Ferrite, RA: Retained austenite, B: Bainite, FM: Newly formed martensite. As shown in Table 3 above, in the case of an inventive example that satisfies the alloy composition and manufacturing conditions of the present invention, the fine microstructure characteristics proposed by the present invention can also be satisfied, and the physical properties of the present invention can also be ensured.

[0106] Furthermore, Examples 1 and 4 are examples where, although the physical properties of the cold-rolled steel sheet proposed in this invention were achieved, the dew point temperature deviated from the range proposed in this invention during continuous annealing, resulting in peeling after plating. It can be confirmed that when the dew point temperature deviates from the range proposed in this invention during continuous annealing, the adhesion of the plating on the plated steel sheet deteriorates.

[0107] On the other hand, Comparative Examples 1 to 5 are examples that do not satisfy Relation 1 proposed in this invention. As a result, excessive ferrite formation fails to ensure the target level of strength.

[0108] In Comparative Example 6, the termination temperature exceeded the proposed range during the first cooling. As a result, the ferrite fraction was insufficient, and the elongation deteriorated.

[0109] In Comparative Example 7, the termination temperature during secondary cooling exceeded the proposed range, and the temperature during reheating also exceeded the proposed range. As a result, excessive formation of new martensite occurred in the final microstructure, leading to a deterioration in elongation.

[0110] Comparative Examples 8 to 10 are examples that do not satisfy Relation 2 proposed in this invention. As a result, the proposed fine tissue fraction was not adequately ensured, and the target level of elongation was not ensured.

[0111] In Comparative Example 11, the temperature did not reach the proposed range during reheating, and bainite failed to form sufficiently. As a result, the elongation decreased.

[0112] The present invention has been described in detail above through embodiments, but other embodiments are also possible. Therefore, the technical concept and scope of the claims set forth below are not limited to the embodiments.

Claims

1. A cold-rolled steel sheet, comprising, by weight percent: C: 0.050-0.250%, Si: 0.10-3.00%, Al: 0.005-3.000%, Mn: 1.00-3.00%, P: less than 0.0400%, S: less than 0.0100%, N: less than 0.0100%, with the balance being Fe and other unavoidable impurities. In relation 1 below, the R1 value is defined as 0.35 or higher. In relation 2 below, the R² value is defined as 0.59 or less. The microstructure, by area percentage, comprises 50.0-85.0% ferrite, 0.5-10.0% retained austenite, 3.0-30.0% bainite, and less than 30.0% newly formed martensite. [Relation 1] R1 = [C] + 0.15 × [Mn] In the formula, [C] and [Mn] are the weight percentages of each element. [Relation 2] R² = [C] + 0.2333 × [Mn] In the formula, [C] and [Mn] are the weights of the elements.

2. The cold-rolled steel sheet according to claim 1, wherein, The cold-rolled steel sheet further comprises, by weight percent, one or more of the following: Cu: less than 0.1%, Ni: less than 0.1%, Mo: less than 0.300%, and Cr: less than 0.200%.

3. The cold-rolled steel sheet according to claim 1, wherein, The cold-rolled steel sheet further comprises, by weight percent, one or more of Nb, Ti and V totaling less than 0.100%.

4. The cold-rolled steel sheet according to claim 1, wherein, The cold-rolled steel sheet has a tensile strength of 690 MPa or higher and an elongation of 25% or higher.

5. The cold-rolled steel sheet according to claim 1, wherein, The cold-rolled steel sheet further comprises a galvanized layer on at least one side.

6. A method for manufacturing cold-rolled steel sheet, comprising: The step of heating the steel billet, by weight%, the steel billet contains: C: 0.050-0.250%, Si: 0.10-3.00%, Al: 0.005-3.000%, Mn: 1.00-3.00%, P: less than 0.0400%, S: less than 0.0100%, N: less than 0.0100%, balance Fe and other unavoidable impurities, and the R1 value defined by the following relation 1 is 0.35 or more, and the R2 value defined by the following relation 2 is 0.59 or less; The step of hot finishing rolling the heated steel billet; The step of coiling the hot-rolled steel plate; The step of cold rolling the coiled steel sheet; The step of continuously annealing the cold-rolled steel sheet in a temperature range of 800-900℃; A single cooling step involves cooling the continuously annealed steel sheet to a temperature range of 550-720°C at an average cooling rate of 1.0°C / second or higher. A secondary cooling step involves cooling the steel plate, which has undergone the first cooling, to a temperature range of 150-480°C at an average cooling rate of 10.0°C / second or higher; and... The step of reheating the steel plate that has undergone secondary cooling to a temperature range of 350-480°C [Relation 1] R1 = [C] + 0.15 × [Mn] In the formula, [C] and [Mn] are the weight percentages of each element. [Relation 2] R² = [C] + 0.2333 × [Mn] In the formula, [C] and [Mn] are the weights of the elements.

7. The method for manufacturing cold-rolled steel sheet according to claim 6, wherein, The steel billet, by weight percent, further comprises one or more of the following: Cu: less than 0.1%, Ni: less than 0.1%, Mo: less than 0.300%, and Cr: less than 0.200%.

8. The method for manufacturing cold-rolled steel sheet according to claim 6, wherein, The billet further comprises, by weight percent, one or more of Nb, Ti and V totaling less than 0.100%.

9. The method for manufacturing cold-rolled steel sheet according to claim 6, wherein, The heating step is performed in a temperature range of 1150-1250℃. The hot finishing rolling step is carried out in a temperature range of 830-980℃. The winding step is performed at an average cooling rate of 10-100℃ / second, followed by a temperature range of 450-700℃. The cold rolling step is carried out with a cold rolling reduction rate of 30-60%.

10. The method for manufacturing cold-rolled steel sheet according to claim 6, wherein the manufacturing method further comprises: The step of coating the reheated steel sheet in a zinc plating bath at 450-470°C; as well as The step of performing alloying heat treatment on the plated steel sheet in a temperature range of 470-550°C.

11. The method for manufacturing cold-rolled steel sheet according to claim 6, wherein, The continuous annealing step is carried out in an atmosphere with a dew point temperature ranging from -15.0°C to 30.0°C.

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