High-strength cold-rolled steel sheet and method for producing same

Abstract

According to one aspect of the present invention, a high-strength cold-rolled steel sheet is provided. According to an embodiment of the present invention, the high-strength cold-rolled steel sheet contains, by weight%, 0.1%-0.3% of carbon (C), 1.0%-2.0% of silicon (Si), 1.5%-3.0% of manganese (Mn), 0.01%-0.05% of aluminum (Al), 0.02% or less of phosphorus (P), 0.005% or less of sulfur (S), and a remainder of iron (Fe) and other unavoidable impurities. According to an embodiment of the present invention, a microstructure of the high-strength cold-rolled steel sheet includes 25%-35% of ferrite, 10%-18% of retained austenite, 5% or less of M-A (martensite-austenite composite phase), and a remainder of martensite in an area ratio.

Description

Technical Field

[0001] The present invention relates to a cold-rolled steel sheet, and more particularly to a high-strength cold-rolled steel sheet having a high strength-elongation balance and excellent flatness, and a method for manufacturing the same. Background Art

[0002] Recently, various environmental and energy usage regulations have led to a demand for high-strength steel sheets to improve fuel efficiency and durability. For steel sheets used in vehicles, it is important to increase strength to ensure user safety and reduce body weight, and to ensure elongation for ease of processing. Among the commonly used ultra-high strength steels at present, there are dual-phase steels that ensure strength and elongation through two phases of ferrite and martensite, and transformation-induced plasticity steels that ensure strength and elongation simultaneously through the phase transformation of retained austenite remaining in the structure during the plastic deformation process. The transformation-induced plasticity steel ensures strength by forming a martensite matrix and ensures elongation by forming retained austenite within the martensite matrix. A method of using so-called quenching and partitioning has been proposed to produce a steel having improved mechanical properties and good formability as a method for preparing a martensite-based high-strength transformation-induced plasticity steel. To ensure high strength and high formability, an appropriate amount of retained austenite and the stability of the retained austenite are required. The transformation-induced plasticity steel usually contains bainite, so it is difficult to ensure elongation. In addition, to improve the stability during operation, it is necessary to control the flatness of the steel sheet below a certain level by suppressing the deformation that often occurs in the steel sheet during the quenching process. As a related technical document, there is Korean Patent Application Publication No. 10-2020-0099752. Summary of the Invention

[0003] Technical Problem

[0004] Therefore, the present invention is dedicated to solving the above problems, and an object of the present invention is to provide a cold-rolled steel sheet and a method for manufacturing the same, the cold-rolled steel sheet having excellent flatness and simultaneously improving the mechanical properties of the steel sheet by increasing the stability of retained austenite. It will be understood that the technical problem is provided only as an example, and the technical idea of the present application is not limited thereto.

[0005] Technical Solution

[0006] According to one aspect of the present invention, the above object and other objects can be achieved by providing a high-strength cold-rolled steel sheet.

[0007] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet contains by weight %: carbon (C): 0.1% to 0.3%; silicon (Si): 1.0% to 2.0%; manganese (Mn): 1.5% to 3.0%; aluminum (Al): 0.01% to 0.05% or less; phosphorus (P): 0.02% or less; sulfur (S): 0.005% or less; and the balance of iron (Fe) and other inevitable impurities.

[0008] According to an embodiment of the present invention, the microstructure of the high-strength cold-rolled steel sheet includes, by area ratio, 25% to 35% of ferrite, 10% to 18% of retained austenite, 5% or less of M-A (martensite-austenite complex phase), and the balance of martensite.

[0009] According to an embodiment of the present invention, the flatness (deformation height) of the high-strength cold-rolled steel sheet is 3.0 mm or less.

[0010] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet may further contain one or more of niobium (Nb), titanium (Ti), and vanadium (V), and the total content of niobium (Nb), titanium (Ti), and vanadium (V) is 0.05% or less (greater than 0).

[0011] According to an embodiment of the present invention, the carbon concentration in the retained austenite may be 1.1% to 1.4% by weight %.

[0012] According to an embodiment of the present invention, the high-strength cold-rolled steel sheet may have a yield strength (YS) of 550 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation index (EI) of 20% or greater, and a tensile strength * elongation index of 20000 MPa% or greater.

[0013] According to an embodiment of the present invention, the martensite may include fresh martensite and tempered martensite, and the value (FM / TM) obtained by dividing the area ratio of fresh martensite (FM) by the area ratio of tempered martensite (TM) may range from 0.1 to 0.6.

[0014] According to an embodiment of the present invention, the area ratio of retained austenite having an aspect ratio (obtained by dividing the major axis length by the minor axis length of the retained austenite) of 3 or greater may range from 3% to 8%.

[0015] According to an embodiment of the present invention, the value obtained by dividing the total area ratio of retained austenite by the area ratio of retained austenite having an aspect ratio of 3 or greater may range from 0.5 to 0.8.

[0016] According to another aspect of the present invention, a method for manufacturing a high-strength cold-rolled steel sheet is provided.

[0017] According to an embodiment of the present invention, a method for manufacturing a high-strength hot-rolled steel sheet includes: manufacturing a hot-rolled steel sheet by hot-rolling a steel material, the steel material containing 0.1% to 0.3% of carbon (C), 1.0% to 2.0% of silicon (Si), 1.5% to 3.0% of manganese (Mn), 0.01% to 0.05% or less of aluminum (Al), 0.02% or less of phosphorus (P), 0.005% or less of sulfur (S), the balance of iron (Fe) and other inevitable impurities by weight%; manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; annealing the cold-rolled steel sheet within a duplex temperature range of austenite and ferrite; performing a first cooling on the annealed cold-rolled steel sheet at a first cooling rate within a first cooling end temperature range of 650°C to 800°C; performing a second cooling on the cold-rolled steel sheet after the first cooling at a cooling rate higher than the first cooling rate of the first cooling; performing partitioning by reheating the cold-rolled steel sheet after the second cooling and immediately performing a third cooling at a cooling rate of 0.07°C / s to 0.21°C / s when a target temperature of 350°C to 460°C is reached.

[0018] According to an embodiment of the present invention, the second cooling may include a first rapid cooling and a second rapid cooling, wherein the first rapid cooling is performed by cooling at a cooling rate of 70°C / s to 110°C / s to a first rapid cooling end temperature of Ms (martensite transformation start temperature) or higher, and the second rapid cooling is performed by cooling at a cooling rate of 30°C / s or higher and lower than 70°C / s to a second rapid cooling end temperature lower than Ms and higher than Mf (martensite transformation end temperature).

[0019] According to an embodiment of the present invention, the range of the first rapid cooling end temperature may be 320°C to 350°C, and the range of the second rapid cooling end temperature may be 200°C to 260°C.

[0020] According to an embodiment of the present invention, the partitioning may be performed for a time ranging from 30 seconds to 600 seconds.

[0021] According to an embodiment of the present invention, the duplex temperature range may be 780°C to 860°C.

[0022] According to an embodiment of the present invention, the range of the first cooling end temperature may be 680°C to 800°C.

[0023] According to an embodiment of the present invention, the range of the third cooling end temperature during the third cooling may be 340°C to 400°C.

[0024] Advantageous Effects

[0025] According to the technical idea of the present invention, the stability of retained austenite can be improved by controlling the cooling rate in the partitioning step, thereby providing a cold-rolled steel sheet having excellent strength and elongation index characteristics. In addition, the deformation of the steel sheet that usually occurs during rapid cooling can be suppressed by controlling the cooling rate in stages during the cooling step, so that the flatness of the final steel sheet can be controlled to be 3.0 mm or less, thereby providing a cold-rolled steel sheet having an excellent shape and improving the operation stability. The effects of the present invention are described as examples, and the scope of the present invention is not limited by these effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 A flowchart for gradually showing a method for manufacturing a high-strength cold-rolled steel sheet according to an embodiment of the present invention.

[0027] Figure 2 Shows a heat treatment process according to an embodiment of the present invention.

[0028] Figure 3 Shows the carbon concentration in retained austenite (RA) depending on the cooling rate during the third cooling process of the partitioning step.

[0029] Figure 4 Shows a photograph of the microstructure of a cold-rolled steel sheet according to an embodiment of the present invention observed using a scanning electron microscope. DETAILED DESCRIPTION

[0030] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. The embodiments of the present application are provided to more completely explain the technical idea of the present application to those skilled in the art, and the following embodiments can be modified in various different forms, but the scope of the technical idea of the present application is not limited to the following embodiments. On the contrary, the embodiments are provided to make the present application complete and sufficient, and to fully convey the technical idea of the present application to those skilled in the art. In the specification, the same reference numerals denote the same components. In addition, various components and regions in the drawings are schematically drawn. Therefore, the technical idea of the present invention is not limited by the relative sizes or spacings drawn in the drawings.

[0031] In the present specification and the appended claims, the term "phase fraction" refers to the area ratio obtained from a microstructure photograph using an image analyzer. In addition, unless otherwise specified, the content or concentration of a specific component is in wt%.

[0032] The high-strength cold-rolled steel sheet according to an embodiment of the present invention contains, by weight %: carbon (C): 0.1% to 0.3%; silicon (Si): 1.0% to 2.0%; manganese (Mn): 1.5% to 3.0%; aluminum (Al): 0.01% to 0.05% or less; phosphorus (P): 0.02% or less; sulfur (S): 0.005% or less; the balance being iron (Fe) and other inevitable impurities. Further, one or more of niobium (Nb), titanium (Ti), and vanadium (V) may be further contained. Here, the total content of niobium (Nb), titanium (Ti), and vanadium (V) may be limited to 0.05% or less (greater than 0).

[0033] The functions and contents of the respective components contained in the high-strength cold-rolled steel sheet according to the present invention are described as follows. Here, the content of the component element refers to the weight % of the entire steel sheet.

[0034] Carbon (C): 0.1% to 0.3%

[0035] Carbon is added to ensure the strength of the steel, especially to increase the strength of the martensite structure. Further, during the partitioning step, carbon diffuses into the retained austenite to stabilize the retained austenite, thereby contributing to ensuring elongation via the TRIP effect. The content range of carbon may be 0.1 wt% to 0.3 wt%, for example 0.18 wt% to 0.22 wt%. If the carbon content is less than 0.1 wt%, it is difficult to obtain the target strength; if the carbon content exceeds 0.3%, weldability is impaired.

[0036] Silicon (Si): 1.0% to 2.0%

[0037] Silicon is a ferrite stabilizing element that delays the formation of carbides in ferrite and has a solid solution strengthening effect. The content range of silicon may be 1.0% to 2.0%, for example 1.5% to 1.9%. When the silicon content is less than 1.0%, the effect is minimal; when the silicon content is greater than 2.0%, oxides (such as Mn2SiO4) are formed during the manufacturing process, hindering the plating performance and increasing the carbon equivalent, thereby reducing weldability.

[0038] Manganese (Mn): 1.5% to 3.0%

[0039] Manganese has a solid solution strengthening effect and contributes to improving strength by increasing hardenability. The content range of manganese may be 1.5% to 3.0%, for example 1.8% to 2.2%. When the manganese content is less than 1.5%, sufficient retained austenite is not ensured and the transformation-induced plasticity effect is also insufficient, so it is difficult to ensure strength. When the manganese content exceeds 3.0%, workability is reduced and resistance to delayed fracture is also reduced due to the formation or segregation of inclusions (such as MnS), and the carbon equivalent increases, which may reduce weldability.

[0040] Aluminum (Al): 0.01% to 0.05% or less

[0041] Aluminum is used as a deoxidizer and, similar to silicon, can help inhibit the formation of carbides. The content range of aluminum is from 0.01% to 0.05%. When the aluminum content is less than 0.01%, the deoxidation effect may be insufficient, while when the aluminum content exceeds 0.05%, AlN may be formed during slab manufacturing, which may cause cracks during casting or hot rolling.

[0042] Phosphorus (P): greater than 0% and 0.02% or less

[0043] Phosphorus is an impurity contained in the steel manufacturing process, and it is desirable to limit it to 0.02% or less. When phosphorus is added, it can help improve strength by improving solidification, but when more than 0.02% is added, it may cause low-temperature brittleness.

[0044] Sulfur (S): greater than 0% and 0.005% or less

[0045] Sulfur is an impurity contained in the steel manufacturing process, and it is desirable to limit it to 0.005% or less. Sulfur is limited to 0.005% or less because it forms non-metallic inclusions (such as FeS and MnS), which reduce toughness and weldability.

[0046] Total content of niobium (Nb), titanium (Ti) and vanadium (V): greater than 0% and 0.05% or less

[0047] Optionally, one or more elements among titanium, niobium, and vanadium may be additionally added. Titanium, niobium, and vanadium can precipitate in the steel in the form of carbides or nitrides, causing precipitation hardening and contributing to grain refinement. However, the precipitation hardening caused by the precipitates is not the object of the present invention. When added in large amounts, there are disadvantages such as a decrease in elongation and an increase in manufacturing cost, so the total amount of titanium, niobium, and vanadium is limited to 0.05% or less.

[0048] The remaining component of the high-strength cold-rolled steel sheet is iron (Fe). However, since it may be inevitably mixed with accidental impurities from raw materials or the surrounding environment during the normal steelmaking process, this situation cannot be excluded. These impurities are known to those skilled in the art of any manufacturing field, so they are not particularly mentioned in this specification.

[0049] Microstructure of the steel sheet

[0050] The cold-rolled steel sheet according to an embodiment of the present invention includes, by area ratio, 25% to 35% ferrite, 10% to 18% retained austenite, 5% or less M-A (martensite-austenite complex phase), and the balance martensite. The martensite includes at least one of fresh martensite (FM) and tempered martensite (TM).

[0051] The range of the area ratio of fresh martensite (FM) divided by the area ratio of tempered martensite (which can be expressed as FM / TM) is from 0.1 to 0.6.

[0052] In the case of retained austenite, the total area ratio of retained austenite can be expressed as RA T , and the area ratio of retained austenite with an aspect ratio (major axis length divided by minor axis length) of 3 or more can be expressed as RA L . The RA of the cold-rolled steel sheet according to an embodiment of the present invention L can be 3% or more, for example, 3% to 8%, and RA L divided by RA T (which can be expressed as RA L / RA T ) ranges from 0.5 to 0.8.

[0053] The cold-rolled steel sheet according to an embodiment of the present invention can ensure stability by increasing the carbon concentration in the retained austenite. The range of the carbon concentration in the retained austenite by weight% can be from 1.1% to 1.4%.

[0054] Mechanical properties of the steel sheet

[0055] The cold-rolled steel sheet of the present invention simultaneously satisfies the following values: the yield strength (YS) is 550 MPa or more, the tensile strength (TS) is 980 MPa or more, the elongation index (EI) is 20% or more, and the tensile strength * elongation index is 20000 MPa% or more. For example, the range of the yield strength can be from 550 MPa to 700 MPa, the range of the tensile strength can be from 980 MPa to 1200 MPa, the range of the elongation index can be from 20% to 25%, and the range of the tensile strength * elongation index can be from 20000 MPa% to 25000 MPa%. In addition, the flatness (deformation height) of the steel sheet is controlled to be 3.0 mm or less.

[0056] Next, a method for manufacturing a high-strength cold-rolled steel sheet having the above-described microstructure and physical properties within the above composition range according to an embodiment of the present invention will be described with reference to the accompanying drawings.

[0057] Method for manufacturing a high-strength cold-rolled steel sheet

[0058] In the manufacturing method according to the present invention, the semi-finished product to be hot-rolled can be, for example, a slab. After obtaining molten steel having a certain composition through a steelmaking process, a slab in a semi-finished state can be obtained through a continuous casting process.

[0059] Figure 1 is a flowchart for gradually showing the method for manufacturing a high-strength cold-rolled steel sheet according to an embodiment of the present invention.

[0060] A method for manufacturing a high-strength cold-rolled steel sheet according to an embodiment of the present invention includes: a step of manufacturing a hot-rolled steel sheet by hot-rolling a steel material having a composition; a step of manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; a step of annealing the cold-rolled steel sheet; a step of performing multi-stage cooling on the annealed cold-rolled steel sheet; and a step of partitioning the cold-rolled steel sheet that has undergone multi-stage cooling.

[0061] Steps for manufacturing a hot-rolled steel sheet

[0062] The steel billet having the above alloy composition is reheated at 1150°C to 1250°C. This reheating process homogenizes the composition segregation generated during the casting process and prepares for hot-rolling. When the slab reheating temperature is lower than 1150°C, there is a problem of restricted reuse of segregation, while when the slab reheating temperature exceeds 1250°C, the austenite grain size increases, and the process cost may increase due to the temperature rise. The slab reheating time is preferably 1 hour to 4 hours. When the reheating time is less than 1 hour, the homogenization of segregation is insufficient, while when the reheating time exceeds 4 hours, the austenite grain size may increase, and the process cost may increase, which is similar to heating above 1250°C.

[0063] After reheating, hot-rolling is performed in a general manner, and then finish hot-rolling is performed within the finish rolling delivery temperature (FDT) range of 850°C to 1000°C to manufacture a hot-rolled steel sheet. When the finish rolling delivery temperature is lower than 850°C, ferrite or pearlite may be generated. When the finish rolling delivery temperature exceeds 1000°C, the formation of scale may increase, the grain size may become coarser, and it may be difficult to achieve microstructural uniformity of the structure.

[0064] Next, the hot-rolled steel sheet is cooled to a coiling temperature of 700°C or less (for example, 500°C to 700°C), and then coiled. The cooling can be performed by air cooling or water cooling, and can be performed at a cooling rate of, for example, 10°C / s to 30°C / s.

[0065] Steps for manufacturing a cold-rolled steel sheet

[0066] The hot-rolled steel sheet is pickled to remove the surface oxide layer. Next, the hot-rolled steel sheet is cold-rolled to form a cold-rolled steel sheet. The reduction ratio during cold-rolling is not specifically defined, but is appropriately implemented to meet the final product specifications, usually specified in the range of 40% to 80%.

[0067] After completion of cold-rolling, the cold-rolled steel sheet is heat-treated, and the heat treatment includes annealing, cooling, and partitioning steps. Figure 2 A heat treatment process according to an embodiment of the present invention is shown. Hereinafter, the heat treatment after cold-rolling will be described step by step with reference to Figure 2 Describe the heat treatment after cold-rolling step by step.

[0068] Annealing step

[0069] The cold-rolled steel sheet is annealed in a continuous annealing furnace with a general slow cooling section ( Figure 2 annealing in). The cold-rolled steel sheet that has been cold-rolled is heated to the annealing temperature at a heating rate of 3 °C / s or higher (for example, at a heating rate in the range of 3 °C / s to 10 °C / s). When the heating rate is less than 3 °C / s, it takes a long time to reach the target annealing temperature, thereby reducing production efficiency and possibly increasing the grain size.

[0070] The annealing should be carried out in a duplex temperature range where austenite and ferrite coexist to produce the final structure, namely tempered martensite, retained austenite, and ferrite. If it is set to austenite single phase, it enters the duplex section during slow cooling, and there is a possibility of a phase transformation from austenite to ferrite, but due to the short time and minimal transformation, sufficient ferrite required by the present invention cannot be formed. When annealing is carried out in the duplex temperature range, in addition to ensuring ferrite during the annealing process, ferrite formed during slow cooling can also be ensured, making it easy to form the final microstructure. Therefore, the annealing temperature according to the present invention is a duplex temperature range of 780 °C to 860 °C, for example, 800 °C to 840 °C. The holding time at the annealing temperature can be 30 seconds to 180 seconds.

[0071] Multi-stage cooling step

[0072] The cold-rolled steel sheet that has been annealed at the annealing temperature is cooled. The cooling step can be carried out in two steps: the first cooling and the second cooling.

[0073] The first cooling is a section where, after annealing, it is slowly cooled to the first cooling end temperature at a cooling rate of 1 °C / s to 10 °C / s ( Figure 2 slow cooling in). Here, the first cooling end temperature can be 680 °C or higher, for example, 680 °C to 800 °C. When the first cooling end temperature is lower than 680 °C, the ferrite transformation accelerates, increasing the elongation index but reducing the strength accordingly, so the target strength may not be achieved.

[0074] The second cooling is a section where it is rapidly cooled after the first cooling, aiming to ensure the final properties by transforming austenite in the microstructure into martensite after slow cooling. When the second cooling is carried out, austenite will transform into martensite during the rapid temperature change. Therefore, during the second cooling, the shape of the steel sheet may be deformed due to volume changes caused by thermal deformation and phase transformation. This change in the shape of the steel sheet will have a negative impact on the quality of the steel sheet and make the normal operation in subsequent stages difficult.

[0075] In the present invention, the second cooling is carried out step by step, divided into a first rapid cooling step and a second rapid cooling step (cooling 1 and cooling 2 in Figure 2 ) to suppress the shape deformation of the steel sheet that often occurs during the second cooling. That is, after the first cooling is completed, the first rapid cooling in the second cooling is carried out, and the second rapid cooling is started at the temperature after the first rapid cooling is completed. Figure 2 After the first cooling is completed, the first rapid cooling in the second cooling is carried out, and the second rapid cooling is started at the temperature after the first rapid cooling is completed.

[0076] The cooling rate of the first rapid cooling is in the range of 70 °C / s to 110 °C / s, for example, 80 °C / s to 100 °C / s. The end temperature of the first rapid cooling is at or above the martensite start temperature (Ms), and for example, the range can be 320 °C to 350 °C.

[0077] The end temperature of the first rapid cooling corresponds to the start temperature of the second rapid cooling. The end temperature of the second rapid cooling is lower than Ms and higher than Mf (martensite finish temperature), and for example, the range can be 200 °C to 260 °C.

[0078] The cooling rate of the second rapid cooling is lower than that of the first rapid cooling. For example, it is 30 °C / s or higher and less than 70 °C / s, or 30 °C / s to 50 °C / s. When the cooling rate of the second rapid cooling is higher than 70 °C / s, the shape deformation of the steel sheet cannot be effectively suppressed. When it is less than 30 °C / s, the transformation amount of austenite to martensite cannot meet the target value, and the content of retained austenite may become too high.

[0079] The second rapid cooling step may include a step of holding the cold-rolled steel sheet that has reached the end temperature of the second rapid cooling for 5 seconds to 120 seconds (see "holding" in Figure 2 ). In the holding section at the end temperature of the second rapid cooling, part of the austenite transforms into bainite, and precipitates such as metal carbides may form in the martensite formed in the second cooling step or the bainite formed in the holding section at the end temperature of the second rapid cooling. Figure 2 In the holding section at the end temperature of the second rapid cooling, part of the austenite transforms into bainite, and precipitates such as metal carbides may form in the martensite formed in the second cooling step or the bainite formed in the holding section at the end temperature of the second rapid cooling.

[0080] Partitioning step

[0081] After the cooling step is completed, the cold-rolled steel sheet is reheated and then heat-treated within a specified temperature range through a partitioning step (partitioning in Figure 2 ). Figure 2 After the cooling step is completed, the cold-rolled steel sheet is reheated and then heat-treated within a specified temperature range through a partitioning step (partitioning in Figure 2 ).

[0082] The partitioning step stabilizes the retained austenite by diffusing and concentrating carbon into the retained austenite. Generally, the partitioning step is carried out by reheating to reach the target temperature and then isothermally holding for a certain time at the target temperature. However, in the present invention, considering the T0 composition (the carbon concentration in austenite when the free energies of austenite (FCC structure) and ferrite (BCC structure) become the same and the driving force for bainite transformation becomes 0), in the partitioning stepFigure 2 in the partitioning) is slowly cooled instead of isothermally held. Thus, the stability of austenite is further increased.

[0083] Specifically, the cold-rolled steel sheet that has undergone a multi-stage cooling step is reheated at a heating rate in the range of 3°C / s to 20°C / s to reach a target temperature in the range of 350°C to 460°C. If the temperature reached is lower than 350°C, the partitioning effect due to carbon diffusion may be insufficient, while if the temperature reached exceeds 460°C, upper bainite may transform, affecting the properties and elongation index.

[0084] Cooling is immediately started after reaching the target temperature ( Figure 2 cooling 3) in). The cooling performed in the partitioning step is called the third cooling. Thus, the target temperature corresponds to the starting temperature of the third cooling. The range of the cooling rate in the third cooling step can be 0.07°C / s to 0.21°C / s, preferably 0.09°C / s to 0.19°C / s. In addition, the range of the ending temperature of the third cooling can be 340°C to 400°C.

[0085] The partitioning time ranges from 30 seconds to 600 seconds, preferably from 200 seconds to 300 seconds.

[0086] Regarding the mechanism by which slow cooling improves the stability of austenite compared to isothermal holding, in the case of isothermal holding, when the carbon concentration in the untransformed austenite reaches the T0 composition at the isothermal holding temperature, the bainite transformation stops. In the case of slow cooling, the T0 composition gradually increases with the temperature decrease caused by slow cooling, such that the C concentration of the untransformed austenite is higher than that in the case of isothermal holding. Therefore, it can be determined that the stability of the untransformed austenite is improved.

[0087] Experimental example

[0088] Next, preferred experimental examples are shown to help understand the present invention. However, the following experimental examples are only for helping to understand the present invention, and the present invention is not limited to the following experimental examples.

[0089] The cold-rolled steel sheet is manufactured by hot rolling and cold rolling a slab, and the composition (unit: wt%) of the slab is shown in Table 1 below. The manufactured cold-rolled steel sheet is heat-treated under the conditions shown in Table 2. The reheating temperature in Table 2 corresponds to the starting temperature of the third cooling, which is the target temperature for partitioning.

[0090] [Table 1]

[0091]

[0092] [Table 2]

[0093]

[0094]

[0095] The deformation trend of the steel plate is observed according to the cooling rate of the second rapid cooling, and the cooling rate of the second rapid cooling varies within the range of 30 °C / s to 100 °C / s. In order to evaluate the deformation of the steel plate, after the second rapid cooling is completed, the deformation height, which is an index of the flatness of the steel plate, is measured. On the other hand, while changing the third cooling rate in the partitioning step, the carbon content in the retained austenite is measured. For the cold-rolled steel plate that has undergone partitioning, the yield strength, tensile strength, and elongation index are measured.

[0096] Table 3 shows the properties of the cold-rolled steel plate according to the process conditions in Table 2, namely, the deformation height, the carbon concentration in the retained austenite, and the mechanical properties (yield strength, tensile strength, and elongation index).

[0097] [Table 3]

[0098]

[0099] Referring to Tables 2 and 3, Inventive Examples 1 to 5 (where the cooling rate of the second rapid cooling is within the range of 30 °C / s to 50 °C / s) show excellent flatness, and the deformation height is 3.0 mm or less. However, in the case of Comparative Examples 1 and 2 (where the cooling rate of the second rapid cooling is within the range of 70 °C / s to 100 °C / s), the deformation height is significantly greater than 3.0 mm, confirming that the shape is severely deformed during the second rapid cooling.

[0100] Figure 3 Shows the carbon concentration in the retained austenite (RA) depending on the cooling rate during the third cooling in the partitioning stage.

[0101] Referring to Tables 2 and 3 and Figure 3 , it can be seen that the carbon concentration in the retained austenite of Inventive Examples 1 to 5 is within the range of 1.1 wt% to 1.4 wt%, while Comparative Examples 3 to 9 all show values less than 1%. It can be confirmed that Inventive Examples 1 to 5 satisfy the cooling rate range of the third cooling of 0.07 °C / s to 0.21 °C / s, but Comparative Examples 3 to 9 are not within this range. In particular, in the case of Comparative Example 3, the carbon concentration in the retained austenite is 0.77%, which is significantly lower than that of the inventive examples, due to maintaining isothermal in the partitioning step.

[0102] Referring to Table 3, all inventive examples simultaneously satisfy the following values: the yield strength (YS) is 550 MPa or greater, the tensile strength (TS) is 980 MPa or greater, the elongation index (EI) is 20% or greater, and the tensile strength * elongation index is 20000 MPa% or greater. In particular, the inventive examples exhibit a high elongation index of 20% or greater because the carbon concentration in the retained austenite is 1.1% or higher through heat treatment in the partitioning step.

[0103] Figure 4 The microstructure of an exemplary specimen corresponding to Inventive Example 1 observed by an electron microscope is shown, and Table 4 shows the phase analysis results of the specimen. The phase fractions shown in Table 4 are in area ratio (area %). In Table 4, F, RA, M-A, FM, and TM represent ferrite, retained austenite, martensite-austenite complex phase, fresh martensite, and tempered martensite, respectively.

[0104] Refer to Figure 4 Table 4, Inventive Example 1 shows the phase fractions corresponding to the microstructure of the present invention. In addition, the RA L / RA T value is 0.6, satisfying the RA L / RA T value of the present invention.

[0105] [Table 4]

[0106]

[0107] It will be apparent to those skilled in the art to which the technical idea of the present invention pertains that the above technical idea of the present invention is not limited to the above embodiments and drawings, and various substitutions, modifications, and changes can be made within the scope of the technical idea of the present invention.

Claims

1. A high-strength cold-rolled steel sheet, which contains by weight %: carbon (C): 0.1% to 0.3%; silicon (Si): 1.0% to 2.0%; manganese (Mn): 1.5% to 3.0%; aluminum (Al): 0.01% to 0.05% or less; phosphorus (P): 0.02% or less; sulfur (S): 0.005% or less; the balance being iron (Fe) and other inevitable impurities, Among them, The microstructure of the high-strength cold-rolled steel sheet includes, by area ratio, 25% to 35% ferrite, 10% to 18% retained austenite, 5% or less of M-A (martensite-austenite complex phase), and the balance being martensite, The flatness (deformation height) of the high-strength cold-rolled steel sheet is 3.0 mm or less.

2. The high-strength cold-rolled steel sheet according to claim 1, which further contains one or more of niobium (Nb), titanium (Ti), and vanadium (V), Among them, The total content of niobium (Nb), titanium (Ti), and vanadium (V) is 0.05% or less (greater than 0).

3. The high-strength cold-rolled steel sheet according to claim 1, wherein, The carbon concentration in the retained austenite is 1.1% to 1.4% by weight %.

4. The high-strength cold-rolled steel sheet according to claim 1, wherein, The yield strength (YS) of the high-strength cold-rolled steel sheet is 550 MPa or greater, the tensile strength (TS) is 980 MPa or greater, the elongation index (EI) is 20% or greater, and the tensile strength * elongation index is 20000 MPa% or greater.

5. The high-strength cold-rolled steel sheet according to claim 1, wherein, The martensite includes fresh martensite and tempered martensite, The value (FM / TM) obtained by dividing the area ratio of fresh martensite (FM) by the area ratio of tempered martensite (TM) ranges from 0.1 to 0.

6.

6. The high-strength cold-rolled steel sheet according to claim 1, wherein, The area ratio of retained austenite with an aspect ratio of 3 or greater ranges from 3% to 8%, and the aspect ratio is obtained by dividing the major axis length of the retained austenite by its minor axis length.

7. The high-strength cold-rolled steel sheet according to claim 6, wherein, The value obtained by dividing the total area ratio of retained austenite by the area ratio of retained austenite with an aspect ratio of 3 or greater ranges from 0.5 to 0.

8.

8. A method for manufacturing a high-strength cold-rolled steel sheet, the method comprising: Manufacturing a hot-rolled steel sheet by hot-rolling a steel material, the steel material containing by weight %: carbon (C): 0.1% to 0.3%; Silicon (Si): 1.0% to 2.0%; manganese (Mn): 1.5% to 3.0%; aluminum (Al): 0.01% to 0.05% or less; phosphorus (P): 0.02% or less; sulfur (S): 0.005% or less; the balance being iron (Fe) and other inevitable impurities; Manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet; Annealing the cold-rolled steel sheet within the duplex temperature range of austenite and ferrite; Performing a first cooling on the annealed cold-rolled steel sheet at a first cooling rate within a first cooling end temperature of 650°C to 800°C; Performing a second cooling on the cold-rolled steel sheet after the first cooling at a cooling rate higher than the first cooling rate of the first cooling; Performing partitioning by reheating the cold-rolled steel sheet after the second cooling, and immediately performing a third cooling at a cooling rate of 0.07°C / s to 0.21°C / s when the target temperature of 350°C to 460°C is reached.

9. The method according to claim 8, wherein, The high-strength hot-rolled steel sheet further contains one or more of niobium (Nb), titanium (Ti), and vanadium (V), wherein the total content of niobium (Nb), titanium (Ti), and vanadium (V) is 0.05% or less (greater than 0).

10. The method according to claim 8, wherein, The second cooling includes first rapid cooling and second rapid cooling, wherein the first rapid cooling is carried out by cooling at a cooling rate of 70°C / s to 110°C / s to a first rapid cooling end temperature of Ms (martensite transformation start temperature) or higher, and the second rapid cooling is carried out by cooling at a cooling rate of 30°C / s or higher and lower than 70°C / s to a second rapid cooling end temperature lower than Ms and higher than Mf (martensite transformation end temperature).

11. The method according to claim 10, wherein, The range of the first rapid cooling end temperature is 320°C to 350°C, and the range of the second rapid cooling end temperature is 200°C to 260°C.

12. The method according to claim 8, wherein Partitioning is carried out for a time ranging from 30 seconds to 600 seconds.

13. The method according to claim 8, wherein, The range of the duplex temperature is 780°C to 860°C.

14. The method according to claim 8, wherein The range of the first cooling end temperature is 680°C to 800°C.

15. The method according to claim 8, wherein The range of the third cooling end temperature during the third cooling is 340°C to 400°C.

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