High strength cold rolled steel sheet and method for manufacturing the same

Abstract

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

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 balance of strength and elongation and excellent flatness, and a method for producing the same. [Background technology]

[0002] Recent environmental and energy regulations have led to a demand for high-strength steel sheets to improve fuel efficiency and durability. Automotive steel sheets require increased strength for user safety and vehicle weight reduction, as well as elongation for ease of processing. Common ultra-high-strength steels currently in use include dual-phase steels, which achieve both strength and elongation through the two phases of ferrite and martensite, and transformation-induced plasticity steels, which achieve both strength and elongation through the phase transformation of retained austenite during plastic deformation. Transformation-induced plasticity steels maintain a martensite matrix to ensure strength, while retained austenite is generated internally to achieve elongation. Quenching and partitioning have been proposed as methods for producing martensite-based high-strength transformation-induced plasticity steels, resulting in steels with improved mechanical properties and good formability. Achieving high strength and formability requires both an appropriate fraction of retained austenite and its stability. Transformation-induced plasticity steel generally contains bainite, which makes it difficult to ensure elongation. To improve operational stability, it is necessary to suppress deformation of the steel sheet, which often occurs during quenching, and control the flatness of the steel sheet to a predetermined height or less. Prior art documents include Korean Patent Application No. 10-2020-0099752. Summary of the Invention [Problem to be solved by the invention]

[0003] The technical problem to be solved by the technical concept of the present invention is to provide a cold-rolled steel sheet having excellent flatness and improved mechanical properties by improving the stability of retained austenite, and a method for manufacturing the same, but this problem is merely an example and the technical concept of the present invention is not limited thereto. [Means for solving the problem]

[0004] According to one aspect of the present invention, a high strength cold rolled steel sheet is provided.

[0005] According to one embodiment of the present invention, the high-strength cold-rolled steel sheet contains, in weight percent, 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 being iron (Fe) and other unavoidable impurities.

[0006] According to one embodiment of the present invention, the microstructure of the high-strength cold-rolled steel sheet is composed of, by area ratio, 25 to 35% ferrite, 10 to 18% retained austenite, 5% or less MA (martensite-austenite composite phase), and the remainder martensite.

[0007] According to one embodiment of the present invention, the flatness of the high strength cold rolled steel sheet has a deformation height of 3.0 mm or less.

[0008] According to one embodiment of the present invention, the high-strength cold-rolled steel sheet further contains one or more of niobium (Nb), titanium (Ti), and vanadium (V), and the total content of niobium (Nb), titanium (Ti), and vanadium (V) is in the range of 0.05% or less (more than 0).

[0009] According to one embodiment of the present invention, the carbon concentration in the retained austenite is in the range of 1.1% to 1.4% by weight.

[0010] According to one embodiment of the present invention, the cold-rolled steel sheet may have a yield strength (YS) of 550 MPa or more, a tensile strength (TS) of 980 MPa or more, an elongation (EI) of 20% or more, and a tensile strength x elongation value of 20,000 MPa% or more.

[0011] According to one embodiment of the present invention, the martensite includes fresh martensite and tempered martensite, and the value (FM / TM) obtained by dividing the area fraction of fresh martensite (FM) by the area fraction of tempered martensite (TM) may be in the range of 0.1 to 0.6.

[0012] According to an embodiment of the present invention, the area ratio of retained austenite having an aspect ratio (long axis length divided by short axis length) of 3 or more may be in the range of 3% to 8%.

[0013] According to one embodiment of the present invention, the value obtained by dividing the total area fraction of retained austenite by the area fraction of retained austenite having an aspect ratio of 3 or more may be in the range of 0.5 to 0.8.

[0014] According to another aspect of the present invention, there is provided a method for producing a high strength cold rolled steel sheet.

[0015] According to one embodiment of the present invention, a method for producing a high strength cold rolled steel sheet includes the steps of: hot rolling a steel material containing 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 being iron (Fe) and other inevitable impurities, to produce a hot rolled steel sheet;

[0016] The method may include the steps of cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet, annealing the cold-rolled steel sheet in a temperature range of austenite and ferrite two-phase region, primarily cooling the annealed cold-rolled steel sheet at a first cooling rate to a primary cooling end temperature in the range of 650°C to 800°C, secondary cooling the primarily cooled cold-rolled steel sheet at a cooling rate higher than the cooling rate of the primary cooling, and partitioning the cold-rolled steel sheet after secondary cooling to a target temperature in the range of 350°C to 460°C by reheating the cold-rolled steel sheet after secondary cooling, and immediately after reaching the target temperature in the range of 350°C to 460°C, performing tertiary cooling at a cooling rate of 0.07°C / sec to 0.21°C / sec.

[0017] According to one embodiment of the present invention, the secondary cooling includes a primary quenching step and a secondary quenching step, in which the primary quenching step is performed at a cooling rate of 70°C / sec to 110°C / sec to a primary quenching end temperature of Ms or higher, and the secondary quenching step is performed at a cooling rate of 30°C / sec to less than 70°C / sec to a secondary quenching end temperature of Mf or higher but lower than Ms.

[0018] According to one embodiment of the present invention, the first quenching end temperature may be in the range of 320°C to 350°C, and the second quenching end temperature may be in the range of 200°C to 260°C.

[0019] According to one embodiment of the present invention, the partitioning step may take place in the range of 30 seconds to 600 seconds.

[0020] According to one embodiment of the present invention, the two-phase temperature range may range from 780°C to 860°C.

[0021] According to an embodiment of the present invention, the primary cooling end temperature may be in the range of 680°C to 800°C.

[0022] According to an embodiment of the present invention, the tertiary cooling end temperature may be in the range of 340°C to 400°C. [Effects of the Invention]

[0023] According to the technical concept of the present invention, by controlling the cooling rate in the partitioning step, the stability of retained austenite can be improved, thereby providing a cold-rolled steel sheet with excellent strength and elongation properties. Furthermore, by controlling the cooling rate in each section in the cooling step, deformation of the steel sheet that typically occurs during rapid cooling can be suppressed, and the flatness of the final steel sheet can be controlled to 3.0 mm or less, thereby providing a cold-rolled steel sheet with an excellent shape and improving operational stability. The above-mentioned effects of the present invention are described by way of example, and the scope of the present invention is not limited by these effects. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a flowchart illustrating steps in a method for manufacturing a high-strength cold-rolled steel sheet according to an embodiment of the present invention. [Figure 2] 1 shows a heat treatment history according to an embodiment of the present invention. [Figure 3] 1 is a graph showing the carbon concentration in retained austenite (RA) depending on the cooling rate during tertiary cooling in the partitioning step. [Figure 4] 1 shows the results of observing the microstructure of a cold-rolled steel sheet according to an example of the present invention using a scanning electron microscope. DETAILED DESCRIPTION OF THE INVENTION

[0025] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The present embodiments are provided to more completely explain the technical concept of the present invention to those skilled in the art. The following embodiments may be modified into various other forms, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more complete and complete and to fully convey the technical concept of the present invention to those skilled in the art. Throughout this specification, the same reference numerals refer to the same elements. Furthermore, various elements and regions in the drawings are shown schematically. Therefore, the technical concept of the present invention is not limited by the relative sizes and spacings shown in the accompanying drawings.

[0026] In this specification and claims, the phase fraction refers to the area ratio obtained from a microstructure photograph using an image analyzer. The content or concentration of a specific component refers to weight percent unless otherwise specified.

[0027] A high-strength cold-rolled steel sheet according to one embodiment of the present invention contains, by weight, 0.1% to 0.3% carbon (C), 1.0% to 2.0% silicon (Si), 1.5% to 3.0% manganese (Mn), 0.01% to 0.05% or less aluminum (Al), 0.02% or less phosphorus (P), 0.005% or less sulfur (S), and the balance being iron (Fe) and other unavoidable impurities. Additionally, the steel sheet may further contain one or more of niobium (Nb), titanium (Ti), and vanadium (V). In this case, the total content of niobium (Nb), titanium (Ti), and vanadium (V) may be limited to 0.05% or less (greater than 0).

[0028] The role and content of each component contained in the high-strength cold-rolled steel sheet according to the present invention will be described below. Here, the contents of all component elements are expressed as wt% based on the total weight of the steel sheet.

[0029] Carbon (C): 0.1%~0.3%

[0030] Carbon is added to ensure the strength of steel, particularly by increasing the strength of the martensite structure. It also diffuses into the retained austenite during the partitioning step to stabilize the retained austenite, thereby contributing to ensuring elongation through the TRIP effect. The carbon content can be in the range of 0.1% to 0.3%, for example, 0.18% to 0.22%. If the carbon content is less than 0.1% by weight, it is difficult to achieve the target strength, and if it exceeds 0.3%, it is detrimental to weldability.

[0031] Silicon (Si): 1.0% to 2.0%

[0032] Silicon is a ferrite stabilizing element that retards the formation of carbides in ferrite and has the effect of solid solution strengthening. The silicon content can be in the range of 1.0% to 2.0%, for example, 1.5% to 1.9%. If it is less than 1.0%, the effect is very small, but if it is 2.0% or more, it may form oxides such as Mn2SiO4 during the manufacturing process, which may hinder galvanization and increase the carbon equivalent, which may reduce weldability.

[0033] Manganese (Mn): 1.5% to 3.0%

[0034] Manganese has a solid solution strengthening effect, increasing hardenability and contributing to improved strength. The manganese content can be in the range of 1.5% to 3.0%, for example, 1.8% to 2.2%. If the manganese content is less than 1.5%, retained austenite and the transformation-induced plasticity effect are insufficient, making it difficult to ensure strength. If the manganese content exceeds 3.0%, the formation and segregation of inclusions such as MnS can cause deterioration in workability and delayed fracture resistance, and the carbon equivalent can be increased, resulting in reduced weldability.

[0035] Aluminum (Al): 0.01% to 0.05%

[0036] Aluminum is used as a deoxidizer and can help suppress the formation of carbides similarly to silicon. The aluminum content ranges from 0.01% to 0.05%. If the aluminum content is less than 0.01%, the deoxidizing effect may be insufficient, and if it exceeds 0.05%, AlN may form during the production of slabs, which may induce cracks during casting or hot rolling.

[0037] Phosphorus (P): More than 0% and less than 0.02%

[0038] Phosphorus is an impurity contained in the steel manufacturing process and is preferably limited to 0.02% or less. Although the addition of phosphorus can help improve strength through solid solution strengthening, adding more than 0.02% can cause low-temperature embrittlement.

[0039] Sulfur (S): More than 0% but less than 0.005%

[0040] Sulfur is an impurity contained in the steel during the manufacturing process and is preferably limited to 0.005% or less. Sulfur forms non-metallic inclusions such as FeS and MnS, which reduces toughness and weldability, so it is limited to 0.005% or less.

[0041] Sum of niobium (Nb), titanium (Ti) and vanadium (V): over 0% and not more than 0.05%

[0042] Optionally, one or more of titanium, niobium, and vanadium may be further added. Titanium, niobium, and vanadium precipitate as carbides or nitrides in steel to cause precipitation hardening and contribute to grain refinement. However, in the present invention, precipitation hardening by precipitation is not the objective, and adding large amounts of these elements can have drawbacks such as reduced elongation and increased manufacturing costs. Therefore, the total amount of titanium, niobium, and vanadium is limited to 0.05% or less.

[0043] The remaining component of the high-strength cold-rolled steel sheet is iron (Fe). However, in a typical steelmaking process, unintended impurities are inevitably mixed in from raw materials or the surrounding environment, and these cannot be eliminated. These impurities are known to any engineer of a typical manufacturing process, and therefore, the full details of these impurities will not be specifically mentioned in this specification.

[0044] Microstructure of steel plate

[0045] A cold-rolled steel sheet according to an embodiment of the present invention is composed of, by area ratio, 25 to 35% ferrite, 10 to 18% retained austenite, 5% or less MA (martensite-austenite composite phase), and the balance martensite, where the martensite includes at least one of fresh martensite (FM) and tempered martensite (TM).

[0046] The value obtained by dividing the area fraction of fresh martensite (FM) by the area fraction of tempered martensite (which can be expressed as FM / TM) is in the range of 0.1 to 0.6.

[0047] In the case of retained austenite, the total area fraction of retained austenite is defined as RA T The area ratio of retained austenite with an aspect ratio of 3 or more, calculated by dividing the major axis length by the minor axis length, is RA. L The cold rolled steel sheet according to an embodiment of the present invention can be expressed as follows: L may be 3% or more, for example, 3% to 8%, and RA L RA T The value divided by (RA L / RA T (which can be expressed as) has a range of 0.5 to 0.8.

[0048] 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 carbon concentration in the retained austenite may be in the range of 1.1 to 1.4% by weight.

[0049] Steel plate mechanical properties

[0050] The cold-rolled steel sheet of the present invention simultaneously satisfies a yield strength (YS) of 550 MPa or more, a tensile strength (TS) of 980 MPa or more, an elongation (EI) of 20% or more, and a tensile strength × elongation value of 20,000 MPa% or more. For example, the yield strength may be 550 to 700 MPa, the tensile strength may be 980 to 1200 MPa, the elongation may be 20 to 25%, and the tensile strength × elongation value may be in the range of 20,000 to 25,000 MPa%. In addition, the flatness (deformed height) of the steel sheet is controlled to 3.0 mm or less.

[0051] Hereinafter, a method for manufacturing a high-strength cold-rolled steel sheet according to an embodiment of the present invention, which has the above-described microstructure and physical properties in the above-described composition range, will be described with reference to the accompanying drawings.

[0052] Manufacturing method for high strength cold rolled steel sheet

[0053] In the manufacturing method according to the present invention, the semi-finished product to be subjected to the hot rolling process may be, for example, a slab. The semi-finished slab can be obtained through a continuous casting process after molten steel having a predetermined composition is obtained through a steelmaking process.

[0054] FIG. 1 is a flowchart showing the steps of a method for manufacturing a high-strength cold-rolled steel sheet according to an embodiment of the present invention.

[0055] A method for manufacturing a high-strength cold-rolled steel sheet according to an embodiment of the present invention includes the steps of hot-rolling a steel material having the above-described composition to manufacture a hot-rolled steel sheet, cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet, annealing the cold-rolled steel sheet, multi-stage cooling the annealed cold-rolled steel sheet, and partitioning the multi-stage cooled cold-rolled steel sheet.

[0056] Hot-rolled steel sheet manufacturing steps

[0057] A steel slab having the above-described alloy composition is reheated to a temperature of 1150°C to 1250°C. This reheating process homogenizes the element segregation that occurred during the casting process, making it ready for hot rolling. If the slab is reheated at a temperature below 1150°C, there is a problem that the re-dissolution of the segregation is limited. If it exceeds 1250°C, the austenite grain size increases, and the process cost increases due to the increase in temperature. The slab reheating time is preferably 1 to 4 hours. If it is less than 1 hour, the degree of segregation homogenization is insufficient. If it exceeds 4 hours, the austenite grain size increases, and the process cost increases, similar to heating above 1250°C.

[0058] After the reheating, hot rolling is performed by a conventional method at a finish delivery temperature (FDT) in the range of 850°C to 1000°C to produce a hot-rolled steel sheet. If the finish delivery temperature is less than 850°C, ferrite or pearlite may be formed. If the finish delivery temperature exceeds 1000°C, scale formation increases, the grain size becomes coarse, and it may become difficult to achieve a fine and uniform structure.

[0059] Next, the hot-rolled steel sheet is cooled to a coiling temperature of 700°C or lower, for example, in the range of 500°C to 700°C, and then coiled. The cooling can be either air cooling or water cooling, and can be performed at a cooling rate of, for example, 10°C / sec to 30°C / sec.

[0060] Cold-rolled steel sheet manufacturing steps

[0061] The hot-rolled steel sheet is subjected to pickling treatment by washing with acid to remove the surface scale layer. The hot-rolled steel sheet is then cold-rolled to form a cold-rolled steel sheet. The reduction ratio during cold rolling is not particularly specified, but is appropriately set to meet the final product specifications and is generally specified to be in the range of 40% to 80%.

[0062] After cold rolling, the cold-rolled steel sheet undergoes a heat treatment process including an annealing heat treatment step, a cooling step, and a partitioning step. Figure 2 shows a heat treatment process according to an embodiment of the present invention. Hereinafter, the heat treatment process after cold rolling will be described step by step with reference to Figure 2.

[0063] Annealing heat treatment step

[0064] The cold-rolled steel sheet is subjected to annealing in a continuous annealing furnace having a normal slow cooling section (annealing in FIG. 2). When the cold-rolled steel sheet is heated to the annealing temperature after cold rolling, it is heated at a heating rate of 3°C / sec or more, for example, in the range of 3°C / sec to 10°C / sec. If the heating rate is less than 3°C / sec, it takes a long time to reach the target annealing temperature, which may reduce production efficiency and increase the grain size.

[0065] Annealing heat treatment must be performed within the two-phase temperature range, where austenite and ferrite coexist, to produce the final structure of tempered martensite, retained austenite, and ferrite. If the temperature is set to a single-phase austenite region, the alloy enters the two-phase region during slow cooling, allowing for a phase transformation from austenite to ferrite. However, the transformation time is short and the transformation is slight, making it difficult to form the sufficient amount of ferrite targeted in the present invention. Annealing within the two-phase temperature range facilitates the formation of the final microstructure, since additional ferrite formed during slow cooling can be secured in addition to the ferrite secured during annealing. Therefore, in the present invention, the annealing temperature is within the two-phase temperature range, ranging from 780°C to 860°C, for example, from 800°C to 840°C. The holding time at the annealing temperature can be within the range of 30 seconds to 180 seconds.

[0066] Multi-stage cooling steps

[0067] The cold-rolled steel sheet that has been subjected to the annealing heat treatment at the annealing temperature is cooled. The cooling step may be performed in two steps, namely, primary cooling and secondary cooling.

[0068] The primary cooling is a section in which, after the annealing heat treatment, the steel is slowly cooled to the primary cooling end temperature at a cooling rate of 1°C / sec to 10°C / sec (slow cooling in FIG. 2). In this case, the primary cooling end temperature can be a temperature of 680°C or higher, for example, in the range of 680°C to 800°C. If the primary cooling end temperature is less than 680°C, the ferrite transformation is accelerated and elongation increases, but this leads to a decrease in strength, and the target strength may not be achieved.

[0069] Secondary cooling is a rapid cooling step performed after primary cooling. It is intended to transform austenite in the microstructure after slow cooling into martensite to ensure final properties. During secondary cooling, austenite transforms into martensite due to a sudden temperature change. Therefore, due to volumetric changes resulting from these thermal changes and phase transformations, the shape of the steel sheet may become distorted during secondary cooling. Such shape changes adversely affect the quality of the steel sheet and make it difficult to properly operate subsequent processes.

[0070] In the present invention, in order to suppress deformation of the shape of the steel sheet that often occurs during such secondary cooling, the secondary cooling is divided into a primary quenching step and a secondary quenching step and is carried out in stages (Cooling 1 and Cooling 2 in FIG. 2). That is, after the primary cooling is completed, the primary quenching is carried out during the secondary cooling, and the secondary quenching starts at the temperature at which the primary quenching ends.

[0071] The cooling rate of the first quenching is in the range of 70°C / sec to 110°C / sec, for example, in the range of 80°C / sec to 100°C / sec. The end temperature of the first quenching is a temperature equal to or higher than Ms (martensitic transformation start temperature), and can be in the range of, for example, 320°C to 350°C.

[0072] The primary quenching end temperature corresponds to the secondary quenching start temperature, which is a temperature lower than Ms and equal to or higher than Mf (martensitic transformation end temperature), and may be in the range of, for example, 200°C to 260°C.

[0073] The cooling rate of the secondary quenching is smaller than that of the primary quenching, for example, 30°C / sec or more and less than 70°C / sec, for example, in the range of 30°C / sec to 50°C / sec. If the cooling rate of the secondary quenching is 70°C / sec or more, deformation of the shape of the steel sheet cannot be effectively suppressed, and if it is less than 30°C / sec, the amount of austenite transformed into martensite will not meet the target value, and the content of retained austenite may become excessively high.

[0074] The secondary quenching step may include a step of maintaining the cold-rolled steel sheet that has reached the secondary quenching end temperature for 5 to 120 seconds (see "Maintain" in Figure 2). During the secondary quenching end temperature maintenance period, a portion of the austenite transforms into bainite, and precipitates such as metal carbides may be formed in the martensite produced in the secondary cooling step or in the bainite produced during the secondary quenching end temperature maintenance period.

[0075] Partitioning step

[0076] After the cooling step is completed, the cold-rolled steel sheet is reheated and then subjected to a partitioning step (partitioning in FIG. 2) in which heat treatment is performed within a predetermined temperature range.

[0077] In the partitioning step, carbon diffuses and concentrates within the retained austenite, stabilizing it. Typically, the partitioning step involves reheating to a target temperature and then isothermal holding at the target temperature for a certain period of time. However, in the present invention, slow cooling rather than isothermal holding is performed in the partitioning step (partitioning in FIG. 2 ) in consideration of the TO composition (the carbon concentration in austenite when the free energies of austenite (FCC structure) and ferrite (BCC structure) are equal and the driving force for bainite transformation becomes zero). This further enhances the stability of austenite.

[0078] Specifically, the cold-rolled steel sheet that has undergone the multi-stage cooling step is reheated at a heating rate of 3 to 20°C / second to reach a target temperature of 350 to 460°C. If the temperature reached is less than 350°C, the partitioning effect due to the carbon diffusion effect may be insufficient, and if the temperature reaches more than 460°C, upper bainite transformation may occur, which may affect the physical properties and elongation.

[0079] Cooling begins immediately after the target temperature is reached (cooling 3 in FIG. 2). The cooling performed in the partitioning step is called tertiary cooling. Therefore, the target temperature corresponds to the tertiary cooling start temperature. The cooling rate in the tertiary cooling step can be in the range of 0.07°C / sec to 0.21°C / sec, preferably 0.09°C / sec to 0.19°C / sec. The tertiary cooling end temperature can be in the range of 340°C to 400°C.

[0080] The partitioning time is in the range of 30 to 600 seconds, preferably in the range of 200 to 300 seconds.

[0081] Regarding the mechanism by which slow cooling improves the stability of austenite compared to isothermal holding, in the case of isothermal holding, the bainite transformation stops when the carbon concentration in the untransformed austenite reaches the TO composition at the isothermal holding temperature. On the other hand, in the case of slow cooling, the TO composition gradually increases as the temperature decreases during slow cooling, so the C concentration in the untransformed austenite becomes higher than in the case of isothermal holding. As a result, it can be concluded that the stability of the untransformed austenite is further improved.

[0082] Experimental example

[0083] In the following, preferred experimental examples are presented to aid in understanding the present invention. However, the following experimental examples are merely provided to aid in understanding the present invention, and the present invention is not limited to the following experimental examples.

[0084] A slab having the composition (unit: wt%) shown in Table 1 below was hot-rolled and cold-rolled to produce a cold-rolled steel sheet. The produced cold-rolled steel sheet was heat-treated under the conditions shown in Table 2. The reheating temperature in Table 2 is the target temperature for partitioning and corresponds to the tertiary cooling start temperature.

[0085] [Table 1]

[0086] [Table 2]

[0087] To observe the tendency of steel sheet deformation depending on the cooling rate of the secondary quenching, the cooling rate of the secondary quenching was varied in the range of 30°C / s to 100°C / s. To evaluate the deformation of the steel sheet, the deformation height was measured as an index of the flatness of the steel sheet after the completion of the secondary quenching. Meanwhile, the carbon content in the retained austenite was measured while varying the tertiary cooling rate in the partitioning step. The yield strength, tensile strength, and elongation of the cold-rolled steel sheet after partitioning were measured.

[0088] Table 3 shows the physical properties of the cold-rolled steel sheets obtained under the process conditions in Table 2, including the deformation height, the carbon concentration in the retained austenite, and the mechanical properties (yield strength, tensile strength, and elongation).

[0089] [Table 3]

[0090] Referring to Tables 2 and 3, Examples 1 to 5, in which the cooling rate of the secondary quenching was in the range of 30°C / sec to 50°C / sec, had a deformation height of 3.0 mm or less, indicating excellent flatness. However, in Comparative Examples 1 and 2, in which the cooling rate of the secondary quenching was in the range of 70°C / sec to 100°C / sec, the deformation height was significantly greater than 3.0 mm, confirming that the shape was severely deformed during the secondary quenching.

[0091] FIG. 3 is a graph showing the carbon concentration in retained austenite (RA) depending on the cooling rate during the tertiary cooling in the partitioning step.

[0092] 3, it can be seen that in Examples 1 to 5, the carbon concentration in the retained austenite was in the range of 1.1% to 1.4% by weight, while in Comparative Examples 3 to 9, it was less than 1%. In Examples 1 to 5, the cooling rate of the tertiary cooling was in the range of 0.07°C / sec to 0.21°C / sec, but in Comparative Examples 3 to 9, it was outside this range. In particular, in Comparative Example 3, which was isothermally held in the partitioning step, the carbon concentration in the retained austenite was 0.77%, which was significantly lower than that of the Examples.

[0093] Referring to Table 3, all of the inventive examples simultaneously satisfy the following values: yield strength (YS) of 550 MPa or more, tensile strength (TS) of 980 MPa or more, elongation (EI) of 20% or more, and tensile strength x elongation of 20,000 MPa% or more. In particular, the inventive examples were able to achieve high elongation of 20% or more by using the cooling treatment in the partitioning step to ensure that the carbon concentration in the retained austenite was 1.1% or more.

[0094] FIG. 4 exemplarily shows the results of observing the microstructure of a test specimen corresponding to Invention Example 1 with an electron microscope, and Table 4 shows the results of phase analysis of the test specimen. The phase fractions shown in Table 4 are area fractions (area %). In Table 4, F, RA, MA, FM, and TM mean ferrite, retained austenite, a martensite-austenite composite phase, fresh martensite, and tempered martensite, respectively.

[0095] Referring to FIG. 4 and Table 4, Example 1 exhibited a phase fraction corresponding to the microstructure of the present invention. L / RA T The RA value of the present invention is also 0.6. L / RA T Value met.

[0096] [Table 4]

[0097] It will be apparent to those skilled in the art to which the technical idea of ​​the present invention pertains that the technical idea of ​​the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and that various substitutions, modifications and changes are possible within the scope of the technical idea of ​​the present invention.

Claims

1. A cold-rolled steel sheet 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, and the balance being iron (Fe) and other inevitable impurities, The microstructure is composed of, by area ratio, 25 to 35% ferrite, 10 to 18% retained austenite, 5% or less M-A (a martensite-austenite composite phase), and the balance martensite; A high-strength cold-rolled steel sheet having a flatness of 3.0 mm or less.

2. Further containing any one or more of niobium (Nb), titanium (Ti), and vanadium (V), The high-strength cold-rolled steel sheet according to claim 1, wherein the sum of niobium (Nb), titanium (Ti), and vanadium (V) is in the range of 0.05% or less (more than 0).

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

4. 2. The high strength cold rolled steel sheet according to claim 1, wherein the cold rolled steel sheet has a yield strength (YS) of 550 MPa or more, a tensile strength (TS) of 980 MPa or more, an elongation (EI) of 20% or more, and a tensile strength x elongation value of 20,000 MPa% or more.

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

6.

6. The high-strength cold-rolled steel sheet according to claim 1, wherein an area fraction of retained austenite having an aspect ratio (aspect ratio) of 3 or more obtained by dividing the major axis length by the minor axis length is in the range of 3% to 8%.

7. The high-strength cold-rolled steel sheet according to claim 6, wherein a value obtained by dividing the total area fraction of retained austenite by the area fraction of retained austenite having an aspect ratio of 3 or more is in the range of 0.5 to 0.

8.

8. a step of manufacturing a hot-rolled steel sheet by hot-rolling a steel material containing 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 being iron (Fe) and other inevitable impurities; cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; Annealing the cold-rolled steel sheet in a temperature range of austenite and ferrite two-phase region; A step of primarily cooling the annealed cold-rolled steel sheet at a first cooling rate to a primary cooling end temperature in the range of 650°C to 800°C; A step of performing secondary cooling on the primarily cooled cold-rolled steel sheet at a cooling rate higher than the cooling rate of the primary cooling; and a partitioning step of reheating the cold-rolled steel sheet after secondary cooling to reach a target temperature in the range of 350°C to 460°C, and then performing tertiary cooling at a cooling rate of 0.07°C / sec to 0.21°C / sec.

9. Further containing any one or more of niobium (Nb), titanium (Ti), and vanadium (V), The method for producing a high-strength cold-rolled steel sheet according to claim 8, wherein the total amount of niobium (Nb), titanium (Ti), and vanadium (V) is in the range of 0.05% or less (more than 0).

10. The secondary cooling includes a primary quenching step and a secondary quenching step, The primary quenching step is performed by cooling the material at a cooling rate of 70°C / sec to 110°C / sec to a primary quenching end temperature of Ms or higher; The method for producing a high strength cold rolled steel sheet according to claim 8, wherein the secondary quenching step comprises cooling to a secondary quenching end temperature lower than Ms and higher than Mf at a cooling rate of 30°C / sec or higher and lower than 70°C / sec.

11. The primary quenching end temperature is in the range of 320°C to 350°C, The method for producing a high-strength cold-rolled steel sheet according to claim 10, wherein the secondary quenching end temperature is in the range of 200°C to 260°C.

12. The method for producing a high-strength cold-rolled steel sheet according to claim 8, wherein the partitioning step is carried out for a period of time ranging from 30 seconds to 600 seconds.

13. The method for producing a high-strength cold-rolled steel sheet according to claim 8, wherein the two-phase temperature range is 780°C to 860°C.

14. The method for producing a high-strength cold-rolled steel sheet according to claim 8, wherein the primary cooling finish temperature is in the range of 680°C to 800°C.

15. The method for producing a high-strength cold-rolled steel sheet according to claim 8, wherein a tertiary cooling end temperature during tertiary cooling is in the range of 340 ° C to 400 ° C.

Images