Ultra-high strength cold-rolled steel sheet and its manufacturing method

Abstract

According to one aspect of the present invention, there is provided an ultra-high strength cold-rolled steel sheet. The ultra-high strength cold-rolled steel sheet contains, by weight percent, 0.28% to 0.4% carbon (C), 1.0% to 2.0% silicon (Si), 1.0% to 3.0% manganese (Mn), 0.01% to 0.3% aluminum (Al), 0.01% to 0.05% niobium (Nb), more than 0% to 0.02% phosphorus (P), more than 0% to 0.003% sulfur (S), 0.001% to 0.005% boron (B), and the balance being iron (Fe) and other inevitable impurities. According to one embodiment of the present invention, the microstructure of the ultra-high strength cold-rolled steel sheet contains, by area fraction, 40 to 60% lath ferrite, 20 to 30% lath martensite (MA), 15% or less (more than 0) blocky MA, and 15 to 25% retained austenite.

Description

[Technical Field]

[0001] The technical concept of the present invention relates to a cold-rolled steel sheet, and more particularly to an ultra-high strength cold-rolled steel sheet having high strength and excellent formability, and a method for producing the same. [Background technology]

[0002] Ultra-high strength steels for automotive steel sheets are being developed to meet two requirements: vehicle weight reduction due to stricter energy resource and environmental regulations, and ensuring collision stability due to stricter safety regulations. However, there is a trade-off between strength and elongation, and as the strength of steel materials increases, the difficulty of forming the steel materials increases. Therefore, various research efforts are being conducted to develop ultra-high strength steels with excellent formability.

[0003] TRIP steel, which partially utilizes the transformation-induced plasticity (TRIP) mechanism in a composite structure, whereby retained austenite in the microstructure transforms into martensite when steel is deformed by external force, is being developed as a third-generation steel sheet that can achieve both ultra-high strength and high elongation.

[0004] The mechanical properties of TRIP steel vary significantly depending on the characteristics and phase fraction of the retained austenite where the TRIP phenomenon occurs. Therefore, ensuring optimal phase stability and phase fraction of the retained austenite is important in manufacturing the steel. Conventional third-generation steel sheets for car bodies have a tensile strength-elongation product (TS × EL), an index of mechanical properties, of 20,000 MPa% to 22,000 MPa%. However, in order to apply ultra-high-strength steel with a tensile strength of 1 GPa or higher to structural parts with more complex shapes, there is a growing demand for the development of steel grades with a tensile strength-elongation product (TS × EL) of 30,000 MPa% or higher.

[0005] Korean Patent Publication No. 10-2014-0068207 proposes a method for manufacturing steel containing tempered martensite and retained austenite (Quenching and Partitioning, Q&P) through annealing, rapid cooling, and partitioning heat treatment of steel. This patent proposes a method for manufacturing ultra-high strength, highly formable steel with a tensile strength x elongation product of 27,000 MPa% or more. However, the application of the Q&P method has the problem of reduced reproducibility of properties during steel manufacturing due to the narrow range of process temperatures in which the properties can be realized.

[0006] Prior art documents include Korean Patent Publication No. 10-2014-0068207. Summary of the Invention [Problem to be solved by the invention]

[0007] The technical problem to be achieved by the technical concept of the present invention is to provide an ultra-high strength cold rolled steel sheet having high strength and elongation by controlling the microstructure, and a manufacturing method thereof.

[0008] However, these problems are merely examples, and the technical idea of ​​the present invention is not limited to these. [Means for solving the problem]

[0009] According to one aspect of the present invention, there is provided an ultra-high strength cold rolled steel sheet.

[0010] According to one embodiment of the present invention, the cold-rolled steel sheet contains, by weight, 0.28% to 0.4% carbon (C), 1.0% to 2.0% silicon (Si), 1.0% to 3.0% manganese (Mn), 0.01% to 0.3% aluminum (Al), 0.01% to 0.05% niobium (Nb), more than 0% to 0.02% phosphorus (P), more than 0% to 0.003% sulfur (S), 0.001% to 0.005% boron (B), and the balance being iron (Fe) and other inevitable impurities. The microstructure contains, by area fraction, 40 to 60% lath-shaped ferrite, 20 to 30% lath-shaped MA, 15% or less (more than 0) blocky MA, and 15 to 25% retained austenite.

[0011] According to one embodiment of the present invention, the cold-rolled steel sheet may further contain, by weight percent, one or more of chromium (Cr) greater than 0% and not greater than 2.0%, and molybdenum (Mo) greater than 0% and not greater than 0.2%, and the sum of chromium (Cr) and molybdenum (Mo) may be 3.0% or less.

[0012] According to one embodiment of the present invention, the cold-rolled steel sheet further includes one or more of more than 0% and not more than 5% polygonal ferrite, more than 0% and not more than 5% martensite, and more than 0% and not more than 5% other structures, wherein the martensite includes one or more of fresh martensite and tempered martensite, and the other structures may include one or more of pearlite and austenite decomposed structures.

[0013] According to one embodiment of the present invention, in the cold-rolled steel sheet, the lath-shaped ferrite and lath-shaped MA may have an aspect ratio, which is the ratio of the major axis length divided by the minor axis length (major axis length / minor axis length), of 3 or more.

[0014] According to one embodiment of the present invention, in the cold-rolled steel sheet, the lath-shaped ferrite and the lath-shaped MA may have a lamellar shape formed so as to be alternately arranged in a direction perpendicular to the longitudinal direction of either the lath-shaped ferrite or the lath-shaped MA.

[0015] According to one embodiment of the present invention, the cold-rolled steel sheet may have a tensile strength (TS) of 980 MPa or more and 1300 MPa or less, an elongation (El) of 25% or more and 45% or less, and a product of tensile strength (TS) and elongation (EL) (TS×EL) of 30,000% or more and 45,000 MPa or less.

[0016] According to another aspect of the present invention, there is provided a method for producing an ultra-high strength cold-rolled steel sheet, the method comprising the steps of: (i) preparing a high strength cold-rolled steel sheet from a steel sheet containing, by weight percent, 0.28% to 0.4% carbon (C), 1.0% to 2.0% silicon (Si), 1.0% to 3.0% manganese (Mn), 0.01% to 0.3% aluminum (Al), 0.01% to 0.05% niobium (Nb), more than 0% to 0.02% phosphorus (P), more than 0% to 0.02% sulfur (S), and 0.001% to 0.0% boron (B). The method includes the steps of: hot-rolling a steel material containing 0.05% of iron (Fe), and the remainder being iron (Fe) and other unavoidable impurities, to produce a hot-rolled steel sheet; cold-rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; subjecting the cold-rolled steel sheet to a first annealing heat treatment; subjecting the cold-rolled steel sheet that has been subjected to the first annealing heat treatment to a second annealing heat treatment; and overaging the cold-rolled steel sheet that has been subjected to the second annealing heat treatment.

[0017] According to one embodiment of the present invention, the first annealing heat treatment step includes a step of performing a first annealing treatment at a first annealing temperature of Ac3 or higher, followed by a step of cooling to a temperature of Ms or lower, the second annealing heat treatment step includes a step of performing a second annealing treatment at a second annealing temperature of (Ac1+Ac3) / 2 or higher and lower than Ac3, followed by a step of cooling to a temperature in the range of (2×Ms+3×Bs) / 5-40°C to (2×Ms+3×Bs) / 5+20°C, and the overaging step may be performed at a temperature in the range of (2×Ms+3×Bs) / 5-40°C to (2×Ms+3×Bs) / 5+20°C.

[0018] According to one embodiment of the present invention, the step of cooling to a temperature equal to or lower than Ms after the first annealing treatment may include: a step of primarily cooling the cold-rolled steel sheet that has been subjected to the first annealing treatment to a primary cooling end temperature in the range of 700°C to 800°C at a first cooling rate; and a step of secondly cooling, after the primary cooling, to a secondary cooling end temperature that is equal to or lower than Ms at a second cooling rate that is faster than the first cooling rate.

[0019] According to an embodiment of the present invention, the first cooling rate may be in the range of 5° C. / sec to 20° C. / sec.

[0020] According to an embodiment of the present invention, the second cooling rate may be in the range of 15° C. / sec to 300° C. / sec.

[0021] According to one embodiment of the present invention, the cold-rolled steel sheet that has undergone the first annealing heat treatment has a microstructure with an area ratio of less than 5% ferrite and the remainder being a low-temperature phase, and the low-temperature phase may include at least one of martensite and bainite.

[0022] According to one embodiment of the present invention, the step of cooling after the second annealing treatment may include: a step of tertiary cooling the cold-rolled steel sheet that has been subjected to the second annealing treatment at a third cooling rate to a tertiary cooling end temperature in the range of 700°C to 800°C; and a step of fourthly cooling, after the tertiary cooling, at a fourth cooling rate faster than the third cooling rate to a fourth cooling end temperature in the range of (2×Ms+3×Bs) / 5-40°C to (2×Ms+3×Bs) / 5+20°C.

[0023] According to an embodiment of the present invention, the third cooling rate may be in the range of 5° C. / sec to 10° C. / sec.

[0024] According to an embodiment of the present invention, the fourth cooling rate may be in the range of 15° C. / sec to 100° C. / sec.

[0025] According to one embodiment of the present invention, the overaging step may be performed for a period of time ranging from 30 to 300 seconds. [Effects of the Invention]

[0026] According to the technical concept of the present invention, the homogeneity of the microstructure of a high alloy steel can be improved by utilizing the first annealing heat treatment and the second annealing heat treatment, and a lath-shaped microstructure can be sufficiently secured, thereby realizing an ultra-high strength cold rolled steel sheet having an excellent balance of strength and elongation. The above-mentioned effects of the present invention are described by way of example only, and the scope of the present invention is not limited by these effects. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a flowchart illustrating steps in a method for manufacturing an ultra-high strength cold-rolled steel sheet according to an embodiment of the present invention.

[0028] [Figures 2A-2D] 1 shows the results of observing the microstructures of the examples and comparative examples after hot rolling, softening heat treatment, and first annealing heat treatment using a scanning electron microscope.

[0029] [Figure 3A-3B] 1 shows the results of observing the microstructure of an example after the second annealing heat treatment with a scanning electron microscope.

[0030] [Figures 4A-4D] 1 shows the results of observing the microstructures of the example and comparative example after the second annealing heat treatment using a scanning electron microscope.

[0031] [Figure 5] Figure 3A shows the specimen analyzed at a magnified analysis magnification.

[0032] [Figures 6A-6B] 1 shows temperature-time histories for a first annealing heat treatment and a second annealing heat treatment for a cold-rolled steel sheet according to an embodiment of the present invention.

[0033] [Figures 7A-7B] The results of analyzing the C content of each phase shown in FIG. 5 by FE-EPMA are shown. BEST MODE FOR CARRYING OUT THE INVENTION

[0034] 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.

[0035] In this specification and claims, the phase fraction refers to the area ratio (area %) derived from a microstructure photograph using an image analyzer. The content or concentration of a specific component refers to weight % unless otherwise specified.

[0036] The technical idea of ​​the present invention is to provide an ultra-high strength cold-rolled steel sheet with excellent workability, which has a tensile strength of 980 MPa or more, an elongation of 25% or more, and a tensile strength x elongation (the product of strength and elongation) of 30,000 MPa% or more, and a method for producing such a cold-rolled steel sheet.

[0037] Hereinafter, the ultra-high strength cold rolled steel sheet according to the technical idea of ​​the present invention will be described in detail.

[0038] An ultra-high strength cold-rolled steel sheet according to one embodiment of the present invention contains, by weight, 0.28% to 0.4% carbon (C), 1.0% to 2.0% silicon (Si), 1.0% to 3.0% manganese (Mn), 0.01% to 0.3% aluminum (Al), 0.01% to 0.05% niobium (Nb), more than 0% to 0.02% phosphorus (P), more than 0% to 0.003% sulfur (S), 0.001% to 0.005% boron (B), and the balance being iron (Fe) and other unavoidable impurities. Optionally, the steel sheet may further contain one or more of more than 0% to 2.0% chromium (Cr) and more than 0% to 0.2% molybdenum (Mo). In this case, the total of chromium (Cr) and molybdenum (Mo) is 3.0% or less.

[0039] The role and content of each component contained in the ultra-high strength cold rolled steel sheet according to the present invention will be described below. Here, the contents of the component elements are all expressed in wt% relative to the total weight of the steel sheet.

[0040] Carbon (C): 0.28%~0.4%

[0041] Carbon is added to ensure the appropriate fraction and stability of retained austenite. The carbon content ranges from 0.28% to 0.40% by weight. If the carbon content is less than 0.28%, the fraction of retained austenite in the final microstructure is insufficient, making it difficult to achieve the desired ductility. On the other hand, if the carbon content exceeds 0.4%, it may be detrimental to weldability.

[0042] Silicon (Si): 1.0% to 2.0%

[0043] Silicon is a ferrite stabilizing element that delays the formation of carbides in ferrite and has the effect of solid solution strengthening. The silicon content ranges from 1.0% to 2.0% by weight. If the silicon content is less than 1.0%, it is difficult to achieve the above-mentioned effects. On the other hand, if the silicon content exceeds 2.0%, oxides such as Mn2SiO4 are formed during the manufacturing process, which can hinder galvanization and increase the carbon equivalent, which can reduce weldability.

[0044] Manganese (Mn): 1.0% to 3.0%

[0045] Manganese has a solid solution strengthening effect, increasing hardenability and contributing to improved strength. The manganese content ranges from 1.0% to 3.0% by weight. If the manganese content is less than 1.0%, the effect is 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 a decrease in workability and a decrease in delayed fracture resistance, and the increase in carbon equivalent can reduce weldability.

[0046] Aluminum (Al): 0.01% to 0.3%

[0047] Aluminum is used as a deoxidizer and can help purify ferrite. The aluminum content ranges from 0.01% to 0.3% by weight. If the aluminum content is less than 0.01%, the deoxidizing effect is insufficient. On the other hand, if the aluminum content exceeds 0.30%, AlN is formed during the continuous casting step to produce slabs, which can cause cracks during continuous casting or hot rolling.

[0048] Niobium (Nb): 0.01% to 0.05%

[0049] Niobium is a precipitation strengthening element that improves strength by precipitating as precipitates such as nitrides. The niobium content ranges from 0.01% to 0.05% by weight. If the niobium content is less than 0.01%, the precipitation strengthening effect is not achieved, while if it exceeds 0.05%, the strength becomes excessively high and the ductility decreases.

[0050] Phosphorus (P): More than 0% and less than 0.02%

[0051] Phosphorus is an impurity contained in steel during manufacturing and can help improve strength through solid solution strengthening, but if contained in large amounts, it can cause low-temperature brittleness. Therefore, the phosphorus content is preferably limited to 0.02% by weight or less.

[0052] Sulfur (S): More than 0% but less than 0.003%

[0053] Sulfur is an impurity contained in steel during the manufacturing process and can form non-metallic inclusions such as FeS and MnS, which can reduce toughness and weldability. Therefore, the sulfur content is preferably limited to 0.003% by weight or less.

[0054] Boron (B): 0.001% to 0.005% or less

[0055] Boron is a hardenable element that suppresses the formation of polygonal ferrite. The boron content ranges from 0.001% to 0.005% by weight. If the boron content is less than 0.001%, the effect is slight, while if it exceeds 0.005%, workability decreases.

[0056] Chromium (Cr): Over 0% and up to 2.0%

[0057] Chromium, like manganese, has a solid solution strengthening effect, increasing hardenability and contributing to improved strength. The chromium content is in the range of more than 0% and not more than 2.0% by weight. If the chromium content exceeds 2.0%, the hardenability becomes excessive, the fraction of retained austenite decreases, the fraction of martensite increases, and ductility decreases. The total of chromium and manganese is preferably not more than 3.0% by weight.

[0058] Molybdenum (Mo): Over 0% and up to 0.2%

[0059] Molybdenum increases hardenability and inhibits the formation of pearlite. The molybdenum content ranges from more than 0% to 0.2% by weight. If the molybdenum content exceeds 0.2%, martensite becomes excessive and ductility decreases.

[0060] The remaining component of the ultra-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.

[0061] Microstructure of steel plate

[0062] The microstructure of the ultra-high strength cold-rolled steel sheet according to the technical concept of the present invention includes a lath-type structure, a bulk-type structure, and retained austenite. The lath-type structure includes a mixed structure of lath ferrite and lath MA. The bulky structure includes MA. The MA means a composite phase of martensite and austenite.

[0063] To distinguish between lath and blocky phase shapes, the aspect ratio, which is the ratio of the major axis length of a phase divided by the minor axis length of the phase (major axis length / minor axis length), is used as a criterion. That is, if the aspect ratio is greater than 3, it is defined as lath, and if it is less than 3, it is defined as blocky. In blocky structures, if the size is less than 2 μm, it is classified as blocky MA, and if it is 2 μm or more, it is classified as martensite.

[0064] In the present invention, the lath ferrite and lath MA constituting the lath structure have the long axis directions of each phase extending in substantially the same or similar directions and are arranged adjacent to each other. Adjacent lath ferrite and lath MA have a lamellar structure formed so that they are alternately arranged in a direction perpendicular to the long axis direction. The cold-rolled steel sheet of the present invention can ensure superior elongation compared to conventional blocky structures by forming a mixed structure of lath ferrite and lath MA in a lamellar structure.

[0065] The retained austenite contributes to the elongation properties of steel through transformation induced plasticity (TRIP).

[0066] The ultra-high strength cold rolled steel sheet of the present invention may contain, in area percentage (area %), 40 to 60% lath ferrite, 20 to 30% lath MA, more than 0% but not more than 15% blocky MA, and 15 to 25% retained austenite. Additionally, it may contain polygonal ferrite, martensite, and other structures.

[0067] Polygonal ferrite, when present in large quantities, is disadvantageous in ensuring tensile strength, so its area fraction is limited to more than 0% and not more than 5%. Martensite, when present in large quantities, reduces ductility, so its area fraction is limited to more than 0% and not more than 5%. The martensite includes one or more of fresh martensite and tempered martensite. Other structures include one or more of pearlite and austenite decomposition structures, which may deteriorate the mechanical properties of the steel, so their area fraction is limited to more than 0% and not more than 5%.

[0068] The ultra-high strength cold rolled steel sheet of the present invention can satisfy, for example, a tensile strength of 980 MPa or more, an elongation of 25% or more, and a product of tensile strength (TS) and elongation (EL) (TS×EL) of 30,000 MPa% or more.

[0069] For example, the tensile strength (TS) can be in the range of 980 MPa to 1300 MPa, the elongation (El) can be in the range of 25% to 45%, and the product of the tensile strength (TS) and the elongation (EL) (TS×EL) can be in the range of 30,000 to 45,000 MPa%.

[0070] Hereinafter, a method for producing a cold-rolled steel sheet according to the technical idea of ​​the present invention, which is an ultra-high strength cold-rolled steel sheet having the composition range described above, will be described with reference to the accompanying drawings.

[0071] Cold-rolled steel sheet manufacturing method

[0072] 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.

[0073] FIG. 1 is a flowchart showing the steps of a method for manufacturing an ultra-high strength cold rolled steel sheet according to an embodiment of the present invention.

[0074] A method for manufacturing an ultra-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 (S100), cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet (S300), subjecting the cold-rolled steel sheet to a first annealing heat treatment (S400), and subjecting the cold-rolled steel sheet that has been subjected to the first annealing heat treatment to a second annealing heat treatment (S500).

[0075] Optionally, a softening heat treatment step (S200) may be further performed between the step of producing the hot-rolled steel sheet (S100) and the step of producing the cold-rolled steel sheet (S300).

[0076] Hot-rolled steel sheet manufacturing steps

[0077] A steel slab having the above-mentioned alloy composition is reheated at a temperature of 1180-1300°C. Slabs are produced in the form of semi-finished products by continuously casting molten steel obtained through a steelmaking process. The reheating process homogenizes the elemental segregation that occurred during the casting process, preparing the slab for hot rolling. If the slab reheating temperature (SRT) is less than 1180°C, the segregation in the slab may not be fully redissolved. If it exceeds 1300°C, the austenite grain size may increase, increasing process costs. The slab can be reheated for 1-2 hours. If the reheating time is less than 1 hour, the segregation bands may not be reduced sufficiently. If it exceeds 2 hours, the grain size may increase, increasing process costs.

[0078] After the reheating, hot rolling can be performed by a conventional method, for example, hot finish rolling at a finish delivery temperature (FDT) in the range of 850°C to 950°C to produce a hot rolled steel sheet. If the finish rolling temperature is lower than 850°C, the rolling load increases sharply, reducing productivity, while if it exceeds 950°C, the grain size increases, reducing strength.

[0079] After hot rolling, the steel sheet is cooled to a temperature of 450 to 650°C and then coiled. If the coiling temperature is less than 450°C, the strength increases, which increases the rolling load during cold rolling. If the coiling temperature exceeds 650°C, defects may occur in subsequent processes due to surface oxidation, etc.

[0080] Softening heat treatment step

[0081] The manufactured hot-rolled steel sheet can be optionally softened at a temperature below Ac1. Hot-rolled steel sheet coiled after hot rolling has high steel strength, so the rolling load during cold rolling is large. To alleviate this, softening can be performed to reduce the strength of the steel. The softening temperature is below Ac1. If the softening temperature is above Ac1, retained austenite may remain after softening, hindering the cold-rolled shape and the final properties of the cold-rolled steel sheet.

[0082] Cold-rolled steel sheet manufacturing steps

[0083] The hot-rolled steel sheet is subjected to pickling treatment by washing with acid to remove a surface scale layer. Next, the hot-rolled steel sheet is cold-rolled at an average reduction of, for example, 30% to 80% to form a cold-rolled steel sheet. The higher the average reduction, the more effective it is in improving formability due to the effect of refining the structure. If the average reduction is less than 30%, it is difficult to obtain a uniform fine structure. If the average reduction exceeds 80%, the rolling force increases, increasing the process load. The structure of the cold-rolled steel sheet may have a shape obtained by elongating the structure of the hot-rolled steel sheet.

[0084] After the cold rolling is completed, the cold-rolled steel sheet is subjected to a first annealing heat treatment and a second annealing heat treatment consecutively, two times in total.

[0085] FIG. 6 shows the temperature-time history of the first annealing heat treatment and the second annealing heat treatment for the cold-rolled steel sheet according to one embodiment of the present invention.

[0086] Hereinafter, the heat treatment after cold rolling will be described step by step with reference to FIG.

[0087] First annealing heat treatment step

[0088] 6A shows the first annealing heat treatment step. Referring to FIG. 6A, the first annealing heat treatment includes a first annealing treatment and a multi-stage cooling step.

[0089] Referring to FIG. 6A, the first annealing heat treatment involves reheating (S11) a cold-rolled steel sheet to an annealing temperature equal to or higher than Ac3, which is an austenite single phase region, for example, a temperature equal to or higher than Ac3 and lower than Ac3+30°C, and then holding the annealing temperature for a certain period of time (S12).

[0090] The temperature rise rate from room temperature to the annealing temperature range is not particularly limited and follows the temperature rise rate of a typical heating furnace. After the annealing temperature is reached, the annealing temperature is maintained for a predetermined time to perform the first annealing treatment. The time maintained at the annealing temperature can be, for example, in the range of 30 to 120 seconds. If it is less than 30 seconds, the microstructure will not be sufficiently homogenized, and if it exceeds 120 seconds, productivity may decrease.

[0091] After the first annealing treatment, the cold-rolled steel sheet is subjected to primary cooling at a first cooling rate (S13). When the primary cooling is performed, the primary cooling end temperature may be 700°C or higher, for example, in the range of 700 to 800°C, in order to suppress the formation of pearlite. If the temperature is lower than 700°C, pearlite is formed, making it difficult to obtain a lath structure in the subsequent second annealing heat treatment.

[0092] In this case, the first cooling rate is not particularly limited, but the cooling rate can be preferably 5° C. / sec or more, for example, in the range of 5 to 20° C. / sec, so as not to generate a large amount of polygonal ferrite during cooling. The pearlite structure transforms into massive austenite during the annealing process of the second annealing heat treatment, making it difficult to obtain lath ferrite and lath MA structure, so its generation must be suppressed as much as possible.

[0093] After the primary cooling is completed, secondary cooling is performed (S14), in which the material is cooled to a temperature below Ms (martensitic transformation start temperature), for example, in the range of Ms to room temperature. During the secondary cooling, the second cooling rate is a quenching step that is faster than the first cooling rate. The second cooling rate is 15°C / sec or more, for example, in the range of 15°C / sec to 300°C / sec.

[0094] If the secondary cooling end temperature exceeds Ms, carbides are precipitated, making it difficult to obtain a lath structure in the second annealing heat treatment step, and if the cooling rate is less than 15°C / sec, a large amount of polygonal ferrite is generated during cooling, which is disadvantageous in ensuring tensile strength. In the present invention, Ms and Bs (bainite transformation start temperatures) are determined by the following formulas (1) and (2).

[0095] Formula (1): Ms(℃)=491.1-302.6C-14.5Si-30.6Mn-16.6Ni-8.9Cr+2.4Mo-11.3Cu+8.58Co+7.4W

[0096] Formula (2): Bs(℃)=656-57.7C-75Si-35Mn-15.3Ni-34Cr-41.2Mo

[0097] The microstructure created by the first annealing heat treatment is called the prior structure. The prior structure consists of less than 5% ferrite by area and the remainder is the low-temperature phase. Here, the low-temperature phase is a general term for both martensite and bainite.

[0098] The first annealing heat treatment is carried out for the following two purposes.

[0099] The first objective is to homogenize the cold-rolled full hard structure. Steels satisfying the steel composition requirements proposed in the present invention may require an additional softening heat treatment after hot rolling due to concerns about rolling load during cold rolling. The cold-rolled full hard microstructure formed through the softening heat treatment process may contain spheroidized cementite. This spheroidized cementite, which is not redissolved in the annealing heat treatment section of a conventional continuous annealing line, can cause problems with the final mechanical properties. Therefore, in the present invention, a first annealing heat treatment is performed to address the cause of material degradation due to the cold-rolled full hard structure.

[0100] The second purpose is to create a favorable precursor structure for forming lath-type MA in the subsequent second annealing heat treatment. During the second annealing, the low-temperature phase structure reverse transforms to austenite, resulting in lath-type ferrite, lath-type MA, and retained austenite in a lamellar structure, which allows for higher elongation than the blocky structure formed by conventional heat treatment processes.

[0101] Since the low-temperature phase structure obtained in the first annealing heat treatment develops into a lath-like structure in the second annealing heat treatment, it is important to maintain the low-temperature phase structure as much as possible. Furthermore, since polygonal ferrite present after the first annealing heat treatment remains after the second annealing heat treatment and reduces tensile properties, the first annealing temperature is preferably Ac3 or higher. If the first annealing temperature is lower than Ac3, the spheroidized cementite present in the ferrite in the two-phase region will not be completely redissolved, adversely affecting the final material quality.

[0102] Conversely, if the first annealing temperature exceeds Ac3+30°C, the austenite grain size becomes coarse, making it difficult to achieve the target tensile properties. In addition, it is difficult to apply the resulting steel to a conventional continuous annealing line, resulting in reduced productivity. Therefore, the first annealing temperature is preferably Ac3 to Ac3+30°C.

[0103] Second annealing heat treatment step

[0104] 6B, the cold-rolled steel sheet that has undergone the first annealing heat treatment is subjected to a second annealing heat treatment, which includes a second annealing treatment and a multi-stage cooling step.

[0105] The cold-rolled steel sheet that has undergone the first annealing heat treatment is heated from room temperature to the second annealing temperature (S21). The heating rate from room temperature to the second annealing temperature range is not particularly limited and follows the heating rate of a typical heating furnace.

[0106] Second annealing is performed for 30 to 120 seconds at a second annealing temperature of (Ac1+Ac3) / 2 or more and less than Ac3, which is the two-phase region (S22). The second annealing is a step in which the low-temperature phase in the prior structure generated in the first annealing heat treatment undergoes reverse transformation to form lath ferrite and lath MA.

[0107] If the second annealing temperature is less than (Ac1+Ac3) / 2, the area ratio of ferrite exceeds 70%, making it impossible to ensure appropriate strength. Therefore, the second annealing temperature must be (Ac1+Ac3) / 2 or more.

[0108] If the second annealing temperature is Ac3 or higher, all of the low-temperature phases of the previous structure will be reverse transformed to austenite, creating a blocky structure, and it will be impossible to obtain the lath ferrite and lath MA structure that the present invention aims to achieve. Therefore, the second annealing temperature must be lower than Ac3.

[0109] During the second annealing treatment, the low-temperature phase in the prior structure undergoes reverse transformation, resulting in the redistribution of C and Mn within the austenite. To achieve sufficient reverse transformation and redistribution of alloying elements, the longer the annealing time, the better. However, if the annealing time is too long, productivity may decrease, so the annealing time is limited to 30 to 120 seconds.

[0110] After the second annealing, the cold-rolled steel sheet is subjected to tertiary cooling at a third cooling rate (S23). When tertiary cooling is performed, the tertiary cooling end temperature is 700°C or higher, for example, in the range of 700°C to 800°C, in order to suppress the formation of pearlite. The pearlite structure traps carbon within cementite, hindering the mechanism of concentrating carbon in austenite and stabilizing austenite in the subsequent overaging step, making it difficult to achieve the material properties desired by the present invention. The cooling rate is not particularly limited, but may be preferably 5°C / sec or higher, for example, in the range of 5 to 10°C / sec, in order to prevent the formation of a large amount of polygonal ferrite during cooling.

[0111] The third-cooled cold-rolled steel sheet is then subjected to a fourth cooling step (S24) in which it is cooled to a temperature of (2×Ms+3×Bs) / 5-40°C to (2×Ms+3×Bs) / 5+20°C at a fourth cooling rate of 15°C / sec or more, and an overaging step (S25) in which it is overaged at the fourth cooling end temperature for 30 to 300 seconds. Here, Ms and Bs can be calculated using the above-mentioned formulas (1) and (2). In this step, the second-annealed steel sheet is cooled to a temperature of (2×Ms+3×Bs) / 5-40°C to (2×Ms+3×Bs) / 5+20°C and held at this temperature range for 30 to 300 seconds to induce redistribution of C and Mn alloying elements and increase the phase stability of the retained austenite.

[0112] If the fourth cooling rate during the fourth cooling is less than 15°C / sec, polygonal ferrite or pearlite will be generated during cooling, resulting in poor tensile properties of the final steel. Therefore, the cooling rate is set to 15°C / sec or more, for example, in the range of 15 to 100°C / sec.

[0113] In steels having a composition that satisfies the range of the present invention, the nose temperature of the bainite transformation curve (referred to as the bainite nose temperature) is defined as (2×Ms+3×Bs) / 5, and the closer to the bainite nose temperature, the more accelerated the bainite transformation. Therefore, it is preferable to perform overaging in the range of (2×Ms+3×Bs) / 5-40°C to (2×Ms+3×Bs) / 5+20°C, which includes the bainite nose temperature. In other words, the overaging temperature range corresponds to the range of the end temperature of the fourth cooling.

[0114] If the fourth cooling end temperature is less than (2 × Ms + 3 × Bs) / 5 - 40°C, the bainite transformation during the overaging time will be insufficient, the stability of austenite will not be sufficiently ensured, and the material desired by the present invention will not be obtained. Furthermore, if the fourth cooling end temperature is less than Ms, martensitic transformation and tempering will occur, making it impossible to ensure sufficient elongation. Therefore, the fourth cooling end temperature or overaging temperature must be Ms or higher.

[0115] If the fourth cooling end temperature exceeds (2 × Ms + 3 × Bs) / 5 + 20°C, the bainite transformation is insufficient and the stability of austenite cannot be sufficiently ensured. The higher the fourth cooling end temperature, the more pearlite is generated during overaging, causing a decrease in strength and elongation.

[0116] During overaging, the temperature may be maintained isothermally, but is not necessarily limited to this, and may include a cooling interval during overaging. To prevent the formation of fresh martensite during overaging, the overaging temperature must be equal to or greater than Ms.

[0117] Overaging can be performed for 30 to 300 seconds. If the overaging time is less than 30 seconds, sufficient diffusion of C and Mn cannot be induced. If it exceeds 300 seconds, redistribution of C and Mn becomes excessive, and the sufficient strength that is the aim of the present invention cannot be obtained.

[0118] After the overaging step (S25) is completed, the steel sheet is cooled to room temperature. The cooling rate to room temperature is not particularly limited, but is preferably 10°C / second or more for productivity reasons.

[0119] The redistribution effect of C and Mn during overaging varies depending on the shape of the austenite, with the redistribution rate being faster in lath than in blocky. This is because the diffusion distance of C and Mn is shorter in lath, so diffusion occurs more easily within the same time period. As a result, the phase after final cooling changes depending on the shape of the austenite. Blocky austenite transforms into blocky martensite, which is unfavorable for maintaining elongation, while lath austenite remains as MA (a composite phase of martensite and austenite), and improved elongation can be expected during deformation due to the retained austenite.

[0120] Experimental Example

[0121] 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.

[0122] Regarding the analysis and measurements for this experimental example, the microstructure was analyzed using a scanning electron microscope (SEM), and XRD analysis was used to analyze the fraction of retained austenite and the carbon content in the retained austenite. Mechanical properties were evaluated by tensile testing using a Zwick / Roell Corp Z100 according to the KS No. 5 standard.

[0123] Steel having the composition (unit: wt %) shown in Table 1 below was prepared, and subjected to the prescribed hot rolling, cold rolling and heat treatment processes to prepare cold-rolled steel sheets according to the examples and comparative examples. The balance was iron (Fe).

[0124] [Table 1]

[0125] Table 2 shows the bainite start temperature (Bs), martensite start temperature (Ms), and bainite nose temperature for each steel type, which were derived through empirical formulas. Ms and Bs were calculated using the above-mentioned formulas (1) and (2). Bainite nose temperature was calculated using the formula (2 × Ms + 3 × Bs) / 5.

[0126] [Table 2]

[0127] Slabs of the above-mentioned steel types were reheated to 1200°C and held there for 3 hours, then hot rolled to a thickness of 2.4 mm at a finish rolling temperature of 950°C, and coiled at 550°C. They were then subjected to softening heat treatment at 600°C. For example, Figure 2A shows the microstructure of Steel Type A after hot rolling, and Figure 2B shows the microstructure of Steel Type A after softening heat treatment, both observed with a scanning microscope.

[0128] After the softening heat treatment, the steel sheets were pickled to remove surface scale, and then cold-rolled to produce cold-rolled steel sheets with a thickness of 1.2 mm. Subsequently, a first annealing heat treatment was performed under the conditions shown in Table 3.

[0129] [Table 3]

[0130] Table 4 shows the results of analyzing the microstructure of the cold-rolled steel sheet after the first annealing heat treatment. In Table 4, F means ferrite, and the low-temperature phase is a general term for both martensite and bainite. The values ​​listed in Table 4 indicate the area ratio (area %) of each phase.

[0131] [Table 4]

[0132] Referring to Table 4, after the first annealing heat treatment, Examples 1 to 4 and Comparative Examples 1 and 2 all exhibited a microstructure with an area fraction of 4% or less of ferrite and the remainder being low-temperature phase. This confirms that the steel sheets maintained in the austenite single-phase region in the first annealing heat treatment step transformed most of the austenite into low-temperature phase and a portion of it into ferrite after multi-stage cooling. Figure 2C shows the results of scanning electron microscopy observation of the microstructure of Example 3 after the first annealing heat treatment. Referring to Figure 2C, it can be confirmed that the phase structure consists of an area fraction of 95% or more of low-temperature phase (gray area) and less than 5% of ferrite (black area), satisfying the microstructure configuration requirements of the present invention.

[0133] In Comparative Example 3, the annealing temperature in the first annealing heat treatment step was 850°C, which was in the austenite-ferrite two-phase region below Ac3, and the primary cooling end temperature was 600°C, both of which are below the range of the present invention. Therefore, the ferrite content was 15% in area fraction, which was higher than in the Examples, the area fraction of the low temperature phase was 78%, and the pearlite fraction was 7%.

[0134] Fig. 2D shows the results of scanning electron microscopy of the microstructure after the first annealing heat treatment of Comparative Example 3. Referring to Fig. 2D, it can be seen that the finish temperature of the first cooling, which is the slow cooling step, is low in the first annealing heat treatment, so that a large amount of polygonal ferrite and pearlite is generated during the first cooling, and the microstructure requirements of the present invention are not met after the first annealing heat treatment of the present invention.

[0135] Comparative Example 4 was not subjected to the first annealing heat treatment, and therefore corresponds to a microstructure that was cold-rolled after softening heat treatment. As a result, in the case of Comparative Example 4, ferrite was the main structure with an area ratio of 93%, and some spheroidized cementite was mixed in.

[0136] In the case of Comparative Example 5, in the first annealing heat treatment step, the primary cooling step of slowly cooling from the annealing temperature was omitted, and the secondary cooling step of immediately rapidly cooling to room temperature was performed, resulting in a fine structure in which the low-temperature phase accounted for 99% of the area and 1% was ferrite.

[0137] After the first annealing heat treatment was completed, the second annealing heat treatment was carried out under the conditions shown in Table 5.

[0138] [Table 5]

[0139] Referring to Table 5, Examples 1 to 4 satisfy the conditions for the first annealing heat treatment and the second annealing heat treatment proposed in the present invention, and all of the target values ​​for mechanical properties were achieved, as shown in Table 7 below.

[0140] In Comparative Example 1, the fourth cooling end temperature in the second annealing heat treatment step exceeded the fourth cooling end temperature range of 396°C to 456°C in the case of Steel Type A according to the present invention.

[0141] In Comparative Example 2, the overaging time performed after the fourth cooling finish temperature was reached in the second annealing heat treatment step was 15 seconds, which was shorter than the overaging time range of 30 to 300 seconds of the present invention.

[0142] In Comparative Example 4, the first annealing heat treatment was not performed, and only the second annealing heat treatment was performed, which means that the annealing heat treatment was performed only once as in the conventional case.

[0143] In Comparative Example 5, the annealing temperature of the second annealing heat treatment was 880°C, which is Ac3 or higher and exceeds the range of (Ac1+Ac3) / 2 to Ac3 of the present invention. In addition, the tertiary cooling end temperature during the second annealing heat treatment was 650°C, which is lower than the tertiary cooling end temperature range of 700°C or higher of the present invention.

[0144] Table 6 shows the results of analyzing the microstructure of the cold-rolled steel sheet after the second annealing heat treatment. In Table 6, Lath α represents lath ferrite, Lath MA represents lath MA (a composite phase of martensite and austenite), PF represents polygonal ferrite, Blocky MA represents blocky MA, RA represents retained austenite, α' represents martensite (including both fresh martensite and tempered martensite), and the other structures represent pearlite or austenite decomposition structures. Among the values ​​listed in Table 6, those related to phase fractions refer to the area fraction (area %) of each phase. Furthermore, the carbon concentration is the concentration of carbon contained in the retained austenite in weight percent.

[0145] [Table 6]

[0146] Table 7 shows the physical and mechanical properties of the cold-rolled steel sheets finally produced after the first and second annealing heat treatments, such as tensile strength (TS), elongation (EL), and the product of tensile strength and elongation (TS × EL).

[0147] [Table 7]

[0148] Referring to Table 6, Examples 1 to 4 satisfy the microstructural composition of the present invention, including, in area percentages (area %), 40 to 60% lath ferrite, 20 to 30% lath MA, 1 to 15% blocky MA, 15 to 25% retained austenite, 5% or less polygonal ferrite, 5% or less martensite (including one or more of fresh martensite and tempered martensite), and 5% or less other structures (including one or more of pearlite and austenite decomposition structures). Furthermore, referring to Table 7, it can be confirmed that Examples 1 to 4 satisfy the target values ​​of the present invention, namely, a tensile strength of 980 MPa or more, an elongation of 25% or more, and a TS×EL of 30,000 MPa or more. Specifically, Examples 1 and 2 exhibit a tensile strength of 980 MPa or more and an elongation of 35%, while Examples 3 and 4 exhibit tensile properties of 1200 MPa or more and an elongation of 25% or more.

[0149] That is, by realizing an optimal microstructure through a manufacturing process including first and second annealing heat treatments within the composition range according to the technical concept of the present invention, it is possible to manufacture an ultra-high strength cold rolled steel sheet having excellent tensile strength and formability.

[0150] Fig. 3A shows the microstructure of Example 1 after the second annealing heat treatment, observed with a scanning electron microscope, and Fig. 5 shows an enlarged view of the microstructure. Referring to Fig. 3A and Fig. 5, the main structure is a mixture of lath ferrite and lath MA (Lath F+MA in Fig. 5), with some polygonal ferrite, blocky MA, and retained austenite also observed.

[0151] In the mixed structure of lath ferrite and lath MA, the lath ferrite (black regions) is formed between the lath MA (gray regions). The lath ferrite and lath MA have long axes extending in almost the same or similar directions, and therefore have a lamellar structure in which the lath ferrite and lath MA are alternately arranged in a direction perpendicular to the long axes.

[0152] The redistribution of C and Mn during the overaging step occurs more easily in the lath structure than in the block structure. This is because the diffusion distance of C and Mn is shorter in the lath structure than in the block structure. Figure 7 shows the results of FE-EPMA analysis of the C content of each phase shown in Figure 5. Figure 7A shows the lath structure and its surrounding matrix, and Figure 7B shows the microstructure and C distribution of the block structure and its surrounding matrix. Comparing the C content distribution of the lath structure and the block structure with the surrounding matrix, it was confirmed that greater C enrichment occurs in the lath structure.

[0153] Figure 3B shows the results of scanning electron microscopy of the microstructure of Example 3 after the second annealing. Referring to Figure 3B, the microstructure is predominantly lath-shaped, similar to that of Example 1, with some polygonal ferrite and retained austenite. Compared to Examples 1 and 2, Example 3 showed a tendency for increased tensile strength due to a decrease in lath-shaped ferrite and an increase in blocky MA in the microstructure.

[0154] Referring to Table 7, the comparative examples were unable to achieve the mechanical properties targeted by the present invention.

[0155] In Comparative Example 1, the fourth cooling end temperature in the second annealing heat treatment step was higher than the range of the present invention, and the austenite stability was not sufficiently ensured in the overaging step after the fourth cooling end, so the elongation of 25% or more and TS×EL of 30,000 MPa% or more, which are the targets of the present invention, were not achieved. Figure 4A shows the results of observation of the microstructure of Comparative Example 1 using a scanning electron microscope after the second annealing heat treatment.

[0156] 4A and Tables 6 and 7, in Comparative Example 1, the overaging temperature (i.e., the fourth cooling end temperature) was high and the bainite transformation was insufficient, resulting in a ferrite fraction of 36.7%, which is below the range of the present invention, and a blocky MA fraction of 16.8%, which exceeds the range of the present invention. Compared with Examples 3 and 4, the lath ferrite was insufficient and the blocky MA fraction was excessive, resulting in high strength and low elongation.

[0157] In the case of Comparative Example 2, the overaging time in the second annealing heat treatment step was 15 seconds, which was shorter than the range of the present invention, and therefore the stability of austenite was not sufficiently ensured. As a result, the strength was higher than that of Examples 1 and 2, but the elongation was lower.

[0158] In Comparative Example 3, although the second annealing step was performed within the appropriate range of the present invention, the first annealing step was performed outside the appropriate annealing temperature range, and as a result, the tensile strength and elongation of the final cold-rolled steel sheet were both lower than the target values ​​of the present invention.

[0159] FIG. 4B shows the microstructure of Comparative Example 3 after the second annealing using a scanning electron microscope. Referring to FIG. 4B, it can be seen that the polygonal ferrite formed in the first annealing still remains after the second annealing. Furthermore, the pearlite formed in the first annealing transforms into blocky austenite during the second annealing, resulting in low austenite phase stability. Therefore, in the third cooling step, which is a slow cooling section, the austenite transforms into pearlite or decomposes into cementite. When the third cooling end temperature is low, the transformation into polygonal ferrite and pearlite and the decomposition of austenite prevent the formation of a lath structure and a sufficient amount of retained austenite, resulting in lower tensile strength and elongation than the target values ​​intended by the present invention.

[0160] In Comparative Example 4, the first annealing heat treatment was not performed, and therefore a single annealing heat treatment was performed as in the conventional case, and the tensile strength and elongation showed values ​​lower than the target values ​​that the present invention aims to achieve.

[0161] Figure 4C shows the results of scanning electron microscopy of the microstructure of Comparative Example 4 after the second annealing. Referring to Figure 4C, a large amount of spheroidized cementite is present in the microstructure. This is because the spheroidized cementite present in the microstructure after the softening heat treatment did not redissolve and remained after the annealing heat treatment. Figure 2B shows the microstructure of a hot-rolled steel sheet of Steel Type A after softening heat treatment, revealing the presence of a large amount of spheroidized cementite. As a result, the tensile strength and elongation of Comparative Example 4 were both lower than the target values ​​sought by the present invention, compared to Examples 3 and 4.

[0162] In Comparative Example 5, the second annealing temperature during the second annealing heat treatment was Ac3 or higher, which is outside the range of the present invention. When the second annealing temperature is Ac3 or higher, massive austenite develops during annealing, and polygonal ferrite is partially formed during cooling, with the remaining austenite transforming into massive martensite. This makes it impossible to obtain a lath-shaped microstructure, thereby negating the benefit of performing the two annealing heat treatments. Figure 4D shows the results of scanning electron microscopy observation of the microstructure of Comparative Example 5 after the second annealing heat treatment. Referring to Figure 4D, martensite and polygonal ferrite were the main components of the microstructure, and the specimen exhibited high tensile strength, but the elongation was lower than the target value sought by the present invention.

[0163] As explained above, it has been found that the mechanical properties of the present invention, which are balanced between tensile strength and elongation, can be obtained by ensuring a large amount of low-temperature phase structure in the first annealing heat treatment and by appropriately ensuring lath ferrite, MA, blocky MA, and retained austenite in the second annealing heat treatment. Furthermore, it has been found that if the first and second annealing heat treatments proposed in the present invention are not performed, it is difficult to redissolve the spheroidized cementite present in the initial structure, making it difficult to achieve the tensile properties desired in the present invention.

[0164] 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): 0.28% to 0.4%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.0% to 3.0%, aluminum (Al): 0.01% to 0.3%, niobium (Nb): 0.01% to 0.05%, phosphorus (P): more than 0% and 0.02% or less, sulfur (S): more than 0% and 0.003% or less, boron (B): 0.001% to 0.005%, and the balance being iron (Fe) and other unavoidable impurities, The ultra-high strength cold-rolled steel sheet has a microstructure containing, by area ratio, 40 to 60% lath ferrite, 20 to 30% lath MA, 15% or less (more than 0) block MA, and 15 to 25% retained austenite.

2. 2. The ultra-high strength cold rolled steel sheet according to claim 1, further comprising, by weight%, one or more of more than 0% and not more than 2.0% chromium (Cr) and more than 0% and not more than 0.2% molybdenum (Mo), the sum of chromium (Cr) and molybdenum (Mo) being not more than 3.0%.

3. Further comprising one or more of: more than 0% and not more than 5% polygonal ferrite; more than 0% and not more than 5% martensite; and more than 0% and not more than 5% other structures; 2. The ultrahigh strength cold rolled steel sheet according to claim 1, wherein the martensite includes at least one of fresh martensite and tempered martensite, and the other structure includes at least one of pearlite and austenite decomposed structure.

4. The lath-shaped ferrite and the lath-shaped MA have an aspect ratio, which is the ratio of the major axis length divided by the minor axis length (major axis length / minor axis length), of 3 or more. Ultra-high strength cold-rolled steel sheet according to claim 1.

5. 5. The ultra-high strength cold rolled steel sheet according to claim 4, wherein the lath-shaped ferrite and the lath-shaped MA have a lamellar morphology formed to be alternately arranged in a direction perpendicular to a major axis direction of either the lath-shaped ferrite or the lath-shaped MA.

6. 2. The ultrahigh strength cold rolled steel sheet according to claim 1, wherein the tensile strength (TS) is in the range of 980 MPa or more and 1300 MPa or less, the elongation (El) is in the range of 25% or more and 45% or less, and the product of the tensile strength (TS) and the elongation (EL) (TS × EL) is in the range of 30,000 or more and 45,000 MPa% or less.

7. hot-rolling a steel material containing, by weight, 0.28% to 0.4% carbon (C), 1.0% to 2.0% silicon (Si), 1.0% to 3.0% manganese (Mn), 0.01% to 0.3% aluminum (Al), 0.01% to 0.05% niobium (Nb), more than 0% to 0.02% phosphorus (P), more than 0% to 0.003% sulfur (S), 0.001% to 0.005% boron (B), and the balance being iron (Fe) and other inevitable impurities to produce a hot-rolled steel sheet; cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; A step of subjecting the cold-rolled steel sheet to a first annealing heat treatment; A step of subjecting the cold-rolled steel sheet that has been subjected to the first annealing heat treatment to a second annealing heat treatment; and overaging the cold-rolled steel sheet that has been subjected to the second annealing heat treatment, The first annealing heat treatment step includes a step of performing a first annealing treatment at a first annealing temperature of Ac3 or higher, and then cooling to a temperature of Ms or lower, the second annealing heat treatment step includes a step of performing second annealing treatment at a second annealing temperature of (Ac1+Ac3) / 2 or more and less than Ac3, and then cooling to a temperature in the range of (2×Ms+3×Bs) / 5−40°C to (2×Ms+3×Bs) / 5+20°C; The method for producing an ultra-high strength cold-rolled steel sheet, wherein the overaging step is carried out at a temperature in the range of (2×Ms+3×Bs) / 5−40°C to (2×Ms+3×Bs) / 5+20°C.

8. 8. The method for producing an ultrahigh strength cold rolled steel sheet according to claim 7, further comprising one or more of more than 0% and not more than 2.0% chromium (Cr) and more than 0% and not more than 0.2% molybdenum (Mo), wherein the sum of chromium (Cr) and molybdenum (Mo) is 3.0% or less.

9. The step of cooling to a temperature equal to or lower than Ms after the first annealing treatment includes: a step of primarily cooling the cold-rolled steel sheet that has been subjected to the first annealing treatment to a primary cooling end temperature in the range of 700°C to 800°C at a first cooling rate; 8. The method for producing an ultrahigh strength cold rolled steel sheet according to claim 7, further comprising: after the primary cooling, performing secondary cooling at a second cooling rate faster than the first cooling rate to a secondary cooling end temperature that is a temperature not higher than Ms.

10. The method for producing an ultrahigh strength cold rolled steel sheet according to claim 9, wherein the first cooling rate is in the range of 5°C / sec to 20°C / sec.

11. The method for producing an ultra-high strength cold rolled steel sheet according to claim 9, wherein the second cooling rate is in the range of 15°C / sec to 300°C / sec.

12. The cold-rolled steel sheet having undergone the first annealing heat treatment has a microstructure in which, in terms of area ratio, it is ferrite of less than 5% and the remainder is a low-temperature phase, The method for producing an ultra-high strength cold rolled steel sheet according to claim 9, wherein the low temperature phase includes at least one of martensite and bainite.

13. The step of cooling after the second annealing treatment includes: Tertiary cooling the cold-rolled steel sheet that has been subjected to the second annealing treatment at a third cooling rate to a third cooling end temperature in the range of 700°C to 800°C; and after the tertiary cooling, fourth cooling is performed at a fourth cooling rate faster than the third cooling rate to a fourth cooling end temperature in the range of (2×Ms+3×Bs) / 5−40°C to (2×Ms+3×Bs) / 5+20°C.

14. The method for producing an ultrahigh strength cold rolled steel sheet according to claim 13, wherein the third cooling rate is in the range of 5°C / sec to 10°C / sec.

15. The method for producing an ultrahigh strength cold rolled steel sheet according to claim 13, wherein the fourth cooling rate has a range of 15°C / sec to 100°C / sec.

16. The method for producing an ultrahigh strength cold rolled steel sheet according to claim 7, wherein the overaging step is carried out in a range of 30 to 300 seconds.

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