Cold-rolled steel sheet and method for producing the same

Abstract

The present invention provides carbon (C): 0.15% to 0.20% by weight, silicon (Si): 1.0% to 2.0% by weight, manganese (Mn): 1.5% to 3.0% by weight, phosphorus (P): more than 0% to 0.02% by weight, sulfur (S): more than 0% to 0.003% by weight, aluminum (Al): 0.01% to 0.3% by weight, nitrogen (N): more than 0% to 0.01% by weight, titanium (Ti): 48 / 14·[N]% to 0.1% by weight (where [N] is the weight percentage value of nitrogen), the remainder being iron (Fe) and other unavoidable impurities. The present invention provides a cold-rolled steel sheet comprising a pure steel, a final microstructure consisting of ferrite, acicular retained austenite, a martensite / austenite composite structure and blocky martensite, an area fraction of the ferrite being 30% to 60%, an area fraction of the acicular retained austenite being 5% to 12%, an area fraction of the martensite / austenite composite structure being 25% to 50%, an area fraction of the blocky martensite being 5% to 12%, and a carbon enrichment amount in the retained austenite being 1.1% by weight or more.

Description

[Technical field]

[0001] The present invention relates to a cold-rolled steel sheet and a manufacturing method thereof, and more particularly to a cold-rolled ultra-high strength, low carbon steel sheet having excellent formability and a manufacturing method thereof. [Background technology]

[0002] Ultra-high strength steels for automotive steel sheets have been developed to meet two requirements: vehicle weight reduction in response to environmental regulatory issues and stronger collision stability standards due to strengthened safety regulations. However, there is a trade-off between strength and elongation, and the problem of reduced formability due to increased strength has emerged, so much research has been conducted to ensure the formability of high strength steels.

[0003] TRIP-aided steel, which uses the TRIPS (Transformation Induced Plasticity) phenomenon in which retained austenite in the microstructure transforms into martensite during deformation, has been developed as a third generation steel sheet that can ensure both high strength and high elongation. Since the physical properties of such TRIP-aided steel are determined by the phase stability and fraction of the retained austenite that causes the TRIP phenomenon, it is important in manufacturing the steel to ensure stable retained austenite in the microstructure.

[0004] Related prior art includes Korean Patent Application No. 2018-0033119. Summary of the Invention [Problem to be solved by the invention]

[0005] The technical problem to be solved by the present invention is to provide a cold-rolled ultra-high strength, low carbon steel sheet having excellent formability and a manufacturing method thereof. [Means for solving the problem]

[0006] In order to solve the above problems, a cold rolled steel sheet according to an embodiment of the present invention has carbon (C): 0.15% by weight to 0.20% by weight, silicon (Si): 1.0% by weight to 2.0% by weight, manganese (Mn): 1.5% by weight to 3.0% by weight, phosphorus (P): more than 0% by weight to 0.02% by weight or less, sulfur (S): more than 0% by weight to 0.003% by weight or less, aluminum (Al): 0.01% by weight to 0.3% by weight, nitrogen (N): more than 0% by weight to 0.01% by weight or less, titanium (Ti): 48 / 14·[N]% by weight to 0.1% by weight (where [N] is the weight percentage value of nitrogen). ), the remainder being iron (Fe) and other unavoidable impurities, and the final microstructure is composed of ferrite, acicular retained austenite, a martensite / austenite composite structure and massive martensite, the area fraction of the ferrite is 30% to 60%, the area fraction of the acicular retained austenite is 5% to 12%, the area fraction of the martensite / austenite composite structure is 25% to 50%, the area fraction of the massive martensite is 5% to 12%, and the enrichment of carbon in the retained austenite is 1.1% by weight or more.

[0007] In the cold-rolled steel sheet, the ferrite may be composed of polygonal ferrite and acicular ferrite, and an area fraction of the acicular ferrite in the ferrite may be 40% or more.

[0008] The cold-rolled steel sheet may have a tensile strength (TS) of 980 MPa to 1180 MPa and an elongation (El) of 23% to 25%.

[0009] A method for producing a cold rolled steel sheet according to an embodiment of the present invention for solving the above problems includes the steps of: (a) reheating a steel material containing 0.15 to 0.20 weight% of carbon (C), 1.0 to 2.0 weight% of silicon (Si), 1.5 to 3.0 weight% of manganese (Mn), more than 0 weight% and not more than 0.02 weight% of phosphorus (P), more than 0 weight% and not more than 0.003 weight% of sulfur (S), 0.01 to 0.3 weight% of aluminum (Al), more than 0 weight% and not more than 0.01 weight% of nitrogen (N), 48 / 14·[N] weight% to 0.1 weight% of titanium (Ti) (where [N] is the weight% value of nitrogen), the remainder being iron (Fe) and other unavoidable impurities; and (b) reheating the steel material containing 0.15 to 0.20 weight% of carbon (C), 1.0 to 2.0 weight% of silicon (Si), 1.5 to 3.0 weight% of manganese (Mn), more than 0 weight% and not more than 0.02 weight% of phosphorus (P), more than 0 weight% and not more than 0.003 weight% of sulfur (S), more than 0 weight% and not more than 0.003 weight% of aluminum (Al), more than 0 weight% and not more than 0.01 weight% of nitrogen (N), and 48 / 14·[N] weight% to 0.1 weight% of titanium (Ti) (where [N] is the weight% value of nitrogen), more than 0 weight% and not more than 0.01 weight% of titanium (Ti), and the remaining iron (Fe) and other unavoidable impurities. (c) hot-rolling the heated steel; (d) a first annealing heat treatment step including a step of maintaining the cold-rolled steel at a first annealing temperature of (Ac1+30°C) to (Ac3-30°C) and then cooling to a cooling end temperature of 340°C or less; and (e) a second annealing heat treatment step including a step of maintaining the steel at a second annealing temperature of Ac1 to (Ac3-30°C) and then cooling to a cooling end temperature of martensite transformation start temperature (Ms) to (bainite transformation start temperature (Bs)-15°C) and then overaging, wherein the second annealing temperature is lower than the first annealing temperature.

[0010] In the method for producing the cold-rolled steel sheet, the step (a) includes a step of reheating the steel material at 1180°C to 1300°C, the step (b) includes a step of hot rolling under conditions of a finish rolling temperature of 850°C to 950°C and a coiling temperature of 450°C to 650°C, and the step (c) includes a step of cold rolling at a reduction ratio of 40% to 70%.

[0011] In the method for producing the cold-rolled steel sheet, the step (d) may include a step of maintaining the cold-rolled steel at the first annealing temperature for 30 to 120 seconds, and then cooling the cold-rolled steel at a cooling rate of 15°C / s or more to a cooling end temperature of 340°C or less.

[0012] In the method for producing the cold rolled steel sheet, after performing step (d), the area fraction of ferrite in the microstructure of the steel may be 30% to 50%.

[0013] In the method for manufacturing the cold-rolled steel sheet, the step (e) may include a process of maintaining the steel at the second annealing temperature for 30 to 120 seconds, cooling the steel at a cooling rate of 15°C / s or more to a cooling end temperature of a martensite transformation start temperature (Ms) or more (a bainite transformation start temperature (Bs) - 15°C) or less, and then overaging the steel for 30 to 300 seconds.

[0014] In the method for manufacturing the cold rolled steel sheet, after performing step (e), the microstructure of the steel material may be composed of ferrite, acicular retained austenite, a martensite / austenite composite structure, and blocky martensite, and the area fraction of the ferrite may be 30% to 60%, the area fraction of the acicular retained austenite may be 5% to 12%, the area fraction of the martensite / austenite composite structure may be 25% to 50%, and the area fraction of the blocky martensite may be 5% to 12%. Effect of the Invention

[0015] According to an embodiment of the present invention, a cold-rolled ultra-high strength, low carbon steel sheet having excellent formability and a manufacturing method thereof can be realized. Specifically, a low carbon steel containing 0.2 wt% or less of carbon is used to ensure excellent weldability, and a sufficient amount of carbon and manganese is concentrated in austenite through multiple alloy element redistribution steps during the heat treatment process to realize a balance between excellent strength and elongation, thereby realizing a cold-rolled ultra-high strength steel having excellent formability and a tensile strength of 980 MPa or more and an elongation of 23% or more.

[0016] Of course, such effects do not limit the scope of the present invention. [Brief description of the drawings]

[0017] FIG. 1 is a flow chart that outlines a method for producing a cold-rolled steel sheet according to one embodiment of the present invention.

[0018] FIG. 2 is a diagram illustrating an overview of (a) the first annealing heat treatment step and (b) the second annealing heat treatment step in the method for producing a cold-rolled steel sheet according to one embodiment of the present invention.

[0019] FIG. 3 is a photograph of the microstructure after the first annealing heat treatment in Example 1 of the Experimental Examples.

[0020] FIG. 4 is a photograph of the microstructure after the second annealing heat treatment in Example 1 of the Experimental Examples.

[0021] FIG. 5 is a photograph of the microstructure after the first annealing heat treatment in Comparative Example 6 of the experimental examples, FIG. 6 is a photograph of the final microstructure in Comparative Example 7 of the experimental examples, FIG. 7 is a photograph of the final microstructure in Comparative Example 8 of the experimental examples, FIG. 8 is a photograph of the final microstructure in Comparative Example 9 of the experimental examples, and FIG. 9 is a photograph of (a) an acicular type structure and (b) a blocky type structure after the second annealing heat treatment and overaging. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] A cold-rolled steel sheet and a manufacturing method thereof according to an embodiment of the present invention will be described in detail. The terms used below are appropriately selected in consideration of the functions in the present invention, and such terms should be defined based on the entire contents of this specification. The following provides specific details of a cold-rolled ultra-high strength, low carbon steel sheet having excellent formability and a manufacturing method thereof.

[0023] Korean Patent Application No. 2018-0033119 presents a method for manufacturing Quenching and Partitioning (Q&P) steel containing tempered martensite and retained austenite through annealing the steel, followed by rapid cooling and partitioning heat treatment. Q&P steel has the advantage that it can achieve physical properties of tensile strength of 980 MPa or more and elongation of 21% or more even with 0.2 wt% carbon steel, but the window for processing temperature is narrow and the deviation in ductility is large, making it difficult to stably secure the high elongation targeted in the present invention.

[0024] Korean Patent Publication No. 2017-0113858 proposes a two-step annealing heat treatment process as a method for securing lath-shaped ferrite and retained austenite and for securing a fine structure (pre-structure) before final annealing to increase the ductility of steel. However, after the first annealing, single-phase annealing is performed to secure a low-temperature structure with a volume fraction of 90% or more, and in steel with a low carbon content, a tensile strength of 980 MPa or more cannot be stably secured, and high-temperature annealing is also required, which may shorten the life of the furnace.

[0025] The present invention discloses a cold-rolled ultra-high strength steel sheet having an excellent elongation ratio, which has a tensile strength of 980 MPa or more and an elongation ratio of 23% or more and is applicable to automobile parts, and a manufacturing method thereof. The microstructure of the cold-rolled steel sheet is composed of polygonal ferrite with an area fraction of 20% to 50%, acicular ferrite with an area fraction of 40% or more, acicular retained austenite with an area fraction of 5% or more and 12% or less, a composite structure of martensite / austenite with an area fraction of 5% or more, and the balance being bainite. The present invention discloses the alloy amounts and heat treatment conditions suitable for securing the targeted yield strength, tensile strength, and elongation ratio.

[0026] steel plate

[0027] The cold rolled steel sheet according to one embodiment of the present invention comprises carbon (C): 0.15% to 0.20% by weight, silicon (Si): 1.0% to 2.0% by weight, manganese (Mn): 1.5% to 3.0% by weight, phosphorus (P): more than 0% to 0.02% by weight, sulfur (S): more than 0% to 0.003% by weight, aluminum (Al): 0.01% to 0.3% by weight, nitrogen (N): more than 0% to 0.01% by weight, titanium (Ti): 48 / 14·[N]% to 0.1% by weight (where [N] is the weight percentage value of nitrogen), with the remainder being iron (Fe) and other unavoidable impurities.

[0028] Hereinafter, the role and content of each component contained in the cold rolled steel sheet will be described.

[0029] Carbon (C)

[0030] Carbon (C) is added to ensure the strength of steel, and the strength increases as the carbon content in the martensite structure increases. Furthermore, carbon combines with elements such as iron to form carbides, improving strength and hardness. Carbon (C) may be added in a content ratio of 0.15 to 0.20 wt% of the total weight in the cold rolled steel sheet according to an embodiment of the present invention. If the carbon content is less than 0.15 wt% of the total weight, there may be a problem that the above-mentioned effects cannot be realized and sufficient strength cannot be secured, while if the carbon content exceeds 0.20 wt% of the total weight, there may be a problem that weldability and workability are reduced.

[0031] Silicon (Si)

[0032] Silicon (Si) is an element added to increase strength and inhibit carbide formation by the effect of ferrite solid solution strengthening. Silicon is also well known as a ferrite stabilizing element, and can increase the fraction of ferrite during cooling to increase ductility, and is also known as an element that can ensure strength by promoting martensite formation through the concentration of austenite carbon. Meanwhile, silicon is added as a deoxidizer to remove oxygen in steel during the steelmaking process together with aluminum, and can also have a solid solution strengthening effect. The silicon may be added in a content ratio of 1.0 to 2.0 wt % of the total weight in the cold rolled steel sheet according to an embodiment of the present invention. If the silicon content is less than 1.0 wt % of the total weight, ductility cannot be ensured, and the above-mentioned silicon addition effect cannot be reliably exhibited. On the other hand, if a large amount of silicon is added in an amount exceeding 2.0 wt % of the total weight, Mn may be added during the manufacturing process. 2 SiO 4 There are problems in that the formation of oxides such as these impairs plating properties, increases the carbon equivalent and may reduce weldability, generates red scale during reheating and hot rolling, which may cause problems in surface quality, and reduces toughness and plastic workability.

[0033] Manganese (Mn)

[0034] Manganese (Mn) is an element that contributes to improving strength by increasing hardenability, facilitates the formation of a low-temperature transformation phase, and provides the effect of increasing strength through solid solution strengthening. Manganese may be added to the cold-rolled steel sheet according to an embodiment of the present invention at a content ratio of 1.5 to 3.0 wt% of the total weight. If the manganese content is less than 1.5 wt%, the above-mentioned effect of ensuring strength cannot be fully exhibited. Also, if the manganese content exceeds 3.0 wt%, inclusions such as MnS are formed or segregated, resulting in a decrease in workability and a decrease in delayed fracture resistance, and the increase in carbon equivalent may result in a decrease in weldability.

[0035] Rin(P)

[0036] Phosphorus (P) can increase strength through solid solution strengthening and suppress the formation of carbides. The phosphorus may be added to the cold rolled steel sheet according to an embodiment of the present invention in an amount of more than 0% and not more than 0.02% by weight based on the total weight. If the phosphorus content exceeds 0.02% by weight, problems may occur in that the weld is brominated, low-temperature brittleness is induced, press formability is deteriorated, and impact resistance is deteriorated.

[0037] Sulfur (S)

[0038] Sulfur (S) improves the machinability of steel by combining with manganese, titanium, etc., and forms fine MnS precipitates to improve workability, but is generally an element that impairs ductility and weldability. The sulfur may be added to the cold rolled steel sheet according to an embodiment of the present invention at a content ratio of more than 0% and not more than 0.003% by weight of the total weight. If the sulfur content exceeds 0.003% by weight, the number of FeS inclusions or MnS inclusions increases, toughness and weldability decrease, workability becomes poor, and high-temperature cracks occur due to segregation during continuous casting solidification.

[0039] Aluminum (Al)

[0040] Aluminum (Al) is an element mainly used as a deoxidizer, and promotes ferrite formation, improves elongation, inhibits carbide formation, and stabilizes austenite by increasing the concentration of carbon in austenite. The aluminum (Al) is preferably added in a content ratio of 0.01 to 0.3% by weight of the total weight in the cold rolled steel sheet according to an embodiment of the present invention. When the content of aluminum (Al) is less than 0.01% by weight, the above-mentioned effect of adding aluminum can be reliably exhibited. On the other hand, when the content of aluminum (Al) is excessively added in an amount exceeding 0.3% by weight, there are problems in that the continuous castability is reduced due to an increase in aluminum inclusions, the galvanization is reduced due to concentration on the surface of the steel sheet, and hot rolling cracks are induced by forming AlN in the slab.

[0041] Nitrogen (N)

[0042] Nitrogen (N) is a solution strengthening element that can increase the strength of steel plate and is generally mixed in from the atmosphere. Its content must be controlled in the degassing process of the steelmaking process. If the nitrogen content exceeds 0.01% by weight, problems such as bromide in the welded joint, induction of low-temperature brittleness, deterioration of press formability, and deterioration of impact resistance may occur.

[0043] Titanium (Ti)

[0044] Titanium (Ti) is a precipitate-forming element that has the effect of precipitating TiN and refining crystal grains. In particular, it can reduce the nitrogen content inside the steel through the precipitation of TiN. Titanium is preferably added at 48 / 14·[N]wt% to 0.1wt%, and if it is less than 48 / 14·[N]wt%, the amount of TiC precipitated is small and the effect of adding Ti is insufficient, and if it is added in excess of 0.1wt%, it reduces the carbon solid solubility in the base material and makes it difficult to ensure strength.

[0045] The cold-rolled steel sheet according to an embodiment of the present invention having the above-mentioned alloy element composition may be a cold-rolled ultra-high strength steel sheet having a tensile strength of 980 MPa or more and an elongation of 23% or more. For example, the cold-rolled steel sheet may have a tensile strength (TS): 980 MPa to 1180 MPa and an elongation (El): 23% to 25%.

[0046] The final microstructure of the cold rolled steel sheet is composed of ferrite, acicular retained austenite, a martensite / austenite composite structure, and blocky martensite, the area fraction of the ferrite is 30% to 60%, the area fraction of the acicular retained austenite is 5% to 12%, the area fraction of the martensite / austenite composite structure is 25% to 50%, the area fraction of the blocky martensite is 5% to 12%, and the amount of carbon enrichment in the retained austenite is 1.1 wt % or more. The ferrite may be composed of polygonal ferrite and acicular ferrite, and the area fraction of the acicular ferrite in the ferrite may be 40% or more.

[0047] A method for manufacturing a cold rolled steel sheet having the above-mentioned composition and microstructure according to an embodiment of the present invention will be described below.

[0048] Steel sheet manufacturing method

[0049] FIG. 1 is a flow chart that outlines a method for producing a cold-rolled steel sheet according to one embodiment of the present invention.

[0050] Referring to FIG. 1, a method for manufacturing a steel sheet according to an embodiment of the present invention includes the steps of: (a) reheating a steel material containing 0.15 to 0.20 weight percent carbon (C), 1.0 to 2.0 weight percent silicon (Si), 1.5 to 3.0 weight percent manganese (Mn), 0 to 0.02 weight percent phosphorus (P), 0 to 0.003 weight percent sulfur (S), 0.01 to 0.3 weight percent aluminum (Al), 0 to 0.01 weight percent nitrogen (N), 48 / 14·[N] weight percent to 0.1 weight percent titanium (Ti) (where [N] is the weight percent value of nitrogen), the remainder being iron (Fe) and other unavoidable impurities (S100); (c) hot rolling the reheated steel (S200); (d) cold rolling the hot rolled steel (S300); (d) a first annealing heat treatment step (S400) including a step of maintaining the cold rolled steel at a first annealing temperature of (Ac1+30°C) to (Ac3-30°C) and then cooling to a cooling end temperature of 340°C or less; and (e) a second annealing heat treatment step (S500) including a step of maintaining the steel at a second annealing temperature of Ac1 to (Ac3-30°C) and then cooling to a cooling end temperature of martensite transformation start temperature (Ms) to (bainite transformation start temperature (Bs)-15°C) and then overaging.

[0051] The step (a) (S100) may include a step of reheating the slab steel material having the above composition at 1180°C to 1300°C. The slab is manufactured in a semi-finished form by continuously casting molten steel obtained through a steelmaking process, and the segregation of components generated during the casting process is homogenized through the reheating process to prepare a state for hot rolling. If the slab reheating temperature (SRT) is 1180°C or less, there is a problem that the segregation of the slab is not sufficiently re-dissolved, while if it exceeds 1300°C, the size of the austenite grains may increase, and the process cost may increase. The slab may be reheated for 1 to 4 hours. If the reheating time is less than 1 hour, the reduction of the segregation zone is insufficient, while if it exceeds 4 hours, the size of the grains may increase, and the process cost may increase.

[0052] The step (b) (S200) is a step of hot rolling the reheated slab. The hot rolling is performed at a finish delivery temperature (FDT) of 850°C to 950°C. If the finish delivery temperature is lower than 850°C, the rolling load increases rapidly and the productivity decreases, while if it exceeds 950°C, the grain size increases and the strength may decrease. After hot rolling, the steel sheet is cooled to a temperature of 450°C to 650°C and then coiled. If the coiling temperature is lower than 450°C, the shape of the hot rolled coil becomes non-uniform and the strength increases, and the rolling load during cold rolling increases, while if it exceeds 650°C, defects may occur in the subsequent process due to surface oxidation, etc., and problems may occur such as oxidation of the inside of the grain boundary due to the difference in cooling speed between the center and edge parts of the steel sheet.

[0053] The step (c) is a step of removing the surface scale layer by pickling the hot rolled coil and performing cold rolling. During cold rolling, the thickness reduction is about 40% to 70%. The higher the reduction, the more effective it is in improving formability due to the effect of refining the structure. When the reduction is less than 40% in cold rolling, it is difficult to obtain a uniform fine structure, while when it is designed to exceed 70%, the roll force becomes high and the process load becomes high.

[0054] After cold rolling, the first annealing heat treatment process and the second annealing heat treatment process are sequentially performed. That is, the cold-rolled steel sheet is annealed twice in total by the first annealing and the second annealing. The heating rate from room temperature to the first or second annealing temperature range is not limited, and may follow the heating rate of a typical heating furnace.

[0055] The step (d) is a first annealing heat treatment step, which includes a step of maintaining the cold-rolled steel at a first annealing temperature of (Ac1+30°C) or more and (Ac3-30°C) or less, and then cooling the cold-rolled steel to a cooling end temperature of 340°C or less.

[0056] The step (d) is a step of performing a dual-phase annealing heat treatment in a first annealing temperature range of (Ac1+30°C) to (Ac3-30°C) for 30 to 120 seconds to secure a dual-phase structure of ferrite and low-temperature phase. The first annealing heat treatment process is a process of forming a preferable prior structure for securing a lath-shaped acicular ferrite and austenite structure in the second annealing heat treatment process. In this specification, the term "prior structure" refers to the microstructure of the steel material manufactured by the first annealing heat treatment (S400). The low-temperature phase structure in the second annealing heat treatment process is reverse transformed into austenite to form a lath-shaped ferrite and austenite microstructure, where the low-temperature phase structure refers to a martensite or bainite phase. Such a lath-shaped structure has the characteristic of being able to secure both high strength and high ductility. When annealing in the two-phase temperature region, a primary redistribution of carbon and manganese occurs, concentrating carbon and manganese in the austenite region and increasing the phase stability of austenite.

[0057] In order to satisfy all the tensile properties of tensile strength of 980 MPa or more and elongation of 23% or more in the hardness of 0.2 wt% or less proposed in the present invention, it is necessary to redistribute carbon and manganese to austenite more than in conventional heat treatment, increase the strength of martensite, ensure sufficient tensile strength, and increase the phase stability of retained austenite to ensure sufficient ductility, so it is preferable to perform the first annealing in the two-phase temperature range. If the first annealing temperature exceeds Ac3, the austenite crystals become coarse during high-temperature annealing, and a large amount of austenite with low carbon and manganese contents is generated, making it difficult to ensure the tensile properties of the final steel. On the other hand, if the first annealing temperature is less than (Ac1+30°C) even if it is Ac1 or more, the fraction of ferrite in the microstructure after the first annealing heat treatment process exceeds 50%, and soft and coarse polygonal ferrite increases in the final microstructure, making it difficult to ensure the tensile properties of the steel.

[0058] Therefore, preferably, the microstructure after the first annealing heat treatment process should appear as a dual phase (DP) structure composed of ferrite and a low temperature phase, and more preferably, the fraction of ferrite may be limited to 30% to 50% in terms of area fraction in order to balance strength and ductility. In cooling the steel sheet subjected to the above-mentioned first annealing heat treatment to room temperature, the steel sheet may be cooled at 15° C. / s or more, preferably 25° C. / s or more, in order to suppress the generation of polygonal ferrite, which adversely affects physical properties during cooling, and to secure a low temperature martensite structure.

[0059] Meanwhile, in a modified embodiment of the present invention, when the microstructure after the first annealing heat treatment process is a dual phase (DP) structure composed of ferrite and a low temperature phase, and the ferrite fraction is limited to 30% to 50% in terms of area fraction, the heat treatment temperature of the first annealing can be limited to (Ac1+30°C) or more and (Ac3-30°C) or less.

[0060] The step (e) is a second annealing heat treatment step including a process of maintaining the steel material at a second annealing temperature of AAc1 or more (Ac3-30°C or less), cooling to a cooling end point temperature of martensite transformation start temperature (Ms) or more (bainite transformation start temperature (Bs)-15°C) or less, and then overaging.

[0061] In step (e), the martensite structure formed in the first annealing heat treatment process is reverse transformed to form lath-shaped ferrite and austenite. During annealing, the primary low-temperature phase is reverse transformed and carbon and manganese are redistributed to the austenite, so the longer the annealing time, the better for sufficient reverse transformation and alloy element redistribution. However, if the annealing time is too long, productivity may decrease, so the annealing maintenance time is limited to within 30 to 120 seconds.

[0062] In this step, the steel sheet that has been subjected to the secondary annealing is cooled to a temperature between the martensite transformation start temperature (Ms) and the bainite transformation start temperature (Bs), and the temperature is maintained for 30 to 300 seconds to induce redistribution of carbon and manganese alloy elements and increase the phase stability of the retained austenite. If the cooling rate is less than 15°C / s when cooling to a cooling end point temperature that is equal to or higher than the martensite transformation start temperature (Ms) (or equal to or lower than the bainite transformation start temperature (Bs) -15°C), polygonal ferrite is generated during cooling, which may deteriorate the tensile properties of the final steel. Therefore, the cooling rate is set to 15°C / s or more, and preferably 25°C / s or more. If the cooling end temperature is higher than (bainite transformation start temperature (Bs)-15°C), ferrite or pearlite is generated during the maintenance phase, which causes a decrease in strength and elongation rate. If the cooling end temperature is a temperature just below the bainite transformation start temperature (Bs), bainite transformation and carbon redistribution do not occur balanced in the high-temperature bainite formation zone. On the other hand, if the cooling end temperature is lower than the martensite transformation start temperature (Ms), fresh martensite is generated by cooling, and the strength of the steel increases significantly, but the amount of retained austenite decreases, making it impossible to ensure a sufficient elongation rate of 23% or more, which is the target of the present invention. Therefore, it is preferable to determine the cooling end temperature as a temperature between the martensite transformation start temperature (Ms) and (bainite transformation start temperature (Bs)-15°C). After cooling to the cooling end temperature, the temperature is maintained for 30 to 300 seconds for additional redistribution of carbon and manganese, and then cooled to room temperature. At this time, the cooling rate to room temperature is not particularly limited, but is preferably 10° C. / s or more for productivity.

[0063] Furthermore, in the method for producing a cold-rolled steel sheet according to the present invention, the second annealing temperature is lower than the first annealing temperature. When the second annealing temperature is higher than the first annealing temperature, the fraction of austenite generated in the second annealing heat treatment (S500) becomes higher than the fraction of the low-temperature phase of the structure after the first annealing heat treatment (S400). The austenite reverse transformed in the low-temperature phase appears as a lamellar structure of acicular ferrite and austenite, but the austenite generated in excess due to the high annealing temperature develops into a blocky type, resulting in an increased fraction of blocky martensite in the final microstructure, which significantly increases the tensile strength of the steel while decreasing the elongation rate.

[0064] The microstructure of the steel material finally realized through the above-mentioned heat treatment process is composed of ferrite, acicular retained austenite, a martensite / austenite composite structure, and blocky martensite, and the area fraction of the ferrite may be 30% to 60%, the area fraction of the acicular retained austenite may be 5% to 12%, the area fraction of the martensite / austenite composite structure may be 25% to 50%, and the area fraction of the blocky martensite may be 5% to 12%.

[0065] Within the range of the composition system described in the present invention, the above-mentioned heat treatment process and the resulting microstructure can realize a low carbon cold rolled ultra high strength steel sheet with excellent formability, having a tensile strength (TS) of 980 MPa to 1180 MPa and an elongation (El) of 23% to 25%.

[0066] The above-mentioned annealing heat treatment process will be described below with reference to the drawings.

[0067] FIG. 2 is a diagram illustrating an overview of (a) the first annealing heat treatment step (S400) and (b) the second annealing heat treatment step (S500) in a manufacturing method for a cold-rolled steel sheet according to one embodiment of the present invention.

[0068] First annealing heat treatment (S400)

[0069] 2(a), section ab corresponds to a step of maintaining the first annealing temperature at (Ac1+30°C) or more and (Ac3-30°C) or less, section bc corresponds to a slow cooling step as the first half of the cooling section, section cd corresponds to a rapid cooling step as the second half of the cooling section, and section de corresponds to an overaging step. In a modified embodiment of the present invention, the slow cooling step in section bc and the overaging step in section de can be omitted.

[0070] The first annealing heat treatment process may be maintained at a first annealing temperature of (Ac1+30°C) or more (Ac3-30°C) or less for 30 to 120 seconds. When the carbon content proposed in this embodiment is 0.2 wt% or less, the redistribution of alloy elements must be increased compared to existing hard types in order to ensure the tensile strength of the steel, so the first annealing heat treatment process is performed in a two-phase temperature range to induce primary redistribution of alloy elements. At this time, if the annealing temperature is excessively low, a large amount of polygonal ferrite is formed in the microstructure after the first annealing heat treatment process, making it difficult to ensure sufficient tensile strength, while if it exceeds (Ac3-30°C), the grains become coarse during high-temperature annealing and the fraction of lean austenite with low alloy element content increases, making it difficult to achieve the target tensile properties. If the maintenance time exceeds 120 seconds, the grain size becomes coarse, which may reduce productivity. The annealed cold-rolled steel is cooled to a temperature of 340° C. or less at a cooling rate of 15° C. / s or more. If the cooling end temperature exceeds 340° C., it is difficult to obtain a lath-shaped structure in the second annealing heat treatment process due to carbide precipitation, while if the cooling rate is less than 15° C. / s, a large amount of polygonal ferrite is generated during cooling, which is disadvantageous in ensuring tensile strength.

[0071] The bc section is a step of slowly cooling the annealed heat-treated steel sheet. In cooling the annealed heat-treated steel sheet, a slow cooling section may be included depending on the heat treatment equipment. When the slow cooling section is included, the slow cooling end temperature and the cooling rate are not particularly limited, but the slow cooling end temperature may be 740° C. or more and the cooling rate may be -5° C. / s or more so that a large amount of polygonal ferrite is not generated during cooling.

[0072] The cd section corresponds to a quenching step as the latter half of the cooling section, and is a step in which the steel sheet cooled in the slow cooling step is cooled to a temperature of 340°C or less. In cooling the steel sheet subjected to the primary annealing heat treatment or the primary cooling, the steel sheet is cooled to -15°C / s or more, preferably 25°C / s or more, in order to suppress the formation of polygonal ferrite that adversely affects physical properties and to form bainite or martensite, which is a low-temperature phase. The cooling rate is maintained to a temperature below the martensite transformation start temperature (Ms) represented by the following formula (1), and then cooled to room temperature through the overaging section of the equipment. Alternatively, the overaging section may be omitted and the steel sheet may be cooled directly to room temperature.

[0073] Number(1)

[0074] Ms(℃)=491.1-302.6[C]-14.5[Si]-30.6[Mn]-16.6[Ni]-8.9[Cr]+2.4[Mo]-11.3[Cu]+8.58[Co]+7.4[W]

[0075] Here, [C], [Si], [Mn], [Ni], [Cr], [Mo], [Cu], [Co], and [W] are the mass percentages of carbon, silicon, manganese, nickel, chromium, molybdenum, copper, cobalt, and tungsten in the steel.

[0076] Second annealing heat treatment (S500)

[0077] 2(b), the pq section corresponds to a stage where the second annealing temperature is maintained at Ac1 or more (Ac3-30°C or less), the qr section corresponds to a slow cooling section as the first half of the cooling section, the rs section corresponds to a rapid cooling section as the second half of the cooling section, and the st section corresponds to an overaging section. Meanwhile, the dotted line profile between the bainite transformation start temperature (Bs) and the martensite transformation start temperature (Ms) in the second annealing heat treatment process disclosed in FIG. 2(b) corresponds to the case where the plating process is performed in a plating bath.

[0078] The second annealing heat treatment process may be maintained at a second annealing temperature of Ac1 or more (Ac3-30°C) or less for 30 to 120 seconds. The second annealing temperature is lower than the first annealing temperature. A two-phase region annealing heat treatment is performed at a temperature of Ac1 or more and less than the first annealing temperature for 30 to 120 seconds. This step is a step in which the low-temperature phase structure generated in the first annealing heat treatment (S400) is reverse transformed to form lath-shaped ferrite and austenite. During annealing, the redistribution of carbon (C) and manganese (Mn) to austenite occurs along with the reverse transformation of the primary low-temperature phase, so the longer the annealing time, the better for sufficient reverse transformation and alloy element redistribution. However, if the annealing time is excessively long, productivity may decrease, so the annealing maintenance time is limited to within 30 to 120 seconds. If the annealing temperature of the second annealing heat treatment (S500) is equal to or higher than the annealing temperature of the first annealing heat treatment (S400), the fraction of austenite generated in the second annealing is greater than the fraction of the low temperature phase in the previous structure. As a result, the development of lath-type austenite reversely transformed in the low temperature phase is inhibited, and an excess fraction of massive austenite is generated. Such massive austenite reduces carbon (C) and manganese (Mn) redistributed to the lath-type austenite, thereby lowering the phase stability of the austenite. This inhibits the effect of the first annealing heat treatment (S400) which generates a lath-type structure required to ensure the elongation ratio to be achieved in the present invention. Therefore, it is preferable to perform the second annealing heat treatment (S500) at a temperature lower than that of the first annealing heat treatment (S400).

[0079] Then, the material is cooled at a cooling rate of 15°C / s or more to a cooling end temperature equal to or higher than the martensite transformation start temperature (Ms) (or equal to or lower than the bainite transformation start temperature (Bs) -15°C), and then maintained for 30 to 300 seconds (overaging) to induce redistribution of the carbon (C) and manganese (Mn) alloying elements and increase the phase stability of the retained austenite.

[0080] The bainite transformation start temperature (Bs) can be expressed by the following formula (2).

[0081] Number(2)

[0082] Bs(℃)=656-57.7[C]-75[Si]-35[Mn]-15.3[Ni]-34[Cr]-41.2[Mo]

[0083] Here, the above [C], [Si], [Mn], [Ni], [Cr], and [Mo] are the mass percentage values ​​of carbon, silicon, manganese, nickel, chromium, and molybdenum in the steel.

[0084] When cooling to the martensite transformation start temperature (Ms) or higher (the bainite transformation start temperature (Bs) - 15°C) or lower, if the cooling rate is less than 15°C / s, polygonal ferrite will be generated during cooling, and the tensile properties of the final steel may be poor, so the cooling rate is set to 15°C / s or higher, preferably 25°C / s or higher.

[0085] If the cooling end temperature is below the bainite transformation start temperature (Bs) (bainite transformation start temperature (Bs) - 15 ° C), austenite is transformed into ferrite or pearlite during the holding stage, which causes a decrease in strength and elongation rate, and if it is just below the bainite transformation start temperature (Bs), carbon redistribution is insufficient, making it difficult to ensure the phase stability of the retained austenite. On the other hand, if the cooling end temperature is below the martensite transformation start temperature (Ms), fresh martensite is generated, and while the strength of the steel increases significantly, the retained austenite decreases, making it impossible to ensure a sufficient elongation rate of 23% or more, which is the target of the present invention. In addition, if the holding time is less than 30 seconds, the redistribution time is insufficient, and the redistribution effect decreases, while if it exceeds 300 seconds, productivity may decrease.

[0086] After cooling to the cooling end temperature, the steel is overaged for 30 to 300 seconds for redistribution of carbon (C) and manganese (Mn), and then cooled to room temperature. The temperature during overaging does not need to be maintained isothermal to the cooling end temperature, and can be cooled as necessary, but the temperature must be Ms or higher to prevent the formation of fresh martensite. The cooling rate to room temperature is not particularly limited, but is preferably 10°C / s or higher for productivity. The redistribution effect of carbon (C) and manganese (Mn) during overaging varies depending on the shape of austenite, and is greater in acicular shapes than in blocky shapes. This is because the diffusion distance of carbon (C) and manganese (Mn) in the acicular shape is shorter, so diffusion occurs more easily in the same time, and as a result of analyzing the carbon (C) and manganese (Mn) contents in the acicular shape and blocky phase structures after overaging as shown in Figure 9 and Table 1, it can be confirmed that more carbon (C) and manganese (Mn) are concentrated in the acicular shape. As a result, in the microstructure after final cooling, the acicular austenite remains as a martensite / austenite composite structure, and the blocky austenite remains as blocky martensite.

[0087] [Table 1]

[0088] According to the above-mentioned cold-rolled steel sheet and manufacturing method thereof according to the technical concept of the present invention, a low-carbon steel type containing 0.2 wt % or less of carbon is used to ensure excellent weldability, and a sufficient amount of carbon (C) and manganese (Mn) is concentrated in austenite through multiple alloy element redistribution steps during the heat treatment process to realize an excellent balance between strength and elongation, thereby providing a cold-rolled ultra-high strength steel material with excellent workability that ensures a tensile strength of 980 MPa or more and an elongation of 23% or more.

[0089] Experimental Example

[0090] In the following, preferred experimental examples are presented to facilitate understanding of the present invention. However, the following experimental examples are merely intended to facilitate understanding of the present invention, and the present invention is not limited to the following experimental examples.

[0091] 1. Preparation of test specimens

[0092] In this experimental example, test pieces having the alloy element compositions (unit: weight %) shown in Table 2 were provided.

[0093] [Table 2]

[0094] The composition system of Table 2 satisfies the composition of the cold rolled steel sheet according to one embodiment of the present invention, which is carbon (C): 0.15 weight % to 0.20 weight %, silicon (Si): 1.0 weight % to 2.0 weight %, manganese (Mn): 1.5 weight % to 3.0 weight %, phosphorus (P): more than 0 weight % to 0.02 weight %, sulfur (S): more than 0 weight % to 0.003 weight %, aluminum (Al): 0.01 weight % to 0.3 weight %, nitrogen (N): more than 0 weight % to 0.01 weight %, titanium (Ti): 48 / 14·[N] weight % to 0.1 weight % (where [N] is the weight % value of nitrogen), and the remaining iron (Fe). According to the composition of Table 2, the bainite transformation start temperature (Bs) is calculated as 437.6°C, and the martensitic transformation start temperature (Ms) is calculated as 341.6°C. The temperatures are calculated according to the following relational expressions.

[0095] Bs(℃)=656-57.7[C]-75[Si]-35[Mn]-15.3[Ni]-34[Cr]-41.2[Mo]

[0096] Ms(℃)=491.1-302.6[C]-14.5[Si]-30.6[Mn]-16.6[Ni]-8.9[Cr]+2.4[Mo]-11.3[Cu]+8.58[Co]+7.4[W]

[0097] On the other hand, according to the composition in Table 2, the Ac1 temperature is 754°C and the Ac3 temperature is 900°C.

[0098] In the experimental examples of the present invention, the steel material having the above composition was reheated at 1250°C for 4 hours, hot rolled to a thickness of 3.5 mm under the condition of a finish rolling temperature (FDT) of 850°C, and then coiled at a coiling temperature of 600°C. Then, the surface oxide scale was removed by pickling, and the steel was cold rolled to a thickness of 1.2 mm. Then, the cold rolled steel sheet was subjected to two consecutive heat treatments according to the configuration disclosed in FIG. 2.

[0099] 2. Process conditions and physical property evaluation

[0100] Table 3 shows the process conditions of the first annealing heat treatment and the second annealing heat treatment applied in the experimental examples of the present invention.

[0101] [Table 3]

[0102] In Table 3, item A corresponds to the annealing temperature in the ab section in FIG. 2(a) as the annealing temperature in the first annealing heat treatment step (S400), and corresponds to the process time in the ab section in FIG. 2(a) as the annealing time in the first annealing heat treatment step (S400), and corresponds to the process time in the ab section in FIG. 2(a) as the annealing end temperature in the first annealing heat treatment step (S400), and corresponds to the temperature at point c which is the end temperature of the annealing step in the bc section in FIG. 2(a) as the quenching end temperature in the first annealing heat treatment step (S400), and corresponds to the temperature at point d which is the end temperature of the quenching step in the cd section in FIG. 2(a) as the quenching end temperature in the first annealing heat treatment step (S400), and corresponds to the process time of the overaging step in the de section in FIG. 2(a) as the overaging time in the first annealing heat treatment step (S400). In addition, in Table 3, item F corresponds to the annealing temperature in the pq section in FIG. 2(b) as the annealing temperature in the second annealing heat treatment process (S500), and corresponds to the process time in the pq section in FIG. 2(b) as the annealing time in the second annealing heat treatment process (S500), and corresponds to the process time in the pq section in FIG. 2(b) as the quenching end temperature in the second annealing heat treatment process (S500), and corresponds to the temperature at point s which is the end temperature of the quenching process in the rs section in FIG. 2(b) as the quenching end temperature in the second annealing heat treatment process (S500), and corresponds to the temperature at point t which is the end temperature of the overaging process in the st section in FIG. 2(b) as the overaging end temperature in the second annealing heat treatment process (S500), and corresponds to the process time of the overaging process in the st section in FIG. 2(b) as the overaging time in the second annealing heat treatment process (S500).

[0103] [Table 4]

[0104] Table 4 shows the area fraction of the microstructure (unit: %) and the amount of carbon enrichment in the retained austenite (unit: weight %) in the experimental examples of the present invention. The microstructure was analyzed using a scanning electron microscope (SEM), and the fraction of the retained austenite and the carbon content in the retained austenite were analyzed using XRD analysis. In Table 4, item A is the area fraction of the ferrite phase formed after the first annealing heat treatment, item B is the area fraction of the low temperature phase formed after the first annealing heat treatment, item C is the area fraction of the ferrite phase formed after the second annealing heat treatment, item D is the area fraction of the polygonal ferrite phase among the ferrite formed after the second annealing heat treatment, item E is the area fraction of the acicular ferrite phase among the ferrite formed after the second annealing heat treatment, item F is the area fraction of the acicular retained austenite phase formed after the second annealing heat treatment, item G is the area fraction of the martensite / austenite composite phase formed after the second annealing heat treatment, item H is the area fraction of the blocky martensite phase formed after the second annealing heat treatment, and item I is the concentration of carbon in the retained austenite formed after the second annealing heat treatment.

[0105] Table 5 shows the tensile properties of the experimental examples of the present invention. The tensile properties were evaluated by KS 5 tensile testing using Zwick / Roell Corp Z100.

[0106] In Table 5, the TS item indicates tensile strength (unit: MPa), the T.El item indicates elongation rate (unit: %), and the TS × T.El item indicates the product of the tensile strength and elongation rate (unit: MPa%).

[0107] [Table 5]

[0108] Referring to Tables 2 to 5, Example 1, Example 2, Example 3, and Example 4 satisfy the tensile properties of 980 MPa or more (e.g., 980 MPa to 1180 MPa), elongation of 23% or more (e.g., 23% to 25%), and TS x El of 22,000 MPa% or more that are aimed to be achieved by the present invention by appropriately carrying out the first annealing heat treatment (S400) and the second annealing heat treatment (S500) proposed in the present invention. Referring to FIG. 3, the structure after the first annealing heat treatment (S400) in Example 1, i.e., the pre-structure, is composed of 43% ferrite and 57% low temperature phase as area fractions, which satisfies the condition of the present invention (area fraction of ferrite: 30% to 50%). The microstructure of Example 1 after the second annealing heat treatment (S500) is shown in FIG. 4, and it can be confirmed that the microstructure is composed of ferrite, acicular retained austenite, a composite structure of martensite / austenite, and blocky martensite in the target fractions of the present invention.

[0109] In Comparative Examples 1, 2, 3, and 4, the first annealing heat treatment (S400) was performed at a two-phase temperature of 850°C, and a sufficient amount of ferrite of 45% was secured in the microstructure after the first annealing heat treatment. However, the redistribution of alloy elements was not smoothly performed in the second annealing heat treatment (S500), and the fraction and phase stability of retained austenite were not sufficiently secured. Therefore, although the tensile strength was sufficiently high at 1000 MPa or more, the elongation rate was far below the 23% that the present invention aims to achieve.

[0110] Specifically, in Comparative Example 1 and Comparative Example 2, the cooling end temperature in the second annealing heat treatment (S500) was equal to or higher than the bainite transformation start temperature (Bs), and redistribution of carbon (C) and manganese (Mn) over the maintenance time after the cooling end point was ineffective, so the elongation did not reach the target value (23% or more).

[0111] In Comparative Example 3, like Examples 1 and 2, cooling to an appropriate temperature equal to or higher than the martensite transformation start temperature (Ms) (equal to or lower than the bainite transformation start temperature (Bs) -15°C) was completed, but the maintenance time (overaging time) was short at less than 30 seconds, and sufficient redistribution did not occur, so the elongation did not reach the target value (23% or more). Example 2, in which the maintenance time in the second annealing heat treatment (S500) was longer than in Comparative Example 3, caused sufficient redistribution of carbon (C) and manganese (Mn), greatly increasing the elongation, and it is understood that a sufficient maintenance time is necessary to increase the elongation.

[0112] In Comparative Example 4, the cooling end temperature in the second annealing heat treatment (S500) was lower than the martensitic transformation start temperature (Ms), so that martensite was formed at the end of cooling, the austenite fraction was reduced, and the redistribution of carbon (C) and manganese (Mn) was ineffective due to the low temperature, so the elongation did not reach the target value (23% or more).

[0113] In Comparative Examples 5, 6, and 7, the annealing temperature in the first annealing heat treatment (S400) was high, so the fraction of ferrite in the microstructure after the first annealing heat treatment was 6% and 0%, respectively, which is below the range proposed in the present invention (30% to 50%).

[0114] 5 showing the prior structure after the first annealing heat treatment (S400) of Comparative Example 6, it can be seen that the prior structure after the first annealing heat treatment (S400) is entirely composed of low temperature phases after single-phase annealing. As the annealing temperature of the first annealing heat treatment (S400) increases, the fraction of ferrite in the prior structure decreases and the fraction of the low temperature phase increases, while the polygonal ferrite in the final microstructure after the second annealing heat treatment (S500) decreases, the fractions of acicular ferrite and retained austenite increase, and the fraction of blocky martensite decreases.

[0115] Referring to FIG. 6 showing the final microstructure of Comparative Example 7, the microstructure is generally composed of acicular ferrite, a composite structure of martensite / austenite, and retained austenite, and the amount of blocky martensite that contributes to increased strength is very small. Therefore, in Comparative Examples 5 and 7, the elongation rate satisfies the target value of the present invention (23% or more), but the tensile strength does not satisfy the target value of the present invention.

[0116] In Comparative Example 6, in the second annealing heat treatment (S500), the cooling end temperature was set to 440°C, which exceeds the bainite transformation start temperature (Bs), and the redistribution of carbon (C) and manganese (Mn) after cooling was reduced, and the tensile strength was sufficiently secured to be 1022 MPa or more. However, the redistribution was insufficient, and the elongation did not reach the target value (23% or more).

[0117] In Comparative Example 8, the annealing temperature (second annealing temperature) of the second annealing heat treatment (S500) is higher than the annealing temperature (first annealing temperature) of the first annealing heat treatment (S400), which is contrary to the heat treatment method proposed in the present invention. If the second annealing temperature is higher than the first annealing temperature, the fraction of austenite generated in the second annealing heat treatment (S500) is higher than the fraction of the low temperature phase in the structure after the first annealing heat treatment (S400). The austenite reverse transformed in the low temperature phase appears as an acicular type of ferrite and austenite lamellar structure, but the austenite generated in excess due to the high annealing temperature develops into a blocky type, resulting in an increase in the fraction of blocky martensite in the final microstructure, which greatly increases the tensile strength of the steel while decreasing the elongation (see FIG. 7).

[0118] Comparative Example 9 was subjected to only the conventional single annealing heat treatment, and a microstructure composed of massive bainite, martensite, and ferrite appears as shown in Figure 8. Due to the high fraction of massive martensite, the composite structure of martensite / austenite, and the low fraction of retained austenite, the specimen exhibits high tensile strength and low elongation.

[0119] From the experimental examples described above, it was confirmed that if a two-phase structure consisting of ferrite and a low temperature phase is not obtained in the first annealing heat treatment (S400), or if acicular ferrite and austenite are not adequately obtained in the second annealing heat treatment (S500), it is difficult to obtain the physical properties in which the tensile strength and elongation are balanced as targeted in the present invention.

[0120] Although the present invention has been described above with a focus on the preferred embodiment, various modifications and variations can be made by those skilled in the art. Such modifications and variations are within the scope of the present invention. Therefore, the scope of the present invention should be determined based on the following claims.

Claims

1. carbon (C): 0.15% to 0.20% by weight, silicon (Si): 1.0% to 2.0% by weight, manganese (Mn): 1.5% to 3.0% by weight, phosphorus (P): more than 0% to 0.02% by weight, sulfur (S): more than 0% to 0.003% by weight, aluminum (Al): 0.01% to 0.3% by weight, nitrogen (N): more than 0% to 0.01% by weight, titanium (Ti): 48 / 14·[N]% to 0.1% by weight (the [N] being the weight percentage value of nitrogen), the remainder being iron (Fe) and other unavoidable impurities, The final microstructure consists of ferrite, acicular retained austenite, a composite structure of martensite / austenite, and blocky martensite; The area fraction of the ferrite is 30% to 60%, the area fraction of the acicular retained austenite is 5% to 12%, the area fraction of the martensite / austenite composite structure is 25% to 50%, and the area fraction of the blocky martensite is 5% to 12%, A cold-rolled steel sheet, characterized in that the amount of carbon enrichment in retained austenite is 1.1 wt % or more.

2. The ferrite is composed of polygonal ferrite and acicular ferrite, The cold rolled steel sheet according to claim 1, wherein an area fraction of the acicular ferrite in the ferrite is 40% or more.

3. The cold rolled steel sheet according to claim 1, characterized in that it has a tensile strength (TS) of 980 MPa to 1180 MPa and an elongation (El) of 23% to 25%.

4. (a) reheating the steel material containing carbon (C): 0.15% to 0.20% by weight, silicon (Si): 1.0% to 2.0% by weight, manganese (Mn): 1.5% to 3.0% by weight, phosphorus (P): more than 0% to 0.02% by weight, sulfur (S): more than 0% to 0.003% by weight, aluminum (Al): 0.01% to 0.3% by weight, nitrogen (N): more than 0% to 0.01% by weight, titanium (Ti): 48 / 14·[N]% to 0.1% by weight (where [N] is the weight percentage value of nitrogen), the remainder being iron (Fe) and other unavoidable impurities; (b) hot rolling the reheated steel product; (c) cold rolling the hot rolled steel material; (d) a first annealing heat treatment step including a step of maintaining the cold-rolled steel at a first annealing temperature of (Ac1+30°C) or more and (Ac3-30°C) or less, and then cooling the steel to a cooling end temperature of 340°C or less; and (e) a second annealing heat treatment step including a step of maintaining the steel material at a second annealing temperature of Ac1 or more (Ac3-30°C) or less, cooling to a cooling end point temperature of martensite transformation start temperature (Ms) or more (bainite transformation start temperature (Bs)-15°C) or less, and then overaging; The method for producing a cold-rolled steel sheet, wherein the second annealing temperature is lower than the first annealing temperature.

5. The step (a) includes reheating the steel material at 1180°C to 1300°C, The step (b) includes hot rolling under conditions of a finish rolling temperature of 850° C. to 950° C. and a coiling temperature of 450° C. to 650° C., The method of claim 4, wherein step (c) comprises cold rolling at a rolling reduction of 40% to 70%.

6. 5. The method of claim 4, wherein step (d) comprises maintaining the cold-rolled steel at the first annealing temperature for 30 to 120 seconds, and then cooling the cold-rolled steel at a cooling rate of 15°C / s or more to a cooling end temperature of 340°C or less.

7. 7. The method of claim 6, wherein after step (d) is performed, an area fraction of ferrite in the microstructure of the steel is 30% to 50%.

8. 5. The method of claim 4, wherein step (e) comprises maintaining the steel at the second annealing temperature for 30 to 120 seconds, cooling the steel at a cooling rate of 15° C. / s or more to a cooling end temperature of a martensite transformation start temperature (Ms) or more (a bainite transformation start temperature (Bs)−15° C.) or less, and then overaging the steel for 30 to 300 seconds.

9. After performing step (e), the microstructure of the steel material comprises ferrite, acicular retained austenite, a composite structure of martensite / austenite, and blocky martensite; 9. The method for manufacturing a cold rolled steel sheet according to claim 8, wherein an area fraction of the ferrite is 30% to 60%, an area fraction of the acicular retained austenite is 5% to 12%, an area fraction of the martensite / austenite composite structure is 25% to 50%, and an area fraction of the blocky martensite is 5% to 12%.

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