Cold rolled steel sheet and method for manufacturing same

Abstract

The present invention relates to an ultra-high strength cold rolled steel sheet of 1470 MPa grade or higher, having excellent shape and weldability suitable for use as a steel material for automobile reinforcements such as bumper beams, sill side beams, etc., or as a steel material for protecting electric vehicle battery cases such as in side frames, cross members, etc.

Description

Cold-rolled steel sheet and method of manufacturing the same

[0001] The present invention relates to an ultra-high strength cold-rolled steel sheet and a method for manufacturing the same, and more specifically, to an ultra-high strength cold-rolled steel sheet of 1470 MPa or higher, a galvanized steel sheet, and a method for manufacturing the same, which have excellent shape and weldability suitable for use as steel for automotive reinforcement materials such as bumper beams and sill side beams, or steel for protecting electric vehicle battery cases such as side frames and cross members.

[0002] For steel materials primarily used as reinforcement components related to the crash safety of automobile passengers, there is a need to develop ultra-high-strength steel with excellent processing characteristics, particularly bendability, when manufactured using cold forming techniques. To this end, research is actively underway on ultra-high-strength steel with a tensile strength of 1470 MPa or higher and its manufacturing methods utilizing a single martensite phase. Recently, the Hot Press Forming (HPF) method has been developed, which secures the required strength by forming the material at high temperatures where forming is easy, followed by water cooling between the die and the material. Since the HPF method can secure high strength relative to the same thickness, it is widely used in component manufacturing; however, due to problems with its application caused by excessive equipment investment costs and increased process costs, there is a need to develop materials for cold stamping and roll forming. Therefore, there is a need to develop ultra-high-strength cold-rolled steel sheets that are suitable for use as materials for cold stamping and roll forming, possess high strength and high yield ratio to ensure impact performance, and exhibit excellent corrosion resistance for bending characteristics for part forming, spot weldability for part assembly, and long part life.

[0003] Representative prior art of this method includes Patent Document 1 and Patent Document 2. Patent Document 1 describes a method of manufacturing a single-phase martensite structure comprising, in weight%, C: 0.25~0.4%, Si: 1.0% or less, Mn: 1.5~2.5%, P: 0.02% or less, S: 0.003% or less, Al: 0.01~0.1%, N: 0.005% or less, B: 0.0005~0.005%, and also including Ti: 0.005~0.1%, Nb: 0.005~0.1%, and a total of 0.005~0.1%, by heating and maintaining the material in a temperature range from the Ae3 transformation point up to 900°C, then rapidly cooling it to 200°C or lower at an average cooling rate of 300°C / s or more, and subsequently tempering it at 250°C or lower. However, in the case of Patent Document 1, there is a problem in that defects occur during molding because the shape (flatness) is degraded due to water cooling.

[0004] The above patent document 2 relates to a thin steel sheet having a weight percentage comprising C: 0.05% or more and 0.35% or less, Si: 0.01% or more and 2.0% or less, Mn: 0.8% or more and 3.0% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.005% or more and 0.10% or less, and N: 0.0060% or less, having a ferrite area ratio of 0% or more and 90% or less, a bainite area ratio of 5% or less (including 0%), a martensite and tempered martensite area ratio of 10% or more (including 100%), and a retained austenite area ratio of 2.0% or less (including 0%), having a standard deviation of yield strength in the width direction of 30 MPa or less, and a maximum bending amount of the steel sheet when sheared at a length of 1 m of 10 mm or less. However, even in the case of Patent Document 2, there is a problem in that shape defects occur due to rapid cooling after annealing. Therefore, it is necessary to develop ultra-high strength cold-rolled steel sheets and galvanized steel sheets of 1470 MPa or higher with excellent bendability and hydrogen embrittlement resistance, which are suitable for use as steel for automotive reinforcement materials such as bumper beams and sill side beams, or as steel for protecting electric vehicle battery cases such as side frames and cross members.

[0005] [Prior Art Literature]

[0006] [Patent Literature]

[0007] (Patent Document 1) Japanese Patent Publication No. JP 2010-248565

[0008] (Patent Document 2) Japanese Patent Publication No. JP 2020-019992

[0009] According to one embodiment of the present invention, in response to strong demands for passenger safety and electric vehicle battery protection, ultra-high strength cold-rolled steel sheets, galvanized steel sheets, and a method for manufacturing the same can be provided, which have a tensile strength of 1470 MPa or more and excellent shape and weldability.

[0010] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0011] A cold-rolled steel sheet according to one embodiment of the present invention comprises, in weight%, carbon (C): 0.18~0.32%, silicon (Si): 0.02~0.40%, manganese (Mn): 0.5~1.5%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.0035% or less (excluding 0%), aluminum (Al): 0.010~0.10%, chromium (Cr): 0.005~0.8%, molybdenum (Mo): 0.001~0.35%, niobium (Nb): 0.05% or less, titanium (Ti): 0.005~0.05%, copper (Cu): 0.003~0.3%, nickel (Ni): 0.003~0.3%, boron (B): 0.0005~0.005%, nitrogen (N): It contains 0.01% or less (excluding 0%) and the remainder being Fe and other unavoidable impurities, and satisfies the following Equations 1 to 3, wherein the microstructure of the core of the steel sheet contains, in area %, a sum of one or more of ferrite and bainite: 5% or less (including 0%) and the remainder being one or more of martensite and tempered martensite, and the microstructure of the surface layer contains, in area %, a sum of one or more of ferrite and bainite: 10% or less (excluding 0%) and the remainder being one or more of martensite and tempered martensite, and when the R3 value of Equation 3 in the surface layer of the steel sheet is denoted as R3S and the R3 value in the core of the steel sheet is denoted as R3C, it can satisfy R3C / R3S≥0.45.

[0012] [Relationship 1]

[0013] R1 = 0.36 ≤ C + Si / 30 + Mn / 20 + (Cr + Mo) / 5 ≤ 0.44

[0014] [Relationship 2]

[0015] R2 = 0.25 ≤ C + 0.04Mn + 0.03Si ≤ 0.35

[0016] [Relationship 3]

[0017] R3 = 14 ≤ 10.1169 + 6.1985Mn + 14.3269P - 10.3715S + 11.7499Si + 3.8815Cu + 2.9755Ni + 5.5696Cr ≤ 21

[0018] (Here, the surface layer refers to the region extending 20㎛ in the thickness direction from the surface of the steel, and the center refers to the region excluding the surface layer. Also, in the above Equations 1 to 3, each alloy element represents the content (weight%) of that element.)

[0019] The above cold-rolled steel sheet may have a yield strength (YS): 1200 MPa or more, tensile strength (TS): 1470 MPa or more, yield ratio (TS / YS): 0.89 or less, elongation: 4% or more, bendability (R / t): 3.5 or less, and hole expansion (HER): 30% or more.

[0020] The edge flatness [ΔH] (mm) of the above cold-rolled steel sheet may be 5mm or less.

[0021] An electro-galvanized layer may be formed on one surface of the above cold-rolled steel sheet.

[0022] A cold-rolled steel sheet according to another embodiment of the present invention comprises the steps of: preparing a steel slab satisfying the steel composition components and satisfying Equations 1 to 3, and then heating it at 1100 to 1300°C; finishing hot-rolling the heated steel slab at Ar3 to Ar3+120°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at Ms to 700°C; cold-rolling the coiled hot-rolled steel sheet at a cold reduction rate of 45 to 70% to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet at Ac3+20°C to Ac3+80°C; and first cooling the continuously annealed cold-rolled steel sheet to a first cooling end temperature of 670 to 750°C at an average cooling rate of 1 to 6°C / s. The method includes the step of secondarily cooling the first cooled cold-rolled steel sheet to a second cooling end temperature (Tf) of 110 to 300°C at an average cooling rate of 30 to 300°C / s; and the step of reheating the second cooled cold-rolled steel sheet to an overaging treatment temperature (H) of 100 to 250°C, and then overaging treatment for 5 to 12 minutes, wherein the Mf-Tf temperature difference during the second cooling is controlled to be 130°C or less, and the overaging treatment temperature (H) - second cooling end temperature (Tf) is controlled to be 100°C or less.

[0023] [Relationship 1]

[0024] R1 = 0.36 ≤ C + Si / 30 + Mn / 20 + (Cr + Mo) / 5 ≤ 0.44

[0025] [Relationship 2]

[0026] R2 = 0.25 ≤ C + 0.04Mn + 0.03Si ≤ 0.35

[0027] [Relationship 3]

[0028] R3 = 10.1169 + 6.1985Mn + 14.3269P - 10.3715S + 11.7499Si + 3.8815Cu + 2.9755Ni + 5.5696Cr

[0029] (In the above Equations 1 to 3, each alloying element represents the content (weight%) of that element)

[0030] In addition, the above-mentioned over-aged cold-rolled steel sheet may additionally include a step of temper rolling with a rolling force of 500 to 1000 tons.

[0031] In addition, the method may further include a step of tension leveling the temper-rolled cold-rolled steel sheet with an elongation of 0.50% or less. In addition, the method may further include a step of forming an electro-galvanized layer on the surface of the tension-leveled cold-rolled steel sheet.

[0032] According to the present invention, ultra-high strength cold-rolled steel sheets and galvanized steel sheets with excellent shape and weldability and a tensile strength of 1470 MPa or higher can be provided. Specifically, the cold-rolled steel sheet may have a yield strength (YS) perpendicular to the rolling direction of 1200 MPa or higher, more preferably 1210 MPa or higher, a tensile strength (TS) perpendicular to the rolling direction of 1470 MPa or higher, more preferably 1500 MPa or higher, a yield ratio (TS / YS) of 0.89 or lower, more preferably 0.85 or lower, an elongation of 4% or higher, bendability (R / t) of 3.5 or lower, more preferably 3.0 or lower, hole expansion (HER) of 30% or higher, more preferably 35% or higher, and a flatness [ΔH] (mm) of the steel sheet edge of 5 mm or lower.

[0033] Figure 1 is a diagram schematically showing the distribution of Invention Example 1-4 and Comparative Example 1-4 with respect to the R1 value of Equation 1 and the R2 value of Equation 2 in an embodiment of the present invention.

[0034] FIG. 2 is a diagram schematically showing the distribution of Invention Example 1-4 and Comparative Example 1-4 with respect to the R1 value of Equation 1 and the R3 value of Equation 3 in an embodiment of the present invention.

[0035] Figure 3 is a photograph showing the degree of flatness of Invention Example 1 and Comparative Example 5 in an embodiment of the present invention.

[0036] Preferred embodiments of the present invention will be described below with reference to the attached drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0037] In addition, embodiments of the present invention are provided to more fully explain the present invention to those with average knowledge in the relevant technical field.

[0038] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0039] In describing the embodiments of the present invention, if it is determined that a detailed description of known technology related to the present invention may unnecessarily obscure the essence of the present invention, such detailed description will be omitted. Furthermore, the terms described below are defined considering their functions in the present invention, and these may vary depending on the intentions or conventions of the user or operator. Therefore, such definitions should be based on the content throughout this specification. The terms used in the detailed description are merely for describing the embodiments of the present invention and should not be limited in any way. Unless explicitly stated otherwise, expressions in the singular form include the meaning of the plural form.

[0040] In this description, expressions such as “include” or “equipped” are intended to refer to certain characteristics, numbers, steps, actions, elements, parts or combinations thereof, and should not be interpreted to exclude the existence or possibility of one or more other characteristics, numbers, steps, actions, elements, parts or combinations thereof other than those described.

[0041] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0042] The present invention will be described in detail below through each embodiment or example of the invention. It should be noted that each embodiment or example described in this specification is not limited to a single embodiment or example, but may also be combined with other embodiments or examples. Accordingly, the citation of claims in the patent claims is merely an example of an embodiment, and the technical concept of the present invention should not be interpreted as being limited only to a combination with the cited claims; rather, combinations with various claims are also included within the scope of the technical concept of the present invention.

[0043] It should be noted that, although not essential, the technical solution according to each aspect of the present invention may be usefully applied to other aspects of the technical solution. Furthermore, the composition and various useful parameters according to each aspect of the present invention can be appropriately combined with other aspects to obtain advantageous effects.

[0044] The inventors recognized that by controlling the alloy composition and manufacturing conditions, they could appropriately control the microstructure, surface work hardening, etc., to produce an ultra-high strength cold-rolled steel sheet with a tensile strength of 1470 MPa or higher, which has excellent shape, bending characteristics, and hole expansion properties, and thus completed the present invention.

[0045] Hereinafter, a cold-rolled steel sheet according to one embodiment of the present invention will be described. First, the alloy composition of the present invention and the reasons for limiting its content, etc., will be explained.

[0046] Carbon (C): 0.18~0.32%

[0047] Carbon (C) is an interstitial solid solution element and is the most effective and important element for improving the strength of steel. Furthermore, it is an element that must be added to ensure the strength of martensitic steel. If the C content is less than 0.18%, it may be difficult to obtain the yield ratio and tensile strength targeted in this invention. If the C content exceeds 0.32%, excessive martensite is formed during cooling due to a rapid increase in hardenability; consequently, the strength increases rapidly, which may lead to inferior elongation. Additionally, weldability may be compromised. Therefore, it is preferable for the C content to be in the range of 0.18% to 0.32%. It is more preferable for the lower limit of the C content to be 0.20%. It is more preferable for the upper limit of the C content to be 0.30%.

[0048] Silicon (Si): 0.02~0.40%

[0049] Si plays a role in suppressing the formation of carbides and controlling their size during the reheating and overaging treatment steps performed after continuous annealing and cooling. If the Si content is less than 0.02%, it may be difficult to sufficiently obtain the aforementioned effects. If the Si content exceeds 0.40%, ferrite may be formed after continuous annealing and cooling, potentially weakening the strength of the steel. Furthermore, as Si is an element that increases resistivity, resistance spot weldability may be compromised. Therefore, it is desirable for the Si content to be in the range of 0.02% to 0.40%. It is more preferable for the lower limit of the Si content to be 0.05%. It is more preferable for the upper limit of the Si content to be 0.35%.

[0050] Manganese (Mn): 0.5~1.5%

[0051] Manganese (Mn) is an element added to ensure strength. If the Mn content is less than 0.5%, the hardenability is low; consequently, if the cooling rate after continuous annealing is not sufficiently fast, martensite is not formed, making it difficult to secure the strength level targeted by the present invention. If the Mn content exceeds 1.5%, the Ms temperature decreases during cooling after continuous annealing, resulting in a lower final cooling temperature and consequently poor shape of the steel sheet. Furthermore, it is difficult to secure an initial martensite structure. In addition, Mn-based segregation zones occur along the longitudinal direction of the slab during steelmaking and continuous casting operations, degrading bendability. Specifically, as manganese is segregated along the thickness direction and forms manganese bands (Mn bands) within the slab, cracks occur during continuous casting, and there is a problem of increased defect occurrence during the rolling process. Therefore, it is desirable for the Mn content to be in the range of 0.5% to 1.5%. It is more preferable for the lower limit of the Mn content to be 0.4%. It is more preferable that the upper limit of the above Mn content is 1.4%.

[0052] Phosphorus (P): 0.03% or less (excluding 0%)

[0053] P is an impurity element contained in steel; if the content of P exceeds 0.03%, weldability deteriorates and there is a risk of brittleness occurring in the steel. Meanwhile, although it is advantageous to have a smaller amount of P added to the steel, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is preferable that the content of P be 0.03% or less (excluding 0%). It is more preferable that the content of P be 0.025% or less.

[0054] Sulfur (S): 0.0035% or less (excluding 0%)

[0055] S, like P, is an impurity element contained in steel; if the S content exceeds 0.0035%, it may impair ductility and weldability, and a large amount of MnS precipitates may be formed, resulting in inferior bendability. Meanwhile, although it is advantageous to have a smaller amount of S added to the steel, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is preferable that the S content be 0.0035% (excluding 0%) or less. It is more preferable that the S content be 0.0030% or less, and even more preferable that it be 0.0020% or less.

[0056] Aluminum (Al): 0.01~0.10%

[0057] Al may be added to remove oxygen from the molten steel. If the Al content is less than 0.01%, deoxidation is not sufficiently achieved, which impairs the cleanliness of the steel. If the Al content exceeds 0.10%, not only does the castability of the slab deteriorate, but the temperature required for single-phase heating during continuous annealing also increases, which may lead to production and equipment problems. Therefore, it is desirable for the Al content to be in the range of 0.01 to 0.10%. It is more preferable for the upper limit of the Al content to be 0.075%.

[0058] Chrome (Cr): 0.005~0.80%

[0059] Cr is an element that facilitates securing a low-temperature transformation structure by suppressing ferrite transformation. Furthermore, when utilizing a continuous annealing process involving slow cooling, as in the present invention, there is an advantage in suppressing ferrite formation. If the Cr content is less than 0.005%, the hardenability is low; consequently, if the cooling rate is not sufficiently fast during cooling after continuous annealing, martensite is not formed, making it difficult to secure the strength level targeted by the present invention. On the other hand, if the Cr content exceeds 0.80%, resistance to delayed fracture may deteriorate, carbides such as CrC may form, impairing bendability, and increasing costs due to excessive alloy input. Therefore, it is preferable for the Cr content to be in the range of 0.005 to 0.80%. It is more preferable for the lower limit of the Cr content to be 0.01%. It is more preferable for the upper limit of the Cr content to be 0.60%.

[0060] Molybdenum (Mo): 0.001~0.35%

[0061] Mo is an element that exhibits effects such as improving the quenching properties of steel, generating Mo-based fine carbides that serve as hydrogen trap sites, and improving resistance to delayed fracture through martensite refinement. If the Mo content is less than 0.001%, it may be difficult to sufficiently obtain the aforementioned effects. If the Mo content exceeds 0.35%, the aforementioned effects do not increase significantly compared to the cost increase resulting from the addition of expensive alloying elements. Therefore, it is desirable for the Mo content to be in the range of 0.001% to 0.35%. It is more desirable for the lower limit of the Mo content to be 0.003%. It is more desirable for the upper limit of the Mo content to be 0.30%.

[0062] Niobium (Nb): 0.05% or less

[0063] Nb is an element that segregates at austenite grain boundaries, suppresses the coarsening of austenite grains during continuous annealing, and contributes to strength improvement by forming fine precipitates. If the Nb content exceeds 0.05%, the precipitation of coarse carbonitrides increases, and there is a risk that strength and elongation will decrease due to the reduction in carbon content in the steel. In addition, there are problems such as reduced workability of the base material and increased manufacturing costs. Therefore, it is desirable for the Nb content to be in the range of 0.05% or less. It is more desirable to set the lower limit of the Nb content to 0.003%. It is more desirable for the upper limit of the Nb content to be 0.04%.

[0064] Titanium (Ti): 0.005~0.05%

[0065] Ti is a nitride-forming element that scavenges dissolved nitrogen by precipitating it as TiN. If the Ti content is less than 0.005%, not only is it difficult to obtain a strength-increasing effect, but the scavenging effect of dissolved nitrogen is also reduced, leading to the formation of a large amount of AlN, which may cause cracks during continuous casting. If the Ti content exceeds 0.05%, the strength of the martensite may decrease due to the precipitation of additional carbides in addition to the removal of dissolved nitrogen, and the hole expansion and bendability may be impaired due to the excessive formation of carbides and nitrides such as TiC and TiN. Therefore, it is desirable for the Ti content to be in the range of 0.005 to 0.05%. It is more desirable for the lower limit of the Ti content to be 0.01%. It is more desirable for the upper limit of the Ti content to be 0.04%.

[0066] Copper (Cu): 0.003~0.3%

[0067] Cu improves corrosion resistance in the operating environment of automobiles and also has the effect of suppressing hydrogen intrusion into the steel sheet by coating the surface of the steel sheet with corrosion products. Furthermore, as an element incorporated when utilizing scrap as a raw material, allowing the incorporation of Cu enables the use of recycled materials as raw materials, thereby reducing manufacturing costs. From this perspective, it is desirable to contain 0.003% or more of Cu, and from the perspective of improving delayed fracture resistance, it is more desirable to contain 0.005% or more of Cu. However, since an excessive amount of Cu leads to the occurrence of surface defects, it is desirable to keep the Cu amount at 0.3% or less, more desirable at 0.2% or less, and even more desirable at 0.08% or less.

[0068] Nickel (Ni): 0.003~0.3%

[0069] Ni, like Cu, is an element that improves corrosion resistance. For this reason, it is desirable to include at least 0.003% of Ni. However, if the amount of Ni becomes excessive, scale formation within the furnace becomes uneven, which can actually cause surface defects. It also leads to increased costs. Therefore, it is desirable to keep the amount of Ni at 0.3% or less, more desirable at 0.2% or less, and even more desirable at 0.08% or less.

[0070] Boron (B): 0.0005~0.005%

[0071] B is an element that inhibits ferrite formation; accordingly, the present invention has the advantage of inhibiting the formation of ferrite during cooling after continuous annealing. If the content of B is less than 0.0005%, there is no hardenability effect at all, making it impossible to secure the strength targeted by the present invention. Furthermore, excessive ferrite is formed in the surface layer, leading to a problem of inferior bendability. If the content of B exceeds 0.005%, ductility may be significantly reduced. Therefore, it is preferable for the content of B to have a range of 0.0005% to 0.005%. It is more preferable for the lower limit of the B content to be 0.0007%. It is more preferable for the upper limit of the B content to be 0.004%.

[0072] Nitrogen (N): 0.01% or less (excluding 0%)

[0073] N is an impurity element, and if its content exceeds 0.01%, it significantly increases the risk of cracking during continuous casting due to the formation of AlN, etc. Considering cases where the N content is unavoidably included in the manufacturing process, 0% is excluded from the above N content. Therefore, it is desirable for the above N content to have a range of 0.01% or less (excluding 0%). It is more desirable for the above N content to be 0.008% or less, and even more desirable for it to be 0.006% or less.

[0074] It is preferable that the cold-rolled steel sheet of the present invention satisfies the aforementioned alloy composition and simultaneously satisfies the following equations 1 to 3.

[0075] Equation 1

[0076] [Relationship 1]

[0077] R1 = 0.36 ≤ C + Si / 30 + Mn / 20 + (Cr + Mo) / 5 ≤ 0.44

[0078] (In the above Equation 1, each alloying element represents the content (weight%) of that element)

[0079] The above Equation 1 is a component relationship closely related to weld hardness. If the value of R1 is less than 0.36, it may be difficult to sufficiently secure weld hardness and strength. On the other hand, if the value of R1 exceeds 0.44, the weld hardness becomes excessively high, increasing the risk of brittle fracture, which may lead to a decline in impact stability. Therefore, it is desirable for the value of R1 to have a range of 0.36 to 0.44. It is more desirable for the lower limit of the R1 value to be 0.37. It is more desirable for the upper limit of the R1 value to be 0.43.

[0080] Equation 2

[0081] [Relationship 2]

[0082] R2 = 0.25 ≤ C + 0.04Mn + 0.03Si ≤ 0.35

[0083] (In the above Equation 2, each alloying element represents the content (weight%) of that element)

[0084] The above Equation 2 is a component relationship related to strength. If the value of R2 is less than 0.25, the strength of the martensite structure is low, making it difficult to secure the target strength. If the value of R2 exceeds 0.35, the strength becomes excessively high, making it difficult to secure the target elongation and bending, which may cause processing cracks during forming and increase the cost of the ferroalloy. Therefore, it is desirable for the value of R2 to have a range of 0.25 to 0.35. It is more desirable for the lower limit of the R2 value to be 0.26. It is more desirable for the upper limit of the R2 value to be 0.34.

[0085] Equation 3

[0086] [Relationship 3]

[0087] R3 = 10.1169 + 6.1985Mn + 14.3269P - 10.3715S + 11.7499Si + 3.8815Cu + 2.9755Ni + 5.5696Cr

[0088] (In the above Equation 3, each alloying element represents the content (weight%) of that element)

[0089] The above Equation 3 is a component relationship equation related to resistivity. In the present invention, it is preferable to manage the value of R3 within the range of 14 to 21. If the value of R3 is less than 14, it is difficult to secure the target welding current range. If the value of R3 exceeds 21, the cost of the ferroalloy may increase excessively. Therefore, it is preferable for the value of R3 to have a range of 14 to 21. It is more preferable that the lower limit of the value of R2 be 14.5. It is more preferable that the upper limit of the value of R2 be 20.5.

[0090] R3C / R3S≥0.45

[0091] Meanwhile, in the present invention, when the R3 value of Equation 3 at the surface layer of the steel plate is denoted as R3S and the R3 value at the center of the steel plate is denoted as R3C, it is desirable to satisfy R3C / R3S ≥ 0.45. If the content of Cu and Ni is too low, the amount of enrichment in the surface layer decreases, and the R3C / R3S ratio becomes less than 0.45, making it difficult to secure the desired welding current range. Therefore, it is desirable to set the R3C / R3S ratio to 0.45 or higher, and more preferably to 0.46 or higher. Here, the surface layer refers to the region extending 20㎛ in the thickness direction from the surface of the steel material, and the center refers to the region excluding the surface layer. The remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the normal manufacturing process, they cannot be excluded. Since these impurities are known to any skilled person in the normal manufacturing process, all details thereof are not specifically mentioned in this specification.

[0092] Meanwhile, the microstructure of the steel plate according to the present invention may vary depending on the center and the surface layer. Here, the surface layer refers to the region extending up to 20 μm in the thickness direction from the surface of the steel, and the center refers to the region excluding the surface layer.

[0093] Specifically, the microstructure of the core of the steel plate of the present invention comprises, in area %, a total of one or more of ferrite and bainite: 5% or less (including 0%), and the remainder being one or more of martensite and tempered martensite, and the microstructure of the surface layer comprises, in area %, a total of one or more of ferrite and bainite: 10% or less (excluding 0%), and the remainder being one or more of martensite and tempered martensite.

[0094] The above-mentioned core microstructure is preferably composed of one or more types of martensite and tempered martensite. The martensite and tempered martensite are structures that are highly advantageous for securing the strength, hole expandability, bending characteristics, and weldability targeted by the present invention. However, one or more types of ferrite and bainite may inevitably be formed during the manufacturing process, and if the total fraction of one or more types of ferrite and bainite exceeds 5%, it may be difficult to secure the physical properties intended by the present invention. It is more preferable that the total fraction of one or more types of ferrite and bainite is 3% or less.

[0095] It is preferable that the main phase of the microstructure of the surface layer comprises one or more types of martensite and tempered martensite. In addition, it is preferable that the total fraction of one or more types of ferrite and bainite is 10% or less (excluding 0%). Since the ferrite and bainite are softer than martensite and tempered martensite, forming an appropriate level of ferrite and bainite in the surface layer can further improve bending properties. If the total fraction of one or more types of ferrite and bainite exceeds 10%, it is difficult to secure sufficient strength, and bending properties may deteriorate. It is more preferable that the total fraction of one or more types of ferrite and bainite is 7% or less.

[0096] The cold-rolled steel sheet of the present invention having the alloy composition and microstructure as described above can satisfy the following: yield strength: 1200 MPa or more, more preferably 1210 MPa or more; tensile strength: 1470 MPa or more, more preferably 1500 MPa or more; yield ratio: 0.85 or less, more preferably 0.80 or less; elongation: 4% or more; bendability (R / t): 3.5 or less, more preferably 3.0 or less; hole expansion (HER): 30% or more, more preferably 35% or more; and edge flatness [ΔH] (mm) of 5 mm or less.

[0097] Meanwhile, the thickness of the cold-rolled steel sheet of the present invention may be 0.6 to 2.3 mm. The lower limit of the thickness of the cold-rolled steel sheet is more preferably 0.7 mm, and more preferably 0.8 mm. The upper limit of the thickness of the cold-rolled steel sheet is more preferably 2.2 mm, and more preferably 2.1 mm.

[0098] In addition, the cold-rolled steel sheet of the present invention may have an electro-galvanized layer formed on at least one surface. The present invention does not specifically limit the type of electro-galvanized layer, and any type of electro-galvanized layer commonly used in the relevant technical field may be formed.

[0099] Next, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0100] The method for manufacturing a cold-rolled steel sheet according to the present invention comprises the steps of: preparing a steel slab satisfying the steel composition and satisfying Equations 1 to 3, and then heating it at 1100 to 1300°C; finishing hot-rolling the heated steel slab at Ar3 to Ar3+120°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at Ms to 700°C; cold-rolling the coiled hot-rolled steel sheet at a cold reduction rate of 45 to 70% to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet at Ac3+20°C; and first cooling the continuously annealed cold-rolled steel sheet to a first cooling end temperature of 670 to 750°C at an average cooling rate of 1 to 6°C / s. The method includes a step of secondarily cooling the first cooled cold-rolled steel sheet to a second cooling end temperature (Tf) of 110 to 300°C at an average cooling rate of 30 to 300°C / s; a step of reheating the second cooled cold-rolled steel sheet to an overaging treatment temperature (H) of 100 to 250°C and then overaging for 5 to 12 minutes, wherein the Mf-Tf temperature difference during the second cooling is controlled to be 130°C or less, and the overaging treatment temperature (H) - second cooling end temperature (Tf) is controlled to be 100°C or less.

[0101] [Slab Heating]

[0102] First, a steel slab satisfying the aforementioned alloy composition and equations 1 to 3 is heated at 1100 to 1300°C.

[0103] Meanwhile, the steel slab used in the manufacturing method of the present invention may be refined and cast through a converter process or an electric furnace process.

[0104] In the converter process, molten iron supplied from a blast furnace is primarily used; however, depending on the supply and demand status of hot metal, some scrap or other iron sources may be added for refining to produce molten steel. In particular, when implementing low HMR operations that reduce the amount of molten iron used to meet requirements such as carbon neutrality, the amount of scrap used may increase, and as a result, elements not intended in this invention may be included in the molten steel within the allowable limits.

[0105] In the electric furnace process, molten steel is obtained by primarily charging scrap, melting it using arc heat, and refining it. In some cases, molten iron may be added in addition to the scrap. As a result of the large amount of scrap included in this manner, elements not intended by the present invention may be present in the molten steel within permissible limits. Examples of such elements include Cr, Cu, Ni, Sn, and Mo.

[0106] Molten steel that has undergone the converter or electric furnace process may undergo an additional refining (secondary refining) process to adjust its composition and other properties.

[0107] The above slab heating process is performed to facilitate the subsequent hot rolling process and to sufficiently obtain the target physical properties of the steel plate. If the above slab heating temperature is less than 1100℃, a problem arises in which the hot rolling load increases rapidly. If the above slab heating temperature exceeds 1300℃, the amount of surface scale increases, and the yield of the material decreases. The lower limit of the above slab heating temperature is more preferably 1110℃, more preferably 1120℃, and most preferably 1130℃. The upper limit of the above slab heating temperature is more preferably 1290℃, more preferably 1280℃, and most preferably 1270℃.

[0108] [Finishing Hot Rolling]

[0109] Subsequently, the heated slab is finished hot-rolled at Ar3 to Ar3+120℃ to obtain a hot-rolled steel sheet. If the finish hot-rolling temperature is below Ar3, rolling occurs in a two-phase region of ferrite + austenite or in a ferrite region, resulting in a mixed grain structure, and plate fracture may occur due to fluctuations in the hot-rolling load. If the finish hot-rolling temperature exceeds Ar3+120℃, a large amount of surface scale may form, which may degrade the surface quality. The lower limit of the finish hot-rolling temperature is more preferably Ar3+10℃, more preferably Ar3+20℃, and most preferably Ar3+30℃. The upper limit of the finish hot-rolling temperature is more preferably Ar3+110℃, more preferably Ar3+100℃, and most preferably Ar3+90℃. Meanwhile, Ar3 can be defined by the following Equation 1.

[0110] [Equation 1]

[0111] Ar3(℃)= 910 - 203√C + 44.7Si + 31.5Mo

[0112] [Record]

[0113] Subsequently, in the present invention, the hot-rolled steel sheet is coiled at Ms~600℃. If the coiling temperature exceeds 700℃, internal oxidation occurs on the surface of the steel sheet, causing the microstructure formed in the surface layer to become non-uniform, and consequently, the bending characteristics may be degraded. Meanwhile, in the present invention, it is desirable to manage the coiling temperature low to ensure material uniformity across the entire length and width by forming the microstructure of the hot-rolled steel sheet as a single-phase structure rather than a composite structure as much as possible. However, if the coiling temperature is below Ms, the strength of the hot-rolled steel sheet becomes excessively high, which may increase the rolling load during the subsequent cold rolling process, making actual production impossible. It is more preferable that the lower limit of the coiling temperature be Ms+50℃. It is more preferable that the upper limit of the coiling temperature be 650℃. Meanwhile, Ms can be defined by the following Equation 2.

[0114] [Equation 2]

[0115] Ms(℃)= 539 - 423C - 30.4Mn - 7.5Si + 30Al - 17.7Ni - 12.1Cr - 7.5Mo

[0116] Meanwhile, after the above coiling, it can be cooled by air cooling or water cooling. In addition, after the above cooling, a pickling process can be performed to remove the oxide layer formed on the surface of the hot-rolled steel sheet.

[0117] [Cold Rolled]

[0118] Next, in the present invention, the coiled hot-rolled steel sheet is cold-rolled at a cold reduction rate of 45 to 70% to obtain a cold-rolled steel sheet. If the cold reduction rate is less than 45%, not only is it difficult to secure the thickness desired in the present invention, but there is also a concern that austenite may be generated during annealing heat treatment due to the persistence of crystal grains formed during hot rolling, which may affect the final physical properties. Additionally, bending characteristics may deteriorate. If the cold reduction rate exceeds 70%, the reduction amount in the length and width directions may become uneven due to work hardening occurring during cold rolling, which may lead to material variation in the steel sheet. Furthermore, it may be difficult to secure the thickness desired in the present invention due to the rolling load. The lower limit of the cold reduction rate is more preferably 46%, more preferably 47%, and most preferably 48%. The upper limit of the above cold rolling rate is more preferably 68%, more preferably 66%, and most preferably 64%.

[0119] [Continuous Annealing]

[0120] Subsequently, in the present invention, the cold-rolled steel sheet is continuously annealed at Ac3+20℃ to Ac3+80℃. If the continuous annealing temperature is less than Ac3+20℃, a mixed grain structure may be formed as two-phase annealing, rather than a single-phase annealing, occurs over the entire length of the steel sheet. Consequently, it is difficult to secure the physical properties targeted by the present invention, and in particular, the difference in hardness between phases becomes large, which may result in significantly inferior hole expansion properties. On the other hand, if the continuous annealing temperature exceeds Ac3+80℃, equipment troubles may occur due to overloading of the annealing furnace. The lower limit of the continuous annealing temperature is more preferably Ac3+21℃, more preferably Ac3+24℃, and most preferably Ac3+25℃. The upper limit of the continuous annealing temperature is more preferably Ac3+70℃, more preferably Ac3+60℃, and most preferably Ac3+50℃. Meanwhile, the above Ac3 can be defined by the following Equation 3.

[0121] [Equation 3]

[0122] Ac3(℃) = 875 - 206 C + 26.2 Si - 25.0 Mn - 12.3 Cr + 9.12 Mo + 50.2 Nb + 148 Ti - 131 B

[0123] [1st Cooling]

[0124] In addition, in the present invention, the continuously annealed cold-rolled steel sheet is first cooled at an average cooling rate of 1 to 6°C / s to a first cooling end temperature (T1) of 670 to 750°C. If the first cooling end temperature (T1) is less than 670°C, a large amount of soft ferrite and bainite other than martensite is formed during the cooling process, causing the surface layer structure to become non-uniform and potentially degrading the bending characteristics. If the first cooling end temperature (T1) exceeds 750°C, the temperature difference between the first cooling end temperature (T1) and the second cooling end temperature (T2) becomes severe, causing rapid phase transformation and potentially resulting in defective product shape. It is more preferable that the lower limit of the first cooling end temperature is 680°C. It is more preferable that the upper limit of the first cooling end temperature is 740°C.

[0125] If the above-mentioned first average cooling rate is less than 1℃ / s, ferrite is formed during cooling, making it impossible to secure the level of strength targeted by the present invention. If the above-mentioned first average cooling rate exceeds 6℃ / s, the average cooling rate during the subsequent second cooling decreases, increasing the fraction of low-temperature transformation phases other than martensite, making it impossible to secure the level of strength targeted by the present invention. It is more preferable that the lower limit of the above-mentioned first average cooling rate is 2℃ / s. It is more preferable that the upper limit of the above-mentioned first average cooling rate is 5℃ / s.

[0126] [Secondary Cooling]

[0127] Subsequently, in the present invention, the firstly cooled cold-rolled steel sheet is secondarily cooled at an average cooling rate of 30 to 300°C / s to a second cooling end temperature (Tf) of 110 to 300°C. The second cooling is intended to secure one or more of the main phases of the present invention, namely martensite and tempered martensite. If the second cooling end temperature (Tf) is less than 110°C, shape defects are caused by rapid phase transformation, and there is a disadvantage that continuous production is difficult due to strip meandering problems. If the second cooling end temperature (Tf) exceeds 300°C, it may be difficult to secure the strength targeted by the present invention. The lower limit of the second cooling end temperature is more preferably 115°C, more preferably 120°C, and most preferably 130°C. The upper limit of the above secondary cooling end temperature is more preferably 290℃, more preferably 280℃, and most preferably 27℃.

[0128] If the above secondary average cooling rate is less than 30℃ / s, a soft ferrite transformation occurs during cooling, making it difficult to secure the target strength. If the above secondary average cooling rate exceeds 300℃ / s, the product shape may become defective due to rapid phase transformation. The lower limit of the above secondary average cooling rate is more preferably 35℃ / s, more preferably 40℃ / s, and most preferably 45℃ / s. The upper limit of the above secondary average cooling rate is more preferably 290℃ / s, more preferably 280℃ / s, and most preferably 270℃ / s.

[0129] Meanwhile, in the present invention, it is preferable to perform secondary cooling in which the Mf-Tf temperature difference is controlled to 130°C or less during the secondary cooling. By controlling the Tf temperature appropriately, the desired yield strength and tensile strength can be secured. If the Mf-Tf temperature difference exceeds 130°C, shape defects are caused by rapid phase transformation, and there is a disadvantage that continuous production is difficult due to strip meandering problems. In the present invention, Mf can be defined by the following Equation 4.

[0130] [Equation 4]

[0131] Mf(℃) = 431 - 412 C - 17.4 Si - 47.4 Mn - 20.9 Cr - 17.0 Mo + 49.2 Nb + 95.0 Ti + 202 B

[0132] [Statute of Limitations]

[0133] Subsequently, in the present invention, the secondary cooled cold-rolled steel sheet is reheated to an overaging treatment temperature (H) of 100 to 250°C, and then overaged for 5 to 12 minutes. Through the reheating and overaging treatment, the martensite obtained by the aforementioned rapid cooling process is transformed into tempered martensite, thereby increasing the yield strength. If the reheating temperature and the overaging treatment temperature are less than 100°C, there is a disadvantage in that tempering is not sufficiently performed, resulting in low yield strength and inability to secure sufficient toughness. If the reheating temperature and the overaging treatment temperature exceed 250°C, there is a disadvantage in that bendability is degraded due to the precipitation and coarsening of a large amount of carbides. The lower limit of the reheating temperature and the overaging treatment temperature is more preferably 110°C, more preferably 120°C, and most preferably 130°C. The upper limit of the above reheating temperature and overaging treatment temperature is more preferably 245℃, more preferably 240℃, and most preferably 235℃.

[0134] If the above overaging treatment time is less than 5 minutes, tempering is not sufficiently performed, and the yield strength may be lowered. If the above overaging treatment time exceeds 12 minutes, carbides may coarsen due to excessive tempering, and bending characteristics may deteriorate. The lower limit of the above overaging treatment time is more preferably 5.5 minutes, more preferably 6.0 minutes, and most preferably 6.5 minutes. The upper limit of the above overaging treatment time is more preferably 11.5 minutes, more preferably 11 minutes, and most preferably 10.5 minutes.

[0135] Meanwhile, in the present invention, it is preferable to control the difference between the over-aging treatment temperature (H) and the secondary cooling end temperature (Tf) to be 100°C or less. If the difference between the over-aging treatment temperature (H) and the secondary cooling end temperature (Tf) exceeds 100°C, carbides may coarsen due to excessive martensite tempering, which may lead to an excessive increase in yield strength and a decline in bendability. [Temper Rolling]

[0136] Furthermore, the present invention may additionally include a step of temper rolling (SPM) the over-aged cold-rolled steel sheet with a rolling force of 500 to 1,000 tons. When the rolling force during temper rolling is less than 500 tons, the load is low, making it difficult to control the surface roughness (Rsk); when it exceeds 1,000 tons, the surface work hardening is severe, which may result in inferior bending characteristics. It is more preferable that the lower limit of the rolling force during temper rolling is 550 tons, and more preferable that it is 600 tons. It is more preferable that the upper limit of the rolling force during temper rolling is 950 tons, and more preferable that it is 900 tons.

[0137] [Tension Leveling]

[0138] Furthermore, the present invention may additionally include a step of tension leveling (T / L) the temper-rolled cold-rolled steel sheet with an elongation of 0.50% or less. The tension leveling is intended to correct the shape of the steel sheet. If the elongation exceeds 0.50% during the tension leveling, work hardening becomes severe, which may result in deteriorated bending characteristics. It is more preferable that the upper limit of the elongation during the tension leveling is 0.35%.

[0139] Meanwhile, after the tension leveling above, the step of forming an electro-galvanized layer on at least one surface of the cold-rolled steel sheet may be additionally included. The present invention does not specifically limit the method of forming the electro-galvanized layer, and any method commonly used in the relevant technical field may be used.

[0140] The present invention will be explained in more detail below through examples. However, the description of these examples is merely for illustrating the implementation of the present invention and does not limit the present invention. This is because the scope of the rights of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0141] (Example)

[0142] A slab having the alloy composition listed in Table 1-2 below was heated to 1200°C, and then the heated slab was finished hot-rolled at 900°C to obtain a hot-rolled steel sheet, which was coiled at 450–700°C. Subsequently, the coiled hot-rolled steel sheet was cold-rolled at a cold reduction rate of 56% to obtain a cold-rolled steel sheet with a thickness of 1.4 mm. Subsequently, a cold-rolled steel sheet was manufactured by continuous annealing, primary cooling, secondary cooling, reheating, overaging treatment, temper rolling, and tension leveling under the conditions listed in Table 3 below. Meanwhile, the conditions listed in Table 3 below were based on the surface temperature of the steel sheet.

[0143] The microstructure and mechanical properties of the cold-rolled steel sheets manufactured in this manner were measured, and the results are shown in Tables 4 and 5 below.

[0144] The microstructure was observed using a scanning electron microscope (SEM) and an optical microscope (OM) at the surface layer of the steel plate (at a position 20 μm in the thickness direction from the surface) and at a position of 1 / 4 t (t: thickness of the steel), and the fraction of each phase was analyzed three times through image analysis to calculate the average value.

[0145] And the R3C / R3S ratio was measured using discharge emission spectroscopy (GDS).

[0146] Yield strength, tensile strength, yield ratio, and total elongation were measured by processing cold-rolled steel sheets into specimens of JIS standard (gauge length width × length: 25 × 50 mm, total specimen length: 200~260 mm) and then performing a tensile test under conditions of a test speed of 28 mm / min.

[0147] The bending workability (R / t) was measured by processing a cold-rolled steel sheet into a specimen with a width of 100 mm × a length of 30 mm, performing a 90° bending test at a test speed of 800 mm / min, checking for the occurrence of cracks in the bending section using a stereoscopic microscope, and calculating the R / t value by dividing the minimum bending radius (R value of the die) at which no cracks occurred by the thickness (mm) of the specimen.

[0148] The hole expansion rate (HER) was measured according to the ISO 16330 standard, and the hole was sheared with a clearance of 12% using a 10mm diameter punch.

[0149] Flatness was measured by measuring the wave height (ΔH) of the edge portion that appeared after cutting a cold-rolled steel sheet into a specimen of 1000 mm in length.

[0150] Spot welding was performed under conditions of Force: 4.95KN, Welding time: 35 cycles, and Holding time: 1 cycle, with a welding current in the range of 4.0 to 12.0kA. Then, the minimum welding current satisfying the minimum nugget diameter (4.25√t: thickness of the steel) and the maximum welding current range where no explosion occurs were derived.

[0151] Steel Type Steel Composition (Weight%) Remarks CSI Mn PS AlCrMoNbTiBNN iCu 10.27 0.10.96 0.03 0.00 11 0.014 0.23 0.16 -0.02 0.002 0.0025 0.05 0.10 Invention Steel 20.26 0.10.87 0.03 0.00 08 0.012 0.25 0.09 -0 .020.00200.00330.060.0830.260.080.870.020.00150.0120.250.200.020.030.00240.00410.040.0940.280.110.960.0250.00120.0150.180.15-0.030.002 00.00300.070.1050.230.131.700.030.00110.0120.090.080.030.030.00200.00410.010.0260.200.251.730.030.00230.0150.100.10-0.020.00180.004 50.020.0470.220.201.750.030.00150.0100.160.050.020.020.00250.00510.010.0580.190.101.800.030.00160.0130.100.10-0.040.00250.00380.030.02

[0152] The remaining components in Table 1 are Fe and unavoidable impurities.

[0153] Steel Type Relationship Formula Satisfaction Remarks Relationship Formula 1 Relationship Formula 2 Relationship Formula 3 1○○○Invention Steel 2○○○3○○○4○○○5XXXComparison Steel 6XXX7XXX8XXX

[0154] In Table 2, [Equation 1] R1 = 0.36 ≤ C + Si / 30 + Mn / 20 + (Cr + Mo) / 5 ≤ 0.44

[0155] [Equation 2] R2 = 0.25 ≤ C + 0.04Mn + 0.03Si ≤ 0.35

[0156] [Relationship 3] R3 = 14 ≤ 10.1169 + 6.1985Mn + 14.3269P - 10.3715S + 11.7499Si + 3.8815Cu + 2.9755Ni + 5.5696Cr ≤ 21

[0157]

[0158]

[0159] In Table 4, F represents ferrite, B represents bainite, M represents fresh martensite, and TM represents tempered martensite.

[0160] As shown in Tables 1 to 4 above, in the case of Invention Examples 1 to 4, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, it can be seen that all target properties are satisfied by securing the mechanical properties, microstructure, welding current range, flatness, etc. that the present invention aims to obtain.

[0161] In contrast, it can be seen that in the case of Comparative Examples 1 to 4, which do not satisfy the alloy composition proposed by the present invention, the tensile strength is inferior and the desired welding current range is not obtained.

[0162] In addition, in the case of Comparative Examples 5 to 12, which do not satisfy the manufacturing conditions proposed by the present invention, it can be seen that the physical properties and shape are inferior because the microstructure fraction, flatness, welding current range, and bendability are not secured.

[0163] More specifically, in the case of Comparative Example 5, the annealing temperature and the first cooling rate were too low, so ferrite was incorporated into the microstructure, and the desired tensile strength could not be secured.

[0164] In the case of Comparative Example 6, the first cooling temperature was too low, so the average cooling rate during the subsequent second cooling was reduced, and the fraction of low-temperature transformation phases other than martensite increased, making it impossible to secure the level of strength targeted by the present invention.

[0165] In the case of Comparative Example 7, the secondary cooling temperature was too high, making it difficult to secure the strength targeted by the present invention.

[0166] In the case of Comparative Example 8, the secondary cooling rate was too low, so soft ferrite transformation occurred during cooling, making it difficult to secure the target strength.

[0167] In the case of Comparative Example 9, the overaging temperature was too low so that tempering was not sufficiently performed, resulting in low yield strength and insufficient toughness.

[0168] In the case of Comparative Example 10, the overaging temperature was too high, which resulted in a problem where the bending workability was poor due to the precipitation and coarsening of a large amount of carbides.

[0169] Meanwhile, FIG. 1 is a diagram schematically showing the distribution of Invention Example 1-4 and Comparative Example 1-4 with respect to the R1 value of Equation 1 and the R2 value of Equation 2 in an embodiment of the present invention.

[0170] FIG. 2 is a diagram schematically showing the distribution of Invention Example 1-4 and Comparative Example 1-4 with respect to the R1 value of Equation 1 and the R3 value of Equation 3 in an embodiment of the present invention.

[0171] Figure 3 is a photograph showing the degree of flatness of Invention Example 1 and Comparative Example 5 in an embodiment of the present invention.

[0172] More specifically, in FIG. 1, it can be seen that in Inventive Example 1-4, both R1 and R2 values ​​satisfy the criteria, but in Comparative Example 1-4, they do not, and in FIG. 2, it can be seen that in Inventive Example 1-4, both R1 and R3 values ​​satisfy the criteria, but in Comparative Example 1-4, they do not. Also, in FIG. 3, it can be confirmed that Inventive Example 1 can secure superior flatness compared to Comparative Example 5.

Claims

1. In wt%, Carbon (C): 0.18–0.32%, Silicon (Si): 0.02–0.40%, Manganese (Mn): 0.5–1.5%, Phosphorus (P): 0.03% or less (excluding 0%), Sulfur (S): 0.0035% or less (excluding 0%), Aluminum (Al): 0.010–0.10%, Chromium (Cr): 0.005–0.8%, Molybdenum (Mo): 0.001–0.35%, Niobium (Nb): 0.05% or less, Titanium (Ti): 0.005–0.05%, Copper (Cu): 0.003–0.3%, Nickel (Ni): 0.003–0.3%, Boron (B): 0.0005–0.005%, Nitrogen (N): 0.01% or less A cold-rolled steel sheet comprising (excluding 0%), the remainder being Fe and other unavoidable impurities, satisfying the following Equations 1 to 3, wherein the microstructure of the core of the steel sheet comprises, in area %, a total of one or more of ferrite and bainite: 5% or less (including 0%), and the remainder being one or more of martensite and tempered martensite, and the microstructure of the surface layer comprises, in area %, a total of one or more of ferrite and bainite: 10% or less (excluding 0%), and the remainder being one or more of martensite and tempered martensite, and wherein, when the R3 value of Equation 3 in the surface layer of the steel sheet is denoted as R3S and the R3 value in the core of the steel sheet is denoted as R3C, R3C / R3S ≥ 0.

45. [Relationship 1] R1 = 0.36 ≤ C + Si / 30 + Mn / 20 + (Cr + Mo) / 5 ≤ 0.44 [Relationship 2] R2 = 0.25 ≤ C + 0.04Mn + 0.03Si ≤ 0.35 [Relationship 3] R3 = 14 ≤ 10.1169 + 6.1985Mn + 14.3269P - 10.3715S + 11.7499Si + 3.8815Cu + 2.9755Ni + 5.5696Cr ≤ 21 (Here, the surface layer refers to the region extending 20㎛ in the thickness direction from the surface of the steel, and the center refers to the region excluding the surface layer. In addition, in the above Equations 1 to 3, each alloy element represents the content (weight%) of that element.) 2. In Paragraph 1, The above cold-rolled steel sheet has a yield strength (YS): 1200 MPa or more, tensile strength (TS): 1470 MPa or more, yield ratio (TS / YS): 0.89 or less, elongation: 4% or more, bendability (R / t): 3.5 or less, and hole expansion (HER): 30% or more.

3. In Paragraph 1, A cold-rolled steel sheet having an edge flatness [ΔH] (mm) of 5mm or less.

4. In Paragraph 1, A cold-rolled steel sheet having an electro-galvanized layer formed on one surface of the above cold-rolled steel sheet.

5. In wt%, Carbon (C): 0.18–0.32%, Silicon (Si): 0.02–0.40%, Manganese (Mn): 0.5–1.5%, Phosphorus (P): 0.03% or less (excluding 0%), Sulfur (S): 0.0035% or less (excluding 0%), Aluminum (Al): 0.010–0.10%, Chromium (Cr): 0.005–0.8%, Molybdenum (Mo): 0.001–0.35%, Niobium (Nb): 0.05% or less, Titanium (Ti): 0.005–0.05%, Copper (Cu): 0.003–0.3%, Nickel (Ni): 0.003–0.3%, Boron (B): 0.0005–0.005%, Nitrogen (N): 0.01% or less A step of preparing a steel slab containing (excluding 0%), the remainder being Fe and other unavoidable impurities, and satisfying the following Equations 1 to 3, and then heating at 1100~1300℃; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above-mentioned heated steel slab at Ar3~Ar3+120℃; a step of coiling the above-mentioned hot-rolled steel sheet at Ms~700℃; A step of obtaining a cold-rolled steel sheet by cold-rolling the above-mentioned coiled hot-rolled steel sheet at a cold reduction rate of 45~70%; A step of continuously annealing the above cold-rolled steel sheet at Ac3+20℃ to Ac3+80℃; A step of first cooling the above continuously annealed cold-rolled steel sheet to a first cooling end temperature of 670 to 750℃ at an average cooling rate of 1 to 6℃ / s; A step of secondarily cooling the above first cooled cold-rolled steel sheet to a second cooling end temperature (Tf) of 110 to 300℃ at an average cooling rate of 30 to 300℃ / s; and The method includes the step of reheating the above secondary cooled cold-rolled steel sheet to an over-aging treatment temperature (H) of 100 to 250°C, and then over-aging treatment for 5 to 12 minutes. A method for manufacturing a cold-rolled steel sheet, wherein the Mf-Tf temperature difference during the second cooling is controlled to be 130℃ or less, and the over-aging treatment temperature (H) - second cooling end temperature (Tf) is controlled to be 100℃ or less. [Relationship 1] R1 = 0.36 ≤ C + Si / 30 + Mn / 20 + (Cr + Mo) / 5 ≤ 0.44 [Relationship 2] R2 = 0.25 ≤ C + 0.04Mn + 0.03Si ≤ 0.35 [Relationship 3] R3 = 14 ≤ 10.1169 + 6.1985Mn + 14.3269P - 10.3715S + 11.7499Si + 3.8815Cu + 2.9755Ni + 5.5696Cr ≤ 21 (In the above equations 1 to 3, each alloy element represents the content (weight%) of that element) 6. In Paragraph 5, A method for manufacturing a cold-rolled steel sheet, further comprising the step of temper-rolling the above-mentioned over-aged cold-rolled steel sheet with a rolling force of 500 to 1000 tons.

7. In Paragraph 6, A method for manufacturing a cold-rolled steel sheet, further comprising the step of tension leveling the above-mentioned temper-rolled cold-rolled steel sheet to an elongation of 0.50% or less.

8. In Paragraph 7, A method for manufacturing a cold-rolled steel sheet, further comprising the step of forming an electro-galvanized layer on the surface of the tension-leveled cold-rolled steel sheet.

9. In Paragraph 5, A method for manufacturing a cold-rolled steel sheet, wherein the R3 value of the above relationship 3 at the surface layer of the above cold-rolled steel sheet is denoted as R3S and the R3 value at the center of the steel sheet is denoted as R3C, satisfying R3C / R3S ≥ 0.

45. (Here, the surface layer refers to the region extending up to 20㎛ in the thickness direction from the surface of the steel, and the center refers to the region excluding the surface layer.)

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