Steel sheet and method for manufacturing same

Abstract

The present invention relates to a steel material used as a steel material for a vehicle reinforcement, such as a bumper beam and a sill side beam, or a steel material for protecting an electric vehicle battery case, such as a side frame and a cross member, and a method for manufacturing same.

Description

Steel plate and method of manufacturing the same

[0001] The present invention relates to steel used as automotive reinforcement material, such as bumper beams and sill side beams, or steel used for protecting electric vehicle battery cases, such as side frames and cross members, and a method for manufacturing the same.

[0002] For steel materials used for reinforcing components related to the crash safety of automobile passengers or for protecting electric vehicle battery cases, high processing characteristics, particularly excellent bendability, are required when processed using cold forming techniques, and at the same time, ultra-high strength is required. To this end, research is actively being conducted on ultra-high strength steel with a tensile strength of 1470 MPa or higher having a single martensitic phase and methods for manufacturing the same.

[0003] Recently, the Hot Press Forming (HPF) method has been developed, which involves heating and forming a material using a die in a high-temperature environment conducive to forming, followed by water cooling to secure the required strength and processability. Since this method allows for the achievement of high strength relative to the same thickness, it is widely used in the manufacturing of parts. However, the HPF method has the disadvantage of requiring excessive equipment investment and increased process costs. Consequently, there is a need to develop materials for cold stamping and roll forming. Specifically, there is a demand for the development of steel sheets that are suitable for cold stamping and roll forming, possess ultra-high strength and a high yield ratio to ensure impact performance, and exhibit excellent bending characteristics and shape for part forming.

[0004] Patent Document 1 is a prior art of such a method. Patent Document 1 describes an ultra-high strength cold-rolled steel sheet utilizing a martensite single-phase structure comprising, in mass%, 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 comprising Ti: 0.005~0.1%, Nb: 0.005~0.1%, and a total of 0.005~0.1%. To manufacture the same, a process is disclosed in which the sheet is heated and stored in a temperature range above the Ae3 transformation point and below 900°C, then rapidly cooled to below 200°C at an average cooling rate of 300°C / s or more, and subsequently tempered at below 250°C. 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.

[0005] Patent Document 2 relates to a thin steel sheet having a composition in mass% including 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, in the case of Patent Document 2, there is a problem that shape defects occur due to rapid cooling after annealing.

[0006] Patent Document 3 comprises, in weight percent, C: 0.2–0.4%, Si: 0.5% or less (excluding 0%), Mn: 1.0–2.0%, P: 0.03% or less (excluding 0%), S: 0.015% or less (excluding 0%), Al: 0.1% or less (excluding 0%), Cr: 0.5% or less (excluding 0%), Mo: less than 0.2% (excluding 0%), Ti: 0.1% or less (excluding 0%), Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; the microstructure consists of a tempered martensite single-phase structure or a mixed structure of martensite and tempered martensite, and the microstructure contains F per unit area of ​​45㎛ × 45㎛ HAGB A is 60% or more of the area, and L HAGB This invention relates to an ultra-high strength cold-rolled steel sheet with excellent hole expansion properties of 8 mm or more. However, it does not provide bending properties that are essential for forming ultra-high strength steel parts, improving impact characteristics and hydrogen embrittlement.

[0007] Meanwhile, to manufacture ultra-high-strength steel with a tensile strength of 1470 MPa or higher, it is essential to introduce martensite or some bainite. However, the steel is prone to brittle fracture caused by hydrogen remaining within the material or introduced from the outside, a phenomenon referred to as hydrogen embrittlement. Hydrogen embrittlement manifests at a strength lower than the fracture strength, meaning the material can fracture due to hydrogen embrittlement even under applied stresses that are very small compared to the actual fracture strength of the material. In particular, sensitivity to hydrogen embrittlement increases as the strength of the steel increases. Furthermore, since resistance to hydrogen embrittlement improves with superior bending characteristics even with the same initial amount of hydrogen in the steel, improvement in bending characteristics is necessary.

[0008] (Patent Document 1) Japanese Patent Publication No. JP 2010-248565

[0009] (Patent Document 2) Japanese Patent Publication No. JP 2020-019992

[0010] (Patent Document 3) Korean Patent Publication No. 10-2023-0043267

[0011] According to one embodiment of the present invention, a steel plate having excellent bending characteristics and shape while securing high strength and a method for manufacturing the same may be provided.

[0012] 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.

[0013] A steel sheet according to one embodiment of the present invention comprises, in weight percent, carbon (C): 0.16~0.33%, silicon (Si): 0.02~0.60%, manganese (Mn): 0.3~2.3%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.0050% or less (excluding 0%), aluminum (Al): 0.005~0.08%, chromium (Cr): 0.005~0.50%, boron (B): 0.0005~0.005%, nitrogen (N): 0.01% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities.

[0014] The surface layer of the steel plate includes a decarburization layer, and the depth (Dave.) of the decarburization layer is 20 to 75 μm, and

[0015] When the total number fraction of the region where the KAM (Kernel Average Misorientation) value is 0~5.0° is set to 1%, the number fraction of the region where the KAM value is 0~1° at the center of the steel plate (KAM0~1°) is 0.18~0.35%, and

[0016] The average value of KAM (KAMave.) is 1.30–1.70°, and

[0017] The average GND value (GNDave.) is 260~305 (×10 12 m -2 ) and,

[0018] The following [Relation 6] can be satisfied.

[0019] [Relationship 6]

[0020] 1.2 ≤ (Cs*KAMave.*GNDave.) / Dave. ≤ 6.0 (%·°·10 12 m -2 ·㎛ -1 )

[0021] (Cs = C + 0.03Mn + 0.02Si, each alloying element symbol represents the content (weight%) of each alloying element)

[0022] The above steel plate can satisfy the following [Relationship 5].

[0023] [Relationship 5]

[0024] 95 ≤ Cs*KAMave.*GNDave. ≤ 200, (%·°·10 12 m -2 )

[0025] (Cs = C + 0.03Mn + 0.02Si, each alloying element symbol represents the content (weight%) of each alloying element)

[0026] The above steel plate may further include one or more of molybdenum (Mo): 0.001~0.35%, niobium (Nb): 0.003~0.05%, titanium (Ti): 0.005~0.25%, copper (Cu): 0.003~0.3%, and nickel (Ni): 0.003~0.3%.

[0027] The above steel plate may satisfy one or more of the following [Equation 1] to [Equation 4].

[0028] [Relationship 1]

[0029] 50% ≤ H = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb + 10000B + 14.5Ni + 9.6Cu ≤220%

[0030] [Relationship 2]

[0031] 0.20% ≤ Cs = C + 0.03Mn + 0.02Si ≤ 0.45%

[0032] [Relationship 3]

[0033] 0.0013 ≤ Cs / H ≤ 0.008

[0034] [Relationship 4]

[0035] M = Cr + Mo + Ni + Cu ≤ 0.70%

[0036] (In the above Equations 1 to 4, each alloy element symbol represents the content (weight%) of each element, and the units in Equations 1, 2, and 4 are weight%)

[0037] The microstructure of the above steel plate may contain at least 95% of one or more of martensite and tempered martensite.

[0038] The above steel plate may have a yield strength (YS) perpendicular to the rolling direction of 1200 MPa or more and a tensile strength (TS) perpendicular to the rolling direction of 1470 MPa or more.

[0039] The above steel plate may have a Total_EL (total elongation, T_EL) of 4% or more, a Uniform_EL (uniform elongation, U_EL) of 2.5% or more, and a Post_EL (T_EL - U_EL) of 2~6%.

[0040] The maximum angle of three-point bending of the above steel plate may be 60 to 100°.

[0041] The above steel plate has a Post_EL × 3-point bending angle of 180~300(%·°) and (Post_EL × 3-point bending angle) / TS is 0.10~0.30(%·°·MPa -1 It can be.

[0042] The flatness of the above steel plate may be 10㎛ or less.

[0043] The thickness of the above steel plate may be 0.6 to 2.5 mm.

[0044] The above steel plate may include an electro-galvanized layer formed on at least one surface.

[0045] A method for manufacturing a steel sheet according to another embodiment of the present invention comprises the step of providing a cold-rolled steel sheet comprising, in weight percent, carbon (C): 0.16~0.33%, silicon (Si): 0.02~0.60%, manganese (Mn): 0.3~2.3%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.0050% or less (excluding 0%), aluminum (Al): 0.005~0.08%, chromium (Cr): 0.005~0.50%, boron (B): 0.0005~0.005%, nitrogen (N): 0.01% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities;

[0046] A step of annealing the above cold-rolled steel sheet in a temperature range of Ac3+20℃ to Ac3+90℃;

[0047] A step of primary cooling at an average cooling rate of 0.5 to 6℃ / s to a primary cooling end temperature (T1) of 650 to 800℃ after the above annealing;

[0048] A step of secondary cooling at an average cooling rate (CR) of 40 to 300℃ / s to a secondary cooling end temperature (T2) of 40 to 250℃ after the above primary cooling;

[0049] After the above secondary cooling, a step of reheating to an overaging treatment temperature (OAT) of 140–260°C and overaging treatment for 3–14 minutes; and

[0050] A step of tension leveling (T / L) with an elongation rate of 0.05~0.55% after the above overaging treatment;

[0051] It includes, and the secondary cooling may be a method satisfying the following [Equation 7] and [Equation 8].

[0052] [Relationship 7]

[0053] Mf-T2 ≥ 20℃

[0054] (Mf is the martensite transformation end temperature (°C), and T2 is the secondary cooling end temperature.)

[0055] [Relationship 8]

[0056] 0.0005 ≤ TL_EL / CR ≤ 0.006 (%ㆍ(℃ / sec) -1 )

[0057] (TL_EL is the tension leveling elongation (%), and CR is the cooling rate (°C / sec) between the first cooling end temperature (T1) and the second cooling end temperature (T2))

[0058] The above steel plate may further include one or more of molybdenum (Mo): 0.001~0.35%, niobium (Nb): 0.003~0.05%, titanium (Ti): 0.005~0.25%, copper (Cu): 0.003~0.3%, and nickel (Ni): 0.003~0.3%.

[0059] The above cold-rolled steel sheet may include one or more of the following [Equation 1] to [Equation 4].

[0060] [Relationship 1]

[0061] 50% ≤ H = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb + 10000B + 14.5Ni + 9.6Cu ≤220%

[0062] [Relationship 2]

[0063] 0.20% ≤ Cs = C + 0.03Mn + 0.02Si ≤ 0.45%

[0064] [Relationship 3]

[0065] 0.0013 ≤ Cs / H ≤ 0.008

[0066] [Relationship 4]

[0067] M = Cr + Mo + Ni + Cu ≤ 0.70%

[0068] (In the above Equations 1 to 4, each alloy element symbol represents the content (weight%) of each element, and the units in Equations 1, 2, and 4 are weight%)

[0069] The method for preparing the above cold-rolled steel sheet is,

[0070] A step of heating the steel slab to a temperature range of 1100~1300℃;

[0071] A step of hot rolling the above heated steel slab by finishing hot rolling at Ar3~Ar3+120℃;

[0072] A step of coiling at a temperature range of Ms~650℃ after the above hot rolling; and

[0073] Cold rolling step with a cold reduction rate of 45~70%

[0074] It may include.

[0075] The above annealing can be performed for 50 to 250 seconds.

[0076] The dew point temperature of the atmosphere during the above annealing can be -30 to 20℃.

[0077] The atmosphere during the above annealing may contain moist nitrogen (N2+H2O).

[0078] After the above overaging treatment, temper rolling can be performed.

[0079] The method may further include the step of forming an electro-galvanized layer on the surface of the steel plate.

[0080] According to the present invention, it is possible to provide a steel plate having an ultra-high strength of 1470 MPa or more, which has excellent shape and excellent bending characteristics.

[0081] The various and beneficial advantages and effects of the present invention are not limited to those described above and will be more easily understood in the process of explaining specific embodiments of the present invention.

[0082] FIG. 1 is a schematic diagram showing a flatness measurement method for securing shape characteristics in an embodiment of the present invention.

[0083] Figure 2 shows the number fraction (%) of KAM (Kernel Average Misorientation) for Invention Example 1 and Comparative Example 1 in an embodiment of the present invention.

[0084] Figure 3 shows the number fraction (%) of GND (Geometrically Necessary Dislocations) for Invention Example 1 and Comparative Example 1 in an embodiment of the present invention.

[0085] The embodiments of the present invention are provided to more fully explain the invention to those with average knowledge in the art. Meanwhile, the shapes and sizes of the elements in the drawings may be exaggerated for clearer explanation.

[0086] 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.

[0087] 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.

[0088] 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.

[0089] Hereinafter, a steel plate according to one embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. Unless otherwise noted, the unit % in the alloy composition described below refers to weight % (wt.%).

[0090] Carbon (C): 0.16~0.33%

[0091] The above-mentioned 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 content of C is less than 0.16%, it may be difficult to secure the strength targeted in the present invention. If the content of C exceeds 0.33%, the strength increases rapidly, which may result in inferior elongation. Additionally, coarse carbides may form, leading to reduced hydrogen embrittlement resistance and inferior weldability. Therefore, it is preferable for the content of C to be in the range of 0.16 to 0.33%. It is more preferable for the lower limit of the C content to be 0.17%. It is more preferable for the upper limit of the C content to be 0.32%.

[0092] Silicon (Si): 0.02~0.60%

[0093] The above Si plays a role in suppressing the formation of carbides and controlling the size of carbides 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 0.60%, ferrite may be formed after continuous annealing and cooling, which may weaken the strength of the steel. Furthermore, as Si is an element that increases resistivity, resistance spot weldability may be compromised. Therefore, it is preferable that the Si content be between 0.02% and 0.60%. It is more preferable that the lower limit of the Si content be 0.03%. It is more preferable that the upper limit of the Si content be 0.50%.

[0094] Manganese (Mn): 0.3~2.3%

[0095] The above Mn is an element added to ensure strength. If the Mn content is less than 0.3%, 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 level of strength targeted in the present invention. If the Mn content exceeds 2.3%, 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, during steelmaking / continuous casting operations, Mn-based segregation zones occur along the longitudinal direction of the slab, degrading bendability and hydrogen embrittlement resistance. That is, as Mn is segregated along the thickness direction and manganese bands (Mn bands) are formed within the slab, there is a problem in that cracks occur during continuous casting and the occurrence of defects increases during the rolling process. Therefore, it is desirable for the Mn content to be in the range of 0.3 to 2.3%. It is more preferable that the lower limit of the Mn content is 0.4%. It is more preferable that the upper limit of the Mn content is 2.2%.

[0096] Phosphorus (P): 0.03% or less (excluding 0%)

[0097] The above-mentioned P is an impurity element contained in steel. If the P content exceeds 0.03%, weldability deteriorates, segregation at grain boundaries occurs easily, leading to intergranular embrittlement, and the grain boundaries become susceptible to fracture by hydrogen, thereby adversely affecting hydrogen embrittlement resistance. Meanwhile, while 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 P content be 0.03% or less (excluding 0%). It is more preferable that the P content be 0.02% or less.

[0098] Sulfur (S): 0.0050% or less (excluding 0%)

[0099] The above-mentioned S is an impurity element included in steel, similar to P. If the content of S exceeds 0.0050%, it may impair ductility and weldability, and a large amount of MnS precipitates may be formed, resulting in inferior bendability and hydrogen embrittlement resistance. 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 content of S be 0.0050% (excluding 0%) or less. It is more preferable that the content of S be 0.0030% or less, and even more preferable that it be 0.0020% or less.

[0100] Aluminum (Al): 0.005~0.08%

[0101] The above Al may be added to remove oxygen from the molten steel. If the Al content is less than 0.005%, deoxidation is not sufficiently achieved, which impairs the cleanliness of the steel. If the Al content exceeds 0.08%, 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 0.005 to 0.08%. It is more desirable for the lower limit of the Al content to be 0.01%. It is more desirable for the upper limit of the Al content to be 0.07%.

[0102] Chrome (Cr): 0.005~0.50%

[0103] The above 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. If the Cr content exceeds 0.50%, 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 0.50%. It is more preferable for the lower limit of the Cr content to be 0.007%. It is more preferable for the upper limit of the Cr content to be 0.40%.

[0104] Molybdenum (Mo): 0.001~0.35%

[0105] The above Mo is an element that exhibits the effects of improving the quenching properties of steel, generating Mo-based fine carbides that serve as hydrogen trap sites, and improving delayed fracture resistance by strengthening austenite grain boundaries to suppress hydrogen intrusion, similar to B. If the content of the above Mo is less than 0.001%, it may be difficult to sufficiently obtain the aforementioned effects. If the content of the above Mo 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 preferable for the content of the above Mo to be in the range of 0.001% to 0.35%. It is more preferable for the lower limit of the above Mo content to be 0.002%. It is more preferable for the upper limit of the above Mo content to be 0.25%.

[0106] Niobium (Nb): 0.003~0.05%

[0107] The above Nb is an element that segregates at austenite grain boundaries, suppresses the coarsening of austenite grains during the continuous annealing process, and contributes to strength improvement by forming fine precipitates. If the Nb content is less than 0.003%, sufficient effects of austenite grain refinement and precipitation strengthening cannot be obtained. 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 0.003 to 0.05%. It is more desirable for the lower limit of the Nb content to be 0.004%. It is more desirable for the upper limit of the Nb content to be 0.04%.

[0108] Titanium (Ti): 0.005~0.25%

[0109] The above 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 0.25%, 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 0.25%. 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 0.15%.

[0110] Boron (B): 0.0005~0.005%

[0111] The above B is an element that inhibits ferrite formation. Accordingly, the present invention has the advantage of inhibiting ferrite formation during cooling after continuous annealing, and strengthens the austenite grain boundaries to suppress hydrogen intrusion, thereby increasing resistance to hydrogen embrittlement. If the content of the above B is less than 0.0005%, there is no hardenability effect at all, so not only can the strength targeted by the present invention not be secured, but excessive ferrite is formed in the surface layer, leading to a problem of inferior bendability. If the content of the above B exceeds 0.005%, ductility may be significantly reduced. Therefore, it is preferable that the content of the above B be in the range of 0.0005 to 0.005%. It is more preferable that the lower limit of the above B content be 0.0007%. It is more preferable that the upper limit of the above B content be 0.004%.

[0112] Nitrogen (N): 0.01% or less (excluding 0%)

[0113] The above N is an impurity element, and if its content exceeds 0.01%, it significantly increases the risk of cracking during continuous casting due to AlN formation, etc. Considering cases where the above N content is unavoidably included in the manufacturing process, 0% is excluded. 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.

[0114] Copper (Cu): 0.003~0.3%

[0115] The above Cu improves corrosion resistance in the operating environment of automobiles and also has the effect of suppressing hydrogen intrusion into the steel plate by coating the surface of the steel plate 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.

[0116] Nickel (Ni): 0.003~0.3%

[0117] Like Cu, Ni is an element that improves corrosion resistance. For this reason, it is desirable to include 0.003% or more 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 to keep it at 0.2% or less, and even more desirable to keep it at 0.08% or less.

[0118] The remaining component is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. As these impurities are known to any skilled person in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0119] The steel plate of the present invention may satisfy one or more of the following [Equation 1 to Equation 4].

[0120] [Relationship 1]

[0121] 50% ≤ H = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb + 10000B + 14.5Ni + 9.6Cu ≤220%

[0122] [Relationship 2]

[0123] 0.20% ≤ Cs = C + 0.03Mn + 0.02Si ≤ 0.45%

[0124] [Relationship 3]

[0125] 0.0013 ≤ Cs / H ≤ 0.008

[0126] [Relationship 4]

[0127] M = Cr + Mo + Ni + Cu ≤ 0.70%

[0128] (In the above Equations 1 to 4, each alloy element symbol represents the content (weight%) of each element, and the units in Equations 1, 2, and 4 are weight%.)

[0129] The above Equation 1 is a compositional relationship related to hardenability that is effective in securing the microstructure targeted in the present invention. If the value of H is less than 50%, the soft ferrite and bainite structures transform during cooling, making it difficult to secure the target strength. If the value of H exceeds 220%, the strength becomes excessively high, making it difficult to secure the target elongation, which may cause processing cracks during forming and increase the cost of the ferroalloy. Therefore, it is desirable for the value of H to have a range of 50 to 220%. It is more desirable for the lower limit of the value of H to be 120%. It is more desirable for the upper limit of the value of H to be 200%.

[0130] The above Equation 2 is a compositional relationship effective for increasing strength. If the value of Cs is less than 0.20%, the strength of the martensitic structure is low, making it difficult to secure the target strength. If the value of Cs exceeds 0.45%, 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 Cs to have a range of 0.20 to 0.45%. It is more desirable for the lower limit of the H value to be 0.25%. It is more desirable for the upper limit of the H value to be 0.40%.

[0131] The above Equation 3 is an effective compositional relationship for simultaneously securing the microstructure and strength targeted in the present invention. If the Cs / H value is less than 0.0013, it is a region where the H value is high and the Cs value is low; while the hardenability is high and the formation of a hard structure (martensite, bainite) is easy, the inherent strength of the martensite is low, making it difficult to secure the target strength. If the Cs / H value exceeds 0.008, it is a region where the H value is low and the Cs value is high; although the cost of the ferroalloy is lowered by adding expensive elements (Cr, Mo, etc.), the addition of large amounts of C, Mn, and Si components results in excessively high inherent strength of the martensite, making it difficult to secure the target elongation and bending. Therefore, it is desirable for the Cs / H value to have a range of 0.0013 to 0.008. It is more preferable for the lower limit of the Cs / H value to be 0.0014. It is more preferable that the upper limit of the above Cs / H value be 0.006.

[0132] The above equation 4 is a compositional equation for Cr, Mo, Ni, and Cu. Since these elements are expensive, adding more of these elements is advantageous in terms of securing hardenability and corrosion resistance, but it may increase manufacturing costs due to the increase in the cost of ferroalloys. Therefore, it is desirable to have 0.70% or less, and even more desirable to have 0.65% or less.

[0133] The surface layer of the steel plate may include a decarburization layer. For example, the surface layer refers to an area extending from the surface to 75 μm in the thickness direction, and the decarburization layer refers to an area containing C at a lower content than the C content contained in the steel plate, and for example, may be an area with an average C content of 95% or less relative to the center of the steel plate. The decarburization layer does not always coincide with the surface layer.

[0134] Based on the cross-section, the thickness of the decarburization layer may be in the range of 20 to 75 μm in the thickness direction from the surface of the steel plate, and preferably in the range of 25 to 70 μm. If the decarburization layer is too thin, such that the region is less than 25 μm from the surface, it is difficult to secure a sufficient soft phase, making it difficult to secure the target bending characteristics (R / t). On the other hand, if the thickness exceeds 75 μm and is excessively thick, an excessive soft phase is formed, making it difficult to secure the target strength and potentially resulting in inferior fatigue characteristics.

[0135] It is preferable that the microstructure of the center of the above steel plate consists of at least one type of martensite and tempered martensite. This enables the securing of ultra-high strength with a tensile strength of 1470 MPa or higher. As a preferred example, the area fraction may be 95% or more (including 100%). The center includes the entire region excluding the decarburized layer.

[0136] It has been confirmed that the composition and fraction of the microstructure are highly correlated with dislocation density, thereby affecting physical properties such as the strength of the steel sheet. As an example of a method to verify the characteristics of the aforementioned dislocation density, the steel sheet can be measured using Electron Backscattered Diffraction (EBSD) equipment, and the Kernel Average Misorientation (KAM) and Geometrically Necessary Dislocations (GND) values ​​related to dislocation density can be quantified using Orientation Imaging Microscopy (OIM) Analysis software provided by TSL. The above KAM refers to the orientation difference; more specifically, KAM is the average value of the amount of crystal rotation (crystal orientation difference) between the target measurement point and surrounding measurement points. A higher value indicates the presence of a highly deformed structure (martensite) within the crystal, signifying higher strength. In addition, GND is a value that can be calculated from the KAM value and quantifies the dislocation density using TSL's OIM Analysis software; a higher value indicates a higher dislocation density.

[0137] When measuring the steel sheet of the present invention using EBSD, it is preferable that the number fraction of the region with a KAM value of 0 to 1° in the center (KAM0~1°) be 0.18~0.35%, when the total number fraction of the region with a KAM value of 0~5.0° is set to 1%. If the KAM0~1° in the center exceeds 0.35%, it may be difficult to secure the target tensile strength because it contains a large amount of microstructure with low deformation. If the KAM0~1° in the center is 0.18% or less, the strength is high due to a microstructure composition with high dislocation density, which may result in inferior elongation and shape. Therefore, it is preferable that the KAM0~1° be 0.18~0.35%, and more preferable that it be 0.19~0.34%.

[0138] It is preferable that the average value of KAM (KAMave.) be 1.30 to 1.70°. If KAMave. is 1.30° or less, it may be difficult to secure the target tensile strength because it contains a large amount of microstructure with low deformation. If KAMave. exceeds 1.70°, the strength is high due to the microstructure composition with high dislocation density, but the elongation and shape may be inferior. Therefore, it is preferable that KAMave. be 1.30 to 1.70°, and more preferable that it be 1.32 to 1.68°.

[0139] In addition, the average GND value (GNDave.), which quantifies the potential density, is 260~305 (×10 12 m -2 It is desirable that ) GNDave. is 260(×10 12 m -2 If it is less than ), it contains a large amount of microstructure with low deformation, making it difficult to secure the target tensile strength. GNDave. is 305(×10 12 m -2 If it exceeds ), the high dislocation density results in a high strength, which may lead to inferior elongation and shape. Therefore, GNDave. should be 260~305(×10 12 m -2 It is desirable that ) is 265~300(×10 12 m -2 It is more desirable to be ).

[0140] The following [Equation 5] is the Cs component relationship related to strength and the product of KAMave. and GNDave. related to dislocation density; the higher this value, the higher the strength, and to secure the target material properties, 95~200(%·°·10 12 m -2 It is desirable that this value be 95(%ㆍ°ㆍ10 12 m -2 If it is less than ), it is difficult to secure the target strength, and 200(%ㆍ°ㆍ10 12 m -2 If it exceeds ), the strength becomes too high, making it difficult to secure the target elongation and bending.

[0141] [Relationship 5]

[0142] 95 ≤ Cs*KAMave.*GNDave. ≤ 200, (%·°·10 12 m -2 )

[0143] (Cs = C + 0.03Mn + 0.02Si, each alloying element symbol represents the content (weight%) of each alloying element)

[0144] The following [Equation 6] is a correlation equation between the component, dislocation density, and decarburization layer depth for simultaneously securing strength and bending, ranging from 1.2 to 6.0 (%ㆍ°ㆍ10 12 m -2 ㆍ㎛ -1 It is desirable that this value be 1.2(%ㆍ°ㆍ10 12 m -2 ㆍ㎛ -1 If it is less than ), it is difficult to secure the target strength, and 6.0(%ㆍ°ㆍ10 12 m -2 ㆍ㎛ -1 If it exceeds ), the depth of the decarburization layer is small, making it difficult to secure the target bend.

[0145] [Relationship 6]

[0146] 1.2 ≤ (Cs*KAMave.*GNDave.) / Dave. ≤ 6.0 (%·°·10 12 m -2 ·㎛ -1 )

[0147] (Cs = C + 0.03Mn + 0.02Si, each alloying element symbol represents the content (weight%) of each element, the decarburization depth Ave. is the average value of the top and bottom of the cross-section in the thickness direction)

[0148] The steel sheet of the present invention may have a yield strength (YS) perpendicular to the rolling direction of 1200 MPa or more, more preferably 1250 MPa or more, and a tensile strength (TS) perpendicular to the rolling direction of 1470 MPa or more, more preferably 1500 MPa or more.

[0149] The above steel plate may have a Total_EL (total elongation, T_EL) of 4% or more, more preferably 5% or more, and a Uniform_EL (uniform elongation, U_EL) of 2.5% or more, more preferably 3.0% or more. In addition, Post_EL is the value obtained by subtracting U_EL from T_EL (T_EL - U_EL), which is 2~6%, and more preferably 2.5~5.5%.

[0150] The above steel plate has a maximum three-point bending angle of 60 to 100°, and preferably 65 to 95°.

[0151] The above steel plate may have a Post_EL X 3-point bending angle of 180 to 300 (%·°), more preferably 190 to 290 (%·°), and (Post_EL X 3-point bending angle) / TS is 0.10 to 0.30 (%·°·MPa -1 ) and more preferably 0.13~0.28(%·°·MPa -1 It can be.

[0152] The above steel plate has a flatness of 10㎛ or less, and more preferably 8㎛ or less.

[0153] The thickness of the above steel plate may be 0.6 to 2.5 mm. The lower limit of the thickness of the above cold-rolled steel plate is more preferably 0.7 mm, and more preferably 0.8 mm. The upper limit of the thickness of the above cold-rolled steel plate is more preferably 2.4 mm, and more preferably 2.3 mm.

[0154] The above steel plate may include an electro-galvanized layer formed on at least one surface. In the present invention, the type of electro-galvanized layer is not specifically limited, and any type of electro-galvanized layer commonly used in the relevant technical field may be formed.

[0155] Hereinafter, a method for manufacturing a steel plate according to one embodiment of the present invention will be described.

[0156] The above manufacturing method can be produced by providing a cold-rolled steel sheet satisfying the alloy composition and [Equations 1 to 4] described above, and by performing annealing heat treatment, cooling, overaging treatment, and tension leveling. This will be explained in more detail below.

[0157] Provision of cold-rolled steel sheets

[0158] As an example of a method for providing the above-described cold-rolled steel sheet, a steel slab satisfying the alloy composition and [Equations 1 to 4] described above is heated at 1100 to 1300°C. The steel slab heating process is performed to facilitate the subsequent hot rolling process and to sufficiently obtain the target physical properties of the steel sheet. If the slab heating temperature is below 1100°C, a problem arises in which the hot rolling load increases rapidly. If the slab heating temperature exceeds 1300°C, the amount of surface scale increases, thereby reducing the yield of the material. The lower limit of the slab heating temperature is more preferably 1110°C, more preferably 1120°C, and most preferably 1130°C. The upper limit of the slab heating temperature is more preferably 1290°C, more preferably 1280°C, and most preferably 1270°C.

[0159] Meanwhile, the steel slab may be refined and cast through a converter process or an electric furnace process.

[0160] 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.

[0161] In the electric furnace process, molten steel can be 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 including a large amount of scrap in this manner, elements not intended in this invention may be included in the molten steel within permissible limits.

[0162] 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.

[0163] 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, the above Ar3 refers to the temperature at which austenite begins to transform into ferrite upon cooling, and can be calculated using the following Equation 1.

[0164] [Equation 1] Ar3(°C) = 910 - 203√C + 44.7Si + 31.5Mo

[0165] (In Formula 1 above, each alloying element symbol represents the content (weight%) of each element.)

[0166] Subsequently, the hot-rolled steel sheet is coiled at Ms~650℃. If the coiling temperature (CT) exceeds 650℃, 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 deteriorate. Meanwhile, it is desirable to manage the coiling temperature at a low level to ensure material uniformity across the entire length and width by forming the microstructure of the hot-rolled steel sheet into 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 make actual production impossible due to the increased rolling load during the subsequent cold rolling process. It is more preferable that the lower limit of the coiling temperature be Ms+10℃. It is more preferable that the upper limit of the coiling temperature be 600℃. Ms refers to the temperature at which austenite begins to transform into martensite upon cooling, and can be calculated using Equation 2 below.

[0167] [Equation 2] Ms(℃) = 539 - 423C - 30.4Mn - 7.5Si + 30Al - 17.7Ni - 12.1Cr - 7.5Mo

[0168] (In Equation 2 above, each alloying element symbol represents the content (weight%) of each element.)

[0169] After the above coiling, the material 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.

[0170] Subsequently, 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%, it is difficult to secure the thickness desired in the present invention, and furthermore, due to the persistence of crystal grains formed during hot rolling, austenite may be generated during annealing heat treatment, potentially affecting the final physical properties. 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. Additionally, 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%.

[0171] Annealing

[0172] The above cold-rolled steel sheet is annealed at Ac3+20℃ to Ac3+90℃. A continuous annealing process may be used for the annealing. If the annealing temperature is less than Ac3+20℃, a mixed grain structure may be formed as two-phase annealing occurs over the entire length of the steel sheet instead of a single-phase annealing, and it may be difficult to secure a sufficient decarburization layer on the surface layer, making it difficult to secure the physical properties targeted by the present invention. If the annealing temperature exceeds Ac3+90℃, equipment trouble may occur due to overloading of the annealing furnace. It is more preferable that the lower limit of the annealing temperature be Ac3+30℃. It is more preferable that the upper limit of the annealing temperature be Ac3+80℃. Meanwhile, Ac3 refers to the temperature at which the austenite transformation is completed upon heating and can be calculated using Equation 3 below.

[0173] [Equation 3] Ac3(℃) = 900 - 206C + 26.2Si - 25.0Mn - 12.3Cr + 9.12Mo + 50.2Nb + 148Ti - 131B

[0174] (In Equation 3 above, each alloying element symbol represents the content (weight%) of each element.)

[0175] The above annealing can be performed for 50 to 250 seconds. If the annealing time is less than 50 seconds, it is difficult to secure a single-phase austenite structure, and undissolved carbides remain, causing coarsening, which may result in inferior bending and hydrogen embrittlement resistance, and also prevent the formation of a sufficient decarburization layer. If the above annealing time exceeds 250 seconds, the austenite size becomes coarsened, which has the disadvantage of making it difficult to secure strength. It is more preferable that the lower limit of the above annealing time is 70 seconds. It is more preferable that the upper limit of the above annealing time is 240 seconds.

[0176] In the present invention, when performing annealing treatment under the conditions described above, it is preferable to manage the dew point temperature inside the annealing furnace to -30 to 20°C, and more preferably to -25 to 15°C. By controlling the dew point temperature in this way, a decarburized layer can be formed on the surface of the steel during the annealing process. Typically, the dew point inside the annealing furnace is about -40 to -50°C, and the atmosphere inside the furnace can be controlled by adjusting the hydrogen concentration using humid nitrogen (Nx Gas), DX Gas, etc. For example, if humid nitrogen (N2+H2O) is introduced to raise the dew point temperature above -30°C, the oxygen partial pressure increases, and carbon (C) in the steel meets oxygen (O) inside the annealing furnace to be released as CO gas, causing decarburization to occur on the surface layer. If the dew point temperature inside the annealing furnace is below -30°C, a sufficient decarburized layer cannot be formed on the surface of the steel, whereas if it exceeds 20°C, there are problems with reduced equipment lifespan and productivity.

[0177] Primary cooling

[0178] The above annealed cold-rolled steel sheet is first cooled to a first cooling end temperature (T1) of 650 to 800°C at an average cooling rate of 0.5 to 6°C / s.

[0179] If the above first cooling end temperature (T1) is less than 650°C, it is difficult to secure the target strength because a large amount of soft ferrite and bainite other than martensite is formed during the cooling process. If the above first cooling end temperature (T1) exceeds 800°C, the temperature difference between the first cooling end temperature (T1) and the second cooling end temperature (T2) becomes severe, causing a rapid phase transformation and potentially resulting in defective product shape. It is more preferable that the lower limit of the above first cooling end temperature be 680°C. It is more preferable that the upper limit of the above first cooling end temperature be 780°C.

[0180] If the above-mentioned first average cooling rate is less than 0.5℃ / s, the first cooling end temperature (T1) is considerably high, and the temperature difference between the first cooling end temperature (T1) and the second cooling end temperature (T2) becomes severe, causing a rapid phase transformation and potentially resulting in defective product shape. 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 1℃ / s. It is more preferable that the upper limit of the above-mentioned first average cooling rate is 5℃ / s.

[0181] Secondary cooling

[0182] The above first cooled cold-rolled steel sheet is second cooled to a second cooling end temperature (T2) of 40 to 250°C at an average cooling rate (CR) of 40 to 300°C / s.

[0183] The above secondary cooling is intended to secure one or more of the main phases of the present invention, namely martensite and tempered martensite. If the secondary cooling end temperature (T2) is less than 40°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 secondary cooling end temperature (T2) exceeds 250°C, it may be difficult to secure the strength targeted by the present invention. The lower limit of the secondary cooling end temperature is more preferably 50°C, more preferably 55°C, and most preferably 60°C. The upper limit of the secondary cooling end temperature is more preferably 240°C, more preferably 230°C, and most preferably 220°C.

[0184] If the above secondary average cooling rate is less than 40℃ / 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 45℃ / s, more preferably 50℃ / s, and most preferably 55℃ / s. The upper limit of the above secondary average cooling rate is more preferably 280℃ / s, more preferably 260℃ / s, and most preferably 240℃ / s.

[0185] Meanwhile, as shown in [Equation 7] below, it is desirable to manage the temperature difference of Mf-T2 to be 20°C or more, and more preferably to be 30°C or more. If the above temperature difference is less than 20°C, the martensite transformation may not occur sufficiently, making it difficult to secure the target strength. Mf can be calculated through Equation 4 above.

[0186] [Relationship 7]

[0187] Mf-T2 ≥ 20℃

[0188] (Mf is the martensite transformation end temperature (°C), and T2 is the secondary cooling end temperature.)

[0189] [Equation 4] Mf(°C) = 371 - 412C - 17.4Si - 47.4Mn - 20.9Cr - 17.0Mo + 49.2Nb + 95.0Ti + 202B

[0190] (In Equation 4 above, each alloying element symbol represents the content (weight%) of each element.)

[0191] Statute of limitations processing

[0192] The above secondary cooled cold-rolled steel sheet is reheated to an overaging treatment temperature (OAT) of 140 to 260°C and then overaged for 3 to 14 minutes. Through the above reheating and overaging treatment, the martensite obtained by the aforementioned cooling process is transformed into tempered martensite, thereby increasing the yield strength. If the above reheating temperature and overaging treatment temperature are below 140°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 above reheating temperature and overaging treatment temperature exceed 260°C, there is a disadvantage in that bendability and hydrogen embrittlement resistance are inferior due to the precipitation and coarsening of large amounts of carbides. The lower limit of the above reheating temperature and overaging treatment temperature is more preferably 145°C, more preferably 150°C, and most preferably 155°C. The upper limit of the above reheating temperature and overaging treatment temperature is more preferably 255℃, more preferably 250℃, and most preferably 245℃.

[0193] If the above overaging treatment time is less than 3 minutes, tempering is not sufficiently performed, and the yield strength may be lowered. If the above overaging treatment time exceeds 14 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 3.5 minutes, more preferably 4.0 minutes, and most preferably 4.5 minutes. The upper limit of the above overaging treatment time is more preferably 13.5 minutes, more preferably 12 minutes, and most preferably 11.5 minutes.

[0194] Tension leveling

[0195] After the above overaging treatment, tension leveling (T / L) is performed with an elongation rate of 0.05~0.55%.

[0196] Meanwhile, prior to the tension leveling above, a skin pass mill (SPM) may be performed if necessary. The skin pass mill enables control of surface roughness (Rsk). The skin pass mill may be performed with a rolling force of 500 to 1,000 tons. If the rolling force during the skin pass mill is less than 500 tons, the load is low, making it difficult to control the surface roughness (Rsk); if it exceeds 1,000 tons, severe work hardening of the surface may occur, which could actually lead to inferior bending characteristics. It is more preferable that the lower limit of the rolling force during the skin pass mill be 550 tons, and more preferable that it be 600 tons. It is more preferable that the upper limit of the rolling force during the skin pass mill be 950 tons, and more preferable that it be 900 tons.

[0197] The above tension leveling is intended to correct the shape of the steel plate. If the elongation is less than 0.05% during the above tension leveling, shape correction may be difficult. If the elongation exceeds 0.55% during the above tension leveling, work hardening may become severe, leading to deterioration in bending characteristics, and the difference in yield strength between the vertical and horizontal directions relative to the rolling direction may become significant, which may adversely affect dimensional accuracy during part processing. It is more preferable that the lower limit of the elongation during the above tension leveling be 0.075%, and more preferable that it be 0.10%. It is more preferable that the upper limit of the elongation during the above tension leveling be 0.50%, and more preferable that it be 0.45%.

[0198] [Equation 7] is a relationship related to manufacturing conditions for simultaneously securing shape and material, 0.0005~0.006(%·(℃ / sec) -1 It is desirable that ) is , and more desirable that it is 0.0006 to 0.0045. This value is 0.0005(%·(℃ / sec) -1 If this value is less than ), the cooling rate is fast and the elongation of the tension leveling is low, which may result in shape degradation. If this value is 0.006 (%·(℃ / sec)) -1 If it exceeds ), the cooling rate may be low, making it difficult to secure the target strength, and the tension leveling may be excessive, leading to severe surface work hardening, making it difficult to secure the target elongation and bending.

[0199] [Equation 7] 0.0005 ≤ TL_EL / CR ≤ 0.006 (%ㆍ(℃ / sec) -1 )

[0200] (TL_EL is the tension leveling elongation (%), and CR is the cooling rate (°C / sec) between the first cooling end temperature (T1) and the second cooling end temperature (T2))

[0201] plating

[0202] 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.

[0203] The present invention will be described in detail below through examples. However, it should be noted that the examples described below are intended merely to illustrate and embody the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the patent claims and matters reasonably inferred therefrom.

[0204] (Example)

[0205] A steel slab having the composition of Table 1 (weight%, the remainder being Fe and unavoidable impurities) and the conditions of Table 2 was heated at 1200°C, and then the heated steel slab was finished hot-rolled at 900°C to obtain a hot-rolled steel sheet, and coiled at 450°C. The hot-rolled steel sheet was cold-rolled to obtain a cold-rolled steel sheet having the thickness of Table 3 below. The cold-rolled steel sheet was subjected to annealing heat treatment, primary cooling, secondary cooling, overaging treatment, and tension leveling under the conditions disclosed in Tables 3 and 4 to manufacture a steel sheet. The temperatures in Tables 3 and 4 were based on the surface of each specimen.

[0206]

[0207]

[0208] In Table 2 above, the following relationships 1 to 4 are derived, and Ar3 and Mf are derived by the following equations 3 and 4.

[0209] [Relationship 1]

[0210] 50% ≤ H = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb + 10000B + 14.5Ni + 9.6Cu ≤ 220%

[0211] [Relationship 2]

[0212] 0.20% ≤ Cs = C + 0.03Mn + 0.02Si ≤ 0.45%

[0213] [Relationship 3]

[0214] 0.0013 ≤ Cs / H ≤ 0.008

[0215] [Relationship 4]

[0216] M = Cr + Mo + Ni + Cu ≤ 0.70%

[0217] (In the above Equations 1 to 4, each alloy element symbol represents the content (weight%) of each element, and the units in Equations 1, 2, and 4 are weight%.)

[0218] [Equation 3]

[0219] Ac3(℃) = 900 - 206C + 26.2Si - 25.0Mn - 12.3Cr + 9.12Mo + 50.2Nb + 148Ti - 131B

[0220] [Equation 4]

[0221] Mf(℃) = 371 - 412C - 17.4Si - 47.4Mn - 20.9Cr - 17.0Mo + 49.2Nb + 95.0Ti + 202B

[0222] (In the above Equations 3 and 4, each alloying element symbol represents the content (weight%) of each element.)

[0223]

[0224]

[0225] For the steel plate manufactured as described above, the microstructure, mechanical properties, etc. were measured, and the results are shown in Tables 5 and 6 below.

[0226] The average depth of the upper and lower decarburization layers (Dave.) in the thickness-direction cross-section was measured using a GDS (Glow discharge spectrometer).

[0227] The moisture fraction, KAMave., and GNDave. (Geometrically Necessary Dislocations) in the region where the KAM (Kernel Average Misorientation) value is 0–1.0° were measured three times at 2000x magnification using EBSD (Backscattered Electron Diffraction Pattern Analyzer) at the center (t / 4, where t is the thickness of the steel plate) (Confidence Index (CI) ≥ 0.3, measurement area: 45×45㎛, step size: 80nm), and the average was derived by quantifying them using OIM (Orientation Imaging Microscopy) Analysis software provided by TSL. Specifically, for the KAM calculation, the Nearest 3rd and Maximum 5 were used, and for the GND calculation, Phase Iron (Alpha), Preset Slip Systemps BCC Slip, Nearest 1st, and Maximum 5 were used. At this time, the total sum of the moisture fraction (Number Fraction) in the region where the KAM value is 0–5.0° during the above EBSD measurement is It was considered to be 1%.

[0228] Yield strength, tensile strength, and elongation were measured by processing cold-rolled steel sheets according to JIS standards perpendicular to the rolling direction and then performing a tensile test under conditions of a test speed of 28 mm / min.

[0229] The maximum angle of 3-point bending was measured 5 times under VDA conditions (0.4R) and the average value was calculated.

[0230] Flatness was measured as shown in Fig. 1 by shearing a full-width specimen in the rolling direction, with the width and length each set to 1.5 m, and the height was measured at 8 points each on the upper and lower surfaces of the steel plate and the average value was obtained.

[0231]

[0232]

[0233] As can be seen from Tables 1 to 6 above, in the case of Invention Examples 1 to 5, which satisfy the alloy composition, component relationship formulas 1 to 4 and manufacturing conditions proposed by the present invention, all of the tensile properties, bending, and flatness sought by the present invention were satisfied by securing the microstructure (decarburized layer, KAM, GND) sought by the present invention.

[0234] Meanwhile, Figures 2 and 3 show the number fractions (%) of KAM and GND for Inventive Example 1 and Comparative Example 1. As can be seen from these results, Inventive Example 1 has high KAMave. and GNDave. values, and KAM 0~1° and GND 0~200(×10 12 m -2 Although the number fraction of ) is low, Comparative Example 1 has low KAMave. and GNDave. values, and KAM 0~1° and GND 0~200(×10 12 m -2 The number fraction of ) was high, and the dislocation density was low, so the target strength could not be secured.

[0235] Comparative Examples 1 to 3 did not satisfy the C, Mn, Cr, and Mo components and component relationship equations 1 to 4 proposed by the present invention, and thus did not satisfy the KAM and GND values ​​related to dislocation density, so the target tensile properties were not secured.

[0236] Comparative Example 4 did not satisfy the C, Mn, Cr, and Mo components and component relationship equations 1 to 4, and the dew point temperature, cooling rate, and tension leveler elongation were not satisfied, resulting in inferior elongation and bending characteristics.

[0237] Comparative Example 5 did not satisfy the dew point temperature, so the target decarburization layer thickness was not secured, and the bending characteristics were inferior.

[0238] Comparative Example 6 had a low cooling rate and failed to satisfy the KAM and GND values ​​related to dislocation density, resulting in insufficient yield strength and tensile strength.

[0239] Comparative Example 7 exceeded the dew point temperature, resulting in excessive decarburization and failing to meet the target strength.

[0240] Comparative Example 8 had a low dew point temperature, so the target decarburization layer thickness was not secured, and the bending characteristics were inferior.

[0241] Comparative Example 9 had a low cooling rate and failed to satisfy the KAM and GND values ​​related to dislocation density, resulting in insufficient yield strength and tensile strength.

[0242] Comparative Example 10 had a low dew point temperature and a fast cooling rate, so the KAM and GND values ​​related to the target decarburization layer thickness and dislocation density were exceeded, resulting in inferior bending characteristics and flatness.

[0243] In Comparative Example 11, the secondary cooling end temperature (T2) and the reheat / overaging treatment temperature were exceeded, so the KAM and GDS values ​​related to dislocation density were not satisfied, and the target strength was not secured.

[0244] Comparative Example 12 had a low dew point temperature and tension leveling elongation, resulting in inferior bending characteristics and flatness.

[0245] Comparative Example 13 had a low dew point temperature and excessive tension leveling elongation, resulting in severe surface work hardening and inferior elongation and bending characteristics.

Claims

1. In wt%, carbon (C): 0.16–0.33%, silicon (Si): 0.02–0.60%, manganese (Mn): 0.3–2.3%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.0050% or less (excluding 0%), aluminum (Al): 0.005–0.08%, chromium (Cr): 0.005–0.50%, boron (B): 0.0005–0.005%, nitrogen (N): 0.01% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities, The surface layer of the steel plate includes a decarburization layer, and the depth (Dave.) of the decarburization layer is 20 to 75 μm, and When the total number fraction of the region where the KAM (Kernel Average Misorientation) value is 0~5.0° is set to 1%, the number fraction of the region where the KAM value is 0~1° at the center of the steel plate (KAM0~1°) is 0.18~0.35%, and The average value of KAM (KAMave.) is 1.30–1.70°, and The average GND value (GNDave.) is 260~305 (×10 12 m -2 ) and, A steel plate satisfying the following [Relationship 6]. [Relationship 6] 1.2 ≤ (Cs*KAMave.*GNDave.) / Dave. ≤ 6.0 (%·°·10 12 m -2 ·㎛ -1 ) (Cs = C + 0.03Mn + 0.02Si, each alloying element symbol represents the content (weight%) of each alloying element) 2. In Paragraph 1, The above steel plate is a steel plate satisfying the following [Relationship 5]. [Relationship 5] 95 ≤ Cs*KMave.*GNDave. ≤ 200, (%·°·1 12 m -2 ) (Cs = C + 0.03Mn + 0.02Si, each alloying element symbol represents the content (weight%) of each alloying element) 3. In Paragraph 1, The above steel plate further comprises one or more of molybdenum (Mo): 0.001~0.35%, niobium (Nb): 0.003~0.05%, titanium (Ti): 0.005~0.25%, copper (Cu): 0.003~0.3%, and nickel (Ni): 0.003~0.3%.

4. In Paragraph 3, The above steel plate is a steel plate satisfying one or more of the following [Equation 1] to [Equation 4]. [Relationship 1] 50% ≤ H = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb + 10000B + 14.5Ni + 9.6Cu ≤220% [Relationship 2] 0.20% ≤ Cs = C + 0.03Mn + 0.02Si ≤ 0.45% [Relationship 3] 0.0013 ≤ Cs / H ≤ 0.008 [Relationship 4] M = Cr + Mo + Ni + Cu ≤ 0.70% (In the above Equations 1 to 4, each alloy element symbol represents the content (weight%) of each element, and the units in Equations 1, 2, and 4 are weight%) 5. In Paragraph 1, A steel plate whose microstructure comprises at least 95% of one or more of martensite and tempered martensite.

6. In Paragraph 1, The above steel plate has a yield strength (YS) perpendicular to the rolling direction of 1200 MPa or more and a tensile strength (TS) perpendicular to the rolling direction of 1470 MPa or more.

7. In Paragraph 1, The above steel plate is a steel plate having a Total_EL (total elongation, T_EL) of 4% or more, a Uniform_EL (uniform elongation, U_EL) of 2.5% or more, and a Post_EL (T_EL - U_EL) of 2~6%.

8. In Paragraph 1, The above steel plate is a steel plate having a maximum 3-point bending angle of 60 to 100°.

9. In Paragraph 1, The above steel plate has a Post_EL × 3-point bending angle of 180~300(%·°) and (Post_EL × 3-point bending angle) / TS is 0.10~0.30(%·°·MPa -1 )person, steel plate.

10. In Paragraph 1, A steel plate having a flatness of 10㎛ or less.

11. In Paragraph 1, A steel plate having a thickness of 0.6 to 2.5 mm.

12. In Paragraph 1, The above steel plate comprises an electro-galvanized layer formed on at least one surface.

13. A step of providing a cold-rolled steel sheet comprising, in wt%, carbon (C): 0.16~0.33%, silicon (Si): 0.02~0.60%, manganese (Mn): 0.3~2.3%, phosphorus (P): 0.03% or less (excluding 0%), sulfur (S): 0.0050% or less (excluding 0%), aluminum (Al): 0.005~0.08%, chromium (Cr): 0.005~0.50%, boron (B): 0.0005~0.005%, nitrogen (N): 0.01% or less (excluding 0%), and the remainder being Fe and other unavoidable impurities; A step of annealing the above cold-rolled steel sheet in a temperature range of Ac3+20℃ to Ac3+90℃; A step of primary cooling at an average cooling rate of 0.5 to 6℃ / s to a primary cooling end temperature (T1) of 650 to 800℃ after the above annealing; A step of secondary cooling at an average cooling rate (CR) of 40 to 300℃ / s to a secondary cooling end temperature (T2) of 40 to 250℃ after the above primary cooling; After the above secondary cooling, a step of reheating to an overaging treatment temperature (OAT) of 140–260°C and overaging treatment for 3–14 minutes; and A step of tension leveling (T / L) with an elongation rate of 0.05~0.55% after the above overaging treatment; A method for manufacturing a steel plate, comprising, wherein the secondary cooling satisfies the following [Equation 7] and [Equation 8]. [Relationship 7] Mf-T2 ≥ 20℃ (Mf is the martensite transformation end temperature (°C), and T2 is the secondary cooling end temperature.) [Relationship 8] 0.0005 ≤ TL_EL / CR ≤ 0.006 (%ㆍ(℃ / sec) -1 ) (TL_EL is the tension leveling elongation (%), and CR is the cooling rate (°C / sec) between the first cooling end temperature (T1) and the second cooling end temperature (T2)) 14. In Paragraph 13, A method for manufacturing a steel sheet in which the above cold-rolled steel sheet further comprises one or more of molybdenum (Mo): 0.001~0.35%, niobium (Nb): 0.003~0.05%, titanium (Ti): 0.005~0.25%, copper (Cu): 0.003~0.3%, and nickel (Ni): 0.003~0.3%.

15. In Paragraph 14, The above cold-rolled steel sheet is a method for manufacturing a steel sheet that satisfies one or more of the following [Equation 1] to [Equation 4]. [Relationship 1] 50% ≤ H = 48.8 + 49logC + 35.1Mn + 25.9Si + 76.5Cr + 105.9Mo + 1325Nb + 10000B + 14.5Ni + 9.6Cu ≤220% [Relationship 2] 0.20% ≤ Cs = C + 0.03Mn + 0.02Si ≤ 0.45% [Relationship 3] 0.0013 ≤ Cs / H ≤ 0.008 [Relationship 4] M = Cr + Mo + Ni + Cu ≤ 0.70% (In the above Equations 1 to 4, each alloy element symbol represents the content (weight%) of each element, and the units in Equations 1, 2, and 4 are weight%) 16. In Paragraph 13, The method for preparing the above cold-rolled steel sheet is, A step of heating the steel slab to a temperature range of 1100~1300℃; A step of hot rolling the above heated steel slab by finishing hot rolling at Ar3~Ar3+120℃; A step of coiling at a temperature range of Ms~650℃ after the above hot rolling; and Cold rolling step with a cold reduction rate of 45~70% A method for manufacturing a steel plate, comprising 17. In Paragraph 13, A method for manufacturing a steel plate in which the above annealing is performed for 50 to 250 seconds.

18. In Paragraph 13, A method for manufacturing a steel plate in which the dew point temperature of the atmosphere during the above-mentioned annealing is -30 to 20℃.

19. In Paragraph 18, A method for manufacturing a steel sheet in which the atmosphere during annealing above contains moist nitrogen (N2+H2O).

20. In Paragraph 13, A method for manufacturing a steel plate by performing temper rolling after the above-mentioned overaging treatment.

21. In Paragraph 13, A method for manufacturing a steel plate, further comprising the step of forming an electro-galvanized layer on the surface of the steel plate.

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