Cold-rolled steel sheet and method for manufacturing same

Abstract

The present invention relates to a cold-rolled steel sheet and a method for manufacturing same and, more particularly, to a cold-rolled steel sheet and a method for manufacturing same, the cold-rolled steel sheet being suitable for use as a steel material for vehicle reinforcements, such as bumper beams, sill side beams, pillars, and the like, or as a steel material for side frames, cross members, and the like.

Description

Cold-rolled steel sheet and method of manufacturing the same

[0001] The present invention relates to a cold-rolled steel sheet and a method for manufacturing the same, and more specifically, to a cold-rolled steel sheet suitable for use as steel for automotive reinforcing materials such as bumper beams, sill side beams, and pillars, or as steel for side frames, cross members, etc., and a method for manufacturing the same.

[0002] For steel materials primarily used as reinforcement components related to the crash safety of automobile passengers, it is required to have excellent processing characteristics, particularly bending characteristics, and ultra-high strength. To this end, research is actively underway to develop ultra-high strength steel with a tensile strength of 1470 MPa or higher using a single martensitic phase.

[0003] Meanwhile, the Hot Press Forming (HPF) method has recently been developed, which involves forming the material using a die in a high-temperature environment conducive to forming, followed by water cooling to secure the required strength. Since the HPF method allows for securing high strength relative to the same thickness, it is widely used in parts manufacturing; however, this 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 ultra-high-strength cold-rolled steel sheets that are suitable for cold stamping and roll forming, possess high strength and high yield ratios to ensure impact performance, and exhibit excellent bending characteristics for part forming, spot weldability for part assembly, and corrosion resistance for long-term part lifespan.

[0004] Representative prior art of this method includes Patent Documents 1 and 2.

[0005] Patent Document 1 discloses that the steel has a single-phase martensitic structure comprising 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 Ti: 0.005~0.1%, Nb: 0.005~0.1%, and a total of 0.005~0.1%, and can be obtained by heating and maintaining in a temperature range above the Ae3 transformation point and below 900°C, then rapidly cooling to below 200°C at an average cooling rate of 300°C / s, and subsequently tempering at below 250°C. However, in the case of Patent Document 1, there is a problem in that the shape (flatness) deteriorates due to rapid cooling (water cooling), causing defects during molding.

[0006] Patent Document 2 relates to a thin steel plate having a microstructure 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, with 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%), a standard deviation of yield strength in the width direction of 30 MPa or less, and a maximum bending amount of the steel plate when sheared to 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.

[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. In this case, brittle fracture is prone to occur due to hydrogen remaining within the steel or introduced from the outside, a phenomenon referred to as hydrogen embrittlement. Hydrogen embrittlement manifests as fracture at a strength lower than the fracture strength; the material can fracture due to hydrogen embrittlement even at applied stresses that are very small compared to the actual fracture strength of the material. In particular, this hydrogen embrittlement becomes more sensitive as the strength of the steel increases. Therefore, in ultra-high-strength steel, it is necessary to control the initial amount of hydrogen remaining in the material to prevent such hydrogen embrittlement.

[0008] Therefore, in order to solve the aforementioned problems, it is necessary to develop an ultra-high strength cold-rolled steel sheet with a tensile strength of 1470 MPa or higher that has excellent shape and excellent resistance to hydrogen embrittlement.

[0009] [Prior Art Literature]

[0010] (Patent Document 1) Japanese Patent Publication No. JP 2010-248565

[0011] (Patent Document 2) Japanese Patent Publication No. JP 2020-019992

[0012] One aspect of the present invention is to provide a cold-rolled steel sheet and a method for manufacturing the same.

[0013] A preferred aspect of the present invention is to provide an ultra-high strength cold-rolled steel sheet with excellent hydrogen embrittlement resistance and a method for manufacturing the same.

[0014] 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.

[0015] One embodiment of the present invention comprises, in weight%, carbon (C): 0.160~0.350%, silicon (Si): 0.010~0.80%, manganese (Mn): 0.30~2.60%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.0050% or less (excluding 0%), aluminum (Al): 0.0030~0.080%, chromium (Cr): 0.0010~0.60%, molybdenum (Mo): 0.0010~0.40%, niobium (Nb): 0.0010~0.10%, titanium (Ti): 0.0050~0.20%, boron (B): 0.00050~0.0070%, nitrogen (N): 0.010% or less (excluding 0%), copper (Cu): It is divided into a core containing 0.0010~0.30%, nickel (Ni): 0.0010~0.30%, and the remainder being Fe and other unavoidable impurities; and a surface layer formed on the outer side of the core in the thickness direction; wherein the boundary density of the core is 1.95~3.55mm -1 And, a cold-rolled steel sheet is provided in which X__0.1, expressed by the following Equation 1 at a point 0.1㎛ away from the surface in the thickness direction, is 3.0 to 25.0.

[0016] [Equation 1] 2

[0017] (However, the above surface layer refers to the region extending up to 80㎛ in the thickness direction from the surface of the steel plate, the above center refers to the region outside the above surface layer, and in Equation 1, [] represents the content of each alloying element.)

[0018] The above cold-rolled steel sheet can satisfy the following equations 1 to 3.

[0019] [Relational Expression 1] 1.0 ≤ 2≤ 6.0

[0020] [Equation 2] Y = 100([Cr]+0.8[C]+0.07([Si]+[Mn])-0.1[Mo]-50[B]-70[P]-30[S])+100 ≤ 120

[0021] [Equation 3] 3.0 ≤ Y / X ≤ 80.0

[0022] (However, in the above equations 1 to 3, [] represents the content of each alloying element.)

[0023] The microstructure of the above-mentioned core may include, 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 fresh martensite and tempered martensite.

[0024] The above cold-rolled steel sheet can satisfy the following relationship 4.

[0025] [Relational Expression 4] 1.70 ≤ Z = X_0.1 / Boundary density ≤ 10.0

[0026] The above cold-rolled steel sheet may have a yield strength (YS): 1200~1900MPa, a tensile strength (TS): 1470~2000MPa, and an elongation (EL): 3.0~12.0%.

[0027] The above cold-rolled steel sheet may have a corrosion loss of 13% or less.

[0028] The above cold-rolled steel sheet may have a plating layer formed on at least one surface.

[0029] Another embodiment of the present invention comprises, in weight%, carbon (C): 0.160~0.350%, silicon (Si): 0.010~0.80%, manganese (Mn): 0.30~2.60%, phosphorus (P): 0.030% or less (excluding 0%), sulfur (S): 0.0050% or less (excluding 0%), aluminum (Al): 0.0030~0.080%, chromium (Cr): 0.0010~0.60%, molybdenum (Mo): 0.0010~0.40%, niobium (Nb): 0.0010~0.10%, titanium (Ti): 0.0050~0.20%, boron (B): 0.00050~0.0070%, nitrogen (N): 0.010% or less (excluding 0%), copper (Cu): A step of heating a slab containing 0.0010~0.30%, nickel (Ni): 0.0010~0.30%, the remainder being Fe and other unavoidable impurities; a step of finishing hot rolling the heated slab to obtain a hot-rolled steel sheet; a step of coiling the hot-rolled steel sheet; a step of cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet; a step of continuously annealing the cold-rolled steel sheet for 50~200 seconds at a continuous annealing temperature (SS) of Ac3-5℃~Ac3+65℃; a step of first cooling the continuously annealed cold-rolled steel sheet; a step of second cooling the first-cooled cold-rolled steel sheet to a cooling end temperature (T2) of Mf_1-150℃~Mf_1-20℃ at an average cooling rate (CR2) of 60~350℃ / s; The present invention provides a method for manufacturing a cold-rolled steel sheet having a Ms_1 of 356℃ or less, comprising the step of reheating the secondarily cooled cold-rolled steel sheet and then over-aging it.

[0030] [Relational Expression 1] 1.0 ≤ 2 ≤ 6.0

[0031] [Equation 2] Y = 100([Cr]+0.8[C]+0.07([Si]+[Mn])-0.1[Mo]-50[B]-70[P]-30[S])+100 ≤ 120

[0032] [Equation 3] 3.0 ≤ Y / X ≤ 80.0

[0033] (However, Mf_1 above means Mf+0.26SS-205, and Mf above means 371-412[C]-17.4[Si]-47.4[Mn]-20.9[Cr]-17[Mo]+49.2[Nb]+95[Ti]+202[B], Ms_1 above means Ms+0.26SS-205, and Ms above means 521-379[C]-15.1[Si]-43.9[Mn]-19.5[Cr]-14.7[Mo]+43.7[Nb]+91.9[Ti]+169[B], and [] indicates the content of each alloying element.)

[0034] The above slab can satisfy the following relationships 1 to 3.

[0035] [Relational Expression 1] 1.0 ≤ 2 ≤ 6.0

[0036] [Equation 2] Y = 100([Cr]+0.8[C]+0.07([Si]+[Mn])-0.1[Mo]-50[B]-70[P]-30[S])+100 ≤ 120

[0037] [Equation 3] 3.0 ≤ Y / X ≤ 80.0

[0038] (However, in the above equations 1 to 3, [] represents the content of each alloying element.)

[0039] The heating of the above slab can be carried out at 1100~1300℃.

[0040] The above finishing hot rolling can be performed at Ar3 to Ar3+120℃.

[0041] The above winding can be performed at Ms~700℃.

[0042] The boundary density of the steel sheet after coiling and before cold rolling is 3.0 mm -1It is less than or equal to, and the yield strength may be 1000 MPa or less.

[0043] The above cold rolling can be performed with a cold reduction rate of 40 to 70 percent.

[0044] The above first cooling can be carried out at an average cooling rate (CR1) of 0.5 to 6.0°C / s until a cooling end temperature (T1) of 670 to 785°C.

[0045] The above over-aging treatment can be performed at 90 to 270°C for 180 to 900 seconds.

[0046] During the above over-aging treatment, the over-aging treatment temperature (HT) - secondary cooling end temperature (T2) can be controlled to be 15℃ or higher.

[0047] After the above overaging treatment, the method may further include a step of forming a plating layer on at least one surface of the cold-rolled steel sheet.

[0048] According to one aspect of the present invention, a cold-rolled steel sheet and a method for manufacturing the same can be provided.

[0049] According to a preferred aspect of the present invention, an ultra-high strength cold-rolled steel sheet with excellent hydrogen embrittlement resistance and a method for manufacturing the same can be provided.

[0050] Figure 1 shows the distribution of boundary density and X_0.1 for Inventive Examples 1 to 5 and Comparative Examples 1 to 11 of the present invention.

[0051] Preferred embodiments of the present invention are described below. 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] Unless otherwise specifically defined in the specification of the present invention, % units mean weight %.

[0056] 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.

[0057] 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 will be described. Unless otherwise specified, the alloy composition described below refers to weight percent.

[0058] Carbon (C): 0.160~0.350%

[0059] C is an interstitial solid solution element and is the most effective and important element for improving the strength of steel. If the content of C is less than 0.160%, it may be difficult to obtain the strength targeted in the present invention. If the content of C exceeds 0.350%, the strength increases rapidly, and the elongation may be inferior. In addition, hydrogen embrittlement resistance may decrease, and weldability may be inferior. Therefore, it is advantageous for the content of C to have a range of 0.160 to 0.350%. The lower limit of the C content is more advantageous at 0.170%, and more advantageous at 0.180%. The upper limit of the C content is more advantageous at 0.340%, and more advantageous at 0.330%.

[0060] Silicon (Si): 0.010~0.80%

[0061] Si is an element effective for improving resistance to tempering softening, and is also an element effective for improving strength through solid solution strengthening. If the Si content is less than 0.010%, it may be difficult to sufficiently obtain the aforementioned effects. If the Si content exceeds 0.80%, there is a risk that excessive ferrite will be generated after continuous annealing and cooling, thereby weakening the strength of the steel. Furthermore, as Si is an element that increases resistivity, resistance spot weldability may be inferior. Therefore, it is advantageous for the Si content to be in the range of 0.010 to 0.80%. The lower limit of the Si content is more advantageous at 0.020%, and more advantageous at 0.030%. The upper limit of the Si content is more advantageous at 0.70%, and more advantageous at 0.60%.

[0062] Manganese (Mn): 0.30~2.60%

[0063] Mn is an element added to ensure strength. If the Mn content is less than 0.30%, 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 2.60%, the Ms temperature decreases during cooling after continuous annealing, and as the temperature at which cooling must end decreases, the shape of the steel sheet becomes defective. Furthermore, it is difficult to secure a martensite structure. In addition, Mn-based segregation zones occur along the longitudinal direction of the slab during steelmaking / continuous casting operations, degrading bending characteristics. That is, as manganese bands (Mn bands) are formed within the slab, cracks occur during continuous casting, and there is a problem of increased defect occurrence during the rolling process. Therefore, it is advantageous for the Mn content to be in the range of 0.30 to 2.60%. The lower limit of the above Mn content is more advantageous for being 0.40%, and 0.50% is more advantageous. The upper limit of the above Mn content is more advantageous for being 2.50%, and 2.40% is more advantageous.

[0064] Phosphorus (P): 0.030% or less (excluding 0%)

[0065] P is an impurity element contained in steel. If the content of P exceeds 0.030%, weldability deteriorates, and it is prone to segregation at grain boundaries, leading to intergranular embrittlement. Furthermore, the grain boundaries are prone to fracture due to hydrogen in the steel, raising concerns about brittleness in the steel. Meanwhile, while it is advantageous to exclude P from the steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is advantageous for the content of P to be 0.030% or less (excluding 0%). It is even more advantageous for the content of P to be 0.020% or less.

[0066] Sulfur (S): 0.0050% or less (excluding 0%)

[0067] S is an impurity element included in steel, similar to P. If the content of S exceeds 0.0050%, it can impair ductility and weldability, and a large amount of MnS precipitates may be formed, which can lead to inferior bending properties. Meanwhile, it is advantageous for S not to be included in steel as much as possible, but 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is advantageous for the content of S to be 0.0050% or less (excluding 0%). It is more advantageous for the content of S to be 0.0030% or less, and even more advantageous for it to be 0.0020% or less.

[0068] Aluminum (Al): 0.0030~0.080%

[0069] Al can be added to remove oxygen from the molten steel. If the Al content is less than 0.0030%, deoxidation is not sufficiently achieved, which impairs the cleanliness of the steel. If the Al content exceeds 0.080%, not only does the castability of the slab deteriorate, but the temperature required for single-phase heating during continuous annealing also increases, which may cause production and equipment problems. Therefore, it is advantageous for the Al content to be in the range of 0.0030% to 0.080%. The lower limit of the Al content is more advantageous at 0.0050%, and more advantageous at 0.010%. The upper limit of the Al content is more advantageous at 0.070%, and more advantageous at 0.060%.

[0070] Chrome (Cr): 0.0010~0.60%

[0071] Cr is an element that facilitates securing a low-temperature transformation structure by suppressing ferrite transformation. Additionally, 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 0.0010%, 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 in the present invention. If the Cr content exceeds 0.60%, resistance to delayed fracture may deteriorate, carbides such as CrC may form to reduce bending characteristics, manufacturing costs may increase due to excessive alloy input, and through-corrosion may be accelerated. Therefore, it is advantageous for the Cr content to be in the range of 0.0010% to 0.60%. The lower limit of the Cr content is more advantageous at 0.0020%, and even more advantageous at 0.0030%. The upper limit of the above Cr content is more advantageous at 0.50%, and more advantageous at 0.40%.

[0072] Molybdenum (Mo): 0.0010~0.40%

[0073] Mo is an element that exhibits effects such as improving the hardenability of steel, generating Mo-based fine carbides that serve as hydrogen trap sites, and improving resistance to delayed fracture through martensite refinement. If the content of Mo is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the content of Mo exceeds 0.40%, the aforementioned effects do not increase significantly compared to the cost increase resulting from the addition of expensive alloying elements. Therefore, it is advantageous for the content of Mo to be in the range of 0.0010% to 0.40%. The lower limit of the Mo content is more advantageous at 0.0020%, and more advantageous at 0.0030%. The upper limit of the Mo content is more advantageous at 0.350%, and more advantageous at 0.30%.

[0074] Niobium (Nb): 0.0010~0.10%

[0075] 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 is less than 0.0010%, sufficient austenite grain refinement and precipitation strengthening effects cannot be obtained. If the Nb content exceeds 0.10%, the precipitation of coarse carbonitrides increases, and there is a risk that strength will decrease due to the reduction in carbon content in the steel, and hydrogen embrittlement may be inferior. In addition, there are problems such as reduced workability of the base material and increased manufacturing costs. Therefore, it is advantageous for the Nb content to be in the range of 0.0010 to 0.10%. It is more advantageous for the lower limit of the Nb content to be 0.0020%, and even more advantageous for it to be 0.0030%. The upper limit of the above Nb content is more advantageous at 0.080%, and more advantageous at 0.050%.

[0076] Titanium (Ti): 0.0050~0.20%

[0077] Ti is a nitride-forming element that scavenges dissolved N by precipitating it as TiN. If the Ti content is less than 0.0050%, it is difficult to obtain the effect of increasing strength, and the scavenging effect of dissolved N is reduced, leading to the formation of a large amount of AlN, which may cause cracks during continuous casting. If the Ti content exceeds 0.20%, the strength of the martensite may decrease as additional carbides are precipitated in addition to the removal of dissolved N, and hydrogen embrittlement may be inferior due to the inhibition of hole expansion and bending characteristics caused by the excessive formation of carbonitrides such as TiC and TiN. Therefore, it is advantageous for the Ti content to be in the range of 0.0050 to 0.20%. The lower limit of the Ti content is more advantageous at 0.0070%, and even more advantageous at 0.010%. The upper limit of the above Ti content is more advantageous at 0.170%, and more advantageous at 0.150%.

[0078] Boron (B): 0.00050~0.0070%

[0079] 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 strengthening austenite grain boundaries to inhibit hydrogen intrusion, thereby increasing resistance to hydrogen embrittlement. If the content of B is less than 0.00050%, it is difficult to obtain the aforementioned effects sufficiently, and there is no hardenability effect at all, making it difficult to secure the strength targeted by the present invention. If the content of B exceeds 0.0070%, ductility may be significantly reduced. Therefore, it is advantageous for the content of B to have a range of 0.00050% to 0.0070%. The lower limit of the B content is more advantageous at 0.00070%, and more advantageous at 0.0010%. The upper limit of the B content is more advantageous at 0.0060%, and more advantageous at 0.00550%.

[0080] Nitrogen (N): 0.010% or less (excluding 0%)

[0081] N is an impurity element, and if its content exceeds 0.010%, it significantly increases the risk of cracking during continuous casting due to the formation of AlN, etc. Although it is advantageous for the above N content to be excluded from the steel as much as possible, 0% is excluded to account for cases where it is unavoidably included during the manufacturing process. Therefore, it is advantageous for the above N content to have a range of 0.010% or less (excluding 0%). It is more advantageous for the above N content to be 0.0080% or less, and even more advantageous for it to be 0.0060% or less.

[0082] Copper (Cu): 0.0010~0.30%

[0083] Cu improves corrosion resistance and has the effect of suppressing hydrogen intrusion into the steel plate by coating the surface of the steel plate with corrosion products. In addition, as an element incorporated when utilizing scrap as a raw material, it allows for the use of recycled materials as raw materials, thereby reducing manufacturing costs. If the content of Cu is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the content of Cu exceeds 0.30%, it may lead to the occurrence of surface defects. Therefore, it is advantageous for the content of Cu to have a range of 0.0010% to 0.30%. The lower limit of the Cu content is more advantageous at 0.0050%, and even more advantageous at 0.0070%. The upper limit of the Cu content is more advantageous at 0.250%, and even more advantageous at 0.20%.

[0084] Nickel (Ni): 0.0010~0.30%

[0085] Ni is an element that, like Cu, acts to improve corrosion resistance. Additionally, as an element incorporated when scrap is used as a raw material, it allows for the use of recycled materials, thereby reducing manufacturing costs. If the Ni content is less than 0.0010%, it may be difficult to sufficiently obtain the aforementioned effects. If the Ni content exceeds 0.30%, it may lead to the occurrence of surface defects. Therefore, it is advantageous for the Ni content to be in the range of 0.0010% to 0.30%. The lower limit of the Ni content is more advantageous at 0.0050%, and more advantageous at 0.0070%. The upper limit of the Ni content is more advantageous at 0.250%, and more advantageous at 0.20%.

[0086] 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.

[0087] The cold-rolled steel sheet of the present invention satisfies the aforementioned alloy composition and simultaneously satisfies the following equations 1 to 3. However, in the following equations 1 to 3, [] represents the content of each alloy element.

[0088] [Relational Expression 1] 1.0 ≤ 2 ≤ 6.0

[0089] The above Equation 1 is a component relationship related to corrosion resistance and is also closely related to hydrogen embrittlement. If the value of X is less than 1.0, corrosion resistance is inferior, leading to a large amount of corrosion loss in the steel sheet, which may result in inferior hydrogen embrittlement. If the value of X exceeds 6.0, excessive addition of Cu increases costs, and the surface quality of slabs and hot-rolled steel sheets may be inferior. Therefore, the value of X may have a range of 1.0 to 6.0. A lower limit of X of 1.20 is more advantageous, and 1.40 is more advantageous. An upper limit of X of 5.80 is more advantageous, and 5.50 is more advantageous.

[0090] [Equation 2] Y = 100([Cr]+0.8[C]+0.07([Si]+[Mn])-0.1[Mo]-50[B]-70[P]-30[S])+100 ≤ 120

[0091] The above Equation 2 is a component relationship related to through-corrosion. If the value of Y exceeds 120, local through-corrosion is accelerated, resulting in a large amount of corrosion loss of the steel plate and potentially inferior hydrogen embrittlement. An upper limit of 110 for the above Y value is more advantageous, and 100 is more advantageous. In the present invention, since a lower Y value is more advantageous from the perspective of through-corrosion, no lower limit is imposed. However, if a small amount of hardenable element is added, it may be difficult to secure strength due to the introduction of a soft phase during cooling; therefore, in this regard, the lower limit of the above Y value may be 20. It is more advantageous for the lower limit of the above Y value to be 40.

[0092] [Equation 3] 3.0 ≤ Y / X ≤ 80.0

[0093] The above Equation 3 is a compositional equation designed to improve the corrosion resistance of the steel plate and simultaneously secure excellent hydrogen embrittlement and strength. If the value of Y / X is less than 3.0, it is a region where the Y value is low and the X value is high; while this is advantageous in terms of securing strength due to the fine martensitic substructure, the corrosion loss of the steel plate is high, which may result in inferior hydrogen embrittlement. If the value of Y / X exceeds 80.0, it is a region where the Y value is high and the X value is low; while this is advantageous from the perspective of corrosion resistance, securing strength may be difficult due to the introduction of a soft phase during cooling because less hardenable element is added. Therefore, it is advantageous for the value of Y / X to have a range of 3.0 to 80.0. The lower limit of the Y / X value is more advantageous at 4.0, and even more advantageous at 5.0. The upper limit of the Y / X value is more advantageous at 70.0, and even more advantageous at 60.0.

[0094] The cold-rolled steel sheet of the present invention may be divided into a core portion in terms of microstructure, boundary density, decarburization, etc., and a surface portion formed on the outer side of the core portion in the thickness direction. Meanwhile, since the depth of the surface portion may change depending on the thickness of the steel sheet, it is not specifically limited thereto; however, as an example, the surface portion may be an area up to 80 μm in the thickness direction from the surface of the steel sheet, and the core portion may be an area outside the surface portion. Meanwhile, the thickness of the surface portion may be affected by an annealing process, etc., during the manufacturing process.

[0095] The above-mentioned core microstructure may comprise, in area %, a total of one or more types of ferrite and bainite: 5% or less (including 0%), and the remainder being one or more types of fresh martensite and tempered martensite. That is, the total of one or more types of fresh martensite and tempered martensite may be 95% or more (including 100%). The above-mentioned fresh martensite and tempered martensite are structures that are highly advantageous for securing the strength, bending characteristics, and excellent hydrogen embrittlement 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. Therefore, it is advantageous for the total fraction of one or more types of ferrite and bainite to be 5% or less, and more advantageous for it to be 3% or less. The above tempered martensite may include auto-tempered martensite formed during the cooling process. Meanwhile, the surface microstructure may include one or more types of fresh martensite and tempered martensite, and may include one or more types of ferrite and bainite that are inevitably formed during the manufacturing process. The total fraction of one or more types of ferrite and bainite may be 10% or less (excluding 0%).

[0096] The boundary density of the center above is 1.95~3.55mm -1 It may be. The above boundary density is 1.95mm -1 If it is less than that, the substructure of the martensite (packets, blocks, prior austenite) becomes coarse, making it difficult to secure the target strength, and resistance to hydrogen embrittlement may also be low. The above boundary density is 3.55 mm -1If it exceeds this value, the fine grain size may be advantageous in terms of strength and hydrogen embrittlement; however, since large amounts of expensive elements such as Nb, Ti, and Mo must be added to refine the grain, it may be economically inferior due to increased manufacturing costs. The lower limit of the above boundary density is 2.0 mm. -1 It is more desirable that it is 2.1mm -1 It is more desirable that... The upper limit of the above boundary density is 3.50 mm -1 It is more advantageous to be 3.40mm -1 It is more advantageous to be.

[0097] Meanwhile, the above boundary density can be obtained through Electron Backscatter Diffraction (EBSD) analysis. In the above EBSD analysis, boundary density is a concept representing the amount of grain boundaries per unit area, and it can be used to quantitatively express how many grain boundaries exist in the microstructure of a material. The above boundary density is an important factor in analyzing the characteristics of the microstructure and evaluating its influence on the mechanical properties and hydrogen embrittlement of the material. That is, the higher the boundary density, the more grain boundaries exist; this contributes to suppressing dislocation movement and thereby increasing strength, and contributes to improving resistance to hydrogen embrittlement by acting as hydrogen trap sites. Meanwhile, in the present invention, the above boundary density may refer to grain boundaries having high-angle grain boundaries of 15° or more.

[0098] In the cold-rolled steel sheet of the present invention, X_0.1, expressed by the following Equation 1, may be 3.0 to 25.0 at a point 0.1 μm away from the surface in the thickness direction. The following X_0.1 is a value obtained by substituting the alloy composition at a point 0.1 μm away from the surface in the thickness direction into X of the aforementioned Equation 1. In the cold-rolled steel sheet of the present invention, X_0.1 at a point 0.1 μm away from the surface in the thickness direction may be higher than the X value defined in the aforementioned Equation 1. If the above X_0.1 is less than 3.0, the concentration of components affecting corrosion resistance is low, resulting in a large amount of corrosion loss of the steel sheet and potentially inferior hydrogen embrittlement. If the above X_0.1 exceeds 25.0, the corrosion loss of the steel sheet is low due to improved corrosion resistance, which may be advantageous for hydrogen embrittlement; however, excessive addition of Cu increases costs, and the surface quality of the slab and hot-rolled steel sheet may be inferior. The lower limit of X_0.1 is advantageously 4.0, 5.0 is more advantageous, and exceeding 6.0 is most advantageous. The upper limit of X_0.1 is advantageously 20.0, and 15.0 is more advantageous. X_0.1 can be obtained by performing a compositional analysis in the thickness direction from the surface of the steel plate using a Glow Discharge Spectrometer (GDS). Meanwhile, although it is advantageous to perform the compositional analysis of the surface layer on the surface of the steel plate, the accuracy of the analysis may be reduced because impurities may be present on the surface of the steel plate. Accordingly, in the present invention, the X value was measured at a point 0.1 μm away from the surface in the thickness direction.

[0099] [Equation 1] 2

[0100] (However, in Equation 1 above, [] represents the content of each alloying element.)

[0101] The cold-rolled steel sheet of the present invention can satisfy the following equation 4. If the value of Z below is less than 1.70, the substructure of the martensite (packet, block, prior austenite) is fine, so the boundary density is high, and the target strength can be secured; however, the concentration of components with excellent corrosion resistance in the surface layer is low, so the corrosion loss of the steel sheet increases, and hydrogen embrittlement may be inferior. If the value of Z below exceeds 10.0, the concentration of components with excellent corrosion resistance in the surface layer of the steel sheet is high, so the corrosion loss of the steel sheet is low, which is advantageous from the perspective of hydrogen embrittlement; however, the substructure of the martensite (packet, block, prior austenite) is coarse, so it may be difficult to secure the target strength. The lower limit of the above Z value is more advantageous at 2.0, and is more advantageous at 2.5. The upper limit of the above Z value is more advantageous at 8.0, and is more advantageous at 7.5.

[0102] [Relational Expression 4] 1.70 ≤ Z = X_0.1 / Boundary density ≤ 10.0

[0103] As described above, the cold-rolled steel sheet of the present invention may have a yield strength (YS): 1200~1900 MPa, a tensile strength (TS): 1470~2000 MPa, and an elongation (EL): 3.0~12.0%. More advantageously, the yield strength may be 1210~1850 MPa, and more advantageously, 1220~1800 MPa. More advantageously, the tensile strength may be 1480~1950 MPa, and more advantageously, 1500~1900 MPa. More advantageously, the elongation may be 4.0~11.0%, and more advantageously, 5.0~10.0%. In addition, the cold-rolled steel sheet of the present invention may have a corrosion loss of 13% or less. The above corrosion loss amount may more advantageously be 12% or less, and more advantageously be 11% or less. In addition, the cold-rolled steel sheet of the present invention may not exhibit hydrogen embrittlement.

[0104] The thickness of the cold-rolled steel sheet of the present invention may be 0.6 to 2.5 mm. The lower limit of the thickness of the cold-rolled steel sheet is more advantageously 0.7 mm, and 0.8 mm. The upper limit of the thickness of the cold-rolled steel sheet is more advantageously 2.4 mm, and 2.3 mm.

[0105] The cold-rolled steel sheet of the present invention may have a plating layer formed on at least one surface. The present invention does not specifically limit the type of plating layer, and any type of plating layer commonly used in the relevant technical field may be formed. However, as an example, the plating layer may be an electro-galvanized layer.

[0106] Hereinafter, a method for manufacturing a cold-rolled steel sheet according to one embodiment of the present invention will be described.

[0107] First, a slab satisfying the aforementioned alloy composition is heated. The slab may satisfy the above-described equations 1 to 3. The slab heating process is performed to facilitate the subsequent hot rolling process and to sufficiently obtain the target physical properties of the steel sheet. The heating of the slab may be performed at 1100 to 1300°C. If the slab heating temperature is below 1100°C, a problem may occur in which the hot rolling load increases rapidly. If the slab heating temperature exceeds 1300°C, the amount of surface scale increases, which may lower the yield of the material. The lower limit of the slab heating temperature is more advantageous at 1110°C, more advantageous at 1120°C, and most advantageous at 1130°C. The upper limit of the above slab heating temperature is more advantageous at 1290℃, more advantageous at 1280℃, and most advantageous at 1270℃.

[0108] Subsequently, the heated slab is finished hot-rolled to obtain a hot-rolled steel sheet. The finish hot-rolling can be performed at a temperature of Ar3 to Ar3+120℃. 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 breakage 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 advantageous at Ar3+10℃, more advantageous at Ar3+20℃, and most advantageous at Ar3+30℃. The upper limit of the finish hot-rolling temperature is more advantageous at Ar3+110℃, more advantageous at Ar3+100℃, and most advantageous at 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 2.

[0109] [Equation 2] Ar3(°C) = 910 - 203√C + 44.7Si + 31.5Mo

[0110] Subsequently, the hot-rolled steel sheet is coiled. The coiling may be performed at Ms to 700°C. If the coiling temperature exceeds 700°C, 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 advantageous 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 increase the rolling load during the subsequent cold rolling process, making actual production impossible. It is more advantageous for the lower limit of the coiling temperature to be Ms + 10°C. It is more advantageous for the upper limit of the coiling temperature to be 600°C. The above Ms represents the temperature at which austenite begins to transform into martensite upon cooling, and Mf represents the temperature at which the transformation of austenite into martensite upon cooling is completed, and can be calculated using the following Equations 3 and 4, respectively.

[0111] [Equation 3] Ms(℃) = 521-379C-15.1Si-43.9Mn-19.5Cr-14.7Mo+43.7Nb+91.9Ti+169B

[0112] [Equation 4] Mf(°C) = 371 - 412C - 17.4Si - 47.4Mn - 20.9Cr - 17.0Mo + 49.2Nb + 95.0Ti + 202B

[0113] 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.

[0114] Meanwhile, the boundary density of the steel sheet after the above coiling and before the following cold rolling is 3.0 mm -1 It is less than or equal to, and the yield strength may be 1000 MPa or less. That is, the boundary density of the hot-rolled steel sheet is 3.0 mm -1If it exceeds, excessive martensitic transformation occurs and the substructure of the martensite is refined, causing the yield strength of the hot-rolled steel sheet to rise to a level exceeding 1000 MPa, which may make cold rolling difficult. The boundary density of the above hot-rolled steel sheet is 2.8 mm -1 It is more advantageous to be less than or equal to 2.6mm -1 It is more advantageous for it to be less than or equal to 900 MPa. It is more advantageous for the yield strength of the above hot-rolled steel sheet to be 900 MPa or less, and more advantageous for it to be 800 MPa or less. In the present invention, the boundary density and the lower limit of the yield strength of the above hot-rolled steel sheet are not specifically limited, but as an example, 0.5 mm each -1 It can be 500MPa.

[0115] Subsequently, the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The cold rolling may be performed with a cold reduction rate of 40 to 70%. If the cold reduction rate is less than 40%, it is difficult to secure the thickness desired in the present invention, and there is a concern that non-uniform 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. If the cold reduction rate exceeds 70%, the reduction amount in the length and width directions may become non-uniform due to work hardening occurring during cold rolling, and this may lead to material variation in the steel sheet. In addition, 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 advantageous at 41%, more advantageous at 42%, and more advantageous at 43%. The upper limit of the above cold rolling rate is more advantageous at 69%, more advantageous at 68%, and more advantageous at 67%.

[0116] Afterwards, the above cold-rolled steel sheet is continuously annealed at a continuous annealing temperature (SS) of Ac3-5℃ to Ac3+65℃ for 50 to 200 seconds.

[0117] If the above continuous annealing temperature is less than Ac3-5℃, a mixed structure may be formed as two-phase annealing occurs over the entire length of the steel sheet instead of a single-phase annealing. Consequently, it is difficult to secure the physical properties targeted by the present invention; in particular, the difference in hardness between phases becomes large, which may significantly reduce hole expansion properties, and the concentration of components favorable for corrosion resistance in the surface layer may decrease. If the above continuous annealing temperature exceeds Ac3+65℃, equipment troubles may occur due to overloading of the annealing furnace, and it may be difficult to secure the target strength as the substructure of the martensite (packet, block, prior austenite) becomes coarse. The lower limit of the above continuous annealing temperature is more advantageous at Ac3℃, and it is more advantageous at Ac3+5℃. The upper limit of the above continuous annealing temperature is more advantageous at Ac3+60℃, and it is more advantageous at Ac3+55℃. Meanwhile, the above Ac3 refers to the temperature at which 100% transformation of ferrite into austenite is completed upon heating, and can be calculated using Equation 5 below. The above continuous annealing can be performed for 50 to 200 seconds. If the above continuous annealing time is less than 50 seconds, it may be difficult to secure a single-phase austenite structure, and as undissolved carbides remain and coarsen, bending characteristics and hydrogen embrittlement resistance may be reduced, and it may be difficult to sufficiently form a component concentration favorable for corrosion resistance in the extreme surface layer of the steel sheet. If the above continuous annealing time exceeds 200 seconds, the austenite size coarsen, and the martensite substructure formed after cooling also coarsen, resulting in the disadvantage of difficulty in securing strength. The lower limit of the above continuous annealing time is more advantageous at 60 seconds, and 70 seconds is more advantageous. The upper limit of the above continuous annealing time is more advantageous at 190 seconds, and 180 seconds is more advantageous.

[0118] [Equation 5] Ac3(℃) = 900-206C+26.2Si-25Mn-12.3Cr+9.12Mo+50.2Nb+148Ti-131B

[0119] Subsequently, the continuously annealed cold-rolled steel sheet is cooled first. The first cooling may be performed at an average cooling rate (CR1) of 0.5 to 6.0°C / s until a cooling end temperature (T1) of 670 to 785°C. If the 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, making it difficult to secure the target strength and potentially degrading bending characteristics. If the cooling end temperature (T1) exceeds 785°C, the temperature difference between the first cooling end temperature and the second cooling end temperature becomes severe, causing rapid phase transformation and potentially resulting in defective product shape. It is more advantageous for the lower limit of the cooling end temperature (T1) to be 680°C, and even more advantageous for it to be 690°C. The upper limit of the above cooling end temperature (T1) is more advantageously 770°C, and 750°C is more advantageous. If the above average cooling rate (CR1) is less than 0.5°C / s, ferrite is formed during cooling, making it impossible to secure the level of strength targeted by the present invention. If the above average cooling rate (CR1) exceeds 6.0°C / s, the average cooling rate during the subsequent secondary 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. The lower limit of the above average cooling rate (CR1) is more advantageously 1°C / s. The upper limit of the above average cooling rate (CR1) is more advantageously 5°C / s.

[0120] Subsequently, the first-cooled cold-rolled steel sheet is subjected to a second cooling process at an average cooling rate (CR2) of 60 to 350°C / s to a cooling end temperature (T2) of Mf_1-150°C to Mf_1-20°C. Additionally, it is advantageous to control the Ms_1 below to be 356°C or lower. Since the cooling end temperature (T2) is closely related to the martensite transformation, precise control is required to secure the strength targeted in the present invention. The inventors recognized that the martensite transformation start and end temperatures are influenced by the alloy composition and the austenite size, and that while the austenite size is influenced by the alloy composition, it is also predominantly influenced by the annealing temperature. Therefore, it is necessary to consider the annealing temperature, the martensite transformation start temperature, and the transformation end temperature during cooling, and Mf_1 and Ms_1 can be obtained from the following Equations 6 and 7, respectively. If the above cooling end temperature (T2) is less than Mf_1-150℃, 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 above cooling end temperature (T2) exceeds Mf_1-20℃, sufficient martensite transformation does not occur, which may make it difficult to secure the strength targeted in the present invention. The lower limit of the above cooling end temperature (T2) is more advantageous at Mf_1-140℃, and more advantageous at Mf_1-130℃. The upper limit of the above cooling end temperature (T2) is more advantageous at Mf_1-30℃, and more advantageous at Mf_1-40℃. If the above average cooling rate (CR2) is less than 60℃ / s, soft ferrite transformation occurs during cooling, making it difficult to secure the target strength. If the above average cooling rate (CR2) exceeds 350℃ / s, the product shape may become defective due to rapid phase transformation. The lower limit of the above average cooling rate (CR2) is more advantageous at 70℃ / s, and 80℃ / s is more advantageous. The upper limit of the above average cooling rate (CR2) is more advantageous at 300℃ / s, and 250℃ / s is more advantageous.If the following Ms_1 exceeds 356℃, there may be difficulties in securing the strength targeted by the present invention because the precipitation of carbides increases. In the present invention, the lower limit of the above Ms_1 is not specifically limited, but as an example, it may be 250℃.

[0121] [Equation 6] Mf_1 = Mf+0.26SS-205

[0122] [Equation 7] Ms_1 = Ms+0.26SS-205

[0123] Subsequently, the above-mentioned secondary cooled cold-rolled steel sheet is reheated and subjected to overaging treatment. 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. The overaging treatment may be performed at 90 to 270°C for 180 to 900 seconds. If the overaging treatment temperature is below 90°C, sufficient tempering may not occur, resulting in low yield strength and difficulty in securing sufficient toughness. If the overaging treatment temperature exceeds 270°C, bendability may be compromised due to the precipitation and coarsening of large amounts of carbides. The lower limit of the overaging treatment temperature is more advantageous at 100°C, more advantageous at 110°C, and most advantageous at 120°C. The upper limit of the above overaging treatment temperature is more advantageous at 265°C, more advantageous at 260°C, and most advantageous at 255°C. If the above overaging treatment time is less than 180 seconds, tempering is not sufficiently performed, and the yield strength may be lowered. If the above overaging treatment time exceeds 900 seconds, carbides may coarsen due to excessive tempering, and bending characteristics may deteriorate. The lower limit of the above overaging treatment time is more advantageous at 185 seconds, more advantageous at 190 seconds, and most advantageous at 195 seconds. The upper limit of the above overaging treatment time is more advantageous at 895 seconds, more advantageous at 890 seconds, and most advantageous at 885 seconds.

[0124] When the overaging treatment is performed, the overaging treatment temperature (HT) - secondary cooling end temperature (T2) can be controlled to be 15°C or higher. If the overaging treatment temperature - secondary cooling end temperature is less than 15°C, sufficient tempering may not occur, making it difficult to secure the target yield strength. It is more advantageous for the overaging treatment temperature (HT) - secondary cooling end temperature (T2) to be 20°C or higher.

[0125] After the above overaging treatment, the method may additionally include a step of forming a plating layer on at least one surface of the cold-rolled steel sheet. The present invention does not specifically limit the method of forming the plating layer, and any method commonly used in the relevant technical field may be used. However, as an example, the plating layer may be an electro-galvanized layer.

[0126] 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.

[0127] (Example)

[0128] A slab having the alloy composition listed in Tables 1 and 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 under the conditions listed in Table 3 below. The boundary density and yield strength of the hot-rolled steel sheet obtained in this way were measured, and the results are shown in Table 3 below. Subsequently, the hot-rolled steel sheet was cold-rolled, continuously annealed, first-stage cooling, second-stage cooling, and reheated / over-aged under the conditions listed in Tables 4 and 5 to produce a cold-rolled steel sheet with a thickness of 1.2 to 2.0 mm. Meanwhile, the conditions listed in Tables 4 and 5 below were based on the surface temperature of the steel sheet.

[0129] The microstructure and mechanical properties of the cold-rolled steel sheets manufactured in this manner were measured, and the results are shown in Tables 6 and 7 below.

[0130] The types and fractions of the microstructure at the center of the steel plate were observed using a scanning electron microscope (SEM) and an optical microscope (OM) at a position t / 4 (t: thickness of the steel plate) in the thickness direction of the steel plate, and the fractions of each phase were analyzed 10 times through image analysis to calculate the average value.

[0131] The boundary density of hot-rolled and cold-rolled steel sheets was measured three times at 2000x magnification using EBSD (Backscattered Electron Diffraction Pattern Analyzer) at a 1 / 4 position in the thickness direction of the steel sheet (Confidence Index (CI)≥0.3, measurement area: 45×45㎛, Step size: 80nm), and calculated by quantifying using OIM (Orientation Imaging Microscopy) Analysis software. The boundary density was measured for high-angle grain boundaries of 15° or more.

[0132] X_0.1 was calculated by measuring the alloy composition at a point 0.1 μm away from the surface of the steel plate in the thickness direction using GDS.

[0133] Yield strength (YS), tensile strength (TS), and elongation (EL) were measured by processing the steel plate according to JIS standards in a direction perpendicular to the rolling direction and then performing a tensile test under conditions of a test speed of 28 mm / min.

[0134] The corrosion loss was calculated by cutting a steel plate into 60×60mm pieces, immersing it in a 0.1N HCl solution for 100 hours, and measuring the weight change before and after immersion.

[0135] For the hydrogen embrittlement evaluation, a steel plate was cut to 30×100mm to obtain a specimen, a 20Φ hole was machined in the specimen using a laser, and then baked at 170±10℃ for 20 minutes. After 4-point bending according to the ASTM G39 evaluation method, the specimen was evaluated for 7 days in the order of Salt spray (35℃, 95%RH, 4hr), Drying (70℃, 30%RH, 2hr), Wetting (50℃, 95%RH, 2hr), Drying (25℃, 60%RH, 1.5hr), and Low Temp. (-20℃, 2.5h) to visually determine whether cracks occurred.

[0136] Steel Grade No. Alloy Composition (Wt%) CSI Mn PS Al Cr Mo Nb 10.3 10.10 1.9 20.01 00.00 100.03 00.00 50.00 10.03 520.2 10.15 2.3 00.01 10.00 90.02 40.05 00.04 00.029 30.26 0.05 2.1 00.00 80.00 150.025 0.12 00.08 00.019 40.29 0.15 2.000.01 00.00 100.03 50.04 00.05 00.025 50.3 10.2 11.800.0120.000120.0350.1200.0200.02560.220.051.500.0100.00110.0250.0500.0520.04570.220.201.300.0110.00180.0210.0100.0110.04180.320.153.000.0120.00090.0350.3500.0050.02990.300.051.500.0100.00100.0340.8000.0050.025

[0137] Steel Grade No. Alloy Composition (Wt%) TiBNNiCuXYY / X10.0200.00250.00400.1500.1604.0353.9313.6320.0210.00250.00360.2100.0602.6246.3517.6930.0310.00210.00410.1700.1003.1576.0524.1440.0250.00190.00420.0500.1804.1759.2514.2150.0350.0 0210.00410.0700.1003.1455.8117.7760.0250.00150.00250.0050.0050.4652.13113.3370.0310.00350.00240.0110.0 080.7529.0938.7980.0250.00150.00510.0120.0010.9288.4096.0990.0280.00140.00500.0120.0151.63134.8082.70X = 26.01[Cu]+3.88[Ni]+1.20[Cr]+1.49[Si]+17.28[P]-7.29[Cu][Ni]-9.10[Ni][P]-33.39[Cu] 2 Y = 100([Cr]+0.8[C]+0.07([Si]+[Mn])-0.1[Mo]-50[B]-70[P]-30[S])+100, where Y / X is the value calculated by rounding the significant figures of X and Y to 2 decimal places.

[0138] Classification Steel Grade No. Ar3(°C) Ms(°C) Mf(°C) Coiling Temperature(°C) Hot-rolled Steel Sheet Boundary density(mm² -1Yield Strength (YS) (MPa) Invention Example 1 17913211555051.39698 Invention Example 2 28153401754701.16654 Invention Example 3 38013301645001.19665 Invention Example 4 47993231564601.42702 Invention Example 5 57973231564501.52715 Comparative Example 1 68093742114501.12645 Comparative Example 2 78143822215501.09625 Comparative Example 3 8792263914002. 56956 Comparative Example 497913291634231.68780 Comparative Example 528153401753003.151185 Comparative Example 628153401754501.41701 Comparative Example 728153401754251.46712 Comparative Example 828153401754361.39695 Comparative Example 928153401754561.30684 Comparative Example 1028153401754511.31695 Comparative Example 1128153401754451.25652

[0139] Classification Steel Grade No. Thickness (mm) Cold Reduction Rate (%) Ac3 (°C) Annealing Temperature (°C) Annealing Time (sec) Primary Cooling End Temperature (T1) (°C) Primary Average Cooling Rate (CR1) (°C / s) Invention Example 1 1.45 279 58 251 257 253 Invention Example 2 2.1.45 280 7840 1327 313 Invention Example 3 3.1.45 280 08351 257 263 Invention Example 4 4.1.45 279 9830 1307 452 Invention Example 5 5.2.04 280 28251 057 542 Comparative Example 1 6.1.45 28248351 257 253 Comparative Example 2 7.1.45 28348451457363 Comparative Example 381.4527648001256653 Comparative Example 491.4527978251246453 Comparative Example 521.4288078901167412 Comparative Example 621.4528077801257152 Comparative Example 721.4528078951327213 Comparative Example 821.452807825257353 Comparative Example 921.2568078313507253 Comparative Example 1021.2568078251357353 Comparative Example 1121.2568078351257253

[0140] Classification Steel Grade No. Ms_1(°C) Mf_1(°C) Secondary Cooling Termination Temperature (T2)(°C) Secondary Average Cooling Rate (CR2)(°C / s) Reheating / Overaging Treatment Temperature (HT)(°C) Overaging Treatment Time (sec) HT-T2(°C) Invention Example 1 13 23 16 49 0 125 20 5 48 6 115 Invention Example 2 23 45 18 89 5 13 5 18 9 5 22 94 Invention Example 3 33 34 17 6 ​​89 14 5 19 25 10 103 Invention Example 4 43 26 16 79 5 15 21 78 48 683 Invention Example 5 53 25 16 6 90 120 17 9 48 089 Comparative Example 1 63 78 223 126 14 5 18 5 48 659 Comparative Example 2 7 38923513714219450457 Comparative Example 382589465146205486140 Comparative Example 4933017297145198522101 Comparative Example 523582019514017861283 Comparative Example 6233017310514519551090 Comparative Example 7235920211015218254672 Comparative Example 8234118595145197486102 Comparative Example 923431869514018248087 Comparative Example 10234118520595199522-6 Comparative Example 1123441871653019253427

[0141] Classification Microstructure (Weight%) Boundary density (mm²) -1)X_0.1ZF+BFM+TM Invention Example 101002.8610.523.6 Invention Example 201002.568.523.33 Invention Example 301002.659.253.49 Invention Example 401002.8211.253.99 Invention Example 501002.879.503.31 Comparative Example 101002.351.250.53 Comparative Example 201002.251.520.68 Comparative Example 301003.191. 960.61 Comparative Example 401002.894.251.47 Comparative Example 501001.948.264.26 Comparative Example 625751.655.653.42 Comparative Example 701001.937.563.92 Comparative Example 815851.752.951.69 Comparative Example 901001.938.524.41 Comparative Example 1010901.797.564.22 Comparative Example 1101001.856.523.52F: Ferrite, B: Bainite, FM: Fresh Martensite, TM: Tempered Martensite X_0.1 = 26.01[Cu]+3.88[Ni]+1.20[Cr]+1.49[Si]+17.28[P]-7.29[Cu][Ni]-9.10[Ni][P]-33.39[Cu] 2 Z = X_0.1 / boundary density

[0142] Classification Yield Strength (YS) (MPa) Tensile Strength (TS) (MPa) Elongation (EL) (%) Corrosion Loss (%) Hydrogen Embrittlement Crack Occurrence Invention Example 1 149 5177 57.16.2× Invention Example 2 129 5158 47.56.8× Invention Example 3 1350 168 57.26.5× Invention Example 4 148 5175 77.34.2× Invention Example 5 150 5178 96.95.9× Comparative Example 1 125 2152 17.615.2○ Comparative Example 2 123 4150 5 7.5 14.2 ○ Comparative Example 3 1590 185 25.6 14.1 ○ Comparative Example 4 148 51 77 56.8 13.6 ○ Comparative Example 5 119 51 46 77.8 6.9 × Comparative Example 6 105 21 40 25.6 7.1 × Comparative Example 7 118 61 46 47.6 5.9 × Comparative Example 8 108 91 436 7.9 13.2 ○ Comparative Example 9 1130 1 480 7.7 6.2 × Comparative Example 10 110 21 44 57.5 9.8 × Comparative Example 11 115 01 47 57.6 6.8 ×

[0143] Figure 1 shows the distribution of boundary density and X_0.1 for Inventive Examples 1 to 5 and Comparative Examples 1 to 11 of the present invention. As can be seen from Tables 1 to 7 and Figure 1, Inventive Examples 1 to 5, which satisfy the alloy composition and manufacturing conditions proposed by the present invention, satisfy the microstructure, boundary density, X_0.1, and Z proposed by the present invention, and thus not only are the mechanical properties excellent, but the corrosion resistance and hydrogen embrittlement resistance are also excellent.

[0144] Comparative Example 1 does not satisfy X, Y / X, and Ms_1, and thus does not satisfy X_0.1 and Z proposed by the present invention, so it can be seen that the corrosion resistance and hydrogen bromination resistance are insufficient.

[0145] Comparative Example 2 does not satisfy X and Ms_1, and thus does not satisfy X_0.1 and Z proposed by the present invention, so it can be seen that the corrosion resistance and hydrogen bromination resistance are insufficient.

[0146] Comparative Example 3 does not satisfy the Mn content, X, Y / X, and first cooling end temperature, and thus does not satisfy X_0.1 and Z proposed by the present invention, so it can be seen that the corrosion resistance and hydrogen breakage resistance are insufficient.

[0147] Comparative Example 4 does not satisfy Cr content, Y, Y / X, and primary cooling end temperature, and thus does not satisfy Z proposed by the present invention, so it can be seen that the corrosion resistance and hydrogen breakage resistance are insufficient.

[0148] It can be seen that Comparative Example 5 does not satisfy the cold rolling rate, annealing temperature, and Ms_1, and thus does not satisfy the boundary density proposed by the present invention, resulting in insufficient yield strength and tensile strength. In particular, it can be seen that the yield strength is at a high level because it does not satisfy the coiling temperature, and thus does not satisfy the boundary density of the hot-rolled steel sheet proposed by the present invention.

[0149] Comparative Example 6 does not satisfy the annealing temperature and therefore does not satisfy the microstructure and boundary density proposed by the present invention, and thus it can be seen that the yield strength and tensile strength are insufficient.

[0150] Comparative Example 7 does not satisfy the annealing temperature, Ms_1, and thus does not satisfy the boundary density proposed by the present invention, so it can be seen that the yield strength and tensile strength are insufficient.

[0151] Comparative Example 8 has an excessively short annealing time, so it does not satisfy the microstructure, boundary density, X_0.1, and Z proposed by the present invention, and thus it can be seen that the yield strength, tensile strength, corrosion resistance, and hydrogen bromination resistance are insufficient.

[0152] Comparative Example 9 shows that the yield strength is insufficient because the annealing time is excessively long and it does not satisfy the boundary density proposed by the present invention.

[0153] Comparative Example 10 does not satisfy the secondary cooling end temperature, HT-T2, and thus does not satisfy the microstructure and boundary density proposed by the present invention, and thus it can be seen that the yield strength and tensile strength are insufficient.

[0154] Comparative Example 11 does not satisfy the second average cooling rate and thus does not satisfy the boundary density proposed by the present invention, so it can be seen that the yield strength is insufficient.

Claims

In wt%, Carbon (C): 0.160–0.350%, Silicon (Si): 0.010–0.80%, Manganese (Mn): 0.30–2.60%, Phosphorus (P): ≤0.030% (excluding 0%), Sulfur (S): ≤0.0050% (excluding 0%), Aluminum (Al): 0.0030–0.080%, Chromium (Cr): 0.0010–0.60%, Molybdenum (Mo): 0.0010–0.40%, Niobium (Nb): 0.0010–0.10%, Titanium (Ti): 0.0050–0.20%, Boron (B): 0.00050–0.0070%, Nitrogen (N): ≤0.010% (excluding 0%), Copper (Cu): 0.0010~0.30%, Nickel (Ni): 0.0010~0.30%, containing the remainder Fe and other unavoidable impurities, It is divided into a central part; and a surface layer formed on the outer side of the thickness direction of the central part. The boundary density of the center above is 1.95~3.55mm -1 And, A cold-rolled steel sheet having X_0.1, expressed by Equation 1 below, at a point 0.1㎛ away from the surface in the thickness direction, with X_0.1 being 3.0 to 25.

0. [식 1] X_0.1 = 26.01[Cu]+3.88[Ni]+1.20[Cr]+1.49[Si]+17.28[P]-7.29[Cu][Ni]-9.10[Ni][P]-33.39[Cu] 2 (However, the above surface layer refers to the region extending up to 80㎛ in the thickness direction from the surface of the steel plate, the above center refers to the region outside the above surface layer, and in Equation 1, [] represents the content of each alloying element.) In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet satisfying the following equations 1 to 3. [Relationship Equation 1] 1.0 ≤ X = 26.01[Cu]+3.88[Ni]+1.20[Cr]+1.49[Si]+17.28[P]-7.29[Cu][Ni]-9.10[Ni][P]-33.39[Cu] 2 ≤ 6.0 [Equation 2] Y = 100([Cr]+0.8[C]+0.07([Si]+[Mn])-0.1[Mo]-50[B]-70[P]-30[S])+100 ≤ 120 [Equation 3] 3.0 ≤ Y / X ≤ 80.0 (However, in the above equations 1 to 3, [] represents the content of each alloying element.) In paragraph 1, A cold-rolled steel sheet in which the microstructure of the above-mentioned center 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 fresh martensite and tempered martensite. In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet satisfying the following relationship 4. [Relational Expression 4] 1.70 ≤ Z = X_0.1 / boundary density ≤ 10. 0 In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet having a yield strength (YS): 1200~1900MPa, a tensile strength (TS): 1470~2000MPa, and an elongation (EL): 3.0~12.0%. In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet with a corrosion loss of 13% or less. In paragraph 1, The above cold-rolled steel sheet is a cold-rolled steel sheet having a plating layer formed on at least one surface. In wt%, Carbon (C): 0.160–0.350%, Silicon (Si): 0.010–0.80%, Manganese (Mn): 0.30–2.60%, Phosphorus (P): ≤0.030% (excluding 0%), Sulfur (S): ≤0.0050% (excluding 0%), Aluminum (Al): 0.0030–0.080%, Chromium (Cr): 0.0010–0.60%, Molybdenum (Mo): 0.0010–0.40%, Niobium (Nb): 0.0010–0.10%, Titanium (Ti): 0.0050–0.20%, Boron (B): 0.00050–0.0070%, Nitrogen (N): ≤0.010% (excluding 0%), Copper (Cu): A step of heating a slab containing 0.0010~0.30% nickel (Ni): 0.0010~0.30%, the remainder being Fe and other unavoidable impurities; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above heated slab; Step of winding the above hot-rolled steel sheet; A step of obtaining a cold-rolled steel sheet by cold-rolling the above-mentioned coiled hot-rolled steel sheet; A step of continuously annealing the above cold-rolled steel sheet at a continuous annealing temperature (SS) of Ac3-5℃ to Ac3+65℃ for 50 to 200 seconds; A step of first cooling the above continuously annealed cold-rolled steel sheet; A second cooling step of cooling the above first cooled cold-rolled steel sheet to a cooling end temperature (T2) of Mf_1-150℃~Mf_1-20℃ at an average cooling rate (CR2) of 60~350℃ / s; and The method includes the step of reheating the above secondary cooled cold-rolled steel sheet and then performing over-aging treatment; A method for manufacturing cold-rolled steel sheets with Ms_1 of 356℃ or less. (However, Mf_1 above means Mf+0.26SS-205, and Mf above means 371-412[C]-17.4[Si]-47.4[Mn]-20.9[Cr]-17[Mo]+49.2[Nb]+95[Ti]+202[B], Ms_1 above means Ms+0.26SS-205, and Ms above means 521-379[C]-15.1[Si]-43.9[Mn]-19.5[Cr]-14.7[Mo]+43.7[Nb]+91.9[Ti]+169[B], and [] indicates the content of each alloying element.) In paragraph 8, The above slab is a method for manufacturing a cold-rolled steel sheet satisfying the following equations 1 to 3. [Relationship Equation 1] 1.0 ≤ X = 26.01[Cu]+3.88[Ni]+1.20[Cr]+1.49[Si]+17.28[P]-7.29[Cu][Ni]-9.10[Ni][P]-33.39[Cu] 2 ≤ 6.0 [Equation 2] Y = 100([Cr]+0.8[C]+0.07([Si]+[Mn])-0.1[Mo]-50[B]-70[P]-30[S])+100 ≤ 120 [Equation 3] 3.0 ≤ Y / X ≤ 80.0 (However, in the above equations 1 to 3, [] represents the content of each alloying element.) In paragraph 8, A method for manufacturing cold-rolled steel sheets in which the heating of the above slab is performed at 1100~1300℃. In paragraph 8, The above finishing hot rolling is a method for manufacturing cold-rolled steel sheets performed at Ar3 to Ar3+120℃. In paragraph 8, The above coiling is a method for manufacturing cold-rolled steel sheets performed at Ms~700℃. In paragraph 8, The boundary density of the steel sheet after coiling and before cold rolling is 3.0 mm -1 A method for manufacturing cold-rolled steel sheets having a yield strength of 1000 MPa or less and less than or equal to the following. In paragraph 8, The above cold rolling is a method for manufacturing a cold-rolled steel sheet, performed with a cold reduction rate of 40 to 70 percent. In paragraph 8, A method for manufacturing a cold-rolled steel sheet, wherein the above first cooling is performed at an average cooling rate (CR1) of 0.5 to 6.0℃ / s until the cooling end temperature (T1) of 670 to 785℃. In paragraph 8, The above over-aging treatment is a method for manufacturing cold-rolled steel sheets, performed at 90 to 270°C for 180 to 900 seconds. In paragraph 8, A method for manufacturing a cold-rolled steel sheet in which, during the above overaging treatment, the overaging treatment temperature (HT) - secondary cooling end temperature (T2) is controlled to be 15℃ or higher. In paragraph 8, A method for manufacturing a cold-rolled steel sheet, further comprising the step of forming a plating layer on at least one surface of the cold-rolled steel sheet after the above overaging treatment.

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