Steel sheet and manufacturing method thereof

Abstract

According to one aspect of the present invention, provided are a steel sheet suitable for cold forming due to having excellent bendability while having ultra-high strength and high ductility, and a manufacturing method thereof.

Description

Steel plate and method of manufacturing the same

[0001] The present invention relates to a steel plate suitable for use as an automobile material and a method for manufacturing the same.

[0002] Automakers are continuously pursuing vehicle lightweighting due to carbon dioxide emission regulations related to environmental issues. To reduce vehicle body weight, the thickness of the steel sheets used for the body must be decreased; however, since passenger safety must be ensured simultaneously, the adoption of ultra-high-strength steel is essential. Nevertheless, steel materials generally tend to exhibit decreased elongation as strength increases, posing a problem that limits the use of ultra-high-strength steel in parts requiring complex forming processes.

[0003] As a solution to overcome the aforementioned problems, a method utilizing steel obtained by hot press forming (hot-press forming steel) is proposed. Hot-press forming steel refers to steel that is imparted with ultra-high strength for parts requiring complex forming by heating steel sheets provided by steel companies to high temperatures, forming them, and cooling them to introduce a low-temperature transformation phase into the steel sheet. As an example, hot-press forming steel with a tensile strength of 1.5 GPa is applied and utilized as a material for automotive structural components that require complex forming and impact resistance, such as the A-pillar of an automobile.

[0004] However, manufacturers of automotive parts inevitably have to invest in hot forming equipment to obtain hot-formed steel, and there is a disadvantage in that the manufacturing cost of parts increases due to high-temperature heat treatment.

[0005] To overcome this, research is continuously being conducted on high-strength steel that can be cold-formed to replace hot-formed steel.

[0006] For example, Patent Document 1 discloses a steel sheet containing C and Mn in amounts exceeding a certain level, which has ultra-high strength and excellent impact resistance and bendability. However, while the steel sheet obtained from this technology has ultra-high strength, its elongation is less than 8%, so it can only be used for parts with relatively simple shapes formed by cold roll forming.

[0007] Patent Document 2 discloses a steel plate having ultra-high strength and a yield ratio of 0.7 or higher by adding a large amount of Mn of 6% or more. However, according to this technology, an increase in alloy costs is inevitable, and there is a risk that continuous casting and spot weldability will be reduced due to excessive Mn.

[0008] Accordingly, in providing cold-formed steel to replace hot-formed steel, there is a need to develop technology capable of securing high ductility along with ultra-high strength.

[0009] (Patent Document 1) Korean Published Patent Application No. 2014-0097332

[0010] (Patent Document 2) Korean Published Patent Application No. 2020-0027387

[0011] According to one aspect of the present invention, the aim is to provide a steel sheet suitable for cold forming that has high ductility along with ultra-high strength and excellent bendability, and a method for manufacturing the same.

[0012] The problems of the present invention are not limited to those described above. The problems of the present invention can be understood from the entirety of this specification, and those skilled in the art will have no difficulty understanding additional problems of the present invention.

[0013] One aspect of the present invention provides a steel sheet comprising, in weight percent, carbon (C): 0.20~0.30%, silicon (Si): 1.00~3.00%, manganese (Mn): 2.50~4.00%, aluminum (Al): 0.50% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.50% or less, titanium (Ti): 0.20% or less, vanadium (V): 0.10% or less, boron (B): 0.0100% or less, phosphorus (P): 0.050% or less (excluding 0%), sulfur (S): 0.020% or less (excluding 0%), nitrogen (N): 0.020% or less, and the remainder being Fe and other unavoidable impurities, satisfying the following equation 1.

[0014] [Relationship 1]

[0015] ((10×C)+Mn) / (Si+Al) ≤ 3.10

[0016] (In Equation 1, each element represents weight content.)

[0017] In one embodiment of the present invention, the surface layer of the steel plate up to a depth of 50 μm in the thickness direction from the surface may include a decarburized layer.

[0018] In one embodiment of the present invention, the decarburized layer may satisfy a slope value of 0.02 to 0.07 when linear regression analysis is performed on the graph of the carbon (C) content ratio measured according to the depth in the thickness direction.

[0019] In one embodiment of the present invention, the surface layer may comprise, in terms of area fraction, ferrite: 10% or more (including 100%), and the remainder being one or more of bainite, fresh martensite, and tempered martensite.

[0020] In one embodiment of the present invention, the central region, which is the remaining area excluding the surface layer, may comprise, in terms of area fraction, ferrite: 5% or less, fresh martensite: 10% or less, retained austenite: 5-20%, and the remainder bainite and tempered martensite.

[0021] In one embodiment of the present invention, the steel plate may have a yield strength of 1100 MPa or more, a tensile strength of 1470 MPa or more, and an elongation of 12.0% or more.

[0022] In one embodiment of the present invention, the steel plate has a thickness of 0.6 to 2.5 mm, and the relationship between the bending angle and the thickness during a three-point bending test can satisfy the following equation 2.

[0023] [Relationship 2]

[0024] (3-point bending angle (°) / thickness (mm) ≥ 42° / ㎜

[0025] In this way, by optimizing the alloy composition and the content of specific elements, along with satisfying the manufacturing conditions described below, a decarburized layer having a certain amount of ferrite can be formed on the surface layer of the steel sheet. Accordingly, the steel sheet according to the present invention has the effect of being suitable for cold forming, as it possesses ultra-high strength while having excellent bendability and formability.

[0026] In one embodiment of the present invention, the steel plate may include a plating layer on at least one surface.

[0027] Another aspect of the present invention comprises the steps of: preparing a steel slab satisfying the aforementioned alloy composition and Equation 1; heating the steel slab to a temperature range of 1050 to 1300°C; finishing hot rolling the heated steel slab at a temperature range of 800 to 1000°C to obtain a hot-rolled steel sheet; coiling the hot-rolled steel sheet at a temperature range of 400 to 700°C and cooling it to room temperature; cold rolling the coiled and cooled hot-rolled steel sheet at a cold reduction rate of 20 to 70% to obtain a cold-rolled steel sheet; continuously annealing the cold-rolled steel sheet at a temperature range of 810 to 880°C for at least 30 seconds; and cooling the continuously annealed cold-rolled steel sheet to a temperature range of 150 to 250°C. The present invention provides a method for manufacturing a steel plate comprising the step of reheating the cooled cold-rolled steel plate to a temperature range of 300 to 450°C and maintaining it for 100 to 1000 seconds.

[0028] In one embodiment of the present invention, the continuous annealing step may be performed in an atmosphere where the dew point temperature is maintained at -10 to 20°C.

[0029] In one embodiment of the present invention, the step of cooling the continuously annealed cold-rolled steel sheet can be performed at a cooling rate of 2 to 30°C / s.

[0030] In one embodiment of the present invention, the step of cooling the continuously annealed cold-rolled steel sheet can be performed by dividing it into a slow cooling section and a rapid cooling section.

[0031] In one embodiment of the present invention, the step of plating after reheating and holding may be further included, and as one example, the plating may be performed by hot-dip galvanizing in a plating bath at 430 to 490°C.

[0032] According to the present invention, it is possible to provide a steel plate having high ductility along with ultra-high strength and excellent bendability. In particular, the steel plate according to the present invention has the advantage of excellent impact resistance while possessing a level of formability suitable as a material for parts requiring difficult forming.

[0033] In addition, according to the present invention, since it can replace conventional hot-formed steel, it also has the economic effect of reducing the manufacturing cost of parts.

[0034] Figure 1 shows a photograph of the cross-sectional structure of the surface layer of a steel plate (Invention Example 1) according to one embodiment of the present invention, observed using SEM.

[0035] FIG. 2 shows a photograph of the cross-sectional structure of the surface layer of a steel plate (Comparative Example 3) according to another embodiment of the present invention, observed by SEM.

[0036] FIG. 3 is a graph showing the results of measuring GDOES on the surface of a steel plate (Invention Examples 2 and 3) according to one embodiment of the present invention, where the x-axis represents the depth in the thickness direction from the surface of the steel plate, and the y-axis represents the ratio of the C content measured at each depth in the thickness direction to the average C content (nominal carbon content) of the steel plate (C content at each depth / nominal C content).

[0037] Preferred embodiments of the present invention will be described below with reference to the attached drawings. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0038] 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.

[0039] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0040] 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.

[0041] 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.

[0042] In addition, in the present invention, the term "steel plate" refers to a coil or sheet material that has not yet been processed into a specific shape, and the term "part" refers to a part that has been processed into a non-plate shape through a forming process.

[0043] It should be noted that in the present invention, when expressing the content of each element, the basis is weight (weight%) unless specifically otherwise specified. Furthermore, the proportion of crystals or structures is based on area (area%) unless specifically otherwise expressed, and the gas content is based on volume unless specifically otherwise expressed.

[0044] 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.

[0045] The inventors of the present invention have completed the present invention as a result of conducting in-depth research to provide a cold forming steel that can replace existing hot forming steel, ensures mechanical properties equivalent to or better than those of said steel, and enables cost reduction during the manufacturing of parts.

[0046] The present invention will be described in detail below.

[0047] A steel sheet according to one aspect of the present invention may contain, in weight percent, carbon (C): 0.20~0.30%, silicon (Si): 1.00~3.00%, manganese (Mn): 2.50~4.00%, aluminum (Al): 0.50% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.50% or less, titanium (Ti): 0.20% or less, vanadium (V): 0.10% or less, boron (B): 0.0100% or less, phosphorus (P): 0.050% or less (excluding 0%), sulfur (S): 0.020% or less (excluding 0%), and nitrogen (N): 0.020% or less.

[0048] The reasons for limiting the alloy composition of the steel sheet according to one aspect of the present invention will be explained in detail below. Meanwhile, unless specifically stated otherwise, the content of each element is based on weight, and the ratio of the microstructure is based on area.

[0049] Carbon (C): 0.20~0.30%

[0050] Carbon (C) is an element effective for strengthening steel and can be added to control the stability of austenite and secure strength. In one embodiment of the present invention, C may be included in an amount of 0.20% or more to obtain the aforementioned effect. However, if the content of C exceeds 0.30%, there is a risk that weldability will be reduced.

[0051] Accordingly, in one embodiment of the present invention, C may be included in an amount of 0.20 to 0.30%. In another embodiment of the present invention, C may be included in an amount of 0.21% or more and 0.22% or more, and in yet another embodiment, C may be 0.29% or less and 0.28% or less.

[0052] Silicon (Si): 1.00~3.00%

[0053] Silicon (Si) is a major element of TRIP (Transformation Induced Plasticity) steel, which acts to increase the fraction of the retained austenite phase and improve elongation by inhibiting the precipitation of cementite. In one embodiment of the present invention, if the Si content is less than 1.00%, the retained austenite phase is almost non-existent, and the elongation of the steel is significantly reduced. On the other hand, if the content exceeds 3.00%, it causes LME cracking, making it impossible to prevent the deterioration of the weldment properties. In addition, the surface characteristics and plating properties of the steel may be impaired.

[0054] Accordingly, in one embodiment of the present invention, Si may be included in an amount of 1.00 to 3.00%. In another embodiment of the present invention, the Si may be included in an amount of 1.05% or more and 1.10% or more, and in yet another embodiment, the Si may be 2.90% or less and 2.80% or less.

[0055] Manganese (Mn): 2.50~4.00%

[0056] Manganese (Mn) is an element added to ensure the strength of steel. In one embodiment of the present invention, if the Mn content is less than 2.5%, the target strength cannot be secured. On the other hand, if the content exceeds 4.0%, the phase transformation rate during the steel manufacturing process slows down, and as fresh martensite is excessively formed, there is a risk that formability will be reduced. In addition, as a band structure is formed due to Mn segregation, the material uniformity and formability of the steel are reduced.

[0057] Accordingly, in one embodiment of the present invention, Mn may be included in an amount of 2.50 to 4.00%. In another embodiment of the present invention, the Mn may be included in an amount of 2.55% or more and 2.60% or more, and in yet another embodiment, the Mn may be 3.90% or less and 3.80% or less.

[0058] Aluminum (Al): 0.50% or less

[0059] Aluminum (Al) is an element that contributes to the stabilization of residual austenite by suppressing the formation of carbides within ferrite, and can also be added for the deoxidation of steel. In one embodiment of the present invention, if the Al content exceeds 0.50%, the strength of the steel is reduced, and a sound slab cannot be produced through reaction with mold flux during casting. In addition, there is a problem of impeding plating properties by forming surface oxides.

[0060] Accordingly, in one embodiment of the present invention, Al may be included in an amount of 0.50% or less. However, 0.00% is excluded from the content of Al to account for the level inevitably added during the steel manufacturing process. In another embodiment of the present invention, Al may be 0.48% or less, or 0.45% or less. In yet another embodiment, Al may be 0.01% or more, or 0.03% or more.

[0061] Chrome (Cr): 1.0% or less

[0062] Chromium (Cr) is an effective element for ensuring the strength and hardenability of steel. When Cr is added, the problem of having to add an excessive amount of Mn when Mn is added alone in steel can be resolved. In one embodiment of the present invention, if the Cr content exceeds 1.0%, localized corrosion is impaired and oxides are formed on the surface, which causes phosphate treatment to be hindered.

[0063] Accordingly, in one embodiment of the present invention, Cr may be included in an amount of 1.0% or less. Meanwhile, in the present invention, the target physical properties can be obtained even without intentionally adding the above Cr, so it is acceptable even if the content is 0.0%.

[0064] In another embodiment of the present invention, the Cr may be 0.9% or less and 0.8% or less. In yet another embodiment, the Cr may be 0.1% or more and 0.15% or more.

[0065] Molybdenum (Mo): 0.50% or less

[0066] Molybdenum (Mo) is an element that forms carbides in steel, and when combined with carbide-nitride forming elements such as Ti, Nb, and V, it plays a role in improving the yield strength and tensile strength of steel by maintaining the size of the formed precipitates finely. In one embodiment of the present invention, if the Mo content exceeds 0.50%, the aforementioned effect becomes saturated and instead causes an increase in manufacturing costs.

[0067] Accordingly, in one embodiment of the present invention, Mo may be included in an amount of 0.50% or less. Meanwhile, in the present invention, the target physical properties can be obtained even without intentionally adding the above Mo, so the content may be 0.00%.

[0068] In another embodiment of the present invention, the Mo may be 0.45% or less and 0.40% or less. In yet another embodiment, the Mo may be 0.01% or more and 0.03% or more.

[0069] Titanium (Ti): 0.20% or less

[0070] Titanium (Ti) is an element that forms fine carbides in steel, contributing to the improvement of the yield strength and tensile strength of steel. In addition, the above-mentioned Ti is an element that forms nitrides, and by precipitating N in the steel as TiN, it has the effect of suppressing the precipitation of AlN, thereby having the advantage of reducing the risk of crack formation during continuous casting. In one embodiment of the present invention, if the Ti content exceeds 0.20%, not only are coarse carbides precipitated, but there is also a risk that the strength and ductility of the steel will decrease as the carbon content in the steel is reduced. Furthermore, it may cause nozzle clogging during continuous casting.

[0071] Accordingly, in one embodiment of the present invention, Ti may be included in an amount of 0.20% or less. Meanwhile, in the present invention, the target physical properties can be obtained even without intentionally adding the Ti, so the content may be 0.00%.

[0072] In another embodiment of the present invention, the Ti may be 0.19% or less and 0.18% or less. In yet another embodiment, the Ti may be 0.01% or more and 0.03% or more.

[0073] Vanadium (V): 0.10% or less

[0074] Vanadium (V) is also an element that reacts with carbon or nitrogen in steel to form carbon or nitrides, and plays an important role in increasing the yield strength of steel by forming fine precipitates at low temperatures. In one embodiment of the present invention, if the V content exceeds 0.10%, not only are coarse precipitates formed, but there is also a risk that the strength and ductility of the steel will decrease as the carbon content in the steel is reduced. In addition, V is an expensive element, and if its content is excessive, it can increase manufacturing costs.

[0075] Accordingly, in one embodiment of the present invention, V may be included in an amount of 0.10% or less. Meanwhile, in the present invention, the target physical properties can be obtained even without intentionally adding V, so the content may be 0.00%.

[0076] In another embodiment of the present invention, V may be 0.09% or less and 0.08% or less. In yet another embodiment, V may be 0.01% or more and 0.02% or more.

[0077] Boron (B): 0.0100% or less

[0078] Boron (B) is an element that delays the transformation of austenite into ferrite during the cooling process after heat treatment of steel, and contributes to improving hardenability by promoting the formation of martensite. In one embodiment of the present invention, if the content of B exceeds 0.0100%, B may become excessively concentrated on the steel surface, thereby impairing plating adhesion.

[0079] Accordingly, in one embodiment of the present invention, B may be included in an amount of 0.0100% or less. Meanwhile, in the present invention, the target physical properties can be obtained even without intentionally adding B, so the content may be 0.0000%.

[0080] In another embodiment of the present invention, B may be 0.0090% or less, 0.0080% or less. In yet another embodiment, B may be 0.001% or more.

[0081] Phosphorus (P): 0.050% or less (excluding 0%)

[0082] Phosphorus (P) is an element that contributes to the improvement of steel strength through solid solution strengthening, but if its content exceeds 0.050%, it impairs the weldability of the steel and increases the risk of brittleness.

[0083] Accordingly, in one embodiment of the present invention, P may be included in an amount of 0.050% or less. However, 0.000% is excluded to account for the level inevitably introduced during the steel manufacturing process. In another embodiment of the present invention, the P may be 0.040% or less, or 0.020% or less.

[0084] Sulfur (S): 0.020% or less (excluding 0%)

[0085] Sulfur (S) is an impurity inevitably contained during the steel manufacturing process and is an element that impairs the ductility and weldability of steel. In one embodiment of the present invention, if the S content exceeds 0.020%, the likelihood of impairing the ductility and weldability of steel increases.

[0086] Accordingly, in one embodiment of the present invention, S may be included in an amount of 0.020% or less. However, 0.000% is excluded to account for the level inevitably introduced during the steel manufacturing process. In another embodiment of the present invention, the S may be 0.015% or less, or 0.010% or less.

[0087] Nitrogen (N): 0.020% or less

[0088] Nitrogen (N) is an element that contributes to the improvement of steel strength through solid solution strengthening, but if its content exceeds 0.020%, it increases the risk of brittleness in steel and can impair continuous casting quality by combining with Al in the steel to precipitate AlN excessively.

[0089] Accordingly, in one embodiment of the present invention, N may be included in an amount of 0.020% or less. However, 0.000% is excluded to account for the level inevitably introduced during the steel manufacturing process. In another embodiment of the present invention, N may be 0.015% or less, or 0.010% or less.

[0090] The remaining component of the present invention 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 person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0091] The content relationship of C, Mn, Si, and Al among the alloying elements constituting the steel plate according to one embodiment of the present invention can satisfy the following Equation 1.

[0092] [Relationship 1]

[0093] ((10×C)+Mn) / (Si+Al) ≤ 3.1

[0094] (In Equation 1, each element represents weight content.)

[0095] As will be explained in detail below, a steel plate according to one embodiment of the present invention may include a decarburized layer on its surface having a lower carbon content relative to the carbon content of the steel plate (which may be referred to as 'norminal content'). The structure of the decarburized layer is mainly composed of ferrite, and from this, the ductility, bendability, formability, etc. of the steel plate can be improved.

[0096] Meanwhile, in one embodiment of the present invention, the decarburization layer may be formed during the annealing heat treatment of a cold-rolled steel sheet obtained by cold rolling, which is part of a series of processes for manufacturing a steel sheet. Although the annealing heat treatment conditions will be described in detail below, a decarburization reaction can be induced on the surface of the steel sheet (cold-rolled steel sheet) by controlling the dew point temperature inside the annealing furnace, and at this time, the decarburization reaction is influenced by the alloying elements of the steel sheet. The inventors of the present invention have conducted in-depth research on methods to promote the decarburization reaction during the annealing heat treatment process of a steel sheet (cold-rolled steel sheet), and discovered that when the steel sheet is heated for annealing heat treatment, austenite with high carbon solubility appears at a higher temperature; in other words, it is advantageous as the appearance of austenite is delayed. Along with this, recognizing that among the alloying elements constituting the steel sheet, C and Mn play a role in lowering the austenite transformation temperature, while Si and Al play a role in raising the austenite appearance temperature, they came to derive the aforementioned relationship Equation 1.

[0097] That is, by satisfying Equation 1 according to one embodiment of the present invention, it is possible to induce the decarburization reaction on the surface of the steel plate to be promoted during the annealing heat treatment process in the process of manufacturing the steel plate.

[0098] At this time, the smaller the value of the above relationship 1, the better the decarburization reaction can occur. More specifically, cooling is performed after the annealing heat treatment, and in order for ferrite transformation to occur well in the decarburized layer formed on the surface of the steel sheet during this cooling process, the hardenability must be low. C and Mn are elements that increase hardenability, while Si and Al are elements that promote the formation of ferrite. Therefore, the smaller the value of the above relationship 1, the better the decarburization reaction occurs during the annealing heat treatment process, and at the same time, the formation of ferrite in the decarburized region can be further promoted during the subsequent cooling process.

[0099] In one embodiment of the present invention, if the value of the above relationship 1 exceeds 3.1, the decarburization reaction is delayed, and as the formation of ferrite in the decarburized region during cooling is not smooth, the effect of improving the bendability of the steel sheet cannot be sufficiently obtained.

[0100] As previously mentioned, a steel plate according to one embodiment of the present invention includes a decarburization layer on its surface, and specifically, when the area from the surface to a depth of 50 μm in the thickness direction is referred to as the surface layer region, the decarburization layer exists within this surface layer region.

[0101] In one embodiment of the present invention, when linear regression analysis (curve fitting) is performed on the graph of the carbon (C) content ratio according to the depth in the thickness direction of the decarburized layer, the slope value can satisfy 0.02 to 0.07.

[0102] Referring to the drawings, FIG. 3 is a graph showing data (points) measured by a GDOES (Glow discharge optical emission spectrometer) on the surface of a steel plate (Invention Examples 2 and 3) according to one embodiment of the present invention, and a slope (straight line) derived by linear regression analysis. The GDOES concentration profile can measure the concentration (content) of carbon (C) contained in the steel plate at intervals of 5 to 10 nm in the thickness direction from the surface of the steel plate. As one example, the C concentration can be measured at intervals of 7 nm in the thickness direction of the steel plate. As shown in FIG. 3, the x-axis represents the depth in the thickness direction from the surface of the steel plate, and the y-axis represents the ratio of the C content measured at each depth in the thickness direction to the average C content (nominal carbon content) of the steel plate (C content at each depth / nominal carbon content). At this time, in order to remove noise values ​​from the extreme surface of the steel plate, values ​​where the ratio of C content (y-axis) is less than 0.1 are excluded, and all data corresponding to 0.1 or greater and 1.0 or less are taken (the points shown in Fig. 3 correspond to each data). In this way, all data in the region where the ratio of C content is 0.1 to 1.0 are taken, and the slope can be obtained through linear regression analysis by least squares fitting on the data. As a result, in the case of Invention Example 2, the slope value is 0.03, and in the case of Invention Example 3, the slope value is 0.07, and it can be seen that the decarburized layer of Invention Example 2, which has a smaller slope value, is formed more deeply.

[0103] In one embodiment of the present invention, when measuring the carbon (C) content according to the depth in the thickness direction from the surface of the steel plate, if the inclination value of the decarburization layer present in the surface layer exceeds 0.07, the decarburization layer is hardly formed in the surface layer, and thus the formability and bendability of the steel plate cannot be secured. On the other hand, if the value is less than 0.02, the decarburization layer is formed too excessively relative to the total thickness of the steel plate, and thus the target level of strength, such as ultra-high strength exceeding 1400 MPa tensile strength, cannot be secured.

[0104] Meanwhile, according to one embodiment of the present invention, the surface layer region including the decarburization layer has a microstructure mainly composed of ferrite, and the fraction of said ferrite is not particularly limited. However, in order to secure an elongation of the steel sheet above a certain level, the ferrite within the surface layer region may be included in an area fraction of 10% or more, and in this case, the fraction of said ferrite may be 100%. The remaining structure excluding said ferrite is not particularly limited, but may be one or more of bainite, fresh martensite, and tempered martensite.

[0105] A steel plate according to one embodiment of the present invention includes a central region as the remaining region excluding the aforementioned surface region based on the thickness direction, and the central region may have a microstructure in terms of area fraction of ferrite: 5% or less, fresh martensite: 10% or less, retained austenite: 5~20%, and the remainder being bainite and tempered martensite.

[0106] In one embodiment of the present invention, in order to secure ultra-high strength, it is advantageous for the microstructure of the central region based on the thickness direction of the steel plate to be mainly composed of bainite and tempered martensite. The fractions of each phase of bainite and tempered martensite are not specifically limited, and if bainite and tempered martensite exist in combination while satisfying the respective phase fractions shown below, it shall be considered to be within the scope of the present invention.

[0107] In one embodiment of the present invention, the ferrite is a structure advantageous for securing the elongation of the steel sheet and may be included up to 5%, but if it exceeds 5%, it becomes impossible to secure the target level of strength. At this time, the ferrite may be 0%.

[0108] In one embodiment of the present invention, the residual austenite is also a structure advantageous for securing the elongation of the steel sheet and may be included in an amount of 5 to 20%. If the fraction of the residual austenite is less than 5%, the target elongation cannot be secured, and thus, improvement in formability cannot be achieved. On the other hand, if the fraction exceeds 20%, the mechanical stability of the residual austenite is lowered, and it rapidly transforms into martensite even under low stress during forming, thereby reducing the effect of contributing to the elongation.

[0109] Meanwhile, a steel plate according to one embodiment of the present invention may partially contain a fresh martensite phase, wherein the fresh martensite phase is a structure advantageous for securing strength and may be included up to 10%, or 0%. If the fresh martensite phase is included in an amount exceeding 10%, while it is advantageous for improving strength, the steel plate becomes brittle and prone to breaking, or the elongation is significantly reduced, making it impossible to secure formability, bendability, etc.

[0110] A steel plate according to one embodiment of the present invention not only has ultra-high strength, but also excellent formability and bendability, and can have an elongation rate of at least a certain amount suitable for this.

[0111] As one example, the steel plate may have a yield strength of 1100 MPa or more, a tensile strength of 1470 MPa or more, and an elongation of 12.0% or more. In addition, the relationship between the bending angle and the thickness of the steel plate during a three-point bending test may satisfy the following Equation 2. At this time, the steel plate may have a thickness of 0.6 to 2.5 mm.

[0112] [Relationship 2]

[0113] (3-point bending angle (°) / thickness (mm) ≥ 42° / ㎜

[0114] In one embodiment of the present invention, Equation 2 is a physical property equation representing the formability and bendability of a steel plate, and it can be determined that the greater the three-point bending angle while the thickness of the steel plate is constant, the better the bendability, and together with this, the better the formability.

[0115] Meanwhile, a steel plate according to one embodiment of the present invention may include a plating layer on its surface. Here, the term "surface" refers to the surface of a surface layer where a decarburization layer is formed, and the plating layer may exist on one or both sides of the steel plate.

[0116] In one embodiment of the present invention, the type of plating layer is not specifically limited, but as one example, it may be a zinc-based plating layer. As a non-limiting example, the zinc-based plating layer may be one of a molten zinc plating layer (GI), an alloyed molten zinc plating layer (GA), and an electro-zinc plating layer (EG).

[0117] Hereinafter, a method for manufacturing a steel sheet according to another aspect of the present invention will be described in detail. It should be noted that the following manufacturing method corresponds to one example for manufacturing a steel sheet according to one embodiment of the present invention, particularly a cold-rolled steel sheet, and a galvanized steel sheet using said cold-rolled steel sheet.

[0118] A steel plate according to one embodiment of the present invention can be manufactured by subjecting a prepared steel slab to the [heating - hot rolling - coiling - cold rolling - annealing - cooling] process, and each process step is described in detail below.

[0119] [Heating Steel Slabs]

[0120] First, a steel slab is prepared. Afterward, the prepared steel slab can be heated. The heating process of the steel slab is a process to facilitate the hot rolling process described later and to sufficiently obtain the target physical properties of the steel plate.

[0121] As one example, the above steel slab may have the same alloy composition and alloy component relationship formula (Relationship Formula 1) as the steel plate according to one embodiment of the present invention, and the description of each alloy element and the description of the component relationship formula are replaced by the aforementioned matters.

[0122] In one embodiment of the present invention, the heating of the steel slab can be performed in a temperature range of 1050 to 1300°C. If the heating temperature is less than 1050°C, there is a problem that the load increases rapidly during subsequent hot rolling. On the other hand, if the temperature exceeds 1300°C, there is a concern that excessive surface scale will be generated, leading to material loss and increased energy costs.

[0123] [Hot Rolled]

[0124] A hot-rolled steel sheet can be obtained by hot-rolling the above heated steel slab.

[0125] In one embodiment of the present invention, a hot-rolled steel sheet can be manufactured by performing a finishing hot rolling in a temperature range of 800 to 1000°C during the hot rolling. In one embodiment of the present invention, if the temperature during the finishing hot rolling is less than 800°C, there is a problem that the rolling load increases significantly. On the other hand, if the temperature exceeds 1000°C, surface defects may occur due to scale, and there is a risk of causing a shortened lifespan of the rolling rolls.

[0126] [Winding and Cooling]

[0127] The hot-rolled steel sheet manufactured above can be cooled after being wound.

[0128] In one embodiment of the present invention, the coiling process can be performed in a temperature range of 400 to 700°C. If the coiling temperature is below 400°C, the strength of the hot-rolled steel sheet becomes excessively high, which may cause a rolling load during subsequent cold rolling. In addition, cooling is required to lower the temperature of the steel sheet, and processes such as spraying cooling water are required, which may lead to an increase in unnecessary process costs. On the other hand, if the temperature exceeds 700°C, excessive scale may form on the surface of the hot-rolled steel sheet, causing defects, which is a cause of impeding plating properties.

[0129] Meanwhile, in one embodiment of the present invention, cooling may be performed after winding, and the cooling may be achieved by leaving the wound state as is, rather than by using a specific facility. Accordingly, the cooling speed during cooling is not specifically limited, and the cooling may be performed down to room temperature.

[0130] [Cold Rolled]

[0131] The above-mentioned coiled and cooled hot-rolled steel sheet can be cold-rolled to produce a cold-rolled steel sheet. The above-mentioned cold rolling is a process performed to control the shape and thickness of the steel sheet.

[0132] In one embodiment of the present invention, cold rolling can be performed at a cold reduction rate of 20 to 70%. If the cold reduction rate is less than 20%, it is difficult to secure the target thickness, and it is also difficult to correct the shape of the steel sheet. On the other hand, if the cold reduction rate exceeds 70%, the likelihood of cracks occurring at the edge of the steel sheet increases, and there is a problem of causing a cold rolling load.

[0133] Meanwhile, prior to performing the above cold rolling, the hot-rolled steel sheet may be pickled to remove the oxide layer formed on its surface. At this time, the pickling treatment may be performed under normal conditions, and such conditions are not specifically limited.

[0134] [Continuous Annealing]

[0135] The above-mentioned cold-rolled steel sheet can be annealed, and as an example, this can be done in a continuous annealing furnace.

[0136] In one embodiment of the present invention, the continuous annealing may be a process of heating the cold-rolled steel sheet to a temperature range of 810 to 880°C and maintaining it for 30 seconds or more. That is, by heating the steel sheet to an austenite single-phase region, austenite close to 100% is formed, and then a phase transformation can be induced in a subsequent process.

[0137] In one embodiment of the present invention, if the temperature during continuous annealing is less than 810°C, the austenite reverse transformation is not sufficiently achieved, and the ferrite phase may be formed in excess of 5% during the cooling process after annealing, in which case the target strength cannot be secured. On the other hand, if the temperature exceeds 880°C, surface oxides may be excessively formed, which may lower the surface quality and productivity of the steel sheet. In addition, the elongation and strength of the steel sheet may be reduced due to the formation of coarse austenite.

[0138] Meanwhile, as previously mentioned, the continuous annealing described above can be performed in a continuous annealing furnace, and by controlling the dew point temperature of the continuous annealing furnace, a decarburization reaction can be induced on the surface of the steel sheet. In one embodiment of the present invention, the dew point temperature of the continuous annealing furnace can be controlled to -10 to 20°C, and by performing annealing in an annealing furnace with such a controlled dew point temperature, a decarburization reaction occurs in which carbon on the surface of the steel sheet is discharged into the atmosphere of the annealing furnace. Ferrite may be generated on the surface of the steel sheet where the decarburization reaction has occurred, and if the amount of decarburization reaction increases, the fraction of ferrite also increases. Furthermore, as decarburization occurs at a high temperature, the crystal grains on the surface of the steel sheet may grow and become coarser.

[0139] In one embodiment of the present invention, if the dew point temperature in the annealing furnace is less than -10°C, the decarburization reaction is insufficient, making it impossible to sufficiently form a decarburized layer, and consequently, the bendability of the steel sheet cannot be improved. In particular, when performing linear regression analysis after measuring the carbon (C) content according to the depth in the thickness direction from the surface of the final steel sheet, the slope may exceed 0.07. On the other hand, if the temperature exceeds 20°C, the decarburized layer may be excessively formed, and in this case, the ratio of the thickness of the decarburized layer containing ferrite as the main structure to the total thickness of the steel sheet increases, making it impossible to secure the target strength.

[0140] [cooling]

[0141] Cooling can be performed after continuous annealing according to the above.

[0142] In one embodiment of the present invention, the cooling may be performed in a temperature range of 150 to 250°C. That is, by cooling the continuously annealed steel sheet to a temperature lower than the martensite transformation start temperature (Ms), a martensite transformation can be induced during the cooling process. The martensite phase formed at this time can be finally transformed into tempered martensite by undergoing a subsequent reheating process.

[0143] In one embodiment of the present invention, if the cooling end temperature is less than 150°C, the martensite undergoes excessive transformation, resulting in an insufficient fraction of the retained austenite phase in the final structure, which may lower the elongation of the steel sheet. Additionally, the yield strength may become excessively high, making forming difficult. On the other hand, if the cooling end temperature exceeds 250°C, the martensite phase is not sufficiently formed during the cooling process, making it impossible to secure the stability of the retained austenite phase. Furthermore, the fraction of the fresh martensite phase becomes excessive, resulting in a significantly higher tensile strength, while the yield strength and elongation cannot be obtained at the target level.

[0144] In one embodiment of the present invention, when cooling to the aforementioned cooling end temperature, the cooling rate may satisfy 2 to 30°C / s. If the cooling rate is less than 2°C / s, there is a problem in that ferrite is excessively formed in the central region excluding the surface layer during the cooling process, thereby reducing the strength of the steel sheet. On the other hand, if the cooling rate exceeds 30°C / s, the formation of ferrite is suppressed in the decarburized region of the surface, and there is a risk that the bendability of the steel sheet will be degraded.

[0145] Meanwhile, in one embodiment of the present invention, in order to perform cooling to the aforementioned cooling end temperature, the cooling may be divided into a slow cooling section with a relatively slow cooling rate and a rapid cooling section with a relatively fast cooling rate. At this time, the order of the slow cooling section and the rapid cooling section is not specifically limited, but it is preferable that the cooling rate in each section be controlled to 2 to 30℃ / s.

[0146] [Reheating and Maintaining]

[0147] Reheating can be performed after the above cooling.

[0148] In one embodiment of the present invention, a steel plate that has been continuously annealed and cooled according to the above can be heated to a temperature range of 300 to 450°C and maintained for 100 to 1000 seconds. Through this reheating process, the distribution of carbon between phases necessary for stabilizing residual austenite can be induced, and bainite transformation can be additionally induced.

[0149] In one embodiment of the present invention, if the reheating temperature is less than 300°C, the carbon of the martensite generated during the cooling process is not sufficiently redistributed to the surrounding austenite, making it difficult to secure the stability of the retained austenite. In addition, the tempering of the martensite becomes insufficient, resulting in a problem where the strength of the final steel sheet becomes excessively high and the elongation becomes inferior. On the other hand, if the temperature exceeds 450°C, the tempering of the martensite becomes excessive, making it impossible to secure the target strength of the final steel sheet. Furthermore, the elongation of the final steel sheet may decrease as the stability of the retained austenite is reduced due to the precipitation of cementite.

[0150] The steel sheet manufactured through the aforementioned series of processes includes a decarburized layer in the surface layer, so it not only has excellent bendability and formability, but also has a microstructure in the core mainly composed of bainite and tempered martensite and contains a suitable fraction of retained austenite, thereby possessing ultra-high strength and high ductility.

[0151] In one embodiment of the present invention, a plating step after reheating and holding may be further included. That is, a plated steel plate can be obtained by plating a steel plate that has been reheated and held according to the above.

[0152] In one embodiment of the present invention, the plating process is not specifically limited, but as one example, it may be a process of molten zinc plating in a plating bath at 430 to 490°C. The composition of the plating bath is not specifically limited and may be a pure zinc plating bath or a zinc-based alloy plating bath containing Si, Al, Mg, etc.

[0153] Additionally, the process may further include a step of alloying heat treatment after the above-mentioned hot-dip galvanizing treatment, from which an alloyed hot-dip galvanized steel sheet can be obtained. The conditions of the alloying heat treatment process are not specifically limited, and normal conditions are acceptable. As an example, the alloying heat treatment process may be performed in a temperature range of 480 to 600°C.

[0154] 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.

[0155] (Example)

[0156] After preparing steel slabs having the alloy composition system shown in Table 1 below, each steel slab was held at a temperature of 1200°C for 1 hour, and then hot-rolled to produce hot-rolled steel sheets. At this time, finishing hot rolling was performed at 900°C, and subsequently, the hot-rolled steel sheets were loaded into a furnace preheated to 600°C and held for 1 hour, after which furnace cooling was performed to simulate hot-rolled coiling. Subsequently, each hot-rolled steel sheet was cooled to room temperature and then cold-rolled with a cold reduction rate of 40% to obtain cold-rolled steel sheets. Afterward, continuous annealing, cooling, reheating, and holding processes were performed according to the conditions shown in Table 2 below. At this time, the continuous annealing temperature was held for 300 seconds, and the steel was also held for 300 seconds after reheating to obtain the final steel sheets.

[0157] Tensile tests were performed on each steel plate manufactured according to the above to measure tensile properties (yield strength, tensile strength and elongation), and the measurements were taken using a universal tensile testing machine in accordance with ISO 6892 standards with JIS-5 specimens.

[0158] In addition, to measure the microstructure of each steel plate, the collected specimens were etched using the Nital etching method, and then the types and fractions of each phase were measured using a scanning electron microscope (SEM) and an image analyzer. At this time, the microstructure of the surface layer (the region from the surface to a thickness of 50 μm) and the center (the 1 / 4t point, where t represents the steel plate thickness (mm)) of each steel plate was observed. It should be noted that the microstructure results measured at the 1 / 4t point in the steel plate thickness direction represent the microstructure of all regions except the surface layer. Meanwhile, the microstructure results (fractions of each phase) in the surface layer were not described.

[0159] Then, a three-point bending test was performed on each steel plate. Specifically, the bending angle at maximum load was measured according to the VDA238-100 test method. Equation 2 was calculated from this bending angle and the thickness of the steel plate.

[0160] Meanwhile, to evaluate whether a sufficient decarburization layer was formed on the surface of the steel plates, the carbon (C) content along the thickness direction depth from the surface of each steel plate was measured using GDEOS, and linear regression analysis was performed on the graph of the measurement results to obtain the slope value.

[0161] The results of each test are shown in Table 3 below.

[0162] Relationship between Steel Grade, Alloy Composition (Weight%) 1 CS i M n Al Cr Mo Ti V B PS NA 0.25 1.2 2.7 0.40 -0.2 0.1 0.0 5 0.00 2 0.0 1 0.0 5 0.00 2 3.25 B 0.25 2.0 3.5 0.0 2 -0.1 0.1 0.0 5 0.00 2 0.0 8 0.0 5 0.00 3 2.97 C 0.25 2 .53.00.02--0.100.050.00200.0110.0090.0022.18D0.272.03.00.02--0.02- 0.00250.0090.0060.0042.82E0.272.03.30.02--0.02-0.00250.0120.0090.0 032.97F0.272.02.80.02-0.100.10-0.00250.0090.0020.0042.72G0.252.03 .00.020.40.100.04-0.00200.0090.0050.0042.72H0.252.03.50.02-0.100.0 4-0.00200.0100.0070.0022.97I0.252.03.50.020.40.100.04-0.00200.0090 .0060.0042.97J0.252.02.60.10-0.200.100.050.00200.0120.0040.0042.43

[0163] Steel Grade Continuous Annealing (°C) Cooling Termination Temperature (°C) Reheating Temperature (°C) Classification Dew Point Temperature Annealing Temperature A 10 850 200 300 Comparative Example 1 B 11 850 160 380 Invention Example 1 B 11 850 120 380 Comparative Example 2 B 11 850 160 425 Invention Example 2 C 4 880 180 380 Invention Example 3 C 8880 160 410 Invention Example 4 D 4 1860 160 350 Comparative Example 3 D 7860 200 350 Invention Example 5 E 8860 180 350 Invention Example 6 E 9860 160 380 Invention Example 7 E 9800 180 380 Comparative Example 4 F 12 860 220 380 Invention Example 8 F 12 860 160 470 Comparative Example 5G10850200350Invention Example 9G11880200350Invention Example 10G10850270350Comparative Example 6H15850200350Invention Example 11H13880230350Invention Example 12H25880200350Comparative Example 7I5830120350Comparative Example 8I7830180350Invention Example 13I8850180350Invention Example 14I-31850200350Comparative Example 9J10850200350Invention Example 15

[0164] Classification Thickness (mm) Mechanical Properties Decarburization Gradient Microstructure (Area Fraction, %) YS(MPa) TS(MPa) El(%) Relationship 2(° / mm) TM+BFFMRA Comparative Example 1 1.2 1 1 50 1 49 9 1 2.4 38 0.0 8 79 38 10 Inventive Example 1 1.4 1 39 9 1 56 5 1 2.9 5 1 0.0 2 78 37 12 Comparative Example 2 1.4 1 45 0 1 55 8 1 0.6 39 0.0 38 8 35 4 Inventive Example 2 1.4 1 42 7 1 53 0 1 2.1 44 0.0 38 0 27 11 Inventive Example 3 1.4 1 34 1 1 51 7 1 2.7 43 0.0 78 3-89 Inventive Example 41.41348148112.5470.0583188 Comparative Example 31.4128615428.7390.0983278 Inventive Example 51.41233148812.6490.04792712 Inventive Example 61.41169148214.2430.0480-911 Inventive Example 71.41282148812.4470.0383-710 Comparative Example 41.2857145814.0380.046615154 Inventive Example 81.21348149413.2500.0381289 Comparative Example 51.2149615786.4350.0489353 Invention Example 91.21234151012.9430.05801514 Invention Example 101.21212149212.8470.05772615 Comparative Example 61.2850151112.8430.05692254 Invention Example 111.21159149614.1440.0580-713 Invention Example 121.21143148013.8490.0585-411 Comparative Example 71.21072145614.5460.008802711 Comparative Example 81.2138015598.9430.0590-64 Invention Example 131.21241154812.3470.0480-317 Invention Example 141.21246153112.9510.06802711 Comparative Example 91.21102153013.6350.0880299 Invention Example 151.21188147515.0440.0580389 In Table 3, TM represents tempered martensite, B represents bainite, F represents ferrite, FM represents fresh martensite, and RA represents retained austenite phase.

[0165] As shown in Tables 1 to 3 above, Invention Examples 1 to 15, which satisfy both the alloy composition system and manufacturing conditions according to one embodiment of the present invention, secure high ductility along with the target ultra-high strength. Accordingly, the steel sheet according to one embodiment of the present invention can be suitablely applied to cold forming (cold press forming) by replacing conventional hot forming steel.

[0166] Meanwhile, Comparative Examples 1 to 9, which did not satisfy one or more of the alloy composition system and manufacturing conditions according to one embodiment of the present invention, could not secure the target physical properties.

[0167] Among these, Comparative Example 1 could not secure bendability (Equation 2) because the decarburization layer was not sufficiently formed due to failure to satisfy Equation 1.

[0168] Comparative Examples 2 to 9, excluding Comparative Example 1, are examples where the alloy composition system satisfies the present invention but the manufacturing conditions are unsatisfactory. In Comparative Examples 3 and 9, the dew point temperature was excessively low, resulting in insufficient formation of a decarburization layer and thus inferior bendability. In Comparative Example 7, the dew point temperature was excessively high, causing the decarburization layer to form too deeply, which prevented the achievement of ultra-high strength. Comparative Example 4 was a case where the temperature during continuous annealing was low; as annealing in a dual-phase region occurred, an excessive amount of ferrite was formed in the core structure, which prevented the achievement of ultra-high strength. In Comparative Examples 2, 6, and 8, the cooling end temperature after continuous annealing was excessively low or high; consequently, the stability of the retained austenite was insufficient, making it impossible to secure a sufficient fraction of the retained austenite phase, and consequently, the elongation was insufficient. In Comparative Example 5, the temperature during reheating was excessively high; decomposition of the retained austenite occurred, resulting in inferior elongation and an excessively high yield strength.

[0169] Figure 1 shows a photograph of the cross-sectional structure of the surface layer of Invention Example 1 observed by SEM, and it can be confirmed that a decarburized layer containing ferrite with an area fraction of 10% or more is formed from the surface to a depth of 50 μm.

[0170] Figure 2 shows an SEM image of the cross-sectional structure of the surface layer of Comparative Example 3, showing that the fraction of ferrite from the surface to a depth of 50 μm is less than 10% as the decarburization layer was not formed.

[0171] Figure 3 is a graph showing the results of measuring GDOES on the surfaces of Invention Examples 2 and 3, where the slope values ​​were calculated as 0.03 and 0.07, respectively, when linear regression analysis was performed on the measured values ​​for the invention examples.

[0172] Although the invention has been described with reference to the above embodiments, those skilled in the art will understand that various modifications and changes can be made to the invention without departing from the spirit and scope of the invention as described in the following claims.

Claims

1. In wt%, containing Carbon (C): 0.20–0.30%, Silicon (Si): 1.00–3.00%, Manganese (Mn): 2.50–4.00%, Aluminum (Al): 0.50% or less, Chromium (Cr): 1.0% or less, Molybdenum (Mo): 0.50% or less, Titanium (Ti): 0.20% or less, Vanadium (V): 0.10% or less, Boron (B): 0.0100% or less, Phosphorus (P): 0.050% or less (excluding 0%), Sulfur (S): 0.020% or less (excluding 0%), Nitrogen (N): 0.020% or less, and the remainder being Fe and other unavoidable impurities, satisfying the following Equation 1, A steel plate in which the surface layer extending to a depth of 50㎛ in the thickness direction from the surface includes a decarburized layer, and when linear regression analysis is performed on the graph of the carbon (C) content ratio according to the depth in the thickness direction of the decarburized layer, the slope value is 0.02 to 0.

07. [Relationship 1] ((10×C)+Mn) / (Si+Al) ≤ 3.10 (In Equation 1, each element represents weight content.) 2. In Paragraph 1, A steel plate comprising, in terms of area fraction, ferrite: 10% or more (including 100%), and the remainder being one or more of bainite, fresh martensite, and tempered martensite.

3. In Paragraph 1, When the area remaining excluding the surface layer in the thickness direction of the above steel plate is referred to as the center, The above-mentioned core is a steel sheet comprising, in terms of area fraction, ferrite: 5% or less, fresh martensite: 10% or less, retained austenite: 5~20%, and the remainder being bainite and tempered martensite.

4. In Paragraph 1, The above steel plate is a steel plate having a yield strength of 1100 MPa or more, a tensile strength of 1470 MPa or more, and an elongation of 12.0% or more.

5. In Paragraph 1, The above steel plate has a thickness of 0.6 to 2.5 mm, and A steel plate in which the relationship between the bending angle and thickness during a 3-point bending test satisfies the following Equation 2. [Relationship 2] (3-point bending angle (°) / thickness (mm) ≥ 42° / ㎜ 6. In Paragraph 1, The above steel plate is a steel plate comprising a plating layer on at least one surface.

7. A step of preparing a steel slab satisfying the following equation 1, comprising, in wt%, carbon (C): 0.20~0.30%, silicon (Si): 1.00~3.00%, manganese (Mn): 2.50~4.00%, aluminum (Al): 0.50% or less, chromium (Cr): 1.0% or less, molybdenum (Mo): 0.50% or less, titanium (Ti): 0.20% or less, vanadium (V): 0.10% or less, boron (B): 0.0100% or less, phosphorus (P): 0.050% or less (excluding 0%), sulfur (S): 0.020% or less (excluding 0%), nitrogen (N): 0.020% or less, and the remainder being Fe and other unavoidable impurities; A step of heating the above steel slab to a temperature range of 1050 to 1300℃; A step of obtaining a hot-rolled steel sheet by finishing hot-rolling the above heated steel slab at a temperature range of 800 to 1000℃; A step of coiling the above hot-rolled steel sheet in a temperature range of 400 to 700°C and then cooling it to room temperature; A step of obtaining a cold-rolled steel sheet by cold-rolling the above-mentioned coiled and cooled hot-rolled steel sheet at a cold reduction rate of 20 to 70%; A step of continuously annealing the above cold-rolled steel sheet at a temperature range of 810 to 880°C for 30 seconds or more; A step of cooling the above continuously annealed cold-rolled steel sheet to a temperature range of 150 to 250°C; and A method for manufacturing a steel sheet comprising the step of reheating the cooled cold-rolled steel sheet to a temperature range of 300 to 450°C and maintaining it for 100 to 1000 seconds. [Relationship 1] ((10×C)+Mn) / (Si+Al) ≤ 3.1 (In Equation 1, each element represents weight content.) 8. In Paragraph 7, A method for manufacturing a steel sheet in which the above continuous annealing step is performed in an atmosphere where the dew point temperature is maintained at -10 to 20℃.

9. In Paragraph 7, A method for manufacturing a steel sheet in which the step of cooling the continuously annealed cold-rolled steel sheet is performed at a cooling rate of 2 to 30℃ / s.

10. In Paragraph 7, A method for manufacturing a steel sheet in which the step of cooling the continuously annealed cold-rolled steel sheet is divided into a slow cooling section and a rapid cooling section.

11. In Paragraph 7, It further includes the step of plating after the above-mentioned reheating and holding, A method for manufacturing a steel sheet in which the plating is performed by hot-dip galvanizing in a plating bath at 430 to 490°C.

12. In Paragraph 11, A method for manufacturing a steel plate comprising the additional step of alloying heat treatment after the above-mentioned hot-dip galvanizing treatment.

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