Steel sheet and manufacturing method therefor

Abstract

The present invention relates to a steel sheet that can be used for structural members such as vehicle bodies, and a method for manufacturing same.

Description

Steel plate and method of manufacturing the same

[0001] The present invention relates to a steel plate that can be used in structural members, such as a vehicle body, and a method for manufacturing the same.

[0002] Recently, efforts are being made to secure manufacturing technology for high-strength steel plates to simultaneously ensure lightweighting and safety in automobiles. In particular, there is a growing demand for high-strength steel of 1180 MPa or higher, as steel plates with high tensile strength require a high load capacity before fracture occurs in the event of a collision.

[0003] Previously, many attempts were made to improve the strength of steel, but this was accompanied by a problem of inferior formability. Therefore, manufacturing steel sheets that combine high strength and formability can increase productivity during vehicle production, offering economic advantages, and is expected to result in improved safety of final components.

[0004] Meanwhile, in order to achieve high strength of 1180 MPa or higher, it is essential to utilize a large amount of low-temperature transformation phases such as bainite, martensite, tempered martensite, and retained austenite. The steel manufacturing process involves several stages, including steelmaking, continuous casting, rolling, and annealing. Due to minute variations in process conditions occurring in each stage, differences in the fraction of the aforementioned low-temperature transformation phases arise, causing the strength of the steel sheet to fluctuate sensitively, which leads to the problem of difficulty in securing stable material properties.

[0005] To date, various technologies have been proposed to improve the strength and formability of steel sheets for use in applications such as automotive structural materials.

[0006] For example, Patent Document 1 presents a method for manufacturing a steel with an excellent strength-elongation balance, comprising 3.0 to 15.0% of retained austenite by volume fraction, 2.5 to 50.0% of martensite and bainite in total, and 90.0% or more of the total of retained austenite, martensite, tempered martensite, bainite, and ferrite. However, Patent Document 1 does not consider a method for improving hole expansion, which plays an important role in the crushing impact mode during the forming of the flange portion when forming parts.

[0007] Patent Document 2 presents a method for manufacturing high-strength steel with excellent elongation and hole expandability, in which the product of tensile strength and hole expandability is 38,000 MPa·% or more, by controlling the area fraction and average crystal grain size of the ferrite and bainite phases, and containing residual austenite with an area fraction of 2 to 10%. However, while securing high strength of 1,180 MPa or higher, it fails to secure hole expandability of 32% or more, so there is a problem that the hole expandability is insufficient to realize complex part shapes.

[0008] Therefore, in order to ensure the lightweighting and stability of automobiles, it is necessary to develop steel materials that not only provide excellent crash performance through high tensile and yield strengths but also possess excellent formability, such as elongation and hole expansion, to facilitate the forming of parts.

[0009] (Patent Document 1) Japanese Patent Publication No. 2020-059919

[0010] (Patent Document 2) Korean Patent Publication No. 10-2015-0048885

[0011] One aspect of the present invention is to provide a steel plate having excellent impact performance due to high tensile strength and yield strength, as well as excellent elongation and hole expansion properties, and a method for manufacturing the same.

[0012] The problems of the present invention are not limited to those described above. A person skilled in the art to which the present invention pertains will have no difficulty understanding additional problems of the present invention from the overall contents of this specification.

[0013] A steel sheet according to one embodiment of the present invention comprises, in weight percent, carbon (C): 0.15~0.28%, silicon (Si): 0.5~2.0%, manganese (Mn): 1.5~3.5%, aluminum (Al): 0.1~1.0%, boron (B): 0.0005~0.005%, phosphorus (P): 0.0001~0.05%, sulfur (S): 0.0001~0.05%, nitrogen (N): 0.0001~0.02%, and the remainder being Fe and other unavoidable impurities.

[0014] The microstructure comprises a total of 85–92 area% of one or more types of tempered martensite and bainite as the matrix structure, and 8–15 area% of retained austenite as the secondary phase.

[0015] The internal carbon content of the residual austenite may be 1.20 weight% or more.

[0016] The above steel plate may further include one or more of the following: chromium (Cr): 0.01~1.5%, molybdenum (Mo): 0.01~1.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, copper (Cu): 0.001~0.5%, nickel (Ni): 0.001~0.5%, and tin (Sn): 0.0001~0.1%.

[0017] The above steel plate is C of the following [Relationship 1] G The value can be 1.20 or higher.

[0018] [Relationship 1]

[0019] C G =(a G -3.572-0.00120×[Mn]+0.00102×[Si]-0.00600×[Al]) / 0.033

[0020] Here, a Gθ is the lattice constant (Å) of the retained austenite measured by X-ray diffraction, and each element represents the content (weight%).

[0021] Among the above microstructures, tempered martensite may account for 50% or more of the total area.

[0022] The above microstructure may contain one or more of granular ferrite, fresh martensite, and carbides in an area of ​​5% or less.

[0023] The above steel plate may have a yield strength of 750 MPa or more, a tensile strength of 1180 MPa or more, an elongation of 14% or more, and a hole expansion of 45% or more.

[0024] The above steel plate may include a plating layer.

[0025] A method for manufacturing a steel sheet according to another embodiment of the present invention comprises the step of providing a cold-rolled steel sheet comprising, in weight percent, carbon (C): 0.15~0.28%, silicon (Si): 0.5~2.0%, manganese (Mn): 1.5~3.5%, aluminum (Al): 0.1~1.0%, boron (B): 0.0005~0.005%, phosphorus (P): 0.0001~0.05%, sulfur (S): 0.0001~0.05%, nitrogen (N): 0.0001~0.02%, and the remainder being Fe and other unavoidable impurities;

[0026] A step of first heating the above cold-rolled steel sheet to a temperature range of Ac3+5℃ to Ac3+80℃;

[0027] A step of first cooling the above first heated steel plate to a temperature range of 560 to 700°C (T1) and second cooling to a temperature range of 200 to 380°C (T2); and

[0028] The method includes the step of reheating the cooled steel plate to a temperature range of 350 to 480°C (T3) and maintaining it for at least 10 seconds (t1),

[0029] The conditions of [Relation 2] through [Relation 4] below can be satisfied.

[0030] [Relationship 2]

[0031] X RM (T2)≤X B (T3,t1)

[0032] Here, X RM (T2) is defined by the following [Relation 3], and X B (T3,t1) represents the value defined by the following [Relationship 4].

[0033] [Relationship 3]

[0034]

[0035] Here, Ms is the temperature (°C) at which the formation of martensite begins by cooling, and T2 is the second cooling end temperature.

[0036] [Relationship 4]

[0037]

[0038] Here,

[0039] k o It is calculated as = 4,768,000 - 25,800 × [C] - 13,250,000 × [Si] + 8,350,000 × [Al] - 2,652,000 × [Mn] - 12,950,000 × [Cr] - 15,256,000 × [Mo], where each element represents the content (weight%). Additionally, T3 is the reheating temperature, and t1 is the holding time.

[0040] The above cold-rolled steel sheet may further include one or more of the following: chromium (Cr): 0.01~1.5%, molybdenum (Mo): 0.01~1.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, copper (Cu): 0.001~0.5%, nickel (Ni): 0.001~0.5%, and tin (Sn): 0.0001~0.1%.

[0041] The above cold-rolled steel sheet is,

[0042] Step of preparing the slab;

[0043] A step of reheating the above slab at 1100~1350℃;

[0044] A step of finishing hot rolling the above heated slab at a finishing rolling temperature (FDT) range of 800 to 1050°C;

[0045] A step of cooling to a temperature range of 700℃ or lower at an average cooling rate of 10 to 100℃ / sec after the above finishing hot rolling;

[0046] A step of winding the cooled hot-rolled steel sheet and cooling it to room temperature; and

[0047] It may be manufactured by cold rolling with a reduction rate of 30~65%.

[0048] The holding time after the first heating mentioned above can be 5 seconds or more.

[0049] The average cooling rate of the above first cooling can be performed at 10℃ / sec or less.

[0050] The average cooling rate of the above secondary cooling can be performed at 10℃ / sec or higher.

[0051] The above reheating can be performed at a heating rate of 20℃ / sec or less.

[0052] The above hot rolling can be performed with the reduction amount of the final 2 passes being 10 to 40% of the total reduction amount.

[0053] The step of plating after the above reheating may be further included.

[0054] The step of performing the above temper rolling may be further included.

[0055] According to the present invention, the steel plate has high yield strength and tensile strength, resulting in excellent impact performance, while also having excellent elongation and hole expansion properties, thereby improving formability.

[0056] In particular, according to the present invention, the problem of securing a stable material caused by differences in the fraction of the low-temperature transformation phase due to minute variations in process conditions can be solved.

[0057] The various and beneficial advantages and effects of the present invention are not limited to those described above and will be more easily understood in the process of explaining specific embodiments of the present invention.

[0058] Figures 1 (a) and (b) are schematic images of the image quality of backscattered electron diffraction analysis (EBSD) of the microstructures of Comparative Example 6 and Invention Example 1, respectively, in the examples.

[0059] Figures 2(a) and 2(b) show the Pole Figure and Inverse Pole Figure derived after measuring the specimen of Invention Example 2 with EBSD.

[0060] 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.

[0061] 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.

[0062] In drawings, the shapes and sizes of elements may be exaggerated for clearer explanation.

[0063] 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.

[0064] 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.

[0065] In this specification, terms such as 'top', 'upper', 'upper surface', 'lower', 'lower surface', 'lower surface', and 'side surface' are based on the drawings and may actually vary depending on the direction in which the elements or components are arranged.

[0066] Additionally, throughout the specification, when it is said that one part is 'connected' to another part, this includes not only cases where they are 'directly connected,' but also cases where they are 'indirectly connected' with other elements in between.

[0067] 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.

[0068] Hereinafter, a steel plate according to one embodiment of the present invention will be described. First, the alloy composition of the present invention will be described. Unless otherwise specified, the unit % in the alloy composition described below refers to weight % (or indicated as wt.%).

[0069] The steel sheet of the present invention may contain, in weight percent, carbon (C): 0.15~0.28%, silicon (Si): 0.5~2.0%, manganese (Mn): 1.5~3.5%, aluminum (Al): 0.1~1.0%, boron (B): 0.0005~0.005%, phosphorus (P): 0.0001~0.05%, sulfur (S): 0.0001~0.05%, and nitrogen (N): 0.0001~0.02%.

[0070] Carbon (C): 0.15 to 0.28%

[0071] Carbon (C) is the most economical and effective element for strengthening steel. In addition, it plays a role in improving elongation through the Transformation Induced Transformation (TRIP) phenomenon by stabilizing austenite. If the content of C is less than 0.15%, it may be difficult to secure sufficient elongation due to insufficient fraction of retained austenite. On the other hand, if the content of C exceeds 0.28%, there is a problem in that it is difficult to secure sufficient weld strength due to brittle fracture of the weldment during spot welding. Therefore, the content of C may be 0.15 to 0.28%, and more specifically, 0.16 to 0.25%.

[0072] Silicon (Si): 0.5 to 2.0%

[0073] Silicon (Si) is an element that improves the hardenability of steel and can improve strength through solid solution strengthening effects. In addition, the above Si can improve elongation by securing residual austenite through delaying the formation of carbides and preventing the formation of pearlite. If the content of the above Si is less than 0.5%, there is a problem in that the elongation is inferior because the formation of pearlite cannot be effectively prevented. On the other hand, if the content of the above Si exceeds 2.0%, Fe-Si-based complex oxides are formed on the surface of the slab upon reheating, which not only degrades the surface quality of the steel plate but also reduces weldability. Therefore, the content of the above Si may be 0.5 to 2.0%, and more specifically, 0.6 to 0.1.5%.

[0074] Manganese (Mn): 1.5 to 3.5%

[0075] Manganese (Mn) is an element that improves the hardenability of steel, and by preventing the formation of ferrite and bainite during the cooling process after annealing, it can improve the strength of the steel by securing the martensite fraction. If the Mn content is less than 1.5%, the hardenability of the steel is insufficient, and ferrite and high-temperature bainite are formed during the cooling process after annealing, making it difficult to secure sufficient strength. On the other hand, if the Mn content exceeds 3.5%, the T2 temperature (the secondary cooling end temperature described later) required to secure the martensite fraction becomes excessively low, making precise control difficult, and the time required to complete the bainite transformation at the T3 temperature (the reheating temperature described later) becomes excessively long, which may result in inferior productivity. Therefore, the Mn content may be 1.5 to 3.5%, and more specifically, 1.8 to 3.0%.

[0076] Aluminum (Al): 0.1 to 1.0%

[0077] Aluminum (Al) is an element added for deoxidation, but in the present invention, it may be added for the purpose of improving the stability of residual austenite. Similar to Si, Al prevents the formation of pearlite, thereby facilitating the formation of residual austenite, and can play a role in increasing the lattice constant of austenite, which increases the solid solubility of carbon within the austenite. If the content of Al is less than 0.1%, the austenite stabilization effect is insufficient, and hole expansion performance may be inferior. On the other hand, if the content of Al exceeds 1.0%, there is a problem where the annealing temperature for single-phase austenite annealing becomes excessively high, which may reduce productivity and the surface quality of the steel sheet. Therefore, the content of Al may be 0.1 to 1.0%, and more specifically, 0.2 to 0.8%.

[0078] Boron (B): 0.0005 to 0.005%

[0079] Boron is an element that improves the hardenability of steel by reducing grain boundary energy through enrichment at the austenite grain boundaries. In the present invention, it plays a role in ensuring that the main phase consists of tempered martensite and bainite by suppressing the phase transformation of ferrite and upper bainite, where nucleation of phase transformation occurs through diffusion transformation at the austenite grain boundaries. If the content of B is less than 0.0005%, the hardenability of the steel is insufficient, and ferrite and high-temperature bainite are formed during the cooling process after annealing, making it difficult to secure sufficient strength. On the other hand, if the content exceeds 0.005%, the fluidity of the molten steel increases excessively, which degrades continuous casting performance and may degrade surface quality by forming oxides on the surface. Therefore, the content of B may be 0.0005% to 0.005%, and more specifically, 0.0010% to 0.0030%.

[0080] Phosphorus (P): 0.0001 to 0.05%

[0081] Phosphorus (P) is an impurity inevitably contained in steel and can be a major cause of reduced workability of steel due to segregation; therefore, the lower its content, the more effective it can be for the workability of steel sheets. The lower limit of the above P content may be 0%, but considering limitations in the manufacturing process or excessive increases in manufacturing costs, the above P content may be 0.0001% or more, and more specifically, 0.0002% or more. In addition, since the workability of steel sheets may be reduced if the above P content exceeds 0.05%, the above P content may be 0.050% or less, more specifically 0.045% or less, and even more specifically 0.040% or less.

[0082] Sulfur (S): 0.0001 to 0.05%

[0083] Sulfur (S) is an impurity inevitably contained in steel and can be a major cause of reduced workability in steel by forming non-metallic inclusions through combination with Mn, etc. Therefore, the lower the content, the more effective it can be for the workability of steel sheets. The lower limit of the S content may be 0%, but considering limitations in the manufacturing process or excessive increases in manufacturing costs, the S content may be 0.0001% or more, and more specifically, 0.0002% or more. In addition, since the workability of steel sheets may be reduced if the S content exceeds 0.05%, the S content may be 0.05% or less, and more specifically, 0.045% or less.

[0084] Nitrogen (N): 0.0001~0.02%

[0085] Nitrogen (N) is an impurity inevitably contained in steel and can reduce the workability of steel by reacting with Al and others to precipitate nitrides; therefore, the lower the content, the more effective it can be for the workability of steel sheets. The lower limit of the N content may be 0%, but considering limitations in the manufacturing process or an excessive increase in manufacturing costs, the N content may be 0.0001% or more, and more specifically, 0.0002% or more. In addition, since the workability of steel sheets may be reduced if the N content exceeds 0.02%, the N content may be 0.02% or less, specifically 0.018% or less, and more specifically 0.015% or less.

[0086] In addition to the alloy composition described above, the steel sheet of the present invention may further include one or more of chromium (Cr): 0.01~1.5%, molybdenum (Mo): 0.01~1.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, copper (Cu): 0.001~0.5%, nickel (Ni): 0.001~0.5%, and tin (Sn): 0.0001~0.1%.

[0087] Chromium (Cr): 0.01 to 1.5%

[0088] Chromium (Cr) is an element that improves the hardenability of steel, and by preventing the formation of ferrite and bainite during the cooling process after annealing, it can improve the strength of the steel by securing the martensite fraction. If the content of Cr is less than 0.01%, the above-described effect cannot be sufficiently obtained. In addition, if the content of Cr exceeds 1.5%, a stable oxide layer is formed on the surface after hot rolling, which degrades pickling ability and may degrade the surface quality of the steel sheet. Therefore, the content of Cr may be 0.01 to 1.5%, and more specifically, 0.1 to 0.8%.

[0089] Molybdenum (Mo): 0.01 to 1.5%

[0090] Molybdenum (Mo) is an element that significantly improves the hardenability of steel, and by preventing the formation of ferrite and bainite during the cooling process after annealing, it can improve the strength of steel by securing the martensite fraction. If the content of Mo is less than 0.01%, the above-described effect cannot be sufficiently obtained. In addition, if the content of Mo exceeds 1.5%, the time required to complete the bainite transformation at the T3 temperature becomes excessively long, which may result in inferior productivity. Therefore, the content of Mo may be 0.01 to 1.5%, and more specifically, 0.05 to 0.5%.

[0091] Titanium (Ti): 0.001 to 0.25%

[0092] Titanium (Ti) is an element that forms carbonitrides and is widely used to secure the strength of steel by inducing the formation of precipitates. In addition, it can be used to control the grain size of austenite during annealing, along with the role of removing nitrogen (N) in the steel to suppress the formation of BN, thereby allowing boron (B) to be enriched at the austenite grain boundaries. If the Ti content is less than 0.001%, the above-described effects cannot be sufficiently obtained. Furthermore, if the Ti content exceeds 0.25%, the coarse carbonitrides generated during the casting stage become excessively stable, which prevents sufficient dissolution during the slab reheating stage, resulting in a problem of inferior formability of the steel sheet. Therefore, the Ti content may be 0.001 to 0.25%, and more specifically, 0.010 to 0.050%.

[0093] Niobium (Nb): 0.001 to 0.25%

[0094] Niobium (Nb) is an element that forms carbonitrides and can be widely used to secure the strength of steel by inducing the formation of precipitates in this way, and can also be used to control the austenite grain size during annealing. If the content of Nb is less than 0.001%, the above-described effect cannot be sufficiently obtained. In addition, if the content of Nb exceeds 0.25%, the austenite grain size becomes excessively fine, so the Ms temperature becomes excessively low, which causes a problem in that it is difficult to precisely control the matrix structure fraction. Therefore, the content of Nb may be 0.001 to 0.25%, and more specifically, 0.005 to 0.070%.

[0095] Copper (Cu): 0.001 to 0.5%

[0096] Copper (Cu) is an element that stabilizes austenite and can play a role in improving elongation and hole expansion. If the content of Cu is less than 0.001%, the above-described effect cannot be sufficiently obtained. In addition, if the content of Cu exceeds 0.5%, it may cause surface cracks during hot rolling, thereby degrading the surface quality of the steel sheet. Therefore, the content of Cu may be 0.001 to 0.5%, and more specifically, 0.005 to 0.20%.

[0097] Nickel (Ni): 0.001 to 0.5%

[0098] Nickel (Ni) is an element that stabilizes austenite and can play a role in improving elongation and hole expansion. If the content of Ni is less than 0.001%, the above-described effect cannot be sufficiently obtained. In addition, if the content of Ni exceeds 0.5%, a highly viscous oxide is formed on the surface in the reheating furnace, which can impair descaling properties during hot rolling and impair the surface quality of the steel sheet. Therefore, the content of Ni may be 0.001 to 0.5%, and more specifically, 0.005 to 0.20%.

[0099] Tin (Sn): 0.0001 to 0.1%

[0100] Tin (Sn) is an impurity contained in iron scrap that can become concentrated on the surface during annealing and is a major cause of the deterioration of the surface quality of steel. The lower limit of the Sn content may be 0%, but considering limitations in the manufacturing process or an excessive increase in manufacturing costs, the Sn content may be 0.0001% or higher. On the other hand, if the Sn content exceeds 0.1%, the surface quality of the steel sheet may deteriorate, so the Sn content may be 0.1% or lower. Accordingly, the Sn content may be 0.0001 to 0.1%, and more specifically, 0.005 to 0.05%.

[0101] The steel plate may include iron (Fe) as a remaining component. Furthermore, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the normal manufacturing process, they cannot be excluded. Because these impurities are known to any skilled person in the normal manufacturing process, all details thereof are not specifically mentioned in this specification.

[0102] The microstructure of the steel sheet of the present invention comprises a total of 85 to 92 area% of one or more types of tempered martensite and bainite as the matrix structure, and 8 to 15 area% of retained austenite as the secondary phase. Meanwhile, other structures may include granular ferrite, fresh martensite, carbides, etc., in an area of ​​5 area% or less.

[0103] The above-mentioned tempered martensite phase is a structure formed by tempering a martensite structure generated at a temperature below Ms during the secondary cooling process, during the subsequent process of increasing and maintaining the temperature. This tempered martensite phase possesses favorable conditions for securing a high level of strength due to the presence of a high level of dislocation density, over-saturated carbon, and fine carbides within a dense lath structure. Furthermore, the aforementioned dislocations, over-saturated carbon, and fine carbides present within the tempered martensite are uniformly distributed, resulting in excellent resistance to crack initiation and propagation, thereby providing excellent hole expansion properties.

[0104] The above bainite phase is formed as untransformed austenite undergoes a phase transformation while being maintained at the T3 temperature (the reheating temperature described later), and exhibits microstructural characteristics similar to tempered martensite. Since the effects on strength and hole expansion are similar due to similar microstructural characteristics, the area fraction of the two phases can be managed as a sum.

[0105] If the sum of the area fractions of the tempered martensite and bainite is less than 85%, it may be difficult to secure the target yield strength and tensile strength. On the other hand, if the sum of the area fractions of the tempered martensite and bainite exceeds 92%, there may be a problem in that it is difficult to secure sufficient elongation due to insufficient residual austenite fraction.

[0106] The area fraction of the tempered martensite may be 50% or more of the total area fraction of the microstructure. The martensite structure formed at Ms temperature or below during the secondary cooling process plays a role in promoting the bainite phase transformation during the subsequent heating and holding process. If the fraction of the martensite structure formed during the cooling process at Ms temperature or below is less than 50%, the bainite phase transformation cannot be smoothly promoted, and the stability of the austenite cannot be sufficiently secured, so fresh martensite is formed during the final cooling process to room temperature, which may result in poor hole expansion performance.

[0107] Meanwhile, during the secondary cooling, heating, and holding processes, some of the austenite formed at the annealing temperature does not undergo phase transformation into martensite and bainite. During the holding process at the reheating temperature (T3), the formation of bainite occurs along with the diffusion of carbon atoms into the austenite, causing the carbon concentration inside the austenite to increase. Subsequently, even after cooling to room temperature, it does not undergo phase transformation into martensite and remains in the structure, existing as a second phase. The above-mentioned residual austenite can improve the elongation of the steel sheet through the TRIP effect. If the area fraction of the above-mentioned residual austenite is less than 8%, there is a problem in that it is difficult to secure the target elongation. On the other hand, if the area fraction of the above-mentioned residual austenite exceeds 15%, there is a problem in that it is difficult to secure the target yield strength and tensile strength due to insufficient area fraction of the matrix structure.

[0108] The carbon content within the retained austenite may be 1.00 wt% or more. As previously explained, the carbon concentration within the austenite increases. The carbon content present within the retained austenite affects phase stability. Preferably, it may be 1.10 wt% or more, and more preferably, 1.20 wt% or more. There are various methods for deriving the carbon content within the retained austenite. As an example, C in the following Equation 1 G It can be derived through.

[0109] [Relationship 1]

[0110] C G =(a G -3.572-0.00120×[Mn]+0.00102×[Si]-0.00600×[Al]) / 0.033

[0111] Here, a G θ is the lattice constant (Å) of the retained austenite measured by X-ray diffraction, and each element represents the content (weight%).

[0112] Meanwhile, as an example, X-ray diffraction (XRD) was used to determine the lattice constant of the retained austenite. Specifically, an X-ray beam diffracted from a Cu target was irradiated onto the specimen to scan the range of 40–102° at 0.02° intervals, and then the range of 49.3–52.0° corresponding to the (200) plane of the austenite was fitted to a Gaussian Peak Function to calculate the center point of the peak, Xc. Subsequently, the lattice constant a of the austenite was determined by applying the following relationship. G was calculated using Equation 1 below. Here, λ is the wavelength of the X-ray beam, which is 0.15418 Å.

[0113] [Equation 1]

[0114]

[0115] It was found that when the residual austenite internal carbon content CG is less than 1.00 wt%, it is difficult to satisfy the target hole expansion. Effectively, it can be 1.10 wt% or more, and more effectively, 1.20 wt% or more. Although there is no need to limit the upper limit of the residual austenite internal carbon content, in order to secure a residual austenite internal carbon concentration exceeding 1.50 wt%, it is necessary to add an excessive amount of Al and set the reheating temperature (T3) low and maintain it for a long time, so in terms of securing productivity, it can be managed to be less than 1.50 wt%.

[0116] In addition to the above base structure and the second phase, granular ferrite, fresh martensite, carbides, etc. may inevitably be partially formed.

[0117] If the annealing temperature is somewhat low during annealing, or if the hardenability is somewhat insufficient during cooling after annealing, some granular ferrite may be formed within the microstructure. Granular ferrite is formed by diffusion transformation and is characterized by a low dislocation density within the structure; since there is a large difference in hardness compared to the matrix structure of tempered martensite and bainite, it impairs hole expandability. Meanwhile, while conventional granular ferrite is characterized by low strength, in the present invention, it was confirmed that when included at 5% or less, the dislocation density within the granular ferrite is maintained at a high level to accommodate the grain deformation generated during phase transformation into martensite and bainite, thereby not significantly reducing the hole expandability of the steel. On the other hand, since its presence exceeding 5% significantly reduces hole expandability, it is desirable to manage the upper limit to 5% or less.

[0118] If the bainite transformation is not completed while maintaining at the reheating temperature (T3), the austenite is not sufficiently stabilized, and martensite may be formed during the final cooling process to room temperature. The fresh martensite formed at this time has a high internal carbon content, which is effective for increasing strength, but it has a high hardness difference from the matrix structure, tempered martensite and bainite, resulting in inferior hole expansion performance. It is preferable that the fraction of fresh martensite be 0 area%, and in the present invention, it is preferable to manage it at a maximum of 2.5% to secure the target hole expansion performance.

[0119] Iron carbides may be formed along with the diffusion of carbon into austenite during the transformation into tempered martensite and bainite. In the present invention, it is important to improve elongation and hole expansion by suppressing the formation of iron carbides to enhance the fraction and stability of residual austenite; therefore, excessive formation of iron carbides can impair formability. Meanwhile, when Ti and Nb are added, alloy carbonitrides may be present. In this case, additional strengthening effects due to grain refinement can be expected, but since coarse carbides reduce the toughness of the steel, it is desirable to manage the carbides present in the steel to an area fraction of 5% or less.

[0120] Even if the above granular ferrite, fresh martensite, carbides, etc. are inevitably formed, it is desirable that the total amount does not exceed 5 area%.

[0121] The steel plate of the present invention has a yield strength of 750 MPa or more, a tensile strength of 1180 MPa or more, an elongation of 14% or more, and a hole expansion of 45% or more, thereby securing excellent impact performance.

[0122] The steel sheet of the present invention may include a plating layer. The plating layer may be a molten zinc plating layer formed by a molten zinc plating method. The present invention does not particularly limit the composition of the molten zinc plating layer, and any molten zinc plating layer commonly applied in the relevant technical field may be preferably applied to the present invention. Furthermore, the molten zinc plating layer may be an alloyed molten zinc plating layer that is alloyed with some alloy components of the steel sheet.

[0123] A zinc plating layer formed by an electro-galvanizing method may be formed on at least one surface of the high-strength steel plate according to the present invention. The present invention does not particularly limit the composition of the zinc plating layer, and any zinc plating layer commonly applied in the relevant technical field may be preferably applied to the present invention.

[0124]

[0125] Next, a method for manufacturing a steel plate, which is another embodiment of the present invention, will be described in detail.

[0126] The method for manufacturing the above steel plate can be achieved by providing a cold-rolled steel plate and manufacturing it through an annealing heat treatment - cooling - reheating method.

[0127] The method for providing the above cold-rolled steel sheet is not specifically limited, but as an example, it can be manufactured by heating a steel slab - hot rolling - cooling and coiling - cold rolling.

[0128] Each process is described in detail below.

[0129] [Slab Preparation and Heating]

[0130] A slab satisfying the alloy composition described above may be prepared, and the slab may be heated to a temperature range of 1100 to 1350°C to perform homogenization treatment. If the heating temperature is less than 1100°C, the homogenization of the alloy elements may not be sufficiently performed. In addition, if the heating temperature exceeds 1350°C, an excessive amount of oxide may be formed on the surface of the slab, which may degrade the surface quality of the steel plate. Therefore, the heating temperature may be 1100 to 1350°C, and more specifically, 1150 to 1300°C.

[0131] The slab used in the manufacturing method of the present invention may be refined and cast through a converter process or an electric furnace process.

[0132] In the converter process, molten iron supplied from a blast furnace is primarily used; however, depending on the supply and demand status of hot metal, some scrap or other iron sources may be added for refining to produce molten steel. In particular, when implementing low HMR operations that reduce the amount of molten iron used to meet requirements such as carbon neutrality, the amount of scrap used may increase, and as a result, elements not intended in this invention may be included in the molten steel within the allowable limits.

[0133] In the electric furnace process, molten steel can be obtained by primarily charging scrap, melting it using arc heat, and refining it. In some cases, molten iron may be added in addition to the scrap. As a result of including a large amount of scrap in this manner, elements not intended in this invention may be included in the molten steel within permissible limits.

[0134] Molten steel that has undergone the converter or electric furnace process may undergo an additional refining (secondary refining) process to adjust its composition and other properties.

[0135] [Hot Rolled]

[0136] Hot-rolled steel sheets can be obtained by hot-rolling the above heated slab.

[0137] In the step of obtaining the hot-rolled steel sheet, the rolling end temperature (FDT) may be 800 to 1050°C. If the FDT is less than 800°C, the rolling load may increase excessively, leading to a problem of reduced workability. Additionally, if the FDT exceeds 1050°C, oxides may be excessively formed on the surface of the steel sheet after rolling and may not be effectively removed even after pickling, resulting in a deterioration of the surface quality of the steel sheet. That is, the FDT may be 800 to 1050°C, and more specifically, 820 to 1100°C.

[0138] In the step of obtaining the hot-rolled steel sheet, the reduction amount of the final two passes may be 10 to 40% of the total reduction amount. When hot rolling is performed by multi-stage rolling, the rolling load can be reduced and the thickness can be precisely controlled. If the reduction rate of the final two passes is less than 10%, the reduction rates of the previous passes may be somewhat high, resulting in shape defects, or the temperature of the steel sheet may drop rapidly, leading to reduced workability. Furthermore, if the reduction rate of the final two passes exceeds 40%, the rolling load of the final two passes may increase excessively, causing a problem where workability deteriorates. That is, the reduction rate of the final two passes may be 10 to 40% of the total reduction amount.

[0139] [Cooling and winding stage]

[0140] The above hot-rolled steel sheet is cooled to a coiling temperature of 700°C or lower at an average cooling rate of 10 to 100°C / s, and then coiled. Phase transformation occurs after cooling and / or coiling following hot rolling, and the microstructure generated by this phase transformation may vary depending on the coiling temperature. In the coiling temperature range of 580 to 700°C, granular ferrite and pearlite structures are formed, which has the advantage of reducing the rolling load in the subsequent cold rolling process. In the coiling temperature range of 400 to 580°C, bainite structures are formed, which allows for securing a uniform microstructure in the subsequent cold rolling and continuous annealing processes. In the coiling temperature range of 400°C or lower, martensite structures are formed, which has the advantage of securing excellent material uniformity across the entire length and width of the coil. If the above coiling temperature exceeds 700℃, an internal oxide layer is formed on the surface of the hot-rolled steel sheet and remains even after annealing, degrading the surface quality of the steel sheet. Although there is no need to specifically limit the lower limit of the above coiling temperature, since cooling after hot rolling is performed by water cooling, the lower limit can be managed at room temperature, which is the same as the temperature of the cooling water. To reduce the rolling load of the cold rolling process after coiling, a soft nitriding heat treatment may be performed.

[0141] [Cold Rolled]

[0142] A cold-rolled steel sheet can be obtained by rolling the above-mentioned coiled hot-rolled steel sheet at a reduction rate of 30 to 65%. If the cold reduction rate is less than 30%, it may be difficult to secure the target thickness precision, and there may be a problem in that it becomes difficult to correct the shape of the steel sheet. On the other hand, if the cold reduction rate exceeds 65%, the cold rolling load increases excessively, leading to a problem of inferior productivity. Therefore, it is desirable to limit the cold reduction rate in the cold rolling stage to 30 to 65%.

[0143] [Sodun]

[0144] The above cold-rolled steel sheet may be annealed in a temperature range of Ac3+5℃ to Ac3+80℃ (hereinafter also referred to as the 'annealing temperature'). In the present invention, this can be referred to as the first heating. The purpose of the annealing step is to ensure hole expansion by heating to the austenite single-phase region to remove untransformed ferrite, and to form austenite with an area fraction close to 100% to secure the intended microstructure through subsequent processes. If the annealing temperature is less than Ac3+5℃, sufficient austenite transformation does not occur, and thus the desired tempered martensite and bainite fractions cannot be secured after annealing. On the other hand, if the annealing temperature exceeds Ac3+80℃, productivity decreases, and oxides may be formed on the surface, which can degrade the surface quality of the steel sheet. In the present invention, the Ac3 temperature may use a value defined by the following [Equation 1].

[0145] [Equation 2]

[0146] Ac3 = 912-206×[C]+26.2×[Si]-25.8×[Mn]+43.8×[Al]-6.4×[Cr]+9.12×[Mo]-20.1×[Cu]-30.2×[Ni]+148×[Ti]+50.2×[Nb]-131×[B]

[0147] Each element symbol represents the content (weight%) of the element.

[0148] As an example, the above annealing can be carried out in a continuous alloying molten plating furnace.

[0149] [Primary and Secondary Cooling]

[0150] The above annealed or first heated steel plate is first cooled to a first cooling end temperature of 560 to 700°C (hereinafter also referred to as 'T1'), and then secondarily cooled to a second cooling end temperature of 200 to 380°C (hereinafter also referred to as 'T2') which is below the Ms temperature, thereby introducing martensite into the microstructure of the steel plate.

[0151] Here, the first cooling end temperature (T1) can be defined as the same as the second cooling start temperature, as it is the point at which rapid cooling is initiated by additionally applying rapid cooling equipment that was not applied in the first cooling. Through the first cooling process, the temperature distribution of the steel plate can be made uniform, thereby reducing the final temperature and material deviation. The first cooling is performed slowly at an average cooling rate of 10℃ / s or less, and the cooling end temperature may be in the temperature range of 560 to 700℃. If the first cooling end temperature is lower than 560℃, ferrite and high-temperature bainite, which are microstructures not intended in the present invention, may be formed, which may result in inferior strength and hole expansion. On the other hand, if it exceeds 700℃, an excessive load is applied to the subsequent second cooling, which may result in inferior productivity.

[0152] Secondary cooling can be performed at an average cooling rate of 10°C / s or higher. At this time, a rapid cooling facility that was not applied in the primary cooling may be additionally applied, and as a preferred embodiment, a hydrogen rapid cooling facility using H2 gas may be used, but is not limited thereto. The cooling end temperature (T2) of the secondary cooling is preferably performed between 200°C and 380°C so that the martensite fraction generated during cooling is 50°C to 92°C, that is, so that the fraction of untransformed austenite is 8°C to 50°C. If the fraction of untransformed austenite is less than 8°C, there is a problem in that the elongation rate becomes inferior because it is not possible to secure a residual austenite of 8°C or more in terms of area fraction even after the subsequent heating and holding process. On the other hand, in the subsequent heating and holding period where the fraction of untransformed austenite exceeds 50%, the bainite phase transformation is not smooth, resulting in the formation of fresh martensite in the final microstructure, which can impair hole expansion ability.

[0153] The above untransformed austenite fraction (X RM ) can be defined by [Equation 3] below by measuring the austenite grain size and the martensite transformation start temperature.

[0154] [Relationship 3]

[0155]

[0156] Here, Ms is the temperature (°C) at which the formation of martensite begins by cooling, and T2 is the second cooling end temperature.

[0157] [Equation 3]

[0158] Ms = 550-330×[C]-41×[Mn]-20×[Si]-20×[Cr]-10×[Mo]+30×[Al]-3.7*EXP(0.23*(20-D))

[0159] Here, each element represents the content (weight%), and D represents the original austenite diameter (μm) during the annealing stage.

[0160] [Reheating and Maintaining]

[0161] The above secondary cooled steel plate is reheated again to a temperature range of 350 to 480°C (hereinafter also referred to as 'T3') and maintained for a predetermined time (t1) to temper the martensite obtained in the previous step, induce bainite transformation, and concentrate carbon in the austenite. At this time, reheating can be performed at a heating rate of 20°C / sec or less. It is preferable that the holding time be 10 seconds or more.

[0162] Conventional bainite transformation occurs through nucleation and growth, and it is known to exhibit the fastest phase transformation rate within a specific temperature range (hereinafter also referred to as the 'Nose temperature'). This is because the driving force for phase transformation is low at temperatures above the Nose temperature, resulting in a slow nucleation rate, while the growth rate slows down at temperatures below the Nose temperature. Meanwhile, as in the present invention, when more than 50% martensite is generated during the secondary cooling process, the interface between the existing martensite and austenite acts as a strong nucleation site, exhibiting the characteristic that the phase transformation rate does not decrease even when bainite phase transformation occurs in the high-temperature region. When more than 50% martensite is present, the fraction of bainite (X) generated during a holding time t1 (sec) at a reheating temperature T3 (°C) B ) can be defined by the following [Relation 4].

[0163] [Relationship 4]

[0164]

[0165] k o = 4,768,000 - 25,800 × [C] - 13,250 × [Si] + 8,350,000 × [Al] - 2,652,000 × [Mn] - 129,500 × [Cr] - 152,560 × [Mo] (Here, each element represents the content (weight%).)

[0166] In order to secure excellent hole expansion in the present invention, fresh martensite is excluded from the microstructure along with the prevention of the formation of granular ferrite structure. Accordingly, under temperature and time conditions where the fraction of bainite that can be formed when maintaining the elevated temperature is greater than the amount of untransformed austenite remaining after secondary cooling, bainite transformation can be completed and austenite stabilized, thereby effectively excluding the formation of fresh martensite during the final cooling process. This condition can be expressed by the following [Equation 2].

[0167] [Relationship 2]

[0168] X RM (T2)≤X B (T3,t1)

[0169] In a continuous production process, since the holding time t1 is a value determined by the length of the production process and the passing speed, it is possible to secure the intended microstructure by controlling the temperature T3 considering the martensite fraction generated at T2. If the T3 temperature is below 350℃, there is a problem where productivity is inferior because the time required to complete the bainite transformation becomes excessively long. On the other hand, if the T3 temperature exceeds 480℃, the stability of austenite decreases, which may lead to inferior hole expansion, and the elongation may be inferior because the fraction of retained austenite decreases due to the generation of excessive carbides. Therefore, it is desirable to set the T3 temperature to 350~480℃.

[0170] Meanwhile, plating can be performed on the reheated steel plate. As an example, a hot-dip galvanizing treatment can be performed in a temperature range of 480 to 540°C to form a hot-dip galvanized layer on at least one surface of the steel plate.

[0171] In addition, if necessary, to obtain an alloyed molten zinc plating layer, the material can be cooled to room temperature after performing an alloying heat treatment following the molten zinc plating process.

[0172] In addition, to correct the shape of the steel plate and adjust the yield strength, further temper rolling may be performed after cooling to room temperature. At this time, the reduction rate may be less than 1%.

[0173] 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.

[0174] (Example)

[0175] A slab having the alloy composition shown in Table 1 below (wherein the remainder of the alloy composition is Fe and unavoidable impurities) was prepared and manufactured into a cold-rolled steel sheet. At this time, the reheating temperature of the slab was set to 1220℃, the thickness of the steel sheet after hot rolling was set to 2.6mm, the rolling end temperature was set to 920℃, and the total reduction amount for the final 2 passes during finish rolling was applied uniformly as 25%. After hot rolling, the average cooling rate was applied uniformly as 40℃ / s and coiled at 550℃. Subsequently, a cold-rolled steel sheet with a thickness of 1.4mm was manufactured by applying a cold reduction rate of 46%.

[0176]

[0177] The above cold-rolled steel sheet was subjected to annealing heat treatment (first heating), first cooling, and second cooling according to the manufacturing conditions of Table 2. Subsequently, a reheating and holding process was performed.

[0178] The above first cooling end temperature (T1) was set to 650℃, the first average cooling rate was set to 8℃ / s, and the second average cooling rate was set to 25℃ / s. Meanwhile, the heating rate during reheating was set to 10℃ / s.

[0179] The austenite equivalent diameter D heated to the annealing temperature was measured at the 1 / 4 thickness point of the specimen using Electron Back Scatter Diffraction (EBSD) attached to a Scanning Electron Microscopy (SEM) by the following method. At this time, the electron beam scan area was 50 µm x 50 µm, and scans were performed at intervals of 0.9 µm. After measuring the specimen containing residual austenite with EBSD following the completion of continuous annealing, cooling, heating, and holding, only pixels having an austenite crystal structure were isolated to obtain a Pole Figure as shown in Fig. 2(a). Fig. 2 shows the specimen of Invention Example 2. Subsequently, different colors were applied to each of the five grains, and they can be displayed in the Inverse Pole Figure as shown in Fig. 2(b). Through pixel counting in the Inverse Pole Figure, the proportion of the aforementioned five grains in the total austenite could be measured (20.8% in the case of Fig. 2). This allows for the inference of the number of austenite grains contained within the total scan area (5 x 100% / 19.3% = 24.0 in the case of Fig. 2), and the scan area is 2500 µm 2 Considering this, the average area of ​​a single grain can be calculated (in the case of Fig. 2, 2500㎛). 2 / 24.0 = 104㎛ 2 The austenite equivalent diameter can be estimated through the average grain area (11.5 μm in the case of Fig. 2).

[0180]

[0181] For the above-mentioned Invention Examples 1 to 10 and Comparative Examples 1 to 11, the microstructure fraction, internal carbon concentration of residual austenite, yield strength (YS), tensile strength (TS), elongation (El), and hole expansion (HER) were measured and are listed in Table 3 below.

[0182] The microstructural fractions were measured at the 1 / 4 thickness of the specimen using backscattered electron diffraction analysis according to the following method. The austenite phase fraction was calculated by isolating pixels with an FCC structure. For granular ferrite, utilizing the characteristic of low dislocation density within the microstructure, pixels exhibiting a Kernal Average Misorientation (KAM) value of less than 0.6 were isolated and displayed. Since fresh martensite is characterized by high dislocation density and a dense structure, pixels satisfying an Image Quality (IQ) value of 15,000 or higher and a Confidence Index (CI) value of 0.05 or higher were isolated and displayed to distinguish it from grain boundaries. The remaining structure was classified into the matrix structure, tempered martensite, and bainite.

[0183] The internal carbon concentration of the retained austenite was calculated through X-ray diffraction (XRD) measurements. An X-ray beam diffracted from a Cu target was irradiated onto the specimen and scanned in the range of 40–102° at 0.02° intervals. Subsequently, the range of 49.3–52.0°, corresponding to the (200) plane of the austenite, was fitted to the Gaussian Peak Function to calculate the center point of the peak, Xc. Thereafter, the lattice constant a of the austenite was determined by applying the following relationship. G was calculated using Equation 1 below. Here, λ is the wavelength of the X-ray beam, which is 0.15418 Å.

[0184] [Equation 1]

[0185]

[0186] Afterwards, a G The carbon content C in the austenite by applying the [Mn], [Si], and [Al] content added to the alloy G was calculated using the following relationship 1.

[0187] [Relationship 1]

[0188] C G =(a G-3.572-0.00120×[Mn]+0.00102×[Si]-0.00600×[Al]) / 0.033

[0189] Yield strength, tensile strength, and elongation were determined by testing specimens of JIS-5 standard specimens taken in a direction perpendicular to the rolling direction. At this time, the yield strength and elongation represent the 0.2% off-set yield strength and fracture elongation, respectively.

[0190] Hole expandability was measured according to the ISO 16630 standard using a specimen of 120 mm x 120 mm, and the hole was sheared with a 10 mm diameter punch to a clearance of 12%.

[0191]

[0192] As seen in the results of Tables 1 to 3 above, Invention Examples 1 to 10 satisfy all the alloy compositions and manufacturing conditions proposed in the present invention, and by securing an appropriate matrix structure and the type and fraction of secondary phases, it can be seen that a yield strength of 750 MPa or more, a tensile strength of 1180 MPa or more, an elongation of 14% or more, and a hole expansion of 45% or more can be secured.

[0193] Figures 1(a) and 1(b) are schematic images of the image quality of backscattered electron diffraction (EBSD) of the microstructures of Comparative Example 6 and Inventive Example 1, respectively, in the above examples. According to Figure 1, it can be seen that Comparative Example 6 contains a large amount of unintended fresh martensite. On the other hand, it was confirmed that Inventive Example 1 has fresh martensite, bainite, and retained austenite appropriately formed.

[0194] Meanwhile, it was confirmed that Comparative Example 1 had insufficient carbon (C) content, making it impossible to secure more than 8% of retained austenite in terms of area fraction, resulting in a low elongation. Comparative Example 2 had insufficient manganese (Mn) content, which led to the formation of granular ferrite during cooling after annealing. Consequently, granular ferrite exceeding 5% in area fraction was included in the microstructure, resulting in low strength and low hole expansion. Comparative Example 3 had insufficient aluminum (Al) content, which resulted in insufficient stability of retained austenite and inferior hole expansion.

[0195] Comparative Example 4 was found to have low strength and inferior hole expandability because the annealing temperature was low, failing to reach single-phase annealing, and granular ferrite exceeding 5% in area fraction was present in the microstructure. Comparative Example 5 was found to have low strength and inferior hole expandability because the annealing temperature was low, containing granular ferrite in the microstructure, and failing to satisfy Equation 2, resulting in the formation of fresh martensite during final cooling. Comparative Example 6 was found to have inferior hole expandability because the martensite fraction formed during secondary cooling after annealing was less than 50%, and the bainite fraction formed during reheating and holding time was insufficient, resulting in the formation of fresh martensite during the final cooling stage. Comparative Example 7 was found to have inferior hole expandability because the heating temperature after secondary cooling was excessively low, failing to satisfy Equation 2, and resulting in the formation of fresh martensite during final cooling. Comparative Example 8 had an excessively short holding time after heating, so it did not satisfy Equation 2, resulting in the formation of fresh martensite during final cooling and inferior hole expansion performance. Comparative Example 9 had an excessively high heating temperature after secondary cooling, so the stability of the retained austenite was insufficient, resulting in inferior elongation and hole expansion performance.

Claims

1. In wt%, carbon (C): 0.15–0.28%, silicon (Si): 0.5–2.0%, manganese (Mn): 1.5–3.5%, aluminum (Al): 0.1–1.0%, boron (B): 0.0005–0.005%, phosphorus (P): 0.0001–0.05%, sulfur (S): 0.0001–0.05%, nitrogen (N): 0.0001–0.02%, and the remainder being Fe and other unavoidable impurities, The microstructure comprises a total of 85–92 area% of one or more types of tempered martensite and bainite as the matrix structure, and 8–15 area% of retained austenite as the secondary phase. A steel plate having a residual austenite internal carbon content of 1.00 weight% or more.

2. In Claim 1, The above steel plate further comprises one or more of the following: chromium (Cr): 0.01~1.5%, molybdenum (Mo): 0.01~1.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, copper (Cu): 0.001~0.5%, nickel (Ni): 0.001~0.5%, and tin (Sn): 0.0001~0.1%.

3. In Claim 1, The above steel plate is C of the following [Relationship 1] G Steel plate with a value of 1.00 or higher. [Relationship 1] C G =(a G -3.572-0.00120×[Mn]+0.00102×[Si]-0.00600×[Al]) / 0.033 Here, a G θ is the lattice constant (Å) of the retained austenite measured by X-ray diffraction, and each element represents the content (weight%).

4. In Claim 1, A steel plate in which tempered martensite in the above microstructure accounts for 50% or more of the total area.

5. In Claim 1, A steel plate in which the above microstructure contains at least one of granular ferrite, fresh martensite, and carbides in an area of ​​5% or less.

6. In Claim 1, The above steel plate has a yield strength of 750 MPa or more, a tensile strength of 1180 MPa or more, an elongation of 14% or more, and a hole expansion of 45% or more.

7. In Claim 1, The above steel plate is a steel plate comprising a plating layer.

8. A step of providing a cold-rolled steel sheet comprising, in wt%, carbon (C): 0.15~0.28%, silicon (Si): 0.5~2.0%, manganese (Mn): 1.5~3.5%, aluminum (Al): 0.1~1.0%, boron (B): 0.0005~0.005%, phosphorus (P): 0.0001~0.05%, sulfur (S): 0.0001~0.05%, nitrogen (N): 0.0001~0.02%, and the remainder being Fe and other unavoidable impurities; A step of first heating the above cold-rolled steel sheet to a temperature range of Ac3+5℃ to Ac3+80℃; A step of first cooling the above first heated steel plate to a temperature range of 560 to 700°C (T1) and second cooling to a temperature range of 200 to 380°C (T2); and The method includes the step of reheating the cooled steel plate to a temperature range of 350 to 480°C (T3) and maintaining it for at least 10 seconds (t1), A method for manufacturing a steel plate that satisfies the conditions of [Equation 2] to [Equation 4] below. [Relationship 2] X RM (T2)≤X B (T3,t1) Here, X RM (T2) is defined by the following [Relation 3], and X B (T3,t1) represents the value defined by the following [Relationship 4]. [Relationship 3] Here, Ms is the temperature (°C) at which the formation of martensite begins by cooling, and T2 is the second cooling end temperature. [Relationship 4] Here, k o It is calculated as = 4,768,000 - 25,800 × [C] - 13,250,000 × [Si] + 8,350,000 × [Al] - 2,652,000 × [Mn] - 12,950,000 × [Cr] - 15,256,000 × [Mo], where each element represents the content (weight%). Additionally, T3 is the reheating temperature, and t1 is the holding time.

9. In Claim 8, A method for manufacturing a steel sheet, wherein the above cold-rolled steel sheet further comprises one or more of the following: chromium (Cr): 0.01~1.5%, molybdenum (Mo): 0.01~1.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, copper (Cu): 0.001~0.5%, nickel (Ni): 0.001~0.5%, and tin (Sn): 0.0001~0.1%.

10. In claim 8, The above cold-rolled steel sheet is, Step of preparing the slab; A step of reheating the above slab at 1100~1350℃; A step of finishing hot rolling the above heated slab at a finishing rolling temperature (FDT) range of 800 to 1050°C; A step of cooling to a temperature range of 700℃ or lower at an average cooling rate of 10 to 100℃ / sec after the above finishing hot rolling; A step of winding the cooled hot-rolled steel sheet and cooling it to room temperature; and Manufactured by cold rolling with a reduction rate of 30~65%, Method for manufacturing steel plates.

11. In Claim 8, A method for manufacturing a steel plate characterized by the holding time after the first heating being 5 seconds or more.

12. In claim 8, A method for manufacturing a steel plate, wherein the average cooling rate of the above-mentioned first cooling is 10℃ / sec or less.

13. In Claim 8, A method for manufacturing a steel plate, wherein the average cooling rate of the above secondary cooling is 10℃ / sec or higher.

14. In Claim 8, A method for manufacturing a steel plate, wherein the above reheating is performed at a heating rate of 20℃ / sec or less.

15. In Claim 10, The above hot rolling is a method for manufacturing a steel plate in which the reduction amount of the final 2 passes is 10 to 40% of the total reduction amount.

16. In Claim 8, A method for manufacturing a steel plate, further comprising the step of plating after the above-mentioned reheat.

17. In Claim 8, A method for manufacturing a steel plate, further comprising the step of performing the above-mentioned temper rolling.

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