Steel sheet and method for manufacturing same

Abstract

The present invention relates to a hot-rolled steel sheet that can be used for automotive chassis structural members and the like, and a method for manufacturing same.

Description

Steel plate and method of manufacturing the same

[0001] The present invention relates to a hot-rolled steel sheet that can be used in automobile chassis structural members, etc., and a method for manufacturing the same.

[0002] Recently, the automotive market has been transitioning from internal combustion engine vehicles to eco-friendly vehicles, including electric vehicles, in order to mitigate global warming. As a result of this transition, the types of components constituting automobiles have changed, leading to changes in vehicle weight. Specifically, it is known that the weight of electric vehicles increases by approximately the weight of the battery compared to internal combustion engine vehicles.

[0003] Among the components that make up an automobile, chassis components play a role in supporting the vehicle body and are important for ensuring ride comfort and driving safety by absorbing vibrations and shocks from the road surface while driving. As the weight of the vehicle increases, the fatigue load applied to chassis components increases, so steel materials for chassis components of electric vehicles are required to have excellent fatigue strength.

[0004] It is generally known that the fatigue strength of steel is proportional to its tensile strength and / or yield strength. Therefore, for steel sheets used in automotive chassis components, measures to improve tensile strength and / or yield strength are necessary to enhance fatigue strength.

[0005] Furthermore, chassis components are primarily manufactured by press forming. While the application of high-strength steel allows for component weight reduction by decreasing sheet thickness, the component's shape cannot be significantly altered; therefore, ensuring formability suitable for press forming is paramount in the manufacturing of high-strength steel. In particular, considering the current widespread application of both draw forming, where elongation is critical, and foam forming, which requires hole expansion, it is necessary to simultaneously secure both elongation and hole expansion.

[0006] In order to simultaneously secure a high level of strength and hole expansion, steel plates applied for conventional chassis parts have either applied precipitation-strengthened steel that forms a large amount of fine precipitates inside the ferrite or applied ferrite-bainite composite steel that introduces a secondary phase of bainite into the ferrite matrix structure.

[0007] Patent Document 1 presents a method for manufacturing a hot-rolled steel sheet with excellent tensile strength of 780 MPa or more and hole expansion properties by coiling a steel containing 0.08 mass% or more and 0.25 mass% or less of Ti in a temperature range between 550 and 680°C, thereby causing fine Ti carbides with an average grain size of less than 10 nm to precipitate in ferrite with an area fraction of more than 95%. However, when coiling at high temperatures, grain boundary precipitation of precipitates is promoted, creating a region of non-precipitate formation near the grain boundaries, and during actual part forming, deformation is concentrated in the region of non-precipitate formation near the grain boundaries, resulting in poor shear formability and a problem where cracks occur on the shear surface.

[0008] Patent Document 2 presents a method for manufacturing hot-rolled steel sheets that secures strength and minimizes the deterioration of hole expansion properties by including 20 to 48% of bainite, which has a low interphase hardness difference with ferrite, the matrix structure, as an area fraction. However, it does not consider the problem that, as a large amount of bainite is introduced to secure strength, the fraction of the matrix structure decreases, resulting in excellent hole expansion properties but difficulty in securing elongation. Furthermore, in order to secure the above bainite, cooling must be terminated and coiled in a temperature range of 380 to 520°C; however, since the temperature range of 380 to 520°C corresponds to a boiling transition zone where the boiling phenomenon of water cooling fluctuates rapidly, it is difficult to precisely control the temperature of the steel sheet, leading to a problem where the yield rate of producing hot-rolled steel sheets with secured material properties decreases.

[0009] Therefore, in order to ensure driving stability of chassis components for eco-friendly vehicles such as electric vehicles, it is necessary to develop steel materials that not only possess excellent fatigue life due to high tensile and yield strengths, but also exhibit superior formability—such as elongation and hole expansion—to facilitate press forming, and are easy to produce to secure price competitiveness.

[0010] (Patent Document 1) Korean Patent Publication No. 10-2013-0012081

[0011] (Patent Document 2) Korean Patent Publication No. 10-2003-0076430

[0012] One aspect of the present invention is to provide a hot-rolled steel sheet having excellent fatigue performance with high tensile strength and yield strength, as well as excellent elongation and hole expansion properties, and a method for manufacturing the same.

[0013] 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.

[0014] A steel plate according to one embodiment of the present invention is,

[0015] In weight percent, it contains carbon (C): 0.04~0.15%, silicon (Si): 0.001~1.2%, manganese (Mn): 0.8~2.5%, aluminum (Al): 0.001~1.0%, 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 contains 85~97 area percent of bainic ferrite as the matrix structure, and 3~15 area percent of one or more of lower bainite and martensite as secondary phases, and the martensite in the secondary phase may account for 80% or more of the total secondary phase fraction.

[0016] The above steel plate may contain one or more of chromium (Cr): 0.01~0.8%, molybdenum (Mo): 0.01~0.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, and vanadium (V): 0.001~0.25%.

[0017] The average circular diameter of the above secondary phase may be 3.0㎛ or less.

[0018] The above martensite may be lath martensite and plate martensite.

[0019] The average spacing between the above secondary phases may be 2㎛ or more.

[0020] The above steel plate may have a hole expansion rate of 55% or more.

[0021] The above steel plate may have a yield strength of 670 MPa or more, a tensile strength of 780 MPa or more, and an elongation of 15% or more.

[0022] A method for manufacturing a steel plate according to another embodiment of the present invention is,

[0023] A step of reheating a slab containing, in weight percent, carbon (C): 0.04~0.15%, silicon (Si): 0.001~1.2%, manganese (Mn): 0.8~2.5%, aluminum (Al): 0.001~1.0%, 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, at 1100~1350℃;

[0024] A step of obtaining a hot-rolled steel sheet by hot-rolling the above-mentioned heated slab;

[0025] A step of first cooling the above hot-rolled steel plate to a first cooling end temperature of Bs-30℃ or lower at a cooling rate of 50℃ / s or more;

[0026] A step of secondarily cooling the above first cooled steel plate to a second cooling end temperature of (Bs+Ms) / 2 or higher at a cooling rate of 20℃ / s or less;

[0027] A step of tertiarily cooling the above secondary cooled steel plate to a tertiary cooling end temperature of 120℃ or lower at a cooling rate of 30℃ / s or more;

[0028] The step of winding the above third-cooled steel plate; and

[0029] The above-mentioned wound steel plate may include a step of cooling it to room temperature.

[0030] The step of obtaining the above hot-rolled steel sheet can be performed in a rolling end temperature (FDT) range of 800 to 1150°C such that the Du value of the following Equation 1 satisfies 3 to 10.

[0031] [Relationship 1]

[0032] Du = [(FDT + 7.4×[C] - 24.7×[Si] - 4.7×[Mn] - 3.9×[Cr] - 5.2×[Mo] - 480×[Ti] - 1200×[Nb])×0.049] - 34.2

[0033] (FDT refers to the temperature (°C) of the hot-rolled plate immediately after hot rolling, and each element symbol indicates the content (weight%).)

[0034] In the step of obtaining the above hot-rolled steel sheet, the total reduction amount of the final 2 passes can be 10 to 40%.

[0035] The above secondary cooling step can be performed for a time (ts) satisfying the following relationship 2.

[0036] [Relationship 2]

[0037] 0.85 ≤ 1-exp[-k(T) x (ts) 2 ] ≤ 0.97

[0038] (Here, k(T) represents the value defined by the following Equation 3.)

[0039] [Relationship 3]

[0040]

[0041] (Here, T1 is the first cooling end temperature, T2 is the second cooling end temperature, and each element represents the content (weight%).)

[0042] The above-mentioned manufactured steel plate may further include the steps of pickling and oiling.

[0043] The above-mentioned manufactured steel plate may further include the step of pickling and heating to a temperature range of 400 to 750°C to perform molten plating.

[0044] According to one aspect of the present invention, the steel plate has high yield strength and tensile strength, resulting in excellent fatigue performance, while also having excellent elongation and hole expansion properties, thereby improving formability during press forming.

[0045] The various and beneficial advantages and effects of the present invention are not limited to those described above, and may be more easily understood in the process of explaining specific embodiments of the present invention.

[0046] Figures 1 (a) to (c) are scanning electron microscope (SEM) images of the microstructures of Invention Example 1, Comparative Example 4, and Comparative Example 5, respectively, in the examples.

[0047] Embodiments of the present invention are provided to more fully explain the invention to those with average knowledge in the art. Meanwhile, the shapes and sizes of elements in the drawings may be exaggerated for clearer explanation.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] There is a precipitation-strengthened steel in which precipitation-forming elements such as titanium (Ti), niobium (Nb), and vanadium (V) are added to the steel composition to secure strength and hole expansion properties, and fine precipitates of several nanometers in size are dispersed within the ferrite microstructure by coiling at a high temperature of around 600°C. Although precipitation-strengthened steel was able to improve strength without significantly deteriorating the excellent formability of ferrite, when the distribution of precipitates within the microstructure is uneven, deformation is unevenly distributed within the microstructure during the forming process for producing parts, and a phenomenon was observed in which cracks easily propagate along the regions where deformation is concentrated. Due to this phenomenon, cracks may form on the shear surface of the steel sheet during the final trimming stage of the multi-stage forming process. Therefore, even if the formability measured in the original state is excellent, there is a problem in that the formability under the condition where pre-deformation is applied during the multi-stage forming process for producing actual parts becomes significantly inferior.

[0052] Meanwhile, there is a ferrite-bainite composite structure steel containing bainite in a ferrite matrix. Although the composite structure steel does not pose a risk of shear plane cracking that occurs in the precipitation-strengthened steel mentioned above, a process step of terminating cooling and coiling at the bainite transformation temperature range is essential to secure bainite as a secondary phase. The above cooling is carried out through water cooling, but the temperature range of 400 to 500°C, where bainite transformation occurs, corresponds to the transition boiling zone where the boiling phenomenon caused by water cooling changes rapidly, and thus corresponds to a temperature range where the cooling capacity of the steel sheet by water injection fluctuates significantly. Consequently, a problem arises where the accuracy of the cooling termination temperature required to stably secure bainite is low, which results in an inferior yield of steel sheets with guaranteed quality and an increase in manufacturing costs.

[0053] Accordingly, the inventors of the present invention sought to develop a steel plate and a method for manufacturing the same that can reduce manufacturing costs by being easy to manufacture, while simultaneously securing a high level of strength and hole determinability.

[0054] The technology for manufacturing ferrite-martensite dual-phase steel (DP steel, Dual-Phase steel) can be considered as a method to produce high-strength hot-rolled steel sheets while avoiding the risk of shear plane cracking in precipitation-strengthened steel produced by high-temperature coiling at around 600°C and avoiding the difficulty of controlling the steel sheet temperature in the coiling temperature range of 400 to 500°C. Generally, to secure a ferrite-martensite dual structure, polygonal ferrite, which is the matrix structure, is formed by air cooling for a few seconds in the temperature range of 650 to 750°C where ferrite is generated, and then the steel sheet is rapidly cooled to room temperature to form a martensite phase as a secondary phase. However, in this case, there is a problem with the yield strength of the steel sheet being low due to mobile dislocations introduced into the matrix structure during martensite transformation, and there is a problem with poor hole expansion performance due to the large difference in hardness between the polygonal ferrite and martensite phases.

[0055] On the other hand, it was confirmed that when a large amount of dislocations are introduced into the matrix structure to strengthen the matrix structure, a high level of yield strength can be secured, and excellent hole expansion can be secured by reducing the difference in hardness between the matrix structure and the secondary phase, and this was investigated in depth.

[0056] One method to introduce a large number of dislocations into the matrix structure is to consider work hardening by applying deformation after or during the phase transformation of the matrix structure. However, while an improvement in yield strength can be expected in this case, it was confirmed that the anisotropy was inferior due to increased material differences depending on the forming direction caused by the formation of the texture, and the workability was inferior because hot rolling at low temperatures was required.

[0057] Accordingly, the inventors of the present invention conceived of a composite structure composed of bainitic ferrite generated by displacive phase transformation without bainite diffusion, and secondary products produced by the subsequent diffusion of interstitial alloying elements such as carbon. At this time, since bainitic ferrite is generated by displacive phase transformation, screw dislocations are regularly arranged within the structure to reduce the amount of shear deformation generated during the transformation. Additionally, edge dislocations generated to accommodate the volume expansion accompanying the formation of bainitic ferrite within the austenite accumulate within the bainitic ferrite matrix even after the transformation has progressed. Consequently, immediately after the transformation is completed, the dislocation density of the bainitic ferrite reaches a high level, making it suitable for improving the yield strength of steel without the problem of degraded anisotropy. However, since the density of dislocations generated inside bainitic ferrite varies depending on the transformation temperature, it was realized that it is important to manage the transformation temperature and time to maintain the dislocation density inside bainitic ferrite at an appropriate level, and this was studied in depth.

[0058] As a result, it was recognized that by managing the alloy composition range of the steel plate and the hot rolling and cooling conditions, and optimizing the fraction, type, and size of the matrix structure and secondary phases of the microstructure, it is possible to obtain a steel plate that is easy to manufacture while simultaneously securing a high level of strength and hole expansion, and thus the present invention was completed.

[0059] 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.%).

[0060] The steel sheet of the present invention may contain, in weight percent, carbon (C): 0.04~0.15%, silicon (Si): 0.001~1.2%, manganese (Mn): 0.8~2.5%, aluminum (Al): 0.001~1.0%, 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.

[0061] Carbon (C): 0.04 to 0.15%

[0062] Carbon (C) is the most economical and effective element for strengthening steel. As the content of C increases, the formation of polygonal ferrite during cooling is suppressed, which can increase the fraction of bainitic ferrite structure. At the same time, it lowers the bainite transformation temperature, thereby maintaining a high dislocation density within the bainitic ferrite. Additionally, C diffuses into austenite during the bainitic ferrite transformation to stabilize the austenite, which then transforms into the secondary phase, low-temperature bainite and / or martensite, during the final cooling process, contributing to an improvement in tensile strength. If the content of C is less than 0.04%, hardenability is insufficient, resulting in a low fraction of bainitic ferrite and secondary phases, making it difficult to secure sufficient tensile strength and / or yield strength. On the other hand, if the content of C exceeds 0.15%, the fraction of secondary phases increases excessively, making it difficult to secure elongation and potentially reducing weldability. That is, the content of C may be 0.04 to 0.15%, specifically 0.045 to 0.10%, and more specifically 0.05 to 0.08%.

[0063] Silicon (Si): 0.001 to 1.2%

[0064] 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 yield strength and / or tensile strength by delaying the formation of carbides and preventing the formation of pearlite, thereby causing transformation into secondary phases such as low-temperature bainite and / or martensite. If the content of the above Si is less than 0.001%, it may be difficult to secure the effect of strength improvement through solid solution strengthening. On the other hand, if the content of the above Si exceeds 1.2%, 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 causes problems such as reduced weldability. The content of the above Si may be 0.001 to 1.2%, specifically 0.005 to 0.80%, and more specifically 0.01 to 0.60%.

[0065] Manganese (Mn): 0.8 to 2.5%

[0066] Manganese (Mn) is an element that improves the hardenability of steel and can facilitate the formation of a low-temperature transformation structure by preventing the formation of polygonal ferrite during cooling after finish rolling. If the Mn content is less than 0.8%, it may be difficult to secure sufficient strength due to insufficient fractions of bainitic ferrite, which is a low-temperature transformation structure, and secondary phases. If the Mn content exceeds 2.5%, the hardenability increases significantly, and the transformation of bainitic ferrite does not occur smoothly in the cooling zone, which can lead to an excessive increase in the time required to secure the fraction of the matrix structure and reduce the elongation. The Mn content may be 0.8 to 2.5%, specifically 1.0 to 2.4%, and more specifically 1.2 to 2.2%.

[0067] Aluminum (Al): 0.001 to 1.0%

[0068] Aluminum (Al) is an element added for deoxidation, but it can also improve strength through solid solution strengthening. If the content of Al is less than 0.001%, it may be difficult to sufficiently secure the effect of strength improvement through solid solution strengthening. In addition, if the content of Al exceeds 1.0%, it may lead to an increase in oxide and / or nitride inclusions in the steel, thereby reducing the formability of the steel sheet. That is, the content of Al may be 0.001 to 1.0%, specifically 0.005 to 0.8%, and more specifically 0.01 to 0.5%.

[0069] Phosphorus (P): 0.0001 to 0.05%

[0070] 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.

[0071] Sulfur (S): 0.0001 to 0.05%

[0072] 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.

[0073] Nitrogen (N): 0.0001~0.02%

[0074] 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.

[0075] Meanwhile, in addition to the alloy composition described above, it may further include one or more of chromium (Cr): 0.01~0.8%, molybdenum (Mo): 0.01~0.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, and vanadium (V): 0.001~0.25%.

[0076] Chromium (Cr): 0.01 to 0.8%

[0077] Chromium (Cr) is an element that improves the hardenability of steel and can facilitate the formation of a low-temperature transformation structure by suppressing the formation of ferrite during primary cooling after finish rolling. If the content of Cr is less than 0.01%, the above-described effect cannot be sufficiently obtained. Furthermore, if the content of Cr exceeds 0.8%, the hardenability increases excessively, and since bainite transformation does not occur smoothly in the cooling zone, the time required to secure the fraction of bainitic ferrite, which is the matrix structure, increases excessively, which may reduce the elongation. That is, the content of Cr may be 0.01 to 0.8%, specifically 0.01 to 0.7%, and more specifically 0.05 to 0.6%.

[0078] Molybdenum (Mo): 0.01 to 0.5%

[0079] Molybdenum (Mo) is an element that significantly improves the hardenability of steel and can improve strength through solid solution strengthening effects. In addition, the Mo can facilitate the formation of a low-temperature transformation structure by suppressing the formation of ferrite during cooling after finish rolling. If the content of the Mo is less than 0.01%, the above-described effects cannot be sufficiently obtained. Furthermore, if the content of the Mo exceeds 0.5%, there is a problem where the alloy cost increases significantly, resulting in poor economic efficiency. Additionally, because the hardenability increases excessively, the bainite transformation does not occur smoothly in the cooling zone, which can lead to an excessive increase in the time required to secure the fraction of bainitic ferrite, the matrix structure, thereby reducing the elongation. That is, the content of the Mo may be 0.01 to 0.5%, specifically 0.01 to 0.4%, more specifically 0.02 to 0.3%, and even more specifically 0.05 to 0.2%.

[0080] Titanium (Ti): 0.001 to 0.25%

[0081] Titanium (Ti) is an element that forms carbonitrides, and by delaying the recrystallization of steel during hot rolling, it refines the grain size of austenite, promotes the transformation into bainite in the cooling zone, and refines the grain size of martensite in the microstructure, thereby improving the strength of the steel. If the content of Ti is less than 0.001%, the above-described effect cannot be sufficiently obtained. Furthermore, if the content of Ti exceeds 0.25%, the coarse carbonitrides generated during the casting stage become excessively stable, and there is a problem in that they are not sufficiently dissolved during the slab reheating stage, thereby degrading the formability of the steel sheet. That is, the content of Ti may be 0.001 to 0.25%, specifically 0.001 to 0.20%, more specifically 0.01 to 0.15%, and even more specifically 0.03 to 0.13%.

[0082] Niobium (Nb): 0.001 to 0.25%

[0083] Niobium (Nb) delays the recrystallization behavior of steel during hot rolling, thereby allowing control of the austenite grain size. If the content of Nb is less than 0.001%, the above-described effect cannot be sufficiently obtained. Furthermore, if the content of Nb exceeds 0.25%, the austenite grain size becomes excessively fine, which makes it difficult to precisely control the bainitic ferrite fraction. That is, the content of Nb may be 0.001 to 0.25%, specifically 0.001 to 0.20%, more specifically 0.01 to 0.15%, and even more specifically 0.02 to 0.08%.

[0084] Vanadium (V): 0.001 to 0.25%

[0085] Vanadium (V) delays the recrystallization behavior of steel during hot rolling, thereby allowing control of the austenite grain size. If the content of V is less than 0.001%, the above-described effect cannot be sufficiently obtained. Furthermore, if the content of V exceeds 0.25%, the austenite grain size becomes excessively fine, which makes it difficult to precisely control the bainitic ferrite fraction. The content of V may be 0.001 to 0.25%, specifically 0.001 to 0.20%, more specifically 0.01 to 0.15%, and even more specifically 0.02 to 0.08%.

[0086] In addition to the above components, iron (Fe) may be included as a remaining component. Furthermore, since unintended impurities from raw materials or the surrounding environment may inevitably be incorporated during the ordinary manufacturing process, they cannot be excluded. Because these impurities are known to any person skilled in the ordinary manufacturing process, all details thereof are not specifically mentioned in this specification.

[0087] Next, the microstructure of the steel sheet of the present invention will be described. The steel sheet may include bainitic ferrite as a matrix phase and lower bainite and / or martensite as a secondary phase.

[0088] The above bainitic ferrite may contain 85 to 97 area percent.

[0089] The above steel sheet avoids polygonal ferrite transformation during the first cooling after hot rolling and is cooled to a temperature below Bs (bainite transformation initiation temperature), and then undergoes bainite transformation by slow cooling during the subsequent second cooling. At this time, the bainite transformation involves the formation of bainitic ferrite and carbon diffusion into untransformed austenite, and the carbon-enriched austenite transforms into low-temperature bainite and / or martensite, which are secondary phases, during the subsequent cooling process.

[0090] A large number of dislocations are generated within the bainitic ferrite produced by shear transformation. These dislocations remain within the microstructure even after the subsequent third cooling, enhancing the strength of the matrix structure and playing a role in providing excellent strength and hole expansion. Meanwhile, a significant amount of carbon atoms existing in a super-occupied state within the bainitic ferrite immediately after bainite transformation diffuse into austenite, but some diffuse into the internal dislocations of the bainitic ferrite and exist in a solid solution state; this plays a role in increasing the strength of the bainitic ferrite after the third cooling. In other words, the strength of the bainitic ferrite matrix can be secured by the large number of dislocations present within the microstructure and the presence of solid solution carbons near the dislocations.

[0091] Meanwhile, although the strength of bainitic ferrite is higher than that of polygonal ferrite, it has lower strength and superior ductility compared to the secondary phase martensite described later. Therefore, in the composite structure composition of the present invention, the area fraction of bainitic ferrite can be managed importantly from the perspective of securing elongation.

[0092] If the area fraction of the above bainitic ferrite is less than 85%, it may be difficult to secure sufficient elongation. In addition, if the area fraction of the above bainitic ferrite exceeds 97%, the fraction of low-temperature transformation structures (e.g., martensite, etc.) that play a role in improving strength is insufficient, so it may be difficult to secure appropriate yield strength and / or tensile strength (e.g., yield strength of 670 Pa or more and / or tensile strength of 780 MPa or more). That is, the area fraction of the above bainitic ferrite may be 85 to 97%, and specifically may be 88.0 to 95.0%.

[0093] The above secondary phase may include 3 to 15 area percent.

[0094] The secondary phase may include one or more of low-temperature bainite and martensite. During secondary cooling, as carbon diffusion into the untransformed austenite proceeds along with the formation of bainitic ferrite, carbon enrichment may occur in the untransformed austenite. During an additional cooling process after secondary cooling, the untransformed austenite may undergo transformation into the secondary phase, which is low-temperature bainite and / or martensite.

[0095] When bainitic ferrite is formed during secondary cooling, the size of the untransformed austenite distributed within the microstructure varies depending on the location. Since the carbon content within the austenite varies according to the size of the austenite, the type of secondary phase may also vary. For example, relatively large untransformed austenite has a low carbon content, so it may transform into low-temperature bainite during cooling to the coiling temperature. Additionally, for example, relatively small austenite has high stability, so it may not transform into low-temperature bainite but instead transform into martensite at a lower temperature. Depending on the carbon content of the austenite before transformation, the type of martensite may be lath martensite or plate martensite.

[0096] If the area fraction of the above secondary phase is less than 3%, it may be difficult to secure sufficient yield strength and tensile strength. In addition, if the area fraction of the above secondary phase exceeds 15%, it may be difficult to secure sufficient elongation because the fraction of the matrix structure decreases. That is, the area fraction of the secondary phase may be 3 to 15%, and specifically 5.0 to 10.0%.

[0097] The average equivalent diameter of the secondary phase may be 3.0 μm or less. Since the secondary phase improves the tensile strength of steel through mechanisms such as dispersion strengthening, the strengthening efficiency increases when it is evenly dispersed in fine sizes. Furthermore, it has been confirmed in the present invention that the smaller the size of the secondary phase, the more uniformly the amount of deformation is distributed, thereby preventing the occurrence of cracks due to local deformation concentration and resulting in superior hole expansion properties. To prevent local deformation concentration, it is preferable that the average equivalent diameter of the secondary phase be 3.0 μm or less, and to prevent deformation concentrations occurring at different locations from overlapping, it is preferable that the average spacing between the secondary phases be 2 μm or more.

[0098] Meanwhile, the fraction of martensite among the above secondary phases may be 80% or more of the total fraction of secondary phases. Both low-temperature bainite and martensite exhibit a strengthening effect as they have higher strength than the matrix structure, but the strengthening effect of martensite is higher than that of low-temperature bainite. When the fraction of martensite among the secondary phases is less than 20%, it is difficult to secure a tensile strength of 780 MPa or more, so the tertiary cooling end temperature described later is managed to be 120°C or lower.

[0099] In addition to the above base structure and secondary phase, one or more of polygonal ferrite, residual austenite, and carbide may be included in an area of ​​less than 5%.

[0100] Polygonal ferrite generated during cooling after finish rolling is typically characterized by low strength as it is formed by diffusion transformation; however, the present invention has confirmed that the strength of the steel is not significantly reduced when the polygonal ferrite is included in an amount of less than 5%. In the present invention, it was found that the dislocation density within the polygonal ferrite is maintained at a high level to accommodate the grain deformation generated as austenite undergoes phase transformation into bainitic ferrite, thereby not significantly reducing the strength of the steel sheet. Based on this confirmation, the polygonal ferrite may be included in an amount of less than 5%. However, since the strength of the steel is reduced when the polygonal ferrite is 5% or more, it is necessary to manage the upper limit to be less than 5%.

[0101] Meanwhile, during secondary cooling in the cooling zone, carbon diffuses and migrates from bainitic ferrite to untransformed austenite. As the carbon diffusion coefficient within the austenite drops significantly, the carbon concentration within the austenite has a non-uniform distribution; in areas where carbon is locally excessively concentrated, it may fail to transform into martensite during cooling to room temperature and remain as austenite. Since the phase stability of this retained austenite is not high, even if it is observed as austenite after the manufacture of the steel sheet, it mostly transforms into martensite through strain-induced martensite transformation during the stage where stress and strain are applied while manufacturing the part, thereby playing a role in increasing the tensile strength of the steel. Therefore, in the present invention, the fraction of retained austenite is not managed separately. However, if the content of such retained austenite is excessively high, the yield strength of the steel sheet may decrease due to stress-induced martensite transformation or cause embrittlement associated with hydrogen accumulation, so it is desirable to manage the upper limit to less than 5%.

[0102] During low-temperature bainite transformation, iron carbides may be formed along with the diffusion of carbon into austenite. Since the present invention improves strength by utilizing solid solution carbon within the bainitic ferrite matrix and the secondary phases of low-temperature bainite and / or martensite, the excessive formation of iron carbides hinders the strengthening effect intended by the present invention. 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.

[0103] The steel plate of the present invention has a yield strength of 670 MPa or more, a tensile strength of 780 MPa or more, an elongation of 15% or more, and a hole expansion of 55% or more, thereby securing excellent impact performance.

[0104] Hereinafter, a method for manufacturing a hot-rolled steel sheet according to one embodiment of the present invention will be described.

[0105] A steel plate can be manufactured by performing a series of processes including heating, hot rolling, cooling, and coiling a slab having the alloy composition described above. The conditions for each of the above processes will be explained in detail below.

[0106] Slab heating

[0107] Before performing hot rolling, a homogenization treatment can be performed by heating a slab satisfying the aforementioned alloy composition. The heating temperature in the heating step may be 1100 to 1350°C. 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 galvanized steel sheet. That is, the heating temperature may be 1100 to 1350°C, more specifically 1150 to 1300°C, and even more specifically 1180 to 1250°C.

[0108] The steel slab used in the manufacturing method of the present invention may be refined and cast through a converter process or an electric furnace process.

[0109] 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.

[0110] 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.

[0111] 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.

[0112] Hot rolling

[0113] Hot-rolled steel sheets can be obtained by hot-rolling a heated slab. After hot rolling, the austenite grain size is influenced by the alloy composition, rolling end temperature, and / or reduction amount, which can affect the formation behavior of polygonal ferrite and bainite during the subsequent cooling process and the composition of the final microstructure. Additionally, the average equivalent diameter of the secondary phase, which is one of the factors determining hole expansion, is influenced by the austenite grain size after hot rolling. Although the average equivalent diameter of the secondary phase is influenced by the nucleation behavior during bainite transformation, due to the characteristics of shear transformation, the average equivalent diameter of the secondary phase cannot increase beyond the size of the austenite before transformation; therefore, the average equivalent diameter of the secondary phase can be controlled more effectively by controlling the austenite grain size after hot rolling.

[0114] Accordingly, the inventors were able to manufacture a hot-rolled steel sheet that appropriately controls the final microstructure and secures the desired strength and formability by controlling the rolling end temperature, the content of alloying elements contained in the slab, the effective grain size of austenite that can be calculated from the rolling end temperature, and the total reduction amount of the final two passes.

[0115] In the step of obtaining the hot-rolled steel sheet, the rolling end temperature (FDT) may be 800 to 1150°C. If the FDT is less than 800°C, the rolling load may increase excessively, leading to reduced workability or deterioration of anisotropy. Additionally, if the FDT exceeds 1150°C, oxides may be excessively formed on the surface of the steel sheet after rolling, and since they are not effectively removed even after pickling, the surface quality of the steel sheet may deteriorate. That is, the FDT may be 800 to 1150°C, more specifically 820 to 1100°C, and even more specifically 850 to 950°C.

[0116] In the step of obtaining the hot-rolled steel sheet, the heated slab may be hot-rolled such that the value of the following Equation 1 within the FDT range satisfies 3 to 10. When the value of Du below is 3 to 10, the average equivalent diameter of the secondary phase is controlled to 3 μm, thereby obtaining excellent hole expansion performance. On the other hand, if the rolling end temperature is excessively low and the value of Du below is less than 3 (i.e., the austenite grain size becomes excessively fine), the phase transformation during primary cooling is excessively promoted, and the primary cooling rate required to avoid the formation of polygonal ferrite may become excessively high, and there is a risk that material variation may increase due to the resulting non-uniform cooling. Furthermore, if the value of Du below exceeds 10, the austenite grain size becomes excessively coarse, and the nucleation of the bainite phase transformation occurs unevenly, causing the average equivalent diameter of the secondary phase to exceed 3 μm, which may result in a problem of inferior hole expansion performance.

[0117] [Relationship 1]

[0118] Du = [(FDT + 7.4×[C] - 24.7×[Si] - 4.7×[Mn] - 3.9×[Cr] - 5.2×[Mo] - 480×[Ti] - 1200×[Nb])×0.049] - 34.2

[0119] (Here, Du is an indicator representing the effective grain size of austenite immediately before the first cooling after hot rolling, FDT refers to the temperature (°C) of the hot-rolled sheet immediately after hot rolling, and each element represents the content (weight%).)

[0120] In the step of obtaining the hot-rolled steel sheet, the total reduction amount of the final two passes may be 10 to 40%. When hot rolling is performed by multi-stage rolling, the rolling load can be reduced and the thickness can be precisely controlled. If the total 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 total 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 total reduction rate of the final two passes may be 10 to 40%.

[0121] cooling

[0122] The above hot-rolled steel sheet can be cooled in stages. Specifically, the above hot-rolled steel sheet can be first cooled to a temperature of Bs-30℃ or lower at a cooling rate of 50℃ / s or higher, and secondly cooled to a temperature of (Bs+Ms) / 2 or higher at a cooling rate of 20℃ / s or lower. Here, Bs is the temperature (℃) at which bainite formation begins, and can be calculated using the formula {Bs = 830-320×[C]-90×[Mn]-35×[Si]-70×[Cr]-120×[Mo]} (where each element represents the content (weight%)). In addition, Ms is the temperature (°C) at which the formation of martensite begins upon cooling, and can be calculated using the formula {Ms = 550-330×[C]-41×[Mn]-20×[Si]-20×[Cr]-10×[Mo]+30×[Al]} (where each element symbol represents the content (weight%)).

[0123] Through a first cooling step, the steel plate is cooled to a temperature of Bs-30℃ or lower, thereby avoiding the formation of polygonal ferrite, and subsequently, through a second cooling step, the matrix structure can be secured at 85 to 97 area% through the growth of bainitic ferrite.

[0124] In the first cooling step, the first cooling rate may be 50℃ / s or higher. In this case, the phase transformation of polygonal ferrite generated during cooling can be prevented, thereby preventing a decrease in strength. Although there is no specific upper limit for the first cooling rate, the first cooling rate may be 200℃ / s or lower, as the shape of the steel plate may warp if it is cooled rapidly.

[0125] In the first cooling step, if the cooling end temperature exceeds Bs-30℃, there is a problem in that the dislocation density generated by shear transformation is insufficient, resulting in low strength of the matrix structure and inferior yield strength and hole expansion. That is, the first cooling end temperature may be Bs-30℃ or lower, and specifically may be 650℃ or lower to secure an appropriate amount of dislocation density.

[0126] In the second cooling step, if the second cooling end temperature is less than (Bs+Ms) / 2, a problem may occur where the dislocation density inside the bainitic ferrite during transformation is excessive and heat is degraded. That is, the second cooling end temperature may be (Bs+Ms) / 2 or higher, and specifically, may be 500°C or higher.

[0127] The secondary cooling step can be performed for a time (ts) satisfying the following Equation 2. k(T) in the following Equation 2 is an indicator representing the growth rate of bainitic ferrite and may be influenced by the alloy composition of the steel, the phase transformation temperature, and the austenite grain size after hot rolling. When the following Equation 2, which represents the relationship between k(T) and the holding time of secondary cooling, is satisfied, the area fraction of bainitic ferrite can be secured at 85 to 97%.

[0128] [Relationship 2]

[0129] 0.85 ≤ 1-exp[-k(T) x (ts) 2 ] ≤ 0.97

[0130] (Here, k(T) represents the value defined by the following Equation 3.)

[0131] [Relationship 3]

[0132]

[0133] (Here, T1 is the first cooling end temperature, T2 is the second cooling end temperature, and each element represents the content (weight%).)

[0134] If the value of the above Equation 2 is less than 0.85, the matrix structure (bainitic ferrite) is not sufficiently formed, making it difficult to secure sufficient elongation. In addition, if the value of the following Equation 2 exceeds 0.97, the fraction of the matrix structure (bainitic ferrite) is excessive, so while the elongation is excellent, it may be difficult to secure yield strength and tensile strength because a sufficient fraction of secondary phases is not secured.

[0135] In the second cooling step, the second cooling rate may be 20℃ / s or less. During the second cooling, the temperature may increase due to transformation heat generated by the bainite phase transformation, but the dislocation density may decrease excessively due to excessive heat generation. That is, the temperature of the steel plate may be maintained isothermally so that it does not increase due to transformation heat generation, or the second cooling may be performed at a cooling rate of 20℃ / s or less. If the second cooling rate exceeds 20℃ / s, the phase transformation proceeds actively, and the shape of the plate may become warped.

[0136] After the second cooling is completed, the steel plate can be 3rd cooled to 120℃ or lower at a cooling rate of 30℃ / s or more and the 3rd cooled steel plate can be wound.

[0137] In the third cooling step, the third cooling end temperature may be 120°C or lower. During the second cooling, the austenite stabilizes and the Ms temperature substantially decreases; however, when the composition and microstructure composition ratio of the present invention are satisfied, the Ms temperature of the carbon-enriched austenite is maintained at 120°C or higher, so most of the austenite can be transformed into low-temperature bainite and martensite during the third cooling step. The lower limit of the third cooling end temperature is not specifically limited, but as an example, it may be cooled to room temperature.

[0138] In the third cooling step, the third cooling rate may be 30℃ / s or higher. In this case, the formation of additional high-temperature bainite can be avoided to secure a secondary phase. Although there is no specific upper limit for the above third cooling rate, the third cooling rate may be 100℃ / s or lower to prevent distortion of the plate shape.

[0139] Kwon Chi

[0140] The coiling step can be performed immediately after the third cooling. If the coiling temperature is 120°C or higher, carbon diffusion is facilitated, and the strength of the secondary phase, low-temperature bainite and / or martensite, may decrease due to tempering. That is, the upper limit of the coiling temperature may be 120°C.

[0141] After completing the cooling and coiling processes according to the above, the target hot-rolled steel sheet can be obtained by performing final cooling. At this time, the final cooling can be completed by performing air cooling.

[0142] The steel plate after the final cooling is completed can be further pickled and oiled. In addition, after pickling, it can be heated to a temperature range of 400 to 750°C to apply a hot-dip galvanizing process.

[0143] The above-mentioned molten plating process may use zinc (Zn)-based, aluminum (Al)-based, and zinc (Zn)-magnesium (Mg)-aluminum (Al) ternary plating baths, and the alloy composition within the plating bath is not specifically limited.

[0144] 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.

[0145] (Example)

[0146] A slab having the alloy composition of Table 1 below (unit weight %, the remaining components being Fe and unavoidable impurities) was prepared, and the slab was manufactured into a hot-rolled steel sheet according to the manufacturing conditions of Table 2. 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, and the total reduction amount for the final 2 passes during finish rolling was applied uniformly as 25%. In addition, the first cooling rate was applied uniformly as 70℃ / s and the third cooling rate as 45℃ / s.

[0147]

[0148]

[0149] For the above-mentioned Invention Examples 1 to 10 and Comparative Examples 1 to 8, the fraction of the microstructure, the average equivalent diameter of the secondary phase, the yield strength (YS), tensile strength (TS), elongation (El), and hole expansion (HER) were measured and are listed in Table 3 below.

[0150] The fraction of the microstructure and the average equivalent diameter of the secondary phase were measured by analyzing the specimen at a magnification of 5,000x using a scanning electron microscope and an image analyzer after etching the specimen with the Nital etching method.

[0151] 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.

[0152] Hole expandability (HER) was measured by punching a hole with an initial diameter (d0) of 10 mm and expanding the hole by raising a conical punch at 60°. At this time, the punch was stopped rising the moment the crack penetrated the thickness of the steel plate, and the diameter (d) of the punched hole after the crack penetrated was measured. Subsequently, HER(%) was calculated as = ((d - d0) / d0) × 100. The average value was calculated after conducting three tests for each steel grade.

[0153]

[0154] As shown in Tables 1 to 3 above, it was confirmed that Invention Examples 1 to 10, which satisfy all the alloy compositions and manufacturing conditions proposed in the present invention, contain bainitic ferrite as the matrix in an area fraction of 85 to 97% and low-temperature bainite and / or martensite as the secondary phase in an area fraction of 3 to 15%. In addition, it was confirmed that the average equivalent diameter of the secondary phase is 3 μm or less, and the proportion of martensite in the total area of ​​the secondary phase is 80% or more.

[0155] That is, it can be seen that Invention Examples 1 to 10 satisfy all the alloy composition and manufacturing conditions proposed in the present invention, and by securing an appropriate matrix structure and the type and fraction of secondary phases, a yield strength of 670 MPa or more, a tensile strength of 780 MPa or more, an elongation of 15% or more, and a hole expansion of 55% or more can be secured.

[0156] Meanwhile, it was confirmed that Comparative Example 1 had insufficient carbon (C) content, so it was not possible to secure a secondary phase of 3% or more in area fraction, and thus the yield strength and tensile strength were measured to be low.

[0157] In addition, it was confirmed that Comparative Example 2 had insufficient manganese (Mn) content, so it was not possible to secure a secondary phase of 3% or more in area fraction, and thus the yield strength and tensile strength were measured to be low.

[0158] In addition, Comparative Example 3 had an excessive carbon (C) content, so it was not possible to secure a matrix structure of 85% or more in terms of area fraction, resulting in inferior elongation. It was also confirmed that the yield strength and tensile strength were excessively high, and thus the hole expansion ability was also inferior.

[0159] In addition, Comparative Example 4 had excessively high first cooling end temperatures and second cooling end temperatures, so it was not possible to secure sufficient dislocations within the matrix structure, and thus could not secure a yield strength of 670 MPa or higher, and the hole expansion ability was also inferior due to the high difference in hardness between the matrix structure and the secondary phase.

[0160] In addition, it can be confirmed that in Comparative Examples 5 and 6, the third cooling end temperature exceeds 120°C, so the proportion of low-temperature bainite in the secondary phase is excessively high, and the yield strength and tensile strength are measured to be low.

[0161] In addition, considering that Comparative Example 7 was calculated to have a value exceeding 0.97 in relational equation 2, it was found that a sufficient fraction of the secondary phase was not secured due to the excessive secondary cooling time, and as a result, the tensile strength was inferior.

[0162] In addition, considering that Comparative Example 8 is calculated to have a value of less than 0.85 in relational equation 2, it can be confirmed that a sufficient fraction of the base structure was not secured due to insufficient secondary cooling time, and as a result, the elongation is insufficient.

[0163] Meanwhile, FIGS. 1(a), (b), and (c) are scanning electron microscope images of the microstructures of Inventive Example 1, Comparative Example 4, and Comparative Example 5, respectively. Specifically, FIG. 1(a) is a photograph of Example 1, showing that the matrix structure and secondary phase intended in the present invention were appropriately formed. In contrast, FIG. 1(b) is a photograph of Comparative Example 4, showing that polygonal ferrite was excessively formed, and as a result, the yield strength and hole expandability were measured to be low. Meanwhile, FIG. 1(c) is a photograph of Comparative Example 5, showing that the tensile strength was measured to be low because the tertiary cooling end temperature was high and martensite was not formed as a secondary phase. Additionally, it was observed that a large amount of precipitates were formed within the matrix structure, and the strengthening effect of solid solution carbon present within the matrix structure was lost, confirming that the yield strength was also low.

[0164] The above embodiments are merely examples, and the present invention is not limited thereto. Any configuration having substantially the same structure as the technical concept described in the claims of the present invention and achieving the same functional effect is included within the technical scope of the present invention.

Claims

1. In wt%, carbon (C): 0.04–0.15%, silicon (Si): 0.001–1.2%, manganese (Mn): 0.8–2.5%, aluminum (Al): 0.001–1.0%, 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 85–97 area% of bainic ferrite as the matrix structure and 3–15 area% of one or more of lower bainite and martensite as the secondary phase, and A steel plate in which martensite in the above secondary phase constitutes 80% or more of the total secondary phase fraction.

2. In Paragraph 1, The above steel plate is a steel plate containing one or more of chromium (Cr): 0.01~0.8%, molybdenum (Mo): 0.01~0.5%, titanium (Ti): 0.001~0.25%, niobium (Nb): 0.001~0.25%, and vanadium (V): 0.001~0.25%.

3. In Paragraph 1, A steel plate having an average circular diameter of the above secondary phase of 3.0㎛ or less.

4. In Paragraph 1, The above martensite is a steel plate consisting of lath martensite and plate martensite.

5. In Paragraph 1, A steel plate having an average spacing between the above secondary phases of 2㎛ or more.

6. In Paragraph 1, The above steel plate is a steel plate having a hole expansion rate of 55% or more.

7. In Paragraph 1, The above steel plate has a yield strength of 670 MPa or more, a tensile strength of 780 MPa or more, and an elongation of 15% or more.

8. A step of reheating a slab containing, in wt%, carbon (C): 0.04~0.15%, silicon (Si): 0.001~1.2%, manganese (Mn): 0.8~2.5%, aluminum (Al): 0.001~1.0%, 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, at 1100~1350℃; A step of obtaining a hot-rolled steel sheet by hot-rolling the above-mentioned heated slab; A step of first cooling the above hot-rolled steel plate to a first cooling end temperature of Bs-30℃ or lower at a cooling rate of 50℃ / s or more; A step of secondarily cooling the above first cooled steel plate to a second cooling end temperature of (Bs+Ms) / 2 or higher at a cooling rate of 20℃ / s or less; A step of tertiarily cooling the above secondary cooled steel plate to a tertiary cooling end temperature of 120℃ or lower at a cooling rate of 30℃ / s or more; The step of winding the above third-cooled steel plate; and Step of cooling the above-mentioned wound steel plate to room temperature A method for manufacturing a steel plate including 9. In Paragraph 8, A method for manufacturing a steel sheet, wherein the step of obtaining the above hot-rolled steel sheet is performed in a rolling end temperature (FDT) range of 800 to 1150°C such that the Du value of the following Equation 1 satisfies 3 to 10. [Relationship 1] Du = [(FDT + 7.4×[C] - 24.7×[Si] - 4.7ㅠ[Mn] - 3.9×[Cr] - 5.2×[Mo] - 480×[Ti] - 1200×[Nb])×0.049] - 34.2 (Here, FDT refers to the temperature (°C) of the hot-rolled plate immediately after hot rolling, and each element represents the content (weight%).) 10. In Paragraph 8, A method for manufacturing a steel sheet in which, in the step of obtaining the hot-rolled steel sheet, the total reduction amount of the final two passes is 10 to 40%.

11. In Paragraph 8, A method for manufacturing a hot-rolled steel sheet in which the above secondary cooling step is performed for a time (ts) satisfying the following relationship 2. [Relationship 2] 0.85 ≤ 1-exp[-k(T) x (ts) 2 ] ≤ 0.97 (Here, k(T) represents the value defined by the following Equation 3.) [Relationship 3] (Here, T1 is the first cooling end temperature, T2 is the second cooling end temperature, and each element represents the content (weight%).) 12. In Paragraph 8, A method for manufacturing a steel plate comprising the additional steps of pickling and oiling the above-mentioned manufactured steel plate.

13. In Paragraph 8, A method for manufacturing a steel plate, further comprising the step of pickling the steel plate manufactured above and heating it to a temperature range of 400 to 750°C to perform molten plating.

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