STEEL SHEET THAT HAS HIGH STRENGTH AND HIGH FORMABILITY AND METHOD FOR MANUFACTURING THE SAME.

Abstract

A steel sheet having high strength and high formability according to one aspect of the present invention comprises, by weight percent, 0.05–0.15% carbon (C), greater than 0 and 0.4% or less silicon (Si), 4.0–9.0% manganese (Mn), greater than 0 and 0.3% or less aluminum (Al), 0.02% or less phosphorus (P), 0.005% or less sulfur (S), 0.006% or less nitrogen (N), and the remainder iron (Fe) and other unavoidable impurities. The steel sheet has a microstructure consisting of ferrite and residual austenite. The grain size of the microstructure is 3 µm or less. The steel sheet has a strain limit (YS) of 800 MPa or more, a tensile strength (TS) of 980 MPa or more, an elongation (EL) of 25% or more, and a hole expansion ratio (HER) of 20% or more.

Description

STEEL SHEET HAVING HIGH STRENGTH AND HIGH FORMABILITY AND METHOD FOR MANUFACTURING IT FIELD OF INVENTION The present invention relates to a steel sheet and a method for manufacturing the same, and, more particularly, to a steel sheet having high strength and high formability and a method for manufacturing the same. BACKGROUND OF THE INVENTION In recent years, from the perspective of automotive safety and weight reduction, the development of high-strength automotive steel sheets has progressed rapidly. To ensure passenger safety, steel sheets used for automotive structural members need sufficient impact resistance, which requires increased strength and hardness. Furthermore, these steel sheets need adequate formability for automotive applications, and reducing the vehicle body's weight is essential for improving fuel efficiency. Therefore, ongoing research has focused on significantly strengthening automotive steel sheets and increasing their formability. Currently, since high-strength steel sheets for automobiles possess the aforementioned characteristics, a dual-phase steel has been proposed, which has strength and elongation ensured by two phases, ferrite and martensite phases, and transformation-induced plasticity steel, which has its strength and elongation ensured by means of the phase transformation of austenite retained in the final structure during plastic deformation. The technologies related to it include Korean patent application number 102016-0077463 (titled “Ultra-High-Strength, High-Ductility Steel Sheet Having Excellent Yield Strength and Method for Manufacturing the Same”). SUMMARY OF THE INVENTION Technical Problem One problem that the present invention must solve is to provide a steel sheet that has high formability and high strength, and a method for manufacturing the same. Technical Solution In one aspect of the present invention, a steel sheet is provided that has high strength and high formability, comprising: by weight, 0.05 to 0.15% carbon (C), 0.4% silicon (Si), 4.0 to 9.0% manganese (Mn), 0.3% aluminum (Al), 0.02% or less phosphorus (P), 0.005% or less sulfur (S), 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other unavoidable impurities. The steel sheet has a microstructure consisting of ferrite and retained austenite. The microstructure has a grain size of 3 μm or less. The steel sheet has a strain limit (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater. In an exemplary form, the steel blade may also include one or more of the following: niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), each of which may be included in an amount greater than 0 or less than or equal to 0.02% by weight. In an exemplary form, the steel blade may also include more than 0 and less than or equal to 0.001% by weight of boron (B). In an exemplary modality, the volume fraction of austenite retained in the microstructure can be 10 to 30% by volume. In one aspect of the present invention, a method is provided for manufacturing a steel sheet having high strength and high formability, comprising the steps of: (a) manufacturing a heat-rolled steel sheet from a steel slab comprising, by weight, an amount of 0.05 to 0.15% of carbon (C), an amount greater than 0 or less than or equal to 0.4% of silicon (Si), an amount of 4.0 to 9.0% of manganese (Mn), an amount greater than 0 or less than or equal to 0.3% of aluminum (Al), an amount of 0.02% or less of phosphorus (P), an amount of 0.005% or less of sulfur (S), an amount of 0.(a) producing a cold-rolled steel sheet by cold-rolling the heat-rolled steel sheet; (c) subjecting the cold-rolled steel sheet for the first heat treatment to a temperature of AC3 to (AC3 + 15) °C; and (d) subjecting the cold-rolled steel sheet, subjected to the first heat treatment, to a second heat treatment at an intercritical temperature. The cold-rolled steel sheet after step (d) has a microstructure consisting of ferrite and retained austenite. In an exemplary form, the steel slab may also include one or more of the following: niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), each of which may be included in an amount greater than 0 or less than or equal to 0.02% by weight. In an exemplary form, the steel slab may also include an amount greater than 0 and less than or equal to 0.001% by weight of boron (B). In an exemplary embodiment, step (c) may include a cooling step of the cold-rolled, heat-treated steel sheet to a temperature of 350 to 450 °C at a cooling rate of 4 to 10 °C / s. In an exemplary embodiment, step (d) may include a cooling step of the cold-rolled, heat-treated steel sheet to a temperature of 350 to 450 °C at 4 to 10 °C / s. In an exemplary embodiment, step (a) may include the steps of: (a1) reheating the steel slab to a temperature of 1,150 to 1,250 °C; (a2) heat-rolling the preheated steel slab to a finished delivery temperature of 925 to 975 °C; and (a3) ​​cooling the heat-rolled steel slab rqc Lnn / zznz / B / Yi to a temperature of 700 °C to 800 °C at a cooling rate of 10 to 30 °C / s, followed by winding. In an exemplary embodiment, the method may also include, between steps (a) and (b), a step of subjecting the heat-rolled steel sheet to a softening heat treatment at a temperature of 550 °C to 650 °C. In an exemplary embodiment, the cold-rolled steel sheet after stage (d) may have a strain limit (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater. In an exemplary form, the cold-rolled steel sheet after stage (d) can have a grain size of 3 pm or less. Advantageous Effects According to the present invention, it is possible to manufacture a steel sheet having a microstructure consisting of ultrafine granulated ferrite and retained austenite through control of the component system and process conditions. Due to the fine granulated ferrite, the steel sheet can have high strength, and due to the retained austenite present in an amount of 10 to 30% by volume in the microstructure, the steel sheet can have high strength and elongation. Furthermore, the steel sheet can have a high hole expansion ratio (HER) as a result of controlling the shape of the microstructure. As a result, it is possible to effectively obtain a steel sheet with high strength and high formability. BRIEF DESCRIPTION OF THE FIGURES Fig. 1 is a process flow diagram that schematically shows a method for manufacturing a steel sheet that has high strength and high formability according to an exemplary embodiment of the present invention. Fig. 2 shows the results of a high temperature stress test for a sample of a comparative component system of the present invention. Fig. 3 shows the results of a high temperature stress test for a sample component system implementing the present invention. Fig. 4 is a photograph showing the microstructure of a high-strength steel sheet according to an exemplary embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION Hereafter, the present invention will be described in detail with reference to the accompanying figures, so that it may be easily implemented by those skilled in the art to which the present invention pertains. The present invention can be embodied in a variety of different forms and is not limited to the embodiments described herein. Reference numbers RQf Lnn / zznz / E / Yi and similar components are provided through this descriptive specification. Furthermore, detailed descriptions of known functions and configurations will be omitted when they unnecessarily obscure the subject matter of the present invention. According to an exemplary embodiment of the present invention, a steel sheet having high strength and high formability can have a final microstructure consisting of fine-grained ferrite and retained austenite in an amount of 10 to 30% by volume. In this way, the steel sheet can have high strength, high elongation, and a high hole expansion ratio (HOR). First, to achieve high elongation in the steel sheet, it contains a sufficient amount of retained austenite, at a level of 10 to 30% by volume. Retained austenite can significantly improve the elongation of the steel sheet, similar to how it does in conventionally plasticized steel. To ensure the required fraction of retained austenite, an austenite stabilizing agent can be added to the steel sheet as described later. Furthermore, as described later, the first and second annealing heat treatments can be performed continuously, with the second annealing heat treatment taking place at an intercritical temperature. Subsequently, to achieve a high hole expansion ratio in the steel sheet, the phase boundary between a hard and a soft phase, which can act as a cracking site, is reduced. To this end, the steel sheet may not contain hard phases, such as martensite and bainite, in its final microstructure. Furthermore, to achieve a high hole expansion ratio, the interfaces between precipitates and grains are reduced. This can be accomplished by controlling the content of precipitate-forming elements, such as titanium, niobium, and vanadium, and precipitate-growth-inhibiting elements, such as molybdenum. Additionally, to achieve a high hole expansion ratio, the fraction of high-angle grain boundaries (HABG) in the final structure can be increased.As an example, high-angle grain boundaries can refer to grain boundaries where the angle between adjacent grains is 15° or greater. Furthermore, the shape of the microstructure can be controlled so that the steel sheet has a high proportion of hole expansion. In order to increase the fraction of high-angle grain boundaries and control the shape of the microstructure, as will be described later, the annealing heat treatment can be carried out in two stages: a first heat treatment and a second heat treatment. Next, to ensure the steel sheet has high strength, the grains of the final microstructure are refined. Although the annealing heat treatment described above is performed in two stages, the sizes of the ferrite and retained austenite grains can be controlled to 3 pm or less. rqc Lnn / zznz / B / Yi Furthermore, the first annealing heat treatment can be carried out at a temperature of AC3 to (AC3 + 15) °C. Hereafter, the steel sheet having high strength and high formability according to an exemplary embodiment of the present invention having the characteristics described above will be described in more detail. Steel sheet that has high strength and high formability A high-strength steel sheet according to an exemplary embodiment of the present invention includes, by weight, 0.05 to 0.15% carbon (C), 0.4% silicon (Si), 4.0 to 9.0% manganese (Mn), 0.3% aluminum (Al), 0.02% or less phosphorus (P), 0.005% or less sulfur (S), 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other unavoidable impurities. In addition, the high-strength steel sheet further includes one or more of the following: niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), each of which may be included in an amount greater than 0.02% or less by weight. Furthermore, the high-strength steel sheet may also include more than 0 and less than or equal to 0.001% by weight of boron (B). Hereafter, the function and content of each component included in the cold-rolled high-strength steel sheet according to an exemplary embodiment of the present invention will be described in detail (the content of each component is given in % by weight based on the total weight of the steel sheet and will hereafter be expressed in %). Carbon (C): 0.05% to 0.15% Carbon (C) is the most important alloying element in steelmaking and is used primarily to provide the basic strengthening and stabilizing austenite in the present invention. A high concentration of carbon (C) in austenite improves its stability, making it easier to ensure adequate austenite for material property enhancement. However, an excessively high carbon (C) content can result in decreased weldability due to the increased carbon equivalent, and numerous precipitated cementite structures, such as pearlite, may form during cooling. For this reason, carbon (C) is preferably added in an amount of 0.05 to 0.15% of the total weight of the steel sheet. If the carbon content is less than 0.05%, it may be difficult to ensure the strength of the steel sheet, and when the carbon content is greater than 0.05%, the steel sheet may not be strong enough.15%, can deteriorate the hardness and ductility of the steel sheet. Silicon (Si): more than O v less than or equal to 0.4% Silicon (Si) is an element that suppresses carbide formation in ferrite and increases the austenite diffusion rate by increasing carbon (C) activity. Silicon (Si) is also known as a ferrite stabilizing element, which increases ductility by increasing the ferrite fraction during cooling. Furthermore, silicon has a very high ability to suppress carbide formation and, consequently, is a necessary element to ensure the TRIP effect by increasing the carbon concentration in austenite retained during bainite formation. However, if silicon (Si) is added in an amount greater than 0.4%, it can form silicon dioxide (SiO2) on the surface of the steel sheet during the process, increase the load on the roll during heat rolling, and generate a large amount of red scale. Therefore, silicon (Si) is preferably added in an amount of 0.4% of the total weight of the steel sheet. Manganese (Mn): 4.0% to 9.0% Manganese (Mn) is an austenite-stabilizing element. As manganese (Mn) is added, the Ms, which is the martensite formation start temperature, gradually decreases, thus showing the effect of the increased fraction of austenite retained after heat treatment. Manganese is included in an amount of 4.0 to 9.0% of the total weight of the steel sheet. If manganese is added in an amount less than 4.0%, the effect described above cannot be sufficiently guaranteed. On the other hand, if manganese is added in an amount greater than 9.0%, weldability may decrease due to an increase in the carbon equivalent, and manganese oxide (MnO) may form on the surface of the steel sheet during the process, resulting in decreased plate formation due to reduced wetting of the affected area. Aluminum (Al): more than 0 v less than or equal to 0.3% Aluminum (Al) is known as a ferrite-stabilizing element that inhibits carbide formation, such as silicon (Si). Furthermore, aluminum raises the equilibrium temperature, thus broadening the suitable heat treatment temperature range. However, excessive addition of aluminum (beyond 0.3%) can lead to problems in continuous melting due to Al precipitation. Therefore, aluminum should be added in amounts greater than 0.3% and less than or equal to 0.3% of the total weight of the steel sheet. At least one of the following: niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo): more than 0 and less than or equal to 0.2% for each Niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo) can be optionally included in steel. First, niobium (Nb), titanium (Ti), and vanadium (V) are elements that precipitate as carbides in steel and are added to ensure strength through carbide precipitation. Titanium (Ti) can suppress crack formation during continuous casting by inhibiting the formation of alloying elements (AIN). However, if niobium (Nb), titanium (Ti), and vanadium (V) are each added in amounts greater than 0.2%, they can form harsh precipitates. This leads to disadvantages because the amount of carbon in the steel is reduced, degrading the material's properties, and increasing manufacturing costs due to the addition of niobium (Nb), titanium (Ti), and vanadium (V). Furthermore, if titanium is added in excess, it can cause nozzle clogging during continuous casting.Therefore, when at least one of the following is added: niobium (Nb), titanium (Ti) and vanadium (V), each of the following: niobium (Nb), titanium (Ti) and vanadium (V) may be added in an amount greater than 0 and less than or equal to 0.2% of the total weight of the steel sheet. Furthermore, molybdenum (Mo) can be used to control carbide size by suppressing carbide growth. However, if molybdenum is added in amounts greater than 0.2%, there are disadvantages, as the aforementioned effect becomes saturated and manufacturing costs increase. Boron (B) Boron (B) can be optionally added to the steel sheet and can act as a grain boundary strengthening agent. Boron can be added in amounts greater than 0 and less than or equal to 0.001% of the total weight of the steel sheet. If boron is added in amounts greater than 0.001%, it can decrease the high-temperature ductility of the steel sheet by forming a nitride, such as BN. Other elements Phosphorus (P), sulfur (S), and nitrogen (N) are inevitably added to steel during the steelmaking process. Ideally, these elements should be included, but they may be present in some quantities since their complete removal is difficult with current manufacturing processes. Phosphorus (P) can have a similar function to silicon in steel. However, if phosphorus is added in an amount greater than 0.02% of the total weight of the steel sheet, it can reduce the weldability of the steel sheet and increase its brittleness, consequently leading to a deterioration of the material's properties. Therefore, the amount of phosphorus added should be controlled to 0.02% or less of the total weight of the steel sheet. Sulfur (S) can inhibit the hardness and weldability of steel, and consequently, its content can be controlled to 0.005% or less of the total weight of the steel sheet. If nitrogen (N) is present in excessive amounts in steel, a large quantity of nitride can precipitate, resulting in a deterioration of the steel sheet's ductility. Therefore, the nitrogen (N) content can be controlled to 0.006% or less of the total weight of the steel sheet. The high-strength steel sheet of the present invention, which has the alloying components described above, has a microstructure consisting of ferrite and retained austenite. In this case, the volume fraction of retained austenite in the microstructure can be 10 to 30% by volume. The grains of the high-strength steel sheet can be fine grains with a size of 3 µm or less. The fraction of high-angle grain boundaries between the grains can be 70% or greater. High-strength steel sheet may have material properties including a strain limit (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater. rqc Lnn / zznz / E / Yi Therefore, the high-strength steel sheet according to the embodiment of the present invention can be applied to fields that require high strength and high formability. The high-strength steel sheet described above, according to the embodiment of the present invention, can be manufactured by a method of an exemplary embodiment, as shown below. The present invention aims to provide a steel sheet that has excellent elongation, hole expansion ratio, and strength as a result of using alloying components with suitably controlled composition ratios and undergoing a two-stage annealing heat treatment after a hot rolling process and a cold rolling process, and a method for manufacturing the same. Method for manufacturing a steel sheet that has high strength and high formability Fig. 1 is a process flow diagram that schematically shows a method for manufacturing a steel sheet that has high strength and high formability according to an exemplary embodiment of the present invention. Referring to Fig. 1, the method for making a steel sheet includes the steps of: (S110) reheating a steel slab; (S120) making a heat-rolled steel sheet by heat-rolling the steel slab; (S130) cold-rolling the heat-rolled steel sheet; and (S140) subjecting the cold-rolled steel sheet to the annealing heat treatment. First, the step (S110) of reheating a steel slab is a step to prepare a steel slab that includes: % by weight, an amount of 0.05 to 0.15% of carbon (C), an amount greater than 0 or less than or equal to 0.4% of silicon (Si), an amount of 4.0 to 9.0% of manganese (Mn), an amount greater than 0 and less than or equal to 0.3% of aluminum (Al), an amount of 0.02% or less of phosphorus (P), an amount of 0.005% or less of sulfur (S), an amount of 0.006% or less of nitrogen (N), and the remainder being iron (Fe) and other unavoidable impurities, and reheating the steel slab to redissolve the components segregated during smelting and homogenize them as casting components. Meanwhile, the steel slab may also include one or more of the following: niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), each of which may be included in an amount less than 0 or less than or equal to 0.02% by weight.Furthermore, the steel slab may also include more than 0 and less than or equal to 0.001% by weight of boron (B). The preferred reheating temperature for the steel slab is approximately 1,150 to 1,250 °C to ensure a normal hot delivery temperature. If the reheating temperature is lower than 1,150 °C, problems may arise because the heat load on the rolled product increases rapidly. Conversely, if the reheating temperature is higher than 1,250 °C, it may be difficult to guarantee the strength of the final manufactured steel sheet due to the thickening of the initial austenite grains. Subsequently, the heat rolling stage (S120) is performed after reheating the steel slab. This stage forms a heat-rolled steel sheet by performing heat rolling using a conventional method and then a final rolling at a temperature from 925 to 975 °C. Since the steel slab of the present invention has high alloying element content, such as manganese, the final rolling can be performed at a high temperature of 925 to 975 °C. After the final rolling, the heat-rolled steel sheet is cooled to a temperature of 700 to 800 °C at a cooling rate of 10 to 30 °C / s and then wound into coils. The cooling method can be performed using a waterless quenching method. The heat-rolled steel sheet can have a martensitic structure after cooling. According to some exemplary methods, before cold rolling a heat-rolled steel sheet with a fully martensitic structure, a softening heat treatment can be performed to reduce the rolling load during cold rolling. This softening heat treatment can be carried out at a temperature of 550 to 650 °C. If the softening heat treatment temperature is lower than 550 °C, recrystallization of the martensite produced after heat rolling may not occur, and only quenching may take place. Consequently, supersaturated carbon may form in the structure as cementite and spheroids. In such a case, given the increased brittleness of the martensite, fracture of the steel sheet may occur during cold rolling.On the other hand, if the softening heat treatment temperature exceeds 650 °C, excessive austenite formation may occur, and martensite may form from the austenite during cooling, rendering the softening heat treatment ineffective. Through softening heat treatment performed within the above temperature range, the martensitic structure after heat rolling is transformed into a ferrite-retained austenite structure. Subsequently, the cold rolling stage (S130) is a cold rolling stage of the heat-rolled steel sheet after pickling. Cold rolling can be carried out under conditions where the heat-rolled steel sheet is cold rolled at a reduction ratio of 40 to 60%. Through cold rolling, the ferrite-austenite composite structure retained after the softening heat treatment can be transformed into a ferrite-martensite composition. Subsequently, the annealing heat treatment stage (S140) may include a stage of subjecting the cold-rolled steel sheet to a first heat treatment at a temperature of AC3 to (AC3 + 15) °C, and a stage of subjecting the cold-rolled steel sheet, after the first heat treatment, to a second heat treatment at an intercritical temperature. The temperature of AC3 to (AC3 + 15) °C in the first heat treatment stage may be, for example, 735 to 750 °C. The intercritical temperature in the second heat treatment stage may be, for example, 640 to 660 °C. In one exemplary instance, the first heat treatment can transform a ferrite-martensite composition in the cold-rolled steel sheet into a martensitic structure. In this first heat treatment, the cold-rolled steel sheet is heated to a specific temperature of 735 to 750 °C at a heating rate of 1 to 3 °C / s and held at that temperature for 40 to 120 seconds. If the heat treatment temperature is below 735 °C, it is not possible to ensure that the austenite grains are of sufficient size at the specified temperature, and a martensite-ferrite structure may form after heat treatment. Consequently, the strength and ductility of the final structure after annealing may increase. On the other hand, if the heat treatment temperature is above 750 °C, the austenite grain size at the specified temperature may increase excessively, which is detrimental to ensuring austenite stabilization in the final structure after annealing, potentially resulting in lower strength in the steel sheet. Furthermore, if the heating rate is less than 1 °C / s, the holding time at the specific temperature of 735 to 750 °C may exceed the upper limit of the 40 to 120 second range, potentially leading to an excessive increase in the austenite grain size at that temperature. Conversely, if the heating rate is greater than 3 °C / s, the holding time at the specific temperature of 735 to 750 °C may be shorter than the upper limit of the 40 to 120 second range, making it impossible to ensure that the austenite grains are of sufficient size at that temperature. Subsequently, the cold-rolled, heat-treated steel sheet is cooled to a temperature of 350 to 450 °C at a cooling rate of 4 to 10 °C / s. In one example, the cold-rolled steel sheet cooled to the above temperature can be cured for 120 to 330 seconds. The cold-rolled steel sheet that has undergone the first heat treatment can be continuously subjected to a second heat treatment. In one example, the second heat treatment involves heating the cold-rolled steel sheet to a specific temperature of 640 to 660 °C at a heating rate of 1 to 3 °C / s and holding the sheet at that temperature for 40 to 120 seconds. As the second heat treatment is performed at an intercritical temperature corresponding to the specified temperature range, the martensitic structure following the first heat treatment can be transformed into a structure consisting of ferrite and retained austenite. In this case, the volume fraction of retained austenite can be 10 to 30% by volume. If the temperature of the second heat treatment is below 640 °C, an excessive amount of austenite structures may form at the specified temperature, and the stability of the austenite may increase. Consequently, the austenite in the microstructure after cooling may not exhibit phase transformation during plastic deformation, and the strength and ductility of the steel sheet may decrease. On the other hand, if the temperature of the second heat treatment is above 660 °C, an excessive amount of austenite structures may form at the specific temperature, and the stability of the austenite may decrease. Consequently, martensite may form in the microstructure after cooling, resulting in a decrease in the ductility and hole expansion ratio of the steel sheet. If the heating rate is less than 1 °C / s, spheroidization may occur before the cold-rolled sheet material reaches the intercritical temperature range described above, resulting in deterioration of the steel sheet's material properties. If the heating rate is greater than 3 °C / s, the steel sheet may not be held within the specified temperature range for 40 to 120 seconds, making it impossible to ensure a sufficient fraction of retained austenite in the final structure. Subsequently, the cold-rolled, heat-treated steel sheet is cooled to a temperature of 350 to 450 °C at a cooling rate of 4 to 10 °C / s. In one example, the cold-rolled steel sheet cooled to the above temperature can be cured for 120 to 330 seconds. Through the method described above, a steel sheet can be manufactured that has high strength and high formability according to an exemplary embodiment of the present invention. The steel sheet of the present invention, manufactured by the process described above, can have a strain limit (YS) of 800 MPa or greater, a breaking stress (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater. As described above, in the manufacturing method according to an exemplary embodiment of the present invention, austenite stabilizing elements can be added to the steel slab in predetermined quantities as described above. Furthermore, as the first and second annealing heat treatments are performed continuously, the steel sheet can have a final microstructure consisting of fine-grained ferrite and 10 to 30% by volume of retained austenite. Because the steel sheet has a sufficient fraction of retained austenite, it can exhibit a high elongation of 25% or more due to its transformation-induced plasticity properties. Furthermore, the phase boundary between the hard and soft phases can be reduced by excluding hard phases, such as martensite and bainite, from the final microstructure described above. Additionally, the interfaces between precipitates and grains can be minimized by controlling the content of precipitate-forming elements, such as titanium, niobium, and vanadium, and precipitate-growth-inhibiting elements, such as molybdenum, in the steel slab component system. Moreover, the proportion of high-angle grain boundaries (HAGBs) in the final structure can be increased by performing a two-stage split annealing heat treatment, consisting of the first and second treatment stages, at predetermined temperature ranges.In the first heat treatment, due to the high dislocation density present in the martensite formed by the cold rolling process, recrystallization can occur actively before the martensite irreversibly transforms into austenite. In the second heat treatment, the martensite formed through the first heat treatment is heated, and consequently, recrystallization is relatively suppressed before the martensite irreversibly transforms into austenite. This can increase the fraction of high-angle grain boundaries in the final microstructure to 70% or more. As a result, the steel sheet can have a high hole expansion ratio of 20% or more. Subsequently, the grains of the final microstructure can be refined to give the steel sheet high strength. In particular, the grain size of the initial austenite can be optimized by performing the first heat treatment at a temperature of AC3 to (AC3 + 15) °C. Furthermore, the ferrite grain sizes and the retained austenite in the final microstructure can be controlled to 3 pm or less through the second heat treatment performed in the intercritical temperature range. Mode of Invention The configuration and effects of the present invention will be described in more detail hereafter with reference to preferred examples of the present invention. However, the following examples are provided to aid in understanding the present invention, and the scope of the present invention is not limited to the following examples. Example 1 The steel slabs having the comparative component system and the implementation component system in Table 1 below are produced through a continuous casting process. A sample was prepared for each of the steel slabs and subjected to a high-temperature tensile test. In the case of the comparative component system, the silicon and aluminum contents were higher compared to the upper limits of the silicon and aluminum content ranges according to an exemplary embodiment of the present invention. rqc Lnn / zznz / E / Yi Table 1 Component System (% by weight) C Si Mn Al PSN Comparative Component System 0.09 0.78 6.01 0.521 0.006 0.002 0.004 Implementation Component System 0.0772 0.081 6.385 0.266 0.0066 0.0008 0.004 Figure 2 shows the results of a high-temperature stress test for a sample of the comparative component system of the present invention, and Figure 3 shows the results of a high-temperature stress test for the sample of the implementing component system of the present invention. Specifically, the stress test results are those obtained by heating each of the sample of the comparative component system and the sample of the implementing component system to temperatures of 700 °C, 750 °C, 800 °C, 850 °C, 900 °C, 950 °C, 1000 °C, and 1100 °C, and then subjecting each sample to a tensile test at the above temperatures. Regarding the high-temperature tensile test, Figure 3 shows graph 201 obtained by heating the sample to a temperature higher than 1100 °C and then cooling the sample to each of the tensile test temperatures at a cooling rate of -1 °C / s, along with graph 202 obtained by cooling the sample to each of the tensile test temperatures at a cooling rate of -20 °C / s. In general, when the area reduction rate at a predetermined temperature is 50% or greater, it can be determined that ductility at that predetermined temperature is assured. Referring to Fig. 2, in the case of the comparative component system sample, the area reduction rate at 1100 °C was 55%, the area reduction rate in the temperature range of 700 to 800 °C was 50%, and the area reduction rate in the temperature range of 800 to 1050 °C was less than 50%, which is the specified value. On the other hand, referring to Fig. 3, the area reduction rate in the temperature range of 800 to 1100 °C exceeded 50%, which is the specified value. Referring to Figs. 2 and 3, in the case of the comparative component system sample, unlike the component system sample implementing the modality of the present invention, high temperature ductility is not assured in the high temperature range equal to or greater than 800 °C, in which continuous casting is carried out according to the modality of the present invention, and, consequently, cracks may occur during continuous casting, making it impossible to ensure a good slab. Table 2 below shows the rolling force for each step calculated by stimulating rolling by heat according to an exemplary embodiment of the present invention for both the sample of the comparative component system and the sample of the implementation component system. rqc Lnn / zznz / B / Yi Table 2 Rolling Step No. 1 No. 2 No. 3 No. 4 No. 5 No. 6 No. 7 Reduction Ratio 35.6% 34.5% 26.3% 30% 39.8% 50.8% 51.7% Comparative Component System Rolling Force (tons) 158.8 177.6 146.7 193.1 282.1 387.8 419.9 Implementation Component System Rolling Force (tons) 92.6 147.6 128.0 166.7 235.8 320.8 334.6 Referring to Table 2 above, it can be observed that a greater rolling force must be applied to the comparator component system sample compared to that applied to the implementation component system sample in order to generate the same reduction ratio for each rolling pass. That is, it can be confirmed that a relatively high load is applied to a rolling mill during the heat rolling of the comparator component system sample. Example 2 The sample prepared from the implementation component system shown in Table 1 above was subjected to the first and second annealing heat treatment processes according to Table 3 below. In the case of Comparative Examples 1 and 3, the second annealing temperature was less than 640 °C, which is the lower limit of the second annealing temperature according to the embodiment of the present invention. In the case of Comparative Examples 2 and 4, the second annealing temperature was greater than 660 °C, which is the upper limit of the second annealing temperature according to the embodiment of the present invention. In the case of Comparative Examples 5 through 7, the first annealing temperature was greater than 750 °C, which is the upper limit of the first annealing temperature according to the embodiment of the present invention.Furthermore, in the case of Comparative Example 7, the second annealing temperature was greater than 660 °C, which is the upper limit of the second annealing temperature according to the embodiment of the present invention. In the case of Comparative Examples 8 to 11, the first annealing heat treatment was performed, and only the second annealing heat treatment was performed. Furthermore, in the case of Comparative Example 11, the second annealing temperature was greater than 660 °C, which is the upper limit of the second annealing temperature according to the embodiment of the present invention. rqc Lnn / zznz / B / Yi Table 3 First annealing Second annealing Annealing temperature (°C) Cooling rate (°C / s) Cooling completion temperature (°C) Annealing temperature (°C) Cooling rate (°C / s) Cooling completion temperature (°C) Comparative Example 1 735 6 400 630 6 400 Example 1 735 6 400 640 6 400 Example 2 735 6 400 650 6 400 Example 3 735 6 400 660 6 400 Comparative Example 2 735 6 400 670 6 400 Comparative Example 3 750 6 400 630 6 400 Example 4 750 6 400 640 6 400 Example 5 750 6 400 650 6 400 Example 6 750 6 400 660 6 400 Comparative Example 4 750 6 400 670 6 400 Comparative Example 5 850 6 400 650 6 400 Comparative Example 6 850 6 400 660 6 400 Comparative Example 7 850 6 400 670 6 400 Comparative Example 8 640 6 400 Comparative Example 9 650 6 400 Comparative Example 10 660 6 400 Comparative Example 11 670 6 400 rqc ίηη / ζζηζ / Β / γι Table 4 shows the results of the material property evaluation of the samples from Comparative Examples 1 to 11 and Examples 1 to 6 subjected to the annealing heat treatment according to Table 3. rqc Lnn / zznz / E / Yi Table 4 Material Properties Deformation Limit (MPa) Tensile Strength (MPa) Elongation (7°) Tensile Elongation X (MPa·%) Re-dyed Austenite (7°) Average Grain Size (pm) HABG Fraction (7°) Hole Expansion Ratio (%) Material Properties Achieved 800 or greater 980 or greater 250 greater 10 to 30 2 or less 70 or greater 200 greater Comparative Example 1 915 988 23 22,724 17 2 or less 65 21 Not achieved Example 1 882 1,003 25 25,075 21 2 or less 72 28 Achieved Example 2 871 1,031 26 26,806 24 2 or less 77 27 Achieved Example 3 823 1,027 25 25,675 20 1 or less 70 23 Achieved Comparative Example 2 808 1,071 21 22,491 18 1 or less 71 22 Not achieved Comparative Example 3 911 946 24 22,704 15 2 or less 62 22 Not achieved Example 4 921 990 26 25,740 23 2 or less 75 27 Achieved Example 5 883 1,007 25 25,175 22 3 or less 73 30 Achieved Example 6 851 1,022 25 25,550 21 2 or less 69 22 I got Example 801 1,071 21 22,491 17 3 or 68 21 I don't know, Comparative 4 lower achieved Example Comparative 5 813 882 23 20,286 20 6 or lower 65 22 Not achieved Example Comparative 6 793 906 24 21,744 21 7 or lower 61 21 Not achieved Example Comparative 7 732 942 21 19,782 14 7 or lower 62 19 Not achieved Example Comparative 8 828 969 20 19,380 14 2 or lower 45 21 Not achieved Example Comparative 9 1,045 1,019 24 24,456 22 2 or lower 51 16 Not achieved Example Comparative 10 997 1,050 23 24,150 20 1 or lower 47 15 Not achieved Comparative Example 11 976 1,115 18 20,070 18 2 or less 48 14 Not achieved rqc Lnn / zznz / E / Yi The specified material properties of the high-strength steel sheet according to an exemplary embodiment of the present invention are a strain limit of 800 MPa or greater, a tensile strength of 980 MPa or greater, an elongation of 25% or greater, a retained austenite volume fraction of 10 to 30%, a high-angle grain boundary (HAGB) fraction of 70% or greater, and a hole expansion ratio of 20% or greater. The samples of Examples 1 through 6 met all of the above specified values. In the case of Comparative Example 1, the elongation and the high-angle grain boundary (HAGB) fraction were below the specified values. In the case of Comparative Example 2, the elongation was below the specified value. In the case of Comparative Example 3, the breaking stress, elongation, and high angle grain boundary fraction (HAGB) are below the specified values.In Comparative Example 4, the elongation, elongation at tensile strength x, average grain size, and high-angle grain boundary fraction (HAGB) are below the specified values. In Comparative Example 5, the tensile strength, elongation, average grain size, and high-angle grain boundary fraction (HAGB) are below the specified values. In Comparative Examples 6 and 7, the strain limit, tensile strength, elongation, average grain size, and high-angle grain boundary fraction (HAGB) are below the specified values. In Comparative Example 8, the tensile strength, elongation, and high-angle grain boundary fraction (HAGB) are below the specified values.In the case of Comparative Example 9 to 11, the elongation, the high angle grain boundary fraction (HAGB), and the hole expansion ratio are below the specified values. Figure 4 is a photograph showing the microstructure of the high-strength steel sheet according to an exemplary embodiment of the present invention. Specifically, Figure 4 is a photograph of the microstructure of the sample from Example 1. Referring to Table 4 and Figure 4, retained austenite having a volume fraction of 17% and the remainder being ferrite was observed in the sample from Example 1. Example 3 The sample prepared from the implementation component system shown in Table 1 above was subjected to the first and second annealing heat treatment processes according to Table 5 below. RQf Lnn / zznz / E / Yi Table 5 First annealing Second annealing Heating rate (°C / s) Annealing temperature (°C) Assimilation time (s) Cooling rate (°C / s) Cooling termination temperature (°C) Annealing temperature (°C) Cooling rate nto (°C / s) Cooling termination temperature (°C) Comp Example 12 4.0 750 31 13 400 660 6 400 Example 7 3.0 750 43 10 400 660 6 400 Example 8 2.0 750 59 7 400 660 6 400 Example 9 1.5 750 80 6 400 660 6 400 Example 1.0 750 118 4 400 660 6 400 lo 10 Comp Example lo 13 0.5 750 236 2 400 660 6 400 Comp Example lo 14 0.03 750 3,600 660 6 400 rqc Lnn / zznz / E / This Referring to Table 5 above, in the case of Comparative Example 12, the heating rate during the first annealing heat treatment was greater than 3 °C / s, which is the upper limit of the heating rate during the first annealing heat treatment according to an exemplary embodiment of the present invention, and the first annealing hold time did not satisfy 40 seconds or more. In the case of Comparative Example 13, the heating rate during the first annealing heat treatment was less than 1 °C / s, which is the lower limit of the heating rate during the first annealing heat treatment according to an exemplary embodiment of the present invention, and the first annealing hold time exceeded 120 seconds, which is the upper limit.In the case of Comparative Example 14, the heating rate during the first annealing heat treatment was less than 1 °C / s, which is the lower limit of the heating rate during the annealing heat treatment according to an exemplary embodiment of the present invention, and the first annealing hold time exceeded 120 seconds, which is the upper limit. Furthermore, the cooling rate was less than 4 °C / s, which is the lower limit. Examples 7 through 10 satisfied the conditions for both the first and second annealing heat treatments according to an exemplary embodiment of the present invention. Table 6 below shows the results of the material property evaluation of the samples from Comparative Examples 12 to 14 and Examples 7 to 10 subjected to the annealing heat treatment according to Table 5 above. Table 6 Material Properties Deformation Limit (MPa) Tensile Strength (MPa) Elongation (%) Elongation at Tensile Strength X (MPa%) Average Grain Size (pm) Whether the material properties were achieved 800 or 980 or 25 or greater - 2 or less Greater Greater Comp. Example 12 881 932 23 21,436 2 or less Not achieved Example 7 899 982 25 24,550 2 or less Achieved Example 8 874 1,011 25 25,275 2 or less Achieved Example 9 849 1,003 26 26,078 2 or less Achieved Example 10 865 993 26 25,818 3 or less Achieved Comp. Example 13 888 955 24 22,920 4 or less Not achieved Comp. Example 14 755 888 21 18,648 10 or less Not achieved rqc Lnn / zznz / B / Yi Referring to Table 6 above, in the case of Comparative Example 12, the specified values ​​for tensile strength and elongation were not achieved. In the case of Comparative Example 13, the specified values ​​for tensile strength, elongation, and average grain size were not achieved. In the case of Comparative Example 14, the specified values ​​for strain limit, tensile strength, elongation, and average grain size were not achieved. Examples 7 through 10 satisfied all the specified values ​​for the material properties according to the embodiment of the present invention. Example 4 The sample prepared from the implementation component system shown in Table 1 above was each subjected to the first and second annealing heat treatment processes. Table 7 First annealing Second annealing Annealing temperature (°C) Cooling rate (°C / s) Cooling termination temperature (°C) Heating rate (°C / s) Annealing temperature (°C) Holding time (°C) Cooling rate (°C / s) Cooling termination temperature (°C) Comp Example 15 735 6 400 4.0 660 31 13 400 Example 735 6 400 3.0 660 43 10 400 Example 11 Example 12 735 6 400 2.0 660 59 7 400 Example 13 735 6 400 1.5 660 80 6 400 Example 14 735 6 400 1.0 660 118 4 400 Comp Example 16 735 6 400 0.5 660 236 2 400 rqc Lnn / zznz / E / Yi Referring to Table 7 above, in the case of Comparative Example 15, the heating rate during the second annealing heat treatment exceeded 3 °C / s, which is the upper limit of the heating rate during the second annealing according to an exemplary embodiment of the present invention, and the second annealing hold time did not meet the requirement of 40 seconds or more. In the case of Comparative Example 16, the heating rate during the second annealing was less than 1 °C / s, which is the lower limit of the heating rate during the second annealing according to an exemplary embodiment of the present invention, and the second annealing hold time exceeded 120 seconds, which is the upper limit. Examples 11 to 14 met the conditions for both the first and second annealing heat treatments according to an exemplary embodiment of the present invention. Table 8 below shows the results of the material property evaluation of the samples from Comparative Examples 15 and 14 and Examples 11 to 10 subjected to the annealing heat treatment according to Table 7. Table 8 Material Properties Deformation Limit (MPa) Tensile Strength (MPa) Elongation (%) Elongation at Tensile Strength X (MPa%) Average Grain Size (pm) Material Properties Achieved 800 or greater 980 or greater 25 or greater - 2 or less Comp. Example 15 895 972 24 23,328 2 or less Not Achieved Example 11 881 1,030 25 25,750 1 or less Achieved Example 12 900 1,028 25 25,700 2 or less Achieved Example 13 928 1,039 25 25,975 1 or less Achieved Example 14 915 1,016 26 26,416 2 or less Achieved Comp. Example 16 876 1,001 24 24,024 2 or less Not achieved rqc Lnn / zznz / E / Yi Referring to Table 8, in the case of Comparative Example 15, the specified values ​​for tensile strength and elongation were not achieved. In the case of Comparative Example 16, the specified value for elongation was not achieved. Examples 11 to 14 satisfied all the specified values ​​for the material properties according to the exemplary embodiment of the present invention. Although the foregoing description has been given with reference to exemplary embodiments of the present invention, those skilled in the art may make various changes or modifications. Such changes and modifications may be considered to be within the scope of the present invention unless they depart from it. Accordingly, the scope of the present invention shall be determined by means of the appended claims. Experts in the field can implement simple modifications and changes, and such modifications and changes can be considered to be included within the scope of the present invention.

Claims

1. A steel sheet having high strength and high formability comprising: % by weight, an amount of 0.05 to 0.15% of carbon (C), an amount greater than 0 or less than or equal to 0.4% of silicon (Si), an amount of 4.0 to 9.0% of manganese (Mn), an amount greater than 0 and less than or equal to 0.3% of aluminum (Al), an amount of 0.02% or less of phosphorus (P), an amount of 0.005% or less of sulfur (S), an amount of 0.006% or less of nitrogen (N), and the remainder being iron (Fe) and other unavoidable impurities. where the microstructure has a grain size of 3 µm or less, and the steel sheet has a strain limit (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.

2. The steel sheet according to claim 1, comprising one or more of the following: niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo), each of which is included in an amount greater than 0 or less than or equal to 0.02% by weight.

3. The steel sheet according to claim 1, further comprising an amount greater than 0 and less than or equal to 0.001% by weight of boron (B).

4. The steel sheet according to claim 1, wherein the volume fraction of austenite retained in the microstructure is 10 to 30% by volume.

5. A method for manufacturing a steel sheet having high strength and high formability, the method comprising the steps of: (a) manufacturing a heat-rolled steel sheet from the steel slab comprising: % by weight, an amount of 0.05 to 0.15% of carbon (C), an amount greater than 0 or less than or equal to 0.4% of silicon (Si), an amount of 4.0 to 9.0% of manganese (Mn), an amount greater than 0 and less than or equal to 0.3% of aluminum (Al), an amount of 0.02% or less of phosphorus (P), an amount of 0.005% or less of sulfur (S), an amount of 0.006% or less of nitrogen (N), and the remainder being iron (Fe) and other unavoidable impurities.(b) manufacturing a cold-rolled steel sheet by cold-rolling the heat-rolled steel sheet; (c) subjecting the cold-rolled steel sheet to a first heat treatment at a temperature of AC3 to (AC3 + 15) °C; and (d) subjecting the cold-rolled steel sheet, subjected to the first heat treatment, to a second heat treatment at an intercritical temperature, wherein the cold-rolled steel sheet after step (d) has a microstructure consisting of ferrite and austenite. rqc Lnn / zznz / E / Yi.

6. The method according to claim 5, wherein the steel slab includes one or more of niobium (Nb), titanium (Ti), vanadium (V), and molybdenum (Mo), each of which is included in an amount greater than 0 or less than or equal to 0.02% by weight.

7. The method according to claim 5, wherein the steel slab further comprises an amount greater than 0 and less than or equal to 0.001% by weight of boron (B).

8. The method according to claim 5, wherein the volume fraction of austenite retained in the microstructure is 10 to 30% by volume.

9. The method according to claim 5, wherein step (c) comprises a cooling step of the heat-treated cold-rolled steel sheet to a temperature of 350 to 450 °C at a cooling rate of 4 to 10 °C / s.

10. The method according to claim 9, wherein step (d) comprises a cooling step of the heat-treated cold-rolled steel sheet to a temperature of 350 to 450 °C at a cooling rate of 4 to 10 °C / s.

11. The method according to claim 5, wherein step (a) comprises the steps of: (a1) reheating the steel slab to a temperature of 1,150 to 1,250 °C; (a2) heat-rolling the reheated steel slab to a final delivery temperature of 925 to 975 °C; and (a3) ​​heat-cooling the rolled steel sheet to a temperature of 700 °C to 800 °C at a cooling rate of 10 to 30 °C / s, followed by winding.

12. The method according to claim 5, further comprising, between steps (a) and (b), a step of subjecting the heat-rolled steel sheet to a softening heat treatment at a temperature of 550 °C to 650 °C.

13. The method according to claim 5, wherein the cold-rolled steel sheet after step (d) has a strain limit (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.

14. The method according to claim 5, wherein the cold-rolled steel sheet after step (d) has a grain size of 3 pm or less.