High-strength and highly formable steel plate and manufacturing method thereof

Abstract

The present invention provides a high-strength and highly formable steel sheet and a manufacturing method thereof. The high-strength and highly formable steel sheet according to an embodiment of the present invention contains, in weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% to 0.05%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.005%, nitrogen (N): more than 0% to 0.006%, and the balance being iron (Fe) and other inevitable impurities, and the contents of the carbon (C), manganese (Mn), and silicon (Si) are X C、重量% +0.066×X Si、重量% +0.043×X Mn、重量% ≦0.4, and satisfies the following requirements: yield strength (YS): 500 MPa or more, tensile strength (TS): 980 MPa or more, total elongation (T.EL): 23% or more, and the product of tensile strength and elongation: 23,000 MPa% or more.

Description

[Technical Field]

[0001] The technical concept of the present invention relates to a steel material, and more particularly to a steel plate having high strength and high formability, and a method for manufacturing the same. [Background technology]

[0002] Automotive steel sheets have been developed with a focus on increasing strength to ensure user safety and reduce vehicle weight, and ensuring elongation for ease of processing. Currently, typical ultra-high-strength steels include dual-phase steels, which maintain elongation through the two phases of ferrite and martensite, and transformation-induced plasticity steels (TRIP), which achieve strength and elongation through the phase transformation of retained austenite in the final structure during plastic deformation. However, development based on dual-phase steels, which cannot overcome the limitations of the Rule of Mixture (ROM), and transformation-induced plasticity steels, which have relatively low strength due to their predominantly bainite matrix, has reached its limit. Therefore, steelmakers are focusing on developing next-generation ultra-high-strength automotive steel sheets that achieve ultra-high strength and high formability by improving the microstructure of transformation-induced plasticity steels. Prior art documents include Korean Patent Application No. 10-2016-0077463. Summary of the Invention [Problem to be solved by the invention]

[0003] The technical problem that the technical concept of the present invention aims to achieve is to provide a high strength and highly formable steel sheet and a method for manufacturing the same.

[0004] However, these problems are merely examples, and the technical idea of ​​the present invention is not limited to these. [Means for solving the problem]

[0005] According to one aspect of the present invention, a high strength and highly formable steel plate and a method for manufacturing the same are provided.

[0006] According to one embodiment of the present invention, the high-strength and high-formability steel plate contains, in weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% to 0.05%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.005%, nitrogen (N): more than 0% to 0.006%, and the balance being iron (Fe) and other inevitable impurities, and the contents of the carbon (C), the manganese (Mn), and the silicon (Si) are X C、重量% +0.066×X Si、重量% +0.043×X Mn、重量% ≦0.4, and may satisfy yield strength (YS): 500 MPa or more, tensile strength (TS): 980 MPa or more, total elongation (T.EL): 23% or more, and the product of tensile strength and elongation: 23,000 MPa% or more.

[0007] According to one embodiment of the present invention, the high-strength and high-formability steel sheet may have a mixed structure of retained austenite, ferrite, and martensite / tempered martensite, where the area fraction of the ferrite is 20% to 50%, the area fraction of the retained austenite is 5% to 20%, and the area fraction of the martensite / tempered martensite is the remaining area fraction. Here, "martensite / tempered martensite" is defined as a general term for both fresh martensite and tempered martensite.

[0008] According to an embodiment of the present invention, the high-strength and high-formability steel sheet may have a ratio of uniform elongation / total elongation of 0.7 or more and less than 1.

[0009] According to one embodiment of the present invention, when a plastic deformation of 5% is applied to the high-strength and high-formability steel plate in a direction perpendicular to the rolling direction, the reduction rate of the area fraction of retained austenite in the high-strength and high-formability steel plate before and after the application may be more than 0% and 50% or less.

[0010] According to one embodiment of the present invention, the high-strength and high-formability steel plate may further contain, by weight percent, titanium (Ti), niobium (Nb), and vanadium (V): more than 0% to 0.05% in total.

[0011] According to one embodiment of the present invention, a method for producing the high-strength and highly formable steel sheet includes the steps of hot-rolling a steel material containing, in weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% to 0.05%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.005%, nitrogen (N): more than 0% to 0.006%, and the balance being iron (Fe) and other inevitable impurities to produce a hot-rolled steel sheet; a step of annealing the cold-rolled steel sheet by heating the cold-rolled steel sheet to more than 780°C and less than 840°C at a heating rate of 1°C / sec to 10°C / sec and holding the temperature for 50 seconds to 110 seconds; a step of multi-stage cooling the cold-rolled steel sheet; and a post-heat treatment step of heating the cold-rolled steel sheet to more than 380°C and less than 450°C at a heating rate of 20°C / sec or more and holding the temperature for 10 seconds to 240 seconds; C、重量% +0.066×X Si、重量% +0.043×X Mn、重量% The relationship may be ≦0.4.

[0012] According to an embodiment of the present invention, the step of producing the hot-rolled steel sheet may include the steps of reheating a steel material having the alloy composition at 1,150 to 1,250°C, and hot-rolling the reheated steel material at a finish rolling end temperature of 850 to 1,000°C, with the finish rolling being performed at a cumulative reduction of 70% to 90%, to produce a hot-rolled steel sheet, cooling the hot-rolled steel sheet to 500 to 700°C at a cooling rate of 10 to 30°C / sec, and coiling the hot-rolled steel sheet at 500 to 700°C, and the hot-rolled steel sheet may have a mixed structure of ferrite and pearlite, and the area fraction of the ferrite may be in a range of 20 to 50%, and the area fraction of the pearlite may be the remaining area fraction.

[0013] According to one embodiment of the present invention, the multi-stage cooling step may include a step of primarily cooling the cold-rolled steel sheet to 550°C to 750°C at a cooling rate of 1°C / sec to 10°C / sec, and a step of secondary cooling the cold-rolled steel sheet to more than 180°C to less than 240°C at a cooling rate of 50°C / sec or more, and holding the temperature for 5 to 20 seconds.

[0014] According to one embodiment of the present invention, the high-strength and high-formability steel sheet manufactured by the manufacturing method of the high-strength and high-formability steel sheet satisfies a yield strength (YS): 500 MPa or more, a tensile strength (TS): 980 MPa or more, a total elongation (T.EL): 23% or more, and a product of tensile strength and elongation: 23,000 MPa% or more, and has a mixed structure of retained austenite, ferrite, and martensite / tempered martensite, wherein the area fraction of the ferrite is 20% to 50%, the area fraction of the retained austenite is 5% to 20%, and the area fraction of the martensite / tempered martensite is the remaining area fraction, and a ratio of uniform elongation / total elongation is 0.7 or more and less than 1. When a plastic deformation of 5% is applied to the high-strength and high-formability steel sheet in a direction perpendicular to the rolling direction, a decrease in the area fraction of the retained austenite of the high-strength and high-formability steel sheet before and after the application may be more than 0% and less than 50%. [Effects of the Invention]

[0015] According to the present invention, by controlling the chemical composition and process conditions, it is possible to produce a high-strength, highly formable steel sheet having a microstructure consisting of retained austenite, ferrite, and martensite / tempered martensite, and having a yield strength (YS) of 500 MPa or more, a tensile strength (TS) of 980 MPa or more, a total elongation (T.EL) of 23% or more, and a product of tensile strength and elongation of 23,000 MPa% or more. The effect of the present invention is to provide an ultra-high tensile strength, highly formable steel sheet and process condition design that can maintain the final microstructure compared to cold-rolled steel sheets.

[0016] The effects of the present invention described above are merely examples, and the scope of the present invention is not limited to these effects. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a process flowchart illustrating a method for manufacturing a high-strength and highly formable steel plate according to an embodiment of the present invention. [Figure 2] 1 is a photograph showing the microstructure of a high-strength and highly formable steel plate according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings. The present embodiments are provided to more completely explain the technical concept of the present invention to those skilled in the art. The following embodiments may be modified into various other forms, and the scope of the technical concept of the present invention is not limited to the following embodiments. Rather, these embodiments are provided to make the present disclosure more complete and complete and to fully convey the technical concept of the present invention to those skilled in the art. Throughout this specification, the same reference numerals refer to the same elements. Furthermore, various elements and regions in the drawings are shown schematically. Therefore, the technical concept of the present invention is not limited by the relative sizes and spacings shown in the accompanying drawings.

[0019] To overcome the limitations of the mechanical properties of existing transformation-induced plasticity steels, steelmakers are focusing on the development of next-generation ultra-high-strength automotive steels that achieve high strength and adequate elongation by replacing bainite with martensite as the main matrix. Conventional techniques, such as forming a composite microstructure of ferrite, annealed martensite, and retained austenite, have achieved high strength and elongation, but the low ferrite fraction results in a high yield ratio, resulting in poor formability. Other conventional techniques have increased the ferrite volume fraction to ensure formability, but have not achieved the required tensile strength of 1000 MPa or more, elongation of 20% or more, or a product of tensile strength and elongation of 20,000 MPa or more. While other conventional techniques have achieved high strength and adequate formability and workability, the high carbon content results in poor weldability. Another prior art technology has been developed: a high-strength cold-rolled steel sheet with excellent burring properties due to a composite structure of ferrite, annealed martensite, retained austenite, and bainite. However, due to restrictions on heat treatment conditions, it is difficult to produce in a conventional CGL (Continuous Galvanized Line). For example, the overaging period takes a longer time than in a conventional CGL.

[0020] The fractures that occur during the forming of existing ultra-high strength materials for car body parts can be explained by evaluation criteria such as cup formability and biaxial stretchability, which can be seen in general forming limit diagrams, and hole expansion ratio, which cannot be seen in forming limit diagrams. Generally, the better the formability evaluation results of steel sheets used to process car body parts, the more complex the forming structure they can be. This formability index is an important factor in the forming of car body parts, which is mainly performed by pressing. Ultra-high strength materials generally exhibit a tendency for elongation to decrease as strength increases. To form these ultra-high strength materials, special forming processes or steels with deformation mechanisms that further ensure formability are being developed.

[0021] In the case of conventional ultra-high-strength steels with a dual-phase microstructure composed of ferrite and martensite, plastic deformation occurs through the basic deformation mechanism in which dislocations form and move within the microstructure when the steel undergoes plastic deformation. These dislocation movements create and grow defects, leading to fracture. Under the influence of this deformation mechanism, hard phases such as martensite and bainite are formed to ensure strength, but elongation decreases as the fraction of the hard phase increases. To compensate for this elongation, a soft phase called ferrite is formed within the microstructure. In the case of ultra-high-strength steels with this final microstructure, strength and elongation follow the rule of mixture (ROM), making it difficult to improve physical properties beyond the rule of mixture.

[0022] Transformation-induced plasticity steel (TIPS) has been developed to improve the ultra-high-strength steel with a dual-phase structure composed of ferrite and martensite. TIPS steel maintains retained austenite in the final structure and ensures strength and elongation through the phase transformation of the retained austenite that occurs during plastic deformation. However, the area fraction of the retained austenite in the final microstructure of TIPS steel is small, making it difficult to achieve significant improvements in formability.

[0023] The present invention aims to improve the formability of ultra-high strength steel by ensuring retained austenite in the final microstructure, thereby achieving improved formability compared to conventional ultra-high strength steels by utilizing a three-phase microstructure consisting of ferrite, retained austenite, and martensite / tempered martensite. Additionally, the present invention aims to develop a cold-rolled steel sheet that simultaneously satisfies material requirements while maintaining the microstructures of ferrite, retained austenite, and martensite / tempered martensite through control of the composition and heat treatment. The results of simulation tests allowed the development of a range of the composition and heat treatment process.

[0024] The retained austenite is a structure that easily ensures the strength, elongation, and formability of the steel sheet through the transformation-induced plasticity mechanism, but if it is contained in excess, excessive alloying elements may be required to ensure the stability for implementing the transformation-induced plasticity mechanism, and hydrogen embrittlement resistance may be reduced. Therefore, the fraction of the retained austenite is preferably 5% to 20%, the fraction of ferrite is 20% to 50%, and the remaining fraction may be composed of the sum of tempered martensite and martensite.

[0025] Therefore, the method proposed in the present invention for realizing the above-mentioned microstructure and ensuring yield strength, tensile strength, elongation, and hole expandability can be summarized as follows.

[0026] (1) After annealing, in order to ensure retained austenite in the final microstructure, steelmaking, continuous casting, hot rolling, and cold rolling are carried out using a composition system that optimally controls the austenite-stabilizing elements carbon and manganese, and silicon, which suppresses carbide formation and adjusts carbon redistribution behavior.

[0027] (2) The obtained cold-rolled coil is utilized to obtain the fine structure proposed in the present invention through the control of the two-phase annealing-quenching-reheating heat treatment.

[0028] Regarding the design direction (1), in securing elongation and hole expandability, strength that may not be achieved is secured by utilizing the transformation-induced plasticity of tempered martensite and retained austenite.

[0029] Regarding design direction (2), in order to ensure elongation, the plan aims to secure improved elongation by simultaneously applying two methods: a plan to secure elongation by ensuring a soft phase (ferrite) in the final microstructure in existing ultra-high strength steels, and a plan to secure more retained austenite in the final microstructure, which is utilized in transformation-induced plasticity steels.

[0030] According to the technical idea of ​​the present invention, in order to overcome the limitations in the mechanical properties of conventional transformation-induced plasticity steel (TRIP steel), the main matrix of transformation-induced plasticity steel is replaced by martensite instead of bainite, thereby providing a next-generation ultra-high strength automotive steel sheet that ensures high strength, high elongation, and excellent formability.

[0031] Hereinafter, a high-strength and highly formable steel sheet according to one embodiment of the present invention will be described.

[0032] The high-strength and highly formable steel sheet according to the technical idea of ​​the present invention can stably secure high tensile strength and elongation by controlling the final microstructure through process conditions that allow mass production, and is characterized by having excellent elongation despite its high strength.

[0033] High strength and highly formable steel plates

[0034] A high-strength and highly formable steel plate according to one embodiment of the present invention contains, in weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% to 0.05%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.005%, nitrogen (N): more than 0% to 0.006%, and the balance being iron (Fe) and other unavoidable impurities.

[0035] The high-strength and high-formability steel plate may further contain more than 0% to 0.05% of the total of titanium (Ti), niobium (Nb) and vanadium (V).

[0036] The role and content of each component contained in the high-strength and highly formable steel sheet according to the present invention will be described below. Here, the contents of all component elements are expressed as wt% based on the total weight of the steel sheet.

[0037] Carbon (C): 0.1%~0.3%

[0038] Carbon is the most important alloying element in steelmaking, primarily serving to strengthen the steel and stabilize austenite. A high carbon concentration in austenite improves the stability of austenite and facilitates the creation of adequate austenite for improved mechanical properties. If the carbon content is less than 0.1%, it is difficult to achieve the desired yield strength and elongation. If the carbon content exceeds 0.3%, the increase in carbon equivalent may result in a decrease in weldability. Therefore, the carbon content is preferably 0.1% to 0.3% of the total weight of the steel plate.

[0039] Silicon (Si): 1.0% to 2.0%

[0040] Silicon is a ferrite stabilizer that inhibits the formation of carbides (e.g., Fe3C) in ferrite, increases carbon activity, and accelerates the diffusion rate of austenite. Silicon is also known as a ferrite stabilizer, increasing the ferrite fraction during cooling and improving ductility. If the silicon content is less than 1.0%, the effect of adding silicon is insufficient. If the silicon content exceeds 2.0%, oxide (SiO2) may form on the surface of the steel sheet during processing, which may result in poor wettability and reduced galvanic properties. Therefore, it is preferable to add silicon in an amount of 1.0% to 2.0% of the total weight of the steel sheet.

[0041] Manganese (Mn): 1.5% to 3.0%

[0042] Manganese is an austenite-stabilizing element, and its addition gradually lowers the martensite formation start temperature (Ms) and can increase the fraction of retained austenite during continuous annealing. If the manganese content is less than 1.5%, the effect of adding manganese is insufficient. If the manganese content exceeds 3.0%, the carbon equivalent increases, significantly reducing weldability. Furthermore, oxides (MnO) form on the steel sheet surface during processing, which can lead to poor wettability and reduced galvanic properties. Therefore, it is preferable to add manganese in an amount of 1.5% to 3.0% of the total weight of the steel sheet.

[0043] Aluminum (Al): Over 0% to 0.05%

[0044] Aluminum is used as a deoxidizer and, like silicon, stabilizes ferrite and retained austenite, and also plays a role in solid solution strengthening and suppressing the formation of carbides. If the aluminum content exceeds 0.05%, AlN may form during slab production, which may induce cracks during casting or hot rolling. Therefore, it is preferable to add aluminum in an amount greater than 0% to 0.05% of the total weight of the steel sheet.

[0045] Sum of titanium (Ti), niobium (Nb) and vanadium (V): over 0% to 0.05%

[0046] Titanium, vanadium, and niobium are the main elements that precipitate in the form of carbides in steel. The purpose of adding titanium, vanadium, and niobium is to ensure the stability of retained austenite and improve strength by refining primary austenite grains through the formation of precipitates, and to refine ferrite grains and promote precipitation hardening through the presence of precipitates in ferrite. If the total content of titanium, vanadium, and niobium exceeds 0.05%, this can result in a deterioration in material quality and an increase in manufacturing costs. Therefore, when selectively included, the total content of titanium, niobium, and vanadium is preferably greater than 0% and up to 0.05% of the total weight of the steel sheet.

[0047] The steel sheet may selectively contain at least one of titanium, niobium, and vanadium, whereby the titanium may be 0% to 0.05% of the total weight of the steel sheet, the niobium may be 0% to 0.05% of the total weight of the steel sheet, and the vanadium may be 0% to 0.05% of the total weight of the steel sheet.

[0048] Phosphorus (P): Over 0% to 0.02%

[0049] Phosphorus is an impurity contained in the steel manufacturing process and can help improve strength through solid solution strengthening, but if contained in large amounts, it can cause low-temperature embrittlement. Therefore, it is preferable to limit the phosphorus content to more than 0% to 0.02% of the total weight of the steel sheet.

[0050] Sulfur (S): More than 0%~0.005%

[0051] Sulfur is an impurity contained in the steel manufacturing process and can form non-metallic inclusions such as FeS, MnS, etc., which can reduce toughness and weldability. Therefore, it is preferable to limit the sulfur content to more than 0% to 0.005% of the total weight of the steel plate.

[0052] Nitrogen (N): Over 0% to 0.006%

[0053] Nitrogen is an element that is inevitably contained during steel production, and if present in excess, large amounts of nitrides will precipitate, which can deteriorate ductility. Therefore, the nitrogen content is preferably limited to more than 0% to 0.006% of the total weight of the steel sheet.

[0054] The remaining component of the high-strength and highly formable steel sheet is iron (Fe). However, in a typical steelmaking process, unintended impurities are inevitably mixed in from raw materials or the surrounding environment, and it is not possible to eliminate these impurities. These impurities are known to any engineer of a typical manufacturing process, and therefore, the details of these impurities will not be specifically mentioned in this specification.

[0055] The carbon (C), manganese (Mn), and silicon (Si) contents of the high-strength and high-formability steel plate are X C、重量% +0.066×X Si、重量% +0.043×X Mn、重量% The relationship of ≦0.4 is satisfied.

[0056] A high-strength and highly formable steel sheet manufactured by controlling the specific components and their content ranges of the alloy composition described above and using the steel sheet manufacturing method described below can satisfy yield strength (YS): 500 MPa or more, tensile strength (TS): 980 MPa or more, total elongation (T.EL): 23% or more, and the product of tensile strength and elongation: 23,000 MPa% or more. For example, the high-strength and highly formable steel sheet can satisfy yield strength (YS): 500 MPa to 760 MPa, tensile strength (TS): 980 MPa to 1180 MPa, total elongation (T.EL): 23% to 30%, and the product of tensile strength and elongation: 23,000 MPa% to 35,400 MPa%. In addition, the ratio of uniform elongation / total elongation can be 0.7 or more to less than 1. This is the ratio of the area where uniform elongation is improved through transformation-induced plasticity behavior to the entire material.

[0057] Factors that affect the properties of the high-strength and highly formable steel sheets include ensuring strength and elongation through the phase transformation of retained austenite due to the transformation-induced plasticity phenomenon, ensuring the stability of retained austenite, ensuring elongation through ferrite, increasing strength through tempered martensite itself, which is the basic matrix, and increasing strength through refinement of crystal grains and precipitation hardening.The high-strength and highly formable steel sheets have a product of tensile strength and elongation of 23,000 MPa% or more, which is superior to the value generally proposed for the ultra-high strength strength level.

[0058] When a 5% plastic deformation is applied to the high-strength and high-formability steel sheet in a direction perpendicular to the rolling direction, the reduction rate of the area fraction of retained austenite in the high-strength and high-formability steel sheet before and after the application is more than 0% and not more than 50%. When it is 50% or less, transformation-induced plasticity behavior does not appear in the early stage, and the transformation-induced plasticity phenomenon continues until the middle of the deformation, which can help improve elongation.

[0059] The high-strength and high-formability steel sheet may have a mixed structure of retained austenite, ferrite, and martensite / tempered martensite. The area fraction of ferrite may have a significant effect on the overall mechanical properties and may be, for example, 20% to 50%. If the area fraction of ferrite is less than 20%, the yield ratio may be high, resulting in reduced workability and difficulty in ensuring elongation. If the area fraction of ferrite is more than 50%, the fraction of tempered martensite, which is the matrix structure, may be reduced, making it difficult to ensure sufficient yield strength and tensile strength. The area fraction of retained austenite may be, for example, 5% to 20%. The area fraction of martensite / tempered martensite may be the remaining fraction and may be, for example, 30% to 75%. The area fraction refers to an area ratio calculated from a microstructure photograph using an image analyzer.

[0060] Here, the term "martensite / tempered martensite" does not distinguish between martensite and tempered martensite, but refers to both of them combined.

[0061] In addition, in the high-strength and high-formability steel sheet, it is preferable to suppress the formation of cementite and pearlite as much as possible, thereby eliminating cementite and pearlite in the microstructure.

[0062] Hereinafter, a method for manufacturing a high-strength and highly formable steel plate according to the present invention will be described with reference to the accompanying drawings.

[0063] Manufacturing method for high strength and highly formable steel plate

[0064] FIG. 1 is a process flow chart that schematically illustrates a method for manufacturing a high-strength and highly formable steel plate according to one embodiment of the present invention.

[0065] Referring to FIG. 1, the method for manufacturing the high-strength and highly formable steel sheet includes a step of hot-rolling a steel material to manufacture a hot-rolled steel sheet (S110), a step of cold-rolling the hot-rolled steel sheet to manufacture a cold-rolled steel sheet (S120), a step of annealing the cold-rolled steel sheet (S130), and a step of multi-stage cooling the cold-rolled steel sheet (S140).

[0066] The method for manufacturing a high-strength and highly formable steel sheet may further include a step (S150) of post-heat treating the cold-rolled steel sheet.

[0067] Hot-rolled steel sheet manufacturing step (S110)

[0068] In the hot-rolled steel sheet manufacturing step (S110), a steel material containing, by weight, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): greater than 0% to 0.05%, phosphorus (P): greater than 0% to 0.02%, sulfur (S): greater than 0% to 0.005%, nitrogen (N): greater than 0% to 0.006%, and the remainder being iron (Fe) and other unavoidable impurities is prepared.

[0069] The steel material may further contain, by weight percent, more than 0% to 0.05% of the total of titanium (Ti), niobium (Nb), and vanadium (V).

[0070] In the manufacturing method according to the present invention, the semi-finished product to be subjected to the hot rolling process may be, for example, a slab. The semi-finished slab can be obtained through a continuous casting process after molten steel having a predetermined composition is obtained through a steelmaking process.

[0071] The steel is reheated at a temperature above Ac3, for example, at a slab reheating temperature (SRT) in the range of 1,150°C to 1,250°C. This reheating can cause re-dissolution of elements that segregated during casting and re-dissolution of precipitates. If the reheating temperature is less than 1,150°C, a problem of a sudden increase in hot rolling load may occur. If the reheating temperature is more than 1,250°C, slab warpage may make it difficult to charge and discharge the slab from a heating furnace, and coarsening of primary austenite grains may make it difficult to ensure the strength of the final steel sheet.

[0072] The reheated steel is then hot-rolled after heating to adjust its shape. The hot-rolling can be performed successively through width rolling, rough rolling, and finish rolling. Through the hot-rolling step, the steel can be formed into a hot-rolled steel plate.

[0073] In the hot rolling, the steel material may be finish-rolled at a finish delivery temperature (FDT) in the range of, for example, 850°C to 1,000°C. If the finish delivery temperature is less than 850°C, the grains become finer and the strength increases, but edge fractures may occur, resulting in an increase in the rolling load and a decrease in productivity. If the finish delivery temperature exceeds 1,000°C, the quality of the steel sheet may be reduced due to the generation of surface scale on the steel sheet.

[0074] The finish rolling may be performed at a cumulative reduction rate of 70% to 90%. In the present invention, after hot rolling is completed, the microstructure is made to be a mixed structure of pearlite and ferrite. To achieve this, in the present invention, the cumulative reduction rate is increased during the finish hot rolling, thereby promoting the transformation of austenite to pearlite and ferrite through plastic deformation.

[0075] When austenite-phase steel is subjected to plastic deformation due to an external force during cooling, the onset of the ferrite-pearlite phase transformation is accelerated, allowing for the formation of more ferrite and pearlite. Therefore, in the present invention, a sufficient cumulative reduction of 70% or more is provided in the finish rolling step to more easily produce a hot-rolled steel sheet with a pearlite-ferrite mixed structure. If the cumulative reduction is less than 70%, the ferrite-pearlite phase transformation does not begin quickly enough, making it difficult to form the desired amounts of ferrite and pearlite. However, if the cumulative reduction exceeds 90%, excessive load may be applied to the rolling equipment, making the process unreasonable.

[0076] Next, the hot-rolled steel material is cooled to a predetermined coiling temperature. The cooling can be either air-cooling or water-cooling, and can be performed at a cooling rate of, for example, 10°C / sec to 30°C / sec. The cooling can be performed, for example, to a coiling temperature of 500°C to 700°C, for example, to 550°C to 650°C. If the cooling rate is less than 10°C / sec, the average particle size of precipitates increases, making it difficult to ensure strength. Conversely, if the cooling rate exceeds 30°C / sec, the structure of the steel material becomes hard, and impact toughness can decrease.

[0077] Next, the hot-rolled steel sheet is coiled at a coiling temperature (CT) in the range of, for example, 550°C to 650°C. If the coiling temperature is less than 500°C, the surface quality of the steel material may deteriorate due to a sudden difference between the finish rolling temperature and the coiling temperature, and the strength may increase, which may increase the rolling load during cold rolling. If the coiling temperature exceeds 700°C, the carbonitride elements may not be maintained in a solid solution state and may form as undesired precipitates, which may cause defects in subsequent processes due to surface oxidation, etc. The coiled steel material may be cooled to room temperature. Preferably, the coiling temperature may be 550°C to 650°C.

[0078] The hot-rolled steel sheet may have a mixed structure of ferrite and pearlite. The area fraction of the ferrite may be, for example, in the range of 20% to 50%. The area fraction of the pearlite may be the remaining area fraction, for example, in the range of 50% to 80%.

[0079] Cold-rolled steel sheet manufacturing step (S120)

[0080] The cold-rolled steel sheet manufacturing step (S120) is performed to adjust the thickness of the final steel sheet using the hot-rolled steel sheet. The coiled hot-rolled steel sheet is then subjected to a pickling treatment, in which the sheet is washed with acid. The pickled hot-rolled steel sheet is then cold-rolled at a cold reduction of 40% to 60% to form a cold-rolled steel sheet. If the cold reduction is less than 40%, the amount of nuclei generated for recrystallization during the subsequent soaking treatment is small, which can lead to excessive grain growth during the soaking treatment and a rapid decrease in strength. If the cold reduction is more than 60%, the amount of nuclei generated is excessive and the grains formed during the soaking treatment are excessively fine, which can reduce ductility and formability.

[0081] The microstructure of the cold-rolled steel sheet has an elongated shape of the structure of the hot-rolled steel sheet, and the microstructure of the steel sheet that is finally produced is determined in the subsequent heat treatment.

[0082] Annealing heat treatment step (S130)

[0083] In the annealing step (S130), the cold-rolled steel sheet may be heat-treated in a continuous annealing furnace having a conventional slow cooling section. The annealing may be performed in the austenite-ferrite two-phase temperature range. This is to ensure the appropriate shape and fraction of ferrite and to obtain the desired final properties of the steel sheet through a microstructure in which retained austenite, ferrite, and tempered martensite are mixed together in the final microstructure.

[0084] In the annealing heat treatment, the cold-rolled steel sheet is heated at a temperature rising rate of, for example, 1°C / sec to 10°C / sec, for example, 1°C / sec to 5°C / sec, and is held at a two-phase temperature, for example, a temperature higher than 780°C and lower than 840°C, for example, for 40 seconds to 120 seconds, for example, 50 seconds to 110 seconds.

[0085] If the heating rate is slow, the austenite will remain in the transformation region for a long time, causing some recrystallization and the average size of the initial austenite grains to grow, which will have a negative effect on the material. Therefore, it is advantageous to increase the heating rate.

[0086] If the annealing temperature is 780°C or lower, it may be difficult to form sufficient austenite, making it difficult to achieve the target strength and elongation.If the annealing temperature is 840°C or higher, the fraction of ferrite decreases, making it impossible to obtain sufficient elongation.

[0087] Multi-stage cooling step (S140)

[0088] In the multi-stage cooling step (S140), the annealed cold-rolled steel sheet is subjected to primary cooling at a cooling rate of, for example, 1°C / sec to 10°C / sec, for example, 5°C / sec to 10°C / sec, to a temperature of, for example, 550°C to 750°C, for example, 600°C to 700°C. The primary cooling can be referred to as a slow cooling step. The primary cooling is performed to ensure a certain amount of ferrite in the final microstructure to ensure plasticity. The primary cooling is also performed to ensure an appropriate fraction of retained austenite through the shape and fraction of ferrite formed during the heat treatment process.

[0089] Next, the cold-rolled steel sheet is subjected to secondary cooling, for example, at a cooling rate of 50°C / sec or more, for example, 70°C / sec or more, for example, 50°C / sec to 150°C / sec, to, for example, the Ms temperature or lower, for example, above 180°C to below 240°C, and held for, for example, 5 seconds to 20 seconds, for example, 10 seconds. The secondary cooling can be referred to as a quenching step. The secondary cooling is performed to control the quenching end temperature and transform austenite in the microstructure after slow cooling to martensite, thereby making it easier to ensure the final material quality. Furthermore, the cooling rate is 50°C / sec or more to suppress phase transformation that may occur during the quenching process.

[0090] Post-heat treatment step (S150)

[0091] In the post-heat treatment step (S150), the cold-rolled steel sheet is heated at a heating rate of, for example, 20°C / sec or more, for example, 30°C / sec or more, for example, 20°C / sec to 50°C / sec, and held at, for example, the Ms temperature or higher, for example, a temperature higher than 380°C and lower than 450°C, for example, 10 seconds or more, for example, 10 seconds to 240 seconds. The post-heat treatment concentrates carbon in the retained austenite and forms tempered martensite through tempering of martensite, thereby ensuring high strength and elongation and maintaining the final microstructure. After the post-heat treatment, the cold-rolled steel sheet is cooled to room temperature, for example, 0°C to 40°C.

[0092] The cold-rolled steel sheet produced by the method of the present invention can have a mixed structure of retained austenite, ferrite, and martensite / tempered martensite.

[0093] The present invention is characterized in that the hot-rolled structure is formed into a two-phase structure consisting of 20% to 50% ferrite and the remainder pearlite, and no softening heat treatment is performed before cold rolling.

[0094] If as many austenite formation sites as possible are formed through dislocations formed during cold rolling, austenite is formed locally and in multiple locations during the subsequent annealing heat treatment, making it possible to control the size of the primary austenite grains relatively small. If the hot-rolled structure is formed into a low-temperature phase structure such as martensite or bainite, a softening heat treatment process is required before cold rolling, and since dislocations are restored by the softening heat treatment, the dislocation density decreases even after cold rolling, making it difficult to reduce the size of the primary austenite grains.

[0095] On the other hand, if the hot-rolled structure is formed into a two-phase structure of ferrite and pearlite, a softening heat treatment process is not performed, and dislocations accumulate, maintaining a relatively high dislocation density after cold rolling, allowing austenite to be formed small. Also, by pre-forming ferrite before post-heat treatment, austenite-stabilizing elements such as carbon and manganese can be contained in the pearlite region at a relatively high content, thereby more stably securing retained austenite after post-heat treatment, thereby achieving the target material having a good balance of strength and elongation.

[0096] Furthermore, the present invention is characterized in that, during heat treatment, the temperature is raised relatively quickly from the Ac1 temperature (austenite phase transformation start temperature, approximately 730°C) within the austenite phase transformation interval to the holding temperature (below the austenite phase transformation completion temperature, approximately 880°C). If the temperature is raised slowly, the austenite is left in the transformation interval for a long time, which can cause some of the austenite to recrystallize, thereby increasing the average size of the initial austenite grains and causing deterioration of the material.

[0097] Experimental example

[0098] Below, preferred experimental examples are presented to aid in understanding the present invention. However, the following experimental examples are merely provided to aid in understanding the present invention, and the present invention is not limited to these experimental examples. Contents not described here can be fully inferred by those skilled in the art, and therefore, explanations thereof will be omitted.

[0099] Table 1 shows the composition of a high-strength and highly formable steel sheet according to one embodiment of the present invention. In Table 1, the balance consists of iron (Fe) and impurities that are inevitably contained in the steelmaking process. The content of each component is expressed in weight percent.

[0100] [Table 1]

[0101] The composition parameters listed in Table 1 are the relationship between the carbon (C), manganese (Mn), and silicon (Si) contents of the high strength and high formability steel plate. C、重量% +0.066×X Si、重量% +0.043×X Mn、重量% where X C、重量% is the carbon composition of the steel sheet in weight percent, and X Si、重量% is the silicon composition of the steel sheet in weight percent, and X Mn、重量% is the manganese composition of the steel sheet expressed in wt%.

[0102] In the examples, the value of the composition parameter satisfies 0.4 or less (X C、重量% +0.066×X Si、重量% +0.043×X Mn、重量% ≦0.4), on the other hand, it can be seen that in Comparative Examples 3 and 4, the values ​​of the composition parameters exceed 0.4.

[0103] Slabs having the compositions of the comparative examples and examples described above were reheated to 1200°C and then subjected to rolling simulation to produce hot-rolled steel sheets. The cumulative reduction during finish rolling was controlled to 80%. After finish rolling at 900°C, the hot-rolled steel sheets were cooled to the coiling temperature and then loaded into a coiling furnace at 600°C. After holding for 2 hours, they were cooled in the furnace, and then cooled in the furnace to simulate the actual coiling and cooling process. The produced hot-rolled steel sheets were then rolled under typical cold rolling conditions to produce cold-rolled steel sheets. The produced cold-rolled steel sheets were annealed at 800°C, a two-phase temperature, for 60 seconds, primarily cooled to 680°C at a cooling rate of 3°C / s, and then secondary cooled to a temperature below Ms at a cooling rate of 50°C / s or more. After secondary cooling, the cold-rolled steel sheets were loaded into a heat treatment furnace, heated, and post-heat treated at 400°C for 60 seconds.

[0104] Table 2 shows the mechanical properties of a high-strength and highly formable steel plate according to an embodiment of the present invention.

[0105] [Table 2]

[0106] Table 3 shows the microstructure of a high-strength and highly formable steel sheet according to an embodiment of the present invention.

[0107] [Table 3]

[0108] Referring to Tables 2 and 3, the yield strength, tensile strength, elongation, and tensile strength × elongation values ​​of the examples satisfy the ranges set forth in the present invention. In addition, the reduction rate of the area fraction of retained austenite after 5% plastic deformation is 50% or less, which satisfies the range set forth in the present invention.

[0109] In Comparative Example 1, the total elongation does not satisfy the 23% or more range proposed by the present invention, the uniform elongation ratio (U.EL / T.EL) does not satisfy the range, and the reduction rate of the area fraction of retained austenite does not satisfy the range proposed by the present invention.

[0110] In Comparative Example 2, the total elongation does not satisfy the requirement of 23% or more proposed by the present invention.

[0111] Comparative Examples 3 and 4 do not satisfy the composition parameters, and it is expected that the carbon equivalent increases due to the increased carbon content, resulting in decreased spot weldability.

[0112] The results of the comparative examples show that elongation increases as the carbon content increases, so if the carbon redistribution behavior to austenite is controlled while the strength is reduced, the elongation can be improved. Therefore, in the examples of the present invention, the silicon content was increased compared to Comparative Example 2, and the carbon and manganese contents were adjusted to reduce the strength.

[0113] FIG. 2 is a photograph showing the microstructure of the high-strength and highly formable steel plate according to Example 1.

[0114] 2, it can be seen that the high-strength and high-formability steel sheet has a mixed structure of ferrite, retained austenite, and martensite / tempered martensite. The area fraction of the retained austenite was measured to be about 8%, the area fraction of the ferrite was measured to be 44%, and the area fraction of the martensite / tempered martensite was measured to be 48%.

[0115] It will be apparent to those skilled in the art to which the technical idea of ​​the present invention pertains that the technical idea of ​​the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and that various substitutions, modifications and changes are possible within the scope of the technical idea of ​​the present invention.

Claims

1. In weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% to 0.05%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.005%, nitrogen (N): more than 0% to 0.006%, and the balance including iron (Fe) and other unavoidable impurities, The carbon (C), manganese (Mn), and silicon (Si) contents are Ｃ、重量％ +0.066 x X Ｓｉ、重量％ +0.043 x X Ｍｎ、重量％ ≦0.4, A high-strength and highly formable steel plate that satisfies the following requirements: yield strength (YS): 500 MPa or more; tensile strength (TS): 980 MPa or more; total elongation (T.EL): 23% or more; and the product of tensile strength and elongation: 23,000 MPa% or more.

2. The high strength and highly formable steel plate is It has a mixed structure in which retained austenite, ferrite, and martensite / tempered martensite are mixed, the area fraction of the ferrite is 20% to 50%; The area fraction of the retained austenite is 5% to 20%; and The high strength and highly formable steel plate according to claim 1 , wherein the area fraction of martensite / tempered martensite is the remaining area fraction.

3. 2. The high-strength and highly formable steel plate according to claim 1, wherein the high-strength and highly formable steel plate has a ratio of uniform elongation / total elongation of 0.7 or more and less than 1.

4. 2. The high-strength and high-formability steel plate according to claim 1, wherein, when a plastic deformation of 5% is applied to the high-strength and high-formability steel plate in a direction perpendicular to the rolling direction, a reduction rate of the area fraction of retained austenite in the high-strength and high-formability steel plate before and after the application is more than 0% to 50% or less.

5. The high-strength and highly formable steel plate according to claim 1, further comprising, by weight%, the total of titanium (Ti), niobium (Nb), and vanadium (V): more than 0% to 0.05%.

6. A step of manufacturing a hot-rolled steel sheet by hot-rolling a steel material containing, in weight percent, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% to 0.05%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.005%, nitrogen (N): more than 0% to 0.006%, and the balance being iron (Fe) and other unavoidable impurities; cold rolling the hot-rolled steel sheet to produce a cold-rolled steel sheet; A step of annealing the cold-rolled steel sheet by heating it to more than 780°C and less than 840°C at a heating rate of 1°C / sec to 10°C / sec and holding it for 50 seconds to 110 seconds; A step of multi-stage cooling the cold-rolled steel sheet; A post-heat treatment step of heating the cold-rolled steel sheet to more than 380°C and less than 450°C at a temperature rising rate of 20°C / sec or more and holding the temperature for 10 seconds to 240 seconds, The carbon (C), manganese (Mn), and silicon (Si) contents are Ｃ、重量％ +0.066 x X Ｓｉ、重量％ +0.043 x X Ｍｎ、重量％ 1. A method for producing a high-strength and highly formable steel plate, which satisfies the relationship of .gtoreq.0.

4.

7. The step of manufacturing the hot-rolled steel sheet includes: Reheating the steel material having the alloy composition at 1,150 to 1,250°C; hot rolling the reheated steel material at a finish rolling end temperature of 850°C to 1,000°C, with a cumulative rolling reduction of 70% to 90%, to produce a hot-rolled steel sheet; Cooling the hot-rolled steel sheet to 500°C to 700°C at a cooling rate of 10°C / sec to 30°C / sec; The hot-rolled steel sheet is coiled at 500°C to 700°C.

7. The method for producing a high-strength and highly formable steel plate according to claim 6, wherein the hot-rolled steel plate has a mixed structure of ferrite and pearlite, an area fraction of the ferrite is in a range of 20% to 50%, and an area fraction of the pearlite is the remaining area fraction.

8. The multi-stage cooling step includes: A step of primarily cooling the cold-rolled steel sheet to 550°C to 750°C at a cooling rate of 1°C / sec to 10°C / sec; The method for producing a high-strength and highly formable steel plate according to claim 6, further comprising a step of secondary cooling the cold-rolled steel plate to a temperature of from more than 180 ° C. to less than 240 ° C. at a cooling rate of 50 ° C. / second or more and holding the temperature for 5 to 20 seconds.

9. The high-strength and highly formable steel plate manufactured by the manufacturing method of the high-strength and highly formable steel plate is Yield strength (YS): 500 MPa or more, tensile strength (TS): 980 MPa or more, total elongation (T.EL): 23% or more, and the product of tensile strength and elongation: 23,000 MPa% or more; The steel has a mixed structure containing a mixture of retained austenite, ferrite, and martensite / tempered martensite, wherein the area fraction of the ferrite is 20% to 50%, the area fraction of the retained austenite is 5% to 20%, and the area fraction of the martensite / tempered martensite is the remaining area fraction; The ratio of uniform elongation / total elongation is 0.7 or more and less than 1, 7. The method for producing a high-strength and highly formable steel plate according to claim 6, wherein when a plastic deformation of 5% is applied in a direction perpendicular to the rolling direction of the high-strength and highly formable steel plate, a reduction rate of the area fraction of retained austenite in the high-strength and highly formable steel plate before and after the application is more than 0% to 50% or less.

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