Ultra-high strength steel plate and its manufacturing method

Abstract

The present invention provides an ultra-high strength steel sheet characterized by comprising, by weight, carbon (C): 0.1% to 0.30%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% but not more than 0.05%, the sum of at least one element selected from niobium (Nb), titanium (Ti), and vanadium (V): more than 0% but not more than 0.05%, phosphorus (P): more than 0% but not more than 0.02%, sulfur (S): more than 0% but not more than 0.005%, nitrogen (N): more than 0% but not more than 0.006%, and the balance being iron (Fe) and other unavoidable impurities, and having a final microstructure consisting of tempered martensite, martensite, ferrite, and retained austenite, and having a yield strength (YP): 850 MPa or more, a tensile strength (TS): 1180 MPa or more, and an elongation (El): 14% or more.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a manufacturing method thereof, and more particularly to an ultra-high strength steel sheet having an excellent elongation ratio and a manufacturing method thereof. [Background technology]

[0002] Automotive steel sheets have been developed with a focus on achieving high elongation rates to increase strength and facilitate processing for vehicle weight reduction and safety. Common ultra-high-strength steels currently in use include dual-phase steels, which achieve elongation through the use of two phases: ferrite and martensite, and transformation-induced plasticity steels, which achieve strength and elongation through the phase transformation of retained austenite in the final structure during plastic deformation. However, development of dual-phase steels, which cannot deviate from the Rule of Mixture and transformation-induced plasticity steels, which suffer from relatively low strength due to their predominantly bainite matrix, has reached its limit. Therefore, steelmakers are focusing their efforts on developing next-generation ultra-high-strength automotive steel sheets that improve the microstructure of transformation-induced plasticity steels to ensure ultra-high strength and high formability.

[0003] Related prior art includes Japanese Patent Application Laid-Open No. 2005-105367. Summary of the Invention [Problem to be solved by the invention]

[0004] The technical problem to be achieved by the present invention is to provide an ultra-high strength steel sheet having an excellent elongation ratio and a method for producing the same. [Means for solving the problem]

[0005] The present invention provides an ultra-high strength steel sheet that achieves the above object, comprising, by weight, 0.1% to 0.30% carbon (C), 1.0% to 2.0% silicon (Si), 1.5% to 3.0% manganese (Mn), more than 0% but not more than 0.05% aluminum (Al), more than 0% but not more than 0.05% total of at least one element selected from niobium (Nb), titanium (Ti), and vanadium (V), more than 0% but not more than 0.02% phosphorus (P), more than 0% but not more than 0.005% sulfur (S), more than 0% but not more than 0.006% nitrogen (N), and the balance being iron (Fe) and other unavoidable impurities. The final microstructure of the steel sheet is composed of tempered martensite, martensite, ferrite, and retained austenite, and has a yield strength (YP) of 850 MPa or more, a tensile strength (TS) of 1180 MPa or more, and an elongation (El) of 14% or more.

[0006] In the ultra-high strength steel plate, the final microstructure has a martensite matrix that is not bainite.

[0007] In the ultra-high strength steel plate, the area fraction of the ferrite may be 10% to 20%, the area fraction of the retained austenite may be 10% to 20%, and the area fractions of the tempered martensite and martensite may be included as the remaining fractions.

[0008] In the ultra-high strength steel plate, the ferrite contains carbides, and the carbides have an average size of 50 nm or more and a density of 20 particles / μm 2 It can be the following:

[0009] In the ultra-high strength steel sheet, the value of the product of the tensile strength and the elongation rate may not be less than 15,000 (MPa·%).

[0010] In order to solve the above problems, a method for producing an ultra-high strength steel sheet according to one embodiment of the present invention includes, in weight percent, carbon (C): 0.1% to 0.30%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% and 0.05% or less, the sum of at least one element selected from niobium (Nb), titanium (Ti), and vanadium (V): more than 0% and 0.05% or less, phosphorus (P): more than 0% and 0.02%, sulfur (S): more than 0% and 0.005%, and nitrogen (N): more than 0% and 0.006%. and the remainder being iron (Fe) and other unavoidable impurities; softening the hot-rolled steel sheet by maintaining the temperature at a constant range of 500°C to 650°C; cold-rolling the softened steel sheet to produce a cold-rolled steel sheet; annealing the cold-rolled steel sheet at a temperature in the range of 820°C to 860°C; cooling the cold-rolled steel sheet in multiple stages to a temperature in the range of 200°C to 260°C; and partitioning the cooled cold-rolled steel sheet at a temperature in the range of 370°C to 460°C.

[0011] In the method for manufacturing the ultra-high strength steel sheet, the step of manufacturing the hot-rolled steel sheet may include the steps of preparing a steel slab having the alloy composition; reheating the steel slab at a temperature in the range of 1150°C to 1250°C; hot-finish rolling the reheated steel slab at a finish rolling temperature in the range of 850°C to 1000°C to manufacture a hot-rolled steel sheet; and coiling the hot-rolled steel sheet at a temperature in the range of 500°C to 700°C.

[0012] In the method for manufacturing an ultra-high strength steel sheet, the step of annealing the cold-rolled steel sheet may be performed by heating the cold-rolled steel sheet at a temperature increase rate of 1 to 10°C / s and maintaining the temperature at 820°C to 860°C for 40 to 120 seconds.

[0013] In the method for manufacturing the ultra-high strength steel sheet, the step of multi-stage cooling the cold-rolled steel sheet may include a first cooling step of slowly cooling the cold-rolled steel sheet at a cooling rate of 1 to 10°C / s to an end temperature of 550°C to 750°C; and a second cooling step of rapidly cooling the cold-rolled steel sheet at a cooling rate of 50°C / s or more to an end temperature of 200°C to 260°C.

[0014] In the method for manufacturing the ultra-high strength steel sheet, the partitioning heat treatment may be performed by heating the cooled cold-rolled steel sheet at a temperature increase rate of 20°C / s or more and maintaining the temperature in a range of 370°C to 460°C for a time period in a range of 10 seconds to 240 seconds. [Effects of the Invention]

[0015] According to the embodiments of the present invention, an ultra-high strength steel sheet having an excellent elongation rate and a manufacturing method thereof can be realized.

[0016] Of course, the scope of the present invention is not limited by such effects. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a photograph of the final microstructure of an ultra-high strength steel sheet having an excellent elongation rate according to an embodiment of the present invention. [Figure 2] 1 is a photograph showing the density of carbides in ferrite in the final microstructure of an ultra-high strength steel sheet having excellent elongation according to an embodiment of the present invention. [Figure 3] 1 is a flowchart illustrating a method for manufacturing an ultra-high strength steel sheet having an excellent elongation rate according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] An ultra-high strength steel sheet having excellent elongation and a manufacturing method thereof according to an embodiment of the present invention will now be described in detail. The terms used below are appropriately selected in consideration of the functions of the present invention, and these terms should be defined based on the overall content of this specification.

[0019] Automotive steel sheets have been developed with a focus on ensuring elongation to increase strength and facilitate processing for ensuring user safety and reducing vehicle weight. Common ultra-high-strength steels currently in use include dual-phase steels, which achieve elongation through the use of two phases: ferrite and martensite, and transformation-induced plasticity steels, which achieve strength and elongation through the phase transformation of retained austenite in the final structure during plastic deformation. However, development of dual-phase steels, which cannot deviate from the rule of mixtures, and transformation-induced plasticity steels, which suffer from 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 improve the microstructure of transformation-induced plasticity steels to ensure ultra-high strength and high formability.

[0020] In order to overcome the limitations of the mechanical properties of existing TRIP steel, the present invention provides a next-generation ultra-high strength coated steel sheet for automobiles that ensures high strength and an appropriate elongation rate by replacing the main matrix of TRIP steel with martensite instead of bainite.

[0021] Hereinafter, an ultra-high strength steel sheet having excellent elongation according to an embodiment of the present invention will be described in detail.

[0022] steel plate

[0023] According to one embodiment of the present invention, an ultra-high strength steel sheet with excellent elongation includes a steel sheet containing, by weight, 0.1% to 0.30% carbon (C), 1.0% to 2.0% silicon (Si), 1.5% to 3.0% manganese (Mn), more than 0% but not more than 0.05% aluminum (Al), more than 0% but not more than 0.05% total of at least one element selected from niobium (Nb), titanium (Ti), and vanadium (V), more than 0% but not more than 0.02% phosphorus (P), more than 0% but not more than 0.005% sulfur (S), more than 0% but not more than 0.006% nitrogen (N), and the balance being iron (Fe) and other unavoidable impurities. The role and content of each element contained in the steel sheet will be described below.

[0024] Carbon (C)

[0025] Carbon (C) is the most important alloying element in steelmaking, and its primary purpose in the present invention is to strengthen the steel and stabilize austenite. A high carbon concentration in austenite improves austenite stability and facilitates the creation of adequate austenite for improved mechanical properties. If the carbon content is less than 0.1 wt% of the total weight, the above effects cannot be achieved and sufficient strength cannot be achieved. Conversely, if the carbon content exceeds 0.3 wt% of the total weight, the increased carbon equivalent can lead to problems such as reduced weldability and workability.

[0026] Silicon (Si)

[0027] Silicon (Si) is an element that suppresses carbide formation, and in particular, it plays a role in preventing deterioration of material properties due to the formation of Fe3C. Silicon is also well known as a ferrite stabilizing element, increasing the ferrite fraction during cooling and increasing softness. It is also known as an element that promotes martensite formation by concentrating carbon in austenite, thereby ensuring strength. Meanwhile, silicon, along with aluminum, is added as a deoxidizer to remove oxygen from steel during the steelmaking process, and can also have a solid solution strengthening effect.

[0028] In the steel sheet constituting the ultra-high strength steel sheet with excellent elongation according to one embodiment of the present invention, silicon may be added in a content ratio of 1.0 wt% to 2.0 wt% of the total weight. If the silicon content is less than 1.0 wt% of the total weight, the softness cannot be ensured and the above-mentioned effects of adding silicon cannot be reliably achieved. Conversely, if the silicon content is greater than 2.0 wt%, oxide (SiO2) is formed on the surface of the steel sheet, which may reduce wettability in the steel sheet and result in reduced galvanic properties. It may also cause problems with surface quality by forming red scale during reheating and hot rolling, and may also cause problems such as reduced toughness and plastic workability, and may also reduce the weldability of the steel.

[0029] Manganese (Mn)

[0030] Manganese (Mn) is a key element that stabilizes austenite. Its addition gradually lowers the martensite formation temperature (Ms) and increases the fraction of retained austenite during the continuous annealing process. Manganese also facilitates the formation of low-temperature transformation phases and increases strength through solid-solution strengthening. While manganese impairs the acid and oxidation resistance of steel, it refines pearlite and solid-solution strengthens ferrite, improving yield strength.

[0031] Manganese may be added in a content ratio of 1.5 wt% to 3.0 wt% of the total weight of the steel sheet constituting the ultra-high strength steel sheet with excellent elongation according to one embodiment of the present invention. If the manganese content is less than 1.5 wt%, the aforementioned effect of ensuring strength cannot be fully achieved. Furthermore, if the manganese content exceeds 3.0 wt%, the carbon equivalent increases, which can lead to a decrease in weldability. Furthermore, oxides (MnO) may form on the surface of the steel sheet during processing, which can lead to a decrease in galvanizability due to poor wettability in the corresponding areas. Furthermore, internal and external segregation zones may form in the continuously cast slab and steel sheet, which can induce crack initiation and propagation, thereby reducing bendability.

[0032] Aluminum (Al)

[0033] Aluminum (Al) acts similarly to silicon (Si), primarily contributing to solid solution strengthening and the inhibition of carbide formation. Aluminum is also used primarily as a deoxidizer, promoting ferrite formation, improving elongation, and stabilizing austenite by increasing the carbon concentration in austenite. Aluminum also acts as a layer between the iron and zinc coating layer, improving galvanizability and is effective in inhibiting the formation of manganese bands in hot-rolled coils.

[0034] In the steel sheet constituting the ultra-high strength steel sheet with excellent elongation according to one embodiment of the present invention, the aluminum (Al) is preferably added in a content ratio of more than 0 wt % and not more than 0.05 wt % of the total weight. If the aluminum (Al) content is excessive, exceeding 0.05 wt %, there are problems such as an increase in aluminum inclusions, which reduces continuous castability, concentration on the surface of the steel sheet, which reduces galvanic properties, and formation of AlN in the slab, which induces hot rolling cracks.

[0035] At least one selected from niobium (Nb), vanadium (V) and titanium (Ti)

[0036] Niobium (Nb), vanadium (V) and / or titanium (Ti) are the main elements that precipitate in the form of carbides in steel. The present invention aims to ensure the stability of retained austenite and improve strength by refining the initial austenite grains through the formation of precipitates, and to achieve precipitation hardening through the refinement of ferrite grains and the presence of precipitates in ferrite.

[0037] In the steel sheet constituting the ultra-high strength steel sheet having excellent elongation according to one embodiment of the present invention, the sum of at least one selected from niobium (Nb), vanadium (V), and titanium (Ti) is preferably added in a content ratio of more than 0 wt% to 0.05 wt% of the total weight. If the sum of at least one selected from niobium (Nb), vanadium (V), and titanium (Ti) exceeds 0.05 wt%, problems such as deterioration of material properties and increased manufacturing costs, coarsening of grains due to the formation of coarse carbides, and an excessive increase in recrystallization temperature can cause problems such as an inhomogeneous structure.

[0038] Rin (P)

[0039] Phosphorus (P) plays a role similar to that of silicon (Si), increasing strength through solid solution strengthening and suppressing the formation of carbides. The phosphorus may be added to a steel sheet constituting an ultra-high strength steel sheet with excellent elongation according to an embodiment of the present invention in a content ratio of more than 0 wt% to 0.02 wt% of the total weight. If the phosphorus content exceeds 0.02 wt%, problems such as embrittlement of the welded joint, reduced press formability, and reduced impact resistance may occur. Therefore, in the present invention, it is necessary to control the phosphorus content as low as possible.

[0040] Sulfur (S)

[0041] Sulfur (S) improves the machinability of steel by bonding with manganese, titanium, etc., and improves workability by forming fine MnS precipitates. However, it generally impairs softening and weldability. The sulfur content may be greater than 0 wt. % and less than 0.005 wt. % of the total weight of a steel sheet constituting an ultra-high strength steel sheet with excellent elongation according to one embodiment of the present invention. If the sulfur content exceeds 0.005 wt. %, the number of MnS inclusions increases, deteriorating workability, and high-temperature cracks may occur due to segregation during continuous casting solidification. Therefore, in the present invention, it is necessary to control the sulfur content as low as possible.

[0042] Nitrogen (N)

[0043] Nitrogen (N) is a solid solution strengthening element that can increase the strength of steel sheets and is generally introduced from the atmosphere. Its content must be controlled during the degassing process of the steelmaking process. If the nitrogen content exceeds 0.006 wt%, problems such as embrittlement of welds, induction of low-temperature brittleness, reduced press formability, and reduced impact resistance may occur. Therefore, in the present invention, it is necessary to control the nitrogen content as low as possible.

[0044] In an ultra-high strength steel sheet having excellent elongation according to an embodiment of the present invention, the steel sheet may have a yield strength (YP) of 850 MPa or more, a tensile strength (TS) of 1180 MPa or more, and an elongation (El) of 14% or more. For example, the steel sheet may have a yield strength (YP) of 850 MPa to 1070 MPa, a tensile strength (TS) of 1180 MPa to 1250 MPa, and an elongation (El) of 14% to 20%. Furthermore, the product of tensile strength and elongation may not be less than 15,000 (MPa·%). In the present invention, the product of tensile strength and elongation is at a level of 15,000 (MPa·%) or more, which is more than twice the value of the product of tensile strength and total elongation at this strength level, 7,000 (MPa·%), and therefore it can be seen that the steel sheet may have better formability than existing ultra-high strength steels having the same strength.

[0045] FIG. 1 is a photograph of the final microstructure of an ultra-high strength steel sheet having an excellent elongation rate according to an embodiment of the present invention, and FIG. 2 is a photograph showing the density of carbides in ferrite in the final microstructure of an ultra-high strength steel sheet having an excellent elongation rate according to an embodiment of the present invention.

[0046] 1 and 2, the final microstructure of the steel sheet is composed of tempered martensite, martensite, ferrite, and retained austenite. The final microstructure is characterized in that the matrix is ​​martensite, not bainite. The area fraction of the ferrite is 10% to 20%, the area fraction of the retained austenite is 10% to 20%, and the area fractions of the tempered martensite and martensite may be included as the remaining fractions. Furthermore, the ferrite contains carbides, and the carbides have an average size of 50 nm or more and a density of 20 particles / μm 2 It can be the following:

[0047] Hereinafter, a method for manufacturing an ultra-high strength steel sheet having excellent elongation ratio according to one embodiment of the present invention, which has the above-mentioned composition, properties and microstructure, will be described.

[0048] Steel plate manufacturing method

[0049] FIG. 3 is a flowchart showing a method for manufacturing an ultra-high strength steel sheet having an excellent elongation ratio according to one embodiment of the present invention.

[0050] Referring to FIG. 3, a method for manufacturing a steel sheet according to an embodiment of the present invention includes, in weight percent, carbon (C): 0.1% to 0.30%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% and 0.05% or less, the sum of at least one element selected from niobium (Nb), titanium (Ti), and vanadium (V): more than 0% and 0.05% or less, phosphorus (P): more than 0% and 0.02%, sulfur (S): more than 0% and 0.005%, nitrogen (N): more than 0% and 0.006%, and the balance being iron (Fe) and other impurities. The method includes the steps of: (S100) producing a hot-rolled steel sheet containing unavoidable impurities; (S200) subjecting the hot-rolled steel sheet to softening heat treatment by maintaining the temperature at a constant value in the range of 500°C to 650°C; (S300) cold-rolling the softened steel sheet to produce a cold-rolled steel sheet; (S400) subjecting the cold-rolled steel sheet to annealing heat treatment at a temperature in the range of 820°C to 860°C; (S500) multi-stage cooling the cold-rolled steel sheet to a temperature in the range of 200°C to 260°C; and (S600) subjecting the cooled cold-rolled steel sheet to partitioning heat treatment at a temperature in the range of 370°C to 460°C.

[0051] The step (S100) of producing the hot-rolled steel sheet may include the steps of preparing a steel slab having the alloy composition; reheating the steel slab at a temperature in the range of 1150°C to 1250°C; hot-finish rolling the reheated steel slab at a finish rolling temperature in the range of 850°C to 1000°C to produce a hot-rolled steel sheet; and coiling the hot-rolled steel sheet at a temperature in the range of 500°C to 700°C.

[0052] The step of manufacturing the hot-rolled steel sheet (S100) may include reheating the steel at 1150°C to 1250°C. A slab of steel having the composition of the present invention is reheated at a temperature of Ac3 or higher to redissolve the components at the time of casting. A low reheating temperature may increase the hot rolling load, while a high reheating temperature may cause warpage of the slab, making it difficult to load and discharge into a heating furnace. When the steel is reheated at the above temperatures, components segregated during the continuous casting process may be redissolved. To improve strength through precipitation and solution strengthening, strengthening elements prior to hot rolling must be fully dissolved in austenite, and for this reason, the steel must be heated at 1150°C or higher. A reheating temperature lower than 1150°C may increase the hot rolling load, may not fully dissolve various carbides, and may result in insufficient uniform distribution of the components segregated during the continuous casting process.

[0053] However, if the reheating temperature exceeds 1250°C, adverse effects such as austenite coarsening and decarburization occur, making it impossible to obtain the desired strength. That is, if the reheating temperature exceeds 1250°C, very coarse austenite grains are formed, making it difficult to ensure strength. Furthermore, if the reheating temperature exceeds 1250°C, slab warpage can make it difficult to charge and discharge the slab into a heating furnace, increasing heating costs and process time, which can lead to increased manufacturing costs and reduced productivity.

[0054] The step (S100) of manufacturing the hot-rolled steel sheet may include a step of hot-rolling under conditions of a finish rolling temperature (FDT) of 850°C to 1000°C, a cooling rate of 10 to 30°C / s, and a coiling temperature (CT) of 500°C to 700°C. Because this is a high-alloy steel, it is necessary to minimize edge cracking and rolling load to ensure mass productivity, so the rolling finish temperature and coiling temperature can be set in a high temperature range. The finish rolling temperature (FDT) is a very important factor affecting the final material properties, and rolling at 850°C to 1000°C can refine the austenite.

[0055] However, if the hot rolling temperature is lower than 850°C, the rolling load increases during rolling, which can lead to the formation of a duplex structure at the edge. Furthermore, rolling at temperatures above 1000°C results in coarsening of the grains, making it impossible to obtain the desired mechanical properties. After hot rolling, cooling is carried out at a rate of 10 to 30°C / s, with faster cooling rates being more effective at reducing the average grain size.

[0056] On the other hand, if the coiling temperature is lower than 500°C, the shape of the hot-rolled coil becomes non-uniform, increasing the cold rolling load.If the coiling temperature is higher than 700°C, the difference in cooling rate between the center and edge of the steel sheet can cause a non-uniform microstructure, leading to oxidation of the inside of the grain boundaries.

[0057] In the method for manufacturing an ultra-high strength steel sheet, the step of annealing the cold-rolled steel sheet may be performed by heating the cold-rolled steel sheet at a temperature increase rate of 1 to 10°C / s and maintaining the temperature at 820°C to 860°C for 40 to 120 seconds.

[0058] In the method for manufacturing the ultra-high strength steel sheet, the step of multi-stage cooling the cold-rolled steel sheet may include a first cooling step of slowly cooling the cold-rolled steel sheet at a cooling rate of 1 to 10°C / s to an end temperature of 550°C to 750°C; and a second cooling step of rapidly cooling the cold-rolled steel sheet at a cooling rate of 50°C / s or more to an end temperature of 200°C to 260°C.

[0059] In the method for manufacturing the ultra-high strength steel sheet, the partitioning heat treatment may be performed by heating the cooled cold-rolled steel sheet at a temperature increase rate of 20°C / s or more and maintaining the temperature in a range of 370°C to 460°C for a time period in a range of 10 seconds to 240 seconds.

[0060] After performing the step of producing a hot-rolled steel sheet (S100), the step of cold-rolling the softened steel sheet to produce a cold-rolled steel sheet (S300) may be followed by a step of softening the hot-rolled steel sheet by maintaining the temperature at a constant range of 500°C to 650°C (S200).

[0061] The composition system used in the present invention is a high-alloy steel containing a large amount of alloying elements such as manganese, which retards the hardening ability of the material. Therefore, during the completion of hot rolling and cooling to room temperature, a hard martensite phase is formed without the formation of soft ferrite and pearlite phases. This results in a hot-rolled material with very high strength. Therefore, a softening heat treatment is performed after hot rolling. This is because if the strength of the hot-rolled sheet material is high, problems such as thickness hunting and shape defects may occur during cold rolling. Therefore, the hot-rolled sheet is softened through the softening heat treatment to improve the efficiency of the cold rolling process and ensure cold-rollability.

[0062] Specific process conditions for the softening heat treatment step (S200) are shown in Table 1. Referring to Table 1, the softening heat treatment step (S200) may include, for example, heating up to 500 to 650°C at a heating rate of 30 to 80°C / hr, maintaining the temperature at a constant level for 2 to 16 hours, and then cooling down to room temperature at a cooling rate of 30 to 60°C / hr.

[0063] [Table 1]

[0064] The softening heat treatment is performed at a temperature between 500°C and 650°C for a sufficient time. If the softening heat treatment temperature is lower than 500°C, the martensite formed after hot rolling does not recrystallize, and only tempering occurs. The carbon supersaturated in the structure is formed and spheroidized in the form of cementite (θ). This can lead to the brittleness of martensite, which can cause safety issues such as sheet breakage during cold rolling. If the softening heat treatment temperature is higher than 650°C, excessive austenite is formed during the softening heat treatment, and martensite is formed during cooling, preventing effective strength reduction. Therefore, the strength of the hot-rolled material can be reduced by controlling the temperature range to 500°C to 650°C. The step (S300) of cold-rolling the softened steel sheet to produce a cold-rolled steel sheet is performed to adjust the thickness of the hot-rolled material to the final production steel sheet. A pickling process can also be performed before cold rolling. The cold rolling process is performed with a reduction rate that matches the final product specifications and can be controlled to, for example, 40% to 60%. The higher the cold rolling reduction rate, the more effective it is in improving formability due to the effect of refining the structure. If the cold rolling reduction rate is less than 40%, it is difficult to obtain a uniformly fine structure, and if it is designed to exceed 60%, the roll force becomes high and the process load becomes heavy.

[0065] The step of annealing the cold-rolled steel sheet (S400) may include heating the cold-rolled steel sheet at a temperature increase rate of 1 to 10°C / s and maintaining the annealing temperature at 820 to 860°C for 40 to 120 seconds. The annealing start temperature and annealing maintenance period in the annealing heat treatment process are performed under conditions in the austenite-ferrite two-phase region. In this example, the heat treatment is performed in the two-phase region to ensure an appropriate fraction of ferrite and to obtain the desired final properties of the steel sheet by ensuring ideal ferrite, tempered martensite, and retained austenite in the final microstructure.

[0066] The annealed cold-rolled steel sheet is subjected to a step (S500) of multi-stage cooling to a temperature in the range of 200°C to 260°C. The step (S500) of multi-stage cooling of the cold-rolled steel sheet may include a first cooling step of slowly cooling the cold-rolled steel sheet at a cooling rate of 1 to 10°C / s to an end temperature of 550°C to 750°C; and a second cooling step of rapidly cooling the cold-rolled steel sheet at a cooling rate of 50°C / s or more to an end temperature of 200°C to 260°C.

[0067] The primary cooling step is a step of slowly cooling after annealing heat treatment, which aims to ensure the plasticity of the final microstructure by ensuring a certain amount of ferrite in the final microstructure during the heat treatment process. The secondary cooling step can include a step of rapidly cooling the material after the primary cooling step, for example, to a quenching end temperature of 200°C to less than 260°C at a cooling rate of 70°C / s or more. This is to transform austenite in the microstructure into martensite after slow cooling by controlling the quenching end temperature, thereby facilitating the final material quality. A cooling rate of 70°C / s or more is required to suppress phase transformation that may occur during the quenching process.

[0068] The multi-stage cooled cold-rolled steel sheet is subjected to a partitioning heat treatment (S600) at a temperature ranging from 370°C to 460°C. The partitioning heat treatment (S600) can be performed by heating the cooled cold-rolled steel sheet at a heating rate of 20°C / s or more and maintaining the temperature ranging from 370°C to 460°C for a time ranging from 10 seconds to 240 seconds. The partitioning heat treatment (S600) aims to ensure strength and elongation through carbon enrichment in retained austenite and martensite tempering, and ultimately to maintain the final microstructure. The partitioning heat treatment (S600) can include, for example, maintaining the temperature in a reheating range of more than 400°C but less than 460°C for 60 seconds or less.

[0069] After the partitioning heat treatment step (S600), the steel sheet is immersed in a Zn-Mg-Al plating bath and then cooled to room temperature to obtain the final steel sheet.

[0070] The ultra-high strength steel sheet of the present invention, realized through the above steps, may have a yield strength (YP) of 850 MPa or more, a tensile strength (TS) of 1180 MPa or more, and an elongation (El) of 14% or more. For example, the yield strength (YP) may be 850 MPa to 1070 MPa, the tensile strength (TS) may be 1180 MPa to 1250 MPa, and the elongation (El) may be 14% to 20%. Furthermore, the product of tensile strength and elongation may not be less than 15,000 (MPa·%). In the present invention, the product of tensile strength and elongation is at a level of 15,000 (MPa·%) or more, which is more than twice the value of the product of tensile strength and total elongation at this strength level, 7,000 (MPa·%), and therefore may have better formability than existing ultra-high strength steels of the same strength.

[0071] In conventional forming of body parts, fractures of ultra-high strength materials can be explained by evaluation criteria such as drawability and biaxial stretchability, which can be confirmed by a typical forming limit diagram, and hole expansion ratio, which cannot be confirmed by a forming limit diagram. Generally, the better the formability evaluation results of a sheet material used to process body parts, the more complex the formed structure it can be. These formability indices are important factors in forming body parts, which is primarily performed by pressing. Generally, ultra-high strength materials tend to have a lower elongation rate as their strength increases. To form such ultra-high strength materials, special forming processes or deformation mechanisms that can further ensure formability are being developed.

[0072] The present invention aims to secure improved formability compared to existing ultra-high strength steels by utilizing a microstructure 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 a microstructure of ferrite, retained austenite, and martensite / tempered martensite through chemical composition and heat treatment control. Based on the results of a replica test, the present invention was able to propose the chemical composition and heat treatment process range.

[0073] In the case of existing ultra-high strength steels with a dual-phase structure of ferrite and martensite, plastic deformation occurs through a basic deformation mechanism in which electrical potential is formed and transferred within the structure when the steel undergoes plastic deformation. This transfer of electrical potential causes defects to form and grow, resulting in fracture. To ensure strength below this deformation mechanism, hard phases such as martensite and bainite are formed to ensure strength. However, increasing the hard phase fraction to ensure strength inevitably reduces the elongation rate, so soft phases such as ferrite are formed within the structure to compensate for the elongation rate. In the case of ultra-high strength steels with this final microstructure, strength and elongation follow the Rule of Mixture (ROM), which has the disadvantage that it is difficult to improve material quality beyond the Rule of Mixture.

[0074] A steel type developed to improve on this ultra-high-tensile steel with a dual-phase structure of ferrite and martensite is transformation-induced plasticity steel, which secures retained austenite in the final structure and ensures strength and elongation through the retained austenite phase transformation that occurs during plastic deformation. However, transformation-induced plasticity steel has a drawback in that it is difficult to dramatically improve formability because the area fraction of retained austenite contained in the final microstructure is small.

[0075] Therefore, in the present invention, retained austenite is secured in the final microstructure to improve the formability of ultra-high strength steel, and the final microstructure of the manufactured steel sheet is composed of ferrite, retained austenite, and tempered martensite (see FIG. 1). 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 stability for realizing the transformation-induced plasticity mechanism. Therefore, it is preferable to limit the amount of retained austenite to 10% to 20%, and it is preferable to have 10% to 20% ferrite and the remainder be martensite / tempered martensite.

[0076] Therefore, the method proposed in the present invention for realizing the microstructure and ensuring the yield strength, tensile strength, and elongation ratio is summarized as follows.

[0077] (1) First Plan: After annealing and plating heat treatment, in order to secure retained austenite in the final microstructure, steelmaking, continuous casting, hot rolling, and cold rolling are carried out using a composition system in which the austenite stabilizing elements carbon and manganese are optimally controlled.

[0078] (2) Second method: The obtained cold rolled coil is used to obtain the microstructure proposed in the present invention through the control of two-phase annealing, quenching, and reheating treatment.

[0079] In the present invention, in order to ensure high formability, the final microstructure is set based on the following principle, and the process optimization to realize this is carried out through a replica test.

[0080] (1) First design direction: In order to ensure the elongation rate and hole expandability, the strength that may not be achieved is ensured by utilizing the transformation-induced plasticity of martensite / tempered martensite and retained austenite.

[0081] (2) Second design direction: In order to secure the elongation rate, we aim to secure an improved elongation rate by simultaneously applying two methods: a method to secure the elongation rate by securing the soft phase (ferrite) in the final microstructure in existing ultra-high tensile steel, and a method to secure more retained austenite in the final microstructure, which is used in transformation-induced plasticity steel.

[0082] (3) Third design direction: Additionally, to optimally secure the stability of the retained austenite, the density of carbides in the final microstructure is controlled to achieve the desired properties.

[0083] In order to form the above-mentioned microstructure, the present invention employs two-phase annealing, rapid cooling, reheating, and continuous plating processes, thereby establishing heat treatment process conditions for a cold-rolled steel sheet having a final microstructure of ferrite, retained austenite, and tempered martensite.

[0084] To construct the final microstructure associated with the first and second design directions, two-phase annealing, rapid cooling, and reheat treatments are required.

[0085] First, it is important to ensure a sufficient amount of ferrite in the final microstructure through intercritical annealing. Ferrite in the final microstructure can be generated in two stages during the heat treatment process: annealing and slow cooling. If the annealing zone is set to the single-phase zone, the austenite-to-ferrite phase transformation occurs during slow cooling, but the time is so short that the amount of ferrite is negligible, making it impossible to form the sufficient amount of ferrite targeted by the present invention. Conversely, if the annealing zone is set to the two-phase zone, ferrite can be secured during slow cooling in addition to the ferrite secured in the two-phase zone, ensuring the desired ferrite fraction and facilitating the formation of the final microstructure. In the present invention, taking into consideration the controllability of a conventional continuous annealing line, the annealing temperature is set to the two-phase region, and the slow cooling end temperature is set to more than 550°C and less than 750°C. Preferably, the annealing temperature can be set to 820°C to 860°C, and the slow cooling end temperature can be set to 600°C to 700°C.

[0086] Second, it is important to ensure the retention of retained austenite through the rapid cooling process after slow cooling. If the steel sheet is cooled at a rapid rate after slow cooling without interfering with hardening ability, the austenite present outside the ferrite is transformed into martensite and austenite through rapid cooling. This allows the fraction of martensite, which facilitates ensuring strength in the final microstructure, and the fraction of retained austenite, which facilitates ensuring formability, to be determined before the reheating process. Therefore, in determining these fractions, it is important to find an appropriate temperature range between the cooling rate and the martensite formation start and end temperatures. In the present invention, taking into consideration the controllability of the continuous annealing line, the rapid cooling rate is specified to be 70°C / s or more, and the rapid cooling end temperature is specified to be greater than 200°C and less than 260°C. More preferably, the rapid cooling end temperature can be specified to be between 210°C and 250°C.

[0087] Third, reheating (partitioning heat treatment) induces carbon redistribution between phases within the microstructure, stabilizing the formed austenite and simultaneously softening the martensite structure. The austenite stabilized after the reheating process does not undergo further phase transformation even when cooled to room temperature, and is called retained austenite. This retained austenite induces transformation-induced plasticity during subsequent plastic deformation, acting as a major base for simultaneously ensuring the formability and strength of the steel sheet. The reheating temperature and time increase the amount of redistributed carbon, and the reheating process can be varied depending on the final target material. In the present invention, the reheating temperature can be specified to be greater than 370°C and less than 460°C, more preferably 400°C to 430°C.

[0088] Finally, the effects of the annealing temperature rise rate, maintenance time, slow cooling rate, and reheating temperature rise rate and maintenance time were examined. If the total annealing time is too long or too short, excessive phase transformation toward the target structure occurs in the unit process, resulting in recrystallization and grain growth, or insufficient phase transformation, making it impossible to achieve the desired properties. In the present invention, the annealing temperature rise rate and maintenance time are specified as 1-10°C / s (40-120 seconds), the slow cooling rate as 1-10°C / s, and the reheating temperature rise rate and maintenance time as 20°C / s or more (10 seconds or more). Preferably, the annealing temperature rise rate and maintenance time are specified as 1-5°C / s (50-110 seconds), the slow cooling rate as 5-10°C / s, and the reheating temperature rise rate and maintenance time as 30°C / s or more (10-240 seconds).

[0089] In relation to the third design direction, in order to ensure optimum retained austenite stability, it is necessary to control the carbides formed in the final microstructure.

[0090] The present invention incorporates the concepts of ensuring the final microstructure of the first and second design directions, while at the same time controlling the stability of the retained austenite, which is the main structure that ensures the material properties, thereby controlling the phase transformation due to plastic deformation behavior and ensuring the target material properties. It is important to incorporate an appropriate amount of austenite stabilizing elements into the austenite structure during a series of heat treatment processes, particularly the annealing and reheating processes. To achieve this, by suppressing the reduction in the amount of austenite stabilizing elements diffused due to the formation of unnecessary inclusions, retained austenite with sound stability is formed in the final microstructure after heat treatment.

[0091] In the present invention, carbon is used as an austenite stabilizer to form retained austenite in the final microstructure. In this case, it is necessary to suppress the formation of unnecessary carbides due to carbon diffusion into austenite. Typically, carbides are found primarily in martensite and ferrite phases. In particular, in the present invention, significant carbide formation and growth occurs during the reheating process after annealing and cooling. Carbide formation occurs primarily due to two factors: 1) partial transformation of austenite to bainite during the reheating process after quenching, and 2) tempering of martensite during the reheating process. In this case, the addition of silicon to the steel minimizes carbide formation. However, this is also effective in the growth of carbides previously formed (during hot rolling and softening heat treatment). Ultimately, controlling the amount of previously formed carbides may have the effect of promoting carbon diffusion into austenite, along with the effect of controlling the composition.

[0092] Therefore, the present invention aims to minimize the amount of carbides formed in the final microstructure, thereby ensuring the stability of the retained austenite. As a result of the experimental analysis of the present invention, it was found that the density of ferrite carbides (average size of 50 nm or more) was 20 particles / μm 2 It was confirmed that the optimum material can be secured in the following cases (see Figure 2). Additionally, each carbide in the ferrite (soft phase) can act as a crack formation site when deformation is applied, which can be detrimental to formability.

[0093] The ultra-high strength steel sheet and manufacturing method thereof according to the technical concept of the present invention described above ensures strength and elongation by utilizing a strengthening mechanism based on transformation-induced plasticity steel and carbide control. Unlike existing ultra-high strength steels, which secure soft phases in the final microstructure to achieve elongation, the transformation-induced plasticity steel secures more retained austenite in the final microstructure, thereby achieving improved elongation. Furthermore, to optimally control the stability of retained austenite and achieve the desired material properties, a carbide standard was established to control the diffusion of carbon, an austenite-stabilizing element.

[0094] Experimental example

[0095] In the following, preferred experimental examples are presented to facilitate understanding of the present invention, but the following experimental examples are merely for the purpose of facilitating understanding of the present invention and are not intended to limit the present invention.

[0096] 1. Composition of test specimen

[0097] In this experimental example, a test piece having the alloy element composition shown in Table 2 was provided.

[0098] [Table 2]

[0099] The chemical composition of Table 2, which is a composition constituting an ultra-high strength steel sheet with excellent elongation according to one embodiment of the present invention, satisfies the following composition by weight: carbon (C): 0.1% to 0.30%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% and 0.05% or less, the sum of at least one element selected from niobium (Nb), titanium (Ti), and vanadium (V): more than 0% and 0.05% or less, phosphorus (P): more than 0% and 0.02%, sulfur (S): more than 0% and 0.005%, nitrogen (N): more than 0% and 0.006%, and the balance being iron (Fe).

[0100] 2. Process conditions and physical property evaluation

[0101] Table 3 shows the process conditions applied in this experimental example having the alloy composition of Table 2. The softening heat treatment in Table 3 corresponds to the softening heat treatment step (S200) shown in Figure 3, the annealing corresponds to the annealing heat treatment step (S400) shown in Figure 3, the slow cooling and rapid cooling correspond to the multi-stage cooling step (S500) shown in Figure 3, and the reheating corresponds to the partitioning heat treatment step (S600) shown in Figure 3. Other process conditions were the same as those described in the manufacturing method of the ultra-high strength steel sheet of the present invention above, within the range of process conditions.

[0102] Table 4 shows the microstructure and physical properties derived from the present experimental examples according to Tables 2 and 3. In Table 4, the criteria for whether or not the material was achieved are: yield strength (YP): 850 MPa or more, tensile strength (TS): 1180 MPa or more, elongation (El): 14% or more, area fraction of ferrite: 10% to 20%, area fraction of retained austenite: 10% to 20%, area fraction of tempered martensite / martensite: remaining fraction, density of carbides in ferrite with an average size of 50 nm or more: 20 / μm 2 The following is the result.

[0103] [Table 3]

[0104] [Table 4]

[0105] Referring to Tables 3 and 4, Experimental Examples 1, 2, 4, 6, and 8 are examples of the present invention, and are characterized by softening heat treatment temperature of 500°C to 650°C, annealing temperature rise rate of 1 to 10°C / s, annealing temperature of 820°C to 860°C, annealing maintenance time of 40 to 120 seconds, slow cooling rate of 1 to 10°C / s, slow cooling end temperature of 550°C to 750°C, rapid cooling rate of 50°C / s or more, rapid cooling end temperature of 200°C to 260°C, reheating temperature rise rate of 20°C / s or more, reheating temperature of 370°C to 460°C, reheating maintenance time of 40 to 120 seconds. The process conditions of holding time: 10 seconds to 240 seconds are satisfied, and thereby the final microstructure is composed of tempered martensite, martensite, ferrite, and retained austenite, the area fraction of the ferrite is 10% to 20%, the area fraction of the retained austenite is 10% to 20%, the area fraction of the tempered martensite and martensite satisfy the remaining fractions, the ferrite contains carbides, and the carbides have an average size of 50 nm or more and a carbide density of 20 particles / μm 2 The following conditions were satisfied: yield strength (YP): 850 MPa to 1070 MPa, tensile strength (TS): 1180 MPa to 1250 MPa, elongation (El): 14% to 20%, and the product of tensile strength and elongation was 15,000 (MPa·%) or more. In contrast, Experimental Example 3, a comparative example of the present invention, had an annealing temperature above but not in the range of 820°C to 860°C, which resulted in a ferrite area fraction below but not in the range of 10% to 20%, a yield strength (YP) above but not in the range of 850 MPa to 1070 MPa, an elongation (El) below but not in the range of 14% to 20%, and a product of tensile strength and elongation below but not in the range of 15,000 (MPa·%) or more. That is, when the annealing zone is set to the single-phase zone, the steel enters the two-phase zone during slow cooling, and austenite-to-ferrite phase transformation is possible, but because the time is very short, the amount of transformation is negligible. In this case, it becomes impossible to form a sufficient amount of ferrite, which is the target of the present invention, and the elongation rate decreases.

[0106] Experimental Example 5 is a comparative example of the present invention, in which the slow cooling end temperature was above the range of 550°C to 750°C and failed to satisfy the range, and as a result, the ferrite area fraction was below the range of 10% to 20%, and failed to satisfy the range, the yield strength (YP) was above the range of 850 MPa to 1070 MPa, the elongation (El) was below the range of 14% to 20%, and failed to satisfy the range, and the value of the product of tensile strength and elongation was below the range of 15,000 (MPa·%) or more.

[0107] Experimental Example 7 is a comparative example of the present invention, and the quenching end temperature exceeds the range of 200°C to 260°C, which results in the area fraction of retained austenite falling below the range of 10% to 20%, which results in the yield strength (YP) falling below the range of 850 MPa to 1070 MPa.

[0108] Experimental Example 9 is a comparative example of the present invention, in which the reheating temperature was above the range of 370°C to 460°C and thus the yield strength (YP) was above the range of 850 MPa to 1070 MPa and thus the elongation (El) was below the range of 14% to 20% and thus the product of tensile strength and elongation was below the range of 15,000 (MPa·%) or more ... above the range of 15,000 (MPa·%) or more and thus the product of tensile strength and elongation was below the range of 15,000 (MPa·%) or more and thus the product of tensile strength and

[0109] Experimental Examples 10 and 11 are comparative examples of the present invention. The softening heat treatment temperature was below the range of 500°C to 650°C, and as a result, the density of carbides with an average size of 50 nm or more in the ferrite was 20 / μm. 2 The following ranges cannot be satisfied and are exceeded, and the elongation ratio (El) cannot be satisfied and is below the range of 14% to 20%.

[0110] Although the present invention has been described above with reference to exemplary embodiments, various modifications and variations may be made by those skilled in the art. Such modifications and variations are within the scope of the present invention. Therefore, the scope of the present invention should be determined by the following claims.

Claims

1. The alloy contains, by weight, carbon (C): 0.1% to 0.30%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% and 0.05% or less, the sum of at least one element selected from niobium (Nb), titanium (Ti), and vanadium (V): more than 0% and 0.05% or less, 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, The final microstructure consists of tempered martensite, martensite, ferrite, and retained austenite; An ultra-high strength steel plate having a yield strength (YP): 850 MPa or more, a tensile strength (TS): 1180 MPa or more, and an elongation rate (El): 14% or more.

2. The ultra-high strength steel plate according to claim 1, wherein the final microstructure has a martensite matrix that is not bainite.

3. The area fraction of the ferrite is 10% to 20%, The area fraction of the retained austenite is 10% to 20%, The ultra-high strength steel plate according to claim 1 , wherein the area fraction of the tempered martensite and the martensite is included as a remainder fraction.

4. the ferrite includes carbides; The carbides have an average size of 50 nm or more and a density of 20 particles / μm ２ 2. The ultra-high strength steel plate according to claim 1, wherein:

5. 2. The ultra-high strength steel plate according to claim 1, wherein the product of tensile strength and elongation is not less than 15,000 (MPa·%).

6. a step of manufacturing a hot-rolled steel sheet containing, by weight, carbon (C): 0.1% to 0.30%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): more than 0% and 0.05% or less, the sum of at least one element selected from niobium (Nb), titanium (Ti), and vanadium (V): more than 0% and 0.05% or less, 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; softening the hot-rolled steel sheet by maintaining the temperature at a constant range of 500°C to 650°C; cold-rolling the softening heat-treated steel sheet to produce a cold-rolled steel sheet; Annealing the cold-rolled steel sheet at a temperature in the range of 820°C to 860°C; cooling the cold-rolled steel sheet in multiple stages to a temperature in the range of 200°C to 260°C; and and subjecting the cooled cold-rolled steel sheet to a partitioning heat treatment at a temperature in the range of 370°C to 460°C.

7. The step of manufacturing the hot-rolled steel sheet comprises: providing a steel slab having said alloy composition; reheating the steel slab to a temperature in the range of 1150°C to 1250°C; hot finish rolling the reheated steel slab at a finish rolling temperature in the range of 850°C to 1000°C to produce a hot rolled steel sheet; and The method for producing an ultra-high strength steel sheet according to claim 6, further comprising the step of: coiling the hot-rolled steel sheet at a temperature in the range of 500°C to 700°C.

8. The step of annealing the cold-rolled steel sheet comprises: The method for producing an ultrahigh strength steel sheet according to claim 6, wherein the cold-rolled steel sheet is heated at a temperature rising rate in a range of 1 to 10 ° C. / s and maintained at a temperature in a range of 820 ° C. to 860 ° C. for a time in a range of 40 seconds to 120 seconds.

9. The step of multi-stage cooling the cold-rolled steel sheet includes: A primary cooling step in which the cold-rolled steel sheet is slowly cooled at a cooling rate of 1 to 10 ° C. / s to a final temperature of 550 ° C. to 750 ° C.; and The method for producing an ultra-high strength steel sheet according to claim 6, further comprising: a secondary cooling step of quenching the cold-rolled steel sheet at a cooling rate of 50°C / s or more to a final temperature of 200°C to 260°C.

10. The partitioning heat treatment step includes: The method for producing an ultrahigh strength steel sheet according to claim 6, wherein the cooled cold-rolled steel sheet is heated at a temperature rising rate of 20°C / s or more and maintained at a temperature in the range of 370°C to 460°C for a time in the range of 10 seconds to 240 seconds.

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