High-strength steel plate with excellent workability and manufacturing method thereof

Abstract

The present invention relates to a steel sheet that can be used for automobile parts, etc., and relates to a steel sheet that has an excellent balance between strength and ductility, a good balance between strength and hole expandability, and an excellent yield ratio evaluation index, and a method for manufacturing the same.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet that can be used for automobile parts, etc., and relates to a steel sheet that has high strength properties and excellent workability, and a method for manufacturing the same. [Background technology]

[0002] Recently, the automotive industry has been focusing on methods to reduce the weight of materials while simultaneously ensuring passenger stability in order to protect the global environment. To meet this demand for stability and weight reduction, the use of high-strength steel sheets has rapidly increased. It is generally known that the higher the strength of a steel sheet, the lower its formability. Therefore, there is a demand for steel sheets for automotive parts that have high strength properties while also having excellent formability, such as ductility and hole expandability.

[0003] TRIP (Transformation Induced Plasticity) steel, which utilizes the transformation-induced plasticity of retained austenite, has a complex microstructure consisting of ferrite, bainite, martensite, and retained austenite, and is known to have high strength properties while also maintaining a certain level of workability.

[0004] Patent Documents 1 and 2 disclose methods of utilizing tempered martensite as a technique for further improving the workability of steel sheets. Tempered martensite, which is produced by tempering hard martensite, is softened martensite, and therefore there is a difference in strength between tempered martensite and conventional untempered martensite (fresh martensite). Therefore, by suppressing fresh martensite and forming tempered martensite, workability can be improved.

[0005] However, in the techniques disclosed in Patent Documents 1 and 2, the balance between tensile strength and elongation (TS 2 *EL 1 / 2) is 3.0*10 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) range, which means that it is difficult to secure a steel plate that is excellent in both strength and ductility.

[0006] Meanwhile, as another technique for improving the workability of steel sheets, a method of inducing the formation of bainite by adding boron (B) is disclosed in Patent Document 3. When boron (B) is added, the ferrite-pearlite transformation is suppressed and the formation of bainite is induced, so that both strength and workability can be achieved.

[0007] However, in the technology disclosed in Patent Document 3, 3.0*10 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) balance of tensile strength and elongation (B TE ), 6.0*10 6 ~11.5*10 6 (MPa 2 % 1 / 2 ) balance of tensile strength and hole expansion rate (B TH ) and yield ratio evaluation index (I YR ) cannot be secured simultaneously, which means that it is difficult to secure a steel sheet that is excellent in all of strength, hole expandability, ductility, and yield ratio.

[0008] In other words, the balance between tensile strength and elongation (B TE ), balance between tensile strength and hole expansion rate (B TH ) and yield ratio evaluation index (I YR ) do not all meet the requirements for excellent steel sheets. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Korean Patent Publication No. 10-2006-0118602 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-019258 [Patent Document 3] Japanese Patent Application Laid-Open No. 2016-216808 Summary of the Invention [Problem to be solved by the invention]

[0010] According to one aspect of the present invention, a steel sheet having an excellent balance between tensile strength and elongation, a balance between tensile strength and hole expansion rate, and a yield ratio evaluation index, all of which are excellent by optimizing the composition and microstructure of the steel sheet, and a method for manufacturing the same, can be provided.

[0011] The object of the present invention is not limited to the above-mentioned matters. Further object of the present invention is described in the entire content of the specification, and a person having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the further object of the present invention from the content described in the specification of the present invention. [Means for solving the problem]

[0012] A high-strength steel plate with excellent formability according to one aspect of the present invention contains, by weight %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, with the remainder being Fe and unavoidable impurities, and the steel plate contains, as a microstructure, bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures, and can satisfy the following [Relational Formula 1] and [Relational Formula 2]. [Equation 1] 0.03≦[B] FM / [B] TM ≦0.55 In the above relational expression 1, [B] FM is the boron (B) content (wt%) in fresh martensite, [B] TM is the boron (B) content (wt%) contained in the tempered martensite. [Equation 2] T(γ) / V(γ)≧0.08 In the above relational expression 2, T(γ) is the fraction (volume %) of tempered retained austenite in the steel sheet, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0013] The steel sheet may further contain, by weight %, any one or more of the following (1) to (8): (1) One or more of Ti: 0-0.5%, Nb: 0-0.5%, and V: 0-0.5% (2) At least one of Cr: 0-3.0% and Mo: 0-3.0% (3) One or more of Cu: 0-4.0% and Ni: 0-4.0% (4) One or more of Ca: 0-0.05%, REM excluding Y: 0-0.05%, and Mg: 0-0.05% (5) One or more of W: 0-0.5% and Zr: 0-0.5% (6) One or more of Sb: 0-0.5% and Sn: 0-0.5% (7) One or more of Y: 0-0.2% and Hf: 0-0.2% (8) Co: 0-1.5%

[0014] The microstructure of the above steel plate can contain, by volume fraction, 10 to 30% bainite, 50 to 70% tempered martensite, 10 to 30% fresh martensite, 2 to 10% retained austenite, and 5% or less (including 0%) ferrite.

[0015] The above steel sheet has a balance between tensile strength and elongation (B TE ) is 3.0*10 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 ~11.5*10 6 (MPa 2 % 1 / 2 ) and the yield ratio evaluation index (I YR) can satisfy 0.15 to 0.42. [Equation 3] B TE = [tensile strength (TS, MPa)] 2 *[Elongation rate (El, %)] 1 / 2 [Equation 4] B TH = [tensile strength (TS, MPa)] 2 *[Hole expansion rate (HER, %)] 1 / 2 [Equation 5] I YR =1-[yield ratio (YR)]

[0016] According to one aspect of the present invention, a method for producing a high-strength steel sheet having excellent workability includes the steps of: providing a cold-rolled steel sheet containing, by weight, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, and the remainder Fe and unavoidable impurities; and heating the cold-rolled steel sheet to 700°C at an average heating rate of 5°C / s or more (first heating), heating to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / s or less (second heating), and maintaining the temperature for 50 to 1200 seconds (first maintenance). The process may include steps of: cooling the steel sheet after the first cooling step to a temperature range of 200 to 400°C at an average cooling rate of 2 to 100°C / s (first cooling); heating the steel sheet after the first cooling step to a temperature range of 400 to 600°C at an average heating rate of 5 to 100°C / s (third heating) and maintaining the temperature for 10 to 1800 seconds (second maintaining); cooling the steel sheet after the second cooling step to a temperature range of 300 to 500°C at an average cooling rate of 1 to 100°C / s (second cooling) and maintaining the temperature for 10 to 1800 seconds (third maintaining); and cooling the steel sheet after the third cooling step to room temperature at an average cooling rate of 1°C / s or more (third cooling).

[0017] The steel slab may further include any one or more of the following (1) to (8): (1) One or more of Ti: 0-0.5%, Nb: 0-0.5%, and V: 0-0.5% (2) At least one of Cr: 0-3.0% and Mo: 0-3.0% (3) One or more of Cu: 0-4.0% and Ni: 0-4.0% (4) One or more of Ca: 0-0.05%, REM excluding Y: 0-0.05%, and Mg: 0-0.05% (5) One or more of W: 0-0.5% and Zr: 0-0.5% (6) One or more of Sb: 0-0.5% and Sn: 0-0.5% (7) One or more of Y: 0-0.2% and Hf: 0-0.2% (8) Co: 0-1.5%

[0018] The cold-rolled steel sheet can be provided through the steps of heating a steel slab to 1000 to 1350°C; finish hot rolling at a temperature range of 800 to 1000°C; coiling the hot-rolled steel sheet at a temperature range of 350 to 650°C; pickling the coiled steel sheet; and cold-rolling the pickled steel sheet at a reduction ratio of 30 to 90%.

[0019] The cooling rate Vc1 of the primary cooling and the cooling rate Vc2 of the secondary cooling can satisfy the relationship Vc1>Vc2. [Effects of the Invention]

[0020] According to a preferred aspect of the present invention, there is provided a steel sheet having an excellent balance between tensile strength and ductility, a balance between tensile strength and hole expandability, and a yield ratio evaluation index, and suitable for use in automobile parts, etc., and a method for manufacturing the same. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present invention relates to a high-strength steel plate having excellent formability and a manufacturing method thereof. Preferred embodiments of the present invention will be described below. The embodiments of the present invention may be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. The present embodiments are provided to further explain the present invention to those skilled in the art to which the present invention pertains.

[0022] The present inventors have recognized that in boron (B)-added transformation-induced plasticity (TRIP) steels containing bainite, tempered martensite, fresh martensite, and retained austenite, if the microstructure fractions of tempered martensite, fresh martensite, and retained austenite are controlled within certain ranges, the boron (B) content in the tempered martensite and fresh martensite is controlled within certain ranges, and the shape and size of the retained austenite are controlled within certain ranges, it is possible to simultaneously achieve an excellent balance between tensile strength and ductility, an excellent balance between tensile strength and hole expandability, and an excellent yield ratio rating index. Based on this understanding, they devised a method for effectively achieving excellent strength, yield ratio, ductility, and hole expandability, resulting in the present invention.

[0023] Hereinafter, a high-strength steel sheet with excellent formability according to one aspect of the present invention will be described in more detail.

[0024] A high-strength steel plate with excellent formability according to one aspect of the present invention contains, by weight %, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, with the remainder being Fe and unavoidable impurities, and the steel plate contains, as a microstructure, bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures, and can satisfy the following [Relational Formula 1] and [Relational Formula 2]. [Equation 1] 0.03≦[B] FM / [B]TM ≦0.55 In the above relational expression 1, [B] FM is the boron (B) content (wt%) in fresh martensite, [B] TM is the boron (B) content (wt%) contained in the tempered martensite. [Equation 2] T(γ) / V(γ)≧0.08 In the above relational expression 2, T(γ) is the fraction (volume %) of tempered retained austenite in the steel sheet, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0025] The steel composition of the present invention will be described in more detail below. Unless otherwise specified, % representing the content of each element is based on weight.

[0026] A high-strength steel sheet with excellent formability according to one aspect of the present invention contains, by weight percent, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, with the balance being Fe and unavoidable impurities. In addition, it may further contain one or more of Ti: 0.5% or less (0% inclusive), Nb: 0.5% or less (0% inclusive), V: 0.5% or less (0% inclusive), Cr: 3.0% or less (0% inclusive), Mo: 3.0% or less (0% inclusive), Cu: 4.0% or less (0% inclusive), Ni: 4.0% or less (0% inclusive), Ca: 0.05% or less (0% inclusive), REM excluding Y: 0.05% or less (0% inclusive), Mg: 0.05% or less (0% inclusive), W: 0.5% or less (0% inclusive), Zr: 0.5% or less (0% inclusive), Sb: 0.5% or less (0% inclusive), Sn: 0.5% or less (0% inclusive), Y: 0.2% or less (0% inclusive), Hf: 0.2% or less (0% inclusive), and Co: 1.5% or less (0% inclusive).

[0027] Carbon (C): 0.1~0.25% Carbon (C) is an essential element for ensuring the strength of steel sheet, and also stabilizes retained austenite, which contributes to improving the ductility of steel sheet. Therefore, the present invention may contain 0.1% or more of carbon (C) to achieve this effect. A preferred carbon (C) content may be greater than 0.1%, 0.11% or more, or 0.12% or more. On the other hand, if the carbon (C) content exceeds a certain level, an excessive increase in strength may result in a decrease in ductility and deterioration of weldability. Therefore, the present invention may limit the upper limit of the carbon (C) content to 0.25%. The carbon (C) content may be 0.24% or less, and more preferably 0.23% or less.

[0028] Silicon (Si): 0.01 to 1.5% or less Silicon (Si) is an element that contributes to improving strength through solid solution strengthening and to improving workability by homogenizing the microstructure. Silicon (Si) also inhibits cementite precipitation and contributes to the formation of retained austenite. Therefore, to achieve these effects, the present invention adds 0.01% or more of silicon (Si). The preferred silicon (Si) content is 0.02% or more, and more preferably 0.04% or more. However, if the silicon (Si) content exceeds a certain level, it can cause coating defects such as undercoating during the coating process and reduce the weldability of the steel sheet. Therefore, the present invention limits the silicon (Si) content to 1.5%. The preferred upper limit of the silicon (Si) content is 1.48%, and more preferably 1.46%.

[0029] Manganese (Mn): 1.0-4.0% Manganese (Mn) is an element useful for increasing both strength and ductility. Therefore, in the present invention, 1.0% or more of manganese (Mn) can be added to achieve this effect. A preferred lower limit of the manganese (Mn) content can be 1.2%, and a more preferred lower limit of the manganese (Mn) content can be 1.4%. However, excessive addition of manganese (Mn) increases the bainite transformation time and reduces the carbon (C) concentration in austenite, resulting in the problem of not being able to secure the desired austenite fraction. Therefore, in the present invention, the upper limit of the manganese (Mn) content can be limited to 4.0%. A preferred upper limit of the manganese (Mn) content can be 3.9%.

[0030] Aluminum (Al): 0.01 to 1.5% Aluminum (Al) is an element that combines with oxygen in steel to deoxidize it. Similarly to silicon (Si), aluminum (Al) also inhibits cementite precipitation and stabilizes retained austenite. Therefore, to achieve this effect, the present invention allows the addition of 0.01% or more of aluminum (Al). A preferred aluminum (Al) content is 0.03% or more, and a more preferred aluminum (Al) content is 0.05% or more. However, excessive addition of aluminum (Al) not only increases the number of inclusions in the steel sheet but also reduces the workability of the steel sheet. Therefore, the present invention limits the aluminum (Al) content to 1.5%. A preferred upper limit of the aluminum (Al) content is 1.48%.

[0031] Phosphorus (P): 0.15% or less (including 0%) Phosphorus (P) is an element that is contained as an impurity and deteriorates impact toughness, so the phosphorus (P) content is preferably controlled to 0.15% or less.

[0032] Sulfur (S): 0.03% or less (0% included) Sulfur (S) is an element that is contained as an impurity and forms MnS in the steel sheet, which deteriorates ductility, so the sulfur (S) content is preferably 0.03% or less.

[0033] Nitrogen (N): 0.03% or less (including 0%) Nitrogen (N) is an element that is contained as an impurity and forms nitrides during continuous casting, causing cracks in the slab. Therefore, the nitrogen (N) content is preferably 0.03% or less.

[0034] Boron (B): 0.0005 to 0.005% Boron (B) is an element that improves hardenability and strength, and also suppresses grain boundary nucleation. Furthermore, the present invention requires the addition of boron (B) to simultaneously achieve an excellent balance between tensile strength and elongation, an excellent balance between tensile strength and hole expandability, and an excellent yield ratio rating index through the enrichment of boron (B) in tempered martensite. Therefore, the present invention allows the addition of 0.0005% or more of boron (B) to achieve these effects. However, adding boron (B) in excess of a certain level can result in excessive property effects and increased manufacturing costs, so the present invention limits the upper limit of the boron (B) content to 0.005%.

[0035] Meanwhile, the steel sheet of the present invention may contain additional alloy compositions in addition to the above-mentioned alloy components, which will be described in detail below.

[0036] One or more of titanium (Ti): 0-0.5%, niobium (Nb): 0-0.5%, and vanadium (V): 0-0.5% Titanium (Ti), niobium (Nb), and vanadium (V) are elements that form precipitates to refine crystal grains and also contribute to improving the strength and impact toughness of steel sheets. Therefore, the present invention allows the addition of one or more of titanium (Ti), niobium (Nb), and vanadium (V) for these effects. However, if the content of each of titanium (Ti), niobium (Nb), and vanadium (V) exceeds a certain level, excessive precipitates are formed, reducing impact toughness and increasing manufacturing costs. Therefore, the present invention limits the content of titanium (Ti), niobium (Nb), and vanadium (V) to 0.5% or less, respectively.

[0037] At least one of chromium (Cr): 0-3.0% and molybdenum (Mo): 0-3.0% Chromium (Cr) and molybdenum (Mo) are elements that not only inhibit austenite decomposition during alloying but also stabilize austenite, similar to manganese (Mn). Therefore, in the present invention, one or more of chromium (Cr) and molybdenum (Mo) can be added for these effects. However, if the chromium (Cr) and molybdenum (Mo) content exceeds a certain level, the bainite transformation time increases and the amount of carbon (C) enriched in the austenite becomes insufficient, making it impossible to ensure the desired fraction of retained austenite. Therefore, in the present invention, the chromium (Cr) and molybdenum (Mo) content can be limited to 3.0% or less, respectively.

[0038] Copper (Cu): 0-4.0% and Nickel (Ni): 0-4.0% or more Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. Copper (Cu) and nickel (Ni) also concentrate on the surface of the steel sheet, preventing hydrogen penetration into the steel sheet and inhibiting delayed hydrogen fracture. Therefore, in the present invention, one or more of copper (Cu) and nickel (Ni) can be added to achieve these effects. However, if the copper (Cu) and nickel (Ni) content exceeds a certain level, it can cause excessive property effects and increased manufacturing costs. Therefore, in the present invention, the copper (Cu) and nickel (Ni) content can be limited to 4.0% or less, respectively.

[0039] One or more of calcium (Ca): 0-0.05%, magnesium (Mg): 0-0.05%, and rare earth elements (REM) excluding yttrium (Y): 0-0.05% Here, rare earth elements (REM) refer to scandium (Sc), yttrium (Y), and lanthanum group elements. Rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) contribute to improving the ductility of steel sheets by spheroidizing sulfides. For this purpose, the present invention may add one or more of these elements. However, if the content of these elements exceeds a certain level, it may result in excessive property effects and increased manufacturing costs. Therefore, the present invention may limit the content of these elements to 0.05% or less.

[0040] One or more of tungsten (W): 0-0.5% and zirconium (Zr): 0-0.5% Tungsten (W) and zirconium (Zr) are elements that improve hardenability and increase the strength of steel sheets, so the present invention may add at least one of tungsten (W) and zirconium (Zr) for this purpose. However, if the tungsten (W) and zirconium (Zr) content exceeds a certain level, it may not only cause excessive property effects but also increase manufacturing costs, so the present invention may limit the tungsten (W) and zirconium (Zr) content to 0.5% or less, respectively.

[0041] One or more of antimony (Sb): 0-0.5% and tin (Sn): 0-0.5% Antimony (Sb) and tin (Sn) are elements that improve the plating wettability and plating adhesion of steel sheets, and the present invention may add at least one of antimony (Sb) and tin (Sn) to achieve this effect. However, if the antimony (Sb) and tin (Sn) contents exceed a certain level, the brittleness of the steel sheet increases, which may lead to cracks during hot working or cold working. Therefore, the present invention may limit the antimony (Sb) and tin (Sn) contents to 0.5% or less, respectively.

[0042] At least one of yttrium (Y): 0-0.2% and hafnium (Hf): 0-0.2% Yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of steel sheets, and the present invention may add at least one of yttrium (Y) and hafnium (Hf) for this effect. However, if the yttrium (Y) and hafnium (Hf) content exceeds a certain level, the ductility of the steel sheet may be deteriorated, so the present invention may limit the yttrium (Y) and hafnium (Hf) content to 0.2% or less, respectively.

[0043] Cobalt (Co): 0-1.5% Cobalt (Co) is an element that promotes bainite transformation and increases the TRIP effect, and therefore, in the present invention, cobalt (Co) can be added for this effect. However, if the cobalt (Co) content exceeds a certain level, the weldability and ductility of the steel sheet may deteriorate, so in the present invention, the cobalt (Co) content can be limited to 1.5% or less.

[0044] A high-strength steel sheet with excellent formability according to one aspect of the present invention may contain the remaining Fe and other inevitable impurities in addition to the above-mentioned components. However, since unintended impurities may be inevitably mixed in from raw materials or the surrounding environment during normal manufacturing processes, it is not possible to completely eliminate these impurities. These impurities are known to anyone skilled in the art, and therefore, not all of them will be specifically mentioned herein. Furthermore, the addition of additional effective components other than the above-mentioned components is not completely excluded.

[0045] A high-strength steel sheet with excellent workability according to one aspect of the present invention may include bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures as microstructures.

[0046] Both untempered martensite (fresh martensite, FM) and tempered martensite (tempered martensite, TM) are microstructures that improve the strength of steel sheets. However, compared to tempered martensite, fresh martensite has the characteristic of reducing the ductility and burring ability of steel sheets. Furthermore, compared to tempered martensite, fresh martensite tends to reduce the yield ratio of steel sheets. This is because the microstructure of tempered martensite is softened by tempering heat treatment. Therefore, the balance between tensile strength and elongation (TS) that is the objective of this invention is to 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2It is preferable to control the structure fraction of tempered martensite and fresh martensite to ensure a yield ratio rating index (1-YR) of 3.0*10. 6 The balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), 6.0*10 6 The balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 In order to satisfy the yield ratio evaluation index (1-YR) of 0.42 or less, it is preferable to limit the fraction of tempered martensite to 50% by volume or more and the fraction of fresh martensite to 10% by volume or more. A more preferable fraction of tempered martensite can be 52% by volume or more or 54% by volume or more, and a more preferable fraction of fresh martensite can be 12% by volume or more. On the other hand, if tempered martensite or fresh martensite is formed excessively, ductility and burring ability decrease, and ultimately, 6 The balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), 6.0*10 6 The balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and a yield ratio rating index (1-YR) of 0.42 or less cannot be simultaneously satisfied. Therefore, the present invention limits the fraction of tempered martensite to 70 vol% or less and the fraction of fresh martensite to 30 vol% or less. More preferably, the fraction of tempered martensite can be 68 vol% or less or 65 vol% or less, and more preferably, the fraction of fresh martensite can be 25 vol% or less.

[0047] The balance of tensile strength and elongation (TS) 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield ratio rating index (1-YR), optimization of the bainite fraction is necessary. 6The balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), 6.0*10 6 The balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 In order to ensure a tensile strength of 1000 MPa and a yield ratio rating index (1-YR) of 0.42 or less, it is preferable to control the bainite fraction to 10% by volume or more. A more preferable bainite fraction is 12% by volume or more or 14% by volume or more. On the other hand, if bainite is formed excessively, it will result in a decrease in the fraction of tempered martensite, which may affect the desired balance between tensile strength and elongation (TS). 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 In order to ensure a high yield ratio (1-YR), the bainite fraction can be limited to 30% by volume or less. The preferred bainite fraction can be 12% by volume or more, or 14% by volume or more, or 28% by volume or less, or 26% by volume or less.

[0048] Steel sheets containing retained austenite have excellent ductility and workability due to the transformation-induced plasticity that occurs when austenite transforms into martensite during processing. If the fraction of retained austenite is below a certain level, the balance between tensile strength and elongation (TS) is poor. 2 *EL 1 / 2 ) is 3.0*10 6 (MPa 2 % 1 / 2 ), which is not preferable. On the other hand, if the fraction of retained austenite exceeds a certain level, the local elongation may decrease or the spot weldability may deteriorate. Therefore, the present invention aims to achieve a good balance between tensile strength and elongation (TS 2 *EL 1 / 2 In order to obtain a steel sheet excellent in the above-mentioned properties, the fraction of retained austenite can be limited to the range of 2 to 10%. The preferred fraction of retained austenite is 3% by volume or more, or 9% by volume or less.

[0049] The steel sheet of the present invention may contain unavoidable structures such as ferrite, pearlite, and island martensite (Martensite Austenite Constituent, MA). Since excessive ferrite formation can reduce the strength of the steel sheet, the present invention limits the ferrite fraction to 5% by volume (including 0%) or less. Furthermore, since excessive pearlite formation can reduce the workability of the steel sheet or reduce the fraction of retained austenite, the present invention limits the formation of pearlite as much as possible.

[0050] A high-strength steel sheet with excellent formability according to one aspect of the present invention can satisfy the following [Relational Formula 1] and [Relational Formula 2]. [Equation 1] 0.03≦[B] FM / [B] TM ≦0.55 In the above relational expression 1, [B] FM is the boron (B) content (wt%) in fresh martensite, [B] TM is the boron (B) content (wt%) contained in the tempered martensite. [Equation 2] T(γ) / V(γ)≧0.08 In the above relational expression 2, T(γ) is the fraction (volume %) of tempered retained austenite in the steel sheet, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0051] The present invention achieves the desired balance between tensile strength and elongation (TS 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 In order to ensure the required tensile strength and yield ratio index (1-YR), the structural fractions of tempered martensite, fresh martensite, and retained austenite are controlled within a certain range, and the ratio of boron (B) content in the tempered martensite and fresh martensite is controlled within a certain range, thereby controlling the ratio of specific types of retained austenite to the total retained austenite within a certain range.

[0052] The present invention relates to the boron (B) content ([B]) contained in tempered martensite as shown in [Relationship 1]. TM The boron (B) content ([B] FM , weight %) ratio is controlled in the range of 0.03 to 0.55, 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) balance of tensile strength and elongation (B TE ), 6.0*10 6 ~11.5*10 6 (MPa 2 % 1 / 2 ) balance of tensile strength and hole expansion rate (B TH ) and yield ratio evaluation index (I YR ) can be secured simultaneously.

[0053] The inventors of the present invention have conducted in-depth research into methods for ensuring the physical properties of boron (B)-added TRIP steel. Although the theoretical basis has not been clearly identified, they have noted that the desired physical properties of the present invention can be ensured only when the ratio of the boron (B) content in fresh martensite to the boron (B) content in tempered martensite satisfies a certain range. In particular, they have confirmed that the yield ratio of steel sheets shows a certain tendency depending on the ratio of the boron (B) content in tempered martensite to the fresh martensite. Therefore, the present invention limits the ratio of the boron (B) content in fresh martensite to the boron (B) content in tempered martensite to the range of 0.03 to 0.55, as shown in [Relationship 1], thereby achieving the desired balance between tensile strength and elongation (TS 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield ratio evaluation index (1-YR).

[0054] Furthermore, the inventors of the present invention have found that not only the fraction of retained austenite but also the ratio of a specific type of retained austenite to the total retained austenite is an important factor in ensuring strength and workability.

[0055] The greater the proportion of tempered retained austenite in the retained austenite, the more advantageous it is for improving the workability of the steel sheet. Tempered retained austenite is retained austenite that is enriched by the inflow of carbon (C) during heat treatment at the bainite formation temperature, and refers to retained austenite with a carbon (C) content (wt%) that is 1.45 times or more the average carbon (C) content (wt%) of the steel sheet. Tempered retained austenite is relatively enriched in carbon (C), an austenite stabilizing element, which inhibits transformation to martensite. When the proportion of tempered retained austenite is above a certain level, the workability of the steel sheet can be more effectively ensured.

[0056] In the present invention, the fraction (volume%) of tempered retained austenite relative to the fraction (V(γ), volume%) of the total retained austenite contained in the steel sheet is limited to 0.08 or more as shown in [Relationship 2], and therefore, the desired balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) can be effectively ensured.

[0057] The high-strength steel sheet excellent in formability according to one aspect of the present invention has a balance between tensile strength and elongation (B TE ) is 3.0*10 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 ~11.5*10 6 (MPa 2 % 1 / 2 ) and the yield ratio evaluation index (I YR) can satisfy 0.15 to 0.42. [Equation 3] B TE = [tensile strength (TS, MPa)] 2 *[Elongation rate (El, %)] 1 / 2 [Equation 4] B TH = [tensile strength (TS, MPa)] 2 *[Hole expansion rate (HER, %)] 1 / 2 [Equation 5] I YR =1-[yield ratio (YR)]

[0058] An example of a method for producing the steel sheet of the present invention will be described in detail below.

[0059] According to one aspect of the present invention, a method for manufacturing a high-strength steel sheet includes the steps of: heating a cold-rolled steel sheet having a predetermined alloy composition to 700°C at an average heating rate of 5°C / s or more (primary heating); heating the steel sheet to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / s or less (secondary heating); and maintaining the temperature for 50 to 1200 seconds (primary maintenance); cooling the steel sheet after the primary maintenance to a temperature range of 200 to 400°C at an average cooling rate of 2 to 100°C / s (primary cooling); and cooling the steel sheet after the primary cooling. The method may include the steps of heating the steel sheet to a temperature range of 400 to 600°C at an average heating rate of 5 to 100°C / s (tertiary heating) and then maintaining that temperature for 10 to 1800 seconds (secondary maintenance); cooling the steel sheet after the secondary maintenance to a temperature range of 300 to 500°C at an average cooling rate of 1 to 100°C / s (secondary cooling) and then maintaining that temperature for 10 to 1800 seconds (tertiary maintenance); and cooling the steel sheet after the tertiary maintenance to room temperature at an average cooling rate of 1°C / s or more (tertiary cooling).

[0060] The cold-rolled steel sheet can be provided through the steps of heating a steel slab having a predetermined alloy composition to 1000 to 1350°C; finish hot rolling at a temperature range of 800 to 1000°C; coiling the hot-rolled steel sheet at a temperature range of 350 to 650°C; pickling the coiled steel sheet; and cold-rolling the pickled steel sheet at a reduction ratio of 30 to 90%.

[0061] Steel slab preparation and heating A steel slab having a predetermined alloy composition is prepared. Since the steel slab of the present invention has an alloy composition corresponding to the alloy composition of the steel plate described above, the description of the alloy composition of the steel slab replaces the description of the alloy composition of the steel plate described above.

[0062] The prepared steel slab can be heated within a certain temperature range, and the heating temperature of the steel slab can be in the range of 1000 to 1350°C. If the heating temperature of the steel slab is less than 1000°C, there is a risk that the steel slab will be hot rolled in a temperature range below the temperature range of the intended finish hot rolling, and if the heating temperature of the steel slab exceeds 1350°C, there is a risk that the steel will reach its melting point and melt.

[0063] Hot rolling and coiling The heated steel slab can be hot-rolled to provide a hot-rolled steel sheet. The finish hot-rolling temperature during hot-rolling is preferably in the range of 800 to 1000°C. If the finish hot-rolling temperature is less than 800°C, excessive rolling load may become a problem, and if the finish hot-rolling temperature exceeds 1000°C, the crystal grains of the hot-rolled steel sheet may become coarse, which may cause a deterioration in the physical properties of the final steel sheet.

[0064] After hot rolling, the hot-rolled steel sheet can be cooled at an average cooling rate of 10°C / s or more and coiled at a temperature range of 350 to 650°C. If the coiling temperature is less than 350°C, coiling is difficult, and if the coiling temperature exceeds 650°C, surface scale may form inside the hot-rolled steel sheet, making pickling difficult.

[0065] Pickling and cold rolling After the wound hot-rolled coil is uncoiled, the steel sheet may be pickled to remove scale formed on the surface, followed by cold rolling. While the conditions for pickling and cold rolling are not particularly limited in the present invention, it is preferable that cold rolling be performed at a cumulative reduction of 30 to 90%. If the cumulative reduction of cold rolling exceeds 90%, it may be difficult to perform cold rolling in a short time due to the high strength of the steel sheet.

[0066] The cold-rolled steel sheet may be subjected to an annealing heat treatment process to produce an uncoated cold-rolled steel sheet, or may be subjected to a coating process to provide corrosion resistance to produce a coated steel sheet. Coating methods such as hot-dip galvanizing, electrogalvanizing, and hot-dip aluminum coating may be applied, and the method and type are not particularly limited.

[0067] Annealing heat treatment In the present invention, an annealing heat treatment step is carried out in order to simultaneously ensure the strength and workability of the steel sheet.

[0068] The cold-rolled steel sheet is heated to 700°C at an average heating rate of 5°C / s or more (primary heating), then heated to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / s or less (secondary heating), and then maintained at this temperature for 50 to 1200 seconds (primary maintenance).

[0069] If the average heating rate of the first heating to 700°C is less than 5°C / s, massive austenite will form from the ferrite and cementite generated during heating, and as a result, it will be impossible to form fine tempered martensite and retained austenite as the final structure. This will result in the desired balance of T(γ) / V(γ), tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2) cannot be realized. In addition, if the secondary heating rate to the primary holding temperature exceeds 5°C / s, the transformation of cementite generated during heating to austenite is accelerated, forming a large amount of blocky austenite, resulting in coarsening of the final structure and insufficient enrichment of boron (B) in the tempered martensite. This results in [B] FM / [B] TM exceeds 0.55, achieving the desired balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield ratio evaluation index (I YR ) cannot be realized.

[0070] If the primary temperature is below Ac3 (two-phase region), 5% or more by volume of ferrite is formed, and the balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease. In addition, if the first holding time is less than 50 seconds, the structure may not be sufficiently homogenized, which may result in a deterioration in the physical properties of the steel sheet. There are no particular limitations on the upper limits of the first holding temperature and the first holding time, but in order to prevent a decrease in toughness due to grain coarsening, it is preferable to limit the first holding temperature to 920°C or less and the first holding time to 1200 seconds or less.

[0071] After the first hold, the steel can be cooled to the first cooling stop temperature of 200-400°C at an average cooling rate of 2°C / s or more (first cooling). If the average cooling rate for first cooling is less than 2°C / s, the fraction of retained austenite will be insufficient due to slow cooling, and accordingly, the T(γ) / V(γ) and balance of tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2) may decrease. There is no particular need to specify the upper limit of the average cooling rate for the primary cooling, but it is preferable to set it to 100°C / s or less. If the primary cooling stop temperature is less than 200°C, tempered martensite will be formed excessively and retained austenite will be insufficient, resulting in an insufficient balance between T(γ) / V(γ) and tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 On the other hand, if the primary cooling stop temperature exceeds 400°C, excessive bainite is formed and tempered martensite is insufficient, which may result in an imbalance between the tensile strength and elongation rate (TS) of the steel sheet. 2 *EL 1 / 2 ), and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease.

[0072] After the first cooling, the steel sheet can be heated to a temperature range of 400 to 600°C at an average heating rate of 5°C / s or more (third heating), and then maintained for 10 to 1800 seconds (second maintenance). There is no particular upper limit to the average heating rate of the third heating, but it is preferably 100°C / s or less. If the second maintenance temperature is less than 400°C, the balance between the tensile strength and hole expansion rate (TS) of the steel sheet can be improved by using a low heat treatment temperature. 2 *HER 1 / 2 If the secondary maintenance temperature exceeds 600°C, the fraction of retained austenite will be insufficient, resulting in a decrease in T(γ) / V(γ) and the balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 If the secondary holding time is less than 10 seconds, the heat treatment time will be insufficient and the balance between the tensile strength and hole expansion rate (TS 2 *HER 1 / 2 There is no particular need to specify an upper limit for the secondary maintenance time, but it is preferable to set it to 1800 seconds or less.

[0073] After the second hold, the material can be cooled to a temperature range of 300 to 500°C at an average cooling rate of 1°C / s or more (secondary cooling), and then maintained at that temperature for 10 to 1800 seconds (third hold). There is no need to specify an upper limit for the average cooling rate of the secondary cooling, but it is preferable to set it to 100°C / s or less. If the third hold temperature is less than 300°C, the balance between tensile strength and hole expansion rate (TS) can be improved by using a low heat treatment temperature. 2 *HER 1 / 2 On the other hand, if the tertiary maintenance temperature exceeds 500°C, the fraction of retained austenite will be insufficient, resulting in a decrease in T(γ) / V(γ) and the balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 If the third holding time is less than 10 seconds, the heat treatment time will be insufficient and the balance between the tensile strength and hole expansion rate (TS 2 *HER 1 / 2 There is no need to specify an upper limit for the tertiary maintenance time, but it is preferable to set it to 1800 seconds or less.

[0074] The cooling rate Vc1 of the primary cooling and the cooling rate Vc2 of the secondary cooling can satisfy the relationship Vc1>Vc2.

[0075] After the third maintenance, it can be cooled to room temperature (third cooling) at an average cooling rate of 1°C / s or more.

[0076] The high-strength steel plate with excellent workability manufactured by the above-mentioned manufacturing method may contain, as a microstructure, bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures, and as a preferred example, may contain, by volume fraction, 10 to 30% bainite, 50 to 70% tempered martensite, 10 to 30% fresh martensite, 2 to 10% retained austenite, and 5% or less (including 0%) ferrite.

[0077] The steel sheet manufactured by the above-mentioned manufacturing method has a balance between tensile strength and elongation (BTE ) is 3.0*10 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 ~11.5*10 6 (MPa 2 % 1 / 2 ) and the yield ratio evaluation index (I YR ) can satisfy 0.15 to 0.42. [Equation 3] B TE = [tensile strength (TS, MPa)] 2 *[Elongation rate (El, %)] 1 / 2 [Equation 4] B TH = [tensile strength (TS, MPa)] 2 *[Hole expansion rate (HER, %)] 1 / 2 [Equation 5] I YR =1-[yield ratio (YR)] [Example]

[0078] Hereinafter, a high-strength steel sheet with excellent formability and a manufacturing method thereof according to one aspect of the present invention will be described in more detail with reference to specific examples. It should be noted that the following examples are provided to aid in understanding the present invention and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters set forth in the claims and matters that can be reasonably inferred therefrom.

[0079] (Example) A 100 mm thick steel slab having the alloy composition shown in Table 1 below (the remainder being Fe and unavoidable impurities) was manufactured, heated to 1200°C, and then finish hot rolled at 900°C. It was then cooled at an average cooling rate of 30°C / s and coiled at the coiling temperatures shown in Tables 2 and 3 to manufacture a 3 mm thick hot-rolled steel sheet. It was then pickled to remove surface scale and cold rolled to a thickness of 1.5 mm.

[0080] Thereafter, the steel sheets were manufactured by performing heat treatment under the annealing heat treatment conditions shown in the following Tables 2 to 5. In the following Tables 2 and 3, the single-phase region means a temperature range of Ac3 to 920°C, and the two-phase region means a temperature range below Ac3°C.

[0081] The microstructures of the steel sheets manufactured in this manner were observed, and the results are shown in Tables 6 and 7. Ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM), and pearlite (P) were observed via SEM after nital etching of the polished cross-sections of the specimens. After nital etching, the specimens were classified as ferrite if their surfaces were smooth, and as pearlite if they had a lamellar structure of cementite and ferrite. Because bainite (B) and tempered martensite (TM) were observed in lath and block forms and difficult to distinguish, the fractions of bainite and tempered martensite were calculated using a dilatation curve after dilatation evaluation. The fraction of bainite was determined by subtracting the fraction of tempered martensite calculated using a dilatation curve from the fractions of bainite and tempered martensite measured by SEM observation. On the other hand, since it is not easy to distinguish between fresh martensite (FM) and retained austenite (retained γ), the fraction of fresh martensite was determined by subtracting the fraction of retained austenite calculated by X-ray diffraction from the fraction of martensite and retained austenite observed by SEM described above.

[0082] On the other hand, [B] of steel plate FM / [B] TM , T(γ) / V(γ), balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield ratio evaluation index (I YR ) were measured and evaluated, and the results are shown in Tables 8 and 9.

[0083] Boron (B) content in fresh martensite ([B]FM ) and the boron (B) content in tempered martensite ([B] TM The annealed martensite was determined based on the boron (B) concentration measured in the fresh martensite and tempered martensite using an Electron Probe MicroAnalyser (EPMA). The tempered retained austenite was classified based on the carbon (C) content measured in the retained austenite using an EPMA.

[0084] Tensile strength (TS) and elongation (El) were evaluated by tensile testing, using test pieces prepared according to JIS No. 5, with the 90° angle to the rolling direction of the rolled plate material as the reference. Hole expansion ratio (HER) was evaluated by a hole expansion test, where a 10 mm Ψ punch hole (die inner diameter 10.3 mm, clearance 12.5%) was formed, and a conical punch with an apex angle of 60° was inserted into the punch hole with the burr facing outward. The periphery of the punch hole was compressed and expanded at a moving speed of 20 mm / min, and then the HER was calculated using the following [Relationship 6]. [Equation 6] Hole expansion rate (HER, %) = {(D-D0) / D0} × 100 In the above relational expression 6, D means the hole diameter (mm) when the crack penetrates the steel plate along the thickness direction, and D0 means the initial hole diameter (mm).

[0085] [Table 1]

[0086] [Table 2]

[0087] [Table 3]

[0088] [Table 4]

[0089] [Table 5]

[0090] [Table 6]

[0091] [Table 7]

[0092] [Table 8]

[0093] [Table 9]

[0094] As shown in Tables 1 to 9 above, in the case of test specimens that satisfy the conditions presented in the present invention, both [Relationship 1] and [Relationship 2] are satisfied, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6 ~6.2*10 6 (MPa 2 % 1 / 2 ) and balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 ~11.5*10 6 (MPa 2 % 1 / 2 ) and the yield ratio evaluation index (I YR ) satisfies 0.15 to 0.42.

[0095] For specimen 2, the primary average heating rate was less than 5°C / s, resulting in insufficient tempered martensite and retained austenite. As a result, specimen 2 had a T(γ) / V(γ) ratio of less than 0.08 and a poor balance between tensile strength and elongation (B TE ) is 3.0*106 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0096] For specimen 3, the secondary average heating rate was higher than 5°C / s, and massive austenite was formed, preventing boron (B) from concentrating in the tempered martensite. As a result, specimen 3 was [B] FM / [B] TM exceeds 0.55, and the yield ratio evaluation index (I YR ) exceeds 0.42, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0097] Specimen 4 was tested in the two-phase region where the primary temperature was less than Ac3, and the ferrite fraction was exceeded. As a result, Specimen 4 had a poor balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0098] For specimen 5, the primary average cooling rate was less than 2°C / s, resulting in an insufficient fraction of retained austenite. As a result, specimen 5 had a T(γ) / V(γ) ratio of less than 0.08 and a poor balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0099] For specimen 6, the primary cooling stop temperature was less than 200°C, resulting in an excess of tempered martensite and an insufficient fraction of retained austenite. As a result, specimen 6 had a T(γ) / V(γ) ratio of less than 0.08 and a poor balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH) is 6.0*10 6 It was less than.

[0100] For specimen 7, the primary cooling stop temperature was over 400°C, resulting in an excess of bainite and an insufficient tempered martensite fraction. As a result, specimen 7 had a poor balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0101] For test piece 8, the secondary temperature was below 400°C, which meant that the heat treatment temperature was insufficient. As a result, test piece 8 had a poor balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0102] For specimen 9, the third temperature was higher than 600°C, resulting in an insufficient fraction of retained austenite. As a result, specimen 9 had a T(γ) / V(γ) ratio of less than 0.08 and a poor balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0103] The second holding time for test piece 10 was less than 10 seconds, which meant that the heat treatment time was insufficient. As a result, the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0104] For specimen 11, the third temperature was maintained at less than 300°C, which resulted in an insufficient heat treatment temperature. As a result, specimen 11 had a poor balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0105] For specimen 12, the third maintenance temperature was higher than 500°C, resulting in an insufficient fraction of retained austenite. As a result, specimen 12 had a T(γ) / V(γ) ratio of less than 0.08 and a poor balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0106] Test piece 13 was subjected to a third heat treatment with a holding time of less than 10 seconds, resulting in an insufficient heat treatment time. As a result, test piece 13 had a poor balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0107] Test piece 35 has a low carbon (C) content and a good balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0108] Specimen 36 had a high carbon (C) content, resulting in a shortage of tempered martensite and an excess of fresh martensite. As a result, specimen 36 had a poor balance of tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0109] Specimen 37 had a low silicon (Si) content, resulting in an insufficient fraction of retained austenite. As a result, the T(γ) / V(γ) ratio of specimen 37 was less than 0.08, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0110] Specimen 38 had a high silicon (Si) content, resulting in an excess fraction of fresh martensite. As a result, specimen 38 had a poor balance of tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0111] Specimen 39 had a high aluminum (Al) content, which resulted in an excess fraction of fresh martensite. As a result, specimen 39 had a poor balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0112] The manganese (Mn) content of the test piece 40 was low, and the fraction of retained austenite was insufficient due to the formation of pearlite. As a result, the T(γ) / V(γ) ratio of the test piece 40 was less than 0.08, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0113] Specimen 41 had a high manganese (Mn) content, which resulted in an excess fraction of fresh martensite. As a result, specimen 41 had a poor balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0114] Specimen 42 had a high chromium (Cr) content, resulting in an excess fraction of fresh martensite. As a result, specimen 42 had a poor balance of tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0115] Specimen 43 had a high molybdenum (Mo) content, which resulted in an excess fraction of fresh martensite. As a result, specimen 43 had a poor balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0116] The boron (B) content of the test piece 44 was low, and boron (B) could not be concentrated in the tempered martensite. As a result, the test piece 44 was [B] FM / [B] TM exceeds 0.55, and the yield ratio evaluation index (I YR ) exceeded 0.42.

[0117] Specimen 45 had a high boron (B) content, and boron (B) was excessively concentrated in the tempered martensite. As a result, specimen 45 had a [B] FM / [B] TM is less than 0.03, and the yield ratio evaluation index (I YR ) was less than 0.15.

[0118] Although the present invention has been described in detail with reference to the above embodiments, other embodiments are possible, and the technical spirit and scope of the claims set forth below should not be limited to the embodiments.

Claims

1. The alloy contains, by weight, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, and B: 0.0005 to 0.005%, with the remainder being Fe and inevitable impurities; The microstructure includes bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures. The microstructure contains, by volume fraction, at least one of bainite and tempered martensite in an amount of 60% or more, 10 to 30% fresh martensite, and 2 to 10% retained austenite; The following [Relationship 1] and [Relationship 2] are satisfied, A high-strength steel plate with excellent workability, in which the balance between tensile strength and hole expansion rate (B TH ) represented by the following [Relational Formula 4] satisfies 6.0*10 6 to 11.5*10 6 (MPa 2 % 1 / 2 ). [Relationship 1] 0.03≦[B] ＦＭ / [B] ＴＭ ≦0.55 In the above-mentioned relational formula 1, [B] ＦＭ is the boron (B) content (wt%) contained in fresh martensite, [B] ＴＭ is the boron (B) content (wt%) contained in the tempered martensite. [Relationship 2] T(γ) / V(γ)≧0.08 In the above-mentioned relational expression 2, T(γ) is the fraction (volume %) of tempered retained austenite in the steel plate, and V(γ) is the fraction (volume %) of retained austenite in the steel plate. [Relationship 4] B TH = [tensile strength (TS, MPa)] 2 * [hole expansion ratio (HER, %)] 1 / 2

2. The high-strength steel plate with excellent workability according to claim 1, wherein the steel plate further contains, in weight percent, any one or more of the following (1) to (8): (1) One or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5% (2) One or more of Cr: 0 to 3.0% and Mo: 0 to 3.0% (3) One or more of Cu: 0 to 4.0% and Ni: 0 to 4.0% (4) One or more of Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, and Mg: 0 to 0.05% (5) One or more of W: 0 to 0.5% and Zr: 0 to 0.5% (6) One or more of Sb: 0 to 0.5% and Sn: 0 to 0.5% (7) One or more of Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co: 0-1.5%

3. 2. The high-strength steel plate with excellent workability according to claim 1, wherein the microstructure of the steel plate contains, by volume fraction, 10 to 30% bainite, 50 to 70% tempered martensite, and 5% or less (including 0%) ferrite.

4. The steel sheet has a balance between tensile strength and elongation (B ＴＥ ) is 3.0*10 ６ ~6.2*10 ６ (MPa ２ % １／２ ) and the yield ratio evaluation index (I) is expressed by the following [Relationship 5]. ＹＲ 2. The high-strength steel plate having excellent formability according to claim 1, wherein σ is 0.15 to 0.

42. [Relationship 3] B ＴＥ = [tensile strength (TS, MPa)] ２ * [Elongation rate (El,%)] 1 / 2 [Relationship 5] I ＹＲ =1-[yield reduction ratio (YR)]

5. a step of cold rolling a steel slab containing, by weight, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, and B: 0.0005 to 0.005%, with the remainder being Fe and unavoidable impurities; heating the cold-rolled steel sheet to 700°C at an average heating rate of 5°C / s or more (primary heating), heating to a temperature range of Ac3 to 920°C at an average heating rate of 5°C / s or less (secondary heating), and maintaining the temperature for 50 to 1200 seconds (primary maintenance); cooling the steel sheet (primary cooling) at an average cooling rate of 2 to 100°C / s to a temperature range of 200 to 400°C; heating the primarily cooled steel sheet to a temperature range of 400 to 600°C at an average heating rate of 5 to 100°C / s (third heating) and maintaining the temperature for 10 to 1800 seconds (second maintenance); cooling the second-maintained steel sheet to a temperature range of 300 to 500°C at an average cooling rate of 1 to 100°C / s (second cooling), and then maintaining the temperature for 10 to 1800 seconds (third maintenance); and 5. The method for manufacturing a high-strength steel plate with excellent workability according to claim 1, further comprising a step of cooling the tertiary-maintained steel plate to room temperature at an average cooling rate of 1°C / s or more (tertiary cooling).

6. The method for producing a high-strength steel plate with excellent workability according to claim 5, wherein the steel slab further contains any one or more of the following (1) to (8): (1) One or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5% (2) One or more of Cr: 0 to 3.0% and Mo: 0 to 3.0% (3) One or more of Cu: 0 to 4.0% and Ni: 0 to 4.0% (4) One or more of Ca: 0 to 0.05%, REM excluding Y: 0 to 0.05%, and Mg: 0 to 0.05% (5) One or more of W: 0 to 0.5% and Zr: 0 to 0.5% (6) One or more of Sb: 0 to 0.5% and Sn: 0 to 0.5% (7) One or more of Y: 0 to 0.2% and Hf: 0 to 0.2% (8) Co: 0-1.5%

7. The step of cold rolling the steel slab comprises: heating the steel slab to 1000-1350°C; Finish hot rolling in the temperature range of 800 to 1000°C; Coiling the hot-rolled steel sheet at a temperature in the range of 350 to 650°C; pickling the coiled steel sheet; and The method for producing a high-strength steel sheet with excellent workability according to claim 5, further comprising: cold rolling the pickled steel sheet at a reduction ratio of 30 to 90%.

8. 6. The method for producing a high strength steel plate having excellent formability according to claim 5, wherein the cooling rate Vc1 of the primary cooling and the cooling rate Vc2 of the secondary cooling satisfy the relationship Vc1 > Vc2.

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