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 producing the same. [Background technology]

[0002] Recently, the automobile industry has been focusing on methods for reducing the weight of materials in order to protect the global environment while at the same time ensuring the stability of passengers. In order to meet the demand for stability and weight reduction, the use of high-strength steel sheets has been rapidly increasing. It is generally known that the higher the strength of a steel sheet, the lower the workability of the steel sheet. Therefore, there is a demand for steel sheets for automobile parts that have high strength properties while also having excellent workability, such as ductility and hole expandability.

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

[0004] As a technique for further improving the workability of steel sheets, a method of utilizing tempered martensite is disclosed in Patent Documents 1 and 2. Since tempered martensite made by tempering hard martensite is softened martensite, there is a difference in strength between tempered martensite and conventional untempered martensite (fresh martensite). Therefore, if tempered martensite is formed by suppressing fresh martensite, the 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 obtain a steel plate with excellent strength and ductility.

[0006] Meanwhile, as another technique for improving the workability of steel sheet, 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 it is possible to achieve both strength and workability.

[0007] However, in the technology disclosed in Patent Document 3, 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 the yield ratio evaluation index (I YR ), which means that it is difficult to obtain 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 meet the requirements for superior steel sheets. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Korean Patent Publication No. 10-2006-0118602 [Patent Document 2] JP 2009-019258 A [Patent Document 3] JP 2016-216808 A 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 rating index by optimizing the composition and microstructure of the steel sheet, and a method for manufacturing the same may be provided.

[0011] The object of the present invention is not limited to the above. Further object of the present invention is described in the entire 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 contents of the specification of the present invention. [Means for solving the problem]

[0012] A high-strength steel plate with excellent workability according to one embodiment of the present invention contains, by weight, C: 0.1-0.25%, Si: 0.01-1.5%, Mn: 1.0-4.0%, Al: 0.01-1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005-0.005%, with the remainder being Fe and unavoidable impurities, and contains, as a microstructure, bainite, tempered martensite, fresh martensite, retained austenite and other unavoidable structures, and can satisfy the following [Relational Formulas 1] to [Relational Formula 4]. [Equation 1] 0.03≦[B] FM / [B] TM ≦0.55 In the above formula 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] V(1.2μm,γ) / V(γ)≧0.12 In the above Relational Formula 2, V(1.2 μm, γ) is the fraction (volume %) of retained austenite having an average crystal grain size of 1.2 μm or more, and V(γ) is the fraction (volume %) of retained austenite in the steel plate. [Equation 3] V(lath, γ) / V(γ)≧0.5 In the above Relational Expression 3, V(lath, γ) is the fraction (volume %) of the retained austenite in the lath form, and V(γ) is the fraction (volume %) of the retained austenite in the steel plate. [Equation 4] T(γ) / V(γ)≧0.08 In the above Relation 4, 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.

[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 the following: 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 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.

[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 5] B TE =[Tensile strength (TS, MPa)] 2 *[Elongation rate (El,%)] 1 / 2 [Equation 6] B TH =[Tensile strength (TS, MPa)] 2 *[Hole expansion rate (HER,%)] 1 / 2 [Equation 7] I YR =1-[yield ratio (YR)]

[0016] According to one aspect of the present invention, there is provided a method for producing a high strength steel sheet having excellent workability, comprising the steps of: providing a cold-rolled steel sheet containing, by weight percent, C: 0.1-0.25%, Si: 0.01-1.5%, Mn: 1.0-4.0%, Al: 0.01-1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005-0.005%, with the remainder being Fe and unavoidable impurities; heating the 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 after the primary maintenance to a temperature range of 400 to 600°C at an average cooling rate of 2 to 100°C / s (primary cooling), and maintaining the temperature for 5 to 600 seconds (secondary maintenance). cooling the steel plate after the second maintenance to a temperature range of 300-500°C at an average cooling rate of 1-100°C / s (second cooling), and then maintaining the temperature for 5-600 seconds (third maintenance); cooling the steel plate after the third maintenance to a temperature range of 200-400°C at an average cooling rate of 2-100°C / s (third cooling); heating the steel plate after the third cooling to a temperature range of 400-600°C at an average heating rate of 5-100°C / s (third heating), and then maintaining the temperature for 10-1800 seconds (fourth maintenance); cooling the steel plate after the fourth maintenance to a temperature range of 300-500°C at an average cooling rate of 1-100°C / s (fourth cooling), and then maintaining the temperature for 10-1800 seconds (fifth maintenance); and cooling the steel plate after the fifth maintenance to room temperature at an average cooling rate of 1°C / s or more (fifth 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 the following: 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 by the steps of heating a steel slab to 1000-1350°C; finish hot rolling at a temperature range of 800-1000°C; coiling the hot rolled steel sheet at a temperature range of 350-650°C; pickling the coiled steel sheet; and cold rolling the pickled steel sheet at a reduction ratio of 30-90%.

[0019] The cooling rate Vc1 of the primary cooling and the cooling rate Vc4 of the fourth cooling can satisfy the relationship Vc1>Vc4. Effect of the Invention

[0020] According to a preferred embodiment of the present invention, 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 can be provided. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The present invention relates to a high-strength steel plate having excellent workability and a manufacturing method thereof, and 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 in detail to those skilled in the art to which the present invention pertains.

[0022] The inventors of the present invention have come to realize that in a boron (B)-added transformation induced plasticity (TRIP) steel including bainite, tempered martensite, fresh martensite, and retained austenite, when the structural fractions of tempered martensite, fresh martensite, and retained austenite are controlled within a certain range, the boron (B) content contained in the tempered martensite and fresh martensite is controlled within a certain range, and the shape and size of the retained austenite are controlled within a certain range, it is possible to simultaneously secure a balance between excellent tensile strength and ductility, a balance between excellent tensile strength and hole expandability, and an excellent yield ratio evaluation index. After clarifying this, the inventors have devised a method for effectively achieving excellent strength, yield ratio, ductility, and hole expandability, and have arrived at the present invention.

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

[0024] A high-strength steel plate with excellent workability according to one embodiment of the present invention contains, by weight, C: 0.1-0.25%, Si: 0.01-1.5%, Mn: 1.0-4.0%, Al: 0.01-1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005-0.005%, with the remainder being Fe and unavoidable impurities, and contains, as a microstructure, bainite, tempered martensite, fresh martensite, retained austenite and other unavoidable structures, and can satisfy the following [Relational Formulas 1] to [Relational Formula 4]. [Equation 1] 0.03≦[B] FM / [B] TM ≦0.55 In the above formula 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] V(1.2μm,γ) / V(γ)≧0.12 In the above Relational Formula 2, V(1.2 μm, γ) is the fraction (volume %) of retained austenite having an average crystal grain size of 1.2 μm or more, and V(γ) is the fraction (volume %) of retained austenite in the steel plate. [Equation 3] V(lath, γ) / V(γ)≧0.5 In the above Relational Expression 3, V(lath, γ) is the fraction (volume %) of the retained austenite in the lath form, and V(γ) is the fraction (volume %) of the retained austenite in the steel plate. [Equation 4] T(γ) / V(γ)≧0.08 In the above Relation 4, 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.

[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 plate with excellent workability according to one embodiment of the present invention contains, by weight, C: 0.1-0.25%, Si: 0.01-1.5%, Mn: 1.0-4.0%, Al: 0.01-1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005-0.005%, with the balance being Fe and unavoidable impurities. In addition, one or more of the following may be further included: Ti: 0.5% or less (including 0%), Nb: 0.5% or less (including 0%), V: 0.5% or less (including 0%), Cr: 3.0% or less (including 0%), Mo: 3.0% or less (including 0%), Cu: 4.0% or less (including 0%), Ni: 4.0% or less (including 0%), Ca: 0.05% or less (including 0%), REM excluding Y: 0.05% or less (including 0%), Mg: 0.05% or less (including 0%), W: 0.5% or less (including 0%), Zr: 0.5% or less (including 0%), Sb: 0.5% or less (including 0%), Sn: 0.5% or less (including 0%), Y: 0.2% or less (including 0%), Hf: 0.2% or less (including 0%), and Co: 1.5% or less (including 0%).

[0027] Carbon (C): 0.1~0.25% Carbon (C) is an essential element for ensuring the strength of a steel plate, and also an element for stabilizing the retained austenite that contributes to improving the ductility of the steel plate. Therefore, the present invention may contain 0.1% or more of carbon (C) to achieve such an effect. A preferred carbon (C) content may be more 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, the ductility may decrease due to an excessive increase in strength, and the weldability may deteriorate. 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 the carbon (C) content may be 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 also improves workability by homogenizing the structure. Silicon (Si) is also an element that inhibits the precipitation of cementite and contributes to the generation of retained austenite. Therefore, in the present invention, 0.01% or more of silicon (Si) can be added to achieve this effect. A preferred silicon (Si) content can be 0.02% or more, and a more preferred silicon (Si) content can be 0.04% or more. However, if the silicon (Si) content exceeds a certain level, it may not only cause plating defect problems such as non-coating during the plating process, but also reduce the weldability of the steel sheet, so the present invention may limit the upper limit of the silicon (Si) content to 1.5%. A preferred upper limit of the silicon (Si) content can be 1.48%, and a more preferred upper limit of the silicon (Si) content can be 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, in order to achieve such an effect, 1.0% or more of manganese (Mn) can be added. A preferable lower limit of the manganese (Mn) content can be 1.2%, and a more preferable lower limit of the manganese (Mn) content can be 1.4%. On the other hand, when manganese (Mn) is added excessively, there is a problem that the bainite transformation time increases and the carbon (C) concentration in the austenite becomes insufficient, so that the desired austenite fraction cannot be secured. Therefore, in the present invention, the upper limit of the manganese (Mn) content can be limited to 4.0%. A preferable 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 perform deoxidation. In addition, aluminum (Al) is also an element that inhibits cementite precipitation and stabilizes retained austenite, similar to silicon (Si). Therefore, in order to achieve such an effect, the present invention can add aluminum (Al) of 0.01% or more. A preferred aluminum (Al) content can be 0.03% or more, and a more preferred aluminum (Al) content can be 0.05% or more. On the other hand, if aluminum (Al) is added excessively, not only the inclusions in the steel sheet can increase, but also the workability of the steel sheet can be reduced, so the present invention can limit the upper limit of the aluminum (Al) content to 1.5%. A preferred upper limit of the aluminum (Al) content can be 1.48%.

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

[0032] Sulfur (S): 0.03% or less (including 0%) Sulfur (S) is an element that is contained as an impurity and forms MnS in a steel sheet, which deteriorates ductility, and therefore 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 nucleation of grain boundaries. In addition, the present invention requires the addition of boron (B) in order to simultaneously secure a good balance between tensile strength and elongation, a good balance between tensile strength and hole expandability, and a good yield ratio evaluation index by enriching boron (B) in tempered martensite. Therefore, the present invention may add 0.0005% or more of boron (B) to achieve these effects. However, if boron (B) is added in excess of a certain level, it may not only cause excessive property effects but also cause an increase in manufacturing costs, so the present invention may limit the upper limit of the boron (B) content to 0.005%.

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

[0036] 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, so the present invention may add at least one of titanium (Ti), niobium (Nb) and vanadium (V) for this effect. However, if the content of titanium (Ti), niobium (Nb) and vanadium (V) exceeds a certain level, excessive precipitates are formed, which not only reduces impact toughness but also causes an increase in manufacturing costs, so the present invention may limit the content of titanium (Ti), niobium (Nb) and vanadium (V) to 0.5% or less, respectively.

[0037] Chromium (Cr): 0-3.0% and Molybdenum (Mo): 0-3.0% Chromium (Cr) and molybdenum (Mo) are elements that not only suppress austenite decomposition during alloying treatment, but also stabilize austenite like manganese (Mn), so the present invention may add at least one of chromium (Cr) and molybdenum (Mo) for this effect. However, if the chromium (Cr) and molybdenum (Mo) contents exceed 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, the present invention may limit the chromium (Cr) and molybdenum (Mo) contents to 3.0% or less, respectively.

[0038] At least one of copper (Cu): 0-4.0% and nickel (Ni): 0-4.0% Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. Copper (Cu) and nickel (Ni) are also elements that concentrate on the surface of the steel sheet, preventing hydrogen from penetrating 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 for this effect. However, if the copper (Cu) and nickel (Ni) contents exceed a certain level, it not only causes excessive property effects but also increases manufacturing costs, so in the present invention, the copper (Cu) and nickel (Ni) contents can be limited to 4.0% or less, respectively.

[0039] One or more of the following: 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) are elements that contribute to improving the ductility of steel sheets by spheroidizing sulfides, so the present invention may add at least one of rare earth elements (REM) other than calcium (Ca), magnesium (Mg) and yttrium (Y) for this effect. However, if the content of rare earth elements (REM) other than calcium (Ca), magnesium (Mg) and yttrium (Y) exceeds a certain level, it not only causes excessive property effects but also increases manufacturing costs, so the present invention may limit the content of rare earth elements (REM) other than calcium (Ca), magnesium (Mg) and yttrium (Y) to 0.05% or less.

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

[0041] 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, so in the present invention, one or more of antimony (Sb) and tin (Sn) can be added to achieve this effect. However, if the antimony (Sb) and tin (Sn) contents exceed a certain level, the brittleness of the steel sheet increases, and cracks may occur during hot working or cold working, so in the present invention, the antimony (Sb) and tin (Sn) contents can be limited 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, so 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, so the present invention can add cobalt (Co) for this effect. However, if the cobalt (Co) content exceeds a certain level, the weldability and ductility of the steel sheet may deteriorate, so the present invention can limit the cobalt (Co) content to 1.5% or less.

[0044] The high strength steel sheet with excellent workability according to one embodiment 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 exclude them. Since these impurities are known to anyone having ordinary skill in the art, the contents of all of them will not be specifically mentioned in this specification. Furthermore, the addition of additional effective components in addition to the above-mentioned components is not completely excluded.

[0045] The high-strength steel plate having excellent workability according to one embodiment of the present invention may contain 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 is characterized by lowering the ductility and burring ability of steel sheets. Also, compared to tempered martensite, fresh martensite tends to lower the yield ratio of steel sheets. This is because the microstructure of tempered martensite becomes softer due to the tempering heat treatment. Therefore, the balance between tensile strength and elongation (TS) that is the objective of the present invention is 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2It is preferable to control the fraction of tempered martensite and fresh martensite in order to ensure the yield ratio rating index (1-YR). 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 evaluation index (1-YR) of 0.42 or less, it is preferable to limit the fraction of tempered martensite to 50 volume % or more and the fraction of fresh martensite to 10 volume % or more. More preferably, the fraction of tempered martensite can be 52 volume % or more or 54 volume % or more, and more preferably, the fraction of fresh martensite can be 12 volume % or more. On the other hand, if tempered martensite or fresh martensite is formed excessively, ductility and burring ability are reduced, 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 satisfied simultaneously. Therefore, the present invention can limit the fraction of tempered martensite to 70 volume % or less, and the fraction of fresh martensite to 30 volume % or less. More preferably, the fraction of tempered martensite can be 68 volume % or less or 65 volume % or less, and more preferably, the fraction of fresh martensite can be 25 volume % or less.

[0047] The balance between tensile strength and elongation (TS 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield ratio evaluation index (1-YR), optimization of the bainite fraction is required. 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 yield ratio rating index (1-YR) of 0.42 or less, it is preferable to control the fraction of bainite to 10 volume % or more. More preferably, the fraction of bainite is 12 volume % or more or 14 volume % or more. On the other hand, if bainite is excessively formed, it will result in a decrease in the fraction of tempered martensite, which may result in a loss of the desired balance between tensile strength and elongation (TS 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 In order to ensure the desired yield ratio (1-YR), the bainite fraction may be limited to 30 vol.% or less. The preferred bainite fraction may be 12 vol.% or more or 14 vol.% or more, or 28 vol.% or less or 26 vol.% 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. When the fraction of retained austenite is below a certain level, the balance between tensile strength and elongation (TS 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, local elongation may decrease or spot weldability may decrease. Therefore, the present invention provides a 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 the retained austenite can be limited to a range of 2 to 10%. The preferable fraction of the retained austenite can be 3 volume % or more or 8 volume % or less.

[0049] The steel sheet of the present invention may contain ferrite, pearlite, island martensite (Martensite Austenite Constituent, MA), etc. as unavoidable structures. If ferrite is formed excessively, the strength of the steel sheet may be reduced, so the present invention may limit the ferrite fraction to 5 volume % (including 0%) or less. In addition, if pearlite is formed excessively, the workability of the steel sheet may be reduced or the fraction of retained austenite may be reduced, so the present invention limits the formation of pearlite as much as possible.

[0050] A high-strength steel plate with excellent workability according to one aspect of the present invention can satisfy the following [Relational formula 1] to [Relational formula 4]. [Equation 1] 0.03≦[B] FM / [B] TM ≦0.55 In the above formula 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] V(1.2μm,γ) / V(γ)≧0.12 In the above Relational Formula 2, V(1.2 μm, γ) is the fraction (volume %) of retained austenite having an average crystal grain size of 1.2 μm or more, and V(γ) is the fraction (volume %) of retained austenite in the steel plate. [Equation 3] V(lath, γ) / V(γ)≧0.5 In the above Relational Expression 3, V(lath, γ) is the fraction (volume %) of the retained austenite in the lath form, and V(γ) is the fraction (volume %) of the retained austenite in the steel plate. [Equation 4] T(γ) / V(γ)≧0.08 In the above Relation 4, 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.

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

[0052] The present invention relates to a boron (B) content ([B]) contained in tempered martensite as shown in [Relationship 1]. TM The boron (B) content in fresh martensite ([B] FM In order to control the ratio of 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 the 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, and have noted that, although the theoretical basis has not been clearly clarified, the physical properties targeted by 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, it has been confirmed that the yield ratio of the steel sheet shows a certain tendency depending on the ratio of the boron (B) content in tempered martensite and 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 between 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 retained austenite of a specific size, shape and type to the total retained austenite is an important factor in ensuring strength and workability.

[0055] The more the proportion of retained austenite with an average grain size of 1.2 μm or more increases, the more it can help improve the workability of steel sheets. Retained austenite with an average grain size of 1.2 μm or more is retained austenite whose average size has increased due to heat treatment at the bainite formation temperature, and is a structure in which the driving force for transformation into martensite is relatively suppressed compared to retained austenite with an average grain size of 1.2 μm or less. Therefore, since the transformation into martensite is suppressed in retained austenite with an average grain size of 1.2 μm or more, when the proportion of retained austenite with an average grain size of 1.2 μm or more is above a certain level, the workability of steel sheets can be more effectively improved.

[0056] The greater the proportion of lath-type retained austenite among the retained austenite, the more advantageous it is for improving the workability of the steel sheet. Retained austenite is divided into lath-type retained austenite formed between bainite phases and block-type retained austenite formed in areas without bainite phase. Since block-type retained austenite is further transformed into fresh martensite during heat treatment, when the proportion of lath-type retained austenite is above a certain level, the workability of the steel sheet can be more effectively improved.

[0057] The higher the ratio of tempered retained austenite in the retained austenite, the more advantageous it is for improving the workability of the steel sheet. Tempered retained austenite refers to retained austenite that is enriched by the inflow of carbon (C) during heat treatment at a bainite formation temperature, and has a carbon (C) content (wt%) of 1.45 times or more the average carbon (C) content (wt%) of the steel sheet. In tempered retained austenite, carbon (C), an austenite stabilizing element, is relatively enriched, suppressing transformation to martensite, and when the ratio of tempered retained austenite is at a certain level or more, the workability of the steel sheet can be more effectively ensured.

[0058] The retained austenite having an average grain size of 1.2 μm or more, the lath-shaped retained austenite, and the tempered retained austenite are classifications of retained austenite based on their size, shape, and type, and are mutually compatible concepts.

[0059] In the present invention, the ratio of the fraction of retained austenite having an average grain size of 1.2 μm or more (V(1.2 μm, γ), volume%) to the total fraction of retained austenite contained in the steel sheet (V(γ), volume%) is controlled to 0.12 or more as shown in [Relationship 2], the ratio of the fraction of lath-shaped retained austenite (V(lath, γ), volume%) to the total fraction of retained austenite contained in the steel sheet (V(γ), volume%) is controlled to 0.5 or more as shown in [Relationship 3], and the fraction (volume%) of tempered retained austenite to the total fraction of retained austenite contained in the steel sheet (V(γ, volume%)) is limited to 0.08 or more as shown in [Relationship 4], thereby achieving a desired balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) and balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) can be effectively ensured.

[0060] The high-strength steel sheet having excellent workability according to one embodiment 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 5] B TE =[Tensile strength (TS, MPa)] 2 *[Elongation rate (El,%)] 1 / 2 [Equation 6] B TH =[Tensile strength (TS, MPa)] 2 *[Hole expansion rate (HER,%)] 1 / 2 [Equation 7] IYR =1-[yield ratio (YR)]

[0061] An example of a method for producing the steel sheet of the present invention will now be described in detail.

[0062] A method for producing a high-strength steel sheet according to one embodiment of the present invention 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 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 400 to 600°C at an average cooling rate of 2 to 100°C / s (primary cooling), and maintaining the temperature for 5 to 600 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 maintaining the temperature for 5 to 600 seconds. cooling the tertiarily-cooled steel sheet to a temperature range of 200-400°C at an average cooling rate of 2-100°C / s (tertiary cooling); heating the tertiarily-cooled steel sheet to a temperature range of 400-600°C at an average heating rate of 5-100°C / s (tertiary heating) and then maintaining the temperature for 10-1800 seconds (fourth maintenance); cooling the tertiarily-cooled steel sheet to a temperature range of 300-500°C at an average cooling rate of 1-100°C / s (fourth cooling) and then maintaining the temperature for 10-1800 seconds (fifth maintenance); and cooling the 5th-maintained steel sheet to room temperature at an average cooling rate of 1°C / s or more (fifth cooling).

[0063] The cold rolled steel sheet can be provided by the steps of heating a steel slab having a predetermined alloy composition to 1000-1350°C; finish hot rolling at a temperature range of 800-1000°C; coiling the hot rolled steel sheet at a temperature range of 350-650°C; pickling the coiled steel sheet; and cold rolling the pickled steel sheet at a reduction ratio of 30-90%.

[0064] Preparation and heating of steel slabs 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.

[0065] The prepared steel slab may be heated within a certain temperature range, and the heating temperature of the steel slab may 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 may 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 may reach its melting point and melt.

[0066] 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 be coarsely formed, which may cause a deterioration in the physical properties of the final steel sheet.

[0067] The hot-rolled steel sheet after the hot rolling 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 not easy, and if the coiling temperature exceeds 650° C., surface scale is formed inside the hot-rolled steel sheet, which may make pickling difficult.

[0068] 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 thereof, and then cold rolled. Although the pickling and cold rolling conditions are not particularly limited in the present invention, it is preferable that the cold rolling is performed at a cumulative reduction of 30 to 90%. If the cumulative reduction of the cold rolling exceeds 90%, it may be difficult to perform the cold rolling in a short time due to the high strength of the steel sheet.

[0069] The cold-rolled steel sheet may be made into an uncoated cold-rolled steel sheet through an annealing heat treatment process, or may be made into a coated steel sheet through a coating process to impart corrosion resistance. Coating may be performed by a coating method such as hot-dip galvanizing, electrolytic galvanizing, or hot-dip aluminum coating, and the method and type are not particularly limited.

[0070] 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.

[0071] 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 that temperature for 50 to 1200 seconds (primary maintenance).

[0072] If the average heating rate of the first heating to 700°C is less than 5°C / s, blocky 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) and tensile strength and elongation (TS 2 *EL 1 / 2 ) and 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 formed during heating to austenite is accelerated, a large amount of blocky austenite is formed, the final structure becomes coarse, and boron (B) cannot be sufficiently concentrated in the tempered martensite. This causes [B] FM / [B] TM exceeds 0.55, achieving the desired balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield ratio evaluation index (I YR) cannot be realized.

[0073] If the first 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 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 cannot be sufficiently homogenized, and the physical properties of the steel sheet may decrease. 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.

[0074] After the first cooling, the steel plate can be cooled to a temperature range of 400-600°C at an average cooling rate of 2°C / s or more (first cooling), and then maintained at that temperature range for 5 seconds or more (secondary cooling). If the average cooling rate for the first cooling is less than 2°C / s, the fraction of retained austenite will be insufficient due to the slow cooling, and accordingly, the T(γ) / V(γ) and balance of tensile strength and elongation (TS 2 *EL 1 / 2 ) and 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 or less. If the secondary maintenance temperature is less than 400°C, the V(1.2μm,γ) / V(γ) of the steel sheet and the balance between the tensile strength and hole expansion rate (TS 2 *HER 1 / 2 On the other hand, if the secondary maintenance temperature exceeds 600℃, the residual austenite is insufficient, and the balance of T(γ) / V(γ), V(lath,γ) / V(γ), tensile strength and elongation (TS 2 *EL 1 / 2 ) and balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2In addition, if the second holding time is less than 5 seconds, the heat treatment time is insufficient, and the balance between the V(lath, γ) / V(γ), V(1.2μm, γ) / V(γ) and the tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease. There is no need to specify an upper limit for the secondary maintenance time, but it is preferable to limit it to 600 seconds or less.

[0075] After the second hold, the steel sheet 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 range for 5 seconds or more (tertiary hold). There is no particular upper limit to the average cooling rate for secondary cooling, but it is preferably set to 100°C / s or less. When the tertiary hold temperature is less than 300°C, the V(1.2μm,γ) / V(γ) of the steel sheet and the balance between tensile strength and hole expansion rate (TS) can be improved due to the low heat treatment temperature. 2 *HER 1 / 2 On the other hand, if the tertiary maintenance temperature exceeds 500℃, the residual austenite is insufficient, and the balance of T(γ) / V(γ), V(lath,γ) / V(γ), tensile strength and elongation (TS 2 *EL 1 / 2 ), balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 In addition, if the 3rd maintenance time is less than 5 seconds, the heat treatment time is insufficient, and the balance between the V(lath, γ) / V(γ), V(1.2μm, γ) / V(γ) and the tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease. There is no need to specify an upper limit for the tertiary maintenance time, but it is preferable to limit it to 600 seconds or less.

[0076] After the tertiary hold, the steel plate can be cooled to the primary cooling stop temperature of 200-400°C at an average cooling rate of 2°C / s or more (tertiary cooling). If the average cooling rate of the tertiary cooling is less than 2°C / s, the fraction of retained austenite will be insufficient due to the slow cooling, which will result in the steel plate's T(γ) / V(γ) and balance of tensile strength and elongation (TS 2 *EL 1 / 2), 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 of the tertiary cooling, but it is preferable to set it to 100°C / s or less. If the primary cooling end temperature is less than 200°C, tempered martensite is excessively formed and the retained austenite is insufficient, resulting in deterioration of the T(γ) / V(γ), V(1.2μm, γ) / V(γ), and balance of tensile strength and elongation (TS 2 *EL 1 / 2 ) and balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 On the other hand, if the first cooling stop temperature exceeds 400°C, bainite is excessively formed and tempered martensite is insufficient, resulting in an imbalance of the tensile strength and elongation rate (TS) of the steel plate. 2 *EL 1 / 2 ), and the balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease.

[0077] After the tertiary 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 (tertiary heating), and then maintained for 10 to 1800 seconds (fourth maintenance). There is no particular need to specify an upper limit for the average heating rate of the tertiary heating, but it is preferable to set it to 100°C / s or less. If the fourth maintenance temperature is less than 400°C, the V(1.2μm,γ) / V(γ) of the steel sheet and the balance between tensile strength and hole expansion rate (TS) can be improved due to the low heat treatment temperature. 2 *HER 1 / 2 If the fourth maintenance temperature exceeds 600℃, the fraction of retained austenite is insufficient, and the balance of T(γ) / V(γ), V(1.2μm, γ) / V(γ), tensile strength and elongation (TS 2 *EL 1 / 2 ) and balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 If the fourth maintenance time is less than 10 seconds, the heat treatment time is insufficient, and the balance between the V(1.2μm, γ) / V(γ) and the tensile strength and hole expansion rate (TS 2 *HER 1 / 2) may decrease. There is no need to specify an upper limit for the fourth order sustain time, but it is preferable to set it to 1800 seconds or less.

[0078] After the fourth 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 (fourth cooling), and then held for 10 to 1800 seconds (fifth hold). There is no particular upper limit to the average cooling rate in the fourth cooling, but it is preferably 100°C / s or less. When the fifth hold temperature is less than 300°C, the V(1.2μm,γ) / V(γ) and the balance between tensile strength and hole expansion rate (TS) can be improved by the low heat treatment temperature. 2 *HER 1 / 2 On the other hand, if the fifth maintenance temperature exceeds 500°C, the fraction of retained austenite is insufficient, and the balance of T(γ) / V(γ), V(1.2μm, γ) / V(γ), tensile strength and elongation (TS 2 *EL 1 / 2 ) and balance between tensile strength and hole expansion rate (TS 2 *HER 1 / 2 If the fifth maintenance time is less than 10 seconds, the heat treatment time is insufficient, and the balance between the V(1.2μm, γ) / V(γ) and the tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may decrease. There is no need to specify an upper limit for the fifth order sustain time, but it is preferable to set it to 1800 seconds or less.

[0079] The cooling rate Vc1 of the primary cooling and the cooling rate Vc4 of the fourth cooling can satisfy the relationship Vc1>Vc4.

[0080] After the fifth hold, it can be cooled to room temperature (fifth cooling) at an average cooling rate of 1°C / s or more.

[0081] 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.

[0082] The steel sheet manufactured by the above-mentioned manufacturing method 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 5] B TE =[Tensile strength (TS, MPa)] 2 *[Elongation rate (El,%)] 1 / 2 [Equation 6] B TH =[Tensile strength (TS, MPa)] 2 *[Hole expansion rate (HER,%)] 1 / 2 [Equation 7] I YR =1-[yield ratio (YR)] EXAMPLES

[0083] Hereinafter, a high strength steel sheet having excellent workability 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 described in the claims and matters reasonably inferred therefrom.

[0084] (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.

[0085] Thereafter, the steel sheets were produced by performing heat treatment under the annealing heat treatment conditions shown in the following Tables 2 to 7. 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.

[0086] The microstructure of the steel sheets manufactured in this manner was observed, and the results are shown in Tables 8 and 9. Ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM), and pearlite (P) among the microstructures were observed through SEM after nital etching of the polished cross section of the test specimen. After nital etching, the structure without irregularities on the surface of the test specimen was classified as ferrite, and the structure having a lamellar structure of cementite and ferrite was classified as pearlite. Since bainite (B) and tempered martensite (TM) were all observed in lath and block form and difficult to distinguish, the fractions of bainite and tempered martensite were calculated using the dilatation curve after dilatation evaluation. In other words, the fraction of bainite was determined by subtracting the fraction of tempered martensite calculated through the 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.

[0087] On the other hand, [B] of steel plate FM / [B] TM , V(1.2μm,γ) / V(γ), V(lath,γ) / V(γ), T(γ) / V(γ), balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), balance between 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 10 and 11.

[0088] Boron (B) content in fresh martensite ([B] FM ) and the boron (B) content in tempered martensite ([B] TM) was determined by the boron (B) concentration measured in fresh martensite and tempered martensite using EPMA (Electron Probe MicroAnalyser). Retained austenite with an average grain size of 1.2μm or more (V(1.2μm,γ)) and lath-shaped retained austenite (V(lath,γ)) were measured using EBSD (Electron Backscatter Diffraction) phase maps. Tempered retained austenite was classified based on the carbon (C) content measured in the retained austenite using EPMA.

[0089] Tensile strength (TS) and elongation (El) were evaluated by tensile tests, and were measured using test pieces taken based on JIS No. 5 standard, with the 90° direction relative to the rolling direction of the rolled plate material as the reference. Hole expansion ratio (HER) was evaluated by hole expansion tests, and after forming a punched hole of 10 mm Ψ (die inner diameter 10.3 mm, clearance 12.5%), a conical punch with an apex angle of 60° was inserted into the punched hole in the direction in which the burr of the punched hole was on the outside, and the periphery of the punched hole was compressed and expanded at a moving speed of 20 mm / min, and then the HER was calculated using the following [Relationship 8]. [Equation 8] Hole expansion rate (HER,%) = {(D-D0) / D0} x 100 In the above Relational Formula 8, 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).

[0090] [Table 1]

[0091] [Table 2]

[0092] [Table 3]

[0093] [Table 4]

[0094] [Table 5]

[0095] [Table 6]

[0096] [Table 7]

[0097] [Table 8]

[0098] [Table 9]

[0099] [Table 10]

[0100] [Table 11]

[0101] As shown in Tables 1 to 11 above, in the case of test pieces that satisfy the conditions presented in the present invention, all of [Relational formula 1] to [Relational formula 4] 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*106 ~11.5*10 6 (MPa 2 % 1 / 2 ) and the yield ratio evaluation index (I YR ) satisfies 0.15~0.42.

[0102] 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*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0103] For specimen 3, the secondary average heating rate was more than 5°C / s, and blocky 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 of tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0104] Specimen 4 was tested in the two-phase region where the first 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 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0105] For specimen 5, the primary average cooling rate was less than 2°C / s, and the fraction of retained austenite was insufficient. As a result, the T(γ) / V(γ) ratio for specimen 5 was less than 0.08, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0106] For test piece 6, the second maintenance temperature was less than 400°C, and the heat treatment temperature was insufficient. As a result, for test piece 6, V(1.2μm,γ) / V(γ) was less than 0.12, and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0107] For test piece 7, the secondary holding temperature was over 600°C, and the fraction of retained austenite was insufficient. As a result, for test piece 7, V(lath,γ) / V(γ) was less than 0.5, T(γ) / V(γ) was less than 0.08, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0108] For test piece 8, the second hold time was less than 5 seconds, and the heat treatment time was insufficient. As a result, for test piece 8, V(1.2μm,γ) / V(γ) was less than 0.12, V(lath,γ) / V(γ) was less than 0.5, and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0109] For test piece 9, the third maintenance temperature was less than 300°C, and the heat treatment temperature was insufficient. As a result, for test piece 9, V(1.2μm,γ) / V(γ) was less than 0.12, and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0110] For specimen 10, the third sustained temperature was over 500°C, resulting in an insufficient fraction of retained austenite. As a result, specimen 10 had a V(lath,γ) / V(γ) of less than 0.5, a T(γ) / V(γ) of less than 0.08, and a poor balance between tensile strength and elongation (B TE ) is 3.0*10 6Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0111] For test piece 11, the third maintenance time was less than 5 s, and the heat treatment time was insufficient. As a result, for test piece 11, V(1.2 μm, γ) / V(γ) was less than 0.12, V(lath, γ) / V(γ) was less than 0.5, and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0112] For specimen 12, the tertiary average cooling rate was less than 2°C / s, and the fraction of retained austenite was insufficient. As a result, the T(γ) / V(γ) ratio of specimen 12 was less than 0.08, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0113] Test piece 13 is 3 The secondary cooling stop temperature was less than 200°C, so the fraction of tempered martensite was excessive and the fraction of retained austenite was insufficient. As a result, the V(1.2μm,γ) / V(γ) was less than 0.12, the T(γ) / V(γ) was less than 0.08, and the balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0114] The test piece 14 is 3 The cooling stop temperature was over 400°C, so the proportion of bainite was excessive and the proportion of tempered martensite was insufficient. As a result, the balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0115] For test piece 15, the fourth maintenance temperature was less than 400°C, and the heat treatment temperature was insufficient. As a result, for test piece 15, V(1.2μm,γ) / V(γ) was less than 0.12, and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0116] For specimen 16, the fourth sustained temperature was over 600°C, resulting in an insufficient fraction of retained austenite. As a result, specimen 16 had a V(1.2μm,γ) / V(γ) of less than 0.12, a T(γ) / V(γ) of less than 0.08, and a balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0117] For test piece 17, the fourth hold time was less than 10 s, and the heat treatment time was insufficient. As a result, for test piece 17, V(1.2 μm, γ) / V(γ) was less than 0.12, and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0118] For specimen 18, the fifth maintenance temperature was less than 300°C, and the heat treatment temperature was insufficient. As a result, for specimen 18, V(1.2μm,γ) / V(γ) was less than 0.12, and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0119] For specimen 19, the fifth maintenance temperature was over 500°C, and the fraction of retained austenite was insufficient. As a result, specimen 19 had V(1.2μm,γ) / V(γ) of less than 0.12, T(γ) / V(γ) of less than 0.08, and a balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0120] For test piece 20, the fifth maintenance time was less than 10 seconds, and the heat treatment time was insufficient. As a result, for test piece 20, V(1.2 μm, γ) / V(γ) was less than 0.12, and the balance between tensile strength and hole expansion rate (B TH ) is 6.0*10 6 It was less than.

[0121] Test piece 42 has a low carbon (C) content and a good balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0122] Specimen 43 had a high carbon (C) content, and the tempered martensite fraction was insufficient and the fresh martensite fraction was excessive. As a result, specimen 43 had a poor balance of tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0123] The test piece 44 had a low silicon (Si) content and an insufficient fraction of retained austenite. As a result, the test piece 44 had a V(lath, γ) / V(γ) of less than 0.5, a T(γ) / V(γ) of less than 0.08, and a balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0124] Specimen 45 had a high silicon (Si) content and the fraction of fresh martensite was excessive. As a result, specimen 45 had a poor balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0125] Specimen 46 had a high aluminum (Al) content and the fraction of fresh martensite was excessive. As a result, specimen 46 had a poor balance of tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0126] Specimen 47 had a low manganese (Mn) content, and the fraction of retained austenite was insufficient due to the formation of pearlite. As a result, specimen 47 had a V(lath, γ) / V(γ) of less than 0.5, a T(γ) / V(γ) of less than 0.08, and a poor balance between tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0127] Specimen 48 had a high manganese (Mn) content and the fraction of fresh martensite was excessive. As a result, specimen 48 had a poor balance of tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0128] Specimen 49 had a high chromium (Cr) content and the fraction of fresh martensite was excessive. As a result, specimen 49 had a poor balance of tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH ) is 6.0*10 6 It was less than.

[0129] Specimen 50 had a high molybdenum (Mo) content and the fraction of fresh martensite was in excess. As a result, specimen 50 had a poor balance of tensile strength and elongation (B TE ) is 3.0*10 6 Less than 100% of the tensile strength and hole expansion rate balance (B TH) is 6.0*10 6 It was less than.

[0130] The test piece 51 had a low boron (B) content, and boron (B) could not be concentrated in the tempered martensite. As a result, the test piece 51 had a [B] FM / [B] TM exceeds 0.55, and the yield ratio evaluation index (I YR ) exceeded 0.42.

[0131] The test piece 52 had a high boron (B) content, and boron (B) was excessively concentrated in the tempered martensite. As a result, the test piece 52 had a [B] FM / [B] TM is less than 0.03, and the yield ratio evaluation index (I YR ) was less than 0.15.

[0132] Although the present invention has been described in detail with reference to the embodiments, other embodiments are possible and the spirit and scope of the claims set forth below should not be construed as being 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 unavoidable impurities; The microstructure includes bainite, tempered martensite, fresh martensite, retained austenite, and other unavoidable structures. The microstructure contains, 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; A high-strength steel plate with excellent formability that satisfies the following [Relational Formula 1] to [Relational Formula 4]. [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] V(1.2 μm, γ) / V(γ)≧0.12 In the above Relational Formula 2, V(1.2 μm, γ) is the fraction (volume %) of retained austenite having an average crystal grain size of 1.2 μm or more, and V(γ) is the fraction (volume %) of retained austenite in the steel plate. [Relationship 3] V(lath, γ) / V(γ)≧0.5 In the above Relational Expression 3, V(lath, γ) is the fraction (volume %) of the retained austenite in the lath form, and V(γ) is the fraction (volume %) of the retained austenite in the steel sheet. [Relationship 4] T(γ) / V(γ)≧0.08 In the above-mentioned Relational Expression 4, 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.

2. The high-strength steel plate having excellent workability according to claim 1, further comprising, in weight percent, 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) One or more 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%

3. 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 satisfy the balance between the tensile strength and the 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 2. The high-strength steel plate having excellent workability according to claim 1, wherein the ratio of tensile strength to compressibility is 0.15 to 0.

42. [Relationship 5] B ＴＥ = [tensile strength (TS, MPa)] ２ * [Elongation (El,%)] １／２ [Relationship 6] B ＴＨ = [tensile strength (TS, MPa)] ２ * [Hole expansion rate (HER,%)] １／２ [Relationship 7] I ＹＲ =1-[Yield Ratio (YR)]

4. providing a cold rolled steel sheet containing, by weight percent, C: 0.1-0.25%, Si: 0.01-1.5%, Mn: 1.0-4.0%, Al: 0.01-1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, and B: 0.0005-0.005%, 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 (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); cooling the steel sheet after the first cooling step to a temperature range of 400 to 600° C. at an average cooling rate of 2 to 100° C. / s (first cooling), and then maintaining the steel sheet for 5 to 600 seconds (secondary 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 steel sheet for 5 to 600 seconds (third maintenance); cooling the third-maintained steel sheet to a temperature range of 200 to 400° C. at an average cooling rate of 2 to 100° C. / s (third cooling); heating the tertiarily cooled 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 maintaining the temperature for 10 to 1,800 seconds (fourth maintenance); cooling the steel sheet after the fourth cooling step to a temperature range of 300 to 500° C. at an average cooling rate of 1 to 100° C. / s (fourth cooling step), and then maintaining the steel sheet for 10 to 1,800 seconds (fifth maintenance step); and The method for producing a high strength steel plate having excellent workability according to any one of claims 1 to 3, further comprising a step of cooling the steel plate after the fifth cooling to room temperature at an average cooling rate of 1°C / s or more (fifth cooling).

5. The method for producing a high-strength steel plate having excellent workability according to claim 4, wherein the cold-rolled steel plate further contains 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) One or more 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%

6. The cold rolled steel sheet is 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 and cold rolling the pickled steel sheet at a rolling reduction of 30 to 90%.

7. 5. The method for producing a high strength steel plate having excellent workability according to claim 4, wherein the cooling rate Vc1 of the primary cooling and the cooling rate Vc4 of the quaternary cooling satisfy a relationship of Vc1>Vc4.

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