High-strength steel plate with excellent workability and method for producing the same

Abstract

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

Description

Technical Field

[0001] The present invention relates to a steel sheet that can be used for automobile parts and the like, and relates to a steel sheet having high strength characteristics and excellent workability, and a method for producing the same. Background Art

[0002] In recent years, the automotive industry has focused on methods that can achieve material lightweighting while ensuring passenger safety in order to protect the global environment. To meet these safety and lightweighting demands, the use of high-strength steel sheets has increased dramatically. Generally, it is known that as steel sheet strength increases, its workability decreases. Therefore, steel sheets for automotive parts require high strength properties while also exhibiting excellent workability, particularly in terms of ductility and hole expandability.

[0003] Transformation Induced Plasticity (TRIP) steel, which utilizes the transformation induced plasticity of retained austenite, has a complex microstructure composed of ferrite, bainite, martensite, and retained austenite, and therefore has high strength characteristics and a certain level of workability.

[0004] Patent Documents 1 and 2 disclose a method utilizing tempered martensite as a technique for further improving the workability of steel sheets. Tempered martensite, produced by tempering hard martensite, softens the martensite, resulting in a difference in strength between tempered martensite and conventional untempered martensite (fresh martensite). Therefore, suppressing fresh martensite and forming tempered martensite can improve workability.

[0005] However, in the techniques disclosed in Patent Documents 1 and 2, the balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) cannot meet 3.0*10 6 to 6.2*10 6 (MPa 2 % 1 / 2 ), which means that it is difficult to ensure a steel plate with excellent strength and ductility.

[0006] Another technique for improving the workability of steel sheets is a method of inducing the formation of bainite by adding boron (B), as disclosed in Patent Document 3. Adding boron (B) suppresses the ferrite-pearlite transformation and induces the formation of bainite, thereby achieving a balance between strength and workability.

[0007] However, the technology disclosed in Patent Document 3 cannot simultaneously ensure 3.0*10 6 to 6.2*10 6 (MPa2 % 1 / 2 )'s balance of tensile strength and elongation (B TE ), 6.0*10 6 to 11.5*10 6 (MPa 2 % 1 / 2 )'s tensile strength and hole expansion ratio (B TH ) and a yield strength ratio evaluation index (I YR ), which means it is difficult to ensure a steel plate with excellent strength, hole expandability, ductility and yield strength ratio.

[0008] That is, the balance between tensile strength and elongation (B TE ), the balance between tensile strength and hole expansion rate (B TH ) and yield strength ratio evaluation index (I YR ) are in demand for excellent steel plates.

[0009] (Prior art literature)

[0010] (Patent Document 1) Korean Patent Publication No. 10-2006-0118602

[0011] (Patent Document 2) Japanese Patent Publication No. 2009-019258

[0012] (Patent Document 3) Japanese Patent Publication No. 2016-216808 Summary of the Invention

[0013] Technical problems to be solved

[0014] According to one aspect of the present invention, a steel plate having excellent balance between tensile strength and elongation, balance between tensile strength and hole expansion rate, and yield ratio evaluation index by optimizing the composition and microstructure of the steel plate and a method for manufacturing the same can be provided.

[0015] The technical problems of the present invention are not limited to the above-mentioned contents. Additional technical problems of the present invention are described in the entire specification, and those skilled in the art can easily understand the additional technical problems of the present invention through the contents described in the specification.

[0016] Technical Solution

[0017] According to one aspect of the present invention, a high-strength steel sheet with excellent workability may contain, in terms of 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%, and the balance Fe and inevitable impurities. The fine structure may include bainite, tempered martensite, fresh martensite, retained austenite, and other inevitable structures. The steel sheet may satisfy the following [Relationship 1] and [Relationship 2].

[0018] [Equation 1]

[0019] 0.03≤[B] FM / [B] TM ≤0.55

[0020] In the above relational expression 1, [B] FM is the content (weight %) of boron (B) contained in the fresh martensite, [B] TM It is the content (weight %) of boron (B) contained in tempered martensite.

[0021] [Equation 2]

[0022] V(1.2μm,γ) / V(γ)≥0.12

[0023] In the above-mentioned Relational Formula 2, V(1.2 μm, γ) is the fraction (volume %) of retained austenite having an average grain size of 1.2 μm or greater, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0024] The steel sheet may further contain, in wt%, any one or more of the following (1) to (8).

[0025] (1) one or more of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5%,

[0026] (2) one or more of Cr: 0-3.0% and Mo: 0-3.0%,

[0027] (3) Cu: 0-4.0% and Ni: 0-4.0% or more,

[0028] (4) One or more of Ca: 0-0.05%, REM excluding Y: 0-0.05%, and Mg: 0-0.05%,

[0029] (5) one or more of W: 0-0.5% and Zr: 0-0.5%,

[0030] (6) one or more of Sb: 0-0.5% and Sn: 0-0.5%,

[0031] (7) one or more of Y: 0-0.2% and Hf: 0-0.2%,

[0032] (8) Co: 0-1.5%.

[0033] In terms of volume fraction, the microstructure of the steel plate may include 10-30% bainite, 50-70% tempered martensite, 10-30% fresh martensite, 2-10% retained austenite, and 5% or less (including 0%) ferrite.

[0034] In the steel plate, the balance between tensile strength and elongation (B TE ) can meet 3.0*10 6 to 6.2*10 6 (MPa 2 % 1 / 2 ), the balance between tensile strength and hole expansion ratio (B TH ) can meet 6.0*10 6 to 11.5*10 6 (MPa 2 % 1 / 2 ), the yield strength ratio evaluation index (I YR ) can satisfy 0.15 to 0.42.

[0035] [Equation 3]

[0036] B TE =[Tensile Strength (TS, MPa)] 2 *[Elongation (El, %)] 1 / 2

[0037] [Equation 4]

[0038] B TH =[Tensile Strength (TS, MPa)] 2 *[Hole expansion ratio (HER, %)] 1 / 2

[0039] [Equation 5]

[0040] I YR =1-[yield strength ratio (YR)]

[0041] According to one aspect of the present invention, a method for manufacturing a high-strength steel sheet with excellent workability may include the following steps: providing a cold-rolled steel sheet, wherein the steel sheet comprises, in 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%, and the balance 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), and heating to a temperature range of Ac3 to 920° C. at an average heating rate of 5° C. / s or less (secondary heating); The steel plate is heated at an average heating rate of 5-100°C / second to a temperature range of 350-550°C (third heating), and then held for 50-1200 seconds (first holding); the steel plate that has been held once is cooled to a temperature range of 350-550°C at an average cooling rate of 2-100°C / second (first cooling), and then held for 5-600 seconds (secondary holding); the steel plate that has been held twice is cooled to a temperature range of 200-400°C at an average cooling rate of 2-100°C / second (secondary cooling); the steel plate that has been cooled twice is heated to a temperature range of 350-550°C at an average heating rate of 5-100°C / second (third heating), and then held for more than 50 seconds (third holding); the steel plate that has been held three times is cooled to room temperature at an average cooling rate of more than 1°C / second (third cooling).

[0042] The steel billet may further include any one of the following (1) to (8).

[0043] (1) one or more of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5%,

[0044] (2) one or more of Cr: 0-3.0% and Mo: 0-3.0%,

[0045] (3) Cu: 0-4.0% and Ni: 0-4.0% or more,

[0046] (4) One or more of Ca: 0-0.05%, REM excluding Y: 0-0.05%, and Mg: 0-0.05%,

[0047] (5) one or more of W: 0-0.5% and Zr: 0-0.5%,

[0048] (6) one or more of Sb: 0-0.5% and Sn: 0-0.5%,

[0049] (7) one or more of Y: 0-0.2% and Hf: 0-0.2%,

[0050] (8) Co: 0-1.5%.

[0051] The cold-rolled steel plate can be provided by the following steps: heating a steel billet to 1000-1350°C; performing hot finish rolling in a temperature range of 800-1000°C; coiling the hot-rolled steel plate in a temperature range of 350-650°C; pickling the coiled steel plate; and cold rolling the pickled steel plate at a reduction ratio of 30-90%.

[0052] Beneficial effects

[0053] According to a preferred aspect 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 as automobile parts, etc., and a method for producing the same can be provided.

[0054] Best Practice

[0055] The present invention relates to a high-strength steel plate with excellent workability and a method for producing the same. Preferred embodiments of the present invention are described below. The embodiments of the present invention can be modified in various ways, and the scope of the present invention should not be construed as being limited to the embodiments described below. These embodiments are provided to further illustrate the present invention to those skilled in the art.

[0056] The inventors of the present invention have recognized that in boron (B)-added transformation-induced plasticity (TRIP) steels comprising bainite, tempered martensite, fresh martensite, and retained austenite, by controlling the microstructure fractions of tempered martensite, fresh martensite, and retained austenite within certain ranges, controlling the boron (B) content in the tempered martensite and fresh martensite within certain ranges, and controlling the shape and size of the retained austenite 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 evaluation index. Recognizing this, the inventors devised a method that effectively achieves excellent strength, yield ratio, ductility, and hole expandability, thereby completing the present invention.

[0057] Hereinafter, a high-strength steel sheet having excellent workability according to one aspect of the present invention will be described in detail.

[0058] According to one aspect of the present invention, a high-strength steel sheet with excellent workability may contain, in terms of weight%, the following: 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%, and the balance Fe and inevitable impurities. The fine structure may include bainite, tempered martensite, fresh martensite, retained austenite, and other inevitable structures. The steel sheet may satisfy the following [Relationship 1] and [Relationship 2].

[0059] [Equation 1]

[0060] 0.03≤[B] FM / [B] TM ≤0.55

[0061] In the above relational expression 1, [B] FM is the content (weight %) of boron (B) contained in the fresh martensite, [B] TM It is the content (weight %) of boron (B) contained in tempered martensite.

[0062] [Equation 2]

[0063] V(1.2μm,γ) / V(γ)≥0.12

[0064] In the above-mentioned Relational Formula 2, V(1.2 μm, γ) is the fraction (volume %) of retained austenite having an average grain size of 1.2 μm or greater, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0065] The steel composition of the present invention will be described in more detail below. Unless otherwise specified, the percentages representing the content of each element are based on weight.

[0066] A high-strength steel sheet with excellent workability according to one aspect of the present invention comprises, in weight %, 0.1-0.25% C, 0.01-1.5% Si, 1.0-4.0% Mn, 0.01-1.5% Al, 0.15% or less P, 0.03% or less S, 0.03% or less N, 0.0005-0.005% B, and the balance Fe and unavoidable impurities. In addition, it may further include one or more of the following: 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%).

[0067] Carbon (C): 0.1-0.25%

[0068] Carbon (C) is an element that is essential for ensuring the strength of the steel plate, and is an element that stabilizes the retained austenite that helps to improve the ductility of the steel plate. Therefore, in order to achieve the effects described above, the present invention may contain more than 0.1% carbon (C). The preferred carbon (C) content may exceed 0.1%, and may be more than 0.11% or more than 0.12%. On the other hand, when the carbon (C) content exceeds a certain level, due to excessive increase in strength, ductility decreases, and weldability may deteriorate. Therefore, in the present invention, the upper limit of the carbon (C) content may be limited to 0.25%. The carbon (C) content may be less than 0.24%, and a more preferred carbon (C) content may be less than 0.23%.

[0069] Silicon (Si): 0.01-1.5% or less

[0070] Silicon (Si) is an element that contributes to improving strength by solid solution strengthening, and is also an element that improves workability by homogenizing the structure. In addition, silicon (Si) is an element that contributes to the generation of retained austenite by suppressing the precipitation of cementite. Therefore, in order to achieve the effect as described above, silicon (Si) of more than 0.01% can be added in the present invention. Preferred silicon (Si) content can be more than 0.02%, and more preferably silicon (Si) content can be more than 0.04%. However, when silicon (Si) content exceeds a certain level, plating defect problems such as unplated are caused in the plating process, and the weldability of the steel sheet may be reduced, so the upper limit of silicon (Si) content can be limited to 1.5% in the present invention. Preferred silicon (Si) content can be upper limit to 1.48%, and more preferably silicon (Si) content can be upper limit to 1.46%.

[0071] Manganese (Mn): 1.0-4.0%

[0072] Manganese (Mn) is a useful element for improving both strength and ductility. Therefore, in order to achieve the above-mentioned effect, more than 1.0% manganese (Mn) can be added in the present invention. The lower limit of the preferred manganese (Mn) content can be 1.2%, and the more preferred lower limit of the manganese (Mn) content can be 1.4%. On the other hand, when too much manganese (Mn) is added, the carbon (C) enrichment in the austenite is insufficient due to the increased bainite transformation time, and therefore the desired austenite fraction cannot be ensured. Therefore, the upper limit of the manganese (Mn) content can be limited to 4.0% in the present invention. The upper limit of the preferred manganese (Mn) content can be 3.9%.

[0073] Aluminum (Al): 0.01-1.5%

[0074] Aluminum (Al) is an element that acts as a deoxidizer by combining with oxygen in steel. In addition, like silicon (Si), aluminum (Al) is an element that stabilizes retained austenite by suppressing the precipitation of cementite. Therefore, in order to achieve the effect as described above, more than 0.01% of aluminum (Al) can be added in the present invention. The preferred aluminum (Al) content can be more than 0.03%, and the more preferred aluminum (Al) content can be more than 0.05%. On the other hand, when too much aluminum (Al) is added, the inclusions of the steel plate increase and the workability of the steel plate may be reduced, so the upper limit of the aluminum (Al) content can be limited to 1.5% in the present invention. The preferred upper limit of the aluminum (Al) content can be 1.48%.

[0075] Phosphorus (P): 0.15% or less (including 0%)

[0076] Phosphorus (P) is an element contained as an impurity and deteriorates impact toughness. Therefore, the content of phosphorus (P) is preferably controlled to 0.15% or less.

[0077] Sulfur (S): 0.03% or less (including 0%)

[0078] Sulfur (S) is an element contained as an impurity and forms MnS in the steel sheet, thereby deteriorating ductility. Therefore, the sulfur (S) content is preferably 0.03% or less.

[0079] Nitrogen (N): 0.03% or less (including 0%)

[0080] Nitrogen (N) is contained as an impurity and forms nitrides during continuous casting, thereby causing cracks in the slab. Therefore, the nitrogen (N) content is preferably 0.03% or less.

[0081] Boron (B): 0.0005-0.005%

[0082] Boron (B) is an element that increases strength by improving hardenability and is also an element that suppresses nucleation of grain boundaries. In addition, the purpose of the present invention is to simultaneously ensure the balance of excellent tensile strength and elongation, the balance of excellent tensile strength and hole expansion, and the excellent yield strength ratio evaluation index by the enrichment of boron (B) in tempered martensite, so boron (B) must be added in the present invention. Therefore, in order to achieve the above-mentioned effect, more than 0.0005% of boron (B) can be added in the present invention. However, when the added boron (B) exceeds a certain level, not only the characteristic effect is excessive, but also the manufacturing cost increases. Therefore, the upper limit of the content of boron (B) can be limited to 0.005% in the present invention.

[0083] Furthermore, the steel sheet of the present invention may contain further alloy compositions in addition to the above-mentioned alloy components, which will be described in detail below.

[0084] One or more of titanium (Ti): 0-0.5%, niobium (Nb): 0-0.5%, and vanadium (V): 0-0.5%

[0085] Titanium (Ti), niobium (Nb), and vanadium (V) are elements that refine grains by forming precipitates and contribute to improving the strength and impact toughness of steel sheets. Therefore, in the present invention, one or more of titanium (Ti), niobium (Nb), and vanadium (V) may be added to achieve these effects. However, if the content of each of titanium (Ti), niobium (Nb), and vanadium (V) exceeds a certain level, excessive precipitates may form, reducing impact toughness and increasing manufacturing costs. Therefore, in the present invention, the content of each of titanium (Ti), niobium (Nb), and vanadium (V) may be limited to 0.5% or less.

[0086] One or more of chromium (Cr): 0-3.0% and molybdenum (Mo): 0-3.0%

[0087] Chromium (Cr) and molybdenum (Mo) inhibit austenite decomposition during alloying. Like manganese (Mn), chromium (Cr) and molybdenum (Mo) are elements that stabilize austenite. Therefore, in the present invention, one or more of these elements may be added to achieve the aforementioned effects. However, when the chromium (Cr) and molybdenum (Mo) content exceeds a certain level, the bainite transformation time increases, resulting in insufficient carbon (C) enrichment in austenite, making it impossible to achieve the desired retained austenite fraction. Therefore, in the present invention, the chromium (Cr) and molybdenum (Mo) content may be limited to 3.0% or less, respectively.

[0088] Copper (Cu): 0-4.0% and nickel (Ni): 0-4.0% or more

[0089] Copper (Cu) and nickel (Ni) are elements that stabilize austenite and inhibit corrosion. Furthermore, copper (Cu) and nickel (Ni) are elements that accumulate on the surface of the steel sheet and prevent the intrusion of hydrogen that migrates into the steel sheet, thereby inhibiting hydrogen-induced delayed fracture. Therefore, in order to achieve the above-mentioned effects, one or more of copper (Cu) and nickel (Ni) may be added in the present invention. However, when the content of copper (Cu) and nickel (Ni) exceeds a certain level, the characteristic effects are excessive and the manufacturing cost is increased. Therefore, in the present invention, the content of copper (Cu) and nickel (Ni) may be limited to 4.0% or less, respectively.

[0090] One or more of calcium (Ca): 0-0.05%, magnesium (Mg): 0-0.05%, and rare earth elements (REM) other than yttrium (Y): 0-0.05%

[0091] Here, rare earth elements (REM) refer to scandium (Sc), yttrium (Y), and lanthanides. Rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) contribute to improving the ductility of steel sheets by spheroidizing sulfides. Therefore, in the present invention, one or more rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) may be added to achieve the aforementioned effects. However, exceeding a certain level of rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) results in excessive property effects and increases in manufacturing costs. Therefore, in the present invention, the content of each of the rare earth elements (REM) other than calcium (Ca), magnesium (Mg), and yttrium (Y) may be limited to 0.05% or less.

[0092] Tungsten (W): 0-0.5% and zirconium (Zr): 0-0.5% or more

[0093] Tungsten (W) and zirconium (Zr) are elements that increase the strength of steel sheets by improving hardenability. Therefore, in the present invention, one or more of tungsten (W) and zirconium (Zr) may be added to achieve the aforementioned effects. However, when the content of tungsten (W) and zirconium (Zr) exceeds a certain level, the characteristic effects are excessively reduced and manufacturing costs are increased. Therefore, in the present invention, the content of tungsten (W) and zirconium (Zr) may be limited to 0.5% or less, respectively.

[0094] Antimony (Sb): 0-0.5% and tin (Sn): 0-0.5% or more

[0095] Antimony (Sb) and tin (Sn) are elements that improve the plating wettability and plating adhesion of steel sheets. Therefore, in the present invention, one or more of these elements may be added to achieve the aforementioned effects. However, if the content of these elements exceeds certain levels, the steel sheet becomes more brittle, potentially causing cracks during hot or cold working. Therefore, in the present invention, the content of each of these elements may be limited to 0.5% or less.

[0096] One or more of yttrium (Y): 0-0.2% and hafnium (Hf): 0-0.2%

[0097] Yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of steel sheets. Therefore, in the present invention, one or more of these elements may be added to achieve the aforementioned effects. However, if the yttrium (Y) and hafnium (Hf) content exceeds a certain level, the ductility of the steel sheet may deteriorate. Therefore, in the present invention, the yttrium (Y) and hafnium (Hf) content may be limited to 0.2% or less, respectively.

[0098] Cobalt (Co): 0-1.5%

[0099] Cobalt (Co) is an element that enhances the TRIP effect by promoting bainite transformation. Therefore, in the present invention, cobalt (Co) may be added to achieve the above-mentioned effect. However, if the cobalt (Co) content exceeds a certain level, the weldability and ductility of the steel sheet may deteriorate. Therefore, in the present invention, the cobalt (Co) content may be limited to 1.5% or less.

[0100] The high-strength steel sheet with excellent workability according to one aspect of the present invention may contain, in addition to the above-mentioned components, a balance of Fe and other unavoidable impurities. However, during typical manufacturing processes, unwanted impurities inevitably enter from raw materials or the surrounding environment, and therefore cannot be completely eliminated. These impurities are well known to those skilled in the art, and therefore, not all of them are specifically mentioned in this specification. Furthermore, the addition of active ingredients other than the above-mentioned components is not completely excluded.

[0101] In the high-strength steel sheet having excellent workability according to one aspect of the present invention, the fine structure may include bainite, tempered martensite, fresh martensite, retained austenite, and other inevitable structures.

[0102] Both untempered martensite (fresh martensite, FM) and tempered martensite (tempered martensite, TM) are microstructures that increase the strength of the steel sheet. However, fresh martensite has the characteristic of reducing the ductility and burr resistance of the steel sheet compared to tempered martensite. In addition, fresh martensite tends to reduce the yield strength ratio of the steel sheet compared to tempered martensite. This is because the microstructure of tempered martensite is softened by the tempering heat treatment. Therefore, in order to ensure the desired balance of tensile strength and elongation (TS) of the present invention, 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield ratio evaluation index (1-YR), it is preferred to control the microstructure fraction of tempered martensite and fresh martensite. In order to meet 3.0*10 6 The balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), 6.0*10 6 The balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2) and a yield ratio evaluation index (1-YR) of 0.42 or less, the fraction of tempered martensite is preferably limited to 50 volume % or more, and the fraction of fresh martensite is preferably limited to 10 volume % or more. A more preferred fraction of tempered martensite may be 52 volume % or more or 54 volume % or more, and a more preferred fraction of fresh martensite may be 12 volume % or more. On the other hand, when tempered martensite or fresh martensite is excessively formed, ductility and burr resistance are reduced, and ultimately the requirements of 3.0*10 6 The balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), 6.0*10 6 The balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and a yield ratio evaluation index (1-YR) of 0.42 or less. Therefore, in the present invention, the fraction of tempered martensite can be limited to 70 volume % or less, and the fraction of fresh martensite can be limited to 30 volume % or less. A more preferred fraction of tempered martensite can be 68 volume % or less or 65 volume % or less, and a more preferred fraction of fresh martensite can be 25 volume % or less.

[0103] In order to ensure the desired level of tensile strength and elongation balance (TS 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield strength ratio evaluation index (1-YR), the bainite fraction must be optimized. In order to ensure 3.0*10 6 The balance of tensile strength and elongation (TS 2 *EL 1 / 2 ), 6.0*10 6 The balance of 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 control the bainite fraction to 10 volume % or more. More preferably, the bainite fraction may be 12 volume % or more or 14 volume % or more. On the other hand, when excessive bainite is formed, the tempered martensite fraction will eventually decrease. Therefore, in order to ensure 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 the yield ratio evaluation index (1-YR), the bainite fraction can be limited to 30 volume % or less. The preferred bainite fraction can be 12 volume % or more or 14 volume % or less, or 28 volume % or less or 26 volume % or less.

[0104] Steel sheets containing retained austenite have excellent ductility and workability due to the transformation-induced plasticity generated when transforming from austenite to martensite during processing. When the fraction of retained austenite is less than a certain level, the balance of tensile strength and elongation (TS 2 *EL 1 / 2 ) is less than 3.0*10 6 (MPa 2 % 1 / 2 ), so it is not preferred. In addition, when the fraction of retained austenite exceeds a certain level, the local elongation (Local Elongation) may decrease or the spot weldability may decrease. Therefore, in order to obtain a balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) Steel sheets with excellent performance, the fraction of retained austenite can be limited to a range of 2-10% in the present invention. Preferably, the fraction of retained austenite can be 3% by volume or more or 8% by volume or less.

[0105] The steel sheet of the present invention may contain ferrite, pearlite, island martensite (Martensite Austenite Constituent, MA), and other unavoidable microstructures. Excessive ferrite formation can reduce the strength of the steel sheet. Therefore, the ferrite fraction is limited to 5% by volume (including 0%) or less in the present invention. Furthermore, excessive pearlite formation can reduce the workability of the steel sheet or the fraction of retained austenite. Therefore, the present invention aims to minimize the formation of pearlite.

[0106] The high-strength steel sheet with excellent workability according to one aspect of the present invention can satisfy the following [Relational Expression 1] and [Relational Expression 2].

[0107] [Equation 1]

[0108] 0.03≤[B] FM / [B] TM ≤0.55

[0109] In the above relational expression 1, [B] FM is the content (weight %) of boron (B) contained in the fresh martensite, [B] TM It is the content (weight %) of boron (B) contained in tempered martensite.

[0110] [Equation 2]

[0111] V(1.2μm,γ) / V(γ)≥0.12

[0112] In the above-mentioned Relational Formula 2, V(1.2 μm, γ) is the fraction (volume %) of retained austenite having an average grain size of 1.2 μm or greater, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0113] To ensure 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 (1-YR), in the present invention, the microstructure fractions of tempered martensite, fresh martensite and retained austenite can be controlled within a certain range, and the proportion of boron (B) contained in the tempered martensite and fresh martensite can be controlled within a certain range, and the ratio of retained austenite of a specific size, shape and type to the entire retained austenite can be controlled within a certain range.

[0114] In the present invention, as shown in [Relational Formula 1], the content of boron (B) contained in the newly formed martensite ([B] FM , wt%) and the content of boron (B) contained in tempered martensite ([B] TM , weight %) is controlled in the range of 0.03 to 0.55, thus ensuring 3.0*10 6 to 6.2*10 6 (MPa 2 % 1 / 2 )'s balance of tensile strength and elongation (B TE ), 6.0*10 6 to 11.5*10 6 (MPa 2 % 1 / 2 )'s tensile strength and hole expansion ratio (B TH ) and a yield strength ratio evaluation index (I YR ).

[0115] The inventors of the present invention have conducted in-depth research on methods for ensuring the physical properties of TRIP steel with added boron (B). They have noted that, although the theoretical basis has not yet been clearly elucidated, the desired physical properties of the present invention can be ensured when the ratio of the boron (B) content contained in the fresh martensite to the boron (B) content contained in the tempered martensite falls within 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 contained in the tempered martensite to the fresh martensite. Therefore, in the present invention, as shown in [Relationship 1], the ratio of the boron (B) content contained in the fresh martensite to the boron (B) content contained in the tempered martensite is limited to the range of 0.03 to 0.55, thereby ensuring the desired balance between tensile strength and elongation (TS 2 *EL1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield strength ratio evaluation index (1-YR).

[0116] Furthermore, the inventors of the present invention have recognized that not only the fraction of retained austenite but also the ratio of retained austenite of a specific size to the entire retained austenite is an important factor in ensuring strength and workability.

[0117] Increasing the proportion of retained austenite with an average grain size of 1.2 μm or greater within the retained austenite can help improve the workability of the steel sheet. Retained austenite with an average grain size of 1.2 μm or greater is retained austenite whose average size increases due to heat treatment at a bainite-forming temperature. Compared to retained austenite with an average grain size of 1.2 μm or less, the driving force for the phase transformation to martensite is relatively suppressed. Therefore, the phase transformation of retained austenite with an average grain size of 1.2 μm or greater to martensite is suppressed. Therefore, when the proportion of retained austenite with an average grain size of 1.2 μm or greater is above a certain level, the workability of the steel sheet can be further improved.

[0118] In the present invention, as shown in [Relational Formula 2], 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 fraction of the entire retained austenite contained in the steel sheet (V (γ), volume %) is controlled to be 0.12 or more, thereby effectively ensuring the desired balance between tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance of tensile strength and hole expansion (TS 2 *HER 1 / 2 ).

[0119] In the high-strength steel sheet with excellent workability according to one aspect of the present invention, the balance between tensile strength and elongation (B TE ) can meet 3.0*10 6 to 6.2*10 6 (MPa 2 % 1 / 2 ), and the balance between tensile strength and hole expansion ratio (B TH ) can meet 6.0*10 6 to 11.5*10 6 (MPa 2 % 1 / 2 ), and the yield strength ratio evaluation index (I YR ) can satisfy 0.15 to 0.42.

[0120] [Equation 3]

[0121] B TE =[Tensile Strength (TS, MPa)] 2 *[Elongation (El, %)] 1 / 2

[0122] [Equation 4]

[0123] B TH =[Tensile Strength (TS, MPa)] 2 *[Hole expansion ratio (HER, %)] 1 / 2

[0124] [Equation 5]

[0125] I YR =1-[yield strength ratio (YR)]

[0126] Hereinafter, an example of a method for producing the steel sheet of the present invention will be described in detail.

[0127] According to one aspect of the present invention, a method for manufacturing a high-strength steel plate may include the following steps: heating a cold-rolled steel plate 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 then holding for 50-1200 seconds (primary holding); cooling the steel plate that has been held once to a temperature range of 350-550°C at an average cooling rate of 2-100°C / s (primary cooling), and then holding for 5-600 seconds (secondary holding); cooling the steel plate that has been held twice to a temperature range of 200-400°C at an average cooling rate of 2-100°C / s (secondary cooling); heating the steel plate that has been cooled twice to a temperature range of 350-550°C at an average heating rate of 5-100°C / s (tertiary heating), and then holding for more than 50 seconds (tertiary holding); cooling the steel plate that has been held three times to room temperature at an average cooling rate of 1°C / s or more (tertiary cooling).

[0128] The cold-rolled steel plate can be provided by the following steps: heating a steel billet having a predetermined alloy composition to 1000-1350°C; performing hot finish rolling in a temperature range of 800-1000°C; coiling the hot-rolled steel plate in a temperature range of 350-650°C; pickling the coiled steel plate; and cold rolling the pickled steel plate at a reduction ratio of 30-90%.

[0129] Billet preparation and heating

[0130] A steel slab having a predetermined alloy composition is prepared. The steel slab of the present invention has an alloy composition corresponding to the alloy composition of the steel plate described above, so the alloy composition of the steel slab will be described below instead of the alloy composition of the steel slab.

[0131] The prepared steel slab can be heated to a certain temperature range, and the heating temperature of the steel slab can be in the range of 1000-1350°C. When the heating temperature of the steel slab is lower than 1000°C, hot rolling may be performed in a temperature range below the desired hot finishing rolling temperature range. When the heating temperature of the steel slab exceeds 1350°C, the steel may reach its melting point and melt.

[0132] Hot rolling and coiling

[0133] The heated steel slab can be hot-rolled to provide a hot-rolled steel plate. The hot finishing rolling temperature during hot rolling is preferably in the range of 800-1000°C. If the hot finishing rolling temperature is lower than 800°C, excessive rolling load may become a problem. If the hot finishing rolling temperature exceeds 1000°C, coarse grains are formed in the hot-rolled steel plate, which may cause a decrease in the physical properties of the final steel plate.

[0134] Hot-rolled steel sheets that have completed hot rolling can be cooled at an average cooling rate of 10°C / s or higher and can be coiled within a temperature range of 350-650°C. This is because coiling is difficult if the coiling temperature is lower than 350°C, and when the coiling temperature exceeds 650°C, surface scale forms deep within the hot-rolled steel sheet, making pickling difficult.

[0135] Pickling and cold rolling

[0136] After unwinding the hot-rolled coil, the steel sheet may be pickled to remove scale formed on the surface, followed by cold rolling. While the pickling and cold rolling conditions are not particularly limited in the present invention, cold rolling is preferably performed at a cumulative reduction of 30-90%. If the cumulative reduction exceeds 90%, the high strength of the steel sheet may make cold rolling difficult to perform in a short time.

[0137] The cold-rolled steel sheet can be subjected to an annealing heat treatment process to produce an uncoated cold-rolled steel sheet, or can be subjected to a plating process to produce a plated steel sheet to impart corrosion resistance. The plating method can be hot-dip galvanizing, electro-galvanizing, hot-dip aluminizing, etc., and the method and type are not particularly limited.

[0138] Annealing heat treatment

[0139] In the present invention, in order to simultaneously ensure the strength and workability of the steel sheet, an annealing heat treatment process is performed.

[0140] The cold-rolled steel sheet is heated to 700°C at an average heating rate of 5°C / s or more (primary heating), 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 held for 50-1200 seconds (primary holding).

[0141] When the average heating rate of the primary heating to 700°C is less than 5°C / s, massive austenite is formed from ferrite and cementite formed during the heating process, resulting in failure to form fine tempered martensite and retained austenite as the final structure. Therefore, the desired balance of tensile strength and elongation (TS) cannot be achieved. 2 *EL 1 / 2 ) and the balance of tensile strength and hole expansion (TS 2 *HER 1 / 2 ). In addition, when the secondary heating rate to the primary holding temperature exceeds 5°C / second, the phase transformation from cementite formed during the heating process to austenite is accelerated, a large amount of massive austenite is formed, and the final structure is coarsened, and boron (B) cannot be fully enriched in the tempered martensite. Therefore, [B] FM / [B] TM exceeds 0.55, and the desired level of 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 strength ratio evaluation index (I YR ).

[0142] When the primary holding temperature is less than Ac3 (two-phase region), more than 5 volume % of ferrite is formed, so the balance of tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance of tensile strength and hole expansion (TS 2 *HER 1 / 2 ) may be reduced. Furthermore, when the primary holding time is less than 50 seconds, the structure is not sufficiently homogenized, and thus the physical properties of the steel plate may be reduced. The upper limits of the primary holding temperature and the primary holding time are not particularly limited, but to prevent a decrease in toughness due to grain coarsening, the primary holding temperature is preferably limited to 920°C or less, and the primary holding time is preferably limited to 1200 seconds or less.

[0143] After the primary holding, the steel plate can be cooled to a temperature range of 350-550°C at a primary cooling rate of 2°C / s or more (primary cooling), and then held in the corresponding temperature range for more than 5 seconds (secondary holding). When the average cooling rate of the primary cooling is less than 2°C / s, the fraction of retained austenite becomes insufficient due to slow cooling, and the balance of tensile strength and elongation (TS) of the steel plate is affected.2 *EL 1 / 2 ) and the balance of tensile strength and hole expansion (TS 2 *HER 1 / 2 ) may be reduced. The upper limit of the average cooling rate of the primary cooling does not need to be particularly specified, but it is preferably 100°C or less. When the secondary holding temperature is lower than 350°C, the V(1.2μm,γ) / V(γ) of the steel plate and the balance between tensile strength and hole expansion ratio (TS) are affected due to the low heat treatment temperature. 2 *HER 1 / 2 On the other hand, when the secondary holding temperature exceeds 550℃, the retained austenite is insufficient and the balance of tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance of tensile strength and hole expansion (TS 2 *HER 1 / 2 ) may be reduced. In addition, when the secondary holding time is less than 5 seconds, the heat treatment time is insufficient, so the V (1.2 μm, γ) / V (γ) of the steel plate and the balance of tensile strength and hole expansion ratio (TS 2 *HER 1 / 2 There is no need to specify an upper limit for the secondary holding time, but it is preferably limited to 600 seconds or less.

[0144] After the secondary holding, the steel plate can be cooled to the primary cooling end temperature of 200-400°C at an average cooling rate of 2°C / s or more (secondary cooling). When the average cooling rate of the secondary cooling is less than 2°C / s, the fraction of retained austenite becomes insufficient due to slow cooling, and the balance of tensile strength and elongation (TS) of the steel plate is affected. 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may be reduced. The upper limit of the average cooling rate of the secondary cooling does not need to be particularly specified, but it is preferably 100°C / s or less. When the primary cooling end temperature is lower than 200°C, excessive tempered martensite is formed and insufficient retained austenite is formed, so the balance of V (1.2μm, γ) / V (γ) and tensile strength and elongation (TS) of the steel plate is affected. 2 *EL 1 / 2 ) and the balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 On the other hand, when the primary cooling end temperature exceeds 400°C, excessive bainite is formed and insufficient tempered martensite is formed, so the balance between the tensile strength and elongation of the steel plate (TS 2 *EL 1 / 2 ) and the balance of tensile strength and hole expansion (TS 2 *HER 1 / 2) may be reduced.

[0145] After secondary cooling, it can be heated to a temperature range of 350-550°C at an average heating rate of 5°C / second or more (tertiary heating), and then held for more than 50 seconds (tertiary holding). The upper limit of the average heating rate for the tertiary heating does not need to be specifically specified, but is preferably 100°C / second or less. When the tertiary holding temperature is lower than 350°C or the tertiary holding time is less than 50 seconds, too much tempered martensite is formed, making it difficult to ensure the fraction of retained austenite. As a result, the balance of V(1.2μm, γ) / V(γ), tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance of tensile strength and hole expansion (TS 2 *HER 1 / 2 ) may be reduced. When the three-holding temperature exceeds 550℃ or the three-holding time exceeds 155,000 seconds, the fraction of retained austenite is insufficient, so the balance of V(1.2μm,γ) / V(γ), tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance of tensile strength and hole expansion (TS 2 *HER 1 / 2 ) may be reduced.

[0146] After three holding steps, the material may be cooled to room temperature at an average cooling rate of 1° C. / s or higher (three cooling steps).

[0147] In the high-strength steel plate with excellent workability manufactured by the above-mentioned manufacturing method, the fine structure may include bainite, tempered martensite, fresh martensite, retained austenite and other inevitable structures. As a preferred example, the fine structure may include, by volume fraction, 10-30% of bainite, 50-70% of tempered martensite, 10-30% of fresh martensite, 2-10% of retained austenite, and 5% or less (including 0%) of ferrite.

[0148] In the steel sheet manufactured by the above-mentioned manufacturing method, the balance between tensile strength and elongation (B TE ) can meet 3.0*10 6 to 6.2*10 6 (MPa 2 % 1 / 2 ), the balance between tensile strength and hole expansion ratio (B TH ) can meet 6.0*10 6 to 11.5*10 6 (MPa 2 % 1 / 2 ), the yield strength ratio evaluation index (IYR ) can satisfy 0.15 to 0.42.

[0149] [Equation 3]

[0150] B TE =[Tensile Strength (TS, MPa)] 2 *[Elongation (El, %)] 1 / 2

[0151] [Equation 4]

[0152] B TH =[Tensile Strength (TS, MPa)] 2 *[Hole expansion ratio (HER, %)] 1 / 2

[0153] [Equation 5]

[0154] I YR =1-[yield strength ratio (YR)] DETAILED DESCRIPTION

[0155] The following describes in more detail the high-strength steel sheet with excellent workability and its manufacturing method, one aspect of the present invention, through specific examples. It should be noted that the following examples are intended solely for 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 claims and any reasonable inferences therefrom.

[0156] (Example)

[0157] Steel slabs with a thickness of 100 mm were produced, each having the alloy composition shown in Table 1 below (the balance being Fe and unavoidable impurities). These slabs were heated at 1200°C and then hot-finished rolled at 900°C. Subsequently, the slabs were cooled at an average cooling rate of 30°C / s and coiled at the coiling temperatures shown in Tables 2 and 3 to produce hot-rolled steel sheets with a thickness of 3 mm. These slabs were then pickled to remove surface scale and then cold-rolled to a thickness of 1.5 mm.

[0158] Then, heat treatment was performed under the annealing conditions described in Tables 2 to 5 below to produce steel sheets. In Tables 2 and 3 below, the single-phase region represents the temperature range of Ac3 to 920°C, and the two-phase region represents the temperature range below Ac3°C.

[0159] The microstructure of the steel plates produced as described above was observed, and the results are shown in Tables 6 and 7. Cross-sections of the polished test pieces were etched with Nital, and then SEM was used to observe the microstructures for ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM), and pearlite (P). After Nital etching, the structure with no irregularities on the test piece surface was classified as ferrite, while the structure with a layered structure of cementite and ferrite was classified as pearlite. Because both bainite (B) and tempered martensite (TM) were observed to have lamellar and blocky morphologies, making them difficult to distinguish, the expansion curve was used to calculate the bainite and tempered martensite fractions after expansion evaluation. Specifically, the bainite fraction was determined by subtracting the tempered martensite fraction calculated using the expansion curve from the bainite and tempered martensite fractions measured by SEM observation. Since fresh martensite (FM) and retained austenite (retained γ) are also difficult to distinguish, the fresh martensite fraction was determined by subtracting the retained austenite fraction calculated by X-ray diffraction from the martensite and retained austenite fractions observed by the SEM.

[0160] In addition, the steel plate's [B] FM / [B] TM , V(1.2μm,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 strength ratio evaluation index (I YR ) were measured and evaluated, and the results are shown in Tables 8 and 9.

[0161] The boron (B) content in the fresh martensite ([B] FM ) and the boron (B) content in tempered martensite ([B] TM ), determined by measuring the boron (B) concentration in fresh martensite and tempered martensite using an electron probe microanalyzer (EPMA). Retained austenite (V(1.2μm, γ)) with an average grain size of 1.2μm or greater was measured using a phase map using electron backscatter diffraction (EBSD).

[0162] Tensile strength (TS) and elongation (El) are evaluated by tensile testing. Test pieces are taken and evaluated according to JIS No. 5 standard, with the rolling direction of the rolled plate at 90°. The hole expansion ratio (HER) is evaluated by a hole expansion test. After forming a 10 mm Ψ hole (die inner diameter 10.3 mm, clearance 12.5%), a conical punch with a top angle of 60° is inserted into the hole in the direction where the burr of the hole becomes the outside. The area around the hole is squeezed and expanded at a moving speed of 20 mm / min. The result is then calculated using the following [Equation 6].

[0163] [Equation 6]

[0164] Hole expansion ratio (HER, %) = {(D-D0) / D0} × 100

[0165] In the above-mentioned relational expression 6, D represents the hole diameter (mm) when the crack penetrates the steel plate along the thickness direction, and D0 represents the initial hole diameter (mm).

[0166] [Table 1]

[0167]

[0168] [Table 2]

[0169]

[0170] [Table 3]

[0171]

[0172] [Table 4]

[0173]

[0174] [Table 5]

[0175]

[0176] [Table 6]

[0177]

[0178] [Table 7]

[0179]

[0180] [Table 8]

[0181]

[0182] [Table 9]

[0183]

[0184] As shown in Tables 1 to 9, it can be seen that in the case of the test pieces that meet the conditions proposed in the present invention, [Relationship 1] and [Relationship 2] are satisfied, and the balance between tensile strength and elongation (B TE )Satisfy 3.0*10 6 to 6.2*10 6 (MPa 2 % 1 / 2 ), balance between tensile strength and hole expansion rate (B TH )Meet 6.0*10 6 to 11.5*10 6 (MPa 2 % 1 / 2 ), yield strength ratio evaluation index (I YR ) satisfies 0.15 to 0.42.

[0185] In the test piece 2, the average heating rate is less than 5°C / s, so the tempered martensite and retained austenite are insufficient. As a result, the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0186] In the test piece 3, the secondary average heating rate exceeded 5°C / s, so massive austenite was formed and boron (B) was not concentrated in the tempered martensite. As a result, the [B] of the test piece 3 was FM / [B] TM More than 0.55, the yield strength ratio evaluation index (I YR ) exceeds 0.42, the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0187] In the test piece 4, the temperature was kept below Ac3 in the two-phase region, so the ferrite fraction was excessive. As a result, the balance of tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0188] In the test piece 5, the primary average cooling rate was less than 1°C / s, so the fraction of retained austenite was insufficient. As a result, the balance between tensile strength and elongation (BTE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0189] In the case of the test piece 6, the secondary holding temperature was lower than 350°C, so the heat treatment temperature was insufficient. As a result, the V(1.2μm,γ) / V(γ) of the test piece 6 was less than 0.12, and the balance between the tensile strength and the hole expansion ratio (B TH ) is less than 6.0*10 6 .

[0190] In the test piece 7, the secondary holding temperature exceeded 550°C, so the fraction of retained austenite was insufficient. As a result, the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0191] In the test piece 8, the secondary holding time was less than 5 seconds, so the heat treatment time was insufficient. As a result, the V(1.2μm,γ) / V(γ) of the test piece 8 was less than 0.12, and the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0192] In the test piece 9, the secondary average cooling rate is less than 2°C / s, so the fraction of retained austenite is insufficient. As a result, the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0193] In the test piece 10, the primary cooling end temperature is lower than 200°C, so the fraction of tempered martensite is excessive and the fraction of retained austenite is insufficient. As a result, the V(1.2μm,γ) / V(γ) of the test piece 10 is less than 0.12, and the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0194] In the test piece 11, the primary cooling end temperature exceeded 400°C, so the fraction of bainite was excessive and the fraction of tempered martensite was insufficient. As a result, the balance between tensile strength and elongation (B TE ) is less than 3.0*106 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0195] In the test piece 12, the three-time holding temperature is lower than 350°C, so the fraction of tempered martensite is excessive and the fraction of retained austenite is insufficient. As a result, the V(1.2μm,γ) / V(γ) of the test piece 12 is less than 0.12, and the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0196] In the test piece 13, the three-time holding temperature exceeded 550°C, so the fraction of retained austenite was insufficient. As a result, the V(1.2μm,γ) / V(γ) of the test piece 16 was less than 0.12, and the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0197] In the test piece 14, the three holding times were less than 50 seconds, 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) of the test piece 14 was less than 0.12, and the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0198] In the test piece 15, the three-time holding time exceeded 155,000 seconds, so the fraction of retained austenite was insufficient. As a result, the V(1.2μm,γ) / V(γ) of the test piece 15 was less than 0.12, and the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0199] In the test piece 37, the carbon (C) content is low, so the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0200] In the test piece 38, the carbon (C) content is high, so the fraction of tempered martensite is insufficient, the fraction of fresh martensite is excessive, and the fraction of retained austenite is excessive. As a result, the balance (B) of tensile strength and elongation of the test piece 38 is poor. TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0201] In the test piece 39, the silicon (Si) content is low, so the fraction of retained austenite is insufficient. As a result, the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0202] In the test piece 40, the silicon (Si) content is high, so the fraction of newly formed martensite is high. As a result, the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0203] In the test piece 41, the aluminum (Al) content is high, so the fraction of newly formed martensite is high. As a result, the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0204] In the test piece 42, the manganese (Mn) content is low and pearlite is generated, resulting in an insufficient fraction of retained austenite. As a result, the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0205] In the test piece 43, the manganese (Mn) content is high, so the fraction of newly formed martensite is high. As a result, the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0206] In the test piece 44, the chromium (Cr) content is high, so the fraction of newly formed martensite is high. As a result, the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0207] In the test piece 45, the molybdenum (Mo) content is high, so the fraction of newly formed martensite is high. As a result, the balance between tensile strength and elongation (B TE ) is less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0208] In the test piece 46, the boron (B) content is low, so the boron (B) cannot be enriched in the tempered martensite. As a result, the [B] of the test piece 51 is FM / [B] TM More than 0.55, the yield strength ratio evaluation index (I YR ) exceeds 0.42.

[0209] In the test piece 47, the boron (B) content is high, and the boron (B) is excessively concentrated in the tempered martensite. As a result, the [B] of the test piece 52 is FM / [B] TM Less than 0.03, yield strength ratio evaluation index (I YR ) is less than 0.15.

[0210] The present invention has been described in detail above through the embodiments, but other embodiments are also possible. Therefore, the technical concept and scope of the claims are not limited to the embodiments.

Claims

1. A high-strength steel plate with excellent workability, comprising, in weight percent, 0.1-0.25% C, 0.01-1.5% Si, 1.0-4.0% Mn, 0.01-1.5% Al, 0.15% or less P, 0.03% or less S, 0.03% or less N, 0.0005-0.005% B, and the balance Fe and unavoidable impurities. The microstructure includes bainite, tempered martensite, fresh martensite, retained austenite and other inevitable structures. The steel plate satisfies the following [Relationship 1] and [Relationship 2], [Equation 1] 0.03≤[B] FM / [B] TM ≤0.55 In the above relational expression 1, [B] FM is the weight percent of boron contained in the fresh martensite; [B] TM is the weight % of boron contained in tempered martensite [Equation 2] V(1.2μm,γ) / V(γ)≥0.12 In the above-mentioned Relational Formula 2, V(1.2 μm, γ) is the volume % of retained austenite having an average grain size of 1.2 μm or more, and V(γ) is the volume % of retained austenite in the steel sheet.

2. The high-strength steel sheet having excellent workability according to claim 1, wherein The steel sheet further comprises, in 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) one or more of Cr: 0-3.0% and Mo: 0-3.0%, (3) Cu: 0-4.0% and Ni: 0-4.0% or more, (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 high-strength steel sheet having excellent workability according to claim 1, wherein Calculated by volume fraction, the microstructure of the steel plate comprises: 10-30% of bainite, 50-70% of tempered martensite, 10-30% of fresh martensite, 2-10% of retained austenite, and less than 5% of ferrite.

4. The high-strength steel sheet having excellent workability according to claim 1, wherein In the steel plate, the balance B between tensile strength and elongation expressed by the following [Relationship 3] TE Meet 3.0*10 6 to 6.2*10 6 MPa 2 % 1 / 2 The balance between tensile strength and hole expansion ratio is expressed by the following [Relationship 4] TH Meet 6.0*10 6 to 11.5*10 6 MPa 2 % 1 / 2 , the yield strength ratio evaluation index I expressed by the following [Relationship 5] YR Satisfy 0.15 to 0.42, [Equation 3] B TE =TS 2 *The 1 / 2 In the relational expression 3, TS refers to the tensile strength of the steel plate, and El refers to the elongation of the steel plate. [Equation 4] B TH =TS 2 *HER 1 / 2 In the above equation 4, TS refers to the tensile strength of the steel plate, HER refers to the hole expansion ratio of the steel plate, [Equation 5] I YR =1-YR In the relational expression 5, YR refers to the yield strength ratio of the steel plate.

5. A method for manufacturing a high-strength steel plate having excellent workability, comprising the following steps: A cold-rolled steel sheet is provided, wherein the cold-rolled steel sheet comprises, in weight percent, 0.1-0.25% C, 0.01-1.5% Si, 1.0-4.0% Mn, 0.01-1.5% Al, 0.15% or less P, 0.03% or less S, 0.03% or less N, 0.0005-0.005% B, and the balance Fe and unavoidable impurities. The cold-rolled steel sheet is heated to 700° C. at an average heating rate of 5° C. / s or more, and then heated to a temperature range of Ac3 to 920° C. at an average heating rate of 5° C. / s or less, and then maintained for 50-1200 seconds. Cooling the steel plate held once at an average cooling rate of 2-100° C. / s to a temperature range of 350-550° C., and then holding the steel plate for a second time for 5-600 seconds; Secondarily cooling the steel plate held for the second time at an average cooling rate of 2-100° C. / s to a temperature range of 200-400° C.; Heating the secondary cooled steel plate three times at an average heating rate of 5-100° C. / second to a temperature range of 350-550° C., and then holding the temperature for more than 50 seconds three times; and The steel plate held three times is cooled three times at an average cooling rate of 1° C. / s or more to room temperature.

6. The method for producing a high-strength steel sheet having excellent workability according to claim 5, wherein: The cold-rolled steel sheet further comprises 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) Cu: 0-4.0% and Ni: 0-4.0% or more, (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%.

7. The method for producing a high-strength steel sheet having excellent workability according to claim 5, wherein: The cold-rolled steel sheet is provided by the following steps: Heat the steel billet to 1000-1350℃; Hot finishing rolling is carried out in the temperature range of 800-1000°C; The hot-rolled steel plate is coiled at a temperature of 350-650°C. pickling the coiled steel plate; as well as The pickled steel sheet is cold rolled at a reduction ratio of 30-90%.

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