High-strength steel sheet with excellent workability and method for manufacturing 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 lightweight materials while ensuring passenger stability in order to protect the global environment. To meet this demand for stability and lightweighting, the use of high-strength steel sheets has increased dramatically. Generally, it is known that as the strength of a steel sheet 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 while maintaining 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 to be solved by the present invention are not limited to the above contents. The additional technical problems to be solved by the present invention are described in the full text of the specification, and those skilled in the art can easily understand the additional technical problems to be solved by the present invention from the contents described in the specification of the present invention.

[0016] Technical Solution

[0017] In a high-strength steel sheet having excellent workability according to one aspect of the present invention, the steel sheet 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 being Fe and inevitable impurities. The fine structure may contain bainite, tempered martensite, fresh martensite, retained austenite, and other inevitable structures. The steel sheet may satisfy the following [Relationship 1] to [Relationship 4].

[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] [Equation 3]

[0025] V(lath, γ) / V(γ)≥0.5

[0026] In the above-mentioned Relational Formula 3, V(lath, γ) is the fraction (volume %) of lath-shaped retained austenite, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0027] [Equation 4]

[0028] T(γ) / V(γ)≥0.08

[0029] In the above-mentioned Relational Expression 4, T(γ) is the fraction (volume %) of tempered retained austenite in the steel sheet, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0030] The steel sheet may further contain, in wt%, any one or more of the following (1) to (8).

[0031] (1) one or more of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5%,

[0032] (2) one or more of Cr: 0-3.0% and Mo: 0-3.0%,

[0033] (3) Cu: 0-4.0% and Ni: 0-4.0% or more,

[0034] (4) One or more of Ca: 0-0.05%, REM excluding Y: 0-0.05%, and Mg: 0-0.05%,

[0035] (5) one or more of W: 0-0.5% and Zr: 0-0.5%,

[0036] (6) one or more of Sb: 0-0.5% and Sn: 0-0.5%,

[0037] (7) one or more of Y: 0-0.2% and Hf: 0-0.2%,

[0038] (8) Co: 0-1.5%.

[0039] 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.

[0040] 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.

[0041] [Equation 5]

[0042] B TE =[Tensile Strength (TS, MPa)] 2 *[Elongation (El, %)] 1 / 2

[0043] [Equation 6]

[0044] B TH =[Tensile Strength (TS, MPa)] 2 *[Hole expansion ratio (HER, %)] 1 / 2

[0045] [Equation 7]

[0046] I YR =1-[yield strength ratio (YR)]

[0047] A method for manufacturing a high-strength steel sheet having excellent workability according to one aspect of the present invention may include the following steps: providing a cold-rolled steel sheet, the steel sheet comprising, 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 being Fe and unavoidable impurities; heating the cold-rolled steel sheet to 700° C. at an average heating rate of 5° C. / s or more (primary heating), heating to a temperature range of Ac3 to 920° C. at an average heating rate of 5° C. / s or less (secondary heating), and then holding the steel sheet for 50-1200 seconds (primary holding); cooling the steel sheet after the primary holding to a temperature range of 400-600° C. at an average cooling rate of 2-100° C. / s (primary cooling). , and then hold for 5-600 seconds (secondary holding); cooling the steel plate held for the second time to a temperature range of 300-500° C. at an average cooling rate of 1-100° C. / second (secondary cooling), and then hold for 5-600 seconds (third holding); cooling the steel plate held for the third time to a temperature range of 200-400° C. at an average cooling rate of 2-100° C. / second (third cooling); heating the steel plate cooled for the third time to a temperature range of 400-600° C. at an average heating rate of 5-100° C. / second (third heating), and then hold for 10-1800 seconds (fourth holding); cooling the steel plate held for the fourth time to a temperature range of 300-500° C. at an average cooling rate of 1-100° C. / second (fourth cooling), and then hold for 10-1800 seconds (fifth holding); and cooling the steel plate held for the fifth time to room temperature at an average cooling rate of more than 1° C. / second (fifth cooling).

[0048] The steel billet may further include any one or more of the following (1) to (8).

[0049] (1) one or more of Ti: 0-0.5%, Nb: 0-0.5% and V: 0-0.5%,

[0050] (2) one or more of Cr: 0-3.0% and Mo: 0-3.0%,

[0051] (3) Cu: 0-4.0% and Ni: 0-4.0% or more,

[0052] (4) One or more of Ca: 0-0.05%, REM excluding Y: 0-0.05%, and Mg: 0-0.05%,

[0053] (5) one or more of W: 0-0.5% and Zr: 0-0.5%,

[0054] (6) one or more of Sb: 0-0.5% and Sn: 0-0.5%,

[0055] (7) one or more of Y: 0-0.2% and Hf: 0-0.2%,

[0056] (8) Co: 0-1.5%.

[0057] 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%.

[0058] The cooling rate (Vc1) of the first cooling and the cooling rate (Vc4) of the fourth cooling may satisfy the relationship of Vc1>Vc4.

[0059] Beneficial effects

[0060] According to a preferred aspect of the present invention, a steel sheet suitable for automobile parts and the like can be provided, which has an excellent balance between tensile strength and ductility, a balance between tensile strength and hole expandability, and a yield ratio evaluation index, and a method for producing the same.

[0061] Best Practice

[0062] The present invention relates to a high-strength steel plate with excellent workability and a method for manufacturing 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.

[0063] 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.

[0064] Hereinafter, a high-strength steel sheet having excellent workability according to one aspect of the present invention will be described in detail.

[0065] In a high-strength steel sheet with excellent workability according to one aspect of the present invention, the steel sheet 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 contain bainite, tempered martensite, fresh martensite, retained austenite, and other inevitable structures. The steel sheet may satisfy the following [Relationship 1] to [Relationship 4].

[0066] [Equation 1]

[0067] 0.03≤[B] FM / [B] TM ≤0.55

[0068] 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.

[0069] [Equation 2]

[0070] V(1.2μm,γ) / V(γ)≥0.12

[0071] In the above relational expression 2, V(1.2 μm, γ) is the fraction (volume %) of retained austenite with an average grain size of 1.2 μm or more, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0072] [Equation 3]

[0073] V(lath,γ) / V(γ)≥0.5

[0074] In the above-mentioned Relational Formula 3, V(lath, γ) is the fraction (volume %) of lath-shaped retained austenite, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0075] [Equation 4]

[0076] T(γ) / V(γ)≥0.08

[0077] In the above-mentioned Relational Expression 4, T(γ) is the fraction (volume %) of tempered retained austenite in the steel sheet, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0078] 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.

[0079] In a high-strength steel sheet with excellent workability according to one aspect of the present invention, the steel sheet comprises, in weight %, 0.1-0.25% of C, 0.01-1.5% of Si, 1.0-4.0% of Mn, 0.01-1.5% of Al, 0.15% or less of P, 0.03% or less of S, 0.03% or less of N, 0.0005-0.005% of B, and the balance of 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%).

[0080] Carbon (C): 0.1-0.25%

[0081] Carbon (C) is an essential element for ensuring the strength of the steel plate, and is an element that stabilizes the retained austenite that helps 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, the ductility decreases due to excessive increase in strength, and weldability may deteriorate. Therefore, the upper limit of the carbon (C) content may be limited to 0.25% in the present invention. The carbon (C) content may be less than 0.24%, and the more preferred carbon (C) content may be less than 0.23%.

[0082] Silicon (Si): 0.01-1.5% or less

[0083] 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 formation 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 phenomenon 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%.

[0084] Manganese (Mn): 1.0-4.0%

[0085] 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%.

[0086] Aluminum (Al): 0.01-1.5%

[0087] 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 in the present invention, the upper limit of the aluminum (Al) content can be limited to 1.5%. The preferred upper limit of the aluminum (Al) content can be 1.48%.

[0088] Phosphorus (P): 0.15% or less (including 0%)

[0089] 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.

[0090] Sulfur (S): 0.03% or less (including 0%)

[0091] 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.

[0092] Nitrogen (N): 0.03% or less (including 0%)

[0093] 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.

[0094] Boron (B): 0.0005-0.005%

[0095] 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 enriching 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.

[0096] 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.

[0097] One or more of titanium (Ti): 0-0.5%, niobium (Nb): 0-0.5%, and vanadium (V): 0-0.5%

[0098] 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.

[0099] One or more of chromium (Cr): 0-3.0% and molybdenum (Mo): 0-3.0%

[0100] 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.

[0101] Copper (Cu): 0-4.0% and nickel (Ni): 0-4.0% or more

[0102] 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, it leads to excessive characteristic effects and increases manufacturing costs. Therefore, in the present invention, the content of copper (Cu) and nickel (Ni) may be limited to 4.0% or less, respectively.

[0103] One or more of calcium (Ca): 0-0.05%, magnesium (Mg): 0-0.05%, and rare earth elements (REM) excluding yttrium (Y): 0-0.05%

[0104] 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.

[0105] Tungsten (W): 0-0.5% and zirconium (Zr): 0-0.5% or more

[0106] 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, exceeding a certain level of tungsten (W) or zirconium (Zr) can lead to excessive property effects and increase manufacturing costs. Therefore, in the present invention, the content of tungsten (W) and zirconium (Zr) can be limited to 0.5% or less, respectively.

[0107] Antimony (Sb): 0-0.5% and tin (Sn): 0-0.5% or more

[0108] 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 antimony (Sb) and tin (Sn) may be added to achieve the aforementioned effects. However, when the content of antimony (Sb) and tin (Sn) exceeds a certain level, the brittleness of the steel sheet increases, potentially causing cracks during hot or cold working. Therefore, in the present invention, the content of antimony (Sb) and tin (Sn) can be limited to 0.5% or less, respectively.

[0109] One or more of yttrium (Y): 0-0.2% and hafnium (Hf): 0-0.2%

[0110] 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.

[0111] Cobalt (Co): 0-1.5%

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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 edge workability 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 balance of tensile strength and elongation (TS) desired in 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 edge workability 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.

[0116] 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), it is necessary to optimize the bainite fraction. 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.

[0117] Steel sheets containing retained austenite have excellent ductility and workability due to the transformation-induced plasticity generated when austenite transforms 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 ) For a steel sheet with excellent performance, the retained austenite fraction can be limited to a range of 2-10% in the present invention. Preferably, the retained austenite fraction can be 3% by volume or more or 8% by volume or less.

[0118] 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.

[0119] The high-strength steel sheet with excellent workability according to one aspect of the present invention can satisfy the following [Relational Expression 1] to [Relational Expression 4].

[0120] [Equation 1]

[0121] 0.03≤[B] FM / [B] TM ≤0.55

[0122] 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.

[0123] [Equation 2]

[0124] V(1.2μm,γ) / V(γ)≥0.12

[0125] 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.

[0126] [Equation 3]

[0127] V(lath,γ) / V(γ)≥0.5

[0128] In the above-mentioned Relational Formula 3, V(lath, γ) is the fraction (volume %) of lath-shaped retained austenite, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0129] [Equation 4]

[0130] T(γ) / V(γ)≥0.08

[0131] In the above-mentioned Relational Expression 4, T(γ) is the fraction (volume %) of tempered retained austenite in the steel sheet, and V(γ) is the fraction (volume %) of retained austenite in the steel sheet.

[0132] 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 proportion of retained austenite of a specific size, shape and type relative to the entire retained austenite can be controlled within a certain range.

[0133] 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 (MPa2 % 1 / 2 )'s tensile strength and hole expansion ratio (B TH ) and a yield strength ratio evaluation index (I YR ).

[0134] The inventors of the present invention have conducted in-depth research on methods for ensuring the physical properties of TRIP steels containing boron (B). Although the theoretical basis has not yet been clearly elucidated, it has been noted that the desired physical properties of the present invention can be ensured only 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 and 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 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) and yield strength ratio evaluation index (1-YR).

[0135] 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 relative to the entire retained austenite are important factors in ensuring strength and workability.

[0136] 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, its driving force for transformation to martensite is relatively suppressed. Therefore, by suppressing the transformation of retained austenite with an average grain size of 1.2 μm or greater to martensite, the workability of the steel sheet can be more effectively improved when the proportion of retained austenite with an average grain size of 1.2 μm or greater is above a certain level.

[0137] Increasing the proportion of lath-shaped retained austenite within the retained austenite can improve the workability of the steel sheet. Retained austenite is divided into lath-shaped retained austenite formed between bainite phases and block-shaped retained austenite formed in areas without bainite phases. Block-shaped retained austenite further transforms into fresh martensite during heat treatment. When the proportion of lath-shaped retained austenite exceeds a certain level, the workability of the steel sheet can be more effectively improved.

[0138] Increasing the proportion of tempered retained austenite in retained austenite can improve the workability of the steel sheet. Tempered retained austenite is retained austenite enriched with carbon (C) during heat treatment at a bainite-forming temperature. This refers to retained austenite with a carbon (C) content (weight %) of at least 1.45 times the average carbon (C) content (weight %) of the steel sheet. Tempered retained austenite is relatively rich in carbon (C), an austenite-stabilizing element, thereby suppressing the phase transformation to martensite. When the proportion of tempered retained austenite is above a certain level, the workability of the steel sheet can be more effectively maintained.

[0139] Retained austenite with an average grain size of 1.2 μm or more, lath-shaped retained austenite, and tempered retained austenite are retained austenite distinguished by size, shape, and type, respectively, and are mutually compatible concepts.

[0140] 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 %) relative to the fraction of the entire retained austenite contained in the steel sheet (V(γ), volume %) is controlled to be 0.12 or more. As shown in [Relational Formula 3], the ratio of the fraction of lath-shaped retained austenite (V(lath, γ), volume %) relative to the fraction of the entire retained austenite contained in the steel sheet (V(γ), volume %) is controlled to be 0.5 or more. As shown in [Relational Formula 4], the fraction (volume %) of tempered retained austenite relative to the fraction (V(γ), volume %) contained in the steel sheet is limited to 0.08 or more. Therefore, the desired balance between tensile strength and elongation (TS) can be effectively ensured. 2 *EL 1 / 2 ) and the balance of tensile strength and hole expansion (TS 2 *HER 1 / 2 ).

[0141] 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 (IYR ) can satisfy 0.15 to 0.42.

[0142] [Equation 5]

[0143] B TE =[Tensile Strength (TS, MPa)] 2 *[Elongation (El, %)] 1 / 2

[0144] [Equation 6]

[0145] B TH =[Tensile Strength (TS, MPa)] 2 *[Hole expansion ratio (HER, %)] 1 / 2

[0146] [Equation 7]

[0147] I YR =1-[yield strength ratio (YR)]

[0148] Hereinafter, an example of a method for producing the steel sheet of the present invention will be described in detail.

[0149] The method for manufacturing a high-strength steel plate according to one aspect of the present invention 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. / second or more (primary heating), heating to a temperature range of Ac3 to 920° C. at an average heating rate of 5° C. / second or less (secondary heating), and then holding for 50-1200 seconds (primary holding); cooling the steel plate held in the primary holding to a temperature range of 400-600° C. at an average cooling rate of 2-100° C. / second (primary cooling), and then holding for 5-600 seconds (secondary holding); cooling the steel plate held in the secondary holding to a temperature range of 300-500° C. at an average cooling rate of 1-100° C. / second (secondary cooling), and then holding for 5-600 seconds (secondary holding); the steel plate which has been held three times is cooled at an average cooling rate of 2-100° C. / second to a temperature range of 200-400° C. (three cooling times); the steel plate which has been cooled three times is heated to a temperature range of 400-600° C. at an average heating rate of 5-100° C. / second (three heating times), and then held for 10-1800 seconds (four holding times); the steel plate which has been held four times is cooled at an average cooling rate of 1-100° C. / second to a temperature range of 300-500° C. (four cooling times), and then held for 10-1800 seconds (five holding times); and the steel plate which has been held five times is cooled to room temperature at an average cooling rate of more than 1° C. / second (five cooling times).

[0150] 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%.

[0151] Billet preparation and heating

[0152] 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.

[0153] The prepared steel slab may be heated to a certain temperature range, and the heating temperature of the steel slab may 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 melt due to reaching its melting point.

[0154] Hot rolling and coiling

[0155] 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.

[0156] 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.

[0157] Pickling and cold rolling

[0158] 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 total reduction ratio of 30-90%. If the total reduction ratio exceeds 90%, the high strength of the steel sheet may make cold rolling difficult to perform in a short time.

[0159] 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 in order to impart corrosion resistance. The plating can be performed by hot-dip galvanizing, electrogalvanizing, hot-dip aluminizing, or other plating methods, and the method and type are not particularly limited.

[0160] Annealing heat treatment

[0161] In the present invention, in order to simultaneously ensure the strength and workability of the steel sheet, an annealing heat treatment process is performed.

[0162] 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).

[0163] 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 T(γ) / V(γ), 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 / s, the cementite formed during the heating process is accelerated to transform into austenite, forming a large amount of massive austenite, 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 ).

[0164] 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 cannot be fully 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.

[0165] After the primary holding, the steel plate can be cooled to a temperature range of 400-600°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 T(γ) / V(γ), 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 400°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 600℃, the retained 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 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 (lath, γ) / V (γ), V (1.2 μm, γ) / V (γ) of the steel plate and the balance of tensile strength and hole expansion rate (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.

[0166] After the secondary hold, the steel plate can be cooled to a temperature range of 300-500°C at an average cooling rate of 1°C / s or more (secondary cooling), and then held in the corresponding temperature range for more than 5 seconds (tertiary hold). There is no need to specify an upper limit for the average cooling rate of the secondary cooling, but it is preferably 100°C / s or less. When the tertiary hold temperature is lower than 300°C, the V(1.2μm,γ) / V(γ) of the steel plate and the balance between tensile strength and hole expansion ratio (TS) are improved due to the low heat treatment temperature.2 *HER 1 / 2 On the other hand, when the three-holding temperature exceeds 500℃, the retained austenite is insufficient, so the balance of T(γ) / V(γ), V(lath,γ) / V(γ), tensile strength and elongation (TS 2 *EL 1 / 2 ), balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may be reduced. In addition, when the three-holding time is less than 5 seconds, the heat treatment time is insufficient, so the V (lath, γ) / V (γ), V (1.2 μm, γ) / V (γ) of the steel plate and the balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 There is no need to specify an upper limit for the three-time holding time, but it is preferably limited to 600 seconds or less.

[0167] After three holding times, the steel plate can be cooled to a primary cooling end temperature of 200-400°C at an average cooling rate of 2°C / s or more (tertiary cooling). When the average cooling rate of the three cooling times is less than 2°C / s, the fraction of retained austenite becomes insufficient due to slow cooling, and the balance of T(γ) / V(γ), 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 three coolings 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 T(γ) / V(γ), V(1.2μm,γ) / V(γ), and the balance of tensile strength and elongation (TS) of the steel plate are 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 rate (TS 2 *HER 1 / 2 ) may be reduced.

[0168] After the third cooling, the steel sheet may be heated to a temperature range of 400-600°C at an average heating rate of 5°C / s or more (third heating), and then held for 10-1800 seconds (fourth holding). The upper limit of the average heating rate for the third heating does not need to be particularly specified, but is preferably 100°C / s or less. When the fourth holding temperature is lower than 400°C, the V(1.2μm,γ) / V(γ) of the steel sheet and the balance between tensile strength and hole expansion ratio (TS) are improved due to the low heat treatment temperature. 2 *HER 1 / 2 ) may be reduced. When the four-holding temperature exceeds 600℃, the fraction of retained austenite is insufficient, so the balance of T(γ) / V(γ), 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 four-holding time is less than 10 seconds, the heat treatment time is insufficient, so the V(1.2μm,γ) / V(γ) of the steel plate and the balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 The upper limit of the four-time holding time does not need to be particularly specified, but is preferably 1800 seconds or less.

[0169] After the four holding times, the temperature may be cooled to a range of 300-500°C at an average cooling rate of 1°C / second or more (fourth cooling), and then held for 10-1800 seconds (fiveth holding). The upper limit of the average cooling rate for the four cooling times does not need to be particularly specified, but is preferably 100°C / second or less. When the five-holding temperature is lower than 300°C, the balance between V(1.2μm,γ) / V(γ) and the tensile strength and hole expansion ratio (TS) is improved due to the low heat treatment temperature. 2 *HER 1 / 2 ) may decrease. On the other hand, when the five-time holding temperature exceeds 500℃, the fraction of retained austenite is insufficient, so the balance of T(γ) / V(γ), V(1.2μm,γ) / V(γ), tensile strength and elongation (TS 2 *EL 1 / 2 ) and the balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 ) may be reduced. When the five holding times are less than 10 seconds, the heat treatment time is insufficient, so the V(1.2μm,γ) / V(γ) of the steel plate and the balance of tensile strength and hole expansion rate (TS 2 *HER 1 / 2 The upper limit of the five-time holding time does not need to be particularly specified, but is preferably 1800 seconds or less.

[0170] The cooling rate (Vc1) of the first cooling and the cooling rate (Vc4) of the fourth cooling may satisfy the relationship of Vc1>Vc4.

[0171] After five times of holding, the material may be cooled to room temperature at an average cooling rate of 1° C. / second or more (five times of cooling).

[0172] 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.

[0173] 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 (I YR ) can satisfy 0.15 to 0.42.

[0174] [Equation 5]

[0175] B TE =[Tensile Strength (TS, MPa)] 2 *[Elongation (El, %)] 1 / 2

[0176] [Equation 6]

[0177] B TH =[Tensile Strength (TS, MPa)] 2 *[Hole expansion ratio (HER, %)] 1 / 2

[0178] [Equation 7]

[0179] I YR =1-[yield strength ratio (YR)] DETAILED DESCRIPTION

[0180] 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.

[0181] (Example)

[0182] 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 finish hot-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.

[0183] Then, heat treatment was performed under the annealing conditions described in Tables 2 to 7 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 dual-phase region represents the temperature range below Ac3°C.

[0184] The microstructure of the steel plates produced as described above was observed, and the results are shown in Tables 8 and 9. Cross-sections of the polished test pieces were etched with Nital, and then SEM observations were made of ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM), and pearlite (P). After Nital etching, the surface of the test piece was classified as smooth, as ferrite, while the structure with a layered structure of cementite and ferrite was classified as pearlite. Both bainite (B) and tempered martensite (TM) were observed in lamellar and blocky morphologies, making them difficult to distinguish. Therefore, the fractions of bainite and tempered martensite were calculated using the expansion curve after expansion evaluation. Specifically, the fraction of tempered martensite, calculated using the expansion curve, was subtracted from the fractions of bainite and tempered martensite measured by SEM observation to determine the bainite fraction. Furthermore, since fresh martensite (FM) and retained austenite (retained γ) are also difficult to distinguish, the fraction of fresh martensite was determined by subtracting the fraction of retained austenite calculated by X-ray diffraction from the fractions of martensite and retained austenite observed by the SEM.

[0185] In addition, the steel plate's [B] 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 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 10 and 11.

[0186] Boron (B) content in fresh martensite ([B] FM ) and the boron (B) content in tempered martensite ([B] TM ) is determined as the boron (B) concentration in fresh martensite and tempered martensite measured using an electron probe microanalyzer (EPMA). Retained austenite with an average grain size of 1.2 μm or larger (V(1.2 μm, γ)) and lath-shaped retained austenite (V(lath, γ)) are measured using a phase map using electron backscatter diffraction (EBSD). Tempered retained austenite is differentiated based on the carbon (C) content in the retained austenite measured using EPMA.

[0187] Tensile strength (TS) and elongation (El) are evaluated by tensile testing, with test pieces taken and evaluated in accordance with JIS No. 5 standards at a 90° angle relative to the rolling direction of the rolled plate. Hole expansion ratio (HER) is evaluated by hole expansion testing, where a 10 mm Ψ hole is formed (die inner diameter 10.3 mm, clearance 12.5%), and a conical punch with a 60° apex angle is inserted into the hole in a direction where the burr of the hole becomes outward. 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 8].

[0188] [Equation 8]

[0189] Hole expansion ratio (HER, %) = {(D-D0) / D0} × 100

[0190] In the above relational expression 8, 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).

[0191] [Table 1]

[0192]

[0193] [Table 2]

[0194]

[0195] [Table 3]

[0196]

[0197] [Table 4]

[0198]

[0199] [Table 5]

[0200]

[0201] [Table 6]

[0202]

[0203] [Table 7]

[0204]

[0205] [Table 8]

[0206]

[0207] [Table 9]

[0208]

[0209] [Table 10]

[0210]

[0211] [Table 11]

[0212]

[0213] As shown in Tables 1 to 11, it can be seen that in the case of the test pieces that meet the conditions proposed in the present invention, [Relationship 1] to [Relationship 4] are satisfied, and the balance between tensile strength and elongation (B TE ) meets 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.

[0214] In the case of test piece 2, the average heating rate was less than 5°C / s, so the tempered martensite and retained austenite were insufficient. As a result, the T(γ) / V(γ) of test piece 2 was less than 0.08, 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 .

[0215] 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 .

[0216] In the test piece 4, the test was carried out in the dual phase region where the primary holding temperature was lower than Ac3, so the ferrite fraction exceeded the standard. 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 .

[0217] In the test piece 5, the primary average cooling rate was less than 2°C / s, so the fraction of retained austenite was insufficient. As a result, T(γ) / V(γ) of the test piece 5 was less than 0.08, 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 .

[0218] In the case of the test piece 6, the secondary holding temperature was lower than 400°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 .

[0219] In test piece 7, the secondary holding temperature exceeded 600°C, so the fraction of retained austenite was insufficient. As a result, V(lath, γ) / V(γ) of test piece 7 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 less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0220] In the case of 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, the V(lath,γ) / V(γ) was less than 0.5, and the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0221] In the case of the test piece 9, the temperature was kept below 300°C for three times, so the heat treatment temperature was insufficient. As a result, the V(1.2μm,γ) / V(γ) of the test piece 9 was less than 0.12, and the balance between tensile strength and hole expansion ratio (B TH ) is less than 6.0*10 6 .

[0222] In the test piece 10, the temperature was held over 500°C for three times, so the fraction of retained austenite was insufficient. As a result, the V(lath, γ) / V(γ) of the test piece 10 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 less than 3.0*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0223] In the case of the test piece 11, the three holding times were 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 11 was less than 0.12, the V(lath,γ) / V(γ) was less than 0.5, and the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0224] In the test piece 12, the average cooling rate over three times was less than 2°C / s, so the fraction of retained austenite was insufficient. As a result, the T(γ) / V(γ) of the test piece 12 was less than 0.08, 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 .

[0225] In the case of test piece 13, the primary cooling end temperature was lower than 200°C, so the tempered martensite fraction was excessive and the retained austenite fraction was insufficient. As a result, the V(1.2μm,γ) / V(γ) of test piece 13 was less than 0.12, T(γ) / V(γ) was less than 0.08, 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 .

[0226] In the test piece 14, the primary cooling end temperature exceeded 400°C, so the fraction of bainite exceeded the limit 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*10 6 , the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0227] In the test piece 15, the holding temperature was lower than 400°C four times, so the heat treatment temperature 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 hole expansion ratio (B TH ) is less than 6.0*10 6 .

[0228] In the test piece 16, the holding temperature exceeded 600°C four times, 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, T(γ) / V(γ) was less than 0.08, 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 .

[0229] In the test piece 17, the four holding times were less than 10 seconds, so the heat treatment time was insufficient. As a result, the V(1.2μm,γ) / V(γ) of the test piece 17 was less than 0.12, and the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0230] In the test piece 18, the temperature was kept below 300°C for five times, so the heat treatment temperature was insufficient. As a result, the V(1.2μm,γ) / V(γ) of the test piece 18 was less than 0.12, and the balance between tensile strength and hole expansion ratio (B TH ) is less than 6.0*106 .

[0231] In test piece 19, the holding temperature exceeded 500°C five times, so the fraction of retained austenite was insufficient. As a result, V(1.2μm,γ) / V(γ) of test piece 19 was less than 0.12, T(γ) / V(γ) was less than 0.08, 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 .

[0232] In the test piece 20, the five holding times were less than 10 seconds, so the heat treatment time was insufficient. As a result, the V(1.2μm,γ) / V(γ) of the test piece 20 was less than 0.12, and the balance between tensile strength and hole expansion rate (B TH ) is less than 6.0*10 6 .

[0233] In the test piece 42, 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 .

[0234] In the test piece 43, the carbon (C) content is high, so the fraction of tempered martensite is insufficient and the fraction of newly formed martensite is excessive. 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 .

[0235] In the test piece 44, the silicon (Si) content is low, so the fraction of retained austenite is insufficient. As a result, the V (lath, γ) / V (γ) of the test piece 44 is less than 0.5, T (γ) / V (γ) is less than 0.08, 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 .

[0236] In the test piece 45, the silicon (Si) content is high, so the fraction of newly formed martensite exceeds the standard. 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 .

[0237] In the test piece 46, the aluminum (Al) content is high, so the fraction of newly formed martensite exceeds the standard. 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 .

[0238] In test piece 47, the manganese (Mn) content is low, pearlite is formed, and the fraction of retained austenite is insufficient. As a result, the V (lath, γ) / V (γ) of test piece 47 is less than 0.5, T (γ) / V (γ) is less than 0.08, 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 .

[0239] In the test piece 48, the manganese (Mn) content is high, so the fraction of newly formed martensite exceeds the standard. 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 .

[0240] In the test piece 49, the chromium (Cr) content is high, so the fraction of newly formed martensite exceeds the standard. 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 .

[0241] In the test piece 50, the molybdenum (Mo) content is high, so the fraction of newly formed martensite exceeds the standard. 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 .

[0242] In the test piece 51, the boron (B) content is low, so 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 (IYR ) exceeds 0.42.

[0243] In the specimen 52, the boron (B) content is high, and boron (B) is excessively concentrated in the tempered martensite. As a result, the [B] of the specimen 52 is FM / [B] TM Less than 0.03, yield strength ratio evaluation index (I YR ) is less than 0.15.

[0244] 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. In terms of volume fraction, the microstructure includes: 10-30% bainite, 50-70% tempered martensite, 10-30% new martensite, 2-10% retained austenite and other inevitable structures. The steel plate satisfies the following [Relationship 1] to [Relationship 4], [Equation 1] 0.03≤[B] FM / [B] TM ≤0.55 In the above relational expression 1, [B] FM is the content of boron (B) contained in the fresh martensite, where The unit of content is weight %; [B] TM is the content of boron (B) contained in tempered martensite, wherein the unit of content is weight %. [Equation 2] V(1.2μm,γ) / V(γ)≥0.12 In the above relational expression 2, V(1.2 μm, γ) is the fraction of retained austenite with an average grain size of 1.2 μm or more, wherein the unit of the fraction is volume %, and V(γ) is the fraction of retained austenite in the steel plate, wherein the unit of the fraction is volume %. [Equation 3] V(lath,γ) / V(γ)≥0.5 In the above relational expression 3, V(lath, γ) is the fraction of retained austenite in the form of laths, where the unit of the fraction is volume %, and V(γ) is the fraction of retained austenite in the steel sheet, where the unit of the fraction is volume %. [Equation 4] T(γ) / V(γ)≥0.08 In the relational expression 4, T(γ) is the fraction of tempered retained austenite in the steel plate, where the unit of the fraction is volume %, and V(γ) is the fraction of retained austenite in the steel plate, where the unit of the fraction is volume %.

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 The steel plate has a fine structure containing 5% or less and including 0% of ferrite by volume.

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 5] is 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 6] 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 7] YR Satisfy 0.15 to 0.42, [Equation 5] B TE =[Tensile Strength (TS, MPa)] 2 *[Elongation (El, %)] 1 / 2 [Equation 6] B TH =[Tensile Strength (TS, MPa)] 2 *[Hole expansion ratio (HER, %)] 1 / 2 [Equation 7] I YR =1-[yield strength ratio (YR)].

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 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 400-600° C., and then holding the steel plate for a second time for 5-600 seconds; The steel plate held for the second time is cooled at an average cooling rate of 1-100°C / second to a temperature range of 300-500°C, and then held for 5-600 seconds three times; Cooling the steel plate held three times at an average cooling rate of 2-100° C. / s to a temperature range of 200-400° C.; The steel plate cooled three times is heated three times at an average heating rate of 5-100° C. / s to a temperature range of 400-600° C., and then maintained for 10-1800 seconds four times; Cooling the steel plate held four times at an average cooling rate of 1-100° C. / s to a temperature range of 300-500° C., and then holding the steel plate five times for 10-1800 seconds; and The steel plate held five times was cooled five times at an average cooling rate of 1° C. / s or higher to room temperature.

6. The method for producing a high-strength steel sheet having excellent workability according to claim 5, wherein: The steel billet 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; Coiling the hot-rolled steel plate at a temperature ranging from 350° C. to 650° C. pickling the coiled steel plate; as well as The pickled steel sheet is cold rolled at a reduction ratio of 30-90%.

8. The method for producing a high-strength steel sheet having excellent workability according to claim 5, wherein: The cooling rate Vc1 of the first cooling and the cooling rate Vc4 of the fourth cooling satisfy the relationship of Vc1>Vc4.

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