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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
  • C21D1/20—Isothermal quenching, e.g. bainitic hardening
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C22C—ALLOYS
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  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22C—ALLOYS
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
  • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present invention relates to a steel sheet that may be used for automobile parts and the like, and to a steel sheet having high strength characteristics and excellent workability and a method for manufacturing the same.
• transformation induced plasticity (TRIP) steel using transformation-induced plasticity of retained austenite, has a complex microstructure consisting of ferrite, bainite, martensite, retained austenite, and the like, it is known as having a certain level or more of workability, as well as high strength characteristics.
• Patent Documents 1 and 2 As a technique for further improving the workability of a steel sheet, a method of utilizing tempered martensite is disclosed in Patent Documents 1 and 2. Since tempered martensite made by tempering hard martensite is softened martensite, there is a difference in strength between tempered martensite and existing untempered martensite (fresh martensite). Therefore, when fresh martensite is suppressed and tempered martensite is formed, the workability may increase.
• Patent Document 3 discloses a method for inducing generation of bainite by means of adding boron (B).
• B boron
• a ferrite-pearlite transformation is suppressed, while generation of bainite is induced, whereby coexistence of strength and workability can be achieved.
• a steel sheet having all of an excellent balance of tensile strength and elongation, an excellent balance of tensile strength and a hole expansion ratio, and an excellent yield ratio evaluation index and a method manufacturing the same can be provided, by optimizing a composition and microstructures of the steel sheet.
• An object of the present invention is not limited to the abovementioned contents. Additional objects of the present invention are described in the overall content of the specification, and those of ordinary skill in the art to which the present invention pertains will have no difficulty in understanding the additional objects of the present invention from the contents described in the specification of the present invention.
• a high strength steel sheet having excellent workability may comprise: by wt%, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities; and as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures, wherein the high strength steel sheet may satisfy the following relational expression 1 and relational expression 2: 0.03 ⁇ B FM / B TM ⁇ 0.55 where [B] FM is a content (wt%) of boron (B) contained in the fresh martensite, and [B] TM is a content (wt%) of boron (B) contained in the tempered martensite; and V 1.2 ⁇ m , ⁇ / V ⁇ ⁇
• the steel sheet may further comprise, by wt%, one or more of the following (1) to (8):
• the microstructures of the steel sheet may include, by volume fraction, 10 to 30% of bainite, 50 to 70% of tempered martensite, 10 to 30% of fresh martensite, 2 to 10% of retained austenite, and 5% or less (including 0%) of ferrite.
• a balance (B TE ) of tensile strength and elongation expressed by the following relational expression 3 satisfies 3.0*10 6 to 6.2*10 6 (MPa 2 % 1/2 )
• a balance (B TH ) of tensile strength and a hole expansion ratio expressed by the following relational expression 4 satisfies 6.0*10 6 to 11.5*10 6 (MPa 2 % 1/2 )
• a yield ratio evaluation index (I YR ) expressed by the following relational expression 5 satisfies 0.15 to 0.42:
• B TE Tensile Strength TS , MPa 2 * Elongation El , % 1 / 2 ;
• B TH Tensile Strength TS , MPa 2 * Hole Expansion Ratio HER , % 1 / 2 ;
• I YR 1 ⁇ Yield Ratio YR .
• a method for manufacturing a high strength steel sheet having excellent workability may comprise: providing a cold-rolled steel sheet including, by wt%, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities; heating (primarily heating) the cold-rolled steel sheet to a temperature of 700°C at an average heating rate of 5°C/s or more, heating (secondarily heating) the primarily heated steel sheet to a temperature within a range of Ac3 to 920°C at an average heating rate of 5°C/s or less, and then maintaining (primarily maintaining) the secondarily heated steel sheet for 50 to 1200 seconds; cooling (primarily cooling) the primarily maintained steel sheet to a temperature within a range of 350 to 550°C at an average cooling rate of 2
• the steel slab may further comprise, by wt%, one or more of the following (1) to (8):
• the cold-rolled steel sheet may be provided by: heating a steel slab to 1000 to 1350°C; performing finishing hot rolling at a temperature within a range of 800 to 1000°C; coiling the hot-rolled steel sheet at a temperature within a range of 350 to 600°C; pickling the coiled steel sheet; and cold rolling the pickled steel sheet at a reduction ratio of 30 to 90%.
• a steel sheet that may be used for automobile parts and the like, and to a steel sheet having an excellent balance of tensile strength and ductility, an excellent balance of tensile strength and hole expansion ratio, and an excellent yield ratio evaluation index, and a method for manufacturing the same.
• the present invention relates to a high strength steel sheet having excellent workability and a method for manufacturing the same, and exemplary embodiments of the present invention will hereinafter be described. Exemplary embodiments of the present invention may be modified into various forms, and it is not to be interpreted that the scope of the present invention is limited to exemplary embodiments described below. The present exemplary embodiments are provided in order to further describe the present invention in detail to those skilled in the art to which the present invention pertains.
• the inventors of the present invention recognized that in a boron(B)-added transformation-induced plasticity (TRIP) steel comprising bainite, tempered martensite, fresh martensite and retained austenite, when the fractions of the tempered martensite, the fresh martensite, and the retained austenite are controlled to be within certain ranges, the contents of the boron (B) contained in the tempered martensite and the fresh martensite are controlled to be within certain ranges, and a shape and a size of the retained austenite are controlled to be within certain ranges, it is possible to simultaneously secure an excellent balance of tensile strength and ductility, an excellent balance of tensile strength and a hole expansion ratio, and an excellent yield ratio evaluation index. Based thereon, the present inventors have conceived of the present invention by devising a method such that excellent strength, excellent yield ratio, excellent ductility, and an excellent hole expansion ratio may be simultaneously provided.
• TRIP transformation-induced plasticity
• a high strength steel sheet having excellent workability may comprise: by wt%, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities; and as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures, wherein the high strength steel sheet may satisfy the following relational expression 1 and relational expression 2: 0.03 ⁇ B FM / B TM ⁇ 0.55 where [B] FM is a content (wt%) of boron (B) contained in the fresh martensite, and [B] TM is a content (wt%) of boron (B) contained in the tempered martensite; and V 1.2 ⁇ m , ⁇ / V ⁇ ⁇
• compositions of steel according to the present invention will be described in more detail.
• % indicating a content of each element is based on weight.
• the high strength steel sheet having excellent workability includes: by wt%, C: 0.1 to 0.25%, Si: 0.01 to 1.5%, Mn: 1.0 to 4.0%, Al: 0.01 to 1.5%, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, B: 0.0005 to 0.005%, a balance of Fe, and unavoidable impurities.
• the high strength steel sheet may further include one or more of 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: 0.05% or less (including 0%) excluding Y, 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%).
• Carbon (C) is an unavoidable element for securing strength of a steel sheet, and is also an element for stabilizing retained austenite that contributes to the improvement in ductility of the steel sheet. Accordingly, in the present invention, 0.1% or more of carbon (C) may be added in order to achieve such an effect.
• a preferable content of carbon (C) may be greater than 0.1%, may be 0.11% or more, and may be 0.12% or more.
• an upper limit of the content of carbon (C) of the present invention may be limited to 0.25%.
• the content of carbon (C) may be 0.24% or less, and a more preferable content of carbon (C) may be 0.23% or less.
• Silicon (Si) is an element that contributes to improvement in strength by solid solution strengthening, and is also an element improving workability by homogenizing a structure.
• silicon (Si) is an element contributing to generation of retained austenite by suppressing precipitation of cementite. Therefore, in the present invention, silicon (Si) of 0.01% or more may be added in order to achieve such an effect.
• a preferable content of silicon (Si) may be 0.02% or more, and a more preferable content of silicon (Si) may be 0.04% or more.
• the present invention may limit an upper limit of the content of silicon (Si) to 1.5%.
• a preferable upper limit of the content of silicon (Si) may be 1.48%, and a more preferable upper limit of the content of silicon (Si) may be 1.46%.
• Manganese (Mn) is a useful element for increasing both strength and ductility. Therefore, in the present invention, manganese (Mn) of 1.0% or more may added in order to achieve such an effect.
• a preferable lower limit of the content of manganese (Mn) may be 1.2%, and a more preferable lower limit of the content of manganese (Mn) may be 1.4%.
• an upper limit of the content of manganese (Mn) of the present invention may be limited to 4.0%.
• a preferable upper limit of the content of manganese (Mn) may be 3.9%.
• Aluminum (Al) is an element performing deoxidation by combining with oxygen in steel.
• aluminum (Al) is also an element for stabilizing retained austenite by suppressing precipitation of cementite like silicon (Si). Therefore, in the present invention, aluminum (Al) of 0.01% or more may be added in order to achieve such an effect.
• a preferable content of aluminum (Al) may be 0.03% or more, and a more preferable content of aluminum (Al) may be 0.05% or more.
• the present invention may limit an upper limit of the content of aluminum (Al) to 1.5%.
• a preferable upper limit of the content of aluminum (Al) may be 1.48%.
• Phosphorus (P) is an element which is contained as an impurity and deteriorates impact toughness. Therefore, it is preferable to manage the content of phosphorus (P) to 0.15% or less.
• Sulfur (S) is an element which is contained as an impurity to form MnS in a steel sheet and deteriorate ductility. Therefore, it is preferable that the content of sulfur (S) is 0.03% or less.
• Nitrogen (N) is an element which is contained as an impurity and forms nitride during continuous casting to cause cracks in a slab. Therefore, it is preferable that the content of nitrogen (N) is 0.03% or less.
• Boron (B) is an element improving hardenability to increase strength, and is also an element suppressing nucleation of grain boundaries.
• it is intended to simultaneously secure an excellent balance of tensile strength and elongation, an excellent balance of tensile strength and a hole expansion ratio, and an excellent yield ratio evaluation index, and therefore, boron (B) is to be necessarily added in the present invention. Therefore, in the present invention, 0.0005% or more of boron (B) may be added in order to achieve such an effect.
• the present invention may limit an upper limit of the content of boron (B) to 0.005%.
• the steel sheet of the present invention has an alloy composition that may be additionally included in addition to the above-described alloy components, which will be described in detail below.
• Ti titanium
• Nb niobium
• V vanadium
• Titanium (Ti), niobium (Nb), and vanadium (V) are elements that make precipitates and refine crystal grains, and are elements that also contribute to the improvement in strength and impact toughness of a steel sheet, and therefore, in the present invention, one or more of titanium (Ti), niobium (Nb), and vanadium (V) may be added in order to achieve such an effect.
• titanium (Ti), niobium (Nb), and vanadium (V) exceeds a certain level, excessive precipitates are formed to lower impact toughness and increase manufacturing costs, so the present invention may limit the contents of titanium (Ti), niobium (Nb), and vanadium (V) to 0.5% or less, respectively.
• chromium (Cr) and molybdenum (Mo) are elements that not only suppress austenite decomposition during alloying treatment, but also stabilize austenite like manganese (Mn), in the present invention, one or more of chromium (Cr) and molybdenum (Mo) may be added in order to achieve such an effect.
• the present invention may limit the contents of chromium (Cr) and molybdenum (Mo) to 3.0% or less, respectively.
• Copper (Cu) and nickel (Ni) are elements that stabilize austenite and suppress corrosion.
• copper (Cu) and nickel (Ni) are also elements that are concentrated on a surface of a steel sheet to prevent hydrogen from intruding into the steel sheet, thereby suppressing hydrogen delayed destruction. Therefore, in the present invention, one or more of copper (Cu) and nickel (Ni) may be added in order to achieve such an effect.
• the present invention may limit the contents of copper (Cu) and nickel (Ni) to 4.0% or less, respectively.
• the rare earth element (REM) is scandium (Sc), yttrium (Y), and a lanthanide element. Since calcium (Ca), magnesium (Mg), and the rare earth element (REM) excluding yttrium (Y) are elements that contribute to the improvement in ductility of a steel sheet by spheroidizing sulfides, in the present invention, one or more of calcium (Ca), magnesium (Mg), and the rare earth element (REM) excluding yttrium (Y) may be added in order to achieve such an effect.
• the present invention may limit the contents of calcium (Ca), magnesium (Mg), and the rare earth element (REM) excluding yttrium (Y) to 0.05% or less, respectively.
• tungsten (W) and zirconium (Zr) are elements that increase strength of a steel sheet by improving hardenability
• one or more of tungsten (W) and zirconium (Zr) may be added in order to achieve such an effect.
• the present invention may limit the contents of tungsten (W) and zirconium (Zr) to 0.5% or less, respectively.
• antimony (Sb) and tin (Sn) are elements that improve plating wettability and plating adhesion of a steel sheet
• one or more of antimony (Sb) and tin (Sn) may be added in order to achieve such an effect.
• the present invention may limit the contents of antimony (Sb) and tin (Sn) to 0.5% or less, respectively.
• Y yttrium
• Hf hafnium
• yttrium (Y) and hafnium (Hf) are elements that improve corrosion resistance of a steel sheet
• one or more of the yttrium (Y) and hafnium (Hf) may be added in order to achieve such an effect.
• the present invention may limit the contents of yttrium (Y) and hafnium (Hf) to 0.2% or less, respectively.
• cobalt (Co) is an element that promotes a bainite transformation to increase a TRIP effect
• cobalt (Co) may be added in order to achieve such an effect.
• the present invention may limit the content of cobalt (Co) to 1.5% or less.
• the high strength steel sheet having excellent workability may include a balance of Fe and other unavoidable impurities in addition to the components described above.
• unintended impurities may inevitably be mixed from raw materials or the surrounding environment, and thus, these impurities may not be completely excluded. Since these impurities are known to those skilled in the art, all the contents are not specifically mentioned in the present specification.
• a further addition of effective components other than the above-described components is not entirely excluded.
• the high strength steel sheet having excellent workability may include, as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures.
• Both untempered martensite (fresh martensite, FM) and tempered martensite (TM) are microstructures that improve the strength of a steel sheet.
• fresh martensite has a characteristic of greatly reducing ductility and burring workability of a steel sheet.
• fresh martensite has a tendency of reducing a yield ratio of a steel sheet.
• a balance (TS 2 *EL 1/2 ) of tensile strength and elongation
• a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio
• a balance (TS 2 *EL 1/2 ) of tensile strength and elongation of 3.0*10 6 or more, a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio of 6.0*10 6 or more, and a yield ratio evaluation index (1-YR) of 0.42 or less it is preferable to limit a fraction of the tempered martensite to 50 vol% or more, and to limit a fraction of the fresh martensite to 10 vol% or more.
• a more preferable fraction of tempered martensite may be 52 vol% or more, or 54 vol% or more, and a more preferable fraction of fresh martensite may be 12 vol% or more.
• the present invention may limit a fraction of tempered martensite to 70 vol% or less, and limit a fraction of fresh martensite to 30 vol% or less.
• a more preferable fraction of tempered martensite may be 68 vol% or less, or 65 vol% or less, and more preferable fraction of fresh martensite may be 25 vol% or less.
• a more preferable fraction of bainite may be 12 vol% or more, or 14 vol% or more.
• bainite when bainite is excessively formed, it causes a fraction reduction of tempered martensite, so that a fraction of bainite may be limited to 30 vol% or less, in order to secure a balance (TS 2 *EL 1/2 ) of tensile strength and elongation, a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio, and a yield ratio evaluation index (1-YR) targeted by the present invention.
• a preferable fraction of the bainite may be 12 vol% or more, 14 vol% or more, 28 vol% or less, or 26 vol% or less.
• a steel sheet including retained austenite has excellent ductility and bending workability due to transformation-induced plasticity occurring during transformation from austenite to martensite during processing.
• a balance (TS 2 *EL 1/2 ) of tensile strength and elongation may be less than 3.0*10 6 (MPa 2 % 1/2 ), and is not preferably.
• a fraction of retained austenite exceeds a certain level, local elongation may be lowered, or point weldability may be lowered.
• a fraction of retained austenite may be limited to be in a range of 2 to 10 vol% in order to obtain a steel sheet having an excellent balance (TS 2 *EL 1/2 ) of tensile strength and elongation.
• a preferable fraction of retained austenite is 3 vol% or more, or 8 vol% or less.
• the steel sheet of the present invention may include ferrite, pearlite, martensite austenite constituent (M-A), and the like.
• M-A martensite austenite constituent
• the present invention may limit a fraction of ferrite to 5 vol% (including 0%).
• workability of the steel sheet may be lowered or a fraction of retained austenite may be lowered, so the present invention intends to limit the formation of pearlite as much as possible.
• the high strength steel sheet having excellent workability may satisfy the following relational expression 1 and relational expression 2: 0.03 ⁇ B FM / B TM ⁇ 0.55 where [B] FM is a content (wt%) of boron (B) contained in the fresh martensite, and [B] TM is a content (wt%) of boron (B) contained in the tempered martensite; and V 1.2 ⁇ m , ⁇ / V ⁇ ⁇ 0.12 where V(1.2 ⁇ m, ⁇ ) is a fraction (vol%) of the retained austenite having an average grain size of 1.2 ⁇ m or more, and V( ⁇ ) is a fraction (vol%) of the retained austenite of the steel sheet.
• the present invention not only controls the fractions of the tempered martensite, the fresh martensite, and the retained austenite to be in certain ranges, but also controls the content ratios of the boron (B) contained in the tempered martensite, and the fresh martensite to be in certain ranges, while controlling a ratio of the retained austenite of specific size, shape and type with respect to the entire retained austenite to be in a certain range, in order to secure a balance (TS 2 *EL 1/2 ) of tensile strength and elongation, a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio, and a yield ratio evaluation index (1-YR) targeted thereby.
• the present invention controls a ratio of the content ([B] TM , wt%) of boron (B) contained in the fresh martensite to the content ([B] FM , wt%) of boron (B) contained in the tempered martensite to be in a range of 0.03 to 0.55 as shown in relational expression 1, so as to secure a balance (B TE ) of tensile strength and elongation of 3.0*10 6 to 6.2*10 6 (MPa 2 % 1/2 ), a balance (B TH ) of tensile strength and a hole expansion ratio of 6.0*10 6 to 11.5*10 6 (MPa 2 % 1/2 ), and ha yield ratio evaluation index (I YR ) of 0.15 to 0.42 at the same time.
• the inventors of the present invention conducted in-depth research on a method for securing physical properties of a boron(B)-added TRIP steal, and as a result, noted that the physical properties targeted by the present invention may be secured only when a ratio of a content of boron (B) contained in fresh martensite to a content of boron (B) contained in tempered martensite satisfies a certain range, even though the theoretical basis thereof is not clearly identified.
• a yield ratio of a steel sheet has a constant tendency according to a content ratio of boron (B) contained in tempered martensite and fresh martensite.
• the present invention limits the ratio of the content of boron (B) contained in the fresh martensite to the content of boron (B) contained in the tempered martensite to be in a range of 0.03 to 0.55 as shown in relational expression 1, thereby securing a balance (TS 2 *EL 1/2 ) of tensile strength and elongation, a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio, and a yield ratio evaluation index (1-YR) targeted thereby.
• the inventors of the present invention could know that not only a fraction of retained austenite, a ratio of retained austenite having a specific size to the entire retained austenite is a factor for securing strength and workability.
• Retained austenite having an average grain size of 1.2 ⁇ m or more is retained austenite that is heat-treated at a bainite forming temperature, and has an increased average size, and is a structure in which transformation driving force to martensite is suppressed relative to retained austenite having an average grain size of 1.2 ⁇ m or less.
• the present invention controls a ratio of a fraction (V(1.2 ⁇ m, ⁇ ), vol%) of retained austenite having an average grain size of 1.2 ⁇ m or more to a fraction (V(y), vol%) of the entire retained austenite contained in a steel sheet to be 0.12 or more, and therefore, may effectively secure a balance (TS 2 *EL 1/2 ) of tensile strength and elongation, a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio.
• a balance (B TE ) of tensile strength and elongation expressed by the following relational expression 3 may satisfy 3.0*10 6 to 6.2*10 6 (MPa 2 % 1/2 ), a balance (B TH ) of tensile strength and a hole expansion ratio expressed by the following relational expression 4 may satisfy 6.0*10 6 to 11.5*10 6 (MPa 2 % 1/2 ), and a yield ratio evaluation index (I YR ) expressed by the following relational expression 5 may satisfy 0.15 to 0.42:
• B TE Tensile Strength TS , MPa 2 * Elongation El , % 1 / 2 h ;
• a method for manufacturing a high strength steel sheet having excellent workability may comprises: heating (primarily heating) a cold-rolled steel sheet having a predetermined alloy composition to a temperature of 700°C at an average heating rate of 5°C/s or more, heating (secondarily heating) the primarily heated steel sheet to a temperature within a range of Ac3 to 920°C at an average heating rate of 5°C/s or less, and then maintaining (primarily maintaining) the secondarily heated steel sheet for 50 to 1200 seconds; cooling (primarily cooling) the primarily maintained steel sheet to a temperature within a range of 350 to 550°C at an average cooling rate of 2 to 100°C/s, and maintaining (secondarily maintaining) the primarily cooled steel sheet for 5 to 600 seconds; cooling (secondarily cooling) the secondarily maintained steel sheet to a temperature within a range of 200 to 400°C at an average cooling rate of 2 to 100°C/s; heating (tertiarily heating) the secondarily cooled steel sheet to a temperature within
• the cold-rolled steel sheet may be provided by: heating steel slab having a predetermined alloy composition to 1000 to 1350°C; performing finishing hot rolling at a temperature within a range of 800 to 1000°C; coiling the hot-rolled steel sheet at a temperature within a range of 350 to 600°C; pickling the coiled steel sheet; and cold rolling the pickled steel sheet at a reduction ratio of 30 to 90%.
• a steel slab having a predetermined alloy composition is prepared. Since the steel slab according to the present invention includes an alloy composition corresponding to an alloy composition of the steel sheet described above, the description of the alloy compositions of the slab is replaced by the description of the alloy composition of the steel sheet described above.
• the prepared steel slab may be heated to a temperature within a certain range, and the heating temperature of the steel slab at this time may be in the range of 1000 to 1350°C.
• the steel slab When the heating temperature of the steel slab is less than 1000°C, the steel slab may be hot rolled at a temperature within a range below a desired finish hot rolling temperature range, and when the heating temperature of the steel slab exceeds 1350°C, the temperature reaches a melting point of steel, and thus, the steel slab may be melted.
• the heated steel slab may be hot rolled, and thus, provided as a hot-rolled steel sheet.
• the finish hot rolling temperature is preferably in the range of 800 to 1000°C.
• the finish hot rolling temperature is lower than 800°C, an excessive rolling load may be a problem, and when the finish hot rolling temperature exceeds 1000°C, grains of the hot-rolled steel sheet are coarsely formed, which may cause a deterioration in physical properties of the final steel sheet.
• the hot-rolled steel sheet may be cooled at an average cooling rate of 10°C/s or more, and may be coiled at a temperature within a range of 350 to 650°C.
• the coiling temperature is lower than 350°C, coiling is not easy, and when the coiling temperature exceeds 650°C, surface scale may be formed into the inside of the hot-rolled steel sheet, which may make pickling difficult.
• the pickling After uncoiling the coiled hot-rolled coil, in order to remove the scale generated on the surface of the steel sheet, the pickling may be performed, and the cold rolling may be performed.
• the cold rolling is preferably performed at a cumulative reduction ratio of 30 to 90%. When the cumulative reduction ratio of the cold rolling exceeds 90%, it may be difficult to perform the cold rolling in a short time due to the high strength of the steel sheet.
• the cold-rolled steel sheet may be manufactured as a non-plated cold-rolled steel sheet through the annealing heat treatment process, or may be manufactured as a plated steel sheet through a plating process to impart corrosion resistance.
• plating methods such as hot-dip galvanizing, electro-galvanizing, and hot-dip aluminum plating may be applied, and the method and the type are not particularly limited.
• the annealing heat treatment process is performed.
• the cold-rolled steel sheet is heated (primarily heated) to a temperature of 700°C at an average heating rate of 5°C/s or more, is heated (secondarily heated) to a temperature within a range of Ac3 to 920°C at an average heating rate of 5°C/s or less, and then is maintained (primarily maintained) for 50 to 1200 seconds.
• the primary maintaining temperature When the primary maintaining temperature is lower than Ac3 (two-phase region), 5 vol% or more of ferrite is formed, and therefore, a balance (TS 2 *EL 1/2 ) of tensile strength and elongation and a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio may be lowered.
• the primary maintaining time when the primary maintaining time is less than 50 seconds, the structure may not be sufficiently homogenized and the physical properties of the steel sheet may be lowered.
• Upper limits of the primary maintaining temperature and the primary maintaining time are not particularly limited, but it is preferable that the primary maintaining temperature is limited to 920°C or less, and the primary maintaining time is limited to 1200 seconds or less, in order to prevent toughness reduction due to coarsened grains.
• the primarily maintained steel sheet may be cooled (primarily cooled) to a temperature within a range of 350 to 550°C at a primary cooling rate with an average cooling rate of 2°C or more, and then may be maintained (secondarily maintained) for 5 seconds or more.
• a balance (TS 2 *EL 1/2 ) of tensile strength and elongation, and a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio of the steel sheet may be lowered.
• An upper limit of the average cooling rate of the primary cooling does not need to be particularly specified, but is preferably set to be 100°C or less.
• V(1.2 ⁇ m, ⁇ )/V( ⁇ ) and a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio of the steel sheet may be lowered due to a low heat treatment temperature.
• the secondary maintaining temperature is greater than 550°C, retained austenite becomes insufficient, whereby a balance (TS 2 *EL 1/2 ) of tensile strength and elongation and a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio of the steel sheet may be lowered.
• the secondary maintaining time when the secondary maintaining time is less than 5 seconds, a heat treatment time becomes insufficient, whereby V(1.2 ⁇ m, ⁇ )/V( ⁇ ) and a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio of the steel sheet may be lowered.
• An upper limit of the secondary maintaining time does not need to be particularly specified, but is preferably set to be 600 seconds or less.
• the secondarily maintained steel sheet may be cooled (secondarily cooled) to a primary cooling stop temperature in a range of 200 to 400°C at an average cooling rate of 2°C/s or more.
• a balance (TS 2 *EL 1/2 ) of tensile strength and elongation, and a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio of the steel sheet may be lowered.
• An upper limit of the average cooling rate of the secondary cooling does not need to be particularly specified, but is preferably set to be 100°C/s or less.
• the secondarily cooled steel sheet may be heated (tertiarily heated) to a temperature within a range of 350 to 550°C at an average heating rate of 5°C/s or more, and then the tertiarily heated steel sheet may be maintained (tertiarily maintained) for 50 seconds or more.
• An upper limit of the average heating rate of the tertiary heating does not need to be particularly specified, but is preferably set to be 100°C/s or less.
• V(1.2 ⁇ m, ⁇ )/V( ⁇ ), a balance (TS 2 *EL 1/2 ) of tensile strength and elongation and a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio may be lowered.
• the tertiary maintaining temperature exceeds 550°C or the tertiary maintaining time exceeds 155,000 seconds, a fraction of retained austenite becomes insufficient, and therefore, V(1.2 ⁇ m, ⁇ )/V( ⁇ ), a balance (TS 2 *EL 1/2 ) of tensile strength and elongation of the steel sheet, and a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio of the steel sheet may be lowered.
• the tertiarily maintained steel sheet may be cooled (tertiarily cooled) to room temperature at an average cooling rate of 1°C/s or more.
• the high strength steel sheet having excellent workability manufactured by the aforementioned manufacturing method may comprise, as microstructures, bainite, tempered martensite, fresh martensite, retained austenite and unavoidable structures, and as a preferable example, may comprise, by volume fraction, 10 to 30% of bainite, 50 to 70% of tempered martensite, 10 to 30% of fresh martensite, 2 to 10% of retained austenite, and 5% or less (including 0%) of ferrite.
• a balance (B TE ) of tensile strength and elongation expressed by the following relational expression 3 may satisfy 3.0*10 6 to 6.2*10 6 (MPa 2 % 1/2 ), a balance (B TH ) of tensile strength and a hole expansion ratio expressed by the following relational expression 4 may satisfy 6.0*10 6 to 11.5*10 6 (MPa 2 % 1/2 ), and a yield ratio evaluation index (I YR ) expressed by the following relational expression 5 may satisfy 0.15 to 0.42:
• B TE Tensile Strength TS , MPa 2 * Elongation El , % 1 / 2 ;
• a steel slab having a thickness of 100 mm having alloy compositions (a balance of Fe and unavoidable impurities) shown in Table 1 below was prepared, heated at 1200°C, and then subjected to finish hot rolling at 900°C. Thereafter, the steel slab was cooled at an average cooling rate of 30°C/s, and coiled at a coiling temperature of Tables 2 and 3 to manufacture a hot-rolled steel sheet having a thickness of 3 mm. Thereafter, after removing a surface scale by pickling, cold rolling was performed to a thickness of 1.5 mm.
• the heat treatment was performed under the annealing heat treatment conditions shown in Tables 2 to 5 to manufacture the steel sheet.
• the single-phase region means a temperature range of Ac3 to 920°C
• the two-phase region means a temperature range below Ac3°C.
• microstructure of the thus prepared steel sheet was observed, and the results were shown in Tables 6 and 7.
• ferrite (F), bainite (B), tempered martensite (TM), fresh martensite (FM) and pearlite (P) were observed through SEM after nital-etching a polished specimen cross section. After nital-etching, a structure having no concave-convex portions on a surface of a specimen was classified as ferrite, and a structure having a lamella structure of cementite and ferrite is classified as pearlite.
• [B] FM /[B] TM , V(1.2 ⁇ m, ⁇ )/V( ⁇ ), a balance (TS 2 *EL 1/2 ) of tensile strength and elongation, a balance (TS 2 *HER 1/2 ) of tensile strength and a hole expansion ratio, and a yield ratio evaluation index (I YR ) of the steel sheet were measured and evaluated, and the results thereof were shown in Tables 8 and 9.
• the concentrations of boron (B) in fresh martensite and tempered martensite measured using an electron probe microanalyser (EPMA) were determined as a content ([B] FM ) of boron (B) contained in fresh martensite, and a content ([B] TM ) of boron (B) contained in tempered martensite.
• Tensile strength (TS) and elongation (El) were evaluated through a tensile test, and the tensile strength (TS) and the elongation (El) were measured by evaluating the specimens collected in accordance with JIS No. 5 standard based on a 90° direction with respect to a rolling direction of a rolled sheet.
• D is a hole diameter (mm) when cracks penetrate through the steel sheet along the thickness direction
• Do is the initial hole diameter (mm).
• Steel Type Chemical Components (wt%) C Si Mn P S Al N Cr Mo B Others A 0.15 0.48 2.26 0.012 0.0008 0.41 0.0030 0.0024 B 0.17 0.45 2.38 0.008 0.0010 0.50 0.0029 0.25 0.31 0.0021 C 0.14 0.60 2.19 0.009 0.0011 0.44 0.0032 0.52 0.0025 D 0.13 0.52 1.28 0.011 0.0009 0.82 0.0028 0.82 0.0028 E 0.16 1.44 2.26 0.010 0.0008 0.11 0.0031 0.0043 F 0.22 0.12 1.94 0.009 0.0010 1.45 0.0027 0.0046 G 0.14 0.38 2.46 0.010 0.0012 0.72 0.0028 0.0020 Ti: 0.05 H 0.11 0.36 2.39 0.011 0.0011 0.38 0.0031 0.0035 Nb: 0.04 I 0.19 0.49
• the primary average heating rate was less than 5°C/s, so that tempered martensite and retained austenite were insufficient.
• a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 2.
• the secondary average heating rate was more than 5°C/s, so that lump austenite was formed, and boron (B) was not concentrated into tempered martensite.
• [B] FM /[B] TM was more than 0.55
• a yield ratio evaluation index (I YR ) was more than 0.42
• a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 3.
• the primary average cooling rate was less than 1°C/s, so that a fraction of retained austenite was insufficient.
• a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 5.
• the secondary maintaining temperature was less than 350°C, so that a heat treatment temperature was insufficient.
• V(1.2 ⁇ m, y) was less than 0.12
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 6.
• the secondary maintaining temperature was more than 550°C, so that a fraction of retained austenite was insufficient.
• a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 7.
• the secondary maintaining time was less than 5 seconds, so that a heat treatment time was insufficient.
• V(1.2 ⁇ m, y) was less than 0.12
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 8.
• the secondary average cooling rate was less than 2°C/s, so that a fraction of retained austenite was insufficient.
• a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 9.
• the primary cooling stop temperature was less than 200°C, so that a fraction of tempered martensite was excessive, and a fraction of retained austenite was insufficient.
• V(1.2 ⁇ m, y) was less than 0.12
• a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 10.
• the primary cooling stop temperature was more than 400°C, so that a fraction of bainite was excessive, and a fraction of tempered martensite was insufficient.
• a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 11.
• the tertiary maintaining temperature was less than 350°C, so that a fraction of tempered martensite was excessive, and a fraction of retained austenite was insufficient.
• V(1.2 ⁇ m, y) was less than 0.12
• a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 12.
• the tertiary maintaining temperature was more than 550°C, so that a fraction of retained austenite was insufficient.
• V(1.2 ⁇ m, y) was less than 0.12
• a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 13.
• the tertiary maintaining time was less than 50 seconds, so that a fraction of tempered martensite was excessive, and a fraction of retained austenite was insufficient.
• V(1.2 ⁇ m, y) was less than 0.12
• a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 14.
• the tertiary maintaining time was more than 155,000 seconds, so that a fraction of retained austenite was insufficient.
• V(1.2 ⁇ m, y) was less than 0.12
• a balance (B TE ) of tensile strength and elongation was less than 3.0*10 6
• a balance (B TH ) of tensile strength and a hole expansion ratio was less than 6.0*10 6 , in specimen 15.

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