Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • 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
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/008—Heat treatment of ferrous alloys containing Si
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  • C21—METALLURGY OF IRON
  • 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
  • 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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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  • C21—METALLURGY OF IRON
  • 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
  • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • 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
  • 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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  • C21—METALLURGY OF IRON
  • 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
  • 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
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • 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
  • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • 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
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
  • C21D9/54—Furnaces for treating strips or wire
  • C21D9/68—Furnace coilers; Hot coilers
• • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
• • 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
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • 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
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • 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
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • 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
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • 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
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • 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
  • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
• • 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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/14—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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • 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
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • 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
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • 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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • 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
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • 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
  • C21D2211/005—Ferrite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • 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

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 superior workability and a method for manufacturing same.
• Patent Documents 1 and 2 As a technique for improving workability of a steel sheet, a method of utilizing tempered martensite is disclosed in Patent Documents 1 and 2. Since the tempered martensite made by tempering hard martensite is softened martensite, there is a difference in strength between the tempered martensite and the existing untempered martensite (fresh martensite). Therefore, when fresh martensite is suppressed and the tempered martensite is formed, the workability may be increased.
• TRIP transformation induced plasticity
• Patent Document 3 discloses improving high ductility and workability by including polygonal ferrite, retained austenite, and martensite, but it can be seen that Patent Document 3 uses bainite as a main phase, and thus, the high strength is not secured and the balance (TSXEl) of tensile strength and elongation also does not satisfy 22,000 MPa% or more.
• the present invention provides a high strength steel sheet having superior ductility, bending formability, and hole expansion ratio by optimizing a composition and microstructure of the steel sheet and a method for manufacturing the same.
• An object of the present invention is not limited to the abovementioned contents. Additional problems 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 problems of the present invention from the contents described in the specification of the present invention.
• a high strength steel sheet having superior workability may include: by wt%, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, a balance of Fe, and unavoidable impurities, and include, as microstructures, 30 to 70 vol% of tempered martensite, 10 to 45 vol% of bainite, 10 to 40 vol% of retained austenite, 3 to 20 vol% of ferrite, and unavoidable structures, and may satisfy the following [Relational Expression 1], 1.02 ⁇ Si + Al F / Si + Al av ⁇ 1.45
• [Si+Al] F is an average total content (wt%) of Si and Al included in the ferrite
• [Si+Al] av is an average total content (wt%) of Si and Al included in the retained austenite.
• the steel sheet may further include one or more of the following (1) to (9).
• a total content (Si+Al) of Si and Al may be 1.0 to 6.0 wt%.
• a balance B T ⁇ E of tensile strength and elongation expressed by the following [Relational Expression 2] may be 22,000 (MPa%) or more
• a balance B T ⁇ H of tensile strength and hole expansion ratio expressed by the following [Relational Expression 3] may be 7*10 6 (MPa 2 % 1/2 ) or more
• bendability B R expressed by the following [Relational Expression 4] may be a range of 0.5 to 3.0.
• B T ⁇ E Tensile Strength TS , MPa * Elongation El , %
• B T ⁇ H Tensile Strength TS , MPa 2 * Hole Expansion Ratio HER , % 1 / 2
• B R R / t
• R is a minimum bending radius (mm) at which cracks do not occur after a 90° bending test
• t is a thickness (mm) of the steel sheet.
• a manufacturing method for high strength steel sheet having superior workability may include: heating and hot rolling a steel slab including, by wt%, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, a balance of Fe, and unavoidable impurities; coiling the hot-rolled steel sheet; performing hot-rolled annealing heat treatment on the coiled steel sheet in a temperature within a range of 650 to 850°C for 600 to 1700 seconds; cold rolling the hot-rolled annealing heat-treated steel sheet; heating (primarily heating) the cold-rolled steel sheet to a temperature within a range of Ac1 or higher and less than Ac3 at an average temperature increase rate of 5°C/s or more, and maintaining (primarily maintaining) the primarily heated steel sheet for 50 seconds or more; cooling (primarily cooling) the primarily heated steel sheet to a temperature within
• the steel slab may further include one or more of the following (1) to (9).
• a total content (Si+Al) of Si and Al included in the steel slab may be 1.0 to 6.0 wt%.
• the steel slab may be heated to a temperature within a range of 1000 to 1350°C, and may be subjected to finish hot rolling in a temperature within a range of 800 to 1000°C.
• the hot-rolled steel sheet may be coiled in a temperature within a range of 300 to 600°C.
• a reduction ratio of the cold rolling may be 30 to 90%.
• the cooling rate of the secondary cooling may be 1°C/s or more.
• the steel sheet has superior strength as well as superior workability such as ductility, bending formability, and hole expansion ratio.
• the present invention relates to a high strength steel sheet having superior workability and a method for manufacturing the same, and exemplary embodiments in the present invention will hereinafter be described. Exemplary embodiments in the present invention may be modified into several 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 transformation induced plasticity (TRIP) steel including bainite, tempered martensite, retained austenite, and ferrite, when controlling a ratio of specific components included in the retained austenite and the ferrite to a certain range while promoting stabilization of the retained austenite, it is possible to simultaneously secure workability and strength of a steel sheet by reducing an inter-phase hardness difference of the retained austenite and the ferrite. Based on this, the present inventors have reached the present invention by devising a method capable of improving ductility and workability of the high strength steel sheet.
• TRIP transformation induced plasticity
• a high strength steel sheet having superior workability may include: by wt%, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, a balance of Fe, and unavoidable impurities, and include, as microstructures, 30 to 70 vol% of tempered martensite, 10 to 45 vol% of bainite, 10 to 40 vol% of retained austenite, 3 to 20 vol% of ferrite, and unavoidable structures, and may satisfy the following [Relational Expression 1].
• 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 superior workability includes, by weight, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, 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: 4.5% or less (including 0%), Ni: 4.5% or less (including 0%), B: 0.005% 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%), Co: 1.5% or less (including 0%).
• a total content (Si+Al) of Si and Al may be 1.0 to 6.0%
• Carbon (C) is an unavoidable element for securing strength of a steel sheet, and is also an element for stabilizing the retained austenite that contributes to the improvement in ductility of the steel sheet. Accordingly, the present invention may include 0.25% or more of carbon (C) to achieve such an effect.
• a preferable content of carbon (C) may exceed 0.25%, may be 0.27% or more, and may be 0.30% or more.
• the more preferable content of carbon (C) may be 0.31% or more.
• an upper limit of the content of carbon (C) of the present disclosure may be limited to 0.75%.
• the content of carbon (C) may be 0.70% or less, and the more preferable content of carbon (C) may be 0.67% or less.
• Silicon (Si) is an element that contributes to improvement in strength by solid solution strengthening, and is also an element that improves workability by strengthening ferrite and homogenizing a structure.
• silicon (Si) is an element contributing to a generation of the retained austenite by suppressing precipitation of cementite. Therefore, in the present invention, silicon (Si) may be necessarily added to achieve such an effect.
• the preferable content of silicon (Si) may be 0.02% or more, and the more preferable content of silicon (Si) may be 0.05% or more.
• the present invention may limit the upper limit of the silicon (Si) content to 4.0%.
• the preferable upper limit of the content of silicon (Si) may be 3.8%, and the more preferable upper limit of the content of silicon (Si) may be 3.5%.
• Aluminum (Al) is an element that performs deoxidation by combining with oxygen in steel.
• aluminum (Al) is also an element for stabilizing the retained austenite by suppressing precipitation of cementite like silicon (Si). Therefore, in the present invention, aluminum (Al) may be necessarily added to achieve such an effect.
• a preferable content of aluminum (Al) may be 0.05% or more, and a more preferable content of aluminum (Al) may be 0.1% or more.
• the present invention may limit the upper limit of the content of aluminum (Al) to 5.0%.
• the preferable upper limit of the content of aluminum (Al) may be 4.75%, and the more preferable upper limit of the content of aluminum (Al) may be 4.5%.
• the total content (Si+Al) of silicon (Si) and aluminum (Al) is preferably 1.0 to 6.0%. Since silicon (Si) and aluminum (Al) are components that affect microstructure formation in the present invention, and thus, affect ductility, bending formability, and hole expansion ratio, the total content of silicon (Si) and aluminum (Al) is preferably 1.0 to 6.0%. The more preferable total content (Si+Al) of silicon (Si) and aluminum (Al) may be 1.5% or more, and may be 4.0% or less.
• Manganese (Mn) is a useful element for increasing both strength and ductility. Therefore, in the present disclosure, a lower limit of a content of manganese (Mn) may be limited to 0.9% in order to achieve such an effect. A preferable lower limit of the content of manganese (Mn) may be 1.0%, and a more preferable lower limit of the content of manganese (Mn) may be 1.1%. On the other hand, when manganese (Mn) is excessively added, the bainite transformation time increases and a concentration of carbon (C) in the austenite becomes insufficient, so there is a problem in that the desired austenite fraction may not be secured. Therefore, an upper limit of the content of manganese (Mn) of the present disclosure may be limited to 5.0%. A preferable upper limit of the content of manganese (Mn) may be 4.7%, and a more preferable upper limit of the content of manganese (Mn) may be 4.5%.
• Phosphorus (P) is an element that is included 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 that is included as an impurity to form MnS in a steel sheet and deteriorate ductility. Therefore, the content of sulfur (S) is preferably 0.03% or less.
• Nitrogen (N) is an element that is contained as an impurity and forms nitride during continuous casting to causes cracks of slab. Therefore, the content of nitrogen (N) is preferably 0.03% or less.
• 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 to achieve such an effect.
• titanium (Ti), niobium (Nb), and vanadium (V) exceed a certain level, respectively, excessive precipitates are formed to lower impact toughness and increase manufacturing cost, so the present invention may limit the content of titanium (Ti), niobium (Nb), and vanadium (V) to 0.5% or less, respectively.
• the present invention may add one or more of chromium (Cr) and molybdenum (Mo) to achieve such an effect.
• the content of chromium (Cr) and molybdenum (Mo) exceeds a certain level, the bainite transformation time increases and the concentration of carbon (C) in austenite becomes insufficient, so the desired retained austenite fraction may not be secured. Therefore, the present invention may limit the content 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, to thereby suppress hydrogen delayed destruction. Accordingly, in the present invention, one or more of copper (Cu) and nickel (Ni) may be added to achieve such an effect.
• the present invention may limit the content of copper (Cu) and nickel (Ni) to 4.5% or less, respectively.
• Boron (B) is an element that improves hardenability to increase strength, and is also an element that suppresses nucleation of grain boundaries. Therefore, in the present invention, boron (B) may be added to achieve such an effect. However, when the content of boron (B) exceeds a certain level, not only excessive characteristic effects, but also an increases in manufacturing cost is induced, so the present invention may limit the content of boron (B) to 0.005% or less.
• 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 to achieve such an effect.
• the present invention may limit the content 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 to achieve such an effect.
• the present invention may limit the content 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 to achieve such an effect.
• the present invention may limit the content 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 to achieve such an effect.
• the present invention may limit the content of yttrium (Y) and hafnium (Hf) to 0.2% or less, respectively.
• cobalt (Co) is an element that promotes bainite transformation to increase a TRIP effect
• cobalt (Co) may be added 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 superior 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 a raw material 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. In addition, additional addition of effective components other than the above-described components is not entirely excluded.
• the high strength steel sheet having superior workability according to an aspect of the present invention may include, as microstructures, tempered martensite, bainite, retained austenite, and ferrite.
• the high strength steel sheet having superior workability according to an aspect of the present invention may include, by volume fraction, 30 to 70% of tempered martensite, 10 to 45% of bainite, 10 to 40% of retained austenite, 3 to 20% of ferrite, and an unavoidable structure.
• unavoidable structure of the present invention fresh martensite, perlite, martensite austenite constituent (M-A), and the like may be included. When the fresh martensite or the pearlite is excessively formed, the workability of the steel sheet may be lowered or the fraction of the retained austenite may be lowered.
• the ratio of the average total content ([Si+Al] F , wt%) of silicon (Si) and aluminum (Al) included in the ferrite to the average total content ([Si+Al] av , wt% ) of silicon (Si) and aluminum (Al) included in the steel sheet may satisfy the range of 1.02 to 1.45. 1.02 ⁇ Si + Al F / Si + Al av ⁇ 1.45
• a balance B T ⁇ E of tensile strength and elongation expressed by the following [Relational Expression 2] is 22,000 (MPa%) or more
• a balance B T ⁇ H of tensile strength and hole expansion ratio expressed by the following [Relational Expression 3] is 7*10 6 (MPa 2 % 1/2 ) or more
• bendability B R expressed by the following [Relational Expression 4] satisfies a range of 0.5 to 3.0, it may have a superior balance of strength and ductility, a balance of strength and hole expansion ratio, and superior bending formability.
• R is a minimum bending radius (mm) at which cracks do not occur after a 90° bending test
• t is a thickness (mm) of the steel sheet.
• the present invention it is important to stabilize retained austenite of a steel sheet because it is intended to simultaneously secure superb ductility and bending formability as well as high strength properties.
• carbon (C) is concentrated into austenite by using ferrite, the strength of the steel sheet may be insufficient due to the low strength characteristics of ferrite, and excessive inter-phase hardness difference may occur, thereby reducing the hole expansion ratio (HER). Therefore, the present invention is intended to concentrate carbon (C) and manganese (Mn) into austenite by using the bainite and tempered martensite.
• the hardness of the ferrite increases, so it is possible to effectively reduce an inter-phase hardness difference of ferrite which is a soft structure and tempered martensite, bainite, and retained austenite which are a hard structure.
• the present invention limits a ratio of an average total content ([Si+Al] F , wt%) of silicon (Si) and aluminum (Al) included in the ferrite to an average total content ([Si+Al] av , wt%) of silicon (Si) and aluminum (Al) included in the steel sheet to 1.02 or more, so the inter-phase hardness difference of the soft structure and the hard structure may be effectively reduced.
• the present invention may limit the ratio of the average total content ([Si+Al] F , wt%) of silicon (Si) and aluminum (Al) included in the ferrite to the average total content ([Si+Al] av , wt%) of silicon (Si) and aluminum (Al) included in the steel sheet to 1.45 or more.
• a steel sheet including retained austenite has superb ductility and bending formability due to transformation-induced plasticity that occurs during transformation from austenite to martensite during processing.
• the balance (TSXEl) of tensile strength and elongation may be less than 22,000 MPa%, or the bendability (R/t) may exceed 3.0.
• the fraction of the retained austenite exceeds a certain level, local elongation may be lowered.
• the fraction of the retained austenite may be limited to a range of 10 to 40 vol% in order to obtain a steel sheet having a balance (TSXEl) of tensile strength and elongation and superior bendability (R/t).
• TSXEl balance of tensile strength and elongation and superior bendability
• both untempered martensite (fresh martensite) and tempered martensite are microstructures that improve the strength of the steel sheet.
• fresh martensite has a characteristic of greatly reducing the ductility and the hole expansion ratio of the steel sheet. This is because the microstructure of the tempered martensite is softened by the tempering heat treatment. Therefore, in the present invention, it is preferable to use tempered martensite to provide a steel sheet which is superior in the balance of strength and ductility, the balance of strength and hole expansion ratio, and the bending formability.
• the fraction of the tempered martensite may be limited to 30 to 70 vol% to obtain a steel sheet having the balance (TSXEl) of tensile strength and elongation, the balance (TS 2 XHER 1/2 ) of tensile strength and hole expansion ratio, and superb bendability (R/t).
• bainite is appropriately included as the microstructure. As long as a fraction of bainite is a certain level or more, it is possible to secure the balance (TSXEl) of tensile strength and elongation of 22,000 MPa% or more, the balance (TS 2 XHER 1/2 ) of tensile strength and hole expansion ratio of 7*10 6 (MPa 2 % 1/2 ) or more and the bendability (R/t) of 0.5 to 3.0.
• the present invention may not secure the desired balance (TSXEl) of tensile strength and elongation, the balance (TS 2 XHER 1/2 ) of tensile strength and hole expansion ratio, and bendability (R/t). Accordingly, the present invention may limit the fraction of bainite to a range of 10 to 45 vol%.
• the present invention may secure the desired balance (TSXEl) of tensile strength and elongation, as long as the fraction of ferrite is a certain level or more.
• TSXEl desired balance
• HER hole expansion ratio
• the present invention may not secure the desired balance (TS 2 XHER 1/2 ) of tensile strength and hole expansion ratio. Accordingly, the present invention may limit the fraction of ferrite to a range of 3 to 20 vol%.
• a method for manufacturing a high-strength steel sheet may include: preparing a steel slab having a predetermined component, and heating and hot rolling the steel slab; coiling the hot-rolled steel sheet; performing hot-rolled annealing heat treatment on the coiled steel sheet in a temperature within a range of 650 to 850°C for 600 to 1700 seconds; cold rolling the hot-rolled annealing heat-treated steel sheet; heating (primarily heating) the hot-rolled annealing heat-treated steel sheet to a temperature within a range of Ac1 or higher and less than Ac3 at an average temperature increase rate of 5°C/s or more, and maintaining (primarily maintaining) the primarily heated steel sheet for 50 seconds or more; cooling (primarily cooling) the primarily heated steel sheet to a temperature within a range of 100 to 300°C at an average cooling rate of 1°C/s or more; heating (secondarily heating) the primarily cooled steel sheet to a temperature within a range of 300 to 500°C, and maintaining (secondarily
• a steel slab having a predetermined component 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 certain temperature range, and the heating temperature of the steel slab at this time may be in the range of 1000 to 1350°C. This is because, when the heating temperature of the steel slab is less than 1000°C, the steel slab may be hot rolled in the temperature range below the 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 is 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 less 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 after the hot rolling has been completed may be cooled at an average cooling rate of 10°C/s or more, and may be coiled at a temperature of 300 to 600°C.
• the coiling temperature is less than 300°C, the coiling is not easy, and when the coiling temperature exceeds 600°C, a surface scale is formed to the inside of the hot-rolled steel sheet, which may make pickling difficult.
• the hot-rolled annealing heat treatment may be performed in a temperature within a range of 650 to 850°C for 600 to 1700 seconds.
• the hot-rolled annealing heat treatment temperature is less than 650°C or the hot-rolled annealing heat treatment time is less than 600 seconds, the strength of the hot-rolled annealing heat-treated steel sheet increases, and thus, subsequent cold rolling may not be easy.
• the hot-rolled annealing heat treatment temperature exceeds 850°C or the hot-rolled annealing heat treatment time exceeds 1700 seconds, the pickling may not be easy due to a scale formed deep inside 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 type are not particularly limited.
• the annealing heat treatment process is performed.
• the cold-rolled steel sheet is heated (primarily heated) to a temperature within a range of Ac1 or higher and less than Ac3 (two-phase region), and maintained (primarily maintained) in the temperature range for 50 seconds or more.
• the primary heating or primary maintaining temperature is Ac3 or higher (single-phase region), the desired ferrite structure may not be realized, so the desired level of [Si+Al] F /[Si+Al] av , and the balance (TS 2 XHER 1/2 ) of tensile strength and hole expansion ratio may be implemented.
• the average temperature increase rate of the primary heating may be 5°C /s or more.
• the structure may not be sufficiently homogenized and the physical properties of the steel sheet may be lowered.
• the upper limit of the primary maintaining time is not particularly limited, but the primary heating time is preferably limited to 1200 seconds or less in order to prevent the decrease in toughness due to the coarsening of grains.
• the cold-rolled steel sheet may be cooled (primarily cooled) to a primary cooling stop temperature of 100 to 300°C at a primary cooling rate of an average cooling rate of 1°C/s or more.
• the upper limit of the primary cooling rate does not need to be particularly specified, but is preferably limited to 100°C/s or less.
• the primary cooling stop temperature is less than 100°C, the tempered martensite is excessively formed and the amount of retained austenite formed is insufficient, so [Si+Al] F /[Si+Al] av , the balance (TSXEl) of tensile strength and elongation, and the bendability (R/t) may be lowered.
• the cold-rolled steel sheet may be heated (secondarily heated) to a secondary heating temperature of 300 to 500°C at a secondary heating rate of an average temperature increase rate of 5°C/s or more, and may be maintained (secondarily maintained) for 50 seconds or more in the temperature range.
• the upper limit of the secondary temperature increase rate does not need to be particularly specified, but is preferably limited to 100°C/s or less.
• the secondary heating or secondary maintaining temperature is less than 300°C, or the maintaining time is less than 50 seconds, the tempered martensite is excessively formed and the control of Si and Al content in the steel sheet is insufficient, so the desired fraction of the retained austenite is difficult to secure.
• the cold-rolled steel sheet After the secondary maintaining, it is preferable to cool (secondarily cool) the cold-rolled steel sheet to room temperature at an average cooling rate of 1°C/s or more.
• the high strength steel sheet having superior workability manufactured by the above-described manufacturing method may include, as a microstructure, tempered martensite, bainite, retained austenite, and ferrite, and as a preferred example, may include, by the volume fraction, 30 to 70% of tempered martensite, 10 to 45% of bainite, 10 to 40% of retained austenite, 3 to 20% of ferrite, and unavoidable structures.
• the ratio of the average total content ([Si+Al] F , wt%) of silicon (Si) and aluminum (Al) included in the ferrite to the average total content ([Si+Al] av , wt%) of silicon (Si) and aluminum (Al) included in the steel sheet may satisfy a range of 1.02 to 1.45, and the balance B T ⁇ E of tensile strength and elongation expressed by the following [Relational Expression 2] is 22,000 (MPa%) or more, the balance B T ⁇ H of tensile strength and hole expansion ratio expressed by the following [Relational Expression 3] may be 7*10 6 (MPa 2 % 1/2 ) or more, and the bendability B R expressed by the following [Relational Expression 4] may satisfy the range of 0.5 to 3.0.
• R is a minimum bending radius (mm) at which cracks do not occur after a 90° bending test
• t is a thickness (mm) of the steel sheet.
• 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 was 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.
• the hot-rolled steel sheet was subjected to hot-rolled annealing heat treatment under the conditions of Tables 2 and 3. Thereafter, after removing a surface scale by pickling, cold rolling was performed to a thickness of 1.5 mm.
• the 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), and pearlite (P) were observed through SEM after nital-etching a polished specimen cross section.
• the fractions of bainite and tempered martensite, which are difficult to distinguish among them, were calculated using an expansion curve after evaluation of dilatation.
• fresh martensite (FM) and retained austenite (retained y) are also difficult to distinguish
• a value obtained by subtracting the fraction of retained austenite calculated by X-ray diffraction method from the fraction of martensite and retained austenite observed by the SEM was determined as the fraction of the fresh martensite.
• An average total content ([Si+Al] F , wt%) of silicon (Si) and aluminum (Al) contained in ferrite was measured using an electron probe MicroAnalyser (EPMA), and an average total content ([Si+Al] av , wt%) of silicon (Si) and aluminum (Al) included in the steel sheet was calculated from the content of alloy composition of the steel sheet.
• EPMA electron probe MicroAnalyser
• 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.
• the bendability (R/t) was evaluated by a V-bending test, and calculated by collecting a specimen based on the 90° direction with respect to the rolling direction of the rolled sheet and being determined as a value obtained by dividing a minimum bending radius R, at which cracks do not occur after a 90° bending test, by a thickness t of a sheet.
• the hole expansion ratio (HER) was evaluated through the hole expansion test, and was calculated by the following [Relational Expression 5] by, after forming a punching hole (die inner diameter of 10.3mm, clearance of 12.5%) of 10 mm ⁇ , inserting a conical punch having an apex angle of 60° into a punching hole in a direction in which a burr of a punching hole faces outward, and then compressing and expanding a peripheral portion of the punching hole at a moving speed of 20 mm/min.
• Hole expansion ratio HER , % D ⁇ D 0 / D 0 ⁇ 100
• D is a hole diameter (mm) when cracks penetrate through the steel plate along the thickness direction
• D 0 is the initial hole diameter (mm).
• the primary cooling stop temperature was high, so the bainite was excessively formed and the tempered martensite was formed less.
• the balance (TSXEl) of tensile strength and elongation is less than 22,000 MPa% and the balance (TS 2 XHER 1/2 ) of tensile strength and hole expansion ratio is less than 7*10 6 (MPa 2 % 1/2 ).
• Specimens 40 to 48 may satisfy the manufacturing conditions presented in the present invention, but may be outside the alloy composition range. In these cases, it could be seen that [Si+Al] F /[Si+Al] av , the balance (TSXEl) of tensile strength and elongation, and the balance (TS 2 XHER 1/2 ) of tensile strength and hole expansion ratio of the present invention does not simultaneously satisfy the conditions of 7*10 6 (MPa 2 % 1/2 ) and the bendability (R/t) .

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