Classifications

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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
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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
  • C21D1/22—Martempering
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  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/002—Heat treatment of ferrous alloys containing Cr
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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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
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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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  • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0278—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment 
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  • 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
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • 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
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  • 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
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  • 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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
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  • 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
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with 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
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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  • 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
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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
  • 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
  • C21D2211/003—Cementite
• • C—CHEMISTRY; METALLURGY
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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
  • C21D2211/004—Dispersions; Precipitations
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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
  • C21D2211/005—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 related art includes Japanese Patent Publication No. 2005-105367 .
• the present invention provides an ultra-high-strength cold-rolled steel sheet with high yield ratio and excellent bendability and a method of manufacturing the same, and more particularly, a cold-rolled steel sheet capable of implementing martensite steels with a tensile strength of 1400 MPa or more, and a method of manufacturing the same.
• a cold-rolled steel sheet consisting of carbon (C): 0.23 wt% to 0.35 wt%, silicon (Si): 0.05 wt% to 0.5 wt%, manganese (Mn): 0.3 wt% to 2.3 wt%, phosphorus (P): more than 0 wt% and not more than 0.02 wt%, sulfur (S): more than 0 wt% and not more than 0.005 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): more than 0 wt% and not more than 0.8 wt%, molybdenum (Mo): more than 0 wt% and not more than 0.4 wt%, titanium (Ti): 0.01 wt% to 0.1 wt%, vanadium (V): more than 0 wt% and not more than 0.3 wt%,
• the cementite, the transition carbide, and the fine precipitate may each have an average size of 50 nm or less and an average aspect ratio of 4.0 or less.
• the cementite, the transition carbide, and the fine precipitate may each have an area fraction of more than 0% and not more than 5%.
• the final microstructure may consist of only tempered martensite.
• the final microstructure may consist of tempered martensite, ferrite, and bainite, the tempered martensite having an area fraction of 70% or more and less than 100%, and the ferrite and bainite having an area fraction of more than 0% and not more than 20%.
• a method of manufacturing a cold-rolled steel sheet including (a) hot rolling a steel material consisting of carbon (C): 0.23 wt% to 0.35 wt%, silicon (Si): 0.05 wt% to 0.5 wt%, manganese (Mn): 0.3 wt% to 2.3 wt%, phosphorus (P): more than 0 wt% and not more than 0.02 wt%, sulfur (S): more than 0 wt% and not more than 0.005 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): more than 0 wt% and not more than 0.8 wt%, molybdenum (Mo): more than 0 wt% and not more than 0.4 wt%, titanium (Ti): 0.01 wt% to 0.1 wt%, vanadium (V)
• Step (a) may be performed under conditions of a reheating temperature of 1150 °C to 1300 °C, a finishing delivery temperature of 800 °C to 1000 °C, and a coiling temperature of 500 °C to 650 °C
• step (c) may be performed under conditions of an annealing temperature of 800 °C to 900 °C and a first heat treatment temperature of 100°C to 300 °C
• the second heat treatment may include maintaining a second heat treatment temperature T satisfying Inequality 1 for a second heat treatment holding time t. 3800 ⁇ T + 300 ⁇ 10 + log t ⁇ 5650 (where a unit of T is °C and a unit of t is hours).
• Step (a) may be performed under conditions of a reheating temperature of 1150 °C to 1300 °C, a finishing delivery temperature of 800 °C to 1000 °C, and a coiling temperature of 500 °C to 650 °C
• step (c) may include performing coating and be performed under conditions of an annealing temperature of 800 °C to 900 °C and a first heat treatment temperature of 450 °C to 600 °C
• the second heat treatment may include maintaining a second heat treatment temperature T satisfying Inequality 1 for a second heat treatment holding time t. 3800 ⁇ T + 300 ⁇ 10 + log t ⁇ 5650 (where a unit of T is °C and a unit of t is hours).
• the first heat treatment following the annealing, may be performed after cooling to a first heat treatment temperature.
• step (c) the second heat treatment, following the first heat treatment, may be performed after cooling to room temperature and then raising the temperature.
• an ultra-high-strength cold-rolled steel sheet with high yield ratio and excellent bendability and a method of manufacturing the same may be implemented.
• a high-strength cold-rolled steel sheet with a high tensile strength, a high yield ratio (YP/TS) of more than 70%, and an excellent bendability (R/t) of 4.0 or less may be implemented.
• YP/TS high yield ratio
• R/t excellent bendability
• a cold-rolled steel sheet and a method of manufacturing the same will now be described in detail.
• the terms used herein are selected based on their functions in the present invention, and their definitions should be made in the context of the entire specification.
• a detailed description of an ultra-high-strength cold-rolled steel sheet with high yield ratio and excellent bendability and a method of manufacturing the same will be provided below.
• a cold-rolled steel sheet consists of carbon (C): 0.23 wt% to 0.35 wt%, silicon (Si): 0.05 wt% to 0.5 wt%, manganese (Mn): 0.3 wt% to 2.3 wt%, phosphorus (P): more than 0 wt% and not more than 0.02 wt%, sulfur (S): more than 0 wt% and not more than 0.005 wt%, aluminum (Al): 0.01 wt% to 0.05 wt%, chromium (Cr): more than 0 wt% and not more than 0.8 wt%, molybdenum (Mo): more than 0 wt% and not more than 0.4 wt%, titanium (Ti): 0.01 wt% to 0.1 wt%, vanadium (V): more than 0 wt% and not more than 0.3 wt%, boron
• C is the most effective and important element for increasing the strength of steel.
• C is added and dissolved in austenite to form a martensite structure when quenched.
• C combines with elements such as Fe, Cr, and Mo to form carbides and enhances strength and hardness.
• C may be added at a content ratio of 0.23 wt% to 0.35 wt% of a total weight in a base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention.
• the content of C is less than 0.23 wt% of the total weight, the above-described effect may not be achieved and a sufficient strength may not be ensured.
• the content of C is greater than 0.35 wt% of the total weight, weldability and workability may be reduced.
• Si is an element added to ensure bendability and hydrogen embrittlement resistance by suppressing the formation of cementite.
• Si is also an element added to increase strength and suppress the formation of cementite due to the solid solution strengthening effect in ferrite.
• Si is well known as a ferrite stabilizing element and thus may improve ductility by increasing the fraction of ferrite during cooling.
• Si is also known as an element capable of ensuring strength by promoting the formation of martensite through carbon enrichment in austenite.
• Si may be added together with Al as a deoxidizer for removing oxygen from steel during a steelmaking process, and have a solid solution strengthening effect.
• Si may be added at a content ratio of 0.05 wt% to 0.5 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention.
• content of Si is less than 0.05 wt% of the total weight, ductility may not be ensured and the above-described effects of Si addition may not be properly realized.
• ferrite when the content of Si is greater than 0.5 wt% of the total weight due to excessive addition, ferrite may be excessively formed to reduce strength, oxide may be formed on the surface of the steel sheet to reduce the coatability of the steel sheet, red scale may be formed during reheating and hot rolling to degrade the surface quality, toughness and plasticity may be reduced, and the weldability of steel may also be reduced.
• Mn may be added at a content ratio of 0.3 wt% to 2.3 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention.
• the content of Mn is less than 0.3 wt%, the above-described effect of strength enhancement may not be sufficiently realized.
• the content of Mn is greater than 2.3 wt%, bendability and hydrogen embrittlement resistance may be reduced due to the formation of Mn bands and MnS.
• segregation zones may be formed inside and outside the continuously casted slab and the steel sheet and the formation and propagation of cracks may be caused to reduce bendability. That is, slab quality and weldability may be reduced, and center segregation may occur to reduce the ductility and workability of the base steel sheet.
• P may serve to increase the strength of steel through solid solution strengthening and suppress the formation of carbides.
• P may be added at a content ratio of more than 0 wt% and not more than 0.02 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention.
• the content of P is greater than 0.02 wt%, welded joints may become embrittled, brittleness may be caused by grain boundary segregation, press formability may be reduced, and impact resistance may be lowered.
• S is an element that combines with Mn or Ti to improve the machinability of steel and forms fine MnS precipitates to enhance workability, but generally hinders ductility and weldability.
• S may be added at a content ratio of more than 0 wt% and not more than 0.005 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention.
• the content of S is greater than 0.005 wt%, the number of MnS inclusions may be increased to reduce bendability and hydrogen embrittlement resistance, and segregation may occur during continuous casting solidification to cause high-temperature cracks.
• Al is an element commonly used as a deoxidizer, and prevents slab cracks during the formation of nitrides, promotes the formation of ferrite to enhance elongation, suppresses the formation of carbides, and stabilizes austenite by increasing the concentration of C in austenite.
• Al serves as a layer between Fe and zinc (Zn) coating to enhance coatability, and effectively suppresses the formation of Mn bands in a hot-rolled coil.
• Al may be added at a content ratio of 0.01 wt% to 0.05 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of Al is less than 0.01 wt%, the above-described effects of Al addition may not be properly realized.
• Cr is an element capable of enhancing hardenability and ensuring high strength, and may improve quenchability as an austenite stabilizing element. Cr increases elongation by forming Cr-based precipitates in the grains during annealing. Cr may be added at a content ratio of more than 0 wt% and not more than 0.8 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of Cr is greater than 0.8 wt% due to excessive addition, the saturation effect may occur, laser weldability and ductility may be reduced, and coatability may be hindered.
• Mo is an element added to improve quenchability and ensure strength and toughness, and is also an element capable of enhancing hydrogen embrittlement resistance due to the grain refinement and precipitation effect.
• Mo may be added at a content ratio of more than 0 wt% and not more than 0.4 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention. When the content of Mo is greater than 0.4 wt%, production costs may increase and weldability may decrease.
• Ti contributes to grain refinement and BN formation suppression.
• Ti may be added at a content ratio of 0.01 wt% to 0.1 wt% of the total weight in the base steel sheet for forming the cold-rolled steel sheet according to an embodiment of the present invention.
• the content of Ti is less than 0.01 wt%, a reduction in ductility of the casting slab due to excessive formation of BN precipitates may reduce slab quality and strength.
• the content of Ti is greater than 0.1 wt%, bendability and hydrogen embrittlement resistance may be reduced due to the coarsening of TiN precipitates, and recrystallization temperature may be excessively increased to cause a non-uniform structure.
• the final microstructure of a cold-rolled steel sheet according to another modified embodiment of the present invention may consist of only tempered martensite. In this case, bainite and ferrite are not present.
• the above-described microstructure is based on the result of analyzing a 1/4 point of a thickness direction from a direction perpendicular to the rolling direction with the scanning electron microscope.
• the area fraction of the tempered martensite is less than 70%, the desired strength may not be achieved.
• the ferrite and bainite are inevitably formed due to an insufficient cooling rate and serve as a main factor for reducing strength, the smaller the area fractions thereof, the better.
• the sum of area fractions of the two phases of ferrite and bainite is required not to exceed 20%.
• the cold-rolled steel sheet with the above-described alloying element composition and microstructure according to an embodiment of the present invention includes a cementite-type carbide but may achieve the properties of a yield strength (YP) of 1170 MPa or more, a tensile strength (TS) of 1400 MPa or more, an elongation (EI) of 3.0% or more, a yield ratio of 70% or more, and a bendability (R/t) of 4.0 or less.
• YP yield strength
• TS tensile strength
• EI elongation
• R/t bendability
• the cold-rolled steel sheet according to an embodiment of the present invention may have a YP of 1170 MPa to 1400 MPa, a TS of 1400 MPa to 1700 MPa, an El of 3.0% to 9.0%, a yield ratio of 70% to 90%, and a bendability (R/t) of 2.0 to 4.0.
• R represents the minimum bending radius and t represents the thickness.
• the steel sheet manufacturing method includes (a) hot rolling a steel material consisting of C: 0.23 wt% to 0.35 wt%, Si: 0.05 wt% to 0.5 wt%, Mn: 0.3 wt% to 2.3 wt%, P: more than 0 wt% and not more than 0.02 wt%, S: more than 0 wt% and not more than 0.005 wt%, Al: 0.01 wt% to 0.05 wt%, Cr: more than 0 wt% and not more than 0.8 wt%, Mo: more than 0 wt% and not more than 0.4 wt%, Ti: 0.01 wt% to 0.1 wt%, V: more than 0 wt% and not more than 0.3 wt%, B: 0.001 wt% to 0.005 wt%, and a balance of Fe; (b) cold rolling the hot-rolled steel material; and (c) sequential
• the hot rolling step (a) may be performed under conditions of a reheating temperature of 1150 °C to 1300 °C, a finishing delivery temperature of 800 °C to 1000 °C, and a coiling temperature of 500 °C to 650 °C.
• the reheating temperature when the reheating temperature is higher than 1300 °C, very coarse austenite grains may be formed and thus strength may not be easily ensured.
• the reheating temperature when the reheating temperature is higher than 1300 °C, heating costs and process time may be increased to increase production costs and reduce productivity.
• the finishing delivery temperature is an important factor affecting the final material properties, and rolling at 800 °C to 1000 °C may refine austenite. However, when the hot rolling temperature is lower than 800 °C, the rolling load may increase and a mixed grain structure may occur at the edge. Rolling at a temperature higher than 1000 °C may cause grain coarsening and prevent the desired mechanical properties from being achieved. After hot rolling, cooling is performed at a cooling rate of 1 °C/s to 100 °C/s. The higher the cooling rate, the smaller the average grain size.
• the hot-rolled coil may have a non-uniform shape and the cold rolling load may be increased.
• the coiling temperature is higher than 650 °C, a non-uniform microstructure may occur due to the difference in cooling rate between the center and edge of the steel sheet, and the inside of the grain boundaries may be oxidized.
• the hot rolling may be performed under a condition of a reduction ratio of 35% to 65%.
• the microstructure of the steel material after the hot rolling may include bainite, martensite, and ferrite.
• the cold rolling step (b) may include performing pickling and then performing cold rolling at a reduction ratio of 35% to 65%.
• the higher the reduction ratio the greater the increase in formability due to the microstructural refinement effect.
• the cold rolling is performed at a reduction ratio less than 35%, a uniform microstructure may not be easily obtained, and when the cold rolling is performed at a reduction ratio greater than 65%, the roll force may increase and thus the process load may also increase.
• FIG. 6 is a graph showing a step of sequentially performing annealing, first heat treatment, and second heat treatment in a method of manufacturing a cold-rolled steel sheet, according to an embodiment of the present invention.
• the formation and growth of the cementite and the transition carbide proceed under the conditions proposed by the present invention.
• the formation of transition carbide is a very important factor for ensuring yield strength.
• the desired yield strength may not be easily ensured under the second heat treatment condition of an excessively low temperature or short time.
• the growth of acicular cementite is a factor that deteriorates bendability. Because a relatively high temperature or long time of the second heat treatment accelerates the growth of acicular cementite, an appropriate second heat treatment condition needs to be set to ensure bendability.
• an ultra-high-strength cold-rolled steel sheet with high yield ratio and excellent bendability may be implemented by controlling the range of Inequality 1.
• the present test examples provide samples with the alloying element composition (unit: wt%) of Table 1.
• Table 1 Component System C Si Mn Cr Mo Ti V B Fe A 0.25 0.1 2.0 0.4 0.2 0.03 0.1 0.0025 Bal. B 0.24 0.1 1.9 0.3 0.2 0.06 0.1 0.0025 Bal. C 0.22 0.2 2.2 0.3 0.2 0.03 0.1 0.0022 Bal. D 0.26 0.7 1.8 0.4 0.2 0.03 0.1 0.0020 Bal.
• Component Systems A and B satisfy the composition of the cold-rolled steel sheet according to an embodiment of the present invention, i.e., C: 0.23 wt% to 0.35 wt%, Si: 0.05 wt% to 0.5 wt%, Mn: 0.3 wt% to 2.3 wt%, P: more than 0 wt% and not more than 0.02 wt%, S: more than 0 wt% and not more than 0.005 wt%, Al: 0.01 wt% to 0.05 wt%, Cr: more than 0 wt% and not more than 0.8 wt%, Mo: more than 0 wt% and not more than 0.4 wt%, Ti: 0.01 wt% to 0.1 wt%, V: more than 0 wt% and not more than 0.3 wt%, B: 0.001 wt% to 0.005 wt%, and a balance of Fe.
• Component System C falls below and
• Table 2 shows various heat treatment conditions for samples with the compositions shown in Table 1
• Table 3 shows a result of evaluating the properties after the compositions and heat treatment conditions shown in Tables 1 and 2 are applied.
• 'Component System' represents the composition shown in Table 1
• 'Inequality 1' represents the value obtained by calculating Inequality 1 [(T + 300) ⁇ (10 + log(t))].
• 'YP (MPa)', 'TS (MPa)', and 'EL (%)' represent a yield strength, a tensile strength, and an elongation of the samples, respectively.
• Test Examples 1 and 2 show the difference in properties depending on the annealing temperature.
• Test Examples 1 and 2 which are cold-rolled steel sheets implemented according to embodiments of the present invention and satisfy the annealing temperature range of 800 °C to 900 °C, achieve the properties of a YP of 1170 MPa or more, a TS of 1400 MPa or more, an El of 3.0% or more, a yield ratio of 70% or more, and a bendability (R/t) of 4.0 or less.
• the cementite, the transition carbide, and the fine precipitate each has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of more than 0% and not more than 5%.
• Test Examples 3 to 6 show the difference in properties depending on the first heat treatment temperature.
• Test Examples 3 to 5 which are non-coated cold-rolled steel sheets implemented according to embodiments of the present invention and satisfy the first heat treatment temperature range of 100 °C to 300 °C, achieve the properties of a YP of 1170 MPa or more, a TS of 1400 MPa or more, an El of 3.0% or more, a yield ratio of 70% or more, and a bendability (R/t) of 4.0 or less. Furthermore, in the final microstructure of the cold-rolled steel sheets, the cementite, the transition carbide, and the fine precipitate each has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of more than 0% and not more than 5%.
• Test Example 6 which is a coated cold-rolled steel sheet implemented according to an embodiment of the present invention and satisfies the first heat treatment temperature range of 450 °C to 600 °C, achieves the properties of a YP of 1170 MPa or more, a TS of 1400 MPa or more, an El of 3.0% or more, a yield ratio of 70% or more, and a bendability (R/t) of 4.0 or less. Furthermore, in the final microstructure of the cold-rolled steel sheet, the cementite, the transition carbide, and the fine precipitate each has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of more than 0% and not more than 5%.
• Test Example 24 which is annealed at 350 °C and exceeds and does not satisfy the first heat treatment temperature range of 100 °C to 300 °C, does not achieve the desired properties of a YP of 1170 MPa or more and a TS of 1400 MPa or more, and does not satisfy a carbide average size range of 50 nm or less and a carbide average aspect ratio range of 4.0 or less.
• the first heat treatment temperature is maintained in the range between 300 °C and 450 °C, strength degradation is caused by transformation heat.
• the first heat treatment temperature satisfies the range of 450 °C to 600 °C as in Test Example 6 as in Test Example 6, transformation may be suppressed and thus the material properties may be ensured.
• Inequality 1 the difference in properties depending on the second heat treatment condition is shown.
• the value of Inequality 1 shown in Table 2 represents the value of [(T + 300) ⁇ (10 + log(t))] based on a second heat treatment temperature T and a second heat treatment holding time t.
• the unit of the second heat treatment temperature T is °C
• the unit of the second heat treatment holding time t is hours.
• Test Examples 1 to 6, 11, and 20 to 24 are applied with conditions of a second heat treatment temperature T of 150 °C and a second heat treatment holding time t of 6 hours
• Test Example 7 is applied with conditions of a second heat treatment temperature T of 25 °C and a second heat treatment holding time t of 6 hours
• Test Example 8 is applied with conditions of a second heat treatment temperature T of 50 °C and a second heat treatment holding time t of 6 hours
• Test Example 9 is applied with conditions of a second heat treatment temperature T of 100 °C and a second heat treatment holding time t of 6 hours
• Test Example 10 is applied with conditions of a second heat treatment temperature T of 130 °C and a second heat treatment holding time t of 6 hours
• Test Example 12 is applied with conditions of a second heat treatment temperature T of 180 °C and a second heat treatment holding time t of 6 hours
• Test Example 13 is applied with conditions of a second heat treatment temperature T of 200 °C and a second heat treatment holding time t of 6 hours
• Test Examples 1 to 6 show that, although the annealing temperature and the first heat treatment conditions are changed within the ranges proposed by the present invention, when the second heat treatment temperature T and the second heat treatment holding time t are applied in such a manner that the value of Inequality 1 [(T + 300) ⁇ (10 + log(t))] given as the second heat treatment condition satisfies the range between 3800 and 5650, the desired properties are ensured.
• Test Examples 7 and 8 show that, when the value of Inequality 1 given as the second heat treatment condition is lower than 3800, the transition carbide is not formed in the final microstructure and thus the YP fails to reach the desired value (1170 MPa or more).
• Test Examples 9 to 13, 18, and 19 show that, when the value of Inequality 1 given as the second heat treatment condition is between 3800 and 5650, a YP of 1170 MPa or more, a TS of 1400 MPa or more, an El of 3.0% or more, a yield ratio of 70% or more, and a bendability (R/t) of 4.0 or less are satisfied, and the cementite, the transition carbide, and the fine precipitate each has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of more than 0% and not more than 5%.
• Test Examples 14 to 17 show that, when the value of Inequality 1 given as the second heat treatment condition is higher than 5650, deterioration in bendability occurs due to the increase in size and aspect ratio of carbides. That is, the desired property of a bendability (R/t) of 4.0 or less is not achieved and the average aspect ratio of carbides does not satisfy the range of 4.0 or less. In other words, the desired bendability (R/t) of 4.0 or less is not satisfied due to carbide shape defects.
• Test Examples 20 to 23 show the difference in properties depending on the alloy composition.
• Test Examples 20 and 21 which are cold-rolled steel sheets implemented according to embodiments of the present invention and satisfy the composition of C: 0.23 wt% to 0.35 wt%, Si: 0.05 wt% to 0.5 wt%, Mn: 0.3 wt% to 2.3 wt%, P: more than 0 wt% and not more than 0.02 wt%, S: more than 0 wt% and not more than 0.005 wt%, Al: 0.01 wt% to 0.05 wt%, Cr: more than 0 wt% and not more than 0.8 wt%, Mo: more than 0 wt% and not more than 0.4 wt%, Ti: 0.01 wt% to 0.1 wt%, V: more than 0 wt% and not more than 0.3 wt%, B: 0.00
• the cementite, the transition carbide, and the fine precipitate each has an average size of 50 nm or less, an average aspect ratio of 4.0 or less, and an area fraction of more than 0% and not more than 5%.
• Test Example 22 which satisfies the annealing, first heat treatment, and second heat treatment conditions of the present invention but falls below and does not satisfy the composition range of C: 0.23 wt% to 0.35 wt%, shows that the desired properties of a YP of 1170 MPa or more and a TS of 1400 MPa or more are not achieved.
• Test Example 23 which satisfies the annealing, first heat treatment, and second heat treatment conditions of the present invention but exceeds and does not satisfy the composition range of Si: 0.05 wt% to 0.5 wt%, the desired property of a YP of 1170 MPa or more is not achieved due to the transformation of intermediate phases such as ferrite and bainite.
• Test Example 23 which satisfies the carbide size and carbide aspect ratio characteristics, ensures bendability but does not achieve the desired YP because ferrite is formed by more than 10%.
• a cold-rolled steel sheet and a method of manufacturing the same, according to embodiments of the present invention, have been described above.
• a high-strength cold-rolled steel sheet with a high tensile strength, a high yield ratio (YP/TS) of more than 70%, and an excellent bendability (R/t) of 4.0 or less may be implemented.
• YP/TS high yield ratio
• R/t excellent bendability

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