High-strength cold rolled steel sheet with excellent yield strength and ductility, coated steel plate, and method for manufacturing same

Abstract

Provided is a high-strength steel sheet for use in lightening materials for electronic products and materials for vehicles including automobiles, trains and ships and, more particularly, to a high-strength cold rolled steel sheet, a coated steel plate, and a method for manufacturing the same, wherein yield strength and ductility are improved by controlling the internal oxidation depth and the amount of residual austenite after first annealing operation, and the high-strength cold rolled steel sheet and the coated steel plate is stably manufactured and provided without any dent defects during the manufacturing thereof.

1 . A high-strength cold rolled steel sheet, excellent in terms of yield strength and ductility, comprising, by weight, 0.1% to 0.3% of carbon (C), 0.1% to 2.0% of silicon (Si), 0.005% to 1.5% of aluminum (Al), 1.5% to 3.0% of manganese (Mn), 0.04% or less (excluding 0%) of phosphorus (P), 0.015% or less (excluding 0%) of sulfur (S), 0.02% or less (excluding 0%) of nitrogen (N), 0.01% to 0.1% of antimony (Sb), a remainder of iron (Fe) and unavoidable impurities, and the sum of Si and Al (Si+Al, wt %) satisfies 1% to 3.5%,
 wherein a microstructure comprises, by an area fraction, 5% or less of polygonal ferrite having a ratio of a short axis to a long axis of more than 0.4, 70% or less of acicular ferrite having a ratio of a short axis to a long axis of 0.4 or less, 0.6% to 25% of retained austenite, and a remainder of martensite,   an internal oxidation depth is 1 μm or less from a surface.   
     
     
         2 . The high-strength cold rolled steel sheet according to  claim 1 , wherein, in the cold rolled steel sheet, the microstructure before the second annealing operation comprises 0.6% or more of acicular retained austenite, and the remainder is composed of one or more of bainite, martensite and tempered martensite. 
     
     
         3 . The high-strength cold rolled steel sheet according to  claim 1 , wherein the cold rolled steel sheet further comprises one or more selected from the group consisting of 0.005% to 0.1% of titanium (Ti), 0.005% to 0.1% of niobium (Nb), 0.005% to 0.1% of vanadium (V), 0.005% to 0.1% of zirconium (Zr), and 0.005% to 0.5% of tungsten (W). 
     
     
         4 . The high-strength cold rolled steel sheet according to  claim 1 , wherein the cold rolled steel sheet further comprises one or more selected from the group consisting of 1% or less (excluding 0%) of molybdenum (Mo), 1% or less (excluding 0%) of nickel (Ni), 0.5% or less (excluding 0%) of copper (Cu), and 1% or less (excluding 0%) of chromium (Cr). 
     
     
         5 . The high-strength cold rolled steel sheet according to  claim 1 , wherein the cold rolled steel sheet further comprises one or more selected from the group consisting of 0.01% or less (excluding 0%) of calcium (Ca), 0.1% or less (excluding 0%) of bismuth (Bi), and 0.01% or less (excluding 0%) of boron (B). 
     
     
         6 . The high-strength cold rolled steel sheet according to  claim 1 , wherein tensile strength of the cold rolled steel sheet is 780 MPa or greater. 
     
     
         7 . The high-strength cold rolled steel sheet according to  claim 1 , wherein one of a hot-dip galvanized layer, a hot-dip galvannealed layer, an aluminum-silicon plated layer and a zinc-magnesium-aluminum plated layer is formed on a surface of the cold rolled steel sheet. 
     
     
         8 . A method of manufacturing a high-strength cold rolled steel sheet, excellent in terms of yield strength and ductility, comprising:
 heating a steel slab comprising, by weight, 0.1% to 0.3% of carbon (C), 0.1% to 2.0% of silicon (Si), 0.005% to 1.5% of aluminum (Al), 1.5% to 3.0% of manganese (Mn), 0.04% or less (excluding 0%) of phosphorus (P), 0.015% or less (excluding 0%) of sulfur (S), 0.02% or less (excluding 0%) of nitrogen (N), 0.01% to 0.1% of antimony (Sb), a remainder of iron (Fe) and unavoidable impurities, and the sum of Si and Al (Si+Al, wt %) satisfies 1% to 3.5%, at a temperature of 1,000° C. to 1,300° C.;   hot rolling the heated steel slab at a temperature of 800° C. to 950° C. to produce a hot rolled steel sheet;   coiling the hot rolled steel sheet at a temperature of 750° C. or lower;   cold rolling the coiled hot rolled steel sheet to produce a cold rolled steel sheet;   performing a first annealing operation in which the cold rolled steel sheet is annealed at a temperature of Ac 3  or higher and cooled at an average cooling rate of 25° C./sec or lower; and   performing a second annealing operation in which, after the first annealing operation, the first annealed cold rolled steel sheet is heated and maintained at a temperature of Ac 1  to Ac 3 , cooled in an average cooling rate of lower than 20° C./sec to a temperature of 500° C. or lower, maintained at the temperature of 500° C. or lower 1 second or more, and then cooled.   
     
     
         9 . The method according to  claim 8 , wherein the steel slab further comprises one or more selected from the group consisting of 0.005% to 0.1% of titanium (Ti), 0.005% to 0.1% of niobium (Nb), 0.005% to 0.1% of vanadium (V), 0.005% to 0.1% of zirconium (Zr), and 0.005% to 0.5% of tungsten (W). 
     
     
         10 . The method according to  claim 9 , wherein the steel slab further comprises one or more selected from the group consisting of 1% or less (excluding 0%) of molybdenum (Mo), 1% or less (excluding 0%) of nickel (Ni), 0.5% or less (excluding 0%) of copper (Cu), and 1% or less (excluding 0%) of chromium (Cr). 
     
     
         11 . The method according to  claim 9 , wherein the steel slab further comprises one or more selected from the group consisting of 0.01% or less (excluding 0%) of calcium (Ca), 0.1% or less (excluding 0%) of bismuth (Bi), and 0.01% or less (excluding 0%) of boron (B). 
     
     
         12 . The method according to  claim 8 , further comprising forming one of a hot-dip galvanized layer, a hot-dip galvannealed layer, an aluminum-silicon plated layer and a zinc-magnesium-aluminum plated layer, on a surface of the cold rolled steel sheet after the second annealing operation.