Steel sheet having high strength and high formability and method for manufacturing same

Abstract

Provided herein is a steel sheet having high strength and high formability according to an aspect of the present invention including, % by weight, an amount of 0.05 to 0.15% of carbon (C), an amount greater than 0 and 0.4% or less of silicon (Si), an amount of 4.0-9.0% of manganese (Mn), an amount of greater than 0 and 0.3% or less of aluminum (Al), an amount of 0.02% or less of phosphorus (P), an amount of 0.005% or less of sulfur (S), an amount of 0.006% or less of nitrogen (N), and the remainder of iron (Fe) and other inevitable impurities. The steel sheet has a microstructure consisting of ferrite and residual austenite. The grain size of the microstructure is 3 μm or less. The steel sheet has a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.

1 . A steel sheet having high strength and high formability, comprising, % by weight, an amount of 0.05 to 0.15% carbon (C), an amount greater than 0 and less than or equal to 0.4% silicon (Si), an amount of 4.0 to 9.0% manganese (Mn), an amount of greater than 0 and less than or equal to 0.3% aluminum (Al), an amount of 0.02% or less phosphorus (P), an amount of 0.005% or less sulfur (S), an amount of 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities, wherein the steel sheet comprises a microstructure consisting of ferrite and retained austenite,
 wherein the microstructure has a grain size of 3 μm or less, and the steel sheet has a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater.   
     
     
         2 . The steel sheet of  claim 1 , comprising one or more components of niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo), each of which is included in an amount greater than 0 and less than or equal to 0.02 wt %. 
     
     
         3 . The steel sheet of  claim 1 , further comprising an amount greater than 0 and less than or equal to 0.001 wt % boron (B). 
     
     
         4 . The steel sheet of  claim 1 , wherein a volume fraction of the retained austenite in the microstructure is 10 to 30 vol %. 
     
     
         5 . A method for manufacturing a steel sheet having high strength and high formability, the method comprising steps of:
 (a) manufacturing a hot-rolled steel sheet from a steel slab comprising: % by weight, an amount of 0.05 to 0.15% carbon (C), an amount greater than 0 and less than or equal to 0.4% silicon (Si), an amount of 4.0 to 9.0% manganese (Mn), an amount greater than 0 and less than or equal to 0.3% aluminum (Al), an amount of 0.02% or less phosphorus (P), an amount of 0.005% or less sulfur (S), an amount of 0.006% or less nitrogen (N), and the remainder being iron (Fe) and other inevitable impurities;   (b) manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet;   (c) subjecting the cold-rolled steel sheet to first heat treatment at a temperature of AC3 to (AC3+15) ° C.; and   (d) subjecting the cold-rolled steel sheet, subjected to the first heat treatment, to second heat treatment at an intercritical temperature,   wherein the cold-rolled steel sheet after step (d) has a microstructure consisting of ferrite and austenite.   
     
     
         6 . The method of  claim 5 , wherein the steel slab includes one or more of niobium (Nb), titanium (Ti), vanadium (V) and molybdenum (Mo), each which is included in an amount of greater than 0 and less than or equal to 0.02 wt %. 
     
     
         7 . The method of  claim 5 , wherein the steel slab further comprises an amount greater than 0 and less than or equal to 0.001 wt % boron (B). 
     
     
         8 . The method of  claim 5 , wherein a volume fraction of the retained austenite in the microstructure is 10 to 30 vol %. 
     
     
         9 . The method of  claim 5 , wherein step (c) comprises a step of cooling the cold-rolled steel sheet, subjected to the heat treatment, to a temperature of 350 to 450° C. at a cooling rate of 4 to 10° C./s. 
     
     
         10 . The method of  claim 9 , wherein step (d) comprises a step of cooling the cold-rolled steel sheet, subjected to the heat treatment, to a temperature of 350 to 450° C. at a cooling rate of 4 to 10° C./s. 
     
     
         11 . The method of  claim 5 , wherein step (a) comprises steps of:
 (a1) reheating the steel slab to a temperature of 1,150 to 1,250° C.;   (a2) hot-rolling the reheated steel slab to a finish delivery temperature of 925 to 975° C.; and   (a3) cooling the hot-rolled steel sheet to a temperature of 700° C. to 800° C. at a cooling rate of 10 to 30° C./s, followed by coiling.   
     
     
         12 . The method of  claim 5 , further comprising, between steps (a) and (b), a step of subjecting the hot-rolled steel sheet to softening heat treatment at a temperature of 550° C. to 650° C. 
     
     
         13 . The method of  claim 5 , wherein the cold-rolled steel sheet after step (d) has a yield strength (YS) of 800 MPa or greater, a tensile strength (TS) of 980 MPa or greater, an elongation (EL) of 25% or greater, and a hole expansion ratio (HER) of 20% or greater. 
     
     
         14 . The method of  claim 5 , wherein the cold-rolled steel sheet after step (d) has a grain size of 3 μm or less.