Eco-friendly steel sheet having high strength and high formability, and method for manufacturing same

Abstract

The present invention relates to a steel sheet used in automobiles, etc., and to a steel sheet that has high strength and high formability and is eco-friendly manufactured, and a manufacturing method therefor.

1 . An eco-friendly steel sheet having high strength and high formability, comprising: by wt %, C: 0.05 to 0.10%, Si: 0.3% or less (excluding 0%), Mn: 2.0 to 2.5%, Ti: 0.05% or less (excluding 0%), Nb: 0.1% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), P: 0.1% or less, S: 0.01% or less, and a balance of Fe, and inevitable impurity elements,
 wherein a microstructure comprises, in area fraction, 15 to 35% of a hard phase and 65 to 85% of a soft phase.   
     
     
         2 . The eco-friendly steel sheet having high strength and high formability of  claim 1 , wherein the soft phase comprises, in area fraction, 60% or more of recrystallized ferrite and 5% or less of non-recrystallized ferrite. 
     
     
         3 . The eco-friendly steel sheet having high strength and high formability of  claim 1 , wherein the hard phase comprises martensite or a mixed structure of martensite and a trace amount of bainite. 
     
     
         4 . The eco-friendly steel sheet having high strength and high formability of  claim 1 , wherein an aspect ratio of the hard phase is 1.2 or less. 
     
     
         5 . The eco-friendly steel sheet having high strength and high formability of  claim 1 , wherein the steel sheet has tensile strength TS of 780 MPa or more and an elongation El of 18% or more. 
     
     
         6 . A method for manufacturing an eco-friendly steel sheet having high strength and high formability, the method comprising:
 manufacturing a hot-rolled steel sheet using a steel slab comprising, by wt %, C: 0.05 to 0.10%, Si: 0.3% or less (excluding 0%), Mn: 2.0 to 2.5%, Ti: 0.05% or less (excluding 0%), Nb: 0.1% or less (excluding 0%), Cr: 1.5% or less (excluding 0%), P: 0.1% or less, S: 0.01% or less, and a balance of Fe, and inevitable impurity elements;   manufacturing a cold-rolled steel sheet by cold-rolling the hot-rolled steel sheet at a reduction ratio of 70 to 90%;   heating and maintaining the cold-rolled steel sheet to a temperature within a range of Ac1 to Ac1+50° C.; and   cooling the cold-rolled steel sheet slowly at an average cooling rate of 1 to 10° C./s to a temperature within a range of 650 to 700° C., and then rapidly cooling the cold-rolled steel sheet to a temperature within a range of 300 to 580° C. at an average cooling rate of 5 to 50° C./s.   
     
     
         7 . The method for manufacturing an eco-friendly steel sheet having high strength and high formability of  claim 6 , wherein the hot-rolled steel sheet comprises:
 heating the steel slab to a temperature within a range of 1100 to 1300° C.;   performing hot rolling the steel slab by performing finish rolling on the heated steel slab at a temperature within a range of Ar3 to 1000° C.; and   after the hot rolling, coiling a hot-rolled steel at a temperature within a range of 400 to 700° C. and cooling a wound hot-rolled steel at a cooling rate of 0.1° C./s or less.   
     
     
         8 . The method for manufacturing an eco-friendly steel sheet having high strength and high formability of  claim 6 , further comprising:
 performing an over aging treatment for 200 to 800 seconds after the rapid cooling.