Ultra-high strength cold-rolled steel sheet having excellent hole-expandability and method for manufacturing same

Abstract

Provided is an ultra-high strength cold-rolled steel sheet having excellent hole expandability and a method for manufacturing same and, more specifically, to an ultra-high strength cold-rolled steel sheet having excellent hole expandability which can be mainly used for structural members and to ensure safety in the event of an automobile collision, and to a method for manufacturing same.

1 . An ultra-high strength cold-rolled steel sheet having excellent hole expandability, comprising by weight:
 C: 0.2-0.4%, Si: 0.5% or less (excluding 0%), Mn: 1.0-2.0%, P: 0.03% or less (excluding 0%), S: 0.015% or less (excluding 0%), Al: 0.1% or less (excluding 0%), Cr: 0.5% or less (excluding 0%), Mo: less than 0.2% (excluding 0%), Ti: 0.1% or less (excluding 0%), Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), with a remainder of Fe and other unavoidable impurities,   wherein a microstructure comprises a single-phase structure of tempered martensite or a mixed structure of martensite and tempered martensite,   wherein the microstructure has an F HAGB  of 60% or more by area and L HAGB  of 8 mm or more, per unit area of 45 μm×45 μm,   where F HAGB  is a fraction of grains with high-hardness angle grain boundaries, and L HAGB  is a total length of grain boundaries with high-hardness angle grain boundaries, wherein the high-hardness grain boundaries refer to grain boundaries with a mismatch angle between adjacent grains of 15° or more.   
     
     
         2 . The ultra-high strength cold-rolled steel sheet having excellent hole expandability of  claim 1 , wherein the cold-rolled steel sheet further comprises one or more of Cu: 0.5% or less and Ni: 0.5% or less. 
     
     
         3 . The ultra-high strength cold-rolled steel sheet having excellent hole expandability of  claim 1 , wherein the cold-rolled steel sheet further comprises Sb: 0.05% or less. 
     
     
         4 . The ultra-high strength cold-rolled steel sheet having excellent hole expandability of  claim 1 , wherein the cold-rolled steel sheet has an average particle size of prior austenite of 6 μm or less. 
     
     
         5 . The ultra-high strength cold-rolled steel sheet having excellent hole expandability of  claim 1 , wherein the cold-rolled steel sheet has a tensile strength (TS) of 1470 MPa or more and a value of tensile strength (TS) (MPa) and hole expansion rate (HER) (%) of 73500 MPa % or more. 
     
     
         6 . A method for manufacturing an ultra-high cold-rolled steel sheet having excellent hole expandability, comprising operations of:
 heating a steel slab including by weight: C: 0.2-0.4%, Si: 0.5% or less (excluding 0%), Mn: 1.0-2.0%, P: 0.03% or less (excluding 0%), S: 0.015% or less (0%) (excluding 0%), Al: 0.1% or less (excluding 0%), Cr: 0.5% or less (excluding 0%), Mo: less than 0.2% (excluding 0%), Ti: 0.1% or less (excluding 0%), Nb: 0.1% or less (excluding 0%), B: 0.005% or less (excluding 0%), N: 0.01% or less (excluding 0%), with a remainder of Fe and other unavoidable impurities, to a temperature in a range of 1100 to 1300° C.;   finish hot rolling the heated steel slab at a temperature of Ar3 or higher;   coiling the hot-rolled steel sheet at a temperature of 720° C. or lower;   cold rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet;   performing an annealing heat treatment on the cold-rolled steel sheet at a temperature in a range of 780 to 900° C.;   slowly cooling the annealed heat-treated cold-rolled steel sheet to a temperature in a range of 650 to 750° C. at a cooling rate of 5° C./s or less;   rapidly cooling the slowly-cooled cold-rolled steel sheet to a temperature of 150° C. or lower at a cooling rate of 40° C./s or more; and   reheating and performing an overaging heat treatment on the rapidly-cooled cold-rolled steel sheet at a temperature in a range of 180 to 240° C.   
     
     
         7 . The method for manufacturing an ultra-high cold-rolled steel sheet having excellent hole expandability of  claim 6 , wherein the steel slab further comprises one or more of Cu: 0.5% or less and Ni: 0.5% or less. 
     
     
         8 . The method for manufacturing an ultra-high cold-rolled steel sheet having excellent hole expandability of  claim 6 , wherein the steel slab further comprises, Sb: 0.05% or less. 
     
     
         9 . The method for manufacturing an ultra-high cold-rolled steel sheet having excellent hole expandability of  claim 6 , wherein the overaging heat treatment is performed for more than 400 seconds.