Ultra high-strength steel sheet having excellent hole expandability and manufacturing method therefor

Abstract

Provided is an ultra high-strength steel sheet having excellent hole expandability. The ultra high-strength steel sheet includes: 0.15 to 0.30% of C, 1.0 to 3.0% of Si, 3.0 to 5.0% of Mn, 0.020% or less of P, 0.010% or less of S, 0.01 to 3.0% of Al, and 0.020% or less of N (excluding 0%) by weight, with the remainder consisting of Fe and other inevitable impurities, wherein a microstructure contains 5% to 20% by area fraction of retained austenite with the remainder including ferrite, bainite and fresh martensite.

1 . An ultra high-strength steel sheet having excellent hole expandability, the ultra high-strength steel sheet comprising: 0.15 to 0.30% of C, 1.0 to 3.0% of Si, 3.0 to 5.0% of Mn, 0.020% or less of P, 0.010% or less of S, 0.01 to 3.0% of Al, and 0.020% or less of N (excluding 0%) by weight, with a remainder of Fe and other inevitable impurities,
 wherein a microstructure contains 5% to 20% by area fraction of retained austenite with the remainder including ferrite, bainite and fresh martensite.   
     
     
         2 . The ultra high-strength steel sheet of  claim 1 , wherein the steel sheet further comprises at least one of 1.5% or less of Cr (excluding 0%), 0.005 to 0.3% of Nb, 0.005 to 0.3% of V and 0.05 to 0.3% of Mo, by weight. 
     
     
         3 . The ultra high-strength steel sheet of  claim 1 , wherein the sum of the ferrite and bainite is 20% or less by area. 
     
     
         4 . The ultra high-strength steel sheet of  claim 1 , wherein the steel sheet has a yield strength of 850 MPa or more, a tensile strength of 1200 MPa or more, and a hole expandability of 15% or more. 
     
     
         5 . The ultra high-strength steel sheet of  claim 1 , wherein the steel sheet has a yield ratio of 0.7 or more. 
     
     
         6 . The ultra high-strength steel sheet of  claim 1 , wherein the steel sheet has a hot-dip galvanized layer formed on a surface of the steel sheet. 
     
     
         7 . The ultra high-strength steel sheet of  claim 1 , wherein the steel sheet has transformed martensite of 15% or less by area in a tensile test. 
     
     
         8 . A method of manufacturing an ultra high-strength steel sheet having excellent hole expandability, the method comprising:
 heating a steel slab including: 0.15 to 0.30% of C, 1.0 to 3.0% of Si, 3.0 to 5.0% of Mn, 0.020% or less of P, 0.010% or less of S, 0.01 to 3.0% of Al, and 0.020% or less of N (excluding 0%) by weight, with a remainder of Fe and other inevitable impurities, to 1000 to 1250° C.;   hot-rolling the heated steel slab such that a temperature at a finish-rolled outlet side thereof is 500 to 950° C. to obtain a hot-rolled steel sheet;   winding the hot-rolled steel sheet at a temperature of 750° C. or less;   cold-rolling the wound hot-rolled steel sheet at a reduction ratio of 30% to 80% to obtain a cold-rolled steel sheet;   annealing the cold-rolled steel sheet in a temperature range of 750° C. to 950° C.;   cooling the annealed cold-rolled steel sheet to a cooling end temperature of Mf to Ms−90° C.; and   heat-treating the cooled cold-rolled steel sheet at a temperature of Ms+100° C. or more for 350 seconds or more.   
     
     
         9 . The method of  claim 8 , wherein said Ms is obtained by the following relational expression (1):
   M s =547.6−596.9C−28.4Mn−13.1Si−17.7Cr+8.8Al  [Relational Expression 1]
   
       (In the relational expression 1, each element symbol represents the content of each element as weight %, the unit of Ms is ° C., and an element not included is calculated as 0). 
     
     
         10 . The method of  claim 8 , wherein the steel slab further comprises at least one of 1.5% or less of Cr (excluding 0%), 0.005 to 0.3% of Ti, 0.005 to 0.3% of Nb, 0.005 to 0.3% of V and 0.05 to 0.3% of Mo, by weight. 
     
     
         11 . The method of  claim 8 , further comprising: dipping the steel sheet in a galvanizing bath to form a hot-dip galvanized layer, after the heat-treating.