Ultra high strength cold rolled steel sheet having excellent ductility and delayed fracture resistance and method for manufacturing the same

Abstract

An ultra.-high-strength cold-rolled steel sheet with excellent ductility and delayed fracture resistance includes 0.15% to 0.75 C. 1.0% to 3.0% Si, 1.5% to 2.5% Mn, 0.05% or less P, 0.02% or less 5, 0.01% to 0.05% Al, and less than 0.005% N on a mass ratio, the remainder being Fe and =avoidable impurities, the ultra-high-strength cold-rolled steel sheet having a metal microstructure including 40% to 85% of a tempered martensite phase and 15% to 60% of a ferrite phase on a volume fraction basis and a tensile strength of 1320 Mtn or more.

1 . An ultra-high-strength cold-rolled steel sheet with excellent ductility and delayed fracture resistance, comprising 0.15% to 0.25% C, 1.0% to 3.0% Si, 1.5% to 2.5% Mn, 0.05% or less P, 0.02% or less S, 0.01% to 0.05% Al, and less than 0.005% N on a mass ratio, the remainder being Fe and unavoidable impurities, the ultra-high-strength cold-rolled steel sheet having a microstructure comprising 40% to 85% of a tempered martensite phase and 15% to 60% of a ferrite phase on a volume fraction basis and a tensile strength of 1320 MPa or more. 
     
     
         2 . The cold-rolled steel sheet according to  claim 1 , further comprising one or more of 0.1% or less Nb, 0.1% or less Ti, and 5 ppm to 30 ppm B on a mass ratio. 
     
     
         3 . The cold-rolled steel sheet according to  claim 1 , having a total elongation of 12% or more. 
     
     
         4 . A method for manufacturing an ulta-high-strengh cold-rolled steel sheet having excellent ductility and delayed fracture resistance, comprising;
 heating a steel slab having the chemical composition specified in  claim 1  to 1200° C. or higher;   hot-rolling the steel slab at a finish rolling end temperature of 800° C. or higher;   pickling the steel;   cold-rolling the steel;   continuously annealing the steel such that the steel is held at a temperature ranging from the Ac 1  transformation temperature to Ac 3  transformation temperature thereof for 30 a to 1200 s, cooled to a temperature of 600° C. to 800° C. at an average cooling rate of 100° C./s or less, and then cooled to 100° C. or lower at an average cooling rate of 100° C./s to 1000° C./s; and   tempering the steel such that the steel is reheated and held at a temperature of 100° C. to 300° C. for 120 s to 1800 s.   
     
     
         5 . The cold-rolled steel sheet according to  claim 2 , having a total elongation of 12% or more. 
     
     
         6 . A method for manufacturing an ultra-high-strength cold-rolled steel sheet having excellent ductility and delayed fracture resistance, comprising;
 heating a steel slab having the chemical composition specified in  claims 2  to  1200 ° C. or higher;   hot-rolling the steel slab at a finish roiling end temperature of800° C. or higher;   pickling the steel;   cold-rolling the steel;   continuously annealing the steel such that the steel is held at a temperature ranging from the Ac1 transformation temperature to Ac3 transformation temperature thereof for 30 s to 1200 s, cooled to a temperature of 600° C. to 800′C at an average cooling rate of 100° Cis or less, and then cooled to 100° C. or lower at an average cooling rate of 100° C/.s to 1000° C./s; and   tempering the steel such that the steel is reheated and held at a temperature of 100° C. to 300° C. for 120 s to 1800 a.