Ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties and method for manufacturing same

Abstract

Provided is an ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties, which can be used as a structural component for automobiles such as a member, a seat rail, and a pillar, and a method for manufacturing same. The steel sheet includes: by weight: C: 0.03 to 0.12%, Si: 0.03 to 0.3 Mn: 2.1 to 2.9%, Al: 0.005 to 0.07%, Nb: 0.01 to 0.08%, Ti: 0.005 to 0.08%, B: 0.0005 to 0.005%, Cr: 0.7 to 1.4%, Mo: 0.005 to 0.10%, N: 0.008% or less (excluding 0%), with balance of Fe and inevitable impurities. A microstructure thereof includes by area, fresh martensite: 4 to 19%, a sum of tempered martensite and bainite: 78 to 95%, and retained austenite: 0.2 to 2.0%. An average grain size of the microstructure is 0.5 to 6 μm.

1 . An ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties, comprising by weight:
 C: 0.03 to 0.12%, Si: 0.03 to 0.30%, Mn: 2.1 to 2.9%, Al: 0.005 to 0.07%, Nb: 0.01 to 0.08%, Ti: 0.005 to 0.08%, B: 0.0005 to 0.005%, Cr: 0.7 to 1.4%, Mb: 0.005 to 0.10%, N: 0.008% or less (excluding 0%), with a balance of Fe and inevitable impurities, satisfying the following Relational Expressions 1 to 3,   wherein a microstructure thereof includes by area,   fresh martensite: 4 to 19%, a sum of tempered martensite and bainite: 78 to 95%, and retained austenite: 0.2 to 2.0%,   wherein an average grain size of the microstructure is 0.3 to 6 μm,
   [Relational Expression 1] 
   0.18≤C+Si/30+Mn/20+2P+4S≤0.30
 
   [Relational Expression 2] 
   180≤48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb≤270
 
   [Relational Expression 3] 
   700≤48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb/C+Si/30+Mn/20+2P+4S≤1200
 
   where, a content of alloy components described in the Relational Expressions 1 to 3 refers to weight %.   
     
     
         2 . The ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties of  claim 1 , wherein the impurities further comprise: P: 0.04% or less (excluding 0%) and S: 0.005% or less (excluding 0%). 
     
     
         3 . The ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties of  claim 1 , wherein the impurities comprise at least one of Sb, Mg, Sn, Sb, Zn, and Pb, and a total content thereof is 0.1% or less by weight. 
     
     
         4 . The ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties of  claim 1 , wherein the microstructure further comprises 10% or less of ferrite. 
     
     
         5 . The ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties of  claim 1 , wherein the cold-rolled steel sheet has a yield strength (YS) of 800 to 980 MPa, a tensile strength (TS) of 980 to 1180 MPa, an elongation (EL) of 4 to 12%, a yield ratio (YS/TS) of 0.70 to 0.95, a hole expansion ratio (HER) of 35 to 80%, R/t of 0.8% or less, and a 3-point bending maximum angle of 90 to 140°. 
     
     
         6 . The ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties of  claim 1 , wherein the cold-rolled steel sheet has hardness (HvBM) of 300 to 400 Hv, a hardness of a fusion zone (HvFZ) of a weld zone formed after welding of 350 to 450 Hv, wherein HvFZ/HvBM is 1.30 or less. 
     
     
         7 . A method for manufacturing an ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties, comprising operations of:
 heating a slab including, by weight: 0.03 to 0.12%, Si: 0.03 to 0.30%, Mn: 2.1 to 2.9%, Al: 0.005 to 0.07%, Nb: 0.1 to 0.08%, Ti: 0.005 to 0.08%, B. 0.0005 to 0.005%, Cr: 0.7 to 1.4%, Mo: 0.005 to 0.10%, N: 0.008% or less (excluding 0%), with a balance of Fe and inevitable impurities, satisfying the following Relational Expressions 1 to 3;   finish hot rolling the heated slab so that a finish roiling exit temperature is Ar3+50° C. to Ar3+150° C.;   cooling the hot-rolled steel sheet to a temperature within a range of Ms+50° C. to Ms+300° C. and winding the same;   cold rolling the wound hot-rolled steel sheet to obtain a cold-rolled steel sheet;   continuously annealing the cold-rolled steel sheet at a temperature within a range of Ar3+10° C. to Ar3+70° C.;   soaking the continuously annealed cold-roiled steel sheet for 50 to 200 seconds;   primarily cooling the soaked cold-rolled steel sheet to a temperature of 620 to 700° C. at a cooling rate of 1 to 10° C./s;   secondarily cooling the primarily-cooled cold-rolled steel sheet to a temperature of 360 to 420° C. at a cooling rate of 5 to 50° C./sec;   overaging the secondarily-cooled cold-rolled steel sheet for 250 to 650 seconds, and then terminating the same at 320 to 400° C.,   wherein the following Relational Expressions 4 to 6 are satisfied during the secondary cooling and the overaging treatment,
   [Relational. Expression 1] 
   0.18≤C+Si/30+Mn/20+2P+4S≤0.30
 
   [Relational Expression 2] 
   180≤48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb≤270
 
   [Relational Expression 3] 
   700≤48.8+49logC+35.1Mn+25.9Si+76.5Cr+105.9Mo+1325Nb/C+Si/30+Mn/20+2P+4S≤1200
 
   [Relational Expression 4] 
   10≤A≤70
 
   [Relational Expression 5] 
   30≤B≤100
 
   [Relational Expression 6] 
   2.5≤overaging treatment time/B≤14
 
   where, in the above Relational Expressions 1 to 3, a content of alloy components described is weight %, and in the above Relational Expressions 4 to 6, A is a Ms-secondary cooling end temperature (° C.), and B is an overaging end temperature (° C.).   
     
     
         8 . The method for manufacturing an ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties of  claim 7 , wherein the slab is performed at a temperature within a range of 1100 to 1300° C. 
     
     
         9 . The method for manufacturing an ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties of  claim 7 , wherein the slab has a thickness of 230 to 270 mm. 
     
     
         10 . The method for manufacturing an ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties of  claim 7 , further comprising
 cooling the wound hot-rolled steel sheet to room temperature at a cooling rate of 0.1° C./s or less.   
     
     
         11 . The method for manufacturing an ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties of  claim 7 , wherein the cold rolling is performed at a reduction ratio of 40 to 70%. 
     
     
         12 . The method for manufacturing an ultra-high-strength cold-rolled steel sheet having excellent yield strength and bending properties of  claim 7 , further comprising
 temper rolling the overaged cold-rolled steel sheet at an elongation of 0.1 to 2.0%, after the averaging treatment.