Steel sheet and manufacturing method therefor

Abstract

The present invention relates to a high-strength steel sheet having excellent formability and a high yield ratio, and a manufacturing method therefor, and more particularly, to a high-strength steel sheet having excellent formability and a high yield ratio, and a manufacturing method therefor, the steel sheet having various uses such as that of vehicle parts.

1 . A steel sheet, comprising by weight:
 0.05 to 0.25% of C, 0.7% or less (excluding 0%) of si, 0.46 to 1.8% of Mn, 0.7% or less (excluding 0%) of Al, 0.05% or less (excluding 0%) of P, 0.03% or less (excluding 0%) of S, 0.03% or less (excluding 0%) of N, 0.22% or less of a total amount of at least one of Ti, Nb, and V, with a remainder of Fe and other unavoidable impurities,   wherein a microstructure includes, by area: 1 to 13% of unrecrystallized ferrite, 67 to 98% of recrystallized ferrite, and 1 to 20% of cementite.   
     
     
         2 . The steel sheet of  claim 1 , wherein the steel sheet further comprises a total amount of at least one of Cr and Mo of 0.8% or less. 
     
     
         3 . The steel sheet of  claim 1 , wherein the steel sheet further comprises a total amount of at least one of Cu and Ni of 0.8% or less. 
     
     
         4 . The steel sheet of  claim 1 , wherein the steel sheet further comprises 0.005% or less of B. 
     
     
         5 . The steel sheet of  claim 1 , wherein the steel sheet further comprises a total amount of at least one of Ca, REM (excluding Y), and Mg of 0.05% or less. 
     
     
         6 . The steel sheet of  claim 1 , wherein the steel sheet further comprises a total amount of at least one of W and Zr of 0.5% or less. 
     
     
         7 . The steel sheet of  claim 1 , wherein the steel sheet further comprises a total amount of at least one of Sb and Sn of 0.5% or less. 
     
     
         8 . The steel sheet of  claim 1 , wherein the steel sheet further comprises a total amount of at least one of Y and Hf of 0.2% or less. 
     
     
         9 . The steel sheet of  claim 1 , wherein the steel sheet has a tensile strength (TS) of 610 MPa or more, a yield ratio (YR) of 0.8 to 0.95, a tensile strength (TS) 2 ×√ elongation (EL) of 1.8×10 6  to 2.3×10 6  MPa 2 % 0.5 , and a tensile strength (TS) 2 ×√ Hole expandability (HER) of 2.5×10 6  to 3.8×10 6  MPa 2 % 0.5 . 
     
     
         10 . A manufacturing method for a steel sheet, comprising:
 heating a steel ingot or slab, including by weight: 0.05 to 0.25% of C, 0.7% or less (excluding 0%) of Si, 0.46 to 1.8% of Mn, 0.7% or less (excluding 0%) of Al, 0.05% or less (excluding 0%) of P, 0.03% or less (excluding 0%) of S, 0.03% or less (excluding 0%) of N, 0.22% or less of a total amount of at least one of Ti, Nb, and V, with a remainder of Fe and other unavoidable impurities, at a temperature within a range of 1000 to 1350° C.;   hot rolling the heated steel ingot or slab at a finish rolling temperature of 800 to 1000° C. to obtain a hot-rolled steel sheet;   coiling the hot-rolled steel sheet at a temperature within a range of 300 to 600° C.;   heat treating the coiled hot-rolled steel sheet at a temperature within a range of 650 to 800° C. for 600 to 1700 seconds;   cold rolling the heat treated hot-rolled steel sheet at a cold rolling reduction rate of 30 to 90% to obtain a cold-rolled steel sheet;   reheating the cold-rolled steel sheet at a temperature within a range of 720 to 860° C. and maintained for 50 seconds or more;   primarily cooling the reheated and maintained cold-rolled steel sheet to a temperature within a range of 600 to 760° C. at an average cooling rate of 1° C./s or more;   secondarily cooling the primarily cooled cold-rolled steel sheet to a temperature within a range of 450 to 550° C. at an average cooling rate of 2° C./s or more and maintained for 50 seconds or more; and   tertiarily cooling the secondarily cooled and maintained cold-rolled steel sheet to room temperature.   
     
     
         11 . The manufacturing method for a steel sheet of  claim 10 , wherein the steel ingot or slab further comprises a total amount of at least one of Cr and Mo of 0.8% or less. 
     
     
         12 . The manufacturing method for a steel sheet of  claim 10 , wherein the steel ingot or slab further comprises a total amount of at least one of Cu and Ni of 0.8% or less. 
     
     
         13 . The manufacturing method for a steel sheet of  claim 10 , wherein the steel ingot or slab further comprises 0.005% or less of B. 
     
     
         14 . The manufacturing method for a steel sheet of  claim 10 , wherein the steel ingot or slab further comprises a total amount of at least one of Ca, REM (excluding Y), and Mg of 0.05% or less. 
     
     
         15 . The manufacturing method for a steel sheet of  claim 10 , wherein the steel ingot or slab further comprises a total amount of at least one of W and Zr of 0.5% or less. 
     
     
         16 . The manufacturing method for a steel sheet of  claim 10 , wherein the steel ingot or slab further comprises a total amount of at least one of Sb and Sn of 0.5% or less. 
     
     
         17 . The manufacturing method for a steel sheet of  claim 10 , wherein the steel ingot or slab further comprises a total amount of at least one of Y and Hf of 0.2% or less. 
     
     
         18 . The manufacturing method for a steel sheet of  claim 10 , further comprising, after the finish rolling:
 cooling the hot-rolled steel sheet to a coiling temperature at an average cooling rate of 10° C./s or more.