High strength steel sheet having superior workability and method for manufacturing same

Abstract

Provided is a method for manufacturing a steel steel sheet having superior workability. The method includes: providing a cold-rolled steel sheet; primarily heating the cold-rolled steel sheet to Ac1-Ac3, and primarily maintaining the primarily heated steel sheet for 50 seconds or more; primarily cooling the primarily heated steel sheet to 600-850° C. at an average cooling rate of 1° C./s or more; secondarily cooling the primarily cooled steel sheet to 300-500° C. at an average cooling rate of 2° C./s or more, and secondarily maintaining the secondarily cooled steel sheet for 5 seconds or more; tertiarily cooling the secondarily cooled steel sheet to 100-300° C. at an average cooling rate of 2° C./s or more; secondarily heating the tertiarily cooled steel sheet to 300-500° C. at an average temperature increase rate of 5° C./s or more, and tertiarily maintaining the secondarily heated steel sheet for 50 seconds or more; and quaternarily cooling to room temperature.

1 . A method for manufacturing a steel strength steel sheet having superior workability, the method comprising:
 providing a cold-rolled steel sheet including, by wt %, C: 0.25 to 0.75%, Si: 4.0% or less, Mn: 0.9 to 5.0%, Al: 5.0% or less, P: 0.15% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and unavoidable impurities;   heating (primarily heating) the cold-rolled steel sheet to a temperature within a range of Ac1 or higher and less than Ac3, and maintaining (primarily maintaining) the primarily heated steel sheet for 50 seconds or more;   cooling (primarily cooling) the primarily heated steel sheet to a temperature within a range (primarily cooling stop temperature) of 600 to 850° C. at an average cooling rate of 1° C./s or more;   cooling (secondarily cooling) the primarily cooled steel sheet to a temperature within a range of 300 to 500° C. at an average cooling rate of 2° C./s or more, and maintaining (secondarily maintaining) the secondarily cooled steel sheet in the temperature within a range for 5 seconds or more;   cooling (tertiarily cooling) the secondarily cooled steel sheet to a temperature within a range (secondarily cooling stop temperature) of 100 to 300° C. at an average cooling rate of 2° C./s or more;   heating (secondarily heating) the tertiarily cooled steel sheet to a temperature within a range of 300 to 500° C. at an average temperature increase rate of 5° C./s or more, and maintaining (tertiarily maintaining) the secondarily heated steel sheet in the temperature within a range for 50 seconds or more; and   cooling (quaternarily cooling) the secondarily heated steel sheet to room temperature.   
     
     
         2 . The high strength steel sheet of  claim 1 , wherein the cold-rolled steel sheet further includes any one or more of the following (1) to (9):
 (1) one or more of Ti: 0 to 0.5%, Nb: 0 to 0.5%, and V: 0 to 0.5%,   (2) one or more of Cr: 0 to 3.0% and Mo: 0 to 3.0%,   (3) one or more of Cu: 0 to 4.5% and Ni: 0 to 4.5%,   (4) B: 0 to 0.005%,   (5) one or more of Ca: 0 to 0.05%, REM: 0 to 0.05% excluding Y, and Mg: 0 to 0.05%,   (6) one or more of W: 0 to 0.5% and Zr: 0 to 0.5%,   (7) one or more of Sb: 0 to 0.5% and Sn: 0 to 0.5%,   (8) one or more of Y: 0 to 0.2% and Hf: 0 to 0.2%, and   (9) Co: 0 to 1.5%.   
     
     
         3 . The high strength steel sheet of  claim 1 , wherein a total content (Si+Al) of Si and Al included in the cold-rolled steel sheet is 1.0 to 6.0 wt %. 
     
     
         4 . The method of  claim 1 , wherein the cold-rolled steel sheet is provided by:
 heating a steel slab to 1000 to 1350° C.;   performing finishing hot rolling in a temperature within a range of 800 to 1000° C.;   coiling the hot-rolled steel sheet in a temperature within a range of 300 to 600° C.;   performing hot-rolled annealing heat treatment on the coiled steel sheet in a temperature within a range of 650 to 850° C. for 600 to 1700 seconds; and   cold rolling the hot-rolled annealing heat-treated steel sheet at a reduction ratio of 30 to 90%.   
     
     
         5 . The high strength steel sheet of  claim 1 , wherein a cooling rate Vc1 of the primary cooling and a cooling rate Vc2 of the secondary cooling satisfy a relationship of Vc1<Vc2.