| 摘要 |
<p>Provided are an ultra-high strength cold rolled steel sheet comprising in wt%, 0.12-0.2% of C, 0.5% or less of Si (0% is excluded), 3.0-4.0% of Mn, 0.03% or less of P (0% is excluded), 0.015% or less of S (0% is excluded), 0.1% or less of Al (0% is excluded), 1% or less of Cr (0% is excluded), 48/14*′N′-0.1% of Ti, 0.1% or less of Nb (0% is excluded), 0.005% or less of B (0% is excluded), 0.01% or less of N (0% is excluded), and residues comprising Fe and other unavoidable impurities, and a method for manufacturing the same. A content of C, Mn, Cr, and Ti satisfies a relational expression 1 (3756′C′+ 324′Mn′+ 218′Cr′- 619′Ti′> 1690). A microstructure includes 90% or more of martensite and 10% or less of ferrite and bainite in terms of volume fraction. According to the present invention, ultra-high strength steel having tensile strength of 1400 MPa or more can be manufactured by using a general continuous annealing furnace where a slow cooling section exists. Moreover, a cold rolled steel sheet having the excellent form quality compared to the martensite steel manufactured by using water cooling can be provided.</p> |
