| 摘要 |
<p>Disclosed are an ultra-high strength cold-rolled steel sheet having a tensile strength of 1470 MPa or more and an elongation of 5% and a method for manufacturing the same. According to the present invention, the method for manufacturing the ultra-high strength cold-rolled steel sheet includes: a step of cold-rolling a hot-rolled steel sheet consisting of 0.15 to 0.25 wt% of carbon (C), 0.2 wt% or less of silicon (Si), 2.0 to 3.0 wt% of manganese (Mn), 0.03 wt% or less of phosphorous (P), 0.003 wt% or less of sulfur (S), 0.05 wt% or less of soluble aluminum (S_Al), 0.4 wt% or less of chrome (Cr), 0.1 wt% or less of molybdenum (Mo), 0.02 wt% or less of titanium (Ti), 0.06 wt% or less of niobium (Nb), 0.004 wt% or less of boron (B), 0.006 wt% or less of nitrogen (N), remainder iron (Fe) and inevitable impurities; a step of annealing the cold-rolled steel sheet in a single phase range of austenite by heating the cold rolled steel sheet; a step of cooling the annealed steel sheet at an average cooling speed of 20 to 50°C/sec up to a temperature range of martensite; and a step of over-aging the primarily cooled steel sheet in the temperature range of martensite for 30 seconds or more.</p> |
