| 摘要 |
The present invention provides an ultra-high strength cold rolled steel sheet having a high yield ratio, and a method to manufacture the same. According to an embodiment of the present invention, the ultra-high strength cold rolled steel sheet having a high yield ratio comprises: 0.12-0.17 wt% of carbon (C); 0.3 wt% or less (excluding 0 wt%) of silicon (Si); 2.5-3.2 wt% of manganese (Mn); 0.5-1.2 wt% of chromium (Cr); 0.0010-0.0050 wt% of boron (B); 0.01-0.05 wt% of titanium (Ti); 0.01-0.05 wt% of niobium (Nb); 0.01-0.10 wt% of soluble aluminum (Sol.Al); and the remaining consisting of iron (Fe) and inevitable foreign substances including tempered martensite with a ratio of fraction of area, which is 90% or more (including 100%), wherein tempered martensite has an average grain diameter of 3μm or less (excluding 0μm). The ultra-high strength cold rolled steel sheet has less variation of material, a high yield ratio, and a superior expandability. |
