SUPER-HIGH STRENGTH COLD-ROLLED STEEL SHEET HAVING EXCELLENT BENDING PROPERTIES

摘要

The invention provides an ultra high strength cold rolled steel sheet with a small thickness which exhibits excellent bendability and delayed fracture resistance. The ultra high strength cold rolled steel sheet with excellent bendability contains C at 0.15 to 0.30%, Si at 0.01 to 1.8%, Mn at 1.5 to 3.0%, P at not more than 0.05%, S at not more than 0.005%, Al at 0.005 to 0.05% and N at not more than 0.005%, with the balance being represented by Fe and inevitable impurities, and has a steel sheet superficial soft portion satisfying the following equations: $\[ \mathrm{Hv}\left(\mathrm{S}\right)\mathrm{/}\mathrm{Hv}\left(\mathrm{C}\right)\mathrm{\le }\mathrm{0.8} \]$wherein Hv(S) is the hardness of the steel sheet superficial soft portion, and Hv(C) is the hardness of a steel sheet core portion, $\[ \mathrm{0.10}\mathrm{\le }\mathrm{t}\left(\mathrm{S}\right)\mathrm{/}\mathrm{t}\mathrm{\le }\mathrm{0.30} \]$wherein t(S) is the thickness of the steel sheet superficial soft portion, and t is the sheet thickness,the steel sheet superficial soft portion containing tempered-martensite at a volume fraction of not less than 90%, the microstructure of the steel sheet core portion including tempered-martensite, the ultra high strength cold rolled steel sheet having a tensile strength of not less than 1270 MPa.