Patent ID: 8951367
Filing Date: 2015-02-10
Classification: C21D,C22C

Abstract:
1. An ultra high strength cold rolled steel sheet with excellent bendability which comprises, in terms of mass %, 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 Equations (1) and (2): wherein Hv(S) is hardness of the steel sheet superficial soft portion, and Hv(C) is hardness of a steel sheet core portion, wherein t(S) is a thickness of the steel sheet superficial soft portion, and t is sheet thickness, the steel sheet superficial soft portion containing tempered-martensite at a volume fraction of not less than 90%, a 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.