Ultra high strength cold rolled steel sheet having excellent bendability

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):

Hv ( S )/ Hv ( C )≦0.8  (1)

wherein Hv(S) is hardness of the steel sheet superficial soft portion, and Hv(C) is hardness of a steel sheet core portion,

0.10≦ t ( S )/ t≦ 0.30  (2)

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.

2. The steel sheet according to claim 1 , further comprising, in terms of mass %, one or more selected from Ti: 0.001 to 0.10%, Nb: 0.001 to 0.10% and V: 0.01 to 0.50%.

3. The steel sheet according to claim 1 , further comprising, in terms of mass %, B at 0.0001 to 0.005%.

4. The steel sheet according to claim 1 , further comprising, in terms of mass %, one or more selected from Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.50% and Cr: 0.01 to 0.50%.

5. The steel sheet according to claim 2 , further comprising, in terms of mass %, B at 0.0001 to 0.005%.

6. The steel sheet according to claim 2 , further comprising, in terms of mass %, one or more selected from Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.50% and Cr: 0.01 to 0.50%.

7. The steel sheet according to claim 3 , further comprising, in terms of mass %, one or more selected from Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.50% and Cr: 0.01 to 0.50%.