| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength steel sheet having the maximum tensile strength of≥900 MPa and having satisfactory ductility and delayed fracture toughness, and to provide a method for producing it. SOLUTION: The high strength steel sheet having the maximum tensile strength of≥900 MPa and having satisfactory ductility and delayed fracture toughness has a composition comprising, by mass, 0.07 to 0.25% C, 0.3 to 2.50% Si, 1.5 to 3.0% Mn, 0.005 to 0.09% Ti, 0.0001 to 0.01% B, 0.001 to 0.03% P, 0.0001 to 0.01% S,≤2.5% Al, 0.0005 to 0.0100% N and 0.0005 to 0.007% O, and the balance iron with inevitable impurities, wherein a steel sheet structure consists essentially of ferrite, and includes martensite composed of block sizes of≤1μm, and the volume ratio of the ferrite is≥50%, the C concentration in the martensite is 0.3 to 0.9%, and a Yield ratio (YR) is≤0.75. COPYRIGHT: (C)2011,JPO&INPIT |
