| 摘要 |
PROBLEM TO BE SOLVED: To provide a high ductility hot rolled steel sheet which has high strength satisfying TS≥780 MPa, and in which the balance of strength-elongation and the balance of fatigue properties (endurance ratio) and stretch flange properties are excellent. SOLUTION: In the steel comprising 0.05 to 0.3% Ti as an essential component, and having a structure composed of the main phase of ferrite and a second phase consisting mainly of bainite and retained austenite, the volume occupancy ratio of the ferrite as the main phase to the whole of the steel structure is controlled to≥60%, similarly, the volume occupancy ratio of the second phase is controlled to≥10%, also, the volume occupancy ratio of the retained austenite in the second phase is controlled to≥8%, further, the piece ratio of the ones with a crystal grain size of≤4μm in the ferrite as the main phase is controlled to≥80%, the average crystal grain size in the whole of the ferrite is controlled to≤3μm, the piece number of the ones with a crystal grain size of≤3μm in the retained austenite in the second phase is controlled to≥60%, also, the average crystal grain size in the whole of the retained austenite is controlled to≤2.5μm, and further, the difference in hardness between the ferrite as the main phase and the second phase is controlled to≤1.5 on the average. COPYRIGHT: (C)2006,JPO&NCIPI
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