| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength steel sheet excellent in deep drawability and secondary working embrittlement resistance, which is useful for an automobile steel sheet or the like and has a high strength satisfying the tensile strength (TS) of≥440 MPa and a high r value (the average r value is≥1.3). SOLUTION: In the steel components, particularly, regarding C and Nb, a composition satisfying 0.0005 to 0.04% C and 0.04 to 0.1% Nb is prepared. Further, the volume ratio of recrystallized ferrite and worked ferrite occupied in the steel structure is controlled to≥8% and≥5%, respectively, and, a Vickers hardness ratio of the worked ferrite to the recrystallized ferrite when being measured under the condition where a test load in a Vickers hardness test is 0.2942N is controlled to≥1.2. Further, the average colony size of the worked ferrite is controlled to≤15μm. COPYRIGHT: (C)2008,JPO&INPIT
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