| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength hot dip galvanized steel sheet having excellent stretch flange formability, and whose tensile strength is ≥590 MPa, and to provide a method for producing the same. SOLUTION: The steel sheet has a componential composition including, by mass, 0.06 to 0.09% C, and contains ≥0.010% Ti and ≥0.030% Nb by 0.040 to 0.100% in total. In the structure, the average crystal grain size of a ferritic phase is 2 to 10 μm; the average crystal grain size of a martensite phase is 1 to 5 μm; the volume fraction of the ferrite phase is 80 to 95%; the volume fraction of the martensite phase is 5 to 20%; also, the ratio of the martensite phase present in the ferrite grain boundaries occupying in the circumferential length of the ferrite grains is 1 to 40%; and further, the hardness of the martensite is ≥3.0 times of the hardness of the ferrite phase. For forming the above structure, the production method is prescribed primarily with a heat pattern after cold rolling. COPYRIGHT: (C)2010,JPO&INPIT
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