| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength dual phase steel plate improved in forming limit and excellent in press workability by improving work hardenability in a high strain region. SOLUTION: This steel plate is a dual-phase steel plate containing ferrite as main phase and also containing martensite. In this steel plate, the mass percentage of Mn in the steel, represented by Mn}, is regulated to 1.0 to 2.8% and plural layers dissimilar in Mn content are laminated in a plate thickness direction. Further, when the local Mn content by mass percentage in each layer is represented by [Mn], the sum A of respective thicknesses of the layers satisfying [Mn]>=1.1×Mn} and the sum B of respective thicknesses of the layers satisfying [Mn]<= Mn} satisfy the relation of 4<B/A<=90. By this method, the nonuniformity of strength on a larger scale than the scale in the crystal grain boundary can be attained, and as a result, high work hardenability can be maintained as far as the high strain region.
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