HOT-ROLLED, COLD ROLLED, AND PLATED STEEL SHEET HAVING IMPROVED UNIFORM AND LOCAL DUCTILITY AT A HIGH STRAIN RATE

摘要

A multi-phase hot-rolled steel sheet has a metallurgical structure having a main phase of ferrite with an average grain diameter of at most 3.0 mum and a second phase including at least one of martensite, bainite, and austenite. In the surface layer, the average grain diameter of the second phase is at most 2.0 mum, the difference (DeltanHav) between the average nanohardness of the main phase (nHalphaav) and the average nanohardness of the second phase (nH2nd av) is 6.0-10.0 GPa, the difference (DeltasigmanH) of the standard deviation of the nanohardness of the second phase from the standard deviation of the nanohardness of the main phase is at most 1.5 GPa, and in the central portion, the difference (DeltanHav) between the average nanohardnesses is at least 3.5 GPa to at most 6.0 GPa and the difference (DeltasigmanH) between the standard deviations of the nanohardnesses is at least 1.5 GPa.