HOT-ROLLED STEEL SHEET, COLD-ROLLED STEEL SHEET, AND PLATED STEEL SHEET EACH HAVING EXELLENT UNIFORM DUCTILITY AND LOCAL DUCTILITY IN HIGH-SPEED DEFORMATION

摘要

The present invention relates to a hot-rolled steel sheet, a cold-rolled steel sheet, and a plated steel sheet having improved uniform ductility and local ductility at a high strain rate. A multi-phase hot-rolled steel sheet according to one mode of the present invention has a metallurgical structure having a main phase of ferrite with an average grain diameter of at most 3.0 µm 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 µm, the difference (”nH av ) between the average nanohardness of the main phase (nH ±av ) and the average nanohardness of the second phase (nH 2nd av ) is 6.0 - 10.0 GPa, the difference (”ÃnH) 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 (”nH av ) between the average nanohardnesses is at least 3.5 GPa to at most 6.0 GPa and the difference (”ÃnH) between the standard deviations of the nanohardnesses is at least 1.5 GPa.