HIGH-STRENGTH GALVANNEALED STEEL SHEET OF HIGH BAKE HARDENABILITY, HIGH-STRENGTH ALLOYED GALVANNEALED STEEL SHEET, AND METHOD FOR MANUFACTURING SAME

摘要

A steel contains each of C, Si, Mn, P, S, Al, N, O, with the balance made up of Fe and inevitable impurities, at a range from 1/8 thickness centered around a 1/4 sheet thickness from a surface to 3/8 thickness centered around the 1/4 sheet thickness from the surface at a base steel sheet, a structure of the base steel sheet contains, in volume fraction, 3% or more of a retained austenite phase, 50% or less of a ferrite phase, and 40% or more of a hard phase, an average dislocation density is 5 × 10 13 /m 2 or more, a solid-solution C amount contained in the retained austenite phase is in mass% 0.70 to 1.00%, an X-ray random intensity ratio of FCC iron in an texture of the retained austenite phase is 3.0 or less, a ratio between a grain diameter relative to a rolling direction and a grain diameter relative to a sheet width direction of the retained austenite phase is 0.75 to 1.33, further, a hot-dip galvanized layer is formed at the surface of the base steel sheet and the sheet thickness becomes 0.6 to 5.0 mm.