Patent ID: 10287661

Abstract:
A hot-rolled steel sheet is provided having high strength and excellent toughness and ductility includes a composition that contains, on a mass percent basis, 0.04% or more and 0.15% or less of C, 0.01% or more and 0.55% or less of Si, 1.0% or more and 3.0% or less of Mn, 0.03% or less P, 0.01% or less S, 0.003% or more and 0.1% or less of Al, 0.006% or less N, 0.035% or more and 0.1% or less Nb, 0.001% or more and 0.1% or less of V, 0.001% or more and 0.1% or less Ti, and the balance being Fe and incidental impurities, in which the hot-rolled steel sheet includes a microstructure in which the proportion of precipitated Nb to the total amount of Nb is 35% or more and 80% or less, the volume fraction of tempered martensite and/or tempered bainite having a lath interval of 0.2 μm or more and 1.6 μm or less is 95% or more at a position 1.0 mm from a surface of the sheet in the thickness direction, and the volume fraction of ferrite having a lath interval of 0.2 μm or more and 1.6 μm or less at the center position of the sheet in the thickness direction is 95% or more.