Patent ID: 8828154
Filing Date: 2014-09-09
Classification: C21D,C22C

Abstract:
1. A hot-rolled steel sheet comprising, in terms of percent by mass, C: 0.01 to 0.2%, Si: 2.0% or less Mn: 1.5 to 3.0%, P: 0.1% or less S: 0.02% or less, Al: 0.1% or less N: 0.02% or less, the balance being Fe and inevitable impurities, wherein a non-tempered martensite phase is dominant phase, a ferrite phase as a second phase is contained in the range of 3% or more and 20% or less in terms of area ratio, the average grain size of the ferrite phase being 2.8 μm to 20 μm, and an amount of solute carbon is 0.01 percent by mass or more; wherein the hot-rolled steel sheet has a strain aging property ΔTS of 100 MPa or more, ΔTS being an increase in tensile strength by strain aging corresponding to the difference between a tensile strength of the steel sheet after strain aging and a tensile strength of the steel sheet before strain aging; and wherein the hot-rolled steel sheet has a fatigue strength ratio FL′/TS of 0.8 or more, FL′/TS being a ratio of a fatigue limit under pulsating tension FL′ to a tensile strength TS of the steel sheet before strain aging.