Hot-rolled high strength steel sheet having excellent ductility, and tensile fatigue properties and method for producing the same

Abstract

A hot-rolled high strength steel sheet in which, without using expensive Mo, and by effectively using Ti, which is an expensive element, the amount of precipitation hardening of which is large, both ductility and stretch-flangeability are improved at a tensile strength of 780 MPa or higher, and desirable tensile fatigue properties are exhibited. The hot-rolled high strength steel sheet has a composition including, in percent by mass, C: 0.06% to 0.15%, Si: 1.2% or less, Mn: 0.5% to 1.6%, P: 0.04% or less, S: 0.005% or less, Al: 0.05% or less, and Ti: 0.03% to 0.20%, the balance being Fe and incidental impurities, wherein the steel sheet has a structure in which the volume fraction of ferrite is 50% to 90%, the balance is substantially bainite, the total volume fraction of ferrite and bainite is 95% or more, precipitates containing Ti are precipitated in the ferrite, and the precipitates have an average diameter of 20 nm or less; and 80% or more of the Ti content in the steel is precipitated.

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