| 摘要 |
The present invention provides a hot-rolled high strength steel sheet in which, without using expensive Mo, by effectively using Ti which is an inexpensive element and 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 excellent tensile fatigue properties are exhibited; and a method for producing the hot-rolled high strength steel sheet. A hot-rolled high strength steel sheet having 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. |
