High-tensile strength hot-rolled steel sheet excellent in elongation properties and stretch flangeability, and producing method thereof

摘要

A high-tensile strength hot-rolled steel sheet that satisfies, in addition to TS of 780 MPa or more or furthermore 980 MPa or more, TS X EL ≥ 20000 MPa% and TS x lambda ≥ 82000 MPa%, and a method of producing the same are provided. Specific resolution means are as follows. That is, a high-tensile strength hot-rolled steel sheet including a composition that includes C of 0.04% by mass or more and 0.25% by mass or less; Si of 0.4% by mass or more and 2.0% by mass or less; Mn of 3.0% by mass or less; Al of 0.2% by mass or less; S of 0.007% by mass or less; Ti of 0.08% by mass or more and 0.3% by mass or less; and the balance of Fe and inevitable impurities, in the above, the contents of the C, the Si and the Ti satisfing (Ä%CÜ/12 - Ä%TiÜ/48)/(Ä%SiÜ/28) ≤ 0.4; and a microstructure that includes ferrite; bainite; and retained austenite; in the above a fraction of the ferrite in an entire microstructure is 40% or more and an average grain size of the ferrite is 5 mu m or less; a fraction of the bainite is in the range of 20% to 48% with respect to an entire microstructure; and a fraction of the retained austenite is in the range of 2% to 7% with respect to an entire microstructure is produced.