HIGH-STRENGTH HOT-ROLLED STEEL SHEET AND PROCESS FOR PRODUCTION THEREOF

摘要

A high-strength hot-rolled steel sheet which has a tensile strength (TS) of 540 to 780MPa, a small scatter of strength, and excellent uniformity of strength is provided by using an inexpensive general-purpose Ti steel sheet. The composition contains by mass C: 0.05 to 0.12%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.030% or less, S: 0.01% or less, Al: 0.005 to 0.1%, N: 0.01% or less, and Ti: 0.030 to 0.080% with the balance consisting of Fe and unavoidable impurities. The structure comprises at least 70% of bainitic ferrite in terms of fraction and the quantity of Ti present in precipitates having sizes of less than 20nm is not less than 50% of the Ti* value calculated by formula (1): Ti* = [Ti] - 48 ÷ 14 × [N] ... (1) wherein [Ti] and [N] are the respective contents (mass%) of Ti and N in the steel sheet.