HIGH-STRENGTH HOT-ROLLED STEEL SHEET AND METHOD FOR PRODUCING SAME

After low-temperature finish rolling has been performed as hot rolling on a steel material having a chemical composition containing, by mass%, C: 0.07% to 0.20%, Si: 1.50% or less, Mn: 1.0% to 4.0%, P: 0.030% or less, S: 0.0030% or less, Al: 0.010% to 1.000%, cooling is performed at an average cooling rate of 10°C/s or higher to a temperature of 500°C, rapid cooling is further performed in a temperature range from a Ms temperature to a temperature of (Ms temperature - 200°C), coiling is thereafter performed in a low temperature range of 250°C or lower, and the coiled steel sheet is uncoiled and further subjected to rolling with a certain amount or more of rolling load per unit width and the like. Consequently, it is possible to obtain a high-strength hot-rolled steel sheet excellent in terms of delayed fracture resistance having a microstructure including, in terms of area fraction, 95% or more of a martensite phase at a position located at 1/4 of the thickness of the steel sheet, in which an average aspect ratio of prior austenite grains is 3.0 or more, an amount of stress relaxation after a lapse of 5 min of 20 MPa or lower in a stress relaxation test with an applied stress of 400 MPa, and a tensile strength of 1180 MPa or higher.