HIGH-STRENGTH HOT-ROLLED STEEL SHEET HAVING EXCELLENT BENDING CHARACTERISTICS AND LOW-TEMPERATURE TOUGHNESS AND METHOD FOR PRODUCING SAME

The invention provides high-strength hot rolled steel sheets suited for large-sized construction and industrial machinery structural members. A steel with a chemical composition including C: 0.08 to 0.25%, Si: 0.01 to 1.0%, Mn: 0.8 to 2.1% and appropriately controlled amounts of P, S and Al is heated to a temperature of 1100 to 1250°C, rough rolled, and finish rolled in such a manner that the cumulative reduction ratio in the partially recrystallized ³ region and the non-recrystallized ³ region divided by the cumulative reduction ratio in the recrystallized ³ region becomes 0 to 0.2. Immediately after the completion of the finish rolling, cooling is initiated and the steel sheet is cooled to a cooling termination temperature that is not more than Ms transformation temperature + 150°C within 30 seconds from the initiation of the cooling, the average cooling rate in the temperature range of 750°C to 500°C being not less than the critical cooling rate for the occurrence of martensite formation. The steel sheet is then held at a temperature in the range of the cooling termination temperature ± 100°C for 5 to 60 seconds, and is coiled into a coil at a coiling temperature in the range of the cooling termination temperature ± 100°C. In this manner, hot rolled steel sheets may be obtained which have high strength with a yield strength YS of 960 MPa or above and high toughness as well as excellent bendability and which have a microstructure in which the main phase is a tempered martensite phase or a lower temperature-transformed bainite phase, the average grain diameter of prior ³ grains is not more than 20 µm in a cross section parallel to the rolling direction and the average grain diameter of prior ³ grains is not more than 15 µm in a cross section perpendicular to the rolling direction.