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

摘要

Provided is a high-strength hot-rolled steel sheet that is suitable for structural members of large construction/industrial machinery. A steel material having a composition that comprises 0.08 to 0.25% of C, 0.01 to 1.0% of Si, and 0.8 to 2.1% of Mn and in which P, S, and Al are adjusted to appropriate ranges is: heated to a temperature of 1100 to 1250°C; subjected to rough rolling; subjected to finish rolling such that the value found by dividing the cumulative rolling-reduction rate in the partial recrystallization gamma region and the non-recrystallization gamma region by the cumulative rolling-reduction rate in the recrystallization gamma region is from 0 to 0.2; immediately subjected to cooling after completion of said finish rolling and cooled to a cooling stop temperature, which is lower than or equal to 150°C above the Ms point, within 30 seconds from the start of cooling at a cooling rate that is higher than or equal to the martensite-generating critical cooling rate in terms of average cooling rate within the temperature range of 750°C to 500°C; kept for 5 to 60 seconds within the temperature range of ±100°C of said cooling stop temperature; and wound up into a coil shape at a winding temperature within the range of ±100°C of said cooling stop temperature. Thus, it is possible to obtain a hot-rolled steel sheet that: comprises a tempered martensite phase or a low-temperature transformation bainite phase as the main phase; has a structure in which the average grain size of prior gamma grains in a cross section parallel to the rolling direction is 20 µm or less and the average grain size of prior gamma grains in a cross section orthogonal to the rolling direction is 15 µm or less; and has high toughness and high strength with a yield strength YS of 960 MPa or higher and also has excellent bending characteristics.