High-strength hot rolled steel sheet with excellent bendability and low-temperature toughness, and method for manufacturing the same

A method of manufacturing high-strength hot rolled steel sheets with excellent bendability and low-temperature toughness, comprising:

• subjecting a steel to a series of sequential steps including a heating step of heating the steel,
• a hot rolling step of subjecting the heated steel to hot rolling including rough rolling and finish rolling,
• a cooling step, and
• a coiling step, thereby producing a hot rolled steel sheet,
• wherein the steel has a chemical composition including, in mass %,
• C:
• 0.08 to 0.25%, Si:
• 0.01 to 1.0%,
• Mn:
• 0.8 to 2.1%, P:
• not more than 0.025%,
• S:
• not more than 0.005% and Al:
• 0.005 to 0.10%,
• the balance comprising Fe and inevitable impurities, and wherein
• the heating step is a step in which the steel is heated to a temperature of 1100 to 1250° C.,
• the rough rolling in the hot rolling step is rolling of the steel heated in the heating step into a sheet bar, and the finish rolling in the hot rolling step is rolling of the sheet bar such that a cumulative reduction ratio in a partially recrystallized austenite region and a non-recrystallized austenite region divided by the cumulative reduction ratio in the recrystallized austenite region becomes 0 to 0.2,
• the cooling step includes a cooling treatment in which cooling is initiated immediately after completion of the finish rolling 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 initiation of the cooling, the average cooling rate in a temperature range of 750° C. to 500° C. being not less than a critical cooling rate for occurrence of martensite formation, and a holding treatment in which after the cooling treatment is terminated, the steel sheet is held at a temperature of the cooling termination temperature±100° C. for 5 to 60 seconds, and
• the coiling step is a step in which the steel sheet is coiled into a coil at a coiling temperature of (cooling termination temperature±100° C.).