MANUFACTURING METHOD OF HOT ROLLED STEEL SHEET FOR RECTANGULAR STEEL PIPE WITH LOW YIELD RATIO AND MANUFACTURING METHOD OF RECTANGULAR STEEL PIPE WITH LOW YIELD RATIO

摘要

PROBLEM TO BE SOLVED: To provide a manufacturing method of a hot rolled steel sheet suitable as a material for a rectangular steel pipe having sufficient strength, low yield ratio and low temperature toughness even in the case that the thickness exceeds 25 mm.SOLUTION: The manufacturing method of the hot rolled steel sheet according to the present invention, in which the hot rolled sheet is obtained from a steel stock having a specific component composition by applying rough rolling at a rough rolling finish temperature of 1150-950°C after heating the steel stock to 1100-1300°C, and finish rolling at a finish rolling start temperature of 1100- 850°C and a finish rolling finish temperature of 900-750°C, comprises: a hot rolling step having a pre-finish rolling cooling step, ia hot rolling step having a pre-finish rolling cooling step, which includes cooling of both surfaces, performed between each pass of the rough rolling and/or after finishing of the rough rolling and before start of the finish rolling, to a cooling stop temperature of 600°C or lower at a cooling rate that average cooling rate from start of cooling to stop of cooling is not less than 30°C/s, and subsequent temperature recuperation; a cooling step after hot rolling in which cooling is performed to a cooling stop temperature of 580°C or lower measured in the center of the thickness at an average cooling rate from start of cooling to stop of cooling of 4-25°C/s; and a coiling step in which coiling at a coiling temperature of 580°C or lower and subsequent cooling in air are performed.SELECTED DRAWING: None