ROLLED H-SECTION STEEL WITH LOW YIELD RATIO SUPERIOR IN FIRE RESISTANCE, AND MANUFACTURING METHOD THEREFOR

摘要

PROBLEM TO BE SOLVED: To provide a rolled H-section steel having a low yield ratio and superior fire resistance, and to provide a manufacturing method therefor.;SOLUTION: The rolled H-section steel has a composition comprising 0.10-1.0% Mo, preferably, 0.01-0.20% C, 0.6% or less Si, 0.05-1.6% Mn and 0.1% or less Al. The manufacturing method comprises reheating a base steel material having the above composition to 1,000 to 1,350°C; hot-rolling it at a finishing temperature not lower than the Ar₃ transformation temperature to obtain the H-section steel; subsequently cooling the outer or inner surface of a flange to a cooling-stopping temperature in the range of 100 to 650°C with an average cooling rate of 5 to 80°C/s; then stopping cooling; and recuperating it to 200 to 700°C. Thereby, the rolled H-section steel has a flange of which the inner or outer surface layer has a hard layer thereon and the rest surface layer has a soft layer thereon, of which the soft layer includes ferrite with an average particle diameter of 5 to 40 μm, and which has a structure containing a hard phase of 20 to 80% by a volume fraction by an average value in a flange thickness direction; has such a hot strength as to show a yield strength of 176 MPa or higher at 600°C while maintaining a low yield ratio of 80% or lower; and consequently shows superior fire resistance.;COPYRIGHT: (C)2006,JPO&NCIPI