| 摘要 |
PROBLEM TO BE SOLVED: To provide fire resistant rolled shape steel excellent in resistance to local buckling caused in the beam member of a steel structure and to destruction caused by welding defects in the beam edges or the like in case of a big earthquake. SOLUTION: The composition of this steel is composed of, by weight, 0.05 to 0.18% C, <=0.6% Si, 0.5 to 1.6% Mn, 0.2 to 0.7% Mo, <=0.020% P, <=0.015% S, 0.01 to 0.05% Al, <0.005% Nb, <=0.0060% N, <=4 ppm H, and further, Ceq: <=0.40%, and the balance substantially Fe with inevitable impurities, and the metallic structure of a flange is formed of the mixed one constituting of a ferritic phase as a soft phase an d a hard phase containing a bainitic phase, and in which, the volume ratio of the soft phase is controlled to >50 to 80%, the average grain size to >=10μm, the aspect ratio of the hard phase is controlled to <=3, yield elongation to 0.8 to 3.0%, yield ratio to <=70%, and strain hardening index (n value) in nominal strain of 2 to 5% is controlled to >=0.20.
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