To check the propagation of brittle fracture in a steel in the generation of an earthquake or the like and to prevent its fracture.
A steel having a compsn. contg., by weight, 0.04 to 0.20% C, 0.01 to 0.50% Si, 0.3 to 1.8% Mn, ≤0.02% Ti, ≤0.060% Al and ≤0.025 Nb, in which carbon equivalent Ceq is regulated to ≤0.40%, and the balance Fe is subjected to hot rolling. By intermediate rolling before finish rolling, cooling is executed till the temp. in ≥50% of the total width of the flange from the center of the flange width toward the direction of both edges and also in the range of 10 to 50% of the total thickness of the flange from the outside face of the flange toward the center of the thickness reaches the Ar3 transformation point or below, it is recuperated till the temp. of the outside face of the flange reaches 600°C to the Ar3 point, and the grain size thereof is regulated to ≤5μm by the average grain size equivalent to a circle. Furthermore, its structure into formed into the one in which the intensity in the (100) plane is regulated ≥two times the intensity in the X-ray plane by a random standard sample.
MABUCHI HIDESATO
YAMAMOTO KOICHI
