| 摘要 |
PROBLEM TO BE SOLVED: To prevent the development of corner-welding crack without needing straight-cutting and beveling, and to obtain a box column having high toughness at HAZ(heat-affected zone) by welding a flat steel as-roll and assembling to manufacture the box column for building structure. SOLUTION: In the flat steel, since the center segregation is separated to the center side from the edge surfaces in the width direction, a segregation part exposed on a beveling surface is reduced in comparison with a straight-cut steel plate and the sensibility of the corner-welding crack is extremely lowered. It is desirable that both edge sides in the width direction of the flat steel are roll-formed into a bevel-shape for corner welding. Chemical composition of the flat steel is desirable to be the following composition by wt.%. This is composed of 0.08-0.18 C, <=0.6 Si, 0.8-1.8 Mn, <=0.015 P, <=0.008 S, 0.002-0.05 Al, 0.03-0.15 V, 0.005-0.015 N and one or two kinds among 0.1-0.7 Cu, 0.05-0.6 Ni, 0.05-0.5 Cr, 0.03-0.2 Mo, 0.005-0.04 Nb, 0.003-0.02 Ti, 0.001-0.01 Ca and 0.001-0.01 REM.
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