A method of fabricating box section from steel with walls of non-uniform thickness along its periphery. A starting product in the form of round tubing with a wall that is uniformly thick along its circumference is manufactured and rolled into box section with walls of uniform thickness along its periphery. A temperature difference is generated between the flange midsections and the web midsections of the box section with walls of uniform thickness along its periphery such that the flange midsections are at least 600.degree. C. hotter than the web midsections, the flanges are edged and thickened peripendicular to the longitudinal axis of the box section, with the edge length being shortened, by rollers, and the box section is finally cooled with air and/or water subject to standardization conditions.