Black Hollow Section Carbon Steel Q235 Square or Square Tubes are cold-formed and welded by hot rolling, cold rolling, pre-galvanized or stainless steel. In order to form a square steel profile, it is necessary to first form an appropriate parent tube, namely a round steel tube. The rollers used in the circular tube gradually press the circular tube into a square hollow part. This is all done inline. Square and rectangular steel tubes have the advantage of robustness.