Home Flats, Angles, Squares & Rounds Flat Steel Plain Black Flat Steel Plain Black Flat Steel United Steel stocks and distributes a wide variety of Plain Black Mild Steel Flat Bars. Sizes of Flat Bar range from 10mmx3.0mm up to 300mmx25mm Standard stock lengths are 6.0 metre Sub Categories 3mm 5mm 6mm 8mm 10mm 16mm 20mm 25mm