As you may have already guessed, hot rolled steel is steel that is in fact rolled while hot.
Hot rolled steel profiles are manufactured by heating a particular steel alloy until it is red-hot at which point the molten material is squeezed through rollers (aka: mills) which impart a particular shape and size (just like your old Play-Doh Fun Factory).
Through this process of heating and milling the steel takes on a mill scale finish. Bluish-black in color with an uneven & flaky texture, mill scale is iron oxide that coats the surface of hot rolled steel. Some folks like the industrial look of the mill scale and there are ways to protect that character from corrosion with a topcoat finish. If your project requires a finish other than the factory mill scale- paint, powder coat, blackening, etc- the mill scale will have to be removed. Some methods we have employed to removed mill scale include pickling and oiling, sand blasting, and/or DA sanding prior to applying a final finish.
Hot rolled steel is readily available in a wide variety of shapes and sizes– from flat bar to pipe, tubing, sheet, channel, angle, wide flanges and bar stock– the list goes on. One thing to note about hot rolled steel forms is that similar to the aforementioned Play-Doh, it’s near impossible to get a nice crisp, perfectly square edge. Squeezing the red-hot steel through the rollers produces forms with slightly eased edges which can be cut or ground square, but if square is your goal you may want to consider cold rolled steel.