Key steel is often used in engineering especially for couplings or fixing things to round shafts. By using key steel it means that you are not just replying on the friction from the taper fitting gripping the bar. Key steel is often used with spider couplings, taper bushes and other types of coupling that need a high torque connection to a round steel bar. Key steel is called key bar in some places and is available in sizes from about 2mm to 50mm, it is also available in imperial sizes. Fitting the key steel into the key way is normally fairly easy and the piece of key steel can easily be shaped using an engineers file.