A chromed and induction hardened C1045 carbon steel rod / bar. You will struggle to cut to length with an ordinary hacksaw so best to use an abrasive cut off say. It's just the surface that has been case / surface hardened so once you are through that, the going gets easier.
These hardened bars are intended for use with linear bearings where if you use silver steel for example, the hardened ball bearings will quickly score the surface of the bar.