Hardened steel can be tough to cut. According to this video from Ultimate Handyman, that’s because hardened steel is often harder than the steel in hacksaw blades and similar toothed saw blades.
In fact, your best bet for cutting hardened steel may be an angle grinder with a cutoff wheel.
This video, in addition to showing you how to cut hardened steel with an angle grinder, has some valuable safety tips. These tips include how to cut with an abrasive blade¹ without having the blade shatter or break on you. Of course, you don’t have to worry about your blade shattering, breaking or grenading if you cut with a Safety Blade or Safety Blade Grinder/Cutter, because they’re made of solid steel (up to 14″) to prevent such accidents.
¹ = When we say “abrasive blade”, we don’t mean “abrasive grinding wheel”. Never cut with an abrasive grinding wheel. It will shatter or explode on you if you do.