Most any "cold chisel" found at a fleamarket will work fine cutting hot steel. Hot steel is not too particular. When you want to modify them is when you don't like how they cut.
Exp: you may want a special chisel when you want to slit hot steel and so need a thin chisel that will stay hard even when buried in hot steel. Blacksmiths often use high alloy steels like H13 for making slitters as it has high heat hardness.
Note that hardening and tempering a tool that will be buried in hot steel can be rather a waste of time unless it's a high alloy steel. I generally just normalize my cutters, drifts and punches that are plain carbon steels to make sure there is no brittleness in them.
Making tools into what you need is one of the joys of smithing!
My last chisel I re-worked was an S1 Pharmaceutical punch that I reforged into a slitting chisel and did heat treat as it's an alloy that would profit from it. It is short and it has a depressed section that a pair of special tongs grab with a tong ring to hold it. Short tools don't tend to incline in use and transmit more force into the work. The tongs keep the hand away from the hot metal. I've used it to slit hawk's made from old mining drill shafts using my screw press and it does a lovely job.
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Thomas
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