The limitation on Materials is in the 'Item' Tab. Open the Perk, double-click where it says 'Modifying Tempering', and in the next window click 'Item'. There you will see a list of Keywords. Presumably, if you remove those Keywords the effect will be applied to all items you can temper.
Alternatively, you can open the other Smithing Perks, go to the same Tab, and right-click. That will open a small menu, and you can click 'Copy All Conditions'. Return to Steel Smithing, and right-click again, this time selecting 'Paste Conditions'. Do that for each one.