That's odd, as steel works fine for me.
First, can you double check that you have researched steel? Just like normal machines, the recipe won't appear until you've researched it.
Next, make sure you're clicking on one of the big furnaces and on the Intermediate Products tab? Steel should be right next to copper and iron if you've researched it.
If it isn't one of those two, my next best guess is that it is some other mod that is messing with it. Can you try deactivating mods until it starts working again to try to figure out which mod my mod might be in conflict with?