I left my stainless steel frying pan on the hob empty for a while by mistake. It got very hot indeed, I ran it carefully under the tap to cool it down, which took a while - lots of steam. Now, there's a kind of black residue inside, that wasn't there before (it was clean while it was on the hob). I can't remove it even with the toughest of detergents and pan scourers, is it safe to cook with this pan any more?