Steel detection in Lemmings is somewhat of a can of worms. Its inaccuracy is well known. Consider, for example, that if you are not standing on steel area and you explode, the explosion will take out all pixels, steel or not. Conversely, if you are standing on steel, the explosion doesn't take out anything, even if there are non-steel pixels nearby.
When I have time, I'll PM you with the way it works in the actual game (well, the DOS version at least; it's of note that some versions, such as the Mac which is hi-res, is known to have slightly different steel detection), and you can decide how exactly you want to follow the game. For reasons I'll mention in the PM, you might also want to actually consider adjusting the steel areas used in the game so that they align exactly with the actual steel plate terrain pieces.