You only gain skill from smelting after so many levels (up to level 20 i believe). I suspect your above that now and must use your iron ingots to gain skill.
At 76 I needed 330 attempts to gain skill. How many times you attempt is more relative to skillgain than how many sheets you use. If it takes you 330 tries making something that takes 5 sheets will mean you use 1650 sheets till your next level. Maybe they changed it since then, but thats my experience.
well considering that the newest thing I can make is flame helmet, and the one before that was steel hats: I should be able to gain skill from smithing steel hats.
Making helments is your best bet. It makes no sense that some thing that takes 4 ingots gives you less skill than some thing that takes 1. Visored helmets dude....