You can easily make steel with a lead burning box, it just takes ages. 
The Crucible only cools down over time if you dont actively heat it up, so even if you inserted 1HU/t you could eventually reach thousands of degrees, until the thing inevitably melts down.
All you need to have is the Crucible at the right Temperature (see NEI) and all the ingredients inside of the Crucible at the same time. Some Ingredients like the aforementioned Air will escape the Crucible if not used up instantly.
If you hypothetically stopped blowing Air halfway through conversion you might get weird fractions of Steel that cant fully convert to Nuggets, since a 1000th of a Steel Ingot is typically not something that has an Item form.