I'm guessing that everyone who models a steel mill on their layout does a lot of compromising and simplifying. These complexes are huge with all kinds of trackage, buildings, and equipment. Robin mentioned some of the range of rolling stock.
I was originally planning to only model outgoing rolled steel but grew dissatisfied with the way I was simulating loads in 40 ft gondolas. They didn't look the way I wanted. I really wanted to spend a fortune on covered coil cars but decided against it.
Then I decided to move my blast furnace, which had been just a back drop structure, in front of some tracks to hide them and break up the appearance of the loop of track the whole mill was located in. The main line tunnels through part of one building but this is in the back and not obvious since the entrances are visually blocked by other structures...it was a way of making the best of available space.
I'm not attempting to model the operation of the blast furnace since I don't have any of the specialized cars to do this nor have I modelled a "high line" where coke and ore is deposted. What I did instead was to use the three track yard as the "materials yard" where my numerous open hoppers of coal can be spotted on track 1 and 2 while my gondolas of scrap metal are set on track 3. I've made a large pile of scrap next to the track to suggest some unloading there. I also placed some dummy tracks in the corner of the layout that go into a building to suggest there is more activity. I see no reason why you can't have a single spur. Decide what sorts of cars you want to spot there and let your creative imagination do the rest...you could even hide the end of the spur behind a building to make it look like it is the lead track to all the rest of the complex track of the mill. Here's my track plan...