The Steel Mill Trail is actually the northern extension of the Bridgeway Trail that runs from the Day's Dam picnic area located in E. 31st Street 41-26.569N and 082-06.492W up to Colorado Avenue at 41-27.698N and 082-07.392 W. There is paved parking at each end of this short trail although the parking lot at Day's Dam is considerably nicer and well-maintained while the parking lot on Colorado Avenue appears to be a park at your own risk lot.
Starting at Day's Dam lot, cross E. 31st Street and follow the trail north through a short patch of woods until you emerge within view of the US Steel plant. The trail then crosses Black River on the first of two bridges and runs between the river and US Steels slag fields where mountains of steelmaking slag are dumped. Continuing north, the trail crosses another nice bridge, ducks into the woods where there is a nice observation area of the river, and finally emerges on Colorado Avenue near a small marina and paved parking area. Round trip from Day's Dam to Colorado Avenue and back is 4.25 miles.
While I would not consider this to be a scenic trail by any means, it is a nice ride and would appeal especially to those interested in the steel industry. Steelmaking IS interesting to me so I actually enjoyed seeing this area.
In my opinion, I would ride this trail only in conjunction with the far more scenic Bridgeway Trail. That is, if I was going to park at Day's Dam to ride a trail, then I'd prefer to ride south along woods and rivers than north along slag heaps...but I'd ride north to extend my ride.
See my review of the Bridgeway Trail for greater elaboration. I'll post some pictures for you.