Although I'm not such a big fan of industrial sites, and I don't really know a lot about the process of iron- and steel-making, I did enjoy my trip to the Völklingen Ironworks. Not only was there an interesting exhibition on the Inca in a part of the complex when I visited in August 2017, you can also wander around the site freely and explore all nooks and crannies on your own. After some walking around, you will come to the area where hard hats are required to climb various staircases up to the viewing platform 50 or so metres above ground - this is really the highlight of any visit. Völklingen is just a 10-minute train ride from Saarbrücken, and the ironworks is maybe a 5-minute walk from the station. You can't see it very well from the train if coming from Saarbrücken, but it should be perfectly visible if coming from the other direction (e.g. Trier).