I think you schould implement new material "Carbon" witch either could be obtained by mineing COAl (and further procedures), or like gems and minerals (Carbon is more common than water btw, so pls not), or hundred other solutions cuz carbon is in almost everything,
but most realistic way is by just heating it and cooling it, cuz steel was made by proces of carbonizin carbon from charcoal on witch iron was placed would mix with Iron and thats how you get steel. high quality steel schould be pain in the ass only 90+ smelter can produce that
Options are limitless smelting have so much space to work with.
You schould add more alloys and much more materials,