Steel
To the dwarves, primarily a category of magnetic iron materials that serve a purpose, giving the metal a mechanical benefit: harder, more workable, more resistant, or a combination of these.
Dwarven steel
Most steels vary somewhat in carbon, so modern categories are only a rough approximation of the dwarves' own. Steel is primarily a category that dwarves recognize as magnetic, and they have no use for iron alloys that serve no purpose. Hardcast is really iron, but its silicate inclusions give it useful mechanical properties, and the dwarves call it a kind of steel. What they call iron is mild steel; they do not seek to produce or value true pure iron, which has no qualities they appreciate.
Grades
Medium steel is weapon steel, the material for common use and for performance Actinate-clad steels. It is also the preferred base for silver-plating and for enhancement with gleam. Dwarves know of spring steel, usually made from star-steel by a special heat treatment, but it is largely a curiosity. Star-steel and the rumored nickel steels are rare and somewhat experimental.
Limits
Dwarves have not yet reached the ability to make Damascus steels, though they have most of the metallurgical skills to do so.
See also: Hardcast, Star-steel, Nickel, Actinate, Gleam