"Steel mill articles" refers to large bulk and generally unfinished raw metal. Billet, pipe, round or square bar stock, wire, sheet and railroad rail. (Not sure if this includes powderized metals like those used in press-sintering or 3D-printing.)
It seems to mention steel a lot, but little or nothing about aluminum, nickel or other common industry metals, oddly.
"Parts" are different because value has been added to them by manufacturing.