Nether the less, the discs are cast iron.
Having had a spot of braking bother with some guzzi discs,
where the discs actually snapped in half !,
I can vouch that the discs are indeed (brittle) iron.
O, and iron discs have a better coefficient of friction than steel, graphite content not withstanding, or so many auto braking applications wouldn't use it...
Its a strange form of cast iron that would actually mark a sheet of clean white paper. You may get a smear of rust if the disc hasn't been used recently.