I had to go look it up to confirm but, indeed, motorcycle disc brake rotors are made from specially formulated and heat-treated stainless steel:
"Conventionally, SUS420J1 (13%Cr-0.2%C) and SUS420J2 (13%Cr-0.3%C) had mainly been used as rotor materials. However, while these steels have excellent heat resistance, a 2-stage heat treatment process comprising quenching and tempering is necessary to obtain the required hardness, and their corrosion resistance is somewhat inadequate... At present, low carbon martensitic stainless steel, as represented by JFE410DB, is mainly used, because proper hardness is obtained by quenching treatment only... and corrosion resistance is also excellent..."
taken from a JFE Steel technical report: Martensitic Stainless Steel “JFE410DB-ER” with Excellent Heat Resistance for Motorcycle Brake Disks
found at: http://www.jfe-steel.co.jp/en/research/report/012/pdf/012-11.pdf