AFAIK no manufacturers (even Nissan) warrant rotors. They are a consumable and are subject to misuse and abuse.
Also, I don't believe that you can 'turn' slotted rotors.
I agree with what the others have said....this sounds like pad deposit build-up. Another driving technique to consider is to keep your foot off the brake after you have come to a complete stop. This will reduce the heat soak and deposit build-up, especially after hard driving.