For this achievement you need to win all the races on at least steel difficulty.
This is the easiest difficulty of the races and to be honest I would directly play the races on silver or bronze. If you are an experienced racer you can race on platinum and then tune down to gold for the more difficult races if you want to go through the races as quickly as possibe. And then go back to the ones you skipped.
But racing just on steel is maybe a waste of time. If you manage to finish all the races on a higher difficulty this one will unlock as well.