BUDAPEST, Sept 23 (Reuters) - German carmaker Audi AG aims to produce 1.5 million cars a year by 2015, aiming to be the biggest manufacturer in the premium category, Audi said in a statement on Thursday. "With the expansion of our factory in Gyor (in Hungary) we expand our international competitiveness and create the best conditions to achieve our further ambitious goals," Audi Chairman Rupert Stadler said in the statement. Audi announced on Thursday that it would spend 900 million euros