Abstract: Taking two kinds of low carbon steel with different chemical compositions and same microstrucute of warm-rolled ferrite and pearlite as research objects, the austenite was refined by selecting reasonable deformation parameters during austenitizing, and the refinement mechanism was analyzed by combing the true stress-true strain curve and microstructure. The results show that submicron crystal austenite grains were prepared successfully by thermal-mechanical coupling method. It was found that the cooperative competition of dynamic austenite transformation, dynamic recrystallization of ferrite and work hardening during deformation was crucial for submicron-crystallized austenite.