Instead of off-line quenching and tempering(QT),on-line ausforming(non-recrystallization controlled roling)and direct quenching(DQ)was employed to improve the toughness of low alloy steel with yield strength of 900 MPa Grade.Low carbon content ensured a high level of upper shelf energy,while the fine-grained martensite substructures decreased the ductile-tobrittle transition temperature and compensated the strength loss due to carbon reduction.Two mechanisms for the refinement of martensite substructure were proposed: one was the austenite grain refinement in the thickness direction,and the other was the self-accommodation of martensite variants due to austenite grain hardening.More than 200 J of Charpy V-notch impact absorbed energy at 233 K was obtained in the industrial ausformed plate,which was about the double in the traditional QT steel with the same strength grade.