Grain Refinement and Toughening of Low Carbon Low Alloy Martensitic Steel with Yield Strength 900 MPa Grade by Ausforming

Abstract

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-to-brittle 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.