Microstructural evolution and mechanical properties of 9Mn steel during warm/cold rolling and subsequent intercritical annealing

Abstract

The microstructural evolution of 9Mn steel during warm rolling in the intercritical region and cold rolling, both followed by subsequent intercritical annealing was studied. A significantly different microstructure was obtained by warm rolling and cold rolling, the former contained austenite and deformed ferrite/martensite and the latter consisted of deformed ferrite and transformed martensite, which significantly impacted the microstructure after intercritical annealing. The existence of austenite in warm-rolled sample reduced the elemental content in the deformed matrix, weakening the kinetics of austenite formation during subsequent annealing. In addition, the occurrence of dynamic recovery during warm rolling led to a low storage energy of recrystallization, resulting in a partial recrystallization of ferrite during IA. However, a slightly larger volume fraction as well as more heterogeneous austenite morphology with a wide austenite size distribution was obtained by combining both warm rolling and annealing. These austenite grains could transform to martensite in a more sustainable way, increasing the TRIP-related strain. Consequently, the warm-rolled + annealed steel showed a better combination of ultimate tensile strength (1380 MPa) and total elongation (29.11%).