Bainite transformation behavior, microstructural feature and mechanical properties of nanostructured bainitic steel subjected to ausforming with different strain

The interplay between ausforming strain level applied prior to austempering step and bai-nite transformation kinetic as well as plastic deformation mechanism of nanostructured bainitic steel was essentially investigated. The results indicated that the bainite transforma-tion proceeded rapidly with increasing ausforming strain owing to the added heterogeneous nucleation sites and overall storage energy. Nevertheless, when the ausforming strain fur-ther increased to 40%, the acceleration effect became less pronounced attributing to the counteraction mechanism by inhibition effect of retarded bainite growth. Increasing aus-forming strain could effectively refine bainite sheaves and minimize the dimension of blocky retained austenite, yet strengthen undercooled austenite, leading to a decreased volume fraction of bainitic ferrite at the termination of transformation. Ausforming strain had a positive effect on mechanical properties, particularly, the sample ausformed at 300 degrees C for 40% strain achieved the largest tensile strength of 1900 MPa and the best total elonga-tion of 24%. This enhanced strength-ductility synergy mainly ascribed to the benefit of the refined bainitic ferrite lath and the increased dislocation density together with transforma-tion induced plasticity effect. Unfortunately, the apparent variant selection brought about by ausforming could offset the increased impact toughness resulting from microstructure refinement when ausforming strain was higher than 10%, which leaded to an intensive deterioration of impact property. (C) 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)..