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

Abstract

The interplay between ausforming strain level applied prior to austempering step and bainite transformation kinetic as well as plastic deformation mechanism of nanostructured bainitic steel was essentially investigated. The results indicated that the bainite transformation proceeded rapidly with increasing ausforming strain owing to the added heterogeneous nucleation sites and overall storage energy. Nevertheless, when the ausforming strain further increased to 40%, the acceleration effect became less pronounced attributing to the counteraction mechanism by inhibition effect of retarded bainite growth. Increasing ausforming 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 °C for 40% strain achieved the largest tensile strength of 1900 MPa and the best total elongation 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 transformation 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.

Keywords

• Nanostructured bainitic steel
• Ausforming strain
• Retained austenite
• Tensile performance
• Impact toughness