Influencing TRIP threshold and variant pairing through minor cold and cryo-rolling in bainitic steel

Abstract

An attempt has been made to compare the effects of small reduction cold and cryo-rolling on the microstructural evolution and stability of retained austenite in a 3rd generation advanced high-strength bainitic steel. The steel was heat treated and isothermally held at 300 °C for 2 h above M_s temperature to obtain bainitic microstructure. The heat-treated steel was subsequently subjected to cold-rolling and cryo-rolling (10 % reduction in thickness). The substructure of bainite consists of packets, blocks, and sub-blocks. Both the crystallite size and micro-strain of bainitic ferrite shows a decreasing trend after rolling compared to the undeformed condition. However, cryo-rolling induces higher micro-strain in retained austenite, which enhances its mechanical stability requiring higher stress to trigger the transformation induced plasticity (TRIP) effect, thus contributing to increased overall strength. Crystallographic variant analysis showed an increase in the boundary density and frequency of specific V1-V3(V5) (high angle block boundaries) and V1-V4 (sub-block boundaries) variant pairs after cryo-rolling following the K-S orientation relationship. While the heat-treated, undeformed state exhibited all the variants without preference for any particular variant pair. The results suggest that the mechanical properties of the steel after minor cryo-rolling are significantly influenced by crystallographic variant pairing and microstrain. Additionally, minor cryo-rolling proved to be superior to cold rolling in bainitic steel as it increased the threshold strain required for the TRIP effect during rolling.