Variant Pairing of Lath Bainite and Martensite in an Ultra-High-Strength Steel

The mixed structure of lath bainite and martensite was obtained by isothermal transformation tests at 698 K, 673 K and 623 K in ultra-high-strength steel. The interweaving mode of lath bainite and martensite was revealed by colored metallography and band contrast maps based on Gaussian fit. The crystallographic characteristics were analyzed in terms of variant pairing. The twin-related V1/V2 variant pairs were found in the region with high band contrast (BC) values, inferred as lath bainite. While in the low-BC value region, which was inferred as lath martensite, besides the dominated twin-related V1/V2 variant pair, V1/V4 variant pairs with low-angle grain boundaries were occasionally revealed. According to the classification by Takayama et al., both variant pairings of lath bainite and martensite in this work correspond to a type II mode. The present work has confirmed, even with complex microstructure, i.e., mixed lath bainite and martensite were formed depending on the processing route, bainite and martensite still follow their own rules of variant pairing.