Variant selection of lath martensite and bainite transformation in low carbon steel by ausforming

Variant selection of lath martensite and bainite transformed in an ausformed low-alloy low-carbon steel is analyzed quantitatively by applying reconstruction method of austenite orientation. It was found that transformation texture having {1 1 1}_α and {0 1 1}_α along compression axis is developed by ausforming both in martensite and bainite transformation. Development of such texture can be explained qualitatively from deformation texture in austenite without taking variant selection into account. However, their intensities cannot be reproduced quantitatively, indicating that specific variants are favorably selected by ausforming. Quantitative analyses of variant fractions revealed that martensite variants whose habit planes are nearly parallel to the primary and secondary slip planes in austenite are formed preferentially. Similar variant selection rule is observed in ausformed bainite while it is not as strong as in ausformed martensite. A preferential nucleation model is proposed for variant selection by ausforming in which variants with habit planes nearly parallel to the primary or secondary slip planes in austenite are nucleated on the microband boundaries in austenite introduced by compression.