Local strength of bainitic and ferritic HSLA steel constituents understood using correlative electron microscopy and microcompression testing

HSLA steels with different polygonal ferrite and granular bainite contents resulting from two different cooling rates were investigated. Micropillar compression tests, electron channeling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) experiments were performed to reveal microscopic strength differences and their origin. The obtained results indicate that a higher cooling rate caused a smaller granular bainite substructure size and a higher dislocation density for both ferrite and bainite. In addition, the critical resolved shear stress (CRSS) values for both phases were found to be higher for the faster cooling process. This is ascribed to the increased dislocation density for faster cooling rather than the grain size as will be discussed in the manuscript. Interestingly, the macroscopic yield strength can be closely estimated by the CRSS obtained from micropillar compression considering the corresponding phase fractions. The achieved results can be used in future as input variables for crystal plasticity models.