Characterisation of bimodal grain structures in HSLA steels

High strength low alloy (HSLA) steels can show varying degrees of bimodality in their grain size distributions following rolling and also in the reheated condition, which can have significant effects on their toughness. Current methods of measuring bimodality work well for distinguishing between structures with significantly different levels and/or types of bimodality. However, these are not as good at consistently quantifying small differences between microstructures of, for example, steels processed under different conditions. This paper suggests a new method to construct the grain size distributions (in area-percent versus linear scale of equivalent circle diameter grain size) and to quantify bimodality in HSLA steels based on two parameters (peak height ratio, PHR, and peak grain size range, PGSR) measured from such distributions. The parameters were found to be simple, easy to measure, less subjective and more consistent for these steels compared to the standard and non-standard parameters used in the literature.