The role of fibrous morphology on the Charpy impact properties of low carbon ferrite-bainite dual phase steel

Primarily ferrite-pearlite microstructure having coarse ferrite grain size (24 µm) and high pearlite fraction (42%) offered YS ∼ 575 MPa with poor impact properties such as, upper shelf energy (USE) of only 30 J and ductile brittle transition temperature (DBTT) as high as 27 °C in an industrially hot-rolled plate of 0.25 wt% C steel. In order to improve the strength along with the impact properties by developing ferrite-bainite microstructures, two different types of heat-treatments, namely step-cooling (SC) and intermediate cooling (IC) treatments, were carried out on the as-received material. Significant improvement in strength along with the impact toughness (with YS of 740 MPa, USE of 222 J and DBTT of − 57 °C) has been achieved by developing fibrous microstructure, with alternate thin-films (2-4 µm thick) of ferrite and bainite through intermediate cooling (IC) treatment. Fine film-like structure with large orientation difference across the ferrite-bainite interface boundaries not only increased the strength but also resulted in frequent deflection in cleavage crack propagation path which improved the low-temperature impact toughness and reduced the DBTT.