Effect of initial phase transformation temperature on the microstructural characteristics and strength-toughness of bainitic steel

This study investigates the strengthening and toughening mechanisms in bainitic steels with different microstructural constituents. A continuous cooling process with varying initial phase transformation temperatures was employed, and microstructural characterization was conducted using XRD, TEM, and SEM. Based on microstructural features, the samples were categorized into three types: Type I, consisting of pre-formed martensite (PM), lath bainite (LB), and retained austenite (RA); Type II, composed of LB and RA; and Type III, comprising granular bainite (GB), LB, RA, and martensite/austenite (M/A) constituents. With increasing initial transformation temperature, the strength initially decreased, then increased, and finally decreased again, while toughness first increased and then decreased. The reduction in strength was attributed to decreased microstructural refinement and lower dislocation density. The introduction of an appropriate amount of PM improved toughness, whereas the presence of M/A constituents led to a deterioration in both ductility and toughness. Additionally, a positive linear correlation was observed between elongation and RA content. The transformation behavior of RA with different morphologies during tensile and impact processes was examined, revealing distinct coordination mechanisms between RA and LB under dynamic and static loading conditions. This behavior explains how the samples achieved high strength and toughness while maintaining considerable ductility.