Effect of thermomechanical processing on hardenability and tensile fracture of dual-phase steel

Thermomechanical processing of various rolling reductions was applied at intercritical temperature of 790 °C on a low alloy steel containing 0.09% C, followed by quenching in the iced brine solution. Ferrite was grown epitaxially in rolled specimens due to increasing the interfacial area of ferrite/austenite, decreasing the hardenability of the steel. Rolling caused an anisotropic increase in tensile strength and little change in ductility. Up to 20% rolling reduction ferrite was strengthen by work hardening, larger reductions then reduced the strength of ferrite, anisotropically, due to increasing recovery. The increase in strength after rolling was attributed to the subgrain formation in the ferrite. Microvoids were found to nucleate and grow in the necks of tensile specimens by fracture of martensite and by decohesion of the martensite/ferrite interface. The density of the microvoids was an increasing function of strain in the neck. The growth of voids appeared to be interrupted when a critical stress was reached at which the ferrite cleaved. Total ductility (true strain at fracture) and the extent of ductile dimpling on fracture surfaces were determined by the extent of void growth and coalescence prior to interruption by cleavage.