Microstructural Characterization of Dual-Phase Low-Carbon Steel as Function of Inter-critical Annealing

Ferritic-martensitic low-carbon steels can be produced by inter-critical treatment with fast cooling, and are commonly referred to as dual-phase steels. The soft ferrite with hard martensite provides higher mechanical resistance than normalized steel. In this work, specimens from a low-carbon steel with 0.158%C were submitted to inter-critical heat treatments in three temperatures (770 °C, 750 °C and 730 °C) followed by rapid cooling. The martensite and ferrite phases were quantified by light optical microscopy and electron backscattered scanning diffraction using band slope, with good match between the results from both techniques. Martensite derived from the austenite phase present in the inter-critical annealing temperature, which can be predicted by computational thermodynamics. In agreement, the martensite volume fraction increased with the increase of the inter-critical annealing temperature. Fine carbides were observed by high-resolution scanning electron microscopy in the ferrite phase, for all heat treatment conditions. Kernel average misorientation (KAM) and geometrically necessary dislocation (GND) analysis were also calculated by electron backscattered scanning diffraction. Impact toughness, hardness and microhardness were determined, and the results were correlated to the microstructural features. The mixture rule was not verified with the microhardness tests in specimens treated at 730 °C and 750 °C, but matched the experimental values for the specimen treated at 770 °C. The effects of tempering at 300 °C on the microstructure (precipitation, KAM and GND) were investigated and correlated to the mechanical properties.