Isothermal Recrystallization Behavior of Cold-deformed Martensite in an Ultra-low-carbon Microalloyed Steel

One of the most promising ways to produce a grain-refined microstructure in some steel materials is the thermomechanical processing route of subcritical recrystallization annealing of a cold-deformed martensite structure. In the present study, the microstructural evolutions and the associated recrystallization kinetics under various subcritical annealing heat treatment conditions are explored in an API X120 grade, advanced, High-Strength, Low-Alloy (HSLA) steel with an initial cold-deformed martensite microstructure. The steel sheet was the subject of a conventional cold rolling process for moderate true strain of 60% followed by isothermal recrystallization for different temperature-time combinations. Optical microscopy and scanning electron microscopy were used to characterize the microstructural evolutions, and the recrystallization kinetics was evaluated by hardness measurements with the aid of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) relationship. The experimental results indicated that annealing at 948 K (675 °C) for 18 h is the optimum condition to achieve a grain-refined ferrite microstructure with an averaged grain size of 5.2 µm. The slow kinetics of recrystallization was also revealed by JMAK model as the Avrami exponent was calculated around one for all of the experiments. These observations are rationalized in part by the possible formation of microalloying elements carbides during the annealing process in association with the existence of the inhomogeneously deformed initial microstructure. This results in the appearance of a continuous regime for the recrystallization nucleation along with the sluggish movement of recrystallization fronts.