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A Study of Recrystallization Behavior of Two Ni-Cu–Containing High-Strength Steels with Different Mn Contents

The Defence Metallurgical Research Laboratory (DMRL) has been investigating several high-strength and high-toughness steels in the past few decades for various defense applications. As a part of this work, two novel low-carbon steels—each containing 4–6 wt% Ni + Cu of similar strength level, one with high Mn (∼1 wt%) and one with low Mn (∼0.3 wt%)—were investigated. In order to optimize the rolling schedules during production, it is necessary to understand the recrystallization behavior of these two steels during hot deformation. Usually, the recrystallization behavior of materials that do not undergo phase transformations (viz. aluminum, copper, etc.) is commonly studied using microstructural techniques for identification of fraction of recrystallized grains. But, as the steels in the present study undergo phase transformation wherein the austenitic structure at deformation temperature is transformed to martensitic/bainitic structure during cooling, it is challenging to track the recrystallization process directly from the microstructure. Therefore, the recrystallization behavior was investigated by conducting double-hit compression tests (DHCTs) using Gleeble. Fractional softening values were determined from DHCT data using various methods, such as the offset method, back-extrapolation method, and mean flow stress method. The mean flow stress method was found to be more reliable and thus was adopted in this study. The effects of different deformation temperature (850°C–1,050°C) and intermediate static recrystallization time (ISRT) (10, 100, and 500 s) on fractional softening were studied. Temperature for 50 % recrystallization (T0.5), determined as the temperature for 50 % fractional softening, was seen to reduce with an increase in ISRT. Steel with lower manganese content showed relatively lower fractional softening values and thus relatively higher T0.5, as compared with that for the steel with higher manganese content, for all three ISRTs.