Dynamic measurement of recovery and recrystallisation in interstitial free steel using a high temperature electromagnetic sensor

Interstitial free (IF) steel grades are used In many applications as they offer excellent formability and resistance to thinning, due to their favourable microstructures of uniform fine ferrite grains. IF steels are produced by hot then cold rolling, followed by an annealing heat treatment to soften the steel through the processes of recovery and recrystallisation. It is desirable to be able to monitor microstructural development during annealing to ensure full recrystallisation is achieved and to determine whether grain growth occurs. Introduction of an accurate in-situ dynamic monitoring system that directly assesses the microstructural state, allowing feedback control, would improve the steel manufacturing process, reducing costs and saving energy and time. This paper discusses the use of a laboratory based high temperature laboratory electromagnetic (EM) sensor to monitor recovery and recrystallisation in IF steels in-situ during annealing heat treatments. Interrupted heat treatments to reveal the microstructure and correlated to the change in EM sensor signal (Inductance) showed that inductance is sensitive to the recovery and recrystallisation processes. It was found that the different rates of recovery and recrystallisation due to annealing at different temperatures (from 375 to 700°C) were clearly revealed by the in-situ inductance measurements.