Measurement and modelling of the EMspec® sensor response to phase transformation in 2.25 Cr-Mo steel


Electromagnetic (EM) sensors are sensitive to changes in permeability and resistivity, which in turn are determined by microstructure, composition and temperature. Recently a new industrial EMspec® system, consisting of three sensor heads positioned at different points along the run-out table (ROT) has been installed in the Tata Steel Hot Strip Mill in IJmuiden to monitor the phase transformation in strip steels during cooling on the ROT prior to coiling. Another EMspec® system (consisting of one sensor head of identical design) has been installed in a lab based, bespoke furnace and run out table cooling system. This has the option to cool the reheated steel samples using either natural air, forced air or water sprays whilst continuously monitoring the microstructure of the steel using the EMspec® signal. A full 3D FE model has been developed to allow the EMspec® output (Zero Crossing Frequency, ZCF) to be related to the steel microstructure (for example phase fraction) using the known relationships between permeability and resistivity with microstructure and temperature. Using the lab based EMspec® system, the transformation of a 2.25 Cr-Mo steel from austenite to bainite has been monitored during cooling on the ROT. This paper reports on the relationship between the ZCF and transformation fraction during cooling (obtained independently using dilatometry with the cooling rate measured for the 2.25 Cr-Mo steel plate on the ROT during natural cooling) and compares the 3D FE model results for the predicted ZCF with the measured values. The EMspec® ZCF clearly shows the transformation and there is excellent agreement between the predicted and measured ZCF values. The results have shown the potential for the EMspec® system with model to be able to map full steel transformation below the Curie temperature.