The development of a multifrequency electromagnetic instrument for monitoring the phase transformation of hot strip steel

This paper describes the development of an instrument to analyze the phase transformations of hot strip steel using an electromagnetic sensor. The sensor exploits variations in the,electrical conductivity and magnetic permeability of the steel to monitor microstructure evolution during processing. The sensor is an inductive device based on an H-shaped ferrite core, which is interrogated with a multifrequency impedance analyzer containing a digital signal processor. Online fast Fourier transform was performed to abstract the multifrequency inductance changes due to the microstructural evolution of the sample. An overview of the instrument and measurements from a range of carbon steel samples are presented. The results verify the ability of the instrument both to monitor the microstructural changes and to reject variations in liftoff distance between the sensor and the hot strip. It is believed that this is the first time this result has been reported with tests on hot steel samples.