Quantification of Microstructure in Plate Steels Using EM Sensor Technology

The ability to non-destructively characterise metal microstructures in-situ during hot processing offers enormous advantages to metals producers for dynamic feedback control and optimisation. Hot rolling and controlled cooling during steel production is used to develop the desired microstructures and currently feedback is achieved via temperature measurement coupled to mill models to indirectly determine microstructure development. The ability to directly measure microstructural parameters on-line therefore represents a significant step forwards; in this project electromagnetic (EM) sensors for on-line inspection are considered. EM sensor signals are sensitive to changes in the relative permeability of a steel sample, where the relative permeability is determined by the steel microstructure, temperature and presence of applied or residual stresses. EM based sensors are already used in the steel industry to assess the condition of strip steels off-line or in mill locations where the strip is cold. Primetals Technology Limited is currently developing an EM system for hot steel inspection, with systems being trialled in the cooling zone after hot strip rolling for phase transformation fraction measurement.
The key to developing this technique in the industrial environment for a broader range of microstructural characterisation is to understand and interpret the meaning of the EM sensor signal in the context of the steel microstructure. This is not straightforward, as it has been demonstrated that a number of microstructure constituents influence the measured signal. For example grain size, precipitates, texture, phase balance and distribution. In addition, when considering plate and sections the influence of non-uniform through thickness microstructure (and temperature for in-situ testing during hot deformation or cooling) needs to be considered. This project will assess the role of microstructure on the EM sensor signals. Factors such as residual stress, through thickness variations in microstructure and/or temperature will be considered.
The project is aligned with the EPSRC research themes of: 'sensors and instrumentation' in supporting the design of new sensor system and interpretation of the signals with respect the steel microstructures; 'materials engineering - metals and alloys' through use of the sensor system to understand the development of steel microstructure during processing; and 'manufacturing technologies' as the sensors will provide dynamic feedback for rolling mill optimization to produce steel products with consistent microstructures and hence properties.