Imaging the transformation of hot strip steel using magnetic techniques

In the production of steel strip, the temperature distribution and cooling rates along the mill run-out table have a significant effect on the steel microstructure and hence on final material properties, such as yield strength, tensile strength and ductility. Consequently, demands for improved product consistency and a greater diversity of sophisticated steel grades have increased the requirements for tighter control of these process conditions. Non-contacting optical temperature sensors are typically used to implement feedback control of cooling. Unfortunately water spray variations and surface emissivity irregularities can adversely affect these sensors. In addition, temperature is only used as an assumed indicator of microstructure and only the surface of the steel is measured. Ideally, the control of cooling path should take account of the progress of dynamic transformation at required points rather than the strip temperature alone. There are a number of reports describing the use of magnetic sensors to monitor transformation. These sensors exploit the change in the electromagnetic properties as the steel progresses through transformation, for example the austenitic phase is paramagnetic and the ferritic phase is ferromagnetic below the Curie point. Previous work has concentrated on the operation and design of individual transformation sensors. This paper now describes the novel use of an array of electromagnetic sensors to image the progression of transformation along a sample steel block on a pilot scale industrial mill. The paper will describe the underlying physical principles, the design of the system and present images showing the progress of transformation along one surface of the sample.