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-but table have a significant effect on the steel m icrostructure and hence on final material properties, e.g., yield strength, tensile strength, and ductility. Noncontacting, optical temperature sensor s are typically used to implement feedback control of cooling, but water sp ray and surface emissivity irregularities can adversely affect these sensor s. Ideally, the control of cooling path should account for the progress of dynamic transformation! at required points rather than the strip temperatur e alone. There are several reports describing the use of magnetic sensors t o monitor transformation. These sensors exploit the change in the electroma gnetic properties as the steel progresses through transformation, for examp le the austenitic phase is paramagnetic and the ferritic phase is ferromagn etic below the Curie point. Previous work has concentrated on the operation and design of individual transformation sensors. This paper now describes the 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. (C) 2001 SPIE and IS&T.