Online Material Characterisation of Steel Sheets in Hot-dip Galvanizing Lines by Means of Harmonic Analysis of Eddy Current Signals


Cold rolled flat steel is still the classic material for the automotive industry and as quality demands ever increase it becomes necessary to characterize not only singular test samples at isolated positions of the steel strip but to provide immediate information on the produced steel sheet quality over the entire length of the steel strip. Since such a online testing of a product can for obvious reasons only be provided by nondestructive and non-contact testing methods, a measuring system was developed which uses the harmonic analysis of eddy current signals. It is based on the fact that the material condition, that is its arrangement of structure, grain size, alloy composition , dislocation density etc. define not only the mechanical-technological characteristic values like yield strength R p0.2 and tensile strength R m but also the electrical and magnetical properties of a material. This indirect relationship is used to compute e.g. the R p0.2-and R m-values solely from harmonic measuring values using multi-dimensional regression analysis. The online implementation of this system, the Harmonic-Analysis-Coil-Online-Measuring system HACOM, makes possible the monitoring of characteristic material values like the R p0.2-and the R m-values in order to maintain the highest possible product quality. The HACOM-system has been in operation in the two Salzgitter-Flachstahl GmbH SZFG´s hot-dip galvanizing lines for more than 3 years monitoring more than 55000 steel strips from a large variety of steel qualities over their entire length. It reliably provides highly accurate information on the named material characteristics as well as precise indication of changes of the material structure which may be caused by variations of furnace temperature, degree of skin-passing as well as degree of levelling. Introduction: Cold rolled flat steel and galvanized steel sheets are classic materials for the automotive industry and one of the major products of steelmaking plants. To ensure the qualtity of the products it becomes necessary to characterize not only test samples of the beginning and the end of a produced steel strip but to provide information on the produced steel sheet quality over the entire length of the steel strip. Destructive testing systems are obviously not the appropriate means to this task of online quality checks. These systems are not only requiring too much time including a time-lag but are also destroying the test samples during the tests, i.e. in destructive quality checks the entire product is destroyed. Therefore, a nondestructive and non-contact measuring system was …

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